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13 Charge Carrier Recombination in Bulk Heterojunction Organic Solar Cells Gytis Juška and Kęstutis Arlauskas Vilnius University Lithuania 1. Introduction Photovoltaic phenomenon was first observed by E. Becquerel (Becquerel) in 1839. He observed the electric current-lit silver electrode, immersed in the electrolyte. In 1894, taking advantage of the observed photoconductivity phenomenon in amorphous selenium the semiconductor solar cell was developed. The very first silicon p-n junction solar cell was made in 1954, energy conversion efficiency of which was 6% and the energy price $200/W did not seem promising for wide application. Later, the development of satellites needed to provide sustainable energy sources and the cadmium sulfid, cadmium telluride, gallium arsenide and more efficient solar cells of other materials were created. The first solar cell breakthrough was something like of the 1970 year, feeling the lack of oil, which oncreased interest in alternative energy sources. The basic raw materials, in addition to crystalline silicon, a polycrystalline silicon, were also amorphous silicon and other, suitable for thin solar cells, materials. Although, due to the high cost of these energy sources, extracted energy was only a small part of total energy production, but the lending spread as energy sources in various areas of small devices such as mobile phone, calculators, meteorogical instruments, watches and so on. A solar powered cars and even solar powered aircraft were constructed. Major Solar cells used for the purification of salt water, as well as supply power to isolated objects: mountains, islands or jungle living population. The second and much greater solar energy use breakthrough occurred in the first decade of the twenty-first century. This is caused by the earth's climate warming due to the increasing threat of thermal energy and the increasing CO2 in the atmosphere. Many governments in many ways stimulated the solar energy lending. Germany in the decade from 1994 to 2004, installed as much as 70 times more solar energy equipment, and now is installed more than 1GW: produced over 3TWh energy, which cost around 0.5 €/kWh. In Japan solar power energy is less costly than the heat. The main price of solar energy is caused by the installation consts - ~ 1€/W. Till 2004 there have already been installed over 1GW, while in 2006, the world's installed 6.5 GW. In 2007, the European Union in the fight against climate warming threat committed by 2030 to achieve that 25% of the total energy from alternative sources, mainly from the Sun. It should be around 1200 GW, the cost should not exceed 0.1 €/kWh. Another reason for the needed alternative energy sources is projected oil and gas resource depletion. Source: Solar Energy, Book edited by: Radu D. Rugescu, ISBN 978-953-307-052-0, pp. 432, February 2010, INTECH, Croatia, downloaded from SCIYO.COM www.intechopen.com 294 Solar Energy Crystalline silicon still remains the unrivaled leader in the development of solar cells. However, the demant of renewable energy sources stimulated a search for a new, low-cost technologies and materials. Hydrogenated amorphous silicon (a-Si:H) has long been regarded as one of the most promising materials for development of cheap, lightweight and technologicall solar cells. However, a Si:H solar cells degraded in high intensity-light. Thus, forward-looking, more efficient microcrystalline (μc-Si:H) and nanocrystalline silicon (nc-Si:H) solar cells began to compete successfully with a-Si:H. The first organic materials were investigated for more than a hundred years ago and for a in 1977 A. J. Heeger, A. G. MacDiarmid and H. Shirakawa showed that the π-conjugated long time the widest application, in scope of optoelectronics, was electrography. However, polymers can be doped, and change the properties of substances. This work demonstrated 2000 was awarded the Nobel Prize. During the period from 1977 on the base of π-conjugated the possibility use polymers to create optoelectrical devices, resulted in huge interest and in effect transistors, sensors, photodiodes, etc. On 1993 - 2003 years π-conjugated polymers polymers has been built a number of electronic and optoelectronic devices: diodes, field have been investigated in order to create a light-emitting diodes (OLED) and their systems, and these studies culminated in the creation of a colour OLED matrix, which is adapted to different types of displays. Recently, organic polymers mainly involved studies of organic solar cells and other organic electronics appliances, effectiveness of which is determined by the drift and recombination of charge carriers. In order to develop efficient solar cells it is necessary the maximum possible the light absorption, the carrier photogeneration quantum efficiency, and that all photogenerated carriers be collected in a solar cell electrodes. The collection of charge carriers depends on their mobility and recombination. Thus, the investigations of carrier mobility and their density dependencies on the electric field, temperature and material structure are essential for the formation of understanding of charge carrier transport in these materials, which is essential to find effective new inorganic and organic materials and to development of new optoelectronic structures. One of the main factors limiting efficiency of organic solar cells (OSC) is charge carrier recombination. In crystals, where the carrier location uncertain, recombination is caused by the probability to transfere energy: or emit photon - radiation recombination, or to another electron - Auger recombination, or induction phonons through the deep states. The latter depends on the density of deep states. In disordered structures, with a lot of localized states, should be very rapid recombination, but there recombinationis caused by the meeting probability of electron and hole in space, as the only their meeting at a distance closer than the Coulomb radius causes their recombination (named Langevin), likely as gemini recombination. It is valid only if the energy dissipation or jump distance is less than the Coulomb radius. Thus, the Langevin bimolecular recombination is ordained by the mutual Coulomb attraction drift time, because under this attraction electron is moving toward the nearest hole, while at the same time, due to diffusion, with equal probability in any direction. The Langevin recombination time can be expressed as: εε 0 τL = ∫ = r ( μ n + μp )F e( μ n + μp )n dx (1) 0 Here μn , μ p are electron and hole mobility, respectively; F = e / 4πεε 0 x 2 is strength of Coulomb electric field; n is density of charge carriers ( 1 / n = 4π r 3 / 3 ), and r is a mean www.intechopen.com Charge Carrier Recombination in Bulk Heterojunction Organic Solar Cells 295 recombination coefficient BL = e(μn + μp) /εε0 it is clearly seen that recombination is caused distance between electrone and hole. Thus, from the expression of Langevin bimolecular by the features of charge carrier transport. In bulk heterojunction organic solar cells the reduced Langevin recombination is observed. In this work we describe methods of investigation of charge carrier recombination in disordered structures, where stochastic transport of charge carriers complicates interpretation of experimental results: integral time of flight (i-TOF) (Juška et, 1995), using of which allows easily estimate the temperature dependence of recombination coefficient; charge carriers extraction by linearly increasing voltage (CELIV) (Juška et, 2000, a), which allows independently measure relaxation of density and mobility of photoexcited charge carriers; double injection current transient (DoI) (Juška et, 2005; Juška et, 2007), which is additional method of investigation of charge carrier recombination and, which allows to measure dependence of recombination coefficient on electric field. In this study we represent how using current transient methods may be cleared up the materials. The microcrystalline silicon and π-conjugated polymers have been investigated as features of charge carrier transport and recombination in disordered inorganic and organic a typical inorganic and organic material. 2. Investigation methods The disordered structure of material causes that mobility of charge carriers is low, because their motion is slowed down by the interaction with spectrum of the local states. Thus, the classical investigation methods: the Hall and magnetoresistance measurements are invalid. The carrier transport in disordered inorganic and organic materials, conductivity (σ) of which is low, is studied using time-of-flight (TOF) method. However, the conductivity of many π-conjugated polymers is high and does not fulfill the latter condition. Thus, for their investigation has been adapted and refined microcrystalline hydrogenated silicon (μc-Si:H) used the extraction of charge carriers by linearly increasing voltage (CELIV) method. The latter method allows to investigate the transport properties of charge carriers both in conductive and low conductivity materials. For investigation of charge carrier transport and recombination the double injection current (DoI) transient method is promising as well. 2.1 TOF method Time-of-flight method is widely used for investigation of transport, trapping-retrapping and recombination of charge carriers in disordered materials and structures. This method is applicable only for investigation of low conductivity materials, i.e. where the Maxwell relaxation time exceeds the duration of transit time (ttr) of charge carriers through the interelectrode distance (d): εε 0 τσ = >> ttr = d2 σ μU (2) TOF method is based on the current transient measurement when photogenerated of the same sign charge carriers is moving in the electric field (E) created in the interelctrode distance (d) of the sample and during a drift time (ttr)the package achieves an opposite mobility (μ), trapping (τt) and lifetime of charge carriers (τ) in low conductivity (τσ > ttr) electrode. The simplicity and efficiency of method meant that it is a widely used for study of www.intechopen.com 296 Solar Energy materials. Low conductivity of material ensures that during the drift of photogenerated charge carriers through interelectrod distance the density equilibrium charge carriers will be too low to redistribute the electric field inside the sample, and the electric field will be steady at the moment of charge carrier photogeneration, i.e. RC < tdel < τ σ ttr (here R is total resistance of measurement system and sample elctrodes, C is geometric capacitance of sample). TOF method, dependently on amount of initial injected charge (Q0) and, also, on characteristic time RC of measurement system, is devided into a number of regimes. Small charge drift currents (SCDC). This regime is ensured when an amount of photogenerated charge is much less than an amount of charge on sample electrodes at given voltage (U0), i.e. eL = Q0 << CU0. Here L amount of charge carriers photogenerated by pulse a. current (diferencial) regime (ttr > RC). In case of strong absorption of light (αd >> 1, α is of light. In this regime there are a few cases: absorption coefficient) and nondispersive transport, the shape of pulse of photocurrent transient is close to rectangular, duration of which is ttr (Fig. 1a, L = 0,3), and form the (αd << 1), charge carriers are photogenerated in the bulk of sample, thus, the shape of area of current transient the Q0 can be estimated. In case of weak absorption of light photocurrent pulse is triangular, which's duration is ttr, and area is equal Q0/2. The dispersive transport of charge carriers, due to dependence of charge carrier mobility on form which will be possible to estimate ttr (even if αd >> 1). In this case, if the current time, causes that pulse of current transient did not demonstrate obvious break points, transient is represented by a double-log scale (lgj = f(lgt)), the turning point corresponds the ttr. b. charge (integral) regime (ttr < RC). Even in case of strong absorption of light and nondispersive transport of charge carriers the shape of photocurrent pulse is not so informative as in current regime (Fig. 1a, L = 0,3): the drift time of charge carrier package is estimated as halftime (t1/2) of rise time of photocurrent pulse, i.e. ttr = 2 t1/2. The magnitude of photocurrent pulse is equal to amount of charge (Q) collected onto the sample electrodes during the charge carrier drift time. In case of bulk absorption of light, the magnitude of photocurrent pulse is equal Q/2, and ttr = 3,41 t1/2 If the voltage of backward direction is applied onto solar cell electrodes and, by short pulse of light the charge carrier pairs are photogenerated, the photocurrent pulse of their drift is observed, from which's duration (ttr) the mobility of the charge carriers of the same polarity as illuminated electrode is estimated. An amount of drifting charge carriers is estimated from the area of photocurrent pulse, from which, when amount of absorbed quanta of light In case of trapping with characterstic trapping time τ or in case of stochastic transport, after is known, the quantum efficiency is evaluated (Fig.1). photogeneration, the shape of photocurrent pulse is decreasing, and, from the area of voltage follows Hecht's dependence (Eg. (3)). From the latter dependence the μτ-product, photocurrent pulse, estimated dependence of amount of photogenerated charge carriers on which determines both the diffusion and drift lengths of charge carries, and causes effectiveness of solar cell, is estimated. N μτE ⎛ ⎛ ⎞⎞ ⎜1 − exp⎜ − d ⎟ ⎟ = ⎜ μτE ⎟ ⎟ d ⎜ ⎝ ⎝ ⎠⎠ (3) N0 www.intechopen.com Charge Carrier Recombination in Bulk Heterojunction Organic Solar Cells 297 Space charge limited photocurrent (SCLP). In this case an amount of phtogenerated charge pulse depends on Q0 (Fig. 1a), and strongly absorbed light (αd >> 1) creates reservoir of is higher than charge on sample electrodes at U0, i.e. Q0 >> CU0. The shape of photocurent charge carriers at the illuminated elektrode, from which not more than CU0 charge package can drift to the opposite elektrode. This package is moving in growing electric field, thus, in case of nondispersive transport and when ttr > RC, drift time is tSCLC = 0,78 ttr, which is estimated from the spike of current transient (Fig. 1a). When t > tSCLC, current flows until the whole charge is extracted from reservoir and the second turning point, at extraction time (te), appears on the pulse of photocurrent. An amount of charge extracted from the reservoir (Qe), as well as te, depend on recombination speed of charge carriers in reservoir. a) Β/ΒL= 0.01 Β /ΒL= 1 1,5 30 1,5 L = 30 b) 10 Standard SCLC 1,0 3 1,0 j / jSCLC j/j SCLC 1 0,5 0,5 L = 0.3 1 3 10 0.3 0,0 0,0 0 1 2 3 4 0 1 2 3 4 t / ttr t/t tr Fig. 1. Numerically modelled photocurrent transients of charge carrier drift dependence on exciting light intensity in case when B/BL = 0.01 (a), and when Langevin recombination prevails (b). Density of photogenerated charge carriers is normalised to amount of charge on sample electrodes in SCLC regime For investigation of charge carrier recombination by photocurrent transient methods the dependence of collected onto sample electrodes charge on intensity of photoexciting light pulse is measured (Pivrikas et, 2005). When, due to increasing intensity of light pulse, the amount of photogenerated charge achieves amount of charge carriers on sample electrodes (Q0 = CU0), the TOF regime changes from small charge drift current (SCDC) to space charge limited current (SCLC) (Fig. 2a). Further increase of light pulse intensity not follows by increase of photocurrent, but increases the duration (te ≥ ttr ) of photocurrent pulse, which is caused by the extraction of charge carriers from reservoir. The faster charge carrier recombination in reservoir, the shorter extraction time (te), and, when recombination is very fast, te → ttr. Thus, the dependence of te on intensity of exciting light pulse L gives te(L) ≈ lnL indicates that monomolecular recombination prevails; if, at high intensity of light information about recombination process in charge carrier resrvoir: dependence as pulse, te saturates with L, than the bimolecular or of higher order of charge carrier recombination prevails. www.intechopen.com 298 Solar Energy 0,10 300 0,10 300 10 30 a) Β/ΒL=1 b) 100 100 Β /BL= 0.01 30 0,08 3 0,08 10 3 0,06 1 0,06 j j 1 0,04 0,04 0,02 0.3 0,02 0.3 0.1 0.1 0,00 0,00 0 10 20 30 40 0t 10 20 30 40 tr t/ttr t/ttr Fig. 2. Numerically modelled integral TOF current transients (RC = 10 ttr) In organic polymers the bimolecular recombination typically is of Langevine-type. The photocurrent transients of this case are shown in Fig. 2b, and the maximal amount of extracted charge is estimated as: ⎛ eL ⎞ = 1 − exp ⎜ − ⎟ Q ⎝ CU ⎠ (4) CU The maximal amount of extracted charge Q = CU. When the bimolecular recombination is weaker than Langevin's one, from the saturation of extraction time, which is estimated as difference of photocurrent pulse halwidths at space charge limited and at small charge regimes, i.e. te = t1/2 (L>1) – t1/2(L<<1), the ratio of bimolecular recombination coefficient (B) with Langevin's one according to expression: = 2 1 BL te (t e + ttr ) α d B ttr (5) Β/ΒL=0,01 Β /ΒL= 0,01 30 Β/ΒL=1 25 Β /ΒL= 1 1 20 Q/CU t1/2 15 b) 10 0,1 a) 5 0,1 1 10 100 0,1 1 10 100 Q0/CU Q0/CU Fig. 3. Dependencies of amount of extracted charge Q (a) and of photocurrent halfwidth (b) on amount of photogenerated charge in case of Langevin and reduced bimolecular recombination www.intechopen.com Charge Carrier Recombination in Bulk Heterojunction Organic Solar Cells 299 Here α is absorption coefficient. However, it is easer to measure the recombination coefficient using integral TOF when RC > ttr (Pivrikas et al, 2005). The examples of numerically modelled transients are demonstrated in Fig.2. Using this method the coefficient of bimolecular recombination is estimated as (Fig. 3): B= edS (6) t eQ 2.2 Charge carrier extraction by linearly increasing voltage (CELIV) method. Method has the advantage that it is suitable for investigation of both high and low conductivity materials (Juška et al, 2000 a; Juška et al, 2004). After the triangular voltage pulse is connected to the sample electrodes in backward direction, the current caused by geometric capacitance of sample (j(0)) and conductivity current Δj are observed (Fig. 4). U t Δj t1/2 j(0) tmax 0 ttr t Fig. 4. Voltage pulse and current density observed by CELIV method The measuring device is very simple: triangular pulse voltage generator and oscilloscope. Another advantage is that after triangular pulse of voltage is applied onto sample electrode, there is no initial, caused by capacitance, current peak, which disturb to monitor drift current in conductive materials. The current transients were calculated by using standard solution method from continuity, current and Poisson equations in case when one of electrodes is blocking: Schottky or p-i density of equilibrium carriers is n0 the extraction depth l ( 0 ≤ l ≤ d ) is estimated: barrier, or even special structure with isolating sublayer. From Poisson equation when = = E(0, t ) − E( d , t ) , Q(t ) en0 l(t ) εε 0 εε 0 (7) here Q is the amount of extracted charge, E(0,t) and E(d,t) are the magnitudes of electric field at the front and back electrodes correspondingly. From the continuity equation: www.intechopen.com 300 Solar Energy = jd = σ 0 E( d , t ) , dQ (8) dt In case of linearly increasing voltage U = A t and ∫ Edx = At = E(d , t ) ⋅ d + E(0, t ) − E( d , t ) ⋅ l(t ) , d 0 2 the Rikati equation is obtained for l(t) σ 2 μ At + l (t ) = dl(t ) 2εε 0 d , dt d Then the curent transient is εε o A σ ⎛ l(t ) ⎞ ⎛ μ At σ ⎞ j( t ) = + ⎜1 − ⎟⎜ − ⋅ l2 (t ) ⎟ , μ⎝ d ⎠⎝ d 2εε o d ⎠ (9) d When τσ = εεo/σ >> ttr The first component is caused by capacitance, and second one by conductivity. A⎡ ⎛ μ At 2 ⎞ ⎤ j( t ) = ⎢εε o + σ t ⎜ 1 − ⎟ ⎥ , when t < d = ttr , 2 d⎣ ⎝ 2d2 ⎠⎦ μA (10a) j( t ) = ⋅ εε o = j ( 0 ) , when t > ttr A (10b) d From experimentally observed current transient the thickness of sample and/or dielectric permitivity may be estimated: εε 0 = j(0) , (11) A d The dielectric relaxation time may be estimated as ⋅ tmaxτσ = 2 j(0) Δj , (12) 3 The mobility of equilibrium charge carriers can be estimated as μ= if Δj ≤ j(0), i.e. τ σ ≥ ttr 2d2 2 3 Atmax τσ d2 (13) μ= 3 if Δj >> j(0), i.e. τ σ << ttr Atmax The bulk conductivity of sample follows from: σ bulk = εε 0 dj / j0 . (14) dt t =0 www.intechopen.com Charge Carrier Recombination in Bulk Heterojunction Organic Solar Cells 301 The density of charge may be calculated from: ed ∫ ∞ p0 = Δjdt . 2 (15) 0 In Fig. 5 there are demonstrated the results of modelling without trapping and with single tmax(A) ≈ A-0.5 and A-0.33, predicted by Eq. (13). When trapping is accounted then in both trap level (Juška et al, 2000, b). For high and low A the modelling very well reproduces limiting cases the same expressions for tmax and Δj like without trapping are obtained, if one substitute μ by(μf), where f is the trapping factor or for single trap f = τC/(τC + τR), where τR is the release time. Fig. 5. Numerical modelling results of dependencies of Δj, tmax, j(0) on A. A = 1, when ttr = σ. Bold lines demonstrate dependencies when shallow trapping is accounted ( C = 1, R = 100); lines correspond case when shallow trapping is absent. Density of current is normalized to magnitude of j(0), when A = 1, and time is normalized to σ The basic measurable parameters of CELIV Δj, tmax depend on charge carrier interaction with trapping states, and this is reflected in dependencies of tmax and Δj (Fig. 6). Numerical N(E) ~ exp(-E2/2δ 2), demonstrate that measurements of Δj(A) ~ Aβ and tmax(A) ~ Aγ modelling (Juška et al, 2000 b), taking into account energy distribution of trapping states as dependencies in various temperatures and electric fields, while choosing such A that Δj ≅ j(0), and estimating the rates of change as coefficients β = d(ln j ) d(ln A) Δj = j(0) and γ= d(lnt max ) d(lnA) Δj = j (0) , the nature of charge carrier interaction with trapping states can be 1. if μ(F) dependence is caused by stochastic transport, then (β - γ) = 1, (β + γ) < 0; cleared up, i.e., which charge carrier transport model is prevailing (Fig. 6): 2. if μ(E) dependence is caused by Poole-Frenkel type dependence of micromobility on electric field ( μ ~ exp(a E ) ) then (β - γ) > 1, and (β + γ) < 1, and the latter is independent or decreases with increasing a (T decreases); τ R ~ exp( −b E) then, when b increases, (β - γ) > 0, and (β + γ) increases or even changes 3. if the characteristic release from trapping states time R depends on electric field, i.e. the sign. www.intechopen.com 302 Solar Energy Fig. 6. Numerical modelling results of (β + γ) and (β - γ) dependencies on: (a) parameter δ/kT b (line) when δ/kT = 3 of Gaussian distribution of localized states; (b) Poole-Frenkel parameters a (doted line) and 2.3 Photo-CELIV method Photo-CELIV method demonstrate even more opportunities where, by short pulse of light, photogenerated charge carriers are extracted by delayed (delay time tdU) triangular pulse of voltage (Fig. 7) (Österbacka et al, 2004). Measurements of amount of extracted charge dependence on the delay time tdU allow investigation of the relaxation of charge carrier density and mobility, independently. The latter are important in case of stochastic transport. Fig. 7. Time chart of photo-CELIV method 150 100 I [μA] 50 0 0 2 4 t [μs] Fig. 8. Photocurrent transients of photo – CELIV for different tdL www.intechopen.com Charge Carrier Recombination in Bulk Heterojunction Organic Solar Cells 303 Fig. 9. Photocurrent transients of photo-CELIV at different intensity of pulse of light and fixed delay time (a), and at different delay time tdU and fixed intensity of pulse of light (b) in RRa-PHT layer 2.4 Double injection current transient (DoI) method. After the voltage is applied onto solar cell's electrodes in forward direction, the double injection current is observed. When the dielectric relaxation time is longer than charge carrier drift time ( σ = εε0/ >> ttr=d/μE), in case of bimolecular Langevin recombination, the whole injected charge carriers recombine while moving through interelectrode distance, and the observed current transient matches space charge limited current transient in case of sum of mobilities of both sign carriers. When the recombination is weaker, then, after the drift time (tsl) of slower charge carriers, an amount of injected charge carriers and, at the same time, current increases till saturates, due to recombination. Thus, the dependence of saturated density of current on voltage is: j=2 ( ) eεε 0 μ n μp μ n + μp U 2 = 2εε 0 L μ n μp 3 . , when τσ >> ttr; B U2 (16) 3 B d B d From the shape of current transient pulse it is possible evaluate whether recombination is of Langevin-type or weaker. In Fig. 10 there are shown measurable parameters, from which the transport and recombination values is estimated. The sum of mobilities of both sign charge carriers as μ n + μp = 0.8 d2 (17) Utsc www.intechopen.com 304 Solar Energy μn/μp=10 10 js/2 B/BL=10-3 t1/2 j(t) j/jSCLC B/BL= 1 1 tsc dj/dt tsl 0,1 1 10 100 t/ttr Fig. 10. Transients of double injection currents into dielectric in cases of Langevin and reduced bimolecular recombination or in other case μ n + μp ≅ jSCLC d 3 εε 0U 2 . (18) The mobility of slower charge carriers μsl = 0.8 d2 . (19) Utsl The coefficient of bimolecular recombination B= ln3 2 ( e μ n + μp U jst1/2 d ). (20) In case of plasma injection into semiconductor, when dielectric relaxation time is shorter than charge carrier transit time ( σ = εε0/ >> ttr=d/μE), after ambipolar transit time of charge carriers (ta= tm), an amount of injected plasma, and, thereby, the current, increases till, due to recombination, saturates. ⎧ ⎪ σ E⎛ 1 − 2 t ⎞ , − 1 ⎜ ⎟ t < ta ⎪ 2 5 j( t ) = ⎨ ⎝ 3 ta ⎠ 6 ⎪3 ⎛ ⎞ ⎛ ⎛ 5 ⎞⎞ ⎪ σ E + ⎜ jS − σ E ⎟ tanh ⎜ BΔnS ⎜ t − t a ⎟ ⎟ , t > t a 3 5 ⎩ 2 ⎝ 2 ⎠ ⎝ ⎝ 6 ⎠⎠ 6 www.intechopen.com Charge Carrier Recombination in Bulk Heterojunction Organic Solar Cells 305 js = 8 (μ ) + μn μp μ n ( n0 − p0 ) U 3/2 8 U ⋅ 2 = ⋅ εε 0 L ⋅ 1 , when τσ << ta. B τ σ ta p B e (21) 9 B d 9 d mobility (μa) is estimated: From the maximum of differential of current transient, using Eq. (22), the ambipolar ⎛ dj ⎞ tm ⎜ ⎜ dt ⎟ = ta = ⋅ ⎟ 5 d2 ⎝ max ⎠ 6 μ aU . (22) Using Eq. (23), the coefficient of bimolecular recombination is: τ σ tm B / BL = 0.45 . (23) t12/ 2 From shape of double injection current pulse the information about charge carrier trapping is obtained (Fig. 11) (Juška et al, 2008). During the trapping of the slower charges, after the transit time, through the interelectrode distance, of faster charge carriers, the space charge limited current is flowing through the sample till the whole trapping states are filled in by slower charge carriers ("hole trapping" in Fig. 11). When the trapping of faster charge carriers is dominating, the current is decreasing and begins to increase after trapping states are filled in ("electron trapping" in Fig. 11). Thus, the integration of current until time when the current starts to rise, allows evaluate density of trapping states. 10 μn/μp=100 without trapping j / jsclc 1 hole trapping electron trapping 0,1 0,1 1 10 100 1000 t/ttr Fig. 11. Numerical modelling of double injection current transients when trapping is absent and when faster or slower charge carriers are trapped The high capacitance of thin solar cells, immediately after application onto electrodes of rectangular pulse of voltage, causes high initial spikes of current, which complicates measurement and analysis of double injection current transients. To around the latter problem is possible by modification of DoI method, i.e. immediately after forward voltage pulse to apply the pulse of backward direction, and to measure the extraction current transient (Fig. 12) (Juška et al, 2006). www.intechopen.com 306 Solar Energy U a) tp U Uoff t j b) js t Qex Fig. 12. Time charts of voltage pulse (a) and transients of double injection and extraction currents The integral of extraction current gives an amount of extracted charge, from which's dependence on duration of injecting voltage (Fig. 13), the charge carrier mobility and bimolecular recombination coefficient can be estimated as: B= ln 3 edS . (24) 2 t1/2Qs Β /ΒL=10 Qs -3 μn/μp=10 Q 10 Qs/2 Qex/CU 1 dQ/dt tsc tslow t1/2 0,1 1 10 100 tp/ttr Fig. 13. Dependence of extracted charge on duration of injecting voltage pulse 3. Experimental results 3.1 Amorphous and microcrystalline silicon Light-induced degradation of amorphous hydrogenated silicon (a-Si:H) is a serious problem of a-Si:H based photovoltaic solar cells. The most probable driving force for a-Si:H degradation is the energy (more than 1 eV) released during nonradiative bimolecular recombination of electron-hole pairs (which prevails at high light intensity) and that is why the discovery of mechanism of this recombination is of great importance. www.intechopen.com Charge Carrier Recombination in Bulk Heterojunction Organic Solar Cells 307 For the study of bimolecular recombination coefficient (B) we have proposed the photoelectrical method (Juška et al, 1995), which is based on the measurement of extraction time (te) of the charge carrier reservoir using the space-charge limited photocurrent (SCLP) transient method. This method gives a possibility to estimate the monomolecular recombination time from the shape of the te dependence on the light intensity (L) and the bimolecular recombination coefficient B from the saturated value of te. These photoelectrical carrier density is approximately 1017 cm-3, and B ≅ 10-9 cm3/s. measurements demonstrated that the bimolecular recombination begins to prevail if charge In a-Si:H layers it was observed the reduced bimolecular recombination, which, possibly, is reduced because electron and hole, immediately after photogeneration, are separated by internal random potential field. Fig. 14 demonstrates that the bimolecular recombination coefficient is lower in a-Si:H layers, which are deposited at high grow speeds (internal random field is greater), and that temperature dependence of B is stronger than that of high- quality amorphous silicon layers. on temperature in: high grade a-Si:H (×, ●, ○), high deposition rate a-Si:H (+) and μc-Si:H (□) Fig. 14. Dependence of bimolecular recombination coefficient of electrons (●) and holes (○) layers. Temperature dependencies of Langevin recombination coefficient (BL) Fig. 15. Dependencies of bimolecular recombination coefficient B and dispersion parameter, μc-Si:H layer estimated as t1/2/tmax from CELIV (Fig. 4), on substrate temperature during deposition of www.intechopen.com 308 Solar Energy In similar way the internal random potential influences bimolecular recombination in microcrystalline hydrogenated silicon (μc-Si:H). The temperature of substrate during deposition of μc-Si:H strongly influences the magnitude of internal random potential, and, through the latter, influences dispersion of charge carrier transport. Thus, decreasing of the substrate temperature leads to increase of dispersion of charge carrier transport, but decreases coefficient of bimolecular recombination (Fig. 15). 3.2 π-conjugated polymers Recently, the great opportunity to create enough effective, large area and low-cost organic solar cells (OSC) increased interest in π-conjugated polymers, but also has raised several problems. First of all, in disordered materials, which include π-conjugated polymers, the mobility of charge carriers, due to hopping, is low (μ << 1 cm2/Vs) in comparison with crystalline materials, and the mean hopping distance of charge carriers is shorter than Coulomb radius. In turn, the low charge carriers jump distance results in a low photogeneration quantum efficiency and conditions the diffusion-controlled, Langevin type charge carrier recombination. Latter is caused by, the diffusion and Coulomb inter-traction field controlled, meeting probability of electron and hole in space. On the other hand, in order to OSC current density would be comparable with the crystalline semiconductors the density of photogenerated charge carriers should be much higher than in the crystalline solar cells. because τ = (Bnf) -1. However, for effective OSC, it is necessary that the lifetime of charge The density of charge carriers, due to bimolecular recombination, causes small their lifetime carriers should be higher than their drift time through the interelectrode distance in intrinsic electric field, i.e. τ > ttr = d 2 / μUi (Ui is intrinsic potential). Thus, for higher than 5% i efficiency of OSC, when open circuit voltage is ~ 0.5 V, thickness of sample is 300 nm, it is μBL/B > 5×10-3 cm2/Vs. Thus, the bimolecular recombination limits efficiency of organic necessary that density of photocurrent will be higher than 15 mA/cm2, and solar cell in region of high intensity light, and ratio of bimolecular recombination coefficient with Langevin's one allows evaluate effectiveness of materials and structures. As a model material for investigation of features of bimolecular recombination was chosen π-conjugated polymer RRa PHT. In RRa PHT layer the TOF current transients were nondispersive at low intensity of light pulses. With increase of intensity of the light pulse, the shape of photocurrent transient changes to the classic SCLC kinetics, and, at a very high intensity of light, its shape stopped to change (Fig. 16). An amount of extracted charge linearly increased with intensity of light and saturated in the region of high light intensity, when Qe/CU0 = 1. Such saturation of the Qe(L) dependence is the consequence of Langevin recombination (See Eq. (4)). To assess the coefficient of Langevin recombination, it is necessary to know the charge carriers mobility, when the electric field is zero, because in the depth of photogeneration the electric field is shielded by the carriers. By measuring the hole mobility dependence on μp(E = 0) = 6.5×10 -6 cm2/Vs, and, considering that the mobility of electrons at least by one electric field strength and by extrapolation to E = 0, the hole mobility was evaluated as order lower than of the holes, BL = 4×10 -12 cm3/s was estimated. www.intechopen.com Charge Carrier Recombination in Bulk Heterojunction Organic Solar Cells 309 RRa PHT layer is 8 μm, E = 105 V/cm Fig. 16. TOF transients of photocurrent for different intensity pulse of light. Thickness of For direct measurements of photogenerated charge carrier density and mobility relaxation on time, the photo-CELIV method was used. From the duration tmax (see Fig. 4) the mobility e∫ ∞ Δjdt , here Δj is density of 1 of charge carriers and from integral of conductivity current ( 0 conductivity current) the density of charge carriers (p) at given tdU (see Fig. 7) are estimated. The presence in the structure of intrinsic electric field has been compensated by offset voltage. Possible inaccuracy of this method can be caused by spatially distributed intrinsic electric field, which separates photogenerated charge carriers, thereby, decreasing recombination. -5 10 RRaPHT μ (cm /Vs) 10 -6 0.42 2 -7 10 0.58 p (cm ) -3 15 10 14 10 -5 -4 -3 -2 -1 10 10 10 10 10 tdU (s) according B(t ) = eμ (t ) / εε 0 in case of Langevin recombination Fig. 17. Charge carrier mobility and density dependencies on tdU. Solid line is p(t) results www.intechopen.com 310 Solar Energy demonstrated. Fitting of mobility relaxation as μ = at - 0,42 gives that Langevin recombination In Fig. 17 the typical for RRa PHT mobility and density dependencies of hole on time are coefficient changes with time too, i.e. B(t) = eμ (t)/εε0. Thus, charge carrier density follows expression p(t ) = p(0) 1 + p(0)B∫ μ (t )dt t . (25) 0 Therefore ⎛ 1 e at 0.58 ⎞ −1 p(t ) = ⎜ + ⋅ ⎟ . ⎝ p(0) εε 0 0.58 ⎠ (26) which is shown by solid line in Fig. 17. The coincidence of experimental results and theory confirms that, in low mobility organic material, bimolecular recombination is of Langevin- type. So, the same result has been obtained from the saturation of SCLC transients with intensity of light (Pivrikas et al, 2005 [5]). 3.3 Recombination in conjugated polymer/fullerene bulk heterojunction solar cells One of possibilities reduce bimolecular recombination is to make junction of two organic material layers, in one of which are mobile the electrons and in another one the holes. The excitons, immediately after photoexcitation, are destroyed by electric field of heterojunction and separated electrons and holes are moving each of its transport material to sample electrodes. However, in organic polymers the diffusion distance not exceed 100 nm. Thus, an efficiency of such solar cell would be low because the thickness of solar cell would be approximately 100 nm and absorption of light weak. From Fig. 18a follows, that, oppositely to MEH PPV (poly(2-methoxy-5-(2'-ethylhexykoxy)-1,4-phenylenevinylene) layer, the heterojunction of MEH PPV/perilene more effectively separates photogenerated pairs, i.e. the charge carrier reservoir is created and an amount of collected charge approximately twice exceeds CU0 (Fig. 18b). Supporting the latter experimental result, the numerical modelling, taking into account the Langevin recombination, demonstrates that, in case of bulk absorption of light and of small resistor, causing extraction current, an amount of extracted charge can exceed CU a few times, too. Thus, the obtained experimental results do not deny the Langevin recombination in heterojunction. Another charge carriers bimolecular recombination reduction method has been identified investigating a-Si:H and μc-Si:H layers: separate photogenerated charge carriers by an internal random field in space, so, that they move towards the electrodes in different ways. This method has been used for organic semiconductor structures: layer cast mixing transporting materials of holes and electrons. Such a bulk heterojunction blends allow to expect a significant reduction of bimolecular recombination, as in the bulk of samples created excitons are in the vicinity to heterojunctions. When they disintegrate, resulting electrons and holes moving towards each of its material to the contrary of the electrodes, i.e. separated in space. Experimentally there were investigated bulk heterojunctions of various organic polymers with PCBM. www.intechopen.com Charge Carrier Recombination in Bulk Heterojunction Organic Solar Cells 311 a) b) the intensity of light pulse: × - MEH PPV, ▲ – perilene/MEH PPV junction, Fig. 18. Dependencies of TOF current transient halfwidth (a) and of collected charge (b) on O - MEH PPV:PCBM blend Time-dependent mobility and recombination in the blend of poly[2-methoxy-5-(3,7- dimethyloctyloxy)-phenylene vinylene] (MDMO-PPV) and 1-(3-methoxycarbonyl)propyl-1- phenyl-(6,6)-C61 (PCBM) is studied simultaneously using the photoinduced charge carrier extraction by linearly increasing voltage technique (Mozer et al, 2005). Photo-CELIV transient at various delay times, light intensities and applied voltages have been recorded, and the charge carrier mobility and lifetime simultaneously studied. It is found that, shortly after photoexcitation, both the charge mobility and the recombination are time-dependent (dispersive) processes, which is attributed to the initial relaxation of the confirm that the recombination dynamics within the studied μs - ms time scale is a charge carriers towards the tails states of the density of states distribution. The results thermally activated process rather than a temperature independent tunneling. The obtained time-dependent mobility values are used to directly describe the recombination dynamics (see Fig. 19 and Fig. 20). Density decay of charge carriers fitted according to Eq. (25). Therefore results suggest that the recombination dynamics is nearly Langevin-type, i.e. controlled by diffusion of the charge carriers towards each other. 3.4 Recombination in P3HT:PCBM Bulk Heterojunction Organic Solar Cells In poly(3-hexylthiophene): 1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61 (P3HT:PCBM) bulk heterojunction solar cells, a reduction of the Langevin recombination is commonly observed after thermal treatment. This treatment has been shown to modify significantly the nanomorphology of the photoactive composite, inducing a crystallization of both the donor and the acceptor phases (Pivrikas et al, 2007). In Fig. 21 the experimentally measured results using integral TOF SCLC regime are presented. By comparing experimentally measured BL, it was shown that B/BL ≅ 10 - 3. bimolecular recombination coefficient B with calculated Langevin recombination coefficient caused by random potential, the bimolecular recombination has follow B ≅ BL exp(-ΔE/kT) According to (Adriaenssens et al, 1997), if the reduction of bimolecular recombination is dependence on temperature (here ΔE is mean random potential energy). Thus, the activation energy of B has to be higher than one of BL, while experimentally it is obtained an opposite result. In case if bimolecular recombination is caused by tunnelling, the B/BL ratio will www.intechopen.com 312 Solar Energy Fig. 19. Photo-CELIV transients recorded at 300 K at (a) various delay times at fixed light intensity; (b) varying illumination intensities attenuated using optical density filters at fixed 5 μs delay time. The voltage rise speed A was 4 V/10 μs. The insets show the calculated dispersion parameters t1/2 to tmax versus delay time and the concentration of the extracted charge carriers, respectively. Fig. 20. Mobility (a) and the density (b) of extracted charge carriers versus the delay time for samples with different active layer thickness. Charge carrier mobility and density measured for the 360 nm device. Density relaxation of charge carriers fitted according to Eq. (25) www.intechopen.com Charge Carrier Recombination in Bulk Heterojunction Organic Solar Cells 313 Fig. 21. Dependencies of integral TOF photocurrent transients (a), and of Qe/CU0 and t1/2 on intensity of light pulse in RRP3HT:PCBM bulk heterojunction. Horizontal dotted line (b) corresponds Qe = CU0 demonstrate strong dependence on electric field. However, as is obvious from experimental results, this is not a case. The increasing of random potential also did not cause reduction of B/BL ratio. In blend of segmented electron and hole transporting materials the meeting of charge carriers of opposite sign may be limited by charge carriers of lower mobility. However, this is insufficient to explain such big reduction of bimolecular recombination in RRP3HT:PCBM blend. This can be explained by, that the interface between polymer and acceptor materials, decreases Coulomb interaction, which suppress gemini and bimolecular recombination as it was proposed in (Arkhipov Heremans & Bässler, 2003). Furthermore, double injection current transients (DoI) and photo-CELIV measurements revealed, that the reduced B depends on the charge carrier density as in the case of Auger recombination (Juška et al, 2008). The same conclusion followed from transient photo voltage and transient photo absorption spectroscopy experiments (Shuttle et al, 2008). In the recent transient absorption spectroscopy experiments (Nelson, 2003) it was suggested that this type of relaxation is caused by a stochastic transport attributed to an exponential tail of localized states. However photo-CELIV and TOF experiments showed that the photocurrent relaxation is caused by the charge carrier's recombination (Pivrikas et al, 2005). In this work, we are demonstrating that the reduction of B and its dependence on charge carriers density is caused by the two dimensional Langevin recombination (Juška et al, 2009). Sirringhaus (Sirringhaus et al, 1999) showed that the mobility across and along the lamellar structure differs more than 100 times, which led to the fact that the recombination of charge between lamellas l << rm, rm is determined by π rm l = 1 / n . Then the recombination carriers is mainly taking place in the two-dimensional lamellar structure. When spacing 2 probability www.intechopen.com 314 Solar Energy 1 3 π e( μ n + μp ) f 2D = = ⋅ (l ⋅ n)3/2 = γ 2D n 3/2 . εε 0 (27) tm 4 where γ2D is 2D recombination parameter. Hence, in two dimensional case, the bimolecular recombination coefficient will be reduced in comparison with one of the three dimensional case as B2D 3 π 3/2 1/2 = l n . B3D 4 For RRP3HT l ≅ 1.6 nm (Sirringhaus et al, 1999) and, for example, when n = 1016cm-3, β2D/β3D = 6×10-3, and that is close to experimental results of Ref. (Pivrikas et al, 2007). In the 2D recombination case, using Eq. (27), the equation governing the decay of the charge carriers is: = G − γ 2D n 5/2 . dn (28) dt observed experimentally: dn/dt ∝ n2/6 (Shuttle et al, 2008). According to Eq. (28), after where G is the rate of the photogeneration or double injection. Similar dependencies were excitation by short pulse of light, the decay of the density of charge carriers is described as: ⎛ − ⎞ ∝ t −2/3 −2/3 n(t ) = ⎜ n0 3/2 + γ 2Dt ⎟ n0 →∞ 3 ⎝ ⎠ , (29) 2 here n0 is initial density of photogenerated charge carriers. The similar dependence is observed using photo-CELIV technique (Fig.22). In the case of 3D Langevin recombination n(t)∝ t -1. It is worth to notice, that the slower than n(t)∝ t -1 dependence can be observed due to the mobility dependence on time (stochastic transport), as it was shown in regiorandom poly(3-hexylthiophene) (Pivrikas et al, 2007). However, it is established by photo-CELIV that the mobility does not depend on the delay time after excitation (Fig. 22). That is why this explanation is not valid for the RRP3HT:PCBM blends. Another technique, which allows investigation of recombination process, is DoI current transient technique (Juška et al, 2007). 17 10 n -2 μ 10 μ [cm V s ] -1 -1 n [cm ] 16 -3 10 2 -3 10 15 10 -6 -5 -4 -3 10 10 10 10 t [s] Fig. 22. Measured using photo-CELIV dependencies of mobility and density of electrons on time in annealed RR-P3HT:PCBM bulk heterojunction. www.intechopen.com Charge Carrier Recombination in Bulk Heterojunction Organic Solar Cells 315 In the case of 3D Langevin recombination volt-ampere characteristics and DoI current transients corresponds to the sum of the space charge limited currents of electrons and holes, because the injected charge carriers will recombine completely within the interelectrode distance. In Fig. 23a numerically modelled DoI current transients are shown between fast and slow charge carriers mobilities μf /μsl. for the both 3D and 2D Langevin recombination cases for different distance l and ratios recombination (1) and 2D Langevin recombination for [(2) l = 10 nm, (3) l = 1 nm and μf /μ sl Fig. 23. Numerically modelled DoI current transients: a) in the case of 3D Langevin = 10, (4) l = 1 nm and μf /μsl = 1]; both the time scale and current are normalized to transit – measurements of RRP3HT:PCBM bulk heterojunction (d = 1.4 μm, U = 9 V) and time ttr and SCLC of faster charge carriers, respectively. (b) DoI current transients: solid lines TiO2/RRP3HT (d = 0.6 μm, U = 4 V) structure; dashed line - numerically modelled DoI current transient for RRP3HT:PCBM structure (μn = 10-2 cm2/Vs, μp = 2.5×10-3cm2/Vs, l = 1.6 nm) 100 -4/3 9/5 10 0 tr~U js~U -1 10 j [A/cm ] 2 tr [μs] -2 10 10 j0~U 2 -3 10 -4 10 1 1 10 U [V] Fig. 24. The dependencies of initial current j0, saturation current js and recombination time tr on voltage in the case of double injection. from Eq. (16) in the same way as in Ref. (Juška et al, 2006): j ∝ U9/5 /d13/5, while the In the case of 2D Langevin recombination the current-voltage characteristics can be obtained saturation time of the DoI current transient is tr ∝ U-6/5. The observed experimental results (Juška et al, 2007) are very close to these dependencies. modelled using μn = 10-2 cm2/Vs, μp = 2.5×10-3 cm2/Vs and l = 1.6 nm (obtained from X-rays The comparison of DoI current transients obtained experimentally and numerically www.intechopen.com 316 Solar Energy studies (Sirringhause et al, 1999) of RRP3HT:PCBM blend is presented in Fig. 23b. The ratios between initial and stationary currents are in good agreement with experiment giving the same l value, and the discrepancy in current rise times could be explained by not taken into account dispersion of the charge carrier transport. So, the observed increase of DoI current in RRP3HT:PCBM bulk heterojunction and TiO2/RRP3HT structures and good fit of numerical modelling with experimental data in RRP3HT:PCBM, proves that recombination takes place in RRP3HT. The slower rise of current in TiO2/RRP3HT structure is caused by the lower electron mobility and deep trapping (Juška et al, 2008). By the integral mode TOF method, where the RC time constant of the measurement setup is recombination parameter γ2D in lamellas and its temperature dependencies in more much larger than the transit time of the charge carriers (RC >> ttr) we can determine the 2D convenient (Pivrikas et al, 2005) and straightforward way because it is independent on material's parameters. In the case of 3D Langevin recombination, the current transient CU (when αd >> 1; Qex = CU, therefore tex = 0). In the case of 2D Langevin recombination, saturates as a function of light-intensity and the amount of extracted charge slightly exceeds the charge carrier extraction time tex, when collected charge saturates with light intensity, is estimated in the similar way as in the case of reduced bimolecular recombination (Juška et al, 1995): ⎛ 2 ⎞ ⎛ ed ⎞ tex = ⎜ ⎟ ⎜ ⎟ ∝ jex , 2/5 3/5 ⎝ 3γ 2D ⎠ ⎝ jex ⎠ -3/5 (30) where jex is extraction current, which could be varied by changing loading resistor or applied voltage. In the case of the reduced bimolecular recombination tex ∝ jex . -1/2 In Fig. 25b the extraction time as a function of the density of extraction current in different structures containing RRP3HT is shown. Since tex shows the same dependence on the extraction current density jex, it can be concluded that the recombination is taking place in RRP3HT and it is governed by the 2D Langevin recombination. Fig. 25. Current transients of charge carrier extraction (a) observed by integral TOF: small charge drift current (1, 3) and transient of saturated on light intensity photocurrent (2, 4) in TiO2/RRP3HT structures and RRP3HT:PCBM bulk heterojunction, respectively. Measurement of extraction time is indicated. Dependencies of extraction time (b) on the extraction current density in TiO2/RRP3HT structures and RRP3HT/PCBM bulk heterojunction www.intechopen.com Charge Carrier Recombination in Bulk Heterojunction Organic Solar Cells 317 4. Conclusion In this work there are demonstrated the methods of investigation of charge carrier recombination in organic solar cells, where stochastic transport of charge carriers complicates interpretation of experimental results: charge carriers extraction by linearly increasing voltage (CELIV), which allows independently measure relaxation of density and mobility of photoexcited charge carriers; double injection current transient, which is additional method of investigation of charge carrier recombination and, which allows to measure dependence of recombination coefficient on electric field; integral time of flight (SCLC), using of which allows easily estimate the temperature dependence of recombination coefficient. Experimentally it is shown, that the decay of the density of photogenerated charge carriers in the blend of MDMO-PPV:PCBM is of 3D Langevin-type, which is typical for organic materials, and in annealed samples of RRP3HT and bulk heterojunction solar cells of RRP3HT:PCBM it is of 2D Langevin-type recombination in the lamellar structure. 5. Referencies Adriaenssens, G. J. & Arhipov, V. I. Non-Langevin recombination in disordered materials with random potential distributions. Solid State Communications, Vol. 103, Issue 9 (September 1997) 541- 543. ISSN 0038-1098. Arkhipov, V. I.; Heremans, P. & Bässler, H., Why is exciton dissociation so efficient at the interface between a conjugated polymer and an electron acceptor? Applied Physics Letters, Vol. 82, Issue 25 (June 2003) 4605 1 - 3, ISSN 0003 6951. Juška, G. ; Viliūnas, M.; Arlauskas, K.; Kočka, J. Space-charge-limited photocurrent transients: The influence of bimolecular recombination. Physical Review B, Vol. 51, No. 23 (June 1995) 16668 – 16676, ISSN 1098-0121. Recombination in a-Si:H and μc-Si:H. Physica status solidi (a), Vol. 171, No. 2 Juška, G.; Viliūnas, M.; Arlauskas, K.; Stuchlik, J. & J. Kočka. Ultrafast Charge Carrier (February 1999) 539 - 547, ISSN 1682-6300. Juška, G.; Arlauskas, ; Viliūnas, M. & Kočka, J.. Extraction Current Transients: new method of study of charge transport in microcrystalline silicon. Physical Review Letters, Vol. 84, No. 21, (May 2000) 4946-4949, ISSN 0031–9007, a. Juška, G.; Arlauskas, K.; Viliūnas, M.; Genevičius, K.; Österbacka, R. & Stubb, H. Charge transport in π-conjugated polymers from extraction current transients. Physical Review B, Vol. 62, No. 24 (December 2000) 16235-16238, ISSN 1098–0121, b. carrier concentration and mobility in π-conjugated polymers. Macromolecular Juška, G.; Genevičius, K.; Arlauskas, K.; Österbacka, R. & Stubb, H.. Features of charge Symposia, Vol. 212, No. 1 (May 2004) 209-217, ISSN 1022–1360. Juška, G.; Arlauskas, K.; Sliaužys, G.; Pivrikas, A.; Mozer, A. J.; Sariciftci, N. S.; Scharber, M. & Österbacka, R. Double injection as a technique to study charge carrier transport and recombination in bulk-heterojunction solar cells. Applied Physics Letters, Vol. 87, No. 22 (November 2005) 222110 1—3, ISSN 0003–6951. Juška, G.; Sliaužys, G.; Genevičius, K.; Arlauskas, K.; Pivrikas, A.; Scharber, M.; Dennler, G.; Sariciftci, N. S. & Österbacka R. Charge-carrier transport and recombination in thin insulating films studied via extraction of injected plasma. Physical Review B, Vol. 74, No. 11 (September 2006) 115314 1-5, ISSN 1098–0121. www.intechopen.com 318 Solar Energy Juška, G.; Genevičius, K.; Sliaužys, G.; Pivrikas, A.; Scharber, M. & Österbacka, R. Double- injection current transients as a way of measuring transport in insulating organic films. Journal of Applied Physics, Vol. 101, No. 11 (June 2007) 114505 1-5, ISSN 0021- 8979. Juška, G.; Genevičius, K.; Sliaužys, G.; Nekrašas, N. & Österbacka, R.. Double injection in organic bulk-heterojunction. Journal of Non-Crystalline Solids, Vol. 354, Issues 19-25 (May 2008) 2858-2861, ISSN 0022-3093. Juška, G.; Genevičius, K.; Nekrašas, N.; Sliaužys, G. & Österbacka, R.. Two dimensional Langevin recombination in regioregular poly(3-hexylthiophene), Applied Physics Letters, Vol. 95, No. 1 (July 2009), 013303 1-3, ISSN 0003-6951. Mozer, A. J.; Dennler, G.; Sariciftci, N. S.; Westerling, M.; Pivrikas, A.; Österbacka, R. & Juska, G. Time-dependent mobility and recombination of the photoinduced charge carriers in conjugated polymer/fullerene bulk heterojunction solar cells, Physical Review B, Vol. 72, No. 3 (July 2005), 035217 1-10, ISSN 1098-0121. Nelson, J. Diffusion-limited recombination in polymer-fullerene blends and its influence on photocurrent collection, Physical Review B, Vol. 67, No. 15 (April 2003) 155209 1-10, ISSN 1098–0121. Österbacka, R.; Pivrikas, A.; Juška, G.; Genevičius, K.; Arlauskas, K. & Stubb, H. Mobility and density relaxation of photogenerated charge carriers in organic materials. Current Applied Physics, Vol. 4, No. 5 (August 2004) 534-538, ISSN 1567-1739. Pivrikas, A.; Juška, G.; Österbacka, R.; Westerling, M.; Viliūnas, M.; Arlauskas, K. & Stubb, H. Langevin recombination and space-charge-perturbed current transients in regiorandom poly(3-hexylthiophene). Physical Review B, Vol. 71, No. 12, (March 2005) 125205 1-5, ISSN 1098–0121. Pivrikas, A.; Juška, ; Mozer, A. J.; Scharber, M.; Arlauskas, K.; Sariciftci, N. S.; Stubb, H. & Österbacka, R. Bimolecular recombination coefficient as a sensitive testing parameter for low-mobility solar-cell materials. Physical Review Letters, Vol. 94, No. 17, (May 2005) 176806 1 - 4, ISSN 0031-9007. Pivrikas, A.; Sariciftci, N. S.; Juška, G. & Österbacka, R. A review of charge transport and recombination in polymer/fullerene organic solar cells. Progress in Photovoltaics:. Research and Applications , Vol. 15 (July 2007) 677-696, ISSN 1062-7995. Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meljer, E. W.; Herwig, P. & de Leeuw, D. M. Two dimensional charge transport in self-organized, high mobility conjugated polymers. Nature, Vol. 401 (October 1999), 685 - 688, ISSN 0028-0836. Shuttle, G.; O'Regan, B.; Ballantyne, A. M.; Nelson, J.; Bradley, D. D. C.; de Mello, J. & Durrant, J. R. Experimental determination of the rate law for charge carrier decay in a polythiophene: Fullerene solar cell. Applied Physics Letters, Vol. 92, No. 9, (March 2008), 093311 1 - 3, ISSN 0003-6951. www.intechopen.com Solar Energy Edited by Radu D Rugescu ISBN 978-953-307-052-0 Hard cover, 432 pages Publisher InTech Published online 01, February, 2010 Published in print edition February, 2010 The present "Solar Energy" science book hopefully opens a series of other first-hand texts in new technologies with practical impact and subsequent interest. They might include the ecological combustion of fossil fuels, space technology in the benefit of local and remote communities, new trends in the development of secure Internet Communications on an interplanetary scale, new breakthroughs in the propulsion technology and others. The editors will be pleased to see that the present book is open to debate and they will wait for the readers' reaction with great interest. Critics and proposals will be equally welcomed. How to reference In order to correctly reference this scholarly work, feel free to copy and paste the following: Gytis Juška and Kęstutis Arlauskas (2010). Charge Carrier Recombination in Bulk Heterojunction Organic Solar Cells, Solar Energy, Radu D Rugescu (Ed.), ISBN: 978-953-307-052-0, InTech, Available from: http://www.intechopen.com/books/solar-energy/charge-carrier-recombination-in-bulk-heterojunction-organic- solar-cells InTech Europe InTech China University Campus STeP Ri Unit 405, Office Block, Hotel Equatorial Shanghai Slavka Krautzeka 83/A No.65, Yan An Road (West), Shanghai, 200040, China 51000 Rijeka, Croatia Phone: +385 (51) 770 447 Phone: +86-21-62489820 Fax: +385 (51) 686 166 Fax: +86-21-62489821 www.intechopen.com
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in News
Tonight on Crush Radio:
Depression and the pharmacutical drugs that make it worse!
-Natural Treatments and Remedies
The truth about the food pyramid, the lies exposed!
-Learn to eat for your benefit and health
Health Care and it's agents of deception
Book Review:
Forks over Knives
Urban Therapy with Sun #125 Taking A Beating with Tanisha A. Cunningham
in Health
Many Black people will agree that spanking is an accepted form of punishment in our community but most of us are in denial of the long lasting effects that severe beatings by parents can have. Child abuse in the Black community is real and it's time that we deal with that reality and get a better understanding of how it can effect our emotional development.
This is where Tanisha A. Cunningham? who is the author of the book A Rose Can't Grow In A Garden Of Weeds comes in.
On Wednesday we will review her very revealing autobiography that is sure to engage and enlighten all who follow her life story. Some of the highlights of this show will deal with:
Physical and emotional child abuse in the Black community.
Foster Care. Corruption issues, abuse, and advocacy.
Group Home violence and at risk youth labeling.
Perseverance from turmoil against all odds.
Tanisha's book is an astounding personal account of the many struggles that a young person may have to overcome just to be able to be functional in the world. There is a good chance that there is a little bit of our story in every chapter of her work. Her story is extremely candid, easy to understand and has a very personal feel to it that almost forces the reader to feel where she is coming from and want to cheer her on. It's a great read and this Wednesday we will break it on down.
Please pick up the book that you won't be able to put down: A Rose Can't Grow In A Garden Of Weeds by Tanisha A Cunningham on www.amazon.com and visit her website at www.tanishacunningham.com. Your life will be changed for the better and you will not be disappointed.
Ask the Sun How the 2015 Spring Solstice Energy Will Support You!
in Spirituality
Karen Cressman - a Spiritual Interpreter, Self Empowerment Coach, Author & Speaker - who brings forth wisdom from other realms, will have a conversation with the Sun to learn what the current weather patterns and energy changes are bringing to us. Find out if there will be a shift in the energies that will support our abilities to move forward with the 2015 Spring Solstice. Call in or type questions into the chat room to ask our source of light, heat and energy, how YOU can move forward in your life this Spring!
Urban Therapy with Sun #123 Kids In The Bed With Their Parents
in Lifestyle
How long should parents allow their children to sleep in the bed with them? Should they ever let them sleep in the same bed with them? Maybe as a newborn? An Infant? A toddler? When does the time come to teach children to be a little more independent? Is it the child that needs to sleep with the parents or is it the parents that need the kids to sleep with them? What about 2 parent families? Why would a mom and dad want their children to sleep in the bed with them? When would they have sex?
There are some studies that show that children that sleep with their parents actually become more independent. They also say that children that sleep with their parents share more information and become closer to their parents. Do you think that that's possible?
Are space issues the reason that some parents allow their kids to sleep in the bed with them? Is that understandable? If there are more people in the house than there are beds, does it make sense that parents would have their kids sleep in the bed with them?
Do you think that children who sleep with their parents develop insecurities like thumb sucking, needing security blankets, being afraid of the dark etc.?
Do you think that parents who have fears transfer their fears to their children and make them fear the same things. Ex. If mommy and daddy are afraid of mice, do you think that their children may also be afraid of mice as well? Do these things develop more because of children sleeping with their parents?
The Helios Biblios(the book of the Sun) Hour/ Tetrad" Four Blood Moon
Tetrad" Four Blood Moon: So….you may have heard on the news that there is a prophesy that the world is going to end...Blood Moon keep falling in Aries and Libra..Does the Moon symbolizes the Right Woman, and the Sun symbolizes the Right Man. the Moon has no light of its own but reflects the light of the Sun. Similarly, the Right Woman reflects the Light of her Right Man. Libra is the peacemaker, the one who sees both sides of the situation. Aries is usually portrayed as the warrior but it's also the initiator, the energy that breaths passion into whatever it focuses on. it signals a time when all of us can use the fiery energy of the sign to make changes in our life...What are your theory on the blood is something big happening with the currency, or is the New World Order will be finalize is moon prophecy or hype? Beliefs in ancient symbolism, secret societal occultism, and ritual magic affects us all regardless of religious beliefs or practices.
Leaders of the world, including religious leaders like the Pope, answer to other philosophies secretly while outwardly they combine new age and biblical symbolism to exercise absolute control over the world. What is your spin on the blood moon?
- ...
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Battlefield Hardline VGA graphics performance review
OCZ Vector 180 - 480 and 960GB SSD review
AMD FreeSync Review With the Acer XG270HU Monitor
Nvidia GeForce GTX Titan X Review
Corsair H80i GT review
Plextor M6e Black Edition PCIe SSD Review
Corsair Carbide 100R Silent review
ECS Z97I-Drone motherboard review
USB 3.1 review: 10 Gbps USB with the ASUS Z97-A
Guru3D Rig of the Month - February 2015
ForceWare 77.76 Win2000|XP
Posted by: Hilbert Hagedoorn on: 07/20/2005 06:49 AM [ 0 comment(s) ]
Windows 2000/XP Official Beta
NVIDIA released ForceWare 77.76 today. This driver is supposed to have all kinds of improvements and bug fixes. This set has not been WHQL qualified just yet. All GeForce models are supported with this driver release.

NVIDIA has now enabled SLI functionality with GeForce cards all the way down to the new sub-$100 GeForce 6600 LE, even though they are not equipped with the same SLI connector along the top of the card.

Release Highlights:

SLI support for GeForce 6600 and GeForce 6600 LE

New SLI rendering mode (SLI Antialiasing) for increased antialiasing performance

Broader SLI optimizations and validation for the latest games

Issues resolved since ForceWare 77.72

"Overbright" color issue affecting video playback

GeForce 7800 GTX clock reduction when using extremely cold temperatures to cool the GPU

For a full list of other issues resolved, please view the driver release notes.

Microsoft® DirectX® 9.0c and OpenGL® 1.5 support

For GeForce 6600 and 6600 LE cards, SLI communications are instead funneled through the PCI Express bus. On top of this, NVIDIA is also unveiling two new anti-aliasing modes, available to owners of an SLI configuration: SLI 8X AA and SLI 16X AA.

Release Date: July 20, 2005
BETA Driver

Tagged as: ForceWare
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D2 Potion After Shave Lotion Spray 100ml
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'A Science of Seduction' Dean and Dan Caten. An essence project inspired by the labortories of the past. A potion of ingredients having a rich heritage. An artful mix of raw materials involving years of research and sensory evaluation.
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The Story
Weather-resistant polystyrene translates the beautiful grain and warm tones of natural Brazilian ipe wood into a stylish collection designed for the great outdoors. Variegated slats of faux wood line up with a hint of drape at the edges, revealing lifelike, end-cut grain. Clean-lined, lightweight aluminum frame, is topped with a smart charcoal powdercoat finish. Easy-care Sunbrella® acrylic cushion in chic charcoal is fade-, water- and mildew-resistant. Rocha lounge collection also available.
Part of the Rocha Dining Chair with Sunbrella ® Cushion Collection
Details and Dimensions
Rocha Dining Chair with Cushion 22.75"Wx22"Dx34"H
- Rustproof aluminum with powdercoat finish
- Extruded polystyrene with UV and antioxidant protection
- For indoor or outdoor use
- Stacks up to 4 high for easy storage
- Cover or store inside during inclement weather
- Cushion is fade- and mildew-resistant Sunbrella® acrylic with polyurethane foam fill
- Cushion is secured with fabric tab fasteners
- Spot clean cushion
- Made in China and USA
Arm HeightHeight: 26" Seat HeightHeight: 18.25" Seat Height W/CushionHeight: 19.75" Overall DimensionsWidth: 22.75" Depth: 22" Height: 34"
- Do not leave spills unattended.
- For outdoor use.
- Do not use abrasive cleaner.
- Outdoor fabric is mold, mildew and fade resistant.
- Spot clean with mild detergent and water.
- Cover or store indoors during the winter and/or extreme weather.
- Do not dry clean.
- Remove or stand wet cushions to aid in drying time.
- Care for nicks and scrapes immediately with Rustoleum.
- We recommend using a soft brush to remove dirt and dust.
- To prevent mildew during storage,fabric and furniture must be dry.
- Clean surfaces with a dry or damp soft cloth.
- We recommend the use of our outdoor furniture covers.
- If the powdercoat finish is chipped, the exposed area should be
- sealed to prevent moisture from getting under the paint.
- We recommend the use of our Indoor Outdoor Fabric Guard.
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- In light rain Sunbrella® will repel water. When there is heavy
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- In extreme weather we recommend storing cushions indoors.


text:
A process of enhancing hydrocarbon production from wells previously hydraulically fractured with a polymer based fluid(s). An in-situ combustion process is initiated in the reservoir for a short duration wherein thermal and chemical processes act to reduce the viscosity of unbroken gels and other fluids retained in the propped hydraulic fracture and immediate reservoir matrix vicinity.
1. A method for restimulating a previously fractured hydrocarbon reservoir comprising the steps of:
a) shutting in a producing well bore in communication with said previously fractured hydrocarbon reservoir to prevent flow;
b) displacing at least a portion of a hydrocarbon product existing in said producing well bore with a fluid;
c) injecting air down said hydrocarbon well bore and into the hydrocarbon reservoir to create an in-situ combustion reaction in said previously fractured hydrocarbon reservoir;
d) shutting in the hydrocarbon well bore for a period of at least about 12 hours; and
e) producing the hydrocarbon products from said well bore.
2. The method of
3. The method of
4. The method of
5. The method of
6. The method of
7. The method of
8. The method of
9. The method of
10. The method of
11. The method of
12. The method of
13. The method of
a) injecting an air mixture at a surface rate of between about 50,000 to 100,000 standard cubic feet per day per foot of perforations;
b) restricting injection pressure below a reservoir fracture pressure; and
c) maintaining a continuous injection of said air mixture for a time period not less than about 5 hours.
14. A method of improving the productivity of a previously fractured hydrocarbon reservoir, comprising the steps of:
a) providing a well bore in communication with the previously fractured hydrocarbon reservoir having an API fluid gravity of at least about 35 degrees;
b) shutting in said producing well bore;
c) injecting a non-combustible fluid down said well bore to evacuate said well bore of substantially any combustible hydrocarbons;
d) injecting a predetermined volume of air down said well bore and into said previously fractured hydrocarbon reservoir to initiate an in-situ combustion reaction and generate heat;
e) shutting in said well bore for a predetermined period of time; and
f) producing a hydrocarbon product from said previously fractured hydrocarbon reservoir and said well bore.
15. The method of
16. The method of
17. The method of
18. The method of
19. The method of
20. A method for restimulating a previously fractured hydrocarbon reservoir comprising the steps of:
a) shutting in a producing well bore in communication with said previously fractured hydrocarbon reservoir to prevent flow;
b) displacing at least a portion of a hydrocarbon product existing in said producing well bore with a fluid;
c) injecting air down said hydrocarbon well bore and into the hydrocarbon reservoir to create an in-situ combustion reaction;
d) shutting in the hydrocarbon well bore for a period of at least 12 hours; and
e) producing the hydrocarbon products from said well bore.
[0001] This application claims priority of U.S. provisional patent application Serial No. 60/273,294 having a filing date of Mar. 1, 2001 and is incorporated herein in its entirety by reference.
[0002] This process generally relates to a process for enhancing the recovery of hydrocarbons from a producing hydrocarbon zone. More specifically, it relates to enhancing permeability in an already propped fracture by removing unbroken or residual polymer based gel fracturing fluids. When removed, flow from previously undrained portions of the hydrocarbon reservoir is established or enhanced.
[0003] Many different processes have been employed to enhance the recovery of hydrocarbons (liquids and/or gases) from subterranean formations/reservoirs. This includes improving the rate of flow and/or the ultimate recovery of hydrocarbons from a producing hydrocarbon reservoir. Hydraulic fracturing is one such method known in the petroleum industry to accomplish this goal. This process if most often plied where transmissibility (kh/μ) is low, most frequently in low permeability gas formations. Typical fracturing fluids used to enhance reservoir productivity during hydraulic fracturing techniques include, but are not limited to 1) water-based systems which are normally comprised of polymer gels; 2) oil-based systems which are often gelled; 3) foam based systems; and 4) alcohol based systems.
[0004] Early water and oil based systems have been extensively referenced in the literature for their ineffectiveness due to the inherent inability of the gels not to �break�, i.e., become less viscous and ultimately produced from the reservoir. In some cases, unbroken gels have been inadvertently produced from these well bores years later, long after they should have broken. The original design emphasis on these gel systems was reduced surface tension and increased viscosity to reduce friction and maximize sand carrying capacity. Unfortunately, breaking these systems was not well understood nor accomplished.
[0005] Recognizing the benefits of oxidation in gel degradation, remediation attempts by injecting strong oxidizers such as chlorine bleach have previously been attempted. The outcome was usually unsuccessful, due primarily to the inability of an injected fluid phase contacting the damaged area. Thus, the petroleum industry is plagued worldwide with thousands of damaged hydrocarbon reservoirs with extensively propped hydraulic fractures that are non-productive due to residual polymer gels, thus resulting in low productivity and the inherent loss of recoverable hydrocarbon reserves and significant revenue.
[0006] The prior art has generally addressed the problem of low productivity in hydrocarbon reservoirs by focusing on heavy crude oil (<35 API gravity) reservoirs. Thus, attempts have previously been made to establish a combustion zone in a carbonaceous stratum by the injection of air, oxygen enriched air, or pure oxygen through an independent injection well. As the combustion supporting gas is injected, products of combustion and other heated fluids are forced away from the point of injection within the producing reservoir formation toward one or more producing wells where they are withdrawn to the surface. Associated production increases result from 1) increased pressure around a designated injection well driving reservoir fluids toward a producing well; or 2) an associated reduction in oil viscosity achieved through an increase in temperatures of the oil and the surrounding rock matrix. Examples of previous attempts at downhole combustion may be found generally in U.S. Pat. No. 4,566,536 to Holmes, U.S. Pat. No. 5,868,202 to Hsu, U.S. Pat. No. 4,274,487 to Hollingsworth, U.S. Pat. No. 4,418,751 to Emery, U.S. Pat. No. 4,042,026 to Pusch and U.S. Pat. No. 4,557,329 to Savard.
[0007] Typical heavy crude oil reservoirs are generally shallow (<3500 ft. depth) with low (<120� F.) bottom hole temperatures. The thermodynamic properties of these viscous oils require long injection times (weeks to months) and high temperatures (>700� F.) to achieve in-situ ignition. These problems were addressed in part by utilizing oxygen enriched air or pure oxygen as the injection stream, and at times used in combination with bottom-hole ignition systems. However, the injection of pure oxygen creates safety issues due to the hazardous nature of uncontrolled reactions or explosions associated with hydrocarbon products exposed to extreme heat. U.S. Pat. No. 4,598,772 to Holmes and U.S. Pat. No. 4,440,227 to Holmes are examples of attempts to mitigate unintended ignition and manage well bores within safe operating levels.
[0008] Those skilled in the art will appreciate that low temperature oxidation (�LTO�) processes cause reservoir oil to become partially oxidized and thereby form volatile oxygenated compounds and unstable hydro peroxide intermediates. Their decomposition releases significant heat. As the temperature produced by such reactions is raised, intermediate temperature reactions (�ITR�) cause distillation and thermal cracking which produces hydrogen gas and light hydrocarbons that are left as carbonaceous residue on solid matrix materials in the formation. As the reservoir temperature continues to rise to a minimum active combustion temperature (�MACT�), a high temperature oxidation (�HTO�) reaction occurs between the gaseous oxygen phase and the deposited carbonaceous residue. Ignition in heavy oils generally occurs in the HTO region wherein temperatures reach levels high enough to ignite and burn the deposited carbonaceous material. For reasons not fully understood, light oils and gas condensates will ignite and burn in the LTO region. Accelerated Rate Calorimeter tests performed on a specific light oil (50 API gravity) supported ignition within hours versus days or weeks, with a MACT of less than 500� F. These lower generated temperatures for the combustion of light oils allows In-situ combustion for most existing producing wells without requiring prohibitive surface or bottom-hole design changes.
[0009] Hydrocarbon oxidation by transition metal compounds is recognized in the refining and processing of oil and gas. The addition of such compounds can significantly increase oxidation reactions, and thus shorten the time to ignition. For In-situ combustion processes, this is important where the thermodynamic qualities of the target oil and reservoir temperature would require unreasonably long periods of oxygen injection to achieve ignition. There is thus a significant need for a cost effective process for stimulating previously fractured well bores with damage caused from polymer gels in a safe, effective manner using an in-situ combustion process.
[0010] Successful implementation of the process requires maintaining a combustion front through the reservoir at some distance from the injection well, a problem that has rendered a majority of heavy oil thermal recovery projects unsuccessful. The process of the present invention injects air for a short duration to achieve in-situ combustion, and advance the combustion front radially a small distance from the hydraulic fracture. The well is then shut-in for a predetermined period to allow for maximum oxygen utilization, bottom-hole temperature abatement to within safe operating levels, and CO2 miscibility with reservoir oil.
[0011] It is thus one aspect of the present invention to employ an in-situ combustion process in a hydrocarbon reservoir to remediate production impairment in general and to improve the productivity and total recovery of produced hydrocarbons from a producing hydrocarbon reservoir. In one embodiment of the present invention the process is specifically designed to reduce the production impairment caused by the previous use of a polymer based hydraulic fracturing fluids. The formation of, and reduced conductivities created by polymer gel residues and unbroken gels are some of the more common forms of damage caused by fracture fluids. Thus, the gel residues and unbroken gels act as barriers to fluid flow in a formation that is damaged in this manner.
[0012] In another aspect of the present invention, a process is provided which is preferably carried out through a single well bore that has been previously treated with any form of a polymer gel based system. One primary benefit of the process described herein is the degradation and removal of unbroken gels in the existing propped hydraulic fracture system. Combustion processes used in this application have several novel characteristics which contribute to polymer gel degradation, including but not limited to:
[0013] 1) Oxidation through contact by injected air;
[0014] 2) Elevated temperatures between 400� F. to 700� F.;
[0015] 3) Organic acids generated as a by-product of combustion distillation; and
[0016] 4) Low PH water generated as a by-product of combustion distillation.
[0017] It is a further aspect of the present invention to facilitate the removal of localized connate water and/or retrograde condensation through reduced surface tensions created by elevated temperatures. Further, the treatment creates an effective fracture having higher fracture conductivity and/or which penetrates an area of higher pore pressure than the previous fracture and thus enhances productivity and the ultimate recovery from the hydrocarbon reservoir.
[0018] It is a further aspect of the present invention to provide a process which can be practiced at a fraction of the cost of conventional hydraulic fracturing techniques and which utilizes equipment well known in the Petroleum Industry, and which can be effectively implemented in a period of days as opposed to weeks or months. Thus, in one aspect of the present invention, conventional compressors for the injection of high pressure air are utilized, as well as commonly used high pressure nitrogen gas.
[0019] Thus, in one embodiment of the present invention, a method for restimulating a previously fractured hydrocarbon reservoir is provided, and comprises the steps of:
[0020] a) shutting in a producing well bore in communication with said previously fractured hydrocarbon reservoir to prevent flow;
[0021] b) displacing at least a portion of a hydrocarbon product existing in said producing well bore with a fluid;
[0022] c) injecting air down said hydrocarbon well bore and into the hydrocarbon reservoir to create an in-situ combustion reaction in said previously fractured hydrocarbon reservoir;
[0023] d) shutting in the hydrocarbon well bore for a period of at least about 12 hours; and
[0024] e) producing the hydrocarbon products from said hydrocarbon reservoir.
[0025]FIG. 1 is a top plan view of a theoretical well bore which has been hydraulically fractured and identifying an effective drained area and a fracture created during the stimulation process;
[0026]FIG. 2 is a top plan view of a well bore identifying the geometry of the actual hydraulic fracture and showing the effective area of drainage;
[0027]FIG. 3 is a top plan view of a well bore with a pre-treatment area of drainage and a post-treatment area of drainage and depicting the subsequent improvement in productivity based on the method of the present invention;
[0028]FIG. 4 is a front elevation view of a typical well bore of the present invention and identifying the flow of air into the hydrocarbon producing reservoir and showing the propagation of heat into the surrounding rock matrix;
[0029]FIG. 5 is a front elevation view of the well bore shown in FIG. 4 after treatment and identifying the production of hydrocarbons into the well bore and the increase in the effective area of drainage;
[0030]FIG. 6 is a top plan view of a wellhead in the present invention and identifying the flow of air and nitrogen and subsequent production of well bore fluids into a separator or flat tank; and
[0031]FIG. 7 is a front elevation view of the well bore of the present invention and identifying the producing reservoir and the injection of air and nitrogen and the subsequent production of produced fluids through the production tubing.
[0032] Referring now to the drawings, FIGS. 1-7 depict various views of a hydraulically fractured reservoir, the penetrating well bore, and the equipment associated with the present invention. More specifically, FIG. 1 depicts a top plan view of a theoretical well bore 6 and associated reservoir which has been hydraulically fractured and further showing the effective drained area 8. FIG. 1 represents a hypothetical or ideal plan view wherein the fracture wings generated by the hydraulic treatment are shown extending horizontally into the hydrocarbon reservoir 2. These fractures are �propped� open by materials such as sand or ceramic beads, thus increasing the permeability and transmissibility of the reservoir fluids and the rate of flow entering the well bore.
[0033]FIG. 2 is a plan view of a well bore 6 and identifying the actual geometry of a typical hydraulic fracture 4 extending into a hydrocarbon reservoir 2. The darker regions of the fracture wings 4 depict a zone damaged by residual polymers remaining after hydraulic fracturing and thus not effective for the production of hydrocarbons. More specifically, as the distance increases away from the well bore 16 the fractures become ineffective due to the presence of unbroken, highly viscous fracture fluids and residues. This creates a decreased pressure drop away from the well bore 6 which combines to make the fluids immobile and impermeable to oil and gas production past the effective fracture zone, thus resulting in reduced production outside of the effective drained area 8.
[0034]FIG. 3 depicts a plan view of the well bore 6 shown in FIG. 2 and the associated hydrocarbon reservoir 2 of the present invention and identifies an increased post treatment area of drainage 10 once the residual polymers and other well bore damage is removed from the hydraulically fractured area. Accordingly, a more effective fracture and improved permeability in the rock matrix is obtained in the treated hydrocarbon reservoir 2.
[0035] Referring now to FIG. 4, a front elevation view of a typical well bore 4 of the present invention is provided herein and which shows casing 12 penetrating a hydrocarbon reservoir 2. In this particular schematic, the injected air is shown being introduced through the perforations 14 and into the hydrocarbon reservoir 2 to create an oxidation process which generates significant heat. The heat and combustion front is advanced and replenished by the cooler injected air, and the propagation of the heat continues along the hydraulic fracture 4 network into the surrounding rock matrix as it is propelled by the available air and oxygen. This effectively creates carbon dioxide gases, low ph water and peroxide byproducts and distillation which assist in the thermal degradation of entrained fracture fluids and the mobilization of the broken gelled fluids and the improved productivity of the hydrocarbon reservoir 2.
[0036]FIG. 5 is a depiction of the well bore 6 shown in FIG. 4, but identifies the production of hydrocarbons after the injection process and treatment of the present invention is completed. More specifically, after the air injection has been discontinued, the bottom hole temperature is allowed to cool to a specified level and the well returned to production. The rate of production is enhanced due to the increased effective fracture length, resulting from the breakdown of viscous polymer gels, which allows contact with higher reservoir pressure and improved permeabilities in the fracture and surrounding rock matrix, hence improving production rates and total recovery of hydrocarbon fluids.
[0037]FIG. 6 is a top plan view identifying equipment utilized in one aspect of the present invention. More specifically, a compressor 15 used to inject the air necessary for combustion, while a nitrogen source maybe used for injection down the annulus between the production tubing 16 and casing 12 and the subsequent necessity of a separator for use during production after treatment. Further, an open top flat tank may additionally be used for sales or the removal of water or other byproducts.
[0038]FIG. 7 is a front elevation view of one aspect of the present invention and further identifies the hydrocarbon reservoir 2, well bore casing 12, production tubing 16, injection lines necessary for the injection of the air and nitrogen, as well as the valves and associated piping required for the present method of treatment. Additionally, a thermocoupler is shown which may be used to monitor the bottom hole temperature throughout the process of the present invention to assure temperatures do not exceed design limitations. FIG. 7 further depicts the well bore 6 extending from the surface of the earth through the overburden and extending into the target hydrocarbon producing formation from which hydrocarbons are recovered by an in-situ combustion process of the present invention.
[0039] In one aspect of the present invention, the operation of the system is as follows. A predetermined amount of water is pumped down the tubing in sufficient volume to kill the well and evacuate the lower portion of oil, condensate or other hydrocarbon products. If metallic salts are required as a combustion catalyst, a predetermined volume is mixed with the water prior to injection. These metallic salts may include, but are not limited to Ammonium, Magnesium, Bismuth, Manganese, Calcium, Nickel, Cobalt, Potassium, Copper, Silver, Iron, Sodium or other transition metal complexes as identified on a periodic chart. Nitrogen is then injected down the casing 12 and production tubing 16 in sufficient quantity to over-flush the associated well bore 6 volume from surface to the base of the perforations 14. Air or oxygen enriched air is then injected into the hydrocarbon reservoir 2 via the tubing 16 at a predetermined constant rate until ignition is indicated by a significant increase in bottom-hole temperature as indicated by the thermocoupler or sudden increases in surface injection pressures. Air injection is continued until a predetermined amount of air volume has been injected or pressures meet certain limits that exceed hydraulic fracture pressures for the Rock Matrix of the hydrocarbon reservoir 2. It is intended that air be injected at pressures that allow diffusion down the previous hydraulic fracture 4 and into the rock matrix, but not to create additional fractures which would diminish the effectiveness of the present treatment.
[0040] In one embodiment of the present invention, air injection is immediately followed by nitrogen gas in sufficient volumes to over-displace the tubing volume by at least 50%. As an inert gas, the Nitrogen gas serves to protect the well bore 6 from propagation of the combustion front back into the well bore and the inherent problems associated with excessive temperatures. As appreciated by one skilled in the art, other inert gases may also be used for the same purpose. The well bore 6 is then shut-in and bottom-hole temperatures are monitored by the thermocoupler. The well is then produced to an open top flat tank and flue gases vented until the bottom-hole temperatures have subsided to within safe operational levels, which typically occurs in a period of about 24 hours. Flue gases are additionally monitored until O2 and N2 levels are deemed to be within accepted pipeline levels, at which time the well is returned to sales.
[0041] As one example of the present invention, the following actual procedure is described below for a hydrocarbon reservoir having a depth of 6700 ft. which was penetrated by 4.5″ casing 12 and 2⅜″ production tubing 16:
[0042] Procedure:
[0043] Embodiment 1�All Perforated Intervals to Be Treated
[0044] 1. Shut-in Offset producing wells
[0045] 2. Pressure test casing to rated minimum wellhead pressure of 3000 psia.
[0046] 3. Flush tubing with enough water to void the well bore volume from the top of the perforations to the bottom of the hole.
[0047] Note: In one preferred embodiment of the present invention, a combination of substances (chemical soups, principally comprised of various metallic salt compounds) are added to the water prior to placement in the well bore, wherein combustion ignition times from certain reservoir temperatures, pressures and air flux rates are known.
[0048] 4. Place wireline thermocoupler down the tubing or casing/tubing annulus to the base of the tubing to monitor bottom hole temperature.
[0049] 5. Inject Nitrogen down the annulus at low rates and a total volume of 1.5 times the annular tubing volume.
[0050] 6. Inject nitrogen down tubing at low rates and a total volume of 1.5 times the tubing volume.
[0051] 7. Immediately follow Nitrogen injection down tubing with air injection. Adjust air injection rates such that monitored pressures are either 1) below maximum rated wellhead pressures or 2) below fracture initiation pressures of the formation.
[0052] 8. Continuously monitor surface pressures for a rid rise indicative of combustion ignition.
[0053] 9. Periodically suspend air injection and monitor bottom hole temperatures for an increase above baseline temperatures indicating combustion ignition.
[0054] Note: If bottom hole temperatures exceed 800 degrees F.; 1) immediately resume air injection and if they do not immediately subside, 2) cease air injection and immediately follow with nitrogen injection down tubing. Shut-in well until bottom hold temperatures subside to 500 degrees F.; resume air injection.
[0055] 10. Continue air injection for a predetermined period of time following combustion ignition (usually 3 to 48 hours).
[0056] 11. Inject nitrogen down tubing with a total volume of 1.5 times tubing volume.
[0057] 12. Shut-in the well for a predetermined period of time (usually 2-5 days).
[0058] 13. Pull thermocoupler from the well bore.
[0059] 14. Flowback well to an open top flat tank. Swab if necessary.
[0060] 15. Monitor oxygen and nitrogen levels of the vented gas until levels have fallen below the designated containment levels for the sales line.
[0061] 16. Return production to the sales line.
[0062] Embodiment 2�Selected Perforated Intervals to Be Treated
[0063] 1. The process is as above with a packer positioned above a bridge plug to isolate the perforation target intervals. This step eliminates the annulus and the need to inject nitrogen in step 5 above.
[0064] To assist in the understanding of the present invention, the following components and numberings associated with the drawings are provided herein:
[0065] The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commenced here with the above teachings and the skill or knowledge of the relevant art are within the scope in the present invention. The embodiments described herein above are further extended to explain best modes known for practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments or various modifications required by the particular applications or uses of present invention. It is intended that the dependent claims be construed to include all possible embodiments to the extent permitted by the prior art.
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