the 
 degree of mixing of fullerene acceptors with conjugated polymer 
 donors used for thin film bhj solar cells has significant implications 
 for determining morphologies and overall device performance . blends of pbttt and pcbm are an ideal bhj system for understanding 
 how molecular mixing influences the outcomes of photovoltaic processes . pbttt chains readily assemble into well - ordered -stacked 
 lamellar crystalline structures , essential 
 for establishing multidimensional charge and energy transfer pathways . 
 extensive x - ray scattering and diffraction , solid - state nmr spectroscopy , differential scanning calorimetry ( dsc ) , and ir spectroscopy studies on pbttt / pcbm blends have demonstrated 
 the preponderance of fullerene intercalation into the polymer alkyl 
 side groups resulting in bimolecular cocrystals following annealing 
 treatments . pbttt lattice spacing and paracrystalline 
 disorder in the -stacking direction increase with fullerene 
 intercalation that is marked by broadening in x - ray diffraction peaks 
 and increased amounts of the gauche alkyl side group conformation . the pbttt / pcbm system has proven 
 a useful model for predicting 
 optimal blend stoichiometries and morphologies , however , solar cell 
 power conversion efficiencies are < 3% , well below current benchmarks . this result is surprising 
 considering the relatively high charge mobilities of pristine pbttt 
 ( > 0.001 cm / v / s at low charge densities ) , yet it underscores the need to better understand how ground state 
 structure and interactions influence excited state photophysical processes . ultrafast pump  probe transient absorption 
 spectroscopy ( tas ) of pbttt / pcbm blends show evidence of charge separation 
 on time scales < 1 ps followed by efficient geminate recombination 
 of separated charge carriers occurring on time scales of 200 
 ps . these results are consistent with 
 a well - mixed phase favoring efficient charge generation but separated 
 carriers lack sufficient transport pathways and can not escape coulomb 
 attractions thus recombining on fast time scales . importantly , x - ray spectromicroscopy studies of pbttt / pcbm blends 
 produced estimates of acceptor miscibility of 42 wt % implying that tas dynamics are dominated by a well - mixed 
 phase but it is difficult to determine if this corresponds solely 
 to bimolecular crystals . furthermore , spectroscopic and charge transport 
 studies have shown evidence of structurally similar pbttt conformational 
 polymorphs ( conformers ) with energy differences of 0.1 ev . pcbm intercalation is expected to disrupt pbttt conformation and 
 packing in blends but connecting specific ground state structures 
 and interactions to the outcomes of excited state photovotlaic processes 
 remains more elusive . here , we use resonance raman spectroscopy 
 to identify signatures 
 of structurally distinct pbttt conformers in pcbm blends and track 
 their evolution by varying processing conditions , composition and 
 excitation energies . resonance raman and photocurrent imaging are 
 then used to spatially map and correlate their contributions to local 
 material performance in model pbttt / pcbm solar cells . we show that 
 the pbttt alkyl - thiophene symmetric c = c stretching vibration 
 ( 14901500 cm ) can be decomposed 
 into contributions from ordered ( lower energy , c = c = 1489 cm ) and disordered ( higher 
 energy , c = c = 1500 cm ) pbttt conformers which are not evident in optical spectra . we propose 
 that both ordered and disordered pbttt species are well - mixed with 
 pcbm but only the former exist only in the bimolecular crystal phase . density functional theory ( dft ) raman simulations and excitation energy 
 dependent raman spectra indicate that ordering of alkyl - thiophene 
 rings defines the two observed pbttt forms and the relative amounts 
 of each are determined by the pcbm loading and film processing conditions . raman excitation profiles further demonstrate that annealing converts 
 the disordered form to bimolecular crystals suggesting that it is 
 in fact a precursor kinetic phase . well - resolved overtone and combination 
 transition intensities ( up to 4 quanta )  chiefly involving 
 the dominant pbttt cc symmetric skeletal vibrations ( 14001600 
 cm )  are also apparent and intensities 
 show much smaller sensitivity to pcbm loading indicating pbttt excitations 
 are electronically localized for both species . raman images 
 demonstrate that a significant fraction of ordered 
 pbttt chains reside in pcbm - rich regions confirming these are indeed 
 bimolecular crystals . however , corresponding photocurrent images show 
 substantial losses in these regions , likely due to efficient geminate 
 charge recombination . intensity modulated photocurrent spectroscopy 
 ( imps ) measurements of annealed pbttt / pcbm devices also display positive 
 phase shifts at low modulation frequencies ( i.e. , photocurrent leads 
 the modulation frequency ) , which is a signature of nongeminate charge 
 recombination becoming operative . imps images 
 of these model pbttt / pcbm solar cells reveal that this process is 
 most prevalent at boundaries of ordered / disordered pbttt and pcbm 
 aggregates . the c14 variant of pbttt ( aldrich , mw=49 kda , pdi = 
 2.3 ) and pcbm ( aldrich ) were dissolved separately in anhydrous o - dichlorobenzene 
 ( 5 and 20 mg / ml , respectively ) inside a nitrogen circulating glovebox . samples were heated ( 100 c ) and stirred for over 8 h to facilitate 
 dissolution and filtered prior to deposition . blend thin films were 
 produced by adding pcbm and pbttt solutions in varying weight / weight 
 fractions , i.e. , 1:1 up to 1:8 ( pbttt / pcbm ) and spin - casting at speeds 
 of 600 rpm for 120s onto rigorously cleaned glass coverslips . blend 
 solutions were prepared and used immediately after heating to avoid 
 precipitate formation or gelling . optical absorption spectra of solutions 
 and thin films were recorded on a uv / vis / nir instrument ( hitachi u4100 ) 
 and raman spectra were measured on a home - built scanning microscope 
 spectrometer system described in detail previously . argon- and krypton - ion laser sources ( exc = 458647 
 nm ; 2.711.92 ev ) as well as a nir laser diode ( exc = 785 nm , 1.58 ev ) were used to selectively excite pbttt 
 in pcbm blends . single molecule images and spectra of pbttt diluted 
 into a polystyrene host matrix were also measured using the above 
 confocal microscope with single photon counting techniques . scattered 
 excitation light was removed by rayleigh rejection filters ( semrock ) 
 prior to entering the polychromator / ccd detector system ( andor ) . relative 
 raman intensity profiles were constructed from comparing excitation 
 energy - dependent raman spectra and comparing to an in situ external 
 sapphire standard . model 
 pbttt / pcbm solar cells were fabricated using the optimal 
 loading ratio reported in the literature ( 1:4 w / w ) and were spun on top of pedot : pss ( bayrtron ) 
 coated indium  tin oxide ( ito ) substrates ( metavac ) that had 
 previously been heated to 150 c for 30 min to evaporate residual 
 water . aluminum was next deposited on top of the pbttt / pcbm blend 
 under high vacuum to complete the device and current voltage 
 ( i  v ) characterization was 
 carried out in the glovebox in the dark and under am1.5 illumination 
 ( newport ) . resonance raman spectroscopic- and 
 photocurrent imaging was performed on as - cast and annealed pbttt / pcbm 
 devices using the above - mentioned raman imaging spectrometer . pbttt / pcbm 
 films were annealed for 30 min at 110 c prior to aluminum 
 deposition in order to generate sufficient phase separation affording 
 better contrast in raman and photocurrent images . devices were studied 
 within a home - built inert gas flow cell and no evidence of material 
 or device degradation was observed in the course of spectral image 
 acquisition or in subsequent images collected over the same scan area . quasi - dc photocurrent imaging was initially performed by modulating 
 the laser excitation ( 100200 hz ) using a mechanical 
 chopper and current was detected using a lock - in amplifier . photocurrent 
 images were generated using substantially lower excitation intensities 
 than raman images ( i.e. , 10w / cm vs 1 kw / cm , respectively ) to avoid nonlinear effects , such as exciton exciton 
 annihilation or other photon - induced bimolecular processes and raman 
 images were constructed using a procedure described previously . imps spectra and images were next recorded using 
 a laser diode source ( exc = 488 nm , omicron phoxx ) 
 modulated by the reference output of the phase - sensitive detector 
 and the frequency was swept in the range of 1 hz up to 20 
 khz . modulation depths were 10% or slightly less to ensure 
 linear response over the entire frequency range . ensemble imps spectra 
 were measured in a widefield configuration whereas imps images were 
 acquired in a confocal geometry and the modulation frequency was held 
 constant over the entire imaging scan . a silicon photodiode with a 
 known frequency response was used as a reference for all imps experiments . raman spectra 
 of a model pbttt monomer were calculated at the b3lyp / def-2sv(d ) level 
 of theory using the orca computational suite . the c14 side 
 groups were replaced with ethyl ( c2 ) groups and a fully 
 planar bttt - c2 monomer and a twisted local minima variant 
 in which the center fused ring is rotated with respect to the two 
 thiophene rings by 29 degrees were simulated . resolution of identity ( ri - j ) and chain of spheres ( cosx ) approximations were utilized with the equivalent 
 quality auxiliary basis set . in 
 general , conjugated polymers become more disordered as fullerene loading 
 increases due to disruption of packing arrangements and reduced planarity 
 between monomers . in crystalline polymers , such 
 as the archetype poly(3-hexylthiophene ) ( p3ht ) system , addition of 
 pcbm breaks up -stacked lamellar chains in aggregates leading 
 to increases in a solution - like amorphous component with higher energy 
 and featureless absorption lineshapes . the ability 
 of pcbm to intercalate into pbttt chains presents an intriguing case 
 challenging traditional views of order / disorder transitions . spectroscopic 
 and electrical imaging approaches are used here to ( i ) identify spectroscopic 
 markers of intercalated pbttt chains in pcbm blends , ( ii ) assess how 
 ground state structures and interactions affect excited state processes , 
 and ( iii ) spatially correlate local composition and order / disorder 
 characteristics to material performance in functioning solar cells . we begin by re - examining the effect of variable pcbm content in 
 pbttt blend thin film absorption spectra which will be useful later 
 for selection of resonance raman excitation schemes ( vide infra ) . figure 1 displays as - cast ( solid traces ) and 
 annealed at 140 c for 20 min ( dotted traces ) pbttt / pcbm blend 
 thin films with 1:1 , 1:2 , 1:4 , and 1:8 w / w ratios deposited on clean 
 glass substrates . improved resolution of vibronic structure near the 
 pbttt absorption onset is observed in addition to a slight red - shift 
 ( 0.04 ev ) relative to the pristine pbttt line shape ( see figure 1 inset ) . as pcbm loading increases , the pbttt contribution 
 decreases concomitantly with an increase of a broad and overlapping 
 higher energy component consistent with the pcbm absorption line shape . spectra from annealed blends show relatively small changes compared 
 to as - cast films suggesting that conversion from a kinetic mixed ( as - cast ) 
 phase to the bimolecular crystal phase is not as efficient as reported 
 previously for higher annealing temperatures ( i.e. , approaching the 
 liquid crystalline transition of pbttt ) . optical absorption spectra 
 of as - cast ( solid traces ) and annealed 
 ( dotted traces ) pbttt / pcbm blend thin films at several pcbm loadings . inset : comparison of pristine pbttt and a 1:2 w / w pcbm blend ( offset 
 for clarity ) . pbttt / pcbm blend absorption 
 spectra in figure 1 generally resemble previously 
 published spectra in the singlet 
 exciton onset region but differ from other reports 
 that show better resolution of vibronic structure . this improvement 
 of vibronic resolution upon pcbm addition and annealing has been attributed 
 to bimolecular crystal formation . however , gasperini and sivula also 
 recently showed that higher molecular weight pbttt ( > 40 kda ) leads 
 to entanglement of chains and rougher film textures that may inhibit 
 bimolecular crystal formation in pcbm blends for larger pbttt molecular 
 weight fractions in our samples . single - molecule 
 images and spectra of pbttt diluted into a polystyrene host matrix 
 were also recorded to verify if pbttt chains were preassociating in 
 solution prior to blending with pcbm , which might also inhibit bimolecular 
 crystal formation . images show well - isolated , diffraction - limited 
 spots and areal densities scale linearly with concentration ( see supporting information ) demonstrating that no 
 agglomeration or gelling occurs in our concentration range . we expect 
 that incomplete conversion of a well - mixed , kinetic pbttt / pcbm phase 
 into bimolecular crystals may arise in films with larger pbttt molecular 
 weight and polydispersity . however , this feature is advantageous for 
 our study because the range of accessible pbttt conformations upon 
 pcbm intercalation can be tracked and correlated with their performance 
 attributes . evidence of adverse intercalation - induced disorder 
 effects on electrical 
 properties in pbttt / pcbm blends manifest as over an order of magnitude 
 decrease of charge mobilities . this dramatic decrease 
 in mobility is believed to originate from twisted pbttt backbones 
 and side group disorder due to intercalated fullerenes . solid - state 
 nmr studies of pbttt / pcbm blends support this view and found that c and h signals from the less - ordered gauche 
 alkyl side group conformer , a minority species in pristine pbttt ( < 10% ) , 
 increase with pcbm loading . likewise , 
 a franck  condon analysis of pristine pbttt absorption lineshapes 
 showed evidence of two distinct transitions with electronic origin 
 transitions separated by 0.1 ev . the relatively small difference in energy between apparent intrinsic 
 pbttt structures potentially complicates the use of absorptive spectroscopies 
 for quantifying amounts and properties of these species since lineshapes 
 strongly overlap and become even more congested in pcbm blends . resonance 
 raman spectroscopy can help overcome these issues using selective 
 resonant excitation schemes to target specific electronic excited 
 states corresponding to distinct pcbm intercalation - dependent pbttt 
 conformers . figure 2 presents representative 
 resonance 
 raman spectra of pbttt / pcbm blend thin films excited with 514.5 nm 
 ( 2.41 ev ) light corresponding to the maximum of the pbttt absorption 
 line shape . resonance excitation usually leads to large enhancements 
 ( 1  10 to 1  10 ) in raman 
 scattering cross sections of the resonant chromophore . contributions 
 from pcbm in the excitation wavelength range used are absent demonstrating 
 that pbttt raman cross - section enhancements overwhelm those of pcbm 
 despite that it is usually present in larger concentrations ( e.g. , 
 > 1:1 w / w ) . well - resolved progressions of overtone and combination 
 bands , mainly involving vibrations of pbttt c  c and c = c 
 stretching character ( see table 1 ) , are visible . 
 comparison to ir absorption spectra of pbttt / pcbm blends in the first 
 overtone / combination band region ( 02 ) as well as the appearance 
 of higher order progressions confirm these are not fundamentals of 
 high frequency modes ( i.e. , c table 1 lists mode assignments of 
 both fundamental and overtone / combination bands ( for only the 02 
 region ) . resonance raman spectra of pbttt / pcbm blend thin films ( excitation 
 energy = 2.41 ev ) with overtone / combination band transitions highlighted 
 and shifted to the most intense transition ( inset ) . determined from dft simulations 
 ( see below ) and ref . from figure 2 c 
 and c = c symmetric stretches of the thiophene and thienothiophene 
 backbone rings display pure overtone transitions . higher - order ( > 02 ) 
 overtone / combination band clusters show greater broadening , probably 
 because of dispersion effects ( i.e. , wavepacket broadening ) due to 
 coupling between nuclear motions . weak clusters of combination bands 
 from multiple low frequency vibrations , likely from thiophene - thienothiophene 
 ring bends and librations , are also apparent before the 02 
 region ( 17002500 cm ) . the appearance 
 of multiple apparent progression - forming modes suggests that vibrational 
 displacements are large ( i.e. , large huang rhys factors , s = in1/2i2 , where i is the displacement 
 for mode i. however , pbttt absorption band line widths 
 ( fwhm ) are not significantly different than p3ht or other crystalline 
 polymers ( s  1.0 ) meaning that the total vibrational displacements are probably similar . namely , the weakly resolved 
 vibronic interval can be explained by the franck  condon displacement 
 of multiple low frequency modes causing the  valleys  
 between dominant mode progressions ( i.e. , the pbttt cc backbone symmetric 
 stretches , 37 ) to become filled in . conversely , 
 increased inhomogeneous broadening might also explain larger absorption 
 vibronic line widths in pristine pbttt , however , narrowing of line 
 widths in pcbm blends suggests increased order or longer excited state 
 lifetimes . the latter effect is not expected due to the presence of 
 intimately mixed pcbm electron acceptors . it is also useful to point 
 out that overtone / combination band intensities show less sensitivity 
 with increased pcbm loading ( constant excitation energy ) implying 
 that either disorder effects are not important until longer times 
 ( several vibrational periods , > 100 fs ) or chromophores are spatially 
 localized making them less sensitive to disorder . typically , in large 
 molecules with many displaced modes , overtone / combination intensities 
 are usually extinguished before the first overtone ( 02 ) region 
 because of destructive interference caused by rapid damping from strong 
 coupling to the bath or among chromophores of different energies ( inhomogeneous 
 broadening ) . this effect appears suppressed 
 in pbttt systems and we speculate the persistence of the multimode 
 overtone / combination band transitions in pbttt / pcbm blend raman spectra 
 arises from weak coupling to the phonon bath and small contributions 
 from inhomogeneous broadening effects . the qualitative picture 
 emerging from raman trends reported in 
 figure 2 is that the multidimensional excited 
 state wavepacket survives for longer times allowing sufficient buildup 
 of overlap and overtone / combination intensities . this scenario is 
 most consistent with localized excitations despite the relatively 
 high order of pbttt ( even in pcbm blends ) that intuitively suggest 
 delocalized electronic structures . the implications of localization / delocalization 
 in polymeric solar cells are significant and have been the subject 
 of recent investigations of ultrafast charge separation . for example , jamieson et al . highlighted the importance of fullerene 
 crystallites in promoting charge separation while simultaneously suppressing 
 geminate recombination in several polymer / fullerene systems that show 
 varying degrees of mixing . further insights into the nature of pbttt chromophores pbttt / pcbm 
 blends can be obtained from resonance raman spectra as a function 
 of excitation energy spanning the pbttt optical absorption line shape 
 ( 1.92.7 ev ) . figure 3 displays 
 variable excitation energy raman spectra and are normalized to the 
 thienothiophene ring c = c symmetric stretch ( 1415 cm mode , 4 ) for comparison . raman patterns show significant 
 changes with excitation energy consistent with resonant excitation 
 of distinct pbttt chromophores . in the 01 region , the relative 
 intensity of the 1391 cm mode ( thiophene symmetric 
 c  c stretching character ) decreases and the 14891500 
 cm band region of the symmetric c = c thiophene 
 ring stretch gains in intensity in addition to apparent blue - shifting 
 and broadening with increased excitation energies . comparison of the 
 two pcbm loadings also demonstrates specific interactions with pbttt 
 backbones . for example , a large increase in relative intensity is 
 observed for the 14891500 cm mode 
 in the 1:4 blend for excitation near the pbttt resolved absorption 
 onset ( 1.92 ev ) , suggestive of bimolecular crystals . pbttt / pcbm ( 1:1 and 1:4 
 w / w loadings ) resonance raman spectra as 
 a function of variable excitation energies displayed in the fundamental 
 ( 01 ) and first overtone ( 02 ) regions of the main pbttt 
 backbone stretching modes . corresponding optical absorption spectra chromophore - specific 
 resonance enhancement is more obvious in the 
 first overtone ( 02 ) region where increasing excitation energy 
 causes intensity redistributions toward higher frequencies . residual 
 fluorescence masks overtone / combination bands in the background noise 
 at the lowest excitation energy ( 647 nm , 1.92 ev ) and these spectra 
 were not included . for comparison , we measured raman spectra of pristine 
 pbttt and as - cast 1:1 w / w pbttt / pcbm thin films under nonresonant 
 conditions ( exc = 785 nm , 1.58 ev ) , that show pronounced 
 red - shifts of the main pbttt skeletal stretching vibrations for the 
 blend ( see the supporting information ) . it is likely that nascent bimolecular crystals in the blend become 
 preresonant at this excitation energy , which also gives rise to very 
 weak overtone transitions . we propose that line shape ( intensity ) 
 changes with excitation 
 energy reflect the presence of both ordered and disordered pbttt conformations 
 whose populations are modulated by pcbm loading and annealing . raman 
 excitation profiles ( reps ) are now constructed to test this hypothesis 
 that reveal vibrational mode - specific views of the excited state potential 
 energy landscape . figure 4 shows reps from 
 as - cast pbttt / pcbm films ( solid traces ) for all backbone skeletal 
 vibrations showing appreciable intensity in resonance raman spectra 
 in figures 2 and 3 ( 37 ) and intensities are reported relative to a nonabsorbing 
 external standard ( i.e. , sapphire ) . generally , reps bear similarity 
 to absorption lineshapes provided that raman and absorption transitions 
 involve only a single excited state ( i.e. , single absorber ) . rep lineshapes 
 in figure 4 show noticeable deviations from 
 one - photon absorption spectra ( figure 1 ) confirming 
 contributions from multiple states . in particular , a pronounced dip 
 around 2.35 ev is observed as well as increased activity ( cross 
 sections ) in the higher energy region of the main pbttt absorption 
 line shape . as pcbm concentration increases , the relative intensities 
 of the lower energy feature decrease for all mode - specific reps reported . the dip at 2.35 ev probably results from the crossing of excited state 
 potential energy surfaces of two states leading to destructive quantum 
 interference and intensity de - enhancements . raman 
 excitation profiles ( reps ) of the pbttt backbone symmetric 
 stretching fundamental ( 01 ) region from variable pbttt / pcbm 
 loadings . on the basis of the trends observed 
 here and from previous studies , 
 it is relatively straightforward to assign the low ( high ) energy rep 
 feature to ordered ( disordered ) pbttt chains . because of the amounts 
 of pcbm used , pbttt should always exist in a mixed phase owing to 
 large cohesive energy densities between these molecules and available 
 intercalation sites . we next measured reps of an annealed 
 1:4 w / w pbttt / pcbm blend ( dotted traces , figure 4 ) and compare these to as - cast rep lineshapes . a pronounced decrease 
 of the higher energy rep component is apparent indicating conversion 
 to pbttt chains with improved backbone and side group order consistent 
 with intercalated bimolecular crystals . this result highlights the 
 greater sensitivity of raman techniques to chromophore environments 
 compared to one - photon absorption spectroscopy ( figure 1 ) , which can obscure contributions from closely overlapping 
 states . we now focus on the 14891500 cm region assigned to the c = c symmetric stretch 
 of the thiophene 
 rings ( 6,7 , table 1 ) that 
 are particularly sensitive to pcbm loading and excitation energy ( figure 3 ) . figure 5a presents resonance 
 raman spectra generated from a pbttt / pcbm blend thin film of a 1:2 
 w / w ratio in the thiophene c = c symmetric stretching fundamental 
 region . excitation at the pbttt red absorption onset ( i.e. , 1.92 ev ) 
 selects the 1489 cm ( 6 ) component 
 that subsequently blue - shifts and coalesces into the 1500 
 cm mode ( 7 ) at higher excitation 
 energies ( i.e. , 2.71 ev ) . these trends have been explained previously 
 from theoretical studies of oligothiophenes where lower energy chromophores 
 ( viz . longer conjugation lengths ) show red - shifted raman - active backbone 
 vibrations . the 1500 cm mode is proposed to originate from disordered pbttt c = c thiophene 
 rings , probably because of fullerene intercalation - induced disorder 
 among the alkyl side groups causing twisting of the backbone thiophene 
 rings and greater paracrystalline disorder . likewise , the 1489 cm component of the symmetric thiophene c = c stretch 
 derives intensity from ordered pbttt chains in bimolecular crystals , 
 where side group and backbone order is improved . line widths of both 
 pbttt c = c thiophene forms are also consistent with their proposed 
 structural origins , namely , ordered conformers are 15 cm compared to 25 cm for 
 disordered chains because of heterogeneity . this feature is also consistent 
 with improved vibronic resolution in absorption spectra of pbttt / pcbm 
 blends , which indicates the presence of ordered pbttt chains in bimolecular 
 crystals . the relative amounts of ordered and disordered pbttt conformers ) 
 are estimated by deconvoluting the c = c thiophene band using 
 two line shape functions corresponding to the 6 and 
 7 bands ( table 1 ) . we do 
 not attempt to obtain absolute cross sections for each species but 
 it is expected that these values are similar because of the the structural 
 similarity of ordered and disordered forms ( i.e. , energy difference 
 of 0.1 ev or less ) , which is much subtler than in other crystalline 
 polymers displaying polymorphic behavior , such as p3ht . in this description , the total c = c thiophene raman band intensity is a linear combination 
 of both components and we use this simple model to assess how the 
 amounts of disordered pbttt conformers , ( ( i7)/(i6+i7 ) ) , 
 change with blend film processing conditions . we further speculate 
 that the disordered component is the precursor species to the ordered 
 pbttt form in bimolecular crystals and the evolution of both forms 
 can be revealed from raman spectra as a function of varying pcbm loading 
 and excitation energy . figure 5b presents estimates 
 of the disordered pbttt content in as - cast blends , which increases 
 with pcbm content and excitation energies . for comparison , ( ( i7)/(i6+i7 ) ) values were determined for an annealed blend ( 1:4 
 w / w ) and displayed in figure 5b , which has 
 significantly less of the disordered pbttt because of conversion into 
 bimolecular crystals which is consistent with rep trends in figure 4 and corresponding plots of ( i7)/(i6+i7 ) values confirm this behavior ( not shown ) . ( a ) resonance raman spectra 
 of as - cast pbttt / pcbm blend thin films 
 ( 1:2 w / w ) showing lineshapes of the two distinct pbttt forms ; ordered 
 ( 6 ) and disordered ( 7 ) pbttt chains . ( b ) percent of disordered species present in all blend ratios as a 
 function of excitation energy . the dashed line represents the resonance 
 maximum excitation energy of the disordered form ( 2.71 ev ) . dft 
 raman simulations of a planar and twisted pbttt model monomer system 
 ( bttt - c2 ) are next performed to validate our proposed model 
 and are shown in figure 6 with their respective 
 structures . calculated dft raman line shape trends agree very well 
 with experiment although nonresonant conditions are used in simulations 
 and a scaling factor of 0.95 must be applied to calculated frequencies 
 in order to compare with experiment . the nominal c  c symmetric 
 stretch appearing at 1461 cm for the planar conformation 
 undergoes a blue shift of 7 cm in the 
 twisted variant . most notably , intensity redistributions occur between 
 the inter - ring c = c symmetric stretch ( 1541 cm ) and the thiophene c = c stretch ( 1594 cm ) depending on backbone planarity . for example , the latter increase 
 in intensity for the twisted bttt - c2 variant whereas the 
 former dominate in the planar monomer . previous dft simulations of 
 planar and twisted pbttt trimer structures show similar trends as 
 presented in figure 6(40 ) confirming that at least two pbttt types are present in pcbm blends 
 revealed from pcbm loading- and excitation energy dependent raman 
 spectra . moreover , calculated frequencies and intensities of trimers 
 were very similar to experimental raman spectra , suggesting that excitations 
 are probably localized to a few monomer units . thus far we have 
 shown that the ability of the two species model to decompose key raman 
 bands into separate contributions from morphology - dependent pbttt 
 conformers offers a much simpler means to assess order / disorder transitions 
 in this system . although dft simulations predict observed experimental 
 behavior , they do not allow us to incorporate side group disorder 
 and paracrystallinity . it is also important to note that ordered pbttt 
 chains in bimolecular crystals are twisted but side group disorder 
 should be diminished relative to the kinetic disordered intercalated 
 form . simulated raman spectra of the bttt - c2 monomer and structures . despite the relatively 
 poor performance of pbttt / pcbm , we have thus far demonstrated its 
 value as a model for understanding donor / acceptor interactions and 
 supramolecular organization and their impact material performance . resonance raman spectroscopic and photocurrent imaging techniques 
 are now employed to spatially resolve how ordered and disordered pbttt 
 chains impact local device performance in model solar cell devices . 
 for these experiments , as - cast and annealed pbttt / pcbm blends in 1:4 
 w / w ratios ( the optimal blend ratio reported for solar cells ) were prepared to compare morphology - dependent order / disorder spatial 
 distributions . because these studies emphasize model pbttt / pcbm morphologies , 
 power conversion efficiencies were < 1% but nonetheless show good 
 stability and expected diode - like behavior ( see the supporting information ) . we emphasize annealed devices because 
 of better material performance in addition to better contrast in raman 
 and photocurrent images . figure 7 shows 
 representative resonance raman and photocurrent images of a 1:4 w / w 
 annealed device and appreciable microscopic phase segregation is apparent 
 within the active layer . raman images represent the 14891500 
 cm spectral region of the thiophene c = c 
 stretches and are generated using 458 nm ( 2.71 ev ) excitation light . this excitation energy was specifically chosen to selectively interrogate 
 disordered pbttt chains and their spatial locations relative to ordered 
 chains in bimolecular crystals . because charge transfer and recombination 
 processes are strongly dependent on local polymer ordering and composition 
 we expect significantly different responses for each pbttt form . figure 7a presents the total integrated intensity 
 of the 6,7 modes and corresponding photocurrent 
 over the same area is shown in figure 6b . lower 
 pbttt intensity represents pcbm - rich areas , but an appreciable amount 
 of pbttt still exists in these regions indicating these are most likely 
 bimolecular crystals . because the pcbm loading of these devices is 
 high ( 1:4 w / w ) , it is unlikely that pbttt completely phase separates 
 meaning the entire film corresponds to ordered and disordered mixed 
 phases . photocurrent images ( figure 7b ) show 
 much lower output in putative bimolecular crystal regions probably 
 originating from unbalanced charge transport as expected from increased 
 charge recombination . ( a ) total integrated raman intensity of 
 c = c symmetric stretching 
 mode ( 6,7 ) and ( b ) corresponding photocurrent images 
 of annealed pbttt / pcbm ( 1:4 w / w ) device ( excitation energy = 2.71 ev ) . ( c , d ) ratios and ( e , f ) frequency dispersion of ordered ( 6 ) and disordered ( 7 ) pbttt species , respectively . 
 a  5% tolerance was applied for determining the center frequencies 
 of each pbttt component . the pbttt thiophene c = c stretch is now decomposed 
 into its 
 constituent components and figures 7c , d shows 
 fractional compositions of ordered ( ( i6)/(i6+i7 ) ) 
 and disordered ( ( i7)/(i6+i7 ) ) forms , respectively , 
 using the same procedure described above . comparison of these images 
 with morphology - dependent frequency dispersion characteristics ( figure 7e , f , respectively ) confirm that bimolecular crystals 
 are indeed most concentrated in pcbm - rich regions ( i.e. , lower pbttt 
 intensities , figure 7a ) . although raman signatures 
 for pcbm are absent , direct excitation may lead to photocurrent from 
 hole transfer to pbttt from photoexcited pcbm . we measured photocurrent 
 images on the same devices using 488 nm ( 2.54 ev ) light , which is 
 near the pbttt absorption maximum , but identical behavior is observed 
 ( see the supporting information ) . the effects 
 of excitation intensity and pcbm crystal size ( annealing time ) on 
 local photocurrent production were also investigated and no significant 
 differences were found ( see supporting information ) . we speculate that increased 
 geminate charge recombination dominates in bimolecular crystals , which is consistent with previous 
 tas studies of pbttt / pcbm thin films predominantly in the bimolecular 
 crystal phase . raman and photocurrent images of as - cast pbttt / pcbm 
 ( 1:4 w / w ) devices 
 were also measured under the same conditions as annealed devices ( see 
 the supporting information ) . these devices 
 generally show relatively uniform morphological features and photocurrent 
 is at least an order of magnitude smaller than annealed devices when 
 illuminating with a diffraction - limited laser spot . resonance 
 raman and photocurrent images have so far demonstrated 
 that morphology - dependent variations in material performance are determined 
 not only from the type and amounts of pbttt species but also their 
 spatial distributions in the device active layer . intensity modulated 
 photocurrent spectroscopy ( imps ) and imaging is next used to expose 
 how specific conformers and morphologies impact loss mechanisms in 
 pbttt / pcbm devices , namely , charge recombination . imps uses 
 a small ( 10% ) sinusoidal modulation of the excitation 
 source and the frequency is swept over several decades ( typically , 
 0.1 hz up to 1 mhz ) . figure 8 shows imps ensemble spectra from annealed ( figure 8a , c ) and as - cast ( figure 8b , d ) pbttt / pcbm 
 ( 1:4 w / w ) solar cells recorded by using a widefield configuration 
 that illuminates the entire device active area ( 20 mm ) . ensemble imps sweeps show similar behavior as reported 
 previously in related polymer / fullerene solar cells and a characteristic 
 maximum is observed in photocurrent sweeps and the phase decreases 
 significantly in this region toward its maximum ( 180 ) . photocurrents in the low frequency regime ( < 1 khz ) are almost entirely 
 real and phase shifts are positive ( < 10 ) for annealed devices 
 indicating that charge carriers lead the modulation frequency . on 
 the other hand , as - cast films typically show lower photocurrents and 
 phase shifts start at 0. at larger modulation frequencies , 
 both devices show increasingly negative phase shifts due to charge 
 carriers lagging behind the modulation frequency . imps spectra ( photocurrent 
 and phase shift ,  ) and nyquist 
 ( complex ) plots of ( a , c ) annealed and ( b , d ) as - cast pbttt / pcbm ( 1:4 
 w / w ) solar cells , respectively . seminikhin and co - workers first reported positive phase shifts 
 at low modulation frequencies or , a component in the first quadrant 
 of the complex ( nyquist ) imps plot from p3ht / pcbm devices . this feature has been attributed to nongeminate 
 charge recombination that becomes more pronounced as the device ages . luther and co - workers recently advanced this understanding by systematically 
 studying device aging and preparation conditions and tracking imps 
 responses . these authors showed that 
 first quadrant photocurrent contributions in nyquist plots , result 
 from the formation of deep traps and introduced a drift - diffusion 
 model to account for this behavior . the 
 observation of positive phase shifts in the low - frequency modulation 
 regime demonstrates that nongeminate recombination processes become 
 operative in annealed pbttt / pcbm devices indicating suppression of 
 prevailing geminate recombination . the lack of this signature in as - cast 
 devices ( either fresh or aged ) supports this view because geminate 
 processes occur on faster time scales beyond what is currently accessible 
 by imps techniques . nonetheless , our ability to spatially correlate 
 the specific pbttt phase to local photocurrent production can be leveraged 
 to map recombination sites or zones to morphological boundaries using 
 a hybrid imps imaging approach . imps images were generated using 
 the same high na objective used 
 for raman and quasi - dc photocurrent images and are shown in figure 9 . scan ranges were expanded to 20  20 m 
 and laser modulation frequencies were held fixed throughout the scans 
 using similar power densities as the quasi - dc photocurrent images 
 shown in figure 7 . several modulation frequencies 
 were selected representing different charge transport and recombination 
 regimes observed in ensemble imps spectra , namely , low frequency ( e.g. , 
 1 and 3 khz ) , near the maximum frequency photocurrent ( 7 khz ) 
 and at the high - requency regime ( 9 khz ) . pbttt / pcbm blends 
 were annealed for longer times in order to achieve greater phase separation 
 that allows us to better resolve distinct phase boundaries . similar 
 to quasi - dc raman and photocurrent images shown in figure 7 , bimolecular crystals in figure 9 produce lower photocurrent output than the surrounding mixed 
 phase . from the imps phase shift (  ) images , positive phase 
 shift accumulates on the periphery of these regions . as the modulation 
 frequency increases past the maximum , imps images lose contrast owing 
 to carriers lagging behind the modulation . we infer that boundaries 
 surrounding regions of positive phase shift ( or , relative positive 
 phase shift at higher modulation frequencies ) represent recombination 
 zones for nongeminate processes since separated electron  hole 
 carriers can diffuse away from the interface before becoming trapped . 
 in this case , trap sites are probably located at phase boundaries 
 between ordered and disordered pbttt regions , which is consistent 
 with the current and phase maps ( figure 9 ) . despite loss of resolution at higher modulation frequencies , these 
 results in general show that lateral diffusion effects are probably 
 not significant since features of size scales comparable to the diffraction 
 limit are resolvable . imps photocurrent ( left ) and phase shift (  , right ) 
 images 
 of same area at 1 khz , 3 khz , 7 khz , 9 khz laser modulation frequency 
 of annealed pbttt / pcbm ( 1:4 w / w ) device ( excitation energy = 2.54 
 ev ) . image scan area = 400 m . on the basis of optical and raman spectra of pristine 
 pbttt and 
 pcbm blends , small energetic differences between ordered and disordered 
 pbttt species imply that charge traps are mostly shallow in nature . phenomena 
 occur in pristine pbttt where facile detrapping of charges occurs 
 due to small structural changes ( kbt ) . however , greater disorder in blends probably results in deeper traps , 
 which is evident from imps results . although it is unclear if a similar 
 self - healing mechanism is conceivable in pbttt / pcbm solar cells , effective 
 management of order  disorder boundaries through new molecular - level 
 organization strategies may help overcome detrimental trapping effects . we have shown that resonance 
 raman spectroscopy of pbttt / pcbm blends 
 can be used in a straightforward manner to extract the relative amounts 
 of ordered and disordered pbttt conformers . both ordered and disordered 
 components exist in a mixed phase but , the former are found in bimolecular 
 crystals whereas the latter correspond to precursors of the ordered 
 forms . the larger frequency and line width of disordered chains likely 
 originate from greater side group disorder and highly twisted monomer 
 thiophene rings due to disordered alkyl side groups . dft simulations 
 and excitation energy dependent raman spectra supported this assignment 
 although we point out that previous studies have shown that pbttt 
 chains in the bimolecular crystals also tend to twist around the intercalated 
 fullerene but side group and paracrystalline disorder is reduced . 
 this common structural trait shared between ordered and disordered 
 pbttt chains localizes excitations , hence , the invariance of raman 
 overtone - combination intensities with pcbm loading . resonance raman 
 and photocurrent images next expose the morphology dependence of intercalation - induced 
 order / disorder and its influence on local current generation . imps 
 spectra and images showed evidence for increased nongeminate recombination 
 at the boundaries between bimolecular crystals and disordered mixed 
 zones . overall , these experiments help bridge the gap in understanding 
 of how ground - state structure and acceptor interactions influence 
 the outcomes of excited state photovoltaic processes .
resonance 
 raman spectroscopy was used to identify ordered and disordered 
 conformers of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene ) ( pbttt ) blended with the electron acceptor 
 [ 6,6]-phenyl c61 butyric acid methyl ester ( pcbm ) in bulk 
 heterojunction ( bhj ) solar cells where pcbm intercalates into pbttt 
 side groups . 
 we show that the pbttt thiophene ring symmetric c = c 
 stretching mode consists of contributions from ordered ( c = c = 1489 cm1 , fwhm  15 
 cm1 ) and disordered ( c = c = 1500 cm1 , fwhm  25 cm1 ) components and their relative amounts are sensitive to pcbm loading , 
 annealing and excitation energy . 
 the 1500 cm1 pbttt 
 component originates from twisted thiophene rings and disordered side 
 groups due to pcbm intercalation in a mixed kinetic phase and thermal 
 annealing promotes ordering of pbttt chains from the formation of 
 bimolecular pbttt / pcbm crystals . 
 density functional theory ( dft ) raman 
 simulations of pbttt monomers support these assignments . 
 resonance 
 raman images of annealed pbttt / pcbm model solar cells confirm that 
 ordered pbttt chains are most concentrated in pcbm - rich bimolecular 
 crystals and corresponding intensity modulated photocurrent spectroscopy 
 ( imps ) and imaging measurements show increased nongeminate charge 
 recombination at the boundaries of ordered / disordered regions .