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Effects of methylamine doping on the stability of triple cation (FA0.95−xMAxCs0.05)PbI3 single crystal perovskites

Despite being promising photovoltaic materials, widespread use of organic–inorganic halide perovskite materials is still hindered by their undesirable stability. To cope with this challenge, methylamine (MA) is doped into triple cation perovskite single crystals of (FA0.95−xMAxCs0.05)PbI3, and cesium-containing triple cation perovskite single crystals with five different MA molar ratios (x = 0, 0.05, 0.10, 0.15, and 0.20) are synthesized and characterized. Among them, (FA0.8MA0.15Cs0.05)PbI3 shows high stability against water-oxygen and light for 60 days, and the thermal decomposition temperature of (FA0.8MA0.15Cs0.05)PbI3 reaches as high as 305 °C. Besides, the carrier lifetime of (FA0.8MA0.15Cs0.05)PbI3 is up to 5.957 μs, which remains as 5.646 μs (95%) after 60 days of light illumination. This work studies the stability of perovskite single crystals based on (FA0.95−xMAxCs0.05)PbI3 compositions and provides a reference for the discovery of novel perovskite photovoltaic devices with high efficiency and long-term stability.


Introduction
Perovskite solar cells (PSCs) have received wide attention due to their easy preparation [1][2][3][4][5][6][7][8][9] and ever-increasing photoelectric conversion efficiency (PCE). [10][11][12][13][14] Currently, the certied highest PCE of PSCs has reached 25.2%. 15 The high light absorption 16 and tunable cation components [17][18][19] of perovskites make them promising for application in PSCs. However, compared with commercial silicon-based solar cells (with more than 20 years of service life), 20 the undesirable stability of PSCs still hinders their rapid development and widespread use. The performance of perovskite photovoltaic devices was threatened by light, heat and water-oxygen, [21][22][23][24][25] and the main reason is the instability of the perovskite material itself. For example, MAPbI 3 perovskite was reported to be sensitive to moisture [26][27][28] and temperature. [29][30][31] FAPbI 3 (ref. 32 and 33) perovskite easily decomposes into yellow phase d-FAPbI 3 at room temperature, which leads to the loss of optical properties. Snaith 34 reported that replacing some of the formamidinium (FA) in FAPbI 3 with methylamine (MA), forming (FA/MA)PbI 3 perovskite, can efficiently inhibit its phase separation and enhance the stability of the perovskite. Besides, doping an appropriate amount of cesium (Cs) ions can reduce trap state density and further improve the thermal stability of (FA/MA)PbI 3 perovskite. The introduction of the Cs ion can inhibit the formation of yellow phase FAPbI 3 due to its smaller radius. However, excessive doping of Cs will lead to the decrease of crystal size and induce phase decomposition forming CsPbI 3 . 32, [35][36][37] According to the literature reports, doping a trace amount of Cs ions would be optimal for (FA/MA) PbI 3 perovskite.
In addition, most of the studies on PSCs are based on polycrystalline lms with pores, grain boundaries and defects, [38][39][40][41][42] resulting in severe hysteresis and environmental sensitivity, 43 which are detrimental to electron transport in perovskites and long-term stability 44 of PSCs. Nowadays, single crystal perovskites have drawn a lot of attention in the development of PSCs, 45 and perovskites with low defect states and suitable optical band gaps benet both the photovoltaic performance and long-term stability of PSCs. Compared with polycrystalline thin lms, FAPbI 3 single crystal perovskite has good thermal resistance, 46 but the optical properties and carrier dri of the FAPbI 3 single crystal material are seriously affected by its easy decomposition into yellow phase d-FAPbI 3 . 47 MAPbI 3 single crystal perovskite is the most widely used because of its excellent photoelectric performance, and the formation of (FA/MA) PbI 3 perovskite can effectively improve the structural stability. 48 And aer doping a small amount of Cs ion into the FAPbI 3 single crystal perovskite, the density of defect states is reduced. 49 Thus, doping a trace amount of Cs ions into (FA/MA) PbI 3 single crystal perovskite would lead to better performance than the polycrystalline counterpart.
In this work, a series of mixed cation perovskites (FA 0.95Àx -MA x Cs 0.05 )PbI 3 were prepared as single crystals, and a systematic study of their long-term stability, including water-oxygen, light and thermal stability was carried out, respectively. Related optimization effects were investigated and discussed in this paper. And the optimal cation doping ratio was determined based on the characterization of the properties of the materials. Cesium-containing triple cation perovskite single crystals with excellent stability and a long carrier lifetime were obtained, and they can be used as light absorption layers in high PCE and stable PSCs.

Growth of perovskite single crystals
A series of (FA 0.95Àx MA x Cs 0.05 )PbI 3 single crystals were prepared using the inverse temperature crystallization (ITC) method. to nd the complete dissolution temperature and the crystallization temperature, and during this process the solution will change from a clear yellow solution to a black suspension, as shown in ESI Video 1. † The growth temperature of each crystal composition can be precisely and efficiently determined by this method. And the perovskite samples were slowly heated from the complete melting temperature to the crystallization temperature with 1 C/30 min. Finally, the perovskite samples were sintered at the crystallization temperature for 3 hours, and all the parameters are listed in Table 1.

Perovskite single crystal alloy characterization
EDS mapping of the perovskite single crystal powders was performed using a eld-emission scanning electron microscope (SEM) (Jeol JEM 6510LV). The X-ray diffraction (XRD) spectra were obtained using a Bruker-AXS D8 Advance X-ray diffractometer. Absorbance was tested using a UV-vis spectrophotometer (UV 3600, Shimadzu). Steady-state photoluminescence (PL) experiments were performed using an FLSP920 spectrometer (Edinburgh Instruments Ltd), and transient PL measurements were carried out using an FLS 980E uorometer. The PL spectra were measured with a 485 nm laser as the excitation light source. Thermogravimetric Analysis (TGA) was carried out on a Mettler-TGA1 to characterize the thermal stability of the samples. And the long-term stability, water and oxygen stability, and light and thermal stability of the perovskite single crystal samples were evaluated, respectively. For the water and oxygen stability tests, the perovskite single crystal powders were put in a brown desiccator with a humidity of 20% and characterized intermittently for 60 days. For the light stability tests, the perovskite single crystal powders were stored in a self-made sealed box lled with argon, and kept in an ultraviolet aging box (Q-Lab, USA) for 60 days to conduct the "Day and Night" simulation test. Aer 12 hour illumination, the samples were kept in the dark for 12 hours, and the above-mentioned treatments were repeated. The device performances were measured under ambient conditions (15% < relative humidity (RH) < 60%) every time.

Results and discussion
Cs containing triple cation (FA 0.95Àx MA x Cs 0.05 )PbI 3 perovskite single crystals were synthesized as shown in Fig. 1. EDS mapping was carried out on the single crystal powders (Fig. 2), and the uniform distribution of the Cs element indicates the successful doping of the Cs element into the (FA 0.8 MA 0.15 Cs 0.05 )PbI 3 perovskite. In order to analyze the crystal structure of the perovskites, the powder samples were subjected to XRD tests. As shown in Fig. 3a, diffraction peaks at 11.6 were detected from (FA 0.95 Cs 0.05 )PbI 3 , (FA 0.9 MA 0.05 Cs 0.05 )PbI 3 and (FA 0.85 MA 0.1 Cs 0.05 ) PbI 3 samples, and can be assigned to the (010) plane of the yellow phase d-FAPbI 3 . And the 11.6 peak disappeared when the MA doping ratio reached 0.15, which suggests that yellow phase d-FAPbI 3 formation can be efficiently inhibited with the addition of the MA cation (Fig. 3a). And the strong peaks at 14.1 , 28.3 and 31.8 , assigned to (110), (220) and (310) planes, proved the successful synthesis of cesium-containing triple cation perovskite single crystals. The magnied XRD pattern of the (110) plane is displayed in Fig. 3b. Obviously, all diffraction peaks demonstrate a shi to a high degree, and this is due to the smaller radius of the MA (2.17 A) ions compared to the FA (2.53 A) ions and the  partial replacement of the FA ions with MA ions in the perovskite structure, causing lattice shrinking. 50 Fig. 4 shows the UV-vis absorption spectra of the perovskite single crystals. A single crystal perovskite with high absorption at 830 nm is obtained. With MA ion doping, the absorption intensity of the perovskite single crystal is signicantly increased, which could be the reason for the formation of a black a-phase perovskite (as shown in Fig. 3a) resulting in higher light absorption. 51 According to the absorption spectra, the band gaps of the perovskite single crystals of (FA 0. 9 (Fig. 5a-d). With the increase of the molar ratio of MA cations, the band gaps of the perovskite single crystals are slightly widened, which can be attributed to the smaller radius of the MA cation compared to that of the FA cation. 52 Doping a smaller quantity of MA into the perovskite crystal structure shrinks the crystal lattice, according to the Bragg equation, and the XRD peaks shi to the right, corresponding to Fig. 3b, and the bandgap is widened. 53,54 Emission peaks of single crystals of (FA 0. 9     nonradiative recombination. 46 Moreover, transient PL measurements (Fig. 6b)    This journal is © The Royal Society of Chemistry 2020 Nanoscale Adv., 2020, 2, 332-339 | 335 Paper (FA 0.9 MA 0.05 Cs 0.05 )PbI 3 , and all parameters are summarized in Table 2). In order to evaluate the water-oxygen stability of the perovskite single crystals, we placed the perovskite powders in a brown dryer and stored them in a dark environment, exposed to water (20% humidity) and oxygen. It is found that the (FA 0.9 MA 0.05 Cs 0.05 )PbI 3 sample gradually turns yellow, and (FA 0.85 MA 0.1 Cs 0.05 )PbI 3 , (FA 0.8 MA 0.15 Cs 0.05 )PbI 3 and (FA 0.75 -MA 0.2 Cs 0.05 )PbI 3 samples still remain black. Aer 60 days of water-oxygen treatment, phase characterization was carried out on all samples by XRD tests (Fig. 7). The peaks at 11.6 suggest d-FAPbI 3 phase separation from (FA 0.9 MA 0.05 Cs 0.05 )PbI 3 and (FA 0.85 MA 0.1 Cs 0.05 )PbI 3 samples. By comparison, (FA 0.8 MA 0.15 -Cs 0.05 )PbI 3 and (FA 0.75 MA 0.2 Cs 0.05 )PbI 3 samples still maintained their pure perovskite single crystal structure. With the incorporation of 15% and 20% MA (molar ratio), the long-term stability of the perovskite samples was efficiently enhanced. Doping a smaller quantity of MA into the perovskite crystal structure enables the promotion of the interaction between the cation and the inorganic octahedron, which reduces the defect state of perovskite crystals thus increasing the long-term wateroxygen stability of the crystals. 20 To investigate the long-term light stability of the perovskite crystals, the samples were placed in a self-made sealed transparent box lled with argon gas and under simulated illumination of day and night. Aer 60 days of "Day and Night" simulation test, the XRD characterization (Fig. 8a) Fig. 9a; the thermal decomposition temperature is 305 C, which is close to that of pure FAPbI 3 , and the weight loss of 20% between 305 and 360 C can be attributed to the removal of organic components. 46,55,56 Lead halide is evaporated above 470 C. By comparison, the removal of the organic component from MAPbI 3 occurs at 150-200 C. [57][58][59] And the initial decomposition temperatures (Fig. 9b and c) of (MA 0.95 Cs 0.05 )PbI 3 and (FA 0.8 MA 0.2 )PbI 3 are 290 C and 300 C, respectively. Compared to the perovskite crystals of (MA 0.95 Cs 0.05 )PbI 3 and (FA 0.8 MA 0.2 )PbI 3 , the better thermal stability of (FA 0.8 MA 0.15 Cs 0.05 )PbI 3 can be explained by the strong interaction between the cation and the inorganic octahedron. The initial decomposition temperature of the crystal without the FA cation decreased by 15 C and the initial decomposition temperature without the Cs cation decreased by only 5 C, which indicate that the FA cation has a stronger inuence on the thermal stability of the crystal than the Cs cation, and this provides a reference for the synthesis of novel perovskite composition with excellent thermal stability.

Conclusions
In summary, we prepared a series of compositions of perovskite single crystals. Among them, (FA 0.8 MA 0.15 Cs 0.05 )PbI 3 perovskite maintains long-term stability aer 60 days of wateroxygen and light treatment. The MA ion doping enabled a strong interaction between the cation and the inorganic octahedron, which efficiently improved the structural stability of the perovskite samples. The band gap of (FA 0.8 MA 0.15 Cs 0.05 ) PbI 3 is 1.45 eV, which is very close to the optimal optical absorption band gap (1.1-1.4 eV); the carrier lifetime of (FA 0.8 MA 0.15 Cs 0.05 )PbI 3 perovskite single crystals reaches 5.957 ms, and the lifetime is still 5.646 ms aer 60 days of light illumination. The thermal decomposition temperature of (FA 0.8 -MA 0.15 Cs 0.05 )PbI 3 is as high as 305 C. This study has provided a reference for the synthesis of perovskite single crystal materials with improved long-term stability and carrier lifetime, which are promising to be applied in high-efficiency and stable perovskite solar cells.

Conflicts of interest
There are no conicts to declare. This journal is © The Royal Society of Chemistry 2020 Nanoscale Adv., 2020, 2, 332-339 | 337 Paper Nanoscale Advances