CH3NH3PbI3‐xClx is a commonly used chemical formula to represent the methylammonium lead halide perovskite fabricated from mixed chlorine‐ and iodine‐containing salt precursors. Despite the rapid progress in improving its photovoltaic efficiency, fundamental questions remain regarding the atomic ratio of Cl in the perovskite as well as the reaction mechanism that leads to its formation and crystallization. In this work we investigated these questions through a combination of chemical, morphological, structural and thermal characterizations. The elemental analyses reveal unambiguously the negligible amount of Cl atoms in the CH3NH3PbI3‐xClx perovskite. By studying the thermal characteristics of methylammonium halides as well as the annealing process in a polymer/perovskite/FTO glass structure, we show that the formation of the CH3NH3PbI3‐xClx perovskite is likely driven by release of gaseous CH3NH3Cl (or other organic chlorides) through an intermediate organometal mixed halide phase. Furthermore, the comparative study on CH3NH3I/PbCl2 and CH3NH3I/PbI2 precursor combinations with different molar ratios suggest that the initial introduction of a CH3NH3+ rich environment is critical to slow down the perovskite formation process and thus improve the growth of the crystal domains during annealing; accordingly, the function of Cl− is to facilitate the release of excess CH3NH3+ at a relatively low annealing temperatures.

The applications of organotin halide perovskites are limited because of their chemical instability under ambient conditions. Upon air exposure, Sn2+ can be rapidly oxidized to Sn4+, causing a large variation in the electronic properties. Here, the role of organic cations in degradation is investigated by comparing methylammonium tin iodide (MASnI3) and formamidinium tin iodide (FASnI3). Through chemical analyses and theoretical calculations, it is found that the organic cation strongly influences the oxidation of Sn2+ and the binding of H2O molecules to the perovskite lattice. On the one hand, Sn2+ can be easily oxidized to Sn4+ in MASnI3, and replacing MA with FA reduces the extent of Sn oxidation; on the other hand, FA forms a stronger hydrogen bond with H2O than does MA, leading to partial expansion of the perovskite network. The two processes compete in determining the material's conductivity. It is noted that the oxidation is a difficult process to prevent, while the water effect can be largely suppressed by reducing the moisture level. As a result, FASnI3‐based conductors and photovoltaic cells exhibit much better reproducibility as compared to MASnI3‐based devices. This study sheds light on the development of stable Pb‐free perovskite optoelectronic devices through new material design.

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Degradation of coevaporated CH 3 NH 3 PbI 3 thin films were investigated with X-ray photoelectron spectroscopy and X-ray diffraction as the films were subjected to exposure of oxygen, low pressure atmospheric air, atmospheric air, or H 2 O. The coevaporated thin films have consistent stoichiometry and crystallinity suitable for detailed surface analysis. The results indicate that CH 3 NH 3 PbI 3 is not sensitive to oxygen. Even after 10 13 Langmuir (L, one L equals 10 −6 Torr s) oxygen exposure, no O atoms could be found on the surface. The film is not sensitive to dry air as well. A reaction threshold of about 2 × 10 10 L is found for H 2 O exposure, below which no CH 3 NH 3 PbI 3 degradation takes place, and the H 2 O acts as an n-dopant. Above the threshold, the film begins to decompose, and the amount of N and I decrease quickly, leaving the surface with PbI 2 , hydrocarbon complex, and O contamination.

1411wileyonlinelibrary.com current-voltage ( I-V ) characteristics in planar perovskite PV structures, where the reverse scanning (from open circuit voltage ( V oc ) to short circuit current( J sc )) current is superior to the forward scanning (from J sc to V oc ) current. [6][7][8][9][10] Three mechanisms have been proposed [ 6 ] to explain the hysteresis behavior of perovskite PV cells: charge trapping at the interface between perovskite and its neighboring charge transport layers, ferroelectric response of perovskites, and migration of interstitial or vacancy defects in perovskites. In earlier studies, ineffi cient electron extraction (i.e., electron trapping) at the perovskite/compact TiO 2 (c-TiO 2 ) interface [ 11 ] was considered as the origin of the I-V hysteresis. It was shown that by replacing c-TiO 2 with mesoporous TiO 2 [ 6,12 ] or by modifying the c-TiO 2 surface with a C 60 self-assemble monolayer, [ 13 ] the hysteresis can be reduced. On the other hand, it was noted that in the hysteretic devices the response time of photocurrent upon voltage change is in the range of seconds or longer, while trapping/detrapping of charges should happen at much faster timescales. [ 9 ] Ferroelectric polarization may occur in the perovskite devices, but this mechanism alone cannot explain the variation of hysteresis upon interface modifi cation [ 13 ] and the absence of hysteresis in some planar PV structures, such as indium tin oxide (ITO) /poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)/ perovskite/phenyl-C61-butyric acid methyl ester (PCBM)/ C 60 /2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/Al. [ 14 ] Recently, migration of ionic defects (e.g., Ii i or V i MA ) has been associated with the hysteresis behavior. [ 9,[15][16][17][18][19][20][21][22][23] Huang and coworkers demonstrated a giant switchable photovoltaic effect in both ITO/PEDOT:PSS/MAPbI 3 /gold (Au) [ 15 ] and lateral Au/ MAPbI 3 /Au [ 17 ] structures and explained the phenomenon by migration of methylammonium ions (MA + ) based on photothermal induced resonance measurement results. [ 17 ] Yang et al. investigated specifi cally the ionic conductivity of lead iodidebased perovskites by employing electrochemical cells, [ 18 ] and they found that the ion conduction is largely contributed by iodide ions (I − ). Eames et al. estimated the active energy of I − migration in MAPbI 3 to be 0.6 eV through chronophotoamperometry measurement of a fl ourine-doped tin oxide (FTO)/cTiO 2 /MAPbI 3 /spiro-OMeTAD/Au structure; [ 19 ]