2, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Hybrid organic-inorganic perovskites (HOIPs) have been demonstrated as a promising candidate for photovoltaic applications. For efficient and stable photovoltaic devices, it is critical to understand the effects of environment on the material. In this work, we investigate the evolution of chemical distribution as a function of temperature and its correlation to lattice distortion. Using time-of-flight secondary ion mass spectrometry (ToF-SIMS), we reveal the chemical gradient of CH3NH3+ and Pb into the bulk at room temperature. As temperature increases, the CH3NH3+ distributes uniformly and Pb2+ gradient does not change. These results prompted a further crystal structure study as our earlier investigations revealed a strong interaction between ion segregation and lattice strain. We performed grazing incidence X-ray diffraction (GIXRD) to explore the lattice change in the direction normal to the sample surface by adjusting grazing incidence angle. As expected, the GIXRD results indicate lattice expansion in (110) and compression in (002) directions with the grazing incidence angle increase corresponding to depth increases. Moreover, both chemical and lattice strain gradients decrease as temperature increases. DFT simulations corroborate these results, suggesting a coupling between temperature, lattice distortion, and chemical distribution. Finally, we demonstrate that temperature, lattice distortion, and chemical distribution simultaneously alter photovoltaic performance. These measurements and results offer an in-depth understanding of the extrinsic and intrinsic chemical effects on device performance.