Abstract
Scientific evidence for warming of the climate system is unequivocal. Developing a renewable, economic and environmentally friendly green energy, such as solar cells, is of particular importance. Among all kinds of solar cells, the hybrid organic-inorganic perovskite solar cell (HOIP) is the fastest-advancing solar technology to date and cheap to manufacture. Solar cell efficiency of devices using these materials has reached 22.7% in late 2017. However, the microscopic mechanism of their high photovoltaic performance is yet to be fully understood. In this work, we present a systematic study on structures, phase transitions, rotational and vibrational dynamics, and optoelectronic properties of two lead iodide HOIPs: methylammonium lead iodide (MAPbI3, where MA represents the CH3NH3+ cation), and formamidinium lead iodide (FAPbI3, where FA represents the HC(NH2)2+ cation).
Even though FAPbI3 has a higher theoretical power conversion efficiency than MAPbI3 due to that its bandgap is better matched to the solar spectrum, it has been studied less compared to MAPbI3 due to its structural phase instability. Upon cooling, FAPbI3 exhibits abrupt suppression of photovoltaic effect as the system undergoes cubic-to-hexagonal transition. The transition temperature and rate were found to vary widely depending on the sample environment. We have systematically studied the structures and phase transitions of FAPbI3 using neutron powder diffraction and synchrotron X-ray diffraction. We have discovered three new phases of FAPbI3, some of which are obtained by thermal quenching. Rietveld refinement is used to determine their structures. Using neutron diffraction and first-principles calculations on formamidinium lead iodide (FAPbI3), we have successfully explained the mysterious hexagonal-to-cubic phase transition with the entropy-driven picture. We show that the entropy contribution to the Gibbs free energy due to isotropic rotations of the organic cation play a crucial role in the cubic-to-hexagonal structural phase transition.
The rotational and vibrational dynamics and their impact on relevant processes such as charge recombination are still poorly understood. We used quasielastic neutron scattering as well as group theoretical analysis to determine the rotational modes of organic cations in different phases of MAPbI3 and FAPbI3. The characteristic relaxation times are found to be 1~10 ps at room temperature. Our findings on the rotational dynamics have important implications for understanding the low exciton binding energy and slow charge recombination rate in HOIPs which are directly relevant for the high solar cell performance. Vibrational dynamics are of great importance to HOIPs. A precise understanding of the phonon dispersion in HOIP materials is crucial for developing quantitative models of ionic transport, and recombination and scattering of photo-generated charges in devices. We used inelastic neutron scattering to measure the phonon spectra of both MAPbI3 and FAPbI3. Our phonon data correlate well with our first-principles phonon calculations based on finite displacement method.
Long carrier lifetime is what makes HOIPs high-performance photovoltaic materials. We have performed measurements and calculations on optoelectronic properties such as optical band structures, electronic band structures, and charge carrier lifetime. Our results reveal that the band-edge carrier lifetime increases when the system transits from a phase with lower rotational entropy to another phase with higher entropy. We show that the screening of band-edge charge carriers by rotation of organic cation molecules can be a major contribution to the prolonged carrier lifetime. These results imply that the recombination of the photo-excited electrons and holes is suppressed by the screening, leading to the formation of polarons and thereby extending the lifetime. Thus, searching for organic-inorganic perovskites with high rotational entropy over a wide range of temperature may be a key to achieve superior solar cell performance.
