Role of disorder, free-carrier recombination kinetics and phonon modes in the performance of CH3NH3PbI3 perovskite films

Apart from broadband absorption of solar radiation, the performance of photovoltaic devices is governed by the density and mobility of photogenerated charge carriers. The latter parameters indicate how many free carriers move away from their origin, and how fast, before loss mechanisms such as carrier recombination occur. However, only lower bounds of these parameters are usually obtained. Here we independently determine both density and mobility of charge carriers in the organometallic halide perovskite film CH3NH3PbI3 by use of time-resolved terahertz (THz) spectroscopy (TRTS). Our data reveal the modification of the free carrier response by strong backscattering expected from these heavily disordered perovskite films. The results for different phases and different temperatures show a change of kinetics from two-body recombination at room temperature to three-body recombination at low temperatures. Our results suggest that perovskite-based solar cells can perform well even at low temperatures as long as the three-body recombination has not become predominant [1]. We also studied the temperature-dependent phonon modes of these films across the THz (0.5 – 3 THz) and temperature (20 – 300 K) ranges, using THz time-domain spectroscopy (THz-TDS). These modes are related to the vibration of the Pb–I bonds. We found that two phonon modes in the room-temperature tetragonal phase split into four modes in the low-temperature orthorhombic phase. The carrier mobility values calculated from the low-temperature phonon mode frequencies, via two theoretical approaches, are found to agree reasonably with the experimental value from our TRTS work. Thus we have established a possible link between THz phonon modes and the transport properties of perovskite-based solar cells [2].Reference:[1] C. La-o-vorakiat, T. Salim, J. Kadro, M-T Khuc, R. Haselsberger, L. Cheng, H. Xia, G. G. Gurzadyan, H. Su, Y. M. Lam, R. A. Marcus, M-E Michel-Beyerle, and Elbert E. M. Chia, Nat. Commun. 6, 7903 (2015).[2] C. La-o-vorakiat, H. Xia, J. Kadro, T. Salim, D. Zhao, T. Ahmed, Y. M. Lam, J.-X. Zhu, R. A. Marcus, M-E Michel-Beyerle, and Elbert E. M. Chia, J. Phys. Chem. Lett. 7, 1-6 (2016).