Relativistic GW calculations on CH3NH3PbI3 and CH3NH3SnI3 Perovskites for Solar Cell Applications

Abstract

Hybrid AMX3 perovskites (A = Cs, CH3NH3; M = Sn, Pb; X = halide) have revolutionized the scenario of emerging photovoltaic technologies, with very recent results demonstrating 15% efficient solar cells. The CH3NH3PbI3/MAPb(I1−xClx)3 perovskites have dominated the field, while the similar CH3NH3SnI3 has not been exploited for photovoltaic applications. Replacement of Pb by Sn would facilitate the large uptake of perovskite-based photovoltaics. Despite the extremely fast progress, the materials electronic properties which are key to the photovoltaic performance are relatively little understood. Density Functional Theory electronic structure methods have so far delivered an unbalanced description of Pb- and Sn-based perovskites. Here we develop an effective GW method incorporating spin-orbit coupling which allows us to accurately model the electronic, optical and transport properties of CH3NH3SnI3 and CH3NH3PbI3, opening the way to new materials design. The different CH3NH3SnI3 and CH3NH3PbI3 electronic properties are discussed in light of their exploitation for solar cells, and found to be dominantly due to relativistic effects. These effects stabilize the CH3NH3PbI3 material towards oxidation, by inducing a deeper valence band edge. Relativistic effects, however, also increase the material band-gap compared to CH3NH3SnI3, due to the valence band energy downshift (~0.7 eV) being only partly compensated by the conduction band downshift (~0.2 eV).

Breathing bands due to molecular order in CH3NH3PbI3

“…The electronic structure calculations on these models were carried out at different levels of approximation, namely DFT with and without spin-orbit coupling corrections, pseudopotential self-interaction corrections and MBPT within the GW approximation. We show that DFT gaps are in good agreement with experimental data only fortuitously; recovering gaps comparable with experiments requires including many-body effects, as already pointed out in Refs [30,32,39]. Besides, we show that the gap size variations may be correlated to interactions between the organic and the inorganic moieties of the system, which give rise to the aforementioned structural distortions.…”

“…At room temperature, it consists of a broad band and does not exhibit peaks reflecting band structure details [25,26]. Calculations within the density functional theory (DFT) and the many-body perturbation theory (MBPT) are consistent with electronic gaps ranging between 1.2 and 1.7 eV [9,[27][28][29][30][31][32], which agree reasonably well with photoemission experiments [33] and with the onset of the optical absorption spectrum. DFT calculations have also pointed out that the electronic gap width depends on the particular orientation of the MA molecules; in particular, larger gaps seem to correspond to systems with lower symmetry in most cases.…”

Strong Quantum Confinement and Fast Photoemission Activation in CH3NH3PbI3 Perovskite Nanocrystals Grown within Periodically Mesostructured Films

“…This set of values for μ, ε r , E B , and R B are in fair agreement with previously reported ones based on theoretical estimations that have been demonstrated to reproduce well the behavior of bulk MAPbI 3 at room temperature. [19,22,[26][27][28] This agreement supports the validity of the use of Brus approximation to describe the observed trends, although it should be taken into account that, in this expression, the effective mass is considered to be independent of the crystal size, an approximation that might not necessarily hold for extremely small crystallites.…”

Perovskite-Inspired Photovoltaic Materials: Toward Best Practices in Materials Characterization and Calculations

“…As an example, the electronic single particle gap of MAPbI 3 has been measured as 1.7±0.1 eV, compared to an optical bandgap of 1.65 eV and 9-17 meV exciton binding energy matching the predictions from GW calculations with corrections for spin-orbit coupling (1.67 eV). 163,175,176 An improvement to this evaluation can be achieved by carefully lining up the experimentally obtained spectra from PES and IPES measurements with simulated spectra from DFT calculations, as described above and demonstrated in Figure 8. A significant refinement of the band onset determination for a range of hybrid perovskite films was reported by Endres et al Therein, the electrical bandgap for MAPbI 3 was determined to be 1.6±0.1 eV, 177 in agreement with optical measurements, 175 and demonstrating greater accuracy than previous electronic bandgap measurements that did not make use of DOS fitting of the IPES spectra.…”