Resolving the Physical Origin of Octahedral Tilting in Halide Perovskites

Hybrid perovskites are currently the fastest growing photovoltaic technology, having reached a solar cell efficiency of over 20%. One possible strategy to further improve the efficiency of perovskite solar cells is to tune the degree of octahedral tilting of the halide frame, since this in turn affects the optical band gap and carrier effective masses. It is commonly accepted that the ion sizes are the main control parameter influencing the degree of tilting in perovskites. Here we re-examine the origin of octahedral tilts in halide perovskites from systematic first-principles calculations. We find that while steric effects dominate the tilt magnitude in inorganic halides, hydrogen bonding between an organic A-cation and the halide frame plays a significant role in hybrids. For example, in the case of MAPbI(3), our calculations suggest that; without the contribution from hydrogen bonding, the octahedra would not tilt at all. These results demonstrate that tuning the degree of hydrogen bonding can be used as an additional control parameter to optimize the photovoltaic properties of perovskites.