First-principles calculations of shift currents for perovskite solar cell materials*

Solar cells using methylammonium lead perovskite as light absorbing materials have achieved an astounding 25.5% conversion efficiency improvement, equivalent to silicon solar cells as of 2022. However, there are concerns about the toxic effects of lead in perovskite solar cell materials on human health and the environment, and there is an urgent need to completely replace lead with a more inert metal. In this study, the shift current in lead-free perovskite solar cell materials is estimated by first-principles calculations to evaluate and predict new lead-free perovskite materials. Shift current is a current generated by the real-space movement of electron clouds in materials due to light irradiation. The evaluation and prediction of materials by analyzing their complex photo-response is expected to be applied not only to solar energy conversion but also to photocatalysts, sensors, and other devices.

Typical lead perovskite solar cell materials CsPbBr3, and CsSnBr3 and CsGeBr3, in which lead is replaced by tin and germanium, were discussed. Brillouin zone integrations were performed on an 8 x 8 x 8 grid for structure optimization and on a 20 x 20 x 20 k-point grid for shift currents. First-principles calculations were performed at the GGA/PBE level using norm-conserving pseudopotentials.

The band gaps of CsPbBr3, CsSnBr3, and CsGeBr3 are obtained as 1.79, 0.84, and 1.02 eV for the direct transition type at the R point, and the imaginary part of the dielectric constant rises from those energy values, respectively, and the optical absorption intensity is CsPbBr3<CsSnBr3<CsGeBr3. Interestingly, in the case of CsSnBr3 and CsGeBr3, the shift current at 1~2 eV is significantly increased compared to CsPbBr3, but the individually integrated shift vectors in that energy region show almost no change for all systems. Therefore, the product of the transition intensity and shift vector, integrated separately for each, does not determine the direction and magnitude of the shift current susceptibility, indicating that the product of each transition and the associated shift vector must be integrated over the band and k points to obtain the shift current. The analysis of the mechanism of shift current onset and its relation to conversion efficiency will be discussed in the poster.