Quantitative study of hot carriers thermalization mechanisms in GAx[Cs0.05(FA0.23MA0.77)0.95]1-xPbIBr2 for hot carrier multi-junction solar cells

The hot carrier multi-junction solar cell (HCMJSC) is an advanced concept solar cell with a theoretical efficiency greater than 65 %, which consists of a thin top junction with a wide bandgap and a thick bottom junction with a narrow bandgap to absorb high and low energy photons, respectively. Wide bandgap perovskites are expected to be a suitable candidate for top absorption layers due to their ease of band engineering. However, the carrier thermalization mechanisms of quaternary A-site perovskites have not been well studied until now. Conventionally, the incorporation of larger A-site cations induces lattice expansion and enhances thermal fluctuations, thereby accelerating carrier cooling. In this work, the incorporation of guanidinium (GA) did not immediately shorten the cooling process but instead exhibited a non-monotonic trend, initially slowing then accelerating the thermalization process. The thermalization coefficients (Q_th) of the quaternary A-site perovskites with different GA doping ratio were obtained from the power dependent-steady state photoluminescence spectra. Moderate addition of GA can prolong the thermalization process of the hot carriers and reduce Q_th, which will improve power conversion efficiency, with the optimal effect achieved at 3 % GA incorporation in the A-site. This work lays the foundation for a theoretical framework that incorporates perovskite materials which may be suitable as the top absorption layer of HCMJSC.