Study of Thermalization Mechanisms of Hot Carriers in BABr-Added MAPbBr3 for the Top Layer of Four-Junction Solar Cells

The hot carrier multi-junction solar cell (HCMJC) is an advanced-concept solar cell with a theoretical efficiency greater than 65%. It combines the advantages of hot carrier solar cells and multi-junction solar cells with higher power conversion efficiency (PCE). The thermalization coefficient (<i>Q_th</i>) has been shown to slow down by an order of magnitude in low-dimensional structures, which will significantly improve PCE. However, there have been no studies calculating the <i>Q_th</i> of MAPbBr₃ quantum dots so far. In this work, the <i>Q_th</i> values of MAPbBr₃ quantum dots and after BABr addition were calculated based on power-dependent steady-state photoluminescence (PD-SSPL). Their peak positions in PD-SSPL increased from 2.37 to 2.71 eV after adding BABr. The fitting shows that, after adding BABr, the <i>Q_th</i> decreased from 2.64 ± 0.29 mW·K^-1·cm^-2 to 2.36 ± 0.25 mW·K^-1·cm^-2, indicating a lower relaxation rate. This is because BABr passivates surface defects, slowing down the carrier thermalization process. This work lays the foundation for the theoretical framework combining perovskite materials, which suggests that the appropriate passivation of BABr has the potential to further reduce <i>Q_th</i> and make MAPbBr₃ QDs with BABr modified more suitable as the top absorption layer of HCMJCs.