Enhancing Defect Tolerance and Phase Stability of High-Bandgap Perovskites via Guanidinium Alloying

The open-circuit voltages (V_OC) of hybrid perovskite (HP) solar cells do not increase sufficiently with increasing bandgap (for Eg > 1.70eV). We study the impact of A⁺ size mismatch induced lattice distortions (in ABX₃ structure) on the optoelectronic quality of high-bandgap HPs and find that the highest quality films have high A-site size-mismatch, where large guanidinium (GA) compensates for small Cs to keep the tolerance factor in the range for the perovskite structure. Specifically, we find that 1.84eV bandgap (FA_0.33GA_0.19Cs_0.47)­Pb­(I_0.66Br_0.34)₃ and 1.75eV bandgap (FA_0.58GA_0.10Cs_0.32)­Pb­(I_0.73Br_0.27)₃ attain quasi-Fermi level splitting of 1.43eV and 1.35eV, respectively, which is >91% of the Shockley-Queisser limit for both cases. Films of 1.75eV bandgap (FA,GA,Cs)­Pb­(I,Br)₃ are then used to fabricate p-i-n photovoltaic devices that have a V_OC of 1.24 V. This V_OC is among the highest V_OC reported for any HPs with similar bandgap (1.7 to 1.8 eV) and a substantial improvement for the p-i-n architecture, which is desirable for tandems with Si, CIGS, or a low-bandgap HP. Collectively, our results show that non-radiative recombination rates are reduced in (FA,GA,Cs)­Pb­(I,Br)₃ films and prove that FA-GA-Cs alloying is a viable route to attain high V_OC in high-bandgap HP solar cells.