Effects of molecular configuration on charge diffusion kinetics within hole-transporting materials for perovskites solar cells

First principles calculations combined with Marcus theory were carried out to investigate the hole diffusion kinetics of two thiophene-based hole-transporting materials 4,4′,5,5′-tetra[4,4′-bis(methoxyphenyl)aminophen-4-yl]-2,2′-bithiophene (H112) and 2,2′,5,5′-tetrakis[N,N-di(4-methoxyphenyl)amino]-3,3′-bithiophene (KTM3) in perovskite solar cells (PSCs). The isomers H112 and KTM3 only differ in the almost planar or swivel-cruciform geometry but give rise to significantly different power conversion efficiency (14.7 and 7.3%). We found that the highest occupied molecular orbitals of H112 and KTM3 are on the same energy level, which explains why the two PSCs exhibit similar open-circuit voltage. We showed that the exciton binding energy of H112 is 23.6% smaller than that of KTM3, which indicates an easier generation of free charge carriers in H112. More importantly, the most stable crystal structure of H112 and KTM3, respectively, belongs to P₂₁₂₁₂₁ and P₂₁ space groups, where the packing pattern is face-to-face and herringbone model. The face-to-face packing pattern leads to stronger hole couplings between the neighboring H112 molecules and therefore results in substantial hole mobility (6.75 × 10^-2 cm²/V s), which is about four hundred times of that in KTM3. This clarifies the obvious enhancement of the short-circuit current density and therefore the overall performance of PSC with H112 as hole-transporting material. Our work has provided new insights into the hole-transporting properties that should be carefully considered for rational design of high-efficiency hole-transporting materials.