Taurine modified hole transport layers for efficient low-bandgap perovskite solar cells

All-perovskite tandem solar cells (TSCs) show an effective way to overcome the efficiency limit of single-junction perovskite solar cells (PSCs). Low-bandgap mixed tin-lead perovskite bottom subcell is a critical component in all perovskite TSCs, which are currently limited by the relatively low efficiency and poor stability of the bottom subcells. Here in this work, we use multifunctional taurine material to dope and modify the PEDOT:PSS hole transport layer (HTL) for better optical transmittance and charge transporting properties, enabling efficient low-bandgap PSCs with highly improved open circuit voltages and short circuit current densities. Taurine has a suitable refractive index to reduce light reflection loss when introduced at the interface of PEDOT:PSS/perovskite film. Moreover, taurine improves the energy level alignments and facilitates charge transfer dynamics in the low-bandgap PSCs when doped into the PEDOT:PSS HTL. Finally, the synergistic effects of the interface modification and bulk doping of PEDOT:PSS with taurine leads to significantly enhanced efficiencies of over 22% and 26% for low-bandgap PSCs and all-perovskite TSCs, respectively.