Ester-Functionalized Polythiophene Interlayers for Enhanced Performance and Stability of Perovskite Solar Cells

Interfacial defects between the organic-inorganic lead halide perovskite and hole-transporting layers are one significant factor limiting the performance of the perovskite solar cells (PSCs). These defects typically result from either lead or halide atom vacancies or under-coordinated Pb2+ atoms, which act as recombination centers for holes. To address this, it is demonstrated that a thin layer of a poly(bithiophene ester) (PBTE) or a poly(terthiophene diester) (PTTDE) introduced between the FA0.92MA0.08Pb(I0.92Br0.08)3 (MA+ = methylammonium; FA+ = formamidinium) perovskite and spiro-OMeTAD hole transporting material (HTM) layers improves the device stability and power-conversion efficiency (PCE). The PCE improvements are primarily associated with enhanced open-circuit voltage and fill factor arising from Lewis-base passivation of under-coordinated Pb2+ through interaction with the polymer carbonyl groups, as well as enhanced charge transfer between the perovskite and HTM layers facilitated by the conjugated thiophene functionalities of the polymeric interlayer. Furthermore, the examined polymers improve the thermal and moisture stability of the devices. Encapsulated PSCs with PBTE and PTTDE retain 89 +/- 1 and 94 +/- 3% of their initial PCE after 1440 h at 65 degrees C and under 0.5-sun irradiation, respectively, while PSCs without a polymeric interlayer lose almost half of the performance. These results can guide future designs of polymeric interlayers for high-performance PSCs. Bifunctional polymer interlayers passivate the under-coordinated Pb2+ defects, facilitate charge transfer, and provide a hydrophobic barrier within perovskite solar cells. image