Engineering a thermally robust hole-selective layer for stable flexible perovskite solar cells

Self-assembled monolayer (SAM) based hole-selective layer (HSL) plays a decisive role in the thermal stability of flexible perovskite solar cells (fPSCs). In particular, the binding between SAMs and the substrate is essential for enhancing the thermal stress resistance of fPSCs. Herein, we employ a indolo[2,3-a]carbazole-11,12-diylbis (propane-3,1-diyl)) bisphosphonic acid (3PAIDCz) SAM with a bisphosphate anchoring group on the nickel oxide (NiO_x) film to form a thermally robust HSL for fPSCs that can withstand at a high temperature. The bisphosphate anchoring group toughens the NiO_x/SAM interface and show less aggregation under thermal stress when compared with monophosphate anchoring groups of [2-(9H-Carbazol-9-yl) ethyl] phosphonic acid (2PACz). Additionally, 3PAIDCz exhibits outstanding band alignment with the perovskite film, reducing the interfacial non-radiative losses. As a result, the champion fPSCs achieve a certified power conversion efficiency (PCE) of 22.90 %, which is one of the highest values for dual HSL based inverted fPSCs. Moreover, the unencapsulated fPSCs exhibit prominent durability under thermal and light stresses with T₈₀ > 600 h (i.e., 85 °C and illumination) following the ISOS-L-2I protocol. The encapsulated fPSCs show a T₈₀ > 1200 h under standard damp-heat stress (i.e., 85 °C and 85 % RH) according to the ISOS-D-3 protocol. This research effectively addresses the key issue of thermal stability for fPSCs by developing a thermally robust HSL.