Various phosphonic acid based self-assembled monolayers (SAMs) have been commonly used for interface modifications in inverted perovskite solar cells. This typically results in significant enhancement of the hole extraction and consequent increase in the power conversion efficiency. However, the surface coverage and packing density of SAM molecules can vary, depending on the chosen SAM material and underlying oxide layer. In addition, different SAM molecules have diverse effects on the interfacial energy level alignment and perovskite film growth, resulting in complex relationships between surface modification, efficiency, and lifetime. Here we show that ethanolamine surface modification combined with [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) results in significant improvement in device stability compared to devices with 2PACz modification only. The significantly smaller size of ethanolamine enables it to fill any gaps in 2PACz coverage and provide improved interfacial defect passivation, while its different chemical structure enables it to provide complementary effects to 2PACz passivation. Consequently, the perovskite films are more stable under illumination (slower photoinduced segregation), and the devices exhibit significant stability enhancement. Despite similar power conversion efficiencies (PCE) between 2PACz only and combined ethanolamine-2PACz modification (PCE of champion devices ∼21.6-22.0% for rigid and ∼20.2-21.0% for flexible devices), the T80 lifetime under simulated solar illumination in ambient is improved more than 15 times for both rigid and flexible devices.
Keywords: flexible solar cells; halide perovskite; interface modification; solar cells; stability testing.