Stable and efficient organic solar cells featuring an ultra-thin and transparent solution-deposited MoO₃ hole extraction layer

The development of non-fullerene acceptor (NFA) based organic solar cells (OSCs) featuring self-assembled monolayers (SAMs) as the transparent hole extraction layers (HELs) has led to power conversion efficiency (PCE) values of over 20%. Unfortunately, SAM-based OSCs exhibit limited operational stability due to their sensitivity to elevated temperature and light stress. Here, we tackled this issue by developing NFA-based OSCs using ultrathin solution-processed molybdenum oxide (s-MoO_x) as the HEL. Devices featuring s-MoO_x exhibited superior stability while retaining a similar PCE to 2PACz-based cells (∼17.3%). The time required for the initial PCE of cells based on ITO/2PACz and the ITO/s-MoO_x to degrade by 20% (T₈₀) under continuous thermal stress at 85 °C in nitrogen was 15 and 600 h, respectively, highlighting the crucial role of HELs in operational stability. Analysis using time-of-flight secondary ion mass spectroscopy (ToF-SIMS) reveals that in cells with ITO/SAM, the diffusion of electrode elements and 2PACz and chemical interactions with the NFA are responsible for the performance degradation observed. Replacing 2PACz with s-MoO_x significantly suppressed the diffusion of ITO and prevented its interaction with the organic semiconductor. Our work revealed the crucial roles of HELs and could help in developing efficient and more stable OSCs.