Strengthening perovskite interfaces with in-situ polymerized self-assembled monolayers

Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) has been widely used as the hole transport layers (HTLs) for perovskite solar cells (PSCs), especially in all-perovskite tandems. However, the energy-level mismatch between PEDOT:PSS and perovskite leads to large voltage deficit in PSCs, and the dopant PSS with high acidity and hygroscopicity conspicuously deteriorates the device stability. Herein, a powerful strategy for constructing self-assembled polymer HTLs is developed by in-situ polymerization of functionalized 3,4-ethylenedioxythiophene with carboxylic acids as side groups. This strategy facilitates the formation of a self-assembled polymer monolayer to be strongly anchored on the glass substrate, and enables the elimination of the dependence of PSS doping for traditional PEDOT. The obtained polymer HTL PEDOT-l-COOH (PTLC) exhibits an appropriate energy-level alignment with the perovskite, which enhances the charge carrier collection at the interfaces. Besides, the self-assembled PTLC with high structural ordering favors the heterogeneous nucleation of perovskite, resulting in the formation of high-quality perovskite films with superior buried interfaces. Consequently, the inverted PSCs based on PTLC demonstrate a champion conversion efficiency of 20.30 % with a high open-circuit voltage of 1.03 V which are much higher than that of PEDOT:PSS-based devices (14.47 %, 0.79 V). More encouragingly, the unsealed devices with PTLC deliver outstanding operational stability by maintaining 90 % of initial efficiency for 950 h under ambient condition with a relative humidity of 30 % ± 5 %. This work opens a new avenue for developing self-assembled PEDOT-based HTLs for optoelectronic devices, and paves the way for further improving the performance of inverted PSCs.