Xingnan Qi, Jiantao Wang, Vishal Yeddu, Tyler Trefz, Yameen Ahmed, Jesse Delmage, I. Teng Cheong, Nicholas Sandor, Dongyang Zhang, Shuang Qiu, Augusto Amaro, Sergey Dayneko, Ori Granot, Wanlong Wang, Weihai Zhang, Hao Chen, Irina Paci, Stefaan De Wolf, Makhsud I. Saidaminov
Self-assembling molecules (SAMs) have emerged as effective hole transport layers, accelerating the progress of inverted perovskite solar cells (PSCs) toward their Shockley-Queisser efficiency limit. Here, we reveal that the commonly used SAM, MeO-2PACz, spontaneously self-aggregates in solution due to its amphiphilic nature, driven by hydrogen bonding between phosphonic groups. Using electrospray ionization mass spectrometry, we provide the first direct experimental evidence of SAM oligomerization, quantitatively resolving dimers, trimers, tetramers, and pentamers, which hinder the formation of a compact, uniform film. To overcome this, we introduce a combination of a small, strong Lewis base (Cl – ) with a hydrogen-bond-forming counterion (PEA + ), which together disrupt the hydrogen-bond network within the SAM solution, suppressing pentamers by over 4-fold. Scanning transmission electron microscopy suggests that PEACl also disrupts the large MeO-2PACz micelles. The resulting SAM film exhibits improved molecular packing, enhanced hole mobility, reduced residual stress, and a favorable energy level alignment with perovskite. Corresponding PSCs achieve an efficiency of 26.3% and retain 86.3% of maximum power output under AM1.5G illumination for 1200 h.