tBu-4PACz is a self-assembled monolayer material featuring a steric di-tert-butylcarbazole terminal, a butyl linker and a phosphonic acid anchor. Developed for high-efficiency perovskite solar cells as a hole transport layer interface between the substrate and the perovskite active layer. The bulky di-tert-butyl groups on the carbazole terminal prevent undesired aggregation of the self-assembled molecules on the interface, which in return facilitates more efficient hole-extraction and suppresses interfacial recombination losses.
The terminal carbazolyl moiety of tBu-4PACz containing di-tert-butyl groups is hydrophobic and this results in wetting issues upon the deposition of the perovskite layer. To overcome the wettability of tBu-4PACz, a second component is introduced to adopt a mixed SAM system as the hole-extraction layer in inverted devices. The second component could either be another self-assembled monolayer material, such as MeO-2PACz, or 1,6-hexylene-diphosphonic acid (6dPA). With MeO-2PACz as the mixed SAM, tBu-4PACz based inverted PSC shows high power conversion efficiency (PCE) of 26.25% (26.21%, certificated) with outstanding fill factors over 86%. Also, the mixed SAM based inverted PSC devices demonstrate great stability, retaining over 94.7% of their initial efficiency after 500 h continuous maximum power-point tracking under simulation 1-sun irradiation.
Serving as hole selective contact for organic solar cells and perovskite solar cells, tBu-4PACz is an alternative to PEDOT:PSS with superior performance and the convenience of solution deposition at low concentration, i.e. 1 mM.