Thieno[3,2 -b]thiophene-based linear dopant-free hole transport materials for efficient perovskite solar cells

Organic hole transport materials (HTMs) are extensively studied in perovskite solar cells (PSCs). However, due to the inherently low charge carrier mobility, chemical dopants are often required to increase mobility and achieve high power conversion efficiency. Unfortunately, the use of dopants not only brings high costs, but also accelerates the degradation of perovskite, severely reducing the stability of perovskite solar cells. Therefore, developing efficient dopant-free HTMs has become a big challenge. Here, two linear molecules DTTTP-DPA and DTTTP-TPA were designed and synthesized, with diethyl 2,5-bis(thieno [3,2-b]thiophen-2-yl)terephthalate (DTTTP) as the core structure, methoxydiphenylamine/methoxytriphenylamine as the end group, respectively. Compared to diphenylamine, the triphenylamine end group endowed DTTTP-TPA with a longer conjugation length and a lower HOMO level. The longer conjugation is more conducive to charge transporting. While the lower HOMO level due to the weaker electron donating ability of triphenylamine, is potential to achieve a higher open-circuit voltage when used in PSCs. In addition, the carbonyl group in molecular structure could passivate the defect at perovskite/hole transport layer (HTL) interface. As a result, the dopant-free DTTTP-TPA device shows a high photovoltaic performance of 21.62%. At room temperature (30% relative humidity), the initial PCE of the unencapsulated device remains above 93% after 1000 h, showing excellent stability. Our work demonstrates that constructing linear type molecular structure with extended conjugation is an efficient way to enhance hole mobility and is promising to obtain dopant-free HTMs.