Open Atmosphere-Processed Stable Perovskite Solar Cells Using Molecular Engineered, Dopant-Free, Highly Hydrophobic Polymeric Hole-Transporting Materials: Influence of Thiophene and Alkyl Chain on Power Conversion Efficiency

Developing efficient and stable perovskite solar cells (PSCs) in open atmosphere desperately requires robust hole-transporting material (HTMs) with high hole conductivity and rich hydrophobicity. Here, we present two dopant-free, highly hydrophobic, donor−π–acceptor conducting polymeric HTMs by interconnecting three monomer units of 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2- b:4,5- b ′]dithiophene, pyrrolo[3,4- c ]pyrrole-1,3-dione, and [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4- b ]thiophenediyl] (TT) named as R1 and two monomer units of [4,8-bis[(2-ethylhexyl)oxy]benzo[1,2- b:4,5- b ′]dithiophene-2,6-diyl] and TT named as R2. These two R1 and R2 HTMs integrated in PSCs exhibit an excellent photovoltaic performance of ∼15.8 and ∼13.5% at open atmospheric conditions, respectively. This distinguished photovoltaic performance is strongly correlated with their hole mobility, solubility, and energetic alignment with perovskite valence band. Briefly, the excess thiophene rings with extended alkyl chains in R1 brought a significant impact on photovoltaic performance due to (i) S-based heterocyclic thiophene strengthening the interaction with the perovskite/HTM interface and increasing its conductivity, (ii) a retarded recombination rate, (iii) high solubility helping to obtain uniform film coverage over perovskite, (iv) the highest occupied molecular orbital level being well aligned with perovskite energy band and (v) increased hydrophobicity. Except the top gold metal contact, the complete PSC device was fabricated in an open atmosphere from a low-temperature solution process and these dopant-free HTMs pave the way for attaining stable and efficient PSCs toward potential applications.