Open atmospheric processed perovskite solar cells using dopant-free, highly hydrophobic hole-transporting materials: Influence of thiophene and selenophene π-spacers on charge transport and recombination properties

We have synthesized three donor-n-acceptor polymers by crosslinking 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b'] and pyrrolo[3,4-c]pyrrole-1,3-dione with thiophene, thieno[3,2-b]thiophene, selenophene as pi-spacer. These polymers have been integrated successfully as dopant-free hole-transporting materials in fully open atmosphere solution-processed lead halide perovskite solar cells. Due to the synergistic impact of linear and long alkyl chains with different n-spacers, these hole transporters retard interfacial charge recombination and afford rich solubility and morphology resulting long carrier lifetime with uniform film coverage. In addition, these hole transporters possess a deeper highest occupied molecular orbitals (-5.40, -5.37 and -5.42 eV) and suitable over potential with perovskite valence band (-5.43 eV), favorable for efficient hole extraction. Further, the large number of S-based heterocycles helps to strengthen the interaction of the perovskite and hole transporting layer interface and high hydrophobicity (similar to 104 degrees) facilitate enhanced stability with insignificant hysteresis. As a result, the power conversion efficiency has reached 14% under AM1.5 G without any additives in fully open atmospheric conditions. Our findings highlight the better energy transfer capabilities of additive-free hole transporters as a promising candidate for developing stable PSCs in the open air.