Porphyrin–thieno[3,2-b]thiophene hole-transport materials enabling the production of long-lived radical ion pairs and high-performance perovskite solar cells

Abstract

Porphyrin-based hole-transporting materials (HTMs) have gained significant attention in perovskite solar cells (PSCs) due to their high efficiency, long-term stability, and excellent charge-transport properties. In this work, we reported a novel zinc porphyrin-based HTM denoted as SPS-PPY-TT for highly efficient and stable PSCs. The molecular design strategy of SPS-PPY-TT involves zinc porphyrin as the central core unit, which is functionalized with 6-fluoro-benzothiadiazole (FBDT) and hexylthieno[3,2-b]thiophene (TT) groups through an ethynyl linkage. The synthesised HTM was comprehensively investigated by using optical absorption spectroscopy, electrochemical methods, and theoretical studies. These studies indicate that the highest occupied molecular orbital (HOMO) energy levels are aligned with the valence band of MAPbI₃ perovskite. As a result, planar PSC devices constructed with SPS-PPY-TT as the HTM exhibit a power conversion efficiency (PCE) of 15.29%, while PSC devices made with Spiro-OMeTAD exhibit a PCE of 16.62%. The similar photovoltaic performance of SPS-PPY-TT and Spiro-OMeTAD might be attributed to the appropriate band alignment, higher hole mobility, and conductivity of SPS-PPY-TT. This work demonstrates that TT is an alternative methoxy-free donor for porphyrin HTMs to achieve highly efficient and stable PSCs. Eventually, the same molecular system, which was axially coordinated with ImC₆₀, was utilised to generate a long-lived charge-separated species with a lifetime of 1.56 ns after irradiation at different wavelengths. The value of k_CS/k_CR was found to be 5.78, revealing charge stabilisation.