Facile synthesis of bipyridine-derived hole transport materials for highly efficient inverted hybrid solar cells

Organic small-molecule hole transport materials are exceptional building blocks for optoelectronics devices owing to their unique properties including versatile chemical structures, adjustable energy levels, as well as simple synthesis and easy purification. In this study, four bipyridine-based organic hole transport materials (P1, P3, P7 and P9) were designed, synthesized, and characterised. The hole transport materials were specifically tailored with a bipyridine moiety as the electron acceptor, a conjugated π bridge of varying lengths, and a substituted triphenylamine group as the electron donor. These molecules exhibit intramolecular charge transfer, confirming their linear donor-acceptor-donor (D-AD) configuration. They have a wide and strong optical response in the UV–visible region, and their energy levels can be modulated to facilitate the charge-transfer process efficiently in hybrid systems. Specifically, the P7 molecule with a large conjugation system exhibited excellent hole transport performance because of enhanced intermolecular π-π packing and π-π interaction. Consequently, the photoelectric conversion efficiency (PCE) of the P7-based inverted hybrid solar cell is up to 5.86 %, outperforming that of the commonly used conjugated polymer hole transport material poly(3-hexylthiophene) (P3HT, PCE: 4.38 %) under the same test conditions. This study provides a comprehensive understanding of the mechanisms by which organic hole transport materials can improve the efficiency and sustainability of photovoltaic devices.