Hydroxylated organic semiconductors for efficient photovoltaics and photocatalytic hydrogen evolution

Abstract

Organic semiconductor (OSC)-based bulk heterojunctions (BHJs) have been widely applied for efficient organic photovoltaics (OPVs) and recently also utilized as photocatalysts for the hydrogen evolution reaction (HER). It is desirable for the BHJ to simultaneously harvest long-wavelength light and possess a high LUMO level which can maximize the theoretical open-circuit voltage (V_oc) for OPVs and provide sufficient overpotential for the HER. However, it remains challenging to obtain OSCs with a narrow bandgap and a shallow LUMO. To overcome this compromise, we designed two hydroxyl-functionalized OSCs (BTP-FOH and BTP-2OH) as acceptors in the BHJ. Compared to hydroxyl-free BTP-4F, the LUMO levels of BTP-FOH and BTP-2OH gradually increase with increased hydroxyl groups. Meanwhile, hydroxyl-induced intermolecular H-bonds augment the molecular alignment leading to red-shifted film absorptions in the long-wavelength region. Moreover, hydroxyl groups result in variable aggregation behavior and hydrophilicity of the molecules, causing their different performances in OPVs and the HER. While the BTP-FOH-based BHJ delivers a higher efficiency of 16.71% in solar cells, the BTP-2OH-based one exhibits a higher hydrogen evolution rate (102.1 mmol h⁻¹ g⁻¹) and EQE (9.17% at 800 nm) in the HER. Our work demonstrates that hydroxylation is effective for designing acceptor materials with long-wavelength light utilization and a high LUMO level for both light-driven applications.