Charge-Transfer State Formation and Recombination Dynamics in Deep Absorbing Porphyrin–PCBM Blends

Abstract

Single-molecule panchromatic chromophores are promising candidates for organic photovoltaics (OPVs), offering advantages in terms of synthetic control, purity, and ease of device fabrication. Donor–acceptor porphyrins are the next-generation candidates for OPVs given their numerous structural degrees of freedom. Recently, zinc porphyrins with designed donor–acceptor structures have been reported to exhibit excellent power conversion efficiencies (up to 10%), efficient charge migration, and enhanced stability up to 200 °C on pairing with PCBM in a bulk heterojunction. Despite their excellent efficiencies, a direct structure–function relationship for tuning device efficiencies is lacking. In this study, we present a detailed spectroscopic analysis of three donor–acceptor porphyrins ZnP-TEH-DPP (ZnP), ZnP2-DPP (ZnP2), and HDDPP-EHTOM (HDDPP) with distinct connectivity and evaluate their performance when combined with PCBM in a bulk heterojunction-like configuration. We observe >80% photoluminescence quenching on pairing the three distinct porphyrins with PCBM as an electron acceptor. With femtosecond-to-microsecond broadband transient absorption spectroscopy, we observe fast recombination in directly connected porphyrin dimers (ZnP2) leading to inefficient charge separation compared to the other two analogues with a single porphyrin unit (ZnP) and a porphyrin dimer with DPP spacer (HDDPP). The trends obtained via our evaluation of the primary charge generation dynamics do not directly correlate with the obtained device efficiencies, providing insights about the optimization of parameters beyond the morphological diversity of these blends.