Direct Observation of Increased Free Carrier Generation Owing to Reduced Exciton Binding Energies in Polymerized Small-Molecule Acceptors

Energy loss caused by exciton binding energy (Eb) has become a key factor that restricts further advancement of organic solar cells (OSCs). Herein, we used transient mid-IR spectroscopy to study direct photo-generation of free charge carriers in small-molecule acceptors (SMAs) Y6 and IDIC as well as polymerized SMAs (PSMAs) PYFT and PZ1. We found that free carrier concentration is higher in PSMAs than in their corresponding SMAs, indicating reduced exciton Eb, which is then confirmed by ultraviolet photoelectron spectroscopy, low-energy inverse photoemission spectroscopy, and film absorption spectra measurements. The measured Eb values of PYFT and PZ1 are 0.24 and 0.37 eV, respectively, smaller than those of Y6 (0.32 eV) and IDIC (0.47 eV). This work not only provides a method to directly monitor the photogenerated free carriers in OSC materials but also demonstrates that polymerization is an effective strategy to reduce the Eb, which is crucial to decrease the energy losses in high-performance OSCs.

Automated summary

In this study, transient mid-IR spectroscopy was used to investigate the direct photo-generation of free charge carriers in different types of organic solar cell materials. It was found that polymerized SMAs have higher free carrier concentration compared to their corresponding SMAs, leading to reduced exciton binding energy and decreased energy losses in high-performance OSCs.