A Universal Conundrum of Determination of Frenkel Exciton Binding Energy

The absorption of a photon by Organic Semiconductors (OSCs) results in the formation of a bound electron-hole pair quasiparticle or Frenkel Exciton. The energy required to separate an exciton into noninteracting electron and hole, Exciton Binding Energy (Eb), is a critical parameter for the purpose of improving the efficiency of optoelectronic devices such as solar cells and light emitting diodes. In the last two decade there have been many efforts to measure the Eb of OSCs using different techniques. However, there are discrepancies in the literature and the reported values are scattered over a large range between a few meV to 1.5eV, even for a specific material. Eb of Frenkel excitons can be estimated as the difference between the transport (Eg) and the optical gap (Eopt), a traditional definition borrowed from the language of Wannier Exciton in inorganic SCs. Here, we explore the Eb of different variants of PBnDT-FTAZ polymer. We focus on the two most common methods used to measure Eg: combination of Ultraviolet Photoelectron Spectroscopy and Inversed Photoelectron Spectroscopy (UPS-IPES) and Solid-State Cyclic Voltammetry (CV). We show that Eb measured by the abovementioned methods are not consistent or correlated with each other. The Eg measured by UPS-IPES technique is comparable with (or even smaller than) Eopt leading to small Eb. On the other hand, CV usually measures larger Eg compared to the Eopt resulting in larger values of Eb that are scattered between 200meV-1eV depending on the molecular structure of the materials. This discrepancy is the result of lack of both an exclusive theoretical and a functional definition of Eb that includes the relaxation effects, an important characteristic of Frenkel excitons. Moreover, due to the nature of each measurement method, they might measure different parameters than the actual properties of the bulk in a photovoltaic device. Our results elucidate the current conundrum on determination of Eb in OSCs and emphasize the importance of establishing standard theoretical and practical guidelines on how to properly estimate Eb.