Exciton Binding Energy in Organic Polymers: Experimental Considerations and Tuning Prospects

Discrepancies in reported values of exciton binding energy (E_b) for organic semiconductors (OSs) necessitate a comprehensive study. Traditionally, E_b is defined as the difference between the transport gap (E_t) and the optical gap (E_opt). Here, the E_b values of PBnDT-TAZ polymer variants are determined using two commonly employed methods: a combination of ultraviolet photoemission spectroscopy and low-energy inverse photoemission spectroscopy (UPS-LEIPS) and solid-state cyclic voltammetry (CV). E_b values obtained by UPS-LEIPS show low dispersion and no clear correlation with the polymer structure and thedielectric properties. In contrast, CV reveals a larger dispersion (200 meV-1 eV) and an apparent qualitative E_b-molecular structure correlation, as the lowest E_b values are observed for oligo-ethylene glycol side chains. This discrepancy is discussed by examining the implications of the traditional definition of E_b. Additionally, the impact of both intrinsic and extrinsic factors contributing to the derived experimental values of E_t is discussed. The differences in intrinsic and extrinsic factors highlight the context-dependent nature of measurement when drawing global conclusions. Notably, the observed E_b trend derived from CV is not intrinsic to the pure materials but likely linked to electrolyte swelling and associated changes in dielectric environment, suggesting that high-efficiency single-material organic photovoltaics with low E_b may be possible via high dielectric materials.