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

Abstract

Discrepancies in reported values of exciton binding energy (Eb) for organic semiconductors (OSs) necessitate a comprehensive study. Traditionally, Eb is defined as the difference between the transport gap (Et) and the optical gap (Eopt). Here, the Eb 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). Eb 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 Eb‐molecular structure correlation, as the lowest Eb values are observed for oligo‐ethylene glycol side chains. This discrepancy is discussed by examining the implications of the traditional definition of Eb. Additionally, the impact of both intrinsic and extrinsic factors contributing to the derived experimental values of Et is discussed. The differences in intrinsic and extrinsic factors highlight the context‐dependent nature of measurement when drawing global conclusions. Notably, the observed Eb 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 Eb may be possible via high dielectric materials.

Intrinsic Defects of Vacancy-Ordered Perovskite Cs2TiIBr5 for Solar Cells

“…Relatively little research has been done on vacancy-ordered perovskites for solar cells. Studies have shown that Cs 2 TiX 6 (X = Br and I) possesses multiple desirable attributes including good stability and high optical absorption coefficients in the visible light ranges. − More recently, Zhao et al and Reyes-Francis et al, respectively, pointed out the ease of synthesis and high power conversion efficiency (PCE) of Cs 2 TiX 6 . , The impact of large exciton binding energy in Cs 2 TiX 6 may potentially be mitigated through atomic substitution or by changing the dielectric environment of the insulating organic layer. − The aforementioned discussions suggest that Cs 2 TiX 6 holds the potential for use in solar cells. However, existing calculations and experiments have proven that both Cs 2 TiBr 6 and Cs 2 TiI 6 have some shortcomings that limit their further applications. , First, these two materials have quasi-direct bandgaps with indirect bandgaps between Γ (valence band maximum, VBM) and X (conduction band minimum, CBM), which are detrimental to their photovoltaic performance .…”