Proceedings Article | 10 September 2019
KEYWORDS: Excitons, Dielectrics, Polymers, Photovoltaic materials, Organic photovoltaics, Organic materials, Solar cells, Quantum efficiency, Fullerenes, Molecules
Owing to the low dielectric constant of organic materials, organic photovoltaic (OPV) is regarded as an excitonic solar cell that excitons are generated upon photo-excitation. Such intrinsic small dielectric constant (ε) in organic materials results in large exciton binding energy (Eb). That becomes a key detrimental factor limiting the further improvement in organic photovoltaic cells. Increasing the material dielectric constant seems to be a straight-forward strategy to reduce the strong coulombic attraction of the photo-generated electron-hole pairs. Despite the matter of importance, there are limited reports in measuring the Eb and ε in organic photovoltaic materials and the correlation between the dielectric constant and the exciton binding energy is unclear. Here, we extend our demonstration by using quantum efficiency measurement [1] and electro-absorption to access the transporting gap and exciton binding energy in pristine organic photovoltaic materials for polymeric donor, fullerene and non-fullerene small molecular acceptors. It is found that Eb varies from 0.3 eV to 1.2 eV in those prototypical materials and it apparently follows a second power law with the inverse of the dielectric constant of the materials, i.e. Eb ∝ 1 / ε2. Instead of widely assumed first-order dependence, this second order dependent relationship is firstly reported. Interestingly, we have also found that the binding energy is more dependent on the molecular-molecular interaction rather than the intrinsic properties of single molecule. In this presentation, we will also demonstrate how the higher dielectric material benefits the exciton dissociation at donor/acceptor interface.
[1]: Ho-Wa Li, Zhiqiang Guan, Yuanhang Cheng, Taili Liu, Qingdan Yang, Chun-Sing Lee, Song Chen, Sai-Wing Tsang, On the Study of Exciton Binding Energy with Direct Charge Generation in Photovoltaic Polymers, Adv. Electron. Mater., 2016, 2 (11), 1600200.
