Critical binding energy for exciton dissociation and its implications for the thermodynamic limit of organic photovoltaics

Abstract

Spectroscopic signatures of strongly bound excitons (binding energy, E_B) in various low dielectric constant materials and reduced dimensional systems, such as CNT transistors, Quantum well lasers, etc. are often erased during room temperature, high-field, high-power-density operation of these devices. Similarly, while pump-probe experiments have been used extensively to explore exciton dynamics in organic photovoltaics (OPVs), one wonders if the exciton bottlenecks would persist under continuous broadband illumination of OPV. In this paper, we use a self-consistent thermodynamic model (involving detailed-balance and energy-conservation) to explicitly model the kinetics of exciton dissociation and corresponding energy balance considerations [1,2]. We find that exciton bottleneck may arise under normal PV operation if and only if the exciton binding energy, E_B > _B(critical) ≡ (p_therm + ΔE)× J_sc/J_opt, where J_sc and J_opt are the short-circuit and maximum power point currents, with the ratio~1; the thermalization per carrier is given by p_therm ≈ P_therm/(J_SC) ≈ ((4+ξ+6/ξ)/(2+ξ)) kT_s (for an ideal blackbody source), with ξ = E_G/kT_s. And, ΔE is the type-II band-discontinuity of a bulk-heterojunction (BHJ) cell. We predict that all signatures of exciton-limited performance of OPV would be eased if E_B <; E_B(critical), and the operation of an OPV would be indistinguishable from their classical counterparts.