Light induced quasi-Fermi level splitting in molecular semiconductor alloys

Abstract

Quasi-Fermi-level (QFL) splitting is a direct measure of the open-circuit voltage (V_OC) in an optically illuminated semiconductor solar cell (SC). The evolution of QFL splitting under 1 sun illumination in ternary blends of Gaussian disordered (GD) excitonic molecular semiconductors (MSs) is a complex process. The experimental diagonal band-gap (E_CT) fitted with Vegard's law provided a bowing parameter as low as 0.05 for the used ternary alloys as a function of the mixing of two n-type semiconductors, which is a hallmark of good mixing without much alloying disorder. An analytical model based on population occupancy in GD systems is used to determine the change in QFLs as a function of the alloy composition in ternary (two n-type and one p-type) MS blends under 1 sun light illumination. The model predicts a remarkable quantitative change in the QFL due to light-induced charge carriers in such alloys to fit the experimental V_OC value. This analytical model, combined with temperature-dependent mobility studies on unipolar devices with various MS alloy compositions, also reveals an interesting observation that a suitable change in QFL is due to the formation of an effective density of states (DOS) between the two n-type MSs. Further, a simpler routinely used double-diode model is also used for comparison with the Gaussian disorder model to fit the V_OC values of ternary-alloy-based organic solar cells (OSCs). We show that, overall, the model is more generalized for use with any binary and ternary MS heterojunction systems being used for photovoltaic applications to determine the QFL splitting.