Interfacial energetic disorder induced by the molecular packing structure at conjugated polymer-based donor/acceptor heterojunctions

Abstract

The energetic disorder in conjugated polymer films plays a crucial role in determining the charge separation and recombination process at donor/acceptor heterojunctions, and thus dominates the photocurrent and photovoltage in organic solar cells. However, how the polymer molecular packing structure influences the energetic disorder is still mysterious, hindering the promotion of device performance in term of morphological control. In this work, we employed a J71/N2200 planar heterojunction model to construct the relationship between the molecular packing structure and interfacial energetic disorder. Both J71 and N2200 films exhibit a remarkable transformation of the molecular orientation from nearly pure face-on to mixed face-on/edge-on by the incorporation of the DIO additive, in which J71 has a larger degree of disorder in the mixed orientation than N2200. The simultaneously increased coherence length (CL) in the J71 crystalline domains with mixed molecular orientation probably creates enlarged grain boundaries and thus an enriched density of morphological defects, while the decreased CL in the N2200 crystalline domains, compared to their J71 counterparts, would produce a relatively small amount of grain boundaries and thus moderately enhanced defects in N2200 films. Therefore, the supreme interfacial energetic disorder can be found in the bilayer device consisting of the J71 component with mixed orientation as revealed by the characterization of the Urbach energy (E_u) and effective bandgap (E^eff_g), which results in severe charge recombination and a shortened charge carrier lifetime. Our results indicate that the pure molecular orientation of the conjugated polymer is vital to the reduction of the interfacial energetic disorder at the heterojunction.