Control of the oxidation level of graphene oxide for high efficiency polymer solar cells

Abstract

Graphene oxides (GOs) have been used as interfacial layers for fabricating more stable organic solar cells (OSCs). However, the influence of the degree of oxidation of GOs on their optoelectronic properties has been ignored. In this article, a series of GOs with different degrees of oxidation were successfully synthesized, by controlling the amount of oxidant KMnO₄ during the oxidation process of graphite. With increasing oxidation level, more oxygenated functional groups were attached to the carbon basal plane and more defects were introduced into the GO sheets, resulting in an increased work function (WF) and a decreased conductivity. Meanwhile, the film-forming property of GOs was improved with increasing oxidation level, which is attributed to the adequate exfoliation of the GO sheets. After carefully controlling the oxidation level of GOs, the OSCs with GOs as the hole transport layer (HTL) show an efficiency value of 3.0%, comparable to that with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (3.2%), originating from the good film-forming property, appropriate work function and high conductivity.