Molecular Regulation Of Graphene Oxide For Polymer Solar Cells

Due to the appropriate photoelectric property and favourable chemical stability, graphene oxides(GOs) have been used as interfacial layers for fabricating more stable polymer solar cells(PSCs). 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 PSCs 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.Then, we have rationally designed and successfully synthesized a reduced graphene oxide functionalized with fluorine atoms(F-rGO). The resultant F-rGO has an excellent dispersibility in dimethylformamide without any surfactants, leading a good film-forming property of F-rGO for structuring a stable interface. The recovery of conjugated C=C bonds in graphene oxide during reduction increases the conductivity of F-rGO, which enhance the short-circuit current density of photovoltaic device from 14.04 to 14.78 mA cm-2. Fluorine group with a high electronegativity allows for a higher work function(5.1 eV) of F-rGO than GO(4.9 eV). The better matched work function with the HOMO level of PTB7-Th(5.22 eV) donor induces an improved energy alignment in devices, resulting in a superior open-circuit voltage of the device(0.793 to 0.802 V). As a result, the device with F-rGO as hole transport layer(HTL) achieves a higher power conversion efficiency(7.7%) with long-term stability than the devices with GO HTL(7.2%), even than PEDOT:PSS control device(7.4%). These results clearly demonstrate that the F-rGO is a promising hole transport material and an ideal replacement for conventional PEDOT: PSS, further promoting the realization of low-cost, solution-processed, highly efficient and stable PSCs.