Potassium functionalized graphene oxide for applications as electron transporting layers in organic photovoltaic devices

Full text: Despite the success of inorganic solar cells, organic photovoltaic devices (OPV) hold great potential for supply the future energy demands due their potential low cost and large-scale fabrication. It is known the OPV performances are dependent on the charge extraction efficiencies and thus, strategic interfacial layers must be engineered to guarantee Ohmic contacts. Thus, organic hole/electron transporting layers (HTL/ETL) are deposited between the photoactive layer and the electrodes. Recently, graphene oxide (GO) has been largely used as HTL in OPVs. However, its intrinsic p-type behavior and high work-function (WF) difficult its use as ETL. Nevertheless, the rich surface functionalization of GO allows the use of chemical strategies to decrease its WF and turn it in a n-type material. Among them, its functionalization with alkaline metals can be used to prepare a material with the requirements of an ETL. In this work, dispersions of GO were chemically functionalized with potassium ions (K-GO) using a simple and environment-friendly strategy. GO powder with huge quantities of carbonyl groups were prepared using modifications of the Hummers' method. K-GO samples were characterized using many spectroscopic techniques (Raman, FT-IR, EDS, NMR, XPS, UPS, etc.). In addition, K-GO thin films were employed as ETL in OPVs based on the bulk heterojunction structure blend of poly(3-hexylthiophene) (P3HT) and the phenyl-C61-butyric acid methyl ester (PCBM) as the active layer, and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as HTL. The results showed that the insertion of K-GO improves the OPV efficiencies in about 65% when compared with the ones without ETL. This improvement is associated with the good alignment between the energy levels of K-GO, active layer and aluminum (Al) cathode, as well as the interface effects between them. The results will be presented and discussed. (author)