Graphene as interface modifier in ITO and ITO-Cr electrodes

We explore graphene as interface modifier for electrodes in optoelectronic organic devices by measuring the electrical properties of ITO/graphene and ITO/Cr/graphene. For this purpose, exfoliated graphene (EG) was electrochemically synthesized and deposited by spray-pyrolysis. The built-in voltage (Vbi) values were 450 mV for the ITO/CuPc/Al reference, 750 mV for ITO/Cr/graphene/CuPc/Al and 1000 mV for ITO/graphene/CuPc/Al device structures. From these results, we estimate the work functions as 3.20 eV, 3.45 eV and 4.75 eV for ITO/ EG, ITO/Cr/EG and ITO. To understand how the work function changes, we carried out first-principles calculations based on density-functional theory (DFT) where Cr work function (similar to 4.2 eV) is not modified by the deposition of pristine graphene; however there is a substantial increase (from 4.2 eV to 5.2 eV), upon deposition of graphene oxide (GO), resulting from a complete transfer of O atoms from the GO sheet to the Cr surface forming a thin layer of chromium oxide.