▎ 摘　　要

In this work, graphene oxide (GO) reduced by thermal annealing was employed as a hole transport layer (HTL) in bulk heterojunction (BHJ) solar cells. Considering the insulating property of nonreduced GO, the annealing temperature plays an important role in recovering the conjugated structure of the graphene sheet, and thereby the conductivity of GO. BHJ solar cells with high-temperature (e. g., 230 degrees C) reduced GO as the HTL showed much larger fill factor (FF) than devices with low-temperature (e. g., 130 degrees C) reduced GO as the HTL, indicating the better conductivity of GO annealed at an elevated temperature due to the removal of oxygen functional groups from the graphene sheet to a much-higher level. On the other hand, the work function of GO may be lowered toward that of graphene (4.5 eV) with increasing the reduction temperature, which results in a decreased open-circuit voltage (V-oc) for the high-temperature reduced GO devices. By further optimizing the concentration and spin-coating speed of GO dispersion, we achieved a power conversion efficiency (PCE) that is 26% higher than devices without any HTL. This is mainly attributed to the increase in FF as a result of the decreased series resistance (R-s). In addition, the PCE of the optimized GO device was similar to 85% of the PCE of the conventional device with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. We anticipate that further optimization of the reduction conditions (e. g., using chemical reductants) will lead to the better performance of GO solar cells. (C) 2012 Elsevier B.V. All rights reserved.