Novel Cu Nanowires/Graphene as the Back Contact for CdTe Solar Cells

Cu-nanowire-doped graphene (Cu NWs/graphene) is successfully incorporated as the back contact in thin-film CdTe solar cells. 1D, single-crystal Cu nanowires (NWs) are prepared by a hydrothermal method at 160 degrees C and 3D, highly crystalline graphene is obtained by ambient-pressure CVD at 1000 degrees C. The Cu NWs/graphene back contact is obtained from fully mixing the Cu nanowires and graphene with poly(vinylidene fluoride) (PVDF) and N-methyl pyrrolidinone (NMP), and then annealing at 185 degrees C for solidification. The back contact possesses a high electrical conductivity of 16.7 S cm-1 and a carrier mobility of 16.2 cm2 V-1 s-1. The efficiency of solar cells with Cu NWs/graphene achieved is up to 12.1%, higher than that of cells with traditional back contacts using Cu-particle-doped graphite (10.5%) or Cu thin films (9.1%). This indicates that the Cu NWs/graphene back contact improves the hole collection ability of CdTe cells due to the percolating network, with the super-high aspect ratio of the Cu nanowires offering enormous electrical transport routes to connect the individual graphene sheets. The cells with Cu NWs/graphene also exhibit an excellent thermal stability, because they can supply an active Cu diffusion source to form an stable intermediate layer of CuTe between the CdTe layer and the back contact.