Improved efficiency of organic photovoltaic cells using tris (8-hydroxy-quinoline) aluminum as a doping material

Abstract An improvement of photovoltaic characteristics has been demonstrated in heterojunction organic photovoltaic (OPV) cells with a ITO/copper phthalocyanine (CuPc)/C60/ bathocuproine (BCP)/LiF/Al configuration by doping tris (8-hydroxyquinoline) aluminum (Alq3) fluorescent dye into the acceptor (or donor) layers. With Alq3 doping, the short-circuit current density (JSC) increased from 8.25 to 12.23 mA/cm2, the open-circuit voltage (VOC) increased from 0.50 to 0.62 V, and the power conversion efficiency (ηp) increased from 1.76 to 2.57% under 100 mW/cm2 AM 1.5 illumination. These improvements can be attributed to the increased light absorption caused by Alq3, which led to more effective exciton formation. The energy difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor in the doped device increased, leading to a larger VOC. There are no photoluminescence (PL) in the Alq3-doped C60 layer, suggesting that the formation of Alq3 excitons was followed by energy transfer or charge transfer to C60 under illumination in the doped layer. The conversion efficiency of OPV cells was thus improved.