Effect of electron transport layer materials on the performance of copper phthalocyanine/fullerene heterojunction with function of organic solar cells

Small-molecule organic solar cells with structure of indium tin oxide-coated glass substrate/Copper phthalocyanine/fullerene/electron transport layer/aluminum were studied. 2-(4-Biphenylyl)-5-(4-tert-butylphenyl)-1, 3, 4-oxadiazole, bathocuproine and tris (8-hydroxy-quinolinato) aluminum were used as electron transport layers. It was found that the power conversion efficiency of the devices with bathocuproine layer is five times than that of devices without the electron transport layer. The current density of short circuit increases from 1.74 mA/cm2 for the devices with 8 nm-thick 2-(4-Biphenylyl)-5-(4-tert-butylphenyl)-1, 3, 4-oxadiazole layer, to 3.4 mA/cm2 for 8 nm-thick bathocuproine layer. This results in the power conversion efficiency of the devices to increase from 0.136% to 0.54%. Optical absorption spectroscopy and Atomic Force Microscopy measurements show that the surface of device with a bathocuproine layer has a better optical absorbency and a smoother surface morphology than that of other electron transport layer materials. This results in generating more electron-hole pairs in devices. Thus, more electrons are guided outside to provide higher power conversion efficiency. Additionally, the energy barrier between the lowest unoccupied molecular orbital level of fullerene and the lowest unoccupied molecular orbital level of bathocuproine are relatively lower than other electron transport layer materials, resulting in more electrons that can cross the energy barrier.