Enhanced efficiency of inverted organic solar cells by using alcohol molecules modified ZnO as an electron transport layer

The zinc oxide (ZnO) film as electron transport layer (ETL) is modified by various alcohol molecules in inverted organic solar cells. The conduction band minimum of the ZnO is reduced by the surface modifiers to enhance charge transfer and electron extraction. The Fourier transform infrared (FTIR) spectroscopy of ZnO shows that alcohol molecules are successfully incorporated with reduced hydroxyl groups for improved hydrophobicity of the ZnO film. The improved roughness of modified ZnO films provides better interfacial adhesion and intimate contact between the ETL and active layer which is deposited subsequently. The atomic force microscopy (AFM) images have shown that the morphology of the active layer on the ZnO/1,3-propanediol (ZnO/PDO) exhibits most moderate phase separation and interpenetrating network structure for better light absorption (external quantum efficiency (EQE) = 79%, improvement of 22%), more favorable for the exciton separation and charge transfer with the highest electron mobility of ~ 1.07 × 10 –2 cm 2 V −1 s −1 and lowest charge transfer resistance of ~ 1334 Ω (decreased by 34%). Therefore, the ZnO/PDO-based device exhibits the highest power conversion efficiency of 7.05% improved by 28%, with 13% and 15% increases in short-circuit current density ( J sc ) and fill factor (FF), respectively.