Tailoring the work-function of ZnO sol-gel derived ZnO films using electron beam irradiation for flexible organic solar cells

Abstract

This study investigates the use of electron beam (EB) irradiation to tune the work-function of ZnO films prepared from EB-irradiated ZnO sol-gel solution for application as electron transporting layers (ETLs) in flexible organic solar cells (OSCs) fabricated on PEN/ITO substrates. ZnO precursor solutions were irradiated at doses from 100 to 500 kGy under nitrogen-purged, vacuum-sealed conditions using a 10 MeV electron accelerator. Ultraviolet photoelectron spectroscopy (UPS) revealed that EB irradiation in the range between 100 and 300 kGy reduced the ZnO work function from 4.06 eV (0 kGy) to 2.78 eV (500 kGy), allowing optimal alignment with the PC₇₁BM LUMO (∼3.9 eV). Surface characterization by atomic force microscopy (AFM) confirmed roughness was reduced at intermediate doses, while X-ray photoelectron spectroscopy (XPS) showed a decrease in hydroxyl species and stabilized lattice oxygen, indicating defect passivation. Flexible OSC devices incorporating the ZnO film prepared from EB-irradiated ZnO sol-gel solution exhibited improved fill factors from 55.72 % at 0 kGy to 60.47 % at 300kGy and increased power conversion efficiencies from 7.61 % to 8.13 %. These results demonstrate that EB irradiation effectively tailors the electronic and interfacial properties of ZnO ETLs, enabling enhanced charge extraction and scalable low-temperature processing for flexible photovoltaics.