Design of Efficient and Stable Non-fullerene Acceptor-based Organic Solar Cells by Buffer Layer Modification

Abstract

The practical applications of organic solar cells (OSCs) are hindered by their insufficient lifetimes. Zinc oxide (ZnO) and tin (IV) oxide (SnO2), which are widely used electron transport layer (ETL) materials in OSCs, suffer from photocatalytic effects and surface defects. These problems limit the power conversion efficiency (PCE) and stability of OSCs and must be solved to realise the practical application of OSCs. This study focuses on solving these problems by modifying ZnO and SnO2 ETL materials with commercial ultraviolet resins based on polyurethane diacrylate and urethane acrylate, whose given names by the manufacturing company were SAR and OCS, respectively. In this study, the OSCs ETL, modification interlayer, and photoactive layer were fabricated by spin coating technique on indium tin oxide (ITO) patterned substrates. The hole transport layer (HTL) and top silver (Ag) electrode were deposited using the thermal evaporation method. The interfacial modification properties, such as hydrophobicity, work function (WF), and surface defects, were studied using contact angle measurement, ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy. Modifying ZnO and SnO2 with SAR and OCS resins improved the contact angles and reduced the surface free energy, thus enhancing the interfacial contact between ETLs and the photoactive layer. UPS analysis showed that SAR and OCS modification reduced the WF of ETLs, resulting in energy-level alignment with the photoactive layer. A significant decrease in the surface defects of ETLs upon SAR and OCS resin modification was observed in XPS and PL analyses. The improved interfacial contact at the ETLs/photoactive layer, reduced work functions, and surface defects contributed to the PCE improvement of the OSCs based on ZnO ETL from 13.6% to 14.6% and those based on SnO2 ETL from 13.8% to 14.4%. The photocatalytic activity of ZnO and SnO2, which decomposes the organic photoactive layer material upon light illumination, was found to be suppressed through SAR and OCS modification. The suppressed photocatalytic effect contributed to the improvement in the photostability of OSCs. In addition, the SAR and OCS resins formed a cross-linked network on ETLs which was resistant to thermal stress, contributing to improved OSC thermal stability. Thus, this study presents a good approach to enhance the performance and stability of OSCs utilising ultraviolet resins.