Enhanced p-doping and efficiency in organic solar cells using Mg and Pd ions at the HTL/PTB7 interface

Abstract

This study investigates the application of new hole transport layers (HTLs) integrating magnesium and palladium metals with the organic polymer poly(styrene sulfonate) (PSS) in organic solar cells (OSCs). When used alone, these HTLs exhibited various drawbacks; however, blending them with the benchmark material PEDOT:PSS mitigated these issues and improved efficiency. Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) measurements provided a detailed understanding of the interfacial energy level alignment, electronic band structure, and band bending at the HTL/PTB7 interface. Single Mg:PSS and Pd:PSS OSCs showed efficiencies of 6.232 and 5.836%, respectively. The relatively low open-circuit voltage (V_OC) and fill factor (FF) were attributed to Auger recombination under light intensity. UPS and XPS also indicated that the hole extraction capability of PTB7 was hindered, leading to recombination at the barrier. By blending with PEDOT:PSS, the efficiencies of Mg:PSS and Pd:PSS were improved to 8.356 and 8.303%, respectively. This improvement was due to reduced current leakage, resulting from higher shunt resistance and lower series resistance, as observed in dark current measurements. Additionally, the formation of ohmic contacts at the HTL/PTB7 interface enhanced hole extraction and reduced recombination. This study underscores the potential of mixed organic-metal HTL structures in OSCs to modulate energy band structures, providing insights into the selection of metal-organic combinations for optimizing OSC efficiency and performance.