Self-Assembled Monolayer Engineered ZnO Electron Transport Layer to Improve the Photostability of Organic Solar Cells

Abstract

The degradation of organic solar cells (OSCs) can occur in any of the layers, underlining the importance of each layer in prolonging their lifetime. To enhance the performance and stability of inverted OSCs (i-OSCs), interfacial modification has been employed. In this context, two self-assembled monolayers (SAMs), namely, octadecanthiol (ODT) and octadecyltrimethoxysilane (OTMS), were utilized to effectively passivate typical surface defects in the ZnO electron transport layer (ETL). The SAM-treated ZnO films were found to be more hydrophobic, which reduced surface defects produced by adsorbed oxygen and hydroxyl groups. Consequently, the power conversion efficiency (PCE) of the i-OSCs comprising an indacenodithieno[3,2-b]thiophene-alt-5,5-di(thiophen-2-yl)-2,2-bithiazole (PIDTT-DTBTz) donor blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) acceptor increased from 4.20% in pristine ZnO- to 5.01 and 5.37% in ODT- and OTMS-treated ZnO-based devices, respectively. In addition, the photostability of the device substantially improved. Hence, devices based on ZnO treated with ODT and OTMS kept 76 and 89% of their initial PCE, respectively, while pristine ZnO-based devices retained only 66% of the initial PCE after 48 h of irradiation. The improved PCE and extended lifetime of the i-OSCs can be attributed to enhanced charge transfer, the reduction in both bimolecular and trap-assisted recombination processes, and the enhanced interface between the ETL and the active layer. Moreover, it has been observed that the OTMS-treated ZnO ETL-based i-OSC offers better stability and more efficient devices compared to the ODT-treated ZnO ETL-based devices. This can be attributed to the favorable dipole moment generated by the increased electrostatic potential at the anchor group, which promotes improved device performance.