Understanding charge recombination and light-induced degradation: an in-depth study of tert-butyl modified carbazole-based self-assembled monolayers for enhanced performance in organic solar cells

Abstract

The self-assembled monolayer (SAM) has been proven to be an efficient hole-selective material in photovoltaic devices. In organic solar cells (OSCs), despite their competitive performance, the mechanism behind device stability remains unclear. This research highlights that although SAM molecules can upshift the work function (WF) of ITO relative to the vacuum level, they may not consistently disperse in monolayers that simultaneously improve the performance and operating stability. Here, we prepared two carbazole-based molecules for a SAM namely CBZC4 and CBZC2 with the introduction of a tert-butyl substituent at 3,6-positions of carbazole to achieve ideal energy-level alignment with the HOMO energy level of the donor, along with improved intermolecular interaction that is beneficial for SAM arrangement on the ITO surface. Light-induced degradation in the SAM-based OSCs is identified through diverse aging tests and comprehensive chemical and electronic characterization. This degradation is caused by the reactivity of the SAM molecules to external oxygen and moisture, which accelerates device degradation. Addressing these challenges, CBZC2 showed superior stability, with only a 0.63% PCE loss after UV exposure, outperforming PEDOT:PSS and MeO-2PACz. The enhanced performance of CBZC2-based OSCs is related to the decreased interface resistance and longer carrier lifetime, and thus, CBZC2 emerges as a potential alternative to hygroscopic PEDOT:PSS.