Analysis of 2PACz Functionalization of Different ITO Layers Using an e-Beam-Based Technique for Work Function Measurement

In recent years, phosphonic acid-carbazole-based self-assembled monolayers (PACz SAMs) have become standard as hole-selective layers in inverted perovskite solar cells (PSCs) and perovskite/silicon tandem devices. PACz SAMs covalently bond to the transparent conductive oxide (TCO) on which they are deposited, typically an indium tin oxide (ITO) layer, thereby increasing the work function (WF) of the ITO surface and enabling hole extraction from the perovskite absorber. The properties of the underlying ITO layer can affect the functionalization of its surface by SAMs, and while there is an increasing number of studies to understand this aspect, the ITO properties required for good functionalization remain unclear. This article investigates the effectiveness of ITO functionalization by 2PACz by using an original electron-beam-based technique that directly reveals the ability of a sample to repel electrons and allows the calculation of the sample WF. We compared two different ITO layers: a commercially available polycrystalline ITO layer and a thin, in-house deposited amorphous ITO layer, which is typically used as a recombination layer in perovskite/silicon tandem solar cells. The e-beam-based measurement reveals significantly different work function shifts after 2PACz functionalization, in agreement with the observed PSC performances. This difference is discussed in terms of the ITO layer properties and surface chemistry. Overall, this work highlights the importance of taking into account optimal carrier selection in the optimization of TCO-based recombination junctions.