Enhanced Self-Assembled Monolayer Surface Coverage by ALD NiO in p-i-n Perovskite Solar Cells

Abstract

Metal halide perovskites have attracted tremendous attention due to their excellent electronic properties. Recent advancements in device performance and stability of perovskite solar cells (PSCs) have been achieved with the application of self-assembled monolayers (SAMs), serving as stand-alone hole transport layers in the p-i-n architecture. Specifically, phosphonic acid SAMs, directly functionalizing indium-tin oxide (ITO), are presently adopted for highly efficient devices. Despite their successes, so far, little is known about the surface coverage of SAMs on ITO used in PSCs application, which can affect the device performance, as non-covered areas can result in shunting or low open-circuit voltage. In this study, we investigate the surface coverage of SAMs on ITO and observe that the SAM of MeO-2PACz ([2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid) inhomogeneously covers the ITO substrate. Instead, when adopting an intermediate layer of NiO between ITO and the SAM, the homogeneity, and hence the surface coverage of the SAM, improve. In this work, NiO is processed by plasma-assisted atomic layer deposition (ALD) with Ni(MeCp)₂ as the precursor and O₂ plasma as the co-reactant. Specifically, the presence of ALD NiO leads to a homogeneous distribution of SAM molecules on the metal oxide area, accompanied by a high shunt resistance in the devices with respect to those with SAM directly processed on ITO. At the same time, the SAM is key to the improvement of the open-circuit voltage of NiO + MeO-2PACz devices compared to those with NiO alone. Thus, the combination of NiO and SAM results in a narrower distribution of device performance reaching a more than 20% efficient champion device. The enhancement of SAM coverage in the presence of NiO is corroborated by several characterization techniques including advanced imaging by transmission electron microscopy (TEM), elemental composition quantification by Rutherford backscattering spectrometry (RBS), and conductive atomic force microscopy (c-AFM) mapping. We believe this finding will further promote the usage of phosphonic acid based SAM molecules in perovskite PV.

Keywords: atomic layer deposition; indium tin oxide; nickel oxide; perovskite solar cells; self-assembled monolayer; surface coverage.

Figure 1

Box charts of PV parameters of devices using ALD NiO with postannealing at 300 °C for 20 min in air, MeO-2PACz, and ALD NiO (without post-treatment) + MeO-2PACz: (a) PCE, (b) open-circuit voltage (V_OC), (c) fill factor (FF), (d) short-circuit current (J_SC). The data are collected from reverse second J–V scan with 100 mV/s scan rate. Box charts represent 25–75%. Whiskers are 5–95%, and the rest are outliers. Data are collected from 36 devices (active area of 0.16 cm²), with each condition reported from several different batches.

Figure 2

High-angle annular dark-field images of a cross section of the (a) ITO/MeO-2PACz device and (b) ITO/NiO + MeO-2PACz device. O 1s spectra of (c) ITO and ITO + SAM and (d) NiO and NiO + SAM samples.

Figure 3

(a) Schematic of c-AFM where the layers are biased at 100 mV and a metal-coated AFM tip is used to collect the current signal from the surface. (b) Height and current map of ITO + MeO-2PACz layer and (c) corresponding line profile along the dashed line. (d) c-AFM height and current maps of NiO + MeO-2PACz layer with line profile plotted in (e).

Figure 4

(a) Dark current density–voltage curves of low- and high-efficiency MeO-2PACz devices and a NiO + MeO-2PACz device. (b) Distribution of shunt resistance of MeO-2PACz devices and NiO + MeO-2PACz devices obtained from the data set in Figure 1.