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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 co

The chemistry of the interface between the metal halide perovskite absorber and the charge transport layer affects the performance and stability of metal halide perovskite solar cells (PSCs). The literature provides several examples of poor PSC conversion efficiency values, when electron transport layers (ETLs), such as SnO2 and TiO2, are processed by atomic layer deposition (ALD) directly on the perovskite absorber. In the present work, we shed light on the chemical modifications occurring at the perovskite surface, during ALD processing of SnO2 and TiO2, in parallel with the evaluation of the PSC cell performance. The ALD processes are carried out on a (Cs,FA)Pb(I,Br)3 perovskite by adopting tetrakis(dimethylamino)tin(IV) and tetrakis(dimethylamino)titanium(IV) as metal precursors and H2O as the coreactant for SnO2 and TiO2, respectively. Perovskite surface modification occurs in the form of an ultrathin PbBr2 layer. Furthermore, in the case of SnO2, halogen molecules are detected at the interface, in parallel with the initial growth of an oxygen-deficient SnO2. Subgap defect states just above the valence band maximum of SnO2 are also detected. These states act as hole traps at the perovskite/SnO2 interface, subsequently promoting charge recombination and deteriorating the performance of the cell. We hypothesize that a redox reaction between the perovskite, or its decomposition products, and the Sn metal center of the ALD precursor takes place: I− and Br− are oxidized to I2 and Br2, respectively, and Sn(IV) is reduced to Sn(II). In contrast, the Ti(IV) metal center does not undergo any redox process, and, as a result, a promising 11% power conversion efficiency is measured with TiO2 as the ETL. This result strongly suggests that TiO2 may be a more suitable ETL, when processed directly on the perovskite absorber.

Moisture-induced degradation in perovskite solar cells was thoroughly investigated by structural (SEM, EDS, XRD and XPS) and device characterization (impedance and intensity modulated photocurrent spectroscopy) techniques. Both the influence of the perovskite composition and the nature of the hole selective material were analyzed. The degradation rate was found to be significantly slower for mixed perovskites and P3HT-based devices. However, for a fixed degradation degree (defined as a 50% drop from the initial photocurrent), all configurations show similar features in small-perturbation analysis. Thus, a new mid-frequency signal appears in the impedance response, which seems to be related to charge accumulation at the interfaces. In addition, faster recombination, with a more important surface contribution, and slower transport were clearly inferred from our results. Both features can be associated with the deterioration of the contacts and the formation of a higher number of grain boundaries.