The success of metal halide perovskite solar cells stems from high absorption combined with a low recombination rate. Despite the fact these properties are inherent to the perovskite material, the choice of selective contacts is critical to achieve high voltages according to experimental evidence. In this work, the impedance and the open-circuit photopotential are measured for two excitation wavelengths (blue and red light), in two illumination directions (back and front), and at different temperatures. The open-circuit recombination characteristics of two different perovskite compositions, i.e., pure MAPbI 3 and mixed ion-based (FAPbI 3 ) 0.85 (MABr 3 ) 0.15 , and with two different hole selective layers (Spiro-OMeTAD and P3HT) have been studied. Our results indicate that, for the studied devices, the recombination process that determines the open-circuit potential is governed by the bulk of the perovskite layer via a trap-limited mechanism, but surface-mediated recombination cannot be ruled out for P3HT contact or degraded devices. Further, we propose a model that provides a general interpretation of the nature of recombination in perovskite solar cells.

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 O 2 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.

Hysteresis is one of the most remarkable features of perovskite solar cells; however, it is also present in other kinds of devices such as dye-sensitized solar cells. Hysteresis is due to underlying slow dynamic processes that interfere with the process of charge separation which depends critically on the selective contacts used. In this work we focus on the low-frequency (0.1-10 Hz) dynamics using impedance and intensity-modulated photocurrent spectroscopy and found that both perovskite solar cells (PSCs) and "viscous electrolyte containing" dye-sensitized solar cells (DSSCs) can be described on the same fundamental grounds. By comparing different electrolyte compositions in DSSCs and both methylammonium and formamidinium-based PSCs, we find a connection between the polar nature of the cations and the low-frequency component of these solar cells. There is evidence that in both cases ion transport and specific chemical interactions with the TiO surface give rise to the slow dynamics and the hysteresis. This is mainly inferred from the slope of the capacitance vs. applied voltage which shows accumulation behavior for the formulations with higher dipole moments only.

Impedance spectroscopy is a powerful electrochemical small‐perturbation technique that provides dynamic electrical data in solar cells. This technique has been widely used to characterize dye‐sensitized solar cells and perovskite solar cells (PSCs). Physical parameters are normally obtained by fitting to an equivalent circuit, composed of electrical elements which theoretically correspond to physical processes involved in the photoconversion process. A variety of equivalent circuits to model the impedance spectra of PSCs are commonly used by different research groups. In this work, we evaluate their performance and adequacy. We demonstrate the analytical and numerical equivalence of impedance expressions for Voight, matryoshka, and hybrid circuits, which are used to fit a typical impedance spectrum of a PSC and compare the resulting parameters to the empirical values obtained without any equivalent circuit. The numerical equivalence can be demonstrated by using two‐ and three‐component impedance spectra. In contrast, Maxwell‐type equivalent circuits reveal parameters that have a more complex relation to empirical values. The presence of inductive effects such as “loops” and “negative tails” in impedance spectra are also discussed in terms of negative values of resistances and capacitances. We propose a general protocol to analyze impedance data of PSCs and to extract useful information from them.