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The remarkable power conversion efficiency of perovskite solar cells (PSCs) is overshadowed by concerns about their stability, and its degradation mechanism remains elusive. PSCs are reported to suffer long-term degradation under real working conditions. In this work, we systematically studied the degradation mechanism of PSCs with various device configurations (planar, meso and inverted device structure) by continuous 12-h day/night cycling tests. The fatigue phenomenon, defined in our previous work, was observed both in planar and meso devices. A relationship between the fatigue instability and device-physics of PSCs is established. Through a comparative analysis of results from day/night cycling tests, bulk/interfacial morphology analysis, and transient photocurrent/photovoltage decay measurements, we identify the fatigue behavior in the day/night cycling tests by a cyclic ion movement mechanism, where ions migrate towards the electrode interfaces under illumination and move back to the bulk in the dark. This cyclic migration of ions generates defects in bulk perovskite without destroying the crystal structure. The present study offers a new approach to evaluate the stability of PSCs, contributing to better understanding of degradation mechanisms that are critically important for applications of this novel photovoltaic technology.

Inorganic hole–transporting materials (HTMs) are a promising class of compounds for improving the long-term stability of perovskite solar cells. In this study, copper(I) thiocyanate (CuSCN) has been applied as an HTM in planar-structured thin film perovskite solar cells based on methylammonium lead(II) triiodide. A common obstacle associated with the deposition of inorganic HTMs in perovskite-based solar cell devices is the damaging effect of polar solvents, required during the solution-processed deposition step, on the underlying perovskite film. Here we describe a novel fabrication method that allows the deposition of a CuCSN layer on perovskite film, achieving a maximum power conversion efficiency of 9.6%. The magnitude of J-Vhysteresis is found to be strongly dependent on the HTM used, with the phenomenon being much more prevalent in the CuSCN- and spiro-OMeTAD-based devices compared to CuI-based devices. Interestingly, CuSCN and CuI showed significantly different J-Vhysteresis behaviors despite their similar physicochemical properties. Further characterization by open circuit voltage decay (OCVD) measurements revealed that the relaxation of the perovskite polarization depends on the light intensity and the adjacent HTM layer. We propose that the stronger J-Vhysteresis in CuSCN compared to CuI is a result of defects generated during the deposition process and possible degradation at the material interfaces while other possibilities are also discussed.

Long-term stability represents a major challenge for the commercial deployment of hybrid perovskite solar cells (PSCs). The stability of solar cells is commonly tested under continuous illumination over extended periods of time, for example, 1000h. We have found that such a method does not adequately reflect the long-term performance of perovskite solar cells under the diurnal solar irradiation cycles experienced in real-world applications. We report a new characterization protocol of multiple 12-h cycles of darkness and illumination, uncovering a unique ‘fatigue’ behavior of PSCs. The PSC efficiency was found to decrease to 50% or less of its maximum value after storage in the dark for 12h under open circuit conditions. The solar cell performance was capable of recovering to its maximum value in the subsequent 12-h illumination period, but the recovery rate slowed significantly with successive illumination/darkness cycles. This fatigue mechanism was strongly dependent on the cell temperature. The identification of this fatigue behavior renders our proposed characterization protocol an essential component of perovskite solar cell testing.

Perovskite solar cells (PSCs) employing planar and mesoscopic architectures have both resulted in high efficiency devices. However, there is presently a limited understanding of the inherent advantages of both systems, particularly in terms of the charge transport and recombination dynamics. In the present study we characterize the relative benefits of the two most prominent CH3NH3PbI3morphologies, primarily through time-resolved microwave conductivity (TRMC) and time-resolved photoluminescence (TRPL) measurements. The comparatively large perovskite grains, typical of planar assemblies, exhibited higher charge mobilities and slower trap-mediated recombination compared to the mesoscopic architectures. These findings reveal the injurious influence of grain boundaries on both charge transport and recombination kinetics, and suggest an innate advantage of planar morphologies. However, through impedance spectroscopy (IS) measurements, mesoscopic architectures were found to limit the interfacial recombination at the transparent conductive oxide (TCO) substrate. The lessons learnt through the characterization measurements were subsequently utilized to produce an optimized cell morphology, resulting in a maximum conversion efficiency of 16%.