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We report a detailed study on APbX3 (A = formamidinium (FA+), Cs+; X = I–, Br–) perovskite quantum dots (PQDs) with combined A- and X-site alloying that exhibits both a wide bandgap and high open-circuit voltage (V oc) for the application of a potential top cell in tandem junction photovoltaic (PV) devices. The nanocrystal alloying affords control over the optical bandgap and is readily achieved by solution-phase cation and anion exchange between previously synthesized FAPbI3 and CsPbBr3 PQDs. Increasing only the Br– content of the PQDs widens the bandgap but results in shorter carrier lifetimes and associated V oc losses in devices. These deleterious effects can be mitigated by replacing Cs+ with FA+, resulting in wide-bandgap PQD absorbers with improved charge-carrier mobility and PVs with higher V oc. Although further device optimization is required, these results demonstrate the potential of FA1–x Cs x Pb­(I1–x Br x )3 PQDs for wide-bandgap perovskite PVs with high V oc.

Negative capacitance in photovoltaic devices has been observed and reported in several cases, but its origin, at low or intermediate frequencies, is under debate. Here we unambiguously demonstrate a direct correlation between the observation of this capacitance and a corresponding decrease in performance of a halide perovskite (HaP; CsPbBr3)-based device, expressed as reduction of open-circuit voltage and fill factor. We have prepared highly stable CsPbBr3 HaPs that do not exhibit any degradation over the duration of the impedance spectroscopy measurements, ruling out degradation as the origin of the observed phenomena. Reconstruction of current–voltage curves from the impedance spectroscopy provided further evidence of the deleterious role of negative capacitance on photoconversion performance.

ABSTRACT:The use of self-assembled monolayers (SAMs) of fullerene derivatives reduces 11 the hysteresis of perovskite solar cells (PSCs

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Metal Halide Perovskites (MHPs) have arisen as promising materials to construct cost-effective photovoltaic and light emission devices. The study of nonlinear optical properties of MHPs is necessary to get similar success in nonlinear photonic devices, which is practically absent in the literature. The determination of the third order nonlinear coefficients is typically done by the Z-scan technique, which is limited by the scattering of polycrystalline thin films. In this work, we have studied nonlinear optical properties of polycrystalline CH3NH3PbX3 (MAPbX3) thin films and colloidal CsPbX3 nanoparticles with three different bandgaps (X3 = I3, Br3, and Br1.5I1.5). Their bright generation of photoluminescence under infrared illumination demonstrates an excellent efficiency of multiphoton absorption. The nonlinear absorption coefficient (β) was studied by analyzing the transmitted light through the samples, observing the expected Eg−3 dependence with values as high as β = 1500 cm/GW. In addition, we proposed the use of a modified Z-scan technique with imaging processing to analyze the nonlinear refraction coefficient (n2) under the laser damage threshold. Our experimental data agree quite well with theoretical predictions, demonstrating the accuracy of the method and potential applications to other thin films. Moreover, n2 parameter reaches values of 3.5 cm2/GW, indicating the suitability of MHPs for nonlinear photonics.

Methylammonium Lead Iodide (CH 3 NH 3 PbI 3 ) is the archetypical active component of perovskite solar cells, which stand out due to their impressive photovoltaic performance. A major drawback of CH 3 NH 3 PbI 3 is its rapid degradation in humid environments.In this work, we fabricate CH 3 NH 3 PbI 3 films and devices by solvent engineering in N 2 and in ambient conditions with different humidities. Their aging and degradation is monitored by optical absorption and impedance spectroscopy measurements under monochromatic illumination with two different wavelengths. Aged devices show a substantial difference between the recombination rate under red and blue light illumination, attributed to enhancement of local recombination routes upon aging. Interestingly, we observe that devices prepared at higher humidity resist better the aging. We explain this by the presence of coordinating water in the films, as detected by XPS measurements. Hence, small amounts of water in the perovskite structure proves to have a beneficial effect against degradation in humid environments.

Halide perovskite derivatives present unprecedented physical phenomena among those materials suitable for photovoltaics, such as a fast ion diffusion coefficient. Here we report how to take benefit from this property during the growth of halide perovskite in order to control the morphological and optoelectronic properties of the final thin film. Using a large enough halide reservoir, the nature of the halides present in the final perovskite layer can be exchanged respect the initial salt used in the two step deposition method. In particular, we report the preparation of methylammonium lead bromide (MAPbBr 3 ) thin film using a two-step method based on the transformation of PbI 2 , PbBr 2 and PbCl 2 salts into MAPbBr 3 perovskite after dipping in a MABr solution. The films prepared from different salts present different properties in terms of morphology and optoelectronic properties, thus providing significantly different performance when they are used for the preparation of photovoltaic devices. Interestingly, the use of PbI 2 and PbCl 2 salts reduce the charge recombination and increases the obtained open circuit potential, especially in the former case. However, the highest photocurrent is obtained when PbBr 2 is used. While for PbI 2 and PbCl 2 salts no traces of the former salt are observed in the obtained MAPbBr 3 layer after 10 minutes of dipping time, the presence of PbBr 2 still been detected when this salt is employed as it has been determined by Xray diffraction.2