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      Efficient luminescent solar cells based on tailored mixed-cation perovskites.

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          Abstract

          We report on a new metal halide perovskite photovoltaic cell that exhibits both very high solar-to-electric power-conversion efficiency and intense electroluminescence. We produce the perovskite films in a single step from a solution containing a mixture of FAI, PbI2, MABr, and PbBr2 (where FA stands for formamidinium cations and MA stands for methylammonium cations). Using mesoporous TiO2 and Spiro-OMeTAD as electron- and hole-specific contacts, respectively, we fabricate perovskite solar cells that achieve a maximum power-conversion efficiency of 20.8% for a PbI2/FAI molar ratio of 1.05 in the precursor solution. Rietveld analysis of x-ray diffraction data reveals that the excess PbI2 content incorporated into such a film is about 3 weight percent. Time-resolved photoluminescence decay measurements show that the small excess of PbI2 suppresses nonradiative charge carrier recombination. This in turn augments the external electroluminescence quantum efficiency to values of about 0.5%, a record for perovskite photovoltaics approaching that of the best silicon solar cells. Correspondingly, the open-circuit photovoltage reaches 1.18 V under AM 1.5 sunlight.

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          Anomalous Hysteresis in Perovskite Solar Cells.

          Perovskite solar cells have rapidly risen to the forefront of emerging photovoltaic technologies, exhibiting rapidly rising efficiencies. This is likely to continue to rise, but in the development of these solar cells there are unusual characteristics that have arisen, specifically an anomalous hysteresis in the current-voltage curves. We identify this phenomenon and show some examples of factors that make the hysteresis more or less extreme. We also demonstrate stabilized power output under working conditions and suggest that this is a useful parameter to present, alongside the current-voltage scan derived power conversion efficiency. We hypothesize three possible origins of the effect and discuss its implications on device efficiency and future research directions. Understanding and resolving the hysteresis is essential for further progress and is likely to lead to a further step improvement in performance.
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            On the origin of the open-circuit voltage of polymer-fullerene solar cells.

            The increasing amount of research on solution-processable, organic donor-acceptor bulk heterojunction photovoltaic systems, based on blends of conjugated polymers and fullerenes has resulted in devices with an overall power-conversion efficiency of 6%. For the best devices, absorbed photon-to-electron quantum efficiencies approaching 100% have been shown. Besides the produced current, the overall efficiency depends critically on the generated photovoltage. Therefore, understanding and optimization of the open-circuit voltage (Voc) of organic solar cells is of high importance. Here, we demonstrate that charge-transfer absorption and emission are shown to be related to each other and Voc in accordance with the assumptions of the detailed balance and quasi-equilibrium theory. We underline the importance of the weak ground-state interaction between the polymer and the fullerene and we confirm that Voc is determined by the formation of these states. Our work further suggests alternative pathways to improve Voc of donor-acceptor devices.
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              Small Photocarrier Effective Masses Featuring Ambipolar Transport in Methylammonium Lead Iodide Perovskite: A Density Functional Analysis.

              Methylammonium lead iodide perovskite (CH3NH3PbI3) plays an important role in light absorption and carrier transport in efficient organic-inorganic perovskite solar cells. In this Letter, we report the first theoretical estimation of effective masses of photocarriers in CH3NH3PbI3. Effective masses of photogenerated electrons and holes were estimated to be me* = 0.23m0 and mh* = 0.29m0, respectively, including spin-orbit coupling effects. This result is consistent with the long-range ambipolar transport property and with the larger diffusion constant for electrons compared with that for holes in the perovskite, which enable efficient photovoltaic conversion.
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                Author and article information

                Journal
                Sci Adv
                Science advances
                American Association for the Advancement of Science (AAAS)
                2375-2548
                2375-2548
                Jan 2016
                : 2
                : 1
                Affiliations
                [1 ] Laboratory of Photomolecular Science, Ecole Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland.
                [2 ] Laboratory of Photonics and Interfaces, Ecole Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland.
                [3 ] Laboratory of Photonics and Interfaces, Ecole Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland.; Group for Molecular Engineering of Functional Materials, Ecole Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland.
                [4 ] Group for Molecular Engineering of Functional Materials, Ecole Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland.
                [5 ] Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland.
                Article
                1501170
                10.1126/sciadv.1501170
                4705040
                26767196
                695dc4b7-1af5-4d35-950d-ebfb06d3a66e
                History

                Applied physics,perovskites,photovoltaics,solar cells
                Applied physics, perovskites, photovoltaics, solar cells

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