3,21,22 The highest efficiency perovskite cells employ mixed-cation lead mixed-halides. 20 Iodide can be replaced with other halides, either entirely or in a mixture. 18,19 There are efforts to replace lead (Pb 2+) with tin (Sn 2+), though this usually comes at the expense of performance and stability. The methylammonium (CH 3NH 3 +) cation has been successfully replaced with formamidinium 16 (NH 2CHNH 2 +) and/or caesium 17 (Cs +) a mixture of these cations can also be used. 14 One of the most commonly used, 15 and indeed longest used, 1 perovskites is methylammonium lead iodide (CH 3NH 3PbI 3), or MAPI. In most perovskite solar cells, the organometal halide perovskite absorber is sandwiched between an electron transporting (hole blocking) layer and a hole transporting (electron blocking) layer. Hysteresis in the current–voltage characteristics of perovskite solar cells was first reported in 2014 8 and confirmed by subsequent studies, 9–12 although hysteresis loops in conductance measurements of bulk perovskite had been reported earlier. 7 measured how the efficiency fell over time for different cell architectures: in the most extreme case the PCE fell from 11.5% to zero. Historically, the credibility of reported PCE values for perovskite solar cells has been undermined by widespread use of a voltage scanning protocol that takes advantage of hysteresis in the current density–voltage ( J– V) characteristics of some cells to obtain high initial PCE values that decrease as the cell is held at the maximum power point (MPP) voltage. 6 Perovskites are the subject of a great deal of research interest due to their potential for easier and lower-cost manufacture than the market-leading silicon-based technologies. Recent reviews of perovskite solar cell technologies are given by Sum et al., 2 Stranks and Snaith, 3 Niu et al., 4 Miyasaka 5 and Park. 1 Introduction Since their invention in 2009, 1 organometal halide perovskite solar cells have reached power conversion efficiencies (PCEs) of over 20%. An activation energy of 0.55 eV is inferred from fitting simulations to measurements made at room temperature. The close fit of the model predictions to the measurements shows that mobile ions in the perovskite layer influence transient behaviour on timescales of up to 50 s. A drift–diffusion model that accounts for slow moving ions as well as electrons and holes acting as charge carriers was used to predict the current transients. These measurements are highly reproducible, in contrast to most other techniques used to investigate perovskite cells. The current decay in response to a sudden change of applied bias up to 1 V has been measured on a methylammonium lead triiodide perovskite solar cell with titania and spiro-OMeTAD transport layers, for temperatures between 258 and 308 K.
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