Metal halide perovskites are of great interest for various high-performance optoelectronic applications. The ability to tune the perovskite bandgap continuously by modifying the chemical composition opens up applications for perovskites as coloured emitters, in building-integrated photovoltaics, and as components of tandem photovoltaics to increase the power conversion efficiency. Nevertheless, performance is limited by non-radiative losses, with luminescence yields in state-of-the-art perovskite solar cells still far from 100 per cent under standard solar illumination conditions. Furthermore, in mixed halide perovskite systems designed for continuous bandgap tunability (bandgaps of approximately 1.7 to 1.9 electronvolts), photoinduced ion segregation leads to bandgap instabilities. Here we demonstrate substantial mitigation of both non-radiative losses and photoinduced ion migration in perovskite films and interfaces by decorating the surfaces and grain boundaries with passivating potassium halide layers. We demonstrate external photoluminescence quantum yields of 66 per cent, which translate to internal yields that exceed 95 per cent. The high luminescence yields are achieved while maintaining high mobilities of more than 40 square centimetres per volt per second, providing the elusive combination of both high luminescence and excellent charge transport. When interfaced with electrodes in a solar cell device stack, the external luminescence yield-a quantity that must be maximized to obtain high efficiency-remains as high as 15 per cent, indicating very clean interfaces. We also demonstrate the inhibition of transient photoinduced ion-migration processes across a wide range of mixed halide perovskite bandgaps in materials that exhibit bandgap instabilities when unpassivated. We validate these results in fully operating solar cells. Our work represents an important advance in the construction of tunable metal halide perovskite films and interfaces that can approach the efficiency limits in tandem solar cells, coloured-light-emitting diodes and other optoelectronic applications.

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We investigate the origin of the broadband visible emission in layered hybrid lead-halide perovskites and its connection with structural and photophysical properties. We study ⟨001⟩ oriented thin films of hexylammonium (HA) lead iodide, (CHN)PbI, and dodecylammonium (DA) lead iodide, (CHN)PbI, by combining first-principles simulations with time-resolved photoluminescence, steady-state absorption and X-ray diffraction measurements on cooling from 300 to 4 K. Ultrafast transient absorption and photoluminescence measurements are used to track the formation and recombination of emissive states. In addition to the excitonic photoluminescence near the absorption edge, we find a red-shifted, broadband (full-width at half-maximum of about 0.4 eV), emission band below 200 K, similar to emission from ⟨110⟩ oriented bromide 2D perovskites at room temperature. The lifetime of this sub-band-gap emission exceeds that of the excitonic transition by orders of magnitude. We use X-ray diffraction measurements to study the changes in crystal lattice with temperature. We report changes in the octahedral tilt and lattice spacing in both materials, together with a phase change around 200 K in DAPbI. DFT simulations of the HAPbI crystal structure indicate that the low-energy emission is due to interstitial iodide and related Frenkel defects. Our results demonstrate that white-light emission is not limited to ⟨110⟩ oriented bromide 2D perovskites but a general property of this class of system, and highlight the importance of defect control for the formation of low-energy emissive sites, which can provide a pathway to design tailored white-light emitters.

Mixed lead–tin halide perovskites have sufficiently low bandgaps (∼1.2 eV) to be promising absorbers for perovskite–perovskite tandem solar cells. Previous reports on lead–tin perovskites have typically shown poor optoelectronic properties compared to neat lead counterparts: short photoluminescence lifetimes (<100 ns) and low photoluminescence quantum efficiencies (<1%). Here, we obtain films with carrier lifetimes exceeding 1 μs and, through addition of small quantities of zinc iodide to the precursor solutions, photoluminescence quantum efficiencies under solar illumination intensities of 2.5%. The zinc additives also substantially enhance the film stability in air, and we use cross-sectional chemical mapping to show that this enhanced stability is because of a reduction in tin-rich clusters. By fabricating field-effect transistors, we observe that the introduction of zinc results in controlled p-doping. Finally, we show that zinc additives also enhance power conversion efficiencies and the stability of solar cells. Our results demonstrate substantially improved low-bandgap perovskites for solar cells and versatile electronic applications.

There is a variety of possible ways to tune the optical properties of 2D perovskites, though the mutual dependence between different tuning parameters hinders our fundamental understanding of their properties. In this work we attempt to address this issue for (C n H 2n+1 NH 3 ) 2 PbI 4 (with n=4,6,8,10,12) using optical spectroscopy in high magnetic fields up to 67 T. Our experimental results, supported by DFT calculations, clearly demonstrate that the exciton reduced mass increases by around 30% in the low temperature phase. This is reflected by a 2-3 fold decrease of the diamagnetic coefficient. Our studies show that the effective mass, which is an essential parameter for optoelectronic device op-eration, can be tuned by the variation of organic spacers and/or moderate cooling achievable using Peltier coolers. Moreover, we show that the complex absorption features visible in absorption/transmission spectra track each other in magnetic field providing strong evidence for the phonon related nature of the observed side bands.The inherent sensitivity of lead-halide perovkites to ambient conditions 1 is the Achilles heel which currently prevents the deployment of their superior properties 2-8 in real world applications. 9 The last few years have witnessed rapid development of numerous perovskite derivatives, [10][11][12][13][14][15][16][17] in an attempt to overcome the environmental stability issue. For example, 2D Ruddlesden-Popper perovskites have already demonstrated power conversion efficiency greater than 10%, with significantly improved stability. 15,18-20 Ruddlesden-Popper 1 arXiv:1909.06061v1 [cond-mat.mes-hall] 13 Sep 2019 halide perovskites are natural type I quantum wells formed by thin layers of halide perovskite separated by organic spacers layers, which act as barriers. 14,21,22 They are described by general formula A' 2 A m−1 M m X m+1 where A' is a monovalent organic cation acting as a spacer, A is a small monovalent cation (MA, FA, Cs), and M a divalent cation that can be Pb 2+ , Sn 2+ , Ge 2+ , Cu 2+ , Cd 2+ , etc., X an anion (Cl − , Br − , I − ) and m=1,2,3... is the number of octahedra layers in the perovskite slab. The hydrophobic nature of the organic spacers significantly increases the stability of these compounds promising an excellent long term performance in photovoltaic and light-emitting applications. 15,[18][19][20]23,24 A unique feature of 2D perovskites is that their properties can be tuned in far more ways than in the case of 3D perovskite semiconductors. The band gap can be tailored by varying chemical composition of the inorganic part 25,26 or by changing the thickness of the octahedral slabs, which significantly impacts the optoelectronic properties. 19,27,28 In addition, many of the 2D perovskite properties can be tuned by an appropriate choice of the building blocks with a plethora of different organic spacers to choose from. 15,20 Varying the organic spacer modifies the dielectric environment of the inorganic slab, affecting the exciton binding energy and the band gap. 14,28,...