Snaith group

Abstract:

Organolead trihalide perovskites are shown to exhibit the best of both worlds: charge-carrier mobilities around 10 cm2 V-1 s -1 and low bi-molecular charge-recombination constants. The ratio of the two is found to defy the Langevin limit of kinetic charge capture by over four orders of magnitude. This mechanism causes long (micrometer) charge-pair diffusion lengths crucial for flat-heterojunction photovoltaics. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

Perovskite-based solar cells have attracted significant recent interest, with power conversion efficiencies in excess of 15% already superceding a number of established thin-film solar cell technologies. Most work has focused on a methylammonium lead trihalide perovskites, with a bandgaps of ∼1.55 eV and greater. Here, we explore the effect of replacing the methylammonium cation in this perovskite, and show that with the slightly larger formamidinium cation, we can synthesise formamidinium lead trihalide perovskites with a bandgap tunable between 1.48 and 2.23 eV. We take the 1.48 eV-bandgap perovskite as most suited for single junction solar cells, and demonstrate long-range electron and hole diffusion lengths in this material, making it suitable for planar heterojunction solar cells. We fabricate such devices, and due to the reduced bandgap we achieve high short-circuit currents of >23 mA cm-2, resulting in power conversion efficiencies of up to 14.2%, the highest efficiency yet for solution processed planar heterojunction perovskite solar cells. Formamidinium lead triiodide is hence promising as a new candidate for this class of solar cell. © 2014 The Royal Society of Chemistry.

Abstract:

The study of the photophysical properties of organic-metallic lead halide perovskites, which demonstrate excellent photovoltaic performance in devices with electron- and hole-accepting layers, helps to understand their charge photogeneration and recombination mechanism and unravels their potential for other optoelectronic applications. We report surprisingly high photoluminescence (PL) quantum efficiencies, up to 70%, in these solution-processed crystalline films. We find that photoexcitation in the pristine CH3NH 3PbI3-xClx perovskite results in free charge carrier formation within 1 ps and that these free charge carriers undergo bimolecular recombination on time scales of 10s to 100s of ns. To exemplify the high luminescence yield of the CH3NH3PbI 3-xClx perovskite, we construct and demonstrate the operation of an optically pumped vertical cavity laser comprising a layer of perovskite between a dielectric mirror and evaporated gold top mirrors. These long carrier lifetimes together with exceptionally high luminescence yield are unprecedented in such simply prepared inorganic semiconductors, and we note that these properties are ideally suited for photovoltaic diode operation. © 2014 American Chemical Society.

Abstract:

The organic-inorganic hybrid perovskites methylammonium lead iodide (CH3NH3PbI3) and the partially chlorine-substituted mixed halide CH3NH3PbI3-xClx emit strong and broad photoluminescence (PL) around their band gap energy of ∼1.6 eV. However, the nature of the radiative decay channels behind the observed emission and, in particular, the spectral broadening mechanisms are still unclear. Here we investigate these processes for high-quality vapor-deposited films of CH3NH3PbI3-xClx using time- and excitation-energy dependent photoluminescence spectroscopy. We show that the PL spectrum is homogenously broadened with a line width of 103 meV most likely as a consequence of phonon coupling effects. Further analysis reveals that defects or trap states play a minor role in radiative decay channels. In terms of possible lasing applications, the emission spectrum of the perovskite is sufficiently broad to have potential for amplification of light pulses below 100 fs pulse duration.