Prof Henry Snaith FRS

Abstract:

We have accurately determined the exciton binding energy and reduced mass of single crystals of methylammonium lead triiodide using magneto-reflectivity at very high magnetic fields. The single crystal has excellent optical properties with a narrow line width of ∼3 meV for the excitonic transitions and a 2s transition that is clearly visible even at zero magnetic field. The exciton binding energy of 16 ± 2 meV in the low-temperature orthorhombic phase is almost identical to the value found in polycrystalline samples, crucially ruling out any possibility that the exciton binding energy depends on the grain size. In the room-temperature tetragonal phase, an upper limit for the exciton binding energy of 12 ± 4 meV is estimated from the evolution of 1s-2s splitting at high magnetic field.

Abstract:

Metal halide perovskite thin films can be crystallized via a broad range of solution-based routes. However, the quality of the final films is strongly dependent upon small changes in solution composition and processing parameters. Here, this study demonstrates that a fractional substitution of PbCl2 with PbI2 in the 3CH3 NH3 I:PbCl2 mixed-halide starting solution has a profound influence upon the ensuing thin-film crystallization. The presence of PbI2 in the precursor induces a uniform distribution of regular quadrilateral-shaped CH3 NH3 PbI3 perovskite crystals in as-cast films, which subsequently grow to form pinhole-free perovskite films with highly crystalline domains. With this new formulation of 3CH3 NH3 I:0.98PbCl2 :0.02PbI2 , this study achieves a 19.1% current-voltage measured power conversion efficiency and a 17.2% stabilized power output in regular planar heterojunction solar cells.

Abstract:

Metal halide perovskites(1) are the subject of intensive research efforts due to the impressive performance achieved in photovoltaic and optoelectronic devices.(2, 3) The attraction toward these materials, hereafter simply perovskites, arises for a multitude of reasons. First, they show optimal primary optoelectronic properties, such as direct band gaps, long carrier diffusion lengths, and low exciton binding energies, resulting in the remarkable power conversion efficiency, over 22%, that these materials already deliver in optimized photovoltaic devices. These properites are accompanied by ease of processing via solution or vapor phase (or a combination of the two) techniques, low cost and abundance of base materials, low temperature of processing leading to versatility in terms of what substrates can be used, and the ability to process multiple layers on top of each other.