Snaith group

Abstract:

We determine high charge-carrier mobilities ≥ 33 cm2 V−1 s−1 and bi-molecular recombination rates about five orders of magnitude below the prediction of Langevin's model for vapour-deposited CH3NH3PbI3−xClx using ultrafast THz spectroscopy. At charge-carrier densities below ∼1017 cm−3 intrinsic diffusion lengths are shown to approach 3 microns, limited by slow mono-molecular decay processes.

Abstract:

Already exhibiting solar to electrical power conversion efficiencies of over 17%, organic-inorganic lead halide perovskite solar cells are one of the most promising emerging contenders in the drive to provide a cheap and clean source of energy. One concern however, is the potential toxicology issue of lead, a key component in the archetypical material. The most likely substitute is tin, which like lead, is also a group 14 metal. While organic-inorganic tin halide perovskites have shown good semiconducting behaviour, the instability of tin in its 2+ oxidation state has thus far proved to be an overwhelming challenge. Here, we report the first completely lead-free, CH3NH 3SnI3 perovskite solar cell processed on a mesoporous TiO2 scaffold, reaching efficiencies of over 6% under 1 sun illumination. Remarkably, we achieve open circuit voltages over 0.88 V from a material which has a 1.23 eV band gap.

Abstract:

Structure control in solution-processed hybrid perovskites is crucial to design and fabricate highly efficient solar cells. Here, we utilize in situ grazing incidence wide-angle X-ray scattering and scanning electron microscopy to investigate the structural evolution and film morphologies of methylammonium lead tri-iodide/chloride (CH3NH3PbI(3-x)Cl(x)) in mesoporous block copolymer derived alumina superstructures during thermal annealing. We show the CH3NH3PbI(3-x)Cl(x) material evolution to be characterized by three distinct structures: a crystalline precursor structure not described previously, a 3D perovskite structure, and a mixture of compounds resulting from degradation. Finally, we demonstrate how understanding the processing parameters provides the foundation needed for optimal perovskite film morphology and coverage, leading to enhanced block copolymer-directed perovskite solar cell performance.

Abstract:

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.

Abstract:

Excitonic solar cells, within which bound electron-hole pairs have a central role in energy harvesting, have represented a hot field of research over the last two decades due to the compelling prospect of low-cost solar energy. However, in such cells, exciton dissociation and charge collection occur with significant losses in energy, essentially due to poor charge screening. Organic-inorganic perovskites show promise for overcoming such limitations. Here, we use optical spectroscopy to estimate the exciton binding energy in the mixed-halide crystal to be in the range of 50 meV. We show that such a value is consistent with almost full ionization of the exciton population under photovoltaic cell operating conditions. However, increasing the total photoexcitation density, excitonic species become dominant, widening the perspective of this material for a host of optoelectronic applications.