Dr Alexandra Ramadan [she/her]

Abstract:

Metal halide perovskites have demonstrated extraordinary potential as materials for next-generation optoelectronics including photovoltaics and light-emitting diodes. Nevertheless, our understanding of this material is still far from complete. One remaining puzzle is the phenomenon of perovskite “photo-brightening”: the increase in photoluminescence during exposure to light in an ambient atmosphere. Here, we propose a comprehensive mechanism for the reactivity of the archetypal perovskite, MAPbI3, in ambient conditions. We establish the formation of lead-oxygen bonds by hydrogen peroxide as the key factor leading to perovskite photo-brightening. We demonstrate that hydrogen peroxide can be applied directly as an effective “post-treatment” to emulate the process and substantially improve photoluminescence quantum efficiencies. Finally, we show that the treatment can be incorporated into photovoltaic devices to give a 50 mV increase in open-circuit voltage, delivering high 19.2% steady-state power conversion efficiencies for inverted perovskite solar cells of the mixed halide, mixed cation perovskite FA0.83Cs0.17Pb(I0.9Br0.1)3.

Abstract:

Perovskite solar cells have achieved the highest power conversion efficiencies on metal oxide n‐type layers, including SnO2 and TiO2. Despite ZnO having superior optoelectronic properties to these metal oxides, such as improved transmittance, higher conductivity, and closer conduction band alignment to methylammonium (MA)PbI3, ZnO is largely overlooked due to a chemical instability when in contact with metal halide perovskites, which leads to rapid decomposition of the perovskite. While surface passivation techniques have somewhat mitigated this instability, investigations as to whether all metal halide perovskites exhibit this instability with ZnO are yet to be undertaken. Experimental methods to elucidate the degradation mechanisms at ZnO–MAPbI3 interfaces are developed. By substituting MA with formamidinium (FA) and cesium (Cs), the stability of the perovskite–ZnO interface is greatly enhanced and it is found that stability compares favorably with SnO2‐based devices after high‐intensity UV irradiation and 85 °C thermal stressing. For devices comprising FA‐ and Cs‐based metal halide perovskite absorber layers on ZnO, a 21.1% scanned power conversion efficiency and 18% steady‐state power output are achieved. This work demonstrates that ZnO appears to be as feasible an n‐type charge extraction layer as SnO2, with many foreseeable advantages, provided that MA cations are avoided.

Abstract:

Raw data associated with paper. AFM images can be opened using the open access software gwyddion. Files concerned with data fitting require origin graphing software to open.

Abstract:

Intrinsic organic-inorganic metal halide perovskites (OIHP) based semiconductors have shown wide applications in optoelectronic devices. There have been several attempts to incorporate heterovalent metal (e.g., Bi3+) ions in the perovskites in an attempt to induce electronic doping and increase the charge carrier density in the semiconductor. It has been reported that inclusion of Bi3+ decreases the band gap of the material considerably. However, contrary to the earlier conclusions, despite a clear change in the appearance of the crystal as observed by eye, here we show that the band gap of MAPbBr3 crystals does not change due the presence of Bi3+ in the growth solution. An increased density of states in the band gap and use of very thick samples for transmission measurements, erroneously give the impression of a band gap shift. These sub band gap states also act as nonradiative recombination centers in the crystals.

Abstract:

© 2018 American Chemical Society. The local structure of the nonplanar phthalocyanine, vanadyl phthalocyanine (VOPc), adsorbed on Cu(111) at a coverage of approximately one-half of a saturated molecular layer, has been investigated by a combination of normal-incidence X-ray standing waves (NIXSW), scanned-energy mode photoelectron diffraction (PhD), and density-functional theory (DFT), complemented by scanning tunnelling microscopy (STM). Qualitative assessment of the NIXSW data clearly shows that both "up" and "down" orientations of the molecule (with V=O pointing out of, and into, the surface) must coexist on the surface. O 1s PhD proves to be inconclusive regarding the molecular orientation. DFT calculations, using two different dispersion correction schemes, show good quantitative agreement with the NIXSW structural results for equal co-occupation of the two different molecular orientations and clearly favor the many body dispersion (MBD) method to deal with long-range dispersion forces. The calculated relative adsorption energies of the differently oriented molecules at the lowest coverage show a strong preference for the "up" orientation, but at higher local coverages, this energetic difference decreases, and mixed orientation phases are almost energetically equivalent to pure "up"-oriented phases. DFT-based Tersoff-Hamann simulations of STM topographs for the two orientations cast some light on the extent to which such images provide a reliable guide to molecular orientation.