Prof Michael Johnston

Abstract:

We exploit the photoconductivity of semiconductor nanowires to achieve ultrafast broad-bandwidth modulation of THz pulses. A modulation depth of -8 dB was exhibited by a polarizer consisting of 14 layers of nanowires encapsulated in polymer.

Abstract:

We exploit the photoconductivity of semiconductor nanowires to achieve ultrafast broad-bandwidth modulation of THz pulses. A modulation depth of -8 dB was exhibited by a polarizer consisting of 14 layers of nanowires encapsulated in polymer.

Abstract:

Semiconductors nanowires have to potential to be building blocks for future nano-optoelectronic devices. We have recently demonstrated high performance THz photonic devices based on GaAs and InP nanowires. These include ultrafast optically switched modulators of THz radiation and single nanowire photoconductive detectors of THz pulses.

Abstract:

Impressive power conversion efficiencies coupled with the relative ease of fabrication have made perovskite solar cells a front runner for next-generation photovoltaics. Although perovskite films and optoelectronic devices have been widely studied, relatively little is known about the chemistry of the precursor solutions. Here, we present a study on the hydrolysis of N,N-dimethylformamide, correlating how pH changes related to its degradation affect the crystallization of MAPbI3xClx perovskite films. By careful manipulation of the pH, and the resulting colloid distribution in precursor solutions, we fabricate perovskite films with greatly improved crystallinity, which when incorporated into photovoltaic devices reproducibly yield efficiencies of over 18%. Extending this method to the mixed cation, mixed halide perovskite FA0.83MA0.17Pb(I0.83Br0.17)3, we obtain power conversion efficiencies of up to 19.9% and open-circuit voltages of 1.21 V for a material with a bandgap of 1.57 eV, achieving the lowest yet reported loss in potential from bandgap to a VOC of only 360 mV.

Abstract:

An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CH3NH3PbI3 are strongly affected by photon re-absorption which masks a much larger intrinsic bimolecular recombination rate constant. By investigating a set of films whose thickness varies between 50nm and 533nm, we find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon re-absorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient, of value 6.8x10(-10)cm(3)s(-1), is common to all samples irrespective of film thickness. Hence we postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples, with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, while thinner films or index-matched surrounding layers can reduce the possibility for photon re-absorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement.