Prof Michael Johnston

Abstract:

Hysteresis in the current–voltage characteristics of vapor-deposited perovskite solar cells is shown to originate from an amorphous region of CH3NH3PbI3 at the interface with the device's electron transport layer. Interface engineering is used to produce highly crystalline perovskite material at this interface which results in hysteresis-free evaporated planar heterojunction solar cells.

Abstract:

Air-stable doping of the n-type fullerene layer in an n-i-p planar heterojunction perovskite device is capable of enhancing device efficiency and improving device stability. Employing a (HC(NH2 )2 )0.83 Cs0.17 Pb(I0.6 Br0.4 )3 perovskite as the photoactive layer, glass-glass laminated devices are reported, which sustain 80% of their "post burn-in" efficiency over 3400 h under full sun illumination in ambient conditions.

Abstract:

Lead mixed halide perovskites are highly promising semiconductors for both multi-junction photovoltaic and light emitting applications due to their tunable band gaps, with emission and absorption energies spanning the UV-visible to near IR regions. However, many such perovskites exhibit unwanted halide segregation under photoillumination, the cause of which is still unclear. In our study, we establish crucial links between crystal phase stability, photostability and optoelectronic properties of the mixed-cation lead mixed-halide perovskite CsyFA(1-y)Pb(BrxI(1-x))3. We demonstrate a region for caesium content between 0.10 < y < 0.30 which features high crystalline quality, long chargecarrier lifetimes and high charge-carrier mobilities. Importantly, we show that for such high-quality perovskites, photoinduced halide segregation is strongly suppressed, suggesting that high crystalline quality is a prerequisite for good optoelectronic quality and band gap stability. We propose that regions of short-range crystalline order aid halide segregation, possibly by releasing lattice strain between iodide rich and bromide rich domains. For an optimized caesium content, we explore the orthogonal halide-variation parameter space for Cs0.17FA0.83Pb(BrxI(1-x))3 perovskites. We demonstrate excellent charge-carrier mobilities (11-40 cm2 V^−1 s^−1) and diffusion lengths (0.8 - 4.4 µm) under solar conditions across the full iodide-bromide tuning range. Therefore, the addition of caesium yields a more photostable perovskite system whose absorption onsets can be tuned for bandgap-optimized tandem solar cells.

Abstract:

We have used optical-pump-terahertz-probe spectroscopy (OPTPS) to study a range of novel of semiconductors including III-V nanowires and metal halide perovskites. We show that OPTPs allows key figures of merit to be extracted in a non-contact manner, including charge mobility, surface recombination velocity, and doping density. Furthermore, the technique allows charge recombination dynamics to be followed on a picosecond time-scale. This knowledge is useful in the design of new optoelectronic devices from lasers to solar cells as well as for the development and optimisation of new semiconductors.

Abstract:

Reliable doping in GaAs nanowires is essential for the development of novel optoelectronic devices. Previously, GaAs nanowires have been shown to exhibit extremely short photoconductivity lifetimes of a few picoseconds due to their high surface recombination velocity, which is detrimental for nanowire devices, such as solar cells and nanowire lasers. Here, we show that, by exploiting engineered band-bending via selective doping, this parasitic surface recombination can be reduced. We utilise non-contact time-resolved terahertz spectroscopy to characterise the doping efficiency in n-type and p-type doped GaAs nanowire8 and show high carrier concentrations of the order of 1018 cm-3. The carrier lifetimes were increased by an order of magnitude from 0.13ns for undoped to 3.8ns and 2.5ns for n-doped and p-doped GaAs nanowires respectively; showing that surface recombination is greatly suppressed as a result of shell doping. We also present a novel effect of p-doping in GaAs nanowires: a rapid decay in photoconductivity within 25ps after photoexcitation. This fast decay is attributed to rapid electron trapping at the nanowire surface due to doping related band bending. Thus, we demonstrate the advantages of selective doping for enhancement of desirable transport properties in GaAs nanowires, as well as highlighting terahertz spectroscopy as a reliable technique for characterising doped GaAs nanowires1.