Alex Green

3 m InAs resonant-cavity-enhanced photodetector

This paper presents the design, fabrication and optoelectronic characterization of an InAs resonant-cavity-enhanced photodetector, intended for the detection of methane gas, incorporating a 10 period GaAs/AlAs distributed-Bragg reflector. The spectrally narrowed responsivity was resonantly enhanced to a value of 34.7 A W−1, at an applied voltage of 3.0 V. This high resonantly enhanced peak in the responsivity was at λ = 3.14 µm, which is only 5% away from the target value.

Analytical method for the optical modelling of resonant-cavity-enhanced photodiodes

We have obtained Airy formulae for the reflectance, transmittance and absorptance of an asymmetric Fabry–Perot cavity, and applied them to the modelling of a variety of resonant-cavity-enhanced photodiode structures. The resulting fully analytical method accurately matches results from a rigorous numerical method. The correspondence, of absorptance calculated at resonance, between the numerical method and the analytical method was exact for thin absorbing-layer structures and the root-mean-square difference was <5% for thick absorbing-layer structures. The analytical method makes use of a new simple analytical approximation for the reflectance spectrum of a distributed-Bragg reflector (DBR), which is based on a single Gaussian function. This approximation is suitable for modelling any DBR system as long as the refractive index contrast between the DBR constituent layers is <~0.5, and the number of periods is low (<~16).

Transient absorption and photovoltage characterization of dye-sensitized solar cells

Interfacial electron transfer dynamics of dye sensitised metal oxide films have been widely studied by transient optical techniques. In this paper, we extend such studies to complete dye sensitised solar cells, and show how such transient optical studies can be correlated both with transient photovoltage studies and current / voltage analyses of device photovoltaic performance.

Kinetic redundancy in dye-sensitized solar cells: the use of high-bandgap metal oxide barrier layers

Control of charge interfacial charge transfer is central to the design of photovoltaic devices. A an elegant approach to control those dynamics, is the use of an insulating metal oxide blocking layer at a nanocrystalline inorganic / organic semiconductor interface. We show that the conformal growth of a ~1 nm thick overlayer of MgO on a preformed nanocrystalline SnO2 film results in a ~4-fold retardation in the rate of charge recombination at such an interface This observation shows a good correlation with the current/voltage characteristics of dye sensitised nanocrystalline solar cells fabricated from such films, with the MgO coating resulting in ~ 50% improvement in overall device efficiency.

Efficient hybrid polymer/TiO2 solar cells using a multilayer structure

This study focuses on systems consisting of high hole-mobility MEHPPV based polymers or a fluorene-bithiophene co-polymer in contact with different nanocrystalline TiO2 films. We use photoluminescence quenching, time of flight mobility measurements and optical spectroscopy to characterize the exciton transport, charge transport and light harvesting properties, respectively, of the polymers, and correlate these material properties with photovoltaic device performance. We find that the polymer properties with greatest influence on device efficiency are the polymer exciton diffusion length and absorption range, followed by the hole mobility. We have also studied the photovoltaic performance of these TiO2/polymer devices as a function of active layer thickness. Device performances are significantly improved by introducing a PEDOT layer between the polymer and the top Au electrode and by reducing the thickness of the active layers. The optimized devices have peak external quantum efficiencies ≈ 40 % at the polymers maximum absorption wavelength and yield short circuit current densities ≥ 2 mA cm-2 for air mass (AM) 1.5 conditions (100 mW cm-2, 1 sun). The AM 1.5 open circuit voltage reaches 0.64 V and the fill factor 0.43, resulting in an overall power conversion efficiency of 0.58 %.