A comprehensive study of the highly lattice mismatched alloy GaAsBi for high efficiency photovoltaic applications

Abstract

In this thesis a comprehensive study of the highly lattice mismatched ternary alloy GaAsBi is presented and its suitability for use in future high efficiency photovoltaic cells assessed. A large non-linear bowing of the band gap in GaAsBi allows the alloy to realise a 1.0 eV band gap with as little as 6% Bi and 0.7% compressive strain, which is highly desirable for next generation quad- junction photovoltaic cells. However, despite the promising electronic properties GaAsBi remains in the early stages of development and further understanding of the affects of Bi incorporation on GaAs is required. Minimisation of extrinsic front surface reflection losses can lead to appreciable performance enhancements in both triple and quad junction solar cells. A small enhancement of 0.7 percentage points in power conversion efficiency is observed when applying an optimised triple layer anti- reflection coating to a triple junction solar cell, however a more appreciable improvement of 1.5 percentage points is demonstrated when the triple layer coating is applied to a potential quad junction design. This demonstrates the importance of minimising extrinsic losses when wishing to realise high efficiency photovoltaic cells in excess of 50%. Samples containing bulk GaAsBi up to 3.7% are examined using time integrated and time resolved photoluminescence spectroscopy. Clear evidence of alloy disorder is observed, with a red-blue-red shift, or ’s-shape’, evident in the temperature dependence of the peak luminescence energy, and a strong spectral dependence of the effective carrier lifetime at low temperature. Selective excitation of the sample is employed to investigate an anomalous low energy emission peak centred at 0.978 eV, which is attributed to defect related recombination in the N+ GaAs sub- strate. This initial study highlights the presence of alloy disorder in GaAsBi and the caution that should be employed when evaluating the optical characteristics of novel semiconductor alloys on doped substrates. The nature of disorder induced localised states resulting from fluctuations in the band edge potential is assessed. The magnitude of the s-shape, is demonstrated to decrease with increasing excitation power, indicative of the filling of localised energy states. Examining the luminescence spectrum lineshape under varying excitation power indicates that a Gaussian distribution with standard deviation σGauss = 100 meV and a localisation energy up to 81 meV best describes the profile of the localised distribution of states extending from the valence band edge. Detailed analysis of the thermal quenching of the luminescence under varying excitation power indicates that the distribution of localised energy states is made up of two Gaussian components with standard deviations σ1 = 62 meV and σ2 = 20 meV. Good agreement between the spectrum lineshape and thermal quenching analysis highlights consistency in the Gaussian profile of the localised density of states, which is in contrast with the classical exponential interpretation and other reports in the literature. The affect of inherent alloy disorder on the voltage performance of prospective GaAsBi based solar cells is investigated. It is shown that the open-circuit voltage may degrade by up to 202 meV with Bi fractions up to 5.5%, indicating that GaAsBi may not be as suitable as initially anticipated as a potential 1.0 eV sub-cell material. In light of this analysis GaAsBi is considered as a potential replacement for InGaAs in future upright metamorphic solar cells. Both abrupt absorption and material limited absorption calculations are presented and demonstrate that GaAsBi with as little as 2.8% Bi, incurring only 0.25% compressive strain on the Ge substrate, can offer improved device performance over the existing InGaAs based architecture under single sun AM0 illumination. This demonstrates the potential of GaAsBi for producing the next generation of highly efficient photovoltaic devices and highlights a new pathway to achieving the next milestone goal of 50% efficient solar cells.