Improvement of GaAsP/SiGe tandem solar cell on silicon by optical characterisation, modelling, analysis, and design

摘要

The GaAsP/SiGe tandem solar cell grown directly on Si has the potential for high efficiency at relatively low cost. However, achieving high efficiency in any photovoltaic device requires careful design based on both optical and electrical material parameters. The III-V alloys selected for lattice- and current- matching in this device have been little studied, with parameters mostly absent from the literature. This work uses optical characterisation, modelling, and analysis to determine these parameters and thereby improve the tandem efficiency. Complex indicies of refraction of previously unstudied III-V alloys are extracted. These enable optical modelling of the device, through which unexpected collection of carriers generated in the window layer is discovered. A double-layer antireflection coating (DARC) is designed and it is demonstrated that co-optimisation of the window layer results in low broadband reflection even in current-matched situations, enabling the use of a simpler structure than commonly considered necessary. The top cell current is increased to >20 mA/cm^2 by experimental application of the DARC. Extracted constants have the potential to improve interpolation of values for additional alloy fractions. The optical constants are also applied to the extraction of previously unmeasured electrical properties through the direct fitting of modelled IQE curves to experimental data on a variety of devices. The results enable the identification of the best materials independent of variations in layer thickness, the determination of collection profiles with varying layer thickness, and the evaluation of design changes to increase top cell current by up to 1 mA/cm^2 and maximise efficiency in tandem structures. New device designs are optimised for performance under concentration. For the first time, the suitability of Si wet etching processes to the presence of III-V materials is considered. Rear surface processes developed on SiGe-on-Si single junction cells are adapted for tandem fabrication. The first outdoor measurements of this device are reported, achieving 17.0% 2T tandem efficiency with fully isolated devices under AM1.5-D. With the material parameters extracted and optimisations performed in this work, the ability of the next generation of devices to exceed 26% efficiency at one sun, and 32% under concentration, is clearly demonstrated.