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An overview of the materials, processing techniques, and characterisation procedures for flexible solar modules is presented. Flexible modules are lightweight, roll-able, and/or foldable for storage and transport. The design approach selected by the Australian National University incorporates very thin, high-efficiency crystalline silicon solar cells embedded between flexible coversheets, and supported by silicone encapsulant and flexible electrical contacts. The modules can be fabricated using a number of approaches including constructing the circuitry separately to the packaging, or using the packaging as both a protective layer and a base for circuitry.
Silicon films of 20 μm thickness have been grown epitaxially on silicon substrates by liquid-phase epitaxy. Solar cells fabricated on such layers display open-circuit voltages as high as 663 mV (AM1.5, 25 °C), a value which exceeds previous data by a large margin. High open-circuit voltages are a prerequisite for thin-film solar cells with high efficiencies. Our result has applications to both space cells and to low-cost terrestrial cells.
Interdigitated back-contact (IBC) solar cells developed in the past two years have efficiencies in the range 24.4%-25.6% As high as these efficiencies are, there are opportunities to increase them further by improving on the light trapping. Silicon solar cells incorporating double-sided pyramidal texture are capable of superior light trapping than cells with texture on just the front. One of the principle losses of double-sided pyramidal texture is the light that escapes after a second pass through the cell when the facet angles are the same on the front and rear. This contribution investigates how this loss might be reduced by changing the facet angle of the rear pyramids. A textured pyramid rounding is introduced to improve the light trapping. The reduction in surface recombination that rounding the facets introduces is also evaluated. With confocal microscopy, spectrophotometry and ray tracing, the rounding etch time required to yield the best light trapping is investigated. With photoconductance lifetime measurements, the surface recombination is found to continue to decrease as the rounding time increases. The spectrophotometry and ray tracing suggests that the double sided textured samples featuring rounded rear pyramids have superior light trapping to the sample with a planar rear surface. The high-efficiency potential of rounded textured pyramids in silicon solar cells is demonstrated by the fabrication of 24% efficient back-contact silicon solar cells.
Thin solar cells based on low-quality silicon are assessed for a range of possible material parameter values and device structures. Device thickness is freely optimized for maximum efficiency for a range of doping densities and numbers of junctions, le ading to results differing markedly from previous investigations. Modelling of conventional and multilayer structures in this paper indicates little difference in efficiency potential on low-lifetime (<50 ns) crystalline silicon layers. Moderate effici encies (>15%) are possible given adequate light trapping. Conventional structures (single and double junction cells) are superior if excellent light trapping is assumed. Thicker multilayer structures are advantageous in the case of poor light trapping or surface passivation. In an optimized cell in low-quality silicon, increasing the number of junctions allows a high current to be maintained, but at the cost of a reduced voltage and fill factor caused by increased junction recombination. Formidable pra ctical difficulties are likely to be encountered to realize the theoretical performances discussed.
In this study the influence of implantation damage on emitter recombination is examined for both boron and phosphorus implanted emitters after thermal processing. Dominant defects are identified and used to describe observed changes in emitter saturation current, J0, as a function of implant fluence. Recombination through defects is identified as the cause of increased J0 compared with simulated values. For P-implanted samples J0n+ is shown to depend on annealing temperature for samples partially amorphised by a 1×1015 cm-2 P implantation at 40 keV. J0n+ of samples completely amorphised by a 3×1015 cm-2 phosphorus implant show much reduced dependence on annealing temperature. For boron implanted samples annealed at 1000 °C, J0p+ was found to be below 25 fA.cm-2 for implant fluence less than 5×1014 cm-2 but to increase significantly for high fluence, where defects such as boron-interstitial clusters and dislocation loops are likely to dominate the observed recombination.
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