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Silicon nanocrystals (Si NCs) embedded in a dielectric matrix is regarded as one of the most promising materials for the third generation photovoltaics, owing to their tunable bandgap that allows fabrication of optimized tandem devices. Previous work has demonstrated fabrication of Si NCs based tandem solar cells by sputter-annealing of thin multi-layers of silicon rich oxide and SiO2. However, these device efficiencies were much lower than expected given that their theoretical values are much higher. Thus, it is necessary to understand the practical conversion efficiency limits for these devices. In this article, practical efficiency limits of Si NC based double junction tandem cells determined by fundamental material properties such as minority carrier, mobility, and lifetime are investigated. The practical conversion efficiency limits for these devices are significantly different from the reported efficiency limits which use Shockley-Queisser assumptions. Results show that the practical efficiency limit of a double junction cell (1.6 eV Si NC top cell and a 25% efficient c-Si PERL cell as the bottom cell) is 32%. Based on these results suggestions for improvement to the performance of Si nanocrystal based tandem solar cells in terms of the different parameters that were simulated are presented. [ABSTRACT FROM AUTHOR]

Measuring the spectral response of photoluminescence (SRPL) in solar cells has recently attracted attention as it can be used as a contactless relative measure of external quantum efficiency (EQE) prior to full device fabrication. However, this technique requires that the monitored PL spectrum originates mainly from a region in the solar cell with uniformly distributed majority carriers. For a stack of thin films with a similar material composition, the slightly different emission spectrum from each layer may lead to the superposition of several luminescence peaks. This letter presents the measurement of the SRPL from GaAsP tandem solar cells and outlines a method for separating the individual layer contributions. Good agreement between measured SRPL and EQE at short wavelengths has been achieved, and the deviations at longer wavelengths have been analyzed. This study also reveals unexpected bandgap narrowing resulting from a variable material composition within the active region. [ABSTRACT FROM AUTHOR]

A GaAsP/SiGe tandem solar cell on Si substrate has been further fabricated using light trapping techniques, such as texturing and adding a back surface reflector, and thinning the Si substrate. This is of importance to increase the Jsc of the Si0.18Ge0.82 bottom cell in this tandem system since bottom cell is current limiting. The Jsc of the bottom cell has been increased by relative 7.4%. This current improvement leads to a predicted efficiency of near 21% for this tandem device, with an absolute efficiency improvement of 0.3% over previous results without light trapping processes. The current of the bottom cell can be further improved by optimizing the bottom cell structure and texturing process, further thinning the Si substrate and increasing Ge concentration. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

We report on the point-contacting by localized dielectric breakdown (PLDB) technique applied to form ohmic contacts at room temperature, to silicon test structures with locally doped surface regions and four dielectrics commonly used for surface passivation in silicon solar cells. Our results show that the statistical distribution of the electric field at breakdown for these samples can be fitted with a Weibull distribution, indicating that a percolation model used for dielectric failure can be used to describe this process. Poole-Frenkel field-assisted hopping conduction was identified as being the dominant conduction mechanism for all of the passivating layers, however, no correlation between leakage current and breakdown field was observed. Despite the lack of a predictive model for the breakdown field, the PLDB technique can be applied to structures with a number of different dielectric layers, delivering a low-resistance ohmic contact without the need for high-temperature processing steps. [ABSTRACT FROM AUTHOR]