Effects of light soaking on the performance and stability of a-Si:H thin-film solar cells

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Thin-film silicon photovoltaic (PV) technology has an enormous potential in the solar power market due to its light weight, flexibility and ease of integration. This enables it to have a wide range of applications like roof top and building integrated installations making it more sustainable. Some of the other key benefits of thin-film silicon PV technology include energy efficient production, less material usage, low temperature coefficient and low manufacturing cost. The relatively low conversion efficiency, however, is the major drawback of thin film silicon solar cells. Hydrogenated amorphous silicon (a-Si:H) grown by plasma-enhanced chemical vapor deposition (PECVD) is one of the extensively employed light absorbers in thin-film silicon solar cells. However, light-induced degradation (LID) of a-Si:H is one of the biggest limiting factors of this technology. LID is a metastable defect creation phenomenon that decreases the conversion efficiency of the a-Si:H solar cell after prolonged exposure to light. This is known as the Staebler-Wronski effect (SWE). SWE has been a subject of research over the past 4 decades. Despite tremendous efforts, the complete suppression of the LID has not been demonstrated yet. Nevertheless, there has been a great progress in characterisation of the defect creation and also the optimisation of solar cells to minimise the effect of LID.
In this thesis, LID of three single junction a-Si:H thin film solar cells with different qualities of a-Si:H absorber layer namely high bandgap a-Si:H, low bandgap a-Si:H and high/low bandgap a-Si:H are studied. Light soaking experiments were carried out for several hundred hours and the performance of the solar cells was analysed through external quantum efficiency (EQE) and illuminated JV characterisations at regular time intervals. Some of the key findings of the experiments include severe LID of high bandgap a-Si:H absorber layer due to increased defect creation, enhanced recombination at the p/i interface in all of the devices and also a significant relationship between the performance and light soaking temperature. Furthermore, the thermal annealing study revealed that, maximum recovery of the performance parameters can be observed at an annealing temperature of 180 ◦C.
Based on the findings of the above mentioned study, additional experiments were carried out to investigate the stability of the devices with different materials as a buffer layer at the p/i interface. The results of the experiments revealed the significance of the buffer layer in the performance of the devices and also its influence on the LID. In addition, the analysis suggests that intrinsic silicon oxide (i-SiOX), intrinsic hydrogenated amorphous silicon (i-a-Si:H), and p-doped hydrogenated amorphous silicon (p-aSi:H) are some of the potential candidates for a buffer layer in an a-Si:H solar cell that could give an improved stabilised performance. Furthermore, the speculation of light induced boron diffusion into
the a-Si:H bulk through the buffer layer was tested by another light soaking study to investigate effects of window layer (p) doping on LID. The study found no correlation between the window layer doping and the p/i interface degradation indicating the possibility of a different defect creation mechanism responsible for the enhanced recombination at the p/i interface.