Shunt Hunt in Thin Film Silicon Solar Cells

Abstract

Thin film silicon solar cells are a type of photovoltaic technology with the advantage of having thin layers of silicon to generate electricity. Unlike traditional crystalline silicon solar cells, which use thick silicon wafers, thin-film silicon solar cells use amorphous silicon that can be made with much thinner layers of silicon. This allows them to be more lightweight, flexible, and potentially less expensive to manufacture. However, their efficiency has traditionally been lower than crystalline silicon solar cells, which are more
commonly used in large-scale installations. This is primarily due to the material property of amorphous silicon, which is laden with defects and voids. Nonetheless, ongoing research and development aim to improve the efficiency and commercial viability of thin-film silicon solar cells.

HyET Solar B.V. is a company based in the Netherlands which employs a Roll to Roll (R2R) technology to produce such flexible solar cells. A temporary aluminum foil is used as the substrate on which the solar cell stack is deposited. The temporary foil is etched away, and the layers are encapsulated in low-cost polymer foils. This thesis is part of the ongoing FlamingoPV (Flexible Lightweight Advanced Materials In Next Generation of PV) project in collaboration between HyET Solar and TU Delft, to develop single, tandem, and triple junction cells with 12, 13, and 14% efficiencies, and a lifetime longer than 35 years. Part of the ongoing research to improve the performance of thin-film silicon solar cells is to understand the difference between the thin films deposited on a rigid glass substrate and flexible aluminum substrate and to investigate why the performance is lower on the aluminum substrate. In
particular, special emphasis is given to the origin of shunts in thin-film silicon solar cells, and conducting a top-down root cause analysis to investigate the origin of these shunts. Mitigation strategies are suggested to improve the solar cells fabricated on foil. Apart from this, the solar cell layers are optimized for improved electrical and optical performance on the glass substrate, to ultimately implement it on the aluminum substrate.

The key takeaways from this research are that aluminum foil was found to be a major culprit for the origin of the shunts. The foil consists of alloying elements of iron and copper, which get exposed to the surface when the foil is cleaned. When the subsequent layers are grown on the foil, the alloying elements were observed to short-circuit the device, thus causing leakage currents. Another major culprit was the formation and accumulation of silicon dust on the samples during the PECVD deposition, which was more prevalent on devices fabricated on aluminum foil. The key takeaways from the optimization experiments are that by the band-gap profiling of i-layer at a lower thickness (230nm) than the standard (300nm), we could maintain the initial electrical and optical properties of the devices. This gives us room to reduce material usage and costs at the same output performance. A permanent degradation was observed in the metal contacts of these devices, apart from the temporary light-induced degradation commonly seen in a-Si:H based solar cells. The tests conducted to improve the electrical performance did give the desired results, with an increase in the efficiency of the devices. The tests conducted to improve the optical performance did not give the desired results, with a decrease in the electrical performance and no significant increase in the optical response of the devices.