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Abstract
This thesis is concerned with the development of metallisation and interconnection schemes for e-beam evaporated polycrystalline silicon thin-film solar cells on glass substrates. Of particular interest is a method for series connection of individual cells to primarily minimise light leakage to the glass substrate and thus to reduce associated current loss. In this thesis, two new metallisation schemes have been developed and investigated.
The first scheme is called the “aligned bifacial interconnected metallisation”. The diode is divided into individual cells by laser scribing an isolation groove, which is then filled with photoresist to act as an isolating dielectric. Aluminium is deposited over the whole device and patterned to form interconnected bifacial mini-modules. The open-circuit voltage of the mini-module was the sum of individual cells' voltages, indicating a successful interconnection. The mini-module short-circuit was slightly higher than the individual cell current due to lower light leakage in the glass substrate when a diffuse back surface reflector is used for light trapping. Current gain of about 2.6 % of a mini-module was achieved through the interconnection of individual cells.
The second scheme investigated is the "self-aligned bifacial interconnected metallisation". In this scheme the emitter grooves are etched down to the glass substrates rather than to the emitter layer as in aligned bifacial scheme. Emitter electrodes are formed by filling the grooves with evaporated aluminium, which does not need alignment. The resulting emitter electrodes were wider, which was expected to reduce series resistance. However, the series resistance of both the individual cells and interconnected mini-modules were found to be higher due to reduced area of contact between the emitter electrodes and the emitter sidewalls. Devices metallised using this self-aligned scheme also recorded low shunt resistances due to Schottky contacts between emitter electrodes and the cell absorber.
E-beam evaporated thin-film cells were successfully interconnected using the aligned bifacial scheme. Additional short-circuit current of the mini-module was reported, due to less current loss from light leakage through the glass substrate. Cells metallised with the self-aligned scheme suffered from series and shunt resistance.