Development of a fabrication process for parallel multijunction thin film silicon solar cells on wafer substrates

The parallel multijunction (PMJ) cell design theoretically enables high efficiency thin film polysilicon solar cells at lower cost. Since its initial proposal in 1994 the PMJ cell has been the subject of a number of theoretical studies, however, no detailed experimental investigation has yet been reported. Any systematic study of the PMJ solar cell will require suitably designed and fabricated devices to serve as experimental test-beds. This paper reports the successful development of a fabrication sequence for PMJ cells in CVD-epilayers on inert single-crystal silicon substrates, producing cells with efficiencies up to 13%. The processing sequence is based on photolithography, anisotropic wet etching, high temperature furnace steps and evaporated metallisation. Full details of the processing sequence are provided, with explanations of particular process choices, including the method of parallel electrical connection of like-polarity layers, the use of a thick photoresist (Shipley SJR5740), avoiding pitfalls, and procedures to minimise cell shunt behaviour. The establishment of this baseline fabrication sequence for PMJ cells opens up a wealth of opportunities for systematic studies of cell performance limiting mechanisms, such as junction recombination, and the implications of various cell design and processing options, particularly those likely to be of more commercial relevance, such as laser scribing, laser doping, rapid thermal processing and electroless metal plating.