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Abstract
The Epitaxy Wrap-Through (EpiWT) solar cell is a new concept that has been developed to apply back contacting to epitaxial crystalline silicon thin-film solar cells, or Wafer-Equivalents. The EpiWT cell consists of thin active layers that are deposited epitaxially onto the front of a low-cost silicon substrate and simultaneously wrapped through an array of via holes to the back side, thus enabling back contacting.
The theoretical concept development phase produced five versions of the EpiWT cell. The two that most closely resemble the better known Emitter Wrap-Through back contact cell concept are the basic EpiWT concept and a similar version with a passivated selective emitter. The third version also has a passivated selective emitter but also introduces a reduced back-side emitter coverage, which aims to prevent associated voltage loss. Finally, there are two versions that implement alternative novel geometries: optimised finger spacing and slit-shaped via holes.
Significant development was required on two of the fabrication processes in order for them to be applicable to producing EpiWT solar cells. The epitaxy-through-holes process was tested extensively to define the process parameters necessary for growing thick and continuous epitaxial layers through the via holes in the substrate. An isolation layer was required for the low back-emitter version of the EpiWT concept; various materials were tested and a screen printable titanium dioxide based diffusion barrier was found to be suitable.
Device fabrication sequences were developed to produce four ‘proof of concept’ batches of EpiWT solar cells (one of each version except for the optimised finger spacing geometry). All structuring was achieved with industrially relevant screen-printing technology. Substantial optimisation was involved in defining the printing processes, wet-chemical processes and contact sintering processes.
The best result of 13.2% efficiency was achieved on the passivated selective emitter version of the cell with a standard back-side emitter fraction. Also, some promising trends were seen in the voltage results of the cells with lower back-side emitter coverage, meaning further improvements are within reach. These achievements are not far behind the best results of other screen-print structured Wafer-Equivalent concepts. Given the early stage of development of the technology, this suggests that the EpiWT cell concept could contribute to improving the performance and lowering the cost of crystalline silicon thin-film photovoltaics in the near future.