Design and numerical optimization of Si/TCO heterostructure solar cells

Abstract

After being the industrial leading products for years in the global photovoltaic market, the aluminum back surface field (BSF) solar cells reached their limited conversion efficiency to around 19%, marking the end of an era in 2017. At present, emerging cell designs like PERC and TOPCon technologies are present in the arena, with efficiencies lying between 24 to more than 26% and costs approaching 0.1 $/Wp. The purpose of the study was to design and numerically investigate the performance of a new heterostructure involving Transparent Conducting Oxides (TCOs) beside Silicon material. The expected performance of this design is to achieve a conversion efficiency in-between that of the conventional BSF and PERC structures, with simple and lower cost processes. The numerical software used for the simulation is SCAPS-1D. The early stage of the work was dedicated to validate the conventional monocrystalline Silicon n+pp+ structure numerical parameters, by referring to two different laboratory experimental solar cell data. Once close agreement was successfully obtained with these later results, it has been possible to go further by examining our proposed silicon-based heterostructure. This means carefully selecting compatible TCO thin films for silicon base material. A number of critical parameters was investigated, such as lattice mismatch, energy band alignment at the TCO/Si interfaces and materials resistivity. It was interesting to assess the performance of TCO materials with different band gap types. P-type Cupric Oxide (CuO) with indirect-band gap, and Cuprous oxide (Cu2O) with direct-band gap were chosen as BSF materials at the back side of the cell. Next, to replace the n+ silicon emitter, Zinc Sulfide (ZnS), a well-known n-type TCO used for the front side, was selected. With a wide band-gap, low lattice mismatch and good band-alignment, it is a verifiable highly-doped silicon replacement. Through numerical simulation, conversion efficiencies of 20.31% and 20.35% were recorded when CuO and Cu2O were introduced as BSF instead of silicon. While reaching 20.49% with the ZnS emitter, compared to the classical all silicon n+pp+ devices with 18.73%. With technological advantages as low temperature and simple deposition processes, serving low cost along with high conversion efficiencies that can reach 22%, the ZnS/Si/CuxO heterostructure can be promoted to a high efficiency class device. At this post-simulation stage, the new designed heterostructure can be ready for direct experimental prototyping in a materials laboratory.