Among the available photovoltaic materials for fabrication of solar cells, Si and GaAs have been the basis for solar cells with relatively high energy conversion efficiency. However, their efficiencies are constrained by the Shockley-Queisser limit for single-junction solar cells, thus tempering the global deployment of solar photovoltaic technology as an alternative renewable energy source. Therefore, the development of higher efficiency devices with low material and fabrication costs for 1-sun applications is highly desirable.
Recently, monolithic multijunction devices with up to 6-junctions have been developed. Unfortunately, high manufacturing costs have hindered widespread deployment of these kind of devices. This has led to the development of heterogeneous integration methods for relaxing the requirement of lattice matching in multijunction devices so that alternate materials, including Si, can be incorporated.
In a paper recently published in the journal Optics Express, Professor Rafael Kleiman and his student Ronan Garrison from McMaster University introduced a new composite-cell current matching concept. They partially or fully replaced the tandem solar cells with a composite stack composed of more than one cell of the same bandgap. Each composite stack was internally current matched and interconnected using tunnel junctions. The overall device perforamacne was explored taking into consideration the number of cells comprising both the top and bottom stack. Additionally, they examined new bandgap combinations capable of giving high efficiencies. While the authors focused on exploring new opportunities associated with the new set of configurations for double junction devices, applications to triple junction design were also discussed. Their work clearly establishes the relationship between 2- and 3-terminal operation in tandem cells and the benefits of composite-cell current matching on improving the efficiency over wide range of bandgaps.
The feasibility of their concept was presented by evaluating the silicon-based multijunction device performance using 3-terminal efficiency as reference. The silicon materials were used to construct the base cells of the tandem device. Considering the high efficiency of a multijunction design with fully optimized bandgaps of individual cells, the authors evaluated the new choice of bandgaps for top cells that become available with the composite-cell current matching approach by using only two cell types. This added flexibility could lead to new low-cost and high-efficiency multijunction devices. For example, the 2.19 eV/Si (3) composite-cell current matching structure constructed by integrating the high bandgap cell with repeated Si cells was considered. A maximum theoretical efficiency of 42.9% that was only a bit less than the 45.1% of an ideal 2-junction tandem cell structure was obtained. Therefore, silicon-based tandem cells with the composite-cell current matching approach are a promising solution for high efficiency in large-scale 1-sun applications.
In summary, composite cell current matching is an interesting concept based on composite cell stacks that provides a new degree of freedom in multijunction device design. The authors’ analysis has shown how the new current matching constraint enabled by the introduction of composite stacks greatly increases efficiency for some bandgap pairs, unleashing the contribution of composite-cell current matching devices in enhancing the performance of 2-terminal devices in terms of efficiencies over a wide range of bandgaps. Altogether, the study will advance multijunction design to realize high efficiency at low cost.
Garrison, R., & Kleiman, R. (2019). Higher efficiency tandem solar cells through composite-cell current matching. Optics Express, 27(8), A543.Go To Optics Express