Feasibility and Costs of II-VI Materials in Multijunction Solar Cells

Solar energy is often hailed as the successor to fossil fuels as the planet’s main source of energy. However, solar cells face various issues affecting widespread adoption including prohibitive costs and low energy conversion rates. Currently, multijunction solar cells are the most efficient. These cells are able to boost conversion rates by employing different junctions of semiconductors that utilize different wavelengths of light. The most commonly used semiconductors belong to the III-V group due to perceived advantages over II-VI group semiconductors. Garland et al (2011) argue that II-VI semiconductors are both more efficient and less expensive than III-V semiconductors. Results from models and initial experimentation indicate that II-VI solar cells are 3–4% more efficient than III-V solar cells.—Alan Hu

Garland, J. W., T. Biegala, M. Carmody, C. Gilmore, and S. Sivananthan. Next-generation Multijunction Solar Cells: The Promise of II-VI Materials. Applied Physics Letters 109, 102423(2011).

Garland and colleagues of EPIR Technologies project III-V and II-VI solar cell output with the commonly accepted “standard” model put forth by Xu et al. (2010) in an earlier paper. The model uses a beta coefficient that takes such factors into account as the dimensions and doping (the molecular makeup) of the semiconductor. The beta is calculated through a best fit line describing modeled data and figures from real world performance of the latest III-V solar cell. Projected efficiency is then computed by dividing the sum of the junction outputs by the power input. The researchers supplement their projected figures with real world figures generated through actual experimentation. Garland and colleagues also grew II-VI semiconductors and collected empirical data on the solar cells.

The results from both projections and empirical observation supported the argument in favor of II-VI solar cells. Calculated efficiency for a III-V solar cell under one sun, a measure of sun intensity, was 43.7% whereas the figure for a II-VI solar cell was 49.7%. Empirical observations agree with these results: III-V solar cells were observed to have achieved 38.6% under one sun whereas II-VI solar cells achieved 44.5% under the same conditions.

The study also argues that II-VI solar cells could bring about significant reductions in manufacturing cost. The authors claim that due to the sturdy nature of silicon wafers used in the production of II-VI solar cells and lower costs of growing II-VI crystals, nearly all associated costs of creating II-VI semiconductors are lower than those of creating III-V semiconductors. Specifically, molecular beam epitaxy (MBE), which is a process for growing crystals, can be replaced by a cheaper production line method of production due to the nature of II-VI materials.

The lower cost of II-VI solar cells means that medium concentration photovoltaics can be used instead of high concentration photovoltaics. III-V solar cells were relatively costly; as such, it was cheaper to have fewer solar cells and instead have a system of mirrors that concentrated sunlight onto a small area of solar cells. This meant the costs of a solar energy field were increased by the installation of such tracking systems. The cheaper II-VI solar cells allow for a relatively larger area of solar cells and less complicated tracking systems.

The search for a commercially viable alternative to fossil fuels continues. As long as solar cells are more expensive and less effective than existing energy sources, the widespread adoption of solar energy is unlikely. However, incremental advances in photovoltaic technology are gradually cheapening the cost of solar energy. The improvement of 3–4% in energy conversion rates brought about by the use of II-VI semiconductors is one small step toward a greener future.

D. Xu, T. Biegala, M. Carmody, J. W. Garland, C. Grein, and S. Sivananthan. Proposed monolithic triple-junction solar cell structures with the potential for ultrahigh efficiencies using II–VI alloys and silicon substrate. Applied Physics Letters. 96, 073508(2010).

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