Multijunction Solar Cells May Exceed 50% Efficiency In The Near Future

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Dear Friends, Visitors/Viewers/Readers,

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Just quickly sharing some of the things I’ve learned from a solar-cells course I’ve been taking recently at Stanford and from www.phys.org.

In the past decade, the best lab examples of traditional silicon solar cells have efficiencies around 25%, while current world record of multi-junction/3-junction solar cells is 43.5% (based on former Professor/Dr. Jim Harris & his group  at Stanford and then Solar Junction ) . Commercial examples of tandem cells are widely available at 30% under one-sun illumination, and improve to around 40% under concentrated sunlight. However, this efficiency is gained at the cost of increased complexity and manufacturing price. To date, their higher price and lower price-to-performance ratio have limited their use to special roles,   such as in aerospace (where their high power-to-weight ratio is desirable).

Sun Seen From Space Station (Credit:STS-129 crew, NASA), available @ Sun Is The Future of http://www.cafepress.com/sunisthefuture

In earthly applications, these multijunction (MJ)  solar cells are used in concentrated photovoltaics (CPV) with operating plants all over the world.

Tandem techniques can also be used to improve the performance of existing cell designs, although there are strict limits in the choice of materials. In particular, the technique can be applied to thin-film solar cells using amorphous silicon to produce a cell with about 10% efficiency that is lightweight and flexible.

At this point, please allow me to share a graph developed by NREL (now available at www.cafepress.com/sunisthefuture), the Best Research-Cell Efficiencies, summarizing the results of various solar cell efficiencies from 1975-2013, below:

Best Research-Cell Efficiencies (by NREL, available at www.cafepress.com/sunisthefuture)

Now, based on the collaboration of researchers from the California Institute of Technology in Pasadena; the National Institute of Standards and Technology in Gaithersburg, Maryland; the University of Maryland in College Park; and ,Boeing-Spectrolab Inc., in Sylmar, California. The team published a paper on their work in a recent issue of Applied Physics Letters.  These scientists have designed a new multijunction solar cell that can achieve an efficiency of 51.8%. This high performance exceeds the current goal of 50% efficiency in multijunction solar cell research as well as the current world record of 43.5% for a 3-junction solar cell. In multijunction solar cells each junction or subcell absorbs and converts sunlight from a specific region of the spectrum. The subcell can be stacked one on top of  another so that sunlight first strikes the highest bandgap subcell (tuned to light with shortest wavelengths or highest frequencies). The longer wavelengths pass through the first subcell and strike the lower band gap subcells. In order to approach its theoretical limit, multijunction solar cells not only need multiple subcells but also optimal semiconductor materials for the subcells to provide a combination of band gaps that cover as much of the solar spectrum as possible. To improve upon the current best 43.5% efficiency level of MJ solar cells, researchers focused on improving the current match between the different subcells, along with using a lattice-matched design. These two factors have previously limited MJ solar cell efficiency. Below is a video clip on new multijunction solar cell during a lecture given by Dr. Vijit Sabnis of Stanford University:

 

The lattice match corresponds to the matching between the crystal unit cells from the different subcells,” lead author Marina Leite, an energy researcher at the National Institute of Standards and Technology, told Phys.org. “By using subcells that are lattice-matched, we can minimize dislocations and other crystal defects that can significantly affect the performance of the device. A current match is required for two-terminal tandem configurations because in this case a single current passes through all the subcells and the voltages are added; therefore, if one subcell has less photocurrent it will limit the current generated by the entire device. The current match is desired so that each individual subcell works at its own maximum power point of operation.”

Solar energy world is definitely dynamic, with constant progress made beating the previous record ! We will look forward to the day when  a new Multijunction Solar Cell will be able to exceed 50% efficiency goal.

~have a bright and sunny day~

Gathered, written, and posted by sunisthefuture-Susan Sun Nunamaker

Any of your comments & suggestions are welcomed at sunisthefuture@gmail.com

Homepage: http://www.sunisthefuture.net

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