Solar cell efficiency reaches new levels, decades ahead of expectations

Posted on 3 Jun 2016 by Doug Neale
Dr Mark Keevers with one of the spectrum splitting, four-junction mini-modules developed at UNSW to improve solar cell efficiency - image courtesy of UNSW.
Dr Mark Keevers with one of the spectrum splitting, four-junction mini-modules developed at UNSW to improve solar cell efficiency - image courtesy of UNSW.

Australian researchers have broken the record of solar cell efficiency by 44%, nearing levels previously expected to be achieved in 2050.

A team of engineers researching solar cell efficiency, and led by Dr Mark Keevers and Professor Martin Green at The University of New South Wales, have created a new solar cell configuration that can extract more energy from light than previously possible.

Since 1961, the Shockley-Queisser Limit established an absolute theoretical limit on traditional solar cell efficiency. According to the theory, a single-layer of silicon cells — the type of cells most widely used in today’s solar panels — has an upper limit of 32%.

The mini-module was recorded hitting a 34.5% sunlight-to-electricity conversion efficiency, that is the percentage of sunlight hitting the solar cell that is converted into electricity. The previous solar cell efficiency record was 24%, achieved by Alta Devices, an American-based company.

This level of solar cell efficiency had not been expected to be reached for many years, according to Professor Green. “A recent study by Germany’s Agora Energiewende think tank set an aggressive target of 35% efficiency by 2050 for a module that uses unconcentrated sunlight, such as the standard ones on family homes.”

Receiving unconcentrated light, light that is not directed, is suitable for household rooftop solar panels. Therefore, advancements such as these are hoped to bring the solar energy cost to parity with coal energy, as is the case now in Chile.

The key to the module’s solar cell efficiency is its use of a triple-junction cell, that targets light into three bands (according to wavelength) and directs them to their most suitable receiver.

A diagram of the spectrum-splitting, four-junction mini-module developed at UNSW - image courtesy of UNSW.
A diagram of the spectrum-splitting, four-junction mini-module developed at UNSW – image courtesy of UNSW.

 

The prototype mini-module, which is only 28cm2, will need to be scale to greater sizes if it is to be useful for rooftops. The challenge for the team will be achieving the same efficiency of the mini-module once it is interconnected between other modules.

The multi-junction solar cell currently comes at a high price since as cells of this type are complex to manufacture. However, efforts are being made to reduce this complexity, allowing cheaper cells to be available for market. Despite this, Professor Green told the Sydney Morning Herald that any commercialisation of the cell will be more than 10 years away.

MIT vs UNSW solar cell efficiency research

Researchers at MIT are also looking at similar research using nanophotonic crystals that can be tuned and configured to emit precisely determined wavelengths of light when heated.

However, the two approaches to enhancing efficiency are very different: UNSW splits the solar spectrum into four different wavelength bands and uses four solar cells, whereas MIT converts the solar spectrum via absorption and thermal emission to one narrow wavelength band and uses one solar cell.

Spectrum splitting via the 3D prism - image courtesy of UNSW
Spectrum splitting via the 3D prism – image courtesy of UNSW.

The UNSW minimodule uses spectrum splitting of unconcentrated sunlight to ensure each of four distinct bands of sunlight are absorbed by the most suitable solar cell: one of the three cells (GaInP/GaInAs/Ge) monolithically stacked in the triple-junction cell, or the separate silicon cell. The prism serves multiple purposes: to mount the spectrum-splitting filter and the triple-junction and silicon cells, and to steer the light to the desired cell – basically creating a four-junction receiver. See image.

The MIT solar thermophotovoltaic (STPV) device absorbs concentrated sunlight in order to heat an absorber and thermal emitter to over 700°C. Then thermal emission, which occurs over a narrower wavelength range than the solar spectrum, is directed towards a suitable (InGaAsSb) single-junction solar cell. See Fig. 1 of the MIT Nature Energy paper.