New efficiency record for solar cells

    California-based startup Alta Devices has developed a 28.2% photocell, which is the new world record for photovoltaic converters. This is close to the fundamental Shockley-Quisser limit, which is 33.7% for a cell with one pn junction, 42% for a two-layer cell, 49% for a three-layer cell and 68% for a hypothetical cell with an infinite number of layers.

    Over the past twenty years, the maximum efficiency for single-stage photoelectric converters was 26.1%, only recently it was possible to increase it to 26.4%, so experts call the result of Alta Devices a real breakthrough.

    From the point of view of quantum chemistry, something like this happens inside the photomultiplier: the electrons of the substance absorb the incoming photons and transfer to new energy levels. Depending on the energy of each individual photon (i.e., on the frequency of light), the electron can go into electricity (i.e. leave the cell), go into thermal radiation and form new photons with lower energy (longer wavelength). These secondary photons trigger the same reaction, and so on, until the wavelength of the photon falls below the red border of the photoelectric effect .

    The approach of Alta Devices to the Shockley-Kweisser limit was made possible thanks to a significant increase in the "utilization" of photons in the photocell. The secret is in adding films from a very high-quality gallium arsenide (GaAs) grating with a thickness of 1 μm to the photomultiplier tubes. The structure is such that it utilizes up to 99% of “secondary” recombinations into photons. Alta Devices co-founder invented a new method of growing GaAs films, with which you can get a better crystal lattice. In addition, Alta Devices increased the reflectivity of the photocell coating, so that photons do not leave the solar cells.

    Representatives of Alta Devices presented their development at the IEEE Photovoltaic Specialist Conference on June 20, 2011.

    Improving the efficiency of solar cells, even by several percent, can significantly reduce their payback period. Although the listed results up to 28.2% are achievable only in laboratory conditions, and when assembling real solar cell modules the efficiency decreases markedly, Alta Devices Executive Director Christopher Norris is quite sure that they can achieve 30% in the laboratory and assemble ready-made modules for commercial use with an efficiency in the region of 26% (in double-layer cells - even higher). In this case, solar energy will be quite competitive with fossil fuels.

    Alta Devices is now trying to build an assembly line for the production of the first batch of solar cells. The company has already raised $ 72 million in investments and grants to improve the process. On the chart, Alta Devices achievement is indicated as 27.6%, because this figure was in the official report at the IEEE Photovoltaic Specialist Conference (apparently, it has already been tested in an independent laboratory), but in fact, shortly after the conference, Alta Devices reported an increase Efficiency up to 28.2%. via IEEE Spectrum

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