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The figure above shows that when the band gap is 0.92eV, the sunlight utilization rate can reach 90%. The following figure shows the structure of the new type of solar cell and the principle of charge separation.
In "PV Japan 2014", the researcher of the Institute of Industrial Science at Osaka University in Japan, Masahiro Ichimura proposed a new solar cell scheme that does not use pn junctions. Ideally, this solar cell's conversion efficiency is expected to reach 70-80%.
The idea of ​​the new principle is to separate the excitons (pairs of electrons and holes) using the polarity within the crystal, which is the internal electric field gradient induced by spontaneous polarization. The material Si commonly used for solar cells is not polar, but the crystals of many compounds are highly polar. When these materials absorb photons to generate excitons under the action of an internal electric field gradient, electrons and holes will spontaneously separate into different directions. According to a specific idea, the solar cell element has a structure in which an InGaN layer having a thickness of 300 nm to 350 nm and a band gap of 0.92 eV is sandwiched between the InN layer and the electrode.
For a typical solar cell, separation of excitons and the transfer of electrons and holes to different electrodes are accomplished by pn junctions. Jiang Cun said that the use of an internal electric field gradient to separate excitons has many advantages. Among them, the biggest advantage is the ability to reduce the recombination of electrons and holes and thermal relaxation.
For example, in general Si-type solar cells, in order to increase the light absorption rate, only the active layer often reaches tens of micrometers or more. This causes most photons with short wavelengths and high energy to become “hot excitons†away from the pn junction. Before arriving at the pn junction to separate into electrons and holes, it has been lost due to recombination and thermal relaxation.
The problem with the conventional single-junction solar cell is that light having a wavelength shorter than the band gap is lost due to thermal relaxation, while light having a longer wavelength is transmitted and cannot be effectively used. This phenomenon is called "Shockley-Queisser limit" and relates to the maximum performance of a single junction solar cell.
The thickness of InGaN used for the photoactive layer of the new solar cell proposed this time is 300 nm to 350 nm. According to Jiangcun, InGaN, unlike Si, is a direct migration type and has a high light absorption rate. “Only a thickness of about 100 nm can absorb 1/2 of the incident light,†and 300 nm can absorb most of the light. The distance from the layer to the electrode is also relatively short relative to the lifetime of the carrier. Therefore, "no phonons are scattered and no thermal relaxation occurs." This eliminates one of the two major causes of loss that determine the "Shockley-Queisser limit."
The transmission loss of long-wavelength electromagnetic waves such as infrared rays still exists. However, by controlling the composition of In in InGaN and reducing the band gap to 0.92 eV, the energy ratio of infrared rays that cannot be used can be reduced to 10% of the total sunlight.
Jiangcun said that even taking into account the losses caused by the 10% and light reflections, the overall loss can be reduced to 20 to 30%. In other words, in an ideal case, a solar cell having a conversion efficiency of 70 to 80% can be realized.
However, as of now, this kind of solar cell still stays in the theoretical stage. Jiangcun said, "In the future, we must actually make components and conduct evaluations."
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