Study On In GaN/GaN Multiple Quantum Well Solar Cells

InGaN/GaN multiple quantum well solar cell is a new type of solar cell. By changing the In component fraction, the band gap of InGaN can be tuned from 0.7eV to 3.4eV, which is basically covered the solar spectrum. By theoretical calculation, it can be manufactured into highly efficient solar cells, so it has received widespread attention. However, the growth of high quality of InGaN is a great challenge for deposition process, and it is difficult to produce high conversion efficiency of InGaN/GaN multiple quantum well solar cells. At present, InGaN/GaN multi quantum well solar cell is still under investigation, and the power conversion efficiency of InGaN/GaN MQW solar cells is between 0.5% and 3%. In this paper, we analyze and optimize the InGaN/GaN multiple quantum well solar cell from two aspects: material and device structure. The basic structure of the InGaN/GaN multiple quantum well solar cell in this paper is p-i-n, with the InGaN/GaN multiple quantum well as i layer, p-GaN as the p layer and n-GaN as the n layer. The main results of this paper are as follows:(1) The effect of the period of InGaN/GaN and the equivalent thickness of InGaN on the quality of crystal and the performance of the device is studied. The results show that by increasing the period of InGaN/GaN and reducing the equivalent thickness of InGaN in the i layer, we can reduce the dislocation density in crystal and suppress the phase separation of InGaN, thus promoting the quality of crystal. But with the decrease of InGaN equivalent thickness, the absorption intensity of the material decreases. By comprising the two factors above the power conversion efficiency of the device is increased by 37.5%.(2) The influence of thickness of the i layer on the material quality and further on the performance of the device is studied. The results show that as the thickness of the i layer reduced, and the transport capability of the i layer can be effectively improved. However, the equivalent thickness of InGaN is reduced at the same time, which weakens the absorption ability of the material. Then we take effort to optimize the i layer thickness so as to minimize the disadvantage. Finally, by reducing the thickness of the i layer, the power conversion efficiency of the device is increased by seven times.(3) The influence of etching depth of n region of solar cells and p electrode pattern on power conversion efficiency is studied. The results show that as the etching depth decreases, more photo electrons can be generated and diffused into the n electrode, which is beneficial to the improvement of the efficiency of solar cells. p electrode pattern includes p fingers and p pads. The results show that the increase of p fingers and p pads can decrease the series resistance, so that the output voltage and current of the device can be improved. However, the p electrode shades the light, which will affect collection of photos for solar cells. So it is necessary to optimize p electrode on GaN/InGaN multiple quantum well solar cells. In this paper, the power conversion efficiency of the device is increased from 0.62% to 0.79% by optimizing etching depth of n region and the p electrode pattern.(4) The solar cell efficiency with the change of the device unit area is studied. The results show that with the decrease of the unit area of the GaN/InGaN MQW solar cells, photo-generated electrons are more easily to diffuse to the n electrode, and the power conversion efficiency of the device is improved. By reducing the unit area of the device, a 1.93 V open circuit voltage could be achieved, and the power conversion efficiency has reached 0.94%, which is approaching the state of the art record.