Fabrication And Characteristics Of InGaN/GaN Multiple Quantum Well Solar Cells

As the third generation semiconductor material, the InGaN alloy has become a hot research field internationally due to its excellent photovoltaic properties recently. The InxGa1-xN alloy has many advantages such as tunable energy bandgap from0.7eV (InN) to3.4eV (GaN)(which cover almost the whole solar spectrum), high absorption coefficient, high electron mobility, high hardness, excellent temperature and radiation resistance, which make InGaN the ideal material for realizing full spectrum solar cells and offer broad application prospects for high efficiency solar cells. The present study concentrates on InGaN/GaN multiple quantum well solar cells and systematically investigates the characterization of the epitaxial material, device fabrication, performance characterization of the device and the impacts of polarization effect, etc. The main results were summarized as follows:(1) The fabricated Ino.2Gao.8N/GaN MQWs solar cells with spectrum reponse extending out to475nm showed excellent photoelectric response characteristics and performance. The open circuit voltage(Voc), short circuit current(Jsc), fill factor(FF),peak external efficiency(EQE), peak internal efficiency(IQE) and conversion efficiency(η) of the devices were2.16V,0.55mA/cm2,60.1%,16.5%,33.2%and0.64%, respectively.(2) The optical and electrical properties of InGaN/GaN MQWs solar cells were investigated. The results of HDXRD and AFM showed that the interfaces between wells and barriers were clear and the entire MQWs region remained good periodicity. It was proposed that the high Voc could be ascribed to good overall quality of InGaN/GaN MQWs. The reflection, transmission and absorption spectrum of the devices showed that the absorption edge was nearly475nm. Combined with the EQE spectrum, we proposed that the absorption layer was not thick enough to absorp all the incident lights, which could affect the Jsc and spectral response of the devices. Based on the I-V curve of the devices, it was proposed that p-type contact could increase the series resistance of the devices, which could influence the Jsc and FF. Based on the results of various characterization techniques of the material and the reverse dark current test, we presented that the V-shaped defects and the screw dislocations of the material was also an important factor affecting the device performance and the key is to improve the material quality.(3) The impacts of polarization effect on InGaN/GaN MQWs solar cells were investigated. An adjustable attenuator was added to the original test system of solar cells. Accroding to the analysis of the I-V curve and EQE spectrum under different concentration conditions, we presented that the polarization effect of InGaN/GaN MQWs was one of the main causes of the "turning point" in I-V curve and could affect its position. As the polarization effect strengthened, the series resistance increased and the FF decreased, the turning point would gradually shift toward the high-voltage area. Through the analysis of the EQE spectra under different applied voltages, it was convinced that the polarization induced electric fields across the MQW active region which was in the opposite direction of the depletion field resulting in a reduction or even a reversal of the net electric field. The energy band of the MQWs tiled and resulted in additional barriers which dramatically reduced carrier collection, leading to the decrease of EQE. By comparing the theoretical and experimental results, it can be found that the nonradiation recombination rate of the carriers was increased by the polariztion effect and resulted in an increase of the reverse satuation current, leading to the drop of Voc. It is of great importance to reduce the influence of polarization effect for improving the performance of InGaN/GaN MQWs solar cells.