Study On The Fabrication And Photovoltaic Characteristic Of Ingan/Gan Multiple Quantum Wells (Mqws) Solar Cells

P-i-n solar cells with InGaN/GaN multiple quantum wells (MQWs) as effective absorption region is a major research topic of III-Nitride based solar cells. However, III-Nitride epi-layers still have comparably high density of defects, which significantly degrade device performances of solar cells. In terms of the device structure and contact schemes, in this thesis, InGaN/GaN MQWs solar cells with high open-circuit voltage (Voc) were obtained. Processing condition of low specific contact resistance of Ni/Au contact to p-GaN was optimized, and its impact on solar cells was studied. In addition, the dependences of photovoltaic behaviour of InGaN/GaN MQWs solar cells on Indium Tin Oxide (ITO) current spreading layer, V-pit density of p-GaN, chip size and sunlight concentration ratio were investigated in detail. The related results were as follows:(1) Contact scheme design of p-GaN and effects on InGaN/GaN MQWs solar cells. The optimized process condition of Ni/Au (5/5nm) to p-GaN was confirmed, and specific contact resistance as low as5.05×10-6Ω·cm2was obtained. The roles of ITO current spreading layer in InGaN/GaN MQWs solar cells were discussed. It was found that ITO current spreading layer can not only enhance the transport of photo-generated carriers and light transmission, but also decrease light reflection loss from18%to less than5%. In terms of this decrement in reflection loss due to ITO, we have observed the enhancement of short-circuit current density (Jsc) up to13%and peak external quantum efficiency (EQE) up to11%in percentage, respectively. (2) This work presents results showing how the impacts of V-pit density of p-GaN on Voc. It was convinced that Vocwas found to be0.81V and1.4V when the V-pit density observed by scanning electron microscope is decreased from1010cm-2to108cm-2. When the V-pit density further decreased to10-7cm2, Voc could keep reasonably high.(3) We successfully fabricated InGaN/GaN MQWs solar cells with comparatively high Voc. The higher Voc could be ascribed to low reverse saturation current density (Jo) and good period repeatability and interface of InGaN/GaN MQWs which are evidenced by high-resolution X-ray diffraction measurement. Effect of chip size on Voc could be very weak even negligible. However, series resistance (Rs) of solar cells was found to be lower when chip size is smaller. The reduction in Rs could lead to an enhancement of Jsc from0.52mA·cm-2to0.6mA·cm-2, that of fill factor from45%to54%and finally that of the overall conversion efficiency from0.59%to0.84%.(4) Photovoltaic properties of InGaN/GaN MQWs solar cells with high Voc under concentrated sunlight condition were studied. An improvement of Voc was found from2.31V (one sun) to2.50V as the sunlight concentration ratio increases to190suns and kept constant till240suns. FF dependence on light intensity shows difference from Voc. Judging from these factors with increasing concentration ratio, we can still see an slight enhancement of conversion efficiency, which implies potential of InGaN/GaN MQWs solar cells in concentrator PV application.