Study On Carrier Transport Of Hetero-structure InGaN Solar Cell

With the fast development of In GaN LED,this material has attracted much attention.For the application of photovoltaic(PV)devices,InGaN alloys are of great potential for high-efficiency multi-junction solar cells due to their tunable energy band gap varying from 0.65 eV to 3.4 eV,which cover almost the whole solar spectrum.In addition,InGaN alloys have many superior photovoltaic characteristics including direct energy band gap,high carrier mobility,high drift velocity,high radiation resistance,and high absorption coefficients of ~105cm-1near the band edge.We investigate the carrier transport in hetero-structure InGaN solar cell in this thesis.Chapter 1 introduces the significance of this work and reviews the study progresses in worldwide scale.Then,we introduce the Silvaco Atlas system,and introduce the principle of heterojunction solar cell.In chapter 2,we investigated the maximum operating wavelength of InGaN MQW solar cell,which has been found much smaller than the wavelength of the sample absorption edge.To explain this phenomenon,we analyzed the photo-generated carriers escape mechanism in MQWs theoretically.Due to the carriers excited by photon with low energy can achieve low energy level,then those carriers need to cross high potential barrier to escape the quantum well,when the excitation photon energy is lower than a certain threshold,the carriers can only recombine and have no contribution to photocurrent.The photon energy threshold of carriers is associated with barrier thickness.So we calculate InGaN/Ga N multiple quantum well solar cell the operating wavelength limit of In GaN/GaN multiple quantum well solar cell with different barrier thickness,and provide a guideline for the design of the InGaN/GaN multiple quantum well solar cell.In chapter 3,Carrier transport via the V-shaped pits(V-pits)in InGaN/GaN multiple-quantum-well(MQW)solar cells is numerically investigated.By simulations,it is found that the V-pits can act as effective escape paths for the photo-generated carriers.Furthermore,the V-pits are found can reduce the recombination losses of carriers due to their screening effect to the dislocations.These findings are helpful for understanding the carrier transport mechanism in the In GaN/GaN MQW and are important for design of the structure of solar cells.In chapter 4,we directly observed the relationship between the degree of carrier leakage and the temperature in InGaN multiple quantum wells by measuring the photocurrent.Photocurrent increases under the same light intensity when temperature increases from room temperature to 360 K.At the same time,it is found that carriers escape more in a lower density,and the increase of photocurrent was related to the emission photon energy.Then,we used the model of quantum well-quantum dot to explain the phenomena.To achieve higher efficiency,InGaN p-i-n solar cells with different n and p interface structure were studied by numerical simulations in chapter 5.We found that the high potential barrier on the N-interface of the p-GaN/InGaN/n-GaN can be greatly reduced by replacing the n-GaN layer with an n-ZnO layer.At the same time,we studied that with optimal In contents of the absorption layer and the p-type layer,respectively,conversion efficiency increases to more than 14%.Also,it is found that after optimizing the thicknesses of p-type layer and absorption layer,the conversion efficiency can further enhance to 15%.At last,we conclude this work and look to the future of In GaN alloy.