Effect of Impact Ionization on the Performance of Thin Film Solar Cells

Abstract

Thin film crystalline silicon solar cells are attractive due to potential to achieve high efficiency along with the reduction in material cost as compared to bulk silicon solar cells. As a result, finding innovative ways to increase the output current of the thin film c-Si solar cells is of vital interest to future PV systems. One such way is to utilize the generated hot carriers that have energy to surpass the bandgap of the material. These hot carriers can start impact ionization that causes a feedback mechanism that increases the overall output current of the solar cells. This photocurrent generation requires these energetic hot carriers to produce a second or more EHPs through impact ionization. In the first part of the thesis, Physics Based TCAD simulation is used to investigate the effect of II on the performance of c-Si thin film solar cell. There are two different type of structures of c-Si solar cell are simulated. One of the structures has a stack of emitter and absorber layer on the top of the silicon substrate while the other structure has P+ pocket implanted in the absorber layer. In both the structure emitter layer is kept thin (~10-30 nm) with a high doping profile for facilitating high electric field of the magnitude ~1 MV/m which can maximize the impact ionization mechanism. The simulation focuses on the simulation models and material parameters used in our work. The dependence of output current density, output voltage, and quantum ef?ciency on key geometric parameters such as wavelength, absorber doping, P+ pocket doping and length were discussed. The simulation results show that high doping density emitter layer, absorber layer and P+ pocket enhances the current density without affecting the voltage. In addition simulation results show that Internal Quantum Efficiency (IQE) exceed unity for high doping density of emitter layer, absorber layer and P+ pocket. In the second part of the thesis, aSi:H(n+)/cSi(P+) HIT solar cells were fabricated to study the effect of a highly doped junction that increase the electric field which further enhances the Impact Ionization and Tunneling rates that could increase the current density, Jsc. Moreover the effect of emitter doping and thickness on the performance of a-Si(n+)/c-Si(P+) is studied. The results show a significant increase in Jsc, Voc, efficiency, Fill Factor (FF) and Internal Quantum Efficiency (EQE) for thinner and higher doped emitter layer.

Date of Award	May 2014
Original language	American English
Supervisor	Ammar Nayfeh (Supervisor)