Effect of deep-trap density of states for a-Si:H layers on HIT cell performances: Modeling Framework
In this work, an aSi and cSi heterojunction solar cell with an intrinsic layer is simulated using AFORS-HET software. The study focuses on the role of the electronic density of states DOS of the amorphous emitter, p type aSiH, in governing the performance of the HIT cell and the electronic transfer across the heterointerface. It is found that the DOS of the p type aSiH emitter can cause an unfavorable recombination and trapping effect, which becomes more severe and difficult to avoid when the bulk dangling-bond density in the a-Si emitter approaches or exceeds the doping concentration. An increase in deep trap density beyond a critical value causes a drop in the built in voltage and shifts the depletion layer into p type aSi. This situation leads to a sharp decline in HIT cell performance, namely the open circuit voltage and the fill factor, and to the appearance of an S shaped IV characteristic. The simulation results suggest that the emitter activation energy needs to be optimized to achieve high efficiency. Under optimal conditions, holes accumulate near the heterojunction interface, leading to the formation of an inversion layer. This paper presents a physical mechanism for recombination processes within the emitter and concludes with the optimized parameters required to achieve a HIT cell efficiency comparable to Sanyo record of 23 percent on a thin silicon wafer.