Mechanism of Trapping Effect in Heterojunction With Intrinsic Thin-Layer Solar Cells: Effect of Density of Defect States

We have simulated the performance of heterojunction with intrinsic thin-layer (HIT) solar cells on n-type c-Si substrates using the numerical simulator automat for simulation of heterostructures (AFORS-HET), with emphasis on the effect of density of defect states (DOS) in both p-type hydrogenated amorphous silicon (a-Si:H) emitter and intrinsic a-Si:H buffer layers. A detailed and accurate DOS distribution, including both bandtail states and deep dangling-bond states, has been established in a-Si:H layers based on existing experimental results of a-Si:H films in the literature. The changes of DOS distribution for differently doped thin a-Si:H layers, depending on their doping concentrations (from intrinsic to highly doped), have explicitly been considered. With this DOS distribution model, we have calculated the charge trapped in defect states, which understands the mechanism of trapping effect, i.e., how the DOS influences the built-in field, space-charge region, and the cell performance within HIT solar cell structures. It is found that the DOS in the p-type a-Si:H emitter layer can cause an unfavorable trapping effect, which becomes more serious and even difficult to avoid in case of high doping concentration beyond 1 × 10²⁰ cm^-3. In contrast, the DOS in the intrinsic a-Si:H buffer layer will not sufficiently suppress the cell output until the density of dangling-bond states reaches 5 × 10¹⁸ cm^-3. This paper presents a clear physical picture for the mechanism of trapping effect and concludes the suggestive DOS required for the high efficiency of HIT cells above 20%.