Optimization of absorber layers' thickness in a Si micromorph solar cell for current matching with intermediate ZnO reflector

Abstract

In a micromorph silicon (Si) solar cell, the optimum performance is decided by the top hydrogenated amorphous Si (a-Si:H) and the bottom microcrystalline Si (μc-Si) absorber layer thicknesses. This paper investigates the performance of the micromorph cell using modeling and simulation studies in matlab. The short circuit current density (Jsc), open circuit voltage (Voc), fill factor, and efficiency (η) of the cells are analysed by varying thicknesses of the top a-Si:H and the bottom μc-Si absorber layers with and wihtout ZnO Intermidiate Reflector (ZIR). ZIR improves current density of top a-Si:H i-layer and hence thickness reduction of top subcell i-layer. Due to this, the light induced degradation can be minimised. For each top a-Si:H layer thickness, the bottom μc-Si layer thickness is varied in steps. The micromorph structure is numerically solved using Newton-Raphson technique and current density-voltage characteristics for top a-Si:H, bottom μc-Si:H sub-cells, and composite cell are obtained. One of the combinations with the top a-Si:H layer with thickness of 300 nm and the bottom μc-Si layer with thickness of 3.25 μm gives the best efficiency of 13.12% with ZIR. The simulation study performed here can be extended to design third generation multijunction Si quantum dots solar cells.