Ray tracer simulations of ITO/crystalline silicon (c-Si) interface for heterojunction solar cells: Effect of c-Si substrate’s thickness.

Abstract

Wide band gap indium tin oxide (ITO) is increasingly used as anti-reflective coating (ARC) and transparent electrode in solar cells. This owes to its high optical transmission and superior electrical conductivity. ITO’s promising ARC characteristics made it a candidate for applications requiring increasedbroadband optical absorption within 300–1100 nm wavelength region. This is due to its stable chemical properties, wide band gap (3.3–4.3 eV) and optical transmission exceeding 80% in the visible wavelength region. ITO is also used to form heterojunction solar cells with p-type c-Si. This is attributed to the natural n-type conductivity of the ITO. In ITO/c-Siheterojunction solar cells, high energy photons in the ultraviolet (UV) region are absorbed by the ITO while middle and low energy photons in the visible and near-infrared regions are absorbed by the c-Si. Also, ITO/c-Si heterojunction architecture is advantageous by eliminating the heavily doped emitter layer (“dead layer”) on the front side of the conventional high temperature doped homojunction solar cells. Furthermore, in ITO/c-Si heterojunction solar cells, larger energy band gap of the ITO (3.3–4.3 eV) with respect to c-Si (1.12 eV) results in lower reverse saturation current density (J0), due to lesser defects at the interface. This results in suppressed interfacerecombination which yields higher Voc when compared to homojunction c-Si solar cells. Despite these advantages, there is no available study on the expected simulated behavior of ITO deposited on c-Si substrate with varying thicknesses for potential heterojunction solar cells. Optimizing substrate thickness is essential for reducing the cost of high efficiency ITO/c-Si heterojunction solar cells. Besides, the substrate thickness is a crucial parameter that impact light absorption, reflection, transmission, and electrical performance of ITO/c-Si heterojunction solar cells. In this study, wafer ray tracer simulation from PV lighthouse is used to investigate the impact of substrate thickness on the optical properties of ITO deposited on c-Si substrates. The effects of varying substrate thickness on light absorption, reflection, and transmission has been analysed. ITO thin film thickness is kept at 90 nm while the c-Si substrate thickness is varied (i.e., 50 µm, 100 µm, 150 µm, 200 µm, 250 µm and 300 µm). With 90 nm of ITO on 200µm thick c-Si, an open-circuit voltage of 741 mV, short-circuit current density of 36.872 mA/cm2, fill factor of 0.80% has been achieved.