Role of electron carrier selective contact layer of lithium fluoride films with wide bandgap and low work function for silicon heterojunction solar cells

Abstract

Wide bandgap with low work function-based materials are widely used as an electron transport layer to achieve high efficiency in silicon heterojunction (SHJ) solar cells. We presented transparent Lithium fluoride (LiFx) films with a wide optical bandgap and various thicknesses for the applications of SHJ solar cells. The high-resolution transmission electron microscopic (HR-TEM) analysis was used to confirm the thickness of LiFx layers. The aluminium electrode's work-function was reduced because of the high dipole moment by the LiFx and lowers an energy barrier at back contact. Enhancement of quantum efficiency (QE) at longer wavelength regions and operating voltage was due to lower energy barrier height can improve the short-circuit current density (Jsc), open-circuit voltage (Voc), and hence solar cell efficiency. Thermally evaporated-based LiFx films showed an optical transmittance of 88 ~ 90% in the visible-NIR (380–1100) nm wavelength region. LiFx/a-Si:H(i)/c-Si/a-Si:H(i)/LiFx structure for 20 nm thick LiFx showed lifetime of 1129.73 μs and implied iVoc of 731.7 mV. SHJ solar cells were fabricated using various thicknesses 1, 10 and 20 nm, respectively. The SHJ solar cells fabricated on 20 nm thick LiFx film yield the maximum efficiency of 17.16% and was used for the baseline condition of simulations. The simulated performance of the SHJ solar cell was improved from 0.098 to 22.58% as a function of variable LiFx work function from 3.2 to 3.9 eV.