DESIGN AND SIMULATIONS OF SiC BASED REAR CONTACT Si AND SiGe SOLAR CELLS FOR STANDALONE AND TANDEM APPLICATIONS

Abstract

The most abundant renewable source of energy on the earth is solar energy, yet its potential has not been exploited efficiently by solar cells present in the market. Affordability of solar energy can be enhanced, either by increasing the efficiency of a solar cell or by reducing its cost. In this thesis, several approaches for improving solar efficiency with careful design have been proposed using Technology Computer Aided Design (TCAD). These approaches attempt to improve the solar cell current, solar cell voltage, light absorption, surface recombination, and ultraviolet (UV) stability. Finally, the mechanically stacked tandem architecture addresses thermalization and lack of absorption losses for improved efficiency. The initial focus is to minimize the issues associated with thin devices such as low absorption and high surface recombination. These issues have been resolved using novel front surface design which consists of Zirconia (ZrO2) based texturing along with Silicon Carbide (SiC) based front surface passivation. Design principle balances out photonic and electronic effects together and resulted in 15.7% efficient rear contact silicon (Si) solar cell, in the sub10 µm-thick regime. The next focus is to minimize the thermalisation losses. 300 microns and 10 microns thick SiC passivated rear-contact solar cell has been placed in four terminal (mechanically stacked) tandem configuration with 20.9% efficient perovskite top subcell. Realistic TCAD analysis has been done for both top and bottom subcell; which resulted in 27.6% and 22.4% efficient tandem devices under single air mass 1.5 (AM 1.5) irradiance. Further, SiC passivated interdigitated back contact silicon heterojunction solar cell (IBCSiHJ) has also been discussed for bottom subcell application under perovskite top subcell since IBC-SiHJ solar cell uses low-temperature fabrication processes and has excellent photovoltaic (PV) performance. The efforts resulted in 29.5% and 23.7% efficient tandem devices which contain 250 µm and 25 µm thick IBC-SiHJ bottom subcells, respectively.

Si as an active material for most of the PV devices, whose absorption coefficient is small at higher wavelengths; therefore, thick silicon wafers are required to obtain greater efficiencies in both standalone as well as tandem configuration. Thicker silicon wafer eventually increases the module cost, and hence, modification in the bandgap (Eg< 1.1 eV) of the Si is required to increase the absorption of sunlight at higher wavelengths while keeping the thickness low. Therefore, Silicon-Germanium (SiGe) material has been introduced to rear contact solar cell designs and investigation is done for both standalone and tandem configuration. The thickness of SiGe based devices reported in this thesis is 10 microns, which is projected to enhance the efficiencies keeping the thickness low. The device exhibits improved higher wavelength absorption without the need of complex texturing schemes and suggested its potential use as a bottom subcell under tandem configuration. 15.4% power conversion efficiency (PCE) is reflected in convention rear contact SiGe solar cell, whereas in interdigitated back contact SiGe heterojunction solar cell (IBC-SiGeHJ) architecture, 15.5% PCE is achieved in a stand-alone configuration, and in combination with perovskite top subcell, further 25.7% PCE is demonstrated.