| Solar cells in thin crystalline-silicon are of much interest because of their potential high efficiencies and low cost. However, many previous attempts to attain thin silicon for photovoltaic applications have involved costly processes, such as high-energy implants and epitaxial growths, and impractical ways of handling the thin films. Hence, development of cost-effective crystalline-silicon solar cells with optimal thickness of 10-100microm has remained an unfulfilled goal for many years. In this work, we report a novel kerfless exfoliation technology capable of producing ultra-thin 25microm flexible monocrystalline silicon foils from thick Si wafers.;We set up an object-oriented 2-D device simulator (FLOODS), and augmented it for reliable physics-based numerical simulation of thin-Si solar cells. This tool provides flexibility for general simulations that commercial tools do not. The setup includes (i) characterization of the electron-hole generation rate, (ii) internal photon reflection, (iii) modeling of SRH and Auger carrier recombination rates, (iv) modeling of carrier mobilities, and (v) physical accounting for energy-bandgap narrowing and Fermi-Dirac statistics in heavily doped regions.;Using FLOODS simulations, we present physical insights into the various recombination mechanisms and how they affect performances of thin BSF cells. We have explored novel design techniques and engineering tradeoffs such as base doping density, local BSF, local back contacts, and contact width and pitch to reduce the recombination losses.;As the crystalline-silicon thickness is reduced to attain substantial cost reduction, excellent surface passivation on both sides is required to fabricate higher-efficiency solar cells. We fabricated back-contact solar cells and a-Si:H/c-Si heterojunction solar cells to achieve this goal. Numerical simulations with FLOODS were used to identify losses in these devices, and optimum device structures were designed, and performance predicted with numerical simulations.;A novel remote-plasma CVD (RPCVD)-based process was developed for fabrication of a-Si:H/c-Si heterojunction (HJ) photovoltaic cells. In the RPCVD system, during the deposition process there is no direct exposure of the sample to the plasma. This can reduce the plasma damage to the silicon surface and improve passivation quality. Very high open-circuit voltage measured from fabricated heterojunction cells suggests that RPCVD is a potential technology for achieving improved passivation in HJ cells. |
