Preparation and modification of interfaces in silicon hetero-junction solar cells

The innovative Interdigitated Back Contact Silicon Hetero-Junction (IBC-SHJ) solar cell combines the advantages of a back contact design with c-Si/a-Si technology to achieve the very high efficiency, > 26% at potentially lower cost. Contacts of IBC-SHJ are formed at the rear sides, which eliminate the front contact shading loss of conventional front junction solar cell devices and allow wider latitude of optical optimization and contact design. In addition, the back contact design allows flexibility in inter-connecting cells into modules. In this thesis, a plasma-enhanced CVD (PECVD) process is used to deposit 5-30nm layers of hydrogenated amorphous silicon (a-Si:H) on c-Si wafers to form heterojunction devices. Unlike traditional diffused junctions requiring temperatures of ~1000°C, PECVD deposited c-Si/a-Si:H heterojunction can be fabricated at temperatures from 150°C-300°C where the low temperature process is compatible with thin c-Si wafers, <50microm. Therefore this IBC-SHJ technology will provide the industry with a new high-efficiency device structure that has the potential to lower the cost c-Si cell.;The performance of IBC-SHJ solar cells is primarily governed by the interface quality between the a-Si and c-Si in terms of the electronic junction and surface recombination mechanisms and a focus of the dissertation is the cleaning and modification of textured c-Si wafer for IBC-SHJ solar cell fabrication. Cell performance is improved by using a thin, ~ 5 nm intrinsic a-Si:H buffer layer as the passivation layer inserted between p-n junction to reduce interface defect density. However, a band-offset barrier between a-Si buffer layer and c-Si substrate inhibits carrier transport through the interface affecting both Voc and FF. This thesis also focuses on the optimization of buffer layer by reducing the bandgap of the a-Si buffer layer where .the properties of the intrinsic a-Si were systematically varied over a wide range of processing conditions for both surface passivation and band-offset in the IBC-SHJ devices. In addition, a-SiGe:H films were also evaluated as the surface passivation layer where the incorporation of Ge reduces Eg and preliminary results showed future potentials. The IBC-SHJ device with lower Eg i-a-Si:H films showed FFs increasing to > 70%. The interface qualities also would degrade during rear-side pattern formation. The last part of the dissertation approached this matter via both interface treatment and process simplification. With interface optimization, the IBC-SHJ devices have experimentally achieved Voc >700mV, FF >70%, Jsc > 38mA/cm2 and overall efficiency >15%.