Microcrvstalline and Mixed Phase Si:H : Preparation. Potential and Properties for Devices

Abstract

We discuss the preparation, properties and device potential of microcrystalline Si:H (μc-Si:H) material deposited using plasma-CVD techniques. We have been able to deposit high quality μc-Si:H using both glow discharge and ECR plasma CVD techniques. In both cases, the critical parameter was the presence of a high flux of H, and high power. The quality of the material was measured using sub-gap photo-conductivity, in both films and devices. We find that the absorption coefficient in μc-Si:H films follows the c-Si curve very closely, with little excess absorption. We have also fabricated p-i-n devices in these materials. We find that the properties of the devices depend critically upon the p-i interface and on the deposition technique used to make the devices. We use quantum efficiency techniques to study the devices in detail. We find that often, a good device characteristic corresponds to a case where the undoped layer is a mixed phase (amorphous-microcrystalline) and not just microcrystalline. In contrast, using just the microcrystalline phase for i layers in devices leads to severe interface problems, which lead to inflexion points in I (V) curves. Using the mixed phase for i layer, we have made solar cells with 0.46 V open circuit voltage, and a fill factor of 0.58. In contrast, using a microcrystalline phase can lead to devices with high voltages (0.6), but poor fill factors. We also discuss the device potential of microcrystalline devices, and we find that it may be possible to fabricate devices with 8% solar conversion efficiency in this material once all the interface problems are solved. Such devices may have applications as low-gap cells in an a-Si:H/μc-Si:H cell structure.