For the far majority of current photovoltaic devices, photons with an energy below 1.1eV are not utilized. Adding a Plasma Enhanced Chemical Vapor Deposition (PECVD) processed germanium bottom cell to a multi-junction device has the potential to provide a low-cost boost in conversion efficiency by utilizing a part of this spectral range below 1.1eV. For this thesis, 89 amorphous/nano-crystalline hydrogenated germanium (a-/nc-Ge:H) films were PECVD processed with the objective of creating a device quality material that is stable, intrinsic and has a high photoresponse. These characteristics are required to enable p-i-n bottom cell integration in a multi-junction device and prevent post-deposition oxidation and carbonation. To this end, the following three approaches were applied: I. Boron doping to increase the activation energy; II. Hydrogen plasma treatment (HPT) to decrease the defect density of the material with hydrogen passivation; III. A decreased electrode gap to improve the photoresponse. These approaches were integrated in the sample preparation steps. Subsequent film characterization was carried out by using vibrational analysis, elemental analysis, and opto-electrical analysis.
The influence of boron doping was considered insignificant and the influence of hydrogen plasma treatment was considered inconclusive. Furthermore, a decreased electrode gap did not result in a higher photoresponse. Reported photo-/dark conductivity ratios were in the range of 1-7. However, the decreased electrode gap resulted in lower deposition rates and higher refractive indices of 4.9-5.0 (compared to 4.1-4.9 for a 20mm electrode gap), indicating denser films. A substantial fraction of the films was processed at an increased temperature of 275°C. It was found that increasing the processing temperature from 200°C to 275°C resulted in considerably denser films with refractive indices of up to 5.2, leading to higher activation energy (up to 314meV) and very low post-deposition oxidation and carbonation. Therefore, by increasing the processing temperature, it was accomplished to produce denser, more stable films, which have a lower bandgap energy and are more intrinsic.