Improving the stability and opto-electrical properties of hydrogenated amorphous and nano-crystalline germanium films
J.E.C. van Dingen (TU Delft - Electrical Engineering, Mathematics and Computer Science)
T. De Vrijer – Mentor (TU Delft - Photovoltaic Materials and Devices)
Arno H.M. Smets – Mentor (TU Delft - Photovoltaic Materials and Devices)
Luana Mazzarella – Graduation committee member (TU Delft - Photovoltaic Materials and Devices)
Jianning Dong – Graduation committee member (TU Delft - DC systems, Energy conversion & Storage)
More Info
expand_more
Other than for strictly personal use, it is not permitted to download, forward or distribute the text or part of it, without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license such as Creative Commons.
Abstract
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.