Development of germanium thin films for solar cell fabrication

Abstract

This thesis aimed to process and characterize hydrogenated germanium (Ge:H) films to use them as a low-bandgap intrinsic absorber layer. The bandgap of Germanium can go down to 0.67 eV. A p-i-n solar cell consisting of intrinsic germanium film produced using Plasma Enhanced Chemical Vapour Deposition (PECVD) can be inexpensively paired with crystalline silicon and other semiconductor materials to produce a multi-junction solar cell with high efficiencies. Such a cell would utilize the lower part of the solar spectrum currently untapped in commercially available mass-produced solar modules. Previous research had deposited Ge:H films using a counterflow PECVD reactor, as compared to a showerhead configuration in the reactor used in this research.

The study consisted of experiments that explored the properties of the germanium film deposited. First, the deposition parameters of PECVD were varied to study its effect on important film properties. Since the germanium layer will be used in a solar cell as the absorber layer, the band-gap and activation energy were the most critical parameters to monitor film intrinsicity. Experiments were performed by depositing germanium layers on glass substrates and silicon wafers to understand the properties of the films when deposited at different temperatures and also when the germane flow was altered. The effect of the power and pressure on the properties of the germanium samples was studied too. In all these experiments, the deposition time and electrode gap are kept constant. The processing window of 3-4.5 mbar and 12-20 W where intrinsic amorphous germanium (a-Ge:H) films can be produced was identified.

Single junction solar cells in p-i-n and n-i-p configuration were then incorporated with the intrinsic amorphous germanium. The p-i-n configuration displayed solar cell characteristics, whereas the n-i-p configuration cells did not, despite the changes in doping of the p-layer. Further experiments were carried out to look into the stability of the material by repeating the J-V curve measurement of the first cell that showed solar cell characteristics, but similar behaviour was not observed. Additionally, Raman spectroscopy was carried out, which revealed that the material is amorphous.

In summary, the results look promising in proving these films as a suitable material for an intrinsic absorber layer. It also strongly suggests that the showerhead PECVD configuration can lead to better control of reaction mechanics, giving films with desired material properties. The layer can be used as an intrinsic layer in a p-i-n structure device, but further investigation is required to look into the interaction of the layers of the solar cells.