Tandem solar cells deposited using hot-wire chemical vapor deposition

Abstract

In this thesis, the application of the hot-wire chemical vapor deposition (HWCVD) technique for the deposition of silicon thin films is described. The HWCVD technique is based on the dissociation of silicon-containing gasses at the catalytic surface of a hot filament. Advantages of this technique are the high deposition rate, the low equipment costs, and the scalability. The main goal of this thesis is the optimization of the material properties of both hydrogenated amorphous silicon and microcrystalline silicon, so that these materials can be incorporated as the absorbing layers in tandem solar cells. Firstly, the influence of specific deposition parameters on the material quality of hydrogenated amorphous silicon was investigated. With the use of tantalum filaments, the deposition temperature could be decreased to moderate temperatures, while the (electronic) properties of the amorphous silicon were improved. However, at these low filament temperatures the silicide formation at the filaments was enhanced, resulting in a decrease in the deposition rate and a deterioration of the material quality over time. For extensive silicide formation, even epitaxial growth on crystalline wafers was observed. By preheating the filaments at elevated temperature before deposition, the influence of silicide formation could be minimized, which resulted in an improvement in the reproducibility of the material quality. Solar cells, in which the absorbing layer was made at moderate temperature, had high open-circuit voltages and high fill factors. The best n-i-p structured cell on plain stainless steel had an initial efficiency of 7.2 %. The incorporation of amorphous silicon in p-i-n structured cells with a textured front contact resulted in a higher short-circuit current density and a higher efficiency. Occasionally, many n-i-p structured cells showed shunting problems. The number of working cells was directly correlated to the age of the filaments. The presence of silicides on the wires resulted in a deterioration of the material quality and in the formation of shunting paths. By annealing the filaments before deposition, most silicon was evaporated from the filaments, and its influence was minimized. Furthermore, the incorporation of a buffer layer between the n- and i-layer resulted in a change of the sticking probability of adverse radicals and a reduced formation of shunting paths. Next, the influence of the hydrogen dilution of the silane gas on the material properties of microcrystalline silicon was investigated. Crystalline growth occurred at high hydrogen dilutions. Different microcrystalline layers were incorporated as the absorbing layer in n-i-p structured solar cells. The best cells were made with material that was deposited at the edge of the transition from the microcrystalline to the amorphous regime. The best cell had an initial efficiency of 4.8 %. Furthermore, the applicability of the HWCVD technique for the deposition of microcrystalline n-doped layers was investigated. The material with the highest conductivity was deposited at moderate temperature, using a high hydrogen dilution. Application of these n-layers in n-i-p structured solar cells resulted in similar open- circuit voltages, but lower short-circuit current densities compared to solar cells with a plasma-enhanced deposited n-layer. Further optimization of the hot-wire n-layers is necessary. Finally, the different intrinsic layers were incorporated in tandem solar cells. It was possible to obtain high open-circuit voltages and high fill factors. Spectral response measurements of n-i-p/n-i-p structured solar cells indicated that a low photocurrent easily leaked through the bottom cell, which did not occur in p-i-n/p-i-n cells. Apparently, the order in which the p- and n-layer in the tunnel-recombination junction were deposited influenced this small leakage effect. This junction needs further optimization in n-i-p structured tandem solar cells. Based on the achieved efficiencies for amorphous and microcrystalline solar cells on untextured stainless steel, efficiencies of well over 10 % can be expected by optimizing the thicknesses of the subcells.