J. Woerdenweber

Influence of base pressure and atmospheric contaminants on a-Si:H solar cell properties

The influence of atmospheric contaminants oxygen and nitrogen on the performance of thin-film hydrogenated amorphous silicon (a-Si:H) solar cells grown by plasma-enhanced chemical vapor deposition at 13.56 MHz was systematically investigated. The question is addressed as to what degree of high base pressures (up to 10−4 Torr) are compatible with the preparation of good quality amorphous silicon based solar cells. The data show that for the intrinsic a-Si:H absorber layer exists critical oxygen and nitrogen contamination levels (about 2×1019 atoms/cm3 and 4×1018 atoms/cm3, respectively). These levels define the minimum impurity concentration that causes a deterioration in solar cell performance. This critical concentration is found to depend little on the applied deposition regime. By enhancing, for example, the flow of process gases, a higher base pressure (and leak rate) can be tolerated before reaching the critical contamination level. The electrical properties of the corresponding films show that incre...

Critical oxygen concentration in hydrogenated amorphous silicon solar cells dependent on the contamination source

For hydrogenated amorphous silicon (a-Si:H) solar cells, the critical concentration of a given impurity defines the lowest concentration which causes a decay of solar cell efficiency. Values of 2–5×1019 cm−3 are commonly found for the critical oxygen concentration (COcrit) of a-Si:H. Here we report a dependence of COcrit on the contamination source. For state-of-the-art a-Si:H solar cells prepared at the same plasma deposition conditions, we obtain with a (controllable) chamber wall leak COcrit ∼2×1019 cm−3 while for a leak in the gas supply line a higher COcrit of ∼2×1020 cm−3 is measured. No such dependence is observed for nitrogen.

In-situ determination of silane gas utilization and deposition rate for different deposition regimes of μc-Si:H using FTIR and OES in-situ

For the deposition of microcrystalline silicon it is important to increase the deposition rate and silane utilization rate. In the past, a method based on optical emission spectroscopy (OES) has been introduced to obtain the transition point from amorphous to crystalline growth in-situ, which is the point for optimum microcrystalline silicon solar cell conditions. The method is based on alternating deposition by a silane/hydrogen plasma and etching by a pure hydrogen plasma. This paper combines OES with Fourier transform infrared (FTIR) spectroscopy in the exhaust line to determine the growth rate in-situ. In this way, the multidimensional space of silane flow, deposition rate and gas utilization rate is determined in-situ in one deposition. It is aimed to increase the gas utilization rate towards 100%.