M.K. van Veen

The influence of the filament temperature on the structure of hot-wire deposited silicon

Exposure to hydrogen significantly cools the filament from the set temperature. This can mainly be explained by the power dissipation due to dissociation of hydrogen. The effect of silane on the filament temperature is more complicated. Below a certain threshold temperature (1850 K for W, 1750 K for Ta) a silicon-rich silicide is deposited on the filament, partially shielding it for further dissociation reactions. A drop in deposition rate accompanies this. Above another but higher threshold temperature (2000 K for W and 1950 K for Ta) the silicon-rich silicide is evaporated from the filament and the dissociation reactions occurred and thus the deposition rate are restored. Below these threshold temperatures (2 2 0) oriented materials can be produced.

Protocrystalline Silicon at High Rate from Undiluted Silane

Hot Wire Chemical Vapor Deposition (HWCVD) is shown to be a fast method for the deposition of protocrystalline silicon films from undiluted silane. Intrinsic silicon-hydrogen films (2 µm thick) have been deposited by HWCVD on plain stainless steel as well as on stainless steel precoated with a n-type doped microcrystalline silicon layer. In X-ray diffraction experiments, the linewidths of the first sharp peak (FSP) were 5.59 ± 0.09 degrees and 5.29 ± 0.11 degrees, respectively, indicating improved medium-range order and a template effect due to the µc-Si:H n-layer. For thinner layers (0.7 µm thick), the linewidths of the FSP were 5.29 ± 0.09 degrees and 5.10 ± 0.09 degrees. These FSPs are as narrow as for optimized i-layers made by H2-diluted plasma deposition, however, at a much higher deposition rate (1 nm/s), at moderate temperature (250 °C), and without the use of H 2 dilution. In accompanying transmission electron micrographs, the layers show a significant concentration of elongated small voids in the growth direction that are not interconnected. Small Angle X-ray Scattering (SAXS) results are consistent with these observations. We suspect that the void nature allows the bulk of the film to be more ordered. The utilization of such layers in n-i-p solar cells on plain stainless steel leads to cells with a remarkably good stability, showing a decrease of the fill factor of less than 10 % during 1500 h of light soaking.

Beneficial effect of a low deposition temperature of hot-wire deposited intrinsic amorphous silicon for solar cells

Hot-wire deposited amorphous silicon is an excellent material for incorporation as the absorbing layer in solar cells. We show the beneficial effect of a low deposition temperature of hot-wire deposited intrinsic amorphous silicon for solar cells. The influence of a few specific deposition parameters on the material properties was investigated. It is shown that both the filament history and the deposition pressure are crucial parameters for the material quality. Optimized material, deposited at 250 °C, was incorporated in efficient single- and multijunction solar cells on flexible stainless steel substrates. The n-i-p structure was used to avoid any influence of TCO- and p-layer degradation, which is otherwise present in p-i-n structures. The cells have a high open-circuit voltage and high fill factor, clearly showing the improved performance of hot-wire deposited amorphous silicon made at moderate temperature.

Incorporation of amorphous and microcrystalline silicon in n-i-p solar cells

Abstract We have investigated the material properties and n–i–p solar cell quality of hot-wire deposited amorphous and microcrystalline silicon. Although it is possible to make high quality amorphous silicon solar cells, occasionally many cells show shunting behavior. Therefore, better control over the variation in cell performance is needed. We prove that this behavior is correlated with the filament age and different methods for improving the reproducibility of the cell performance are presented. Furthermore, the influence of different deposition parameters of microcrystalline silicon layers on the material and solar cell properties was studied. Although some of these microcrystalline layers are porous and oxidize in air, an initial efficiency of 4.8% is obtained for an n–i–p cell on untextured stainless steel.

Understanding shunting behavior in hot-wire-deposited amorphous silicon solar cells

Amorphous silicon solar cells, in which the absorbing layer is deposited using the hot-wire chemical vapor deposition (CVD) technique, have potential advantages over solar cells made with the standard plasma enhanced CVD technique. Although it is possible to make high-quality solar cells, many cells occasionally show shunting behavior and better control over the variation in cell performance must be obtained. In this letter, we prove that this behavior is directly correlated with the filament age, and we present different methods for avoiding shunted cells and for improving the reproducibility of cell performance.

a-Si:H/poly-Si tandem cells deposited by hot-wire CVD

Abstract Innovative multibandgap a-Si:H/poly-Si tandem solar cells have been developed, where the two absorbing layers have been deposited by hot-wire CVD. These n–i–p/n–i–p cells have been deposited on a flexible stainless steel substrate, where the microcrystalline doped layers are made by PECVD. No enhanced back reflector was applied. Although the bottom cell shows a shunting problem under low-light conditions, the best tandem cell has an efficiency of 8.1% under AM-1.5 illumination, a fill factor of 0.60, an open-circuit voltage of 1.18 V, and a short-circuit current density of 11.4 mA / cm 2 . The total thickness of the tandem structure is only 1.1 μm.

Thin Film a-Si/poly-Si Multibandgap Tandem Solar Cells With Both Absorber Layers Deposited by Hot Wire Cvd

The first competitive a-Si/poly-Si multibandgap tandem cells have been made in which the two intrinsic absorber layers are deposited by Hot Wire Chemical Vapor Deposition (HWCVD). These cells consist of two stacked n-i-p type solar cells on a plain stainless steel substrate using plasma deposited n- and p-type doped layers and Hot-Wire deposited intrinsic (i) layers, where the i-layer is either amorphous (band gap 1.8 eV) or polycrystalline (band gap 1.1 eV). In this tandem configuration, all doped layers are microcrystalline and the two intrinsic layers are made by decomposing mixtures of silane and hydrogen at hot filaments in the vicinity of the substrate. For the two layers we used individually optimized parameters, such as gas pressure, hydrogen dilution ratio, substrate temperature, filament temperature, and filament material. The solar cells do not comprise an enhanced back reflector, but feature a natural mechanism for light trapping, due to the texture of the (220) oriented poly-Si absorber layer and the fact that all subsequent layers are deposited conformally. The deposition rate for the throughput limiting step, the poly-Si i-layer, is ≍ 5-6 A/s. This layer also determines the highest substrate temperature required during the preparation of these tandem cells (500 °C). The initial efficiency obtained for these tandem cells is 8.1 %. The total thickness of the silicon nip/nip structure is only 1.1 µm.

Investigation of scaling-up issues in hot-wire CVD of polycrystalline silicon

In order to apply device quality polycrystalline silicon obtained by the hot-wire chemical vapour deposition technique in industrial processes a high deposition rate is desirable. Meanwhile it is important to maintain good thickness uniformity over large areas. To achieve this the catalytic area needs to be increased. The influence of increasing the catalytic area on the deposition rate and material properties was studied. Proportionally increasing the feedstock gas flows leads to an unexpected drop in deposition rate. The dissociation of the gases causes the filament temperature to decrease. Below 1850 K the dissociation process is limited by the silicon deposition on the filament. This leads to a drastic drop in deposition rate. By adjusting the process parameters to the increased catalytic area, device quality polycrystalline silicon is again obtained. The deposition rate of polycrystalline silicon has been increased from 6 to 21 A/s while maintaining the material properties.

Shutterless deposition of phosphorous doped microcrystalline silicon by Cat-CVD

Abstract In this paper we present results on phosphorous-doped μc-Si:H by catalytic chemical vapour deposition in a reactor with an internal arrangement that does not include a shutter. An incubation phase of around 20 nm seems to be the result of the uncontrolled conditions that take place during the first stages of deposition. The optimal deposition conditions found lead to a material with a dark conductivity of 12.8 S/cm, an activation energy of 0.026 eV and a crystalline fraction of 0.86. These values make the layers suitable to be implemented in solar cells.