C.H.M. van der Werf

Understanding light trapping by light scattering textured back electrodes in thin film n‐i‐p-type silicon solar cells

For substrate n‐i‐p-type cells rough reflecting back contacts are used in order to enhance the short-circuit currents. The roughness at the electrode∕silicon interfaces is considered to be the key to efficient light trapping. Root-mean-square (rms) roughness, angular resolved scattering intensity, and haze are normally used to indicate the amount of scattering, but they do not quantitatively correlate with the current enhancement. It is proposed that the lateral dimensions should also be taken into account. Based on fundamental considerations, we have analyzed by atomic force microscopy specific lateral dimensions that are considered to have a high scattering efficiency. Textured back reflectors with widely varying morphologies have been developed by the use of sputtered Ag and Ag:AlOx layers. For these layers we have weighted the rms roughness of the surface with the lateral dimensions of the effective scattering features. A clear correlation is found between the current generation under (infra)red light...

Performance of heterojunction p+ microcrystalline silicon n crystalline silicon solar cells

We have studied by Raman spectroscopy and electro-optical characterization the properties of thin boron doped microcrystalline silicon layers deposited by plasma enhanced chemical vapor deposition (PECVD) on crystalline silicon wafers and on amorphous silicon buffer layers. Thin 20–30 nm p+ μc-Si:H layers with a considerably large crystalline volume fraction (∼22%) and good window properties were deposited on crystalline silicon under moderate PECVD conditions. The performance of heterojunction solar cells incorporating such window layers were critically dependent on the interface quality and the type of buffer layer used. A large improvement of open circuit voltage is observed in these solar cells when a thin 2–3 nm wide band-gap buffer layer of intrinsic a-Si:H deposited at low temperature (∼100 °C) is inserted between the microcrystalline and crystalline silicon [complete solar cell configuration: Al/(n)c-Si/buffer/p+μc-Si:H/ITO/Ag)]. Detailed modeling studies showed that the wide band-gap a-Si:H buffe...

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.

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.

Deposition of HWCVD poly-Si films at a high growth rate

Abstract The process parameters for high growth rate poly-silicon films by hot-wire chemical vapour deposition have been explored. A four-wire hot wire assembly has been employed for this purpose. High silane to hydrogen flow ratios and high wire temperatures are the key process parameters to achieve high growth rate and growth rates higher than 5 nm/s can be achieved. The process conditions to incorporate high hydrogen content into the material for passivation of defects and donor states have been identified as high hydrogen dilution and lower wire temperature. With these procedures poly-Si films deposited at 1.3 nm/s showed high ambipolar diffusion length of 132 nm. Incorporating such poly-Si films as i-layer, n–i–p solar cell on stainless steel substrate without back reflector showed an efficiency of 4.4%.

Above-CMOS a-Si and CIGS solar cells for powering autonomous microsystems

Two types of solar cells are successfully grown on chips from two CMOS generations. The efficiency of amorphous-silicon (a-Si) solar cells reaches 5.2%, copper-indium-gallium-selenide (CIGS) cells 7.1%. CMOS functionality is unaffected. The main integration issues: adhesion, surface topography, metal ion contamination, process temperature, and mechanical stress can be resolved while maintaining standard photovoltaic processing.

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.

Low Temperature Poly-Si Layers Deposited by Hot Wire CVD Yielding a Mobility of 4.0 cm 2 V −1 s −1 in Top Gate Thin Film Transistors

ABSTRACT Direct deposition of polycrystalline silicon (poly-Si) thin films by the Hot Wire CVD method has been used for the first time for the fabrication of poly-Si top gate Thin Film Transistors (TFTs). The TFTs have a high electron mobility in saturation of up to 4 cm 2 V -1 s -1 as well as a remarkably large ON/OFF ratio of up to 6 x 10 5 . INTRODUCTION To increase the size of active-matrix liquid crystal displays (AMLCDs) and to relax the design rules, the trend in this field is the replacement of a-Si:H pixel TFTs by polysilicon TFTs. Poly-Si TFTs have a higher field-effect mobility and allow larger drive currents, so that the pixel aperture ratio can be increased and bright, low power LCDs can be obtained even when they have a large size. By supporting both n-channel and p-channel operation, poly-Si TFTs also enable CMOS circuits for display drivers. High quality poly-Si films can be obtained, e.g., by CVD at high temperature, but high temperatures must be avoided as the cost of LCDs would increase due to the required use of large-area temperature-resistant quartz substrates. Therefore, low temperature approaches have to be developed to create poly-Si thin films on glass. By excimer-laser crystallization, excellent polycrystalline films can be obtained using a-Si:H layers as the starting material [1-3]. Laser crystallization, however, is by definition an extra step and the throughput may be limited. In addition, if the starting material is deposited by Plasma Enhanced CVD (PECVD), the material has to be dehydrogenated carefully in order to avoid explosive evolution of hydrogen. While Hot Wire CVD (or catalytic CVD) has already demonstrated that a-Si:H films with a hydrogen content lower than 1 at.-% can be obtained [4,5], which would make fast single-pulse crystallization possible, the same CVD method has now shown that poly-Si can also be

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.