S.J. Jones

Intermediate order in tetrahedrally coordinated silicon: evidence for chainlike objects

Abstract In this report, we describe the nature of intermediate order in silicon as determined by recent measurements on thin films using transmission electron microscopy (TEM) and Raman scattering. The TEM images show in addition to the expected continuous random network (CRN), the presence of highly ordered quasi-one-dimensional “chain-like objects” (CLOs) that are 1– 2 nm wide and tens of nm long that meander and show some evidence of cross-linking with each other. The presence of these objects correlate to a Raman feature centered at 490 cm −1 whose width is 35– 40 cm −1 , and is used to quantify the heterogeneity in terms of the CLO and CRN (=475 cm −1 scattering) concentrations. The 490 and 35 cm −1 values are consistent with bond angle deviations approaching 0°, and thus reinforces an association with the CLOs. We find that in reference quality a-Si:H (made using pure SiH4), the CLO concentration is about 5 vol % , while in state-of-the-art material using high H2 levels of dilution during processing, it increases to about 15%. Increased stability of such material to light-soaking is thus not mediated by a direct volumetric replacement of poor with high-quality components. Rather, an important characteristic of intermediate order in silicon is the low-dimensional aspect of its order, which allows it to influence more total volume than which it is itself composed. Consistent with these and other recent findings, we propose a tensegrity model of amorphous silicon.

Triple-junction amorphous silicon alloy PV manufacturing plant of 5 MW annual capacity

A spectral-splitting, triple-junction a-Si alloy solar cell processor has been designed, built and optimized. A roll-to-roll process has been used to deposit two layers of back reflector, a triple-cell structure with nine layers of a-Si and a-SiGe alloys and a single layer of antireflection coating consecutively on a half-a-mile roll of stainless steel. The coated web is next slabbed and processed to make a variety of products. The design of the machine and processes used incorporate several key features developed for improving cell efficiency. In order to reduce manufacturing cost, higher deposition rates and thinner cells than are used in R&D have been used. The back reflector also consists of Al/ZnO rather than Ag/ZnO. Large-scale production has begun, and products are being shipped for a wide range of applications.

Correlation of Small-Angle X-Ray Scattering and Solar Cell Performance of a-SiGe:H Alloys

Small-angle x-ray scattering (SAXS) measurements were made on a-SiGe:H alloys to study microstructure on the nanometer scale as a function of Ge content, and the results were compared with representative single-junction solar cell properties. Samples consisting of only the i-layer were used for SAXS. Above a Ge content of 20 %, a significant increase in SAXS was seen. From measurements made with the samples tilted relative to the incident x-ray beam, the increase in scattering is attributed to the appearance of elongated low density regions in the film, modeled as ellipsoidal microvoids, which are preferentially oriented perpendicular to the film surface and may be related to columnar-like microstructure. Flotation density measurements support the presence of low density regions. Initial and light-degraded measurements on corresponding solar cell structures do not show a correlation between SAXS and initial cell properties; there is, however, some evidence that the light-induced degradation is higher for cells with larger amounts of SAXS-detected microstructure and this needs further investigation.

VHF Plasma Deposition of μc-Si p-Layer Materials

Microcrystalline silicon (μc-Si) p-layers have been widely used in amorphous silicon (a-Si) solar cell research and manufacturing to achieve record high solar cell efficiency. In order to further improve the solar cell performance and achieve wider parameter windows for the process conditions, we studied the deposition of high quality μc-Si p-layer material using a very high frequency (VHF) plasma enhanced CVD process. A design of experiment (DOE) approach was used for the exploration and optimization of deposition parameters. The usage of DOE leads to a quick optimization of the deposition process within a short time frame. In addition, by using a modified VHF deposition process, we have improved the solar cell blue response which leads to a 6–10% improvement in the solar cell efficiency. Such an improvement is likely due to an improved microcrystalline formation in the p-layer.

Improved /spl mu/c-Si p-layer and a-Si i-layer materials using VHF plasma deposition

Microcrystalline Si p-layers have been widely used in a-Si solar cell technology to achieve high efficiency. To improve the solar cell performance further, the authors have studied the deposition of high quality /spl mu/c-Si p-layer material using a modified VHF plasma enhanced CVD process and consequently have improved the solar cell current. This improvement was primarily in the blue response which leads to a 6-10% improvement in the overall solar cell efficiency. In addition, they have explored the deposition of a-Si at high rates using VHF plasma and compared these VHF i-layers with RF plasma deposited i-layers. With improved deposition conditions, VHF intrinsic layers deposited at a rate up to 15 /spl Aring//s show similar device performance and light stability to VHF and RF i-layers deposited at low rates and show higher stability than RF i-layers deposited at high rates in the same deposition system. A 10.9% efficient single-junction solar cell was fabricated using a VHF deposited i-layer.

Small-Angle X-Ray Scattering Studies of Glow-Discharge-Produced a-SiGe:H Alloys

Electron density fluctuations associated with microstructural features on a scale from about 1 to 25 nm in glow-discharge-deposited a-Si 1-x Ge x :H films were studied by the technique of small-angle x-ray scattering (SAXS). Films prepared in four different deposition systems (in different laboratories) have been characterized and a general increase in the SAXS signal with increasing x is observed. Density deficiencies determined from film flotation measurements lead to the correlation of the increased scattering intensities with increases in the volume fractions of micro voids. Modeling of the data yields void size distributions that demonstrate significantly more of the larger voids (2 to 6 nm) than found at x=0 (around 1 nm). For the alloys with x>0.4, the scattering at the smallest angles was observed to decrease substantially upon tilting of the sample relative to the x-ray beam. This result contrasts with the small or no changes in SAXS upon tilting device-quality x=0 films. This anisotropie scattering associated with the tilting experiments has been modeled with distributions of ellipsoidal microvoids that are preferentially oriented with their major axes normal to the film plane. This latter result is consistent with a columnar-like microstructure. However, one film with x=0.37 shows no evidence for such microstructure.

Use of a gas jet deposition technique to prepare microcrystalline Si solar cells

A gas jet deposition technique has been used to prepare microcrystalline Si (/spl mu/c-Si) i-layers for nip solar cells at rates of 15 /spl Aring//s. The red light absorbing capabilities make these cells an attractive alternative to a-SiGe in high efficiency multi-junction structures. The high deposition rates allow for fabrication of the required thick /spl mu/c-Si i-layers in a similar amount of time to those used for high quality a-SiGe i-layers (rates of 1-3 /spl Aring//s). Using a 610 nm cutoff filter which only allows red light to strike the device, pre-light soaked short circuit currents of 8-10 mA/cm/sup 2/ and 2.7% red-light efficiencies have been obtained while AM1.5 white light efficiencies are above 7%. These efficiencies on average degrade only by 2% (stabilized efficiencies of 2.6%) after long-term light soaking (1000 hrs.). This small amount of degradation compares with the 15-17% degradation in efficiencies for a-SiGe cells subjected to similar irradiation treatments (final light-soaked red light efficiencies of 3.2%). Using the /spl mu/c-Si nip structure as the bottom cell of an a-Si//spl mu/c-Si tandem-junction cell, pre-light soaking AM1.5 efficiencies of 9.8% have been achieved.

Assessment of the Use of Microcrystalline Silicon Materials Grown at Rates Near 15 Å/s as i-layer Material for Single and Multi-Junction Solar Cells

A microwave-based technique has been used to prepare microcrystalline Si (µc-Si) materials rates near 15 A/s. The use of these materials as intrinsic layers (i-layers) for single and multi-junction devices has been assessed. Since the high deposition rates allow for fabrication of the required thicker µc-Si i-layers in a similar amount of time to that used for high quality a-SiGe i-layers (rates of 1-3 A/s), the materials are attractive, low cost replacements for a-SiGe bottom cell i-layers in a-Si/a-SiGe and a-Si/a-SiGe/a-SiGe multi-junction cells. Single-junction nip, a-Si/µc-Si and a-Si/a-SiGe/µc-Si devices have been fabricated. For these devices, the doped and amorphous layers were deposited using conventional rf glow discharge processes and deposition equipment separate from that used to fabricate the µc-Si materials. 7.0% efficiencies have been achieved for single-junction devices while pre-light soaked 9.8 and 11.4% efficiencies have been achieved for the tandem and triple-junction devices, respectively. The single-junction devices exhibit a degradation of only 0-2% after long term (1000 hrs.) of light soaking demonstrating a high degree of stability. Based on the present status, the µc-Si material prepared at high rates qualifies as a reasonable candidate for the i-layer of a bottom cell of a triple-junction device. Improvements in the performance, particularly the FF, will be needed before use in single-junction and tandem devices can be considered.

High specific power amorphous silicon alloy photovoltaic modules

Amorphous silicon (a-Si) alloy solar cells are attractive for space applications for several reasons, including potential for very low mass. Ultralight a-Si alloy solar modules have been fabricated and modifications to achieve dramatic increases in specific power (W/kg) identified. For modules on stainless steel and Kapton, specific powers of 384 W/kg and 1256 W/kg have been achieved. For 0.5 mil Kapton with thinner bus bars, an extremely high specific power > 2000 W/kg is possible for a module with 10% AM0 efficiency.

Preparation of Microcrystalline Silicon Based Solar Cells at High i-layer Deposition Rates Using a Gas Jet Technique

A Gas Jet technique has been used to prepare microcrystalline silicon (μc-Si) thin films at deposition rates as high as 20 A/s. The films have microcrystal sizes between 80 and 120 A with a heterogeneous microstructure containing regions with columnar growth and other regions with a more randomly oriented microstructure. These materials have been used as i-layers for nip single-junction solar cells. The high deposition rates allow for fabrication of the required thicker μc-Si i-layers in a similar amount of time to those used for high quality a-SiGe:H i-layers (rates of 1-3 A/s). Using a 610nm cutoff filter which only allows red light to strike the device, pre-light soaked short circuit currents of 8-10 mA/cm 2 and 2.7% red-light efficiencies have been obtained while AM1.5 white light efficiencies are above 7%. These efficiencies are higher than those typically obtained for μc-Si cells prepared at the high i-layer growth rates using other deposition techniques. After 1000 h. of light soaking, the efficiencies on average degrade only by 2-5% (stabilized efficiencies of 2.6%) consistent with the expected high stability with the microcrystalline materials. The small amount of degradation compares with the 15-17% degradation in efficiencies for a-SiGe:H cells subjected to similar irradiation treatments (final light-soaked red light efficiencies of 3.2%). Improvements in the cell efficiencies may come through an understanding of the role that columnar microstructure and void structure plays in determining the device performance.