Xixiang Xu

Hydrogen dilution effects on a-Si:H and a-SiGe:H materials properties and solar cell performance

Abstract Effect of H 2 dilution on structural and electronic properties of both a-Si:H and a-SiGe:H films has been investigated. It is found that high H 2 dilution tends to create oriented microstructures which appear to cause hydrogen evolution from the films at low temperature. No correlation between (SiH 2 ) n and the oriented microstructures is, however, observed. In contradiction to results obtained from measuring sub-bandgap absorption of films, solar cells made with high H 2 dilution exhibit improved performance.

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.

High efficiency ultra lightweight a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cells on polymer substrate using roll-to-roll technology

A roll-to-roll manufacturing technology has been developed for fabricating lightweight, high efficiency and flexible triple-junction thin film a-Si:H/a-SiGe:H/a-SiGe:H solar cells on polymer substrate for the emerging space and near-space stratospheric markets. The highest initial (before light soak), 25°C, aperture-area (721.8 cm2), AM0 efficiency attained was 9.7%. The highest initial specific power was ∼1200 W/kg with a proprietary top coating ∼0.25 mil thick. In order to develop the next generation space photovoltaic technology, effort has been made on developing nc-Si:H material. We have fabricated large area a-Si:H/nc-Si:H double-junction solar cells on 5 mil thick steel and 1 mil thick polymer substrates and demonstrated an initial AM0 active-area efficiency of around 10% on a 462 cm2 device.

Progress in triple-junction amorphous silicon alloy solar cells with improved current mismatch in component cells

We have achieved a new world record stable efficiency of 11.8% for amorphous silicon alloy solar cells using a spectrum-splitting, triple-junction structure. In addition to our previously reported key factors leading to high performance multijunction solar cells, we have improved the current matching among the component cells. We have designed the triple structure such that the top cell, which usually exhibits the highest fill factor, remains to be the current-limiting cell in the degraded state. One critical requirement for achieving the desired current matching without sacrificing the triple cell current is to obtain a high quality narrow bandgap bottom cell capable of producing sufficient red current. Details on this narrow bandgap amorphous silicon germanium alloy cell as well as stability data on the triple-junction cell are presented.

Advances in cell efficiency of a-Si:H and nc-Si:H-based multi-junction solar cells for space and near-space applications

We have developed thin film amorphous silicon alloy (a-Si:H) and nanocrystalline silicon (nc-Si:H) based multijunction solar cells on lightweight polymer substrate ∼25 µm thick for space and near-space applications. The baseline cells use an a-Si:H/a-SiGe:H/a-SiGe:H structure deposited by conventional Radio Frequency (RF) plasma enhanced CVD using roll-to-roll deposition. The best initial performance for the baseline cells is aperture-area efficiency 9.84% and specific power ∼1200 W/kg. The baseline cells are available to potential customers in large quantities. In order to increase the solar cell efficiency, we have pursued two new approaches. In the first, we use a Modified Very High Frequency (MVHF) technique to deposit the multijunction a-SiGe:H based cells. In the second, we have investigated nc-Si:H based multijunction cells. In this paper, we present the solar cell efficiency results on the three different device structures.

Coating and interconnect development for a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cells on polymer substrate for space and stratospheric applications

A top coating has been developed for a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cells on polymer substrate for space and stratospheric applications. Several candidate coatings were screened for potential use in the space and stratospheric environments. A proprietary coating performed well; however, the coating exhibited darkening after UV exposure in high vacuum or inert atmosphere. The darkening was reversed with a postexposure treatment. An effective pre-treatment technique has been developed to prevent darkening in UV/vacuum, such as in high-orbit space applications. A method has been developed to interconnect lightweight thin film cells on polymer into strings. Strings have been tested for mechanical robustness and have been repeatedly rolled and unrolled with no degradation in cell performance.

Study of Large Area a-Si:H and nc-Si:H Based Multijunction Solar Cells and Materials

Solar cells based on hydrogenated nanocrystalline silicon (nc-Si:H) have demonstrated significant improvement in the last few years. From the standpoint of commercial viability, good quality nc-Si:H films must be deposited at a high rate. In this paper, we present the results of our investigations on obtaining high quality nc-Si:H and a-Si:H films and solar cells over large areas using high deposition rate. We have employed the modified very high frequency (MVHF) glow discharge technique to realize high-rate deposition. Modeling studies were conducted to attain good spatial uniformity of electric field over a large area (15”×1”) MVHF cathode for nc-Si:H deposition. A comparative study has been carried out between the RF and MVHF plasma deposited a-Si:H and nc-Si:H single-junction and a-Si:H/nc-Si:H double-junction solar cells. By optimizing the nc-Si:H cell and the tunnel/recombination junctions, we have obtained an initial aperture-area (460 cm 2 ) efficiency of 11.9% for a-Si:H/nc-Si:H double-junction cells using conventional RF (13.56 MHz) plasma deposition. The deposition rate was 3 A/sec. Results on solar cells made with MVHF will also be presented.

Effect of ion bombardment during deposition of amorphous silicon and silicon-germanium alloy solar cells

Abstract Experiments of electric bias effect on a-Si:H and a-SiGe:H solar cells are carried out in both diode and triode configurations. In triode mode, positive bias on substrate improves the performance of a-Si:H cells, while zero bias the gives best results for a-SiGe:H cells. In diode mode, there is essentially no effect of bias on the cells observed over the voltage range in our experiments.

High efficiency large area multi-junction solar cells incorporating a-SiGe∶H and nc-Si∶H using MVHF technology

We fabricated five different types of a-SiGe∶H and nc-Si∶H based multi-junction solar cell structures using modified Very High Frequency (MVHF) technology. After optimization, all five structures reached similar initial cell performance, i.e. ∼12% small active-area (0.25 cm2) efficiency and 10.6–10.8% large aperture-area (≥ 400 cm2) efficiency after encapsulation. However, they showed quite different light soaking stability behavior, which can be attributed to the degradation of component cells. We conducted a comparative study between the MVHF deposited solar cells with those deposited by RF. Materials studies were also conducted to understand the mechanism responsible for better stability for the MVHF deposited a-SiGe∶H solar cells. The best stable efficiency achieved for the large-area encapsulated cells is approaching 10% for both a-SiGe∶H and nc-Si∶H based multi-junction cells.

Improvement of a-Si:H and nc-Si:H multi-junction solar cells by optimization of textured back reflectors

The effect of the texture of Ag/ZnO back reflector (BR) on nc-Si:H single-junction solar cell performance has been investigated systematically. Using a high textured BR, a 74% gain in short circuit current density (Jsc) was obtained over a cell made using the same recipe on specular stainless steel. However, the texture reduced the fill factor (FF) from 0.73 to 0.54. Dark current versus voltage measurements showed a significant increase in reverse saturate current when the texture is increased, indicating a poor nc-Si:H material quality in the nc-Si:H cells deposited on highly textured Ag/ZnO BR. In order to maintain both high Jsc and FF, we have optimized the BR texture. With the improved BR, we have achieved initial efficiencies of 9.5% in a nc-Si:H single-junction and 13.4% in an a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells made at 10 Å/s.