Ginger Pietka

High efficiency, multi-junction nc-Si:H based solar cells at high deposition rate

Summary form only given. Hydrogenated nanocrystalline silicon (nc-Si:H) has become a promising candidate to replace hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) in multijunction thin film silicon solar cells due to its superior long-wavelength response and stability against light-induced degradation. Due to the indirect band gap in crystalline silicon, the absorbing nc-Si:H layer needs to be much thicker than the corresponding a-SiGe:H layer. For nc-Si:H based solar cells to be commercially viable, the greatest challenge is to deposit the absorbing layers at a high rate with good spatial uniformity, while maintaining the same superior quality achieved at lower deposition rate. In this paper, we report on the development of our proprietary High Frequency (HF) glow discharge deposition technology to fabricate high efficiency, large area, a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells at a high deposition rate ≥1 nm/s. We have improved our nc-Si:H and a-Si:H processes to fabricate high performance component cells used in the triple-junction solar cells. We have fabricated small area cells (0.25 cm2) and mini module (1.2 cm2) cut out from the large deposited area. We have attained initial, active-area efficiency as high as ~14.0% and light-stabilized, active-area efficiency ~12.8% on these cells. SIMS analysis on the device show low impurity levels in the nc-Si:H absorbing layers. We have also fabricated large area encapsulated modules. We have attained initial aperture-area (~212 cm2) efficiency of ~11.8% on an encapsulated module. These are the highest values measured at United Solar for such high rate samples. Detailed results will be presented at the conference.

12.0% Efficiency on Large-Area, Encapsulated, Multijunction nc-Si:H-Based Solar Cells

Hydrogenated nanocrystalline silicon (nc-Si:H) has become a promising candidate to replace hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) in multijunction thin film silicon solar cells due to its superior long-wavelength response and stability against light-induced degradation. In this paper, we report on the development of our proprietary High Frequency (HF) glow discharge deposition technology for nc-Si:H solar cells that has resulted in high quality nc-Si:H materials with good spatial uniformity. We have studied the HF-deposited nc-Si:H material using various analytical techniques, such as X-ray diffraction, Secondary Ion Mass Spectrometry, and Glow Discharge Mass Spectrometry, and optimized the deposition parameters for best device quality. We conducted a systematic study of the quality and spatial uniformity of nc-Si:H solar cells. We fabricated and optimized a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells deposited on textured Ag/ZnO back reflectors on thin flexible stainless steel substrates using the optimized nc-Si:H component cells. Cells with aperture area ∼400 cm2 and 807 cm2 were fabricated and encapsulated using our proprietary lightweight flexible encapsulants. We sent representative large-area samples to National Renewable Energy Laboratory (NREL) for confirmation of conversion efficiency. NREL has confirmed an initial aperture-area efficiency of 12.0% for cells with aperture area ∼400 cm2. The highest initial efficiency for the encapsulated cells with aperture area ∼807 cm2 is ∼11.9% as measured at United Solar. We light soaked small-area and large-area cells to obtain stable performance. Detailed results will be presented at the conference.

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.

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.

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.

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.

Advances in flexible and ultra lightweight monolithically integrated a-Si∶H based triple-junction PV modules made using roll-to-roll deposition

United Solar Ovonic (USO) has developed a-Si∶H based solar cells on polymer substrate using highthroughput roll-to-roll deposition technology for space and stratospheric missions. In addition, we have advanced two new developments: (1) the monolithically laser-integrated module on flexible polymer substrate to attain an initial specific power as high as 2343 W/kg at the module level; and (2) a new monolithic hybrid module design which marries the advantages of our wire bonded baseline space cell with those of the advanced laser integrated module. It uses standard production cells as the starting material and therefore it is roll-to-roll compatible. With this design, we have fabricated modules having initial AM0 aperture area efficiency ∼9.5%. It operates with high voltage and high specific power. It is flexible, modular and easily scalable. The modules can be interconnected into strings and arrays that can be rolled up into a small volume for stowage.

Advances in cell and module efficiency of a-si:h based triple-junction solar cells made using roll-to-roll deposition

Over the last several years, we have continuously increased the conversion efficiency of solar cells and modules. In this paper, we discuss new developments which led to a ∼5% increase in conversion efficiency of cells and modules made using roll-to-roll deposition. The enhanced efficiency is attributed primarily to the replacement of the conventional Al/ZnO back reflector with a new Ag/ZnO structure. We optimized the deposition parameters to obtain the highest cell efficiency. We used large-area cells to fabricate flexible interconnected modules of dimensions 548.6 cm × 39.4 cm. The initial power output (Pmax) of the modules was 180.2–183.3 W which is the highest obtained for roll-to-roll processing. We conducted light soak studies to determine stable module power output.

Large area nanocrystalline silicon based multi-junction solar cells with superior light soaking stability

We present our progress in attaining high efficiency nc-Si:H solar cells at high deposition rates with superior light soaking stability. We have focused our effort on three areas: i) improving the spatial uniformity and homogeneous properties for nc-Si:H, such as crystallite grain size and volume fraction, ii) optimizing nucleation and seed layer during the initial growth of the nc-Si:H film, and iii) optimizing nc-Si:H bulk growth and grain evolution. We have conducted an extensive study of the effect of process parameters including hydrogen dilution profiling, VHF power, and substrate temperature on the nc-Si:H film properties and component cell characteristics. We also conducted light soaking tests both indoors and outdoors. The a-Si:H/nc-Si:H/nc-Si:H triple-junction cells incorporating the optimized nc-Si:H component cells show significantly higher performance, achieving an 11.2% AM1.5 stabilized efficiency for both encapsulated large-area (464 cm2) cells and inter-connected modules (2320 cm2). To the best of our knowledge, this is the highest stabilized efficiency for a large-area thin-film silicon module.