Process technology for mass production of large-area a-Si solar modules

The production of large-area (0.6 × 1.0 m²) modules takes place in a fully automated semiconductor line and a semi-automated panel finishing line. The a-Si deposition occurs in a multichamber reactor for parallel processing of p-, i- and n-layers which provides for flexibility in the cell design (single- and multijunction structures) without significant loss in throughput. Specific aspects of module construction, namely the properties of commercially available large-area TCO substrates, scale-up of the PECVD process, and patterning for color-neutral semitransparent modules, are discussed. The module production is now based exclusively on-a-Si/a-Si tandem structures. The average power output is initially 36 W, and stabilizes around 15% below that level.     

Production technology of large-area multicrystalline silicon solar cells

In 1996 a conversion efficiency of 17.1% had been obtained on 15 cm×15 cm mc-Si solar cell. In this paper, large-scale production technology of the high-efficiency processing will be discussed. Enlarging reactive ion etching (RIE) equipment size, technology of passivation, and fine contact grid with low resistance by screenprinted metallization, which is firing through PECVD SiN, have been investigated.     

Numerical approach for high-efficiency a-Si solar cells

We have developed the first precise numerical simulator for thin-film solar cells with two-dimensional structures, such as a submicron textured a-Si solar cell. Conventional simulators for thin-film solar cells were all one-dimensional, which made precise simulation of the behavior of light and carrier transport in the cell impossible. Using the 2D simulator, guidelines for cell design, including textured structures, were obtained. One proposal to increase the conversion efficiency of the textured a-Si single-junction solar cell is to make the texture period longer than the film thickness.     

非晶硅太阳电池的原理

非晶硅太阳电池是２０世纪７０年代中期发展起来的一种新型薄膜太阳电池，与其他太阳电池相比，非晶硅电池具有以下突出特点：（１）制作工艺简单，在制备非晶硅薄膜的同时就能制作pin结构。（２）可连续、大面积、自动化批量生产。（３）非晶硅太阳电池的衬底材料可以是玻璃、不锈钢等，因而成本小。（４）可以设计成各种形式，利用集成型结构，可获得更高的输出电压和光电转换效率。（５）薄膜材料是用硅烷（SiH４）等的辉光放电分解得到的，原材料价格低。１非晶硅太阳电池的结构、原理及制备方法非晶硅太阳电池是以玻璃、不锈钢及特种…     

Easy-to-fabricate 20% efficient large-area silicon solar cells

Recently, an innovative silicon solar cell structure has been developed at ISFH which is capable of achieving very high cell efficiencies on industrial-size wafers with a simple photolithography-free processing sequence. As the corresponding solar cells essentially rely on the application of obliquely evaporated contacts they are denoted as OECO cells. In this paper the successful up-scaling of the novel OECO process from 21% efficient 4 cm² laboratory devices to the fabrication of large-area (100 cm²) silicon solar cells is described, and independently confirmed total area efficiencies of 20% are reported for 10×10 cm² OECO-type solar cells fabricated on p-type float-zone silicon.     

Nanoimprint patterning for tunable light trapping in large-area silicon solar cells

We demonstrate the flexibility of UV nanoimprint lithography for effective light trapping in p-i-n a-Si:H/μc-Si:H tandem solar cells. A textured polymeric layer covered with pyramidal transparent conductive oxide structures is shown as an ideal system to promote front light scattering and thus enhanced photocurrent. The double structure incorporated into micromorph tandem thin film silicon solar cells is systematically investigated in order to find a relationship between interface morphology, optical properties and photovoltaic characteristics. To prevent the formation of defects during cell growth, a controllable smoothing of the imprinted texture is developed. Modules grown on polymer structures smoothed via multi-replication show excellent performance reaching a photocurrent of 12.6 mA/cm² and an efficiency of 12.8%.     

Advanced fabrication technologies of integrated-type structure for a-Si solar cells

This paper proposes a new advanced fabrication technology for a low-cost integrated-type a-Si solar cell. Integrated-type cells provide many advantages and have been industrialized with a laser patterning method. However, a higher throughput and more efficient patterning method was required for applying a-Si solar cells to a power generating system. Plasma CVM (Chemical Vaporization Machining) was first applied to advanced patterning because of its advantages of high speed and selectivity. In this method, a plasma generated under high pressure localizes near the wire electrode and concentrates reactive radicals. As a result, we achieved an etching rate of more than 1 μm/s and selective patterning of a 200 μm-wide a-Si layer in 1 s multiline patterning was also developed for large-area modules.     

Damage-free reactive ion etch for high-efficiency large-area multi-crystalline silicon solar cells

Texturing of silicon (Si) wafer surface is a key to enhance light absorption and improve the solar cell performance. While alkaline texturing of single-crystalline Si (sc-Si) wafers was well established, no chemical solution has been successfully developed for multi-crystalline Si (mc-Si) wafers. Reactive-ion-etch (RIE) is a promising technique for effective texturing of both sc-Si and mc-Si wafers, regardless of crystallographic characteristics, and more suitable for thin wafers. However, due to the use of plasma source generated by high power, the wafer surface gets a physical damage during the processing, which requires an additional subsequent damage-removal wet processing. In this work, we developed a damage-free RIE texturing for mc-Si solar cells. An improved self-masking RIE texturing process, developed in this study, produced ∼0.7% absolute efficiency gain on 156×156 mm² mc-Si cells, where the gas ratio and the plasma power density were keys to mitigate the plasma-induced-damage during the RIE processing while maintaining decent surface reflectance. In the self-masking RIE texturing, a mixture of SF₆/Cl₂/O₂ gases was found to significantly affect the surface morphology uniformity and reflectance, where an optimal etch depth was found to be 200-400 nm. We achieved J_sc gain of ∼1.3 mA/cm² while maintaining decent FFs of ∼0.78 without a V_oc loss after optimization of firing conditions.     

Toward stabilized 10% efficiency of large-area (>5000 cm) a-Si/a-SiGe tandem solar cells using high-rate deposition

This paper reviews recent progress in large-area a-Si/a-SiGe tandem solar cells at Sanyo. Optimized hydrogen dilution conditions for high-rate deposition of hydrogenated amorphous silicon (a-Si:H) films and thinner i-layer structures have been systematically investigated for improving both the stabilized efficiency and the process throughput. As a result, a high photosensitivity of 10⁶ for a-Si:H films has been maintained up to the deposition rate of 15 Å/s. Furthermore, the world's highest initial conversion efficiency of 11.2% which corresponds to a stabilized efficiency of about 10% has been achieved for a 8252 cm² a-Si/a-SiGe tandem solar cell by combining the optimized hydrogen dilution and other successful technologies.     

a-Si alloy three-stacked solar cells with high stabilized-efficiency

a-Si alloy three-stacked solar cells have been studied to improve the stabilized efficiency of a-Si: H based solar cells. Based on the analysis by the individual characterization method of the component cells in stacked type cells, the a-Si :H middle cell was replaced with an a-SiGe :H cell. Furthermore, the optical confinement technology was improved to obtain a high-output current with thin i-layer thickness in the a-SiGe :H bottom cell. By this device design, the initial conversion efficiency was improved up to 12.4% and more than a 10% stabilized efficiency was obtained in a-SiC :H/a-SiGe :H/a-SiGe :H three-stacked cells. These cell characteristics were confirmed by measurements at the JQA Organization (the former JMI Institute).     

Development and application of see-through a-Si solar cells

A new type of translucent amorphous silicon (a-Si) solar cell, called the see-through a-Si solar cell, is developed. It has multiple microscopic holes within its effective area to transmit light and it generates electric power. A series of technical data on the fabrication processing with various patterning and photovoltaic performance are presented. Some examples of application systems such as car sunroof and home interior are introduced and discussed on a wide variety of new areas of PV applications. The see-through a-Si solar cell was mounted on a car sunroof to drive the car's ventilating system or to charge its battery. The ventilating system reduced the interior temperature of the car from 61 to 47°C during daytime parking.     

Flexible, long-lived, large-area, organic solar cells

We report herein large area (>10 cm²), interconnected organic solar cell modules both on glass substrates as well as on flexible ultra-high barrier foils, reaching 1.5% and 0.5% overall power conversion efficiency under AM1.5 conditions. Series connection is described, as these modules consist of up to three cells. Using our flexible barrier material, a shelf lifetime of polythiophene-based solar cells of 6000 h could be realized. Furthermore, we compare the photovoltaic performance of efficient conjugated polymer:fullerene solar cell modules with established technologies. Under typical indoor-office lighting, our modules are competitive with these systems.     

Phase engineering of a-Si:H solar cells for optimized performance

Until recently, the advances in hydrogenated amorphous silicon (a-Si:H) solar cell performance and stability have been achieved materials prepared with hydrogen dilution following primarily empirical approaches. This paper discusses the recently obtained insights into the growth, microstructure and nature of these materials. Such protocrystalline Si:H materials are more ordered than the a-Si:H obtained without dilution and evolve with thickness from an amorphous phase into first a mixed amorphous–microcrystalline and subsequently into a single microcrystalline phase. The development of deposition phase diagrams, characterize their microstructural evolution during growth which can be used to guide the fabrication of solar cell structures in a controlled way. Examples are presented and discussed of their application in solar cell fabrication to obtain a fundamental understanding of the properties of the phase transitions as well as the systematic optimization of cell performance.     

A new technique for large-area thin film CdS/CdTe solar cells

The atmospheric pressure CSS method has been developed as a reproducible and efficient process. Thin film CdTe grown under atmospheric pressure has a rough surface morphology. The density of carbon black powder in the graphite carbon paste for screen printing is a key factor in reducing the series resistance of the device with rough surface CdTe. Using graphite carbon paste with 7 wt% carbon black powder has resulted in cells with a relatively low back contact resistance. A highly efficient large-area CdS/CdTe solar cell (11.0%, 5327 cm²) sub-module has been fabricated using the new technique.     

Solar simulator measurement system for large-area solar cells at standard test conditions

A measurement system for solar cells, which can be used for solar cells with a maximum size of 12 × 12 cm² and photocurrents up to 8 A, is described. The measurement facility consists of a commercially available 1000-W solar simulator, a temperature-controlled test chuck, measurement electronics for recording photovoltaic output characteristics of solar cells, and a data acquisition unit. Most emphasis is given to design of measurement electronics, construction of test chuck, beam uniformity and spectral distribution of solar simulator light.     

Modeling of a-Si/poly-Si and a-Si/poly-Si/poly-Si stacked solar cells

A computer model for the poly-Si thin film-related solar cells is established, with which the solar cells with the structure of single junction poly-Si cell, a-Si/poly-Si tandem cell and a-Si/poly-Si/poly-Si triple cell are simulated. The results indicate that the practical structure for poly-Si-related solar cell is a-Si/poly-Si/poly-Si triple cell with the best matched thickness of 0.23/0.95/3 μm and with optical confinement structure, which has the highest simulated efficiency of 22.74%.     

Single-junction a-Si solar cells with over 13% efficiency

Single junction hydrogenated amorphous silicon solar cells having a high conversion efficiency of 13.2% were developed by combining three approaches. First, a new type of p-layer, such as (a-Si/a-C)_n multilayers, was investigated. The high open-circuit voltage was obtained without lowering the short-circuit current and the fill factor. Second, alternately repeating deposition and hydrogen plasma treatment method was applied to the fabrication of an a-SiC or wide gap a-Si : H films for p/i interface layer. High photoconductive and wide bandgap materials were obtained applicable to the p/i buffer layers. Third, the relationships between defect density of films or fill factors of solar cells and hydrogen radical in plasma were investigated. It was suggested that the H^*/SiH^* ratio was an effective parameter to improve the defect and fill factor, and the excess hydrogen radical deteriorated quality of films and cells.     

Limiting carrier effect in a-Si:H solar cells

The limiting carrier effect in a-Si:H p–i–n and n–i–p solar cells has been investigated computationally, by adjusting the values of the carrier band mobilities. Using a realistic optical generation rate profile, it was found that the effect is significant in both types of cell, with the electron identified as the limiting carrier in the p–i–n cell, and the hole in the n–i–p cell. However, using a uniform generation rate profile in the simulation indicated that the limiting carrier effect was much less significant, and that device performance in both cells seemed to be slightly more sensitive to the hole transport properties than to the electron transport properties.     

Optically induced degradation in a-Si:H solar cells

Difference spectra of optically induced changes in the surface photo-voltage response of a-Si:H solar cells show that illumination generates two sets of near midgap states which have energy levels consistent with the two energy states for dangling bond defects in this material. Recombination of photo-generated carriers in these states leads to significant degradation of cell performance for photon energies larger than the bandgap. At low temperature (−160°C), the difference spectrum indicates that although the defect states are still present, such degradation does not occur.     

Formation of porous silicon for large-area silicon solar cells: A new method

Luminescent porous silicon (PS) was prepared for the first time using a spraying set-up, which can diffuse in a homogeneous manner HF solutions, on textured or untextured (1 0 0) oriented monocrystalline silicon substrate. This new method allows us to apply PS onto the front-side surface of silicon solar cells, by supplying very fine HF drops. The front side of N⁺/P monocrystalline silicon solar cells may be treated for long periods without altering the front grid metallic contact. The monocrystalline silicon solar cells (N⁺/P, 78.5 cm²) which has undergone the HF-spraying were made with a very simple and low-cost method, allowing front-side Al contamination. A poor but expected 7.5% conversion efficiency was obtained under AM1 illumination. It was shown that under optimised HF concentration, HF-spraying time and flow HF-spraying rate, Al contamination favours the formation of a thin and homogeneous hydrogen-rich PS layer. It was found that under optimised HF-spraying conditions, the hydrogen-rich PS layer decreases the surface reflectivity up to 3% (i.e., increase light absorption), improves the short circuit current (I_sc), and the fill factor (FF) (i.e., decreases the series resistance), allowing to reach a 12.5% conversion efficiency. The dramatic improvement of the latter is discussed throughout the influence of HF concentration and spraying time on the I–V characteristics and on solar cells parameters. Despite the fact that the thin surfae PS layer acts as a good anti-reflection coating (ARC), it improves the spectral response of the cells, especially in the blue-side of the solar spectrum, where absorption becomes greater, owing to surface band gap widening and conversion of a part of UV and blue light into longer wavelengths (that are more suitable for conversion in a Si cell) throughout quantum confinement into the PS layer.