Polycrystalline Silicon-Film™ Thin-film Solar Cells: Advanced Products

Thin-film polycrystalline silicon has the potential to achieve the cost reduction and performance improvement necessary for large-scale electricity markets. Reduced cost is achieved by capitalizing on the benefits of thin films grown on low-cost, large-area substrates. Improved efficiency is realized, in spite of reduced material quality, by incorporating enhanced optical absorption and back-surface passivation. The cornerstone of AstroPower's thin-film solar cell technology is the Silicon-Film™ process: a method for the manufacture of solar cell-quality, polycrystalline films of silicon on a variety of low-cost, supporting substrates. Three thin-film solar cell designs, based on this technology, are currently under development. This paper presents the key design features of these three products and briefly reviews the current status of the development of the key technologies that comprise the advanced thin-film solar cell products.     

Technology and economics of three advanced silicon solar cells

This paper describes the structure, processing and economics of three difference silicon solar cell technologies developed at the University of New South Wales. The first is the high-efficiency PERL (Passivated Emitter, Rear Locally-diffused) cell technology which has produced the highest ever efficiency silicon solar cells and photovoltaic modules. These cells have a sophisticated cell structure and require high-quality materials and advanced microelectronic-quality technology in their processing. The second is the buried contact solar cell technology, presently the most successfully commercialized solar cell technology developed over the last 15 years. This structure retains the key features of the PERL cell required for high efficiency, while reducing material and processing costs to those comparable to previous commercial screen-printing silicon solar cell approaches. The third technology holds great promise for the future. This multilayer cell technology is based on combining features of the buried contact cell technology with a multijunction thin-film approach to produce thin-film polycrystalline silicon solar cells of potentially high efficiency on low-cost substrates such as glass. © 1998 John Wiley & Sons, Ltd.     

A Novel Semiconductor CIGS Photovoltaic Material and Thin-Film ED Technology

In order to achieve low-cost high-efficiency thin-film solar cells, a novel Semiconductor Photovoltaic (PV) active material CuIn1-xGaxSe2 (CIGS) and thin-film Electro-Deposition (ED) technology is explored. Firstly,the PV materials and technologies is investigated, then the detailed experimental processes of CIGS/Mo/glass structure by using the novel ED technology and the results are reported. These results shows that high quality CIGS polycrystalline thin-films can be obtained by the ED method, in which the polycrystalline CIGS is definitely identified by the (112), (204, 220) characteristic peaks of the tetragonal structure, the continuous CIGS thin-film layers with particle average size of about 2μm of length and around 1.6μm of thickness. The thickness and solargrade quality of CIGS thin-films can be produced with good repeatability. Discussion and analysis on the ED technique, CIGS energy band and sodium (Na) impurity properties, were also performed. The alloy CIGS exhibits not only increasing band-gap with increasing x, but also a change in material properties that is relevant to the device operation. The beneficial impurity Na originating from the low-cost soda-lime glass substrate becomes one prerequisite for high quality CIGS films. These novel material and technology are very useful for low-cost high-efficiency thin-film solar cells and other devices.     

Ni/Cu electroplating,a worthwhile alternative to use instead of Ag screen-printed front side metallization of conventional solar cells

For commercial purposes, it is necessary to manufacture high-efficiency and low-cost solar cells using simple processes. The front contact formation is one of the most critical steps in solar cell processing. Although silver paste screen-printed solar cells are the most widespread on the photovoltaic market, their efficiency is strongly limited as a result of shading and resistive losses, or more precisely the high contact resistance. Cu metallization for crystalline Si solar cells has attracted much attention as an alternative to the screen-printing technology. The low-cost Ni/Cu metal contact is regarded as the next generation of metallization processes to still improve the efficiency with a low specific contact resistance; it is formed using low-cost electroless plating and electroplating. A diffusion barrier should be placed between Cu and Si, to prevent Cu diffusion. Ni is shown to be an adequate barrier to Cu diffusion. For these reasons, geometry optimization of metal contacts of the front face, deposited by commercial processes, is investigated in this paper, in order to improve the spectral response of conventional multicrystalline mc-Si silicon solar cells. Their efficiency variation is analyzed as a function of changes in cell parameters (finger separation distance, height and width of finger, sheet resistance emitter...) using simulation programs in MATLAB, using contours to represent the efficiency evolution in terms of two variables. Efficiency gain of more than 0.7% has been achieved in this study. The simulation results were then compared with experimental data in order to be validated.     

Survey of material options and issues for thin film silicon solar cells

The high production cost of thick high-efficiency crystalline silicon solar cells inhibits widespread application of photovoltaic devices whereas the most developed of thin film cell technologies, that based on amorphous silicon, suffers inherent instability and low efficiency. Crystalline thin-film silicon solar cells offer the potential for a long-term solution for low cost but high-efficiency modules for most applications. This paper reviews the progress in thin-film silicon solar cell development over the last two decades, including progress in thin-film crystal growth, device fabrication, novel cell design, new material development, light trapping and both bulk and surface passivation. Quite promising results have been obtained for both large-grain (>100 μm) polycrystalline silicon material and the recently developed microcrystalline silicon materials. A novel multijunction solar cell design provides a new approach to achieving high-efficiency solar cells from very modest quality and hence low-cost material. Light trapping is essential for high performance from thin-film silicon solar cells. This can be realized by incorporating an appropriate texture on the substrate surface. Both bulk and surface passivation is also important to ensure that the photogenerated carriers can be collected effectively within the thin-film device. © 1998 John Wiley & Sons, Ltd.     

喷墨印制PCB用新型纳米银导电油墨的研发现状及趋势

喷墨打印加成法制造电路板,可明显改善传统减成法效率低、成本高、污染重等缺点,因而受到业内广泛关注。该方法所使用的新型功能材料纳米银导电油墨,因具有烧结温度低、导电性能好等优点,近年来成为全球电子油墨行业的研发热点之一。文章详细介绍了纳米银油墨在印制电子产品电路制作领域的应用,并对喷印纳米金属油墨的技术要求、纳米银油墨的特点及主要不足进行了评述,重点展示了全球纳米银油墨的产品开发动态,同时对新型喷印纳米金属油墨的制备与研发提出了一些建议。     

Recent progress in thin-film cadmium telluride solar cells

Cadmium telluride (CdTe) with a room-temperature bandgap energy of 1.45 eV has been shown to be the most promising low-cost, thin-film photovoltaic material for terrestrial applications. Significant progress has been made during the past several years, and thin-film CdTe solar cells of > 1 cm² area with conversion efficiencies higher than 12% have been prepared by several techniques. Thin-film CdTe photovoltaic modules with 10% efficiency have also been produced. They are of the heterojuntion configuration using a transparent conducting semiconductor (TCS) as the window and p-CdTe as the absorber. In this paper, the potential window materials for thin-film CdTe solar cells are discussed. Thus far, cadmium sulphide (CdS) with a bandgap energy of 2.42 eV at room temperature has been found to be best suited for efficient CdTe solar cells. the deposition techniques for p-CdTe films capable of producing efficient solar cells, including close-spaced sublimation (CSS), electrodeposition, screen printing and spraying, are briefly reviewed, and the characteristics of the resulting solar cells are discussed. It is concluded that the efficiency of thin-film CdTe solar cells can be increased to 18-19% in the near-term, leading to 15-16.5% efficient modules.     

Thin-film solar cells: A unified analysis of their potential

The development and deployment of low-cost thin-film solar cells for the direct conversion of sunlight to electricity can be accelerated by the utilization of loss minimization and cost minimization methodologies. The solar cell is separated into its five constituent layers to provide a common basis for the development of these methodologies. Photovoltaic theory, materials science, and loss analysis are combined to develop the loss minimization methodology which can be used to systematically improve and optimize performance of any solar-cell material system. The techniques of the chemical process industry have been applied to achieve cost minimization. The loss-and cost-minimization methodologies have been combined into a generalized procedure for the accelerated development of all low-cost thin-film photovoltaic material systems.     

Thick-film metallization for solar cell applications

The use of an integral printing technique for the fabrication of silicon solar cells is attractive due to its throughput rate, materials utilization, and modular, automatable design. The transfer of this technology from single crystal to semicrystalline silicon requires a significant amount of process optimization. Processing parameters found to be critical include the optimum glass frit content in the silver-based inks, the silver ink firing temperature, and the formation of the back-surface field using screen-printed aluminum layers. Open-circuit voltages as high as 617 mV have been achieved using a novel BSF approach on 4-in wafers. Important mechanisms controlling ink contact resistance, ink sheet resistivity, and ohmic contact on and silicon materials are discussed in this paper. The solar cell stability is a function of the glass frit and the firing temperature of the silver-based inks. Finally, a simple economic analysis, based on the IPEG technique, indicates that screen printing is a cost-effective option when the cell manufacturing is done on a large scale.     

光诱导镀改善太阳电池填充因子的研究

本文采用常规的太阳电池工艺制备了一批单晶硅电池片,退火后得到了填充因子迥异的结果。比较了光诱导镀前后电池参数尤其是串联电阻,并分析了光诱导镀提升填充因子的机理。利用扫描电镜（SEM）观察了去除体银后的微观结构,证明了差填充因子太阳电池在光诱导镀后填充因子的提高得益于烧结中形成的银晶粒的充分利用。文中还提出了将光诱导镀应用于接触电阻较大的电池,如纳米柱电池和径向结电池的可能性。     

Organic integrated circuits using room-temperature sintered silver nanoparticles as printed electrodes

Organic integrated circuits based on organic thin-film transistor (TFT) devices are fabricated with solution-based electrodes by using dense inks of silver nanoparticles, which can be sintered at room-temperature. The TFT devices fabricated at a sintering temperature of 30 °C exhibit good electrical characteristics. There is a strong relation between the sintering temperature of silver nanoparticle inks and transistor characteristics. A work function of silver electrodes can be controlled by changing the sintering temperature of silver nanoparticle inks, thereby threshold voltage of fabricated TFT devices are shifted accordingly. Fabricated pseudo-CMOS inverter circuits are successfully operated at low voltage with small hysteresis, and large gains are obtained. These results suggest that printed organic TFT devices fabricated with a low-temperature process enable large-area and low-cost integrated circuits by using these techniques in future applications.     

Towards thinner and low bowing silicon solar cells: form the boron and aluminum co‐doped back surface field with thinner metallization film

Using thinner wafers can largely reduce the cost of silicon solar cells. One obstacle of using thinner wafers is that few methods can provide good dopant concentration for the back surface field (BSF) and good ohmic contact while generated only in low bowing. In this paper, we have demonstrated the screening–printing B and Al (B/Al) mixture metallization film technique, making use of the screen‐printing technique and the higher solubility of B in silicon to form a B/Al‐BSF. This technique can raise the carrier concentration in the BSF by more than one order of magnitude and reduce the back surface recombination at a low firing temperature (≤800 °C). We have also shown that through the new technique, the metallization paste thickness at the rear could be reduced largely, which however did not degrade the solar cell efficiency. All these efforts are aiming for pushing forward the application of thinner wafers. Copyright © 2011 John Wiley & Sons, Ltd.     

Silicon solar cells based on all‐laser‐transferred contacts

Crystalline silicon solar cells based on all‐laser‐transferred contacts (ALTC) have been fabricated with both front and rear metallization achieved through laser induced forward transferring. Both the front and rear contacts were laser‐transferred from a glass slide coated with a metal layer to the silicon substrate already processed with emitter formation, surface passivation, and antireflection coating. Ohmic contacts were achieved after this laser transferring. The ALTC solar cells were fabricated on chemically textured p‐type Cz silicon wafers. An initial conversion efficiency of over 15% was achieved on a simple cell structure with full‐area emitter. Further improvements are expected with optimized laser transferring conditions, front grid pattern design, and surface passivation. The ALTC process demonstrates the advantage of laser processing in simplifying the solar cell fabrication by a one‐step metal transferring and firing process. Copyright © 2013 John Wiley & Sons, Ltd.     

A transition solvent strategy to print polymer:fullerene films using halogen-free solvents for solar cell applications

Inkjet printing is a mask-less non-contact deposition technique that is potentially suited for prototyping and manufacturing of thin-film polymer organic semiconductor devices from digital images. However new strategies are needed to achieve films with good macromorphology (i.e., high-fidelity footprint and uniform cross-section) and nanomorphology on unstructured substrates using a conventional ink-jet. Here we report a new transition solvent strategy to provide the desired film macromorphology and ultrafine nanomorphology in regioregular poly(3-hexylthiophene):phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) model films, without using chlorinated solvents. This strategy employs a good volatile solvent in combination with a miscible poor solvent that is much less volatile, which is the reverse of the usual low−high boiling-point solvent method. The good solvent suppresses premature aggregation in the ink head. Its removal by evaporation on the substrate leaves the poor solvent that triggers early π-stacking ordering and/or gelation of the polymer matrix that immobilizes the printed fluid on the substrate, suppressing both contact-line depinning and evaporation-induced solvent flow effects. The resultant donor–acceptor nanomorphology is further improved by vacuum drying at an optimal rate that avoids bubble formation. We have systematically characterized P3HT:PCBM films deposited with different solvents and platen temperatures to identify key macro- and nano-morphology determining processes. High-performance printed P3HT:PCBM solar cells were realized. These findings are applicable also to other printing and coating techniques based on low-viscosity inks.     

Improved Conversion Efficiency of GaN/InGaN Thin-Film Solar Cells

In this letter, we report on the fabrication and photovoltaic characteristics of p-i-n GaN/InGaN thin-film solar cells. The thin-film solar cells were fabricated by removing sapphire using a laser lift-off technique and, then, transferring the remaining p-i-n structure onto a Ti/Ag mirror-coated Si substrate via wafer bonding. The mirror structure is helpful to enhance light absorption for a solar cell with a thin absorption layer. After the thin-film process for a conventional sapphire-based p-i-n solar cell, the device exhibits an enhancement factor of 57.6% in current density and an increment in conversion efficiency from 0.55% to 0.80%. The physical origin for the photocurrent enhancement in the thin-film solar cell is related to multireflection of light by the mirror structure.      

Add‐on laser tailored selective emitter solar cells

An elegant laser tailoring add‐on process for silicon solar cells, leading to selectively doped emitters increases their efficiency η by Δη = 0.5% absolute. Our patented, scanned laser doping add‐on process locally increases the doping under the front side metallization, thus allowing for shallow doping and less Auger recombination between the contacts. The selective laser add‐on process modifies the emitter profile from a shallow error‐function type to Gaussian type and enables excellent contact formation by screen printing, normally difficult to achieve for shallow diffused emitters. The significantly deeper doping profile of the laser irradiated samples widens the process window for the firing of screen printed contacts and avoids metal spiking through the pn‐junction. Copyright © 2010 John Wiley & Sons, Ltd.     

Study of solar cell fabrication using an electrostatic thick-filmprinting method

A novel thick-film circuit printing technique which is based on the electrostatic principle known as noncontact electrostatic thick-film printing was developed for the metallization of edge-defined film-fed growth (EFG) solar cells. The conventional thick-film solar cell inks were modified by adding 10-20% terpineol solvent. The effects of ink viscosity, applied voltages, nozzle diameter, and nozzle-to-substrate distance on line definition and ink-flow rate were investigated. A simple theoretical model was derived for the electrostatic ink ejection. The minimum line width obtained was 3 mm. Multilayer printing was able to be used to raise the line film thickness. The maximum line width obtained was about 20-30 mm for a single run. The system is now completely computercontrolled and capable of printing films onto solar cell substrates reliably, with a high degree of accuracy. Multiple-layer prints can be made with food layer-to-layer registration     

Lightweight tandem GaAs/CuInSe2 solar cells

High-efficiency, ultralightweight, mechanically stacked 4-cm² thin-film tandem solar cells are discussed. The tandem stack consists of a single-crystal, thin-film Ga(Al)As cell fabricated by the cleavage of lateral epitaxy for transfer (CLEFT) process and adhesively bonded to the top of a CdZnS/CuInSe₂ polycrystalline thin-film cell deposited on glass. Maximum tandem efficiency in a four-terminal configuration of 21.6% AM0 have been demonstrated. This represents the highest thin-film cell efficiency reported to date. Individual subcells with efficiencies of 19.5% for CLEFT GaAs and 3.0% for CuInSe₂ have also been achieved. Cell specific power as high as 600 W/kg has been achieved with a 4-cm² cell weight of 188 mg without coverglass, at an efficiency of 20.8% AM0     

Recent progress in silicon-based solid-state solar cells

Silicon (Si)-based solar cells constitute about 90% of the photovoltaic (PV) market, and a drastic reduction in module cost and significant improvement in PV performance have been observed since its first inception in 1941. This article aims to present the comprehensive review of prominent advancements enacted in Si solar cells after the year 2000. Monocrystalline Si solar cell has been the matured technology with the record efficiency (η) of 26.6% achieved so far. As the drive to push η around 30% Schokley–Quiesser limit is foreseeable in the near future, PV community is actively striving to fabricate efficient yet cost-effective devices. Polycrystalline Si solar cells contain small-sized grains, and efforts are underway to enhance the η beyond 21.9% by controlling the recombination at grain boundaries, optimising the passivated interfaces and deposition process. Thin-film amorphous Si technology proffers low-cost fabrication process and η of 13.6% has been recorded for a multijunction solar cell. Employment of sophisticated nanowire-based light trapping schemes and dopant-free carrier-selective layers along with the development of hybrid solar cells of organic and Si materials are among the emerging research trends for Si solar cells.     

Zone-melting recrystallization of silicon thin films for solar cell application

Zone-melting recrystallization (ZMR) has been applied successfully to fabricate a thin-film silicon solar cell with high conversion efficiency that also has the potential to lower the material cost. It is found that seeding from an Si substrate during ZMR is not necessary for high-quality thin-film Si with a low defect density and the dominant (100) crystallographic orientation. This feature is very important because one can separate the thin-film Si from the substrate in order to obtain a flexible solar cell and the substrate can be recycled. Lowering the scanning speed of the upper movable carbon strip heater has proved to be most effective for high-quality crystal. In order to realize thin-film Si solar cells, a 60-μm thick Si active layer is deposited by chemical vapour deposition on recrystallized Si film. Pyramidal shape formation at the surface for light confinement by using (100) orientation and low-energy H⁺ ion irradiation for the passivation of crystal defects has been applied to the fabrication of thin-film Si solar cells and we achieved high conversion efficiencies of more than 14% for a 10 × 10 cm² cell and 16% for a 2 × 2 cm² cell.