PVI Issue

Photovoltaics International Volume 17

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The seventeenth edition of Photovoltaics International applauds new markets emerging to plug the deployment gaps left by countries such as Spain, the Czech Republic and Italy. Profitless prosperity is the way Mark Osborne, Senior News Editor at PV-Tech.org characterises the PV manufacturing supply chain at the moment. In this issue the Fraunhofer ISE presents an overview of MWT technologies and calls on manufacturers to “quickly bring these techniques to industrialisation”. Additionally back contact cells and modules are featured extensively with valuable contributions from IMEC/ECN and the ISFH.

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In this issue...

PVTP Paper
Plant Performance, PV Tech Power Papers
On-site testing | PV project owners are becoming increasingly conscious of the need to understand how their plants are performing in the field. Ben Willis explores how mobile testing units are emerging as an important tool in the early detection of faulty module equipment.
PVI Paper
Market Watch, Photovoltaics International Papers
There has been rapid development of renewables in Europe in the last decade thanks to various support schemes. These are now part of the electricity reality in Europe and will continue driving the energy revolution in the coming years. In the PV sector in particular, feed-in tariffs (FiTs) have proved quite successful: at the beginning of 2010 more than 51GW of PV systems were connected to the grid in the EU, compared to less than 5GW five years earlier. This translates to 2% of the electricity demand being fulfilled by PV systems in the EU27. But all coins are double-sided: FiTs have proved to be too successful in several countries, inducing uncontrolled market development. The time has come to identify how these mechanisms – and in particular support schemes based on pure electricity injection – should evolve in order to manage a sustainable transition to competitiveness.
PVI Paper
Market Watch, Photovoltaics International Papers
In this paper an assessment is made of the impact of causal peer effects found in a recent paper by Bollinger and Gillingham, simulating solar adoption over many markets in the presence of a causal peer effect. Heterogeneity in both the peer effect and the baseline adoption rate is introduced and their interaction assessed. The nature of the heterogeneity and the size of the peer effect both have implications for the resulting diffusion process. Causal peer effects have implications for firms and policymakers, who have the ability to utilize social spillover effects in their marketing activities in order to increase and expedite solar adoption.
PVI Paper
Photovoltaics International Papers, Power Generation
Like all semiconductor photovoltaic devices, cadmium telluride (CdTe) modules have a characteristic response to temperature changes. This paper describes the effects of the temperature coefficient of power, using operational system data to quantify the First Solar CdTe technology energy-yield advantage over typical crystalline silicon technology in high-temperature conditions. This paper also describes the underlying mechanisms of initial stabilization and longterm degradation that influence module efficiency. The processes used to characterize and rate module power output, given these effects, are further discussed. First Solar’s significant experience in building and operating power plants in high-temperature conditions, along with associated system performance data and accelerated lab test data, is reviewed to substantiate the warranty considerations and long-term capability of power plants using CdTe PV modules.
PVI Paper
Photovoltaics International Papers, PV Modules
There has been recent interest in the use of thermoplastic encapsulant materials in photovoltaic modules to replace chemically cross-linked materials, for example ethylene-vinyl acetate. The related motivations include the desire to reduce lamination time or temperature, to use less moisture-permeable materials, and to use materials with better corrosion characteristics or improved electrical resistance. However, the use of any thermoplastic material in a hightemperature environment raises safety and performance concerns, as the standardized tests do not currently include exposure of the modules to temperatures in excess of 85°C, even though fielded modules may experience temperatures above 100°C. Eight pairs of crystalline silicon modules and eight pairs of glass/encapsulation/glass thin-film mock modules were constructed using different encapsulant materials, of which only two were designed to chemically cross-link. One module set with insulation on the back side was exposed outdoors in Arizona in the summer, and an identical set was exposed in environmental chambers. High-precision creep measurements (±20μm) and performance measurements indicated that, despite many of these polymeric materials being in the melt state during outdoor deployment, there was very little creep because of the high viscosity of the materials, the temperature heterogeneity across the modules, and the formation of chemical cross-links in many of the encapsulants as they aged. In the case of the crystalline silicon modules, the physical restraint of the backsheet reduced the creep further.
PVI Paper
Photovoltaics International Papers, PV Modules
Apart from aesthetics, the gain in electrical performance of back-contact solar cells and modules is particularly attractive compared to conventional PV modules. This major benefit results from getting rid of (the majority of ) the metallization at the front, and providing all the cell contacts at the back. An overview is presented here of the different concepts put forward by different institutes and companies around the world for such back-contact modules. The different types of state-of-the-art back-contact cell are first introduced, together with their corresponding contacting and interconnection schemes. Keeping in mind the reference module technology for two-side-contacted cells as a starting point, each module concept is then briefly discussed in terms of technology and level of maturity. Finally, the main technological differences are summarized.
PVI Paper
Photovoltaics International Papers, Thin Film
Thin-film PV modules are one of the most sustainable options for the generation of electricity, with low material consumption and short energy-payback times. Both of these factors are essential for paving the way towards a terawatt PV market. However, the cost-competitive production of PV modules has become extremely difficult, and module producers are facing huge challenges. A rapid technology transfer from research to industry is therefore required in order to introduce innovations for lower production costs and higher conversion efficiencies. At the Competence Centre Thin-Film- and Nanotechnology for Photovoltaics Berlin (PVcomB), founded by the Helmholtz-Zentrum Berlin (HZB) and the Technical University Berlin, two R&D lines for 30 x 30cm2 modules based on thin-film silicon and copper indium gallium (di)selenide (CIGS) respectively are operated. Robust baseline processes on a high efficiency level, combined with advanced process and device analytics, have been established as a basis for the introduction and development of further innovative technology steps, and their transfer to industry.
PVI Paper
Photovoltaics International Papers, Thin Film
Thin-film solar cells (TFSCs) still hold unlocked potential for achieving both high efficiency and low manufacturing costs. The formation of integrated interconnects is a useful way of maintaining high efficiency in small-scale solar cells by their connection in series to form a module. Laser scribing is widely used for scribing a-Si- and CdTe-based TFSCs to form interconnects. The optical properties of the ternary copper-indium-gallium (di)selenide (CIGS) compound are well suited to the solar spectrum, with the potential to achieve a high photoelectrical efficiency. However, since it is a thermally sensitive material, new approaches for the laser-scribing process are required, to eliminate any remaining heating effects. For flexible CIGS solar cells on non-transparent substrates (metal foils or polymer), the scribing process faces additional challenges. This is one reason why ultrashort laser pulses yield better results in terms of scribing quality and selectivity. The modelling of laser energy coupling and an extensive characterization of laser scribes allow approaches to be developed for laser scribing of CIGS solar cells on flexible polymer substrates. The measured high efficiency of the resulting high-speed laser-scribed, integrated CIGS mini-modules proved the capability of this approach.
PVI Paper
Cell Processing, Photovoltaics International Papers
The development of a cost-effective and industrially up-scalable process for p-type Cz monocrystalline silicon solar cells of the passivated emitter, rear locally diffused (PERL) type requires a careful trade-off between the potential benefits that novel process steps can deliver (in terms of improved efficiency and/or process control) and the additional costs involved. The approach chosen by Photovoltech is to limit as much as possible the number of PERL-specific process steps and to fine-tune the processes already in use for our standard full Al back-surface field (Al-BSF) technology in order to satisfy the more stringent requirements of PERL technology. Some of the results of this development are reported in this paper. In particular, the impact of different local BSF pastes on our proprietary extended laser ablation (ELA) rear-contacting technique is investigated, as well as the effect of the wafer resistivity and emitter diffusion/oxidation processes on cell performance. This paper also reports the results of large-batch experiments in which the capability of our optimized PERL process was tested against that of a standard full Al-BSF process. An average efficiency of 19.5% and a top efficiency of 19.7% were demonstrated.
PVI Paper
Cell Processing, Photovoltaics International Papers
The passivated emitter and rear cell (PERC) is considered to be the next generation of industrial-type screen-printed silicon solar cell. However, only a few deposition methods currently exist for rear passivation layers which meet both the high-throughput and low-cost requirements of the PV industry while demonstrating high-quality surface passivation properties. This paper presents an evaluation and the optimization of a novel deposition technique for AlOx passivation layers, applying an inductively coupled plasma (ICP) plasma-enhanced chemical vapour deposition (PECVD) process. High deposition rates of up to 5nm/s, as well as excellent surface recombination velocities below 10cm/s after firing, are possible using this ICP AlOx deposition process. When applied to PERC solar cells the ICP AlOx layer is capped with a PECVD SiNy layer. Independently confirmed conversion efficiencies of up to 20.1% are achieved for large-area 15.6cm x 15.6cm PERC solar cells with screen-printed metal contacts and ICP AlOx/SiNy rear side passivation on standard boron-doped Czochralski-grown silicon wafers. The internal quantum efficiency (IQE) reveals an effective rear surface recombination velocity Srear of 110±30cm/s and an internal rear reflectance Rb of 91±1%, which demonstrates the excellent rear surface passivation of the ICP AlOx/SiNy layer stack. Currently, the ICP AlOx deposition process is being transferred from the ISFH laboratory-type tool to the Singular production tool of Singulus Technologies in order to commercialize this novel deposition process during 2012.
PVI Paper
Cell Processing, Photovoltaics International Papers
This paper reviews metal wrap through (MWT) solar cell and module technology. As MWT solar cells and modules have received more and more attention in recent years, many highly efficient MWT cell types have been presented by research institutes and industry and are summarized herein. The MWT cell structure benefits from a reduced silver consumption compared with a conventional H-pattern cell, and its realization can be easily combined with novel metallization technologies such as dispensing or stencil printing. The introduction of a rear-surface passivation into the MWT structure is feasible with the high-performance MWT (HIP-MWT) concept developed at Fraunhofer ISE. The resulting fabrication sequence includes only one additional process step – laser drilling of vias – compared with an H-pattern passivated emitter and rear cell (PERC). Furthermore, the synergistic effects of MWT and PERC boost the conversion efficiency gain of MWT-PERC-type cells beyond the expected sum of what could be achieved individually from these two approaches. According to the calculations made by Fraunhofer ISE, conversion efficiencies of up to 21.5% (annealed) are feasible for p-type Cz silicon MWT-PERC cells. Because via metallization is one of the challenges in the fabrication of MWT cells, different via pastes are investigated with regard to their series resistance and contact behaviour. With cell-to-module losses in conversion efficiency of only 0.9% abs., both the interconnector-based MWT module technology and the conductive backsheet concept show promising results.
PVI Paper
Cell Processing, Photovoltaics International Papers
This paper presents ISFH’s recent developments and advances in the field of back-contacted silicon solar cells. The efficiency potential of back-contacted solar cells is very high; nevertheless, in industrial production, back-contacted solar cells are decidedly the minority. In the field of back-contacted solar cells, ISFH has developed several cell concepts and new processing techniques, such as laser ablation for silicon structuring, contact opening through passivation layers, and hole drilling for emitter-wrap-through (EWT) solar cells. The latest results are presented regarding ISFH’s work on back-junction back-contacted solar cells and EWT solar cells, as well as on back-contacted solar cells employing an amorphous/crystalline silicon heterojunction. Also discussed are the advances in high-throughput evaporation of aluminium as a low-cost option for the metallization of back-contacted solar cells. Finally, a novel, silver-free cell interconnection technique is presented, which is based on the direct laser welding of a highly conductive, low-cost Al foil, as a cell interconnect, onto the rear side of back-contacted solar cells.
PVI Paper
Materials, Photovoltaics International Papers
In slurry-based wafering of silicon bricks using multi-wire saws, the slurry is subject to significant evolution with time as the grits become worn and the silicon kerf accumulates. A good understanding of this evolution would allow wafer producers to make better-informed decisions on when and how to replenish slurry during wafering. This paper summarizes certain important slurry properties and presents some experimental results regarding their evolution. Sampling the slurry at the front and rear of silicon bricks during wafering has allowed the effect of a single pass through the sawing channel to be studied. The wear on the slurry grit is interpreted in terms of identifying what portion of the particle-size distribution plays the most critical role in wafering, and how this critical region changes as the slurry ages. It is found that in a relatively fresh slurry, the particles around the median size and slightly larger are the most active, while particles more than a few μm below the median play only a small part. As the slurry ages, the active region of the particle-size distribution becomes narrower, and shifts towards larger particles even though there are fewer such particles available. This leads to less slurry–brick interaction and poorer material removal properties.
PVI Paper
Fab & Facilities, Photovoltaics International Papers
The cleaning performance of three different fluorine-containing precursors – sulphur hexafluoride (SF6), nitrogen trifluoride (NF3) and molecular fluorine (F2) – is compared from theoretical, experimental and commercial points of view. Experiments were performed using an Oerlikon Solar KAI Gen 5 (1300mm x 1100mm) R&D platform. For the experiments with F2, an ‘on-site/on-demand’ generator from The Linde Group was installed at the Oerlikon Solar facility in Trübbach, Switzerland. The SF6-based cleaning process was found to be up to 75% less efficient than the corresponding NF3 or F2 process. A comparison between NF3 and F2 indicates that a significantly larger process window is available for reactor cleaning when F2 is used in place of NF3. This leads to both time and gas mass savings, improving productivity and bringing down the cost of ownership of the reactor cleaning process. As a direct consequence, Oerlikon Solar has decided to transfer the process to their production KAI MT plasma-enhanced chemical vapour deposition (PECVD) platforms.

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