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.
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.
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.
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.
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.
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 industrialisationâ€. Additionally back contact cells and modules are featured extensively with valuable contributions from IMEC/ECN and the ISFH.
The selective emitter (SE) concept features two different doping levels at the front surface of the cell. Both doping profiles are tailored individually to best suit their specific purposes, thus achieving both low contact resistance of the emitter electrode and low recombination in the emitter and at the Si/SiNx:H interface. This paper details the experience gained since the first tools for generating an SE structure were installed two years ago. The approach taken is discussed and a presentation given of the physical concept and properties of SE technology, along with the different aspects that have to be considered when integrating SE into an otherwise unchanged production facility.
Low-temperature thermal stresses in a manufactured photovoltaic module (PVM) based on crystalline silicon (Si), before the PVM is fastened into a metal frame, are assessed on the basis of a simple, analytical (mathematical), easy-to-use and physically meaningful predictive stress model. The PVM considered comprises the front glass, ethylene vinyl acetate (EVA) encapsulant (with silicon cells embedded into it) and a laminate backsheet. The stresses addressed include normal stresses that act in the cross sections of the constituent materials and determine their short- and long-term reliability, as well as the interfacial (shearing and peeling) stresses that affect the assembly’s ability to withstand delaminations. The interfacial stresses also determine the cohesive strength of the encapsulant material. The calculated data, based on the developed model, indicate that the induced stresses can be rather high, especially the peeling stress at the encapsulant-glass interface, so that the structural integrity of the module might be compromised, unless the appropriate design-for-reliability (DfR) measures, including stress prediction and accelerated stress testing, are taken. The authors are convinced that reliability assurance of a photovoltaic (PV) product cannot be delayed until it is manufactured – such an assurance should be considered and secured, first of all, at the design stage.
China has become the largest manufacturing base for crystalline silicon modules in the world, and is becoming increasingly reliant on a domestic supply base. This article discusses the emergence of local supply chains and the strategic responses of global suppliers to this domestic competition. It proceeds to review a set of conclusions from four case studies of formulated material supply within China that can apply to supply chain participants in the PV industry, concluding with some strategic considerations for suppliers on the cusp of entering the Chinese market.
India is a unique market. As part of an India-specific strategy on the part of the players, solar capital costs in India have significantly fallen in relation to the global average. This paper describes the trend for lower cost modules and services to be offered by module suppliers and EPC companies in order to capture the greatest share of the Indian market. In this context, more importance is being given to gaining a greater market share than earning a higher return.
