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New manufacturing processes

The ecological burden caused by solar cell production is getting lighter. New processing methods are saving energy and resources, the use of lasers and quick throughput processes enable a reduction in problematic chemicals, the cell production is becoming environmentally friendlier and more cost-effective, and the cells are becoming more powerful.

Newly developed, mass-production methods for high (< 20%) and very high efficiency (> 20%) crystalline silicon solar cells are being increasingly used in manufacturing. With the increasingly thinner wafers, the trend is towards more environmentally friendly inline processes. The researchers are working on one-sided processing stages, laser structuring, e.g. for selective emitters, more efficient passivation and metallization technologies, replacing Ag with Cu and converting from p- to n-substrates. The quality assurance is benefiting from new wafer tracking processes, quick inline measurement methods and integrated process checks. The current research and development work in publicly funded projects will be described here in accordance with the solar cell processing stages. (The institutes and project partners can be accessed online at www.bine.info).

Process chains and purification stages

With the aim of keeping breakage rates with thin wafers to less than 1%, the partners in the ReST project are working on minimising mechanical tension and other loads in the process stages, whereby they are modelling process stages as well as assessing and reducing mechanical and thermally induced tensions in solar cells.

Texture: Structuring wafer surfaces

By means of texturing, light-directing surface structures can be specifically created on crystalline silicon wafers. This enables solar cells to convert a greater proportion of the sunlight into electricity and the efficiency increases considerably. By means of roll-to-roll nanoimprint lithography, Fraunhofer ISE is working on generating defined structures for highly efficient solar cells as part of a throughput process that is attractive for industrial production.

Emitter

The emitter substantially determines the maximum efficiency of a c-Si solar cell. With standard n-emitter/p-base solar cells, the emitter ismanufactured by diffusing phosphorus into the substrate from a phosphorous oxychloride/ oxygen environment. The University of Konstanz is investigating the creation of phosphorus emitters from the gas phase. By optimising the doping profile, electrical losses that previously occurred in the emitter in the form of saturation currents are reduced and the cell efficiency is increased by 0.3% absolute. In a further stage, the efficiency of the cells is increased again by 0.5% absolute by introducing a selective emitter structure, in particular by etching back the cells. A laser doping process developed at the University of Stuttgart is conducted at room temperature under ambient conditions and can be integrated into an existing production line. This has managed to increase the solar cell efficiency by 0.2% absolute. Solar cells with a laser-doped emitter applied to the entire surface achieve an efficiency of 16.2%. Fraunhofer ISE has further developed the laser chemical processing (LCP) method, which is based on a liquid-jet laser that uses a reactive chemical among other things for local n- and p-doping. The ISFH is evaluating the potential of screenprinted aluminium-doped p+ emitters for use in highly efficient industry-level solar cells (stable efficiency above 20%) based on n-type Czochralski silicon.

Passivation layers

With increasingly thinner solar cells, the passivating and optical properties of aluminium conventionally screenprinted on the rear side of cells are no longer sufficient to achieve the desired increases in efficiency.

These require a better passivating and also optically reflective rear side. Fraunhofer ISE and Roth&Rau are therefore looking to apply a SiC passivation layer on the rear side and create local through-contacting with a plasma stage, as is used on the front side for depositing an antireflective layer. QCells is investigating new layers for passivating crystalline silicon solar cells for industrial production, whereby synergies between new passivating technologies and existing cell concepts are utilised, such as standard screen-printed cells.

The ISFH is researching the preparation of silicon interfaces for manufacturing a-Si:H heterojunction solar cells and is developing corresponding cells. The potential provided by aluminium oxide layers deposited with the atomic layer deposition process is also being evaluated. The process enables both thermal- and plasma-supported ALD, whereby stacked layers are also being investigated with PECVD silicon nitride and oxide. Thanks to its high, permanently negative space charge, aluminium oxide is particularly suited for passivating p-type silicon surfaces and boron emitters from n-type solar cells using the field effect.

Metallization

When screen-printing front-side contacts, relatively wide (100 μm) lines are created with relatively wide shading and only average electrical conductivity. With the trend towards larger and more efficient cells, this loss mechanism is becoming increasingly relevant. Fraunhofer ISE and the Gebr. Schmid company are therefore pursuing the idea of applying a very narrow and thin seed layer, which is then thickened using selective silver galvanisation. The conductivity of the applied material is substantially higher than that of the screen-printed material, so that a higher efficiency can be achieved thanks to the improved contact geometry and conductivity. As part of its “Lasercontacted rear sides of industrial, screen-printed solar cells” project, Fraunhofer ISE is developing the basis for the industrial implementation of solar cells with dielectrically passivated rear sides and laser-fired contacts (LFC). It has been able to present Czochralski solar cells using conventional screen-printed front contacts that achieve an efficiency of 18.5%, thus increasing the relative cell efficiency by at least 4% and achieving an economic benefit of more than 2%.

Fraunhofer ISE and Applied Materials are also researching and developing the basis for the industrial implementation of physical gas phase contact deposition technologies. With this high-rate deposition technology, it is planned to demonstrate cell efficiencies of more than 20% on monocrystalline Si wafers. These processes can be used for galvanically thickening both thicker rear-side layers as well as thinner front-side seed layers. To evaluate the inline capability, a pre-commercial prototype is being constructed at Fraunhofer ISE. The ISFH is developing a high-rate vacuum deposition process for inline use as an alternative to the screen-printing processes usually used today.

Edge isolation

The conventional, double-sided diffusion of the phosphorus emitter makes it necessary to interrupt the electrical short circuit between the rear contact and the front emitter. This used to be generally done via plasma etching in a batch oven. Edge isolation via lasers then became popular, which is now in turn being increasingly replaced with one-sided, wet-chemical rear etching of the emitter or with one-sided attachment of the emitter. A comprehensive system for rapid (1s) and damage-reducing edge isolation is now being developed. For this purpose, Manz Automation is contributing a new system for optically recognising data with improved position accuracy, Trumpf Laser is providing an ultrashort-pulse laser beam source, the Laser-Zentrum Hannover has developed the innovative laser edge isolation approach and Schott Solar has provided solar cells and is responsible for evaluating and testing the samples.

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