One of the most efficient ways of reducing the costs of photovoltaic power generation is to increase the efficiency of the solar cells used. Particularly in the case of silicon solar cells, in which the silicon material accounts for a significant proportion of the costs, targeted improvements in solar cell manufacture can yield major cost benefits. Optimizing industrial cell processes and developing new high efficiency cell structures are thus priorities in silicon photovoltaics.

There are two central laboratories available for this purpose at Fraunhofer ISE: On the one hand there is the clean room , in which new cell structures can be developed under ideal conditions, and on the other hand the PVTEC , in which industry-driven technology allows new concepts to be put efficiently into practice. Thanks to the combination of the two research platforms and a wide-ranging pool of characterization and simulation tools, Fraunhofer ISE is in the unique position worldwide of being able to assist its clients with all aspects of the entire chain, from basic development to production line implementation.
In addition to the standard screen printed cell, we work on a very wide and varied range of solar cell concepts:

• Solar cells with dielectric back surface passivation
  A large proportion of the efficiency losses in today's solar cells are due to recombination on the back surface of the cell. In particular in very thin cells, the intention was to replace the currently used aluminum back surface field with a dielectric passivation layer and dot contacts. Using such a cell structure and our patented laser-fired contact technology we were able to achieve the world record for multicrystalline silicon solar cells (see photo).
• Metal wrap through (MWT) solar cells (see photo)
  The cell concept behind an MWT solar cell is very similar to that of a standard industrial cell, except that the busbar is applied to the back surface of the solar cell. As a result, shading losses are reduced and entirely one-sided interconnection in the module is made possible.
• Emitter wrap through (EWT) solar cells
  In these cells, in contrast to MWT cells, all metal contacts are applied to the back surface. Transportation of the current from the front to the back surface is ensured by means of numerous phosphorus-diffused holes.
• Back junction, back contact solar cells
  In this variant of back contact cells, not only the metal contacts but also the collecting pn-junction is applied to the back surface of the cell. This concept has the greatest efficiency potential, but also places the greatest demands on material quality, for which reason n-type silicon is mainly used here.
• Solar cells made from n-type silicon
  In comparison with p-type silicon, n-type silicon exhibits greater tolerance of impurities. Higher efficiency levels may thus, in principle, be achieved with this material. In addition to the back contact cells mentioned above, we are also working on concepts involving an emitter of alloyed aluminum , boron-diffused silicon and amorphous silicon (heterojunction)

Silicon hetero solar cells have the potential for high efficiency levels. They combine the advantages of wafer technology, namely achieving high levels of efficiency by using high quality substrates, with those of thin-film technology, such as the use of large-area (inexpensive) deposition methods and low substrate temperatures.

The aim of work at Fraunhofer ISE is, on the one hand, to develop a process for this type of cell which is fit for industrial application as soon as possible and, on the other hand, to develop novel cell structures and so achieve the highest possible levels of efficiency .
In parallel to this, we are extending our basic understanding of the functioning of this cell type so that further optimization can be achieved.

Research priorities:

• Deposition of ultra-thin amorphous and microcrystalline layers, also on textured surfaces
• Passivation of surfaces by means of amorphous and microcrystalline layers
• Evaluation of new solar cell structures
• Use of higher frequencies during deposition
• Optimization and analysis of surface conditioning before deposition
• Inexpensive transparent conducting oxide layers (TCO) as front contact
• Optimization of TCO/semiconductor junction
• Low temperature metallization
• Alternative dopant sources

Laboratories and equipment:

• Chemistry benches for cleaning and wet chemical conditioning of substrates
• Industrial 3-chamber PECVD system for substrate sizes of up to 30 cm x 30 cm for the deposition of amorphous and microcrystalline silicon layers
• RIE plasma etcher for structuring
• TCO sputtering system
• Vapor deposition system
• Spectrometer for measuring transmittance, reflectance and absorbance
• Ellipsometer for spatially resolved measurement of refractive index and layer thickness
• Measurement setup for the determination of light and dark conductivities at different temperatures
• TLM measurement setup
• Measurement setup for the determination of the spectral response for single and tandem junction cells
• IV curve measurement setup
• Luminescence measurement setup
• Setup for light induced degradation

The physical efficiency limit for silicon solar cells is 29%. The aim of our research is to come technologically as close as possible to this physical limit. This involves investigating numerous different concepts in order to minimize loss mechanisms to the greatest possible extent. Important points include, on the one hand, maximizing silicon light capture (e.g. by using diffractive structures on the rear surface of the solar cell) and, on the other hand, reducing surface recombination.

However, the aim of the work is not only to achieve the highest levels of efficiency, but also to develop future technologies. In addition to conventional technologies such as diffusion and oxidation, more recent technologies are also being investigated. Examples of these are ALD (atomic layer deposition) for depositing ultra-thin passivation layers (e.g. Al ₂ O ₃ ) and PECVD (plasma-enhanced chemical vapor deposition) for depositing amorphous or microcrystalline silicon layers.

Research priorities
At Fraunhofer ISE, work focuses on:

• Analysis, evaluation and assessment of novel silicon solar cell concepts
• Application of new technologies for manufacturing ultra-efficient silicon solar cells
• Manufacture of specially adapted reference solar cells

In the quest for further efficiency increase of solar cells at reduced costs, new cell concepts are getting strongly into focus. Such concepts are often complex and exhibit a large number of processing steps. The numerical modelling of the production processes and the complete solar cell provide here the decisive key to reduce the time to market, as they can dramatically reduce the number of required experimental tests.

Device simulation

With the solar cell simulator “PC1D”, the determination of the performance-limiting factors of a solar cell are often possible simply by performing a simultaneous adjustment to the quantum yield and the current-voltage characteristic. Complex solar cell structures, however, are no longer possible to be described one-dimensionally or even analytically.

Multidimensional properties like point contacts or “wrap-through” emitter can only be precisely investigated with two- and three-dimensional simulation tools. For this purpose, we use, and have developed several years of experience with the semiconductor simulation environment “Sentaurus TCAD” to investigate electronic properties. The use of this complex simulation program makes it especially possible to correctly model and analyze highly doped surface layers. For the electrical modelling, the results of a three-dimensional optical simulation usually serve as input parameters. A constant expansion and improvement of the models in close cooperation with tech­nologists at the institute ensure that novel solar cell concepts can also be described and subjected to a detailed loss analysis

Process simulation

The simulation of diffusion of dopants is of great interest from a technological point of view. For this purpose, at Fraunhofer ISE, models have been developed with the help of “Sentaurus TCAD”. Finding processing conditions for desired dopant depth profiles can be greatly speeded up with the simulations, even for multi-step diffusion conditions. The diffusion and gettering behaviour of metallic impurities, from crystallization up to the finished solar cell, are numerically investigated. An additional important requirement here is the knowledge about the material properties , which is constantly being further improved with the help of the corresponding working groups at Fraunhofer ISE. The models can be combined with each other, as well as with the device simulations.

The simulation activities at Fraunhofer ISE additionally include specific solar cell processes, such as doping with laser-chemical-processing. Both the laser-induced melting and the dopant diffusion in the melt can be simulated for this process, allowing the optimal laser parameters to be found.

Major Fields of Research

■        Analyse and evaluation of novel solar cell concepts

■        Predictive modelling of dopant profiles

■        Simulation and optimization of gettering processes

■        Optimization of laser doping processes

■        Improvement of the physical understanding of solar cell behaviour and identification of the limiting factors

Our main objective is to develop new high efficiency cell structures and to transfer them from the laboratory to industrial fabrication. High levels of efficiency are becoming ever more important, as the silicon wafer accounts for a large proportion of the costs of a finished module and the best possible use should therefore be made of it.
Our work is based on a detailed understanding of the physics of solar cells (multi-dimensional component simulation) (LINK: http://www.ise.fraunhofer.de/geschaeftsfelder-und-marktbereiche/silicium-photovoltaik/industrielle-und-neuartige-solarzellen/bauelement-simulation) and the ability to manufacture a wide range of different high efficiency cell types. We have therefore been able to manufacture solar cells with very high levels of efficiency, such as the world record cell made from multicrystalline silicon on n-type silicon.
Research priorities

At Fraunhofer ISE, work focuses on:

• Development and optimization of high efficiency solar cell structures
• Development, evaluation and optimization of individual processes

Laboratories and equipment
The centerpiece of our laboratory facilities is a clean room comprising a clean room area of 330 m² and a gray room area of 200 m², equipped with the following:

• 3 diffusion furnaces (LINK: http://www.ise.fraunhofer.de/geschaeftsfelder-und-marktbereiche/silicium-photovoltaik/produktionsanlagen-und-prozessentwicklung/diffusion-und-oxidation) (oxidation (wet and dry), phosphorus and boron diffusion)
• Wet chemistry benches (etching and cleaning)
• Automated and manual photoresist processing modules (coating and development)
• Automated photoresist exposure system (mask aligner) http://www.ise.fraunhofer.de/geschaeftsfelder-und-marktbereiche/silicium-photovoltaik/industrielle-und-neuartige-solarzellen/photovoltaik-technologie-evaluations-center-pv-tec
• Atomic layer deposition (PECVD) for deposition of e.g. Al ₂ O ₃
• PECVD Cluster system for manufacturing high efficiency a-Si/c-Si hetero junction solar cells (LINK: http://www.ise.fraunhofer.de/geschaeftsfelder-und-marktbereiche/silicium-photovoltaik/industrielle-und-neuartige-solarzellen/si-hoechsteffizienz-solarzellen)
• 3 metal vapor deposition units (LINK: http://www.ise.fraunhofer.de/geschaeftsfelder-und-marktbereiche/silicium-photovoltaik/produktionsanlagen-und-prozessentwicklung/pvd-verfahren) (Al, Ti, Pd, Ag, electron beam gun and thermal evaporation)
• 1 annealing furnace
• Semi-automatic electroplating unit

We can process any wafer without exception up to a size of 6" (round) or 150 x 150 mm ² (square). We also have the capability to thin wafers down to a thickness of 20 µm with our automatic grinding and polishing unit.

Our clean room is the perfect working environment for developing new solar cell structures and processes. In the next step, these new cell structures are transferred to the PV-TEC industrial production line (LINK: http://www.ise.fraunhofer.de/geschaeftsfelder-und-marktbereiche/silicium-photovoltaik/industrielle-und-neuartige-solarzellen/photovoltaik-technologie-evaluations-center-pv-tec). Here, the emphasis is on simplifying processes and transfer to industrial solar cell production.

The Photovoltaic Technology Evaluation Center PV-TEC at Fraunhofer ISE was established with the assistance of the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) in order to close the gap between laboratory research and industrial application. This major laboratory has access to the latest processing and characterization equipment for developing silicon solar cells.

PV-TEC focuses on the following key topics relating to production technology and metrology for crystalline silicon solar cells:

• Evaluation and development of manufacturing processes and process technology components
• Development and production of advanced industrial solar cell structures
• Characterization and development of materials and solar cells
• Training and professional development in PV technology
• Process transfer with on-site assistance
• Economic cost studies

PV-TEC serves companies right along the PV value chain:

• Solar cell manufacturers / module manufacturers
• Equipment manufacturers
• Material manufacturers (silicon and process materials)

Standard process

At Fraunhofer ISE, we work on developing the standard solar cell manufacturing process and incorporating improvements, such as selective emitters or fine-line contacts. This ensures that a high-quality, continuously optimized baseline process is available at PV-TEC which acts as a reference point for process development. Thanks to standardization of wafer sizes, PV-TEC is also in a position to complete the processing of partially processed solar cells. 125 x 125 mm ² , 156 x 156 mm ² and 210 x 210 mm ² sizes can be handled by all process systems. As a result, semi-finished products can be manufactured for material testing or individual manufacturing steps can be purposefully exchanged or optimized. Please contact us regarding processing of different wafer sizes. We have various adapters for carrier systems or can also carry out short production runs with appropriate systems.

Passivated rear surfaces

Another major step towards higher efficiency is the passivation of solar cell rear surfaces. PV-TEC can offer various cleaning and passivation processes for the back surface of the solar cell. Thermal oxides (dry, wet) as well as PECVD layers and multilayer systems (e.g. aluminum oxide/silicon nitride) may be used for this purpose. Contacting through this rear passivation layer may be effected by means of LFCs (laser fired contacts) punched through the screen printed layers, or by means of PVD deposited layers.

Passivation is of particular significance, especially for thin wafers (see picture). These solar cells achieve efficiency levels distinctly above 18% (stabilized) on industrial Cz silicon. It has proved possible to achieve efficiencies distinctly above 20% with this technology in a clean room environment.

Solar cell concepts / Back surface contact solar cells

At PV-TEC, we are developing MWT (metal wrap through) and EWT (emitter wrap through) solar cells and solar cells with a back surface emitter.

Methods and Equipment