The industrial solar cells group is working on the improvement of industrial manufacturing processes for mono- and multicrystalline silicon solar cells. For this purpose, new concepts and technologies are investigated and, if regarded as technically feasible and economically promising, developed to pilot line level in order to transfer them to industrial production with our partners. The pursued concepts are chosen by their short to medium term applicability and the potential of reducing the cost of photovoltaic electricity. This application-oriented work is supplemented by research activities that aim on a deeper understanding of the devices and individual process steps.
Standard Solar Cells
Over the recent years, we were able to establish a reliable, industry-related standard process with which we achieve solar cell efficiencies above 19% using large area monocrystalline substrates. On multicrystalline solar cells efficiencies of over 17% were attained. This process uses only screen printing technology for the metallization and can be extended by industrially feasible processes in order to improve the solar cell performance.
Selective Emitter (SE)
Reducing the front side losses of a solar cell can be accomplished by implementing a selective emitter. In this concept, only the metallized emitter region is heavily doped in order to ensure a low contact resistance, while the illuminated surface is weakly doped to reduce the recombination losses. A patented etch-back process for the production of selective emitter solar cells was developed in the industrial solar cells group which is currently being marketed by the equipment manufacturer Gebr. Schmid GmbH.
Passivated Rear Side
In order to reduce the recombination losses at the cell back side, the full area aluminum back-surface-field (BSF) of a standard solar cell can be replaced by a dielectric passivation. For contact formation this layer is firstly opened locally, e.g. by laser ablation or masked etching. Afterwards aluminum paste is screen-printed over the full rear surface. During the contact firing step, a local BSF is formed in afore opened areas, thus ensuring a field effect passivation of the highly recombination-active surface.
For surface passivation by aluminum oxide, an Oxford atomic layer deposition (ALD) tool is available. As an inexpensive alternative, the deposition by atmospheric pressure chemical vapor deposition (APCVD) is also subject of the current research activities in the industrial solar cells group.
n-Type Solar Cells
The minority charge carrier recombination in the base constitutes one of the main power losses of standard silicon solar cells. The successful optimization of the individual process steps in the recent years increases the share of the base recombination to the total losses, furthermore most future cell concepts require a higher quality of the base substrate. Up to now the vast majority of industrially manufactured solar cell uses p-type silicon which typically features a minority carrier lifetime of below 1 ms. Especially in boron doped Czochralski (Cz) silicon wafers recombination-active boron-oxygen complexes form under illumination which lead to light-induced degradation of the minority carrier lifetime.
Significantly higher minority carrier lifetimes can be achieved by using phosphorus-doped n-type Cz silicon which has a lower sensitivity for metallic impurities and is only marginally affected by light-induced degradation due to the very low boron content.
The so-called PHOSTOP cell concept is the easiest way produce a solar cell based on n-type silicon. The cell process widely corresponds to the standard p-type solar cell process. The front surface of the n-type base is passivated by a diffused phosphorus front surface field (FSF) while the alloyed aluminum profile creates the n/p junction on the rear side where the charge carriers are separated. The reduced recombination current in the base leads not only to an increase of the open circuit voltage but also the n-type base contributes to the lateral conductivity between the front contact fingers thus increasing the fill factor. In combination with an improved front side passivation, efficiencies of up to 19.5 % were achieved with this cell concept on large area Cz silicon at the University of Konstanz.
Bifacial Solar Cells
Bifacial solar cells have a passivated rear side which is contacted by a grid structure similar to the front side. Since only the contacted area is covered with metal, such a solar cell can absorb light from both sides. Presuming an adequate installation, up to 20% more electricity can be produced over the day. E.g. a vertical mounting with east-west orientation of the surfaces leads to a redistribution of power generation to the morning and afternoon. This can help to compensate the typical midday peak of PV electricity fed into the power grid.
Besides the passivation of the rear surface, a bifacial solar cell usually features a doping profile below with the inverse polarity of the front side doping. This improves not only the lateral conductivity but also reduces the contact resistance and contributes to the electric field effect passivation of the surface. Bifacial solar cells can be produced from p- or n-type silicon. Depending on the polarity of the base and surface doping, the emitter is located on the front or the rear side of the solar cell.
Current research activities of the industrial solar cells group on bifacial solar cells focus on the cost effective and desirably simultaneous generation of the doping profiles as well as the reduction of recombination and series resistance losses associated with contacting boron-doped surfaces.