Your daily dose of Photovoltaic Technology Developments and Solar News

By Hanne Degans, Izabela Kuzma, Guy Beaucarne & J. Poortmans, IMEC, Belgium

ABSTRACT
Thin-film silicon solar cells are a potentially low-cost alternative to solar cells based on bulk silicon that are commonly used in the industry at the present time. However, a major drawback of the current epitaxial semi-industrial screenprinted cells is that they only achieve an efficiency of about 11-12%. By upgrading their efficiency, this kind of solar cell would become more attractive to the photovoltaic industry. The optimization of the front surface texture by dry texturing based on a fluorine plasma and the introduction of an intermediate porous silicon reflector at the epi/ubstrate interface (multiple Bragg reflector) has proven to result in an efficiency boost up to about 14%.

By Dirk Ochs, HÜTTINGER Elektronik GmbH + Co KG, Freiburg, Germany

ABSTRACT
The rapidly-growing photovoltaic market has placed a strong demand on manufacturers to decrease solar cell production costs. For thin-film solar cells, this can be achieved by increasing substrate sizes to achieve a better productivity and by adding more advanced layer stack systems to enhance the solar cell’s efficiency. Nearly all required layers of the prominent thin-film-based solar cell types (a-Si/μc-Si, CdTe and CI(G)S) can be deposited by using plasma processes. On the one hand, plasma-enhanced chemical vapor deposition (PECVD) is used for the deposition of a-Si and μc-Si layers. On the other hand, magnetron sputtering is used for coating with transparent conductive oxides as ITO (indium tin oxide) and ZAO (aluminium-doped zinc oxide), metallic back contact layers such as Ti, Al and Mo, or components of the compound semiconductor layers such as Cu and In. Magnetron sputter processes use direct current (DC) or pulsed DC, whereas radio frequency (RF) power is used for PECVD processes. Of utmost importance to get a reliable, high-efficiency solar cell is a good uniformity of the deposited layers and the need for the layer to be defect-free. Defects such as particles and splashes are created inside the plasma when an unwanted local discharge – a so-called arc – occurs. This arc can be eliminated by switching off the power supply. The faster this is done, and the less energy that is delivered into the arc, the smaller and more insignificant the defect creation will be. For this reason, as well as for precise control of electrical power, advanced, fast-reacting arc management is very important to attain high-quality solar cell coatings.

The next two years will be crucial in determining the market viability and future of what many see as the most promising thin-film photovoltaics technology: copper indium gallium (di)selenide (CIGS) and its gallium-free cousin, CIS. 

James N. Hilfiker & Ron A. Synowicki, J.A. Woollam Co., Inc.

ABSTRACT

Research and production of state-of-the-art solar cells requires accurate thin-film measurements.  Spectroscopic ellipsometry (SE) provides non-destructive characterization by detecting changes in polarization when light reflects from a layered structure (Figure 1).  Measurements can be applied to all types of thin films: dielectrics, semiconductors, thin metals and organics.  For most, SE is used to determine film thickness and optical constants from the interaction between probe beam and the thin films.  Many additional properties can be determined from the changes they produce on optical constants – including composition, crystallinity, porosity, conductivity, surface roughness, and more.