Photovoltaic electricity generation with solar modules, converting sunlight directly into electricity, plays an important role in the worldwide efforts to increase the share of renewable energy production. Silicon (Si) wafers and its processing to solar cells are the most expensive parts of solar module fabrication. The fabrication of thin (10-20 micrometer (um)) high quality micro-crystalline silicon (mc-Si) layers applying liquid-phase crystallization of Si on glass (LPCSG) by line focus laser is a promising method to reduce Si consumption in solar modules by a factor of 10 and to reduce processing costs (see image of LPCSG absorber).
One objective of the cSiOnGlass project executed at the Leibniz-Institute of Photonic Technologies in Jena, Germany, was the fabrication and investigation of barrier layers (BL) like SiO2 and SiNx deposited on the glass before deposition and crystallization of the Si layer to improve mc-Si layer quality.
Next, an objective of the project was to investigate different glasses like Borofloat 33 from Schott and Corning Eagle for implementation in a large area (several sqm) module or as very thin (100-200 um) glass substrate for a so-called wafer equivalent.
Finally, an objective was the development of a contact-less characterization method to determine the electronic material quality of mc-Si layers on glass.
Concluding, in the cSiOnGlass project considerable progress was made in the determination of the electronic quality of the LPCSG absorbers by using the quasi steady-state photoconductance (QSSPC) method. This method determines the effective charge carrier lifetime (indicator for material quality) in function of the illumination intensity exposed to the sample. Lifetime data from the QSSPC method correlates with lifetime data from photoluminescence (PL) decay measured by time-correlated single photon detection. Effective lifetime of more than 400 ns has been measured in LPCSG absorbers indicating an effective charge carrier diffusion length more than the double of the absorber thickness (10 um). In small (mm) solar cells open circuit voltage of 629 mV on Corning Eagle glass and 603 mV on Schott Borofloat 33 are measured. These values confirm the high electronic material quality of the mc-Si layers after hydrogen passivation which was determined from lifetime data. Sputtered and post annealed SiO2 layer in the thickness range from 100 to 300 nm are effective barrier layer to prevent impurity diffusion from the glass. SiNx layers present issues related to the formation of non-homogenous Si/glass interface after laser crystallization. Glass bending after the laser crystallization is an issue for large scale industrial application and should be focus of further research.
