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After successful completion of his master's degree, "regenerative energy systems" at the University of Applied Sciences Berlin (Hochschule für Technik und Wirtschaft Berlin) in June 2008, Stefan Wendlandt assumed a research position at the Photovoltaik Institut Berlin, where he is currently engaged in the analysis of Photovoltaic modules.
In addition to the theoretical content of his bachelor's degree, he developed his interest in scientific practice at the 2004 summer school at the Helmholtz Zentrum für Materialien und Energie (HZB) in Berlin, where he investigated the long-term stability of encapsulated CuInS2 laboratory solar modules. Subsequently, he took up a post at the HZB as a student assistant, working with the evaluation of measured data taken from in-situ EDXRG growth experiments in CuInS2 solar cells. He completed his bachelor's thesis at the Fraunhofer Institute for Solar Energy Systems in Freiburg, where he studied both optoelectronic measurement techniques and the optical evaluation of Fresnel lenses for application in concentrator photovoltaics. Mr. Wendlandt returned to the HZB in the course of his master's studies, taking up a position as a student assistant investigating the electrical and optical properties of CuInS2 solar cells. Drawing on this experience, he completed a seminar paper with the title "Analysis of Cu(In,Ga)Se2 solar cells with silver precoursers". In the course of a work placement, he planned a solar module test line and conducted contact tests for thin-film technology at the firm Q.-cells SE. Mr. Wendlandt researched his master's thesis at the Centre for Renewable Energy Systems Technology at Loughborough University in Great Britain, developing an innovative solar simulator design whilst working on the area of optical simulation. The research trip was financed with a grant from the Leonardo da vinci programme obtained via the Humboldt University Berlin.
"Rapid Hot Spot Analysis Using Photovoltaic Modules."
The goal of the planned doctoral research is the scientific development of a unique in-line capable procedure for the evaluation of the risk of hot-spot developments on PV modules. In short, this involves the scientific assessment of the foundation of the hot-spot effect; process development; evaluation of individual cell and module errors in relation to the risk of a hot spot; and the validation and classification of the procedure.
The necessity of such a procedure is dictated by the dynamic and thus constantly developing photovoltaic market which dictates price reduction in the manufacturing process under conditions of maintained or even improved quality. One possible method of cost reduction involves the use of crystalline solar cells from upgraded metallurgical grade silicium (UMG silicium), also known as photovoltaic grade silicium (PVG silicium). The wafers and cells of UMG silicium are thought to represent a cheaper solution, as the manufacture of polysilicium requires less energy and process complexity. These financial advantages are however balanced by an end product of poorer quality. The increased proportion of impurities per volume produces higher dark currents and a lower breakdown voltage with reverse current. Following from this, partial shading of the module by antennae, trees and/or chimneys can result in local overheating otherwise known as hot spots. Such overheating can lead to irreversible damage to the solar cells and the module capsule and thus module outage. In addition to material quality, the factors increasing the risk of hot spots include cell fracture and cracks as well as material or metallization failure.
With its large-scale coating, integrated series circuitry, high material savings and high flexibility in the realization of innovative module and cell concepts, thin-film photovoltaic technology brings a high potential for cost reduction. The hot spot risk associated with thin-film modules is broadly similar to that of impure silicium in thick-layer technology. This means that the production-related small-grain crystal structure produces poorer diode characteristics in thin-layer cells with reverse current, thus producing a lower blocking effect. Moreover, thin-film technology carries the risk of the development of inhomogenous material and layer characteristics during the deposition of the absorber layer in the manufacturing process, which produces differences in the local optical and electrical properties of the absorber. The parallel resistance of the semiconductor also displays similar patterns. Should this fall below a technologically-dependant value, the result can be local overheating upon shading. In contrast to thick-layer modules, with thin-layer modules, the formation of considerable temperature gradients can result in both electrical failure and glass breakage. This represents a considerable safety hazard.
The doctoral thesis will be supervised by Prof Dr.-Ing habil. Stefan Krauter from Berlin Technical University and conducted at the Photovoltaik Institut Berlin.