Within the last few years Thuringia in Germany has been developing into an important place for photovoltaics. In 2007 more than ten percent of the worldwide sales for photovoltaic devices have been generated by manufacturers in Thuringia. Even in the global economical crisis the solar energy sector continues to see remarkable growth rates. Along with the currently dominating solar modules based on silicon, thin film solar modules are on the advance. As their production is much less energy intensive and the thickness of their layers is only a tenth of conventionally used modules they are much more cost-effective to produce.
Yet there is still a hitch in the efficiency of these modules. Therefore, industry and science are working on the increased efficiency of thin film solar cells. "The pre-requisite are measuring methods which enable us to analyze and describe the quality of the semiconductors", Professor Dr. Carsten Ronning of the Friedrich-Schiller-University Jena (Germany) explains. The director of the Institute for Solid State Physics and his colleagues have now applied such a method and discovered new interrelationships. Thereby, the efficiency of solar cells based on copper, indium, gallium, and selenium (CIGS) should be able to be enhanced in the long term. The Jena scientists have published their results in the renowned scientific journal "Physical Review Letters" which is also available online under http://prl.aps.org/abstract/PRL/v104/i22/e226403.
"Like a lot of electronic devices these solar cells are based on semiconductors", explains Dr. Udo Reislöhner. "Their conductivity is determined by their doping, which means by the appearance of crystal defects", he continues. By "defects" the scientists mean embedded foreign atoms and irregularities in the arrangement of lattice atoms. "Such lattice defects are being held responsible for the fact that the existing CIGS-solar cells don´t yet reach their maximum efficiency".
Temperature is more important than expected
To determine the properties of defects in CIGS-crystals and the degree of doping, the physicists measure the electrical capacitance of the solar cell very precisely by applying a small AC voltage. Whilst doing so, they made a surprising discovery. "We observed that the mobility of charge carriers in CIGS is much more determined by the temperature than expected," Dr. Reislöhner explains. The more the CIGS-semiconductor is cooled down, the less mobile the charge carriers become. "They then perform 'hopping'-like movements from one defect to the next."
According to the result of the Jena research this phenomenon, called hopping conduction, is also responsible for a number of measured effects, which have been attributed to defects in CIGS-crystals in the scientific literature until now. "The hopping conduction can create a signal which is very similar to that of real crystal defects", Professor Ronning says. Thus, many results on the doping and thereby the quality of CIGS-crystals have been assumedly misinterpreted, the semiconductor expert of the Jena University says. Dr. Reislöhner is convinced that a profound knowledge of the electrical characteristics of these materials should only be possible through the new insights. "This will promote the characterization of CIGS thin film solar cells considerably and thus will make an important contribution to the improvement of their efficiency."
Original Publication:
Reislöhner U, Metzner H, Ronning C.: Hopping Conduction Observed in Thermal Admittance Spectroscopy, Phys. Rev. Lett. 104, 226403 (2010)
Contact Details:
Prof. Dr. Carsten Ronning, Dr. Udo Reislöhner
Institute for Solid State Physics of Friedrich-Schiller-University Jena
Helmholtzweg 3
D-07743 Jena
Tel.: 0049 (0) 3641 / 947300, or 947324
E-Mail: ,
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