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What "Hopping" Charge Carriers Tell Us about Solar Cells

Physicists gain new knowledge about the characterization of semiconductors
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03.06.2010

 

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: ,

Meldung vom: 2010-06-03 10:25
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