Quantitative analysis of the separate influences of material composition and local defects on the Voc of photovoltaic devices
Johannes Hepp1,2,3, Andreas Vetter1,2, Bernhard Hofbeck2, Umair Sultan2, Jens A. Hauch1,2,4, Christian Camus1,2,4, Christoph J. Brabec1,2,4
1Materials for Electronics and Energy Technology (iMEET), Erlangen, Germany
/2Bavarian Center for Applied Energy Research (ZAE Bayern), Erlangen, Germany
/3Erlangen Graduate School in Advanced Optical Technologies (SAOT), Erlangen, Germany
/4Helmholtz Institute Erlangen-Nuremberg for Renewable Energy (HI ERN), Erlangen, Germany

Manufactures of photovoltaic technologies are constantly striving further for more detailed analysis of material quality, performance optimizations as well as predictions of solar module output and lifetime. In doing so, optical metrology is an essential tool in order to guide these processes. Within this study various, influences on the open circuit voltage (Voc) of Cu(In,Ga)Se2 devices have been investigated. Two of the major impact factors are the material composition of the absorber and the presence of local defects. They were examined using luminescence spectroscopy as well as infrared (IR) thermography and subsequently correlated with the electrically measured Voc. Both, the acquired luminescence peak wavelength as well as the evaluation of the IR images exhibit a seemingly linear correlation to the Voc. This behavior can be expressed in form of a double linear plane fit. In addition, the effect of varying illumination intensity on the device was investigated. It can be shown that with increasing illumination intensity, the impact of the local defects on module performance drops, while the influence of the material composition remains constant. Hence, the data can be used in order to generate an equation which provides the Voc in dependency on all three of these crucial influences – the luminescence peak wavelength, the impact of the local defects, and the illumination intensity applied on the device.