Deep centers and fill factor losses in the CIGS devices
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Deep centers and fill factor losses in the CIGS devices M. Igalson∗, A. Kubiaczyk, P. Zabierowski Faculty of Physics, Warsaw Uniwersity of Technology, Koszykowa 75, 00-662 Warszawa, Poland
ABSTRACT The influence of non-equilibrium space charge distributions on the fill factor of ZnO/CdS/Cu(In,Ga)Se2 photovoltaic devices has been investigated. Metastable changes of the amount of charge captured in deep levels have been produced by red or white illumination. Subsequent characterization by current-voltage, capacitance-voltage and admittance spectroscopy has been conducted at low temperature. The fill factor increases after a white light soaking and decreases after illumination of the reverse-biased device with red light at low temperature. This effect has been attributed to the metastable change of net doping concentration in the p+ layer of absorber close to interface. Blue light, absorbed in CdS provides holes to that region neutralizing the negative charge and thus improving the fill factor. The defects responsible for the interface-related metastability are in our opinion the same as those causing the persistent increase of the net doping concentration in the bulk of Cu(In,Ga)Se2 and seem to be related to selenium vacancies.
INTRODUCTION ZnO/CdS/Cu(In,Ga)Se2 devices are known for their metastable behavior caused by illumination or voltage bias, which can be divided into two categories [1, 2]. One is related to the bulk of the absorber – it is a persistent increase of the net doping in the bulk, which causes an increase of Voc [3]. The second one is an interface-related phenomenon, which is linked to a “double diode” effect, i.e. a deformation of the illuminated I-V characteristics, particularly pronounced at low temperature and under “red” (absorbed only in CIGS) illumination. For efficient devices the effect disappears if the light spectrum contains shorter wavelengths, absorbed in CdS ("blue” light). In [2] it has been shown, how “blue” light and reverse bias affect the double diode. The results suggested, that a defected p+ layer of the absorber, proposed in a model explaining a cross-over of IV light and dark characteristics [4], is responsible for the fill factor losses. This is in contradiction with the most popular model explaining beneficial effect of the “blue” light by a photodoping of the buffer layer [1,5]. In this paper we will further explore the relations between the space-charge distribution in the junction and I-V characteristics. The characterization of the devices has been conducted at low temperature (around 100 K) in order to avoid disturbance caused by relaxation effects typical for room temperature. The non-equilibrium space-charge distribution within the junction has been produced by use of two methods: light soaking and ”red on reverse bias”. The second method [6] involves illumination of the reverse-biased device by the “red” light at low temperature, which creates a metastable increase of the negative charge in the absorber close to the interface. That charge is only partly removed by f
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