Metastable variations of the fill factor in CIGS thin film solar cells
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1165-M01-08
Metastable variations of the fill factor in CIGS thin film solar cells
Aleksander Urbaniak1*, Małgorzata Igalson1 1
Faculty of Physics, Warsaw University of Technology, Koszykowa 75, PL 00 662 Warszawa, Poland, ABSTRACT We investigate the origin of fill factor changes induced by reverse bias treatment. Evolution of current-voltage characteristics have been measured during application of reverse voltage bias. Two different cell behaviors have been identified. At elevated temperatures one kind of the devices strongly deteriorates and exhibit so called double diode behavior. On the other hand, in the same conditions another cells keep their fill factor almost constant. We correlate the fill factor changes with the kinetics of capacitance and show that although increased number of shallow acceptors itself cannot induce severe FF deterioration, it may strongly influence position of the Fermi level at the heterointerface what in a presence of an electron barrier is crucial for the device behavior. INTRODUCTION Bias-induced metastabilities in Cu(In,Ga)Se2 based devices are one of the phenomena observed from the early days of CIGS technology [1-5]. In particular stressing a device at negative voltage strongly influences the device performance and in some cases may entirely suppress flow of photoinduced current. A level of deterioration varies between the cells and there are samples where such an effect is almost not observable even at elevated temperatures. On the other hand, independently on the changes of current-voltage characteristics persistent changes of the net acceptor density in Cu(In,Ga)Se2 absorbers is always a consequence of reverse bias treatment.. Here we present an attempt to solve this puzzling feature of CIGS devices and discuss the influence of increased acceptor doping of the absorber on the value of fill factor parameter. We show how important for the cell behavior is the position of Fermi level at the heterointerface and how it determines a degree of changes induced in current-voltage characteristics by stressing a sample at reverse bias. EXPERIMENTAL DETAILS The investigated cells are baseline CIGS devices, prepared in standard fabrication process with absorber co-evaporated on Mo-coated soda lime glass. On top of Cu(In,Ga)Se2 layer, CdS and i-ZnO are subsequently deposited. Front, transparent contacts are made from either ITO or Al doped ZnO. Detailed information about device fabrication processes can be found in [6] and [7]. For the investigated cells the evolution of light current-voltage (I-V) characteristics was measured with application of the reverse bias to the cell for increasing periods of time. (Fig.1). Times from one second up to one hour were used at temperatures from 300 K to 350 K. Red light diode of 600 nm wavelength has been used as a light source. In addition to I-V characteristics junction capacitance was monitored during reverse bias stress using Boonton 7200 Capacitance Meter. Simulations of the I-V characteristics were performed with the SCAPS1D software [8].
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