Complex defect behavior in Cu(In,Ga)Se 2 solar cells with different gallium content
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Complex defect behavior in Cu(In,Ga)Se2 solar cells with different gallium content Verena Mertens1, Jürgen Parisi1, Robert Kniese2, Marc Köntges3, Rolf Reineke-Koch3 1
: University of Oldenburg, Institute of Physics, Energy and Semiconductor Research Laboratory, D-26111 Oldenburg, Germany 2 : Center of Solar Energy and Hydrogen Research (ZSW), Industriestr. 6, D-70565 Stuttgart, Germany 3 : Institute of Solar Energy Research Hameln/Emmerthal (ISFH), Am Ohrberg 1, D-31860 Emmerthal, Germany
ABSTRACT
We report on capacitance spectroscopy measurements on Cu(In,Ga)Se2 based solar cells where the gallium content was varied systematically. In all samples we found, depending on the measurement conditions, a minority carrier and/or a majority carrier signal. The minority trap can be attributed to the prominent donor-like state N1 or β already discussed in literature. However, the origin of the majority trap remains unclear.
INTRODUCTION
Photovoltaic devices based on Cu(In,Ga)Se2 absorber layers with a molar gallium to gallium plus indium ratio GGI of about 0.28 exhibit the highest record efficiency demonstrated for thin film approaches at present, 19.2 % on small-scale laboratory cells [1]. In order to further enlarge the band gap and, therefore, the open circuit voltage of the devices, the interest in Cu(In,Ga)Se2 based solar cells with absorber compositions that cover the whole indium-gallium alloy range has increased [2]. To get more insight into the general properties of the indium-gallium alloy system Cu(In,Ga)Se2, we applied deep level transient spectroscopy (DLTS) with variation of different measurement parameters as well as admittance spectroscopy (AS) on solar cells with different gallium content. In this article, we would like to focus on one remarkable feature of the Cu(In,Ga)Se2 defect spectra, that is the prominent trap signal usually called N1 or β in literature [3,4]. We found some interesting properties of the N1 signal that have not been reported so far. EXPERIMENTAL DETAILS
The Cu(In,Ga)Se2 based solar cells investigated had a GGI of 0.00, 0.10, 0.28, 0.50 and 0.75. They were processed by single-stage evaporation of the elementary compounds as described elsewhere in detail [5]. With these samples, we performed temperature dependent AS and various DLTS measurements. The operating principle of these techniques is discussed in [6,7].
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Figure 1. Defect spectrum (i.e. DLTS signal versus temperature) of a Cu(In,Ga)Se2 based solar cell with GGI = 0.75 for three different boxcar frequencies (points: 97.0 Hz, circles: 52.0 Hz, crosses: 14.9 Hz). The measurement parameters are as follows: UR = -1.5 V, U1= 1.5 V and tp = 1 s.
For the frequency and temperature dependent AS measurements, we used a Solatron impedance analyzer. In the frequency range from 100 Hz to 1 MHz, an alternating voltage of 30 mV amplitude was applied to the device under test. The DLTS measurements were carried out with a home built transient depp level transient spectroscopy setup based on a Boonton 7200 capacitance mete
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