This work contributes to the discussion on defect levels in Cu(In,Ga)Se2 photovoltaic material. CuInSe2- and Cu(In,Ga)Se2- based Schottky junctions, solar cells and thin films were investigated using complementary capacitance and current spectroscopic techniques. Depending on the applied technique and type of investigated structure, six different signals were observed. Out of the signals identified, three were ascribed to responses from bulk defects-two electron and one hole trap. The remainder were discussed in light of available in-literature models including carrier mobility freeze-out and non-ohmic back junction.
A potential barrier impeding current flow in the forward direction is observed in some CIGS devices especially at low temperature. In this paper possible origin and location of this barrier is discussed. We have taken into account: a front contact barrier in the window layer, front contact barrier due to the p+ layer, and back contact barrier at CIGS/Mo interface. The discussion is based on currentvoltage, capacitance-voltage and admittance spectroscopy data measured for CIGS cells with CdS and In2S3 buffer layers and with Mo and Pt back electrode. Accumulated evidences point towards a front electrode as a source of the barrier in the CdS-buffered cells. We show that the height of this barrier is influenced not only by properties of the buffer but also by sodium supply.
The persistent increase of conductivity after illumination in CuInGaSe 2 thin films was investigated as a function of the temperature and light intensity at its creation. Experimentally observed dependences were compared with the results of calculations based on the Lany-Zunger (L-Z) model relating the phenomenon to configurational changes of V S e -V C u divacancy. The calculations showed that the description of all results within this model would require, in some cases, unrealistic values of cross section for electron capture by the center in donor configuration. We concluded that the model does not fully account for the observed phenomena, and that additional mechanisms, in particular to explain the temperature effect, are required.Index Terms-Cu(In, Ga)Se 2 (CIGS) thin-film solar cells, defects, photoconductivity.
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