P. Zabierowski

Classification of metastabilities in the electrical characteristics of ZnO/CdS/Cu(In,Ga)Se2 solar cells

Abstract We experimentally distinguish between three types of metastable electrical behavior in Cu(In,Ga)Se 2 -based heterojunction thin-film solar cells. A metastable increase of net acceptor concentration in the bulk of the Cu(In,Ga)Se 2 absorber material is induced by illumination with red light, an increase of net positive charge at the interfacial region is induced by blue light, and a reversible creation of negative charges occurres by the application of reverse bias at temperatures above 340 K. We discuss the physical origin of these metastabilities.

Charge and doping distributions by capacitance profiling in Cu(In,Ga)Se2 solar cells

Doping distributions in the Cu(In,Ga)Se2 solar cells with various gallium contents are analyzed by the use of capacitance-voltage and drive-level capacitance profiling. The influence of deep traps on the evaluation of the spatial-doping distribution in the bulk of Cu(In,Ga)Se2 absorbers is discussed. An analysis is presented, which shows that traps labeled N1, commonly observed in these devices, are interface states or compensating donors and their input to the capacitance is related only to the width of the depleted n-type insulating layer. We attribute the apparent increase of doping density toward the back electrode to the accumulation of the electrostatic charge in deep bulk acceptors with a concentration at an order of magnitude higher than net shallow doping. The metastable changes of doping distributions induced by light or reverse bias are also investigated and interpreted in terms of the Lany–Zunger model of VSe-VCu divacancies with negative-U property. All conclusions have been tested by numeric...

Transient capacitance spectroscopy of defect levels in CIGS devices

The conventional DLTS, deep level transient spectroscopy, reverse-bias DLTS and Laplace transform analysis of capacitance transients have been employed for investigation of defect spectra in the bulk and at the interface of standard ZnO/CdS/Cu(In,Ga)Se2 devices. The results are reviewed with special attention devoted to the persistent changes induced by illumination or bias voltage. In the discussion a model of mobile copper ions and shallow-deep transition of compensating donors are compared. We show, that defect relaxation model can also account for the observed phenomena. One of its consequences is a dipole layer in the interface region, consisting of positively charged interface states and negatively charged thin layer of absorber with persistently removed compensation.

Electrical characterization of ZnO/CdS/Cu(In,Ga)Se2 devices with controlled sodium content

Abstract A set of devices prepared with varying amount of sodium precursor NaF has been characterized by use of junction capacitance techniques. The analysis of the DLTS and admittance spectra shows, that two components contribute to the minority carrier signal–interface states and compensating donors. In Na-free device only a donor level of energy 0.26 eV has been detected. In the other samples both contributions have been found with the values of emission rates from the donor level distorted by electric field-induced tunnel ionization. We conclude, that a fast, tunneling-dominated emission from this state is a mark of the presence of a defected, p + layer adjacent to the heterointerface. Worse photovoltaic performance of the device with not sufficient amount of sodium provided during growth we ascribe to a more pronounced defected layer.

Thermally assisted tunnelling in Cu(In, Ga)Se2-based photovoltaic devices

The effect of thermally assisted tunnelling has been found to explain the commonly observed non-linearities of admittance and RDLTS Arrhenius plots. In terms of this model the light induced increase of the emission rates has been ascribed to the lowering of the tunnelling barrier. We have proposed that the physical picture underlying these metastabilities is essentially the same as used for explanation of the cross-over effect. These statements are confirmed by numerical calculations.

Persistent Photoconductivity in Polycrystalline Cu(In,Ga)Se 2 Thin Films: Experiment Versus Theoretical Predictions

The persistent increase of conductivity after illumination in CuInGaSe2 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 VSe-VCu 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.