M. Igalson

Distinction between bulk and interface states in CuInSe2/CdS/ZnO by space charge spectroscopy

We present a detailed study of admittance spectroscopy and deep level transient spectroscopy on CuInSe2/CdS/ZnO thin film solar cells. The admittance spectra reveal an emission from a distribution of hole traps centered at an activation energy of 280 meV and a shallower level with a sharp activation energy of ∼ 120 meV. After repetitive annealing of the device in air at 200 °C, the activation energy of the latter level increases continuously from 120 to 240 meV, while the 280 meV hole traps remain unaffected. Deep level transient spectroscopy with optical excitation reveals an emission of minority carriers with time constants comparable to those observed for the shallow level in admittance spectroscopy. The shift of the activation energy after annealing also occurs in deep level transient spectroscopy and ascertains that the emissions observed in both techniques have the same origin. The magnitude and continuous shift of the activation energy of the minority carrier emission indicates a distribution of le...

The metastable changes of the trap spectra of CuInSe2‐based photovoltaic devices

Spectra of hole and electron traps of CuInSe2/CdS/ZnO photovoltaic devices have been investigated using deep‐level transient spectroscopy. A decrease of the concentration of shallow electron traps and an increase of the hole trap concentration after an injection of electrons has been observed. The effect is metastable below 200 K. A proposed explanation is based on the idea that both levels belong to the same defect in a different charge state. A resemblance of the phenomena related to that defect and to ‘‘dangling bond’’‐type centers in amorphous semiconductors has been indicated. Some consequences of defect conversion for current transport and performance of photovoltaic devices have been discussed.

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...

Defects in Cu(In, Ga) Se2 semiconductors and their role in the device performance of thin-film solar cells

This contribution is a summary of an international, interdisciplinary workshop dedicated to defects in chalcopyrite semiconductors and their relation to the device characteristics of thin-film solar cells, held on 3–5 June 1996 in Oberstdorf, Germany. Results of different characterization methods were brought together to identify common observations. The comparison of results from electrical defect spectroscopy and luminescence investigations confirmed the presence of energetic distributions of defects throughout the bandgap of chalcopyrite thin films. Electrical defect spectroscopy detects a defect about 280 meV above the valence band edge of Cu(In, Ga)Se2 regardless of the preparation conditions of the sample. In a solar cell the density of this defect depends on the operation conditions. This observation might be related to the migration of copper in an electric field, which occurs even at room temperature. Other defects appear to be related to processing or impurities. Photoluminescence decay measurements yield time constants of several nanoseconds under low injection conditions. Modelling of the current–voltage characteristics of Cu(In, Ga)Se2-based thin-film cells suggests that compensating acceptor states in the CdS or at the heterointerface are responsible for the frequently observed cross-overs between the dark and illuminated curves.© 1997 John Wiley & Sons, Ltd.

The change of the electronic properties of CIGS devices induced by the ‘damp heat’ treatment

Abstract The changes of the electronic properties of the absorber layer in the ZnO/CdS/Cu(In,Ga)Se2 photovoltaic devices induced by the ‘damp heat’ test have been investigated by use of junction capacitance techniques. Deep level transient spectroscopy and admittance spectroscopy have been employed for characterization of the bulk and interface levels in the absorber. Additional information on the transport mechanisms has been provided by the analysis of current–voltage characteristics. We conclude that the ‘damp heat’ treatment introduces deep electron traps, thus increasing the absorber compensation and decreasing Voc of the devices. The same states facilitate transport of carriers by means of trap-assisted tunneling, causing a decrease of the fill factor. OSe is a probable candidate for a defect introduced by the humidity test.

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.

Creation and relaxation of light- and bias-induced metastabilities in Cu(In,Ga)Se2

Metastabilities in Cu(In,Ga)Se2 based solar cells were investigated. Capacitance and conductance transients were measured in order to analyze carrier trapping and emission processes related to the creation and relaxation of metastable states. Our experimental findings support the theoretical predictions of Lany and Zunger [Lany and Zunger, J. Appl. Phys. 100, 113725 (2006)] for a (VSe-VCu) complex, a defect with negative-U energy that can exist in both the donor and acceptor configurations. We show that two different defect reactions induced by either voltage bias or illumination lead to the same acceptor configuration of the defect. The relaxation process is the same for light- and bias-induced metastabilities in devices and thin films. Time constants and activation energies for all investigated processes have been obtained. The results agree very well with the values calculated for (VSe-VCu) divacancy.

The ‘defected layer’ and the mechanism of the interface-related metastable behavior in the ZnO/CdS/Cu(In,Ga)Se2 devices

The electronic properties of the interface region in the ZnO/CdS/Cu(In,Ga)Se2 devices have been investigated in the various metastable states induced by voltage bias and illumination. Capacitance spectroscopy has been used to gain information about the level spectrum in the interface region of absorber and space-charge distribution within the structures. The results of capacitance spectroscopy are analyzed in conjunction with the current–voltage characteristics. We have differentiated between the metastable effect due to the changes of the space-charge distribution in the absorber and a process involving the persistent changes of the Fermi-level position at the interface. We attribute the first one to the electronic processes due to relaxing donor-type VSe centers. The second one in our opinion involves the process of forming a quasi-equilibrium between the positive and negative charges in the immediate vicinity of the interface. In the admittance and DLTS spectra of interface levels a signal belonging to bulk donors (most probably to InCu defects) has been identified.

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.