A. Jasenek

Influence of the Ga-content on the bulk defect densities of Cu(In, Ga)Se2

Abstract We fabricate a series of CIGS absorber layers with Ga-contents x =Ga/(In+Ga) ranging from x =0 (CuInSe 2 ) to 1 (CuGaSe 2 ) by single layer coevaporation. The open circuit voltages V oc of the completed solar cells increase with increasing Ga-content but not proportional to the change of the band gap energy E g of the CIGS-layers. In contrast to the behaviour at Ga-contents exceeding x ≈0.3 the difference E g −q V oc decreases from x =0 to x ≈0.3, thus, having a minimum at x ≈0.3. We determine defect concentrations in the absorber of these cells by admittance spectroscopy. These bulk defects also have a minimum concentration at x ≈0.3. This low concentration of deep defects reduces recombination losses and thus, the difference of E g −q V oc . In addition, we fabricated absorber layers with a three stage process and an average Ga content x of approximately 0.3. The resulting solar cells have a lower defect concentration and a higher open circuit voltage than solar cells from single stage processes with x ≈0.3. We find a correlation between volume defect concentrations and the difference E g −q V oc suggesting that volume defects determine the open circuit voltage of CIGS solar cells in the whole composition range from CuInSe 2 to CuGaSe 2 and when using different absorber processes.

Electrical characterization of Cu(In,Ga)Se2 thin-film solar cells and the role of defects for the device performance

Electrical analysis of Cu(In,Ga)Se2-based heterojunction devices shows that recombination in the space-charge region is the dominant recombination mechanism which determines the open-circuit voltage of these devices. We identify a specific defect as the relevant recombination center. The concentration of this defect varies with the Ga-content in the absorber alloy with the highest concentration in pure CuGaSe2. In this material, tunneling-enhanced recombination plays a major role for recombination.

Electronic loss mechanisms in chalcopyrite based heterojunction solar cells

This article discusses the fundamental limitations imposed on Cu(In,Ga)Se2 based heterostructure solar cells by recombination in the bulk of the absorber and at the interface between the absorber and the CdS buffer layer. Bulk recombination to a certain extent can be minimized by increasing the doping density up to a limit where tunneling currents significantly enhance recombination. We propose simple schemes for the analysis of experimentally gained data. By comparison of theoretical models with experimental data we show that tunneling plays a role for polycrystalline Cu(In,Ga)Se2 in some cases and for CuGaSe2 in general.

Radiation resistance of Cu(In,Ga)Se2 solar cells under 1-MeV electron irradiation

Polycrystalline ZnO/CdS/Cu(In,Ga)Se 2 heterojunction solar cells display a very high radiation resistance under 1-MeV electrons. We irradiated high-efficiency Cu(In,Ga)Se 2 solar cells with 1-MeV electron fluences up to Φ c = 5 × 10 18 cm 2 . The loss in the conversion efficiency, starting at Φ c = 10 17 cm 2 , was caused by the open circuit voltage loss. An analytical model for the open circuit voltage describes the loss by considering the increase in space charge recombination via deep defects introduced by electron irradiation. A reduction of the doping density in the Cu(In,Ga)Se 2 absorber layer upon irradiation was analyzed by capacitance voltage measurements. The rate at which the net doping density decreased was 0.045 cm -1 . Accumulative irradiation shows that partial recovery of the radiation induced damage occurred during our analysis cycle well below 100°C.

Defect annealing in Cu(In,Ga)Se2 heterojunction solar cells after high-energy electron irradiation

Cu(In,Ga)Se2/CdS/ZnO solar cells need at least 1018 cm−2 electrons of an energy of 1 MeV to degrade in their power conversion efficiency by more than 25%. Even after such high irradiation doses, annealing of the irradiated solar cells at temperatures between 130 and 160 °C leads to a full recovery of the device performance. Isochronal annealing experiments unveil that the annealing of the irradiation-induced defects has an activation energy of 1.05 eV.

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.

Interface mixing of CuOx/SiO2 bilayers by swift heavy ions

Abstract Thin films of CuOx (x=0, 0.5, 1) on SiO2-substrates were irradiated with heavy ions in the electronic stopping regime (some MeV/amu) and in the nuclear stopping regime (some keV/amu). The irradiation of the oxide layers with Ar, Kr and Xe ions of 90–260 MeV lead to strong interface mixing. The mixing efficiency seems to correlate with the bandgap of the toplayer and the estimation of the effective diffusion constant indicates interdiffusion in molten ion tracks. No mixing was observed after high energy irradiation of Cu/SiO2, which can be explained by the fact that the critical electronic stopping power for track formation has not been exceeded even with 230 MeV Xe ions. Irradiation of CuOx/SiO2 (x=0, 0.5, 1) with 900 keV Xe in all cases leads to only weak intermixing, which can be explained by the ballistic model.

CuGaSe2-based superstrate solar cells

We grow polycrystalline CuGaSe 2 thin films by simultaneous evaporation of the elements in high vacuum for use as absorber layers in superstrate solar cells. The films are grown on a stack of intrinsic and Al:doped ZnO deposited by sputtering on soda lime glass. We systematically vary the CuGaSe 2 growth temperature T s between 530°C and 630°C and investigate the resulting changes of the electrical properties of the devices. The CuGaSe 2 films are measured by X-ray diffraction. The full width at half maximum of the CuGaSe 2 [112] reflex displays a value of 0.19° for T, = 630°C compared to 0.49° for T, = 530°C. Both the X-ray diffraction and the electrical analysis favor the higher growth temperature. We achieve open circuit voltages above 800 mV which is comparable to the best CuGaSe 2 substrate devices. The efficiency of 3.5% is limited by the low fill factor. Light soaking under AM1.5 illumination increases the fill factor from 27 to 46% with only a small impact on short circuit current and open circuit voltage.

Illumination-induced recovery of Cu(In,Ga)Se2 solar cells after high-energy electron irradiation

Cu(In,Ga)Se2/CdS/ZnO solar cells irradiated with a 1 MeV electron fluence of 1018 cm−2 degrade to about 80% of their initial conversion efficiency. Illumination with white light at an intensity of 100 mW cm−2 for 3 h at room temperature restores more than 90% of the preirradiation efficiency. The healing process is more efficient if the device is kept under open-circuit conditions during illumination than for short-circuit conditions. Injecting minority carriers by voltage bias in the dark, instead of illumination, does not cause enduring device recovery. This behavior of Cu(In,Ga)Se2 is in contrast to illumination-induced defect healing processes reported for other semiconductor materials, like GaAs, InP, or GaP.

Consequence of 3-MeV electron irradiation on the photovoltaic output parameters of Cu(In,Ga)Se2 solar cells

Abstract Irradiation experiments of Cu(In,Ga)Se 2 thin film solar cells with 3-MeV electrons cause a degradation of the short circuit current density j SC starting at fluences φ e =2×10 17 cm −2 and leading to a loss of 90% of the initial j SC at φ e =2×10 18 cm −2 . This decrease of j SC is linked to the generation of a deep defect with an activation energy of approximately 500 meV. Thermal annealing of the 3-MeV electron irradiated samples at 360 K restores the initial j SC and annihilates the deep defect.