Christian A. Kaufmann

Interpretation of admittance, capacitance-voltage, and current-voltage signatures in Cu(In,Ga)Se2 thin film solar cells

A series of Cu(In,Ga)Se2 (CIGS) thin film solar cells with differently prepared heterojunctions has been investigated by admittance spectroscopy, capacitance-voltage (CV) profiling, and temperature dependent current-voltage (IVT) measurements. The devices with different CdS buffer layer thicknesses, with an In2S3 buffer or with a Schottky barrier junction, all show the characteristic admittance step at shallow energies between 40 and 160 meV, which has often been referred to as the N1 defect. No correlation between the buffer layer thickness and the capacitance step is found. IVT measurements show that the dielectric relaxation frequency of charge carriers in the CdS layers is smaller than the N1-resonance frequency at low temperatures where the N1 step in admittance is observed. These results strongly contradict the common assignment of the N1 response to a donor defect at or close to the heterointerface. In contrast, an explanation for the N1 response is proposed, which relates the admittance step to a ...

Influence of Na on Cu(In,Ga)Se2 solar cells grown on polyimide substrates at low temperature: Impact on the Cu(In,Ga)Se2/Mo interface

There are still open questions regarding the nature of the positive effect of the presence of Na on the performance of Cu(In,Ga)Se2 based, chalcopyrite thin film solar cells, especially at low processing temperatures. Studying Cu(In,Ga)Se2 thin film devices fabricated from low-temperature coevaporated absorbers on polyimide substrates by admittance and J-V-T measurements, characteristic properties are identified for different amounts of Na present during the growth. A roll-over behavior can be directly correlated with the Na-content. X-ray photoelectron spectroscopy shows the development of a MoSe2 phase at the back contact of the device. Efficiencies of 15.1% with MgF2 antireflection coating are demonstrated.

Comprehensive comparison of various techniques for the analysis of elemental distributions in thin films

In a recent publication by Abou-Ras et al., various techniques for the analysis of elemental distribution in thin films were compared, using the example of a 2-µm thick Cu(In,Ga)Se2 thin film applied as an absorber material in a solar cell. The authors of this work found that similar relative Ga distributions perpendicular to the substrate across the Cu(In,Ga)Se2 thin film were determined by 18 different techniques, applied on samples from the same identical deposition run. Their spatial and depth resolutions, their measuring speeds, their availabilities, as well as their detection limits were discussed. The present work adds two further techniques to this comparison: laser-induced breakdown spectroscopy and grazing-incidence X-ray fluorescence analysis.The present work shows results on elemental distribution analyses in Cu(In,Ga)Se2 thin films for solar cells performed by use of wavelength-dispersive and energy-dispersive X-ray spectrometry (EDX) in a scanning electron microscope, EDX in a transmission electron microscope, X-ray photoelectron, angle-dependent soft X-ray emission, secondary ion-mass (SIMS), time-of-flight SIMS, sputtered neutral mass, glow-discharge optical emission and glow-discharge mass, Auger electron, and Rutherford backscattering spectrometry, by use of scanning Auger electron microscopy, Raman depth profiling, and Raman mapping, as well as by use of elastic recoil detection analysis, grazing-incidence X-ray and electron backscatter diffraction, and grazing-incidence X-ray fluorescence analysis. The Cu(In,Ga)Se2 thin films used for the present comparison were produced during the same identical deposition run and exhibit thicknesses of about 2 μm. The analysis techniques were compared with respect to their spatial and depth resolutions, measuring speeds, availabilities, and detection limits.

Experimental indication for band gap widening of chalcopyrite solar cell absorbers after potassium fluoride treatment

The implementation of potassium fluoride treatments as a doping and surface modification procedure in chalcopyrite absorber preparation has recently gained much interest since it led to new record efficiencies for this kind of solar cells. In the present work, Cu(In,Ga)Se2 absorbers have been evaporated on alkali containing Mo/soda-lime glass substrates. We report on compositional and electronic changes of the Cu(In,Ga)Se2 absorber surface as a result of a post deposition treatment with KF (KF PDT). In particular, by comparing standard X-ray photoelectron spectroscopy and synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES), we are able to confirm a strong Cu depletion in the absorbers after the KF PDT which is limited to the very near surface region. As a result of the Cu depletion, we find a change of the valence band structure and a shift of the valence band onset by approximately 0.4 eV to lower binding energies which is tentatively explained by a band gap widening as expected for Cu defic...

Characterization of metastabilities in Cu(In,Ga)Se2 thin-film solar cells by capacitance and current-voltage spectroscopy

Metastabilities in Cu(In,Ga)Se2 (CIGS)-based solar cells induced by red light illumination, blue light illumination, and voltage bias treatment are investigated by admittance spectroscopy, capacitance-voltage profiling, thermally stimulated capacitance, and temperature dependent current-voltage (IVT) measurements. It is found that the characteristic N1-admittance response exhibits changes in the activation energy after light soaking and forward bias treatment, whereas no significant change of the activation energy is observed after reverse bias treatment. A roll-over effect is observed in the IVT characteristics for conditions of the sample for which the N1-activation energy is larger than 100 meV. We show that these results are in good agreement with a recently proposed interpretation of admittance spectra, which relates the N1 signature to a back-contact barrier in CIGS solar cells. We also show that, within this model, the changes in the IV characteristics upon light soaking with blue light can be cons...

Lift-off process and rear-side characterization of CuGaSe2 chalcopyrite thin films and solar cells

An alternative approach to the so-called “lift-off” technology is presented, in which a CuGaSe2 solar cell absorber film is detached from a Mo-coated glass substrate. The proposed lift-off takes advantage of an interfacial MoSe2 layer, acting as a sacrificial layer, which forms at the rear contact during the growth of the CuGaSe2 film. No additional processing step is thus required to proceed with the lift-off. The lift-off was carried out in ultrahigh vacuum for quality assessment, and the rear CuGaSe2 and top MoSe2 surfaces were characterized by means of surface-sensitive techniques, namely, Kelvin probe force microscopy and photoelectron spectroscopy. The cleanness of the CuGaSe2 rear surface was confirmed by the absence of Mo remnants, thus demonstrating the suitability of the proposed method for further processing of the absorber film onto alternative substrates. In addition, a quantitative analysis of surface photovoltage, doping concentration, and interface charge at grain boundaries on the absorbe...

Generation-dependent charge carrier transport in Cu(In,Ga)Se2/CdS/ZnO thin-film solar-cells

Cross section electron-beam induced current (EBIC) and illumination-dependent current voltage (IV) measurements show that charge carrier transport in Cu(In,Ga)Se2 (CIGSe)/CdS/ZnO solar-cells is generation-dependent. We perform a detailed analysis of CIGSe solar cells with different CdS layer thicknesses and varying Ga-content in the absorber layer. In conjunction with numerical simulations, EBIC and IV data are used to develop a consistent model for charge and defect distributions with a focus on the heterojunction region. The best model to explain our experimental data is based on a p+ layer at the CIGSe/CdS interface leading to generation-dependent transport in EBIC at room temperature. Acceptor-type defect states at the CdS/ZnO interface cause a significant reduction of the photocurrent in the red-light illuminated IV characteristics at low temperatures (red kink effect). Shallow donor-type defect states at the p+ layer/CdS interface of some grains of the absorber layer are responsible for grain specif...

Formation of the physical vapor deposited CdS∕Cu(In,Ga)Se2 interface in highly efficient thin film solar cells

We report on the buffer/absorber interface formation in highly efficient (14.5%, air mass 1.5) ZnO∕CdS∕Cu(In,Ga)Se2 solar cells with a physical vapor deposited CdS buffer. For Se-decapped Cu(In,Ga)Se2 (CIGSe) absorbers we observe sulfur passivation of the CIGSe grain boundaries during CdS growth and at the interface a thermally stimulated formation of a region with a higher band gap than that of the absorber bulk, determining the height of the potential barrier at the CdS∕CIGSe interface. For air-exposed CIGSe samples the grain boundary passivation is impeded by a native oxide/adsorbate layer at the CIGSe surface determining the thermal stability of the potential barrier height.

Na incorporation into Cu(In,Ga)Se2 thin-film solar cell absorbers deposited on polyimide: Impact on the chemical and electronic surface structure

Na has deliberately been incorporated into Cu(In,Ga)Se2 (“CIGSe”) chalcopyrite thin-film solar cell absorbers deposited on Mo-coated polyimide flexible substrates by adding differently thick layers of NaF in-between CIGSe absorber and Mo back contact. The impact of Na on the chemical and electronic surface structure of CIGSe absorbers with various Cu-contents deposited at comparatively low temperature (420 °C) has been studied using x-ray photoelectron and x-ray excited Auger electron spectroscopy. We observe a higher Na surface content for the Cu-richer CIGSe samples and can distinguish between two different chemical Na environments, best described as selenide-like and oxidized Na species, respectively. Furthermore, we find a Cu-poor surface composition of the CIGSe samples independent of Na content and — for very high Na contents — indications for the formation of a (Cu,Na)–(In,Ga)–Se like compound. With increasing Na surface content, also a shift of the photoemission lines to lower binding energies cou...

Three-dimensional structure of the buffer/absorber interface in CdS/CuGaSe2 based thin film solar cells

The chemical structure of the CdS/CuGaSe2 chalcopyrite solar cell buffer/absorber interface is investigated by combining element depth profiling using elastic recoil detection analysis and surface-near bulk sensitive x-ray emission spectroscopy. Significant Cd and S concentrations (≥0.1 at. %) are found deep in the absorber bulk. The determined high Cd and S diffusion coefficient values at 333 K of 3.6 and 3.4×10−12 cm2/s, respectively, are attributed to diffusion along CuGaSe2 grain boundaries. As a result, a three-dimensional buffer/absorber interface geometry is proposed.