Melanie Nichterwitz

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...

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...

Numerical simulation of cross section electron-beam induced current in thin-film solar-cells for low and high injection conditions

Electron-beam induced current measurements (EBIC) in the cross-section configuration can be used to characterize electronic properties of thin-film solar-cells with a spatial resolution in the submicrometer range. Assuming low injection conditions and complete charge carrier collection in the depletion region, the minority charge-carrier diffusion length and width of the space charge region can be extracted from EBIC data using an analytical expression. In the present work, we evaluate the validity of the assumptions underlying the analytical description by using numerical device simulation to describe EBIC profiles perpendicular to the pn-junction of thin-film solar cells. We find that under low injection conditions, the analytical description provides good results if the minority charge-carrier diffusion length in the absorber layer is significantly larger than the width of the space charge region. On the other hand, the analytical description of the EBIC profiles deviates significantly from the numeric...

Electron-beam-induced current at absorber back surfaces of Cu(In,Ga)Se2 thin-film solar cells

The present work reports on investigations of the influence of the microstructure on electronic properties of Cu(In,Ga)Se2 (CIGSe) thin-film solar cells. For this purpose, ZnO/CdS/CIGSe stacks of these solar cells were lifted off the Mo-coated glass substrates. The exposed CIGSe backsides of these stacks were investigated by means of electron-beam-induced current (EBIC) and cathodoluminescence (CL) measurements as well as by electron backscattered diffraction (EBSD). EBIC and CL profiles across grain boundaries (GBs), which were identified by EBSD, do not show any significant changes at Σ3 GBs. Across non-Σ3 GBs, on the other hand, the CL signals exhibit local minima with varying peak values, while by means of EBIC, decreased and also increased short-circuit current values are measured. Overall, EBIC and CL signals change across non-Σ3 GBs always differently. This complex situation was found in various CIGSe thin films with different [Ga]/([In]+[Ga]) and [Cu]/([In]+[Ga]) ratios. A part of the EBIC profile...

Combined electron backscatter diffraction and cathodoluminescence measurements on CuInS2/Mo/glass stacks and CuInS2 thin-film solar cells

Electron backscatter diffraction (EBSD) and cathodoluminescence (CL) measurements in a scanning electron microscope were performed on cross sections of CuInS2 thin films and ZnO/CdS/CuInS2/Mo/glass thin-film solar cells. The CuInS2 layers analyzed for the present study were grown by a rapid thermal process. The regions of the CuInS2 layers emitting high CL intensity of band-band luminescence are situated near the top surface (or close to the interface with ZnO/CdS). This can be attributed to an enhanced crystal quality of the thin films in this region. The phenomenon may be related to the recrystallization via solid-state reactions with CuxS phases, which is assumed to run from the top to the bottom of the growing CuInS2 layer. The distribution of CL intensities is independent of the sample temperature, the acceleration voltage of the electron beam, and of whether or not the ZnO/CdS window layers are present. When comparing CL images and EBSD maps on identical sample positions, pronounced intragrain CL co...

Compositional dependence of charge carrier transport in kesterite Cu2ZnSnS4 solar cells

Cu2ZnSnS4 solar cells deposited by thermal co-evaporation have been characterized structurally and electronically to determine the dependence of the electronic properties on the elemental composition of the kesterite phase, which can significantly deviate from the total sample composition. To this end, the kesterite phase content and composition were determined by a combination of X-ray fluorescence and X-ray absorption measurements. The electronic properties, such as carrier density and minority carrier diffusion length, were determined by electron beam induced current measurements and capacitance-voltage profiling. The charge-carrier transport properties are found to strongly depend on the Cu/(Sn+Zn) ratio of the kesterite phase. For the Cu-poor sample, a minority carrier diffusion length of 270 nm and a total collection length of approx. 500 nm are deduced, indicating that current collection should not be an issue in thin devices.

Preferred Orientation, Grain Sizes and Grain Boundaries of Chalcopyrite-Type Thin Films

Chalcopyrite-type thin films - CuInS 2 , CuInSe 2 , CuGaSe 2 , and Cu(In,Ga)Se 2 - in various completed solar cells were studied in cross-section by means of electron-backscatter diffraction (EBSD). Valuable information on grain sizes, local grain orientations, film textures, and grain boundaries were extracted from the EBSD linescans and maps. The grain-size distributions from the chalcopyrite-type thin films can be represented well by lognormal distribution functions. The EBSD measurements on CuGaSe 2 thin film reveal a fiber texture, in good agreement with x-ray diffraction texture analysis performed on the same sample. The EBSD maps from all samples studied exhibit considerable twinning in the chalcopyrite-type thin films. Indeed, the most frequent types of grain boundaries in these thin films are (near) Σ3 60°- and 71°- twins. It is shown that rotational 180°- twins (which are symmetrically equivalent to 71°- ) are more frequently found than anion- or cation-terminated 60°- twin boundaries.

Electron backscatter diffraction: Exploring the microstructure in Cu(In,Ga)(S,Se) 2 and CdTe thin-film solar cells

Electron backscatter diffraction (EBSD) in a scanning electron microscope provides access to grain-size and local-orientation distributions, film textures and grain-boundary types of thin-film solar cells. Since EBSD exhibits an information depth of only about 20 nm in Cu(In,Ga)(S,Se)2 and CdTe thin films, the most important issue of the corresponding analyses is an appropriate preparation of layer or cross-section surfaces in order to reduce possible surface roughnesses to a minimum. When EBSD is combined with energy-dispersive X-ray spectrometry (EDX), electron-beam-induced current (EBIC) and cathodoluminescence (CL) measurements, compositional as well as electrical and optoelectronic material properties are obtained, complementarily to the microstructural information.