Orientation distribution mapping of polycrystalline materials by Raman microspectroscopy

Raman microspectroscopy provides the means to obtain local orientations on polycrystalline materials at the submicrometer level. The present work demonstrates how orientation-distribution maps composed of Raman intensity distributions can be acquired on large areas of several hundreds of square micrometers. A polycrystalline CuInSe(2) thin film was used as a model system. The orientation distributions are evidenced by corresponding measurements using electron backscatter diffraction (EBSD) on the same identical specimen positions. The quantitative, local orientation information obtained by means of EBSD was used to calculate the theoretical Raman intensities for specific grain orientations, which agree well with the experimental values. The presented approach establishes new horizons for Raman microspectroscopy as a tool for quantitative, microstructural analysis at submicrometer resolution

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

The following article appeared in Journal of Applied Physics 115.1 (2014): 014504 and may be found at http://scitation.aip.org/content/aip/journal/jap/115/1/10.1063/1.4858393The 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 profiles exhibiting reduced signals across non-Σ3 GBs can be approximated by a simple model based on diffusion of generated charge carriers to the GBs.This work was supported in part by the BMU projects comCIGS and comCIGSII. R.C. acknowledges financial support from Spanish MINECO within the program Ramon y Cajal (RYC-2011-08521)

Metastable behavior of donors in CuGaSe2 under illumination

Several metastable effects have been observed in chalcopyrite solar cells. Recently, they have been related to the amphoteric behavior of the Se vacancy. We give an independent experimental evidence on this amphoteric behavior. By comparing charge carrier densities obtained from Hall effect measurements under illumination and in the dark, we conclude that illumination removes compensating donors

Band alignment at Sb2S3 Cu In,Ga Se2 heterojunctions and electronic characteristics of solar cell devices based on them

Band offsets at Sb2S3 Cu In,Ga Se2 heterojunctions have been studied by x ray and ultraviolet photoemission spectroscopy. The valence and conduction band offset have been estimated to 0.6 0.3 eV and 0.2 0.3 eV , respectively. This result suggests Sb2S3 as a potential buffer layer material for chalcopyrite based solar cells. However, Cu In,Ga Se2 Sb2S3 ZnO solar cells have been investigated. While the open circuit voltage ranged up to amp; 1011;0.4 0.5 V, the short circuit current was limited to amp; 1011;1.8 4.9 mA cm2. A photocurrent of about 30 mA cm2 was found for negative bias. On the basis of bias dependent quantum efficiency measurements and calculations, limiting mechanisms are discussed

Symmetry dependent optoelectronic properties of grain boundaries in polycrystalline Cu In,Ga Se2 thin films

In a correlative study applying electron backscatter diffraction as well as spatially and spectrally resolved cathodoluminescence spectroscopy at low temperatures of about 5 K, the symmetry dependent optoelectronic properties of grain boundaries GBs in Cu In,Ga Se2 thin films were investigated. We find that GBs with lower symmetries tend to show a distinct spectral red shift of about 10 meV and a weak influence on the emission intensity. These behaviors are not detected at high symmetry amp; 931;3 GBs, or at least in a strongly reduced way. The investigations in the present work help to clarify the ambivalent properties reported for GBs in Cu In,Ga Se

Investigation of Cu In,Ga Se2 thin film formation during the multi stage co evaporation process

In order to transfer the potential for the high efficiencies seen for Cu In,Ga Se2 CIGSe thin films from co evaporation processes to cheaper large scale deposition techniques, a more intricate understanding of the CIGSe growth process for high quality material is required. Hence, the growth mechanism for chalcopyrite type thin films when varying the Cu content during a multi stage deposition process is studied. Break off experiments help to understand the intermediate growth stages of the thin film formation. The film structure and morphology are studied by X ray diffraction and scanning electron microscopy. The different phases at the film surface are identified by Raman spectroscopy. Depth resolved compositional analysis is carried out via glow discharge optical emission spectrometry. The experimental results imply an affinity of Na for material phases with a Cu poor composition, affirming a possible interaction of sodium with Cu vacancies mainly via In Ga Cu antisite defects. An efficiency of 12.7 for vacancy compound based devices is obtaine

Evaluating different Na incorporation methods for low temperature grown CIGSe thin film on polyimide foils

The goal of this work is to investigate the influence of the Na incorporation method into CuIn1 xGaxSe2 x Ga In Ga CIGSe based solar cells on polyimide PI foil. In particular we want to compare the effect of a NaF precursor layer with that of NaF co and post deposition. Secondary ion and neutral mass spectroscopies SIMS SNMS are used to study the distribution of the elements through the CIGSe layers. Cross sectional scanning electron microscopy SEM shows the dependence of the absorber microstructure on the method of how Na is supplied with and without Ga present. Adding Ga the device microstructure is generally characterized by smaller CIGSe grains next to the Mo back contact, which indicates the very low process temperature used. The use of a NaF precursor, our standard method for the supply of Na, modifies the growth kinetic of the absorber layer and emphasizes the importance of growth parameters such as the Cu flux and max Cu content during the deposition process. Optimization of the deposition process that uses a NaF precursor, so far led to a max efficiency of 15.9 ta 0.95 cm2 . In the case of NaF post deposition the use of a low process temperature is argued to reduce the Na diffusion throughout the absorber layer, which may reduce the quality of the CIGSe Mo back interface. This may explain the lower FF observed generally for this process in comparison to that where Na is supplied by a precursor laye

Effect of compositional gradients on structural defects in Cu In,Ga Se2 thin films for solar cell

The object of this work is to study the influence of compositional gradients on the microstructure of Cu In,Ga Se2 CIGSe absorber layers for thin film solar cells. The deposition of CIGSe films in a multi stage coevaporation process leads to compositional gradients of gallium and indium, dependent on process parameters such as the integral gallium content. Transmission electron microscopy TEM imaging combined with energy dispersive X ray spectroscopy EDX enables the determination of structural defects and elemental distributions at identical sample positions. The occurrence of linear dislocations and planar defects stacking faults, microtwins was studied by means of TEM images along the lengths of the CIGSe layers. The grain size distribution is given by a electron backscatter diffraction EBSD pattern quality map. Elemental distribution maps obtained by EDX exhibit strong Ga In gradients inside a single large grain along the absorber depth. However, the CIGSe film shows not any lateral compositional inhomogeneities inside the grains or in the vicinity of the grain boundarie