Raman and photoreflectance study of Cu In,Ga S2 films and solar cells

The structural and optical properties of CuIn1 xGaxS2 CIGS , CdS CuIn1 xGaxS2, and ZnO CdS CuIn1 xGaxS2 polycrystalline films, with applications in photovoltaics, were studied by Raman and Photoreflectance PR spectroscopy for two different compositions, [Ga] [In] [Ga] 0.04 and 0.12, of the CuIn1 xGaxS2 absorber. The energy band gap of the absorber film was determined by fitting the PR spectra with a third derivative functional form. Moreover, the thickness of the film was calculated from the interference fringes observed in the PR spectra below band gap energy. Raman scattering was excited by the 514.5nm line of Ar laser and the 647.1nm line of Kr laser. The Raman spectra of the absorber films consist of phononmodes assigned to CuInS2, CuGaS2 and CuS. The results of the present study are discussed together with the results of SEM and XRD studies of the films and the results of electrical measurements performed on solar cells based on the CIGS absorber

Widegap Cu InGa S2 thin film solar cells on transparent substrates

ABSTRACT We have previously demonstrated the potential of Cu In,Ga S2 CIGS for realizing high efficiency wide gap chalcopyrite based solar cells. CIGS absorber films were prepared by multi source evaporation using a Cu poor Cu rich deposition sequence onto soda lime glass substrates coated with different transparent conductive oxides TCOs . The Ga In ratio was adjusted for a band gap of more than 1.6 eV. The properties of films and cells are compared to those deposited onto standard glass Mo substrates. Films with reasonable morphology and good adhesion to the substrate were obtained on most of the investigated TCOs. Also, electrical and optical properties of the TCO typically did not deteriorate strongly during absorber preparation. Our best result so far is a cell with an efficiency of about 8 on indium tin oxide ITO . At an absorber band gap of 1.64 eV this cell exhibited an open circuit voltage of 775 mV, a fill factor of 55 and a short circuit current density of 18.6 mA cm2. To our knowledge, this is currently the best wide gap chalcopyrite based cell on a glass TCO substrate. The cell was slightly shunted but there were no indications for a non ohmic behavior of the ITO CIGS contact Keywords CuInS2 , Thin Film , TCO Transparent Conducting Oxide

The effect of sulphur pressure on the depth distribution of elements in Cu In,Ga S2 films

We have studied the deposition of Cu In,Ga S2 films for thin film solar cells. In this study, Cu In,Ga S2 films have been prepared with varied sulphur pressure. A sequential multi source evaporation process has been employed. Deposition of an In Ga S precursor layer was followed by the diffusion of Cu and S into the precursor layer. The depth profiles of the impurities O and C, and constituent elements In and Ga were investigated by secondary ion mass spectroscopy SIMS . Coevaporated films were measured for comparison and the concentrations of O, C, In, and Ga were almost constant throughout the film. In case of our sequential process, a higher Ga concentration inside the Cu In,Ga S2 layer and a lower one towards the surface has been confirmed. X ray diffraction XRD data also confirm the existence of two Cu In,Ga S2 layers with different Ga content. It has been found that the O concentration was constant in the bulk and maximum at the interface between Ga rich bottom layers and In rich surface layers with a decreased concentration next to the surface. On the other hand, C was also nearly constant in the bulk, but tended to increase in a layer close to the surface. A model of the incorporation of impurities will be proposed. The changes of concentrations of O, C, In, and Ga become less pronounced with increasing S pressure. This indicates that higher S partial pressure may promote the diffusion of the constituent elements and impurities in the film. A solar cell made from one of our Cu In,Ga S2 films showed an efficiency of 9.3 total area, no antireflection coating, effective band gapg1.52 e

Three stage evaporation of Cu In,Ga S2 solar cell absorber films without KCN treatment and Na control

Cu poor Cu In,Ga S2 films have been prepared by three stage sequential evaporation In,Ga S Cu S In,Ga S on Mo covered soda lime glass without Na control. The depth profiles of O and Na in the grown films were investigated by secondary ion mass spectroscopy SIMS . It has been found that the O concentration was constant in the bulk, and decreased close to the surface comparable to the case of the two stage process. The observed depth profile of Na resembles that of O. The correlation of the depth profiles between O and Na is again indicated. The efficiencies of solar cells from the O2 annealed Cu rich Cu In,Ga S2 films did not increase, but the efficiencies of some solar cells prepared from Cu poor Cu In,Ga S2 films increased when the absorber was annealed in oxygen. The best efficiency of a solar cell from our O2 annealed Cu poor Cu In,Ga S2 films was 9.3 no antireflection coating without KCN treatment and Na contro

Band gap grading in multi source evaporated Cu In,Ga S2 thin films

Wide gap chalcopyrite based solar cells with good performance were achieved previously by sequential multi source evaporation of Cu In,Ga S2 absorbers. The gallium incorporation was inhomogeneous with the Ga In Ga ratio being higher close to the back contact. In this work we investigated process modifications which were aimed at gallium enrichment towards the front surface of the absorber in order to realize a V structure band gap grading. It is found that the recrystallization which is an important feature of the sequential process has to be taken into account when designing modified processes. SIMS depth profiling indicates that one of the investigated process modifications achieves the desired gallium distribution. The quantum efficiency confirms the presence of a strong band gap grading in this cas

Unidirectional diagonal order and three-dimensional stacking of charge stripes in orthorhombic Pr1.67Sr0.33NiO4Pr_{1.67}Sr_{0.33}NiO_{4} and Nd1.67Sr0.33NiO4Nd_{1.67}Sr_{0.33}NiO_{4}

The interplay between crystal symmetry and charge stripe order in Pr1.67Sr0.33NiO4 and Nd1.67Sr0.33NiO4 has been studied by means of single crystal x-ray diffraction. In contrast to tetragonal La1.67Sr0.33NiO4, these crystals are orthorhombic. The corresponding distortion of the NiO2 planes is found to dictate the direction of the charge stripes, similar to the case of diagonal spin stripes in the insulating phase of La2-xSrxCuO4. In particular, diagonal stripes seem to always run along the short a-axis, which is the direction of the octahedral tilt axis. In contrast, no influence of the crystal symmetry on the charge stripe ordering temperature itself was observed, with T_CO 240K for La, Pr, and Nd. The coupling between lattice and stripe degrees of freedom allows one to produce macroscopic samples with unidirectional stripe order. In samples with stoichiometric oxygen content and a hole concentration of exactly 1/3, charge stripes exhibit a staggered stacking order with a period of three NiO2 layers, previously only observed with electron microscopy in domains of mesoscopic dimensions. Remarkably, this stacking order starts to melt about 40K below T_CO. The melting process can be described by mixing the ground state, which has a 3-layer stacking period, with an increasing volume fraction with a 2-layer stacking period.Comment: 13 pages, 13 figure