Dielectric and electric studies of the [N(CH3)4][N(C2H5)4]ZnCl4 compound at low temperature

The [N(CH₃)₄][N(C₂H₅)₄]ZnCl₄ compound was prepared and characterized by electrical technique. The temperature dependence of the dielectric permittivity shows that this compound is ferroelectric below T = 268 K. The two semi-circles observed in the complex impedance identify the presence of the grain interior and grain boundary contributions to the electrical response in the material. The equivalent circuit is modeled by a combination series of two parallel R_P–CPE circuits. The frequency dependent conductivity is interpreted in term of Jonscher's law. The modulus plots can be characterized by the empirical Kohlrausch–Williams–Watts (K.W.W.) function: ?(t) = exp [(−t/τ)^β]. The temperature dependence of the alternative current conductivity (σ_p), direct current conductivity (σ_dc) and the relaxation frequency (f_p) confirm the presence of the ferroelectric–paraelectric phase transition.     

Influence of absorber copper concentration on the Cu(In,Ga)Se2/(PVD)In2S3 and Cu(In,Ga)Se2/(CBD)CdS based solar cells performance

The present contribution deals with the influence of the copper concentration in Cu(In,Ga)Se₂ (CIGSe) on the solar cells based on CIGSe/(PVD)In₂S₃ and CIGSe/(CBD)CdS. We find that, depending on the buffer layer, the optimum open circuit voltage (Voc) is not reached for the same copper concentration. The values of Voc for the CIGSe/(CBD)CdS solar cells are higher when the copper content is very close to stoichiometry (25%), whereas, the Voc values for CIGSe/(PVD)In₂S₃ solar cells attain their maximum for lower copper contents. On the other hand, contrary to the case of the (CBD)CdS buffer, the Jsc is strongly hindered for the (PVD)In₂S₃ buffered cells when the copper content is lowered. The study has been made for different absorber gallium contents and the evolution is coherent with the presence of a cliff at the CIGSe/(PVD)In₂S₃ interface.     

Comparative study of Cu(In,Ga)Se2/(PVD)In2S3 and Cu(In,Ga)Se2/(CBD)CdS heterojunction based solar cells by admittance spectroscopy, current-voltage and spectral response measurements

Co-evaporated Cu(In,Ga)Se₂ (CIGSe) based solar cells with Physical Vapour Deposited (PVD) Indium Sulphide (In₂S₃) as buffer layer have been studied by admittance spectroscopy and current-voltage characteristics measurements. The results have been compared to those obtained with a reference CBD-CdS/CIGSe device. In darkness, the PVD-In₂S₃ buffer layer devices exhibit higher densities of trapping defects and low values of shunt resistance. However, under illumination we have observed an important improvement of the In₂S₃/CIGSe electronic transport properties. This behavior seems to be linked to the presence of a metastable defect with activation energy of 0.3 eV.     

Effect of copper-deficiency on multi-stage co-evaporated Cu(In,Ga)S2 absorber layers and solar cells

Solar cell absorber films of Cu(In,Ga)S₂ have been fabricated by multi-stage co-evaporation resulting in compositional ratios [Cu]/([In] + [Ga]) = 0.93-0.99 and [Ga]/([In] + [Ga]) = 0.15. Intentional doping is provided by sodium supplied from NaF precursor layers of different thicknesses. Phases, structure and morphology of the resulting films are investigated by X-ray diffraction (XRD) and scanning electron microscopy. The XRD patterns show CuIn₅S₈ thiospinel formation predominantly at the surface in order to accommodate decreasing Cu content. Correlated with the CuIn₅S₈ formation, a Ga-enrichment of the chalcopyrite phase is seen at the surface. Since no CuS layer is present on the as-deposited films, functioning solar cells with CdS buffer and ZnO window layers were fabricated without KCN etch. The open-circuit voltage of solar cells correlates with the copper content and with the amount of sodium supplied. The highest efficiency cell (open-circuit voltage 738 mV, short-circuit current 19.3 mA/cm², fill factor 65%, efficiency 9.3%) is based on the absorber with the least Cu deficiency, [Cu]/([In] + [Ga]) = 0.99. The activation energy of the diode saturation current density of such a cell is extracted from temperature- and illumination-dependent current-voltage measurements. A value of 1.04 eV, less than the band gap, suggests the heterojunction interface as the dominant recombination zone, just as in cells based on Cu-rich grown Cu(In,Ga)S₂.     

Impact of the Se evaporation rate on the microstructure and texture of Cu(In,Ga)Se2 thin films for solar cells

Coevaporated Cu(In,Ga)Se₂ layers on Mo-coated soda-lime glass substrates were produced by a three-stage process using various Se overpressure conditions during the three stages. Cross-sections of these samples were analyzed by electron backscatter diffraction (EBSD) in a scanning electron microscope in order to reveal the microstructures in the Cu(In,Ga)Se₂ layers. In addition, the preferential orientations of these Cu(In,Ga)Se₂ layers were studied by plan-view EBSD measurements. It was found that Cu(In,Ga)Se₂ exhibits a texture in 110 orientation for Se/(Cu + In + Ga) atomic flux ratios R which are sufficiently large (≥ 4). In one Cu(In,Ga)Se₂ layer produced with approximately R = 4, a large density of (near) Σ3 (twin) boundaries were detected which are oriented preferentially perpendicular to the substrate. By comparison of the local textures of neighboring grains and the theoretically possible changes in orientation by twinning, it is possible to retrace how the twinning occurred.