Search

Abstract: In order to understand the effect of the process temperature on the growth of Cu(In,Ga)Se2 (CIGSe) thin films via the 3-stage co-evaporation process, absorber layers have been fabricated on glass using a set of different maximum process temperatures in the nominal temperature range between 330 and 525°C. Using energy dispersive X-ray analysis, depth profiles could be recorded on cross-sections of finished devices and were correlated to the device performance. The effect of the process temperature on the gallium gradient in the CIGSe layer is evident in the gallium distribution across the depth of the device. [Copyright &y& Elsevier]

In this study, kesterite Cu 2 ZnSnS 4 (CZTS) absorber layers were fabricated by DC magnetron sputtering deposition of metallic Cu-Zn-Sn precursors, followed by an annealing treatment in sulfur vapor atmosphere at 600 °C for 3 min using rapid thermal processing (RTP). Three types of stacked metallic films were prepared and included pre-annealing of Cu-Sn stacks in order to induce preferential Cu-Sn alloying. The chemical composition of the sulfurized films was obtained by X-ray fluorescence (XRF) before and after etching the samples in KCN solution. All CZTS thin films are found to be Cu-poor and Zn-rich. Structural characterizations were performed by X-ray diffraction (XRD) and Raman spectroscopy to investigate the impact of pre-annealing on the structural properties of the precursors and final CZTS films. Glow discharge optical emission spectroscopy (GDOES) shows that pre-annealing of the precursors can improve depth homogeneity of the CZTS films. Photoluminescence spectra and the optical band gap energy values are compatible with literature. Selected samples were processed to solar cells and characterized. [ABSTRACT FROM AUTHOR]

The variation of the sodium content in low-temperature grown Cu(In,Ga)Se 2 (CIGSe) absorbers has a strong influence on the doping level and the photoelectrical properties of the bare CIGSe films and corresponding solar cells. The negative impact of excessive sodium contents is studied for samples from CIGSe processes with co-evaporation of NaF during absorber deposition. By a combined analysis of temperature dependent current–voltage, capacitance-voltage and quantum efficiency measurements we conclude that the dominating recombination takes place in the bulk of the absorber for all samples independent of the sodium content. The optimum sodium content with respect to solar cell performance results from a tradeoff between the beneficial influence of sodium on open circuit voltage ( V OC ) and the negative impact of high sodium contents on short circuit current ( I SC ) and fill factor ( FF ). The change of V OC , I SC and FF can be explained by a concurrent increase of net doping and deep defect density for increasing sodium content and the presence of barriers at the CIGSe/CdS and the CIGSe/Mo interface. [ABSTRACT FROM AUTHOR]

Highlights: [•] Nondestructive chemical quantitative analysis of Cu(In,Ga)Se2 by photoluminescence. [•] Extension to nondestructive Ga depth profile assessment with confocal measurements. [•] Correlation with Raman scattering microcrystalline and phase assessment. [•] Demonstration of methodologies for monitoring of advanced CIGS PV technologies. [Copyright &y& Elsevier]

Abstract: Sodium is known to play an important role for the performance optimization of Cu(In,Ga)Se2 (CIGSe) thin film solar cells, i.e. it is found to significantly increase the open-circuit voltage (V OC). For this work CIGSe absorbers were produced in a low temperature deposition process on polyimide substrates with varying sodium content. In agreement with previous studies we find an increase of V OC and overall solar cell performance with increasing sodium content and a decrease of the overall performance for very high sodium content. At the same time time-resolved photoluminescence measurements indicate that the minority carrier lifetime decreases from 50ns to about 5ns for increasing sodium content in the absorber. This correlation was found to be valid for three different types of low-temperature CIGSe deposition processes. The results can be explained by an increase in deep defect density with increasing Na content, proportional to the increased net charge carrier density as measured by capacitance–voltage profiling. For very high sodium content this leads to an actual decrease in the overall performance of the solar cells. [Copyright &y& Elsevier]

Cu2ZnSnS4 single crystals prepared by the chemical vapor transport technique were investigated by x-ray diffraction, chemical microprobe analysis, and Raman and photoluminescence spectroscopies. Single-crystal x-ray measurements show that Cu2ZnSnS4 crystallizes in the kesterite structure with the space group 74 and unit cell parameters a = 5.4174(2) and c = 10.7903(7) Â at 150 K. Photoluminescence measurements show one broad asymmetric band peaked at 1.29 eV at low temperature. Temperature and excitation intensity dependencies of the luminescence band indicate that the transition is due to a free-to-bound recombination with an activation energy of 140 meV. We attribute this activation energy to the intrinsic acceptor-type CuZn antisite defect in close agreement with results from density-functional theory. It is shown that by a detailed analysis of the temperature dependence of the free-to-bound luminescence transition the majority carrier density can be deduced, which in our case is estimated at p ≈ 1016 cm-3. [ABSTRACT FROM AUTHOR]

Abstract: 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 738mV, short-circuit current 19.3mA/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 1.04eV, 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. [Copyright &y& Elsevier]

Abstract: Cu(In,Ga)Se2-absorber deposition on commonly used soda lime glass is constrained in temperature by the softening of the substrate. To overcome this limitation, a high-temperature resistant glass was employed as a substrate for the growth of Cu(In,Ga)Se2-absorbers by multi-stage coevaporation at standard (530°C) and elevated (610°C) temperatures. Absorbers were investigated using cathodoluminescence and X-ray diffraction and compared to the performance of solar cells fabricated from these absorbers. The higher deposition temperature is shown to lead to an increased homogeneity of the absorber layer both laterally and vertically and strongly enhanced open-circuit voltage. A best certified efficiency of 19.4% is reached. [ABSTRACT FROM AUTHOR]