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Öğe Characteristics of ZnO thin films doped by various elements(Elsevier Science Bv, 2013) Kahraman, S.; Cakmak, H. M.; Cetinkaya, S.; Bayansal, F.; Cetinkara, H. A.; Guder, H. S.We have investigated the effects of Al, K and Co dopant elements on the properties of ZnO thin films deposited by CBD method on glass substrates. Changing in morphology, structural parameters, ionization energies of impurity levels, absorption behavior and optical band gap values were investigated through scanning electron microscopy (SEM), X-Ray diffraction (XRD), resistance-temperature measurement (R-T) and ultraviolet-visible spectroscopy (UV-vis) techniques. From the SEM observations, various morphologies (rod-like, flower-like and rice-like) were observed. Those morphological variations were attributed to the change in stable growth mechanism of intrinsic ZnO, induced by different atomic radius and different electronegativity of dopants. XRD results indicated that all orientations are well indexed to hexagonal phase crystalline ZnO. The impurity level ionization energy values (Delta E) were estimated as 0.32/0.13/0.07 eV; 0.34/0.15 eV; 0.40/0.13 eV and 0.48/0.22 eV for the Al, K, Co doped samples and i-ZnO, respectively. Optical band gap values were found that the doped samples' were higher than the intrinsic one's. This increasing (blue shift) was attributed to a deterioration which occurred in the lattice of the structures after doping. This effect was also supported by the structural results. (C) 2012 Elsevier B.V. All rights reserved.Öğe Characterization of Al/n-ZnO/p-Si/Al structure with low-cost solution-grown ZnO layer(Taylor & Francis Ltd, 2013) Cetinkaya, S.; Cetinkara, H. A.; Kahraman, S.; Bayansal, F.We report the fabrication of Al/n-ZnO/p-Si/Al diode structures with a flower-like ZnO layer. The average grain size, microstrain and dislocation density in the ZnO layer were determined as 25nm, 1.55x10(-3) and 3.23x10(13)cm(-2), respectively. From absorption spectra, the optical band gap was found to be approximate to 3.17eV. A red shift was attributed to non-stoichiometry arising from Zn+2 ions substituting for oxygen vacancies. The ideality factor was determined as 1.55. The barrier height was calculated as 0.71eV from I-V characteristics and 0.73eV using the Norde plots.Öğe A comparative study of Cu2ZnSnS4 thin films growth by successive ionic layer adsorption-reaction and sol-gel methods(Elsevier Science Sa, 2014) Kahraman, S.; Cetinkaya, S.; Cetinkara, H. A.; Guder, H. S.Owing to the high natural abundance and non-toxicity of all the constituents, the copper-zinc-tin-sulfide compound has been attracting attention in recent years for the production of cheap solar absorber materials. Solution-based low-cost approaches are being developed for the deposition of this compound. In this comparative study, we have investigated Cu2ZnSnS4 thin films prepared by using successive ionic layer adsorption-reaction and sol-gel methods. X-ray diffraction studies indicated the polycrystalline nature of the films. No secondary phases were observed. The sol-gel grown Cu2ZnSnS4 film was found to have smaller crystallite size and higher microstrain and dislocation density values. Phase purity and good crystalline quality of the studied films were proved through the Raman studies. From the scanning electron microscopy images, sol-gel grown Cu2ZnSnS4 film was found to have more homogenous and smooth morphology. Possible chemical formula of the films was determined. The optical absorption of the sol-gel grown film covered a wider wavelength range by means of absorbing more visible photons. The optical band gap values were estimated to be 1.45 and 1.40 eV for the successive ionic layer adsorption-reaction and sol-gel grown samples, respectively. It was concluded that both methods are effective and can be used to produce high-quality Cu2ZnSnS4 thin films for cheap and green solar cell applications. (C) 2013 Elsevier B. V. All rights reserved.Öğe The effects of coumarin additive on the properties of ZnO nanostructures(Pergamon-Elsevier Science Ltd, 2013) Kahraman, S.; Cakmak, H. M.; Cetinkaya, S.; Cetinkara, H. A.; Guder, H. S.In this study, we investigated the effects of coumarin presence in the growth solution on the properties of ZnO. Significant changes/improvements were observed in morphology and crystal structure. From the morphological studies, it was seen that the substrates were uniformly covered with rod/rice-like nano-structures and average diameters of the structures decreased with increasing coumarin content in the growth solution. Also coumarin affected the surface morphology, coverage density and uniformity of the shape of the structures. The X-ray diffraction patterns indicated that all diffraction peaks were well indexed to hexagonal phase crystalline ZnO. The greater intensities of (0 0 2) peaks in the patterns revealed c-axis orientation normal to the substrate. The crystallization of the samples was gradually enhanced with increasing coumarin volume up to 5%. When the coumarin volume exceeded 5%, the crystallization was affected negatively. The electrical resistivity values increased with increasing coumarin content and it was attributed to the decrease in carrier concentration originated by carrier traps at the grain boundaries. (C) 2012 Elsevier Ltd. All rights reserved.Öğe Effects of diethanolamine on sol-gel-processed Cu2ZnSnS4 photovoltaic absorber thin films(Pergamon-Elsevier Science Ltd, 2014) Kahraman, S.; Cetinkaya, S.; Cetinkara, H. A.; Guder, H. S.As a promising solar absorber, the Cu2ZnSnS4 compound has been popular recently for the production of green and economical thin-film solar cells owing to the abundancy and non-toxicity of all the constituents. In this study, we have produced Cu2ZnSnS4 films via the sal-gel technique. As a stabilizer, the effects of the diethanolamine on the properties of the films were investigated. The amount of diethanolamine significantly affected the crystal structure, crystallite sizes and phase purity of the films. X-ray diffraction and Raman spectroscopy analyses confirmed the formation of phase-pure CZTS films. It was found that the film produced by using 2 ml of diethanolamine in sol exhibited pure CZTS phase, compact and dense morphology and enhanced photo-sensitivity. Light on/off current ratio of the n-Si/Cu2ZnSnS4 junction was found to be 47 under 100 mW/cm(2) of illumination. Electrical activation energies of the films were investigated and the variations were attributed to delocalized phonon states generating from the presence of other phases and lattice defects. (C) 2013 Elsevier Ltd. All rights reserved.Öğe Effects of the sulfurization temperature on sol gel-processed Cu2ZnSnS4 thin films(Elsevier Sci Ltd, 2013) Kahraman, S.; Cetinkaya, S.; Podlogar, M.; Bernik, S.; Cetinkara, H. A.; Guder, H. S.As a promising and alternative solar absorber material, the copper zinc tin sulfide compound (Cu2ZnSnS4) has been drawing attention in recent years for the production of cheap thin-film solar cells owing to the high natural abundance and non-toxicity of all the constituents, a tunable direct-band-gap energy and a large optical absorption coefficient. In addition, to overcome the problem of expensive vacuum-based methods, solution-based approaches are being developed for Cu2ZnSnS4 deposition. In this study, we have produced Cu2ZnSnS4 thin films via the sol gel technique and subsequent sulfurization. The effects of the sulfurization temperature on the structural, morphological, compositional and optical properties of the films were investigated. X-ray diffraction and Raman spectroscopy analyses confirmed the formation of phase-pure CZTS films. The crystallinity of the films increased with an increasing sulfurization temperature. From the surface images and the results of the composition analysis, it was found that the films are uniform, composed of homogenously distributed grains and have compositions with Cu deficit. The values of the optical absorption coefficients for the films were found to be 10(4) cm(-1) based on absorbance spectroscopy. The optical band-gap values were estimated to be between 1.32 and 2.27 eV depending on the sulfurization temperature. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.Öğe Growth and Characterization of CuO Nanostructures on Si for the Fabrication of CuO/p-Si Schottky Diodes(Hindawi Publishing Corporation, 2013) Cetinkaya, S.; Cetinkara, H. A.; Bayansal, F.; Kahraman, S.CuO interlayers in the CuO/p-Si Schottky diodes were fabricated by using CBD and sol-gel methods. Deposited CuO layers were characterized by SEM and XRD techniques. From the SEM images, it was seen that the film grown by CBD method is denser than the film grown by sol-gel method. This result is compatible with XRD results which show that the crystallization in CBD method is higher than it is in sol-gel method. For the electrical investigations, current-voltage characteristics of the diodes have been studied at room temperature. Conventional I-V and Norde's methods were used in order to determine the ideality factor, barrier height, and series resistance values. It was seen that the morphological and structural analysis are compatible with the results of electrical investigations.Öğe Improved characteristics for chemically grown Cu2SnS3 promising solar absorbers through the use of TritonX-100® surfactant(Elsevier Science Sa, 2015) Yasar, S.; Kahraman, S.; Cetinkaya, S.; Bilican, I.In this paper, we report, for the first time, the results of the TritonX-100 (R) surfactant assisted growth of Cu2SnS3 thin films obtained by using sol-gel spin coating method and a subsequent annealing in a sulfur atmosphere. Structural, morphological, compositional, photo-electrical investigations have been carried out. X-ray diffraction patterns of the samples matched well with the reference Cu2SnS3 pattern and indicated the polycrystalline nature of the films. Crystallite size of the films increased whereas surface roughness of the films decreased with increasing Triton-X100 (R) content. The surface of the samples has been smoother with Triton-X100 (R) inclusion. The photo-sensitivity of the n-Si/CTS structures has been confirmed through photo-transient current measurements. An increment was observed in the photoinduced current values of the samples with increasing Triton-X100 (R) content and was attributed to transport of photo-induced electrons facilitated with decreasing recombination resulted from the surface enhancement of the films. Electrical conduction mechanism of the films was investigated with resistance-temperature measurements. It has been revealed that TritonX-100 (R) surfactant assisted growth is a promising way to improve conversion efficiency of Cu2SnS3 based solar cells. (C) 2014 Elsevier B.V. All rights reserved.Öğe A novel study on ZnO nanostructures: coumarin effect(Taylor & Francis Ltd, 2012) Cakmak, H. M.; Kahraman, S.; Bayansal, F.; Cetinkaya, S.This novel study reports the behaviour of coumarin during the ZnO chemical bath deposition (CBD) process. Scanning electron microscopy studies revealed that grain sizes of the ZnO nanostructures get smaller and become more uniform in size with increasing coumarin content. Length-to-diameter ratio of the structures increased with the incorporation of coumarin. From the X-Ray diffraction results, with increasing coumarin content, significant decrease in grain sizes was determined. It was found that resistivity values of the structures increased with increasing coumarin content. This increase is attributed with a decrease in carrier concentration originated by carrier traps at the grain boundaries. Through slopes of lnR versus 1/T curves, impurity level ionization energy values were calculated as 0.16, 0.06 eV; 0.11, 0.02 eV; and 0.08, 0.06 eV for the structure growth in the solutions containing 0, 1 and 5 at.% coumarin, respectively.Öğe Numerical thickness optimization study of CIGS based solar cells with wxAMPS(Elsevier Gmbh, 2016) Yasar, S.; Kahraman, S.; Cetinkaya, S.; Apaydin, S.; Bilican, I.; Uluer, I.In this paper, a numerical investigation of the effects of thickness of absorber and buffer layers for a typical CIGS based solar cell was carried out for the first time, employing the wxAMPS software which is a new custom-designed software package for solar cell simulation. We have examined how the output parameters change with absorber and buffer thicknesses and suggested four different cell structures. By using the baseline structure, an optimum band gap value of 1.4 eV was determined for the CIGS absorber layer prior to do thickness study. Optimum absorber thickness value has been determined as 2500 nm from the simulation results. Spectral response of the cell decreased with increasing thickness in blue region. The influence of buffer layer thickness was investigated and it was found that most of the photo-generated carriers are collected by a thinner CdS layer. A significant decrease in current density, open circuit voltage and conversion efficiency occurred with increasing buffer layer thickness. This behavior was attributed to a decrease in the number of collected carriers due to the recombination of electron-hole pairs proceeded from the absorbed photons in the CdS bulk for large thickness. As a result, efficiencies around 27% were obtained through optimization while that of baseline structure was 25%. Our results have shown that the suggested structures both produce higher efficiencies than that of baseline structure and may lead to produce more efficient CIGS-based thin film solar cells. (C) 2016 Elsevier GmbH. All rights reserved.Öğe Polyethylene glycol-assisted growth of Cu2SnS3 promising absorbers for thin film solar cell applications(Taylor & Francis Ltd, 2014) Kahraman, S.; Cetinkaya, S.; Yasar, S.; Bilican, I.In this paper, we report, for the first time, the results of the polyethylene glycol- (PEG) assisted preparation and characterization of high-quality and well-crystallized Cu2SnS3 (CTS) thin films obtained using sol-gel spin-coating method and a subsequent annealing in a sulphur atmosphere. Structural, morphological, compositional, electrical and optical investigations were carried out. The X-ray diffraction patterns of the samples proved the polycrystalline nature and preferred crystallization of the films. No peak referring to other binary or ternary phases were detected in the patterns. The intensity of the preferred orientation and crystallite size of the films increased with increasing PEG content. This trend yielded an improvement in photo-transient currents of the PEG-assisted growth of CTS films. The scanning electron microscopy images revealed that the CTS films have continuous, dense and agglomeration-like morphology. Through energy dispersive X-ray spectroscopy studies, it has been deduced that the samples consist of Cu, Sn and S of which atomic percentages were consistent with Cu/Sn and S/metal initial ratios. The agglomerated morphology of the samples has been attributed to increasing PEG content. A remarkable enhancement was observed in photo-transient currents of p-n junction of the produced films along with increasing PEG content. Through resistivity-temperature measurements, three impurity level electrical activation energy values for each film were found. Optical band gap values of the films were estimated via absorbance-wavelength behaviours and decreased with increasing PEG content. It has been revealed that PEG-assisted growth of CTS thin films is a promising way to improve its photovoltaic characteristics.
