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Bayrakl O.,Middle East Technical University | Bayrakl O.,Center for Solar Energy Research and Applications | Bayrakl O.,Ahi Evran University | Terlemezoglu M.,Middle East Technical University | And 6 more authors.
Journal of Alloys and Compounds | Year: 2017

Characterization of Cu2ZnSnSe4 (CZTSe) thin films deposited by thermal evaporation sequentially from the pure elemental sources and in-situ post annealing was carried out at 400 C under Se evaporation atmosphere. Another annealing process was applied in nitrogen atmosphere at 450 °C to get polycrystalline monophase CZTSe film structure. XRD analysis together with Raman spectroscopy was used to determine the structural properties. Spectral optical absorption coefficient evaluated from transmission data showed the band gap value of 1.49 eV for annealed film. Electrical measurements indicated that CZTSe thin films have p-type semiconductor behavior with the carrier density and mobility values of 10−19 cm−3 and 0.70 cm2/(V.s). Illumination effects on the device properties of Ag/n-Si/p-CZTSe/In heterostructure were investigated by analyzing current-voltage(I-V) and frequency dependent capacitance-voltage(C-V) data. Under the illumination, Ag/n-Si/p-CZTSe/In heterostructure showed photodiode behavior having Voc value of 100 mV and Isc value of 27.5 μA. With the illumination, series resistances (Rs), diode ideality factor (n) and barrier height (Φb) decreased and shunt resistance (Rsh) increased. Capacitance value at lower frequency decreased due to the illumination effect. © 2017 Elsevier B.V.


Saleh Z.M.,Center for Solar Energy Research and Applications | Saleh Z.M.,Arab American University - Jenin | Nasser H.,Center for Solar Energy Research and Applications | Ozkol E.,Center for Solar Energy Research and Applications | And 4 more authors.
Physica Status Solidi (C) Current Topics in Solid State Physics | Year: 2015

Plasmonic interfaces integrated to the front, back and both surfaces of photovoltaic thin films show different degrees of enhancement of light trapping. Enhancements in the spectral dependence of photocurrent normalized to the power of excitation light are used as an indicator of enhanced light trapping. In a previous study, we obtained enhancement in the spectral range of 600-700 nm by integrating 100-nm Ag nanoparticles to the back surface of a-Si:H with a critical dependence on the SiNx spacer layer thickness. In this study, we compare the enhancement in photocurrent due to plasmonic interfaces integrated to the front, back and both front and back surfaces of the a-Si:H absorber. Interfaces integrated to the back result in the largest enhancement in photocurrent while those integrated to the front give the lowest. The marginal enhancements due to two interfaces appear to be mainly due to the back interface. While plasmonics effects may not account for the total enhancement in photocurrent, it explains the relative enhancement in the spectral range of 550-700 rather well. For all configurations, the enhancement in the spectral dependence of photocurrent is accompanied by broadening into the red of the localized surface plasmon resonance (LSPR). © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Gullu H.H.,Middle East Technical University | Gullu H.H.,Center for Solar Energy Research and Applications | Coskun E.,Canakkale Onsekiz Mart University | Parlak M.,Middle East Technical University | Parlak M.,Center for Solar Energy Research and Applications
Physica Status Solidi (C) Current Topics in Solid State Physics | Year: 2015

In this work, zinc selenide (ZnSe) thin films were deposited by thermal evaporation method using pure elemental (Zn and Se) sources. The physical properties of the films have been investigated in terms of the structural and optical characterizations depending on the post-annealing process under nitrogen atmosphere in the temperature between 300 and 500 °C for 30 min. The structural and compositional analyses were carried out by means of X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The compositional analysis indicated that the deposited films were nearly stoichiometric whereas there was a decrease in Se and increase in the Zn contents. This implies the segregation and/or reevaporation of Se atoms from the thin film structure. The optical characteristics were studied by using the room temperature transmission measurements. The analysis of transmission values showed that the band gap values changed in between 2.38 and 2.62 eV depending on the annealing temperatures. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Tugay E.,Middle East Technical University | Tugay E.,Center for Solar Energy Research and Applications | Yilmaz E.,Abant Izzet Baysal University | Turan R.,Middle East Technical University | Turan R.,Center for Solar Energy Research and Applications
Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films | Year: 2012

The authors studied effects of gamma radiation on the electrical properties of the MOS capacitor mČadež on SiN x thin films with thickness of 100 nm deposited on p-type (100) silicon wafer using plasma-enhanced chemical vapor deposition method. The authors investigated the chemical bonds and their densities inside the films using Fourier transform infrared (FTIR) spectroscopy. The as-deposited and annealed samples with Al/SiN x/Si structure as metal-insulator-semiconductor (MIS) capacitors were exposed to a 60-Co gamma radiation source with a dose rate of 0.015 Gy/S. The authors performed capacitance-voltage measurements at frequencies 10, 100, and 1000 kHz before and after radiation exposure with doses of up to 40 Gy. It was found that before gamma irradiation compared with as-deposited sample, the annealed samples exhibit less negative flatband voltages (V fb) shift. This indicates the relative reduction in positive charge in the SiN x:H samples. After gamma irradiation, for all samples a negative shift has been observed in V fb, being more pronounced in the samples annealed at 700 °C. The more striking feature is that the amount of shift does not change by increasing radiation dose after first irradiation, in which we attributed what to the radiation hardening in Al/SiN x/Si MIS capacitors. © 2012 American Vacuum Society.


Coskun E.,Middle East Technical University | Coskun E.,Canakkale Onsekiz Mart University | Coskun E.,Center for Solar Energy Research and Applications | Gullu H.H.,Middle East Technical University | And 5 more authors.
Journal of Low Temperature Physics | Year: 2014

The structural properties and electrical conduction mechanisms of Ag–Ga–In–Te thin films deposited by a combination of e-beam and thermal evaporation methods were studied for various annealing temperatures. Structural analysis showed the existence of In(Formula presented.) and In(Formula presented.) binary phases at the early stage of crystallization and monophase of AgGa(Formula presented.) with the main orientation along (112) direction following the post-annealing at 400 (Formula presented.)C. The effects of the structural changes on electrical properties and temperature dependence of the electrical conductivity of Ag–Ga–In–Te thin films were studied in the temperature range of 90–400 K. The analysis of electrical conductivity revealed the Efros–Shklovskii variable range hopping (VRH) mechanism in between 90 and 210 K and Mott VRH mechanisms for the temperature range of 250–400 K for all deposited films. The VRH parameters including average hopping distance, average hopping energy and characteristic temperature coefficient for Efros–Shklovskii and Mott VRH mechanisms were determined and discussed in detail. © 2014, Springer Science+Business Media New York.


Hosseini A.,Middle East Technical University | Hosseini A.,Center for Solar Energy Research and Applications | Icli K.C.,Middle East Technical University | Icli K.C.,Center for Solar Energy Research and Applications | And 4 more authors.
Energy Procedia | Year: 2014

Porous TiO2 films were deposited on glass and ITO coated glass substrates by spin coating for spinning rates of 2000, 5000, 2000 followed by successive run of 5000 rpm (2000-5000). The AFM, SEM, XRD and UV-Vis analysis of the films coated on bare glass substrates for individual and 2000-5000 rpm runs showed amorphous and crystalline behaviours respectively, while TiO2 films deposited on ITO/Glass substrates show crystalline structure. Both substrate nature and thickness of the films influence the formation of the crystallinity of the films. Also AFM images reveal that the roughness values of the films are independent of substrate. © 2014 The Authors.


Gullu H.H.,Middle East Technical University | Gullu H.H.,Center for Solar Energy Research and Applications | Candan I.,Middle East Technical University | Candan I.,Center for Solar Energy Research and Applications | And 4 more authors.
Optik | Year: 2015

Annealing effect on the structural and optical properties of the quaternary Cu-Ag-In-Se thin film deposited by the thermal evaporation has been investigated. The evaporation source was prepared by using vertical Bridgman-Stockbarger crystal growth system. Structural analysis indicated that annealing the films following to the deposition resulted in the changes from amorphous to polycrystalline phase with the preferred orientation along (1 1 2) direction. In order to determine the optical properties of the thin films as a function of annealing temperature, the transmission measurements were carried out in between 300 and 2000 nm. The optical band gap values were lying in between 1.29 and 1.50 eV upon annealing the thin films in the temperature range of 300-500 °C. The refractive indices of the samples were in the range of 2.7-3.8 depending on the wavelength region and annealing temperature by applying the Envelope Method. The other optical constants of the samples were also calculated using Cauchy Method and Single Oscillator Model. © 2015 Elsevier GmbH.


Coskun E.,Middle East Technical University | Coskun E.,Canakkale Onsekiz Mart University | Coskun E.,Center for Solar Energy Research and Applications | Gullu H.H.,Middle East Technical University | And 9 more authors.
Materials Science in Semiconductor Processing | Year: 2015

In this work, p-(Ag-Ga-In-Te) polycrystalline thin films were deposited on soda-lime glass and n-type Si substrates by e-beam evaporation of AgGa0.5In0.5Te2 crystalline powder and the thermal evaporation of Ag powder, sequentially in the same chamber. The carrier concentration and mobility of the Ag-Ga-In-Te (AGIT) film were determined as 5.82×1015 cm-3 and 13.81 cm2/(V s) as a result of Hall Effect measurement. The optical analysis indicated that the band gap values of the samples were around 1.58 eV. The structural analysis was carried out by means of X-ray diffraction. Current-Voltage (I-V) measurements depending on the sample temperature were performed to investigate the device characteristics and the dominant conduction mechanism in an In/p-AGIT/n-Si/Ag structure. The series and shunt resistances were calculated by the help of parasitic resistance analysis as 5.73 and 1.57×104 Ω cm2, respectively at room temperature. The ideality factors and barrier heights were evaluated as a function of sample temperature. In the low bias region, the thermionic emission together with the generation-recombination mechanism was investigated as the dominant transport mechanism; however, in the high bias region, space charge limited current was analyzed as the other effective mechanism in the carrier conduction. The built-in potential of the device was also determined by the help of capacitance-voltage measurements. © 2015 Elsevier Ltd.


Gullu H.H.,Middle East Technical University | Gullu H.H.,Center for Solar Energy Research and Applications | Coskun E.,Middle East Technical University | Coskun E.,Canakkale Onsekiz Mart University | And 3 more authors.
Brazilian Journal of Physics | Year: 2014

In this study, annealing effect on the structural, electrical, and optical characteristics of the quaternary Cu-Ag-In-Se (CAIS) thin films was investigated. These samples were deposited by co-evaporation of the Cu, Ag, In2Se3, and Se sources at the substrate temperature of 300 °C. The structural properties of the thin films were analyzed by means of X-ray diffraction, and the results indicated that all of the films were in the polycrystalline structure with the preferred orientation along (112) direction. From the optical measurements, the band gap values were found to vary between 1.38 and 1.45 eV with annealing processes. The temperature-dependent electrical conductivity of the samples was measured in the temperature range of 90–400 K. The films gained degenerate behavior with increasing annealing temperature. The carrier conduction mechanism was determined at high- and low-temperature regions by comparing thermionic emission and hopping parameters. Photoconductivity of the as-grown film showed that there was an increase in conductivity with increasing illumination intensity. From this measurement, the variation of photocurrent as a function of illumination intensity was determined. © 2014, Sociedade Brasileira de Física.


Coskun E.,Middle East Technical University | Coskun E.,Canakkale Onsekiz Mart University | Coskun E.,Center for Solar Energy Research and Applications | Gullu H.H.,Middle East Technical University | And 3 more authors.
Materials Research Express | Year: 2015

An In/n-AgGa0.5In0.5Se2/p-Si/Al heterostructure was produced by thermal sequential stacked layer deposition method and the device characteristics were investigated. The compositional analysis showed that the depositions of the intended stoichiometric composition of AgGa0.5In0.5Se2 structure were obtainable by controlling and providing the necessary deposition conditions during the deposition processes. By means of the room temperature Hall effect and transmission measurements, the carrier concentration and optical band gap values were determined as 9 × 1015 cm-3 and 1.65 eV, respectively. In addition, temperature-dependent current-voltage (I-V) and the room temperature capacitancevoltage (C-V) measurements of this heterostructure were carried out. The rectification factor was obtained as about 104 at 1.20V for all sample temperatures. Depending on the change in the temperature, the series and shunt resistances were calculated as 101 and 106Ω, respectively. The studies on the current transport mechanisms showed that there were two different mechanisms at two different voltage regions: tunneling enhanced recombination mechanism in the voltage range of 0.08 and 0.30V and the space charge limited current mechanism in the voltage range of 0.30 and 0.60 V. The barrier height, built-in potential and interface states density of the deposited heterostructure were also calculated and discussed. © 2014 IOP Publishing Ltd.

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