[Show abstract][Hide abstract] ABSTRACT: The sputtered-Zn(O,S) and -CdS films were prepared as a buffer layer in CuInSe2 (CIS) in the bottom cell of a Cu(In,Ga)Se2 (CIGS)-tandem-structured solar cell. We characterized the thermal stability and long wavelength response of the CIS solar cell. The thermal stability of CIS solar cell is tested in increasing annealing temperature up to 500 °C in a vacuum for 60 min. The optical response of CIS solar cell was measured by using long-wavelength pass filter to verify availability as a bottom cell. The power conversion efficiency (PCE) of the sputtered-Zn(O,S)/CIS cell showed a slight drop from 9.73% as-fabricated to 9.61% after 60 min of annealing at 300 °C, whereas the 7.97% as-fabricated PCE of the sputtered-CdS/CIS cell fully deteriorated above 300 °C at 60 min of annealing. The PCE of as-fabricated-Zn(O,S)/CIS was 5.46% by using the selective 715 nm long-pass filter. This value remains almost unchanged after post-annealing, whereas the 4.36% PCE of as-fabricated-CdS/CIS solar cell by using the 715 nm long-pass filter continuously dropped after 60 min of 300 °C annealing. The high-temperature-stability of the Zn(O,S) buffer was attributed to the smaller diffusivity of Zn atoms than that of Cd atoms.
Physica Status Solidi (A) Applications and Materials 05/2014; · 1.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We demonstrate the first example of carbon- and oxygen-free Cu(In,Ga)(SSe)2 (CIGSSe) absorber layers prepared by electrospraying a CuInGa (CIG) precursor, followed by annealing, sulfurization, and selenization at elevated temperature. X-ray diffraction and scanning electron microscopy showed that the amorphous as-deposited (CIG) precursor film was converted into polycrystalline CIGSSe with a flat-grained morphology after post-treatment. The optimal post treatment temperature was 300°C for annealing and 500°C for both sulfurization and selenization with a ramp rate of 5°C per min. The carbon impurities in the precursor film were removed by air annealing and oxide formed during annealing was removed by sulfurization. The fabricated CIGSSe solar cell showed a conversion efficiency of 4.63% for a 0.44 cm2 area with Voc = 0.4 V, Jsc = 21 mA/cm2, and FF = 0.53.
[Show abstract][Hide abstract] ABSTRACT: A primary reference solar cell calibration technique recently established at the Korea Institute of Energy Research in Korea is introduced. This calibration technique is an indoor method that uses a highly collimated continuous-type solar simulator and absolute cavity radiometer traceable to the World Radiometric Reference. The results obtained using this calibration technique are shown with a precise uncertainty analysis, and the system configuration and calibration procedures are introduced. The calibration technique avoids overestimating the short-circuit current of a reference solar cell due to multiple reflections of incident simulator light using a novel method. In addition, the uncertainty analysis indicates that the calibration technique has an expanded uncertainty of approximately 0.7% with a coverage factor of k = 2 for a c-Si reference cell calibration. In addition, the developed primary reference solar cell calibration technique was compared with other techniques established in the World Photovoltaic Scale (WPVS) qualified calibration laboratories to verify its validity and reliability.
[Show abstract][Hide abstract] ABSTRACT: The deposition power dependence of visible transmittance and refractive index of room temperature-deposited ZnO:Al thin films by RF magnetron sputtering has been studied. All films exhibited high visible transmittance and near-complete UV absorption. The refractive index of the films decreased continuously with an increase in the RF power at all photon energies in the visible and near-IR region, which has been partially attributed to the decreased packing density of the films. For each film, the refractive index exhibited strong frequency dispersion in the weak-absorption region. The origin of optical dispersion at different RF power has been discussed in the light of a single-oscillator model.
[Show abstract][Hide abstract] ABSTRACT: The kesterite Cu2ZnSnS4 (CZTS) nanocrystals (NCs) were successfully synthesized using a relatively simple and one-step hydrothermal route. The structural, compositional, and optical properties of the kesterite CZTS NCs have been studied in detail. The pH-dependent CZTS phase formation has been elucidated for the first time. The X-ray diffraction and Raman spectroscopy confirmed the formation of a main phase kesterite CZTS structure only at pH 7. However, for pH values (4.3, 5, and 9), the formation of CZTS alongwith few secondary phases like Cu2SnS3 (CTS), Cu2−xS, and SnS2/Sn2S3 have been detected. CZTS NCs of size 10–100 nm were obtained at 200 °C and pH 7. The synthesized NCs showed a pH-dependent variation in optical band gap values from 1.15 to 1.44 eV, which is near optimum value for low cost thin film solar cells.
Physica Status Solidi (A) Applications and Materials 03/2014; · 1.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: CuInS2 (CIS) thin films were prepared by sulfurization of In/Cu stacked precursor films. Prior to sulfurization the stacked metallic precursors were subjected to the soft annealing in Ar atmosphere at different time (10, 30, and 60 min) and temperature (100 °C and 300 °C). The effect of soft annealing condition on the structural, morphological and optical properties of CIS films was investigated. X-ray diffraction, Raman, and X-ray photoelectron spectroscopy studies showed that the sulfurized thin films exhibited a CIS tetragonal structure with minor secondary phases such as Cu2−xS and CuIn5S8. The secondary phases were minimized by introducing soft annealed process in the CIS thin films. Void free CIS microstructures have been observed for soft annealed CIS films. The band gap energy of CIS films were increased from 1.37 to 1.5 eV depending on the soft annealing conditions.
Journal of Crystal Growth 01/2014; 394:49–54. · 1.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Copper lines with widths varying from 150 to 1500 μm were deposited onto crystalline silicon wafers and soda-lime glass plates by cold spraying copper particles with 1 μm average diameter through a mask. This direct deposition method yielded high-aspect-ratio electrodes with minimum shadowing effects and maximum electrode-to-silicon contact area. The copper lines had triangular cross sections with aspect ratios (height/width) ranging from 0.1 to 1.1, depending on the number of spray gun passes. Copper particles were densely packed with increasing the width of the masking slit. This study presents the potential use of the cold spray technology in printing lines as front electrodes in solar cell applications.
Journal of Thermal Spray Technology 10/2013; · 1.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In the present work, CZTS thin films have been prepared by sulfurization
of electrodeposited Cu-Zn-Sn (CZT) precursor. Prior to
sulfurization, as-deposited CZT precursors have been soft annealed in Ar
atmosphere at different temperatures (250-350 °C). The
structural, morphological, compositional and optical properties of the
films have been investigated in detail. It is found that, soft annealing
temperature has a significant impact on the properties of CZTS thin
films. The systematic study on the improvement in the properties of CZTS
films using soft annealing route has been studied and discussed.
[Show abstract][Hide abstract] ABSTRACT: Nontoxic Cu–In solution-processed CuInSe2 absorber thin films and resultant photovoltaic cells have been investigated. Acetate-based Cu–In precursors having different Cu/In ratios of 0.8–1.2 were deposited by spin-coating and then selenized in Se atmosphere up to 550 °C. Single tetragonal CuInSe2 phase was dominantly obtained regardless of Cu/In ratios, with the segregation of Cu2−xSe secondary phase only in the case of Cu-rich films as evidenced by Raman spectra. The films with the 1.1 ratio demonstrated a larger grain size of ~1.06 µm with an increased carrier concentration of ~1.7 × 1018 cm−3 and a decreased band gap of ~1.02 eV, compared to the values obtained for Cu-deficient absorber films. The resultant best cell efficiency was ~3.1% for the absorber having the 1.1 ratio, suggesting a potential of this simple spin-coating method as an alternative to typical vacuum processes.
Journal of Physics D Applied Physics 05/2013; 46(24):245102. · 2.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: CuInSe2 (CISe) absorber layers for thin-film solar cells were fabricated through the selenization of amorphous Cu-In-S nanoparticles, which were prepared by using a low-temperature colloidal process within one minute without any external heating. Two strategies for obtaining highly dense CISe absorber films were used in this work; the first was the modification of nanoparticle surface through chelate complexation with ethanolamine, and the second strategy utilized the lattice expansion that occurred when S atoms in the precursor particles were replaced with Se during selenization. The synergy of these two strategies allowed formation of highly dense CISe thin films, and devices fabricated using the absorber layer demonstrated efficiencies of up to 7.94 % under AM 1.5G illumination without an anti-reflection coating.
[Show abstract][Hide abstract] ABSTRACT: This study guided by device evaluations was conducted to reveal the reasons for the loss of the photo-generated carriers in CIGS cells with the buffer based on In2S3 derivatives. Chemical bath deposited Inx(OOH,S)y films have been employed as a Cd-free buffer layers. When compared to solar cells with CdS buffer layer, the Cu0.9(In0.7,Ga0.3)Se2.1 (Eg = 1.18 eV) cells with the Inx(OOH,S)y buffer exhibited strong voltage-dependent carrier collection and poor spectral response above 500 nm, presumably, due to energy barrier at the junction. In order to improve the charge collection by upward shift of the conduction band minimum of CIGS absorber, Inx(OOH,S)y/Cu0.9(In0.55,Ga0.45)Se2.1 (Eg = 1.30 eV) solar cells were also fabricated and their spectral responses were examined. When compared to the Cu0.9(In0.7,Ga0.3)Se2.1 cells, the improved spectral response and voltage dependent carrier collection were obtained. Nevertheless, considerable loss in charge collection above 500 nm was still observed. The efficiency reached 9.3% while the Cu0.9(In0.7,Ga0.3)Se2.1 cell exhibited only the efficiency of 3.4%. Finally, CIGS (Eg = 1.18 eV) solar cells with n-ZnO/i-ZnO/Inx(OOH,S)y/CdS/CIGS and n-ZnO/i-ZnO/CdS/Inx(OOH,S)y/CIGS configurations were fabricated. The influence of the TCO/buffer interface on the device characteristics was also addressed by means of comparison between the characteristics of two cells employing different interfaces. A 13.0% efficient cell has been achieved from n-ZnO/i-ZnO/CdS/Inx(OOH,S)y/CIGS configuration. The obtained data suggested that the limitation of the device efficiency was mainly related to the i-ZnO/Inx(OOH,S)y interface. The experimental results provide the knowledge base for further optimization of the interface properties to form high-quality p-n junction in the CIGS solar cells employing the CBD In2S3 buffer layer.
Physical Chemistry Chemical Physics 05/2013; · 3.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The influence of the conventional depositing processes on the optical properties of Cu(In,Ga)Se2 (CIGS) thin films in solar cell structures was investigated by measuring the photoluminescence (PL) and Raman spectra of the CIGS layer at each stage of the solar cell deposition process. The intensities of the PL and the Raman A1 mode increase after the CdS buffer layer is deposited, suggesting that the CdS layer either improves the optical quality of the CIGS film or protects it from degradation due to environmental factors. The temperature and excitation power dependences of the PL for the bare CIGS sample are very different from those for the samples with the CdS layer, reflecting different characters of the luminescence centers near the surface of the CIGS layer. On the other hand, the lateral homogeneity, as seen in the micro-PL and micro-Raman images, does not seem to improve. After the ZnO window layer is deposited, the overall PL and Raman intensities do not change much, although the intensity distribution becomes more inhomogeneous.
Thin Solid Films 05/2013; 535:118–121. · 1.60 Impact Factor