bluuebunny/crossref_metadata_2025 huggingface dataset:
Development of Novel Al-Doped Zinc Oxide Films Fabricated on Etched Glass and Their Application to Solar Cells
Authors: Hongsingthong Aswin , Aino Akehiro , Sichanugrist Porponth , Konagai Makoto , Kuramochi Hideto , Akiike Ryo , Iigusa Hitoshi , Utsumi Kentaro , Shibutami Tetsuo
Published: October 2012

We have successfully developed novel aluminum-doped zinc oxide (AZO-X) films with a high haze ratio by the combined use of an etched glass substrate and wet-etched AZO-X films. The effects of the use of an etched glass substrate and wet-chemical etching on the properties of AZO-X films were investigated. The texture size and rms roughness of these films largely increased with glass surface roughening. Post-treatment using wet chemical etching slightly increased the texture size and rms roughness. The etched glass approach has been found to be a promising method for achieving an AZO-coated glass substrate with a high haze ratio. Using high-haze ratio AZO-X films as the front transparent conductive oxide (TCO) layers in solar cells, we improved the quantum efficiency (QE) of these solar cells particularly in the long-wavelength region. Thus, the AZO-X films deposited on etched glass have a high potential for use as front TCO layers in silicon-based thin-film solar cells.
Effect of TiO2 Antireflection Layer with Various Conductivities and Refractive Indices on Performance of Amorphous Silicon/Amorphous Silicon Germanium Tandem Solar Cells
Authors: Kang Dong-Won , Ahn Seh-Won , Lee Heon-Min , Han Min-Koo
Published: October 2012

TiO2 films were prepared by RF magnetron sputtering with process variations of substrate temperature and oxygen dilution ratio to investigate the effect of the optoelectronic properties of TiO2 films on antireflection characteristics in solar cells. With an increase in substrate temperature from RT to 350 °C, the conductivity and refractive index of TiO2 films increased. However, the absorption coefficient also increased. In the case of oxygen-diluted sputtering (0.5–1%), the conductivity and refractive index decreased with the increase in oxygen dilution. On the other hand, the absorption coefficient decreased simultaneously. To evaluate these optoelectronic property variations of TiO2 films in terms of the antireflection effect in solar cells, amorphous silicon (a-Si:H)/amorphous silicon germanium (a-SiGe:H) tandem solar cells with various optoelectronic properties and thicknesses (20–40 nm) of TiO2 films were fabricated. The TiO2 film deposited at 350 °C and 0.5% oxygen dilution showed a high conductivity (∼10-3 Ω-1 cm-1) and refractive index (∼2.56 at 550 nm). The fabricated tandem cell with the TiO2 antireflection layer showed an efficiency of 11.22%, whereas the reference cell without TiO2 exhibited an efficiency of 10.97%.
Microcrystalline Silicon Carbide p-Layer with Wide-Bandgap and Its Application to Single- and Triple-Junction Silicon Thin-Film Solar Cells
Authors: Kim Soohyun , Park Jinhee , Lee Hongchul , Lee Hyun , Ahn Seh-Won , Lee Heon-Min
Published: October 2012

Wide-bandgap, high-quality p-type microcrystalline silicon carbide (p-µc-SiC) films have been prepared by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) technique for use as window layers for single- and triple-junction thin-film silicon solar cells. We have found that the p-µc-SiC films have wider optical bandgaps and lower absorption spectra in the short-wavelength region than the conventional p-µc-Si films. The new p-type doping layer was applied as a window layer of a microcrystalline silicon (µc-Si:H) single-junction cell, and the thin-film solar cell with the new window layer showed higher open circuit voltage (V
oc) and conversion efficiency. In addition, the insertion of an optimized p/i buffer layer was essential for reducing atomic damage at the p/i interface and obtaining a higher conversion efficiency. The optimized p-µc-SiC layer and p/i buffer layer were adopted successfully as a new window layer for the bottom cell within the triple-junction cell structure.
Optical Improvement of ZnO-Coated Glass with New Refractive-Index Matching Layer Inserted at Glass/ZnO Interface
Authors: Janthong Bancha , Hongsingthong Aswin , Moriya Yuki , Sichanugrist Porponth , Wronski Christophe R. , Konagai Makoto
Published: October 2012

We prepared and applied a-SiOxthin films to hydrogenated microcrystalline silicon solar cells (µc-Si:H) as a front antireflection layer (FAL) in order to reduce optical reflection loss. By inserting the optimized SiOxFAL with a refractive index of ∼1.75 into the glass/ZnO interface a relative increase in short-circuit current density (Jsc) by 5% could be obtained which corresponded to an improved spectral response in the 550–950 nm wavelength regions. In addition, this optimized FAL did not deteriorate the properties of the ZnO layer because no significant changes in open-circuit voltage (Voc) and fill factor (FF) were observed. As a result, the cell with an efficiency of as high as 8.28% (Voc=0.495 V,Jsc=25.09 mA/cm2, FF=0.667) could be obtained.
Improvement of the Band Profile of Cu(In,Ga)Se2 Solar Cells with High-Ga Content Prepared Using a Five-Stage Method
Authors: Hirai Yoshiaki , Hidaka Yukiya , Kurokawa Yasuyoshi , Yamada Akira
Published: October 2012

The efficiency of Cu(InGa)Se2 (CIGS) solar cells with high Ga content fabricated by the three-stage method is lower than that with low Ga content in spite of a better matching solar spectrum. Secondary ion mass spectrometry (SIMS) measurement revealed that the band profile of CIGS films with high Ga content had a deep notch around 0.5 µm from the CdS/CIGS interface. In order to decrease the notch depth of the CIGS with high Ga content, the five-stage method was employed instead of the conventional three-stage method. As a result, we successfully obtained the efficiency of 14.9% using the CIGS absorber with an average band gap of 1.40 eV prepared by the five-stage method. Theoretical simulation revealed the effects of the notch location and depth on solar cell performance characteristics.
Optimization of Compositional Ratio of Zn(O,S) Window Layer in CuInS2 Solar Cells
Authors: Hamazaki Ryosuke , Minemoto Takashi , Oda Yusuke , Fukamizu Shohei , Yamamoto Akito , Takakura Hideyuki
Published: October 2012

CuInS2 (CIS) solar cells with Zn(O,S) window layers deposited by co-sputtering were fabricated to optimize the conduction band offset (CBO) of Zn(O,S)/CIS. Zn(O,S) has an advantage in the control of the CBO because the conduction band minimum can be controlled by S/(S+O) compositional ratios. The efficiency of the CIS solar cells with the Zn(O,S) window layers increased with increasing S/(S+O) ratio from 0.19 to 0.50. However, the efficiency decreased at the S/(S+O) ratio of 0.59 owing to the decrease in the fill factor because of the double-diode-like behavior. The maximum efficiency was obtained at the S/(S+O) ratio of 0.50. Also, the CIS solar cell with standard bi-layer buffers of sputtered ZnO and chemical-bath-deposited CdS was fabricated and the performance was compared.
Characterization of Cu(In,Ga)Se2 Solar Cell Fabrication Process by Photoluminescence
Authors: Shirakata Sho , Ohta Hiroyuki , Iwado Naoki
Published: October 2012

Photoluminescence (PL), PL intensity mapping and time-resolved PL (TR-PL) studies have been applied to the Cu(In,Ga)Se2 (CIGS) solar cell fabrication process. Measurements have been done just after the respective cell process for the preparation of the Al/ZnO:Al/ZnO/CdS/CIGS structure, in which CdS has been formed by the chemical-bath deposition (CBD) while undoped and Al-doped ZnO layers were deposited by RF magnetron sputtering. PL intensity does not change by depositions of CdS and undoped ZnO buffer layers. PL intensity decreases by the deposition of the ZnO:Al film due to the cell shunt at the edge. The electrical cell isolation by the mechanical scribing leads to the increase in PL intensity because of the formation of the hetero-junction under the open circuit condition. The decay curves of the as deposited CIGS film, CdS/CIGS and ZnO/CdS/CIGS are non-exponential and composed of dominant fast decay and weak slow decay components. After the ZnO:Al deposition, PL decay is represented by the single exponential curve with long decay time. They are discussed in terms of the junction formation. PL intensity mapping after cell processes has been correlated with the solar cell performance.
Modeling and Optimization of Sub-Wavelength Grating Nanostructures on Cu(In,Ga)Se2 Solar Cell
Authors: Kuo Shou-Yi , Hsieh Ming-Yang , Lai Fang-I , Liao Yu-Kuang , Kao Ming-Hsuan , Kuo Hao-Chung
Published: October 2012

In this study, an optical simulation of Cu(In,Ga)Se2 (CIGS) solar cells by the rigorous coupled-wave analysis (RCWA) method is carried out to investigate the effects of surface morphology on the light absorption and power conversion efficiencies. Various sub-wavelength grating (SWG) nanostructures of periodic ZnO:Al (AZO) on CIGS solar cells were discussed in detail. SWG nanostructures were used as efficient antireflection layers. From the simulation results, AZO structures with nipple arrays effectively suppress the Fresnel reflection compared with nanorod- and cone-shaped AZO structures. The optimized reflectance decreased from 8.44 to 3.02% and the efficiency increased from 14.92 to 16.11% accordingly. The remarkable enhancement in light harvesting is attributed to the gradient refractive index profile between the AZO nanostructures and air.
Correlation between Electrical Properties and Crystal c-Axis Orientation of Zinc Oxide Transparent Conducting Films
Authors: Maejima Keigou , Shibata Hajime , Tampo Hitoshi , Matsubara Koji , Niki Shigeru
Published: October 2012

Al-doped ZnO (AZO) was deposited on an undoped ZnO buffer layer at various deposition temperatures by RF-magnetron sputtering. We controlled the crystalc-axis orientation of AZO using the undoped ZnO buffer layer. With increasing deposition temperature of the undoped ZnO buffer layer, the alignment of thec-axis orientation of the AZO layer improved, and the sheet resistance of the AZO layer decreased. Comparing the electrical properties of the AZO layer with thec-axis orientation, the above correlation was confirmed. It is important to improve the alignment of thec-axis orientation for the improvement of the electrical properties of the AZO layer.
Structure and Composition Analysis of Cu(In,Al)Se2 Thin Films Prepared by Rapid Thermal Selenization
Authors: Lee Shi-Wei , Tseng Bae-Heng
Published: October 2012

Thin films of CuIn1-x
Al
x
Se2 (CIAS) were prepared by stacked elemental precursor layers in an inert ambient. The stacking sequence of precursor layers may affect the kinetics of phase formation. The soda lime glass (SLG)/Cu/Al/In/Se sample heated at 750 °C for 30 s with a temperature ramp rate of 15 °C/s may react to form single-phase CIAS thin films with a chemical composition fairly close to the predetermined value. Transmission electron microscopy (TEM) analysis of a selenized film prepared at 600 °C revealed the segregation of the content of Al to the substrate side and a relatively large variation of Al distributed in a lateral direction as compared with those of other elements. Changing the Cu and Al layer sequence may affect the reaction paths and lead to the formation of a mixture of two CIAS quaternary phases with different compositions.
Surface Stabilities of Various Crystal Faces of CuInSe2 and Related Compounds by First-Principles Calculation
Authors: Shigemi Akio , Wada Takahiro
Published: October 2012

The typical crystal surfaces of CuInSe2 (CISe) and related compounds were studied using density functional theory (DFT) methods. We evaluated energies and surface structures of (112), (1̄1̄2̄), (110), (102), (100), (1̄00), (001), and (001̄) surfaces on CISe. For CISe, the (112) surface had the lowest energy among these surfaces, and the (110) and (102) surfaces had slightly higher energies than the (112) surface. These surface atoms coordinate with the three surrounding atoms. We found that the (112) surface is most stable for CISe, and this result is consistent with the experimental results showing that the (112) surface is most frequently observed in polycrystalline CISe thin films. We also evaluated the surface energies of CuGaSe2 (CGSe), CuAlSe2 (CASe), CuInS2 (CIS), CuInTe2 (CIT), and AgInSe2 (AISe). For CGSe, CIS, and CIT, the (112) surface had the lowest energy as in the case of CISe. However, for CASe and AISe, the (110) surface had the lowest energy.
Effects of Bi Incorporation on Cu(In1-x,Gax)Se2 Thin Films and Solar Cells
Authors: Nakakoba Hiroya , Yatsushiro Yuta , Mise Takahiro , Kobayashi Taizo , Nakada Tokio
Published: October 2012

The effects of bismuth (Bi) incorporation into Cu(In1-x
,Ga
x
)Se2 (CIGS) thin films and solar cells have been investigated. 10–50-nm-thick Bi thin layers were deposited onto Mo-coated soda-lime glass (SLG) and SiO
x
-coated SLG substrates by vacuum evaporation. CIGS thin films were then deposited by a three-stage process at substrate temperatures of 450–550 °C. The grain growth of CIGS thin films was enhanced, and the open-circuit voltage and hence the conversion efficiency was improved by the Bi incorporation when the SLG substrates were used. However, little effect was observed when the alkali barrier SiO
x
layer was deposited on SLG substrates. As a result, we found that the Bi incorporation is beneficial for improving the cell performance when sodium exists simultaneously in CIGS layers.
Structural Study of Cu-Deficient Cu2(1-x)ZnSnSe4 Solar Cell Materials by X-ray Diffraction and X-ray Absorption Fine Structure
Authors: Gao Feng , Yamazoe Seiji , Maeda Tsuyoshi , Wada Takahiro
Published: October 2012

Cu-poor Cu2(1-x)ZnSnSe4 powders were prepared from elemental powders. The crystal structure of Cu-poor Cu2(1-x)ZnSnSe4 was examined by X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) analyses. Kesterite-type Cu2(1-x)ZnSnSe4 could be prepared in the range of 0≤x ≤0.0750. The lattice parameters were refined by the Rietveld analysis of X-ray diffraction data. The lattice constants a and c decreased with a decrease in the Cu/(Zn+Sn) ratio. However, there was little change in c/a value. On the other hand, the position of the Se atom (u parameters) changed considerably. The XAFS study showed that the local structure of Sn in Cu2ZnSnSe4 (CZTSe) changed with a decrease in Cu/(Zn+Sn) ratio and the local structural changes in Cu, Zn, or Se could not be clearly observed. These local structural changes around Sn are due to the disordering of Cu, Zn, and Sn atoms. The diffuse reflectance spectra showed that the band gap of Cu2ZnSnSe4 is 0.98 eV and that the band gaps do not depend on Cu/(Zn+Sn) ratio in the range of 0≤x ≤0.0750.
Structural and Optical Properties of In-Free Cu2ZnSn(S,Se)4 Solar Cell Materials
Authors: Gao Feng , Yamazoe Seiji , Maeda Tsuyoshi , Nakanishi Koji , Wada Takahiro
Published: October 2012

We prepared a Cu2ZnSn(S
x
Se1-x
)4 (CZTSSe) solid solution from elemental powders. The CZTSSe solid solutions were synthesized by heating the elemental mixtures at 550 °C for 5 h in an N2 gas. All CZTSSe materials were analyzed by Rietveld analysis using the kesterite structure with a space group of I4̄ (No. 82). Rietveld analysis showed that the lattice parameters, a and c, monotonically decreased with increasing S content. Moreover, the local structures of the Cu and S atoms were investigated by X-ray absorption near edge structure (XANES). Although the local structure of the S atom in CZTSSe hardly changed in relation to the S/Se ratio, we found that the surface of CZTSSe powders became slightly oxidized. On the other hand, Cu K edge XANES showed that the S/Se ratio could be easily determined from the XANES spectra. The band gap energies of the CZTSSe materials were determined by transmittance and diffuse-reflectance spectra. The transmittance spectra were recorded using CZTSSe films fabricated by a printing and high-pressure sintering (PHS) process. The band gap energy, E
g, monotonically increased from 1.05 eV for CZTSe to 1.51 eV for CZTS.
Prediction of the Band Offsets at the CdS/Cu2ZnSnS4 Interface Based on the First-Principles Calculation
Authors: Bao Wujisiguleng , Ichimura Masaya
Published: October 2012

The valance band offset ΔE
v for the CdS/Cu2ZnSnS4 heterojunction is obtained on the basis of the first-principles pseudopotential method. Cu2ZnSnS4 is considered to crystallize in the kesterite structure. The total density of states and the local density of states for each atoms are calculated for the CdS(001)/Cu2ZnSnS4(001) supercell. There are two inequivalent interfaces between CdS and Cu2ZnSnS4 in the supercell, and different values of ΔE
v are obtained for them, i.e., ΔE
v1 = 1.1 eV and ΔE
v2 = 1.6 eV. The corresponding conduction band offsets ΔE
c are ΔE
c1 = 0.2 eV and ΔE
c2 = 0.7 eV with the conduction band minimum of Cu2ZnSnS4 higher than that of CdS.
Effects of Transition Metal Dichalcogenide Molybdenum Disulfide Layer Formation in Copper–Zinc–Tin–Sulfur Solar Cells from Numerical Analysis
Authors: Chelvanathan Puvaneswaran , Hossain Mohammad Istiaque , Husna Jamilah , Alghoul Mohammad , Sopian Kamaruzzaman , Amin Nowshad
Published: October 2012

This study demonstrates the effects of transition metal dichalcogenide, MoS2 layer formation in between the copper–zinc–tin–sulphide (CZTS) absorber layer and Mo back contact from theoretical study and numerical modeling. The objective of this study is to elucidate the effects of n or p type MoS2 on the overall CZTS solar cell performance. Energy band line-up of Mo/MoS2/CZTS interface is analyzed to elucidate the interface properties. It is found out that p-MoS2 layer in CZTS solar cell induces the same adventitious effect as p-MoSe2 in CIGS solar cell. However, n-MoS2 layer has detrimental effect on the CZTS solar cell by creating an additional back contact diode with p-CZTS layer and an ohmic contact with Mo layer. Thickness, bandgap energy and carrier concentration of n-MoS2 all have been varied in the numerical simulation to observe its effects on the cell performance parameters. The results from numerical simulation show that MoS2 layer as thin as 50 nm is sufficient enough to induce adverse effect on the solar cell performance. This could be caused by the increase in series resistance of the solar cell as n-type MoS2 would inhibit hole current into Mo back contact due to the hole barrier between n-type MoS2 and Mo back contact. The increase in MoS2 bandgap and carrier concentration also results in detrimental effect to the performance of the cell mainly due to the possibility of electrons to drift towards the back contact and recombine.
Fabrication of Three-Dimensional-Structure Solar Cell with Cu2ZnSnS4
Authors: Kurokawa Masato , Tanaka Kunihiko , Moriya Katsuhiko , Uchiki Hisao
Published: October 2012

We fabricated three-dimensional (3D)-structure solar cells with Cu2ZnSnS4 (CZTS). A CdS buffer layer was deposited around nc-TiO2 by the chemical bath deposition (CBD) method. The CdS deposition time was varied from 5 to 30 min at 65 °C. CZTS absorber was deposited by spray pyrolysis deposition (SPD) onto the CdS buffer layer. Cu-, Zn-, Sn-, and S-containing solutions were used in SPD. The metal sources of copper(II) acetate monohydrate, zinc(II) acetate dehydrate, and tin(II) chloride dehydrate, and pure sulfur powder were desolved in N,N-dimethylformamide (DMF) as a solvent and mono-ethanolamine as a stabilizer. The solution-sprayed 3D-structure substrate was annealed in a nitrogen atmosphere at 250 °C for 30 min. A 3D-structure CZTS solar cell presented the best conversion efficiency η of 0.51% with a CdS buffer layer deposition time of 20 min. The dependence of each solar cell characteristic on CdS buffer layer deposition time was measured.
Cu2SnS3Thin-Film Solar Cells from Electroplated Precursors
Authors: Koike Junpei , Chino Kotaro , Aihara Naoya , Araki Hideaki , Nakamura Ryota , Jimbo Kazuo , Katagiri Hironori
Published: October 2012

Cu2SnS3(CTS) contains non-rare metals and it has suitable optical characteristics for the absorber layer of thin-film solar cells. In this study, CTS thin films were fabricated by sulfurizing Cu–Sn precursors deposited by co-electrodeposition. Solar cells with a structure glass/Mo/CTS/CdS/ZnO:Al/Al were fabricated from the films. The best cell had an efficiency of 2.84%. A relatively high conversion efficiency was obtained from films with Cu/Sn≤2.
Improvement of Electrochemically Deposited Cu2O/ZnO Heterojunction Solar Cells by Modulation of Deposition Current
Authors: Song Ying , Ichimura Masaya
Published: October 2012

Cu2O thin films were deposited on indium–tin-oxide-coated glass from an aqueous solution containing CuSO4, lactic acid and KOH by the galvanostatic electrochemical deposition at 40 °C with several different current densities. The photo-absorption of Cu2O was increased and the conduction type was changed from weak p-type to clear p-type by raising the current value. Cu2O(2)/Cu2O(1)/ZnO three-layer heterojunctions were fabricated electrochemically by modulation of deposition current density of Cu2O. The first Cu2O layer Cu2O(1) was deposited at a lower deposition current, and the second one Cu2O(2) at a higher current. Under the optimized condition, the conversion efficiency of a Cu2O(2)/Cu2O(1)/ZnO solar cell was found to be higher than that of a Cu2O(1)/ZnO solar cell.
Evolution of Multijunction Solar Cell Technology for Concentrating Photovoltaics
Authors: Jones Russell K. , Ermer James H. , Fetzer Christopher M. , King Richard R.
Published: October 2012

Multijunction solar cells have evolved from their original development for space missions to displace silicon cells in high concentrating photovoltaic (CPV) systems. Today's three-junction lattice-matched production cells have efficiency of 39–39.5% under high concentration, and there appears to be little opportunity for further efficiency gain with this three-junction technology. Future generations of CPV cells will exploit more than three junctions, with metamorphic subcells, or both technical approaches to achieve efficiencies >45%. As new designs seek closer current matching and further spectral splitting, atmospheric variability will necessitate careful modeling to optimize energy output. These new cells will also be higher cost, which will favor higher CPV system concentration.
Effect of Quantum Well on the Efficiency of Carrier Collection in InGaAs/GaAsP Multiple Quantum Well Solar Cells
Authors: Fujii Hiromasa , Watanabe Kentaroh , Sugiyama Masakazu , Nakano Yoshiaki
Published: October 2012

For a quantitative evaluation of the carrier transport dynamics of multiple quantum well (MQW) solar cells, carrier collection efficiency (CCE) was defined and its measurement procedure was proposed. CCE is essentially the quantum efficiency normalized to the saturation value at a reverse bias. It allows us to know whether substantial carriers are actually extracted at any bias voltage, and to uncover the bottleneck problems of carrier transport that emerge at the operation bias. The advantage of CCE analysis is that the dynamics of photoexcited carriers can be selectively examined independently of the diode characteristics of the devices if the effects of resistance are small enough. In the present study, GaAs p–i–n solar cells including various numbers of InGaAs/GaAsP MQWs with a band gap of 1.2 eV in the i-region of equal thickness were fabricated and characterized. Interfered carrier transport by increasing the well number was quantitatively and directly evaluated. With up to 30 periods of MQWs, the carriers generated, especially in the wells, were less likely to be collected than those generated in the top p-region at a moderate forward bias, but collection of both was found to be degraded severely with 40 periods of MQWs.
Current–Voltage Characteristics of GaAs/AlGaAs Coupled Multiple Quantum Well Solar Cells
Authors: Ding Yi , Noda Takeshi , Mano Takaaki , Jo Masafumi , Kawazu Takuya , Han Liyuan , Sakaki Hiroyuki
Published: October 2012

We have studied how current–voltage (I–V) characteristics of GaAs/AlGaAs multiple quantum well (MQW) solar cells under illumination depend on the wavelength of incoming photons at 6 K. Coupled MQW structures used in this study consist of 4-nm-thick GaAs and 3-nm-thick AlGaAs layers. It is found that I–V curves under illumination measured at 6 K exhibit distinct shapes that depend systematically on the energy E
ph of incoming photons; when E
ph is smaller than the bandgap E
g of AlGaAs, the photocurrent shows a single-step decrease with voltage, while for E
g < E
ph, it decreases in two or three steps. These behaviors are explained in terms of bias-voltage dependent changes in various dynamic processes of photogenerated carriers.
Optimization of Gas-Switching Sequence for InGaAs/GaAsP Superlattice Structures Using In situ Wafer Curvature Monitoring
Authors: Ma ShaoJun , Sodabanlu Hassanet , Watanabe Kentaroh , Sugiyama Masakazu , Nakano Yoshiaki
Published: October 2012

By high-accuracy in situ curvature measurement during the growth of InGaAs/GaAsP superlattice structures by metal organic vapor phase epitaxy, we have successfully observed the effect of thin GaAs insertion layers between InGaAs wells and GaAsP barriers on strain control. By analyzing curvature transients, we found that an inadequate gas-switching sequence induces the carry over of indium from the InGaAs layer to the overlying GaAs insertion layer. The resulting carry-over layer has an estimated thickness of 0.6 nm and adversely affects the average strain of the structure. Through consideration of the kinetics of surface atoms, it has been revealed that an optimized gas-switching sequence with a 1 s hydrogen purge after the growth of InGaAs wells is effective for preventing the carry over.
Effect of Anti-Soiling Layer Coated on Poly(methyl methacrylate) for Concentrator Photovoltaic Modules
Authors: Nabemoto Kenji , Sakurada Yuya , Ota Yasuyuki , Takami Kazuyuki , Nagai Hirokazu , Tamura Kazuyuki , Araki Kenji , Nishioka Kensuke
Published: October 2012

An anti-soiling layer was coated on a poly(methyl methacrylate) (PMMA) substrate. The anti-soiling layer was prepared by coating the acrylic urethane capping layer, the inorganic/organic nano-graded intermediate layer, and the photocatalytic surface layer including modified WO3 and partial hydrolyzed tetraethyl orthosilicate. The layers were coated by spin-coat method. The photocurrent from each subcell of the InGaP/InGaAs/Ge triple-junction solar cell can be calculated by multiplying the solar spectrum of AM1.5D (850 W/m2), the transmittance of the PMMA substrate, and the spectral response of the triple-junction solar cell. After 8 months exposure, the reduction rate of photocurrent of the sample without anti-soiling coat was 9.6%. On the other hand, that with anti-soiling coat could be suppressed to 3.3%.
Characterization Capabilities of Solar Simulators for Concentrator Photovoltaic Modules
Authors: Antón Ignacio , Domínguez César , Victoria Marta , Herrero Rebeca , Askins Steve , Sala Gabriel
Published: October 2012

Solar simulators for characterization of concentrator photovoltaic modules (CPV) are now commercially available, providing an alternative to outdoor performance characterization with clear advantages in terms of repeatability, availability, control of operating conditions, and cost. Nevertheless, optical concentration implies particular characteristics that introduce the need of new performance figures and characterization methods. CPV solar simulators have demonstrated characterization capabilities beyond simple current–voltage (I–V) curve acquisition. This paper summarizes other indoor performance tests developed at the Instituto de Energía Solar (IES-UPM) that have proven to be useful for the optical, mechanical, and spectral characterization and optimization of a CPV module.
Surface Nanostructure Optimization for GaAs Solar Cell Application
Authors: Hong Lei , Yu Rusli , Wang Xincai , Wang Hao , Zheng Hongyu
Published: October 2012

Numerical simulation of optical absorption characteristics of gallium arsenide (GaAs) thin-film solar cells by the three-dimensional finite element method is presented, with emphasis on optimizing geometric parameters for nanowire and nanocone structures to maximize the ultimate photocurrent under AM1.5G illumination. The nanostructure-based GaAs thin-film solar cells have demonstrated a much higher photocurrent than the planar thin films owing to their much suppressed reflection and high light trapping capability. The nanowire structure achieves its highest ultimate photocurrent of 29.43 mA/cm2 with a periodicity (P) of 300 nm and a wire diameter of 180 nm. In contrast, the nanocone array structure offers the best performance with an ultimate photocurrent of 32.14 mA/cm2. The results obtained in this work provide useful guidelines for the design of high-efficiency nanostructure-based GaAs solar cells.
Impact of Strain Accumulation on InGaAs/GaAsP Multiple-Quantum-Well Solar Cells: Direct Correlation between In situ Strain Measurement and Cell Performances
Authors: Sodabanlu Hassanet , Ma Shaojun , Watanabe Kentaroh , Sugiyama Masakazu , Nakano Yoshiaki
Published: October 2012

The effects of accumulating strain inside InGaAs/GaAsP multiple-quantum-well (MQW) solar cells were investigated and their correlation within situwafer curvature measurement was examined. The p–i–n GaAs solar cells, containing 20-period InGaAs/GaAsP MQWs in an i-GaAs layer, were fabricated by metalorganic vapor phase epitaxy. The strain inside MQWs was varied by changing In content in an InGaAs well, while maintaining other parameters. As evidenced by curvature transience, the excessive strain led to lattice relaxation, resulting in defects, dislocations, and poor crystal quality. Consequently, short circuit current density and open circuit voltage deteriorated, and solar cell performance degraded. The highest conversion efficiency was obtained in a strain-balanced MQW solar cell. InGaAs/GaAsP MQWs have a great potential for extending the absorption edge of GaAs cells and for enhancing the efficiency of III/V multijunction solar cells by current matching. Hence, the growth of InGaAs/GaAsP MQWs for photovoltaic application requires a strain monitoring system and careful control such that the accumulating strain is minimized.
N Substitution in GaAs(001) Surface under an Atmosphere of Hydrogen
Authors: Kawano Jun , Kangawa Yoshihiro , Kakimoto Koichi
Published: October 2012

First-principles calculations of GaAsN surface with low nitrogen (N) content grown by chemical beam epitaxy were performed to theoretically analyze the incorporation process of nitrogen and impurities at the atomic scale. As a result, stable surface structures of GaAsN(001) under hydrogen (H) atmosphere were determined. In these structures, N is suggested to readily substitute into surface sites, especially those that bond with H, compared with in the bulk. This indicates that N is incorporated into a thin film together with H. This may generate H-related defects, which may lead to the degradation of its electric properties. These defects are difficult to minimize by post-annealing processes. Therefore, the amount of H attached to the growth surface should be reduced in order to obtain high-quality crystals. The calculated surface phase diagram suggests that a condition in which the extent of the incorporation of H-related defects can be reduced exists.
Visible to Near-Infrared Photoelectric Conversion in a Dye-Sensitized Solar Cell Using Ru(II) Porphyrin with Azopyridine Axial Ligands
Authors: Kinoshita Takumi , Fujisawa Jun-ichi , Nakazaki Jotaro , Uchida Satoshi , Kubo Takaya , Segawa Hiroshi
Published: October 2012

A novel Ru(II) porphyrin sensitizer, bis[4-(4-pyridylazo)-benzoic acid]tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrinato ruthenium(II), [RuTBP(azpyba)2] derived from tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl) Ru(II) porphyrin with azopyridine axial ligands have been synthesized and characterized by UV–vis, emission, nuclear magnetic resonance (NMR), differential pulse voltammetry, density functional theory (DFT) calculation, and subpicosecond time-resolved transient absorption. The RuTBP(azpyba)2 sensitizer shows both the intra-porphyrin π–π* absorption in the visible region and broad charge-transfer absorption from Ru2+ to the axial ligands in the near-IR region. The RuTBP(azpyba)2-sensitized solar cell shows sensitivity in the near-IR region up to ∼1100 nm owing to the charge transfer absorption band. It was indicated that the excited electrons are injected to TiO2 through the azopyridine axial ligand of RuTBP(azpyba)2.
Differences in Dye-Sensitized Photovoltaic Behaviors between SnO2 and TiO2 Electrodes
Authors: Sadamasu Kengo , Ogomi Yuhei , Pandey Shyam S. , Hayase Shuzi
Published: October 2012

A SnO2 electrode is a promising candidate for a bottom electrode of dye-sensitized solar cells. One of the drawbacks is its low fill factor (FF). We clarified the cause of this low FF using our original hybrid cells consisting of carrier generation areas (a bottom layer consisting of TiO2/a dye layer) and carrier transport areas (a top layer consisting of SnO2 with and without a dye layer). A large decrease in FF was observed only when the SnO2 charge transport areas were covered by photo excited dyes, leading to the conclusion that back electron transfer reaction from SnO2 to oxidized dyes is a major route for the charge recombination. This was also confirmed by electron lifetime and dark current measurements.
Influences of Electron-Withdrawing Groups of Organic Dyes on Spectral Property and Photovoltaic Performance in Dye-sensitized Solar Cells Application
Authors: Numata Youhei , Han Liyuan
Published: October 2012

We synthesized five donor–π-spacer–acceptor type organic photosensitizers bearing different types of electron-withdrawing groups (EWGs); trifluoromethyl, o-nitrophenyl, p-nitrophenyl, cyano, and carboxyl groups, on their acceptor part in the aim of observing an influence of the EWGs on spectral and photovoltaic properties from viewpoints of steric structure and π-conjugation. The EWG possessing smaller dihedral angle between the EWG and dye skeleton exhibited larger bathochromic shift in absorptions. Highly planer cyano group presented the most red-shifted absorption at 464 nm, and the highest conversion efficiency of 5.69% was obtained. In contrast, highly distorted o- and p-nitrophenyl groups exhibited blue-shifted absorption at 416 and 422, respectively; however, despite of resemble spectral properties, o- and p-nitrophenyl gave second best and the worst conversion efficiencies of 4.05 and 2.51%, respectively. By combination with computational chemistry, it was indicated that the configuration of the EWG and distance between TiO2 surface and the EWG dominated electron injection efficiency.
Directly Determine an Additive-Induced Shift in Quasi-Fermi Level of TiO2 Films in Dye-Sensitized Solar Cells
Authors: Zhang Shufang , Yanagida Masatoshi , Yang Xudong , Han Liyuan
Published: October 2012

The shifts in quasi-Fermi level (QFL) of TiO2 films affected by additives are directly measured using three-electrode dye-sensitized solar cells (DSCs). The difference in QFL of TiO2 under short circuit and open circuit are also analyzed. It is found that the QFL difference between the electrolyte side of the TiO2 film and the equilibrium redox potential, as well as the QFL difference between the two sides of the TiO2 film shift to higher potential owing to the addition of TBP to the electrolyte. The shift values are influenced by the cations present in the electrolyte. These directly determined changes in QFL in TiO2 films both under open circuit and short circuit may provide valuable information for studying DSC operation mechanisms such as the dynamics of charge separation and charge recombination processes.
Integration of Surfactant-Free Silicon Nanocrystal in Hybrid Solar Cells
Authors: Svrcek Vladimir , Mariotti Davide , Yamanari Toshimi , Matsubara Koiji , Kondo Michio
Published: October 2012

Here, we discuss the results related to improvement of electronic interactions and structural properties of hybrid organic/inorganic composites based on free-standing and surfactant-free silicon nanocrystals (Si-ncs). Performance of Si-ncs in bulk-heterojunction solar cells combined with a polythieno[3,4-b]thiophenebenzodithiophene (PTB7) is studied. Further we demonstrate that three dimensional surface engineering of Si-ncs by low-cost and room temperature DC atmospheric microplasma processing in ethanol considerably enhance the Si-ncs electronic interactions with polymers and enhance the overall external quantum efficiency conversion of bulk heterojunction solar cells without using any surfactant.
Enhancement of CdSe/Poly(3-hexylthiophene) Bulk Heterojunction Solar Cell Efficiency by Surface Ligand Exchange and Thermal Treatment
Authors: Truong Nguyen Tam Nguyen , Park Chinho
Published: October 2012

Bulk heterojunction solar cells were fabricated by blending semiconducting cadmium selenide nanoparticles with various capping ligands and regioregular poly(3-hexylthiophene). The effects of surface ligand modification of CdSe nanoparticles and thermal treatment of fabricated cells on the device performance were investigated. It was found that surface ligands of nanoparticles could affect the device performance by increasing the charge carrier separation at the nanoparticle/polymer interface by quenching photoluminescence. Thermal treatment of fabricated cell structure at 140 °C was found to be optimal for device performance, resulting in a maximum power conversion efficiency of 1.17% under AM1.5G simulated solar irradiation.
Enhanced Conversion Efficiency for Si Nanowire–Organic Hybrid Solar Cells through the Incorporation of Organic Small Molecule
Authors: He Lining , Jiang Changyun , Lai Donny , Wang Hao , Rusli Rusli
Published: October 2012

We demonstrate high-efficiency hybrid solar cells based on heterojunctions formed between n-type silicon nanowires (SiNWs) and p-type organic semiconductors fabricated using a simple solution-based approach. Two types of devices have been fabricated with different organic materials used, namely poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and a small molecule, 2,2',7,7'-tetrakis(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD). The cells are characterized and compared in terms of their physical characteristics and photovoltaic performance. Using SiNWs of the same length of 0.35 µm, it is found that the SiNWs/Spiro cells exhibit a power conversion efficiency of 10.3%, which is higher than the 7.7% of SiNWs/PEDOT cells. The results are interpreted in terms of the ability of the two organic semiconductors to fill the gaps between the SiNWs and the optical reflectance of the samples. The degradation of the SiNWs/Spiro cells is also studied and presented.
Accuracy of Energy Yield Prediction of Photovoltaic Modules
Authors: Roy Jyotirmoy , Betts Thomas Richard , Gottschalg Ralph
Published: October 2012

The uncertainty analysis of irradiance and temperature measurements in relation to the energy yield prediction of the photovoltaic (PV) modules are presented. A Monte Carlo simulation approach is demonstrated separately to propagate the monthly and annual measurement uncertainties of irradiance and temperature to annual energy yield prediction uncertainty for two commercially available PV modules. The annual irradiation uncertainty as measured with a thermopile pyranometer is calculated as ±1.56%. Uncertainty of the annual average of ambient temperature measurement is calculated as ±0.08 °C. Finally, the uncertainty in the energy yield estimation of the PV devices is determined as 2.8 and 15.5% for crystalline silicon (c-Si) and copper indium gallium (di)selenide (CIGS) modules, respectively.
Comparison of Curve Correction Procedures for Current–Voltage Characteristics of Photovoltaic Devices
Authors: Tsuno Yuki , Hishikawa Yoshihiro
Published: October 2012

It is useful to understand the effect of irradiance, temperature, and spectral irradiance on the photovoltaic cell and module performance, to estimate their current–voltage curves under various climate conditions for power rating and energy rating. In this paper, we present the accuracy of the three methods described at the IEC 60891 and investigate the applicability of these methods. Procedure 3 showed good agreements between the calculations and experiments within ±1%. We have also demonstrated that the accuracies of procedures 1 and 2 depend on the device structure. Although both methods can introduce larger deviations at lower irradiance level, these deviations are not negligible for energy yield prediction.
A Methodology for Estimating the Effect of Solar Spectrum on Photovoltaic Module Performance by Using Average Photon Energy and a Water Absorption Band
Authors: Ishii Tetsuyuki , Otani Kenji , Itagaki Akihiko , Utsunomiya Kenji
Published: October 2012

Various photovoltaic (PV) technologies are commercially available today. The effect of solar spectrum on the performance of PV modules should be evaluated quantitatively in order to estimate the module performance with high accuracy and precision. Average photon energy (APE) has been frequently applied to evaluate the effect of solar spectrum. The purpose of this study is to enhance the precision and accuracy by introducing other indexes. In this study, we select solar spectra, the integrated spectral irradiance (ISI) and APE of which are equivalent to those of the standard AM1.5G spectrum. There is a slight difference in shape, although the shapes are approximately similar. We introduce one more index, which defines the spectral irradiance at the atmospheric window or the depth of the water absorption band. The introduction would further improve the accuracy and precision of the evaluation of the effect of solar spectrum.
Modeling of Solar Spectral Irradiance Data from Cloudless to Overcast Skies
Authors: Usami Akira , Kawasaki Norihiro
Published: October 2012

Solar spectral models applicable to partly cloudy and overcast skies as well as cloudless skies are developed by the analyses of standing spectral observation data in Japan. To characterize solar spectral irradiance in detail by decomposing a global spectrum into a direct and diffuse spectrum, not only global spectra but also diffuse spectra have been observed. The average photon energies of the diffuse spectra were changed largely by the cloud cover in contrast to the little dependence in the direct spectra, which confirms the importance of separate observations. Spectral models describing the effects of clouds on the direct and diffuse spectra were derived; these models were demonstrated by regression analyses of the observations. Additionally, we have revealed a relationship associating Rayleigh scattering intensity in diffuse irradiance with the diffuse broadband irradiance and the solar altitude, which permits us to estimate the model parameter from the available parameters.
Output Energy Estimation of Si-Based Photovoltaic Modules Using Clearness Index and Air Mass
Authors: Takei Ryosuke , Minemoto Takashi , Yoshida Shota , Takakura Hideyuki
Published: October 2012

The output energy estimation of photovoltaic (PV) modules was carried out using contour maps as a function of average photon energy (APE), which is an index of solar spectral irradiance and module temperature (T
mod) in our previous report. This method is useful for not only crystalline Si PV modules but also thin film Si PV modules, which have a narrow spectral response. However, this method requires spectrum measurement to obtain APE using a spectroradiometer, which is not widely installed in various locations. In this study, we used clearness index (Kt) and air mass (AM), which have a strong correlation with APE and T
mod. Kt and AM can be calculated easily from irradiance, time, and location. The comparison between the output energy estimation using Kt and AM and that using APE and T
mod was carried out. Results indicate that the accuracy of the output energy estimation using Kt and AM is as high as that using APE and T
mod. The proposed method using Kt and AM is useful for estimating the output energy of the PV modules.
Early Failure Detection of Interconnection with Rapid Thermal Cycling in Photovoltaic Modules
Authors: Aoki Yuichi , Okamoto Manabu , Masuda Atsushi , Doi Takuya , Tanahashi Tadanori
Published: October 2012

To accelerate the degradation with thermal fatigue in crystalline silicon photovoltaic modules, the modules were exposed to dry thermal stress with rapid thermal cycling, and module impedance was monitored in situ during this testing. The spikelike increase in module impedance at a temperature-alteration point was observed in the early stage of this rapid thermal cycling. The pattern of increase in module impedance proceeded step-by-step, from the early stage, via the double-spikelike pattern at two temperature-alteration points (the middle stage), and finally to the successive increases in module impedance in the high-temperature period (the late stage). The nondestructive analyses suggest that the interconnector failures without the defects of photovoltaic cells occurred. From these results, it is suggested that the pattern of increase in module impedance is related to the interconnection degradation of modules, and that the rapid thermal cycling with in situ monitoring of module impedance would be a useful procedure for the earlier detection of interconnection failures in photovoltaic modules.
Defect Evolution in Multiwalled Carbon Nanotube Films Irradiated by Ar Ions
Authors: Honda Shin-ichi , Nanba Shuhei , Hasegawa Yoichi , Nosho Yusuke , Tsukagoshi Akira , Niibe Masahito , Terasawa Mititaka , Hirase Ryuji , Izumi Hirokazu , Yoshioka Hideki , Lee Kuei-Yi , Niwase Keisuke , Taguchi Eiji , Oura Masaki
Published: October 2012

Controlling defect structure in multiwalled carbon nanotube (MWCNT) is essential to realization of MWCNT devices. Here, we show that the diagram of the Raman intensity ratio of the G to D peaks and the G peak width can reveal two damaging stages of MWCNT films. In a transition period, additional peaks appeared in the X-ray absorption spectra, thereby indicating some significant change in the electronic structure. Also, a remarkable increase occurred in the diameter of the MWCNTs in the latter stage, suggesting the formation of dislocation dipoles which may relate to the change in the properties of field-emission devices.
Thermo-Optic Coefficients of 4H-SiC, GaN, and AlN for Ultraviolet to Infrared Regions up to 500 °C
Authors: Watanabe Naoki , Kimoto Tsunenobu , Suda Jun
Published: October 2012

The temperature dependence of the refractive indices of 4H-SiC, GaN, and AlN were determined in a wavelength range from the near band edge (392 nm for 4H-SiC, 367 nm for GaN, and 217 nm for AlN) to infrared (1700 nm) and a temperature range from room temperature to 500 °C. Optical interference measurements with vertical incidence along a c-axis configuration were employed to precisely evaluate ordinary refractive indices. Near the band-edge region, the temperature dependence of the refractive index mainly originates from the temperature change of the bandgap. At 450 nm, the thermo-optic coefficients of 4H-SiC, GaN, and AlN were measured to be 7.8 ×10-5, 1.6 ×10-4, and 3.6 ×10-5 K-1, respectively.