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Abstract: We report the effect of Cr impurity barrier on Cu(In,Ga)Se2 (CIGS) thin-film solar cells prepared on flexible substrates. The Cr films with varying the thickness (t Cr) were deposited on stainless steel substrates using direct-current magnetron sputtering. The solar cell performance was improved by increasing t Cr since the diffusion of Fe impurities from the substrate to CIGS was suppressed. Although the elemental composition, grain size, and strain of CIGS film showed little change with varying Fe content, the fill factor and the short-circuit current density increased as decreasing Fe. The Fe increased the series resistance, shunt paths, and saturation current density. The reduction of Fe caused a steeper bandgap grading in CIGS which enhances current collection due to higher electric fields in bulk CIGS. CIGS solar cells with 1000 nm-thick Cr barrier showed the best conversion efficiency of 9.05%. [Copyright &y& Elsevier]

Abstract: We have studied the influence of growth temperature (T G) in the deposition of an indium tin oxide (ITO) transparent conducting oxide layer on Cu(In,Ga)Se2 (CIGS) thin-film solar cells. The ITO films were deposited on i-ZnO/glass and i-ZnO/CdS/CIGS/Mo/glass substrates using radio-frequency magnetron sputtering at various T G up to 350°C. Both the resistivity of ITO and the interface quality of CdS/CIGS strongly depend on T G. For a T G ≤200°C, a reduction in the series resistance enhanced the solar cell performance, while the p–n interface of the device was found to become deteriorated severely at T G >200°C. CIGS solar cells with ITO deposited at T G =200°C showed the best performance in terms of efficiency. [Copyright &y& Elsevier]

To utilize eco-friendly materials in Cu(In,Ga)Se 2 (CIGS) thin-film solar cells, a Zn-based buffer layer has been adopted to replace the conventional CdS buffer layer. Buffer layers can be grown using a variety of techniques, with chemical bath deposition (CBD) generally used for the preparation of the Zn(O,S) buffer layer. The growth rate of the CBD-Zn(O,S) buffer layer is determined by the dissociation rate of S during chemical reaction between Zn and S ions. Thiourea (TU) is a conventional precursor of S, but it suffers from poor dissociation rate of S, leading to prolonged layer deposition times. In this study, we replaced TU with thioacetamide (TAA), thereby drastically shortening the deposition time from 80 min (TU) to 7 min (TAA). Furthermore, incorporating Zn with S ions is known to be difficult owing to the high-speed release of S ions from TAA. However, by adjusting the molar concentration of ZnSO 4 , we discovered a process to effectively form the Zn–S bonds for the CBD-Zn(O,S) buffer layer. The efficiency of the CIGS thin-film solar cell shows the best performance at a ZnSO 4 concentration of 0.05 M, which rivals the performance of the TU base process. The physical and chemical properties of CBD-Zn(O,S) grown at a high deposition rate using TAA were examined as a function of ZnSO 4 molar concentration. We proposed a growth model that accounts for the changes in film thickness and surface morphology evolution, both of which depend on the ZnSO 4 concentration. Additionally, to investigate the p–n junction properties in terms of device operation, we elucidated the band alignment at the CIGS/CBD-Zn(O,S) interface, concentrating on the interfacial barrier height. It is empirically demonstrated that a lower ΔE C value improves the solar cell operation. [ABSTRACT FROM AUTHOR]

The use of colorful solar cells is necessary for building-integrated photovoltaic (BIPV) applications in urban areas, where aesthetics play an important role. The conventional deposition process of multilayer color filters utilized in solar cells for BIPV applications is expensive because it requires multiple sputter targets. Herein, to address the issue of high cost, a single sputter target was used to produce a color filter consisting of a Zn-based multilayer structure. The oxygen content in the Zn(O,S) films could be controlled by adjusting the amount of oxygen gas supplied during the sputtering process. This allows for control of the optical properties of the films, including the refractive index and extinction coefficient. A color filter with the Zn(O 0.92 ,S 0.08)/ZnS structure repeated three times was applied to Cu(In,Ga)Se 2 thin-film solar cells; the resulting solar cell demonstrated a wide range of color expressions and less than 10% reduction in optical performance. The proposed technology is expected to significantly reduce the cost of optical filter processes and their applications. [ABSTRACT FROM AUTHOR]

Simultaneously enhancing the optical transmittance and electrical conductivity of transparent conductors (TCs) applicable in various optoelectronic devices is a long-standing challenge. Herein, we present an innovative approach for fabricating electroplated Ni and Cu microfiber networks (NiMF and CuMF) as highly conductive TCs to realize high efficiency and desired aesthetics in thin-film solar cells and modules. The metal microfibers (MFs) are fabricated using electrospun polyacrylonitrile nanofibers as the template. The large cross-sectional aspect ratio of the metal MF networks remarkably and concurrently improves their electrical conductivity and optical transmittance. Between the NiMF and CuMF TCs, the CuMF sample exhibits a superior figure of merit owing to its exceptionally low electrical resistivity. The metal MF TC is a promising alternative to conventional patterned grids used in flexible Cu(In,Ga)Se 2 thin-film solar cells, because it effectively reduces the series resistance, which is advantageous for large-area cells. The CuMF can be successfully employed as a ribbon to maintain the solar cell performance in centimeter-scale cells connected in series. The outstanding performance of the metal MF TCs indicates their potential to eliminate complicated monolithic integration processes or front grids and ribbons in flexible thin-film solar cells and modules. • Metal microfiber (MF) transparent conductors (TCs) were fabricated. • Ni and Cu MF TCs were formed by electrospinning and electroplating processes. • MFs show exceptionally low electrical resistivity with high transmittance. • Cu(In,Ga)Se 2 solar cells with MFs outperformed traditional grids. • Proposed MFs are expected to replace grids and ribbons in photovoltaic modules. [ABSTRACT FROM AUTHOR]

Abstract: We have investigated the effect of annealing thin-film CdS/Cu(In,Ga)Se2 (CIGS) on solar cell performance. CdS/CIGS/Mo and CdS deposited on glass substrates were annealed on a hot plate in an air environment. Both the transmittance of the CdS layer at short wavelength and the X-ray diffraction (XRD) intensity of its (200)H peak increased after annealing. The quantum efficiency of the fabricated solar cells with annealed CdS buffer layer was also enhanced at short wavelength. A CIGS solar cell with a CdS buffer annealed at 200 °C for 1 min showed an efficiency of 16.47% (J sc = 34.18 mA/cm2, V oc = 0.642 V and Fill Factor = 75.0%) which is higher than that of an un-annealed solar cell by about 0.6%. We believe that annealing increases the optical transmittance and eliminates recombination centers at the p–n junction. This leads to the improved performance of CIGS solar cells. [Copyright &y& Elsevier]

We report the structural, optical, and electrical properties of an atomic layer deposited ZnO thin films in the temperature range of 80–180 °C. Also, the device characteristics of the CIGS solar cells with the ALD-ZnO buffer layers were investigated. The thickness and grain sizes of the ZnO were increased with an increase of temperature. All ZnO films exhibited the preferred crystal orientation along the (100) plane except the films grown at 80 °C showed along the (002) plane. The optical energy band gap of the ZnO was decreased from 3.30 to 3.29 eV with an increase of temperature. The electrical resistivity decreased from 6.2 to 5.6 × 10-3 Ω cm, while the mobility and the carrier concentrations increased from 1.7 to 21.6 cm2/V.s and from 6.0 × 1018 to 2.0 × 1020 cm−3, respectively. The fabricated CIGS solar cells using a 70 nm-thick ZnO film deposited at 100 °C, exhibited the best device characteristics of the FF , Jsc , and PCE as 63.48%, 28.84 mA/cm2, and 8.59%, respectively. • There is no temperature effect on thickness of the ALD-ZnO between 100 and 180 °C. • ALD-ZnO films showed the preferred crystal orientation along the (100) plane between 100 and 180 °C. • The ZnO grown at 100 °C showed the lowest optical absorption coefficient of 1.46 × 10 5 cm−1. • The CIGS solar cell with a 70 nm-thick ZnO film grown at 100 °C , exhibited the best device performance. [ABSTRACT FROM AUTHOR]

In recent years, amorphous InZnO (a-IZO) film gets recognition for the possibility as a transparent conducting oxide (TCO) layer with outstanding its optical and electrical properties in photovoltaic devices. Generally, the sputtering technique is normally used to deposit a-IZO film, which suffers from plasma damage exerting a negative influence on device performance. Here, we suggest an alternative to deposit a-IZO film with a thermal evaporation technique using a simply customized system to compensate defects by plasma damage. Thermally grown IZO films by controlling two variables of Zn rate and substrate temperature show the distinguishable characters in terms of transparency, conductivity and crystalline structure with a substantial change of Zn content in In 1- x Zn x O. Zn incorporation in IZO film is considered as a critical key to determining the crystal structure and IZO film quality. In the thermal evaporation system, phase transition occurs from crystalline to amorphous; the IZO film indicates the superior property in the amorphous phase than crystalline structure. We acquired the optimized IZO film with amorphous phase in the relative concentration of In 0.73 Zn 0.27 O grown at 200 °C. To demonstrate the feasibility of a-IZO film as a TCO layer, Cu(In,Ga)Se 2 solar cell was practically fabricated, which shows excellent performance. It is attributed to the absence of plasma damage during deposition of the a-IZO film, proved by the decrease of shunt resistance in the solar cell. • The InZnO films were deposited by a customized thermal evaporation system by varying the Zn rate and substrate temperature. • The quality of the InZnO film can be decided by Zn content to determine the crystal structure. • Thermally evaporated InZnO film is satisfied for alternative TCO material transformed into the amorphous phase. • The CIGS solar cell with an amorphous InZnO film as a TCO layer shows the superior cell performance. [ABSTRACT FROM AUTHOR]

With the increasing use of building-integrated photovoltaic technology, it has become necessary for solar panels to blend in with their surroundings. Diffractive nanostructures can be used to reduce optical losses and create colorful solar cells. To make colors on Cu(In,Ga)Se 2 (CIGS) thin-film solar cells, we fabricated diffractive nanostructures by using nanoscale imprinting and transfer lithography methods. And we investigated how the material types and pattern shapes of the nanostructures influence the optical properties of solar cells, such as the short-circuit current density (J SC), coordination of color appearance, and color quality. We used two types of pillars and grating nanostructures of SiO 2 and TiO 2 layers on CIGS thin-film solar cells to produce various colors. The SiO 2 demonstrated an increase in J SC without significant loss of color quality compared to TiO 2. By utilizing a hexagonally arrayed pillar pattern, colors are observed at various angles on an axis different from the incident light, unlike colors observed only at a single angle in the grating structure. The nanoimprinting lithography process allowed us to produce high-quality nanostructures on both rigid glass and flexible stainless-steel substrates. [Display omitted] • Diffractive nanostructures abled color solar cells with minimizing efficiency loss. • Nanoscale imprinting and transfer lithography were used to make the nanostructures. • The coordinates and quality of the color were influenced by the materials and patterns of nanostructures. [ABSTRACT FROM AUTHOR]

Chemical bath deposition (CBD)-ZnS is used as a buffer layer for Cu(In,Ga)Se 2 (CIGS) solar cell and then, plasma damage originated from the negative oxygen ions or neutral particles with high energy is regarded as an important issue during subsequent deposition of window layer by sputtering process. To avoid negative plasma effects, we newly designed sputtering system with dual deposition mode of low damage sputtering (LDS) process and direct sputtering (DS) process, adjusting plasma direction by dual cylindrical targets. It allows to grow bilayer AZO film by sequentially depositing thin LDS AZO film as a protect layer and thick AZO film as a transparent conducting layer to collect electrons. The physical and electrical characteristics of bilayer AZO film were investigated to verify the positive effect against plasma damage. In plasma damage test using GaAs wafer, we observed that increase of sputtering power leads to a falling-off in electrical qualities by plasma damage. For practical solar cell application, we fabricated three types of CIGS/CBD-ZnS solar cells with LDS AZO film grown at 2 kW and 5 kW (by 1500 mm cylindrical targets) and Conventional-AZO film (by 4-in. circular targets), and demonstrated that the performance of CIGS solar cell with LDS AZO is superior to that with Conventional-AZO film due to the effective protection layer to relieve plasma damage. [ABSTRACT FROM AUTHOR]

Cu(In,Ga)Se 2 (CIGS) solar cells were fabricated by varying the film thickness of the cracker-ZnS (c-ZnS) buffer layer from 0 nm to 20 nm, and performance was found to depend on c-ZnS film thickness. The best cell efficiency of approximately 8% was obtained from the CIGS solar cell with an 8 nm thick-c-ZnS buffer layer. To investigate the primary factor to determine the cell performance, we utilized the impedance spectroscopy (IS) reflecting interface qualities, and capacitance-voltage (CV) profiling sensitive to bulk properties. In IS results, an equivalent circuit model including the resistance and capacitance was proposed to interpret cell performance, and carrier lifetime was obtained in connection with recombination probability at p-n junction. In CV profiling, the carrier concentration in the CIGS bulk, the depletion width, and the charge distribution related to the defect states along the depth direction were evaluated. The formation mechanism of c-ZnS buffer layer is suggested by measuring the chemical states, which is closely associated with the IS and CV results. The depletion width substantially increased at c-ZnS film thickness more than 15 nm due to the diffusion of Zn atoms toward CIGS layer, resulting in negative influence on cell performance. From this study, we demonstrated that IS and CV profiling are complementary analysis tools for interpretation of the solar cell operation concerning the interface and bulk properties. [ABSTRACT FROM AUTHOR]

The photoluminescence (PL) of Cu(In,Ga)Se 2 solar cell with sputtered Zn(O,S) buffer layers annealed at various temperatures were measured. As the annealing temperature increased, the peak positions of PL spectra shifted to higher energies. The broad PL spectra were separated two or three emissions, which are suggested to be mostly to donor-acceptor pair transitions with an acceptor due to copper vacancy and donors due to selenium vacancy or III Cu antisite. The changes of PL spectra were discussed from the view of the diffusion of elements, especially sulfur from buffer to absorber during the post-annealing process. [ABSTRACT FROM AUTHOR]

Cu(In,Ga)Se 2 absorbers with several different ratios of Ga/(Ga + In) were deposited by varying the Ga content using co-evaporation. The effects of Ga concentration at the surface of the CIGS on the Zn(O,S)/CIGS photovoltaic performance were investigated. Considering application as the bottom cell in a monolithic tandem structure, the optimum Ga/(Ga + In) ratio of approximately 0.26 led to the best efficiency of 14.56%, and to complete carrier collection at long wavelengths. In addition, the degradation rate in efficiency was lowest at an annealing temperature of 400 °C. The reason for losses of other Ga/(Ga + In) ratios (0.33 and 0.36) at longer wavelengths might be due to interface recombination, possibly aided by conduction band offset with the Ga grading shape by varying the Ga concentration. The results suggested that the decrease in efficiency might be due to recombination at the interface between the Zn(O,S) and CIGS layers before/after annealing. The reverse bias effect during quantum efficiency measurements at long wavelengths was used to determine separate optical and electrical losses. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In the current study, chemical bath deposition (CBD) was used to grow CdS thin films on a Cu(In,Ga)Se2 (CIGS) absorption layer, in order to examine the effects of CdS deposition conditions on the properties of CIGS solar cell devices. The dip time leading up to the start of CdS synthesis is thought to be an important process variable determining the concentration of Cd ions diffused into the CIGS as well as the condition of the CIGS surface. Accordingly, the behavior of the CIGS solar cell efficiency variation was observed while different dip times were applied, at 4, 15 and 30 minutes, respectively. When the dip time was extended, the series resistance (Rs) of the device fell by a substantial margin, leading to improved photoelectric conversion efficiency and enhanced uniformity in device properties. This can be attributed to the effect of CIGS surface cleaning by the NH4OH contained in the reaction solution. [ABSTRACT FROM PUBLISHER]

We report the effect of Na on the electronic properties of Cu(In,Ga)Se2 (CIGS) thin-film solar cells with a structure of grid/ITO/i-ZnO/CdS/CIGS/Mo/SiOx/soda-lime glass (SLG). The diffusion of Na from the SLG into the CIGS layer was systematically controlled by varying the thickness of SiOx. As the Na content increased, the hole concentration of CIGS was enhanced, while the band-gap was nearly constant, which led to a lower Fermi level in the CIGS towards its valence-band edge. The Na-induced increment in the built-in potential (Vbi) across the n-(ITO/i-ZnO/CdS)/p-CIGS junction yielded an increment of open-circuit voltage that well agreed with the calculated Vbi. [ABSTRACT FROM AUTHOR]

Buffer layers in Cu(In,Ga)Se 2 (CIGS) solar cells are highly preferable to be vacuum-compatible and use non-toxic materials not only for the film but also for the manufacturing process. Moreover, high controllability for the energy levels of buffer layers is greatly desirable for achieving higher solar cell efficiency because the buffer layer is formed directly onto the absorber layer which generates carriers. In this study, the S/(O + S) composition ratio of the Zn(O,S) thin films was finely controlled by varying O 2 /(Ar + O 2) gas flow ratio using a reactive sputtering method including an Ar/O 2 mixture gas and a single ZnS sputter target. The structural, electrical, and photovoltaic performance properties of the Zn(O,S) thin films and their CIGS solar cells were investigated at varied S/(O + S) composition ratios. As the S/(O + S) composition ratio of the Zn(O,S) films increased, bandgap bowing occurred with the increase of the conduction band and valence band energy levels. The photovoltaic performance was greatly influenced by the difference of the conduction band energies between the Zn(O,S) buffer and the CIGS absorber. When the conduction band energy of the Zn(O,S) was too high or too low compared to that of the CIGS, it increased the carrier recombinations or series resistance, respectively, which induced losses of the open-circuit voltage and the fill factor. The controlling and optimizing the energy levels at the Zn(O,S)/CIGS interface were crucial for improved solar cell performances. • S/(O + S) ratio of Zn(O,S) buffer was finely controlled using reactive sputtering. • Energy levels of E g , E C , and E V in Zn(O,S) buffer were precisely tuned. • Performance of CIGS solar cells is greatly influenced by Δ E C at the interface. • Too high or too low Δ E C increased carrier recombinations or series resistance. [ABSTRACT FROM AUTHOR]

• We fabricated Zn(O,S) buffered CIGS solar cells with controlled oxygen and sulfur ratio. • Carrier transport is enhanced by tuning the interfacial band offsets between the absorber and the buffer. • Depth-profiling of spatial work function of the devices was examined by Kelvin probe force microscopy. • A spike-like band offsets at the CIGS cell with Zn(O,S) grown by 1.3% O 2 gas ratio show an optimal efficiency. Environment-friendly Cu(In,Ga)Se 2 (CIGS) solar cells requires the replacement of Cd-containing buffers with non-toxic materials. Zn(O,S) buffers have been developed and yielded even better efficiency than CdS-buffered CIGS thin-film solar cells [23.35%, Ref. [6]]. In this work, we studied band offsets of Zn(O,S) and CIGS interfaces. The Cd-free buffer layers were deposited with 1.0%, 1.3%, and 1.6% oxygen (O 2) gas partial pressure during the deposition. Effects of the oxygen partial pressure on the structure and electronic properties of the devices were investigated by micro-Raman scattering spectroscopy and Kelvin probe force microscopy, respectively. We achieved depth-profiling of spatial work function mapping across the interface between the absorbers and the buffers. The best efficiency sample, grown using 1.3% of oxygen, showed 80 mV spike-like band offsets. We propose that the efficiency can be improved through tailoring of the band offsets at the interface as well as improving the absorber and the buffer materials. [ABSTRACT FROM AUTHOR]

We investigated the hydrazine effect on the physical characteristics of zinc sulfide (ZnS) thin films on both the soda-lime glass substrate and the Cu(In,Ga)Se 2 (CIGS) absorber layer as a function of its relative content r c with respect to ammonia as well as the deposition time t d in the chemical bath deposition. As the r c varied from 0 to 1.39, all the deposited ZnS films using hydrazine as a secondary complexing agent showed nearly identical characteristics of amorphous structure with the composition of [S/(S + O)] ~ 0.32 and the direct energy bandgap of E g = 3.54–3.75 eV. However, they exhibited a strong dependence of light transmission on the longer deposition times t d (≥ 70 min), due to the enhancement of the areal density of in-plane grains. By applying the ZnS buffer on top of the CIGS, the heterojunction device showed the best solar cell performance with η = 12.03%, V oc = 0.549 V, J sc = 32.92 mA/cm2, FF = 66.7% from the ZnS buffer grown at r c = 0.28 and t d = 30 min. When compared to only ammonia, the relative hydrazine was found to play a key role in the deposition of ZnS to expedite the in-plane grain growth by 6 times accompanied both with a reduction of in-plane grain size and a denser packing density. [ABSTRACT FROM AUTHOR]

Improving the optical transmittance and electrical conductivity in transparent conductors (TC) has been a critical issue for decades due to their numerous applications. In this paper, we suggest an approach to produce extremely conductive TC material from electroplated Ni microfiber networks (NiMFs) in order to achieve highly efficient and aesthetically superior thin-film solar cells and modules. The high cross-sectional aspect ratio of NiMFs significantly enhanced their electrical conductivity and optical transmittance simultaneously. The TC structure employing NiMFs was a successful substitute for conventional patterned grids in Cu(In,Ga)Se 2 thin-film solar cells because it reduced the series resistance, which is especially advantageous for large-area cells. The NiMF-induced transmittance loss was compensated for by the formation of a light diffusion layer on the NiMF. We propose that the excellent performance of NiMF TC materials enables the elimination or significant reduction of the grids in thin-film solar cells and modules. • Electroplated Ni microfiber networks (NiMFs) were grown with 3 diameters. • NiMF samples were implemented into gridless Cu(In,Ga)Se 2 thin film solar cells. • Various figures-of-merit were determined to assess the solar cells' applicability. • High photovoltaic performance and good appearance were obtained. [ABSTRACT FROM AUTHOR]