ZnS Buffer Layer Research Articles

Cu(In,Ga)Se2 Thin Film Solar Cells with Solution Processed Metal Nanowire Based Transparent Conductor

Since the traditional sputtered metal oxide thin films are not suited for using as transparent electrodes of flexible thin film solar cells and their increasing prices limit development of solar cells industry, high performance, low cost and flexible transparent conductor technology has aroused researchers’ great interests. In this paper, we successfully apply a solution processed silver nanowire composite film as the transparent electrode for Cu(In,Ga)Se2 thin film solar cells with ZnS buffer layers. Compared with traditional sputtered window layer, the solution processed transparent electrode causes no physical damage to the adjacent device layers and junction region, which can greatly decrease the required thickness of the ZnS buffer layer. Properties of the silver nanowire composite transparent conductor and its effects on performance of Cu(In,Ga)Se2/ZnS thin film solar cells are studied.

Effect of ZnS buffer layers in ZnO/ZnS/CdS nanorod array photoelectrode on the photoelectrochemical performance

ZnO/CdS/ZnS and ZnO/ZnS/CdS nanorod array photoelectrodes were constructed and fabricated by an alternative chemical bath deposition of CdS and ZnS layers onto the surface of the ZnO nanorod array. The photoelectrochemical performances of composite photoelectrodes were investigated, and the action mechanism of ZnS buffer layers was also considered. The results show that the ZnO/ZnS/CdS nanorod array photoelectrode possesses enhanced hydrogen production efficiency. The enhancement may be attributed to the ZnS buffer layers, which are advantageous in the separation and transportation of photogenerated electron–hole pairs in ZnO/ZnS/CdS nanorod array photoelectrodes.

Fabrication methods for performance improvement of Cu(In,Ga)Se 2 thin film solar cells

Two advanced fabrication methods are introduced: the hydrogen peroxide (H2O2)-added chemical bath deposition technique and molybdenum (Mo) back-contact formation under 3 kW sputter power. The parameters of the short circuit current density (J SC) and conversion efficiency (η) were improved over standard cells when the ZnS buffer layer was deposited in the H2O2-added chemical solution. Otherwise, the fill-factor and η were best at 3 kW Mo sputtering power conditions. Advanced fabrication methods are realised to improve cell performance without modifying the chemical composition of the Cu(In,Ga)Se2 absorption layer.

Efficient Cu(In,Ga)Se2 thin film solar cells with reduced thickness of ZnS(O,OH) Buffer Layer

The influence of the buffer layer thickness and the thiourea concentration in chemical bath on the cell performance of ZnS(O,OH)/CIGS solar cells was investigated. It was found that a high efficiency CIGS solar cell could be achieved by increasing the thiourea concentration in the chemical bath deposition (CBD) solution even if a very thin ZnS(O,OH) buffer layer (10nm) was used. As a result, the CBD deposition time could be shortened to one six as compared to that of thicker buffer layer thickness of 120nm. The results are discussed in connection with the conduction band offset at the ZnS(O,OH)/CIGS interface.

Studies in CuInS2 based solar cells, including ZnS and In2S3 buffer layers

The influence of the growth conditions on the surface chemistry and on the homogeneity of the chemical composition of CuInS2 (CIS) thin films, prepared by sequential evaporation of metallic precursors in presence of elemental sulfur in a two-stage process, was studied by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). It was found that the growth temperature affects the phase in which this compound grows. The samples deposited at temperatures around 500°C (2nd stage) contain mainly the CuInS2 phase; however, secondary phases like In2S3, Cu2S were additionally identified at the surface and in the bulk of CuInS2 samples deposited at temperatures greater than 550°C. Also, the elemental composition of the layers constituting the Glass/Mo/CuInS2/buffer/ZnO structure was studied through Auger electron spectroscopy (AES) depth profile measurements. AES measurements carried out across the Glass/Mo/CuInS2/buffer/ZnO heterojunction gave evidence of Cu diffusion from the CuInS2 layer towards the rest of the layers constituting the device, and of the formation of a MoS2 layer in the Mo/CuInS2 interface. The performance of CuInS2-based solar cells fabricated using CBD (chemical bath deposition) deposited ZnS as buffer layer was compared to that of cells fabricated using CBD deposited In2S3 as buffer.

Comparison of Cell Performance of ZnS(O,OH)/CIGS Solar Cells With UV-Assisted MOCVD-ZnO:B and Sputter-Deposited ZnO:Al Window Layers

ZnO:B films deposited by ultraviolet light-assisted metal organic chemical vapor deposition (UM-ZnO:B) were applied to CBD-ZnS(O,OH)/CIGS solar cells in order to eliminate plasma damages during the subsequent ZnO sputtering. It was verified that the conversion efficiency of CIGS solar cells with a UM-ZnO:B window layer was higher than that of the device with a sputter-deposited(Sp-) ZnO:Al window layer; in both cases, thick (120 nm) and thin (10 nm) ZnS(O,OH) buffer layers were used. The conversion efficiency of CIGS solar cell was improved from 16.3% to 17.5% upon replacement of the Sp-ZnO:Al by a UM-ZnO:B window layer when the thick ZnS(O,OH) (120nm) buffer layer was used. Notably, the conversion efficiency was remarkably improved from 0.2% to 15.6% by the replacement of the window layer even when the ultra thin ZnS(O,OH) (10nm) buffer layer was used. The temperature dependence of open-circuit voltage revealed that interface recombination decreased owing to the use of a UM-ZnO: B window layer.

Effects of combined heat and light soaking on device performance of Cu(In,Ga)Se2 solar cells with ZnS(O,OH) buffer layer

ABSTRACTThe impacts of air annealing, light soaking (LS), and heat–light soaking (HLS) on cell performances were investigated for ZnS(O,OH)/Cu(In,Ga)Se2 (CIGS) thin‐film solar cells. It was found that the HLS post‐treatment, a combination of LS and air annealing at 130 °C, is the most effective process for improving the cell performances of ZnS(O,OH)/CIGS devices. The best solar cell yielded a total area efficiency of 18.4% after the HLS post‐treatment. X‐ray photoelectron spectroscopy showed that the improved cell performance was attributable to the decreased S/(S + O) atomic ratio, not only in the surface region but also the interface region between the ZnS(O,OH) and CIGS layers, implying the shift to an adequate conduction‐band offset at the ZnS(O,OH)/CIGS interface. Copyright © 2013 John Wiley & Sons, Ltd.

Highly efficient and well-resolved Mn2+ ion emission in MnS/ZnS/CdS quantum dots

We demonstrate a strategy for the growth of Mn2+ ion doped cadmium based II–VI semiconductor quantum dots (QDs) with a designed buffer layer of ZnS (MnS/ZnS/CdS or Mn:CdS QDs), which aims to meet the challenge of obtaining highly efficient and well-resolved Mn2+ ion emission. First, small, high quality MnS cores are obtained by using thiols to replace conventional alkyl amines as capping ligands. Then a buffer layer of ZnS with a tailored thickness is introduced to the QDs before the growth of CdS shells to reduce the size mismatch between the Mn2+ (dopant) and Cd2+ (host) ions. The fabricated MnS/ZnS/CdS core/shell QDs exhibit a high PL QY of up to 68%, which is the highest ever reported for any type of Mn2+ ion doped cadmium based II–VI semiconductor QD. The photoluminescence (PL) of the QDs consists of well-resolved Mn2+ ion emission without any detectable emission from the CdS band edge or surface defects. In addition, our MnS/ZnS/CdS QDs cannot only be made water-soluble, but can also be coated by ligands with short carbon chain lengths, nearly without cost to the PL QY, which could make them strong candidates for practical applications in biology/biomedicine and opto/electronic devices.

To Enhance Performance of Light Soaking Process on ZnS/CuIn1-xGaxSe2Solar Cell

The ZnS/CuInGaSe2heterojunction solar cell fabricated on Mo coated glass is studied. The crystallinity of the CIGS absorber layer is prepared by coevaporated method and the ZnS buffer layer with a band gap of 3.21 eV. The MoS2phase was also found in the CuInGaSe2/Mo system form HRTEM. The light soaking effect of photoactive film for 10 min results in an increase in F.F. from 55.8 to 64%, but series resistivity from 7.4 to 3.8 Ω. The efficiency of the devices improved from 8.12 to 9.50%.

Effects of the Modified TCO Sputtering Condition on ZnS/CIGS Solar Cell

The Cu(In,Ga)Se2(CIGS) thin film solar cells were fabricated using ZnS buffer layer deposited by chemical bath deposition and ZnO/Al-doped ZnO transparent conducting layers deposited by magnetron sputtering with two different configurations of target-to-substrate angle and distance in order to evaluate the effect of plasma damage on device performance. The higher efficiency was obtained from the configuration of the substrate being far away from the target center than face-to-face configuration, suggesting that the excessive bombardment of high energy particles from the plasma on the film surface is responsible for the solar cell performance degradation. It is suggested that the improved open-circuit voltage is ascribed to the increased recombination barrier height.

Improvement of the cell performance in the ZnS/Cu(In,Ga)Se2 solar cells by the sputter deposition of a bilayer ZnO : Al film

ABSTRACTZnS is a candidate to replace CdS as the buffer layer in Cu(In,Ga)Se2 (CIGS) solar cells for Cd‐free commercial product. However, the resistance of ZnS is too large, and the photoconductivity is too small. Therefore, the thickness of the ZnS should be as thin as possible. However, a CIGS solar cell with a very thin ZnS buffer layer is vulnerable to the sputtering power of the ZnO : Al window layer deposition because of plasma damage. To improve the efficiency of CIGS solar cells with a chemical‐bath‐deposited ZnS buffer layer, the effect of the plasma damage by the sputter deposition of the ZnO : Al window layer should be understood. We have found that the efficiency of a CIGS solar cell consistently decreases with an increase in the sputtering power for the ZnO : Al window layer deposition onto the ZnS buffer layer because of plasma damage. To protect the ZnS/CIGS interface, a bilayer ZnO : Al film was developed. It consists of a 50‐nm‐thick ZnO : Al plasma protection layer deposited at a sputtering power of 50 W and a 100‐nm‐thick ZnO : Al conducting layer deposited at a sputtering power of 200 W. The introduction of a 50‐nm‐thick ZnO : Al layer deposited at 50 W prevented plasma damage by sputtering, resulting in a high open‐circuit voltage, a large fill factor, and shunt resistance. The ZnS/CIGS solar cell with the bilayer ZnO : Al film yielded a cell efficiency of 14.68%. Therefore, the application of bilayer ZnO : Al film to the window layer is suitable for CIGS solar cells with a ZnS buffer layer. Copyright © 2012 John Wiley & Sons, Ltd.

Factors affecting the shape of MBE-grown laterally aligned Fe nanowires

Various microstructural and chemical analysis techniques were applied to study two types (type-A and B) of self-assembled laterally aligned Fe nanowires (NWs) fabricated by molecular beam epitaxy on a ZnS buffer layer. The formation of the three-dimensional shapes of these NWs was found to be driven by the principle of surface energy minimization. We have provided phenomenological models to address the factors affecting the observed topological shape of these NWs, including the role of the lattice relationship between the Fe NWs and the underlying buffer layer, growth temperature, Fe nominal coverage and substrate orientation. Magnetic hysteresis measurements were performed at different temperature, demonstrating the Fe NWs possess a coercivity about 30 times larger than that of a Fe thin film. The observed gradual magnetization reversal indicates the magnetization process is accomplished by the rotation of magnetic moments within a single domain.

Characterization of the ZnS thin film buffer layer for Cu(In, Ga)Se2 solar cells deposited by chemical bath deposition process with different solution concentrations

ZnS buffer layer thin films for Cu(In, Ga)Se2 (CIGS) have been prepared by chemical bath deposition (CBD) technique onto the ITO coated glasses at 80°C bath temperature, using zinc sulfate hepta-hydrate (ZnSO4·7H2O), thiourea (SC(NH2)2) and ammonia (NH4OH) as reacting chemicals. The concentrations of thiourea and ammonia were varied while the concentration of ZnSO4 was immobilized at 0.03M. The thickness, transmittance, morphology of ZnS films were measured and the band gap was calculated according to the equation related with the absorption coefficient. Through the analysis of the parameters, we could conclude that the physical and optical properties of different ZnS thin films were influenced severely by the concentration of the two reacting chemicals and then the optimal concentration group of SC(NH2)2 and NH4OH solutions can come to 0.3M and 1.5M, respectively.

Controlled growth of Cu2ZnSnS4 (CZTS) thin films for heterojunction solar-cell applications

Cu2ZnSnS4 (CZTS) thin films (absorber layers) were successfully synthesized on glass substrates by using a RF magnetron sputtering system. The films were rapidly thermally annealed in a nitrogen atmosphere for 20 minutes to improve the crytallinity. The formation of kesterite structures (JCPDS-26-0575) in the film was confirmed using X-ray diffraction (XRD) measurements. The improved crytallinity of the CZTS with a (112) orientation was observed with increasing annealing temperature. The band gaps of all the as-deposited and annealed films were found to be in the range from 1.97 to 1.55 eV. The films’ stoichiometry and morphologies were investigated using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) measurements. The sample annealed at 500 °C showed a uniform granular structure with an elemental composition near stoichiometric CZTS. Next, a buffer layer of ZnS for a heterojuntion solar cell was fabricated using a chemical bath deposition (CBD) technique. The films adhered well, were optically transparent and had band gap energy of 3.6 eV.

Junction formation of Cu3BiS3 investigated by Kelvin probe force microscopy and surface photovoltage measurements

Recently, the compound semiconductor Cu3BiS3 has been demonstrated to have a band gap of ~1.4 eV, well suited for photovoltaic energy harvesting. The preparation of polycrystalline thin films was successfully realized and now the junction formation to the n-type window needs to be developed. We present an investigation of the Cu3BiS3 absorber layer and the junction formation with CdS, ZnS and In2S3 buffer layers. Kelvin probe force microscopy shows the granular structure of the buffer layers with small grains of 20–100 nm, and a considerably smaller work-function distribution for In2S3 compared to that of CdS and ZnS. For In2S3 and CdS buffer layers the KPFM experiments indicate negatively charged Cu3BiS3 grain boundaries resulting from the deposition of the buffer layer. Macroscopic measurements of the surface photovoltage at variable excitation wavelength indicate the influence of defect states below the band gap on charge separation and a surface-defect passivation by the In2S3 buffer layer. Our findings indicate that Cu3BiS3 may become an interesting absorber material for thin-film solar cells; however, for photovoltaic application the band bending at the charge-selective contact has to be increased.

Synthesis of a Cu2ZnSnS4 (CZTS) Absorber Layer and Metal Doped ZnS Buffer Layer for Heterojunction Solar Cell Applications

A novel synthetic strategy was employed to fabricate a Cu2ZnSnS4 (CZTS) absorber layer using a quaternary compound target which has similar properties to that of copper indium gallium selenide (CIGS). A conventional R-F magnetron sputtering system was used to fabricate the CZTS absorber layers on glass substrates. The films were rapidly thermally annealed at 500 °C in a nitrogen atmosphere for 20 minutes to improve their crytallinity. The formation of a kesterite structure was confirmed using X-ray diffraction measurements. The improved crytallinity of the CZTS was observed with (112) oriented phase. The band gap of the as-deposited and annealed films was found to be 1.97 and 1.55 eV respectively. The films stoichiometry and morphology were observed using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) measurements. In addition; buffer layer of ZnS which was doped with metal elements; was fabricated by using a chemical bath deposition (CBD) technique.

Effect of [Zn2+] Concentration on the Formation of Zinc Sulfide Film as a Buffer Layer of Cu(In,Ga)Se2 Thin Film Solar Cells

ZnS buffer layer thin films for Cn(In, Ga)Se2 solar cells have been prepared by chemical bath deposition (CBD) technique with the bath temperature of 80ºC. Zinc sulfate (ZnSO4•7H2O), thiourea (CS(NH2)2) and ammonia (NH4OH) solutions were used to deposit ZnS film onto ITO/Glass. The [Zn2+] (ZnSO4) concentration was varied from 0.006 to 0.04 M, while CS(NH2)2 and NH4OH concentrations were fixed at 0.2 M and 1.5 M respectively. And the thickness and transmittance of different ZnS films were measured and the band gap (Eg) was calculated. The results showed that the thickness and the deposition rate increased as increasing the [Zn2+] concentration. The surface morphology was also analyzed. More ZnS grains were generated and deposited on the substrate so that more ZnS films were covered on the surface of the substrate. However, the optical band gap slightly decreased from 3.87 to 3.85 eV when the [Zn2+] concentration increased.

Study of Structure and Optical Properties of Magnetron Sputtered ZnS Thin Films by Annealing

Nanocrystalline ZnS thin films were deposited on glass substrates at 300W and 0.6Pa by using radio frequency magnetron sputtering, and then annealed at different temperatures. This work investigated the influence of ZnS buffer layer on different annealing temperature, analysed structural, surface topography, and optical properties of ZnS films by using X-ray diffraction (XRD), UV-spectroscopy measurements and scanning electronic microscope (SEM) analysis techniques. Findings showed that the film annealed at 300°C was uniformity and compact, which was zinc blende cubic structure. The film exhibited the optical transparency as high as 85% in the visible region, and its optical band gap was calculated to be 3.56 eV.

Research on ZnS Buffer Layer by RF Magnetron Sputtering for Cu(In,Ga)Se<sub>2</sub> Solar Cells

Nanocrystalline ZnS thin films are prepared on glass substrates under various deposition conditions (radio frequency power, and sputtering pressure) using radio frequency magnetron sputtering at room temperature. Through optimization of deposition process, very thin and uniform ZnS layer was deposited to minimize the light blocking effect in short wavelength region. This paper investigates the influence of ZnS buffer layer by magnetron sputtering,and analyses structure,surface topography,and optical properties of ZnS films by using X-ray diffraction (XRD), UV-spectroscopy measurements and scanning electronic microscope (SEM) analysis techniques. Findings show that the high-quality ZnS thin films with a good crystallinity and optical properties, which is zinc blende cubic structure with a preferred orientation, can be grown by sputtering at 300W and 0.6Pa. The films exhibit the optical transparency as high as 80% in the visible region, and the resultant ZnS thin films have a high crystallinity, low defect concentration and good optical properties with the band gap of 3.50eV.

Alignment of energy levels at the ZnS/Cu(In,Ga)Se2 interface

Further understanding of the electronic structure at the ZnS/Cu(In,Ga)Se2 interface is necessary to enhance the electron injection across the interface in Cu(In,Ga)Se2 solar cells. The valence band structure and shallow core levels were investigated by ultraviolet photoelectron spectroscopy depth profile analysis with He II line excitation. ZnS film was grown by a chemical bath deposition on a Cu(In,Ga)Se2 absorber deposited by the co-evaporation of Cu, In, Ga, and Se elemental sources. The discontinuity of 2.0 eV in the valence band edge at the ZnS/Cu(In0.7Ga0.3)Se2 interface was directly determined. This type of valence band offset yields a spike conduction band alignment of 0.25 eV. The positions of the VBM and the Zn 3d core-level emission of the buffer underwent the substantial shifts of 0.36 eV and 0.64 eV to a lower binding energy levels during the etching process. The shifts are ascribed to the contribution of the band bending in the ZnS buffer layer and its graded chemical composition. This study is the first to determine the small conduction band offset at the interface formed by the chemical bath deposited ZnS layer and the Cu(In0.7Ga0.3)Se2 absorber. Our results also provide information toward the design optimization of the optoelectronic properties of the ZnS/Cu(In0.7Ga0.3)Se2 interface. To enhance the electron injection from Cu(In0.7Ga0.3)Se2 absorber to ZnS layer further lowering of the energy barrier is required. For this purpose, the bandgap of ZnS should be reduced by controlling the crystal structure and composition or its Fermi level should be upward shifted by appropriate doping.