Cu(In,Ga)Se2太阳电池缓冲层ZnS薄膜性质及应用

在含有ZnSO4,SC(NH2)2,NH4OH的水溶液中采用CBD法沉积ZnS薄膜,XRF和热处理前后的XRD测试表明,ZnS沉积薄膜为立方相结构,薄膜含有非晶态的Zn(OH)2.光学透射谱测试表明,制备的薄膜透过率(λ＞500nm)约为90%,薄膜的禁带宽度约为3.51eV.ZnS薄膜沉积时间对Cu(In,Ga)Se2太阳电池影响显著,当薄膜沉积时间在25～35min时,电池的综合性能最好.对比了不同缓冲层的电池性能,采用CBD-CdS为缓冲层的电池转换效率、填充因子、开路电压稍高于CBD-ZnS为缓冲层的无镉电池,但无镉电池的短路电流密度高于前者,两者转换效率相差2%左右.ZnS可以作为CIGS电池的缓冲层,替代CdS,实现电池的无镉化.     

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

ZnS 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.     

High‐efficiency copper indium gallium diselenide (CIGS) solar cells with indium sulfide buffer layers deposited by atomic layer chemical vapor deposition (ALCVD)

This paper presents optimization studies on the formation of indium sulfide buffer layers for high‐efficiency copper indium gallium diselenide (CIGS) thin‐film solar cells with atomic layer chemical vapour deposition (ALCVD) from separate pulses of indium acetylacetonate and hydrogen sulfide. A parametric study of the effect of deposition temperature between 160° and 260°C and thickness (15–30 nm) shows an optimal value at about 220°C for a layer thickness of 30 nm, leading to an efficiency of 16·4%. Analysis of the device shows that indium sulfide layers are characterised by an improvement of the blue response of the cells compared with a standard CdS‐processed cell, due to a high apparent band gap (2·7–2·8 eV), higher open‐circuit voltages (up to 665 mV) and fill factor (78%). This denotes high interface quality. Atomic diffusion processes of sodium and copper in the buffer layer are demonstrated. Copyright © 2003 John Wiley & Sons, Ltd.     

Improved fill factor and open circuit voltage by crystalline selenium at the Cu(In,Ga)Se2/buffer layer interface in thin film solar cells

A surface treatment by evaporated selenium on Cu(In,Ga)Se₂ (CIGS) is shown to improve open circuit voltage, V_oc, and in some cases fill factor, FF, in solar cells with CdS, (Zn,Mg)O or Zn(O,S) buffer layers. V_oc increases with increasing amount of crystalline Se, while FF improves only for small amounts. The improvements are counteracted by a decreasing short circuit current assigned to absorption in hexagonal Se. Improved efficiency is shown for device structures with (Zn,Mg)O and Zn(O,S) buffer layers by atomic layer deposition. Analysis by grazing incidence X‐ray diffraction and photoelectron spectroscopy show partial coverage of the CIGS surface by hexagonal selenium. The effects on device performance from replacing part of the CIGS/buffer interface area by a Se/buffer junction are discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of MoSe2 on the performance of CIGS solar cells

A CuIn1-xGaxSe2 (CIGS) thin film solar cell model with MoSe2 transition layer was established, using SCAPS-1D software. The influence of MoSe2 interface layer formed between absorption layer CIGS and the back contact Mo on the solar cell performance was investigated.By changing the doping concentration,thickness and bandgap of MoSe2 layer, it is found that the MoSe2 and the variation of parameters have a significant effect on the electrical characteristics and photovoltaic parameters of CIGS thin film solar cells. Based on the energy band, the interfaces of Mo/MoSe2 and MoSe2/CIGS are analyzed. It is considered that Mo/MoSe2 is a Schottky contact, MoSe2/CIGS is an ohmic contact. When suitable parameters of MoSe2 layer are formed into the interface, it will provide a new path for designing CIGS solar cells with thinner absorption layer.     

Impacts of surface sulfurization on Cu(In1−x,Gax)Se2 thin‐film solar cells

In this work, the impacts of surface sulfurization of high‐quality Cu(In_1−x,Ga_x)Se₂ (CIGS) thin films deposited by three‐stage process on the film properties and the cell performance were investigated. The CIGS thin films were sulfurized at 550 °C for 30 min using H₂S gas. The X‐ray photoelectron spectroscopy analysis revealed that sulfur atoms diffused into the CIGS surface layer and that the valence band minimum was lowered by the film sulfurization. The open circuit voltage (V_oc) drastically increased from 0.590 to 0.674 V as a result of the sulfurization process. Temperature‐dependent current–voltage and capacitance–frequency measurements also revealed that interface recombination was drastically decreased by the lowering of the defect's activation energy level at the vicinity of the buffer/CIGS interface after the sulfurization. Copyright © 2014 John Wiley & Sons, Ltd.     

Controllable Formation of Ordered Vacancy Compound for High Efficiency Solution Processed Cu(In,Ga)Se2 Solar Cells

Solution processing of Cu(In,Ga)Se₂ (CIGS) absorber makes it cost-competitive in the photovoltaic market. It is reported that copper-poor ordered vacancy compound (OVC) is crucial for high performance CIGS solar cells. However, in solution process method, controllable formation of OVC is unavailable and limited research has been carried out. In this work, the controllable formation of the OVC phase on the CIGS surface is successful by controlling the selenization temperature and intentional variation of Cu/(In+Ga) stoichiometry in precursors for top layers and bulk layers deposition. The effects of OVC contents on the device performance are investigated. The CIGS thin film with OVC phase exhibits a lower valence band position. Meanwhile, the CIGS devices with optimized OVC content show decreased interface defects density and better carrier collection ability. The above advantages translate into a champion PCE of 16.39% for CIGS device with OVC phase, which is the champion performance among non-hydrazine solution-processed CIGS solar cells. The results demonstrate that the controllable formation of OVC phase approach should make a significant contribution to the efficiency promoting of solution processed CIGS solar cells.     

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

The 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.     

Study on ALD In2S3/Cu(In,Ga)Se2 interface formation

The formation of the interface between In₂S₃ grown by atomic layer deposition (ALD) and co‐evaporated Cu(In,Ga)Se₂ (CIGS) has been studied by X‐ray and UV photoelectron spectroscopy. The valence band offset at 160°C ALD substrate temperature was determined as −1·2±0·2 eV for CIGS deposited on soda‐lime glass substrates and −1·4±0·2 eV when a Na barrier substrate was used. Wavelength dependent complex refractive index of In₂S₃ grown directly on glass was determined from inversion of reflectance and transmittance spectra. From these data, an indirect optical bandgap of 2·08±0·05 eV was deduced, independent of film thickness, of substrate temperature and of Na content. CIGS solar cells with ALD In₂S₃ buffer layers were fabricated. Highest device efficiency of 12·1% was obtained at a substrate temperature of 120°C. Using the bandgap obtained for In₂S₃ on glass and a 1·15±0·05 eV bandgap determined for the bulk of the CIGS absorber, the conduction band offset at the buffer interface was estimated as −0·25±0·2 eV (−0·45±0·2 eV) for Na‐containing (Na‐free) CIGS. Copyright © 2005 John Wiley & Sons, Ltd.     

All‐Solution‐Processed Indium‐Free Transparent Composite Electrodes based on Ag Nanowire and Metal Oxide for Thin‐Film Solar Cells

Fully solution‐processed Al‐doped ZnO/silver nanowire (AgNW)/Al‐doped ZnO/ZnO multi‐stacked composite electrodes are introduced as a transparent, conductive window layer for thin‐film solar cells. Unlike conventional sol–gel synthetic pathways, a newly developed combustion reaction‐based sol–gel chemical approach allows dense and uniform composite electrodes at temperatures as low as 200 °C. The resulting composite layer exhibits high transmittance (93.4% at 550 nm) and low sheet resistance (11.3 Ω sq^‐1), which are far superior to those of other solution‐processed transparent electrodes and are comparable to their sputtered counterparts. Conductive atomic force microscopy reveals that the multi‐stacked metal‐oxide layers embedded with the AgNWs enhance the photocarrier collection efficiency by broadening the lateral conduction range. This as‐developed composite electrode is successfully applied in Cu(In_1‐x,Ga_x)S₂ (CIGS) thin‐film solar cells and exhibits a power conversion efficiency of 11.03%. The fully solution‐processed indium‐free composite films demonstrate not only good performance as transparent electrodes but also the potential for applications in various optoelectronic and photovoltaic devices as a cost‐effective and sustainable alternative electrode.     

Role of the CdS buffer layer as an active optical element in Cu(In,Ga)Se2 thin‐film solar cells

ZnO/CdS/Cu(In,Ga)Se₂ (CIGS) thin‐film heterojunction solar cells with CdS buffer layers of thicknesses between 0 and 85 nm are characterized by current–voltage, quantum efficiency, and optical reflection measurements. We investigate the correlation between the short‐circuit current density and the CdS layer thickness, focusing on the counteracting effects of light absorption and reduced optical reflection induced by the CdS layer. Both effects almost compensate each other for CdS layer thicknesses between 0 and 40 nm. Thus, an optimization of the short‐circuit current density is not achieved by omitting the CdS layer, but rather by replacing the CdS buffer with an alternative buffer material with higher bandgap energy and optical constants similar to those of CdS. Copyright © 2002 John Wiley & Sons, Ltd.     

A comparative study of Cd‐ and Zn‐compound buffer layers on Cu(In1−x,Gax)(Sy,Se1−y)2 thin film solar cells

This paper reports a comparative study of Cu(In,Ga)(S,Se)₂ (CIGSSe) thin‐film solar cells with CBD‐CdS, CBD‐ZnS(O,OH) and ALD‐Zn(O,S) buffer layers. Each buffer layer was deposited on CIGSSe absorber layers which were prepared by sulfurization after selenization (SAS) process by Solar Frontier K. K. Cell efficiencies of CBD‐CdS/CIGSSe, CBD‐ZnS(O,OH)/CIGSSe and ALD‐Zn(O,S)/CIGSSe solar cells exceeded 18%, for a cell area of 0.5 cm². The solar cells underwent a heat‐light soaking (HLS) post‐treatment at 170 °C under one‐sun illumination in the air; among the three condtions, the ALD‐Zn(O,S)/CIGSSe solar cells showed the highest cell efficiency of 19.78% with the highest open‐circuit voltage of 0.718 V. Admittance spectroscopy measurements showed a shift of the N₁ defect's energy position toward shallower energy positions for ALD‐Zn(O,S)/CIGSSe solar cells after HLS post‐treatment, which is in good agreement with their higher open‐circuit voltage and smaller interface recombination than that of CBD‐ZnS(O,OH)/CIGSSe solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

Zn1?xMgxO用于 CIGS 太阳电池的研究进展

Zn1-xMgxO透过率高、带隙可调,且与CIGS太阳电池在晶格和能带结构上匹配良好,可用作CIGS太阳电池缓冲层、窗口层,因此制备高质量的Zn1-xMgxO薄膜是提高太阳电池性能的关键。文章介绍了Zn1-xMgxO薄膜的结构特性、光学特性及制备方法;从Mg含量、Zn1-xMgxO膜厚及Zn1-xMgxO/CIGS界面处缺陷密度等方面概述了Zn1-xMgxO用于CIGS太阳电池的研究进展,并比较了Zn1-xMgxO与In2S3,ZnS,CdS等其他材料作缓冲层的CIGS太阳电池性能的差别。     

Locating the electrical junctions in Cu(In,Ga)Se2 and Cu2ZnSnSe4 solar cells by scanning capacitance spectroscopy

We determined the electrical junction (EJ) locations in Cu(In,Ga)Se₂ (CIGS) and Cu₂ZnSnSe₄ (CZTS) solar cells with ~20‐nm accuracy by developing scanning capacitance spectroscopy (SCS) applicable to the thin‐film devices. Cross‐sectional sample preparation for the SCS measurement was developed by high‐energy ion milling at room temperature for polishing the cross section to make it flat, followed by low‐energy ion milling at liquid nitrogen temperature for removing the damaged layer and subsequent annealing for growing a native oxide layer. The SCS shows distinct p‐type, transitional, and n‐type spectra across the devices, and the spectral features change rapidly with location in the depletion region, which results in determining the EJ with ~20‐nm resolution. We found an n‐type CIGS in the region next to the CIGS/CdS interface; thus, the cell is a homojunction. The EJ is ~40 nm from the interface on the CIGS side. In contrast, such an n‐type CZTS was not found in the CZTS/CdS cells. The EJ is ~20 nm from the CZTS/CdS interface, which is consistent with asymmetrical carrier concentrations of the p‐CZTS and n‐CdS in a heterojunction cell. Our results of unambiguously determination of the junction locations contribute significantly to understanding the large open‐circuit voltage difference between CIGS and CZTS. Copyright © 2016 John Wiley & Sons, Ltd.     

Effect of Se flux on CuIn1‐xGaxSe2 film in reactive sputtering process

CuIn_1‐xGa_xSe₂ (CIGS) thin films were grown on Mo/soda lime glass using a reactive sputtering process in which a Se cracker was used to deliver reactive Se molecules. The Cu_0·6 Ga_0·4 and Cu_0·4In_0·6 targets were simultaneously sputtered under the delivery of reactive Se. The effects of Se flux on CIGS film deposition were investigated. The CIGS film growth rate decreased, and the surface roughness of a film increased as the Se flux increased. The [112] crystal orientation was dominant, and metallic crystal phases such as Cu₉Ga₄ and Cu₁₆In₉ in a film were disappearing with increasing Se flux. A solar cell fabricated using a 900‐nm CIGS film showed the power conversion efficiency of 8·6%, the highest value found in a sub‐micron thick CIGS solar cell related to a reactive sputtering process with metallic targets. Copyright © 2011 John Wiley & Sons, Ltd.     

Highly‐efficient Cd‐free CuInS2 thin‐film solar cells and mini‐modules with Zn(S,O) buffer layers prepared by an alternative chemical bath process

Recent progress in fabricating Cd‐ and Se‐free wide‐gap chalcopyrite thin‐film solar devices with Zn(S,O) buffer layers prepared by an alternative chemical bath process (CBD) using thiourea as complexing agent is discussed. Zn(S,O) has a larger band gap (E_g = 3·6–3·8 eV) than the conventional buffer material CdS (E_g = 2·4 eV) currently used in chalcopyrite‐based thin films solar cells. Thus, Zn(S,O) is a potential alternative buffer material, which already results in Cd‐free solar cell devices with increased spectral response in the blue wavelength region if low‐gap chalcopyrites are used. Suitable conditions for reproducible deposition of good‐quality Zn(S,O) thin films on wide‐gap CuInS₂ (‘CIS’) absorbers have been identified for an alternative, low‐temperature chemical route. The thickness of the different Zn(S,O) buffers and the coverage of the CIS absorber by those layers as well as their surface composition were controlled by scanning electron microscopy, X‐ray photoelectron spectroscopy, and X‐ray excited Auger electron spectroscopy. The minimum thickness required for a complete coverage of the rough CIS absorber by a Zn(S,O) layer deposited by this CBD process was estimated to ∼15 nm. The high transparency of this Zn(S,O) buffer layer in the short‐wavelength region leads to an increase of ∼1 mA/cm² in the short‐circuit current density of corresponding CIS‐based solar cells. Active area efficiencies exceeding 11·0% (total area: 10·4%) have been achieved for the first time, with an open circuit voltage of 700·4 mV, a fill factor of 65·8% and a short‐circuit current density of 24·5 mA/cm² (total area: 22·5 mA/cm²). These results are comparable to the performance of CdS buffered reference cells. First integrated series interconnected mini‐modules on 5 × 5 cm² substrates have been prepared and already reach an efficiency (active area: 17·2 cm²) of above 8%. Copyright © 2006 John Wiley & Sons, Ltd.     

Theoretical simulation of performances in CIGS thin-film solar cells with cadmium-free buffer layer

Copper indium gallium selenium (CIGS) thin film solar cells have become one of the hottest topics in solar energy due to their high photoelectric transformation efficiency. To real applications, CIGS thin film is covered by the buffer layer and absorption layer. Traditionally, cadmium sulfide (CdS) is inserted into the middle of the window layer (ZnO) and absorption layer (CIGS) as a buffer layer. However, the application of the GIGS/CdS thin film solar cells has been limited because of the environmental pollution resulting from the toxic cadmium atom. Although zinc sulfide (ZnS) has been proposed to be one of the candidates, the performance of such battery cells has not been investigated. Here, in this paper, we systematically study the possibility of using zinc sulfide (ZnS) as a buffer layer. By including the effects of thickness, concentration of a buffer layer, intrinsic layer and the absorbing layer, we find that photoelectric transformation efficiency of ZnO/ZnS(n)/CIGS(i)/CIGS(p) solar cell is about 17.22%, which is qualified as a commercial solar cell. Moreover, we also find that the open-circuit voltage is~0.60 V, the short-circuit current is~36.99 mA/cm² and the filled factor is~77.44%. Therefore, our results suggest that zinc sulfide may be the potential candidate of CdS as a buffer layer.     

Atomic layer deposition of Zn1−xMgxO buffer layers for Cu(In,Ga)Se2 solar cells

Fabrication of Zn_1−xMg_xO films by atomic layer deposition (ALD) has been studied for use as buffer layers in Cu(In,Ga)Se₂ (CIGS)‐based solar cell devices. The Zn_1−xMg_xO films were grown using diethyl zinc, bis‐cyclopentadienyl magnesium and water as precursors in the temperature range from 105 to 180°C. Single‐phase ZnO‐like films were obtained for x < 0·2, followed by a two phase region of ZnO‐ and MgO‐like structures for higher Mg concentrations. Increasing optical band gaps of up to above 3·8 eV were obtained for Zn_1−xMg_xO with increasing x. It was found that the composition of the Zn_1−xMg_xO films varied as an effect of deposition temperature as well as by increasing the relative amount of magnesium precursor pulses during film growth. Completely Cd‐free CIGS‐based solar cells devices with ALD‐Zn_1−xMg_xO buffer layers were fabricated and showed efficiencies of up to 14·1%, which was higher than that of the CdS references. Copyright © 2006 John Wiley & Sons, Ltd.     

Structure of chemically deposited Zn(S,O,OH) buffer layer and the effects on the performance of Cu(in,Ga)Se2 solar cell

The effects of the immersion into a NH₃ aqueous solution on the structural characteristics of the chemically deposited Zn(S,O,OH) layer and photovoltaic performance of the CIGS/Zn(S,O,OH) solar cells were investigated with structural and electrical characterizations. The as‐deposited‐Zn(S,O,OH) layer possessed a layered structure of upper Zn(OH)₂ and Zn(S,O) layers, and the upper Zn(OH)₂ layer was removed by the immersion. The conversion efficiency for the CIGS solar cell was improved from 6.8% to 13.7% by removing the upper Zn(OH)₂ layer during the immersion. Copyright © 2015 John Wiley & Sons, Ltd.     

Gallium gradients in Cu(In,Ga)Se2 thin‐film solar cells

The gallium gradient in Cu(In,Ga)Se₂ (CIGS) layers, which forms during the two industrially relevant deposition routes, the sequential and co‐evaporation processes, plays a key role in the device performance of CIGS thin‐film modules. In this contribution, we present a comprehensive study on the formation, nature, and consequences of gallium gradients in CIGS solar cells. The formation of gallium gradients is analyzed in real time during a rapid selenization process by in situ X‐ray measurements. In addition, the gallium grading of a CIGS layer grown with an in‐line co‐evaporation process is analyzed by means of depth profiling with mass spectrometry. This gallium gradient of a real solar cell served as input data for device simulations. Depth‐dependent occurrence of lateral inhomogeneities on the µm scale in CIGS deposited by the co‐evaporation process was investigated by highly spatially resolved luminescence measurements on etched CIGS samples, which revealed a dependence of the optical bandgap, the quasi‐Fermi level splitting, transition levels, and the vertical gallium gradient. Transmission electron microscopy analyses of CIGS cross‐sections point to a difference in gallium content in the near surface region of neighboring grains. Migration barriers for a copper‐vacancy‐mediated indium and gallium diffusion in CuInSe₂ and CuGaSe₂ were calculated using density functional theory. The migration barrier for the In_Cu antisite in CuGaSe₂ is significantly lower compared with the Ga_Cu antisite in CuInSe₂, which is in accordance with the experimentally observed Ga gradients in CIGS layers grown by co‐evaporation and selenization processes. Copyright © 2014 John Wiley & Sons, Ltd.