柔性不锈钢衬底CIGS薄膜太阳电池

以轻质柔性不锈钢材料为衬底,利用三步共蒸发法制备较高质量的四元化合物Cu(In,Ga)Se_2薄膜,CIGS层在Mo导电层上具有很强的附着力。利用XRD和XRF分别分析了所制备薄膜的晶相和组分。以ZnO:Al/i-ZnO/ CdS/CIGS/Mo/Stainless steel结构为基础得到最高转换效率为9.39%的柔性太阳电池。最后讨论了衬底粗糙度、有害杂质的扩散和不含有Na元素等不利因素对于电池性能的影响。     

Improved CIGS thin-film solar cells by surface sulfurization using In2S3 and sulfur vapor

Surface sulfurization of Cu(In,Ga)Se₂ (CIGS) thin films was carried out using two alternative techniques that do not utilize toxic H₂S gas; a sequential evaporation of In₂S₃ after CIGS deposition and the annealing of CIGS thin films in sulfur vapor. A Cu(In,Ga) (S,Se)₂ thin layer was grown on the surface of the CIGS thin film after sulfurization using In₂S₃, whereas this layer was not observed for CIGS thin films after sulfurization using sulfur vapor, although a trace quantity of S was confirmed by AES analysis. In spite of the difference in the surface modification techniques, the cell performance and process yield of the ZnO:Al/CdS/CIGS/Mo/glass thin-film solar cells were remarkably improved by using both surface sulfurization techniques.     

Improved Cu(In,Ga)(S,Se)2 thin film solar cells by surface sulfurization

Surface sulfurization was developed as a technique for fabricating efficient ZnO : Al/CdS/graded Cu(In,Ga)(S,Se)₂/ Mo/glass solar cells. Prior to the sulfurization, single-graded Cu(In,Ga)Se₂ (CIGS) films were deposited by a multi-stage process. The sulfurization of CIGS films was carried out using a H₂S---Ar mixture at elevated temperatures. The crystallographic and compositional properties of the absorber layers were investigated by XRD, SEM and AES analyses. After sulfurization, sulfur atoms were substituted for selenium atoms at the surface layer of CIGS films to form a Cu(In,Ga)(S,Se)₂ absorber layer. The diffusion of sulfur depends strongly on the grain structure of CIGS film. The cell efficiency of the 8–11% range before sulfurization was improved dramatically to 14.3% with V_oc = 528 mV, J_sc = 39.9 mA/cm² and FF = 0.68 after the sulfurization process.     

Preparation of Cu(In,Ga)Se2 thin films from Cu–Se/In–Ga–Se precursors for high-efficiency solar cells

Improved preparation process of a device quality Cu(In,Ga)Se₂ (CIGS) thin film was proposed for production of CIGS solar cells. In–Ga–Se layer were deposited on Mo-coated soda-lime glass, and then the layer was exposed to Cu and Se fluxes to form Cu–Se/In–Ga–Se precursor film at substrate temperature of over 200°C. The precursor film was annealed in Se flux at substrate temperature of over 500°C to obtain high-quality CIGS film. The solar cell with a MgF₂/ITO/ZnO/CdS/CIGS/Mo/glass structure showed an efficiency of 17.5% (V_oc=0.634 V, J_sc=36.4 mA/cm², FF=0.756).     

Improved compositional flexibility of Cu(In,Ga)Se2-based thin film solar cells by sodium control technique

The effects of sodium on off-stoichiometric Cu(In,Ga)Se₂ (CIGS)-based thin films and solar cells were investigated. The CIGS-based films were deposited with intentionally incorporated Na₂Se on Mo-coated SiO_x/soda-lime glass substrates by a multi-step process. By sodium control technique high-efficiency ZnO : Al/CdS/CIGS solar cells with efficiencies of 10–13.5% range were obtained over an extremely wide Cu/(In + Ga) ratio range of 0.51–0.96, which has great merit for the large-area manufacturing process. The improved efficiency in the off-stoichiometric regions is mainly attributed to the increased acceptor concentration and the formation of the Cu(In,Ga)₃Se₅ phase films with p-type conductvity. A new type of solar cell with p-type Cu(In,Ga)₃Se₅ phase absorber materials is also suggested.     

Cu(In,Ga)Se2 solar cells with superstrate structure using lift-off process

Cu(In,Ga)Se₂ (CIGS) solar cells with a superstrate structure were fabricated using a lift-off process. To widen the variety of substrate choices for CIGS solar cells, a lift-off process was developed without an intentional sacrificial layer between the CIGS and Mo back-contact layers. The CIGS solar cells fabricated on Mo/soda-lime glass (SLG) were transferred to an alternative SLG substrate. The conversion efficiency of the CIGS solar cells with the superstrate structure was 5.1%, which was almost half that of the CIGS solar cells with a substrate structure prior to the lift-off process. The low conversion efficiency was caused by the high series resistance and low shunt resistance, which would be due to the junction resistance between the CIGS/back contact and cracks introduced during the lift-off process, respectively.     

Electrical properties of the Cu(In,Ga)Se2/ MoSe2/Mo structure

We investigated the electrical properties of the Cu(In,Ga)Se₂/MoSe₂/Mo structure. CIGS/Mo heterocontact including the MoSe₂ layer is not Schottky-type but a favorable ohmic-type contact by the evaluation of dark I–V measurement at low temperature. A characteristic peak at 870 nm is observed in differential quantum efficiency of a solar cell with a CIGS thickness of 0.5 μm. This peak is considered with relating to the absorption of the MoSe₂ layer. The band gap of MoSe₂ is calculated to be 1.41 eV from the absorption peak. The band diagram is discussed on the basis of the electrical point of view.     

Control of conduction band offset in wide-gap Cu(In,Ga)Se2 solar cells

The effects of conduction band offset of window/Cu(In,Ga)Se₂ (CIGS) layers in wide-gap CIGS based solar cells are investigated. In order to control the conduction band offset, a Zn_1−xMg_xO film was utilized as the window layer. We fabricated CIGS solar cells consisting of an ITO/Zn_1−xMg_xO/CdS/CIGS/Mo/glass structure with various CIGS band gaps (E_g≈0.97–1.43 eV). The solar cells with CIGS band gaps wider than 1.15 eV showed higher open circuit voltages and fill factors than those of conventional ZnO/CdS/CIGS solar cells. The improvement is attributed to the reduction of the CdS/CIGS interface recombination, and it is also supported by the theoretical analysis using device simulation.     

Fabrication of Cu(In,Ga)Se2 thin films solar cell by selenization process with Se vapor

CIGS films were prepared on Mo-coated glass by sputtering and selenization processes. The metallic precursors were selenized under higher pressure in selenium vapor instead of H₂Se. In order to improve the performance of CIGS thin film solar cells, the morphologies of CIGS thin films were studied carefully by various temperature profiles. The relationship between temperature decrease rate and fill factor (FF) of solar cells was investigated thoroughly. On the other hand the value of open circuit voltage (V_oc) was improved by increasing the gallium content near the surface of CIGS thin film. A glass/Mo/CIGS/CdS/ZnO cell was fabricated and the conversion efficiency of 9.4% was obtained without antireflective film.     

Cu(In,Ga)Se2 based photovoltaic structure by electrodeposition and processing

CuIn_1−xGa_xSe₂ (CIGS) thin films were formed from an electrodeposited CuInSe₂ (CIS) precursor by thermal processing in vacuum in which the film stoichiometry was adjusted by adding In, Ga and Se. The structure, composition, morphology and opto-electronic properties of the as-deposited and selenized CIS precursors were characterized by various techniques. A 9.8% CIGS based thin film solar cell was developed using the electrodeposited and processed film. The cell structure consisted of Mo/CIGS/CdS/ZnO/MgF₂. The cell parameters such as J_sc, V_oc, FF and η were determined from I–V characterization of the cell.     

Improved Jsc in CIGS thin film solar cells using a transparent conducting ZnO:B window layer

A comparative study of the cell performance of CIGS thin-film solar cells fabricated using ZnO:Al and ZnO:B window layers has been carried out. ZnO:B films were deposited by RF magnetron sputtering using an undoped ZnO target in a B₂H₆–Ar gas mixture. The short-circuit current (J_sc) was found to improve upon the replacement of the ZnO:Al layer with ZnO:B layers. This improvement in J_sc is attributed to an increase in quantum efficiency due to the higher optical transmission of the ZnO:B layer in the near-infrared region. The best cell fabricated with a MgF₂/ZnO:B/i-ZnO/CdS/CIGS/Mo structure yielded an active area efficiency of 18.0% with V_oc=0.645 V, J_sc=36.8 mA/cm², FF=0.76, and an active area of 0.2 cm² under AM 1.5 illumination.     

Investigation of pulsed non-melt laser annealing on the film properties and performance of Cu(In,Ga)Se2 solar cells

Pulsed non-melt laser annealing (NLA) has been used for the first time to modify near-surface defects and related junction properties in Cu(In,Ga)Se₂ (CIGS) solar cells. CIGS films deposited on Mo/glass substrates were annealed using a 25 ns pulsed 248 nm laser beam at selected laser energy density in the range 20–60 mJ/cm² and pulse number in the range 5–20 pulses. XRD peak narrowing and SEM surface feature size increase suggest near-surface structure changes. Dual-beam optical modulation (DBOM) and Hall-effect measurements indicate NLA treatment increases the effective carrier lifetime and mobility along with the sheet resistance. In addition, several annealed CdS/CIGS films processed by NLA were fabricated into solar cells and characterized by photo- and dark-J–V and quantum efficiency (QE) measurements. The results show significant improvement in the overall cell performance when compared to unannealed cells. The results suggest that an optimal NLA energy density and pulse number for a 25 ns pulse width are approximately 30 mJ/cm² and 5 pulses, respectively. The NLA results reveal that overall cell efficiency of a cell processed from an unannealed film increased from 7.69% to 13.41% and 12.22% after annealing 2 different samples at the best condition prior to device processing.     

A 4.2% efficient flexible dye-sensitized TiO2 solar cells using stainless steel substrate

An efficient flexible dye-sensitized solar cells (DSSCs) using stainless steel supporting substrate for fabricating nanocrystalline TiO₂ film electrodes were developed, intending to improve the photoelectrochemical properties of plastic substrate-based DSSCs. The most important advantage of a stainless steel-based TiO₂ film electrode over a plastic-based electrode lies in its high-temperature sinterability. Optimal photovoltaic properties were obtained with a cell where the TiO₂ film was coated on both ITO- and SiO_x-sputtered stainless steel (denoted as TiO₂/ITO/SiO_x/StSt). The photocurrent of the flexible cells with a TiO₂/ITO/SiO_x/StSt electrode increased significantly, leading to a much higher overall solar conversion efficiency η of 4.2% at 100 mW/cm², based on short-circuit photocurrent density, open-circuit voltage and fill factor of 11.2 mA/cm², 0.61 and 0.61 V, respectively, than those reported for cells with plastic substrates.     

Fabrication of wide-gap Cu(In1−xGax)Se2 thin film solar cells: a study on the correlation of cell performance with highly resistive i-ZnO layer thickness

The correlation of the cell performance of wide-gap Cu(In_1−xGa_x)Se₂ (CIGS) solar cells with the thickness of highly resistive i-ZnO layers, which are commonly introduced between the buffer layer and the transparent conductive oxide (TCO) layer in CIGS solar cell devices, was studied. It was found that cell parameters, in particular, the fill factor (F.F.) varied with the thickness of the i-ZnO layers and the variation of the F.F. was directly related to cell efficiency. A 16%-efficiency was achieved without use of an anti-reflection coating from wide-gap (E_g1.3 eV) CIGS solar cells by adjusting the deposition conditions of the i-ZnO layers.     

Large area CIGS2 thin film solar cells on foils: nucleus of a pilot plant

In earlier research, conversion efficiency of 10.4% (AM1.5) and 9.9% (AM0) has been achieved on small area CuIn_xGa_1−xS₂ (CIGS2) solar cell on 127 μm thick stainless steel substrate. The area of research is mainly focused on studying CIGS2 thin films as solar cell absorber material and growing high efficiency cells on ultralightweight and flexible metallic foils such as 127 μm thick stainless steel and SiO₂ coated 25 μm thick Ti foils. This paper presents the scaling up process of CIGS2 thin film substrate from 2.5 × 2.5 cm² to 10 × 10 cm². Initial scaling up efforts focused on achieving uniform thickness and stress-free films. Process of scaling up consisted of refurbishment of selenization/sulfurization furnace, design and fabrication of scrubber and enlargement of new CdS deposition setup. The scaling up from 2.5 × 2.5 cm² to 10 × 10 cm² substrate size has laid the foundation for PV Materials Lab of Florida Solar Energy Center becoming the nucleus of a pilot plant.     

High-efficiency Cu(In,Ga)Se2 thin-film solar cells with a novel In(OH)3:Zn buffer layer

We propose the inclusion of a novel In(OH)₃:Zn²⁺ buffer layer for fabricating high-efficiency CIGS solar cells. This buffer layer was deposited using a solution consisting of ZnCl₂, InCl₃·4H₂O, and thiourea. The In(OH)₃:Zn²⁺ films showed high resistivities of 2.1×10⁸ Ω cm and transmittance of above 95% in the visible range. We expected two effects due to this new buffer layer: first is the formation of a passivation layer on the CIGS surface and the second is Zn-doping into CIGS layer, resulting in the formation of a buried junction. A cell efficiency of 14.0% (V_oc: 0.575 V, J_sc: 32.1 mA/cm², FF: 0.758) was achieved by using an In(OH)₃:Zn²⁺ buffer layer, without the light soaking effect.     

Analysis of heterointerface recombination by Zn1−xMgxO for window layer of Cu(In,Ga)Se2 solar cells

An adjustment of a conduction band offset (CBO) of a window/absorber heterointerface is important for high efficiency Cu(In,Ga)Se₂ (CIGS) solar cells. In this study, the heterointerface recombination was characterized by the reduction of the thickness of a CdS layer and the adjustment of a CBO value by a Zn_1−xMg_xO (ZMO) layer. In ZnO/CdS/CIGS solar cells, open-circuit voltage (V_oc) and shunt resistance (R_sh) decreased with reducing the CdS thickness. In constant, significant reductions of V_oc and R_sh were not observed in ZMO/CdS/CIGS solar cells. With decreasing the CdS thickness, the CBO of (ZnO or ZMO)/CIGS become dominant for recombination. Also, the dominant mechanisms of recombination of the CIGS solar cells are discussed by the estimation of an activation energy obtained from temperature-dependent current-voltage measurements.     

Near-band-edge photoluminescnce in Cu(In,Ga)Se2 solar cells

Photoluminescence (PL) and PL decay characteristics of the near-band-edge (NBE) PL at room temperature have been studied on the Cu(In,Ga)Se₂ (CIGS) solar cells. The carrier recombination process has been discussed with emphasis on the photovoltaic properties of the solar cell. It has been found that: (i) PL intensity of the CIGS solar cells is much stronger than that in the corresponding CIGS thin films, (ii) the PL decay time of the cell is longer than that of the CIGS film, and (iii) the PL decay time of the CIGS solar cell exhibits strong dependence on the PL excitation intensity. In the CIGS solar cell, intense PL is obtained under the open circuit condition (oc), in contrast to the very low PL yield under the short circuit (sc) condition. The PL decay time under the sc condition is much shorter than that under the oc condition. Excitation intensity dependence of PL intensity and the PL decay time have been studied, and they are discussed with relation to the photo-voltage due to the PL excitation light. PL and injection EL under the external DC bias have been studied. The mapping image of NBE-PL intensity has been compared with that of the laser beam induced current (LBIC), and the PL intensity image reflects the photovoltaic properties of the CIGS solar cells. We demonstrated that NBE-PL of the CIGS solar cell reflects the photovoltaic effect, and it can be utilized as a powerful characterization method.     

Impact of Zn1-xMgxO:Al transparent electrode for buffer-less Cu(In,Ga)Se2 solar cells

Buffer layers such as CdS and ZnS are used in high efficiency Cu(In,Ga)Se₂ (CIGS) thin film solar cells. Eliminating buffer layer is attractive to realize low-cost thin film solar cells by reducing fabrication process. However, the elimination of the buffer layers leads to shunting due to the interface recombination between transparent conductive oxide (TCO) and CIGS layers. To reduce the interface recombination, the control of conduction band offset (CBO) is effective. In this study, we fabricated Zn_1−xMg_xO:Al (ZMO:Al) as the TCO for the CBO control. ZMO:Al was prepared by co-sputtering of ZnO:Al₂O₃ (ZnO:Al) and MgO:Al₂O₃ targets. ZMO:Al shows high transmittance in visible region and the band gap energy widen with the addition of Mg to ZnO:Al. Buffer-less CIGS solar cells with an Al/NiCr/TCO/CIGS/Mo/soda-lime glass structure using ZMO:Al and ZnO:Al were fabricated. For comparison, ZnO/CdS buffered cell was also fabricated. Current density-voltage characteristics of the devices showed the cell with ZMO:Al film achieved higher efficiency compared to the buffer-less cell with ZnO:Al. This result suggested that the control of CBO is important to reduce interface recombination between TCO layer and CIGS absorber.     

Investigation of rapid thermal annealing on Cu(In,Ga)Se2 films and solar cells

Rapid thermal annealing (RTA), with fast ramp rate, was performed on several Cu(In,Ga)Se₂ (CIGS) films and solar cells under various peak annealing temperatures and holding times. Characterizations were made on CIGS films and cells before and after RTA treatments to study effects of RTA on the CIGS film properties and cell performance. In addition, AMPS-1D device simulation program was used to study effects of defect density on the cell performance by fitting the experimental data of RTA-treated CIGS cells. The results show that RTA treatments under optimal annealing condition can provide significant improvements in the electrical properties of CIGS films and cell performance while preserving the film composition and microstructure morphology.