Microstructural properties of (In,Ga)2Se3 precursor layers for efficient CIGS thin-film solar cells

(In,Ga)₂Se₃ thin films were deposited on Mo-coated glass substrates by a conventional MBE system. To control the preferred orientation of Cu(In,Ga)Se₂ (CIGS) layers, the deposition temperature dependence T_depo of the (In,Ga)₂Se₃ layer was investigated including observations of both surface morphology and cross-sectional structure, Raman scattering and preferred orientation in the range 50–500 °C. γ-phase (In,Ga)₂Se₃ films exhibited (1 1 0) and (3 0 0) X-ray diffraction lines with a little or no (0 0 6) line contribution for T_depo>300 °C. It was revealed that a (3 0 0) preferred orientation of the (In,Ga)₂Se₃ layer could promote a (2 2 0/2 0 4) orientation of subsequently grown CIGS films, which were obtained only at the moderate temperatures of 300–400 °C during (In,Ga)₂Se₃ deposition.     

Cu(In,Ga)Se2 thin film solar cells with buffer layer alternative to CdS

Progress in fabricating Cu(In,Ga)Se₂ (CIGS) solar cells with ZnS(O,OH) buffer layers prepared by chemical bath deposition (CBD) is discussed in this paper. Such buffer layers could potentially replace CdS in the CIGS solar cell. Total-area conversion efficiency of up to 18.6% has been reported previously using ZnS(O,OH) prepared by CBD. The reported 100 nm CBD ZnS(O,OH) layer was prepared by at least three consecutive depositions, which would make it a relatively expensive replacement for CdS. The recent development of a ZnS(O,OH) layer that enabled to obtain high-efficiency devices using a single-layer CBD is reported in this paper. A 14.4%-efficient device is obtained by using one-layer CBD ZnS(O,OH) on commercial-grade Shell Solar Cu(In,Ga)(S,Se)₂ (CIGSS) absorber and an up to 17.4% device is obtained by using two-layer CBD ZnS(O,OH) on an NREL CIGS absorber.     

Cu(In,Ga)Se2 solar cells on stainless steel substrates covered with insulating layers

We have developed the flexible Cu(In,Ga)Se₂ (CIGS) solar cells on the stainless steel substrates with the insulating layer for the fabrication of the integrated module. The CIGS films have strong adhesion to the Mo films with insulating layers. An efficiency of 12.3% was achieved by the flexible CIGS solar cell with a structure of ITO/ZnO/CdS/CIGS/Mo/SiO₂/stainless steel. The insertion of the SiO₂ insulating layer did not have an influence on the formation of the CIGS film and solar cell performances.     

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.     

Effect of composition gradient in Cu(In,Al)Se2 solar cells

Cu(In,Al)Se₂ (CIAS) thin films were prepared by a three-stage evaporation process. In this experiment, the composition ratio of Cu/(In+Al) at the end of the second stage (Cu/III_2nd) was changed from 1.1 to 1.7. The CIAS films showed an Al distribution with a V-shape profile. The valley depth of the V-shape from the surface increased with increasing the Cu/III_2nd ratio. The valleys of the V-shape for the films with the Cu/III_2nd ratio of 1.1–1.7 were located at approximately 0.3–1.0 μm from the film surface, respectively. The rms surface roughness increased from 40 nm for Cu/III_2nd=1.1 to 90 nm at Cu/III_2nd=1.3 and then saturated for greater Cu/III_2nd ratios. Solar cells with the Al/ITO/ZnO/CdS/CIAS/Mo/soda-lime glass structure were fabricated. The fill factor was seen to decrease while the product of short-circuit current and open-circuit voltage remained constant. The reverse saturation current increased when the Cu/III_2nd ratio is greater than 1.3 which is a behavior of the surface roughness. Cu/III_2nd ratios greater than 1.3 lead to the distant position of V-shape from the surface and the increase in surface roughness.     

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.     

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.     

Analysis of Cu(In,Ga)(S,Se)2 thin-film solar cells by means of electron microscopy

The present work gives an overview of how electron microscopy and its related techniques are used to analyze individual layers and their interfaces in Cu(In,Ga)(S,Se)₂ thin-film solar cells. Imaging of samples can be performed at scales of down to the (sub)angstroms range. At similar spatial resolutions, information on composition can be gathered by means of energy-dispersive X-ray spectroscopy (EDX) and on spatial distributions of electrostatic Coulomb potentials in the specimen by applying electron holography. Microstructural and compositional properties as well as charge-carrier collection and radiative recombination behavior of the individual layers are accessible by use of electron backscatter diffraction, EDX, electron-beam-induced current (EBIC) and cathodoluminescence measurements, available in scanning electron microscopy. The present contribution gives an overview of the various scanning and transmission electron microscopy techniques applied on Cu(In,Ga)(S,Se)₂ thin-film solar cells, examples from case studies, and also demonstrates how these techniques may be combined in order to improve the analysis. Particularly, EBIC results show a reduced charge-carrier collection at Cu(In,Ga)Se₂ grain boundaries, while no indication was found for a charge accumulation at the grain boundaries by electron holography.     

效率为12.1%的Cu(In,Ga)Se2薄膜太阳电池

利用共蒸发的三步法制备了较高质量的四元化合物Cu(In，Ga)Se2(CIGS)薄膜，并采用Mo／CIGS／CdS／ZnO结构为基础做出转换效率超过10％的薄膜太阳电池，其最高转换效率达到12．1％(测试条件为：AM1．5，Global 1000W／m^2)。通过与国际最高水平的CIGS太阳电池各参数的比较，分析了我们所制备的CIGS太阳电池在工艺和物理方面存在的问题。     

Investigation of Al2O3 diffusion barrier layer fabricated by atomic layer deposition for flexible Cu(In,Ga)Se2 solar cells

The use of Al₂O₃ fabricated by atomic layer deposition (ALD) as a metal diffusion barrier between the stainless steel substrate and the back contact layer in flexible Cu(In,Ga)Se₂ (CIGS) photovoltaic (PV) devices was found to reduce metal ion diffusion from the substrate and reduce the number of defects at the CIGS absorber layer, as determined from the secondary ion mass spectrometry (SIMS) depth profile and quantitative defect analysis using C–V measurements. Cells with Al₂O₃ barrier layers were found to show higher efficiency and uniformity compared to cells with ZnO barrier layers. XRD pattern analysis showed the Al₂O₃ barrier layer's amorphous characteristic which can form a complex diffusion path. In addition, quantum efficiency (QE) analysis of the cells showed that the main advantage of using an Al₂O₃ barrier layer is derived from the increase in the current density due to the decrease in the number of recombination sites resulting from the decrease in the number of defects due to the amorphous nature of the layer. Therefore, cells with an Al₂O₃ barrier layer fabricated by ALD showed better average conversion efficiency and uniformity (11.23 ± 1.86%) compared to cells with a ZnO barrier layer fabricated by sputtering. Ongoing advancements in ALD processes make the use of Al₂O₃ barrier layers promising for obtaining large-scale flexible solar cells.     

Reversible changes of the fill factor in the ZnO/CdS/Cu(In,Ga)Se2 solar cells

The reversible persistent changes of the fill factor (FF) induced by the illumination and voltage bias along with changes in the electronic properties of the ZnO/CdS/Cu(In,Ga)Se₂ photovoltaic devices have been studied. Admittance spectroscopy and capacitance–voltage characterization reveal a correlation between the FF and the space charge distribution within the absorber. Our experiments provide evidence that a major source of FF loss in efficient devices is caused by excess negative charge close to the interface. We explain the persistent changes in the net acceptor concentration in the interface region by the relaxation effects due to compensating donors—the same mechanism, which leads to metastable changes of the doping level in the bulk of the absorber.     

Quantum efficiency and admittance spectroscopy on Cu(In,Ga)Se2 solar cells

We investigate the electronic transport properties of Cu(In,Ga)Se₂ solar cells by means of quantum efficiency and temperature dependent admittance spectroscopy. A simple evaluation scheme of quantum efficiency data is introduced which accounts for recombinatoric losses in the Us buffer layer and in the Cu(In,Ga)Se₂ absorber. By admittance spectroscopy, we find that the controlled incorporation of Na into the absorber material leads to a shallow acceptor state at about 75 meV above the valence band.     

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.     

Temperature-dependent quantum efficiency analysis of graded-gap Cu(In,Ga)Se2 solar cells

Despite the high solar cell efficiencies achieved with Cu(In,Ga)Se₂ (CIGS) absorbers, key parameters such as the carrier diffusion length and recombination lifetime are still under investigation. Here, we extract lifetime and diffusion length from temperature-dependent internal quantum efficiency (IQET) spectra of state of the art high efficiency CIGS solar cells. Two-parameter fits to the measured IQE curves using a model for double-graded gap solar cells show very good agreement in the studied temperature range T=146–293 K, allowing the extraction of the electron recombination lifetime in the absorber and the collection probability in the front region of the cell. The obtained results agree with current literature values obtained by other characterization techniques. Furthermore, the temperature dependence of the recombination lifetime is explained by Shockley–Read–Hall recombination through a single bulk defect level with an activation energy of 200 meV.     

Depth profiling of Cu(In,Ga)Se2 thin films grown at low temperatures

In order to understand the effect of the process temperature on the growth of Cu(In,Ga)Se₂ (CIGSe) thin films via the 3-stage co-evaporation process, absorber layers have been fabricated on glass using a set of different maximum process temperatures in the nominal temperature range between 330 and 525 °C. Using energy dispersive X-ray analysis, depth profiles could be recorded on cross-sections of finished devices and were correlated to the device performance. The effect of the process temperature on the gallium gradient in the CIGSe layer is evident in the gallium distribution across the depth of the device.     

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 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.     

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.     

Cu(In,Ga)Se2 thin-film solar cells with an efficiency of 18%

An efficiency of over 18% have been achieved in Cu(In,Ga)Se₂ (CIGS) thin-film solar cells. Solar cell parameters were estimated for the cells with efficiencies of more and less than 18%. A diode quality factor n and forward current (saturated current) J₀ of the cell with over 18% efficiency are lower than those with below 18% efficiency. This would be attributed to sufficient coverage of the CdS film with excellent uniformity as a buffer and/or window layer over the CIGS film because the process of CdS film formation was improved.     

Improved performance of Cu(InGa)Se2 thin-film solar cells using evaporated Cd-free buffer layers

Polycrystalline Cu(InGa)Se₂ (CIGS) thin-film solar cells using evaporated In_xSe_y and ZnIn_xSe_y buffer layers are prepared. The purpose of this work is to replace the chemical bath deposited CdS buffer layer with a continuously evaporated buffer layer. In this study, a major effort is made to improve the performance of CIGS thin-film solar cells with these buffer layers. The relationship between the cell performance and the substrate temperature for these buffer layers is demonstrated. Even at the high substrate temperature of about 550°C for the buffer layer, efficiencies of more than 11% were obtained. Furthermore, the I−V characteristics of the cells using these buffer layers are compared with cells using CdS buffer layers fabricated by chemical bath deposition method. We have achieved relatively high efficiencies of over 15% using both the ZnIn_xSe_y and the CdS buffer layers.