Analysis of laser scribes at CIGS thin-film solar cells by localized electrical and optical measurements

Laser patterning of thin-film solar cells is essential to perform external serial and integrated monolithic interconnections for module application and has recently received increasing attention. Current investigations show, however, that the efficiency of thin-film Cu(In,Ga)Se₂ (CIGS) modules is reduced due to laser scribing also with ultrashort laser pulses. Hence, to investigate the reasons of the laser-induced material modifications, thin-film CIGS solar cells were laser-scribed with femto- and picosecond laser pulses using different scribing procedures and laser processing parameters. Besides standard electrical current voltage (I–V) measurements, additional electrical and optical analysis were performed such as laser beam-induced current (LBIC), dark lock-in thermography (DLIT), and electroluminescence (EL) measurements to characterize and localize electrical losses due to material removal/modifications at the scribes that effecting the electrical solar cell properties. Both localized as well as distributed shunts were found at laser scribe edges whereas the laser spot intensity distribution affecting the shunt formation. Already laser irradiation below the ablation threshold of the TCO film causes material modification inside the thin film solar cell stack resulting in shunt formation as a result of materials melting near the TCO/CIGS interface that probably induces the damage of the pn-junction.     

Thickness study of Al:ZnO film for application as a window layer in Cu(In1−xGax)Se2 thin film solar cell

Structural, electrical and optical properties of Al doped ZnO (Al:ZnO) thin film of various thicknesses, grown by radio-frequency magnetron sputtering system were studied in relation to the application as a window layer in Cu(In_1−xGa_x)Se₂ (CIGS) thin film solar cell. It was found that the electrical and structural properties of Al:ZnO film improved with increasing its thickness, however, the optical properties degraded. The short circuit current density, J_sc of the fabricated CIGS based solar cells was significantly influenced by the variation of the Al:ZnO window layer thickness. Best efficiency was obtained when CIGS solar cell was fabricated with electrically and optically optimized Al:ZnO window layer.     

Junction configurations and their impacts on Cu(In,Ga)Se2 based solar cells performances

One dimension solar cells simulator package (SCAPS) is used to study the possibility of carrying out thin CIGS solar cells with high and stable efficiency. In the first step, we modified the conventional ZnO:B/i-ZnO/CdS/SDL/CIGS/Mo structure by substituting the SDL layer with the P?+?layer, having a wide bandgap from 1 to l.12?eV. Then, we simulated the J-V characteristics of this new structure and showed how the electrical parameters are affected. Conversion efficiency of 18.46% is founded by using 1.1?μm of P?+?layer thickness. Secondly, we analyze the effect of increase thickness and doping density of CIGS, CdS and P?+?layers on the electric parameters of this new structure. We show that only the short-circuit current density (J_SC) and efficiency are improved, reaching respectively 34.68?mA/cm² and 18.85%, with increasing of the acceptors density. Finally, we introduced 10?nm of various electron reflectors at the CIGS/Mo interface in the new structure to reduce the recombination of minority carriers at the back contact. High conversion efficiency of 23.34% and better stability are obtained when wide band-gap BSF is used.     

低温超薄高效Cu(In,Ga)Se2太阳电池的实现

衬底温度保持恒定, 在Se气氛下按照一定的元素配比顺序蒸发Ga, In, Cu制备厚度约为0.7 μrm的Cu(In_0.7Ga_0.3)Se₂ (CIGS)薄膜. 利用X射线衍射仪分析薄膜的晶体结构及物相组成, 扫描电子显微镜表征薄膜形貌及结晶质量, 二次离子质谱仪测试薄膜内部元素分布, 拉曼散射谱 分析薄膜表面构成, 带积分球附件的分光光度计测量薄膜光学性能. 研究发现在Ga-In-Se预制层内, In主要通过晶界扩散引起Ga/(Ga+In)分布均匀化. 衬底温度高于450 ℃时, 薄膜呈现单一的Cu(In_0.7Ga_0.3)Se₂相; 低于400℃, 薄膜存在严重的Ga的两相分离现象, 且高含Ga相主要存在于薄膜的上下表面; 低于300 ℃, 薄膜结晶质量进一步恶化. 薄膜表层的高含Ga相Cu(In_0.5Ga_0.5)Se₂以小晶粒形式均匀分布于薄膜表面, 增加了薄膜的粗糙度, 在电池内形成陷光结构, 提高了超薄电池对光的吸收. 加上带隙值较小的低含Ga相的存在, 使电池短路电流密度得到较大改善. 衬底温度在550 ℃–350 ℃变化时, 短路电流密度J_SC是影响超薄电池转换效率的主要因素; 而衬底温度T_sub低于300 ℃时, 开路电压V_OC和填充因子FF降低已成为电池性能减退的主要原因. T_sub为350 ℃时制备的0.7 μm左右的超薄CIGS电池转换效率达到了10.3%. 关键词： 2薄膜')" href="#">Cu(In,Ga)Se₂薄膜 衬底温度 超薄 太阳电池     

Effects of Cu2−xS phase removal on surface potential of Cu2ZnSnS4 thin-films grown by electroplating

Cu₂ZnSnS₄ (CZTS) has an optical band gap of 1.4–1.5 eV, which is similar to that of Cu(In,Ga)Se₂ (CIGS), and a high absorption coefficient (>10⁴ cm⁻¹) in the visible light region. In previous reports, CIGS thin-film solar cells have been shown to improve the performance of the device since the secondary phase is removed by Potassium cyanide (KCN) etching treatment. Therefore, in this study we applied a KCN etching treatment on CZTS and measured the effects. We confirmed the removal of Cu_2−xS via Kelvin probe force microscopy (KPFM) and Raman scattering spectroscopy. The effects of the experiment indicate that we can define with precision the location of the secondary phases, and therefore the control of the secondary phases will be easier and more efficient. Such capabilities could improve the solar cell performance of CZTS thin-films.     

Buffer-less Cu(In,Ga)Se2 solar cells by band offset control using novel transparent electrode

Cu(In,Ga)Se₂ (CIGS) solar cells without buffer layers have been demonstrated. Currently, CdS, Zn(O,S,OH), ZnS, or InS buffer layers are used in high efficiency CIGS solar cells to suppress interface recombination. One of the important parameters to reduce the recombination is the conduction band offset (CBO) between the buffer and CIGS layers. In this study, we have proposed the use of a novel transparent conductive oxide (TCO) which can control the CBO to reduce interface recombination and eliminate the buffer layers. The device simulation was used to verify the effect of CBO control theoretically. Then, the novel TCO material of ZnO_1?xS_x:Al prepared by co-sputtering of ZnO:Al₂O₃ and ZnS targets was fabricated to verify the CBO effect experimentally. The efficiency of a CIGS solar cell with a ZnO:Al/CIGS/Mo/soda-lime glass structure, i.e. buffer-less structure using a conventional TCO, was significantly low because of severe shunting. In contrast, the use of ZnO_1-xS_x:Al instead of ZnO:Al increased the shunt resistance of the CIGS solar cell, resulting in higher open-circuit voltage and efficiency. The result is the first proof of the concept of the buffer-less CIGS solar cells.     

Ga梯度分布对Cu(In,Ga)Se_2薄膜及太阳电池的影响

传统制备Cu(In,Ga)Se2(CIGS)手段之一是共蒸发三步法,工艺中通过Cu,In,Ga,Se 4种元素相互扩散、作用形成抛物线形的Ga梯度分布.本文通过调整Ga源温度制备了Ga梯度分布不同的CIGS薄膜及电池.利用多种测试方法,研究了Ga梯度分布不同对CIGS薄膜表面及背面结构性质及电性质的影响,计算分析了表面导带失调值及背面电场对电池性能的影响,从而获得了合适的Ga梯度分布,提高了电池光谱相应,获得了较好的电池性能参数.     

Rapid treatment of CIGS particles by intense pulsed light

Intense pulsed light (IPL) technique has been proposed to make large grains Cu(In_0.7Ga_0.3)Se₂ (CIGS) film using CIGS particles. The proposed process is non-vacuum based and performed at room temperature without selenization treatment. Melting and recrystallization of CIGS particles to larger grains without structural deformation and phase transformation are proved with adequate characterization evidences. X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion analysis (EDS) were used to characterize the prepared films. Melting of the CIGS particles and recrystallization to larger grains by light energy in 20 ms short reaction time could be the reason for no structural deformation and secondary phase generation during the process. The CIGS film prepared from its constituent nanoparticles by IPL treatment has great potential for use as absorber layer for solar cell application and is expected to have large impact on cell fabrication process in terms of cost reduction and simplified processing.     

CIGS solar cell devices on steel substrates coated with Na containing AlPO4

Flexible copper indium gallium diselenide (CIGS)-based solar cells are developed on stainless steel (STS) substrates covered with an insulating layer. The Na containing AlPO₄ (“Na-AlPO₄”) material is processed using the slot-die coating method. The coated film is analyzed using various spectroscopic methods including scanning electron microscopy, energy dispersive X-ray spectrometry, transmission electron microscopy, secondary-ion mass spectrometry, X-ray diffraction, and 3D profiler. The characteristics of the solar cells fabricated on these insulating films are also evaluated. The application of the Na-AlPO₄ layer on the STS substrates is compared with the electrical performance of the CIGS solar cells fabricated on metal foil. Although the insertion of the insulating layer does not influence the formation of the CIGS film and solar cell performance, a better uniformity in the current–voltage curve is obtained.     

Computational analysis of the effect of the surface defect layer (SDL) properties on Cu(In,Ga)Se2-based solar cell performances

In this article, the performances of Cu(In,Ga)Se₂ (CIGS) solar cells have been modelled and numerically simulated using the one-dimensional simulation program Solar Cell Capacitance Simulator in 1 Dimension (SCAPS-1D), and a detailed analysis of the effect of surface defect layer (SDL) thickness, band gap and carrier mobility with Fermi level pinning is presented. Furthermore, donor-type defect state density in the SDL has been investigated, and their effect on device performances has been presented. Based on the simulation results, optimal properties of the SDL for the CIGS solar cell are proposed. The simulated results show that the optimal thickness of the SDL to optimise the solar cells is in the range of 100–200 nm. The increase in the band gap of the SDL >1.3 eV improves the device performance by enhancing the open-circuit voltage (V_oc), fill factor (FF) and conversion efficiency due to the larger quasi-Fermi energy-level splitting, and optimal band offset between the SDL and the buffer layer (CdS). The simulation results suggest that the SDL defect density as well as carrier mobilities are the critical parameters for the limitation of the performances for the CIGS solar cells. All these results show that the SDL plays an important role in designing high-efficiency and high-performance CIGS-based solar cells.     

Cu元素对Cu(In,Ga)Se_2薄膜及太阳电池的影响

Cu元素成分对Cu(In,Ga)Se2(简称CIGS)薄膜材料的电学性质及其电池器件性能有很重要的影响.本文利用蒸发法制备了贫Cu和富Cu的CIGS吸收层(0.7Cu/(Ga+In)1.15)及相应的电池器件.扫描电镜和Hall测试发现,富Cu材料的结构特性(晶粒大、结晶状态好)和电学特性(电阻率低、迁移率高等)优于贫Cu材料,而性能测试表明贫Cu器件的效率优于富Cu器件.变温性能测试分析表明,贫Cu器件的主要复合路径是体复合,激活能与CIGS禁带宽度相当;富Cu器件的主要复合路径是界面复合,其激活能远小于CIGS禁带宽度,这大大降低了开路电压Voc,从而降低了电池效率.最后利用蒸发三步法制备了体材料稍富Cu表面贫Cu的CIGS吸收层,降低了短路电流和开路电压的损失,获得了超过15%的电池效率.     

Effects of heavy alkali elements in Cu(In,Ga)Se2 solar cells with efficiencies up to 22.6%

We report on the use and effect of the alkali elements rubidium and caesium in the place of sodium and potassium in the alkali post deposition treatment (PDT) as applied to Cu(In,Ga)Se₂ (CIGS) solar cell absorbers. In order to study the effects of the different alkali elements, we have produced a large number of CIGS solar cells with high efficiencies resulting in a good experimental resolution to detect even small differences in performance. We examine the electrical device parameters of these fully functional devices and observe a positive trend in the I –V parameters when moving from devices without PDT to KF‐, RbF‐, and eventually to CsF‐PDT. A diode analysis reveals an improved diode quality for cells treat‐ed with heavier alkalis. Furthermore, secondary ion mass spectrometry (SIMS) measurements reveal a competitive mechanism induced within the class of alkali elements in the CIGS absorber induced by the alkali post deposition treatment. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Effect of Co doping on CIS2 thin films

In recent years, substantial scientific attention has been focused on renewable energy resources, which utilize natural resources for the production of electrical energy. Chalcopyrite semiconductors are used as one of the alternatives, Cu(In,Ga)Se₂ (CIGS) and CuInS₂ (CIS) are used for the fabrication of solar cells. These materials possess various properties Viz. ideal band gap (1.5?eV), high optical absorption, low light degradation, high radiation resistance, etc., hence they are suitable in the fabrication of solar cells. In contrast to other chalcopyrates, CuInS₂ is nontoxic, low-cost and easy to prepare by simple deposition techniques. Several impurities were doped to CuInS₂ bulks, to control conduction and also to obtain low resistivity. In this context, the structural, morphological and optical properties are reported for cobalt-doped CuInS₂ (CIS₂) thin films prepared by electro-deposition technique at room temperature. In the present study, we have used different cobalt concentration in the range of 0–5?wt.%. Doping of cobalt does not lead to the formation of any secondary phase, either in the form of metallic clusters or impurity complexes. However, with increase in cobalt concentration a decrease in the optical band gap, from 2.10 to 1.53?eV, is observed. In addition, implantation of cobalt in the CIS₂ gave changes in structural and surface properties of the thin films obtained. These thin films are also subjected to elemental analysis using EDAX.     

Cu（In,Ga）Se2 Thin Films on Flexible Polyimide Sheet： Structural and Electrical Properties versus Composition

The structural and electrical properties of Cu（In,Ga）Se2 （CIGS） films grown on polyimide （PI） sheet using the three-stage co-evaporation process are investigated by x-ray diffraction spectra （XRD）, scanning electron microscopy （SEM）, Raman spectra, and Hall effect measurements, respectively. The results show that the properties of CIGS films on PI sheet are strongly dependent on the compositional ratio of Cu/（In＋Oa） （Cu/Ⅲ）. In contrast to the non-stoichiometric CIGS films, stoichiometric CIGS films show better structural and electrical properties, such as a relatively larger grain size, lower resistivity and higher carrier concentration. The flexible CIGS solar cells on PI sheet with the conversion efficiencies of 9.7% and 6.6% are demonstrated for the CIGS absorber layer with Cu/Ⅲ of 0.96 and 0.76, respectively （active area, 0.20cm^2）. The cell efficiency for Cu-poor CIGS films is limited by a relatively lower open circuit voltage and fill factor.     

Influence of shunt conduction on determining the dominant recombination processes in CIGS thin-film solar cells

We investigated the transport and photovoltaic properties of Cu(In_1-xGa_x)Se₂ (CIGS) thin-film solar cells. The shunt-current-eliminated diode current could be obtained from the current–voltage characteristics by subtracting the parasitic shunt leakage current from the total current. The temperature dependence of the open-circuit voltage, extracted from the shunt-eliminated (total) current, suggested that the recombination activation energy is comparable to (much less than) the CIGS bandgap. The low-temperature characteristics of the diode ideality factor supported bulk-dominated recombination in the same cell. This suggests that shunt-current subtraction can provide the proper diode parameters of CIGS solar cells.     

Extending absorption of near-infrared wavelength range for high efficiency CIGS solar cell via adjusting energy band

The efficient photon harvesting in near infrared wavelength range is still a challenging problem for high performance Cu(In_1-x, Ga_x)Se₂ (CIGS) solar cell. Herein, adjusting the energy band distribution of CIGS solar cell could provide significant academic guidance for devices with superior output electric power. To understand the role of each functional layer, the optimal 3000 nm CIGS absorber layer with 1.3 eV bandgap and 30 nm CdS buffer layer were firstly obtained via simulating the uniform band-gap structures. By introducing CIGS absorber layer with a double grading Ga/(Ga+In) profile, the power conversion efficiency of the double gradient band gap cell is superior to that of uniform band-gap cell through extending absorption of near-infrared wavelength range. Upon optimization, the best power conversion efficiency of CIGS with a double gradient band gap solar cell is improved significantly to 24.90%, among the best values reported in literatures, which is an 8.17% relative increase compared with that of the uniform band-gap cell. Our findings provide a theoretical guide toward the design of high performance solar cells and enrich the understandings of the energy band engineering for developing of novel semiconductor devices.     

Improving the performance of pure sulfide Cu(InGa)S2 solar cells via injection annealing system

In this study, we present an effective method of improving the performance of pure sulfide Cu(InGa)S₂ (CIGS) solar cells via injection annealing system. The injection annealing system can perform annealing at desired temperatures, and therefore, the CIGS thin film passed over the temperature range in which secondary phases occurs. Via the injection annealing system, secondary phase InS_x was effectively removed from the surface of the CIGS thin films at the temperatures over 550°C. This resulted in the formation of good-quality P–N junction CIGS devices, thereby improving significantly the performance of the CIGS solar cell. In addition, the open-circuit-voltage (V_OC) and fill factor (FF) of the CIGS devices increased gradually with increasing annealing temperature in the range of 550–640°C. It is speculated that the bulk defects were decreased as the annealing temperature increased. Finally, via injection annealing system, a pure sulfide CIGS solar cell with an efficiency of 12.16% was achieved.     

Performance improvement of CdS/Cu（In,Ga）Se2 solar cells after rapid thermal annealing

In this paper, we investigated the effect of rapid thermal annealing (RTA) on solar cell performance. An opto-electric conversion efficiency of 11.75% (Voc=0.64 V, Jsc = 25.88mA/cm2 , FF=72.08%) was obtained under AM 1.5G when the cell was annealed at 300℃ for 30s. The annealed solar cell showed an average absolute efficiency 1.5% higher than that of the as-deposited one. For the microstructure analysis and the physical phase confirmation, X-ray diffraction (XRD), Raman spectra, front surface reflection (FSR), internal quantum efficiency (IQE), and X-ray photoelectron spectroscopy (XPS) were respectively applied to distinguish the causes inducing the efficiency variation. All experimental results implied that the RTA eliminated recombination centers at the p-n junction, reduced the surface optical losses, enhanced the blue response of the CdS buffer layer, and improved the ohmic contact between Mo and Cu(In, Ga)Se2 (CIGS) layers. This leaded to the improved performance of CIGS solar cell.     

Inkjet printed Cu(In,Ga)S<Subscript>2</Subscript> nanoparticles for low-cost solar cells

Cu(In,Ga)Se₂ (CIGSe) thin film solar cells were fabricated by direct inkjet printing of Cu(In,Ga)S₂ (CIGS) nanoparticles followed by rapid thermal annealing under selenium vapor. Inkjet printing is a low-cost, low-waste, and flexible patterning method which can be used for deposition of solution-based or nanoparticle-based CIGS films with high throughput. XRD and Raman spectra indicate that no secondary phase is formed in the as-deposited CIGS film since quaternary chalcopyrite nanoparticles are used as the base solution for printing. Besides, CIGSe films with various Cu/(In + Ga) ratios could be obtained by finely tuning the composition of CIGS nanoparticles contained in the ink, which was found to strongly influence the devices performance and film morphology. To date, this is the first successful fabrication of a solar device by inkjet printing of CIGS nanoparticles.     

Cu(In,Ga)Se2 thin film solar cells from nanoparticle precursors

A non-vacuum process for Cu(In,Ga)Se₂ (CIGS) thin film solar cells from nanoparticle precursors was described in this work. CIGS nanoparticle precursors was prepared by a low temperature colloidal route by reacting the starting materials (CuI, InI₃, GaI₃ and Na₂Se) in organic solvents, by which fine CIGS nanoparticles of about 15 nm in diameter were obtained. The nanoparticle precursors were then deposited onto Mo/glass substrate by the doctor blade technique. After heat treating the CIGS/Mo/glass layers in Se gas atmosphere, a complete solar cell structure was fabricated by depositing the other layers including CdS buffer layer, ZnO window layer and Al electrodes by conventional methods. The resultant solar cell showed a conversion efficiency of 0.5%.