Surface modification of CIGS film by annealing and its effect on the band structure and photovoltaic properties of CIGS solar cells

The composition of Cu(In,Ga)Se₂ (CIGS) films employed in CIGS solar cells is Cu deficient. There can be point defects, including Cu vacancies, Se vacancies, and metal anti-site defects. The surface composition and defects are not well controlled right after CIGS film fabrication with a three-stage co-evaporation process. This fabrication technique can result in a large variation in cell efficiency. In order to control the CIGS film in a reproducible way, we annealed the CIGS film in air, S, or Se. With this annealing procedure, the Cu content of the CIGS surface was significantly reduced and Ga content was strongly increased. An intrinsic CIGS layer with a lower valence-band maximum and a wider ban gap was formed at the surface. By annealing the CIGS film, the open-circuit voltage and fill factor were significantly improved, which indicates that the surface intrinsic layer acts as a hole-blocking layer so that the surface recombination rate is suppressed. In addition to CIGS film annealing, with subsequent annealing of the completed devices using rapid thermal annealing, the efficiency and reproducibility of CIGS solar cells were markedly improved.     

Cu(In,Ga)Se2 薄膜在共蒸发"三步法"中的相变过程

CIGS薄膜的结晶相是制备高质量薄膜的关键问题. 本文采用共蒸发"三步法"工艺沉积Gu(In, Ga)Se₂ (CIGS) 薄膜, 通过X射线衍射仪 (XRD) 和X射线荧光光谱仪 (XRF)、扫描电镜 (SEM) 结合的方法详细研究了"三步法"工艺的相变过程, 并制备出转换效率超过15% 的 CIGS 薄膜太阳电池. 关键词： CIGS薄膜 共蒸发三步法 相变过程     

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.     

A new method of generating Ga slope in Cu(In,Ga)Se2 film by controlling Se content in a multi-stacked precursor

An appropriate Ga slope is required in Cu(In,Ga)Se₂ (CIGS) film to enhance the cell performance of CIGS thin-film solar cells. In the conventional three-stage-co-evaporation process, the Ga slope was obtained by controlling Ga/In flux during deposition process. However, in two-step process, where a precursor was deposited first and then annealed in a Se environment for mass production, the desirable Ga slope was not achievable with the Ga/In flux control. We observed that the Ga/(Ga + In) ratio was nearly flat in CIGS film for Se-rich precursor and the ratio was nearly zero at surface and very high on bottom side of CISG film for Se-deficient precursor. We were able to generate a CIGS film with a Ga non-zero Ga surface and desired slope in the bulk by devising a precursor with Se-rich layer on top and Se-deficient layer on bottom, resulting in the enhancement of Cell performance.     

Flexible Cu（In,Ga）Se2 Thin-Film Solar Cells on Polyimide Substrate by Low-Temperature Deposition Process

The electrical and structural properties of polycrystalline Cu（In, Ga）Se2 films grown on polyimide （PI） substrates below 400℃ via one-stage and three-stage co-evaporation process have been investigated by x-ray diffraction spectra （XRD）, scanning electron microscopy （SEM） and Hall effect measurement. As shown by XRD spectra, the stoichiometric CIGS films obtained by one-stage process exhibit the characteristic diffraction peaks of the （In0.68Ga0.32）2Se3 and Cu（In0.7Ga0.3）2Se. It is also found that the film structures indicate more columnar and compact than the three-stage process films from SEM images. The stoichiometric CIGS films obtained by three-stage process exhibit the coexistence of the secondary phase of （In0.68Ga0.32）2Se3, Cu2-xSe and Cu（In0.7Ga0.3）2Se. High net carrier concentration and sheet conductivity are also observed for this kind of film, related to the presence of Cu2-xSe phase. As a result, when the CIGS film growth temperature is below 400℃, the three-stage process is inefficient for solar cells. By using the one-stage co-evaporation process, the flexible CIGS solar cell on a PI substrate with the best conversion efficiency of 6.38% is demonstrated （active area 0.16cm^2）.     

Growth mechanism of thermally processed Cu(In,Ga)S2 precursors for printed Cu(In,Ga)(S,Se)2 solar cells

We investigate a process used for the selenisation of particle‐based precursors to prepare low‐cost Cu(In,Ga)(S,Se)₂ (CIGS) solar cells. It is suitable for high throughput with a short optimum selenisation duration of 3–5 min and employs a rapid thermal annealing system with elemental selenium vapour. Homogeneous crack‐free Cu(In,Ga)S₂ precursor films of up to 1 µm are obtained via doctor blading. The high selenium vapour pressure in the selenisation reaction chamber results in the formation of a compact Cu(In,Ga)(S,Se)₂ layer on top of a carbon‐rich underlayer. In order to investigate the phase development in the film, the selenisation process was interrupted at different stages and the samples were monitored via XRD and surface‐sensitive Raman measurements. We find the formation of a polycrystalline Cu(In,Ga)Se₂ phase already after 1 s at the target temperature of 550 °C. Furthermore, the effect of initial precursor thickness on solar cell parameters is discussed. Complete solar cells are prepared by conventional methods, leading to conversion efficiencies well above 8%. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation and characterization of Cu(In,Ga)(Se,S)2 films without selenization by co-sputtering from Cu(In,Ga)Se2 quaternary and In2S3 targets

In this study, Cu(In,Ga)(Se,S)₂ (CIGSS) thin films were deposited onto a bi-layer Mo coated soda-lime glass by co-sputtering a chalcopyrite Cu(In,Ga)Se₂ (CIGS) quaternary alloy target and an In₂S₃ binary target. A one-stage annealing process was performed to form CIGSS chalcopyrite phase without post-selenization. Experimental results show that CIGSS films were prepared by the proposed co-sputter process via CIGS (70 W by radio frequency) and In₂S₃ (30 W by direct current) with a substrate temperature of 373 K, working pressure of 0.67 Pa, and one-stage annealing at 798 K for 30 min. The stoichiometry ratios of the CIGSS film were Cu/(In + Ga) = 0.92, Ga/(In + Ga) = 0.26, and Se/(S) = 0.49 that approached device-quality stoichiometry ratio (Cu/(In + Ga) < 0.95, Ga/(In + Ga) < 0.3, and (Se/S) ≈ 0.5). The resistivity of the sample was 14.8 Ω cm, with a carrier concentration of 3.4 × 10¹⁷ cm⁻³ and mobility of 1.2 cm² V⁻¹ s⁻¹. The resulting film exhibited p-type conductivity with a double graded band-gap structure.     

Post-annealing effect on the reactively sputter-grown CIGS thin films and its influence to solar cell performance

Using a reactive co-sputtering from Cu_0.6Ga_0.4 and Cu_0.4In_0.6 alloy targets, we prepared CuIn_1−xGa_xSe₂ (CIGS) thin films on Mo/soda-lime glass (SLG) in association with a thermal cracker for elemental atomic Se radicals. The film growth was performed at 500 °C for 90 min. To achieve the composition ratio of CIGS absorber layer, Cu_0.6Ga_0.4 target was set at RF power of 50 W, 60 W, 70 W, and 80 W while keeping at 100 W for Cu_0.4In_0.6 alloy target. Post-annealing was done for all the CIGS films at 550 °C for 30 min. The composition ratio of [Cu]/[In + Ga] and [Ga]/[In + Ga] was increased with RF power but showed no change after post-annealing. X-ray diffraction analysis revealed all the samples has grown dominantly in the [112] crystal orientation. We found the Cu_2−xSe and (InGa)_2−xSe₃ defect phase both at the surface and in the bulk, and developed with post-annealing. From the devices fabricated in the structure of grid/ITO/i-ZnO/CdS/CIGS/Mo/soda-lime glass (SLG), the external quantum efficiency (EQE) was observed to improve in the wavelength, λ ≥ 550 nm in the samples treated with annealing. In the current–voltage (J–V) measurements, the solar cell showed the best performance of FF = 54.1%, V_oc = 0.48 V, J_sc = 33.1 mA/cm² and η = 8.5% in the sample with [Cu]/[In + Ga] = 0.84 that improved largely from η = 4.6% for the solar cell with an as-grown CIGS films.     

A non-selenization technology by co-sputtering deposition for solar cell applications

This work presents a novel method to form polycrystalline Cu(In(1-x)Ga(x))Se(2) (CIGS) thin film by co-sputtering of In─Se and Cu─Ga alloy targets without an additional selenization process. An attempt was also made to thoroughly elucidate the surface morphology, crystalline phases, physical properties, and chemical properties of the CIGS films by using material analysis methods. Experimental results indicate that CIGS thin films featured densely packed grains and chalcopyrite phase peaks of (112), (220), (204), (312), and (116). Raman spectroscopy analysis revealed chalcopyrite CIGS phase with Raman shift at 175 cm(-1), while no signal at 258 cm(-1) indicated the exclusion of Cu(2-x)Se phase. Hall effect measurements confirmed the polycrystalline Cu(In,Ga)Se₂ thin film to be of p type semiconductor with a film resistivity and mobility of 2.19×10(2) Ω cm and 88 cm(2)/V s, respectively.     

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%.     

硒化前后电沉积贫铜和富铜的Cu(In1-xGax)Se2薄膜成分及结构的比较

采用电沉积法获得了接近化学计量比的贫铜和富铜的Cu（In_1-xGa_x）Se₂（CIGS）预置层,研究比较了两种预置层及其硒化处理后的成分和结构特性.得到了明确的实验证据证明,硒化后富铜薄膜中的Cu_xSe相会聚集凝结成结晶颗粒分散在表面.研究表明：在固态源硒化处理后,薄膜成分基本不变;当预置层中原子比Cu/（In+Ga）<11时,硒化后薄膜表面存在大量的裂纹;而当Cu/（In+Ga） >12时,可以消除裂纹的产生,形成等轴状小晶粒;富铜预置层硒化时蒸发沉积少量In,Ga和Se后,电池效率已达到68%;而贫铜预置层硒化后直接制备的电池效率大于2%,值得进一步深入研究. 关键词： 1-xGa_x）Se₂薄膜')" href="#">Cu（In_1-xGa_x）Se₂薄膜 电沉积 硒化处理 贫铜或富铜薄膜     

Structural and optical properties of CdS/Cu(In,Ga)Se<Subscript>2</Subscript> heterostructures irradiated by high-energy electrons*

Thin films of Cu(In, Ga)Se₂ (CIGS) with a Ga/(Ga + In) ratio of ~0.27 corresponding to the standard elemental composition for solar-energy transducers were grown on Mo-coated glass substrates by the Cu, In, Ga, and Se co-evaporation technique from different sources. Transmission (T), photoluminescence (PL), and photoluminescence excitation (PLE) spectra at 4.2 K were used to analyze electronic properties in the asgrown and electron-irradiated CIGS films. The band-gap energy (E_g) of the CIGS films measured using both transmission and PLE methods was found to be about 1.28 eV at 4.2 K. Two deep bands in the PL spectra of the irradiated CIGS films, P₁ at ~0.91 eV and P₂ at ~0.77 eV, have been detected. These bands are tentatively associated with copper atoms substituting indium (Cu_In) and indium vacancies V_In, respectively, as the simplest radiation-induced defects.     

Effect of post annealing on the characteristics of In2S3 buffer layer grown by chemical bath deposition on a CIGS substrate

In₂S₃ as an alternative Cd-free buffer in Cu(In,Ga)Se₂ (CIGS) solar cells was deposited on CIGS substrate by a chemical bath deposition and characterized after post annealing to optimize film properties for CIGS solar cells. A uniform and pinhole-free In₂S₃ film was deposited on a CIGS substrate by H₂O₂ treatment prior to chemical bath deposition. The In₂S₃ layer was an amorphous state due to the co-existence of In–S, In–O, and In–OH bonds. Annealing at 200 °C induced copper diffusion from CIGS into In₂S₃ layer and lowered the band gap from 3.3 to 1.9 eV, leading to phase change from amorphous state to crystalline state. The conduction band alignment at the In₂S₃/CIGS interface can be controlled by the post annealing. The shunt current through In₂S₃ film was prevented down to the thickness of 30 nm and a 1.15 eV shallow defect was eliminated by the annealing. The results indicated that post annealing in air is a critical to fabricate CIGS solar cells with a sub-30 nm CBD-In₂S₃ buffer layer.     

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.     

Structural analysis of Cu(In,Ga)Se2 films fabricated by using sputtering and post-selenization

We made Cu(In,Ga)Se₂ (CIGS) films by using sputtering and post-selenization. First, we deposited stacked metallic films by using the sputtering method and then carried out post-selenization for the precursor stacked metals with different amounts of Se powder, 0.160 g, 0.321 g, 0.642 g, and 0.964 g. We found that with a small amount of Se, separated CuInSe₂ and CuGaSe₂ layers were formed, which was confirmed by X-ray diffraction (XRD), Raman spectroscopy, and energy-dispersive X-ray analysis. For larger Se amounts, the CIGS phase was observed in the results of XRD and Raman spectroscopy. These results indicate that the amount or the partial pressure of Se plays an important role in the reaction kinetics for stacked precursor metals to form the CIGS phase.     

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     

Band-gap grading in Cu(In,Ga)Se2 solar cells

The quaternary system Cu(In,Ga)Se₂ (CIGS) allows the band gap of the semiconductor to be adjusted over a range of 1.04-1.67 eV. Using a non-uniform Ga/In ratio throughout the film thickness, additional fields can be built into p-type CIGS-based solar cells, and some researchers have asserted that these fields can enhance performance. The experimental evidence that grading improves device performance, however, has not been compelling, mostly because the addition of Ga itself improves device performance and hence a consistent separation of the grading benefit has not always been achieved. Numerical modeling tools are used in this contribution to show that (1) there can be a beneficial effect of grading, (2) in standard thickness CIGS cells the benefit is smaller than commonly believed, (3) there is also the strong possibility of reduced rather than of increased device performance, and (4) thin-absorber cells derive more substantial benefit.     

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.     

衬底对Cu(In,Ga)Se2薄膜织构的影响

分别在苏打石灰玻璃、Mo箔、无择优取向的Mo薄膜以及(110)择优取向的Mo薄膜四种不同衬底上，采用共蒸发工艺沉积约2 μm厚的Cu(In,Ga)Se₂薄膜，用X射线衍射仪测量薄膜的织构，研究衬底对Cu(In,Ga)Se₂薄膜织构的影响.在以上四种衬底上沉积的Cu(In,Ga)Se₂薄膜的(112)衍射峰强度依次逐渐减弱，(220/204)衍射峰从无到有且强度逐渐增强.在苏打石灰玻璃和Mo箔衬底上的Cu(In,Ga)Se_{2关键词： 择优取向 Cu(In 2}薄膜')" href="#">Ga)Se₂薄膜 太阳电池     

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.