Enhancement of photo‐conversion efficiency in Cu2ZnSn(S,Se)4 thin‐film solar cells by control of ZnS precursor‐layer thickness

CZTSSe thin‐film absorbers were grown by stacked ZnS/SnS/Cu sputtering with compound targets, and the precursors were annealed in a furnace with a Se atmosphere. We controlled the thickness of the ZnS precursor layer for the CZTSSe thin films in order to reduce the secondary phases and to improve the performance of the devices. The optimal value of the ZnS precursor thickness was determined for the CZTSSe absorbers, and this configuration showed an efficiency of up to 9.1%. In this study, we investigated the depth profiles of the samples in order to determine the presence of secondary phases in the CZTSSe thin films by Raman spectroscopy and Kelvin probe force microscopy. Cu₂SnSe₃, ZnSe, and MoSe₂ secondary phases appeared near the back contact, and the work function distribution of the CZTSSe thin‐film surface and the secondary phase distribution were different depending on the depths of the absorber layer. This phase characterization allows us to describe the effects that changes in the thickness of the ZnS precursor can have on the performance of the CZTSSe thin‐film solar cells. Although it is important to identify the phases, the effects of secondary phases and point defects are not yet fully understood, even in optimal devices. Therefore, phase identification that is based on the work function and the results obtained from the Raman spectra in terms of the depth profile are instrumental to improve the surface and interface of CZTSSe thin‐film solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

Effects of different precursors on Cu2ZnSnS4 thin film solar cells prepared by sputtering method

In this study, the impacts of different precursors on Cu₂ZnSnS₄ thin film solar cells were investigated. The two kinds of precursors of (Cu+Sn)/Zn and (Cu+Sn)/ZnS were deposited on Mo-coated soda lime glasses by magnetron sputtering. Cu₂ZnSnS₄ (CZTS) films based on different precursors were fabricated by soft annealing and following two-step sulfurization in sulphur vapour. The crystal structure, phase purity, surface morphology, composition and optical properties of CZTS films from different precursors were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM), energy dispersive spectrometry (EDS) and UV–vis–NIR spectroscopy, respectively. As a result, the CZTS thin films with smooth surface and uniform compositional ratio distribution were obtained from the precursors of (Cu+Sn)/ZnS. The best conversion efficiency of the fabricated CZTS film solar cell based on (Cu+Sn)/ZnS precursors was 3.36%.     

Effects of Cu incorporation on physical properties of ZnTe thin films deposited by thermal evaporation

The present paper reports on a systematic study of the Cu doping effect on the optical, electrical and structural properties of ZnTe:Cu (Cu=0, 6, 8, and 10 at%) thin films. Polycrystalline Cu-doped ZnTe thin films were deposited on glass substrates at room temperature by thermal evaporation. A detailed characterization of the Cu-doped ZnTe films were performed by X-ray diffraction (XRD), Spectrophotometry, Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. XRD of the as-deposited Cu-doped ZnTe films belong to single-phase cubic structure of ZnTe with preferential orientation along (111) planes revealed minor effect of Cu content. The interference pattern in optical transmission spectra was analyzed to determine energy band gap, refractive index, extinction coefficient and thickness of the films. Wemple–DiDomenico and Tauc's relation were used for the determination and comparison of optical band gap values. The formation of ZnTe and Cu-doped ZnTe phase was confirmed by FT-IR. AC conductivity in a frequency range of 0–7 MHz has been studied for investigation of the carriers hoping dynamics in the films. Raman spectra indicated merely typical longitudinal optical (LO) phonon mode of the cubic structure ZnTe thin film at 194 cm⁻¹ because the excitation energy is well above of the optical band-gap of the material and exhibited a blue-shift from 194 to 203 cm⁻¹ with Cu which could be associated to the substitution of Zn atom with Cu at the lattice sites.     

基于纳米颗粒的铜锌锡硫硒薄膜制备

采用热注入法制备了Cu2ZnSnS4(CZTS)纳米颗粒,并形成高分散、稳定的"墨水",采用滴注方法形成CZTS前驱体薄膜。利用X射线衍射(XRD)、拉曼光谱(Raman)、透射电子显微镜(TEM)和紫外-可见光谱(UV-VIS)对CZTS纳米颗粒的晶体结构、表面形貌和带隙进行了表征。Raman数据显示合成的纳米颗粒为纯的CZTS,不存在ZnS和Cu2SnS3等杂相。傅里叶红外光谱(FTIR)和UV-VIS表明合成的CZTS纳米颗粒表面被油胺(OLA)包覆,并且其带隙为1.52 eV。对CZTS前驱体薄膜在硫化氢气氛和固态硒气氛中退火处理,得到铜锌锡硫硒(CZTSSe)薄膜。结果表明,经硫化氢处理后薄膜表面平整但CZTS晶粒并没长大,而经过固态硒处理后得到了结晶质量较好的CZTSSe薄膜。     

铜含量变化对Cu(In,Ga)Se2薄膜微结构的影响

报道了不同的铜含量(Cu/(Ga+In)=0.748~0.982)对Cu(In,Ga)Se2 (CIGS)薄膜微结构的影响.文章中的CIGS薄膜采用磁控溅射金属预置层后硒化的方法制备, 其X射线衍射谱(XRD)中一系列黄铜矿结构CIGS(CH-CIGS)相的衍射峰确认了CH-CIGS相的存在.对CIGS薄膜拉曼光谱的分析表明, 随着铜含量的上升, CIGS薄膜经历了CH-CIGS和有序缺陷化合物(OVC)混合相、CH-CIGS单相、CH-CIGS和CuxSe混合相三种状态.进一步的分析显示, CIGS薄膜拉曼峰的半高宽随铜含量变化, 并在Cu/(Ga+In)=0.9附近时达到最小值, 这说明此时CIGS薄膜具有更好的结晶度和更少的无序性.此外还得到了CIGS薄膜拉曼峰半高宽与铜含量的经验关系公式.这些研究表明拉曼光谱能比XRD更加灵敏地探测CIGS薄膜的微结构, 可望作为一种无损和快速测量方法, 用于对CIGS薄膜晶相和铜含量的初步估计.     

Effect of preheating and annealing temperature on the microstructure and optoelectronic properties of CZTS films prepared by sol-gel method

The effects of different preheating and annealing temperatures on the surface morphology, microstructure, and optical properties of Cu2ZnSnS4 (CZTS) thin films are investigated by controlling the preheating and annealing temperatures. The prepared thin films were characterized using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and ultra-violet-visible (UV-Vis) spectroscopy techniques. XRD and Raman spectroscopy showed that a Kesterite structure with a selective orientation along the (112) peak was generated, and the thin films produced at a preheating temperature of 300 °C and annealing temperature of 570 °C had fewer secondary phases, which was beneficial for improving the performance of the solar cells. SEM confirms that the crystallite size increases and then decreases as the temperature increases, and the largest and most uniform crystallite size with the smoothest surface is generated at the above preheating and annealing temperatures. UV-Vis measurements show that the thin films generated at the above temperature have the lowest transmittance and the lowest optical band gap value of 1.46 eV, which is close to the optimal band gap value for solar cells and is suitable as an absorber layer material.     

Effects of K ions doping on the structure, morphology and optical properties of Cu2FeSnS4 thin films prepared by blade-coating process

Quaternary chalcogenide Cu2FeSnS4 (CFTS) nanoparticles, as a kind of potential absorber layer material in thin film solar cells (TFSCs), were successfully synthesized by using a convenient solvothermal method. Alkali element K is incorporated into CFTS thin films in order to further improve the surface morphology and the optical properties of related films. X-ray diffraction (XRD), Raman spectroscopy and field emission scanning electron microscopy (FESEM) were used to characterize the phase purity, morphology and composition of CFTS particles and thin films. The results show that the particle elemental ratios of Cu/(Fe+Sn) and Fe/Sn are 1.2 and 0.9, respectively, which are close to the characteristics of stoichiometric CFTS. The band gaps of CFTS films before and after doping K ions are estimated to be 1.44 eV and 1.4 eV with an error of ±0.02 eV. This work has been supported by National Natural Science Foundation of China (No. 51674026), and the Fundamental Research Funds for the Central Universities in 2015 (No.FRF-BD-15-004A). E-mail:chywang@yeah.net      

Fabrication and Characterization of CZTS Thin Films Prepared by the Sulfurization of RF-Sputtered Stacked Metal Precursors

In this work, Cu₂ZnSnS₄ (CZTS) thin films were prepared by the sulfurization of metal precursors deposited sequentially via radio frequency magnetron sputtering on Mo-coated soda-lime glass. The stack order of the precursors was Mo/Zn/Sn/Cu. Sputtered precursors were annealed in sulfur atmosphere with nine different conditions to study the impact of sulfurization time and substrate temperature on the structural, morphological, and optical properties of the final CZTS films. X-ray fluorescence was used to determine the elemental composition ratio of the metal precursors. Final CZTS films were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). XRD and EDS were combined to investigate the films’ structure and to identify the presence of secondary phases. XRD analysis indicated an improvement in film crystallinity with an increase of the substrate temperature and annealing times. Also indicated was the minimization and/or elimination of secondary phases when the films experienced longer annealing time. EDS revealed slight Sn loss in films sulfurized at 550°C; however, an increase of the sulfurization temperature to 600°C did not confirm these results. SEM study showed that films treated with higher temperatures exhibited dense morphology, indicating the completion of the sulfurization process. The estimated absorption coefficient was on the order of 10⁴ cm^?1 for all CZTS films, and the values obtained for the optical bandgap energy of the films were between 1.33 eV and 1.52 eV.     

Voc deficit in kesterite solar cells

As containing earth-abundant elements,kesterite (Cu2Zn-Sn(S,Se)4,CZTSSe) semiconductors have great potential to be low-cost and environmental-friendly inorganic absorbers.However,the record power conversion efficiency (PCE) for CZTSSe solar cells is only 12.6％[1,2],much lower than that for Cu(In,Ga)Se2 (CIGS) solar cells (23.35％)[3].The key issue for kes-terite solar cells is the large open-circuit voltage deficit(Voc,def,the gap between Voc and Shockley-Queisser limit VocsQ) or small Voc gain (Voc/VocSQ).The Voc/VocSQ is higher than 85％ for high-performance CIGS solar cells but only 61％for current world-record CZTSSe device[2].Many factors may cause the Voc loss of kesterite:(1) the narrow phase stability makes it difficult to achieve highly uniform absorber composi-tion,which can result in bandgap fluctuation and secondary phases;(2) the similar ionic size of Cu and Zn leads to high con-centration of Cu-Zn antisite defects (Cu-Zn disorder),which may cause electrostatic potential fluctuation and band tail-ing;(3) the multi-element composition and the variable valence of Sn lead to complicated defect property,causing seri-ous recombination in absorber bulk and interfaces[4-8].Identi-fy the most critical one and its origin is crucial for further im-proving device efficiency.     

Cu2ZnSnS4 thin film solar cells by fast coevaporation

Solar cells based on kesterite‐type Cu₂ZnSnS₄ (CZTS) were fabricated on molybdenum coated soda lime glass by evaporation using ZnS, Sn, Cu, and S sources. The coevaporation process was performed at a nominal substrate temperature of 550°C and at a sulfur partial pressure of 2–3 × 10⁻³ Pa leading to polycrystalline CZTS thin films with promising electronic properties. The CZTS absorber layers were grown copper‐rich, requiring a KCN etch step to remove excess copper sulfide. The compositional ratios as determined by energy‐dispersive X‐ray spectroscopy (EDX) after the KCN etch are Cu/(Zn + Sn): 1.0 and Zn/Sn: 1.0. A solar cell with an efficiency of 4.1% and an open‐circuit voltage of 541 mV was obtained. Copyright © 2010 John Wiley & Sons, Ltd.     

Microstructure and Raman Scattering of Cu<Subscript>2</Subscript>ZnSnSe<Subscript>4</Subscript> Thin Films Deposited onto Flexible Metal Substrates

Cu₂ZnSnSe₄ thin films are produced by selenizing electrochemically layer-by-layer deposited and preliminarily annealed Cu–Zn–Sn precursors. For flexible metal substrates, Mo and Ta foils are used. The morphology, elemental and phase compositions, and crystal structure of Cu₂ZnSnSe₄ films are studied by scanning electron microscopy, X-ray spectral microanalysis, X-ray phase analysis, and Raman spectroscopy.     

XPS analysis and characterization of thin films Cu2ZnSnS4 grown using a novel solution based route

A new technique to grow single phase Cu₂ZnSnS₄ (CZTS) thin films for solar cells applications using a chemical route is presented; this consist in sequential deposition of Cu₂SnS₃ (CTS) and ZnS thin films followed by annealing at 550 °C in nitrogen atmosphere, where the CTS compound is prepared in one step process by simultaneous precipitation of Cu₂S and SnS₂ performed by diffusion membranes assisted CBD (chemical bath deposition) technique and ZnS by conventional CBD technique.Measurements of X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM) were used to identify the phases present in the CTS and CZTS films as well as to study their structural and morphological properties. Further, the oxidation states and the chemical composition homogeneity in the volume were studied by X-ray photoelectron spectroscopy (XPS) analysis. Oxidation states and results regarding structural and morphological characterization of CZTS films prepared using the novel technique are compared with those results obtained from single phase CZTS films prepared by sequential evaporation of metallic precursors in presence of elemental sulfur. XRD and Raman spectroscopy studies were used to verify that the CZTS films prepared by the novel method do not present secondary phases.     

Intermetallic phase formation in thin solid-liquid diffusion couples

Conducting joints with low fabrication temperatures and high thermal stability are useful in modern electronics. This paper discusses the potential use of intermetallic phases in making such joints. Thin interconnection layers that consist entirely of intermetallic phases have been produced by joining planar Cu substrates that are coated with thin films of Sn. Thin layers (1-5 μm) of intermetallic phase are produced at temperatures slightly above the melting temperature of Sn in a process similar to reflow soldering. Metallography and x-ray analysis are used to characterize the formation mechanism of the intermetallic. Cu dissolves into the liquid Sn by diffusion along narrow channels between grains of the growing η-Cu₆Sn₆ intermetallic phase. Tensile tests were used to measure mechanical properties. The joint strength increased with reaction time. The joint fails in a ductile mode as long as unreacted Sn is present, but fractures along interphase boundaries when the joint is completely intermetallic.     

Investigation of Cu(In,Ga)Se2 thin‐film formation during the multi‐stage co‐evaporation process

In order to transfer the potential for the high efficiencies seen for Cu(In,Ga)Se₂ (CIGSe) thin films from co‐evaporation processes to cheaper large‐scale deposition techniques, a more intricate understanding of the CIGSe growth process for high‐quality material is required. Hence, the growth mechanism for chalcopyrite‐type thin films when varying the Cu content during a multi‐stage deposition process is studied. Break‐off experiments help to understand the intermediate growth stages of the thin‐film formation. The film structure and morphology are studied by X‐ray diffraction and scanning electron microscopy. The different phases at the film surface are identified by Raman spectroscopy. Depth‐resolved compositional analysis is carried out via glow discharge optical emission spectrometry. The experimental results imply an affinity of Na for material phases with a Cu‐poor composition, affirming a possible interaction of sodium with Cu vacancies mainly via In(Ga)_Cu antisite defects. An efficiency of 12.7% for vacancy compound‐based devices is obtained. Copyright © 2011 John Wiley & Sons, Ltd.     

Rapid annealing of reactively sputtered precursors for Cu2ZnSnS4 solar cells

Cu₂ZnSnS₄ (CZTS) is a promising thin‐film absorber material that presents some interesting challenges in fabrication when compared with Cu(In,Ga)Se₂. We introduce a two‐step process for fabrication of CZTS films, involving reactive sputtering of a Cu‐Zn‐Sn‐S precursor followed by rapid annealing. X‐ray diffraction and Raman measurements of the sputtered precursor suggest that it is in a disordered, metastable CZTS phase, similar to the high‐temperature cubic modification reported for CZTS. A few minutes of annealing at 550 °C are sufficient to produce crystalline CZTS films with grain sizes in the micrometer range. The first reported device using this approach has an AM1.5 efficiency of 4.6%, with J_sc and V_oc both appearing to be limited by interface recombination. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of sputter deposited Zn precursor film thickness and annealing time on the properties of Cu2ZnSnS4 thin films deposited by sequential reactive sputtering of metal targets

Cu₂ZnSnS₄ (CZTS) is made of earth abundant elements and also have suitable optical properties for solar cell applications. But, in phase diagram, CZTS exists in a narrow range of temperature and composition. Therefore, optimizing the elemental composition and annealing time is very important for obtaining phase pure CZTS. In this study, the effects of elemental composition and short annealing time on the structural and optical properties of reactively sputtered CZTS thin films are reported. Thin films were deposited by reactive sputtering of Cu: Sn (60:40 wt%), Sn and Zn targets sequentially in the presence of H₂S at room temperature. Amount of Zn precursor was varied by changing the sputter time for Zn. The films were rapidly annealed in inert atmosphere for varying time. The band gap of sample changed with change in the composition as well as annealing time. Sample with higher Zn content showed better crystallinity. With increase in the annealing time the crystallinity of samples improved. Sample annealed for 12 min at 550 °C was phase pure. Obtaining good quality film even for very short anneal time is the novelty of reactive sputtering method as all the elements are already mixed and short annealing is required only for crystal growth. Through detailed experiments, the optimum composition and annealing time required for the growth of phase pure CZTS has been established.     

Cu掺杂对sol-gel法制备的ZnO:Co薄膜发光特性的影响

采用溶胶-凝胶法制备了Cu、Co共掺的Zn0.95-xCo0.05CuxO(x=0,0.01,0.03,0.05)薄膜,并用金相显微镜和X射线衍射(XRD)研究了样品薄膜的形貌和结构,结果发现掺杂量影响着衍射峰的强度和位置。测量了样品的室温光致发光谱(PL谱),所有样品均观察到紫外发光带和蓝光发光带,同时伴随有较弱的绿光发光带。微量Cu掺杂能够显著提高ZnO∶Co薄膜的发光强度,对样品的发光机制进行了讨论。     

Impact of capping during the formation of Cu2ZnSn(S,Se)4 thin films

Major challenge for the fabrication of kesterite absorber thin films such as CZTSSe (Cu₂ZnSn(S,Se)₄) is the volatility of chalcogens. Material loss and poor morphology are two key issues during high temperature annealing, carried out for the formation of CZTSSe thin film absorber layers. The purpose of the present study is to investigate the influence of capping during the crystallization of precursor to CZTSSe films via annealing. In this work, initial precursor was synthesized from elemental constituents by ball milling. CZTSSe films were deposited by doctor's blade process. Annealing was carried out in two different atmosphere viz. vacuum and inert gas. Both sets of samples were annealed with and without capping. We found significant changes for different annealing atmospheres. Capping has a positive influence on the film properties, revealed by structural, morphological and compositional analysis. Capping reduced material loss of volatile constituents and resulted compact crystalline films.     

以低价态钛氧化物制备TiO2薄膜研究

试验以Ti2O3,Ti3O5和TiO2作为初始膜料,在ZZS700-6/G型真空镀膜机上采用O2-离子束辅助蒸发制备氧化钛薄膜.用XRD检测方法确定各种膜料和薄膜的相成分,并全面地分析了各种膜料的蒸发特性和薄膜;用分光光度计测量薄膜的透射率,并分析薄膜的光学性能.试验表明,在采用Ti2O3,Ti3O5和TiO2作为蒸发制备氧化钛薄膜时,钛的氧化物中存在Ti3O5固态同一蒸发相;各种膜料在蒸发时,发生分解,熔池中的物质成分逐渐转变成同一蒸发相成分,最终完全转变成同一蒸发相.     

Annealing effects on physical properties of doped CdTe thin films for photovoltaic applications

Polycrystalline Cadmium Telluride (CdTe) thin films were prepared on glass substrates by thermal evaporation at the chamber ambient temperature and then annealed for an hour in vacuum ~1×10⁻⁵ mbar at 400 °C. These annealed thin films were doped with copper (Cu) via ion exchange by immersing these films in Cu (NO₃)₂ solution (1 g/1000 ml) for 20 min. Further these films were again annealed at different temperatures for better diffusion of dopant species. The physical properties of an as doped sample and samples annealed at different temperatures after doping were determined by using energy dispersive x-ray analysis (EDX), x-ray diffraction (XRD), Raman spectroscopy, transmission spectra analysis, photoconductivity response and hot probe for conductivity type. The optical band gap of these thermally evaporated Cu doped CdTe thin films was determined from the transmission spectra and was found to be in the range 1.42–1.75 eV. The direct energy band gap was found annealing temperatures dependent. The absorption coefficient was >10⁴ cm⁻¹ for incident photons having energy greater than the band gap energy. Optical density was observed also dependent on postdoping annealing temperature. All samples were found having p-type conductivity. These films are strong potential candidates for photovoltaic applications like solar cells.