Nanoscale observation of surface potential and carrier transport in Cu2ZnSn(S,Se)4 thin films grown by sputtering-based two-step process

Stacked precursors of Cu-Zn-Sn-S were grown by radio frequency sputtering and annealed in a furnace with Se metals to form thin-film solar cell materials of Cu₂ZnSn(S,Se)₄ (CZTSSe). The samples have different absorber layer thickness of 1 to 2 μm and show conversion efficiencies up to 8.06%. Conductive atomic force microscopy and Kelvin probe force microscopy were used to explore the local electrical properties of the surface of CZTSSe thin films. The high-efficiency CZTSSe thin film exhibits significantly positive bending of surface potential around the grain boundaries. Dominant current paths along the grain boundaries are also observed. The surface electrical parameters of potential and current lead to potential solar cell applications using CZTSSe thin films, which may be an alternative choice of Cu(In,Ga)Se₂.PACS number: 08.37.-d; 61.72.Mm; 71.35.-y     

不同旋涂方式对铜锌锡硫硒薄膜及相应器件性能的影响

晶体质量是决定铜锌锡硫硒(Cu₂ZnSn(S,Se)₄, CZTSSe)吸收层薄膜吸收效率的关键,旋涂是溶液法制备CZTSSe吸收层的第一步,因此旋涂方式的选择至关重要。为了探究不同旋涂方式对CZTSSe吸收层薄膜质量和相应器件性能的影响,分别采用三组不同的旋涂方式制备铜锌锡硫(Cu₂ZnSnS₄, CZTS)前驱体薄膜及CZTSSe吸收层薄膜,并利用X射线衍射仪(XRD)、能谱仪(EDS)、显微拉曼光谱仪(Raman)、场发射扫描电子显微镜(FE-SEM)分析了不同旋涂方式对所制备的CZTSSe吸收层薄膜晶体结构、元素成分、相纯度、表面形貌的影响。同时,采用电流密度-电压(J-V)测试和外量子效率(EQE)测试对CZTSSe吸收层薄膜太阳电池的光电特性进行了表征。结果表明:旋涂7周期,且第一周期烘烤之前旋涂2次的效果最好,所制备的CZTS前驱体薄膜均匀,无裂纹,CZTSSe吸收层薄膜结晶度更高,薄膜表面更平整致密,晶粒大小更均匀,实现了9.63%的光电转换效率。通过对采用不同旋涂方式制备的器件的性能参数进行统计分析,得出新的旋涂方式可以提高CZTSSe薄膜太阳电池的可重复性,为将来可能的大规模商业化应用做铺垫。     

Structural,optical and electrical properties of indium doped Cu2ZnSn(S,Se)4 thin films synthesized by the DC and RF reactive magnetron cosputtering

Element doping into the Cu₂ZnSn(S,Se)₄ (CZTSSe) absorber is an effective method to optimize the performance of thin film solar cells. In this study, the Cu₂In_xZn_1-xSn(S,Se)₄ (CIZTSSe) precursor film was deposited by magnetron cosputtering technique using indium (In) and quaternary Cu₂ZnSnS₄ (CZTS) as targets. Meanwhile, the In content was controlled using the direct current (DC) power on In target (P_In). A single kesterite CIZTSSe alloy was formed by successfully doping a small number of In³⁺ into the main lattice of CZTSSe. The partial Zn²⁺ cations were substituted by In³⁺ ions, resulting in improving properties of CZTSSe films. Morphological analysis showed that large grain CIZTSSe films could be obtained by doping In. The well-distributed, smooth, and dense film was obtained when the P_In was 30 W. The band gap of CIZTSSe could be continuously adjusted from 1.27 to 1.05 eV as P_In increased from 0 to 40 W. In addition, the CIZTSSe alloy thin film at P_In = 30 W exhibited the best p-type conductivity with Hall mobility of 6.87 cm²V^?1s^?1, which is a potential material as the absorption layer of high-performance solar cells.     

Influence of H2Se concentration on Se-rich CZTSSe absorbers sputtered with a ceramic quaternary target

Se-rich CZTSSe absorbers with large grains are important contributors to the performances of CZTSSe solar cells and can be fabricated by the selenization of as-sputtered CZTS precursors. To explore the effects of the H₂Se concentration in the annealing atmosphere on the growth of CZTSSe phases, sputtered CZTS precursors were subjected to annealing with a mixed gas of Ar and H₂Se at different concentrations. A series of characterization techniques were employed to investigate the morphologies, phase structures, surface states, and elemental compositions of the annealed samples. The results demonstrate that the H₂Se concentration in the annealing atmosphere can significantly affect the grain size, suppressing the decomposition of CZTSSe absorbers. When the H₂Se concentration in the annealing atmosphere reaches 4.5 vol%, a selenium-enriched CZTSSe absorber that is composed of the pure kesterite structure and that has densely packed large grains and a high concentration of selenium was obtained. The highest efficiency of 10.12% of CZTSSe solar cells was achieved herein.     

Solution transformation of SnS into Cu2ZnSnS4xSe4(1-x) for solar water splitting

Cu₂ZnSnS_4xSe_4(1-x) (CZTSSe) has recently attracted much attention for solar water splitting due to its high absorption efficient and tunable bandgap. However, high quality CZTSSe thin films with good quality and adherence with the substrate are mainly based on physical deposition method, such as magnetron sputtering, which is expensive and energy consuming processes. Here, we have developed a novel and low-cost solution fabrication method and CZTSSe electrodes were synthesized at low temperature by hydrothermal treatment of chemical bath deposited SnS films. The quality and phase purity of CZTSSe thin films were greatly improved after annealing process and the effect of SnS thickness to the physical and photoelectrochemical (PEC) properties of CZTSSe thin films were detailedly studied. The fabrication of FTO/CZTSSe/CdS/TiO₂/Pt photocathode improved the PEC properties of CZTSSe thin films greatly and the highest water splitting photocurrents of 7.2 mA/cm⁻² had been achieved under simulated solar illumination. Furthermore, the electrode showed good stability and had a good incident photon-to-current efficiency (IPCE) response in the visible light range.     

Synthesis and investigation of environmental protection and earth-abundant kesterite Cu2MgxZn1-xSn(S,Se)4 thin films for solar cells

We have synthesized Cu₂Mg_xZn_1–xSn(S,Se)₄ (0?≤?x?≤?0.6) thin films by a facile sol-gel method, and studied the influence of Mg concentration on the crystal structure, surface morphology and photoelectric performance of Cu₂Mg_xZn_1–xSn(S,Se)₄ thin films systematically. It was shown that the smaller Zn²⁺ in Kesterite phase Cu₂ZnSn(S,Se)₄ will be replaced by larger Mg²⁺, forming uniform pure phase Cu₂Mg_xZn_1–xSn(S,Se)₄. The band gap of Cu₂Mg_xZn_1–xSn(S,Se)₄ films can be adjusted from 1.12 to 0.88?eV as the x value changes from 0 to 0.6. Furthermore, the Cu₂Mg_xZn_1–xSn(S,Se)₄ thin films with large grain size, smooth surface and less grain boundaries was obtained at an optimized condition of x?=?0.2. The carrier concentration of Cu₂Mg_xZn_1–xSn(S,Se)₄ thin film reaches the maximum 6.47?×?10¹⁸ cm^?3 at x?=?0.2, which is a potential material to be the absorption layer of high efficiency solar cells.     

Effect of TiN diffusion barrier layer on residual stress and carrier transport in flexible CZTSSe solar cells

The major drawback of flexible Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells is the inevitable residual stress in CZTSSe that considerably limits the efficiency and flexibility of these cells. Hence, in this work, TiN layers with varying thicknesses were sputtered between flexible Ti substrates and back contact Mo layers as diffusion barriers. The TiN barrier layer relieved residual stress, facilitated grain growth, and decreased the porosity of CZTSSe, thereby effectively suppressing the formation of carrier recombination paths and improving the mechanical strength of CZTSSe. Meanwhile, the band alignment of the CZTSSe/CdS heterojunction could be significantly tailored, leading to an improved ‘‘cliff-like’’ conduction band offset from ?0.49 eV to ?0.33 eV. Under the optimized TiN layer thickness of 50 nm, the power conversion efficiency of the fabricated flexible CZTSSe solar cell increased considerably from 3.43% to 4.85% along with high bending stability. Therefore, introducing the TiN diffusion barrier into traditional flexible CZTSSe solar cells improves the efficiency and flexibility of these devices. Moreover, this method could be a promising pathway for the large-scale production of smart, flexible, and portable electronic devices.     

Effect of annealing on grain growth and Y segregation behavior in tetragonal ZrO2 thin film

In order to fabricate tetragonal yttria stabilized zirconia samples with large grain size, 3 mol% Y₂O₃ doped zirconia thin films were grown on (0001) α-Al₂O₃ substrate by pulsed laser deposition (PLD) followed by subsequent high temperature annealing. The thin film samples were annealed at 1200°C, 1250°C, 1300°C, and 1350°C in order to obtain larger grain size without Y segregation. The microstructure and chemical composition of these annealed films were analyzed using atomic force microscopy, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The as-grown thin film was found to be composed of [111]-oriented grains of ∼100 nm connected with small-angle tilt boundaries. Based on analysis of annealed thin films, it was revealed that grain growth of tetragonal zirconia occurred anisotropically. Cross section scanning transmission electron microscopy observations revealed that such grain growth behavior is affected by the step-terrace structures of the sapphire substrate. Energy-dispersive X-ray spectroscopy showed that Y was found to distribute almost uniformly below 1300°C but to segregate at the grain boundaries at 1350°C. As a conclusion, the 1300°C-annealed sample shows the largest grain size with homogeneous Y distributions.     

TiO2 films prepared by micro-plasma oxidation method for dye-sensitized solar cell

Nanocrystalline TiO₂ films are widely investigated as the electrodes of dye-sensitized solar cell(s) with different preparation methods. In this paper, thin titanium dioxide films have been prepared on titanium plates by the micro-plasma oxidation method in the sulfuric acid solution. The thin TiO₂ films were sensitized with a cis-RuL₂(SCN)₂·2H₂O (L = cis-2,2′-bipyridine-4,4′-dicarboxylic acid) ruthenium complex and implemented into a dye-sensitized solar cell configuration. The influence of reaction current density (10, 15, 20, 25 and 30 A dm⁻²) on the structural and the surface morphology of the films was investigated by X-ray diffraction, scanning electron microscopy, atom force microscopy and X-ray photoelectricity spectroscopy. Impedance analysis for dye-sensitized solar cells was carried out by electrochemical impedance spectroscopy. The results show that the rise of current density leads to the increase in the amount of rutile and the thickness of the TiO₂ film, which makes the TiO₂ films have different photovoltages and photocurrents. The relatively higher photoelectricity properties were obtained in the TiO₂ films prepared at a current density of 20 A dm⁻². The open-circuit voltage and the short-circuit current are 605 mV and 165 μA cm⁻², respectively.     

Effects of ceric ammonium nitrate (CAN) additive in HNO3 solution on the electrochemical behaviour of ruthenium for CMP processes

In order to develop a new chemical mechanical polishing process for ruthenium (Ru), the present work deals with the effect of a ceric ammonium nitrate (CAN) additive on the electrochemical behaviour of physical vapour deposited Ru films in a 1 M HNO₃ solution employing electrochemical methods and surface analytical techniques. By adding CAN to HNO₃ solution, the polarisation curves showed an increase in the corrosion potential and current, suggesting that Ru is anodically polarised by CAN as an oxidising additive. To characterise the influence of CAN, open-circuit potential (OCP) and potentiostatic anodic current transient curves were examined in CAN-containing HNO₃ solution and the resulting surfaces were then characterised by scanning electron microscopy and X-ray photoelectron spectroscopy. It is proposed that Ru is oxidised to heterogeneous Ru₂O₃ and RuO₂ films on the Ru surface in CAN-containing HNO₃ solution and galvanic corrosion occurs at grain boundaries, caused by the difference in OCP between the grain interiors and boundaries. The grain boundaries are oxidised to RuO₄, a volatile species, resulting in a roughened and porous structure.     

Influence of the selenization time on the properties of (Na0.1Cu0.9)2ZnSn(S,Se)4 thin films and their photovoltaic solar cells

(Na_0.1Cu_0.9)₂ZnSn(S,Se)₄ thin films with a single kesterite phase were synthesized using a sol-gel spin-coating method accompanied by rapid post-annealing. In this study, we investigated the effect of selenization time on the crystal quality and photoelectric performance of the (Na_0.1Cu_0.9)₂ZnSn(S,Se)₄ films. It was found that the crystallinity and morphology of the films was enhanced, and some of bigger Se substituted for the S site in (Na_0.1Cu_0.9)₂ZnSn(S,Se)₄ with increasing the selenization time. The bandgap of the film can be regulated from 1.04 eV to 0.99 eV by varying the selenization time. In addition, all films showed p-type conductive characteristics, and films with optimal electrical performance could be obtained by optimizing the selenization time. Finally, the (Na_0.1Cu_0.9)₂ZnSn(S,Se)₄ thin film with the best crystal quality and optical-electrical characteristics was obtained at an optimized selenization time of 15 min. A high power conversion efficiency (PCE) of 3.92% was obtained for the (Na_0.1Cu_0.9)₂ZnSn(S,Se)₄ device, which is 42% higher compared to that of the undoped Cu₂ZnSn(S,Se)₄ (CZTSSe) device.     

Preparation and electrical properties of Sm-doped Bi2Ti2O7 thin films prepared on Pt (111) substrates

Crack-free Sm-doped Bi₂Ti₂O₇ (Sm:BTO) thin films with strong (111) orientation have been prepared on Pt (111) substrates using a chemical solution deposition (CSD) method. The structural properties and crystallizations were studied by X-ray diffraction. The surface morphology and quality were examined using atomic force microscopy (AFM). The insulating and dielectric properties were also evaluated at room temperature. The results demonstrate that the Sm:BTO films exhibit improved electrical performances as compared to the pure Bi₂Ti₂O₇ thin films and suggest a strong potential for utilization in microelectronics devices.     

Novel synthesis method for quaternary Cd(Cu,Zn)Se thin films and its characterizations

Nowadays, the search for novel compounds by chemical synthesis is in trend. Herein, we report the deposition of Cd_1-x-yZn_xCu_ySe (0.025 ≤ x = y ≤ 0.15) films by facile, industry-oriented chemical synthesis. The Cd_1-x-yZn_xCu_ySe thin films were deposited at the optimized growth conditions (temperature = 70 ± 0.1 °C, pH = 10.3 ± 0.1, substrate rotation speed = 70 ± 2 rpm and time = 100 min). As-synthesized thin films were characterized for physical, chemical, topographical and electrical attributes. The study of vibrational modes in Cd_1-x-yZn_xCu_ySe thin films was done with the help of Raman spectroscopy. Improvement in surface topography with the integration of Cu²⁺ and Zn²⁺ into the CdSe lattice has been noticed by the atomic force microscopy (AFM). The electrochemical impedance spectroscopy revealed lower values of R_s and R_ct for x = y = 0.05 composition. Chemical deposition of Cd_1-x-yZn_xCu_ySe thin films may offer an excellent way to fabricate quaternary chalcogenide-based absorber materials for solar cells.     

Nonvacuum Solution Synthesis of (Ag,Cu)(In,Ga)Se2 Absorbers for Applications in Thin‐Film Solar Cells

The effects of selenization temperatures on the phase formation and the photovoltaic properties of silver‐ion‐doped Cu(In,Ga)Se₂ (ACIGS) films were investigated. Cu_2?xSe phase coexisted with CuInSe₂ phase in the films as the selenization temperature was relatively low. Increasing the selenization temperatures promoted the formation of the chalcopyrite phase and increased the grain size. Upon increasing the selenization temperature to 600°C, single‐phased ACIGS films with a grain size of 2.1–2.2 μm were successfully synthesized. The incorporation of Ag⁺ and Ga³⁺ ions into CuInSe₂ during the phase formation of ACIGS elevated the band gaps of the films, thereby improving the open‐circuit voltage (V_oc) of the solar cells. The grain growth on raising the selenization temperatures also elevated the short‐circuit current (J_sc) values owing to the suppression of the electron‐hole recombination at grain boundaries. In the diode analysis, the facilitated phase formation suppressed the shunt path, decreasing the values of the diode factor (A), shunt conductance (G), and saturated current (J_o), thereby improving the cell performance. In this study, ACIGS solar cells with an efficiency of 7.21% prepared via the nonvacuum process were first demonstrated.     

In situ characterization of Ni-rich NMC811 thin films prepared by sol-gel method using different scanning probe microscopy techniques

In this paper, LiNi_0.8Co_0.1Mn_0.1O₂ (NMC811) thin films were prepared by the sol-gel method based on different parameter conditions, and the microstructure and electrochemical properties of the films were compared to obtain the film sample under the best preparation parameters. In order to explain the formation mechanism of cell battery performance based on the NMC811 cathode at the nano scale, electrochemical strain microscopy, conductive atomic force microscopy and Kelvin probe force microscopy are specifically utilized to study the changes of NMC811 thin film in electrochemical deformation, conductivity, and potential distribution under different electrical and thermal fields. The results indicate that the local amplitude, conductivity, and electrical potential of thin film in the scanning region increases with the voltage from 0 V to 3 V and the temperature from 25 °C to 150 °C. However, the electrochemical deformation, conductivity, and potential of the film decrease significantly when the voltage is applied to 4 V or the temperature rises to 200 °C.These results provide visual evidence for the influence of external field on the phase structure change and lithium-ion diffusion behavior of NMC811 thin film.     

Flexible CZTSSe thin film solar cells fabricated at low temperature with relieved residual stress by Sb incorporation

The troublesome residual stress is always a stumbling block that drags the progress pace of flexible CZTSSe thin film solar cells, which urgently needs to be noticed and solved. In this paper, low-temperature prepared CZTSSe absorber with relieved residual stress (0.558 GPa) is realized by Sb incorporation. Owing to the evaporated 20 nm Sb layer under CZTS precursor, the crystalline quality and band mismatching of CZTSSe/CdS interface are simultaneously improved. Additionally, the spatial potential fluctuation extracted from the PL results is found to decrease from 63.26 meV to 41.57 meV, indicating a reduction in band tailing and disorder of CZTSSe absorber. Compared with the general solar cells fabricated at 580 °C, flexible devices with Sb incorporation can maintain a slightly higher performance at a lower temperature about 60 °C. The best power conversion efficiency (PCE) of 4.41% is obtained in the solar cell with 550 °C-selenized CZTSSe absorber after incorporating 20 nm Sb layer, featuring 351.20 mV Voc, 25.73 mA/cm² Jsc and 48.79% FF. Finally, low-temperature prepared flexible CZTSSe thin film solar cell can retain over 83% of the original PCE after bending at 180° for 40 cycles. The mechanical durability paves a promising way for flexible CZTSSe thin film solar cell in roll-to-roll production.     

The Structural,Optical and Electrical Properties of Spin Coated WO<Subscript><Emphasis Type="Bold">3</Emphasis></Subscript> Thin Films Using Organic Acids

Tungsten trioxide (WO₃) thin films were prepared incorporating various organic acid additives by the sol-gel spin coating technique. They were characterized by X-ray diffraction (XRD), UV-Visible analysis, scanning electron microscopy (SEM) and dc electrical conductivity. From XRD, the crystal phase, average grain size and structural parameters of WO₃ thin films were found to vary owing to different water dissolved organic acid additives. The variation of optical conductivity and band gap energy was calculated from the UV-Visible analysis. The SEM studies revealed that the organic acids influenced the surface morphology of the microsized plates of tungsten oxides. The electrical conductivity at various temperatures correlated with the average grain size of the nanocrystallites of WO₃ thin films.     

Annealing-induced changes in the nanoscale electrical homogeneity of bismuth ferrite dielectric thin films

In this study, we investigated the effects of forming gas (7% H₂ + 93% Ar) annealing (FGA) and recovery annealing (RA) in ambient oxygen on the structure and electrical properties of BiFeO₃ (BFO) thin films. X-ray diffraction results indicate that BFO remains in the perovskite phase following FGA. However, the spatial distribution of current maps obtained by conductive atomic force microscopy shows that FGA-treated BFO thin films are less electrically insulating than those without prepared thermal annealing. Recovery annealing improves the structural and chemical homogeneity of the FGA-treated films, thereby increasing the electrical resistance of the films.     

Effect of sulfur annealing on the morphological,structural, optical and electrical properties of iron pyrite thin films formed from FeS<Subscript>2</Subscript> nano-powder

Iron pyrite (FeS₂) thin films were fabricated by spin coating the solution of FeS₂ nanocrystals of ～40 nm in size on glass substrates, followed by annealing in a sulfur environment at different temperatures. The effect of sulfurization temperature on the morphology, structural, optical and electrical properties was investigated. With increase of the sulfurization temperature, the grain size and crystallinity of the films was improved, although some cracks and voids were observed on the surface of thin films. The band gap of the FeS₂ films was decreased at higher sulfurization temperature. The electrical properties were also changed, including the increasing in resistivity and the decrease in Hall mobility, with increase of sulfurization temperature. The change in the optical and electrical properties of the FeS2 thin films was explained based on the changes of phase, morphology, surface, and grain boundary property.     

Self-cleaning performance of sol–gel-derived TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> double-layer thin films

The self-cleaning properties of the TiO₂/SiO₂ double-layer films prepared by sol–gel method were investigated. Thin films were prepared by spin coating onto glass and then thermally treated at different temperatures, and characterized using X-ray diffraction, atomic force microscopy, field emission scanning electron microscopy, and UV–visible spectroscopy. The cross-sectional structure of the films was observed by FESEM. The surface roughness of the films was characterized by AFM. The root mean square surface roughness of the thin films was below 2 nm, which should enhance their optical transparency. The photo-induced hydrophilicity of the films was evaluated by water contact angle measurement in air. The photocatalytic activity of the films was studied by the photocatalytic degradation of methylene blue under UV light irradiations. The TiO₂/SiO₂ double-layer thin films are plausibly applicable to developing self-cleaning materials in various applications such as windows, solar panels, cement, and paints.