CZTSe Formation Research Articles

Influence of the precursor composition on the resulting absorber properties and defect concentration in Cu2ZnSnSe4 absorbers

In this paper the influence of composition variations during the growth of Cu2ZnSnSe4 (CZTSe) layers on defect concentrations and final absorber properties is analyzed. The samples are prepared from sputtered metallic and alloyed Zn/Cu–Sn/(Sn)/Zn precursors. The variation of sputtered Sn content facilitates different forms for the embedded Sn (Cu3Sn, Cu6Sn5, elemental Sn). The predominating formation reaction during the subsequent annealing is tuned by the ratio of the alloys and the overall chemical composition in the precursor. During the high temperature stage of CZTSe formation the layer undergoes an in-process composition shift (Sn content variation), after which the final absorber composition is reached. As all absorbers are reaching a comparable end composition, in this paper the influence of the different precursor compositions and corresponding reaction pathways on the resulting defects and optoelectronic properties of the absorbers are addressed. Spectroscopic investigations were used to extract the defect configuration of the absorbers. The results are complemented with device performance measurements. Preferable precursor compositions leading to lower defect concentration and improved photoluminescence quantum-yield is identified. The obtained Urbach energy and VOC deficit are among the most promising results measured in our laboratories.

Reaction Pathway for Efficient Cu2ZnSnSe4 Solar Cells from Alloyed CuSn Precursor via a Cu‐Rich Selenization Stage

The selenization of stacked elemental metallic layers (CuSn–Zn) is a commonly reported approach in kesterite Cu2ZnSnSe4 (CZTSe) processing. CZTSe formation via this approach usually involves a reaction route containing binary selenides, such as SnSe2−x. The high volatility of these phases at the necessary annealing temperatures (500–550 °C) makes this reaction pathway prone to Sn loss, which makes it challenging to control the composition and quality of the grown material. Herein, an approach based on stacked elemental and alloyed precursors is reported, and the benefits of using a Zn/CuSn/Zn configuration are discussed. The absence of nonalloyed elemental Sn helps in suppressing the formation and subsequent evaporation of SnSe2−x phases, preventing Sn loss from the film during selenization. This reaction pathway involves a process scheme which 1) starts with the growth of CZTSe in a “Cu‐rich” environment, 2) includes a shift of the composition by supply of SnSe2−x vapor, and 3) terminates in the “Cu‐poor” regime, leading to device efficiencies above 10%. This composition shift in the presented process appears similar to the final stage of the commonly known CIGSe three‐stage coevaporation.

Vacuum evaporated FTO/(Cu, Ag)2ZnSnSe4 thin films and its electrochemical analysis

Cu2ZnSnSe4 (CZTSe) and Ag2ZnSnSe4 (AZTSe) thin films were deposited on FTO substrates by the thermal evaporation method and the films were annealed at 300 °C for 2 h. The XRD analysis confirms the formation of tetragonal structure for both CZTSe and AZTSe thin films. The introduction of Ag instead of Cu is confirmed by the major crystalline peak (112) shift towards lower angle 2θ = 26.1° (for AZTSe). In the Raman spectra, the noticeable vibrations appeared at 186 cm−1 and 183 cm−1 confirm the formation of CZTSe and AZTSe thin films. Raman technique also found the presence of SnSe as a secondary phase. By comparing the optical spectra, the AZTSe film has higher absorption in the visible region than the CZTSe thin films which is attributed to the surface plasmon resonance of Ag. AFM images show a pyramidal structure for both CZTSe and AZTSe films. The Mott-Schottky plot shows p-type conductivity for FTO/CZTSe while there is n-type conductivity for a FTO/AZTSe thin film. The variation in the conductivity is due to replacing Cu with Ag. Photoelectrochemical cell (PEC) performance was employed in the presence of a polysulfide electrode by a standard three-electrode system. By comparing the value of Voc, the AZTSe film shows higher Voc than CZTSe films which is important for photovoltaic applications. The Photoconversion efficiency (PCE) is measured to be 0.31% for FTO/AZTSe thin films, while it is low about 0.29% for FTO/CZTSe. The impedance plot shows the semicircular nature for both the FTO/CZTSe and FTO/AZTSe films.

Reaction pathway to CZTSe formation in CdI2

The chemical reactions between CuSe, SnSe and ZnSe and their enthalpies in molten CdI2 in closed vacuum ampoules were investigated. Differential thermal analysis (DTA) was used for the determination of the temperatures of processes occurring in the studied mixtures. X-ray diffraction and Raman spectroscopy were performed to discover the phase changes in the long-term annealed samples at higher than peak temperatures in DTA. The occurring chemical reactions and their enthalpies were derived. The recrystallization of CZTSe in CdI2 was investigated. The results confirmed the formation of Cu2SnSe3, Cu2SnSe4, CdSe, Cu2CdSnSe4, Cu2ZnSnSe4, and Zn1−xCdxSe compounds and solid solutions.

Insights into the Formation Pathways of Cu2ZnSnSe4 Using Rapid Thermal Processes

Recent advances in Cu2ZnSn(S,Se)4 (CZTSe) thin film photovoltaics open the possibility for the future industrialization of this technology. Nevertheless, major progresses in CZTSe have been achieved using conventional thermal processing annealing routes (CTP), which rely on time-consuming processes with tubular furnaces, incompatible with the requisites of fast methodologies for the Industry. Changing from conventional to rapid thermal processes (RTP) using halogen lamps as heating method is not at all obvious, since the system becomes kinetically controlled, and the CZTSe formation mechanisms as well as crystallization pathways can drastically change. In this work we present the transfer of our kesterite production baseline (Cu2ZnSnSe4:Ge) from a conventional thermal process using a tubular furnace toward a rapid thermal process using an adapted system, by comparing them and analyzing the differences between both processes in terms of formation mechanisms as well as photovoltaic absorber properties. For ...

Depth-resolved and temperature dependent analysis of phase formation processes in Cu–Zn–Sn–Se films on ZnO substrates

The constitution of secondary phases in kesterite Cu2ZnSnSe4 (CZTSe) thin films is still a limiting factor for their application in solar cells. Therefore an enhanced understanding of phase formation processes during the fabrication of CZTSe films is required. In this study we present a temperature and film-depth dependent phase analysis of Zn/Sn/Cu precursors on ZnO substrates selenized at different temperatures. A special sample preparation step using a focused ion beam was applied to prepare shallow angle cross sections for depth-resolved Raman profiling of the thin films. At low selenization temperatures multiphase structures are demonstrated and a first formation of CZTSe besides secondary phases at only 250 °C is detected. At high selenization temperatures an accumulation of ZnSe at the interface of CZTSe and ZnO substrates is observed. Furthermore indications for the formation of a thin SnO2 interface layer were found by X-ray diffraction, secondary electron microscopy and energy dispersive X-ray spectrometry.

Thermodynamics and kinetics insight into reaction mechanism of Cu2ZnSnSe4 nanoink based on binary metal-amine complexes in polyetheramine-synthesized process

This paper reports on the reaction mechanism of Cu2ZnSnSe4 (CZTSe) nanoink via a solvent-thermal reflux method using copper (Cu), zinc (Zn), tin (Sn), and selenium (Se) powders as precursors and polyetheramine as a reaction solvent. The formation of CZTSe nanoparticles in polyetheramine began with the formation of binary phase CuSe and CuSe2 due to the strong catalysis provided by polyetheramine. Finally, ternary crystals of Cu2SnSe3 transformed into well-dispersed nanocrystals of Cu2ZnSnSe4. The size of the crystals was shown to decrease with reaction time due to the emulsification effect of the polyetheramine epoxy group. The PH value-reaction time curves for single Cu, Zn elements and CZTSe from all participants elements reacted together have a relationship just reversed each other and both multistage feature were observed, which indicates that the CZTSe reaction was dominated by copper and zinc elements. The PH-temperature mechanism demonstrates that the reaction was controlled by the formation of metal-amine complexes, especially, after heating the PH-time variation manner is the same for pure element and all four elements reacted together. To the best of our knowledge, this is the first study on the mechanism underlying CZTSe formation based on the reactivity and stability of reaction species.

Reaction enthalpies of Cu2ZnSnSe4 synthesis in KI

The phase transitions and chemical reactions in the formation process of Cu2ZnSnSe4 (CZTSe) from CuSe, SnSe, and ZnSe in the presence of KI were studied using the DTA method for the determination of thermal effects. XRD and Raman spectroscopy were performed for the phase analysis in samples after heating them for prolonged time to several degrees of temperatures higher than peak-top temperatures of DTA effects in heating and cooling curves. The chemical pathways of the CZTSe formation before and during the melting of KI were clarified. The reaction enthalpies were determined using pure KI as a substance for the calibration of enthalpy values. In the mixtures, the melting of KI is always accompanied by some dissolution of the precursor compounds and lowering of the melting temperature in comparison with pure KI. The initiator of the process of the partial CZTSe formation at around 400 °C is elemental Se. The CZTSe formation of binary precursors in liquid phase of KI takes place with the experimentally determined enthalpy of −36 ± 2 kJ mol−1. The heat of fusion of formed CZTSe is 4 kJ mol−1 at 788 °C.

Growth of Cu2ZnSnSe4 by cosputtering and reactive annealing atmosphere

Cu2ZnSnSe4 (CZTSe) thin films were synthesized by a two-step process involving co-sputtering metallic precursor followed by reactive annealing. Two different techniques of annealing were used, i.e. graphite box and three-zone furnace. Very important differences were observed between the two techniques in terms of Se diffusion through the precursor and reaction path leading to CZTSe formation. Thermodynamic analysis was used in order to explain the reaction mechanism and kinetics of CZTSe formation during the annealing step. The films exhibit large grains with high crystalline quality and solar cells with conversion efficiency of 7.1% were synthesized.

Cu2ZnSnSe4 formation and reaction enthalpies in molten NaI starting from binary chalcogenides

The present study deals with chemical reactions and enthalpies during the synthesis of Cu2ZnSnSe4 (CZTSe) from CuSe, SnSe, and ZnSe in molten NaI as flux material in closed degassed ampoules. Differential thermal analysis (DTA) at heating rates 5 °C min−1 and cooling rates 10 °C min−1 were used for the determination of temperatures of phase transitions and/or chemical reactions. XRD and Raman analyses confirmed that the formation of CZTSe starts already at 380 °C after the melting of Se that deliberates from the transformation of CuSe to Cu1.8Se, and the CZTSe formation process impedes to a great extent due to the presence of solid NaI. After the melting of NaI, the formation of CZTSe is completed. For the determination of enthalpy values, the calibration with pure NaI was performed. The thermal effects and enthalpies were compared with the available known thermodynamical values. The specific enthalpy of exothermic Cu2ZnSnSe4 formation at 661 °C in NaI −36 ± 3 kJ mol−1 was determined experimentally for the first time. Ternary compound Na2SnSe3 was formed during the synthesis process. NaI·2H2O, if present in NaI, was found to be a critical issue in the synthesis process of CZTSe monograin powders in molten NaI—it gave rise to the formation of oxygen-containing by-products Na2SeO4 and Na2Cu(OH)4. The complete dehydration of NaI·2H2O at T ≤ 70 °C in vacuum is necessary to avoid the formation of oxygen-containing compounds.

Synthesis and characterization of CZTSe nanoinks using polyetheramine as solvent

This study developed a novel one-step method for the synthesis of well-dispersed CZTSe nanoparticles from Cu, Zn, Sn, Se elemental powders using a solvent-thermal reflux technique. Polyetheramine was used as a solvent and chelating agent, which provided two NH2 bonding sites at the ends as well as long, continuous epoxy chains in the center, making it ideally suited to the formation of CZTSe for use in ink printing. X-ray diffraction (XRD) and optical Raman spectroscopy were used to identify structural and phase information; transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to confirm the morphology of the film as well as the shape and composition of the particles, and UV-Vis was used to characterize the band gap. We also investigated the composition and morphology of the CZTSe nanoparticles depending on different solvents polyetheramine and isophorondiamine. The polyetheramine-based process produced particles with far more Cu-poor and Zn-rich composition CZTSe phases, a smoother and closed packed morphology, and a more spherical crystal shape. In contrast, the isophorondiamine-based process produced nanoparticles with an irregular sheet-like shape. These results demonstrate that the solvent-thermal reflux method depends mainly on the structure of the chelating agent. The proposed technique is a low-cost and environmentally friendly solution-synthesis method applicable to the large-scale production of CZTSe for photovoltaic devices.

Synthesis of Cu<sub>2</sub>ZnSnSe<sub>4</sub> Compounds Using Solvo-Thermal Method

We reported a fabrication of Cu2ZnSnSe4 (CZTSe) via an elemental solvo-thermal method. To suppress the sublimation of Se, we made use of binary compounds of high vaporization temperature by first converting solid Se into a liquid-like alloy. Single-phase CZTSe compounds with the band gap of 1.32 eV were successfully synthesized and characterized. The effect of synthesizing temperature on the formation of CZTSe was also investigated.

Phase formation in Cu2ZnSnSe4 thin films deposited with multi-stage co-evaporation processes

The deposition of Cu2ZnSnSe4 (CZTSe) thin films with a two-stage co-evaporation process was investigated with in situ laser light scattering (LLS) and ex situ characterization methods. As a result of the process conditions as well as intrinsic reactions, the film growth is divided into distinct growth phases with characteristic phase formation processes in each phase. In a first phase, under supply of Cu, Zn, Sn and Se, only Cu2−xSe and ZnSe are deposited on the substrate. A film with several hundred nanometer thickness is necessary to initiate the CZTSe formation. Then, in a second phase, CZTSe grows by incorporation of Cu and Sn in ZnSe while Cu2−xSe remains as secondary phase. In the second process stage with terminated Cu evaporation, the remaining Cu2−xSe is consumed and the film stoichiometry turns from Cu-rich to Cu-poor. A substantial influence on the growth process is found for the Mo substrate, CZTSe growth is hindered by a detrimental interaction between Mo and CZTSe. During the whole process, the Cu2−xSe phase plays an important role in the grain growth process and supports the formation of large CZTSe grains.

Real-time observation of Cu2ZnSn(S,Se)4 solar cell absorber layer formation from nanoparticle precursors

The selenization of Cu-Zn-Sn-S nanocrystals is a promising route for the fabrication of low-cost thin film solar cells. However, the reaction pathway of this process is not completely understood. Here, the evolution of phase formation, grain size, and elemental distributions is investigated during the selenization of Cu-Zn-Sn-S nanoparticle precursor thin films by synchrotron-based in situ energy-dispersive X-ray diffraction and fluorescence analysis as well as by ex situ electron microscopy. The precursor films are heated in a closed volume inside a vacuum chamber in the presence of selenium vapor while diffraction and fluorescence signals are recorded. The presented results reveal that during the selenization the cations diffuse to the surface to form large grains on top of the nanoparticle layer and the selenization of the film takes place through two simultaneous reactions: (1) a direct and fast formation of large grained selenides, starting with copper selenide which is subsequently transformed into Cu2ZnSnSe4; and (2) a slower selenization of the remaining nanoparticles. As a consequence of the initial formation of copper selenides at the surface, the subsequent formation of CZTSe starts under Cu-rich conditions despite an overall Cu-poor composition of the film. The implications of this process path for the film quality are discussed. Additionally, the proposed growth model provides an explanation for the previously observed accumulation of carbon from the nanoparticle precursor beneath the large grained layer.