Cu2ZnSnS4 Thin Films Research Articles

Assessing the Impact of Na and Sb Doping in Solution Processed Cu2ZnSnS4 Thin-Film Solar Cells

Thin-film inorganic solar cells based on Cu2ZnSn(SxSe1-x)4 absorbers remain as one of the most promising PV technologies featuring Earth abundant and low toxic elements. These devices conventionally adopt the so-called substrate configuration which characterises commercial CIGS technologies. Interestingly, similar record cell efficiencies have been achieved using solution processing (12.6%)1 and sputtering based methods (13.0%).2 It is widely recognised that cell voltage is the key limiting factor in these devices, yet the physical origin of voltage losses remains to be elucidated.The composition complexity of these materials can lead to a variety of point defects which have been extensively investigated from the experimental and computational point of view. For instance, the loss of voltage has been linked to Cu/Zn antisites which can give rise to tails in the band edge energies (so-called Urbach tails).3 Furthermore, we have also observed Sn disorder in the bulk3 and at the surface of these compounds,4 which are responsible for deep states highly detrimental to cell performance.Numerous studies have reported improvement in Cu2ZnSn(SxSe1-x)4 cells performance upon doping and/or alloying with a variety of alkali, group IV and V elements.5 The introduction of dopants can have multiple effects such as supressing elemental disorder, doping, passivation of recombination sites, changes in grain size and so forth. The material complexity makes rather challenging the establishment of meaningful correlations between composition and device properties. In this contribution, we uncover the effects of Na and Sb doping in the bulk and interfacial electronic properties of solution-processed Cu2ZnSnS4 (CZTS) thin films, which manifest itself by an increase in power conversion efficiency from 3.2 to 5.7%.6 I will first show how the surface electronic landscape of CZTS thin films evolves upon doing, employing high-resolution energy-filtered photoemission of electron microscopy (EF-PEEM). This technique allows mapping fluctuations in the local effective work function (LEWF) with sub-micron resolution.4,7 We uncover unprecedented correlations between surface electronic properties with key device parameters such as the activation energy for the predominant recombination pathway and Urbach tails. We will also discuss new data correlating variable temperature photoluminescence and admittance spectroscopy, demonstrating the fundamental role of Sb in diminishing Sn disorder.

Preparation of Cu2CdxZn1−xSnS4 thin films by nanoparticle ink method

Because of their excellent optical and electrical properties, Cu2ZnSnS4 (CZTS) thin films have been considered as a potential absorber layer for solar cells. However, CZTS suffers from some problems, such as secondary phases, antisite defects, and band tailing issues. Introducing Cd into CZTS to form Cu2CdxZn1−xSnS4 (CCZTS) is an effective method for alleviating the limitations of CZTS. In this study, the nanoparticle ink method was used to fabricate CCZTS thin films, and the effect of Cd content on the properties of CCZTS thin films was studied. The results showed that the as-prepared thin films had high purity, good crystallinity and photocurrent response. The photosensitivity and power conversion efficiency increased at first and then decreased with increasing Cd content. Due to the relatively excellent crystallization and photoresponse characteristics, the highest conversion efficiency of 0.72% was obtained when x = 0.3.

Influencing mechanism of post-sulfurization with sulfur flakes on phase evolution and Schottky diode characteristic of Cu2ZnSnS4 thin films sputter deposited from a single target

While single target sputter deposition appears very attractive to prepare kesterite Cu2ZnSnS4 (CZTS) thin films for photovoltaic applications, the growth of secondary phases arising as a consequence of stoichiometric deviation stemming from variation in sputter yield of elements must be prevented. Here, we demonstrate growth of phase pure kesterite CZTS thin films by RF magnetron sputtering of a single target by intuitively manipulating the target composition and post-sulfurization process carried out in a quasi-open environment using sulfur flakes. The influencing mechanism of post-sulfurization process was elucidated from systematic variation in the dwell time, temperature and the sulfur amount. A high temperature or a shorter dwell time yielded a small-grained microstructure associated with the presence of secondary phases. Phase pure CZTS films with better microstructural features were obtained for sulfurization at 500 °C for 60 min with 1.0 g of sulfur flakes. This film exhibited an optical bandgap of ∼1.58 eV indicating its photovoltaic potential. A device in the Mo/CZTS/Ag configuration showed typical features of a Schottky junction. The obtained current–voltage characteristic was analyzed to estimate saturation current, ideality factor and series resistance in correlation with the properties of the CZTS film.

Blue-shift in optical bandgap of sprayed nanocrystalline Cu2ZnSnS4 thin films induced by 200 MeV Xe swift heavy ions irradiation

Blue-shift in optical bandgap of sprayed nanocrystalline Cu2ZnSnS4 thin films induced by 200 MeV Xe swift heavy ions irradiation

Synthesis and Characterization of Cu2ZnSnS4 Thin Films Obtained by Combined Magnetron Sputtering and Pulsed Laser Deposition.

Cu2ZnSnS4 (CZTS) is a complex quaternary material, and obtaining a single-phase CZTS with no secondary phases is known to be challenging and dependent on the production technique. This work involves the synthesis and characterization of CZTS absorber layers for solar cells. Thin films were deposited on Si and glass substrates by a combined magnetron sputtering (MS) and pulsed laser deposition (PLD) hybrid system, followed by annealing without and with sulfur powder at 500 °C under argon (Ar) flow. Three different Cu2S, SnS2, and ZnS targets were used each time, employing a different target for PLD and the two others for MS. The effect of the different target arrangements and the role of annealing and/or sulfurization treatment were investigated. The characterization of the absorber films was performed by grazing incidence X-ray diffraction (GIXRD), X-ray reflectometry (XRR), Raman spectroscopy, scanning electron microscopy, and regular transmission spectroscopy. The film with ZnS deposited by PLD and SnS2 and Cu2S by MS was found to be the best for obtaining a single CZTS phase, with uniform surface morphology, a nearly stoichiometric composition, and an optimal band gap of 1.40 eV. These results show that a new method that combines the advantages of both MS and PLD techniques was successfully used to obtain single-phase Cu2ZnSnS4 films for solar cell applications.

Cu2ZnSnS4 thin film as a counter electrode in zinc stannate-based dye-sensitized solar cells

In this study, we deposited Cu2ZnSnS4 (CZTS) thin films with various thicknesses using electrodeposition and sputtering methods to exploit them as counter electrodes (CEs) in Zn2SnO4 -based dye-sensitized solar cells (DSSCs). The ternary Zn2SnO4 compound with wide bandgap energy, large corrosive resistivity, high electron mobility, and an appropriate conduction band edge position concerning the dye molecules (N719) can be an outstanding alternative for photoanode besides CZTS CE in DSSCs. On the other hand, CZTS is an impressive candidate as a CE material, but its electrocatalytic activity for the recovery of ionic species can be different depending on the synthesis process. Our results indicated the creation of porous morphology in the solution-based deposited films, which yields higher electrocatalytic activity in the CE performance in comparison with the physical deposition method, resulting in short circuit current densities of 9.77 and 8.40 mA cm−2 and open-circuit voltages of 633 and 583 mV for the DSSCs prepared by the respective techniques. The large active area and highly crystallized CZTS films, prepared by the electrodeposition method, led to an around 50% efficiency improvement compared to the widely used, more expensive platinum.

The Influence of Cd and Ba Incorporation on Microstructure and Photovoltaic Properties in Cu2ZnSnS4 Thin Film

Cd-doping and Ba-doping Cu2ZnSnS4 (CZTS) thin films solar cells were fabricated and compared. Precursors of Cu2ZnSnS4, Cu2(CdZn)SnS4 (CCZTS) and Cu2(BaZn)SnS4 (CBZTS) absorber layer were prepared by the spin coating method and annealed in a powdered sulfur atmosphere. The theoretical Cd or Ba composition was 0.3 according to the element ratio in the solution. The microstructure, morphology, photovoltaic performance of Cd/Ba doping CZTS were analyzed. Ba and Cd oxidation states can be found in the doping samples by XPS. The preferred (112) diffraction peak in CCZTS shifted to low angle apparently due to the increase of the lattice constant with the larger Cd2+ cations. The actual Ba-doping amount was low because Ba alkaline-earth metal easily reacts with H2O in the solution method. Raman spectra showed that the major peaks of CCZTS appeared a red-shift because of Cd-substitution for Zn, while CBZST displayed a blue-shift which could be related to the trigonal structure of CBTS. The stacks structure ZnO/ITO/CdS/CZTS/Mo with Cd/Ba doping solar cells were investigated. Through Cd incorporation the short-circuit current improved from 10.64 to 14.86 mA/cm−2 which is relate to the reduction of the grain boundary defects states, thus achieved conversion efficiency of 3.65%. CBZTS solar cells with Ba incorporation demonstrated the worst performance with inferior short circuit current and open circuit voltage, which originated from optical absorption losses and rough CBZTS/CdS interface with a great deal of recombination centers.

Impact of Solvent type on the Structural, Surface morphology and Optical energy gap of Kesterite Cu2ZnSnS4 thin films deposited by solvothermal method

In present work kesterite quaternary Cu2ZnSnS4 (CZTS) semiconductor thin films were prepared on fluorine-doped tin oxide (FTO) substrate by solvothermal method using different solvent types Ethylene glycol (EG), polyethylene glycol (PEG), and Ethylenediamine (ED). Crystal structure, surface morphology and optical characteristics of the prepared thin film were characterized by x-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS) and UV–Vis absorption spectroscopies. X-ray diffraction results reveal that the deposited films have polycrystalline nature and in a kesterite phase (tetragonal structure). the crystallite size is in the rang (20-27) nm and the largest one is for the prepared by using PEG solvent. The films surface morphology shows smooth and uniform surface with well distributed spherical particles with largest particles for the films deposited by EG solvent. The EDS spectra confirmed the the existence of the elements Cu, Zn, Sn and S of the films composition. The direct optical energy gap shows different values caused by using different solvent type, where the lower band gap is about 1.56 eV and the largest is about 1.9 eV for the films using ED and EG respectively.

Impact of stacking order and annealing temperature on properties of CZTS thin films and solar cell performance

In this study, Cu2ZnSnS4 (CZTS) thin films were synthesized by a two-stage process. In the first stage, CuSn/Zn/Cu (E-type) and CuSn/ZnS/Cu (B-type) stacked films were formed using the sputtering method. In the second stage, precursor films were annealed in sulfur atmosphere utilizing various annealing temperatures (500, 525, 550 and 575 °C) employing the Rapid Thermal Processing (RTP) method. The EDX measurements demonstrated that almost all the samples had Cu-poor and Zn-rich compositions, as targeted. The XRD patterns of all the CZTS samples were dominated by diffraction peaks of the kesterite CZTS phase. In addition to CZTS phase, Cu–S/Sn–S based secondary phases in all E-type CZTS thin films and some B-type CZTS samples annealed at lower temperatures (500 and 525 °C) were observed. The samples annealed at above 525 °C revealed purer crystal structure in terms of secondary phases and they have more promising crystallite size in both types of CZTS thin films. The Raman spectroscopy measurements confirmed the formation of kesterite CZTS phase and distinguished the formation of Cu2SnS3 (CTS) phase for some samples. The samples annealed at 550 °C presented purer structure for potential solar cell application. The SEM surface and cross-section images of all CZTS samples displayed dense and polycrystalline structures but samples annealed at 550 °C presented a larger-grained surface and cross-section structure in both types of CZTS films. PL spectra of the B-type CZTS samples exhibited a purer band structure with respect to E type CZTS samples according to their PL band values. The best solar cell performance was achieved with CZTS thin film prepared using CuSn/ZnS/Cu stack annealed at 550 °C temperature with 252 mV, 32 mA/cm2, and 3.79% parameters.

Influence of hydrogen plasma treatment on secondary phases in CZTS thin films for energy harvesting

Cu2ZnSnS4 (CZTS) thin films deposited using direct current magnetron sputtering and sulfurized at argon atmosphere pressures of 950, 460, and 50 mbar are studied in a view of solar cell absorber fabrication. The main novelty in the work is the influence of the radio frequency (RF) electromagnetic field treatment on the Cu2−xS secondary phase. The treatment reduces the amount of the Cu2−xS secondary phase, which is confirmed by Raman spectroscopy. The RF effect is long-term, at least one year later. So, this treatment is a promising technique to achieve higher purities of CZTS absorber layers for solar cells.

Preparation of CZTS thin films for the fabrication of ZnO nanorods based superstrate solar cells

Intensive studies have recently been performed to produce low-cost, environmentally friendly and high-efficiency cells. Some of these studies are associated with the incorporation of one-dimensional nanostructures into thin film based solar cell architecture. The aim of this work was therefore to build one of these structures by incorporating ZnO nanorod into the conventional Cu2ZnSnS4 (CZTS) thin film system. To achieve this, conformally CdS-coated vertically-aligned ZnO nanorods (NRs) were first successfully synthesized on ITO-coated substrates by using the hydrothermal technique. ZnO NRs were then decorated with 600 nm thick CZTS thin layer by a one-step thermal evaporation route to produce a SLG/ITO/ZnO-NR/CdS/CZTS/Ag superstrate solar cell architecture. The findings revealed the formation of Cu-poor mono-phase kesterite CZTS thin film with an optical band gap of 1.45 eV. The performance of the fabricated solar cell was measured both in the dark and under irradiation of (AM) 1.5 G at room temperature. The superstrate solar cell had a short-circuit current density of ~20.80 mA/cm2, an open-circuit voltage of ~0.46 V, a fill factor of ~29.5%, providing ~2.82% of the energy conversion efficiency.

Investigation of Cu2ZnSnS4 thin film with preannealing process and ZnS buffer layer prepared by magnetron sputtering deposition

Unlike traditional methods for fabricating a CdS buffer layer through chemical bath deposition in thin-film solar cells, this study fabricated a ZnS thin film by sulfurizing the Zn film that was deposited using a magnetron sputtering system at varied temperatures from 500° to 560 °C in a sulfurization furnace. The ZnS thin film have been used as a buffer layer on the Cu2ZnSnS4 absorption layer. In this study, the Cu2ZnSnS4 absorber layer was prepared by first depositing a copper–zinc–tin (CZT) precursor film with a metal stack order of Zn–Sn–Cu–Zn through magnetron sputtering, followed by a preannealing process on the CZT precursor, to promote the interdiffusion of elements for the transfer of the metal phase into the alloy phase. Experimental results revealed the increased crystallite size in the SEM observation, an enhanced Raman intensity at 336 cm−1, and a reduced FWHM value in the XRD study for the Cu2ZnSnS4 thin film fabricated at a sulfurization temperature of 510 °C for 20 min with a preannealing treatment of the CZT precursor at a temperature of 420 °C for 30 min. In addition, the improved optical transmittance to 81.3% and an increased optical band gap value to 3.28 eV due to the reduced grain boundaries was obtained for the ZnS film with improved crystallinity quality fabricated at a sulfurization temperature of 560 °C.

Influence of H2S on the growth of sputter deposited Cu2ZnSnS4 thin film

Cu2ZnSnS4 (CZTS) is considered as a suitable absorber layer for thin film solar cells. In the present study, the effect of flow rates of H2S on the fabrication of CZTS thin films are studied in detail. RF magnetron sputtering in the presence of Ar and H2S gas was adopted to deposit sulphides of Cu, Sn and Zn, followed by annealing at higher temperature in H2S ambience in the same chamber to complete the crystallization process. Raman spectra showed peaks corresponding to kesterite CZTS and new impurity peaks observed while the H2S flow rate increases. XRD result helped to identify various secondary phases present along with CZTS phase. It was revealed by FESEM images that the samples prepared at different H2S flow rates exhibiting different nano shapes. Compositional study indicated that the Zn/Sn ratio decreases with H2S flow rate. Surface roughness was calculated by AFM technique and big sulphur aggregates were present on the surface of the sample prepared at high H2S flow rate. Optical bandgap value of the CZTS films was varied from 1.28 to 1.74 eV and electrical parameters were calculated for samples prepared at lower H2S flow rate. This study presents the importance of choosing appropriate H2S flow rates for the fabrication of CZTS thin films.

Growth of Cu2ZnSnS4 thin films by hybrid chemical approach

Thin films of Cu2ZnSnS4 (CZTS), an emerging solar cell absorber layer, were prepared by a hybrid chemical process followed by annealing. In this process, two binary layers, SnS and ZnS were grown sequentially using chemical bath deposition method (CBD) onto soda-lime glass substrates. Subsequently the ZnS layer was partly converted into CuxS layer through an ion-exchange process by immersing the stack glass/SnS/ZnS in CuCl2 solution. The resulting stack, namely glass/SnS/ZnS/CuS, was annealed in sulphur atmosphere at 450 °C for an hour to promote inter-diffusion of the layers leading to the formation of CZTS film through the solid-state reaction path. The stack with a ZnS layer of 470 nm thick and 270 nm thick SnS led finally to the desired CZTS phase formation. XRD analysis of these films showed CZTS peaks which are close to the Cu2SnS3 (CTS) phase. Raman spectra recorded to distinguish CTS and CZTS phases showed modes at 291, 338 cm−1 corresponding to CZTS phase confirming its formation. The band gap of CZTS films is found to be 1.42 eV. The films were found to be p-type in nature. An attempt is made to fabricate heterojunction solar cell.

Effect of sulfurization temperature on Cu-Zn disorder for non-stoichiometric spray pyrolyzed Cu2ZnSnS4 thin films

Cu2ZnSnS4(CZTS) thin-films were synthesized using chemical spray pyrolysis technique followed by sulfurization at different temperatures via the chemical vapor deposition technique. The Cu-Zn disorder for the films sulfurized at various temperatures was investigated using the near-resonance Raman spectra with an excitation wavelength of 785 nm. The sulfurized films exhibited optimal non-stoichiometric Cu-poor and Zn- rich composition to achieve higher solar cell efficiency. The CZTS films sulfurized at a temperature of 550 °C exhibited higher Cu-Zn order.

Study on the Optimization of Cu-Zn-Sn-O to Prepare Cu2ZnSnS4 Thin Film via a Nano Ink Coating Method.

To reduce the formation of the impurity phase, a buffer volume can be used to expands and smooths the surface of Cu2ZnSnS4(CZTS) thin film. In this study, a Cu-Zn-Sn-O(CZTO) precursor was synthesized through the process of coprecipitation-calcination-ball milling-spin coating. The influence of pH, temperature, and PVP on the constituent of hydroxides was investigated in the process of coprecipitation. Cu-Zn-Sn-O with appropriate compositions could be obtained by regulating the temperature and preservation time of the calcination stage. After ball milling to form a nano ink, and then spin coating, SEM images proved the generation of CZTO precursors, which effectively promoted the formation of Cu2ZnSnS4 thin films. Finally, the phase, microstructure, chemical composition, and optical properties of the Cu2ZnSnS4 thin films prepared by sulfurized annealing CZTO precursors were characterized by EDX, XRD, Raman, FESEM, Hall effect, and UV methods. The prepared CZTS thin film demonstrated a band gap of 1.30 eV, which was suitable for improving the performance of CZTS thin film solar cells.

Effect of sodium-doping on the performance of CZTS absorb layer: Single and bifacial sodium-incorporation method

In current work, sodium doped CZTS films are deposited onto Mo-coated stainless-steel foil for fabrication of flexible solar cells. New strategies for sodium incorporation of Cu2ZnSnS4 (CZTS) thin films are introduced and investigated. In detail, bilayer NaF film was deposited at the bottom and on the top of CZTS films to obtain a uniformly doped CZTS film. For comparison, CZTS films were also doped with a single layer NaF from bottom and top, respectively. Secondary ion mass spectroscopy (SIMS) is applied to investigate the distribution of different elements along the depth, X-ray diffraction and Raman spectroscopy are used to identify the crystallinity and secondary phases. Finally, solar cells are assembled based on the prepared CZTS films with an efficiency of 2.3%. Although the obtained devices show a far lower efficiency compared with other reports, current research provides a feasible path for sodium incorporation when fabricating CZTS films on Na-free substrates.

Sol-gel prepared Cu2ZnSnS4 (CZTS) semiconductor thin films: Role of solvent removal processing temperature

The conditions under which thin films are deposited are the most important parameter determining the physical performance of devices. In this work, we present the effect of preheating temperatures under N2 ambient conditions on the quality of Cu2ZnSnS4 (CZTS) thin films. Herein we are interested in the structural, morphological, vibrational, optical and electrical characteristics of CZTS layers, synthesized using the sol-gel method. CZTS layers were deposited at preheating temperatures of 200 °C, 250 °C and 300 °C under N2 ambient atmosphere prior to a 1-h sulphuration process at 580 °C. XRD results confirm the dependence of the crystallite size on temperature. All the deposited layers are in stannite phase, confirmed by Raman spectroscopy, and the electrical properties show that all films obtained are p-type semiconductors with a carrier charge concentration of 4.06–9.48 × 1020 cm−3. CZTS films preheated at 300 °C show better crystallinity, larger grains, more suitable band gap, higher electrical conductivity and improved surface morphology compared to the rest of the thin films, which indicates that the method proposed here can be used to deposited high quality absorber layers of CZTS for solar cells applications.

Influence of Na2S treatment on CZTS/Mo interface in Cu2ZnSnS4 solar cells annealed in sulfur-free atmosphere

The Na2S-treated CZTS/Mo interface in back contact of Cu2ZnSnS4 (CZTS) thin films solar cell was reported firstly. The CZTS thin films were deposited on Mo/Glass substrates by radio frequency magnetron sputtering using a copper-zinc-tin-sulfur composite target and annealed in a sulfur-free atmosphere. The element composition of excess-sulfur CZTS target was modified to Cu:Zn:Sn:S=4.12:1.61:1:17.65. The microstructure, morphology, optical and electrical properties of Cu2ZnSnS4 were analyzed. It was found that the sulfur-free annealing suppressed the formation of resistance MoS2 layer at CZTS/Mo interface. The CZTS/Mo interface was treated in a Na2S solution which promoted a better crystallization of CZTS absorbing layer.The optical band gap (Eg) value were found to be approximately ~1.47 eV and ~1.50 eV for CZTS and Na:CZTS samples. Finally, the photovoltaic properties of ZnO/ITO/CdS/CZTS/Mo solar cells were investigated. The open circuit voltage increased from 510.28 to 622.16 mV which ascribes to the recombination center diminishing through Na passivation. The conversion efficiency was improved from 2.37% to 3.26% after Na2S-treated method.

Band alignment tuning at Mo/CZTS back contact interface through surface oxidation states control of Mo substrate

The molybdenum (Mo) back contact is one of the crucial factor that affects the performance of kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. Thick MoS2 which is formed during sulfurization process of CZTS is known to be harmful for the device performance. MoOx on surface of Mo substrate with proper thickness can enhance the efficiency of CZTS solar cells through reducing the thickness of MoS2 layer and tailoring the work function of back contact. However, the role of oxidation states of Mo in MoOx on the performance of solar cells is unclear. Herein the effect of MoOx with different oxidation states on CZTS absorber and intermediate MoS2 layer were investigated. Our results shown that the efficiency of CZTS solar cells on Mo substrate with proper surface oxidation status (MoO2.03) can be as high as 2.86 times of the one fabricated on higher oxidized Mo (MoO2.21), although the thinnest MoS2 layer was present in the latter case. This work discloses the origin of the oxidation states of Mo for the efficiency enhancement of kesterite structured solar cells. The results will be also helpful to improve the performance of compound semiconductor by oxidation states control strategy.