Mo-coated Glass Research Articles

Comparative study of Cu 2 ZnSnS 4 thin film solar cells fabricated by direct current and pulse reverse co-electrodeposition

Abstract Cu2ZnSnS4 (CZTS) thin films have been fabricated by sulfurization of direct current and pulse reverse co-electrodeposited CZTS precursors on Mo-coated glass substrates. The CZTS thin film synthesized using pulse reverse co-electrodeposition exhibits homogeneous and large grains, Cu-poor and Zn-rich compositions and free of Cu2S secondary phase compared with that obtained from direct current co-electrodeposition. By using pulse reverse co-electrodeposition, the performance of p–n junction is improved with low diode quality factor (A) and reverse saturation current density (J0), and the CZTS device first reaches an efficiency of 6.28% (Voc = 609 mV, Jsc = 18.4 mA/cm2, FF = 56.1%) threshold, which is higher than that of 4.69% (Voc = 561 mV, Jsc = 16.4 mA/cm2, FF = 50.9%) using direct current co-electrodeposition. This result offers a novel research direction for preparing high-efficiency CZTS thin film solar cells.

Probing diffusion of In and Ga in CuInSe2/CuGaSe2 bilayer thin films by x-ray diffraction

The bilayer thin films of CuInSe2 (CIS) and CuGaSe2 (CGS) are fabricated by a molecular beam deposition (MBD) technique by sequential depositions of CGS followed by CIS and vise versa on Mo-coated soda-lime glass (SLG) and Mo/Al2O3-coated (Na blocking) SLG substrates. The thicknesses of CIS/CGS layers are adjusted to have the overall x=[Ga]/([In]+[Ga]) at approximately 0.4 similar to what is generally used in CIGS thin film solar cells. The growth temperature and the Cu-ratio y=[Cu]/([In]+[Ga]) of the CIS and CGS layers are systematically varied for Cu-rich (y>1) and Cu-poor (y<1) conditions. X-ray diffraction (XRD) technique is used to analyze the shift of diffraction peaks of the preferred orientations of the bilayers compared to those of the single-layer CIS, CGS and CIGS thin films. The XRD results show that higher Ga diffusion is observed in the Cu-rich CIS/CGS bilayer rather than others. The results suggest that the diffusion of Ga is enhanced by the excess Cu–Se phase that is dependent upon the substrate temperature. The Na from the SLG substrate is one of the important factors affecting Ga diffusion. The results show alloying CIGS patterns rather than separated CIS and CGS patterns as observed in the bilayers with Na.

Phase segregations and thickness of the Mo(S,Se)2 layer in Cu2ZnSn(S,Se)4 solar cells at different sulfurization temperatures

The appearance of phase segregations in the kesterite Cu2ZnSn(Sx,S1−x)4 (CZTSSe) absorber layer and formation of the interfacial Mo(S,Se)2 layer during the annealing process is a challenge and limitation for the inderstrization of highly efficiency and low-cost thin film solar cells (TFSCs). These two factors are effectively controlled by sulfurization temperature. In this study, kesterite CZTSSe thin films were successfully prepared via the sulfurization of co-evaporated Cu–Zn–Sn–Se precursor, which was deposited on Mo-coated glass substrates at sulfurization temperatures ranging from 520°C to 580°C. The structure and phase of the sulfurized CZTSSe films were characterized by conventional XRD and Raman analyses. The thickness of the Mo(S,Se)2 layer was estimated based on cross-sectional FE-SEM images. Advanced characterizations, such as STEM images and STEM EDS analyses of the sulfurized CZTSSe films were performed to detect the sulfurization temperature dependent phase segregation and localized fluctuations in the elemental composition of the sulfurized CZTSSe films. The formation of secondary phases was clearly evident at the bottom region in the CZTSSe absorber thin films prepared over sulfurization temperature of 540°C. The performances of the devices were strongly related to the sulfurization temperature, thickness of the interfacial Mo(S,Se)2 layer and proportion of secondary phases in the kesterite CZTSSe absorber layers. The highest power conversion efficiency was 5.52% and was achieved by the CZTSSe absorber layer prepared at 540°C (Voc:0.575V, Jsc:18.14mA/cm2 and FF: 52.94%).

Cu(In,Ga)(Se,Te)2 pentenary thin films formed by reaction of precursor layers

Sodium doped CuIn(1−x)Gax(Se(1−y)Tey)2 (CIGST) thin films were grown on Mo-coated soda lime glass substrates by a two-stage technique. The effects of Te content and annealing temperature on the microstructural characteristics and phase segregation in CIGST thin films were studied. It was observed that presence of Ga in these films affected the kinetics of the reaction and phase formation and largely eliminated the separation of the Te-rich phase towards the Mo surface, which was previously observed under the two-stage processing conditions in films containing only Cu, In, Se and Te. Microprobe measurements showed that the smaller grain material visible through the gaps within the larger grain material had higher Ga and Se content compared to larger grains, which were richer in Te and In. For CIGS film formation, an annealing regime that involved a low temperature partial reaction step followed by a high temperature reaction step yielded better films compared to a process with a single high temperature step. However, for films containing both Se and Te, the two temperature processing approach caused excessive phase separation.

Sulfurization temperature dependence of the structural transition in Cu2FeSnS4-based thin films

Cu2FeSnS4 (CFTS)-based thin films have been prepared on Mo-coated glass substrates by magnetron sputtering technique combined with a post-sulfurization processing. Energy dispersive X-ray measurement indicates that the Sn content decreases with the increasing sulfurization temperature, and all samples are in Cu-poor and Fe-rich states. X-ray photoelectron spectroscopy and X-ray diffraction show that all phases belong to the CFTS and no impure phase, e.g. Cu2SnS3, can be discovered in thin films. However, samples exhibit a phase transition behavior from rhodostannite to stannite structure with the increasing sulfurization temperature. The frequency of A1 mode of CFTS with rhodostannite structure is estimated to be 323.8cm−1 by using the empirical model, which coincides with the experimental value as measured by Raman spectra analysis. These results are helpful to understand the properties of CFTS-based thin films.

Effect of UV irradiation on Cu2ZnSnS4 thin films prepared by the sol–gel sulfurization method

Cu2ZnSnS4 (CZTS) thin films were fabricated on Mo-coated soda lime glass substrates by the sol–gel sulfurization method, which is a non-vacuum process. UV irradiation was introduced to the drying process, resulting in a significant increase in the grain size and density as well as a remarkable improvement in the crystallinity of the CZTS films. In addition, sulfurization of the Mo substrate was suppressed due to the increased density. We confirmed that the carbon/metal ratio in the precursor increased as a result of the UV irradiation.

Suppression of the &lt;inline-formula&gt;&lt;tex-math&gt;${\hbox{Cu}}_{2-x}{\hbox{S}}$&lt;/tex-math&gt;&lt;/inline-formula&gt; Secondary Phases in CZTS Films Through Controlling the Film Elemental Composition

Kesterite Cu2ZnSnS4 (CZTS) thin films were grown by the sulfurization of stacked metal precursors deposited using radio-frequency magnetron sputtering on Mo-coated soda-lime glass substrates. In this paper, we report the role of the film chemical composition in the evolution of ${\hbox{Cu}}_{2-x}{\hbox{S}}$ phases and how to avoid their development through controlling the film composition. Furthermore, the effect of the elemental concentration on the structural and morphological properties of the final CZTS films has been investigated. The prepared CZTS films have a composition ratio M = Cu/(Zn + Sn) varying from 0.81 (Cu-poor) to 1.05 (Cu-rich). X-ray diffraction and Raman scattering studies revealed the presence of ${\hbox{Cu}}_{2-x}{\hbox{S}}$ phases in films with a Cu/(Zn + Sn) ratio higher than 1.00 and/or in films with a Sn/Cu ratio close to or less than the stoichiometric value of 0.50. However, ${\hbox{Cu}}_{2-x}{\hbox{S}}$ -phases-free CZTS films were achieved with Sn/Cu ratios sufficiently above 50% without regard to the Cu/(Zn + Sn) ratio. Plan and cross-sectional scanning electron microscopy showed compact films, in general. Electron back-scattered diffraction revealed randomly oriented CZTS films.

Cu(In,Ga)(S,Se)₂ thin film solar cell with 10.7% conversion efficiency obtained by selenization of the Na-doped spray-pyrolyzed sulfide precursor film.

Selenium-rich Cu(In,Ga)(S,Se)2 (CIGSSe) thin films on an Mo-coated soda-lime glass substrate were fabricated by spray pyrolysis of an aqueous precursor solution containing Cu(NO3)2, In(NO3)3, Ga(NO3)3, and thiourea followed by selenization at 560 °C for 10 min. We studied the effects of intentional sodium addition on the structural and morphological properties of the fabricated CIGSSe films by dissolving NaNO3 in the aqueous precursor solution. The addition of sodium was found to affect the morphology of the final CIGSSe film: the film had denser morphology than that of the CIGSSe film obtained without addition of NaNO3. Photoelectrochemical measurements also revealed that the acceptor density of the nondoped CIGSSe film was relatively high (N(a) = 7.2 × 10(17) cm(-3)) and the addition of sodium led to a more favorable value for solar cell application (N(a) = 1.8 × 10(17) cm(-3)). As a result, a solar cell based on the sodium-modified CIGSSe film exhibited maximum conversion efficiency of 8.8%, which was significantly higher than that of the cell based on nondoped CIGSSe (4.4%). In addition, by applying MgF2 antireflection coating to the device, the maximum efficiency was further improved to 10.7%.

Characterization of in-situ annealed sub-micron thick Cu(In,Ga)Se2 thin films

Sub-micron thick Cu(In,Ga)Se2 (CIGS) thin films were deposited on Mo-coated soda-lime glass substrates under various conditions by single-stage co-evaporation. Generally, the short circuit current (Jsc) decreased with the decreasing thickness of the absorber layer. However, in this study, Jsc was nearly unchanged with decreasing thickness, while the open circuit voltage (Voc) and fill factor (FF) decreased by 31.9 and 31.1%, respectively. We believe that the remarkable change of Voc and FF can be attributed to the difference in the total amount of injected thermal energy. Using scanning electron microscopy, we confirmed that the surface morphology becomes smooth and the grain size increased after the annealing process. In the X-ray diffraction patterns, the CIGS thin film also showed an improved crystal quality. We observed that the electric properties were improved by the in-situ annealing of CIGS thin films. The reverse saturation current density of the annealed CIGS solar cell was 100 times smaller than that of reference solar cell. Thus, sub-micron CIGS thin films annealed under a constant Se rate showed a 64.7% improvement in efficiency.

Study of alternative back contacts for thin film Cu2ZnSnSe4-based solar cells

Cu2ZnSnSe4 thin film solar cells are usually fabricated on a soda lime glass substrate with a molybdenum (Mo) back contact. It is suspected that degradation in electrical performance occurs due to the formation of a barrier between the absorber and Mo back contact. To overcome such degradation, Titanium Nitride (TiN), Titanium Tungsten (TiW), Chromium (Cr), Titanium (Ti) and Aluminum (Al) deposited on Mo-coated glass substrates are investigated as alternative back contact materials. Physical and electrical characterization as well as photoluminescence measurements are performed. Compositional analysis of the absorber layer on the metallized substrates identifies Mo, TiN and TiW as being the most inert during the formation of Cu2ZnSnSe4. On the other hand, Ti and Cr reacted with Se during selenization, thereby affecting the growth of the absorber, leading to low conversion efficiency. For Al, the absorber layer was etched after the standard potassium cyanide etch, hence, cannot be used as a back contact. The best device efficiencies obtained are 8.8% on TiN, 7.5% on Mo and 5.9% on TiW, respectively. The TiN back contact provides the lowest barrier value of about 15 meV which could be considered as a good ohmic contact.

Electrical characterization of Cu(In,Ga)Se2 thin films peeled off from Mo-coated soda-lime glass substrate by AC Hall measurement

We have developed a new evaluation method for electrical properties of Cu(In,Ga)Se2 (CIGS) grown on a Mo-coated soda-lime glass (SLG). The method consists of the peel-off process and the AC Hall measurement, which enables us to evaluate CIGS films grown on the Mo electrode. It was found, from the measurement, that the hole concentration of CIGS grown on a Mo-coated SLG was approximately two orders of magnitude higher than that on a SLG, suggesting the Na-doping effect. Furthermore, the hole mobility of 0.47 cm2/(V·s) was simultaneously measured, even though the film was deposited on the Mo electrode.

Microstructural and chemical properties of ZnO films formed using electrodeposition.

We investigated the effect of bath temperature and electrodeposition potential on the microstructural and chemical properties of ZnO films formed on Mo-coated soda-lime glass substrates using electrodeposition. The electrodeposition was performed using an electrolytic solution containing 0.05 M Zn(NO3)2 for 6 min. The ZnO islands grew larger to impinge with other islands until the bath temperature was increased up to 40 degrees C, above which continuous ZnO film was eventually formed. An increase in the electrodeposition potential resulted in enhancement of the growth rate of the electrodeposited ZnO film with the facilitation of film texturation. The c-axis was perpendicular to surface, which could be associated with the preferential orientation along the (002) direction. At the electrodeposition potential of -1.3 V (vs. a saturated calomel electrode), significant amounts of hydrogen bubbles that electrochemically evolved near the surface of the working electrode hampered the homogenous growth of the ZnO film, which could be responsible for morphological degradation of the ZnO film.

Effects of photo-assisted electrodeposited on CuInSe2 thin films.

Photo-assisted one-step electrodeposition has been applied to help in forming smooth and dense CuInSe2 films. The difference in surface morphology and crystalline quality between CuInSe2 films with various photo-assistance has been investigated. In the photo-assisted electrodeposition process, the many kinds of lamps providing maximum light intensity at about 380 to 620 nm were used as light source to be irradiated onto the surface of Mo-coated soda-lime glass substrates. The results suggested effects of photo-assistance including activating surface diffusion and growing high-crystalline quality films with reduced defects during electrodeposition.

Preparation of Cu2ZnSnS4 thin films using spin-coating method with thermolysis and annealing

Cu2ZnSnS4 (CZTS) thin films are prepared using a spin-coating method with thermolysis and annealing on Mo-coated soda lime glass substrates. The thermogravimetric analysis, differential scanning calorimetry and Fourier transform infrared spectra of precursors are measured to identify the pyrolytic temperature of CZTS thin films. It is found that the decompositions of organics mainly occur before 280 °C. The phase compositions of prepared thin films are identified to be CZTS kesterites without other binary or ternary phase by X-ray diffractometer and Raman spectroscopy. It is observed through the scanning electron microscopy that there are not obvious carbon-rich layer in the interface and the CZTS thin films are composed of large densely packed grains. In addition, the structures of CZTS thin films are analyzed further by high-resolution transmission electron microscope and X-ray photoelectron spectroscopy. The resistivity and photoelectric response of CZTS thin film are also characterized, which demonstrate its potential for application in solar cells.

In-situ monitoring of CuInSe2 thin films growth by light scattering

Light scattering spectroscopy, mainly developed for Cu(In,Ga)Se2 thin films growth, has been demonstrated to be an efficient in-situ monitoring tool. It consists of illuminating the substrate surface using a light source and collecting the diffused light with a spectrometer. The on-line information allows monitoring the point of stoichiometry of the grown layer. In this work, we propose an alternative approach of this method, replacing the spectrometer by a standard webcam. This system allows to precisely choose the analyzed area on the substrate surface, which may be the whole sample surface or just a specific zone. We have grown CuInSe2 (CIS) thin films by co-evaporation under vacuum using the three-stage process. We have simultaneously performed CIS growth on both Mo-coated and bare glass substrates within the same growth run, while, on purpose, the scattered light was collected from the Mo-coated substrate. The CIS thin films were characterized by X-ray diffraction, atomic force microscopy, Hall effect measurements, absorption spectroscopy and energy dispersive spectroscopy for composition. By interrupting the CIS growth at different stages of the growth process, we have correlated the scattered light features with the Cu-rich/In-rich transitions and we observe the reproducibility of the scattered light spectra structures.

Influence of precursor thin films stacking order on the properties of Cu2ZnSnS4 thin films fabricated by electrochemical deposition method

We fabricated Cu2ZnSnS4 (CZTS) thin films by electrochemically depositing precursor stacks on Mo-coated glass in a variety of orders: Cu/Sn/Cu/Zn, Cu/Zn/Cu/Sn, Zn/Cu/Sn/Cu, and Sn/Cu/Zn/Cu. Using Raman spectroscopy and X-ray diffraction, we found that for all stacking orders the annealed film was composed of a single CZTS phase with good crystallinity and strong (112) orientation. For the Cu/Sn/Cu/Zn stack, field-emission scanning electron microscopy revealed a homogeneous, compact surface morphology and large columnar grains. This stack also had an optical absorption coefficient of >104cm−1 and an optical band gap of 1.51eV. We fabricated a solar cell with the structure SLG substrate/Mo/Cu2ZnSnS4/CdS/i-ZnO Al:ZnO/Al, which achieved a conversion efficiency of 2.3%.

Effect of complexing agents on the electrodeposition of Cu–Zn–Sn metal precursors and corresponding Cu2ZnSnS4-based solar cells

Copper zinc tin sulfide (Cu2ZnSnS4; CZTS) precursors were electrochemically deposited on Mo-coated glass substrates from aqueous solutions following by annealing. The effect of three complexing agents, trisodium citrate, ethylenediamine tetraacetic acid and tartaric acid, on the structural, morphological and compositional properties of CZTS thin films was investigated. The annealed thin films were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy, and energy-dispersive X-ray spectroscopy (EDS), UV–vis absorption and Raman scattering spectroscopies. XRD patterns of the synthesized films revealed the preferred orientation of the (112), (220), (200) and (312) planes, confirming the kesterite structure of CZTS. The film prepared without a complexing agent was uneven with some pores and cracks, whereas those prepared using complexing agents exhibited good coverage with some overgrown particles on the surface of the films. EDS analysis revealed that the films deposited using trisodium citrate were nearly stoichiometric for CZTS. Voltammetric studies suggested that trisodium citrate is a suitable complexing agent because it helps to align the deposition potentials of Cu, Zn, and Sn. UV–vis absorption spectra confirmed the films possessed a direct bandgap between 1.33 and 1.47eV, which is quite close to the optimum value for a semiconductor material as an absorber in solar cells. We fabricated solar cells with the structure soda-lime glass/Mo/CZTS/CdS/i-ZnO/ZnO:Al/Al with a maximum conversion efficiency of ∼2.9%.

Galvanostatically-electrodeposited Cu–Zn–Sn multilayers as precursors for crystallising kesterite Cu2ZnSnS4 thin films

Metallic stacked layers containing Cu, Zn and Sn on Mo-coated glass substrates were prepared by galvanostatic electrodeposition as precursors for crystallising kesterite Cu2ZnSnS4 films. Due to the nature of the MoOx passivating layer on the Mo surface, initial trials were particularly addressed to deposit strong adhesive precursors by firstly depositing Cu layers on Mo-coated substrates from acidic as well as alkaline solutions. It was found that the acidic Cu solutions were not appropriate for the Cu deposition on Mo-coated glass substrates, since they led to an inhomogeneous coverage of the Cu deposits in addition to extremely poor adhesion. In contrast, utilising an alkaline Cu solution yielded a homogeneous coverage of the Cu deposits, having strong adhesion to the Mo-coated glass substrates, which was essential for the subsequent acidic Zn or Sn galvanostatic electrodepositions. The crystallisation of kesterite Cu2ZnSnS4 from metallic stacked Cu/Sn/Zn precursors was investigated as well. The sulfurisation process crystallises the Cu–Zn–Sn precursor films into films exhibiting predominant kesterite phase. This result demonstrates that the established galvanostatic electrodeposition technique for metals has a technological potential for preparing kesterite precursors.

Influence of substrate temperature on the properties of electrodeposited kesterite Cu2ZnSnS4 (CZTS) thin films for photovoltaic applications

We prepared Cu2ZnSnS4 (CZTS) films on Mo-coated glass substrates by using electrodeposition in an acidic electrolyte containing Cu, Zn, and Sn species. We examined how the substrate temperature influenced the compositional, structural, morphological, and electrical properties of the CZTS films by using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), Raman spectroscopy, energy dispersive spectroscopy, and ultraviolet–visible absorption spectroscopy. The film sulfurized at a substrate temperature of 580 °C exhibited the best characteristics because of its large grain size and low number of voids. At a sufficiently high temperature, this fabrication process yielded single-phase CZTS with no secondary phases. The grain size increased with deposition temperature: the size at 580 °C was about twice that at 460 °C. All the CZTS films were Cu-rich and S-poor, with the composition changing at 580 °C. XRD revealed that the films had nanocrystalline kesterite structures, indicated by intense, sharp (112), (200), (220), and (312) diffraction peaks. The crystallite size was 34.6–57.3 nm, increasing with substrate temperature. FESEM indicated that the morphology improved with substrate temperature. The optical band gaps of the films were 1.36–1.49 eV, and their absorption coefficients were on the order of 10−4 cm−1, making them quite suitable for use in solar cells. We fabricated solar cells with a structure of soda-lime glass/Mo/Cu2ZnSnS4/CdS/i-ZnO/ZnO:Al/Al and achieved a maximum conversion efficiency of ~2.69 %.

Chemical bath deposition route for the synthesis of ultra-thin CuIn(S,Se)2 based solar cells

CuIn(S,Se)2 (CISSe) photovoltaic grade thin films are usually grown by expensive vacuum based methods or chemical routes that require highly toxic precursors. In this work, we present the synthesis of CISSe absorbers by a simple chemical bath deposition (CBD) route. In the first step, In2S3/Cu2−xS stack was deposited as a precursor by CBD on Mo-coated soda lime glass substrates, using respectively thioacetamide and N,N′-dimethylthiourea as S source. Then the CISSe thin films were synthesized by the precursor's selenization at 450°C. The obtained films were characterized by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). The tetragonal chalcopyrite structure of CISSe was identified by XRD and Raman, confirming that the major part of S was replaced by Se. SEM images show a compact and homogeneous film and by cross-section the thickness was estimated to be around 700nm. Solar cells prepared with these absorbers exhibit an open circuit voltage of 369mV, a short circuit current density of 13.7mA/cm2, a fill factor of 45% and an efficiency of 2.3%.