CTS Solar Cells Research Articles

Influence of thiourea concentration during deposition of a CdS buffer layer on the electric properties of Cu2SnS3 solar cells

The p–n interface state and depletion layer region of solar cells were investigated using electrochemical impedance spectroscopy (EIS) measurements to elucidate the effect of varying thiourea (TU) concentrations in chemical bath deposition (CBD) on the photovoltaic properties of Cu2SnS3 (CTS) solar cells. The photoconversion efficiency (PCE) increased as the TU concentration increased, peaking at 0.16 M and decreasing thereafter. Additionally, the constant phase element-p value, which is indicative of the p–n interface state in CTS solar cells, exhibited a similar trend to that of the PCE of CTS solar cells with increasing TU concentration. The decrease in the PCE was due to the electron recombination that occurred at the p–n interface of CTS solar cells with increasing TU concentrations. Therefore, to improve the PCE of the CTS solar cells, the TU concentration in CBD must be optimized because the p–n interface state is sensitive to changes in the growth conditions of CdS films. By studying the effects on the p–n interface state and depletion layer in CTS solar cells using EIS measurements, useful knowledge can be obtained to increase the PCE of all-sulfide-compound solar cells using n-type CdS films as well as CTS.

Exploring the correlation between the bandgap engineering and defect density toward high CTS solar cell efficiency

Ternary chalcogenide Cu2SnS3 (CTS) has emerged as a relevant compound for solar energy harvesting owing to its favorable optoelectronic properties. The aim of this study was to conduct a numerical investigation using SCAPS-1D software to determine the optimal conditions for an efficient CTS solar cell. The research focused on how the bandgap (Eg) design affects the optical properties and photovoltaic performance (PV) of a CTS solar cell. The correlation between the Eg width and bulk defect density (Nt), as well as the CTS/CdS interface defect density (Nit) of CTS thin films, was also investigated. The results revealed that the increase in Eg significantly improves the external quantum efficiency (EQE), power conversion efficiency (PCE), and open-circuit voltage (Voc) of the solar cells. In addition, a remarkable decrease in recombination rate was observed. For Eg values of 1.1 eV and 1.18 eV, the solar cell demonstrated a minimal recombination rate of 5 × 1020 cm−3 and a high PCE of 5%. The contour plot of Nt as a function of Eg confirmed that Eg and Nt values of 1.18 eV and 5 × 1016 cm−3, respectively, provide higher efficiency. Furthermore, the results highlighted the importance of limiting Nit to below 5 × 1014 cm−3 at the CTS/CdS junction. Under optimum conditions, the output parameters of the CTS solar cell were calculated to be 6.30% for the PCE, 46.64% for the fill factor (FF), 33.89 mA/cm2 for the short-circuit current (Jsc), and 398.2 mV for Voc.

Correlation between some physical properties of pure and Sb doped Cu2SnS3 thin films under the effect of sulfur amount for solar cell application

Adjusting of sulfur content and amount of doping in Cu2SnS3 (CTS), as absorber layer for solar cell, is a promising approach to fabricate thin film solar cells with high performance. In this work, the effect of different amounts of sulfur in the sulfurization process and the percentage of Sb doping in the films were studied. The physical properties of un-doped and doped CTS thin films were investigated. The pure and doped CTS thin films showed a monoclinic structure, with a larger grain size under the influence of Sb doping. XPS analysis demonstrated that Cu, Sn, S, and Sb oxidation states were +1, +4, −2, and +3, respectively. The Sb-doped sample for 25 min (25Sb) has a higher Cu+1/Cu+2 ratio than other samples and no satellite peak of Sn+2 was observed as compared to other levels of doping samples. This concluded that the Cu species in the 25Sb film experienced a more Cu-rich chemical environment. Both PL and UV–Vis spectroscopy were used to obtain the optical properties. Hall Effect was applied to measure the electrical properties of un-doped and doped CTS films. The pure and Sb-doped CTS solar cells were fabricated by optimization of Sb doping time and sulfur amount. The conversion efficiency of the prepared solar cell was 2.13% when fabricated with CTS layer that treated with 25 min of Sb doping and 100 mg of sulfur, whereas the efficiency of the un-doped CTS solar cell at the same sulfur amount was just 0.615%.

Effect of Li Doping on Cu2SnS3/n‐Si Heterojunction Solar Cells: Experiments and Simulation

AbstractCu2SnS3 (CTS) semiconductor material owns very interesting absorbing properties that allow it to be effectively utilized in many thin‐film‐based heterojunction solar cells. In this work, using the ball milling method, a mixture of pure elemental powders is used to synthesize CTS material. Homemade undoped and Li‐doped target pellets are used to form CTS‐doped and CTS‐undoped thin films. The crystallinity investigation of synthesized targets and thin films confirms the formation of CTS tetragonal phase. Compositional, morphologic, and optic studies are also made to examine the effect of Li atom incorporation on different properties of CTS thin films. These CTS thin films are also incorporated into heterojunction solar cells (Al/n‐Si/CTS/Ag and Al/n‐Si/Li0.03CTS/Ag). In addition, J–V characteristics of both CTS and Li0.03CTS solar cells are demonstrated and discussed in details. The effect of CTS thin films' thickness on the performance is studied using Solar Cell Capacitance Simulator (SCAPS‐1D) program. The results of the calculation are discussed in details and compared with the experimental results. An increase in efficiency is achieved in solar cells based on Li‐doped CTS thin films, and it is discussed in detail along with SCAPS‐1D simulation that is carried out depending on the thickness of CTS thin films.

Elucidation of electrical properties of undoped and Sb-induced Cu2SnS3 (CTS) thin films and degradation properties on CTS thin films and solar cells by intentional proton irradiation

The fundamental electrical properties of undoped and Sb-induced Cu2SnS3 (CTS) thin films were evaluated. Furthermore, the relationship between defect properties during intentional degradation and thin film/solar cell properties was investigated. The carrier concentration decreased after Sb induction in the CTS film, and the resistivity increased by one order of magnitude. These values were independent of the Sb volume. These results imply that a small quantity of Sb atoms passivates the defects, such as Sb atoms at Sn or Cu sites that compensate for the intrinsic acceptors at Cu vacancies. In addition, the number of defects around the grain boundary tended to decrease with Sb induction because of passivation. The carrier concentration of the CTS layer remained unchanged following proton irradiation at 1 × 1014 cm−2. Furthermore, the number of defects increased, independent of the Sb induction.

MODELLING OF THE SOLAR CELL BASED ON Cu2SnS3 THIN FILM PRODUCED BY SPRAY PYROLYSIS

Cu2SnS3 (CTS) thin film has been produced for 30 ccm sulphur flux rate at 30 minutes annealing durations at 550 oC temperature. CTS thin film’s crystalline structure has been investigated and crystalline size, lattice parameters, dislocation density and microstrain, crystalline number have also been determined. The CTS thin film’s morphological and optical properties have been examined and thoroughly interpreted. Mo/CTS/CdS/AZO solar cell has been modelled based on CTS thin film produced at the present work, using SCAPS-1D simulation programme. Voc, Jsc, FF, conversion efficiency and photovoltaic parameters have been determined depending on neutral defect density at the interface, coefficient of radiative recombination, Auger electron/hole capture’s coefficient and operation temperature of CTS solar cell. As a consequence of simulation study, ideal efficiency of CTS solar cell has been determined to be 3.72 % and all the data obtained in this study have been presented, interpreted and concluded to be original results.

Effect of annealing temperature on p–n junction formation in Cu2SnS3 thin-film solar cells fabricated via the co-evaporation of elemental precursors

Cu2SnS3 (CTS) thin-film solar cells were fabricated by the co-evaporation of the precursors, and the effect of annealing in N2 atmosphere on their photovoltaic properties was investigated by varying the annealing temperature after the chemical bath deposition of CdS. The characteristics of the solar cells improved as the annealing temperature was increased in the 250 °C–275 °C range (annealing time: 30 min). However, annealing temperatures exceeding 275 °C caused the deterioration of the device characteristics. Therefore, annealing in the 250 °C–275 °C range after CdS deposition is important for forming an optimum p–n junction at the CTS/CdS interface for manufacturing the CTS solar cells evaluated in this study. The best-performing solar cell fabricated using a CTS film annealed at 275 °C after CdS deposition exhibited an open circuit voltage of 0.181 V, with a short circuit current density of 20.9 mA cm−2, fill factor of 0.462, and power conversion efficiency of 1.74%.

Fabrication of Cu2SnS3 thin film solar cells via a sol-gel technique in air

Cu2SnS3 (CTS) thin films are prepared by sulfurizing CTS precursor films which are obtained using a sol-gel technique in air atmosphere. The effect of the sulfurization time on the properties of CTS thin films and solar cells is investigated. The uniform and dense CZT thin film is obtained by sulfuring at 575°Cfor 20 min. The corresponding solar cell shows the highest power conversion efficiency (3.68%), with a short-circuit current density (Jsc) of 30.25 mA/cm2, an open circuit voltage (Voc) of 254 mV, a fill factor (FF) of 47.9%, and a peak external quantum efficiency (EQE) of 70%.

Influence of Sb inclusion on morphologies and carrier concentration properties of CTS thin films grown by sulfurization of Cu–Sn precursors

The impact of antimony (Sb) inclusion in Cu2SnS3 (CTS) films grown by sulfurization of Sb/Cu–Sn precursors and the growth mechanism of Sb-containing CTS were examined. The CTS grain size depended on the volume of the included Sb. During the sulfurization of the Cu–Sn precursor, S atoms first reacted with the Cu or Sn atoms and formed intermediate phases, such as CuS and SnS2. In addition, Sb acted to form a large-sized CuS grain during the low-temperature sulfurization up to 350 °C. The excess Sb tended to form Sb2S3 during the sulfurization, most of which disappeared from the upper layer of the CTS films due to re-evaporation. The carrier concentration of the Sb-included CTS tended to decrease from compensation with Cu vacancies. The experimental results obtained in this study may serve as a first step toward achieving the high-quality large grain CTS thin films and high-efficiency CTS solar cells.

Na induction effects for J–V properties of Cu2SnS3 (CTS) solar cells and fabrication of a CTS solar cell over-5.2% efficiency

The effects of Na on Cu2SnS3 (CTS) solar cells were investigated by focusing on the electrical properties obtained using electrochemical impedance spectroscopy and Hall measurements to clarify which factors and regions are strongly affected by sulfurization with a NaF layer. The increase in the depletion layer width (xdep) suggested that the number of non-radiative recombination centers decreased at the p–n interface of the CTS solar cell owing to the sulfurization process with the NaF layer. Thus, the increase in Jsc indicated that the carrier collection width widened as xdep increased. Furthermore, Voc increased because the CTS lifetime was lengthened. The performance of the CTS solar cells was improved by these factors. Moreover, as a result of an investigation of the dark and light J–V properties, suppression of the crossover behavior was accomplished by sulfurization with the NaF layer. This technique improved the CTS/Mo and CdS/CTS interfaces owing to the Na content, leading to increases in all desirable solar cell properties. Therefore, the induction of Na from a soda-lime glass substrate as well as sulfurization with a NaF layer is required to obtain highly efficient CTS solar cells. As a result of this study, a CTS solar cell with an efficiency higher than 5.24% was achieved.

Impact of Na and/or Sb on the CTS thin films and solar cell performance

Cu2SnS3 (CTS) thin films were synthesized onto a soda lime glass substrate coated molybdenum by sulfurization the Cu–Sn stacks were prepared using RF sputtering. The influence of Na-doped CTS, Sb-doped CTS, and Sb+Na co-doped CTS films on their physical properties has been investigated. The undoped and doped films have been illustrated in a monoclinic structure with no shifts in peak position. Larger grain size has been obtained with Sb doping compared to undoped CTS. Enhancement in photoluminescence sharpening and yield spectra were indicated in the existence of Na and/or Sb. The power conversion efficiency was enhanced by the addition of Na and/or Sb to the CTS absorber layer, from 0.32% for undoped CTS to 1.54, 1.89 and 0.76% for Na, Sb, and co-doping CTS, respectively. As a result, the influence of doping has improved all cell efficiency with an increasing ratio of 381, 490, 137% with Na, Sb, and co-doping, respectively.

Role of defect density in absorber layer of ternary chalcogenide Cu2SnS3 solar cell

Ternary Chalcogenide Cu 2 SnS 3 (CTS) has emerged as a potential candidate for light absorbing material for photovoltaic solar cells owing to its tuneable bandgap, abundance in earth's crust and low toxicity. The PV performance of CTS solar cells has been assessed using Solar Cell Capacitance Simulator (SCAPS-1D) in the present study. The influence of defects in ascertaining the CTS solar cells' photovoltaic performance has been described. The impact of various type of defects and their density on V oc , J sc , FF and η has been evaluated. Change in energy band diagrams due to change in defect density has been analysed. Quantum Efficiency curves for different defect densities in CTS solar cells have been investigated. The increment in defect density in the CTS absorber has been found to degrade CTS solar cell performance regardless of the type of defect. • The PV performance of ternary chalcogenide CTS solar cells has been evaluated by using SCAPS-1D. • The increase in defect density in the CTS absorber layer has been found to degrade the performance of CTS solar cell. • With increase in defect densities, the quantum efficiency decreases. • Optimum defect density in CTS layer has been found to be 10 14 cm − 3 , above which the solar cell performance deteriorates.

Emission properties of intrinsic and extrinsic defects in Cu2SnS3 thin films and solar cells

Intrinsic and extrinsic defects around the p–n interface in Cu2SnS3 (CTS) solar cells were evaluated using low-temperature photoluminescence (LT-PL) measurements. The intrinsic defects were investigated based on the PL-dependence of CTS films on the excitation power and temperature. Donor–acceptor pair recombination was observed with shallow acceptors (copper vacancies, V Cu) located approximately 18 meV above the valence band maximum and typical donors located 72 and 112 meV below the conduction band minimum (CBM). The PL spectra of various CTS solar cell structures were measured to identify the Cd-related defects formed by Cd diffusion from the CdS layer. A new LT-PL peak was observed at 0.87 eV for the CdS/CTS solar cells, corresponding to D–A pair recombination with Cd on Cu site donors located 62 meV below the CBM. A p–n homojunction may form in CTS by V Cu passivation by Cd diffusion and suppressed interface recombination.

Effect of rapid thermal annealing on sprayed Cu2SnS3 thin films for solar-cell application

Ternary compound Cu2SnS3 (CTS) thin films were synthesized using two simple and non-vacuum processes: chemical spray pyrolysis and rapid thermal annealing (RTA). The crystal quality of CTS films improved and the band tailing, shown in Tauc’s plot, tended to vanish after annealing in a nitrogen atmosphere using RTA. Although ethanol was used as a solvent in the spray pyrolysis, the CTS film was not contaminated by carbon and oxygen. The bandgap of CTS was changed via sulfurization. The optimization of the RTA process affected the composition of the CTS thin films, modifying material defects, and improving the crystal quality. These results represent the initial step toward the practical application of CTS solar cells using an inexpensive fabrication process.

Phase composition of the earth-abundant Cu2SnS3 thin films with different annealing temperature and its effects on the performance of the related solar cells

Earth-abundant ternary Cu2SnS3 (CTS) thin films have attracted lots of attention as promising solar cell materials. However, the performances of the related devices were limited by the coexistence of different polymorphic phases and the poor quality of the sulfurized CTS films. It is well known that these two key factors are mainly dependent on the heat treatment temperatures. In view of this, CTS films were prepared via a simple sol-gel method followed by rapid sulfuration process and the effects of the annealing temperature on the crystal structure, morphologies and the photoelectrical properties of samples were discussed in detail. The results indicate that higher quality films with suitable properties can be obtained by annealing at 560 °C. More importantly, a phase transition from cation-disordered cubic structure to cation-ordered monoclinic structure was observed with the increasing temperature, which led to a reduction of the deep-level defects in the bulks of the films. Benefit from both of these effects, the efficiency of the CTS solar cells showed a significant improvement from 1.78% to 3.16% due to the enhanced open-circuit voltage (Voc) and fill factor (FF).

Impact of Urbach energy on open-circuit voltage deficit of thin-film solar cells

Correlation between Urbach energy (EU) and open-circuit voltage deficit (VOC,def) among conventional as well as emerging thin-film solar cells, i.e. Cu(In,Ga)Se2 (CIGSe), Cu(In,Ga)(S,Se)2 (CIGSSe), Cu2SnS3 (CTS), Cu2Sn1-xGexS3 (CTGS), SnS, Cu2ZnSnSxSe4-x (CZTSSe), perovskite (PVK), Cu(In,Ga)S2 (CIGS), and CdTe thin-film solar cells, is examined. The EU is estimated from an exponential tail in a long-wavelength edge of an external quantum efficiency. It is determined that the relation between short-circuit current density (JSC) and EU is not clearly shown because the JSC is primarily influenced by the bandgap of the thin-film absorbers. On the other hand, EU demonstrates a close relation with VOC,def, where the reduction of the EU leads to the decrease in the VOC,def, implying that the EU can be used as an indicator of the absorber quality. The EU of over 20 meV, observed in the CIGS, CTS, CTGS, SnS, and CZTSSe solar cells, gives rise to the high VOC,def and the low conversion efficiency (η) values. The EU of below 20 meV, seen in the CIGSe, CIGSSe, PVK, and CdTe solar cells, leads to the low VOC,def and high η values. Ultimately, the thin-film absorbers with the EU values of below 20 meV, which is lower than thermal energy under room temperature, is of vital to realize high photovoltaic performances.

Effect of Selenium Doping on the Performance of Flexible Cu2SnS3(CTS) Thin Film Solar Cells

Due to its favorable optical properties, Cu2SnS3 (CTS) is a promising material for thin film solar cells. Doping, which modifies the absorber properties, is one way to improve the conversion efficiency of CTS solar cells. In this work, CTS solar cells with selenium doping were fabricated on a flexible substrate using sputtering method and the effect of doping on the properties of CTS solar cells was investigated. In XRD analysis, a shift in the CTS peaks can be observed due to the doped selenium. XRF analysis confirmed the different ratios of Cu/Sn and (S+Se)/(Cu+Sn) depending on the amount of selenium doping. Selenium doping can help to lower the chemical potential of sulfur. This effectively reduces the point defects of CTS thin films. Overall improved electrical properties were observed in the CTS solar cell with a small amount of selenium doping, and a notable conversion efficiency of 1.02 % was achieved in the CTS solar cell doped with 1 at% of selenium.

Influence of Ge/(Ge+Sn) composition ratio in Cu2Sn1-xGexS3 thin-film solar cells on their physical properties and photovoltaic performances

Effect of Ge/(Ge + Sn) composition ratio of Cu2Sn1-xGexS3 (CTGS) films is examined. The CTGS films are fabricated by sulfurization of the 700-nm-thick Cu–SnS2/Ge stacked precursors on Mo-coated soda-lime glass (SLG) substrates. It is revealed that the thickness of Ge precursor (Ge thickness) from 0 to 420 nm enhances the Ge/(Ge + Sn) composition ratio in the CTGS thin films from 0 to about 0.58, thereby increasing bandgap energy (Eg) from approximately 0.97 (Cu2SnS3: CTS) to 1.23 eV (CTGS), respectively. Not only does Ge precursor act as Ge source to form the CTGS films but also it prevents the formation of undesirable MoS2 secondary phase. In addition, the CTGS solar cells with a structure of SLG/Mo/CTGS/CdS/ZnO/ZnO:Al/Ni/Al are fabricated with the Ge thickness from 0 to 420 nm. It is found that the optimized Ge thickness of 200 nm resulting in the Ge/(Ge + Sn) composition ratio of about 0.32 eliminates the excessive MoS2 secondary phase and increases the Eg, thereby increasing conversation efficiency (η) to 5.6% of the CTGS solar cell, higher than that (3.6%) of the CTS solar cell. However, the η is reduced when the Ge thickness over 200 nm because CTGS quality observed by Urbach energy is deteriorated and the conduction band offset at the CdS/CTGS interface demonstrates the large cliff type heterostructure, thus increasing the open-circuit voltage deficit.

Investigation of carrier recombination of Na-doped Cu2SnS3 solar cell for its improved conversion efficiency of 5.1%

Na-doped Cu2SnS3 (CTS) solar cells are fabricated and their carrier recombination is examined. The CTS absorbers on Mo-coated soda-lime glass substrates are grown by the sulfurization of NaF/Cu–SnS2 (900 nm) precursors. The NaF thickness is varied from 0 to 100 nm to vary the Na content in the resulting CTS absorbers. It is disclosed that the large grain with the uniformity of the material distribution (Cu, Sn, and S) in the Na-doped CTS absorber is obtained. With optimal Na doping (NaF thickness of 60 nm), the open-circuit voltage deficit (VOC,def) is obviously reduced, whereas short-circuit current density deficit (JSC,def) is not varied much. The reduction of VOC,def is attributable to the decrease in the carrier recombination across the device. The conversion efficiency (η) is consequently increased to approximately 4.7%. However, with severe Na doping (NaF thickness of over 75 nm), the VOC,def is clearly increased owing to the increase in the carrier recombination, thereby reducing the η. This is occurs because the Sn2S3 and Na2S secondary phases near the surface of the CTS film are formed with the severe Na doping. To further increase the η, the JSC,def is reduced through the decrease in CdS buffer thickness. Ultimately, the 5.1%-efficient CTS solar cell is obtained with the Na-doped CTS absorber prepared with the optimal NaF thickness of 60 nm.

The effect of metal-chelate complex in Cu2SnS3 thin film solar cells and their characteristics, photovoltaic performance, and defect analysis

Hybrid inks with a chelating agent were prepared and coated by a spin-coating method to form Cu2SnS3 (CTS) thin films. After the coating, a subsequent sulfurizing process via rapid thermal annealing was performed. During the sulfurization, the Cu and Sn precursors in the hybrid inks exist in complex forms with chelates and these complexes help to form the CTS thin films by controlling the reaction rate of the metal precursors. Additionally, even though the complexes with chelates were formed, the oxidation numbers of the metal precursors were affected by the ionization tendency of each metal in the hybrid inks to form the semiconducting CTS thin films. After obtaining the optimum sulfurizing condition by controlling the reaction pressure and temperature, the CTS thin films were characterized and CTS solar cells were fabricated under these conditions. The best conversion efficiency of the fabricated cells was 2.953% and the temperature-dependent photovoltaic performances were also examined to investigate the carrier transport mechanisms of the devices. According to admittance spectroscopy, the dominant defect energy level was determined as 0.09 eV above the valence band minimum, which accords with the copper vacancy (VCu) level. In addition, capacitance–voltage measurements and drive-level capacitance profiling were applied to demonstrate the carrier densities and defect behaviors.