Cu2ZnSnS4 Thin Film Solar Cells Research Articles

Preparation of Cu2ZnSnS4-based thin film solar cells by a combustion method

A novel route that fabricate Cu2ZnSnS4 solar cells by a combustion method has been successfully conducted. The probable formation mechanism of Cu2ZnSnS4 for metal oxides system during the sulfurization process is analyzed in detail through the phase analysis in several continuous temperature ranges in combination with thermodynamic calculation. The effects of annealing temperature on the structural, morphological, compositional and optical properties of absorber layer were studied. The UV–vis spectra revealed the band gap of as-prepared thin films varying from 1.48 to 1.34eV along with rising the sulfurization temperature. It was found that secondary phase with narrow band gap significantly affects the band gap of Cu2ZnSnS4. The best performance of the Cu2ZnSnS4 thin film solar cells prepared by the combustion method achieved a powder conversion efficiency (PCE) of 1.6% with a short circuit current density of 10.5mA/cm2 and an open circuit voltage of 505mV. This low-cost non-vacuum technique has a promising application in synthesis of light-absorbing layers for photovoltaic devices, which can vastly reduce the costs and simplify the preparation process.

Boosting the efficiency of pure sulfide CZTS solar cells using the In/Cd-based hybrid buffers

The large open circuit voltage (Voc) deficit (Eg/q-Voc) is the key factor limiting the current efficiency of pure sulfide Cu2ZnSnS4 (CZTS) thin film solar cells. This present work reports CZTS devices with a Voc of over 710mV by utilizing either In2S3 buffer or In2S3/CdS hybrid buffer. The improvement in Voc mainly results from (i) an increased carrier concentration in CZTS due to In-doping when the In2S3 buffer is adjacent to the CZTS absorber and (ii) favorable conduction band alignments at the CZTS/buffer interface. However, devices with In2S3 buffers suffer from having a lower Fill Factor (FF) and short circuit current density (Jsc) when compared with those with CdS buffers. In contrast devices with In2S3/CdS hybrid buffers not only yield an enhancement in Voc, but also give the same level of FF and an even higher Jsc relative to CdS buffers, thereby increasing the efficiency from 5.5% to 6.6%. These hybrid In2S3/CdS buffers provide a promising way to reduce the Voc deficit and further boost the efficiency of pure sulfide CZTS solar cells.

Improvement of Jsc in a Cu2ZnSnS4 Solar Cell by Using a Thin Carbon Intermediate Layer at the Cu2ZnSnS4/Mo Interface

Back contact modification plays an important role in improving energy conversion efficiency of Cu2ZnSnS4 (CZTS) thin film solar cells. In this paper, an ultrathin carbon layer is introduced on molybdenum (Mo)-coated soda lime glass (SLG) prior to the deposition of CZTS precursor to improve the back contact and therefore enhance CZTS solar cell efficiency. By introducing this layer, the short circuit current (Jsc) and device conversion efficiency increase for both nonvacuum (sol-gel) and vacuum (sputtering) methods. Specifically, for the sol-gel based process, Jsc increases from 13.60 to 16.96 mA/cm(2) and efficiency from 4.47% to 5.52%, while for the sputtering based process, Jsc increases from 17.50 to 20.50 mA/cm(2) and efficiency from 4.10% to 5.20%. Furthermore, introduction of this layer does not lead to any deterioration of either open circuit voltage (Voc) or fill factor (FF).

Two different effects of Na on Cu2ZnSnS4 thin-film solar cells

Among the many factors that affect the characteristics of Cu2ZnSnS4 thin-film solar cells, the effects of Na were examined. Because Na passivates the grain boundaries (GBs) in absorber layers, the GBs increase the minority carrier collection by providing a current pathway through which the minority carriers can reach the buffer and window layer and be collected. However, excess Na remains in the absorber layer and diffuses into the buffer and window layers, where it acts as an impurity and traps the minority carriers, thus decreasing the solar cell characteristics. By applying an annealing process after the deposition of the buffer and window layers, the Na content could be decreased. By improving the solar cell characteristics, a power conversion efficiency of 6.43% was achieved.

Chemical Consequences of Alkali Inhomogeneity in Cu2ZnSnS4 Thin‐Film Solar Cells

This study offers new insight into the role of Na in Cu2ZnSnS4 (CZTS) thin film solar cells by studying samples with a spatially varying alkali distribution. This is achieved by omitting a diffusion barrier between the soda‐lime glass substrate and the Mo back contact, where compositional variations of the glass inherently result in non‐uniform alkali distributions in the CZTS. By correlating light beam induced current (LBIC) maps with secondary ion mass spectrometry composition maps, it is shown that samples containing regions of higher Na concentration (“hot spots”) have corresponding LBIC hot spots on comparable length scales. Samples containing an alkali diffusion barrier have lower LBIC dispersion; thus, LBIC can be used to evaluate non‐uniformity in CZTS devices, where a common cause is Na inhomogeneity. Moreover, it is shown that the Na hot spots are strongly correlated with other compositional variations in the device, including increased Cu in‐diffusion with the underlying MoS2 layer and decreased diffusion of Cd to the back contact. Neither of these effects are well understood in CZTS devices, and neither have previously been correlated with the presence or absence of Na.

Phase evolution of Cu2ZnSnS4 (CZTS) kesterite thin films during the sulfurization process

The development of high efficiency Cu2ZnSnS4 (CZTS) thin film solar cells depends on the synthesis of phase pure CZTS films. Due to the narrow phase window, it is difficult to prepare CZTS films with a pure kesterite phase (i.e., a film that lacks traces of detrimental secondary phases). For this, phase evolution of kesterite CZTS in films must be systematically studied. In the present study detailed phase evolution of kesterite CZTS via the sulfurization of stacked Cu/SnS2/ZnS precursor was examined. The phase evolution of kesterite CZTS was analyzed during the temperature-dependent sulfurization of stacked precursors. In addition to traditional X-ray diffraction and Raman analysis, transmission electron microscopy (TEM) characterization was used to study the phase evolution and trace locations of secondary phases in the sulfurized films. Furthermore, based on the experimental evidences the reaction pathways for the formation of CZTS kesterite phase and plausible phase evolution mechanism are proposed.

Fabrication of Cu2ZnSnS4 thin film solar cells by sulfurization of electrodeposited stacked binary Cu–Zn and Cu–Sn alloy layers

Kesterite Cu2ZnSnS4 (CZTS) thin films have been fabricated by an electrodeposition route via sulfurizing sequentially electrodeposited Cu–Zn and Cu–Sn stacked binary alloy layers. The CZTS films exhibit densely packed grains with only minor ZnS secondary phase. Photovoltaic device with the structure of ITO/i-ZnO/CdS/CZTS/Mo/Glass shows the highest power conversion efficiency of 3.67%. The limiting factors of the efficiency are the high series resistance and recombination rate. The ZnS phase, voids at the Mo/CZTS interface and high resistance MoS2 layer have been considered to lead to high series resistance and thereby low JSC and FF.

The effect of ZnS segregation on Zn-rich CZTS thin film solar cells

Analysis of ZnS segregation behaviour and its influence on the device performance has been made on the Zn-rich Cu2ZnSnS4 thin film solar cells. Cross-sectional transmission electron microscopy images reveal that ZnS is the main secondary phase in the Cu2ZnSnS4 layer obtained from a sulphurised Zn/CuSn metallic stack. The excess Zn diffuses from back contact region to top surface of Cu2ZnSnS4 layer accumulating in the form of ZnS. The solar cell with a higher Zn concentration shows a large quantity of isolated ZnS grains at Cu2ZnSnS4 top surface which is close to CdS/Cu2ZnSnS4 heterojunction interface. Soft X-ray absorption spectroscopy indicates a large spike-like conduction band offset between Cu2ZnSnS4 and ZnS. Consequently, such much ZnS precipitates would increase series resistance and generate lower short-circuit current and external quantum efficiency. However, appropriate amount of ZnS at the space charge region of the solar cell has beneficial effects by reducing the heterojunction interface recombination. Therefore, an improved open-circuit voltage and a higher shunt resistance are achieved. This paper provides a possible method to intentionally segregate ZnS at the space charge region by depositing the Zn layer at the bottom of co-sputtered CuSn layer. Although it is difficult to synthesis a pure phase Cu2ZnSnS4 absorber, we can utilise the ZnS secondary phase to improve the Cu2ZnSnS4 solar performance by controlling the Zn-excess amount.

The effect of dopant concentration on properties of transparent conducting Al-doped ZnO thin films for efficient Cu2ZnSnS4 thin-film solar cells prepared by electrodeposition method

Al-doped ZnO (AZO) thin films were potentiostatically deposited on indium tin oxide substrates. The influence of the doping level of the ZnO:Al films was investigated. The results of the X-ray diffraction and scanning electron microscopy analysis revealed that the structural properties of the AZO films were found polycrystalline with a hexagonal wurtzite-type structure along the (002) plane. The grain size of the AZO films was observed as approximately 3 μm in the film doping with 4 mol% ZnO:Al concentration. The thin films also exhibited an optical transmittance as high as 90 % in the wavelength range of 100–1,000 nm. The optical band gap increased from 3.33 to 3.45 eV. Based on the Hall studies, the lowest resistivity (4.78 × 10−3 Ω cm) was observed in the film doping with 3 mol% ZnO:Al concentration. The sheet resistant, carrier concentration and Hall mobility values were found as 10.78 Ω/ square, 9.03 × 1018 cm−3 and 22.01 cm2/v s, respectively, which showed improvements in the properties of AZO thin films. The ZnO:Al thin films were used as a buffer layer in thin-film solar cells with the structure of soda-lime glass/Mo/Cu2ZnSnS4/ZnS/ZnO/Al grid. The best solar cell efficiency was 2.3 % with V OC of 0.430 V, J SC of 8.24 mA cm−2 and FF of 68.1 %.

Cu2ZnSnS4 thin film solar cells from coated nanocrystals ink

Earth abundant Cu2ZnSnS4 (CZTS) has triggered great attention in the field of low cost thin film solar cells. High-quality CZTS nanocrystals were synthesized by a facile solution method, with an average particle diameter about 12 nm and a direct band gap of 1.49 eV. And these nanocrystals were readily dispersed to form a colloidal ink, which was then used to prepare thin films by doctor blading approach. Effects of sulfurization temperature on those films’ composition, morphology and phase were studied. The obtained thin film, annealed under flow S/Ar atmosphere at 550 °C for 30 min, shows a dense smooth surface, a slightly Cu-poor and Zn-rich composition and the kesterite CZTS structure. Besides, this 550 °C annealed CZTS film was then fabricated into full photovoltaic device and a power conversion efficiency of 2.29 % has been achieved. Directions for further improvements have also been discussed and proposed.

Effects of Sulfurization Pressure on the Conversion Efficiency of Cosputtered Cu2ZnSnS4Thin Film Solar Cells

We report herein Cu2ZnSnS4(CZTS) thin film solar cells with 6.75% conversion efficiency, without an antireflection coating. The CZTS precursors have been prepared by cosputtering using three different targets on Mo-coated substrates: copper (Cu), tin sulfide (SnS), and zinc (Zn). The postsulfurization was carried out at different pressures in a H2S/N2environment at 550°C for one hour. A comparative study on the performances of solar cells with CZTS absorber layers prepared at different sulfurization pressures was carried out. The device efficiency of 1.67% using CZTS absorber and low pressure sulfurization is drastically improved, to an efficiency of 6.75% with atmospheric pressure sulfurization.

Characteristics of In-Substituted CZTS Thin Film and Bifacial Solar Cell

Implementing bifacial photovoltaic devices based on transparent conducting oxides (TCO) as the front and back contacts is highly appealing to improve the efficiency of kesterite solar cells. The p-type In substituted Cu2ZnSnS4 (CZTIS) thin-film solar cell absorber has been fabricated on ITO glass by sulfurizing coelectroplated Cu-Zn-Sn-S precursors in H2S (5 vol %) atmosphere at 520 °C for 30 min. Experimental proof, including X-ray diffraction, Raman spectroscopy, UV-vis-NIR transmission/reflection spectra, PL spectra, and electron microscopies, is presented for the interfacial reaction between the ITO back contact and CZTS absorber. This aggressive reaction due to thermal processing contributes to substitutional diffusion of In into CZTS, formation of secondary phases and electrically conductive degradation of ITO back contact. The structural, lattice vibrational, optical absorption, and defective properties of the CZTIS alloy absorber layer have been analyzed and discussed. The new dopant In is desirably capable of improving the open circuit voltage deficit of kesterite device. However, the nonohmic back contact in the bifacial device negatively limits the open circuit voltage and fill factor, evidencing by illumination-/temperature-dependent J-V and frequency-dependent capacitance-voltage (C-V-f) measurements. A 3.4% efficient solar cell is demonstrated under simultaneously bifacial illumination from both sides of TCO front and back contacts.

Cu2ZnSnS4 thin film solar cell fabricated by co-electrodeposited metallic precursor

Copper, tin and zinc have been co-electrodeposited on Mo-coated glass substrates via an electrochemical process with the aim at preparing kesterite absorber for thin film solar cells. The co-electrodeposition behavior of Cu–Sn–Zn (CZT) thin film has been studied by using linear voltammetery technique. The as-electrodeposited, pre-annealed and sulfurized films were characterised by scanning electron microscopy and energy dispersive X-ray spectroscopy, X-ray diffraction and Raman spectroscopy. To obtain suitable composition and high crystallinity, deposition potential and sulfurization temperature have been optimized. As the results, the solar cell fabricated with the CZTS thin film sulfurized at 560 °C showed a conversion efficiency of 3.74 %. The estimated band-gap energy from the external quantum efficiency measurements is about 1.48 eV.

Trap and recombination centers study in sprayed Cu2ZnSnS4 thin films

In this work, a study of trap and recombination center properties in polycrystalline Cu2ZnSnS4 thin films is carried out in order to understand the poor performance in Cu2ZnSnS4 thin film solar cells. Thermally stimulated current has been studied in Cu2ZnSnS4 deposited by pneumatic spray pyrolysis method using various heating rates, in order to gain information about trap centers and/or deep levels present within the band-gap of this material. A set of temperature-dependent current curves revealed three levels with activation energy of 126 ± 10, 476 ± 25, and 1100 ± 100 meV. The possible nature of the three levels found is presented, in which the first one is likely to be related to CuZn antisites, while second and third to Sn vacancies and SnCu antisites, respectively. The values of frequency factor, capture cross section, and trap concentration have been determined for each center.

Cu2ZnSnS4thin film solar cell utilizing rapid thermal process of precursors sputtered from a quaternary target: a promising application in industrial processes

Download options Please wait... Article information DOI https://doi.org/10.1039/C4RA07748B Article type Paper Submitted 29 Jul 2014 Accepted 04 Sep 2014 First published 04 Sep 2014 Download Citation RSC Adv., 2014,4, 43080-43086 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Cu2ZnSnS4 thin film solar cell utilizing rapid thermal process of precursors sputtered from a quaternary target: a promising application in industrial processes J. He, L. Sun, Y. Chen, J. Jiang, P. Yang and J. Chu, RSC Adv., 2014, 4, 43080 DOI: 10.1039/C4RA07748B To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Jun He Lin Sun Ye Chen Jinchun Jiang Pingxiong Yang Junhao Chu

Effects of chlorine and carbon on Cu2ZnSnS4 thin film solar cells prepared by spray pyrolysis deposition

Cu2ZnSnS4 (CZTS) thin film solar cells were fabricated under non-vacuum conditions, wherein the absorption layer was prepared by the spray pyrolysis deposition method. By using a chlorine (Cl)-free solution instead of a Cl-containing solution, the solar cell efficiency was improved from 0.18% to 0.35%. On the CZTS surface deposited from the Cl-containing solution, Cl-containing precipitates with size of 10μm were present and were found to degrade the solar cell efficiency. The still low efficiency of 0.35% was caused by a layer containing small grains that existed between the CZTS layer and the Mo electrode, which included a significant amount of carbon. By optimizing the pre-annealing process, the small-grained layer became thinner and the efficiency was improved to 0.63%. Finally, by adjusting the chemical composition of the Cl-free solution and the CdS buffer layer deposition conditions, the cell efficiency was improved to 0.94%.

Optimization of sputtered ZnS buffer for Cu2ZnSnS4 thin film solar cells

Cu2ZnSnS4 (CZTS) thin film solar cells with sputtered zinc sulfide (ZnS) buffer layers are fabricated. The effect of ZnS (buffer layer) film thicknesses along with a CZTS absorber layer prepared by sputtered metallic precursor film followed by sulfurization on the solar cell properties is investigated. The impact of the sputtering power on the structural, morphological, compositional and optical properties of the ZnS buffer layers is studied. The optimized ZnS buffer layer with a thickness of ~30nm in CZTS based solar cell structure exhibits the best solar cell conversion efficiency of 2.11% with open-circuit voltage of 311mV, short-circuit current density of 12.16mA/cm2 and fill factor of 0.55.

Inhibiting MoS2 formation by introducing a ZnO intermediate layer for Cu2ZnSnS4 solar cells

A 10nm ZnO layer have been employed as an intermediate layer between absorber and back contact in Cu2ZnSnS4 thin film solar cells prepared by sulfurization of the metallic stacking layers. By introducing this layer, the device conversion efficiency increases from 2.1% up to 3.3%, open-circuit voltage increases from 476mV to 657mV while short-circuit current decreases from 14.52mA/cm2 to 10.72mA/cm2. The ZnO layer can significantly suppress the reaction of Mo back contact with S and facilitate the reaction between S and metallic stacking layers. It results in the MoS2 thickness decrease from 300nm to 80nm and less unreacted elemental Sn left in the CZTS absorber after sulfurization annealing as shown in transmission electron microscopy images. However, due to the Sn loss through a path of SnS at high annealing temperature, the reacted and eliminated elemental Sn results in a higher density of voids formed at the bottom of sulfurized stacking layers. The improvement in the cell efficiency and open-circuit voltage could be related to both reduced MoS2 thickness and removal of shunting path generated by elemental Sn. However, the voids formation at back contact deteriorates the carrier transportation and decreases short-circuit current.

Impact of rapid thermal annealing of Mo coated soda lime glass substrate on device performance of evaporated Cu2ZnSnS4 thin film solar cells

An industry compatible annealing method-rapid thermal annealing (RTA) of Mo coated soda lime glass substrate has been investigated to improve evaporated Cu2ZnSnS4 (CZTS) thin film solar cell efficiency. It indicates that the RTA treatment makes the Mo film dense, facilitates the column growth of the Mo film and the growth of the CZTS absorber though it enhances the formation of voids and secondary phases in the meantime. Also, this treatment causes low absorption of the absorber. Consequently, the treatment enhances open circuit voltage (VOC) as well as efficiency dramatically, improves fill factor (FF) mildly; though lowers the short circuit current density (JSC) slightly.

Fabrication of Cu2ZnSnS4 solar cell on a flexible glass substrate

Cu2ZnSnS4 (CZTS) thin film solar cells were fabricated on 100μm thick flexible glass (FG) substrates coated with Molybdenum (Mo) anodes. The CZTS thin films were deposited by using magnetron sputtering followed by annealing in sulfur vapor. The fabricated cell structure was FG/Mo/CZTS/CdS/ZnO/ITO/Ag, where cadmium sulfide (CdS), zinc oxide (ZnO), tin-doped indium oxide (ITO) and silver (Ag) were used as an n-type window layer, resistive layer, transparent conductor and finger electrode, respectively. The best conversion efficiency achieved of the flexible CZTS solar cell was 3.08%, with an active area of 0.45cm2. The conversion efficiency was reduced by 20% when the cell was bent to a 50mm radius.