SLG Substrate Research Articles

Enhanced the performance of CZTSSe solar cells by optimized i:ZnO window layer

We have systematically investigated the effect of i:ZnO window layer with different thickness on the performance of CZTSSe solar cells. The i:ZnO films were deposited on SLG substrate by adjusting the supttering time, and the accurate thickness was determined by XRR spectra. GIXRD, AFM images and absorption spectra show that the crystalline quality is stable and increases with the increase of thickness, while transmittivity declines. Furthermore, CZTSSe solar cells with the structure of SLG/Mo/CZTSSe/CdS/i:ZnO/ITO/Al were fabricated. It is found that Voc and FF of solar cell are affected by the thickness of i:ZnO layer obviously. The i:ZnO window layer plays an important role in reducing the channels of current leakage. Meanwhile, a compromise among the parallel conductance, series resistance and transmittivity are needed. By optimized the process, the efficiency of CZTSSe solar cell was significantly improved to 10.74 % when the thickness of i:ZnO was about 26 nm.

Radiation hardness of Cu2ZnSn(S,Se)4 thin film solar cells under 10 MeV proton irradiation

The radiation response of kesterite Cu2ZnSn(S,Se)4 solar cells was investigated by 10 MeV high-energy protons irradiation. The optoelectronic characteristics of the CZTSSe photovoltaic device with proton irradiation were characterized by EQE, IV, SIMS and TRPL, and the influencing factors of its performance were systematically analyzed. The degradation curves of the irradiated samples were plotted as a function of the proton irradiation fluences. It was found that the radiation hardness of the CZTSSe solar cells was better than that of the traditional solar cells. The main factor for the device performance degradation after proton irradiation was originate from the excess of sodium diffusion from SLG substrate. These results are the first step toward realizing practical application of CZTSSe solar cells in space.

Interplay between Li and Na amid co-doped solution-processed Cu2ZnSn(S,Se)4 absorbers for solar cells

Alkali doping and alloying are well-known strategies to improve the performance of Cu 2 ZnSn(S,Se) 4 (CZTSSe) absorber based thin film solar cells. The effects of individual light alkali elements such as Li and Na have been thoroughly investigated, with both dopants resulting in significant improvements in performance of CZTSSe solar cells. Here, the combined effects of Li and Na are investigated in a so-called co-doping approach to capture the benefits from both elements simultaneously. In order to do that, various concentrations of Li and Na between 0.0 M and 0.5 M are added to the solution used for spin coating of the precursor layer. After annealing under Se-enriched atmosphere, the two alkali elements displayed mutual dependency in terms of their concentrations in the CZTSSe absorber layer. Furthermore, both, Li and Na showed signs of forming alloys with the CZTSSe phase. The efficiencies of the best Li–Na co-doped solar cells are above 10%, slightly above the Li baseline thanks to increased open-circuit-voltage and short-circuit current. A non-negligible Na incorporation was observed even in nominally Na-free devices, likely from indirect contamination from the SLG substrate. Further work is needed to better understand Na-poor compositions and draw conclusions relevant to Na-free substrates. • Cu 2 ZnSn(S,Se) 4 absorbers are co-doped using Li and Na in a solution-based approach. • After annealing, the Li and Na concentrations mutually depend on each other. • Both, Li and Na show a tendency to be alloyed into the lattice. • Na from the SLG substrate overcomes the alkali diffusion barrier via the gas phase. • A champion device with a power conversion efficiency of 10.5% was fabricated.

Development of 2nd Generation Thin Film Photovoltaics Based on CZTS Absorber Layer and ZnS Window Layer

Among the second-generation solar cells, thin-film solar cells based on CIGS, CZTS and CdTe are well known due to their higher efficiencies, low cost and simple fabrication techniques. In this proposed work, we are committed to developing CZTS and ZnS thin films for second-generation thin-film photovoltaics. CZTS and ZnS have energy band gap of 1.4-1.5 eV and 3.6 eV, while their light absorption coefficients are 104 cm-1 and 3.3×104 cm-1 respectively. In thin film solar cells, absorber layer consists of photovoltaic material that should have optimal bandgap close to 1.34 eV. Moreover, a wide band gap window layer acts as p-n junction with absorber layer. In this work, we are reporting the synthesis of CZTS and ZnS thin films via wet chemistry route using spin coating method. Deposition of thin films on SLG substrate were carried out at specific process parameters such as spinning speed, concentration of precursors and annealing temperature to get the optimized thin film suitable for photovoltaic applications. Surface morphology and elemental compositional analysis investigated through SEM and EDX spectroscopy respectively. Electrical and optical properties were examined via Hall effect measurements and UV-VIS NIR spectrophotometry respectively. In addition micro-chemical and functional group analysis were conducted by FRIT spectroscopy. Keywords: Thin film PV, wet-chemistry, CZTS, ZnS

Structural, morphological and optical properties of spray-formed silver-doped zinc sulphide thin films

The study focused on the qualities of spray-formed Silver-doped Zinc sulphide (ZnS) thin films sprayed on soda-lime glass (slg) substrate. Silver-doped and undoped ZnS thin films with 0%, 1%, 3% and 5% Silver concentrations were deposited. The qualities of the synthesized films were investigated using x-ray diffractometry, scanning electron microscopy, Fourier transforms infrared (FTIR) spectrometry, UV-VIS spectrophotometry, Raman spectrometry and contact angle techniques. The x-ray diffraction data identified cubic structures forthin films. Scanning electron microscopy shows the presence of agglomerates of nanoparticles and pores in the thin films. The thin films’ crystallite size ranges between 3.107 and 4.103 nm. FTIR revealed the chemical bonds in the film. The transmittance of the thin films is between 42.35 and 81.86% at 550 nm, the energy gap is observed within a range of 3.11 and 3.60 eV while the indices of refraction are in the range of 1.52–3.81 at 550 nm wavelength. Photoluminescence result shows Sulphur vacancies. A hydrophilic surface feature of the film was revealed by the contact angle measurement.

Spin-coated Cu2CrSnS4 thin film: A potential candidate for thin film solar cells

In this regard for the first time, the multicomponent chalcogenide Cu2(Zn1-xCrx)SnS4 (CZCrTS) nanocrystals at (0 ≤x ≤ 1) were synthesized by the solvothermal method. Improvement of structural, optical and electrical properties in Cr-doped Cu2ZnSnS4 thin film deposited on SLG substrate by spin coating technique has been investigated. It was observed that introduced the Cr element into the Cu2ZnSnS4 nanocrystal led to increasing the disorder of cations distribution within the unit cell through increasing the tensile strain inside the unit cell resulting in a decline the unit cell volume accompanied by a decrease the particle size and also the crystallite size. It has been found that the optical band gap of the CZCrTS nanocrystals ranges from 1.54 to 1.35 eV with increasing the partial substitution of Zn element with Cr element which in turn also led to a significant improvement in the crystallinity of the Cu2ZnSnS4 film, a facilitates the grain growth and also increases the grain size where accompanied this process post-sulfurization at 450 ℃ for 30 min. In addition, it was observed that the number of grains in the Cu2CrSnS4 film (x = 1), 2.79 (nm−2), were lower than that, 6.8 (nm−2) of the Cu2ZnSnS4 film. Also, the dislocation density in the Cu2CrSnS4 film (x = 1), 0.012 (nm−2), was lower than that, 0.024 (nm−2) of the Cu2ZnSnS4 film, which resulted in decreased the defects in the grain boundaries and increasing the current from 0.34 mA for the Cu2ZnSnS4 film to 0.42 mA for the Cu2CrSnS4 film. Due to a higher absorption coefficient (>104 cm−1), a higher optical conductivity (> 23 *1012 S−1) and lower transmittance have been reported for the Cu2CrSnS4 thin film (x = 1), we believe the Cu2CrSnS4 thin film may be a good candidate can be used as an absorbent layer to sunlight in thin film solar cell.

Development of a CZTS solar cell with CdS buffer layer deposited by RF magnetron sputtering

A thin film solar cell device having the architecture Ni/Al/Al:ZnO/i-ZnO/CdS/CZTS/Mo/SLG with a RF-sputtered CdS buffer layer was reported in this study. CZTS absorber layers were prepared on SLG and Mo-coated SLG substrates by sulphurization of sputtered stack precursor films. Structural, morphological, optical and electrical characteristics of absorber layer and interfaces of the layers in the cell structure were investigated in detail. It was determined that CZTS films were crystallized in kesterite structure and had a direct band gap of 1.4 eV and an absorbtion coefficient over 104 cm−1. A depth profile of the structure showed that the Na ions diffused quite deeply from SLG substrate through the Mo back contact layer into the CZTS absorber layer. The fabricated solar cell was characterized by current-voltage (I-V) measurements under simulated AM1.5 G spectrum at 0.1 W/cm2 illumination. An efficiency of 1.64% was obtained from the solar cell with an active area of 0.41 cm2. The open-circuit voltage (VOC), the short circuit current (ISC) and the fill factor (FF) values of the solar cell were 0.55 V, 9.95 mA and 0.37, respectively. The low FF value was attributed to high series resistance of the cell, the formation of MoS2 interfacial layer at Mo/CZTS interface and secondary phases in the absorber layer.

Influence of deposition pressure and power on characteristics of RF-Sputtered Mo films and investigation of sodium diffusion in the films

Mo films deposited by DC sputtering are widely used as back contact in CIGS and CZTS based thin film solar cells. However, there have been only a few studies on the deposition of Mo films by RF sputtering method. In this context, Mo films on SLG substrates were prepared as a function of deposition pressure and power by using RF magnetron sputtering method to contribute to this shortcoming. Mo films were deposited at 250 °C substrate temperature by using 20, 15, 10 mTorr Ar pressures at 120 W RF power and 10 mTorr Ar pressure at 100 W RF power. Structural, morphological and reflectivity properties of RF-sputtered Mo films were clarified by XRD, AFM, FE-SEM and UV–Vis measurements. In addition, due to sodium incorporation from SLG substrate to the absorber layer through Mo back contact layer is so essential in terms of improving the conversion efficiency values of CIGS and CZTS thin film solar cell devices, the effects of Na diffusion in the films were analyzed with SIMS depth profile. The electrical properties of the films such as mobility, carrier density and resistivity were determined by Hall Effect measurements. It was found that Mo films prepared at 120 W, 10 mtorr and 250 °C substrate temperature and then annealed at 500 °C for 30 min, had resistivity as low as 10−5 Ω cm, as well as higher amount of Na incorporation than other films.

Intense pulsed light annealing of copper zinc tin sulfide nanocrystal coatings

A promising method for forming the absorber layer in copper zinc tin sulfide [Cu2ZnSnS4 (CZTS)] thin film solar cells is thermal annealing of coatings cast from dispersions of CZTS nanocrystals. Intense pulsed light (IPL) annealing utilizing xenon flash lamps is a potential high-throughput, low-cost, roll-to-roll manufacturing compatible alternative to thermal annealing in conventional furnaces. The authors studied the effects of flash energy density (3.9–11.6 J/cm2) and number of flashes (1–400) during IPL annealing on the microstructure of CZTS nanocrystal coatings cast on molybdenum-coated soda lime glass substrates (Mo-coated SLG). The annealed coatings exhibited cracks with two distinct linear crack densities, 0.01 and 0.2 μm−1, depending on the flash intensity and total number of flashes. Low density cracking (0.01 μm−1, ∼1 crack per 100 μm) is caused by decomposition of CZTS at the Mo-coating interface. Vapor decomposition products at the interface cause blisters as they escape the coating. Residual decomposition products within the blisters were imaged using confocal Raman spectroscopy. In support of this hypothesis, replacing the Mo-coated SLG substrate with quartz eliminated blistering and low-density cracking. High density cracking is caused by rapid thermal expansion and contraction of the coating constricted on the substrate as it is heated and cooled during IPL annealing. Finite element modeling showed that CZTS coatings on low thermal diffusivity materials (i.e., SLG) underwent significant differential heating with respect to the substrate with rapid rises and falls of the coating temperature as the flash is turned on and off, possibly causing a build-up of tensile stress within the coating prompting cracking. Use of a high thermal diffusivity substrate, such as a molybdenum foil (Mo foil), reduces this differential heating and eliminates the high-density cracking. IPL annealing in presence of sulfur vapor prevented both low- and high-density cracking as well as blistering. However, grain growth was limited even after annealing with 400 flashes. This lack of grain growth is attributed to a difficulty of maintaining high sulfur vapor pressure and absence of alkali metal impurities when Mo foil substrates are used.

Controlled substitution of S by Se in reactively sputtered CZTSSe thin films for solar cells

CZTSSe thin films are considered to be future of flexible thin film solar cell. Amount of S and Se in the CZTSSe thin films controls the band gap. Therefore, control over the concentration of S/Se is very important for enhancing the efficiency. CZTS thin films were deposited on SLG substrate by sequential reactive sputtering of metal targets. The films were annealed in Se atmosphere at 550 °C for 10–25 min in order to obtain CZTSSe thin films. Elemental, surface morphological, optical and structural properties of CZTSSe thin films have been investigated in detail. Optical band gap energy of CZTSSe thin films were in the range of 1.1–1.35 eV. XRD and Raman analysis were used to analyze the structure and phase purity of CZTSSe thin films. High intense peak at 27.9° in the XRD pattern corresponding to (112) plane was observed. High intensity of XRD peak corresponding to (112) direction revealed that the growth is oriented along (112) direction. Raman analysis also showed the presence of two peaks related to Se and S anions in CZTSSe thin films. In the Raman spectra of CZTSSe thin films; peaks corresponding to Se anions were observed at 190–206 cm−1 and peaks corresponding to S anions were observed at 329–338 cm−1. Elemental analysis results showed that S/Se ratio did not change much with increase in annealing time. To the best of our knowledge, this is the first report on the deposition of CZTSSe by sequential reactive sputtering of metal targets and controlled selenization.

Photoluminescence characterization of surface degradation mechanism in Cu(In,Ga)Se2 thin films grown on Mo/soda lime glass substrate

Intensity and decay time of a near-band-edge photoluminescence (PL) of a Cu(In,Ga)Se2 (CIGS) thin film grown on a Mo/soda lime glass (SLG) substrate decreased with air exposure time, showing the degradation of the CIGS film surface. The dependence of Mo sputtering conditions on intensity and decay time of PL was studied in CIGS films deposited on Mo/SLG substrates. The Mo/SiO2 (20 nm)/SLG substrate was used to control Na diffusion during CIGS deposition. Intensity and decay time of PL with time in air changed depending on the Mo-film sputtering temperature and pressure. Based on the secondary ion mass spectroscopy, the in-depth profile of Na in the CIGS film is correlated with those of H and O, which depended on the Mo-film sputtering condition and air exposure time. The degradation of the CIGS film in view of PL is discussed in terms of the Na close to the CIGS surface, which reacted with O2 and H2O. CIGS solar cells were examined in relation to Mo sputtering conditions and the SiO2 sodium barrier. The control of Na diffusion was found to be important for both reducing the rate of CIGS film surface degradation and the realization of a CIGS solar cell with a uniform PL intensity image.

In situ ellipsometric study of the three‐stage process in CuInSe2 film deposition

AbstractIn Situ ellipsometric study has been carried out during the deposition of the CuInSe2 thin film by means of the three‐stage process. A rotator analyzing ellipsomerter using a 632.8 nm He‐Ne laser was used. Ellipsometric parameters (Ψ and Δ) and reflectivity R was obtained during the entire deposition stages, in which a complex reflection coefficient is ρ=tanΨ exp(iΔ). Cu, In and Se were deposited on the Mo‐coated SLG substrate. At the first‐stage (In‐Se deposition), the In‐Se film deposition rate and its reflactive index has been obtained on the basis of the light interference. At the second‐stage, changes in both Ψ and Δ are observed. Based on the X‐ray diffraction measurement, these changes are related to the stoichiometric composition of the film from In‐rich to Cu‐rich. This signal was utilized as a process switch from the second‐stage to the third‐stage. At the third stage, the weak change in Ψ has been observed showing the change of the stoichiometric composition from the Cu‐ rich to the In‐rich. By means of the in situ ellipsometry‐controlled three‐stage process, the CuInSe2 layer with single phase chalcopyrite structure has successfully been prepared, which exhibits an intense near‐band‐edge photoluminescence at 0.998 eV at room temperature. The preliminary fabricated ZnO/CdS/CuInSe2 solar cell exhibited a conversion efficiency of 5.6%. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cu(In,Ga)Se 2 solar cells with superstrate structure using lift-off process

Cu(In,Ga)Se 2 (CIGS) solar cells with a superstrate structure were fabricated using a lift-off process. To widen the variety of substrate choices for CIGS solar cells, a lift-off process was developed without an intentional sacrificial layer between the CIGS and Mo back-contact layers. The CIGS solar cells fabricated on Mo/soda-lime glass (SLG) were transferred to an alternative SLG substrate. The conversion efficiency of the CIGS solar cells with the superstrate structure was 5.1%, which was almost half that of the CIGS solar cells with a substrate structure prior to the lift-off process. The low conversion efficiency was caused by the high series resistance and low shunt resistance, which would be due to the junction resistance between the CIGS/back contact and cracks introduced during the lift-off process, respectively.