Numerical Study and Optimization of CZTS-Based Thin-Film Solar Cell Structure with Different Novel Buffer-Layer Materials Using SCAPS-1D Software

This study explored the performances of CZTS-based thin-film solar cell with three novel buffer layer materials ZnS, CdS, and CdZnS, as well as with variation in thickness of buffer and absorber-layer, doping concentrations of absorber-layer material and operating temperature. Our aims focused to identify the most optimal thin-film solar cell structure that offers high efficiency and lower toxicity which are desirable for sustainable and eco-friendly energy sources globally. SCAPS-1D, widely used software for modeling and simulating solar cells, has been used and solar cell fundamental performance parameters such as open-circuited voltage (), short-circuited current density (), fill-factor() and efficiency() have been optimized in this study. Based on our simulation results, it was found that CZTS solar cell with Cd0.4Zn0.6S as buffer-layer offers the most optimal combination of high efficiency and lower toxicity in comparison to other structure investigated in our study. Although the efficiency of Cd0.4Zn0.6S, ZnS and CdS are comparable, Cd0.4Zn0.6S is preferable to use as buffer-layer for its non-toxic property. In addition, evaluation of performance as a function of buffer-layer thickness for Cd0.4Zn0.6S, ZnS and CdS showed that optimum buffer-layer thickness for Cd0.4Zn0.6S was in the range from 50 to 150nm while ZnS offered only 50 – 75 nm. Furthermore, the temperature dependence performance parameters evaluation revealed that it is better to operate solar cell at temperature 290K for stable operation with optimum performances. This study would provide valuable insights into design and optimization of nanotechnology-based solar energy technology for minimizing global energy crisis and developing eco-friendly energy sources sustainable and simultaneously.

Research on the optimum hydrogenated silicon thin films for application in solar cells

Hydrogenated silicon （Si：H） thin films for application in solar ceils were deposited by using very high frequency plasma enhanced chemical vapour deposition （VHF PECVD） at a substrate temperature of about 170 ℃, The electrical, structural, and optical properties of the films were investigated. The deposited films were then applied as i-layers for p-i-n single junction solar cells. The current-voltage （I - V） characteristics of the cells were measured before and after the light soaking. The results suggest that the films deposited near the transition region have an optimum properties for application in solar cells. The cell with an i-layer prepared near the transition region shows the best stable performance.

Absorption enhancement in thin film a-Si solar cells with double-sided SiO_2 particle layers

Light absorption enhancement is very important for improving the power conversion efficiency of a thin film a-Si solar cell. In this paper, a thin-film a-Si solar cell model with double-sided SiO2 particle layers is designed, and then the underlying mechanism of absorption enhancement is investigated by finite difference time domain（FDTD） simulation;finally the feasible experimental scheme for preparing the SiO2 particle layer is discussed. It is found that the top and bottom SiO2 particle layers play an important role in anti-reflection and light trapping, respectively. The light absorption of the cell with double-sided SiO2 layers greatly increases in a wavelength range of 300 nm-800 nm, and the ultimate efficiency increases more than 22% compared with that of the flat device. The cell model with double-sided SiO2 particle layers reported here can be used in varieties of thin film solar cells to further improve their performances.

First-principles study on the alkali chalcogenide secondary compounds in Cu(In,Ga)Se_2 and Cu_2ZnSn(S,Se)_4 thin film solar cells

The beneficial effect of the alkali metals such as Na and K on the Cu（In.Ga）Se2 （CIGS） and Cu2ZnSn（S,Se）4 （CZTSSe） solar cells has been extensively investigated in the past two decades, however, in most of the studies the alkali metals were treated as dopants. Several recent studies have showed that the alkali metals may not only act as dopants but also form secondary phases in the absorber layer or on the surfaces of the films. Using the first-principles calculations, we screened out the most probable secondary phases of Na and K in CIGS and CZTSSe, and studied their electronic structures and optical properties. We found that all these alkali chalcogenide compounds have larger band gaps and lower VBM levels than CIGS and CZTSSe, because the existence of strong p-d coupling in CIS and CZTS pushes the valence band maximum （VBM） level up and reduces the band-gaps, while there is no such p-d coupling in these alkali chalcogenides. This band alignment repels the photo-generated holes from the secondary phases and prevents the electron-hole recombination. Moreover, the study on the optical properties of the secondary phases showed that the absorption coefficients of these alkali chalcogenides are much lower than those of CIGS and CZTSSe in the energy range of 0-3.4eV, which means that the alkali chalcogenides may not influence the absorption of solar light. Since the alkali metal dopants can passivate the grain boundaries and increase the hole carrier concentration, and meanwhile their related secondary phases have innocuous effect on the optical absorption and band alignment, we can understand why the alkali metal dopants can improve the CIGS and CZTSSe solar cell performance.

Design of periodic metal-insulator-metal waveguide back structures for the enhancement of light absorption in thin-film solar cells

To increase the absorption in a thin layer of absorbing material （amorphous silicon, a-Si）, a light trapping design is presented. The designed structure incorporates periodic metal-insulator-metal waveguides to enhance the optical path length of light within the solar cells. The new design can result in broadband optical absorption enhancement not only for transverse magnetic （TM）-polarized light, but also for transverse electric （TE）-polarized light. No plasmonic modes can be excited in TE-polarization, but because of the coupling into the a-Si planar waveguide guiding modes and the diffraction of light by the bottom periodic structures into higher diffraction orders, the total absorption in the active region is also increased. The results from rigorous coupled wave analysis show that the overall optical absorption in the active layer can be greatly enhanced by up to 40%. The designed structures presented in this paper can be integrated with back contact technology to potentially produce high-efficiency thin-film solar cell devices.

Impact of oxygen incorporation on interface optimization and defect suppression for efficiency enhancement in Cu_(2)ZnSn(S,Se)_(4)solar cells

Kesterite Cu_(2)ZnSn(S,Se)_(4)(CZTSSe)solar cells suffer from severe carrier recombination,limiting the photovoltaic performance.Unfavorable energy band alignment at the p-n junction and defective front interface are two main causes.Herein,oxygen incorporation in CZTSSe via absorber air-annealing was developed as a strategy to optimize its surface photoelectric property and reduce the defects.With optimized oxygen incorporation conditions,the carrier separation and collection behavior at the front interface of the device is improved.In particular,it is found that oxygen incorporated absorber exhibits increased band bending,larger depletion region width,and suppressed absorber defects.These indicate the dynamic factors for carrier separation become stronger.Meanwhile,the increased potential difference between grain boundaries and intra grains combined with the decreased concentration of interface deep level defect in the absorber provide a better path for carrier transport.As a consequence,the champion efficiency of CZTSSe solar cells has been improved from 9.74%to 12.04%with significantly improved open-circuit voltage after optimized air-annealing condition.This work provides a new insight for interface engineering to improve the photoelectric conversion efficiency of CZTSSe devices.

Microstructure and optical properties of sprayed γ-CuI thin films for CuInS_2 solar cells

γ-CuI thin films were prepared by a spraying method using acetonitrile as a solvent,CuI and iodine as reagents.The influences of substrate temperature on the structure,topography,and optical properties of CuI films were investigated.Scanning electron microscope（SEM） photos revealed that the shape and grain size of CuI grains were related to substrate temperature.X-ray diffraction results showed that substrate temperature affected the crystalline quality of CuI films.When the substrate temperature was 110°C,CuI thin films showed γ-phase zinkblende structure with（111） preferred orientation.The dimension of the globular CuI crystallite was approximately 35 nm,the energy band gap was 2.97 eV,the maximum transmittance was 87.3% in the part of the visible region,and the open circuit voltage was close to 380 mV.This opened a route for a cadmium-free buffer layer for CuInS2 solar cells.

Photovoltaic properties of Cu_2O-based heterojunction solar cells using n-type oxide semiconductor nano thin films prepared by low damage magnetron sputtering method

We improved the photovoltaic properties of Cu_2O-based heterojunction solar cells using n-type oxide semiconductor thin films prepared by a sputtering apparatus with our newly developed multi-chamber system. We also obtained the highest efficiency(3.21%) in an AZO/p-Cu_2O heterojunction solar cell prepared with optimized pre-sputtering conditions using our newly developed multi-chamber sputtering system. This value achieves the same or higher characteristics than AZO/Cu_2O solar cells with a similar structure prepared by the pulse laser deposition method.

Preparation and characterization of Cd_(1-x)Zn_xS buffer layers for thin film solar cells

Cd1_xZnxS （x = 0, 0.1, 0.2, 0.3, 1.0） thin films have been grown successfully on soda-lime glass substrates by chemical bath deposition technique as a very promising buffer layer material for optoelectronic device applications. The composition, structural properties, surface morphol- ogy, and optical properties of Cd~_xZnxS thin films were characterized by energy dispersive analysis of X-ray tech- nique （EDAX）, X-ray diffraction （XRD）, scanning electron microscopy （SEM）, and UV-Vis spectrophotometer tech- niques, respectively. The annealed films were observed to possess the deficient sulfur composition. The results of XRD show that the Cdl_xZnxS （x = 0. l） thin film annealed at 450 ~C forms hexagonal （wurtzite） structure with lattice parameters a = 0.408814 nm, c ： 0.666059 nm, and its average grain size is 24.9902 nm. The diffraction peaks become strong with the increasing annealing temperatures. The surface of Cdl_~ZnxS （x = 0.1） thin film annealed at 450 ~C is uninterrupted and homogenous as compared to other temperatures. From optical properties, it is observed that the presence of small amount of Zn results in marked changes in the optical band gap of CdS. The band gaps of the Cdl_xZnxS thin films vary from 2.42 to 3.51 eV as composition varies from x = 0.0 to 1.0.

Synthesis of the CuInSe_2 thin film for solar cells using the electrodeposition technique and Taguchi method

The Taguchi method was used to obtain the optimum electrodeposition parameters for the synthesis of the CuInSe2 thin film for solar cells. The parameters consist of annealing temperature, current density, CuCl2 concentration, FeCl3 concentration, H2SeO3 concentration, TEA amount, pH value, and deposition time. The experiments were carried out according to an L18（2^13^7） table An X-ray diffractometer （XRD） and a scanning electron microscope （SEM） were respectively used to analyze the phases and observe the microstructure and the grain size of the CuInSe2 film before and after annealing treatment. The results showed that the CuInSe2 phase was deposited with a preferred plane （112） parallel to the substrate surface. The optimum parameters are as follows： current density, 7 mA/cm^2; CuCl2 concentration, 10 mM; FeCl3 concentration, 50 mM; H2SeO3 concentration, 15 mM; TEA amount, 0 mL; pH value, 1.65; deposition time, 10 min; and annealing temperature, 500℃.

Enhanced optical absorption by Ag nanoparticles in a thin film Si solar cell

Thin film solar cells have the potential to significantly reduce the cost of photovoltaics. Light trapping is crucial to such a thin film silicon solar cell because of a low absorption coefficient due to its indirect band gap. In this paper, we investigate the suitability of surface plasmon resonance Ag nanoparticles for enhancing optical absorption in the thin film solar cell. For evaluating the transmittance capability of Ag nanoparticles and the conventional antireflection film, an enhanced transmittance factor is introduced. We find that under the solar spectrum AM1.5, the transmittance of Ag nanoparticles with radius over 160 nm is equivalent to that of conventional textured antireflection film, and its effect is better than that of the planar antireflection film. The influence of the surrounding medium is also discussed.

In-situ growth of a CdS window layer by vacuum thermal evaporation for CIGS thin film solar cell applications

Highly crystalline and transparent CdS films are grown by utilizing the vacuum thermal evaporation （VTE） method. The structural, surface morphological, and optical properties of the films are studied and compared with those prepared by chemical bath deposition （CBD）. It is found that the films deposited at a high substrate temperature （200 ℃） have a preferential orientation along （002） which is consistent with CBD-grown films. Absorption spectra reveal that the films are highly transparent and the optical band gap values are found to be in a range of 2.44 eV-2.56 eV. Culnl_xGaxSe2 （CIGS） solar cells with in-situ VTE-grown CdS films exhibit higher values of Voc together with smaller values of Jsc than those from CBD. Eventually the conversion efficiency and fill factor become slightly better than those from the CBD method. Our work suggests that the in-situ thermal evaporation method can be a competitive alternative to the CBD method, particularly in the physical- and vacuum-based CIGS technology.

Effect of emitter layer doping concentration on the performance of a silicon thin film heterojunction solar cell

A novel type of n/i/i/p heterojunction solar cell with a-Si：H（15 nm）/a-Si：H（10 nm）/epitaxial c-Si（47 p.m）/epitaxial c-Si（3 um） structure is fabricated by using the layer transfer technique, and the emitter layer is deposited by hot wire chemical vapour deposition. The effect of the doping concentration of the emitter layer Sd （Sd=PH3/（PH3 ＋SiH4＋H2）） on the performance of the solar cell is studied by means of current density-voltage and external quantum efficiency. The results show that the conversion efficiency of the solar cell first increases to a maximum value and then decreases with Sd increasing from 0.1% to 0.4%. The best performance of the solar cell is obtained at Sd = 0.2% with an open circuit voltage of 534 mV, a short circuit current density of 23.35 mA/cm2, a fill factor of 63.3%, and a conversion efficiency of 7.9%.

A NEW CONCEPT TOWARD INDUSTRIALIZATION OF Cu-III-VI_2 THIN FILM SOLAR CELLS AND SOME PRELIMINARY EXPERIMENT RESULTS

A new concept of full vacuum manufacturing for Cu-III-IV2 thin-film solar cells has been discussed. Cu-III-IV2 thin-film solar cells manufactured using full in- line reactive sputtering will result in lower cost than that of the conventional method with CdS layer fabricated with chemical bath deposition (CBS) method. Us ing reactive sputtering process with organo- metallic gases, the compositions a nd electronic properties of Cu-III-IV2 thin-film can be fine-tuned and precisely controlled. n-type Cu-III-IV2 film and ZnS suffer layer can also be deposited u sing the in-line sputtering instead of using the CdS layer. The environmental po llution problems arising from using CdS can be eliminated and the ultimate goal of full in-line process development can then be realized. Some preliminary exper imental results on a modal solar cell fabricated by the new technique in the new concept have been presented.

Observation on Surface and Cross Section of Thin Film Solar Cells Using Atomic Force Microscope

Atomic force microscope (AFM) is able to produce three-dimensional digital data in both force-mode and height-mode and its applications are not limited to map the surfaces of conducting materials. It can use the force-mode to image the repulsive and attractive force patterns. The cross sections of polycrystalline CdS/CdTe and amorphous silicon heterojunction solar cells are observed with AFM. In case of short circuit, the microstructures of different layers in the samples are clearly displayed. When the cells are open circuit, the topographical images are altered, the potential outline due to the space charge in junction region is observed. Obviously, AFM can be employed to investigate experimentally built-in potential in junction of semiconductor devices, such as solar cells.

Evaluation of electrical and optical characteristics of ZnO/CdS/CIS thin film solar cell

In this study, device modeling and simulation are conducted to explain the effects of each layer thickness and temperature on the performance of ZnO/CdS/CIS thin film solar cells. Also, the thicknesses of the CIS and CdS absorber layers are considered in this work theoretically and experimentally. The calculations of solar cell performances are based on the solutions of the well-known three coupling equations： the continuity equation for holes and electrons and the Poisson equation. Our simulated results show that the efficiency increases by reducing the CdS thickness. Increasing the CIS thickness can increase the efficiency but it needs more materials. The efficiency is more than 19% for a CIS layer with a thickness of 2 μm. CIS nanoparticles are prepared via the polyol route and purified through centrifugation and precipitation processes.Then nanoparticles are dispersed to obtain stable inks that could be directly used for thin-film deposition via spin coating.We also obtain x-ray diffraction（XRD） peak intensities and absorption spectra for CIS experimentally. Finally, absorption spectra for the CdS window layer in several deposition times are investigated experimentally.

Optimizing a Single-Absorption-Layer Thin-Film Solar Cell1 Model to Achieve 31% Efficiency

This research builds upon the authors’ previous work that introduced and modeled a novel Gallium-Arsenide, Emitterless, Back-surface Alternating Contact (GaAs-EBAC) thin-film solar cell to achieve >30% power conversion efficiency. Key design parameters are optimized under an Air-Mass (AM) 1.5 spectrum to improve performance and approach the 33.5% theoretical efficiency limit. A second optimization is performed under an AM0 spectrum to examine the cell’s potential for space applications. This research demonstrates the feasibility and potential of a new thin-film solar cell design for terrestrial and space applications. Results suggest that the straight-forward design may be an inexpensive alternative to multi-junction solar cells.

Texture ZnO Thin-Films and their Application as Front Electrode in Solar Cells

In this paper, three kinds of textured ZnO thin-films (the first kind has the textured structure with both columnar and polygon, the second posses pyramid-like textured structure only, and the third has the textured structure with both crater-like and pyramid-like), were prepared by three kinds of methods, and the application of these ZnO thin-films as a front electrode in solar cell was studied, respectively. In the first method with negative bias voltage and appropriate sputtering parameters, the textured structure with columnar and polygon on the surface of ZnO thin-film are both existence for the sample prepared by direct magnetron sputtering. Using as a front electrode in solar cell, the photoelectric conversion efficiency Eff of 7.00% was obtained. The second method is that by sputtering on the ZnO:Al self-supporting substrate, and the distribution of pyramid-like was gained. Moreover, the higher (8.25%) photoelectric conversion efficiency of solar cell was got. The last method is that by acid-etching the as-deposited ZnO thin-film which possesses mainly both columnar and polygon structure, and the textured ZnO thin-film with both crater-like and pyramid-like structure was obtained, and the photoelectric conversion efficiency of solar cell is 7.10% when using it as front electrode. These results show that the textured ZnO thin-film prepared on self-supporting substrate is more suitable for using as a front electrode in amorphous silicon cells.

Nickel Antimony Sulphide Thin Films for Solar Cell Application: Study of Optical Constants

Chemical bath deposition technique has been used to deposit Ni-doped Sb2S3 thin films onto glass substrate. Doping was carried out by adding 1, 3 and 5 wt% of Ni. Bath temperature was kept as 10℃ and films were annealed at 250℃ under vacuum. Polycrystalline nature of films with an orthorhombic phase was analyzed by X-ray diffraction technique. Scanning electron microscopy was used for morphological study which shows that grains are spherical. Optical measurements using transmittance data indicated that films have a direct band gap of 1.00 - 2.60 eV with an absorption coefficient of ~104 cm-1 in visible range. The average value of electrical conductivity was calculated as 1.66, 1.11 and 1.06 (Ω·cm)-1 for as-deposited films and 1.90, 2.08 and 1.15 (Ω·cm)-1 for annealed films while refractive indices were found as 2.18 - 3.38 and 1.91 - 3.74 respectively. The obtained films can be used for solar cell applications due to their good absorbing properties like higher absorption coefficient and refractive index values.

A Novel Thin-Film, Single-Junction Solar Cell Design1 to Achieve Power Conversion Efficiency above 30 Percent

The record efficiency for a thin-film, single-junction solar cell has remained static at 28.8% since 2012. This research presents a unique design that demonstrates potential to exceed record efficiency and approach the theoretical efficiency limit of ~33.5%. The findings of this study are significant, from an efficiency standpoint, and also because the cell design can be realized using existing fabrication methods that do not require complex, post-processing steps. In this study, a benchmark simulation is developed that closely resembles a high-efficiency, front-and-back contact cell. Intrinsic performance limiters are overcome by moving the emitter and front-contact to the back of the cell to eliminate electrical grid shading and improve optical performance. To further improve performance, the P-N junction formed by the emitter layer is removed from the model to allow selective Ohmic contacts to accept (reject) minority (majority) carriers as required. The design modifications improve open-circuit voltage, short-circuit current, and fill-factor which collectively boost efficiency above 30%-primarily due to a 2% gain of incident irradiance and improved optical performance.