High-efficiency Thin-film Solar Cells Research Articles

Simulating the performance of a highly efficient CuBi2O4-based thin-film solar cell

In this study, copper bismuth oxide (CuBi2O4) absorber-based thin film heterojunction solar cell structure consisting of Al/FTO/CdS/CuBi2O4/Ni has been proposed. The proposed solar cell device structure has been modeled and analyzed by using the solar cell capacitance simulator in one dimension (SCAPS-1D) software program. The performance of the proposed photovoltaic device is evaluated numerically by varying thickness, doping concentrations, defect density, operating temperature, back metal contact work function, series and shunt resistances. The current density–voltage behaviors at dark and under illumination are investigated. To realize the high efficiency CuBi2O4-based solar cell, the thickness, acceptor and donor densities, defect densities of different layers have been optimized. The present work reveals that the power conversion efficiency can be enhanced by increasing the absorber layer thickness. The efficiency of 26.0% with open-circuit voltage of 0.97 V, short-circuit current density of 31.61 mA/cm2, and fill-factor of 84.58% is achieved for the proposed solar cell at the optimum 2.0-μm-thick CuBi2O4 absorber layer. It is suggested that the p-type CuBi2O4 material proposed in the present study can be employed as a promising absorber layer for applications in the low cost and high efficiency thin-film solar cells.

High-efficiency CdTe-based thin-film solar cells with unltrathin CdS:O window layer and processes with post annealing

The thickness of the CdS:O window layer plays an important role in CdTe-based thin-film solar cell performance. Normally with thinner CdS:O layer, the cells’ short wavelength response can be enhanced to achieve higher short-circuit current density (Jsc), while the open-circuit voltage (Voc) and fill factor (FF) would degrade. Previous research finds that reducing the CdS:O layer thickness to less than 40 nm would decrease the cells’ efficiency (Eff.). In this work, 18% efficiency CdTe thin-film solar cell with only 30 nm CdS:O window layer is demonstrated. A post annealing process is developed to fabricate high efficiency cells with ultrathin CdS:O layers. Our results indicate that the high defect density in CdTe absorber, low built-in potential (Vbi) and high back contact barrier (Φb) are partially responsible for the reduced Voc and FF of cells with ultrathin CdS:O layers. The post annealing process can increase the carrier concentration (NCV) and Vbi, and decrease the Φb, in which way improves Voc and FF for higher cell efficiency.

Performance enhancement of Sb2Se3 solar cell using a back surface field layer: A numerical simulation approach

In the present work, antimony selenide (Sb2Se3)-based solar cell with a back surface field (BSF) layer has been designed and studied. The purpose of this research is to improve the performance of the Sb2Se3-based solar cell by introducing low-cost and widely available barium silicide (BaSi2) material as the BSF layer into the basic Sb2Se3-based heterojunction solar structure. A comparative study on the performance of the conventional Sb2Se3 solar cell structure consisting of Al/FTO/CdS/Sb2Se3/Mo and the proposed structure of Al/FTO/CdS/Sb2Se3/BaSi2/Mo is made. The photovoltaic parameters such as open circuit voltage, short-circuit current density, fill-factor, power conversion efficiency, and quantum efficiency of heterojunction structures are analyzed intensively by using the Solar Cell Capacitance Simulator in One Dimension (SCAPS-1D) program. To optimize the device, the thickness of Sb2Se3 absorber layer is varied from 0.1 to 2 μm. In addition, the effects of acceptor ion and bulk defect densities in Sb2Se3 absorber layer, interface defect density of buffer/absorber and absorber/BSF, back surface recombination velocity, operating temperature, and cell resistances on the overall performances are investigated. The thicknesses of BaSi2 BSF and Sb2Se3 absorber layers are optimized to be 0.3 μm and 1 μm, respectively. The efficiency of the proposed solar structure with a thin 1 μm Sb2Se3 absorber layer is obtained to be 29.35%. The simulation results lead to suggest that the BaSi2 material as a BSF layer would be effective to fabricate low cost and high-efficient Sb2Se3-based thin-film solar cells.

First-principles study of the stability, electronic and optical properties of CdTe under hydrostatic pressure

CdTe is a promising candidate material for stable and high efficiency thin-film solar cells. In the present study, we systematically investigated the effect of hydrostatic pressure on the stability, electronic and optical properties of CdTe based on density functional theory (DFT). From the calculations of phonon dispersions, CdTe is still dynamically stable at high pressure (5.76 GPa). An increase in the band gap is found for CdTe under different pressure conditions, which agrees well with the available experimental results. Moreover, the optical absorption coefficient of CdTe with under pressure is reduced in the visible light region.

Performance improvement of Perovskite/CZTS tandem solar cell using low-cost ZnS/Ag/ITO multilayer spectrum splitter

In this paper, a new approach based on strategic combination between Dielectric1/Metal/Dielectric2 multilayer aspect and Particle Swarm Optimization (PSO) technique is proposed to design an efficient low-cost spectrum splitter for tandem solar cell applications. This approach allows selecting the suitable dielectric materials, noble metal and geometry of each sub-layer, providing high transparency, while maintaining a superior reflectance within well selected spectral ranges. It is revealed that the optimized ZnS/Ag/ITO multilayered structure offers 92% of transparency and a high reflectance of 89% with an optimum cut-off wavelength of 700 nm. These outstanding results are mainly attributed to the role of the adopted PSO approach in promoting an enhanced light-management. Subsequently, the influence of the optimized spectrum splitter on the solar cell efficiency is analyzed by implementing it in a Perovskite/CZTS tandem cell. It is found that the investigated cell exhibits a high efficiency exceeding 16%, far surpassing that of the conventional monolithic tandem photovoltaic device. This is due to the reduced optical and recombination losses enabled by the use of an optimized and low-cost spectrum splitter system. Therefore, the proposed strategy based on combined Dielectric1/Metal/Dielectric2 spectrum splitting and PSO metaheuristic technique pinpoints a new pathway towards reaching improved efficiencies, making it potential alternative for designing high-efficiency and cost-effective thin-film tandem solar cells.

Partial and Total Substitution of Zn by Mg in the Cu2ZnSnS4 Structure

Cu 2 ZnSnS 4 (CZTS) is a quaternary semiconductor that has emerged as a promising component in solar absorber materials due to its excellent optical properties such as band-gap energy of ca. 1.5 eV and significant absorption coefficient in the order of 10 4 cm − 1 . Nevertheless, the energy conversion efficiency of CZTS-based devices has not reached the theoretical limits yet, possibly due to the existence of antisite defects (such as Cu Zn or Zn Cu ) and secondary phases. Based on electronic similarities with Zn, Mg has been proposed for Zn substitution in the CZTS structure in the design of alternative semiconductors for thin-film solar cell applications. This work aims to study the properties of the CZTS having Mg incorporated in the structure replacing Zn, with the following stoichiometry: x = 0, 0.25, 0.5, 0.75, and 1 in the formula Cu 2 Zn 1 − x Mg x SnS 4 (CZ-MTS). The semiconductor was prepared by the hot injection method, using oleylamine (OLA) as both surfactant and solvent. The presence and concentration of incorporated Mg allowed the fine-tuning of the CZ-MTS semiconductor’s structural and optical properties. Furthermore, it was observed that the inclusion of Mg in the CZTS structure leads to a better embodiment ratio of the Zn during the synthesis, thus reducing the excess of starting precursors. In summary, CZ-MTS is a promising candidate to fabricate high efficient and cost-effective thin-film solar cells made of earth-abundant elements.

Maximize CdTe solar cell performance through copper activation engineering

Abstract The incorporation of copper (Cu) is one of the critical processes for fabricating high-efficiency CdTe thin-film solar cells. However, due to its high mobility in CdTe, the distribution and concentration of Cu must be carefully engineered to reduce the compensative donor-like interstitial defects in CdTe bulk and the recombination centers at the buffer layer/CdTe interface to maximize device performances. Here, a cuprous chloride (CuCl) solution treatment and a rapid thermal annealing (RTA) process are used to control the concentration and distribution of Cu in CdTe absorbers, enabling a champion CdTe thin-film solar cell with a power conversion efficiency of 17.5% without selenium incorporation. The results demonstrate that the use of a CuCl solution can substantially reduce the amount of Cu needed in CdTe and the RTA process is a viable approach to engineer the Cu distribution in CdTe solar cells.

Point defects in BaSi2 thin films for photovoltaic applications studied by positron annihilation spectroscopy

Barium di-silicide (BaSi2) is a very promising absorber material for high-efficiency thin-film solar cells, due to its suitable bandgap, high light absorption coefficient, and long minority-carrier lifetime. In this study, we compare the nanostructure, layer composition, and point defects of BaSi2 thin films deposited by Radio Frequency (RF) sputtering, Thermal Evaporation (TE), and Molecular Beam Epitaxy (MBE), using Doppler Broadening Positron Annihilation Spectroscopy (DB-PAS) depth profiling, Raman spectroscopy, and x-ray diffraction. Our DB-PAS study on thermally annealed RF-sputter deposited and on TE-deposited BaSi2 layers, in a comparison with high quality BaSi2 films produced by MBE, points to the presence of vacancy-oxygen complexes and Si or Ba mono-vacancies, respectively, in the (poly)crystalline BaSi2 films. The degree of near-surface oxidation increases, going from MBE and TE to the industrially applicable RF-sputtered deposition synthesis. The use of a-Si capping layers on the thermally annealed RF-sputtered BaSi2 films leads to a clear reduction in sub-surface oxidation and improves the quality of the BaSi2 films, as judged from DB-PAS.

Energy, temperature, and deposition angle dependence of Cd and Te2 deposited on CdTe

Cadmium Telluride (CdTe) is an important material for the production of high-efficiency thin-film solar cells. While sputter deposition has been used to create CdTe-based solar cells, two other important methods of growth are close-spaced sublimation and vapor deposition. In these methods, the depositing clusters correspond to Cd atoms and Te2 dimers while the deposition energies are relatively low. In addition, depending on vapor pressure, deposition method, and target-substrate distance, deposition may occur at relatively large angles with respect to the substrate normal. Here we investigate the dependence of the attachment probability and deposition site for Cd and Te2 clusters deposited on Cd-terminated and Te-terminated (100) and (111) surfaces of zincblende CdTe on deposition conditions. In general, we find that the deposition of Cd atoms and/or Te2 dimers on the oppositely terminated surface leads to an attachment probability which is close to 1 and relatively independent of deposition conditions for both the (100) and (111) orientations. In contrast, deposition on the same terminated surface leads to a significantly lower attachment probability which generally decreases with increasing deposition angle, energy, and substrate temperature. Our results also indicate that deposition on the (111) surface leads to a significant excess Te sticking probability. In contrast, the excess Te attachment probability for deposition on the (100) surface is typically significantly smaller, and in some cases may even be negative. We also find, for both deposition on the (111) surface as well as opposite termination deposition on the (100) surface, that the dominant deposition mode corresponds to sitting on top of the surface which corresponds to growth of the next layer. In contrast, for same termination deposition on the (100) surface the dominant deposition mode corresponds to joining the first layer. These results imply that even for low deposition energies and substrate temperatures, deposition on the (100) surface is likely to create interstitials in the surface layer.

High optoelectronic performance of ZnO films co-doped with ternary functional elements of F, Al and Mg

For use as front electrodes in high-efficiency thin-film solar cells, ZnO transparent conductive films must have both broad spectral transmission and high conductivity. However, these characteristics are difficult to achieve simultaneously. In this work, F, Al, and Mg co-doped ZnO (FAMZO) films with superior electrical and optical properties were fabricated by RF magnetron sputtering and rapid thermal annealing (RTA). F and Al co-doping enables the film to maintain low resistivity and high transmittance in the visible and near-infrared regions, while Mg doping widens the optical band gap. Morphological characterization shows that the FAMZO films undergo recrystallization and growth during RTA, which further improves their electrical and optical properties. Our FAMZO films achieved a resistivity of 4.07 × 10−4 Ω cm, a mobility of 47.85 cm2/V, a carrier concentration of 3.21 × 1020 cm−3, and 90.54% average transmittance between 400 and 1400 nm. The excellent performance of our ZnO films makes them suitable for a broad range of applications in high-efficiency thin-film solar cells.

Determination of band alignment at the CdTe/SnTe heterojunction interface for CdTe thin-film solar cells

The Ohmic back contact for CdTe is a key issue to realize high-efficiency CdTe thin film solar cells because of the high work function of CdTe. CdTe/SnTe heterojunctions (HJs) have been implemented to address this issue which shows promising potential, but the band alignment at the HJs is unknown. The valence band offsets in MBE-grown cadmium telluride (CdTe)/tin telluride (SnTe) (111) heterostructures were measured with X-ray photoelectron spectroscopy (XPS). The XPS results show that the heterostructure has an ideal type-I band structure for CdTe solar cells applications, with a valence band offset of and a conduction band offset of , which expedites hole transport from the CdTe absorber to the hole electrode and improves the Ohmic contact for CdTe. Experimental determination of the band structure of CdTe/SnTe HJs can help improve the photovoltaic performance of CdTe thin film solar cells and facilitate the design and fabrication of CdTe/SnTe related devices. Furthermore, we inserted a SnTe back contact buffer layer into the CdTe thin film solar cells, and it was compared with the cell structure without the SnTe buffer layer. The feasibility of using SnTe as a solar cell back contact is confirmed.

First-principles study of the band gap tuning and doping control in CdSexTe1−x alloy for high efficiency solar cell**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0700700), the National Natural Science Foundation of China (Grant Nos. 51672023, 11847043, 11634003, and U1530401), and the Science Challenge Project (Grant Nos. TZ2016003 and TZ2018004).

CdTe is one of the leading materials for low cost, high efficiency thin-film solar cells with a nearly ideal band gap of 1.48 eV. However, its solar to electricity power conversion efficiency (PCE) is hindered by the relatively low open circuit voltage (VOC) due to intrinsic defect related issues. Here, we propose that alloying CdTe with CdSe could possibly improve the solar cell performance by reducing the “ideal” band gap of CdTe to gain more short-circuit current from long-wavelength absorption without sacrificing much VOC. Using the hybrid functional calculation, we find that the minimum band gap of the CdTe1−xSex alloy can be reduced from 1.48 eV at x = 0 to 1.39 eV at , and most of the change come from the lowering of the conduction band minimum. We also show that the formation of the alloy can improve the p-type doping of CuCd impurity based on the reduced effective formation energy and nearly constant effective transition energy level, thus possibly enhance VOC, thus PCE.

Improved power conversion efficiency in radial junction thin film solar cells based on amorphous silicon germanium alloys

Advanced material combining with novel 3D radial junction design is an attractive strategy to fabricate low-cost high-efficiency thin film solar cells. For improving the power conversion efficiency of traditional a-Si:H thin film solar cells, a conceptually new method incorporating Ge doping technology with 3D radial junction architecture is proposed here. By doping Ge with the optimal amount, the eternal quantum efficiency response of a-Si:Ge alloy thin film solar cell is greatly improved in the near-infrared wavelength. This enhanced spectrum response contributes to the improved power conversion efficiency according to the experimental results and theoretical simulations. On the other hand, by introducing the 3D radial junction architecture, both the excellent light trapping effect and the fast carrier separation contribute to the significantly improved short current density. With the optimal Ge doping amount, the radial junction a-Si:Ge alloy thin film solar cell achieves an open circuit voltage of 0.58 V, short current density of 20.14 mA/cm2, and fill factor of 53.7%, which contributes to the high power conversion efficiency up to 6.26%. From beginning to end, the fabrication temperature is lower than 400 °C and the whole fabrication process shows remarkable compatibility with current photovoltaic industry application. All above results suggest a potential route to the next generation of stable and highly efficient a-Si:H thin film solar cells.

Tailoring the Post-Etching of Room-Temperature-Fabricated H and Ti Co-Doped ZnO Films for Efficient Harvested Light Trapping

Chemical etching is widely applied to texture the surface of sputter-deposited zinc oxide films for efficient light-scattering in silicon thin-film solar cells. Based on experimental findings from our previous studies, two types of experimental techniques, a combination of HCl/NaOH and NaOH/HCl etching modes as well as a multi-step etching process, were considered to improve the light-trapping ability of room-temperature-deposited H and Ti co-doped ZnO (HTZO) films. The effects of the employed experimental procedures on the optical, electrical, and morphological properties of the HTZO films were systematically investigated. After optimization, higher haze values of 96.2%, 90.6%, and 74% at 550 nm, 800 nm, and 1100 nm, as well as a higher average haze value of 90.1% in the range of 400–1100 nm were achieved, which are better than those of the reference conventional HCl-etched Al-doped ZnO (AZO), high-light-trapping AZO, and SnO2:F films. These findings reveal a method to develop high-performance ZnO films for the front electrodes in high-efficiency Si thin-film solar cells.

8% Efficiency Cu2ZnSn(S,Se)4 (CZTSSe) Thin Film Solar Cells on Flexible and Lightweight Molybdenum Foil Substrates.

The use of flexible and highly conducting molybdenum (Mo) foil as a substrate offers several advantages such as a high thermal stability, smooth surface, and chemical inertness for the fabrication of high-efficiency thin film solar cells (TFSCs) by lowering the manufacturing costs. Here, we report a record preliminary efficiency of ∼8% for sputtered-grown Cu2ZnSn(S,Se)4 (CZTSSe) TFSCs on flexible and lightweight Mo foils. Careful studies were focused on identifying the role of preparative parameters such as annealing temperature, absorber composition, and post-preparative optimization to bridge the obtained record efficiency of ∼8% to a previous record efficiency of 7.04% for Na-incorporated CZTSSe sputter-based TFSCs. Interestingly, the preliminary record efficiency of ∼8% for our CZTSSe device grown via a scalable sputtering method was achieved by optimizing the absorber quality and post-preparative device optimization. While our preliminary results with a record efficiency demonstrate the potential of sputtering method, there is much scope for further improvement in the device efficiency by thoroughly understanding alkali element doping in the absorber layer.

High photoresponsivity in a GaAs film synthesized on glass using a pseudo-single-crystal Ge seed layer

Research to synthesize a high-quality GaAs film on an inexpensive substrate has been continuing for decades in the quest to develop a solar cell that achieves both high efficiency and low-cost. Here, we applied a large-grained Ge layer on glass, formed by Al-induced layer exchange, to an epitaxial template for a GaAs film. The GaAs film, grown epitaxially from the Ge seed layer at 520 °C, became a pseudosingle crystal (grain size > 100 μm) with high (111) orientation. Reflecting the large grain size, the internal quantum efficiency reached 70% under a bias voltage of 1.0 V. This value approaches that of a simultaneously formed GaAs film on a single-crystal Ge wafer and is the highest for a GaAs film synthesized on glass at a low temperature. The application of a Ge seed layer formed by layer exchange offers excellent potential to develop high-efficiency thin-film solar cells with III–V compound semiconductors based on low-cost glass substrates.

Atmospherically induced defects in (FASnI3)0.6(MAPbI3−3xCl3x)0.4 perovskites

The effects of brief atmospheric exposure are assessed for mixed composition perovskites comprised of a combination of formamidinium tin iodide (FASnI3) and methylammonium lead iodide + chloride (MAPbI3−3xCl3x) as well as MA0.7FA0.3PbI3 perovskite for use as absorber layers in high-efficiency thin film solar cells. Photothermal deflection spectroscopy (PDS) is employed to probe the relative defect density of all films by way of extracting the Urbach energy characterizing the width of the sub-bandgap absorption tail. Exposure to atmosphere for ~3 min prior to PDS measurement is found to degrade mixed composition films such that exposed films had an average increase of 36% in Urbach energies compared to those that are fully kept out of atmosphere through immersion in inert C6F14 liquid. By contrast, the same exposure to atmosphere induced no change in Urbach energy for MA0.7FA0.3PbI3. These results demonstrate the influence of ambient exposure on perovskites, particularly those containing Sn, and highlight the fact that handling procedure itself throughout all fabrication and subsequent characterization steps influences the measured results.

Bandgap tunable CdS:O as efficient electron buffer layer for high-performance Sb2Se3 thin film solar cells

Antimony selenide (Sb2Se3) is a promising low-cost and low-toxicity photovoltaic material. The electron buffer layer is of great significance for superstrate Sb2Se3 thin film solar cells. Herein, high-performance Sb2Se3 solar cells were fabricated by using sputtered, bandgap-tunable oxygenated cadmium sulfide (CdS:O) as a novel class of electron buffer layers. The optical transparency and energy band levels of CdS:O buffers were precisely adjusted by controlling the oxygen content in CdS:O layers. With the incorporation of an optimized wide band-gap CdS:O electron buffer layer, the device parameters JSC, VOC and FF were all improved. An efficiency of as high as 6.29% was achieved. It's found that, besides the reduced light absorption in the CdS:O layer, the high-quality CdS:O/Sb2Se3 junction played a vital role in the efficiency enhancement. The optimization of interfacial band alignment and defects passivation by atomic oxygen both contribute to the improved quality of CdS:O/Sb2Se3 heterojunction. This study demonstrates that the sputtered CdS:O is a promising electron buffer layer for preparation of high-efficiency and large-area Sb2Se3 thin film solar cells.

Fabrication of CIGS/CZTS Thin Films Solar Cells by Non-vacuum Process

Inorganic semiconductor compounds, e.g., CIGS and CZTS, are promising materials for thin film solar cells because of their high light absorption coefficient and stability. Research on thin film solar cells using this compound has made remarkable progress in the last two decades. Vacuum-based processes, e.g., co-evaporation and sputtering, are well established to obtain high-efficiency CIGS and/or CZTS thin film solar cells with over 20% of power conversion. However, because the vacuum-based processes need high cost equipment, they pose technological barriers to producing low-cost and large area photovoltaic cells. Recently, non-vacuum based processes, for example the solution/nanoparticle precursor process, the electrodeposition method, or the polymer-capped precursors process, have been intensively studied to reduce capital expenditure. Lately, over 17% of energy conversion efficiency has been reported by solution precursors methods in CIGS solar cells. This article reviews the status of non-vacuum techniques that are used to fabricate CIGS and CZTS thin films solar cells.

Oxidation-Induced Structure Transformation: Thin-Film Synthesis and Interface Investigations of Barium Disilicide toward Potential Photovoltaic Applications.

Barium disilicide (BaSi2) has been regarded as a promising absorber material for high-efficiency thin-film solar cells. However, it has confronted issues related to material synthesis and quality control. Here, we fabricate BaSi2 thin films via an industrially applicable sputtering process and uncovered the mechanism of structure transformation. Polycrystalline BaSi2 thin films are obtained through the sputtering process followed by a postannealing treatment. The crystalline quality and phase composition of sputtered BaSi2 are characterized by Raman spectroscopy and X-ray diffraction (XRD). A higher annealing temperature can promote crystallization of BaSi2, but also causes an intensive surface oxidation and BaSi2/SiO2 interfacial diffusion. As a consequence, an inhomogeneous and layered structure of BaSi2 is revealed by Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). The thick oxide layer in such an inhomogeneous structure hinders further both optical and electrical characterizations of sputtered BaSi2. The structural transformation process of sputtered BaSi2 films then is studied by the Raman depth-profiling method, and all of the above observations come to an oxidation-induced structure transformation mechanism. It interprets interfacial phenomena including surface oxidation and BaSi2/SiO2 interdiffusion, which lead to the inhomogeneous and layered structure of sputtered BaSi2. The mechanism can also be extended to epitaxial and evaporated BaSi2 films. In addition, a glimpse toward future developments in both material and device levels is presented. Such fundamental knowledge on structural transformations and complex interfacial activities is significant for further quality control and interface engineering on BaSi2 films toward high-efficiency solar cells.