Efficient Thin Film Solar Cells Research Articles

Towards highly efficient thin-film solar cells with a graded-bandgap CZTSSe layer. Part II: piecewise-homogeneous bandgap grading

Abstract In Part I, we optoelectronically optimized a thin-film solar cell with a graded-bandgap CZTSSe photon-absorbing layer and
a periodically corrugated backreflector, using the hybridizable discontinuous
Galerkin (HDG) scheme to solve the drift-diffusion equations.
The efficiency increase due
to periodic corrugation was minimal, but significant improvement was achieved
with a nonlinearly graded bandgap. Due to occasional failures of the HDG
scheme from negative carrier densities, we developed a new computational scheme using the finite-difference
method, which also reduced the overall computational cost of optimization. Later, a normalization error
was discovered in the electrical submodel in Part I, but it did not alter the overall conclusions. We have
now re-optimized the solar cells with (i) a homogeneous bandgap, (ii) a linearly graded bandgap, or (iii)
a nonlinearly graded bandgap, and (iv) piecewise-homogeneous bandgap which is easier to implement than a
continuously graded bandgap.
An efficiency of 13.53\% is predicted with a three-layered piecewise-homogeneous CZTSSe 
layer. Furthermore, concentrating sunlight by a factor of one hundred can boost the efficiency to 16.70\%
 with the
piecewise-homogeneous bandgap.

Efficiency Investigation and Modeling of an Ultra Thin-film CIGS Solar Cell using WxAMPS

The highly efficient CIGS thin-film solar cell is numerically investigated in this paper using solar simulator software wxAMPS (Analysis of Microelectronic and Photonic Structures). WxAMPS is a customized simulation software package mainly used for photovoltaic cells, which supports quick data input and enhanced visualization with its improved user interface. Copper–indium–gallium–diselenide Cu(In, Ga)Se2 (CIGS) is a semiconductor material with chalcopyrite crystal structure that has high conversion efficiency and structural stability. CIGS model- ZnO: Al/ZnO/CdS/CIGS/Mo/substrate is mainly experimental research that considers the physical characteristics, dimensions, and thicknesses of the different layers. The bilayer window and absorber layer with different thicknesses are the critical factors that influence solar cell performance. The bilayer window concept can assist in reducing the loss at the window layer. In this paper, through the numerical simulation, the highest conversion efficiency was achieved, 18.7%, with an optimum bandgap of 1.12 eV with 3000 nm thickness of the absorber layer under AM 1.5G.

3D/2D lead-free halide perovskite CsSnBr3/MoX (X = Se2, SSe, S2) heterostructures for high-efficiency solar cell: Interface engineering strategies and transition of band alignments

The rapid progress in the fabrication of van der Waals (vdW) heterostructures based on perovskite has significantly advanced the fields of optoelectronics and solar cells by integrating the exceptional properties of different materials. Overall, perovskite-based vdW heterostructures display unique functional characteristics due to their varied type-I, type-II, and type-III energy band configurations. However, it is a challenge to achieve flexible regulation of band edge alignment in heterostructures for different applications. Using first-principles density functional theory, we systematically study the electronic and optical properties of vdW heterostructures system consisting of three-dimensional lead-free all-inorganic halide perovskite materials CsSnBr3 and MoX (X = Se2, SSe, S2). The research results show that in the CsSnBr3/MoX vdW heterostructures, type I, II, and III transitions of band arrangements are achieved through interface engineering strategy to adapt to different applications. Meanwhile, by calculating the light absorption efficiency of the constructed 3D/2D vdW heterostructures, it was found that the interface effect enhances the optical absorption range and intensity of the perovskite-based heterostructures. The theoretically estimated power conversion efficiency (PCE) of the heterostructure thin-film solar cell reaches 21.4 %. The results of our research indicate that the controllable band alignment of CsSnBr3/MoX heterostructures has significant potential for future applications in optoelectronics.

Facile Tailor on the Surface of Mo Foil Toward High-Efficient Flexible CZTSSe Solar Cells.

Flexible Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is attracting much attention because of their enormous application prospects. However, current flexible CZTSSe solar cells with Mo foil as substrate still suffer from severe back interface problems due to the complexity of substrate surfaces. Herein, a facile approach to tailor the surface of the flexible substrate and modify the back interface between CZTSSe and Mo foil is proposed. The study discloses that a simple polishing can not only improve the wettability of the precursor solution on the substrate unexpectedly and thus improve the quality of the CZTSSe film, but also increase the mechanical stability of the absorber layer grown on Mo foil. The subsequent UV-ozone treatment helps to form a favorite MoO3 layer for efficient CZTSSe devices. Surprisingly, the quasi-ohmic contact is formed between CZTSSe/Mo foil by such combined treatments and thus promoting the carrier collection. Consequently, the efficiency of the flexible CZTSSe solar cell is significantly improved from 4.94% to 10.32% without anti-reflection layer. The bending durability of the cell fabricated on the treated Mo foil is increased greatly. This work discloses that back contact interface is very important for the carrier collection and thus the highly efficient flexible thin film solar cells.

An electro-thermal finite element method (FEM) model for local hotspot kinetics in Cu(In, Ga)Se2 thin-film solar modules

Partial shading can significantly impair the efficiency of thin-film solar cells. When exposed to partial shading, cells within the array tend to become reverse biased, leading to thermal runaway events and the emergence of hotspots. In Cu(In,Ga)Se2 (CIGS) solar cells such hotspots are also associated with so-called worm-like defects. Both theoretical and experimental studies have shown that in CIGS, a positive-feedback loop leads to instability and thermal runaway events. However, we observe an inconsistency between published simulation results and recently published experimental work. In a recent experimental study, it was shown that under certain conditions, a hotspot develops within 1ms, showing signs of melting of the CIGS in an area with a 5μm radius. However, in published simulation results, the time for such high temperatures to develop is in the order of seconds, a discrepancy of three orders of magnitude. In this work, we argue that this discrepancy is explained by the size of the seed defect, demonstrating that the origin of these experimentally observed, fast-developing hotspots is likely microscopic defects. To this end, we developed an electro-thermal finite element model, with very high temporal and spatial resolution. We demonstrate that, assuming a seed defect with a 10nm radius, we can reproduce the experimental results with respect to the size of the defect and the time it took to develop.

Boosting the performance of SnSe-based solar cells through electron and hole transport layers: A qualitative study and perspectives

The SnSe compound is garnering considerable interest as a potential solar absorber for developing highly efficient thin-film solar cells. Using one experimental report as a foundation and employing the one-dimensional solar cell capacitance simulator (SCAPS-1D), we examined the elements that impact the efficiency of solar cells utilizing tin selenide as the base material. The base structure consists of Anode/SnSe/CdS/i-ZnO/ZnO:Al/Cathode. A 0.5 μm thick SnSe film demonstrated an efficiency of 2.51 %. In the initial phase, we optimized the device efficiency to 18.65 % through parameter adjustments, utilizing toxic CdS as a buffer layer. In the subsequent phase, earth-abundant and non-toxic Zn1-xMgxO, SnO2, and TiO2 alternatives were explored as electron transport materials (ETL) to replace CdS. Zn1-xMgxO (with x = 0.1875) exhibited the highest efficiency at 19.03 %. In the final phase, various hole transport materials (HTL) were studied to enhance the SnSe-based solar cell's performance. Among the HTL materials investigated, NiO yielded the best efficiency of 20.59 % when using Zn1-xMgxO (with x = 0.1875) as a buffer layer.

Enhancing the photovoltaic efficiency of CZTSSe thin-film solar cells via Ag-doping induced defect modulation

Defect modulation is one of the key factors in determining the performance of solar cells, and the presence of defects tends to adversely affect carrier recombination and transport, thus having a direct impact on both cell stability and efficiency. Ag doping can modulate the defects, but in-depth studies on the mechanism of the effect are needed. Therefore, an in-depth study of the effect of Ag incorporation on the defect modulation mechanism of Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is of great practical significance for understanding the mechanism of action of Ag doping and further optimising the cell performance. In this study, (AgxCu1-x)2ZnSn(S,Se)4 (0 ≤ x ≤ 1) (ACZTSSe) solar cells were successfully prepared by N, N-dimethylformamide (DMF) solution, which have the characteristics of reducing defects and enhancing photovoltaic performance. After the introduction of Ag, the absorber formed a crystal structure with few holes and penetrating top and bottom. And the electrical performance of the solar cell is significantly improved, J0 is reduced from 4.6 × 10−5 A/cm2 to 2.7 × 10−6 A/cm2, and its contribution to the photovoltaic conversion efficiency (PCE) improvement reaches 142%. This indicates that the introduction of Ag helps to optimize the quality of the absorber, thereby reducing the concentration of interfacial and bulk defects. It also reduces carrier recombination and improves carrier lifetime from 0.86 ms to 1.75 ms. Ultimately increasing our cell efficiency from 7.55% to 10.64% when the Ag content reached 5%. These results clearly show that we have successfully achieved the modulation of defects in CZTSSe solar cells by Ag doping.

A Review of CIGS Thin Film Semiconductor Deposition via Sputtering and Thermal Evaporation for Solar Cell Applications

Over the last two decades, thin film solar cell technology has made notable progress, presenting a competitive alternative to silicon-based solar counterparts. CIGS (CuIn1−xGaxSe2) solar cells, leveraging the tunable optoelectronic properties of the CIGS absorber layer, currently stand out with the highest power conversion efficiency among second-generation solar cells. Various deposition techniques, such as co-evaporation using Cu, In, Ga, and Se elemental sources, the sequential selenization/sulfidation of sputtered metallic precursors (Cu, In, and Ga), or non-vacuum methods involving the application of specialized inks onto a substrate followed by annealing, can be employed to form CIGS films as light absorbers. While co-evaporation demonstrates exceptional qualities in CIGS thin film production, challenges persist in controlling composition and scaling up the technology. On the other hand, magnetron sputtering techniques show promise in addressing these issues, with ongoing research emphasizing the adoption of simplified and safe manufacturing processes while maintaining high-quality CIGS film production. This review delves into the evolution of CIGS thin films for solar applications, specifically examining their development through physical vapor deposition methods including thermal evaporation and magnetron sputtering. The first section elucidates the structure and characteristics of CIGS-based solar cells, followed by an exploration of the challenges associated with employing solution-based deposition techniques for CIGS fabrication. The second part of this review focuses on the intricacies of controlling the properties of CIGS-absorbing materials deposited via various processes and the subsequent impact on energy conversion performance. This analysis extends to a detailed examination of the deposition processes involved in co-evaporation and magnetron sputtering, encompassing one-stage, two-stage, three-stage, one-step, and two-step methodologies. At the end, this review discusses the prospective next-generation strategies aimed at improving the performance of CIGS-based solar cells. This paper provides an overview of the present research state of CIGS solar cells, with an emphasis on deposition techniques, allowing for a better understanding of the relationship between CIGS thin film properties and solar cell efficiency. Thus, a roadmap for selecting the most appropriate deposition technique is created. By analyzing existing research, this review can assist researchers in this field in identifying gaps, which can then be used as inspiration for future research.

Optimization of CdSe Thin-Film Photoelectrochemical Cells: Effects of NaOH/Na2S/S Redox Couple Concentration and Activity on Cell Efficiency

This study investigates the relationships among redox couple activity, electrolyte concentration, and efficiency in CdSe thin-film photoelectrochemical solar cells. A CdSe photo-electrode was prepared using the electro-depositing technique to produce well-staged layering of CdSe, followed by chemical bath deposition to produce a layer with an acceptable thickness to absorb enough photons to create a suitable amount of photocurrent. The CdSe photo-electrochemical cell was tested under various concentrations of a NaOH/Na2S/S electrolyte solution. The results showed that the activity of the redox couple greatly affected the efficiencies of the solar cells. Correlation plots between ionic strength and PEC efficiency with the Debye–Hückel equation yielded an R² value of 0.96, while those between ionic strength and photocurrent density had an R² value of 0.92. The correlation between concentration and PEC efficiency was much weaker. This paper highlights how optimal ionic activity increases the performance of photoelectrochemical solar cells, which consequently improves the conversion efficiency of solar energy.

(Invited) Semiconductor Nanostructures for Light Energy Conversion

This talk will review some of the key developments of semiconductor quantum dots based light energy conversion. Semiconductor nanostructures are finding new ways to design light energy conversion devices (e.g., thin film solar cells and light emitting devices). The decreased consumption of energy during the manufacture and the lessened use of semiconductor materials lowers the overall carbon footprint with energy payback time less than a year for such devices. The early studies which focused on the synthesis of various semiconductor nanostructures and exploration of their size dependent optical and electronic properties have led to their their integration in high efficiency thin film solar cells.. Unlike solar cell devices, photocatalytic processes are yet to deliver conversion efficiencies and stability that can make them compatible for practical applications. Understanding the limitations imposed by interfacial charge transfer processes remains a key to further advance photocatalytic systems for chemical energy storage. Recent developments in utilizing semiconductor nanostructures for light energy conversion and storage will be discussed.

Fabrication of efficient methylammonium-free perovskite thin-film solar cells without post-treatment and antisolvent

Recently, there has been a remarkable improvement in the efficiency of organic-inorganic perovskite solar cells (PSCs). However, the complexity of the fabrication process for these PSCs has increased due to the post-processing of the perovskite layer and the use of antisolvents, among others. In this investigation, a straightforward method for fabricating high-performance and CH3NH3+-free [CH(NH2)2PbI3]0.95[CsPbBr3]0.05 solar cells using fluorinated phosphoric acid passivating additives in combination with mixed solvents was presented. This study required neither post-treatment nor anti-solvent for solar cell fabrication. Fluorinated phosphoric acid was introduced into the perovskite layer, which then self-organized on the perovskite surface and exhibited a passivating effect. Planar PSCs fabricated at reduced temperatures without fluorinated phosphoric acid exhibited power conversion efficiencies (PCEs) of 19.9 % and 17.3 % during reverse and forward scanning, respectively. Conversely, PSCs fabricated with fluorinated phosphoric acid as an additive exhibited a remarkable PCE of 20.5 % without hysteresis and a stabilized power output of 20.4 %. This study will facilitate the fabrication of high-efficient PSCs in future research efforts, thereby promoting the advancement of these devices.

Low-temperature influence on the properties and efficiency of thin-film perovskite solar cells fabricated by the PVco-D technique

The study discussed in this publication aimed to develop a perovskite solar cell adapted to operating conditions at reduced temperature and pressure and less than 1 μm thick. The physical vapor co-deposition technique was proposed and successfully used to create a solar cell structure with a thin layer of hybrid perovskite as an energy-collecting material. A comprehensive analysis of methylammonium lead iodide behavior in a wide temperature range from 10 K to 320 K was carried out to implement this project. The initial phase included assessing the degradation of the perovskite material layer after cooling to the temperature of liquid helium and then re-heating it to room temperature. Then, using spectroscopic techniques, the characteristics of the layers' optical and electrical properties were determined. The obtained results allowed the design of the complete structure of a thin-film perovskite cell, which was made using only the vacuum sublimation process. Simulations using the SCAPS-1D program and experimental results showed high agreement and allowed for obtaining an efficiency of approximately 18.5 % in the interested temperature range. Perovskite solar cell stability tests over six months confirmed the positive impact of the proposed technique for depositing the complete cell structure on its temporal stability. The research results are optimistic for the applications of perovskite cells in space.

Point-junction and alkali-assisted surface selenium diffusion: Unveiling a two-step method for enhancing the efficiency of VTD-SnS thin-film solar cells

Surface and interface engineering in thin-film solar cells (TFSCs) is vital for optimizing charge carrier dynamics, reducing recombination, enhancing material properties, and ultimately improving efficiency and overall performance. In this study, e-beam evaporation was used to deposit selenium (Se) onto tin sulfide (SnS) absorber layers with a thickness of 10 nm to 50 nm. The Se layer optimization was validated using various characterization techniques, which confirmed the formation of micro-sized openings via Se droplet deposition. This Se passivation-plus-local opening geometry plays a crucial role in forming point-junctions with the n-type CdS during cell fabrication, presenting a novel approach in state-of-the-art technology. The results revealed a significant enhancement in the conversion efficiency of the SLG/Mo/SnS/Se/CdS/i-ZnO/AZO/Al device to 3.01 % (VOC = 0.326 V, JSC = 20.21 mA cm−2, and FF = 0.45) with 30 nm Se layer deposition. Furthermore, a 30 nm-thick NaF-assisted Se layer was deposited to improve device performance. The diffusion of both Se and Na into the SnS substrate was validated using XRD, SEM, EDS, AFM, and XPS. The power conversion efficiency (PCE) of the SLG/Mo/SnSSe-NaF/CdS/i-ZnO/AZO/Al device was further enhanced to 3.70 % (VOC = 0.340 V, JSC = 22.56 mA cm−2, and FF = 0.48). This enhancement is attributed to the creation of localized junctions via Se surface passivation, and the concurrent SnSSe formation induces band gap tuning and increases the grain size, contributing to additional PCE improvement during Na-assisted Se diffusion.

Optimizing thin-film silicon solar cells with nanostructured TiO2 and a silver back reflector for enhanced energy conversion efficiency

This paper investigates the improvement of energy conversion efficiency in thin-film silicon solar cells by employing periodic nanostructures of TiO2 on the silicon active layer and a back reflector featuring periodic nanostructures of silver. The objective is to increase the optical path length, enhance absorption probability for longer wavelengths, and subsequently improve solar cell performance. Three silicon-based solar cell configurations are proposed and simulated using the finite difference time domain (FDTD) method to assess their performance. Electrical characteristics are obtained through the drift-diffusion method. The resulting short-circuit current density increased from 40.93 to 65.28 to 95.373mA/cm2 for the three cells, leading to significant improvements in conversion efficiency with observed values of 20.39%, 33.26%, and 47.28%, respectively, in the optimized structures. Furthermore, we compare the simulation results of the three structures with those of a reference structure and several structures previously proposed in the literature.

Designing novel plasmonic architectures for highly efficient CIGS solar cells

CIGS solar cells is one of the best thin film solar cells which have efficiency up to 22.6 %. In the recent decades, a lot of attention has been drawn to the methods of increasing solar cells efficiencies. One of the emerging technology for enhancing thin film solar cell efficiencies are using plasmonic effects in the visible or infrared part of the electromagnetic spectrum. Therefore, the primary goal of this study is to use plasmonic nanostructures in the active layer to enhance the absorption and consequently the efficiency of a thin film CIGS solar cell with a 1000 nm thickness. Different factors such as type of material, size and surface occupied with nanostructure can be effect on CIGS solar cell performance. In this research, the solar cell has been optically and electrically characterized in the presence of plasmonic nanostructure by simulations. The efficiency was determined by characteristics parameters such as absorption, quantum efficiency, electron-hole generation rate, current–voltage curve and power-voltage curve. Based on our simulations, the maximum efficiency of 25.61 % was achieved using gold nanocubic structures with dimensions of 100 nm, Occupied Factor of 0.16, positioned at the top of the active layer. This increase is due to increase in optical path of length of scattered light and consequently its absorption in the active layer. Moreover, enhancing carrier generation, decreasing recombination and improving carrier separation can be occurred due to localized surface plasmon resonance and the near-field distribution effect, which effectively scatter light into the active layer.

Nickel-doped silver nanoclusters as a mechanism to capture photons

Narrow width of optical absorption of conducting polymers and photons energy losses have been the challenges for fabricating highly efficient thin-film organic solar cell. Nickel-doped silver nanoclusters (Ni/Ag NCs) are employed here to capture more photons using polymers blend solar absorber medium to improve solar cell performances. The poly-3-hexylthiophene and (6-6)phenyl-C61-butyric acid methyl ester molecules blend were used a solar absorber layer in this investigation. The solar cells fabricated with NCs exhibited enhanced opt-electronic properties compared to the reference solar cell. Consequently, the experimental results suggest that the power conversion efficiency (PCE) has substantially increased with the incorporations of NCs in absorber layer, which is dependent on the concentrations of NCs in the medium. The maximum PCE achieved, in this work, is η\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\eta $$\\end{document} = 6.2% at 2% of NCs by weight, which has exhibited to the lowest energy losses compared to other doping levels. This improvement in PCE is attributed to the occurrence of local surface plasmon resonance effect due to the inclusion of Ni/Ag NCs in polymer matrix. The results provide valuable insights on the use of Ni/Ag NCs for efficient photons capture in thin-film polymers blend medium.

Hierarchical pore structure with a confined resonant mode for improving the solar energy utilizing efficiency of ultra-thin perovskite solar cells

The perovskite solar cell (PSC) has the benefits of flexibility, inexpensiveness, and high efficiency, and has important prospective applications. However, serious optical losing and low solar energy-utilizing efficiency remain a challenge for the ultra-thin PSCs because of the interface reflection of traditional planar structure. In this study, a hierarchical pore structure with a confined resonant mode is introduced and optimized by electromagnetic theory to improve the solar energy absorbing and utilizing efficiency of ultra-thin PSCs. The large pores in the top layer that support a whispering gallery mode can focus and guide the incident light into the solar cell. The small pores in the bottom layer enable backward scattering of the unabsorbed light and can improve the effective absorption of active layer. The finite-difference time-domain method is employed to optimize the geometric parameters of hierarchical pore structure to improve the light absorption of PSCs. The proposed resonant hierarchical pore structure can greatly improve sunlight absorption of ultra-thin PSCs, and the effective light absorption and photocurrent of PSCs with a hierarchical pore structure is 20.7% higher than that of PSCs with traditional planar structure. This work can offer a beneficial guideline for improving solar energy utilizing efficiency of various thin-film solar cells.

Buried junction and efficient carrier separation in CdSexTe1−x/CdTe solar cells

Alloying CdTe absorbers with Se is a critical advancement for the fabrication of highly efficient CdTe thin-film solar cells. Herein, the role of the Se concentration gradient in CdSexTe1−x/CdTe solar cells is stressed in addition to the decreased bandgap and passivation effect of Se. The buried graded CdSexTe1−x at the front interface of the device was stripped and analyzed by quasi in situ x-ray photoelectron spectroscopy depth profiling. A serial shift of Fermi level toward the valence band was probed as the Se concentration decreased in the graded CdSexTe1−x absorber, revealing the presence of n-type CdSexTe1−x region near the front contact. Kelvin probe force microscopy characterizations and voltage-dependent photocurrent collection analysis further confirmed the existence of a buried junction in the graded CdSexTe1−x absorber. The CdS-free CdTe solar cell with a graded CdSexTe1−x/CdTe absorber fabricated in this study showed an efficiency of 17.6%. These results indicated that the non-uniform distribution of Se introduced a built-in field in the graded CdSexTe1−x absorber and enabled efficient separation of photogenerated carriers, yielding high-performance CdTe solar cells in the absence of a conventional n-type CdS window layer. This study deepened the understanding of the performance enhancement in Se-containing CdTe solar cells and provided new ideas for further performance optimization.

Regulating the crystal orientation of vapor-transport-deposited GeSe thin films by a post-annealing treatment

Recently, GeSe has emerged as a highly promising photovoltaic absorber material due to its excellent optoelectronic properties, nontoxicity, and high stability. Although many advantages make GeSe well suited for thin-film solar cells, the power conversion efficiency of the GeSe thin-film solar cell is still much below the theoretical maximum efficiency. One of the challenges lies in controlling the crystal orientation of GeSe to enhance solar cell performance. The two-step preparation of GeSe thin films has not yet been reported to grow along the [111] orientation. In this work, we study the effect of a post-annealing treatment on the GeSe thin films and the performance of the solar cells. It was found that amorphous GeSe films can be converted into polycrystalline films with different orientations by changing the post-annealing temperature. [111]-oriented and [100]-oriented GeSe thin films were successfully prepared on the same substrate by optimizing the annealing conditions. With the structure of Au/GeSe/CdS/ITO cell devices, PCEs of 0.14% and 0.16% were ultimately achieved.

ZnxCd1–xSySe1–y as an effective electron transport layer for improving the efficiency of Sb2S3 and Sb2Se3 thin-film solar cells

ZnxCd1–xSySe1–y as an effective electron transport layer for improving the efficiency of Sb2S3 and Sb2Se3 thin-film solar cells