Absorber Material For Solar Cells Research Articles

Flexible Cu2znsn(S, Se)4 Solar Cells over 7% Power Conversion Efficiency

The development of flexible solar cells is necessary for achieving market competitiveness through the implementation of low cost solar cells and for applying customized business models, such as Building Integrated Photovoltaics (BIPV). Cu2ZnSn(S,Se)S4 (CZTSSe) thin films have attracted attention as low-cost absorber materials for solar cells; however, further studies are required to develop flexible solar cells from this material and to achieve a high power conversion efficiency. In this work, the carrier separation characteristics of flexible CZTSSe thin-film solar cells on Mo foil substrates were identified. The surface electrical characteristics of the CZTSSe absorber layer indicate that the topography, surface potential, and local current are proportional to one another, in contrast to the characteristics exhibited by CIGS cells. For the CZTSSe absorber layer, an upward band bending-like band structure formed at and near the GBs, and a current path formed along the IGs as holes collected toward the GBs, and electrons collected toward the IGs. In contrast to that in CIGS cells, the carrier separation induced by the formation of the upward band bending-like band structure at the GBs is crucial in CZTSSe thin-film solar cells. Furthermore, the positive potential distribution at the IGs and average potential difference showed a proportional relationship to the local current and PCE. Through the implementation of these carrier separation characteristics, a flexible CZTSSe thin-film solar cell on a Mo foil substrate achieved a PCE of over 7%. Acknowledge: This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) (No. 20173010012980) and by the DGIST R&D Programs of the Ministry of Science, ICT & Future Planning of Korea (19-BD-05).

Novel synthesis method for quaternary Cd(Cu, Zn)Se thin films and its characterizations

Nowadays, the search for novel compounds by chemical synthesis is in trend. Herein, we report the deposition of Cd1-x-yZnxCuySe (0.025 ≤ x = y ≤ 0.15) films by facile, industry-oriented chemical synthesis. The Cd1-x-yZnxCuySe thin films were deposited at the optimized growth conditions (temperature = 70 ± 0.1 °C, pH = 10.3 ± 0.1, substrate rotation speed = 70 ± 2 rpm and time = 100 min). As-synthesized thin films were characterized for physical, chemical, topographical and electrical attributes. The study of vibrational modes in Cd1-x-yZnxCuySe thin films was done with the help of Raman spectroscopy. Improvement in surface topography with the integration of Cu2+ and Zn2+ into the CdSe lattice has been noticed by the atomic force microscopy (AFM). The electrochemical impedance spectroscopy revealed lower values of Rs and Rct for x = y = 0.05 composition. Chemical deposition of Cd1-x-yZnxCuySe thin films may offer an excellent way to fabricate quaternary chalcogenide-based absorber materials for solar cells.

Observation of photoluminescence edge emission in CuSbSe2 absorber material for photovoltaic applications

CuSbSe2 has great potential for being an earth-abundant absorber material in efficient thin film solar cells. In this study, the radiative recombination in CuSbSe2 polycrystalline material was studied by temperature and laser power dependent photoluminescence spectroscopy which revealed the band-to-band and band-to-deep acceptor transitions, with the last one involving a deep acceptor defect with an activation energy of 165 ± 6 meV. The observation of the edge emission in CuSbSe2 refers to the high quality of the synthesized material.

Highly efficient and stable planar heterojunction solar cell based on sputtered and post-selenized Sb2Se3 thin film

Antimony selenide (Sb2Se3) is regarded as one of the key alternative absorber materials for conventional thin film solar cells due to its excellent optical and electrical properties. Here, we proposed a Sb2Se3 thin film solar cell fabricated using a two-step process magnetron sputtering followed by a post-selenization treatment, which enabled us to optimize the best quality of both the Sb2Se3 thin film and the Sb2Se3/CdS heterojunction interface. By tuning the selenization parameters, a Sb2Se3 thin film solar cell with high efficiency of 6.06% was achieved, the highest reported power conversion efficiency of sputtered Sb2Se3 planar heterojunction solar cells. Moreover, our device presented an outstanding open circuit voltage (VOC) of 494 mV which is superior to those reported Sb2Se3 solar cells. State and density of defects showed that proper selenization temperature could effectively passivate deep defects for the films and thus improve the device performance.

Free-electron effects on the optical absorption of the hybrid perovskite CH3NH3PbI3 from first principles

Hybrid organic-inorganic perovskites, such as methyl-ammonium lead tri-iodide (MAPbI$_3$), are interesting candidates for efficient absorber materials in next-generation solar cells, partly due to an unusual combination of low exciton binding energy and strong optical absorption. Excitonic effects in this material have been subject to debate both for experiment and theory, indicating a need for better understanding of the screening mechanisms that act upon the electron-hole interaction. Here we use cutting-edge first-principles theoretical spectroscopy, based on density-functional and many-body perturbation theory, to study atomic geometries, electronic structure, and optical properties of three MAPbI$_3$ polymorphs and find good agreement with earlier results and experiment. We then study the influence of free electrons on the electron-hole interaction and show that this explains consistently smaller exciton binding energies, compared to those in the material without free electrons. Interestingly, we also find that the absorption line shape strongly resembles that of the spectrum without free electrons up to high free electron concentrations. We explain this unexpected behavior by formation of Mahan excitons that dominate the absorption edge, making it robust against free-electron induced changes observed in other semiconductors.

Controlled thickness of a chemical-bath-deposited CdS buffer layer for a SnS thin film solar cell with more than 3% efficiency

Tin monosulfide (SnS) is considered a promising earth-abundant absorber material for thin film solar cells (TFSCs) with a theoretical power conversion efficiency (PCE) as high as 30%. However, the maximum PCE reported thus far is just 4.36%. This inferior device performance is attributed to low values of open-circuit voltage. In addition to the dense and pinhole-free surface morphology of the phase-pure p-type SnS thin film absorber, a controlled growth of the n-type buffer layer is a prerequisite for enhanced device performance. In this study, we explore the manner in which the thickness of a chemical-bath-deposited n-type CdS buffer layer influences the performance of SnS/CdS TFSCs. The thickness of the buffer layer was adjusted by varying the deposition time from 15 to 25 min, which resulted in thicknesses of 30–80 nm. Initially, the performance parameters of the TFSCs improved with the buffer layer thickness, but later deteriorated with thicker CdS layers. The highest PCE achieved was 3.05% at an optimized CdS buffer layer thickness of approximately 42 nm. This performance was accompanied by a significant decrease in reverse saturation current and shunt conductance, as evidenced by the current density–voltage characteristics under dark conditions.

Research progress in lead-less or lead-free three-dimensional perovskite absorber materials for solar cells

The trend toward lead-free or lead-less perovskite solar cells (PSCs) has attracted increasing attention over the past few years because the toxicity of lead (Pb) is one of the substantial restrictions for large-scale applications. Researchers have investigated the viability of substituting Pb with other elements (group 14 elements, group 2 elements, transition-metal elements, and group 13 and 15 elements) in the three-dimensional (3D) perovskites by theoretical calculations and experimental explorations. In this paper, recent research progress in Pb-less and Pb-free PSCs on the perovskite compositions, deposition methods, and device structures are summarized and the main problems that hinder the enhancement of device efficiency and stability are discussed in detail. To date, the fully Sn-based PSCs have shown a power conversion efficiency (PCE) of 8.12% and poor device stability. However, lead-less PSCs have shown higher PCE and a better stability. In addition, the introduction of double-perovskite materials also draws researchers’ attention. We believe that the engineering of elemental composition, perovskite deposition methods, and interfacial modification are critical for the future development of Pb-less and Pb-free PSCs.

Triplet-Sensitization by Lead Halide Perovskite Thin Films for Near-Infrared-to-Visible Upconversion

Lead halide-based perovskite thin films have attracted great attention due to the rapid increase in perovskite solar cell efficiencies. The same optoelectronic properties that make perovskites ideal absorber materials in solar cells are also beneficial in other light-harvesting applications and make them prime candidates as triplet sensitizers in upconversion via triplet–triplet annihilation in rubrene. In this contribution, we take advantage of long carrier lifetimes and carrier diffusion lengths in perovskite thin films, their high absorption cross-sections throughout the visible spectrum, and the strong spin–orbit coupling owing to the abundance of heavy atoms to sensitize the upconverter rubrene. Employing bulk perovskite thin films as the absorber layer and spin-mixer in inorganic/organic heterojunction upconversion devices allows us to forego the passivating ligands required for colloidal sensitizers, which can hinder exciton transport through large scale arrays and reduce the triplet transfer effic...

Highly oriented GeSe thin film: self-assembly growth via the sandwiching post-annealing treatment and its solar cell performance.

GeSe is considered as a potential absorber material for thin film solar cells owing to its ideal band gap, strong light absorption, remarkable air durability, Earth-abundance and non-toxic constituents. However, the high vapor pressure of GeSe at a temperature below its melting point makes it difficult to synthesize a high-quality GeSe film. To alleviate this limitation, in this work, a thermal evaporation combining a novel sandwiching post-annealing method was introduced to deposit high quality GeSe thin films with (100)-orientation. The self-assembling mechanism of the highly oriented GeSe film was carefully investigated by the systematic experiments and confirmed by the lowest total energy of the (100) crystal plane. Finally, the fully-inorganic, low-cost and non-toxic planar device with the superstrate configuration of FTO/TiO2/GeSe/carbon/Ag was also successfully fabricated. Notably, as a result, an impressive open circuit voltage (VOC) of 340 mV (maximum: 456 mV) was achieved, which is the highest VOC of GeSe solar cells reported so far. Furthermore, through current-voltage, capacitance-voltage profiling and drive level capacitance profiling measurements, it was demonstrated that the limiting factors of the GeSe solar cell performance were the narrow depletion width (138 nm) and the drastic recombination at the TiO2/GeSe interface.

Highly oriented CuSbS2 thin films by rapid thermal processing of pre-annealed Sb2S3-Cu layers for PV applications

Copper antimony sulfide (CuSbS2) is an emerging absorber material in solar cells due to its nearly ideal optoelectronic properties, earth abundancy and environmentally benign composed elements. In the present work, highly oriented CuSbS2 thin films have been prepared by annealing layered structures of chemical bath deposited Sb2S3 and thermal evaporated Cu, followed by rapid thermal processing (RTP). A systematic study was performed by varying the processing time. Crystalline structure, morphology, elemental composition, chemical state, optical and electrical properties of the films formed at different conditions were analyzed using various techniques. X-Ray diffraction analysis showed that the post RTP treatment resulted CuSbS2 thin films with preferential orientation along (301) planes perpendicular to the substrate surface, on glass as well as Mo coated glass substrates. Optical properties showed that CuSbS2 has a direct bandgap of 1.6 eV supported by photo-current sensitivity at different wavelengths, which is nearly the ideal bandgap for an absorber layer in solar cell.

Modification of Mo Back Contact with MoO3−x Layer and its Effect to Enhance the Performance of Cu2ZnSnS4 Solar Cells

Kesterite Cu2ZnSnS4 (CZTS) has been investigated intensively as a promising absorber material for thin film solar cells. However, the reported best power conversion efficiency (PCE) is still low due to the intrinsic limitations of CZTS defects and the unfavorable front and back contact interfaces. In this study, an intermediate MoO3−x layer is introduced as a primary back contact prior to the Cd‐doped Cu2ZnSnS4 (CZCTS) absorber layer growth. It has been demonstrated that the insertion of the MoO3−x layer can suppress the decomposition reaction between CZCTS and Mo, reducing secondary phases and voids and improving the rear interface quality. The MoO3−x layer can also accelerate potassium diffusion into the CZCTS layer and interfaces, which facilitates the grain growth, passivates interfaces and hence yields better crystallinity. Moreover, the band alignment at the back contact is modified by the MoO3−x layer, resulting in better minority carrier collection and improvement of open‐circuit voltage (VOC). With the optimal MoO3−x layer, the PCE of CZCTS solar cells has increased from ≈5.43 to ≈7%, largely attributed to the ≈60 mV VOC increment. Through the modification of the CZCTS precursor and optimizing the anti‐reflection coating layer, the best PCE achieved is 8.98%, with an active area efficiency of 9.26%.

Nanospheres and nanoflowers of copper bismuth sulphide (Cu3BiS3): Colloidal synthesis, structural, optical and electrical characterization

Ternary copper-bismuth-sulphide (CBS) semiconducting nanocrystals (NCs), with optimal band gap values of ∼1.5 eV and optical absorption coefficient α > 104 cm−1, have been investigated in recent years as an absorber material for thin film solar cells. We report on a facile hot injection method to synthesize phase-pure CBS NCs of Cu3BiS3 in the form of nanospheres and nanoflowers. The precursor, capping ligands, and reaction temperature play a key role in stabilizing the structural phase and morphology of the synthesized nanocrystals. The resultant nanocrystals films show distinct photo-response characteristics, which makes them attractive as absorber materials for use in thin film solar cells.

Optical and Electrical Properties of Copper Zinc Tin Sulfide Films As Solar Cell Absorber Materials Prepared by a Compacting Process

The copper zinc tin sulfide (CZTS) thin films are prepared by a promising non vacuum compacting process based on ball milling and coating technique. The results reveal that a well-formed crystalline structure of kesterite CZTS are observed after sintering at 450°C. The optical band gap is 1.49, 1.59, 1.33, and 1.21 eV for the films with Cu/Zn/Sn atomic radio of 1.00 : 0.50 : 0.52, 1.00 : 0.44 : 0.41, 1.00 : 0.60 : 0.49, and 1.00 : 0.49 : 0.58, respectively. The compacting process can further improve the performances of CZTS film. For the CZTS thin film with Cu/Zn/Sn atomic radio of 1.00 : 0.60 : 0.49, the surface morphology of the film is significantly improved by a compacting process applied to the coating films before sintering, and the resistivity decreased from 23.67 to 1.18 Ω cm with the compacting pressure increasing from 0 to 200 MPa at room temperature, which showing a promising application of the compacting process method in fabricating CZTS film for solar cells.

Full-inorganic Sb2(S,Se)3 solar cells using carbon as both hole selection material and electrode

Sb2(S,Se)3 are considered as a promising absorber material for solar cells owing to its earth-abundant, nontoxic constituent, simple phase and long-term durability. In this work, a facile chemical bath deposition followed by post-selenization was introduced to prepare Sb2(S,Se)3 thin film, which leads to a graded band gap structure. The sulfoselenide Sb2(S,Se)3 thin films with smooth morphology were obtained through optimizing the selenization process. Meanwhile, a low cost and substitutable carbon electrode was used as hole selection material. Sequentially, the full-inorganic Sb2(S,Se)3 device with efficiency of 2.64% was also fabricated according to structure of FTO/CdS/Sb2(S,Se)3/Carbon, which is the highest value in CBD processed FTO/CdS/Sb2(S, Se)3/Carbon solar cells.

Optical and structural properties of orthorhombic and tetragonal polymorphs of Cu2CdGeSe4

The quaternary Cu2CdGeSe4 (CCGSe) is a potential absorber material for thin film solar cells. This study presents the results of photoluminescence (PL) and X-ray diffraction analysis of high-temperature and low-temperature CCGSe polycrystals with orthorhombic (space group Pmn21) and tetragonal (space group I4¯2 m) crystal structure, respectively. Detailed PL analysis revealed different dominating radiative recombination mechanisms for orthorhombic and tetragonal CCGSe.

Identifying Short-Range Disorder in Crystalline Bulk Cu2SnS3 Phases: A Solid-State Nuclear Magnetic Resonance Spectroscopic Investigation

Cu2SnS3 has recently attracted attention as a low-cost, earth-abundant absorber material for thin-film solar cells. Unfortunately, multiple Cu2SnS3 structures exist and the conditions under which different structures form are poorly defined. The structures of tetragonal and monoclinic Cu2SnS3 phases are clarified by use of solid-state nuclear magnetic resonance spectroscopy (ssNMR). The monoclinic Cu2SnS3 phase is shown to form under Sn-rich, Cu-poor conditions and the tetragonal Cu2SnS3 phase forms under Cu-rich, Sn-poor conditions, indicating that there is a composition difference between the structures that has not previously been reported. Furthermore, the monoclinic phase is destabilized at temperatures between 200 and 345 °C, when the material becomes tetragonal.

Improved efficiency of Sb2Se3/CdS thin-film solar cells: The effect of low-temperature pre-annealing of the absorbers

Sb2Se3 is one of the alternative absorber materials for thin-film solar cells (TFSCs) comprising non-toxic and low-cost elements. The duration of pre-annealing at 100 °C prior to rapid thermal annealing (RTA) influenced the final morphology, crystallinity, and preferred orientation of thermally evaporated Sb2Se3. Pre-annealing for 40 min promoted the crystallization of pure Sb2Se3 phase following RTA; however, a long pre-annealing time such as 100 min rendered morphology similar to that of the reference absorber that was not pre-annealed, as ascertained by detailed scanning electron microscopy, X-ray diffraction, and Raman spectroscopic analyses. Furthermore, Sb2Se3/CdS TFSCs were fabricated with Sb2Se3 pre-annealed for various durations. The highest efficiency of 1.47% was achieved for the optimized 40-min pre-annealed absorber, enabling a smooth current flow. Our results emphasize the detailed optimization of pre-annealing time for achieving efficient TFSCs.

Band alignment of Cd-free (Zn, Mg)O layer with Cu2ZnSn(S,Se)4 and its effect on the photovoltaic properties

Cu2ZnSn(S,Se)4 (CZTSSe) is an interesting absorber material for thin film solar cells. However, one of the key challenges for the kesterite-based solar cells is to improve the open-circuit voltage (Voc) deficit, which is resultant of recombination at the interface of buffer/absorber. In this work, Cd-free n-type buffer layers with two different Mg-doped ZnO layers (Mg0.26Zn0.74O, Mg0.30Zn0.70O) have been examined using ultraviolet photoelectron spectroscopy. The most important electronic properties which are essential for the band offset study, ie. fermi level location, valence and conduction band offsets at the interface in the CZTSSe substrate, have been determined. The conduction band offset values for Mg0.26Zn0.74O, Mg0.30Zn0.70O buffer layers has been calculated experimentally. We have also established the correlation between device parameters and performances for dual ion beam sputtered ZnO buffer/CZTSSe-based heterojunction solar cells as a function of conduction band offset and the energy distribution of interface defects, to gain deeper understanding about the Voc-deficit behavior from a high recombination rate at the buffer/kesterite interface using simulation study. From the simulation study, the values of the solar cell efficiency with Mg0.26Zn0.74O and Mg0.30Zn0.70O buffer layers are 10.18 and 10.25%, respectively, which are higher in comparison to those obtained by using conventional CdS buffer layer.

Secondary phases and temperature effect on the synthesis and sulfurization of CZTS

Copper, Zinc and Tin sulfides were synthesized by solution method employing Na2S as sulfur source. Individual sulfides were sulfurized in non-vacuum condition at 350°, 450°, 550° and 650 °C to obtain CZTS. Structural, optical and morphological analysis indicated major portion of CZTS along with secondary phases such as SnS2, SnS and Cu3SnS4 at 350° and 450 °C whereas the secondary phases vanishing significantly at 550° and 650 °C. The optical studies indicated high absorption coefficient for CZTS in the range of 104–105 cm−1. Electrochemical analysis demonstrated that the CZTS samples prepared by the current methodology could be very well used as absorber material in thinfilm solar cells.

Investigation of single phase Cu2ZnSnxSb1−xS4 compounds processed by mechanochemical synthesis

The copper zinc tin sulfide (CZTS) compound is a promising candidate as an alternative absorber material for thin-film solar cells. In this study, we investigate the direct formation of $\mathrm{C}{\mathrm{u}}_{1.92}\mathrm{ZnS}{\mathrm{n}}_{x}(\mathrm{S}{\mathrm{b}}_{1\ensuremath{-}x}){\mathrm{S}}_{4}$ compounds $\text{[CZT}(A)\text{S]}$, with $x=1$, 0.85, 0.70, and 0.50, via a mechanochemical synthesis (MCS) approach, starting from powders of the corresponding metals, zinc sulfide, and sulfur. The thermal stability of the $\text{CZT}(A)\text{S}$ compounds was evaluated in detail by in situ synchrotron high-energy x-ray diffraction measurements up to 700 \ifmmode^\circ\else\textdegree\fi{}C. The $\text{CZT}(A)\text{S}$ compounds prepared via MCS revealed a sphalerite-type crystal structure with strong structural stability over the studied temperature range. The contribution of the MCS to the formation of such a structure at room temperature is analyzed in detail. Additionally, this study provides insights into the MCS of CZTS-based compounds: the possibility of a large-scale substitution of Sn by Sb and the production of single phase $\text{CZT}(A)\text{S}$ with a Cu-poor/Zn-poor composition. A slight increase in the band gap from 1.45 to 1.49--1.51 eV was observed with the incorporation of Sb, indicating that these novel compounds can be further explored for thin-film solar cells.