Narrow Band Gap Research Articles

Tuning optical and electronic properties of indanthrene derivatives with electron-withdrawing groups (-Cl, -NO₂ and -C≡N) for enhanced organic solar cell performance: a DFT approach

Abstract In this study, we focused on improving the electronic and optical properties of the reference compound indanthrene (R) by introducing electron-withdrawing groups (−Cl, −C≡N, −NO₂). density functional theory and time-dependent density functional theory calculations were performed using Gaussian 16 software to study indanthrene derivatives as electron acceptor materials. The electronic and optical properties of the designed molecules (M1-M8) were investigated through frontier molecular orbital analysis, UV-visible absorption spectra, density of states, and transition density matrix analysis, utilizing GaussView 6.0 and Multiwfn 3.8 software. The designed molecules exhibit absorption across the entire visible spectrum, with maxima extending up to ~737 nm. Their electron affinity values range from 3.0 to 4.0 eV, surpassing the fullerene-based acceptor material. Among the designed molecules, M5 stands out with superior photovoltaic parameters, including a narrow optical band gap (~1.68 eV), exciton binding energy (0.25 eV), higher electron affinity (3.35 eV), an extended excited state lifetime (17.0 ns) owing to its low electron and hole reorganization energies (λe ~ 0.190 eV, and λh ~ 0.134 eV), and improved short-circuit current density of ~ 15.7 mA/cm². The photovoltaic parameters and power conversion efficiency were assessed using the Scharber and Alharbi models. The calculated device parameters, including light harvesting energy and photovoltaic characteristics (Voc, FF, Jsc, and PCE), suggest that these molecules, with electron-withdrawing groups, show great potential for use as acceptors in organic solar cells, particularly in terms of stability and power conversion efficiency. Notably, M2 and M5, with their PCE values exceeding 16%, emerge as outstanding candidates for device performance, impressively outperforming the other designed molecules.

Highly Efficient Lightweight Flexible Cu(In,Ga)Se2 Solar Cells with a Narrow Bandgap Fabricated on Polyimide Substrates: Impact of Ag Alloying, Cs and Na Doping, and Front Shallow Ga Grading on Cell Performance

Herein, lightweight, flexible Cu(In,Ga)Se2 (CIGS) solar cells with a narrow bandgap of ≈1 eV are grown on polyimide substrates. The poor performance of the CIGS solar cells owing to a low growth temperature (≈400 °C) is considerably improved via Ag alloying, Na doping using alkali‐silicate‐glass thin layers (ASTLs) and the CsF postdeposition treatment (CsF‐PDT), and front shallow Ga grading (surface field; SF). Along with improved device process, a notably high conversion efficiency of 21.2%, low VOC deficit of 0.346 V, and high JSC of ≈40 mA cm−2 are achieved. Ag alloying and Na doping using ASTLs predominantly improve the CIGS bulk quality, while the CsF‐PDT and SF reduce carrier recombination at the CIGS/CdS interface and vicinity. Device simulations reveal that the SF increases the electrical field at the CIGS surface under the forward bias voltage close to VOC owing to electron injection from the CdS side, which increases the chemical potential. Thus, the SF effectively repulses holes and improves the interfacial property. Device simulations also reveal that a high CIGS absorber's quality is prerequisite to benefit from the SF. Thus, CIGS solar cells showing improved bulk quality due to optimum alkali doping and Ag alloying considerably benefit from the SF.

Heterocoronenes Containing Pyridine and 1,2-Azaborine Units.

Several coronenes containing pyridine and azaborine units have been readily prepared and structurally confirmed by X-ray crystallographic analysis. The codoping results in interesting findings and properties such as the first observation of BN-H---NPy hydrogen bonds in crystals of BN-PAHs, short π-π stacking distances, lowered HOMO-LUMO levels, narrow band gap, and unique dual response to fluoride ion and proton in solution.

Unlocking high-performance near-infrared photodetection: polaron-assisted organic integer charge transfer hybrids

Room temperature femtowatt sensitivity remains a sought-after attribute, even among commercial inorganic infrared (IR) photodetectors (PDs). While organic IR PDs are poised to emerge as a pivotal sensor technology in the forthcoming Fourth-Generation Industrial Era, their performance lags behind that of their inorganic counterparts. This discrepancy primarily stems from poor external quantum efficiencies (EQE), driven by inadequate exciton dissociation (high exciton binding energy) within organic IR materials, exacerbated by pronounced non-radiative recombination at narrow bandgaps. Here, we unveil a high-performance organic Near-IR (NIR) PD via integer charge transfer between Poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (C-14PBTTT) donor (D) and Tetrafluorotetracyanoquinodimethane (TCNQF4) acceptor (A) molecules, showcasing strong low-energy subgap absorptions up to 2.5 µm. We observe that specifically, polaron excitation in these radical and neutral D-A blended molecules enables bound charges to exceed the Coulombic attraction to their counterions, leading to an elevated EQE (polaron absorption region) compared to Frenkel excitons. As a result, our devices achieve a high EQE of ∼107%, femtowatt sensitivity (NEP) of ~0.12 fW Hz-1/2 along a response time of ~81 ms, at room temperature for a wavelength of 1.0 µm. Our innovative utilization of polarons highlights their potential as alternatives to Frenkel excitons in high-performance organic IR PDs.

Colloidal quantum dots as solution-based nanomaterials for infrared technologies.

This review focuses on the recent progress of wet-chemistry-based synthesis methods for infrared (IR) colloidal quantum dots (CQD), semiconductor nanocrystals with a narrow energy bandgap that absorbs and/or emits infrared photos covering from 0.7 to 25 micrometers. The sections of the review are colloidal synthesis, precursor reactivity, cation exchange, doping and de-doping, surface passivation and ligand exchange, intraband transitions, quenching and purification, and future directions. The colloidal synthesis section is organized based on precursors employed: toxic substances such as mercury- and lead-based metals and non-toxic substances such as indium- and silver-based metal precursors. CQDs are prepared by wet-chemical methods that offer advantages such as precise spectral tunability by adjusting particle size or particle composition, easy fabrication and integration of solution-based CQDs (as inks) with complementary metal-oxide-semiconductors, reduced cost of material manufacturing, and good performances of IR CQD-made optoelectronic devices for non-military applications. These advantages may allow facile and materials' cost-reduced device fabrications that make CQD-based infrared technologies accessible compared to optoelectronic devices utilizing epitaxially grown semiconductors. However, precursor libraries should be advanced to improve colloidal infrared quantum dot synthesis, enabling CQD-based IR technologies to be available to consumer electronics. As the attention of academia and industry to CQDs continues to proliferate, the progress of precursor chemistry for IR CQDs could be rapid.

Enhancing thermoelectric efficiency of highly conductive CuSbTe2 alloy by reducing electrical thermal conductivity via heavy Se doping.

Recently, CuSbTe2, one of the I-V-VI-based compounds, has received attention as a promising thermoelectric (TE) material that exhibits a narrow bandgap with high electrical conductivity. In this study, the evolution of electrical and thermal transport properties of CuSbTe2 by heavy Se doping was investigated by synthesizing a series of CuSb(Te1-xSex)2 (x = 0, 0.1, 0.2, 0.3, and 0.4) compositions. The high electrical conductivity of CuSbTe2 (5400 S/cm) is gradually decreased to 1800 S/cm by Se doping with x = 0.4 at 300K with decreased carrier concentration and mobility. Due to this large reduction in electrical conductivity, the power factor of pristine CuSbTe2 significantly decreased to 0.98 mW/mK2 for x = 0.4 by 25%, along with reduced density-of-states effective mass at 550K. Nevertheless, the lattice thermal conductivity was reduced by 5%, and the electrical thermal conductivity was significantly reduced by 67% for x = 0.4 at 550K. Consequently, the total thermal conductivity of pristine CuSbTe2 (2.76 W/mK) is significantly reduced to 1.65 W/mK for x = 0.4 by 40%, mainly owing to the significant reduction of electrical thermal conductivity, which originates from the reduced electrical conductivity. Therefore, an enhanced TE figure of merit (zT) of 0.33 at 550K is observed for CuSb(Te0.6Se0.4)2 (x = 0.4), which was 26% higher than that of CuSbTe2. In addition, the expected zT for various carrier concentrations is calculated by using a single parabolic band model. It was found that the zT could be further enhanced by reducing the carrier concentration, which can be achieved by further doping of electrons.

Facile Synthesis of Bandgap-Engineered N-Doped MgO/g-C3N4 Nanocomposites for Enhanced Sunlight-Driven Photocatalytic Degradation of Organic Dyes

This study tests the theory of using a p-n junction to inhibit the recombination of photo-generated electrons and holes in N-MgO/g-C3N4 for the photocatalytic degradation of methylene blue (MB) dye. While the use of n-type semiconductor junctions with g-C3N4 for bandgap narrowing is well-studied in dye degradation, this research explores the innovative application of nitrogen (N)-doped MgO to create a p-type junction with n-type g-C3N4. The N-MgO/g-C3N4 nanocomposites were designed and synthesized to leverage both the bandgap narrowing effect and the formation of a p-n junction. The results demonstrate that these nanocomposites can effectively degrade MB dye under sunlight, achieving over 90% degradation efficiency with a bandgap of 2.63 eV. Under high pH conditions, the degradation efficiency further increased to 94%. The enhanced photocatalytic activity of the N-MgO/g-C3N4 composite is attributed to the formation of a heterojunction, which facilitates efficient separation and transmission of photo-induced charge carriers. This study discusses the detailed mechanism of this enhanced activity, emphasizing the role of the p-n junction in promoting effective charge separation and transfer, thereby improving the photocatalytic performance under sunlight.

Buried hole-selective interface engineering for high-efficiency tin-lead perovskite solar cells with enhanced interfacial chemical stability

Mixed Sn-Pb perovskites are attracting significant attention due to their narrow bandgap and consequent potential for all-perovskite tandem solar cells. However, the conventional hole transport materials can lead to band misalignment or induce degradation at the buried interface of perovskite. Here we designed a self-assembled material 4-(9H-carbozol-9-yl)phenylboronic acid (4PBA) for the surface modification of the substrate as the hole-selective contact. It incorporates an electron-rich carbazole group and conjugated phenyl group, which contribute to a substantial interfacial dipole moment and tune the substrate’s energy levels for better alignment with the Sn-Pb perovskite energy levels, thereby promoting hole extraction. Meanwhile, enhanced perovskite crystallization and improved contact at bottom of the perovskite minimized defects within perovskite bulk and at the buried interface, suppressing non-radiative recombination. Consequently, Sn-Pb perovskite solar cells using 4PBA achieved efficiencies of up to 23.45%. Remarkably, 4PBA layer provided superior interfacial chemical stability, and effectively mitigated device degradation. Unencapsulated devices retained 93.5% of their initial efficiency after 2000 h of shelf storage.

Sonocatalytic degradation of furazolidone using a heterogeneous triple ion synergy system of ZnFe2O4@ZIF-67 nanostructures: Physicochemical Characterization and degradation pathway

Heterogeneous sonocatalysis coupled with peroxydisulfate (PDS) activation is considered as a rapid advanced oxidation process for the degradation of emerging pharmaceutical pollutants. ZnFe2O4@ZIF-67 nanostructures (NSs) with commendable features including a high crystallinity, a narrow bandgap, and an outstanding specific surface, showed excellent sonocatalytic performance, PDS activation and stability. A novel triple ion synergistic system was used for the swift degradation of 60 mg/L furazolidone (FZD) from real contaminated water sources. A superior sonocatalytic degradation performance of FZD (94 %) by ZnFe2O4@ZIF-67/PDS/Ultrasound system was achieved under the optimal conditions of 20 mmol/L PDS and 0.4 g/L of the nanocomposite within 60 min. The e-/h+ pairs produced as a result of the sonoluminescence phenomenon in the ZnFe2O4@ZIF-67 NSs, with a bandgap of 1.86 eV, accelerated the redox reduction transform cycle of the Zn2+ /Zn, Fe3+/ Fe2+ and Co3+/ Co2+ pairs by generating a large number of oxidative radicals, leading to the enhanced degradation efficiency. Quenching tests illustrated that SO4•-,1O2, •OH radicals and surface oxidation–reduction reactions played a crucial role during FZD degradation. FZD degradation by-products were put forward using the LC-MS technique, and a plausible degradation pathway was suggested. The mineralization tests and the toxicity of generated intermediates of FZD degradation were also examined. Also, the stability and reusability results underscored the substantial proficiency of the above system to remediate real-pharmaceutical-contaminated-water sources, providing a practical approach for its industrial applications.

Enhanced degradation of levofloxacin using photocatalysis and peroxymonosulfate oxidation processes by Z-scheme BiVO4/g-C3N4 heterojunction

Constructing a direct Z-scheme heterostructure is an effective way to improve photogenerated charge separation efficiency. In this work, a novel BiVO4/g-C3N4 (BVO/CN) Z-scheme heterojunction was successfully fabricated through a liquid film combustion method and ultrasonic desperation technique. The obtained samples were analyzed by serials characterization techniques, such as XRD, FT-IR, XPS, SEM, HRTEM, EDS, BET, UV-vis, and PL. The results demonstrated a strong integration between CN and BVO, as evidenced by the absorption edge shifting to 520nm and revealing a narrow band gap (Eg=2.47eV), thereby enhancing light absorption capacity. Under visible light irradiation, the removal efficiency of levofloxacin (LVFX) reached 97.7% within 40min using 20 BVO/CN, which was respectively 2.03 and 2.21 times higher than that achieved with BVO or CN alone. The mechanism of the BVO/CN Z-scheme heterojunction was elucidated based on the ESR test and radical trapping experiment. Additionally, the potential degradation pathway of LVFX was analyzed using HPLC-MS. This study demonstrates the feasibility of constructing a Photocatalyst/Visible-light/peroxymonosulfate (PMS) system for mitigating antibiotic-induced ecological pollution.

Construction of Fe and g-C3N4 codoped magnetic bamboo charcoal for enhanced catalytic degradation of tetracycline: Mechanism, degradation pathway, and ecological toxicity.

The well-designed bamboo charcoal (BC) composite Fe-g-C3N4/BC with multi-active sites of FeOx, FeNx, and g-C3N4, was fabricated in-situ by calcining Fe-melamine loaded bamboo charcoal (Fe-Me-BC) under nitrogen atmosphere. The as-synthesized Fe-g-C3N4/BC(550) exhibited a mesoporous structure with a large specific surface area of 108.23 m2/g. The adsorption of tetracycline (TCL) on Fe-g-C3N4/BC(550) was calculated following the Langmuir isotherm model, and showed a maximum adsorption capacity of 19.92 mg/g. Furthermore, the pseudo-second-order kinetic model showed a good fit for the TCL adsorption process on Fe-g-C3N4/BC(550). The Fe-g-C3N4/BC(550)/H2O2 system exhibited excellent photo-Fenton catalytic performance in degrading TCL with a degradation efficiency reaching up to 98.9% within 5 min under visible-light. The effects of initial pH value and coexisting anions on TCL degradation were determined. As narrow band gap semiconductors, g-C3N4, Fe3O4, and Fe2O3 in the Fe-g-C3N4/BC exhibited good visible-light-driven photocatalytic activity. Moreover, photogenerated electrons could further activate H2O2 to produce high concentrations of ∙OH radicals. This outstanding photo-Fenton catalytic performance can be ascribed to the synergistic effect of g-C3N4/Fe3O4-Fe2O3/FexN multi-active sites as well as the excellent adsorption ability and conductivity provided by bamboo charcoal. This work presents a convenient approach for constructing economical catalysts for environmental remediation through g-C3N4 and Fe-N codoped BC.

A novel mesoporous Bi2MoO6/g-C3N4 nanocomposite as an effective photocatalyst against toxic organic pollutants

This study used microwave irradiation technique to synthesize a sequence of Bi2MoO6@g-C3N4 nanocomposite as photocatalysts. The two narrow band gap nanomaterials used were g-C3N4 and Bi2MoO6. The nanocomposites were thoroughly characterized by XRD, FESEM, EDS, HRTEM, Raman spectroscopy, UV–visible spectra, PL and XPS. Moreover, using sunlight radiation, two distinct dyes including Malachite Green (MG) and Acid Blue 113 (AB 113) were degraded using as synthesized photocatalyst. The outcomes indicated that the photocatalytic degradation efficiency of Bi2MoO6@ g-C3N4 was higher than that of pure g-C3N4. A 95 % degradation efficiency towards Malachite Green and an 84 % degradation efficiency towards Acid Blue 113 shown the great effectiveness of Bi2MoO6@ g-C3N4. This enhanced efficiency is attributed to the increased visible light absorption, effective charge separation due to the heterojunction interface between Bi2MoO6 and g-C3N4, and the increased surface area facilitating greater active site availability. According to these results, the synthesized Bi2MoO6@ g-C3N4 would be very beneficial for the removal of organic contaminants in a variety of industrial environments.

Oxygen vacancies and octahedral distortion induced bandgap narrowing in [KNbO3](1−x)–[Ba(Ni0.1Zn0.3Nb0.6)O3−δ]x perovskites for visible-light photocatalysis: a combined experimental and theoretical study

Oxygen vacancies and octahedral distortion induced bandgap narrowing in [KNbO3](1−x)–[Ba(Ni0.1Zn0.3Nb0.6)O3−δ]x perovskites for visible-light photocatalysis: a combined experimental and theoretical study

Polymerized non-fullerene small molecular acceptor as a versatile electron-transport material for 24.50 % efficiency inverted perovskite solar cells with extended spectral response

Development of novel non-fullerene electron-transport materials (ETMs) with excellent optoelectronic properties, good thermal and chemical stability, and defect passivation capability is of great significance for improving the device performance of inverted perovskite solar cells (PSCs). In this work, a polymerized non-fullerene small molecular acceptor with narrow bandgap named PY-DFT is rationally designed and synthesized. The resultant materials not only exhibits great hydrophobicity, thermal and chemical stability, but also maintains the excellent electron-transport properties of Y-type small molecule. Moreover, the electron-rich units in PY-DFT finely passivates the surface defects in perovskite. Consequently, an exceptional PCE of 24.50 % has been achieved for the champion device based on PY-DFT ETM, which is the highest efficiency for inverted PSCs based on non-fullerene ETMs to date. Notably, PY-DFT provides an extra current density of 0.25 mA/cm2 in the near-infrared light response region, where the intrinsic perovskite film has no spectral response, demonstrating attractive advantages in rationally utilization of solar spectrum. The target device without encapsulation also demonstrates good long-term operational stability. This work paves a new avenue for designing photo-active non-fullerene electron-acceptor towards efficient and stable inverted PSCs.

Covalent triazine frameworks as particle electrode for three-dimensional photoelectrocatalytic degradation of oxytetracycline: Synergy effects, pathway, and mechanism.

Photoelectrocatalysis has been widely employed for degrading antibiotics due to its high efficiency. However, the application is significantly impeded by the rapid recombination of photogenerated charge carriers and the limited surface areas of photoelectrodes. In the study, high crystallinity covalent triazine frameworks were fabricated at low temperature of 150°C and firstly used as particle photoelectrode in the three-dimensional photoelectrochemical reactor to degrade oxytetracycline (OTC). SEM, TEM, XRD, XPS, and FT-IR confirmed the successful synthesis of high crystallinity covalent triazine frameworks. Compared to CTF-120 (71.2%) and CTF-180 (46.9%), CTF-150 exhibited excellent OTC removal. Electrochemical impedance, UV-vis absorption spectra, and Mott-Schottky tests showed that CTF-150 demonstrated more wide light absorption range of 501nm and narrow bandgap of 2.52eV, and smaller Rct value under illumination, in comparing to CTF-120 and CTF-180. When the initial concentration of OTC was 50mgL-1, the 86.2% of OTC removal and 62.7% of mineralization were obtained under light irradiation (λ>420nm), current of 10mA, pH of 6.4, electrolyte of 0.1M Na2SO4. The synergy effect between photocatalytic and electrocatalytic processes of CTF-150 not only enhanced by 38.5% current efficiency but also reduced energy consumption to 1.90 kWh m-3. CTF-150 had a wide range of acid-base application and displayed resistance on coexisting ions. Electron spin resonance detection, quenching experiments, and probe experiments illustrated that h+, •O2-, 1O2, and •OH contributed to the degradation of OTC and the generation pathways of •O2-, 1O2, and •OH were verified. Moreover, •O2-, 1O2, and h+ were the main reactive species responsible for OTC removal, while 1O2 was for OTC mineralization. Based on high-performance liquid chromatography-tandem mass spectrometry detection, OTC with benzene ring was decomposed to opening ring products. The acute toxicity, developmental toxicity, bioaccumulation factor and mutagenicity of OTC and its intermediates using T.E.S.T. showed the toxicity of 82.35% degradation products decreased.

Thermal Shielding Gd3TaO7-Based Thermal Barrier Ceramic with Ultralow NIR Transmittance.

Most thermal barrier coating materials exhibit transparent/semi-transparent properties at higher temperatures, causing the surface heat flow to directly heat the substrate with infrared radiation, which significantly reduces the thermal barrier effectiveness. Herein, composite ceramic materials composed of GdFeO3 diffusely dispersed within the Gd3TaO7 are produced. Specifically, the 0.9Gd3TaO7/0.1GdFeO3 composition demonstrates an ultra-low near-infrared (NIR) transmittance of less than 0.1% across the 400-2500nm range. The introduction of variable-valence behavior and oxygen vacancies contribute to the narrow bandgap of GdFeO3. The incorporated GdFeO3 produces extra multimode vibrations, resulting in the strong infrared emissivity of Gd3TaO7/GdFeO3 composite ceramics. Besides, the refractive index difference between Gd3TaO7 and GdFeO3 can contribute to the potential photons scattering within the composites, severely impeding the infrared radiation penetration. The upward curvature of the thermal conductivity curve of 0.9Gd3TaO7/0.1GdFeO3 at high temperature is significantly suppressed, confirming its excellent IR radiation shielding properties. Moreover, the Gd3TaO7/ GdFeO3 composite ceramic exhibits thermal suitability, with a thermal expansion coefficient (9.47-9.67 × 10-6 K-1 at 1400°C) comparable to YSZ. These combined benefits position the Gd3TaO7/ GdFeO3 composite ceramic system as a promising candidate for the development of infrared radiation shielding and high-emissivity thermal radiation materials.

Fabrication of C and N co-doped CdS semiconductor photocatalysts derived from a novel Cd-MOF for enhancing photocatalytic performance

The narrow bandgap value and long lifetime of photo generated charges make CdS exhibit higher photocatalytic activity compared to other semiconductor catalysts. Given the excellent optoelectronic properties of CdS, a novel 2D cadmium-organic framework (Cd-MOF) was obtained by solvothermal method, which was served as a precursor template in this paper. C and N co-doped CdS-T (T = 600/800/1000) semiconductor materials were successfully prepared by chemical deposition method under different high temperatures. The as-prepared materials had been characterized in detail. Furthermore, the Cd-MOF and CdS-T were used to conduct the photocatalytic activity of organic dyes. Among them, CdS-800 showed excellent photocatalytic degradation effects on five organic dyes, especially on RB with the photocatalytic degradation efficiency of 98.87 %, which was significantly improved 6.5 times compared to Cd-MOF (58.23 %). Free radical capture experiments have shown that •O2− play a dominant role in the photocatalytic process. In addition, UV–vis diffuse reflection, photoluminescence (PL), photoelectrochemical (PEC) testing and density functional theory (DFT) calculations had shown that CdS-800 could effectively delay the recombination of photo generated charges, improve the rapid migration and separation of photo generated carriers, and enhance photocatalytic activity. This work provides new ideas for the preparation of efficient and recyclable CdS semiconductor photocatalytic materials.

Highly efficient BODIPY-based covalent triazine frameworks for photocatalytic H2O2 production and photodegradation

Photocatalysts based on covalent triazine frameworks (CTFs) showcased excellent prospect in treating wastewater contaminated with organic compounds. However, effectively combining adsorption and degradation processes presents a significant hurdle for enhancing photocatalytic efficiency. This work focuses on the elegant design of an advanced photocatalyst based on sulfonated A-D-A-type CTFs, with enhanced photosynthesis of H2O2, and photodegradation efficiency of 99 % degradation of 10 ppm BPA in just 15 min and 25 ppm RhB in only 5 min. This enhancement can be ascribed to heightened photocurrent mobility, significant reduction in electrochemical impedance, narrowed band gap, and enhanced dispersibility compared to the non-sulfonated counterpart of the CTF-based photocatalyst. Mechanistic investigations suggested that the photodegradation of BPA and RhB are dominated by ·O2– and 1O2, respectively. This finding will provide a useful guide to the design of high-performance photocatalyst for wastewater remediation.

Highly efficient narrow bandgap Cu(In,Ga)Se2 solar cells with enhanced open circuit voltage for tandem application

Although an ideal bandgap matching with 0.96 eV and 1.62 eV for a double-junction tandem is hard to realize practically, among all mature photovoltaic systems, Cu(In,Ga)Se2 (CIGSe) can provide the closest bandgap of 1.00 eV for the bottom sub-cell by adjusting its composition. However, pure CuInSe2 (CISe) solar cell suffers strong interfacial carrier recombination. We hereby present approaches to introduce appropriate Ga gradients in both the back and front parts of absorber while maintaining the absorption spectrum close to CISe. With an appropriate front Ga gradient, the open circuit voltage can be enhanced by ~30 mV. With a pre-deposited CIGSe layer and a high copper excess deposition during absorber growth, the Ga diffusion can be well suppressed and a wide U-shaped Ga grading with a minimum bandgap of 1.01 eV has been created. Our optimized narrow-bandgap CIGSe solar cell has achieved a certified record PCE of 20.26%, with a record-low open circuit voltage deficit of 368 mV and a record-high contribution of 10% absolute efficiency to a four-terminal tandem. This work demonstrates the potential of controlling gallium diffusion to improve the performance of narrow bandgap CIGSe solar cells for tandem applications.

Electronic and optical properties of Sb2Se3 and Sb2S3: theoretical investigations

Abstract In recent developments in solar energy research, Sb2Se3 and Sb2S3 emerge as environment friendly photovoltaic absorber materials, distinguished by their narrow bandgap and high absorption coefficient. Theoretical investigations to determine the electronic structure, effective density of states, dielectric function, and absorption coefficient of Sb2Se3 and Sb2S3 crystals have been performed using first-principle methods. The results reveal band gap values of about 0.822 and 1.757 eV (PBE method), 1.114 and 1.778 eV (HSE06 method) for Sb2Se3 and Sb2S3, respectively. The valence band and conduction band edges are primarily formed by Se 4p, S 3p, and Sb 5p hybridized orbitals. The effective density of states (DOS) exhibit magnitudes on the order of 1019 cm−3. Notably, anisotropic characteristics are observed in the real and imaginary parts of the dielectric function. Furthermore, the absorption coefficient surpasses 105 cm−1 at 1 and 1.2 eV for both Sb2Se3 and Sb2S3. The result indicates that these highly efficient absorber materials are suitable in collecting solar energy.