Electron Transport Material Research Articles

Effect of substrate temperature and oxygen plasma treatment on the properties of magnetron-sputtered CdS for solar cell applications

Cadmium sulfide (CdS) is an n-type semiconductor with excellent electrical conductivity that is widely used as an electron transport material (ETM) in solar cells. At present, numerous methods for preparing CdS thin films have emerged, among which magnetron sputtering (MS) is one of the most commonly used vacuum techniques. For this type of technique, the substrate temperature is one of the key deposition parameters that affects the interfacial properties between the target film and substrate, determining the specific growth habits of the films. Herein, the effect of substrate temperature on the microstructure and electrical properties of magnetron-sputtered CdS (MS-CdS) films was studied and applied for the first time in hydrothermally deposited antimony selenosulfide (Sb<sub>2</sub>(S,Se)<sub>3</sub>) solar cells. Adjusting the substrate temperature not only results in the design of the flat and dense film with enhanced crystallinity but also leads to the formation of an energy level arrangement with a Sb<sub>2</sub>(S,Se)<sub>3</sub> layer that is more favorable for electron transfer. In addition, we developed an oxygen plasma treatment for CdS, reducing the parasitic absorption of the device and resulting in an increase in the short-circuit current density of the solar cell. This study demonstrates the feasibility of MS-CdS in the fabrication of hydrothermal Sb<sub>2</sub>(S,Se)<sub>3</sub> solar cells and provides interface optimization strategies to improve device performance.

Structural modification of fullerene derivates for high-performance inverted perovskite solar cells

This review focuses on the design strategies of fullerenes and their derivatives as electron transport materials in inverted PSCs, and the effects of different application forms on the photovoltaic performance and stability of the devices.

Synthesis and structures of cobalt-expanded zirconium- and cerium-oxo clusters as precursors for mixed-metal oxide thin films.

Transforming current complementary metal-oxide-semiconductor (CMOS) technology to fabricate memory chips and microprocessors into environmentally friendlier electronics requires the development of new approaches to resource- and energy-efficient electron transport and switching materials. Metal and multi-metal oxide layers play a key role in high-end technical applications. However, these layers are commonly produced through high-energy and high-temperature procedures. Herein, we demonstrate our first attempts to obtain stimuli-responsive mixed-metal oxide thin films from solution-processed molecular precursors under milder conditions. The molecular compounds of interest were prepared by one-pot reactions of a CoII carboxylate complex, triethylamine (Et3N), N-butyldiethanolamine (H2bda), and a hexanuclear complex [Ce6O4(OH)4(piv)12] (Hpiv = pivalic acid) or [Zr6O4(OH)4(ib)12(H2O)]·3Hib (Hib = isobutyric acid) in acetonitrile solution. The resulting charge-neutral, heterometallic coordination compounds display a ligand-supported pentanuclear {CeIV3CoIII2} core (in 1) and a dodecanuclear {ZrIV6CoII6} core (in 2), exhibiting thermal stability up to ca. 100 °C in air. Compound 2 was deposited and analyzed on Au(111) and SiO2/Si(100) surfaces to explore its potential as a single-molecule precursor for the preparation of atomically precise, complex mixed-metal oxide thin films. The adsorption characteristics of it demonstrate the ability to form stable agglomerates on the investigated surfaces.

Electroluminescence efficiency and stability of near ultraviolet organic light-emitting diodes based on BCPO luminous materials

To date, in the traditional method of obtaining near-ultraviolet (NUV) light, mercury atoms, which can create a highly toxic heavy metal contaminant, have been used. Therefore, it is an important issue to obtain NUV light by using new environmentally friendly devices. In the last decade, the fabrication of near ultraviolet organic light-emitting diodes (NUV-OLEDs) has become a research hotspot in the field of organic electronics. However, when the electroluminescence wavelength is extended to shorter than 400 nm, higher requirements are put forward for the materials used for each functional layer in these devices. In this work, a wide bandgap small molecule material of BCPO is used as the luminescent layer. The electron-transporting and hole-transporting materials are determined based on the overlaps between absorption spectra of these materials and emission spectrum of BCPO. And NUV-OLEDs with electroluminescent peak wavelength at 384 nm are prepared. By using the optimal device structure, the maximum external quantum efficiency of the device reaches 2.98%, and the maximum radiance of the device reaches 38.2 mW/cm<sup>2</sup>. In the electroluminescence spectrum, NUV light with wavelengths below 400 nm accounts for 57% of the light emission. In addition, the device demonstrates good stability when biased at two different constant voltage modes. The multiple key factors which affect the stability of the device are analyzed in detail. Firstly, it is found that the high glass transition temperature (<i>T</i><sub>g</sub>) of hole-transporting material is very important for the long-time stability of this device. The poor device stability is closely related to the low <i>T</i><sub>g</sub> temperature of hole-transporting material. Secondly, due to the widespread use of PEDOT:PSS as hole injection material in OLEDs, the electron leakage from the hole-transpor layer into the PEDOT:PSS layer may cause significant damage to the conducting polymer. When bombarded with low energy electrons, bond breakage occurs on the surface of PEDOT:PSS, followed by the release of oxygen and sulfur, resulting in changes in conductivity and oxidation reactions with molecules of hole transport material. Thirdly, the photoelectrical stability of organic molecules is the most fundamental reason that restricts the device lifetime. The aging process of material or device is directly relevant to the bond dissociation energy (BDE) of organic molecule. Generally, the BDE value of organic molecule is not high enough. As a result, molecules are prone to chemical bond breakage during electrochemical or photochemical aging. In summary, highly stable NUV-OLEDs should be fabricated by using hole-transporting materials with high <i>T</i><sub>g</sub> temperature, sufficient electron-blocking capacity, and large BDE value.

An open-cage bis[60]fulleroid as an electron transport material for tin halide perovskite solar cells.

An open-cage bis[60]fulleroid (OC) was applied as an electron transport material (ETM) in tin (Sn) halide perovskite solar cells (PSCs). Due to the reduced offset between the energy levels of Sn-based perovskites and ETMs, the power conversion efficiency (PCE) of Sn-based PSCs with OC reached 9.6% with an open-circuit voltage (VOC) of 0.72 V. Additionally, OC exhibited superior thermal stability and provided 75% of the material without decomposition after vacuum deposition. The PSC using vacuum-deposited OC as the ETM could afford a PCE of 7.6%, which is a big leap forward compared with previous results using vacuum-deposited fullerene derivatives as ETMs.

Evaluating the influence of novel charge transport materials on the photovoltaic properties of MASnI3 solar cells through SCAPS-1D modelling.

In recent decades, substantial advancements have been made in photovoltaic technologies, leading to impressive power conversion efficiencies (PCE) exceeding 25% in perovskite solar cells (PSCs). Tin-based perovskite materials, characterized by their low band gap (1.3 eV), exceptional optical absorption and high carrier mobility, have emerged as promising absorber layers in PSCs. Achieving high performance and stability in PSCs critically depends on the careful selection of suitable charge transport layers (CTLs). This research investigates the effects of five copper-based hole transport materials and two carbon-based electron transport materials in combination with methyl ammonium tin iodide (MASnI3) through numerical modelling in SCAPS-1D. The carbon-based CTLs exhibit excellent thermal conductivity and mechanical strength, while the copper-based CTLs demonstrate high electrical conductivity. The study comprehensively analyses the influence of these CTLs on PSC performance, including band alignment, quantum efficiency, thickness, doping concentration, defects and thermal stability. Furthermore, a comparative analysis is conducted on PSC structures employing both p-i-n and n-i-p configurations. The highest-performing PSCs are observed in the inverted structures of CuSCN/MASnI3/C60 and CuAlO2/MASnI3/C60, achieving PCE of 23.48% and 25.18%, respectively. Notably, the planar structures of Cu2O/MASnI3/C60 and CuSbS2/MASnI3/C60 also exhibit substantial PCE, reaching 20.67% and 20.70%, respectively.

Synergistic Effect of CNT-TiO2 Catalyst and 3D Electrode Architecture for Electrochemical Performance in Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) are a potential electrochemical storage system for the future with high capacity (1675 mAh g−1) and energy density (∼2600 Wh kg−1). The poor conductivity of sulfur, polysulfide shuttle effect, and volume expansion of the sulfur cathode are the main hurdles to their commercialization. To mitigate these issues, this work represents a rational composite of hybrid multiwalled carbon nanotubes (MWCNT) -TiO2 high surface area carbon-sulfur composite (CTHS) onto a 3D carbon fiber (CF) based free-standing electrode (CTHS@CF) architecture. TiO2 can effectively anchor the polysulfides by chemical bonding and improve cyclability. MWCNTs and CFs are the effective electron transport materials that accelerate the redox kinetics of polysulfides. The electrochemistry of CTHS@CF reveals an excellent discharge capacity of 910 mAh g−1 (1st cycle) at 100 mA g−1 compared to the conventional aluminum-coated (CTHS@Al) of 532 mAh g−1. The CTHS@CF (at 300 mA g−1) displays 514 mAh g−1 (initial discharge) capacity with 83% capacity retention up to 100 cycles, whereas CTHS@Al shows 394 mAh g−1 with 44% capacity retention. Combining 3D electrode architecture with the metal oxide (TiO2) plays a vital role in the electrochemistry of LSBs by improving the stability of the battery’s cycle life and overall energy density.

Self-assembled molecular network with waterwheel-like architecture: experimental and theoretical evaluation toward electron transport capabilities for optoelectronic devices.

In recent times, self-assembled electron transport materials for optoelectronic devices, both solar cells and organic light-emitting diodes (OLEDs), have been gaining much interest as they help in fabricating high-efficiency devices. However, designing organic small molecular materials with star-shaped self-assembled networks is a challenge. To achieve this sort of target, we chose triazine and benzene-1,3,5-tricarbonyl cores for developing such architecture, and we developed four molecular systems, vizTCpCN, TCmCN, TmCN, and TpCN. Successful isolation of single crystals followed by structural analysis of TmCN revealed interesting molecular arrangements in the solid state resulting in the formation of a waterwheel type architecture with an extended network bearing characteristic voids. Theoretical calculations was carried out to check their electron transportability. The natural transition orbital calculation helped in understanding the locally excited and charge transfer excited states. The low electron reorganization energies of these molecules indicated that these materials may have potential to be used in electron transport layers of optoelectronic devices, particularly in OLEDs. Moreover, the assembled networks have a relatively wide surface area and linked structures, which are advantageous for the conduction of carriers with poor electron recombination inside the ETL, and these may offer a straightforward channel for electron conduction to the emissive layer. Finally, the fabricated electron-only device indicated that the synthesized materials may be used as ETMs in the electron transport layer of optoelectronic devices.

A pyrimidine end-capped electron transport material interacted with silver for improving electron-injection and long-term stability in OLEDs

We developed a non-Phen electron-transporter named DPmPy-BP. With Ag as an air-stable EIL, DPmPy-BP exhibited superior electron-injection, achieving EQE of 20% and LT50 of 17 000 h at 1000 cd m−2, outperforming the corresponding Phen/Ag device.

Improving electron transportation and operational lifetime of full color organic light emitting diodes through a "weak hydrogen bonding cage" structure.

Efficient electron-transporting materials (ETMs) are critical to achieving excellent performance of organic light-emitting diodes (OLEDs), yet developing such materials remains a major long-term challenge, particularly ETMs with high electron mobilities (μeles). Herein, we report a short conjugated ETM molecule (PICN) with a dipolar phenanthroimidazole group, which exhibits an electron mobility of up to 1.52 × 10-4 cm2 (V-1 s-1). The origin of this high μele is long-ranged, regulated special cage-like interactions with C-H⋯N radii, which are also favorable for the excellent efficiency stability and operational stability in OLEDs. It is worth noting that the green phosphorescent OLED operation half-lifetimes can reach up to 630 h under unencapsulation, which is 20 times longer than that based on the commonly used commercial ETM TPBi.

UV-Durable Fully-Printed Carbon-Based Multi-Porous-Layered-Electrode Perovskite Solar Cells Using SrTiO3 Electron Transport Layer

Carbon-based multi-porous-layered-electrode perovskite solar cells (MPLE-PSC) using porous carbon as the back electrode have excellent performance stability against high temperature and high humidity environments at a low cost. However, the stability of MPLE-PSC under ultraviolet light has not improved yet. This is due to the photocatalytic effect of TiO2, which is commonly used as an electron transport layer in MPLE-PSC. In this study, we investigated the relationship between the electron transport material and ultraviolet light stability in MPLE-PSC. MPLE-PSC with TiO2 or SrTiO3 as the electron transport layer were fabricated and photoelectric conversion performance of them were compared. As a result, the performance of solar cells using TiO2 decreased to 80 % of the initial performance after 2 hours of ultraviolet light irradiation. In contrast, no performance degradation was observed in solar cells using SrTiO3. Therefore, the SrTiO3 electron transport layer can be proposed as a solution to improve the ultraviolet light stability of solar cells.

(Invited) Advances in Photon-Harvesting Technologies for Perovskite Absorbers and Water Splitting Reactions

Hybrid halide perovskites have made significant progress in power conversion efficiencies (> 25 %) and high stability achieved through chemical engineering of the AMX3 composition. Compositional engineering of perovskite precursor inks minimizes batch-to-batch variations and fluctuation in PCE values, however the understanding of perovskite inks and solution behavior remains a key step towards the exploitation of solution-processed perovskite devices. Herein, a comprehensive 207Pb nuclear magnetic resonance spectroscopy (NMR) study performed on various lead perovskite precursor inks will be presented. The NMR data provide fresh insights in identifying molecular species that are invariably and predominantly formed in precursor mixtures to coexist with larger networks and colloids. The solution chemistry of PbI2+I- is dominated by the type of connectivity as edge- face- and corner-sharing (PbI6)-octahedra and their dimensionality. The trends in the 207Pb chemical shifts revealed dynamic equilibria in the inks that can be controlled by stoichiometry, nature of the solvents and temperature. Furthermore, perovskite precursor ink development can be applied to fabricate devices beyond the planar device structures, such as direct electrospinning of the three major perovskite solar cell components, namely, photo absorber, hole, and electron transport materials, as continuous single triaxial fibers of μm radial dimensions. These perovskite fibers lay the foundation of materials engineering for fabricating tiny solar cells, which can either be woven into fabrics or incorporated as single fibers to power wearables and a variety of devices or sensors, forming the internet of things. They are also suited for tandem PV-PEC set-up for solar-assisted photoelectrochemical production of hydrogen by water-splitting reactions.

Poly(acrylic acid)‐Modified SnO2 Electron Transport Layer for Perovskite Solar Cells

AbstractSnO2 is one of the most used inorganic electron transport materials for organic‐inorganic halide perovskite solar cells. However, the surface defects of SnO2 will block the electron transport at the SnO2/perovskite buried interface and limit the device performance. Here, poly(acrylic acid) (PAA) is employed to modify the SnO2 electron transport layer. Carboxyl groups can react with KOH in the commercial SnO2 aqueous colloidal dispersion to produce potassium polyacrylate, the −COO− can combine not only with oxygen and Sn4+ on SnO2, but also can chelate with uncoordinated Pb2+ defects on perovskite. Moreover, carboxyl groups can form a chemical linker between SnO2 and perovskite via the esterification reaction, leading to the reduction of surface hydroxyl group defects of SnO2. The optimal solar cells with PAA modification achieve an enhanced power conversion efficiency of 18.88 % and improved stability.

Blue emitting exciplex for yellow and white organic light-emitting diodes

White organic light-emitting diodes (WOLEDs) have several desirable features, but their commercialization is hindered by the poor stability of blue light emitters and high production costs due to complicated device structures. Herein, we investigate a standard blue emitting hole transporting material (HTM) N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)benzidine (NPB) and its exciplex emission upon combining with a suitable electron transporting material (ETM), 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ). Blue and yellow OLEDs with simple device structures are developed by using a blend layer, NPB:TAZ, as a blue emitter as well as a host for yellow phosphorescent dopant iridium (III) bis(4-phenylthieno[3,2-c]pyridinato-N,C2')acetylacetonate (PO-01). Strategic device design then exploits the ambipolar charge transport properties of tetracene as a spacer layer to connect these blue and yellow emitting units. The tetracene-linked device demonstrates more promising results compared to those using a conventional charge generation layer (CGL). Judicious choice of the spacer prevents exciton diffusion from the blue emitter unit, yet facilitates charge carrier transport to the yellow emitter unit to enable additional exciplex formation. This complementary behavior of the spacer improves the blue emission properties concomitantly yielding reasonable yellow emission. The overall white light emission properties are enhanced, achieving CIE coordinates (0.36, 0.39) and color temperature (4643 K) similar to daylight. Employing intermolecular exciplex emission in OLEDs simplifies the device architecture via its dual functionality as a host and as an emitter.Graphical abstract

Hybrid Steroid‐[60]Fullerene as n‐Type Material for Organic Photovoltaics

AbstractFullerene derivatives have been used as electron acceptor and transport materials in organic photovoltaics as well as in perovskite solar cells. Among them, [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC61BM) has been one of the most widely used, in combination with poly‐3‐hexylthiophene (P3HT) as an electron donor semiconducting polymer, for the fabrication of organic bulk heterojunction solar cells (BHJ‐OSC). In this work, a steroid‐fullerene hybrid was synthesized through the Bingel–Hirsch reaction to compare its properties with PC61BM as the reference structure. Different spectroscopic techniques were employed to characterize the new fullerene derivative, namely 1D and 2D nuclear magnetic resonance (NMR), Fourier Transform Infrared Spectroscopy (FTIR), and high‐resolution mass spectrometry (HRMS). In addition, thermogravimetric analysis (TGA), and cyclic voltammetry were also performed. The characterization was completed by transmission electron microscopy (TEM), which explore the hybrid‘s morphology, where the presence of the steroid moiety increases the solubility of the fullerene material. DFT/PBE−D3(BJ)/6‐311G(d,p) calculations for both the steroid‐fullerene hybrid and PC61BM allowed to obtain geometry optimizations and electronic structure data. A comparative study to analyze the nature of the electronic properties of the synthesized steroid‐[60]fullerene hybrid and PC61BM with an oligomer of ten thiophene units, used in the active layer of organic solar cells, was carried out. Electron density maps using the [CIS|CNDOL/21] approach illustrate the photoexcitation of the donor followed by an electron‐charge transfer to the acceptor fullerene.

Evaporable Fullerene Indanones with Controlled Amorphous Morphology as Electron Transport Layers for Inverted Perovskite Solar Cells.

Fullerenes are among the most commonly used electron-transporting materials (ETMs) in inverted perovskite solar cells (IPSCs). Although versatile functionalized fullerene derivatives have shown excellent performance in IPSCs, pristine [60]fullerene (C60) is still the most widely used in devices mainly because of its uniform morphology by thermal deposition. However, thermally evaporable fullerene derivatives have not yet been achieved. Herein, we developed a series of evaporable fullerene derivatives, referred to as fullerene indanones (FIDOs), affording IPSCs with high power conversion efficiency (PCE) and long-term storage stability. The FIDOs were designed with a unique architecture in which the fullerene moiety and a benzene ring moiety are linked via a five-membered carbon ring in benzene ring plane. This molecular arrangement affords exceptional thermal stability, allowing the FIDOs to withstand harsh thermal deposition conditions. Moreover, by manipulating the steric bulk of the functional groups, we could control the state of the organic film from crystalline to amorphous. Subsequently, we used FIDOs as an electron transport layer (ETL) in IPSCs. Thanks to the suitable energy level and dual-passivation effect of FIDOs compared with a reference ETL using C60, the device using FIDOs achieved an open-circuit voltage of 1.16 V and a fill factor of 0.77. As a result, the PCE reached 22.11%, which is superior to 20.45% of the best-performing reference device. Most importantly, the FIDO-based IPSC devices exhibited exceptional stability in comparison to the reference device due to the stability of the amorphous ETL films.

Aqueous solution processed electron-transporting layer based on organosilica nanodots for highly efficient and stable inverted organic solar cells

In this work, we synthesis a new electron-transporting layer material for inverted organic solar cells(i-OSCs) based on organosilica nanodots. The material is synthesized by a simple one-step hydrothermal reaction of 3-[2-(2-aminoethylamino)ethylamino] propyl-trimethoxysilane (AEEA) with carminic acid in water solvent (OSi-CA). Compared with ZnO-based devices, OSi-CA based devices show improved carrier extraction, higher PCE, and greatly increased device stability under UV irradiation. With PM6:Y6 as the active layer, OSi-CA based device has a PCE of 16.01 %. In addition, the OSi-CA electron-transporting layer significantly improves the photostability of OSCs. After 600 min of continuous UV lamp (5 mW cm−2) irradiation, the OSi-CA based i-OSC retained 95.5 % of its initial performance, whereas the ZnO-based device had only 71.2 %.

Facile Synthesis of Asymmetric aza-Boron Dipyrromethene Analogues Bearing Quinoxaline Moiety.

An asymmetric aza-BODIPY analogue bearing quinoxaline moiety was synthesized via a titanium tetrachloride-mediated Schiff-base-forming reaction of 6,7-dimethyl-1,4-dihydroquinoxaline-2,3-dione and benzo[d]thiazol-2-amine. This novel aza-BODIPY analogue forms a complementary hydrogen-bonded dimer due to the quinoxaline moiety in the crystal structure. It also shows intense absorption and fluorescence, with fluorescence quantum yields close to unity. The electrochemical measurements and the DFT calculations revealed the presence of the low-lying HOMO, which benefits their potential applications as an electron-transporting material.

Metallaaromatic Complexes as Candidates for Future Molecular Materials and Electronic Devices: Recent Advancements.

In recent years, the field of organometallic chemistry has made a great progress and diverse types of metallaaromatics have successively been reported. In those studies, incorporation of ligated osmium centers into metallaaromatic systems played a prominent role. The reviewed literature documents that certain metallaaromatics with unconventional photophysical properties, redox and electronic transport properties and magnetism, have potential to be widely used in diverse practical applications, with selected examples of amino acid and fluoride anion identification, photothermal effects, functional materials, photodynamic therapy (PDT) in biomedicine, single-molecule junction conductors, and electron-transport layer materials (ETLs) in solar cells.

Effective passivation of defects in high-performance tin oxide-based perovskite solar cells using guanidinium phosphate additives

Perovskite solar cells (PSCs) have been extensively studied owing to their excellent photovoltaic performance. Tin oxide (SnO2) is an ideal electron transport material for PSCs. However, owing to nanoparticle agglomeration and defects, the excellent performance of SnO2 cannot be effectively utilized. In this article, a multifunctional additive guanidinium phosphate (GP) is introduced into SnO2. The modification of GP not only improves the surface properties of the electron transport layer (ETL) and the crystalline quality of the perovskite layer (PVK), but also passivates the double-sided defects and suppresses the non-radiative carrier recombination, as well as enhances the electron extraction ability and harmonizes the energy level alignment between the ETL and PVK. Consequently, the GP-modified device achieves a power conversion efficiency (PCE) of 23.91 % and a fill factor (FF) of up to 83.45 %. In addition, the unencapsulated device maintains over 90 % of its starting efficiency after being aged under 25 ℃ and 40 ± 10 % relative humidity (RH) for 1000 h. The results demonstrate that the introduction of GP is an effective strategy to enhance the performance and stability of PSCs.