Charge Transport In Materials Research Articles

Charge transport materials, bismuth and copper-based perovskite solar cells: A review

Improving perovskite solar cell (PSC) efficiencies would not have been possible without discovering and incorporating novel materials. More significant than materials usage is the compatibility of various material components in the entire device. Charge transport materials have been at the heart of this discussion to decide a PSC's functioning fundamentally. This review highlights various high-efficiency examples using alternate charge transport materials, bringing us one step closer to commercializing this technology. The article also elaborates on recent innovations in Bismuth and Copper-based PSCs. These are possible candidates to replace the conventional materials used in a standard PSC and affirmatively yield favorable results through extensive research.

Influence of absorption, energy band alignment, electric field, recombination, layer thickness, doping concentration, temperature, reflection and defect densities on MAGeI3 perovskite solar cells with Kesterite HTLs

Over the past decade, perovskite materials have emerged as a promising absorber layer in photo voltaic (PV) cells. Germanium (Ge) based perovskite layers have attracted the attention of scientists because of its excellent photovoltaic properties and nontoxic nature. Selecting the right material for charge transport layers (CTL) can further enhance the performance and stability of the cell. Kesterite materials are chalcogenides quaternary compound with high conductivity and tunable bandgap. They have exhibited excellent performance when employed in thin film PV cells as the active material and hence emerged as an option to be used as hole transport layer (HTL) in perovskite solar cells. In this work the Ge based perovskite solar cell (PSC) of methyl ammonium germanium tri-iodide (MAGeI3) is numerically modelled with 6 kesterite quaternary compounds as HTLs and 4 oxide/sulphide materials as electron transport layers (ETL). Henceforth, a total of 24 unique structures are numerically modelled and optimized using SCAPS-1D. A systematic methodology is adopted to analyze the effect of the charge transport materials on the absorption, quantum efficiency, energy band alignment, electric field intensity, recombination rate, carrier density, thickness, doping concentration, temperature, reflection and interface defect densities of the PSC in detail. The optimization of the PSC structures enhanced the performance of the cells up to 7% more. Based on the simulation results the best performing perovskite structures were TiO2/Per/CZTS and SnO2/Per/CZTS with PCE of 24.57% and 24.87%, Jsc of 16.13 mA cm−2 and 16.32 mA cm−2, Voc of 1.730 V and 1.733 V and F.F of 88.10% and 88.101% respectively.

Theoretical study of perovskite solar cell for enhancement of device performance using SCAPS-1D

Perovskite solar cells are a pioneering photovoltaic technology that has significantly improved performance in current years. The fundamental n-i-p planar heterojunction structure of solar cells is structured and simulated in the present work. The device configuration Glass/ITO/WS2/CH3NH3PbI3/P3HT/Au was investigated using the Solar Cell Capacitance Simulator (SCAPS-1D) program. To increase the performance of the photovoltaic solar cell thickness, bandgap, doping concentration and temperature have been varied. Further, using the optimal value of the different parameters, the performance of the photo-voltaic device such as power conversion efficiency (PCE) and Fill Factor (FF) are obtained as 27.02%, and 85.44%, respectively. Also, Open-circuit Voltage (VOC) of 1.46 V and Short-circuit current density (JSC) of 21.56 mA cm−2 were achieved. The influence of donor concentrations has been studied by varying its value from 1 × 10−12 cm−3 to 1 × 10−20 cm−3 for the proposed device. Thus, using different charge transport materials, the power convergence efficiency of the perovskite solar cell has been enhanced. Our simulation study reveals that the proposed configuration could be used to fabricate a device for the improvement of the efficient perovskite solar cell.

Injection-limited and space charge-limited currents in organic semiconductor devices with nanopatterned metal electrodes

Charge injection at metal-organic interfaces often limits the electric current in organic light-emitting diodes without additional injection layers. Integrated nanopatterned electrodes may provide a way to overcome this current injection limit by local field enhancements leading to locally space charge-limited currents. We compare electrical characteristics of planar and nanopatterned hole-only devices based on the charge transport material NPB with different thicknesses in order to investigate the nanopattern’s effect on the current limitation mechanism. Integration of a periodic nanograting into the metal electrode yields a current increase of about 1.5–4 times, depending on thickness and operating voltage. To verify the experimental results, we implement a finite element simulation model that solves the coupled Poisson and drift-diffusion equations in a weak form. It includes space charges, drift and diffusion currents, nonlinear mobility, and charge injection at the boundaries. We find in experiment and simulation that the planar devices exhibit injection-limited currents, whereas the currents in the nanopatterned devices are dominated by space charge effects, overcoming the planar injection limit. The simulations show space charge accumulations at the corners of the nanopattern, confirming the idea of locally space charge-limited currents.

Side chain engineering and film uniformity: two key parameters for the rational design of dopant-free polymeric hole transport materials for efficient and stable perovskite solar cells

Herein, we introduce a series of new (X–DADAD)n-type conjugated polymers comprised of benzo[1,2-b:4,5-b′]dithiophene (X), benzo[c] [1,2,5]thiadiazole (acceptor, A), and thiophene (donor, D) units as highly promising hole transport materials for perovskite solar cells (PSCs). The rational engineering of the backbone structure and the side chains enabled high power conversion efficiencies of up to 20% in n-i-p perovskite solar cells in combination with impressive operational stability: the devices preserved initial performance after >2500 h of continuous illumination at open-circuit conditions. The origins of the observed long-term perovskite solar cells stability enabled by the newly designed polymeric hole transport materials have been revealed through the use of infrared scattering-type scanning near-field optical microscopy and further elucidated by density functional theory calculations. The presented findings point to novel strategies of designing advanced charge transport materials to simultaneously endow the PSCs with high performance and durability.

Recent Advances in Inverted Perovskite Solar Cells: Designing and Fabrication.

Inverted perovskite solar cells (PSCs) have been extensively studied by reason of their negligible hysteresis effect, easy fabrication, flexible PSCs and good stability. The certified photoelectric conversion efficiency (PCE) achieved 23.5% owing to the formed lead−sulfur (Pb−S) bonds through the surface sulfidation process of perovskite film, which gradually approaches the performance of traditional upright structure PSCs and indicates their industrial application potential. However, the fabricated devices are severely affected by moisture, high temperature and ultraviolet light due to the application of organic materials. Depending on nitrogen, cost of protection may increase, especially for the industrial production in the future. In addition, the inverted PSCs are found with a series of issues compared with the traditional upright PSCs, such as nonradiative recombination of carriers, inferior stability and costly charge transport materials. Thus, the development of inverted PSCs is systematically reviewed in this paper. The design and fabrication of charge transport materials and perovskite materials, enhancement strategies (e.g., interface modification and doping) and the development of all−inorganic inverted devices are discussed to present the indicator for development of efficient and stable inverted PSCs.

Performance enhancement of (FAPbI3)1-x(MAPbBr3)x perovskite solar cell with an optimized design

In this paper, an optimized design of (FAPbI3)1-x(MAPbBr3)x perovskite solar cell is numerically investigated using SCAPS-1D software package. A variety of potential charge transport materials are investigated. Cu2O as HTL and ZnO as ETL outperform other choices; they are therefore considered as the best candidates. The impact of the electronic properties of both ZnO/perovskite and Perovskite/Cu2O interfaces on the solar cell performance is thoroughly investigated. We discovered that appropriate values of the conduction band offset (CBO+ = 0.29) and valence band offset (VBO+ = 0.09) assure a “spike-type” band alignment at both interfaces. This choice lowers the unwanted interfacial recombination mechanism, resulting in a challenging PCE. In addition, the impact of the work function of back contact is also investigated. According to simulation findings, Ni back electrodes with a work function of 5.04 eV is appropriate for Zn0.8Mg0.2O/(FAPbI3)0.85(MAPbB3)0.15/Cu2O perovskite solar cell. The optimized FTO/MgZnO/(FAPbI3)0.85(MAPbBr3)0.15/Cu2O/Ni PSC reaches a conversion efficiency as high as 25.86%. These findings will pave the way for the design of low-cost, high-efficiency solar cells.

Configurable Organic Charge Carriers toward Stable Perovskite Photovoltaics.

Due to their solution processability and unique photoelectric characteristics, perovskite solar cells (PSCs) have shown considerable promise in the area of renewable energy. Although their power conversion efficiency (PCE) has risen from 3.8% to 25.7% in only a few years, their short lifetime and high material prices continue to be key roadblocks to commercial viability. Charge transporting materials (CTMs), such as hole/electron transporting materials, are critical components in PSCs because they not only govern hole or electron extraction and transporting from the perovskite layer to the electrodes but also protect the perovskite from direct contact with the ambient environment. CTMs are split into two types: inorganic CTMs (ICTMs) and organic CTMs (OCTMs). Because of their inexpensive prices, well-adjusted energy levels, and low temperature solution-processed features, OCTMs have been more frequently explored and employed than ICTMs. Various forms of OCTMs with more straightforward synthetic pathways and better performance have been thoroughly researched. Recent achievements in the development of OCTMs will be discussed and evaluated on a molecular level in this study, which will include a systematic categorization of OCTMs based on molecular functionalization techniques. In order to achieve highly efficient and stable PSCs, we will present insights on the structure-property relationship in the design of OCTMs as well as device stability. We hope that this analysis will serve as a comprehensive reference to molecular design guidelines for various types of OCTMs, spurring greater research toward designing highly efficient and OCTMs for stable PSCs.

High-Efficiency Tandem White Perovskite Light-Emitting Diodes by Using an Organic/Inorganic Intermediate Connector

The performance of monochromatic perovskite light-emitting diodes (PeLEDs) has undergone rapid development in recent years, while white PeLEDs are still in their infancy. The tandem devices prepared by connecting light-emitting units of different colors with intermediate connectors comprise one of the promising approaches for realizing white light emission. The intermediate connector plays a crucial role in determining the effectiveness of the light-emitting units. In this work, all-perovskite-based white LEDs are assembled by employing sky-blue (490 nm) and red (690 nm) perovskite emitters as active layers. With mixed p-type and n-type organic charge transport materials as the intermediate connector, the highest external quantum efficiency (EQE) of the device was 1.4%, the electroluminescence (EL) peak position of the blue light part was shifted to 512 nm, and the red light part was shifted to 673 nm. In contrast, with organic and inorganic charge transport materials as the intermediate connector, the devices exhibited better charge generation/injection capabilities and a record EQE of 8.5%. The EL peak position of the device prepared from the stored film was not shifted, implying that the organic/inorganic layer effectively suppresses halide migration. Our work paves the way for highly efficient tandem white PeLEDs.

Ultrathin Self-Assembly Two-Dimensional Metal–Organic Framework Films as Hole Transport Layers in Ideal-Bandgap Perovskite Solar Cells

Two-dimensional conjugated metal–organic framework (2D c-MOF) Cu3(HHTT)2 (2,3,7,8,12,13-hexahydroxytetraazanaphthotetraphene, HHTT) is found to be an ideal hole transport material for Pb–Sn perovskite solar cells (PSCs) for the first time. 2D c-MOF films are conveniently prepared by a self-assembly deposition method and used in ideal-bandgap Pb–Sn mixed PSCs. The ultrasmooth surface of the Cu3(HHTT)2 film can facilitate perovskite growth and enable defect passivation on the perovskite surface. The self-assembly approach is suitable for preparing large-area films conformally on a substrate, promising the application of the film in large-area devices. Encouragingly, an efficiency over 22% is obtained from ideal-bandgap PSCs. Moreover, an efficiency of 19.86% for large-area ideal-bandgap PSCs is achieved from devices with an area of 1 cm2. This work demonstrates that 2D c-MOFs are promising charge transport materials for high-efficiency and large-area PSCs.

Nanocomposite system of a discotic liquid crystal doped with thiol capped gold nanoparticles

In the pursuit of novel charge transporting materials, composite systems have always proved to be a promising option. While it seems cumbersome and ponderous to molecularly engineer a material, combining and customizing two exclusive systems to obtain a new material, with desired properties appears far simpler and more yielding. One such developing field of research is the study of discotic liquid crystal (DLC) – nanocomposite systems for applicability as a charge transport material in the form of one-dimensional nanowires or two-dimensional conducting matrix layers. In this work, we present a detailed study of composite systems of gold nanoparticles doped in Hexa-hexylthiotriphenylene DLC. In addition to an appreciable increase in conductivity, we also observe a decrease in lattice parameters by using small-angle X-ray scattering (SAXS). An increase in the enthalpy of transition observed by using thermodynamic study and formation of dendritic textures observed by using polarized light microscopy (PLM) study signifies the formation of perfect hexagonal structures despite doping. Overall, the different studies suggest that the doped nanoparticles have very well settled in between host molecules without disturbing the molecular ordering of the hosts. Such results are encouraging and indicative of the potential applicability of the nanocomposite systems in portable electronic devices as a charge transport layer.

Boronic Acid-Based n-Type Semiconductor for Electronic Device Application

Electron transporting, or n-type, semiconductors can serve as charge-transport materials, and are ideal for use in organic electronic devices. Boron-based small organic molecules have garnered immense research attention as the heteroatom can effectively alter the electronic structures leading to excellent photophysical and electrochemical properties. A luminescent Schiff base (E)-(4-((2-(2-hydroxybenzoyl)hydrazono)methyl)phenyl)boronic acid (SHB) was prepared by a one-pot condensation reaction between salicyloyl hydrazide and formylphenylboronic acid. The synthesized molecule was chemically characterized by infrared spectroscopy, nuclear magnetic resonance spectroscopy, and mass spectrometry. The blue-emitting boronic acid-derived molecule displayed intramolecular charge transfer, high carrier concentration, good thermal stability, a reversible reduction tendency and formation of uniform amorphous thin films. A diode was successfully fabricated via a solution processing technique with an ideality factor of 7.76. Further, AC conductivity, dielectric constant, dielectric loss, and capacitance values in a frequency range of 10–1000 Hz were extracted from dielectric studies. The dielectric constant of SHB was found to be 9.71 with an AC conductivity of 6.34 × 10−9 Ω−1 cm−1 at 1000 Hz.Graphical

Understanding interfacial energy structures in organic solar cells using photoelectron spectroscopy: A review

Organic solar cells (OSCs) have received considerable attention as a promising clean energy-generating technology because of their low cost and great potential for large-scale commercial manufacturing. With significant advances in new charge-transport material design, interfacial engineering, and their operating conditions, power conversion efficiencies of OSCs have continued to increase. However, a fundamental understanding of charge carrier transport and especially how ionic moieties affect carrier transport is still lacking in OSCs. In this regard, photoelectron spectroscopy has provided valuable information about interfacial electronic structures. The interfacial electronic structure of OSC interlayers greatly impacts charge extraction and recombination, controls energy level alignment, guides active layer morphology, improves material’s compatibility, and plays a critical role in the resulting power conversion efficiency of OSCs. Interfacial engineering incorporating inorganic, organic, and hybrid materials can effectively enhance the performance of organic photovoltaic devices by reducing energy barriers for charge transport and injection while improving compatibility between metal oxides and donor–acceptor based active layers or transparent conducting electrodes. This article provides a review of recent developments in interfacial engineering underlying organic photovoltaic devices of donor–acceptor interfaces.

그라비어 인쇄기술을 이용한 페로브스카이트 태양전지 Roll-to-Roll 제작 공정

Perovskite solar cells (PSCs) are attracting attention as a next-generation thin-film solar cell based on their high efficiency. Technologies to commercialize PSCs for power generation and special purposes have been being developed. Since the PSCs are manufactured using a solution process, the development of a roll-to-roll (R2R) process to mass-produce flexible solar cells at low cost can be a stepping stone for commercialization. In order to manufacture high-quality perovskite solar cells through the R2R process, the perovskite material and charge transport materials must be formed in a roll-based continuous and scalable method. Gravure printing has the advantage of being able to directly form patterns on a substrate. The R2R process has limitations that were not considered in the existing sheet-to-sheet process to fabricate unit cells in terms of process temperature, process time, and environmental management. In this paper, we share the current status of R2R gravure printing process development for perovskite solar cells and discuss issues to be considered and problems to be solved during R2R process design.

In-situ TiO2-Coated CsPbBr3 quantum dots with enhanced stability, photoluminescence quantum yields, and charge transport properties

Cesium lead bromide quantum dots (CsPbBr3 QDs) have suffered from instability against the environmental factors in broad applications due to the inherent perovskite properties. Here, TiO2 coating on CsPbBr3 QDs by ex-situ and in-situ methods was performed to prevent the agglomeration of the nanocrystals and to improve the stability of CsPbBr3 QDs without heat treatment at high temperature. By focusing on the in-situ coating method, this work also reported the effects of injection temperature of titanium precursor into the QDs solution from 25 °C to 170 °C on the morphological, structural, and optical properties of the obtained TiO2-coated CsPbBr3 QDs. The in-situ coated samples were non-agglomerated and remained the original shape and crystal structure of CsPbBr3 QDs, whereas the ex-situ coated samples were completely agglomerated. The in-situ samples exhibited a significant stability improvement against toluene, ultrasonication treatment in water, and light illumination (all with the photoluminescence intensity greater than 50%). Furthermore, the in-situ TiO2-coated CsPbBr3 QDs were proved to use as a good charge transport material in the photoelectrochemical cell. The results demonstrated the remarkable enhancement of photocurrent, owing to a suitable alignment of energy levels of TiO2 and CsPbBr3 and a stable structure of QDs, when compared to that of the as-synthesized CsPbBr3 QDs. The present study provides a new perspective to utilize stable TiO2-coated CsPbBr3 QDs in various photoelectrochemical applications.

Computational study of optoelectronic properties of oxadiazole-based compounds for organic light emitting diodes

The electronic, charge transport properties and UV-visible electronic transitions of a series of oxadiazole-based compounds 1–8 were calculated using DFT and TD-DFT methods. HOMO–LUMO gaps, reorganisation energies, and injection barriers were calculated at the B3LYP/6-31G(d,p) level of theory. The photophysical properties were predicted at the TD-PBE0/6-31G(d,p) computational level. The obtained results put in evidence that the designed oxadiazole derivatives 2–8 are better charge transport materials compared to the experimental derivative 1 and the reference materials Alq3 and TPD. The calculated reorganisation energies show that derivatives 6 and 4 are predicted as the best electron transport materials; whereas 4 and 5 are predicted as the best hole transport materials. The low values of the calculated ionisation and hole extraction potentials suggest that 4 is the best molecule for hole injection; whereas the high values of the calculated electron affinities and electron extraction potentials reveal that 6 is the most suitable derivative for electron injection. Absorption and fluorescence transitions involve mainly LUMOs and HOMOs. The designed fluorophore 8, which has the highest Stokes shift values, could be used in OLED light-emission layers.

Hydrogen bonding drives the self-assembling of carbazole-based hole-transport material for enhanced efficiency and stability of perovskite solar cells

Designing a hole-transport material (HTM) that guarantees effective hole transport while self-assembling at the perovskite|HTM interface with the formation of an ordered interlayer, has recently emerged as a promising strategy for high-performance and stable perovskite solar cells (PSCs). Hydrogen bonding (HB) is a versatile multi-functional tool for the design of small molecular HTMs. However, to date, its employment is mostly limited to p-i-n inverted PSCs. This study demonstrates the advantages of a novel HTM design that can self-assemble into a long-range ordered interlayer on the perovskite surface via HB association. A hydro-functional HTM (O1) is compared to a reference HTM (O2) that cannot form HB due to the replacement of the amide group of O1 with a plain butyl alkyl chain in O2. As a result, O1-based n-i-p PSCs display enhanced hole extraction reaction, suppressed interfacial charge recombination, reduced hysteresis effect, and an increase in Voc (by 60 mV), FF (>11% increase), and overall power conversion efficiency, PCE (32% increase) compared to the case of HB-free O2-based devices. Remarkable stability is observed for unencapsulated O1 cells, with a T80 lifetime of 35.5 h under continuous maximum power point tracking in air. This work emphasizes the role of HB-directed self-assembling in simultaneously enhancing both the PCE and stability of popular n-i-p PSCs. This study paves the way for the development of new hydro-functional charge-transport material designs for efficient and stable PSCs.

Mechanical Manipulation of Archimedean Spirals of an Achiral Pyrazinacene for Chiral Assemblies

AbstractThe fabrication of chiral assemblies from achiral molecules has attracted considerable interest since it provides insight into the structures of biological systems. It is also of general importance from the point‐of‐view of the various factors affecting self‐assembly processes especially supramolecular chirality. Here, the dynamic interfacial assembly strategy is used to obtain unusual chiral assemblies from achiral linear nitrogen‐rich acene derivatives, the pyrazinacenes. 1D supramolecular J‐aggregate tapes of homogeneous width are coiled to form Archimedean spirals. Moreover, the dynamic assembly process further permits the controlled transformation of the Archimedean spirals into tightly coiled toroids and chiral concentric vortices. Grazing incidence X‐ray diffraction study using synchrotron radiation reveals that J‐aggregate monolayer deposited using the layer‐by‐layer technique forms a lamellar columnar oblique mesophase‐like structure. Molecular packing in J‐aggregates provides a significant channel for the occurrence of charge transport exhibiting carrier mobility of 0.045 cm2 V−1 s−1 due to enhanced electronic coupling. This study illustrates an intriguing approach for achieving chiral supramolecular assemblies from achiral molecules and also reveals the potential of the pyrazinacenes as charge transport materials for molecular electronics.

Fluorinated copper phthalocyanine as an electron transport material in perovskite solar cell

The state-of-art charge transport materials, TiO2 and Spiro-OMeTAD suffer from various drawbacks such as instability, lengthy fabrication procedure and low charge mobilities. In this paper, we have studied copper phthalocyanine (CuPc) and halogen-substituted copper phthalocyanine (X16CuPc; X = F, Cl, Br, I) through density functional theory (DFT) and determined their ground state properties, excited state properties and solubility in DMSO (widely used solvent in the fabrication of perovskite solar cell [PSC]). We have shown that fluorinated copper phthalocyanine(F16CuPc) acts as an electron transport layer (ETL) and CuPc acts as a hole transport layer (HTL) in MAPbI3-based PSC. The moisture stability of CuPc and F16CuPc has been established and it has been shown that CuPc and F16CuPc will protect the perovskite layer from degradation due to the presence of moisture in the surroundings. Using the properties obtained via DFT, PSC has been simulated using SCAPS, and it is established that CuPc as hole transport material and F16CuPc as electron transport material gives better efficiency compared with the state of art charge transport materials (TiO2 and Spiro-OMeTAD). The device has been optimized with respect to (w.r.t.) thickness, doping concentration, defect density and interface defect density. We have studied the effect of series resistance, shunt resistance and temperature on the performance of PSC and this configuration has shown good thermal stability. Carbon has been proposed as an alternative to gold as the back contact thereby making our device more economical. The optimized device showed quantum efficiency of (81.59% to 98.69%) in the visible region. A fill factor of 79.01%, power conversion efficiency of 22.30% has been obtained.

Density functional theory and time‐dependent density functional theory studies on optoelectronic properties of fused heterocycles with cyclooctatetraene

AbstractWe establish a structure–property relation for the system having a cyclooctatetraene (COT) ring fused with four five‐membered heterocyclic rings. The two five‐membered heterocycles selected are furan and selenophene and their fused structure with COT is known as cyclooctatetrafuran (COF) and cyclooctatetraselenophene (COSe), respectively. We found 15 geometrical isomers of each COF and COSe and their structural stability and optoelectronic properties have been evaluated by quantum chemical simulations. The density functional theory (DFT) and time‐dependent DFT simulations were employed for a systematic review of all isomers. Electronic excitations, hole reorganization energies, electron reorganization energies, ionization potentials, and electron affinities, of all the isomers, were reported. Based on our comparative study, it is shown that one of the isomers is suggested as a better charge transport material.