Carrier Transport Materials Research Articles

A review on the current status of dye‐sensitized solar cells: Toward sustainable energy

AbstractDye‐sensitized solar cells (DSSCs) are among the most attractive third‐generation photovoltaic technologies due to their low toxicity, versatility, roll‐to‐roll compatibility, ultralightness, and attractive power conversion efficiencies (PCEs). However, their transition from the laboratory scale to the industrial scale has been slow due to their inability to compete with silicon‐based cells in terms of efficiencies and stabilities. Research activities on DSSCs have been ongoing for several decades to improve the efficiency and cost‐effectiveness of photovoltaics but these attempts are still inadequate. Their chemical and physical properties must be refined to increase efficiency and commercialization. This review provides a concise overview of the recent advances taking place in the DSSCs research field, including molecular engineering technologies, the quest for superior carrier transport materials (CTMs), efficient sensitizers, and better electrodes. Also, this review compiles knowledge of the historical development of DSSCs, the current advancements such as control of surface morphologies, doping strategies, modeling and simulation, characterization, and recent cutting‐edge research happenings in photovoltaic research. Finally, nanostructured materials that have been used as photoelectrodes and the practical applications of DSSCs in internet of things (IoT) and portable electronics are examined to identify challenges and future advancements. The main aim of this work is to be a pathfinder for scientific researchers in this field exploring various energy harvesting materials and optimization strategies of different components of DSSCs.

Effect of the Glass Transition Temperature of Organic Materials on Exciton Recombination Region of Deep Blue OLED under Thermal Stress

The carrier transport and the exciton recombination region, which are strongly affected by temperature, are essential to organic light‐emitting diodes (OLEDs) under thermal stress. A different charge carrier transport materials with different glass transition temperature (Tg) is selected to investigate their effect on the thermal stability of OLEDs. By calculating the carrier mobility at different annealing temperatures and comparing the simulated values by the Arrhenius equation, the effect of the thermal stress on the performance of OLEDs is analyzed. Then, using the luminescent dye as a probe adjacent to the emitting layer and charge transporting layer to identify the exciton recombination region at different temperatures and driving voltages, it is revealed that replacing the functional material with low Tg with a high Tg one could effectively increase the thermal stability of the device to 100 °C. The optimized devices have a reduced turn‐on voltage, and the electron mobility is increased by more than one order of magnitude, which was conducive to exciton recombination. The work promotes the application of OLEDs in extreme environments by improving their thermal stability.

Comprehensive Review on Thin Film Homojunction Solar Cells: Technologies, Progress and Challenges

With the aim of achieving high efficiency, cost-effectiveness, and reliability of solar cells, several technologies have been studied. Recently, emerging materials have appeared to replace Si-based cells, seeking economic fabrication of solar cells. Thin-film solar cells (TFSCs) are considered strong candidates for this mission, specifically perovskite-based solar cells, reporting competitive power convergence efficiencies reaching up to 25.7%. Substantial efforts have been invested in experimental and research work to surpass the Si-based cells performance. Simulation analysis is a major tool in achieving this target by detecting design problems and providing possible solutions. Usually, a TFSC adopts p-i-n heterojunction architecture by employing carrier transport materials along with the absorber material in order to extract the photogenerated electrons and holes by realizing a built-in electric field. Eventually, this dependency of conventional heterojunction TFSCs on carrier transport layers results in cost-ineffective cells and increases the possibility of device instability and interface problems. Thus, the design of p-n homojunction TFSCs is highly desirable as an essential direction of structural innovation to realize efficient solar cell operation. In this review, a summary of the fundamentals of TFSC materials, recent design and technology progress, and methodologies for improving the device performance using experimental research studies will be discussed. Further, simulation analysis will be provided by demonstrating the latest research work outcomes, highlighting the major achievements and the most common challenges facing thin film homojunction solar cell structures and the methods to improve them.

Toward High-Performances of Halide Light-Emitting Diodes: The Importance of Ligands Engineering

Halide perovskite light-emitting diodes (PeLEDs) have attracted great attention because of their superior optical properties, such as extremely high photoluminescence (quantum yield up to nearly 100%) of active layers with tunable wavelengths over the entire visible spectral range. With a suitable modification of halide perovskites, carrier transport materials, and their interfaces, external quantum efficiencies exceeding 10%, 25%, and 20% have been achieved for blue-colored (465 nm), green-colored (512 nm), and red-colored (640 nm) LEDs, respectively. Many strategies for pursuing high performances of devices have been successfully demonstrated, among which ligand engineering has always played an important role in the active layer. Herein, we present a perspective to illustrate the effects and roles of the ligands in cesium lead bromide light-emitting diodes. This perspective is mainly classified into three parts: (1) ligands for CsPbBr3 LEDs could improve radiative recombination of perovskites and contribute to better efficiency of LEDs; (2) ligands could confine CsPbBr3 growth for blue emission of LEDs; (3) stabilities of materials and devices become better with ligand engineering. Finally, the summary and perspective on PeLEDs are highlighted and possible solutions are provided.

Near-ultraviolet organic light emitting diodes using melem

This study fabricates near-ultraviolet (NUV)-organic light-emitting diodes (NUV-OELDs) using 2,5,8-triamino-tri-s-triazine (melem) as the emitting layer. Melem exhibits NUV emission owing to thermally activated delayed fluorescence. However, the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) gap of melem is large, thus rendering difficulty in carrier injection from the electrodes and causing the carrier injection layer or carrier transport layer to emit light. Therefore, in this study, the device structure of the NUV-OLED is optimized to obtain melem-derived luminescence using carrier transport materials with a large HOMO–LUMO gap. Consequently, NUV-OLED with emission in the NUV region is successfully realized.

Designing an efficient lead-free perovskite solar cell with green-synthesized CuCrO2 and CeO2 as carrier transport materials.

With increased efficiency, simplicity in manufacturing, adaptability, and flexibility, solar cells constructed from organic metal halide perovskite (PVK) have recently attained great eminence. Lead, a poisonous substance, present in a conventional PVK impacts the environment and prevents commercialization. To deal with this issue, a number of toxicity-free PVK-constructed solar cells have been suggested. Nevertheless, inherent losses mean the efficiency conversion accomplished from these devices is inadequate. Therefore, a thorough theoretical investigation is indispensable for comprehending the losses to improve efficiency. The findings of a unique modelling method for organic lead-free solar cells, namely methylammonium tin iodide (MASnI3), are investigated to reach the maximum practical efficiencies. The layer pertinent to MASnI3 was constructed as a sandwich between a bio-synthesized electron transport layer (ETL) of CeO2 and a hole transport layer (HTL) of CuCrO2 in the designed perovskite solar cells (PSCs). In this study, the use of algae-synthesized Au in the back contacts has been proposed. To obtain the maximum performance, the devices are further analyzed and optimized for active layer thickness, working temperature, total and interface defect density analysis, impedance analysis (Z'-Z), and capacitance-voltage (C-V), respectively. An optimal conversion efficiency of 26.60% has been attained for an MASnI3-constructed PSC. The study findings may open the door to a lead-free PSC through improved conversion efficiencies.

Optical gain properties of interfacial material PFN-Br and its application potentials in future electrically pumped organic lasers

In this paper, the optical gain properties of the water/alcohol soluble conjugated polyelectrolyte (Poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]) (PFN-Br) and its potential applications in future electrically pumped organic lasers are revealed and systematically studied. To the best of our knowledge, no studies on the optical gain properties of PFN-Br or its prototype, poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] have been reported before. These conjugated polyelectrolytes are widely used as the interlayers in organic light emitting diodes or organic solar cells. The thickness of such an interlayer is usually less than 10 nm, which is considered not sufficient for supporting light waveguiding. Therefore, the thickness of the PFN-Br layer used in this work is increased to more than 100 nm. Through careful study, the polymer is found to possess a low threshold of amplified spontaneous emission (ASE) (~11 μJ/cm<sup>2</sup>) and a small ASE cutoff thickness (<50 nm). It is an efficient blue emission (~456 nm) gain medium. The ASE peak of the PFN-Br film is red-shifted as the thickness increases from 50 to 220 nm. By utilizing the great resistance of PFN-Br against the organic solvent, such as toluene, PFN-Br/F8BT bilayer devices on quartz and PFN-Br/MEH-PPV bilayer devices on ITO glass are fabricated and characterized. In the PFN-Br/F8BT bilayer devices, it is found that the PFN-Br interlayer has very limited influence on F8BT. The ASE threshold of F8BT increases only twice, compared with that of F8BT monolayer device, when 100-nm-thick PFN-Br layer is introduced beneath the F8BT film. No significant change in optical gain or loss is observed. Most of the extra losses in F8BT due to the introduction of PFN-Br are attributed to the larger refractive index of PFN-Br than that of quartz substrate. Furthermore, in the PFN-Br/MEH-PPV bilayer devices on ITO glass, introducing PFN-Br interlayer resulting in optimal ASE performance of MEH-PPV compared with that on bare ITO surface. The ASE threshold of MEH-PPV is reduced as much as 60% (from 402 μJ/cm<sup>2</sup> to 160 μJ/cm<sup>2</sup>) while the PFN-Br layer is sandwiched between ITO and MEH-PPV. The PFN-Br layer modifies the waveguiding modes, and reduces the interaction between excitons and ITO electrodes. As a result, the ASE performance of MEH-PPV is improved. The findings of this report indicate that the PFN-Br is not only a good carrier transport material but also a highly-efficient gain medium. PFN-Br, combined with its advantages in different fields, is expected to play various roles in future organic electrically pumped lasers.

Identification of Asymmetric Interfacial Recombination in Perovskite Solar Cells through Impedance Spectroscopy

Nonradiative carrier recombination plays a major role in affecting solar cells’ performance and power conversion efficiency (PCE). Herein, we utilize illumination-side-dependent impedance spectroscopy (ISD-IS) to study the different carriers’ recombination in complete perovskite solar cells (PSCs) with three different commonly used electron transport materials (ETMs). We have conducted the ISD-IS measurements of PSCs subjected to different illumination intensities and under different operation conditions (i.e., open circuit and short circuit). The results point to unbalanced interfacial recombination in the tested devices, whereby higher recombination occurs near one of the interfaces of the perovskite layer. By changing the illumination intensity and/or operation point, the dominant site of recombination (i.e., interface or bulk) inside the PSC changes depending on the carrier transport materials of the PSC structure. Overall, the ISD-IS measurements of the PSCs reveal the effects of different ETMs on the losses and recombination in PSCs.

Green InP-based quantum dots and electroluminescent light-emitting diodes

With the advancement toward commercialization of quantum dots (QDs) in the field of lighting and display, improving the performance of Cd-free QDs and related quantum dot light-emitting diodes (QLEDs) becomes necessary. Thus far, the performance of ZnTeSe- and InP-based blue and red QLEDs has been significantly improved by optimizing QDs emitting materials and device structure. However, as one of the three primary color sources, the performance of green InP-based QLEDs still lags behind that of blue and red Cd-free QLEDs. Herein, this review discusses the latest progress of green InP-based emitting materials and corresponding QLEDs, covering the engineering of InP core, the optimization of nanostructure and surface ligands of core/shell QDs, as well as the majorization of device architecture and carrier transport materials. Finally, some challenges and possible development directions of green InP-based QDs and related QLEDs are also identified, which may speed up the commercialization process of Cd-free QDs and corresponding QLEDs.

Investigation of Carrier Transport Materials for Performance Assessment of Lead-Free Perovskite Solar Cells

This present work emphasizes the numerical modeling of lead-free methylammonium tin tri-iodide (MASnI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> )-based perovskite solar cells (PSCs) under optimizing preconditions. The prior selection of the perovskite material of MASnI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> is feasible for a more extended absorption spectrum due to a smaller bandgap of 1.3 eV than higher bandgap methylammonium lead tri-iodide (MAPbI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> )-based PSC and the factor of lesser toxicity. Furthermore, to enhance the efficiency of the device, selecting potentially steadier and superior carrier transport materials (CTMs) is among the most effective approaches for optimizing device outputs. Among the proposed materials, a prior selection of copper antimony sulfide (CuSbS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) and zinc oxide (ZnO) as CTMs with an optimized thickness of MASnI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> material has offered a higher power conversion efficiency (PCE) of 22.16% under the photoillumination AM1.5. Furthermore, the less-defective PSC device can also be helpful for further device optimization and futuristic development.

Doping Free and Amorphous NiOx Film via UV Irradiation for Efficient Inverted Perovskite Solar Cells.

High crystallization and conductivity are always required for inorganic carrier transport materials for cheap and high‐performance inverted perovskite solar cells (PSCs). High temperature and external doping are inevitably introduced and thus greatly hamper the applications of inorganic materials for mass production of flexible and tandem devices. Here, an amorphous and dopant‐free inorganic material, Ni3+‐rich NiOx, is reported to be fabricated by a novel UV irradiation strategy, which is facile, easily scaled‐up, and energy‐saving because all the processing temperatures are below 82 ℃. The as‐prepared NiOx film shows highly improved conductivity and hole extraction ability. The rigid and flexible PSCs present the champion efficiencies of 22.45% and 19.7%, respectively. This work fills the gap of preparing metal oxide films at the temperature below 150 °C for inverted PSCs with the high efficiency of >22%. More importantly, this work upgrades the substantial understanding about inorganic materials to function well as efficient carrier transport layers without external doping and high crystallization.

Toward Commercialization of Efficient and Stable Perovskite Solar Modules

The commercialization of perovskite photovoltaic technology is dependent on the development of high‐efficiency, stable, and large‐area solar modules. Despite the rapid rise in efficiencies of laboratory‐scale perovskite solar cells (PSCs), there is still a big gap in the transition from small‐area devices to large‐area perovskite solar modules (PSMs). Herein, recent progresses on scaling‐up PSMs are reviewed: first, multifarious scalable preparation methods, solvent engineering, and corresponding morphology control strategies for large‐area homogeneous perovskite films are summarized. Various charge carrier transport materials, electrode materials, and their scaling methods for high‐efficiency and stable PSMs are then outlined and the device structure design of PSMs is discussed. Finally, the current strategies for optimizing the environmental stability of devices are highlighted, and packaging for reducing lead leakage during operation is discussed.

Dehydration-Reaction-Based Low-Temperature Synthesis of Amorphous SnOx for High-Performance Perovskite Solar Cells.

The development of methodologies for synthesizing carrier-transporting materials is critical for optoelectronic device fabrication. Amorphous metal oxides have emerged as potential carrier transport materials for perovskite tandem solar cells and flexible electronics due to their ease of fabrication and excellent electronic properties. However, perovskite solar cells employing amorphous metal oxides as the electron-transporting layers (ETLs) remain inefficient. This research describes a moderate dehydration reaction for the low-temperature synthesis of amorphous SnOx. We investigated this amorphous SnOx as the ETL for perovskite solar cells and demonstrated a maximum power conversion efficiency (PCE) of 20.4%, the greatest efficiency ever attained with an amorphous metal oxide ETL produced below 100 °C. Remarkably, the device maintained 85% of its initial efficiency for more than 4800 h. Furthermore, flexible perovskite solar cells based on this amorphous SnOx have a maximum PCE of 11.7%. Finally, this amorphous SnOx was used to fabricate LEDs and exhibited a maximum external quantum efficiency of over 3%.

Numerical simulation study for efficiency enhancement of doubly graded perovskite solar cell

In this paper, optimization of perovskite solar cell (PSC) with a computational simulation approach of double graded active layers for attaining an excellent power conversion efficiency (PCE) has been studied. The device is designed before the fluency in charge carriers with the implication of the double carrier transport materials (ETMs and HTMs) in the simulated PSC architecture. The present study further facilitates that the perovskite active materials with optimized thickness are stacked having a bandgap of 1.6 eV for achieving an improved short circuit current density (JSC) of about 20.29 mA/cm2, exhibiting a high open-circuit voltage (VOC) of 1.1 V, fill factor (FF) about an average of 90% and an excellent power conversion efficiency (PCE) of 19.20%, respectively. Furthermore, the highest external quantum efficiency (EQE), averaging 82%, is also observed for the PSC, precisely in the wavelength span of 380 nm–800 nm. Hence, the static combination of the double carrier transport materials having double graded active layers effectively enhances the optoelectronic parameters and provides a significant pathway for further device optimization in the future.

63‐4: Development of Ink‐jet Printing Process for 55‐inch UHD AMQLED Display

In this study, by selecting the carrier injection and transport materials and optimizing the thickness of each functional layer in the device, we have solved the problem of unbalanced carrier injection in quantum dot light‐emitting diodes (QLED). The current efficiencies of red, green and blue spin‐coating QLED devices reach 30 cd/A, 70 cd/A, and 8 cd/A, respectively. By continuous optimization of the ink‐jet printing and film‐forming process, the suitable volume of printed ink drops and drying process conditions are obtained, and completed the fabrication of 55‐inch UHD AMQLED display.

Deep learning for ultra-fast and high precision screening of energy materials

Semiconductor materials for energy storage are the core and foundation of modern information society and play important roles in photovoltaic system, integrated circuit, spacecraft technology, lighting applications, and other fields. Unfortunately, due to the long experiment period and high calculation cost, the high-precision band gap (the basic characteristic parameter) of semiconductor is difficult to obtain, which hinders the development of new semiconductor materials. Since the traditional Perdew–Burke–Ernzerhof (PBE) functional not only requires a long calculation time, but also significantly underestimates the band gap, we developed a deep learning model that can predict the more precise Heyd–Scuseria–Ernzerhof (HSE06) band gaps in milliseconds for 1,503 binary metallic oxides, nitrides, and sulfides, with a mean absolute error (MAE) of 0.35 eV, a mean squared error (MSE) of 0.21 eV, and a coefficient of determination (R2) of 0.98. Based on transfer learning, only < 5% of the data set (64 structures) was required to train the model and predict the band gaps of the remaining 1,439 structures. From the 1,503 candidate materials, we quickly identified 75 carrier transport materials, 33 electrode and electrocatalytic materials, 299 power switching materials, and 114 sensing materials. This work is the first to demonstrate the feasibility of transfer learning in band gap prediction, from the low-level PBE to the high-level HSE06 calculation, with a computation speed at least 104 times faster than the ab initio calculation. The proposed method could be further expanded to incorporate entire organic/inorganic crystal materials databases (> 106 crystals), which is of great significance for the screening and discovery of new semiconductor energy storage materials.

Optimization of carrier transport materials for the performance enhancement of the MAGeI3 based perovskite solar cell

The perovskite solar cells have turned into one of the most promising devices due to their exclusive properties in the photovoltaic field. But in recent years, the lead-free perovskite (PVK) based PSC is extensively carried out for the advantages of toxic-less and high stability factor in challenging commercial production. In this paper, numerical modeling of MAGeI3 based PSC device is considered to investigate the photovoltaic parameters under the illuminance of AM1.5. Moreover, the effect of distinct carrier transport layers (CTLs) on PSC parameters is shown for obtaining suitable device parameters. As, the better carrier mobility and band alignment for the ZnO and Spiro-OMeTAD exhibiting much improved Jsc of 16.27 mAcm−2, and effective quantum efficiency (EQE) about 85% in the wavelength region of 300–680 nm respectively. Furthermore, this combination also offers a much-improved power conversion efficiency (PCE) up to 18.3%. Thus, the insertion of ZnO and Spiro-OMeTAD as ETL and HTL in the PSC design possibly offer a novel design for the MAGeI3 based PSC structures.

E-beam evaporated hydrophobic metal oxide thin films as carrier transport materials for large scale perovskite solar cells

ABSTRACT E-beam evaporated metal-oxide thin films have been studied with different stoichiometries as carrier transport materials for large-scale perovskite solar cell devices. The thin films are deposited through a reactive evaporation of pure metals in various oxygen atmosphere pressures. The measured optical and electrical properties were used in numerical modelling using SCAPS-1D software to demonstrate a power conversion efficiency in excess of 26%. X-ray photoelectron spectroscopy and optical characterization confirmed the variation of the oxygen concentration into these films with respect to the deposition pressures. The electrical measurements using hall probes complies with the reasonable data range. The field emission scanning electron microscopy demonstrated the deposition of dense, homogeneous and pin-hole-free uniform films throughout the substrate’s surface. Hydrophobic surface was found for the films grown with nano-porous structure. In particular, large-scale vapour deposition of perovskite solar cells becomes feasible due to such fabrication technique without breaking the vacuum.

Simultaneous Passivation of Surface Vacancies and Enhancement in Charge Transfer Property of ZnO Electron Transport Layer for Inverted Organic Solar Cells

The design and development of a charge carrier transport material are decisive parameters for controlling the organic solar cell's power conversion efficiency (PCE). ZnO is one of the most suitable electron transport materials used in inverted bulk heterojunction polymer solar cells. However, the solution‐processed ZnO has surface defects, which hinders the power conversation efficiency of the device. Herein, it is designed and demonstrated that the 2D NO2 group functionalized reduced graphene oxide (rGN) sheet coated on top of the 1D ZnO nanoridges not only passivates the surface but also enhances the charge transport property of the electron transport layer (ETL), thereby improving the overall PCE by 31%. Atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), and optical measurements reveal that the highly transparent bilayer ZnO/rGN ETL has uniform film formation and, thereby, improved ohmic contact between the cathode and the photoactive layer. Due to the improved electron transport from the photoanode (PTB7‐Th:PC71BM) to the buffer layer, a photoinduced current density of 20.05 mA cm−2 is achieved. This interface modification by rGN can be an effective strategy to passivate the surface and retards the recombination rate to enhance the efficiency of organic photovoltaic cells.

Band Alignment as the Method for Modifying Electronic Structure of Metal-Organic Frameworks.

Electronic-level ordering in metal-organic frameworks (MOFs) is a route to modulate their electronic properties such as optical absorption, band alignment, work function, charge separation, charge carrier lifetimes, and ground- or excited-state conductivity. A systematic application of this approach requires the knowledge on how a MOF chemical composition affects its electronic structure. In this work, the fundamental principles for selecting MOF components to achieve targeted level alignment are considered. Correlations between the electronic parameters of building blocks and MOF band structure are analyzed. The factors affecting the energy position of constituents are discussed. In particular, the impact of the chemical composition of ligands, including the structure of its scaffold and side groups, on their energy positions in MOFs is addressed. Besides, the effect of the choice of reference potential and surface termination on the band alignment is investigated. The performance of several density functionals in the computation of absolute band positions is assessed. Finally, general principles for the modification of the MOF electronic structure are formulated and the routes to achieve an appropriate band alignment with carrier-transporting materials, co-catalysts, and redox reaction potentials are suggested.