Electron Transport Material Research Articles

Structurally Nontraditional Benzo[c]cinnoline-Based Electron-Transporting Materials with 3D Molecular Interaction Architecture.

The academically widely used electron-transporting materials (ETMs) typically suffer from low glass transition temperatures (Tg ) that could lead to poor device stability. Considering practical applications, we herein put forward a "3D molecular interaction architecture" strategy to design high-performance ETMs. As a proof-of-concept, a type of structurally nontraditional ETMs with the benzo[c]cinnoline (BZC) skeleton have been proposed and synthesized by the C-H/C-H homo-coupling of N-acylaniline as the key step. 2,9-diphenylbenzo[c]cinnoline (DPBZC) exhibits strong intermolecular interactions that feature a 3D architecture, which boosts Tg to exceedingly high 218 °C with a fast electron mobility (μe ) of 6.4×10-4 cm2 V-1 s-1 . DPBZC-based fluorescent organic light-emitting diodes show outstanding electroluminescent performances with an external quantum efficiency of 20.1 % and a power efficiency as high as 70.6 lm W-1 , which are superior to those of the devices with the commonly used ETMs.

Structurally Nontraditional Benzo[c]cinnoline‐Based Electron‐Transporting Materials with 3D Molecular Interaction Architecture

AbstractThe academically widely used electron‐transporting materials (ETMs) typically suffer from low glass transition temperatures (Tg) that could lead to poor device stability. Considering practical applications, we herein put forward a “3D molecular interaction architecture” strategy to design high‐performance ETMs. As a proof‐of‐concept, a type of structurally nontraditional ETMs with the benzo[c]cinnoline (BZC) skeleton have been proposed and synthesized by the C−H/C−H homo‐coupling of N‐acylaniline as the key step. 2,9‐diphenylbenzo[c]cinnoline (DPBZC) exhibits strong intermolecular interactions that feature a 3D architecture, which boosts Tg to exceedingly high 218 °C with a fast electron mobility (μe) of 6.4×10−4 cm2 V−1 s−1. DPBZC‐based fluorescent organic light‐emitting diodes show outstanding electroluminescent performances with an external quantum efficiency of 20.1 % and a power efficiency as high as 70.6 lm W−1, which are superior to those of the devices with the commonly used ETMs.

New insights in construction of three-dimensional donor/acceptor interface for high performance perovskite solar cells: The preparation of wolf tooth stick-like TiO2

Organic-inorganic hybrid perovskite solar cells (PSCs) have recently been considered as emerging candidates for the new generation of photovoltaic devices due to their outstanding properties. However, the challenge in carrier transport such as the unfavorable nonradiative recombination in the active layer and the finite stability at device interfaces limit their further commercialization. One of the major obstacles is that the metastable state at donor/acceptor (D/A) interfaces limits the device stability. Thus, how to improve the stability of the interface is one of the main concern. Herein, a new strategy is proposed to building stable D/A interface by introducing the three-dimensional (3-D) structure in PSCs, where a novel wolf tooth stick-like TiO2 (wts-TiO2) is prepared as electron transport material (ETM). The as-prepared devices exhibit excellent performance with accelerated charge transport, reduced nonradiative recombination, expanded D/A interface, and shortened internal photoexcitons diffusion distance. As a result, a top PCE of 19.6% was obtained. Moreover, the wts-TiO2 based solar cells showed excellent long-term stability, which maintained over 81.5% of the initial efficiency after 30 days. Our work provided new insights in fabrication of high-performance perovskite solar cells.

Improved Charge Balance in Green Perovskite Light-Emitting Diodes with Atomic-Layer-Deposited Al2O3

Perovskite light-emitting diodes (LEDs) have experienced a rapid increase in efficiency over the last several years and are now regarded as promising low-cost devices for displays and communication systems. However, it is often challenging to employ ZnO, a well-studied electron transport material, in perovskite LEDs due to chemical instability at the ZnO/perovskite interface and charge injection imbalance caused by the relatively high conductivity of ZnO. In this work, we address these problems by depositing an ultrathin Al2O3 interlayer at the ZnO/perovskite interface, allowing the fabrication of green-emitting perovskite LEDs with a maximum luminance of 21 815 cd/m2. Using atomic layer deposition, we can precisely control the Al2O3 thickness and thus fine-tune the electron injection from ZnO, allowing us to enhance the efficiency and operational stability of our LEDs.

Effective passivation of TiO2/Si by interlayer SiOx controlled by scanning zone annealing for perovskite/Si tandem solar cell

A 2-terminal tandem solar cell that utilized organometal halide perovskite as the top cell and crystalline silicon (Si) as the bottom cell has attracted much attention in recent years for its high theoretical energy conversion efficiency. Among many different approaches, high energy conversion efficiency with relatively simple device architecture can be achieved by direct deposition of electron transport material of a perovskite top cell on a Si bottom cell. To maximize the amount of extractable current from this type of solar cell, precise control of surface recombination at the interface between the electron transport layer and Si is a crucial factor. In this study, we proposed the utilization of Scanning Zone Annealing (SZA), a unique heating treatment developed by our research group, to reduce surface recombination by selective treatment of a TiO2/Si interface. This method was proven effective to increase the minority carrier lifetime of a sputtered-TiO2/Si interface from 6.3 μs to 355 μs for p-type float-zone Si, from 7.9 μs to 310 μs for n-type float-zone Si, and from 9.3 μs to 271 μs for p-type Czochralski Si. Furthermore, the origin behind the reduction in surface recombination at a TiO2/Si interface by SZA was proposed based on direct observation of the interface. Here, we found that the thickness of a natively formed SiOx interlayer is crucial to the reduction of the surface recombination at a TiO2/Si interface and is controllable at nanometer scale by adjusting the amount of heat transferred during SZA treatment.

Through-Space Conjugated Electron Transport Materials for Improving Efficiency and Lifetime of Organic Light-Emitting Diodes.

Thermally stable electron transport (ET) materials with high electron mobility and high triplet state energy level are highly desired for the fabrication of efficient and stable organic light‐emitting diodes (OLEDs). Herein, a new design strategy of constructing through‐space conjugated folded configuration is proposed to explore robust ET materials, opposite to the widely used planar configuration. By bonding two quinolines to the 9,10‐positions of phenanthrene, two novel folded molecules with high thermal and morphological stabilities and high triplet state energy levels (>2.7 eV) are created. These folded molecules possess excellent ET ability with electron mobilities of three orders of magnitude higher than those of linear and planar counterparts. Theoretical calculation and crystallography analysis demonstrate the through‐space conjugated folded configuration has not only reduced reorganization energy but also enlarged charge transfer integral at various dimensions, bringing about efficient multi‐dimensional ET, independent of molecular orientation. By adopting the folded molecule as ET layers, OLEDs with no matter delayed fluorescence or phosphorescence emitters can achieve high external quantum efficiencies and long operational lifetimes simultaneously. This work paves a new avenue towards robust ET materials to improve efficiency and stability of OLEDs.

Self-Enhancement of Efficiency and Self-Attenuation of Hysteretic Behavior of Perovskite Solar Cells with Aging

Spontaneous enhancement of the photovoltaic performance of perovskite solar cells (PSCs) after aging has been reported, but the underlying reasons for such behavior are still under debate. Herein, we demonstrate that this spontaneous improvement effect accompanied by self-attenuation of hysteresis in the current-voltage characteristics is time- and temperature-dependent. Moreover, it is universal to PSCs based on a range of mixed-ion perovskites and coupled to different hole- and electron-transporting materials. Time-resolved confocal fluorescence microscopy and other characterization techniques suggest that the "self-healing" phenomenon is accompanied by the homogenization and enhancement of the charge extraction efficiency as well as suppressed recombination throughout cm2-scale perovskite layers. These dynamic effects need to be accounted for when assessing the initial and stabilized performance of PSCs.

Planar perovskite solar cells: Plasmonic nanoparticles‐modified ZnO as an electron transport layer for enhancing the device performance and stability at ambient conditions

The interfacial engineering of inorganic electron transport materials is to implement stable interfaces with perovskites that have received tremendous momentum in the field of photovoltaics. The viability of perovskite solar cells at ambient conditions and their possible commercialization is one of the recent interests among photovoltaic researchers. In this work, planar perovskite solar cells (PSCs) are fabricated using plasmonic nanoparticles (Ag and Au) incorporated flake-like ZnO nanostructured material that acts as an electron transport/interfacial layer. The power conversion efficiency and stability of the fabricated perovskite solar cells are evaluated at ambient conditions. The power conversion efficiency (PCE) of the constructed PSCs namely ZnO, ZnO/Ag, ZnO/Au nanostructured-based electron transport layer (ETL) is 4.68%, 4.92% and 5.50% respectively whereas the interfacial layered ZnO/Ag and ZnO/Au has achieved a maximum PCE of about 6.35% and 6.41%. The stability of all the fabricated devices is studied under AM 1.5 G illumination and UV light conditions in which the ZnO/Au interfacial layered device exhibits significant enhancement retaining ~93% and ~82% stability of initial power conversion efficiency.

Reducing Energy Disorder in Perovskite Solar Cells by Chelation.

In inverted perovskite solar cells (PSCs), the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is a widely used electron transport material. However, a high degree of energy disorder and inadequate passivation of PCBM limit the efficiency of devices, and severe self-aggregation and unstable morphology limit the lifespan of devices. Here, we design a series of fullerene dyads FP-Cn (n = 4, 8, 12) to replace PCBM as an electron transport layer, where [60]fullerene is linked with a terpyridine chelating group via a flexible alkyl chain of different lengths as a spacer. Among three fullerene dyads, FP-C8 shows the most enhanced molecule ordering and adhesion with the perovskite surface due to the balanced decoupling between the chelation effect from terpyridine and the self-assembly of fullerene, leading to lower energy disorder and higher morphological stability relative to PCBM. The FP-C8/C60-based devices using Cs0.05FA0.90MA0.05PbI2.85Br0.15 as a light absorber show a power conversion efficiency of 21.69%, higher than that of PCBM/C60 (20.09%), benefiting from improved electron extraction and transport as well as reduced charge recombination loss. When employing FAPbI3 as a light absorber, the FP-C8/C60-based devices exhibit an efficiency of 23.08%, which is the champion value of inverted PSCs with solution-processed fullerene derivatives. Moreover, the FP-C8/C60-based devices show better moisture and thermal stability than PCBM/C60-based devices and maintain 96% of their original efficiency after 1200 h of operation, while their counterpart PCBM/C60 maintains 60% after 670 h.

Facile Synthesis of 1,7-Phenanthroline Derivatives and Evaluation of Their Properties as Hole-Blocking Materials in Organic Light-Emitting Diodes

Abstract Phenanthroline derivatives are typical simple aza-polycyclic aromatic hydrocarbons that have been used as hole-blocking and electron-transporting materials in organic light-emitting diodes (OLEDs). In contrast to the widely used 1,10-phenanthroline derivatives, other isomers, including 1,7-phenanthrolines, have been explored less, partly due to the lack of effective and selective synthesis methods. This study demonstrates the facile synthesis of 1,7-phenanthroline derivatives bearing various substituents via a multicomponent reaction based on the aza-Diels-Alder reaction. By investigating the correlation between the substituents and their performance as hole-blocking materials in OLEDs, we gained insight into the molecular design of 1,7-phenanthroline derivatives for material applications.

Interface barrier strategy for perovskite solar cells realized by In-situ synthesized polyionic layer

The harmonious coordination between the perovskite and the charge transport layer is the prerequisite for the high efficiency and stability of perovskite solar cells (PSCs). The excellent photoelectric and solution processable SnO2 electron transport material opens the most commercial potential of planar PSCs. However, SnO2 has rich surface activity and photocatalytic properties, which will deteriorate perovskite after absorbing UV light and seriously affect the stability and efficiency of PSCs. Here, the interface barrier strategy is introduced to insert an anchored-then-rinsed lead polystyrene sulfonate (ARPSS-Pb) polyionic layer between SnO2 and perovskite via in-situ synthesis method to construct a new stable interface, which can effectively block the phase transition of perovskite driven by photocatalytic SnO2. In addition, ARPSS-Pb also has the functions of passivating interfacial defects, relaxing interfacial stress and guiding crystallization of perovskite. PSCs based on ARPSS-Pb achieved a power conversion efficiency (PCE) of 22.26%, much higher than 19.60% of the control devices. More importantly, after 1000 h of storage under continuous humidity conditions, up to 96.32% of the initial PCE is maintained. A high open circuit voltage (VOC) as high as 1.190 V has been obtained for ARPSS-Pb based devices. To our best knowledge, this is the highest VOC achieved in the system of perovskite with 1.56 eV energy gap.

In situ-formed tetrahedrally coordinated double-helical metal complexes for improved coordination-activated n-doping

In situ coordination-activated n-doping by air-stable metals in electron-transport organic ligands has proven to be a viable method to achieve Ohmic electron injection for organic optoelectronics. However, the mutual exclusion of ligands with high nucleophilic quality and strong electron affinity limits the injection efficiency. Here, we propose meta-linkage diphenanthroline-type ligands, which not only possess high electron affinity and good electron transport ability but also favour the formation of tetrahedrally coordinated double-helical metal complexes to decrease the ionization energy of air-stable metals. An electron injection layer (EIL) compatible with various cathodes and electron transport materials is developed with silver as an n-dopant, and the injection efficiency outperforms conventional EILs such as lithium compounds. A deep-blue organic light-emitting diode with an optimized EIL achieves a high current efficiency calibrated by the y colour coordinate (0.045) of 237 cd A−1 and a superb LT95 of 104.1 h at 5000 cd m−2.

Synthesis and research of a kind of perylene imide discoid molecule

Discotic liquid crystal molecules are excellent organic semiconductor materials due to their high carrier mobility. Dibenzocoronene derivatives obtained by nuclear expansion with perylene diimide as a matrix are one of the discotic molecules. The key factor for the application of this type of molecule is that it can form stable and long-range ordered organic nano-scale thin films. It can be used as an efficient carrier transport channel. This paper intends to use the “channel effect” to obtain the corresponding long-range orderly ideal film. The “channeling effect” referred to in this article is to bond functional discoid molecules on the substrate firstly, than generate the corresponding self-assembled monomolecular membranes (SAMs) to form an ordered channel on the surface which strongly induces and restricts the discoid molecules that arranged in parallel and orderly with each other along the “channels” created on the surface of the SAMs. Perylene diimide derivatives are a kind of good electron transport materials, which are characterized by high carrier mobility, low processing cost, and good thermal stability. However, it has the large rigid core and the melting point is relatively high. In this paper, a monobenzocoperylene diimide derivative is designed and synthesized, which will have a strong effect on the surface of the silicon substrate, and reduce the molecular melting point by reducing the size of the perylene imide discotic molecular core expansion.

Tantalum Oxide as an Efficient Alternative Electron Transporting Layer for Perovskite Solar Cells.

Electron transporting layers facilitating electron extraction and suppressing hole recombination at the cathode are crucial components in any thin-film solar cell geometry, including that of metal–halide perovskite solar cells. Amorphous tantalum oxide (Ta2O5) deposited by spin coating was explored as an electron transport material for perovskite solar cells, achieving power conversion efficiency (PCE) up to ~14%. Ultraviolet photoelectron spectroscopy (UPS) measurements revealed that the extraction of photogenerated electrons is facilitated due to proper alignment of bandgap energies. Steady-state photoluminescence spectroscopy (PL) verified efficient charge transport from perovskite absorber film to thin Ta2O5 layer. Our findings suggest that tantalum oxide as an n-type semiconductor with a calculated carrier density of ~7 × 1018/cm3 in amorphous Ta2O5 films, is a potentially competitive candidate for an electron transport material in perovskite solar cells.

Electron transport interface engineering with pyridine functionalized perylene diimide-based material for inverted perovskite solar cell

Charge-transport layer engineering is proved to be as important as the optimization of perovskite light harvesting layer in achieving high power conversion efficiency for the inverted perovskite solar cells (p-i-n PSCs). This work mainly focuses on electron transport interface engineering and demonstrates a pyridine functionalized perylene diimide-based electron transport material (ETM), termed SFX-Py-PDI2. Benefiting from the high electron mobility, suitable energy arrangement and defect passivating effect, p-i-n PSCs with SFX-Py-PDI2 as ETM produce a desired PCE over 19% surpassing PC61BM based devices. Especially noteworthy is that introducing SFX-Py-PDI2 into perovskite film through anti-solvent treatment together with PC61BM as ETM further boost the PCE of p-i-n PSCs over 21%. Moreover, the SFX-Py-PDI2 based device showed impressive long-term stability at unencapsulated condition, maintaining over 80% initial PCE after aging for 1000 h in 65 ℃, 50–60% RH environment.

Performance Analysis of Calcium-Doped Titania (TiO2) as an Effective Electron Transport Layer (ETL) for Perovskite Solar Cells

Ca-doped TiO2 films were synthesized by the modified sol-gel method and employed as the electron transport material of perovskite solar cells (PSCs). Morphological, optoelectronic, thermal, and electrical studies of thin films were investigated through XRD, RAMAN, SEM, AFM, UV-Vis, FTIR, and IV characteristics. Ca doping was detected with the help of structural properties while morphological analysis revealed that thin films based on Ca-doped titania are crack-free, homogenous, and uniformly distributed. Further optoelectronic properties have shown a promising conversion efficiency of 9.79% for 2% Ca-doped titania followed by 1% Ca-doped titania, while 3% have shown the lowest conversion efficiency among these prepared samples. The 2% an optimized doping of Ca has shown an almost two-fold increase in conversion efficiency in comparison to pristine TiO2, along with an increase in current density from 15 mA⋅cm−2 to 19.3 mA⋅cm−2. Improved energy efficiency and higher current density are attributed to faster electron transportation; moreover, the optimized percentage of Ca doping seems to be an effective approach to improve the PSCs’ performance.

Improving stability of perovskite solar cells using fullerene-polymer composite electron transport layer

Perovskite solar cells (PSCs) have attracted significant attention due to their high efficiency and potential for low-cost manufacturing, but their commercialization is strongly impeded by low operational stability. The engineering of charge-transport layer materials have been recognized as an effective strategy to improve both stability and performance of PSCs. Here, we introduce a pyrrolo[3,4-c]pyrrole-1,4-dione-based n-type copolymer as an electron transport material for perovskite solar cells. Using a composite of this polymer with the fullerene derivative [60]PCBM delivered an efficiency of 16.4% and enabled long-term operational stability of p-i-n perovskite solar cells.

Design and simulation of efficient tin based perovskite solar cells through optimization of selective layers: Theoretical insights

Within a decade, Perovskite solar cells (PSCs) have attained an efficiency of 25.8%. PSCs lack commercialization due to two main factors: (i) poor device stability (ii) toxicity. The tin (Sn) is a possible candidate to substitute toxic lead (Pb) in traditional PSCs; however, the performance of Sn-based PSCs is 13%, much lower than Pb-based PSCs due to several aspects. In this work, we have used drift-diffusion simulation to propose a pathway to improve the performance of lead-free PSCs using MASnI3 as a light harvester. The conventional charge selective layer, such as TiO2, requires high-temperature processing, which also is a major concern. As reported earlier, traditional transport layers contribute actively to the degradation of PSCs, affecting their stability. Therefore, we have used ZnO nanoparticles (NP) as electron transport material (ETM), and conducted an analysis using various organic and inorganic hole transport materials (HTMs) with spike and cliff structures. The impact of HTM thickness on the performance parameters of MASnI3 based PSCs with constant and variable charge acceptor concentrations was investigated. This work indicates that organic HTMs exhibit more thickness-dependent behavior than inorganic HTM for MASnI3 based PSCs. Furthermore, the suitable valence band offset (VBO) and barrier height at Perovskite/HTM and HTM/Back contact interfaces were studied for efficient energy level mismatch with absorber layer to boost the performance of MASnI3 based PSCs. The concept of selecting back contact for Sn-based PSCs using different HTMs was analyzed. The influence of temperature variation on the performance of PSCs using different HTM was analyzed. The hybrid configuration devices face the issue of interfacial recombination; thus, the impact of defect density at the HTM/MASnI3 interface on the performance of PSCs with various HTMs was investigated. Lastly, MASnI3 based PSC showed a power conversion efficiency (PCE) of 23.51% using the unmodified CuSCN based HTM and, CuI can also deliver an efficiency of 23.31% employed modified valence band energy level.

Inverted Perovskite Solar Cells: The Emergence of a Highly Stable and Efficient Architecture

Perovskite solar cells (PSCs) have experienced a rapid development during the past decade. For regular PSCs, device efficiency has reached already a power conversion efficiency (PCE) of 25.5%. Inverted PSCs have been attracting increasing attention owing to their easy fabrication, cost‐effectiveness, and suppressed hysteresis characteristics. A maximum PCE of 23.72% for these types of devices is already achieved and some recent progresses in their performance suggest that inverted PSCs are still far from being fully optimized. Great opportunities for reaching simultaneously high stability and efficiency are forecast. Within this work, the most recent developments concerning solar device structure, fabrication of perovskite films, hole and electron transport materials, and electrode contacts in inverted PSCs are overviewed. This work also focuses on stability issues, fabrication methods for large‐area inverted PSCs, encapsulation techniques, economic assessment, and environmental opportunities for hastening inverted PSCs for commercialization.

Study of CZTSSe-Based Solar Cells with Different ETMs by SCAPS

Third-generation thin-film solar cells based on CZTSSe are highly promising because of their excellent optoelectrical properties, earth-abundant, and non-toxicity of their constituent elements. In this work, the performance of CZTSSe-based solar cells with TiO2, CdS, and ZnSe as electron transporting materials (ETMs) was numerically investigated using the Solar Cell Capacitance Simulator (SCAPS). The effect of the active layer’s thickness and electron affinity, different buffer layers, and the contour plot of the operating temperature versus thickness of the CdS buffer layer were studied. The results show that the optimum power conversion efficiency for CdS, TiO2, and ZnSe, as the ETMs, is 23.16%, 23.13%, and 22.42%, respectively.