Balanced Charge Carrier Research Articles

Hybrid nonfullerene acceptors based on thieno[2′,3′:4,5]thieno[3,2-b]indole for efficient organic solar cells

The emergence of non-fullerene acceptors (NFAs) has revived organic solar cells (OSCs) and promoted its industrialization significantly. Nowadays there are two main kinds of fused-ring electron acceptors: ITIC series and Y-series NFAs. Herein, two hybrid NFAs TIT-H and TIT-EH integrated ITIC series and Y-series NFAs have been developed, which based on thieno[2′,3′:4,5]thieno[3,2-b]indole-thiophene with one spiro group. TIT-H and TIT-EH both exhibited excellent thermal stability, low optical bandgaps of ∼1.5 eV, planar molecular geometries, and suitable energy levels. As a result, OSCs based on PM6:TIT-EH afforded a decent PCE of 13.32 %, with an VOC of 0.856 V, a JSC of 23.50 mA/cm2 and a FF of 65.91 %, which was due to the suitable intermolecular interactions of TIT-EH, balanced charge carrier mobilities and face-on orientation in blend films. As comparison, PM6:TIT-H cell exhibited lower PCE of 12.26 %. These results demonstrated that hybrid NFAs are promising for OSC applications.

Polymer Based on Asymmetrically Halogenated Benzotriazole Enables High Performance Organic Solar Cells Prepared in Nonhalogenated Solvent.

Halogenation on the A unit of the D-π-A-type polymer donor has been proven as an effective strategy to improve the performance of organic solar cells (OSCs). Compared with fluorination, chlorination usually increases the open-circuit voltage because of the downward shift of energy levels, but decreases the charge transport ability due to the large steric hindrance of the chlorine atom. We reported herein a method to balance the energy loss and charge transport through asymmetric halogenation on the benzotriazole (BTA) unit of the polymer. The designed PE3-FCl based on the BTA unit containing fluorine and chlorine atoms rendered the highest power conversion efficiency (PCE) of 17.83% when eC9-2Cl-γ and o-xylene were used as the electron acceptor and solvent, respectively. The performance is obviously higher than that of the polymer PE3 containing a difluorinated BTA unit (16.65%) and polymer PE3-2Cl with dichlorinated BTA (14.65%) due to the manipulated morphology by preaggregation, improved and more balanced charge carrier transport, and reduced recombination loss. Notably, this PCE is a breakthrough for the BTA-based polymers processed by nonhalogenated solvent. This work gives deep insight into the asymmetric halogenation of polymer donors for high-performance green solvent-processed OSCs.

Improved Charge Recombination Efficiency in Organic Light‐Emitting Transistors via Luminescent Radicals

AbstractLuminescent radicals are attracting attention as emitters in electroluminescent devices thanks to the exploitation of doublet excitons. Recent studies reveal that exciton formation in radical organic light‐emitting diodes (OLEDs) primarily occurs through a charge trapping mechanism. Although typically detrimental for OLEDs, this might be a key process to elucidate light emission in organic light‐emitting transistors (OLETs). Here, a unipolar n‐type architecture suitable for the implementation of radical emitters is introduced, designed based on computational calculations. The operation of the as‐realized devices incorporating the newly synthesized [2,6‐dichloro‐4‐(2,6‐dimethoxyphenyl)phenyl](3,5‐dichloro‐4‐pyridyl) (2,4,6‐trichlorophenyl)methyl radical is investigated via transient electroluminescence measurements to demonstrate the occurrence of long‐living emission ascribed to the charge trapping mechanisms. Moreover, a comprehensive understanding of the processes governing radical‐OLET is obtained by recording complete 2D maps of both optical and electrical response of the device as a function of applied voltages. Notably, the trapping of electrons by radical moieties is demonstrated to generate a negative charge density in the emissive layer that facilitates holes to be injected: increasing the balance of opposite charge carriers, a tenfold enhancement of the external quantum efficiency (EQE) at the proper source‐drain and source‐gate voltage conditions is reported to reach a maximum EQE value of 0.2%.

Over 19 % efficienct ternary organic solar cells enabled by tuning charge behaviors through quinoxalineimide-based Y-type acceptors

Attaining a balanced charge carrier generation is a significant challenge in organic photovoltaics (OPVs). To enhance device performance, the ternary strategy has emerged as an effective approach. In this study, two Y6 derivatives, QIP-4F and QIP-4Cl, containing quinoxalineimide (QI) building blocks fused with thienylthiophene as the central unit were incorporated into the PM6:L8-BO system. The quinoxalineimide units reduce the energy losses in charge carrier extraction and non-radiative charge recombination. Furthermore, the increasing steric hindrance in the central block impacts phase separation and ordered intermolecular packing, which enhances the charge transport and minimizes the charge recombination. Ultimately, the PM6:L8-BO:QIP-4Cl device achieved a champion efficiency of 19.05%, offering a promising direction for enhancing the photovoltaic performance of OPVs.

Enhanced performance of ambient-air processed CsPbBr3 perovskite light-emitting electrochemical cells via synergistic incorporation of dual additives

Metal halide perovskite light-emitting electrochemical cells (MHP-LECs) are promising due to their facile solution processability and exceptional optoelectrical properties. However, commercialization is hindered by poor thin-film coverage and the need for glovebox processing. This study addresses these issues by incorporating poly(ethylene oxide) (PEO) and potassium hexafluorophosphate (KPF6) into cesium lead bromide perovskite (CsPbBr3) emitting layers (EML), processed under ambient-air conditions. These perovskite composite thin films were spin-coated on a preheated substrate, and the device structure was ITO/PEDOT:PSS/EML/Al. Notably, the MHP-LEC based on CsPbBr3:PEO:KPF6 (100:65:0.2, weight ratio) exhibited pure-green emission with a peak at 524 nm and a narrow full width at half-maximum of 27 nm. Compared to a reference device (CsPbBr3:PEO (100:65, weight ratio)), the new device showed 1.5-, 4-, and 5-fold improvements in current density, electroluminescence (EL) intensity, and lifetime, respectively. These enhancements are attributed to reduced non-radiative current leakage, improved film surface coverage and passivation, and balanced charge carrier injection and transportation. This study offers a straightforward approach for processing CsPbBr3 thin films under ambient-air conditions, enhancing the EL performance of MHP-LECs.

Synthesis, Photoluminescence, and Electroluminescence of Phosphorescent Dipyrido[3,2-a;2'3'-c]phenazine-Platinum(II) Complexes Bearing Hole-Transporting Acetylide Ligands.

In this study, novel phosphorescent dipyrido[3,2-a;2'3'-c]phenazine (dppz)-platinum(II)-phenylacetylide complexes were developed to fabricate non-doped organic light-emitting diodes (OLED) by solution-processing. To facilitate the charge carrier injection into the emitting layer (EML), 3,6-di-tert-butylcarbazole-functinalized phenylacetylides were employed. As for the dppz ligand, 9,9-dihexylfluoren-2-yl and 4-hexylthiophen-2-yl side-arms were introduced to the 2,7-positions, which led to reddish orange and red photoluminescence (PL), respectively, in solution and film states (PL wavelength: ca. 600 and ca. 625 nm, respectively). The carbazole-appended phenylacetylide ligands hardly affected the emission color, although unsubstituted phenylacetylides gave rise to aggregate- or excimer-based near-infrared PL with a low quantum yield. Two types of non-doped OLEDs were fabricated: single-layer and multilayer devices. In both devices, the organic layers were fabricated by spin-coating, and the EML consisted of a neat film of the corresponding platinum(II) complex. Therein, electroluminescence spectra corresponding to those of PL were observed. The single-layer devices exhibited low device efficiencies due to a deteriorated charge carrier balance. The multilayer devices possessed hole- and electron-transporting layers on the anode and cathode sides of the EML, respectively. Owing to an improved charge carrier balance, the multilayer devices exhibited higher device performance, affording considerably improved values of luminance and external quantum efficiency.

Designing delafossite CuAl1-xTMxO2 solid solutions: the role of 3d transition metal spin states in photo(electro)catalytic performance

This study delves into the effects of 3d transition metal (TM) spin states on the structural and electronic properties of CuAl1-xTMxO2 solid solutions, focusing on their implications for photo(electro)catalytic performance. The research reveals that the Jahn–Teller distortion, associated with TM3+, is a critical factor in determining the lattice parameters and photo(electro)catalytic performances. Solid solutions without Jahn–Teller distortion adhere to Vegard's law, whereas those with strong distortion exhibit deviations, indicating the influence of TMO6 octahedral distortion on solubility and lattice parameters. The electronic structure of solid solutions with weak Jahn–Teller distortion is governed by the O–Cu–O and TMO6 crystal fields, which leads to a narrowed bandgap and reduced conduction band minimum (CBM), impacting the hydrogen evolution potential. In particular, the CuAl0.5Cr0.5O2 shows a significant enhancement in photocurrent density and hydrogen production rate due to its balanced light absorption and effective charge carrier separation. In contrast, the weak Jahn–Teller distortion in CuAl1-xFexO2 results in localized electronic states at CBM, leading to diminished carrier mobility. Solid solutions with strong Jahn–Teller distortion, such as CuAl1-xTMxO2 (TM = Mn and Ni), display a range of electronic properties from semiconductor to semimetallic, with the semimetallic CuAl0.9Mn0.1O2 capable of infrared light absorption and efficient photocatalytic hydrogen and oxygen production.

Negative corona discharge strategy for efficient quantum dot light-emitting diodes.

In colloid quantum dot light-emitting diodes (QLEDs), the control of interface states between ZnO and quantum dots (QDs) plays a vital role. We present a straightforward and efficient method using a negative corona discharge to modify the QD film, creating a dipole moment at the interface of QDs and magnesium-doped ZnO (ZnMgO) for balanced charge carrier distribution within the QDs. This process boosts external quantum efficiencies in red, green, and blue QLEDs to 17.71%, 14.53%, and 9.04% respectively. Notably, optimized devices exhibit significant enhancements, especially at lower brightness levels (1000 to 10,000 cd·m-2), vital for applications in mobile displays, TV screens, and indoor lighting.

Exploring the optimal design space of transparent perovskite solar cells for four-terminal tandem applications through Pareto front optimization

Machine learning algorithms can enhance the design and experimental processing of solar cells, resulting in increased conversion efficiency. In this study, we introduce a novel machine learning-based methodology for optimizing the Pareto front of four-terminal (4T) perovskite-copper indium selenide (CIS) tandem solar cells (TSCs). By training a neural network using the Bayesian regularization-backpropagation algorithm via Hammersley sampling, we achieve high prediction accuracy when testing with unseen data through random sampling. This surrogate model not only reduces computational costs but also potentially enhances device performance, increasing from 29.4% to 30.4% while simultaneously reducing material costs for fabrication by 50%. Comparing experimentally fabricated cells with the predicted optimal cells, the latter show a thinner front contact electrode, charge-carrier transport layer, and back contact electrode. Highly efficient perovskite cells identified from the Pareto front have a perovskite layer thickness ranging from 420 to 580 nm. Further analysis reveals the front contact electrode needs to be thin, while the back contact electrode can have a thickness ranging from 100 to 145 nm and still achieve high efficiency. The charge-carrier transport layers play a crucial role in minimizing interface recombination and ensuring unidirectional current flow. The optimal design space suggests thinner electron and hole transport layer thicknesses of 7 nm, down from 23 to 10 nm, respectively. It indicates a balanced charge-carrier extraction is crucial for an optimized perovskite cell. Overall, the presented methodology and optimized design parameters have the potential to enhance the performance of 4T perovskite/CIS TSC while reducing material fabrication costs.

Enhanced Performance of Inverted Perovskite Quantum Dot Light-Emitting Diode Using Electron Suppression Layer and Surface Morphology Control.

The energy level offset at inorganic layer-organic layer interfaces and the mismatch of hole/electron mobilities of the individual layers greatly limit the establishment of balanced charge carrier injection inside the emissive layer of halide perovskite light-emitting diodes (PeQLEDs). In contrast with other types of light-emitting devices, namely OLEDs and QLEDs, various techniques such as inserting an electron suppression layer between the emissive and electron transport layer have been employed as a means of establishing charge carrier injection into their respective emissive layers. Hence, in this study, we report the use of a thin layer of Poly(4-vinylpyridine) (PVPy) (an electron suppression material) placed between the emissive and electron transport layer of a halide PeQLEDs fabricated with an inverted configuration. With ZnO as the electron transport material, devices fabricated with a thin PVPy interlayer between the ZnO ETL and CsPbBr3 -based green QDs emissive layer yielded a 4.5-fold increase in the maximum observed luminance and about a 10-fold increase in external quantum efficiency (EQE) when compared to ones fabricated without PVPy. Furthermore, the concentration and coating process conditions of CsPbBr3 QDs were altered to produce various thicknesses and film properties which resulted in improved EQE values for devices fabricated with QDs thin films of lower surface root-mean-square (RMS) values. These results show that inhibiting the excessive injection of electrons and adjusting QDs layer thickness in perovskite-inverted QLEDs is an effective way to improve device luminescence and efficiency, thereby improving the carrier injection balance.

Highly Soluble Ambipolar anti-Perylene-3,4 : 9,10-bis(benzimidazole)s Stabilize a Room-Temperature Columnar Hexagonal Phase.

A new series of highly soluble perylene anti-bis(4,5-dialkoxybenzimidazole)s bearing branched flexible chains stabilizing room temperature columnar hexagonal phase and with balanced ambipolar charge carrier mobility is reported for the first time. Only the anti isomer was successfully separated and characterized. These compounds have a high extinction coefficient, small optical band gap and wide absorption range, thus making them a promising class of ambipolar organic semiconductors capable of self-organizing.

Correlation of Functional Coumarin Dye Structure with Molecular Packing and Organic Solar Cells Performance

Nowadays, ternary organic solar cells (OSCs) based on all‐small molecules have the convenient strategy toward high‐performance and stable devices. Herein, two coumarin‐based dyes C4 and C4–CN are designed and synthesized with a simple synthetic route. The molecules display different intra‐ and intermolecular interactions in the solid state originating from acceptor substitution, as revealed by the single‐crystal structure analysis. The introduction of dicyanomethylene acceptor group in C4–CN plays a synergistic role in molecular packing and orientation in the solid state, thus fostering the elongation of exciton lifetime and improved charge transport in photoactive layer. The optimized C4–CN:AQx–3‐based binary OSCs show a power conversion efficiency (PCE) of 15.07%, which is appreciably higher than the C4:AQx–3‐based OSC (11.39%) due to the appropriate energy‐level alignment, balanced charge transport, favorable phase separation, and lower charge recombination. Furthermore, by introduction of C4–CN as the third component into C4:AQx–3 blend, the ternary OSC shows an improved PCE of 15.34%, which is linked with the enhanced crystallization, optimized morphology, balanced charge carrier mobility, and suppressed recombination with low energy loss.

Non-fused nonfullerene acceptors based on thieno[2',3':4,5]thieno[3,2-b]indole for efficient organic solar cells

Thieno[3,2-b]indole (TI) has attracted more and more attention in organic electronics. Herein, thieno[2',3':4,5]thieno[3,2-b]indole (TTI), as a tetracyclic derivative of TI, was been developed by fused another thiophene unit. Furtherly, two non-fused NFAs TTI-2F and TTI-2Cl based on TTI have been synthesized via simple and easy synthetic routes. TTI-2F and TTI-2Cl both exhibited excellent thermal stability, low optical bandgaps of ∼1.5 eV, nearly planar molecular geometries, and suitable energy levels. As a result, PM6:TTI-2F solar cells afforded a decent PCE of 10.67 %, with an VOC of 0.935 V, a JSC of 19.21 mA/cm2 and a FF of 59.40 %, which was due to the suitable intermolecular interactions, higher HOMO energy level, better exciton dissociation, suppressed trap-assisted recombination, and balanced charge carrier mobilities. As comparison, PM6:TTI-2Cl cell exhibited much lower PCE of 8.34 %. These results demonstrated that NFAs based on TTI are promising for industrialized organic photovoltaic applications due to the simple synthesis and decent photovoltaic performance.

Ambipolar hosts for blue TADF OLEDs: Assessment of the device performance and lifetime

Designing host materials suitable for thermally activated delayed fluorescence (TADF) based OLEDs emitting in the blue spectral range imposes great challenge, since several important factors including high compound stability, high triplet energy and balanced charge carrier mobility must be fulfilled. In attempt to address the issue, herein, we employed ambipolar hosts (ATRZ and tert-butyl modified ATRZtBu) composed of acridine donors and triazine acceptor linked in a bridgeless configuration and exhibiting triplet energies above 3 eV. The performance of blue TADF OLEDs containing the ambipolar hosts was thoroughly assessed and compared with that featuring benchmark host mCBP. A special emphasis was placed on the device operational lifetime, which was found to be at least twice as long for ATRZ device compared to the reference mCBP device. The enhanced device stability was attributed to broadened/shifted recombination zone from EML/HBL interface towards the center of EML resulting in a suppressed degradation due to the reduced triplet exciton concentration.

Prolonging blue TADF-OLED lifetime through ytterbium doping of electron transport layer

Balanced charge carrier transport and broad carrier recombination zone are essential features for reducing triplet exciton concentration and suppressing triplet-mediated annihilation processes, which affect the performance and stability of blue-emitting organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) phenomenon. Doping of organic layers is among the most effective methods to control these features. Herein, the impact of ytterbium (Yb) doping of electron transport layer on the TADF-OLED performance, and particularly the lifetime, was thoroughly assessed. It was found that the lower doping concentrations of Yb (<10wt%) cause electron trapping and reduced current density, whereas higher Yb concentrations facilitate electron transport in the device. Although the introduction of Yb somewhat deteriorated device efficiency, it extended operational lifetime of blue-emitting device by 2 orders of magnitude (up to 46 hours at 1000 cd/m2). This was shown to occur as a result of Yb-assisted non-radiative triplet exciton quenching at the electron transport and emission layer interface, thus diminishing detrimental exciton annihilation processes.

Novel Spiro Core‐Based Hole Transport Materials for Stable Deep‐Blue OLEDs with LT95 Over 420 h

AbstractFlexible, lightweight, and cost‐effective organic light‐emitting diodes (OLEDs) show great promise for use in displays and lighting applications. However, creating highly efficient and stable deep‐blue OLEDs remains a challenge due to a lack of efficient functional materials, specifically hole transport materials (HTM). In this report, two new spiro‐HTMs are introduced with fluorene (FBP‐SFX) or dibenzofuran (DBF‐SFX) peripheral substituents. This study finds that the DBF‐SFX variant produces highly efficient deep‐blue OLEDs with a narrow emission (full‐width at half‐maximum FWHM of 25 nm) and the highest external quantum efficiency (EQE) of 33.2% among the FBP‐SFX (12.2%) and NPB (11.9%) variants. The superior performance of DBF‐SFX is attributed to its deeper highest occupied molecular orbital level (HOMO) and small trap density, which results in reduced band‐edge offset, lower turn‐on voltages, and balanced charge carrier densities. Importantly, the blue fluorescence OLED with DBF‐SFX displays exceptional device stability with LT95 over 420 h with an initial luminance of 1000 cd m−2. This is due to the stronger bond dissociation energy (BDE) and suppressed Exciton–Polaron Annihilation (EPA) using the dibenzofuran substituent. This work demonstrates a new HTM design approach using a spiro‐core architecture and dibenzofuran substituents to create efficient and long‐lasting deep‐blue OLEDs.

Ionic Density Control of Conjugated Polyelectrolytes via Postpolymerization Modification to Enhance Hole‐Blocking Property for Highly Efficient PLEDs with Fast Response Times

AbstractFor an ideal electron interlayer, both electron injection and hole‐blocking properties are important to achieve better polymer light‐emitting devices (PLEDs) performance. Conjugated polyelectrolytes (CPEs) are applied widely in PLEDs to enhance charge injection. Understanding the role of backbone structures and energetic matching between the CPEs and emitters can benefit charge injection and balance. Herein, a postpolymerization approach to introduce varying amounts of alkyl sulfonate groups onto the backbone of a copolymer of 5‐fluoro‐2,1,3‐benzothiadiazole and 9,9′‐dioctylfluorene is utilized. This study finds that device performance is dependent on the percentage of sulfonate groups incorporated, with the optimal copolymer (CPE‐50%) maintaining efficient ohmic electron injection and gaining enhanced hole‐blocking properties, thereby achieving the most balanced hole/electron current. Therefore, the PLED with CPE‐50% interlayer exhibits the highest efficiency (20.3 cd A−1, 20.2 lm W−1) and the fastest response time (4.3 µs), which is the highest efficiency among conventional thin (70 nm) F8BT PLEDs with CPEs. These results highlight the importance of balanced charge carrier density in CPEs and highlight that postpolymerization modification is a useful method for fine‐tuning ionic content.

Two compatible non-fullerene acceptors towards efficient ternary organic photovoltaics

The composition of organic active layers plays significant role in determining the photovoltaic performance of organic solar cells (OSCs). Ternary strategy has shown promising advantages in fabrication for highly efficient OSCs due to its simple process and various donor and acceptor materials. Herein, a famous small molecule acceptor (ITIC) was employed as an assistant acceptor to make efficient ternary organic solar cells (TOSCs). The use of ITIC shows good compatibility and leads to the formation of alloyed state with the host acceptor (BTP-eC9). The middle bandgap of ITIC is also complementary with those of the host active materials (BTP-eC9 and PM6), which will be beneficial for the light-harvesting and thus enhance the photocurrent for OSC devices. Furthermore, the introduction of ITIC can fine-tune the film morphology with continuous nanofiber interpenetrating network, which is great for more balanced charge carrier transport and efficiently suppressed charge recombination. Consequently, a prominent power-conversion-efficiency (PCE) was achieved up to 18.25% with all-around enhancement photovoltaic parameters for the PM6:BTP-eC9:ITIC-based TOSCs.

Turning the energy channel of side-chain TADF polymers by monomer optimization for high-efficiency solution-processed white OLEDs

Suppressing non-radiative channel in luminescent polymer is the key factor for high device efficiency of organic light-emitting diodes (OLEDs). In this paper, we synthesized three side-chain TADF polymers, PTRZ-TRZ, PTRZ-TPA and PTRZ-Cz, to deeply investigate the relationship between energy transfer and molecular structure. The photophysical measurements show that the copolymerized monomer has significant effect on the exciton deexcitation process of TADF fragment. The rigid carbazole unit can efficiently separate the TADF units to inhibit the aggregation-caused quenching (ACQ) of polymer, while electron-donating triphenylamine unit will induce the severe charge transfer state to form low energy exciplex, which lead to severe energy leakage of the excited TADF. Moreover, according to the single-charge devices test, the ideal separation unit also can improve the capability of charge injection and carrier balance. As a result, the solution-processed OLEDs host with PTRZ-Cz achieved the external quantum efficiency (EQEmax) of 14.2%, which was much higher than that of PTRZ-TPA and PTRZ-TRZ. By doping red phosphorescent emitter Ir(MDQ)2(acac), the hybrid white OLED devices (T-P WOLEDs) based on PTRZ-Cz achieved the maximum current efficiency (CEmax), power efficiency (PEmax) and EQEmax of 46.8 cd/A, 32.6 lm/W and 20.7%, respectively, with a high CRI of 80.6. The outcomes obtained in this study can provide useful insights into the structure−property relationship for further development of efficient TADF polymer hosts for solution-processable OLEDs.

Organic- inorganic nanoparticle composite as an electron injection/hole blocking layer in organic light emitting diodes for large area lighting applications

The charge carrier balance in organic light-emitting diodes (OLEDs) is crucial for optimizing external quantum efficiency (EQE). This is typically achieved by using charge injection and blocking layers deposited by vacuum deposition, which increases the fabrication time and cost. This study investigates the potential of polyethylenimine ethoxylated (PEIE), Zinc oxide (ZnO) nanoparticles and a PEIE-ZnO nanocomposite as functional layers for OLEDs with improved electron injection and hole blocking properties fabricated by solution processing. The PEIE-ZnO nanocomposite improves OLED performance: (a) a 28% increase in current density and enhanced electron mobility, (b) two times increase in electron mobility of the electron injection/transport layer, and (c) a reduction in surface and interface trap density. Consequently, the PEIE-ZnO nanocomposite shows an 84% increase in electroluminescence, 52.5% increase in luminous efficacy, 35.5% increment in external quantum efficiency, and increased stability as measured by degradation studies. Employing these interlayers fabricated by the novel and roll-to-roll compatible Spray-on-Screen deposition technique show a comparable OLED performance as that of spin coated and vacuum deposited electron injection layers, which makes the large area fabrication of optoelectronic devices with high efficiencies, high output, low cost, and long lifetime a possible reality.