Unbalanced Carrier Research Articles

Organic bulk heterojunction photodetectors based on polymer blends with unbalanced mobilities

We investigate bulk heterojunction organic photodetectors with unbalanced charge carrier mobilities in the active layer. We present an experimental example of an organic photodiode with very unbalanced mobilities but achieving state-of-the-art parameters in terms of external quantum efficiency exceeding 75% (at illumination 638 nm), and of bandwidth exceeding 1 MHz. The active layer of the photodiode is a 50 nm thick bulk heterojunction made of a blend of PCBM and copolymer of 5,6-difluoro2,1,3-benzothiadiazole, with mobility of slower charge carriers of order of 5 × 10−7 , estimated using space-charge-limited photocurrent technique. To understand why such a low mobility does not have deteriorating effects on the photodetecting performance of the photodiode, we performed drift-diffusion simulations of bulk heterojunction photodiodes with Langevin recombination. According to the simulation, the bandwidth of the photodiode is approximately independent of the mobility of slower charge carriers in the blend, and the negative effect of low mobility on the responsivity can be compensated by increasing the reverse bias. Our study shows that well-performing organic photodetectors can be fabricated using organic semiconductors having too low mobility for photovoltaic applications.

Bulk heterojunction perovskite solar cells incorporated with solution-processed TiOx nanoparticles as the electron acceptors

In the past ten years, perovskite solar cells were rapidly developed, but the intrinsic unbalanced charge carrier diffusion lengths within perovskite materials were not fully addressed by either a planar heterojunction or meso-superstructured perovskite solar cells. In this study, we report bulk heterojunction perovskite solar cells, where perovskite materials CH3NH3PbI3 is blended with solution-processed n-type TiOx nanoparticles as the photoactive layer. Studies indicate that one-step solution-processed CH3NH3PbI3:TiOx bulk-heterojunction thin film possesses enhanced and balanced charge carrier mobilities, superior film morphology with enlarged crystal sizes, and suppressed trap-induced charge recombination. Thus, bulk heterojunction perovskite solar cells by CH3NH3PbI3 mixed with 5 wt% of TiOx, which is processed by one-step method rather than typical two-step method, show a short-circuit current density of 20.93 mA/cm2, an open-circuit voltage of 0.90 V, a fill factor of 80% and with a corresponding power conversion efficiency of 14.91%, which is more than 30% enhancement as compared with that of perovskite solar cells with a planar heterojunction device structure. Moreover, bulk heterojunction perovskite solar cells possess enhanced device stability. All these results demonstrate that perovskite solar cells with a bulk heterojunction device structure are one of apparent approaches to boost device performance.

Ultrasensitive and high gain solution-processed perovskite photodetectors by CH3NH3PbI2.55Br0.45:Zn2SnO4 bulk heterojunction composite

Hybrid perovskite photodetectors, the novel alternative devices transform incident light into electrical signal, have been rapidly developed in the past years. However, intrinsic unbalanced charge carrier transport within hybrid perovskite materials, restricting further boosting device performance of perovskite photodetectors, has rarely been addressed. In this study, we report room temperature–operated solution-processed bulk heterojunction perovskite photodetectors with ultrahigh sensitivity and great photo-gain. It is found that the introduction of Zn2SnO4 nanoparticles into CH3NH3PbI2.55Br0.45 perovskites results in enlarged photocurrent and suppressed dark current for bulk heterojunction perovskite photodetectors. As a result, a responsivity of over 1500 mAW−1 and projected detectivity of approximatively 1014 Jones (1 Jones = cm Hz1/2 W−1) from 380 to 760 nm are observed from bulk heterojunction perovskite photodetectors. In addition, bulk heterojunction perovskite photodetectors exhibit an excellent linear dynamic range of 124 dB and a great photo-gain of 535. All these results indicate that high-performance perovskite photodetectors could be realized through novel bulk heterojunction device structure.

Bulk Heterojunction Perovskite Solar Cells Incorporated with Zn2SnO4 Nanoparticles as the Electron Acceptors.

Perovskite solar cells were developed very fast in the past decade, but hybrid perovskite materials with unbalanced charge carrier diffusion lengths were not fully addressed by either conventional or planar heterojunction device structures. In this study, high-performance perovskite solar cells with bulk heterojunction device structures where CH3NH3PbI2.55Br0.45 is blended with an n-type high-electron-mobility Zn2SnO4 nanoparticle as the photoactive layer are reported. Systematic studies indicate that the CH3NH3PbI2.55Br0.45:Zn2SnO4 bulk heterojunction thin film possesses enhanced and balanced charge carrier mobilities, superior film morphology with enlarged crystal sizes, and suppressed trap density. Photoluminescence and time-resolved photoluminescence studies further demonstrate that there is an efficient photoinduced charge carrier transfer between CH3NH3PbI2.55Br0.45 and Zn2SnO4 nanoparticles. Thus, bulk heterojunction perovskite solar cells by the CH3NH3PbI2.55Br0.45:Zn2SnO4 thin film exhibit over 21.07% power conversion efficiency, which is more than 12% enhancement as compared to that (18.74%) observed from planar heterojunction perovskite solar cells by the pristine CH3NH3PbI2.55Br0.45 thin film. Moreover, bulk heterojunction perovskite solar cells possess significantly suppressed photocurrent hysteresis, dramatically enhanced device stability, and reproducibility. All these results demonstrate that high-performance perovskite solar cells can be realized through bulk heterojunction device structures.

Solvent treatment induced interface dipole and defect passivation for efficient and bright red quantum dot light-emitting diodes

Abstract Quantum dot light-emitting diodes (QLEDs) have been widely recognized as one of the potential candidates for next-generation displays. In QLEDs, unbalanced carrier distribution (lacking holes) is an essential factor limiting device efficiency. Herein, we propose the solvent treatment of hole transporting layer (HTL) and quantum dot (QD) emission layer (EML) to enhance hole injection from HTL and reduce the defects in QD EML. Scanning Kelvin probe microscopy measurements reveal that the surface potential of HTL is lowered by solvent treatment with dimethyl formamide (DMF), which is due to the formation of an interface dipole layer between QD EML and HTL, and thus the hole injection barrier from HTL to QD EML is reduced. Furthermore, steady and transient photoluminescence (PL) measurements reveal that the DMF treated QD film, in comparison with the standard QD film, exhibits higher PL intensity and reduced non-radiative recombination induced by the defects. These improvements result in a 25% boost in the current efficiency of the QLEDs with DMF treated QD EML as compared to the QLED with unmodified EML, leading to a high current efficiency of 14.81 cd/A and a maximum brightness of 113 373 cd/m2. Furthermore, the solvent treatment enables the QLEDs possessing sub-bandgap turn-on voltage, which is 1.8 V (the optical bandgap of 2.1 eV) for the QLEDs with DMF treated QD EML.

Electrically Injected Hybrid Organic/Inorganic III-Nitride White Light-Emitting Diodes Based on Rubrene/(InGaN/GaN) Multiple-Quantum-Wells P-N Junction

It is known that the commercialized white light-emitting diodes (WLEDs) depend on phosphor conversion, which is limited by the unbalanced carrier injection in multiple quantum wells (MQWs) and low down-conversion efficiency. To solve these problems, hybrid organic/inorganic III-nitride WLEDs were designed and fabricated based on Rubrene/(InGaN/GaN) MQWs p-n junction. In such new structure, Rubrene can act as yellow–red emitter and play an important role in hole transport layer owing to its high carrier mobility. By optimizing the hybrid LED structure, the charge balance and recombination zone in radiative region were significantly improved. In this work, the optimal hybrid WLED with color coordinates of (0.31, 0.33) shows a maximum current efficiency of 15.22 cd/A with excellent long lifetime stability. The hybrid WLEDs based on organic/inorganic p-n junction were demonstrated with a good performance, which provides an attractive route to achieve high performance phosphor-free WLED sources for microdisplay or large-area WLED applications.

Improved performance of quantum dot light-emitting diodes by hybrid electron transport layer comprised of ZnO nanoparticles doped organic small molecule

Abstract ZnO nanoparticles (NPs) are widely employed as the electron transport layer (ETL) material in quantum dot light-emitting diodes (QLEDs) due to their excellent electron transport properties and inherent stability. However, the unbalanced carrier injection caused by the redundant electron injection is a common phenomenon existing in QLEDs with ZnO ETL. Here, based on the lower electron mobility and shallower lowest unoccupied molecular orbit (LUMO) of most organic electron transport ·materials than ZnO NPs, we report a strategy to modulate the electron transporting properties of ZnO NPs by doping organic small molecular electron transport materials. Several kinds of small molecular dopants including LiQ (lithium 8-quinolate), TPBi (1,3,5-tris(N-phenylbenzimidazol-2,yl) benzene), and BPhen (4,7-diphenyl-1,10-phenanthroline) are separately introduced to modify ZnO. The resulting device with LiQ-doped ZnO ETL at a doping ratio of 7 wt% exhibits the optimal performance at peak current efficiency (CE) and external quantum efficiency (EQE) up to 8.07 cd/A and 7.74%, which are about 1.93- and 1.94-fold higher than 4.19 cd/A and 3.98% of the device with pristine ZnO ETL, respectively. The improvement of performances can be mainly attributed to the inhibition of electron injection and interfacial exciton quenching by the use of LiQ-doped ZnO ETLs. This work may offer a promising method for fabricating high performance QLEDs and promote their application development in the display fields.

47-Fold EQE improvement in CsPbBr3 perovskite light-emitting diodes via double-additives assistance

All-inorganic cesium halide perovskite materials are intensively investigated due to their excellent photoelectric characteristics, but poor film coverage, unbalanced charge carrier transporting and insufficient exciton harvesting are remaining challenges to be overcome in order to achieve high electroluminescent (EL) performance in perovskite light-emitting diodes (PeLEDs). Here we show that these problems can be solved by introducing both a phosphorescent sensitizer (FIrpic) and an electron transporting material (TmPyPB) into all-inorganic cesium halide perovskite (CsPbBr 3 ). And highly efficient green PeLEDs with full coverage CsPbBr 3 :TmPyPB:FIrpic film as the emissive layer have been achieved, exhibiting the maximum luminance of 37784 cd/m 2 , the maximum current efficiency of 22.6 cd/A and the corresponding maximum external quantum efficiency (EQE) of 5.85%, where the EQE is 48-fold to that of bare CsPbBr 3 PeLEDs. The enhanced EL performance is attributed to less non-radiative current leakage, better film surface passivation effect, balanced charge carrier injection and transportation, as well as better exciton harvesting. It is believed that our work will provide an effective strategy to further improve EL performance of PeLEDs via better exciton harvesting. • Excellent enhanced electroluminescent performances were realized in CsPbBr 3 :TmPyPB:FIrpic based PeLEDs. • Pinhole-free and continuous CsPbBr 3 :TmPyPB:FIrpic perovskite films was achieved. • The current leakage was largely reduced. • More balanced charge carrier transporting behavior in perovskite emitting layer was reached. • Improved exciton harvesting was realized due to Förster and Dexter energy transfer processes.

High-performance solution-processed white organic light-emitting diodes based on silica-coated silver nanocubes.

Solution-processed white organic light-emitting diodes (WOLEDs) with silica-coated silver nanocubes (Ag@SiO2 NCs) incorporated at the interface of a hole transporting layer and emission layer are studied. The concentration of Ag@SiO2 NCs is varied to investigate the effect of Ag@SiO2 NCs on the performances of WOLEDs. Owing to the sharp edges and corners, Ag NCs greatly improve the radiative rate and emission intensity of nearby blue excitons. The blue emission at different Ag@SiO2 NC concentrations determines the performance of the WOLEDs. The emission of the orange excitons is strengthened by the high concentration of Ag@SiO2 NCs, which slightly influences the device performance. On the other hand, the SiO2 shell and some SiO2 nanospheres coexisting with Ag NCs reduce the hole transporting, improving the carrier balance in the WOLEDs. The experimental and simulated results also show that excessive Ag@SiO2 NCs may cause a rough film surface, unbalanced carrier injection, and fluorescence quenching, which decreases the device performance. The optimized WOLED with a proper concentration of Ag@SiO2 NCs has a peak current efficiency of 53.9cd/A, acquiring a significant enhancement factor of 77.3% compared to the control device without Ag@SiO2 NCs.

Tunable Emission of Bluish Zn-Cu-Ga-S Quantum Dots by Mn Doping and Their Electroluminescence.

On the basis of bluish-emitting double-shelled quantum dots (QDs) of Zn-Cu-Ga-S (ZCGS)/ZnS/ZnS, Mn doping into ZCGS host with different Mn/Cu concentrations is implemented via surface adsorption and lattice diffusion. The resulting double-shelled Mn-doped ZCGS (ZCGS/Mn) QDs exhibit a distinct Mn2+4T1-6A1 emission as a consequence of effective lattice incorporation simultaneously with host intragap states-involving emissions of free-to-bound and donor-acceptor pair recombinations. The relative contribution of Mn emission to the overall photoluminescence (PL) is consistently proportional to its concentration, resulting in tunable PL from bluish, white, to reddish white. Regardless of Mn doping and its concentration, all QDs possess high PL quantum yield levels of 74-79%. Those undoped and doped QDs are then employed as an emitting layer (EML) of all-solution-processed QD-light-emitting diodes (QLEDs) with hybrid charge transport layers and their electroluminescence (EL) is compared. Compared to undoped QDs, doped analogues give rise to a huge spectral disparity of EL versus PL, specifically showing a near-complete quenching of Mn2+ EL. This unexpected observation is rationalized primarily by considering unbalanced carrier injection to QD EML on the basis of energetic alignment of the present QLED and rapid trapping of holes injected at intragap states of QDs.

Balanced carrier injection of quantum dots light-emitting diodes: the case of interface barrier of bilayer ZnO electron transport layer

Unbalanced carrier injection is one of the most important reasons for the efficiency roll-off in quantum dot light-emitting diodes. Reducing the electron injection can effectively balance the carrier transport and improve the optoelectronic performance of the device. In this work, a bilayer ZnO electron transport layer was fabricated by twice spin-coating and annealing methods. More than 60% of electrons are effectively blocked by the ZnO interface barrier compared with the standard device, resulting in increasing the maximum luminance of the device from 25 390 to 48 220 cd m−2 and the current efficiency from 1.5 to 3.2 cd A−1.

Organic Salt Semiconductor with High Photoconductivity and Long Carrier Lifetime

AbstractIntrinsic photogeneration of charge carriers in organic semiconductors is generally attributed to high energy ionization or exciton dissociation by a strong electric field. Here, high bulk photoconductivity is reported in pristine pentamethine cyanine films with photocurrent onset at the band‐edge of the organic semiconductor. Single‐layer cyanine diodes with selective hole and electron contacts show linear dependence of photocurrent with reverse voltage and light intensity. Numerical drift‐diffusion simulations reveal that the linear resistor behavior stems from low and unbalanced carrier mobilities giving rise to negative space charge. Slow bimolecular recombination kinetics of photoinduced charges obtained by time delayed charge extraction measurements show strongly reduced Langevin recombination with long carrier lifetime of the order of a millisecond. Such reduced charge carrier recombination puts forward a materials concept to be exploited in photodiodes and more generally in optoelectronic devices.

The effects of SQ additive on charge carrier transport and recombination in PCDTBT:PC71BM based ternary organic solar cells

The ternary solar cell has been proved as an elegant strategy to realize efficient bulk heterojunction (BHJ) organic solar cells (OSCs) by harvesting a larger range of photons than binary devices. In order to design such an efficient system, the charge carrier transport and recombination in ternary OPVs must be considered. We fabricated various ternary devices by adding squaraine derivatives (SQ-1, SQ-2, SQ-3, SQ-4 and SQ-5) with complementary absorption spectrum in near infrared region as the additional donor material into Poly [[9-(1-octylnonyl)-9H-carbazole-2, 7-diyl]-2, 5-thiophenediyl-2, 1, 3-benzothiadiazole-4, 7-diyl-2, 5-thiophenediyl] (PCDTBT): [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) host binary blend film in this paper. Through the charge carrier mobility and transient photovoltage (TPV) measurements, it is found that the incorporation of 10% SQ-5 with shallower Highest Occupied Molecular Orbital (HOMO) level hinders holes in transferring from SQ to PCDTBT, which leads to unbalanced charge carrier transport, significant charge recombination, and decrease of performance. On the contrary, the performance of ternary devices incorporated with 10% SQ-3 with deeper HOMO energy level is improved by 21% compared with binary devices, which can be attributed to increased photon harvesting and changeless charge carrier characteristics.

Imaging the Anomalous Charge Distribution Inside CsPbBr3 Perovskite Quantum Dots Sensitized Solar Cells.

Highly luminescent CsPbBr3 perovskite quantum dots (QDs) have gained huge attention in research due to their various applications in optoelectronics, including as a light absorber in photovoltaic solar cells. To improve the performances of such devices, it requires a deeper knowledge on the charge transport dynamics inside the solar cell, which are related to its power-conversion efficiency. Here, we report the successful fabrication of an all-inorganic CsPbBr3 perovskite QD sensitized solar cell and the imaging of anomalous electrical potential distribution across the layers of the cell under different illuminations using Kelvin probe force microscopy. Carrier generation, separation, and transport capacity inside the cells are dependent on the light illumination. Large differences in surface potential between electron and hole transport layers with unbalanced carrier separation at the junction have been observed under white light (full solar spectrum) illumination. However, under monochromatic light (single wavelength of solar spectrum) illumination, poor charge transport occurred across the junction as a consequence of less difference in surface potential between the active layers. The outcome of this study provides a clear idea on the carrier dynamic processes inside the cells and corresponding surface potential across the layers under the illumination of different wavelengths of light to understand the functioning of the solar cells and ultimately for the improvement of their photovoltaic performances.

Highly Efficient Nondoped Near‐Ultraviolet Electroluminescence with an External Quantum Efficiency Greater Than 6.5% Based on a Carbazole–Triazole Hybrid Molecule with High and Balanced Charge Mobility

AbstractLow charge injection and unbalanced carrier transport are unfavorable factors for highly efficient near‐ultraviolet (NUV) electroluminescence (EL) emitters. The construction of weak donor‐ and acceptor‐containing fluorophores has been utilized as a common solution to these issues. Few fluorescent emitters simultaneously exhibiting NUV emission and high/balanced charge‐transport properties have been reported. Herein, a new 1D donor–π–acceptor molecule 9‐(4′‐(4,5‐diphenyl‐4H‐1,2,4‐triazol‐3‐yl)‐[1,1′‐biphenyl]‐4‐yl)‐9H‐carbazole named PCZTZ consisting of 9‐phenylcarbazole as a donor and 2,3,4‐triphenyl‐1,2,4‐triazole as an acceptor with a biphenyl π‐bridge is constructed. The backbone‐twisted PCZTZ exhibits not only strong solid‐state NUV fluorescence but also high and balanced hole and electron mobilities (both up to 10−4 cm2 V−1 s−1), benefiting from the 3D supramolecular H‐bonding network containing strong CH⋯N interactions. Remarkably, a fabricated nondoped light‐emitting diode (OLED) emits 408 nm bright NUV light with a maximal external quantum efficiency of 6.57% and Commission Internationale de L'Eclairage coordinates of (0.17, 0.07). Thus, this OLED is the most efficient NUV OLED reported thus far. These results provide a valuable strategy for the design of highly efficient NUV EL emitters by modulating the linking positions of carbazole and triazole on a biphenyl bridge.

Highly Efficient Three Primary Color Organic Single‐Crystal Light‐Emitting Devices with Balanced Carrier Injection and Transport

Organic single crystals have a great potential in the field of organic optoelectronics because of their advantages of high carrier mobility and high thermal stability. However, the application of the organic single crystals in light‐emitting devices (OLEDs) has been limited by single‐layered structure with unbalanced carrier injection and transport. Here, fabrication of a multilayered‐structure crystal‐based OLED constitutes a major step toward balanced carrier injection and transport by introducing an anodic buffer layer and electron transport layer into the device structure. Three primary color single‐crystal‐based OLEDs based on the multilayered structure and molecular doping exhibit a maximum luminance and current efficiency of 820 cd cm−2 and 0.9 cd A−1, respectively, which are the highest performance to date for organic single‐crystal‐based OLEDs. This work paves the way toward high‐performance organic optoelectronic devices based on the organic single crystals.

Effects of unbalanced carrier injection on the performance characteristics of InGaN light-emitting diodes

Two kinds of InGaN-based light-emitting diodes (LEDs) having different electron concentrations in the n-GaN injection layer are investigated in order to understand the effects of unbalanced carrier injection on LED performance characteristics. Electrical and optical characteristics such as capacitance–voltage, current–voltage, external quantum efficiency, and electroluminescence spectrum are compared and analyzed. It is shown that the unbalanced carrier distribution in multiple quantum wells affects the forward operating voltage since a large disparity of injection rate between electrons and holes can induce a small effective active volume, thus leading to the severe overflow of electrons to the p-(Al)GaN layer in the LED devices.

Bulk heterojunction perovskite hybrid solar cells with large fill factor

A bulk heterojunction device structure was developed to address the unbalanced charge carrier extraction efficiencies in perovskite hybrid solar cells.

Efficient Phosphorescent Organic Light Emitting Diodes Using F4TCNQ as the Indium-Tin-Oxide Modification Layer

A phosphorescent organic light emitting diode by using tetrafluorotetracyanoquinodimethane (F4TCNQ) as the indium-tin-oxide modification layer and 4,4′-bis(carbazol-9-yl)biphenyl (CBP) as the hole transporting layer is reported. CBP doped with a green phosphorescent dopant, tris(2-(p-tolyl)pyridine) iridium(III) (Ir(mppy)3) is used as the emission layer in this device, and the maximum current efficiency of 31.3 cd/A is achieved. Furthermore, low efficiency roll-off of 10.4% is observed with device luminance increasing from 100 cd/m2 (29.7 cd/A) to 10000 cd/m2 (26.5 cd/A). It is demonstrated that a charge-generation area is formed at F4TCNQ/CBP interface, which will benefit hole injection into the hole transporting layer. Moreover, use of the CBP hole transporting layer will benefit the low efficiency roll-off by broadening triplet exciton formation, as well as by avoiding accumulation of unbalanced carrier at the hole transporting layer/emission layer interface.

Effect of light incidence angle on optical absorption characteristics of low bandgap polymer-based bulk heterojunction organic solar cells

The bulk heterojunction organic solar cell based on thieno[3,4-b]thiophene/benzodithiophene (PTB7) is one of an alternative candidate for traditional silicon-based solar cells owing to its advantages of ease of manufacture, low cost, and flexibility. Currently, many research studies of these devices focus on power conversion efficiency (PCE) enhancement with only normal sunlight incidence. In this study, we have experimentally verified that PCE markedly decreased from 5.51 to 3.47% as incidence angle was changed from 0 to 60°. Using the finite-difference time-domain method, we found that the degeneration of optical absorption is caused by the decreased electrical field intensity in the photoactive layer over the entire wavelength range due to the optical interference profile change. In addition, we confirmed that a higher incidence angle also results in unbalanced charge carrier transport characteristics, resulting in further decrease in solar cell efficiency.