Charge Carrier Transport In Layers Research Articles

A Review on Pulsed Laser Preparation of Quantum Dots in Colloids for the Optimization of Perovskite Solar Cells: Advantages, Challenges, and Prospects.

In recent years, academic research on perovskite solar cells (PSCs) has attracted remarkable attention, and one of the most crucial issues is promoting the power conversion efficiency (PCE) and operational stability of PSCs. Generally, modification of the electron or hole transport layers between the perovskite layers and electrodes via surface engineering is considered an effective strategy because the inherent structural defects between charge carrier transport layers and perovskite layers can be reshaped and modified by adopting the functional nanomaterials, and thus the charge recombination rate can be naturally decreased. At present, large amounts of available nanomaterials for surface modification of the perovskite films are extensively investigated, mainly including nanocrystals, nanorods, nanoarrays, and even colloidal quantum dots (QDs). In particular, as unique size-dependent nanomaterials, the diverse quantum properties of colloidal QDs are different from other nanomaterials, such as their quantum confinement effects, quantum-tunable effects, and quantum surface effects, which display great potential in promoting the PCE and operational stability of PSCs as the charge carriers in perovskite layers can be effectively tuned by these quantum effects. However, preparing QDs with a neat and desirable size remains a technical difficulty, even though the present chemical engineering is highly advanced. Fortunately, the rapid advances in laser technology have provided new insight into the precise preparation of QDs. In this review, we introduce a new approach for preparing the QDs, namely pulsed laser irradiation in colloids (PLIC), and briefly highlight the innovative works on PLIC-prepared QDs for the optimization of PSCs. This review not only highlights the advantages of PLIC for QD preparation but also critically points out the challenges and prospects of QD-based PSCs.

Optimization of the Power Conversion Efficiency of CsPbIxBr3−x-Based Perovskite Photovoltaic Solar Cells Using ZnO and NiOx as an Inorganic Charge Transport Layer

In this study, we analyzed the maximum power conversion efficiency (PCE) of a photovoltaic cell with an ITO/ZnO/CsPbIxBr3−x/NiOx/Au structure, using ZnO and NiOx as the inorganic charge transport layers and CsPbIxBr3−x as an absorption layer. We optimized the thickness of each layer and investigated the effects of the defect density and interface defect density. To achieve the highest PCE, the optimal thicknesses were 300 nm for the electron transport layer (ZnO), 60 nm for the hole transport layer (NiOx), and 1000 nm for the absorption layer. The absorber defect density was maintained at approximately 1015 cm−3, and the interface defect density was approximately 1011 cm−3. The highest PCE obtained through optimization of each of these factors was 23.07%. These results are expected to contribute to the performance optimization of perovskite solar cells that use inorganic charge carrier transport layers.

Physical Passivation of Grain Boundaries and Defects in Perovskite Solar Cells by an Isolating Thin Polymer

Passivation and interlayer engineering are important approaches to increase the efficiency and stability of perovskite solar cells. Thin insulating dielectric films at the interface between the perovskite and the charge carrier transport layers have been suggested to passivate surface defects. Here, we analyze the effect of depositing poly(methyl methacrylate) (PMMA) from a very low-concentration solution. Spatial- and time-resolved photoluminescence and atomic force microscopy analyses of samples with diverse morphologies demonstrate the preferential deposition of PMMA in topographic depressions of the perovskite layer, such as grain and domain boundaries. This treatment results in an increase in the fill factor of more than 4% and an absolute efficiency boost exceeding 1%, with a maximum efficiency of 20.4%. Based on these results, we propose a physical isolation mechanism rather than a chemical passivation of perovskite defects, which explains not only the data of this study but also most results found in earlier works.

Improving the performance of OLEDs by controlling the molecular orientation in charge carrier transport layers.

The transition dipole moment (TDM) orientation in the emission layer (EML) of organic light-emitting diodes (OLEDs) have attracted increasing attention from many researchers. But the study point at the molecular orientation in the hole transport layer (HTL) and electron transport layer (ETL) was not reported widely. In this paper, the molecular orientation of HTLs and ETLs were controlled by the deposition rate. The angle-dependent PL spectra and the variable angle spectroscopic ellipsometry (VASE) were used for evaluating the molecular orientation of B3PYMPM and TAPC, respectively. We found that fast deposition rate can boost preferentially vertical molecular orientation in both molecules and facilitate the hole and electron mobility, which was tested by the current density-voltage and capacitance-voltage curves of HODs and EODs. Moreover, the HTLs and ETLs were employed in OLED devices to verify the influence of molecular orientation on charge carrier mobility, which determined the performance of OLEDs significantly.

23.3: Charge Injection Control of Cadmium‐Free Quantum Dot Light‐Emitting Diodes

Within recent years, cadmium‐free quantum dots have been gaining attention in order to produce large‐scale environmentfriendly displays. Here, we present high performance, electroluminescent Cd‐free quantum dot light‐emitting diode designs that enhance stability, and efficiencies between 1.6–3.5 times through novel charge carrier transport layers compared to references.

51‐1: Invited Paper: Charge Injection Control of Cadmium‐Free Quantum Dot Light‐Emitting Diodes

Within recent years, cadmium‐free quantum dots have been gaining attention in order to produce large‐scale environment‐friendly displays. Here, we present high performance, electroluminescent Cd‐free quantum dot light‐emitting diode designs that enhance stability, and efficiencies between 1.6‐3.5 times through novel charge carrier transport layers compared to references.

In Situ Growth of All-Inorganic Perovskite Single Crystal Arrays on Electron Transport Layer.

Directly growing perovskite single crystals on charge carrier transport layers will unravel a promising route for the development of emerging optoelectronic devices. Herein, in situ growth of high‐quality all‐inorganic perovskite (CsPbBr3) single crystal arrays (PeSCAs) on cubic zinc oxide (c‐ZnO) is reported, which is used as an inorganic electron transport layer in optoelectronic devices, via a facile spin‐coating method. The PeSCAs consist of rectangular thin microplatelets of 6–10 µm in length and 2–3 µm in width. The deposited c‐ZnO enables the formation of phase‐pure and highly crystallized cubic perovskites via an epitaxial lattice coherence of (100)CsPbBr3∥(100)c‐ZnO, which is further confirmed by grazing incidence wide‐angle X‐ray scattering. The PeSCAs demonstrate a significant structural stability of 26 days with a 9 days excellent photoluminescence stability in ambient environment, which is much superior to the perovskite nanocrystals (PeNCs). The high crystallinity of the PeSCAs allows for a lower density of trap states, longer carrier lifetimes, and narrower energetic disorder for excitons, which leads to a faster diffusion rate than PeNCs. These results unravel the possibility of creating the interface toward c‐ZnO heterogeneous layer, which is a major step for the realization of a better integration of perovskites and charge carrier transport layers.

Recent Advances of Device Components toward Efficient Flexible Perovskite Solar Cells

Flexible solar cells have launched potential applications in diverse areas, such as civil engineering, consumer electronics, electric automobile, and aerospace use. In recent years, perovskite solar cells (PSCs) have been successful with a rocketing power conversion efficiency (PCE) from 3.8% to 25.2% in the last decade. Due to its material flexibility, together with low‐cost and facile fabrication processes, perovskites have certainly been considered as a promising candidate for the next generation of flexible solar cells. So far, the PCE of single‐junction flexible perovskite solar cells (FPSCs) has reached 19.51%, which retains researchers' great enthusiasm for further development and applications of FPSCs. Herein, a brief review of the recent advances in the structural components of FPSCs is presented, including perovskite absorber layers, flexible substrates, transparent bottom electrodes, and charge carrier transport layers. In particular, the focus is on the development of low‐temperature fabrication processes of the aforementioned compositional layer structures. Finally, noticeable achievements and annual milestones are discussed and summarized, and suggestions for further improvement of FPSCs and their ways toward commercialization are presented.

Rotational design of charge carrier transport layers for optimal antimony trisulfide solar cells and its integration in tandem devices

Sb2S3 thin-film solar cells have recently gained attention due to their low cost, low toxicity, and simple fabrication. However, there is still plenty of room to improve their performance. It is known that efficient carrier transport is essential for high performance Sb2S3 solar cells, which, unfortunately, is difficult to characterize by conventional testing methods. Therefore, the carrier transport process in Sb2S3 solar cells was studied here using a theoretical simulation. The results show that high solar performances can be achieved with a wide parameter window for selecting the electron transport layer as well as the hole transport layer, viz., with a conduction band minimum of the electron transport layer (−4.4 eV < CBM < −3.2 eV), and a valence band maximum of the hole transport layer (−5.2 eV > VBM > −6.4 eV). Here the interfacial potential barrier become negligible and as a consequence electrons and holes cross at ease, which guarantee the good device performance. Indeed, a Sb2S3 solar cell with a high power conversion efficiency (PCE) can be obtained by ensuring that the carrier transport and collection are unimpeded in the device, i.e., the Sb2S3-based single junction solar cells shows high efficiency of 19.53%. Furthermore, we found that optimized Sb2S3 solar cells are particularly suitable for use as the top cell of tandem structure solar cells. Thus, a Sb2S3/Sb2Se3 double junction solar cell structure was proposed. With a 0.5 μm thick Sb2S3 absorber, double junction solar cells could achieve a theoretical efficiency as high as 26.64%. Our results based on the rotational design of bandgap alignment provide a general guide rule for selecting the optimal electron transport layer as well as the hole transport layer to boost the power conversion efficiency for Sb2S3 solar cells up to its theoretical limit.

Communication—Structural Modulation of Macrocyclic Materials for Charge Carrier Transport Layers in Organic Light-Emitting Devices

A cyclo-meta-phenylene (CMP) macrocycle with donor/acceptor substituents was designed. After optimization of the synthesis routes, the CMP congener was synthesized via [3 + 3] assembly of phenylene panels in moderate yield. The molecule showed a highly crystalline nature, and a single crystal formed upon sublimation. The molecule was transparent in the visible light region and was thermally robust with a decomposition temperature of 583°C. The effect of the donor/acceptor substituents was evaluated for charge carrier transport layers in emitter-doped phosphorescent OLEDs, which showed a dramatic improvement in its electron transporting ability as well as power efficiency in devices.

C70/Pentacene Organic Heterojunction as Charge Generator to Realize Highly Efficient Charge Carrier Injection in Organic Light‐Emitting Diodes: Performance and Mechanism Analysis

Highly efficient charge carrier injection from electrodes is basically required for high‐performance organic light‐emitting diodes (OLEDs). However, the mismatch of metal work function and energy level of charge carrier transport layers inevitably introduces the injection barrier and also greatly restricts the choice of the electrode metals. Here, a planar organic heterojunction (OHJ) C70/pentacene is used as charge generator to realize the highly efficient injection of charge carriers in OLEDs, not only achieving the high efficiency but also significantly improving the stability. More importantly, the resulting devices show an electrode work function‐independent efficiency property, greatly broadening the selectivity of electrode metals. Detailed studies are presented on the transport properties of electrons and holes in single‐carrier devices based on C70/pentacene OHJ by current density–voltage (J–V) measurements at various temperatures. It can be seen that Fowler–Nordheim (F–N) tunnel model can be used to well demonstrate the J–V properties, indicating that the charge injection based on OHJ is a tunnel process. The impedance measurements found that the lower lowest unoccupied molecular orbit energy level and higher electron mobility properties are very necessary for the used electron‐transport materials to reduce the space charge region width, thus realizing highly efficient electron injection.

Model of diffusion-drift charge carrier transport in layers with a fractal structure

A new theoretical (semiempirical) model of diffusion-drift charge carrier transport in layers with a fractal structure based on a partial differential equation with a fractional time derivative has been proposed. It has been shown by numerical calculations that a decrease in the order of the fractional derivative in the presence of the external electric field leads to broadening and asymmetry of the spatial distributions of charge carriers, which physically corresponds to intensification of scattering processes.

Efficiency enhancement of fluorescent blue organic light-emitting diodes using doped hole transport and emissive layers

The electroluminescent characteristics of blue organic light-emitting diodes (BOLEDs) fabricated with doped charge carrier transport layers are analyzed. The fluorescent blue dopant BCzVBi is doped in an emissive layer, hole transport layer (HTL) and electron transport layer (ETL), respectively, to optimize the probability of exciton generation in the BOLEDs. The luminance and luminous efficiency of BOLEDs made with BCzVBi-doped HTL and ETL increase by 22% and 17% from 11,683 cd/m2 at 8.5 V and 6.08 cd/A at 4.0 V to 14,264 cd/m2 at 8.5 V and 7.13 cd/A at 4.0 V while CIE coordinates of (0.15, 0.15) of both types of BOLEDs remained unchanged. The electron mobility of BCzVBi is estimated to be 1.02×10−5 cm2/Vs by TOF.

High Efficiency Blue Organic Light Emitting Devices doped by BCzVBi in Hole and Electron Transport Layer

ABSTRACTThe electroluminescent characteristics of blue organic light-emitting diodes(BOLEDs) were fabricated with single emitting layer using host-dopant system and doped charge carrier transport layers. The structure of the high efficiency BOLED device was; NPB(600Å)/NPB:BCzVBi-7%(100Å)/ADN:BCzVBi-7%(300Å)/BAlq:BCzVBi-7%(100Å)/BAlq(200Å)/Liq(20Å)/Al(1200Å) to optimize probability of exciton generation by doping BCzVBi in emitting layer and hole/electron transport layers(HTL/ETL) as well. Luminance and luminous efficiency of BOLED doped BCzVBi in EML and HTL/ETL improved from 10090 cd/m2 at 9.5V and 6.44 cd/A at 4.0V to 13190 cd/m2 at 9.5V and 7.64 cd/A at 4.0V about 30% and 18%, respectively, with CIE coordinates of (0.14, 0.17) comparing to BOLED doped BCzVBi in EML only

Organic solar cells incorporating buffer layers from indium doped zinc oxide nanoparticles

Monodisperse, indium doped zinc oxide (IZO) nanoparticles were prepared via the polyol-mediated synthesis and incorporated into regular and inverted poly-(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C 61-butyric acid methyl ester organic photovoltaic devices as buffer layers between the active layer and the cathode. Efficient hole blocking at the particle buffer layers leads to an enhanced open-circuit voltage of the solar cells. This effect is even more pronounced for inverted device architectures. Device degradation studies revealed a solar cell performance reduction upon sample exposition to ambient atmosphere. However, this degradation is fully reversible under UV illumination. In addition, the n-doped IZO particles form suitable charge carrier transport layers for an efficient recombination in an intermediate recombination zone in tandem solar cells. Accordingly we have fabricated fully solution-processed tandem solar cells and investigated their optoelectronic properties.

Charge carrier transport in layers of discotic liquid crystals as studied by transient photocurrents

Transient photocurrent measurements were performed for a series of layers of discotic liquid crystals and of perylene derivative prepared in different ways with an aim to characterize the charge carrier transport in configurations corresponding to photovoltaic devices and field effect transistors. The course of the transient photocurrent was found to be strongly dependent on the layer preparation procedure: the non-dispersive transport, and well defined transit times, were found only for “squeezed” sandwich-type sample of triphenylene derivative in which homeotropic orientation of the discotic columns was induced assuring formation of continuous paths connecting the opposite electrodes. In all other cases dispersive transport of charge carriers was observed, due to the spatial and/or energy disorder of hopping states and fast trapping by localized states. From the asymmetry of the transient photocurrents for different polarity of the illuminated electrode it was concluded, that holes are more mobile in the hexabenzocoronene derivatives and in the triphenylene derivative, while electron transport dominates in the perylene derivative.

29.1: Full Color Active Matrix OLED Displays with High Aperture Ratio

AbstractFor the integration of organic light emitting diodes (OLEDs) on an active matrix backplane, an efficient top‐emitting OLED is essential since the TFT‐circuitry covers a large space of the pixel aperture. Moreover, for the integration on n‐channel transistors used in amorphous silicon technology an inverted (anode on top) OLED setup is necessary.We here present a way to manufacture efficient top emitting inverted OLEDs on active matrix substrates by applying our proprietary technology of intentionally doped charge carrier transport layers. We demonstrate their integration in full color active matrix displays with QVGA and VGA resolution.