PVK Films Research Articles

Gas Molecule Assisted All-Inorganic Dual-Interface Passivation Strategy for High-Performance Perovskite Solar Cells.

The trap states at both the upper and bottom interfaces of perovskite layers significantly impact non-radiative carrier recombination. The widely used solvent-based passivation methods result in the disordered distribution of surface components, posing challenges for the commercial application of large-area perovskite solar cells (PSCs). To address this issue, a novel NH3 gas-assisted all-inorganic dual-interfaces passivation strategy is proposed. Through the gas treatment of the perovskite surface, NH3 molecules significantly enhanced the iodine vacancy formation energy (1.54eV) and bonded with uncoordinated Pb2+ to achieve non-destructive passivation. Meanwhile, the reduction of the film defect states is accompanied by a decrease in the work function, which promotes carrier transport between the interface. Further, a stable passivation layer is constructed to manage the bottom interfacial defects using inorganic potassium tripolyphosphate (PT), whose ─P═O group effectively mitigated the charged defects and lowered the carrier transport barriers and nucleation barriers of PVK, while the gradient distribution of K+ improved the crystalline quality of PVK film. Based on the dual-interface synergistic effect, the optimal MA-contained PSCs with an effective area of 0.1 cm2 achieved an efficiency of 24.51% and can maintain 90% of the initial value after aging (10-20% RH and 20°C) for 2000h.

Chelating Dual Interface for Efficient and Stable Crystal Growth and Iodine Defect Management in Sn-Pb Perovskite Solar Cells.

Suppressing Sn2+ oxidation and rationally controlling the crystallization process of tin-lead perovskite (Sn-Pb PVK) films by suitable bonding methods have emerged as key approaches to achieving efficient and stable Sn-Pb perovskite solar cells (PSCs). Herein, the chelating coordination is performed at the top and bottom interfaces of Sn-Pb PVK films. The chelation strength is stronger toward Sn2+ than Pb2+ by introducing oligomeric proanthocyanidins (OPC) at the bottom interface. This difference in chelation strength resulted in a spontaneous gradient distribution of Sn/Pb within the perovskite layer during crystallization, particularly enhancing the enrichment of Sn2+ at the bottom interface and facilitating the extraction and separation of photogenerated charge carriers in PSCs. Simultaneously, this top-down distribution of gradually increasing Sn content slowed down the crystallization rate of Sn-Pb PVK films, forming higher-quality films. On the top interface of the PVK, trifluoroacetamidine (TFA) was used to inhibit the generation of iodine vacancies (VI) through chelating with surface-uncoordinated Pb2+/Sn2+, further passivating defects while suppressing the oxidation of Sn2+. Ultimately, the PSCs with simultaneous chelation at both top and bottom interfaces achieved a power conversion efficiency (PCE) of 23.31% and an open-circuit voltage (VOC) exceeding 0.90 V. The stability of unencapsulated target devices in different environments also improved.

Buried interface modification toward efficient perovskite solar cell based on PVK hole transport layer

Poly (9-vinylcarbazole) (PVK) is an attractive hole transport material for organometallic-halide perovskite solar cell (PSC) because of its excellent hole transport ability and great film-forming property. Nowadays, the PVK hole-transport-layer (HTL) based PSC suffered from low power conversion efficiency (PCE), which mainly caused by bad interface contact between PVK and perovskite film, including the energy level mismatch. Doing PVK is an effective and facial method to modify the buried interface of perovskite film. In this work, 1,1-bis [(di-4-tolylamino)phenyl]cyclohexane (TAPC) was added into PVK HTL. Experimental results show that a highest occupied molecular orbital (HOMO) level elevation was achieved after doping TAPC. Meanwhile perovskite films with increased crystallinity, reduced defect density and enhanced light absorbance were obtained. Finally, the PCE of PSCs were enhanced from 10.81% of the control device to 15.65% of PVK + 40% TAPC device. The reason that responsible for the energy level lifting, morphology improvement and performance enhancement were thoroughly investigated and analyzed. This work provides an alternative option for the dopant in PVK to improve the buried interface of perovskite film, and therefore, the performance of PSCs.

Organic LEDs Based on Bis(8-hydroxyquinoline) Zinc Derivatives with a Styryl Group.

For the first time, organic light-emitting diodes (OLEDs) based on bis(8-hydroxyquinoline) zinc with a styryl group (ZnStq) dispersed in poly(N-vinylcarbazole) matrix (ZnStq_R:PVK, where R = H, Cl, OCH3) were fabricated. The ZnStq_R:PVK films made via the spin-coating method were used as the active layer in these devices. The produced OLEDs showed strong electroluminescence with yellow emissions at 590, 587 and 578 nm for the ZnStq_H:PVK, ZnStq_Cl:PVK and ZnStq_OCH3:PVK, respectively. For all the studied thin films, the main photoluminescence emission bands were observed between 565 and 571 nm. The OLED with the ZnStq_OCH3:PVK layer with a narrow electroluminescence spectrum was found to have sufficient color purity to produce ultra-high-resolution displays with reduced power consumption (full width at half maximum of 59 nm, maximum brightness of 2244 cd/m2 and maximum current efficiency of 1.24 cd/A, with a turn-on voltage of 6.94 V and a threshold voltage of 7.35 V). To characterize the photophysical properties of the active layer, the ZnStq_R:PVK layers samples were additionally deposited on glass and silicon substrates. We found that the obtained results predestine ZnStq_R:PVK layers for use in the lighting industry in the future.

Counter electrode corrosion mechanism in Sn-contained perovskite solar cells

In the recently researched Sn-contained perovskite solar cells (PSCs), limited work is focused on corrosion of a metal electrode, which is recognized as the obstacle of efficient and stable devices. Here, it is found that a pure Sn based PSC shows the most serious and quick corrosion of an Ag electrode, compared with Pb and Sn–Pb PSCs. I2, SnOX, and AgI are the main products of the aged Ag electrode in Sn based PSCs, which greatly differed from Pb and Sn–Pb PSCs. Oxidation of FASnI3 under O2/H2O produces aggressive I2, together with gaseous FA and diffusive I−, degrading the PVK film and Ag electrode greatly. This work highlights the corrosion velocity and mechanism of Ag electrodes in Sn-contained PSCs, which may help to find out possible solutions to circumvent the instable issue.

Improved properties of high molar mass poly(9-vinylcarbazole) and performance as a light emitter compared with the commercial PVK

Poly(9-vinylcarbazole), PVK, has been the object of study due to its versatility in polymerization methods. It presents high thermal and photochemical stability and high charge mobility. In the present work, different bimodal high molar mass PVKs were synthesized by cationic polymerization with high yields, above 80 %, and the polymer properties were investigated. The obtained PVKs presented degradation temperatures above 460 °C. The syndiotacticity was around 60 %, and the polymers were amorphous. The glass transition temperature achieved 238 °C. The samples revealed a bandgap value of around 3.49 eV, lower than that reported in the literature. The AFM analysis shows that all the PVK films are smooth, with good substrate coverage and low roughness values. The polymers were evaluated as emitting materials in OLEDs assembled using PVK as a single electroluminescent layer, and the performance was compared to an OLED containing a commercially available PVK. The photoluminescence spectrum presented only a broad emission peak near the visible region at 360-a480 nm. The turn-on voltage (Von) of all synthesized polymers was lower, while luminance (Lmax) and current efficiency values (ηcurr) were higher than those obtained using the commercial PVK.

Influence of charge transport layer on the crystallinity and charge extraction of pure tin-based halide perovskite film

Tin-based halide PVK films grown on organic CTLs have better crystallization than inorganic ones and photogenerated holes seem easier to be extracted than electrons from the film.

Aggregation-Induced Emission Luminogens Sensitized Quasi-2D Hybrid Perovskites with Unique Photoluminescence and High Stability for Fabricating White Light-Emitting Diodes.

In order to endow quasi‐2D organic‐inorganic hybrid metal halide perovskites (quasi‐2D‐PVK) with superior performance, an aromatic organic ligand with aggregation‐induced emission (AIE) features is rationally designed and utilized for constructing distinctive quasi‐2D‐PVK materials. This AIE‐active ligand, TTPy‐NH2, well fits into the lattices of quasi‐2D‐PVK and leaves hydrophobic tails surrounding PVK layers, making the presented TTPy‐NH2/PVK film extraordinary in terms of both luminescence and stability. Benefiting from the prominent sensitization function and AIE tendency of TTPy‐NH2, the presented TTPy‐NH2/PVK film exhibits a high quantum yield of 62.2%, unique blue‐red dual‐emission property of both blue and red, high stability with the remnant of more than 94% fluorescence intensity remnant after 21 days. As a result, TTPy‐NH2/PVK film is capable of constituting high‐performance white light‐emitting diodes, with its color gamut reaching 138% of the National Television System Committee （NTSC） standard and the maximum efficiency is 105 lm W−1 at 20 mA. Evidently, a win‐win effect is achieved by the integration of AIE‐active ligands and quasi‐2D‐PVK, which are two of the most reputable solid‐state luminogens. This developed protocol thus opens up a new avenue for exploring the next generation of luminescent devices.

Enhancing the electroluminescence efficiency by controlling the migration of excited states to quenching sites in a truxene-based oligomer

Control of unwanted low-energy emission originating due to trap states is crucial for improving the emission efficiency and for maintaining the color purity in blue light-emitting diodes. Here, a truxene-based emitter (T-pyrene) that is a good candidate for the blue emitter is investigated using photophysical studies. Steady-state absorption of the emitter reveals the presence of a charge-transfer state, and the photoluminescence spectrum suggests an occurrence of a strong interchain/exciplex state. Time-resolved photoluminescence (TRPL) and fluence-dependent measurements of the material in the dilute solution show the presence of multiple excitonic processes contributing to the delayed fluorescence. Similar studies on the pristine film suggest that the delayed fluorescence is dominated by the presence of the traps. By blending the oligomer with a higher bandgap polymer matrix, poly(9-vinyl carbazole) (PVK), a type I heterojunction at the T-pyrene:PVK interface is obtained that confines the generated excitons in T-pyrene. With this architecture, device efficiencies with T-pyrene (20 wt. %):PVK show significantly enhanced efficiency than those of the pristine device. We elucidate the relevance of the matrix:T-pyrene system using TRPL and fluence-dependent studies on T-pyrene (20 wt. %):PVK. Time-resolved emission spectra on the T-pyrene (20 wt. %):PVK films do not show red-shifted emission with time delays, which suggests the passivation of the quenching sites due to the dispersion of T-pyrene in PVK to prevent the intermolecular interactions.

Opto-electrically and electro-optically controllable diaphragm aperture in a poly(N-vinylcarbazole) film-coated tandem-90°-twisted nematic liquid crystal cell

The opto-electrically and electro-optically controllable diaphragm aperture in a poly(N-vinylcarbazole) (PVK) film-coated tandem-90°-twisted nematic liquid crystal (TNLC) cell is investigated. The tandem-90°-TNLC cell assembled by two common 45°-TNLC cells can be adopted for incident linearly polarized lights at any linear polarization direction. The variations of diaphragm aperture made by a PVK film-coated tandem-90°-TNLC cell are demonstrated opto-electrically and electro-optically. The differences in the degree of linear polarization between the tandem-90°-TNLC and single-90°-TNLC cells for demonstrating diaphragm apertures are investigated. The dynamic transmittance, related to orientation of LCs resulting from the built-in electric fields induced by the diffused positive and negative ions in LC bulk, the induced electric fields generated by the diffused ions in LC bulk and conductive PVK films, and the ion-moving induced force, of the PVK film-coated tandem-90°-TNLC cell is studied, and the possible mechanism based on dynamic distribution of ions (impurities) in LC bulk is also proposed. The proposed mechanism of the dynamic transmittance phenomenon can be a useful reference to the developments of other PVK film-coated LC cells. Moreover, the PVK film-coated tandem-90°-TNLC cell can be considered a controllable diaphragm aperture shutter if the above ion-induced issues are overcome.

Measuring Elastic and Plastic Properties of PVK and CBP Thin Films using Triangular Pyramid Indenter

Measuring Elastic and Plastic Properties of PVK and CBP Thin Films using Triangular Pyramid Indenter

The application of a high boiling point dissolution solvent on a poly(N-vinylcarbazole) host toward improving the performance of blue electrophosphorescent devices via a solution process

ODCB triggers the formation of a p-PVK conformation, a low content PVK electromer, enhancing the performance of blue phosphorescent s-PhOLEDs.

Mechanisms of charge transport and resistive switching in composite films of semiconducting polymers with nanoparticles of graphene and graphene oxide

We have investigated the effect of the resistive switching in the composite films based on polyfunctional polymers – PVK, PFD and PVC mixed with particles of Gr and GO with the concentration of ∼ 1 - 3 wt.%. We have developed the solution processed hybrid memory structures based on PVK and GO particles composite films. The effect of the resistive switching in Al/PVK(PFD; PVC):Gr(GO)/ITO/PET structures manifests itself as a sharp change of the electrical resistance from a low-conducting state to a relatively high-conducting state when applying a bias to Al–ITO electrodes of ∼ 0.2–0.4 V. It has been established that a sharp conductivity jump characterized by S-shaped current-voltage curves and the presence of their hysteresis occurs upon applying a voltage pulse to the Au/PVK(PFD; PVC):Gr(GO)/ITO/PET structures, with the switching time in the range from 1 to 30 μs. The mechanism of resistive switching associated with the processes of capture and accumulation of charge carriers by Gr(GO) particles introduced into the matrixes of the PVK polymer due to the reduction/oxidation processes. The possible mechanisms of energy transfer between organic and inorganic components in PVK(PFD; PVC):GO(Gr) films causes increase mobility are discussed. Incorporating of Gr (GO) particles into the polymer matrix is a promising route to enhance the performance of hybrid memory structures, as well as it is an effective medium for memory cells.

Effect of pre-drying treatments on solution-coated organic thin films for active-matrix organic light-emitting diodes

Due to capillary rise, organic thin films fabricated by solution coating exhibit the concave thickness profile. It is found that the thickness and emission uniformities within pixels vary depending sensitively on the pre-drying treatment that has been done before hard bake. We investigate its effect on the film quality by varying the temperature, time, pressure, fluid flow-related solute concentration, and evaporation-related solvent. To this end, we carry out spin coatings of a non-aqueous poly(N-vinylcarbazole) (PVK) for a hole transporting blanket layer. With a low-boiling-point (BP) organic solvent, the pre-drying makes no significant impact on the thickness profiles. With a high-BP organic solvent, the PVK films pre-dried in a vacuum for a sufficient time exhibit very uniform light emission in the central region, but non-emission phenomenon near the perimeter of pixels. It is addressed that such a non-emission phenomenon can be suppressed to some extent by decreasing the vacuum pressure. However, the rapid evaporation by heat conduction during the pre-drying degrades the thickness uniformity due to a rapid microflow of solute from the edge to the center. No further enhancement in the thickness uniformity is obtained by varying the solute concentration and using a mixture of low- and high-BP solvents.

Stimulated emission and optical properties of pyranyliden fragment containing compounds in PVK matrix

Organic solid state lasers are thoughtfully investigated due to their potential applications in communication, sensors, biomedicine, etc. Low amplified spontaneous emission (ASE) excitation threshold value is essential for further use of the material in devices. Intramolecular interaction limits high molecule density load in the matrix. It is the case of the well-known red light emitting laser dye - 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM). The lowest ASE threshold value of the mentioned laser dye could be obtained within the concentration range between 2 and 4wt%. At higher concentration threshold energy drastically increases. In this work optical and ASE properties of three original DCM derivatives in poly(N-vinylcarbazole) (PVK) at various concentrations will be discussed. One of the derivatives is modified DCM dye in which the methyl substituents in the electron donor part have been replaced with bulky trityloxyethyl groups (DWK-1). These sterically significant functional groups do not influence electron transitions in the dye but prevent aggregation of the molecules. The chemical structure of the second investigated compound is similar to DWK-1 where the methyl group is replaced with the tert-butyl substituent (DWK-1TB). The third derivative (DWK-2) consists of two N,N-di(trityloxyethyl)amino electron donor groups. All results were compared with DCM:PVK system. Photoluminescence quantum yield (PLQY) is up to ten times larger for DWK-1TB with respect to DCM systems. Bulky trityloxyethyl groups prevent aggregation of the molecules thus decreasing interaction between dyes and amount of non-radiative decays. The red shift of the photoluminescence and amplified spontaneous emission at higher concentrations were observed due to the solid state solvation effect. The increase of the investigated dye density in the matrix with a smaller reduction in PLQY resulted in low ASE threshold energy. The lowest threshold value was obtained around 21μJ/cm2 (2.1kW/cm2) in DWK-1TB:PVK films.

Poly(9-vinylcarbazole) as a hole transport material for efficient and stable inverted planar heterojunction perovskite solar cells

Conjugated polymer poly(9-vinylcarbazole) (PVK) has been explored as an efficient hole transport material (HTM) to replace poly(3,4-ethylenedio-xythiophene): poly(styrenesulfonate) (PEDOT:PSS) for the preparation of p-i-n type, inverted planar heterojunction perovskite solar cells. The PVK film can be facilely deposited by room-temperature solution-casting without any doping or post treatments. Using MAPbI3 as the photoactive layer, a 15.8% stabilized power conversion efficiency was achieved in ITO/PVK/MAPbI3/PCBM/Ag devices, which is a significant improvement compared with 12.1% from devices using PEDOT:PSS as the HTM. The perovskite photoactive layer cast on PVK HTM has higher crystallinity and less PbI2 residues, leading to a higher charge recombination resistance of the device. After a storage of 1000h, the PCE of PVK- and PEDOT:PSS- based devices dropped to 82.5% and 56% respectively, and demonstrated that the PVK-based perovskite devices is more stable compared with the PEDOT:PSS-based devices.

Effect of mixed hole transporting host on the mobility, Gaussian density of states and efficiencies of a heterojunction phosphorescent organic light emitting diode

We present an in-depth study of the hole transport in poly(vinylcarbazole) PVK films blended with small molecule tris(4-carbazoyl-9-ylphenyl)amine (TcTa). Doping TcTa in PVK introduces shallow hole traps when the doping concentration is lower than 20 wt%. It becomes percolative at higher concentrations. The energetic disorder σ of the blended system reduces from ~72 meV at 0 wt% TcTa to ~41 meV at 50 wt% TcTa. A correlation between σ and the film morphologies suggests that the blending of TcTa molecules in the film does not only change the film homogeneity and roughness but also the energetic disorder. In addition to the mobility study, we fabricated a red phosphorescent organic light emitting diode with the same blending system. By doping merely 5 wt% of TcTa into PVK as mixed hole-transporting hosts, the efficiency of the deep red heterojunction phosphorescent organic light emitting diode increased from 2 cd A−1 to 4 cd A−1, suggesting that TcTa molecules assist in hole injection.

The influence of thickness on memory characteristic based on nonvolatile tuning behavior in poly(N-vinylcarbazole) films

The memory characteristic based on nonvolatile tuning behavior in indium tin oxide/poly(N-vinylcarbazole)/aluminum (ITO/PVK/Al) was investigated, the different memory behaviors were first observed in PVK film as the film thickness changing. By control of PVK film thickness with different spinning speeds, the nonvolatile behavior of ITO/PVK/Al sandwich structure can be tuned in a controlled manner. Obviously different nonvolatile behaviors, such as (i) flash memory behavior and (ii) write-once-read-many times (WORM) memory behavior are from the current–voltage (I–V) characteristics of the PVK films. The results suggest that the film thickness plays a key part in determining the memory type of the PVK.

Paper No P03: Optical Switching of Scattering Mode Liquid Crystal Light Shutters

AbstractWe report that the isothermal phase transition induced by photoisomerization of azobenzene liquid crystals (azo‐LCs) from trans‐ to cis‐isomers results in the dissolution of poly(N‐vinylcarbazole) (PVK) into azo‐LCs. All‐optically controllable and highly efficient scattering mode light modulators based on PVK films and its re‐form mechanism were demonstrated.

Broadband cholesteric liquid crystal devices with poly(N-vinyl carbazole) microstructures

The particular thermally induced phase separation (TIPS) method was developed by using poly(N-vinyl carbazole) (PVK) dissolved in liquid crystal (LC) layer recently. Permanent scattering is produced via the particular TIPS due to the formation of poly-domain LCs. In this work, we applied the particular TIPS to a conventional cholesteric LC (CLC) device. A broadband CLC device with a poly-domain structure was demonstrated by using the particular TIPS of CLC and PVK. The CLC device fabricated by two indium tin oxide glass substrates coated with PVK films generated the rough PVK layers and microstructures when the device was cooled from a temperature higher than the clear point of CLCs. The produced poly-domain planar texture of CLCs still preserved the bistability. A high cooling rate and PVK concentration both increased the reflection bandwidth of the CLC devices.