Muddasir Hanif

Tight intermolecular packing through supramolecular interactions in crystals of cyano substituted oligo(para-phenylene vinylene): a key factor for aggregation-induced emission

Strong supramolecular interactions, which induced tight packing and rigid molecules in crystals of cyano substituent oligo(para-phenylene vinylene) (CN-DPDSB), are the key factor for the high luminescence efficiency of its crystals; opposite to its isolated molecules in solution which have very low luminescence efficiency.

The Origin of the Improved Efficiency and Stability of Triphenylamine-Substituted Anthracene Derivatives for OLEDs: A Theoretical Investigation†

Herein, we describe the molecular electronic structure, optical, and charge-transport properties of anthracene derivatives computationally using density functional theory to understand the factors responsible for the improved efficiency and stability of organic light-emitting diodes (OLEDs) with triphenylamine (TPA)-substituted anthracene derivatives. The high performance of OLEDs with TPA-substituted anthracene is revealed to derive from three original features in comparison with aryl-substituted anthracene derivatives: 1) the HOMO and LUMO are localized separately on TPA and anthracene moieties, respectively, which leads to better stability of the OLEDs due to the more stable cation of TPA under a hole majority-carrier environment; 2) the more balanceable hole and electron transport together with the easier hole injection leads to a larger rate of hole-electron recombination, which corresponds to the higher electroluminescence efficiency; and 3) the increasing reorganization energy for both hole and electron transport and the higher HOMO energy level provide a stable potential well for hole trapping, and then trapped holes induce a built-in electric field to prompt the balance of charge-carrier injection.

The Counter Anionic Size Effects on Electrochemical, Morphological, and Luminescence Properties of Electrochemically Deposited Luminescent Films

Luminescent films electrodeposited (ED) on indium tin oxide glass are considered for application in low-cost luminescent devices. During ED processes, the anions play an important role in the quality of resultant ED films. Three species of supporting electrolytes TBA-X (TBA = n-tetrabutylammonium, X = BF - 4 , PF - 6 , and AsF - 6 ) with different size of anions were used for the preparation of ED films from a branched carbazyl luminescent precursor TCPC. Increasing the size of anions (BF - 4 < PF - 6 < AsF - 6 ) resulted in an increase of growth rate and improved the morphology of ED films. The ED film prepared using TBA-AsF 6 as supporting electrolyte showed strong fluorescence with efficiency of 65%, which was significantly higher than that of ED films from the smaller anions BF - 4 and PF - 6 . By utilizing the ED films as emitting layer, the light-emitting device also showed improved performance with increasing size of anions in the electrolytes. The origin of observed counter anionic size effects on electrochemical, morphological, and luminescent properties of ED films can be attributed to the difference in interaction between the carbazyl cation and anions, which induced an easier (i.e., for the larger size ions) or difficult (i.e., for the smaller size ions) dedoping process.

Electrochemical deposition of patterning and highly luminescent organic films for light emitting diodes

This paper describes a simple electrochemical deposition (ED) technique to prepare luminescent and patterned films for light-emitting devices (LEDs). The luminescent films are deposited directly on the patterned ITO (indium tin oxide) electrodes through an oxidation coupling reaction of an electroactive and luminescent precursor. The ED films deposited on the ITO strips (width of 200 µm) exhibit smooth surface morphology (roughness of morphology surface of 3.1 nm), small roughness in electrode edge of 1–2 µm and high fluorescence quantum efficiency (>60%). The LEDs with structure ITO/ED film/Ba/Al show pure blue emission (CIE coordinates of (0.16, 0.07)), high brightness of 3080 cd m −2 and the maximum external quantum efficiency of 0.60%. (Some figures in this article are in colour only in the electronic version)

Tunable electroluminescent color for 2, 5-diphenyl -1, 4-distyrylbenzene with two trans-double bonds

Exciplex emission is observed in electroluminescent (EL) spectrum of an organic light-emitting device (OLED), where 2, 5-diphenyl -1, 4-distyrylbenzene with two trans-double bonds (trans-DPDSB), (8-hydroxyquinoline) aluminum, and N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) are used as light-emitting, electron-transporting, and hole-transporting layers, respectively. This emission can be dramatically weakened by inserting a hole-injecting layer of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) between the hole-transporting layer and the anode. Consequently, EL color of this OLED is tuned from white to blue. This phenomenon may result from the improvement of hole injection, which shifts the major recombination zone from the NPB/trans-DPDSB interface to the trans-DPDSB layer.

Effect of cyano-substitution in distyrylbenzene derivatives on their fluorescence and electroluminescence properties

Efficient organic electroluminescent materials with both high solid-state fluorescence efficiency and high excitons usage efficiency for use in organic light-emitting diodes (OLEDs) are relatively rare. We report two isomeric-phenylethylene compounds with different positions and spatial orientation of the –CN substituent on the vinylene groups, namely α-CN-APV and β-CN-APV. The synthesis, characterization, crystal structure, optical, electrochemical, thermal, and electroluminescence (EL) properties of the two compounds are discussed in detail. The crystal structure of the β-CN-APV shows tight solid-state organization because of two, oppositely pointed, vertically aligned, hydrogen bonds between the two cyano groups and the two vinylene hydrogens (CN⋯H–CC), while the α-CN-APV showed more flexible molecular structure with absence of vinylene hydrogen intermolecular interactions in the crystal. Such tight intermolecular stacking of β-CN-APV ensures the high solid-state fluorescence quantum efficiency. Both the compounds exhibit hybrid local and charge transfer (HLCT) excited states, which facilitate the population of singlet excitons through the reverse intersystem crossing (RISC) process from the high lying triplet states. Experimental and theoretical investigation indicated that the β-CN-APV, when compared to the α-CN-APV, showed higher solid-state fluorescence quantum efficiency and also higher excitons usage efficiency, which eventually provided much higher external quantum efficiency and brightness in the corresponding EL devices.

Perylene bisimide as the cathode modifier in organic photovoltaics: the role of aggregation morphology on the interlayer performance

Nanorods and nanoparticles of perylene bisimides (PBI-1) were prepared and applied as cathode interlayer in organic photovoltaic devices. The device performance showed an important relationship with the morphology of the interlayer.

An ultra-thin gold nanoparticle layer modified cathode for the enhanced performance of polymer light emitting diodes

The enhanced performance of polymer light emitting diodes has been achieved by embedding an ultra-thin gold nanoparticle layer on the cathode. The performance of devices could be optimized simply by adjusting the thickness of gold, which exhibited the improved brightness from 17k cd m−2 to 20k cd m−2 and the enhanced luminous efficiency from 15.4 cd A−1 to 18.3 cd A−1, when a greenish emissive polymer P-PPV was applied as an emissive layer. The experimental results show that it is mainly attributed to the decreased holes and the increased electrons, resulting in the balanced electron–hole recombination and the shifted light emitting profile to the anode. The results demonstrate that gold nanoparticles constitute a feasible and effective route for achieving high-performance polymer optoelectronic devices through their electric properties.

Construction of Layered Structure of Anion–Cations To Tune the Work Function of Aluminum-Doped Zinc Oxide for Inverted Polymer Solar Cells

Suitable work function (WF) of the cathode in polymer solar cells (PSCs) is of essential importance for the efficient electron extraction and collection to boost the power conversion efficiency. Herein, we report a facile and efficient method to tune the surface WF of aluminum-doped zinc oxide (AZO) through building of a definite interfacial dipole, which is realized by the construction of a layered structure of positive and negative ionized species. A cross-linked perylene bisimide (poly-PBI) thin film is deposited onto the AZO surface first, and then it is reduced to the radical anion state (poly-PBI•-) in an electrochemical cell, using tetraoctylammonium (TOA+), a bulky cation, as a counter ion. Owing to the huge volume of TOA+, it is absorbed on the surface of the cross-linked PBI•- thin film through Coulomb force, and thus a definite interface dipole is formed between the two ionized layers. Because of the definite interface dipole, the surface WF of the electrode modified with ionized layers is decreased dramatically to 3.9 eV, which is much lower than that of the electrode modified with the neutral PBI layer (4.5 eV). By using this novel cathode interlayer with a definite interface dipole in PSCs, a significantly increased open-circuit voltage ( VOC) is obtained. The results indicate that it is a facile and unique method by the construction of a definite interface dipole to tune the surface WF of the electrode for the application in organic electronic devices.

Preparation and electronic characteristics of anionic perylene bisimide films

Neutral perylene bisimides (PBI) are well-known n-type organic semiconductors, with number of challenging electronic properties in their neutral and reduced states. We report the characteristic electronic properties of PBI anionic films. We unexpectedly discovered that pristine PBI dianion film showed p-type character, while oxidized dianion film (dominant neutral state with few radical anions) showed normal n-type semiconductor character based on Seebeck effect measurements. Both kinds of films exhibit high electrical conductivity with a potential for thermoelectric applications. The mechanism of polarity reversal is proposed.