Imran Murtaza

A Unique Blend of 2-Fluorenyl-2-anthracene and 2-Anthryl-2-anthracence Showing White Emission and High Charge Mobility

White-light-emitting materials with high mobility are necessary for organic white-light-emitting transistors, which can be used for self-driven OLED displays or OLED lighting. In this study, we combined two materials with similar structures-2-fluorenyl-2-anthracene (FlAnt) with blue emission and 2-anthryl-2-anthracence (2A) with greenish-yellow emission-to fabricate OLED devices, which showed unusual solid-state white-light emission with the CIE coordinates (0.33, 0.34) at 10 V. The similar crystal structures ensured that the OTFTs based on mixed FlAnt and 2A showed high mobility of 1.56 cm2  V-1  s-1 . This simple method provides new insight into the design of high-performance white-emitting transistor materials and structures.

Highly Simplified Tandem Organic Light-Emitting Devices Incorporating a Green Phosphorescence Ultrathin Emitter within a Novel Interface Exciplex for High Efficiency

Herein we report a novel design philosophy of tandem OLEDs incorporating a doping-free green phosphorescent bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)iridium(III) (Ir(ppy)2(acac)) as an ultrathin emissive layer (UEML) into a novel interface-exciplex-forming structure of 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TmPyPB). Particularly, relatively low working voltage and remarkable efficiency are achieved and the designed tandem OLEDs exhibit a peak current efficiency of 135.74 cd/A (EQE = 36.85%) which is two times higher than 66.2 cd/A (EQE = 17.97%) of the device with a single emitter unit. This might be one of the highest efficiencies of OLEDs applying ultrathin emitters without light extraction. Moreover, with the proposed structure, the color gamut of the displays can be effectively increased from 76% to 82% NTSC if the same red and blue emissions as those in the NTSC are applied. A novel form of harmonious fusion among interface exciplex, UEML, and tandem structure is successfully realized, which sheds light on further development of ideal OLED structure with high efficiency, simplified fabrication, low power consumption, low cost, and improved color gamut, simultaneously.

Molecular phase engineering of organic semiconductors based on a [1]benzothieno[3,2-b][1]benzothiophene core

Two environmentally and thermally stable [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives, 2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT) and 2-(4-hexylphenyl)[1]benzothieno[3,2-b][1]benzothiophene (C6-Ph-BTBT), are prepared by Suzuki coupling. Organic thin-film transistors with a top-contact and bottom-gate based on BTBT, Ph-BTBT and C6-Ph-BTBT are fabricated by vacuum-deposition on octyltrichlorosilane treated Si/SiO2 substrates. Experimental results show that the thin-film based on BTBT sublimes instantly after the deposition of electrodes, and no semiconductor signal is detected. Ph-BTBT shows a mobility of 0.034 cm2 V−1 s−1. Furthermore, C6-Ph-BTBT exhibits three liquid crystal phases (Sm A, Sm E and Sm K or H) and achieves the highest hole mobility of 4.6 cm2 V−1 s−1 with an on/off ratio of 2.2 × 107 for polycrystalline organic thin-film transistors in ambient air. The present study exemplifies that the introduction of mesoscopic order in molecular design provides a general approach for the high electronic performance of organic semiconductors.

Highly transmissive blue electrochromic polymers based on thieno[3,2-b]thiophene

A series of three polymers based on a thieno[3,2-b]thiophene core is synthesized and polymerized via electrochemical polymerization. The addition of benzene and thiophene rings as two different types of substituents on the 3,6-position of the thieno[3,2-b]thiophene core brings about a variance in color changing, optical contrast and morphological demeanor. Electrochromical studies demonstrate that P1 and P2 switch between deep blue neutral and colorless transparent oxidized states, while P3 switches between violet and light green transmissive states. Amid the three polymers, P1 shows the highest optical contrast (71%) in the visible region with complete coloring and bleaching in just 1.10 s and 1.80 s, respectively. In addition, all three polymers reveal about 60% of the transmittance change in the near-IR region, which endows them with commendable applications in NIR electrochromic devices. AFM images depict an augmented surface roughness due to the introduction of alkyl chains in the thieno[3,2-b]thiophene core, which gives rise to a better stability of the polymer thin film.

Unlocking the potential of diketopyrrolopyrrole-based solar cells by a pre-solvent annealing method in all-solution processing

In this work, we report a solution processible diketopyrrolopyrrole anthracene based diblock copolymer synthesized via a rapid direct C–H arylation coupling method. After optimizing the combination of solvent and thermol annealing process, we investigated the characteristics of this di-block low band gap copolymer used as a donor part in bulk hetero-junction solar cells. The influence of the polymer:fullerene ratio (PDPP-ANT:PC61BM) and annealing conditions on the photo active film nanomorphology and device performance has been studied in detail. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) reveal that the resulting film morphology strongly depends on the fullerene ratio before heat treatment. Power conversion efficiencies increase, more than four-fold, to 1.7% for regular architectures with high Voc (0.73 V), Jsc (4.75 mA cm−2) and FF (49.0%), compared with the unannealed device 0.37% (with Voc (0.54 V), Jsc (1.83 mA cm−2), FF (37.8%)). Furthermore, the two broad absorption bands of the PDPP-ANT materials show their potential to be applied in ternary polymer solar cells and tandem polymer solar cells.

Thin film transistors based on two dimensional graphene and graphene/semiconductor heterojunctions

During the past few years, two-dimensional (2D) layered materials have emerged as the most fundamental building blocks of a wide variety of optoelectronic devices. The weak van der Waals (vdW) interlayer forces allow the 2D monolayers to isolate and restack into arbitrary stacking heterojunctions. The recently developed chemical vapor deposition (CVD) technique shows great promise for the production of large domain building blocks of 2D heterostructures with vertical and lateral stacking and much better device performance. This review is the first of its kind to discuss the research progress of flexible FETs based on graphene/semiconductor heterostructures, in which graphene acts as both electrode and semiconductor material.

An efficient and thickness insensitive cathode interface material for high performance inverted perovskite solar cells with 17.27% efficiency

In inverted planar heterojunction (PHJ) perovskite solar cells (PSCs), the interface layer between the electron transporting layer and the cathode plays a very important role in achieving high power efficiency. Herein, we present the synthesis of a new water/alcohol soluble small molecular cationic compound based on the naphthalene diimide functionalized pyridinium salt, N,N′-bis(1-n-hexylpyridinium-4-ylmethyl)-1,4,5,8-naphthalenetetracarboxydiimide (PN6). PN6 exhibits a large band gap (2.95 eV), high electron mobility (7.1 × 10−5 cm2 V−1 s−1), deep LUMO energy level (−4.07 eV) and can easily form highly transparent thin films. By introducing it as a cathode interface layer in PHJ PSCs, PN6 can effectively decrease the work function of the cathode, boosting all the photovoltaic parameters of the fabricated devices. By using PN6 in the ITO/NiOx/perovskite/PC61BM/PN6/Ag device structure, a power conversion efficiency (PCE) of 17.27% is obtained. Moreover, it is observed that the performance of PHJ PSCs with the PN6 interface layer is insensitive to the interlayer thickness and even at a thickness of 30 nm; the PCE of the device can still reach up to 15.37%, which makes it compatible with large-area roll-to-roll processing technology. These results demonstrate that the naphthalene diimide functionalized pyridinium salt can be a new category of water/alcohol soluble cathode interface material for high performance PHJ PSCs.

Highly responsive phototransistors based on 2,6-bis(4-methoxyphenyl)anthracene single crystal

Herein, thin film and single crystal phototransistors based on 2,6-bis(4-methoxyphenyl)anthracene (BOPAnt) are systematically studied. High quality BOPAnt single crystals are grown via the PVT (physical vapor transport) method to fabricate bottom gate top contact phototransistors. The thin film phototransistor shows a photoresponsivity of 9.75 A W−1 under 1 mW cm−2 blue LED illumination, whereas under the same conditions the single crystal based phototransistor displays a photoresponsivity of 414 A W−1. Furthermore, using low power monochromatic light with the wavelength of 350 nm, the photoresponsivity of 3100 A W−1 under 0.11 mW cm−2 illumination and EQE of 9.5 × 105% are obtained for the BOPAnt single crystal phototransistor with the channel length of 65 μm. The much higher photoresponsivity of the single crystal based phototransistor is due to its much higher exciton diffusion length compared to that of the thin film device, which is also evident by the large Von shift towards the positive voltage direction. The photoswitching behaviors of the single crystal based phototransistor are also studied. It is observed that after a short warming up period, the single crystal based phototransistor shows stable switching behavior.

Thermal and Optical Modulation of the Carrier Mobility in OTFTs Based on an Azo-anthracene Liquid Crystal Organic Semiconductor

One of the most striking features of organic semiconductors compared with their corresponding inorganic counterparts is their molecular diversity. The major challenge in organic semiconductor material technology is creating molecular structural motifs to develop multifunctional materials in order to achieve the desired functionalities yet to optimize the specific device performance. Azo-compounds, because of their special photoresponsive property, have attracted extensive interest in photonic and optoelectronic applications; if incorporated wisely in the organic semiconductor groups, they can be innovatively utilized in advanced smart electronic applications, where thermal and photo modulation is applied to tune the electronic properties. On the basis of this aspiration, a novel azo-functionalized liquid crystal semiconductor material, (E)-1-(4-(anthracen-2-yl)phenyl)-2-(4-(decyloxy)phenyl)diazene (APDPD), is designed and synthesized for application in organic thin-film transistors (OTFTs). The UV-vis spectra of APDPD exhibit reversible photoisomerizaton upon photoexcitation, and the thin films of APDPD show a long-range orientational order based on its liquid crystal phase. The performance of OTFTs based on this material as well as the effects of thermal treatment and UV-irradiation on mobility are investigated. The molecular structure, stability of the material, and morphology of the thin films are characterized by thermal gravimetric analysis (TGA), polarizing optical microscopy (POM), (differential scanning calorimetry (DSC), UV-vis spectroscopy, atomic force microscopy (AFM), and scanning tunneling microscopy (STM). This study reveals that our new material has the potential to be applied in optical sensors, memories, logic circuits, and functional switches.

Exploring the electrochromic properties of poly(thieno[3,2-b]thiophene)s decorated with electron-deficient side groups

Two novel thieno[3,2-b]thiophene (TT)/3,4-ethylenedioxythiophene (EDOT)-based compounds of 2,5-(EDOT–TT–EDOT) type bearing electron-withdrawing side groups (4-cyanophenyl or 4-pyridyl) at 3,6-positions of the TT moiety have been synthesized. Their electropolymerization leads to electroactive conjugated polymers, P(CNPh-ETTE) and P(Py-ETTE), which possess electrochromic properties changing the color from purple to pale grey-blue or from sand brown to pale grey-green, respectively. Cyclic voltammetry and spectroelectrochemical experiments reveal that functionalization with electron-withdrawing side groups decreases the HOMO and LUMO energy levels and contracts the band gap of materials. Both new polymers demonstrated extremely short response times of 0.9–1.1 s for bleaching and 0.34–0.35 s for coloring. P(CNPh-ETTE) and P(Py-ETTE) polymers showed reasonably good contrast (16–23%) and coloration efficiency (120–190 cm2 C−1) in the visible region (at the maxima of their π–π* transitions, 540/570 nm), and high contrast and coloration efficiency in the near-infrared region (50–62% and 324–440 cm2 C−1 at 1500 nm, respectively). While the stability of the pyridine-functionalized polymer, P(Py-ETTE), was shown to be low (with unstable charge–discharge characteristics, presumably due to the protonation of the pyridine ring during the redox process), P(CNPh-ETTE) demonstrated superior electrochromic performance retaining 91–96% of its electroactivity after 2000 cycles between −0.5 and +1.0 V. DFT calculations on these and related EDOT–TT–EDOT polymers reported by us early have been presented and analyzed to understand the structure–property relationships in this class of electrochromic polymers.