Ashu K. Bansal

Wearable organic optoelectronic sensors for medicine

This work was supported by Engineering and Physical Sciences Research Council Programme Grant “Challenging the limits of photonics: Structured light” (grant number EP/J01771X/1) and MRC.

Tuning the Emission of Cationic Iridium (III) Complexes Towards the Red Through Methoxy Substitution of the Cyclometalating Ligand

The synthesis, characterization and evaluation in solid-state devices of a series of 8 cationic iridium complexes bearing different numbers of methoxy groups on the cyclometallating ligands are reported. The optoelectronic characterization showed a dramatic red shift in the absorption and the emission and a reduction of the electrochemical gap of the complexes when a methoxy group was introduced para to the Ir-C bond. The addition of a second or third methoxy group did not lead to a significant further red shift in these spectra. Emission maxima over the series ranged from 595 to 730 nm. All complexes possessing a motif with a methoxy group at the 3-position of the cyclometalating ligands showed very short emission lifetimes and poor photoluminescence quantum yields whereas complexes having a methoxy group at the 4-position were slightly blue shifted compared to the unsubstituted parent complexes, resulting from the inductively electron withdrawing nature of this directing group on the Ir-C bond. Light-emitting electrochemical cells were fabricated and evaluated. These deep red emitters generally showed poor performance with electroluminescence mirroring photoluminescence. DFT calculations accurately modelled the observed photophysical and electrochemical behavior of the complexes and point to an emission from a mixed charge transfer state.

Enhancing the photoluminescence quantum yields of blue-emitting cationic iridium(III) complexes bearing bisphosphine ligands

Herein we present a structure–property relationship study of thirteen cationic iridium(III) complexes of the form of [Ir(C^N)2(P^P)]PF6 in both solution and the solid state through systematic evaluation of six bisphosphine (P^P) ligands (xantphos, dpephos, dppe, Dppe, nixantphos and isopropxantphos). All of the complexes are sky-blue emissive, but their photoluminescence quantum yields (ΦPL) are generally low. However, strong and long-lived blue luminescence (λem = 471 nm; ΦPL = 52%; τe = 13.5 μs) can be obtained by combining the reduced bite angle of the 1,2-bis-diphenylphosphinoethene (dppe) chelate with the bulky 2-(4,6-difluorophenyl)-4-mesitylpyridinato (dFmesppy) cyclometalating ligand. To the best of our knowledge this is the highest ΦPL and the longest τe reported for cyclometalated iridium(III) complexes bearing bisphosphine ligands. Light-emitting electrochemical cells (LEECs) were fabricated using lead complexes from this study, however due in part to the irreversible electrochemistry, no functional LEEC was achieved. Organic light-emitting diodes were successfully fabricated but only attained maximum external quantum efficiencies of 0.25%.

Synthesis and characterisation of new diindenodithienothiophene (DITT) based materials

Three new diindenodithienothiophene (DITT) based materials were synthesised and their electrochemical properties investigated. The HOMO–LUMO gaps were observed to be 3.33, 3.48 and 2.81 eV, respectively. Cyclic voltammetry results indicate increased stability for the alkylated derivatives. The dioxide exhibits strong photoluminescence, giving a photoluminescence quantum yield of 0.72 in solution and 0.14 in the solid state. Hole mobility measurements were carried out on the non-alkylated derivative and the corresponding values were ∼10−4 cm2 V−1 s−1.

Mercaptophosphonic acids as efficient linkers in quantum dot sensitized solar cells

Control over the deposition of quantum dots (QDs) on nanostructured semiconductors is very important for the photovoltaic performance of QD sensitized solar cells. The best control is typically achieved using bifunctional molecular linkers, such as mercaptopropionic acid (MPA), to attach the QDs to metal oxides in a specific manner; however some materials, such as ZnO, are not compatible with these molecules due to their pH sensitivity. We have developed new linkers, mercaptophosphonic acids of different length, which allow efficient functionalization of ZnO nanowires and also mesoporous TiO2 without damaging their surface. Detailed XPS and contact angle studies of the mechanism of self-assembly of these acids show that their strong chelation of the oxide surface prevents protonic attack and etching. Using these linkers, we show that colloidal ternary quantum dots, CuInS2, can be conformally and homogeneously deposited on the functionalized metal oxides. Photophysical studies by means of time-resolved photoluminescence spectroscopy confirm efficient electron transfer from the QDs to the metal oxides with the rate and efficiency scaling with respect to the linker length and nature. The efficiency of the QD sensitized solar cells fabricated with such assemblies also strongly depends on the linkers used and follows the trends observed for the charge transfer.

Photophysical and structural characterisation of in situ formed quantum dots

Conjugated polymer-semiconductor quantum dot (QD) composites are attracting increasing attention due to the complementary properties of the two classes of materials. We report a convenient method for in situ formation of QDs, and explore the conditions required for light emission of nanocomposite blends. In particular we explore the properties of nanocomposites of the blue emitting polymer poly[9,9-bis(3,5-di-tert-butylphenyl)-9H-fluorene] together with cadmium sulphide (CdS) and cadmium selenide (CdSe) precursors. We show the formation of emissive quantum dots of CdSe from thermally decomposed precursor. The dots are formed inside the polymer matrix and have a photoluminescence quantum yield of 7.5%. Our results show the importance of appropriate energy level alignment, and are relevant to the application of organic-inorganic systems in optoelectronic devices.

Formation of quantum dots from precursors in polymeric films by ps-laser

Quantum dots (QDs) of semiconductors are promising materials for light emission applications due to their size-tunable optoelectronic properties. We present results of direct quantum dot (QD) formation from precursors inside a polymer matrix using laser irradiation. The method is important because it provides a simple means of patterning nanocomposite material within selected regions of a polymer, as required for device design. Several combinations of polymer/precursors films were treated with a picosecond laser at wavelength of 266 nm in order to verify the formation of the QDs inside the polymeric matrix. Precursors for CdS and CdSe QDs were used in experiments. The structural studies of laser-irradiated samples carried out by means of transmission electron microscopy (TEM) showed the QD formation. The size of QDs and the clusters depended on the laser irradiation dose transferred to the film. The QDs were collected to clusters including 10-60 QDs of different size. The mean size of QDs was less than 10nm. The optical analysis carried out by means of UV-VIS and optical microscopy confirmed the formation of the QDs after laser processing. The time-resolved photoluminescence revealed the energy transfer from the organic host to QDs. However, the charge separation was present due to a certain energy level alignment. Modification of the polymer/precursor blends is still required to prevent imbalance of carrier injection to QDs. Photo-luminescent spectroscopy and fluorescence microscopy have revealed that even if the QDs are not emissive, in certain polymer/QDs combinations the PL emission of the polymer is restored after laser treatment.

An Organic Vortex Laser

Optical vortex beams are at the heart of a number of novel research directions, both as carriers of information and for the investigation of optical activity and chiral molecules. Optical vortex beams are beams of light with a helical wavefront and associated orbital angular momentum. They are typically generated using bulk optics methods or by a passive element such as a forked grating or a metasurface to imprint the required phase distribution onto an incident beam. Since many applications benefit from further miniaturization, a more integrated yet scalable method is highly desirable. Here, we demonstrate the generation of an azimuthally polarized vortex beam directly by an organic semiconductor laser that meets these requirements. The organic vortex laser uses a spiral grating as a feedback element that gives control over phase, handedness, and degree of helicity of the emitted beam. We demonstrate vortex beams up to an azimuthal index l = 3 that can be readily multiplexed into an array configuration.

An organic optoelectronic muscle contraction sensor for prosthetics

Non-invasive wearable sensors are desirable for a wide range of medical measurements, and especially for ubiquitous monitoring of a patient’s health. They have applications in the treatment of neurological disorders, depression, drug addiction, and even for rehabilitation. Our work in organic optoelectronic devices suggests a new class of wearable sensors for medicine and sports. Organic semiconductor devices have the attractive properties of tuneable light emission, simple fabrication on flexible substrates with scope for miniaturization, and the ability to generate and detect light.1 We have shown that organic lightemitting diodes (OLEDs) and organic photodiodes (OPDs) can be combined to make compact, non-invasive, flexible sensors for medical applications by simple solution-processingmethods. Specifically, we developed a potentially low-cost muscle contraction sensor that can measure signals from intact muscles to control the movement of active prosthetic devices such as artificial limbs.2 Moreover, we demonstrated the feasibility of this all-organic, optoelectronic sensor by controlling a robotic arm so that it mimicked the motion of a healthy volunteer’s arm for two types of muscle contractions. Our contraction sensor works by measuring light backscattered by skeletal muscle tissue.3 Specifically, our device measures the difference between the light travelling parallel and perpendicular to themuscles. Inside the device, four organic photodiodes surround an organic LED light source. When the muscle contracts, the light scattering alters and the sensor detects the change. Fabricating the sensor is simple: we used semiconducting polymers, which are flexible and can be deposited from solution. Figure 1 shows a photograph of the actual Figure 1. Photograph of the thin flexible organic optoelectronic sensor.