Dinesh Kabra

Efficient Single‐Layer Polymer Light‐Emitting Diodes

Organic/polymeric light emitting diodes (LEDs) have been actively investigated in recent years for display and solid-state lighting due to their rapidly improving effi ciency and performance. [ 1 ] Amongst other critical aspects, the interfaces between the electrodes and emissive semiconductors play important roles in determining their operating characteristics and stability. [ 2 , 3 ] We and others have shown that ZnO can provide adequate electron injection into F8BT [ 4–10 ] Coating of ZnO with a thin layer of Cs 2 CO 3 has been shown to further improve the current effi ciency in hybrid PLEDs. [ 8 ] Ohmic hole injection into the deep HOMO level of F8BT ( ∼ 5.8 eV, chemical structure shown in Figure 1a ) is diffi cult using high work function metals or a layer of the conducting polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). Recent work on hybrid PLEDs has demonstrated that MoO 3 is a potential candidate to achieve good hole injection into F8BT. [ 4 , 6 , 7 ] It has recently been shown in photoelectron spectroscopy study that the work-function of MoO 3 is as large as 6.9 eV. [ 11 ] This enables ohmic hole injection into materials with ionisation potentials signifi cantly deeper than that for F8BT. [ 12 ] MoO 3 has also been utilized in OLEDs [ 13 ] and in transistor [ 14 ] structures for improved hole injection. The hybrid inverted PLED structures, as shown in Figure 1b , have metal-oxides as charge transporting and injection layers. Most of the studies in this area are focused on the bottom n-type metal-oxides layer for example compact TiO 2 , [ 4 , 6 ] mesoporous TiO 2 , [ 4 , 9 , 15 ] ZnO [ 4 , 5 , 7 , 8 , 16 ]

Ag-nanowire films coated with ZnO nanoparticles as a transparent electrode for solar cells

We demonstrate that solution-processible silver-nanowire films coated with zinc-oxide-nanoparticles (ZnO-NPs) can be used as transparent electrodes in organic photovoltaic devices. The ZnO-NP coating acts as electron extraction layer and as encapsulating agent, protecting the wires from oxidation and improving their mechanical stability. Scanning photocurrent microscopy showed photocurrent generation to be more efficient at the active material surrounding the wires. Ultra-violet illumination as present in the solar spectrum was found to enhance photocurrent by improving the ZnO in-layer conductivity through the photoconductive effect. Inverted polythiophene:fullerene devices using ZnO-NP coated silver-nanowires or indium-tin-oxide as transparent electrode reached power conversion efficiencies of 2.4%.

Surface-Directed Spinodal Decomposition in Poly[3-hexylthiophene] and C61-Butyric Acid Methyl Ester Blends

Demixed blends of poly[3-hexylthiophene] (P3HT) and C₆₁-butyric acid methyl ester (PCBM) are widely used in photovoltaic diodes (PV) and show excellent quantum efficiency and charge collection properties. We find the empirically optimized literature process conditions give rise to demixing during solvent (chlorobenzene) evaporation by spinodal decomposition. Ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS) results are consistent with the formation of 1-2 nm thick surface layers on both interfaces, which trigger the formation of surface-directed waves emanating from both film surfaces. This observation is evidence that spinodal demixing (leading to a bicontinuous phase morphology) precedes the crystallization of the two components. We propose a model for the interplay of demixing and crystallization which explains the broadly similar PV performance for devices made with the bottom electrodes either as hole or electron collector. The process regime of temporal separation of demixing and crystallization is attractive because it provides a way to control the morphology and thereby the efficiency of PV devices.

Improved photoinduced charge carriers separation in organic-inorganic hybrid photovoltaic devices

We demonstrate enhanced performance of a hybrid photovoltaic device, where poly[3-hexylthiophene] (P3HT) is used as active material and a solution-processed thin flat film of ZnO modified by a self-assembled monolayer (SAM) of phenyl-C61-butyric acid (PCBA) is used as electron extracting electrode. Ultraviolet photoemission spectroscopy measurements reveal an increase in the substrate work function from 3.6 to 4.1 eV upon PCBA SAM deposition due to an interfacial dipole pointing away from the ZnO. External quantum efficiency (EQE) of the SAM modified devices reached 9%, greatly improved over the 3% EQE of the unmodified devices. This corresponds to full charge separation of all photoexcitations generated in the P3HT within an exciton diffusion range from the interface.

Band Gap Tuning of CH3NH3Pb(Br1–xClx)3 Hybrid Perovskite for Blue Electroluminescence

We report on the structural, morphological and optical properties of AB(Br(1-x)Cl(x))3 (where, A = CH3NH3(+), B = Pb(2+) and x = 0 to 1) perovskite semiconductor and their successful demonstration in green and blue emissive perovskite light emitting diodes at room temperature. The bandgap of perovskite thin film is tuned from 2.42 to 3.16 eV. The onset of optical absorption is dominated by excitonic effects. The coulomb field of the exciton influences the absorption at the band edge. Hence, it is necessary to explicitly account for the enhancement of the absorption through the Sommerfield factor. This enables us to correctly extract the exciton binding energy and the electronic bandgap. We also show that the lattice constant varies linearly with the fractional chlorine content satisfying Vegards law.

Highly Efficient Single-Layer Polymer Ambipolar Light-Emitting Field-Effect Transistors

Single-layer polymer light-emitting field-effect transistors (LEFETs) that yield EQEs of >8% and luminance efficiencies >28 cd A(-1) are demonstrated. These values are the highest reported for LEFETs and amongst the highest values for fluorescent OLEDs. Due to the electrostatics of the ambipolar LEFET channel, LEFETs provide an inherent advantage over OLEDs in terms of minimizing exciton-polaron quenching.

Collective osmotic shock in ordered materials

Osmotic shock in a vesicle or cell is the stress build-up and subsequent rupture of the phospholipid membrane that occurs when a relatively high concentration of salt is unable to cross the membrane and instead an inflow of water alleviates the salt concentration gradient. This is a well-known failure mechanism for cells and vesicles (for example, hypotonic shock) and metal alloys (for example, hydrogen embrittlement). We propose the concept of collective osmotic shock, whereby a coordinated explosive fracture resulting from multiplexing the singular effects of osmotic shock at discrete sites within an ordered material results in regular bicontinuous structures. The concept is demonstrated here using self-assembled block copolymer micelles, yet it is applicable to organized heterogeneous materials where a minority component can be selectively degraded and solvated whilst ensconced in a matrix capable of plastic deformation. We discuss the application of these self-supported, perforated multilayer materials in photonics, nanofiltration and optoelectronics.

Semiconducting-polymer-based position-sensitive detectors

We demonstrate the utility of the organic semiconducting polymers as active media for light-sensitive position-sensitive detectors (PSDs). The characteristics of these PSDs include reasonable linearity and photoresponsivity over a large range of distance, photovoltaic mode of operation, and other advantages such as nonrigid substrates and absence of any other transporting-conducting coating layer. These devices utilize the formation of Al-polymer Schottky-type photoactive interface as a common backcontact. We demonstrate results for poly(3-hexylthiophene)-based PSDs of different interelectrode spacings where the photovoltaic signals as a function of incident-beam position are linear over the entire range.

On the Uniqueness of Ideality Factor and Voltage Exponent of Perovskite-Based Solar Cells

Perovskite-based solar cells have attracted much recent research interest with efficiency approaching 20%. While various combinations of material parameters and processing conditions are attempted for improved performance, there is still a lack of understanding in terms of the basic device physics and functional parameters that control the efficiency. Here we show that perovskite-based solar cells have two universal features: an ideality factor close to two and a space-charge-limited current regime. Through detailed numerical modeling, we identify the mechanisms that lead to these universal features. Our model predictions are supported by experimental results on solar cells fabricated at five different laboratories using different materials and processing conditions. Indeed, this work unravels the fundamental operation principle of perovskite-based solar cells, suggests ways to improve the eventual performance, and serves as a benchmark to which experimental results from various laboratories can be compared.

A review on triphenylamine (TPA) based organic hole transport materials (HTMs) for dye sensitized solar cells (DSSCs) and perovskite solar cells (PSCs): evolution and molecular engineering

In this review article, the important features (optical, thermal and electrochemical) of triphenylamine (TPA) based organic hole transport materials (HTMs) in accordance with their structural diversities are discussed from their evolution to recent advancements. The literature here covers past and ongoing work mostly relevant to HTMs used in DSSCs and PSCs. Besides the good optical properties and high hole mobility, the stability of the amorphous state of the HTM layer is a crucial factor in the commercialization of PSCs. The stability of the amorphous glassy state of HTMs is defined by an important physical parameter, i.e., the glass transition temperature (Tg) which is discussed on the basis of the molecular structure of HTMs.