Sumit Chaudhary

Trilayer hybrid polymer-quantum dot light-emitting diodes

We report a trilayer hybrid polymer-quantum-dot light-emitting diode fabricated by sandwiching a CdSe-ZnS core–shell quantum-dot (QD) layer, a few monolayers thick, between films of polyvinylcarbazole (PVK) and an oxadiazole derivative (butyl-PBD). All the layers have been deposited by a controlled spin-coating technique. Stable aqueous dispersion of QDs has been prepared to make possible the spin coating of multiple layers without affecting the layer underneath. Our device shows 20 times the quantum efficiency (0.2%) and less than half the threshold voltage (5 V) of a single-layer device made from the PVK-QD-PBD blend. This is attributed to balanced carrier conduction, enhanced recombination, and reduced quenching of emission due to a better electro-optical device design.

A New Architecture for Transparent Electrodes: Relieving the Trade‐Off Between Electrical Conductivity and Optical Transmittance

Transparent conducting electrodes with the combination of high optical transmission and good electrical conductivity are essential and desirable in solar energy harvesting and electric lighting devices including organic solar cells and light-emitting diodes (LEDs) as well as in their inorganic counterparts. Currently, indium tin oxide (ITO) coated glass is most often used because ITO has relatively high transparency to visible light and low sheet resistance for electrical current conduction. However, ITO is costly due to limited resources, is brittle, [ 1 ] and has poor chemical compatibility with the active organic materials. [ 2 ] These disadvantages have motivated the search for other conducting electrodes with similar or better optical and electrical properties. In recent research efforts, carbon nanotube networks, unpatterned thin metal fi lms, random silver metal nanowire meshes, graphene fi lms, and patterned metal nanowire grids have been evaluated as potential replacements for ITO electrodes. [ 3‐11 ] Although these alternative transparent electrode approaches do have the potential to replace ITO, they still suffer from the classic trade-off between the optical transmittance and electrical conductivity. Thicker layers offer higher conductivity, but this comes at the expense of optical transmittance, and vice versa. Here, we report a new architecture for transparent electrodes, which leads to quasi-elimination of this tradeoff. This architecture consists of high-aspect-ratio metallic ribbons with nanoscale thickness and microscale width, spaced at desired periodicities and held in place by a polymer matrix to provide a fl at top surface for fabrication of active layers in solar cells or LEDs. By design, the light path is only obstructed by the nanoscale thickness of the ribbons, thus decoupling the conductivity and transmittance properties from each other. Catrysse and Fan performed theoretical investigations on similar nanopatterned metallic structures, and their simulations indicate that such structures have excellent optical and electrical properties for potential use as transparent conductive electrodes. [ 12 ] Our experimental results show that the novel structure is very promising for such applications.

On realizing higher efficiency polymer solar cells using a textured substrate platform.

and emergence of new conjugated polymers with tailored energy levels. [ 4–6 ] Power conversion effi ciency (PCE) exceeding 7% has recently been achieved. [ 4 ] The state-of-the-art devices are so called bulk-heterojunction (BHJ) type in which the PV activelayer is coated from a blend of donor and acceptor species. The nanoscale nature of phase separation between the donors and acceptors in a BHJ active-layer alleviates the mismatch between exciton diffusion length ( ∼ 10 nm) and optical absorption length ( > 100 nm). However, there still exists a mismatch between optical absorption length and charge transport scale. BHJ activelayers tend to suffer from cul-de-sacs in the charge transport pathways, and hole mobilities in conjugated polymers remain low. Both of these factors lead to recombination losses, higher series resistances and lower fi ll-factors. [ 7 ] Thus, it is imperative to develop fabrication methodologies that can enable effi cient optical absorption in fi lms thinner than optical absorption length. The most desirable methodology would be one which can also substantially improve absorption at the band edge of conjugated polymers, which usually lies in the red/near-infrared region, and where signifi cant amount of solar fl ux is also located. It is more so important because the charge carriers photoexcited at the band edge were found to have a higher dissociation effi ciency than the ones excited at higher energies. [ 8 ]

Multicolour hybrid nanoprobes of molecular beacon conjugated quantum dots: FRET and gel electrophoresis assisted target DNA detection

We have developed multicolour hybrid DNA probes employing green, yellow and orange colour quantum dot conjugated molecular beacons with black hole quencher 2. Optical and electrophoretic characterization revealed fluorescent energy transfer that follows the FRET mechanism with single nucleotide discrimination. Target DNA identification was observed to be highly sensitive up to 8 ng in gel electrophoresis. Comparison with the conventional organic dye SYBR Gold™ showed that our hybrid nanoprobes exhibit more stable performance with less background signal.

Enhanced charge separation in organic photovoltaic films doped with ferroelectric dipoles

A key requirement for realizing efficient organic photovoltaic (OPV) cells is the dissociation of photogenerated electron-hole pairs (singlet-excitons) in the donor polymer, and charge-transfer-excitons at the donor–acceptor interface. However, in modern OPVs, these excitons are typically not sufficiently harnessed due to their high binding energy. Here, we show that doping the OPV active-layers with a ferroelectric polymer leads to localized enhancements of electric field, which in turn leads to more efficient dissociation of singlet-excitons and charge-transfer-excitons. Bulk-heterojunction OPVs based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester are fabricated. Upon incorporating a ferroelectric polymer as additive in the active-layer, power conversion efficiencies increase by nearly 50%, and internal quantum efficiencies approach 100% – indicating complete exciton dissociation at certain photon energies. Similar enhancements in bilayer-heterojunctions, and direct influence of ferroelectric poling on device behavior show that improved dissociation is due to ferroelectric dipoles rather than any morphological change. Enhanced singlet-exciton dissociation is also revealed by photoluminescence lifetime measurements, and predicted by simulations using a numerical device model.

Memristive Behavior in Thin Anodic Titania

A common material in creating memristors is titanium dioxide (TiO2), grown by atomic layer deposition, sputtering, or sol-gel process. In this letter, we study the memristive behavior in thin TiO2 films fabricated by brief electrochemical anodization of titanium. The effects of different anodization times and annealing are explored. We discover that inherent oxygen-vacancies at the bottom Ti/TiO2 interface naturally lead to memristive switching in nonannealed films. Annealing induces extra oxygen vacancies near the top metal/oxide interface, which leads to symmetric and ohmic current-voltage characteristics with a collapse of memristive switching. No clear dependence on anodization time was observed for times between 1 s and 1 min.

Polythiophene‐Fullerene Based Photodetectors: Tuning of Spectral Response and Application in Photoluminescence Based (Bio)Chemical Sensors

A photoluminescence (PL)-based oxygen and glucose sensor utilizing inorganic or organic light emitting diode as the light source, and polythiophene: fullerene type bulk-heterojunction devices as photodetectors, for both intensity and decay-time based monitoring of the sensing elements PL. The sensing element is based on the oxygen-sensitive dye Pt-octaethylporphyrin embedded in a polystyrene matrix.

Growth rate dependent trap density in polythiophene-fullerene solar cells and its implications

To understand the effect of processing conditions such as spin coating speed and drying rate on the density of defects; poly(3-hexylthiophene):fullerene-derivative solar cells A, B, and C were fabricated with solvent drying times of ∼40 min, 7 min, and 1 min, respectively. We show that slowest grown device A has one order of magnitude less subband gap traps than device C. The open circuit voltage and its light intensity dependence was strongly affected by interfacial recombination of carriers at subgap defect states. The losses due to trap-assisted recombination can even dominate over bimolecular recombination, depending on the density of defect states

The identification, characterization and mitigation of defect states in organic photovoltaic devices: a review and outlook

In any microelectronic device, fundamental physical parameters must be well understood for successful electronic optimization. One such prominent parameter is energetic trap states, which are well-known to plague amorphous or otherwise impure semiconducting materials. Organic semiconductors are no strangers to such states and their electronic properties are evidently tied to these defects. Herein, this article discusses the identification, characterization and mitigation of bandgap residing trap levels in organic photovoltaic devices. A compilation of select studies to date is given and a general outlook is proposed. Organic photovoltaic materials are depicted as multiple trap-level systems with a seemingly continuous distribution of electronic states throughout the bandgap. Some elucidations as to the origins of these electronic states as well as recent works centered on defect removal are also presented.

On accurate capacitance characterization of organic photovoltaic cells

Capacitance measurements, widely used to characterize numerous semiconductor properties, have been recently adopted to characterize organic photovoltaic (OPV) devices. It is known that certain challenges are associated with capacitance measurements. Of upmost importance is the employment of a proper measurement model (series or parallel). Owing to larger capacitive impedances and low series resistances, the parallel model is typically employed in inorganics. However, we find that for characteristically thinner organic films, a hybrid model should be used. We highlight the inconsistencies in OPV literature due to indiscriminate usage of parallel model and show how proper model selection can rectify any artifacts.