Kanwar S. Nalwa

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 ]

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

Dependence of recombination mechanisms and strength on processing conditions in polymer solar cells

Charge carrier recombination due to carrier trapping is not often considered in polymer based solar cells, except in those using non-fullerene acceptors or new donor polymers with limited short-range order. However, we show that even for the canonical poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester (P3HT:PCBM) system, relative strengths of bimolecular and trap-assisted recombination are strongly dependent on processing conditions. For slow-grown active-layers, bimolecular recombination is indeed the major loss mechanism under one sun illumination. However, for fast-grown active-layers, trap-assisted recombination dominates over bimolecular recombination by an order of magnitude, and recombination strength at short-circuit condition is 3-4 times higher, leading to loss of photocurrent and lowering of fill factor.

Design of light-trapping microscale-textured surfaces for efficient organic solar cells

Organic photovoltaic (OPV) cells suffer from low charge carrier mobilities of polymers, which renders it important to achieve complete optical absorption in active layers thinner than optical absorption length. Active layers conformally deposited on light-trapping, microscale textured, grating-type surfaces is one possible approach to achieve this objective. In this report, we analyze the design of such grating-type OPV cells using finite element method simulations. The energy dissipation of electromagnetic field in the active layer is studied as a function of active layer thickness, and pitch and height of the underlying textures. The superiority of textured geometry in terms of light trapping is clearly demonstrated by the simulation results. We observe 40% increase in photonic absorption in 150 nm thick active layer, for textures with 2 microm pitch and 1.5 microm height.

Fabrication of metallic nanowires and nanoribbons using laser interference lithography and shadow lithography

Ordered and free-standing metallic nanowires were fabricated by e-beam deposition on patterned polymer templates made by interference lithography. The dimensions of the nanowires can be controlled through adjustment of deposition conditions and polymer templates. Grain size, polarized optical transmission and electrical resistivity were measured with ordered and free-standing nanowires.

Polymer-based photodetectors for structurally integrated photoluminescence based oxygen sensors

A photoluminescence (PL)-based O2 sensor utilizing inorganic light emitting diode (LED) as the light source and a polymer-based photodetector (PD) is demonstrated. The device structure is compact and the sensor integrates the sensing element, light source, and organic PD as thin films that are attached such that the sensing element is sandwiched between the LED and the PD. The sensing elements are based on the oxygen-sensitive dyes Pt-octaethylporphyrin embedded in a polystyrene matrix. A green inorganic LED (peak emission ~525 nm) light source was used to excite the porphyrin dye, which emits at ~640 nm. This emission can be measured using P3HT:PCBM bulk heterojunction photodiodes, which have been shown earlier to have efficient photodetection at this wavelength if the active layer is sufficiently thick. The time constant associated with sweeping out the photogenerated carriers is found to be ~ 10μs. Such a fast decay of photocurrent is useful for oxygen monitoring, determined by measuring the Pl decay time rather than the PL intensity, of the sensing film. This approach can eliminate the need for frequent sensor calibration and optical filters (as pulsed LED excitation is employed in this mode) which lead to bulkier design.