Organic photovoltaic (OPV) technology is an attractive solarelectric conversion paradigm due to the promise of low cost roll-to-roll production and amenability to fl exible substrates. Substantial progress has been made over the last 5 years, by virtue of optimization of materials processing parameters [1][2][3] and emergence of new conjugated polymers with tailored energy levels. [4][5][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 ] Textured substrate based light-trapping schemes are a commonplace in traditional inorganic photovoltaic (PV) cells. However, they have not been successfully applied to polymer based PVs due to an obvious problem of solution-processing nanoscale thick and conformal PV layers on topographical surfaces. In this communication, we show that realization of such conformal layers is indeed possible, if the underlying topography has suitable dimensions. We fabricated poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) based BHJ PV cells on grating-type textured substrates possessing several sub-micrometer and micrometer scale topographical dimensions. We discovered that if the height of the underlying topographical features is reduced to sub-micrometer regime (e.g. 300 nm) and the pitch is increased to more than a micrometer (e.g. 2 μ m), the textured surface becomes amenable to coating a conformal PV active-layer. The resultant PV cells showed 100% increase in average light absorption near the band edge due to trapping of higher wavelength photons, and 20% improvement in PCE as compared with the fl at PV cell.Till date, a few light management techniques in ray optics regime have been investigated for enhancement of optical absorption in OPVs, namely, collec...

Dissipationless current in topologically protected states is promising for disorder-tolerant electronics and quantum computation. Here, we show giant anisotropic terahertz (THz) photocurrents with vanishing scattering driven by femtosecond laser-induced coherent phonons of broken inversion symmetry (IS) in a centrosymmetric Dirac material ZrTe 5 . Our work suggests that this phononic THz symmetry switching leads to Weyl point (WP) formation,

We demonstrate enhanced absorption in solar cells and enhanced light emission in OLEDs by light interaction with a periodically structured microlens array. We simulate n-i-p perovskite solar cells with a microlens at the air-glass interface, with rigorous scattering matrix simulations. The microlens focuses light in nanoscale regions within the absorber layer enhancing the solar cell. Optimal period of ~700 nm and microlens height of ~800-1000 nm, provides absorption (photocurrent) enhancement of 6% (6.3%). An external polymer microlens array on the air-glass side of the OLED generates experimental and theoretical enhancements >100%, by outcoupling trapped modes in the glass substrate.

Over the last decade, polymer solar cells (PSCs) have attracted a lot of attention and highest power conversion efficiencies (PCE) are now close to 10%. Here we employ an optical structure - the microlens array (MLA) - to increase light absorption inside the active layer, and PCE of PSCs increased even for optimized devices. Normal incident light rays are refracted at the MLA and travel longer optical paths inside the active layers. Two PSC systems - poly(3-hexylthiophene-2,5-diyl):(6,6)-phenyl C61 butyric acid methyl ester (P3HT:PCBM) and poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]:(6,6)-phenyl C71 butyric acid methyl ester (PCDTBT:PC70BM) - were investigated. In the P3HT:PCBM system, MLA increased the absorption, absolute external quantum efficiency, and the PCE of an optimized device by ∼4.3%. In the PCDTBT:PC70BM system, MLA increased the absorption, absolute external quantum efficiency, and PCE by more than 10%. In addition, simulations incorporating optical parameters of all structural layers were performed and they support the enhancement of absorption in the active layer with the assistance of MLA. Our results show that utilizing MLA is an effective strategy to further increase light absorption in PSCs, in which optical losses account for ∼40% of total losses. MLA also does not pose materials processing challenges to the active layers since it is on the other side of the transparent substrate.

electronics-based (bio)chemical sensing and biotechnology applications. [ 1 ] As examples, luminescent conjugated polymers have been used to gain insight into biology and pathology of protein aggregation diseases, [ 2 ] and for designing electrochemical switches and ion pumps for cell biology studies. [ 3 , 4 ] Organic thin fi lm transistors (OTFTs) were implemented to develop cost-effective and label-free DNA or protein sensor chips, [ 5 ] and organic light-emitting diodes (OLEDs) have been evaluated as excitation sources in photo luminescence (PL)-based sensing of analytes such as oxygen, ethanol, glucose, lactate, and cholesterol. [ 1 , 6-13 ] Other examples of the use of OLEDs (including polymer LEDs (PLEDs)) in sensing applications include an integrated PL-based oxygen and pH sensor, utilizing an OLED as the light source and an organic photodetector (PD); [ 13 -16 ] two polarizers were used for separating the PL and the OLED's electroluminescence (EL). [ 13 ] OLEDs were used also for fl uorescence detection of proteins [ 17 ] and PLEDs were used as an integrated excitation source for microfabricated capillary electrophoresis. [ 18 ] The use of PLEDs for monitoring biomolecules labeled with fl uorescent dyes by monitoring shifts in the PLED's EL [ 19 ] and a surface plasmon resonance sensor utilizing an OLED and a metallic sensing layer were also reported. [ 20 ] A nanotextured OLED-based chemical sensor for label-free detection of methanol and ethanol was demonstrated. [ 21 ] The detection was based on monitoring analyte-induced changes in the OLED turn-on voltage and EL intensity. In another example, a refractometer with an integrated OLED light source and dual organic PDs (OPDs) were used for sensitive analyte detection by monitoring the change in light fl ux from the OLED to the PD that resulted from changes in refractive index of the analyte solution relative to a reference solution. [ 16 ] Good detection sensitivities are often obtained using OTFTand OLED-based sensors, and the issue of the long-term stability that affects the organic devices is often less important in their sensing platforms, as the sensing probes are often shorter lived than the OLEDs. Moreover, as the cost of OTFTs and OLEDs is expected to drop, they are promising for use in disposable sensors. is used for directional PL scattering toward the photodetector, which leads to a ∼ 2.1-3.8 fold enhancement of the PL signal. This behavior is shown for oxygen sensing, which is the basis for sensing of bioanalytes such as glucose, lactate, ethanol, cholesterol, and uric acid.

The performance of organic photovoltaic devices is improving steadily and efficiencies have now exceeded 10%. However, the incident solar spectrum still largely remains poorly absorbed. To reduce optical losses, we employed a microlens array (MLA) layer on the side of the glass substrate facing the incident light; this approach does not interfere with the processing of the active-layer. We observed up to 10% enhancement in the short circuit current of poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl}):(6,6)-phenyl C71-butyric acid methyl ester (PTB7:PC71BM) OPV cells. Theoretically and experimentally investigating several MLA dimensions, we found that photocurrent increases with the ratio of the height to the pitch size of MLA. Simulations reveal the enhancement mechanisms: MLA focuses light, and also increases the light path within the active-layer by diffraction. Photocurrent enhancements increase for a polymer system with thinner active-layers, as demonstrated in poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT):PC71BM OPVs with 17% improvement in short circuit current.

We discover hidden Rashba fine structure in CH3NH3PbI3 and demonstrate its quantum control by vibrational coherence through symmetry-selective vibronic (electron-phonon) coupling. Above a critical threshold of a single-cycle terahertz pump field, a Raman phonon mode distinctly modulates the middle excitonic states with persistent coherence for more than ten times longer than the ones on two sides that predominately couple to infrared phonons. These vibronic quantum beats, together with first-principles modeling of phonon periodically modulated Rashba parameters, identify a threefold excitonic fine structure splitting, i.e., optically forbidden, degenerate dark states in between two bright ones with a narrow, similar to 3 nm splitting. Harnessing of vibronic quantum coherence and symmetry inspires lightperovskite quantum control and sub-THz-cycle "Rashba engineering" of spin-split bands for ultimate multifunction device.

Thin microporous films were formed by dropcasting a toluene solution containing various ratios of polystyrene:polyethylene glycol blends on a glass substrate, with OLEDs on the ITO that coated the opposite side of that substrate. We demonstrate for the first time that such easily-fabricated films with surface and bulk micropores in the index-matching polystyrene can serve as random microlens-like arrays to improve forward OLED light extraction by up to ~60%. A theoretical interpretation of the angular emission profile of the device, considering the geometrical change at the substrate/air interface and the scattering by the pores within the films, was established in excellent agreement with the experiments. The use of such blended thin films provides an economical method, independent of the OLED fabrication technique, for improving the outcoupling efficiency.