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Infrared-to-visible photon upconversion has potential applications in photovoltaics, sensing, and bioimaging. We demonstrate a solid-state thin-film device that utilizes sensitized triplet-triplet exciton annihilation, converting infrared photons absorbed by colloidal lead sulfide nanocrystals (NCs) into visible photons emitted from a luminescent dopant in rubrene at low incident light intensities. A typical bilayer device consisting of a monolayer of NCs and a doped film of rubrene is limited by low infrared absorption in the thin NC film. Here, we augment the bilayer with an optical spacer layer and a silver-film back reflector, resulting in interference effects that enhance the optical field and thus the absorption in the NC film. The interference-enhanced device shows an order-of-magnitude increase in the upconverted emission at the wavelength of λ = 610 nm when excited at λ = 980 nm. At incident light intensities above 1.1 W/cm2, the device attains maximum efficiency, converting (1.6 ± 0.2)% of absor...

We demonstrate organic light emitting devices (OLEDs) with a charge trap layer that show memory behavior. These OLEDs demonstrate that organic heterojunction structures can controllably trap and release electronic charges. The trap layer is either 5-nm-thick clustered silver islands, or a 10-nm-thick organic laser dye DCM2 ([2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[i,j]quinolizin-9-yl)-ethenyl]-4H-pyran-4-ylidene] propane-dinitrile) doped into TPD (N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine). Predictions of the energy band structure indicate that both DCM2 sites and the metal islands can trap charge, consistent with the measured current–voltage characteristics. Trap sites are charged by applying reverse bias over the OLEDs. For devices with DCM2 traps we observe quenching of DCM2 photoluminescence upon charging, which allows us to quantify the charged trap density as approximately 10% of the trap sites or 1018cm−3. From time resolved measurements we observe that the charge rete...

Organic photodetectors incorporating an ultrathin (⩾5 A) donor–acceptor alternating multilayer stack as the optically active region exhibit external quantum efficiencies of 75% across the visible spectrum, and have subnanosecond response times. Photogenerated excitons efficiently dissociate into free electrons and holes by rapid charge-transfer across the several closely spaced organic-layer interfaces. The dependence of the quantum efficiency on applied voltage and layer thickness suggests that escape of photogenerated carriers from potential wells formed by the multilayers due to tunneling prior to recombination leads to the high efficiencies observed. The impulse response of the highest-bandwidth devices is characterized by a full width at half maximum of (720±70) ps. The performance of these devices makes them a useful building block for molecular organic photonic integrated circuits.

We demonstrate a method for efficient photon harvesting in organic thin films, thereby increasing the efficiency of organic photovoltaic cells. By incorporating an exciton-blocking layer (EBL) inserted between the photoactive organic layers and the metal cathode, we achieved an external power conversion efficiency of 2.4%±0.3% in vacuum-deposited ultrathin organic bilayer photovoltaic (PV) cells employed in a simple light trapping geometry. Ultrathin (∼100 A) cells incorporating the transparent, conductive EBL have an internal quantum efficiency as high as 33%±4% over a spectral region matched to the solar spectrum. The very thin organic layers have a low series resistance, allowing for efficient power conversion in organic PV cells under intense (>15 suns) AM1.5 illumination. This device structure demonstrates that control of exciton diffusion in solid-state organic devices leads to a significant increase in the photon-to-carrier conversion efficiency.

Characteristics of lasing in optically pumped, vacuum deposited organic semiconductor thin films of tris-(8-hydroxyquinoline) aluminum doped with DCM laser dye are studied as a function of DCM concentration and ambient temperature. In contrast to inorganic laser diodes, the lasing wavelength, output power, differential quantum efficiency, and threshold pump energy of organic lasers are found to be independent of temperature in the range from 0 to 140 °C. In addition, no degradation in laser performance was observed at temperatures approaching 160 °C.

We report the demonstration of transparent organic light emitting devices (OLEDs) which are ∼70% transparent throughout the visible spectrum when switched off, and emit light from both sides with a total external quantum efficiency of ∼0.1% when turned on. The devices are Alq3‐based single heterostructure OLEDs grown on an ITO‐coated glass substrate with a top electrode composed of a very thin layer of Mg–Ag and an overlaying ITO film. The top electrode is both electron injecting and transparent. The transparent OLEDs are expected to be useful in high‐resolution full‐color displays, as well as for helmet‐mounted, windshield‐mounted, or other ‘‘head‐up’’ display applications.

Photoluminescence of 1.3 μm wavelength strained multiple quantum well InGaAsP/InP laser structures has been used to understand the excess carrier redistribution between the quantum well and waveguide regions at a high level of excitation. A model is developed to describe the experimental results. The model suggests that space charge barriers play a significant role in the electron confinement in quantum wells at the high excitation range typical of laser diode operation.

We present a simple encapsulation technique for organic light emitting devices (OLEDs). By studying the degradation of a population of OLEDs, we show that the lifetime of encapsulated devices is increased by more than two orders of magnitude over that of unencapsulated devices. In both cases, degradation is primarily due to the formation of nonemissive regions, or dark spot defects. By studying the structure and evolution of the dark spots, we infer that the growth of electrode defects limits device lifetime. Hermetic packaging of OLEDs is essential if they are to be used in commercially viable flat panel displays.

An electrically tunable organic vertical-cavity surface-emitting laser (VCSEL) is demonstrated and characterized. A lasing wavelength tunability of Δλ = 10 nm with 6 V actuation is shown for a red laser emission tuned between λ = 637 nm and λ = 628 nm. Wavelength tuning of the VCSEL structure is enabled by electrostatic deflection of a reflective flexible membrane that is suspended over an air gap and a dielectric mirror, forming a 3λ lasing cavity. The lasing gain medium consists of an evaporated organic thin film coated on a reflective membrane, which is then additively placed over a patterned substrate containing the dielectric mirror to fabricate an array of air-gap-VCSEL structures, each 100 μm in diameter. Beyond the electrostatic actuation of these tunable lasers, the VCSEL array geometry also has the potential to be used as pressure sensors with an all-optical remote excitation and readout and a pressure sensitivity of 64 Pa/nm in the demonstrated configuration.

Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract W911NF-07-D-0004)

Vacuum and solution processed versions of poly(3,4-ethylenedioxythiophene) (PEDOT) are used as cathode interlayers in inverted organic photovoltaic cells comprising tetraphenyldibenzoperiflanthene as the electron donor and C60 as the electron acceptor. Chemical treatment of the as-deposited PEDOT layers with tetrakis(dimethylamino)ethylene or cesium carbonate reduces the work function by up to 0.8 eV. Inserting these PEDOT layers at the indium tin oxide cathode results in improved electron collection and efficiencies of up to 2.3 ± 0.2%, approaching the 3.2 ± 0.3% of the conventional device. This illustrates the potential for efficient polymer cathode materials and inverted device architectures compatible with either solution or vacuum processing.

We fabricate near-infrared absorbing organic photovoltaics that are highly transparent to visible light. By optimizing near-infrared optical-interference, we demonstrate power efficiencies of 1.3±0.1% with simultaneous average visible transmission of >65%. Subsequent incorporation of near-infrared distributed-Bragg-reflector mirrors leads to an increase in the efficiency to 1.7±0.1%, approaching the 2.4±0.2% efficiency of the opaque cell, while maintaining high visible-transparency of >55%. Finally, we demonstrate that a series-integrated array of these transparent cells is capable of powering electronic devices under near-ambient lighting. This architecture suggests strategies for high-efficiency power-generating windows and highlights an application uniquely benefiting from excitonic electronics.

We demonstrate a heterojunction photodetector of lateral geometry that utilizes an evaporated film of the hole-transporting molecular material N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-9,9-spirobifluorene (spiro-TPD) as a charge transport layer and that is sensitized across visible wavelengths by a thin film of colloidal CdSe nanocrystal quantum dots (QDs). High photon-to-electron quantum conversion efficiencies are obtained at room temperature as a result of photoconductive gain. With an electric field of 3.0×105 V/cm applied across the electrodes, we measure the external quantum efficiency at the first QD absorption peak (at wavelength λ=590 nm) to be 13%, corresponding to an internal quantum efficiency of approximately 80%. The operating mechanism of these devices is discussed, noting that the optical response is dominated by the QD absorption spectrum while the charge transport nearly exclusively takes place in the spiro-TPD.

We demonstrate a two-terminal, lateral organic bilayer photoconductor that generates an external quantum efficiency of (12±1)%, with an internal quantum efficiency of (140±2)% indicative of photon-to-electron conversion gain. The photoconductor incorporates a heterojunction between N,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (TPD) and 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI). Excitons generated with photoexcitation of PTCBI dissociate at the PTCBI/TPD interface and raise the charge carrier concentration in TPD, increasing device conductance. The exposed top surface enables interaction with chemical analytes in the environment, motivating the use of the photoconductor as a chemical sensor that transduces chemical signals into amplified changes in the electrical response.

The authors demonstrate a subtractive stamping technique for patterning metal electrodes of organic light emitting devices. Patterning is achieved by placing a relief patterned polydimethylsiloxane stamp in contact with a planar metal electrode film and subsequently peeling off the stamp. A fast peel rate increases the weak adhesion energy of the stamp to the metal so that no surface treatment, pressure, or temperature control is necessary to lift-off the metal film in contact with the stamp. Patterning is dependent on metal film thickness, geometry of the features, and peel direction of stamp release. The minimum feature size patterned is 13μm wide stripes within

Light emission from single colloidal CdSe∕ZnS (core/shell) nanocrystals embedded in electrically driven organic light emitting devices is demonstrated at room temperature. Spectral diffusion and blinking from individual quantum dots were observed both in electro- and photoluminescence. The authors propose a model in which the nanocrystals act as seeds for the formation of current channels that lead to enhanced exciton recombination in the vicinity of the quantum dots. This work demonstrates that individual semiconductor nanocrystals can serve as emissive probes in organic light emitting devices and that they can be used to manipulate device structure and properties at the nanometer scale.

Charged interface states are introduced by UV-ozone treatment of a polymer gate dielectric, parylene, prior to deposition of the organic semiconductor, pentacene, thereby modifying the organic field effect transistor (OFET) operation from enhancement to depletion mode. Quasistatic capacitance-voltage measurements and the corresponding current-voltage characteristics show that the threshold voltage VT and flatband voltage VFB can be shifted by over +50V, depending on the ozone exposure time. This work demonstrates that careful control of the semiconductor-insulator interface state densities is essential to VT and VFB control and the fabrication of reliable OFET integrated circuits.

We demonstrate photodetectors of sandwich geometry active in the visible spectrum in which the active layer is a 200 nm thick film of CdSe quantum dots (QDs). The solution-phase treatment of the QD film with n-butylamine after casting greatly increases the exciton dissociation efficiency and charge-transport properties of the film. Under 110mW∕cm2 illumination with light at λ=514nm, the photocurrent to dark current ratio, Iphoto∕Idark, is 103 at V=0V, and the 3 dB frequency is ∼50kHz. At room temperature, we observe zero-bias external quantum efficiencies (EQE) from 0.08% to 0.23% in the wavelength range λ=350nm to λ=575nm, corresponding to an internal quantum efficiency (IQE) of 0.6±0.1% across the tested spectrum. At V=−6V, EQE ranges from 15% to 24%, corresponding to an IQE of 70±10%.

We demonstrate a vacuum-deposited organic light emitting device which emits from its top surface through a transparent indium-tin-oxide anode. This device employs a novel protective cap layer which prevents damage to the organic layers during sputter deposition of the anode, while also improving hole injection. Mechanisms of current transport and carrier injection from the contacts are investigated. This device configuration allows for integration of organic light emitting devices with n-channel field effect transistors used in display active matrix backplanes.