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Semitransparent organic solar cells (ST-OSCs) have garnered strong interest for building integrated photovoltaics (PV) and wearable electronics. Although seen as an appealing low-cost PV technology...

Metal halide perovskites are generating enormous excitement for use in solar cells and light-emission applications, but devices still show substantial non-radiative losses. Here, we show that by combining light and atmospheric treatments, we can increase the internal luminescence quantum efficiencies of polycrystalline perovskite films from 1% to 89%, with carrier lifetimes of 32 μs and diffusion lengths of 77 μm, comparable with perovskite single crystals. Remarkably, the surface recombination velocity of holes in the treated films is 0.4 cm/s, approaching the values for fully passivated crystalline silicon, which has the lowest values for any semiconductor to date. The enhancements translate to solar cell power-conversion efficiencies of 19.2%, with a near-instant rise to stabilized power output, consistent with suppression of ion migration. We propose a mechanism in which light creates superoxide species from oxygen that remove shallow surface states. The work reveals an industrially scalable post-treatment capable of producing state-of-the-art semiconducting films. Metal halide perovskites are exciting materials for low-cost optoelectronic devices such as solar cells and LEDs. In order to reach the theoretical efficiency limits for both applications, any parasitic non-radiative charge-carrier recombination losses, such as those mediated by carrier trapping, must be eliminated. At present, perovskite materials still suffer from substantial non-radiative decay, particularly under solar illumination conditions, and are therefore yet to reach their full potential. Perovskite single crystals have very low trap concentrations but their controlled growth into devices does not lend themselves to the advantages offered by solution-processing thin films such as roll-to-roll depositions. Here, we demonstrate the use of light and atmospheric treatments on polycrystalline perovskite films, resulting in minimal non-radiative losses and properties approaching those of perovskite single crystals and even the best crystalline semiconductors reported to date. The authors demonstrate the use of light and atmospheric treatments on polycrystalline perovskite thin films, resulting in properties approaching those of the best crystalline semiconductors reported to date. The results translate to exceptional photovoltaic device performances with rapid rises to stabilized power output consistent with an inhibition of ionic migration.

Self-absorption of internally radiated photons (``photon recycling'') is common in high-quality, direct-gap semiconductors, and can yield increased photovoltage in solar cells and enhanced emission efficiency in LEDs. For perovskite semiconductors, photon recycling is relatively unexplored, especially for devices under operating conditions. The authors develop a model to quantify the extent of photon recycling in state-of-the-art perovskite solar cells of varying nonradiative loss and geometry. They present clear experimental targets for material optoelectronic quality to harness photon recycling in solar cells, with major implications for achieving low-threshold lasing and efficient LEDs.

Polycrystalline CsPbBr3 thin films are deposited by vacuum co-evaporation of cesium halide and lead halide precursors, leading to uniform pinhole-free morphology and precise control over the film thickness and precursor stoichiometry. By utilizing the organic hole and electron transport layers, all-vacuum-deposited perovskite LEDs are fabricated. The resulting devices exhibit a maximum luminance of 1800 cd/m2, a 531 nm emission wavelength peak with a spectral linewidth of 21 nm, and an external quantum efficiency of 1.1%.

International audience; State-of-the-art light emitting diodes (LEDs) are made from high-purity alloys of III-V semiconductors or small molecules, but high fabrication cost and conplicated synthetic process have limited their widespread use for large area solid-state lighting applications. Here we report efficient and stable LEDs processed from solution with tunable color enabled by using phase-pure two-dimensional (2D) Ruddlesden-Popper (RP) layered perovskites with a formula (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n=3-5 in this report). By using controlled vertically oriented of crystal in the thin-films that facilitate efficient charge injection and transport, we obtain efficient electroluminescence with a radiance of 35 W Sr-1 cm-2 at 744 nm with an ultra-low turn-on voltage of 1V. Finally, operational stability tests suggest that phase purity is strongly correlated to stability. Phase-pure 2D perovskites exhibit >14 hours of stable operation at peak operating conditions with no droop at current-densities of several Amperes/cm2 in comparison to mixtures of 2D/3D mixture or 3D perovskites, which degrade within minutes.

We report an ultra-bright, highly efficient, low roll-off, inverted quantum dot-based red light emitting device (QLED) using solution-processed zinc oxide nanoparticles and cesium carbonate films as the electron injection and hole blocking layers, respectively. Record luminance of 165,000 Cd/m2 has been obtained at a current density of 1000 mA/cm2 with a low driving voltage of 5.8 V for deep red device with CIE coordinates of (0.69, 0.31).

The relationship between charge-carrier lifetime and the tolerance of lead halide perovskite (LHP) solar cells to intrinsic point defects has drawn much attention by helping to explain rapid improvements in device efficiencies. However, little is known about how charge-carrier lifetime and solar cell performance in LHPs are affected by extrinsic defects (i.e., impurities), including those that are common in manufacturing environments and known to introduce deep levels in other semiconductors. Here, we evaluate the tolerance of LHP solar cells to iron introduced via intentional contamination of the feedstock and examine the root causes of the resulting efficiency losses. We find that comparable efficiency losses occur in LHPs at feedstock iron concentrations approximately 100 times higher than those in p-type silicon devices. Photoluminescence measurements correlate iron concentration with nonradiative recombination, which we attribute to the presence of deep-level iron interstitials, as calculated from first-principles, as well as iron-rich particles detected by synchrotron-based X-ray fluorescence microscopy. At moderate contamination levels, we witness prominent recovery of device efficiencies to near-baseline values after biasing at 1.4 V for 60 s in the dark. We theorize that this temporary effect arises from improved charge-carrier collection enhanced by electric fields strengthened from ion migration toward interfaces. Our results demonstrate that extrinsic defect tolerance contributes to high efficiencies in LHP solar cells, which inspires further investigation into potential large-scale manufacturing cost savings as well as the degree of overlap between intrinsic and extrinsic defect tolerance in LHPs and "perovskite-inspired" lead-free stable alternatives.

We report an ultracompact low-threshold laser with an Alq3:DCM host:guest molecular organic thin film gain layer. The device uses a photonic crystal nanobeam cavity which provides a high quality factor to mode volume (Q/V) ratio and increased spontaneous emission factor along with a small footprint. Lasing is observed with a threshold of 4.2 μJ/cm(2) when pumped by femtosecond pulses of λ = 400 nm wavelength light. We also model the dynamics of the laser and show good agreement with the experimental data. The inherent waveguide geometry of the structure enables easy on-chip integration with potential applications in biochemical sensing, inertial sensors, and data communication.

Sub-50 mV operation of nano-electro-mechanical relays is demonstrated for the first time, enabled by an anti-stiction molecular coating. Specifically, a self-assembled monolayer of perfluorodecyltriethoxysilane (PFDTES) is shown to be effective for reducing the switching hysteresis voltage, without dramatically increasing its ON-state resistance, enabling stable device operation at very low voltages.

The mainstream commercialization of colloidal quantum dots (QDs) for light-emitting applications has begun: Sony televisions emitting QD-enhanced colors are now on sale. The bright and uniquely size-tunable colors of solution-processable semiconducting QDs highlight the potential of electroluminescent QD light-emitting devices (QLEDs) for use in energy-efficient, high-color-quality thin-film display and solid-state lighting applications. Indeed, this year’s report of record-efficiency electrically driven QLEDs rivaling the most efficient molecular organic LEDs, together with the emergence of full-color QLED displays, foreshadow QD technologies that will transcend the optically excited QD-enhanced products already available. In this article, we discuss the key advantages of using QDs as luminophores in LEDs and outline the 19-year evolution of four types of QLEDs that have seen efficiencies rise from less than 0.01% to 18%. With an emphasis on the latest advances, we identify the key scientific and technological challenges facing the commercialization of QLEDs. A quantitative analysis, based on published small-scale synthetic procedures, allows us to estimate the material costs of QDs typical in light-emitting applications when produced in large quantities and to assess their commercial viability.

Red quantum-dot light-emitting diodes with an external quantum efficiency of 18%, close to the theoretical maximum of 20%, are reported. Using a layer of zinc oxide nanocrystals provides highly effective electron transport, resulting in devices with a low operating voltage and a high luminous power efficiency of 25 lm W−1.

This Review article summarizes the key advantages of using quantum dots (QDs) as luminophores in light-emitting devices (LEDs) and outlines the operating mechanisms of four types of QD-LED. The key scientific and technological challenges facing QD-LED commercialization are identified, together with on-going strategies to overcome these challenges.

Growth of semiconducting nanostructures on graphene would open up opportunities for the development of flexible optoelectronic devices, but challenges remain in preserving the structural and electrical properties of graphene during this process. We demonstrate growth of highly uniform and well-aligned ZnO nanowire arrays on graphene by modifying the graphene surface with conductive polymer interlayers. On the basis of this structure, we then demonstrate graphene cathode-based hybrid solar cells using two different photoactive materials, PbS quantum dots and the conjugated polymer P3HT, with AM 1.5G power conversion efficiencies of 4.2% and 0.5%, respectively, approaching the performance of ITO-based devices with similar architectures. Our method preserves beneficial properties of graphene and demonstrates that it can serve as a viable replacement for ITO in various photovoltaic device configurations.

For the successful integration of graphene as a transparent conducting electrode in organic solar cells, proper energy level alignment at the interface between the graphene and the adjacent organic layer is critical. The role of a hole transporting layer (HTL) thus becomes more significant due to the generally lower work function of graphene compared to ITO. A commonly used HTL material with ITO anodes is poly(3,4-ethylenedioxythiophene) (PEDOT) with poly(styrenesulfonate) (PSS) as the solid-state dopant. However, graphene's hydrophobic surface renders uniform coverage of PEDOT:PSS (aqueous solution) by spin-casting very challenging. Here, we introduce a novel, yet simple, vapor printing method for creating patterned HTL PEDOT layers directly onto the graphene surface. Vapor printing represents the implementation of shadow masking in combination with oxidative chemical vapor deposition (oCVD). The oCVD method was developed for the formation of blanket (i.e., unpatterened) layers of pure PEDOT (i.e., no PSS) with systematically variable work function. In the unmasked regions, vapor printing produces complete, uniform, smooth layers of pure PEDOT over graphene. Graphene electrodes were synthesized under low-pressure chemical vapor deposition (LPCVD) using a copper catalyst. The use of another electron donor material, tetraphenyldibenzoperiflanthene, instead of copper phthalocyanine in the organic solar cells also improves the power conversion efficiency. With the vapor printed HTL, the devices using graphene electrodes yield comparable performances to the ITO reference devices (η(p,LPCVD) = 3.01%, and η(p,ITO) = 3.20%).

The high degree of morphological and energetic disorder inherent to many nanosized materials places limitations on charge injection into and transport rates through thin films of these materials. We demonstrate electroluminescence achieved by local generation of charge that eliminates the need for injection of charge carriers from the device electrodes. We show electroluminescence from thin films of nanoscale materials that do not support direct current excitation and suggest a mechanism for the charge generation and electroluminescence that is consistent with our time-averaged and time-resolved observations.

Analog & digital circuits implemented in a dual threshold voltage (VT) p-channel organic technology are presented. The dual VT organic technology is compatible with large-area and mechanically flexible substrates due to its low processing temperature (≤ 95°C) and scalable patterning techniques. We demonstrate the first analog & digital organic integrated circuits produced by a dual-gate metal process. The analog circuits are powered by a 5-V supply and include a differential amplifier and a two-stage uncompensated operational amplifier (op-amp). A dynamic comparator is measured to have an input offset voltage of 200 mV and latching time of 119 ms. Both the comparator and the op-amp dissipate 5 nW or less. Area-minimized digital logic is presented. Inverters powered by a 3-V supply were measured to have positive noise margins and consumed picowatts of power. An 11-stage ring oscillator, also powered by a 3-V supply, swings near rail to rail at 1.7 Hz. These results demonstrate dual threshold voltage process feasibility for large-area flexible mixed-signal organic integrated circuits.

Quantum dot light emitting devices (QD-LEDs) con-sist of a monolayer of QDs sandwiched between a hole transporting layer (HTL) and electron transporting layer (ETL) of organic materials. These hybrid devices emit with the narrow bandwidth characteristic of the QDs. The precise position of the QD layer, relative to the interface between the ETL and HTL, can affect the quantum efficiency of the device on the scale of 10 nm or less. Motivated by this observation, the exact nature of the morphology of contact printed and self-assembled QDs on typical organic materials is investigated. The QDs are substantially pressed into the organic material, to a somewhat greater extent when contact printed compared to self-assembled structures. Measured device characteristics from samples made with the two methods are consistent with these observations (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A fully photolithographic dual threshold voltage (VT) organic thin-film transistor (OTFT) process suitable for flexible large-area integrated circuits is presented. The nearroom-temperature (

Colloidal quantum dot light-emitting devices (QD-LEDs) have generated considerable interest for applications such as thin film displays with improved color saturation and white lighting with a high color rendering index (CRI). We review the key advantages of using quantum dots (QDs) in display and lighting applications, including their color purity, solution processability, and stability. After highlighting the main developments in QD-LED technology in the past 15 years, we describe the three mechanisms for exciting QDs - optical excitation, Fo¨ rster energy transfer, and direct charge injection - that have been leveraged to create QD-LEDs. We outline the challenges facing QDLED development, such as QD charging and QD luminescence quenching in QD thin films. We describe how optical downconversion schemes have enabled researchers to overcome these challenges and develop commercial lighting products that incorporate QDs to achieve desirable color temperature and a high CRI while maintaining efficiencies comparable to inorganic white LEDs (>65 lumens per Watt). We conclude by discussing some current directions in QD research that focus on achieving higher efficiency and air-stable QD-LEDs using electrical excitation of the luminescent QDs. Keywords: nanocrystals; optoelectronics; displays; lighting (Published: 7 July 2010) Citation: Nano Reviews 2010, 1: 5202 - DOI: 10.3402/nano.v1i0.5202

The effects of bias stress in integrated pentacene organic transistors are studied and modeled for different stress conditions. It is found that the effects of bias stress can be expressed in terms of the shift in applied gate voltage ?V for a given current. An empirical equation describing ?V in terms of different gate and drain bias stress measurements and stress times is presented and verified. In the measured devices, ?V saturates at 14 V, independent of the gate bias-stress condition. A model based on carrier trapping rate equation that accounts for this ?V saturation is developed. The model suggests that the ?V saturation is due to the small density of traps compared to the channel carrier density.

Perovskite solar cells have attained incredible power conversion efficiencies but it is still unclear whether photogenerated carriers are free or excitonic in nature. Originally, it is believed that they are exciton-based devices, similar to organic or dye-sensitized solar cells. However, the emergence of efficient planar devices as well as measurements of exciton binding energy in the range of 10–100 meV suggest that they may be free carrier-based. In this work, the free carrier model is confirmed by building graphene/perovskite Schottky barrier solar cells, analogous to conventional metal/semiconductor Schottky barrier solar cells. To address the challenges of building such devices, solution-processing techniques are extensively investigated for depositing perovskite films directly onto graphene in order to obtain an intimate contact between the graphene and perovskite. Interestingly, these graphene/perovskite Schottky barrier devices have reasonably good efficiency—up to 10.6%—and short circuit current densities only slightly lower than control devices. Furthermore, devices with neither a hole transport layer nor an electron transport layers have power conversion efficiencies of up to 6%. These results provide convincing evidence supporting the free carrier model for methylammonium lead iodide perovskites and offer insights on potential alternative designs for perovskite solar cells.

We report a low temperature ( ~ 100°C) lithographic method for fabricating hybrid metal oxide/organic field-effect transistors (FETs) that combine a zinc-indium-oxide (ZIO) semiconductor channel and organic, parylene, dielectric layer. The transistors show a field-effect mobility of (12±0.8) cm2 V-1 s-1, on/off ratio of 108 and turn-off voltage of Voff = -1 V. This work demonstrates that organic and inorganic layers can be deposited and patterned using a low temperature budget, integrated lithographic process to make FETs suitable for large area electronic applications.

We report a novel unipolar light-emitting device architecture that operates using direct-current, field-driven electroluminescence of colloidally synthesized quantum dots (QDs). This device architecture, which is based only on transparent ceramics and QDs, enables emission from different color QDs and, for the first time, constant QD electroluminescence during extended operation in air, unpackaged.

We investigate the effect of the electronic energy level positioning, conductivity, and morphology of metal oxide charge transport layers on the performance of light emitting devices (LEDs) that consist of a colloidally synthesized quantum dot (QD) luminescent film embedded between electron and hole injecting ceramic layers. We demonstrate that understanding of these material properties and their effect on charging processes in QDs enables the systematic design of higher efficiency QD-LEDs and excitation of QDs with different emission colors using the same device structure.

We demonstrate a solvent-free contact printing process for deposition of patterned and unpatterned colloidal quantum dot (QD) thin films as the electroluminescent layers within hybrid organic-QD light-emitting devices (QD-LEDs). Our method benefits from the simplicity, low cost, and high throughput of solution-processing methods, while eliminating exposure of device structures to solvents. Because the charge transport layers in hybrid organic/inorganic QD-LEDs consist of solvent-sensitive organic thin films, the ability to avoid solvent exposure during device growth, as presented in this study, provides a new flexibility in choosing organic materials for improved device performance. In addition, our method allows us to fabricate both monochrome and red-green-blue patterned electroluminescent structures with 25 microm critical dimension, corresponding to 1000 ppi (pixels-per-inch) print resolution.

We define a metric of useful operating lifetime of an organic light-emitting device (OLED) display and relate it to the commonly measured half-life of constituent OLED pixels. We enumerate sources of OLED operational instability and propose an optical feedback solution in a novel integrated configuration to counter pixel aging and maintain stable light output across all of the pixels of an OLED display. Such optical feedback can correct pixel imperfections in both active matrix and passive matrix OLED displays. As an example, we analyze lifetime data previously published by Kwong et al., in 2002, and demonstrate that our optical feedback technique could maintain 100 cd/m2 display light output within a 2% brightness accuracy for more than 25 000 hours of continuous use for this specific OLED system. From this example we draw conclusions generally applicable to extending stable operating lifetime of other OLED structures.

In this paper, a proof of concept 4 x 4 active-matrix imager fabricated at near room temperature (< 95 degC) is presented. Conventional photolithography and inkjet printing were used to pattern integrated organic FETs and photoconductors. The design and characterization of a pixel circuit is described. A simple first-order calibration technique is used to partially compensate for fixed pattern noise. Following the calibration, the imager is shown to correctly image a "T" pattern.

We review our research efforts to develop solid state integrated devices that operate in the strong coupling limit of cavity quantum electrodynamics (QED) for eventual application in high speed optical switching, optical computing, and quantum computing. Our devices contain J-aggregates of (organic) cyanine dyes which, by virtue of their molecular arrangement and strong dipolar coupling, exhibit a collective narrow linewidth high oscillator strength optical transition. Using J-aggregates, the strong coupling limit can be reached at room temperature with large coupling strengths (Rabi-splitting >250 meV) in exciton–polariton microcavity structures. We demonstrate that high quality nanoscale thick J-aggregate films can be uniformly deposited over macroscopic substrates, engineered at the molecular level, and patterned into single or multi-dimensional photonic bandgap structures. Our unique methods for depositing J-aggregates enabled us to structure light emitting devices that demonstrated the first ever electrically pumped polariton emission, uniquely accomplished in room temperature operation. Additionally, we demonstrated critically coupled resonators that concentrate nearly all of the incident light into 5 nm thick J-aggregate films, yielding a record high effective absorption constant of 6.8 · 10 6 cm � 1 for films with thickness that is less than 1% of the incident light wavelength. Such remarkable optical properties, enabled by scalable deposition techniques, suggest that J-aggregates are a unique materials platform on which to demonstrate integrated exciton–polariton devices with the far reaching properties of polaritons in the optical domain. � 2007 Elsevier B.V. All rights reserved. PACS: 71.36.+c; 71.35.Aa; 85.60.Jb; 78.40.Me

Nanoelectromechanical (NEM) switches have emerged as a promising competing technology to the conventional metal-oxide semiconductor (MOS) transistors. NEM switches exhibit abrupt switching behavior with large on-off current ratios, near-zero off-state leakage currents and sub-threshold slopes below the 60 mV/decade theoretical limit of conventional MOS devices [1]. However, current NEM switches commonly operate at relatively high actuation voltages exceeding 1 V and suffer from failure due to stiction [1]. Reducing the switching gap is a common approach utilized to lower the operating voltage. However, the decrease in the gap size further increases the surface adhesion forces and consequently the possibility of stiction-induced failure.

Using solvent-free composite membrane transfer, we demonstrate an electrically tunable organic visible light-emitting laser with reversible tuning range of 10 nm under 6 V actuation. Large-area scalability of utilized fabrication methods suggests potential use in all-optical pressure-sensing surfaces.

We demonstrate a hybrid inorganic/organic light-emitting device composed of a CdSe/ZnS core/shell semiconductor quantum-dot emissive layer sandwiched between p-type NiO and tris-(8-hydroxyquinoline) aluminum (Alq3), as hole and electron transporting layers, respectively. A maximum external electroluminescence quantum efficiency of 0.18% is achieved by tuning the resistivity of the NiO layer to balance the electron and hole densities at quantum-dot sites.

Semiconductor nanocrystals (NCs) or quantum dots (QDs) show great promise for use in QD-LED (quantum dot lightemitting device) displays, owing to their unique optical properties and the continual development of new core and core–shell structures to meet specific color needs. This in combination with the recent development of more efficient and saturated QD-LEDs as well as new QD-LED fabrication techniques, suggests that QD-LEDs have the potential to become an alternative flat-panel display technology. The ideal red, green, and blue emission spectrum of an LED for a display application should have a narrow bandwidth and a wavelength such that its color coordinates on the Commission Internationale de l+Eclairage (CIE) chromaticity diagram lie outside the current National Television System Committee (NTSC) standard color triangle (see Figure 2). For a Gaussian emission spectrum with a full width at half maximum (FWHM) of 30 nm and a maximized perceived power, the optimal peak wavelength for display applications is l= 610– 620 nm for red, l= 525–530 nm for green, and l= 460– 470 nm for blue. For the red pixels, wavelengths longer than l= 620 nm become difficult for the human eye to perceive, while those shorter than l= 610 nm have coordinates that lie inside the standard NTSC color triangle. Optimization of wavelength for the blue pixels follows the same arguments as for the red pixel, but at the other extreme of the visible spectrum. For green pixels, l= 525–530 nm provides a color triangle with the largest area on the CIE chromaticity diagram (and therefore the largest number of colors accessible by a display). Wavelengths longer than l= 530 nm make some of the blue/green area of the triangle inaccessible. Wavelengths shorter than l= 525 nm compromise the yellow display emissions. To date, efficient red-emitting QD-LEDs with a peak emission wavelength optimized for display applications have been realized using (CdSe)ZnS core–shell NCs, while blue QD-LEDs with a peak wavelength of emission optimized for display applications have been realized with a (CdS)ZnS core–shell material. To date, although efficient green-emitting core–shell semiconductor NCs that emit at l= 525 nm have been synthesized, they have not been successfully incorporated into a QD-LED suitable for display applications. Previous work using (CdSe)ZnS core–shell NCs gave QD-LEDs that emit at wavelengths no shorter than l= 540–560 nm. 14] Using (CdSe)ZnS core–shell NCs to achieve l= 525 nm emission requires making small CdSe cores ( 2.5 nm in diameter). 16] Such small CdSe semiconductor NCs can be difficult to synthesize with narrow size distributions and high quantum efficiencies, and are also more difficult to process and overcoat with a higher-band-gap inorganic semiconductor, which is necessary for incorporation into solid-state structures. A core–shell composite, rather than an organically passivated NC, is desirable in a solid-state QD-LED device owing to the enhanced photoluminescence and electroluminescence (EL) quantum efficiencies of core– shell NCs and their greater tolerance to the processing conditions necessary for device fabrication. Larger NCs are also more desirable for use in QD-LEDs because the absorption cross section of NCs scales with size. Larger NCs with larger absorption cross sections lead to an increase in the efficiency of F?rster energy transfer from electroluminescing organic molecules to NCs in a working QD-LED, which in turn leads to more efficient devices. Herein, we report the synthesis of a CdxZn1 xSe alloy core on which we then grew a CdyZn1 yS shell to create a core– shell NC material with the ideal spectral characteristics for green emission in a QD-LED display and with a size large enough for fabricating a working QD-LED. Our CdxZn1 xSe core synthesis was based on work recently published, in which Cd and Se precursors were slowly introduced into a growth solution of ZnSe NCs. A three-step synthetic route was employed to prepare the (CdxZn1 xSe)CdyZn1 yS core–shell NCs. In the first step, ZnSe NCs were prepared by rapidly injecting 0.7 mmol of diethylzinc (Strem) and 1 mL of tri-noctylphosphine selenide (TOPSe; 1m) dispersed in 5 mL of tri-n-octylphosphine (TOP; 97% Strem) into a round-bottom flask containing 7 grams of degassed hexadecylamine (distilled from 90% Sigma–Aldrich) at 310 8C and by then growing the NCs at 270 8C for 90 min. The second step consisted of transferring 8 mL of the above ZnSe NC growth solution, at 160 8C, into a degassed solution of 16 grams of trin-octylphosphine oxide (TOPO; distilled from 90% Sigma– Aldrich) and 4 mmol of hexylphosphonic acid (HPA; Alfa Aesar), also at 160 8C. A solution of 1.1 mmol of dimethylcadmium (Strem) and 1.2 mL of TOPSe (1m) dispersed in 8 mL of TOP (97% Strem) was then introduced dropwise [*] Dr. J. S. Steckel, Dr. P. Snee, Dr. J. P. Zimmer, J. E. Halpert, Prof. M. G. Bawendi Massachusetts Institute of Technology Department of Chemistry Center for Materials Science and Engineering and The Institute for Soldier Nanotechnologies 77 Massachusetts Avenue, Room 6-221 Cambridge, MA 02139 (USA) Fax: (+1)617-253-7030 E-mail: mgb@mit.edu Dr. S. Coe-Sullivan, P. Anikeeva, L.-A. Kim, Prof. V. Bulovic Massachusetts Institute of Technology Laboratory of Organic Optics and Electronics Department of Electrical Engineering and Computer Science Cambridge, MA 02139 (USA) [] These authors contributed equally to this work.

This paper reports a photolithographic process for fabricating organic field-effect transistors which provides two layers of metal with arbitrary via placement, and optionally allows for subtractive lithographic patterning of the transistor active layer. The demonstrated pentacene transistors have a field-effect mobility of 0.1/spl plusmn/0.05 cm/sup 2//(V/spl middot/s). Parylene-C is used both as the gate dielectric and an encapsulation layer which allows for subtractive lithographic patterning. Also demonstrated is a PMOS inverter without level shifting circuitry and level-restoring V/sub High/ and V/sub Low/. This work demonstrates a high definition, multilayer, integrated photolithographic process which creates organic field effect transistors suitable for use in integrated circuit applications such as a display backplanes.

Blue poly(phenylene-ethynylene) (PPE) electroluminescence is achieved in a single layer organic light emitting device. The polymeric system consists of an oxadiazole grafted PPE, which combines the necessary charge transport properties while maintaining the desirable efficient, narrow light-emitting properties of the PPE. Incorporation of a pentiptycene scaffold within the PPE structure prevents ground-state and excited-state interactions between the pendent oxadiazole units and the conjugated backbone.

The luminescence of inorganic core-shell semiconductor nanocrystal quantum dots (QDs) can be tuned across much of the visible spectrum by changing the size of the QDs while preserving a spectral full width at half maximum (FWHM) as narrow as 30 nm and photoluminescence efficiency of 50% [Journal of Physical Chemistry B 101 (46) (1997) 9463] [1] . Organic capping groups, surrounding the QD lumophores, facilitate processing in organic solvents and their incorporation into organic thin film light-emitting device (LED) structures [Nature 370 (6488) (1994) 354] [2] . A recent study has shown that hybrid organic/inorganic QD-LEDs can indeed be fabricated with high brightness and small spectral FWHM, utilizing a phase segregation process which self-assembles CdSe(ZnS) core(shell) QDs onto an organic thin film surface [Nature 420 (6917) (2002) 800] [3] . We now demonstrate that the phase segregation process can be generally applied to the fabrication of QD-LEDs containing a wide range of CdSe particle sizes and ZnS overcoating thicknesses. By varying the QD core diameter from 32 A to 58 A, we show that peak electroluminescence is tuned from 540 nm to 635 nm. Increase in the QD shell thickness to 2.5 monolayers (∼0.5 nm) improves the LED external quantum efficiency, consistent with a Forster energy transfer mechanism of generating QD excited states. In this work we also identify the challenges in designing devices with very thin (∼5 nm thick) emissive layers [Chemical Physics Letters 178 (5–6) (1991) 488] [4] , emphasizing the increased need for precise exciton confinement. In both QD-LEDs and archetypical all-organic LEDs with thin emissive layers, we show that there is an increase in the exciton recombination region width as the drive current density is increased. Overall, our study demonstrates that integration of QDs into organic LEDs has the potential to enhance the performance of thin film light emitters, and promises to be a rich field of scientific endeavor.

We demonstrate an unexpected tenfold reduction in the lasing threshold of an organic vertical microcavity under subpicosecond optical excitation. In contrast to conventional theory of lasing, we find that the lasing threshold depends on the rate at which excitons are created rather than the total energy delivered within the exciton lifetime. The threshold reduction is discussed in the context of microcavity-enhanced super-radiant coupling between the excitons. The interpretation of super-radiance is supported by the temporal relaxation dynamics of the microcavity emission, which follows the super-radiance time rather than the cavity lifetime. This demonstration suggests that room-temperature super-radiant effects could generally lower the threshold in four-level lasing systems of similar relaxation dynamics.

Triplet excitons are ubiquitous in organic optoelectronics, but they are often an undesirable energy sink because they are spin-forbidden from emitting light and their high binding energy hinders the generation of free electron-hole pairs. Harvesting their energy is consequently an important technological challenge. Here, we demonstrate direct excitonic energy transfer from 'dark' triplets in the organic semiconductor tetracene to colloidal PbS nanocrystals, thereby successfully harnessing molecular triplet excitons in the near infrared. Steady-state excitation spectra, supported by transient photoluminescence studies, demonstrate that the transfer efficiency is at least (90 ± 13)%. The mechanism is a Dexter hopping process consisting of the simultaneous exchange of two electrons. Triplet exciton transfer to nanocrystals is expected to be broadly applicable in solar and near-infrared light-emitting applications, where effective molecular phosphors are lacking at present. In particular, this route to 'brighten' low-energy molecular triplet excitons may permit singlet exciton fission sensitization of conventional silicon solar cells.

Colloidal quantum dots (QDs) are promising materials for use in solar cells, light-emitting diodes, lasers, and photodetectors, but the mechanism and length of exciton transport in QD materials is not well understood. We use time-resolved optical microscopy to spatially visualize exciton transport in CdSe/ZnCdS core/shell QD assemblies. We find that the exciton diffusion length, which exceeds 30 nm in some cases, can be tuned by adjusting the inorganic shell thickness and organic ligand length, offering a powerful strategy for controlling exciton movement. Moreover, we show experimentally and through kinetic Monte Carlo simulations that exciton diffusion in QD solids does not occur by a random-walk process; instead, energetic disorder within the inhomogeneously broadened ensemble causes the exciton diffusivity to decrease over time. These findings reveal new insights into exciton dynamics in disordered systems and demonstrate the flexibility of QD materials for photonic and optoelectronic applications.

A quantum-dot (QD) p-i-n heterojunction solar cell with an increased depletion region is demonstrated by depleting the QD layer from both the front and back junctions. Due to a combination of improved charged extraction and increased light absorption, a 120% increase in the short-circuit current is achieved compared with that of conventional ZnO/QD devices.

A method for studying the degradation of organic light emitting devices (OLEDs) in real time is described. Transparent OLEDs allow for the spatial correlation of cathode topographic images with optical images (transmission, photoluminescence, and electroluminescence) of the devices throughout the degradation process. In this study we focused on the evolution of nonemissive, “dark” spots during device operation. We conclude that the electroluminescent dark spots originate as nonconductive regions at the cathode/organic interface and expand or grow as a result of exposure to atmosphere. We propose a mechanism of dark spot growth involving aerobic oxidation of the cathode/organic interfacial region, leading to a highly resistive, carrier blocking interface at the dark spot locations. No initial defects on the cathode surface, which might be responsible for the formation of dark spots, were detected by atomic force microscopy. Structural changes, such as degradation of organic materials and the cathode surfac...

We demonstrate a scheme to pattern vacuum-deposited, small molecular weight organic light emitting diodes (OLEDs). Both the organic thin films and the metallic electrodes are patterned by shifting the position of a single shadow mask which accompanies the substrate throughout the deposition process. A full color, stacked OLED (SOLED) has been fabricated using this technique. The substrate movement relative to the mask was manually controlled to a root mean square accuracy of ±8 μm using a mask translating fixture. The performance of the patterned SOLED is comparable with that of devices fabricated by conventional, low tolerance methods. The limits to the display fill factor and resolution using this technique are discussed. The technique is generally applicable to all structures requiring precise patterning of vacuum-deposited thin films.

We present a study of optically pumped waveguide and microcavity lasers based on vacuum-deposited thin films of small molecular weight organic semiconductors. Lasing action in waveguide lasers is characterized by high output peak power (50 W), high differential quantum efficiency (70%), low lasing threshold (1 μJ/cm2), and long operational lifetime (>106 laser pulses at 100 times the threshold pump power). Microcavity laser characteristics include 3 W peak output power, 300 μJ/cm2 lasing threshold, and lifetimes of >106 pump laser pulses (operating at 6 times the threshold power). We demonstrate wavelength variability from 460 to 700 nm by changing the composition of the organic films. The confinement of excitations on the dopant molecules leads to quantum dot-like behavior such as high temperature stability of the lasing threshold, output power, and emission wavelength in the temperature range from 0 to 140 °C. The linewidth of laser emission from microcavity structures is found to be 0.2±0.1 A and is tr...

We describe a novel organic light-emitting device (OLED) in which color, grey scale and intensity can be independently achieved. The device consists of three separately contacted, red-, green-and blue-emitting OLEDs placed in a vertical stack and separated by transparent, conducting electrodes. The stacked OLED (or SOLED) is shown to efficiently generate a high-intensity optical output whose color is a linear superposition of spectra of the individual emitting elements in the device. Optical microcavity effects are found to distort the output spectrum in non-optimized SOLEDs. To our knowledge, the SOLED architecture provides the highest efficiency and resolution, as well as the simplest means for fabricating full-color displays based on small molecular weight organic materials demonstrated to date.

We review recent results in the field of organic light-emitting devices (OLED's), with particular attention to the application of organic light-emitting devices to ultra-lightweight, full color, flat-panel displays. We show that OLED brightness, efficiency, operating voltage, and lifetime is sufficient to compete with other flat-panel display technologies such as backlit liquid crystal displays. We describe a novel, tunable OLED consisting of vertically stacked, transparent light-emitting devices which can serve as a color-tunable element in high-resolution full-color display., In addition, the unique physical properties of organic thin films allow for flexible, conformable, or foldable displays which are unobtainable with conventional, inorganic semiconductor technologies.

Transport of nanoscale energy in the form of excitons is at the core of photosynthesis and the operation of a wide range of nanostructured optoelectronic devices such as solar cells, light-emitting diodes and excitonic transistors. Of particular importance is the relationship between exciton transport and nanoscale disorder, the defining characteristic of molecular and nanostructured materials. Here we report a spatial, temporal and spectral visualization of exciton transport in molecular crystals and disordered thin films. Using tetracene as an archetype molecular crystal, the imaging reveals that exciton transport occurs by random walk diffusion, with a transition to subdiffusion as excitons become trapped. By controlling the morphology of the thin film, we show that this transition to subdiffusive transport occurs at earlier times as disorder is increased. Our findings demonstrate that the mechanism of exciton transport depends strongly on the nanoscale morphology, which has wide implications for the design of excitonic materials and devices.

PbS colloidal quantum dot heterojunction solar cells have shown significant improvements in performance, mostly based on devices that use high-temperature annealed transition metal oxides to create rectifying junctions with quantum dot thin films. Here, we demonstrate a solar cell based on the heterojunction formed between PbS colloidal quantum dot layers and CdS thin films that are deposited via a solution process at 80 °C. The resultant device, employing a 1,2-ethanedithiol ligand exchange scheme, exhibits an average power conversion efficiency of 3.5%. Through a combination of thickness-dependent current density–voltage characteristics, optical modeling, and capacitance measurements, the combined diffusion length and depletion width in the PbS quantum dot layer is found to be approximately 170 nm.

Ultraviolet (UV) optical pumping followed by fluorescence wavelength downconversion of thin film organic light emitting materials deposited directly on the surface of Si p‐n junction diodes is found to be an accurate and rapid means to determine the film internal fluorescence efficiency. By measuring the photoresponse of the Si detectors in the UV, we find that the organic light emitting films of aluminum tris‐(8‐hydroxyquinoline) (Alq3), N,N′‐diphenyl‐N,N′‐bis‐(3‐methylphenyl)‐ 1,1′‐biphenyl‐4,4′‐diamine, and bis‐(8‐hydroxyquinaldine)‐chlorogallium (Gaq2′Cl), have internal fluorescence efficiencies of 0.30±0.05, 0.35±0.03, and 0.36±0.03, respectively. It has also been found that the organic films can be grown to a thickness which optimizes UV light conversion and can, at the same time, serve as antireflection coatings in the visible spectral region, thereby resulting in enhanced Si photodiode sensitivity extending from the UV to the infrared.