Organic light emitting devices with enhanced outcoupling via microlenses fabricated by imprint lithography

High efficiency white organic light emitting devices (WOLEDs) with optical outcoupling enhanced by hexagonal polymethylmethacrylate microlens arrays fabricated by imprint lithography on a glass substrate are demonstrated. Monte Carlo and finite difference time domain simulations of the emitted light are used to optimize the microlens design. The measured enhancement of light outcoupling and the angular dependence of the extracted light intensity are in agreement with the simulation. Using microlens arrays, we demonstrate a fluorescent/phosphorescent WOLED with a maximum external quantum efficiency of (14.3±0.3)%(14.3±0.3)% at 900 cd/m2900cd∕m2 and power efficiency of 21.6±0.5 lm/W21.6±0.5lm∕W at 220 cd/m2220cd∕m2. The electroluminescent spectra at viewing angles from normal to the substrate plane, to 60° off normal, remain almost unchanged, giving a color rendering index of 87.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87744/2/073106_1.pd

Thermal analysis of high intensity organic light-emitting diodes based on a transmission matrix approach

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98717/1/JApplPhys_110_124516.pd

White organic light-emitting device based on a compound fluorescent-phosphor-sensitized-fluorescent emission layer

The authors demonstrate a combination fluorescent and phosphor-sensitized-fluorescent white organic light-emitting device (WOLED), employing the conductive host material, 4,4′4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′1,1′-biphenyl, doped with the phosphorescent green, and the fluorescent red and blue emitters, fac-tris(2-phenylpyridinato-N,C2′N,C2′) iridium (III), 4-(dicyanomethylene)-2-t2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H4H-pyran, and 4,4′4,4′-bis (9-ethy-3-carbazolvinylene)-1-1′1-1′-biphenyl, respectively. Although two fluorescent dopants are employed along with only a single phosphor, this simple structure can, in principle, achieve 100% internal quantum efficiency. In the prototype, the phosphor-sensitized WOLED exhibits total external quantum and power efficiencies of ηext,tot = 13.1±0.5%ηext,tot=13.1±0.5% and ηp,tot = 20.2±0.7 lm/Wηp,tot=20.2±0.7lm∕W, respectively, at a luminance of 800 cd/m2800cd∕m2 with Commission Internationale de L’Eclairage chromaticity coordinates of (x = 0.38(x=0.38, y = 0.42y=0.42) and a color rendering index of 79.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87798/2/143516_1.pd

25.1: Invited Paper : Achieving Efficient Solid State Lighting Using Organic Light Emitting Devices

A significant challenge facing human kind in the 21st Century is how to address the ever decreasing supply of depletable and renewable energy. One approach to this problem is to decrease our usage. for this reason, considerable attention has been focused on more efficient means for room lighting which currently consumes approximately 20% of the total energy used. Organic light emitting devices (OLEDs) provide a unique opportunity to provide this high efficiency solid state lighting at very low cost. In this talk, I will discuss several strategies for achieving very high efficiency white light emission at high brightnesses for the next generation of efficient solid state lighting sources based on small molecular weight, vapor deposited OLED structures. Key to our approach is the use of electrophosphorescence as a means for converting all electrical into optical energy. We show that the highest luminance efficiencies can be obtained by a combination of fluorescence and phosphorescence in a unique OLED structure. Furthermore, the highest brightnesses are achieved (without a significant loss in power efficiency) by stacking several fluorescent/phosphorescent elements in a single OLED structure (called a SOLED), with each emitting element in the stack separated by a transparent charge generation layer. Prospects for OLEDs as the next practical generation of interior illumination sources will be reviewed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/92000/1/1.2785501.pd

Mechanisms for current-induced conductivity changes in a conducting polymer

A layer of polyethylene dioxythiophene:polystyrene sulfonic acid (PEDT:PSS) spun onto the surface of an inorganic semiconductor forms a highly asymmetric rectifying junction when a small current is applied and can be permanently open circuited with application of a high current density. This allows the polymer/semiconductor junction to function as a write-once-read-many-times memory element. We use x-ray photoelectron spectroscopy and temperature induced conductivity measurements to study the morphological and chemical changes responsible for the large current-induced conductivity changes. It is found that by applying a large current to the organic-inorganic semiconductor rectifying heterojunction structure Au/PEDT:PSS/SiAu∕PEDT:PSS∕Si, the ratio of PEDT+PEDT+ to PSS−PSS− near the interface changes due to phase segregation in the presence of both high electric field (>105 V/cm)(>105V∕cm) and temperature. This leads to a decrease in film conductivity by up to six orders of magnitude from its value in the conductive state.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87796/2/142109_1.pd

Semitransparent organic photovoltaic cells

We demonstrate semitransparent, small molecular weight organic solar cells employing a thin silver/indium tin oxide compound cathode with a maximum transmission of (60±6)%(60±6)% averaged over the visible spectral range and with a power conversion efficiency, ηp = (0.28±0.03)%ηp=(0.28±0.03)% under simulated, AM1.5G, 1 sun illumination. By increasing the Ag thickness, an average transmission of (26±3)%(26±3)% is achieved with ηp = (0.62±0.06)%ηp=(0.62±0.06)%, a value approximately half of that obtained for the same structure employing a conventional, reflective, and thick Ag cathode. A semitransparent tandem organic solar cell with ηp = (0.48±0.02)%ηp=(0.48±0.02)% and an average transmission of (44±4)%(44±4)% is also demonstrated. Semitransparent organic photovoltaic cells have potential uses as tinted and power-generating thin-film coatings on architectural surfaces, such as windows and walls. The use of a transparent top electrode also significantly simplifies the design of tandem cells, relaxing requirements for the placement of different absorbing materials at the maxima of optical fields introduced by reflective cathodes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87783/2/233502_1.pd

Method of fabricating multiwavelength infrared focal plane array detector

A multiwavelength local plane array infrared detector is included on a common substrate having formed on its top face a plurality of In.sub.x Ga.sub.1-x As (x.ltoreq.0.53) absorption layers, between each pair of which a plurality of InAs.sub.y P.sub.1-y (y.ltoreq.1) buffer layers are formed having substantially increasing lattice parameters, respectively, relative to said substrate, for preventing lattice mismatch dislocations from propagating through successive ones of the absorption layers of decreasing bandgap relative to said substrate, whereby a plurality of detectors for detecting different wavelengths of light for a given pixel are provided by removing material above given areas of successive ones of the absorption layers, which areas are doped to form a pn junction with the surrounding unexposed portions of associated absorption layers, respectively, with metal contacts being formed on a portion of each of the exposed areas, and on the bottom of the substrate for facilitating electrical connections thereto

Solvent-Annealed Crystalline Squaraine: PC 70 BM (1:6) Solar Cells

No AbstractPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/83478/1/184_ftp.pd

Improved power conversion efficiency of InP solar cells using organic window layers

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98676/1/ApplPhysLett_98_053504.pd

Stark Tuning of Donor Electron Spins in Silicon

We report Stark shift measurements for 121Sb donor electron spins in silicon using pulsed electron spin resonance. Interdigitated metal gates on top of a Sb-implanted 28Si epi-layer are used to apply electric fields. Two Stark effects are resolved: a decrease of the hyperfine coupling between electron and nuclear spins of the donor and a decrease in electron Zeeman g-factor. The hyperfine term prevails at X-band magnetic fields of 0.35T, while the g-factor term is expected to dominate at higher magnetic fields. A significant linear Stark effect is also resolved presumably arising from strain.Comment: 10 pages, 4 figures, to be submitted to PR