We report on the performance of green phosphorescent organic
light-emitting diodes (OLEDs) based on the well-known
device structure of a hole-transport layer, an emissive layer with host 4,4'-di(carbazol-9-yl)-biphenyl [CBP] and the
green phosphor emitter
<i>fac</i> tris(2-phenylpyridinato-N,C<sup>2,</sup>) iridium [Ir(ppy)<sub>3</sub>], a hole-blocking layer of 2,9-dimethyl-4,7-
diphenyl-1,10-phenanthroline [BCP] and and
tris-(8-hydroxyquinolinato-N,O) aluminum [Alq<sub>3</sub>] as an
layer. Using spin-coated hole-injection/transport layers with increasing ionization potentials and decreasing hole
mobilities, external quantum efficiencies of up to 18.1% at 100 cd/m<sup>2</sup> were measured in such devices. Furthermore, by
removing the electron-transport layer of Alq<sub>3</sub> and increasing the thickness of BCP, devices with efficiencies of 21.2%
and 72 cd/A at 100 cd/m<sup>2</sup> were obtained. Achieving such high efficiencies with a simplified hybrid structure in which
one layer is processed from solution and only two other organic layers are deposited from the vapor phase is desirable
for the fabrication of low-cost OLEDs.
Methods for scalable output voltage and encapsulation of organic photovoltaic cells are addressed in this paper. To
obtain scalable output voltages, integrated photovoltaic modules comprised of a bulk heterojunction of poly(3-
hexylthiophene) (P3HT) and a soluble C<sub>70</sub> derivative, [6,6]-phenyl C<sub>71</sub> butyric acid methyl ester (PCBM-70), were
fabricated. Power conversion efficiency of individual P3HT/PCBM-70 cells was estimated to be 4.1 % for AM1.5 G
illumination. Modules of one to four cells connected in series produced open-circuit voltages V<sub>OC</sub> that linearly depend on
the number of cells <i>N</i> as <i>V<sub>OC</sub></i> = <i>N</i> × 0.621 V with a nearly constant short-circuit current of 1.4 ± 0.1 mA. Separately,
shelf lifetimes of more than one year were achieved for pentacene/C<sub>60</sub> solar cells by encapsulation with a 200-nm-thick
layer of Al<sub>2</sub>O<sub>3</sub> deposited by atomic layer deposition (ALD). In addition, the ALD process improved the open-circuit
voltage and power conversion efficiency of the solar cells by thermal annealing that occurs during the process.
We report on high performance field-effect transistors fabricated with pentacene as an active material and Al<sub>2</sub>O<sub>3</sub> as a gate
dielectric material grown by atomic layer deposition (ALD). These transistors were operated in enhancement mode with
a zero turn-on voltage and exhibited a low threshold voltage (< -10 V) as well as a low subthreshold slope (< 1
V/decade) and an on/off current ratio larger than 10<sup>6</sup>. Hole mobility values of 1.5 ± 0.2 cm<sup>2</sup>/Vs were obtained when
using heavily n-doped silicon (n<sup>+</sup>-Si) as gate electrodes and substrates. Atomic force microscopy (AFM) images of
pentacene films on Al<sub>2</sub>O<sub>3</sub> treated with octadecyltrichlorosilane (OTS) revealed well-ordered island formation, and X-ray
diffraction patterns showed characteristics of a "thin film" phase. Compared with thermally-grown SiO<sub>2</sub>, Al<sub>2</sub>O<sub>3</sub> gate
insulators have lower surface trap density and higher capacitance density, to which the high performance of pentacene
field-effect transistors can be attributed.
We report on highly efficient organic solar cells based on polycrystalline thin films of pentacene, a material that has been widely investigated for p-type transport layers in organic field-effect transistors (OFET). The spectral measurement of external quantum efficiencies (EQE) shows that these high efficiencies are due to the efficient light-harvesting occurring throughout the visible spectrum. In particular, the peak EQE of 69% has been measured at a wavelength of 668 nm where most of the excitons are generated inside the pentacene layer. This suggests that the polycrystalline nature of pentacene films leading to high field-effect mobilities in OFET also results in relatively large exciton diffusion lengths which are desired in multilayer organic solar cells. In an effort to understand these devices, we model the external quantum efficiencies as a function of wavelength based on the exciton diffusion model using the complex indices (<i>n, k</i>) of participating materials. This study provides information on the correlation of optical properties of photoactive materials to the spectral responses and allows one to estimate exciton diffusion lengths. Based on this information, we discuss the optimization of layer structures that can lead to maximization of the photocurrent under standard illumination condition.
Significant progress has been made in the area of p-type organic field-effect transistors while progress in developing n-type materials and devices has been comparatively lacking, a limiting factor in the pursuit to develop complementary organic electronic circuits. Given the need for n-type organic semiconductors we have carried out studies using two different fullerene molecules, C<sub>60</sub> and C<sub>70</sub>. Here, we report mobilities for C<sub>60</sub> ranging from 0.02 cm<sup>2</sup>/Vs up to 0.65 cm<sup>2</sup>/Vs (depending on channel length), and mobilities from 0.003 cm<sup>2</sup>/Vs up to 0.066 cm<sup>2</sup>/Vs for C<sub>70</sub>. All devices were fabricated with organic films deposited under high vacuum but tested at ambient pressures under nitrogen.
Organic photovoltaic cells exhibiting an ideal diode behavior with large fill factor (FF) are presented. It is demonstrated that the current-voltage characteristics can be well described using the equivalent circuit model that is also used for inorganic solar cells. Resistances associated with the cells and other diode parameters are extracted by fitting the experimental electrical characteristics using the equivalent circuit model. The effects of these parameters on FF are quantitatively described. Changes in these parameters under different illumination conditions are presented and compared to those occurring in inorganic pn-junction solar cells.
A series of soluble arylamine-based hole transporting molecules with a fluorene core and with various ionization potentials have been synthesized. The transport properties of these molecules doped into polystyrene have been measured by time-of-flight experiments and compared to those of analogous compounds with a biphenyl core (TPD). Reorganization energies between the neutral molecules and their cations have been calculated using density functional theory. The effects of bond length and geometry relaxations on the overall reorganization energy in these two classes of molecules are discussed. Molecules from both classes have been doped into polystyrene and used as hole-transport layers (HTLs) in multi-layer light-emitting diodes with the structure ITO/HTL/AlQ<sub>3</sub>/Mg:Ag [ITO = indium tin oxide, AlQ<sub>3</sub> = tris(8-hydroxyquinolinato)aluminum]. The electroluminescent properties and lifetime measurements at constant current have been evaluated. Significant variations in lifetime when using different substituents have been observed.
We report on the photovoltaic properties of solar cells containing a new discotic liquid crystalline material (DL-CuPc) based on copper phthalocyanine. In addition to being soluble, these materials can self-organize into highly ordered structures which can lead to good transport properties that can potentially be superior to those of amorphous materials. Increase in short-circuit current density and fill factor was obtained by thermal annealing of spin-coated DL-CuPc layer in bi-layer solar cells based on junction between DL-CuPc and C<sub>60</sub>. These improvements are explained by change in structure and morphology upon thermal annealing.
Hole mobilities in substituted N, N'-bis-(m-tolyl)-N-N'-diphenyl-1,1'-biphenyl-4,4'-diamine (TPD) derivatives doped in polystyrene (PS), were analyzed by the time-of-flight technique to determine the effect of altering the geometric and electronic structure of TPD. Data were collected as a function of applied field and temperature to yield the energetic and positional disorder parameters defined in the disorder formalism. The impact of the molecular dipole moment on transport properties was also evaluated. The larger molecular dipole moments of the derivatives lead to an increase in the energetic disorder, which contributes to their lower mobilities. However, the dipolar disorder contribution was found to account only partially for the large differences in mobility.
A series of soluble arylamine-based hole transporting polymers with glass transition temperatures in the range of 97-108 degree(s)C have been synthesized. The synthetic methodology allows substitution of the aryl groups on the amine with electron-withdrawing and electron-donating moieties, which permits tuning of the redox potential of the polymer. The TPD-based monomers have been copolymerized with cinnamate-based moieties to obtain photo-crosslinkable polymers. These polymers have been used as hole-transport layers (HTLs) in multi-layer light-emitting diodes ITO/HTL/AlQ<SUB>3</SUB>/Mg:Ag [ITO=indium tin oxide, AlQ<SUB>3</SUB>=tris(8-hydroxyquinolinato)aluminum]. The maximum external quantum efficiency of the device increases as the redox potential of the HTL is increased. A fluorinated hole- transport polymer with a relatively high oxidation potential (390 mV vs ferrocenium/ferrocene) yielded the device with the highest external quantum efficiency and the longest lifetime under constant current operation. UV cross-linking was optimized to obtain an insoluble hole-transport layer with stable performance. Processing of these materials is compatible with a standard mask aligner used for photolithography. Electroluminescent devices have also been fabricated by spinning a blend of polystyrene and AlQ<SUB>3</SUB> on top of the crosslinked hole-transport layer.
SC336: Organic Light-Emitting Displays: Materials, Devices, and Applications
This course will cover the multidisciplinary aspects of the emerging display technology based on organic light-emitting devices (OLEDs) fabricated from molecules and polymers. It will review the basic concepts of polymer chemistry necessary to understand current organic photonic materials, and describe the physical processes of charge injection, charge transport, and light-emission in organic amorphous thin films. Then, different classes of organic electroluminescent devices and their properties will be described. The state-of-the-art in device performance and the properties of different display technologies will be discussed. Projections for future performance levels will be made based on the current theoretical understanding of organic materials.