Electrical doping of organic semiconductors increases conductivity and reduces injection barriers from electrode materials, both of which effects can improve the performance of organic light-emitting diodes (OLEDs). However, the low electron affinities of typical OLED electron-transport materials make the identification of suitable n-dopants particularly challenging; electropositive metals such as the alkali metals are not easily handled and form monoatomic ions that are rather mobile in host materials, whereas molecular dopants that operate as simple one-electron reductants must have low ionization energies, which leads to severe air sensitivity. This presentation will discuss approaches to circumventing this issue by coupling electron transfer to other chemical reactivity. In particular, dimers formed by certain highly reducing organometallic sandwich compounds and organic radicals can be handled in air, yet have effective reducing potentials, corresponding to formation of the corresponding monomeric cations and contribution of two electrons to the semiconductor, of ca. –2.0 V vs. ferrocene. These values fall a little short of what is required for typical OLED materials; approaches to further extending the doping reach of these dimers will be described. One such approach involving photoirradiation of a dimer:semiconductor blend leads to metastable doping of a material with a redox potential of –2.24 V, which allows the fabrication of efficient OLEDs in which even high-workfunction electrodes, such as indium tin oxide, can be used as electron-injection contacts.
We report on the electronic structure of freshly evaporated and air-exposed Molybdenum tri-oxide (MoO3) and the energy-level alignment between this compound and a hole-transport material [e.g., N,N′-diphenyl-N,N′-bis (1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD)]. Ultraviolet and inverse photoelectron spectroscopy show that freshly evaporated MoO3 exhibits deep-lying electronic states with an electron affinity (EA) of 6.7 eV and ionization energy (IE) of 9.7 eV. Air exposure reduces EA and IE by ∼1 eV, to 5.5 and 8.6 eV, respectively, but does not affect the hole-injection efficiency, which is confirmed by device studies. Thus, MoO3 can be applied in low-vacuum environment, which is particularly important for low-cost manufacturing processes. Our findings of the energy-level alignment between MoO3 and α-NPD also leads to a revised interpretation of the charge-injection mechanism, whereby the hole-injection corresponds to an electron extraction from the organic highest-occupied molecular orbital (HOMO) level via the MoO3 conduction band.
The energy level alignment at interfaces between poly(9,9'-dioctylfluorene) (F8), poly(9,9'-dioctylfluorene-<i>co</i>-bis-<i>N</i>,<i>N</i>'-(4-butylphenyl)-diphenylamine) (TFB) and poly(9,9'-dioctylfluorene-<i>co</i>-bis-<i>N</i>,<i>N</i>'-(4-butylphenyl)-bis-<i>N</i>,<i>N</i>'-phenyl-1,4-phenylenediamine) (PFB) and substrates with work function ranging from 4.3 eV to 5.1 eV is investigated via ultra-violet photoemission spectroscopy. Vacuum level alignment with flat bands away from the interface is found when the interface hole barrier is 0.6 eV or larger. Band bending that moves the filled states away from the Fermi level occurs when the hole barrier is smaller than 0.4 eV. This is presumably due to the accumulation of excess interface charges on the polymer side when the interfacial barrier is small. The resulting field shifts the polymer levels in a way that limits charge penetration in the bulk of the film. We also study metal-on-polymer interfaces. Different metals exhibit different growth modes. While Pt shows complete layer-by-layer type of growth, Al shows island type of growth. Current-voltage measurement shows the presence of hole traps in the Au-on top-contact device, suggesting diffusion of small Au clusters into the polymer film. Furthermore, metal-on-polymer interfaces frequently present different interface energetics than their polymer-on-metal counterpart. e.g. a 0.3 - 0.4 eV higher hole injection barrier for Pt-on-TFB than TFB on Pt.
Ultra-violet and X-ray photoemission spectroscopy and current- voltage measurements were used to investigate the fundamental mechanisms responsible for the improvement of hole injection between modified indium-tin-oxide (ITO) surfaces and the hole- transport layer (HTL) of an organic light emitting diode. Two ITO surface modification techniques were investigated: oxygen- plasma treatment and deposition of an ultra-thin organic interlayer between the ITO and the HTL. We demonstrate that the improvement in injection is due to an increase in surface work function of ITO mediated by the presence of an oxygen radical in the first case, and to the presence of an intermediate energy level between the ITO Fermi level and the HTL highest occupied molecular orbital in the second.
The electronic structure and chemical properties of organic/organic and organic/metal interfaces involving molecular semiconductors are investigated via photoemission spectroscopy. The alignment of electronic levels, electron and hole injection barriers, and interface dipoles are measured for each interface. Chemical reactions and interdiffusion dominate metal-on-organic contacts, whereas organic-on-metal and organic/organic interfaces are more abrupt. The rule of vacuum level alignment, expected to hold for organic molecular interfaces, breaks down for all metal/organic and several organic/organic interfaces, showing that electronic gap states and other interface effects cannot be neglected at these interfaces.
Active matrix displays that are lightweight, rugged and bendable are a key DoD need for applications ranging from panoramic displays for aircraft cockpits to foldable maps. To achieve such displays compatible substrates, TFT backplanes, and light valve/light emissive materials systems must be developed. Advances toward this goal achieved in the joint Penn State/Princeton Display Program are discussed.
In an effort to raise the efficiency and speedup the rate of technology transfer from its university funded research programs, DARPA has ben encouraging the formation of industry/university teams to accelerate the development of backplane thin-film electronics for AMLCD displays. The effort among its university researchers has been carried forward through voluntary participation in a series of workshops cosponsored by DARPA and the Electric Power Research Institute. Evidence of the effectiveness of the teaming arrangement is shown by the many collaborations entered by the display industry participants.
A solution to the thin film silicon transistor gate metallization problem in active matrix liquid crystal displays is demonstrated in the form of a self-passivation process for copper. Bottom-level copper (Cu) lines are passivated by a self-aligned chromium oxide encapsulation formed by surface segregation of chromium (Cr) from dilute Cu<SUB>1-x</SUB>Cr<SUB>x</SUB> alloys at 400 degrees C. The encapsulation is an efficient barrier for Cu diffusion into the SiN<SUB>x</SUB> gate insulator during the plasma deposition and transistor processing, and solves the problems of oxidation and adhesion to the glass substrate without introducing additional mask steps into the manufacturing process. Gate line resistivities of 4.5 (mu) (Omega) cm are obtained. The performance of self-passivated Cu-gate thin film transistors is comparable to that of transistors with refractory metal gates.