X. Y. Hou

Interfacial electronic structures in an organic light-emitting diode

Direct measurements of electronic properties have been made for the metal/organic and organic/organic interfaces in a multilayer organic light-emitting diode (LED) using ultraviolet photoemission spectroscopy. The device configuration considered is indium–tin oxide (ITO)/copper phthalocyanine (CuPc)/N,N′-bis-(1-naphyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamne (NPB)/8-hydroxyquinoline aluminum (Alq)/Mg. For the material interfaces considered here, our result indicates that the traditional concept of vacuum-level alignment, though not valid for metal/organic interfaces, still holds at organic/organic interfaces. This implies that little charge transfer occurs at the interfaces due to the small interaction between organic molecules. The largest band offsets are observed between the lowest unoccupied state levels of the organic molecules. Based on the directly measured energy-level alignments, a model is proposed to explain the improved efficiency of multilayer organic LEDs, as compared to those with a single ...

Energy level alignment at Alq/metal interfaces

The energy level alignment for both Mg/8-hydroxyquinoline aluminum (Alq) and Au/Alq interfaces has been determined by the ultraviolet photoemission measurements. For both interfaces, the difference between the Fermi level and the low-energy edge of the highest occupied molecular orbital (HOMO) is around 1.7 eV. This implies that the Fermi level with respect to the HOMO edge of Alq is independent of the work function of Mg and Au despite a large difference in the metal work function. A Fermi level alignment model is proposed, invoking a charge transfer between the metal and Alq and the formation of a dipolar layer at the metal/Alq interface.

Wide-band “black silicon” based on porous silicon

Solar cells and optical detection devices often incorporate antireflective surfaces to reduce undesired reflection and enhance optical absorption. This letter reports a “black silicon” structure of antireflective porous silicon fabricated by using electrochemical etching. The sample has a gradient-index multilayer structure, i.e., the refraction indices of the structure increase from the top (near the air) to the bottom (near the Si substrate). Reflectance below 5% is obtained over a broad wave number range (3000–28000cm−1) and the depression mechanism of the optical reflectance is analyzed by simulating the structure with the transfer matrix method. The simulated result fits the measured spectra well.

Study of the Raman peak shift and the linewidth of light‐emitting porous silicon

The correlation between the Raman peak shift and the linewidth of porous silicon is studied. The experimental result does not fit with the relationship predicted by the phonon confinement model. By taking into account both the phonon confinement and the effect of strain, the calculated Raman line shape coincides fairly well with the measured spectrum. The built‐in strain of porous silicon varies with the porosity of the sample and is on the order of 10−3.

Modification of the hole injection barrier in organic light-emitting devices studied by ultraviolet photoelectron spectroscopy

Ultraviolet photoelectron spectroscopy has been applied to the investigation of modified hole injection barriers in organic light-emitting devices (OLEDs). Different from those reported previously, the indium tin oxide (ITO) surface treated in situ by oxygen plasma possesses a work function of 5.2 eV, and the organic ITO interface thereafter formed shows a 0.5 eV smaller hole injection barrier compared to that on untreated ITO. Insertion of an ultrathin SiO2 layer between the organic and ITO results in a similar reduction of the barrier. This indicates that improved hole injection favors efficient operation of OLEDs, as manifested by enhanced efficiency by the SiO2 insertion.

Positive and negative fluorescent imaging induced by naphthalimide polymers

Two novel polymers with naphthalimide pendant groups have been prepared. In poly(2-methylacrylic acid 2-[6-(4-methylpiperazin-1-yl)-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-2-yl]ethyl ester-co-methyl methacrylate) (copolymer 1), an alkylated tertiary amine acts as an electron donor and quenches the fluorescence of the naphthalimide fluorophore via the photo-induced electron transfer (PET) process. Protonation of the alkylated tertiary amine by an acid generated by a photoacid generator (PAG) can switch off the PET path and the fluorescence of the naphthalimide fluorophore would be recovered and enhanced. For poly(2-methylacrylic acid 2-[6-(4-methylpiperazin-1-yl)-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-2-yl]ethyl ester) (polymer 3), protonation both of the aromatic amine and the alkylated amine by the acids generated from may result in significant fluorescence quenching and alter the fluorophore. When protonation processes occur, the fluorescence enhancement (×2) of copolymer 1 or the fluorescence quenching (×15) of polymer 3 would be applied to generate positive or negative fluorescent patterned images on their net films.

Degradation of small-molecule organic solar cells

Small-molecule organic solar cells with a structure of indium tin oxide (ITO)\tris-8-hydroxy-quinolinato aluminum (Alq3) (2nm)\fullerene (C60) (40nm)\copper phthalocyanine (CuPc) (32nm)\Au (40nm) were fabricated. The shelf lifetime of unencapsulated devices was over 1500h, and the power conversion efficiency reached 0.76% under AM1.5G (air mass 1.5 global) 75mW∕cm2. The long lifetime was attributed to the inverted structure compared to the conventional ITO\CuPc\C60\buffer\Al structure since the former could effectively protect C60 from the diffusion of oxygen and modify interfacial electrical properties. The introduction of a 2nm Alq3 layer into the cells enhanced the power conversion efficiency by more than 20 times. The presence of the thin Alq3 film on the ITO substrate lowered the substrate work function and hence increased the electric field in the organic layers, which was beneficial to the collection of free carriers. The reasons for the degradation of such kind of organic solar cells are analyzed ...

ENHANCED HOLE INJECTION IN A BILAYER VACUUM-DEPOSITED ORGANIC LIGHT-EMITTING DEVICE USING A P-TYPE DOPED SILICON ANODE

We report the fabrication of a vacuum-deposited light-emitting device which emits light from its top surface through an Al cathode using p-type doped silicon as the anode material. Enhanced hole injection is clearly demonstrated from the p-Si anode as compared to the indium–tin–oxide (ITO) anode. The mechanisms of hole injection from both the p-Si and ITO anodes into the organic layer are investigated and a possible model based on anode surface band bending is proposed. During the operation of the organic light-emitting device, the surface band bending of the anode plays a very important role in modifying the interfacial barrier height between the anode and the organic layer.

1,8-Naphthalimides for non-doping OLEDs: the tunable emission color from blue, green to red

Substitution at the 4-position of 1,8-naphthalimide with electron-donating groups can increase fluorescent quantum yields and change emissive wavelengths from blue to red. Based on this molecular design concept, novel naphthalimide derivatives containing Schiff base moiety were prepared by condensing 4-hydrazino-1,8-naphthalimides with the aldehydes. Amino conjugation between the 4-amino-1,8-naphthalimide and the substituted moiety resulted in red shift of the absorption and fluorescence maximum wavelengths in the acetonitrile solution and in the net solid film. In the meantime, concentration-quenching effect of fluorescence for common luminescent materials was avoided. Some of these dyes emit brilliant red fluorescence in solid films and were used as non-doping emissive materials to fabricate electroluminescence devices. Based on these results, guidelines for the molecular design of non-doping red emissive materials for OLED applications are presented in this paper.

Buffer-layer-induced barrier reduction: Role of tunneling in organic light-emitting devices

Based on the WKB approximation of the tunneling model, we calculate the J–V characteristics of organic light-emitting devices (OLEDs) having buffer layers of different thickness. The results show how the insertion of a buffer layer with proper thickness lowers the OLED turn-on voltage. Further calculation suggests some parameters, such as the resistivity ratio and the position of the conduction band minimum of the buffer layer relative to the lowest unoccupied molecular orbital of the organic layer, are important in selecting a buffer material. A quantitative estimation of the optimal buffer layer thickness is also presented to serve as a guide to device design. The model is validated by comparison of its predictions to experimental results.