Lillian Sze Man Lam

Optical properties of copper phthalocyanine

Optical functions of copper phthalocyanine (CuPc) have been determined using spectroscopic ellipsometry in the spectral range from 1.55 to 4.1 eV. The samples have been prepared by evaporation on glass substrates. CuPc deposited on unheated and heated (100 °C) substrates have been investigated. The optical functions data were determined by point-to-point fit, as well as the conventional oscillator model and modified oscillator model. Atomic force microscopy images revealed considerable surface roughness of the films and hence surface roughness correction in determination of the optical functions has been performed. Good agreement with experimental spectra has been obtained with all three methods for the sample grown on unheated substrate. However, for the CuPc film deposited on heated substrate the conventional oscillator model failed to achieve good agreement with the experimental data. Also, there are indications that the isotropic model may not be applicable to CuPc films grown on heated substrate.

Spectroscopic ellipsometry of the optical functions of tris (8-hydroxyquinoline) aluminum (Alq3)

Abstract Optical functions of tris (8-hydroxyquinoline) aluminum (Alq 3 ) have been studied in the spectral range from 1.55 to 5 eV using spectroscopic ellipsometry. The samples have been deposited by thermal evaporation on glass substrates. Optical functions of Alq 3 deposited on unheated substrates and on substrates kept at 100 °C have been determined. The influence of atmosphere exposure to the optical properties of the Alq 3 films has been investigated by photoluminescence measurements, absorption measurements, and spectroscopic ellipsometry. It has been found that the substrate temperature during evaporation affects the environmental stability of the films.

Low-band-gap, sublimable rhenium(I) diimine complex for efficient bulk heterojunction photovoltaic devices

The use of chlorotricarbonyl rhenium(I) diimine complex as photosensitizing molecule in photovoltaic cells is demonstrated. Unlike other transition-metal-based sensitizers, the complex is sublimable and has low band gap. It also exhibits bipolar charge transport character with relatively high carrier mobilities on the order of 10−3 cm2 V−1 s−1. Multilayer heterojunction and bulk heterojunction devices with fullerene as the electron accepting molecule were prepared. For the bulk heterojunction devices, the fill factor and power conversion efficiency under simulated solar light illumination were 0.56 and 0.48%, respectively. Atomic force microscopic images showed that the complex dispersed evenly with fullerene molecules in solid state.

Spectroscopic ellipsometry of metal phthalocyanine thin films

Optical functions of cobalt phthalocyanine, nickel phthalocyanine (NiPc), and iron phthalocyanine (FePc) have been determined by use of spectroscopic ellipsometry in the spectral range 1.55-4.1 eV (300-800 nm). The samples were prepared by evaporation onto glass and silicon substrates. The optical functions were determined by point-to-point fit. Absorption spectra were also measured. The index-of-refraction data for NiPc and FePc are reported for the first time to our knowledge. Good agreement with the experimental spectra was obtained for all three materials.

Photosensitizing properties of some rhenium(I) tricarbonyl diimine complexes

The title complexes were doped into polycarbonate matrices together with triphenylamine, a hole-transport compound, in order to study their photosensitizing properties. Complexes based on 1 exhibited strong absorption in the visible spectrum due to the presence of a metal-ligand charge-transfer transition and were able to enhance the sensitivity in this region; the best enhancement was found when Ar is a p-flurophenyl group.

Study of charge generation process in some photosensitizing rhenium diimine complexes

Abstract The photosensitizing properties of different polymer composites based on triphenylamine, polycarbonate, and chlorotricarbonyl rhenium(I) diimine complexes with various substituents are reported. The quantum efficiency increases with increasing applied electric field, following closely the Onsager theory with a thermalization distance of 13–18 A. The primary yield shows a great dependence on the structure of the complexes, with the fluoro-substituted ligand exhibit the highest primary yield of 0.80, which is one order of magnitude higher than those of other complexes. When fabricated into a photovoltaic device, the quantum efficiency and fill factor were measured to be 2% and 0.18, respectively.

Optical properties of doped polycarbonate layers

Optical properties of polycarbonate layers doped with triphenylamine and chlorotricarbonyl diimine rhenium complex have been investigated. The samples have been prepared by spin coating on quartz glass and indium tin oxide (ITO) coated glass substrates. Root mean square surface roughness of the films obtained by atomic force microscopy is low (∼3 A). Optical functions of ITO and doped polymer layer for two different dopant concentrations have been determined by spectroscopic ellipsometry in the spectral range from 1.55 to 4.1 eV. The absorption spectra of the chlorotricarbonyl diimine rhenium complex dissolved in chloroform and toluene have also been measured.

A novel type of metal-containing polyimides based on tricarbonylrhenium(I) diimine complexes

A series of rhenium containing polyimides was prepared by the polymerization between a tricarbonylrhenium diimine complex with various aromatic dianhydrides. Electronic absorption spectra of the polymers showed a very strong intramolecular charge transfer transition which obscures the transfer transition band due to the rhenium complex in the visible region. A large photocurrent response was also observed when the polyimide was irradiated at 514 nm.

Evolution of optical properties of tris (8-hydroxyquinoline) aluminum (Alq3) with atmosphere exposure

Tris (8-hydroxyquinoline) aluminum (Alq3) represents a material of significant interest for electron transport and/or light emitting layer applications in organic light emitting diodes (OLEDs). In spite of advances in Alq3 based devices, the knowledge and understanding of the optical properties of Alq3 and its chemical and environmental stability is still limited. With the reports of decreased turn-on voltage and increased efficiency of OLEDs, the issues of lifetime and stability of those devices are attracting increasing attention. The degradation of Alq3 based OLEDs and dark spots formation and growth have been intensively studied. The studies on degradation of optical properties of Alq3 itself remain scarce. We have investigated effects of atmosphere exposure to properties of tris (8-hydroxyquinoline) aluminum (Alq3) thin films by photoluminescence (PL) and absorption measurements. Alq3 films were evaportated on glass substrates at different temperatures. The influence of annealing to the environmental stability of the films has also been investigated. It has been found that deposition at higher substrate temperature and annealing of the samples deposited at room temperature yields improvement in environmental stability of the films, i.e. less decrease of the PL intensity over time with atmosphere exposure, as well as increased PL intensity. To investigate further effects of the air exposure, films deposited at room temprature were stored for four days in air, nitrogen, and oxygen. No decrease in PL intensity has been found for storage in nitrogen, while decrease for the film stored in oxygen was smaller than that for film stored in air, indicating that both humidity and oxygen play a role in PL intensity decrease in Alq3 thin films.

Phthalocyanine based Schottky solar cells

Phthalocyanine (Pc) materials are commonly used in organic solar cells. Four different phthalocyanines, nickel phthalocyanine (NiPc), copper phthalocyanine (CuPc), iron phthalocyanine (FePc), and cobalt phthalocyanine (CoPc) have been investigated for organic solar cell applications. The devices consisted of indium tin oxide (ITO) coated lass substrate, Pc layer, and aluminum (al) electrode. It has been found that ITO/CuPc/Al Schottky cell exhibits the best performance. To investigate the influence of the active layer thickness on the cell performance, cells with several different thicknesses were fabricated and optimal value was found. Schottky cell exhibits optimal performance with one ohmic and one barrier contact. However, it is suspected that ITO/CuPc contact is not ohmic. Therefore, we have investigated various ITO surface treatments for improving the performance of CuPc based Schottky solar cell. We have found that cell on ITO treated with HCl and UV-ozone exhibits the best performance. AM1 power conversion efficiency can be improved by 30% compared to cell made with untreated ITO substrate. To improve power conversion efficiency, double or multiplayer structure are required, and it is expected that suitable ITO treatments for those devices will further improve their performance by improving the contact between ITO and phthalocyanine layer.