C.Y. Kwong

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.

Indium-tin-oxide surface treatments: Influence on the performance of CuPc/C60 solar cells

In this work, we investigate the influence of different indium tin oxide (ITO) surface treatments on the performance of organic solar cells. ITO substrates have been characterized by Hall measurements, Seebeck coefficient measurements, surface sheet resistance measurements, and surface probe microscopy. Single layer (ITO/copper phthalocyanine (CuPc)/Al) and double layer (ITO/CuPc/C60/Al) solar cells were fabricated. It was found that the surface treatments changed the parameters of the ITO (work function, carrier concentration, sheet resistance, surface roughness) and significantly influenced the solar cell performance. The AM1 power conversion efficiency of the ITO/CuPc/C60/Al cell with optimal surface treatment (∼0.1%) is 1 order of magnitude larger than the power conversion efficiency of the solar cell fabricated on untreated ITO substrate (∼0.01%). The AM1 power conversion efficiency can be further enhanced with improved device structures. Obtained AM1 power conversion efficiency for a three layer structure ITO/CuPc/CuPc:C60 (1:1)/C60/Al was measured to be 0.16%.In this work, we investigate the influence of different indium tin oxide (ITO) surface treatments on the performance of organic solar cells. ITO substrates have been characterized by Hall measurements, Seebeck coefficient measurements, surface sheet resistance measurements, and surface probe microscopy. Single layer (ITO/copper phthalocyanine (CuPc)/Al) and double layer (ITO/CuPc/C60/Al) solar cells were fabricated. It was found that the surface treatments changed the parameters of the ITO (work function, carrier concentration, sheet resistance, surface roughness) and significantly influenced the solar cell performance. The AM1 power conversion efficiency of the ITO/CuPc/C60/Al cell with optimal surface treatment (∼0.1%) is 1 order of magnitude larger than the power conversion efficiency of the solar cell fabricated on untreated ITO substrate (∼0.01%). The AM1 power conversion efficiency can be further enhanced with improved device structures. Obtained AM1 power conversion efficiency for a three layer st...

Response to “Comment on ‘Change of the emission spectra in organic light-emitting diodes by layer thickness modification’” [Appl. Phys. Lett. 86, 186101 (2005)]

In our recent paper, 1 we presented the study of the emission spectra of triss8-hydroxyquinolined aluminum sAlqd based organic light-emitting diodes sOLEDsd as a function of organic layer thickness. Both calculations and experimental results were presented. The discrepancy between the calculated and measured emission spectra was noted, and possible reasons were discussed. Finally, it was concluded that further study is needed to conclusively establish whether any other phenomena in addition to simple interference play a role in the obtained results. In his recent comment, 2 Shore claimed that our experimental data can be entirely explained by simple interference phenomena, and presented calculations which qualitatively “reproduce” basic behavior of our devices. However, the calculations in our work fFig. 1sbdg also show similar behavior as those presented in the comment by Shore sFig. 2d. Since Shore 2 changed only the Alq layer thickness, while we considered devices with different N, N8-disnaphthalene-1-yl d-N,N8-diphenylbenzidinesNPBd and Alq thicknesses, direct comparison can only be made for the device with 65 nm NPB and 139 nm Alq. It is obvious from Fig. 1sbd in our letter and Fig. 2 in the recent comment 2 that both calculations show essentially the same features. Yet, quantitative agreement between the experimental data and the calculated results is lacking. Since the thicknesses of organic layers were verified by step profiler and spectroscopic ellipsometry after deposition, thickness errors in the fabricated devices are unlikely. Another possible reason for the discrepancy, as correctly identified by Shore, 2 is different emission region thickness. Figure 1 illustrates the influence of the assumed emission region thickness on the calculated electroluminescence sELd spectra. Further studies with confined emitting layers of known thickness are in progress to experimentally establish the influence of the emitting layer thickness. We would also like to point out that, regardless of the assumed emission layer thickness, the calculated spectra always show two peaks, while some of the experimental spectra showed clear shoulders in addition to two peaks. Comparison between the calculated and the experimental spectra for the 65/153s65 nm NPB and 153 nm Alq d device is shown in Fig. 2sad. It can be observed that the calculated spectrum, exhibiting a two-peak structure, does not describe the experimental spectrum well. The calculation for the same

Optical functions of tris (8-hydroxyquinoline) aluminum (Alq3)

In this work we report spectroscopic ellipsometry study of the optical functions of tris (8-hydroxyquinoline) aluminum (Alq3) thin films on glass substrates in the spectral range from 1.55 eV to 5 eV. Optical functions of Alq3 deposited on unheated substrates and on substrates kept at 100 degree(s)C have been determined. The influence of atmosphere exposure to the optical properties has been investigated. Studies on degradation of OLEDs usually focus on changes due to crystallization of Alq3 or cathode degradation during operation (in atmosphere or encapsulated). In this work we show that exposure to atmosphere of an Alq3 layer even without deposition of metal cathode results in change of the optical properties of the layer. It has been found that deposition at higher substrate 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. Evolution of the optical functions obtained by spectroscopic ellipsometry, absorption, and photoluminescence spectra are presented and discussed.

Angular dependence of the emission from low Q-factor organic microcavity light emitting diodes

Microcavity light emitting diodes based on materials with broad emission spectrum, such as tris (8-hydroxyquinoline) aluminum (Alq3), typically exhibit a significant blue shift of the emission wavelength with increasing viewing angle. In this work, we investigate the influence of the organic layer thickness on the angular dependence of the emission spectrum in low Q-factor microcavity OLEDs. We demonstrate that for different organic layer thicknesses qualitatively different emission wavelength dependences on the viewing angle can be obtained in microcavity devices consisting of two organic layers, N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (NPB) as a hole transport layer and Alq3 as emitting layer. The devices with different organic layer thickness were characterized by electroluminescence, photoluminescence, reflectance, and transmission measurements and results compared with the model.