P.C. Chui

Poly(3-hexylthiophene):TiO2 nanocomposites for solar cell applications

The properties of organic/inorganic poly(3-hexylthiophene) (P3HT):TiO2 nanocomposite films and nanocomposite based solar cells as a function of TiO2 concentration and the solvent used for the film fabrication were studied. For low nanoparticle concentration (20?30%) the device performance was worse compared to pure P3HT, while for nanoparticle concentration of 50% and 60% significant improvements were obtained. P3HT photoluminescence quenching in 600?800?nm spectral region changes by a factor of two for the increase in TiO2 concentration from 20% to 60%, while the AM1 power conversion efficiency increases times. Photoluminescence quenching and solar cell efficiency were found to be strongly dependent not only on nanoparticle concentration but also on the solvent used for spin-coating. The changes in the film and device properties were explained by the change in the film morphology. For optimal fabrication conditions, external quantum efficiency up to 15% and AM1 power conversion efficiency of 0.42% were obtained.

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...

Comprehensive analysis and optimal design of top-emitting organic light-emitting devices

int 0 of 0.25, the maximum zero-degree luminance and EL efficiency can be achieved by locating the emitters around the first antinode of the microcavity while for phosphorescent material with int 0 =1.0, the maximum zero-degree luminance and EL efficiency are around the second antinode. Through relaxing the efficiency by 10%‐20%, the angular intensity distribution can be even better than the Lambertian distribution; meanwhile, the color shows only a small variation over an angle range of 150°. Our results, which are in good agreement with experiments, show that the Purcell effect on TOLED performances is significant and should be carefully examined in studying TOLEDs.

Highly efficient fluorescence of a fluorescing nanoparticle with a silver shell.

Spontaneous emission (SE) rate and the fluorescence efficiency of a bare fluorescing nanoparticle and the nanoparticle with a silver nanoshell are analyzed rigorously by using a classical electromagnetic approach with the consideration of the nonlocal effect of the silver nano-shell. The dependences of the SE rate and the fluorescence efficiency on the core-shell structure are carefully studied and the physical interpretations of the results are addressed. The results show that the SE rate of a bare nanoparticle is much slower than that in the infinite medium by almost an order of magnitude and consequently the fluorescence efficiency is usually low. However, by encapsulating the nanoparticle with a silver shell, highly efficient fluorescence can be achieved as a result of a large Purcell enhancement and high out-coupling efficiency for a well-designed core-shell structure. We also show that a higher SE rate may not offer a larger fluorescence efficiency since the fluorescence efficiency not only depends on the internal quantum yield but also the out-coupling efficiency.

Voltage-controlled colour-tunable microcavity OLEDs with enhanced colour purity

The emission spectrum of single-unit voltage-controlled colour-tunable organic light emitting devices (OLEDs) has been theoretically and experimentally studied. Our results show that by introducing the microcavity structure, the colour purity of not only the destination colour but also the colour-tunable route can be enhanced, while colour purity is still an issue in typical single-unit voltage-controlled colour-tunable OLEDs. With the consideration of the periodical cycling of resonant wavelength and absorption loss of the metal electrodes, the appropriate change in the thickness of the microcavity structure has been utilized to achieve voltage-controlled red-to-green and red-to-blue colour-tunable OLEDs without adding dyes or other organic materials to the OLEDs.

CuPc/C60 Solar Cells -- Influence of the Indium Tin Oxide Substrate and Device Architecture on the Solar Cell Performance

We investigated the influence of different indium tin oxide (ITO) surface treatments on the performance of organic solar cells with different device architectures. Two types of devices (CuPc in contact with ITO treated with different treatments and C60 in contact with ITO treated with different treatments) were fabricated. The surfaces of CuPc and C60 layers deposited on ITO substrates treated with different surface treatments were examined using atomic force microscopy. The devices were characterized by measuring current-voltage characteristics in the dark and under AM1 illumination. We found that a one order of magnitude improvement in the AM1 power conversion efficiency for ITO/CuPc/C60/Al cells can be achieved for optimal ITO surface treatment, while ITO/C60/CuPc/Cu devices exhibit less sensitivity to surface treatments. Moreover, these devices exhibit better performance compared to ITO/CuPc/C60/Al devices. The observed differences in sensitivity to surface treatments were attributed to difference in the dependence of the film surface on ITO surface morphology for CuPc and C60.

Device Optimization of Tris-Aluminum (Alq3) Based Bilayer Organic Light Emitting Diode Structures

In this work we present detailed analysis of the emitted radiation spectrum from tris(8-hydroxyquinoline) aluminum (Alq3) based bilayer OLEDs as a function of: the choice of cathode, the thickness of organic layers, and the position of the hole transport layer/Alq3 interface. The calculations fully take into account dispersion in the glass substrate, the indium tin oxide anode, and in the organic layers, as well as the dispersion in the metal cathode. Influence of the incoherent transparent substrate (1 mm glass substrate) is also fully accounted for. Four cathode structures have been considered: Mg/Ag, Ca/Ag, LiF/Al, and Ag. For the hole transport layer, N,N-diphenyl-N,N-(3-methylphenyl)-1,1-biphenyl-4,4-diamine (TPD) and N,N-di(naphthalene-1-yl)-N,N-diphenylbenzidine (NPB) were considered. As expected, emitted radiation is strongly dependent on the position of the emissive layer inside the cavity and its distance from the metal cathode. Although our optical model for an OLED does not explicitly include exciton quenching in vicinity of the metal cathode, designs placing the emissive layer near the cathode are excluded to avoid unrealistic results. Guidelines for designing devices with optimum emission efficiency are presented. Finally, several different devices were fabricated and characterized and experimental and calculated emission spectra were compared.

The Purcell effect of silver nanoshell on the fluorescence of nanoparticles

The Purcell effect on the spontaneously emission rate and fluorescence efficiency of nanoparticles with and without a silver nanoshell will be investigated which are important for nanoparticle applications in biomedical diagnostics, information storage and optoelectronics.

Influence of the device architecture to the ITO surface treatment effects on organic solar cell performance

In this work, we investigate the influence of different indium tin oxide (ITO) surface treatments on the performance of organic solar cells with different device architectures. Two layer cells with different layer hierarchy (ITO/copper phthalocyanine (CuPc)/fullerene (C60)/Al and ITO/C60/CuPc/Cu) and three layer cells with mixed layer inserted between CuPc and C60 were fabricated. We found that in all cases the short circuit current was the parameter which was most significantly affected by ITO surface treatment. However, the performance of the cells with C60 layer in contact with ITO was markedly less sensitive to the ITO surface treatments compared to the cells with CuPc in contact with ITO. The cells with C60 layer in contact with ITO also exhibited higher efficiency compared to the cells with CuPc in contact with ITO. We also fabricated two layer cells with structures ITO/CuPc/ perylene tetracarboxylic acid diimide (PTCDI)/Al and ITO/PTCDI/CuPc/Cu. In this case, we also obtain higher efficiency and lower sensitivity to ITO properties when “n type” material is in contact with ITO. The best obtained AM1 power conversion efficiency was 0.4% for ITO/PTCDI/CuPc/Cu cell and ITO/C60/CuPc:C60/CuPc/Cu cells.

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