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 glass 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 multilayer 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.published_or_final_versio

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 evaporated 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 temperature 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.published_or_final_versio

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

The fabrication of efficient bulk heterojunction photovoltaic cells using the chlorotricarbonyl rhenium(I) diimine complex as photosensitizer was discussed. The complex has a lower band gap, which can be adjusted easily by changing the structure of the ligand. The electric properties of the complex were studied using time-of-flight analysis. It was observed that the rhenium complex showed bipolar charge transport character with relatively high electron and hole mobilities.published_or_final_versio

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. © 2003 Optical Society of America.published_or_final_versio

Photosensitizing properties of some rhenium(i) tricarbonyl diimine complexes

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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. © WILEY-VCH Verlag GmbH 2000.link_to_subscribed_fulltex

Influence of atmospheric exposure of tris (8-hydroxyquinoline) aluminum (Alq3): A photoluminescence and absorption study

We have investigated the effects of atmospheric exposure on the properties of tris (8-hydroxyquinoline) aluminum (Alq3) thin films by photoluminescence (PL) and UV-Vis absorption measurements. Alq3 films were evaporated on glass substrates at different temperatures. The influence of annealing on 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 yield an improvement in the environmental stability of the films, i.e. less decrease in the PL intensity over time with exposure to atmosphere, as well as increased PL intensity. To investigate further the effects of the air exposure, films deposited at room temperature were stored for four days in air, nitrogen, and oxygen. No decrease in PL intensity has been found for storage in nitrogen, while the decrease for the film stored in oxygen was smaller than that for the film stored in air, indicating that both humidity and oxygen play a role in the PL intensity decrease in Alq3 thin films.link_to_subscribed_fulltex

Synthesis of a diazacrown ether containing polymers and formation of complexes with alkaline earth metals

We report the synthesis of a polymer that contains the diaza-18-crown-6 moiety on the polymer main chain. Metal containing polymers were obtained when the respective barium or strontium complexes were used as monomers. These polymers emitted greenish blue light in solution, and the metal had little influence on absorption and emission energies. In the solid state, the position of the emission peaks was different probably due to a different degree of aggregation. © WILEY-VCH Verlag GmbH, 2000.link_to_subscribed_fulltex

Study of charge generation process in some photosensitizing rhenium diimine complexes

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 Å. 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. © 2002 Elsevier Science B.V. All rights reserved.link_to_subscribed_fulltex

The role of ruthenium and rhenium diimine complexes in conjugated polymers that exhibit interesting opto-electronic properties

This paper reports the synthesis and opto-electronic properties of different conjugated polymers that contain the diimine complexes of ruthenium or rhenium. Conjugated poly(phenylene vinylene)s that contain aromatic 1,3,4-oxadiazole and 2,2′-bipyridine units on the main chain were synthesized by the palladium catalyzed olefinic coupling reaction. Other types of polymers based on 1,10-phenanthroline bis(2,2-bipyridyl) ruthenium(II) or chlorotricarbonyl rhenium(I) complexes were also synthesized by the same reaction. In general, these polymers exhibit two absorption bands due to the π-π* transition of the conjugated main chain and the d-π* metal-to-ligand charge-transfer transition of the metal complex. As a result, the photosensitivity of the polymers beyond 500 nm was enhanced. Charge-carrier mobility measurements showed that the presence of metal complexes could facilitate the charge-transport process, and the enhancement in carrier mobility was dependent on the metal content in the polymer. In addition, we have also demonstrated that the ruthenium complex could act as both photosensitizer and light emitter. Photovoltaic cells were constructed, and they were subjected to irradiation with a xenon arc lamp. Under illumination, the short circuit current and the open circuit voltage were measured to be 0.05 mAcm-2 and 0.35 V, respectively. The polymers were fabricated into single-layer emitting devices, and light emission was observed when the device was subjected to forward bias. The maximum luminance was determined to be 300 cdm-2, and the external quantum efficiency was approximately 0.05 to 0.2%. Although the efficiency was relatively low when compared with other devices based on organic materials, we have demonstrated the first examples of using transition metal complexes for both photovoltaic and light-emitting applications.link_to_subscribed_fulltex