Bilayer Heterojunction Research Articles

Ambipolar organic light-emitting transistors employing heterojunctions of n-type andp-type materials as the active layer

The realization of organic light-emitting transistors (OLETs) with high quantum efficiencyand fast switching time is crucial for the development of highly integrated organicoptoelectronic systems. In order to reach such a goal, fabrication of devices withambipolar transport and high charge mobility values is needed. At present, organicmaterials having intrinsically ambipolar transport are restricted in number and showpoor performance. This limits their use in efficient and fast switching single layerambipolar OLETs. In this framework, we have taken the approach of combiningp-type and n-type materials in two complementary configurations. The first one isbased on realizing layered structures (bi-layer heterojunction) where materials aresequentially deposited. The second one is based on simultaneously evaporatingtwo materials with variable composition (bulk heterojunction) to form a mixedfilm. In this paper, the charge transport and electroluminescence properties ofOLETs based on these heterostructures are presented. A correlation betweenthe active layer structure and the electrical performances has been obtained bymeans of laser scanning confocal microscopy, here employed for morphology andspectroscopy analysis. Differences between the two approaches are critically discussed.

On the use of cyanine dyes as low-bandgap materials in bulk heterojunction photovoltaic devices

Cyanine dyes with absorption edges of almost 1000 nm were used in combination with MEH-PPV for the fabrication of organic solar cells. For blended thin films, a pronounced phase separation between the two components occurred and resulted in photocurrents with different signs for bilayer and bulk heterojunction devices. Absorption spectra and selective dissolution experiments were used to illustrate the process of vertical phase segregation, with the preferential wetting of the polar anode by the cyanines while maintaining percolating carrier pathways between the electrodes. For a cyanine with long alkyl side chains, the compatibility with the polymer matrix was increased and the development of the effective inverted bilayer configuration was not observed. The generally low oxidative photocurrents were explained with unfavourable shifts of the highest occupied molecular orbital (HOMO) dye energy levels in the solid state.

Triphenylamine−Thienylenevinylene Hybrid Systems with Internal Charge Transfer as Donor Materials for Heterojunction Solar Cells

Star-shaped molecules based on a triphenylamine core derivatized with various combinations of thienylenevinylene conjugated branches and electron-withdrawing indanedione or dicyanovinyl groups have been synthesized. UV-vis absorption and fluorescence emission data show that the introduction of the electron-acceptor groups induces an intramolecular charge transfer that results in a shift of the absorption onset toward longer wavelengths and a quenching of photoluminescence. Cyclic voltammetry shows that all compounds present a reversible first oxidation process whose potential increases with the number of electron-withdrawing groups in the structure. Prototype bulk and bilayer heterojunction solar cells have been realized using fullerene C60 derivatives as acceptor material. The results obtained with both kinds of devices show that the introduction of electron-acceptor groups in the donor structure induces an extension of the photoresponse in the visible spectral region, an increase of the maximum external quantum efficiency, and an increase of the open-circuit voltage under white light illumination. These synergistic effects allow reaching power conversion efficiencies of approximately 1.20% under simulated AM 1.5 solar irradiation at 100 mW cm(-2).

Organic Photovoltaics Interdigitated on the Molecular Scale

Phosphonated porphyrin frameworks that are porous to an electron-accepting perylenediimide are systematically interdigitated with a phosphonated form of the diimide to form bulk heterojunctions. When employed in photovoltaics, these molecularly interlaced heterojunctions provide large junction areas while retaining the phase connectivity of traditional bilayer heterojunctions. Zirconium phosphonate linkages facilitate layer-by-layer chromophore assembly from solution under ambient conditions. The dependence of photovoltaic performance on heterojunction architecture is investigated.

Molecular and supramolecular engineering of π-conjugated systems for photovoltaic conversion

Various series of conjugated systems have been used as donor in hetero-junction solar cells. The results obtained with EDOT-containing π-conjugated oligomers show that besides their direct effect on the electronic properties of the conjugated chain, the number and position of the EDOT units control to a large extent the orientation of the molecules on the substrate and hence the performances of the derived solar cells. The characteristics of the cells based on oligothienylenevinylenes donors show that in this case structural control of orientation can be achieved by means of substitution of the conjugated structure by alkyl chains. Donors based on star-shaped oligothiophenes provide another example of control of horizontal molecular orientation by design. The photovoltaic characteristics of bi-layer hetero-junctions show that the combined effects of controlled molecular orientation and planarization of the structure by the use of a fused tri-thienobenzene central core lead to power conversion efficiencies approaching 1%. In order to avoid the need to control molecular orientation first examples of bulk hetero-junctions based on PCBM and three-dimensional conjugated systems with isotropic electronic properties have been realized. A power conversion efficiency of 0.30% has been obtained under 79 mW cm − 2 white light illumination. This result obtained with a 3D donor based on a terthiophene conjugated chain with very limited absorption of the visible spectrum demonstrates the large potentialities of this novel concept.

Bi-layer photovoltaic devices with PPQ as the electron acceptor layer

Abstract Photovoltaic cells made from bi-layer thin-film heterojunctions having poly(3-octylthiophene) (P3OT) as the electron donor and poly(phenylquinoxaline) (PPQ) as the electron acceptor have been investigated. The photovoltaic characteristics of the heterojunction device are considerably enhanced from those in a single-layer device. The photocurrent is improved by almost two orders of magnitude, and the open-circuit voltage corresponds to the estimated energy level difference of the two polymers. The observed enhancement of the heterojunction device has been discussed in terms of photoinduced charge transfer between P3OT and PPQ. The results showed that PPQ can be regarded as an efficient electron acceptor material.

Vapor deposited solar cells based on heterojunction or interpenetrating networks of zinc phthalocyanine and C60

In this paper, we point out the technical requirements necessary to achieve, with a co-evaporation set up, photovoltaic cells based on heterojunction (single or multiple) or interpenetrating networks of organic molecules. We describe the evaporation cell design of our home made experimental coevaporation bench and show the limitation of such experiment. Finally, ITO/ZnPc-C 60 /Al diodes were achieved, the active layer being a single heterojunction bilayer or a coevaporated layer. In the case of heterojunctions, the poor reproducibility of the results was attributed to the presence of short circuit pathways due to the C 60 layer inhomogeneity. The coevaporated layers have performances comparable to those obtain in literature without doping or additive layers.

Vapor deposited solar cells based on heterojunction or interpenetrating networks of zinc phthalocyanine and C 60

In this paper, we point out the technical requirements necessary to achieve, with a co-evaporation set up, photovoltaic cells based on heterojunction (single or multiple) or interpenetrating networks of organic molecules. We describe the evaporation cell design of our home made experimental coevaporation bench and show the limitation of such experiment. Finally, ITO/ZnPc-C 60/Al diodes were achieved, the active layer being a single heterojunction bilayer or a coevaporated layer. In the case of heterojunctions, the poor reproducibility of the results was attributed to the presence of short circuit pathways due to the C 60 layer inhomogeneity. The coevaporated layers have performances comparable to those obtain in literature without doping or additive layers.

Electrochemical fabrication of aligned microtubular heterojunctions of poly(p-phenylene) and polythiophene

Aligned microtubular heterojunctions of poly(p-phenylene) (PPP) and polythiophene (PTh) were fabricated by successive electrochemical oxidations of benzene and thiophene in freshly distilled boron trifluoride–diethyl ether (BFEE) solution and using a microporous alumina membrane with pores of 200 nm diameter as the template. The morphology and chain structure of the microtubular heterojunctions were studied by scanning and transmission electron microscopies and Raman spectroscopy. The current (I)–voltage (V) curves revealed that the aligned microtubular heterojunctions had a better rectification effect than that of the normal PPP/PTh bilayer heterojunction.

High electron injection currents from a nanotube composite in an organic heterojunction

Organic bilayer heterojunctions have been fabricated from an organic polymer-nanotube composite and an insulating polymer. Using poly(m-phenlyenevinylene-co-2,5-dioctyloxy-p-phenylenevinylene) highly doped with multiwalled carbon nanotubes as an injection layer, extremely high electron injection currents through a poly-methylmethacrylate (PMMA) layer were observed. These currents are up to 10 6 times greater than those through PMMA alone. Examination of the AC and DC characteristics of these heterojunctions are presented in this paper. Also examined is the effect of an aluminium layer at the interface of the heterojunction on the electrical characteristics.

Organic Light-Emitting Diodes Based on New Oxadiazole and Pyrazoline Derivatives

The bilayer heterojunction devices were fabricated successfully by using a novel oxadiazole derivative: 2, 2'-(2,5-thiophenediyl) bis (5-(4-methyl) phenyl-1,3,4-oxadiazole) (T-OXD) as the electron-transporting layer (ETL) and a pyrazoline derivative:1-phenyl-3-(dimethylamino)styryl-5-(p-(dimethylamino) phenyl)pyrazoline (PDP) as the light-emitting layer and the hole-transporting layer. The emission at 500 nm was derived from PDP layer. In comparison with the bilayer device of tris (8-hydroxyquinoline) aluminum (Alq) as the ETL, the luminous efficiency of the PDP/T-OXD heterojunction device was enhanced by 104 times.

Electroluminescence of Multicomponent Conjugated Polymers. 1. Roles of Polymer/Polymer Interfaces in Emission Enhancement and Voltage-Tunable Multicolor Emission in Semiconducting Polymer/Polymer Heterojunctions

Effects of the electronic structure of polymer/polymer interfaces on the electroluminescence efficiency and tunable multicolor emission of polymer heterojunction light-emitting diodes were explored by a series of 16 n-type conjugated polymers with varying electron affinities and ionization potentials in conjunction with poly(p-phenylenevinylene). Efficiency and luminance of diodes of the type indium−tin oxide/poly(p-phenylenevinylene)/n-type polymer/aluminum were maximized and were as high as 3% photons/electron and 820 cd/m2, respectively, when the energetics at the polymer/polymer interface favored electron transfer while disfavoring hole transfer. Energetic barrier to electron transfer at the polymer/polymer interface was more important to electroluminescence efficiency and diode luminance than injection barrier at the cathode/polymer interface. By a judicious choice of the relative layer thicknesses and the components of the bilayer heterojunctions, the rate of both electron and hole transfer across t...

Exciplex emission in bilayer polymer light-emitting devices

Photoluminescent and electroluminescent studies of bilayer heterojunctions formed from a poly(pyridyl vinylene phenylene vinylene) (PPyVPV) derivative and poly(vinyl carbazole) (PVK) show an emission peak which cannot be ascribed to either the PPyVPV derivative or PVK layer. Through studies of absorption and photoluminescence excitation (PLE) spectra we demonstrate that the additional feature results from an exciplex at the bilayer interface. The photoluminescence efficiency of the exciplex is greater than 20%. The electroluminescence spectrum from the bilayer devices is entirely due to exciplex emission, with internal efficiencies initially achieved exceeding 0.1%.