Interest in organic photocells was significantly renewed a few years ago due to discovery of extremely fast photo-induced charge transfer (CT) from conducting polymer (CP) to fullerene C/sub 60/ molecules in CP(C/sub 60/) composites. This allowed creation of interfacial junction photocells with relatively high efficiencies. It has been recognized that those are not conventional p/n type semiconducting junction devices, but are so called donor-acceptor (D A) photocells, reminiscent of reaction centers of natural photosynthetic systems. Even more efficient photovoltaic devices have recently been fabricated with interpenetrating D-A networks which allow electron-hole separation to take place throughout the bulk of the device. In the present paper we demonstrate how these photocells can be further improved; we describe new experimental results, give general discussion of the efficiency of such devices, and analyze strategies for design of better D-A type photocells. Our recent experimental results on sensitization by inserting an excitonic layer between donor and acceptor layers multilayered structures, and by doping the D-layer by A-molecules in D(A)/A cells are briefly discussed. The organic photocell, which consists of the multilayer structure: ITO/poly(2,5-dioctyloxy-phenylenvinylene) (OOPPV)/octaethylporphine (OEP)/C/sub 60//Al, has been fabricated. Photocurrent yield spectra are interpreted by light absorption at OEP layer, exciton migration, and charge generation at both organic heterojunctions. That is, double heterojunctions of OOPPV/OEP and OEP/C/sub 60/ contribute to the charge generation by excitonic dissociation. We also report new type of "fractal network" photovoltaic devices made of two conducting polymers, particularly D and A derivatives of poly(phenylene vinylene). Conceptual theoretical work and modeling have been carried out to understand the ways for improvement of the device performance. Strategies to improve each step involving selective doping have been suggested to improve each step. Selective doping can provide low serial resistance and create internal electric fields for the collection of charges, while in the undoped parts excitons are effectively photogenerated. Ways to increase the effective use of absorbed photon energy and filling factor are also suggested.

Novel photovoltaic devices based on donor-acceptor molecular and conducting polymer systems

A detailed investigation both of the DC and of the AC electrical properties of the Schottky barrier formed between aluminium and electrodeposited poly(3-methylthiophene) is reported. The devices show rectification ratios up to 2*104 which can be increased further after post-metal annealing. The reverse characteristics of the devices follow predictions based on the image-force lowering of the Schottky barrier, from which the doping density can be estimated. As the forward voltage increases, the device current is limited by the bulk resistance of the polymer with some evidence for injection limitation at the gold counter-electrode at high bias. In the bulk-limited regime, the device current is thermally activated near room temperature with an activation energy in the range 0.2-0.3 eV. Below about 150 K the device current is almost independent of temperature. Capacitance voltage plots obtained at frequencies well below the device relaxation frequency indicate the presence of two distinct acceptor states. A set of shallow acceptor states are active in forward bias and are believed to determine the bulk conductivity of the polymer. A set of deeper acceptors are active only for very small forward voltages and for all reverse voltages, namely when band banding causes the Fermi energy to cross these states. The density of these deeper states is approximately an order of magnitude greater than that of the shallow states. Evidence is presented also for the influence of fabrication conditions on the formation of an insulating interfacial layer at the rectifying interface. The presence of such a layer leads to inversion at the polymer surface and a modification of the I-V characteristics.

https://iopscience.iop.org/article/10.1088/0022-3727/28/12/025/pdf

Electrical characterization of the rectifying contact between aluminium and electrodeposited poly(3-methylthiophene)

Electrically Rewritable Memory Cells from Poly(3-hexylthiophene) Schottky Diodes

Scanning-tunneling-microscopy of alkylated oligothiophenes at interfaces with graphite

A new polyphenylene-co-furan (PPF) and a polyhenylene-co-thiophene (PPT) have been synthesized utilizing the Stille coupling reaction and their luminescence properties characterized. The UV−vis, electroluminescence (EL), photoluminescence (PL), and current−field (I−F) characteristics, as well as the stability of the emission of light, were studied in detail for both polymer LED devices. Comparisons of the EL and PL characteristics between PPT and PPF suggest that the quantum efficiency of PPF is higher than that of PPT. However, it was also found that PPF is photochemically unstable and its electroluminescent intensity decays faster than that of PPT.

Synthesis of a Polyphenylene-co-furan and Polyphenylene-co-thiophene and Comparison of Their Electroluminescent Properties

Schottky barrier diodes have been fabricated using poly(3‐hexylthiophene)(P3HT) as the semiconductor and indium as the metal. P3HT was doped with FeCl3 at room temperature to form a p‐type semiconductor. The Schottky junctions of In on FeCl3‐doped P3HT using pressure contact exhibit rectification ratios ranging from 104:1 to 106:1 at a bias of ±1 V.

Schottky diodes using poly(3‐hexylthiophene)

Schottky diodes of poly(3-undecylthiophene) containing fullerene C60

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Schottky diodes using poly(3‐hexylthiophene)

Schottky diodes of poly(3-undecylthiophene) containing fullerene C60