Esther M. Conwell

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

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.

Theory of second-harmonic generation in optical waveguides

A theory for the initial rate of second-harmonic generation (SHG) in planar optical waveguides for phase-matched and nonphase-matched cases is derived. The derivation is carried out both by a ray treatment and by a wave treatment and the results of the two are compared. Although formally and from the point of view of physical interpretation the two treatments look very different, the final results are the same if allowance is made for the Goos-Haenchen shift. Dependences of the amplitude of the second harmonic on the amplitude of the fundamental A ω and on distance are quite similar to those for bulk SHG even though the phase-matching condition is quite different. Determining the characteristics of some of the phase-matched modes, we find that, for the same A ω , it is possible to obtain a generation rate in waveguide comparable to that in the bulk. The highest generation rate is achieved, as expected, with both fundamental and harmonic having mode number 0. Phase matching can be achieved in this case without birefringence and with all three media having normal dispersion. The decrease in generation rate in going to mode pairs other than 0,0 is less than a factor 2 in some cases.

The effect of temperature on libron frequencies in molecular crystals: Implications for TTF-TCNQ

Abstract Results for the effect of pressure and oemperature on lattice-phonon frequencies in six crystals (As 4 S 4 , S 8 , S 4 N 4 , C 10 H 8 , C 14 H 10 , C 16 H 10 ) have been analyzed to separate the phonon-excitation-driven (“explicit”) and the thermal-expansion-driven (“implicit”) contributions to d v /d T . The data establish that the volume-driven implicit effect dominates the temperature dependence of external-mode frequencies in molecular crystals . This finding has a bearing upon recent proposals for the role of librons in the electron transport of TTF-TCNQ.

EFFECT OF INTERCHAIN COUPLING ON CONDUCTING POLYMER LUMINESCENCE : EXCIMERS IN DERIVATIVES OF POLY(PHENYLENE VINYLENE)

Optical excitation of a chain in a polymer film may result in formation of an excimer, a superposition of on-chain excitons and charge-transfer excitons on the originally excited chain and a neighboring chain. The excimer emission is red-shifted compared to that of an on-chain exciton by an amount depending on the interchain coupling

Modes in anisotropic optical waveguides formed by diffusion

t_perp

Statistics for polymers whose excitations are polarons, bipolarons, electrons and holes

. Setting up the excimer wavefunction and calculating the red shift, we determine average

WKB approximation for optical guide modes in a medium with exponentially varying index

t_perp

Charge transfer change with pressure in TTF-TCNQ (Tetrathiofulvalene Tetracyanoquinodimethane)

values, referred to a monomer, of 0.52 eV and 0.16 eV for poly(2,5-hexyloxy

Statistical mechanics for solitons in the presence of electrons and their contributions to photovoltage in polyacetylene schottky barrier diodes

p

Optical guided waves in CdSxSe1−x diffused layers

-phenylene cyanovinylene), CN-PPV, and poly[2-methoxy, 5-(2-ethyl-hexyloxy)-1, 4 p-phenylene vinylene], MEH-PPV, respectively, and use them to determine the effect of interchain distance on the emission.