J.R. Heflin

Control of excited state dynamics in ionically self-assembled monolayers of conjugated molecules

We report the fabrication of thin organic layers and photovoltaic devices made from them. Building thin layers of organic materials via the method of ionically self-assembled monolayers (ISAM) provides control over the layer thickness and composition of multilayer structures on a nanometer scale. This allows to accurately dope a photoluminescent host material with energy or charge accepting guests, changing the emissive character of the pure photoluminescent host film to a predominantly non-emissive, charge generating structure. We show that by varying the concentration of the guest Copper phthalocyanine and C60(OH)24 in poly-(para-phenylene-vinylene) we can measure the energy migration as well as dissociation of the exciton and can determine the lifetime and the diffusion radius of the exciton. Increasing the number of dopands in the host material, the photoluminescence emission spectra shift and decrease in intensity reflecting a decrease in the number of excitons transferring to neighboring chains or conjugation segments. For high dopand concentrations the recombination of excitons only happens on the same chain as the generation. Building a device to achieve the optimal guest/host ratio for optimal exciton dissociation is one important step in the design of high efficiency photovoltaic devices.

Photocarrier generation quantum yield for ionically self-assembled monolayers

We have fabricated photovoltaic cells from organic donor/acceptor couples arranged as ionically self assembled monolayers. Making use of the morphological control on the nanometer level we were able to study the influence of the device structure and layer composition on the quantum yield of charge carrier photogeneration. The photoluminescence quenching and the photocurrent spectra reveal sample composition, charge carrier photogeneration and transport properties independently.

Enhanced photovoltaic efficiency in polymer-fullerene composites by thermally controlled interdiffusion

The interface between polymer and fullerene in organic photovoltaic devices is improved by thermally induced interdiffusion. Starting from a bilayer of 2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene copolymer (MEH-PPV) and the Buckminsterfullerene (C60) devices are heated in the vicinity of the glass transition temperature creating a gradient bulk-heterojunction. Interdiffused devices show photoluminescence quenching with concomitant improvements in photocurrents. Variation of the polymer layer thickness shows an increase in photocurrents with decreasing layer thickness within the examined thickness regime as transport of the separated charges out of the device is improved. The interdiffusion was observed in situ by monitoring the photocurrents during the heating step. Cross-sectional transmission electron microscopy reveals C60 clusters of up to 30 nm in diameter in the interdiffused devices. The clustering of the fullerene molecules puts a significant constraint on the interdiffusion process.

Characterization of the purity and stability of commercially available dichlorotriazine chromophores used in nonlinear optical materials

Nonlinear optical (NLO) materials consisting of organic chromophores have unique optical properties due to the polar molecular structure and noncentrosymmetric ordering of the chromophores. Chromophores with dichlorotriazine moieties can be incorporated with a consistent polar ordering into layer-by-layer films via covalent reaction with a nucleophilic polymeric species. While the manufacture of optical materials traditionally requires highly pure reagents, commercially available azo dyes originally manufactured for textile industry use are usually impure. The purities of three commercially available chromophores, Reactive Red 2, Reactive Orange 4, and Reactive Brown 23, that are being used in NLO materials research were determined by HPLC and found to be highly variable. MALDI-TOF MS analysis of collected HPLC peaks was used to confirm the identity of the chromophores and impurities that resulted from basic hydrolysis. Additionally, since noncentrosymmetric deposition of these chromophores into NLO films occurs at basic pH and hydrolyzed chromophores are not reactive, the stability of the chromophores at pH 10.5 was determined. Base hydrolysis experiments also revealed whether other impurities in the dyes were reactive. Because the layer-by-layer method of making NLO films relies on covalent reaction, the presence of non-reactive impurities may not be a critical issue. However, reactive impurities can take up space and inhibit incorporation of the primary chromophore in the film, and if their dipole moment orientation is different, they may cancel out the molecular ordering of the primary chromophore and further decrease NLO performance.

Modelling of external quantum efficiency spectra as a function of varying P3OT thickness in P3OT-C 60 polymer photovoltaic devices

In this paper we present a study of thermally interdiffused poly (3-octylthiophene) (P3OT) - C60 photovoltaic devices of varying polymer and fullerene layer thicknesses. It is found that overall device performance and external quantum efficiencies (EQEs) in the wavelength region of peak absorption are enhanced for thinner polymer as well as thinner fullerene layers. We present a simple, first level model for EQE curves that accounts for the effects of absorption throughout the film. The model considers 3 distinct regions; pure donor, interdiffused donor and acceptor, and pure acceptor. It is found that this model reproduces quite effectively the experimentally observed shapes and relative magnitudes for varying donor thickness when compared with a similar experimental study. It also reproduces the shapes of the EQE curves but is not as effective in reproducing the relative magnitudes for varying fullerene thickness devices.

Organic electro-optic films fabricated by hybrid covalent/ionic self-assembly

By alternating the deposition interaction between covalent and ionic binding, organic self-assembled films are fabricated with electro-optic coefficients of 20 pm/V. The rapidly deposited multilayer films exhibit excellent temporal and thermal stability.

Photovoltaic response in conducting polymer/fullerene self-assembled multilayers

We present results of a detailed study on thin film photovoltaic devices fabricated by the ionically self-assembled monolayer (ISAM) technique. The method allows for precise deposition of the electron accepting materials, such as copper phthalocyanine or fullerene derivatives within the polymer film. We demonstrate in this paper that it is possible to create ultrathin films (100 nm) of photovoltaic material that have enhanced efficiencies.

Sensitivity of long period fiber gratings to nanoscale ionic self-assembled multilayers

We have shown that ionic self-assembled multilayers (ISAMs) deposited on optical fiber long period gratings (LPGs) yield dramatic resonant-wavelength shifts, even with nanometer-thick films. Precise control of the refractive index and the thickness of these films was achieved by altering the relative fraction of the anionic and cationic materials combined with layer-by-layer deposition. We demonstrate the feasibility of this highly controllable deposition-technique for fine-tuning grating properties for grating applications. In addition, we confirm theoretically that the resonant wavelength shift can result from either the variation of the thickness of the film and/or the variation of its refractive index. Finally, we demonstrate that ISAMs adsorbed on LPGs function effectively as biosensors. These simulations and experimental results confirm that ISAM-coated-LPGs provide a thermally-stable, reusable, robust, and attractive platform for building efficient fiber optic sensors and devices.

Hybrid covalent/ionic self-assembly of second order nonlinear optical films

A multilayer deposition scheme has been developed based on alternating covalent and ionic binding between successive layers to yield organic films with inherent polar order. Electro-optic coefficients of 20 pm/V have been observed