Polymer Bilayer Devices

Fi ure 15-17. Configuration of a ITO/CuPc(250A)/PV(450A)/Ag cell (reproduced by permission of the American Institute of Physics from Ref. [14]). 

15 Conjugated Polymer Based Plastic Solar Cells 

Yoshino and co-workers also reported the optical response of a heterojimction device comprising a P3OT and Ceo bilayer [90]. The photoresponse of these devices shows a broad excitation profile ranging from 750 nm into die UV. 

Comparison of the spectral response and of the power efficiency of these first conjugated polymer/fullerene bilayer devices with single layer pure conjugated polymer devices showed that the large potential of the photoinduced charge transfer of a donor-acceptor system was not fully exploited in the bilayers. The devices still suffer from antibatic behavior as well as from a low power conversion efficiency. However, the diode behavior, i.e. the rectification of these devices, was excellent. 

An approach for improving the response of conjugated polymer/fullerene bilayer devices, which is based on an additional excitonic middle layer inserted into the D-A interface, was suggested by Yoshino et al. [94]. In the middle layer light absorption produces electron-hole pairs, which migrate towards the interface 

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The emission spectrum of some PT and PBD polymer bilayer devices cannot be explained by a linear combination of emissions of the components. Thus, white emission of the PLEDs ITO/422/PBD/A1 showed Hof 0.3% at 7 V, and consisted of blue (410 nm), green (530 nm), and red-orange (620 nm) bands. Whereas the first and the last EL peaks are due to the EL from the PBD and the PT layers, respectively, the green emission probably originates from a transition between electronic states in the PBD layer and hole states in the polymer... 

Figure 19. (1) Oxidized polypyrrole (PPy) film electrogenented on a steel electrode. (2) A tape was fastened to the dry polypyrrole film (A). B is doublesided tape and C is a protective sheet of paper. (3) The bilayer device with a protective film is removed from the electrode. (4) The protective sheet is peeled off and the bilayer is ready to work. (Reprinted from Handbook of Organic Conductive Molecules ami Polymers, H.S. Nalwa, ed.,Vol. 4,1997, Figs. 10.13, 10.15a, 10.18, 10.36. Reproduced with permission of John Wiley Sons, Ltd., Chichester. UK.)...

Jang et al. [131] reported high electron affinity perfluorobiphenyl-substituted PPV 78. This polymer was synthesized by the thermoconversion method. Single-layer PLED ITO/ 78/Al showed 64 times higher EL efficiency than that fabricated with unsubstituted PPV 1. A further (380-fold) increase of efficiency was achieved in a bilayer device ITO/1/78/A1. 

Such bilayers can conveniently be built up by successive electropolymerization of complexes containing ligands with vinyl substituents, e.g. 4-vinylpyridine or 4-vinyl-4 -methyl-2,2 -bipyridyl. The films may be deposited on metallic or semiconductor electrodes (e.g. Pt, glassy carbon, Sn02, Ti02). More efficient metailation of the films is obtained by polymerization of coordinated ligand than by subsequent metailation of a preformed polymer film. An alternative to discrete films would be a copolymer with distinct redox sites, or a combination of a single polymer film with a copolymer film in a bilayer device. 

The formation of polymersomes from water in- oil-in-water drops. Initially, a double emulsion consisting of single aqueous drops within drops of a volatile organic solvent ( oil ) is prepared using a microcapillary device. Amphiphilic diblock copolymers dissolved in the middle phase assemble into monolayers at the oil-water interfaces. Evaporation of the solvent then leads to the formation of polymer bilayers (polymersomes). 

Fig. 6.1. Cross-section of (a) an ITO/polymer/Al device and (b) an ITO/polymer/C6o/Al device. This exemplifies the advantages of using bilayer devices as far as the position of the active region is concerned (stippled area of the polymer), with the maximum optical field distribution inside the device due to the node at the mirror electrode in Al. The bulk of the polymer is gray and the C60 molecule is white...

The generation of photoexcited species at a particular position in the film structure has been shown in (6.19) and (6.20) to be proportional to the product of the modulus squared of the electric field, the refractive index, and the absorption coefficient. The optical electric field is strongly influenced by the mirror electrode. In order to illustrate the difference between single (ITO/polymer/Al) and bilayer (ITO/polymer/Ceo/Al) devices, hypothetical distributions of the optical field inside the device are indicated by the gray dashed line in Fig. 6.1. Simulation of a bilayer diode (Fig. 6.1b) clearly demonstrates that geometries may now be chosen to optimize the device, by moving the dissociation region from the node at the metal contact to the heterojunction. Since the exciton dissociation in bilayer devices occurs near the interface of the photoactive materials with distinct electroaffinity values, the boundary condition imposed by the mirror electrode can be used to maximize the optical electric field E 2 at this interface [17]. 

Fig. 6.9. Short-circuit action spectra of bilayer devices constructed with neat polymer and blends, (a) PBOPT, BEHP-PPV and PBOPT BEHP-PPV blend (1 1). (b) P3HT, BEHP-PPV and P3HT BEHP-PPV blend (1 1). (c) PTOPT, BEHP-PPV and PTOPT BEHP-PPV blend (1 1)...

In addition to vacuum-deposited small-molecule structures, bilayer devices have been fabricated from conjugated polymers and other solution-processible materials. One study reports a 1.9% efficient device fabricated by lamination of two spin-coated polymer layers (Granstrom et al, 1998). In principle, polymer bilayers can also be fabricated by spin-coating of successive layers using incompatible solvents. A more practical route to planar bilayers is the deposition of successive layers that have previously been spin-coated and removed from the substrate by a float-off technique (Ramsdale et al, 2002). 

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