Interfaces polymer heterojunctions

X. Zhang and S.A. Jenekhe, Electroluminescence of multicomponent conjugated polymers. 1. Roles of polymer/polymer interfaces in emission enhancement and voltage-tunable multicolor emission in semiconducting polymer/polymer heterojunction, Macromolecules, 33 2069-2082, 2000. 

Morteani et al. demonstrated that after photoexcitation and subsequent dissociation of an exciton at the polymer-polymer heterojunction, an intermediate bound geminate polaron pair is formed across the interface [56,57]. These geminate pairs may either dissociate into free charge carriers or collapse into an exciplex state, and either contribute to red-shifted photoliuni-nescence or may be endothermically back-transferred to form a bulk exciton again [57]. In photovoltaic operation the first route is desired, whereas the second route is an imwanted loss channel. Figure 54 displays the potential energy ciu ves for the different states. 

The origin of the EL emission is provided by the formation of excitons, or electron-hole pairs, which are formed at defect sites, i.e. donor/acceptor sites, in the material and at the interface of a heterojunction of two semiconducting polymer layers. Little is known at this time about the mobility of the electrons, holes, and excitons at the interface of heterojunctions. Limited exciton diffusion lengths in the materials and the interfacial nature of the photogeneration process could explain these exotic transport properties in terms of topological constraints. 

The use of interpenetrating donor-acceptor heterojunctions, such as PPVs/C60 composites, polymer/CdS composites, and interpenetrating polymer networks, substantially improves photoconductivity, and thus the quantum efficiency, of polymer-based photo-voltaics. In these devices, an exciton is photogenerated in the active material, diffuses toward the donor-acceptor interface, and dissociates via charge transfer across the interface. The internal electric field set up by the difference between the electrode energy levels, along with the donor-acceptor morphology, controls the quantum efficiency of the PV cell (Fig. 51). 

The heterojunctions of the polyacetylene were realized not only with inorganic photoconductors but also with organic polymers [139]. The results obtained show good similarity with barrier and heterojunction characteristics for inorganic semiconductors. Photoelectrochemical cell for solar energy conversion with polyacetylene electrodes and Na2S, electrolyte had an efficiency of 1 % at 2.4 eV [140], The complicated phenomena take place at the electrodeelectrolyte interface. 

Potje-Kamloth, K. (2001) Conducting polymer based Schottky and heterojunctions diodes and their sensor application. In Nalwa, H.S. (Ed.) Handbook of Surfaces and Interfaces of Materials, Vol. 5. Academic Press, p. 445—494. 

In bulk heterojunction solar cells, the metal/semiconductor interface is even more complex. Now the metal comes into contact with two semiconductors, one p-type (typically the polymer) and one n-type (typically the fullerene) semiconductor. A classical electrical characterization technique for studying the occurrence of charged states in the bulk or at the interface of a solar cell is admittance spectroscopy. If a solar cell is considered as a capacitor with capacitance C, the complex admittance Y is given by... 

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]. 

Just as described for LEDs in the preceding section, the area of the heterojunction in solar cells with the planar structure shown in Fig. 10.3(a) is defined by the dimensions of the cell. Similarly, this area is dramatically increased by the use of blends of immiscible electron- and hole-transporting polymers (Moons, 2002). Carriers generated at the dispersed interface between the two polymers will be able to diffuse to the electrodes if the network contains... 

Recently, geminate polaron pairs have been proposed for polymer-polymer [35,56,57] and polymer-fullerene [58,59] blends as photoinduced intermediates. Here the hole and electron remain coulombically bound across the interface of the donor-acceptor heterojunction. Only via an electric field and/or a temperature-assisted secondary process, these geminate polaron pairs are dissociated, leading to free charge carriers. This can have a considerable effect on the achievable charge separation efficiencies, since the geminate... 

In the following sections, we investigate blend films spun from chloroform solution at a weight ratio of 50 50 (if not indicated differently). In films prepared in this way, the two polymers are well intermixed and hence the film contains a high density of interface sites. This makes it easier to detect signals due to the heterojunction. The thickness of the polymer films was —170 nm. 

Exciplex states only exist at the interface between the two dissimilar polymers in the blend. Reducing the density of these interfaces in the polymer blend is expected to reduce the amount of exciplex observed. For example, annealing mobilizes the polymers and causes the film to move closer to thermodynamic equilibrium, i.e. the two polymers phase separate and the density of heterojunction sites decreases. Indeed, we observe that the amount of exciplex emission is reduced by the annealing treatment [30]. 

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