Charge polymer heterojunctions

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. 

Additional semiconductor layers (charge-transport layers) may be included between the emissive layer and the electrode to facilitate transport of charges of one polarity, while impeding charges of the opposite polarity, thereby encouraging radiative recombination within the emissive layer, as shown in Fig. 11, for a polymer heterojunction device [30]. 

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). 

Pal SK, Kesti T, Maiti M, Zhang EL, Inganas O, Hellstrom S, Andersson MR, Oswald F, Langa F, Osterman T, Pascher T, Yartsev A, Sundstrom V (2010) Gemmate charge recombination in polymer/fullerene bulk heterojunction films and implications for solar cell function. J Am Chem Soc 132 12440... 

Scheme 5.8 Energy level alignment of bulk heterojunction components (conjugated polymer and semiconductor nanocrystals) facilitating the dissociation of excitons and charge separation. Left panel Case describing excitons formed in the nanocrystal phase. Right panel case describing excitons formed in the polymer phase.

It is the purpose of this chapter to introduce photoinduced charge transfer phenomena in bulk heterojunction composites, i.e., blends of conjugated polymers and fullerenes. Phenomena found in other organic solar cells such as pristine fullerene cells [11,12], dye sensitised liquid electrolyte [13] or solid state polymer electrolyte cells [14], pure dye cells [15,16] or small molecule cells [17], mostly based on heterojunctions between phthalocyanines and perylenes [18] or other bilayer systems will not be discussed here, but in the corresponding chapters of this book. 

Fig. 5.6. Schematic drawing of a bulk heterojunction device. Charge generation occurs throughout the bulk, but the quality of the two transport networks (p-and n-type channels) is essential for the functionality of the blend as an intrinsic, ambipolar semiconductor. Light emission occurs at the semi-transparent ITO electrode. Electron transport on the fullerenes is marked by full arrows and hole transport along the polymer by dotted arrows...

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... 

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