Conjugated heterojunctions

The aim of this chapter is to give a state-of-the-art report on the plastic solar cells based on conjugated polymers. Results from other organic solar cells like pristine fullerene cells [7, 8], dye-sensitized liquid electrolyte [9], or solid state polymer electrolyte cells [10], pure dye cells [11, 12], or small molecule cells [13], mostly based on heterojunctions between phthaocyanines and perylenes [14], will not be discussed. Extensive literature exists on the fabrication of solar cells based on small molecular dyes with donor-acceptor systems (see for example [2, 3] and references therein). 

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. 

One of the most promising uses of C60 involves its potential application, when mixed with 7r-conjligated polymers, in polymer solar cells. Most often the so-called bulk heterojunction configuration is used, in which the active layer consists of a blend of electron-donating materials, for example, p-type conjugated polymers, and an electron-accepting material (n-type), such as (6,6)-phenyl-Cgi -butyric acid methyl ester (PCBM, Scheme 9.6).38... 

Besides ruthenium complexes, rhenium complexes were also used as the photosensitizers in photovoltaic cells. Bulk heterojunction photovoltaic cells fabricated from sublimable rhenium complexes exhibited a power conversion efficiency of 1.7%.75,76 The same rhenium complex moiety was incorporated into conjugated polymer chains such as polymer 16a c (Scheme 9). Fabrication of devices based on conjugated rhenium containing polymers 17a c and SPAN by the LbL deposition method was reported.77 The efficiencies of the devices are on the order of 10 4%. 

The incorporation of siloles in polymers is of interest and importance in chemistry and functionalities. Some optoelectronic properties, impossible to obtain in silole small molecules, may be realized with silole-containing polymers (SCPs). The first synthesis of SCPs was reported in 1992.21 Since then, different types of SCPs, such as main chain type 7r-conjugated SCPs catenated through the aromatic carbon of a silole, main chain type cr-conjugated SCPs catenated through the silicon atom of a silole, SCPs with silole pendants, and hyperbranched or dendritic SCPs (Fig. 2), have been synthesized.10 In this chapter, the functionalities of SCPs, such as band gap, photoluminescence, electroluminescence, bulk-heterojunction solar cells, field effect transistors, aggregation-induced emission, chemosensors, conductivity, and optical limiting, are summarized. 

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.

Scheme 5.9 Scheme of a hybrid photovoltaic cell with an active layer consisting of a composite of a conjugated polymer and semiconductor nanocrystals (so-called bulk heterojunction). 

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. 

The previous section gave an overview of the transport and junction properties of conjugated materials regarding their importance for photovoltaic devices. In this chapter, the bulk heterojunction device itself will be in the spotlight. Device properties will be discussed and evaluated as for classical inorganic solar cells, concentrating on the short-circuit current /sc, the open-circuit voltage Foc, the fill factor FF, and the spectral sensitivity. 

A significant increase in the forward current and in the FF is observed for conjugated polymer/fullerene bulk heterojunction solar cells upon insertion of a thin layer of LiF between the organic layer and the Al electrode (negative electrode of the solar cell), as shown in Fig. 5.37a and b. 

The observed experimental result that Voc decreases linearly for bulk heterojunction solar cells allows us to conclude that, at least in the high temperature range (T > 200 K), these solar cells may be described by a diode model with Ip exp(E/kT). Here E is a parameter analogous to Eg for conventional semiconductors. For conjugated polymer/fullerene bulk heterojunction solar cells, E should correspond to the energy difference between the HOMO level of the donor and the LUMO level of the acceptor components of the active layer [as also suggested by the extrapolated value of V oc(0 K)]. 

A similar temperature dependence of Isc, Voc, and r) is also reported for the lower mobility generation of solar cells, based on interpenetrating networks of conjugated polymers with fullerenes, but processed from solvents so that the initial efficiency is < 1% [156]. This behavior is discussed extensively in the section dealing with Isc. A positive temperature coefficient is also observed for the efficiency of Cgo single-crystal photoelectrochemical cells [160]. Finally, a temperature dependence of Isc qualitatively similar to that shown in Fig. 5.47a and 5.48 is also observed for organic solar cells based on Zn-phthalocyanine (ZnPc)/perylene (MPP) heterojunctions [161]. 

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