Chapter 5 Polyfluorenes

Several organics, e.g. pristine poly(3-octylthiophene), polyfluorene, bifunctional spiro compounds and polyphenyleneethynylene derivative, have been used for fabricating photOFETs. Responsivity as high as 0.5-1 A/W has been achieved in some of these transistors. We have already discussed the bulk heterojunction concept in Chapter 5. The bulk heterojunctions are fabricated using acceptor materials with high electron affinity (such as C<5o or soluble derivatives of C6o) mixed with conjugated polymers as electron donors. PhotOFETs based on conjugated polymer/fullerene blends are expected to show... 

With this collection of short review papers we would like to present a broad overview of research on polyfluorenes and related heteroanalogues over the last two decades. The collection begins with papers on the synthesis of polyfluorenes and related polyheteroarenes, then reports photophysical properties of this class of conjugated polymers both at the ensemble and the single chain level, continues with a discussion of the rich solid state structures of polyfluorenes, and finally switches to device applications (e.g. in OLEDs). In addition, two chapters are devoted to defined oligofluorenes as low molecular weight model systems for polyfluorenes and also to degradation studies. 

The photophysics of jt-conjugated polymers are reviewed in detail in other chapters of this book. (See, for example, Chapter 3.) Here, we focus on the electronic and photophysical phenomena that occur at the heterojunction between two different semiconductor polymers. The heteroj unctions are formed by combining four different polyfluorene copolymers in blend or bilayer thin films and are investigated using time-resolved and steady-state, temperature- and elec-tric-field-dependent photoluminescence measurements as well as electroluminescence and time-resolved spectroscopy. We review a body of work carried out in our laboratories over the last few years, and published in numerous journal articles (see refs. [13-17]). 

The aim of this chapter is to elucidate the various electronic and optical processes that occur at heterojunctions between two semiconductor polymers. Most of the results presented are related to the presence of localized electronic states at heterojunctions between different polyfluorenes. These have an analog in solution systems of small molecules where they are called exciplex states. Here, we give an overview of the theories that have been developed for small-molecule solution systems (for more details see also [24] and [25]). In Section 2.1.3, we then discuss if and how these are applicable to solid-state films of blended conjugated polymers. 

The polymers used in this chapter are all polyfluorene derivatives. They are ABAB copolymers with A being the fluorene group shown in Fig. 2.4(a). Due to their high chemical stability, flexible chemistry and good charge transport and luminescence properties, polyfluorenes are very common materials for polymer optoelectronics and have been used to make highly efficient LEDs [26, 13] as well as good photovoltaic diodes [27, 28] and transistors [29]. Table 2.1 lists the abbreviated and full names of all the polymers that appear in this chapter. The chemical structures of each of the polymers are displayed in Fig. 2.4. 

Finally, white-emitting fluorescent/phosphorescent diodes using a polyfluorene host have been reported by Gong et al. Details about the composition and the performance of these devices are contained in Chapter 4. 

Polyfluorenes are described in Chapter 5. Various PPPs with heterocyclic side rings (e.f., poly(quinoxaline-5,8-diyl)s with a pyrazine side rings) have also been synthesized. These polymers will be described in Chapters 14 and 18. This chapter also deals with some PPPs with heterocyclic side rings. 

---