Photoluminescence polymer heterojunctions

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. 

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]). 

In Section 2.1.4, the electronic and photophysical properties of donor-acceptor or type-II heterojunctions between two semiconductor polymers were investigated. Exciplex formation was found to be a general phenomenon for type-II heterojunctions between polyfluorene copolymers. These are among the most widely used materials in polymer optoelectronics, but the presence of exciplexes had not yet been fully appreciated. This is because the exciplex population is often depopulated via efficient endothermic transfer towards the exciton, which leads to bulk exciton instead of exciplex emission. However, via time- and temperature-depen-dent photoluminescence (PL) spectroscopy the exciplex states can be identified. Employing a relationship known from small-molecule solution systems that relates the relative HOMO and LUMO levels of the molecules to the exciplex emis-... 

The electronic structure of interfacial excitations was recently described by Beljonne, Herz, Friend, and coworkers [38,88]. Static dipoles in the ground state of one polymer chain are considered to modulate the exciton energy on the neighboring polymer chains by Coulomb interactions across the heterojunction. Depending on the relative position of the chains, attractive or repulsive interaction were found, which respectively stabilize or obstruct charge-transfer state formation [88]. This implies that the density of states of interfacial excitons is broader than that for bulk excitons. Indeed, this was experimentally observed by femtosecond time-resolved photoluminescence spectroscopy in the same study [88]. The broader density of states at the heterojunction is consistent with the fast directional motion toward the interface described above (see Section 13.3.1) [87]). Excitons could transfer quickly into lower energy interfacial sites when they approach the interfacial region. 

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