Polymers with Large Spin-Orbit Coupling

Abstract

The dynamics of spin singlet and triplet excitons in p-conjugated polymers define their performance as optically active layer in organic light-emitting diodes (OLEDs) and organic photovoltaic (OPV) cells. As an example, if both triplet and singlet excitons can be used in OLEDs to convert electrical energy to electroluminescence (EL) emission, then the fraction of excitons that potentially can emit light may reach 100 % [1]. Similarly in OPV based on donor/acceptor (D–A) blends, the photogenerated singlet exciton in the polymer donor domains may recombine before reaching the D–A interface, because of its relatively short lifetime (~100 ps). In contrast, because of the much longer lifetime (~5 ms), triplet excitons could reach the D–A interface with larger probability and thus could potentially be the answer to this loss mechanism [2]. Therefore, both OLED and OPV technologies may substantially benefit from the proper use of the spin triplet states. Alas, because the spin-orbit coupling (SOC) in polymers is typically very weak (<5 Gauss), triplet excitons cannot be efficiently photogenerated via intersystem crossing (ISC) in most donor polymers for OPVenhancement and, similarly, cannot efficiently emit light in OLEDs. The SOC however can be enhanced by embedding heavy atoms such as platinum (Pt) in the polymer backbone chains. Recently a series of polymers and oligomers in which platinum (Pt) atoms are inserted in the organic polymer’s building blocks have been synthesized [3, 4]. The Pt atom has large SOC and thus increases the effective SOC of the polymer chain. In fact, the Pt atoms may be inserted into the polymer chain with variable inter-Pt distance (i.e., between adjacent intrachain Pt atoms), and this tunes the effective SOC of the polymers [5, 6]. Such enhanced SOC, in turn, may increase the ISC rate from the singlet to the triplet manifold, which would make triplet excitons more viable for OPV applications. In addition, the enhanced SOC may also trigger substantive phosphorescence (PH) emission from the lowest triplet state [3–6], and thus the emission spectrum from such semiconductor polymers may contain both fluorescence (FL) and PH bands (Fig. 1). In fact, these two bands span the visible spectral range and therefore may potentially be used in designing “white” electroluminescence emission from the same polymer in OLEDs (dubbed WOLED), with internal quantum efficiency approaching 100 % [7]. Encyclopedia of Polymeric Nanomaterials DOI 10.1007/978-3-642-36199-9_170-1 # Springer-Verlag Berlin Heidelberg 2014