Conjugated polymers device applications

Grimsdale, A.C., Leok Chan, K., Martin, R.E., Jokisz, EG., Holmes, A.B., 2009. Synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Chem. Rev. 109,897-1091. 

A. C. Grimsdale, Leok K. Chan, R. E. Martin, P. G. Jokisz, A. B. Holmes, Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices. Chem. Rev. 2009,109,897-1091. 

Polyfluorene is one of the most attractive conjugated polymers for application in light-emitting devices [36-38], It is characterized by low electron affinity and, in consequence, several authors have inserted electron-deficient moieties into the aromatic chain to modify the position of the frontier orbital and increase the electron affinity in order to achieve a better balanced hole and electron injection in electroluminescent diodes. Among the electron-deficient groups, thienyl-5,5-dioxide and dibenzothienyl-5,5-dioxide have also been employed [16-18, 20-25, 27, 28],... 

Therefore rational design and synthesis of the monomer structure and the electropolymerisation experimental conditions play an important role in tailoring the properties of the conjugated polymers for application as electrochromic and fluorescent materials. Several general principles should be kept in mind, including band gap and absorption in the visible region behaviour, HOMO and LUMO energy levels and presence of side chains to enhance the solubility and processability when desirable. These factors are dependent on each other and must be comprehensively considered in pursuit of ideal polymers for application in optical devices. 

Polyacetylene is considered to be the prototypical low band-gap polymer, but its potential uses in device applications have been hampered by its sensitivity to both oxygen and moisture in its pristine and doped states. Poly(thienylene vinylene) 2 has been extensively studied because it shares many of the useful attributes of polyacetylene but shows considerably improved environmental stability. The low band gap of PTV and its derivatives lends itself to potential applications in both its pristine and highly conductive doped state. Furthermore, the vinylene spacers between thiophene units allow substitution on the thiophene ring without disrupting the conjugation along the polymer backbone. 

In low-dimensional systems, such as quantum-confined. semiconductors and conjugated polymers, the first step of optical absorption is the creation of bound electron-hole pairs, known as excitons [34). Charge photogcncration (CPG) occurs when excitons break into positive and negative carriers. This process is of essential importance both for the understanding of the fundamental physics of these materials and for applications in photovoltaic devices and photodctcctors. Since exciton dissociation can be affected by an external electric field, field-induced spectroscopy is a powerful tool for studying CPG. 

The linear and nonlinear optical properties of one-dimensional conjugated polymers contain a wealth of information closely related to the structure and dynamics of the ir-electron distribution and to their interaction with the lattice distorsions. The existing values of the nonlinear susceptibilities indicate that these materials are strong candidates for nonlinear optical devices in different applications. However their time response may be limited by the diffusion time of intrinsic conjugation defects and the electron-phonon coupling. Since these defects arise from competition of resonant chemical structures the possible remedy is to control this competition without affecting the delocalization. The understanding of the polymerisation process is consequently essential. 

Up to now, many conjugated polymers have been found to possess large and very rapid third-order nonlinear optical response, which originates from the one-dimensionally delocalized ji-conjugation system along the polymer chain. Their application to the all optical signal processing devices has been expected. 

Although conjugated polymers can be both n-doped and p-doped - and thus, in principle, be capable of behaving either as negative or as positive electrodes - the majority of applications have been confined to the p-doping, positive side. Conductive polymers have been proposed and tested in a variety of advanced electrochemical devices. Due to lack of space, we will confine our attention to the description of the most illustrative examples which are rechargeable lithium batteries and multi-chromic optical displays. 

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