The Influence of Ordering on the Photoinduced Charge Transfer in Composites of Phenyl-type Substituted Polythiophenes wi
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BSTRACT Regioregular polythiophenes baring 3-(p-methoxyethoxyethoxy)-phenyl substituents (PEOPT) show high photoluminescence efficiencies. Exposing thin films of this polymer to vapors of chloroform or annealing them by heat treatment results in a red shift of the absorption maximum due to solvent or heat induced ordering which gives rise to more planar conformations. The fact, that annealed thin films of PEOPT exhibit absorption edges at relatively low energies and thus have an enhanced spectral range makes them suitable for use in photodiodes / solar cells. The photoinduced charge generation efficiency in PEOPT is significantly enhanced by the addition of a strong electron acceptor like fullerene, as observed by quenching of the luminescence and by photoinduced absorption measurements in the infrared and UV-Vis regime. The efficiency of the photoinduced charge transfer from PEOPT to a methanofullerene is found to depend on the ordering of PEOPT in thin films.
INTRODUCTION Polythiophenes were among the first conjugated polymers modified by attaching long solubilizing side chains to improve their processing properties. For some processible polythiophenes, among them the poly(3-alkylthiophenes), a thermochromic change was found rather early between a red and a yellow phase [1]. The origin of this thermochromism was extensively studied from structural point of view especially with
respect to the interplay between the effective conjugation length of the thiophene backbone and the conformational change of the alkyl side chains (i. e. flat trans vs. twisted gauche structures) [2, 3, 4]. Recently, emphasis was put again on the investigation of the two different phases in substituted polythiophenes, demonstrating photoinduced phase transformation in polyalkylthiophenes within the ns regime [5]. The regioregularity of the polythiophenes has an important influence on the effective conjugation length. For regioregular thiophenes, the π conjugation seems to be longer since the thiophene rings are less twisted due to steric hindrance [6]. Regioregular polythiophenes with enhanced absorption in the near infrared regime are also interesting for a second application. Since the discovery of an ultrafast photoinduced electron transfer [7] from many conjugated polymers onto fullerenes, this effect has been used to develop efficient photovoltaic devices [8]. Prototypes of photovoltaic devices based on a polymeric donor / acceptor networks show solar energy conversion efficiencies of around 1% [9]. A clear handicap of these cells is the relatively large band gap of the conjugated polymers, typically > 2.2. eV. Regioregular thiophenes with extended absorption in the < 2 eV regime are expected to enhance the efficiency of polymeric photovoltaic devices significantly. In this paper we investigated a novel class of regioregular phenyl substituted polythiophenes [10]which show structural ordering upon heating (or solvent vapor treating), on the occurrence of photoinduced charge transfer with fullerenes. The difference between these two ph
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