Photoconductive organic materials for the near-IR radiation range

Abstract

Polymer composition films based on polystyrene and containing electron donors, viz., substituted tetrathiafulvalenes, an electron acceptor, viz., 2,4,5,7 tetranitrofluoren 9 one, and sensitizers, viz., a cationic polymethine dye, 1,3,3 trimethyl 2 [3 (1,3 dihydro 1,3,3 trimethyl 2H indol 2 ylidene)prop 1 en 1 yl] 3H indolium tetrafluoroborate, and a neutral mero cyanine dye, 5 {3 [(1,3 dihydro 1,3,3 trimethyl 2H indol 2 ylidene)ethylidene] 2 phenyl cyclopent 1 en 1 ylmethylene} 2 thioxodihydropyrimidine 4,6(1H,5H ) dione, were prepared. The internal photoeffect upon film irradiation in the near IR range is provided by low ioniza tion potentials of the donors and the high electron affinity of the acceptor. The photoconduc tivity increases upon replacement of a cationic dye by a neutral one, mainly because of photogeneration of mobile charge carriers of both signs and a decrease in the activation energy for the photoconduction current. The decrease in the activation energy for the photocurrent is due to the fact that in the case of a neutral dye, the mobile charge carriers move away from each other during their separation, while in the case of a cationic dye, the colorless counterion strongly helds the photogenerated charge carrier.The design of the materials that conduct electric cur rent on exposure to near IR radiation is a fairly topical task for photoelectric solar energy converters. 1-3 Along with inorganic semiconductors, the use of organic poly mers and their compositions for this purpose appears promising. 1,4-6 These are selected or developed with al lowance for the requirements to their films placed be tween the electrical contacts in the sandwich structures of photoelectric converters. 7 Generally, the photoconduc tivity of polymer films is provided by the presence of three types of molecules: donors, acceptors, and photo generation centers of charge carriers. 8 If the donor mol ecules are characterized by the ionization potential E i,d (proportional to the HOMO energy) and the acceptor molecules, by the electron affinity A e,a (proportional to the LUMO energy), 9 the molecules of the photogeneration centers are selected to meet the conditions |E i,c | > |E i,d | and |A e,c | < |A e,a |, where E i,c and A e,c are the ionization potential and the electron affinity of the photogeneration center molecule, respectively. If these conditions are ful filled, the absorption of a light quantum gives an elec tron-hole pair (EHP). Dissociation of the EHP results in free charge carriers.An important condition for the photoconduction is the transport of charge carriers within the bulk of the polymer film toward the electrical contacts without cap ture or recombination. 10-15 These structural features and properties of polymeric materials have already been imple mented in films based on carbazolyl containing polymers and polymers with conjugated bonds, 5 which use either intermolecular charge transfer complexes (CTC) or or ganic dyes and their aggregates as IR absorption and charge carrier photogeneration centers.How...

Photoconductive Nile red cyclopalladated metallomesogens

“…28 The data reported in the inset of Fig. 4 show that the s/I value for IV is in the order of 3 Â 10 À8 S cm W À1 at 25 V mm À1 , which is a very high value when compared to the photoconductivity of other organic photoconductors, 29 especially because this result is obtained without addition of dopants and without any annealing process to increase the order. This result represents a tremendous step forward in the development of this class of photoconductors since in previously analyzed cyclopalladated complexes, at the same applied electric field, the s/I value was of the order of 10 À12 S cm W À1 for annealed cyclopalladated discotics (complexes I and II), 12 while it was of ca.…”

Design and Photonics of Merocyanine Dyes

“…In order to create polymer composites with photoconductivity in the NIR region, merocyanines 107 and 108 were developed. [264,265] Due to the optimal donor-acceptor properties of the end-groups, the electronic structure of these deeplycolored chromophores closely approaches to the ideal polymethine structure A2 in medium-polarity solvents, resulting in narrow and intense absorption bands.…”

Photoinduced hole-transfer in semiconducting polymer/low-bandgap cyanine dye blends: evidence for unit charge separation quantum yield

“…Different from most small molecules, cyanines are easily processed from solution which makes them attractive for technological applications. Recently, low-bandgap cyanine dyes have been investigated for photoconductive 16 and photovoltaic applications up to 800 nm. 15 Their photophysical properties have been extensively investigated in solution [17][18][19] or when anchored to silver halide 20 or TiO 2 surfaces, 21 however, solid organic films and blends have received little attention.…”