Poly optical bandgap

Andersson and coworkers have prepared solar cells based on blends of poly(2,7-(9-(2 -ethylhexyl)-9-hexyl-fluorene)-fl/t-5,5-(4, 7 -di-2-thienyl-2, l, 3 -benzothiadiazole) (223) and PCBM [416]. The polymer shows a Amax (545 nm) with a broad optical absorption in the visible spectrum and an efficiency of 2.2% has been measured under simulated solar light. The same group has also reported the synthesis of low bandgap polymers 200 (1 = 1.25 eV) and 224 (1 = 1.46 eV) which have been blended with a soluble pyrazolino[70]fiillerene and PCBM, respectively, to form bulk heterojunction solar cells of PCE of 0.7% [417] and 0.9% [418]. Incorporation of an electron-delident silole moiety in a polyfluorene chain affords an alternating conjugated copolymer (225) with an optical bandgap of 2.08 eV. A solar cell based on a mixture 1 4 of 225 and PCBM exhibits 2.01% of PCE [419]. 

Huang s group has systematically studied the structure-property relationships of fluorene-thiophene-based conjugated polymers 57-60 [95, 96], In contrast to poly thiophene homopolymers, the regiochemistry of substitution in bithiophene fragments in the studied copolymers shows little effect on the optical bandgap (59 and 60, respectively Eg = 2.49 and 2.58 eV [96] or 2.57 and 2.60 eV [97, 98]) or the emission maxima, but the head-to-head copolymer 60 was significantly more thermally stable. 

Matzger and coworkers [25] reported the synthesis of alkyl substituted poly(T32hTs) and studied their optical properties. It was found that 3,6-dinonyl substitution leads to a drastic increase in bandgap due to the twisting of adjacent T32bT units away from planarity. 

P. R. Surjdn, M. Kertesz, Electronic structure and optical absorption of poly(biisothianaphthene-methine) and poly(isonaphthothiophene-thiophene) two low-bandgap polymers, J. Am. Chem. Soc., 113, 9865-9867 (1991) (j) J. KUrti, P. R. Suijan, Quinoid vs aromatic structure of polyisothianaphthene, J. Chem. Phys., 92, 3247-3248 (1990) (k) E. Faulques, W. WaUnbfer, H. Kuzmany, Vibrational analysis of heterocyclic polymers a com-... 

It is easy to tailor their optical properties by modifying the polythiophenes (PTs) via simple substitution on the main chain, at least for bandgaps varying from 1 to 3 eV [31, 32], The chemical control of bandgaps is not easily achieved for the poly(/ -phenylenevinylene) (PPV) [33] or poly(p-phenylene) (PPP) [34] systems that represent two of the other processable polymer families. 

Because of their high solubility and desirable electrical and optical properties, poly(3-alkoxythiophene)s have been among the most popular derivatives chosen for study. Zotti etal. [54] and Tourillon [55] observed that the poly(3-alkoxythiophene)s possess a lower bandgap and a lower oxidation potential due to the electron-donating effect of the alkoxy substituent, resulting in a polymeric material with high stability in the doped form compared with poly(3-alkylthiophene)s. 

Removal of the solubilizing groups (tert-butyl) has been achieved, and unsubstituted oligorylenes obtained and characterized. These oligomers have outstanding optical and electronic properties. Pentarylene shows a max at 745 nm. Poly(peri-naphthalene) has been predicted to have a bandgap of about 1.0 eV. 

Leclerc et al. [35] have compared POT and PMT with poly(2-ethylaniline) and poly(3-ethylaniline). Poly-(ethylanilines) show a similar electrochromic behavior to polytoluidines. They also exhibit similar optical properties as found for polytoluidines. Poly(ethylani-lines) exhibit multiple and reversible color changes (pale yellow-green-blue violet) depending on the oxidation state and the pH. Poly(2-ethyIaniline) shows a higher conductivity (1 S cm" ) compared to that of polytoluidines (0.3 Scm" ). Ryoo et al. [75] have compared the photoconductivity of POT with those of PANI, poly(o-ethylaniline) and poly(o-anisidine). The photoconductivity spectra of the polymers resemble their electronic absorption spectra. Therefore the band structures of POT and other PANI derivatives are similar to that of PANI. A major contribution to photoconductivity of these polymers comes from the n— n transition while the polaron—tt transition contributes to a lesser extent. Ryoo et al. [75] have proposed that photoexcitation leads to the oxidation of the polymer due to the removal of electrons. The bandgap energies of POT are the lowest (3.03 and 2.48 eV) while those of PANI are the highest (3.45 and 2.82 eV). 

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