Band gap polymers

When doped, low band-gap polymers have optical transitions in the infrared region of the spectmm, and therefore transmit more visible light in the conducting form than in the insulating form. This feature enables this class of conducting polymers to be investigated for a number of optical appHcations where both electrical conductivity and optical transparency are desired. 

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. 

There is some interest in the spectral properties of copper complexes of thieno[2,3-d]-pyrimidines <88ICA165>, and of polymers of thieno[2,3-6]pyrazines <92CC1672, 93SM960). The latter have been investigated as low band-gap polymers. 

Fig. 9.16 Structures of (a) polyisothianaphthalene, (b) the dicyano substituted analogue and (c) a model small band gap polymer.

Nowadays, a great number of different monomers are known that form conducting polymers upon chemical or electrochemical preparation techniques. Such starting systems include substituted derivatives of the already mentioned hydrocarbons and heterocycles as well as specially tailored novel compounds, such as for the generation of small band-gap polymers with high transparency in the conducting state (Fig. 1). 

In combination with a C70 fullerene derivative—PTPF70—the system yielded improved power conversion efficiencies of 0.7% due the increased absorption of the C70 derivative [208,210]. Using similar polyfluorene derivatives, power conversion efficiencies of about 0.9% (APFO-Green2) [212[ and 2.2% (APFO-Green5) [213] were achieved. Other low band gap polymers, with absorption spectra extending up to 1100 run, yielded efficiencies of aroimd 1% [214-217]. 

Recently, the Konarka group achieved power conversion efficiencies of 5.2% for a low band gap polymer-fuUerene bulk heterojunction solar cell, as confirmed by NREL (National Renewable Energy Laboratory, USA). This encourages the practical use of this concept for low cost, large area production of photovoltaic devices. 

Colladet K, Nicolas M, Goris L, Lutsen L, Vanderzande D (2004) Low-band gap polymers for photovoltaic applications. Thin Solid Films 451-452 7... 

Campos LM, Tontcheva A, Giines S, Sonmez G, Neugebauer H, Sariciftci NS, Wudl F (2005) Extended photocurrent spectrum of a low band gap polymer in a bulk heterojunction solar cell. Chem Mater 17 4031... 

Rasmussen has prepared low band-gap polymers, 139, by electropolymerization of thieno[3,4-ri pyrazinc.91 These polymers are anticipated to coordinate to metals via the iV-donors of the pyrazines to offer environments akin to terpy ligands.92... 

We have shown nanomorphology of the composite layer strongly influences the BHJphotovoltaic performanceinRR-P3HT PCBMsystem. This is also truefor the low band-gap polymer system when C70-PCBM is used. Because C70-PCBM... 

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