Conjugated polymers attracted great interest as photovoltaic materials in the recent years [1, 2]. Photoinduced electron transfer from conjugated polymers to fullerene acceptors [3] and the bulk heterojunction concept [4] offer an efficient way for charge separation. Consequent engineering of polymer/fullerene composite morphology and the contacts of the devices resulted in an improvement of the power conversion efficiency of these solar cells up to h > 2.5 % under AM1.5 illumination [5].
The limiting parameter of this system is the spectral mismatch between the absorption (E_g > 2.2 eV) of the active layer and the terrestrial sunlight spectrum. For efficient light harvesting, photovoltaic materials with bandgaps E_g < 1.8 eV are needed. For improved spectral coverage of the available sunlight, additional absorbers for the range < 2 eV have to be introduced.
In this contribution we present photovoltaic devices made of the novel low bandgap polymer PTPTB [6,7], chemical structure is presented as insert in figure 1a, which has an absorption maximum near the IR spectral region. Furthermore, we successfully sensitized this cell with highly absorbing organic dye's. For the observed contribution of the organic dye to the photocurrent, an energy transfer from the dye to the polymer is presumed.
Devices are prepared in sandwich configuration. ITO/PEDOT-PSS serves as anode substrate and LiF/Al as top cathode. The active layers are spincast from polymer/PCBM 1/3 toluene solutions. The structure of PTPTB is shown as inset in figure 1a. For the dye sensitised solar cells, 10-weight % dye is added to the polymer. The structure of Nile Red is shown as insert in figure 1b.

Figure 1 (a) I-V curve of a PTPTB/PCBM cell and (b) PTPTB/PCBM + nile red in the dark (---) and under AM 1.5 conditions (——)

The bandgap of PTPTB is determined optically and electrochemically as ~1.6 eV. Figure 1a shows the I-V curve of a PTPTB/PCBM in the dark and under AM 1.5 simulated illumination. The open circuit voltage is just 100 mV below the one of MDMO-PPV cells, after a bandgap reduction of > 600 meV. The low fill factor may originate from the poor film quality giving rise to many shunts. Photoactivity of the polymer is shown with the IPCE measurements, figure 2a.

Figure 2 IPCE (——) and % of absorbed photons in transmission (---) of (a) PTPTB/PCBM and (b) PTPTB/PCBM + nile red

Energy transfer in dye/polymer mixtures is investigated by photoluminescence studies. In the two-component blend of the wide bandgap dye nile red, the structure is shown as insert in figure 1b, and the low bandgap polymer PTPTB, the luminescence of the dye is quenched, whereas the polymer luminescence is increased. After addition of PCBM, no luminescence is detectable.
Devices from the three component blend shows high rectification and a clear photoeffect under illumination, shown in figure 1b. The IPCE match the absorption spectrum.

References

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[6]	A. Dhanabalan, P.A. Van-Hal, J.K.J. Van-Duren, J.L.J. Van-Dongen,R.A.J. Janssen, Synthetic Metals 119, No.1-3, 169 (2001).
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