MDMO optical absorption

Fig. 5.23. (a) Optical absorption spectra of 100 nm thick MDMO PPV PCBM films (1 4 by wt.) spin-coated onto glass substrates from either toluene (dash-dotted line) or chlorobenzene (solid line) solutions, (b) IPCE spectra for photovoltaic devices using these films as the active layer... 

Fig. 1.18. Spectrally resolved pump-probe spectrum of pristine MDMO-PPV compared to highly fullerene-loaded MDMO-PPV/PCBM composites at various delay times, (a) Absorption spectrum of a pure MDMO-PPV film (solid line) and AT/T spectrum at 200 fs pump-probe delay (dashed line), (b) AT/T spectra of the MDMO-PPV/PCBM blend (1 3 wt. ratio) at various time delays following resonant photoexcitation by a sub-10-fs optical pulse. The CW PA of the blend ( ) was measured at 80 K and 10-5 mbar. Excitation was provided by the 488 nm line of an argon ion laser, chopped at 273 Hz...

A further improvement of MDMO-PPV based bulk heterojunctions was achieved by the application of a new C70 fullerene derivative, which was substituted with the same side chains as PCBM and is therefore called [70]PCBM [170]. Due to the reduced symmetry of C70 as compared to the football sphere (icosahedral symmetry) of Ceo. more optical transitions are allowed and thus the visible hght absorption is considerably increased for [70]PCBM. This led to an improved external quantum efficiency (EQE) of MDMO-PPV based solar cells reaching up to 66% (Fig. 30). As a result the power conversion efficiency was boosted to 3% under AM 1.5 solar simulation at 1000 W/m [170]. 

Figure 10.10. The onset of the absorption of PTPTB is around 780 nm, which corresponds to an optical band gap of 1.6 eV. When mixed with the electron acceptor PCBM, the characteristic spectroscopic signatures of photoinduced charges are observed [74]. Spectrally resolved photocurrent measurements of a device based on PTPTB/PCBM show clearly that the spectral sensitivity is extended to longer wavelength by almost 200 nm as compared to the MDMO-PPV-PCBM mixture. The power conversion efficiency of 1% is limited by the low FF.

---