Blend MDMO-PPV/PCBM

Fig. 16 Parameters for defining the charge-transfer state energy cx in organic solar cells. Charge-transfer state energy for MDMO-PPV PCBM blend device determined by Fourier transform photocurrent spectroscopy and electroluminescence measurements. Reprinted figure with permission from [188]. Copyright 2010 by the American Physical Society...

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...

Fig. 1.21. (a) Light-induced ESR intensity as a function of the 3-factor in an MDMO-PPV/PCBM blend, = 9.5 GHz, T = 100 K, Aexc = 488 nm, P = 20 lW, 20 mW, and 200 mW. (b) A doubly integrated LESR signal of the prompt contribution as a function of the excitation power dependence. Squares correspond to the positive polaron signal and circles to Cg0... 

Fig. 1.22. High-field LESR in an MDMO-PPV/PCBM blend, = 95 Hz, T = 100 K, Aexc = 448 nm, = 10 mW...

Fig. 5.20. AFM images (acquired in the tapping mode) showing the surface morphology of MDMO-PPV PCBM blend films (1 4 by wt.) with a thickness of approximately 100 nm and the corresponding cross-sections, (a) Film spin-coated from a toluene solution, (b) Film spin-coated from a chlorobenzene solution. The images show the first derivative of the actual surface heights. The cross-sections of the true surface heights for the films were taken horizontally along the dashed lines...

FIG. 5.17. TEM images of MDMO-PPV/PCBM blends with different weight percentages of PCBM as indicated on the upper right corner [136]. 

Hoppe et al. studied MDMO-PPV PCBM solar cells for decoding the different nanophases within these MDMO-PPV PCBM blends cast from the two solvents (toluene and chlorobenzene) [55]. A large difference in the scale of phase separation could be identified as a major difference between toluene and chlorobenzene cast blends (see Fig. 23), but this could not directly explain the observed differences in photocurrent generation [55]. 

The commonly observed larger scale of phase separation of the toluene cast MDMO-PPV PCBM blends has generally been interpreted as the main reason for the reduced photocurrents in comparison to those of the chlorobenzene cast blends. It can then be expected that a lower charge carrier generation efficiency may result when exciton diffusion lengths of 10-20 nm are well exceeded by the PCBM cluster size (200-500 nm), since many exci-tons are generated within these clusters. Experimentally, it has been identified that indeed some unquenched photoexcitations give rise to residual PCBM photoluminescence in toluene cast blends, whereas in chlorobenzene cast... 

Fig. 24 SEM cross sections of cMorobenzene (a, b) and toluene (c, d) based MDMO-PPV PCBM blends. Whereas chlorobenzene based blends are rather homogeneous, toluene cast blends reveal large PCBM clusters embedded in a polymer-rich matrix or skin layer. Small features—referred to as nanospheres —are visible in all cases and can be attributed to the polymer in a coiled conformation. The blending ratio is depicted in the lower right corner. (Reproduced from [55] with permission, 2004, Wiley-VCH)...

Fig. 26 Differences in the chlorobenzene (a) and toluene (b) based MDMO-PPV PCBM blend film morphologies are shown schematically. In a both the polymer nanospheres and the fullerene phase offer percolated pathways for the transport of holes and electrons, respectively. In b electrons and holes suffer recombination, as the percolation is not sufficient. (Reprinted from [61], 2005, with permission from Elsevier)...

Fig. 27 Compositional dependence of electron and hole mobilities in MDMO-PPV PCBM blend films as obtained by space charge limited diode currents. Clearly the mobility for holes is increased upon addition of the fullerene. (Reproduced from [144] with permission, 2005, Wiley-VCH)...

For example, controlled annealing of MDMO-PPV/PCBM blend films causes the differential phase crystallization. As a result, the PCBM single crystals grow gradually with annealing time and stick out of the film plane [428]. 

Figure 2.7 AFM topography images of MDMO-PPV/PCBM blend films (MDMO-PPV/PCBM = 1 4, w/w) in situ recorded upon annealing at 130°C for (a) pristine film ...

Figure 49 SEM cross-section images of MDMO-PPV PCBM blend films cast on ITO-glass from (a) chlorobenzene and (b) toluene solution. The brighter objects (a) are polymer nanospheres, whereas the darker embedments are PCBM clusters. Schematic of film morphology of (c) chlorobenzene- and (d) toluene-cast MDMO-PPV PCBM blend active layws. (Reproduced from Ref. 160. American Chemical Society, 2005.)...

Figure 9.6 Morphology of MDMO-PPV PCBM blends, (a) BHJ thin-film processed from toluene solution (b) BHJ thin-film processed from chlorobenzene solution. A clear difference in morphology is seen. Reprinted from ref. 57 with permission from Wiley-VCH.

Chambon, S. Rivaton, A. Gardette, J.-L. and Firon, M., Photo- and thermal degradation of MDMO-PPV PCBM blends,5o/ur Energy Mater. Solar Celh,9 (5), 394-398 (2007). 

---