PCBM crystallization

The growth of large micron-sized PCBM crystal domains depended on the initial concentration of PCBM in the blend as well as on the duration of the... 

The authors also correlated the increase in device efficiency with an increased crystallinity in the P3HT phase, but in contrast to Kim et aL they did not detect any PCBM crystals in the blend film [181]. Interestingly, the observed increase in optical absorption in the range of 1.9 to 3.0 eV upon annealing is directly correlated to the crystallization-induced ordering of P3HT in these blends (see with Fig. 36) [182]. 

Figure 2.4 A comparison of electron diffraction pattern of superposed diffractions from iarger PCBM crystals (left part) and diffraction rings from nanocrystaiiine PCBM (right part). (Reprinted with permission from Ref [10], Copyright 2010, Wiley-VCH VeriagCmbH.)...

The choice of casting solvents became a clear issue in 2001 when Shaheen et al. published an article showing a PCE increase firom 1.1 to 2.5 % for MDMO-PPV PCBM OPV devices cast from toluene and chlorobenzene, respectively [36]. Subsequent studies showed that toluene was a better solvent for the MDMO-PPV than PCBM, so the PCBM crystallized out of solution with large domain sizes (>1 pm) that reduced the device quality [69]. However, chlorobenzene and dichlorobenzene are equally good solvents for both polymer and fullerene so the polymer and fullerene remain miscible to higher concentrations and the resulting domain sizes are much smaller (tens of nanometers) [69]. 

Also, heating to elevated temperatiu es increases the diffusion rate of PCBM in P3HT, which leads to the formation of extended PCBM crystals and reduced device... 

Fig. 35 Diffraction diagram (grazing incidence) of untreated and annealed P3HT PCBM blend films deposited on glass/ITO/PEDOT PSS substrates left). The corresponding a-axis orientation of the crystals is shown on the right. (Reproduced with permission from [181], 2005, Wiley-VCH)...

A simple BHJ solar cell was fabricated with C6PCH2 and PCBM by spin coating. For a 60 40 wt% blend of C6PCH2 and PCBM, power conversion efficiency (PCE) showed 3.1 % and >70 % of EQE was observed at Q-band region. Studies on the binary mixtures of C6PCH2 and PCBM indicate that the addition of PCBM into C6PCH2 leads to deformation of columnar lattice in the Col mesophase with decrease of isotropization temperature. At ca. 25 mol % of PCBM, one can see the eutectic point where the Col mesophase disappears. This molar ratio implies in the crystal blend that chain-like arrangement of PCBM is positioned... 

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]. 

M.T. Rispens, et al, Influence of the solvent on the crystal structure of PCBM and the efficiency of MDMO-PPV PCBM plastic solar cells. Chemical Communications, 2003 p. 2116-2118. 

Figure 2.6 Bright-field TEM images demonstrating the formation of PCBM single crystals with time from 80wt% MDMO-PPV/PCBM composite films upon annealing at 130°C under different spatial confinements ...

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