Chapter 5 A synergetic effect of molecular weight and fluorine in all-PSCs
5.2 Results and discussion
5.2.3 Morphological Properties
Atomic force microscopy (AFM) was used to obtain insight into the blend film morphology (Figure 5.4). All AMF images of the 3 3 matrix of the blend films exhibit small-length scale microstructures with a root-mean-square roughness in the range of 0.632.01 nm. Although the domain sizes and surface features are somewhat sensitive to both Mn and F variations, the AFM images have limitations in resolving the systematic changes as a function of Mn and F content. Therefore, we performed bright- field transmission electron microscopy (TEM) and grazing incident wide-angle X-ray scattering (GIWAXS) to better understand the synergetic effects of Mn and F substitution on the morphology, molecular orientation, and crystallinity. Figure 5.5 shows the TEM images of the polymer-polymer blend films 3 3 matrix. The TEM images in vertical columns (i.e., varying F content of the donor polymer within the given acceptor sets) reveals a change in phase-separated domains sizes. The domain sizes exhibit the following order: TQ TQ-F TQ-FF. TQ-based blend films show homogeneous morphology and no distinct phase separation while the phase-separated microstructures are observed in the two fluorinated samples. Note that the TQ-F-based blends have smaller and better-interconnected phases compared to the TQ-FF with aggressive coarser domains. The fine nanoscale phase separation with interpenetrating network is known to be beneficial for generating efficient exciton dissociation and charge transport to achieve high Jsc and FF.127
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Figure 5.4 AFM height images of the blend films.
Figure 5.5 TEM images of blend films: (i) TQ:L-P(NDI2OD-T2), (ii) TQ:M-P(NDI2OD-T2), (iii) TQ:H- P(NDI2OD-T2), (iv) TQ-F:L-P(NDI2OD-T2), (v) TQ-F:M-P(NDI2OD-T2), (vi) TQ-F:H-P(NDI2OD-T2), (vii) TQ-FF:L-P(NDI2OD-T2), (viii) TQ-FF:M-P(NDI2OD-T2), and (ix) TQ-FF:H-P(NDI2OD-T2). Scale bars are 20 nm.
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And the degree pf phase separation in BHJ morphologies could be partly associated with the interfacial tension (γ) value between the donor and acceptor estimated via contact angle measurement (Table 5.2), that generally lower γ value in blends leads to smaller phase-separated domains.128 On the other hand, a clear contrast is not observed in the TEM images in horizontal (i.e., varying Mn of the acceptor polymer within the given donor sets).
Table 5.2 Contact angles and calculated surface tensions of neat polymer and interfacial tensions between the donor and acceptor polymer
Polymer θglycerol (deg) θwater (deg) Surface tension (mN m-1)
Interfacial tension (mN m-1)
TQ 85.53 99.26 24.21 0.22a) 0.39b) 0.65c)
TQ-F 87.61 99.91 22.76 0.54a) 0.64b) 1.02c)
TQ-FF 86.99 98.48 22.85 1.08a) 1.18b) 1.70c)
L-P(NDI2OD-T2) 88.57 102.80 23.18 - - -
M-P(NDI2OD-T2) 91.50 105.06 21.67 - - -
H-P(NDI2OD-T2) 91.75 105.83 21.88 - - -
a)The interfacial tension values of each donor polymer: L-P(NDI2OD-T2); b)The interfacial tension values of each donor polymer: M-P(NDI2OD-T2); c)The interfacial tension values of each donor polymer: H-P(NDI2OD-T2).
In addition to the degree of phase separation, all-PSCs performance is also closely related to the polymer packing structures in neat and blend film. As shown in Figure 5.7, for all neat donor films, as the number of F substituents increases, the intensities of both out-of-plane (010) peaks and in-plane (100) peaks are notably enhanced; at the same time, the intensities of the out-of-plane (100) peak gradually fade away.
Figure 5.6 DFT-optimized geometries and calculated dihedral angles of TQ, TQ-F, and TQ-FF donor polymers using Gaussian 09 package with the nonlocal hybrid Becke three-parameter Lee–Yang–Parr (B3LYP) function and 6-31G* basis set.
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These results imply that both fluorinated donor polymers preferentially adopt a face-on orientation toward the substrate with a higher degree of lamellar ordering and stacking. We observe that the degree of F substituents has a large influence on ππ stacking in the crystalline regions. For example, the ππ stacking distances (d(010)) are 4.2 Å for TQ and 3.7 Å for the two fluorinated polymers (TQ-F and TQ-FF). It is thus confirmed that adding F substituents to the donor polymers can enhance the inter- chain π-π stacking attributed to the relatively high co-planarity of the backbones induced by intramolecular interactions. The dihedral angles for unimer models based on the three donor polymers are calculated as 22.77o, 16.68o, and 18.71o by using density functional theory simulation at the B3LYP/6-31G* level, providing a further evidence for improved planar conformations in fluorinated polymers (Figure 5.6). Besides, a change in the molecular packing of the acceptor polymers as a function of Mn is also observed. For example, the multiple out-of-plane (h00) diffraction peaks become fainter and transfer into the in-plane direction with increasing Mns. This indicates that a relatively high degree of crystalline ordering with a preferential face-on orientation is achieved for H-P(NDI2OD-T2).
A similar crystalline behavior caused by Mn variation was also reported by Kim et al recently.111
Figure 5.7 2D GIWAXS images of neat polymers.
All blend films tested in all-PSCs were examined and are provided in Figure 3.1.8, where the arrangement of vertical and horizontal series leads us to visualize the changing trends as a function of Mn and F substitution, respectively. By comparing the blend films 3 3 grid, we can summarize the effects as follows: (i) From the vertical columns, the diffraction intensities in both out-of-plane (010)
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and in-plane (100) peaks are gradually increased from top to bottom, verifying the preference for face- on orientation as the number of F substituents increases, as observed from the scattering patterns of the neat donor films. In particular, the TQ-F-based blend films have slightly smaller d(010) spacings (3.73.8 Å) compared to the TQ and TQ-FF blend sets. (ii) The horizontal rows from left to right signify the impact of the acceptor Mns on the given donor polymer series. For TQ-based blend films, increasing the acceptor Mns leads to not only the increase of the intensity of the (010) peaks but also the decrease of the d(010) spacings, indicating that a strong crystallization process occurs in TQ:H-P(NDI2OD-T2).
On the whole, simultaneous turning of Mn and F content can not only alter the degree of phase separation but also the molecular packing and orientation in the blends; the nanoscale continuous phase separation together with preferred face-on π–π stacking make TQ-F:H-P(NDI2OD-T2) show the highest photovoltaic performance.
Figure 5.8 2D GIWAXS images of blend films: (i) TQ:L-P(NDI2OD-T2), (ii) TQ:M-P(NDI2OD-T2), (iii) TQ:H- P(NDI2OD-T2), (iv) TQ-F:L-P(NDI2OD-T2), (v) TQ-F:M-P(NDI2OD-T2), (vi) TQ-F:H-P(NDI2OD-T2), (vii) TQ-FF:L-P(NDI2OD-T2), (viii) TQ-FF:M-P(NDI2OD-T2), and (ix) TQ-FF:H-P(NDI2OD-T2).
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