Chapter 3. Morphology Control using Macromolecular Additives
3.2. Results and Discussion
3.2.2 Optical Properties and Film Morphologies
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to 7.55%, which is beyond the hitherto reported highest PCE of the DTGe(FBTTh2)2-based OSCs even using an optical spacer.10 Among the macromolecular additives studied, both PC61BM- and PC71BM-based devices processed with PMMA showed the largest extent of performance improvement, respectively.
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and therefore could be ignored. For the BHJ films processed with macromolecular additives, there was no significant change in packing orientations when compared with pristine films. This indicates that both films with and without macromolecular additives are similarly semi-crystalline, adopting edge-on packing relative to the substrates.
On the other hand, together with the presence of intensified peaks around 1.35 Å-1 assigned to PCBM derivatives, both the PC61BM- and PC71BM-based samples with DIO not only exhibited more intense and sharper (001) peaks along qz, but also a strong π-π stacking peak near the qz plane was clearly observed and this suggested improved microstructural ordering with either the co-existence of face-on and edge-on orientations or the tilted orientation.
Figure 3.4. GIWAXS patterns and π-π CCLs for the best OSCs processed with or without additives. (a ̴ e) DTGe(FBTTh2)2:PC61BM and (f ̴ j) DTGe(FBTTh2)2:PC71BM GIWAXS images, (k) and (l) π-π CCL values respectively.
One may conclude that the introduction of DIO enhances the crystallinity of both the edge-on and face-on DTGe(FBTTh2)2 lamellae, with reduced edge-on crystallites and an increased population of face-on ones.
This is most likely attributed to the superior PCEs in the devices with DIO, since the face-on geometry is postulated to be the preferred orientation in BHJs because the average charge transport direction is
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commensurate with device geometry.13 It should be noted that a closer look at the line-cut profiles along qxy reveals another interesting feature in the PMMA-treated DTGe(FBTTh2)2:PC61BM - a splitting of the π- π stacking (141) is observed, which indicates there is a double population of π-π stacking distances. This is identification of one of the specific points of interaction between PMMA and DTGe(FBTTh2)2 and reasonably attributed to the high PCE obtained from DTGe(FBTTh2)2:PC61BM devices with PMMA.
We also calculated crystallite correlation length (CCL) of π-π stacking lattice planes along qxy by using Debye Scherrer’s equation,14 where Gaussian fitting is used to obtain full widths at half-maximum (FWHM) values (Figure 3.4 k, l). All DTGe(FBTTh2)2:PC61BM films with macromolecular additives had nearly similar CCL values in the range of 20±4 nm, while a larger CCL (32±1nm) was observed when using the DIO additive. Note that the calculated CCL for PMMA-treated DTGe(FBTTh2)2:PC61BM would be somewhat smaller than the real value because of the existence of π-π stacking peak splitting, as mentioned earlier. In the cases of DTGe(FBTTh2)2:PC71BM, similar variation trends were found. These results confirm that BHJ films processed with DIO as a small additive versus macromolecular additives have larger π-π CCL values, likely because of increased π-π interactions dominating film formation. This would therefore allow DTGe(FBTTh2)2 molecules to pack with a longer-range order and is in solid agreement with what is observed in the GIWAXS profiles of DIO treated films.15
This matched well with the present study’s AFM observations (Figure 3.5); the root mean square (RMS) roughness (Rq) values do not vary significantly by using different macromolecular additives, while DIO results in larger surface roughness compared to the corresponding non-additive treated films. As a point of interest, in a close up of the phase images (section 3.5, Figure 3.10), the films proceeded with macromolecular additives, especially those containing PDMS, and have a pronounced nano-fibril formation relative to the DIO systems. Such nano-fibrillar features have been previously reported for other small molecule-based BHJ systems processed with PDMS.9b
Figure 3.5. Morphology analysis of the best OSCs processed with or without additives by AFM (1×1 µm).
(Top: a ̴ e) DTGe(FBTTh2)2:PC61BM and (Bottom: f ̴ j) DTGe(FBTTh2)2:PC71BM.
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Next, HR-TEM was measured to monitor the morphological features of the blend films more directly.
Consistent with the AFM morphology, in HR-TEM images (Figure 3.6), the pristine films without additives do not show obvious contrast that would emanate from largely homogenous morphology, while in the additive-treated samples, wire-like structures with nanoscale phase separation are evident from the contrast deviation.
Figure 3.6. Morphology analysis of the best OSCs processed with or without additives by HR-TEM (Scale bar is 0.2 µm). (Top: a,c~f) DTGe(FBTTh2)2:PC61BM and (Bottom: b,g~j) DTGe(FBTTh2)2:PC71BM.
Compared with the TEM images of films with macromolecular additives, the wire-like structures are more vivid with DIO. These results correlate well with the morphology trends induced by macromolecular additives versus DIO observed with AFM and GIWAXS. Note that there is subtle difference in a closer
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look of the TEM images between PC61BM- and PC71BM-based blend films proceeded with macromolecular additives, while both PC61BM- and PC71BM-based films with DIO have similar features. In addition, the observed better crystallinity of DTGe(FBTTh2)2:PC61BM films compared to the corresponding PC71BM- based analogues is one important reason that can contribute to the enhanced FF values, though this cannot be solely the result of the changes seen in BHJ OSCs. It is worth highlighting that similar crystalline domain sizes (length = 30±10 nm, width = 10±3 nm) are observed in films with PMMA (Figure 3.6 e) or DIO (Figure 3.6 f,j), being beneficial for exciton separation and charge transport.16 This is likely the cause of the higher PCEs in the PMMA- and DIO-treated devices.
Even though all results described here are associated with molecular packing/crystallinity and cannot provide solid microscopic clues towards understanding the largely improved PCEs by adding PMMA, it is still believed that favorable morphology, as in the DIO case, comes about with assistance from the addition of the optimized content of PMMA, and therefore, the OSCs with either PMMA or DIO outperform those of other additives. Another possible reason is that friendly active materials-PMMA interactions may lead to a beneficial effect on several key device metrics.