Chapter 3 Mapping suitable donor-acceptor couples in NF-PSCs
3.1 Modulating the molecular packing and nanophase blending via random terpolymerization
3.1.2 Results and discussion
3.1.2.3 Morphology Characterization
To understand the correlation between the structure and performance difference of NF-PSCs, grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were first performed on the neat films.
The neat PTPTI-T100 film exhibits strong (010) diffraction of - stacking in the out-of-plane direction (Figure 3.1.3), indicating a preferable face-on molecular packing orientation with respect to the substrate. However, the neat PTPTI-T50 and PTPTI-T30 polymers seem to possess mostly edge-on packing, as evidenced by the distinct (010) - diffraction in the in-plane direction and invisible (010) diffraction in the out-of-plane direction. An intriguing feature is that PTPTI-T70 crystallites clearly
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display both the edge-on and face-on orientations, a so-called mixed orientation. The change in the local molecular - stacking observed in the neat polymer films probably originates from the different pre- aggregation behavior in the solvent or the morphological evolution during film drying. Moreover, the intense (010) - diffraction observed in the m-ITIC neat film in the out-of-plane direction indicates its predominant face-on orientation.
Figure 3.1.3 (a) GIWAXS images of the neat material films: (i) PTPTI-T100, (ii) PTPTI-T70, (iii) PTPTI-T50, (iv) PTPTI-T30, (v) m-ITIC. (b) Corresponding line cuts of the GIWAXS images.
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Figure 3.1.4 (a) GIWAXS images of the PTPTI-Tx:m-ITIC blend films: (i) PTPTI-T100:m-ITIC, (ii) PTPTI- T70:m-ITIC, (iii) PTPTI-T50:m-ITIC, (iv) PTPTI-T30:m-ITIC. (b) Corresponding in-plane and out-of-plane line cuts of the GIWAXS images of PTPTI-Tx:m-ITIC blend films. (c) Pole figure plots from the (100) lamellar diffraction in the PTPTI-Tx:m-ITIC blend films, the fraction values in parentheses are the ratios of integrated 0‒45°
and 135‒180° (Axy) to 45‒135° (Az) area. (d) Corresponding d-spacings and coherence lengths estimated from the face-on (010) diffraction in the PTPTI-Tx:m-ITIC blend films.
Upon blending the PTPTI-Tx polymers with m-ITIC, the (100) lamellar diffraction appeared along both the out-of-plane and in-plane axes, implying the coexistence of the edge-on and face-on crystallites in the blend films (Figure 3.1.4a, b). The relative change in the orientation distribution of crystallites in the blend films was quantified by pole figure extraction from the (100) diffraction (Figure 3.1.4c).123 The integrated intensities within the azimuthal angle (χ) are defined as fractions of face-on and edge-
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on crystallites, respectively. A spatially averaged value of the Axy to Az ratio (Axy/Az) was used to assess the ratio of face-on to edge-on crystallites. The Axy/Az values of PTPTI-T100, PTPTI-T70, PTPTI-T50, and PTPTI-T30 blend films were found to be 0.41, 0.54, 0.30, and 0.17, respectively, indicating a relatively larger population of the face-on crystallites in the best-performing PTPTI-T70:m-ITIC system.
It can be observed that the variation in the Axy/Az values has the same trend as that in the JSC and PCE values obtained from PTPTI-Tx-based NF-PSCs. The obtained findings agree with an actual consensus on the well-developed conjecture, that the face-on orientation geometry is more favorable for photovoltaic applications owing to the existence of vertical charge-transportation channels. [12a, 13a]
Considering this, the origin of the different performance of PTPTI-Tx:m-ITIC systems can be understood from the viewpoint of the different packing orientation as one key factor.
Figure 3.1.5 (a) AFM height images and (b) TEM images of the PTPTI-Tx:m-ITIC blend films: (i) PTPTI-T100:m- ITIC, (ii) PTPTI-T70:m-ITIC, (iii) PTPTI-T50:m-ITIC, (iv) PTPTI-T30:m-ITIC.
Moreover, the d-spacings and coherence lengths (CCLs)124 of the face-on (010) diffraction peaks in the blend films (Figure 3.1.4d) were calculated. It is interesting that the PTPTI-T70 blend exhibits a lower CCL(010) value of 23.58 Å compared with the other samples, indicating a relatively smaller crystallite size; however, there is no obvious difference in the d-spacing values among the blend films. Even though the correlation of CCL with the photovoltaic performance is exceptional, other parameters such as local phase segregation and domain size must be considered to establish valid structureperformance relationships for PTPTI-Tx-based NF-PSCs.
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Therefore, the top surface and bulk morphology of the blend films were further investigated via atomic force spectroscopy (AFM) and transmission electron microscopy (TEM), respectively. A relatively smooth surface (Figure 3.1.5a) with a low root-mean-square (Rq) value of 1.48 nm was observed in PTPTI-T70:m-ITIC, which implies that the miscibility between the donor and acceptor components improved within the blend. This is in good agreement with the previously discussed CCL results.
Furthermore, the TEM images of the blend films reveal partially visible phase-separated regions with fibril-like networks (Figure 3.1.5b). Notably, the PTPTI-T70:m-ITIC blend exhibits more uniform feature with a smaller domain size of around 10‒20 nm, which is similar to that observed for other high- performance PSCs [4,14]. In addition, the PL of PTPTI-T70:m-ITIC is markedly quenched through efficient exciton dissociation and energy transfer process, providing additional evidence of the absence of aggressive polymer aggregates, which correlates with the enhanced JSC in this blend system. The well-intermixed morphology with smaller phase-separated domains for the PTPTI-T70:m-ITIC blend could be partly associated with the relatively lower interfacial tension (γ) value between PTPTI-T70 and m-ITIC, that was estimated by contact angle measurement (Table 3.1.3).
Table 3.1.3 Contact angles and calculated surface tensions of PTPTI-Tx and m-ITIC and the interfacial tensions in PTPTI-Tx:m-ITIC
Polymer θwater [deg] θglycerol [deg] Surface tension [mN m-1]
Interfacial tension [mN m-1]a)
PTPTI-T100 101.88 88.33 22.94 3.60
PTPTI-T70 100.75 88.32 22.49 2.41
PTPTI-T50 100.36 87.45 23.04 2.64
PTPTI-T30 99.74 85.66 24.31 3.35
m-ITIC 89.17 80.35 26.24 -
a)The interfacial tension values of PTPTI-Tx:m-ITIC blends.
Collectively, charge transport and/or exciton dissociation characteristics are improved in the PTPTI- T70:m-ITIC blend, which is attributed to the favorable face-on oriented packing and the small-scale phase separation with well-distributed microstructure, ultimatelyleading to the improvements of both FF and JSC in the best PTPTI-T70:m-ITIC combination.