Ground-state structural properties

The shapes and curvatures of the backbones of the D–A copolymers and SMAs are controlled by the sizes and shapes of the donor and acceptor units, conforma- tional preferences, and resulting torsions between the neighboring units[111, 178].

Among the studied compounds, the GS geometries of the D–A oligomers have in- deed different shapes and degrees of the curvature (Figures 4.1 and 4.2). Most of them have the sine wave-shaped backbones, except for eD-type DTB-EF-T, whose backbone have a zigzag-shape, and for eD-type BDT-TFQ and eA-type NDI2OD- T2 (Figure 4.3), which have helical structures. The backbone of the eD copolymer

PTQ including the phenyl side groups in the quinoxaline unit has been observed to have a helical shape, as well[150]. However, in this work, the phenyl groups in the studied TQ oligomers have been replaced by hydrogen atoms to ensure the planar backbones (see Section 3.1.3).

In addition to the shapes of the backbone units, the weak non-bonding intramolec- ular interactions between particular atoms (e.g. oxygen, sulfur, nitrogen, hydrogen) of the neighboring backbone units impact the planarity and define the conforma- tional preferences of the D–A copolymers[111]. In Publication I, the global hy- brid functional B3LYP predicts that BDCPDT-TPD and BDT-TPD, which consist of only alternating donor and acceptor units, have planar (0–3^◦) or nearly planar (1–11^◦) backbones, respectively (Figure 4.1). The results obtained with the non- tuned LRC functional CAM-B3LYP for these oligomers are the same. Both com- pounds prefer thesyn-conformation due to non-traditional hydrogen bonding[111]

predicted between the carbonyl oxygen of the TPD acceptor unit and hydrogen of the neighboring donor unit, which enhances the planarity of the backbones. Fur- thermore, the ladder-type donor unit in BDCPDT-TPD increases the planarity com- pared with BDT-TPD, which includes smaller BDT donor unit. In the case of these eD oligomers, the conformation of backbone is not observed to have much effect on their planarity or optoelectronic properties. The GS geometry of BDT-TPD pre- dicted here is inline with the previous theoretical studies of the same compound[8, 9].

BDCPDT-TPD

BDT-TPD

BDT-TFQ

Figure 4.1 Optimized GS geometries of the most stable conformations of the oligomeric models of the eD-type PSC copolymers calculated at the B3LYP/6-31G** level of theory in vacuum. The hydrogens are not shown in the side-view figures (on the right) for the clarity.

As mentioned in Section 2.1.1, the planar backbones of the D–A copolymers have been originally aimed for to ensure the high efficiencies of the corresponding PSCs.

In the case of BDT-TPD, high PCEs (ca. 8.5%) have been predicted for the PSCs based on the corresponding copolymer and PC₇₁BM. However, this is not the case with the ladder-type BDCPDT-TPD, as the PCEs predicted for the device incorpo- rating the corresponding copolymer and PC₇₁BM have been only ca. 6.6%. Thus, the backbone of BDCPDT-TPD might be too rigid leading to the poorer perfor- mance compared to BDT-TPD. However, it should be noted that the efficiency of a PSC and optimal morphology of the active layer are not governed entirely by the backbones of the D–A copolymers, but the choice of the side chains, processing con- ditions, and eA compound may play role, as well.

The backbones of the eD-type oligomers consisting of additional thiophene spac- ers between the donor and acceptor units, i.e. BDT-TFQ, BDT-TzBI, DTB-EF-T, and BDB-T-2F, have larger torsional twists (3–41^◦in vacuum, see the original Pub- lications II and IV for the definitions of the dihedral angles) between the units as predicted by B3LYP and the non-tuned LRC and OT-LRC functionals (Figures 4.1 and 4.2). This is due to the larger degrees of freedom between the units induced by the thiophene spacers. In these compounds, the BDT donor and thiophene units areantito each other due to steric repulsion between the neighboring sulfur atoms,

BDB-T-2F DTB-EF-T BDT-TzBI

Figure 4.2 Optimized GS geometries of the most stable conformations of the oligomeric models of the eD-type NF PSC copolymers calculated at the OT-ωB97X-D/6-31G** level of theory in blend. The hydrogens are not shown in the side-view figures (on the right) for the clarity.

while the thiophene and acceptor units can be eithersyn(BDT-TFQ , BDT-TzBI, and BDB-T-2F) oranti(DTB-EF-T) to each other depending on the possible attrac- tive and repulsive interactions between the neighboring heteroatoms (i.e. sulfur, ni- trogen, oxygen, and fluorine). In line with similar TQ[150], a helical backbone is predicted for BDT-TFQ, which is most probably due to the relative orientations of the backbone units and metoxyphenyl side groups in the quinoxaline acceptor units.

The GS geometry of BDB-T-2F is also consistent with the previous theoretical char- acterizations[16, 19]. As the PSCs employing the corresponding D–A copolymers have had high PCEs of ca. 8.0% for PBDT-TFQ–PC₇₁BM[179]and 9.2–14.2% for the NF PSCs[25, 145, 147], it can be concluded that this type of twisted backbone with thiophene spacers may be more beneficial for the performance of the PSC than the planar, rigid backbone predicted for BDCPDT-TPD.

Among the studied eA compounds, the SMA compounds ITIC and its derivatives, ITIC-2Cl and ITIC-4F, are planar (0^◦, see the original Publications I, II, and IV for the

ITIC NDI2OD-T2

Figure 4.3 Optimized GS geometries of the most stable conformations of the eA-type NF PSC compounds calculated at the OT-ωB97X-D/6-31G** level of theory in blend. The hydrogens are not shown in the side-view figures (on the right) for the clarity.

Table 4.2 Dihedral angles (in degrees)¹and BLA values (in Å)²of the GS geometries of the oligomeric and periodic (in parenthesis) models of PBDT-TFQ predicted with different functionals.

Functional α β₁ β₂ BLA

B3LYP 163 (158) -10 (-15) -7 (-10) 0.036 (0.039) B3LYP-D 160 (-) -12 (-) -10 (-) 0.037 (-) ωB97X 157 (155) -16 (-18) -20 (-19) 0.059 (0.060) ωB97X-D 156 (-) -15 (-) -17 (-) 0.052 (-) OT-ωB97X 161 (160) -21 (-21) -21 (-19) 0.043 (0.044) OT-ωB97X-D 159 (-) -18 (-) -13 (-) 0.042 (-)

1 Between the donor and first thiophene (α), first thiophene and ac- ceptor (β₁), and acceptor and second thiophene (β₂) in the CRU of BDT-TFQ. See Publication II for the definitions of the dihedral an- gles.

2 The oligomeric BLA values were calculated from the innermost CRU of the trimer.

D functional predicts that thesyn-conformation is the most stable for ITIC, i.e. the sulfurs of the IT donor core unit and neighboring carbonyl oxygen of the INCN end-groups prefer to be on the same side. The similar findings have been observed in the previous theoretical calculations with the B3LYP/6-31G** level of theory[180]

and experiments[181], although in the latter case, somewhat larger torsions (4–5^◦) have been predicted for the crystal structures of ITIC by X-ray diffraction analysis.

The differences between the calculated and experimental results may be due to the truncated side chains used here, as the full side chains and the presence of neighboring molecules in the solid state most probably induce additional torsion between the backbone units.

On the contrary, the backbone of eA-type oligomer NDI2OD-2T calculated with ωB97X-D has relatively large twists (ca. 29–60^◦, see the original Publications I, II, and IV for the definitions of the dihedral angles) from planarity (Figure 4.3). This is consistent with the previous findings, although the dihedral angles between the donor thiophene and acceptor NDI units (ca. 60^◦) predicted here are somewhat larger than those obtained both theoretically at the B3LYP/6-31G** level of theory (42^◦and 138^◦) and experimentally (37^◦and 142^◦) via IR and reflection–absorption IR spectroscopy measurements[182]. The differences in the calculated results may be due to the different functionals, while similar to the ITIC derivatives, the differences between the calculations and experiments may originate from the absence of the full side chains in the NDI acceptor and neighboring molecules. Like for ITIC, thesyn- conformation is observed also for NDI2OD-2T, i.e. sulfur atoms of the thiophene donors and the carbonyl oxygens of the neighboring acceptor units are on the same side. This is opposite to some of previous theoretical studies using the global hybrid functionals[182]. However, thesyn-conformation has been predicted also by the (non-tuned) LRC CAM-B3LYP[183].

So far, the GS structural properties of the studied PSC compounds have been pre- sented in the general level without discussing the effect of the functional. The di- hedral angles of the GS geometries of BDT-TFQ predicted by the different func- tionals are in the same range, although the functional has some effect on them (Ta- ble 4.2). The global hybrid functional B3LYP predicts the deviations of 7–17^◦from planarity. The inclusion of the dispersion corrections in B3LYP-D, increases the tor- sions slightly up to 10–20^◦. Both the non-tuned LRC and OT-LRC functionals pre- dict the similar deviations from planarity of 15–24^◦and 13–21^◦, respectively, which are also slightly larger than those of B3LYP. The BLA (middle) values increases in the order of the global hybrid<OT-LRC<non-tuned LRC functionals indicating that the non-tuned LRC functionals predict the less delocalized backbone for BDT- TFQ than other functionals due to the larger amount of HF exchange in them[14].

Similar trends in the effect of the functional on the dihedral angles and BLA values have been predicted by Niskanen and Hukka[14].