a Calculated by CV measurements.b Derived from DFT calculations.
Furthermore, cyclic voltammetry (CV) was employed to estimate the energy
levels of the NFREAs in solid state. The highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy
levels derived from the oxidation/reduction onsets of the CV curves
(Figure S1) are −5.62/−4.00, −5.49/−3.90, and −5.57/−3.97 eV for
NoCA-17, NoCA-18, and NoCA-19, respectively (Figure 1d and Table 1).
Moreover, the trend of DFT calculated E HOMO/LUMOvalues for three NFREAs is well consistent with CV results (Figure S2).
The optimized molecular geometries of the three NFREAs were investigated
by performing density function theory (DFT) calculations at the
B3LYP/6-31G(d,p) level. The intramolecular distances between the sulfur
atom of the thiophene ring and oxygen atom of the phenylalkoxy motif are
found to be ~2.71 Å for all these three acceptors
(Figure 2a), much shorter than the sum of the van der Waals radius
(r w, S···O = 3.25 Å), indicating the existence of
S···O NoCLs. The dihedral angles between the central phenyl core and the
adjacent thiophene ring are 7.1° and 7.4° for NoCA-17 and NoCA-18,
respectively. In the case of NoCA-19, the dihedral angles decrease
slightly to 7.1/6.6°, indicating a more coplanar π -conjugated
backbone due to asymmetric substitution of end-groups.
The packing behaviors in neat films were explored by measuring the
two-dimensional grazing incidence wide-angle X-ray scattering
(2D-GIWAXS). In the in-plane (IP) direction, the neat NoCA-17 film
exhibits a lamellar packing diffraction peak atq xy = 0.30 Å−1 (d~ 22.67 Å), as shown in Figures 1b and S3. Additionally,
the π -π stacking diffraction signal is observed in the
out-of-plane (OOP) direction at q z = 1.61
Å–1 (d ~ 3.90 Å), suggesting
the predominant face-on orientation of NoCA-17. In the neat NoCA-18
film, the lamellar packing diffraction peak is also observed atq xy = 0.30 Å−1 in the IP
direction, while the π -π diffraction peak is located atq z = 1.78 Å–1 (d~ 3.53 Å), indicating that the end-group NC-2F can
enhance intermolecular interactions and achieve tighter packing.
Surprisingly, the neat NoCA-19 film exhibits a balancedπ -π stacking distance (q z = 1.74
Å–1, d ~ 3.62 Å), which is
closer than NoCA-17 and looser than NoCA-18. The results demonstrate
that the asymmetric end-group engineering can effectively regulate the
packing behaviors in the neat films, enabling NoCA-19 to possess
moderate π -π stacking distance and appropriate
crystallinity.
Photovoltaic properties
To investigate the photovoltaic performance of the three NFREAs, OSC
devices were fabricated with a conventional structure of
ITO/PEDOT:PSS/active layer/PDINO/Al based on J52:NoCA-series acceptors
blends. The current density-voltage (J –V ) characteristics
are shown in Figure 3a. The detailed device parameters and the
optimization process can be found in Tables 2 and S1-S3. The optimized
device based on J52:NoCA-17 exhibits an inferior PCE of 9.50%, with an
open circuit voltage (V oc) of 0.804 V, a short
circuit current density (J sc) of 21.55 mA
cm-2, and a fill factor (FF) of 54.81%. Furthermore,
the device based on J52:NoCA-18 shows a decrease inV oc (0.792 V) but an improvement inJ sc (22.72 mA cm-2) and FF
(65.51%), resulting in a higher PCE of 11.77%. Impressively, the
NoCA-19 based device exhibits the champion PCE of 12.26% with an much
higher J s (23.11 mA cm-2) and
FF (67.11%). External quantum efficiency (EQE) measurements were
carried out to verify the above J sc values
(Figure 3b). The integrated J sc values of the
devices based on J52:NoCA-17, J52:NoCA-18, and J52:NoCA-19 are 20.80 mA
cm−2, 21.90 mA cm−2, and 22.16 mA
cm−2, respectively, The corresponding mismatches are
less than 5%, as compared to the values from J –Vcharacteristics.
Space charge limited current method was used to evaluate charge
transport properties in the three blend films, as shown in Figure S4.
The hole mobility (μ h) and electron mobility
(μ e) were calculated to be 8.74 ×
10−4 and 6.69 × 10−4cm2 V−1 s−1 for
the NoCA-19 blend film, respectively. These values were found to be much
higher than those of the NoCA-17-based blend film (6.42 ×
10−4 and 4.10 × 10−4cm2 V−1 s−1) and
NoCA-18-based blend film (7.81 × 10−4 and 5.85 ×
10−4 cm2 V−1s−1). Furthermore, the J52:NoCA-19 blend film
exhibited more balanced charge carrier mobilities
(μ h/μ e = 1.31) compared to
the other two blends, which may contribute to the observed improvement
in FF.
Table 2 Photovoltaic parameters of OSCs based on PM6:NFREAs
under AM1.5G illumination, 100 mW cm-2.