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 (JV ) 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 JVcharacteristics.
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.