Low-bandgap nonfullerene acceptor based on thieno[3,2-b]indole core for highly efficient binary and ternary organic solar cells

A low-bandgap nonfullerene acceptor (NFA) TIT-2FIC based on thieno[3,2-b]indole-thiophenes core has been developed. Compared with the analogue NFAs DTC(4Ph)-4FIC and IT-4F, TIT-2FIC exhibited remarkably redshifted absorption, and up-shifted HOMO energy level. In addition, TIT-2FIC showed interesting universal miscibility with the donors nonfluorinated PBDB-T and fluorinated PM6, therefore the corresponding organic solar cells achieved promising power conversion efficiencies (PCEs) of 11.80% and 13.00%, respectively, which are higher compared to the counterpart IT-4F based cells. Furthermore, the ternary PM6:TIT-2FIC:Y6 cell pronounced a high PCE of 17.22%, being significantly improved from that of 16.04% for the binary PM6:Y6 cell. Similar improvement in PCEs from 13.41% to 14.46% was also observed in the ternary PM6:TIT-2FIC:IT-4F cell with TIT-2FIC as the third component. These results indicated that TIT-2FIC is universally applicable as an acceptor with nonfluorinated or fluorinated polymer donor materials in both binary and ternary cells.

As one of the three most important parameters for OSC devices, open-circuit voltage (V OC ) is roughly proportional to the difference between the highest occupied molecular orbitals (HOMO) of the donor materials and the lowest unoccupied molecular orbitals (LUMO) of the acceptor materials [19]. Therefore, high LUMO energy levels of NFAs should be maintained for achieving high V OC . On the other hand, broad absorption of the active layers is prerequisites for high short-circuit current (J SC ), and thus raising of HOMO energy levels is an excellent strategy to reduce the optical bandgaps of NFAs and extend their absorption spectra. Further, studies have shown that HOMO energy levels of donor-acceptor (D-A) molecules are mainly determined by the donor segments [19,20]. It was well known that enhancing the electrondonating ability of core unit can enhance intramolecular charge transfer (ICT) effect and reduce the optical bandgaps of NFAs [21][22][23][24]. In 2017, Jen et al. replaced the indacenodithieno [3,2-b]thiophene (IDTT) core of the innovative NFA ITIC with more electron-rich thiophenethieno [3,2-b]thiophene-thiophene (4T), which endows the acceptor 4TIC with ~100 nm red-shifted absorption spectrum, much higher HOMO energy level, enhanced photocurrent, and consequently better photovoltaic performance in the resulting OSCs [22]. In 2018, Zhan et al. synthesized a fused-ring electron acceptor (IOIC3) based on naphtho [1,2-b:5,6-b ′ ]dithiophene core with hexyloxy side-chains. Compared with hexyl substituted IOIC2, IOIC3 exhibits a narrower bandgap, redshifted absorption spectra, and higher electron mobility. Thus the OSCs based on the PTB7-Th:IOIC3 show 40% higher PCE than the PTB7-Th:IOIC2-based devices (13.1% vs 9.33%) [23]. Recently, Ding et al. developed a series of A-D-A NFAs based on pentacyclic, hexacyclic, heptacyclic, and octacyclic CO-bridged units. Due to the stronger electron-donating capability and enhanced backbone planarity of donor units, these NFAs exhibited broader absorption, stronger lightharvesting capability, higher HOMO energy levels, and much higher photovoltaic performance compared to the IDT-based counterparts [25][26][27][28][29].

Optical properties
The UV-vis-NIR absorption spectra of TIT-2FIC are shown in Fig. 2a, Figure S4a and the detailed data are summarized in Table S1. In dilute chloroform solutions (10 -5 M), TIT-2FIC exhibited a strong main peak at 753 nm with extinction coefficient (ε) of 2.4 × 10 5 M − 1 cm − 1 , which is ca. 10% higher than those of carbazole analogue DTC(4Ph)-4FIC (2.2 × 10 5 M − 1 cm − 1 ) [33] and benchmark NFA IT-4F (2.1 × 10 5 M − 1 cm − 1 ) [50]. From solution to film, the peak is red-shifted to 806 nm with ~70 nm wider absorption range, indicating strong intermolecular interaction of TIT-2FIC in the solid state [51][52][53]. Calculated from the absorption edge (882 nm), the optical bandgap of TIT-2FIC film is 1.41 eV, which is much smaller than those of DTC(4Ph)-4FIC (1.61 eV) and IT-4F (1.53 eV). It is clear that TIT-2FIC exhibited complementary absorption spectrum with known polymers PBDB-T and PM6. More importantly, TIT-2FIC blend films possessed broader and more effective absorption than IT-4F blend films, especially in the range of 730-950 nm ( Figure S4b), which is important to achieve high photocurrent for the resulting devices based on TIT-2FIC.

Electrochemical properties and theoretical calculations
As shown in Figure S5, cyclic voltammetry (CV) was employed to investigate the electrochemical properties of TIT-2FIC, and the electrochemical data are listed in Table S1. The oxidation onset potential (E ox on ) of TIT-2FIC is 0.66 V, while the reduction onset potential (E red on ) is − 0.82 V. According to the empirical formula [54][55][56][57], the HOMO/ LUMO energy levels of TIT-2FIC are estimated to be − 5.46 eV and − 3.98 eV, respectively. Compared with carbazole-based NFAs [33,37], TIT-2FIC possessed ~0.34-0.40 eV higher HOMO energy level and approximate LUMO energy level. The up-shifted HOMO energy level resulted from the stronger electron-donating ability of thieno [3,2-b] indole in contrast with carbazole, and this should be responsible for the narrower bandgap of TIT-2FIC. More importantly, the replacement of carbazole core with stronger electron-donating thieno [3,2-b]indole could only raise the HOMO energy level of the target NFAs without affecting the LUMO level and the V OC of the resulting devices. It is noteworthy that HOMO offset between TIT-2FIC and PM6 is extremely small, which however should not be regarded as a hurdle for the photovoltaic performance since recent study has shown HOMO offset between donor and acceptor materials is not necessary for efficient OSCs [58]. In addition, TIT-2FIC possessed higher LUMO energy level and much higher HOMO energy level when compared with IT-4F, which leads to a narrower optical bandgap and is beneficial for higher V OC and J SC in the TIT-2FIC based devices.
Density functional theory (DFT) calculations at the ωB97XD/def2-SVP level were carried out to investigate optimal geometric configurations and molecular frontier orbitals for TIT-2FIC in comparison with DTC(4Ph)-4FIC and IT-4F ( Fig. 2c and Figure S6). All these three NFAs have planar conjugated backbones with comparable conjugation path length in the central cores. However, it should be noted that different from centrosymmetric IT-4F and axisymmetric DTC(4Ph)-4FIC, TIT-2FIC is a totally asymmetric molecule. The energy levels calculated at the B3LYP-D3(BJ)/def2-SVP and ωB97XD/def2-SVP levels are both included in Table S1 for comparison, based on the optimized geometries at the corresponding level of theory. The calculated HOMO and LUMO levels at the B3LYP-D3(BJ)/def2-SVP level are − 5.52 eV and − 3.56 eV for TIT-2FIC, − 5.70 eV and − 3.54 eV for DTC(4Ph)-4FIC, and − 5.66 eV and − 3.61 eV for IT-4F, respectively, which are consistent well with the values deduced from the CV data. Compared to DTC(4Ph)-4FIC, TIT-2FIC exhibited much up-shifted HOMO energy levels for the narrower energy gaps, resulting from the replacement of carbazole core with thieno [3,2-b]indole. In addition, the calculated HOMO and LUMO levels at the ωB97XD/def2-SVP level also showed the same trends with the CV data (Table S1). For all the molecules, the HOMOs are mostly located on the electron-donating cores, while the LUMOs are slightly more localized on the electron-accepting units.
Time-dependent density functional theory (TD-DFT) calculations at the ωB97XD/def2-SVP level suggest the lowest excitations from the ground state (S 0 ) to the excited state (S 1 ) correspond to π-π* transitions with over 65% dominated by the HOMO → LUMO transitions for all the three molecules (Table S2). As shown in Table S3 and Figure S7, electron-hole analysis [59,60] of the transitions from the ground state to the excited states reveals that these NFAs feature high overlap of well distributed holes and electrons of the transitions, which may favor a higher oscillator strength of the corresponding transition, and the S 0 → S 1 transitions have the highest oscillator strengths among all transitions of the same systems. The lowest excited-state vertical transition energies show a similar trend to the HOMO-LUMO gaps. As shown in Figure S8, we further simulated the UV-vis absorption of these NFA molecules in the gas phase under vacuum, and in solution with linear response polarizable continuum model (LR-PCM) [61] or state-specific polarizable continuum model (SS-PCM) models [62]. In these three models, the lowest excited-state vertical transition energies all followed the same trend: TIT-2FIC < IT-4F < DTC(4Ph)-4FIC, which are consistent with the optical data. It can be found that DTC(4Ph)-4FIC belongs to C 2v symmetry and IT-4F belongs to C 2h symmetry, while TIT-2FIC belongs to C 1 symmetry. The variation in molecular symmetries can result in different molecular dipole moments. Intermolecular dipole interactions between conjugated molecules can drive neighboring molecules to form aggregates, particularly when the chromophore possesses a large dipole moment. To evaluate the molecular dipole moments of the studied structures more accurately, single point calculations at the level of PBE0/aug-cc-pVDZ were further performed based on the ωB97XD/def2-SVP optimized geometry. Calculated data show that compared to IT-4F with a molecular dipole moment of 0.000 D and DTC(4Ph)-4FIC with a molecular dipole moment of 0.417 D, the asymmetric TIT-2FIC has a much larger molecular dipole moment of 1.452 D, which is expected to favor charge transport through enhanced intermolecular interactions. It is also interesting to note that the frontier molecular orbitals ( Figure S6) for these NFA molecules are strongly affected by the symmetries of the molecular structures, which are also observed in both the isosurface maps of electrostatic potential (ESP) on the van der Waals (vdW) surface and the isosurface maps of the vdW potential as shown in Figure S9. The ESP analysis shows that both TIT-2FIC and DTC(4Ph)-4FIC have a greater electronegativity difference over IT-4F and can form stronger electrostatic interaction to other molecules, enhancing their intermolecular interaction. For all the three NFAs, the isosurface map of the vdW potential evaluated by the Lennard-Jones 12-6 potential shows the regions ( Figure S9, blue isosurface) where the dispersion attraction effect surpasses the exchange-repulsion effect are favorable physical adsorption zones for intermolecular packing and these regions are mainly facing to the plane of the conjugated backbones, beneficial to π-π stacking, and the most favorable adsorption sites are close to the core units of the NFA molecules, as a result of their backbone planarity and symmetry. Moreover, IRI-π analysis based on the recently proposed interaction region indicator (IRI) clearly indicates that these NFAs are ideal for π-π stacking, in consistence with the ESP and vdW analyses.
From the above discussion of molecular dipole moments, electrostatic interaction, van der Waals interaction and IRI-π analyses, it is expected that the asymmetric TIT-2FIC can have enhanced intermolecular interaction, molecular packing and thus improved photovoltaic performance.

Photovoltaic performance of binary OSCs
To investigate the photovoltaic performance of NFA TIT-2FIC, binary OSCs with the device structure of ITO/PEDOT:PSS/active layer/PFN-Br/ Al were firstly fabricated and the known polymer donor materials PBDB-T and PM6 were chosen. For comparison, the commercial NFA IT-4F with heptacyclic core was also used as acceptor for the fabrication of OSCs. After optimization, solar cells based on PBDB-T:TIT-2FIC achieved a decent PCE of 11.80%, with a V OC of 0.805 V, a J SC of 21.16 mA cm − 2 and a fill factor (FF) of 69.34% (Fig. 3a and Table 1). Meanwhile, the control devices based on PBDB-T:IT-4F just exhibited a ~20% lower PCE of 9.56%, with a V OC of 0.727 V, a J SC of 19.07 mA cm − 2 and a FF of 68.91%. Compared with the PBDB-T:IT-4F cell, a much higher V OC of PBDB-T:TIT-2FIC cell may originate from the higher LUMO energy level of TIT-2FIC. Meanwhile, better absorption property of TIT-2FIC is beneficial for the much higher photocurrent of PBDB-T:TIT-2FIC cell. These trend also have been maintained for the cells using fluorinated PM6 as the donor. As a result, the PM6:TIT-2FIC device exhibited higher V OC (0.905 vs 0.858 V) and J SC (20.69 vs 19.95 mA cm − 2 ) when compared with the PM6:IT-4F device. Finally, a slightly lower FF of 69.42% in the case of PM6:TIT-2FIC led to comparable PCEs (13.00% vs 13.03%). The advantages of TIT-2FIC as acceptor was clearly observed as it can always show higher V OC and J SC in the resulting devices compared to the state-of-the-art acceptor IT-4F. Besides, it also exhibited much higher J SC than its carbazole-based NFAs counterpart DTC(4Ph)-4FIC [33], due to the remarkable red-shift absorption.
The enhanced J SC of TIT-2FIC based cells were confirmed by the external quantum efficiency (EQE) spectra as compared to the IT-4F based cells. As shown in Fig. 3b, solar cells based on PBDB-T:TIT-2FIC and PM6:TIT-2FIC both exhibited ~70 nm broader EQE spectra, thus higher calculated currents of 20.35 and 19.83 mA cm − 2 were obtained. These boarder EQE response are consistent well with the absorption spectra of blend films ( Figure S4b). In contrast, the calculated currents of PBDB-T:IT-4F and PM6:IT-4F cells are 18.66 and 19.64 mA cm − 2 , respectively, which are in good agreement with the J SC obtained from the J-V measurements. In addition, PBDB-T:TIT-2FIC cells showed slightly higher EQE response than PM6:TIT-2FIC cells in the range 560 nm to 840 nm due to the better absorption and higher photoluminescence (PL) quenching efficiency ( Figure S4b and Figure S10).
To reveal the charge extraction and exciton dissociation properties, the photocurrent density (J ph ) versus the effective voltage (V eff ) measurements were conducted. As shown in Fig. 3c and Table S4, the exciton dissociation probabilities (P diss ), defined by P diss = J ph */J sat under the short-circuit current condition [41,63], are calculated to be 96.4% and 98.1% for PBDB-T:IT-4F and PBDB-T:TIT-2FIC cells, respectively. PBDB-T:TIT-2FIC cell also exhibited much higher charge collection probability (P coll , defined by J ph & /J sat ) of 90.7%. These values are consistent well with the higher FF for PBDB-T:TIT-2FIC cells, suggesting more efficient exciton dissociation compared to the PBDB-T:IT-4F cell [41,64]. On the other hand, the PM6:IT-4F cell possessed higher P diss and P coll for higher FF of 76.18% as compared to the PM6:TIT-2FIC cells. The light-intensity (P in ) dependence of J SC was examined to investigate the bimolecular recombination behavior on current extraction. As shown in Fig. 3d recombination and more efficient current extraction [41]. The lightintensity dependent V OC measurement indicated TIT-2FIC based devices possessed much less trap-assisted recombination than those of IT-4F based counterparts with smaller V OC -P in slope [41,65]. As a result, TIT-2FIC exhibited excellent photovoltaic performance with the minimal bimolecular and trap-assisted recombination when combining PBDB-T or PM6.
As measured by the space-charge limited current (SCLC), the electron and hole mobilities of PBDB-T:TIT-2FIC blend film are 3.83 × 10 -4 and 2.65 × 10 -4 cm 2 V − 1 s − 1 , respectively ( Figure S11 and Table S5). These values are 50-70% higher than those of PBDB-T:IT-4F film. Similarly, PM6:TIT-2FIC blend film also exhibited higher charge carrier mobility than PM6:IT-4F film, probably originating from the larger molecular dipole moment and stronger intermolecular interactions of asymmetric NFAs [66]. Thus, higher photocurrents for the corresponding cells based on TIT-2FIC were achieved. The surface morphologies of the blend films were investigated by atomic force microscopy (AFM) as shown in Figure S12. PBDB-T:TIT-2FIC and PM6:TIT-2FIC blend films showed root-mean-square (RMS) roughnesses of 3.71 and 2.71 nm, respectively, while the roughness of PBDB-T:IT-4F and PM6:IT-4F blend films are 2.46 and 1.91 nm, respectively. These four blend films all exhibited uniform film morphology supporting their decent photovoltaic performance.
As discussed above, when combining with fluorinated PM6 or nonfluorinated PBDB-T, TIT-2FIC based OSCs achieved decent PCEs of 11.80% or 13.00%, indicating its universal compatibility. This point was proved by contact angle measurements and the Flory-Huggins model [67,68]. As shown in Figure S13 and Table S6, the surface free energy (γ s ) measured from the contact angles on water and diiodomethane (DIM) are 31.9, 34.1, 58.5, and 53.5 mN m − 1 for PBDB-T, PM6, IT-4F, and TIT-2FIC, respectively. Further, the Flory-Huggins interaction parameters (χ) are calculated to be 3.59 for χ PBDB-T/IT-4F , 2.53 for χ PBDB-T/ TIT-2FIC , 3.26 for χ PM6/IT-4F , and 2.16 for χ PM6/TIT-2FIC . It is clearly that TIT-2FIC shows the better miscibility with both PBDB-T and PM6 than IT-4F, according to AFM and charge carrier mobility results, and photovoltaics performance. Besides, χ PBDB-T/IT-4F is the largest among the four pairs, which may be one of the main reasons for its low PCE of 9.56%.

Photovoltaic performance of ternary OSCs
The ternary OSCs with the device structure of ITO/PEDOT:PSS/PM6: TIT-2FIC:Y6/PDIN/Al were fabricated to further investigate the favorable role of TIT-2FIC in ternary devices as the third component. As shown in Fig. 4a and Table 2, various weight ratios of TIT-2FIC (0%, 5%, 10%, 15%, 100% by wt.) over the total acceptor weight were used to construct the ternary active layers and the total D/A weight ratio was kept as 1:1.2. For comparison, the control binary solar cells based on PM6:Y6 were fabricated and a decent PCE of 16.04% was achieved, consisting with the reported results [8]. After the addition of 5%~15% TIT-2FIC, all ternary OSCs based on PM6:TIT-2FIC:Y6 exhibited much better photovoltaic performance (16.69%~17.22%). Especially, the ternary OSCs with 10% TIT-2FIC achieved the highest PCE of 17.22%, with a V OC of 0.870 V, a J SC of 26.49 mA cm − 2 and a FF of 74.73%. This significant improvement mainly originated from the enhanced photocurrent, which was confirmed by EQE spectra. As shown in Fig. 4b, the blend film containing 10% TIT-2FIC exhibited better EQE response than  PM6:Y6 binary film almost in the whole absorption range, and the blend film possessed similar absorption spectra, especially for the enhanced absorption in the range of 660-820 nm ( Figure S14). As a result, the calculated current from the EQE curve was improved to 25.32 mA cm − 2 from 24.60 mA cm − 2 of the PM6:Y6 binary cell. Meanwhile, the PM6: TIT-2FIC cells showed ~10% lower and ~35 nm narrower EQE response than other cells, leading to much lower photocurrent of 20.18 mA cm − 2 and PCE of 13.05% eventually. We also employed the photocurrent density (J ph ) versus the effective voltage (V eff ) measurements to investigate the charge extraction and exciton dissociation properties of ternary OSCs. As shown in Fig. 4c, the exciton dissociation probabilities (P diss ) are 97.6%, 98.3%, and 96.3% for PM6:Y6, PM6:10%TIT-2FIC:Y6 and PM6:TIT-2FIC cells, respectively. Then, P coll of these three cells are 85.7%, 88.7%, and 81.0%, respectively. The higher P diss and P coll indicating efficient exciton dissociation are therefore beneficial for higher FF for the ternary OSCs with 10%TIT-2FIC. The light-intensity dependence of J SC and V OC measurement indicated that ternary OSCs with 10%TIT-2FIC exhibited negligible bimolecular and trap-assisted recombination, due to the α value of 0.990 and n value of 1.02 (Fig. 4d). These mean that ternary OSCs with 10%TIT-2FIC worked much better than the other two binary cells, which is in good agreement with its higher J SC and FF obtained from the J-V measurements. As shown in Figure S15 in the Supporting Information, the ternary PM6:10%TIT-2FIC:Y6 films exhibited clear fibrillar structure and moderate RMS roughness of 1.03 nm when compared with the two binary films, which is beneficial to exciton dissociation and charge transport, and thus high FF in the optimized ternary OSCs [11,69].
For organic photovoltaics, the E loss can be divided into three components: The qV OC SQ is the maximum voltage based on the Shockley-Queisser (SQ) limit, where it is assumed there is no absorption below the optical bandgap of the solar cell. The first term of (E gap − qV OC SQ ) is unavoidable and comes from the radiative recombination originating from the absorption above the bandgap. The second term of (qV OC SQ − qV OC rad ) is an additional radiative recombination and comes from the absorption below the bandgap. The third term of (qV OC rad − qV OC ) comes from the nonradiative loss and the nonradiative channel has a clear contribution to the total voltage loss [70]. To explore the role of TIT-2FIC in the PM6:Y6 based ternary device, the nonradiative loss was further studied by EQE of electroluminescence (EL) (EQE EL ), which is the radiative quantum efficiency of solar cells when charges are   (Fig. 5a). The nonradiative loss can be calculated by: q ΔV OC non-rad = -kT ln(EQE EL ) Compared with the binary PM6:Y6 cell, the ternary device (with 10% TIT-2FIC) exhibited slightly larger non-radiative voltage loss of 0.28 V (vs 0.26 V for the binary cells), but the ternary cell possessed higher V OC (0.870 vs 0.857 V) and lower E loss . These results suggested that the enhancement of V OC in the PM6:10%TIT-2FIC:Y6 cell came from the significantly reduced radiative loss and the loss from radiative channel was suppressed by the addition of the third component TIT-2FIC. Such conclusion could also be confirmed by the steady-state PL spectra (690 nm light excitation). As shown in Fig. 5b and Figure S16a, Y6:10%TIT-2FIC blend film exhibited ~35% higher PL emission intensity at 940 nm than that of Y6 film, and the emission peak of TIT-2FIC at 880 nm is apparently quenched in the blend film [69]. These results indicated there was efficient energy transfer process from TIT-2FIC to Y6, which was also supported by the overlap between absorption spectrum of Y6 and PL spectrum of TIT-2FIC (Fig. 5c). Additionally, acceptor-only devices were fabricated and Y6:10%TIT-2FIC device exhibited lower J SC than that of Y6 based devices ( Figure S16b), implying that no significant charge transfer occurs between TIT-2FIC and Y6 ( Figure S16c) [63,69].
As shown in Fig. 5d-5f and Figure S17, grazing incidence wide-angle X-ray scattering (GIWAXS) was employed to investigate the molecular orientation of the ternary and binary blend films. These three blend films all exhibited obvious π-π stacking diffraction in the out-of-plane (OOP) direction with q value of ~1.7 Å − 1 , indicating preferential face-on orientation. The PM6:Y6 and PM6:10%TIT-2FIC:Y6 blend films shown the same level of intensity response along the OOP direction, indicating that the addition of 10%TIT-2FIC does not interrupt the molecular stacking of PM6 and Y6, even though TIT-2FIC possesses significantly different chemical structure from Y6. In contrast, PM6:TIT-2FIC blend film exhibited significantly reduced (0 1 0) scattering peak in the OOP direction, showing weak molecular stacking, which partially explained the resulting relatively poor photovoltaic performance of 13.05%.
Furthermore, TIT-2FIC was used to improve the photovoltaic performance of PM6:IT-4F cells ( Figure S18 and Table S7). After adding 30% or 50% TIT-2FIC, the PCEs of ternary were improved to 14.14% or 14.46% from 13.41% of the PM6:IT-4F binary cell, respectively. Similarly, the enhanced photocurrent of ~2 mA cm − 2 mainly accounts for the improved photovoltaic performance.

Conclusion
In summary, using thieno [3,2-b]indole fused with two thiophenes as the core, and 2FIC as the end-groups, a NFA TIT-2FIC has been designed and successfully synthesized. Compared with NFA DTC(4Ph)-4FIC based on carbazole and IT-4F, TIT-2FIC exhibited over 80 nm red-shift absorption, low bandgap of 1.41 eV, and much suitable LUMO energy level. Using PBDB-T and PM6 as the donor materials, the corresponding binary OSCs exhibited decent PCEs of 11.80% and 13.00%, respectively. On the other hand, the PBDB-T:IT-4F and PM6:IT-4F cells showed PCEs of 9.56% and 13.03%, respectively. These binary OSCs results indicated that TIT-2FIC possess universal miscibility with both fluorinated and non-fluorinated polymer donors. Furthermore, 10%TIT-2FIC was added to the PM6:Y6 binary cells to construct ternary OSCs and the photovoltaic performance was improved to an impressive PCE of 17.22% from 16.04%, due to the better EQE response, more efficient charge extraction and exciton dissociation, and low voltage loss. Interestingly, it was found that the ternary OSCs based on PBDB-T:TIT-2FIC:IT-4F also achieved higher PCE than that of PBDB-T:IT-4F cells. Therefore, TIT-2FIC could be a commonly used additive for fabricating efficient ternary OSCs. Our results demonstrated that TIT-2FIC is a promising NFA with good compatibility with different donors and a robust third component for construction of ternary OSCs.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.