Background and Originality Content
Organic solar cells (OSCs) have made continuous progress in recent years due to the breakthrough in organic semiconducting materials[1-11]. Especially, the emerging A-DAꞌD-A type small molecule acceptors (SMAs), Y6 and its analogs[2, 12], promote the efficiency of OSCs to 15%-20% through the modification of the fused DAꞌD backbone, electron-withdrawing end groups and side chains[13-30]. Meanwhile, some A-DAꞌD-A acceptors with ultra-narrow bandgap (ultra-NBG) have been reported to realize highJ SC for high-performance tandem or semi-transparent OSCs[31-36]. Extending the fused backbone and modulating the push-pull effects of molecules are the general methods to narrow the molecular optical bandgap (E gopt)[16, 37-40]. And the molecular absorption onset could even be redshifted to exceeding 1000 nm[31, 36]. However, the synthetic cost is increased due to the increased synthetic complexity and reduced yield.
Side-chain engineering is one of the simplest methods to subtly tune the molecular properties and device performance, which can effectively reduce the synthetic difficulty and cost[21, 41-42]. Particularly, the side chains with functional heteroatoms could regulate the intra/intermolecular interactions due to the electronegativity and conjugative/ inductive effects, therefore having the potential in modulating the molecular packing and absorption properties. For instance, alkoxy chains are widely used in electron-donating materials for broadening the absorption spectra[43-45]. However, alkoxy substituents on the SMAs usually cause abnormal blue-shifted absorption[46-47].
In our most recent work, two efficient A-DA’D-A type pentacyclic acceptors, BZ4F-O-2 and BZ4F-O-3, have been reported by moving the oxygen atoms in the alkoxy side chains to the second or third position for the first time[48]. Through the simple side-chain engineering strategy, the molecular absorption is obviously redshifted. And the BZ4F-O-3 achieves one of the highestJ SCs among the binary OSCs based on the pentacyclic acceptors. Therefore, it is significant to extend the strategy into heptacyclic acceptors for high-performance ultra-NBG SMAs with excellent J SC.
Accordingly, we insert oxygen atoms into the second position of the side chains on our reported heptacyclic acceptor (Y11[49]) to synthesize BZO-4F. As predicted, BZO-4F shows a redshifted absorption onset of 960 nm than that of Y11 (932 nm, Figure S1). By replacing the fluorine atoms on the end groups with chlorine atoms, BZO-4Cl is obtained with a further redshifted absorption onset of 990 nm. Choosing the low-cost polymer PBDB-T as the donor, both the PBDB-T: BZO-4F and PBDB-T: BZO-4Cl-based devices exhibit a moderate photovoltaic efficiency of ~12.4% with a poor V OC. However, the J SCof the PBDB-T: BZO-4Cl based device is increased due to the redshifted absorption. To further optimize the energy level arrangement, light absorption and blend morphology, BZ4F-O-1 with high lowest unoccupied molecular orbital (LUMO) level and complementary absorption is introduced as a third component into the PBDB-T: BZO-4Cl blend. As a result, the PBDB-T: BZ4F-O-1: BZO-4Cl-based device achieves a higher PCE of 15.51% with the simultaneously boosted V OC of 0.77 V, J SC of 27.85 mA cm-2and FF of 0.71. As far as we know, this is the best efficiencies of single-junction OSCs based on PBDB-T and SMAs. This work highlights the importance of a synergetic alkoxy side-chain and chlorine-contained end group strategy to synthesize heptacyclic A-DA’D-A type ultra-NBG SMAs for high-performance OSCs.
Results and Discussion
Figure 1a shows the chemical structures of PBDB-T, BZ4F-O-1, BZO-4F and BZO-4Cl. PBDB-T is purchased from Solarmer Materials Inc., BZ4F-O-1 is synthesized according to our previous work. The synthetic procedures of two new acceptors, BZO-4F and BZO-4Cl are shown in Scheme S1 in the supporting information (SI). Alkoxyl-substituted thieno[3,2-b]thiophene unit (compound 4) is one of the key intermediates that is synthesized through three-step reactions, including Bouveault aldehyde synthesis, reduction reaction by sodium borohydride and Williamson reaction. Subsequently, BZO-4F and BZO-4Cl are obtained by the similar synthetic method of BZ4F-O-1. Chemical structures of the intermediate molecules and target SMAs are determined by nuclear magnetic resonance hydrogen and carbon spectra (1H and 13C NMR) and mass spectra (MS), as shown in Figures S12-S24. BZO-4F and BZO-4Cl have good solubility in common processing solvents, eg. , chloroform, chlorobenzene and toluene at room temperature. Thermogravimetric analysis (TGA) is employed to evaluate the thermal stability of the two acceptors, displayed in Figure S2. BZO-4F and BZO-4Cl exhibit high decomposition temperatures of 305 oC and 296oC (T d, 5% weight loss), respectively, revealing their good thermal stability.
The ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectra of BZO-4F and BZO-4Cl in chloroform solution are displayed in Figure S3. BZO-4F shows an optical absorption region in 670-830 nm, while both the absorption peak and onset of BZO-4Cl are redshifted by ~20 nm. Similarly, in the film state, BZO-4Cl has a bathochromic absorption spectrum with an ultra-NBG of 1.25 eV, 30 nm redshift than that of BZO-4F (Figure 1b and Table 1). Electrochemical energy levels of BZO-4F and BZO-4Cl were investigated by cyclic voltammetry (CV) measurement. The CV curves are shown in Figure S4. The highest occupied molecular orbital (HOMO)/ LUMO energy levels of BZO-4F and BZO-4Cl are calculated as -5.61/-3.92 eV and -5.69/-3.93 eV, respectively (Figure 1c and Table 1). The redshifted absorption from BZO-4F to BZO-4Cl could be attributed to the enhanced ICT effect and the π-π stacking caused by the chlorine atoms. Different from the generally lowered LUMO energy level of chlorinated SMA than that of the fluorinated one, BZO-4Cl has a similar LUMO level with BZO-4F. Therefore, the BZO-4Cl-based OSCs have the potential to achieve highJ SC without the sacrifice ofV OC.
Density functional theory (DFT) is employed to further explore the molecular conformation and energy levels of BZO-4F and BZO-4Cl at the B3LYP/6-31G* level. Undecyl and 2-ethylhexyl onto the thiophene/ benzotriazole units are simplified with methyl groups, while the 2-ethylhexyl side chains onto the pyrrole units are retained to reflect the relatively true molecular conformation. The top/side view of BZO-4F and BZO-4Cl in Figure S5 illustrates a planar molecular conformation with a slightly twisted backbone that results from the steric hindrance of the side chains in the pyrrole units. The twisted structure is beneficial for limiting excessive aggregation. The HOMO/LUMO energy levels of BZO-4F and BZO-4Cl are calculated as -5.37/ -3.45 eV and -5.43/ -3.53 eV, respectively, which is consistent with the CV results. Besides, electrochemical energy levels of BZO-4F and BZO-4Cl in dilute chloroform solution further verify the theoretical calculation (Figure S6).