Abstract:

As reported previously, rhodanine and thiazolidine-2, 4-dione units have been widely used as the terminal group to construct the efficient non-fullerene small molecular acceptors with the structure of A₁-A₂-D-A₂-A₁. Compared with the acceptor using thiazolidine-2, 4-dione unit as the terminal group, the acceptor with rhodanine unit as the terminal electron-withdrawing group usually showed the improved short circuit current density (J_sc) and fill factor (FF) as well as the higher power conversion efficiency (PCE), regardless of the lower open circuit voltage (V_oc). However, the causes of difference are still not very clear. Therefore, in this work, an unsymmetrical organic acceptor (IDT-2) has been designed and synthesized with rhodanine and thiazolidine- 2, 4-dione units as the electron-withdrawing terminal groups to connect an indacenodithiophene (IDT) central core, respectively. By comparing with the two analogues of the symmetrical organic acceptors based on rhodamine unit (IDT-1) or thiazolidine-2, 4-dione unit (IDT-3) as the terminal group, the structure-property relationship has been investigated for this series of acceptors. It is found that as two rhodamine terminal groups are replaced step by step with the thiazolidine-2, 4-dione unit from IDT-1 to IDT-3, the ICT absorption of these small molecular acceptors is significantly blue-shifted from 633 (soln)/656 (film), 618/645 to 603/625 nm, and the corresponding optical band gap (E_g^opt) is also gradually widened from 1.68, 1.71 to 1.77 eV for IDT-1, IDT-2 and IDT-3, respectively, which can be attributed to the introduction of thiazolidine-2, 4-dione unit to reduce the stability of quinoid structure of the conjugation backbone. At the same time, the LUMO/HOMO (the lowest unoccupied molecular orbital/the highest occupied molecular orbital) energy levels of the molecules are gradually uplifted to be -3.62/-5.58, -3.60/-5.56, and -3.57/-5.53 eV, respectively, which is generally beneficial for the improvement of the V_oc due to the upshifted LUMO energy levels of the acceptors. Considering the complementary absorption and well-matched energy levels of the donor and acceptor, the regioregular poly(3-hexylthiophene) (P3HT) has been chosen as a donor to fabricate the devices with three small molecular acceptors, respectively, and the corresponding photovoltaic performances have been evaluated and compared. The device based on IDT-1 with two rhodamine terminal groups gives the best PCE of 4.52% with the lowest V_oc of 0.87 V, the highest FF of 70.66% and J_sc of 7.37 mA·cm^-2, while the device based on IDT-3 with two thiazolidine-2, 4-dione terminal groups shows the poorest PCE of 3.40% with the highest V_oc of 0.98 V but the lowest FF of 59.70% and J_sc of 5.82 mA·cm^-2. As for IDT-2 with an unsymmetrical structure, it contains a thiazolidine-2, 4-dione terminal group and a rhodamine terminal group at the two sides of the molecule. It can be seen that the IDT-2 based device just shows a PCE of 4.07% with a V_oc of 0.91 V, a FF of 64.65% and a J_sc of 6.81 mA·cm^-2, all of which are between those of the devices based on IDT-1 and IDT-3. These results indicate that the thiazolidine-2, 4-dione unit is an effective terminal group to enhance the V_oc of the device but is not beneficial to the improvement of the J_sc and FF. Furthermore, when designing the structure of the acceptors, it is very important to maintain the balance of all the three parameters to maximize the PCE in the OSCs.

Key words: Unsymmetric structure, Rhodanine unit, Thiazolidine-2, 4-dione unit, Small molecule acceptor, Organic solar cells