Simple Bithiophene–Rhodanine‐Based Small Molecule Acceptor for Use in Additive‐Free Nonfullerene OPVs with Low Energy Loss of 0.51 eV

Abstract

The introduction of rigid and extended ladder‐type fused‐ring cores, such as indacenodithiophene, has enabled the synthesis of a variety of nonfullerene small molecules for use as electron acceptors in high‐performance organic photovoltaic cells. Contrasting with recent trends, a very simple‐structured nonfullerene acceptor (NFA), T2‐ORH, consisting of a bithiophene core and octyl‐substituted rhodanine ends, is synthesized in two steps from inexpensive commercially available raw materials. Its relatively short π‐conjugation results in a wide bandgap and a blue‐shifted UV–vis absorption profile complementary to those of poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐co‐3‐fluorothieno[3,4‐b]thiophene‐2‐carboxylate] (PTB7‐Th). Despite a sufficient offset between T2‐ORH and PTB7‐Th, the lowest unoccupied molecular orbital (LUMO) energy level of T2‐ORH is still higher than the LUMOs of other NFAs (e.g., ITIC). Therefore, the PTB7‐Th:T2‐ORH blend film exhibits an efficiency of 9.33% with a high open‐circuit voltage of 1.07 V and a short‐circuit current of 14.72 mA cm−2 in an additive‐free single‐junction cell. Importantly, the optimized device displays a remarkably low energy loss of 0.51 eV, in which bimolecular and monomolecular charge recombination is effectively suppressed by solvent vapor annealing treatment. The blend film has a very smooth and homogeneous morphology, providing both vertical and parallel charge transport in the devices.

A Nonfullerene Acceptor Containing Rhodanine and Barbituric Acid End Groups for Use in Organic Photovoltaic Devices

“…9,15 The Knoevenagel condensation between Br-T-CHO and OBAR resulted in Br-T-OBAR, and B-T-ORH was obtained in a similar manner using B-T-CHO and ORH. A rod-shaped A-D-A-type ORH-T2-OBAR was designed to have octylsubstituted asymmetrical electron-withdrawing end groups of RH and BAR, enabling it to function as an acceptor when used in OPV devices.…”

“…The cyclic voltammograms were calibrated using a ferrocenium/ferrocene standard with a redox potential of −4.8 eV. 9,15 As shown in the energy diagram in Figure 1(e), the HOMO and LUMO energy levels of the PTB7-Th polymer donor were at −5.23 and −3.19 eV, respectively. 9,15 As shown in the energy diagram in Figure 1(e), the HOMO and LUMO energy levels of the PTB7-Th polymer donor were at −5.23 and −3.19 eV, respectively.…”

“…The device fabricated with ORH-T2-OBAR without extra treatment displayed a PCE of 0.91% and an open-circuit voltage (V OC ) of 0.81 V. In addition, the energy loss (E loss ), defined as E loss = E g,opt -eV OC , was calculated to be 0.78 eV. 9 Molecular structure modifications to control the aggregation types of NFAs are currently in progress. 14 The maximum EQE response of the ascast film (16%) was observed at 440 nm, indicating a greater contribution of ORH-T2-OBAR (via hole-transfer, so-called channel II) compared with that of PTB7-Th (via electron transfer).…”

“…9,15 Br-T-OBAR (1), B-T-ORH (2), and ORH-T2-OBAR were synthesized through Knoevenagel condensation and Suzuki coupling reactions according to procedures described in the literature. All materials were used without further purification and all chemical reactions were carried out using anhydrous solvents under a nitrogen atmosphere.…”

“…3 However, the disadvantages of fullerene acceptors, which include weak UV-Vis absorption, difficulty of purification, high synthesis cost, and difficult-to-control energy levels, 4,5 necessitate the development of nonfullerene acceptors (NFAs) that can overcome the aforementioned drawbacks of fullerene derivatives. 8,9 The NFAs described in recent studies have been incorporated into acceptor-donor-acceptor (A-D-A) structures, which has enabled optimization of their physical properties (e.g., optical and electrochemical properties and aggregation behaviors) to achieve high efficiencies when NFAs are combined with suitable polymer donors. 6,7 Furthermore, investigations of simple and inexpensive synthesis methods, techniques to manufacture large-area devices, and devices with improved stability are considered important for the commercialization of OPV devices.…”