Completely non-fused electron acceptor with 3D-interpenetrated crystalline structure enables efficient and stable organic solar cell

Non-fullerene acceptors (NFAs) based on non-fused conjugated structures have more potential to realize low-cost organic photovoltaic (OPV) cells. However, their power conversion efficiencies (PCEs) are much lower than those of the fused-ring NFAs. Herein, a new bithiophene-based non-fused core (TT-Pi) featuring good planarity as well as large steric hindrance was designed, based on which a completely non-fused NFA, A4T-16, was developed. The single-crystal result of A4T-16 reveals that a three-dimensional interpenetrating network can be formed due to the compact π–π stacking between the adjacent end-capping groups. A high PCE of 15.2% is achieved based on PBDB-TF:A4T-16, which is the highest value for the cells based on the non-fused NFAs. Notably, the device retains ~84% of its initial PCE after 1300 h under the simulated AM 1.5 G illumination (100 mW cm−2). Overall, this work provides insight into molecule design of the non-fused NFAs from the aspect of molecular geometry control.


Materials characterization
The molecular energy levels were measured by utilizing CHI650D Electrochemical Workstation via square wave voltammetry methods. The working electrode, counter electrode, and reference electrode were Glassy carbon disk, Pt wire, and an Ag/Ag + electrode respectively with the ferrocene/ferrocenium (Fc/Fc + ) as an external standard in the measurement. HOMO/LUMO = -e (φox/φred + 4.80-φFc/Fc + ) (eV). Absorption spectra were measured on a Hitachi UH4150 UV-vis spectrophotometer. 1 HNMR, 13 CNMR spectra were recorded on BRUKER Fourier 300 and 400 spectrometer.

Solubility measurement
To 100 mg of non-fused acceptors, 1 mL of o-xylene was added. The solution was stirred for 3 h at room temperature. The solution was filtered by centrifugation. To 0.5 mL of the filtrate, methanol was added until all solids precipitated out. The precipitate was extracted and dried under vacuum overnight. The solubility of the acceptors can be calculated by the following equation.

Single-Crystal growth.
A solution prepared from ~1 mg A4T-16 in ~0.5 mL toluene was transferred into a 4 mL vial, which was capped with a bottle cap. The 4 mL vial was then placed in a 20 mL vial containing ~3 mL petroleum ether. The 20 mL vial was then tightly sealed, and left standing for a few days (5-7 days) to give rodshaped crystal clusters.

Device fabrication
A conventional structure of ITO/PEDOT:PSS/active layer/PF3N-Br/Ag was used to fabricate the OSCs.

Device characterization and measurement
The J−V measurement was performed via a XES-70S1 (SAN-EI Electric Co., Ltd.) solar simulator (AAA grade) whose intensity was calibrated by a certified standard silicon solar cell (SRC-2020,

DFT calculation
The molecular geometries were optimized by Gaussian 09 with a functional of B3LYP and a basis set of 6-31G(d,p). [4] The long alkyl chains were replaced by methyl for saving computation time. The ESP analysis was carried out by a wavefunction analysis tool Multiwfn. [5]