Bromination and increasing the molecular conjugation length of the non-fullerene small-molecule acceptor based on benzotriazole for efficient organic photovoltaics

Two novel non-fullerene acceptors, namely BZIC-2Br and Y9-2Br, were synthesized by employing a ladder-type electron-deficient-based fused ring central with a benzotriazole core. Y9-2Br is obtained by extending the conjugate length of BZIC-2Br. Compared with BZIC-2Br, Y9-2Br possesses a lower optical bandgap of 1.32 eV with an absorption edge of 937 nm, exhibiting broader and stronger absorption band from 600 to 900 nm. Moreover, Y9-2Br exhibits excellent photovoltaic properties with Voc of 0.84 V, Jsc of 21.38 mA cm−2 and FF of 67.11%, which achieves an impressive PCE of 12.05%. Our study demonstrates that bromination and effective extension of the conjugate length can modulate performance from different aspects to optimize photovoltaic characteristics.


Characterization of information
1 H Nuclear Magnetic Resonance ( 1 H NMR) was recorded using Bruker DMX-400 spectrometer with deuterated chloroform as solvent at 293 K. Chemical shifts were reported as δ values (ppm) with tetramethylsilane (TMS) andCHCl 3 (Chemical shift δ = 7.26 ppm for 1 H NMR) as the internal references. UV-Vis absorption spectra were recorded on the Shimadzu UV-2600 spectrophotometer. Cyclic voltammetry (CV) was recorded with a computer controlled CHI 660E electrochemical workstation using BZIC-2Br (Y9-2Br) film on platinum electrode (1.0 cm 2 ) as the working electrode, a platinum wire as the counter electrode and Ag/AgCl (0.1 M) was used as reference electrode in an anhydrous and argon-saturated solution of 0.1 M of tetrabutylammonium hexafluorophosphate (Bu 4 NPF 6 ) in acetonitrile at a scan rate of 20 mV s -1 . The morphologies of the PBDB-T:BZIC-2Br(PBDB-T:Y9-2Br)blend films were investigated by atomic force microscopy (AFM, Agilent Technologies, 5500 AFM/SPM System, USA) in contacting mode with a 5 μm scanner. Transmission electron microscope (TEM) measurements were performed in a JEM-2100F.

Fabrication of devices
Solar cells were fabricated in the configuration of the traditional structure with an indium tin oxide (ITO) glass positive electrode and a metal Al negative electrode. The ITO glass was cleaned by sequential ultrasonic treatment in detergent, deionized water, acetone and isopropanol, and UV-treated in ultraviolet-ozone chamber (Jelight Company, USA) for 20 min. The PEDOT:PSS (poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate)) (Baytron PVP Al 4083, Germany) was filtered through a 0.45 μm filter and spin coated at 2500 rpm for 50 s on the ITO substrate. The PBDB-T:BZIC-2Br (PBDB-T:Y9-2Br) (D:A = 1:1.2, ω/ω; 16 mgmL −1 in total) were dissolved in chloroform (CF) and spin-cast at 3500 rpm for 30 s onto the PEDOT:PSS layer. It was then annealed at 100°Cfor 5 min. After cooling to room temperature, methanol solution of PDINO at a concentration of 1.0 mgmL -1 was deposited atop the active layer at 2500 rpm for 50 s to afford a thickness of 15 nm. Finally, top Al electrode was deposited in vacuum onto the cathode buffer layer at a pressure of 1×10 -5 Pa. The active area of the device was 5 mm 2 .
The J−V measurement was performed via the solar simulator (SS-F5-3A, Enlitech) along with AM 1.5 G spectra whose intensity was calibrated by the certified standard silicon solar cell (SRC-2020, Enlitech) at 100 mWcm -2 . The external quantum efficiency (EQE) values were measured by Solar Cell Spectral Response Measurement System QE-R3-011 (Enli Technology Co., Ltd., Taiwan). The light intensity at each wavelength was calibrated with a standard single-crystal Si photovoltaic cell.

Hole mobility and electron mobility measurements
The device structures of hole (µ h ) and electron (µ e ) mobilities are ITO/PEDOT:PSS/active layer/MoO 3 /Ag and ITO/ZnO/active layer/PDINO/Al, respectively. The hole and electron mobilities are calculated according to the space charge limited current (SCLC) equation: J = 9µɛ r ɛ 0 V 2 /8d 3 , where J is the current density, µ is the hole or electron mobility, V is the internal voltage in the device, ε r is the relative dielectric constant of active layer material, ε 0 is the permittivity of empty space, and d is the thickness of the active layer.

Mass spectrum and thermogravimetric analysis
The molecular mass was confirmed by using an Autoflex III matrix-assisted Laser desorption ionization mass spectrometer (MALDI-TOF-MS). Thermogravimetric analysis (TGA) was conducted on a Perkin-Elmer TGA-7 with a heating rate of 10 K min -1 under nitrogen.

Compound 3:
In ice-water bath circumstance, concentrated nitric acid(HNO 3 )(57.982 g) were dropwise added to the 500mL three-necked flask with concentrated sulphuric acid(H 2 SO 4 )(98%,360.781 g).Compound2 were dropwise added to system under 50℃ and mixture reacted with stirring 4 h. When reaction completed, solution was transferred to 1000mL beaker with ice-water. And then, the reaction mixture was extracted with dichloromethane (DCM),the organic layer was removed by a rotary evaporator and the residue was purified by silica gel column using DCM/PE (1/3, v/v) to getcompound3(19.601 g，88.9 %).

Compound 4a and 4b:
Compound4a has the same equivalent as compound4b. Compound 5a and 5b: Compound5a has the same equivalent as compound5b.