Chlorinated Benzo[1,2‐b:4,5‐c′]dithiophene‐4,8‐dione Polymer Donor: A Small Atom Makes a Big Difference

Abstract The position of a chlorine atom in a charge carrier of polymer solar cells (PSCs) is important to boost their photovoltaic performance. Herein, two chlorinated D‐A conjugated polymers PBBD‐Cl‐α and PBBD‐Cl‐β are synthesized based on two new building blocks (TTO‐Cl‐α and TTO‐Cl‐β) respectively by introducing the chlorine atom into α or β position of the upper thiophene of the highly electron‐deficient benzo[1,2‐b:4,5‐c′]dithiophene‐4,8‐dione moiety. Single‐crystal analysis demonstrates that the chlorine‐free TTO shows a π‐π stacking distance (d π‐π) of 3.55 Å. When H atom at the α position of thiophene of TTO is replaced by Cl, both π‐π stacking distance (d π‐π = 3.48 Å) and Cl···S distance (d Cl‐S = 4.4 Å) are simultaneously reduced for TTO‐Cl‐α compared with TTO. TTO‐Cl‐β then showed the Cl···S non‐covalent interaction can further shorten the intermolecular π‐π stacking separation to 3.23 Å, much smaller than that of TTO‐Cl‐α and TTO. After blending with BTP‐eC9, PBBD‐Cl‐β:BTP‐eC9‐based PSCs achieved an outstanding power conversion efficiency (PCE) of 16.20%, much higher than PBBD:BTP‐eC9 (10.06%) and PBBD‐Cl‐α:BTP‐eC9 (13.35%) based devices. These results provide an effective strategy for design and synthesis of highly efficient donor polymers by precise positioning of the chlorine substitution.


Experimental Section
Measurements: 1 H NMR and 13 C NMR spectra were recorded on Bruker Avance-400/500 spectrometers. Mass spectra (high resolution mass spectrometer (HRMS)) were determined on an Autoflex III matrix-assisted laser desorption ionization mass spectrometer (MALDI-TOF-MS). Gel permeation chromatography (GPC) was performed on Agilent Technologies 1260 infinityⅡhigh temperature GPC system using 1,2,4-trichlorobenzene (TCB) as eluent at 150 o C. Solution and thin film optical absorption spectra were measured with a UV-Vis spectrophotometer (Shimadzu, UV3600). The thin films of the polymers were spin-coated from their solutions in chloroform, and then the film absorption spectra were measured. The electrochemical cyclic voltammetry (CV) was carried out on a CHI 660E Electrochemicacl Workstation (Shanghai Chenhua Instrumental Co., Ltd. China), with glassy carbon disk, Pt wire and Ag/Ag+ electrode as working electrode, counter electrode and reference electrode in an acetonitrile solution of 0.1 mol L −1 Tetrabutylammonium phosphorus hexafluoride (n-Bu 4 NPF 6 ) at a potential scan rate of 100 mV s -1 under a argon atmosphere. Tapping mode atom force microscopy (TM-AFM) images were taken on a NanoScope IIIa controller (Veeco Metrology Group/Digital Instruments, Sant a Barbara, CA), using built-in software (version V6.13R1) to capture images. Transmission electron microscopy (TEM) images were acquired using a HITACHI H-7650 electron microscope operating at an acceleration voltage of 100 kV. The thickness of the blend films was determined by a Dektak 6 M surface profilometer. All J-V curves were captured under an AAA solar simulator S-4 (SAN-EI) calibrated by a standard single-crystal Si photovoltaic cell (certificated by National Institute of Metrology).

Device Fabrication and Testing
The device structure was ITO/PEDOT:PSS/polymer:BTP-eC9/PNDIT-F3N/Ag. ITO-coated glass substrates were cleaned with deionized water, acetone and isopropyl alcohol for 30 minutes once time and dried in the drying oven at 80 o C for 12 h before used. A PEDOT:PSS interlayer from a precursor solution was spin-coated onto the pre-cleaned and UV-treated ITO substrates, then heated at 150 o C for 10 min. The ITO glass was then placed in the UV-ozone for 15 minutes and the sol-gel-derived ZnO films was spin-coated onto the ITO sustrated followed by thermal treatment at 200 °C for 30 min and cooled to room temperature under vacuum. The mixture of polymer/BTP-eC9 (1:1.2 by wt/wt ratio) was dissolved in chlorobenzene to obtain 10 mg mL −1 of solution. The blend was stirred in the glove box for overnight. The active layer was spin-coating at 2000 rpm for 60 s to get the blend film. A 100 nm Ag layer were subsequently evaporated through a shadow mask to define the active area of the devices. The power conversion efficiencies (PCEs) were tested under AM 1.5G irradiation with the intensity of 100 mW cm -2 (Enlitech.Inc) which was calibrated by a NREL certified standard silicon cell (4 cm -2 ). The J-V curves were recorded with the computer-controlled Keithley 2400 sourcemeter in a dry box under an inert atmosphere. The external quantum efficiency (EQE) spectra were measured through the measurement of solar cell spectral response measurement system QE-R3011 (Enli Technology Ltd., Taiwan).
The mobility of electrons was tested by fitting the current-bias characteristics in the dark utilizing a field-independent space charge limited current (SCLC) model following the Mott-Gurney law fitting the current density-voltage curves, recorded under dark conditions, with the Mott-Gurney equation. The mobility was determined by fitting the dark current to the model of a single carrier SCLC, which is described by the equation where J is the current, μ h is the zero-field mobility, ε 0 is the permittivity of free space, ε r is the relative permittivity of the material, d is the thickness of the active layer, and V is the effective voltage. The effective voltage can be obtained by subtracting the built-in voltage (V bi ) and the voltage drop (V s ) from the substrate's series resistance from the applied voltage (V appl ), V = V appl -V bi -V s. The photo-ability of device was determined by the long-time stable LED white light soaking test system (Enlitech.Inc). The light source area is 10 cm*10 cm with favorable uniformity. The light intensity was outputted in 100 mW cm -2 by control system. The operating temperature of setup is about 50 °C, the humidity is 10% in glove box.

Materials
All chemicals and solvents were reagent grades and purchased from Aldrich, Energy, Derthon and solarmer, respectively. All starting reagents were obtained commercially as analytical grade and used directly without any purification unless stated otherwise. Toluene and THF were distilled over sodium/benzophenone and calcium hydride under N 2 prior to use. S-6
The organic phase was dried over anhydrous sodium sulfate and concentrated by a rotary evaporator. Further purification was carried out by reduced pressure distillation to obtain the production as a colorless oil (30.59 g, 70.01%
After the solid dissolved completely, N-bromosuccinimide (NBS) (1.14 g, 6.39 mmol) was added in one portion. The reaction mixture was stirred at room temperature for 4 h, water was added into the mixture, the mixture was extracted with ethyl acetate , and the organic layer was washed with brine and dried over anhydrous magnesium sulfate.