High‐Performance Polymer Solar Cells Based on a Wide‐Bandgap Polymer Containing Pyrrolo[3,4‐f]benzotriazole‐5,7‐dione with a Power Conversion Efficiency of 8.63%

A novel donor–acceptor type conjugated polymer based on a building block of 4,8‐di(thien‐2‐yl)‐6‐octyl‐2‐octyl‐5H‐pyrrolo[3,4‐f]benzotriazole‐5,7(6H)‐dione (TZBI) as the acceptor unit and 4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene as the donor unit, named as PTZBIBDT, is developed and used as an electron‐donating material in bulk‐heterojunction polymer solar cells. The resulting copolymer exhibits a wide bandgap of 1.81 eV along with relatively deep highest occupied molecular orbital energy level of −5.34 eV. Based on the optimized processing conditions, including thermal annealing, and the use of a water/alcohol cathode interlayer, the single‐junction polymer solar cell based on PTZBIBDT:PC71BM ([6,6]‐phenyl‐C71‐butyric acid methyl ester) blend film affords a power conversion efficiency of 8.63% with an open‐circuit voltage of 0.87 V, a short circuit current of 13.50 mA cm−2, and a fill factor of 73.95%, which is among the highest values reported for wide‐bandgap polymers‐based single‐junction organic solar cells. The morphology studies on the PTZBIBDT:PC71BM blend film indicate that a fibrillar network can be formed and the extent of phase separation can be manipulated by thermal annealing. These results indicate that the TZBI unit is a very promising building block for the synthesis of wide‐bandgap polymers for high‐performance single‐junction and tandem (or multijunction) organic solar cells.

(M n ), weight-average molecular weights (M w ) and polydispersity index (PDI) of copolymers were determined at 150 °C by a PL-GPC 220 type in 1,2,4-trichlorobenzene using a calibration curve with standard polystyrene as a reference. The differential scan calorimetry (DSC) were measured on a Netzsch DSC 204 under N 2 flow at a heating rate of 10 °C/min and cooling rate of 20 °C/min. Thermogravimetric analysis (TGA) was performed on Netzsch TG 209 in nitrogen, with a heating rate of 20 ºC min -1 . UV-vis absorption spectra were recorded on a HP 8453 spectrophotometer. Cyclic voltammograms measurements (CV) were recorded on a CHI 660A electrochemical workstation. The measurements were carried out in a nitrogen-saturated solution of 0.1 M tetra-n-butylammonium hexafluorophospate (n-Bu 4 NPF 6 ) in acetonitrile with platinum electrode against Ag/AgCl referenc electrode. The scan rate was 50 mV s -1 . A platinum electrode coated with thin copolymer film was used as the working electrode. Tapping-mode atomic force microscopy (AFM) images were obtained using a NanoScope NS3A system (Digital Instrument) or a Bruker Multi-mode 8 system.
Transmission electron microscope (TEM) images were characterized with a JEM-2100F instrument. The external quantum efficiency (EQE) data were recorded with a QE-R test system from Enli technology company (Taiwan).
Fabrication and characterization of PSCs. Patterned indium tin oxide (ITO)-glass substrates were used as the anode in the polymer solar cells. The ITO coated glass substrates were cleaned by sonication in detergent, deionized water, acetone and isopropyl alcohol; and then dried in a nitrogen stream, followed by an oxygen plasma treatment. Then the surface of the ITO substrate was modified by spin-coating the conducting poly(3,4ethylenedioythiophene): poly(styrene sulfonic acid) (PEDOT:PSS) (Clevios P4083) layer with a thickness of 40 nm, followed by baking at 150 °C for 10 minutes under ambient conditions. The substrates were then transferred into an nitrogen-filled glove box. The copolymers were blended with PC61BM or PC71BM and dissolved in 1,2-dichlorobenzene (o-DCB). The solutions were then spin-coated onto the PEDOT:PSS layer at 800-1000 rpm. The 3 thicknesses of the active layer were about 80-90 nm. Thermal annealing of the blend films was carried out by placing them onto a hot plate with different temperatures for 15 minutes in a nitrogen atmosphere. A 5 nm PFN or PFN-Br layer was then spin-coated from methanol solution in presence of a trace amount of acetic acid onto the active layer. Subsequently, the films were transferred into a vacuum evaporator and 80 nm of Al were deposited as cathode.
The effective area of a device was 0.04 cm 2 as determined by the shadow mask used during deposition of Al cathode. min under ambient conditions. The substrates were then transferred into an argon-filled glovebox. Subsequently, the BHJ composite films were prepared on ITO/PEDOT:PSS substrates using the same method as that for solar cell device fabrication. Finally, MoO 3 (~10 nm) and Al (~80 nm) were sequentially thermally deposited on the top of the active layer in a vacuum system. The mobility was determined by fitting the dark current to the model of a single carrier SCLC, described by the equation: J SCLC = (9/8)ε 0 ε r μ 0 (V 2 /L 3 ), where J was the current, μ 0 is the zero-field mobility, ε 0 was the permittivity of free space, ε r was the relative permittivity of the material, L was the thickness of the active layer, and V was 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 hole-mobility can be calculated from the slope of the J 1/2 -V curves.
GIXD characterization. Grazing incidence X-ray scattering characterization of the thin films was performed at the Advanced Light Source on beamline 7.3.3, Lawrence Berkeley National Lab (LBNL). Thin film samples were spin-casted on to PEDOT:PSS covered SiO 2 wafers. SiO 2 wafers were cleaned by sonication in detergent, deionized water, acetone and isopropyl alcohol. Then the surface of the SiO 2 wafers was modified by spin-coating a PEDOT:PSS layer with a thickness of 40 nm followed by drying at 120 o C for 30 min. After that, the BHJ films were spin-casted on SiO 2 /PEDOT:PSS substrates under exactly the same conditions as those for the fabrication of solar cell devices. The scattering signal was recorded on a 2D detector (Pilatus 2M) with a pixel size of 0.172 mm by 0.172 mm. The samples were ≈15 mm long in the direction of the beam path, and the detector was located at a distance of ≈300 mm from the sample center (distance calibrated using a silver behenate standard). The incidence angle of 0.16° was chosen which gave the optimized signal-to-background ratio.
The beam energy was 10 keV, operating at top-off mode. Typically, 30 s exposure time was used to collect diffraction signals. All GIXD experiments were done in helium atmosphere.
The data was processed and analyzed using Nika software package.
SiO 2 wafers were cleaned by sonication in detergent, deionized water, acetone and isopropyl alcohol. Then the surface of the SiO 2 wafers was modified by spin-coating a PEDOT:PSS layer with a thickness of 40 nm followed by drying at 120 o C for 30 min. After that, the BHJ films were spin-casted on SiO 2 /PEDOT:PSS substrates under exactly the same conditions as those for the fabrication of solar cell devices. The scattering signals were collected in vacuum using Princeton Instrument PI-MTE CCD camera.
The mixture was extracted with CH 2 Cl 2 , the combined organics were dried with MgSO 4 , filtered, and concentrated under vacuum to give product as a pale yellow solid (1.7 g, crude yield = 86%). The crude product was used directly in the next reaction without further purification.

Synthesis of PTZBIBDT
To a two-necked round-bottomed flask (25 mL) was added Br 2 -TZBI (124.8 mg, 0.17mmol), Sn 2 -BDT (153.5 mg, 0.17 mmol), anhydrous toluene (6 mL) and anhydrous DMF (1 mL) under argon. The mixture was purged with argon for 15 min. Then catalyst Pd(PPh 3 ) 4 (7 mg) was quickly added under a stream of argon, and the mixture was purged with argon for another 15 min. Subsequently, the reaction mixture was heated to reflux for 48 h with stirring.
Then the reaction mixture was cooled to ambient temperature and precipitated into methanol.