Synergy of Liquid‐Crystalline Small‐Molecule and Polymeric Donors Delivers Uncommon Morphology Evolution and 16.6% Efficiency Organic Photovoltaics

Abstract Achieving an ideal morphology is an imperative avenue for enhancing key parameters toward high‐performing organic solar cells (OSCs). Among a myriad of morphological‐control methods, the strategy of incorporating a third component with structural similarity and crystallinity difference to construct ternary OSCs has emerged as an effective approach to regulate morphology. A nematic liquid‐crystalline benzodithiophene terthiophene rhodamine (BTR) molecule, which possesses the same alkylthio‐thienyl‐substituted benzo moiety but obviously stronger crystallinity compared to classical medium‐bandgap polymeric donor PM6, is employed as a third component to construct ternary OSCs based on a PM6:BTR:Y6 system. The doping of BTR (5 wt%) is found to be enough to improve the OSC morphology—significantly enhancing the crystallinity of the photoactive layer while slightly reducing the donor/acceptor phase separation scale simultaneously. Rarely is such a morphology evolution reported. It positively affects the electronic properties of the device—prolongs the carrier lifetime, shortens the photocurrent decay time, facilitates exciton dissociation, charge transport, and collection, and ultimately boosts the power conversion efficiency from 15.7% to 16.6%. This result demonstrates that the successful synergy of liquid‐crystalline small‐molecule and polymeric donors delicately adjusts the active‐layer morphology and refines device performance, which brings vibrancy to the OSC research field.


General information
All solvents and reagents were used as received from commercial sources and used without further purification. PBDB-T, BTR and Y6 were purchased from commercial source.

Morphology characterization
AFM measurements were obtained by using a Dimension Icon AFM (Bruker) in a tapping mode.
Grazing incidence X-ray diffraction (GIXD) 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 Si wafers. Si wafers were cleaned by sonication in detergent, deionized water, acetone and isopropyl alcohol. After that, the BHJ composite films were prepared on Si substrates using the same method as that for solar cell device fabrication. 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.

Solar cell fabrication and characterization
Solar cells were fabricated in a conventional device configuration of ITO/PEDOT:PSS/active layers/PFNBr/Ag. The ITO substrates were first scrubbed by detergent and then sonicated with deionized water, acetone and isopropanol subsequently, and dried overnight in an oven. The glass substrates were treated by UV-Ozone for 30 min before use. PEDOT:PSS (Heraeus Clevios P VP AI 4083) was spin-cast onto the ITO substrates at 4000 rpm for 30 s, and then dried at 150 °C for 15 min in air. The donor/acceptor blends were dissolved in chloroform (the total concentration of blend solutions were 16 mg mL -1 for all blends) with 0.5 % 1-CN, and stirred overnight on a hotplate at 40°C in a nitrogen-filled glove box. The blend solution was spin-cast at 3000 rpm for 30 s. A thin PFNBr layer (0.5 mg mL -1 in methanol, 3000rpm for 30 s, about 15 nm) was coated on the active layer, followed by the deposition of Ag (100 nm) (evaporated under 5×10 -5 Pa through a shadow mask). The optimal active layer thickness measured by a Bruker Dektak XT stylus profilometer was about 100 nm. The current density-voltage (J-V) curves of devices were measured using a Keithley 2400 Source Meter in glove box under AM 1.5G (100 mW cm-2) using a Enlitech solar simulator. A 2×2 cm 2 monocrystalline silicon reference cell (SRC-1000-TC-QZ) was purchased from VLSI Standards Inc. The EQE spectra were measured using a Solar Cell Spectral Response Measurement System QE-R3011 (Enlitech Co., Ltd.). The light intensity at each wavelength was calibrated using a standard single crystal Si photovoltaic cell.

SCLC measurements
The electron and hole mobility were measured by using the method of space-charge limited current (SCLC) for electron-only devices with the structure of ITO/ZnO/active layer/PFNBr/Al and hole-only devices with the structure of ITO/MoO x /active layers/MoO x /Al. The charge carrier mobility was determined by fitting the dark current to the model of a single carrier SCLC according to the equation: J = 9ε 0 ε r μV 2 /8d 3 , where J is the current density, d is the film thickness of the active layer, μ is the charge carrier mobility, ε r is the relative dielectric constant of the transport medium, and ε 0 is the permittivity of free space. V = V app -V bi , where V app is the applied voltage, V bi is the offset voltage. The carrier mobility can be calculated from the slope of the J 1/2 ~ V curves.