Isomerization of Perylene Diimide Based Acceptors Enabling High‐Performance Nonfullerene Organic Solar Cells with Excellent Fill Factor

Abstract A strategy that employs the central‐core regiochemistry to develop two isomeric perylene diimide (PDI)‐based small molecular acceptors (SMAs), BPT‐Se and BPT‐Se1, is introduced, and the effect of the central‐core regiochemistry on the optical, electronic, charge‐transport, photovoltaic, and morphological properties of the molecules and their devices is investigated. The PDBT‐T1:BPT‐Se1‐based device delivers a power conversion efficiency (PCE) of 9.54% with an excellent fill factor (FF) of 73.2%, while the BPT‐Se‐based device yields a PCE of 7.78%. The large improvement of PCE upon isomerization of BPT‐Se should be ascribed to the concurrent enhancement of FF, short circuit current ( J SC), and open circuit voltage (V OC) of the PDBT‐T1:BPT‐Se1 devices. The higher FF of the organic solar cells (OSCs) based on PDBT‐T1:BPT‐Se1 can be attributed to the higher charge dissociation and charge collection efficiency, less bimolecular combination, more balanced µ h/µ e, better molecular packing and a more favorable morphology. It is worth mentioning that the FF of 73.2% is the highest value for PDI‐based SMAs OSCs to date. The result shows that regiochemistry of the central core in PDI‐based SMAs greatly affects the physicochemical properties and photovoltaic performance. The success of the isomerization strategy offers exciting prospects for the molecular design of PDI‐based SMAs.


Device fabrication and characterization
The PSCs were fabricated with a structure of ITO/PEDOT: PSS /active layers/ZrAcAc/Al. Source-Measure Unit. Oriel Sol3A Class AAA Solar Simulator (model, Newport 94023A) with a 450 W xenon lamp and an air mass (AM) 1.5 filter was used as the light source.
The light intensity was calibrated to 100 Mw cm −2 by a Newport Oriel 91150V reference cell. The input photon to converted current efficiency (IPCE) was 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. Optical microscope (Olympus BX51) was used to defined the active area (4.5 mm 2 ) of the device. Masks made using laser beam cutting technology to have a S4 well-defined area of 3.14 mm 2 were attached to define the effective area for accurate measurement. All the masked and unmasked tests gave consistent results with relative errors within 0.5%. All the device measurements were undertaken in a nitrogen glovebox.

Mobility Measurements
Hole and electron mobility were measured using the space charge limited current (SCLC) method. Device structures are ITO/MoOx/active layer/MoOx/Al for hole-only devices and ITO/ZnO/active layer/Zracac/Al for electron-only devices. The SCLC mobilities were calculated by MOTT-Gurney equation: J = 9ε 0 ε r μV 2 /8L 3 . Where J is the current density, ε r is the relative dieletiric constant of active layer material usually 2-4 for organic semiconductor, herein we use a relative dielectric constant of 3, ε 0 is the permittivity of empty space, µ is the mobility of hole or electron and L is the thickness of the active layer, V is the internal voltage in the device, and V = V Applied -V Built-in (in the hole-only and the electron-only devices, the V bi values are 0.2 V and 0 V respectively), where V Applied is the voltage applied to the device, and V Built-in is the built-in voltage resulting from the relative work function difference between the two electrodes.

GIWAXS measurement
GIWAXS measurement were carried out with a Xeuss 2.0 SAXS/WAXS laboratory beamline using a Cu X-ray source (8.05 keV, 1.54 Å) and a Pilatus3R 300K detector. The incidence angle is 0.2°. The samples for GIWAXS measurements are fabricated on silicon substrates using the same recipe for the devices.