Closely packed, low reorganization energy π-extended postfullerene acceptors for efficient polymer solar cells

Abstract

New organic semiconductors are essential for developing inexpensive, high-efficiency, solution-processable polymer solar cells (PSCs). PSC photoactive layers are typically fabricated by film-casting a donor polymer and a fullerene acceptor blend, with ensuing solvent evaporation and phase separation creating discrete conduits for photogenerated holes and electrons. Until recently, n-type fullerene acceptors dominated the PSC literature; however, indacenodithienothiophene (IDTT)-based acceptors have recently enabled remarkable PSC performance metrics, for reasons that are not entirely obvious. We report two isomeric IDTT-based acceptors 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-benz-(5, 6)indanone))-5,5,11,11-tetrakis(4-nonylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']di-thiophene (ITN-C9) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-benz(6,7)indanone))-5,5,11,11-tetrakis(4-nonylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITzN-C9) that shed light on the exceptional IDTT properties vis-à-vis fullerenes. The neat acceptors and blends with fluoropolymer donor poly{[4,8-bis[5-(2- ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-]dithiophene2,6-diyl]--[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo4H,8H-benzo[1,2-:4,5-]dithiophene-1,3-diyl]]} (PBDB-TF) are investigated by optical spectroscopy, cyclic voltammetry, thermogravimetric analysis, differential scanning calorimetry, single-crystal X-ray diffraction, photovoltaic response, space-charge-limited current transport, atomic force microscopy, grazing incidence wide-angle X-ray scattering, and density functional theory-level quantum chemical analysis. The data reveal that ITN-C9 and ITzN-C9 organize such that the lowest unoccupied molecular orbital-rich end groups have intermolecular π-π distances as close as 3.31(1) Å, with electronic coupling integrals as large as 38 meV, and internal reorganization energies as small as 0.133 eV, comparable to or superior to those in fullerenes. ITN-C9 and ITzN-C9 have broad solar-relevant optical absorption, and, when blended with PBDB-TF, afford devices with power conversion efficiencies near 10%. Performance differences between ITN-C9 and ITzN-C9 are understandable in terms of molecular and electronic structure distinctions via the influences on molecular packing and orientation with respect to the electrode.

Unprecedented Nucleophilic Attack of Piperidine on the Electron Acceptor during the Synthesis of Push‐Pull Dyes by a Knoevenagel Reaction

“…It has to be noticed that only few materials have been designed with Napht‐EA , despites its extended core that makes it an ideal electron‐withdrawing candidate for the design of materials for photovoltaic applications. In this field, the preliminary results revealed Napht‐EA to be promising as a non‐fullerene electron acceptor for the design of push‐pull materials for energy conversion, justifying the development of new structures . The low availability of Napht‐EA ‐based push‐pull dyes also originates from the difficult access to Napht‐EA so that a careful optimization of the reaction conditions was required to prepare the different dyes.…”

Fused Cyclopentadithienothiophene Acceptor Enables Ultrahigh Short‐Circuit Current and High Efficiency >11% in As‐Cast Organic Solar Cells

“…CDTTIC is also predicted to have a bent or banana like shape, due to the preferential orientation of the CO bond of the indanone toward the sulfur of the CDTT core. We were unable to grow a suitable single crystal of CDTTIC to confirm this prediction but note that similar endgroups have been shown to orientate in this manner, due to stabilizing noncovalent CO…S interactions . Thin film diffraction measurements, vide infra, suggest that CDTTIC is highly crystalline.…”

Recent Progress in Molecular Design of Fused Ring Electron Acceptors for Organic Solar Cells

“…Marks and co‐workers recently utilized similar strategy and they reported two FREAs, one where benzene is replaced with NC ( ITN‐C9 , A75 , Figure ), and another with isomeric structure ( ITzN‐C9 , A76 , Figure ) . Due to stronger intermolecular interaction both the FREAs favored “face‐to‐face” slip‐stacked columns.…”