Building Blocks for High‐Efficiency Organic Photovoltaics: Interplay of Molecular, Crystal, and Electronic Properties in Post‐Fullerene ITIC Ensembles

Abstract

Accurate single-crystal X-ray diffraction data offer a unique opportunity to compare and contrast the atomistic details of bulk heterojunction photovoltaic small-molecule acceptor structure and packing, as well as provide an essential starting point for computational electronic structure and charge transport analysis. Herein, we report diffraction-derived crystal structures and computational analyses on the n-type semiconductors which enable some of the highest efficiency organic solar cells produced to date, 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-

Fluorinating π‐Extended Molecular Acceptors Yields Highly Connected Crystal Structures and Low Reorganization Energies for Efficient Solar Cells

“…Note that this distance is considerably smaller than was observed for the fluorine‐free counterpart ITzN‐C9 (3.31–3.39 Å) [ 25 ] and is in accord with our previous observation that end group fluorination contracts π–π distances due to increased dispersive interactions. [ 17 ] Similar to ITzN‐C9, the angled geometry of the ITzN‐F4 end group promotes additional π–π and CN⋯H‐Ar hydrogen bonding interactions [ 46,47 ] between columns of molecules (Figure 4C,D), stabilizing π‐face‐to‐face packing and facilitating electron transport between columns of molecules (vide infra).…”

“…The grid‐like packing motif in the ITN‐F4 crystal structure is qualitatively different than what has been observed in most IDTT crystal structures. Rather than forming separated columns of slip‐stacked, π‐face‐to‐face interacting molecules that are segregated by their alkyl chains [ 17 ] which is typical of IDTT crystal structures (including ITN‐C9), ITN‐F4 packs such that there are no distinct columns of molecules. Rather, ITN‐F4 packs in an interconnected web of very strongly electronically coupling (vide infra) π‐face‐to‐face molecules.…”

“…[ 12–16 ] Despite these advances, an atomic level understanding of the molecular organization patterns of high‐performing NFAs and how their packing motifs affect charge transport are underexplored. [ 17 ] Although thin film characterization techniques [ 18 ] and thermodynamic studies [ 19–21 ] are critical for correlating and understanding nanoscale/mesoscale organization, phase separation, charge transport properties, and OSC performance metrics, they do not precisely elucidate the intra‐ and intermolecular forces that drive donor and acceptor self‐assembly in active layers. This impedes the rational design and synthesis of next generation NFAs and solar cells, as the role of NFA functionalities in packing and charge transport are not clearly understood.…”

“…Thus, a focus of this laboratory has been to investigate the intra‐ and intermolecular forces that drive BHJ assembly at the atomic level, [ 22 ] with crystal structures providing key insights into the packing motifs adopted by NFAs and the interactions driving these motifs. [ 17 ] As seen in charge transport of biomolecules, [ 23,24 ] NFA packing motifs that utilize transport networks with greater dimensionality than 1D exhibit superior performance. [ 25,26 ] Here we use a combined experimental and computational approach to create and characterize new indacenodithienothiophene (IDTT) containing NFAs having both π‐extension [ 17,25 ] ( Figure 1 B) and fluorination [ 17,27,28 ] (Figure 1C).…”

“…[ 17 ] As seen in charge transport of biomolecules, [ 23,24 ] NFA packing motifs that utilize transport networks with greater dimensionality than 1D exhibit superior performance. [ 25,26 ] Here we use a combined experimental and computational approach to create and characterize new indacenodithienothiophene (IDTT) containing NFAs having both π‐extension [ 17,25 ] ( Figure 1 B) and fluorination [ 17,27,28 ] (Figure 1C). These systems were chosen because of their relevance to the rational design of high‐performing NFAs, as the IDTT core is among the most widely used and highest performing of NFA scaffolds, [ 14 ] with ITIC [ 29 ] (Figure 1D) being the most well‐known example.…”