Effects of Crystal Morphology on Singlet Exciton Fission in Diketopyrrolopyrrole Thin Films

Abstract

Singlet exciton fission (SF) is a promising strategy for increasing photovoltaic efficiency, but in order for SF to be useful in solar cells, it should take place in a chromophore that is air-stable, highly absorptive, solution processable, and inexpensive. Unlike many SF chromophores, diketopyrrolopyrrole (DPP) conforms to these criteria, and here we investigate SF in DPP for the first time. SF yields in thin films of DPP derivatives, which are widely used in organic electronics and photovoltaics, are shown to depend critically on crystal morphology. Time-resolved spectroscopy of three DPP derivatives with phenyl (3,6-diphenylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, PhDPP), thienyl (3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, TDPP), and phenylthienyl (3,6-di(5-phenylthiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione, PhTDPP) aromatic substituents in 100-200 nm thin films reveals that efficient SF occurs only in TDPP and PhTDPP (τSF = 220 ± 20 ps), despite the fact that SF is most exoergic in PhDPP. This result correlates well with the greater degree of π-overlap and closer π-stacking in TDPP (3.50 Å) and PhTDPP (3.59 Å) relative to PhDPP (3.90 Å) and demonstrates that SF in DPP is highly sensitive to the electronic coupling between adjacent chromophores. The triplet yield in PhTDPP films is determined to be 210 ± 35% by the singlet depletion method and 165 ± 30% by the energy transfer method, showing that SF is nearly quantitative in these films and that DPP derivatives are a promising class of SF chromophores for enhancing photovoltaic performance.

Unified model for singlet fission within a non-conjugated covalent pentacene dimer

“…The SF rate depends on the free energy of the process and the electronic coupling matrix elements between the chromophores for the particular states involved in this process23. Several families of molecular building blocks have been identified that fulfil the energetic requirements for efficient SF, including polyacenes456789, diphenylisobenzofurans10, carotenoids1112, rylenes13 and other chromophores14, although the scope of SF molecules still remains rather limited, as discussed in reviews by Smith and Michl23.…”

Liquid Crystallinity as a Self‐Assembly Motif for High‐Efficiency, Solution‐Processed, Solid‐State Singlet Fission Materials

“…[3] Although the mechanism of SF is not yet fully understood and is still the subject of intense research, it is generally accepted that the single excitation converts to a triplet pair state ( 1 TT), either coherently or incoherently, which later separates into two independent triplets (2 × T 1 ). [6] For SF the energy of the singlet state (E(S 1 )) should be greater than or equal to twice the energy of the triplet state (E(T 1 )), i.e., E(S 1 ) ≥ 2E(T 1 ). [4] The design of a chromophore capable of supporting SF requires optimization of both energetics and intermolecular electronic coupling, so there are only a limited number of reported SF systems.…”

“…In this study, we investigate the use of hexa-peri-hexabenzocoronene (HBC) as a self-organizing core to promote singlet fission in amorphous films. [6] These properties make DPP derivatives good candidates for OPVs. Soluble HBC derivatives show strong self-association in solution and discotic columnar structures with ordered morphology in films.…”

“…HBC is a polycyclic aromatic hydrocarbon made up of thirteen fused six membered rings, which forms a mesophase. [6,37] Their studies on substituent variation in crystalline diketopyrrolopyrroles demonstrated that the torsion induced on substitution of thienyl (TDPP) by phenyl (PDPP) is sufficient to increase the π-π stacking distance and completely shut down SF, so SF in DPP is very sensitive to local-order (intermolecular coupling), which is not ideal for inclusion in OPV devices. [35] The HBC core shows strong π−π intermolecular association and can show high charge mobilities (≈1 cm 2 V −1 s −1 in certain silyl-substituted HBCs using pulse-radiolysis time resolved microwave conductivity measurements).…”

Donor- or Acceptor-type 9,9′-Bifluorenylidene Derivatives for Attaining Singlet Fission Character in Organic Photovoltaics

“…Several singlet fission materials have been reported, such as 1,3-diphenylisobenzofuran, 3,4 zeaxanthin, 5 diketopyrrolopyrrole, 6,7 poly(3-dodecylthienylenevinylene), 8 and donor/ acceptor-type polymers with thiophene-1,1-dioxide repeat units. 9 However, as long as the singlet fission materials are limited to the materials used in actual OPV devices, only polycyclic aromatic compounds based on tetracene, pentacene, hexacene, or perylene structure have been reported, 2,1012 except our recent report on oligothienoquinoid compounds.…”