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

Abstract

Donor-type and acceptor-type 9,9¤-bifluorenylidene derivatives, MOBF and CNBF, were synthesized. Suitable energy levels on the excited states of the compounds for singlet fission were revealed by density functional theory calculation. Thermal and optoelectronic properties of the compounds were investigated, and the OPV devices with the donor-type MOBF and the acceptor-type CNBF were fabricated. MOBF showed large dependence of photocurrent on magnetic field, suggesting the singlet fission behavior in the OPV device. On the other hand, CNBF did not show any magnetic field dependence of photocurrent, probably because of poor morphology. Singlet fission has recently attracted much attention as a multiexciton generation mechanism of organic molecules to enhance efficiency of organic photovoltaics (OPV).1 Singlet fission is the conversion process of one singlet exciton into two triplet excitons through an intermediate of two neighboring molecules. If the triplet excitons via singlet fission can dissociate into free charges at the donor/acceptor (D/A) interface, the upper limit of external quantum efficiency (EQE) of the device could reach ca. 200%. Recently, Baldo et al. reported EQE values of higher than 100% for OPV devices based on the singlet fission of pentacene, which was used as the donor material. 2In order to exhibit singlet fission character, excited energy level of molecules are required to satisfy the following condition: the lowest energy of the triplet state (T 1 ) has to be equal to or less than half the lowest energy of the excited singlet state (S 1 ), i.e. E(S 1 ) ² 2E(T 1 ), assuring exothermic conversion process.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. 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.13 This is because, in addition to the energy condition for singlet fission, molecules also have to satisfy the OPV device conditions: broad and intense light absorption, high charge mobility, appropriate energy levels of the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO), as well as sufficient chemical stability.14 It has been known that E(T 1 ) of the oligoacenes decreases with increasing number of condensed aromatic rings, due to the increased biradical nature of the oligoacenes. Therefore, it is important to endow the compounds with slight biradical nature to reduce E(T 1 ) moderately. Recently, we reported that 9,9¤-bifluorenylidene (BF) has suitable excited energy levels for singlet fission.15 In the bifluorenylidene structure, two fluorene moieties are connected through a double bond at each 9-position of the fluo...

Singlet fission and its application in organic solar cells

“…These materials were used as donors in solar cells to match with acceptors with different LUMO levels. EQE spectra indicated that the excitons dissociated into charge carriers with the acceptor PDIF-CN2 but not with C 60 , as E CT for ThQs/C 60 is higher than E(T 1 ) of ThQs [44] . Two 9,9-bifluorenylidene derivatives, p-type material MOBF and n-type material CNBF (Fig.…”