Enhanced Photovoltaic Performance of Low‐Bandgap Polymers with Deep LUMO Levels

Abstract

Mind the gap! Polymers synthesized by replacing the benzene unit in the acceptor 4,7‐dithien‐2‐yl‐2,1,3‐benzothiadiazole (DTBT) with a π‐electron‐deficient pyridine unit (DTPyT) show a smaller bandgap than their DTBT counterparts. Power conversion efficiencies of over 6 % are demonstrated in solar cell studies (see picture; PNDT, PQDT, and PBnDT represent dithiophene derivatives as weak donors).

Incremental optimization in donor polymers for bulk heterojunction organic solar cells exhibiting high performance

“…For example, the thin-film absorption spectrum of PBnDT-DTPyT saw a remarkable red-shifting relative to the solution spectrum owing to the enhanced molecular stacking in the solid-state arising from the symmetric nature of the BnDT unit. 11 Similarly, higher absorptivity and red-shifting of the absorption peak was observed in a PBTTPD:PC 71 BM films, when cast from CHCl 3 containing 0.5 vol % of diiodooctane (DIO), due to the enhanced miscibility of the polymer with the fullerene, thereby promoting ordering within the PBTTPD chains (Tuning the Morphology: Effect of Additives section). 59 Likewise, owing to the planar nature of the TPD unit, the PCPDTTPD polymer exhibited enhanced interchain interactions and high charge-carrier mobility.…”

“…The introduction of strong electron acceptors into the polymer backbones impart a low HOMO level and hence a lower band gap of the polymers. 1,2,4 BT is one of the widely used electron-deficient units, 6,8,9 and recently several other electron-deficient moieties such as PyT, 11 thieno [3,4-c]pyrrole-4,6-dione (TPD), 13,17,18 NT, 7 and isoindigo 5 have been reported. In addition, fluorine-substituted BT, 12 TT, 1-4,21 and TAZ 13 have also been explored (Fig.…”

“…17 Consequently, several other donor-acceptor polymers were developed using similar strategies. 16,22,[11][12][13]14 Suzuki coupling, which involves coupling of a diboronic ester-substituted aromatic ring and dibrominated aromatic unit in the presence of palladium catalyst and base, has also been used to synthesize donor-acceptor polymers. For example, PCDTBT was synthesized by Suzuki coupling of a dithiophenyl benzothiadiazole (DBT) and a heptadecanyl carbazole.…”

“…53 You et al also investigated the effect of varying electronics of the acceptor unit for a series of copolymers with BDT as a donor unit. 11,58 The replacement of the DTBT unit containing a benzene ring, with a DTPyT unit containing an electron-deficient pyridine unit led to a lowered HOMO energy level (À5.33 ! À5.47 eV) with simultaneous increase in the V OC (0.83 !…”

“…0.85 V) for devices using PC 61 BM as an acceptor. 11,58,62,63 Similarly, upon replacement of the two hydrogen atoms at the 5-and 6-positions in the DTBT unit with two fluorine atoms in DTffBT on going from PBnDT-DTBT to PBnDT-DTffBT, the HOMO energy level decreased (À5.40 ! À5.54 V) owing to the high electronegativity of the fluorine atom.…”