Development of Fluorinated Benzothiadiazole as a Structural Unit for a Polymer Solar Cell of 7 % Efficiency

Abstract

Fluorinated organic molecules exhibit a series of unique features such as great thermal and oxidative stability, [1] elevated resistance to degradation, [2] enhanced hydrophobicity, high lipophobicity of perfluorinated substances, [3] and inverted charge density distribution in fluorinated aromatic compounds.[4] These special features are related to the unique properties of the fluorine atom:[5] a) fluorine is the most electronegative element, with a Pauling electronegativity of 4.0, which is much larger than that of hydrogen (2.2); b) fluorine is the smallest electron-withdrawing group (van der Waals radius, r = 1.35 , only slightly larger than hydrogen, r = 1.2 ). Furthermore, these fluorine atoms often have a great influence on inter-and intramolecular interactions through C-F···H, F···S, and C-F···p F interactions. [2,6] As a result, fluorinated conjugated materials have been explored for their applications in organic field-effect transistors (OFET) [7] and organic light-emitting diodes (OLED). [4,8] However, there are only a few examples of applying fluorinated compounds in organic photovoltaics, [9] especially as p-type semiconductors in bulk heterojunction (BHJ) polymer solar cells.Since the fluorine atom is a strong electron-withdrawing substituent, the introduction of F into the conjugated backbone would lower both the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of the conjugated polymers, as demonstrated by Heeger and BrØdas in a theoretical study of poly(phenylene vinylene) having various substituents. [10] Experimentally, Yu et al. confirmed the electronic effect of the fluorine substituent in their study of a series of benzodithiophene thieno[3,4-b]thiophene copolymers.[9b] After one fluorine atom was substituted onto the thieno [3,4-b]thiophene unit, the copolymer exhibited decreased LUMO and HOMO energy levels, but with a similar band gap, as compared with those of the nonfluorinated analogue. A larger open-circuit voltage (V oc ) was observed from the BHJ device based on the F-substituted polymer, and this difference is largely because of the lower HOMO energy level. Moreover, the short-circuit current (J sc ) and the fill factor (FF) were noticeably increased by judicious selection of solvent and additives, [11] possibly because of an optimized film morphology facilitated by these F atoms. A similar enhancement on the morphology by employing F atoms was observed by Kim et al. in their study of poly(3-hexylthiophene) (P3HT) having various end-groups.[9a] The CF 3 end-group-modified P3HT showed significant improvement in both the J sc and FF values for its BHJ devices, thus leading to a 40 % increase in the efficiency (h). The much improved morphology of the polymer/PC 61 BM blend was attributed to the decreased surface energy of the fluorine-containing polymer. However, there has been no precedent study on the photovoltaic properties of F-containing low-band-gap polymers constructed using the donor-acceptor strategy, [12] which is a comm...

Location, Location, Location ‐ Strategic Positioning of 2,1,3‐Benzothiadiazole Units within Trigonal Quaterfluorene‐Truxene Star‐Shaped Structures

“…[ 23 ] BT has, moreover, been combined with other thiophene based units to achieve smaller optical gap solar cell polymers and molecular light absorbers for dye-sensitized solar cells. [24][25][26][27] A greater understanding of the electronic interplay between the BT and neighboring units in a given structure would be highly benefi cial to the organic electronics community -particularly at the most fundamental level in the structural design of new semiconductors. To this end, the series of structural isomers T4BT-A to T4BT-E allows a thorough investigation of the effect of subtle variations in environment on the infl uence of BT addition to a 9,9-dialkylfl uorene chain.…”

“…The lowest energy of these (labelled III in Figure 2 ) involves a LUMO that is spatially localized on the BT unit and it is generally considered to possess a degree of charge-transfer character [ 28 , 30 ] though not as much as the HOMO-LUMO transition in solar cell materials wherein the BT is fl anked by more strongly electron donating thiophene containing moieties. [23][24][25][26] The second (labelled II in Figure 2 ) links the HOMO to higher lying delocalized unoccupied molecular orbitals that are reported either to be centered on 9,9-dialkylfl uorene units [ 11 ] or more fully delocalized, involving the BT unit as well. [ 30 ] This transition is similar in character to the delocalized-delocalized π -π * HOMO-LUMO transition of T4 [ 34 ] and poly(9,9-dialkylfl uorene)s. [ 11 ] We have undertaken further DFT/TD-DFT calculations to gain a clearer insight into the specifi c electronic structure of our T4BT-X compounds.…”

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

“…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. 5).…”

“…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.…”

Benzo[1,2-b:4,5-b′]dithiophene and benzotriazole based small molecule for solution-processed organic solar cells

“…Intensive interdisciplinary efforts have been dedicated to improving the power conversion efficiencies (PCEs) of solution-processed polymer bulk heterojunction (BHJ) solar cells [4][5][6][7][8][9][10]. More recently, BHJ OSCs using solution-processed small molecules as donors have also attracted increasing attentions [11][12][13][14][15].…”