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...

Impact of Backbone Fluorination on π-Conjugated Polymers in Organic Photovoltaic Devices: A Review

“…Indeed, the fluorine atoms need to be introduced before the functionalization step [10,[19][20][21][22][23][24][25]. For instance, if we consider two of the most common building blocks in the conjugated polymer field, namely the thiophene electron-donating group [26][27][28][29][30][31][32] and the 2,1,3-benzothiadiazole [11,[32][33][34][35][36][37][38][39][40][41][42] electron-accepting group, their fluorinated derivatives requires a minimum of three and one additional steps, respectively (see Scheme 1).…”

“…Polymers 2016, 8,11 3 of 27 [11,[32][33][34][35][36][37][38][39][40][41][42] electron-accepting group, their fluorinated derivatives requires a minimum of three and one additional steps, respectively (see Scheme 1).…”

Manipulating backbone structure with various conjugated spacers to enhance photovoltaic performance of D–A-type two-dimensional copolymers

“…By fluoration of quinoxaline, it is able to reduce the HOMO energy levels of its polymers due to the additional electron-withdrawing effect of fluorine [32][33][34][35][36][37][38][39]. The decreased HOMO energy level is suggested to be available to increase V oc and probably lead to a higher PCE.…”

Cascade-type excitation energy relay in organic thin-film solar cells

“…Increasing research efforts have steadily improved the power conversion efficiencies (PCEs) of small-molecule-based organic [3][4][5][6] and polymer-based [6][7][8][9][10] PV cells since Tang reported the first efficient bilayer PV device [11]. Because structurally well-defined small-molecule materials avoid the inherent batch-to-batch variations in the physical properties of polymers, they provide better reproducibility.…”