Effect of electron-withdrawing fluorine and cyano substituents on photovoltaic properties of two-dimensional quinoxaline-based polymers

Abstract

In this study, strong electron-withdrawing fluorine (F) and cyano (CN) substituents are selectively incorporated into the quinoxaline unit of two-dimensional (2D) D-A-type polymers to investigate their effects on the photovoltaic properties of the polymers. To construct the 2D polymeric structure, electron-donating benzodithiophene and methoxy-substituted triphenylamine are directly linked to the horizontal and vertical directions of the quinoxaline acceptor, respectively. After analyzing the structural, optical, and electrochemical properties of the resultant F- and CN-substituted polymers, labeled as PBCl-MTQF and PBCl-MTQCN, respectively, inverted-type polymer solar cells with a non-fullerene Y6 acceptor are fabricated to investigate the photovoltaic performances of the polymers. It is discovered that the maximum power conversion efficiency of PBCl-MTQF is 7.48%, whereas that of PBCl-MTQCN is limited to 3.52%. This significantly reduced PCE of the device based on PBCl-MTQCN is ascribed to the formation of irregular, large aggregates in the active layer, which can readily aggravate the charge recombination and charge transport kinetics of the device. Therefore, the photovoltaic performance of 2D quinoxaline-based D-A-type polymers is significantly affected by the type of electron-withdrawing substituent.

Figure 1

Synthesis of monomers and polymers: (i) CuI, KOH, 1,10-phenanthroline, toluene, 90 °C, 24 h; (ii) AlCl₃, oxalyl chloride, dichloromethane, 0 °C, 1 h, and then at room temperature for 12 h; (iii) zinc, acetic acid, 80 °C, 4 h, and then 2, acetic acid, overnight; (iv) Pd₂(dba)₃, chlorobenzene/dimethylformamide, 110 °C, 48 h.

Figure 2

(a) UV–Vis spectra of polymers in chloroform solution and (b) polymers and Y6 in film on glass substrate. (c) CV curves of polymers.

Figure 3

Optimized geometries and frontier molecular orbitals of two-repeating unit calculated energy levels at B3LYP/6-31G** level for (a) PBCl-MTQF and (b) PBCl-MTQCN.

Figure 4

(a) Structure, (b) energy band diagram, (c) J–V curves and (d) EQE curves of inverted-type device based on PBCl-MTQF and PBCl-MTQCN as polymeric donor.

Figure 5

(a) Jsc vs. light intensity plots and (b) Voc vs. light intensity plots of devices. (c) J_ph–V_eff curves of polymers.

Figure 6

Tapping-mode AFM images of blend films of (a) PBCl-MTQF and (b) PBCl-MTQCN with Y6.

Figure 7

GIWAXS patterns of (a,b) neat films and (c,d) blend films of PBCl-MTQF and PBCl-MTQCN. (e) Line-cut profiles of neat and blend films (red line: out-of-plane; black line: in-plane).