Comprehensive Study of the Effect of DPE Additive on Photovoltaic Performance of 5,6-Difluoro-benzo[1,2,5] thiadiazole Based Donor-acceptor Copolymers

Three donor-acceptor polymers PBT2F-TT-a, PBT2F-TT and PBT2F-Se were synthesized via Stille cross-coupling reaction with 5,6-difluoro-benzo[1,2,5] thiadiazole as the acceptor, and 2,5-bis-(2-octyl-dodecyloxy)-1,4di(thieno[3,2-b] thiophen-2-yl)-benzene and 2,5-bis-(2-octyl-dodecyloxy)-1,4-di(selenophen-2-yl)-benzene as the donors. The conjugated backbones of these polymers were decorated with alkoxyl groups to achieve the chain planarity through S. O and Se. O intramolecular interactions. Furthermore, the intramolecular interaction was helpful to minimize the torsional angles. These three polymers were characterized by UV-vis absorption spectroscopy, thermal gravimetric analysis, cyclic voltammetry, gel permeation chromatography and elemental analysis. All the polymers showed intense absorption in the visible range and demonstrated suitable E-LUMO and E-HOMO, which match well with the fullerene-based acceptor. Thus these three polymers were applied as donors and incorporated with PC71BM as active materials in bulk heterojunction polymer solar cells (PSCs). Polymers PBT2F-TT-a and PBT2F-TT are constructed with the same monomers and the latter has a high molecular weight. A notable enhancement in PCE from 3.48% to 4.22% was observed as the molecular weight was increased from 6.79 kDa (PBT2F-TT-a) to 10.36 kDa (PBT2F-TT). The effect of diphenyl ether (DPE) additive on photovoltaic performance of PSCs based on these polymers has been comprehensively investigated by means of atomic force microscopy (AFM), transmission electron microscopy (TEM), alternating current impedance spectrometry (ACIS), space-charge-limited current (SCLC) analysis and short circuit current density -light intensity (J(SC)-P-light) measurement. As revealed by AFM and TEM measurement, PBT2F-Se: PC71BM blend with DPE exhibited a nanoscale phase separation dominated with fibrillar structures. On the other hand, the charge carrier mobilities of these blend films were greatly increased after the addition of DPE, giving rise to enhanced photovoltaic performances. Furthermore, the dramatic increase in J(SC) of PBT2F-Se-based devices should also benefit from the well-balanced hole and electron mobilities as revealed by SCLC results. Interestingly, JSC-Plight measurement suggested the weak bimolecular recombination for these devices without and with the addition of DPE. PSCs based on PBT2F-TT-a, PBT2F-TT, and PBT2F-Se showed good power conversion efficiency. Particularly, PBT2F-Se: PC71BM blend with DPE exhibited a good device performance with J(SC) of 10.75 mA/cm(2), V-OC of 0.72 V and PCE of 4.18%.