•Two novel D-A-D small molecules are synthesized using fluorinated benzene or 2,2′-bithiophene as the central donor units.•The effect of fluorination central donor units on photovoltaic performance is investigated.•Introducing fluorine atoms onto rigidly central donor units can improve photovoltaic properties for SMs.•The DFDT(DPP)2 with A-D-A framework presents the outstanding photovoltaic performance with a PCE of 7.87%.For the purpose of exploring the subtle photophysical property on fluorine atom influence, in this work, two simple conjugated small molecules (SMs) with an acceptor-donor-acceptor (A-D-A) molecular backbone, DFPh(DPP)2 and DFDT(DPP)2, based on fluorinated benzene (DFPh) or fluorinated 2,2′-bithiophene (DFDT) as the central donor units, and the electron deficient diketopyrrolopyrrole (DPP) as the terminal acceptor moieties, were designed and synthesized. Due to introducing noncovalent attractive interactions between the fluorine and sulfur atoms (F⋯S), both SMs exhibited a strong absorption in the wavelength region of 300–800 nm, deep HOMO energy level of −5.39 ∼ −5.50 eV, and narrow optical band gap of 1.60–1.66 eV in comparison with those of non-fluorinated analogues. After optimization, the DFDT(DPP)2/PCB71M-based device presented a higher PCE of 7.87% with Voc of 0.86 V, Jsc of 12.6 mA cm−2, and FF of 72.6% in comparison with the PCE of 6.45% with Voc of 0.97 V, Jsc of 10.50 mA cm−2, and FF of 63.8% for DFPh(DPP)2/PCB71M-based device. Notably, the PCE of 7.87% is one of the highest PCEs reported for the A-D-A type SMs-based on DPP as terminal acceptor groups. These primary studies indicated that the incorporating fluorine atom on rigidity skeleton central units is an efficient approach to improve photovoltaic performance of A-D-A type SMs.Two novel A-D-A type small molecules of DFPh(DPP)2 and DFDT(DPP)2 were designed and synthesized. The significant effect of the fluorinated central core is observed on the optical, electrochemical, morphological and photovoltaic properties of SMs. The photovoltaic devices based on SMs have been effectively improved due to the elevation of Js, Voc and FF synchronously.Download high-res image (306KB)Download full-size image

Two novel wide bandgap copolymers based on quinoxalino[6,5-f]quinoxaline (NQx) acceptor block, PBDT–NQx and PBDTS–NQx, are successfully synthesized for efficient nonfullerene polymer solar cells (PSCs). The attached conjugated side chains on both benzodithiophene (BDT) and NQx endow the resulting copolymers with low-lying highest occupied molecular orbital (HOMO) levels. The sulfur atom insertion further reduces the HOMO level of PBDTS–NQx to −5.31 eV, contributing to a high open-circuit voltage, Voc, of 0.91 V. Conjugated n-octylthienyl side chains attached on the NQx skeletons also significantly improve the π–π* transitions and optical absorptions of the copolymers in the region of short wavelengths, which induce a good complementary absorption when blending with the low bandgap small molecular acceptor of 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene. The wide absorption range makes the active blends absorb more photons, giving rise to a high short-circuit current density, Jsc, value of 15.62 mA cm−2. The sulfur atom insertion also enhances the crystallinity of PBDTS–NQx and presents its blend film with a favorable nanophase separation, resulting in improved Jsc and fill factor (FF) values with a high power conversion efficiency of 11.47%. This work not only provides a new fused ring acceptor block (NQx) for constructing high-performance wide bandgap copolymers but also provides the NQx-based copolymers for achieving highly efficient nonfullerene PSCs.

Two novel D(A-Ar)2 type small molecules (SMs) of 3T(DPP-BTI)2 and 4T(DPP-BTI)2 were synthesized and characterized, which contain diketopyrrolopyrrole (DPP) arms, 6-dodecyl-6H-benzo[4,5]thieno[2,3-b]indole (BTI) terminals, and oligomerthiophene donor unit (3T=trithiophene, 4T=tetrathiophene). The relation-ship between structure and property was investigated. Compared with their corresponding parent A-D-A type SMs, both D(A-Ar)2 type SMs exhibited a significantly increasing circuit current density (Jsc) by 1.39∼1.59 times and hole mobility (μh) by one order of magnitude, as well as a comparable power conversion efficiency (PCE) in their OPV and hole-only devices. The better photovoltaic properties with PCE of 3.02% and Jsc of 10.41 mA cm−2 were obtained in the 3T(DPP-BTI)2 based OPV cells. It indicates that appending the BTI terminal unit into A-D-A-type SMs is beneficial to increasing the Jsc and μh values of its resulting D(A-Ar)2 type SMs in their corresponding devices.Two novel D(A-Ar)2 type small molecules (SMs) of 3T(DPP-BTI)2 and 4T(DPP-BTI)2 with oligothiophene, diketopyrrolopyrrole, benzo[4,5]thieno[2,3-b]indole units were synthesized. Compared to their parent A-D-A type SMs of 3TDPP2 and 4TDPP2, both D(A-Ar)2 type SMs exhibited a significantly increasing Jsc by 1.5–1.7 times and hole mobility (Mh) by one order of magnitude, as well as a comparable PCE in their corresponding OPV and hole-only devices.