Asymmetric Electron-Donating 4-Alkyl-8-alkoxybenzo[1,2-b:4,5-b′]dithiophene Unit for Use in High-Efficiency Bulk Heterojunction Polymer Solar Cells

Abstract

A series of new conjugated polymers based on the asymmetric benzo[1,2-b:4,5-b′]dithiophene (BDT) unit were designed and synthesized for use in bulk-heterojunction polymer solar cells. Each side chain of the BDT was tuned by introducing alkyl and alkoxy groups. The best solar cell efficiency was achieved in an asymmetric polymer device based on 4-octyl-8-octyloxy-BDT (7.64% PCE), which performed better than devices based on the symmetric dioctyl-BDT (6.48% PCE) or dioctyloxy-BDT (7.18% PCE). Further modification of the side chains, replacing octyloxy with butoxydiethoxy, improved the PCE to 8.12% due to the enhanced hole mobility, hole/electron mobility balance, and formation of tight contacts with the PEDOT:PSS layer. The effects of the side chains on the polymer HOMO energy levels and photovoltaic parameters were investigated. ■ INTRODUCTIONBenzo[1,2-b:4,5-b′]dithiophene (BDT) derivatives are among the most widely used electron-donating units in the polymer backbones of polymer solar cells (PSCs). A variety of electronaccepting units including thieno [3,4-c]pyrrole-4,6-dione, 1 2,1,3-benzothiadiazole, 2,3 quinoxaline, 4 diketopyrrolopyrrole, 5 and thieno[3,4-b]thiophene 6−11 can provide photoconversion efficiencies (PCEs) exceeding 8% through the use of alternating copolymers by coupling with BDT unit. The most commonly used derivative in the BDT family is 4,8-dialkoxy-substituted BDT. This group has been used because its synthesis is easy and some dialkoxy-BDT polymers provide excellent PCEs. 12−17 Polymers derived from the copolymerization of fluorinated thieno[3,4-b]thiophene (TT) and BDT, TT-BDT, provide good OPV performances in various device structures. 6,9,18−24 These finding led us to use the TT-BDT backbone as a structural platform for investigating the structure/property relationship in an effort to identify new polymers that can improve solar cell performances. 25 The photovoltaic performances of a polymer may be improved by structurally modifying BDT through the introduction of novel substituents, as demonstrated in many research groups (see Figure 1). Liang et al. achieved a HOMO level that was 0.1 eV deeper than the level obtained from the dioctyloxy-BDT polymer by introducing dioctyl-BDT into the polymer backbone; thus, a 0.1 V higher open circuit voltage (V oc ) was achieved in the solar cell device. 15 Lee et al. replaced oxygen atoms with sulfur atoms in the side chains to reduce the HOMO energy level of the conjugated polymer from −5.31 to −5.41 eV and increase V oc from 0.83 to 0.99 V. 26 Bathula et al. polymerized TIPS-substituted BDT with a thieno[3,4-b]-thiophene ester to obtain a deeper HOMO level, thereby increasing the V oc of the resulting PSC to 0.82 V. 27 Wung et al. characterized a polymer of DPP, prepared with unsubstituted-BDT, to achieve a HOMO energy level of −5.46 eV, which was deeper than the −5.29 eV level of the dialkoxy-BDT. 28 Several groups have reported obtaining high V oc s and PCEs by replacing alkoxy side chains with aryl groups. 2,11,29,30 Alkyl side chains offer a...

Side‐Chain Engineering on Dopant‐Free Hole‐Transporting Polymers toward Highly Efficient Perovskite Solar Cells (20.19%)

“…To this end, we tune the polarity of DTB polymer in this work through the side‐chain engineering and introduce diethylene glycol (DEG) groups to replace part of the alkyl groups in the side chains. Contrary to the previous reports, here the π–π stacking is unexpectedly weakened while the lamellar packing is strengthened after the introduction of DEG groups. The properties of DTB polymers (noted as DTB( x DEG), Scheme ) do not monotonically vary with the percentage of the DEG group.…”

Benzodithiophene‐Based Polymers Containing Alkylthiophenyl Side Chains with Lowered HOMO Energy Levels for Organic Solar Cells

“…Absorption at short wavelengths forP 1a nd P2 were observed at around 350 nm, whicha re attributed to the delocalized excitonic p-p* transitions. [14] Compared with the UV-vis absorption spectra in solution,t he UV-vis absorption spectra of P1 and P2 in thin films show ac lear redshift in the absorption maximum and onset because of p-p intermolecular interactions in the film and aggregation of the polymer backbones. [13] The UV-vis absorptions pectrum band of P3 is broader because of enhanced intermolecular interactions in the solid state.…”

“…[13] The UV-vis absorptions pectrum band of P3 is broader because of enhanced intermolecular interactions in the solid state. [14] The optical band gaps (Eopt g) werec alculated from the absorption onsets of the thin films, and the results are 1.73 eV,1 .73 eV, and 1.44 eV forP1-P3,respectively.…”

Structural Engineering of Luminogens with High Emission Efficiency Both in Solution and in the Solid State

“…Such acrystalline effect would naturally enhance the emission via the reduction of molecular vibration and rotation. This is the reason why the emission of powder 1 is stronger than the emission of its amorphous aggregates.T o further discern the stacking mechanism of powder 1,w e applied grazing incidence wide-angle X-ray scattering (GiWAXS, Figure 4f)tocharacterize the packing orientation and d spacing.The in-plane (010) diffraction near q z = 1.5 À1 corresponding to the p-p stacking distance of the solid [26] is very weak. Meanwhile,s trong intermolecular interactions in q xy direction verify that the distorted CS structure could restrict the molecular vibration and rotation, thereby improving the radiation transition process of excited-state electrons.…”