Nonhalogenated-Solvent-Processed Efficient Polymer Solar Cells Enabled by Medium-Band-Gap A−π–D−π–A Small-Molecule Acceptors Based on a 6,12-Dihydro-diindolo[1,2-b:10,20-e]pyrazine Unit

Abstract

In this contribution, a series of A−π−D−π−A small molecules (SMs), IPY-T-IC, IPY-T-ICCl, and IPY-T-ICF, containing the central donor unit (D) of 6,12-dihydrodiindolo[1,2-b:10,20-e]pyrazine (IPY), the π-conjugated bridge of thiophene, and the end-accepting group (A) of 3-(dic yanomethylidene)indol-1-one, 5,6-dichloro-3-(dicyanomethylidene)indol-1-one, or 5,6-difluoro-3-(dicyanomethylene)indol-1-one, were developed, characterized, and employed as the acceptor materials for polymer solar cells (PSCs). Influences of the different end-accepting groups on thermal properties, spectral absorption, energy levels, photovoltaic performance, and film morphology of these small-molecule acceptors (SMAs) were investigated in detail. These SMAs exhibit an excellent thermal stability and strong crystallization. The absorption spectra of these SMs mainly locate the wavelength between 400 and 700 nm, associated with the optical band gaps in the range of 1.75−1.90 eV. Compared with nonhalogenated IPY-T-IC, the halogenated SMAs IPY-T-ICCl and IPY-T-ICF present better absorption abilities, wider absorption region, and downshifted highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels.With regard to the complementary spectral absorption and matched HOMO/LUMO levels, PTB7-Th as a low-band gap polymer was chosen to be an electron donor to pair with these SMAs for fabricating bulk-heterojuntion PSCs. Under optimized conditions, among these SMAs, the PTB7-Th:IPY-T-IC-based PSC processed from a halogenated solvent system (chlorobenzene + 1-chloronaphthalene) delivers the best power conversion efficiency (PCE) of 7.32%, mainly because of more complementary spectral absorption, upper-lying LUMO level, higher and balanced carrier mobility, more efficiently suppressed trap-assisted recombination, better charge collection property, and blend morphology. Encouragingly, an improved PCE of up to 7.68% is achieved when the IPY-T-IC-based solar cell was processed from a nonhalogenated solvent system (oxylene + 2-methylnaphthalene). In view of the large band gap of these IPY-based SMAs, the PCE of over 7.5% is notable and attractive for the related community. Our study argues that the IPY moiety is a potential electron-donating building moiety to develop medium-band-gap high-performance A−π−D−π−A SMAs for nonhalogenated-solvent-processed photovoltaic devices.

Development of A–DA′D–A Small-Molecular Acceptors Based on a 6,12-Dihydro-diindolo[1,2-b:10,20-e]pyrazine Unit for Efficient As-Cast Polymer Solar Cells

“…The cooling crystallization phenomenon induces the enhancement of the aggregation of side chains and the molecular backbone, which could result in an obvious endothermic peak at a higher temperature for IPYT-IC (294 and 307 °C). The results indicate that even four 4-hexylbenzene bulk groups are dispersed from the conjugated plane of the nonacyclic IPYT backbone, and these IPYT-based SMAs still have a great tendency to form crystalline aggregates. , …”

“…The synthetic route to the two IPYT-based target molecules is shown in Scheme . Compounds 1 – 4 were synthesized according to the previous literature . IPY-based ester (compound 5 ) was prepared by a Suzuki coupling reaction between compound 5 and ethyl 2-bromo-3-thiophenecarboxylate using Pd­(PPh 3 ) 4 as the catalyst in 89% yield.…”

“…After comprehensive optimizations, the as-cast PSCs based on PTB7-Th/ IPYT-ICF exhibited PCEs of 6.91 and 7.00% with CB and o -xylene (XY) as the processing solvent without additive and any post-treatment, respectively, both of which are higher than those of PTB7-Th/ IPYT-IC (6.19 and 4.50%). Furthermore, PCE values of 6.91 and 7.00% are also higher than those of the A−π–A′−π–A type counterpart IPY-T-ICF (5.81% in CB and 3.81% in XY) under the same processing solvent . Additionally, using the medium band gap polymer PBDB-T as the electron donor, PSCs based on these SMAs present the highest V oc of up to 0.98 V, which is obviously higher than that of Y-series SMAs benefitting from the higher-lying LUMO levels of these IPYT-based acceptors because of the weakly electron-deficient IPY unit.…”

“…The synthetic route to the two IPYT -based SMAs is outlined in Scheme . Compounds 1 – 4 were synthesized as described in our previous report . Synthetic details of other compounds including two target SMAs are presented in the Supporting Information.…”

“…Very recently, we have reported a series of A−π–A′−π–A SMAs named IPY-T-IC , IPY-T-ICCl, and IPY-T-ICF (see Figure ) with the weak electron-deficient group 6,12-dihydro-diindolo­[1,2- b :10,20- e ]­pyrazine (IPY) as the central core A′, thiophene as the π-conjugated bridge, and IC and its halogenated derivatives (ICCl and ICF) as the terminal A groups . Results showed that these IPY-based SMAs possessed a medium optical band gap (1.75–1.90 eV) with an absorption range mainly located at 400–700 nm.…”