Nanoscale Characterization and Unexpected Photovoltaic Behavior of Low Band Gap Sulfur-Overrich-Thiophene/Benzothiadiazole Decamers and Polymers

Abstract

Highly soluble air-stable conjugated decamers and polymers (compounds 1–6) with alternating “sulfur-overrich” bis­(3,4′-S-alkyl)-2,2′-bithiophenes as the donor units and 2,1,3-benzothiadiazoles as the acceptor units were designed and expediently synthesized with the aid of microwave and ultrasound enabling technologies. Solid-state cyclovoltammetry showed that 1–6 had oxidation and reduction potentials in the range 0.6–0.9 V and −1/–1.2 V (vs SCE), respectively, with energy gaps below 2 eV. The electronic properties of spin-coated films of pure 1–6 were investigated with nanoscale resolution by Kelvin probe force microscopy (KPFM). KPFM measurements showed that charge generation and separation were obtained for all films under illumination. Consequently, 1–6 were tested on single-material organic solar cells (SMOCs). In agreement with KPFM results, photovoltaic behavior was observed for all compounds with power conversion efficiencies in line with the best results obtained so far for the few donor–acceptor molecules already shown to perform in single-component solar cells. To our knowledge, this is the first time in which thiophene–benzothiadiazole co-oligomers and copolymers are shown to be photoactive materials in SMOCs.

The Dawn of Single Material Organic Solar Cells

“…As shown in Table , the best results were obtained with decamer 39c with ethylhexyl substituents which give a J sc of 2.36 mA cm −2 and a PCE of 0.54%. This compound also gives the best results when mixed with PC 61 BM in conventional BHJ (PCE = 1.80%) …”

“…The solution spectra of compounds 37 are practically identical to that of 36c, indicating that even without alkyl spacer, the electronic interactions between the π-conjugated donor chain and the PBI acceptor are negligible in 36c, probably because of a large twist angle around the connecting bond due to steric interactions. [121] Di Maria et al have synthesized low bandgap decamers and polymers combining 3-alkylsulfanylthiophenes and benzothiadiazole [123] with the purpose of investigating the effects of introduction of additional sulfur in the structure (Scheme 11). [122] As shown in Figure 8 Table 2, the three compounds lead to rather comparable results with PCE values in the range of 2.0%, consistent with the EQE responses.…”

“…Compound 37b leads to the best results with a J sc of 4.82 mA cm −2 and a PCE of 2.33%. [123] Examination of the above discussed various molecular D-A structures shows that besides the variety of D and A units, these compounds differ by the degree of electronic communication of D and A between fully isolated D and A in the case of flexible linkers, to partially or even fully conjugated structures, as in 33-41. The highest hole mobility of 2.5 × 10 −5 cm 2 V −1 s −1 was obtained with compound 37b, which is consistent with the highest FF value and with the maximum intensity of the aggregation peak in the optical spectrum.…”

“…Di Maria et al have synthesized low bandgap decamers and polymers combining 3‐alkylsulfanylthiophenes and benzothiadiazole with the purpose of investigating the effects of introduction of additional sulfur in the structure ( Scheme ). Two series of compounds substituted by linear and branched substituents were synthesized in good yields.…”

“…Reproduced with permission [123]. b) Dimer and c,d) two different views of the 3D periodic crystal structure.…”