Atomistic Band Gap Engineering in Donor–Acceptor Polymers

Abstract

We have synthesized a series of cyclopentadithiophene-benzochalcogenodiazole donor-acceptor (D-A) copolymers, wherein a single atom in the benzochalcogenodiazole unit is varied from sulfur to selenium to tellurium, which allows us to explicitly study sulfur to selenium to tellurium substitution in D-A copolymers for the first time. The synthesis of S- and Se-containing polymers is straightforward; however, Te-containing polymers must be prepared by postpolymerization single atom substitution. All of the polymers have the representative dual-band optical absorption profile, consisting of both a low- and high-energy optical transition. Optical spectroscopy reveals that heavy atom substitution leads to a red-shift in the low-energy transition, while the high-energy band remains relatively constant in energy. The red-shift in the low-energy transition leads to optical band gap values of 1.59, 1.46, and 1.06 eV for the S-, Se-, and Te-containing polymers, respectively. Additionally, the strength of the low-energy band decreases, while the high-energy band remains constant. These trends cannot be explained by the present D and A theory where optical properties are governed exclusively by the strength of D and A units. A series of optical spectroscopy experiments, solvatochromism studies, density functional theory (DFT) calculations, and time-dependent DFT calculations are used to understand these trends. The red-shift in low-energy absorption is likely due to both a decrease in ionization potential and an increase in bond length and decrease in acceptor aromaticity. The loss of intensity of the low-energy band is likely the result of a loss of electronegativity and the acceptor unit's ability to separate charge. Overall, in addition to the established theory that difference in electron density of the D and A units controls the band gap, single atom substitution at key positions can be used to control the band gap of D-A copolymers.

Benzochalcogenodiazole‐Based Donor–Acceptor–Acceptor Molecular Donors for Organic Solar Cells

“…[39][40][41] On the other hand, the 2,1,3-benzoselenodiazole (BS) acceptor has also shown great promise as a building block for the development of photovoltaic materials as BS-containing systems generally have smaller optical bandgaps than their BTbased counterparts. [42][43][44][45] Along these lines, we report here the synthesis of four D-A-A molecular donors that bear BO …”

Synthesis and characterization of regioregular poly(3‐dodecyltellurophene)

“…Because tellurium atoms are more electronrich than sulfur atoms, aromatic rings containing tellurium are more easily delocalized than those containing sulfur atoms. 18 The optical bandgap of the polymer thin film was determined from the UV-visible absorption onsets in the solid state (E g 5 1240/k onset eV). The optical bandgap (E g ) of P3DDTe in the thin films was 1.48 eV, which was significantly lower than that of P3DDT (1.74 eV).…”

“…These results demonstrated that the replacement of thiophene with tellurophene resulted in a significant reduction in the bandgap, because the tellurium atoms acted to stabilize the LUMO level, whose results are well corresponding with previous reported work. 18 Thin Film Analysis The X-ray diffraction pattern of the P3DDTe film (which was cast from chloroform solution under evaporation in air) showed the first and second order reflection at 2h angle of 4.4 and 8.7 (Supporting Information Fig. S2).…”

Understanding the colorimetric properties of quinoxaline-based pi-conjugated copolymers by tuning their acceptor strength: a joint theoretical and experimental approach

“…Several examples of acceptors bonded to thiophene-based donor have been reported: benzothiadiazole, 8 benzooxadiazole 10 and quinoxaline derivatives. 9 The modication of the acceptor groups can also be done either by chemical modication of benzothiadiazole 16 or using functionalized quinoxalines. [17][18][19] The use of acceptors based on diphenyl-quinoxaline which possesses two distinct phenyl rings is one of the widely investigated units because of its easy synthesis and versatility.…”