Synthesis and Characterization of Bridged Bithiophene-Based Conjugated Polymers for Photovoltaic Applications: Acceptor Strength and Ternary Blends

Abstract

Six of three-component donor-acceptor random copolymers P1-P6, symbolized as (thiophene donor) m -(thiophene acceptor) n , were rationally designed and successfully synthesized by the palladium-catalyzed Stille coupling. The 4H-cyclopenta[2,1-b:3,4-b 0 ]dithiophene (CPDT) unit serves as the donor for P1-P4, while the benzothiadiazole (BT), quinoxaline (QU), dithienoquinoxaline, and thienopyrazine (TP) units are used as the acceptor for P1, P2, P3, and P4, respectively. P5 and P6 are structurally analogous to P1 and P2 except for using the dithieno[3,2-b:2 0 ,3 0 -d]silole (DTS) unit as the donor. Because the band gap lowering ability of the acceptor units in the polymer is in the order TP > BT > QU presumably due to the quinoid form population in the polymers, the optical band gaps can be well adjusted to be 1.2, 1.6, and 1.8 eV for P4, P1, and P2, respectively. It is found that the two bridged bithiophene units, CPDT and DTS, have similar steric and electronic effects on the P1 and P5 as well as P2 and P6, respectively, leading to comparable intrinsic properties and device performances. Bulk heterojunction photovoltaic cells based on ITO/PEDOT:PSS/polymer:PC 71 BM/Ca/Al configuration were fabricated and characterized. Although P4 exhibits the lowest optical band gap, broadest absorption spectrum, and highest mobility, the too low-lying LUMO level hinders the efficient exciton dissociation, resulting in a low PCE of 0.7%. Compared with poly [2,6-(4,4-bis(2-ethylhexyl)(PCPDTBT), random copolymer P1 shows more blue-shifted, broader absorption spectrum, comparable mobility, and a higher PCE of 2.0%. In view of the fact that P1 shows a higher band gap with strong absorption in visible region, while PCPDTBT has a lower band gap to mainly absorb NIR light, a BHJ device with the active layer containing ternary blend of PCPDTBT/P1/PC 71 BM was investigated and achieved an enhanced PCE of 2.5%, which outperforms the devices based on the binary blending systems of PCPDTBT/ PC 71 BM (PCE = 1.4%) or P1/PC 71 BM (PCE = 2.0%) under the identical conditions. Such an improvement is ascribed to the complementary absorption and compatible structure of P1 and PCPDTBT polymers.

Neutral‐State Green Conjugated Polymers from Pyrrole Bis‐Substituted Benzothiadiazole and Benzoselenadiazole for Electrochromic Devices

“…Contrary to BSe incorporated P2, benzothiadiazole inserted P1 exhibited lower monomer and polymer oxidation potentials due to strong donoracceptor match between pyrrole and BTd units. [26] On the other side, both P1 and P2 have lower oxidation potentials due to the participation of the electron rich pyrrole units to the conjugation. Hence, during the n-type doping of these polymers, higher negative peak potentials for the formation of polymer radical-anions were required which were behind the ACN/TBAPF 6 potential window.…”

Plasma modification of poly(2-heptadecyl-4-vinylthieno[3,4-d]thiazole) low bandgap polymer and its application in solar cells

“…These results are similar to those of Chen et al, where at lowest optical bandgap low PCE was observed and high PCE was observed at high bandgap. 47 The polymer modified with the nitrogen plasma showed more red-shift, a broader absorption spectrum, high mobility, high bandgap and a higher PCE of 5.14%. In view of this fact, the polymer modified with the nitrogen plasma showed a higher bandgap with intense absorption in the visible region.…”

Synthesis and characterization of low band gap random copolymers based on cyclopentadithiophene and thiophene carboxylates for photovoltaic applications

“…The precursors 2,6-dibromo-4,4-diethylhexylcyclopenta[2,1-b:3,4-b 0 ]dithiophene (2) and alkyl 2,5-dibromo thiophene 3-carboxylates (5) were prepared as per reported in the literature [34,35]. Characterization 1 H NMR spectra were recorded on a Bruker 500 MHz spectrometer using deuterated solvents with tetramethylsilane (TMS) as a reference.…”