Linking Group Influences Charge Separation and Recombination in All‐Conjugated Block Copolymer Photovoltaics

Abstract

All‐conjugated block copolymers bring together hole‐ and electron‐conductive polymers and can be used as the active layer of solution‐processed photovoltaic devices, but it remains unclear how molecular structure, morphology, and electronic properties influence performance. Here, the role of the chemical linker is investigated through analysis of two donor–linker–acceptor block copolymers that differ in the chemistry of the linking group. Device studies show that power conversion efficiencies differ by a factor of 40 between the two polymers, and ultrafast transient absorption measurements reveal charge separation only in block copolymers that contain a wide bandgap monomer at the donor–acceptor interface. Optical measurements reveal the formation of a low‐energy excited state when donor and acceptor blocks are directly linked without this wide bandgap monomer. For both samples studied, it is found that the rate of charge recombination in these systems is faster than in poly­mer–polymer and polymer–fullerene blends. This work demonstrates that the linking group chemistry influences charge separation in all‐conjugated block copolymer systems, and further improvement of photovoltaic performance may be possible through optimization of the linking group. These results also suggest that all‐conjugated block copolymers can be used as model systems for the donor–acceptor interface in bulk heterojunction blends.

The synthesis and purification of amphiphilic conjugated donor–acceptor block copolymers

“…In addition, an intriguing aspect of these BCPs is that the interface between donor and acceptor is defined covalently and so allows rational design for optimized charge transfer. 33,34 Homopolymer contaminants dilute this capability. Despite these considerations, difficulties with purification have meant that analysis of fully conjugated BCPs has typically involved polymeric mixtures.…”

Role of Solution Structure in Self-Assembly of Conjugated Block Copolymer Thin Films

“…improved donor-acceptor pairs, or if the covalent linkage of the donor and acceptor in the BCP leads to detrimental recombination or interfacial electronic states. 54,[59][60][61] Additionally, there are also significant questions concerning the role of purity of the donor and acceptor domains in BHJs on charge recombination. 62,63 A fundamental understanding of how the nanostructure and molecular order in CBCPs evolves will help to answer these questions.…”

The Dawn of Single Material Organic Solar Cells

“…[82] Copyright 2017, American Chemical Society. [87] Thus, a new sample of 11 (P3HT-PTBTF) with MW of 20 kDa and a P3HT content of 70 wt% was compared to a parent system in which the P3HT donor block and the dithienyl benzothiadiazole acceptor block were connected by a fluorene unit (13). [84,85] The copolymers were synthesized by GRIM and Suzuki-Miyaura polycondensation.…”

“…Guo et al synthesized all-conjugated poly(3-hexylthiophene)block-poly( (9,9- [4,4-bis(thiophen-5yl)-2,1,3-benzothiadiazole]-2′,2″-diyl block copolymers (11) as active material for SMOSCs (Scheme 5). [87] Although both copolymers present rather similar absorption spectra, the insertion of the fluorene bridge in the structure was reported to produce a large increase of PCE to 2.24%, a result attributed to more efficient charge separation in 13. GRIM is first carried out to give a bromo end-functionalized P3HT macroreagent which is subsequently submitted to Suzuki-Miyaura polycondensation with various diboronic esters.…”

“…[84,85] The copolymers were synthesized by GRIM and Suzuki-Miyaura polycondensation. [87] Lombeck et al have synthesized conjugated D-A block copolymers with carbazole donor units and dithienyl benzothiadiazole acceptor blocks (14) in order to investigate the effects of substitution of the two thienyl rings on phase separation and conversion efficiency. [84] The copolymer containing dioctylfluorene (11) has a total weight-averaged MW of 29 kDa with 56% weight P3HT.…”