Effect of Replacing Alkyl Side Chains with Triethylene Glycols on Photovoltaic Properties of Easily Accessible Fluorene-Based Non-Fullerene Molecular Acceptors: Improve or Deteriorate?

Abstract

Hydrophilic oligo(ethylene glycol) (OEG) side chains are more flexible than alkyl chains and can facilitate the π−π stacking of conjugated polymer backbones, which has been proven to be a key structural feature leading to better device performance in polymer:fullerene blend polymer solar cells (PSCs). So far, little has been known about the influence of OEG side chains on the performance of non-fullerene acceptors in polymer:non-fullerene acceptor blends. Based on an easily accessible conjugated backbone of dicyanomethylene indanone−thiophene−fluorene−thiophene−dicyanometnylene indanone (DICTF), two non-fullerene molecular acceptors were synthesized with decyl (DICTF-C10) and triethylene glycol monoether (DICTF-TEG) side chains to blend with the polymer donor of PTB7-Th for PSCs. Replacing the decyl side chains with the TEGs on DICTF does improve the π−π stacking between the conjugated backbones as expected. For DICTF-TEG, nevertheless, forming the BHJ blend with the PTB7-Th donor leads to unfavorable film morphology structure of the active blend and deteriorated device performance with much lower PCE of 3.09% compared to the blend of PTB7-Th:DICTF-C10 with the maximum PCE of 6.93% at optimized conditions. It is found that charge generation, transport, and recombination within the PSC devices of the two acceptors were greatly impacted by introducing TEG side chains instead of alkyls into the conjugated backbone of DICTF. This comparative study on understanding the side chain effect on device performance allows the establishment of guidance on molecular engineering of non-fullerene molecular acceptors to obtain high performance PSCs.

Glycolated Thiophene‐Tetrafluorophenylene Copolymers for Bioelectronic Applications: Synthesis by Direct Heteroarylation Polymerisation

“…Due to the strong positive mesomeric effect of the oxygen atom attached directly to the thiophene, an increase in the push‐pull effect can be expected, resulting in a higher rigidity of the conjugated backbone. Furthermore, in comparison with commonly used alkyl chains, glycol chains are more flexible and possess constant dipole; both factors can result in a shortening of the π‐π stacking distance between the polymer backbones . Another essential property is that the glycol chains have a high affinity for polar solvents and ions, allowing their penetration from the aqueous electrolyte into the polymer thin film, resulting in the possibility of mixed conduction of both electrical charges and ions in the solid state .…”

“…Furthermore, in comparison with commonly used alkyl chains, glycol chains are more flexible and possess constant dipole; [37] both factors can result in a shortening of the π-π stacking distance between the polymer backbones. [38,39] Another essential property is that the glycol chains have a high affinity for polar solvents and ions, allowing their penetration from the aqueous electrolyte into the polymer thin film, resulting in the possibility of mixed conduction of both electrical charges and ions in the solid state. [40] Ion conduction is a crucial prerequisite for creating a functioning organic electrochemical transistor (OECT).…”

Resolving Donor–Acceptor Interfaces and Charge Carrier Energy Levels of Organic Semiconductors with Polar Side Chains

“…According to the present findings, we speculate that PTEG-1 is predicted to decrease the open-circuit voltage (V OC ), given the same blend, due to increased electrostatic-induced disorder of the interfacial energy levels. [57] Such a decrease in V OC was previously reported upon EG-functionalization of polymers [7] and nonfullerene acceptors, [58] and upon cyano-functionalization [59] of polymers, the latter being another way of introducing permanent dipole moments in organic semiconductors. [14] Broadening of charge carrier energy levels can also lead to lower mobilities.…”

Non‐Fullerene Small Molecule Acceptors Containing Barbituric Acid End Groups for Use in High‐performance OPVs

“…[1][2][3] Electron-deficient groups, such as rhodanine (RH), 4,5 1,3-indandione (IN), 6,7 and 1,1-dicyanomethylene-3-indanone (CNIN) 8,9 have been widely used as acceptor units, together with fused-ring-based electron-rich groups, such as carbazole (Cz), 10,11 fluorene (Flu), 12,13 indacenodithiophene (IDT), 14,15 and indacenodithienothiophene (IDTT). Non-fullerene acceptors (NFAs) composed of donor (D) and acceptor (A) units have been widely reported due to their high power conversion efficiencies (PCEs) exceeding 14%.…”

“…They have several advantages, including their simple synthesis, strong and broad absorption in the UV–Vis region, and easily tunable energy levels . Electron‐deficient groups, such as rhodanine (RH), 1,3‐indandione (IN), and 1,1‐dicyanomethylene‐3‐indanone (CNIN) have been widely used as acceptor units, together with fused‐ring‐based electron‐rich groups, such as carbazole (Cz), fluorene (Flu), indacenodithiophene (IDT), and indacenodithienothiophene (IDTT) …”