Molecular Weight-Dependent Physical and Photovoltaic Properties of Poly(3-alkylthiophene)s with Butyl, Hexyl, and Octyl Side-Chains

Abstract

A series of poly-3-alkylthiophenes (P3ATs) with butyl (P3BT), hexyl (P3HT), and octyl (P3OT) sidechains and well-defined molecular weights (MWs) were synthesized using Grignard metathesis polymerization. The MWs of P3HTs and P3OTs obtained via gel permeation chromatography agreed well with the calculated MWs ranging from approximately 10 to 70 kDa. Differential scanning calorimetry results showed that the crystalline melting temperature increased with increasing MWs and decreasing alkyl side-chain length, whereas the crystallinity of the P3ATs increased with the growth of MWs. An MW-dependent red shift was observed in the UV–Vis and photoluminiscence spectra of the P3ATs in solution, which might be a strong evidence for the extended effective conjugation occurring in polymers with longer chain lengths. The photoluminescence quantum yields of pristine films in all polymers were lower than those of the diluted solutions, whereas they were higher than those of the phenyl-C61-butyric acid methyl ester-blended films. The UV–Vis spectra of the films showed fine structures with pronounced red shifts, and the interchain interaction-induced features were weakly dependent on the MW but significantly dependent on the alkyl side-chain length. The photovoltaic device performances of the P3BT and P3HT samples significantly improved upon blending with a fullerene derivative and subsequent annealing, whereas those of P3OTs mostly degraded, particularly after annealing. The optimal power conversion efficiencies of P3BT, P3HT, and P3OT were 2.4%, 3.6%, and 1.5%, respectively, after annealing with MWs of ~11, ~39, and ~38 kDa, respectively.

Influence of Block Ratio on Thermal, Optical, and Photovoltaic Properties of Poly(3-hexylthiophene)-b-poly(3-butylthiophene)-b-poly(3-octylthiophene)

“…All parameters used to calculate the yields are listed in Table S1 in the supplementary Materials part . The polymerization degree increased with increasing alkyl side-chain length, which can be attributed to the better solubility of the copolymers containing longer alkyl chains in the reaction solvent of THF [ 10 ]. With the high yield of monomer conversion, it can be confirmed that the actual ratio of each P3AT block in the triblock copolymers was virtually the same as the feeding molar ratio of the starting monomers.…”

“…The PCE value of approximately 5% was significantly higher than those of poly(3-butylthiophene) (P3BT) and poly(3-octylthiophene) (P3OT) [ 9 ]. This was attributed to the higher charge carrier extraction capability of the P3HT active layer which is blended with a proper fullerene derivative, owing to its preferable molecular morphology after undergoing the annealing process [ 10 , 11 ]. P3BT might be a suitable material for organic photovoltaic (OPV) applications owing to its higher crystallization facility [ 12 ].…”

“…One possible synthetic method to achieve higher solubility is the incorporation of longer alkyl side-chain thiophene moieties into P3BT to form block copolymers. Thus, P3HT and P3OT were selected because of their better solubility compared to the P3BT homopolymer and the possibly higher charge carrier mobility, as reported previously [ 10 ]. However, random copolymerization must hinder the achievement of a sufficiently high regioregularity of P3ATs, which must cause unfavorable disorder effects by forming active layers of devices with an optimized nanomorphology.…”

A Review on Solution-Processed Organic Phototransistors and Their Recent Developments

“…This study was very promising to select particular functional materials in the design of photovoltaic cells to produce high yield. Molecular weight and alkyl side chain length were investigated for a series of poly(3-alkyl-thiophenes) (P3AT) to elucidate their effect on photovoltaics [127]. P3AT with butyl (C 4 H 9 ), hexyl (C 6 H 13 ), and octyl (C 8 H 17 ) groups were synthesized by adjusting Ni(dppp)Cl 2 catalyst.…”

A Review on Solution-Processed Organic Phototransistors and Their Recent Developments

“…This study was very promising to select particular functional materials in the design of photovoltaic Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 6 December 2021 cells to produce high yield. Molecular weight and alkyl side chain length were investigated for a series of poly(3-alkylthiophenes) (P3AT) to elucidate their effect on photovoltaics[127]. P3AT with butyl (C4H9), hexyl (C6H13), and octyl (C8H17) groups were synthesized by adjusting Ni(dppp)Cl2 catalyst.…”