Fullerene-Grafted Copolymers Exhibiting High Electron Mobility without Nanocrystal Formation

Abstract

Well-soluble fullerene-grafted copolymers PPCBMs with high contents of pendant phenyl-C61-butyric acid methyl ester (PCBM) between 30 and 64 wt % are reported. Herein, the tailor-made precursor copolymers poly(4-methoxystyrene-stat-4-tert-butoxystyrene) obtained by reversible addition–fragmentation chain transfer (RAFT) polymerization are functionalized via an efficient polymer-analogous esterification. The synthesized acceptor copolymers retain the optical and electrochemical properties of the incorporated PCBM independent of their fullerene weight fraction. Their electron transport properties are studied by the space-charge limited current (SCLC) method. The maximum electron mobility μe of 1 × 10–4 cm2 V–1 s–1 is achieved for 37 wt % of incorporated PCBM. Below 50 wt % of PCBM, the acceptor polymers exhibit exceptional high charge carrier mobility compared to the corresponding blends of molecular PCBM and precursor copolymer. Detailed structural studies using AFM, TEM, and XRD are performed. We confirm amorphous morphology both in thin films and in bulk for the PPCBMs, which clearly indicates the absence of PCBM nanocrystals. Thus, an efficient charge carrier percolation is facilitated by the homogeneous distribution of PCBM in the copolymer. Additionally, the absence of nanocrystal formation was demonstrated during thermal annealing.

Donor–acceptor block copolymers carrying pendant PC71BM fullerenes with an ordered nanoscale morphology

“…15 To quantify the extent of fullerene grafting, we favor matrix assisted laser desorption ionization mass spectrometry with time of flight detection (MALDI-TOF MS) as a very reliable method. 17 The mass spectra of the precursor copolymer PS OH and the grafted copolymer PPC 71 BM are depicted in Fig. 3a.…”

“…This is in very good agreement with the MALDI-TOF MS result since the content of PCBM in TGA is usually overestimated, likely due to incomplete decomposition. 17 The optical absorption of PPC 71 BM ranges from the UV region to 650 nm and, hence, has an improved light harvesting capability with respect to optical density and wavelength range compared to the known C 60 fullerene polymers. This was confirmed by UV-vis spectroscopy in solution (Fig.…”

“…16 Just recently, we could demonstrate a well-defined synthesis method for fullerene-grafted copolymers carrying phenyl-C 61 -butyric methyl ester (PC 61 BM) which shows high electron mobilities up to 1 × 10 −4 cm V −1 s −1 without the formation of nanocrystals. 17 Many of the synthetic strategies which aim at a covalent attachment of fullerenes to the block copolymer rely on reactions with unmodified C 60 fullerenes. For example, atom transfer radical addition (ATRA), 18 [3 + 2]-cycloaddition with azides 19,20 or routes using tosylhydrazone addition 21 to C 60 have been reported.…”

Simultaneous morphological stability and high charge carrier mobilities in donor-acceptor block copolymer/PCBM blends

“…While P3HT is an established ptype semiconductor with a maximum reported hole mobility in the range of 0.01 cm 2 Figure 2 and in an earlier report. 39 While the absolute mobility values between SCLC and OFET often differ due to surface effects, anisotropy, varying charge carrier densities, or electrical field strengths, both methods usually represent the same trends in charge carrier mobility. Since the SCLC device configuration must individually be adopted to the type of charge carrier, the OFET method is well suited to study the charge transport of ambipolar materials such as donor-acceptor BCPs or blends since hole and electron mobility can be determined in a single device structure.…”

“…For example, the fullerene grafting density in side-chain functionalized acceptor polymers such as PCBM-grafted polystyrene (PPCBM) is limited due to solubility issues, and in consequence, the resulting electron mobility is less compared to pristine PCBM. 39,40 A feasible route to close the gap of hole and electron mobility in such single-component donor-acceptor BCP is the addition of a small molecule (or polymeric) semiconductors with high mobility. 34,41,42 We favor a blend with the BCP as the majority phase to preserve the nanoscale and stable BCP morphology by enriching one polymer block with small molecules.…”