Synthesis of photosensitizing diblock copolymers for functionalizationof carbon nanotubes and their applications

Abstract

Block copolymers containing pendant pyrene, terpyridine and poly(3-

hexylthiophene) moieties with different block ratios and chain lengths were

synthesized by reversible addition-fragmentation chain transfer (RAFT)

polymerization. The block copolymers obtained had narrow molecular weight

distribution. The applications of these polymers for non-covalent functionalization

of carbon nanotubes and in photovoltaic devices were studied.

The molecular weight distribution and block sizes of the block copolymers

could be controlled quite well. The polydispersities measured were below 1.25.

The block copolymers could be functionalized on the surface of CNTs. The

functionalized CNTs had an improved dispersing ability and a maximum

dispersing ability of 0.30 mgmL-1 in DMF was achieved. The photosensitizing

properties of an individual functionalized CNT were studied by conductive atomic

force microscopy. In the presence of the photosensitizing unit, the photocurrent

was measured to be 6.4 nAμW-1 at 580 nm. This suggests the role of metal

complexes in the photosensitizing process in the block copolymer.

Poly(3-hexylthiophene)-block-pendant pyrene copolymers were synthesized by

Grignard metathesis and RAFT polymerization. Different loadings of the block

copolymers functionalized CNT were employed as the electron accepting

materials in bulk heterojunction photovoltaic devices. A maximum power

conversion efficiency of 0.77 × 10-3 % was achieved for the poly(3-

hexylthiophene): 0.5% polymer functionalized CNT devices. The poor efficiency

was attributed to the low CNT loadings that limited the electron transport in the

devices.

The poly(3-hexylthiophene)-block-pendant pyrene copolymer were employed as

compatibilizer for poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl

ester (PCBM) bulk heterojunction photovoltaic devices. With the addition of 20

% of the block copolymer, a maximum power conversion efficiency of 1.62 %

could be achieved. The long term stability of the encapsulated photovoltaic

devices was studied. There was more than 30 % reduction in the degradation of

performance after 30 days when the block copolymer was added as compatibilizer.

These results suggested the role of the block copolymer compatibilizers in

improving both the photovoltaic performances and stability of the devices.

Differential scanning calorimetry results suggested that the improved photovoltaic

performances may be attributed to the enhanced compatibility between poly(3-

hexylthiophene) and PCBM.