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Chapter 7
7. Conclusions
In this research, three different routes have been explored to obtain a dispersion of CdSe QDs within a polymer matrix. These involve either the controlled growth and/or the stabilization of pre-formed CdSe nanoparticles by using templating or stabilizing polymers.
In particular, the following three different templating/stabilizing systems have been investigated:
1. A hybrid material consisting of multi-walled carbon nanotubes (MWNTs) with grafted poly(acrylonitrile) (PAN) chains, the grafted PAN acting as the templating/stabilizing .
2. A templating block copolymer of P3HT and PAN;
3. A polymeric dispersant consisting of a rr-P3HT bearing a functional phosphonic acid chain end.
Synthetic procedures for the preparation of the above materials and for the in-situ synthesis (case 1 and 2) or controlled combination (case 3) of CdSe within templating/stabilizing systems were developed.
The resulting hybrid/nanocomposite materials thus obtained are expected to be easily dispersible in a matrix consisting of a conjugated polymer such as pure P3HT or any other conjugated polymer deemed suitable as one of the two components (the other being CdSe) of a hybrid, bulk heterojunction-type active layer for a PV cell.
Different synthetic techniques were explored and optimized to obtain the target materials. Among them:
i) the controlled RAFT and ATRP polymerization of acrylonitrile to achieve “grafting from” functionalization of carbon nanotubes or to produce an azido-end-functional PAN, respectively;
ii) the controlled Grignard metathesis (GRIM) polymerization of 2,5-dibromo-3-hexylthiophene to obtain an allyl/Br asymmetrically end-functional rr-P3HT, in which the brominated chain end was used for further functional modification to
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obtain either a polymeric dispersant or a reactive alkynyl-terminated precursor for the “click” coupling into a block copolymer structure.
The composition, morphology, and topological distribution of the inorganic CdSe nanocrystals within the nanocomposite materials obtained by either controlled nucleation and growth (within a templating PAN chain) or controlled replacement of the original low-molecular weight dispersant with a polymeric one (the phosphonic acid-terminated P3HT) were investigated by different spectroscopic, thermal, microscopic, and nano-electrical surface probe techniques.
Formation of controlled size CdSe nanocrystals in the range of 2∼6 nm were achieved, depending on the specific system adopted.
The results obtained have shown that a variety of synthetic and processing approaches may be considered for the preparation of interdispersed hybrid systems in which the characteristic domain size of the P3HT donor and of the CdSe acceptor may be controlled down to a few nanometer size, thus matching the requirements for a bulk heterojunction-type system in which highly efficient photocharge generation and separation could take place.
The design and fabrication of completely formulated thin films by blending with pure rr-P3HT or other suitable conjugated polymer matrix, and their investigation as possible active layers in PV devices will be the obvious continuation of this work.
