Title: Semiconductor Anisotropic Nanocomposites Obtained by Directly Coupling Conjugated Polymers with Quantum Rods**
Authors: Lei Zhao, Xinchang Pang, Ramkrishna Adhikary, Jacob W. Petrich, and Zhiqun Lin
Journal: Angewandte Chemie International Ed.
Organic-inorganic hybrid solar cells using combinations of conjugated polymers and semiconductor nanocrystals (quantum dots) offer many advantages over silicon solar cells, such as solution processability, as well as over all-organic solar cells due to the higher charge mobility of inorganic crystals. The electronic energy levels of both the organic conjugated polymer and the quantum dots can be tuned independently to both harvest the maximum amount of solar energy and complement each other for efficient charge transfer between materials. In order to make these hybrid materials, typically the quantum dots are physically mixed with the conjugated polymers to create a composite material with domains containing nanocrystals and domains consisting of polymers known as a bulk heterojunction. Alternatively, after synthesis of the quantum dots a ligand exchange reaction can attach conjugated polymers to the surface of the quantum dots; however, ligand exchange reactions do not result in full surface coverage. In this paper, the authors physically attach the well-known conjugated polymer P3HT (poly(3-hexylthiophene)) to the surface of cadmium selenide (CdSe) quantum dots using catalyst-free “click chemistry,” the 1,3-dipolar cycloaddition of an alkyne to an azide. This is the first paper to covalently link the conjugated polymer directly to the quantum dot without doing a ligand exchange reaction.
The authors show that the absorption spectrum of the covalently-modified quantum dot is the absorption spectrum of the unmodified quantum dot added to the absorption spectrum of the conjugated polymer, suggesting successful attachment of the components. Additionally, photoluminescence (PL) measurements showed that the fluorescence lifetime was decreased for the click-coupled P3HT-CdSe compared to physically mixed P3HT and CdSe, which suggests that coupling the materials improves charge transfer between the conjugated polymer and the quantum dot. Photoluminescence measurements probe how quickly an electron excited by a light pulse returns to ground state after recombining with the hole it left behind. A shorter PL lifetime means the luminescence has been quenched; in other words, the electron is transferred to another species (in this case, the quantum dot) and therefore cannot recombine with the hole. Therefore, the shorter PL lifetime of the click-coupled P3HT-CdSe suggests that charge transfer happens more rapidly when the conjugated polymer is directly attached to the quantum dot than when the polymer is simply physically mixed with quantum dots. This is advantageous for solar applications because the exciton (electron-hole pair) needs to be rapidly separated into two different materials in order to harvest the sun’s energy as electricity. In sum, this article provided a new method of direct attachment of conjugated polymers to the surface of quantum dots using click chemistry, which allowed for a higher surface density of conjugated polymers than can be achieved through ligand exchange and which showed promising properties for light harvesting in solar cells.