Semiconductor Anisotropic Nanocomposites Obtained by Directly Coupling Conjugated Polymers with Quantum Rods

Abstract

Conjugated polymers (CPs) have received considerable attention as promising materials for use in organic photovoltaics, light-emitting diodes (LEDs), thin film transistors, and biosensors. Among various types of CPs, poly(3hexylthiopene) (P3HT) is one of the most widely studied organic semiconductors. P3HT possesses excellent solution processability, environmental stability, high charge-carrier mobility, and tailorable electrochemical properties. Owing to their quantum-confined nature, for quantum dots (QDs) such as cadmium selenide (CdSe), variation of the nanocrystal size provides continuous and predictable changes in fluorescence emission, thus rendering them useful for a wide range of applications in photovoltaic cells, 7] LEDs, biosensors, and bio-imaging. CP-based organic/inorganic hybrid solar cells (e.g., CP/QD composites) are favorable alternatives to inorganic solar cells as they have many advantages peculiar to CPs, such as light weight, flexibility, processability, roll-to-roll production, low cost, and large area. However, the CP/QD composites are most often prepared by simply physically mixing the CPs and QDs. This procedure, however, suffers from several severe problems, including microscopic phase separation and the existence of insulating interfacial layers, thereby reducing the interfacial area between CPs and QDs and thus limiting the performance of the resulting devices. Recently, various methods have been utilized to overcome these problems, such as the use of cosolvent mixtures or binary solvent mixtures and surface modification of QDs. The most elegant approach is to chemically tether CPs on the QD surface (i.e., preparing CP–QD nanocomposites), hence enabling direct electronic coupling between CPs and QDs. Notably, this strategy has only recently been developed and has been primarily implemented by ligand exchange, which permits the derivatization of the composite with a broad range of functional groups. However, ligandexchange chemistry suffers from incomplete surface coverage. In this context, recently P3HT–CdSe-QD nanocomposites have been synthesized by directly grafting vinyl-terminated P3HT onto a [(4-bromophenyl)methyl]dioctylphosphine oxide(DOPO–Br)-functionalized CdSe QD surface by a mild palladium-catalyzed Heck coupling without the need for ligand exchange. The ability to manipulate the shape of nanocrystals has led to quantum rods (hereafter referred to as nanorods; NRs) with diameters that range from 2 to 10 nm and lengths ranging from 5 to 100 nm. Owing to their intrinsic structural anisotropy, NRs possess many unique properties that make them potentially better nanocrystals than QDs for photovoltaics and biomedical applications. Photovoltaic cells made of NRs and CPs show an improved optical absorption in the red and near-infrared ranges that originates from the NRs. Moreover, the long axis of the NRs provides continuous paths for the transport of electrons, an advantage over QDs, in which electron hopping between QDs is required. The performance of photovoltaic cells can be further improved if NRs are vertically aligned between two electrodes to minimize the carrier transport pathways. It is worth noting that although CP–NR nanocomposites were recently produced by ligand exchange of CPs with insulating ligands that were initially attached to the NR surface, direct grafting of CPs onto anisotropic nanocrystals has not yet been explored. Herein, we report one simple yet robust route to CP–NR nanocomposites that displaces the need for ligand-exchange chemistry. In this strategy, the catalyst-free alkyne–azide cycloaddition, which belongs to the emerging field of click chemistry, was utilized in the preparation of P3HT–CdSeNR nanocomposites. As shown in Scheme 1, CdSe NRs were passivated with bromobenzylphosphonic acid (BBPA), which not only induced elongated growth but also functionalized the CdSe NR surface and led to the formation of BBPA–CdSeNRs. Subsequently, the aryl bromide groups of BBPA were converted into azide groups, thus forming N3–BPA–CdSe NRs. Finally, catalyst-free Huisgen 1,3-dipolar cycloaddition between ethynyl-terminated P3HT and N3–BPA–CdSe NRs successfully gave P3HT–CdSe NR nanocomposites without the introduction of any deleterious metallic impurity. Click reactions possess several attractive features, including an extremely versatile bond-formation process, no need for protecting groups, good selectivity, nearly complete conversion, and generally no need for purification. As such, it stands out as a promising method to simplify the synthetic procedure and opens opportunities to increase the grafting density for large-scale synthesis. The charge transfer occurred at the P3HT/CdSe NR interface and was confirmed by [*] L. Zhao, Dr. X. Pang, Prof. Z. Lin Department of Materials Science and Engineering Iowa State University, Ames, IA 50011 (USA) and School of Materials Science and Engineering Georgia Institute of Technology, Atlanta GA 30332 (USA) Fax: (+1)515-294-7202 E-mail: zqlin@iastate.edu