Poly(3-hexylthiophene)-CdSe Quantum Dot Bulk Heterojunction Solar Cells: Influence of the Functional End-Group of the Polymer

The pending global energy crisis requires the development of new technologies that exploit the potential of renewable sources of energy, such as solar power. For example, inorganic semiconductor-based photovoltaic technology has reached the performance level of converting 30% solar energy into electric power. Despite the high performance, inorganic photovoltaics based on crystalline silicon are still too expensive to compete with the conventional sources of electricity. While extensive research in the field of inorganic photovoltaics is expected to result in a decrease in their fabrication cost, polymer-based photovoltaics represent a very attractive alternative for low-cost, lightweight, large-area, and flexible solar panels. The most used conjugated polymers in photovoltaic structures are regioregular poly (3-alkylthiophenes) and alkoxy-substituted poly(phenylenevinylenes), such as poly[2-methoxy-5-(20-ethylhexyloxy)-1,4-phenylenevinylene] and poly[2-methoxy-5-(30,70-dimethyloctyloxy)-pphenylenevinylene]. Because of their solubility in organic solvents, these polymers are suitable for casting from solution using wet-processing techniques, such as spin-casting, dip-coating, ink jet printing, screen printing, and micromolding. Blending of twomaterials having donor and acceptor properties results in the formation of a bulk heterojunction. Research has been directed toward four important types of bulk heterojunctions. The first type consists of a polymer-polymer heterojunction obtained by mixing of two conjugated polymers with offset energy levels. The second type is obtained by blending a conjugated polymer with (6,6)-phenyl-C61-butyric acid methyl ester (PCBM)as a soluble electron acceptor, which currently shows the best performance. Polymer/titania (TiO2) photovoltaic cell represents the third type of bulk heterojunction, which has received attention due to the possibility of TiO2 patterning into a continuous network for electron transport. Conjugated polymer quantum dots can be considered the fourth type of bulk heterojunction solar cells. For example, CdSe nanocrystals with an electron affinity in the range 3.8-4.5 eV are suitable materials to act as electron acceptors when combined with conjugated polymers. The band gap for quantum dots is controlled simply by adjusting the size of the dots. Semiconductor quantum dots (QDs) have attracted enormous interest in the past two decades due to their tunable optical and electronic properties. Remarkable efforts have been devoted to the synthesis of high-quality, defect-free QDs with narrow size distribution (<5%). These interesting properties of QDs have been employed for various applications including biosensing, light-emitting diodes, and photovoltaics. Poly(3-alkylthiophenes) are one the most attractive candidates for photovoltaic applications due to their opto-electronic properties, stability, and solution processability. Poly(3-hexylthiophene) (P3HT) has shown hole mobilities as high as 0.1 cm V s and crystallinities as a function of the processing conditions. These are important parameters when considering photovoltaic applications where the effect of charge recombination should be minimized. Solar cells incorporating inorganic nanocrystals and organic semiconducting polymers have some potential advantages over silicon-based cells, such as low cost, solution processability, and the possibility of obtaining flexible thin-film solar cells. The ability to tune the band gap of QDs by simply controlling their size makes them promising candidates for photovoltaic applications. Power efficiency as high as 2.8% have been reported using tetrapod-shaped CdSe with a poly(phenylenevinylene) derivative. 27 The dispersion of QDs in the polymer matrix, however, has been a major issue for an efficient charge transfer. The presence of functional groups such as amines can provide intimate contact between the polymer and the CdSe through covalent interactions. Such amine-functionalizedP3HThave been blendedwithCdSe nanorods and showed a maximum efficiency of 1.4%. Here for the first time we report the synthesis of H/thiolterminated P3HT from Br/allyl-terminated P3HT precursor (Scheme 1). Br/allyl-terminated regioregular poly(3-hexylthiophene) was synthesized by in situ end-functionalization of the living nickel-terminated polymer, as previously reported. Br/ hydroxypropyl-terminated regioregular poly(3-hexylthiophene) was synthesized by hydroboration/oxidation of the corresponding Br/allyl-terminated polymer. Br/hydoxypropyl-terminated P3HT was converted to an acetyl-protected thiolpropylterminated P3HT by a Mitsunobu reaction, which subsequently was reduced with LiAlH4. 35 H NMR (Figure 1) and MALDITOF MS (Supporting Information) confirmed the successful end-group transformation. Thiols are known to have strong interactions with metals such as Au, Ag, and Cd. Thiol end-groups were expected to interact with the CdSe surface, providing a better contact with the semiconducting polymer. We studied the photovoltaic response of blends of H/thiolterminated P3HT with spherical CdSe QDs and compared the experimental results with regioregular H/Br and Br/allyl-terminated P3HT. All the P3HT polymers used in this study have similar molecular weights (DPn= 60). Prior to our work, several attempts have been made to directly modify QDs with functionalized P3HT. Even though these reports have shown the successful modification of the QDs, the photovoltaic response of these materials have not been studied in detail. Nearly monodispersed CdSe quantum dots have been synthesized using a reported procedure with slight modifications (experimental procedure given in the Supporting Information). The CdSe QDs were treated with pyridine to replace trioctylphosphine oxide (TOPO) ligand and stored in pyridine for blend preparation. Solution UV-vis data showed no shift in absorption of the CdSe QDs, indicating that there is no oxidation or aggregation after the pyridine ligand exchange treatment (see Supporting Information for experimental details). The particle size of CdSe QDs was estimated to be ∼4 nm from *Corresponding author. E-mail: mci071000@utdallas.edu. D ow nl oa de d by K IN G A B D U L A Z IZ C IT Y S C I& T E C H o n A ug us t 9 , 2 00 9