Published: July 19, 2011 r2011 American Chemical Society 3460 dx.doi.org/10.1021/nl201974s | Nano Lett. 2011, 11, 3460–3467 LETTER pubs.acs.org/NanoLett Enhancement of Interfacial Polymer Crystallinity Using Chromism in Single Inorganic NanowireÀPolymer Nanohybrids for Photovoltaic Applications Christopher M. Rodd and Ritesh Agarwal* Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States T he push for low-cost, high-efficiency solar cells has created a tremendous amount of interest in organic/inorganic bulk heterojunction solar cells for the easy processability gained from their organic constituents coupled with the higher carrier mobi- lity of their inorganic components. Solar cells based on these hybrid organicÀinorganic systems operate on the similar pÀn junction principle as inorganic photovoltaics but in this case there is an electron donor that donates charge carriers to an electron acceptor. 1 Bulk heterojunction solar cells are much easier to handle from a manufacturing perspective since the processing is solution-phase and generally does not require the high temperature or vacuum techniques that lead to the high manufacturing costs inherent in crystalline inorganic solar cells. The organic component of the heterojunction is generally either low molecular weight semiconducting molecules, for example, pentacene (p-type) or polymers of aromatic compounds such as thiophene. 2 Polymerization of thiophene compounds produces polymers with conjugated backbones, such as poly(3-hexylthio- phene-2,5-dikyl) (P3HT), which allows for delocalization of the π orbitals along large sections of the polymer chain. The inorganic component is typically an n-type semiconductor such as ZnO, InP, PbS, or CdS, although organic materials such as PCBM (fullerene derivative phenyl-C61-butyric acid methyl ester) can also serve as electron acceptor in the heterojunction. 3À14 In solar cell applications, the incident light generates excitons in the photoactive material, which need to rapidly diffuse to the interfacial region between the p and n components that split this electronÀhole pair by the difference in their ionization potentials and electron affinities of the organic and inorganic components. The electron and holes are carried by the inorganic and organic components to their respective contacts. Care must be taken to create a percolating network in both the polymer phase and the semiconducting electron acceptor phase else the photovoltaic response generated by the cell will be lost to carrier recombination. In P3HT:PCBM solar cells, this is mainly accomplished by annealing the cells after spin-casting the active layer onto the contacts and changing the concentration of PCBM in the system. These optimization routes increase the crystallinity of the P3HT domains while attempting to create interpenetrat- ing networks of the PCBM phase 9,15À19 The difficulty in con- trolling the phase behavior of P3HT:PCBM leads to the desire to incorporate larger nanostructures, such as nanocrystals, nano- rods, nanotubes, or nanowires to improve the percolation path- ways in the active layer of the cell. 12,13,20À28 In this regard, single crystalline semiconducting nanowires are believed to be an excellent electron acceptor morphology in bulk heterojunction junctions because they allow directed electron transport to the cathode especially when grown directly from the cathode in a vertical manner. 11,12,29 The ease of fabrication efficiency gained by incorporating the inorganic material into the low-cost organic material is compro- mised, however, by the relatively low efficiencies of bulk hetero- junction photovoltaics, typically lying in the range of 1À3% with some extremes to 6%. 30 Invariably, the organicÀinorganic interface Received: June 12, 2011 Revised: July 2, 2011 ABSTRACT: Interfaces play an important role in bulk heterojunc- tion organic/inorganic hybrid photovoltaic devices, but directly probing the interface in order to improve device characteristics is exceedingly difficult. We report on a method to form coreÀshell inorganic nanowireÀpolymer hybrids of a conducting polymer, poly(3-hexlthiophene-2,5-diyl) (P3HT), and a semiconducting nano- wire, cadmium sulfide (CdS), using solution processing to create the polymer shell around the nanowire in order to study the poly- merÀnanowire interface directly without interference from bulk effects. We have used the rodÀcoil transition (chromism) in P3HT to seed and enhance the crystallinity at the polymerÀnanowire interface. We have shown that creating more order within the P3HT main chain, by controlling the temperature and the solvent quality, can increase the extent of polymer crystallinity present at the polymerÀnanowire interface. We believe using the rodÀcoil transition to create more order in P3HT and the resulting polymerÀnanowire interface will provide a facile pathway for designing future organicÀinorganic photovoltaic devices. KEYWORDS: P3HT, chromism, rodÀcoil transition, polymerÀnanowire interfaces, polymer crystallinity, photovoltaics