Session B43: Focus Session: Polymers for Energy Storage and Conversion -- Nanoscale Structure in Polymer-based Photovoltaics

Association of P3HT and PCBM in solution

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Organic solar cells: a theoretical study of the effects of polymer side-chains

Organic photovoltaic cells received a great interest in the last few years as they offer an environmentally clean and low-cost solution to the world's rising energy needs. One of the main problems limiting the efficiency of an organic solar cell device is the strong binding energy of the excitons, typically situated about a few hundreds of meV, which is ten to one hundred times more than in inorganic devices. Another limiting factor can be the misalignment of the the HOMO (Highest Occupied Molecular Orbital) and the LUMO (Lowest Unoccupied Molecular Orbital) energy level of the different components of the solar cell. In this presentation, we will discuss how different modifications on organic polymers' side-chains can affect and improve their electronic properties. Our calculations, based on density-functional theory using the B3LYP functional, indicate a HOMO and LUMO lowering of more than 1 eV in various organic polymers like poly-isothianaphtene (pITN) and poly-thienothiophene (pTT), and various side-chains like aldehyde-based ones. Preliminary calculations on oligothiophenes also show a lowering of the exciton binding energy.   

Perylene Diimide Based ``Nanofabric'' Thin Films for Organic Photovoltaic Cells

We report progress in using a perylene diimide (PDI) nanofabric as an effective electron accepting nanostructure for organic photovoltaics (OPV). A key challenge in OPV continues to be the recovery of electrons after charge separation due to the relatively poor mobility of C60 and related materials. A series of PDI compounds and complexes have been synthesized and used to fabricate nanofibers and thin films using solution and vacuum deposition techniques. Overlaping PDI-based nanofibers form a fast electron-transporting ``nanofabric'' that has been characterized (AFM, PL, UV-vis, etc.) and can be blended with electron donating materials. A solution-processible OPV configuration containing a nanofabric heterojunction (FHJ) of poly(3-hexylthiophene) and the PDI nanofabric was investigated. We observed a significant improvement in power-conversion efficiency due in part to expansion of the interfacial area and the presence of high mobility electron pathways to the LiF/Al electrode.   

Influence of Annealing and Blending of Photoactive Polymers on Their Crystalline Structure

Thin photoactive polymer films of poly(3-octylthiophene-2,5-diyl) (P3OT) and poly(2,5-di(hexyloxy)cyanoterephthalylidene) (CN-PPV) are investigated. With X-ray reflectivity measurements, linear concentration-thickness dependence is found for both. Grazing incidence wide-angle X-ray scattering (GIWAXS) is used to probe the crystallinity of thin films and to determine characteristic length scales of the crystalline structure. Moreover, the orientation of the crystalline parts regarding the substrate of both the homopolymer and the blended films is probed with GIWAXS. Temperature annealing is found to improve the crystallization for both homopolymers. In addition, reorientation of the predominant crystalline structures takes place. Blending both polymers reduces or even suppresses the crystallization during spin coating as well as temperature annealing. Absorption measurements complement the structural investigations [1]. \\[4pt] [1] M.A. Ruderer et al. J. Phys. Chem. B (2010), doi:10.1021/jp106972s.   

Bio-Inspired electro-photonic structure for organic and dye sensitized solar cells

A major challenge in solar cell technology dwells in achieving an efficient absorption of photons with an effective carrier extraction. In all cases, light absorption considerations call for thicker modules while carrier transport would benefit from thinner ones. This dichotomy is a fundamental problem limiting the efficiencies of most photovoltaics. One pathway to overcome this problem is to decouple light absorption from carrier collection. We present solutions to this problem applying bio-inspired nanostructures to two different types of systems: organic photovoltaic (OPV) and dye sensitized solar cells (DSSC). For OPV devices based on poly-3-hexylthiophene:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM), we describe a 2-D photonic crystal geometry that enhances the absorption of polymer-fullerene photonic cells $\sim $ 20{\%} relative to conventional planar cells. Remarkably, the photonic crystal cell offers the possibility to increase photocurrents by improvements in optical absorption and carrier extraction simultaneously, and particularly through the excitation of photonic resonant modes near the band edge of organic PV materials. We also present an optical method to extract charge transport lengths from device photoactive layers. For DSSCs we introduce a new structural motif for the photoanode in which the traditional random nanoparticle oxide network is replaced by vertically aligned bundles of oxide nanocrystals. We have used a pulsed laser deposition system to ablate titanium oxide targets to obtain the porous and vertically aligned structures for enhanced photoelectrochemical performance. Absorption studies show that in optimized structures for titanium oxide, there is a 1.4 times enhancement of surface area compared to the best sol-gel films, Incident-Photon-Conversion-Efficiency values are better than 3 times thicker sol-gel films, and $\sim $ 92{\%} Absorbed-Photon-Conversion-Efficiency values have been observed when sensitizing with the N3 dye (Ru(dcbpyH)2(NCS)2). The direct pathways provided by the vertical structures appear to indeed provide for enhanced collection efficiency for carriers generated throughout the device.   

The Poly(3-hexylthiophene) / ZnO (10-10) interface: structure and energetics

The poly(3-hexylthiophene) (P3HT) polymer on ZnO system is of significant interest for hybrid nanoscale solar energy research and applications. Using density functional theory and periodic supercells, we study the P3HT/ZnO interface where sulfur atoms on the P3HT side chains are used to anchor the polymer onto the ZnO (10-10) surface. We discuss the structure and energetics of the binding modes for low and high polymer coverage. We then apply the Frenkel-Kontorova model to study the likely polymer structures in practice (e.g. dislocation formation to release strain energy). We end with a discussion of the band energy alignment across the interface.   

Photovoltaic Device Performance Enhancement by Interfacial Decoration of Bulk-Heterojunctions with Semiconducting Nanocrystals

We have developed a facile method for decorating the donor-acceptor interface of organic bulk-heterojunctions (BHJs) with semiconducting nanocrystals (NCs). Using nano-scale phase separation of a poly(3-hexylthiophene)/polystyrene copolymer blend, followed by selective removal of the polystyrene, we are able to expose a nano-scale network of poly(3-hexylthiophene) [P3HT] fibers. These fibers are subsequently decorated with cadmium selenide (CdSe) NCs prior to back filling the structure with thermally evaporated C$_{60}$. Optical characterization techniques have confirmed that NCs located at the donor-acceptor interface show enhanced charge transfer to the surrounding medium compared to NCs randomly dispersed in similar BHJs. Photovoltaic (PV) devices made using this technique show improved external efficiencies compared to similar planar PV structures. This technique provides an elegant mechanism for improving the performance of organic BHJs by tailoring their spectral absorption using semiconducting NCs.   

Semiconducting Nanocomposites via Directly Grafting Conjugated Polymer onto Quantum Rods

Nanocomposites of poly(3-hexylthiophene) (P3HT)-cadmium selenide (CdSe) nanorod (NR) were synthesized by directly grafting P3HT onto bromobenzylphosphonic acid (BBPA) functionalized CdSe NR, dispensing with the need for ligand exchange chemistry. The grafting was accomplished by Heck coupling as well as a newly developed catalyst-free click reaction. The resulting P3HT-CdSe NR nanocomposites possess a well-defined interface, thereby significantly promoting the dispersion of CdSe within the P3HT matrix and facilitating the electronic interaction between them. The success of grafting was confirmed by the NMR and DLS, and the occurrence of charge transfer at P3HT/CdSe NR interface was demonstrated by the UV-vis absorption and photoluminescence (PL) measurements as well as the time-resolved PL study. Similar grafting density was yielded using these two methods. The nanocomposites prepared by the catalyst-free click reaction was found to exhibit a faster charge transfer. To the best of our knowledge, this is the first study of grafting conjugated copolymer directly onto the elongated semiconductor nanomaterials. As such, it provides insight into rational design and fabrication of organic-inorganic nanohybrid solar cells with improved power conversion efficiency.   

Directed assembly of core-shell hybrid nanomaterials for polymer photovoltaics

The creation of large-area aligned nanohybrid films/arrays remains a challenge in the fabrication of ordered heterojunction photovoltaics. We demonstrate a bottom-up approach based on the directed assembly of lyotropic inorganic-organic core-shell nanohybrids. Semiconductor nanowires are prepared by solvothermal synthesis. Diameter and length of the nanowires are controlled by various reaction parameters. Core-shell nanohybrids are prepared by grafting conjugated polymers onto the nanowires. Effect of the nanowire diameter on the polymer coating is demonstrated. We show that high aspect ratio nanohybrids spontaneously form nematic phases in liquid media. These systems show isotropic, bi-phasic and nematic phases on increasing concentration in reasonable agreement with Onsager's theory for rigid rods. Suspensions are readily processed to produce films with large-area monodomains. With a decrease of nanowire diameter, the polymers in the nanohybrids tend to form ordered crystalline layers, in which the conjugated backbone is aligned along the nanowire long axis. The corresponding optoelectronic properties are discussed.   

Interconnected and nano-perforated lamellar sheets of metal oxides produced using novel block copolymer templates

Recently, our group has investigated the development of a novel bicontinuous nanostructure using block copolymers. This has lead to the creation of various bicontinuous, mesoporous, and interconnected metal oxides sheets. The high surface area produced by the 3D nanostructure has shown a considerable improvement in efficiency in the method of preparation. The main transformations from polymer scaffold to inorganic matrices produced by our group include the use of oxide of titanium, and a number of other sol-gel transformations. The application of these nanostructures is shown in the development of photovoltaic devices. We highlight future applications in electronic, memory, energy storage and production devices.