Spectroscopic Studies of Charge and Energy Transfer Processes in Self-Organizing Heterogeneous Photovoltaic Materials

1. Spectroscopic Studies of Charge and Energy Transfer Processes in Self-Organizing Heterogeneous Photovoltaic Materials Michael Kelley, Volodimyr Duzhko, Kenneth SingerDepartment of Physics. Case Western Reserve University, Cleveland, Ohio, 44106 Chloroform Toluene Chlorobenzene S2 Original Silicon Architecture J-type Donor material Acceptor material Acceptor material Acceptor material LUMO S1 α LUMO T1 θ HOMO HOMO S0 Bulk Heterojunction E=hc/λ Processes: S2 S1 – internal conversion (<ps) S1 S0 – radiative decay (5 ns) S1 S0 – internal conversion (?) S1 T1 – Inter System Crossing (?) T1 So – internal conversion (100 s) Photovoltaic Cells Blended Spectra Sample Nanostructures Kasha’s Theory* “1-D” Architecture Traditional photovoltaic cells consist of a junction of doped P-type (positively charged) and N-type (negatively charged) inorganic semiconducting crystals, mostly silicon. The surplus charge on each plate generates an electric field drawing charges toward the junction. Photons strike electrons at the junction, exciting them. And generating free electrons in the semiconducting band and holes. These electrons and holes then pass through the junction in opposite directions and are carried through the plates to electrodes, generating electric current through a load. According to Kasha’s molecular exciton theory, the energy stored in a dimer depends on arrangement of molecules and can be described as the potential interaction energy between electric dipole moments of two molecules. This energy can vary between the extremes of the maximum where the molecules’ transition moment vectors are parallel (H-type aggregate) and the minimum where the transition moment vectors are in line (J-type aggregate). Spectroscopy of energy levels of a given nanostructure versus those of single molecules in solution reveals information on molecular orientation in the nanostructure. Below are simulated graphs of the interaction energies between molecules of the two sample materials versus their relative orientations. Desirable Interaction Undesirable Interaction H-type Dipole Energy Equation and Monomer-Dimer Energy Diagrams for Extreme Orientations Organic Photovoltaics *M. Kasha et al. Pure Appl. Chem. 2, 371 (1965). The manufacturing costs of today’s silicon-based photovoltaic cells are very high, rendering the large-scale production of solar panels prohibitively expensive. A potential solution for this problem is the development of photovoltaic cells from cost-effective organic semiconductors. However, the bilayer p-n junction, typical device architecture for inorganic photovoltaic cells, is inefficient due to the small diffusion length of excitons (bound electron and hole pairs) in organic materials. A solution to this problem is to make a nanostructured one-dimensional donor-acceptor device architecture. When light excites the electrons in such blends, excitons are created. At the junctions between the p-type and n-type materials, these excitons dissociate into free electrons and holes, with the electrons traveling through the n-type material and the holes traveling through the p-type material to the electrodes. The geometry of nanostructures needs to be optimized to improve charge generation and transportation. Self-assembly of discotic molecules was used to fabricate two types of nanofibers with electron (n-type) and hole (p-type) conductivities, and to embed the nanofibers into one-dimensional donor-acceptor blends. Interaction Energies vs. Orientation λex= 525 nm Measurement of Molecular Orientation in Aggregates Donor-Acceptor Molecular Electronic Transitions In order to gain information about the specific orientations of the molecules of the nanostructures of each species, temperature-dependent absorbance spectrum and photoluminescence spectrum measurements of the two species were taken. Shifts or new features in the spectra of the nanostructures and single molecule samples can reveal the characteristic formation patterns of the nanostructures. Measurements were performed while varying the concentration of the material in the sample, the temperature of the sample, and the type of solvent. • Requirements to self-assembled structures for efficient exciton photo-excitation and interfacial dissociation: • Long fibers length (1 µm) to maximize light absorption • Distance to p-n junction less than exciton diffusion • length ( 10 nm) • Homeotropic alignment (perpendicular to electrodes) Conclusions Concentration –Dependent Measurements of H2PC-OC8 and PTCBI-C13 in Chloroform The data reveal that upon cooling and high concentrations , the spectra of indvidual peaks gives way to plateaus of blended features and the appearance of additional features confirms the transition from single molecules in a solution to the formation of stacked nanostructures of particular orientations. The absence of additional features in the blended absorbance spectrum of single molecules compared to the spectrum of the sum confirms the absence of ground-state charge transfer. The quenching of the blended photoluminescence spectrum compared to the spectrum of the sum confirms excited-state charge transfer. Absorbance and Photoluminescence Spectroscopy At the p-n junction, upon exciton dissociation, the free electron is transferred from the lowest unoccupied molecular orbital (LUMO) of the donor material to the LUMO of the acceptor material. UV-vis-NIR absorption spectroscopy and photoluminescence spectroscopy are two versatile and non-destructive methods of probing the electronic configuration of a material and can be used to determine how charge carriers behave in different materials Absorption spectroscopy analyzes the structure of a material’s electronic energy levels by measuring the intensity of light emitted by deexcited electrons in a material compared to the wavelength of the light transmitted to excite the material. Photoluminescence spectroscopy measures the intensity of light emitted by a material over a spectrum of wavelengths after being excited by a monochromatic light source. The absence or presence of photoluminescence features can determine whether excited electrons transfer between molecules different materials or not. This research was concerned with the correlated measurements of the structural and charge transfer properties in self-assembled nanofibers and donor-acceptor architectures of phthalocyanine (p-type) and perylene-dicarboximide (n-type) derivatives using UV-vis-NIR optical absorption and photo-emission spectroscopy. Acknowledgements I would like to extend my thanks to my research mentor, Dr. Volodimyr Duzhko, for his guidance throughout the summer, and to Dr. Kenneth D. Singer, the primary investigator of the laboratory. I would also like to extend my thanks to SOURCE, the Case Alumni Association, and to the Dominion foundation for the generous funding and for the opportunity to participate in this project. N,N′-bis(1-hexylheptyl)-perylene-3,4:9,10-bis-(dicarboximide) (PeryleneDiimide derivative, PBI) Electron Acceptor Spectra of Each Species and Blend 2,3,9,10,16,17,23,24-Octakis(octyloxy)-29H,31H-phthalocyanine (Phthalocyanine derivative, H2PC) Electron Donor If excitons efficiently dissociate between molecules of different species, the evidence appears in the absorbance and photoluminescence spectra. When the individual absorbance and photoluminescence spectra of the two species are summed together, and that sum is compared the spectra of the samples blended together, if the two spectra demonstrate distinct differences, charge in the ground state (absorption spectra) or in the first excited state (photoluminescence spectra) is being transferred. References N.S. Lewis, G. Crabtree, A.J. Nozik, and M.R. Wasielewski. Basic Research Needs for Solar Energy Utilization: Report of the Basic Energy Sciences Workshop on Solar Energy Utilization. United States Department of Energy: Office of Basic Energy Sciences, OSTI ID: 899136, 2005 Tang, C.W. Two-layer Organic Photovoltaic Cell. Applied Physics Letters. 48: 183-185 (1986). Hoppe, H.; Sariciftci, N. Organic Solar Cells: An Overview. J. Mater. Res. 19(7): 1924-1945 (2004). Duzhko, V.; Aqad, E.; Imam, M.R.; Peterca, M.; Percec, V.; Singer, K.D. Long-Range Electron Transport in a Self-Organizing N-Type Organic Material. Applied Physics Letters 92:113312 (3 pages) (2008). Kasha, M.; Rawls, H.R.; Ashraf El-Bayoumi, M. The Exction Model in Molecular Spectroscopy. Pure Applied Chemistry 11(3-4): 371-392 (1965). Temperature-Dependent Measurements of PTCBI-C13 in Various Solvents Photoluminescence Spectroscopy UV-vis-NIR Spectroscopy