Senior Project Thesis Senior Project Committee, Department of Physics, Case Western Reserve University

Transcription

1 Liquid Crystal Semiconductor Nanostructures Richard Metzger,Department of Physics, Case Western Reserve University Nick Lind, Department of Physics, Case Western Reserve University Professor K. Singer, Department of Physics, Case Western Reserve University Senior Project Thesis Senior Project Committee, Department of Physics, Case Western Reserve University 5/3/2013

2 Abstract: The development of photovoltaic cells in the application of solar power is increasingly more important, given current energy projections. The focus of research is to develop cheap, efficient methods. Organic semiconductors offer a solution to the more expensive inorganic silicon cells used today. We will study self-assembly of discotic organic molecules, through the process of aggregation in solution. These assemble into fibrous morphologies via π-π interaction, which allow for conductive planar alignment. Observations of structure and properties will be made via UV-vis spectroscopy and photoluminescence. The growth patterns of the molecules will be studied over varying temperatures and conditions and will be correlated with spectroscopic results. Introduction: As the world s supply of natural gas and oil are depleted, it has been in the interests of both countries and private companies to find sources of renewable energy. These include solar, wind, hydro, biomass, and geothermal. Of these the most abundant, by a large amount, is solar power. The Earth receives yearly from the Sun about 174PW of energy. In comparison there is about 1.7 PW available from wind power. In 2008 the global consumption of energy was 15TW; with a high efficient solar cell producing 150W/square meter, this could be offsetted by using 0.02% of the Earth s surface. The graph below shows a projection of energy use by source; the peak for non-renewable sources is around 2030.

3 Figure 1 There are challenges facing solar power. Solar cells are dependent upon the weather and are expensive to manufacture. Traditional photovoltaic cells are composed of inorganic semiconductors such as silicon; these have a high cost associated with them. To combat this, organic semiconductors are being created, which are cheaper to produce. Currently the efficiencies of organic photovoltaics are around 5%, much smaller than the 30% for their inorganic counterparts. The efficiency of organic photovoltaics must be increased in order to be feasibly marketable.

4 Theory: To understand how energy is obtained in a solar cell we must look briefly into the physics of semiconductors. A semi-conductor can be thought of as being the intermediate between an insulator and a metal. In an insulator there is a large energy separation between occupied orbitals (valence band) and unoccupied orbitals (conduction band), this separation is known as the band gap. In a metal there is no such band gap and the electrons can move nearly freely between energy levels. Semiconductors have a band gap that is small enough such that an appreciable number of electrons can exist in the conduction band. Figure 2 The Fermi level is a state with a 50% probability of being occupied at a given temperature When given energy greater than or equal to the band gap, an electron will move from the valence band to the conduction band. This process leaves behind a hole in the valence band, seen as having positive charge. The number of these charge carriers can be increased by a process

5 known as doping. Doping is the adding of impurity atoms to a pure semiconductor to help facilitate conduction. When a semiconductor is positively doped it is known as a p-type semiconductor, a negatively doped semiconductor is known as an n-type. An inorganic photovoltaic cell is composed of a layer of n-type and a layer of p-type semiconductors. Where the two layers meet is the p-n junction (seen as yellow in figure 3). In this area electrons from the n layer diffuse into the p layer creating positively charged ions in the n layer, whereas holes in the p layer diffuse into the n layer creating negatively charged ions in the p layer. This interface region creates an electric field that opposes further diffusion and equilibrium is achieved. The voltage difference in the cell forces electrons to move in only one direction. Figure 3

6 If a photon interacts with an atom in the n layer, a free electron hole pair that moves in opposite directions will be created. The electron hole pair has enough energy to cross the junction. Power can be generated if a wire is connected to the p and n type semiconductors. For an organic semiconductor the electron hole pair is bound, this is known as an exciton. The exciton is formed from the interaction of a photon with an electron in the highest occupied molecular orbital (HOMO), which creates an electron in the lowest unoccupied molecular orbital (LUMO). The relation between the HOMO and LUMO in organic molecules is the same as that between the valence band and conduction band in inorganic crystals. As seen in figure 4, an exciton will dissociate when it comes in contact with the LUMO of the acceptor material because the electron prefers a lower energy. Figure 4 Due to Coulombic interaction inherent in the exciton, the bound pair will only exist for a short period of time before they recombine (diffuse) back into the HOMO. This time is known as the diffusion length, and it is much shorter than that of an inorganic semiconductor. In order for

7 current to be created the excitons must reach the acceptor material before they diffuse. A new structure of the photovoltaic cell is required to incorporate this reduced diffusion length. This new structure is called a bulk heterojunction. The bulk heterojunction consists of dispersed p and n-type material, such that the distances between donor and acceptor molecules are reduced and are comparable to the diffusion length. The bulk heterojunction is seen in figure 5. Figure 5 The bulk heterojunction is not very efficient though. Its inefficiency is related to the randomness of the placement of the acceptor material. Many of the dissociated electrons will not be conducted to the substrate because the material they dissociated into is not connected to the substrate. Only material connected to the substrate will contribute to power generation of the cell. The new design should have the acceptor material connected to the substrate, be small in diameter comparable to the diffusion length, and be long enough to ensure maximum energy

8 output through more interactions with photons. In short, the acceptor material should be fibrous and ordered. The new design is shown in figure 6. Figure 6 Objectives: The goal of the project was to understand the process of aggregation and coaggregation of the fibrous molecules. The first objective was to determine the conditions for nanostructure assembly. This included temperature and concentration dependence. Next, was perform microscopy on suspended solutions and surface deposited crystals. A scanning electron microscope would be used to study the surface structure of the fibers. From there spectrophotometry would be done on solutions. In particular UV-VIS absorption with varying temperature and concentration.

9 Procedure: For construction of the fibrous material it was necessary to choose a molecule that could self assemble in solution. This self assembly mechanism can be described by π-π interactions of the molecular orbitals causing columnar stacking, as seen in figure 7. Figure 7 Upon heating, the molecules dissolve in solution. When cooled the molecules fall out of solution and the self assembly begins, creating fibrous liquid crystals. The discotic shape of the molecules

10 creates channels for charge carriers and increases the diffusion length, allowing conduction to the substrate. The two materials chosen for the donor and acceptor were: Donor: phthalocynine (H 2 Pc-OC 4 ) Acceptor: perylene diimide (PTCBI-C 13 ) Four different solvents were chosen for the construction of the nano-fibers these are: a) Chloroform b) Toluene c) Di-Chlorobenzene d) Chlorobenzene

11 Each of which were tested with varying concentrations to determine the most suitable for the fiber dimensions needed. These dimensions are: diameter 10nm length 1µm, which are dependent upon the diffusion length and size of cell for maximum absorption. Ramping of temperature through the cooling (formation of fibers) period was also done; again this was to ensure proper dimensions of the fibers. Solutions of phthalocynine in Toluene were made for absorption measurements. Concentrations of 5e-4M, 5e-5M, 5e-6M, 5e-7M were created for such purposes. UV-VIS was done via two different instruments; Varian CARY spectrophotometer and the Ocean Optics spectrophotometer in the MORE center. The Ocean Optics was connected to a cuvette holder with temperature control. Absorption was measured from 20C to 105C for 5e-5M and 5e-6M.

12 Results: UV-VIS It was the goal to reproduce the results of reference 1 shown below: Figure 8 UV-VIS optical absorption of H2Pc-OC8 in various concentrations of Toluene The flattening we see at higher concentrations (where the trimer/dimer arrows point) is a main feature that we were looking for. This is due to the formation of dimers and trimmers, aka the start of aggregation.

13 Figure 9 Multiple concentrations using CARY spectrophotometer This doesn t tell us much except for the smoothness associated with the trimer and dimer areas. We see this for the higher concentrations.

14 Figure 10 Corrected with extinction coefficient This graph closely resembles that of figure 8. It is unknown as to why the end points do mesh together as in figure 8. Figure 11 Temperature dependence for 5e-5M using Ocean Optics From this we see as temperature is increased the highly concentrated solution (5e-5M) starts to resemble that of a lower concentration. This is indicative of the lessening of the dimer and trimers for a more pronounced monomer peak. Thus, as heat is added to the system the fibers break down into single molecules.

15 Figure 12 Comparison between change in temperature and concentration Figure 11 is made clearer by showing the comparison of a high temperature high concentration with that of a low temperature low concentration. Future Work: A continuation of this project should consider further investigation into spectrographic data. Absorption spectra of Perylene diimide should be taken. From this and together with the phthalocynine data; conditions, such as temperature and concentration, for which they both begin aggregation can be determined. After this it would be natural to create a combined solution of the donor and acceptor and study the charge transfer characteristics.

16 Acknowledgements: I would like to thank the following people for their help. Nicholas Lind for his assistance in all parts of procedure and analysis. Dr. Kenneth Singer for his guidance and instruction throughout the project. Michael Patrick for his instruction in measurements. Dr. Ina Martin for her assistance while operating in the MORE center. References: 1. Self-Assembled Fibers of a Discotic Phyhalocyanine Derivative. Volodimir Duzhko, Kenneth Singer. JPLC Self-Assembled Solar Cells with Nanostructured Architectures. Michael Usowicz. Department of Physics Case Western Reserve University Spectroscopic Studies of Charge and Energy Transfer Processes in Self-Organizing Heterogeneous Photovoltaic Materials. Michael Kelley. Department of Physics Case Western Reserve University Long-range electron transport in a self-organizing n-type organic material. Volodimir Duzhko. APL Tailored One- and Two-Dimensional Self-Assembly of a Perylene Diimide Derivative in Organic Solvents. Usowicz, Kelley, Duzhko, Singer. 2011

17