Characteristics of P3HT Modified with Lithium Aluminum Hydride

Proceedings of The National Conference On Undergraduate Research (NCUR) 2016 University of North Carolina Asheville Asheville, North Carolina April 7 April 9, 2016 Characteristics of P3HT Modified with Lithium Aluminum Hydride Manuel Mangual Department of Physics and Electronics University of Puerto Rico at Humacao Call Box 860 Humacao, Puerto Rico 00792 Faculty Advisors: Dr. Ezio Fasoli, Dr. Josee Vedrine-Pauléus Abstract The research is focused on the modification of poly (3-hexylthiophene) (P3HT) with lithium aluminum hydride (LAH) to understand polymer absorption and stability characteristics. LAH is used as a reducing agent to modify P3HT and to give it more flexibility for improved durability as a result of reduced polymer oxidation. P3HT is an organic polymer and modification with LAH partially reduces its double bonds in thiophene rings. In this work, we chemically modify and functionalize P3HT with LAH. We prepared thin films of P3HT and functionalized polymer P3HT_LAH, and we measured the absorption and basic crystalline properties of the reduced polymer by ultra violet-visible spectroscopy (UV- VIS) and X-Ray Diffraction (XRD) techniques. The UV-VIS spectroscopy showed that films composed P3HT modified with LAH depicted a lower overall absorption than the unmodified P3HT. Both P3HT and P3HT_LAH exhibited the three inherent peaks at 520 nm, 550 nm, and 620 nm, but the P3HT_LAH showed peaks with a broadening of the shoulder at 620 nm. The XRD analysis revealed peaks at 5 P3HT_LAH films. The peaks of the P3HT_LAH were less pronounced than peaks of the P3HT confirming a decrease in the degree of crystallinity when this modification occurs. P3HT_LAH increases its flexibility in the polymer chain reducing the overall crystallinity of the polymer. This is because LAH modification reduces the double bonds linked to carbon and carbon into a single bond in the physical structure of P3HT. Keywords: organic polymer, P3HT, LAH, solar cells 1. Introduction Organic solar cells have the tendency to degrade and lose their efficiency over a period of time as a result of photodegradation and thermodegradation of the active layers. Organic polymers have been shown to have tunable conductive properties based on their chemistries. As an approach to provide energy through these organic polymers several combinations or blends have been researched, since the power generated from organic solar cells is low compared to inorganic. But the amounts of energy that can be produced depends on polymer blend ratios, the processing conditions, and the fundamental properties of the donor and acceptor components, amongst other factors. For bulk heterojunction (BHJ) devices, the active layer is composed of a poly(3 -hexylthiophene) (P3HT) and phenyl-c61-butyric acid methyl ester (PCBM) blend, where the electron donor is P3HT and PCBM is the electron acceptor. P3HT and PCBM blends, once deposited onto a substrates often must be sealed with additional barrier layers placed between the deposited metal cathode, such as aluminum, and the active layer. Some experimental work have found that adding a barrier layer such as lithium fluoride (LiF) does not provide sufficient insulation, and this addition often decreases the voltage drop at the interface, reducing the overall efficiency of solar cell devices. In this research, we sought to find an alternative approach that chemically modifies the properties of the donor polymer poly(3-hexylthiophene) to reduce oxidation, that would aid in mitigating degradation of the polymer, and help to extend the lifetime use of a solar cell device. In addition, the

modification of P3HT by lithium aluminum hydride, LAH, a well-known reducing agent used in organic synthesis processes, has been shown to decrease the defect density or defect sites of the polymer. Similar methods, also known as defect engineering in materials science, was used by Liang and co-workers to modify P3HT, and we have adapted their approach to study the properties of P3HT modified with LAH using UV-Vis and X-ray diffraction analyses. This research is based on organic polymers and how we understand the differences and contributions between modified and unmodified P3HT, to possibly improve their stability in applications such as solar cell and other organic electronic devices. Poly (3-hexythiophene), (P3HT), electron donor (p-type) polymer is ubiquitous in hybrid solar cell organic polymer devices and often combined with electron acceptor such as n-type PCBM. Here, we modify P3HT with lithium aluminum hydride (LAH) with the aim of partially reducing the double bonds in thiophene rings. The lithium aluminum hydride has been shown to cause substantial improvements in chemical stability and electrical properties. This modification gives the polymer added flexibility, and reduces polymer oxidation resulting in extended lifetime for enhanced device performance. The modification by LAH, as depicted on the left side of the reaction in Figure 1, makes the P3HT more flexible because of the reduction of double bonds. As described in the work by Z. Liang et al., this addition rehybridizes an electroactive sp 2 carbon to an electro-inactive sp 3 state. Figure 1. The chemical reaction model of our research based on the experiment work developed by authors Z. Liang, et al., Chem. Mater. 2009, 21, 4914 4919. 2. Materials and Methods We completed the reaction under nitrogen atmosphere, and reactants and products were protected from ambient light using aluminum foils in all the steps throughout the experimentation process. 100.7 mg of P3HT was dissolved in 40 ml of anhydrous THF and stirred for 2 h, at room temperature under nitrogen atmosphere. Seven milliliters of LAH (1 M in THF) was added under ice and it was allowed to react for 24 h. Seven milliliters of acetic acid were added dropwise, to quench the reaction followed by 25 ml of methanol. The solid precipitate was filtered under vacuum, washed with methanol, dissolved in 90 ml of chloroform and filtered with a Teflon 100 m filter to remove residual impurities. The filtered chloroform solution was evaporated using a rotatory evaporator and the polymer was left to dry under vacuum for 48 h. P3HT Indium Tin Oxide Glass P3HT LAH Indium Tin Oxide Glass Figure 2. Schematic showing ITO glass-coated films of P3HT (left), and P3HT_LAH (right) prepared by spin-coating process to developed thin films for UV-Vis and XRD analysis. 1640

Figure 2 depicts a schematic of the thin films deposited on transparent indium tin oxide conducting substrates. P3HT was purchased from Rieke Metals, Inc. (RMI-001E, 96% regioregular) and LAH from Sigma Aldrich, Inc. (1 M in THF). Separate solution sets composed of either P3HT, or P3HT_LAH (2 mg each) were dissolved in 1 ml of dichlorobenzene solvent. The mixtures were left stirring for three days at a room temperature. Indium tin oxide (ITO) transparent coated glass (10 Ω/sq.) substrates cut down to 2x2.5 sq-in were sonicated in acetone for 5 min., and then rinsed with ionized water, at a room temperature. We deposited polymer solutions of P3HT, and of P3HT_LAH on ITO substrates using a spin coater programmed at 600 rpm for 60 seconds (~100 nm thick film). We then placed substrates of P3HT, and of P3HT_LAH on a hot plate for annealing inside a nitrogen chamber at 140 C for 5 min. 3. Results and Discussions We completed UV-Vis spectrums for the polymers in solid state. The UV-Vis spectrum results for P3HT, and P3HT modified with LAH thin films is shown in Figure 3, and show us that the reaction occurred as well as the modification of P3HT_LAH. The P3HT only film depicts a higher absorbance than modified P3HT_LAH. This result was consistent for all samples deposited on ITO glass substrates. We also observed that both P3HT and P3HT_LAH peaks occurred at or very near the same wavelength, illustrating that our modification maintained the fundamental electrical properties of the P3HT polymer. Figure 3. UV-Vis absorbance spectra of P3HT and P3HT_LAH thin film samples annealed under nitrogen atmosphere at 140 C for 5 min. We detected the peaks appearing at the same wavelengths for both samples, since P3HT and modified P3HT_LAH have the same the general structural properties and they are maintained as very few double bond break post LAH reduction. For the unmodified P3HT, sharper and narrower peaks appear; this is observed especially between 583 and 600 nm signifying that of a more crystalline structure because of the carbon-carbon double bonds contained in the P3HT. Since 1641

the P3HT_LAH have less double bonds, (only 1 in 10 4-10 5 of the polymer sites reacts according to Wang, D, et al.) the sharpness of the peaks is diminished and does not participate in the interaction with single bonds. Nevertheless, both showed three distinctive peaks that characterize the P3HT polymer. These peaks are the 520 nm, 550 nm, and 620 nm. We can see that both samples show these peaks, but the P3HT_LAH shows peaks with decreased intensity, resulting in almost a flattening effect. We also notice a broadening at the shoulder near 620 nm. We think that this is attributed to the reduction of the double bonds as a result of the LAH across the polymers chain resulting in increased flexibility of the modified P3HT_LAH. P3HT P3HT_LAH Figure 4. XRD diffractograms of P3HT and P3HT_LAH thin films spin-coated on ITO glass substrates. We conducted X-Ray diffraction (XRD) analysis of P3HT and P3HT_LAH thin films on ITO glass substrates to compare the effect of LAH modification on the overall crystallinity of the P3HT polymer. As shown in Figure 4, we observe three distinct peaks at 5, ~22 and at ~31 (2-Theta). The dominant peak is at 5 with the higher intentisity peak corresponding to P3HT (black) and the lower intensity peak corresponding to P3HT_LAH (red). The two other peaks observed at ~22 and at ~31 (2-Theta) are distintive to the ITO substates. 4. Conclusions Our experimental data showed that the modification of the P3HT polymer was successful. On the UV-VIS spectroscopy we can see that the P3HT had a higher absorbance than the P3HT_LAH, which could be caused by the reduction of the double bonds in the polymer chain. We also evaluate similar effects on the XRD that showed an increase in the peak of P3HT at 5 (2θ) while the P3HT_LAH shows the same peak but at a lower magnitude. This analysis confirms the degree of crystanillity of the P3HT, and we can also verify that modification of the LAH increases the flexibility of the polymer 1642

chain reducing the overall crystanillity of the polymer. The data confirms that the modification of the polymer did occur. In our future work, we intend to fabricate organic solar devices to analyze their working efficiency with functionalized P3HT. 5. Acknowledgements The author wishes to express his appreciation to the NASA-IDEAS Grant NASA NNX10AM80H/NNX13AB22A, the National Science Foundation Grant Awards NSF-IFN-100240, and NSF-DMR-1523463. Finally, I would like to thank my colleagues Gabriel Calderon Ortiz, Jorge Marcano, and Anamaris Meléndez (PREM technician) for experimentation support. 6. References 1. Ambrosi, A., Chua, C. K., Bonanni, A., & Pumera, M. Chemistry of Materials. 2012, 24, (12), 2292-2298. 2. Calderón-Ortiz, G., Vedrine-Pauléus, J., Carrasco, H.; 2015, J Nano Ener & Poli Research, (3), 1 4. 3. Calderón-Ortiz, G., Vedrine-Pauléus, J., 2012, National Conference on Undergraduate Research (NCUR) Proceedings, 1559. 4. Duraisamy, N., Muhammad, N. M., Hyun, M. T., & Choi, K. H. Materials Letters. 2013, 92, 227-230. 5. Karagiannidis, P. G., Kassavetis, S., Pitsalidis, C., & Logothetidis, S. Thin Solid Films. 2011, 519, (12), 4105-4109. 6. Kim, S. S., Na, S. I., Jo, J., Tae, G., & Kim, D. Y. Advanced Materials. 2007, 19, (24), 4410-4415. 7. Lee, D., & Jang, D. J. Polymer. 2014, 55, (21), 5469-5476. 8. Liang, Z.; Nardes, A.; Wang, D.; Berry, J. J.; Gregg, B. A. Chem. Mater. 2009, 21, 4914-4919. 9. Liang, Z.; Reese, M. O.; Gregg, B. A. Appl. Mater Interfaces. 2011,3, 2042-2050. 10. Ma, W., Yang, C., Gong, X., Lee, K., & Heeger, A. J. Advanced Functional Materials. 2005, 15, (10), 1617-1622. 11. Urrego, S., Otálora, C. A., Chamorro, W., Duarte, J., Rodriguez, O., Romero, E., & Gordillo, G. Revista Colombiana de Física. 2013, 45, (3). 12. Vanlaeke, P., Swinnen, A., Haeldermans, I., Vanhoyland, G., Aernouts, T., Cheyns, D., & Manca, J. V. Solar energy materials and solar cells. 2006, 90, (14), 2150-2158. 13. Wang, D.; Kopidakis, N.; Reese, M. O.; Gregg, B. A. Chem. Mater. 2008, 20, 6307 6309. 1643