Part I: Stable Hetero-Acene Analogs of Heptacene: The Synthesis and Study of their Conductive Properties in Organic Transistors.
The surge of organic electronics in the last two decades has augmented the necessity for the development of high performance devices with superior and less expensive processability. Polycyclic Aromatic Hydrocarbons (PAHs) have been widely investigated for this purpose2-5. With advancement in this field, PAH-based organic semiconductors, in some instances, have superseded Si in performance6-7. They exhibit hole mobilities as high8 as 5.5 cm2V-1S-1. Pentacene, for example, has become a benchmark for organic molecular semiconductors8. Use of such materials for fabrication of electronic devices, however, is limited due to their extreme lack of stability under ambient conditions. Therefore, efforts have been made to develop materials that exhibit comparable mobilities, and also are air-stable. Heterocyclic aromatic hydrocarbons were first developed as viable materials in this. However, though they are quite stable under ambient conditions, their low hole mobilities as compared to their PAH counterparts are an issue. Recently, heterocyclic aromatic hydrocarbons with high air-stable mobilities have been reported. Dinaptho[2,3-b:2,3-f]thieno[3,2-b]-thiophene (DNTT)9-11, for example, showed mobilities as high as 3.1 cm2V-1S-1. It is believed that materials with additional fused rings fused in such systems will allow higher stacking and better molecular packing, leading to higher mobilities. Hence, we propose the synthesis of dianthra[2,3-b:2,3-b]thiophene (DAT) for applications in organic electronics. Preliminary theoretical calculations on DAT suggest that such a material will have hole mobilities as high as ~ 3 cm2V-1S-1. DAT, therefore, will be promising for applications in organic electronics, namely OFETs, OLEDs, etc. We have synthesized this material, developed collaboration with a research group at Stanford to make devices with it, and detailed studies on its device performances are underway.
References:
1. Sheats, J. R. J. Mater. Res.2004, 19, 1974.
2. Butko, V. Y.; Chi, X.; Lang, D. V.; Ramirez. A. P.; Appl. Phys. Lett.2003, 83, 4773.
3. Zhang, Y.; Petta, J. R.; Ambily, S.; Shen, Y.; Ralph, D. C.; Malliaras, G. C. Adv. Mater.2003, 15, 1632.
4. Dimitrakopoulos, C. D.; Kymissis, I.; Purushothaman, S.; Neumayer, D. A.; Duncombe, P. R.; Laibowitz, R. B.; Adv. Mater.1999, 11, 1372.
5. Odom, S. A.; Parkin, S. R.; Anthony, J. E. Org. Lett.2003, 5, 4245.
6. Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.; Gershenson, M. E.; Rodgers, J. A. Science2004, 303, 1644.
7. Reese, C.; Chung, W. J.; Ling, M. M.; Roberts, M.; Bao, Z. N. Appl. Phys. Lett.2006, 89, 202108.
8. Lee, S.; Koo, B.; Shin, J.; Lee, E.; Park, H.; Kim, H. Appl. Phys. Lett.2006, 88, 162109.
9. Yamamoto, T.; Takimiya, K. J. Am. Chem. Soc.2007, 129, 2224.
10. Yamamoto, T.; Takimiya, K. J. Photopolym. Sci. Technol.2007, 20, 57.
11. Sanchez-carrera, R. S.; Atahan, S.; Schrier, J.; Aspuru-Guzik, A. J. Phys. Chem. C 2010, 114, 2334.
Part II: The Photo-induced Formation of Quantum Dot – Conductive Polymers (QD:CP) for Application in Photovoltaics.
The worlds increasing population and energy consumption has led to a serious issue in dealing with energy supply1,2. A recent study3 suggests that, with current world-wide socio-economic growth, the global energy demand increases at a rate of 2% a year, and an additional 10 terawatts (TW) of energy is needed to sustain the worlds population by 2050. While the present supply of energy is not going to be exhausted in the near future, a shortfall in the supply of energy is impending considering the rapid growth in demand4.
Solar energy stands out as the most viable alternative energy resource available because the amount of sunlight that reaches the surface of the Earth each hour is approximately as much as today's society uses in an entire year. However, the greatest challenge lies in harnessing the sunlight and its conversion to energy3. In order to harness this power, we need to find materials that are both more efficient at converting light into energy and cheap to produce.The solar cells prevalent today are typically made of inorganic semiconductors, mostly silicon. Silicon solar cells are expensive—in both energy and cost—to produce. Therefore, one of the approaches taken toward meeting the clean energy demand combines inorganic semiconductors such as quantum dots (QD) with polymer materials. The inorganic semiconductor in such composites provides high efficiency, and the presence of the polymer ensures both higher conductivity as well as cheap fabrication cost5,6. However, physical blending of the components does not produce the polymer-QD composites because QD aggregation poses a major problem for its preparation7. Hence, achieving a homogeneous dispersion of QD in the polymer matrix is necessary8. There are two popular methods for achieving this: grafting on and grafting from methods. In the grafting from method the polymer chain grows from a QD surface and therefore allows better electronic interaction between QD and the polymer9,10. Such methods usually involve complex reaction and strategies. Here we discuss the preparation of QD – polymer composites through the photo-induced polymerization process, where no photo-initiator was used. This is the first report of preparing such composites using the photo-induced polymerization method. We used 5-mercapto-2, 2-bithiophene (BTSH) for surface modification of QD. We observe an efficient hole transfer caused by visible light irradiation from a CdSe quantum dot (QD) core to ligand in BTSH functionalized QD (BTSCdSe); while such hole transfer is ten times slower in 5-(5-mercaptopentyl)-2, 2-bithiophene functionalized QD (BTC5SCdSe). The interspersing methylene groups inhibit hole transfers in the case of BTC5SCdSe. The radical cation+BTSCdSe-, formed via hole transfer in BTSCdSe, acts as a building block for photoinitiated polymerization, with no conventional photo-initiator required. The three dimensional composite structure of BTSCdSe and its visible light initiated polymerization is confirmed from the SEM and TEM images. Hole transfer mechanisms are studied using femtosecond and nanosecond transient absorption measurements. Such a polymer-QD composite is expected to be useful in the fabrication of photovolatics such as polymer-inorganic hybrid solar cells.
Reference:
1. Weisz, P. B. Phys. Today, 2004.
2. Bartlett, A. A. Am. J. Phys. 1986, 54, 398.
3. Kamat, P. V. J. Phys. Chem. C2007, 111, 2834.
4. Campbell, C. J. Environ. 2002, 24, 193.
5. Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353.
6. Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science, 2002, 295, 2425.
7. Greenham, M. C. ; Peng, X. G. ; Alivisatos, A. P. Phys. Rev. B: Condensed Matter1996, 54, 17628.
8. Balazs, A. C. ; Emrick, T. ; Russel, T. P. Science, 2006, 314, 1107.
9. Skaff, H.; Sill, K.; Emrick, T. Polym. Prepr.2004, 45, 615.
10. Skaff, H.; Sill, K.; Emrick, T. J. Am. Chem. Soc.2004, 126, 11322.
11. Cai, X.; De, P. K.; Anyaogu, K. C.; Adhikari, R. M.; Palayangoda, S. S.; Neckers, D. C. Chem. Commun.2009, 13, 1694.