2.5 Applications Development
2.5.2 Organic photodetector
The operation mechanism behind the organic photo detector is similar to the organic solar cell. The photodetector can classified by spectral regions, ranging from ultra violet to near infra-red (NIR) light. A successful photo detector requires not only low dark current but also high photo current. Several parameters are essential in evaluating the performance of a photodetector, which are summarised in T able(2.2).
Parameter units comments
Responsivity A/W sensitivity
Spectral Response Range nm range of detectable wavelength Normalized Detectivity J ones sensitivity
Linear dynamic range dB
Response time s the time of rising to 63.2% of the final state
Quantum efficiency % the number ratio of photo-excited excitons and charge carriers to the incident photon
References
[1] H. L. Dong, H. F. Zhu, Q. Meng, X. Gong and W. P. Hu, “Organic photoresponse materials and devices”Chemical Society Review, vol. 41, no. 5, pp. 1754-1808, 2012. [2] H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang and A. J. Heeger, “Syn-
thesis of electrically conducting organic polymers: halogen derivatives of polyacetylene, (CH)x,”Journal of the Chemical Society, Chemical Communications, no. 16, pp. 578-580, 1977.
[3] X. G. Zhao, X. W. Zhan, “Electron transporting semiconducting polymers in organic electronics,”Chemical Society Review, vol. 40, no. 7, pp. 3728-3743, 2011.
[4] G. Masson, M. Latour, M. Rekinger, I. T. Theologitis and M. Pap, “Global Market Out- look for Photovoltaics 20132017”, European Photovoltaic Industry Association, 2013. [5] C. W. Tang, “Two layer organic photovoltaic cell,”Applied Physics Letters, vol. 48, no.
2, pp. 183, 1986.
[6] A. J. Heeger, N. S. Sariciftci, “Conjugated polymer-acceptor heterojunctions; diodes, photodiodes, and photovoltaic cells,”U.S. Patent 5331183 A, 19 July 1994.
[7] G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, “Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunc- tions,”Science, vol. 270, pp. 1789-1791, 1995.
[8] J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C. Moratti, and A. B. Holmes, “Efficient photodiodes from interpenetrating polymer net- works,”Nature, vol 376, pp. 498-500, 2002.
[9] C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes,”Applied Physics Letters, vol 51, no. 12, pp. 913, 1987.
[10] M. Hiramoto, H. Fujiwara and M. Yokoyama, “Threelayered organic solar cell with a photoactive interlayer of codeposited pigments,”Applied Physics Letters, vol. 58, no. 10, pp. 1062-1064, 1991.
[11] M. Hiromoto, H. Fujiwara and M. Yokoyama, “Organic solar cell based on multistep charge separation system,”Applied Physics Letters, vol. 61, no. 21, pp. 2580-2582, 1992. [12] L. Schmidt-Mende, A. Fechtenk¨otter, K. M¨ullen, E. Moons, R. H. Friend, and J. D. MacKenzie,“Self-Organized Discotic Liquid Crystals for High-Efficiency Organic Photo- voltaics,”Science, vol. 293, pp. 1119-1122, 2001.
[13] G. Yu, K. Pakbaz, and A. J. Heeger, “Semiconducting polymer diodes: Large size, low cost photodetectors with excellent visibleultraviolet sensitivity,”Applied Physics Letters, vol. 64, pp. 3422, 1994.
[14] S. E. Sean, B. J. Christoph, S. N. Serdar, P. Franz, F. Thomas, and H. C. Jan, “2.5 % efficient organic plastic solar cells ,”Applied Physics Letters, vol. 78, no. 6, pp. 841-843, 2001.
[15] X. H. Yang, A. Uddin, “Effect of thermal annealing on P3HT:PCBM bulk-heterojunction organic solar cell: A critical review”, Renewable and Sustainable Energy Reviews, vol. 30, pp. 324-336, 2014.
[16] K. Foe, G. Namkoong, M. Samson, E. M. Younes, I. Nam, and T. M. Abdel-Fattah1, “Stability of High Band Gap P3HT:PCBM Organic Solar Cells Using TiOx Interfacial Layer,”International Journal of Photoenergy, Vol. 2014, 2014.
[17] H. B¨assler, and A. K¨ohler, “Charge transport in organic semiconductors,”Topics in Current Chemistry, vol. 312, pp. 1-65, 2012.
[18] Heliatek reaches efficiency record with 40% transparent organic solar cells, http :
//www.heliatek.com/newscenter/latestnews/heliatek −erzielt−ef f izienzrekord−
mit−40−transparenten−organischen−solarzellen/
[19] N. Sedghi, D. Donaghy, M. Raja, S. Badriya, S. J. Higgins, W. Eccleston, “Experimen- tal observation of the density of localized trapping levels in organic semiconductors”, Journal of Non-crystal Solids, vol. 352, pp. 1641-1643, 2006.
[20] K. J. Baeg, M. Binda, D. Natali, M. Caironi, and Y. Y. Noh, “Organic Light Detectors: Photodiodes and Phototransistors,”Advanced Materials, vol. 25, no. 31, pp. 4267-4295, 2013.
[21] L. Dou, J. You, Z. Hong, Z. Xu, G. Li, R. A. Street, and Y. Yang, “25th Anniversary Article: A Decade of Organic/Polymeric Photovoltaic Research,”Advanced Materials, vol. 25, no. 46, pp. 6642-6671, 2013.
[22] B. Y. Qi,J. Z. Wang, “Open-circuit voltage in organic solar cells,”Journal of Material Chemistry, vol. 22, pp. 24315-24325, 2012.
Chapter 2 Reiew of conducting polymers
[23] J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C. Moratti and A. B. Holmes, “Efficient photodiodes from interpenetrating polymer net- works Nature,”, vol. 376, pp. 498-500, 2002.
Organic Thin Film Transistors
This chapter explores the characteristics of P3HT Thin Film Transistors (TFTs) with low-K gate dielectrics (SiO2). The OTFTs with bottom gate structure were fabricated.
The devices were characterized in non-vacuum conditions. In addition, the influence of the solution concentration is investigated so as to optimize the performance of the devices.
Chapter 3 Organic Thin Film Transistor
3.1
Introduction
The organic thin film transistor (OTFT) has gained considerable popularity in the applica- tion of low cost, and flexible substrate devices such as RFID tags, and displays.[1] However, due to low mobility, large contact resistance and poor ambient and bias stabilities, polymers are not thought to be an alternative to conventional material such as amorphous silicon (a- Si:H). Nonetheless, its unique characteristics such as solution processing, and transparency, underlines the interest in the polymer device in spite of the limitations. The deposition of the polymer thin film in the liquid phase means that the device can be fabricated cost-effectively in large scale. Also with the possibility for integration of organic electrode material, or- ganic dielectric and transparent flexible substrates (i.e. polyethylene naphthalate (PEN) and polyethylene terephthalate (PET)), makes it more attractive over conventional material. This chapter focuses on the study of P3HT OTFT. The structure diagram of P3HT is shown in
F igure.(3.1). And with the aid of the output and transfer characteristics, useful parameters are extracted and subsequently used it to fit the developed organic models. Moreover the effect of variation of the P3HT concentration on device performance is also examined.
Figure 3.1: Diagram of the structure of poly(3-hexylthiophene) (P3HT)