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| Fig. 1: Flexibility of OLED displays. (Source: Wikimedia Commons) |
Thinner, lighter, and brighter displays are big selling points for consumer device manufacturers, but differentiation is the key to new markets. This year alone, the fascination with OLEDs can be seen with the introduction of Qualcomm's smart-watch and LG's G Flex smart-phone. The phone features a curved OLED display, a design not possible with LCDs. [1] Other advantages of OLED displays include: their basic properties as active light-emitters, flexibility, transparency, and scalability. Fig. 1 shows the flexibility of OLED displays. Fig. 2 shows the basic structure of a bilayer OLED which a stack of an electron transporting and emitting layer (ETL) and a hole-transporting layer (HTL) sandwiched between a low work function cathode and a transparent anode. [1] ITO and along with PDOT:PSS, for flexible OLEDs, is commonly used transparent anode. The emission process, shown in Fig. 3, begins with electron-hole pair recombination that results in neutral exited states of 25% singlet (S) and 75% triplet (T) exitons: [1]
Fluorescence is a result of emissive transition from neutral exited states to ground states, but singlet exitons can also decay via thermal deactivation. Fig. 3 also shows 15% of T-T annihilation can result in delayed fluorescence. [1] Thus the maximum EL quantum efficiency can be 40% at most. Multilayer structure improves device performance by lowering the barrier for hole injection and by moving the electron-hole recombination region from the cathode interface (high defect density) into the bulk region with a lower defect density.
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| Fig. 2: Basic stacking on an OLED. |
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| Fig. 3: Schematic of the electron-hole recombination. |
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| Fig. 4: Bulk heterojunction type OPV. |
The corresponding energy values associated with absorbance, EL, and PL plots were calculated by determining the wavelength were peaks are observed and relating this value to Planck's constant using
where h, c, and λ are Planck's constant, the speed of light, and wavelength, respectively.
Cleaning and materials selection play an important role in the fabrication of OLEDs. One such combination was placing the sample under UV-Ozone then immediately spincoating the PDOT:PSS. The UV-Ozone treatment results in a hydrophilic surface for the coating of PDOT:PSS. The PDOT:PSS layer planerizes the ITO surface, matches the ITO interface energy with the active layer, and acts as a hole only transport layer. [4] G. F. Wang et al. characterized three OLEDs and the samples with spincoated PDOT:PSS had lower turn-on voltages and higher luminance compared to the sample without a PDOT:PSS layer. [5] The energy barrier for hole injection may have be smaller for the sample with PDOT:PSS because it has a higher work function of 5.0 eV compared to 4.7 eV for the sample without PDOT:PSS, resulting in lower turn-on voltage.
Currently, inorganic photovoltaic devices perform with higher photoelectric conversion efficiency (PCE) and stability, than organic photovoltaic devices. [6] The power conversion efficiency of OPVs was only 3% in 2005 compared to 24% for a single-crystal silicon solar cell. However, inorganic photovoltaic devices still have deficiencies, such as high manufacturing cost and solid construction, which hampers their application as cheap consumables and flexible electronic products. [6] The development of organic photovoltaic (OPV) devices may play a key role in overcoming the deficiencies of inorganic photovoltaic devices, because they offer several advantages such as: lower energy and material consumption during the manufacturing process, lower cost, and lower temperature processes during fabrication.
The first types of OPVs were Schottky-type devices. This device consisted of a metal-organic-metal sandwich that had a rectifying contact at one of the organic-electrode interfaces. [6] However, these devices are inefficient as charge photogeneration occurs only in a thin layer near the metal-organic interface, thus limiting the quantum yield of charge photogeneration. Second types of OPV, shown in Fig. 4, are known as organic bulk heterojunction (BHJ) photovoltaics.
In heterojunction OPVs charge generation occurs at the interface between two different organic semiconductors that transport either holes or electrons. A donor type conjugated polymer and an acceptor type fullerene (or fullerene derivatives, such as [6,6]-phenyl-C61-butyric acid methyl ester, PCBM) are mixed to form the photoactive layer. Upon illumination, light is absorbed by the photoactive layer and results in the formation of an exciton. Free carriers can be generated by exciton dissociation at the donor- acceptor interface, leaving the electron on the acceptor's lowest unoccupied molecular orbital (LUMO) energy level and the hole on the conjugated polymer highest occupied molecular level (HOMO). [7] The result is an increase in the system's electro-chemical potential.
© Paul Ditiangkin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. Shinar and V. Savvateev, eds., Organic Light-Emitting Devices, A Survey (Springer, 2004), pp. 1-50.
[2] T. Tsutsui, "Electroluminescence in Small Molecules", in Organic Electroluminescence, ed. by Z. H. Kafafi (CRC Press, 2005), p. 1.
[3] J. Yang et al., "mBeRFP, an Improved Large Stokes Shift Red Fluorescent Protein" PLoS ONE 8, e64849 (2013).
[4] G.-F. Wang, X.-M. Tao and R.-X. Wang, "Fabrication and Characterization of OLEDs using PEDOT:PSS and MWCNT Nanocomposites," Composites Sci. Technol. 68, 2837 (2008).
[5] W. Kim et al., "Effect of Annealing on the Electrical Properties and Morphology of a Conducting Polymer used as an Anode in Organic Light-Emitting Devices," J. Polymer Science B 41, 2522 (2003).
[6] S.-S. Sun and N. S. Sariciffci, eds., Organic Photovoltaics: Mechanisms, Materials, and Devices (CRC Press, 2005), pp. 50- 97.
[7] C. J. Brabec et al., eds., Organic Photovoltaics Concept and Realization (Springer, 2003), pp. 1-80.
