Category Principles of Solar Cells,. LEDs and Diodes
A common thiophene-based donor material is poly(3-hexylthiophene), or P3HT. It is soluble in several organic solvents, which makes it compatible with low-cost solution processing. Another donor material is poly(3,3///-didodecyl quaterthiophene), or PQT-12. Their molecular structures are shown in Figure 6.34. The optical absorption spectra of these compounds (Figure 6.35) show that their absorption bands are limited to the green and red parts of the solar spectrum.
Acceptor materials must provide good electron conductivity but optical absorption is not desired. A popular material is a C60 derivative. C60 is a fullerene, or a molecule composed entirely of carbon...Read More
The absorption of sunlight in a molecular organic semiconductor results in the formation of molecular excitons, and in accordance with the dipole process and the discussion in Sections 3.6 and 3.7 the ground-state singlet is excited into an excited singlet molecular exciton. This exciton is localized to a single molecule, which is generally on the nanometer length scale. Unless the exciton can be dissociated and its hole and electron extracted no current can result. In contrast to this, photon absorption in inorganic semiconductors used in solar cells results in separated holes and electrons that are free to flow independently of each other and hence directly contribute to current flow.
A key challenge in the development of organic solar cells is to overcome the localization and pairing i...Read More
Among the highest efficiency dopants are the iridium organometallic complexes. They have a short triplet lifetime of 1-100 ps, which means that radiative recombination is assisted since the normally forbidden radiation from the triplet exciton is somewhat allowed due to spin-orbital interaction in the molecule. This relaxes the requirement that spin is invariant during the transition and triplet excitons become allowed radiative transitions. High-efficiency phosphorescence results. Iridium, a transition metal with an unfilled inner shell and a net angular momentum, provides the needed spin-orbit interaction.
Examples of red, green and blue iridium-based emitters are shown in Figure 6...Read More
The requirements for full-colour display applications of OLEDs include red, green and blue emitters with colour coordinates close to the following values: for green emitters, x = 0.3 and у = 0.6; for red x = 0.62 and у = 0.37; for blue x = 0.14 and у = 0.10. Fluorescent dopants emitting with approximately these colour coordinates are required.
Figure 6.23 Hole transport hosts CBP and CDBP Chemical Structure reproduced from Organic light-emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis
An example of a green dopant is based on a coumarin dye molecule such as C-545TB, shown in Figure 6.24. This dopant yields saturated green emission with colour coordinates x = 0.3, y = 0.64, with a luminescent efficiency of 12...Read More
Suitable host materials must exhibit good electron and/or hole conduction to ensure the recombination of charge carriers and the effective formation of excitons. Their LUMO and HOMO levels must suit the guest molecules: Compared to the guest LUMO level the host LUMO level should lie less deep in energy (closer to the vacuum level). Compared to the guest HOMO level the host HOMO level should lie deeper (further from the vacuum level) to ensure effective energy transfer. The host and guest molecules must exhibit good miscibility to maintain a stable solution without the tendency for precipitation, which will decrease energy transfer efficiency. Finally energy transfer processes should occur quickly.
TAZ1 109 LUMO: -2.6 eV HOMO: -6.6 eV *T1: XXX
Figure 6...Read More
Obviously a key material for successful OLED operation, the LEM must be amenable to a high-quality deposition technique such as vacuum deposition. It also requires the capability to transport both holes and electrons to enable the recombination of these carriers. Moreover, it must effectively allow for the creation of excitons and their decay to generate photons and it must remain stable at the electric fields needed to transport the holes and electrons and the migration of molecules must be minimized for device stability.
In OLED operation, electrons injected from the cathode and holes injected from the anode combine to form molecular excitons, which were discussed in Chapter 3...Read More
Materials for the electron transport layer (ETL) have been investigated intensively and several families of candidate materials are known. Intermolecular transport occurs by electron hopping, and a LUMO level that is similar in energy to the workfunction of the cathode and the electron-conducting level in the EIL is required, as shown in Figure 6.14.
The ETL should have a mobility of at least 10-6 cm2V-1s-1, which is one to two orders of magnitude smaller than the mobility range of HTL materials. Improving this low mobility has been one key target of the intensive investigation of these materials...Read More
The basic requirement of the HTL is good hole conductivity. In conjugated polymers hole conductivity arises through conjugated bonding, and in small-molecule hole transport
Liq LiMeq Liph LiOXD
Figure 6.17 Organo-metallic complexes may also be used for the electron injection layer. Examples are shown consisting of some lithium-quinolate complexes. Liq, LiMeq, Liph and LiOXD. Chemical Structure reproduced from Organic light-emitting materials and devices, ed by Z. Li and H. Meng 9781574445749 (2007) Taylor and Francis
materials the same mechanism applies, combined with the transfer of charge between HTL molecules. As shown in Figure 6...Read More