Exciton-blocking layer

FIGURE 3.23 External quantum efficiencies of PtOEP/CBP and PtOEP/Alq3 devices as a function of current with and without a BCP blocking layer (left). Emission spectra of CBP-based electroluminescent devices with and without a BCP exciton blocking layer (right). (From O Brien, D.F., Baldo, M.A., Thompson, M.E., and Forrest, S.R., Appl. Phys. Lett., 74, 442, 1999. With permission.)... 

M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, and Y. Taga, Highly efficient phosphorescence from organic light-emitting devices with an exciton-block layer, Appl. Phys. Lett., 79 156-158 (2001). 

Kim, S. H Jang, J. Lee, J. Y. (2007). High efficiency phosphorescent organic light-emitting diodes usine carbazole-type triplet exciton blocking layer. Applied Physics Letters, Vol. 90, pp. 223505-223507. 

While the discussion in this section has focused on the need to control (prevent) carrier (hole) and exciton diffusion from the emissive layer to the ETL, the same issues are relevant at the interface between the HTL and the emissive layer. If the HTL material has a LUMO energy that is lower than that of the emissive layer, or a triplet energy that is too low, an electron-exciton-blocking layer may be needed to prevent carrier and exciton loss at that interface. A description of the best materials to act as electron-exciton-blocking materials for this interface will be considered in a later section (see Section 12.04.2.1.4). 

Attempts have been made to deposit TIPS-pentacene from solution as the functional layer in a pentacene/C60 bilayer photovoltaic device. Careful optimization of deposition conditions, optimal concentration of mobile ion dopants, thermal postfabrication annealing, and the addition of an exciton-blocking layer yielded a device with a moderate white-light PCE of 0.52% [41]. Since TIPS-pentacene derivatives rapidly undergo a Diels-Alder reaction with fiillerene, the assembly of potentially more efficient bulk-heterojunction photovoltaic devices from TIPS-pentacene and fiillerene derivatives were not possible [42]. The energy levels of the TIPS-pentacene-PCBM adduct (PCBM is [6,6]-phenyl C61-butyric acid methyl ester) ineffectively supports the photoinduced charge transfer. 

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