In this paper, we demonstrated that controlled charge trapping, both at the emission layer and charge transport layer with
energy level alignment, is essential for charge-balanced and effective electrophosphorescent organic light-emitting
device (OLED). Conditions for enhanced of efficiency and lifetime of OLED were obtained with graded doping profile
at the light-emission layers (varied host-dopant concentration) and different hole (exciton) blocking materials.
Conceptual device physics presented in this study can be applied at an initial design of charge-confined, balanced
structure of highly efficient electrophosphorescent devices.
Thermal annealing has been widely used to improve device performances of organic solar cells with regioregular (RR)
poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction blends.
Especially, short-circuit current density (Jsc) of the thermally-annealed device is significantly increased compared to
that of the non-annealed one. The Jsc is proportional to the product of the carrier mobility and the number of
photogenerated carriers which depends on the photocarrier generation efficiency and carrier recombination lifetime.
Therefore, the enhanced Jsc implies that the thermal annealing can increase either the mobility and/or lifetime of the
photogenerated carriers. In order to understand which parameter is more affected by thermal annealing, we compared
the temperature dependence of the Jsc and carrier mobility of P3HT:PCBM (1:1, weight%) blend solar cells. The carrier
mobility, measured from a time-of-flight photoconductivity (TOF-PC) measurement, increases from about 10-5 cm2/Vs
to the order of 10-4 cm2/Vs as the temperature increases from 300 K to 360 K and then saturates above 360 K up to 400 K. This behavior is very similar to the temperature dependence of the current density of the P3HT:PCBM solar cell
devices with the same blend ratio. Therefore, this correlation indicates that the thermal annealing increases the carrier
mobility by improving morphological order of the blend film and thereby enhances the Jsc of the P3HT:PCBM blend
solar cells.
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