M. Gross et al., Improving the performance of doped pi-conjugated polymers for use in organic light-emitting diodes, NATURE, 405(6787), 2000, pp. 661-665
Organic light-emitting diodes (OLEDs) represent a promising technology for
large, flexible, lightweight, flat-panel displays(1-3). Such devices consis
t of one or several semiconducting organic layer(s) sandwiched between two
electrodes. When an electric field is applied, electrons are injected by th
e cathode into the lowest unoccupied molecular orbital of the adjacent mole
cules (simultaneously, holes are injected by the anode into the highest occ
upied molecular orbital). The two types of carriers migrate towards each ot
her and a fraction of them recombine to form excitons, some of which decay
radiatively to the ground state by spontaneous emission. Doped pi-conjugate
d polymer layers improve the injection of holes in OLED devices(4-9); this
is thought to result from the more favourable work function of these inject
ion layers compared with the more commonly used layer material (indium tin
oxide). Here we demonstrate that by increasing the doping level of such pol
ymers, the barrier to hole injection can be continuously reduced. The use o
f combinatorial devices allows us to quickly screen for the optimum doping
level. We apply this concept in OLED devices with hole-limited electrolumin
escence (such as polyfluorene-based systems(10-12)), finding that it is pos
sible to significantly reduce the operating voltage while improving the lig
ht output and efficiency.