  Based on the assumption that the contact barrier height determines the
current flow in organic semiconductor-based electronic devices, charge
injection at metal-organic (MO) interfaces has been extensively investigated,
while space-charge conduction in organic bulk is generally overlooked. Recent
theoretical modeling and simulation have pointed out that such a simplification
is questionable due to the hopping nature of charge injection and
hopping-related space-charge conduction. Here we show experimentally that
charge transport in metal-organic diodes is complex interplay between
injection-limited current (ILC) and space-charge-limited current (SCLC). We
report the experimental observation of ILC-to-SCLC transition in
Ag/pentacene/Ag diodes as a function of temperature.
