Abstract
We have been working to characterise the electronic excitations created by charge injection into conjugated polymers such as poly(
p-phenylenevinylene) in electroluminescent devices. We report on the optical probing of electronic states injected into an indium-tin oxide/PPV/calcium electroluminescent device. We monitor changes in absorption in the polymer film due to charge carrier injection into the device by detecting the reflection of a probe beam off the back metal contact layer. These injected carriers are expected to reside in states with energy levels within the band gap and hence give rise to a change in the sub-band gap optical properties. We observe absorption features centred around 1.38 eV, 1.55 eV, 0.65 eV. These states closely resemble those seen in photoinduced absorption and have been assigned to optical transitions involving bipolarons (0.65 and 1.55 eV) and a triplet-exciton (1.38 eV).