OPTICAL PROBES OF ELECTRONIC STATES INJECTED INTO POLY(P-PHENYLENEVINYLENE) ELECTROLUMINESCENT DEVICES

We have been working to characterise the electronic excitations create d by charge injection into conjugated polymers such as poly(p-phenylen evinylene) in electroluminescent devices. We report on the optical pro bing of electronic states injected into an indium-tin oxide/PPV/calciu m electroluminescent device. We monitor changes in absorption in the p olymer film due to charge carrier injection into the device by detecti ng the reflection of a probe beam off the back metal contact layer. Th ese injected carriers are expected to reside in states with energy lev els within the band gap and hence give rise to a change in the sub-ban d gap optical properties. We observe absorption features centred aroun d 1.38 eV, 1.55 eV, 0.65 eV. These states closely resemble those seen in photoinduced absorption and have been assigned to optical transitio ns involving bipolarons (0.65 and 1.55 eV) and a triplet-exciton (1.38 eV).