Ultrafast excitation cross-correlation photoconductivity in polyfluorene photodiodes

Two-pulse excitation cross-correlation photocurrent is measured in single-layer poly(9,9Dioctylfluorene) light-emitting diodes. Off-resonant pulses show a three-photon resonance to ahigher lying state responsible for charge generation, mediated by some low-lying excited singletstate S2 (of even symmetry). Resonant excitation shows a two-step transition to the chargegeneration states mediated by a vibrationally hot S1 state (of odd symmetry) which introduces anultrafast timescale suitable for application. Basic knowledge on charge generation and excited staterelaxation is obtained. Possible application to pulse characterization in the blue spectral region isalso demonstrated.