Ž . Synthetic Metals 111–112 2000 493–496 www.elsevier.comrlocatersynmet Femtosecond photovoltage excitation cross-correlation on a ladder-type polymer G. Lanzani a, ) , C. Zenz a,b , G. Cerullo c , W. Graupner d , G. Leising b , U. Scherf e , S. De Silvestri b a Istituto Nazionale per la Fisica della Materia, Istituto di Matematica e Fisica, UniÕersita di Sassari, Sassari I-07100, Italy ` b Institut fur Festkorperphysik, Technische UniÕersitat Graz, Graz A-8010, Austria ¨ ¨ ¨ c Istituto Nazionale per la Fisica della Materia, Dipartimento di Fisica, Politecnico di Milano, Milan I-20133, Italy d Department of Physics, Virginia Tech Blacksburg, VA, USA e Max-Planck Institut fur Polymerforschung, Mainz, Germany Abstract Ž . Femtosecond excitation cross-correlation EXC photovoltage experiments are carried out in methyl-substituted poly-para-phenylene Ž . ladder-type polymer light-emitting devices LEDs . The first excitation pulse at 390 nm generates singlet excited states in the polymer film. The second pulse at 780 nm is in resonance with the excited state absorption. We found that the probability of charge generation is enhanced in doubly excited chains only within a short period of time after excitation. The result is interpreted on the base of diffusion-driven exciton dissociation model in which the active sites are defects or impurities localized close to the Al electrode. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Ultrafast spectroscopy; Conjugated polymer; Organic light-emitting diodes; Photovoltage 1. Introduction Conjugated polymers are a new and promising class of semiconductors, with potential application in electronic wx and optoelectronic devices 1 . The research activity in this area has been growing at a high pace in the last decade, and important results have been achieved in the construc- tion of prototypes and sometime close-to-the-market de- vices. Parallel to the applied research, there is a long- standing effort in fundamental research towards clarifica- tion of the excitation processes in conjugated polymers. The availability of good quality devices, besides the promise for future technology, has boosted the basic re- search because it allows testing the physical properties of polymer semiconductors in working conditions. Examples are methods of electric-field-assisted spectroscopy, such as wx charge injection spectroscopy 2 , and field-assisted tran- w x sient spectroscopy 3,4 . In this paper, we report on a new investigation tech- Ž . nique, based on the excitation cross-correlation EXC scheme, carried out in blue-emitting light-emitting devices ) Corresponding author. Ž . E-mail address: guglielmo.lanzani@fisi.polimi.it G. Lanzani . Ž . LEDs based on methyl-substituted poly-para-phenylene Ž . wx ladder-type polymer mLPPP 5 . The method has similar- ity to the correlation techniques widely used for measuring wx femtosecond and picosecond laser pulses 6 . In the past, it has been used, albeit in different configurations, to study w x inorganic and organic semiconductors 7–10 . Two excita- tion pulses of different colors are spatially coincident on the LED device and delayed in time. The change in photovoltaic response caused by the double excitation is detected by a standard time-integrating system. In this way, the dynamic information is obtained as a correlation function, and not as genuine temporal variation. The ad- vantage is, however, the simplicity of the experiment and the easy access to non-linear effects. 2. Experimental We used single-layer devices, consisting of 100-nm- thick films of mLPPP sandwiched between indium–tin– Ž . oxide ITO and aluminum electrodes. In all the experi- ments, the device was not biased by external voltage. The experimental set-up is shown in Fig. 1. Red pump pulses at 780 nm, 150 fs time duration, 1 kHz repetition rate are obtained from a chirped pulse-amplified Ti:Al O laser 2 3 0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. Ž . PII: S0379-6779 99 00426-9