ABSTRACT
CONTENTS 3.1 Introduction ............................................................................................118 3.2 Transient and Steady-State Photoconductivity in Molecular
Crystal Tetracene................................................................................... 121 3.2.1 Sample Preparation and Measuring Techniques ................. 121 3.2.2 Experimental Results and Discussion ................................... 122 3.2.3 Conclusions ................................................................................ 126
3.3 Detailed Studies of the Photogeneration of Charged Polarons Using All-Optical Ultrafast Carrier Density Measurements ......... 127 3.3.1 Introduction ............................................................................... 127 3.3.2 Sample Preparation and Measuring Technique ................... 128 3.3.3 Experimental Results and Discussion ................................... 129 3.3.4 Conclusions ................................................................................ 134
3.4 The Role of Electron Photoemission in the “Photoconductivity” of Semiconductor Polymers ........................... 135 3.4.1 Introduction ............................................................................... 135 3.4.2 Sample Preparation and Measuring Technique ................... 135 3.4.3 Results and Discussion ............................................................ 136 3.4.4 Conclusions ................................................................................ 140
3.5 Charge Carrier Photoexcitation and Relaxation Dynamics in Highly Ordered Poly(p-Phenylene Vinylene) ................................... 140 3.5.1 Introductory Remarks .............................................................. 140 3.5.2 Details of the Experiment ........................................................ 142 3.5.3 Results and Discussion ............................................................ 143
3.5.3.1 Bimolecular versus Monomolecular Carrier Recombination ............................................................ 143
3.5.3.2 Carrier Density Dynamics Probed by Ultrafast Photoinduced IRAV ................................................... 144
Unraveling the nature of photoexcitations in π-conjugated polymers and molecular crystals is of interest for gaining fundamental understanding of classes of materials that are essential for designing efficient organic optoelectronic devices such as light emitting diodes (LEDs), field effect transistors (FETs), and photovoltaic (PV) cells. A particular research focus in these materials has been the mechanism of carrier photogeneration, which has been recognized as a sensitive gauge for the extent of the photocarrier delocalization.1-3 Models for carrier photogeneration mechanisms were established initially on the basis of measurements on oligo-acenes3,4 and later applied to π-conjugated polymers as well.5-7 These models generally emphasize the localized nature of photoexcitations and describe carrier photogeneration as a secondary process involving exciton dissociation. However, various experimental observations obtained from single crystal anthracene8 and tetracene,1 as well as π-conjugated polymers,2,9-11 are at variance with those models. In fact, they indicate that carrier photogeneration quantum efficiency in these systems appears independent of temperature, photon energy, and light intensity, thus featuring the hallmarks of direct interband carrier photogeneration and coherent prompt carrier transport within delocalized electronic bands.
