ABSTRACT
CONTENTS 1.1 Introduction ............................................................................................... 2
1.1.1 Basic Properties of p -Conjugated Polymers.............................. 2 1.1.2 Optical Transitions of Photoexcitations in
p -Conjugated Polymers ............................................................... 8 1.1.2.1 Optical Transitions of Solitons in Polymers
with Degenerate Ground State Structure ................... 9 1.1.2.2 Optical Transitions of Charged Excitations in
NDGS Polymers ........................................................... 10 1.1.2.3 Optical Transitions of Neutral Excitations in
NDGS Polymers ........................................................... 11 1.1.2.4 Photoinduced Infrared Active Vibrational
Modes ............................................................................. 14 1.1.2.5 Nonlinear Optic Spectroscopy Related to
Exciton Transient Response ........................................ 16 1.1.3 Properties of Intrachain Excitons in p -Conjugated
Polymers ....................................................................................... 18 1.1.4 Experimental Setup for Measuring Transient
and CW Responses ..................................................................... 21 1.1.4.1 Low-Intensity Femtosecond Laser System ............... 21 1.1.4.2 High-Intensity Femtosecond Laser System .............. 22 1.1.4.3 Continuous Wave Optical Measurements ................ 22
1.2 Ultrafast Dynamics of p -Conjugated Polymer Films and Solutions with NDGS Backbone Structure ......................................... 24 1.2.1 Exciton Dynamics in DOO-PPV and PPVD0 Polymers ........ 24
1.2.1.1 Ground and Excited States ......................................... 24 1.2.1.2 Excited States Relaxation Dynamics ......................... 27 1.2.1.3 Three-Beam Spectroscopy .......................................... 32
1.2.2 Photoexcitation Dynamics in Pristine and C60-Doped MEH-PPV ..................................................................................... 37 1.2.2.1 Pristine MEH-PPV Films and Solutions ................... 37
1.1.1 Basic Properties of p -Conjugated Polymers
The photophysics of p -conjugated polymers have been intensively studied during the last three decades. These organic compounds form a new class of semiconductor electronic materials with potential applications such as organic light-emitting diodes (OLEDs) [1-3], thin-film transistors (TFTs) [4], organic photovoltaic cells [5], organic spin-valve devices [6], and optical switches and modulators [7]. As polymers, these organic semiconductors have a highly anisotropic quasi-one-dimensional electronic structure that is fundamentally different from the structures of conventional inorganic semiconductors. This has two consequences: First, their chainlike structure leads to strong coupling of the electronic states to conformational excitations peculiar to the one-dimensional system [8] and, second, the relatively weak interchain binding allows diffusion of dopant molecules into the structure (between chains), whereas the strong intrachain carbon-carbon bond maintains the integrity of the polymer [8]. In their neutral form, these polymers are intrinsic semiconductors with an optical gap of ≈2 eV. However, they can be easily doped with various p and n type dopants, increasing their conductivity by many orders of magnitude; conductivities in the range of ≈103−104 S/cm are not unusual [9]. The ability to dope these organic semiconductors to metallic conductivities resulted in award of the 2000 Nobel Prize in chemistry to Alan Heeger, Alan McDiarmid, and Hideki Shirakawa.
