J. Phys. Chem. B 2007, 111, 6303-6321
6303
FEATURE ARTICLE Excited State and Charge Photogeneration Dynamics in Conjugated Polymers Ivan G. Scheblykin,† Arkady Yartsev,† Tonu Pullerits,† Vidmantas Gulbinas,‡ and Villy Sundstro1 m*,† Department of Chemical Physics, Lund UniVersity, Box 124, 221 00 Lund, Sweden, and Institute of Physics, SaVanoriu 231, 02300 Vilnius, Lithuania ReceiVed: December 22, 2006
Conjugated polymers are becoming interesting materials for a range of optoelectronic applications. However, their often complex electronic and structural properties prevent establishment of straightforward propertyfunction relationships. In this paper, we summarize recent results on the photophysics and excited state dynamics of conjugated polymers, in order to paint a picture of exciton formation, quenching, and generation of charge carriers.
1. Introduction Conjugated polymers are attracting great interest as new materials for applications in, e.g., solar cells, light-emitting diodes (LEDs), lasers, displays, sensors, and transistors. Traditional inorganic and novel organic semiconductors are used in very much the same applications, based on their light-tocharge (e.g., solar cell) and charge-to-light (e.g., LED) converting properties, but the underlying mechanisms are very different. Due to their intermediate standing between traditional semiconductors and molecules and their heterogeneous nanostructure, the properties and processes of conjugated polymers are highly complex and in many cases far from being understood. However, a great deal of progress has been achieved during the past several years, as a result of combining new and powerful experimental and theoretical methods. As an example, ultrafast spectroscopy has been widely applied and gives direct access to the time scale of elementary processes such as energy transfer and charge photogeneration. Similarly, single molecule fluorescence spectroscopy is providing an observation window that allows us to see behind the obscuring curtain of molecular heterogeneity. Quantum chemical calculations and modeling of excited state dynamics afford valuable links between observed energetics, dynamics, and microscopic properties. In this account, we will focus on light-polymer interactions and discuss the processes initiated by light absorption in a conjugated polymer material, from the formation of the initially excited state to conversion of the light energy into more longlived excitations. This includes both “useful” conversions into charged species as well as quenching of excited states where the excitation energy is wasted. In this description, we will adopt a sequential view and discuss phenomena in a chronological order as they occur, following light absorption. As the notion organic semiconductor may suggest, and as has in fact been proposed,1,2 light absorption could be described * Corresponding author. E-mail:
[email protected]. † Lund University. ‡ Institute of Physics.
by a semiconductor band picture and thought of as a direct charge generating process, without any excited state intermediates. However, as we will discuss in the following, a large body of experimental and theoretical results show that a molecular picture is more appropriate and that the major primary photoexcitation in conjugated polymers is a neutral strongly bound exciton. Following the formation of an exciton by light absorption, this exciton can migrate along the polymer chain on which it was formed. If many polymer chains are in close contact, like in a solid state film of a neat conjugated polymer, the exciton may as well jump to a neighboring chain. Time resolved fluorescence spectral diffusion and time resolved fluorescence or absorption anisotropy, combined with Monte Carlo simulations, have proven to be particularly powerful tools to study energy transfer on both isolated chains and thin polymer films.3,4 As we will discuss in detail, both intra- and interchain transfer are efficient and proceed on the sub-picosecond to tens (and hundreds) of picoseconds time scale. Downhill energy transfer from high energy segments (chromophores) to segments at lower energy is fastest, ∼1 ps or less, while transfer among segments at the bottom of the density of states can be much slower (∼10100 ps). Interchain transfer in a solid film can be very fast (
