Interchain effects in the ultrafast photophysics of a semiconducting polymer:THztime-domain spectroscopy of thin films and isolated chains in solution

Abstract

We compare the generation and decay dynamics of charges and excitons in a model polymer semiconductor ͑MEH-PPV͒ in solution and drop-cast thin films, by recording the sub-ps transient complex conductivity using THz time-domain spectroscopy. The results show that the quantum efficiency of charge generation is two orders of magnitude smaller in solution ͑ϳ10−5 ͒ than in the solid film ͑ϳ10 −3 ͒. The proximity of neighboring chains in the films apparently facilitates ͑hot͒ exciton dissociation, presumably by allowing the electron and hole to separate on different polymer strands. For both samples, photoexcitation leads to the predominant formation of bound charge pairs ͑excitons͒ that can be detected through their polarizability. Surprisingly, the polarizability per absorbed photon is a factor of 3 larger in solution than in the film, suggesting that interchain interactions in the film do not result in a substantial delocalization of the exciton wave function.

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

“…It should be noted that the general case, where ð! ; Þ varies on a time scale that is shorter than the THz pulse duration, such a simple separation does not apply and the temporal evolution of the THz fields may be quite complex (Kindt and Schmuttenmaer, 1999;Nemec et al, 2002;Hendry et al, 2005;Nemec, F. Kadlec, C. Kadlec et al, 2005;Nemec, Kadlec, Surendran et al, 2005;Nienhuys and Sundstrom, 2005b;Schins, 2010), as different temporal segments of the THz pulses probe different sample properties.…”

“…This is representative of photoexcitaion in a bulk crystalline semiconductor by an ultrafast laser pulse. The transient current, calculated according to Hendry et al (2005), is seen to rise exponentially with a time constant determined by the electron scattering rate. Thus, the time evolution of the current has a signature of the type of photospecies, and the transport mechanisms and population dynamics in the material of study.…”

“…Frequently, the mobility of electrons exceeds that of the holes, and the data can be described by the Drude model with one carrier type. Hence the two key conduction parameters, the plasma frequency and scattering rate, (Beard et al, 2000;Hendry, Wang, Shan et al, 2004;Hendry, Koeberg, Pijpers, and Bonn, 2007;), can be determined by simultaneously fitting both the real and imaginary parts of the frequency dependence of the conductivity.…”

On the mechanism of charge transport in pentacene

“…[5][6][7][8]20 We only consider data recorded after 0.7 ps following photoexcitation since determining the conductivity immediately after photoexcitation ͑when a "jump" in photoconductivity occurs͒ is not straightforward. 24 In Ref. 25, it has been shown for rubrene that optically active, ground state vibrational modes in the terahertz region can give rise to resonances in the conductivity spectrum.…”

“…This means that care must be taken to extract the conductivity at short delay times. 24 We follow the procedure described in Ref. 24, which involves reading out the modulation of every point of the terahertz field at constant time after the photogeneration of charge carriers, i.e., ⌬E trans ͑t , Ј͒ for constant values of Ј= + t. Typical shapes of transmission and modulation traces are shown in Fig.…”