Understanding the photoinduced ultrafast charge transfer (CT) dynamics across the donor/acceptor interface is a prerequisite for optimizing the performance of organic photovoltaic devices. Time-resolved second harmonic generation, an interface-sensitive probe with femtosecond temporal resolution, is applied to investigate the well-defined single heterojunction C 60 /P3HT. The de-excitation of hot singlet excitons in the conduction bands of the polymer into localized excitonic states is observed. In the presence of the electron acceptor, the ultrafast population of a CT state is identified as the dominating relaxation channel. Interestingly, the charge transfer yield correlates with the excitation wavelength and rises with increasing excess energy.
Perylene-based organic semiconductors are widely used in organic electronic devices. Here, we studied the ultrafast excited-state dynamics in diindenoperylene (DIP) and dicyanoperylene-bis(dicarboximide) (PDIR-CN 2 ) thin films, respectively, after optical excitation using femtosecond (fs) time-resolved second harmonic generation in combination with large scale quantum chemical calculations. In DIP, the initial optical excitation leads to the formation of delocalized excitons, which localize on dimers on a ultrafast time scale of <50−150 fs depending on the excitation energy. In contrast, in PDIR-CN 2 , the optical excitation directly generates localized excitons on monomers or dimers. In both DIP and PDIR-CN 2 , localized excitons decay within hundreds of fs into Frenkel-like trap sites. The relaxation to the ground state occurs in DIP on a time scale of 600 ± 110 ps. In PDIR-CN 2 , this relaxation time is 1 order of magnitude faster (62 ± 1.8 ps). The differences in the exciton formation and decay dynamics in DIP and PDIR-CN 2 are attributed to differences in the aggregation as well as to the respective structural and energetic disorder within the materials. Our study provides important insights into the exciton formation and decay dynamics in perylene-based organic compounds, which is essential for the understanding of the photophysics of these molecules in thin films.
Diindenoperylene (DIP) is an interesting organic molecular semiconductor as a component in organic solar cells. Here, we studied the ultrafast excited state dynamics of DIP after optical excitation in thin films on sapphire and SiO2 substrates, respectively, using femtosecond (fs) time-resolved second harmonic generation (TR-SHG). Sapphire, an inert and noninteracting substrate, is known to deliver no noteworthily contribution to the SHG-signal; thus, the pure response of DIP to the electronic excitation can be resolved in contrast to the measurements on SiO2. For DIP/sapphire, the initial optical excitation leads to the generation of delocalized excitons, which localize within approximately 100 fs in order to generate singlet molecular excitons (Frenkel excitons) or excitons localized on dimers. These excitons have a lifetime of 470 ± 100 fs. In a subsequent step, they form excimer states, which decay on a time scale of 680 ± 110 ps. For DIP/SiO2, the molecular excitons decay on a faster time scale of 210 ± 95 fs and populate either substrate-mediated trap states or excimer states. The present study provides important insights in the excited states dynamics of DIP on the so far unresolved ultrashort time scale, essential for understanding the photophysics of DIP.
Combining photochromism and nonlinear optical (NLO) properties of molecular switches-functionalized self-assembled monolayers (SAMs) represents a promising concept toward novel photonic and optoelectronic devices. Using second harmonic generation, density functional theory, and correlated wave function methods, we studied the switching abilities as well as the NLO contrasts between different molecular states of various fulgimidecontaining SAMs on Si(111). Controlled variations of the linker systems as well as of the fulgimides enabled us to demonstrate very efficient reversible photoinduced ring-opening/closure reactions between the open and closed forms of the fulgimides. Thus, effective cross sections on the order of 10 −18 cm −2 are observed. Moreover, the reversible switching is accompanied by pronounced NLO contrasts up to 32%. Further molecular engineering of the photochromic switches and the linker systems may even increase the NLO contrast upon switching.
A synchronously pumped passive ring resonator, containing an optical Gber as a nonlinear element, is studied experimentally and numerically. Period doubling cascades up to period 32 and chaos are observed in the sequence of pulse energies emerging from the resonator. We provide evidence that beyond this instability, individual pulses encounter an instability of their temporal profile. Pulse shapes develop a substructure that may be stationary, periodic, or chaotic. The full problem thus actually involves formation of a spatiotemporal structure. In contrast to many other spatiotemporal instabilities studied in optics, we deal here with strictly one-dimensional, longitudinal spatial structure.PACS number(s): 42.50. Ne, 42.65.Re, 42.50.Rh
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