Seminars Archive

Exciton dynamics in hybrid organic-inorganic semiconducting systems

Abstract
Hybrid organic-inorganic semiconducting systems promise to lead to a new generation of light-harvesting and optoelectronic devices that combine strong light matter coupling and wide tunability with high carrier mobilities and charge injection (or ejection) efficiency. Furthermore, a careful choice of the two compounds that optimizes the coupling of the localized molecular excitons with the delocalized excitations in the inorganic, could lead to novel physical properties and device functionality that go beyond the mere combination of each material’s attributes. The efficiency of hybrid devices relies on charge or energy transfer occurring at the interface before a significant amount of electronic and vibrational excess energy is lost in competing processes such as vibrational relaxation, (radiative) exciton recombination, bulk diffusion, etc. The relative balance of these processes’ time scales is thus a fundamental aspect in the design of such heterojunctions. We investigate the exciton relaxation dynamics in the hybrid SP6 (2,7-bis(biphenyl-4-yl)-2’,7’- ditertbutyl-9.9’-spirobifluorene)–ZnO(10-10) system by means of linear and non-linear optical pump-probe techniques based on a compressed (' 20 fs) white-light continuum as a probe pulse. Time resolved excited state transmission (tr-EST) identifies two excited states of the dye with lifetimes on the order of the luminescence timescale [1]. While both states decay via luminescence and intersystem crossing, only one is affected by charge transfer to the ZnO. The comparison of luminescence spectra on ZnO and on an inert substrate exhibits contributions from both excited states and reveals that both states share a common ground state. This finding indicates that exciton-vibron coupling plays a relevant role in determining the efficiency of charge transfer at this interface. In order to specifically address the processes occurring at organic inorganic hybrid interface we also developed a setup for time resolved electronic resonant sum frequency generation (tr-ESFG), by using the white light continuum as resonant field. As a second order non-linear optical effect, SFG occurs only in systems without inversion symmetry and can thus lead to interface specificity. Further, it is enhanced if one of the electric fields matches an electronic transition in the system. As a proof of principle we apply the technique to the investigation of non-centrosymmetric ZnO around the optical band gap. To our knowledge it is the first time tr-ESFG is successfully used to study the photoinduced dynamics in solid state systems. The spectra clearly reveal resonances from the free excitonic states connected to the three split ZnO valence bands. All the resonances show dynamics related to the relaxation of hot carriers to the band edges on a ps time-scale. Furthermore, we associated the increase of transient signal observed on 200 ps to the formation of bound excitons. [1] S. Blumstengel et al., Phys. Rev. B 77, 085323 (2008).