Ultrafast interfacial carrier dynamics

Charge carrier dynamics at interfaces is of fundamental interest for a broad range of applications such as photovoltaics and optoelectronics. The interface between different materials can behave as a charge separating boundary but also control the recombination of electron hole pairs. The impact on the specific application can be completely contrastive and interfacial engineering is often necessary to develop the materials towards their typical designation. Femtosecond real time measurements provide relevant information of the ultrafast processes occurring at interfaces in inorganic or organic compounds and help to enhance the performance of the specific interface when incorporated into a functional device. ZnO is a promising semiconductor material intended for use as a charge transport electrode in low cost nano composite solar cells and may eventually replace the widely used colloidal TiO2 in such devices in the future [1]. Electrodes can be perpared as amp; 956;m thick films consisting of ordered crystalline ZnO nanorods [2] with a mean diameter much larger than their colloidal counterparts and thus allowing for a cross sectional potential field gradient [3]. This and also the absence of grain boundaries in nanorod layers is expected to improve the carrier transport properties in comparison to colloidal semiconductor films. However, ZnO based solar cells have not yet met the long standing efficiency values obtained with dye sensitized solar cells on the basis of TiO2. Charge separation at the heterogeneous dye ZnO interface is still poorly understood and interfacial carrier dynamics at TiO2 and ZnO nmstructured electrodes differ considerably in the femtosecond time window Fig. 1,2 . Perylene was used as a model sensitizer chromophore adsorbed to the semiconductor surface via a propionic or acrylic acid group where the latter acts as a spacer binding group. Fs transient absorption fs TA, Fig. 1 and time resolved two photon photoemission TR 2PPE Fig. 2 spectroscopy was applied for monitoring interfacial charge transfer using 20 fs laser pulses supplied by novel type low power NOPAs operating at 150KHz repetition rate. 40

Electron relaxation dynamics at the In rich 100 surface of InP

The temporal relaxation of optically excited electrons at the In rich reconstructed InP 100 surface was studied using time resolved two photon photoemission spectroscopy TR 2PPE . High energy carriers were generated at laser pump energies chosen to populate hot electron bulk states or the well known C2 surface state via resonant direct optical excitation. The different relaxation pathways arising from these population schemes involve amp; 915; L amp; 915; intervalley scattering and the transient occupation of an additional surface state, C1. The dynamics of these processes were recorded with a novel experimental setup using two ultra low power 150 KHz repetition rate sub 20 fs NOPAs enabling two colour pump probe experiments in the linear regime. These experiments provide useful information in understanding the dynamics of devices on the basis of this semiconductor medium such as solar cells and high speed switching circuit

The ultrafast temporal and spectral characterization of electron injection from perylene derivatives into ZnO and TiO2 colloidal films

The ultrafast injection dynamics, early recombination dynamics, and spectral signatures of four systematically varied dye metal oxide hybrid systems were investigated using transient absorption spectroscopy techniques. First, photoinduced electron transfer from two different perylene derivatives into zinc oxide ZnO colloidal films is reported. Here, the electronic coupling of the perylene chromophore 2,5 Di tert butyl perylene 9 ylpropionic acid 1 to the ZnO colloids was weaker than the electronic coupling of the chromophore 2,5 Ditert butyl perylene 9 yl acrylic acid 2 . Second, the photoinduced electron transfer of the same two molecules attached to TiO2 colloids was measured and compared to the results for the ZnO colloids using the same techniques. The temporal traces at both the excited state and the cationic state of the chromophores attached to the semiconductor surfaces were measured simultaneously and showed very good agreement, which indicated a direct injection into the semiconductor. The overall injection times for the ZnO samples was as short as 190 fs, which suggested a strong electronic coupling element for these systems. This injection time is short compared to reports on similar ZnO hybrid systems, but it is still longer than the injection times reported for the TiO2 hybrid systems. The transient absorption spectra of molecule 2 attached to TiO2 showed a large negative signal at 530 550 nm, which indicated the presence of a direct charge transfer state contribution in this syste