In this study, we place a strong emphasis on understanding the ultrafast dynamics of carrier recombination pathways in p-type ZnO, especially in the midgap region. Synthesizing and controlling the properties of p-type ZnO remains a pivotal yet challenging task for numerous optoelectronic and spintronic applications due to intrinsic midgap (defect) states. Through an advanced sol-gel process, we have successfully produced ZnO quantum dots (QD), eliminating unreacted molecules that decrease the excitonic emission. This refined method supports the generation of ZnO with p-type characteristics, primarily attributed to zinc vacancies in oxygen-rich scenarios. Notably, our analysis across timescales from femtoseconds to microseconds unveiled carrier lifetimes at room temperature, and associated long-lasting carriers with zinc vacancy defects, corroborating the p-type nature of our synthesized ZnO QDs.
We have used a femtosecond-resolved spectroscopic technique based on the Stark effect (electromodulated
differential absorption) in order to investigate free charge generation and charge drift in solar cell devices of
neat conjugated polymer pBTTT and in its 1:1 (by weight) blend with PCBM. In the latter, the fullerene
molecules intercalate between the polymer side-chains, yielding a co-crystal phase. Our results show that free
charge generation in both materials is ultrafast and strongly dependent on the applied reverse bias. Charge
drift to the electrodes (under strong reverse bias) occurs with comparable dynamics on the 1.2 ns time scale
for neat pBTTT and the blend, and is probably dominated by hole transport within/between polymer chains.
The plant pigment betanin is investigated as a dye-sensitizer on TiO2 with regard to its potential to undergo twoelectron
oxidation following one-photon excitation. Electrochemical, spectroelectrochemical and transient absorption
measurements provide evidence for two-electron proton-coupled photo-oxidation leading to a quinone methide intermediate
which rearranges to 2-decarboxy-2,3-dehydrobetanin. Time-resolved spectroscopy measurements of betanin on
nanocrystalline TiO2 and ZrO2 films were performed on femtosecond and nanosecond time-scales and provide evidence
for transient species with absorption bands in the blue and the red. The results shed light on previous reports of high
quantum efficiencies for electron injection and point the way to improved solar conversion efficiency of organic dyesensitized
solar cells.
Organic-inorganic hybrid systems based on lead halide compounds have recently encountered considerable success as
light absorbers in solid-state solar cells. Herein we show how fundamental mechanistic processes in mesoporous oxide
films impregnated with CH3NH3PbI3 can be investigated by time resolved techniques. In particular, charge separation reactions such as electron injection into the titanium dioxide film and hole injection into the hole transporting material spiro-OMeTAD as well as the corresponding charge recombination reactions were scrutinized. Femtosecond transient absorption spectroscopy and time-resolved terahertz spectroscopy were applied to CH3NH3PbI3 deposited either on TiO2
or Al2O3 mesoporous films and infiltrated with the hole transporting material spiro-OMeTAD.
The effect of electronic and nuclear factors on the dynamics of dye-to-semiconductor electron transfer was studied employing RuII(terpy)(NCS)3 sensitizers grafted onto transparent films made of titanium dioxide nanoparticles. Various approaches were strived to understand the dependence of the kinetics of charge injection and recombination processes upon the distance separating the dye molecules and the redox active surface. A series of bridged sensitizers containing p-phenylene spacers of various lengths and phosphonic anchoring groups were adsorbed onto TiO2 films. The kinetics of interfacial charge transfer was recorded by use of time-resolved spectroscopy in the fs-ps domain. The electron injection process was found to be biphasic with a clear exponential distance dependence of the fast kinetic component. The slower part of the kinetics was essentially unaffected by the length of the spacer bridge and was attributed to sensitizer molecules that are weakly bound to the surface with no direct contact of the anchoring group with the semiconductor. In a second approach, the kinetics of both forward- and back-electron transfer across a layer of insulating Al2O3 deposited onto TiO2 nanocrystalline particles was investigated. Efficient charge injection was observed over distances up to 3 nm.
The electron transfer (ET) from organic dye molecules to semiconductor-colloidal systems is characterized by a special energetic situation with a charge transfer reaction from a system of discrete donor levels to a continuum of acceptor states. If these systems show a strong electronic coupling they are amongst the fastest known ET systems with transfer times of less than 10 fs. In the first part a detailed discussion of the direct observation of an ET reaction with a time constant of about 6 fs will be given, with an accompanying argumentation concerning possible artifacts or other interfering signal contributions. In a second part we will try to give a simple picture for the scenario of such superfast ET reactions and one main focus will be the discussion of electronic dephasing and its consequences for the ET reaction. The actual ET process can be understood as a kind of dispersion process of the initially located electron into the colloid representing a real motion of charge density from the alizarin to the colloid.
Alexandre Acovic, Philippe Buffat, Paul Brander, Peter Jacob, Oliver Jeandupeux, Vittorio Marsico, Daniel Rosenfeld, Jacques Moser, Markus Kohli, Roger Fluckiger, Karim Belkacem, Pierre Fazan
A large variety of physical analysis techniques are used in the semiconductor industry to identify defects impacting yield or reliability. Identification of a defect often requires the combined use of several techniques to give a clear understanding of the defect nature. In the present study, several microscopy techniques (SEM, TEM, Analytical-TEM, AFM and FIB) have been intensively used to identify the origin of residues observed on the edge of large active areas in a low power CMOS technology. A KLA automatic inspection system has been used for locating and quantifying the defects. It has been shown that the defects are related to amorphous silicon residues whose origin is related to the gate deposition process. In the process, the polysilicon gate is deposited in two steps. A first thin amorphous silicon layer is deposited, through which the Vt implant is done, followed by the deposition of a thick polysilicon layer. Analysis of defaults showed that the residues are related to a non-uniform thin oxide layer located between the thick polysilicon layer and the underlying thin amorphous silicon, which halts the polysilicon gate etch. Thicker native oxide on amorphous silicon due to humidity or drying spots is the presumed source of the thin non-uniform oxide. Increasing the HF dip before the polysilicon deposition eliminated almost all residues. No negative effect on the oxide quality or other electrical parameter has been observed. Eliminating altogether the amorphous-Si gate deposition process is an even more robust solution.
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