Ultrafast Snapshots: Understanding the photoinduced charge transfer in donor–(bridge)–acceptor systems

Photoinduced charge transfer (CT) is a fundamental photophysical process that prevails in nature but also plays a pivotal role in various technological fields. This thesis describes spectroscopic studies on photoinduced CT in donor-bridge-acceptor (DBA) systems and their derivatives. Throughout this work, we employ femtosecond transient...

Traps in the spotlight: How traps affect the charge carrier dynamics in Cs₂AgBiBr₆ perovskite

Suitable optoelectronic properties of lead halide perovskites make these materials interesting semiconductors for many applications. Toxic lead can be substituted by combining monovalent and trivalent cations, such as in Cs₂AgBiBr₆. However, efficiencies of Cs₂AgBiBr₆-based photovoltaics are still...

Zero-Threshold Optical Gain in Electrochemically Doped Nanoplatelets and the Physics behind It

Colloidal nanoplatelets (NPLs) are promising materials for lasing applications. The properties are usually discussed in the framework of 2D materials, where strong excitonic effects dominate the optical properties near the band edge. At the same time, NPLs have finite lateral dimensions such that NPLs are not true extended 2D structures. Here...

Triplet Dynamics in Crystalline Perylene Diimides

Conjugated organic materials are interesting for application in opto-electronic devices where they can act as a light absorbing layer, for instance in solar cells or as a light emitting layer in light-emitting diodes. In addition, they can also be used as the semiconducting materials in for instance field-effect transistors. Conjugated organic...

Effect of Structural Defects and Impurities on the Excited State Dynamics of 2D BA₂PbI₄ Perovskite

In this work, we show that the quality of the precursor and the thin film preparation strongly affect the optoelectronic properties of the 2D perovskite BA₂PbI₄. 2D perovskites with alkylammonium organic cations such as butylammonium (BA) are relatively soft structures that exhibit large dynamic disorder and phase...

Quantitative Electrochemical Control over Optical Gain in Quantum-Dot Solids

Solution-processed quantum dot (QD) lasers are one of the holy grails of nanoscience. They are not yet commercialized because the lasing threshold is too high: one needs >1 exciton per QD, which is difficult to achieve because of fast nonradiative Auger recombination. The threshold can, however, be reduced by electronic doping of the QDs,...

Spectroscopic Evidence for the Contribution of Holes to the Bleach of Cd-Chalcogenide Quantum Dots

In transient absorption (TA) measurements on Cd-chalcogenide quantum dots (QDs), the presence of a band-edge (BE) bleach signal is commonly attributed entirely to conduction-band electrons in the 1S(e) state, neglecting contributions from BE holes. While this has been the accepted view for more than 20 years, and has often been used to...

Carrier multiplication and cooling in semiconductor quantum dots

In semiconductor quantum dots (QDs), charge carrier cooling is in direct competition with carrier multiplication (CM), a process in which one absorbed photon excites two or more electrons that may improve the light conversion efficiency of photovoltaic devices. CM by an initially hot charge carrier occurs in competition with cooling, with the...

Asymmetric Optical Transitions Determine the Onset of Carrier Multiplication in Lead Chalcogenide Quantum Confined and Bulk Crystals

Carrier multiplication is a process in which one absorbed photon excites two or more electrons. This is of great promise to increase the efficiency of photovoltaic devices. Until now, the factors that determine the onset energy of carrier multiplication have not been convincingly explained. We show experimentally that the onset of carrier...

Broadband Cooling Spectra of Hot Electrons and Holes in PbSe Quantum Dots

Understanding cooling of hot charge carriers in semiconductor quantum dots (QDs) is of fundamental interest and useful to enhance the performance of QDs in photovoltaics. We study electron and hole cooling dynamics in PbSe QDs up to high energies where carrier multiplication occurs. We characterize distinct cooling steps of hot electrons and...

Mechanism and Dynamics of Charge Transfer in Donor-Bridge-Acceptor Systems

Photoinduced charge transfer in organic materials is a fundamental process in various biological and technological areas. Donor-bridge-acceptor (DBA) molecules are used as model systems in numerous theoretical and experimental work to systematically study and unravel the underlying mechanisms of charge transfer. Despite the respectable successes...

All-optical wavelength conversion by picosecond burst absorption in colloidal PbS quantum dots

All-optical approaches to change the wavelength of a data signal are considered more energy-and cost-effective than current wavelength conversion schemes that rely on back and forth switching between the electrical and optical domains. However, the lack of cost-effective materials with sufficiently adequate optoelectronic properties hampers...

Charge Injection, Charge Trapping and Charge Transfer in Quantum-Dot Solids

This study reports on fundamental processes in Quantum-Dot Solids, after light absorption. Transient Absorption and Time-resolved Photoluminescence spectrocopy reveal the dynamics of charge transfer and charge trapping processes. Typically, both occur on a picosecond time scale and compete with each other. We find that the efficiency of these...