Energy and Charge Transfer Cascade in Methylammonium Lead Bromide Perovskite Nanoparticle Aggregates

Highly photoluminescent hybrid lead halide perovskite nanoparticles have recently attracted a wide interest in the context of high-stake applications, such as light emitting diodes (LEDs), light emitting transistors and lasers. In addition, they constitute ideal model systems to explore energy and charge transport phenomena occurring at the boundaries of nanocrystalline grains forming thin films in high-efficiency perovskite solar cells (PSCs). Here we report a complete photophysical study of CH3NH3PbBr3 perovskite nanoparticles suspended in chlorobenzene and highlight some important interaction properties. Colloidal suspension under study were constituted of dispersed aggregates of quasi-2D platelets of a range of thicknesses, decorated with 3D-like spherical nanoparticles. These types of nanostructures possess different optical properties that afford a handle for probing them individually. The photophysics of the colloidal particles was studied by femtosecond pump-probe spectroscopy and time-correlated single-photon counting. We show here that a cascade of energy and exciton-mediated charge transfer occurs between nanostructures: Upon photoexcitation, localized excitons within one nanostructure can either recombine on a ps timescale, yielding a short-lived emission, or form charge- transfer states (CTSs) across adjacent domains, resulting in longer-lived photoluminescence in the millisecond timescale. Furthermore, CTSs exhibit a clear signature in the form of a strong photoinduced electroabsorption evidenced in femtosecond transient absorption measurements. Charge transfer dynamics at the surface of the nanoparticles have been studied with various quenchers in solution. Efficient hole transfer to N,N,N’,N’-tetrakis(4-methoxyphenyl)benzidine (MeO- TPD) and 1,4-bis(diphenyl-amino)benzene (BDB) donors was attested by the quenching of the nanoparticles emission. The charge transfer rate was limited by the organic layer used to stabilize the nanoparticles, which acted as a wide spacer between reactants. The forward charge transfer was found to take place in the sub-microsecond time-scale in competition with slow carrier recombination, while back transfer was shown to occur with a time-constant t = 25 ms.

Dynamics of nanoscale systems probed by time-resolved STM-induced luminescence

Anna Maria Roslawska

The efficiency and peculiarities of processes such as surface adsorption or electron-to-photon energy conversion in organic and inorganic structures are determined by the dynamics at the scale of individual molecules, atoms and charges. The timescales of such effects reach down to the ps regime, which can be routinely probed using ultrafast optics methods. However, these far-field techniques suffer from ensemble-averaging that obscures the information encoded at the atomic level. These limits can be overcome by combining scanning tunnelling microscopy (STM) with time-resolved light detection so that pm and ps scales can be reached simultaneously. STM-induced luminescence (STML) methodologies are employed in this thesis to explore the nanoscale dynamics of different systems: light emitting defects in C60 thin films, H2 molecules weakly physisorbed on Au(111), metal-metal tunnel junctions and single atom contacts. First, we study the dynamics associated with the radiative decay of single excitons at defect-related emission centres localized in the C60 molecular film. Such emission centres serve as the smallest possible model of an organic light emitting diode where a single electron and hole recombine producing a photon. Using time-resolved STML (TR-STML) excited by voltage pulses, we follow the formation of an individual exciton and map this process in real space with ns resolution. Besides the excitonic emission, on C60 thin films we observe luminescence due to the radiative decay of plasmons. We study the interplay of these two processes and create a nanoscale bimodal source whose emission can be dynamically tuned by controlling the charge injection rate. A kinetic model is employed to thoroughly describe the charge and exciton dynamics in this system. The developed kinetic model can be also used to analyse the motion of hydrogen molecules adsorbed on an Au(111) crystal. The weak interaction with the metal and the continuous charge injection by the tip leads to recurrent excursions of the molecule inside and outside from the tunnelling junction, which chop the plasmonic luminescence intensity. These dynamical changes in the light intensity can be followed by correlation spectroscopy and allow extraction of the molecule's residence and excursion times. In the next step, we probe the dynamics of a generic metal-metal tunnel junction. Interestingly, when a high bias is applied, the emission from the junction exhibits photon bunching on a time scale below 50 ps as evidenced by Hanbury Brown-Twiss interferometry. Such a process is a result of a distribution of the energy of one electron between two photons, which may possibly be entangled. The opposite process may occur as well, under conditions where two electrons interact supplying energy to one photon. This overbias emission is a feature of single atom contacts and is used here to probe the fluctuations of plasmonic enhancement driven by the motion of single atoms in the vicinity of such a contact.

EPFL2019

Nanoparticle sensitisation of solid-state nanocrystalline solar cell

Robert Plass

Over the past fifteen years dye-sensitised nanocrystalline solar cells have been the subject of intense research and development efforts. These systems provide a technically and economically credible alternative to classical p-n junction solar cells, reaching over 10 % certified efficiency under standard solar illumination conditions (AM 1.5, 1000 W/m2). Recently, the liquid electrolyte, commonly used in these dye-sensitised solar cells, could successfully be replaced by a novel solid hole-conducting material (spiro-OMeTAD). The absence of volatile solvents and corrosive components like iodine presents a clear advantage of these solid-state devices over their photoelectrochemical counterparts. Yet, their maximum overall efficiency of about 3 % clearly lacks behind the performance features of classical dye-sensitized solar cells. The objective of this work is the replacement of the usually used sensitiser (transition metal complexes and organic dyes) by quantum-dots. It was motivated by the possibility to achieve panchromatic light absorption due to the size-dependent properties of the quantum-dots. Some studies have been published using quantum-dots of inorganic low-bandgap semiconductors as sensitisers for mesoporous wide bandgap semiconductor electrodes in conjunction with liquid electrolytes. These systems often suffer from corrosion and photo-corrosion mainly due to the aggressive nature of the electrolyte employed. Solid-state organic-inorganic heterojunctions might provide the desired environment for quantum-dot sensitisation, as the environment is much less aggressive. A large variety of inorganic quantum-dot materials like PbS, CdS, PbSe and CdSe were scrutinized for their use as sensitisers. All particles were synthesised in situ on the TiO2 surface using two different techniques: dip-coating and chemical bath deposition. Lead sulphide was the most investigated due to its superior photovoltaic characteristics. High Resolution Transmission Electron Microscopy (HRTEM) of PbS sensitised TiO2 electrodes fabricated via dip-coating clearly showed the distinct particles on the surface of the semiconductor. The electron transfer dynamics following optical excitation of quantum-dot sensitized TiO2/spiro-OMeTAD heterojunctions were thoroughly studied. Fluorescence spectroscopy was used to prove the possible electron injection from the PbS into the TiO2. Nanosecond laser spectroscopy was applied to monitor the interfacial recombination of the injected electron and the oxidised hole-conductor. Femtosecond laser spectroscopy allowed measuring the ultra-fast kinetics in the system, including electron trapping, exciton recombination and PbS regeneration. An overall efficiency of 0.5 % at 0.1 sun (AM 1.5) has been reached with such systems. Chemical bath deposition (CBD) of PbS leads to the formation of a layer rather than distinct particles on the TiO2. Solar cells fabricated via CBD exhibited open-circuit voltages which were generally 100 mV higher then those of comparable devices made via dip-coating. The PbS layer acts as blocking layer, hindering the contact between TiO2 and the hole-conductor. This technique allowed achieving devices highly linear with respect to the illumination power. Different strategies were pursued to impede interfacial recombination, among which the introduction of self-assembled monolayers at the interface between the organic and inorganic phases was proven to be the most efficient. Nanosecond laser spectroscopy showed a strong decrease of the interfacial recombination kinetic rates in the presence of hexadecylmalonic acid or decylphosphonic acid monolayers. Short-circuit currents could be raised by a factor two to four using such type of molecules. The overall efficiency of the device could be strongly increased, reaching 1 % at 0.1 Sun (AM 1.5). CBD was also used to deposit PbSe and CdSe layers at the surface of TiO2. Fluorescence spectroscopy was used to probe electron injection from CdSe quantum dots to TiO2. Generally it was observed that metal sulfides were more efficient in sensitizing TiO2/OMeTAD heterojunctions then the respective selenides.

EPFL2004

Light Generation and Harvesting in a van der Waals Heterostructure

Esther Alarcon Llado, Andras Kis, Volodymyr Koman, Oriol López Sánchez, Aleksandra Radenovic

Two-dimensional (2D) materials are a new type of materials under intense study because of their interesting physical properties and wide range of potential applications from nanoelectronics to sensing and photonics. Monolayers of semiconducting transition metal dichalcogenides MoS2 or WSe2 have been proposed as promising channel materials for field-effect transistors. Their high mechanical flexibility, stability, and quality coupled with potentially inexpensive production methods offer potential advantages compared to organic and crystalline bulk semiconductors. Due to quantum mechanical confinement, the band gap in monolayer MoS2 is direct in nature, leading to a strong interaction with light that can be exploited for building phototransistors and ultrasensitive photodetectors. Here, we report on the realization of light-emitting diodes based on vertical heterojunctions composed of n-type monolayer MoS2 and p-type silicon. Careful interface engineering allows us to realize diodes showing rectification and light emission from the entire surface of the heterojunction. Electroluminescence spectra show clear signs of direct excitons related to the optical transitions between the conduction and valence bands. Our p-n diodes can also operate as solar cells, with typical external quantum efficiency exceeding 4%. Our work opens up the way to more sophisticated optoelectronic devices such as lasers and heterostructure solar cells based on hybrids of 2D semiconductors and silicon.