The research of the Moser Group concentrates on the preparation and characterization of donor-acceptor heterojunction photovoltaic systems and aims at the understanding of the mechanisms and dynamics of light-induced and relaxation processes at the basis of their functioning.
Systems currently under investigation encompass dye-sensitized nanodispersed metal oxides, lead halide perovskite nanoparticles and thin films of various composition and dimensionality, and small molecules-based organic semiconductors.
Ultrafast time-resolved techniques are mainly used to scrutinize the dynamics of fundamental phenomena, which prevail in nanostructured materials and at their interfaces. Identification of reaction intermediates and quantification of the kinetics of the numerous steps involved in photoinduced charge separation are achieved by a combination of ultrafast transient absorption, fluorescence up-conversion, and time-resolved THz spectroscopies.
Specific investigation methods, such as ultrafast diffuse reflectance, time-resolved electroabsorption, ultra-broadband THz, and optical pump/IR push/THz probe spectroscopies are developed in our lab to address important scientific questions related to photonic materials and systems close to working conditions.
Scientific questions and experimental approaches
Dynamics of light-induced interfacial electron transfer
Photoinduced electron transfer (PET) at molecular-bulk interfaces and donor-acceptor materials heterojunctions are pertinent to a variety of photovoltaic, photocatalytic and photoelectochemical systems. With the help of modern time-resolved spectroscopic methods, our research expands from studying molecular donor/acceptor conjugates over charge transport through molecular wires to PET processes in novel organic and inorganic-organic hybrid solar cell devices.
Solid-state donor-acceptor heterojunction solar cells generally rely on a molecular or semiconductor light-absorber sandwiched between two specific contacts. Phenomena taking place at the interface with electron- and hole-transport materials are rather complex. As these solar energy conversion systems rely on the kinetic competition between fast carrier relaxation, separation and transport and interfacial electron transfer, a thorough understanding of the dynamics of the latter processes is crucial for the optimization of photovoltaic cells performances.
Early carrier and quasiparticle dynamics in lead halide perovskites
Time-resolved terahertz spectroscopy (TRTS) is a method of choice to study the carrier dynamics in semiconductors and at heterojunctions that are central to optoelectronic systems. The technique allows in particular to probe the dynamics of the complex conductivity of a material. Combined with transient absorption spectroscopy, able to provide a direct measurement of the carrier density, TRTS affords a handle for monitoring the time-evolution of charge mobility and to distinguish between free carriers, excitons, interfacial charge transfer states, polarons, and trapped electrons and holes.
Lead halide perovskite thin films of various compositions and dimensionalities coated on quartz or polymer substrates are ideally investigated by TRTS. The photoconductivity of the active semiconductor layer arises almost exclusively from free photocarriers. The effect of the excitation wavelength upon the transient THz absorption amplitude and spectrum allows evidencing the formation of excitons, polarons, and bipolarons during the first picoseconds following pulsed excitation of the perovskite material.
Electric field dependence of carrier association and transport
A number of fundamental aspects of the photophysics of OPV and perovskite solar cells remains to be understood. Open questions in particular are concerned with the mechanism of charge generation from optically created charge transfer states and excitons and transport within the active layer under an external electric field. The electromodulated ultrafast differential absorption technique (EDA) is a method designed to optically probe electric field-induced changes of transient absorption (Stark and Franz–Keldysh–Aspnes effects). The method allows for measuring carrier and quasiparticle dynamics and mobility on a wide range of distances and is complementary to time-resolved THz spectroscopy.
Light absorption by charge-transfer (CT) excitons, occurring from the Coulomb interactions of an electron and a hole separated by an interface, for instance, are characterized by a change in the permanent dipole moment and, hence, induce an electric field perturbation to the surrounding materials. This perturbation is at the origin of a photoinduced electrobsorption (Stark effect), whose spectro-temporal evolution is exploited to probe CT exciton dynamics.
Small molecules-based organic photovoltaic systems
In the broad field of organic photovoltaics (OPV), small soluble molecules are gaining increasing interest. Compared to polymers, small molecules offer a large panel of advantages. In particular, dye molecules, such as cyanines, squaraines, or DPP, are characterized by a large extinction coefficient, enabling efficient harvesting of the incident light in thin, submicron solid films. This decisive advantage allows for building solar cells with a simple planar bilayer architecture and for throwing off the shackles of donor-acceptor bulk heterojunction morphology control inherent to polymer-based systems.
Ultrafast charge transfer at donor-acceptor heterojunctions in OPV systems is monitored by means of fs transient absorption spectroscopy. Light induced interfacial charge transfer typically leads to the formation of charge transfer states (or CT excitons) formed by electrostatically-bound electron-hole pairs astride the material junction. The EDA technique (see here above) allows for directly monitoring the dynamics of free electron formation during the dissociation of interfacial CT states. Motions of electrons and holes is scrutinized separately by selectively probing the Stark shift dynamics at selected wavelengths, thus enabling a spatio-temporal analysis of charge and quasiparticle dynamics in planar bilayer OPV cells.
Symmetry-breaking photoinduced charge separation
Symmetry breaking photoinduced charge separation is a process in which light absorption by a molecule or a material leads to the separation of charges across a region of broken symmetry. This process is important in many natural and artificial systems, including photosynthesis and solar cells.
When a molecule or a material absorbs light, it undergoes an electronic excitation, which generates an excited state. In some cases, the excited state has a different symmetry than the ground state, which can lead to a redistribution of charges and a breaking of symmetry. This breaking of symmetry creates a spatial separation of positive and negative charges, leading to the formation of a charge-separated state.
The process of symmetry breaking photoinduced charge separation can be enhanced by several factors, including the presence of electron-donating and electron-accepting groups in the molecule or material, an electric or magnetic field, as well as the orientation and ordering of the molecules in a material.
Time-resolved THz spectroscopy (TRTS)
The exact mechanism by which electrons and holes overcome trapping and fast recombination to yield free carriers in materials at the base of OPV and perovskite devices is still debated. Increasing evidence points to the critical role of large polarons and hot charge transfer excitons in assisting this process. The precise properties of incoherent excitonic and polaronic species, as well as trapped carrier populations are at the focus of current research efforts of the Moser group.
Conventional experimental techniques, such as ultrafast transient absorption and broadband fluorescence up-conversion, are supplemented by direct quasi-particle probing using a combination of time-resolved electroabsorption and ultra-broadband time-resolved terahertz spectroscopies. EDA and TREAS will be used to investigate the electron and hole drift mobilities in organic photovoltaic devices and perovskite solar cells and provide the necessary insight into charge trapping and detrapping processes and the possible implication of CTEs at grain boundaries. TRTS will allow to distinguish between free carriers and excitonic species involved in the various recombination pathways. This technique will also enable the identification of phonons involved in indirect transitions and in the formation of polarons through the direct monitoring of the time-evolution of their resonant bands.
Pump-push-probe and pump-dump-probe spectroscopy techniques have been used to investigate the properties of excitons and charge-transfer states of conjugated polymers. A real-time view of bound charge states formation and relaxation is provided for dye- sensitized, small molecule-based OPV, and perovskite solar cells systems using a comparable approach. A time-resolved optical pump-IR push-terahertz probe spectroscopy (PPTPS) scheme is employed to scrutinize excitons, CTE, polaron, and trapped carrier dynamics in various systems and conditions and monitor directly the time evolution of their binding energy.