Ultrafast spectroscopy; Time-resolved THz spectroscopy; Perovskite solar cells; Photoinduced charge transfer; Charge carrier dynamics; Donor-acceptor heterojunctions; Semiconductors under illumination; Dye-sensitized solar cells; Small-molecules organic photovoltaics
Huber Robert, Dworak Lars, Moser Jacques-E., Grätzel M., Wachtveitl J. (2016), Beyond Vibrationally Mediated Electron Transfer: Coherent Phenomena Induced by Ultrafast Charge Separation, in The Journal of Physical Chemistry C
Paraecattil Arun, De Jonghe-Risse Jelissa, Pranculis Vytenis, Teuscher Joël, Moser Jacques-E. (2016), Dynamics of photocarrier separation in MAPbI3 perovskite multigrain films under a quasistatic electric field, in The Journal of Physical Chemistry C
Causa' Martina, De Jonghe-Risse Jelissa, Scarongella Mariateresa, Brauer Jan C., Buchaca-Domingo Ester, Moser Jacques-E., Stingelin Natalie, Banerji Natalie (2016), The fate of electron-hole pairs in polymer:fullerene blends for organic photovoltaics, in Nature Communications
, (12556), 1-10.
Devižis Andrius, De Jonghe-Risse Jelissa, Hany Roland, Nüesch Frank, Jenatsch Sandra, Gulbinas Vidmantas, Moser Jacques-E. (2015), Dissociation of charge transfer states and carriers separation in bilayer organic solar cells - A time-resolved electroabsorption spectroscopy study, in Journal of the American Chemical Society
, 137, 8192-8198.
Brauer Jan C., Marchioro Arianna, Paraecattil Arun A., Oskouei Ahmad A., Moser Jacques-E. (2015), Dynamics of Interfacial Charge Transfer States and Carriers Separation in Dye-Sensitized Solar Cells: A Time-Resolved Terahertz Spectroscopy Study, in Journal of Physical Chemistry C
, 119, 26266-26274.
Knorr Fritz J., McHale Jeanne L., Clark Aurora E., Marchioro A., Moser Jacques-E. (2015), Dynamics of Interfacial Electron Transfer from Betanin to Nanocrystalline TiO2: The Pursuit of Two-Electron Injection, in The Journal of Physical Chemistry C
De Jonghe-Risse Jelissa, Heier Jakob, Nüesch Frank, Moser Jacques-E. (2015), Ultrafast charge transfer in solid-state films of pristine cyanine borate and blends with fullerene, in J. Mater. Chem. A
Devižis Andrius, Hertel Dietrich, Meerholtz Klaus, Gulbinas Vidmantas, Moser Jacques-E. (2014), Time-independent, high electron mobility in thin PC61BM films: Relevance to organic photovoltaics, in Organic Electronics
, 15(12), 3729-3734.
The present proposal aims to investigate salient features and details of photoinduced interfacial charge transfer processes in small molecule-based organic, perovskite-based hybrid organic-inorganic, and dye-sensitized solar cells. These systems belong to a new generation of non-conventional photovoltaic converters making use of ultrafast carrier injection at donor-acceptor interfaces and diffusion in selective transporter materials to separate charges. Their working principle, thus, distinguishes markedly from conventional p-n junction devices relying on the migration of carriers in an electric field.Excitons are quasi-particles that form when Coulomb-interacting electrons and holes in semiconductors are bound into pair states. Excitonic signatures commonly appear in the optical absorption and emission of direct gap semiconductor systems and molecular absorbers. The reduced dielectric constant characterizing organic materials at the base of third generation photovoltaics also allows electrostatic interactions between charge carriers separated at a junction and the formation of donor-acceptor charge transfer states (CTS) or charge transfer excitons. Fast geminate recombination of carriers in these interfacial CTS is believed to be one of the main loss processes in this type of devices and to control in a large extend the power conversion efficiency. The precise properties of incoherent exciton populations are difficult to determine and are still the subject of intense debate. Here we propose to supplement conventional experimental techniques by direct quasi-particle spectroscopy using the relatively newly available terahertz light sources. A combination of state of the art ultrafast transient vis-NIR absorption- and time-resolved terahertz spectroscopies will be applied to elucidate the fundamental factors that govern the dynamics of excitons and charge transfer separation processes at donor-acceptor heterojunctions. Time-resolved THz (optical-pump THz-probe, OPTP) spectroscopy is a powerful tool to temporally resolve phenomena at the timescale of nuclear and electronic motion. This method has been selected as it can measure any change in the complex conductivity of a sample induced by a pump laser pulse in the fs-ps time range. The conductivity of a sample is directly proportional to the population of charge carriers and to their intrinsic mobility. Combined with ultrafast optical spectroscopy, which can provide the necessary information regarding carriers’ transient population, terahertz spectroscopy then constitutes an ideal experimental tool to probe mobility changes occurring during charge trapping, selective electron or hole injection at heterojunctions, association of opposite charge carriers in CTS, and charge separation and recombination processes.The exact mechanism by which electrons and holes overcome Coulomb trapping to yield free carriers is still unsolved, but increasing evidence points to the critical role of hot charge transfer states in assisting this process. Within the proposed study, we will provide a real-time view of hot CTS formation and relaxation using a novel approach. We will build a time-resolved two photons optical pump-terahertz probe spectroscopy (PPTPS) setup to scrutinize excitons and CTS dynamics in various systems and conditions. In a typical pump-push-THz probe experiment, a first fs pulse of visible light (h?1) will excite the absorber material to yield excitons or CTS at an interface. After a controlled time delay, a second ultrashort pulse in the NIR or IR spectral range (h?2) will excite the bound electron-hole pairs to produce hot quasi-particles, which will eventually relax or split, yielding mobile charges, which will be readily detected by THz absorption. Tuning h?2 will provide direct information on exciton and CTS binding energy and the involvement of out-of-equilibrium vibrations in the carriers’ generation process.PPTPS will be applied to model systems constituted of dye-sensitized mesoscopic TiO2, cyanine and merocyanine |C60 bilayers, and hybrid organic-inorganic perovskite-based solid-state photovoltaic systems. In the first case, various RuII-complexes and organic dye molecules will be used, that are characterized by different energy levels of their lowest unoccupied molecular orbitals (LUMO) and different distances separating the surface of the oxide semiconductor from the molecular moiety carrying the highest occupied molecular orbital (HOMO). The position of the LUMO level will define the excess energy of electrons injected in the conduction band of the oxide. While the geometry of the molecule on the surface is expected to influence the distance at which injected electrons and holes localized in the oxidized dye molecule will interact and the strength of the Coulomb attraction. In the case of solar cells based on a liquid junction, the dielectric constant and viscosity of the solvent could influence the rate at which dipole reorganization and, concomitantly, interfacial charge screening can take place. The presence of adsorbed small cations at the interface, be it liquid or solid, is also expected to have a strong effect on the dynamics of CTS by affecting electron trapping on the surface.Our current understanding of photovoltaic conversion in model OPV systems, such as cyanine|C60 bilayers, is that ultrafast exciton splitting at the donor-acceptor junction results in the generation of both interfacial charge transfer states and mobile polaron species. Upon high-energy excitation, hot vibronic states of the absorber material should convert into hot interfacial CTS that effectively contribute to free carrier generation. Application of PPTPS time-resolved spectroscopy on these systems will provide an additional insight on the dynamics of CTS and the intimate mechanism of charge separation. The influence of the cyanine counter-ions and other doping ionic species able to migrate in the materials and adsorb at the heterojunction will also be scrutinized.Hybrid organic-inorganic CH3NH3PbI3 perovskite-based solar cells are novel photovoltaic systems. Efficient devices are built by depositing the perovskite light-absorber on an acceptor material (ETM) and by covering it on the other side by an organic hole-transporter material (HTM). Charge separation in this system is expected to take place simultaneously at the two heterojunctions ETM | perovskite | HTM. Fast experimental developments have occurred so far without a sound knowledge of the mechanisms of fundamental processes governing their functioning. By application of ultrafast time-resolved optical spectroscopy and PPTPS measurements, we aim at obtaining conclusive experimental results, allowing to assess exciton and charge separation dynamics in various versions of perovskite-based devices described to date.