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Time-resolved THz spectroscopy of photoinduced charge separation at donor-acceptor heterojunctions

English title Time-resolved THz spectroscopy of photoinduced charge separation at donor-acceptor heterojunctions
Applicant Moser Jacques-Edouard
Number 157215
Funding scheme Project funding (Div. I-III)
Research institution Institut des sciences et ingénierie chimiques EPFL - SB - ISIC
Institution of higher education EPF Lausanne - EPFL
Main discipline Physical Chemistry
Start/End 01.10.2014 - 30.09.2016
Approved amount 158'230.00
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All Disciplines (3)

Discipline
Physical Chemistry
Other disciplines of Engineering Sciences
Condensed Matter Physics

Keywords (9)

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

Lay Summary (French)

Lead
Les panneaux solaires au silicium cristallin dominent actuellement le marché du photovoltaïque. Plusieurs technologies alternatives de piles solaires de troisième génération, moins onéreuses et dont la fabrication requière beaucoup moins d’énergie, sont activement explorées. Parmi ces technologies, on trouve les piles solaires à colorant, les systèmes photovoltaïques organiques et les piles hybrides, dites « perovskite, aux performances particulièrement prometteuses. Le principe de fonctionnement commun de ces trois types de piles solaires repose sur un processus de séparation de charges induit par la lumière à l’interface entre deux matériaux aux propriétés électroniques opposées. Le mécanisme exact par lequel des charges opposées parviennent à vaincre les forces d’attraction électrostatiques et à se séparer en diffusant en sens contraires est encore mal compris, empêchant l'accomplissement de progrès décisifs dans le développement de tels systèmes simples et efficaces.
Lay summary

Spectroscopie THz résolue en temps de la séparation de charges induite par la lumière aux hétéro-jonctions donneur-accepteur

Ce projet de recherche a pour objectif d’étudier les principales caractéristiques et les détails des mécanismes de transfert de charge induits par la lumière dans plusieurs types de piles solaires appartenant à une nouvelle génération de convertisseurs photovoltaïques non-conventionnels. Les charges y sont séparées par injection ultra-rapide à travers une hétérojonction donneur-accepteur et diffusion dans des matériaux sélectifs. Leur principe de fonctionnement se distingue nettement de celui des cellules solaires conventionnelles basées sur une jonction p-n et la migration des porteurs de charge dans un champ électrique. 

La spectroscopie térahertz (THz) résolue en temps utilise un domaine de radiations dont les longueurs d’onde sont comprises entre celles de la lumière infrarouge et celles des micro-ondes. Elle constitue un outil particulièrement puissant, capable de mesurer la variation de la conductibilité complexe d’un milieu avec une résolution temporelle d’un dixième de picoseconde, à l’échelle des mouvements des électrons et des noyaux atomiques dans la matière. Une version améliorée de cette technique sera utilisée pour élucider les facteurs fondamentaux gouvernant la dynamique des porteurs de charge générés par la lumière, de leur association en paires excitoniques, de leur piégeage, et de leur séparation à la jonction et à travers l’ensemble du système donneur-accepteur.

Les résultats devraient pouvoir être utilisés directement dans le développement de cellules solaires photovoltaïques de haut rendement et participer ainsi à l’effort nécessaire au niveau mondial vers une transition énergétique.

Direct link to Lay Summary Last update: 13.10.2014

Responsible applicant and co-applicants

Employees

Publications

Publication
Beyond Vibrationally Mediated Electron Transfer: Coherent Phenomena Induced by Ultrafast Charge Separation
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, 8534-8539.
Dynamics of photocarrier separation in MAPbI3 perovskite multigrain films under a quasistatic electric field
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, 19595-19602.
The fate of electron-hole pairs in polymer:fullerene blends for organic photovoltaics
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.
Dissociation of charge transfer states and carriers separation in bilayer organic solar cells - A time-resolved electroabsorption spectroscopy study
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.
Dynamics of Interfacial Charge Transfer States and Carriers Separation in Dye-Sensitized Solar Cells: A Time-Resolved Terahertz Spectroscopy Study
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.
Dynamics of Interfacial Electron Transfer from Betanin to Nanocrystalline TiO2: The Pursuit of Two-Electron Injection
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, 19030-19041.
Ultrafast charge transfer in solid-state films of pristine cyanine borate and blends with fullerene
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, 10935-10941.
Time-independent, high electron mobility in thin PC61BM films: Relevance to organic photovoltaics
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.

Collaboration

Group / person Country
Types of collaboration
Prof. Natalie Banerji, Université de Fribourg Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Exchange of personnel
Prof. Frank A. Nüesch, Laboratory for functional polymers, EMPA Switzerland (Europe)
- Publication
- Research Infrastructure
Prof. Jeanne L. McHale, Washington State University United States of America (North America)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Research Infrastructure
- Exchange of personnel
Prof. Anders Hagfeldt, EPF Lausanne Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Research Infrastructure
Prof. Vidmantas Gulbinas, Center for Physical Sciences and Technology Vilnius Lithuania (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Exchange of personnel

Scientific events

Active participation

Title Type of contribution Title of article or contribution Date Place Persons involved
Fall Meeting of the Swiss Chemical Society Poster Electro-modulated differential absorption spectroscopy: Investigating charge carrier dynamics under applied electric fields 15.09.2016 Zürich, Switzerland Paraecattil Arun Aby;
Swiss Symposium and Summer School 2016: Solar Light to Chemical Energy Conversion Talk given at a conference Dynamics of photoinduced charge separation in solar energy conversion systems 29.08.2016 Les Diablerets, Switzerland Moser Jacques-Edouard;
Summer School "Perspective on photonics: Investigating the route from materials to devices" Talk given at a conference Ultrafast Spectroscopy Insights into 
Donor-Acceptor Heterojunction Solar Cells 25.07.2016 Saanenmöser, Switzerland Moser Jacques-Edouard;
Summer school "Perspectives on photonics: Investigating the route from materials to devices" Poster Ultrafast Stark and THz spectroscopy of methylammonium lead iodide perovskite thin films : Evidence for carriers trapping and accumulation at the surface 25.07.2016 Gstaad/ Saanenmöser, Switzerland Paraecattil Arun Aby;
International Workshop on Organic and Hybrid Electronics and Photovoltaics (IWOHEPV) Talk given at a conference Dynamics of photoinduced charge separation in organic and hybrid solar cells 22.05.2016 Île de Porquerolles, France Moser Jacques-Edouard;
Perovskite thin film photovoltaics (ABXPV 2016) Poster Ultrafast electro-modulated differential absorption spectroscopy and THz spectroscopic investigation of carrier dynamics in CH3NH3PbI3 perovskite films 03.03.2016 Barcelona, Spain Paraecattil Arun Aby;
NCCR MUST Annual Meeting Talk given at a conference Dynamics of charge carrier generation in bilayer organic and organo-lead iodide perovskite thin films photovoltaic cells 11.01.2016 Engelberg, Switzerland Paraecattil Arun Aby;
1st International Conference on Perovskite Solar Cells & Optoelectronics (PSCO-2015) Talk given at a conference Dynamics of charge separation in pristine CH3NH3PbI3 perovskite and bilayers 27.09.2015 Lausanne, Switzerland Moser Jacques-Edouard;
1st International Conference on Perovskite Solar Cells and Optoelectronics (PSCO-2015) Poster Ultrafast transient Stark and THz spectroscopic investigation of carrier dynamics in CH3NH3PbI3 perovskite films 27.09.2015 Lausanne, Switzerland Paraecattil Arun Aby;
8th International Conference on Molecular Electronics & Bioelectronics (M&BE8) Talk given at a conference Dynamics of photoinduced charge separation at donor-acceptor heterojunctions 22.06.2015 Funabori, Tokyo, Japan Moser Jacques-Edouard;
Hybrid and Organic Photovoltaics Conference (HOPV 15) Poster Ultrafast transient Stark and THz spectroscopic investigation of carrier dynamics in perovskite single crystals and films 10.05.2015 Roma, Italy Paraecattil Arun Aby;
Conférences mensuelles du Département d’optique ultrarapide et de nanophotonique Individual talk Dynamics of charge separation at organic and hybrid donor-acceptor heterojunctions 17.04.2015 Université de Strasbourg, France Moser Jacques-Edouard;
NCCR MUST Annual Meeting Poster Ultrafast transient Stark spectroscopy of perovskite thin films 12.01.2015 Engelberg, Switzerland Paraecattil Arun Aby;


Self-organised

Title Date Place

Associated projects

Number Title Start Funding scheme
175729 Carrier and exciton dynamics in donor-acceptor heterojunction photovoltaic systems 01.11.2017 Project funding (Div. I-III)
125760 NCCR MUST: Molecular Ultrafast Science and Technology (phase I) 01.07.2010 National Centres of Competence in Research (NCCRs)

Abstract

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.
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