Time-independent, high electron mobility in thin PC61BM films: Relevance to organic photovoltaics

Jacques-Edouard Moser
Elsevier, 2014
Article

Ultrafast optical probing of electric field by means of electroabsorption combined with conventional photocurrent measurements was employed to investigate the drift and mobility dynamics of photo-generated charge carriers in the pristine PC61BM film and in the blend with a merocyanine dye. Electrons passed a 40 nm thick PC61BM film within a few picoseconds with time-independent and weakly dispersive mobility. The electron mobility is 1 cm2/(V s) at 1 MV/cm and an estimate of the zero-field mobility yields 5 ⋅ 10−2 cm2/(V s). The initial electron mobility in the blend is of the order of 10−2 cm2/(V s) and decreases rapidly. We conclude that electron motion in PC61BM based organic bulk hetero-junction solar cells is limited by barriers between PC61BM domains rather than by intrinsic PC61BM properties.

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Jacques-Edouard Moser
Elsevier, 2014
Article

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https://infoscience.epfl.ch/record/203068?ln=fr

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Dissociation of charge transfer states and carriers separation in bilayer organic solar cells - A time-resolved electroabsorption spectroscopy study

Roland Hany, Jacques-Edouard Moser, Jelissa Risse

Ultrafast optical probing of the electric field by means of Stark effect in planar heterojunction cyanine dye / fullerene organic solar cells enables to directly monitor the dynamics of free electron formation during the dissociation of interfacial charge transfer (CT) states. Motions of electrons and holes is scrutinized separately by selectively probing the Stark shift dynamics at selected wavelengths. It is shown that only charge pairs with an effective electron-hole separation distance of less than 4 nm are created during the dissociation of Frenkel excitons. Dissociation of the Coulombically bound charge pairs is identified as the major rate-limiting step for charge carriers’ generation. Interfacial CT states split into free charges on the time-scale of tens to hundreds of picoseconds, mainly by electron escape from the Coulomb potential over a barrier that is lowered by the electric field. The motion of holes in the small molecule donor material during the charge separation time is found to be insignificant.

Amer Chemical Soc2015

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Dynamics of Photocarrier Separation in MAPbI3 Perovskite Multigrain Films under a Quasistatic Electric Field

Jacques-Edouard Moser, Arun Aby Paraecattil, Vytenis Pranculis, Jelissa Risse, Joël Teuscher

Applying time-resolved electroabsorption spectroscopy for the first time to methylammonium lead triiodide perovskite (MAPbI3) thin films under reverse bias, we monitored optically the ultrafast evolution of the local counter-electric field produced by the drift of photogenerated electrons and holes in opposite directions. Under an externally applied electric field of |E| < 10^5 V cm–1, the carriers were found to reach a separation of 40 nm within ∼1 ps. This distance corresponds to the average dimensions of crystalline grains in the active film, at the boundaries of which charges were trapped. An intragrain average carrier drift mobility of μ± = 23 cm^2 V–1 s–1 was inferred. Subsequent charge detrapping, migration through the entire film, and accumulation at its insulated surfaces caused a blue shift of the perovskite absorption edge that arose within tens of picoseconds, owing to a trap-limited electron drift mobility μn = 6 cm^2 V–1 s–1. Charge recombination was entirely suppressed between field-separated photocarriers generated at initial densities of n0 ≤ 2 × 10^16 cm–3. Accumulation of electrons at the interface between a mesoporous TiO2 electron-transport layer and a multigrain MAPbI3 film was also observed, which was indicative of delayed charge injection through a poor contact junction.

American Chemical Society2016

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Correlated states in twisted double bilayer graphene

Na Li, Jian Tang, Quansheng Wu, Rong Yang, Oleg Yazyev

Electron-electron interactions play an important role in graphene and related systems and can induce exotic quantum states, especially in a stacked bilayer with a small twist angle(1-7). For bilayer graphene where the two layers are twisted by the 'magic angle', flat band and strong many-body effects lead to correlated insulating states and superconductivity(4-7). In contrast to monolayer graphene, the band structure of untwisted bilayer graphene can be further tuned by a displacement field(8-10), providing an extra degree of freedom to control the flat band that should appear when two bilayers are stacked on top of each other. Here, we report the discovery and characterization of displacement field-tunable electronic phases in twisted double bilayer graphene. We observe insulating states at a half-filled conduction band in an intermediate range of displacement fields. Furthermore, the resistance gap in the correlated insulator increases with respect to the in-plane magnetic fields and we find that the g factor, according to the spin Zeeman effect, is 2, indicating spin polarization at half-filling. These results establish twisted double bilayer graphene as an easily tunable platform for exploring quantum many-body states. Placing two Bernal-stacked graphene bilayers on top of each other with a small twist angle gives correlated states. As the band structure can be tuned by an electric field, this platform is a more varied setting to study correlated electrons.

NATURE PUBLISHING GROUP2020

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