Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS

Abstract

The ultrafast dynamics of photon-to-charge conversion in an organic light-harvesting system is studied by femtosecond time-resolved X-ray photoemission spectroscopy (TR-XPS) at the free-electron laser FLASH. This novel experimental technique provides site-specific information about charge separation and enables the monitoring of free charge carrier generation dynamics on their natural timescale, here applied to the model donor-acceptor system CuPc:C60. A previously unobserved channel for exciton dissociation into mobile charge carriers is identified, providing the first direct, real-time characterization of the timescale and efficiency of charge generation from low-energy charge-transfer states in an organic heterojunction. The findings give strong support to the emerging realization that charge separation even from energetically disfavored excitonic states is contributing significantly, indicating new options for light harvesting in organic heterojunctions.

Inverse photochromism in viologen–tetraarylborate ion-pair complexes: optical write/microwave erase switching in polymer matrices

“…The evolution of the PCS state to the FSC state proceeds as a result of interplay between local magnetic fields at each radical site, and the electron spin exchange interaction. 2,[23][24][25] Of special relevance to the ion-pair complex systems used in this work are the non-covalent interactions involving aromatic groups, such as - stacking, cation- and anion- interactions, which play important roles in the physiochemical properties of radicals. 7,20,[25][26][27][28] In this paper, we will mostly focus on the photoinduced phenomena using visible and microwave irradiation to promote the photochemical changes.…”

Direct observation of ultrafast exciton localization in an organic semiconductor with soft X-ray transient absorption spectroscopy

“…Pioneering works using these sources have demonstrated time-resolved XAS spectroscopy (TR-XAS) of gas phase molecules at the carbon K-edge 25 – 29 and at the M and L edges of heavier elements in semiconductors and metals 30 – 33 . More recently, femtosecond x-ray photoemission spectroscopy has been applied to track charge separation in a organic heterojunction 34 .…”

Influence of Stacking Order of Phthalocyanine and Fullerene Layers on the Photoexcited Carrier Dynamics in Model Organic Solar Cell

“…Phthalocyanine and fullerene are p-type and n-type semiconductors and are often used as electron donor and acceptor molecules, respectively, in the bilayer OPVs, a type of which was first proposed by Tang in the 1980s . Although bulk heterojunction is a major structure for the active layer of OPVs, alternating nanolayered structures, which are an extended type of the bilayer structure, also work well. , A number of studies have been conducted to clarify the carrier dynamics in the phthalocyanine-fullerene bilayer OPVs from the generation and dissociation processes of excitons − to a free-carrier quenching process by electron–hole recombination. ,− A general consensus of the carrier dynamics is as follows: Photogenerated excitons diffuse to the donor/acceptor heterojunction to form charge-transfer excitons in femtoseconds after light absorption. A fraction of the charge-transfer excitons is quenched by fast electron–hole recombination, while the rest is dissociated into the free carriers, whose lifetime ranges from nanoseconds to microseconds.…”

“…However, pinpointing the location of these carriers experimentally is a rather difficult task. Time-resolved X-ray photoelectron spectroscopy (TRXPS) utilizing a pump–probe technique has been proved to be a powerful tool to investigate the excited states of the individual composite molecules via a temporal chemical shift of core levels. , For a bilayer OPV composed of copper phthalocyanine (CuPc) and C 60 fullerene layers with several monolayer thicknesses fabricated on the Si wafer, the green pump laser (532 nm = 2.33 eV) stimulates the shift of the C 1s peak of C 60 toward lower binding energies. − Although the expected C 1s shift has not been detected in CuPc, the observation is a clear indication that the photoexcited electrons reside in the C 60 layer. On the other hand, a different carrier dynamics has been suggested when the layer thicknesses are reduced to monolayer ones and the higher pump laser of 3.06 eV is used; , both C 1s peaks of C 60 and CuPc move to higher binding energies upon laser irradiation on the C 60 /CuPc layered system fabricated on a rutile TiO 2 (110) surface.…”