Efficient charge generation from triplet excitons in metal-organic heterojunctions

Abstract

The success of many emerging molecular electronics concepts hinges on an atomistic understanding of the underlying electronic dynamics. We employ picosecond time-resolved Xray Photoemission Spectroscopy (tr-XPS) to elucidate the roles of singlet and triplet excitons for photoinduced charge generation at a copper-phthalocyanine-C 60 heterojunction. Contrary to common belief, fast intersystem crossing to triplet excitons after photoexcitation is not a loss channel but contributes to a significantly larger extent to the time-integrated interfacial charge generation than the initially excited singlet excitons. The tr-XPS data provide direct access to the diffusivity of the triplet excitons D CuPc = (1.8 ± 1.2) x 10-5 cm 2 /s and their diffusion length L diff = (8 ± 3) nm. Main Text Metal-organic heterojunctions, such as metal-phthalocyanine (MePc) donor-C 60 acceptor systems [1-5], provide an important platform to advance understanding the electronic dynamics underlying many emerging molecular electronics concepts for photochemical and photovoltaic applications [6-16]. The prevailing picture for efficient charge generation is through singlet exciton dissociation at the donor-acceptor interface within less than 1 ps after photoexcitation in order to avoid photon energy loss to non-dissociating triplet excitons by fast intersystem crossing [17,18]. We employ picosecond time-resolved X-ray Photoemission Spectroscopy (tr-XPS), a technique of choice for measuring electronic structure with elemental and chemical site-specificity [19,20], to directly determine the triplet exciton diffusivity and the ratio of singlet and triplet exciton dissociation efficiencies in a copper-phthalocyanine (CuPc)-C 60 planar heterojunction (PHJ). Contrary to common belief, ultrafast intersystem crossing from

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

“…These observations agree qualitatively with our previous, picosecond TR-XPS experiments on planar CuPc:C 60 heterojunctions 16 , 17 and extend them into the femtosecond regime. In addition, a sideband feature (SB) appears at ≈206.5 eV KE when pump and probe pulses overlap in time (Fig.…”

“…Recently, we established time-resolved X-ray photoelectron spectroscopy (TR-XPS) as a unique tool to detect the presence of charge transfer electrons in C 60 (refs. 16 , 17 ). To the best of our knowledge, this is the first femtosecond time-resolved core-level photoemission spectroscopy study of Pc:C 60 systems.…”

“…The technique is described in the methods section, as well as in more detail in our previous publications 16 , 17 . Even though our earlier experiments were limited to a temporal resolution of ~70 ps, they clearly demonstrated that triplet states (T 1 ) substantially contribute to the generation of separated charges in the CuPc:C 60 system.…”

“…Even though our earlier experiments were limited to a temporal resolution of ~70 ps, they clearly demonstrated that triplet states (T 1 ) substantially contribute to the generation of separated charges in the CuPc:C 60 system. Integrated over all timescales from picoseconds to nanoseconds, the triplet states even generate an order of magnitude more charges than any other state 17 . Their long lifetimes more than compensate for their small diffusivities, leading to large diffusion lengths and efficient charge generation at the interface to the C 60 .…”

From atoms to aerosols: probing clusters and nanoparticles with synchrotron based mass spectrometry and X-ray spectroscopy

“…175 This method of pump-probe spectroscopy spearheaded by Oliver Gessner at Berkeley lab has now led to an understanding of the electronic and lattice contributions of CuO 176 with X-ray transient absorption and triplet excitons in metal-organic heterojunctions with X-Ray photoelectron spectroscopy. 177 Recently Smith and Saykally 135 reviewed the field of soft X-ray absorption spectroscopy of liquids and solutions, and it was interesting to note that there was not a single example of a two color experiment probing the excited state at a synchrotron. In contrast, a perspective on ultrafast photoelectron spectroscopy of aqueous liquids, showcases examples of pump-probe experiments of redox reactions and charge transfer to solvent reactions using lab based HHG liquid microjet methods.…”