Site-specific probing of charge transfer dynamics in organic photovoltaics

Abstract

We report the site-specific probing of charge-transfer dynamics in a prototype system for organic photovoltaics (OPV) by picosecond time-resolved X-ray photoelectron spectroscopy. A layered system consisting of approximately two monolayers of C60 deposited on top of a thin film of CopperPhthalocyanine (CuPC) is excited by an optical pump pulse and the induced electronic dynamics are probed with 590 eV X-ray pulses. Charge transfer from the electron donor (CuPC) to the acceptor (C60) and subsequent charge carrier dynamics are monitored by recording the time-dependent C 1s core level photoemission spectrum of the system. The arrival of electrons in the C60 layer is readily observed as a completely reversible, transient shift of the C60 associated C 1s core level, while the C 1s level of the CuPC remains unchanged. The capability to probe charge transfer and recombination dynamics in OPV assemblies directly in the time domain and from the perspective of well-defined domains is expected to open additional pathways to better understand and optimize the performance of this emerging technology.PACS numbers: 88.40.jr,79.60.-i,78.47.da, 78.20.-e Research on materials for organic photovoltaic (OPV) applications is driven mostly by two figures of merit: the light-to-current conversion efficiency and the cost of production, which would make the technology suitable for widespread implementation. So far, OPVs meet the second criterion, as they can easily be prepared by deposition from solutions, or by low-cost printing techniques, rather than using more demanding vacuum deposition methods. However, the relatively low conversion efficiency of OPVs remains a significant challenge. It is generally accepted that recombination of the photo-induced electron-hole pair or excitonic state in the photoreceptive material is one of the major loss mechanisms. Consequently, many research efforts focus on the identification of methods to quench the recombination process. Empirically, the admixture of C 60 into organic photoreceptor films has led to significant progress in this respect [1][2][3][4][5][6][7][8]. Based on observations of intrinsic fluorescence quenching [1, 2] as well as from photoemission studies of the band alignment in thin film systems [9], it has been concluded that after the initial electron-hole production in the chromophore, the electron is captured by the C 60 molecule, thus spatially separating the electron from the hole in the chromophore molecule. This spatial separation prevents direct electron-hole recombination. Studies by photoelectron spectroscopy (PES) have shown that the charge transfer process is energetically possible, both in layered systems [4,5] as well as in heterogeneous mixtures [10]. Here, we concentrate on the material combination of the organic photoreceptor copper phthalocyanine (CuPC), which is a ubiquitous component of photochemical and light harvesting applications [1][2][3], with C 60 molecules as electron acceptors.A recent report on CuPC-C 60 interfaces indicates that, when CuPC is depos...

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