Photoinduced Charge Transfer at Hybrid Semiconductor Interfaces
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1003-O02-03
Photoinduced Charge Transfer at Hybrid Semiconductor Interfaces Juan Cabanillas-Gonzalez, Hans Joachim Egelhaaf, Guglielmo Lanzani, Alberto Brambilla, Lamberto DuÚ, Marco Finazzi, and Franco Ciccacci Physics, Politecnico di Milano, Piazza Leonardo da Vinci 32, Milano, 20133, Italy
ABSTRACT We monitor in real time photoinduced charge injection at the interface between a fluorinated copper phthalocyanine layer (CuPcF16) deposited by thermal evaporation on top of a p ñ doped GaAs (100) wafer. Literature data on the electron affinity of CuPcF16 (5.2 eV respect to vacuum level) combined with photoemission measurements indicates an energy offset of 1.1 eV for the GaAs conduction band respect to the CuPcF16 LUMO level. This suggests that charge transfer at the organic - inorganic interface is feasible. We study bilayers of GaAs and CuPcF16 thin films (25 nm) by pump - probe spectroscopy with 200 fs time resolution. Pump photons at 780 nm excites the CuPcF16 layer whereas probe photons in the visible range, reflected by the GaAs surface, monitor induced changes at the interface. We observe a strong photoinduced absorption band centered around 560 nm which appears during the pulse duration, shows a buildup dynamics and persists beyond 0.2 ns. This band cannot be attributed to single material contribution, as demonstrated by test experiments with single layers. By applying steady state (CW) electromodulated spectroscopy we identify charge state absorption in CuPcF16 in the same spectral region as the photoinduced absorption band. We thus assign our transient dynamics to formation of CuPcF16 ions at the interface, following charge injection. On account of the rapid charge formation we identify this system as a potential candidate for the fabrication of hybrid photodiodes.
INTRODUCTION Understanding the photophysics at organic ñ inorganic interfaces is of paramount importance for improving charge injection in optoelectronic devices. It has been observed in FETs and OLEDs that modification of the energy level configuration at the metal - organic interface affects significantly the performance of the devices [1]. Concerning photovoltaic applications, the use of solar cells and photodiodes based on organic ñ inorganic semiconducting interfaces have attracted a lot of attention in the recent years for several reasons [2]. First, solar cells containing hybrid heterojunctions allow for an increase in charge photogeneration efficiency respect to the inorganic bulk provided that there is an energy offset at the interface large enough to overcome the exciton binding energy [3]. The correct balance in electron and hole transport in the organic and inorganic layer prevent losses due to non geminate electron ñ hole recombination leading to large collection efficiencies [4]. Moreover, enlargement of the spectral response in inorganic solar cells can be achieved by deposition of organic semiconductors which absorb in different parts of the visible and near - IR spectrum [5]. Combination of organic and inorganic semiconductors
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