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ScienceDirect Energy Procedia 60 (2014) 48 – 56

E-MRS Spring Meeting 2014 Symposium Y “Advanced materials and characterization techniques for solar cells II”, 26-30 May 2014, Lille, France

Dual Mode Kelvin Probe: featuring Ambient Pressure Photoemission Spectroscopy and Contact Potential Difference Iain D. Baikie*, Angela C. Grain, James Sutherland, Jamie Law KP Technology Ltd, 12 Burn St,Wick, KW1 5EH, UK

† Abstract We describe a novel dual-mode Kelvin probe featuring ambient pressure Photoemission Spectroscopy (PES), which yields information on the absolute work function (ĭ) of a metal and the Ionisation Potential (IP) of a semiconductor, coupled with a high resolution Contact Potential Difference capability which can be extended to Surface Photovoltage measurements. The relative energy resolution are 50 meV for PES and 1-3 meV for CPD. To surmount the limitation of electron scattering in air the incident photon energy is rastered rather than applying a variable retarding electric field as is used UPS. We propose a mechanism of atmospheric ion generation and show that for the metal photoresponse obeys Fowler Theory. The relationship between CPD and photoelectric threshold is a useful tool in characterizing the electrical behavior of materials. We illustrate this with native oxide covered Cu and n-type Si. Further we show that the photoresponse can be used to generate the near Fermi-level Density of States (DOS) in Iron and Nickel-Phthalocyanine. © Published by Elsevier Ltd. This © 2014 2014The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of the SiliconPV 2014 conference. Peer-review under responsibility of The European Materials Research Society (E-MRS) Keywords: ambient photoemission spectroscopy; Kelvin Probe; CPD; SPV;PES; Fowler theory; Cu2O; NiPc;FePc

* Corresponding author. Tel.: +44-1955-602-777; fax: +44-1955-602-555. E-mail address: [email protected]

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of The European Materials Research Society (E-MRS) doi:10.1016/j.egypro.2014.12.341

Iain D. Baikie et al. / Energy Procedia 60 (2014) 48 – 56

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1. Introduction The realization of atmospheric pressure photoemission spectroscopy (PES) would create a very useful tool for monitoring the absolute work function (Ɏ) and local density of states (LDOS) of surfaces. It is however not selfevident that PES measurements are feasible due to inelastic scattering of the photo-ejected electrons in air. Recently Roy and Gopinath [1] have used combined He I/Al KĮ source to study the oxidation of polycrystalline Copper surfaces at pressures up to 0.3 mbar. We show here that using a rastered photon energy approach rather than a retarding field, atmospheric pressure PES is achievable. We propose a mechanism by which the electronic charge is conserved when atmospheric ions such as N2- and O2- are generated near the metal surface and we show that the photo-emission response (R) for metals is consistent with Fowler Theory [2]. The detection system is a Kelvin probe (KP) [3], a versatile tool normally utilized for measuring differences in work function (Δĭ) or contact potential (CPD) between a vibrating metallic reference electrode and a metallic or semiconducting sample. Traditionally the Kelvin method has been applied for in-situ characterization of metals and semiconductors [3-6]. More recently it has been utilized in ambient surface photovoltage (SPV) characterization of bulk heterojunction organic solar cells [7]; p-type doping of P3HT with F4TCNQ [8] and ĭ-tuning of graphene [9] and ITO [10]. This method can be extended to include surface photovoltage spectroscopy (SPS), providing information on non-neutral surface space-charge regions (SCR) [11]. CPD is a difference method, thus to calculate absolute Φ data, assumptions are required on the work function of the Kelvin probe tip ΦKP and its stability under experimental conditions. Baikie et al [12] have described a UHV calibration method using a retarding field configuration however this arrangement will not work at atmospheric pressure. Previous studies of metal solders [13] and ITO [14] have been performed using a combination of vacuum UPS and ambient CPD, with each analysis being performed at geographically separated laboratories. In this paper we report, for the first time, a combination of ambient pressure PES and CPD using a Kelvin probe. In both measurements the same tip electrode is used as the current collector and the two measurements can be conducted quasi-simultaneously. The principal objective was to study the correlation between the CPD and PE threshold of metallic and semiconducting surfaces and to explore any further information available via LDOS or, for semiconductor surfaces, SPV. 2. Experimental Techniques 2.1. CPD/SPV The CPD gene

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