http://nathan.instras.com/documentDB/paper-44.pdf

Surface photovoltage phenomena: theory, experiment, and applications

We present a concise, although admittedly non-exhaustive, but hopefully didactic review and discussion of some of the central and basic concepts related to the energetics of surfaces and interfaces of solids. This is of particular importance for surfaces and interfaces that involve organic molecules and molecular films. It attempts to pull together different views and terminologies used in the solid state, electrochemistry, and electronic device communities, regarding key concepts of local and absolute vacuum level, surface dipole, work function, electron affinity, and ionization energy. Finally, it describes how standard techniques like photoemission spectroscopy can be used to measure such quantities.

Electron Energetics at Surfaces and Interfaces: Concepts and Experiments†

TiO2-based nanomaterials play currently a major role in the development of novel photochemical systems and devices. One of the key parameters determining the photoactivity of TiO2-based materials is the position of the band edges. Although its knowledge is an important prerequisite for understanding and optimizing the performance of photochemical systems, it has been often rather neglected in recent research, particularly in the field of heterogeneous photocatalysis. This paper provides a concise account of main methods for the determination of the position of the band edges, particularly those suitable for measurements on nanostructured materials. In the first part, a survey of key photophysical and photochemical concepts necessary for understanding the energetics at the semiconductor/solution interface is provided. This is followed by a detailed discussion of several electrochemical, photoelectrochemical, and spectroelectrochemical methods that can be applied for the determination of band edge positions in compact and nanocrystalline thin films, as well as in nanocrystalline powders.

/pdf/photo-electrochemical-methods-for-the-determination-of-the-40nxx1b2nv.pdf

(Photo)electrochemical Methods for the Determination of the Band Edge Positions of TiO2-Based Nanomaterials

The possibility of obtaining a detailed picture of the electronic structure makes surface photovoltage spectroscopy (SPS) eminently suitable for bridging the gap between the chemical, physical, optical and electrical properties of semiconductors. In SPS, changes in band bending (both at the free semiconductor surface and at buried interfaces) are monitored as a function of external illumination. Surface photovoltage spectroscopy can provide detailed, quantitative information on bulk properties (e.g. bandgap and type, carrier diffusion length and lifetime) and can be used for complete construction of surface and interface band diagrams, including the measurement of energy levels in quantum structures. A particular strength is that a comprehensive analysis of surface and bulk defect state distributions and properties is made possible. Measurements using SPS are contactless and non-destructive. In addition, they can be performed both in situ and ex situ, at any reasonable temperature, on any semiconducting material, at any ambient and at any lateral resolution down to the atomic scale. This review starts with an overview of SPS-related surface and interface theory, describes the SPS experimental set-up and presents applications for surface and interface characterization of a wide variety of materials and structures, cross-correlating them with other methodologies. Copyright © 2001 John Wiley & Sons, Ltd.

Surface photovoltage spectroscopy of semiconductor structures: at the crossroads of physics, chemistry and electrical engineering

Surface modification of indium tin oxide (ITO)-coated substrates through the use of self-assembled monolayers (SAMs) of molecules with permanent dipole moments has been used to control the anode work function and device performance in molecular solar cells based on a CuPc:C60 (CuPc: copper phthalocyanine) heterojunction. Use of SAMs increases both the short-circuit current density (Jsc) and fill factor, increasing the power-conversion efficiency by up to an order of magnitude. This improvement is attributed primarily to an enhanced interfacial charge transfer rate at the anode, due to both a decrease in the interfacial energy step between the anode work function and the highest occupied molecular orbital (HOMO) level of the organic layer, and a better compatibility of the SAM-modified electrodes with the initial CuPc layers, which leads to a higher density of active sites for charge transfer. An additional factor may be the influence of increasing electric field at the heterojunction on the exciton-dissociation efficiency. This is supported by calculations of the electric potential distribution for the structures. Work-function modification has virtually no effect on the open-circuit voltage (Voc), in accordance with the idea that Voc is controlled primarily by the energy levels of the donor and acceptor materials.

Using Self‐Assembling Dipole Molecules to Improve Charge Collection in Molecular Solar Cells

A convenient piezoelectric driven Kelvin probe for the measurement of work function changes is described. The probe has a simple construction, is small, bakeable, and can be mounted on any 35‐mm‐i.d. UHV flange. As the piezoelectric device is inside the vacuum chamber, only one electrical feedthrough is needed for operating the probe. The distance between reference electrode and sample can be varied within ±0.5 mm simply by applying a dc voltage to the piezoelectric device in addition to the ac oscillator‐driving voltage, not requiring any mechanical driving and feedthrough provisions. The probe was tested studying the adsorption of O2 on Pt(111). The sensitivity for contact potential measurements is better than 0.1 mV at a response time of 10 sec.

Piezoelectric driven Kelvin probe for contact potential difference studies

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Piezoelectric driven Kelvin probe for contact potential difference studies