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"XYZ ...for the Rest of Us"

Understanding Non-Conventional Photovoltaic Cells

by Laurie Peter

Sunday, October 7, 2007| Washington, DC

Laurie PeterThe physical principles underlying conventional solid state p-n junction photovoltaic devices are well known and can be found in many excellent text books. However, recent years have seen the development of several unconventional types of solar cell that, at first sight at least, appear to differ substantially from normal solid state cells. Two examples of these non-conventional PV cells are dye-sensitized nanocrystalline solar cells (DSC or Grätzel cells) and bulk heterojunction organic solar cells (for example, polymer/fullerene cells). The common feature of these cells is an extended three-dimensional nanostructured junction formed by interpenetration of electron and hole conducting phases. In both types of cell, the primary step in the process of converting solar energy into electrical power involves absorption of photons to create a molecular excited state that can be described as a tightly bound exciton (the cells are commonly referred to as excitonic solar cells for this reason). The subsequent separation of charge across the interfacial region between the two phases (exciton dissociation) leads to a long-lived charge separated state, with the electron in one phase and the hole in the other. The generation of a photovoltage at the cell contacts reflects the deviation in the local electron and hole concentrations from their (generally very low) values in thermal equilibrium, and this in turn depends on the balance between the rate of electron hole pair generation by light and the rate of electron hole recombination across the nanostructured interface. By contrast, efficient generation of a photocurrent requires not only selective contacts (i.e. contacts that that allow extraction of one carrier type while preventing transfer of the other) but also carrier transit times that are shorter than typical recombination lifetimes. Remarkably, these cells can satisfy these two criteria in spite of the enormous internal surface area at which electron-hole recombination can occur.

In the DSC, the interface is formed between an electronically conducting porous nanocrystalline oxide matrix (usually titanium dioxide) and an ionically conducting redox electrolyte or other ‘hole conducting’ medium. The high ionic strength electrolyte that permeates the porous oxide film effectively screens electronic charge in the oxide, largely eliminating macroscopic electrical fields and space charge effects. The driving force for the separation of electrons and ‘holes’ is therefore related primarily to the concentration gradients set up when carriers are extracted at the contacts. The extent to which this may also be true in the case of bulk heterojunction solar cells is still controversial.

In spite of the apparent differences between conventional solid state devices and these non-conventional systems, the physical description of both types of cells is essentially identical. The main problem is one of language and not of underlying physical principles. For example, the physicist’s Fermi level is equivalent to the electrochemical potential of electrons encountered in physical chemistry. The objective of the ZYZ talk will be to show how non-conventional cells work and to explain how they can be described within a simple unified theoretical framework that is accessible to physicists and chemists alike and which allows definition of strategies for device optimization.

Laurie Peter is currently Professor of Physical Chemistry at the University of Bath in the South West of the United Kingdom. He received both his B.Sc. and Ph.D. from the University of Southampton, where his Ph.D. research under the supervision of Sir Graham Hills was concerned with electrode kinetics in aprotic media. Prof. Peter was introduced to photophysics and semiconductor electrochemistry during the five years (1969-1974) that he spent working in the late Professor Heinz Gerischer’s laboratory at the Fritz Haber Institute in Berlin, first as a CIBA Postdoctoral Fellow and then as a staff member. Returning to his almer mata in 1974, Prof. Peter became lecturer at the University of Southampton and was promoted to a Chair 1993. During this period at Southampton, he worked on a range of topics including electrocrystallization, anodic films on metals, conducting polymers, semiconductor electrochemistry and porous silicon. It was in Southampton that Prof. Peter developed the technique of intensity modulated photocurrent spectroscopy (IMPS), the first of several related experimental techniques for characterizing the kinetics of photoelectrochemical reactions. The IMPS method, which involves perturbing a system with an intensity-modulated light and analyzing the frequency-dependent photocurrent response, was applied initially to study photoelectrochemical reactions at single-crystal semiconductor electrodes and later to characterize electron transport in dye-sensitized nanocrystalline solar cells (DSC). In a series of papers, Prof. Peter showed how IMPS could be used to obtain kinetic information about surface recombination as well as about reactions that give rise to photocurrent multiplication, for example during oxygen reduction at p-type semiconductors or the photoanodic dissolution of n-type silicon in fluoride solutions.

In 1993, Prof. Peter took up his present position in Bath, where his more recent work has developed a strong practical focus on the application of electrochemistry and photoelectrochemistry for the fabrication and characterization of thin film solar cells. This work has two main themes. The first is low cost electrochemical synthesis and in situ characterization of absorber materials (CdTe, CuInSe2) for thin film solar cells. The characterization techniques being used in this work include electrolyte electroreflectance spectroscopy and photocurrent spectroscopy. This research is being carried out as part of the EPSRC SUPERGEN Consortium project PV materials for the 21st century. The second theme, which is part of the EPSRC SUPERGEN Excitonic Solar Cell Consortium project, is concerned with characterization and modelling of DSC. Prof. Peter has developed a number of experimental methods to characterize the dynamic behavior of photoelectrochemical systems, including DSC. These include potential and light-modulated microwave reflectivity measurements of semiconductor electrodes, optical detection of photo-injected electrons in DSC and the charge extraction method for analyzing trap distributions in nanocrystalline systems.

Prof. Peter has published around 230 scientific papers as well as several book chapters. He was awarded the Electrochemistry Medal of the Royal Society of Chemistry in 1992 and the Pergamon Medal of the International Society of Electrochemistry in 1997. He has been a Leverhulme Research Fellow and a Vice President of the International Society of Electrochemistry. Until recently, he was a long-serving Editor of the Journal of Electroanalytical Chemistry.

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