Photoeffects at Semiconductor-Electrolyte Interfaces

419 Pages·1981·6.219 MB·English

by  Arthur J. Nozik (Eds.)

Photoeffect
Electrolyte Interfaces
In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Photoeffects at Semiconductor-
Electrolyte Interfaces
Arthur J. Nozik, EDITOR
Solar Energy Research Institute
Based on a symposium
sponsored by the Division
of Colloid and Surface Chemistry
at the 179th Meeting of the
American Chemical Society,
Houston, Texas,
March 25-26, 1980.
146
ACS SYMPOSIUM SERIES
AMERICAN CHEMICAL SOCIETY
WASHINGTON, D. C. 1981
In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
Library of Congress CIP Data
Photoeffects at semiconductor-electrolyte interfaces.
(ACS symposium series; 146 ISSN 0097-6156)
Includes bibliographies and index.
1. Photochemistry—Congresses. 2. Photoelectricity—
Congresses. 3. Semiconductors—Congresses.
I. Nozik, Arthur J., 1936- . II. American Chemi
cal Society. Division of Colloid and Surface Chemistry.
III. Series: American Chemical Society. ACS sympo
sium series; 146.
QD701.P47 541.3'5 80-27773
ISBN 0-8412-0604-X ASCMC8 146 1-416 1981
Copyright © 1981
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In PhotoeffecWts aast Sheimngictoonnd,u cDto.Cr-.E le2ct0ro0l3yt6e Interfaces; Nozik, A.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
ACS Symposium Series
M. Joan Comstock, Series Editor
Advisory Board
David L. Allara James P. Lodge
Kenneth B. Bischoff Marvin Margoshes
Donald D. Dollberg Leon Petrakis
Robert E. Feeney Theodore Provder
Jack Halpern F. Sherwood Rowland
Brian M. Harney Dennis Schuetzle
W. Jeffrey Howe Davis L. Temple, Jr.
James D. Idol, Jr. Gunter Zweig
In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
FOREWORD
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a medium for publishin
format of the Series parallels that of the continuing ADVANCES
IN CHEMISTRY SERIES except that in order to save time the
papers are not typeset but are reproduced as they are sub­
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In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
PREFACE
This volume, based on the symposium "Photoeffects at Semiconductor-
Electrolyte Interfaces," consists of 25 invited and contributed papers.
Although the emphasis of the symposium was on the more basic aspects
of research in photoelectrochemistry, the covered topics included applied
research on photoelectrochemical cells. This is natural since it is clear
that the driving force for intens interes activit i
electrochemistry is the potentia
for solar energy conversion. These versatile cells can be designed either
to produce electricity (electrochemical photovoltaic cells) or to produce
fuels and chemicals (photoelectrosynthetic cells).
The first 12 chapters of this volume are concerned with the vital sub­
ject of the effects of surface properties on photoelectrochemical behavior.
This includes work on the effects of the chemical modification of semi­
conductor electrode surfaces either through molecular derivatization or
ionic adsorption; the effects of surface structural defects and surface elec­
tronic states on photo-induced charge transfer across semiconductor-elec­
trolyte interfaces; the kinetics of competing chemical reactions on semicon­
ductor electrode surfaces; catalytic effects on semiconductor surfaces; and
the problems of photocorrosion of semiconductor electrodes. Chapters
13-15 deal with new electrode materials (oxide semiconductors) and
structures (protective layers and interfacial chlorophyll layers). Chapters
16 and 17 relate to the basic energetics of the semiconductor-electrolyte
interface (potential distribution and the effects of charge inversion leading
to band-edge unpinning), while Chapters 18-22 present results on new
chemical systems and phenomena associated with photoelectrochemistry.
These latter include luminescence studies, surfactant assemblies, a new
model based on the effects of ionic desorption, studies of carbanion oxi­
dation on semiconductor surfaces, and the behavior of molten-salt electro­
lytes. Finally, the volume concludes with three chapters on electrochemical
photovoltaic cell devices dealing with models for current-voltage charac­
teristics, stability performance, and solid electrolytes.
The exceptional interest and ferment in photoelectrochemistry has
been manifested in 1980 by the appearance of at least five major confer­
ences, symposia, or workshops in the field with international participation,
including the present symposium. It is apparent from these meetings, as
well as from the burgeoning amount of published literature, that photo–
ix
In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
electrochemistry is a vital, productive, and stimulating field of science in
which much significant and exciting scientific progress can be expected for
a long time. Although very rapid progress has also been made in the
applications of photoelectrochemistry, it is still too early to predict what
impact photoelectrochemistry will have on practical solar energy conver­
sion systems. We can be confident, however, that basic research in photo­
electrochemistry will continue to produce important new knowledge, and
that attractive potential applications to our energy problems will continue
to provide the impetus for the work.
Solar Energy Research Institute ARTHUR J. NOZIK
Golden, Colorado 80401
October 3, 1980
x
In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
1
The Influence of Surface Orientation and Crystal
Imperfections on Photoelectrochemical Reactions
at Semiconductor Electrodes
HEINZ GERISCHER
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6,
D-1000 Berlin 33, West Germany
It is well known tha
and imperfections in the surface, like grain boundaries or dislo­
cations, affect largely the reaction rates at electrodes made of
metals or semiconductors. Such effects are most pronounced in
those reactions where atoms leave their position in a crystal lat­
tice or have to be incorporated into such one. These processes are
connected with activation barriers which are particularly high for
semiconductors where the chemical bonds between the components of
the crystal lattice are highly directed and localized. If we con­
sider photoelectrochemical reactions at semiconductors we have ad­
ditionally to discuss the influence of these factors on light ab­
sorption and its consequences.
Factors which control the yield of photocurrents
Photocurrents and photovoltages are induced by the generation
of excessive mobile charge carriers. In a semiconductor these are
electron hole pairs generated by light absorption. The size of the
effects is largely dependent on how far the recombination between
electrons and holes can be prevented. An efficient separation of
electron hole pairs occurs only in the space charge layer beneath
the semiconductor/electrolyte contact. Large efficiencies can be
reached if this space charge layer forms a high enough energy bar­
rier for the two charge carriers to encounter each other. Such a
situation is found in a depletion layer of n-type or p-type semi­
conductors or in an inversion layer (1,2). Here we shall not con­
sider insulating materials where one can use high external elec­
tric fields to obtain charge separation (3).
Assuming that we have such a situation favorable for charge
separation, we have to consider what factors influencing the effi­
ciency of charge separation in an illuminated semiconductor elec­
trode are affected by crystal orientation or crystal imperfec­
tions. Five such factors are listed in the following table:
0097-6156/81/0146-0001$05.00/0
© 1981 American Chemical Society
In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
2 PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE INTERFACES
(1) Schottky barrier height : Δψ
(2) Schottky barrier extension: tP
(3) rate of surface recombination: r
g
(k) mean diffusion length of minorities: L
(5) penetration depth of light: —
with α = absorptivity
The quantum yield of the photocurrent for an electrode illu­
minated from the front side can be calculated from a simple model
described by Gartner (h) provided some simplifying assumptions are
applicable. This model is shown in Figure 1. If surface recombina­
tion can be neglected, the quantum yield φ is obtained as
I
Φ = 1 - 1 aL exp (- aW) 0)
The equation above can be approximated i f the width of the space
charge layer is small compare
light 1/a by
* * w f ï i f « . V « 1 (2)
The width of the space charge layer depends on the height of the
Schottky barrier according to
J/ 2ε ε Δφ 11 ^
TWT = ο /se (3)
I
e · Ν
1 ο
where Ν is the donor or acceptor concentration, ε, ε and e have
their usual meaning of permittivity and elementary e2ectric°charge.
These equations contain four parameters of Table 1 and indicate
how the quantum yield is affected by these factors. The surface
recombination rate is only important i f the barrier height is low
and can otherwise be neglected. This was assumed in the derivation
of Equation (1) which requires a high enough barrier height.
The main effect of crystal orientation is caused by different
barrier heights on different crystal faces. It is well known that
Volta-potential differences are dependent on crystal orientation
because the surface dipole differs for different faces. In the
case of a semiconductor electrode this means that the flat band
potential which can be determined experimentally (5_,6.,χ) depends
on surface orientation. Consequently, the band bending at the same
position of the Fermi level in the bulk of the semiconductor, i.e.
at the same electrode potential, differs for different faces.
Figure 2 gives a schematic picture for such differences.
Besides the effects of different surface dipoles, the con­
centration and energy position of surface states depend also
largely on surface orientation with the result that the electric
excess charge in surface states can be very different on different
surfaces. This is indicated in Figure 3 by a comparison between the
flat band situation and the situation at equal electrode potential
for different surfaces. Case (a) is a surface free of surface
In Photoeffects at Semiconductor-Electrolyte Interfaces; Nozik, A.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1981.