FRIEDMAN MARK M (IL)
HODES GARY (IL)
US4544470A | 1985-10-01 | |||
US4637969A | 1987-01-20 |
JOURNAL OF POWER SOURCES, September 1988, Vol. 24, N. KHARE, "A Cadmium Sulphide Solid Electrolyte Photoelectrochemical Solar Cell", pp. 121-125.
1. | A cell, comprising:(a) a substrate; and(b) a nanocrystalline semiconductor layer located on one or both sides of said substrate, said layer having thickness calculated to minimize recombination losses. |
2. | A cell as in claim 1 wherein said substrate is conductive. |
3. | A cell as in claim 1 wherein said semiconductor layer is porous. |
4. | A cell as in claim 2 wherein said conductive substrate is conductive glass. |
5. | A cell as in claim 1 wherein said semiconductor layer is of a thickness of from about 100 nm to about 1000 nm. |
6. | A cell as in claim 1 wherein said semiconductor layer is of a thickness of from about 200 nm to about 500 nm. |
7. | A cell as in claim 1 wherein said nanocrystalline semiconductor layer is made of crystals of average size of from about 3 to about 100 nm. |
8. | A cell as in claim 1 wherein said nanocrystalline semiconductor layer is made of crystals of average size of from about 4 to about 30 nm. |
9. |
10. | 8 A cell as in claim 1 wherein said nanocrystalline semiconductor layer is made of crystals of average size of from about 5 to about 20 nm. |
11. | A cell as in claim 1 further comprising a second nanocrystalline semiconductor layer located on the other side of said substrate. |
12. | 11 A cell as in claim 1 further comprising an electrolyte in contact with said conductive substrate and said porous nanocrystalline semiconductor layer. |
13. | 12 A cell as in claim 11 wherein said electrolyte is in the liquid form. |
14. | 13 A cell as in claim 11 wherein said electrolyte is in the solid form. |
15. | 14 A cell as in claim 13 wherein said electrolyte is applied in the liquid form and subsequently solidifies. |
16. | 15 A cell as in any of claims 11-14 further comprising a second electrode in contact with said electrolyte. |
17. | 16 A cell as in claim 1 wherein said semiconductor layer is fabricated by chemical solution deposition. |
18. | 17 A cell as in claim 1 wherein said semiconductor layer is fabricated by electrochemical deposition. |
19. | 18 A cell as in claim 1 wherein said semiconductor layer is fabricated by vacuum deposition. |
20. | 19 A cell as in claim 1 wherein said semiconductor layer is fabricated by successive immersion of said substrate in solution of the ions making up said semiconductor. |
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to a photovoltaic or photochemical cell
and, more particularly, to a photovoltaic or photochemical cell made of up
of nanocrystals.
All photovoltaic (PV) cells depend on two principles for their
operation. First, the creation of charges (e-/h+ formation) by photon
absorption. Next, the separation in space by a built-in electric field (the
space charge layer). The first step requires only a semiconductor with a
suitable bandgap. The second step requires a built-in electric field in the
semiconductor. This is formed by generation of a junction which may be
produced in various ways as follows.
One way applies for a p-n junction cell which is used in most types
of photovoltaic cells. Here, either the surface is doped to make it the
opposite conductivity type of the bulk semiconductor (e.g., p-n Si cells) or
a layer of opposite conductivity type, and usually a different
semiconductor, is deposited onto the first semiconductor forming a
heterojunction cell, such as CdS/CdTe.
Another way involves the deposition of a metal onto a
semiconductor of one conductivity type. A careful choice of both metal
and deposition method is needed in these Schottky cells.
A third way calls for the immersion of the semiconductor in an
electrolyte, which is usually a liquid but may be a solid, forming a
photoelectrochemical cell (PEC). A liquid electrolyte has the advantages
of ease of formation and good contact between electrolyte and
semiconductor, but has potential problems in terms of stability and sealing.
Electron/hole recombination in the space charge region is the cause
for loss of quantum efficiency, and can occur by bulk and surface
recombination. The latter is particularly important for polycrystalline
semiconductors where the number of grain boundaries in the direction of
charge transport is inversely proportional to crystal size. For this reason,
polycrystalline films are sought with crystallite size as large as possible,
usually greater than 1 micron.
Semiconductor films having small crystal size are much easier and
cheaper to produce since, for example, they eliminate the high costs
involved with using energy-consuming high temperatures. However, they
normally exhibit prohibitively large recombination losses in photovoltaic
cells. Small crystal size and low recombination losses are normally
considered to be mutually exclusive properties.
Photochemical reactions on semiconductor colloids do show high
quantum efficiencies in many cases, and here the small size is an
advantage, since bulk recombination is negligible. See J. Moser and M.
Gratzel, Helv. Chim. Acta, 65, 1436 (1982), which is incorporated by
reference as if fully set forth herein. However, each colloidal crystal
behaves as a complete \'cell\' with both oxidizing and reducing reactions
occurring on the same crystal. They cannot be used to produce electricity
as in a regular PEC, except in the case of slurry electrodes for
semiconductor suspensions which exhibit low quantum efficiencies since,
according to the mechanism postulated for these systems, the charged
crystallites must make electrical contact with a collection grid before
recombination occurs. See W. W. Dunn, Y. Kikawa and A. J. Bard, J.
Am. Chem. Soc, 103, 3456 (1981), which is incorporated by reference as
if fully set forth herein. Also, semiconductor colloids used for
photochemical reactions are normally dispersed as a dilute phase in a
liquid.
SUMMARY OF THE INVENTION
According to the present invention there is provided a cell,
comprising: a substrate and a nanocrystalline semiconductor layer located
on one or both sides of said substrate, said layer having thickness
calculated to minimize recombination losses.
According to further features in preferred embodiments of the
invention described below, especially where the device is used for the
conversion of light into electricity, the conductive substrate is conductive.
The substrate may be made of many suitable materials, such as conductive
glass, and the like.
According to still further features in the described preferred
embodiments, the semiconductor layer is porous and has a thickness of
from about 100 nm to about 1000 nm and, preferably, from about 200 nm
to about 500 nm. The nanocrystalline semiconductor layer is made of
crystals of average size of from about 3 to about 100 nm, preferably from
about 4 to about 30 nm, most preferably from about 5 to about 20 nm.
According to another embodiment the cell also includes a second
nanocrystalline semiconductor layer located on the other side of the
substrate.
One embodiment of a device according to the present invention
successfully addresses the shortcomings of the presently known
configurations by providing a nanocrystalline photovoltaic cell which
features significantly lower recombination losses than has been heretofore
possible.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is herein described, by way of example only, with
reference to the accompanying drawings, wherein:
FIG. 1 is a photoresponse curve (normalized so that each spectrum
gives approximately the same peak response) as a function of wavelength
for a PEC using a front-wall illuminated, chemically deposited CdSe film
grown at 30°C on conducting glass under illumination from a quartz-iodine
lamp (approximately equivalent to AM2 illumination) on conducting glass
for difference thicknesses of CdSe;
FIG. 2 is as described with regards to Figure 1 but for back-wall
illumination;
FIG. 3 shows the I-N characteristics under solar illumination (920
W/m 2 ) of a 0.9 cm 2 CdSe film (deposited on both sides of double-sided
conducting glass at 30°C under illumination during deposition; the total
thickness of CdSe is about 120 nm) in a liquid junction photovoltaic cell
configuration using an electrolyte of 1 M Νa 2 S and 0.1 M S in water. The
solid line indicates the illuminated I-V plot; the broken line indicates the
dark I-V; the inset shows the transmission spectrum of the film;
FIG. 4 is a schematic model of a porous nanocrystalline film
showing electrolyte contact with individual crystallites; illumination is
shown to produce an electron/hole pair in one crystallite; the hole transfers
to the electrolyte and the electron is shown traversing several crystallites
before reaching the substrate;
FIG. 5 is a plot showing the absorption of blue and red light by a
thin film in both front-wall (FW) and back-wall (BW) modes; the use of
blue and red light were selected to illustrate long and short wavelength
light, respectively;
FIG. 6 is as in Figure 5 but using a relatively thick film.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
, The present invention is of a photovoltaic or photochemical cell
which uses a semiconductor film made up of nanocrystals and yet exhibits
low recombination losses. Throughout, the term \'cell\' is used to indicate
a device for converting light energy into either electrical or chemical
energy, or which uses light energy to catalyze a chemical reaction.
Porous nanocrystalline semiconductor films of CdS and CdSe can
be used as photoelectrodes in photovoltaic cells with relatively low
recombination losses. Spectral response measurements have been used to
show how the recombination losses depend on film thickness. These
nanocrystalline photovoltaic cells are shown to operate due to charge
separation at the semiconductor-electrolyte interface rather than by a
built-in space charge layer as normally occurs in photovoltaic or
photoelectrochemical cells. The rapid removal of one charge by the
electrolyte explains the low recombination loss.
The electrolyte used may be either liquid or solid. When the
electrolyte is solid, the electrolyte is preferably applied to the
nanocrystalline layer as a liquid, which subsequently solidifies. An
example of this would be the use of polymer electrolytes which are formed
from liquid precursors and subsequently polymerized. See M. Morita, T.
Fukumasa, M. Motoda, W. Tsutsumi, Y. Matsuda, T. Takahashi and H.
Ashitaka, J. Electrochem. Soc, 137, 3401 (1990), and references cited
therein, which is incorporated by reference as if fully set forth herein.
The principles underlying a cell according to the present invention
can be better understood with reference to the accompanying Figures.
Without in any way limiting the scope of the present invention, the
mechanism which gives the photovoltaic cells according to the present
invention their unique and desirable characteristics, and the manifestation
of the mechanism in measurable phenomena, are discussed below, after an
example of the preparation of illustrative photovoltaic cells according to the
present invention.
It is to be noted that in all photovoltaic cells light is absorbed in the
semiconductor. This is technologically distinct from photogalvanic cells
wherein light is absorbed by a sensitizer which then transfers charge to a
semiconductor. Recent advances in the latter technology have been made
whereby the underlying semiconductor layer is highly porous. The porous
semiconductor layers described in N. Vlachopoulous, P. Liska, J.
Augustynski and M. Gratzel, J.A.C.S., JJO, 1216 (1988), which is
incorporated by reference as if fully set forth herein, may be used as
substrate in the devices according to the present invention.
The semiconductor films described here may be used also for
photochemical cells, where a chemical reaction is driven or catalyzed by
light. In such cells, the substrate need not be electrically conducting and
may even advantageously be porous, allowing flow of solution through the
substrate. As for the photovoltaic cells described herein, the
nanocrystallinity of the semiconductor deposited on, or distributed through,
the substrate is of critical importance to the present invention.
According to the present invention, porous, nanocrystalline
semiconductor films can be used as photoelectrodes with relatively low
recombination losses, in spite of the nanocrystalline structure, i.e., crystal
size on the order of 6 nm. Spectral response measurements show that
above a certain layer thickness, which depends on crystallite size,
recombination increases, and this increase depends strongly on the direction
of illumination, whether it is front-wall or back-wall.
Charge separation in these photoelectrodes takes place at the
semiconductor-electrolyte interface which are found throughout the
thickness of the porous film, rather than by a built-in space charge field.
The relatively low recombination losses are due to the rapid removal of
one of the charges or holes allowing electrons to pass through a number of
grain boundaries with a small chance of meeting a hole on its journey to
the substrate. In some cases, the films show \'p-type\' photoresponse
behavior as would be expected if the direction of current flow were
determined by the semiconductor surface/electrolyte boundary rather than
bv a field in the bulk of the semiconductor.
EXAMPLES
A film according to the present invention was prepared by
electrodepositing CdS according to the method of Baranski et al. (A.S.
Baranski, W.R. Fawcett, A.C. McDonald, R.M. de Nobriga and J.R.
McDonald, J. Electochem. Soc. 128, 963 (1981)), which is incorporated by
reference as if fully set forth herein, from a DMSO solution of CdCl 2 (0.05
M) and S (0.1 M) under a constant current density of 1 mA cm "2 onto a Ti
substrate at 120°C. The CdSe film electrode was immersed in an aqueous
solution of 1 M each KOH, Na 2 S and S. A second electrode of Pt was
immersed in the same solution. When illuminated with tungsten-halogen
light of 100 mW/cm 2 intensity, the photovoltage between the two
electrodes was measured to be 0.26 V and the short circuit photocurrent
was 0.19 mA/cm 2 .
In another example, CdSe was chemically deposited onto SnO 2
coated glass ( <10Ω) from an aqueous solution containing 80 mM CdS0 4
nitriloacetate (KNTA) and 80 mM sodium selenosulphate. The Cd NTA
complex solution was adjusted to a pH of 9.5 (±1.5) with KOH solution.
This solution and the Na 2 S0 3 solution (made by dissolving 0.2 M black Se
in 0.4 M Na 2 SO 3 for several hours at about 60°C) were brought to the
desired temperature and then mixed.
The substrates were put into the solution, which was then placed in
a thermostatic bath under illumination from an ELH lamp roughly
equivalent to sunlight (AM2). Deposition occurred over several hours and
the film was removed from the bath when a total thickness of 120 nm (60
nm per side) was reached. The film was made into a liquid junction
photovoltaic cell as in the first example, but using an electrolyte 1 M in
Na 2 S and 0.1 M in S. When illuminated in sunlight (920 W/m 2 ), an open
circuit voltage of 0.47 V and a short circuit current of 1.45 mA/cm 2 was
generated.
To understand the results described above, it is convenient to start
with the model of the junction between the nanocrystalline semiconductor
layer and the electrolyte junction described previously, as shown in Figure
4. A photon is absorbed by one nanocrystal, forming an
electron hole pair. Since the direction of photocurrent flow is, in most
cases, that of an n-type semiconductor, this means that the hole goes into
solution where it oxidizes an electron donor, while the electron reaches the
back contact, typically a conducting glass. Clearly, the farther the
electron/hole pair is generated from the back contact, the greater the chance
of the pair being lost by recombination.
This visualization of the phenomenon lends itself to the creating of
a model which can be made for photocurrent generation as a function of
average distance of charge generation from the back contact. An
illustration of the model is shown schematically in Figures 5 and 6.
Figure 5 illustrates a thin film of CdSe, where a thin film is taken
to mean a film of a thickness approximately equal to the absorption
coefficient (α) for blue light.
For a thin film of CdSe (Figure 5), FW illumination with blue light
leads to absorption of most of the light relatively close to the substrate
because the film is thin. For red light, the situation is similar except that
the absorption of the light is not as strong. No appreciable difference is
expected for BW illumination, as is evident from Figure 5.
For a thick film, defined as one which is considerably larger than α
for blue light, FW illumination with blue light results in most of the charge
carriers being generated relatively far from the substrate, and the electrons
being as a result more susceptible to recombination loss. With FW red
light illumination, more carriers are generated closer to the substrate. This
should lead to a poor photocurrent response for short wavelength light,
which improves with increase in wavelength until the band edge is reached,
at which point the response drops again. Also, the long wavelength
response should improve with increasing thickness (up to a certain point)
since more of the long wavelength light is absorbed.
Considering now BW illumination of the thick film, the situation is
reversed compared with FW illumination. In the case of BW illumination,
blue light is absorbed close to the substrate while red light is absorbed
further away. This should result in a good response for blue light, with the
close to bandgap response poorer than for FW illumination, since some of
this light is absorbed relatively far from the substrate.
CdSe films deposited on conducting transparent glass were used,
thus allowing illumination from either side of the film, i.e., either front-
wall [FW] or back-wall [BW] illumination.
Crystallite sizes can be measured by electron microscopy or
estimated from the transmission spectra. Using the latter method, it is
estimated that the crystal sizes for the samples used in Figures 1 and 2 are
about 6.5 nm.
The results described above demonstrate that in order to obtain
optimum photocurrents from photovoltaic cells based on these
nanocrystalline films, the optimum configuration will often be a thin film
deposited on both sides of double sided conducting glass. Figure 3 shows
I-V characteristics, both dark and illuminated, of such a CdSe bi-film with
the transmission spectrum of the film shown in the inset.
It is to be noted that attempts to make a solid state Schottky junction
with these films resulted in a high resistance I-V characteristic, which was
virtually collinear with the voltage axis, and little, if any, photoeffect, as
described in Hodes et al.
Two methods were employed to control crystal size in these films.
One way was by controlling the deposition temperature. Another involved
the use of illumination during deposition. An increase in either the
deposition temperature or the illumination during deposition led to an
effective increase in crystal size.
When illumination was used during deposition, the spectral
responses shown in Figures 1 and 2 were obtained. Quantum efficiency
measurements for these films in the region of strong absoφtion and flat
photocurrent response gave values typically between 0.6 and 0.7.
The behavior to be expected based on the above model corresponds
closely to the results in Figures 1 and 2 in all aspects, which attests to the
accuracy of the model. In addition, the thickness at which the peak in the
spectra first begins to appear gives an indication of the thickness of the
active layer of CdSe. For thicknesses greater than this thickness, light
absorbed in the region farthest from the substrate (blue for FW and red for
BW illumination) will be subject to increasing recombination and resistance
losses. For the films with crystal size of about 5.0 nm, this thickness is
about 150 nm, while for the film in Figures 1 and 2 (crystal size of about
6.5 nm), it is about 300 nm.
While these thicknesses may seem thin if close to total absoφtion
of light is desired, as would be the case for solar cell use, it should be
noted that the oscillator strength, and therefore the absoφtion coefficient
of these size-quantized films may be enhanced relative to bulk material.
The α in the region of strong absoφtion is about 2xl0 5 cm "1 for the
approximately 5.0 nm crystallites and about lxlO 5 cm "1 for the 6.5 nm
crystallites. This is to be compared with approximately lxl 0 5 cm \'1 for
\'normal\' CdSe. Hence, preferable film thicknesses for maximum
absoφtion would be range from approximately two or approximately three
times those thicknesses at which peak behavior for FW illumination start
to be exhibited. Glass with a conducting layer on both sides, and with the
semiconductor deposited on both sides, would come close to fulfilling this
requirement.
There are two loss mechanisms in films such as those described
herein as the thickness increases. First, there is a resistance loss, leading
to loss in photovoltage and fill factor. Second, there is a recombination
loss, presumably occurring at crystal boundaries, which leads to loss in
photocurrent. These losses are in addition to the possibility of indirect
recombination by electron injection into the electrolyte, which is probably
the predominant loss mechanism for thin films. These recombination
losses amount to 30-40% reduction in efficiency, as described earlier.
Direct recombination is relatively low for LTs in contrast to a solid
junction because the holes are removed rapidly by the electrolyte, which
lowers the recombination probability.
The mechanism for charge separation described herein is quite
different from that in a conventional PEC or PV cell. In effect, these films
behave as colloidal semiconductors. It is generally accepted that the
existence of an appreciable space charge layer in small lightly doped
semiconductor particles is unlikely, since the particle is too small to
support a field in its bulk. See A. J. Bard, J. Phys. Chem., 86, 172 (1982),
which is incoφorated by reference as if fully set forth herein.
In such a case charge separation occurs at the particle surface by
transfer into the electrolyte. For a colloidal system, such a charge transfer
is measured by the degree of chemical reaction occurring due to either or
both hole and electron transfer. The two reactions are different, resulting
in a non-regenerative system. If the reduction and oxidation reactions were
the same, forming a regenerative system, no net change, other than heat
production, would occur.
For the films described here, however, both regenerative and
non-regenerative reactions can be used, and in the former case, the system
acts as a PV cell with electricity being produced. However, in contrast to
other PV systems, and in common with dispersed colloidal photochemical
systems, charge separation occurs not by a space charge layer, but rather
by differing rates of electron and hole transfer into solution.
It will be clear that the methods used to prepare films according to
the present invention may include most of the methods commonly used to
deposit semiconductor films, and, in addition, some less commonly used
methods. Because it is desired to produce crystals of small size, in contrast
with the large crystal size normally required for photovoltaic cells, the
preparation techniques can be based on low temperature techniques, using
temperatures on the order of 0 to 200°C, rather than on the more common
but more demanding higher temperature techniques taking place at
temperatures of 400 to 600°C. Use of low temperature techniques
dramatically the energy consumption and thus the cost of manufacture.
Besides the methods of electrochemical deposition and chemical
solution deposition described in the examples above, vacuum evaporation
and sputtering (vacuum deposition) onto substrates which are at
approximately room temperature or even cooler will give small crystal size.
The presence of an inert gas under low pressure during vacuum
evaporation will help to form the small crystal sizes required. However,
a large variety of low temperature methods, including, but not limited to,
electrophoresis, anodization, and those mentioned above, can effectively be
used. Another fabrication technique involves the successive immersion of
the substrate in solution of the ions making up the semiconductor.
The porosity of the film may be controlled by depositing a
composite of the desired semiconductor together with an easily dissolvable
material. This may be desirable to increase the rate of diffusion of solution
species in the porous film. Thus, as example, CdSe could be evaporated
or sputtered together with Al (from separate sources) and the Al
subsequently dissolved by an alkali metal hydroxide solution (which will
not dissolved the CdSe). As long as the concentration of Al is not too high
(in which case the film would disintegrate) this will trace a film of porosity
depending on the ratio of CdSe to Al.
For porous substrates (which, for photochemical reactions, may be
either conductive or non-conductive and preferably transparent to the
illumination), simple successive immersions of the substrate in solutions of
the ions making up the semiconductor may be sufficient. For example, to
prepare CdS on a porous substrate, the substrate may be dipped first in a
solution of a cadmium salt, then in a sulfide solution (or vice versa).
While the focus throughout the above description has been on use
of devices according to the present invention as photovoltaic cells for the
production of electrical power, it will be clear that nanocrystalline
semiconductor layers according to the present invention may also be used
for both photocatalytic and photoconversion reactions, when the system is
non-regenerative.
Thus, instead of the production of electricity being the primary
product, chemical energy, in the form of desired chemical products,
becomes the primary product. While ostensibly a radical departure from
use of devices according to the present invention as solar cells, it should
be appreciated that the only difference is that, in the case of electricity
production, the electrolyte is chosen so that whatever is oxidized (reduced)
at the semiconductor electrode is reduced (oxidized) back to the original
reactants at the second electrode, while in the case of chemical production,
the electrolyte is so chosen that the reactions at the two electrodes are
different and there is a net production of new chemical products.
An example of this is the photoelectrolysis of water. If dissolved
oxygen is present at the non-semiconductor electrode, the oxygen is
reduced to water, exactly balancing out the evolution of oxygen at the n-
type semiconductor electrode. In this case the only net production is of
electrons. However, if oxygen is absent, hydrogen may be evolved instead,
making the device a chemical cell.
An example of a photocatalytic reaction where such films may be
of used is the photo-oxidation of small amounts of organochlorine or
cyanide contaminants in water. See C. Korman, D. W. Bahnemann and M.
R. Hoffman, Environmental Sci. & Tech., 25, 494 (1991), which is
incoφorated by reference as if fully set forth herein.
The advantage of the porous layers over more usually proposed
suspensions or colloids of the semiconductor is that the semiconductor is
present in a convenient, immobilized form on a substrate which can be
easily immersed and removed from the water, unlike the case with the
removal of a dispersed suspension.
For such puφoses, the substrate itself may also be porous and of
high surface area. A preferred embodiment in this case might be a flow-
through \'membrane\' whereby the water to be purified flows through the
illuminated porous semiconductor and porous substrate.
While the invention has been described with respect to a small
number of preferred embodiments, it will be appreciated that many
variations, modifications and other applications of the invention may be
made.
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