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Electrolyte modified photoelectrochemical solar cells
I
Solar Energy Materials and Solar Cells
ELSEVIER
Solar Energy Materials and Solar Cells 38 (1995) 305-319
Electrolyte modified photoelectrochemical solar cells Stuart Licht Clark University, Department of Chemistry, Worcester, MA 01610, USA
Abstract Rationale electrolyte modification of photoelectrochemical systems can be used to (i) enhance facile charge transfer, (ij) suppress competing reactions and suppress both (iii) electrode and (iv) electrolyte decomposition products, as well as (v) substantially effect the open-circuit photovoltage. Studies on polysulfide, ferrocyanide, polyselenide and polyiodide electrolyte modification of photoelectrochemical solar cells are discussed. Electrolyte modification of semiconductor/electrolyte systems entails investigation of the primary photo-redox species, the nature of the counter ion, the distribution of species in solution, and related competing reactions. The examples presented emphasize the fundamental and practical importance of probing the active electrolytic constituents pertinent to overall photoelectrochernical energy conversion.
1. Introduction
In operational photovoltaic devices (PVs) an illuminated semiconductor is in intimate contact with other solids, either semiconductors or metal. Photoelectrochemical cells (PECs) bear a resemblance to PVs. However, in PEe devices the solid contact is replaced by an electrolyte containing an appropriate redox couple. For both PECs or PVs, sufficiently energetic light can activate charge separation at a semiconductor, driving electronic charge into the semiconductor bulk. A significant application of illuminated solid photovoltaic and liquid photoelectrochemical semiconductor junction devices is for solar to electrical energy conversion as discussed by Chandra [1]. In ideal regenerative photoelectrochemical solar cells illumination does not change the composition of the cell electrolyte, and the only product is electrical energy. In illuminated semiconductor systems the absorption of photons generates excited electronic states. These excited states have lifetimes of limited duration. Without a mechanism of charge separation their intrinsic energy would be lost through relaxation (recombination). Several distinct mechanisms of charge separa0927-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDl 0927-0248(94)00229-0
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S. Licht / Solar Energy Matenals and Solar Cells 38 (1995) 305-319
Carrier Generation Under Illumination
..------t\ Lood I t - - - f \
"-type ,emlc:onductor
redox
counter-
electroll,Ite
electrode
A Semiconductor! Electrolyte Interface
..------t\ Lood
"-type dye semiconductor
lr---f\
redox electrol yte
counter ... electrode
B Semiconductor! Dye Sensitizer! Electrolyte Interface
Fig. 1. Carrier generation under illumination arising at (A) the semiconductor/liquid interface and (B) the semiconductor/dye sensitizer/liquid interface.
tion have been considered in designing efficient photoelectrochemical systems. At illuminated semiconductor/liquid interfaces, an electric field (the space charge layer) occurs concurrent with charge/ion redistribution at the interface. Upon photogeneration of electron/hole pairs this electric field impedes recombinative processes by oppositely accelerating and separating these charges, resulting in minority carrier injection into the electrolytic redox couple. This concept of carrier generation is illustrated in Fig. lA (for an n-type PEC) and has been the theoretical basis for several efficient semiconductor/redox couple PECs. Excitation can also occur in molecules directly adsorbed and acting as a mediator at the semiconductor interface. In this dye sensitization mode, the function of light absorption is separated from charge carrier transport. Photoexcitation occurs at the dye and photo-generated charge is then injected into a wide bandgap semiconductor. This alternative carrier generation mode also can lead to effective charge separation as illustrated in Fig. IB. The first efficient example of such a device was demonstrated by Gditzel et al. [2] through the use of a novel high surface area (nanostructured thin film) n-TiO z, coated with a well matched trimeric ruthenium complex dye immersed in an aqueous polyiodide electrolyte. The unusually high surface area of the transparent semiconductor coupled to the well matched spectral characteristics of the dye leads to a device which harvests a high proportion of insolation. PECs can generate not only electrical but also electrochemical energy. Fig. 2 presents one configuration of a PEC combining in-situ electrochemical storage and solar conversion capabilities; providing continuous output insensitive to daily variations in illumination. A high solar to electric conversion efficiency cell configuration of this type was demonstrated by Licht et al. [3J and utilized a Cd(Se,Te)/S; conversion half cell and a Sn/SnS storage system resulting in a
S. Licht / Solar Energy Materials and Solar Cells 38 (]995) 305-319
307
R
B Fig. 2. Schematic of a photoelectrochemical solar cell combining both sol'lr conversion and storage capabilities. (A) Under illumination; (B) in the dark.
solar cell with a continuous output. Under illumination, as seen in Fig. 2A, the photocurrent drives an external load. Simultaneously, a portion of the photocurrent is used in the direct electrochemical reduction of metal cations in the device storage half cell. In darkness or below a certain level of light, the storage compartment spontaneously delivers power, as seen in Fig. 2B, by metal oxidation. The PEC utilizes light to carry out a chemical reaction, converting light to chemical energy. This fundamental difference of the PV solid/solid interface and the PEC solid/liquid interface has several ramifications in cell function and application. Hence, photoelectrochemical systems may facilitate not only solar to electrical energy conversion, but also has led to investigations in photoelectrochemical synthesis, photoelectrochemical production of fuels and photoelectro-
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S. Licht / Solar Energy Materials and Solar Cells 38 (1995) 305-319
chemical detoxification of pollutants. A system exemplifying photoelectrochemical synthesis is water photoelectrolysis to generate hydrogen. An early demonstration of water photoelectrolysis was presented by Fujishima and Honda [4] utilized Ti0 2 (band gap 3.0 eV) and was capable of photoelectrolysis at .... 0.1 % solar to chemical energy conversion efficiency. Bard et al. [5] have tested various semiconductor particles for photoelectrochemical breakdown of inorganics. Gerischer and Heller [6] have investigated the theoretical consequences of photocatalytic oxidation of organic materials at Ti0 2 particles by sunlight in aerated waters, and application to pollutant detoxification is being carried by Fujishima and a variety of research groups [7]. In 1987 Licht [8] presented a chemical mechanism for photoelectrochemical solar to electrical energy conversion which emphasized understanding of the distribution of species in photoelectrochemical electrolytes. The approach systematically modifies obsetved photoelectrochemical kinetic and thermodynamic phenomena. This paper probes our utilization of this systematic approach in studies on polysulfide, ferrocyanide, polyselenide and polyiodide photoelectrochemical solar cells. The results indicate that specific design of the distribution of species in an electrolyte can enhance open-circuit voltage, and be used to (i) enhance facile charge transfer, (ij) suppress competing reactions and suppress both (iii) electrode and (iv) electrolyte decomposition products, as well as (v) substantially effect the open-circuit voltage. 2. Experiment Details of the experiments discussed in this review are referred to in each of the individual study's reference. In several of the studies molar concentration units were used and are denoted as "molar" or "M". Alternately in several of the studies molal concentration units were used and are denoted as "molal" or "m". 3. Results and discussion 3.1. n-cadmium chaicogenide / aqueous poiysu/fide photoelectrochemistry
In 1976 the first regenerative pbotoelectrochemical solar cells with substantial and sustained solar to electrical conversion efficiency were demonstrated. These PECs are based on n-type cadmium cbalcogenide ($, Se or Te) electrodes immersed in aqueous poly-cbalcogenide electrolytes. The cells were introduced by Hodes, Cahen and Manassen [9], Wrighton et al. [10], and Heller and Miller [11] and were capable of converting up to 7% of insolation to electrical energy. Most investigations of these systems focused on solid state and interfacial aspects of these PECs and photodriven oxidation of polysulfide at the photoelectrode was represented:
S= +2hv
n - Cd(Se,Te) ~ S
+ 2e-.
(1)
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S. Licht / Solar Energy Materials and Solar Cells 38 (J995) 305-319
Photoelectrochemlcal Solar Cell Solution Phase Studies hv
;-H
Photoalectroda
hv
hv
InsoluUon
InsolUllon
Fe{CN)&"" HxFe(CN)&(X-4, KyFe{CN1&(y-4,
H25e H5e5e= 5112= 51l3= 5e4= OHH+
I
InsoluUon H~
H55=
Hx~Fe(CN)sIY·X-4'
52"
HxFe{CN)&(X-3, KyFe{CN)s(y-3, HxKyFe{CN)&(y·X-3, CNOH-
~-
5,= 54" 5s"
ot{-
H+
H+
' - ~ Counter electrode
Aqueous Polysulftde PEe
?•
I
Aqueous Ferrocyanide PEe
Aqueous Polyselenide PEe
Fig. 3. Chemical species coexisting in polysulfide (left), ferrocyanide (middle) and polyselenide (right) aqueous electrolytes used in regenerative photoelectrochemical solar cells.
However, as represented in the right hand portion of Fig. 3, the number and type of species in polysulfide photoelectrolytes is considerably more complex than represented by Eq. (1). The addition to water of a simple soluble sulfide salt and sulfur to water gives rise to a wide distribution of species in solution. The equilibria constraining this distribution has been investigated by Licht et al. [12-17] Gigenbach [18] and Teder [19]. We have shown that modification of the distribution of species in aqueous polysulfide solution can be correlated to variations in transparency, conductivity, activity, and cadmium chalcogenide photo electrochemistry. Specifically, the separate effects of hydroxide and pH modification [20], sulfur [21] and sulfide [22] and cation [23,24] were investigated in terms of speciation and photoelectrochemical phenomena. Previously cadmium chaicogenide polysulfide PECs had generally employed electrolytes composed 1 molar in sodium sulfide, sulfur and sodium hydroxide and resulted in solar conversion efficiencies of approximately 7 percent [9-11]. However, added hydroxide is to be qtinimized in these cells cesium is the preferred cation, and a sulfur to sulfide ratio of approximately 1.5 to 1 resulted in a near doubling of the conversion efficiency [25]. Fig. 4 presents n-Cd(Se,Te) photocurrents measured at various applied potentials in a traditional and modified electrolyte. As seen in the figure, the cumulative effect of polysulfide electrolyte modifications on photo electrochemical solar to electrical energy conversion by n-Cd(Se,Te)/aqueous polysulfide PECs can be considerable. A greater percentage of photogene rated holes utilized in constructive oxidation of polysulfide results in enhanced photocurrents. This has the additional benefit of
S. Licht / Solar Energy Materials and Solar Cells 38 (J995) 305-319
310
n-CdSeo.6sTeo.3S in aqueous polysulfide
."",,---------
1/ 7.796 efficiency
1.6
«
E
.;
......c CD
:::I 0
...
I I
0
0.6 .c0 a.
I
I
-AM 1 illumination
I Voltage, mV
Fig. 4. Potentiostatic photocurrent-voltage characteristics for a 0.099 cm 2 single crystal n-CdSe o.65TeO.35 photoelectrode immersed in either of two types of aqueous polysulfide electrolyte. The top curve is an electrolyte 1.8 M CS 2 S and 3 M sulfur: The bottom curve is an electrolyte 1 M NaOH, 1 M Na 2 S and 1 M sulfur. The photocurrent voltage curves were obtained outdoors and solar to electrical conversion efficiencies are indicated.
fewer oxidizing holes available for attack on the semiconductor (photocorrosion). As seen in Fig. 5, this results in enhanced photocurrent stability of the PEe. This study showed that with solution optimization not only the photocurrent, but also the polysulfide electrolyte exhibits enhanced lifetime, both approaching one year operation outdoors [26]. 3.2. n-CdSe / aqueous /e"ocyanide photoelectrochemistry Reichman and Russak [27] and Freeze [28] had reported high (12 to 14 percent) conversion efficiencies for n-CdSe/(Fe(CN)~-/4-) solar cells in electrolytes containing a 1:1 ratio of aqueous Fe(CNn- to Fe(CN)~- salts in a highly alkaline environment. However, Rubin et al. [29] had shown that these nCdSe/Fe(CN)~-/4- solar cells can have a limited stability and photocurrents decrease within the order of hours. Their attempts to enhance the photoconversion stability of these systems have included cationic surface modification of the n-CdSe photoelectrode and isolation of the wavelength dependence of the surface instability [29,30]. In these studies, photo-oxidative processes in n-CdSe/Fe(CN)~-/4PECs were typically represented: 4- n-CdSe
Fe ( CN ) 6
-
(
Fe CN
)36
+ e-.
(2)
The role of the various redox active constituents in constrammg nCdSe/Fe(CN)~-/4- photo-oxidative energy conversion had not been previously
S. Licht / Solar Energy Materials and Solar Cells 38 (J995) 305-319
311
O.~,...----r-------r----""
~----o
-0(
o
....
0:: .... > 0 .... 0 l::J
-0( I-
.... o
> eo a :I:
~
JAN
JULY
JULY
JAN
TIME Fig. 5. Long-term outdoor stability curves of thin film CdSeo.7sTeo.3s in electrolytes of either 1.8 M S (solid line) or 2.0 M KOH, 1.4 M Na2S, and 2.6 M S (shaded region). The sodium/potassium electrolyte cell results are averaged results over separate cells, with initial conversion efficiencies from 3.5 to 4.5%. The cesium electrolyte cell had an 4.6% conversion efficiency potential measured under a load chosen for initial maximum power.
Pholoeleclrochemislry of n-CdSe in [Fe(CN)s-Lj3-/2°aq
10 N
E o
L
«
a) -CN-
if 5
b) -NH3
:::J
o
c) -N02°
.c
d) -Fe(CN)s3-
E
...~
o
o
D.
-+--I''-f---..l2:.......---- e) -NO-
o
0.0 0.4 Potential, Volts vs PI Fig. 6. Potentiostatic photocurrent-voltage curves for an 0.17 cm 2 single crystal n-CdSe, illuminated by tungsten-halogen light in several aqueous modified ferro/ferricyanide electrolytes. The electrolytes contain [Fe(CN)s - LP- and [Fe(CN)s - Lj2- with different ligand, L. L = [Fe(CN)sP-, -NH3' -CN-, -NOi or NO+. Voltage swept at 5 mV /sec. The oxidized component of the redox electrolyte is generated in-situ by electrolytic oxidation of a portion of the reduced [Fe(CN)s - LP- species. Electrolytes with L = [Fe(CN)sp-, -CN- or -NOi have a common composition and contain 0.25 m [Fe(CN)soLP-, 0.0125 m [Fe(CN)s-Lj2-, 0.1 m KOH. In the case of L = -NH), the electrolyte contains 0.25 m [Fe(CN)s-NH 3P-, 0.0125 m [Fe(CN)s-NH)12-, 0.1 m NH 4 Cl in 3.5 M ammonia_ For L =-NO+, the electrolyte composition is 0.25 m [Fe(CN)s-NO]2-, 0.6 m [Fe(CN)s-NO]-, pH = 4 buffer. 0
0.8
0
312
S. Licht / Solar Energy Materials and Solar Cells 38 (J995) 305-319
.
20
40mW/cm2 illuminated single crystal n-CdSe in 0.5 mK4Fe(CN)6. 25 mmK3Fe(CN)6. 0.5 mKOH without (dashed) or with (solid line) 0.' mKCN ~------
E
~
E
15
!i1 r
-> ~
iii
z
W
10
Q
d
l'i
~
Z W II: II: :::I
:==========i11
I E I g
,
I §~--~--~----~J1 I 00 1 TIME, days 3
5
(J
,
/
No illumination
~_~J-_....-:o;..---
o -1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
VOLTAGE, volts vs Pt
Fig. 7. Photocurrent-voltage curves of illuminated single crystal n-CdSe immersed in alkaline potassium ferrocyanide electrolytes with and without added cyanide. Inset: Photocurrent stability in several electrolytes. "e" is the only electrolyte with cyanide. Electrolytes "d" and "e" contain low ferricyanide. Electrolyte "b" contains high ferricyanide. Specifically, e: 0.25 m K 4 Fe(CN)6' 0.01 m K 3 Fe(CN)6' 0.5 m KOH 0.1 m KCN; d: 0.25 m K 4 Fe(CN)6' 0.01 m K 3 Fe(CN)6' 0.5 m KOH; b: 0.25 m K 4 Fe(CN)6' 0.25 m K 3 Fe(CN)6' 0.5 m KOH. In an electrolyte comparable to "b" but with pH = 3.8, photocurrent diminished within seconds.
investigated, and ferricyanide and ferrocyanide salts exhibits a complex aqueous equilibria. However, in a manner analogous to the polysulfide electrolytes, and as illustrated in the middle of Fig. 3, these salts exhibit a complex aqueous equilibria resulting in a variety of Fe(II) and Fe(II!) species. As with the polysulfide electrolytes, we have probed the equilibria constraining the distribution of species in these solutions. This speciation can be controlled and substantially effects n-CdSe photo-oxidative energy conversion. We have studied the photoelectrochemical effect and solution stability effects of pH, cation modification and ratio of ferrocyanide to ferricyanide, Licht and Peramunage [31-34]. In addition we have studied systematic variation of the primary photo-oxidized species. As seen in Fig. 6, substitution of a single ligand in the hexacyanoferrate species has a substantial effect on photovoltage and photocurrent, Licht and Peramunage [35]. As seen in Fig. 7 addition of potassium cyanide to an alkaline ferrocyanide electrolyte substantially enhances both photocurrent stability (figure inset) and photovoltage (figure outset), Licht and Peramunage [31]. It was proposed by Bocarlsy et al. [36] that in addition to Fe(CN)~-, CN- was also photooxidized (to CNO-) by the photoelectrode. Our subsequent investigations indicated that whereas cyanide may be chemically oxidized to cyanate, no cyanide was photoelectrochemically oxidized by the semiconductor, Licht and Peramunage [32-34].
S. Licht I Solar Energy Materials and Solar Cells 38 (]995) 305-319
313
3.3. n-GaAs / aqueous polyselenide photoelectrochemistry
n-GaAs/aqueous polyselenide photoelectrochemical solar cells (PECs) have shown stable efficient solar to electrical conversion. Photodriven oxidation of polyselenide had been written: Se= +2hv
n-GaAs ~
Se + 2e-,
(3)
or 2Se= +2hv
n-GaAs ~
Sei + 2e-.
(4)
Parkinson et al. [37] and Lewis et al. [38] have shown that metal ion (Ru3+, OS3+) treatment of the n-GaAs surface leads to high solar to electrical conversion efficiencies in these cells (with up to 15 percent conversion efficiencies reported). Lewis et al. [39] have further investigated O.IM to l.OM concentration K 2 Se electrolytes. The first and second acid dissociation constant of hydrogen selenide of pK 1 = 3.9 and pK 2 = 13.0 have been well characterized by several investigators including Myers et al. However, as represented in the right hand portion of Fig. 2, the fundamental equilibria constraining the distribution of polyselenide species was not characterized. Measurements of these equilibria are a first step in determining the regenerative reactive species in polyselenide medium and probing how they effect photoelectrochemical energy conversion. The equilibria constraining the formation of diselenide, triselenide, and tetraselenide may be written: Sej + Se= 2Se;
~
2Sei ,
+ Se= ~ 3Sej ,
(5) (6)
with equilibrium constants for the equilibrium given by Kn and K 34 , respectively. These equilibria were probed by study of rest potential variation with solution composition and by isolation of the near UV absorption peaks of the various polyselenide species, and yield equilibrium constants of pK 23 = -0.65 and pK 34 = -4.2. When combined with Kl and K2 these provide a description of polyselenide speciation. Using the understanding of polyselenide speciation in solution, we have been investigating rationale electrolyte modification of aqueous polyselenide photoelectrochemical solar cells. n-GaAs photocurrent, photovoltage and photopower are affected by the distribution of hydroselenide, selenide and polyselenide in solution. Polyselenide electrolytes containing 1 to 2 molar hydroxide and 1 molar selenide as K 2 Se) enhance n-GaAs photocurrent density (Reported as "I" in Fig. 8) stability while photocurrent is unstable at either 0.5 molar KOH or 3.0 molar KOH. The presence of polyselenide species in low concentrations (as 0.01 molar dissolved selenium) are important for desorption of selenium from the n-GaAs. Furthermore, electrolytes enriched in dissolved selenium, (as up to 0.2 molar dissolved selenium) enhance n-GaAs photovoltage. As seen in Fig. 9, the photopower (the product of the photocurrent and the applied potential) as well as the photopoten-
314
S. Licht / Solar Energy Materials and Solar Cells 38 (1995) 305-319
r---.----,---.----,.--..,...----r-----,
19
J.~ble
~---------J. Auctuating (Corrosion)
J. Dimiristlng
j 15
~_~_~~_~_~~_~_~
o
__
~
__-J
3
2
4
[KOH].M
Fig. 8. Short-circuit photocurrent density, l"" for a 75 mW/cm 2 tungsten-halogen illuminated single crystal n-GaAs immersed 1 M K 2 Se, 0.01 M Se and in varying KOH concentration.
tial improves with an increase in selenium. However, these electrolytes diminish electrolyte transmittance necessitating use of an alternate back wall cell configuration discussed in Licht and Forouzan [40-42].
3.0
.---.,..---,----.----r--......- - - - r - - - - ,
2.5
• • ~
0.01 M Sa 0.02 M Sa 0.20 M Sa
2.0
1.5 1.0
0.5 0.0
0.0
0.2
0.4
0.6
Photovoltage. volts
Fig. 9. Photopower variation of a 92 mW/cm 2 tungsten-halogen illuminated backwall n-GaAS/polyselenide PEe as a function of increasing selenium concentration.
S. Licht / Solar Energy Materials and Solar Cells 38 (1995) 305-319
315
3.4. Aqueous polyiodide photoelectrochemistry
The aqueous polyiodide 0-/1 3) is the only redox couple shown to be compatible with efficient oxidative photoelectrochemistry at n-type transition metal dichalcogenide including WSe 2, MoS 2, WS 2, MoSe 2. Following the pioneering work of Tributsch et al. [43], several groups including Parkinson et al. [44] and Tenne and Wold [45] have reported n-Wse2 or n-MoSe 2/aqueous polyiodide solar to electrical energy conversion of over 10% and/or photocurrent stability in excess of 10 5 coulombs/cm 2. The photodriven oxidation of iodine at tungsten diselenide . has been described in one of two manners:
(7) or
31-+ 2hv
n- WSe 2 ~
13 + 2e-
(8)
Aqueous Polyiodide Speciation 0.01 M iodine in the indicated concentration of sodium iodide total iodine
=0.01
m
=[121 + [I; I + [I: I
total iodide = [Nail = [I •J + [I ~
iii .-o
-
60%
~
40%
o
'ECD CD
•
I + 2[1 : I
[I; Illotal iodine
D..
o
[I 21110tal iodine
•
total iodine I total iodide
12
4 [Nail. m 8 2
4
6
8
10
12
[Nail, molal
Fig. 10. The fractions of 1:3 and 14' (compared to total iodine) in aqueous polyiodide solutions as a function of added iodide concentration. Inset: The fractions of solution phase iodine (compared to total iodine) and total iodine (compared to total iodide).
S. Licht / Solar Energy Materials and Solar Cells 38 (J995) 305-319
316
Significant species in aqueous polyiodide solution can include: OH-, and H+. Constrained by pH and the equilbria: 1-+ 12 +2 13 13
r, 13,
K3 = 723,
I~-,
r, (9)
+ r+2 I~- K4 = 0.20.
(10)
To a lesser extent the solution phase species 103, HIO, H 2 0 1+, 15' and I~- can also occur. Fig. 10 presents the relative variation of polyiodide speciation in solutions containing 0.01 molal iodine and 1 to 12 molal iodide and which may be compatible with n-WSe 2 regenerative photoelectrochemistry. As emphasized in the figure inset, in these electrolytes the concentration of added iodide dominates iodine in these solutions. The distribution of species in these solutions is calculated in accordance with Eqs. (9) and (10). As can be seen in the figure inset little (less than 0.3%) of the added iodine exists as solution phase iodine, and the maximum [1 2 ] is «1 X 10- 4 m. As seen in the figure outset, the bulk (greater than 99.7%)
illuminated n-WSe 2
in 6 m iodide. 0.01 m iodine
1.0
0.8
Gi ~
~
0.6
CD
~
.;
a:
0.4
0.2
0.0
in KI
in Nal •
without silver
•
without silver
o
with 0.7 m silver nitrate
o
with 1.0 m silver nitrate
·0.6
-0.4
-0.2
0.0 -0.6
-0.4
-0.2
0.0
Potential. V vs Pt
Fig. 11. The effect of dissolved silver on the relative photopower-voltage characteristics for illuminated single crystals of n-WSe 2 • As indicated on the figure, four polyiodide electrolytes are used consisting of either: (6 m KI, 0.01 m 12 ) or (6 m KI,O.oI m 12 , 1.0 m AgNO J ) or (6 m Nal. O.oI m 12 ) or (6 m Nal, 0.01 m 12 .0.7 m AgNO J ) as indicated. Relative power is detennined by comparison to the maximum photopower measured in the silver bearing electrolyte.
S. Licht / Solar Energy Materials and Solar Cells 38 (1995) 305-319
317
of the added iodine resides as the polyiodide species 13" and 14". In iodide concentrations less than 5 m, 13" is the predominant polyiodide, whereas at higher iodide concentrations I~- predominates. Longer chain species, I; and I~-, if they exist, have upper limit concentrations of 1 X 10- 6 m and 1 X 10- 8 m, respectively. Hence, in these aqueous polyiodide electrolytes, 12' I; and I~- are not at substantial concentrations in these photoelectrolytes, and a primary oxidizable ion is r and a primary reducible ion is either 13" or I~-. Choice of cation in aqueous polyiodide solution can effect n-tungsten dichalcogenide photoclectrochemistry. The alkali cations do not substantially interact with solution phase iodide. However, other metal cations including Ag+, or Zn 2+ or Cd 2+, will create a series of complexes with dissolved iodide including: AgI2", AgI~-, ZnI3", ZnI~-, CdI~ etc. These complexes can shift rest potentials or either enhance or diminish charge transfer from the semiconductor surface. Due to the limitations on silver iodide solubility (Ksp(AgI) = 10- 16 ) high (molal) level concentrations of silver will dissolve only in solutions which facilitate complex formation such in concentrated NaI or KI solutions. In particular, we have shown that the presence of high concentrations of dissolved silver is advantageous to n-tungsten diselenide photoelectrochemistry. As seen in Fig. 11 silver, dissolved as AgN0 3 , enhances the voltage at maximum point. The extent of the improvement varies with the initial condition of the individual n-WSe 2 crystal, and general improvements are a 15 to 45 mV increase in the voltage of maximum power, Vmax , and a 5 to 20% relative increase in power. These improvements are sustained in the silver bearing electrolytes. Upon returning the electrode to the polyiodide electrolyte without silver, the PEC gradually (on the order of hours) returns to the silver free PEC behavior. This improvement appears to provide a mechanism for long lasting suppression of n-WSe 2 exposed edges and recombination sites as further discussed by Licht and Myung [46]. 4. Conclusion Along with conventional parameters constraining photovoltaic devices, modification of the electrolyte (solution phase) chemistry is a key to understanding the mechanism of energy conversion and device characteristics of photoelectrochemical solar cells. The fundamental importance of modification of the electrolyte in terms of the distribution of species in photoelectrolytes and the pragmatic importance in terms of enhanced solar to electrical conversion efficiency is reiterated by studies in polysulfide, ferrocyanide, polyselenide and polyiodide electrolyte regenerative photoelectrochemical solar cells. Acknowledgements The author is grateful to his coworkers and collaborators who participated in the studies reviewed in this manuscript including D. Peramunage, F. Forouzan, N.
318
S. Licht / Solar Energy Materials and Solar Cells 38 (1995) 305-319
Myung, G. Hodes and R. Tenne. The author also acknowledges the financial support of this work by the Carl Julius and Anna Carlson Chair in Chemistry, The Petroleum Research Fund, The Dreyfus Foundation, the National Science Foundation and NRELjDOE.
References [1] S. Chandra, Photoelectrochernical Solar Cells (Gordon Breach Publishers, NY, 1985).
[2] [3] [4] [5] [6] [7]
B. O'Regan and M. Griitzel, Nature 353 (1991) 737. S. Licht, G. Hodes, R. Tenne and J. Manassen, Nature 326 (1987) 863. A Fujishirna and K. Honda, Nature 238 (1972) 37. J. Frank and AJ. Bard, J. Am. Chern. Soc. 99 (1977) 303. H. Gerischer and A Heller, J. Electrochern. Soc. 139 (1992) 113. G. Calzaferri (Ed.), Abstracts of the 10th Int. Conf. on Photochemical Conversion and Storage of Solar Energy, Interlaken, Switzerland, July 24-29, 1994. [8] S. Licht, Nature 330 (1987) 1481. [9] G. Hodes, J. Manassen and D. Cahen, Nature 261 (1976) 403. [10] AB. Ellis, S.W. Kaiser and M.S. Wrighton, J. Am. Chern. Soc. 98 (1976) 1635. [11] B. Miller and A Heller, Nature 262 (1976) 680. [12] S. Licht, G. Hodes and J. Manassen, Inorg. Chern. 25 (1986) 2486. [13] S. Licht and J. Manassen, J. Electrochern. Soc. 134 (1987) 918. [14] S. Licht, J. Electrochern. Soc. 135 (1988) 2971. [15] F. Forouzan, K. Longo, D. Perarnunage and S. Licht, Anal. Chern. 62 (1990) 1356. [16] K. Longo, F. Farouzan, D. Peramunage and S. Licht, J. Electroanal. Interfac. Electrochern. 318 (1991) 111. [17] D. Peramunage, F. Forouzan and S. Licht, Anal. Chern. 66 (1994) 378. [18] W. Giggenbach, Inorg. Chern. 10 (1971) 1333. [19] A Teder, Sr. Papperstodn 72 (1969) 245. [20] S. Licht and Joost Manassen, J. Electrochem. Soc. 132 (1985) 1077. [21] S. Licht, G. Hodes and Joost Manassen, J. Electrochem. Soc. 133 (1986) 272. [22] S. Licht and J. Manassen, J. Electrochem. Soc. 133 (1986) 277. [23] S. Licht, R. Tenne, H. Flaisher and J. Manassen, J. Electrochem. Soc. 133 (1986) 52. [24] S. Licht, R. Tenne, H. Flaisher and J. Manassen, J. Electrochem. Soc. 131 (1984) 5920. [25] S. Licht, R. Tenne, G. Dagan, J. Manassen, D. Cahen, R. Triboulet, J. Rioux and C. Levy·Clement, App. Phys Lett. 46 (1985) 608. [26] S. Licht, J. Phys. Chern. 90 (1986) 1096. [27] J. Reichman and M. Russak, J. Electrochern. Soc. 131 (1984) 796-798. [28] K. W. Freeze, Jr., Appl. Phys. Lett. 40 (1982) 275-277. [29] H.D. Rubin, B.D. Humphrey and AB. Bocarsley, Nature 308 (1984) 339-341. [30] H.D. Rubin, DJ. Arent and AB. Bocarsley, J. Electrochern. Soc. 132 (1985) 523-524. [31] S. Licht and D. Peramunage, Nature 345 (1990) 330. [32] S. Licht and D. Peramunage, Nature 354 (1991) 440. [33] S. Licht and D. Peramunage, J. Electrochem. Soc. 139 (1992) L23. [34] S. Licht and D. Perarnunage, J. Electrochem. Soc. 139 (1992) 1791. [35] S. Licht and D. Peramunage, Solar Energy 52 (1994) 197. [36] G. Seshardi, J.K.M. Chun and AB. Bocarsly, Nature 352 (1991) 508. [37] B.A. Parkinson, A Heller and B. Miller, J. Electrochem. Soc. 126 (1979) 954. [38] B.J. Tufts, I.L. Abrahams, P.G. Santengelo, N.G. Ryba, L.G. Casasgrande and N.S. Lewis, Nature 326 (1987) 861. [39] L.G. Casasgrand, B.J. Tufts and N.S. Lewis, J. Phys. Chem. 95 (1991) 1373. [40] D.E. Levy and RJ. Myers, J. Phys. Chem. 94 (1990) 7842.
S. Licht / Solar Energy Materials and Solar Cells 38 (1995) 305-319 [41] [42] [43] [44] [45] [46]
S. Licht and F. Forouzan, Electrochem. Soc. Ext. Abs. 93-1 (1993) 1343. S. Licht and F. Forouzan, (1995) in press. W. Kautek, Gerischer and H. Tributsch, J. Electrochem. Soc. 128 (1981) 247l. B.A. Parkinson, A Heller and B. MiIler, Appl. Phys. Lett. 33 (1978) 52l. R. Tenne and A Wold, App. Phys. Lett. 47 (1985) 707. S. Licht and N. Myung, Electrochem. Soc. Ext. Abs. 94-1 (1994).
319
Solar Energy Materials and Solar Cells
ELSEVIER
Solar Energy Materials and Solar Cells 38 (1995) 305-319
Electrolyte modified photoelectrochemical solar cells Stuart Licht Clark University, Department of Chemistry, Worcester, MA 01610, USA
Abstract Rationale electrolyte modification of photoelectrochemical systems can be used to (i) enhance facile charge transfer, (ij) suppress competing reactions and suppress both (iii) electrode and (iv) electrolyte decomposition products, as well as (v) substantially effect the open-circuit photovoltage. Studies on polysulfide, ferrocyanide, polyselenide and polyiodide electrolyte modification of photoelectrochemical solar cells are discussed. Electrolyte modification of semiconductor/electrolyte systems entails investigation of the primary photo-redox species, the nature of the counter ion, the distribution of species in solution, and related competing reactions. The examples presented emphasize the fundamental and practical importance of probing the active electrolytic constituents pertinent to overall photoelectrochernical energy conversion.
1. Introduction
In operational photovoltaic devices (PVs) an illuminated semiconductor is in intimate contact with other solids, either semiconductors or metal. Photoelectrochemical cells (PECs) bear a resemblance to PVs. However, in PEe devices the solid contact is replaced by an electrolyte containing an appropriate redox couple. For both PECs or PVs, sufficiently energetic light can activate charge separation at a semiconductor, driving electronic charge into the semiconductor bulk. A significant application of illuminated solid photovoltaic and liquid photoelectrochemical semiconductor junction devices is for solar to electrical energy conversion as discussed by Chandra [1]. In ideal regenerative photoelectrochemical solar cells illumination does not change the composition of the cell electrolyte, and the only product is electrical energy. In illuminated semiconductor systems the absorption of photons generates excited electronic states. These excited states have lifetimes of limited duration. Without a mechanism of charge separation their intrinsic energy would be lost through relaxation (recombination). Several distinct mechanisms of charge separa0927-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDl 0927-0248(94)00229-0
306
S. Licht / Solar Energy Matenals and Solar Cells 38 (1995) 305-319
Carrier Generation Under Illumination
..------t\ Lood I t - - - f \
"-type ,emlc:onductor
redox
counter-
electroll,Ite
electrode
A Semiconductor! Electrolyte Interface
..------t\ Lood
"-type dye semiconductor
lr---f\
redox electrol yte
counter ... electrode
B Semiconductor! Dye Sensitizer! Electrolyte Interface
Fig. 1. Carrier generation under illumination arising at (A) the semiconductor/liquid interface and (B) the semiconductor/dye sensitizer/liquid interface.
tion have been considered in designing efficient photoelectrochemical systems. At illuminated semiconductor/liquid interfaces, an electric field (the space charge layer) occurs concurrent with charge/ion redistribution at the interface. Upon photogeneration of electron/hole pairs this electric field impedes recombinative processes by oppositely accelerating and separating these charges, resulting in minority carrier injection into the electrolytic redox couple. This concept of carrier generation is illustrated in Fig. lA (for an n-type PEC) and has been the theoretical basis for several efficient semiconductor/redox couple PECs. Excitation can also occur in molecules directly adsorbed and acting as a mediator at the semiconductor interface. In this dye sensitization mode, the function of light absorption is separated from charge carrier transport. Photoexcitation occurs at the dye and photo-generated charge is then injected into a wide bandgap semiconductor. This alternative carrier generation mode also can lead to effective charge separation as illustrated in Fig. IB. The first efficient example of such a device was demonstrated by Gditzel et al. [2] through the use of a novel high surface area (nanostructured thin film) n-TiO z, coated with a well matched trimeric ruthenium complex dye immersed in an aqueous polyiodide electrolyte. The unusually high surface area of the transparent semiconductor coupled to the well matched spectral characteristics of the dye leads to a device which harvests a high proportion of insolation. PECs can generate not only electrical but also electrochemical energy. Fig. 2 presents one configuration of a PEC combining in-situ electrochemical storage and solar conversion capabilities; providing continuous output insensitive to daily variations in illumination. A high solar to electric conversion efficiency cell configuration of this type was demonstrated by Licht et al. [3J and utilized a Cd(Se,Te)/S; conversion half cell and a Sn/SnS storage system resulting in a
S. Licht / Solar Energy Materials and Solar Cells 38 (]995) 305-319
307
R
B Fig. 2. Schematic of a photoelectrochemical solar cell combining both sol'lr conversion and storage capabilities. (A) Under illumination; (B) in the dark.
solar cell with a continuous output. Under illumination, as seen in Fig. 2A, the photocurrent drives an external load. Simultaneously, a portion of the photocurrent is used in the direct electrochemical reduction of metal cations in the device storage half cell. In darkness or below a certain level of light, the storage compartment spontaneously delivers power, as seen in Fig. 2B, by metal oxidation. The PEC utilizes light to carry out a chemical reaction, converting light to chemical energy. This fundamental difference of the PV solid/solid interface and the PEC solid/liquid interface has several ramifications in cell function and application. Hence, photoelectrochemical systems may facilitate not only solar to electrical energy conversion, but also has led to investigations in photoelectrochemical synthesis, photoelectrochemical production of fuels and photoelectro-
308
S. Licht / Solar Energy Materials and Solar Cells 38 (1995) 305-319
chemical detoxification of pollutants. A system exemplifying photoelectrochemical synthesis is water photoelectrolysis to generate hydrogen. An early demonstration of water photoelectrolysis was presented by Fujishima and Honda [4] utilized Ti0 2 (band gap 3.0 eV) and was capable of photoelectrolysis at .... 0.1 % solar to chemical energy conversion efficiency. Bard et al. [5] have tested various semiconductor particles for photoelectrochemical breakdown of inorganics. Gerischer and Heller [6] have investigated the theoretical consequences of photocatalytic oxidation of organic materials at Ti0 2 particles by sunlight in aerated waters, and application to pollutant detoxification is being carried by Fujishima and a variety of research groups [7]. In 1987 Licht [8] presented a chemical mechanism for photoelectrochemical solar to electrical energy conversion which emphasized understanding of the distribution of species in photoelectrochemical electrolytes. The approach systematically modifies obsetved photoelectrochemical kinetic and thermodynamic phenomena. This paper probes our utilization of this systematic approach in studies on polysulfide, ferrocyanide, polyselenide and polyiodide photoelectrochemical solar cells. The results indicate that specific design of the distribution of species in an electrolyte can enhance open-circuit voltage, and be used to (i) enhance facile charge transfer, (ij) suppress competing reactions and suppress both (iii) electrode and (iv) electrolyte decomposition products, as well as (v) substantially effect the open-circuit voltage. 2. Experiment Details of the experiments discussed in this review are referred to in each of the individual study's reference. In several of the studies molar concentration units were used and are denoted as "molar" or "M". Alternately in several of the studies molal concentration units were used and are denoted as "molal" or "m". 3. Results and discussion 3.1. n-cadmium chaicogenide / aqueous poiysu/fide photoelectrochemistry
In 1976 the first regenerative pbotoelectrochemical solar cells with substantial and sustained solar to electrical conversion efficiency were demonstrated. These PECs are based on n-type cadmium cbalcogenide ($, Se or Te) electrodes immersed in aqueous poly-cbalcogenide electrolytes. The cells were introduced by Hodes, Cahen and Manassen [9], Wrighton et al. [10], and Heller and Miller [11] and were capable of converting up to 7% of insolation to electrical energy. Most investigations of these systems focused on solid state and interfacial aspects of these PECs and photodriven oxidation of polysulfide at the photoelectrode was represented:
S= +2hv
n - Cd(Se,Te) ~ S
+ 2e-.
(1)
309
S. Licht / Solar Energy Materials and Solar Cells 38 (J995) 305-319
Photoelectrochemlcal Solar Cell Solution Phase Studies hv
;-H
Photoalectroda
hv
hv
InsoluUon
InsolUllon
Fe{CN)&"" HxFe(CN)&(X-4, KyFe{CN1&(y-4,
H25e H5e5e= 5112= 51l3= 5e4= OHH+
I
InsoluUon H~
H55=
Hx~Fe(CN)sIY·X-4'
52"
HxFe{CN)&(X-3, KyFe{CN)s(y-3, HxKyFe{CN)&(y·X-3, CNOH-
~-
5,= 54" 5s"
ot{-
H+
H+
' - ~ Counter electrode
Aqueous Polysulftde PEe
?•
I
Aqueous Ferrocyanide PEe
Aqueous Polyselenide PEe
Fig. 3. Chemical species coexisting in polysulfide (left), ferrocyanide (middle) and polyselenide (right) aqueous electrolytes used in regenerative photoelectrochemical solar cells.
However, as represented in the right hand portion of Fig. 3, the number and type of species in polysulfide photoelectrolytes is considerably more complex than represented by Eq. (1). The addition to water of a simple soluble sulfide salt and sulfur to water gives rise to a wide distribution of species in solution. The equilibria constraining this distribution has been investigated by Licht et al. [12-17] Gigenbach [18] and Teder [19]. We have shown that modification of the distribution of species in aqueous polysulfide solution can be correlated to variations in transparency, conductivity, activity, and cadmium chalcogenide photo electrochemistry. Specifically, the separate effects of hydroxide and pH modification [20], sulfur [21] and sulfide [22] and cation [23,24] were investigated in terms of speciation and photoelectrochemical phenomena. Previously cadmium chaicogenide polysulfide PECs had generally employed electrolytes composed 1 molar in sodium sulfide, sulfur and sodium hydroxide and resulted in solar conversion efficiencies of approximately 7 percent [9-11]. However, added hydroxide is to be qtinimized in these cells cesium is the preferred cation, and a sulfur to sulfide ratio of approximately 1.5 to 1 resulted in a near doubling of the conversion efficiency [25]. Fig. 4 presents n-Cd(Se,Te) photocurrents measured at various applied potentials in a traditional and modified electrolyte. As seen in the figure, the cumulative effect of polysulfide electrolyte modifications on photo electrochemical solar to electrical energy conversion by n-Cd(Se,Te)/aqueous polysulfide PECs can be considerable. A greater percentage of photogene rated holes utilized in constructive oxidation of polysulfide results in enhanced photocurrents. This has the additional benefit of
S. Licht / Solar Energy Materials and Solar Cells 38 (J995) 305-319
310
n-CdSeo.6sTeo.3S in aqueous polysulfide
."",,---------
1/ 7.796 efficiency
1.6
«
E
.;
......c CD
:::I 0
...
I I
0
0.6 .c0 a.
I
I
-AM 1 illumination
I Voltage, mV
Fig. 4. Potentiostatic photocurrent-voltage characteristics for a 0.099 cm 2 single crystal n-CdSe o.65TeO.35 photoelectrode immersed in either of two types of aqueous polysulfide electrolyte. The top curve is an electrolyte 1.8 M CS 2 S and 3 M sulfur: The bottom curve is an electrolyte 1 M NaOH, 1 M Na 2 S and 1 M sulfur. The photocurrent voltage curves were obtained outdoors and solar to electrical conversion efficiencies are indicated.
fewer oxidizing holes available for attack on the semiconductor (photocorrosion). As seen in Fig. 5, this results in enhanced photocurrent stability of the PEe. This study showed that with solution optimization not only the photocurrent, but also the polysulfide electrolyte exhibits enhanced lifetime, both approaching one year operation outdoors [26]. 3.2. n-CdSe / aqueous /e"ocyanide photoelectrochemistry Reichman and Russak [27] and Freeze [28] had reported high (12 to 14 percent) conversion efficiencies for n-CdSe/(Fe(CN)~-/4-) solar cells in electrolytes containing a 1:1 ratio of aqueous Fe(CNn- to Fe(CN)~- salts in a highly alkaline environment. However, Rubin et al. [29] had shown that these nCdSe/Fe(CN)~-/4- solar cells can have a limited stability and photocurrents decrease within the order of hours. Their attempts to enhance the photoconversion stability of these systems have included cationic surface modification of the n-CdSe photoelectrode and isolation of the wavelength dependence of the surface instability [29,30]. In these studies, photo-oxidative processes in n-CdSe/Fe(CN)~-/4PECs were typically represented: 4- n-CdSe
Fe ( CN ) 6
-
(
Fe CN
)36
+ e-.
(2)
The role of the various redox active constituents in constrammg nCdSe/Fe(CN)~-/4- photo-oxidative energy conversion had not been previously
S. Licht / Solar Energy Materials and Solar Cells 38 (J995) 305-319
311
O.~,...----r-------r----""
~----o
-0(
o
....
0:: .... > 0 .... 0 l::J
-0( I-
.... o
> eo a :I:
~
JAN
JULY
JULY
JAN
TIME Fig. 5. Long-term outdoor stability curves of thin film CdSeo.7sTeo.3s in electrolytes of either 1.8 M S (solid line) or 2.0 M KOH, 1.4 M Na2S, and 2.6 M S (shaded region). The sodium/potassium electrolyte cell results are averaged results over separate cells, with initial conversion efficiencies from 3.5 to 4.5%. The cesium electrolyte cell had an 4.6% conversion efficiency potential measured under a load chosen for initial maximum power.
Pholoeleclrochemislry of n-CdSe in [Fe(CN)s-Lj3-/2°aq
10 N
E o
L
«
a) -CN-
if 5
b) -NH3
:::J
o
c) -N02°
.c
d) -Fe(CN)s3-
E
...~
o
o
D.
-+--I''-f---..l2:.......---- e) -NO-
o
0.0 0.4 Potential, Volts vs PI Fig. 6. Potentiostatic photocurrent-voltage curves for an 0.17 cm 2 single crystal n-CdSe, illuminated by tungsten-halogen light in several aqueous modified ferro/ferricyanide electrolytes. The electrolytes contain [Fe(CN)s - LP- and [Fe(CN)s - Lj2- with different ligand, L. L = [Fe(CN)sP-, -NH3' -CN-, -NOi or NO+. Voltage swept at 5 mV /sec. The oxidized component of the redox electrolyte is generated in-situ by electrolytic oxidation of a portion of the reduced [Fe(CN)s - LP- species. Electrolytes with L = [Fe(CN)sp-, -CN- or -NOi have a common composition and contain 0.25 m [Fe(CN)soLP-, 0.0125 m [Fe(CN)s-Lj2-, 0.1 m KOH. In the case of L = -NH), the electrolyte contains 0.25 m [Fe(CN)s-NH 3P-, 0.0125 m [Fe(CN)s-NH)12-, 0.1 m NH 4 Cl in 3.5 M ammonia_ For L =-NO+, the electrolyte composition is 0.25 m [Fe(CN)s-NO]2-, 0.6 m [Fe(CN)s-NO]-, pH = 4 buffer. 0
0.8
0
312
S. Licht / Solar Energy Materials and Solar Cells 38 (J995) 305-319
.
20
40mW/cm2 illuminated single crystal n-CdSe in 0.5 mK4Fe(CN)6. 25 mmK3Fe(CN)6. 0.5 mKOH without (dashed) or with (solid line) 0.' mKCN ~------
E
~
E
15
!i1 r
-> ~
iii
z
W
10
Q
d
l'i
~
Z W II: II: :::I
:==========i11
I E I g
,
I §~--~--~----~J1 I 00 1 TIME, days 3
5
(J
,
/
No illumination
~_~J-_....-:o;..---
o -1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
VOLTAGE, volts vs Pt
Fig. 7. Photocurrent-voltage curves of illuminated single crystal n-CdSe immersed in alkaline potassium ferrocyanide electrolytes with and without added cyanide. Inset: Photocurrent stability in several electrolytes. "e" is the only electrolyte with cyanide. Electrolytes "d" and "e" contain low ferricyanide. Electrolyte "b" contains high ferricyanide. Specifically, e: 0.25 m K 4 Fe(CN)6' 0.01 m K 3 Fe(CN)6' 0.5 m KOH 0.1 m KCN; d: 0.25 m K 4 Fe(CN)6' 0.01 m K 3 Fe(CN)6' 0.5 m KOH; b: 0.25 m K 4 Fe(CN)6' 0.25 m K 3 Fe(CN)6' 0.5 m KOH. In an electrolyte comparable to "b" but with pH = 3.8, photocurrent diminished within seconds.
investigated, and ferricyanide and ferrocyanide salts exhibits a complex aqueous equilibria. However, in a manner analogous to the polysulfide electrolytes, and as illustrated in the middle of Fig. 3, these salts exhibit a complex aqueous equilibria resulting in a variety of Fe(II) and Fe(II!) species. As with the polysulfide electrolytes, we have probed the equilibria constraining the distribution of species in these solutions. This speciation can be controlled and substantially effects n-CdSe photo-oxidative energy conversion. We have studied the photoelectrochemical effect and solution stability effects of pH, cation modification and ratio of ferrocyanide to ferricyanide, Licht and Peramunage [31-34]. In addition we have studied systematic variation of the primary photo-oxidized species. As seen in Fig. 6, substitution of a single ligand in the hexacyanoferrate species has a substantial effect on photovoltage and photocurrent, Licht and Peramunage [35]. As seen in Fig. 7 addition of potassium cyanide to an alkaline ferrocyanide electrolyte substantially enhances both photocurrent stability (figure inset) and photovoltage (figure outset), Licht and Peramunage [31]. It was proposed by Bocarlsy et al. [36] that in addition to Fe(CN)~-, CN- was also photooxidized (to CNO-) by the photoelectrode. Our subsequent investigations indicated that whereas cyanide may be chemically oxidized to cyanate, no cyanide was photoelectrochemically oxidized by the semiconductor, Licht and Peramunage [32-34].
S. Licht I Solar Energy Materials and Solar Cells 38 (]995) 305-319
313
3.3. n-GaAs / aqueous polyselenide photoelectrochemistry
n-GaAs/aqueous polyselenide photoelectrochemical solar cells (PECs) have shown stable efficient solar to electrical conversion. Photodriven oxidation of polyselenide had been written: Se= +2hv
n-GaAs ~
Se + 2e-,
(3)
or 2Se= +2hv
n-GaAs ~
Sei + 2e-.
(4)
Parkinson et al. [37] and Lewis et al. [38] have shown that metal ion (Ru3+, OS3+) treatment of the n-GaAs surface leads to high solar to electrical conversion efficiencies in these cells (with up to 15 percent conversion efficiencies reported). Lewis et al. [39] have further investigated O.IM to l.OM concentration K 2 Se electrolytes. The first and second acid dissociation constant of hydrogen selenide of pK 1 = 3.9 and pK 2 = 13.0 have been well characterized by several investigators including Myers et al. However, as represented in the right hand portion of Fig. 2, the fundamental equilibria constraining the distribution of polyselenide species was not characterized. Measurements of these equilibria are a first step in determining the regenerative reactive species in polyselenide medium and probing how they effect photoelectrochemical energy conversion. The equilibria constraining the formation of diselenide, triselenide, and tetraselenide may be written: Sej + Se= 2Se;
~
2Sei ,
+ Se= ~ 3Sej ,
(5) (6)
with equilibrium constants for the equilibrium given by Kn and K 34 , respectively. These equilibria were probed by study of rest potential variation with solution composition and by isolation of the near UV absorption peaks of the various polyselenide species, and yield equilibrium constants of pK 23 = -0.65 and pK 34 = -4.2. When combined with Kl and K2 these provide a description of polyselenide speciation. Using the understanding of polyselenide speciation in solution, we have been investigating rationale electrolyte modification of aqueous polyselenide photoelectrochemical solar cells. n-GaAs photocurrent, photovoltage and photopower are affected by the distribution of hydroselenide, selenide and polyselenide in solution. Polyselenide electrolytes containing 1 to 2 molar hydroxide and 1 molar selenide as K 2 Se) enhance n-GaAs photocurrent density (Reported as "I" in Fig. 8) stability while photocurrent is unstable at either 0.5 molar KOH or 3.0 molar KOH. The presence of polyselenide species in low concentrations (as 0.01 molar dissolved selenium) are important for desorption of selenium from the n-GaAs. Furthermore, electrolytes enriched in dissolved selenium, (as up to 0.2 molar dissolved selenium) enhance n-GaAs photovoltage. As seen in Fig. 9, the photopower (the product of the photocurrent and the applied potential) as well as the photopoten-
314
S. Licht / Solar Energy Materials and Solar Cells 38 (1995) 305-319
r---.----,---.----,.--..,...----r-----,
19
J.~ble
~---------J. Auctuating (Corrosion)
J. Dimiristlng
j 15
~_~_~~_~_~~_~_~
o
__
~
__-J
3
2
4
[KOH].M
Fig. 8. Short-circuit photocurrent density, l"" for a 75 mW/cm 2 tungsten-halogen illuminated single crystal n-GaAs immersed 1 M K 2 Se, 0.01 M Se and in varying KOH concentration.
tial improves with an increase in selenium. However, these electrolytes diminish electrolyte transmittance necessitating use of an alternate back wall cell configuration discussed in Licht and Forouzan [40-42].
3.0
.---.,..---,----.----r--......- - - - r - - - - ,
2.5
• • ~
0.01 M Sa 0.02 M Sa 0.20 M Sa
2.0
1.5 1.0
0.5 0.0
0.0
0.2
0.4
0.6
Photovoltage. volts
Fig. 9. Photopower variation of a 92 mW/cm 2 tungsten-halogen illuminated backwall n-GaAS/polyselenide PEe as a function of increasing selenium concentration.
S. Licht / Solar Energy Materials and Solar Cells 38 (1995) 305-319
315
3.4. Aqueous polyiodide photoelectrochemistry
The aqueous polyiodide 0-/1 3) is the only redox couple shown to be compatible with efficient oxidative photoelectrochemistry at n-type transition metal dichalcogenide including WSe 2, MoS 2, WS 2, MoSe 2. Following the pioneering work of Tributsch et al. [43], several groups including Parkinson et al. [44] and Tenne and Wold [45] have reported n-Wse2 or n-MoSe 2/aqueous polyiodide solar to electrical energy conversion of over 10% and/or photocurrent stability in excess of 10 5 coulombs/cm 2. The photodriven oxidation of iodine at tungsten diselenide . has been described in one of two manners:
(7) or
31-+ 2hv
n- WSe 2 ~
13 + 2e-
(8)
Aqueous Polyiodide Speciation 0.01 M iodine in the indicated concentration of sodium iodide total iodine
=0.01
m
=[121 + [I; I + [I: I
total iodide = [Nail = [I •J + [I ~
iii .-o
-
60%
~
40%
o
'ECD CD
•
I + 2[1 : I
[I; Illotal iodine
D..
o
[I 21110tal iodine
•
total iodine I total iodide
12
4 [Nail. m 8 2
4
6
8
10
12
[Nail, molal
Fig. 10. The fractions of 1:3 and 14' (compared to total iodine) in aqueous polyiodide solutions as a function of added iodide concentration. Inset: The fractions of solution phase iodine (compared to total iodine) and total iodine (compared to total iodide).
S. Licht / Solar Energy Materials and Solar Cells 38 (J995) 305-319
316
Significant species in aqueous polyiodide solution can include: OH-, and H+. Constrained by pH and the equilbria: 1-+ 12 +2 13 13
r, 13,
K3 = 723,
I~-,
r, (9)
+ r+2 I~- K4 = 0.20.
(10)
To a lesser extent the solution phase species 103, HIO, H 2 0 1+, 15' and I~- can also occur. Fig. 10 presents the relative variation of polyiodide speciation in solutions containing 0.01 molal iodine and 1 to 12 molal iodide and which may be compatible with n-WSe 2 regenerative photoelectrochemistry. As emphasized in the figure inset, in these electrolytes the concentration of added iodide dominates iodine in these solutions. The distribution of species in these solutions is calculated in accordance with Eqs. (9) and (10). As can be seen in the figure inset little (less than 0.3%) of the added iodine exists as solution phase iodine, and the maximum [1 2 ] is «1 X 10- 4 m. As seen in the figure outset, the bulk (greater than 99.7%)
illuminated n-WSe 2
in 6 m iodide. 0.01 m iodine
1.0
0.8
Gi ~
~
0.6
CD
~
.;
a:
0.4
0.2
0.0
in KI
in Nal •
without silver
•
without silver
o
with 0.7 m silver nitrate
o
with 1.0 m silver nitrate
·0.6
-0.4
-0.2
0.0 -0.6
-0.4
-0.2
0.0
Potential. V vs Pt
Fig. 11. The effect of dissolved silver on the relative photopower-voltage characteristics for illuminated single crystals of n-WSe 2 • As indicated on the figure, four polyiodide electrolytes are used consisting of either: (6 m KI, 0.01 m 12 ) or (6 m KI,O.oI m 12 , 1.0 m AgNO J ) or (6 m Nal. O.oI m 12 ) or (6 m Nal, 0.01 m 12 .0.7 m AgNO J ) as indicated. Relative power is detennined by comparison to the maximum photopower measured in the silver bearing electrolyte.
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of the added iodine resides as the polyiodide species 13" and 14". In iodide concentrations less than 5 m, 13" is the predominant polyiodide, whereas at higher iodide concentrations I~- predominates. Longer chain species, I; and I~-, if they exist, have upper limit concentrations of 1 X 10- 6 m and 1 X 10- 8 m, respectively. Hence, in these aqueous polyiodide electrolytes, 12' I; and I~- are not at substantial concentrations in these photoelectrolytes, and a primary oxidizable ion is r and a primary reducible ion is either 13" or I~-. Choice of cation in aqueous polyiodide solution can effect n-tungsten dichalcogenide photoclectrochemistry. The alkali cations do not substantially interact with solution phase iodide. However, other metal cations including Ag+, or Zn 2+ or Cd 2+, will create a series of complexes with dissolved iodide including: AgI2", AgI~-, ZnI3", ZnI~-, CdI~ etc. These complexes can shift rest potentials or either enhance or diminish charge transfer from the semiconductor surface. Due to the limitations on silver iodide solubility (Ksp(AgI) = 10- 16 ) high (molal) level concentrations of silver will dissolve only in solutions which facilitate complex formation such in concentrated NaI or KI solutions. In particular, we have shown that the presence of high concentrations of dissolved silver is advantageous to n-tungsten diselenide photoelectrochemistry. As seen in Fig. 11 silver, dissolved as AgN0 3 , enhances the voltage at maximum point. The extent of the improvement varies with the initial condition of the individual n-WSe 2 crystal, and general improvements are a 15 to 45 mV increase in the voltage of maximum power, Vmax , and a 5 to 20% relative increase in power. These improvements are sustained in the silver bearing electrolytes. Upon returning the electrode to the polyiodide electrolyte without silver, the PEC gradually (on the order of hours) returns to the silver free PEC behavior. This improvement appears to provide a mechanism for long lasting suppression of n-WSe 2 exposed edges and recombination sites as further discussed by Licht and Myung [46]. 4. Conclusion Along with conventional parameters constraining photovoltaic devices, modification of the electrolyte (solution phase) chemistry is a key to understanding the mechanism of energy conversion and device characteristics of photoelectrochemical solar cells. The fundamental importance of modification of the electrolyte in terms of the distribution of species in photoelectrolytes and the pragmatic importance in terms of enhanced solar to electrical conversion efficiency is reiterated by studies in polysulfide, ferrocyanide, polyselenide and polyiodide electrolyte regenerative photoelectrochemical solar cells. Acknowledgements The author is grateful to his coworkers and collaborators who participated in the studies reviewed in this manuscript including D. Peramunage, F. Forouzan, N.
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Myung, G. Hodes and R. Tenne. The author also acknowledges the financial support of this work by the Carl Julius and Anna Carlson Chair in Chemistry, The Petroleum Research Fund, The Dreyfus Foundation, the National Science Foundation and NRELjDOE.
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