9995
J. Phys. Chem. 1992,96,9995-loo01
Thermodynamic Stability of n-CdTe In Photoelectmchemlcal Cells Luisa Peraldo Bicelli Dipartimento di Chimica Fisica Applicata del Politecnico, Centro di Studio sui Processi Elettrodici del CNR, Piazza L. da Vinci, 32, 20133 Milano, Italy (Received: April 27, 1992; In Final Form: August 4, 1992)
The thermodynamic stability of n-CdTe phopnodes for photoelectrochrmical solar cells has been considered in several electrolytes (aqueous sulfate solutions, and polyiodide and polychalcogenideselectrolytes) mainly examining bond cleavage and oxidative decomposition photoreactions as well as chemical and photoelectrochemical Te/chalcogen exchange reactions. The F e d level of these reactions has been compared with the Fermi level and the hole quasi-Fermi level of the semiconductor under AM 1 illumination to predict the material stability. Owing to the Te/chalcogen exchange process taking place in both the polysulfide and polyselenide electrolyte, forming an in situ CdS or CdSe layer on the electrode surface even in the dark and in open-circuit conditions, CdTe turns out to be thermodynamically unstable in these electrolytes. Therefore, kinetic factors must be invoked to account for the observed stability of CdTe in the polyselenide electrolyte. In this medium, the relative kinetia of the Te/chalcogen exchange reaction and of the redox-system oxidation is in favor of the latter procas, thus explaining the experimental results. This is not the case in the polysulfide electrolyte where, on the contrary, CdTe was found to be unstable.
Introduction For several years, a relevant interest has been shown in the application of n-type cadmium chalcogenides as photoanodes in regenerative photoelectrochemical (PEC) cells. These materials, particularly CdTe and CdSe, have bandgap energies reasonably matching the solar spectrum and, therefore, a high solar energy power conversion is expected. The research in the field has been reviewed by several authors, so reference is made to the recent book by PleskovIa and to the literature quoted therein. The best results were obtained with singlecrystal photoelectrodes but owing to their prohibitive cost much effort was devoted to develop PEC solar cells based on polycrystalline thin films. Recently, a new ethylene glycol-based bath has been proposed for CdTe electrodeposition,2and the properties of the so obtained thin films have been investigated in view of their application in PEC cells? Besides their performance, problems connected to their photostability in several electrolytes had to be faced. The main purpose of this paper is the systematic analysis of the thermodynamic (TDM) aspects of CdTe photocorrosion according to the method developed by Cahen and Mirovsky? in the line of our previous research on binarySPand ternarysb" chalcogenides. Several authors studied the photooxidation reactions occurring when CdTe is part of a PEC cell (e.g., refs 6-12). Here, we more especially recall the paper by Ellis et a1.6 whose experimental data were applied in our analysis, by Hodes and Miller," and by Lincot and Vedel.I3 Hodes and Miller examined the TDM stability of Cd chalcogenides in the polysulfide electrolyte, taking CdSe as the model system. They assumed the difference between the decomposition potential of the photoreactions and the Nernst potential of the redox couple as an index of the TDM tendency of the material to decompose. Lincot and Vedel mainly studied the mechanism of CdTe photocorrosion in acid and alkaline solutions, as discussed in the following. Thermodynamic Data and Method Table I reports the Gibbs free energy of formation in standard conditions and at 298 K,AGf', for several Cd compounds and ions in aqueous solutions, mainly taken from the basic compilation of Wagman et al.I4 Some data are lacking such as those of the S,Te2- series (n = 1, 2, ...) and were therefore evaluated conS:-, S,,Se2-, Se:-, and SSe:-) sidering the trend of other series ( which allowed some general conclusions to be drawn. First of all, by increasing the total number of atoms in each series, the AGfo values decrease; second, on replacing S with Se atoms, higher values are obtained. Moreover, the AGfo values of the S,,Se2species do not differ very much from those of the equivalent S," species, as already noted by Hodes and Miller.'I Assuming that these rules still hold for the S,Te2- species, we obtain the values reported in Table I. With this choice, the S3Te2-value turns out
TABLE I: St.ad.nl G i b Free Energy of Formation of Several Cd Compounds, Several Iom, rad Water A G O t
compound
-92.0 -137 -156.5 -228.4 -473.6 -20 1.4 -259.4 -315.9 -77.6 -284.4 -363.5 -261.7 -436.9 -392.7 -51.6 -51.4 85.8 79.5 a
A G O ,
kJ/mol ref
compound
14 11 14 14 14 14 14 14 14 14 14 15 15 15 14 14 14 14
kJ/mol 73.7 69.1 129.3 108.3 105 82.4 220.5 162.1 (85)" (75) (70) (68) 80 12.1 44.0 157.5 -157.2 -237.1
ref 14 14 14 4 16 16
15 15 see text see text see text see text 11 14 14 15 14 14
Estimated values are in parentheses.
to be slightly higher than that estimated in ref 17 (70 instead of 63 kJ/mol). However, the difference is not unreasonable, since estimated values may easily be 4-8 kJ/mol in error.Il Tables 11-VI show possible bond cleavage photoreactions and PEC and chemical decomposition reactions in different redox electrolytes of interest for n-CdTe solar cells. The related standard Gibbs free energy change at 298 K,AGO, is also indicated. In the case of PEC reactions,only the oxidative half-reaction involving holes has been considered, the reductive half-reaction being hydrogen evolution whose AGO = 0. So, the calculated CdTe photodecomposition potentials, E"(NHE), refer to the standard hydrogen electrode. For more details, see ref Sb.
ReSults In agreement with the experimental results on Cd Chalcogenides, we assumed that the only species being oxidized during CdTe photodecomposition is tellurium which can be oxidized into Teo or Te(IV); the former can then be dissolved in the polychalcogcnide electrolytes, as discussed below. The reported TDM data refer to standard conditions which are far from experimental ones, particularly as to the reaction products at the very beginning of the measurements? They may be adjusted according to the Nernst equation," and, generally, less stable materials are obtained. Moreover, in the absence of information on the reaction path, the overall process is considered. Bond ckrppge P h o t o " These photoreactions (reported in Table 11) preferably occur at the surface defects of a semi-
0022-36S4/92/2096-9995S03.00/0@ 1992 American Chemical Society
Peraldo Bicelli
9996 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 TABLE II: Bond Cleavage Pbotoreactiona for a-CdTe and Related Thennodydc Quantities
photoreactions
AGO, kJ
+ 2h+ 3Cd2+(aq)+ Teo 2a. CdTe + 2H20 + 2h+ 2Cd(OH)2 + Teo + 2H+(aq) 2b. CdTe + H 2 0 + 2h+ 2CdO + Teo + 2H+(aq) 2c. CdTe + 2H20 + 2h+ 2 Cd02- + Teo + 4H+(aq) 2d. CdTe + 2H20 + 2h+ -% HCd0; + Teo + 3H+(aq) 3. TeO + 2H20 + 4h+ 2HTe02++ 3H+(aq) 4a. CdTe + 80H- + 6h+ 2Cd(OH)2 + Te032-+ 3H20 4b. CdTe + 80H- + 6h+ 2 CdO + Te032-+ 4H20 4c. CdTe + 100H- + 6h+ 2Cd022-+ Te032-+ 5H20 4d. CdTe + 90H- + 6h+ 2 HCdO; + Te0,2- + 4H20 5 . CdTe + 2H20 + 6h+ 2Cd2++ HTe02++ 3H+(aq) 6. CdTe + 30H- + 2h+ 2HCd02- + TeO + H 2 0 7. Teo + 3H20 + 4h+ 3 HTe03- + SH+(aq) 8. CdTe + 3H20 + 6h+ 2Cd2++ HTe0,- + SH+(aq) 1. CdTe
Eo(NHE), V
14.4
0.07
92.6
0.48
100.7
0.52
281.8
1.46
202.7
1.05
212.5
0.55
-228.0
-0.39
-219.9
-0.38
-198.6
-0.34
-197.8
-0.34
226.9
0.39
-37.0
-0.19
274.4
0.71
288.8
0.50
TABLE IIk Oxidative Pbotodecom~itionReactions for n-CdTe in the Polyiodide Electrolyte rad Related “ o d y v l a i c Quantities photoreactions AGO, kJ Eo(NHE), V ~
9a. CdTe + 21- + 2h+ & CdIz + Teo 9b. CdTe + 31- + 2h+
2CdI; + Teo 9c. CdTe + 41- + 2h+ 2CdId2-+ Teo 10. CdTe + (2/3)1,- + (2/3)h+ 2Cd12 + Teo conductor immersed in aqueous electrolytes. Lincot and VedelI3 have already studied CdTe photocorrosion in these media. Both experimental results and TDM calculations showed that the reaction mechanism is different in acid (pH = 0) and alkaline (pH = 14) solutions. In the former, two steps have to be considered: the oxidation of CdTe into Cd2+and Teo (reaction 1 of Table 11) and the oxidation of Teo into HTe02+ (reaction 3). Positive charges accumulated on surface states associated with Teo formation were shown to cause a downward shift of the semiconductor band edges thus allowing the second step to occur. In alkaline solutions, CdTe was directly oxidid into the soluble species (e.g., reaction 4d) and the band edge shift needed to make oxidation possible was much less important since photooxidation left a more stoichiometric surface with less surface states. In Table 11, we also considered the formation of several Cd hydroxides/oxides in acid (reactions 2) and alkaline (reactions 4) solutions, in addition to that of Cd2+ions. It is worth noting that the Cd(OH)2 formation is the most probable reaction among those forming Cd hydroxides/oxides. Moreover, reactions 4 a 4 occur spontaneously with respect to proton reduction and have the lowest AGO and Eo(NHE) values. Reactions 5 and 6 are also reported in the table to evidence the TDM basis of the two different photocorrosion processes. While in acid solutions Te “partial” oxidation is favored over “complete” oxidation (the photodecomposition potential is 0.07 for reaction 1 and 0.39 V vs NHE for reaction 5) in alkaline solutions the reverse occurs (-0.34 for reaction 4d and -0.19 V vs NHE for reaction 6). The same takes place for the other reactions involving Cd hydroxides/oxides. As to neutral solutions containing salts such as Na2S04or K2SO4 where the existence of Cd2+ions is possible, we expect a mechanism similar to that observed in acid solutions, i.e., reaction 1 followed by reaction 7 which involves Teo oxidation to HTe03-. Indeed, according to the diagram by P o u r b a i ~HTe03,~~ is the
-6.2
-0.03
-12.6
-0.06
-17.5
-0.09
-75.1
-1.17
predominant species in pH 5.45-7.74 solutions. The comparison of the TDM quantities for reaction 1 with those for reaction 8 confii that also in this case the two-step oxidation mechanism is the most likely one. Polyiodk HedmIyk This system was often used in connection with ternary chalcogenides, particularly with CuInSe2, sometimm obtaining very positive results (e.g., refs 18-21). In the commonly employed electrolytes, both I- and I< ions are present, the former having the highest concentration (usually 10-100 times). More negative AGO and Eo(NHE) values are obtained if Is- rather than I- ions take part in the decomposition reactions, as shown in the example reported in Table 111. Moreover, the formation of Cd complexes with iodide ions produces a regular decrease of previous TDM quantities. Depending on the pH of the polyiodide electrolyte, the hydroxides/oxides formation reactions already examined in Table I1 (reactions 2 and 4) may also take place. Polycailcogenide Electrolytes. The S2-/S,Z- electrolyte is the most frequently employed for Cd dichalcogenides, but the other two polychalcogenide electrolytes have also been investigated. Three groups of decomposition reactions may be distinguished in these strongly alkaline electrolytes: bond cleavage photoreactions with hydroxides/oxides plus elemental Te formation and subsequent incorporation of the latter in some chalcogenide ions of the solution (the resulting processes are shown in Table IV) as well as chemical (Table V) and PEC (Table VI) Te/chalcogen exchange reactions. The first group of the examined reactions has to be considered as an initial step of the decomposition process since hydroxides/oxides are generally unstable in the presence of chalcogenides/polychalcogenides giving rise to the formation of the corresponding Cd chalcogenides. So, the more probable reactions in these electrolytes are those reported in Table VI. According to Licht et a1.,22besides OH-, the dominant anions in the polysulfide electrolyte are HS-, S42-,and S?-,whereas S?-, S52-,and S2- are less than 1%. The identity of the species in the
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9997
Thermodynamic Stability of n-CdTe
TABLE Iv: Oxidative pktodccampitioa Ractioar for a-CdTe la tbe P d y c h d c e Ekctdyta rad Rdrted Tbmodynmic Qrutitkr Dhotoreactions AGO, kJ Eo(NHE), V
+ S32-+ 20H- + 2h+ & Cd(OH)2 + S3Te211b. CdTe + S3'- + 20H- + 2h+ -% CdO + S3Te2-+ H 2 0 1IC. CdTe + S2- + 40H- + 2h+ 2Cd02- + S3Te2-+ 2H20 1Id. CdTe + S2-+ 30H- + 2h+ 2HCd02- + S3Te2-+ H 2 0 12a. CdTe + Se32-+ 20H- + 2h+ & Cd(OH)2 + Se3Te212b. CdTe + Se2- + 20H- + 2h+ 2CdO + Se3Te2-+ H 2 0 12c. CdTe + Se2- + 40H- + 2h+ 2C d O P + Se3Te2-+ 2H20 HCd02- + Se3Te2-+ H 2 0 12d. CdTe + Se32-+ 30H- + 2h+ 13. CdTe + Se2- + Se2- + 20H- + 4h+ 2Cd(OHI2 + Se3Te2-+ H 2 0 14. CdTe + Te2- + 20H- + 2h+ 2Cd(OH)2 + Te215. CdTe + HTe- + 30H- + 2h+ & Cd(OH)2 + Te2- + H2O 1la. CdTe
-70.9
-0.37
-62.8
4.32
-41.5
-0.22
-40.7
-0.21
-92.2
-0.48
-84.1
-0.44
-62.8
-0.32
-62.0
-0.32
-224.8
-0.58
-125.6
-0.65
-142.5
-0.74
TABLE V chcmicrl Te/Clukogen Exchange Ractiollr for n-CdTe in the Pdychrkgclrkk Electrolytes rad Related Tbmnadyarmic Quratitka chemical reactions AGO, kJ AGo/a,O W/atom
16. CdTe + HS-
+ OH- & CIS + Te2- + H 2 0 17. CdTe + S2- 2CdS + Te218. CdTe + S42-2CdS + S3Te219. CdTe + HSe- + OH- & CdSe + Te2- + H 2 0 20. CdTe + Se2-2CdSe + Te221. CdTe + S e P 2CdSe + Se3Te222. CdTe + HTe- + OH- 2 CdTe + Te2- + H 2 0 'a
64
10.7
70.2
23.4
-63.6
-10.6
51.6
8.6
46.2
15.4
-41.4
-7.9
-16.9
-2.8
is the number of reacting atoms.
TABLE VI: Oxidative Te/S md Te/Se Exchange Photonrctiollsfor n-CdTe in the Polychrlcogenide Electrolytes rad Related Tbcnwdy.rmie
Qurntiw photoreactions
+ (2/3)h+ & CdS + S3Te224. CdTe + 2S2- + 2h+ & CdS + S3Te225. CdTe + S2- + S32-+ 2h+ 2CdS + S3T$26. CdTe + HS- + S2- + OH- + 2h+ & CdS + S3Te2-+ H 2 0 27. CdTe + (4/3)Se32- + (2/3)h+ -% CdSe + Se3Te228. CdTe + 2Se2- + 2h+ & CdSe + Se3Te229. CdTe + Se2- + Se2- + 2h+ 2CdSe + Se3Te230. CdTe + HSe- + Se2- + OH- + 2h+ & CdSe + SeTeBZ- + H20 23. CdTe + (4/3)S2-
polyselenide electrolyte is less certain: so, HSe- and % ,-: with n = 1-4, for which TDM data are available are usually considered. As for the polytelluride electrolyte, the Te2-and Tq2- species were rrcognized.6 Another important point is that Te dissolves to S3T$in the polysulfide electrolyte, the dissolution rate being slow." In the absence of experimental data, according to Hodes and Miller," we assumed that only Se3Te2-is formed in the polyselenide medium, too. TDM calculations show that CdTe is photooxidized to the different hydroxides/oxidcs both in polysulfide and polyselenide (Table IV,reactions 11 and 12) following a trend similar to that observed in aqueous solutions (Table 11, reactions 4) and that
AGO, kJ
Eo(NHE), V
-92.8
-1.44
-153.5
-0.79
-154.0
-0.80
-160.2
-0.83
-105.0
-1.63
-181.6
-0.94
-199.3
-1 -03
-193.9
-1.00
Cd(OH)2 is again the most probable among these reaction products. However, as already underlined, the real final de"position products are in general Cd chalcogenides. As expected on a TDM basis, increasingly negative photodecomposition potentials are obtained in passing from the neutral sulfate to the alkaline polysulfide, polyselenide, or polytelluride solution, but, as shown in the following, a corresponding upward (more negative) shift of the CdTe flatband potential is noted. In the case of the polysulfide electrolyte, type 11 reactions but involving HS- and s?- instead of S32-, thus forming ST& and S4Te2-,respectively, were also considered (though not reported here). AGO values differing no more than 6.5 kl were obtained,
9998 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 TABLE W: n-CdTe Parameters (See Text) sulfate solution, pH = 7 E., eV 1.54 E:, V vs NHE -0.66 E,, V vs NHE 0.88 E, - EFB,V -0.10 Em, V vs NHE -0.56 E”, V vs NHE -0.52 E,(,,, V vs NHE -0.41; 0.82 EF(OC), V vs NHE VdoC), v &*F, V vs NHE 0.33
Peraldo Bicelli
electrolyte
polysulfide electrolyte
polyselenide electrolyte
polytelluride electrolyte
1.54 -0.66 0.88 -0.10 -0.56
1.54 -1.36 0.18 -0.10 -1.26
1.54 -1.51 0.03 -0.10 -1.41
1.54 -1.71 -0.17 -0.10 -1.61
0.49
-0.48 -1.10 0.16 -0.37
-0.71 -1.30 0.1 1 -0.52
-0.81 -1.50 0.1 1 -0.72
polyiodide
0.33
hence, in the approximation range of the estimated free energies of formation of the SnTe2-species. Since Se2- and SeZ2-are expected to be present in the polyselenide solution in nonnegligible quantities, they were also taken into account as reactants (reaction 13). As shown in Table IV, this reaction is the most probable in the polyselenide electrolyte. Table V shows that, in the dark and in rest conditions, chemical Te/S exchange reactions in sulfides (reactions 16 and 17) are not posfible whereas they occur spontaneously in the polysulfide electrolyte (reaction 18, and the reactions not reported here involving SZ2-and S32-which have nearly the same TDM values). Similar results are obtained in the polyselenide electrolyte. In the same Table V, an example is tentatively given of a possible Te/Te exchange reaction in the polytelluride electrolyte (reaction 22). Such a process does not affect the chemical nature of the CdTe electrode but certainly influences its morphology and defective state, chemical bonds weaken and break at the surface, and the number of dangling orbitals increases. So, the material is ever more subject to other possible decomposition reactions. Inspecting the behavior in the three redox systems (Table V), CdTe is seen to be increasingly more unstable in passing from the polytelluride to the polyselenide and to the polysulfide electrolyte. This is even more evident on observing the values of the free energy changes per reacting atom which somehow take the reaction probability into account.23 As shown in Table VI, all PEC Te/S and Te/Se exchange photoreactions have negative free energy changes with smaller values in the polyselenide electrolyte as already found for the PEC oxidative reactions (Table IV). Semiamductor Puametm. The relevant CdTe parameters necessary to build the energy diagram are collected in Table VII. The bandgap energy value, Eg,is taken from ref 3 while all the other experimental data for the sulfate solution and the polychalcogenide electrolytes are taken from the paper by Ellis et a1.6 As previously observed for ternary chalcogenides (e.g., refs 4, 10, 12, 24, and 25), a negative shift of the flatband potential, EFB, with respect to that in the sulfate solution is noted in the three polychalcogenides owing to anion-specific adsorption on the electrode surface. This is also reflected from the flatband potential evaluated from the electronegativity of the CdTe constituent atoms, E*m, supposing there is no net charge across the Helmholtz double While performing these calculations, we assumed the difference between the conduction band edge and the flatband potential at the surface, E, - EFB, equal to -0.10 V, as usual for chalcogenides having a medium to high donor concentration.4V6 The increasingly negative shift of EFBfor CdTe in S2-/Sn2-, Sez-/Se,2-, and Te2-/Tez2-mimics that of the Fermi level of the corresponding redox system, so that the difference EF(cl) - EFB,that is, the maximum output voltage, is nearly the same in the three electrolytes (around 0.8 V). To extend the discuspion to the polyiodide electrolyte for which no experimental data are available, we tentatively assumed that the flatband potential of CdTe in this electrolyte is the same as in the sulfate solution, Le., that no I- ion adsorption takes place, as experimentally observed for example in AgInSSes.27 The open-circuit Fermi level under AM1 illumination, EF(OC), was ~ ) the open-circuit determined from the difference between E F ( and photopotential for CdTe in the various electrolytes given in ref 6. From these values, the open-circuit band bending at the semiconductor surface, Vs(OC),was evaluated as EF(0C) - Em. The
hole quasi-Fermi level at the semiconductor surface in open-circuit conditions under AM 1 illumination, PE*~(OC),was tentatively assumed to differ by -0.55 eV from the valence band edge, E,, as theoretically estimated for CdSe.28 pE*p(Oc) is a very important physical quantity in stability calculations against photocorrosion, since all the anodic procespes occurring at an n-type semiconductor and whose decomposition potential is located inside the bandgap inbetween the Fermi level and the hole quasi-Fermi level at the surface are possible from a TDM point of view.29*30When such condition is satisfied for both the redox system and the photodecomposition reaction (or better for its slowest electrochemical step), the question arises which reaction is thermodynamically favored. The answer lies in the relative position of the two energy levels, the most likely reaction being that having the highest energy value. Consequently, TDM stability and eficiency of a semiamductor/electrolytedevice are anticorrelated since more stable systems on a TDM basis have lower values of the maximum output voltage. In addition, decomposition potentials were shown to shift upward at defect sites,” and so the relating procases become more probable. Indeed, PEC etching, a technique employed to improve the efficiency and output stability of liquid-junction solar cells, is based on the selective attack at perturbed surface sites to eliminate defective layer^.'^ Experimental evidence has shown that the stability of photoelectrodes is often reached by kinetic barriers and by competing reactions preventing a too large splitting of the hole quasi-Fermi level at the surface by rapidly consuming the photogenerated minority carriers. This takes place more easily if the Fermi level of the redox reaction is above the photodecomposition level. However, when this does not occur, a relative stability could be found when the activation barrier of the decomposition path is remarkably higher than that of the redox process, as is often the case.33So,the kinetic basis of stabilization depends on the choice of a redox system which scavenges holes so rapidly that photocorrosion does not occur.34
Discussion Figures 1-5 depict the energy scheme of n-CdTe in the considered electrolytes derived from the data of Table VII. The standard Fermi level of the different redox systems and photodecomposition reactions are also reported. The latter are distinguished by the number which characterizes such reactions in the corresponding tables (Table 11,111, IV, or VI). Owing to the complex behavior of CdTe in aqueous solutions of different pH values with possible shift of the flatband potential under photocorrosion conditions,” we only considered the behavior in a pH 7 solution in flatband conditions (Figure 1). The water redox levels in the figure are those for a neutral solution, and the flatband potential value is the initial one since, also in this electrolyte, band edges unpinning is passible during the photodecomposition process. The electrode will behave in rest conditions as a mixed electrode with a redox potential intermediate between that of the H2/H20 and H 2 0 / 0 2 redox systems but closer to the latter if oxygen is present in the solution. CdTe photodecompositionto Cd2+and formation of a thin Te layer (Table 11, reaction 1) are to be expected, and this even more since holes have a low probability to oxidize water owing to the unfavorable position of the H 2 0 / 0 2 redox level, very close to the valence band edge. The subsequent photooxidization reaction of
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9999
Thermodynamic Stability of n-CdTe V vs. NHE
V vs. NHE
-1.0
E,(OC)
-
-0.8-
26 --;/25
‘24
-0.6- Eg= 1.54eV
-
-0.4
-
-
- s2-/s:lla
,E;(OC)
-llb /llC
-0.2 -
-'lid
-
0.0 -NHE
0.8
-
H20/02,pH=7
F9gwe 1. Energy schemeof n-CdTe in a neutral aqucous K#04 solution in flatband conditions under AM1 illumination. The data are taken from Table VII. The numbered energy levels show the standard photodecomposition potentials relating to some of the different bond cleavage photoreactions reported in Table 11. Water (pH = 7) redox levels are also indicated. V vs. NHE
A
-1.2 -1.0
t I
0.2
---
-
-29
-
-‘28
-0.8-
- E,
-0.6
-
-
0.6
-0.4
0.8
-0.2-
Figme 2. Energy scheme of n-CdTe in the polyiodide electrolyte in
flatband conditionsunder AM1 illumination. The data are taken from Table VII. The numbered energy levels show the standard photodmposition potentials relating to the different oxidative photodecomposition ~ C W ~ ~reported OM in Table 111. The experimental I-/Ic redox level is also indicated?’ elemental tellurium into HTeOy species (reaction 7) is impoaible
u n l a the compound band edges and/or the hole quasi-Fermi level shift downward. In fact, by anodically polarizing the electrode, the density of the photogenerated holes at the surface increasesz9 and other CdTe photooxidation reactions become increasingly possible (reactions 2a,8, and 2b) as well as Teooxidation to Te(IV) (reaction 7). In any case, the two-step (reactions 1 and 7) is more likely than the onastep (reaction 8) photocorrosion mechanism. Finally, nearly all the photodecomposition levels in Figure 1 move upward when actual instcad of standard conditions are considered owing to the smaller activity of the (soluble) reaction products. So, the real TDM situation is even worse than that resulting from Figure 1. In the polyiodide electrolyte (Figure 2 and Table 111). the energy level of the redox system is too low to successfully compete with photodecomposition on a TDM basis and a highly unstable electrode is foreseen. If it is part of a PEC device, its performance
-
-
- 30
- Se27Se;13 _ _ _- - 129
1.54 eV
p$(0c)E12b - 12~,d
Peraldo Bicelli
loo00 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 V vs. NHE
TABLE MII: a-CdS unl a-CdSe Parameters (Sa Text)
n-CdS n-CdSe sulfate sulfate solution, polysulfide solution, polyselenide DH = 7 electrolyte pH = 7 electrolyte E,, eV E,, V V SNHE E,, V vs NHE E, - EFB, v Em, V V SNHE E*m, V vs NHE EF(~I), V vs NHE
2.4 -0.76 1.64 4.10 -0.66 -0.64
2.4 -1.18 1.22 -0.10 -1.08
1.7 -0.56 1.14 -0.10 -0.46 -0.40
1.7 -1.26 0.44 -0.10 -1.16
-0.48
-0.71
V vs. NHE
A -1.2
t -0.2
I
-
-0.8-
I
-
-0.4-
E”
Figure 5. Energy scheme of n-CdTe in the polytelluride electrolyte in flatband and open-circuit conditions under AM1 illumination. The data are taken from Table VII. The numbered energy levels show the standard photodecomposition potentiala relating to the different oxidative photodecomposition reactions reported in Table IV. The experimental TeZ-/Te;- redox level is also indicatede6
0.0 0.4 0.8 -
E, = 1.54 eV E,= 2.40 eV
-
reaction 15). In any case, they are less probable than the Te/ chalcogen exchange photoreactions, in agreement with previous considerations. Moreover, the latter reactions have a higher energy level than the redox process and are located inbetween the Fermi and the hole quasi-Fermi level in the polysulfide (Figure 3, reactions 24-26) and polyselenide (Figure 4, reactions 28-30) electrolyte. So they are possible, the situation being less critical in the latter electrolyte. CdTe in the polytelluride electrolyte is the mentioned exception: due to the practical absence of exchange photoreactions it is expacted (and observed (ref 46))to be stable to photocorrosion (Figure 5). However, CdTe was found to be also stable in the polyselenide electrolyte. Therefore, the relative kinetics of the two possible processes must be invoked to account for the observed behavior. In the polyselenide electrolyte, the kinetics of the Te/chalcogen exchange reaction is different from that of the redox reaction in favor of the latter, thus explaining the stability of CdTe in this medium. This is not the case in the polysulfide electrolyte where CdTe was found to be unstable.36 Therefore, electrolyte concentration and composition (even as far as the cation nature is concerned) are critical, as experimentally n~ticed.~,~~ Depending on the thickness and porosity of the film, an additional heterojunction should be considered when a thin layer of a different c h a l w d e is formed on the s e m i d u c t o r surface. For example, in the case of the CdTe/polychalcogenide systems, a sort of “tandem” cell could be obtained since a widegap material (CdS or CdSe) is formed on a narrow-band semiconductor (CdTe).lb This is particularly important insofar as chemical exchange reactions may take place even in the dark and in rest conditions though at a much lower rate, as previously discussed (Table V). The energy scheme of a CdTe electrode at equilibrium, in the dark in the polysulfide or polyselenide solution after formation of a CdS or CdSe layer, respectively, is depicted in Figure 6, a and b. The data are taken from Tables VI1 and VI11 (experimental v a l w of Ellis et al$ E*w values detmnined according to ref 26). Owing to the mutual disposition of the valence band boundaria, a potential bamer for minority camers exists in both electrolytes at the interface between the two semiconductors. Such barrier hinders the flow of the photogenerated in-depth holes toward the electrode/electrolyte and smaller photocurrents are expected. Indeed, only light-generated holes in the thin outer layer participate in the PEC reaction (of course, excluding possiMe free carrier tunneling through the same layer when its thickness is very small).
1.2 -
-
1.6 -
-
CdTe
CdS
a
s2-/s:-
CdTe
1
CdSe
b
Fipllrr 6. Energy scheme of the n-CdTe/n-CdS (a) and n-CdTc/n-CdSe (b) heterojunction in the polysulfide and polyselenide electrolyte, respectively, at equilibrium in the dark. The data are taken from Tables VI1 and VIII. The experimental Sz-/S.” and Se2-/Se,” redox levels are also indicated!
Conclusions The CdTe/polytellwide system was found to be the most stable among those considered. It also exhibits the highest conversion efficiency6 possibly owing to the high reversibility of the redox process and to the absence of a highly defective restructured surface layer and solid-statejunction as occurring in the polysulfide and polyselenide electrolytes. Unfortunately, polytellurides (as well as polyselenides) are toxic and readily oxidized by the oxygen of air. Furthermore, their solutions have significant absorption in the visible region of the spectrum: Hence, they need hermetically d e d cells having an as small as possible solution layer thickness through which light r e a c h the photoelectrode. Therefore, other redox systems (including organic ones) may be examined both in aqueous and nonaqueous solvents. It is worth recalling that the [Fe(CN)6]b’3- electrolyte has more especially been investigated in recent years, in cOMcction with cds and cdse photoanodes3’ and that the best-so-far conversion efficiency of PEC cells (16.4%) has been claimed for the CdSe/[KFe(CN)6]2-/3-system,)* though the mechanism suggested therein has been criticized.39 The potentialities of CdTe have not completely been exploited, particularly in connection with alternative redox systems, eventually also considering solid solutions of the type CdSe,Tel, (with x between 0.5 and 0.8) whose forbidden bandwidth is close to the optimum onea as well as compositionmodulated thin frlm st” in the Cd-Zn-Te system, analogous with the recently reported Cd-Zn-Se system.4I Research in this direction is highly recommended. Acknowledgment. The author wishes to thank A. B. Ellis, S. W. Kaiser, J. M.Bolts, and M.S.Wrighton for their exjmhental data (ref 6) as well as D. Lincot and J. Vedel for the results they
J. Phys. Chem. 1992,96, 10001-10007
obtained in acid and basic solutions (ref 13). This work was supported in part by the Progetto Finalizzato Materiali of the CNR and the Ministem della UnivmitA e della Ricerca Scientifica e Tecnologica. Registry No. CdTe, 1306-25-8;CdS,1306-23-6;CdSe, 1306-24-7;I-, 20461-54-5;I), 14900-04-0;S32-,12597-05-6;SeIZ-, 12597-25-0;Se”, 22541-48-6;TG-, 22541-49-7;HTe-, 18282-39-8;HS-, 15035-72-0;Sz-, 18496-25-8;SI‘, 12597-07-8;HSe-, 16661-43-1;Sed,’ 12597-27-2;S?-, 16734-12-6;STe2-, 144193-21-5;S2Te2-,144193-22-6;S3Te2-, 1230021-9;S4Te2-, 19489-36-2;S042-, 14808-79-8.
Refer-
and Notes
(1) Pleskov, Yu. V. Solar Energy Conversion; Springler Verlag: Berlin,
1990. (a) pp 128-135; (b) pp 67-70. I21 Gore. R. B.: Pandev. R. K. Thin Solid Films 1988. 164. 225-259. Gore,’ R. B.f Pandey, R. K.f Kulkarni, S.K. Sol. Energy k a t e r : 1989, 18, 159-169; J. Appl. P h p . 1989,65,2693-2698. (3) Pandev. R. K.; Maffi, S.;Razzini, G.; Peraldo Bicelli, L. Manuscript in preparation. (4)Cahen, D.; Mirovsky, Y . J. Phys. Chem. 1985,89,2818-2827. (5)Peraldo Bicelli, L. Electrochim. Acra 1987,32,777-783.(b) Peraldo Bicelli, L. Sol. Energy Mare?. 1987,15,77-98.(c) Peraldo Bicelli, L. J. Phys. Chem. 1988, 92,6991-6997. (6) Ellis, A. B. Kaiser, S.W.; Bolts,J. M.; Wrighton, M. S.J. Am. Chem. Soc. 1977,99,2839-2848.
(7) Lando. D.;Manassen, J.; Hodes, G.; Cahen, D. J. Am. Chem. SOC. 1979, 101,3969-3971. (8) Hodes, G. In Energy Resources Through Photochemistry and Catalysis; Gratzel, M., Ed.; Academic: New York, 1983;Chapter 13,pp 428-434. (9)Sculfort, J. L.; Triboulet, R.; Lemasson, P. J. Electrochem. Soc. 1984,
131, 209-213. (10) Lincot, D.;Vedel, J. J. Electroanal. Chem. 1984,175,207-222;J . Phys. Chem. 1988,92,4103-4110. (11)Hodes, G.; Miller. B. J. Electrochem. Soc. 1986,133, 2177-2180. (12)Lyons, L. E.; Morris, G. C.; Raftery, N. A.; Young, T. L. Ausr. J . Chem. 1987,40,655-666.Lyons, L.; Young, T. L. Ausr. J. Chem. 1987,40, 723-741. (13) Lincot, D.;Vedel, J. J. Crysr. Growth 1985, 72,426-31; J. Electroanal. Chem. 1987, 220, 179-200. (14) Wagman. D.D.;Evans, V. H.; Parker, V. B.; Schumm, R. H.; Halow, I.; Bailey, S. M.; Churney, K. L.; Nuttall, R. L. J. Phys. Chem. Ref. Data 1982, 11, S ~ p p l2., 38, 52, 56, 57,61, 144-146. (1 5) Pourbaix, M. Atlas of Elecrrochemical Equilibria in Aqueous Solutions, 2nd ed.; NACE: Houston, TX, and CEBELCOR, Brussels, 1984. (16) Zaitseva, I. G.; Greiver, T. N. Zh. Prikl. Khim. (Leningrad) 1967,
40, 1683-1686.
lo001
(17)Greiver, T. N.; Zaitseva, I. G. Zh. Prikl. Khim. (Leningrad) 1967, 40, 1920-1923. (18) Menezes, S.;Lewerenz, H. J.; Bachmann, K. J. Nature ( h d o n ) 1983,305,615-616. (19) Cahen, D.;Chen, Y . W. Appl. Phys. Leu. 1984,45,746-748. (20)Razzini, G.; Peraldo Bicelli, L.; Scrosati, B.; Zanotti, L. J. Electrochem. Soc. 1986,133,351-352. (21)Cahen, D.;Chen, Y. W.; Noufi, R.; Ahrenkiel, R., Matson, R.; Tomkiewicz, M.; Shen, W. M. Sol. Cells 1986,16, 529-548. (22) Licht, S.;Manassen, J. J. Electrochem. Soc. 1985,132,1076-1081. Licht, S.;Hodes, G.; Manassen, J. Inorg. Chem. 1986,25,2486-2489. Licht, S.; Tenne, R.; Flaisher, H.; Manassen, J. J. Electrochem. Soc. 1986, 133, 52-59. (23)Wager, J. F.;Jamjoum, 0.;Kazmerski, L. L. Sol. Cells 1983,9, 159-163. (24)Scrosati, B.; Fornarini, L.; Razzini, G.; Peraldo Bicelli, L. J. Elecrrochem. Soc. 1985, 132, 593-598. (25) Shen, W.; Siripala, W.; Tomkiewicz, M.; Cahen, D. J. Electrochem. SOC.1986,133,107-112. (26) Butler, M. A.; Ginley, D. S.J. Electrochem. Soc. 1978,125,228-232. (27) Razzini, G.; Peraldo Bicelli, L.; Arfelli, M.; Scrosati, B. Electrochim. Acra 1986,31, 1293-1298. Peraldo Bicelli, L.; Razzini, G.; Arfelli, M.; Scrosati, B. Sol. Energy Mater. 1987,15,463-474. (28)Tenne, R.; Mirovsky, Y.; Sawatsky, G.; Giriat, W. J. Electrochem. Soc. 1985,132, 1829-1835. (29)Gerischer, H. J. Electroanal. Chem. Interfacial Elecrrochem. 1977, 82, 133-143. (30) Bard, A. J.; Wrighton, M. S. J. Electrochem. Soc. 1977, 124, 1706-1709. (31) Frese, K. W., Jr.; Madou, M. J.; Morrison, S.R. J. Phys. Chem. 1980.84, 3172-3178. (32)Tenne, R.; Hodes, G. Surf. Sci. 1983, 135,453-478. (33)Gerischer, H. Faraday Discuss. Chem. Soc. 1980, 70, 137-151; 255-283. (34)Cahen, D.;Manassen, J.; Hodes, G. Sol. Energy Mater. 1979, 1, 343-355. (35) Noufi, R. N.; Kohl, P. A.; dogers, J. W. Jr.; White, J. M.; Bard, A. J. J. Electrochem. Soc. 1979,126,949-954. (36) Ellis, A. B.; Kaiser, S.W.; Wrighton, M. S.J. Am. Chem. Soc. 1976,
98,6418-6420. (37) Rubin, H. D.;Arent, D. J.; Humphrey, B. D.; Bocarsly, A. B. J. Elecrrochem. Soc. 1987, 134, 93-101, and the references quoted therein. (38) Licht, S.;Peramunage, D. Nature 1990, 345,330-333. (39) Seshandri, G.; Chun, J. K. M.; Bocarsly, A. B. Nature 1991, 352, 508-510. (40)Hodes, G.; Manassen, J.; Cahen, D. J. Am. Chem. Soc. 1980,102, 5962-5964. (41) Krishnan, V.; Ham, D.; Mishra, K. K.; Rajeshwar, K. J. Electrochem. SOC.1992, 139,23-27.
Studies of Solute-Solvent Interactions in Mixtures of Supercritical Fluids Using Fluorescence Spectroscopy Ya-Ping Sun,t Gerald Bennett,* Keith P. Johaston,*J and Marye Anne Fox*lt Departments of Chemistry and Biochemistry and of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712-1167 (Received: April 29, 1992; In Final Form: June 17, 1992)
S o l u t m l v e n t interactions of (dimethy1amino)benzonitrileand ethyl (dimethy1amino)benzoate in mixtures of supercritical trifluoromethane and carbon dioxide are studied using fluorewxnce spectroscopy. T h e density dependence of solvation in the mixtures is similar to that in the pure supercritical fluids. The polar component CHF3in the mixtures clustem preferentially about a solute molecule. This clustering is also density dependent. Bulk and local (microscopic) solvent effects in the mixtures in different density regions are rationalized based on the Onsager reaction field model and on the concepts of local density and composition.
Introduction Supercritical fluids, because of their many unusual properties and important applications, represent a novel medium for studying bulk and local Such studies enrich our fundamental understanding of condensed phase solute-solvent interactions. Soluttsolvent interactions in supercritical fluids are characterized by a higher local dielectric than expected from dielectric ‘Department of Chemistry and Biochemistry. *Department of Chemical Engineering.
0022-3654/92/20961000lS03.OO/O
continuum theory.55 Such unusually large solvent effects are often attributed to an enhanced local solvent namely, that solvent molecules cluster about a solute molecule. Soluttsolvent clustering in supercriticalfluids, though quantitatively solute and solvent-specific, follows a very characteristic pattern which can be approximately accounted for by a threadehsity-region solvation model.5 In the low-density gaslike region of a supercritical fluid, solvation is stoichiometrically discrete and its effect is large. In the high-density liquidlike region, solute-solvent interactions resemble those in liquid solution and generally follow a classic 0 1992 American Chemical Society
