Energetics of the Semiconductor-Electrolyte Interface John A. Turner Solar Energy Institute, 1617 Cole Boulevard, Golden, CO 80401
The use of semiconductors as electrodes for electrochemistry requires an understanding of hoth solid-state physics and electrochemistrv since ohenomena associated with hoth disciplines are seen in se~iconductor/electrolytesystems. The eeneral conceots of solid-state ~ h v s i c have s been presented previously ( l iThis paper will ;on;entrate on the ihterfacial enereetics of these svstems. ~nsolid-statephys&, measurements of the Fermi level and the band edges are made in hieh vacuum where it is convenient to refe; potentials to the-vacuum level of an electron. The reference potential for solution chemistry is the standard hydrogen electrode (SHE). The difference in potential hetween the vacuum level and the SHE is usually taken to he -4.5 eV. althoueh measurements ranee from -4.5 to -4.7 eV (2-4). he refeFence electrodes typi&lly used in photoelectrochemical work are the standard calomel electrode (SCE) or a silver/silver chloride electrode. Potentials are eiven in Fieure 1for some reference electrodes versus b o g the normal hydrogen reference and the vacuum level reference. In this energy scheme, electrons wish t o travel downhill and holes wish to travel up. Further information on reference electrodes can be found in reference (5). In solid-state systems, electrons and holes are collected using metal contacts; for photoelectrochemical devices at least one of the carriers is collected using a redox species. The nature of the redox species used is governed by the type of semiconductor and the energetic position of the hands. Solution species are reduced a t the surface of an illuminated
p-type semiconductor; oxidations occur at illuminated n-type semiconductors. A simplified picture of the bands for hoth nu-tvpe and . .. semiconductors is eiven in Figure 2 under conditions of illumination and solugon redox couples. Reductions for . p-type .. semiconductors will occur from electrons in the conduction band if the solution species lies below the surface conduction hand; oxidations for n-type semiconductors will occur from holes in the valence hand if the solution species lies above the surface valence hand level. The seuaration of charees and their movement toward the surface'arise from the generation of an electric field inside the surface of the semiconductor oictured schematicdv in Fieure 2 as hand bending. The co&ept of hand hending and-the ability to use semiconductors as efficient photoconversion devices lies in the assumption of fixed band edges. As the semiconductor is placed in contact with the electrol.vte, an electrical douhle layer is i h n e d on the solution side I the cmcept of the rlectricul doublc layer h,is dread? heen described (ti),;thi-i douhle layer acts much like a capacirur. The semiconductor alsu has a capacitance aaswiated u,ith it. hut its value is much smaller. Typical wlucs fur duuhlc-layer capacities range from IV-IUOHFrm-2; for the s ~ m i ~ u n d u c t ~ ~ r c n ~ a s i ~ iramee e s from (1.001-1 uF cm- . Since these cauacitm ark in series, [he smallest capacitor will govern the response of the system. For capacitors in series we have (7)
Volume 60 Number 4 April 1983
327
Vacuum Level
Hydrogen Scale
I
4.7
t
n-type
--
0.197 AglAgCl (Sat'd KCI) 0.236 Calomel (Sat'd NaCI) t'0.242 Calomel (Sat'd KCI) Figure 2. A simplified picture of the bands for both n- and p-type semiconductors under conditions of illumination and solution redox couples.
n-type
Accumulation
therefore
The bands in the bulk of the semiconductor will move with changes in applied potential (Fermi level) and the potential J r q WIII ~ :,ppur 111 ;I r r p m l l w r 111t > I I ~ ~ : I ~ Y 9 t 1 111e:CIII~~YW ~lwtcrr.'l'tlii w ~ i t m e t u r ; t l t u ~ ~ dt el r~ in1111t introduction into the field. For more detailed information, the in theibsence of external current flow, the charges build up reader is referred to the literature (8,13-15). and eeuerate a field omosite that of the mace charge layer. Acknowledgment This"wil1 continue until the field in the s&conduFtor disappears and the carriers recombine at a rate equivalent to the This work was supported by the U S . Department of Enphotogeneration rate. The potential one measures is then the ergy, Office of Basic Energy Sciences, Division of Chemical field free potential which is the definition of the flat band Sciences, under Contract EG-77-C-01-4042, ~~~~~
nntantial. r---------~
Determination of the onset of photocurrent involves measurement of the current flow as a function of the voltage, and then comparison of dark and illuminated scans. If the po-
at flat hand there bethe separation current flow becauseisof the ahsence of any field to cause of charges. As one changes the potential, a field is generated in the space charge layer and the charges move under the influence of this field. current now flows in the external circuit. The potential wherethe photocnrrent begins (anodic for n-type, cathodic for p-type) is taken as the flat band potential. Capacity measurements offer perhaps the most information about the system hut are also some of the most difficult measurements to make and interpret. One measures the capacitance of the space charge layer as a function of voltage. This usually involves use of an ac bridge, a lock-in amplifier, ~~
~~
~
Literature Cited
Finklea, H. 0.. J. CHEM EDUC,60,425 (19831. ~ ~ h m s n ,~ ~ n t .u ,i i~~ r s czza.843 h. (1967). 'Trarrtti, S., Adu, Eirctiochem. Electrochem. En&, 10,213 (1977). Gomer.R.,andTlyson,G..J Chem Phys..66,4413 (1977). IV~B.D. J. G.. and J ~ ~ G. Z~ , . , ~ ' ~ s f ~ ~ and e ~P ~~ ~ Ce ~ ~~ ~ Ci ~ ~, " ~ t ~ ~~ d ~d ~ ~ m s sN. ~ W~ o r k1961. , (6) Buckris, J. O'M. J.CHEMEDUC (thisissue). 17) Smith. R. ~J.,"Circuits,Devicesand Systems," J. Wiley and Sons, New Yurk. 1971. ( 8 ) M Y ~ ~ Iv~ A,, " , s n d ~ ~ a s k oYU. v . v., " ~ ~ ~ ~ of semiconductors,"~~enum t ~ ~ ~ h ~ ~ i ~ t Prear,New York, 1967. Geiiseher. H.. in "Topics in Applied Physics? Val. 31: ''Solar Energy Conversion," Seraphin, B. 0..(Editor), Spiinge1-Verlag, New York. 1979. Turner,d.A.,nnd Parkinron,B. A , J. Elrciioonal. Chrm..inpress. Kuhn. P A . , sndBsrd. A. J.,J . Eieclrachrm Soc., 126.59 (19791. Dewald. F., R d S y r i . Tech. J.,39,615 (1960). Morrison. s.R., "Electroehemistry at Semiconductor and Oxidized MetalEiedrudes," PlenumPrear,New York, 1980. No8ik.A. J.,Reo. Piiyi. Chem ,29,189 (1978). R~jeshwar.K.,Singh,P.,and,Dubow. J.,Elecriorhim. ~ & a , 2 3 , 1 1 1 7(1978). (I) (2) (3) (4) (5)
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Number 4
April 1983
329
