An Overview of the Progress in Photoelectrochemical Energy Conversion Bruce Parkinson Solar Energy Research Institute, 1617 Cole Boulevard, Golden, CO 80401
The aim of this article is to provide an overview of the advances that have been made in the field of ~hotoelectrorhr11.i-try, mukin:: one oimme ,,I 111~:rol~tt.pt.; inrroduced hy the p r e ~ i o uartidr.;. ~ Tht mi, It. wll he di\.idrd into three parts. First, a short historical account of the development of the field will be given. Secondly, the state of the art of photoelectrochemical energy conversion will be reviewed, and finally, an attempt will be made to look into the future of this field. Many references will be given to enable the interested reader to find more detailed information on specific topics.
this point in time are of more academic than practical interest because no one has built a cell which shows either high staThe interested reader bility or an efficiency greater than 1%. is referred to the work of Albery and Archer (16-19) for a more detailed and complete analysis of photogalvanic cells. The electrochemical photovoltaic cell makes use of a semiconductor electrode for both light absorption and separation of the photogenerated electron-hole pair. This type of cell is the most easily constructed of d solar cells because all that needs to be done is to immerse a semiconductor electrode and another inert electrode into an aooronriate redox elec.. Historical Background trolyte and connect them through an appropriate load. ElecIn 1839 the French physicist, Edmond Becquerel (not to be tricity is produced when the electron hole pairs created by confused with his son, the discoverer of natural radioactivity, illumination of the semiconductor are separated in the space Henri Becquerel), noticed that a voltage and a current were charee laver near the semiconductor-electrolvte interface with " produced when a silver electrode immersed in a chloride the majority carrier being driven into the bulk of the semielectrolyte was illuminated (I). This was the first observation conductor while the minority carrier is driven to the semiof the photovoltaic effect. The next major advances in phoconductor-electrolyte interface. The minority carrier (holes toelectrochemistry occurred some 125 years later at Bell in an n-type material or electrons in a p-type material) oxiLaboratories, shortly after the invention of the p-n junction dizes or reduces the redox species in the electrolyte while the silicon solar cell, when Brattain and Garrett studied the maiority a load and . . carrier travels around the circuit t h r o u ~ h physics of illuminated semiconductor/electrolyte interfaces ;1;rol11plishe~the opposite reactim at the dark elkrnrde. The ( 2 ) .Subsequent work a t Bell Laboratories by Dewald (3,4), dark electrode i3 orten ca led the counter eltctrude and ran Boddy (5, 6), and Turner ( 7 , 8 ) ; in Germany by Gerischer be constructed of any inert conducting material, i.e., platinum (9-1 I ) and Memming (12,13); and in Russia by Myamlin and or carbon.) The difficulty, for this type of cell, arises because Pleskov (14) resulted in the beeinnine of a fundamental unthe minoritv carriers are usuallv" hiehlv reactive and so the derst.gndil~qoi flit ht.hilvior ut'amiamductor electrodes. semiconductor itself may undergo oxidation or reduction reThe r~mcwtunlIWD whirl1 bruurhr ~ ~ h ~ , t u t l e r r r o c h e ~ i i i s ~ r v actions with the nhoto~eneratedminoritv carrier. This orocess is known as photocorrosion and has been the major dbstacle from the f ~ ~ d a m e n tto a lthe practicai occurred in Japan in 1972 when Fujishima and Honda (15) studied the photooxin the path of the development of photoelectrochemical solar idation of water to oxygen at illuminated semiconducting TiOz cells (20,21). electrodes. They suggested that such a system would be aDDespite the fact that all small band gap semiconductor electrodes are thermodynamically unstable toward photocorrosion, a high degree of kinetic stability has been achieved of sunlight to stored chemical energy. The large bandgap, but in several systems. In Table 1 are listed the most efficient very stable, metal oxide semiconductors (i.e., Ti02, SrTiOs, electrochemical photovoltaic cells which have been conWOs) have the disadvantage that they are capable of constructed from single crystal semiconductor materials and verting only a small fraction of the solar spectrum into electested up to the date of this writing. The amount of charge trical or chemical energy. Since 1974 the main research efforts, which has been passed through each cell is also given. Practical which have been pursued by an increasing number of recells will require a great deal more lifetime testing than has searchers in laboratories a l l over the world, have been to study been given to the laboratorv nrototv~ecells listed in Table 1. semiconductors which have smaller hand gaps. The challenge is that the materials with smaller band .ran. materials (ex.. 1.1 1%)l.Se\',, which arecapntdcd trmwrtiny sunlight touiahle that many of the materials"from which our modern world is t'1ic'rEv ill hikh ci!icit.n,y, tend t i r he t~nst;~l~lv wht n illurninsred constructed are thermodynamically unstable in our atmoin el&trolyie solutions. Table 1. Efficient Electrochemical Photovoltalc Cells Progress in Electricity Producing Cells Constructed from Sinale Crvstal Semiconductor Materials Photoelectrochemical cells, which have only electrical enSunlight Stability ergy as an output, are divided into two main groups: the SemiEffi(coulombs/ Refphutogaluanic cell and the electrochemical photouoltaic cell conductor Redox Electrolyte ciency cm2) erence or semiconductor liquid junction cell. Photogalvanic cells use metal electrodes and produce n-GaAs IMK2Se, 0.01MK2Se2, 1MKOH 12.0% 35,000 (22, 23) p-lnP electricity as a result of light interacting with photosensitive 0.3MV+3. 0.05 VtZ, 5MHCl 11.5% 27,000 (24) n-WSe2 lMKI, O.OIMKIs 10.2% 400,000 (25, 26) chemicals in the solution. The photochemical reaction in son-MoSe2 IMKI, O.OIMKis 9.4% 50.000 (25) lution stores energy as a result of a thermodynamically "upn-CdSe lMNa2S2, IMNaOH 7.5% 20,000 (27) hill" redox reaction and the metal electrodes are used to ex6.0% n-WS2 1MNaBr. 0.01MBr2 (26) tract the energy stored in the solution. Photogalvanic cells at A
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Journal of Chemical Education
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Table 2.
Sunlight Conversion Efficiencies and Preparation Method tor EfficientPolycrystalline Semiconductor Liquid Junction Solar Cells
Semiconductor
Redox Electrolyte
n-CdSea esTeoas "-&As p-i"P n-CdSe
1 M Na& 1M KOH lMKnSe,O.lM K,Se, 1 M KOH 0.3MVi3. 0.05MV+2. 5MHCI 1M Na& 1 M NaOH 1 M Na& 1 M NaOH 1MNa2S2,l M N a O H lMNegSs, l M N a O H
n-CdSe n-CdSe n-CdSe
Sunlight Efficiency
Preparation Technique
7.9% 7.8%
7.0%6 6.5% 6.3%
Reference
Painting Slurry
(28)
CVO* CVOd
129) I301 131) (32) (33) (34)
Vacuum Coevaporation CBOr Electrodeposition Hot Pressed
5.5 5.3
'Chemical vapor Deposition Transient Efficiency Afler Treatment with Agi Chemical Bath Deposition
sphere (i.e., concrete, aluminum, wood, and most plastics). Perhaps the major advantage of a semiconductor liquid junction, over solid state junctions, is that a high percentage of the single crystal efficiency can be retained in a polycrystalline material. This fact is due to the abilitv of the iunction-forming liquid to conform to the uneven surface of the small crystalline grains which make up a polycrystalline material. This is important because the cost of producing thin films of polycrystalline semiconductors is considerably less than that for large single crystals of any given semiconductor. Ultimately, it is the cost per watt of electrical power produced that determines the practical utility of any alternative energy source. Shown in Table 2 are the sunlight conversion efficiencies and the preparation method for the most efficient polycrystalline semiconductor liquid junction solar cells which have been reported in the literature. One feature to notice from Table 2 is that many different preparation techniques, even for the same material, can result in reasonably efficient solar cells. The long-term stability of most of these cells has not been investigated under actual working conditions with the exception of the n-CdSeo.6hTeo.sj ceE. This cell, which was developed by workers in Israel a t the Weizmann Institute for Science, has had extensive testing on the roof of their laboratory for many months (3,5).The Weizmann group has also pioneered the inclusion of a third electrode acting as a battery electrode to store some uortion of the electrical enerev uroduced (36).When there-is no sunlight available the L k e r y electrode can be discharged through the counter electrode and one of the major problems associated with the use of solar energy, that of storage, is solved. Photoelectrosynthetic Cells
Photoelectrosynthetic cells are devices in which a chemical change is brought about a t an electrode-electrolyte interface when the semiconductor electrode is illuminated with light of energy equal to or exceeding the band gap energy of the semiconductor. If the ~hotoinducedchemical reactions a t the cell electrodes result in energy being stored (a positive change in free enerm, AG) the cells are called ohotoelectrolvscs cells. If the opposite is true the cells are calied photocataiytic cells because the light energy is used to overcome an energy barrier of a normally "downhill" reaction. The fact that photocatalytic cells do not result in the net conversion of solar energy to stored energy does not imply that such devices are of no use. Many of the reactions which are carried out on an industrial
i f a phokelectrosynthetic cell requires'an external electrical energy input as well as a light energy input to accomplish the desired electrochemical reactions, the cell would he called a photoassisted electrolysis cell or aphotoassisted electrocatalytic cell. If both electrodes are semiconductors and one is n-type and the other P-tvue, i t would then be called a ohotoelcctrochernical diode a p - n photoelectrosynthetiE cell
Table 3.
Several Laboratory Prototype Photoelectrosynthetic Cells
Electrodes
Type Cell
n-MoSe,, p-lnP(Pt)*B n-MoSe,. p-lnP(Pt)'b n-Moses. n-MoSep, Pt* ' n-WSen, p-SWe2' p-tnP(Pt)* p-GaP p-Gap n-TiOn. p-GaP n-SrTiOa p-LuRhOa, n-TiO,
p-nd p-n PEe P-n PAE' PC0 PE PE PE P-n
Products
HZ, Br, H, I, HZ, S042Hz& HZ. 0, NHI CHSOH. CH,O
Hz. O2 Hz. 0 2 Hz. 0 2
Reactants
Reference
HBr HI H+, SO2 HI Hz0 N, Ht COB Hi H20 H20 Hz0
139) (39) (40) (41) (44 (43) (44. 45) (37. 38) (46) (47)
Electrodes marked with an asterisk (')aresmall band gap materials whicharecapable 01 efficient solar conversion. V~MIUC~S ~imultane~u~ly approximately equal amounts of chemical and electrical enwy. Proposed as pan of a thermal water-nplining cycle. p-n = Photoelectrochemical diode 'PE = Photoelectrolysis cell 'PAE = Photoassistedelectralynis cell PC = Photocafalyficcell
'
(37,381. The advantage of using two semiconductors is that the photovoltages of the two electrodes add such that both the oxidation reaction (at the n-type electrode) and the reduction reaction (at the p-type electrode) are photodriven. This is analogous to photosynthesis in green plants where both the water oxidation and carbon dioxide reduction reactions are light driven. Several laboratory prototype photoelectrosynthetic cells which have achieved respectable efficiencies for the conversion of light to chemical energy are listed in Table 3. Unlike electrochemical ~hotovoltaiccells. comuarisons of efficiencv beergy output to light energy &put is aielatively easy and straightforward measurement, the situation is m i t e different when the output of the cell is chemical energy or, as is the case with photocatalytic cells, activation enerm. Not only must the energy content of the chemical products & considerkd hut also the economic value of the chemicals. In addition, the energy savings over alternative production processes and the ease of recovering the stored energy ought to be considered when evaluating the efficiencv of a ~hotoelectrosvntheticnrocess. Indeed, several different methods for measuring and calculating efficiencies for such cells have appeared (39).The low efficiency (-1%)sometimes quoted for photosynthesis can be misleading unless one appreciates the complex molecules and beautiful physical structures produced by this process. No absolute efficiencies are given in Table 3, yet devices which were constructed from small band gap devices and have the potential to yield high solar efficiencies are indicated with an asterisk (*) Recent Fundamental Advances in Photoelectrochemistry
Much of the fundamental research being conducted in Volume 60
Number 4
A ~ r i l1983
339
photoelectrochemistry is still directed toward solving the ever-present problem of stabilizing smaller band gap semiconductors. Techniques which have been tried include the use of nonaqueous solvents (48-511, applying polymer layers to the surfaces of the semiconductors (52-541, testing new materials with more intrinsic stability (25, 26, 55-57), and attaching catalysts to the semiconductor surface to promote the desirable electrochemical reactions over photocorrosion (58). Recent studies have advanced our understanding of the factors which control the energy positions of the semiconductor hand edges at the semiconductor-electrolyte interface (59-60). Surface states, that is states associated with the atoms at the interface, play an important role in the energetics and chemistrv of the semiconductor, and recent studies have addressed their measurement and chemical nature (23,57, 61) Hot carrier processes a t the semiconductor electrolyte interface have been predicted (62) and observed (63) with several semiconductors. A hot electron or a hot hole is a carrier which is not in thermal equilibrium with the band edges. Theoretical models being developed for the current voltage behavior of ~hotoelectrochemicalcells have been increasing not only in their sophistication but also in their relevance an; auulicabilitv to real svstems (64-66). An evaluation of the Sysical coifigurationi for electrochemical photovoltaic cells has recently appeared (67). Many other significant contributions to this emerging field have been made but are too numerous to be discussed in this article. However. the reader can find more detailed discussions in the many excellent reviews of various aspects of photoelectrochemistry (68-70). Future Outlook I t is clear that the world's long range energy future must contain a substantial contribution from renewable enerev
and better coupling of these catalysts to the energy harveiting semiconductor electrodes to drive more comulex andlor en-
Acknowledgment This work was supported by the US.Department of Energy, Office of Basic Enelgy Sciences, Division of Chemical Sciences, under Contract EG-77-C-01-4042. Literature Cited (I) (2) (3) (4) (5) 16)
Beequerel, E., C. R.,Acad. Sci. Porir, 9,561 (1839). Brattain. W. H., and Garrett, C. G. R.. M i Syslem Tech. J,, 34,129 (1955). Dewald. J.F . , E d System Tech. J 39,615 (1960). Dewald, J. F.. "Semiconducton." Hannry, B., (Editor). Reinhold. NY, 1957, p. 727. Boddy, P. J.. and Brattain, W. H.. J . Electrochem S o i . 109, 1053 (19621. Boddy. P. J.. J. Eirrlrach~rn.Soc.. 115.199 119681.
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Gerischer, H., ' P h y s i d Chemistry: An Advanced T~eatise,"Eyring, H., Henderaon, D.,and Yost, W.. (Editors). AcsdemicPrers, New York, 1970.p.463. Geriseher. H.. J El~rlioonol.Chrm.. 58.263 (19751. ~ , i ~ ~sac.,t 116;~ 785~i i m~) . h ~ ~ . M P ~ ~ R., ~ DJ K Memming, R.. and Schwsndt, G.. Electrachim. Acfa, 13,1299 (1968). Myamlin, V. A , Pleskou, Y. v., 'Electrochemistry of Semiconductms? plenum press, New York. 1967. Fuihishima. A.. and Honda. K.. Nature. 238.37 (19721.
Lioonol. Chem.. 107.23 , .ilSR01. Gerischar, H., J . Electroanol. Chem., 82,153 (1977). Ba7d.A. J.,and Wrighton,M. S., J . Electrochrm. Soc , U4. 1708 (1977). Parkinson.B.A..Holler,A.,and Miller,B.,Appl.Phys Lett., 36,621 (1978). Parkins0n.B. A.,Heilei,A.,and Miller,B., J. Elecirochrm. S o i , 126,955 (1979). Hel1er.A.. Miller,B.,andThiel,F.A.,Appl. Phys. Lett., 38, 282 (1981). Kline. G.. Kam, K., Canfield, D., and Parkinson. B. A , SoiorEnergy Motmini*, 4.301 (1961). (26) Kline. G.. Kam, K.. Ziegiei, R., and Parkinson, B. A,, Solar Energy Materials. 6,337 (1982). I271 Holler, A.,Chang, K. C., andMi1ler.B.. J Electrochem. Sac.. 124,697 (1977). (281 Hodes. G.. Nature, 265,29 (1980). 129) Heiler. A,, I.ewerenz. H.. and Miller, B., Ber. Aunsenpes Phys. Chem., 84, 892
