CA1106484A - Semiconductor liquid junction solar cell using gaas electrode - Google Patents

Abstract

Chang-Heller-Miller 2-5-10 SEMICONDUCTOR LIQUID JUNCTION
SOLAR CELL USING GaAs ELECTRODE

Abstract of the Disclosure Liquld-semiconductor photocells have received much attention recently as candidates for use in solar power conversion devices. According to this inventioninvention, the photosensitive electrode in a photovoltaic cell is made from GaAs and the redox couple is Se?/Se=. The cell produces photocurrent that is extraordinarily stable over extended periods of illumination as compared to previous GaAs cells.
- i -

Description

`- .. 11~)6484 Chang-~ell er-Mlller 2-5-lO

1 Backcrround of thQ Invent~on

2 1. Fleld of the vention

3 This invent~on relates ~enerally to ~emlconductor

4 llquid junct~cn phot ells, and ln partlcular, to such

5 phot~cell~ uslng GaAB as the photoæen~ltive electrode

6 2~ De~cri~tion o~ the Prlor ~rt

7 Concern over the possible depletion o~ fo~æil fuel

8 energy ~curces has generated intense Interest ln recent g yearQ in the search for and development of alternative energy s~urces' Conte~plated alternative energy sources 11 include solar energy utllizea as electricity either dlrectly 12 through photovoltaic devices or lndlrectly through thexmal 13 devices. The latter has not received aB much attention a~ j 14 the former which will, a~ presently contemplated, u~e .
semiconductor devices. The~e devices axe presently t6 relatively expensive, compared to ~ossil ~uel~ power Yource~ ¦:
17 becaq~e the device~ collect light generally ln proportlon to 18 the axe~ o~ the photo#en~itive ~unctlon whic~h must be large ~.
19 to ~enerate useful photocurrents. The C08t of manufacturi"g :20 such devlceR depenas mainly upon the area of the ~1 phokosensitive J~nction an~ is presently too high to permit , :
22 succe~sful ccmmercial explo~tatlon ln other than specializea 23 applications.
24 Considerable time has therefore been expended ln 25 attempting to ~lnd ways to reduce the cost of ~olar energy 2~ obta~ned ~ran ~ conductc~r devlces. Qne approach that has 27 ~ge~eroted much ~nterest~and enthuslasm xecently is a , ~ I
~ 28~ liqu~d-~emlconductor solar cell in whlch t~e active part of t~
.. !

- ~ 10 Chang-~ller-Mlller 2-5-10 1 the cell is a ~unction formed at a liquid-soli~ interface.
z These clevices prcmise to be les~ costly to manufact~re ~han 3 are ~evices in whlch the iunction is formed between ts~o 4 solids as relat~vely costly ~p~taxy or dlffu~ion procedures are not xequire~ to form the ~unction, whlch for~
6 spontaneously in the~e devices at the semiconductor-liqui~
7 interface.
8 A varlety of reasons has prevented thes~ cells frcm

9 being fully exploited at the pre~ent time. One reason i~
that ~ome semiconductor ~aterlals, having bandgap~ of a ~ize t1 favorable for efficient conversion of solar energy lnto 12 e~ectricity and therefore deslrable ~or use as the solld 13 electrode material, ~r~ not stable. That iB, the ef~iciency 14 of the cel~ declines with operatlng time for any of several reasons. ~or example, photoexcitation may produce hole~ at t6 the s~rface which chem~cally reacts with the electrolyte.
17 qhi8 and other processe~ corrode and/or passlvate the 18 semiconductor surface and cau~e degradation of cell 19 efficlency a~ manifested by a decrease in the photocurrent fram the cell a~ the operating t~me of the cell increases.
21~ ~ther mechantsm9 such a~ chemical etching or deposltion of 22 ~mpuritie~ on the semiccnductor ~urface may also be active.
23 GaAn ha~ a bandgap of about 1 .4 ev and, #ince a 24 bandgap of appro~cimately thl~ magnitude theoretically will 25 give the most efflclent photovolta~c conversion of solar 26 p~er ~nto electrlcity, a cell using this materlal and 27 produ d ng a stable photocurrent over an extended time perlod 28 would be extremely de6irable ~r~n a con~nercial point of 29 view. cel}~ ~ln~ GaAs have, however~ appeared eYpecially 30 ~usceptlble to electrode de~radatlon~ with attendan'c decline 31 in photocurrent output, arising fro~n photoche~ical reactions ~ 2 ~

._ _ _ __ _ _ _ _ _ _ _ _ _ _. _ _ _ _ _ _ _ _ _ _ ___ _ . _ _ _ , 1106~18~

of the GaAs electrode with the redox electrolyte. Reported liquid-semiconductor junction photocells using GaAs such as thé ones reported by Gerischer, Journal Electroanalytical Chemistry and Interfacial Electro Chemistry 58 263 (1975) and Wrighton Bulletin of the American Physical Society 22 60 tl977) had lifetimes too short to permit serious consideration of useful commercial exploitation or were unable to support photocurrents of a useful magnitude.
Summary of the Invention According to the present invention there is provided a photocell containing a photovoltaic junction between n-type gallium arsenide and a liquid electrolyte containing a redox couple wherein the redox couple has a ; concentration greater than 0.1 Molar and consists of anions of either selenide or telluride or a mixture thereof.
Brief Description of the Drawing FIG. 1 is a plot of the theoretical energy conversion efficiency for photovoltaic devices using semiconductor materials, taking into account the solar spectrum, as a function of the bandgap of the semiconductor material;
FIG. 2 is a schematic representation of a liquid-semiconductor photocell;
FIG. 3 is a plot of the ratio of the corrosion ::`

~,"~ .

11~'6~
Chan~-~eller-~iller ~-5-lO

1 current to the total curr~n~ as a function ~f sel~nide 2 concentratlon; and 3 ~IG. 4 is a plot of the photocurrent ~ensity as a functi~n of time of operatlon o~ the photocell.
Detailed ~escri Dt ion 6 ~IG. 1 ~s an idealized plot of the effic~ency of 7 the conversion of solar enex~y lnto elect~icity as a 8 function of the semiconductor bandqap wlth the sclar 9 spectr~m taken lnto account. The range of efflciencie~ for each bandgap value results from different at~ospheric 11 conditlons and assumptlons about voltage losse~. ~s can b~
12 seen, GaAs with a bandgap of approxlmately 1.42 ev is clo~e 13 to the most theore~lcally efflcient materlal.
14 The cell structure of ~IG. 2 compri~es a container 20, electrolyte 21, ccunter electrode 22, which ln ~6 our devlces is carbon, althouah other inert ~aterials ~ay be 17 used, and the actlve electrode 23. The electrolyta i8 18 usually aqueouq althou~h nonaqueous electrol~te~ such as 19 propylene carbonate and tetrahydrofuran can be used~
E~ectrode 23 ls in~ulated with epoxy 24 except ~here .
21 illuminated and activated. The container ~.ay he made of any 22 conveniently a~ailable glass or plastic ~aterial, The 23 bottom of the cell, opposihg electrcde 23, i6 transparent to 24 paS8 lncident llght as shc~.
Under illumination, ~n a sultable e~ectrolyte, 26 typlcally an aqueous electrolyte, holes come to ~he surface 27 Of the ~-type GaAs and causes lts oxidative dissolutio~ by 28 the reaction 6h ~ GaA~ Ga(lII) + As(III). If this i~
29 the only reaction, the material photoetches. ~he photoetchln~ reactlon can be suppresse~ 'f a ccm~etin~

31 reaction can ~e found that wlll scaven~e for hol~s and .. ., . , . _. .. . . . . . .. . .. . . . . .... . . . . ........ . . .

. . . . . . , ~ - . . .

11`~6484 - Chang-~eller-~llller ~-5-lo 1 compete c~lrectly with the photoetchinq reaction although lt 2 ~ay be unable to completely suppres~ photoetchinn. Tt has 3 been ~ound that as the redox potential ~eco~es ~ore ne~ative 4 it scavenges more successfully for hole~. ~o~eve~, if the 5 redox ~otential ls not more neqative than -0.5 volt~ lt ls 6 unahle to scaven~e sufficient hole~ to suppress the 7 photocurroslon at GaAs to acceptable levels. Se / Se2 and 8 Te' / ~e2 redox couples satisfy this criterlon and have been ~ f~und to ~uppress photoetchin~ in GaAs cells sufficiently that uqablo cells can be ~ade. The selenium accepts char~e, 11 for example, thr~ugh the reactlcn 2Se- ~ 2h ---Se2 at the 12 ~ll~mlnated electrode, The reaction at the dark electrode 13 is Se2 + 2e r 2Se and there is no chemlcal change in the 14 cell. Suitable redox electrolyte concentratlons range from a maxlmum represented by a saturated solution to a mln~mum 16 o~ approximately 0.1M which represents the mini~um 17 ccncentratlon in an aqueou~ solutlon requir~l to consume 18 sufficlent holes, when llluminated by sunlloht, to prevent 19 unduly rapid photoetching. Gther than aqueous electrolytes ~ay be u~ed but since they generally have a lesser 21 electrlcal conductlvlty, cell efficiency i5 reduced. Por 22 ~igh concentrations, li~ht absorption in the electrolyte can 23 be compen~ated by maklng a thln llquld layer.
24 Dlselenide ion and polyseleni~e ions may be for~ed 2S ~n the solution by passing ~2Se into a ~aslc sol~1tion, such 26 as an aqueo~ 501utlon of KOH, and perm.ittinq air to oxidlze 27 scme of the ~e~ to Se2 or by directly dissolving Se 28 metal. Other ba~es such as ~JaOH and ~4 o~ may also be 2g U6ed.
Photocells as ~ust descrlbed ~ere made with the 31 actlve electrode 21 formed fro~ an n-~y~e ~,aAs sin~le _ 5 _ Chan~-Heller-Mlller 2-5-10 1 c~ystal ~ th a thickness oY 600 ~lcrcns and Ce /5~2 redox 2 electrol~.es, Cells were made with dlfferent selenlde 3 concentraticns all ha~ing a Se /Se2 ratlo of a~,proximately 4 eiaht and the wei~ht lass from the GaAs electrode was s me~ured. Illumination was provlded bv a quartz-halogen 6 lamp o~erating at a level correspondln~ rcughly to 3A~2 7 ~three air ma~s t~o) sun~. ~S2 co-responds a~proximately to 8 noon time ~unlight in middle latltudes. The applled 9 potential cf the GaAs electrode was controlled at -0,4V
versus saturated calomel or 0.5~V posltive of the solution 11 Fermi level. Thls level i~ lV more oxldlzing than that 12 measured at m~ximum power and the rate of photocorrosion 13 under these conditlons ou~ht to be substantially hlaher than 14 that of a cell o~erating at maximum efflciency, althou~h the actual interfaclal potentlal is not known. PIG. 3 i9 a plot t6 of the ratlo of the corrosion current to the total current 17 as a functlon of selenlde concentration. The curve 18 represents the expression ' lc = (1 + 3500C3e) where Cse ls the total molar concentration of selenium in 21 the 601ution8. The ~easured welaht loss of the electrode 22 was c~nverted to corrosion current through the dissolutlon 23 ~toickiometry o~ ~ix electrons per GaA~ ~olecule as 24 conflrmed by the etch rate for zero 6elenide concentration.
Ihe ordlnate repre~ents the ~raction of photocurrent not 26 qoing to the desired regeneratlve solar cell path and thus 27 causing etching~ As can be seen, hlgh selenlde 28 ccncentrat~ons lead to a relatively ~all corrosion current 29 althou~h the corroslon current ls never ccmpletely ~ 6 ~

. .... . . . .. ... .

~, . .

` ~106484 Chan~-Heller-~lller 2-5-l0 1 suppressed.
~ 2 The current voltage characteristics of a typical cell 3 with an n-type GaAs electrode and an aqueous electrolyte con-4 taining KOH and a nominal l molar total selenium concentration and operated under sunlight are as follows. The short fi clrcuit current at an lrradiance of 69 mw~cm2 is 16.5 ~a/cm2 7 which corres~cnds to a quantum efflciency of approximately 8 65 percent for a ~olar AM2 spectrum. ~axl~um power ls 9 dellvered at 0.45V and 13.5 ma/cm2 which yields a flll t0 factor of approxi~ately 57 percent and an efflclency of 11 approximately ~.8~. The c~ystal for thls cell ~as doped 12 wlth Sn and had a free electrcn ccncentration of 13 approxlmately 2 x t017/cm3. Cther materlals that qive n-14 type behavior, e~g., Sl and Te, mlght al80 be us~d as the partlcular doDant used does not appear crucial. The upper 16 llmlt of dopant i8 desirably les~ than 5 x1 o1 8/c~3. Abcve : t7 thls value, the space charge layer ls too t~ln to permlt 18 essentlally all l1ght to be absorbed withln the space charge 19 layer and the resulting recomblnation of carriers reduces cell efflc~ency.
21 A plot of the phctocurrent from this cell ~ersuQ
22 tlme dt~ing an extended run under a quartz-halogen lamp 23 havinq an lntensity equivalent to 3AM2 suns i8 shown in 24 FIG. 4. The photocurrent is essentlally con~tant over a period that exceed~ ~00 hcurs. ic / iT~ for the : 26 approxlmately 1 molar total selenium concentration used, is 27 approxi~ately C~001 glving an avera~e corroslon penetration ; 2~ less than 12 ~.lcrons over the 400 hour pericd. The stable 29 output is attributed not only to suppression of the photoetch~ng reaction but also to the contln~ed inteority of : 31 the semiconductor suxface dt~rln~ cell operatlon. The , :` ~lU~484 Chang-Xeller-l~lller 2 5-10 limited and contro~'ed remo~al of sem~conductor material, 2 while establishlng an ultlmate llfetime ~or the cell, ~, 3 offsets deposltion of ilnpurities whether frc~n the 4 electrolyte or a chemlcal reactic~n ~etween the GaAE; and t:he electrolsrte~ on or near the semiconduc:tor surface and thus 6 malntains i~ integrlty and quality.
7 Altho~gh the cells descrlbed use a s~nale crystal 8 GaAs electrade, the prlnciples that perm~ t a stable 9 photoc:urrent to ~ obtained ~rcm a cell u6ing a single cry~3tal GaAs electroae are also applicabl~ to cell~3 usina 1~ polycrystall~ne Ga~ electr~de!3. 1 12 Although the cell~ have been describe~ with respect 13 to t}~eir utility as solar cell#, 'chey are of ob~ous utlllty 4 for con~er~ng energy fra n llght sources other than the ~un t5 and are therefore more properly called photocell# rather ~6 than sola~ cells~, ' .

.
.

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A photocell containing a photovoltaic junction between n-type gallium arsenide and a liquid electrolyte containing a redox couple wherein the redox couple has a concentration greater than 0.1 Molar and consists of anions of either selenide or telluride or a mixture thereof.

2. A photocell as claimed in claim 1, wherein the electrolyte is propylene carbonate or tetrahydrofuran.

3. A photocell as claimed in claim 1, wherein the GaAs is doped with either Sn or Si or Te.

4. A photocell as claimed in claim 1, 2 or 3 wherein the GaAs electrode comprises a single GaAs crystal.

5. A photocell as claimed in claim 1, 2 or 3 wherein the dopant concentration in the GaAs is less than 5 x 1018/cm3.