Radiation and Temperature Effects in Gallium Arsenide, Indium Phosphide, and Silicon Solar Cells

NASA Technical Memorandum 89870

Radiation and Temperature Effects c in Gallium Arsenide, Indium Phosphide, and Silicon Solar Cells

(NASA-TM-8987c) BADXATIGB AND TEBPEBATUBE N8 7-2 20 9 8 EFPEClS XN GALLliUE ARESEAIIDE, 1OCXfJM FBOSPBXICE AND SILICON SCLAB CElLS (NASA) I 14 p Avail: h9IS BC LIO%/WF A01 CSCL 09C Unclas Hl/33 0075143

I. Weinberg, C.K. Swartz, and R.E. Hart, Jr. Lewis Research Center Cleveland, Ohio

and

R.L. Statler Na Val Research Laboratory Washington, D. C.

Prepared for the 19th Photovoltaic Specialists Conference sponsored by the Institute of Electrical and Electronics Engineers 1 New Orleans, Louisiana, May 4-8, 1987 . Pv- +,-*. r I tddi".'. r?C PACE 13 OF POOR ClJALlTY

RADIATION AND TEMPERATURE EFFECTS IN GALLIUM ARSENIDE, INDIUM PHOSPHIDE, AND SILICON SOLAR CELLS

I. Welnberg, C.K. Swartz, and R.E. Hart, Jr. Natlonal Aeronautlcs and Space Admlnlstratlon Lewls Research Center Cleveland, Ohlo 44135

and

R.L. Statler Naval Research Laboratory Washlngton. D.C.

The effects of radlatlon on performance are speclflc radlatlon envlronment. At present, the determlned for both n*p and ptn GaAs and InP n*p conflguratlon Is preferred excluslvely for cells and for slllcon ntp cells. It 1s found slllcon cells used ln space. Thls follows from a that the radlatlon reslstance of InP 1s greater demonstrated superlor radlatlon reslstance when than that of both GaAs and S1 under 1 MeV elec- compared to the p*n conflguratlonS6 However. tron trrad\at\on. For slllcon. the observed the sltuatlon Is unclear wlth respect to cells decreased radlatlon reslstance wlth decreased based on InP and GaAs. For GaAs, a prellmlnary reslstlvlty 1s attrlbuted to the presence of a comparlson of the two conflguratlons has been con- radlatlon Induced boron-oxygen defect. Compar- ducted after proton lrradlatlon.7 Conslderlng lson of radlatlon damage In both p*n and ntp InP, there Is a scarclty of data In the open llter- GaAs cells ylelds a decreased radlatlon reslst- ature comparing the radlatlon reslstance of the two ance for the n*p cell attrlbutable to conflguratlons. Hence, we have lrradlated InP and lncreased serles reslstance, decreased shunt GaAs solar cells wlth 1 MeV electrons In both the reslstance, and relatlvely greater losses In n+p and ptn conflguratlons and determlned thelr the cell's p-reglon. For InP, the n*p conflg- performance as a functlon of fluence. In addltlon, uratlon 1s found to have greater radlatlon we have determlned temperature dependencles of the reslstance than the ptn cell. The lncreased solar cell parameters for the unlrradlated cells. loss In thls latter cell Is attrlbuted to Our objectlve lles In assesslng the relatlve per- losses In the cell's emltter reglon. Tempera- formance of n*p and ptn InP and GaAs cells and ture dependency results are lnterpreted uslng thus contrlbutlng to a relatlvely scarce data base. a theoretlcal relatlon for dVoc/dT whlch pre- We also sumnarlze the state-of-the-art In AM0 effl- dlcts that lncreased Voc should result In clency achlevements for SI, GaAs, and InP and decreased numerlcal values for dPm/dT. The Include comparatlve radlatlon reslstance data for predicted correlation 1s observed for GaAs but all three cell types. not for InP a result whlch 1s attrlbuted to varlatlons In cell processlng. Exp e r 1menta 1

Detalls of the InP and GaAs solar cells used Introduc t lon In the present radlatlon damage experlments are shown In Flgs. 1 and 2 and Tables I and 11. The Photovoltalc arrays based on slllcon are, at GaAs cells were obtalned from Varlan.8 The n*p present, the major prlmary source of spacecraft cells were processed by the Rensselaer Polytechnlc electrlc power. On the other hand, galllum arse- In~tltute.~whlle the p+n cells were obtalned nlde solar cells are beglnnlng to see llmlted use from Arlzona State Unlverslty. Addltlonal cell In appllcatlons where end-of-llfe array output detalls can be found In the flgures and In Refs. 8 power, In a degrading radlatlon envlronment, Is a to 10. The cells were lrradlated by 1 MeV elec- major deslgn canslderatlon. Compared to these trons In the Naval Research Laboratorles Van de latter two cells, Indlum phosphlde solar cells are Graaf accelerator. Solar cell performance rneasure- In a relatlvely early stage of development. How- ments were carrled out at NASA Lewls. using an alr ever, It has been shown that InP solar cells are mass zero X-25 Xenon arc solar. The cell perform- slgnlflcantly more radlatlon resistant than elther ance data 1s based on total cell area lncludlng GaAs or slllcon when exposed to 1 MeV electron and that covered by the front contacts. Temperature 10 MeV proton 1rradlatlons.l e2 Thelr superlor dependency measurements on unlrradlated cells were radlatlon reslstance, annealablllty.3 and the performed In a nltrogen atmosphere uslng a varlable recent attalnment of 17.9 percent total area AM0 temperature chamber lnto whlch the X-25 slmulator effl~lencles.~wlth feaslblllty lndlcated for beam was lntroduced through a glass port. In a efflclencles greater than 20 per~ent.~make InP later sectlon of the present paper we compare the cells prlme candldates for future use In the space performance of S1, GaAs. and InP cells under 1 MeV radlatlon envlronment. Thls belng the case, It Is electron lrradlatlon. For background lnformatlon concelvable that all three cell types wlll even- to thls comparlson we present In Table 111, the tually be used In space, thelr utlllzatlon depend- performance parameters of those cells havlng the lng on the spacecraft EOL power requlrements In a

1 hlghest AM0 efflclencles. Addltlonal detalls can Level Translent Spectroscopy (DLTS). Evldence be found In Refs. 4. 8, 11, and 12. obtalned from DLTS has led to the concluslon that a radlatlon Induced defect, composed of boron and . Res u 1 t s oxygen, 1s predomlnant In causlng radlatlon Induced degradatlon In boron doped p-type slllc~n.~~*~~ Normallzed efflclencles of the GaAs and InP Of the three major deep level defects observed In cells are shown In Flgs. 3 and 4. From the flgures electron lrradlated slllcon, only the boron-oxygen It Is seen that the radlatlon reslstance of the InP defect exhlblts an lncreasln concentratlon wlth cells Is greater than GaAs cells under 1 MeV elec- decreaslng cell res1 stlvlty.31 Hence, the tron lrradlatlon.1*2 From Flg. 3 It 1s seen that decreased radlatlon reslstance, wlth decreaslng the radlatlon reslstance of the GaAs ptn cells cell base reslstlvlty, noted from Flg. 5, Is are slgnlflcantly greater than that of the ntp attrlbuted to the consequent lncreaslng concentra- L cells under the present lrradlatlons. For InP, the tlon of thls defect. Thls fundamental llmltatlon ntp cells exhlblt hlgher radlatlon reslstance presents a barrler to the use of low reslstlvlty (Flg. 4). The behavlor of the remalnlng cell boron-doped, slllcon solar cells In the space radl- parameters at constant hlgh fluence 1s sumrlzed atlon envlronment. In Tables I11 and IV. Radlatlon Effects In GaAs and InP PrevIous results have shown that, when plotted on a normallzed basls, the radlatlon reslstance of The superlor radlatlon reslstance of n+p InP cells 1s greater than that of elther GaAs or slllcon cells over the ptn conflguratlon has long Sl.2*13 The present data Is found to be In been an establlshed fact. However, the preference agreement wlth thls result (Flg.' 5). In the flg- of one conflguratlon over the other Is far from ure. data for the cell labeled InP (dark) and the establlshed for elther InP or GaAs. GaAs cells are obtalned from the present results. The cell labeled InP (dark) was lrradlated by 1 MeV For galllum arsenlde. the thlnner emltter electrons In the absence of llght. It has been reglon of the present ntp cell could lead to the found that lrradlatlon of InP cells ulth the cell predlctlon of Increased radlatlon reslstance for Illumlnated by llght results In Increased radlatlon thls cell. Thls would seem to follow from prevlous reslstance due to photolnjectlon of mlnorlty results on LPE grown GaAs cells where It was ~arr1ers.l~Thus, the cell labeled InP (llluml- observed that radlatlon reslstance tended to nated) was lrradlated under a llght lntenslty of Increase 4th decreaslng emltter thlckness.z2 70 mW/cm2. Data for thls latter cell was However, desplte the decreased emltter thlckness obtalned from Ref. 14, whlle that for the 10 n-cm the present nip GaAs cell exhlblts slgnlflcantly SI cell was obtalned from the JPL radlatlon hand- decreased radlatlon reslstance. To examlne thls b00k.l~ The low reslstlvlty slllcon cells behavlor In greater detall. we conslder the I-V (0.1 and 0.2 n-cm) are slmllar In deslgn to the curves of the Irradlated and unlrradlated GaAs hlgh efflclency slllcon cells In Table 111. Typl- cells (Flgs. 8 and 9). From Flg. 9. relatlvely cal temperature dependency runs for GaAs and InP small changes are lndlcated, for the pin cell, over a wlde range of temperature are shown In In both serles and shunt reslstance after Irradla- Flgs. 6 and 7. In general, over most of the tem- tlon. However. for the ntp cell, Flg. 8 lndl- perature range, maxlmum power (Pmax), Voc. and flll cates a relatlvely large decrease In shunt factor (FF) decrease wlth lncreaslng temperature reslstance and a relatlvely large Increase In whlle Isc Increases wlth temperature. serles reslstance. These effects result ln decreased solar cell output and thus decreased Dlscusslon efflclency. The relatlvely large decrease In Isc noted for the n+p cell (Table IV) 1s another Radlatlon Effects In Slllcon factor resultlng In decreased efflclency. Thls loss occurs predomlnantly In the cell's base or The decreased radlatlon reslstance wlth p-reglon (Flg. 10) and Is prlmarlly due to a decreaslng cell base reslstlvlty. observed In decrease in dlffuslon length due to radlatlon. Flg. 5, Is typlcal of the behavlor observed for Thus, the decreased radlatlon reslstance of the slllcon solar cells. Thls follows. emplrlcally, GaAs nip cell Is attrlbuted to the radlatlon from the behavlor of the dlffuslon length damage Induced Increased serles reslstance, decreased coefflclent It has been found for shunt reslstance, and decreased short clrcult cells wlth base reslstlvlty pB between 0.1 and current due to defect generatlon In the cell's 20 R-cm that KL Is proportlonal to (l/pg2/3 ). 17,18 p-reglon. Slnce decreaslng values of KL Imply a smaller Conslderlng InP, from Table V, the change In radlatlon Induced reductlon of mlnorlty carrier flll factor, after lrradlatlon, Is relatlvely mall and approxlmately the same for both conflgurat ons . dlf fuslon length, the res lstlvlty dependence r of KL lmplles that for slllcon. In thls resls- Hence relatlve changes In shunt and serles res st- tlvlty range, radlatlon reslstance Increases as ances are not slgnlflcant factors In comparlng the cell base reslstlvlty Increases. The data of radlatlon reslstance of the two conflguratlons Slmllarly, the percentage change In Voc Is Flg. 5 supports thls concluslon. approxl&itely the same for both cell types. On the A more fundamental reason for the reslstlvlty other hand, the percentage loss In Isc Is sllghtly, dependent performance of lrradlated slllcon can be but deflnltely larger for the ptn cell. For further conslderatlon of the loss In Isc we found In the behavlor of defects observed by Deep

2 conslder the normallzed spectral response versus of Voc. To conslder thls latter temperature varla- Wavelength shown In FIgs. 11 and 12. In the flg- tlon In greater detall, we use the relat1onz4 ures, (Isc(A))+ Is short clrcult current at wavelength b and 1 MeV electron fluence +, whlle (Isc(X)), 1s short clrcult current In the unlrradlated cell at the same wavelength. The a posltlon of the p-n junctlon 1s computed from the optlcal path length l/a(b) where a(b) 1s the absorptlon coefflclent of InP at wavelength 1.23 From the flgures It Is seen that most of the current loss occurs In the emltter reglon of the where Eg(T) Is the bandgap at temperature T. q p+n cell and ln the base of the n*p cell. From ls the electronlc charge, k Is the Boltzmann con- prevlous results, It has been demonstrated that, stant, and o and B are constants ln the temper- for n+p InP cells, radlatlon reslstance lncreases ature dependency of Eg(T) wIth as p-dopant concentratton ( ZInc) lncreases .I3 Slnce the zlnc p-dopant concentratlon ln the p*n cell Is at least an order of magnltude greater than (3) the zlnc p-dopant concentratlon In the present n*p InP cell and slnce most of the damge occurs In the where Eg(0) Is the bandgap at 0 K. The parameters p-reglons of both cells one would a prlorl expect requlred In evaluatlng Eg(T) are glven In comparatlvely less radlatlon reslstance In the Table VIII. present n*p cell. Slnce thls 1s not the case for the present cells, the current results are consld- The results of calculations uslng Eqs. (2) ered to be anomalous. In any event. It 1s con- and (3) are shown In Table IX where reasonable cluded that the relatlvely decreased radlatlon agreement 1s found between the experlmental and reslstance of the p*n cell 1s due to compara- calculated values for both InP and 6aAs. Conslder- tlvely greater losses ln the heavlly doped emltter lng the success of Eq. (2) In predlctlng the values reglon. of dVoc/dT, and notlng that the term In Isc Is negllglble, It follows that If dPmax/dT 1s pro- Temperature Dependency portlonal to dVocdT, then Increased values of Voc should result ln lower numerlcal values of To conslder the temperature dependency of dPmax/dT. From Flg. 16, It 1s seen that dPmax/dT unlrradlated InP and GaAs over the wlde range of Is roughly proportlonal to dVoc/dT for GaAs, temperatures shown In Flgs. 6 and 7 would be a whlle for InP no such correlatton exlsts (Flg. 17). hlghly complex undertaking for any large group of from the precedlng. It follows that a correlatlon cells. The present dlscusslon Is slmpllfled by should exlst between Voc and dPmax/dT for GaAs. conslderlng the temperature dependencles of the whlle no such correlatlon should exlst for InP. solar cell parameters at a stngle temperature. That thls 1s the case can be seen from the data of Slnce solar cells In low earth and geosynchronous Flg. 18. Thus, the spread In values In dPmax/dT, orblts operate at temperatures near 60 'C, we chose exhlblted by GaAs In Flg. 13 1s attributed to varl- thIs temperature for the present consIderatIons. atlons In open clrcult voltage.

Data for the temperature varlatlon of Pmax, It 1s noted that, In Flg. 18, the GaAs cells Voc. Isc, and FF at 60 OC are shown In Tables VI are grouped around two values of Voc. The flve and VII. The temperature dependencles of Pmax. cells grouped around the hlghest Voc are AlGaAs/ the cell maxlmum power. are sunmarlzed graphlcally GaAs heteroface cells whlle the remalnlng cells In Flg. 13. The wlde spread ln values, obtalned grouped around the lower Voc are slmple GaAs for thls parameter, emphaslzes the lmportance of homojunctlons. Average values for these cell lndependently determlnlng temperature dependencles groups are shown. together wlth an average value for any group of cells Intended for a speclflc for the InP cells, In Table X. Followlng the pre- rnl sslon. dlctlon based on Eq. (2), It 1s noted that the GaAs cells wlth the hlgnest Voc have the lowest numer- To conslder the relatlve lmportance of the lcal value of dPmax/dT. The absence of even such varlous cell parameters In determlnlng the tempera- a gross correlatlon for the present InP cells Indl- ture dependency of Pmax, the temperature depen- cates that temperature dependent factors, other dencles are expressed In relatlve terms uslng the than dVoc/dT are slgnlflcant in determlnlng the re1 atI on24 temperature dependency of Pmax for these cells. ThIs could arlse from varlatlons In the temperature 1 dPm = 1 dIsc dVoc + 1 dFF + 1 (1) dependencles of factors such as serles and shunt Pm dt Isc dT Voc dT FF dT reslstance whlch In turn could result from varlatlons In cell processlng. 9 Average values, for the cells of Tables VI and VII, for each term In the precedlng relatlon, are shown Concluslon ln Flgs. 14 and 15. Conslderlng terms on the rlght hand slde of Eq. (1). the flgures lndlcate that the It has been shown, In agreement wlth prevlous term In Voc 1s clearly the largest in magnltude results, that the radlatlon reslstance of InP solar for both cell types. Hence, dPmax/dT 1s assumed cells 1s superlor to both GaAs and SI under 1 MeV to be most dependent on the temperature varlatlon electron Irradlatlon. The present n*p Indlum

3 phosphlde cells are found to be sllghtly more radl- 6. J. Mandelkorn, C. McAfee, J. Kesperls. atlon reslstance than p*n cells under 1 MeV L. Schwartz. and W. Pharo, "Fabrlcatlon and electron lrradlatlons. Thls Is attrlbuted to Characterlstlcs of Phosphorus-Dlffused Slllcon degradatlon In the latter cell's heavlly doped Solar Cells." J. Electrochem. SOC., vol. 109, p-type emltter reglon. For GaAs, the ptn conflg- Pp. 313-318, 1962. uratlon Is slgnlflcantly more radlatlon reslstant than the ntp cells. The Increased degradatlon 7. I. Welnberg. C.K. Suartz, and R.E. Hart. Jr.. In thls latter cell can be attrlbuted to radlatlon "Performance and Temperature Dependencles of Induced lncreases In serles reslstance, decreased Proton Irradlated n/P and P/n GaAs n/P SlllCOn shunt reslstance and degradatlon In the p-base Cells," In 18th IEEE Photovoltalc SDeclallsts Conference, IEEE, 1985, pp. 344-349.

Temperature dependency experlments yleld a 8. J.G. Werthen, G.F. Vlrshup, C.W. Ford, theoretlcally predlcted correlatlon between C.R. Lewls, and H.C. Hamaker,' 21% (one sun, dPmax/dT and Voc for GaAs. but none for InP. alr mass zero) 4 cm2 GaAs Space Solar Cells.' The spread In dPmx/dT values observed for the Appl. Phys. Lett., vol. 48, pp. 74-75, 1986. present GaAs cells 1s thus attrlbuted to varlatlons ln open clrcult voltage. The GaAs cells can be 9. K.K. Parat. S. Bothra, J.M. Borrego, and dlvlded Into two groups clusterlng around hlgh Voc S.K. Ghandhl, "Solar Cells In Bulk InP, Hade and low Voc corresponding to low and hlgh numer- by an Open Tube Dlffuslon Process," Solid lcal values respectlvely for dPmax/dT. The hlgh State Electron., vol. 30, pp. 283-287. 1987. Voc group ylelds an average dPmax/dY value of -(4.35+0.35)~10-~mW/crnz K uhlle the low 10. K.Y. Chol and C.C. Shen. "P/N InP Hornojunctlon voltage GaAs group has the average value Solar Cells bv LPE and MOCVD Technlaues.' to -(6.04i0.01)~10-~mW/crn2 K. For InP the average be presented at 19th IEEE Photovoltdlc ' value for 6 cells 1s -(5.3+1.2)xlO-j mW/cm2 K. Speclallsts Conference, New Orleans, LA, The spread In values observed for these cells lndl- Hay 4-9, 1987. cates that great care should be ererclsed In apply- lng publlshed temperature dependency calculatlons 11 M.A. Green, A.W. Blakers, S.R. Wenham, to numerlcal predlctlons of solar cell performance S. Narayanan, H.R. Wllllson, M. Taouk, and In a glven space envlronment. S. Szpltalak, "Improvements In Slllcon Solar Cell Efflclency, In 18th IEEE Photovoltalc References Speclallsts Conference, IEEE. 1987. pp. 39-42.

1. A. Yamamoto, M. Yamaguchl, and C. Uernura. 12 G. Grotty, R. Kachare. and B.E. Anspaugh. 'A "Hlgh Converslon Efflclency and Hlgh Radlatlon New Hlgh-Efflclency Slllcon Solar Cell for Reslstance InP Homojunctlon Solar Cells.' Potentlal Space Appllcatlons," to be presented ADD^. Phys. Lett., vol. 44, pp. 611-613, 1984. at 19th IEEE Photovoltalc Speclallsts Conference, New Orleans, LA, Hay 4-9, 1987. 2. I. Welnberg, C.K. Suartz, and R.E. Hart, Jr., "Potentlal for Use of InP Solar Cells In the 13 M. Yamaguchl, C. Uemura, A. Yammoto, and Space Radlatlon Envlronment." In 18th IEEE A. Shlbukawa. "Electron Irradlatlon Damage In Photovoltalc Speclallsts Conference, IEEE, Radlatlon-Reslstant InP Solar Cells,' Jpn. J. 1985. pp. 1722-1724. Appl. Phys.. vol. 23, pp. 302-307, 1984.

3. M. Yamguchl. K. Ando, A. Yamamoto, and 14 K. Ando and H. Yamaguchl, 'Radlatlon C. Uemura, "Hlnorlty Carrler Injectlon Reslstance of InP Solar Cells Under Llght Anneallnq of Electron Irradlatlon-Induced Illumlnatlon," Appl. Phys. Lett., vol. 47. Defects in InP Solar Cells," Appl. Phys. pp. 846-848. 1985. Lett.. vOl. 44, pp. 432-434, 1984. 15. H.Y. Tada. B.R. Anspaugh, and R.G. Oownlng, 4. M.B. Spltzer, C.V. Keavney, and S.M. Vernon, Solar Cell Radlatlon Handbook, 3rd ed., "Junctlon Formatlon Technlques for Indlum JPL-PUB-82-69, Jet Propulslon Laboratory. Phosphlde Solar Cells." to be presented at the Callfornla Instltute of Technology, Pasadena, 19th Photovoltalc Speclallsts Conference, New CA, 1982. (Also. NASA CR-169662). Orleans, LA, Hay 4-9, 1987. 16. V.G. Welzer. I. Welnberg, and C.K. Suartz, 5. C. Goradla. J.V. Gler, and I. Welnberg. "Radlatlon Tolerance of Low Rejlstlvtty, Hlgh "Modelllng and Deslgn of Hlgh Efflclency Voltage Slllcon Solar Cells," In Radlatlon Tolerant InP Space Solar Cells," to Photovoltalc Research and Technology 1983, be presented at the 19th IEEE Photovoltalc NASA CP-2314. 1983, pp. 74-80. Speclallsts Conference, New Orleans, LA, May 4-9, 1987.

4 17. J.R. Srour, S. Othrner, K.Y. Chlu. and 21. P.H. Hooney. L.J. Cheng, R. Sull. J.D. Gerson, O.L. Curtls. Jr.. "Damage Coefflclents In Lou and J.W. Corbett. "Oefect Energy Levels In Reslstlvlty Slllcon," NASA CR-134768, 1975. Boron-Doped S\l\con Irradated wtlh 1-MeV Electrons." Phys. Rev. B, vol. 15, 18. J.R. Srour. S. Othrner, K.Y. Chlu. and pp. 3836-3843. 1977. O.L. Curtls, Jr., "Electron and Proton Damage Coefftctents tn Low-Resjstlvtty Bulk Slltcon 22. R. Loo, G.S. Kamath. and R.C. Knechtll, and Slllcon Solar Cells," In Hlqh Efflctency "Radlatlon Damage In GaAs Solar Cells." In Slllcon Solar Cell Revlew, NASA TH-X-3326, 14th IEEE Photovoltalc SDeclallsts Conference, 1975. pp. 51-59. IEEE, 1980, pp. 1090-1097.

19. I. Welnberg. S. Hehta, and C.K. Suartz, 23. O.J. Glernbockl, ti. Plllar, "Indlurn Phosphlde "Increased Radlatlon Reslstance In (InP)," In Handbook of Optlcal Constants of Llthlum-Counterdoped Slltcon Solar Cells,' Sollds, E. 0. Pallk, Ed., New York: Academlc App1. Phys. Lett., vol. 44, pp. 1071-1073, Press, 1985, pp. 503-516. 1987. 24. J.C.C. Fan, "Theoretlcal Temperature 20. I. Welnberg, C. Goradla. J.W. Stuplca, and Dependency of Solar Cell Parameters," Solar C.K. Suartz. 'Cell Performance and Defect Cells, 17. pp. 309-315, 1986. Behavlor In Proton-Irradlated Llthlurn- Counterdoped ntp Slllcon Solar Cells,' J- Appl. Phys., vOl. 60, pp. 2179-2181, 1986.

TABLE I. - GaAs PREIRRADIATION PERFORMANCE TABLE 11. - InP PREIRRAOIATION PERFORMANCE PARAMETERS PARAMETERS Cell Efflclency, I V;c.Voc, 1 JSC,JSCV2 1 FF,FF , 1 Efflclency, 1 V;c, 1 JSCS2 I FF, I percent v ma/cmma/cm2 percent I:;;l 1 percent 1 V;c, 1 ma/cmma;IC, /c m2 1 percentp e rFF, c en t 1 n+P 18.8 1.0181 018 30.6 82.8 12.9 0.815 26.3 82.6 19.8 I1:0341.034 I 30.9 1 85 1 12.7 1 .814.a141 26 I 82.3 1 p+n 19.6 1.041 31.8 81.2 P+n 13.9 0.843 32.4 70 19.8 1.044 32.1 81.1 14.7 .858 33 71 c

Cell JSC, Voc, FF, percent ma/cm2 v percent

Inpb 17.9 33.9 868 83.8 GaAsC 21.1 32.3 1050 84 Sid 18.1 44.5 672 82.1 Sie 17.1 44.6 660 81.5

5 TABLE IV. - NORMALIZED CELL PARAMETERS AT CONSTANT FLUENCE

Cell O+/OO (Isc)+/(Isc)~(VOC)+/(VOC)o (FF)+/(FF)o

p+n 0.68 0.9 0.84 0.93 GaAs n+p .52 .72 .81 .85

TABLE V. - NORMALIZED CELL PARAMETERS AT CONSTANT HIGH FLUENCE

[InP: n+p and p'n. + = 1 MeV electron fluence = 3x1015/cm2.1

TABLE VI. - TEMPERATURE VARIATION TERMS - GaAs

Cell dPm/dT dVoc/dT dIsc/dT dFF/dT mW/cm2 K mV/K mA/cm2 K percent /K

GaAs p/n -4.4~10-~ -2 +1.95~10-~-5.4~1 0-2 -4.92~10-2 -2.1 1 .97xl 0-2 -6.55~10-2 -4.28~10-2 -2.05 2.28~10-2 -5.09xlO-2 -4. xl0-2 -1.97 1.91xlO-2 -3.98~10-2 GaAs n/p -4.14~10-2 -1.97 2.71~10-~-5.44~10-2 -6.04~10-2 -2.3 2.03~10-~-8.14~10-2 -6.03~10-2 -2.3 2.09~10-2 -8.42~10-2

6 TABLE VIII. - PARAMETERS USE0 IN CALCULATING BANDGAP TEMPERATURE DEPENDENCY dPm/dT dVoc/dT dIsc/dT I dFF/dT 1 mA/cm2 K percent/K

t3.23~10-2 -2.97~10-2 1 .69xl 0-2 -8.6~10-2 . 2.27~10-2 -1 .16xl0-2 1.44~10-2 -9.78~10-2 -2.38 2.39~10-2 -9.38~10-2 1 .62x10-2 -1.7~10-1

TABLE IX. - COMPARISON OF CALCULATED AND EXPERIMENTAL dVoc/dT VALUES

[T = 60 "C.]

I (a) GaAs I (b) InP dVoc/dT, dVoc/dT, mV K-1 mV K-1

Exper Imen ta1 Calculated Exper Imenta 1 Calculated

-2 -2 -2.31 -2.38 -2.1 -2.1 -2.33 -2.39 -2.05 -2.1 i -2.32 -2.32 -1.97 -2.09 -2.21 -2.34 -2.3 -2.36 -2.38 -2.46 -2.3 -2.29 -2.46 -2.45 -1.97 -2.1

TABLE X. - TEMPERATURE DEPENDENCIES - GaAs and InP

[T = 60 "C.] Cell I No. of 1 Vi;, 1 dVoc/dT, I d Pm/dT I cells mV/K mW/cm2 K I I AlGaAs/GaAs I 5 I 955210 I -2.0220.06 I -(4.3520.35)~10-~ GaAs I I Homojunction I 2 1886211 1-2.3 -( 6.042. 01 ) xl 0-2 ~~ I InP I Homojunction I 6 1744215 I -2.3420.08 I -(5.321.2)~10-~

7 n+D CELL I p+n CELL TYPE THICK- DOPANT TYPE I THICK- I DOFMT NESS- ATOM CONCEN------ClH TRATION. RATION. CH-3 n 0.2 1.5~10~~ GAAS SE P3 ZN 7x10l7 P .5 ZN 8x1Ol8 n

CD-87-26192

FABRICATION ETHOD - MVD FIGURE 1. - GAAS CELL CONFIGURATIONS.

AR COAT r Au

SUBSTRATE

BACK CONTACT: Au/ZN BACK CONTACK: Au/SN GROKTH METHOD: OPEN GROUTH W€THOD - HOCVD TiJBE CAPPED DIFFUSION

FIGURE 2. - InP CELL CONFIGURATION.

1 .o

LU .8

.5 I I 1 1013 1014 1015 1016 1 kVELECTRON FLUENCE FIGURE 4. - NORRALIZED EFFICIENCY AFTER 1 ~EVELECTRON IRRADIATION - 1 P.

8 r InP. DARK rInP, ILLUMINATED

.5 I \ I\ 1013 1014 1015 1 MEV ELECTRON FLUENCE. CM-~ FIGURE 5. - NORMALIZED EFFICIENCIES OF InP. GAAS AND SI AFTER 1 MEV ELECTRON IRRADIATION.

GUS, n+p 45 36 1.8 90

ULL U - 1.0 50

15 I 0.6 30 120 200 280 360 440 TENPERATURE , K FIGURE 6. - VARIATION OF CELL PARAMETERS WITH TERPERATURE FOR GAAS .

120

110 GAAS n+P

InP. n+p 4: 45 2.0 90 & 60 w c* a V N= 35 rN= 2836r- - 70 40

25 a= 20 - 1.0 50 20

voc - 15._ 17 0.6 30 1 120 200 280 360 0 .2 .4 .6 .8 1.0 1.2 TEMPERATURE, K VOLTAGE. V FIGURE 7. - VARIATION OF CELL PARAMETERS WITH TEMPERATURE FIGURE 8. - I-V CURVES FOR n+p GAAS CURVES - IRRADIATION FOR InP. BY 1 MEV ELECTRONS.

9 120

100 1 MEV ELECTRON FLUENCE, CM-~

.5 F 3 .4

W v) z0 a .3 40 Wp:

CT2 I- .2 U W 20 av)

0 .2 .4 .6 .8 1.0 1.2 VOLTAGE. V 0 FIGURE 9.- I-V CURVES FOR p+n GAAs-IRRADIATION BY 1 MEV WAVELENGTH. pM ELECTRONS. FIGURE 10. - SPECTRAL RESPONSE OF GAAS n+P CELL.

- 1.0

1 MEV ELECTRON FLUENCE = 3x1015 CM-~ 1 MEV ELECTRON FLUENCE = 3X1d5/CM2 .9 -

0 .8 h xh

Y Y)u -Y 2 .7 - InP D+n 5 U -v .6

JUNCTION 85% I,, LOSS - 92% CURRENT LOSS ----+ - kp .5 I JUNCTION

.5 .6 .7 .8 .9 WAVELENGTH. pl FIGURE 11. - NORMALIZED SPECTRAL RESPONSE OF fl+p InP CELL. +3x1K3 T = 60 OC

INDIUM I GALLIUM 1 dFF 1 dPM PHOSPHIDE --dVoc -- voc dT FF dT PM dT x I Y x -6 - AA I I f I I E zIn +1 3 I 5 2 I >4 *w 0% -5- 0 OiA W LLON 40I1 ZlJ 4 I 1 W I 3E I s - Dl g -3 P I kW I I I -2 - -3x 10-3 FIGURE 14. - GAAS TEMPERATURE VARIATION TERMS - GVERAGE VALUES.

T = 60 OC

-6.5~10-~ r GAAs. T = 60 OC -6.0 Y+’tY I

Y N E -5.5 3\

I- o ‘3 -5.0 a n

-4.5 L

-4.0

-4~10-~ dVo,/dT. MV/K FIGURE 15.- InP TEMPERATURE VARIATION TERMS AVERAGE FIGURE 16. - MAXIMUM POWER VARIATION VERSUS VALUES, Voc TEMPERATURE VARIATION .

11 InP (T = 60 OC) -7~10-~r 0 0 0

Y -6 N 1

0 0 P

rz9 n . -+-4 t IO -3 -2.1 -2.2 -2.3 -2.4 -2.5 -2.6 dV,,/dT. MU/K

FIGURE 17.- P,,, TEMPERATURE VARIATION VERSUS Voc TEMPERATURE VARIATION.

-3 860 900 940 980 720 740 760 780 v,,, MV FIGURE 18. - P,,, TEMPERATURE VARIATION VESUS V,, TEMPER- ATURE VARIATION GAAS AND InP. NASA 1M-89870 I 4. Title and Subtitle 5. Report Date

Radiation and Temperature Effects in Gallium Arsenide, Indium Phosphide, and Silicon Solar Cells 6. Performing Organization Code 506-41 -1 1 7 Author(s) 8. Performing Organization Report No.

1. Weinberg, C.K. Swartz, R.E. Hart, Jr., and R.L. Statler 10. Work Unit No.

9. Performing Organization Name and Address National Aeronautics and Space Administration Lewis Research Center Cleveland, Ohio 44135 12. Sponsoring Agency Name and Address Technical Memorandum National Aeronautics and Space Administration Washington, D.C. 20546

I 15. Supplementary Notes Prepared for the 19th Photovoltaic Specialists Conference sponsored by the Institute of Electrical and Electronics Engineers, New Orleans, Louisiana, May 4-8, 1987. I.Weinberg, C.K. Swartz, and R.E. Hart, Jr., NASA Lewis Research Center; R.L. Statler, Naval Research Laboratory, Washington, D.C.

I 16. Abstract The effects of radiation on performance are determined for both ntp and ptn GaAs and InP cells and for silicon n+p cells. It is found that the radiation resistance of InP is greater than that of both GaAs and Si under 1 MeV electron irradiation. For silicon, the observed decreased radiation resistance with decreased resistlvity is attributed to the presence of a radiation induced boron- oxygen defect. Comparison of radiation damage in both ptn and n+p GaAs cells yields a decreased radiation resistance for the ntp cell attributable to increased series resistance, decreased shunt resistance, and relatively greater losses in the cell's p-region. For InP, the ntp configuration is found to have greater radiation resistance than the ptn cell. The increased loss in this latter cell is attributed to losses in the cell's emitter region. Temperature dependency results are interpreted using a theoretical relation for dVoc/dT which predicts that increased Voc should result in decreased numerical values for dPm/dT. The predicted correlation is observed for GaAs but not for InP a result which is attributed to variations in cell processing.

17. Key Words (Suggested by Author(s)) 18. Distribution Statement

Solar cells; Radiation damage; Silicon; Unclassified - unlimited Gallium arsenide indium phosphide STAR Category 33

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of pages 22. Price' Unclassified Unc lass if 1ed 14 A02

'For sale by the National Technical Information Service, Springfield, Virginia 22161