INTERNET DOCUMENT INFORMATION FORM A. Report Title: Appliations of Cd2Sn04 Transparent Conduting Oxides in CdS/CdTe Thin-Film Devies B. DATE Report Downloaded From the Internet 8/11/98 Report's Point of Contat: (Name, Organization, Address, Offie Symbol, & Ph #): National Renewable Energy Laboratory 1617 Cole Boulevard Golden, CO 80401-3393 D. Currently Appliable Classifiation Level: Unlassified E. Distribution Statement A: Approved for Publi Release F. The foregoing information was ompiled and provided by: DTIC-OCA, Initials: U rv\ Preparation Date: \L i I ^ Sf The foregoing information should exatly orrespond to the Title, Report Number, and the Date on the aompanying report doument. If there are mismathes, or other questions, ontat the above OCA Representative for resolution. Ägjsswrssd fv rro Wie JO!'- i\q -n qvii-p^(p4
NREL/CP-520-22941 UC Category: 1250 Appliations of Cd2Sn04 Transparent Conduting Oxides in CdS/CdTe Thin-Film Devies X. Wu, P. Sheldon, T.J. Courts, D.H. Rose, and H.R. Moutinho Presented at the 26th IBFFPhotovoltai Speialists Conferene, September 29- Otob er 3, 7997, Anaheim, Ca/ifornia National Renewable Energy Laboratory 1617 Cole Boulevard Golden, Colorado 80401-3393 A national laboratory of the U.S. Department of Energy Managed by Midwest Researh Institute for the U.S. Department of Energy under ontrat No. DE-AC36-83CH10093 Prepared under Task No. PV704201 gf pmjf? INSPECTED 1 September 1997 #4 H ^-//-a^y
APPLICATION OF Cd 2 Sn0 4 TRANSPARENT CONDUCTING OXIDES IN CdS/CdTe THIN-FILM DEVICES X. Wu, P. Sheldon, T.J. Coutts, D.H. Rose, and H.R. Moutinho National Renewable Energy Laboratory (NREL), 1617 Cole Blvd., Golden, CO 80401 ABSTRACT An unusual proessing sheme has been developed for preparing single-phase admium stannate (Cd 2 SnO or CTO) films. Cd 2 Sn0 4 transparent onduting oxide (TCO) films have several signifiant advantages over onventional TCOs when applied to CdS/CdTe thin-film devies. They are more ondutive, more transparent, have lower surfae roughness, are patternable, and are exeptionally stable. Cd 2 Sn0 4 -based CdS/CdTe polyrystalline thin-film solar ells with effiienies of 14% have been fabriated. Preliminary ell results have demonstrated that devie performane an be enhaned by replaing the Sn0 2 layer with a Cd 2 Sn0 4 TCO film. INTRODUCTION Cadmium telluride has long been reognized as a promising photovoltai material for thin-film solar ells beause of its near optimum bandgap of 1.5 ev and its high diret absorption oeffiient. Small-area CdS/CdTe heterojuntion solar ells with effiienies of more than 15% and ommerial-sale modules with effiienies of 9% have been demonstrated [1,2]. In the past, little attention has been paid to the transparent onduting oxide (TCO) layer of this devie struture. Conventional TCOs, primarily Sn0 2 films, typially have been used as the front olletor in CdS/CdTe ells and modules. However, Sn0 2 films, deposited using the ommerially viable SnCI 4 hemistry, have a resistivity of ~ 5-8x10" ß- m. This yields films with an average transmission of 80% and a sheet resistivity of about 10 ß/square. Although this may be satisfatory for small-area devies and firstgeneration modules, it does not provide adequate design latitude when trying to optimize either devie performane or manufaturing osts. For example, reduing the TCO resistivity by a fator of two ould allow for a TCO film that retains the same sheet resistane as the inferior TCO, yet is half as thik. This would improve the transmission and ultimately improve devie performane by inreasing the short-iruit urrent density (J so ). Alternatively, a film with transmission properties similar to those of the inferior TCO, but half the sheet resistane, ould be used. This would have the advantage of reduing the number of interonnets required, thereby improving throughput, reduing interonnet losses, and reduing manufaturing osts. For these reasons, it is desirable to improve the performane of the TCO films for CdS/CdTe solar ell appliations. Nozik and later Haake et al. were the first to report Cd 2 Sn0 4 TCO films deposited by r.f. sputtering [3,4]. Reent improvements in r.f. sputter-deposited Cd 2 Sn0 4 and post-deposition proessing have yielded films with superior properties [5,6]. In this work, we desribe admium stannate films that we have optimized for CdS/CdTe devies. Cadmium stannate films have several signifiant advantages over onventional TCOs. They are more ondutive, more transparent, have lower surfae roughness, are patternable, and are very stable. Cd 2 Sn0 4 -based CdS/CdTe polyrystalline thin-film solar ells with effiienies of 14.0% have been fabriated. Preliminary ell results have demonstrated that devie performane an be enhaned by replaing the Sn0 2 layer with a Cd 2 Sn0 4 TCO film. EXPERIMENTAL Single-phase Cd 2 Sn0 4 films have been prepared using an unusual proessing sheme developed in our laboratory [5,6]. CTO films were prepared by r.f. magnetron sputtering from a ommerial hot-pressedoxide target with a omposition of 33 mol% Sn0 2 and 67 mol% CdO. Coming 7059 glass or soda-lime glass substrates were plaed on a water-ooled sample holder parallel to the target surfae. Deposition was performed at an oxygen partial pressure of 10-20x10" 3 torr at nominally room temperature, and a deposition rate of ~100 A/min. After deposition, all samples were annealed in a tube furnae in an Ar or Ar/CdS atmosphere at elevated temperatures of 580-680 C for 10-30 minutes. The eletrial, optial, and ompositional properties of CTO films were haraterized using Hall effet, optial and infrared spetrosopy, X-ray diffration (XRD), sanning eletron mirosopy (SEM), and atomi fore mirosopy (AFM) measurements. We have also proessed a limited number of CTO-based CdS/CdTe devies. In these devies, the CTO film thikness varied from 2500Ä to 6000Ä, with resistivities of 7 to 3 Q/sq., respetively. The CdS thikness was 800-1000Ä, and the CdTe film thikness was around 10 urn. The details of the CdTe ell fabriation proedure is desribed elsewhere [7]. RESULTS AND DISCUSSION
Cadmium stannate material properties The ondutivity of TCOs an be improved by either inreasing the arrier onentration or the eletron mobility. By optimizing sputtering and post-annealing onditions of CTO films, eletron mobilities as high as 65 m 2 /Vs have been ahieved for a arrier onentration of 2x10 20 m" 3. Even at a arrier onentration of -9x10 20 m" 3, mobilities as high as 55 m 2 /Vs have been observed. These mobilities are 2-3 times higher than those of ommerial Sn0 2 films doped to similar levels. Free arriers are thought to result from oxygen defiienies in the films, aommodated either as oxygen vaanies or admium interstitials, or a ombination of both. This results in resistivities as low as 1.3x10" 4 fl-m in the best CTO samples [7]. This is almost seven times lower than onventional SnCI 4 -based Sn0 2 films and 2.5 times lower than the best Sn0 2 films deposited using tetramethyltinbased preursors (p TMT = 3.3x10" fl-m). Table 1 shows a omparison of eletrial properties of more typial Cd 2 Sn0 4 films prepared at NREL and ommerially available SnCI 4 -based Sn0 2 films. The redued resistivity of CTO films provides a lear advantage. For example, a sheet resistane of ~9 fi/sq. an be ahieved with a CTO film of less than 2000Ä, as ompared to nearly 10.000Ä for the Sn0 2 film. Alternatively, thiker films an be used to redue the devie series resistane, thus improving its fillfator and effiieny. Reduing the series resistane in modules is of partiular interest, beause manufaturers an optimize ell width between laser sribe lines, thereby reduing l 2 R and optial losses. Table 1. Eletrial Properties of CTO and Sn0 2 Films. Sample t (Ä) P (Q-m) Rs (fi/sq.) Sn0 2 (SnCI 4 ) 10000 8.6x 10" 8.6 CT0245 1900 1.8x10" 9.5 CT0229 2550 1.8x10" 6.9 CTO250 3950 2.0x10" 5.0 CT0252 6500 2.2x10" 3.1 12.2% for the Sn0 2 film. This appears to be due to the unusually high eletron mobility of CTO films. The improved transmission and absorbane of CTO films is ideal for the superstrate struture of CdS/CdTe devies and will ultimately yield improved short-iruit urrents. 1.0 -' i i I I «I I i r, " i Cd2Sn04 (1900A, 9.5Q/sq.)" u ^_^ u...^-sf ^t n n 2 (_ir)p<k>a, 10<"Vq) 18 0.8 \ JL r* /\ r»~ ^^\_ ^" n v o /" " " V "\ Xf U) 1 / % ^ ^^. a o.6 1 < I ^ / ^V. a> \ / ^"^* o J& ^^^ C 0.4 - **"* v ^"S_. <8 [ \ t 0.2-1, y' \ '- C 1 (0 1 \ v V _r -^~,v "»_ *- 0.0 "l i i i i...i 1 I.-.I i i i i i i i i il 200 400 600 800 1000 1200 1400 1600 1800 2000 Wavelength (nm) Fig. 1. Transmission and absorbane of a CTO and Sn0 2 film with a sheet resistivity of ~10Q/square. Fig. 2 shows the absorbane data of the four CTO films listed in Table 1. As expeted, the absorbane inreases with an inrease in CTO film thikness. However, more interesting is the shift in optial bandgap to longer wavelengths with inreasing CTO film thikness. The thinnest CTO film has a bandgap that is near that of a Sn0 2 film (see Fig. 1). We do not believe that the bandgap shift is a result of the Moss-Burstein shift, beause the thinnest CTO film has the lowest arrier onentration (the plasma edge is also loated at longer wavelengths). Although we do not ompletely understand this effet, we believe that it may be a result of hanges in film omposition throughout its thikness. X-ray photoeletron spetrosopy (XPS) and XRD analysis is under way to test this hypothesis. i ' i ' i i i ' i ' i i <~T_ t = 6500Ä t = 3950Ä \\ t = 2550A t = 1900Ä ' w. <r.. <,f Cd 2 Sn0 4 films have signifiantly better optial properties than onventional Sn0 2 films. This is, in part, due to the lower resistivities, whih allow thinner films to be used. For example, Fig. 1 shows the transmittane and absorbane of Cd 2 Sn0 4 and Sn0 2 films with similar sheet resistivities (-10 Q/sq.). The CTO film has superior transmission when ompared to the Sn0 2 film, even though the CTO film has a slightly lower sheet resistivity. The absorbane of the CTO film, in the visible range, is muh smaller than that of the Sn0 2 film. For example, at 600 nm, the absorbane of the CTO film is only 0.8%, ompared to 200 400 600 800 1000 1200 1400 1600 1800 2000 Wavelength (nm) Fig. 2. The absorbane of four CTO samples with different thikness.
TCO surfae roughness is also an important parameter when onsidering devie proessing issues. It is well known that in heterojuntion solar ells, higher short-iruit urrents an be obtained by reduing the window layer absorption. In CdS/CdTe ells, this is ahieved by reduing the CdS thikness to yield an enhaned blue spetral reponse. Unfortunately, as the CdS is thinned, pinholes form and ause loalized CdTe/Sn0 2 juntions with inferior open-iruit voltages and fill fators. The probability of forming pinholes likely inreases as the TCO surfae roughness inreases. AFM measurements indiate an average surfae roughness of -210Ä for SnCI 4 -based Sn0 2 films. In ontrast, the Cd 2 Sn0 4 films have a very smooth surfae with an average roughness of -20Ä, an order of magnitude lower than that of the Sn0 2 film [7]. This redution in surfae roughness ould lead to improved devie performane. Cadmium stannate films also have exellent adhesion and thermal stability [7], Our experimental results show that the resistivity of a SnCI 4 -based Sn0 2 film degrades when annealed in an Ar ambient for 20 minutes at a temperature of 500 C. In ontrast, CTO films are stable up to temperatures as high as 650 C [7]. This is partiularly important when onsidering that lose-spaed sublimation (CSS) CdTe deposition temperatures an exeed 600 C. The properties of CTO films desribed above are well orrelated to the mirostruture of the film. For example, Fig. 3 shows typial XRD data for an as-deposited and an Ar/CdS-annealed CTO film. The as-deposited films are amorphous, exhibit extremely high resistivities (-5000 ß- m), have a lower optial bandgap, and have poor transmission harateristis. After the Ar/CdS anneal, the film rystallizes into a single-phase spinel struture with optial and eletrial properties similar to those desribed in Table 1. After rystallization, there is no evidene that seondary phases, suh as CdO, Sn0 2, 311 After CdS/Ar Anneal 2000 Before Annealing * 400 C 1500 3 O o, f 1000 0) 4-«220 500 111 JJ 222 10 20 30 40 26 422 51 1 50 440 1 533 1 620 " 2 Fig. 3. Typial XRD pattern of an as-deposited and an annealed Cd 2 Sn0 4 film. or CdSn0 3, form in our CTO films. We find that Ar/CdS anneals drastially redue the temperature required for film rerystallization. We have demonstrated the same improvement in CTO film properties at 580 C anneals on low-ost, soda-lime glass, ahieving a resistivities as low as 1.84x10" Q-m. This value is similar to values obtained for the higher-temperature proess. Cadmium-stannate-based CdS/CdTe devies The improved material properties of CTO films provide a strong impetus to inorporate them in a superstrate devie struture. We find that proessing parameters that were optimum for Sn0 2 -oated glass are not suitable for CTO-oated glass. This is partiularly evident when omparing the post-cdte-deposition CdCI 2 anneal. CdTe/CdS/Sn0 2 devies that have not been CdCI 2 treated, exhibit open-iruit voltages (V o ) of about 550 mv. To obtain open-iruit voltages of >800 mv, it is neessary to soak the sample in a saturated CdCI 2 :MeOH solution with a onentration of greater than 50% (typially between 50%-100%) and subsequently anneal the film at 400 C. At higher onentrations (near 100%), the films exhibit severe adhesion problems, thereby reduing yields. In ontrast, CdTe/CdS/Cd 2 Sn0 4 devies require a lower CdCI 2 onentration. CdTe/CdS/Cd 2 Sn0 4 devies, whih have not been CdCI 2 -treated, exhibit open-iruit voltages as high as 650 mv (nearly 100 mv higher than the Sn0 2 - based devies). Furthermore, CdCI 2 onentrations as low as 25% are adequate to provide a V«. of >800 mv. To ompare the differenes between Cd 2 Sn0 4 - and Sn0 2 - based CdTe ells, a set of CdTe ells was prepared using idential ell fabriation proedures. The ells were soaked in a saturated 25% CdCI 2 :MeOH solution for zero to 20 minutes and then annealed at 400 C for 30 minutes. As shown in Fig. 4, the effiieny of Cd 2 Sn0 4 -based devies is always higher than that of Sn0 2 -based devies 10-8 -,2 6 - -.4 / t Cd 2 S0 4 4 - ^rz - / Sn0 2-1 1 5 10 15 Soak Time (min) Fig. 4. CdTe ell effiieny as a funtion of soak time for a 25% CdCI 2 :MeOH solution (note: under different proessing onditions, higher effiieny devies have been ahieved both with and without a CdCI 2 treatment). 20
for a given soak time. Earlier work has shown that this is also true when varying the CdCL, onentration and keeping the soak time onstant [7]. This result ould, in part, be aused by the redued surfae roughness of the CTO film. However, it is likely that the hemistry of the CTO/CdS interfae is also an important fator. Fig. 4 also indiates that the effiieny of CTO-based CdS/CdTe devies is less dependent on the CdCI 2 heat-treatment. This result suggests the potential for improved proess reproduibility and yield. In addition to improved devie performane at lower CdCI 2 onentration treatments, CTO-based devies have better adhesion than their Sn0 2 ounterparts. Although the SnCybased devies improve with inreased CdCI 2 onentrations, the adhesion degrades. At 100% CdCI 2 onentrations, most CdTe films will blister and eventually separate from the Sn0 2 superstrate. Although CTO-based devies require lower CdCI 2 onentrations for optimum performane, most devies remain intat after a 100% CdCI 2 treatment. For example, in a set of 16 CTO-based devies treated at 100% CdCI 2, only 2 out of 16 devies resulted in adhesion failure. This ould have signifiant proess yield ramifiations. As desribed earlier, the improved transmission and absorbane of CTO films is ideal for superstrate CdS/CdTe devies and should yield improved devie J so. Fig. 5 shows the relationship between the TCO film transmission and the resulting J s0, for devies deposited on both CTO and Sn0 2 superstrates. It is lear that by replaing the Sn0 2 film with a CTO superstrate, the redued optial losses yield an inrease in J^. For example, replaing the 10.000Ä Sn0 2 film with a 2500Ä CTO film yielded an inrease in J s0 of more than 1.5 ma/m 2. It is important to note that devies fabriated on the thiker CTO films always gave the highest fill fators. Fill fators of 75% have been ahieved using 6000Ä-thik CTO films in ~1 m 2 devies. These films have resulted in devies with o '55 w E 0) (8 0.9 0.8 0.7 i ' r i r r - i i ' _ 24.4 ma/m 2 0.6 11 \ 0.5 _ * Y\ \ 0.4 0.3 23.5 ma/ms 22.8 ma/m! " i s i \\ \ i i \ V %- t\ 0.2 - CTO (2500Ä, 6.9 OJsq.) ij - CTO (6000Ä, 3 fi/sq.) 0.1 - Sn0 2 (10000Ä, 10fl/sq.) 0.0 -i&y i.i.i i 1. 1. - 200 600 1000 1400 1800 Wavelength (nm) Fig. 5. Relationship between J s and transmission of Sn0 2 and CTO films. effiienies of up to 14% (V O0 =0.805 V, J s =23.5 ma/m 2, FF=74%), as measured under standard onditions. Therefore, thiker CTO films, with lower sheet resistivities, may be better suited for module appliations. CONCLUSIONS AND FUTURE WORK Cd 2 Sn0 4 transparent onduting oxides have several signifiant advantages over onventional tin oxides. They are more ondutive, more transparent, have a lower surfae roughness, are patternable, and are exeptionally stable. Cd 2 Sn0 4 -based CdS/CdTe polyrystalline thin-film solar ells with effiienies of 14.0% have been prepared for the first time. The preliminary ell results have demonstrated that, by replaing the onventional Sn0 2 with a Cd 2 Sn0 4 film, CdS/CdTe devie performane an be enhaned. Cd 2 SnO -based CdTe ells yield higher shortiruit urrents and effiienies, and are less dependent on the CdCI 2 treatment than their Sn0 2 ounterparts. We believe that the performane of Cd 2 Sn0 4 -based CdS/CdTe ells is far from optimized. Future work will onentrate on improving the effiieny of Cd 2 Sn0 4 -based CdS/CdTe ells through the optimization of Cd 2 Sn0 4 film preparation and ell fabriation and on developing more manufaturable proesses. ACKNOWLEDGMENT The authors would like to thank Dr. D. Albin and Dr. R. Dhere for useful disussions, R. Ribelin for CdS deposition, and Y. Mahathongdy for CdCI 2 treatments. This work was supported by the U.S. Department of Energy under Contrat No. DE-AC36-83CH10093. REFERENCES [1] J. Britt and C. Ferekides, 'Thin-Film CdS/CdTe Solar Cell With a 15.8% Effiieny" Applied Physis Letters, 62, pp. 2851-2852(1993). [2] K. Zweibel, H. Ullal, and B. von Roedern, "Progress and Issues in Polyrystalline Thin-Film PV," Pro. 25th IEEE Photovoltai Spe. Conf., (1996) pp. 745-750. [3] A.J. Nozik, "Optial and Eletrial Properties of Cd 2 SnO : A Defet Semiondutor," Phys. Rev. B., 6, No. 2, pp. 453-459 (1972). [4] G. Haake, W.E. Mealmaker, and L.A. Siegel, "Sputter Deposition and Charaterization of Cd 2 Sn0 4 Films," Thin Solid Films, 55, pp. 67-81 (1978). [5] T.J. Coutts, X. Wu, W.P. Mulligan, and J.M. Webb, "High-Performane, Transparent Conduting Oxides Based on Cadmium Stannate," J of Eletroni Materials, 25, No. 6, pp. 935-943(1996). [6] X. Wu, W.P. Mulligan, and T.J. Coutts, "Reent Developments in RF-sputtered Cadmium Stannate Films," Thin Solid Films, 286, pp.274-276 (1996). [7] X. Wu, P. Sheldon, T.J. Coutts, D.H. Rose, W.P. Mulligan, and H.R. Moutinho, "CdS/CdTe Thin-Film Devies Using a Cd 2 Sn0 4 Transparent Conduting Oxide," Pro. 14th NREUSNL PV Program Review Meeting, pp.693-702 (1996).