- Email: [email protected]
Research on polycrystalline thin-film photovoltaic devices
658
Journal of Crystal Growth 61(1983> 658—664 North-Holland Publishing Company
RESEARCH ON POLYCRYSTALLINE THIN-FILM PHOTOVOLTAIC DEVICES
*
A.M. HERMANN and L. FABICK Solar Energy Research Institute, 1617 Cole Boulevar4 Golden, Colorado 80401. USA
Received 12 September 1982
Recent results from the United States Department of Energy Polycrystalline Thin-Film Photovoltaic Device Program are presented. The program which is managed by the Solar Energy Research Institute encompasses materials and device research on a variety of compound semiconductors with emphasis on Il—VI compounds and Il—VI ternary analogs. This paper covers preparation (emphasizing thin-film deposition) and characterization of semiconducting materials such as Cu 2_ ~S,Cu2_ ,Se. CdTe and CulnSe2. Photovoltaic device characteristics of these absorbers with heterojunction partners such as CdS are discussed. Excitement in the program has been generated by recent progress in the (Cd, Zn/S)/CuInSe2 device area. An AMI efficiency of 9.93% for a 5 ~smthick 1 cm~cell has been verified at SERI. Unencapsulated (Cd, Zn)S/CuInSe2 cells have been subjected to more than 6000 h of accelerated stability testing with no measureable degradation in photovoltaic performance. Other program highlights include fabrication of a hybrid CdS/Cu25 device (evaporated CdS. sputtered Cu2S) with a 7.2% AMI efficiency, and a 3.9% AMI efficiency all-sputtered CdS/Cu2S cell. Preliminary data are presented on the achievement of intrinsically stable high-efficiency ( > 9%) wet-processed CdS/Cu2S cells. Progress in the less mature research areas is outlined. A 5.3% (AMI) CVD Au/n-CdTe Schottky harrier cell is described, and AM1 cell efficiencies exceeding 4% for vacuum evaporated CdS/Cu2Se and chemically sprayed CdS/CdTe cells are reported. Future research emphasis is discussed. One of the important technical issues which is being addressed in an Increase in the open-circuit voltage of (Cd,Zn)S/CuInSe2 cells. Technical issues being addressed in other areas include film growth and doping studies in CdTe and improvement in the doping profile of CdS/Cu2Se junctions.
1. Cu2S Intensive research in polycrystalline thin-film photovoltaic devices has been performed over the last two decades. Prototype CdS/Cu2S heterojunction cells have been studied most extensively, Efficiencies exceeding2 area 10% (AM 1) have been re(Cd,Zn)S/Cu ported [1] for 1 cm 2S wetprocessed cells (Cu2S formed by chemical reaction of CdS in aqueous cuprous ion solution). Most of these high-efficiency devices, however, have exhibited both intrinsic (electrochemical) and extrinsic degradation. The United States Department of Energy has recently focused its research program in the Cu 2S area on the achievement of intrinsically stable CdS/Cu2S devices and on alternate techniques for their deposition.
*
This paper was presented at the International Conference on
lI—VI Compounds. University of Durham, April 1982.
The degradation of Cu2S based cells due to electrochemical decomposition of Cu2S is well documented [2]. Any Cu2S in local electrical contact with the back electrode causes electrochemical decomposition of the Cu2S when the cell is cxposed to AM 1 illumination at open-circuit voltage. This change in Cu2 S stoichiometry degrades the cell output because of localized copper paths and corresponding internal short-circuiting. The Institute of Energy Conversion of the University of Delaware has shown recently [3] that the use of rigid lattice-matched (Ni/Fe) substrates can reduce the number of has potential defects to a 2, and it furthershunting been shown [4] that few per cm the few remaining shunting defects can be detected and eliminated with laser scanning. Studies have indicated that CdS/Cu2S cells free of shunting defects demonstrate intrinsic stability in all photovoltaic parameters as determined by exposures exceeding 500 h at open-circuit voltage loading in a flowing H2/Ar atmosphere.
0022-0248/82/0000—0000/$03.00 © 1983 North-Holland
A.M. Hermann, L. Fabick
/
Research at Telic Corporation [5] has demonstrated that CdS/Cu2S devices can be formed by sputter-deposition. Cu2S has been reactively sputtered onto vacuum evaporated CdS (hybrid cell) with the achievement of a 7.2% AMI 2, FF efficiency 0.72). Cu(f°~ 0.54 V, J~ 16.2 mA/cm 2S has also been sputtered onto sputterdeposited CdS with the achievement of a 3.94% AMI efficiency. The all-sputtered cell thickness is only 3.6 ~tm, nearly a factor of ten thinner than the thinnest wet-processed cells. Device2,parame~ç ters this FF cell are 12.2 coating). mA/cm 0.527for V, and 0.62 (no AR =
=
659
Research on polycrystalline thin -film photovoltaic devices
the stoichiometry (and hence electronic defect structure) is determined by the relative vapor pressures of the constituent elements, both during evaporation and during adsorption on the heated substrate. of CuInSe2 formed tion from Films the bulk compound are, by as evaporaa result,
=
=
=
=
2. CuInSe2 Copper indium diselenide has a combination of materials characteristics that make it an excellent candidate for photovoltaic applications. It has a direct band gap of 1.04 eV and it is capable of being doped n- and p-type in the range of l015_~l0’~ cm ~ [6,7]. The optical absorption coefficient is greater than l0~cm over most of the solar spectrum [8] and the electron diffusion length in single crystal CuInSe2 is 2.4 ~sm[9]. These factors contribute substantially to the large photocurrents reported for thin film CdS/CuInSe2 devices [10]. Single crystal CuInSe2 heterojunctions formed with evaporated thin-film CdS have been reported with efficiencies exceeding 12% [II]. The high efficiencies of these structures are partly due to the close lattice match (less than 1.2% mismatch) [12] between CdS and CuInSe2. For these devices to be economically feasible for large scale deployment, they must maintain high efficiency when fabricated in thin-film form. Presently, fabrication of this device structure via vacuum evaporation, spray deposition and sputtering is being investigated as part of the SERI polycrystalline thin-film task. Thin-film vacuum evaporated (Cd,Zn)S/CuInSe2 devices with efficiencies of the order of 10% have been reported [13]. Sputter- and spray-deposited [14—16]CdS/CuInSe2 devices with respective efficiencies of 5.1% and 1.7% have also been reported. When a compound semiconductor is evaporated from the bulk compound onto a heated substrate,
copper deficient unless the source temperature is above 1300°C[17]. Moreover, the electrical properties of these films are controllable only by the application of appropriate post-deposition heat treatment Several [18]. techniques have been developed to control the stoichiometry and allow the deposition of CuInSe 2 films with a wide range of electrical parameters. The most successful has been vacuum deposition of CuInSe2 thin films from the elements as implemented at Boeing Aerospace Corporation [19]. This technique independently controls the evaporation rate of the constituent elements in order to vary the electrical properties of the deposited films. The indium and copper rates are simultaneously controlled using an Inficon dual channel co-evaporator Model 200, an Electron Impact Emission Spectroscopy (EIES) feedback sensor shown in fig. 1. The selenium deposition rate is independently controlled using a quartz crystal sensor. A deposition rate for Cu, In and Se of 0.9, 2 and 6.8 A/s, respectively, results in a CulnSe2 growth rate of 8 A/s on a 450°Cceramic
—
Substrate hotter
Se Deposd~onSensor
,/‘\
Se Source
< )
Two W Boats
I d
~
I
EIES Sensor for Cu and In
__________
Sheld Elate Cu Source
b?~~”~
-
W Boat
~ In
Source
-
W Boat
Carbon Block
Fig. I. System configuration for preparing CuInSe2 films using an EIES deposition controller (Boeing Aerospace Company).
660
A.M. Hermann, L. Fabick
/
Research on polycrystalline thin -film photovollaic device.s
(alumina) substrate. The films have resistivities between 5 and 800 kf~/Ufor a 2—3 ~.smthickness, The carrier concentration is typically 5 x bib cm3 2/V. s as determined with a hole mobility of 9 cm The resistivity of the by Hall effect measurements. p-type CuInSe 2 thin films is controlled by varying the relative Cu to In deposition rates. When the copper deposition rate is 1.1 A/s the film resistivity is 1 Q cm. When the copper deposition rate is 0.8 A/s,have theled film resistivity is 500ofS2a cm. factors to the development first These order defect model for CuInSe 2. In this model, In vacancies act as acceptors and Se vacancies act as donors. This contrasts with a previous model which suggests Cu vacancies are the acceptors responsible for p-type doping in CuInSe2 [20]. The CuInSe2 layer used to fabricate high efficiency photovoltaic devices uses these properties in a two-step deposition process. The first step deposits a high conductivity (pt) CuInSe2 layer 2.5 ~sm thick which acts as a low resistivity interface to the molybdenum contact. This is followed by a high resistivity (p) layer 0.8 ~sm thick which forms the heterojunction absorber and prevents the formation of copper nodules at the heteroface. The CdS/CuInSe2 heterojunction is deposited insitu following the resistivity CuInSe2 deposition. consists of a high (20 ~2cm) The layerCdS 0.8 ~sm thick followed by a low resistivity In-doped layer 1.7 ~smthick [21]. The use of a high resistivity CdS layer at the heteroface appears necessary to prevent formation of copper nodules at the interface and may otherwise be related to the achievement of high conversion efficiencies in these cells, The Boeing group using this precise coevaporation deposition technique has deposited a Cd 0 9Zn0 1S/p-CuInSe2 device with the following parameters at SERI: efficiency 9.93%, 0.42 V,measured J 36.3 mA/cm2, FF 0.65, at 2 area. The group is also depositAM1CuInSe for a 1 cm ing 2 films in a planetary evaporation system which allows uniform deposition onto a large number of substrates through rotation of the substrate holders. Spray pyrolysis of CdS/CuInSe 2 devices offers =
=
= =
the possibility of fabricating high efficiency thin film devices at a very low cost. This technique uses 0.001 molar CuCl, InCl n,n-dimethylselenourea solution sprayed onto a 3, molybdenum coated glass substrate at 260°C[22]. As with vacuum evaporation, film resistivities range from 0.2 Q cm to 6 X b0~~2 cm depending on the relative copper concentration in the spray solution. Grain sizes are typically 1 ~sm filmsdiffusion 1 ~tm thick. Hole 2/V’for s with lengths of mobili0.3 ~tm ties of I cm are reported. Equilibrium thermodynamic computer sirnulations have been used extensively to determine the optimum conditions for spray film deposition. Chalcopyrite films have been produced with no detectable second phases (In0Se1, Cu2Se). Thermodynamic considerations also indicate a 6000 H2Se/H2 anneal will reduce the residual oxygen content of the as-sprayed films. Annealing experiments have verified that oxygen can be reduced from 1% at. wt. to below the limit of detectability. Spray pyrolysis of CdS films doped with indium has resulted in low resistivity (less than 0.2 f2 cm) films with grain sizes of 0.3 ~m [23]. Sprayed CdS/CuInSe2 devices fabricated at Stanford Research Institute using these layers havefr~ 1.7%0.23 AM1 2 in area) with V. efficiencies (3 mm J 2, and FF 0.31. 5~Brown 23.3 mA/cm University is investigating direct sputter deposition of CdS/CuInSe-, devices [14]. A very strong dependence of film parameters on substrate temperature has been observed. CuInSe 2 films sputter-deposited with 450°Cor higher substrate temperature are the chalcopyrite phase while films deposited between 50 and 300°C are the sphalerite phase. Films deposited at 480°C have ~em 2/V 0.1 s while grains and hole mobilities of 0.4 cm films deposited at 505°Chave ~sm grains 2/V.0.2—l.0 s. All sputter-dcand holeCulnSe mobilities of 6 cm posited are The p-type resistivities between 0.3 and2 2films 12 cm. bestwith device consists =
=
=
‘
of a sputtered CuInSe2 film and a vacuum evaporated CdS de2 inheterojunction area with ~ partner. 0.45 V, This ~ 22 vice is cm2, 0.2 cm rn/V FF 0.5, and an AMI efficiency of =
=
5.1%.
=
A.M. Hermann, L. Fabick
/
Research on polycrysialline thin -film photovoltaic devices
3. Cu2 ~Se -
Boeing Aerospace Corporation is investigating the properties of CdS/Cu25Se devices [24]. Cu2 Se could offer the high efficiency of CuInSe2 without the indium component and with less of a degradation problem than that of CdS/Cu2S devices. Cu2 ~Se has an indirect bandgap of 1.4 eV and a direct bandgap of 2.2 eV for x 0.2. Two phases of Cu2~Seexist at room temperature depending on the value of x [25]. If x is between 0.2 and 0.25, the crystal structure is face-centered cubic. Initial indications suggest that a second reported hexagonal phase has a deleterious effect on photovoltaic device performance. Vacuum evaporated films deposited from the elements (see vacuum evaporation of CuInSe2) at 150°C substrate temperature ares.p-type with mobilities of the 2/V’ The carrier concentrations order of 10lOIS cmto 1021 cm3 for x values between vary from 0.1 and 0.3 (the Cu 2Se stoichiometry has been measured via the coulometric technique employed with Cu2S). Typical grain sizes are 1 ~smfor a film thickness of 2 p.m. CdS/Cu2~Se devices have demonstrated efficiencies of up2,to and 4.25% I/~C FFwith0.64. 457 mV, J~ 14.4 mA/cm These cells were heat treated in hydrogen for 5 mm at 150°C. —
-
=
=
=
=
4. CdTe Cadmium telluride has a high absorption coefficient (> iO~cm~) [26] due to its direct bandgap at 1.47 eV which is optimum for photovoltaic applications [27,28]. 12% conversion efficiencies have been reported for single crystal CdTe devices with evaporated CdS [29]. An excellent review of the material properties can be found elsewhere [30]. The subcontracted photovoltaics program at SERI emphasizes the development of CdTe-based solar cells fabricated via spray pyrolysis [31], sputter deposition [32], screen printing/sintering [15], chemical vapor deposition (CVD) [33], and hot-wall vacuum evaporation (HWVE) [34]. Dctails of results from the HWVE and CVD programs are presented here.
661
When a compound semiconductor is deposited via thermal evaporation, there are several variables that determine its morphology and stoichiometry. The relative vapor pressures and hence the impingement rate of the constituent elements on the substrate determine the film stoichiometry. The substrate temperature controls the desorption and mobility of atoms on the substrate surface. The temperature of the substrate also controls the number of nuclei formed. If the substrate temperature is below the glass transition temperature of the semiconductor, the film grown will have an amorphous structure. As the substrate temperature is raised past the glass transition temperature, typically small grain polycrystalline films will grow. The small grains are due to the high nucleation rate and low surface mobility on the low temperature In order to grow applications, large grain thin films substrate. suitable for photovoltaic the substrate temperature must be raised further. This has the combined effect of lowering the nucleation rate and raising the surface mobility of the adsorbed atoms or molecules. It also increases the desorption rate of the atoms or molecules and at still higher temperatures of the dcdeposited films takes place.re-evaporation If the constituent ments of a compound semiconductor desorb and/or reevaporate at similar rates, the deposited films will reflect the stoichiometry and electrical structure of the compound evaporant charge. If, however, the constituent elements have a large difference in vapor pressure and/or desorption activation energy, the film will not be stoichiometnc and will have many electrically active defects. This is typically the case with evaporated CdTe films where [~Cd]/[~TeI is 100 or more [25] and as-grown films are highly compensated with deep level defects. If an enclosed hot wall is included in the system (fig. 2), then inclusion of a separate source of the most volatile constituent (e.g., a cadmium source when evaporating CdTe) allows separate control of the relative impingement rate of the individual species [35—41].In addition, if the substrate is maintained at a lower temperature than the hot-wall, then evaporation and desorption from the substrate will not occur. Hence, the addition of a hot wall efffectively decouples the deposition variables thereby allowing more lati-
662
A.M. Hermann, L. Fabick
Thermocouple Position
Research on polycrystalline thin -film photovoltaic devices
Hook (Up and down and/or side-to-side motion)
-
First Zone (Substrate)
/
:
-
Substrate Substrate Carrier (Graphite) Shutter Guide
~Shutter
— -— - -
-- -
/ -
-~
~
Substrate Holder
(Quartz)
::e:r:::a::: Sources
/)
Radiation Shields Heater Wound on Stainless Steel Core Third Source (Cd or Te)
Fourth Zone
~——
-~
Fig. 3. Schematic diagram of HWVE system at SERI.
Threaded Rod Supports
U
ourrO o r
Thermal Links to H C Cooled Baseplate
Fig. 2. Simplified schematic of hot wall retort (Stanford tJniversity).
tern allows the deposition onto a number of sub. strates without breaking vacuum. Preliminary re-
sults from Stanford indicate that indium doping of CdTe thin films reduces the resistivity from io~to 800 12 cm when measured under AMI light. The intergrain barrier height under these conditions is reduced from 0..8 to 0.1 eV. Large grains (10 p.m
tude in the deposition of the thin film, SERI is presently supporting two programs in HWVE of CdTe thin films. The first program at Stanford University employs a reactor design similar to that of Lopez-Otero of the University of Linz, Austria [35,37,39,41], as illustrated in fig. 2. The second program is an in-house research effort at SERI. Fig. 3 illustrates the in-house HWVE apparatus. The apparatus in both programs is similar in that three independently controlled sources respectively containing CdTe, Cd or Te, and a dopant are used to deposit the CdTe films, The SERI design allows independent control of the hot-wall temperature while the Stanford sys-
or larger) are observed in films deposited at 400°C. Subsequent studies will emphasize reproducible pand n-type impurity doping and the development of an optimimized all thin-film n-CdS/p-CdTe photovoltaic device. The chemical vapor deposition (CVD) of CdTe thin films from the elements is being investigated at Southern Methodist University. As shown in fig. 4. the appropriate temperature profile in a seven zone furnace effectively controls the relative vapor pressures of the cadmium and tellurium. p-Type doping is accomplished by the introduction of phosphine or arsine into the hydrogen carrier gas. The deposited CdTe thin films have
AM. Hermann, L. Fabick
/ Research
on polycrystalline thin -film photovoltaic devices
663
— Te
C
0 E
H2 P1* H2S
‘~~2H2
Seven Zone Furnace
,,,/
® Shut-Off Va(ve 0 Metering Valve A,B,C,D,E Flowmeters
To Exhaust
Substrate
\
700
,,,,_......‘
600
-
\.•_____.
________________________________ Temperature Profile
E
. -
500
Fig. 4. Schematic diagram of system used for chemical vapor deposition of CdTe.
grain sizes of the order of 10—40 p.m. Phosphorus and arsenic doped CdTe films (p-type) have lateral
of Delaware on generalized heterojunction devices, and at Drexel University on CdS/CuInSe2 thin-
resistivities as low as 200 12 cm with intragrain diffusion lengths of the order of 3—4 p.m as cornpared to diffusion lengths of 0.1 p.m seen on n-type CdTe films. A Au/n-CdTe Schottky barrier 2 in area) has device formed from CVD CdTe (8 cm demonstrated an AM1 efficiency of 5.3% (J~ 0.40 V, J~ 22 mA/cm2, FF 0.60). n-CdS/pCdTe heterojunction devices have also been fabri-
film devices. Basic studies in the establishment of a phase diagram for CuInSe2 are also being carried out by Stanford Research Institute where thermodynamic solid—vapor equilibrium studies have been mitiated. Current in-house research at SERI is also directed toward co-evaporation of (Cd,Zn) S/CuInSe 2 thin-film devices and corresponding junction characterization with Electron Beam Induced Current scans and CV measurements on annealed and as-deposited devices,
=
=
=
cated. These devices have at present low photoresponse due to high contact resistance at the pCdTe/substrate interface and interdiffusion of the junction during deposition.
Acknowledgement 5. Future research An important issue being addressed in future research is an increase in the open-circuit voltage of (Cd,Zn)S/CuInSe2 cells. Voltages exceeding 0.5 V were obtained previously in single-crystal CuInSe2/evaporated thin-film CdS devices, and further research on this type of cell is being mitiated at Princeton Univeristy. One major atm of this research is the establishment of a full analytical model for the single crystal device. Theoretical modeling is also being performed at the University
This work was supported under DOE Contract No. EG-77-COl-4042,
References [II
RB. Hall, R.W. Birkmire, J.E. Phillips and J.D. Meakin, in: Proc. 15th IEEE Photovoltaic Specialists Conf., 1981,
~
RB. Hall, R.W. Birkmire. J.E. Phillips and J.D. Meakin, AppI. Phys. Letters 38 (1981) 925. [21See, for example, D.T. Bernatowicz and H.W. Brandhorst,
664
A. M. Hermann, L. Fahick
/
Research on polycrystalline thin -film photovoltaic devices
Jr., in: Proc. 8th IEEE Photovoltaic Specialists Conf., 1970, p. 24; W. Palz, J. Beeson, T. Nguyen Duy and J. Vedel, tn: Proc.
Yang, R.H. Bube. LB. Fabick. AL. Fahrenhruch and M.J. Tsai, in: Proc. 13th IEEE Photovoltaic Specialists Conference. 1978.
9th IEEE Photovoltaic Specialists Conf., 1971, P. 69; A.J. Mathiew, K.K. Reinhartz and H. Riekert, in: Proc. 10th IEEE Photovoltaic Specialists Conf.. 1973, p. 93.
[22] C.W. Bates, JR. Mooney, J.M. Rectenwald, L. MacIntosh, R. Lamoreaux, K. Nelson and S. Atig Raza, Thin Solid Films, to be published.
[31J.D.
Meakin, R.W. Birkmire, R.B. Hall and i.E. Phillips, IEC Quarterly Report, June I—Aug. 31. 1981 (SERI Subcontract XS-9-8309-08); see also A.M. Hermann, in: Proc. 5th Annual Photovoltaic Advanced Research and Development Review Meeting, Washington, DC, 1981, in press.
[23] M. Uekita, K. Nelson, C.W. Bates and J. Mooney. Thin Solid Films, to be published. [24] S.P. Sauve, R.D. Mickelsen, J.M. Stewart and W.S. Chen. Final Report, May 21, 1981. SERI Subcontract XG-09216P1. wiih Boeing Aerospace Company, Seaitle. WA. USA.
[4] J.D. Meakin, R.W. Birkmire, R.B. Hall and J.E. Phillips, in: Proc. CdS/Cu 2S and CdS/Cu-Ternary Photovoliaic
[25] A. Tonejo, Z. Ogorelec and B. Mesinik. AppI. Cryst. 8
Cells Sub-Contractors In-Depth Review Meeting (SERI), 1981, p. III; J.E. Phillips, R.W. Birkmire and R.G. Lasswell, in: Proc.
[26] K.W. Mitchell, A.L. Fahenbruch and RH. Rube, J. AppI. Phys. 48(1977)829. [27] J.J. Loferski, Proc. iEEE 51(1963) 667.
(1975) 375.
16th IEEE Photovoltaic Specialists Conf., 1982, preprmt. [5] J.W. Thornton and W.W. Anderson, AppI. Phys. Letters,
[28] K.W. Mitchell. Evaluation of the CdS/CdTe Heterojunclion Solar Cell (Garland. New York, 1979).
in press. [6] H. Neumann, Phys. Status Solidi (a) 56 (1979) K137. [7] H. Neumann and Van Nam, Solid State Commun. 25 (1978) 899.
[29] K. Yamoguchi, N. Nakayama, H. Matsumoto and S. Ikegami, Japan. J. AppI. Phys. 16 (1977) 1203. [30] K. Zanio, in: Semiconductors and Semimetals, Vol. 13 Cadmium Telluride, Eds. R.K. Willardson and A.C. Beer
[8] W. Hong, H. Neumann and H. Sobata, Thin Solid Films 48 (1978) 67. [9] J. Piekoszewski, L. Castaner, J.J. Loferski, J. Beall and W. Giriat, J. AppI. Phys. 51(1980) 5375. [10] R.A. Mickelsen and W. Chen, AppI. Phys. Letters 36 (1980) 371. [II] J.L. Shay, S. Wagner and H. Kasper, AppI. Phys. Letters
(Academic Press, New York, 1978). [31] M.R. Squillante. H.B. Serreze, M. Talbot. R. Turcotte, S. Lie, G. Entine and R.B. Golner, Semiannual Progress Report, September 1981, SERI Subcontract XS-9-8104-3, with Radiation Monitoring Devices Inc.. Watertown, MA, USA. [32] M.B. Das and S.V. Krishnaswamy. Final Report. June
27 (1975) 89. [12] K.A. Jones. J. Crystal Growth 47 (1979) 235. [13] R.A. Mickelson, W. Chen and L. Bulhaupt,
1981. SERI Subcontract XJ-0-9131-l. with Pennsylvania State University, University Park, PA, USA.
Eighth
[33] T.L. Chu, Semiannual Progress Report, January 1982,
Quarterly Report, SERI Subcontract XJ-9-8l2l-l. with
SERI Subcontract ZZ-0-9286, with Southern Methodist
Boeing Aerospace Corporation, Seattle, WA, USA. [14] J.J. Loferski, Final Report, September 1980. SERI Subcontract XI-9-80l2-1. with Brown University, Providence, RI, USA.
115] [16] [17] [18] [19]
J. Mooney, R. Lamoreaux and C.W. Bates, Final Report. February 1981, SERI Subcontract X-9-8104-4, with SRI International, Menlo Park, CA, USA. B. Pamplin and R.S. Feigelson, Thin Solid Films 60(1979) 141. RD. Tomlinson, D. Omeji, J. Parkes and M.J. Hampshire. Thin Solid Films 64 (1979) L3. L.L. Kazmerski, M.S. Ayyogori, FR. White and G.A. Sanborn, J. Vacuum Sci. Technol. 13 (1976) 139. R.A. Mickelson, W. Chen and L. Buldhaupt, Final Report, March 1980, SERI Subcontract XJ-9-8021-1. with Boeing Aerospace Corp., Seattle, WA, USA.
[20] B. Tell and J.L. Shay, J. AppI. Phys. 43 (1972) 2469. [21] H.M. Manasevit, K.L. Hess, P.D. Dapkus. R.P. Ruth. J.J.
University. Dallas. TX. USA. [34] R.H. Rube, A.L. Fahrenbruch. W. Huber. C. Fortmann and T. Thorpe, Progress Report No. 4, SERI Subcontract XW-l-9330-I. with Stanford University. Stanford, CA,
USA. [35] A. Lopez-Otero and W. Huber. J. Crystal Growth 45 (1978) 214. [36] N.G. Dhere and N.R. Panikh, Thin Solid Films 60 (1979) 257. [37] W. Huber and A. Lopez-Otero, Thin Solid Films 58 (1979) 21. 138] I-I. Kinoshita, T. Sakashita and H. Fusiyashi, J. AppI. Phys. 52 (1981) 4. [39] A. Lopez-Otero and W. Huber, Surface Sci. 86 (1979). [40] Yu.K. Yezhovsky and I.P. Kalinkin. Thin Solid Films 18 (1973) 127. [41] A. Lopez-Otero, Thin Solid Films 49 (1978) 3.
Journal of Crystal Growth 61(1983> 658—664 North-Holland Publishing Company
RESEARCH ON POLYCRYSTALLINE THIN-FILM PHOTOVOLTAIC DEVICES
*
A.M. HERMANN and L. FABICK Solar Energy Research Institute, 1617 Cole Boulevar4 Golden, Colorado 80401. USA
Received 12 September 1982
Recent results from the United States Department of Energy Polycrystalline Thin-Film Photovoltaic Device Program are presented. The program which is managed by the Solar Energy Research Institute encompasses materials and device research on a variety of compound semiconductors with emphasis on Il—VI compounds and Il—VI ternary analogs. This paper covers preparation (emphasizing thin-film deposition) and characterization of semiconducting materials such as Cu 2_ ~S,Cu2_ ,Se. CdTe and CulnSe2. Photovoltaic device characteristics of these absorbers with heterojunction partners such as CdS are discussed. Excitement in the program has been generated by recent progress in the (Cd, Zn/S)/CuInSe2 device area. An AMI efficiency of 9.93% for a 5 ~smthick 1 cm~cell has been verified at SERI. Unencapsulated (Cd, Zn)S/CuInSe2 cells have been subjected to more than 6000 h of accelerated stability testing with no measureable degradation in photovoltaic performance. Other program highlights include fabrication of a hybrid CdS/Cu25 device (evaporated CdS. sputtered Cu2S) with a 7.2% AMI efficiency, and a 3.9% AMI efficiency all-sputtered CdS/Cu2S cell. Preliminary data are presented on the achievement of intrinsically stable high-efficiency ( > 9%) wet-processed CdS/Cu2S cells. Progress in the less mature research areas is outlined. A 5.3% (AMI) CVD Au/n-CdTe Schottky harrier cell is described, and AM1 cell efficiencies exceeding 4% for vacuum evaporated CdS/Cu2Se and chemically sprayed CdS/CdTe cells are reported. Future research emphasis is discussed. One of the important technical issues which is being addressed in an Increase in the open-circuit voltage of (Cd,Zn)S/CuInSe2 cells. Technical issues being addressed in other areas include film growth and doping studies in CdTe and improvement in the doping profile of CdS/Cu2Se junctions.
1. Cu2S Intensive research in polycrystalline thin-film photovoltaic devices has been performed over the last two decades. Prototype CdS/Cu2S heterojunction cells have been studied most extensively, Efficiencies exceeding2 area 10% (AM 1) have been re(Cd,Zn)S/Cu ported [1] for 1 cm 2S wetprocessed cells (Cu2S formed by chemical reaction of CdS in aqueous cuprous ion solution). Most of these high-efficiency devices, however, have exhibited both intrinsic (electrochemical) and extrinsic degradation. The United States Department of Energy has recently focused its research program in the Cu 2S area on the achievement of intrinsically stable CdS/Cu2S devices and on alternate techniques for their deposition.
*
This paper was presented at the International Conference on
lI—VI Compounds. University of Durham, April 1982.
The degradation of Cu2S based cells due to electrochemical decomposition of Cu2S is well documented [2]. Any Cu2S in local electrical contact with the back electrode causes electrochemical decomposition of the Cu2S when the cell is cxposed to AM 1 illumination at open-circuit voltage. This change in Cu2 S stoichiometry degrades the cell output because of localized copper paths and corresponding internal short-circuiting. The Institute of Energy Conversion of the University of Delaware has shown recently [3] that the use of rigid lattice-matched (Ni/Fe) substrates can reduce the number of has potential defects to a 2, and it furthershunting been shown [4] that few per cm the few remaining shunting defects can be detected and eliminated with laser scanning. Studies have indicated that CdS/Cu2S cells free of shunting defects demonstrate intrinsic stability in all photovoltaic parameters as determined by exposures exceeding 500 h at open-circuit voltage loading in a flowing H2/Ar atmosphere.
0022-0248/82/0000—0000/$03.00 © 1983 North-Holland
A.M. Hermann, L. Fabick
/
Research at Telic Corporation [5] has demonstrated that CdS/Cu2S devices can be formed by sputter-deposition. Cu2S has been reactively sputtered onto vacuum evaporated CdS (hybrid cell) with the achievement of a 7.2% AMI 2, FF efficiency 0.72). Cu(f°~ 0.54 V, J~ 16.2 mA/cm 2S has also been sputtered onto sputterdeposited CdS with the achievement of a 3.94% AMI efficiency. The all-sputtered cell thickness is only 3.6 ~tm, nearly a factor of ten thinner than the thinnest wet-processed cells. Device2,parame~ç ters this FF cell are 12.2 coating). mA/cm 0.527for V, and 0.62 (no AR =
=
659
Research on polycrystalline thin -film photovoltaic devices
the stoichiometry (and hence electronic defect structure) is determined by the relative vapor pressures of the constituent elements, both during evaporation and during adsorption on the heated substrate. of CuInSe2 formed tion from Films the bulk compound are, by as evaporaa result,
=
=
=
=
2. CuInSe2 Copper indium diselenide has a combination of materials characteristics that make it an excellent candidate for photovoltaic applications. It has a direct band gap of 1.04 eV and it is capable of being doped n- and p-type in the range of l015_~l0’~ cm ~ [6,7]. The optical absorption coefficient is greater than l0~cm over most of the solar spectrum [8] and the electron diffusion length in single crystal CuInSe2 is 2.4 ~sm[9]. These factors contribute substantially to the large photocurrents reported for thin film CdS/CuInSe2 devices [10]. Single crystal CuInSe2 heterojunctions formed with evaporated thin-film CdS have been reported with efficiencies exceeding 12% [II]. The high efficiencies of these structures are partly due to the close lattice match (less than 1.2% mismatch) [12] between CdS and CuInSe2. For these devices to be economically feasible for large scale deployment, they must maintain high efficiency when fabricated in thin-film form. Presently, fabrication of this device structure via vacuum evaporation, spray deposition and sputtering is being investigated as part of the SERI polycrystalline thin-film task. Thin-film vacuum evaporated (Cd,Zn)S/CuInSe2 devices with efficiencies of the order of 10% have been reported [13]. Sputter- and spray-deposited [14—16]CdS/CuInSe2 devices with respective efficiencies of 5.1% and 1.7% have also been reported. When a compound semiconductor is evaporated from the bulk compound onto a heated substrate,
copper deficient unless the source temperature is above 1300°C[17]. Moreover, the electrical properties of these films are controllable only by the application of appropriate post-deposition heat treatment Several [18]. techniques have been developed to control the stoichiometry and allow the deposition of CuInSe 2 films with a wide range of electrical parameters. The most successful has been vacuum deposition of CuInSe2 thin films from the elements as implemented at Boeing Aerospace Corporation [19]. This technique independently controls the evaporation rate of the constituent elements in order to vary the electrical properties of the deposited films. The indium and copper rates are simultaneously controlled using an Inficon dual channel co-evaporator Model 200, an Electron Impact Emission Spectroscopy (EIES) feedback sensor shown in fig. 1. The selenium deposition rate is independently controlled using a quartz crystal sensor. A deposition rate for Cu, In and Se of 0.9, 2 and 6.8 A/s, respectively, results in a CulnSe2 growth rate of 8 A/s on a 450°Cceramic
—
Substrate hotter
Se Deposd~onSensor
,/‘\
Se Source
< )
Two W Boats
I d
~
I
EIES Sensor for Cu and In
__________
Sheld Elate Cu Source
b?~~”~
-
W Boat
~ In
Source
-
W Boat
Carbon Block
Fig. I. System configuration for preparing CuInSe2 films using an EIES deposition controller (Boeing Aerospace Company).
660
A.M. Hermann, L. Fabick
/
Research on polycrystalline thin -film photovollaic device.s
(alumina) substrate. The films have resistivities between 5 and 800 kf~/Ufor a 2—3 ~.smthickness, The carrier concentration is typically 5 x bib cm3 2/V. s as determined with a hole mobility of 9 cm The resistivity of the by Hall effect measurements. p-type CuInSe 2 thin films is controlled by varying the relative Cu to In deposition rates. When the copper deposition rate is 1.1 A/s the film resistivity is 1 Q cm. When the copper deposition rate is 0.8 A/s,have theled film resistivity is 500ofS2a cm. factors to the development first These order defect model for CuInSe 2. In this model, In vacancies act as acceptors and Se vacancies act as donors. This contrasts with a previous model which suggests Cu vacancies are the acceptors responsible for p-type doping in CuInSe2 [20]. The CuInSe2 layer used to fabricate high efficiency photovoltaic devices uses these properties in a two-step deposition process. The first step deposits a high conductivity (pt) CuInSe2 layer 2.5 ~sm thick which acts as a low resistivity interface to the molybdenum contact. This is followed by a high resistivity (p) layer 0.8 ~sm thick which forms the heterojunction absorber and prevents the formation of copper nodules at the heteroface. The CdS/CuInSe2 heterojunction is deposited insitu following the resistivity CuInSe2 deposition. consists of a high (20 ~2cm) The layerCdS 0.8 ~sm thick followed by a low resistivity In-doped layer 1.7 ~smthick [21]. The use of a high resistivity CdS layer at the heteroface appears necessary to prevent formation of copper nodules at the interface and may otherwise be related to the achievement of high conversion efficiencies in these cells, The Boeing group using this precise coevaporation deposition technique has deposited a Cd 0 9Zn0 1S/p-CuInSe2 device with the following parameters at SERI: efficiency 9.93%, 0.42 V,measured J 36.3 mA/cm2, FF 0.65, at 2 area. The group is also depositAM1CuInSe for a 1 cm ing 2 films in a planetary evaporation system which allows uniform deposition onto a large number of substrates through rotation of the substrate holders. Spray pyrolysis of CdS/CuInSe 2 devices offers =
=
= =
the possibility of fabricating high efficiency thin film devices at a very low cost. This technique uses 0.001 molar CuCl, InCl n,n-dimethylselenourea solution sprayed onto a 3, molybdenum coated glass substrate at 260°C[22]. As with vacuum evaporation, film resistivities range from 0.2 Q cm to 6 X b0~~2 cm depending on the relative copper concentration in the spray solution. Grain sizes are typically 1 ~sm filmsdiffusion 1 ~tm thick. Hole 2/V’for s with lengths of mobili0.3 ~tm ties of I cm are reported. Equilibrium thermodynamic computer sirnulations have been used extensively to determine the optimum conditions for spray film deposition. Chalcopyrite films have been produced with no detectable second phases (In0Se1, Cu2Se). Thermodynamic considerations also indicate a 6000 H2Se/H2 anneal will reduce the residual oxygen content of the as-sprayed films. Annealing experiments have verified that oxygen can be reduced from 1% at. wt. to below the limit of detectability. Spray pyrolysis of CdS films doped with indium has resulted in low resistivity (less than 0.2 f2 cm) films with grain sizes of 0.3 ~m [23]. Sprayed CdS/CuInSe2 devices fabricated at Stanford Research Institute using these layers havefr~ 1.7%0.23 AM1 2 in area) with V. efficiencies (3 mm J 2, and FF 0.31. 5~Brown 23.3 mA/cm University is investigating direct sputter deposition of CdS/CuInSe-, devices [14]. A very strong dependence of film parameters on substrate temperature has been observed. CuInSe 2 films sputter-deposited with 450°Cor higher substrate temperature are the chalcopyrite phase while films deposited between 50 and 300°C are the sphalerite phase. Films deposited at 480°C have ~em 2/V 0.1 s while grains and hole mobilities of 0.4 cm films deposited at 505°Chave ~sm grains 2/V.0.2—l.0 s. All sputter-dcand holeCulnSe mobilities of 6 cm posited are The p-type resistivities between 0.3 and2 2films 12 cm. bestwith device consists =
=
=
‘
of a sputtered CuInSe2 film and a vacuum evaporated CdS de2 inheterojunction area with ~ partner. 0.45 V, This ~ 22 vice is cm2, 0.2 cm rn/V FF 0.5, and an AMI efficiency of =
=
5.1%.
=
A.M. Hermann, L. Fabick
/
Research on polycrysialline thin -film photovoltaic devices
3. Cu2 ~Se -
Boeing Aerospace Corporation is investigating the properties of CdS/Cu25Se devices [24]. Cu2 Se could offer the high efficiency of CuInSe2 without the indium component and with less of a degradation problem than that of CdS/Cu2S devices. Cu2 ~Se has an indirect bandgap of 1.4 eV and a direct bandgap of 2.2 eV for x 0.2. Two phases of Cu2~Seexist at room temperature depending on the value of x [25]. If x is between 0.2 and 0.25, the crystal structure is face-centered cubic. Initial indications suggest that a second reported hexagonal phase has a deleterious effect on photovoltaic device performance. Vacuum evaporated films deposited from the elements (see vacuum evaporation of CuInSe2) at 150°C substrate temperature ares.p-type with mobilities of the 2/V’ The carrier concentrations order of 10lOIS cmto 1021 cm3 for x values between vary from 0.1 and 0.3 (the Cu 2Se stoichiometry has been measured via the coulometric technique employed with Cu2S). Typical grain sizes are 1 ~smfor a film thickness of 2 p.m. CdS/Cu2~Se devices have demonstrated efficiencies of up2,to and 4.25% I/~C FFwith0.64. 457 mV, J~ 14.4 mA/cm These cells were heat treated in hydrogen for 5 mm at 150°C. —
-
=
=
=
=
4. CdTe Cadmium telluride has a high absorption coefficient (> iO~cm~) [26] due to its direct bandgap at 1.47 eV which is optimum for photovoltaic applications [27,28]. 12% conversion efficiencies have been reported for single crystal CdTe devices with evaporated CdS [29]. An excellent review of the material properties can be found elsewhere [30]. The subcontracted photovoltaics program at SERI emphasizes the development of CdTe-based solar cells fabricated via spray pyrolysis [31], sputter deposition [32], screen printing/sintering [15], chemical vapor deposition (CVD) [33], and hot-wall vacuum evaporation (HWVE) [34]. Dctails of results from the HWVE and CVD programs are presented here.
661
When a compound semiconductor is deposited via thermal evaporation, there are several variables that determine its morphology and stoichiometry. The relative vapor pressures and hence the impingement rate of the constituent elements on the substrate determine the film stoichiometry. The substrate temperature controls the desorption and mobility of atoms on the substrate surface. The temperature of the substrate also controls the number of nuclei formed. If the substrate temperature is below the glass transition temperature of the semiconductor, the film grown will have an amorphous structure. As the substrate temperature is raised past the glass transition temperature, typically small grain polycrystalline films will grow. The small grains are due to the high nucleation rate and low surface mobility on the low temperature In order to grow applications, large grain thin films substrate. suitable for photovoltaic the substrate temperature must be raised further. This has the combined effect of lowering the nucleation rate and raising the surface mobility of the adsorbed atoms or molecules. It also increases the desorption rate of the atoms or molecules and at still higher temperatures of the dcdeposited films takes place.re-evaporation If the constituent ments of a compound semiconductor desorb and/or reevaporate at similar rates, the deposited films will reflect the stoichiometry and electrical structure of the compound evaporant charge. If, however, the constituent elements have a large difference in vapor pressure and/or desorption activation energy, the film will not be stoichiometnc and will have many electrically active defects. This is typically the case with evaporated CdTe films where [~Cd]/[~TeI is 100 or more [25] and as-grown films are highly compensated with deep level defects. If an enclosed hot wall is included in the system (fig. 2), then inclusion of a separate source of the most volatile constituent (e.g., a cadmium source when evaporating CdTe) allows separate control of the relative impingement rate of the individual species [35—41].In addition, if the substrate is maintained at a lower temperature than the hot-wall, then evaporation and desorption from the substrate will not occur. Hence, the addition of a hot wall efffectively decouples the deposition variables thereby allowing more lati-
662
A.M. Hermann, L. Fabick
Thermocouple Position
Research on polycrystalline thin -film photovoltaic devices
Hook (Up and down and/or side-to-side motion)
-
First Zone (Substrate)
/
:
-
Substrate Substrate Carrier (Graphite) Shutter Guide
~Shutter
— -— - -
-- -
/ -
-~
~
Substrate Holder
(Quartz)
::e:r:::a::: Sources
/)
Radiation Shields Heater Wound on Stainless Steel Core Third Source (Cd or Te)
Fourth Zone
~——
-~
Fig. 3. Schematic diagram of HWVE system at SERI.
Threaded Rod Supports
U
ourrO o r
Thermal Links to H C Cooled Baseplate
Fig. 2. Simplified schematic of hot wall retort (Stanford tJniversity).
tern allows the deposition onto a number of sub. strates without breaking vacuum. Preliminary re-
sults from Stanford indicate that indium doping of CdTe thin films reduces the resistivity from io~to 800 12 cm when measured under AMI light. The intergrain barrier height under these conditions is reduced from 0..8 to 0.1 eV. Large grains (10 p.m
tude in the deposition of the thin film, SERI is presently supporting two programs in HWVE of CdTe thin films. The first program at Stanford University employs a reactor design similar to that of Lopez-Otero of the University of Linz, Austria [35,37,39,41], as illustrated in fig. 2. The second program is an in-house research effort at SERI. Fig. 3 illustrates the in-house HWVE apparatus. The apparatus in both programs is similar in that three independently controlled sources respectively containing CdTe, Cd or Te, and a dopant are used to deposit the CdTe films, The SERI design allows independent control of the hot-wall temperature while the Stanford sys-
or larger) are observed in films deposited at 400°C. Subsequent studies will emphasize reproducible pand n-type impurity doping and the development of an optimimized all thin-film n-CdS/p-CdTe photovoltaic device. The chemical vapor deposition (CVD) of CdTe thin films from the elements is being investigated at Southern Methodist University. As shown in fig. 4. the appropriate temperature profile in a seven zone furnace effectively controls the relative vapor pressures of the cadmium and tellurium. p-Type doping is accomplished by the introduction of phosphine or arsine into the hydrogen carrier gas. The deposited CdTe thin films have
AM. Hermann, L. Fabick
/ Research
on polycrystalline thin -film photovoltaic devices
663
— Te
C
0 E
H2 P1* H2S
‘~~2H2
Seven Zone Furnace
,,,/
® Shut-Off Va(ve 0 Metering Valve A,B,C,D,E Flowmeters
To Exhaust
Substrate
\
700
,,,,_......‘
600
-
\.•_____.
________________________________ Temperature Profile
E
. -
500
Fig. 4. Schematic diagram of system used for chemical vapor deposition of CdTe.
grain sizes of the order of 10—40 p.m. Phosphorus and arsenic doped CdTe films (p-type) have lateral
of Delaware on generalized heterojunction devices, and at Drexel University on CdS/CuInSe2 thin-
resistivities as low as 200 12 cm with intragrain diffusion lengths of the order of 3—4 p.m as cornpared to diffusion lengths of 0.1 p.m seen on n-type CdTe films. A Au/n-CdTe Schottky barrier 2 in area) has device formed from CVD CdTe (8 cm demonstrated an AM1 efficiency of 5.3% (J~ 0.40 V, J~ 22 mA/cm2, FF 0.60). n-CdS/pCdTe heterojunction devices have also been fabri-
film devices. Basic studies in the establishment of a phase diagram for CuInSe2 are also being carried out by Stanford Research Institute where thermodynamic solid—vapor equilibrium studies have been mitiated. Current in-house research at SERI is also directed toward co-evaporation of (Cd,Zn) S/CuInSe 2 thin-film devices and corresponding junction characterization with Electron Beam Induced Current scans and CV measurements on annealed and as-deposited devices,
=
=
=
cated. These devices have at present low photoresponse due to high contact resistance at the pCdTe/substrate interface and interdiffusion of the junction during deposition.
Acknowledgement 5. Future research An important issue being addressed in future research is an increase in the open-circuit voltage of (Cd,Zn)S/CuInSe2 cells. Voltages exceeding 0.5 V were obtained previously in single-crystal CuInSe2/evaporated thin-film CdS devices, and further research on this type of cell is being mitiated at Princeton Univeristy. One major atm of this research is the establishment of a full analytical model for the single crystal device. Theoretical modeling is also being performed at the University
This work was supported under DOE Contract No. EG-77-COl-4042,
References [II
RB. Hall, R.W. Birkmire, J.E. Phillips and J.D. Meakin, in: Proc. 15th IEEE Photovoltaic Specialists Conf., 1981,
~
RB. Hall, R.W. Birkmire. J.E. Phillips and J.D. Meakin, AppI. Phys. Letters 38 (1981) 925. [21See, for example, D.T. Bernatowicz and H.W. Brandhorst,
664
A. M. Hermann, L. Fahick
/
Research on polycrystalline thin -film photovoltaic devices
Jr., in: Proc. 8th IEEE Photovoltaic Specialists Conf., 1970, p. 24; W. Palz, J. Beeson, T. Nguyen Duy and J. Vedel, tn: Proc.
Yang, R.H. Bube. LB. Fabick. AL. Fahrenhruch and M.J. Tsai, in: Proc. 13th IEEE Photovoltaic Specialists Conference. 1978.
9th IEEE Photovoltaic Specialists Conf., 1971, P. 69; A.J. Mathiew, K.K. Reinhartz and H. Riekert, in: Proc. 10th IEEE Photovoltaic Specialists Conf.. 1973, p. 93.
[22] C.W. Bates, JR. Mooney, J.M. Rectenwald, L. MacIntosh, R. Lamoreaux, K. Nelson and S. Atig Raza, Thin Solid Films, to be published.
[31J.D.
Meakin, R.W. Birkmire, R.B. Hall and i.E. Phillips, IEC Quarterly Report, June I—Aug. 31. 1981 (SERI Subcontract XS-9-8309-08); see also A.M. Hermann, in: Proc. 5th Annual Photovoltaic Advanced Research and Development Review Meeting, Washington, DC, 1981, in press.
[23] M. Uekita, K. Nelson, C.W. Bates and J. Mooney. Thin Solid Films, to be published. [24] S.P. Sauve, R.D. Mickelsen, J.M. Stewart and W.S. Chen. Final Report, May 21, 1981. SERI Subcontract XG-09216P1. wiih Boeing Aerospace Company, Seaitle. WA. USA.
[4] J.D. Meakin, R.W. Birkmire, R.B. Hall and J.E. Phillips, in: Proc. CdS/Cu 2S and CdS/Cu-Ternary Photovoliaic
[25] A. Tonejo, Z. Ogorelec and B. Mesinik. AppI. Cryst. 8
Cells Sub-Contractors In-Depth Review Meeting (SERI), 1981, p. III; J.E. Phillips, R.W. Birkmire and R.G. Lasswell, in: Proc.
[26] K.W. Mitchell, A.L. Fahenbruch and RH. Rube, J. AppI. Phys. 48(1977)829. [27] J.J. Loferski, Proc. iEEE 51(1963) 667.
(1975) 375.
16th IEEE Photovoltaic Specialists Conf., 1982, preprmt. [5] J.W. Thornton and W.W. Anderson, AppI. Phys. Letters,
[28] K.W. Mitchell. Evaluation of the CdS/CdTe Heterojunclion Solar Cell (Garland. New York, 1979).
in press. [6] H. Neumann, Phys. Status Solidi (a) 56 (1979) K137. [7] H. Neumann and Van Nam, Solid State Commun. 25 (1978) 899.
[29] K. Yamoguchi, N. Nakayama, H. Matsumoto and S. Ikegami, Japan. J. AppI. Phys. 16 (1977) 1203. [30] K. Zanio, in: Semiconductors and Semimetals, Vol. 13 Cadmium Telluride, Eds. R.K. Willardson and A.C. Beer
[8] W. Hong, H. Neumann and H. Sobata, Thin Solid Films 48 (1978) 67. [9] J. Piekoszewski, L. Castaner, J.J. Loferski, J. Beall and W. Giriat, J. AppI. Phys. 51(1980) 5375. [10] R.A. Mickelsen and W. Chen, AppI. Phys. Letters 36 (1980) 371. [II] J.L. Shay, S. Wagner and H. Kasper, AppI. Phys. Letters
(Academic Press, New York, 1978). [31] M.R. Squillante. H.B. Serreze, M. Talbot. R. Turcotte, S. Lie, G. Entine and R.B. Golner, Semiannual Progress Report, September 1981, SERI Subcontract XS-9-8104-3, with Radiation Monitoring Devices Inc.. Watertown, MA, USA. [32] M.B. Das and S.V. Krishnaswamy. Final Report. June
27 (1975) 89. [12] K.A. Jones. J. Crystal Growth 47 (1979) 235. [13] R.A. Mickelson, W. Chen and L. Bulhaupt,
1981. SERI Subcontract XJ-0-9131-l. with Pennsylvania State University, University Park, PA, USA.
Eighth
[33] T.L. Chu, Semiannual Progress Report, January 1982,
Quarterly Report, SERI Subcontract XJ-9-8l2l-l. with
SERI Subcontract ZZ-0-9286, with Southern Methodist
Boeing Aerospace Corporation, Seattle, WA, USA. [14] J.J. Loferski, Final Report, September 1980. SERI Subcontract XI-9-80l2-1. with Brown University, Providence, RI, USA.
115] [16] [17] [18] [19]
J. Mooney, R. Lamoreaux and C.W. Bates, Final Report. February 1981, SERI Subcontract X-9-8104-4, with SRI International, Menlo Park, CA, USA. B. Pamplin and R.S. Feigelson, Thin Solid Films 60(1979) 141. RD. Tomlinson, D. Omeji, J. Parkes and M.J. Hampshire. Thin Solid Films 64 (1979) L3. L.L. Kazmerski, M.S. Ayyogori, FR. White and G.A. Sanborn, J. Vacuum Sci. Technol. 13 (1976) 139. R.A. Mickelson, W. Chen and L. Buldhaupt, Final Report, March 1980, SERI Subcontract XJ-9-8021-1. with Boeing Aerospace Corp., Seattle, WA, USA.
[20] B. Tell and J.L. Shay, J. AppI. Phys. 43 (1972) 2469. [21] H.M. Manasevit, K.L. Hess, P.D. Dapkus. R.P. Ruth. J.J.
University. Dallas. TX. USA. [34] R.H. Rube, A.L. Fahrenbruch. W. Huber. C. Fortmann and T. Thorpe, Progress Report No. 4, SERI Subcontract XW-l-9330-I. with Stanford University. Stanford, CA,
USA. [35] A. Lopez-Otero and W. Huber. J. Crystal Growth 45 (1978) 214. [36] N.G. Dhere and N.R. Panikh, Thin Solid Films 60 (1979) 257. [37] W. Huber and A. Lopez-Otero, Thin Solid Films 58 (1979) 21. 138] I-I. Kinoshita, T. Sakashita and H. Fusiyashi, J. AppI. Phys. 52 (1981) 4. [39] A. Lopez-Otero and W. Huber, Surface Sci. 86 (1979). [40] Yu.K. Yezhovsky and I.P. Kalinkin. Thin Solid Films 18 (1973) 127. [41] A. Lopez-Otero, Thin Solid Films 49 (1978) 3.