Thin film II–VI photovoltaics

Solid-Stare

Electronics

Vol.

38, No.

Copyright

Pergamon

Printed

in Great

3. pp. 533-549,

t> 1995 Elsevier Britain. 0038-I

All

lOi/95

1995

Science Ltd

rights

reserved

$9.50 + 0.00

INVITED PAPER

THIN FILM II-VI PHOTOVOLTAICS TING L. CHU and SHIRLEY S. CHU Ting L. Chu and Associates, 12 Duncannon Ct. Dallas, TX 75225-1809, U.S.A. (Received

16 September

1994)

Abstract-With the exception of HgSe and HgTe, II-VI compounds are direct gap semiconductors with sharp optical absorption edge and large absorption coefficients at above bandgap wavelengths. Device quality polycrystalline films of II-VI compounds can be prepared from inexpensive raw materials by a number of low-cost methods. They are well-suited for thin film solar cells and provide an economically viable approach to the terrestrial utilization of solar energy. Thin film II-VI solar cells are usually of the heterojunction type consisting of a high bandgap window (or collector) and a lower bandgap absorber. The grain boundary effects in polycrystalline II-VI films are considerably less pronounced than those in III-V films and can be passivated, at least partially, by chemical treatment. The use of CdS, ZnO, ZnSe and Cd, _ .Zn,S as the window and the use of CdTe and Cd, _ .Zn,Te as the absorber are reviewed in this paper. The fabrication and characteristics of a number of the thin film solar cell structures are discussed with emphasis on the thin film CdS/CdTe solar cell.

1.

INTRODUCTION

Binary compounds of group IIB and group VIA elements, commonly referred to as II-VI compounds, have technologically important applications. With the exception of HgSe and HgTe (semimetals), they are direct gap semiconductors with high optical transition probabilities for absorption and emission. The properties of the oxides, sulfides, selenides and tellurides of zinc and cadmium relevant to their optical applications are summarized in Table 1. They have higher bandgap energy than the corresponding III-V compounds due to the larger ionicity in II-VI compounds. Also, the effective mass of carriers in II-VI compounds is relatively high, the radiative carrier lifetime is small, and the carrier diffusion length is short as compared with the III-V compounds. Among these compounds, only cadmium telluride (CdTe) and zinc selenide (ZnSe) can be prepared in both n- and p-type forms. The stable modification of CdS and CdTe at room temperature is the Wurtzite structure, and that of CdTe, CdSe and ZnTe is the zinc blende structure. Zinc sulfide can crystallize in the zinc blende or Wurtzite structure, depending on the preparation conditions. Because of their large optical absorption coefficients at above bandgap wavelengths, a II-VI semiconductor of about 1 pm thickness is sufficient to absorb 99% of the impinging radiation with photon energy higher than the bandgap energy. Thus, they are well-suited for thin film optical devices. The short optical absorption length in II-VI compounds also renders the carrier diffusion length in minority carrier devices relatively unimportant. In addition to the lax

diffusion length requirement, II-VI compounds also have the advantage that they can be prepared in the form of high quality polycrystalline films from inexpensive raw materials by several low-cost methods. Thus, the use of thin film II-VI compounds is an economically viable approach to the terrestrial utilization of solar energy. Polycrystalline thin film II-VI solar cells have been under investigation for about 40 years. All efficient devices are of the heterojunction configuration because of (1) the difficulty in forming a very shallow junction (less than 0.1 pm) with a high conductivity surface layer, and (2) the surface recombination effects. The two semiconductors of opposite conductivity type in a heterojunction solar cell are usually referred to as the “absorber” (bandgap energy EB,), and “collector” or “window” (bandgap energy E,,), respectively, with Eti > Eg, . The radiation is incident on the surface of the window, and the generation of hole-electron pairs by photons with energies between Eg2 and E,, occurs in the absorber. A portion of the radiation with photon energy greater than Eg2 may also reach the absorber, depending on the thickness and absorption coefficient of the window. Since the wide gap semiconductors for windows are usually of n-type conductivity, the absorber is p-type. The photocurrent consists predominately of the electrons generated in the depletion region in the absorber. In practice, the heterojunction solar cells may be classified into frontwall cells and backwall cells according to the manner in which the solar radiation impinges on the cell. In frontwall cells, the window film, the absorber film, and the ohmic contact are 533

Ting L. Chu and Shirley S. Chu

534 Table

I. Prowrties

of selected II-VI comoounds

type

Electron afinity (eV)

Cd0 CdS

2.40 2.42

II n

4.50

Halite Wurtzite

CdSe

1.74

II

4.95

Wurtzite

CdTe zno

*. P n

4.28

3.20

Zincblende Wurzite

ZllS

3.66

n

3.90

Zincblende Wurtzite

Bandgap energy (ev)

Material

1.45

2.67 2.25

ZoSe ZnTe

Conductivity

fl. P P

4.09 3.50

Stable structure

Zincblende Zincblende

deposited successively on a transparent substrate, such as glass, and the solar radiation impinges on the substrate surface. This configuration is also referred to as the superstrate configuration. In the backwall or substrate configuration, the absorber film and the window film are deposited successively on the substrate, where a stable low resistance contact between the absorber and the substrate is essential. The bandgap energy, electron affinity, lattice parameter and thermal expansion coefficient of the window and absorber are important parameters determining the photovoltaic characteristics of the heterojunction solar cell. The conduction and valence band edges of the heterojunction may show a discontinuity due to difference in electron affinities of the two components. When x, > x2 (2, is the electron affinity of the absorber), a conduction band spike is formed, which may interfere with minority carrier transport across the junction. The built-in potential at the junction, and therefore the open-circuit voltage I’,, may decrease when 1, < x2. The V,, is determined by E,, and 1, - x2. The mismatch in the lattice parameter and/or thermal expansion coefficient of the absorber and window always leads to intrinsic defects, or interface states, at the junction. Interface states are also associated with the extrinsic defects introduced during processing, such as contaminants. The interface states usually increase the saturation current density, J,, thereby decreasing V,, and FF. A number of binary and ternary II-VI compounds have been used as windows and as absorbers in thin film solar cells. On the basis of the bandgap energy, CdO, CdS, ZnO, ZnS, ZnSe and certain ternaries are potential window materials, and CdSe, CdTe, and some ternaries, may be used as absorbers. In this paper, the effects of grain boundary in polycrystalhne thin films are briefly discussed, and the use of II-VI compounds in thin film solar cells are reviewed with emphasis on CdS and CdTe. 2.

GRAIN BOUNDARY

EFFECTS

Polycrystalhne semiconductor thin films in solar cells usually consist of micrometer-sized columnar

Lattice parameters (A,

Melting point u C)

Approx. v.p. at m.p. (am)

Thermal expansion coeff. (XI0 “C ‘)

q, = 4.7 1 0,=4.1368 c,=6.7163 a,, = 4.298 (‘”= 7.01 a,, = 6.48 I a,, = 3.25 co = 5.207 o0 = 5.409 u,, = 3.819 ci, = 6.256 a, = 3.669 o,, = 6.104

900 1475

3.80

1239

0.41

1092 1975

0.23 7.80

I830

3.70

6.3

I520 1295

0.53 0.64

6.X 8.4

14.7 13.0 13.7 113.0 4.9 14.8 112.9

grains joined together at grain boundaries. The individual grains may or may not have a preferred crystallographic orientation which usually has no pronounced effects on the photovoltaic characteristics of the solar cell. The structural perfection of the grains depends on the condition of formation. Under optimized conditions, the interior of the grains is usually of good structural perfection with relatively low density of defects, such as dislocations and stacking faults. These defects at densities lower than about lO’cm_ ’ do not act as recombination centers and have negligible effects on junction characteristics and photovohaic performance. However, the grain boundaries are highly disordered, and the periodicity in the crystal lattice is interrupted at the grain surface giving rise to a potential barrier. The potential barriers may affect the series resistance and opencircuit voltage of the solar cell. Atoms at the grain surface may be bonded to impurities giving rise to a high density of surface states, which are effective recombination centers. The impurities along the grain boundaries may reduce the shunt resistance of the solar cell. In thin film II-VI solar cells the photogeneration of carriers occurs mainly in the depletion region of the absorber due to the short optical absorption length and low carrier concentration in the absorber. In general, carriers generated closer to a grain boundary than the junction will have a much higher probability of recombination at the boundary than those generated closer to a junction. The grain boundary effects in III-V compounds are very pronounced, and no effective passivation techniques have been developed[l]. The conversion efficiency of thin film III-V solar cells is considerably less than one-half of that of corresponding single crystalline cells. In II-VI compounds, the grain boundaries can be passivated, at least partially, by chemical treatments such as oxidation. Since the oxidation products such as Cd0 are usually of n-type conductivity due to oxygen deficiencies, the surface of p-absorber grains are converted to n-type conductivity. The grain boundaries then become vertical p-n junctions which are effective in the collection of photogenerated carriers.

Thin film

II-VI photovoltaics

The carriers generated in the immediate vicinity of the vertical junction will be collected at the junction instead of being lost by recombination. The passivation of grain boundaries is important in the fabrication of high efficiency thin film II-VI solar cells. Another effect of grain boundaries is the higher diffusion rate of impurities along grain boundaries. In polycrystalline films with a columnar structure, this effect must be considered when selecting materials and processing temperature in solar cell fabrication.

535

The solution growth technique is most convenient for the low-cost deposition of stoichiometric CdS films and will be considered in some detail here. In this technique, CdS films are deposited on the substrate surface by the reaction between a Cd-salt, an NH.,-salt, ammonia and thiourea [CS(NH,),] in an aqueous solution. Ammonia is a complexing agent, thiourea furnishes S=, and the NH,-salt serves as a buffer. The various reactions involved and their equilibrium constants at room temperature are as follows: NH,+H,O*NH$+OH-

3. CADMIUM SULFIDE

Polycrystalline CdS films have been used as a heterojunction partner in several types of thin film solar cells: CdS/Cu,S, CdS/CuInSe, and CdS/CdTe. The deposition and properties of CdS, CdS/Cu,S solar cells and CdS/CuInSe, solar cells are discussed in this section. The fabrication and characteristics of CdS/CdTe solar cells are discussed in Section 7. Solar cells from CdS and single crystalline absorbers, such as CdS/Si[2], CdS/GaAs[3] and CdS/InP[4], are not included in this paper. 3.1. Deposition of CdS films The deposition of CdS films has been accomplished by a number of processes. The technology of CdS and other transparent conducting semiconductors have been the subject of several reviews[5,6]. The commonly used techniques include vacuum evaporation[7], chemical vapor deposition[8], spray pyrolysis[9], and solution growth[lO]. Other techniques such as sputtering[l 11, electrodeposition[ 121, and screen printing[ 131 can also be used. In the evaporation technique, a CdS (or CdS + S) source is vaporized in a conventional bell jar system to yield Cd and S vapors, which recombine on the surface of heated substrates depositing CdS. The carrier concentration in the deposited film, usually 10’5-10’6cme3 due to the presence of sulfur vacancies, is determined by the parameters of the evaporation process, such as substrate temperature. Highly conductive CdS films (carrier concentration of 10’8-10’9cm~3) can be deposited by the coevaporation of In. Chemical vapor deposition technique using the reaction between a volatile Cd compound (or Cd vapor) with a sulfur compound (or S vapor) on the surface of heated substrates has been used extensively for the growth of CdS films and crystals. The spray pyrolysis technique is well-suited for large scale applications. An aqueous solution containing a cadmium salt (such as CdCI,) and a sulfur compound (such as thiourea) is sprayed onto the surface of heated substrates, where the chemical reaction takes place depositing CdS. The properties of deposited films depend on the substrate temperature, composition of spraying solution, spraying rate and post-spray heat treatment.

(I)

K, = 1.8 x 1O-5 Cd+ + + 20H - * Cd(OH), (s)

(2)

K2 = 1.88 x 10’4 Cd+ + + 4NH, c* Cd(NH,):

+

(3)

K3 = 3.6 x lo6

(NH,),CS + 20H- c* S= + 2H,O + H?CN? Cd++ +S= c*CdS(s)

(4) (5)

K 5= 7.1 x 102*.

Qualitatively, in the presence of sufficient NH,, the Cd salt exists predominantly in the form of Cd(NH3 )4++ . The room temperature equilibrium constant of reaction (4) is very small. When the concentration product of Cd++ and S= in solution exceeds the solubility product of CdS, 1.4 x 10-29, CdS precipitates. The rate of formation of CdS is determined by the concentration of Cd++ provided by Cd(NH,): + and the concentration of S= from the hydrolysis of (NH2)2CS. The rate of hydrolysis of (NH,),CS depends on the pH and temperature of the solution. At 80°C for example, the rate constants of hydrolysis are 3.8 x 10m3and 8.2 x 1O-3 at pH of 13 and 13.7, respectively. These constants become 1.1 x 10m2 and 2.5 x 10e2, respectively, when the solution temperature is increased to 100°C. The presence of an NH,-salt in solution shifts the equilibrium position of reaction (1). increasing the concentration Cd(NH,): + and reducing the concentration of Cd+ + . The equilibrium position of reaction (4) is also shifted, reducing the concentration of S’. As a result, the rate of formation of CdS is reduced. The use of an excess of ammonia increases the pH of the solution, promoting the formation of S=; however, the concentration of Cd(NH,):+ is also increased, reducing the concentration of Cd++ and the rate of CdS formation. Thus, the rate of formation of CdS can be adjusted by varying the concentrations of ammonia and the NH,-salt in solution. Further, the equilibrium constants of all reactions are temperature dependent. The hydrolysis of (CH2)2CS is greatly enhanced as temperature increases. The temperature of the solution can

536

Ting L. Chu and Shirley S. Chu

therefore also be used to control the rate of CdS formation. The formation of CdS can take place heterogeneously on the substrate surface, depositing CdS, or homogeneously in solution, producing CdS precipitate. The homogeneous process is highly undesirable since the adsorption of CdS particles on the substrate surface yields powdery and nonadherent films. The homogeneous process may be suppressed by using conditions for the formation of CdS at low rates such as low concentrations of Cd-salt and thiourea, high concentrations of NH, and NH,-salt, low temperature, etc. In the heterogeneous process, nucleation on the substrate surface results from the preferential adsorption of Cd++ or S= followed by the addition of S= or Cd++ ions. The heterogeneous process may be prompted by preparing the substrate surface conducive to nucleation. Aqueous solutions containing (1) CdAc, or CdCl, in the concentration range of (5-20) x 10m4M, (2) NH,Ac. NH,F or NH,Cl in the concentration range of (2.5-20) x IO-‘M, (3) thiourea in the concentration range of (5-20) x lO-4 M and (4) ammonia in the concentration range of 0.05-0.2 M are suitable for the deposition of device-quality CdS films. The substrates are suspended vertically in the solution. The solution should be vigorously stirred to facilitate the diffusion of Cd++ and S= to the substrate surface. The temperature of the solution should be at SO-90°C during the deposition process. After IO-15 min, the solution becomes slightly yellow. and a thin film deposit appears on the substrate surface. The deposition process is completed after about I h. The average deposition rate is 20-40 &min. The CdS films deposited on thoroughly cleaned glass substrates are highly transparent and highly adherent to the substrate. They are polycrystalline and show very simple X-ray diffraction spectrum, associated with the (002) reflection of hexagonal CdS or the (I I 1) reflection of the cubic modification. The dark lateral resistivity of CdS films is usually 104-IO5 fi cm at room temperature. Under illumination with ELH lamps at 100 mW/cm2, the resistivity is reduced by a factor of 100-1000. The increase in conductivity under illumination is due to the excess carriers in the grain introduced by the absorption of above bandgap radiation and the lowering of potential barriers at grain boundaries. The high photoconductivity ratio indicates that the solution-grown CdS films are of good quality. 3.2. CdSICu,S

solar cells

The CdS/Cu,S solar cell, developed in 1954, is the first polycrystalline thin film cell intended for space and terrestrial applications. The design, fabrication, and performance of CdS/Cu2S solar cells have been reviewed[14,15]. Briefly, the CdS/Cu,S solar cell is commonly of the backwall or substrate configuration

and is fabricated sequence.

by

the

following

processing

(1) The

deposition of 20-30pm of CdS on Zn-coated Cu foil substrates by vacuum evaporation; Zn serves as an ohmic contact to CdS. The conversion of the surface of CdS into (2) Cu,S of 1000-4000~ thickness by dipping into a CuCl solution; Cu,S is a degenerate p-type semiconductor with a bandgap energy of about 1.2 eV (the composition of Cu, S is an important factor affecting the photocurrent, and the chalcocite phase with Cu/S> 1.995 is essential). of the grid contact and anti(3) The deposition reflection coating. It has been postulated that in CdS/Cu,S solar cells, the photocurrent is mainly the electrons generated in the Cu,S with a minor hole contribution from the CdS. The diffusion of photogenerated electrons is controlled by the density of electrons at the junction interface and is determined by the voltage at the junction. The voltage drop is almost entirely across the space charge region in the CdS side of the junction, and the junction has Schottky barrier characteristics yielding a diode-like behavior only close to V,. Near the maximum power point and continuing into the current saturation range, the characteristics are not related to the classical diode theory[ 161. Laboratory CdS/Cu, S solar cells with a conversion efficiency of about 8% at room temperature were demonstrated in 1976. The highest observed J,,, V, and FF were 25 mA/cm2, 0.53 V and 75%, respectively, indicating a potential of about 10% efficiency. By replacing CdS with Cd, ,Zn,S with 0.1 < x < 0.2 (see Section 4 for the deposition and properties of Cd, _ ,Zn,S), the short-circuit current density was comparable to the best observed in CdS/Cu,S solar cells of the same design, about 26 mA/cm’; however, the opencircuit voltage is increased leading to higher conversion efficiencies[ 171. The increased V,, is due to the reduction in the electron affinity mismatch (* 0.2 eV) at the heterojunction interface. The most efficient Cd,,,Zn,,,,S/C&S cell of 0.98 cm’ had VOC and 2 J, and FF of 0.599 V, 18.5mA/cm2 74.8%, respectively, under an outdoor irradiance of 81.2 mW/cm2, corresponding to a conversion efficiency of 10.2%. The CdS/Cu,S solar cells appeared to have lowcost potential for terrestrial applications. A production line with an annual capacity of 0.5 MW was placed in operation[ 181. However, the CdS/Cu,S solar cells showed degradation in J, and FF under illumination due to the electrochemical decomposition of Cu,S at operating voltages of higher than about 0.33 V and the rapid diffusion of Cu through the junction structure. Many man-years of hard work has become a sad memory.

Thin film II-VI photovoltaics

Polycrystalline thin film CdS/Cu, _ .YSesolar cells of the backwall configuration have been made by the successive deposition of CdS and Cu, _ .YSefilms on ITO-coated glass substrates[ 191. The Cur _ .~Se films were deposited by the vacuum co-evaporation of elemental Cu and Se; Cu,,,Se has an indirect bandgap of 1.4 eV and Cu,,Se has an indirect bandgap of 1.9 eV. Under simulated AMI conditions, a typical device of 1 cm* area had V,, J, and FF of 0.457 V, 18.7 mA/cm* and 63%, corresponding to a conversion efficiency of 5.4%. A 10% conversion efficiency was believed to be attainable by optimizing the composition of Cu,_,Se and reducing the optical losses; however, the stability of CdS/Cu,_,Se solar cells has not been studied. 3.3. CdS/CuInSe2

solar cells

Copper indium diselenide, CuInSe,, is a direct gap semiconductor with a room temperature bandgap energy of 1.04eV and an electron affinity of 4.3 eV. It crystallizes in the chalcopyrite structure with lattice parameters a = 5.782 b; and c = 11.62 A. The (112) face of CuInSe, and the basal plane of CdS have a lattice mismatch of about 1.2%, indicating that CdS and CuInSe, are nearly ideal heterojunction partners. A 12% efficient small area CdS/CuInSe, solar cell was made by the evaporation of CdS on the selenium (112) face of single crystalline pCuInSe,[20]. The first efficient thin film CdS/CuInSe, solar cell was reported in 1981[21], and remarkable progress has been made during the past two years by incorporating CuGaSe, into CuInSe, to increase its V,. CuInSe, based thin films for efficient solar cells have been made by two approaches: (1) the coevaporation of Cu, In and Se [and Ga for Cu(In,Ga)Se,] onto heated MO/glass or MO/ceramic substrates; and (2) the selenization of vacuumdeposited Cu-In on MO/glass substrates. 3.3.1. Solar cells by co-evaporation. The coevaporation technique has been used by many ;nvestigators[22-251. The important parameters affecting the composition and microstructure of deposited films are the flux of elemental vapors at the substrate surface and the substrate temperature. A substrate temperature of 550°C or higher is required for the deposition of device quality CuInSe, based films. The resistivity of p-CuInSe, films can be controlled by adjusting the composition of deposited films. The mechanism of CuInSe, formation by the evaporation technique is reasonably well understood. Brietiy, when Cu and In are co-deposited on heated substrates, Cu is more readily selenized than In, and liquid CuSe is formed at temperatures higher than about 55O’C. The growth of CuInSe, in the liquid environment of CuSe accounts for the enhanced grain structure. The formation of CuInSez takes place according to the equation: yCuSe(l) + In(I) + Se(g) -+ CuInSe,(s) + ( y - l)CuSe(l).

537

The evaporation process is flexible in that the composition of deposited films can be precisely controlled. Device quality CuInSe,-based films have also been produced by vacuum evaporation using a two-step process: formation of In,Se, followed by reaction with Cu,Se,. at temperatures above 550°C. Thus, either reaction path (formation of CuSe or InzSez in the first step) can produce device quality CuInSe, films with similar structural and electrical properties. A thin CdS or Cd,_,YZn,S film (5OOA for example), the heterojunction partner, is deposited onto CuInSe, by evaporation or solution growth. This is followed by the successive deposition of a thin film of lightly doped ZnO (about 5OOA) and an Al-doped ZnO film of 5000-6000 8, thickness; the latter serves to reduce the sheet resistance of CdS or Cd , _,ZnS. The device is completed by the deposition of a metal grid on ZnO. The highest reported total area efficiency of a laboratory CdS/CuInSe, device (0.395 cm2) area is 13.2% with V,,, J, and FF of 0.484 V, 36.29 mA/cm* and 75.1%. respectively[24]. The highest reported aperature area efficiency of a large area module (3890 cm*) is 10.5% with V,, I, and FF of 24.5 V, 2.55 A and 65.4%, respectively[26]. The bandgap energy of CuInSe, can be increased by incorporating Ga or S in order to achieve a more optimum match to the AM1.5 spectrum. Considerable success has been reported by several laboratories[22-251. The highest total area efficiency of a laboratory CdS/Cu(GaIn)Se, device (0.437 cm’ area) is 15.9% with V,, J, and FF of 0.649 V, 31.88 mA/cm* and 76.6%, respectively[25]. 3.3.2. Solar cells by selenization. The formation of large-grain CuInSe, films by the selenization of Cu-In films with elemental selenium on appropriate substrates at high temperatures has been known for 10 years[27]. Thin films CdS/CuInSe, solar ceils have been prepared by: (1) the sequential deposition of Cu and In films on MO/glass substrates by vacuum evaporation; (2) the selenization of the Cu-In layer in an Ar + H,Se atmosphere at 400°C; and (3) the successive deposition of a CdS film by solution growth and a ZnO films by sputtering. The AM 1.5 efficiency of a small area device (0.1 cm’) is about 10.9% with V,,, J, and FF of 0.463 V, 35.4 mA/cm’ and 66.6%, respectively[29]. Solar cells of over 10% active area efficiency have also been fabricated from CuInSe, films prepared by the heat treatment of vacuum-deposited Cu. In and Se films[29]. This technique is being developed for the production of large area CuInSe, modules. 4. CADMIUM

ZINC SULFIDE

Cadmium sulfide is used successfully as a window material for thin film CdTe and CuInSe, solar cells. However, the bandgap energy of CdS is only 2.42 eV, and 0.1 pm of CdS absorbs about 63% of the incident radiation with photon energy greater than the

Ting L. Chu and Shirley S. Chu

538 3.5

3.3 2 J

G

3.1 A-

f+ g

2.9 ]

: e

9 a

.

0

425°C

l

375°C

. .

2.1 . 2.5 I.0

I

I

. I

2.0

3.0

4.0

DMCd/DEZn molar ratio Fig. 1. Bandgap energy of Cd, _ ,Zn,S films as a function of DMCdiDEZn molar ratio.

bandgap energy. Thus CdS films of small thicknesses, 3OOA for example, is used to maximize the photocurrent from the absorber. The energy loss in the

window can also be minimized by using a larger bandgap energy material. The bandgap energy of CdS can be increased by the incorporation of ZnS. CdS and ZnS form a continuous series of solid solutions, Cd, _ ,ZrirS. The bandgap energy of Cd,_,Zn,S can be controlled in the range of the binary bandgaps. Thin films of Cd, _ .YZn,S have been deposited by the vacuum evaporation technique using CdS and ZnS sources[30]; the bandgap energy and lattice parameter of Cd, _ .rZn,S have been found to be essentially a linear function of composition, and the resistivity increases from less than 1 Q. cm for CdS to greater than 10” Q . cm for ZnS. The resistivity of Cd, _ .ZrrYS (up to x = 0.3) has been reduced to about 2 R cm by using In as a dopant during the evaporation process[31]. The incorporation of dopants into Cd, _ ,Zn,S becomes increasingly difficult as the ZnS concentration increases. Thin films of Cd,- .Zn,S have also been deposited by: (1) the sublimation of CdS and ZnS in a hydrogen flow[32]; (2) the reaction of cadmium acetate, zinc acetate and thiourea in an ammoniacal solution[33]; (3) the spray pyrolysis technique using Zn-salt, Cdsalt and thiourea; and (4) metal-organic chemical vapor deposition (MOCVD). The MOCVD process is most flexible in controlling the composition and resistivity of Cd, _ .Zn,S films and will be briefly reviewed here. The MOCVD process makes use of the reaction of dimethylcadmium (DMCd), diethylzinc (DEZn) and propyl mercaptan (PM) in a hydrogen flow. The important parameters affecting the deposition rate and properties of Cd, _ ,Zn,S films are the substrate temperature and the composition and flow rate of the reaction mixture. Figure 1 shows the bandgap energy of Cd,_,Zn,S films deposited at 375 and 425°C as a function of the DMCd/DEZn molar ratio in the

reaction mixture. At each temperature, the variation of bandgap energy with the DMCd/DEZn ratio is more pronounced at low DMCd/DEZn ratios, due to the ease of formation of CdS. At a given composition of the reaction mixture, the film deposited at 425°C has a higher bandgap energy than that at 375”C, indicating the increased extent of ZnS formation with increasing temperature. Figure 2 shows the dark lateral resistivity of Cd, _,Zn,S films as a function of the bandgap energy. The dark resistivity is about IO3R . cm for CdS and increases rapidly as the concentration of ZnS increases, exceeding IO8R. cm at a bandgap energy of about 2.8. The VI/II ratio in the reaction mixture affects the bandgap energy of deposited films but has no measurable effect on the lateral resistivity. The photoconductivity of Cd, .rZn,S films depends strongly on their composition. The photoconductivity ratio in films with bandgap energy greater than about 2.8 eV becomes smaller than unity. The lateral resistivity of Cd, _ ,Zn,S films can be reduced by using trimethylaluminum (TMAl) as a dopant during the deposition process. The extent of dopant incorporation depends strongly on the composition of the solid solution. Figure 3 shows the lateral resistivity of Al-doped Cd, _,Zn,S films deposited at 425°C as a function of the bandgap energy of the films. Aluminum can be incorporated into all Cd, _,Zn,S films; however, the incorporation of Al becomes more difficult as the bandgap energy increases. The photoconductivity ratio also decreases in films with bandgap energy higher than about 3 eV and becomes less than unity at still higher bandgap energies. 5. ZINC OXIDE

Thin films of ZnO have been prepared by sputtering, spray pyrolysis and metal-organic chemical vapor deposition. Sputtered ZnO films usually show a strong c-axis orientation perpendicular to the

109

IO’

2.4

2.6

2.8

3.0

3.2

Bandgap energy (eV) Fig. 2. Dark and illuminated resistivity of Cd, _ ,Zn,S films deposited at 375°C as a function of bandgap energy.

Thin film II-VI

102-

2.1

2.9

3.1

3.3

Energy gap (eV

Fig. 3. Dark and illuminated resistivity of aluminum-doped Cd, ,Zn,S films deposted at 425°C.

substrate. Undoped ZnO films deposited by r.f. magnetron sputtering in an Ar-H, atmosphere has an electron concentration of about 1020cm-3, an electron mobility of about 8cm*V’ SK’, and an average transmission of 90% in the visible region[35]. In the spray pyrolysis technique, an aqueous solution containing ZnCl,, ZnCl, + H,02 or Zn-acetate is used, and the hydrolysis of zinc salt takes place on the heated substrate, depositing ZnO. Good optical quality films with a reasonably high conductivity, lo3 (Q. cm))’ have been obtained[36]. Doped zinc oxide films have been produced by spraying an alcohol-water solution of Zn-acetate and indium chloride onto glass substrates at 300-450°C. The as-deposited films have a resistivity of (8-9) x 10m4R . cm and an average transmittance of 85% in the visible region. By heating in a reducing atmosphere or under reduced pressure, the resistivity of In-doped ZnO films is reduced to (3-5) x lO-‘R .cm. The electron concentration and Hall mobility in doped films are 3 x 1020cm-3 and (5-10) cm* V’ s-‘[37]. The use of MOCVD for the deposition of ZnO has the advantages that device-quality films of controlled carrier concentration can be deposited at reasonable rates (200-2000 Ajmin) at relatively low temperatures (200-500°C). The oxidation of diethylzinc with oxygen[38], alcohol[39] or water[rlO] in an inert atmosphere (He or Ar) is the most commonly used process, where organic fluorine or organogallium compounds may be used as a dopant. The deposited films show a high degree of preferred orientation with c-axis perpendicular to the substrate surface. The doped films have electron concentrations of up to 5 x 1020cm.-3 and Hall mobilities of (10-40) cm* V ’ s- ’ Fluorine-doped films with sheet resistances of 4-5 R/l-J have been found to show visible absorption of only 3%, visible transmittance up to 90%, and infrared reflectance of about 85%. The texturing, or surface roughness, of F-doped ZnO films can be controlled by adjusting the deposition

539

photovoltaics

conditions such as substrate temperature and reactant composition. ZnO films have been used as a heterojunction partner in CdTe solar cells[41]. First generation n-ZnO/p-CdTe solar ceils with ZnO prepared by r.f. sputtering or spray pyrolysis show relatively high photocurrent (J, = 19 mA/cm* under AM2 the low photovoltage simulation). However, (VW= 0.37 V) and fill factor (54%) lead to a conversion efficiency of about 4.6%, considerably lower than expected. ZnO film is used in thin film amorphous silicon (a-Si) solar cells as a transparent electrode to a-Si or a-Si alloy. a-Si solar cells can be of the front wall or backwall configuration: Front wall: Ag/ZnO/a-Si or a-Si alloy pin (two or three junctions)/ZnO/glass (substrate). Back wall: ITO/a-Si or a-Si alloy pin (two or three junctions)/ZnO/Ag/steel (substrate). In addition to transparent electrode, ZnO is used in conjunction with Ag as a reflector for efficient light trapping. 6. ZINC SELENIDE

Because of its potential of emitting blue light, zinc selenide has been under investigation since the early sixties. Bulk crystals and epitaxial films have been grown by direct combination of elemental vapors[42] and by close-spaced vapor transport[43]. However, MOCVD is more successful for the epitaxial growth of ZnSe films of controlled carrier concentration at relatively low temperatures[44,45]. The p-type doping of ZnSe by nitrogen using ammonia as a dopant has also been established[46]. Polycrystalline thin films of ZnSe of high conductivity have been deposited by reactive d.c. magnetron sputtering, where zinc is sputtered in an Ar + H,Se atmosphere[47]. The composition of deposited films can be controlled by the zinc sputtering power, substrate temperature and the partial pressure of H,Se. Shallow donors are introduced by co-sputtering Al, Ga or In or by incorporating In in the zinc target. ZnSe films with resistivity as low as 20 R. cm were deposited at substrate temperatures as low as 12OC. The deposition of ZnSe films of 300-1000 8, thickness onto CuInSe, films was used to produce thin film ZnSe/CuInSe, solar cells; a highly conductive ZnO film was deposited onto the ZnSe film to reduce the sheet resistance. Preliminary results were encouraging. Under AM I .5 conditions, the highest V,. J, and r~ were 0.43 V, 37.4 mA/cm’ and 8.5%, respectively. Thin film ZnO/thin ZnSe,‘CuInSe, solar cells of 3.5 cm? with a 10% efficiency have subsequently been reported[48]. Metal-organic chemical vapor deposition is best suited for the deposition of ZnSe films of controlled carrier concentration. The reaction of diethylzinc

Ting L. Chu and Shirley S. Chu

540

AM 1.5 -*/

,n4

I

lY 300

400

350

so0

450

Growth temperature

600

550

(“C)

Fig. 4. Resistivity of ZnSe films deposited at various temperatures from a DESe/DEZn = 0.7 reaction mixture in the dark and under illumination with an ELH lamp at 100 mW/cm*.

(DEZn) and diethylselenium (DESe) on the substrate surface at 400-500°C in a hydrogen atmosphere has been studied in some detail for the deposition of ZnSe films[49]. All films deposited without intentional doping are n-type and have high electrical resistivity in the dark and under illumination with ELH lamps at 100mW/cm2, as shown in Fig. 4. The lateral resistivity of ZnSe films does not vary appreciably with the deposition temperature, and the illuminated resistivity is always higher than the dark resistivity. All ZnSe films deposited under a wide range of reaction mixture composition and substrate temperature show photoconductivity ratios of less than unity. The photoconductivity ratio of ZnSe films can be increased by the introduction of trimethylaluminum (TMAl) as a dopant. For example, Fig. 5 shows the lateral resistivity of Al-doped ZnSe films,

Dark I--+----

1 0

1

I

I

I

I

0.5

1.0

1.5

2.0

2.5

DESelDEZn molar ratio Fig. 5. Resistivity of Al-doped ZnSe films, in the dark and under illumination, deposited at 500°C using a TMAI/DEZn = 0.03, as a function of DESe/DEZn ratio in the reaction mixture.

in the dark and under illumination, deposited at 500°C from reaction mixtures having a TMAl/DEZn ratio of 0.03 and various VI/II ratios. While the dark resistivity is relatively independent of the VI/II ratios, the illuminated resistivity varies strongly with the VI/II ratio, and the maximum photoconductivity ratio of approx. 2000 is observed at a VI/II ratio of about 0.7. The lateral resistivity of Al-doped ZnSe films may also be controlled by varying the concentration of TMAI. The Al-doped ZnSe films with high photoconductivity ratios are suitable as the “window” for thin film II-VI heterojunction solar cells. Such films of lOOO-2OOOA thickness make negligible contribution to the series resistance of the device under illumination. Thin film ZnSe/CdTe solar cells prepared by depositing CdTe films on ZnSe/ZnO: F/glass substrates showed an open-circuit voltage of 740 mV under AM1 5 conditions, considerably lower than the V, of CdS/CdTe heterojunctions, 840-860 mV. This was ascribed to the high interface state density associated with the large lattice mismatch between ZnSe and CdTe. The reaction between a zinc adduct and H,Se with ethyliodide as a dopant has been used for the deposition of n-ZnSe films at 200-25O”C[50]. Thin film ZnO/ZnSe/CuInSe, solar cells of 0.06 cm’ area showed a conversion efficiency of 10.1% with V,, J, and FF of 0.43V, 39mA/cm2 and 60%, respectively. 7. CADMIUM TELLURIDE

The conversion efficiency of thin film CdTe solar cells has advanced significantly during the past several years; CdS/CdTe solar cells of 1 cm2 area with AMl.5 efficiencies approaching 16% have been demonstrated. At present, thin film CdTe solar cells is the most promising low-cost candidate for terrestrial photovoltaic conversion of solar energy; pilot production of CdS/CdTe modules is in progress at several PV manufacturers. A number of review papers have discussed the various stages of thin film CdTe solar cell development[51-541. The first reported thin film CdTe solar cell has the configuration p-Cu,Te/n-CdTe; an n-type CdTe film was deposited on various substrates from the reaction between Cd, CdI, and Te at 450-500°C and followed by brief treatment with a cuprous chloride solution[55]. However, the most efficient thin film CdTe solar cells are of the n-CdS/p-CdTe configuration. In this section, the device-related issues (ohmic contact, improvement of CdTe microstructure and device characterization), the characteristics of n-CdS/p-CdTe solar cells with CdTe prepared by several techniques (close-spaced sublimation, combination of elemental vapors, electrodeposition, screen printing, spray pyrolysis. metal-organic chemical vapor deposition and vacuum evaporation and sputtering), and device stability are discussed.

Thin film II-VI photovoltaics 7.1. Ohmic contact

One major problem associated with the fabrication of efficient CdS/CdTe solar cells is that the formation of stable, low-resistance contact to p-CdTe is difficult due to its large work function. Various techniques used for the contact formation have been extensively reviewed[56]. Two common approaches to the contact formation are: (1) the use of contact materials with a higher work function than p-CdTe, such as low resistivity p-HgTe[57] and p-ZnTe[58]; and (2) the formation of a p +-region under the contact by the reaction or in-diffusion of the contact material to reduce the barrier height; graphite pastes doped with Cu or Hg salts have been found to be suitable since interlayers of Cu,Te and Hg, _,Cd,Te of high carrier concentrations are readily formed. The surface preparation of p-CdTe also appears to be important, etching with a K,Cr,O, + H,S04 or H,PO, + HNO, solution is usually used to provide a Te-rich surface. 7.2. Improvement in microstructure

of CdTe films

The electrical characteristics of thin film CdTe heterojunctions are affected by the microstructure at the interface and at the grain boundaries in the CdTe film. The structural discontinuities at the interface and the shunting effects of grain boundaries lead to excess junction current resulting in low photovoltages and low fill factors. The efficiency of as-deposited CdS/CdTe solar cells varies strongly with the technique of CdTe deposition due to differences in the microstructure of CdTe films. CdTe films deposited by high temperature techniques, such as close spaced sublimation and direct combination of elemental vapors, show the best microstructure, and as-deposited solar cells of 1cm2 area have shown AMI .5 efficiencies of up to 11%. However, CdTe films deposited by other techniques, such as electrodeposition, MOCVD, etc., have inferior microstructure, and the as-deposited solar cells are characterized by efficiencies of less than 3%. The grain structure can be improved and grain boundaries passivated (at least partially) by treatment with a CdCl, solution followed by heating in an oxygen-containing atmosphere. The use of CdCI, has been known to play an important role in the fabrication of efficient CdS/CdTe solar cells for many years[59]. In the screen printing process, for example, CdCl, is used as a flux for the sintering of CdTe and CdS films. The sintering of CdCl>-containing CdS films at 500°C has been found to produce significant densification and grain growth. The CdCl, treatment was used in the processing of nearly all thin film CdTe solar cells with higher than 10% efficiency[60,61]. The effect of CdCl, treatment is particularly pronounced in poor quality CdTe films such as those deposited by electrodeposition, etc. For example, electrodeposited films have average grain size of a few hundred angstroms, and as deposited CdTe/CdS solar cells show conversion efficiencies on the order of 1% or less. The treatment

541

of such structures with a methanol solution of CdClz followed by heating in air at 400°C increased the solar cell efficiency to nearly 9%. This was due mainly to the increase in grain size of CdTe, the improvement of microstructure in the CdS/CdTe junction region, and the increase in barrier height of the CdS/CdTe junction. Heating in an oxygen containing atmosphere results in the oxidation of CdTe along grain boundaries, and this oxide film serves to passivate the grain boundaries to some extent. The CdCl, treatment also induced a reaction between CdS and CdTe at the interface producing a thin layer of CdS.,Te, _ 1. 7.3. Characterization

of thin jlm

solar cells

To characterize a thin film CdTe solar cell, all important diode and photovoltaic parameters should be evaluated. Shunt resistance (&,), series resistance (I?,), and diode quality factor (A ) are most important parameters affecting the photovoltaic characteristics. These parameters are all illumination dependent. Low Rsh leads to reduction in V, and FF. Large R, results in decreased FF and Vmp.Large A value leads to reduction in V, and FF. The A value of thin film cells is always greater than unity in the dark due to carrier recombination; it increases under illumination and increases with decreasing temperature. R,, and R, can be deduced from the J-V relation of the solar cell at low bias (- 20 to + 20 mV for example) and high bias (l-2 V for example), respectively. From the R,, and R, measured in the dark, A and J, can be deduced from the diode equation. Under illumination, A can be deduced from the photodiode equation using the technique developed by Sites and Mauk[62]. When &,, > 3000 0. cm2, the relation is simplified to: dV/dJ = R, + (AkT/q)(J

+ JL)-‘.

The plot of dV/dJ vs (J + JL)-’ yields slope AkT/q and intercept R,. The equation is more complicated if Rsh cannot be neglected. In addition to Rs,,, R,, and A determined in the dark and under illumination, the measurements of photovoltaic parameters (V,, J,, FF and q), spectral response and capacitance complete the characterization of solar cells. 7.4. Close-spaced sublimation (CSS) 7.4.1. Deposition of CdTe jlms. The deposition of CdTe films by the close-spaced sublimation (CSS) technique is based on the reversible dissociation of CdTe at high temperatures:

2CdTe(s) CI Cd(g) + Tez (g). The source material is maintained at a higher temperature than the substrate. The source CdTe dissociates into its elements, which recombine on the substrate surface depositing CdTe films. The dissociation pressure of CdTe at a given temperature is defined by the relation: &R(T)

= (P&)(PR,)“*

542

Ting L.

Chu and Shirley S. Chu

where the p’s are the equilibrium partial pressures at temperature T. The p”s increase exponentially with increasing temperature. The equilibrium partial pressure of TeZ has been calculated to be 2.6 x IO-‘, 7.8 x 10m6and 1.2 x 10e4 atm at 500,600 and 7OO”C, respectively (the equilibrium partial pressure of Cd at a given temperature is twice that of Te,). Thus, the rate of CdTe deposition depends strongly on the source temperature and the gas pressure in the reaction tube. The deposition of CdTe films by the CSS technique and the properties of deposited films have been studied in detail[63]. The CSS technique involves several interrelated parameters, such as the temperatures of the source and the substrate, the separation between the source and the substrate, ambience in the reaction tube, the pressure in the reaction tube, the composition of the source material, etc. The rate of deposition may be controlled in the range of 0.1-10 pm/min by controlling these parameters. The microstructure of CdTe films is determined by the substrate temperature, source-substrate temperature gradient and the crystallinity of the substrate. In general, the grain size increases with increasing substrate temperature and increasing film thickness. The average grain size is 2-5 pm. X-ray diffraction technique indicates that the CSS CdTe films are of random orientation showing no preferred orientation. The electrical resistivity of p-CdTe films may be controlled by using Cd-deficient or Sb-doped source materials. Low resistivity films, 200-300 R . cm, were deposited at 600°C by using source materials containing lOI Sb atoms or 1019Cd vacancies per cm3. At higher dopant concentrations, the resistivity of CdTe films increases due to self-compensation or other defect interactions. 7.4.2. Solar cells. The CSS technique has been used for the successive in situ deposition of CdS and CdTe films onto ITO-coated glass substrates at 550 and 6OO”C, respectively, for the preparation of thin film CdS/CdTe solar cells[64]. Small area (0.1 cm2) devices showed a conversion efficiency of about 10.5% under simulated AM2 illumination at 75 mW/cm’. The CSS CdTe films were also used in conjunction with vacuum evaporated CdS films for the preparation of thin film CdS/CdTe solar cells[65]. Solar cells of 1 cm2 and larger in area showed conversion efficiencies of 10.5-10.6% under global AMl.5 conditions. Thin film CdS/CdTe solar cells of considerably higher efficiency have been produced by using CSS CdTe films and solution-grown CdS films[66-681. The solution-grown CdS is stoichiometric in composition. However, the solution-grown CdS is structurally porous, and the surface of the CdS film is contaminated with adsorbed impurities such as hydroxides. It is thus essential that, prior to the deposition of CdTe films, the CdS film is heated in H, to remove the oxygen-containing species and

30 r 25

-5

-0.2

0

0.2

0.4

0.6

0.8

1.0

Voltage (V) Fig. 6. Current-voltage characteristics of a thin film CdS/CdTe solar cell under global AM1.5 conditions.

to densify the film. Figure 6 shows the currentvoltage characteristics of a solar cell of about 1 cm2 area under global 1.5 illumination, indicating a 15.8% efficiency. The thickness of CdS and CdTe films were 600-800 A and 4-5 pm, respectively, and about 10008, of MgF, was evaporated onto the glass surface as an anti-reflection coating. The quantum efficiency of this solar cell is shown in Fig. 7, where the good photoresponse at wavelengths below 500 nm is associated with the use of a thin CdS film. The relatively high conversion efficiency of CdS/CdTe solar cells appears to be related to the interface reaction between CdS and CdTe. The interdiffusion of CdS and CdTe during the deposition of CdTe at high temperatures results in the formation of CdS,Te,_ 1 which shifts the electrical junction from the metallurgical interface into CdTe, thus improving the electrical characteristics of the junction. The electrical properties of CdS/CdTe heterojunctions have been extensively investigated[69,70].

Wavelength Fig. 7. The quantum

(pm)

efficiency of the CdS/CdTe shown in Fig. 6.

solar cell

Thin film II-VI

The interface cleanness is an important factor affecting the carrier transport across the junction. When the surface of CdS is treated in situ with hydrogen at 400°C before the deposition of CdTe, carrier injection and carrier recombination in the space charge region dominate the current flow. Without in situ cleaning of CdS, tunneling becomes significant near room temperature. Because of the success and ease of scaling-up of the CSS process, Solar Cells, Inc. (Toledo, Ohio) has a program to produce 60 x 120cm photovoltaic modules using successive in situ deposition of CdS and CdTe films on SnO,-coated soda lime glass by a modified CSS process[7 1,721. The major processing steps are: (1) inspection and cleaning of glass superstrate; (2) deposition of CdS; (3) deposition of CdTe; (4) post deposition heat treatment; (5) deposition of Ni-Al contact to CdTe; (6) bus bar application; (7) lead attachment; (8) module testing; and (9) encapsulation. (Three laser scribes at different stages of processing were used for the division of a large area cell and subsequent interconnections.) Conversion efficiencies of 10.5 and 7.4% have been achieved on 0.22 cm’ devices and 7200 cm2 modules, respectively (a 7200 cm2 solar module produces about 53 W of electrical energy under AM15 at 100 mW/cm’ irradiance). Although the throughput rate of the equipment was about 100 kW/yr, the processes are fully compatible with a high throughput automated production line. On an automated line, a production rate of one module per min appears feasible. 7.5. Combination of vapors of elements 7.5. I. Deposition process. The direct combination of Cd and Te, vapors is a flexible technique for the deposition of CdTe films on heated surfaces of substrates in a gas flow system[73]. This technique has the advantages that: (1) a wide range in thickness from several micrometers to hundreds of micrometers can be achieved and controlled; (2) the stoichiometry of deposited films can be precisely controlled; and (3) the dopant concentration and distribution in the deposit can be controlled. Hydrogen or helium is usually used to carry Cd and Te vapors to the surface of the heated substrate where they react to deposit CdTe. The deposition rate of CdTe films is determined by the partial pressures of Cd and Te in the reaction mixture and the substrate temperature; the observed rates are in qualitative agreement with those calculated from the dissociation constants of CdTe. The substrate temperature is also important in that the rate of nucleation and the thickness at which the films become continuous are determined, in part, by the substrate temperature. The use of low substrate temperature facilitates initial nucleation, providing a high density of nuclei, and the film becomes continuous at relatively small thicknesses. However, the average grain size is smaller at lower substrate temperatures.

photovoltaics

543

The composition of deposited films is determined by the Te/Cd molar ratio in the reaction mixture since the elements are soluble in CdTe. The nearly stoichiometric films can be p-type (slightly Cd deficient) or n-type (slightly Te deficient). Empirically, one may determine the composition of the reaction mixture required for the deposition of stoichiometric CdTe films. When the Te/Cd molar ratio in the reaction mixture is increased slightly (less than 1%), the deposit becomes p-type, and n-type CdTe results when the Te/Cd molar ratio was reduced slightly. This transition from p-type CdTe to n-type material occurred over a very narrow range of the Te/Cd molar ratio in the reaction mixture. The nearly stoichiometric n-type and p-type films show high resistivity at room temperature, about lo4 Sz. cm, and the resistivity decreased exponentially with increasing temperature. The resistivity of p-CdTe films may be controlled within a limited range by using reaction mixtures containing Te/Cd molar ratios higher than those required to yield stoichiometric CdTe[74]. The roomtemperature resistivity of nearly stoichiometric p-CdTe deposited at 580°C is about 104Q. cm. As the Cd partial pressure in the reaction mixture is reduced, the resistivity of the film first decreases owing to increased Cd vacancy concentration and reached a minimum, about 200 R. cm, at a Te/Cd molar ratio of about 1.15. Lower resistivity is obtainable at higher substrate temperatures owing to the higher defect concentration. As the Cd partial pressure in the reaction mixture is further reduced, the resistivity increased owing presumably to the formation of defect complexes or self-compensation. At higher Te/Cd molar ratios, the deposited films may have a resistivity as high as lo* fi. cm. The electrical resistivity of p-CdTe films can also be controlled by the addition of dopants (PH, or ASH,) to the reaction mixture during the deposition process. The resistivity vs AsH,/Cd ratio relation also shows a minimum, about 200 R. cm, at a AsH,/Cd ratio of about 2 x 10-l. 7.5.2. Solar cells. The direct combination method is well-suited for the deposition of thin film solar cell structures, since the substrate surface can be cleaned or etched in situ and the resistivity profile of p-CdTe films can be controlled through intrinsic or extrinsic doping. The use of Cd vapor and Te vapor has produced solar cells of the front wall configuration with conversion efficiencies of up to 14% by the atomic layer epitaxy techniquej751. The elemental vapor combination technique is believed to be bestsuited for the manufacture of high efficiency thin film CdTe solar cells. It has the potential to produce 85 W modules on 7200 cm2 substrates, corresponding to a total area efficiency of I I .8%[76]. 7.6. Electrodeposition The electrodeposition of CdTe has been developed to become a promising method for producing efficient

Ting L. Chu and Shirley S. Chu

544

thin film solar cells. This technique is relatively simple in principle. CdTe films of well-defined composition can be deposited cathodically from an acid solution of a cadmium salt (such as CdSO,) and TeO, (HTeO: being the principal species in solution) according to the following reaction: HTeO$ + 3H+ + 4e- + Te + H,O Cd’+ + Te + 2e- + CdTe. CdS and TCS coated glass substrnte is used as the cathode. Both reactions take place simultaneously at cathode potentials (vs the standard calomel electrode) between -0.2. and -0.65 V, just below the deposition potential of metallic Cd. The concentration of HTeOz in solution, limited by the solubility of TeO,, is small and is replenished by switching a Te anode into the circuit. The important process parameters are stirring rate, concentration of HTeO: and temperature of solution. Since the deposition rate of CdTe is limited by the concentration of HTeO$ in the cathode region, the deposition rate is relatively slow, 1-2 pm/h. Dopants can be incorporated into electrodeposited CdTe films by co-electrodeposition or electromigration. Without intentional doping, the as-deposited films are n-type and are converted to p-type by heat treatment[77]. The use of electrodeposited CdTe films for thin film solar cell fabrication has been under investigation for over I5 years. Continued progress has led to the development of heterojunction solar cells with credible efficiencies. High efficiency n-CdS/p-CdTe solar cells have been reported by a number of investigators with CdS films deposited electrolytically or pyrolytically on SnO,/glass substrates[78,79]. Solution-grown CdS films have also been used in conjunction with electrodeposited CdTe films for the preparation of large area thin film CdTe solar cells at BP Solar[80,81]. Solar cells of 1 cm2 area showed a conversion efficiency of 12.7% (V, = 807 mV, J, = 23.8 mA/cm’ and FF = 0.66). and a 30 x 30 cm device consisting of 35 solar cells in series had an aperture area efficiency of 10.1% (active area efficiency of 11.2%, J, = 790 mV, J, = 20.3 mA/cm’ and FF = 0.63). The active area efficiency changes from 12.7 to 11.2% when the area is increased by 700 times, indicating good uniformity of material properties. The efficiency values from plate to plate are reasonably reproducible. The thin film CdTe solar modules are highly reliable under simulated operating conditions. 7.7.

Screen

printing

The screen printing technique utilizes the application of a paste of electronic material through a screen onto a substrate followed by heat treatment. It is a low cost process and has been used successfully for the fabrication of large area thin film CdS/CdTe solar cells. In this process, a CdS film is deposited on

a borosilicate substrate by screen printing of a paste of CdS, CdCI, (flux), and propylene glycol (binder), followed by drying and sintering in a nitrogen atmosphere at about 700°C in a belt furnace. During the sintering process, grain growth of CdS occurs via recrystallization from the flux, and the resulting film, usually 20-30pm in thickness, has grain size of 20-30pm and resistivity of 0.2-0.5 Q .cm. The effects of thickness and sintering conditions of CdS films on the photovoltaic properties of CdS/CdTe solar cells have been studied in detail[82]. The paste for the screen printing of CdTe films consists of an equimolar mixture of Cd and Te (or CdTe) with CdCI, as flux and propylene glycol as binder. This paste is applied to screen-printed CdS/glass structures and sintered at 590-620°C; CdTe is formed by the reaction between Cd and Te with subsequent grain growth. Further, the interface reaction results in the formation of CdSCdTe solid solutions, CdS,Te, ,, and reduces the effects of mismatch between CdS and CdTe to some extent. The important parameters of the screen-printing process are the screen size, composition of the pastes, sintering time and temperature. The short wavelength response of screenprinted CdS/CdTe solar cells is poor because of the large thickness of CdS which absorbs essentially all radiation with wavelength below 500 nm. whereas the photoresponse cut-off wavelength is extended beyond 850 nm due to the interface reaction. Screen-printed CdS/CdTe solar cells manufactured by Matsushita Battery Industrial Co. (Osaka, Japan)]831 have been used in consumer electronics for several years. Conversion efficiencies of screenprinted CdS/CdTe solar cells have been reported to be 11.3% ( V,,, = 797 mV, J,, = 21. I mA/cm’ FF = 0.67 under AMl.5, 100 mW/cmZ)[84] and 8.7% (V,=52.1V,1,=313mA,FF=0.64underAMl5. lOOmW/cm’)[85] for devices of I and 1200cm’. respectively. The reaction between CdS and CdTe during the sintering process has been studied in detail[86]. Significant interdiffusion of S and Te occurs due to the use of CdCl, as a flux, particularly at a high CdTe sintering temperature and high CdClz concentration in the Cd + Te paste. A CdS, ,Te, phase is formed in the CdS layer, degrading the transmission of short wavelength radiation through the window. The absorber contains CdTe, ,S,, reducing the bandgap energy of CdTe. Thus, the spectral response of the solar cell is determined by the optical modifications of the window and absorber layers. Encapsulated CdTe modules show good stability under various loading conditions in outdoor tests. Without water-proof encapsulation, however, gradual degradation of V,, and Is, has been observed. Z 8.

Spray

pyrolysis

Pyrolytic spraying is a truly low-cost technique for the fabrication of large area CdS/CdTe solar cells. In the earlier work, the spraying of an aqueous

Thin film II-VI photovoltaics solution of Cd and Te salts onto a heated substrate was developed for the deposition of CdTe films[87].

Thin film CdS/CdTe solar cells prepared by the successive spray deposition of CdS and CdTe onto IT0 coated glass showed AM1 efficiencies of up to 4%. The use of a CdTe slurry instead of Cd and Te salts has produced solar cells of significantly higher efficiencies[88,89]. The cells are of the conventional configuration and films of SnO,, CdS and CdTe are deposited successively on float glass substrates by spraying. Tin oxide is deposited at 500°C from an aqueous solution of tin dichloride and ammonium bifluoride. The SnO, films have a thickness of 3900 A, a sheet resistance of 7 Q/n and a resistivity of 2.7 x 10m4R. cm. CdS films are deposited at 375-400°C from an aqueous solution of CdCl, and thiourea. Highly transparent CdS films of up to 0.8 pm thickness can be obtained. CdTe films are deposited from a slurry of CdTe and are about 6 pm in thickness. The resulting structure is heated in a closed system to improve the microstructure of CdTe and the CdS/CdTe interface properties. To complete the device, a doped graphite electrode of about 10 pm thickness is deposited on the surface of CdTe, and the contact to SnO, is evaporated Sn of about 1 pm thickness. The best small area (0.3 cm*) device has an efficiency of 12.7% (V, = 799 mV, J, = 26.2 mA/cm* FF = 60.5%). Many small area devices show .Z, higher than 26 mA/cm*. Modules of 1 ft* area have aperature efficiencies of about 8% and active area efficiencies of about 9%. For example, the characteristics of a module of 825 cm2 aperature area and 719 cm2 active area are: V, = 21.1 V, Z, = 0.601 A and FF = 51.6%, aperature area efficiency: 7.9%, and active area efficiency: 9.1%. A module of 4 ft* area show an output of about 27 W. These modules have been subjected to life testing, and no significant degradation was observed. Golden Photon Inc. (Golden, Colorado) has recently constructed a CdTe photovoltaic module manufacturing facility using the spray technology. The throughput rate of this facility is 2mW per year[90]. 7.9 Metal-organic

chemical

vapor

deposition

(MOC VD)

deposition Metal-organic chemical vapor (MOCVD), widely used for the epitaxial growth of compound semiconductors, has also been applied to the deposition of polycrystalline CdTe films[91-931. CdTe films can be deposited, at rates of up to 4pm/b, onto glass or CdS/SnO,/glass substrates at 350-400°C by the reaction between dimethylcadmium (DMCd) and diisopropyltelluride (DIPTe) in a hydrogen atmosphere. The deposited CdTe films are adherent to the substrates and are polycrystalline consisting of densely-packed columnar grains with an average

545

grain size of about 1 pm. The conductivity type of CdTe films is determined by the DMCd/DIPTe molar ratio in the reaction mixture. The nearly stoichiometric films can be p-type due to Cd vacancies or n-type due to Te vacancies. At DMCd/DIPTe molar ratios of about 0.5 or lower, the deposited films are all p-type, the deposited films become n-type at higher DMCd/DIPTe ratios. The change in conductivity type of nearly stoichiometric films takes place over a narrow range of the reactant composition. The dark lateral resistivity of all CdTe films has been found to be lo’-lO*R. cm, with only slight dependence on the DMCd/DIPTe molar ratio in the reaction mixture. The resistivity of MOCVD CdTe films can be reduced by using triethylgallium (TEGa) and arsine (ASH,) as the n- and p-type dopant, respectively. Since gallium occupies the cadmium position in the CdTe lattice, the incorporation of gallium is facilitated by the presence of a high concentration of cadmium vacancies, i.e. by using a reaction mixture with a small DMCd/DIPTe molar ratio, which would yield p-type CdTe without intentional doping. Similarly, a large DMCd/DIPTe molar ratio should be used for the deposition of arsenic-doped CdTe films. Figure 8 shows the dark and illuminated lateral resistivity of gallium-doped CdTe films as a function of the concentration of TEGa in the reaction mixture. The resistivity of CdTe films is significantly reduced by the addition of (l-2)% of TEGa into the reaction Capacitance-voltage measurements of mixture. Ag/CdTe(Ga)/SnO,/glass structures indicate carrier concentrations of ( 1015-2 x 1016)cmm3, depending on the concentration of TEGa. Figure 9 shows the dark and illuminated lateral resistivity of arsenic-doped CdTe films as a function of the concentration of ASH, in the reaction mixture. The incorporation of arsenic into CdTe is highly ineffective compared with the gallium incorporation. Since ASH, is thermally unstable. it is likely that a major fraction of incorpor-

IO’

106

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105

B 3 ‘:

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TEGalDMCd molar ratio (x IO’) Fig. 8. Lateral

resistivity

of Ga-doped

CdTe films.

546

Ting L. Chu and

ated arsenic atoms are electrically inactive owing to compensation and complex formation. Thin film CdS/CdTe solar cells were prepared from MOCVD CdTe films deposited on MOCVD or solution grown CdS films. Preliminary results indicate that solar cells of I cm* or large area showed open-circuit voltage, short-circuit current density, and fill factor of 0.813 V, 19.6 mA/cm*, and 62%, respectively. It is believed that the photocurrent and fill factor can be significantly improved by optimizing the process parameters. 7.10. Vacuum evaporation and sputtering Vacuum evaporation[94] and sputtering techniques[95] have also been used for the fabrication of thin film CdS/CdTe solar cells. By using CdCI, treatment after the deposition of CdTe film, small area solar cells with (IO-1 I)% conversion efficiency have been produced. However, these techniques are unlikely to be used in manufacturing because of the high cost involved. 7. II. Stability of CdSjCdTe

solar cells

Thin film CdS/CdTe solar cells are inherently stable under ordinary operating conditions since their fabrication always involves the use of relatively high temperatures, such as the 400°C CdClz treatment. The concentration and distribution of impurities and defects established at high temperatures are unlikely to change at operating temperatures of 60-80°C. In practice, however, instabilities have been observed due to the lack of proper junction passivations and device encapsulations. When a thin film CdTe solar cell is divided for series-parallel connections, the CdS/CdTe junction surface becomes exposed. Ionic impurities are always present on the junction surface. In a humid environment, water vapor tends to be adsorbed at

Shirley S. Chu

the junction surface, and the ionic species become mobile. The electric field at the junction and the forward biasing of the junction under illumination promote the migration of ionic species across the junction surface. The surface current varies with the humidity in the environment and adds to the other components of the dark current at the junction. When the junction surface is protected from the environment, the surface ions are essentially immobile. Another factor affecting the instability is related to contact degradation. For example, water vapor in the high-humidity environment may react with the contact material and diffuse to the region under the contact. The contact resistance is therefore increased, resulting in device instability. Effective encapsulation is essential to minimize or to eliminate the environment-related degradations.

8. CADMIUM ZINC TELLURIDE

The conversion efficiency of a single-junction polycrystalline thin film solar cell is limited to about 18%. It was thought that by using two solar cells from direct gap semiconductors of appropriate bandgap energies in tandem, the overall conversion efficiency can be improved significantly. The optimum bandgap energies for the upper and lower cells are I .6-l .8 eV and I .0-l. 1 eV, respectively. The CdZnS/CuInSez heterojunction cell is a promising candidate for the lower cell. The upper cell may be prepared from a number of solid solutions of II-VI compounds. For example, CdTe and ZnTe (E, = 2.25eV at 300K) form a continuous series of solid solutions, and the composition of cadmium zinc telluride (Cd, _ ,Zn,Te) may be adjusted to yield the desired bandgap energy. The bandgap energy of Cd, .Zn,Te at 300 K as a function of composition is given by the relation[96]: E,(eV) = (1.510 + 0.005) + (0.606 + 0.010)~ + (0.139 * 0.010)X~.

10’

106 2

.-

AM 1.5 _________~_

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3

0

5

IO

AsH3/DIPTe

15

20

25

30

molar ratio (x 103)

Fig. 9. Lateral resistivity of As-doped

CdTe

films.

In addition to tandem solar cells, Cd, .Zn,Te has many potential applications in solid state devices. For example, the addition of a few atomic percent of ZnTe to CdTe strengthens the bond in CdTe[97], and Cd, ,Zn,Te is better suited as a substrate for the epitaxial growth of mercury cadmium telluride. Cadmium telluride can be doped to show both n- and P-type conductivity; however, ZnTe always exhibits P-type conductivity due to a high degree of self compensation of incorporated donors by native defects. Cd-rich Cd, _ ,Zn,Te can be prepared in both n- and p-type forms; at x > 0.6, only p-type material can be prepared. Polycrystalline Cd, ,Zn,Te films with a bandgap energy of about 1.7eV for tandem solar cells have been deposited on CdS/SnO,/glass substrates by the molecular beam technique using elemental

Thin film II-VI photovoltaics

n 1.6

1.7

1.8

I .9

2.0

Energy (eV) Fig. 10. Optical absorption coefficients of Cd, _ ,Zn,Te films with different concentrations of ZnTe.

sources[98]. The grown films have a uniform composition and sharp interfaces. However, solar cells processed by using the standard CdTe cell fabrication procedure (treatment with CdCl, at 400°C in air) resulted in about 3.6% efficiency, high series resistance, and a bandgap energy reduction to 1.55eV. Without the CdCl, treatment, solar cells from 1.7 eV Cd,_ ,Zn,Te films had efficiencies of about 1%. Polycrystalline Cd, ,Zn,Te films of various stoichiometries have also been prepared by heating sequentially deposited thin layers of Cd, Zn and Te on a substrate[99]. Small area Cd,,,Zn, , Te/CdS solar cells had a conversion efficiency of about 3%. MOCVD is better suited for the deposition of Cd, ,Zn,Te films. The reaction of dimethylcadmium (DMCd), diethylzinc (DEZn) and diisopropyltellurium (DIPTe) in a hydrogen flow has been used, and the resistivity of p-Cd,,,Zn,,,Te films controlled by using arsine as a dopant[lOO]. The important process parameters are the substrate temperature and the flow rates of hydrogen through DMCd, DEZn, DIPTe and the reaction tube. At a substrate temperature of 400 C, the deposition rate of Cd, _ ,Zn,Te films depends on the composition and flow rate of the reaction mixture. Using flow rates of DMCd, DEZn and DIPTe at (1.2-4.5) x 10e5. (0.6443.2) x 10e5 and (2.7-4.9) x 10m5mol/min, respectively, the deposition rate of Cd, ,Zn,Te films was 2-4 pm/h. The bandgap energy of Cd, ~, Zn,Te films at room temperature, determined by optical absorption measurements, is shown in Fig. 10. where the a’vs the photon energy plot for several Cd,- ,Zn,Te films with different concentrations of ZnTe is given. All Cd, ,Zn,Te films deposited with no intentional doping are of p-type conductivity. The dark lateral resistivity of all films is IO’-10’ R . cm, essentially independent of the composition of the film, and can be reduced by adding ASH, to the reaction mixture. Figure I I shows the lateral resistivity of Cd,,Zn, ?Te films as a function of ASH, /DIPTe

547

molar ratio in the reaction mixture. The further addition of ASH, does not reduce significantly the lateral resistivity. Under illumination with ELH the lateral resistivity is lamps at lOOmW/cm*, reduced by a factor of 100 or more, also shown in Fig. 11. This reduction in resistivity may also be associated with the reduction of potential barriers at grain boundaries. The Cd, _,Zn,Te films with higher ZnTe content can be more readily doped; As-doped ZnTe films deposited under similar conditions have resistivities as low as 10 R. cm. Thin film Cd, _ ,Zn,Te heterojunctions solar cells have been prepared by the successive in situ depositions of 2-3 pm of a Cd,_,Zn,Te film and 0.1-0.2 pm of a p+-ZnTe film on a CdS/SnO,/glass substrate; the p +-ZnTe or Cd,-, Zn,S/SnOJglass film was used as an ohmic contact to Cd, _ ,ZnTe. These devices usually show low shunt resistance and high saturation current density (about lo-’ A/cm2), thus limiting their open-circuit voltage. The opencircuit voltage, short-circuit current density and fill factor of a thin film Cd,,Zn,,,S/Cd,,,Zn,2,Te solar cell are 665 mV, 8 mA/cm’ and 42%, respectively, corresponding to a conversion efficiency of 2.25%. The photocurrent is considerably lower than expected from the bandgap energies of Cd,,,Zn,,, S/Cd,,, Zn0,2,Te. The current loss is associated with the carrier recombination in the iunction region and in Cd, _ ,Zn,Te near the junction due to high density of intragrain defects associated with the large difference in the lattice parameters of CdTe (a,, = 6.481 A) and ZnTe (a0 = 6.104 A). 9. CONCLUSION

Extensive research and development during the past 20 years have shown that the polycrystalline thin film structure of CdS/CdTe is well-suited for terrestrial photovoltaic systems. At present, the AMl.5 conversion efficiency of laboratory CdS/CdTe solar

lo7r.

IYk

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I

I

I

5

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lb 20

I 25

molar ratio (x 104)

1I. Lateral resistivity of Cd,,,Zn, ,Te films as a function of ASH, concentration

in the reaction

mixture.

548

Ting L. Chu and Shirley S. Chu

cells is nearly 16% and that of CdS/CdTe modules of 4-8 ft2 area is almost 8%. Further refinement in the manufacturing technology will lead to the production of thin film CdTe photovoltaic modules with conversion efficiencies of 12% or higher. Because of low material costs and relatively simple fabrication processes, thin film CdS/CdTe modules can be produced at less than $1 per peak watt. Other II-VI thin film structures may also be promising for photovoltaic conversion of solar energy; however, considerable work is needed. REFERENCES 1. T. L. Chu, Shirley S. Chu, C. L. Lin and Y. C. Tzeng,

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