Cadmium-telluride - Material for thin film solar cells

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Cadmium-telluride—Material for thin film solar cells Dieter Bonnet ANTEC GmbH, Industriestr. 2, D-65779 Kelkheim, Germany

Peter Meyers ITN Energy Systems, 12401 West 49th Avenue, Wheatridge, Colorado 80033 (Received 2 March 1998; accepted 2 March 1998)

Due to its basic optical, electronic, and chemical properties, CdTe can become the base material for high-efficiency, low-cost thin film solar cells using robust, high-throughput manufacturing techniques. CdTe films suited for photovoltaic energy conversion have been produced by nine different processes. Using n-type CdS as a window-partner, solar cells of up to 16% efficiency have been made in the laboratory. Presently five industrial enterprises are striving to master low cost production processes and integrated modules have been delivered in sizes up to 60 3 120 cm2 , showing efficiencies up to 9%. Stability, health, and environmental issues will not limit the commercial potential of the final product. The technology shows high promise for achieving cost levels of $0.5yWp at 15% efficiency. In order to achieve this goal, scientists will have to develop a more detailed understanding of defect chemistry and device operation of cells, and engineers will have to develop methods for high-throughput manufacturing. I. INTRODUCTION: MATERIALS FOR THIN FILM SOLAR CELLS

Photovoltaics (solid state conversion of light to electricity) has the potential to become a major source of energy and to have a significant and beneficial effect on the global environment. In order for photovoltaics to realize that potential, PV modules will have to be manufactured in quantities measured by square kilometers at costs below 100 $ym2 . This means that mass production on an industrial scale has to be established. PV modules must become standardized products that are inexpensive, durable, and efficient. After the early discovery of the photovoltaic effect in solids more than 100 years ago, and its initial use as a light meter, the suitability of PV for energy conversion has been realized with the advent of the space age and the requirement for long-lasting, self-sufficient energy sources for space vehicles. Silicon semiconductor technology being fairly mature due to its increasing application in the electronic industry, it was natural that the first PV cells would be made of silicon. Over the past 40 years silicon PV technology, along with silicon electronics technology, has become increasingly advanced. From the beginning, however, it has been clear that the low optical absorption coefficient (ø104 cm21 ) and low energy bandgap (1.1 eV) were not ideal for solar PV power generation, and scientists have been developing materials whose electrical and optical properties are better suited for high conversion efficiency and low cost. Due to the advancing mastering of silicon semiconductor technology, though, silicon has been used for photovoltaic conversion with increasing success. 2740

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Until today only three additional semiconductors have shown a definite potential for replacing silicon as the primary material used for PV power generation: amorphous silicon, CdTe, and Cu(In, Ga)Se2 . Various additional materials including Se, Cu2 S, Cu2 O, InP, CdSe, Zn3 P2 , and GaAs—highly suitable from a scientific base—have been studied, but, due to disappointing results or high cost, are no longer being intensely investigated. The search for the ideal PV material is being actively pursued on many fronts, however, and the discussion that follows describes the status and prospects of one major candidate material: CdTe. II. BASIC MATERIALS REQUIREMENTS FOR LOW-COST SOLAR CELLS

Thin film solar cells are large area diodes tailored to enable and maximize the absorption of light within a short distance from the space charge region of a diode. The absorbed photons create electron-hole pairs. The energy of the excited (minority) carrier is converted into electric energy as it is swept through the potential field of the diode. The separation from its opposite (majority) charge carrier develops a voltage, which can drive a current through an external circuit, such as an electric engine. As a minority carrier device, a solar cell requires good electronic base properties, mainly high minority carrier lifetime and mobility, which can be achieved only by good crystalline properties, suitable doping, and contacting. Although there are several varieties of solar cells, the following general description applies most directly to thin film solar cells in which the diode is created by two materials designated as the window layer and the absorber. The field region is generated at the  1998 Materials Research Society

D. Bonnet et al.: Cadmium-telluride — Material for thin film solar cells

interface between window and absorber. In our case CdTe is the absorber. The material requirements of an efficient solar cell are determined by the basic steps involved in the conversion of sunlight to electric power: A. Light absorption

The first requirement is that light reaches the region of the rectifying semiconductor junction. Light is incident through a window layer which is designed to minimize absorption before the light reaches the absorber by having a wide bandgap (.2.4 eV) and by being as thin as possible. The basic properties of optical absorption in a semiconductor are determined by the energy gap of the material. For the given solar spectrum on earth the optimum energy gap lies between 1 and 1.8 eV, ruling out such semiconductors as Ge and CdS. The absorption strength of most semiconductors considered for solar cells is quite high–around or above 105 cm21 . High optical absorption in the absorber allows thin film solar cells of a few (0.5 to 5.0) mm thickness to be made. Of the well-known semiconductors only silicon is characterized by a low average absorptivity over the solar spectrum. This requires cell thicknesses of 50 to 100 mm. It should be noted that the maximum built-in voltage of a junction (discussed below) is limited by the band gap of the absorber, i.e., the higher the band gap the higher the potential cell voltage. On the other hand, photons are absorbed only when their energy is greater than the band gap of the absorber; thus wide band gap materials generate less current than more narrow band gap materials. Based on this consideration and the distribution of photon energies within the solar spectrum, the optimum band gap energy for an absorber is about 1.5 eV. B. Generation of mobile minority charge carriers

Sunlight absorbed by the semiconductor generates electron-hole pairs, i.e., excess charge carriers; in solar cells, the critical charge carriers. The lifetime of these carriers has to be sufficient to enable these carriers to reach and transverse the field region of the collecting diode used. As in most semiconductors electrons have longer lifetimes than holes, it is advantageous to use p-doped semiconductors as absorbers, in which electrons are minority carriers. C. Charge carrier separation by built-in electric field

Minority charge carriers transfer their potential energy into a voltage coupled to an external circuit by transversing a field region, typically generated by a p-n junction. The absorbing semiconductor must therefore

be capable of forming a p-n junction, either as a homojunction (doping only)—such as used in crystalline silicon— or as a heterojunction composed as a junction between p and n-type semiconductors of different chemical composition. The two materials forming the heterojunction must not only have appropriate electron affinities and energy band gaps to create the internal electric field, but also must have chemical and structural properties suitable for excess carriers to transverse the internal field. D. Current output by ohmic contacts

Once it has transversed the electric field region, the minority charge carrier becomes a majority carrier, which has to leave the semiconductor in order to perform useful work. Thus low loss electrical contacts are required between both the p-type and n-type semiconductors and their respective electrodes. These electrodes may be metals or light transmissive degenerate semiconductors. In practice low loss contacts are often achieved by creating a thin, heavily doped or degenerate region between the semiconductor and the electrode. E. Production technologies for low cost

In addition to meeting the electrical and optical requirements discussed above, solar cells must be made in a manner that allows for production of large areas at low cost. Thus researchers have concentrated on methods consistent with the use of low-cost substrates and robust deposition processes which can produce solar modules of high power conversion efficiency at high yield. In the following sections we will discuss the extent to which CdTe thin film solar cells fulfill these requirements and the strategies presently being employed to produce further improvements so that CdTebased thin film solar cells can become a low-cost, high-performance, readily available commercial product. III. CdTe AS A BASE MATERIAL FOR THIN FILM SOLAR CELLS

From its basic physico-chemical properties CdTe is an optimum material for use in thin film solar cells. A. Energy gap

CdTe has an energy gap of 1.45 eV, very well suited to absorb the solar light spectrum. The energy gap is “direct,” resulting in an absorption coefficient for visible light of .105 cm21 so that the absorber layer need only be a few mm thick to absorb .90% of light above the band gap. B. Thermodynamic properties

The phase diagram of CdTe is depicted in Fig. 1.1 It is one of the simplest ones to read; above 500 ±C the stoichiometric compound is the stable solid phase. In

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FIG. 1. Phase diagram of CdTe.1

addition, the constituting elements have a significantly higher vapor pressure than the compound. In the high temperature phase a slight nonstoichiometry (too small to be seen in Fig. 1) is present in the form of a slight Cd deficiency. This perturbation, probably in the form of Cd vacancies, leads to a native p-doping of films. This is fortunate, in that when the absorber, in our case CdTe, is p-type, electrons with their higher mobility are the photo-generated minority carriers. This property makes it relatively easy to produce CdTe films suited for thin film solar cells; no excessive care has to be taken in preparing CdTe films as long as the substrate temperature is sufficiently high. CdTe or Cd 1 Te can be used as starting material. The only requirement is absence of disturbing impurities, which might jeopardize the doping. In practice the compound can be prepared easily in sufficiently high purity, as the elements— Cd and Te—can be easily purified by standard metallurgical processes. C. Crystal lattice

The natural crystal lattice of CdTe (Fig. 2)—formally being cubic—de facto is hexagonal; if viewed perpendicular to the direction of the cubic 111 axis, it appears to consist of stacked planes of hexagonally packed alternating Cd and Te layers. In most cases of film deposition these planes tend to lie on the substrate (the 111 axis being perpendicular to the substrate), leading to columnar growth of crystallites. In many cases quite large crystallites (up to 10 mm in diameter) will grow. Due to its high ionicity, CdTe crystallites will have quite well-passivated grain boundaries. Table I compares the intensities of typical XRD peaks for randomly oriented material (powder) and a film made by close-spaced sublimation. D. Growth and doping of films

As can be deduced from the phase diagram of Fig. 1, CdTe upon heating sublimes congruently, liberating Cd 2742

FIG. 2. Crystal lattice of CdTe.1 TABLE I. Relative intensities of the characteristic x-ray diffraction peaks for CdTe films by CSS and expected intensities for randomly oriented grains.4 hkl

Random grain orientation

CSS films

111 220 311 400 331 422

100 60 30 6 10 10

100 0 1 0 2 2

and Te in equal amounts, the residue remaining stoichiometric CdTe. Upon arrival of Cd and Te on the substrate even in a non 1 : 1 ratio, CdTe condenses stoichiometrically as long as the substrate is heated above approximately 400 ±C. In many cases films deposited at lower temperatures and therefore not necessarily at stoichiometric ratio can be heated to create the single phase stoichiometric compound. This allows numerous film deposition technologies to be applied. CdTe can be doped substitutionally using P or As, but, as the material grows natively p-doped in thin film form, no additional doping has to be introduced. Oxygen is not a critical impurity. It may even enhance p-doping.2 E. Stability

Due to the strong ionicity (72%) of the material,3 the energy of any photon in the solar spectrum is lower than the energy of the chemical bonds (.5 eV) in CdTe or CdS. The strong bonding leads to an extremely high chemical and thermal stability, reducing the risk of degradation of performance or any liberation of Cd to a

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very low level. No degradation intrinsic to the material can be expected. IV. CdTe THIN FILM SOLAR CELLS A. CdTe film deposition technologies

Given the above generous properties of CdTe as thin film material, a general recipe for production of films for thin film solar cells essentially includes the following requirements: (i) Deposit CdTe or/and Cd and Te onto a substrate and (ii) Keep the substrate during this process at a temperature above 400 ±C or afterwards perform a heat treatment at a temperature above 400 ±C. Numerous deposition technologies can be imagined fulfilling this recipe, and indeed about 10 different procedures have been successfully studied. The following gives a brief description of the major techniques that have led to solar cells of above 10% efficiency. 1. Sublimation-Condensation

Many film deposition processes are based on the fact that CdTe not only forms stoichiometric films easily but also upon heating sublimes congruently without melting; if heated sufficiently—typically .600 ±C—CdTe evaporates under dissociation. As can also be seen from the phase diagram, the remaining material stays stoichiometric, so no accumulation of either Cd or Te can take place. The material can be used until complete consumption. The classical film deposition process is high vacuum evaporation; CdTe is evaporated by sublimation from a heated crucible and condenses on a substrate positioned in front of this crucible inside a vacuum vessel. The crucible and source material are kept at a temperature around 700 ±C, and the substrate is heated to temperatures between 200 and 400 ±C. The upper limit on substrate temperature is determined by the ratio or resublimation from the growing film to the rate of material arrival from the crucible. Typical laboratory deposition ˚ rates are ø10 Ays, but this rate is determined primarily by the geometry of the deposition apparatus. Two commercial viable processes are modifications of this basic process, which achieve both higher deposition rate and higher substrate temperature: 2. Close-spaced sublimation (CSS)

The evaporation source is made in the form of a flat plate essentially the same size as the substrate placed in close distance in front of the source such that the sourcesubstrate distance is typically less than 1y10 the substrate length. Due to the confined geometry a large mean free path is not required for the subliming species to reach the substrate and a high vacuum is not needed. An inert gas pressure of 1 mbar or higher can easily be tolerated and good films have even be made at ambient pressure.

CSS has been employed to deposit the CdTe film for the first CdTe thin film solar cell to achieve the benchmark conversion efficiency of 10% in 1982.4 The present world record of 15.8% has also been achieved with this technology, using substrate temperatures of around 600 ±C at 40 mbar.5 Figure 3 illustrates the result. Such a temperature requires a high-temperature substrate, e.g., borosilicate glass. If low-cost soda lime glass is used, lower temperatures of around 500 ±C are required. Soda lime glass has been shown to be feasible 10 years later.6 Using deposition rates of above 10 mmymin, efficiencies of around 12% have been achieved and also 10 3 10 cm2 modules of 10.5% have been fabricated at ANTEC GmbH in Germany. 3. Modified close-spaced sublimation

At temperatures above approximately 530 ±C soda lime glass, the least expensive substrate, becomes soft and has to be supported in order to avoid warping. A process in which the glass is supported by rollers during deposition has been developed by Solar Cells Inc.7,8 The CdTe is evaporated out of semicylindrical troughs positioned above the horizontal substrate, and by suitably structured shields the vapor is directed downward to the substrate. This industrial effort has up to now led to modules of 60 3 120 cm2 area measured at NREL for up to 9.1% efficiency. 4. Chemical spraying

This process was used already in 1966 for depositing CdS films for CdSyCu2 S thin film solar cells9 and since then developed for CdTe deposition. An aerosol of an aqueous solution containing heat decomposable Te- and Cd-compounds is directed onto the substrate kept at a temperature of about 500 ±C. By pyrolysis on the surface Cd and Te are liberated and directly react to form a CdTe film. CdS can be produced in a similar manner. For CdS typically thiourea and CdCl2 are used to supply S and Cd, respectively. The films are somewhat porous, but nevertheless have led to cells of 12.7% efficiency.10 The liberated solvent (water) limits the deposition rate somewhat. The process is simple and does not need a vacuum. This technology is presently developed into an industrial process by Golden Photon Inc. Modules of 60 3 60 cm2 area have been manufactured at efficiencies of around 8%. 5. Galvanic deposition

Both CdTe and CdS can be electrodeposited from an aqueous solution at temperatures of 90 ±C. Due to the low temperature, fluctuations in stoichiometry can be remedied only by thermal annealing at temperatures above 400 ±C. The first cells exhibiting 10% efficiency were made in 1983 by two industrial efforts at

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respectively, plus CdCl2 as flux. Films are then given a heat treatment in a controlled atmosphere at about 700 ±C (for CdS) or 600 ±C (for CdTe) for about an hour to produce large-grained films with thickness from 15 to 30 mm. Efficiencies above 12% have been reported. The process has appeal for manufacturing due to the simplicity of the process and equipment. On the other hand, semiconductor film thickness is a factor of 3 to 63 that of films made by other techniques. The process involves several hours of heat treatment to produce high quality films, and high quality substrates are required. 7. Chemical vapor deposition17,18

FIG. 3. I-V curve of the CdTe thin film solar cell with record efficiency of 15.8%.5

Monosolar Inc. and AMETEK Inc.11,12 using p-type and n-type CdTe, respectively. The early AMETEK devices employed a metallic substrate and had the metal/insulator/metal configuration, but later work adopted the more popular CdTeyCdS heterojunction configuration. In the latter case deposition is onto a glass coated with a transparent conductive oxide (TCO), which has a typical sheet resistance of ø10 ohmsysquare; see Fig. 4. In order to keep the deposition potential constant across large area substrates, deposition must be performed at low current densities (ø0.5 mAycm2 ), resulting in deposition rates of about 10 Ays. After termination of the above-mentioned industrial efforts in the USA, British Petroleum has continued this effort13 and has achieved 14% efficiency on small cells and .8% for 30 3 30 cm2 modules.14 6. Screen printing15,16

CdS and CdTe films can be screen printed from slurries containg CdS and CdTe (or Cd and Te powders),

FIG. 4. Film stack of the typical CdTe thin film solar cell. 2744

CVD has some resemblance to the spraying process, insofar as CdTe is formed by chemical reaction from thermally decomposable compounds. In the case of CVD the compounds are gaseous and are injected into the reactor by a carrier gas, e.g., H2 . Typically metalorganic compounds such as dimethyl cadmium and diethyl tellurium are used as precursors for the reaction. This process has the advantage that doping species, such as P or As, can also be introduced (e.g., in the form of thermally decomposable AsH3 or PH3 ) by a suitable gas mixing system. This process, although slower than the fast physical vapor deposition processes (mmyh vs mmymin), has wide process latitude in gas composition, allowing basic studies to be made. CVD has been used at Georgia Institute of Technology. One interesting result has shown that even under very strong deviations of the CdyTe ratio from 1, device-quality stoichiometric films can be made, again giving proof of the latitude available for CdTe processing. Efficiencies achieved on experimental cells have been well above 10%. Due to the toxicity, high cost, and the low materials efficiency of the metalorganic gases, this process is generally considered less suited for large-scale production of CdTe thin film solar cells. 8. Atomic layer epitaxy

Monolayers of Cd and Te are deposited, alternatively, on the substrate by alternately directing gas streams containing Cd or Te onto the substrate. This allows very stoichiometric and pure films to be grown. Cd and Te are evaporated into the inert gas streams in a closed system at elevated temperatures. The gas streams are of high temperature and are guided inside high temperature tubing to avoid condensation. The substrate is also heated and the deposition is driven by the chemical bonding energy between Cd and Te. Cells of 14% efficiency have been reported and modules of 5 3 5 cm2 area at efficiencies above 10% have been made by Microchemistry Inc.19,20 This process has some similarity to the process used for the very first CdSyCdTe cells around 1970. Here the compound CdTe was evapo-

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rated into an inert gas stream which had been guided onto a substrate at lower temperatures, but still around 500 ±C.21 9. Sputtering

Bombardment with argon ions of a solid target of CdTe leads to emission of Cd and Te from the surface of the target. The atoms move in the ambient vacuum and condense on the substrate, forming proper films at suitable temperatures of up to 300 ±C. This technology, as expected, has led to good results in first experiments at NREL and the University of Toledo.22–24 Deposition rates are typically ,100 nmymin, lower by a factor of 10 than for CSS. For deposition of semiconductor back-contacts, e.g., ZnTe, this process may gain industrial application, as typically such back contacts can be very thin.25 B. Diode structures

There have been many efforts in the past 20 years to design and realize good junctions to CdTe films for extraction and collection of light-generated charge carriers.26 Over the course of time, however, a process of natural selection has taken place so that most research and commercial interest is focused on CdTeyCdS heterojunctions introduced in 1972 by Bonnet and Rabenhorst.21 The rationale for this selection is discussed below, but it should be admitted that they are at least partially empirical (i.e., the CdTeyCdS structure works). For example, the MIS structure produced 8% efficient devices, but current densities were low (ø15 mAycm2 ), presumably due to the low optical transmission of the top contact employed. In the case of thin film CdTe homojunctions, there has been very limited success. The cause is the strong light absorption in the direct semiconductor CdTe coupled to high surface recombination rate which severely limits the minority carrier lifetime and results in low quantum efficiencies. Furthermore, it is difficult to manufacture CdTe p-n junctions in thin film form as the interdiffusion of doping species along grain boundaries degrades and distorts the junction proper. Heterojunctions therefore are the most promising configuration. The first heterojunction was the n-CdTeyp-Cu2 Te junction (which is completely analogous to the CdSyCu2 S solar cell).27 Although efficiencies around 7% have been achieved, stability problems due to the diffusion of Cu stopped further development of this cell structure. As mentioned above, CdTe tends to be p-type and therefore an n-type heterojunction partner is required. In addition, a wider band gap (than CdTe) heterojunction partner allows light to enter the CdTe material more readily; this effect is called the window effect. Around 1970 a new heterojunction was identified

for CdTe by using CdS as the n-partner and has led to much success.21 Like CdTe, CdS has the same strong tendency to form stoichiometric films, but unlike CdTe, CdS films are natively n-doped by a slight nonstoichiometry. Also CdS can be deposited by essentially the same techniques as CdTe, allowing for compatibility of manufacturing. A potential disadvantage is that CdS has a significant lattice mismatch to CdTe. Fortunately, after post-deposition treatments described below, the negative consequences are only mild. The n-CdSyp-CdTe heterojunction solar cell has to be illuminated through the CdS window, so that the light is absorbed in the CdTe close to the junction, as illustrated in Fig. 4. In the preferred fabrication procedure, the n-CdS film is deposited onto a transparent conducting film (typically In2 O3 or SnO2 ; Transparent Conductive Oxide ­ TCO). Next the CdTe is deposited onto the CdS and finally a low-resistance contact is made to the CdTe followed by a back electrode, which can be opaque. It should be pointed out that there are two alternative stack sequences which differ by the direction of light incidence relative to the substrate, either through the glass or from the top of the film stack. Empirically the first alternative has been the most successful. The reason for this appears to be related to the material properties of the films involved. TCO is often deposited at temperatures above 600 ±C and is relatively stable with respect to the typical CdTe device processing. Device quality CdS is readily deposited onto the TCO. CdTe deposition and post processing (which often requires heat treatments above 400 ±C) can be performed with minimal damage to the CdS. In fact, although there is some interdiffusion between CdS and CdTe, there is a miscibility gap between the two compounds which limits the composition of the alloy to a few percent substitution of either chalcogenide. The final step is the fabrication of a low-loss electrical contact to CdTe. Although there are many alternatives, contact fabrication is typically the most delicate step and no contact processing procedures exceed 270 ±C. Thus use of the TCO-superstrate configuration enables use of process steps with decreasing temperatures as the device is fabricated, whereas the alternative deposition sequence requires that the relatively delicate CdTe contact be made early in the fabrication process. C. CdS deposition

As mentioned above, CdS can generally be deposited by the same processes as CdTe, e.g., CSS and its modifications, electrodeposition, spraying, and screen printing. Recently another process has evolved singular to CdS: chemical bath deposition.28 After optimistic expectation, the general opinion for broad application of this

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technology has sobered, due to the fact that the process is slow and too complex for production of large area films in a production environment. D. Activation

In the technology of II-VI compound semiconductors, maximum performance can often be achieved only after special treatments. This is also true for the CdTeyCdS heterojunction thin film solar cell. It has become common practice to activate the cells by using the influence of CdCl2 at elevated temepratures.29,30 Typically the CdCl2 heat treatment is a post-deposition procedure. After deposition the CdTe films are wet with solutions of CdCl2 in methanol, dried, and annealed at temperatures around 400 ±C for 10 to 30 min in air. In other cases, such as screen printing or electrodeposition, Cl ions are supplied during the growth process. The process of electrodeposition uses Cl ions in the galvanic bath, avoiding the step of CdCl2 deposition. After deposition of the back contact, a significantly better performance is observed for “activated” cells.31 Figure 5 shows three typical curves for differently treated and untreated cells. Figure 6 shows the corresponding spectral response curves for differently effective activation steps. Only for the properly activated cell does the classic “window-response” expected for such a heterostructure become obvious. The activation process is not yet completely understood, but it has become obvious that morphological and electronic changes occur.32 Temperature-dependent I-V measurements indicate that in the as-deposited and only

FIG. 5. I-V curves of CdTeyCdS thin film solar cells after different activation steps. 2746

FIG. 6. Spectral response curves of the cells of Fig. 5.

heat-treated samples of Figs. 5 and 6 the current transport is dominated by tunneling recombination processes. In contrast, the CdCl2 -treated cells, which show much improved electrical characteristics, current transport is by thermal emission across the junction, which indicates a reduction in interface recombination rates. It is thus credible that the major effect is the improvement of crystalline quality in the depletion region of the cells. This is consistent with electron microscopy studies which show greatly reduced dislocation and stacking fault densities at the junction. Furthermore, much more homogeneous sensitivity results across the cell surface upon activation.34 In as-deposited cells most of the junction is inactive and reverse bias applied to the cells increases the average sensitivity, but activated cells do show high homogeneous sensitivity independent of bias. More detailed studies on the activation process have shown the density of interface states at the current in the diode. Unfortunately, an increase of defect centers in the space charge region occurs, limiting the improvement.35 This seems to be coupled with a certain intermixing between CdTe and CdS immediately at the physical junction.36 With the requirement of more defined processes suitable to technical upscaling, different methods for activation have been studied. CdCl2 has been deposited using standard thin film technologies, including high vacuum evaporation or close-spaced sublimation. Heat treatments of CdTe films upon which about 100 nm of CdCl2 has been deposited produced results similar to those obtained by using the wet process. An alternative process is to use gaseous chlorides, such as HCl37

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or CdCl2 38 during the heat treatment. Gaseous heat treatments can have the additional advantage of leaving the treated surface free of chloride-containing residues which must otherwise be removed by subsequent processing. As device results appear to be similar, activation using gaseous chlorides may be useful in commercial processes. E. Low resistance electrical contacts to CdTe

There are basically two ways to make a lowresistance ohmic contact to a high-resistivity p-type semiconductor: (1) use a high work-function metal or degenerate semiconductor electrode, which allows flow of holes from the semiconductor to the metal, or (2) generate a highly doped surface region in the p-semiconductor, so that carriers can tunnel through the barrier created by a metal of otherwise unsuited work function. In practice some methods may combine the two approaches. Nevertheless modification of a surface layer of a polycrystalline thin film has proved to be a delicate, often material-dependent process.39,40 Regarding the first approach, efforts have been directed primarily toward three semiconductors: HgTe, ZnTe : Cu, and Te.25,41,42,43 All three materials are p-type conductors with suitable work functions. Furthermore, recent analyses have shown that a complete ohmic contact is not required. At room temperature a contact barrier of 200 meV will not lead to a noticeable reduction of output. Thermally activated reverse currents in such junctions generate sufficiently low series resistances.44 In addition, it is evident not only that each material contains Te, but also that each has the potential for some interdiffusion with CdTe. As Te-rich CdTe is p-type it is possible that tunneling may play some role in formation of these contacts. Regarding the first approach, past efforts have generally used Cu to promote good contacts. Approaches include depositing of a graphite slurry (“aquadag”) containing a Cu salt, depositing Cuymetal, (e.g., AuyCu), or creation of a “diffused Cu” layer on the CdTe.45 While Cu appears to be an almost essential ingredient, it must be used with care. It helps formation of better ohmic contacts by increasing the acceptor density below the surface, but if used in excess it forms recombination centers and shunt paths along grain boundaries through the CdTe film.46 F. Some speculations on device operation

Thin film polycrystalline solar cells are clearly inhomogeneous devices, consisting of grains, grain boundaries, and regions of various degrees of interdiffusion and doping. Thus it seems likely that actual device fabrication and operation are greatly affected by the chemistry of the phases and defects that exist during

fabrication and in the final device. As mentioned above, the CdTeyCdS pseudobinary phase system contains a miscibility gap which limits interdiffusion of the compounds during film growth. Nevertheless, the existence of CdTe12x Sx and CdTey S12y phases in CdTeyCdS solar cells with x , 0.06 and y , 0.1 has been confirmed by various researchers.47–49 Thus it is possible that the major rectifying contact is not an abrupt CdTeyCdS interface but rather a graded CdTeyCdTe12x Sx interface. CdTe12x Sx is expected to be an n-type material with the same (cubic) lattice structure as CdTe and essentially zero lattice mismatch (as x . 0). The existence of devices that are produced using CdS but seem to have no CdS in the final structure50 further supports this interpretation. An important aspect of this model is related to the formation of CdTe12x Sx along grain boundaries. Highly conductive n-type material surrounding the illuminated sides of high resistivity CdTe could significantly enhance the collection efficiency of photogenerated minority carriers (electrons). The fact that the junction is at the CdTeyCdTe12x Sx interface may also explain the relatively low values of Voc (850 mV) obtained for CdTeyCdS devices. Device analysis using EBIC may provide further insight and verification of this model.51 Grain boundaries also may have a major effect on the low resistance back contact to the CdTe. As mentioned above, Cu-doping of CdTe or possibly the formation of Cu2 Te seems to play an important role at the back electrode. It has been pointed out, however, that Cudoping of CdS makes the material highly resistive, and this could have beneficial effects in CdTeyCdS solar cells.52 If one extends the idea slightly to speculate that CdTe12x Sx : Cu may also be highly resistive, a model emerges that is not inconsistent with experimental observations. Low levels of Cu alter the CdTe grain surfaces as described above to produce low resistance contacts along the back sides of the CdTe grains. Cu, which diffuses along grain boundaries to the n-type CdTe12x Sx surfaces, passivates those surfaces and thereby inhibits the formation of a (device shunting) tunnel junction between the heavily doped CdTe12x Sx and CdTe : Cu surfaces; see Fig. 7. Excessive Cu will promote the formation of tunnel junctions along the grain surfaces, thereby shunting the rectifying junction. An interesting experimental observation has been that it has not been possible to directly relate the quality of the final device to the electrical properties of the individual layers. If the model described above is correct, it is clear that the final device properties could be greatly affected by both the CdCl2 heat treatment (which affects the interdiffusion at the CdTeyCdS interface) and by the formation of the low-resistance back contact. Thus the physical and optical properties of the layers may be expected to affect device performance more directly than would the electrical properties.

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FIG. 7. Stylized version of the electrical regions with a polycrystalline CdTeyCdS solar cell. The layer labeled CdS is the source of S for the CdTe12x Sx and may actually be composed to CdS, CdTe12x Sx , or some combination of the two. CdTe12x Sx has a lower band gap than CdS and therefore results in increased absorption by the window layer and decreased Isc .

Clearly, this 3-D model has implications not only for EBIC measurements, but also for interpretation of spectral response, I-V and C-V data. Analysis of experimental data combined with modeling of the results is necessary to evaluate this model.

is improved by partitioning the large cell into small cells and connecting these cells in series in order to produce lower current and higher voltage. One hundred smaller cells connected in series would generate 2 A at 50 V with the same total area. Some loss will be incurred as the interconnection scheme will use some area of the whole surface. Thin film solar cells have the common advantage of permitting the formation of integrated modules of any size. There is a variety of interconnect geometries and fabrication procedures that will produce the desired results. The following is an illustrative example: Small individual cells running parallel to one edge of the substrate are defined and sequentially connected. The top electrode of cell 1 contacts the bottom electrode of cell 2, being separated from the top electrode of cell 2. The top electrode of cell 2 connects to the bottom electrode of cell 3, and so on. Figures 8 and 9 illustrate the principle. Such a behavior is induced by three sets of separation cuts of the different layers of the cells. First the bottom TCO electrode is scribed or separated into bands by N cuts, N being the number of cells in the modules. Then the n-p semiconductor layer is deposited and also cut at a small distance in parallel to the first bottom cut. After deposition of the back contact (which contacts the front contact of the next cell via the cut in

G. Substrates

The substrate onto which thin film solar cells are deposited will to a large extent determine the cost of the final module. Many experiments have been made on borosilicate glass, which is stable at temperatures up to 600 ±C. The cost of this type of glass is about two times that of standard windowpane glass–soda-lime glass, which costs about $5ym2 . Whereas the world record cell has been deposited at low speed and elevated temperature onto borosilicate glass, soda-lime glass under suitably reduced substrate temperatures also has resulted in efficiencies above 13%.53 There exists strong evidence that a certain diffusion of Na from the soda-lime glass into the growing film promotes improved structure and properties. Comparison of cells made on glass 1 TCO and glass 1 Na-diffusion barrier (SiO2 ) 1 TCO show improved results for the first option.54,55 All industrial efforts today use soda-lime standard windowpane glass as substrate.

FIG. 8. Illustration of a thin film module.

H. Modules

Solar cells are by nature high current, low voltage devices, similar to electrochemical batteries. A single square meter solar cell operating at maximum power point in full sunlight might generate a current of 200 A at 0.5 V. As resistive power losses are proportional to the current squared, in practice overall module efficiency 2748

FIG. 9. Illustration of the interconnection of thin film solar cells by suitable cutting of the constituting films.

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the semiconductor film) a third cut is applied, separating the back electrode slightly besides the second cut.56 This principle has been tested, applied, and proved first in amorphous silicon modules. The cuts into and through the individual films are often made by lasers, such as Nd : YAG lasers. In CdTe thin film solar cells another technique can be used: The CdSyCdTe double film adheres quite well to the TCO film, which is very flat and rather hard. CdTe being very brittle allows the second and third cut to be made by mechanical tools, which cut the brittle CdTe and slide on the TCO film without cutting it. The third cut can also be with a mechanical scribe and there is no disadvantage to cutting down to the TCO film. The width of the individual cells has to be optimized so that any voltage loss due to series resistance of the TCO film for the generated photocurrent is minimized. For the typical TCO surface resistances of around 10 V 3 cm and typical current densities around 20 mAycm2 for CdTe cells, a width of 8 to 9 mm is suitable. A module of 1 m length thus will have 110 to 120 cells connected in series. A significant advantage of this arrangement is the generation of a voltage 110 times that of the individual cell (approximately 90 V), which eases adaptation to the utility’s grid. The set of separation cuts will lead to a loss in active area of typically 6% compared to a large area individual cell. I. Open questions—Potential improvements

Polycrystalline thin film PV module fabrication spans a range of scientific and engineering issues including materials science, thin film deposition technology, device analysis, interconnect technology, encapsulation, and stability testing. None of these areas can be said to be mature and therefore much work remains to be done. At present laboratory efficiencies are sufficient for commercial application, but we do not yet have a detailed understanding of how these devices operate. While continued improvement in conversion efficiency can be based on continued empirical process optimization, significant improvements would be facilitated by greater understanding of the factors limiting current collection and especially device voltage. Detailed analyses by Sites and co-workers on different CdTe thin film solar cells57 show that for many cells the current already approaches the value expected from the practical system under study. A significant question, however, deals with the optimum thickness of the CdS layer. Current density can exceed 26 mAycm2 as CdS thickness approaches zero, but in practice most devices with CdS thickness below 80 nm exhibit Voc below 800 mV. While the current generation is generally understood, it is not clear why voltage is significantly lower than expected. The theoretically achievable open circuit voltage lies at 1.05 V

whereas the presently achieved limit is at 850 mV. A voltage of 0.9 V seems a practical goal to strive for, using the technology described. By far the largest remaining voltage loss is due to the reverse saturation current, which continues to be enhanced by junction recombination. Furthermore the highest device efficiencies— 15.8%—have been achieved in one laboratory using one device fabrication technique. While, as discussed below, there are several industrial efforts which have produced excellent results using a variety of techniques, it is important that we understand the key elements of the fabrication process and device operation so that deposition and processing apparatus can be properly designed with proper sensors to provide feedback for process control so that the same high efficiencies can be achieved over large areas (acres) at high throughput (ø1 m2ymin). V. PRESENT INDUSTRIAL EFFORTS AND RESULTS

The technology of the CdTe thin film solar cell has reached a stage in which several industrial groups in different countries are engaged in the industrialization of CdTe thin film solar cells, using a variety of deposition technologies.58 Five are described below and summarized in Table II. 1. Matsushita Corp., Japan Based on early work on the CdSyCu2 S solar cell in the sixties, Matsushita in Japan started earlier than 1977 to apply the process of screen printing to the fabrication of CdTe thin film solar cells. Recently efficiencies of around 10% have been reported for individual cells. Integrated modules of up to 1200 cm2 have been made at 8% efficiency. Small modules are applied in commercial pocket calculators. The total annual production lies around 1 MWp . 2. BP Solar, GB In 1985 BP Solar acquired the process of galvanic deposition from Monosolar Corp. in the USA and has continued the development ever since. As early as 1989 they have presented integrated modules of 30 3 30 cm2 area having efficiencies of up to 10%. Small cells have been reported at 14% efficiency. TABLE II. International pilot-production efforts: Module performance.

Company

Process

Matsushita (Japan) BP Solar

Screen printing Electrodeposition

Golden Photon (USA) Solar Cells, Inc. ANTEC GmbH

Spraying Sublimation Close-spaced sublimation

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Cell area (cm2 )

Efficiency (%)

1200 706 4540 3528 6728 86

8.7 10.1 8.4 7.7 9.1 10.5

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Recently (1996) they have announced the availability of commercial modules in various sizes on the order of 10 W. 3. Golden Photon Inc., USA Similar to Matsushita, this small group (originally under the name Photon Power Inc.) has been studying their process—chemical spraying —for about 20 years starting with the CdSy Cu2 S thin film solar cell. Due at least in part to problems with the long-term stability of CdSyCu2 S cells, they adapted the technology to CdTeyCdS devices. After achieving technical success, they were taken over by COORS Inc. of Golden, Colorado and renamed “Golden Photon Inc.” Work has been continued and intensified, resulting in a pilot-production line generating integrated modules. The highest conversion efficiency reported on a 60 3 60 cm2 module was 7.7%. Small cells have shown efficiencies exceeding 12%. 4. Solar Cells Inc., USA This small company was started in 1987 with the intention of becoming a manufacturer of a –Si modules. After becoming disillusioned with the prospects for this technology, however, they reviewed the available thin film technologies and identified CdTe as the most promising thin film solar cell regarding performance and production cost. In 1990 SCI began a program to develop their own in-house CdTe thin film solar module. They are using a sublimation process conducted within a hot-wall chamber to deposit CdTe films onto large area glass substrates (60 3 120 cm2 ). Recently such modules have been measured by NREL as 9.1% efficient. Several arrays ranging in size from 1 to 20 kWp have been installed at various locations, including two 10 kWp arrays connected to utility grids. Module efficiencies are typically above 7%. The company is aiming at large-scale production. 5. ANTEC GmbH, Germany ANTEC also is a small-sized company founded by former members of Battelle Institut in Frankfurt, having over 20 years of experience in thin film solar cell, including CdTe. They are using the process of close-spaced sublimation and have achieved modules of 10 3 10 cm2 at 10.5% efficiency on soda lime glass substrates. Main aspects of ANTEC’s proprietary process are extremely high deposition rate, insensitivity against parameter fluctuations, such as thickness of films, high materials yield, and low equipment cost due to low vacuum requirements (1 mbar N2 ). ANTEC has secured the budget to develop the production process for 60 3 120 cm2 modules and start production within 3 years at a nominal capacity of 10 MWp per year.

houses, and development countries, can be considered only if they allow energy production to be made at a cost level comparative to today’s energy, without relying on government subsidies. By generally accepted arguments this means a production cost less than $1yWp or 2 ECUyWp . This can be achieved only by mass production of modules in the range of 100,000 m2 p.a. at an efficiency well above 10%. Such a production implies highly automated continuous in-line production with efficient materials utilization and high yield. Once these criteria have been met, minimum module costs are reduced to three basic cost elements: equipment, materials, and labor.59 First estimates and projections have shown that a highly automated manufacturing plant for most of the above-mentioned industrially pursued processes can be built for around $15,000,000. In order to spread out this investment cost, the equipment should be operated 24 h a day in three shifts at staffing of about 30 to 40 persons. Materials cost will take the largest cost share of the product cost, mainly two glass sheets and semiconductor material. This will lead to a cost distribution shown in Fig. 10. Sixty percent of cost is attributable to consumables, mainly glass (substrate and cover) and semiconductor materials, 20% to equipment depreciation, and 20% to staff and rent of site. Thin film solar cells and especially CdTe cells and modules will be characterized by relatively short energy pay-back time, i.e., times over which the module will deliver back the total energy, which has gone into its production. Calculations by detailed process analysis60 have led to values given in Table III. A significant part of energy input, e.g., goes into fabrication of glass substrates, which is included in this calculation. B. Market

The ultimate market for PV is central power generation, large arrays of PV modules that will supply electricity to the utility grid at a cost competitive with fossil fuels but without the need for importing those fuels and without the generation of CO2 and other pollutants.

VI. FUTURE PRODUCT ASPECTS A. Production

CdTe thin film solar cells for use in wide fields of application such as central power stations, individual 2750

FIG. 10. Distribution of cost classes in the product cost.

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TABLE III. Calculated energy pay-back time for solar modules of an efficiency of 10%.60 Material

Pay-back time (years)

CdTe a-Si MIS-Multi-X-Si Multi-X-Si Single-X-Si

1.4 3.2 3.7 4.6 6.1

Central power generation is the market that requires PV costs to drop to less than $1 per installed watt. Even before that goal is reached, however, PV has significant opportunities to grow. Photovoltaic power is cost effective today primarily for applications that are far from the utility grid and when high reliability and low maintenance are required. Thus the present market, approximately 80 MWpyy, includes providing power for space vehicles, telecommunication repeater stations, cathodic protection of pipelines, navigational buoys, and other remote instrumentation. PV is often the power source of choice for water pumping for watering livestock or for irrigation. Another emerging market, especially for inexpensive solar cells, is the stand-alone supply of electricity for remotely located homes. (Extension of utility grids can cost upward of $20,000ymile.) Another major application will be the installation on houses (homes, office buildings, and factories) with grid connection, where no storage systems, such as batteries, are needed. There is no doubt that there is a huge need for solar energy in the industrialized countries and even more in the developing countries. A third of the world’s population, i.e., 1.7 billions of humans, do live without electricity. As utility grids on this scale are often uneconomical and exceed the financial capacity of many countries, an attractive option for providing access to modern conveniences and information technology lies in solar electricity produced by solar cells. Recent studies identify in the developing countries 400 million consumers with potential power consumption of 500 Wp solar electrical power each to a total of 200 GWp . To meet this need a production capacity for solar cells of up to 800 MWp per year would be required. Inexpensive, efficient solar cells, such as CdTeyCdS thin film solar cells, have the potential to achieve this goal. C. Delivery and deployment of CdTe modules and systems

Besides the running production of small screen printed modules at a capacity of 1 MWp per annum, a number of modules and arrays have already been installed from industrial pilot-line production efforts.58

Golden Photon Inc. has installed mainly CdTe systems for water pumping, cumulatively less than 100 kW. Two 25 kW CdTe installations have recently been made at China Lake, CA. The arrays have been supplied by Golden Photon Inc. and Solar Cells Inc. SCI has installed two nominal 10 kW grid-connected arrays at Davies, CA and in Toledo, OH. In addition, SCI has supplied four nominal 1 kW arrays to NREL, Toledo Edison (2 arrays), and Tunisia. D. Stability issues

Purchasers of PV modules are essentially paying for their electricity in advance; thus they have a right to know how long the modules will last. The present goal is 20 years lifetime in the field. The thin film PV industry is keenly aware of this concern and has been proactive in attempting to develop accelerated testing procedures. E.g., NREL in the USA has adopted the development of CdTe PV module stability testing protocol, one goal of its teamed research program. Present activities include field testing of arrays and stress testing of modules and cell, i.e., subjecting cells and modules to conditions of temperature, voltage bias, and illumination above that expected in normal use. While development of the stability testing protocol is still in progress, preliminary results are encouraging. Arrays in the field have been stable for more than a year and minimodules have been stable under illumination and load for more than a year and minimodules have been stable under illumination and load for more than 15,000 h and counting. (A module in the field would be illuminated for 2000 to 3000 hyy.) Both Solar Cells Inc.58 and BP Solar Inc.14 arrays have demonstrated stable performance at various locations. VII. HEALTH AND ENVIRONMENTAL ISSUES

Concerns have been raised about CdTe modules because one of their constituting elements is cadmium, a heavy metal. The concerns are connected to the question of Cd release, either during module manufacturing or deployment, e.g., as a result of an accident or fire. Once again the PV industry in cooperation with independent agencies has taken early action investigating these issues and developing strategies to protect worker health and the environment. Regarding manufacturing, the production of future polycrystalline thin film solar cells basically employs techniques common in the chemical industry. The substances involved are easily manageable by standard processes. Workers in the development laboratories have shown no unusual uptake of Cd under periodic medical scrutiny. Production is possible under existing safety laws without putting into risk the health of the staff. Enviromentally it appears to be technically and economi-

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cally possible to design and operate a factory with zero emission. A number of studies from third parties61–64 show negligible risk under use of CdTe solar modules for the environment and humans even under conditions of exposure to fires. In case of exposure to fire, the substrate- and cover-glass will melt long before the CdTe decomposes, thereby including the semiconductor into the resolidifying glass. There are first indications that the CdTe material is actually dissolved in the glass flux. Actual incineration experiments conducted by BP Solar in cooperation with a fire research institution have led to no detectable emissions. During use a CdTe module compares to laminated glass similar to that used in cars. Thus modules will not easily break and release thier content. At their end of life, modules can be recycled easily by grinding the whole modules and returning the debris to the smelters, who can inject the material into their processes without any additional cost. CdTe thin film solar modules will help to alleviate the environmental burden of fossil and nuclear energy generation. Thus CdTe PV can be expected to have an overall beneficial impact on the environment and by itself be environmentally neutral. VIII. OUTLOOK65,66

CdTe PV module technology shows strong promise of meeting the technical goals of producing 15% efficient modules and cost below $0.50yWp while producing a safer and cleaner environment. In order to achieve that goal scientists will have to develop a more detailed understanding of defect chemistry and device operation of cells, and engineers will have to develop methods for high throughput manufacturing and control. Progress continues to be made on both fronts and prospects for success are high. The history of records for the CdTe thin film solar cell is illustrated in Fig. 11,36 and there is no reason not to expect continued progress.

FIG. 11. History of record efficiencies of CdTe thin film solar cells. 2752

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