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ABSTRACT: The goals of this project are study Cu free back contact alternatives for CdS/CdTe thin film solar cells, and to research dry etching for CdTe surface preparation before contact application. In addition, an attempt has been made to evaluate the stability of some of the contacts researched. The contacts studied in this work include ZnTe/Cu2Te, Sb2Te3, and Ni-P alloys. The ZnTe/Cu2Te contact system is studied as basically an extension of the earlier work done on Cu2Te at USF. RF sputtering from a compound target of ZnTe and Cu2Te respectively deposits these layers on etched CdTe surface. The effect of Cu2Te thickness and deposition temperature on contact and cell performance will be studied with the ZnTe depositions conditions kept constant. C-V measurements to study the effect of contact deposition conditions on CdTe doping will also be performed. These contacts will then be stressed to high temperatures (70-100 degreesC) and their stability with stress time is analyzed. Sb2Te3 will be deposited on glass using RF sputtering, to study film properties with deposition temperature. The Sb2Te3 contact performance will also be studied as a function of the Sb2Te3 deposition temperature and thickness. The suitability of Ni-P alloys for back contacts to CdTe solar cells was studied by forming a colloidal mixture of Ni2P in graphite paste. The Ni-P contacts, painted on Br-methanol etched CdTe surface, will be studied as a function of Ni-P concentration (in the graphite paste), annealing temperature and time. Some of these cells will undergo temperature stress testing to determine contact behavior with time. Dry etching of CdTe will be studied as an alternative for wet etching processes currently used for CdTe solar cells. The CdTe surface is isotropically etched in a barrel reactor in N2, Ar or Ar:O2 ambient. The effect of etching ambient, pressure, plasma power and etch time on contact performance will be studied.
Thesis (Ph.D.)--University of South Florida, 2004.
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DEDICATION This dissertation is dedicated to my parents and sisters for their love and encouragement throughout this dissertation, and my loving wife for her support in the final stages of this work.
ACKNOWLEDGMENTS I would like to express my gratitude to my major professor, Dr. Chris Ferekides, for his invaluable guidance and support during th is project. I also thank Dr. Don Morel for all of his help and guidance over the past years. I would like to thank all the other committee members: Dr. Sarath Witanachchi, Dr. Yun-Leei Chiou, and Dr. Richard Gilbert for their help in the course of the fulfillment of my dissertation, and Dr. A.N.V Rao for chairing my dissertation defense. I also like to thank Sally Asher at NREL for her help with SIMS characterization, Dr Brian McCandless at Georgia State University for his help with GIXRD measurements, and Isaiah Oladeji (University of Central Florida) for SIMS analysis. I would like to express my special thanks to the fellow research assistants at semiconductor lab: B. Te tali, V. Komin, Zhiyong Zhao, V. Palekis, R. Mamazza, S. Jagannathan, P. Selvaraj and N. Rao. Finally I would like to thank all my roommates for their moral support and motivation. This work was supported by National Renewable Energy Laboratory (NREL), US Department of Energy.
viii STUDY OF CU FREE BACK CONTACTS TO THIN FILM CDTE SOLAR CELLS by Vijay Viswanathan ABSTRACT The goals of this project are study Cu free back contact alternatives for CdS/CdTe thin film solar cells, and to research dry etching for CdTe surface preparation before contact application. In addition, an attempt has been made to evaluate the stability of some of the contacts researched. The contacts studied in this work include ZnTe/Cu2Te, Sb2Te3, and Ni-P alloys. The ZnTe/Cu2Te contact system is studied as basically an extension of the earlier work done on Cu2Te at USF. RF sputtering from a compound target of ZnTe and Cu2Te respectively deposits these layers on etched CdTe surface. The effect of Cu2Te thickness and deposition temperature on contact and cell performance will be studied with the ZnTe depositions conditions kept constant. C-V measurements to study the effect of contact deposition conditions on CdTe doping will also be performed. These contacts will then be stressed to high temperatures (70-100 C) and their stability with stress time is analyzed.
ix Sb2Te3 will be deposited on glass using RF sputtering, to study film properties with deposition temperature. The Sb2Te3 contact performance will also be studied as a function of the Sb2Te3 deposition temperature and thickness. The suitability of Ni-P alloys for back contacts to CdTe solar cells was studied by forming a colloidal mixture of Ni2P in graphite paste. The Ni-P contacts, painted on Brmethanol etched CdTe surface, will be studied as a function of Ni-P concentration (in the graphite paste), annealing temperature and time. Some of these cells will undergo temperature stress testing to determine contact behavior with time. Dry etching of CdTe will be studied as an alternative for wet etching processes currently used for CdTe solar cells. The CdTe surface is isotropically etched in a barrel reactor in N2, Ar or Ar:O2 ambient. The effect of etching ambient, pressure, plasma power and etch time on contact performance will be studied.
2 power and wind power have been harnessed, the potential of these forms to satisfy the energy needs is questionable. The Sun has been worshipped as a life-giver to our planet since ancient times. The energy received from the sun on earthÂ’s surface is 1.2 1017 on an average. This means that in less than one hour enough energy is supplied to the earth to satisfy the entire energy need of the human population for the whole year. The conversion of sunlight directly into electricity using the photovoltaic properties of suitable materials is the most elegant energy conversion process. Table 1  at the end of this chapter provides an overall picture of the various alternative resources available with its advantages and shortcomings. 1.2 Solar Cells Â– A Historical Background The photovoltaic effect discovered by Edmund Becquerel in 1839 lies at the heart of the operation of a solar cell. He observed that metal plates immersed into a suitable electrolyte, when exposed to sunlight, produced a small voltage and current. The first solid-state material to exhibit photovoltaic behavior was Selenium in 1876 and later cuprous oxide, indicating that semiconductors would be the material of choice in the future. Major development began with the fabrication of diffused p-n junctions in 1954, while at the same time the initial success was obtained for the Cu2S/CdS heterojunction. With the advent of space exploration, the need for a reliable long lasting power source led to one of the major applications of solar cells, namely the space shuttle. The increasing demand for energy, the crisis in the Middle East in the 1970Â’s, and the oil embargo raised interest in solar cells as an alternative source for terrestrial applications.
4 characterized by a high absorption coefficient due to the direct bandgap. In addition their bandgap closely match the solar spectrum to facilitate efficient utilization. The advantage of possessing a high absorption is the lack of need to grow a thick layer for complete light absorption, as in the case of silicon. Thus the loss of carriers to recombination is reduced and thus places less emphasis on the purity of the material. Another advantage to the use of these materials is the wide range of techniques like MOCVD, MBE, sputtering, evaporation, available for the growth of these films. Also since the films are polycrystalline, low-cost substrates like polymers and glass could be used. Despite the flexibility afforded by these materials, the quality of the films obtained need improvements to facilitate extensive use in terrestrial applications. However novel structures have evolved which provide great promise. In an effort to further reduce the cost of photovoltaics better utilization of the sunlight has evolved over the years. In the first method, spectrum splitting, the sunlight is directed on the appropriate cell by spectrally sensitive mirrors. In the tandem cell approach, semiconductors of different bandgaps are stacked on one another. For instance, a wider band gap material uses the blue part of the solar spectrum whereas the red part with longer wavelengths passes through and is absorbed by the lower cell with a smaller band gap. Another possible approach to reduce the cost of photovoltaic systems is to concentrate the sunlight on the active area of the solar cell using lenses, mirrors and sun tracking systems. The research of the last few decades is making the promise of solar energy a reality with a number of companies investing in research, development, and manufacture of devices. The first decade of the new millennium will decide the future of solar power.
5 1.4 CdTe Solar Cell Technology Cadmium telluride is one of the leading candidates for low cost thin film photovoltaics. Laboratory efficiencies in excess of 15% and module performance of over 10% making it a very promising candidate for large scale applications. In addition, cells with efficiencies of over 11% have been obtained using a variety of deposition methods. In order to make this potential a reality, however, involves understanding of key issues that limit device performance and reliability. Major focus on further development of these devices revolves around the challenge involving fabrication of a good, stable ohmic contact to CdTe. This is encountered primarily due to its high work function. This is further complicated by the inability to obtain sufficient p-type doping in polycrystalline CdTe. Copper, the most widely used contact material, suffers from stability issues believed to be caused by its diffusion into the metallurgical junction with time thereby degrading performance and reliability. Although the theory of copper diffusion is quite speculative and needs more systematic studies to understand and quantify its effects, one of the major areas of research is the search for alternative materials for back contacts. Some of the materials researched earlier include ZnTe:Sb, HgTe, PbTe, Li, Ag, and other high workfunction metals like Ni and even Mo, however their reliability and performance are not adequate. A more detailed account of the problem of CdTe contact formation and various contact materials used are presented in chapter 3. The continued search for alternative materials over the years have provided new materials like Sb2Te3 and Ni:P alloys to be promising candidates. To address this pivotal issue, this work studies the feasibility of these new materials for contact fabrication.
11 the net current saturates at the value Jms, which is independent of the bias voltage. This is the hard or reverse direction of current flow. V E C E Fs E Fm E q V E Fs E C E q q n-type Fm E Semiconductor Metal Figure 2 Schottky Barrier (a) Forward Bias, (b) Reverse Bias 2.3.1.2 The Effect of Surface States In the case of an ideal metal-semiconductor junction the barrier height b depends on the metal work function. However, in practice it is found to be less sensitive to m and under certain conditions b may be almost independent of the choice of the metal. An explanation of this weak dependence on m is due to the effect of surface states. Figure 3(a)&(b) show the effect of surface states on the semiconductor surface and the metal -semiconductor barrier respectively.
14 The reduction of b has the effect of pushing 0 towards Ef, that is, it tends to reduce the positive charge in the surface states. On the other hand, if 0 happens to be below Ef, Qss is negative and Qd must be greater than if surface states were absent. This means that w and b will both be increased and 0 will be pulled towards Ef. The surface states therefore behave like a feedback loop, the error signal of which is the deviation of 0 from Ef. If the density of the surface states becomes very large, the error signal w ill be very small and 0 ~ Ef. The barrier height, in this case, is said to be pinned by the high density of surface states. An alternative way of looking at the effect of surface states is to regard them as screening the semiconductor from the electric field, so that the amount of charge in the depletion region is independent of the work function of the metal. Another consequence of the existence of the surface states is that the bands may not be horizontal near the free surface of the semiconductor even when it is not in contact with another solid. If the fermi level does not coincide with the neutral level, there will be a net charge at the surf ace due to the surface states. This produces an electric field in the semiconductor, which causes bending of the bands. If the surface charge is negative, the bands w ill bend upward to the surface, and the electron concentration at the surface w ill be less than that in the interior of the semiconductor. In this situation the surface is said to be depleted. If the charge is positive, the bands will bend downwards, the electron concentration at the surf ace w ill exceed that in the interior, and the surface is said to be accumulated. If the density of the surface states is large, the fermi level becomes locked to the neutral level and the work function is almost independent of the donor density in the semiconductor.
15 2.3.1.3 Current Transport in MetalSemiconductor Contacts In contrast to p-n junctions, where the current transport is due to minority carriers, majority carriers mainly contribute to the current transport in metal semiconductor contacts. The various ways in which electrons can be transported across a metal semiconductor junction under forward bias is shown schematically in figure 5. The inverse processes occur under reverse bias. The four mechanisms of transport are : V d a b c d b Figure 5 Current Transport Mechanism (Forward Bias) in MetalÂ–Semiconductor Contacts 1) Transport of electrons from the semiconductor into the metal overcoming the potential barrier. 2) Quantum-mechanical tunneling through the barrier.
16 3) Recombination in the space-charge region. 4) Recombinations in the neutral region, that is, hole injection from the metal to the semiconductor. 2.3.1.3.1 Emission Over the Barrier The transport of electrons over the barrier is affected by the supply of the same from the interior of the semiconductor. This in turn is affected by the usual mechanisms of drift and diffusion in the electric field of the barrier. In addition, on arriving at the interface the electron emission into the metal is governed by the number of bloch states that effectively communicate with the states in the semiconductor. Both of the above processes act in series, and hence the one that provides the maximum impediment to the flow of electrons determines the current. According to the diffusion theory the first of these processes control the current flow, whereas the thermionic emission theory suggests that the second process is more important. A brief discussion of the basic mechanisms of emission over the barrier is discussed further. 2.3.1.3.1.1 The Diffusion Theory To obtain the current/voltage characteristic according to the diffusion theory, the current density in the depletion region given by J = qn + qDe dn/dx, where -(1) N is the concentration of electrons in the n-type semiconductor, their mobility, De their diffusion constant, the electric field in the barrier, and Â–q the charge on the electron.
27 overcome by a small voltage. Unlike the case of schottky barrier no depletion layer develops in the semiconductor, as there is an accumulation of majority carriers in the semiconductor. 2.4 P-N Junctions P-N junctions are one of the most basic and widely used device structures. Thus a basic understanding of their behavior is essential. A P-N junction, as the name indicates, is a junction formed between a p-type and a n-type material. P-N junctions are classified into 1) Step junctions 2) Graded junctions A step junction is one, which has uniform p doping on one side of a sharp junction and uniform n doping on the other side, whereas a graded junction is one, which has a graded impurity profile. Before the n and p type materials are joined to form the junction, the n material has a large concentration of electrons and fewer holes and the converse is true for holes. Figure 8(a),(b) show the energy bands of each material separately.
40 2.5.5 Heterojunction Solar Cells Heterojunction solar cells in contrast to the homojunction solar cells are fabricated using two different semiconductors with different bandgaps. The band diagram of a typical heterojunction in thermal equilibrium is shown in figure 11. E E E E E E E E E x xv1 c1 v2 c1 g1 m1 f2Ev c1 f1 c1 c12m2 Figure 16 P-N Heterojunction Light, with energy less than Eg1 but greater than Eg2 passes through the first semiconductor. The first semiconductor acts as a window, the second semiconductor, the absorber. Carriers generated in the depletion region and within a diffusion length of the junction are collected and contribute to the photocurrent. In heterojunctions, the use of two chemically different materials introduces certain problems not encountered in homojunctions such as chemical compatibility and stability, reproducibility of the physical and chemical interface, and the lattice compatib ility at the metallurgical junction. Despite the problems mentioned above, the use of heterojunctions has a few attractive features.
43 Where V is the applied bias, V= V1+V2. From the above equation it is clear that the magnitude of Ec does not affect the transport until the flat-band condition V= Vd1+Vd2. The Anderson model although predicts the bands quite successfully, although the J-V relationship obtained using this model is quite different from the measured values. Thus the model needs to be modified. Another drawback of the Anderson model is the assumption of a perfectly abrupt metallurgical junction. This assumption is not suitable for practical junctions since most junctions, by virtue of being fabricated at high temperatures, are graded junctions due to the intermixing between the constituent materials. This typically causes some of the spikes observed in the conduction or valence band (conduction band in our case) to be smeared to the point where it no longer affects or impedes the carrier flow. Another major modification that is required to model heterojunction theory is to take account of the effect of interface states. A brief description of the nature of interface states is discussed in the next section. 2.5.5.2 The Effect of Interface States Interface defects arise due to the lattice mismatch between the materials that form the heterojunction, and from the impurities or defects introduced during the fabrication of the junction. The difference in the lattice constant causes formation of dangling bonds, which may be electrically active, thereby serving as sites for impurity segregation. The lattice mismatch also produces lattice strain, which beyond a certain critical value causes dislocations to propagate away from the interface along the growth direction.
47 by a layer of interface states where recombination is strong. The expression for the J-V relationship based on this model is given by ] 1 ) / )[exp( / exp(0ÂŠ ÂŠ = AkT qV kT E J J -(31) where J0 and A are slow varying functions of T and V, and 1<2. This model fall short in that it does not explain the temperature Â–ind ependent slopes of the logJ versus V curves seen in many heterojunctions. 2.5.5.3.4 Transport by Interface Recombination The difference between the previous mechanism of transport and the transport bt interface recombination is in the limiting step of the process. While in the previous case it was the thermionic emission over the barrier that limited the curent transport, in the present case the limiting step is interfacial recombination characterized by a recombination velocity Si. The J-V relationship is given by the equation ] 1 ) / )[exp( exp( ) ( ) (0ÂŠ ÂŠ = ÂŠ kT qV kT qV N qS n n qS V Jd D i n n i -(32) 2.5.5.3.5 Tunneling The lack of dependence of the J-V curve with temperature suggests that tunneling through the barrier dominates the current transport in heterojunctions. The probability of this mode of transport increases with the reduction in the thickness of the barrier.
48 Tunneling can be either intraband or interband tunneling. In the former case the tunneling through the conduction band spike is the limiting step for injection into the p-type. In interband tunneling through states in the depletion layer is the limiting step. A further development of tunneling limited transport is the stepwise tunneling and recombination through a staircase of closely spaced states in the interface region. For a more detailed analysis of the various models of heterojunction transport the reader is referred to ref. In the next chapter a detailed overview of the properties of CdTe that affect contact formation is discussed. In addition, a detailed literature review of the various ohmic and pseudo-ohmic contacts fabricated on p-CdTe is presented.
50 CdS is a II-VI semiconductor with a bandgap of 2.42eV at 300K. Unlike elemental semiconductors like Si and Ge the doping of II-VI compounds like CdS, is more complex. This is due to predominance of self-compensation, which is caused by certain native defects like point defect and dislocations. Point defects can be vacancies or interstitial atoms. The conductivity of CdS, as in the case of other II-VI compounds, is influenced by vacancies and defects. The sulfur vacancies act as singly ionized donors  and contribute to the n-type conductivity. As-deposited CdS is highly resistive (of the order of 106 cm  ), which can be reduced by annealing in hydrogen (increases sulfur vacancies) and dopant incorporation. The optical properties of CdS make it an ideal choice for window layers in thin film solar cells. It absorbs light below a wavelength of about 510 nm(a very high absorption coefficient of 105 cm-1), and thus a large portion of the visible solar spectrum is transmitted. The transmission of light in the blue region (<510nm) of the spectrum depends on its thickness. CdS has been a very versatile window layer, in that it has been the junction partner in a number of solar cells, using Cu2S, InP, CIS, CdTe, etc. Of these, the CdS/CdTe heterojunction has aroused the maximum interest. There are several cost-effective techniques used for the preparation of CdS like, chemical bath deposition (CBD), close-spaced sublimation (CSS), screen printing and sputtering. The best results have been obtained with the CBD process, although the use of this method for large-scale production is questionable due to the problems encountered with the disposal of Cd laden wastes. Recent results with cells made using CdS made by CSS technique have proved to be as effective with the added advantage of being more industrially viable.
52 absorption coefficient of 5 104 cm-1 for photon energies greater than its bandgap. Thus a thickness of 1-2 m is enough to absorb 99% of the above bandgap light. This property of CdTe makes it less suitable for homojunction solar cells, although the heterojunction solar cells are capable of high efficiencies. During recent years p-CdTe has gained a great deal of importance as solar cell absorbers in heterojunction solar cells. The best heterojunction partner for CdTe is CdS and very high efficiencies have been achieved using this structure. This is primarily due to the compatability with CdS properties (lattice mismatch of 9.7%)  as compared to other window layers. One of the limitations of CdTe is the difficulty of obtaining ohmic contacts. The discussion of ohmic contacts to CdTe is presented later (chapter 4). 3.3 CdS/CdTe Heterojunction CdTe CdS C V E F E C E Figure 19 Spike at the CdS/CdTe Heterojunction Figure 19 shows the band diagram of a nCdS/ pCdTe heterojunction. The spike at the junction is due to the high electron affinity of CdS, which causes this discontinuity.
55 the conduction band and the fermi level, in the case of p-Cd Te, can be estimated to be 1.38-1.48 eV . Thus the value of the work function is about 5.9eV. In Table 3 some of the most commonly used metal for contacts are presented . Table 3 Metals Work Functions Metal Work function (eV) Mo 4.2 Ag 4.7 Al 4.4 Au 5.1 Co 5.0 Cu 4.65 Ni 5.15 Pd 5.1 Pt 5.6 As is evident, no metal presents a value, that can help reduce the barrier height sufficiently to even make a quasi-ohmic contact and thus most of the contacts to p-CdTe are rectifying. Metals like gold, nickel, carbon (graphite), and plati num have been used, although the contact resistivity is either not low enough or the stability of the contact for solar cell purposes is questionable. The approaches used to overcome this limitation are discussed further.
56 4.1.2 The Doping Problem The inability of obtaining low resistivity CdTe is another limitation. The problem of satisfactory doping limits the maximum Voc obtainable from the device. Other advantages of a higher carrier density are reduction in photoconductivity and in atmospheric insensitivity. This in turn improve s the time stability of electronic properties. CdTe being a defect semiconductor, the ca rrier density is controlled to a large extent by native defects, with or without extrinsic doping. Native defects act as donors and acceptors with cadmium and tellurium vacan cies or interstitials acting as acceptors and donors respectively. Extrinsic dopant s influence the ma terials electrical characteristics if they are present as single i onized impurities or as part of complexes with a native defect. A prominent example is the Vcd-ClTe complex, identified as an acceptor. In single crystalline CdTe(SC) doping concentrations in the range of 1017 1018 cm-3 have been obtained by the indiffusion of both p-ty pe and n-type impurities. By incorporating group III materials like indium, ga llium, and aluminum during crystal growth, donor densities of 2*1018 cm-3 have been obtained, depending on Pcd. Group V elements like phosphorous and arsenic can be readily incorporated during crystal growth and acceptor concentration of 6*1017 cm-3 have been realized. Group IA elements also provide shallow acceptor levels in SC Cd Te. Introduction of s odium and lithium has yielded carrier densities of upto 1019 cm-3 by incorporation during growth or during subsequent incorporation by diffusion. Group IB metals (noble metals) like gold, silver, and copper have also been successfully used to dope CdTe, although their activity is quite complex.
58 success but are questionable for use in CdTe for devices. Previous work done in doping CdTe p-type, at USF, by introduc tion of As in the form of AsH3 by MOCVD were very ineffective due to compensation eff ects and complex formation . From the problems discussed above it is evident that the formation of ohmic contacts to CdTe tends to be mo re of an art than science. 4.2 Contact Approaches Most of the common approaches used to form ohmic or pseudo ohmic contacts fall under four ge neral methods: 1) Utilization of a contact ma terial with the proper work function, to provide a low barrier height at the contact, 2) Heavily doping the semiconductor adj acent to the contact to promote tunneling, 3) Adding recombination centers to th e semiconductor adjacent to the contact, and/or 4) Changing the fermi level pinning at the metal-semic onductor interface. Of the methods mentioned a bove, not all have proved suitable for CdTe. In method 3, recombination centers are created within the junction barrier to promote multi-step tunneling. This is done by hitting the junc tion with a short and intense electrical discharge. The resulting imperfections however cause a decrease in the carrier density. Thus there is no appreciable reduction in Rc.
59 The fourth alternative of altering the fermi level pinning at the surface is a fruitful approach and liquid junctions can provide such an unpinned surface to which ohmic contacts can be made. The use of this approach is unfortunately usef ul only for laboratory characterization and its ultimate stability is open for question. Thus most of the contacts are formed predominantly by the first two methods. As no metal has a work function as high as CdTe, a variation of the first method is used. This involves interposing a layer of another semiconductor at the contact, which forms a pseudo ohmic contact to the metal mo re easily. The important consideration is the match required in the elect ron affinities of CdTe and th e interposed semiconductor. In general most contacts to CdTe are a combination of the alternatives to b choice, doping and surface charge modification. In the second method pseudo-ohmic cont acts are formed by doping the CdTe adjacent to the contact highly p-type to promote tunneling. This is usually done by modifying the CdTe surface to be Cd deficien t, whereby dopants can be incorporated to occupy the vacant Cd sites. 4.3 Specific Contacts to CdTe In this section a detailed review of literature and discussion of some of the typical materials used as back contacts to CdS/CdTe solar cells is presented.
61 junction, where it forms shunting paths, a nd recombination centers. This causes the deterioration of junction pr operties and hence the cell pe rformance. Secondary ion mass spectroscopy(SIMS) of cells with varying c opper thickness corroborates this theory. From the SIMS results it was found that copper pe netration into CdTe had a diffusion-like profile. The diffusion process was aided by the polycrystallinity of the material, and the fact that Cu has an atomic size close to Cd. This suggested Cu diffusion could be along grain boundaries. The diffusion process wa s further enhanced by the out-diffusion of Cd during the metallization process. Further evidence on this grain boundary enhanced diffusion was obtained, when CdTe of different crystallinity (different grain sizes) exhibited different diffusion profiles. SIMS re sults comparing the Cu profiles in CdTe of different crystallinity is shown in figure 20. Thus the conclusion derived from the work was that, the copper seemed to play a dual role in these contacts. On the one hand, copper plays a very important role by acting as a substitutional acceptor for cadmium thereby increasing the doping concentration near the surface of p-type CdTe. On the ot her hand it proves detrimental to device performance by forming interstitials or defect complexes, which act as recombination centers.
63 addition, fast and slow diffusi ng components due to the Cui + and complex formation respectively have been identified. Other researchers argue that the high diffusion coefficients are due to Te precipitates or mo re generally due to ex tended defects present in defective crystals. In PC CdTe, the diffusion of Cu is expected to be higher due to the presence of grain boundaries. The grain boundari es allow for easier diffusion than the bulk due to weaker bonds formed as a result of incomplete coordi nation. Another factor affecting the diffusion of Cu is the presence of an electric field as one finds in p-n junctions. Thus polycrystalline CdS/ CdTe so lar cells using copper contacts are affected in this regard. Despite the extensive research and the low RcÂ’s obtained using the copper/gold contact, its effectiv eness is not predictable and is very unstable. This has led to the search of other re liable contacting materials. 4.3.2 Li-Diffused Gold Contact Lithium, a group I (alkali) metal acts as a p-type dopant in CdTe, and is thus a potential contact material. It has been deposited either using MOCVD (n-butyl lithium) or by vacuum deposition by evaporating lithium chromate or nitrate. The work of Bube et al, on the performance of lithium contact in SC CdTe, indicates that the contact resistivities of 0.01 cm2 can be obtained using this contact . It was found that the final resistivities were quite independent of the starting CdTe resistivity, suggesting that a highly p-type layer is formed at the contact interface. The contact was found to be ohmic at room temperature but deterior ated at lower temperatures (about 80K) due to the probable freezing out of holes and hence the tunneling current.
64 Although the Li contact was quite effective on fa brication its time stability left a lot to be desired. This effect is shown in figure 21, where the contact and sheet resistivities are plotted against time. Figure 21 Contact and Sheet Resistiv ity for Li Diffused Contact on CdTe osheet resistivity & K2Cr2O7 Bromine in methanol This deterioration was attributed to th e loss of substitutional Li impurities through interaction with lattice defect s or precipitation, compounded by the diffusion of the same from the contact interface. 4.3.3 ZnTe:Cu Contact As suggested earlier, an interlayer of another suitable se miconductor could help provide a good ohmic contact. For this app lication, the interpos ed semiconductor must provide both a negligibly sma ll valence band discontinuity w ith the p-CdTe, and also be easily doped heavily p-type at the outer metal contact, to promote tunneling.
66 A number of research group have fabricated solar cells with ZnTe:Cu contacts, exhibiting conversion efficienci es greater than 10%. T.A.Gesse rt et al, at the National Renewable Energy laboratory (N REL) obtained cells with efficiencies of 12.1%. The Cu doped ZnTe films were fabr icated by RF sputtering from a compound target of ZnTe and Cu where the Cu concentration was controlled. Studies on the electrical and compositional properties of the sputtered films indicated that a composition of Te/(Cu + Zn) = 1 provided films with lowest resistivities. At higher or lower values excess Cu or Te reduced the electrical quality of the film. For use in CdS/CdTe solar cell contact fabrication, a Cu concentration of 6 atm% provided the best re sults. It is believed that an alloy formation of CuxZn1-xTe where x = 0.6 possesses impr oved intra-grain quality compared to the films that contain excess Cu or Te. It was also observed that although the films had the highest electrical quality the efficiency of Cu incorporation was less than 50%. This meant that bulk of the Cu was electrical inactive. Thus the possibility of Cu diffusion toward the CdS/CdTe junction with time becomes imminent. J.Tang et al proposed a vacuum evapor ation method of for ming ZnTe/ Cu/ metal contacts with conversion effici encies as high as 12.9%. The effect of ZnTe and Cu thickness, the post deposition anneal and the effect of the final metal contact were studied. It was observed that a copper c oncentration of 4.3 Â– 7.5% provided similar results above which the FF reduced. Post deposition anneals of 200C and below was found to be necessary to obtain high efficiency cells. Ni and Au, the final metals used for fabricating cells, interestingly very different results with the Ni contacted cells always providing lower Voc and FF.
70 It can be seen that fill factors above 70% could be routinely obtained using the above procedure. The variation of current densities wa s attributed to the variation in the CdS thickness and not re lated to the contact. The perform ance of the contact was found to be critically dependent on the CdTe etching procedure and the annealing temperature and time. The etching procedure helps provide a tellurium rich surface to enable contact formation, although the time and concentration of the etch need to be controlled, to prevent excessive etching along the grain boundaries. The contact a nneal dictated the extent of reaction at the CdTe surface in add ition to affecting the diffusion of the contact material into the CdTe. The highest fill f actors obtained using th e above procedure was 76% and the best conversion efficiency wa s 15.8%. The contact, however, is bulky and hence more difficult for use in modules. In addition, it is unstable over long periods of time due to the presence of Cu. 4.3.6 Diffused Copper Contact As is evident from the discussion of some of the contact procedures, copper is used in conjunction with othe r materials to aid contact fo rmation. However, copper can be used as is, for contact fabricati on. The diffused copper contact proposed by McCandless et al , consisted of deposition of elemental Cu layer on the CdTe surface, heat treatment of entire structure, etching in Br/Methanol solution to remove elemental copper, and application of a de sired current Â–carrying contact material like Ni, Cr, Pt, Mo, or ITO.
71 No systematic dependence of device parameters or efficiency with the contact material was found, indicating no influence of contact materi al function. This, together with the low resistance meas ured, indicated that a very conductive surface was obtained after etching and was thus insensitive to the type of the final me tal deposited. Thus novel designs, such as transparent cells, could be fabricated using this approach. Fill factors up to 77% was obtained using the di ffused copper contact process. Stability studies of this contact need to be performed before an evaluation could be made on them, however, since the etchin g procedure is reported to remove any elemental Cu it suggests that these devices could resist degradation to a greater extent. 4.3.7 The Cu2Te Contact Contacts that use elemental Cu are characterized by the formation of a p+-CuxTe layer at the interface between CdTe and the me tal. Previous work undertaken at USF was aimed at forming this layer on CdTe by the process of sputtering from a compound target of Cu2Te as opposed to its formation by post deposition heat treatment. The objective of this method was to limit the amount of free Cu, and as a consequence, reduce the degradation of the junction with time. The EDS analysis of Cu2Te films on glass suggested that stoichiometric compositions were obtained at a de position temperature of 250C. Higher temperatures lead to c opper rich stoichiometries. The contact characteristics, and hence the device results, were influenced by temperature of Cu2Te deposition, thickness, and the post depos ition annealing. The effect of Cu2Te deposition on device performance is shown in table 5 .
72 Table 5 Effect of Cu2Te Deposition Temperature on Cell Performance Substrate Temperature, (C) Voc, (mV) FF, (%) 200 791 65.3 250 825 69.3 300 700 54.7 350 346 34 The results suggest the existence of an optimum temperature of 250C for best device performance, which corresponds to the temperature at which the correct stoichiometry was observed. High er temperature lead to shunt ed devices, probably due to the excess copper diffusion towards the junction. The post deposition anneal was important in that it provided any free Cu, that might be present, the energy to dope the CdTe. The major advantage of the CuxTe/Molybdenum contact was the ease of fabrication, and its ability for large-scal e manufacture . In addition, it was an improvement over the graphite paste contact s used earlier because it provided a better coverage of CdTe. This is illustrated in Figure 24.
74 4.5 Copper-Free Contact Alternatives The first step towards more reliable contact s is to get away from the use of copper in contact fabrication. The development of a Cu-free contact t echnology should thus either involve the incorporation of other materials in CdTe to dope it p-type, or the use of other semiconductor materi als that could provide the pr oper interface between the CdTe and the metal. Group-I elements (transition) can provide p-type doping by replacing Cd in the lattice, whereas Group-V el ements can perform the same by replacing Te in the lattice. Although straightforward in concept, as discussed earlier, the incorporation of these elements in poly crystalline CdTe is difficult. Some of the elements of interest that have been incorporated in single crystalline CdTe during growth include Antimony (Sb), Ar senic (As), Nitrogen (N), or Phosphorous (P). Unfortunately, the translation of these processes to poly crystalline CdTe have not yet been successfully achieved. Some of the cont acting technologies that use materials other than Cu are presented below. 4.5.1 Ni-P Contacts The electrodeless deposition, a technique widely used for deposition of Ni on metal, surface was adopted by B.Ghosh et al for contact fabrication to CdTe. It involved the autocatalytic reduction of Ni in the presence of a hypophosphite. In his study the author observed that the growth of Ni-P layers on CdTe was de pendent on the solution constituents, pH, and the temperature of the solution .
75 Microstructural analysis of the contacting interface using XRD revealed the presence of several peaks corresponding to NiP, NiTe2, NiP2, and P. The effect of annealing on the interface is shown in figure 25. Optimum contact properties were obtained at 250C.The improvement of the contact properties at annealing temperatures of 250C was attri buted to the formation of a favorable phase NiP2 at these temperatures. It is clearly seen from the figure that the peak corresponding to NiP2 becomes more prominent at 250C This phase is believed to posses a work function as high as Au thereby improving contact proper ties. In addition at these temperatures the diffusion of phosphorous in the vicinity of the contact helps to dope the CdTe region more p-type. This fac ilitates the formation of a tunneling contact. Annealing temperatures in excess of 250C however increased th e contact resistance probably due to the precipitation of Ni3P layer. The effect of phosphorus on these contacts is still a speculation and needs careful analysis. Although the Ni-P contacts hold promise solar cells fabricated usin g the Ni-P contacts have yielded VocÂ’s of 600mV and fill factors of 50% only.
79 selectivity between materials. However, the isotropic natures of these etches limited their use in certain applications. This led to th e advent of dry etching which offered the capability of anisotropic or directional etching. Over the years dry etching has gained overwhelming importance due to the additiona l manufacturing advantages of eliminating handling, consumption and disposal of large quantities of acids and other solvents used in wet etching. Wet etching has been predomin antly used to etch CdTe for solar cell applications; however dry etching is gaining increased attention mainly due to the better integration in the assembly during manufactur ing. There are many types of dry etching based on the mechanism of the etching process, like 1) Physical sputtering and Ion beam milling 2) RIE(reactive ion etching) 3) Plasma Etching. Physical sputtering provides a very anisot ropic etch due to the strong directional nature of the energetic ions, although the se lectivity is quite poor. In addition, it also suffers from the accidental re-deposition of nonvolatile species on the substrate. In RIE, the etching mechanism is strictly chemical and thus provides ve ry good selectivity, although they are isotropic. By adding a physical component to a purely chemical mechanism the shortcoming of both the above dry processes can be eliminated. This is the basic idea of the plasma etching process, which is described in greater detail in the next sub section.
82 K2Cr2O7/H2SO4 solutions. The basic mechanism of these etching solutions is the selective removal of cadmium, leaving eith er a tellurium rich layer or a conductive tellurium film. This helps reduce the contact resistance of the resulting contact. In this section a brief review of research done in this regard is presented. In order to illustrate the effect of the CdTe surface composition on the back contact properties Dean Levi et al at NR EL fabricated devices under similar conditions, but for the final surface preparation prior to contact fabrication . In particular he studied the effect of the type of CdCl2 treatment (vapour chlo ride or solution CdCl2) and the effect of nitric acid and N/P etch. The different sample surfaces were analyzed using X-Ray Photoelectron Spectroscopy (XPS) depth profiling, to determine the concentration of the various elemental species like Te, Cd, O2, Cl, and C. The study revealed that the as-deposited samples had a Cd/Te ratio consider ably greater than 1. The ratio reduced to stoichiometric proportions with depth profiling. The higher Cd/Te ratio meant that the surface was probably n-type providing an ex planation for the reason for poor contacts obtained in the absence of a pre-contact etching procedure. The solution grown CdCl2 treated CdTe (SCC) after annealing possessed the highest Cd/Te ratio in addition to the higher oxygen content indicating the substrate surface is probably oxidized considerably. The vapor CdCl2 treated sample (VCC) had considerably less oxygen compared to the SCC. However, irrespective of the sample surface prior to the nitric or N/P etch the etching process results in similar surfaces that are tellurium rich. De vice fabricated using the experimental surfaces were measured for performance and tabulated in table 6 follows.
84 enough to be seen using naked eye. The origin of bubbles was attributed to the formation of gaseous bye-products. At this time the surface was found to change color from dark gray to a silvery gray tinge. Again, with further etching this color became more predominant. The films measured after the etching exhibited considerably lower resistivities. In order to determine the cause SIMS depth profiles were obtained. These indicated the formation of a tellurium rich layer whose thickness depended on the etch time. The XRD patterns of these films confirm the formation of hexagonal Te on the CdTe surface. In order to determine the role of the constituent acids used in the process different compositions were studied. In cases where only one of the constituents were used no discernable changes were observed indicating th at both the acids played an important role in the etching mechanism. Since the main component H3PO4 does not possess a strong oxidizing power as the HNO3, the minor component, the role of HNO3 as an oxidizing agent was quite evident. The H3PO4 merely acted as a prot on source to provide acidic conditions to favor the reactions leading to the desired products. The role of HNO3 as an oxidizing agent was verified by substituting it by another oxidizing agent like H2O2. Although the surface colour change was not clearly observed XRD analysis revealed formation of surface Te (also confirmed by the reduction in the resistivity). The replacement of H3PO4 was studied by replacing it by H2SO4, CH3COOH, and HCl. It was observed that crystalline Te was formed with sheet resistances of 24k /sq when CH3COOH was used to replace H3PO4. The use of HCl to replace H3PO4 was unsuccessful due to uneven et ching, formation of CdTeO3 and partial flaking of the film.
85 When H2SO4 was substituted, the color change was quicker but was temporary. This was explained by the strong oxidizing property of H2SO4, which causes formation of CdTeO3. 4.6.3.2 The Br/Methanol(BM) and Pota ssium Dichromate(KD) Etch A systematic study of the effect of the BM (0.1% Br) and KD etch (K2Cr2O7:H2SO4 :: 1:1) was done by Danaher et al on single crystal and thin film CdTe. The study on the BM etch revealed that the thin film etched more rapidly than the single crystal probably due to enhanced etching along grain boundaries in the thin film. The XPS spectra showed a reduced Cd/Te ratio in both crystal and thin film indicating preferential removal of Te. Depth profiling indicated that the Cd depletion regions in the film were deeper than the single crystal resulting form the predominance of etching along the grain boundaries in the thin film. In addition, the XPS spectra also revealed the formation of etch residue (CdBr2) on both the film and single crystal surface in case where the samples were not washed after etching. On washing in Br/Methanol for 4min the residue reduced significantly. More extensive cleaning was however not beneficial due to the formation of hydroxides on the surface, which prove detrimental to contact formation. Thus it was inferred that th e etch time, washing procedures are critical to obtain the desired surface. The KD etch was found to be more aggressive than the BM etch with deeper Cd depleted regions observe d. The KD etch left a Te rich surface, with TeO2, and elemental Cr as the etch residue, the concentration of which could be significantly reduced by making the etchant sufficiently acidic. The etch time could be altered by varying the K2Cr2O7 concentration in the etchant.
86 4.6.4 Dry Etching of CdTe Although, the use of wet chemical etching is widespread and qu ite effective, it is less preferred for large-scale manufacture. The development of a dry etching procedure for CdTe in solar cell manufacture could help overcome this limitation. Dry etching techniques are part of standard processing in IC manufacture. However, there are few dry etching techniques that have been used for si ngle crystal CdTe. Th e incorporation of dry etching techniques could help development of an all-vacuum process for CdS/CdTe solar cells. 4.7 Motivation From the detailed review of literature presented, it could be realized that the problem of contacting to CdTe is quite complex. The contact containing copper seems to provide the best option, but is not very relia ble for use of the CdTe cells for long-term terrestrial applications. Thus the future of this technology will need to involve new contacting materials that can answer both performance and stability issues. Work done elsewhere on the use of se miconducting materials like Sb2Te3 and Ni-P has provided a new direction in the quest for a suitable contact to CdTe. As promising as the materials are, they need to be researched in greater detail before their suitability to CdTe can be confirmed. The motivation for this work is to perform a detailed analysis on the suitability of these materials as contacts for CdTe cells fabricated at University of South Florida. The work would thus involve the co mprehensive study of th e contact properties, effect on cell performance, and the long-term reliability of these contacts.
88 junction to the front metal contact, is deposited. The TCO used is SnO2, which is deposited by MOCVD. The source of Sn is a metalorganic precursor, tetra methyl tin (TMT), and the halocarbon 13B1 serves as the source of fluorine, which acts as the dopant. Helium and oxygen are the ambient gases for the deposition. The deposition of SnO2 is a bi layer process with the doped and undoped layer deposited sequentially. The final thickness of SnO2 is about 0.8 1 m and the sheet resistivity is about 7 10 /cm2. The window layer, CdS, is deposited using the chemical bath deposition technique. The reactants used for this deposition are as follows. 1) Cadmium acetate, which is the Cd ion source. 2) Thiourea the sulfur ion source. 3) Ammonium acetate(NH4Ac) and Ammonium hydroxide(NH4OH) which acts as the buffer. A mixture consisting of measured amounts of CdAc, NH4Ac and NH4OH solution is prepared. The process involves immersion of the SnO2:F coated substrates contained in a glass holder into a reaction beaker containing specific amount of water. This system is then heated to 90 C and a measured amount of reaction mixture along with a known amount of thiourea is added at periodic intervals of 10 15 min. The temperature is maintained at 90 C at all time during the process. The rate of formation of CdS can be adjusted by varying the concentration of ammonia and NH4 salt in the solution. It is desirable to have a heterogeneous formation of CdS, which provides very adherent films on the substrate as compared to homogeneous formation where the CdS precipitates in the solution forming powdery, and hence non-adherent films.
89 CdTe, the heterojunction partner, used in these cells was deposited using the close spaced sublimation technique. This technique is based on the reversible dissociation of CdTe at high temperature 2 CdTe 2 Cd (g) + Te2 (g) Before the actual deposition, the CdS substrates are annealed in H2 for about 10 min at 400 C. The CdTe source and the substrate are separated by a small distance with the source maintained at a higher temperature than the substrate. The CdTe at the source dissociates into its elements and recombines at the substrate, and the rate of transport is diffusion limited. The important parameters of the CSS techniques are (1) Temperature of source and substrates. (2) Pressure in the reaction tube. (3) Separation of source and substrate. (4) Composition of source material. CdCl2 is essential to improve cell performance. The effect of this treatment is to increase the CdTe grain size, which reduces the grain boundaries, shunting path and recombination and thus causes the betterment of device characteristics. It is also believed to improve the CdS/CdTe interface, thereby enhancing the Voc. CdCl2 deposition is done by the vacuum evaporation of CdCl2. The deposition of CdCl2 is followed a post deposition anneal at 400 C in He and O2, for 45 mins.
90 5.1 Back Contact Formation Before the fabrication of back contact the CdTe surface is modified. This involves the removal of excess CdCl2 by rinsing in methanol. The CdTe surface is then etched using Bromine dissolved in methanol for about 10secs. This provides a tellurium rich surface that is necessary for the making a good low-resistance contact. The contacts experiment in this work were 1) ZnTe/Cu2Te contacts 2) Sb2Te3 contacts 3) Ni2P contacts 5.1.1 The ZnTe/Cu2Te Contact After the Br-methanol etch, the samples were loaded into a vacuum chamber for the deposition of ZnTe by RF sputtering. The substrate temperature during deposition was 300C, and the ZnTe thickness was about 1000-1500. This was followed by the deposition of Cu2Te at substrate temperatures of 250C. Very low sputtering power was used in order to obtain better control of the Cu2Te thickness. After the deposition of Cu2Te cell areas were defined, followed by the deposition of molybdenum at room temperature for a thickness of about 8000. The cells were then subjected to a post deposition heat treatment at 200C for 15mins. The effect of Cu2Te thickness and substrate temperature was studied.
91 5.1.2 Sb2Te3/Moly Contact Before using Sb2Te3 for contact fabrication, depositions were carried out on glass in order to determine the effect of substrate temperature on the stoichiometry of the films. The deposition of Sb2Te3, on CdTe solar cells, was performed using RF sputtering of a compound target of Sb2Te3.The deposition thickness and temperature were varied to study their effect on device performance. The final metal layer deposited was molybdenum, typically 8000 thick. The effect of post deposition anneal of these contacts was also studied in the temperature range of 200-400C. 5.1.3 Ni2P Contact High purity Ni2P powder mixed in graphite paste was used as the contact material. This paste was painted over the etched CdTe surface and dried for a few hours to rid the graphite of its solvent. The cells were then annealed at temperatures ranging between 75300C, to observe the influence on performance as a function of temperature. After the fabrication of the contacts the cells were scribed to expose the SnO2. Indium solder was then applied on this layer to form the front metal contact. Finally, the completed cells were characterized by I-V, C-V, and spectral response measurements. In addition secondary ion mass spectroscopy (SIMS) was performed on certain cells to obtain depth profile measurements.
93 discussed in the previous chapter, ZnTe doped with Cu has been successfully used for back contact fabrication. In this approach, the basic idea was to use the excess Cu, from Cu2Te, for doping the ZnTe. To study the effect of the ZnTe contact, experiments were limited to the study of the influence of Cu2Te thickness and deposition temperature on the cell performance. For this purpose, the ZnTe deposition conditions and thickness were kept constant (approximately 1500). 6.1.1 Effect of Cu2Te Thickness Table 7 gives the variation of Voc and FF as a function of the Cu2Te thickness. Table 7 Effect of Cu2Te Thickness on Cell Performance Sample # Cu2Te thickness () Voc, (mV) FF, (%) 3-12b-4 75 778 56.6 5-20a-2 100 837 65.2 5-20a-3 150 846 71.5 From table 7 it is evident that the Voc and FF increase with the increase in Cu2Te thickness. This is due to the improved contact obtained as a result of the decrease in the series resistance (at Voc), as the thickness of the Cu2Te layer increases. I-V curves of these cells shown in figure 27 indicate this behavior.
100 The stability study of the ZnTe/Cu2Te contact thus suggests that it suffers from problems similar to the Cu contacts, namely, degradation caused by the diffusion of Cu with time away from the back contact region. Thus the focus of this work was directed to finding alternate Cu free strategies 6.2 Sb2Te3 Contact The first attempt to obtain alternatives to copper in back contact applications of CdTe solar cells was the study of Sb2Te3. Experiments to determine the suitability of Sb2Te3 for contacts to CdTe solar cells were performed by varying the Sb2Te3 thickness, and the Sb2Te3 deposition temperature. Before the deposition of Sb2Te3 on cells, depositions were performed on glass substrates to determine the resistivity and film stoichiometry as a function of the deposition temperature. The resistivity was found to decrease with temperature as summarized in Table 10. XRD measurements of films deposited on glass at various temperatures are shown in Figure 31. Sb2Te3 was found to be present in all films. At low temperatures the films appear more amorphous with the crystallinity improving with the temperature of deposition .
104 An interesting observation is that a post-deposition anneal (after the molybdenum deposition) was found to be beneficial for device performance of all cells, in particular the ones with higher Sb2Te3 thickness In order to obtain a better understanding of the effect of Sb2Te3, SIMS measurements were performed on the cell with the best solar cell characteristics. The SIMS analysis was performed at the Materials Characterization Facility at UCF . The profile shows the existence of a distinct Sb2Te3 layer as evidenced by the increase in the Sb and Te profiles in that region. Also, there appears to be no trace of Sb in the CdTe bulk indicating that the Sb does not diffuse into CdTe to help dope it p-type (at least within the detection limits of this technique). This indicates that the Sb2Te3 film is very stable. Further experiments to improve the performance of the contact did not yield any encouraging results, contributed in part by the difficulty in obtaining repeatable results.. The best individual parameters obtained using the Sb2Te3 contact was Voc = 784mV, FF = 59.8%, Jsc = 21.4mA/cm2 corresponding to an efficiency of 10.07% 6.3 Ni-P Contact The objective of this study was to determine the feasibility of Ni-P alloys for use as back contacts to CdTe. For this purpose Ni2P powder was mixed in graphite paste and applied on the etched CdTe surface. The effect of Ni2P concentration, temperature and time of anneal on device performance was characterized.
108 XRD analysis by Brian McCandless at Institute of Energy Conversion (IEC) were unsuccessful due to the overwhelming interference peaks produced by the graphite particles in the contact paste still left on the device even after rigorous cleaning. 6.3.3 Effect of Post-Deposition Anneal Time In order to determine the effect of annealing time on device performance, several devices were annealed at 250C for various anneal times in the range of 15-120 minutes. The dependence of Voc and FF on the post deposition anneal time is summarized in Table 11. Table 11 Effect of Anneal Time on Voc & FF Annealing Time, min Voc, mV FF, % 15 852 60.9 60 848 63.2 90 850 67.7 120 827 56.7 From the table it can be seen that although the Voc remains essentially unchanged, the FF improves with time up to 90 min after which, it deteriorates. The improvement in FF is due to the reduction in rectification, with time of anneal. C-V measurements, performed on these cells yielded no variation in the doping concentration as a function of time. The results indicate that there is no doping effect of P in the CdTe bulk. Thus the changes observed in the cell behavior could be attributed to the CdTe/contact interface.
109 The reduction in FF for anneal times above 90 minutes is due to increased series resistance and the onset of rectification, as shown in figure 37. Interpretation of these results would however, again require analysis of the CdTe surface to determine if different phases of material exist for different anneal times. -0.025 -0.02 -0.015 -0.01 -0.005 0 0.005 0.01 -0.500.51 Voltage, VoltsCurrent, A/cm2 15 min 60 min 90 min 120 min Figure 37 Effect of Annealing Time on Contact Properties 6.3.4 Ni2P Contact Â– Characterization Based on the above-discussed experiments it could be concluded that the contact characteristics are strongly affected by the post-application treatments. However, the reasons for the observed behavior are not yet obvious. In order to explain these behaviors and, in particular the role of nickel and phosphorous in contact formation, SIMS analysis was performed at NREL by Sally Asher.
123 Figure 48 shows the variation of cell characteristics with the power used for the etching process. The pressure was kept constant at 250mT. It can be observed that the JV curve improves as the power is reduced, reaching a maximum at 30W. This is primarily due to the improved contact characteristics, which is initially rectifying at 75W and becoming more ohmic as the power decreases. The improvement in the contact properties at lower powers is attributed to the possible reduction of plasma damage. Argon is a heavy atom and hence more effective than N2 (a lighter atom) in causing damage to the CdTe lattice. This explains the shift to lower energies of the optimum condition in the case of etching using Ar. Experiments to study the effect of further reduction in plasma etch power could not be done because of the difficulty in sustaining the plasma at low powers. Based on this study it appears that the observed improvement in contact properties is due to removal of a surface oxide layer by the process of sputter etching. In order to confirm this hypothesis, the surface etching was performed in a mixture of N2 and O2 keeping the total pressure constant at 250mT. This was done by first setting the N2 flow to the desired level, followed by adjusting the O2 flow meter to read a total pressure of 250mT. The effect of adding O2 to the etching environment is quite clear from the above table. The FF is drastically affected although the change in Voc is within experimental variation. This suggests an degradation of contact properties in the presence of O2 in the chamber. This is shown in figure 49.
125 process is probably enhanced by virtue of the fact that the O2 atoms are probably in a more excited state and hence bond easily bond to the CdTe surface. This explains the degraded contact properties with increased amounts of O2 in the etching ambient. Based on the experiments performed on this study it can be concluded that the dry etching of the CdTe surface for contact formation can be applied successfully for CdTe solar cell fabrication. The process of dry etching is favorable since it lends itself more suitable for large scale manufacture of these devices. The ambient gases, namely Ar and N2, are both suitable for the sputter etching process. The impinging ions help remove surface oxides in CdTe thereby helping contact formation. The presence of any trace amounts of O2 is found to be detrimental to the etching process and needs to be avoided. The complete benefit of the dry etching process can be obtained by using back contact methodologies that can be done in vacuum to minimize the exposure of the CdTe surface to the atmosphere after etching.
127 annealing temperature of 250C is critical for good contact performance. Beyond this temperature there is significant rectification observed, pointing to the possibility of change in phase (from Ni2P to Ni3P) of the Ni-P material as published elsewhere . This behavior could however not be verified due to the limitations of characterization facilities available at the time of this study. The reliability of the Ni-P contact was also explored by temperature stressing of these devices in excess of 1000 hrs. The degradation in FF (about 6.6%) although minimal was more pronounced than Voc(2.2%). It is quite encouraging to note that unlike Copper contacts no rectification was observed with stress time, showing that the contact is quite stable. A more thorough study of the contact mechanism will further help optimize performance and reliability. The best device measured at NREL had a Voc = 833mV, Jsc = 20.34 mA/cm2, and a fill factor = 70.7% corresponding to an efficiency of 12%. Dry Etching of CdTe was also studied as an attempt at a more manufacture friendly process solution for CdTe solar cells. The effect of N2, Ar and O2 on contact performance as studied. The results of these studies indicate that the etch process is mechanical in nature where the surface oxide is removed by the sputter etching. The etching mechanism is more pronounced with Ar(being the bigger element) although the use of N2 has interesting possibilities of also doping the CdTe favorably. Trace amounts of O2 added to the etching ambient results in poor performance possibly due to reoxidation of the etched surface. Further characterization of the etching process will help optimize cell performance and eventually replace the wet etching processes currently being employed in these devices.
128 REFERENCES 1. H.J. Hovel, Semiconductors and Semi-metals, Academic Press, NewYork, 1975. 2. C.S. Ferekides, and J. Britt, Solar Energy Materials and Solar Cells, 35 (1994), 255. 3. E.H. Rhoderick, Metal Semiconductor Contacts, Oxford University Press, NewYork, 1988. 4. G.W. Neudeck, P-N Junction Devices, Addison Wesley, 1989. 5. H.J. Moller, Semiconductors for Solar Cells, Artech House, Boston, USA, 1993. 6. D.M. Oman, K.M. Dugan, J.L. Killian, V. Ceekala, C.S. Ferekides and D.L. Morel, Solar Energy Materials and Solar Cells,, 58, (1999), 361. 7. A.L. Fahrenbruch and R.H. Bube, Fundamentals of Solar Cells, Academic Press, NewYork, 1983. 8. K.M. Dugan, Masters Thesis, Elect. Engg, University of South Florida, USA, 1995. 9. R. Swaminathan, Masters thesis, Elect. Engg, University Of South Florida, USA, 1994. 10. T.C. Anthony, J. Of Appl. Phys., 5 (1985), 1349. 11. T.L. Chu, S.S. Chu, Progress in Photovoltaics res. Appl., 1, (1993) 31. 12. B.R. Tetali, Masters thesis, Elect. Engg, University of South Florida, USA, 1996. 13. E. Janik, R. Triboulet, J. of Physics D: Appl. Phys, 16(1983) 2333. 14. CRC Handbook of Chemistry and Physics, CRC Press, Cleveland, Ohio, (1977). 15. A.L. Fahrenbruch, Solar Cells, 21, (1987), 399. 16. T.C. Anthony, A.L. Fahrenbruch, J. Appl. Phys., 5, (1985), 400.
129 17. T.L. Chu, S.S. Chu, K.D. Han, Y.X. Han, Y.H. Hiu, M.K. Mantravadi, Solar Cells, 24, (1988), 27. 18. T.L. Chu, S.S. Chu, C. Ferekides, J. britt, C.Q. Wu, G. Chen, N. Schultz, Solar Cells, 30, (1991), 123. 19. H.C. Chou, A. Rohatgi, J. Electtrochem. Soc, 142, (1995), 254. 20. H.C. Chou, A. Rohatgi, N.M. Jokerst, E.W. Thomas, and S. Kamra, J. Elect. Mat., 25, (1996), 1093. 21. Chun-Teh Lee, R.H. Bube, J. Appl. Phys., 54, (1983), 7041. 22. T.A.Gessert, A.R. Mason, R.C. Reedy, J. Elect. Mat., 24, (1995), 1443. 23. J. Tang, D. Mao, T.R. Ohno, V. Kaydanov, and J.U. trefny, 26th IEEE Photovoltaic Spec. Conf., Anaheim, CA, Sept 30-Oct 3, (1997), 439. 24. T.A. Gessert, P. Sheldon, X. Li, D. Dunlavy, D. Niles, 26th IEEE Photovoltaic Spec. Conf., Anaheim, CA, Sept 30-Oct 3, (1997), 419. 25. J. Tang, D. Mao, L. Feng, W. Song, J.U. trefny, 25th IEEE Photovoltaic Spec. Conf., May 13-17 Washington D.C USA, (1996), 925. 26. E. Janik, R. Triboulet, J. of Physics D: Appl. Phys, 16(1983) 2333. 27. G. Asa, Y. Nemirovsky, J. Appl. Phys., 77, (1995), 4417. 28. T.L. Chu, S.S. Chu, K.D. Han, M. Mantravadi, 20th IEEE Photovoltaic Spec. Conf., (1988), 1422. 29. T.L. Chu, S.S. Chu, Solid-State Elect., 38, (1995), 533. 30. Britt, J., Ferekides, C.S., Appl. Phy. Lett., 62, (1993), 2851. 31. McCandless, B.E, Y. Qu, R.W. Birkmire, First World Conf. Photovoltaic Energy, Conf., Hawaii, Dec 5-9, (1994), 107. 32. V. Viswanathan, Masters thesis, Elect. Engg, University of South Florida, USA, 1997. 33. C.S. Ferekides, V.Viswanathan, D.L. Morel, 26th IEEE Photovoltaic Spec. Conf., (1997), 423.
130 34. B.Ghosh, S. Purakayastha, P.K. Datta, R.W. Miles, M.J. Carter, R. Hill, Semicond. Sci. Tech., 10 (1995), 71-76. 35. R.W. Miles, B. Ghosh, S. Duke, J.R. Bates, M.J. Carter, P.K.Datta and R. Hill, Journal Of Crystal Growth, 161 (1996), 148. 36. Z.S. El Mandouh, Journal of Material Science, 30, (1995), 1273. 37. B. Roy, B.R. Chakraborty, R. Bhattacharya, A.K. Dutta, Solid State Comm., 25, (1978), 617. 38. N. Romeo, A. Bosio, R. Tedeschi, V. Canevari, Thin Solid Films, 361, (2000), 327. 39. T. Schmidt, K. Durose, C. Rothenhausler, M. Lerch, Thin Solid Films, 361, (2000), 383. 40. D. Levi, D. Albin, D. King, Prog. in Photovoltaic Res. and Appl., 8, (2000), 591. 41. M. Ritala, J. Sarlund, M. Leskela, E. Siponmaa, R. Zilliacus, Solar Energy Materials & Solar Cells, 44, (1996), 177. 42. W.J. Danaher, L.E. Lyons, M. Marychurch, G. C. Morris, Appd. Surface Science, 27, (1986), 338. 43. R. Mamazza, Ph.D, Elect. Engg, University of South Florida, Tampa ,USA. 44. S. Gurumurthy, H. L. Bhat, and Vikram Kumar, Semicond. Sci. Tech., 14, (1999), 909.

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