Possibilities of new materials for solar photovoltaic cells

Solar Energy Materials 1 (1979) 43-57 (~)North-Holland Publishing Company

POSSIBILITIES OF NEW MATERIALS FOR SOLAR PHOTOVOLTAIC CELLS Mauricio SCHOIJET Centro de lnvestioacibn y Estudios Avanzados del Instituto Polit~cnico Nacional Apdo. Postal 14-740, Mi, xico 14,DF, M&rico Received 16 August 1978

The requirements for desirable technological properties of semiconductors for photovoltaic conversion are discussed and two criteria are suggested for the selection of materials that could guide the search towards low melting point compounds made of cheap component elements: (1) the use of a deviation from ideality vs. energy gap diagram for the prediction of energy gaps from the knowledge of melting temperatures or viceversa; (2) the use of the Parth6-Goryunova-MooserPearson criteria for compounds of which only their chemical formula is known but not their structure, in order to guess the structure and therefore be able to use the first criterion. For materials for Schottky devices a third criterion is necessary, derived by Nethercott that relates the Mulliken electronegativities of the component elements of a compound to its electron affinity. This can also be applied in the search of metallic alloys, semimetals or narrow band gap semiconductors that might replace the metal in a Schottky barrier. It is shown that a number of compounds exists which might meet these criteria.

1. Introduction The first requirement for a material to work in a solar photovoltaic energy conversion device is a band gap matching the solar spectrum and also high mobilities and lifetimes of charge carriers. These conditions exist in several materials with which solar cells have been made, such as silicon and gallium arsenide. However, the first applications of solar cells were in space vehicles, in which cost was not an important factor. Therefore the high cost of Si and of GaAs and their undesirable properties from the technological point of view such as high melting points and hardness were not an obstacle for the development of solar photovoltaic cells made of these materials. However, at the present stage of development, in which the massive application of photovoltaic systems is becoming a likely possibility, cost reduction becomes a fundamental problem, and this problem can be dealt by both looking for materials other than silicon or gallium arsenide and by simplifying the manufacturing process. However, some of the proposed solutions which are based in using silicon in other forms such as ribbons or amorphous silicon do not seem satisfactory because they would imply a reduction of device efficiency. We do not know of any basic reasons for the use of silicon or gallium arsenide other than the fact that these materials are fairly 43

44

M. Schoijet New fnaterials lOr solar" photovoltaic ('ells

well known and that solar cells of respectable efficiencies have been made from these materials. So far there has been no systematic effort to look for materials that would have the conditions for converting solar power, such as an adequate band gap, etc., and have also properties which are desirable from the technological point of view, in a way that would combine high efficiency with low cost. The cost of a compound depends on the relative abundance of its components and on their melting points, because for a given relative abundance the melting points determine energy and materials requirements for refining the elements. The cost also depends on the melting point of the compound, because this melting point plus the reactivity of the elements at this temperature determine the energy and materials requirements for crystal growth. Another component of the cost, at least if we think in terms of the manufacturing processes that have already been developed, is the cost of the cutting process, which is related to the hardness of the compound, which determines energy, materials and manpower requirements for this process. If a material is not only softer but cheaper the cost of a device will go down, not only because of reduction of the energy, materials and manpower requirements but also because the loss of material in the cutting process would become less expensive. For a given isoelectronic series of compounds (let us say II-VI or III-¥) hardness is related to the melting point and both decrease with an increase in the average atomic number of the components of.the compound [1]. It seems worth to mention that materials such as InSb, Ge and Si become plastic at temperatures of the order of 2/3 of the melting point [2]. In the case of silicon this means temperatures of the order of 1200 K and therefore it would not seem possible to cut silicon at this temperature. But if this applies to materials other than silicon which melt between 700 and 200°C this would mean temperatures between 300°C and room temperature, which might make possible to cut the material in a plastic state with a possible reduction of energy, materials and manpower needs. In principle the melting point of the compound would also determine the energy and materials requirements and the conditions for the technological processes required for manufacturing the solar cells, since lower melting temperatures imply low diffusion temperatures and less reactive manufacturing conditions, therefore no need for vacuum or inert atmospheres. Therefore we might suggest that an adequate criteria for low cost materials that would not contradict the requirement of high efficiency would be that the materials should be made of relatively abundant low melting point elements and that the compounds themselves should have low melting temperatures. In order to find such materials we would apply the criteria given in ref. [3], i.e. that for given melting temperatures of the elements low melting temperature compounds are those of tetrahedral coordination or even better chain or layer compounds made of elements of the right of the periodic table. In the case of materials that are not known or on which we have incomplete information, we would suggest that compositions, energy gaps and melting points could be estimated from the Parthe-Goryunova criteria that relate valence to structure of compounds [4-7], combined with our diagram relating deviation from ideality to energy gap of semiconductor compounds.

M. Schoijet / New materials Jbr solar photovoltaic cells

45

When we refer to the relative abundance of a material we should keep in mind that the amount of material necessary for a given power amount might vary considerably according to whether we are dealing with materials that have indirect optical transitions, such as silicon, or materials with a direct gap, from which efficient thin film devices could be made, thus reducing the amount of material per unit device area. We also know that for an isoelectronic series, let us say III-V or II-VI, elements of higher atomic weight form compounds with larger mobilities and lifetimes of carriers [8, 9] which is a reason for believing that compounds of low melting point, therefore high average atomic number, would probably have good mobilities and lifetimes of carriers. Of course we should keep in mind that for a material to work in a pn junction device it is not enough that mobilities and lifetimes of carriers be high but it is also necessary that potential barriers of a minimum height could be built into the material. Unfortunately we do not have the information about this last condition for most of the materials that we are going to consider. This applies to the search for new materials for photovoltaic cells based on pn junctions. However, for photovoltaic devices that would operate through Schottky barriers further conditions should be applied. Efficiency of a Schottky barrier device is related to barrier height. Schottky barrier cells have the advantage of a substantially simpler manufacturing process, although experimental efficiencies obtained with Schottky barrier devices are lower than those obtained with pn junctions. This is caused by the fact that barrier heights obtained in Si and GaAs are lower than 0.90 eV [10], i.e. considerably lower than their band gaps which are of 1.1 and 1.43 eV. Besides it should be noted that such barrier heights have been achieved using high cost metal electrodes such as Au and Pt. Efficiencies with such devices are of less than 10~ and even lower for p-type silicon, for which barrier heights are also lower, of the order of 0.58 or less [11]. However, calculations made by Pulfrey [12] predict that if barrier heights in Si or GaAs could be increased up to their band gaps, efficiencies would reach the order of 22 and 25~, respectively. It is known that a Schottky barrier could be formed by either a n semiconductor and a metal of a high work function, such as Au or Pt; or by a p type semiconductor and a metal of a low work function. However, the experiments for systems with low work function metals are rather scarce. We might assume that the reason is that low work function elements are quite reactive (alkaline and alkaline earth metals, lanthanum, cerium, rare earths) and that some of them have high diffusion coefficients even at room temperature [13]. However there is a recent experiment by Scranton et al. [14] which shows that HgSe and (SN)x form higher barriers than Au on GaAs and ZnS, thus suggesting that the high work function metal in a Schottky barrier could be substituted by a high work function semimetal or high electron affinity narrow band gap semiconductor. We would suggest that this would apply also to the case of p type semiconductors that would make a barrier with narrow band gap semiconductors of low electron affinity, that we would expect to be less reactive than low work function metals, Therefore, in order to achieve higher efficiencies we would need Schottky devices with higher barrier heights, which could be achieved in the following ways: a) find

46

M. Schoi/et / New materials/re" solar photovoltaic cell~s

wide band gap semiconductors (i.e. around or below 1.5 ev.) of low electron affinity if they are n type and of high electron affinity if they are p type; bl find high and low electron affinity narrow gap semiconductors or semimetals. An empirical fact that could guide the search for either low or high electron affinity compounds is that low electron affinity compounds are those which have at least a component of a low work function. For instance the lowest electron affinity material found in the literature is Cs3Sb [15] and it is equally found that Eu, La and Yb oxides and sulphides have low electron affinities [16]. These facts are explained by the work of Nethercot [17], who calculated the electron affinities of a number of compounds from the Mulliken electronegativities of the elements and from the work of Michaelson [18] in which he correlated Mulliken electronegativities of the elements to their work functions. All this fits with the above mentioned high barrier heights of HgSe and (SN)x on GaAs and ZnS, and with the fact that the SnO2/Si heterojunction has an efficiency and barrier height higher than the Au/Si Schottky barrier [19]. Therefore, although Nethercot's work was done only for binary compounds we might try to extrapolate his results to ternary or even quaternary compounds. For binary compounds we should be able to calculate directly the electron affinity of a given compound from the known values of the Multiken electronegativities [20]. For ternary or quaternary compounds we should look for high or low electron affinities according to the high or low Mulliken electronegativities of their components.

2. Materials for pn junction devices The compounds for pn junctions should be found among those formed by low melting point elements found mostly on the lower right of the periodic table, such as Hg, Pb, Sn, Sb, Bi, S, I, Se, Te, In, Br. The first condition is an adequate band gap. Not many compounds formed by elements of the right of the periodic table have been studied, and in most cases of materials that look interesting we have a very incomplete information. The information we have is summarized in table 1, in which we list such parameters as band gap, type of absorption, mobilities, resistivity, melting point, structure, etc. Because conditions for materials for Schottky devices are more restrictive than for pn junctions, we should expect that materials that would work in Schottky devices would also in pn junctions, while the reverse is not necessarily true. Therefore we include in table 1 those materials that we would expect to work only in pn junctions. Electron affinities given in the tables have been calculated using the formulas given in refs. [17, 21], and values of the Mulliken electronegativities given in refs. [18, 20]. The values calculated for AB compounds should be considered more reliable than those for AB 2 compounds, because the formula for the former has been tested with excellent results in a large number of cases [17], while in the latter case it has been applied to only three compounds [21].

M. Schoijet / New materials for solar photovoltaic cells

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Table 2 High electron affinity semimetals and narrow gap semiconductors Electron affinity (eVl

Material

Eg (eV)

ReO3 RuO2 VO NbO TiO VO 2 TizS 3 YiST, NbzS3 ~-TaS 2 CoS2 TaSez PtSe2

metal metal metal metal metal <0.2 0.3 0.2-0.3 0.12 0,1 metal 0.25 0.1

MnSe2 CoSe2

0.2 0

4.94 5.30

TiSe2 NiSe~ CuSe2 ~-RhSe2 Col 2 AgzF

0 metal metal metal

4.95 5.34 5.38 5.34 5.80 5.94

PtSb2 FePS (FeCo((~9~,.,)AsS FeAsS FeAsSe CoAsS CoAsSe NiPS NiAsS NiAsSe PtAsSe PtSbSe PtBiSe PdAsSe PdSbSe PdBiSe Cu3SbSe 4 CuSbSe2

metal

0.05 0.25 0.2 0.3 0.1 0.2 metal metal metal metal metal metal metal metal metal 0.2 0.16

Conductivity (~ ~cm 1)

6.24 5.21 5.49 5.13 > 5.78

Melting temperature(C)

Structure

1750 1967

4.97 5.02

layered

5.43 5.48 5.20 5,74

> 1300

104

960 (peritectic) 850 1063

0.1 0.01 4 × 10'~

5.09

515 d.90

1226 10 10q00

layered CdI 2 C2 (pyrite) C2 (pyrite} layered C2 {pyrite) complex f.c. C2 anti-Cdl2

C2 EO7

Sources

Observations

[41] [42] [42] [42] [43] [41, 43] [44] [45] [44] [44] [41, 46] [47] [48] [48, 49] [41, 50, 51, 52, 53] [54] [41, 52, 55] [41, 56] [42] [41, 57] [58] can be electrolitically deposited: decomposes in the ultraviolet [59]

[51, 60] [60]

10 100 > 100

425 460

EOv EO 7 C2 C2 C2 C2 C2 C2 C2 C2 C2 C2 C2 tetrahedral tetrahedral

[60] [60] [51, 60] [51, 603

Altenberg mineral

[5q [51] [51] [51] [51] 151] [51] [51] [51] [61 63] I64-661

Famatinite mineral

49

M. SchoOet/ New materialsfor solar photovoltaic cells

Table 2 (continued)

Material

E, (eV)

Electron Conduc- Melting affinity tivity tempera(eV) (f2 lcm-l) ture(C) Structure Sources

CuAsSe2

415 0.22

687

NaCI

AgBiSe2

0.34

765

NaCI

10~

Eg between0.2 and 1.0 eV [64, 67, Scott [62] gives E~=0.5 68] [61, 63, 64, 69] [70, 71] highly anisotropic metal containing chains of mercury atoms, conductivity varies by 10z from maximumto minimumconductivity direction [66]

Cu3SnSe3

Hgz.s6AsF6 metal

Observations

chain structure

3. Materials for Schottky barrier cells 3.1. Materials that would substitute the metal in the Schottky barrier 3.1.1. High electron affinity semimetals or narrow band gap semiconductors Besides of the already known HgSe and (SN)x, we find a number of semiconductors or semimetals formed by elements that have values of their Mulliken electronegativity as high as those of Au and Pt, and we can find a number of compounds of elements of which their work functions are not known but their Mulliken electronegativities can be easily calculated from available data of first ionization potentials and first electron affinities [18, 20, 39, 40]. Elements that have electronegatives immediately below that of Au are, in increasing order, As, P and Te. Elements of higher values of their electronegativities are, in increasing order, S, Se, I, N, O, Br, Cl, F. Since all these are elements of the right of the periodic table, therefore low melting point elements, we might expect that their compounds would also have lower melting points than those of Au or Pt. Some of the compounds are given in table 2. 3.1.2. Low electron affinity metals, semimetals and narrow gap semiconductors In this case we have to search for compounds of low work function elements such as Mg, Ca, Ce, La, Li, Na, St, Ba and K, which are placed in the order of decreasing Mulliken electronegativities. Unfortunately we have very few data on the electronic properties of semimetals or narrow gap semiconductors formed by these elements, but we might guess a number of compounds that would be in this category from their melting points and crystal lographic structures, i.e. applying the deviation from ideality vs. energy gap criteria. We should mention th/lt Junod and Mooser [72] suggest

511

M. Scho(iet . New materials for solar photovohaic cells

Table 3 Low electron affinity semimetals or narrow gap semiconductors

Material

Eg leV)

Mg2Pb MgzSn HgzK HgLi LiAI

0.1 0.35

Liln

0

Electron affinity (eVI

Melting point ( CI

3.90 3.78 < 3.86 < 3.83 3.10

650 778 270 595 718

3.0

LiTI LiZn

3.10 3.67

NaPb

3.53

NaTI Naln Na3Bi

3.05 2.95

K3Bi CuMg2 CaMg2

metal

metal 4.01 3.55

LaBi CeMg3 Celn 3 Celn Li2Alln LiMgBi LiMgSb LizMgSn

671 568 714

798 3.90 3.61

0

CI C1

CsCI excess tetrahedral 625 635 excess tetrahedral 510 CsCI 490 excess tetrahedrat 368 complex tetrahedral 305 B32 420 B32 (NAT1) 775 DO~8

823

BizMg3 Lain 3 La31n LaTI3 LaMg3 LaSb

Structure

Sources

Observations

[74] [74] [75] [75] [76, 77]

[7(~79] [76, 80] [76, 80] [81] [81] [82] [83.84, 85]

DO18 [83, 84, 86] complex [87] C-14 (Laves [88, 89] phase) D52(LazO3) [90] f.c.c. [91] LI~ [42, 91] h.c.p. [92] DO 3 [93] B1 (NaCI) [94]

ref. [83] is in contradiction with ref. [84], according to which Na3Bi is "violet gray" and K3Bi is "'yellow green"

quoted as superconductor [42] probably metallic, quoted as superconductor [42]

BI (NaCI) [95] 796 BiF3 [96] t215 f.c.c [96] 1160 [96] 750-850 excess [78, 97] tetrahedral antifluorite [98] antifluorite [98] antifluorite [98]

t h a t all I - I I a n d I - I I I c o m p o u n d s s h o u l d be metals o n the basis o f a n a r g u m e n t due to P a u l i n g a b o u t electronic structure. W o r k by Bettett [72] seems to c o n f i r m this to be true for L i G a , LiA1, N a T I a n d Liln. This h o w e v e r d o e s n o t ' i n v a l i d a t e o u r a p p r o a c h , because we m i g h t a d m i t t h a t even if these c o m p o u n d s are m e t a l s they s h o u l d be less reactive t h a n e l e m e n t a l metals. S o m e p o s s i b l e c o m p o u n d s are listed in table 3.

M. Schoijet / New materials.[br solar photovoltaic cells

51

3.2. 144de band gap semiconductors 3.2.1. High electron affinity In trying to identify compounds of a gap around 1.5 eV we should search for layer, chain or tetrahedral compounds of the elements already mentioned in connection with high electron affinity semimetals or narrow gap semiconductors. "High" should be understood in the sense of having an electron affinity probably higher than that of silicon. Some possible compounds are listed in table 4. There are other compounds of which we do not know their band gaps but might guess that they might be wide band gap high electron affinity semiconductors. Let us try to order them according to their structures. We underline those which seem interesting ; the others are quoted because their composition or chemical formula are not far from the former. Tetrahedral: C uAsS (lautite mineral) is black [115, 116]; CuaAsS3 has a NaC1 structure and Eg ~ 1.0 eV [117]. Defect tetrahedral" HgA12S4 and HgAI2Se4 are black; HgIn2Te4, HgIn2Se4 and Cdln2Te4 have the same structure and band gaps of 0.86-1.25, 0.6 and 0.92, respectively; Hgln2Te4 has a carrier mobility of 200 cm2/V.s. [118, 119]. Of ZnaAsIa and Zn3PI3 we do not have any data except their structure. [120]. Pyrite structure: OsS2, OsSe2 and OsTe2 are black [121]; RuSe2 has also a pyrite structure ; RuSe2 and OsSe2 would have band gaps of ~ 1.0 and ~2.0 eV, respectively [122]; OsAs2 has a band gap of 0.9 eV [123]. OsBr4 is black, cubic and decomposes at 350°C [121]. Hg4Sb213 is cubic, dark gray, and forms at 280°C [124, 125]. i ~ 1 4 (MeI~=Zn, Cd, Hg) have a rhomboedral chain structure, are black gray and "non-conducting" and form at 525-610°C [125, 126]. Unknown structure: PdI2 and OslF4 are black and the latter might be a semiconductor of very high electron affinity [127, 128]. 3.2.2. Low electron affinity materials We have to find materials with a gap around 1.5 eV that would contain an important proportion of the elements listed in section 3.1.2. We could find very little information on such compounds. Some possibilities are listed in table 5. 4. Discussion

We have used the criteria of deviation of ideality vs. energy gap, structural criteria and criteria relating the probably high or low electron affinity of a compound to the Mulliken electronegativities of the elements in order to find materials that would have high or low electron affinity and that would be used in pn junctions or Schottky barrier solar devices, that would function as light absorbing semiconductors or materials that would replace the metal in a metal-semiconductor barrier, and would also have the advantages of low cost components and low melting points. We have also included some materials that seem to have the physical requirements but would not meet the low cost requirements and/or low melting point condition.

M. Schoijet / New materials Jor solar photovoltaic (:ells

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Until now the most studied materials for photovottaic applications were materials formed of components symetrically placed with respect to the group IVA elements of the periodic table, such as GaAs and CdTe. By applying the criteria that we have outlined here we find mostly compounds which have components which are asymmetrically placed with respect to the group IVA column. The compounds of high electron affinity are made of elements mostly placed to the right of column IVA, the compounds of low electron affinity of elements placed to the left. Assuming that some of these materials would work better in practice than those already being used such as Si or GaAs, we might have some new problems, i.e. the need to develop new manufacturing techniques might arise, for instance for making contacts, because the ranges of temperatures of operation, diffusion and contact application might become too close. We believe that if our approach is correct it might be used for other connected applications such as for materials for the photoelectrolysis of water, since the efficiency of photoelectrolysis is determined by the band bending at the semiconductorelectrolyte interface, and this band bending depends upon the electron affinity of the semiconductor [135]. Other possibilities might exist in the manufacture of Schottky devices for other applications. Even further, we believe that this approach might work for applications such as electroluminescence, which has an enormous potential for use in television screens. A massive application of this kind would certainly require new materials, since those already known, such as GaAs and GaP, have the disadvantage of a high cost. Unfortunately the information we have on most of the materials that seem to fit our criteria is very limited. However the potential size of the market for solar cells is so large that it would justify a program of research in order to find more about these materials. A calculation of the potential market for solar cells in Mexico at present costs [136] of 16 dollar/W, estimates that only for rural applications the market would have a potential of 1.5 million dollars per year. There can be no doubt that a decrease in costs through the use of new materials would bring with it a large increase of the potential market.

Acknowledgement The author profited from discussions with Dr. I. Chambouleyron.

References [1] N. N. Sirota, in: Semiconductors and Semimetals, eds. R. K. Richardson anti A. S. Beer (Academic Press, New York, 1968) p. 156. [2] J. W. Allen, Phil. Mag. 2 (1957) 1475. [3] M. Schoijet, An empirical relation between thermodynamic and electronic properties of semiconductors, to be published. [4] E. Mooser and W. B. Pearson, J. Chem. Phys. 26 (57) 893. [5] E. Parthe, Z. Krist. 119 (63) 204.

M. Schoijet / New materials Jbr solar photovoltaic cells

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