II–VI compounds in solar energy conversion

Jou;nal of Crystal Growth 39 (1977) 73—9 1 © North-Holland Publishing Company

Il-VI COMPOUNDS IN SOLAR ENERGY CONVERSION Alan L. FAHRENBRUCH Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA Received 19 November 1977

The principal advantages of Il—VI compounds for terrestrial solar photovoltaics are low cost, direct bandgap, and the ease of deposition of good quality films of these materials by a variety of growth methods. Existing solar cell technology shows that a solar efficiency of 10% for all thin film, Il—VI compound cells will be reached within a year or two. This paper outlines the useful H—VI compounds and their preparation in thin films and discusses many of the heterojunctions which show promise. The interaction between the structural parameters of these films such as growth morphology and grain size and the electrical properties of the heterojunctions fabricated from them is crucial to producing effióient cells. Still very little research has been done in this area. The effect of grain boundaries in thin films and heterojunction lattice mismatch is demonstrated by simple models in this paper in order to gain a perspective on the required film properties and cell configurations. Finally experimental results are presented for a number of heterojunctions involving Il—VI compounds, focusing particularly on the CdS/CdTe cell.

1. Introduction

We place considerable emphasis on heterojunction cells when considering direct gap materials since: (1) the heterojunction, backwall configuration can be used to cOnsiderably reduce the effects of surface recombination; (2) low series resistance losses are easier to realize in backwall heterojunctions; (3) in polycrystalline homojunctions rapid diffusion along grain boundaries makes it difficult to obtain n or p layers by diffusion doping [11]. Il—VI materials are especially qualified in this context because of the wide variety of deposition methods and temperatures available. In particular

The use of Il—VI compounds in thin layers (2 to 10 ~m thick) is one of the most promising approaches to economically viable photovoltaic converters. One of the ERDA milestones for establishing the commercial feasibility of terrestrial thin film solar cells is the demonstration of 8 to 10% efficiency (Orono conference [1]). Already, several types of cells involving either H— VI heterojunctions or junctions between Il—VI compounds and other semiconductors have demonstrated efficiencies in the range of 8 to 14% in the film-onsingle crystal form [2—4]. Three of them (Cu2S/CdS

there is considerable commercial or pilot plant experience with CdS deposition by vacuum evaporation, solution spraying [12], silk screen followed by sintering, and other methods. The disposition of the vap6r pressures of the elements above themselves and above their compounds allows congruent evaporation and the stoichiometry of the deposited films is controlled automatically at elevated substrate temperatures [13]. Finally, the prices of Il—VI materials appear to be very competitive. For example, if one uses presentday material prices for 6n pure elements in ~-~lkg lots to get an upper bound estimate, the cost for the elements alone for a Il—VI heterojunction solar cell

[5], CuInSe2/CdS [6], CdTe/CdS [7]) show efficiencies in the 5 to 8% range in all thin film form. Economic constraints prove strictest of all and the advantages of direct gap, thin film solar cells in this regard have been pointed out by many authors (e.g., Hovel [8], Mattox [9], Rappaport [10]). The essence of the argument is that the use of direct gap, high optical absorption coefficient materials places considerably fewer demands on the minority carrier lifetime, the growth of large grains of highly perfect material, and the basic raw material requirements all of which are crucial problem areas. —

73

A.L. Fahrenbruch / II— VI compounds in solar energy conversion

74

(CdTe/CdS, or CdTe/ZnSe) is —$0.08/peak watt, $0.41/peak watt for a GaAs p—n junction, $0.21! peak watt for InP/CdS, and $0.08/peak watt for a Si p—n junction While such economic arguments are very crude, particularly due to the lack of data about the effect of grain boundaries on the efficiency (discussed in section 3), they do suggest the economic viability of the Il—VI materials, Taking a rather basic approach in this paper we concentraLe on several of the less well known junc-

From a device standpoint, the efficiency of an ideal solar cell is approximately proportional to VOCfUL Q(hv) is the quantum efficiency (the ratio of the flux of collected carriers to the incident photon flux at each photon energy, hv). Q(h~)is dependent on the optical absorption coefficient c~(h~) and the minority carrier diffusion length LD. Generally we wish to maximize the product OLD. The fill factor ff is determined by the junction properties (primarily by the ratio of light generated current to the reverse saturation current J0) and by the series resistance.

tions (CdTe/CdS, CdTe/ITO, CdSe/ZnTe, for exampie). The Cu2S/CdS and InP/CdS cells and CdS thin films are covered excellently as special topics elsewhere in this issue. Several additional cells which show promise of even higher efficiencies, such as CdTe/ZnSe, and GaAs/ZnSe, are also discussed. By briefly examining a model cell we establish what material properties are of greatest importance and what constraints must be applied to the physical configuration of an efficient cell. These guiding principles are used to focus on the junctions of greatest interest, After the discussion of several LI—VI compound semiconductors to give a flavor of their general character, we discuss film growth methods of these materials. Following comment on experimental work on several specific cells we conclude by pointing out important future research areas.

The open-circuit voltage V0~is primarily determined by the bandgap of the smaller bandgap material Eg~ and the difference in electron affinities (Xi X2) of the two components of the heterojunction. The general effect of the variation of each of these parameters follows: Egi (absorber bandgap): Increasing Egi increases the diffusion voltage VD of the junction, decreasing J0, and thus increasing Voc and the ff. Decreasing Egi will increase the “window” thereby giving larger light generated currentJL. This tradeoffbetween “window” and J0 is the principal factor in producing~theoften quoted efficiency maximum at —1.4 eV for homounction cells [14]. Eg2: Increasing Eg2 will open the “window” on the high energy side giving larger JL. However, very large Eg2 (over —2.6 eV) generally is accompanied by increasing difficulty in making the semiconductor sufficiently conductive. Electron affinities: For a p-type absorber, the difference in the electron affinities (Xi X2) = should be zero. In this case, ~x> 0 yields a conduction band spike which may interfere with photogenerated carrier transport across the junction while z~x< 0 decreases the V~obtainable. Lattice mismatch. Mismatch introduces interface states which may increase J0 and decrease ~oc and ff and also ~L by interfacial recombination. However,

.

2. Materials and junctions

—

—

2.1. First principles The constraints arising from the basic electronic properties of semiconductors are summarized in this subsection. The bandgaps of the materials determine the portion of the solar spectrum accepted by the cell. Photogenerated carrier diffusion length must be maximized. Therefore, we lean toward the choice of a ptype absorber since electron mobilities are considerably larger than hole mobiities in most semiconductors of interest. Small lattice mismatch between components of a heterojunction cell is desirable to minimize loss due to interfacial recombination.

2 and *

.j

For ideal ~L cell series resistance R5 isin light an current thewith current/voltage relation = jo {exp[q( V— RSJ)/A/CTI — 1 —

}

~ q *

Assuming 10%

efficiency, AM!, and 4 ~im (2 ~zm plus 2 ~m) thickness except for Si, 50 ~zm.

.1

Q(hv) d(hv),

Eg 1

where q is the electronic charge.

rz

cm

A.L. Fahrenbruch III— VI compounds in solar energy conversion

75

Table 1 Selected compounds for Il—Vl heterojunctions Material

Type

Eg (eV)

x

c (eV)

—

Lattice

Therm. exp. (X 10—6/0C)

constant d (A) CdS CdSe CdTe ZnS ZnSe ZnTe ZnSSe 8 ZnCdS a InP GaAs Cu 2S

n n n P n n p n n p (or n) p (or n)

2.42 1.74 1.47

4.5 4.95 4.28

4.137 (5.820) 4.298 (5.078) 6.477

4.0 4.8 5.5

3.66 2.67 2.25 3.12 2.8 1.35 1.43

3.9 4.09

3.814 (5.394) 5.668 6.104 5.545 4.040 (5.713) 5.869 5.654

6.2 7.0 8.2 6.6 4.7 4.5 6.8

p

1.20

3.5 4.0 4.32 4.38 4.07 —

11.88ao

~e 2/V sec) (cm 340 600 1000 80 120 530 130

In, Ga, Cl In, Ga, Cl In,I P,As,Au,Li Al,l Al, Ga Cu,Ag,P

—

In Zn Zn, Ge

—

100 300 1—25

—

Dopants

—

27.3 =b0 Cu2Te CulnS2 CuLnSe2 ITO b

p p (or n) p (or n) n

1.05 1.54 1.04 2.9—3.1

--

—

—

—

—

—.

—

5.782

--

—

—

—

—

200 320 25—100

— —

Sb

~ ZnS~5Se~55and Zn0~35Cd0,65S. b Indium--tin oxide. C Electron affinities from ref. 1181. d (...)equalsa~~~sJ2.

quite efficient junctions with considerable lattice mismatch are common (Cu2S/CdS with —4% and CdTe/CdS with —9%). This point is discussed in detail in section 4. Thermal expansion coefficients. These should be approximately equal to allow high temperature processing. 2.2. II— VI semiconductors Pertinent quantities for the semiconductors of interest for terrestrial solar cells are listed in table 1. This list is not meant to be exhaustive but rather illustrative of the possibilities. All materials cited here have direct bandgaps. The LI—VI compounds crystallize principally in the zincblende and wurtzite structures and polymorphic modifications between the two. Films grown from each of these compounds exhibit several crystalline forms and a variety of orientations with respect to the substrate depending on deposition parameters. For vacuum deposited films on amorphous substrates

the (111) or (0001) axis most commonly points toward the source or is perpendicular to the substrate (the lattice constants given in table 1 are perpendicular to these axes). The compounds show a strong tendency for self compensation when dopants are introduced, especially in the large bandgap materials (Sleger [15],Aven and Devine [16]). Solid solutions of most of the compounds of interest exist over the entire range (Aven and Prener [17], p. 112), although they appear to be somewhat difficult to grow as single crystals. The miscibility of these compounds affords the preparation of intermediate bandgaps, electron effinities, and lattice parameters as well as the possibility of graded structures. Since series resistance must be minimized in an efficient solar cell, the conductivity of the semiconductor components is important Of the n-type, binary *

*

For grid structure collection, material with resistivity ~0.5 52 cm is required. For applications where the contact can cover the entire surface (back of backwall absorber or collection by a transparent conductor) —500 57 cm may be sufficient.

76

AL. Fahrenbruch / Il—V/compounds in solar energy conversion

Il—VI compounds CdS, CdSe, and CdTe can easily be grown sufficiently conductive (<1 &2 cm) either in single-crystal or thin film form by a variety of methods. ZnTe can be easily grown p-type. The remaining binaries (p-CdTe, n-ZnSe, and n-ZnS) present more difficulty. In a thin film the along-the-film electronic mobility !1e is dominated by grain boundary effects and is generally much less than bulk values (see section 4). In this case /1e is a strong function of carrier concentration so that large carrier concentrations (>101S/cm3) may be necessary to achieve reasonable conductivities. 2.3. Properties ofspecific II— VI compounds CdS. We comment only briefly on CdS since it is covered elsewhere in this issue. Doping of CdS is easily accomplished during growth in films and crystals or by heat-treatment of films in H 2 (Mitchell et al. [19]) to obtain resistivities from 0.002 ~2cm to over 106 ~Zcm. Low resistivity ohmic contacts are easily made to CdS with In, Zn, or Al. CdSe. Although CdSe hasn’t much promise as a photovoltaic material alone because of its bandgap and low hole mobility, it might be used in solid solutions with others. It can be made n-type only and its preparation and properties relevant to solar cell purposes closely parallel those of CdS. CdTe. CdTe is the only amphoteric Il—VI binary under ordinary conditions and its bandgap makes it an optimum absorber component. In fact it remains the only real possibility for an efficient absorber among the binary IL—VI compounds. There are of course a number of other excellent absorber materials compatible with the Il—VI compounds including InP, CuInSe2, CuInS2, and GaAs. Single crystals of CdTe are relatively easy to grow (Lorenz [20]; Kyle [21]; Gu [22]) and a number of deposition methods have been used to produce films, including vacuum deposition [23,24], close-spaced vapor transport (CSVT) [25], vapor growth [7], and electrochemical deposition (Kroger [26]). CdTe normally exists in the zincblende cubic structure. Wald has discussed the problem of obtaining sufficiently long minority carrier lifetimes in CdTe [27]. Although material for gamma ray detectors with lifetimes of 106 sec has been prepared, this is highly insulating with few midgap states. For highly doped

n-CdTe lifetimes of iü~ to l0_8 sec are observed. Whether high lifetime material can be prepared in highly conductive form is not yet known. The behavior of dopants in CdTe has been examined by Kroger and De Nobel [28] and others. The n-type material is easy to dope with In, Zn, or I and good electrical activity of the dopant is obtained in both crystals and films. Ohmic contact is easy to make to n-CdTe using In. The p-CdTe is more difficult to produce. Commonly used dopants are P [22,29,30], Au, As, and Li. It is difficult to obtain material with resistivity less than 1 &1 cm (—6 X 1016 holes/cm3). For doping levels above this the electrical activity of the dopant drops sharply and the hole mobility is reduced. Li doping has been used to obtain resistivity <1 ~ cm [31] but Li is unsuitable for solar cell applications because of its rapid atomic diffusivity. In our laboratones As, Au and P have been used to dope CdTe and material with 2 to 3 ~l cm has been obtained with P [31]. Low electrical activity of the dopant was noted however. The difficulty in doping p-CdTe stems from both strong compensation effects and low solubility of the usual dopant species. Films of p-CdTe have been prepared by Kachurin et al. [32] and by vacuum evaporation by Glang, Kren, and Patrick [241, but these were very highly resistive. Chemical transport of CdTe onto single crystal substrates was done by Paorici [33]. Films of pCdTe have been grown epitaxially on single crystal CdS substrates using the CSVT technique in H2 by Vasilchenko [25]. A single crystal, heavily doped with Au and P, was used as a source wafer and large (20 to 60 i.tm) oriented grains and a film resistivity of 12 &~cm were obtained. The optical absorption constant of CdTe has recently been redetermined by a number of workers [34—37] and is much higher than previously thought (fig. 1). Low resistance ohmic contacts to p-CdTe are difficult to make. No metal has a work function which is high enough and the resulting contact is a leaky Schottky barrier. Special treatment must be used to make the barrier thin enough so that extensive tunneling occurs to obtain low contact resistivity, Pc’ Traditionally chemplated Au contacts have been used with some success for electrical measurements. However, for solar cell purposes contact resistivities Pc <

A.L. Fahrenbruch 105

,.

~

d

...,

\~

‘~

‘\

\~c

~

\

0

~

\

\

1

\ 1,1

I

.5

.6 .7 WAVELENGTH (pm)

.8

Fig. 1. Optical absorption coefficient versus wavelength for CdTe: (a) Brown and Brodie [341;(b) Rappaport and Wysocky 1351; (c) Kireev [361;and (d) Mitchell et al. [371.

0.2 ~l cm are desired and the above method is generally unsatisfactory. Evaporation of Ni or Au contacts after a special etching technique, followed by H2 annealing have yielded contacts with Pc < 0.2 &~cm on low bulk resistivity material [38]. The process is somewhat unreliable however. These contacts are destroyed by processing temperatures above —320°C (Au) and —400°C(Ni) [33]. The etching step would probably make this particular method unsuitable for commercial cell application. Ion implantation of As has been used with some success by Chu [39]. ZnTe. Like CdSe, ZnTe is not a particularly useful solar cell material because the intermediate value of its bandgap makes it neither a good window nor an efficient absorber. It may find use as an alloying component however and in fundamental heterojunction studies. Under most conditions it can only be grown high conductivity in p-type. ZnSe. ZnSe is promising with regard to both bandgap and lattice constant which is an excellent match to GaAs. It is somewhat difficult to grow high conductivity single crystals of ZnSe. However, using postanneals in Zn vapor, Wilson and Allen [40] were able to obtain good conductivity. Bouley obtained ==i 17 cm by post-annealing in Al or Ga [411.Despite considerable interest for use in LED devices, thin films of —

77

This is due to strong self compensation effects3 by to native reportthe1Oconi04 ~ defects. cm filmsSleger grownand byMilnes CSVT. [15] To avoid

CdTe

2

in solar energy conversion

ZnSe with low resistivity appear to be quite difficult to produce (Fujita -[42]). Ion implantation has been used to increase the conductivity of films by Shin [43]. J. Aranovich used vacuum evaporation to prepare films on glass with p = 1 to 20 f~cm using a combination of dopant co-evaporation and postannealing in Zn vapor [44]. Type conversion by ion implantation with Li has been reported by Park [45]. ZnS. There has been great interest in ZnS for luminescent devices. Of the binary Il—VI compounds ZnS is perhaps the most difficult to grow with high conductivity, either in single crystal or thin film form.

I

......,~

ioI’

/ II— V/compounds

-

ductivity problem various ternaries with more moderate bandgaps have been used. ZnCdS. Like many of the lI—VI compounds, ZnS and CdS form solid solutions over the complete composition range (Aven and Prener [17], p. 112, and Bonnet [46]) and the Zn/Cd ratio may be varied to obtain larger bandgaps. Vegard’s law holds for these solutions [47] and a composition of Zn 0~3Cd0,7S should have a bandgap of 2.8 eV and an electron affinity of 4.3 eV. The required conductivities may be difficult to obtain however. ZnCdS films have been grown by Chamberlin and Skarman [48] by the solution spray method, Hsieh [49] by reactive sputtering and the Delaware group by vacuum evaporation [5]. ZnSeS. Another of the Il—VI compound solid solutions of interest is ZnSeS. Robinson and Kun [49] grew crystals of ZnSe055S045 by transport in 12 vapor and obtained resistivities of —0.5 17 cm. This material also obeys Vegard’s law and the bandgap is 3.1 eV, the lattice constant of 5.545 A, and x is 4.0 eV for this composition. CuInSe2. The compounds CuInS2, CuInSe2, and CuInTe2 represent a class of pse~ido-II—VI compounds that have direct bandgaps (1.54, 1.04, and 0.96 eV, respectively) favorable for photovoltaic conversion. CuInTe2 can be made n-type and both CulnSe2 and CuInS2 can be made either n- or p-type. In particular CuInSe2 has good lattice and electron affinity matches with CdS. The materials have been grown by a number of workers including Shay et al. [50]. Kazmerski [6] has made both single crystals and thin films of these materials. Kazmerski controlled the conductivity and carrier type by variation of the chal-

(—1.05)

Cu2STe/CdTe

1.47 1.47 1.47 1.47 1.43 1.47 1.47 1.74 1.04 1.20 1.47 1.47

CdTe/CdS

CdS/CdTe p—nCdTe ZnSe/CdTe ZnSe/GaAs ITO/CdTe ZnS/CdTe ZnSe/ZnTe CuInSe2/CdS

Cu2S/ZnCdS (22/78) ZnCdS/CdTe(35/65) 0 +0.28

—

~0

+0.38

—

—0.22 0 +0.19 —0.02

—0.22

—0.12

<0

<0

~x a (eV)

1.22 1.55

—

1.62 0.61 0.84

—

1.02 1.27 1.46 1.21

1.02

1.03

—

<1.0

VD d (eV)

11.5 15.5

—

1.2

0.5

15.3

—

9.7 0 12.5 0.27

9.7

0.27

4

4.6

~ e

(%)

a ~> 0 indicates a conduction band spike. b Configuration: f/f, film-on-film; f/x, film-on-crystal; ARC antireflection coating. C Efficiency from total area. All others from active area. d Assuming Ef is 0.1 V from the conduction (valence) band in the n(p) materials. e ~t2Ois lattice mismatch, ~Exp is mismatch in thermal expansion coefficients.

ZnSSe/CdTe(45/55)

1.35

CdS/InP

-

1.20

Egi (eV)

2S/CdS

Cu

Junction

Table 2 Selected heterojunction solar cells involving LI—VI compounds

—

2

—

—

—

—

—

3

9 0 21

9

12

—

—

~Exp e

(%)

23

6 -15

—

—

17 22—25 21 19

17

17.2

—

16.6

Effic. (%)

Theoretical

[251

[251

—

4.0

5.7

—‘1 12

—

—2

—

—

—

C

7.9 [27] (25] [18]

[25]

C

4 5to6

5.7

>8 7.64 12 6.0 14

Effic. (%)

f/f

f/f

f/x f/x A

f/x

f/x f/f f/x

f/f

f/x f/f f/x f/f f/x

Conf

Observed efficiene

[25]

[501

—

[25]

Ref.

-

A.L. Fahrenbruch / II— VI compounds in solar energy conversion

cogenide over-pressure during vacuum deposition of the films. Homojunction solar cells of modest efficiency have been made in this manner. Transparent conductive metal oxides. Indium—tin oxide (ITO) is representative of several transparent, metal oxides which can be made highly conductive (l0~ to l02 12 cm). All those discussed here are n-type and are deposited in amorphous or small grained polycrystalline thin films, usually by sputtering. These include Cd2SnO4, Sn02, In203, CdO, and ZnO. Their obvious advantages for solar cells are large bandgaps which are variable with composition and high conductivity. Several of them have been shown to make excellent ohmic contact to CdS. Considerable work has been devoted to heterojunctions between Sn02 and Si (essentially a Schottky barrier) [51] and some work on other junctions such as ITO/ p-CdTe by Mitchell [39]. Despite large lattice mismatch these junctions are fairly efficient. It appears that no attempt has been made to grow these materials epitaxially so that little is known about lattice mismatch effects. The use of metal oxide layers is also important as a passive, collecting layer to either replace or augment the metal collecting grid on the light incident side of a solar cell. Here the compromise is between the 5 to 15% shading of a metal grid and the optical transmission (80 to 90%) of ITO for example. A large portion of the transmission loss can be eliminated with an antireflection coating. The most important advantage in the use of a transparent conductor is that the principal current collection transport is through the “window” semiconductor layer rather than along the layer as in the case of grid collection. This allows the use of a much higher resistivity layer (e.g,, 500 12 cm rather than 0.5 12 cm). In regard to ITO, Mehta and Vogel [52] found the composition In0 18Sn0,8202 gave the best optical transmission and conductivity for sputtered films. Praser and Cook [53] obtained ITO films with resistivity of20/cm3 3 X l0~ cm, carrier concentrations of and17 optical transmission of 0.83, 8.6 X l0 most of which is attributable to reflection. ITO films have also been made by solution spraying [54]. Ahrens and Anderson found a bandgap for 1n 203 of 3.1 eV and a refractive index of 1.9 [51]. Junctions. From the criteria just discussed, as well as other more subtle qualities to be discussed later,

79

the junctions listed in table 2 appear to be the most promising for photovoltaics. Electrons are the minority carriers in all cases except n-CdSe/p-ZnTe which is of interest because of its small lattice mismatch. Many of the heterojunctions above will be discussed in detail in section 3. A comment on efficiency calculations is in order here however. Current policy for developmental and commercial devices is to report efficiencies with respect to the total area of the cell including grids (grids usually cover 5 to 15% of the area). These are marked in table 2 in cases where this fact is known. It has been the custom of many, if not most, research workers to report efficiencies with respect to active cell area since grid or contact coverage is not optimized and may account for a large portion of the total area in an experimental cell. 3. Film growth In this section we are interested in relating the character and methods of film growth to their effect on the electronic characteristics of the solar cell. To do this we wish to briefly focus on the commonality of growth methods rather than their differences. How do epitaxy and growth morphology affect solar cell parameters? What are the effects of grain boundaries? Growth methods are then briefly outlined. Notable reviews of Il—VI compound film growth are those by Cusano in Aven and Prener [17], Holt [58], Holt and Abdalla [59], and Yezhousky and Kalinkin [60]. Heteroepitaxial growth (particularly by CVD methods) is the subject of a survey by Manasevit [611, diffusion in thin films is discussed by Balluffi and Blakely [62], molecular beam epitaxy of Il—VI compounds by Smith and Pickhardt [63], and finally Mattox [64] gives a general perspective of thin film technology for solar energy applications. 3.1. Commonality Briefly, the factors common to most growth systems which are of particular importance to solar cell technology are: (1) the effect of the substrate on growth morphology, particularly the differences between epitaxial growth and growth on amorphous substrates; (2) the maintenance of stoichiometry and the inclu-

80

A.L. Fahrenbruch / II— VI compounds in solar-energy conversion

sion of dopants; (3) the effect of grain boundaries on the electrical properties of films; (4) consideration of high temperature processing Sequences. We intuitively feel that the most efficient solar cell will be the one with the least crystalline disorder. In terms of the effect of the substrate for a thin film cell, we expect a homoepitaxial relation to be best, followed by heteroepitaxy where some lattice mismatch is present, the worst case being deposition on an amorphous substrate. Since we are constrained economically to use amorphous substrates we look for ways to minimize the effects of the disorder introduced, Homo- and heteroepitaxy is relatively easy to obtam with Il—VI films on a variety of crystalline substrates (Holt [58], Manasevit [61]). In this case the orientation of the film is determined by the substrate orientation and the grain size depends strongly on preparation variables, ranging from single crystal layers to oriented crystallites fractions of micrometers in size. For many Il—VI materials of interest the epitaxial temperature, below which the orientational influence of the substrate disappears, is in the range of 300 to 500°C(Holt [58]). On amorphous substrates a degree of orientation remains; usually the (111) axis (or the (1000) axis in L:’~agonalsystems) of the crystallites points either towar4 the deposition source or normal to the substrate. This effect is particularly pronounced in Vacuum deposition from a small area souce. In both heteroepitaxy and deposition on an amorphous substrate a transition region exists adjacent to the substraté where crystallites are small and disorder is large (Aleksandrov [65]). The thickness and quality of this disordered layer depends strongly on the preparation conditions of the substrate surface. This transition region would be expected to have a very short carrier lifetime and small diffusion length for photoexcited carriers. Farther from the substrate, the grains become larger and their orientation more uniform. Line and surface defects and grain boundaries are terminated and the layers become more perfect. Although carrier lifetime is crucial in the absorber layer, it is of little consequence in the bulk of the window layer not immediately adjacent to the junction interface. The Il—VI compounds evaporate congruently and

maintenance of stoichiometry is relatively automatic at elevated substrate temperatures since the vapor pressures of the elements are much higher over the pure elements than they are over the compound. For example, at 500°C the pressures of Cd and Se are —15 and —40 Torr over the elements and —3 X l0—~ and —1.5 X l0~Torr over CdSe. Doping of polycrystalline films in general is complicated by rapid diffusion along grain boundaries. For solar cell purposes we are also constrained to use impurities with low diffusivity in order to be sure of long device lifetimes. Doping of Il—VI films can be accomplished by coating the substrate with a thin layer of the dopant before deposition, by applying a layer of dopant after deposition (with a subsequent heat-treatment), and finally, by incorporation during the film deposition. The best results appear to be obtamed using the last method. Doping during chemicalvapor-deposition has proved quite successful for many of the Il—VI compounds (Cusano in Aven and Prener [17], p. 117). The electrical effects of grain boundaries are treated in the following subsection. Finally, high temperature processing sequences must be given careful consideration since ohmic contacts may be unstable at high temperatures, interdiffusion of dopants can change junction properties, and in some cases the formation of intermediate cornpounds can occur. 3.2. Electronic effects ofgrain boundaries Although the electronic effects of grain boundaries are crucial to the operation of thin film solar cells there has been relatively little analysis of the details of these effects. The principal effects of grain boundaries are to act as sinks for minority carriers (thereby effectively reducing carrier diffusion length) and to strongly reduce the mobility of majority carriers across the boundaries. Sosnowski [66] discusses recombination effects in Si and Ge and gives data for the grain boundary recombination velocities. Grain boundary recombination is also discussed by RaiChoudhuri and Hower [67] and by Ettenberg [68]. The effect of grain size on the efficiency of polycrystalline GaAs and Si solar cells is discussed quanti. tatively by Hovel [8] and by Soclof and lies [69]. An excellent study by Kazmerski, et al. [70] relates

A.L. Fahrenbruch / II— VI compounds in solar energy conversion

81

the electrical transport properties of CdS films to preparation variables by using the grain boundary mobility theory of Petritz [71] with surface scattering theory to find values for the grain boundary potentials and to predict electronic mobility in these films. To gain a qualitative understanding of the situa-

recombination. This is true no matter what the diffusion length within the bulk crystal forming each grain. From these considerations it is clear that to maximize cell efficiency the grain size must be large with respect to 1/a. The effective diffusion length Le is limited by grain size, Le ~ ‘y, and by the thickness of the film (if the surface recombination velocity S5 is

tion in a solar cell we examine some simple models. Consider the quantum efficiency Q of the simplest backwall heterojunction where all the useful absorplion takes place in the small bandgap material. Assume for a moment that the cell is all single crystal. If the thickness of the absorbing layer t is large compared with the optical absorption length 1/a the ratio of the number of electrons collected at the interface to the photons absorbed in bulk of the absorber is l/(1 + 1/ OLD). Since fori.e., thelarge material we mobility want LD ~= 1/2 atoisbefixed large; carrier (kTpr/q) (maximized by reducing defect concentrations) and long minority carrier lifetime r. Now consider the polycrystalline film cell geometry shown in fig. 2. We have assumed here that the grain boundaries in the primary layer will propagate through the interface into the second, heteroepitaxial layer. Assume for simplicity: (1) a backwall cell with t ~ 1/a; (2) the grain boundary recombination velocity, Sg, is large; (3) the interface recombination velocity at S 1 at the junction is negligible. We follow in part a model due to Rothwarf [72]. In the absence of fields the photoexcited carriers within the dashed cone can diffuse to junction and be collected while those outside the cone will most likely diffuse to the grain boundaries and be lost through

large), Le ~ t. While the above example is quite naive with respect to its assumptions (in particular the electric fields near the grain boundaries usually are in such a direction so as to promote recombination there) it serves to illustrate the principles involved. A more quantitative treatment of the problem is Shockley’s analysis of carrier lifetime in filaments [73]. The thickness of the film is easy to control. The grain size can by of variation of growth parameters suchbeascontrolled the grain size the primary layer or by recrystallization. The values of Sg and S~are much more difficult to predict and/or control. Very little is known about values of surface recombination velocity in the Il—VI compounds. From these considerations three ways to optimize the efficiency of thin film cells are evident: (1) To optimize the configuration; e.g., backwall versus front wall. Examples of two such configurations are shown in fig. 3a and b. These figures show a typical growth morphology on an amorphous sub-

~-

RECOMBIN~TION —‘

,‘\

1

~

__T—_—~ ~

~

1”

__~

I

ABSORB1M3

~

~

~

~

WINDOW

-

Fig. 2. Relationship of carrier diffusion length and optical absorption length to grain size and layer thickness in polycrystalline film heterojunction.

strate [65,74] at least for the primary layer. (2) To increase the grain size by recrystallization or by adjusting growth parameters to obtain larger primary layer grains. (3) To decrease the effective Sg and S5 by passivation or internal electric fields. Another possible effect of grain boundaries intersecting the junction interface is to increase the interfacial recombination thereby increasing the .Jo and lowering the attainable cell efficiency. Very little, if any, research has been done on this matter in the II— VI materials. In most Il—VI films the transverse (along-thefilm) electronic mobility is dominated by grain boundaries. Only at very high doping concentrations are the potential barriers at the grains thin enough (and/or low enough) for extensive transport so that the mobility can approach its bulk crystal value. For example, see fig. 4. If carriers must be collected by a grid structure on the light incident side then the

AL. Fahrenbruch / I/—V/compounds in solar energy conversion

82

LIGHT

~

LIGHT

a

b

SUBSTRATE

Fig. 3. (a) Polycrystalline thin film heterojunction solar cell schematic. In this configuration heteroepitaxy is definitely advantageous to increase LD as well as decrease Sj. Front wall illumination would also be good given a large aspect ratio (7/t) and low surface recombination velocity. (b) Polycrystalline thin film heterojunction solar cell. Heteroepitaxy would probably be advantageous in this configuration to decrease S 1. Front wall illumination would not be efficient in this configuration.

material there must be ~1 12 cm. If a transparent conductor offering complete coverage can be used this limiting value is about 500 12 cm and we are concerned with lateral mobility which may be much larger,

particularly if the grains are fibrous and run all the way through the film. This is of great advantage if highly resistive materials such as ZnCdS or ZnS are used as window materials. 3.3. Growth methods

io~ -

3

g

10 \\ -

w

to CdTe/CdS and other II—VI compounds; (3) a brief report of solution spray results on CdTe.

\

io° \

3.3.1. Vacuum deposition -

-2

-

10

°

_______________________________ 6 id7 id~ io~

id

Growth methods applicable to Il—VI materials are outlined in table 3 with brief comments. Since most methods are covered extensively in the literature we concentrate on a few pertinent topics from our own work: (1) coevaporation doping of CdS and the effect of doping level on transverse mobility; (2) the close-spaced vapor transport method applied

ri~cm

Fig. 4. Resistivity versus carrier concentration n for vacuum deposited CdS films. Values of n from thermoelectric power measurements. (Open circles denote films co-evaporated with In.)

Deposition of II—VI compounds by vacuum evaporation is perhaps the most widely used technique. These compounds evaporate congruently and dissociate completely on evaporation. Doping during evaporation to control unless a separate dopantis generally source ordifficult flash evaporation is used. Many workers have used a subsequent heat-treatment to improve stoichiometry and crystallrnity and to incorporate dopants. Since a departure from stoichiometry may occur on heating the films in a vacuum, an .

.

.

.

.

A.L. Fahrenbruch / II— VI compounds in solar energy conversion

83

Table 3 Summary of Il—VI compound growth methods Method

Typical substrate temperature (°C)

Typical rate (Jim/mm)

Comment and references

Proven for all II—VI compounds E.g., Cd in S 2 [75] CdS [76] Cd or Zn in H2S High thermal gradients required Least expensive [12,77] Excellent control of deposition parameters Improved stoichiometry, crystallinity [281 Post-anneal required. Used for CdS. CdTe by Bonnet [7]

Vacuum evaporation Reactive evaporation

>180 >180

1

Sputtering Reactive sputtering CSVT Solution spray CVD Hot wall vacuum evaporation Silk screen Vapor transport Electrodeposition aqueous molten salt

any any >350 >300 >600 >300 RT >500

1

<100 >700

inert gas ambient or additional source material is usually present in the heat-treatment chamber. “Hot wall” vacuum evaporation techniques, in which the evaporation—deposition process takes place in a separate enclosure within the vacuum chamber, have been used by Koller and Coghill [79] for ZnS and by Elliot, Halsted, and Coghill for CdTe [23]. This method provides much higher effective fluxes and since impinging atoms have been thermalized to a temperature much closer to that of the substrate, higher substrate temperatures can be used to improve crystallinity and stoichiometry. Dopants are easily introduced as gases or by coevaporation. This technique is clearly very close in principle to chemical vapor deposition. In our research on coevaporation of dopants, CdS films have been deposited on quartz, soft glass, and indium—tin oxide coated glass substrates, as well as on p-CdTe single crystals. A Mo Knudsen cell containing CdS at 885°Cwas used as the source, and films of thickness 1 to 5 pm were deposited at rates of about 0.2 pm/mm typically. Good control of conductivity has been obtained by coevaporation of In donors without subsequent heat-treatment. Films with excellent optical properties are obtainable in this manner withThe bulk resistivities ranging from to 0.002 to 4 12 cm. resistivity can be controlled within i0 a factor of two in most cases by control of the In

—5

—

—

3 0.2 0.2—2 20 —

0.3 — —

CdTe by Kroger 1771 Corrosive to many substrate materials

source temperature. As the concentration of In impurity is increased, the electron mobility in the plane of the film increases strongly and the optical absorption a few hundred Angstroms beyond the absorption edge of CdS actually decreased. Transport properties of several CdS films were analyzed by thermoelectric power vs temperature measurements, with the results given in table 4 [80] (also see fig. 4). The co-evaporation doping techniques have also been used to deposit ZnSe films. To illustrate the strong effect of heteroepitaxy we compare the resistivities of CdS films evaporated simultaneously on (111) oriented CdTe and on soft glass at a substrate temperature of 300°C(also using In co-evaporation) in table 5. 3.3.2. Chemical vapor deposition (CVD) Considerable success has been obtained using CVD deposition of Il—VI compounds (Cusano in Aven and Prener [17]). Early work utilized a reaction of the elements or simple compounds in a gas stream (e.g., Cd + H2S -+ CdS) on a heated substrate. Modern approaches use organometallic complexes as reactants (Manasevit [5]). Stoichiometry and growth parameters are easy to control, growth by rates can be high, and doping is easy to accomplish introduction in the gas or vapor stream. An example is the deposi-

84

A.L. Fahrenbruch

/ Il—V/compounds in solar energy conversion

Table 4 Transport properties of vacuum evaporated CdS films on glass Film description

Resistivity, 300 K (ohm-cm)

No In Co-evaporation Before heat treatment After 400°Cheat treatment in H 2 In Co-evaporation 3 Calculated In density: 1.5 X 1018 cm3 3.1 cm iO’9 cm—3 a Value of ~E for ~ =

l0~ 14

6.3 0.055 0.0087

4.4 X 1016 4.3X 1018

7.4 X 1018 1.1 X 1019 l.7X 1019

Mobility, 300 K (cm2/V-sec)

Mobility activation Energy ~ (eV)

0.013 0.1

0.32 0.04

0.13 10.0 42

0.067 0.027 0.00

~o exp(—~E/kT).

tion of high conductivity n-CdTe by introduction of CdI 2 and Ga by Cusano [81]. A possible disadvantage is the high substrate temperature required, usually >600°C. 3.3.3. Close-spaced vapor transport (CS VT) In the CSVT method, material is transported from a source wafer to a substrate through 0.1 to 0.3 cm of H2 at atmospheric pressure [82—84]. The source and substrate are in contact with carbon blocks (typically) that may be heated either by infrared lamps or by resistance heater elements. The temperatures of the source and substrate are usually independently controlled. The advantages of the method are: (1) the apparatus is quite simple and the essentially planar geometry conserves source material; (2) sintered or powder sources can be used; (3) extremely high deposition rates can be achieved. Table 5 Resistivity (ohm-cm): comparison of CdS films on glass and onCdTe(111) __________________________________________________ Glass CdTe 4000 10.5 0 061

3)

n, 300 K (cm—

0.59 0.0037 0 0025

Its disadvantages are the complexity of the transport process itself and the large temperature gradients that must be maintained (100 to 300°C/mm). In most cases doping is accomplished by incorporation of the dopant into the source wafer and is thus not under immediate control. Considerable information on the growth rate, grain size, and surface morphology of p-CdTe layers deposited on single crystal n-CdS by CSVT has been obtamed by Vasilchenko et al. [25] as a function of the source and substrate temperatures, and the temperature versus time profiles of the source and substrate. Using source and substrate temperatures of —720 and —420°C,p-CdTe films with grain sizes of 40 to 60 pm were obtained with resistivities of 12 12 cm. Deposition rate was —2 pm/mm. In connection with these studies the technique of “temperature profiling” was used (Mitchell et a!. [19]). In this method, the deposition was started with a very high substrate temperature (giving high atomic surface mobility) and a low source temperature (low impingement rate) in order to favor the growth of few but large and well oriented crystallites. As growth continued, the source -

(substrate) temperatures were then gradually raised (lowered) to levels required for high growth rates. Heteroepitaxial crystallite sizes were 20 to 60 pm compared with 7 to 15 pm without profiling. This .

.

-

position techniquemethods might be as well. used to advantage with other de-

AL. Fahrenbruch / II— V/compounds in solar energy conversion

3.3.4. Solution spraying In the spray reaction technique an aqueous solution of the reactants is sprayed on a hot substrate in an inert or reactive atmosphere. For example, Sn02 films were made by Mochel in 1951 [86] by spraying a SnC12 solution in air. Solution spraying has been used extensively for CdS (Chamberlin and Skarman [87] and Jordan [12]) and the technique is readily applicable to mixed compounds such as (ZnCd)S and Cd(S,Se). Doping is easily accomplished by addition to the spray solution. Sprayed films are of generally lower quality than those made by vacuum evaporation or CVD; they are usually structurally complex with small grain sizes. With proper conditions high carrier mobiities can be obtained, however (Ma [881). A strong advantage of the method is its simplicity and that it is perhaps the lowest cost method of CdS deposition. The method of solution spraying (also called spray pyrolysis) has also been applied for the deposition of films of n-CdS on single crystal p-CdTe substrates, as well as on glass. These films are produced by spraying a solution of CdC12 and thiourea onto a heated substrate in air at temperatures 400°C. Solar efficiencies greater than 6% have been initially achieved by cells prepared by solution spraying CdS onto single crystal CdTe, and some of the highest opencircuit voltages observed for this system (up to 0.74 V) have been measured on cells prepared by solution spraying [88]. 4. Types of cells In this section we discuss briefly some fundamentals of current transport in heterojunction structures. Then we present results on a number of different cells involving Il—VI compounds, concentrating principally on the CdS/CdTe cell, 4.1. Fundamental considerations There are two major mechanisms for carrier transacross a photovoltaic heterojunction: injection over the junction barrier from conduction band to conduction band (giving a diode factor of A 1 *) port

*

J = J0 {exp(qv/Akfl

—

i} in the dark.

85

® injection

i~

Recombina

Fig. 5. Current transport mechanisms in forward biased p—n heterojunction.

and transport through recombination centers at or near the junction interface. The latter mechanism may involve either thermal activation or tunneling or both. Although the thermal activation—recombination transport (which usually dominates in heterojunctions at room temperature) is governed by an A 2, suggesting a higher V~,the values of Jo found for this mechanism are also much higher. Thus efficiencies observed for either of the mechanisms aren’t that much different. The mechanisms are shown schematically in fig. 5 and there is a considerable amount of literature about them by this time (e.g., Milnes and Feucht [18]). Intuitively one might expect that an epitaxial relationship between the two components of a heterojunction would play a large part in determining its electrical transport properties. Although varying degress of epitaxy, ranging from oriented small grains to single crystal layers, have been produced experimentally the connection between epitaxy and cell efficiency remains elusive. The effect of impui~tiesat the junction interface may well overwhelm any dependences on the degree of epitaxy. The effects of these parameters will become more clear when single crystal—film heterojunctions made without breaking vacuum are evaluated. Lattice mismatch between epita.xial components introduces dislocations which may act as recombina-

86

A.L. Fahrenbruch / II— VI compounds in solar energy conversion

tion centers in the vicinity of the junction interface (ref. [58], p. 50). According to first principles the total number of these centers should be constant whether they are concentrated at the interface or spread out in a graded junction. In the preparation of heterojunction, however, the condition of the substrate surface is critical and even with zero lattice mismatch, interface states will arise from surface contamination, oxides, and surface defects unless the most scrupulous conditions are met. An exception to this is possibly the case where Cu2S (or Cu2Te) is grown topotaxially “into” the CdS substrate by a replacement reaction. The general effect of recombination centers near the barrier (from whatever cause) is to strongly increase J0 and to increase A to —2. A second effect of the centers is the recombination of photogenerated carriers as they pass over the interface. Since such recombination depends on the electric field at the interface, the collection efficiency of the light current crossing the interface is bias voltage dependent and may cause an appreciable decrease in the fill factor (Rothwarf [89] and Mitchell in ref. [4]). Given a distribution of recombination centers at the interface the doping profiles near ~he junction can have a strong influence on the effectiveness of the centers (Van Ruyven [90], Fahrenbruch and Bube [91]). This may be used to advantage when cells are fabricated. For example, in the Cu2S/CdS cell, heattreatment is used to slightly widen the depletion layer in the CdS by Cu diffusion and thus optimize the cell characteristics. Despite these dire implications, quantum efficiencies of —80% are seen in the Cu2S/CdS system (4% lattice mismatch) and 85% in the CdS/CdTe cell (9.7% lattice mismatch and using a substrate surface polished by wet chemistry techniques). The effect of a conduction band discontinuity is also problematic. Electron affinities are not known well enough in many cases to be certain of the magnitude (or sign) of such discontinuity. Many affinity determinations depend on vacuum/semiconductor interfaces or other junctions than the one of interest, Given the extreme sensitivity of such determinations to surface states one must be cautious in using such values to predict the usefulness of a particular junction. Beyond that, a grading of the junction of more than 0.01 to 0.1 pm can erase the effect of a con-

duction band spike almost entirely (Cheung [92]). Prediction fails here and we must measure carefully preparedjunctions. 4.2. Commentary on particular cells We conclude the paper by taking a closer look at several cells. The Cu2S/CdS and CdS/InP cells are mentioned briefly since they are covered elsewhere in this issue. We focus on the CdS/CdTe, ZnSe/ CdTe, CdS/CuInSe2 and ZnTe/CdSe cells and mention several others. p-Cu2S/n-CdS. At present the Cu2S/CdS cell represents the only large scale demonstration of commercial viability for a cell involving lI—VI cornpounds. In many ways it is also the most complex of the junctions, bringing together a host of complicated phenomena both from the Cu2~Sand the CdS : Cu. The important points in the context of this article follow: (1) The film/film cell demonstrates large quantum and solar efficiencies despite fairly large lattice mismatch (—4%). (2) Its relatively high efficiency in the frontwall mode points to a low effective surface recombination velocity for Cu2S. (3) The Cu2S is usually grown topitaxially “into” the CdS by a replacement reaction, a process which promotes the best epitaxy possible while tending to move foreign contaminants away from the interface. An excellent review of the Cu2S/CdS cell is given by Stanley [93]. n-CdS/p-InF. This has been the highest efficiency single crystal cell involving a Il—VI compound reported to date (Shay [12]). Most interesting is the low lattice mismatch (<0.3%) of this combination which should yield extremely low J0. The cell should not be subject to degradation problems since relatively long heat treatments ca. 600°Care part of its processing. Perhaps its principal disadvantage lies in the difficulty in producing InP. n-CdS/p-CdTe. CdS/CdTe heterojunctions have been prepared by a number of workers including Bonnet and Rabinhorst [7] who used a vapor growth technique to produce film-on-film cells of 5—6% efficiency. n-CdS/p-CdTe heterojunctions have been prepared by closed-spaced vapor transport of p-CdTe films onto single crystal n-CdS [25], by vacuum eva-

A.L. Fahrenbruch III— VI compounds in solar energy conversion

poration of n-CdS films onto single crystal p-CdTe [94,95], and by solution spraying of n-CdS films onto single crystal p-CdTe [88]. The highest solar efficiency to date, L2%, has been obtained with a cell prepared by vapor transport of an n-CdS film onto p-CdTe crystal [99].

Table 6 Photovoltaic properties of n.CdS/p-CdTe heterojunctions

___________________________________________ Ideal junction calculation a

Parameters for cell with highest measured solar efficiency b

A variety of CdS/CdTe heterojunctions have been prepared by the vacuum evaporation of n-CdS films onto single-crystal p-CdTe substrates [4]. Table 6 compares the properties of such a cell with calculations for an ideal junction. Table 7 compares the results depending on the resistivity of the CdTe

—-___________________________________________

2) J A0 (A/cm -‘sc (A/cm2) Voc (V) V~(V)c J~,(A/cm2) C ff Quantum efficiency Solar-efficiency (%) (AkT/q) ln(I 5~/Io)(V)

3.2 x 10—10 2.0 19.8 X i0~ 0.90 0.77 19.3 X 10~ 0.83 1.0 17.0 0.90

87

1.7 x 10—8 1.89 16.1 X 10~ 0.63 0.51 13.3 < ~o—3 0.66 0.82 7.9

substrate used, and on the surface treatment applied to the CdTe before vacuum evaporation. The cells prepared on surfaces chemically polished with a methanol—bromine etchant are significantly better than those prepared on mechanically polished surfaces. These results indicate that mechanically induced defects close to the interface act as recombination centers and that the methanol—bromine etch significantly reduces their concentration.

0.63

a Calculated assuming generation—recombination dominates

Jo,

and neglecting any losses from series resistance, shunt resistance, reflection, less-than-unity quantum efficiency, or less-than-unity area ratio because of electrodes. b Measured values solar illumination ofand 85 2 for a cellfor withsimulated an indium—tin—oxide coating, amW/cm glycerol antireflection coating, with known residual reflection loss of 6%. Values are calculated on the basis of the active cell area; total area = 0.075 cm2, active area = 0.053 cm2.

The for temperature has beenCdTe. measured the cells dependence prepared onof 3J0ohm-cm Above about 294 K, both methanol—bromine etched and mechanically polished cells show a similar variation with Jo J 00 exp(—i~E/kfland L’~.E= 0.60 eV. Joo is consistently lower for the methanol—bromine etched samples than for the mechanically polished samples, indicating that the etching step improves the

c Voltage and current density at the maximum power point.

Table 7 Photovoltaic properties of n-CdS/p-CdTe heterojunctions a Property

Voc (V) 2) efficiency C ~Quantum (mA/cm Fill factor CdS resistivity (ohm-cm) ~ (A/cm2) Diode factor,A Solar efficiency e (%)

3 ohm-cm p-CdTe MB b etch

0.59 13.9 0.85 0.55 2.1 2.8 X 10—8 1.80

5.2

-

133 ohm-cm p-CdTe MB b etch

0.58 13.7 0.84 0.52 0.81 10 ~ 1.48 4.9

3 ohm-cm p-CdTe mech. polished

-

0.53 10.0 0.61 0.52 0.31 l0~ 2.02 3.3

~ Illumination by solar simulator with 85 mW/cm2. b MB stands for methanol—bromine. C Corrected for 17% reflection loss. d Vacuum evaporated CdS without In co-evaporation; heat-treated in H

0.55 10.3 0.63 0.49 0.36 8 X 10—8 2.07 3.2 -

-

2 after deposition. e Not corrected for 17% reflection loss.

133 ohm-cm p-CdTe mech. polished

88

AL. Fahrenbruch /11—V/compounds in solar energy conversion

quality of the junction interface, reducing the density of interface states giving rise te recombination currents. Presumably the 0.60 eV represents the thermal activation energy for holes in the p-CdTe at the interface in order to recombine with electrons via interface states. Below 294 K, transport at the junction appears to be controlled by tunneling through interface states and not by thermal excitation. When the open-circuit voltage of these cells is measured as a function of temperature, it increases with dV0JdT = —2 X i03 V/K with an intercept at 0 K of 1.20 V. The highest actually measured value of V~(at 80 K) is slightly larger than 1 .0 eV. At low temperatures the open-circuit voltage should saturate with increasing light intensity at a value equal to the diffusion potential of the junction. These results show good agreement with calculated values of the diffusion potential. Results obtained when In was co-evaporated with the CdS to produce CdS films of 0.005 ohm-cm resistivity (as measured on the p-CdTe substrate) show values of solar efficiency and fill factor which are somewhat less than those of table 4. This is primarily due to larger values of J 0 which could be caused by diffusion of In into the CdTe bysubstrate the high substrate temperature and/or increasedat tunneling through the much narrower depletion layer in the higher conductivity CdS. The measured efficiency of the cell of table 6, 7.9%, is lower than the ideal efficiency of 17.6%, due primarily to a larger value of J 0 and a voltagedependence of the collection of photocarriers, defined here as the collection function. The collection function not only reduces the quantum efficiency but also significantly reduces the fill factor. The collection function depends on the interface recombination velocity Si, the electric field at the junction ~, the CdTe optical absorption coefficient a, the minority carrier diffusion length LD, and the depletion layer width W. The effect of S1 on the collection function can be significantly reduced by increasing the CdTe acceptor concentration in order to increase e. Values ofLD for this junction of 0.40 pm in the pCdTe and 0.43 pm in the n-CdS were obtained using the electron-beam-induced current technique. The CdS value is in good agreement with Oakes and Greenfield’s data for similar CdS films [96]. Although LD is only 0.40 pm in the CdTe, the

photocarrier collection from the bulk is high due to the large values of a for CdTe. The large quantum efficiency obtained in spite of the small diffusion length in p-CdTe is encouraging evidence that transferral of techniques from single crystal p-CdTe to polycrystalline p-CdTe should not result in appreciable loss of photovoltaic response. The p-CdTe used had a ratio of hole/P-impurity density of about iO~ improvement in this ratio indicates that appreciably better photovoltaic properties are possible with the film-crystal cell, if the diffusion length is, as cxpected, inversely variant with the total P density. Using CdTe with higher acceptor concentrations and longer diffusion lengths, solar efficiencies of 10—15% for the CdS/CdTe heterojunction can be expected. ITO/CdTe Junctions. Indium—tin oxide (ITO) is an excellent candidate for use as the large bandgap window of a heterojunction solar cell since its bandgap of 3.0 eV makes it transparent to a major portion of the solar spectrum. Mitchell (in [39]) deposited ITO onto a single crystal p-CdTe substrate, producing a heterojunction with V 2. 0.66 V and J5~ = 8.7 mA/cm The collection of0~=photogenerated carriers increased markedly with reverse bias; for a reverse bias of —4.0 2. This system, V, the light current was 20 mA/cm therefore, also shows promise for future development. p-CdTe/n-ZnSe. This cell has a large window and a high diffusion voltage which indicates a theoretical efficiency of 21 to 24%. However, the values of electron affinities for the two materials predict a conduction band spike which may impede photogenerated carrier flow. Cells have been made by CSVT but their spectral response indicates interlayers of ZnTe or CdSe formed by alloying at the high substrate ternperatures used (ca. 500°C)[85]. n-ZnSe/p-GaAs. This cell is the analog of the CdS/ InP cell. In theory it holds great promise; the bandgaps are optimum, lattice mismatch is very small (<0.3%), and the electron affinities are perfectly matched. Mimes and Feucht [18] conservatively rate its maximum theoretical efficiency at 19%. This heterojunction has been studied by several workers for potential LED application but photovoltaic results have been disappointing. Wasa and Hayakawa [97] using sputtered ZnSe films found poor photovoltaic response only after heat-treatment under bias voltage. One major difficulty is making the ZnSe films sufficiently conducting.

A.L. Fahrenbruch / Il—V/compounds in solar energy conversion

p-Cu 2Te/n-CdTe. Although this junction is the analog of the Cu2S/CdS cell and it can be made by the same dipping process, its operation appears to be completely different. In the Cu2S/CdS cell nearly all the useful absorption takes place in the Cu2S. In contrast Cusano reports [55] no appreciable photovoltaic response from the Cu2Te. The p-Cu2Te appears to act as a contact to the p-CdTe : Cu adjacent to the junclion. [-Icobtained solar efficiencies of 6% from film! film cells and 7.5% for film-on-single crystal cells, p-CuInSe2/n-CdS. The CuInSe2/CdS system has low lattice mismatch (1.2%) and junctions2 have and yieldV ed 5.7% efficiency with J~= 21 mA/cm 0~= 0.45 V in backwall film/film form (Kazmerski [1,6]) and 12% in film/crystal form (Shay [50]). CulnSe2 is quite easy to grow by vacuum deposition techniques and can readily be doped both n and p-type by heattreatment in 02 or in H2. p-Cu2S/n-ZnCdS. The addition of ZnS offers a means of increasing the V0~of the Cu2S/CdS cell, The Delaware group reports promising results despite 2). Cell efficiencies of somewhat low J~0.68 (8.7 V mA/cm 4.0% with V~of (compared to 0.5 V usually seen in Cu 25/CdS cells) and fill factors of 68% have been obtained using Zno22Cd0,78S [5]. Schottky barrier cells. In principal it should be possible to make efficient Schottky barrier cells with several of the Il—VI compounds. In particular n-CdTe can be used with an Au or Ni semi-transparent contact. The potential barrier height for such a contact appears to be fixed at —0.9 eV above the conduction band by surface states on the CdTe because of its predominantly ionic nature [98]. Thus the potential barrier height is independent of the metal work function. Bell, Serreze, and Wald [27] report Schottky barriers of Au on n-CdTe with measured barrier heights of 0.9 to 1.0 eV. The detailed photovoltaic properties of the resulting barriers are extremely dependent on preparation variables. V0~up to 0.55 V are observed but ,J~and the fill factor are low and the solar efficiency is —1%. Given the simplicity of these junctions and the potentially high efficiencies predicted, this seems a very promising research area almost untouched until now. -

.

5.Summary The obvious strong points of the 11—VI compounds

89

are their direct ba’d-gaps and the ease with which well-structured films can be deposited by a large variety of methods. Most are electronically and structurally complex materials and except for CdS they are relatively not well explored. Existing solar cell technology shows that a 10 to 12% efficiency goal for all-thin-film cells should easily be attainable within a few years at the present rate of research. While the Cu2S/CdS cell is by far the closest to commercial reality, many other relatively unexplored junctions appear to show equal or greater promise in terms of efficiency and stability. A review of the current literature shows trends toward growth at higher substrate temperatures, toward planar geometries, and the blending of deposition methods (e.g., reactive evaporation and CVD). The literature also reveals the emergence of a capability for reproducibly producing epitaxial films with a high degree of perfection under very closely controlled conditions. For the sake of photovoltaics research this capability must films. be coupled with electrical measurements on these Important future research areas are: (1) Investigation of the role of grain boundaries on electronic transport should be expanded. What are typical grain boundary recombination velocities and can passivation reduce them? (2) Measurements of surface and interface recombination velocities are urgently needed. (3) Recrystallization of films may prove a necessity yet the technique is relatively unexplored in the II— VI compounds. (4) Defect studies and minority carrier lifetime measurements are necessary, particularly in highly doped absorber materials.

References .

.

.

-

[1] National Solar Photovoltaic Review Meeting, ERDANSF/RANN, Orono, Me., 1976. [2] J. Dieleman, in: Proc. Intern. Workshop on CdS Solar Cells, University of Delaware, April 30, 1975, p. 92. [31 J.L. Shay and S. Wagner, J. App!. Phys. 47 (1976) 614. [4] A. Fahrenbruch, F. Buch, K.W. Mitchell and R.H. Bube, 12th IEEE Photovoltaic Specialists Conf., Baton Rouge, La.,1976. [51A. Barnett of the University of Delaware at ref. [1]. [6] L.L. Kazmerski, Ternary Compound Thin Film Solar

A.L. Fahrenbruch /1!— VI compounds in solar energy conversion

90

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