Experimental and theoretical studies of Cu2O solar cells

Solar Cells, 7 (1982 - 1983) 247 - 279

247

EXPERIMENTAL AND THEORETICAL STUDIES OF Cu20 SOLAR CELLS L. C. OLSEN, F. W. ADDIS and W. MILLER

Joint Center for Graduate Study, 100 Sprout Road, Richland, WA 99352 (U.S.A.) (Received November 24, 1981; accepted March 2, 1982)

Summary Extensive studies of Cu20 Schottky barrier solar cells have been conducted. Schottky barrier devices based on metals characterized by a wide range of work functions have been investigated. Cell characterization includes electro-optical studies, thermodynamic considerations concerning interface stability and depth-concentration profiles. Cu/Cu20 cells were developed that exhibited active-area air mass 1 values of photocurrent and efficiency of 8.5 mA cm-2 and 1.8% respectively. A detailed photon and carrier loss analysis conducted for Cu/Cu20 cells is used to project the ultimate values of the photocurrent for Cu20 cells to be 12 - 14 mA cm-2. Thermodynamic considerations indicate that thallium is the only metal which can be combined with Cu20 to yield an adequate efficiency. However, T1/Cu20 Schottky barrier cells exhibit properties similar to Cu/Cu20 devices. Depth-concentration profiles show that, although no T1-O bonding exists in the interfacial region, the region is copper rich. It is concluded that the oxygen deficiency occurs because of preferential sputtering of oxygen during the thallium deposition process. As a result of these Cu20 Schottky barrier studies, it is concluded that significant improvements in the efficiency of Cu20 solar cells can be achieved only with a homojunction structure. Thus, an approach to doping Cu20 n type must be developed in order for the potential of this material for low cost photovoltaics to be realized.

1. Introduction The potential for Cu20 to be used in semiconducting devices has been recognized since at least 1920. Most recently, Cu20 has been investigated as a candidate photovoltaic material. Experimental and theoretical studies of Cu20 solar cells have been conducted at the Joint Center for Graduate Study (JCGS) over the past few years. It is the purpose of this paper to present a summary of the JCGS work and, most importantly, to discuss the potential viability of Cu20 for low cost photovoltaic power generation. 0379-6787/82/0000-0000/$02.75

© Elsevier Sequoia/Printed in The Netherlands

248 Background material regarding Cu20 is presented in Section 2. In Section 3, Cu:O substrate fabrication mid characterization are discussed. The results of a detailed analysis concerning the potential Cu20 cell photocurrent are discussed in Section 4. Investigations of Cu20 Schottky barrier solar cells are presented in Section 5, and other types of solar cell structures are considered in Section 6. Finally, conclusions are presented in Section 7. 2. Background material In this section the reasons for investigating Cu20 as a photovoltaic material are discussed, a historical perspective concerning relevant work on Cu20 devices is presented and key technical issues are considered.

2.1. Basis for investigating Cu20 There are four primary reasons for investigating Cu20 as a potential material for low cost photovoltaic power generation, namely (i) theoretical solar cell efficiency, (ii) material abundance, (iii) the simple process for semiconductor layer formation and (iv) its possible use as a material for one cell of a two-cell system or for a low cost luminescent concentration system. Some basic properties of Cu20 are given in Table 1. The band gap is 2.0 eV, which is in the appropriate range for photovoltaics. The material is p type as grown. An approach to doping Cu20 n type has not been determined. The m a x i m u m power conversion efficiency for Cu20 is determined by assuming that the photocurrent acquires the maximum possible value and that parasitic current losses are due only to the injection of minority carriers across the junction formed either by a homojunction or by a heterojunction. The maximum air mass (AM) 1 photocurrent that can be generated by a cell based on Cu20 is 17.3 mA cm -2. If it is assumed that current losses are due to the injection of minority carriers from one side to the other of a homojunction or a heterojunction involving a large band gap n-type material in conjunction with Cu20, then the J0 value can be estimated by [1] J o i ~ l O S e x p ( - - - ~ -) TABLE 1 Properties of Cu20 Band gap Electron affinity Crystal structure Lattice constant Melting point Density Thermal expansion coefficient Conductivity type

2.0 eV (direct forbidden) 3.2 eV Simple c u b i c 4.27 1235 °C 6.0 g cm-3 3.5 × 10 -6 °C-I p type

249

and the maximum AM 1 efficiency is calculated to be approximately 22%. However, it should be noted that J0i for Cu20 is estimated to be 4 X 10 -2s A cm -2. One recombination center per cubic centimeter could be expected to yield J0 ~ 10-17 A cm -2. As a result, the ideal device efficiency estimated on the basis of an injection current is probably meaningless. If it is assumed that current losses are due to depletion region recombination, then the associated J0 value should be of the order of [1]

For C u 2 0 , '-/Or ~ 1 0 - 1 7 A cm -2 and the corresponding maximum efficiency is 14%. Investigations made to date suggest that it is realistic to expect Cu20 solar cells eventually to exhibit a photocurrent of 12 - 14 mA cm -2. To achieve practical effieiencies of 9% - 11%, a J0 value of 10 -17 A cm -2 is required. A key conclusion of this work is that such low values of J0 may only be achieved with a homojunction. The maximum possible efficiencies on the assumption of parasitic losses due to injection or recombination current mechanisms and the estimated efficiencies on the assumption of realistic values for JPH are given in Table 2. In addition to the fact that the potential efficiency is high, another motivating factor concerning studies of Cu20 cells relates to the abundance of copper. Copper is one of the most abundant minerals in the Earth's crust. Furthermore, the cost of copper is sufficiently low that the contribution to the projected cell cost due to raw copper is found to be negligible. Another interesting aspect of Cu20 concerns the growth of the material. The formation of semiconducting layers will be one of the most costly steps in any low cost solar cell fabrication scheme. For Cu20, this step could ultimately be very simple. Copper sheet can be oxidized at 1000 °C to obtain large-grain polycrystalline material. For example, 0.125 mm copper sheet can be allowed to oxidize at 1000 °C to achieve Cu20 sheet with grains several millimeters wide. Copper sheet can also be partially oxidized to obtain Cu20 films with grain sizes several hundred microns wide. A partially oxidized copper substrate results in a Cu20 junction, however. This interface is the basis of the so-called Cu/Cu20 back-wall rectifier. If a Cu20 film grown in TABLE 2 Theoretical and practical efficiencies

JPH

Parasitic current mechanism

Jo (A cm - 2 )

n

(mA cm - 2 )

Efficiency (%)

17.3 17.3 14.0 12.0

Injection Recombination Recombination Recombination

4 × 10 -26 10-17 10-17 10 -17

1 2 2 2

22 14 11 9

250 this manner were utilized for thin film solar cell structures, a method for making this contact ohmic would have to be developed. There is a fourth reason which has motivated photovoltaic investigations of Cu20 , but to a much lesser degree. Since the band gap is approximately 2.0 eV, Cu20 is a candidate material for one cell of a two-cell system. In addition, Cu20 may also be highly compatible with a low cost luminescent concentration system.

2.2. Historical perspective The first device based on Cu20 was a back-wall copper oxide rectifier. This device was fabricated by L. O. Grondahl in 1920. During the 1920s and early 1930s, development efforts were carried out to correlate rectifier characteristics with fabrication techniques. Grondahl summarized these efforts in 1933 [2]. Grondahl also reported preliminary results on a photocell made by reducing the surface of Cu20 to form a Cu/Cu20 Schottky barrier. He reported a photoresponse of about 50% at 500 nm. No effort was made to characterize the device as a solar cell; only the measured photoresponse at 500 nm was mentioned. In the 1940s, efforts were carried out at Bell Laboratories by Bardeen, Brattain and Shockley to develop a basic understanding of the copper oxide rectifier. During this time period, models for Schottky barriers were developed. Interest also began to move to silicon. Again, no effort was made to develop Cu20 solar cells. Brattain published a review of the work done at Bell Laboratories in 1951 [3]. Assimos and Trivich published results concerning the photoresponse of front-wall Cu/Cu20 cells in 1973 [4]. In 1975, two programs specifically aimed at Cu20 solar cell fabrication and characterization were initiated by the National Science Foundation as part of the Research Applied to National Needs Program, one at Wayne State and the other at JCGS. Both programs were relatively small. Both groups investigated front-wall Schottky barrier cells. Power conversion efficiencies had approached 1% by 1978. Additional work conducted by the JCGS group has resulted in an increase in the Cu20 solar cell efficiency to about 1.8%. It should be strongly emphasized that the total integrated effort devoted to Cu20 solar cell research and development is insignificant compared with other materials at present being investigated. The rather crude investigations conducted on back-wall rectifiers in the 1920s and 1930s are often mistakenly regarded as work related to solar cell development. In fact, semiconductor physics had not y e t been developed. The somewhat more recent work of Brattain, published in 1951, concerns strictly back-wall rectifiers fabricated by partially oxidizing copper substrates. These devices are not of primary interest to solar cell studies. 2.3. Key technical issues There are three key technical issues concerning the ultimate viability of CuzO as a photovoltaic material, namely (i) material growth, (ii) photo-

251 current and (iii) junction formation. Procedures for growing large-grain p-type Cu20 substrates have been developed. This work is summarized in Section 3. The improvement of the p.type material is still important, b u t the determination of an approach to the formation of n-type material remains the most important issue concerning material growth. Photocurrent studies are discussed in Section 4. It is shown that shortcircuit currents in the range 12 - 14 mA cm -2 could be expected for Cu20 cells with further development. A value of JPH in this range is adequate for obtaining an efficiency of the order of 10%. Therefore, the need for an increased photocurrent of Cu20 cells is n o t considered to be prohibitive to Cu20 solar cell viability. The ultimate viability of Cu20 solar cells primarily depends on whether or not an adequate junction can be fabricated. To achieve an efficiency of 10%, a Cu20 junction with a built-in potential of the order of 1.4 eV is required. The quality of the junction must be good so that parasitic tunneling currents and recombination processes can be minimized. The work conducted at JCGS concerning junction formation and current mechanisms is discussed in Sections 5 and 6. In particular, studies of Cu20 Schottky barriers are discussed in Section 5, while other cell structures are considered in Section 6.

3. Cu20 substrates The solar cell studies reported here have primarily been carried out with relatively thick polycrystalline wafers. The fabrication and characterization of Cu20 substrates are discussed in this section.

3.1. Growth and doping of Cu20 The baseline fabrication procedure is summarized in Fig. 1. Copper discs cut from high purity copper rods are supported on a bed of crystalline

Ar/O 2

/

W///////J tr~

EXHAUST

IN CRUCIBLE

TUBE FURNACE

Cu20 SUBSTRATEFABRICATION 1. COPPERWAFERS OXIDIZED AT 1050% WITH Ar/02 GAS FLOWING IN CLOSED SYSTEM. 2. MgCl2 HELDAT 100°C TO 400°C DURING GROWTH OF Cu20. 3. AFTEROXIDATION, Cu20 WAFERS POLISHED ON ONE SIDE UNTIL PAST VOID REGION, AND THEN THE OTHER SIDE IS POLISHED AND USED FOR JUNCTION FORMATION.

%1111 ///////

Fig. 1. Approach to the growth and doping of Cu20.

252 MgO lumps which are uniformly distributed across a quartz tray. The crystalline MgO has a purity of 99.95%, while the copper discs were cut from a rod with 99.999% purity. With the tray placed in a tube furnace, the copper discs are oxidized at 1050 °C with a mixture of argon and oxygen gas flowing. Typically, 3/4 in copper rod is used and discs are cut with a thickness of 50 X 10-3 in. Because of the copper disc thickness, complete oxidation requires approximately 72 h. Cu20 reacts to some degree with MgO, but in a tolerable manner. Work reported in the literature indicates that less reaction occurs with BeO [ 5] . It should be noted that Cu20 reacts very strongly with quartz. The oxidation process involves vacancy diffusion and results in the formation o f voids at the center plane of a disc. Figure 2 shows a photomicrograph of a cross section of an as-grown sample; the voids are apparent. As illustrated, samples grow with columnar grains extending from the surface to the center plane. The preparation of a substrate for solar cell fabrication first involves grinding one side past the disordered region. The sample face is then sanded and polished, utilizing progressively finer grit materials; the last step is done with 0.25 pm diamond paste. Cu20 substrates grown in an A r - O2 environment are p type and typically exhibit resistivities in the 1000 - 2000 fZ cm range. A reduction in the resistivity can be achieved by introducing chlorine vapor during the oxidation process. Chlorine vapor is obtained from a source of MgC12 held at a temperature between 100 and 400 °C. A continuous flow of A r - O : transports the chlorine vapor to the sample. Figure 3 gives results for the resistivity v e r s u s MgC12 temperature. Each resistivity value refers to the average of a batch of six to eight samples. The doping agent is apparently chlorine. There are two reasons for this conclusion. T h e r m o d y n a m i c considerations indicate that the chlorine vapor

Fig. 2. Cross section of an as-grown Cu20 sample.

253 [ ,

"1

I000

OX,o \

A

5

o

c

o

o~

o

"

I00 0

COPPER SOURCE , (0.99999 PURITY) •

I

A 10180 B 10180

I

I

2OO

I

J

4OO

I

6OO

MgCI2 TEMPERATURE(C)

Fig. 3. Resistivity

vs.

MgCI2 temperature for doped Cu20.

pressure is significant whereas that for magnesium or MgC12 is not. Furthermore, measurements with an ion microprobe mass analyzer indicate that chlorine is distributed uniformly throughout the Cu:O grains, while magnesium is present only in the form of precipitates at grain boundaries. After considering possible reactions involving MgC12, it is expected that chlorine vapor results from the following reaction: MgC12(s) + ~O2(g) --> MgO(s) + Cl2(g ) It is estimated that, under the conditions of the experiment, a chlorine vapor pressure of the order of 0.01 atm exists during the oxidation of copper. Doping of Cu20 to lower the p-type resistivity has also been achieved using chlorine gas directly. Only a limited study has been devoted to this approach, however. The actual doping mechanism is not y e t clear. Two configurations which could lead to an acceptor level are when chlorine enters the lattice as a neutral atom at an interstitial site or when chlorine substitutes for oxygen to result in a donor level, which in turn could cause copper vacancy production due to charge compensation effects. It is clear that copper purity can have a significant impact on Cu20 resistivity and the solar cell performance. Copper rods obtained from the same vendor, but all characterized by 0.99999 purity, typically yield a different quality of Cu20. In general, copper which yields relatively low resistivity Cu20 also results in solar cells of relatively good quality. The data presented in Fig. 3 illustrate the importance of copper purity. The resistivity data for Cu20 grown from two different production lots of copper are

254 shown. C u 2 0 samples grown f r o m lot A e x h i b i t e d relatively low resistivities and high quality devices. Samples o f c o p p e r f r o m lots A and B were subjected to spark source mass spectrographic analyses. Material f r o m lot B was f o u n d to have i m p u r i t y levels in the parts per million range, while material f r o m lot A was c h a r a c t e r i z e d by i m p u r i t y levels o f the o r d e r o f 0.1 ppm. Clearly, m u c h m o r e e f f o r t is r e q u i r e d c o n c e r n i n g the materials science o f Cu20. 3.2. Optical p r o p e r t i e s

T h e optical a b s o r p t i o n c o e f f i c i e n t of C u 2 0 versus p h o t o n wavelength was r e q u i r e d for p h o t o c u r r e n t calculations. T h e best data for ~ versus ~ in the literature are for wavelengths near the band edge. In particular, the a b s o r p t i o n c o e f f i c i e n t had been o b t a i n e d f r o m optical transmission measurements on large-grain p o l y c r y s t a l l i n e substrates near the band edge. F o r < 6 0 0 nm, ~ had b e e n o b t a i n e d f r o m m e a s u r e m e n t s on o x i d i z e d films on quartz, and the results had a high degree o f u n c e r t a i n t y . T o a u g m e n t the existing i n f o r m a t i o n , ellipsometric m e a s u r e m e n t s were m a d e on p o l y c r y s t a l l i n e substrates similar to those used f o r solar cell fabrication. N a r r o w bandpass filters were used in c o n j u n c t i o n with the ellipsometer light source to obtain m e a s u r e m e n t s at various wavelengths. Results could n o t be o b t a i n e d for wavelengths near the band edge because of the large limits o f e r r o r associated with the m e a s u r e m e n t . T h e values for c~ versus ~ d e t e r m i n e d in this w o r k are given in Fig. 4 t o g e t h e r with results r e p o r t e d in the literature. Studies r e p o r t e d by B a u m e i s t e r

106 i ~

i

105%.~ B

104

-.

\ 103

\

\

\

\

102

I0

L 40O

L

L 500

I ~0I /I

?00

A(nm}

Fig. 4. Cu20 optical absorption coefficient: - - - - - - , assumed values for photocurrent analyses; - - , transmission measurements on large-grain polycrystalline materials (from ref. 6); ©, ellipsometric measurements on large-grain polycrystalline materials (measurements made at JCGS); ~, transmission measurements on oxidized films of copper on quartz (from ref. 7).

255 [6] were carried o u t on samples similar to those used in this study. The results of Brahms and Nikitine [7] were obtained for oxidized films of copper on quartz. The broken line in Fig. 4 indicates values of a versus that were assumed for Cu20 in photoresponse analyses.

4. Photocurrent analysis A detailed analysis has been carried out so that realistic projections could be made concerning the expected ultimate values for the Cu20 solar cell photocurrent. Cu/Cu20 Schottky barrier cells have been utilized as vehicles for conducting this study. It should be emphasized that this study has led to conclusions applicable to any type of Cu20 solar cell. This work involved the determination of the internal photoresponse of Cu/Cu20 Schottky barriers, the conduction of a p h o t o n and carrier loss analysis for Cu20 cells and then the estimation of ultimate values for Jell. Discussions of these three facets of the photocurrent analysis follow. 4.1. Internal photoresponse of Cu/Cu20 cells The approach used to determine the internal photoresponse of Schottky barriers is summarized in Table 3. As indicated, the internal photoresponse S~ is defined according to Q~, = T~S~

where Qx refers to the total photoresponse and T~ the p h o t o n transmittance into Cu20. As a Cu/Cu20 cell is fabricated, a copper film is simultaneously deposited onto a quartz witness. Transmission and reflection measurements versus wavelength are then made on the c o p p e r - q u a r t z sample and utilized to determine the optical constants for that particular film. The optical constants (the real (N) and imaginary (K) parts of the index of refraction) are then utilized to calculate the p h o t o n transmittance T~ into Cu20 for the related Cu/Cu20 S c h o t t k y barrier. The optical constants of the copper films depend on the thickness and the deposition rate. A deposition rate between 10 and 15 A s-1 was used for these studies. The results obtained for copper films were reported previously [8]. The total photoresponse Qx was measured using an approach previously discussed [9]. S~ is then determined according to S~ = Qx/T~. The results for four devices are given in Fig. 5. Two of the cells were fabricated on very lightly doped material and two on more heavily doped substrates. There are interesting differences in S~ at both the low wavelength and the high wavelength ends of the range of ~ where S~ > 0. Let us consider the low wavelength range of the photoresponse curve. The difference between the cells based on substrates lightly doped (as grown) and those based on more heavily doped substrates should be noted. Sx is larger for the low resistivity material. Furthermore, it should be noted that

256 TABLE 3 Summary of the approach for determining the optical constants of copper films and the internal photoresponse of Cu/Cu:O solar cells Internal photoresponse Internal photoresponse SX defined by Qx = TkSx Approach

(1) Simultaneously deposit copper onto Cu20 and quartz (2) Utilize the copper-quartz sample to determine N and K of the copper film (3) Calculate T~ and R x for a Cu/Cu20 cell (4) Use Tx for Q~. analysis and compare calculated and measured Rk

R~I>,

I/// / .

.

.

.

Cu

\iii ¸I ¸¸¸¸¸¸¸¸¸II ¸¸ Cu20

r~ .y

,.,

T}, I),

Sx for cell 78FP35 is midway between those for cells 79FP25 and 79FP27, even though substrates 35 and 27 have similar values of resistivity. The low wavelength effects are m o s t likely to be associated with exciton dissociation. A significant fraction of the p h o t o n absorption in Cu20 is a result of exciton absorption. An exciton must dissociate into a normal e l e c t r o n - h o l e pair before any p h o t o c u r r e n t results. The probability of dissociation is enhanced in the vicinity of certain types of defects. F u r t h e r study of these effects might eventually allow a cell to be fabricated with a large Sx at both ends of the wavelength range. F o u r exciton bands have been identified for Cu20: a yellow band (520 - 650 nm), a green band (480 - 540 nm), a blue band (450 - 490 nm) and a violet band (350 - 500 nm). The lower wavelength blue and violet bands are very sensitive to material processing. For example, Spyridelis f o u n d that an increase in the copper vacancy c o n c e n t r a t i o n enhances the p h o t o c o n d u c t i v i t y [ 1 0 ] . Photoresponse results shown for cells 79FP24 and 79FP27 are consistent with Spyridelis' measurements, since the lower resistivity material probably has a larger c o p p e r vacancy density. Aspects of the material growth and/ or the substrate surface preparation appear also to affect Sx at the low wavelengths. Cells 79FP35 and 79FP27 have similar

257 100

|

I

I

400

450

I

1

I

550

600

I

]

90 80 70 60

u.i 50

40 30

20 10

350

500

650

~(nm)

Fig. 5. Experimental results for the internal photoresponse of Cu/Cu20 solar cells: o, 79FP24 (resistivity, 76 ~ cm); B, 79FP25 (resistivity, 66 ~ cm); v, 79FP27 (resistivity, 305 ~ cm); o, 78FP35 (resistivity, 306 ~ cm).

resistivities b u t yet have significantly different values of Sx in the short wavelength range. Let us consider the results for Sx for photons with 500 - 650 nm. It should be noted that the effective band edge for Cu20 corresponds to )~ = 650 nm or hp ~ 1.91 eV. The yellow exciton band is clearly playing a role in determining the photoresponse in this region. The electron band gap corresponding to normal band-to-band excitation is at about 2.1 eV or )~ = 590 nm. The effect of exciton creation and subsequent dissociation into useful electron-hole pairs gives rise to an effective edge of 1.91 eV (650 nm). A complete analysis of Sx should probably involve exciton transport and recombination effects as well as the usual considerations concerning minority carrier transport. However, it appears that the long wavelength region photoresponse can be explained by assuming p h o t o n absorption results in one electron-hole pair, whether by a normal band-to-band excitation or via

258

exciton creation and subsequent dissociation into a normal electron-hole pair. The internal photoresponse is then interpreted strictly in terms of tho minority Carrier transport. The long wavelength results for Sx were analyzed by assuming that ~L Sx = 1 --exp(--~W) + e x p ( - - c ~ W ) l+c~L where W is the depletion width and L the minority carrier diffusion length. Photoresponse data between ?, = 600 nm and X = 650 nm have been interpreted using the above model for cells 78FP35 and 79FP24. The depletion widths were obtained using capacitance-voltage data taken at 500 kHz. The minority carrier diffusion lengths were determined to be 12.3 pm for 78FP35 and 2.0 pm for 79FP24. These results suggest that with further improvement in the material quality significantly larger diffusion lengths can be obtained.

4.2. Photocurrent o f Cu/Cu20 cells The structure of Cu/Cu20 cells has been optimized to achieve maximum photocurrent. In particular, the optimum values for the antireflection (AR) layer thickness, the AR index of refraction and the copper film thickness were determined. The photocurrent is calculated from JPH = e ; h=0

TxSh¢~ d~.

where ¢~ is the p h o t o n flux per unit wavelength at wavelength h, Tx is the photon transmittance through the AR material and copper layers and S~ is the internal photoresponse. A computer code written for determining the p h o t o n transmittance through multilayered systems was used to calculate T~ versus h. These calculations require a knowledge of the optical constants for each layer. The AR coatings were characterized by K = 0 and a single value of N. The optical constants for copper films were determined as discussed above. Values of Sx determined for cell 78FP35 were used in these studies. Figure 6 describes the calculated values of JpH versus the AR material index of refraction for a copper film thickness of 90 A and an AM 1 solar spectrum appropriate to Phoenix, AZ. As indicated, the m a x i m u m value of JPH is calculated to be approximately 8.6 mA cm -2. No further increase in JPH is achieved by varying the copper film thickness. Thus, the optimum structure for a Cu/Cu20 Schottky barrier cell consists of an AR layer 700 A thick and characterized by an index of refraction of about 1.9 and a copper film thickness of about 90 A. Some experimental results are tabulated in Table 4. Illuminated c u r r e n t voltage (I-V) characteristics were obtained with a simulator based on ELH lamps but shaped by color filters to match the Phoenix AM 1 spectrum for h < 650 nm [8]. The devices are listed in the order in which they were fabricated. The value of JPH increased continually during these studies. The

259 i

f

~

'

I

r

i

,

i

I000

\

900

9.0

8(]o

\

E

70o o~

~8.0 5oo

3oo

7,0--

I

I

L

I

I

I

L

I

2. ~

1.0

2CO

]. 0

AR IND[X OF R[FRACTION

Fig. 6. Calculated photocurrent vs. AR coating index of refraction for Cu/Cu20 cells (Phoenix AM 1 spectrum; S = SEXP (cell 78FP35); copper thickness, 90 A): ~ , photocurrent; - - - - - , AR thickness. TABLE 4 Experimental results for a C u 2 0 cell p h o t o c u r r e n t o n the basis of the active cell area Cell

Substrate resistivity ( ~ cm)

SiO A R thickness (A)

Jsc ( m A c m -2 )

JpH a (mA cm - 2 )

Active area (cm 2 )

79FP24 79FP25 79FP26 79FP27 79FP49-5 79FP55-11 79FP57-9 79FP58-13

76 66 35 305 564 465 574 352

720 720 682 682 705 700 720 702

6.48 6.28 6.90 7.11 7.00 7.83 7.64 8.33

6.55 6.34 6.98 7.51 7.32 8.00 7.92 8.52

0.71 0.71 0.71 0.71 0.060 0.060 0.060 0.060

aDetermined from JPI-I= J at V = --RsI. increase w a s a result o f the c o n t i n u e d i m p r o v e m e n t the o p t i m i z a t i o n p r o c e d u r e s utilized. As indicated, was d e m o n s t r a t e d . E v a p o r a t e d SiO was used as the o f r e f r a c t i o n is 1.9 if the s t o i c h i o m e t r y of the a p p r o a c h e s SiO.

in the device quality and a value o f 8.5 m A c m -2 A R material. T h e index d e p o s i t e d film actually

4.3. Ph o to n and carrier loss analysis A detailed p h o t o n and carrier loss analysis for C u / C u 2 0 cells can be utilized to discuss the possible values o f JPH for a n y C u 2 0 cell. Figure 7

260 PHOENIXAMI SOLARSPECTRUM PHOTONLOSSES

100~ 2 17.2mA/cm

2.3%10.4 mA/cm21 TOTALREFLECTION

,~

15.4%(2.7 mA/cm2)"~1

/ ]

CARRIERLOSSES

\

20.B~ (3.6 mA/cm21

~

~

GWAVELENGTH LON SS

~1.5% ( 2.0 mA/cm2)

ABSORPTIONIN ~ COPPERFILM

SHORTWAVELENGTH LOSS

CTED

II. 7%(3.i mA/cm2)

PHOTONS

EH- PAIRS

32.2~ (5.5 mA/cm2)

50.0~ 18,60 mAlcm2j

Fig. 7. Photon and carrier loss analysis for Cu/Cu20 cells.

indicates where photons and carrier losses occur in an SiO(700 A)/Cu(90 A)/ Cu20 cell structure. If a Phoenix AM 1 spectrum is assumed, the maximum possible photocurrent for Cu20 is 17.2 mA cm -2. First, let us consider the loss of photons by reflection and as a result of p h o t o n absorption in the copper film. As indicated, 2.3% of the photons are reflected. The corresponding lost photocurrent is 0.4 mA cm -2. The p h o t o n flux lost due to absorption in the copper film is much larger, 15.4% of the incident flux. The corresponding lost current is 2.65 mA cm -2. The total lost photocurrent due to reflected and absorbed photons in the front layers is approximately 3.0 mA cm -2. The total p h o t o n flux transmitted into Cu20 is approximately 82.3% of the incident photon flux. If it is assumed that each p h o t o n creates one electron-hole pair, then the remaining possible~photocurrent is 14 mA cm -2. Carrier losses occur primarily at the low wavelength region and near the band edge (in the long wavelength region). If it is assumed that S~ versus X is given by the plot for cell 78FP35 in Fig. 5 and if the calculated values for T~ are utilized, it can be calculated that the uncollected electron-hole pairs created by photons with 500 nm ~< X ~< 650 nm constitute a photocurrent increment of 3.57 mA cm -2 or 20.8% of the m a x i m u m possible photocurrent. Uncollected electron-hole pairs created by photons with k < 500 nm are equivalent to a photocurrent increment of 1.98 mA cm -2 or 11.5% of the maximum photocurrent. The predicted value of JPH is 8.60 mA cm -2. Carrier and p h o t o n losses are equivalent to 5.55 mA cm -2 and 3.05 mA cm -2 respectively.

4.4. Projected values of the photocurrent A key objective of this work has been to understand where the losses of photons and carriers occur in Cu/Cu20 Schottky barriers and to use such information to project possible values of JPH for Cu20 cells. If we refer to

261

Fig. 7, 17.7% of the incident p h o t o n flux is either reflected or absorbed in the copper film. The associated loss in photocurrent is 3.05 mA cm -2. In a well-developed homojunction or heterojunction, a loss of 7%- 10% of the incident p h o t o n flux might be expected because of grid reflection and grid absorption, resulting in a loss of 1.2 mA cm -2 instead of 3.05 mA cm -2. Further improvement in the material quality could lead to larger diffusion lengths and increased values of S~ in the p h o t o n wavelength range of 500 650 nm. It seems reasonable to expect a gain of 2 mA cm -2 from such an improvement. An increased understanding of the photoresponse at short wavelengths could result in a gain of 1 mA cm -2. As a result, JPH could increase until JPH ~ 8.6 + 1.8 + 2 + 1

= 13.4 mA cm -2 Studies of Cu/Cu20 solar cells have provided the basic information useful for projecting the achievable values of JPH. In particular, these studies indicate that it is realistic to expect that further development of Cu20 cells would result in JPH ~ 12 - 14 mA cm -2. Such a value of photocurrent would be adequate for efficient low cost solar cells.

5. Cu20 Schottky barrier cells Most of our studies concerning Cu2 O solar cells have been devoted to Schottky barrier devices. These types of cells are relatively simple to fabricate and to characterize. Homojunctions could n o t be considered since an approach to achieving n-type doping has not been developed. Heterojunction structures have been examined to a limited degree. Questions concerning chemical reactions at the interface between Cu20 and another material are even more difficult to address with heterojunctions than with Schottky barriers, since n-type semiconductors of interest for heterojunctions are compounds. As a result, emphasis was placed on understanding the electronic and chemical properties of Cu20 Schottky barriers. These investigations have led to models for Cu20 metal-semiconductor devices which have implications for other cell structures. Discussions of substrate surface preparation, barrier heights of Cu20 Schottky barriers, Cu/Cu20 solar cells and T1/Cu20 solar cells follow.

5.1. Substrate surface preparation and characterization Procedures used to prepare a Cu20 substrate surface prior to metal deposition have evolved over several years. The following discussion emphasizes the procedures used during the last 2 years of work. It seems clear, however, that further improvements can still be made. As indicated previously, Cu20 wafers grown by oxidizing copper wafers are first sanded on one side until the disordered region is eliminated. This side is eventually used as the back side of a cell. The front side is then

262 TABLE 5 C u 2 0 surface p r e p a r a t i o n p r o c e d u r e

Step

Operation

5 6 7 8 9 10 11 12

E t c h for 5 m i n in H B F 4 Rinse in D H 2 0 Soak for I m i n in m e t h a n o l with a final 20 s in a n u l t r a s o n i c cleaner Soak for 1.5 m i n in 2 v o l . % b r o m i n e - m e t h a n o l s o l u t i o n w i t h a final 30 s in ultrasonic cleaner R e p e a t step 3 R e p e a t step 4 R e p e a t step 3 Soak for 1 m i n in w a r m (40 - 45 °C) 2 v o l . % b r o m i n e - m e t h a n o l s o l u t i o n R e p e a t step 3 R e p e a t step 3 Rinse in D H 2 0 Blow dry w i t h N2

APPROXIMATE DEPTH (A) 15

0

100

90

. . . . . . . . .

" " '

30

! . . . . . . . . .

45

i . . . . . . . . .

60

~ . . . . . . . . .

75

i . . . . . . . . .

i . . . . . . . . .

9C)

105

r . . . . . . . . .

/

.....

I~:

OLSEN

JR3

Cu

7O 0 z _o I,re I"Z

D@TE: 3-J@N-Bi

I

60 ~~ 50

TYPE: AES

-

Z0

40

IE 0

30

- ,-"

20

-

PROFILE

:LEHEHTS: Cu

0

Pr

D-

C 0 C1

f

C

10

~

£l

C

Dr

Cl

(CONCEHTRATIOHS :: I%)

,...,.mm~=~m.m

0

1

2

3 SPUTTER

Fig. 8. D e p t h - c o n c e n t r a t i o n

4

TIME

5

6

(MIN)

p r o f i l e o f a C u 2 0 s u r f a c e p r e p a r e d as i n d i c a t e d in T a b l e 5.

263

polished, starting with 1 pm alumina, then using 1 # m diamond and finally 0.25 tzm diamond. At this point, the Cu20 wafer is approximately 0.5 mm thick. T h e final surface preparation involves etching and cleaning of the front surface as ,indicated in Table 5. The HBF4 etching step provides~ a relatively flat surface. The~ steps involving b r o m i n e - m e t h a n o l result in the removal of excess copper and a very high quality surface. Figure 8 is a depth-concentration profile taken on a Cu20 substrate p r e p a r e d as described above. The estimated sputtering rate is 15 A min -1 . After a d e p t h of essentially one monolayer, the copper-to-oxygen ratio is a b o u t 2 and ,is constant. Depth-concentration profiles obtained for samples prepared by other m e t h o d s indicated that a copper-rich region,existed in the first 100 A. For example, a surface preparation based on a nitric acid etch and a water rinse leads to copper-rich surfaces.

5.2. Barrier height of Cu20 Sehottky barriers Schottky barrier studies have involved device fabrication with metals characterized by a large range of work functions. Figure 9 summarizes the results obtained for barrier heights of Cu20 Schottky barriers using internal photoelectric emission (IPE). The theoretical barrier height is given by CBP =

Es + X - - C m 5 . 2 - Cn~ eV

where the band gap Eg ~ 2.0 eV and the electron affinity × ~ 3.2 eV. The experimental results for CBP do n o t exhibit a dependence on Cm. In fact all the contacts except the A u / C u 2 0 contact exhibit properties similar to Cu/Cu20 cells. Free energy calculations indicate that all t h e metals used in this study, except gold and thallium, can theoretically reduce Cu20. To examine this

2.0

i

\

F

r

1

~_--------~ *BP" 5. 2 - *m

k

\

N

w

l!r

~l.o

Cu

0

I

3.0

I

\

At

Mg

\ L

j

4.O

Au

o 5.0

METALWORK FUNCTION (eV)

Fig. 9. Barrier h e i g h t s of C u 2 0 S c h o t t k y barriers: , theoretical results on the a s s u m p t i o n t h a t X = 3.2 eV a n d Eg = 2.0 eV for c U 2 0 . (All the values o f CBP were

determined by internal pho~electric~ meastrrements except for T l / C u 2 0 devices; in this case, CBP was d e d u c e d f r o m I - V m e a s u r e m e n t s . ) :,

264

possibility, depth-concentration profiles were taken on numerous Schottky barriers. Profiles obtained for M/Cu20 contacts where M - Yb, Mg, Mn or Al indicated that a reduction of C u 2 0 had occurred. In particular, an examination of Auger electron lines showed that in the interfacial region there was significant M-O bonding. In contrast, an examination of A u / C u 2 0 and Tl/Cu20 contacts indicated that no A u - O or TI-O bonding existed at the interface. A depth-concentration profile for a Yb/Cu20 contact is shown in Fig. 10. The depth-concentration profile clearly indicates that oxygen has diffused from the bulk and reacted with ytterbium in the interfacial region. Measured values of the peak energy of the Auger electron transition MsN~N7 are plotted v e r s u s sputtering depth. Near the surface there is clearly considerable Y b - O bonding and the peak energy is about 1500 eV. After about 20 min of sputtering time, the composition is primarily ytterbium and the peak energy is greater than 1505 eV. In the interfacial region the peak energy

100

i

i

i

i

i

i

80 Cu

Z O

Vb

60 ua

7"

8

40

2 20

(a) 1506

'

'o

'

L ~OCATIbN OF Yb MSNTN7 P~AK (.V)Y~ P.ASEI

1505

'

/I

1504

<

,5,,

M~,AL

.o. ,~R,EO

L O×,0E I

1503

Z

T

1502 1501 1500 L

(b)

10

i

2o

L

i

3o

40

SPUTTERING

I

TIME

50

L

~0

I

7o

80

(MIN)

Fig. 10. (a) Atomic concentration and (b) Auger electron energy Yb/Cu20 contacts.

us.

sputtering time for

265

approaches 1500 eV, indicating significant Yb-O bonding. The peak energy of the Auger lines can be resolved to within 0.1 eV. Similar results were obtained for magnesium, manganese and aluminum (see ref. 11 for the results with magnesium). Figures 11 and 12 describe the results obtained for TI/Cu20 and Au/Cu20 contacts. In both cases, concentration profiles for each of the elements change in a rather smooth manner. In fact, no T1-O or Au-O bonding occurs in the interfacial region. The peak energy for the thallium N60404 line is plotted v e r s u s depth in Fig. 11. The surface layer is partially oxidized, resulting in a peak energy below 82 eV. After a depth corresponding to 6 min of sputtering time, the film is clearly primarily thallium. The peak energy of the N60404 line at this depth is greater than 84 eV. In the interfacial region the peak energy of the line remains above 84 eV, giving no indication of T1-O bonding. The copper-to-oxygen ratio is also plotted v e r s u s depth for the T1/Cu20 contact. It should be noted that, even though no reduction of Cu20 occurs, i

i

2.5

r

I

T

=

I

I

TI

_o

~

Cu

2.0 t~

4O < 1.5

(a)

0

I

i

3

6

9

i

i

12 15 18 SPUI"rERTIME(MIN) m ~ i i

I

I

I

3

6

~

i

i

21

24

i

I

2'1

2'4

85

s4 ~_

83

82

(b)

,2 15 ,s SPUTTERTIME(MIN)

Fig. 11. (a) A t o m i c concentration and (b) Auger e]ee~on energy TI/Cu20 contacts.

vs.

sputtering time for

266 100

o

'T000 A

Au

80

g

i 8 4o

C o 5

lo

15

20

25

30

35

40

SPUTTERING TIME ( M I N )

Fig. 12. Atomic concentration vs. sputtering time for Au/Cu20 contacts. apparently a process has occurred whereby the interracial region is still copper rich. The interfacial region is also copper rich for Au/Cu20 contacts. One model that explains these results consists of the assumption that oxygen is preferentially sputtered from the Cu20 substrate surface as the metal is deposited. For an Au/Cu20 contact the barrier height is still very low because of the presence of gold. For T1/Cu20 contacts the copper-rich region and, as discussed below, the presence of oxygen vacancies at the interface prevent the theoretically possible value for the barrier height (1.5 eV) from being achieved. These studies indicate that the barrier height of a Cu20 Schottky barrier is always in the range of 0.7 - 1.0 eV as a result of two phenomena. Most metals reduce Cu20 to form essentially a Cu/Cu20 contact. In addition to a chemical reaction between a metal and Cu20, it appears that evaporated metals can cause preferential sputtering of oxygen from the Cu:O matrix. This latter effect is apparently the reason that T1/Cu20 contacts also exhibit a value of barrier height in the 0.7 - 1.0 eV range. 5.3. C u / C u 2 0 solar cells

A considerable a m o u n t of our effort has been devoted to the study of front-wall Schottky barrier Cu/Cu20 solar cells. Although the ultimate efficiency of the devices was not expected to exceed 3% - 4%, the simple structure provided a useful device for detailed studies. Thus, emphasis has been placed on understanding current mechanisms and their implications concerning the potential efficiency of Cu20 solar cells of any structure. T h e photocurrent~ analysis presented in Section 4 is an example of how results for Cu/Cu20 cells have been used to evaluate the p o ~ n t i a l perfor-

267

mance of Cu20 cells in general. In this section, emphasis is placed on I - V characteristics. Investigations of Cu/Cu20 cells have primarily been done on a substrate similar to that shown in Fig. 13. 19 circular cells are f o r m e d on a Cu20 substrate which is approximately 2 cm in diameter and approximately 20 × 10 -3 in thick. The diameter of each individual cell is 2.9 mm. Since the diameter of these cells is much larger than the minority carrier diffusion length, the results obtained are indicative of the performance that could ultimately be achieved with larger cells. The relatively dark circular area of diameter slightly less than the substrate diameter is an SiO layer. The fabrication of these devices involves the polishing, etching and cleaning of a 2 cm diameter Cu20 substrate, the deposition of a layer of copper approximately 90 A thick through a mask to form small-area cells on Cu20, the deposition of an SiO layer approximately 700 A thick over t h e front of the Cu20 substrate and the formation of a large-area ohmic contact on the back side. The I - V characteristics of individual cells were taken by probing through the SiO layer. The best results obtained to date are described in Fig. 14. On the basis of the total cell area, the efficiency and short-circuit current are 1.62% and 7.69 mA cm -2 respectively. An estimate of the area shadowed by the point probe was made by taking a picture. The shadowed region is estimated to be greater than or equal to 8% of the total area. Conservative estimates of values for the active-area efficiency and short-circuit current are 1.76% and

Fig. 13. Picture of C u 2 0 substrate with small-are~cells (the dark area is due to the SiO A R layer). ~

268 8.33 mA cm -2 respectively. The short-circuit current can be corrected for series resistance effects to obtain the true Jell. It is found that JPH = 8.5 mA cm -2 . To our knowledge, these values of efficiency and photocurrent are the largest values ever reported for Cu20 cells. Several cells have had efficiencies greater than 1.5% and photocurrents in excess of 7.5 mA cm -2. As we noted above, a key objective of the study of Cu/Cu20 cells has been to develop appropriate models which can allow us to address the issue of the viability of Cu20 as a photovoltaic material. The I - V characteristics have been measured at various temperatures and analyzed with appropriate theory. These studies indicate that an interfacial layer exists at the C u - C u 2 0 interface which provides an effective barrier height of the order of 0.95 eV. Figure 15 shows typical results for the transformed dark I - V characteristics of an individual cell. In particular, the measured current is plotted against the junction voltage Vi. Two current mechanisms are operative, one in the voltage range 0 - 0.3 V and another in the range Vi > 0.3 V. The dark I - V characteristics have been interpreted in terms of the theory summarized in Table 6 and illustrated by Fig. 16. Values of I and V are the current through a cell and the voltage across a cell respectively. Vii refers to the voltage across a junction, namely Vj = V - - R s I .

i0-2

,

,

,

,

i0 "3

t0-6o/°/

e

/ Vj (VOLTS)

Fig. 14. I - V characteristics of a C u / C u 2 0 solar cell coupled to simulated AM 1 illumination (cell, 79FP58-13; thickness of copper layer, 83 •; thickness of SiO A R coating, 700 A; cell area, 0.065 cm2; illumination area, 0 . 0 6 0 cm2; temperature T = 28 °C): on the basis of total cell area, Jsc = 7.69 m A crn - 2 with an efficiency of 1.62%; on the basis o f active cell area, Jsc = 8.33 m A cm - 2 with an efficiency o f 1.76%. Fig. 15. Dark I - V characteristics for a C u / C u 2 0 cell ( Vj refers to the junction voltage, the applied voltage corrected for the voltage across the series resistance of the cell) (cell, 79FP61-4; J01 = 3.2 × 10 - s A c m - 2 ; B1 = 7.7; J02 = 2.1 × 10 -9 A c m - 2 ; B2 = 38.7 (n2 = 1.0)): O, measured I v s . V j ; - - - - - - , 11 = l o l { e x p ( B i V j ) - - 1} + Vj/RsH;@ , 12 = I - - 11.

269

TABLE 6 S u m m a r y of the t h e o r y to analyze the dark current--voltage characteristics of C u 2 0 S c h o t t k y barrier cells

l = lol exp(BIVj) + lo2texp ( Vj l -\n2kT] V i = V-- RSI

11÷

RSH

J01 = A o exp - -

n2=l+a (where Xv isin electronvolts and ~ is in ~mgstr6ms) 4~BP = ~'4~BO ÷ (1 - - ~')~o ÷ A ~ F C CBO = Eg + X - - ~bm

eV

eV

7 = (1 + c~)-1 Ds~

a = (0.181)~

Ki

NFC5 A~FC ~ - -

eV

2Ki (~B)PH ~ ~BP + Xv

eV

D s is the density of surface states (1013 cm - 2 eV -1 ). K i is the relative dielectric c o n s t a n t of the i layer. NFC is the density of the fixed positive charge in the i layer (1015 cm - 2 ).

--16~Fig. 16. IllustratingTable 6.

The high voltage mechanism is characterized by J02 ~ 10-9 A c m -2. It should also be noted that n2 ~ I. RSH was determined to be 100 k£~ and Rs had a value of 185 ~ for this cell. If the cell area were 1 c m 2 instead of 0.065 c m 2, Rs would be approximately equal to 12 ~t. The theoretical value

270 TABLE 7 C u r r e n t - v o l t a g e p a r a m e t e r s for cell 80FP14-6

Temperature

B~

(K)

Jol

n2

(A cm - 2 )

J02 (A e m 2 )

RS (~2)

RSH (k~2)

Average error

(%) 321 303 297 282 271 255 242 223

13.8 11.3 12.9 12.8 12.3 11.5 13.9 13.4

1.63 × 10 -5 2.46x10 5 9.75 x 10 - 6 7.29 × 10 - 6 5.72×10 -5 4 . 3 2 × 1 0 -6 1 . 1 3 × 1 0 -6 5.00x10 -7

1.08 1.12 1.13 1.14 1.19 1.25 1.16 1.31

2.92 × 1 0 - s 8 . 5 8 × 1 0 -9 4.18 × 10 - 9 1.08 x 10 - 9 5 . 1 6 × 1 0 -1° 1 . 3 3 × 1 0 -1° 5 . 0 5 × 1 0 -12 2 . 1 4 × 1 0 -12

111 122 130 147 166 209 270 438

9.21 8.38 20.6 27.6 43.1 55.2 85.8 200

0.58 0.14 1.00 0.96 0.53 1.56 1.50 1.70

o f J 0 for a Cu/Cu20 S c h o t t k y barrier is 4.0 × 10 -7 A cm -2 on the assumption that the hole effective mass is 0.84m0 [ 12]. Therefore, the I - V characteristics at larger voltages appear to be a result of a m e t a l - i n s u l a t o r - s e m i c o n d u c t o r (MIS) structure. The effective barrier height is of the order of 0.94 eV. Since n2 ~ 1, the interface states do n o t appear to affect the voltage distribution across the device [ 1 3 ] . The I - V characteristics of cells were investigated as a function of temperature in an ef f or t to understand further the two current mechanisms. Table 7 lists the I - V parameters determined for cell 80FP14-6 at various temperatures. The absolute error, a ve r aged over approximately 15 data points, is indicated. The parameters for the high voltage mechanism are consistent with an MIS structure at lower temperatures as well as at room temperature. The low voltage current c o m p o n e n t of cell 80FP14-6 is p l o t t e d for four temperatures in Fig. 17. In particular, 101 exp(B1Vj) is p l o t t e d versus Vj. The parallel structure of the lines is a result of the fact that B1 is essentially constant with temperature. Thus, the low voltage c o m p o n e n t appears to be a multiple-step tunneling mechanism. A p lo t o f Jol versus 1 0 0 0 / T for this cell is given in Fig. 18 on a semilogarithmic scale. J01 can be written as J01 = constant X exp -Thus, the tunneling mechanism can be interpreted as a thermally activated process. The activation energy for cell 80FP14-6 is 0.31 eV. The results for cell 80FP14-15 on the same substrate are also shown in Fig. 18. In this case the activation energy is 0.38 eV. Analyses have been c o n d u c t e d for numerous devices. All exhibit the low voltage mechanism and are characterized by e ~ 0.3 - 0.4 eV. It shoUld be n o t e d t ha t the magnitude of J0~ varies considerably from cell to cell and is lowest in the m o s t effi ci ent devices. Let us consider the high voltage current mechanism. This c o m p o n e n t can be interpreted in terms o f modified S c h o t t k y barrier t h e o r y as described

271

I0-3

I

,

'

I

'

I

'

I

'

I

r

o,o.O °/dA//'" a/ s • ~ o/O / /• aJ o" ,/ /" / ,eo/O~/, A/ e/• /'" a ,,o / A . / a/

_ I0"4 $B ~>. -- 10.5

,,d

//o/"

o/

/

_6F~ /

,0

I/.o,

/o

/

/a

,°

, 0 ,0F : / 0.1

0.2

,

J

0.3 0.4 Vj(VOLTS)

0.5

J

0.6

Fig. 17. Current due to the low voltage current mechanism is plotted vs. junction voltage for a Cu/Cu20 cell on substrate 80FP14 at four temperatures. Values determined for RS, RSH and the low voltage mechanism parameters, J01 and B 1, are also given. Symbol

T (K)

RS (~)

RSH (k~)

J01 ( A c m -2 )

B1

© A •

321 282 255 223

111 147 209 438

9.21 27.6 55.2 200.0

1.63 7.29 4.32 5.00

13.8 12.8 11.5 13.4

10-3

10 - s 10 -5 x 10 - e x 10 -~ X x

I CELL ~:(eV) o 80FPI4-6 O.311 ~, 80FPI4-15 O.382

"~'~A

iO"4

~O i0-5

I0-6

o 10-7 3.0

i

f

t

i

I 3.5

I

r

f

F

I 4.0

~

~

t

I

4.5

10001T4 K -l) Fig. 18. J01 us. I O O O / T

for C u / C u 2 0

ceils o n substrate 8 0 P P 1 4 .

272 in Table 6 for current mechanism 2. This current mechanism is due to holes thermionically emitted over the built-in barrier which tunnel through the interfacial layer. J02 values clearly indicate that the effective barrier is of the order of 0.95 eV, much larger than the theoretical value for the C u - C u 2 0 contact. The difference can be due to the tunneling factor, the effects of fixed charge or both. J o 2 / T 2 is plotted in Fig. 19 versus 1000/T on a semilogarithmic scale. The slopes of the straight lines give ¢BP, while the intercept as 1000/T goes to zero gives A* exp(xl/26). The effective mass of holes in Cu20 has been reported to be 0.84m0; thus, A* ~ 101. The values determined for CBP and ×vl/26 are given in Fig. 19. Typically, n2 ~ 1.15, implying that a = 0.15 and 3' = 0.87. If we let ¢BO be 0.70, then the values of CBP for cells 80FP14-6 and 80FP14-15 can be interpreted in terms of different values of ¢0 or ACvc. Both parameters, of course, relate to the surface of Cu20. This topic is discussed further as part of the T1/Cu20 Schottky barrier studies. Further information is obtained from the barrier height determined from long wavelength photoresponse or from IPE. Figure 20 shows the results obtained for several cells. The barrier height determined in this manner is typically about 0.95 eV. Since the photocurrent obtained in this measurement is due to electron transitions in the metal and subsequent hole injection into the valence band of Cu,O, we interpret the barrier height to be (¢BhPr: = ~BP + ×v

As a result, X~ can be estimated by combining the I - V and the IPE studies. Values for the barrier height and interracial parameters for several cells are

j0-1l

q,i1!

i

i

i

i

I

i

]

~

cELL I ~0-12

e~

I

+

o

80FPI4~

•

80FPI4-15 0. 600 eV

(~ 7f9 eV

7. 12 0. 9

10-13

°~ 10-i4

%0

\

\

\ i0-15

% \

\ \

to-t6

\ \

10-17 10-t8

\ L

I

t

3.0

I

I

3.5

t

I

t

t

I

4.0

t

J

t

I

4,5

1000/T( K-I) Fig. 19.

J o 2 / T 2 vs.

1000/T

for

Cu/Cu20

cells

on

substrate

80FP14.

273 I

I

$

I

I

• 14

CELL • 12

•

/ /o

80FP14-6

I0

80FP14-15

O,96 eV

80FP15-9

O.eV ? ' /

!;) "

.08

//

.06

#

.04

I/

4¢

.I.

/

.02 0 0.8

I

/'/

i

0.9

, ~ ,.~ ~ 1.0

1.1

1.2

1.3

1.4

hv (eV)

Fig. 20. Internal photoelectric photoresponse vs. photon energy (Q refers to the total collection efficiency and A accounts for the photon absorption in the copper layer). given in Table 8. The results of the electro-optical characterization of CuCu20 cells have significant implications concerning the potential efficiency of Cu20. First, a tunneling current mechanism has been identified which varies considerably in magnitude. If this mechanism can be associated with the substrate microstructure, impurities at the surface or processing steps, it is quite likely that this current component could be eliminated. The elimination of the low voltage mechanism would result in C u - C u 2 0 solar cell efficiencies of 3%. Cell efficiencies could be further improved by increasing JPH and improving the junction associated with the high voltage current mechanism. In particular, if JPn = 12 m A c m -2, Jol = 0 and J02 = 10-9 A cm -2 , the AM 1 efficiency would be approximately 4%. The results of the Cu/Cu20 study indicate that, if Cu20 solar cells are ever to exhibit efficiencies above 5%, the value of J02 must be reduced. This can only be done with a homojunction, heterojunction or Schottky barrier with a larger effective barrier height than 1.0 eV. We have identified the T1/Cu20 contact as the only Schottky barrier structure which has the potential for a significantly larger barrier height than that exhibited by Cu/Cu20. As a result, studies of the T1/Cu20 cells were conducted.

5.4. T l / C u 2 0 S c h o t t k y barriers Thallium is the only low work function metal identified that theoretically would n o t reduce Cu20. Since the work function for thallium is 3.7 eV, a T1/Cu20 Schottky barrier has a very large theoretical efficiency. The theoretical value of J0 is of the order of 10 - i s A cm -2. If a high quality T1-Cu20 junction could be fabricated, it is highly probable that the recom-

23.4

22.2

21.3

24.8

T1/Cu20 80-6-5

T1/Cu20 80-11-15

Tl/Cu20

(°C)

Temperature

Cu/Cu20 80-14-6

Cell

2 . 8 8x 10 - s 2 . 6 3x 10 -6

4.55

1.89×10 -6

9.75 × 10 -6

J01 (Acm -2 )

3.53

5.01

12.9

BI

Cu20 Schottky barrier current-voltage parameters

TABLE 8

1.10

1.03

1.17

1.13

/22

4.92 × 10 -11

9 . 0 3× 10 -1°

1.96x10 v

4.18 × 10- 9

(A c m - 2 )

J02

39.8

8.39

23.0

8.45

(~2 c m 2 )

R s x area

1.04

0.962

5.49 x 106 1.32 × 10 s

0.942

0.923

(~B)effective (eV)

3.65 × 108

1.34 × 103

RSH × area (~2 c m 2 )

1.33 × 10 5 8.28 × 10 4 6.16 x 10 ~

0.753 0.710

6 . 8 1 × 10 4

e x p ( - - X 1/26 )

0.650

0.719

~BP (eV)

From I V characteristics

bO

275 bination current would be the dominant loss mechanism. It would still be expected that an AM 1 efficiency of 10% would be possible, however. Investigations of T1/Cu20 cells have concentrated on the I - V characteristics of thick metal Schottky barriers. The reasons for this emphasis are twofold. First, the main objective of the work has been to characterize the junction with respect to current mechanisms and barrier height. Secondly, evaporated thallium films tend to agglomerate such that the deposition of a 90 A thin film with a reasonable sheet conductance is not possible on a substrate at room temperature. However, we were able to deposit thin films with adequate sheet conductance on cooled substrates. It is clear that thin metal cells can be fabricated. The effort was discontinued when the I - V studies clearly established that the T1/Cu20 barrier represented only a slight improvement over the Cu/Cu20 device. The remainder of this section concerns the dark I - V characteristics of T1/Cu20 cells. The I - V characteristics of T1/Cu20 cells have been analyzed for cell temperatures in the range - - 1 0 0 - +60 °C. As for Cu/Cu20 cells, we find that two current mechanisms are operative; one mechanism is dominant at voltages between 0 and 0.3 V and another mechanism at voltages greater than 0.3 V. The lower voltage mechanism is again apparently due to multiplestep tunneling and the high voltage mechanism can be interpreted as being due to thermionic emission and tunneling through an interfacial layer. The incremental barrier associated with the thallium cells is larger than that determined for C u / C u 2 0 devices. The results for one of the better cells are shown in Fig. 21. A plot of the dark junction current versus junction voltage is given. The Ijct versus V~ct plot clearly shows the two current mechanisms. The high voltage mechanism is characterized by n = 1.1 and J0 = 4.92 X 10 -11 A cm -2. These I - V parameters are impressive for a Cu20 cell b u t still do not approach those expected for a T1/Cu20 contact on the assumption that the thallium work function is 3.7 eV. Typical I - V parameters obtained for Cu/Cu20 and T1/Cu20 cells are tabulated in Table 8. Let us consider the high voltage mechanism. Temperature dependence studies allow the true barrier height CBP and the tunneling factor exp(--×l/2~) associated with the interfacial layer to be determined. It is found that CBP does n o t vary significantly for Cu/Cu20 and T1/Cu20 cells. The main difference between the cells is due to the tunneling factor or incremental barrier and RsH × area. A model for Cu/Cu20 and T1/Cu20 Schottky barriers was formulated with both the electrical and the physical analyses taken into consideration. Figure 22 describes the main features of this model. It is proposed that the barrier height is primarily determined by a high density of donor states approximately 0.7 eV above the valence band. The surface state is most likely to be associated with an oxygen vacancy [ 1 4 ] . An interfacial region consisting of a defective copper-rich Cu20 layer gives rise to the incremental barrier. Thallium cells have a larger incremental barrier because thallium has a lower work function than copper.

276

///.. ///-.

/Y/

/

/-/-/

I0-4

/~// //-/~ //1

/

Tl OR Cu

Cu20 /

/-/-/~ //_/

/

/-//

10-5

///

/

/

./

.7/

=E i.--

lO-6

10-7

/"

VERY DEFECTIVE COPPER-RICH Cu20 REGIONWITH LARGE DENSITYOFACCEPTOR STATES

-9

,.2

/

//~I

/

-¢ 10-8

I 0.1

Cu20 WITH LARGE DENSITY OF O-VACANCIES (DONORS)

i 0.2

I 0.3

I 0.4

DONORLEVELS

~

o

o

o o o

0.5

VjC T ( VOLTS )

Fig. 21. I - V characteristics for a T l / C u 2 0 S c h o t t k y barrier cell on substrate 8 0 F P 2 0 . Fig. 22. Proposed m o d e l for a C u / C u 2 0 and T 1 / C u 2 0 S c h o t t k y barrier cell.

The proposed model for Cu/Cu20 and T1/Cu20 cells suggests that a Cu20 cell with a built-in voltage greater than 1 V will be difficult to fabricate unless a homojunction can be obtained.

6. Other Cu20 solar cells Since no approach to achieving n-type Cu20 is known, a heterojunction structure is clearly of interest for Cu20 solar cells. Limited studies have been conducted by this group and others [15, 16]. Herion e t al. have reported on a fairly detailed study of ZnO/Cu20 devices. The cells exhibited poor performance. In particular, the best values obtained for Voc were of the order of 0.3 V. Depth-concentration profiles were obtained with Auger spectroscopy. The interfacial region was found to be oxygen deficient. Thus, the cell characteristics were clearly influenced by the copper-rich region adjacent to the Cu20 substrate. Thermodynamic considerations indicate that zinc will

277 reduce Cu20. Thus, the copper-rich region could have resulted from the reaction of excess zinc with Cu20 or from the preferential sputtering of oxygen from the Cu20 matrix.

7. Summarizing remarks and conclusions Conclusions can be made concerning four key areas, namely material growth, photocurrent, Cu20 Schottky barriers and future prospects for Cu20 solar cells. P-type Cu20 substrates can be fabricated which have adequate properties for solar cell fabrication, particularly for research purposes. By the oxidation of high purity copper discs, large-grain polycrystalline Cu20 wafers can be obtained. P-type resistivities in the range of 10 - 100 Y~ cm can be obtained by providing a low partial pressure of chlorine during the oxidation of copper. Minority carrier diffusion lengths of the order of 10 pm are observed for this material. A detailed photocurrent analysis has been carried o u t for Cu20 solar cells. Front-wall Cu/Cu20 Schottky barrier devices have been utilized for this study, b u t the conclusions apply to any solar cell structure. On the basis of a p h o t o n and carrier loss analysis of Cu/Cu20 cells, it is concluded that values of JPH in the range of 12 - 14 mA cm -2 could be achieved with a cell structure characterized by a large front-surface p h o t o n transmittance, such as a heterojunction or homojunction. After a detailed analysis of Cu/Cu20 cells had been conducted, the cell structure was optimized to obtain maximum efficiency, As a result, an AM 1 active-area efficiency of 1.767o was achieved. This efficiency was obtained with a thin metal Schottky barrier structure. Further development might lead to 370 Cu20 cells based on the Cu/Cu20 junction but, clearly, solar cell efficiencies in the 670 - 1070 range must be based on another rectifying junction. Front-wall Cu20 Schottky barriers have been fabricated with metals characterized by a wide range of work functions. Cell characterization included electro-optical studies, thermodynamic considerations concerning interface stability and depth-concentration profiles. The calculated free energy changes indicate that all metals will reduce Cu20 except gold, silver and thallium. Gold and silver essentially form ohmic contacts. As a result of the low work function of thallium (3.7 eV), a barrier height of 1.5 eV is predicted for a Tl/Cu20 contact. Analyses of the I - V characteristics of T1/Cu20 cells indicate that the effective barrier height of the devices is of the order of 0.9 eV, essentially the same as that observed for Cu/Cu20 cells. Furthermore, depth-concentration profiles revealed that there was no evidence of T1-O bonding at the interface of T1/Cu20 cells. Nevertheless, the interfacial region was oxygen deficient. It was concluded that, because of the low binding energy associated with C u - O bonds (about 1 eV), preferential sputtering of oxygen must occur as thallium is deposited onto Cu20.

278

The results for T1/Cu20 Schottky barriers allow speculations to be made on the prospects for achieving high efficiencies with Cu20 cells. It appears that any Cu20 solar cell based on a rectifying junction fabricated by depositing a layer of material, either a metal or a compound, would exhibit properties similar to Cu/Cu20 cells because of the formation of a copper-rich region at the metallurgical interfacial region of the junction. Thus, further studies of Cu20 Schottky barrier or heterojunction solar cells do not appear to be appropriate. A homojunction structure remains as the one cell type that could possibly exhibit an efficiency in the 10% range. The development of an approach to obtaining n-type Cu20 is, of course, a prerequisite for homojunction development. More detailed accounts of the work summarized here are given in refs. 17 and 18.

Acknowledgments The authors wish to acknowledge the support of the National Science Foundation and the U.S. Department of Energy through the Solar Energy Research Institute. We have particularly appreciated the support of Dr. Tap Mukerjee and Dr. Richard Burke. Although there has been significant interest in the material for the reasons cited in this paper, much skepticism has been expressed by the photovoltaics community in the past regarding the potential of Cu20 for low cost photovoltaics. However, the skepticism was not based on scientific investigation. Dr. Mukerjee and Dr. Burke agreed with the present authors concerning the need for research aimed at determining whether there are inherent material properties which prevent the fabrication of efficient Cu20 solar cells. A key result of this work is the conclusion that there are such inherent limitations. Without the encouragement and support of Dr. Mukerjee and Dr. Burke, however, the photovoltaics community might still be guessing, whereas now the problems with Cu20 are better understood on a technical basis.

References 1 J. J. Loferski, Theoretical considerations governing the choice of the optimum semiconductor for photovoltaic solar energy conversion, J. Appl. Phys., 27 (1956) 777. 2 L. O. Grondahl, The copper-cuprous-oxide rectifier and photoelectric cell,Rev. Mod. Phys., 5 (1933) 141. 3 W. H. Brattain, The copper oxide rectifier, Rev. Mod. Phys., 23 (1951) 203. 4 J. A. Assimos and D. Trivich, Photovoltaic properties and barrier heights of single crystal and polycrystalline C u 2 0 contacts, J. Appl. Phys., 44 (1973) 1687. 5 R. S. Zucker, Growth of single crystal cuprous oxide from the melt and luminescence of cuprous oxide, J. Electrochem. Soc., 112 (1965) 417. 6 P. W. Baumeister, Optical absorption of cuprous oxide, Phys. Rev., 121 (1961) 359. 7 S. Brahms and S. Nikitine, Intrinsic absorption and reflection of cuprous oxide in the 2.5 to 6.5 eV region, Solid State Commun., 3 (1965) 209.

279

8 L. C. Olsen, F. W. Addis and R. C. Bohara, Investigation of Cu20 solar cells,Proc. 14th Photovoltaic Specialists' Conf., San Diego, CA, January 7 - 10, 1980, IEEE, N e w York, 1980, pp. 462 - 467. 9 H. L. Hampton and L. C. Olsen, Collection efficiency measurements for solar cell research, Proc. 2nd Workshop on Terrestrial Photovoltaic Measurements, Baton Rouge, LA, November 1976, in N A S A Conf. Publ. CP-2010 (1976) (National Aeronautics and Space Administration); Rep. ERDA/NASA-1022/76/10, 1976, p. 363 (Energy Research and Development Administration; National Aeronautics and Space Administration). 10 J. Spyridelis, J. Stoimenos and N. Economou, Optical and photoconductive phenomena in cuprous oxide, Phys. Status Solidi, 20 (1967) 623. 11 L. C. Olsen, R. C. Bohara and M. W. Urie, Explanation for low-efficiency Cu20 Schottky-barrier solar cells,Appl. Phys. Lett., 34 (1979) 47. 12 A. G. Zhilich, J. Halpern and B. P. Zakharchenya, Phys. Rev., 188 (1969) 1294. 13 L. C. Olsen, Model calculations for metal-insulator-semiconductor solar cells,SolidState Electron., 20 (1977) 741. 14 J. Bioem, Philips Res. Rep., 13 (1958) 167. 15 L. Papadimitriou, N. A. Economou and D. Trivich, Heterojunction solar cells on cuprous oxide, Sol. Cells, 3 (1980) 73. 16 J. Herion, A. Niekisch and G. Scharl, Investigation of metal oxide/cuprous oxide heterojunction solar cells, Sol. Energy Mater., 4 (1980) 101. 17 L.C. Olsen, Investigation of solar cells based on Cu20, Final Prog. Rep., May 1, 1979 April 30, 1980 (U.S. Department of Energy Contract ET/32006). 18 L. C. Olsen, Investigation of solar cells based on Cu20, FinaIProg. Rep., June 1, 1980 May 31, 1981 (Solar Energy Research Institute Subcontract XG=0-9190-1).