Review of progress on a-Si alloy solar cell research

Solar Cells, 2 7 ( 1 9 8 9 ) 25 - 37

25

REVIEW OF PROGRESS ON a-Si ALLOY SOLAR CELL RESEARCH A. C A T A L A N O , R. R. A R Y A , M. B E N N E T T , L. Y A N G , J. MORRIS, B. GOLDSTEIN, B. FIESELMAN, J. N E W T O N and S. W I E D E M A N Solarex Thin Film Division, Newtown, P A 18940 (U.S.A.)

Summary Materials and device studies have focused on the development of a-SiGe:H and a-SiC alloys for high performance multijunction solar cells and submodules. O u t d o o r measurements of a-SiC:H/a-Si:H/a-SiGe:H cells have yielded 10.7% conversion efficiency for a triple junction cell which contained a 1.4 eV bandgap b o t t o m cell. a-SiC/a-SiGe submodules with a 2500 A thick b o t t o m junction have shown efficiencies as high as 7.7% (active area 900 cm 2) and single junction submodules have yielded a conversion efficiency of 9.8%. Stacked junction devices show a clear improvement in stability, compared to equivalent single junction devices. Tandem junction devices with a conversion efficiency of 9% have been prepared which exhibit only an 11% performance loss after 1000 hours continuous AM 1.5 illumination at 40 °C. Analysis indicates that the net degradation of a stacked device is the mean of the c o m p o n e n t single junction devices. p-Layers prepared from B(CH3) have been extensively studied. The optical bandgap of the p-layer increases b y approximately 0.1 eV at a given value of conductivity compared to films prepared from diborane. Devices prepared from the feedstock have shown open circuit voltages as high as 0.943 and fill factors as high as 0.74 and unoptimized devices with an efficiency of 11.5% have been measured. In contrast microcrystalline alloys show little or no improvement in open circuit voltages. The steady state photocarrier grating (SSPG), photoconductivity and photothermal defection spectrometry have been used to understand and improve the properties of a-SiGe:H films prepared using a wide variety of feedstocks and deposition conditions. Studies have shown that trace levels of boron have a marked influence on the ambipolar diffusion length of b o t h a-SiGe:H and a-Si:H films. In single chamber systems b o r o n doping occurs due to carryover from the p-layer deposition.

1. Introduction Amorphous silicon (a-Si) alloys with carbon and germanium have emerged as the most useful wide and narrow bandgap materials for multi0379-6787/89/$3.50

© Elsevier S e q u o i a / P r i n t e d in T h e Netherlands

26 junction, thin film devices. Because the optical properties of the amorphous alloys can be varied over a wide range, the device designer has substantial flexibility in constructing devices which offer the best compromise between performance, stability and cost. The Solarex development effort has been comprised of three subtasks: (1) semiconductor research on the a-SiGe and a-SiC alloys, including doped layers, (2) non-semiconductor materials research on contacts used for optical enhancement and (3) submodule research. In this paper, we will summarize the important developments in each area during the past year. Notable among the accomplishments are: (1) single-junction submodules with AM1.5 efficiency of 10.2%; (2) triple-junction submodules with AM1.5 efficiency of 8.6%; (3) triple-junction, a-SiC (1.85 eV)/a-Si (1.7 eV)/a-SiGe (1.4 eV) solar cells with measured o u t d o o r performance of 10.7%; (4) improved a-SiC d o p e d contacts prepared from B(CH3)3, which have resulted in improved open circuit voltages leading to 11.4% conversion efficiency; (5) marked improvements in stability. Tandem devices with 9% conversion efficiency have been tested which show ca. 10% degradation after 1 yr of operation; (6) a two-step laser process for forming submodule interconnections.

2. a-SiGe materials

a-SiGe alloys appear to be the most promising semiconductor thin film for the narrow bandgap portion of double- and triple-junction devices. O p t i m u m performance for a two-terminal tandem device based on a-Sill (1.7 eV) requires a 1.1 eV alloy, whereas the use of a triple-junction arrangement achieves o p t i m u m performance with a b o t t o m cell with approximately a 1.45 eV bandgap. The required energy gap for the b o t t o m cell is raised if a larger bandgap t o p cell is used [1 ]. Studies of the Si-Ge alloy system have taken place at t w o levels: (1) basic materials studies, using photoconductivity, photothermal deflection spectroscopy (PDS), Raman spectroscopy and the steady-state photocarrier grating (SSPG) m e t h o d to characterize and understand the material, and (2) device studies to determine the factors which govern the performance of the devices. An initial survey of the electrical transport properties of the Si-Ge alloys prepared from numerous feedstocks such as GeH4, GeF4, Sill4, SigH6 and silyl germanes did not disclose major differences. In general, increasing the germanium content decreases the # r product deduced from photoconductivity. PDS spectra, however, show little or no increase in the midgap

27

density of states, nor an increase in the tail density of states as disclosed by the Urbach Energy. One of the suggested causes for the poor transport properties of the alloys is the preferential Ge-Ge bonding. This is a plausible explanation as GeH4 decomposes more easily than silane and therefore might be expected to undergo preferential gas-phase polymerization leading to preferential bonding in the solid. Dilution with hydrogen might be expected to reduce this preference. Raman spectroscopy was used to distinguish between homoand heteronuclear bonding. Figure 1 shows the measured Raman ratios (a) IGe_Ge/l$i_Ge and (b) Isi-Ge/Isi--Si as a function of germanium concentration for alloy samples prepared with and without hydrogen dilution. The solid curves show the values expected for a random alloy. These data indicate that preferential bonding is indeed occurring and that hydrogen dilution consistently produces an alloy closer to that expected for homogenous material. By varying deposition conditions, a range of IGe_Ge/Isi_Ge Raman ratios was produced. Figures 2(a) and 2(b) compare the p~ product and diffusion length of 1.45 eV alloys ranging from near-perfect homogeneity (ca. 0.9) to films exhibiting a high degree of preferential Ge-Ge bonding. A clear improvement in the electrical transport properties is seen for the homogeneous alloys.

4.0

~

4.0

(o)

~3.0 00000no H d~lution

?

30~

~ 2.0 ~ 1.0

.

~ .

2

/ .

0

~

!1o

~

0"%.0

(b)

I

0.2

i

L

0.4

h

i

0.6

Ge Content

,

i

0.8

'o.'6 'o.'8 Ge Content

Fig. 1. (a) IGe_Ge/lSi_Ge and (b) ISi-Ge/Isi-si for a-SiGe:H films prepared with (open) squares and without (closed circles) hydrogen dilution. The continuous line is the Raman ratio for the homogenous alloy.

Hydrogen dilution, which produces more homogeneous bonding in the alloy, also improves the device characteristics, as shown in Table 1. A seven-fold increase in pr occurs with hydrogen dilution, and the fill factor of devices increases by nearly 25% over that obtained with the undiluted feedstock. In contrast, the steady-state photocarrier diffusion length appears to decrease slightly for reasons which are unclear. In addition to the improved initial performance, the alloys prepared with hydrogen dilution show less light-induced degradation. After ca. 100 h light exposure hydrogendiluted samples show a 6% loss in performance compared with a 14% loss for the control 1.5 eV device.

28 TABLE 1

Bandgap ( e V )

p~- ( c m 2 V -1)

L d (A)

FF

% Efficiency

Hydrogen diluted GeH4 + Sill4

1.5

7.0 × 10 - s

677

0.482

4.57

Sill4 + GeH4

1.5

1.0 × 10 - s

835

0.388

2.99

Low-level boron doping has been demonstrated to have a major effect on the electrical transport properties of a-Si:H, although conflicting reports of the benefits of its use abound [2 - 7 ]. If hole transport limited collection in a-SiGe alloys, then improvements in device performance might be expected. To understand the effect of low-level boron doping, a-SiGe films were prepared by d.c. glow discharge from silane and germane, using a gasphase boron source. Secondary ion mass spectroscopy (Evans East) was used to quantify the boron content of the solid. The ambipolar diffusion length, Ld, was measured using the SSPG method [8]. The ambipolar diffusion constant, D, defined by Ld = (Dr) 1/2, characterizes the average diffusion process of all photogenerated carriers, including those being deeply trapped, and T is the common steady-state lifetime of both photogenerated electrons and holes. D can be expressed in terms of the average diffusion coefficient of holes (Dp) and electrons, (Dn) by / ~ ----. P n D p Pn

-t- P p / ~ n

(1 )

4- p p

where ~n and /2p are the average electron and hole mobilities. Assuming that transport takes place principally in the extended states,then the average

(o)

1 0 -7

~

¢,i E u v

o

vg 800 g

(b) 0 0

0

o

0

O

,~

-J 400

~'I 0 -"

0.8

1ooo

g

, , 1.0 1.2 Roman Ratio

, 1.4

~ '-'

i 8.

i

1.0

i

1.2

i

1.4

Ramon Ratio: leA-~/Is~-~

Fig. 2. ( a ) / 2 7 p r o d u c t and (b) the d i f f u s i o n length o f a-SiGe samples p l o t t e d as a f u n c t i o n o f the R a m a n r a t i o /Ge-Ge- T h e r a n g e o f G e - G e b o n d c o n c e n t r a t i o n s was o b t a i n e d b y changing d e p o s i t i o n c o n d i t i o n s .

29

drift mobility differs from the free mobility by the ratio of free to total carrier concentration 0n.p so that _

20n0pUn~ p

kT

(2)

q 0n/~n + 0p/~p

and the ambipolar diffusion length is given by

Ld =

k T 2/lnTn/.tpT p I 1/2 q /.tn'r n + #pl"p/

(3)

In contrast, the photoconductivity is given by Oph

= qG(#nrn

+

#prp)

(4)

As low-level doping is not expected to effect free carrier mobilities, changes of the free carrier lifetime can be determined from the photoconductivity. When/./sT n >~ ~/pTp then the electron becomes the majority carrier and Ld = (2kTlJpTp/q) 1/2

(5)

qGI.tnTn

(6)

Oph =

Therefore, the /~r products of the minority and majority carriers can be determined from the ambipolar diffusion length and photoconductivity respectively, as given in eqns. (5) and (6). Figures 3(a) and 3(b) show the ambipolar diffusion length and photoconductivity of boron doped a-Si:H and a-SiGe:H films as a function of the boron doping level of the solid. The diffusion length of the a-Si:H and 1.6 eV a-SiGe:H film exhibit maxima at ca. 0.4 and 0.2 ppm respectively. The photoconductivity data exhibits simultaneous minima. Because the pr product of electrons in undoped a-Si:H is much larger than holes, diffusion length measures essentially holes whereas photoconductivity measures electrons. Hence, the complementary dependence of Ld and photoconductivity may be understood. However, the data indicate that this behavior does not extend beyond concentrations above 0.4 ppm (a-Si:H). The observations are not consistent with the expectation that doping con-

°3

vE

91°

| I

•

/ / ~

0.2

,,,,,, o-Si:H

(_Euffi1.BeV)

N~.~,ILA~, o - SIGe:H

00?--10

z .....

O.

•

.-,

0.5

-~ ...e......o .......

.

-"::-=--_-~:-=-=-=o

1.0

1.5

~v~10 -e

-,

' 2.()

B-doping Level (pprn)

Fig. 3. (a) Diffusion length and (b) photoeonductivity of a-Si and a-Si-Oe alloys as a function of diborane concentration of the solid.

30 tinuously reduces the #T of electrons and increases that of holes at these low concentrations. The only consistent explanation which does not violate the basic assumptions is that doping alters the transport properties of holes and electrons so much that the two types of carriers actually interchange their roles as majority and minority carriers. Therefore, at the higher boron concentrations L d is that of the electron and the photoconductivity is primarily that of the hole. According to this explanation, eqns. (5) and (6) have been used to calculate L d and pT respectively, except at the point corresponding to the extrema where /2nTn = ]ApTp has been assumed. Figures 4 and 5 illustrate the pT dependence for a-Si:H and 1.6 eV bandgap a-SiGe respectively. Excellent agreement between the two methods of calculation is found and the pT of the majority carrier determined from photoconductivity (open squares) is always greater than that of the minority carriers determined from Ld (solid circles). Thus, the pr of electrons is represented by the dashed line and the solid line represents that of holes. It is worthwhile to note that ttr of electrons is approximately two orders of magnitude greater than that of holes for both a-Si:H and the 1.6 eV alloy. However, the values of pT for both carriers are reduced by germanium addition, and ]2T for electrons is reduced more than that for holes. The effect is not seen for the 1.4 eV alloy. Presumably the germanium-induced states are not influenced by boron doping [9 ]. The light intensity dependence of the ambipolar diffusion length has also been measured. Surprisingly, at higher doping levels (/> 1 ppm) diffusion length increases with light intensity. This observation is consistent with other p h o t o c o n d u c t i v i t y studies [ 1 0 - 1 4 ] and indicates that two types of gap states may exist. One type, which is c o m m o n l y associated with neutral dangling bond states located near midgap, traps both electrons and holes. The second type, which is located closer to the valence band edge (0.30.5 eV), traps holes much more effectively than electrons, and acts as a sensitizing center for electrons. At higher doping levels, boron converts a significant fraction of the midgap states into effective electron traps and the diffusion length represents that of electrons. Thus, when sensitizing states become effective at high light intensity, the diffusion length can

10

-61

o-Si:H

Zl°-

>

~'~ 10

o-SiGe:H (E~=l.6eV)

.~

f---o

~ 7

_ _ ......

"~10 -8 10 -8 10 -~o 0.0

""'"O

E ~0- I~'-.

Hole

Electron

....................

4~

i

i

i

i

1.0

2.0

3.0

4.0

B-doping Level (ppm)

J

Hole

.~u10 -' ~ ^ ' " ' ~ / ~ - - ~ - ' - -

tt

1 0 -g

1 0

F

-~o i

0.0

....... "'""'o

,

,

Electron

...............

,

i

,

o ,

0.5 1.0 1.5 B-doping Level (pprn)

Fig. 4. pl" p r o d u c t o f a-Si:H as a f u n c t i o n o f b o r o n c o n c e n t r a t i o n . Fig. 5./~T p r o d u c t o f a-SiGe :H as a f u n c t i o n o f b o r o n c o n c e n t r a t i o n .

,

31 actually increase. In contrast, /~prp, determined from photoconductivity at high doping levels,decreases strongly as a result of the effectivetrapping of holes. Hence, photoconductivity shows the observed weaker light intensity dependence.

3. Doped alloys Investigations into new doped alloys have been made in two areas: (1) microcrystalline n- and p-type alloys, and (2) a-SiC alloys doped with trimethyl boron (TMB). Microcrystalline films have been prepared which rival the conductivity of their polycrystalline counterparts, and several workers have reported both microcrystalline silicon and silicon carbide films [1517]. Improved open circuit voltages have been attributed to the use of microcrystalline Si
32 EFFECT OF ADDING METHANE TO FORM AN o - S I / C p-LAYER ON TAUC BAND GAP AND CONDUCTIVITY

10 -" B(CHs) J and SIH4 Flows ¢2 I "5 I

held constant

I 10- 8;"1 ................ 2()' .2";0" '2'40 TAUC BAND GAP (eV)

Fig. 6. Conductivity vs. bandgap in a-SiC alloys prepared from silane and methane but doped with 'rMB. Also shown is the point representation of a "standard" p-layer used in control devices.

TABLE 2

Solar cell properties Voc TMB Diborane

0.917 0.850

Jsc 16.9 16.8

FF 0.736 0.731

p-layer bulk properties Efficiency 11.4 10.4

B concentration

Bandgap

(atoms cm -3)

(eV)

5 × 1019 5 × 1019

2.14 2.06

tions, the electrical conductivity of a-SiC films d o p e d with TMB exceeds that of the usual films used as the p-layer contact. These results are summarized in Fig. 6. For example, at an optical bandgap of 2.0 eV TMB d o p e d films have one order of magnitude higher conductivity than the "standard" p-layer. Devices prepared with TMB
4. Stability Stacked junction devices exhibit improved stability for at least t w o reasons. First, the very thin (about 1000 A) front junction used in the devices is virtually unaffected b y light exposure and second, the reduced intensity at the second and/or third junctions delays the onset of degradation and reduces its rate. Because one has excellent control of the optical bandgap of these amorphous materials, designs incorporating thin, stable films can be used to optimize b o t h efficiency and stability.

33

Stacked junction devices degrade more slowly because the overall degradation rate of the device is, at long times, approximately the average of the degradation rate of the individual cells. For example, Fig. 7 compares the degradation rate of an a-Si:H/a-Si:H stacked cell with a 2400 A rear junction with its single-junction counterpart and a much thicker {about 6000 A) device. It should be noted that the tandem cell, which has an initial efficiency of 9%, is performing at 90% of its initial value after 1000 h of continuous illumination, equivalent to 1 yr outdoors. The individual 2400 A cell degrades to 83% after 1 yr but because it is coupled with an extremely thin top junction, the net degradation is low [20]. The light-induced degradation of the alloys has not been as thoroughly studied as that of a-Si:H and is clearly more complex. Figure 8 plots the 1.0

~°0o

0.9

0

o

,

O

0

0.8 E

•~

0

O0

O0

0

0

0

0

0

0.7

0.6 0.5

0.4 •~

0.3

0

E 0.2

*****

Thick single junction cell Thin singl¢ junction cell O 0 0 ( ~ T h l n two junction cell

00000

0.1

........

0.0

,

........

,

........

Hou'2

,

1000

Fig. 7. Comparison of the degradation rate of thin a-Si/a-Si stacked junction, thin singlejunction and a thick single-junction device. The initial performance of the a-Si/a-Si tandem is 9%. 1.00

ZkZl

3 0

"1-

0

A

o 0.90

A

0

zs

Comparison of vorlotm S~C~ ;-Io,/e¢ devices to s t a n d a r d Si i - l a y e r c a s e

0 zl

0.80 U ¢..

.~_

~0.70 A

.~ 0.60 0

AAAAA

LI

A

Si i-layer

A

0 ~ ) ~ ) IOOA ~;;~1.5 sV SiGe (H= d i l u t i o n ) CCCX~ IOOA S;~/1.5 eV S;Ge ..... IO00A $i/lOOA S i - S i G e / 1 . 5 eV SiGe ~o~-~,-~2500A 1.5 eV SiGe (Si p - l o y s r )

7° 0.50

•

0

i

1000

.

,

•

,

.

,

•

i

200o 300o 40oo 5ooo I-Ioyer Thickness

•

,

sooo

.

,

70o0

Fig. 8. Comparison o f the degradation rates (100 h) of a-SiGe devices (2000 A) as a function of bandgap.

34 normalized efficiency (100 h) of 2000 A single-junction devices as a f u n c t i o n o f optical bandgap. At low germanium addition, the rate of degradation appears larger than f o r a-Si:H but decreases at higher germanium addition. No degradation is seen in devices with a 1.3 eV bandgap. Initial results with hydrogen-diluted a-SiGe alloys show far lower rates of degradation t h an f o r these films prepared f r o m the undiluted feedstocks. 5. D e v i c e results

Th e p e r f o r m a n c e o f stacked j u n c t i o n cells and modules has been improved b y addressing several issues: (1) minimization o f t u n n e l j u n c t i o n losses; (2) i m p r o v e m e n t of device p e r f o r m a n c e through t he use of graded interfaces and d o p e d layers in a-SiGe:H and a-SiC :H devices; (3) the use o f very high reflectivity ITO/Ag rear contacts. (4) d e v e l o p m e n t of a m e t h o d f or laser scribing ITO/Ag contacts; (5) th e use o f heavily t e x t u r e d , transparent, tin oxide f r o n t contacts. Th e most serious parasitic loss exclusive to t he stacked j u n c t i o n devices and submodules is t he tunnel junction. Losses arise because o f the need to provide a low-resistance i n t e r c o n n e c t i o n between junctions. This requires t h e use o f heavily doped, highly conducting alloys. Calculations have shown t h a t optical losses equivalent to 1.5 m A c m -2 m ay occur in the first tunnel j u n c t i o n in a triple-junction device. Optimizing the doping levels and thickness has allowed substantial i m pr ove m ent s in performance.

TABLE 3 Photovoltaic parameters of amorphous silicon-based solar cells Material

Junction(s)

a-Si(TMB) a-SiC a-SiC a-SiC

Single Single Single Single Single

a-SiGe a-SiGe

Single Single

a-SiGe a-Si/a-Si a-SiC/a-Si a-SiC/a-SiGe a-SiC/Si/SiGe a-SiC/Si/SiGe a-SiC/Si/SiGe

Single Tandem Tandem Tandem Triple Triple Triple

a-Si

aMeasured outdoors.

Eg

Voc

Jsc

(eV)

(mY)

(mA cm -2)

1.7 1.7 1.8 1.85 1.90 1.61 1.55 1.50 1.7/1.7 1.85/1.7 1.85]1.55 1.85/1.7/1.55 1.85/1.7/1.45 1.85/1.7/1.40

0.891 0.917 0.880 0.932 0.952 0.851 0.823 0.752 1.69 1.75 1.72 2.481 2.172 2.38

19.13 16.90 9.59 9.44 9.52 19.34 20.08 19.50 8.3 8.16 9.11 4.99 7.30 7.01

FF

~7

(%) 0.70 0.736 0.704 0.670 0.605 0.654 0.611 0.547 0.710 0.712 0.670 0.669 0.642 0.64

12.0 11.4 5.9 5.9 5.5 10.8 10.2 8.0 10.0 10.2 10.5 8.3 10.2 10.7 a

35 TABLE 4 Photovoltaic parameters of amorphous silicon-based sub-modules Material/junction(s)

a-Si/Single a-Si/Single a-SiC/SiGe/Tandem a-SiC/Si/SiGe/Triple

Size (inches)

No. o f seg.

3 6 12 3

12 35 30 5

x3 X 13 x 13 X3

Voc (V)

Jsc

10.376 29.894 50.918 11.085

17.4 15.05 7.3 6.1

FF

77 (%)

0.677 0.713 0.624 0.641

10.2 9.4 7.7 8.7

( m A cm -2)

MULTI-SOURCE

0.0

iE

5.0

10.0

15.0

8.00

8.0

6.00 -"

6.0

400i

40

| P,

.oo000

"

,

I . . . . . . . .

0 0 . . . . . . . . .

~ . . . . . . . . .

5.0

.......

~ .

.

.

.

.

I"0:0. . . . . . .

VOLTAGE (VOLTS)

.

.

0.0

Voc--11.085 V Jsc 6.1 mAcm -2 F F 64.1% Eft. 8.7% Isc 37.893 mA Pmax 8.683 mW cm - 2 /max 32.544 m A

Vmax --8.271 Jmax 5.249 mA cm -2 Intensity 100.3 mW c m Temp. 24.6 °C Cell area 6.200 cm 2 Segments 5 Total area 31.000 c m 2 Date 05-19-1989 Time 09:43:54

-2

~ 15.0

\

Fig. 9. C u r r e n t - v o l t a g e characteristic of a triple j u n c t i o n stacked j u n c t i o n s u b m o d u l e w i t h t h e structure o f a-SiC/a-Si:H/a-SiGe.

A n o t h e r i m p o r t a n t device design f e a t u r e necessary in h i g h - p e r f o r m a n c e a-SiGe devices is t h e use o f an " i n v e r s e graded i n t e r f a c e " e x t e n d i n g f r o m t h e n a r r o w b a n d g a p a-SiGe:H alloy t o t h e w i d e r a-Si:H at t h e n/i interface, and t h e use o f a n a m o r p h o u s silicon n-layer. T h e l o w e r d o p i n g e f f i c i e n c y o f t h e alloy results in a l o w e r built-in p o t e n t i a l a n d higher series resistance [ 2 1 ] . T a b l e s 3 and 4 s u m m a r i z e t h e p e r f o r m a n c e o f individual cells and subm o d u l e s respectively. F o r cells, c o n v e r s i o n efficiencies 17 as high as 10.7% have b e e n o b t a i n e d in o u t d o o r m e a s u r e m e n t s using a 1.85 eV SiC t o p junct i o n 1.7 eV, a-Si:H m i d d l e j u n c t i o n a n d a 1.4 eV a-SiGe:H b o t t o m j u n c t i o n . T r i p l e - j u n c t i o n s u b m o d u l e s using t h e same device design have b e e n p r e p a r e d w i t h an e f f i c i e n c y o f 8.7% (Fig. 9) using a n e w l y d e v e l o p e d m e t h o d o f scribing I T O / A g rear c o n t a c t s . T a n d e m - j u n c t i o n devices w i t h a s t r u c t u r e a-SiC (1.85 e V ) / a - S i G e : H (1.55 eV) have yielded solar cells w i t h a conver-

36 0.0 20.00

~ m

2.0

4.0

U ± L L I LLI L,, l l i d

, t t L~ a , ,

6.0

8.0

I ~ i , , ~ ~, L ~ l ~ u

10.0 ~t u I ± l

,.11 t i

12.0 zl~

20.0

15.0

15.00

eo E .-j

10.00

_

10.0

5,00

i

5.0

I-z uJ DE CC o

Voc--10.376 V Jsc 17.4 mA cm -2 FF 67.7% Eff. 10.2% Isc 53.131 mA Pmax 10.192 m A c m -2 Imax 45.925 mA Vmax --8.123 Jmax 15.057 m A c m -2 Intensity 100.2 mW cm -2

0.00

-:/ ......... , ......... , ......... ~ t . - , ~ , T F , - m ~ , ~ l ~ 2.0 4.0 6.0

o.o

0.0

.o

Temp. 30.2 °C Cell area 3.050 c m 2 Segment 12 Total area 36.600 c m 2 Date 04-13-1989 Time 12:14:10

VOLTAGE(VOLTS) B-OopingLevel (ppm) Fig. i0, Current-voltage characteristicof a I0.2%, single-junctionmodule.

s i o n e f f i c i e n c y o f 1 0 . 5 % , a n d a 10.2% e f f i c i e n c y has b e e n o b t a i n e d f o r s i n g l e - j u n c t i o n s u b m o d u l e s ( F i g . 10).

Acknowledgments

T h e a u t h o r s t h a n k A. R o t h w a r f a n d C. W r o n s k i f o r h e l p f u l d i s c u s s i o n s a n d Ms. S h a r o n M a r t i n o f o r p r e p a r i n g t h e m a n u s c r i p t . T h i s w o r k w a s supported under Solar Energy Research Institute Subcontract ZB-7-06003-2.

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