Theoretical and experimental considerations for high silicon solar cell performance

Solar Cells, 1 7 (1986) 135-149

135

T H E O R E T I C A L AND EXPERIMENTAL CONSIDERATIONS F O R HIGH SILICON S O L A R CELL P E R F O R M A N C E M. B. SPITZER, C. J. KEAVNEY and L. M. GEOFFROY Spire Corporation, Patriots Park, Bedford, MA 01 730 (U.S.A.) (Received August 25, 1985; accepted August 26, 1985)

Summary This paper reviews ongoing research aimed at the attainment of highly efficient silicon solar cells. The importance of low-recombination highlydoped n + and p+ regions and the manner in which such regions are fabricated are discussed. Theoretical light-trapping considerations are combined with experimental reflectance data to show that high quantum efficiency may be obtained from thin (100 pm) cells. The principal finding of this work is that thin solar cells with conversion efficiencies of over 20% may be fabricated if recombination at the front and back metal/silicon interfaces is reduced. Large-area cells (53 cm 2) with an efficiency of 18% are reported.

1. Introduction The attainment of the highest possible conversion efficiency in silicon solar cells requires that bulk and surface recombination be reduced to extremely low levels. If this requirement can be satisfied, and all other loss mechanisms eliminated, then calculations show that AM 1 conversion efficiency of approximately 27% is attainable [1]. Obviously, cell fabrication at its present state is incapable of meeting these requirements, b u t significant progress toward such goals has recently been made in several laboratories [2-5]. In this paper, we shall review the present status of several aspects of silicon solar cell technology and survey both the technological requirements and ongoing research, with a view to identifying the most important areas for future work. A critical aspect of the design and fabrication of highly efficient cells involves the control of recombination both in heavily-doped regions and at surfaces. We shall show that transparent [6,7 ] highly-doped n ÷ and p+ regions can be combined with surface passivation to improve open-circuit voltage Vo~ and quantum efficiency. By transparent, we mean that the bulk minority carrier lifetime is much longer than the transit time from the junction t o the surface. The transit time of interest is that which characterizes the particular case that obtains when the surface recombination velocity is infinite. By this definition, recombination in the transparent emitter is dom0379-6787/86/$3.50

© Elsevier Sequoia/Printed in The Netherlands

136 inated by the surface unless adequate passivation is employed. The use of passivated transparent regions permits great latitude in cell design, and we will illustrate some of the possibilities that present research is addressing. A second area of importance in reaching the limit to the efficiency comprises the use of optimal junction depth and base thickness. The optimization depends critically on the electronic and optical properties of the silicon and the substructures at its surfaces. For example, optically reflective substructures can make possible the light-trapping concept originally proposed by Redfield [8]. Light-trapping is one technique that would, if perfected, free solar cell design from thickness constraints imposed by absorption considerations. We will discuss a cell design based on light-trapping and present data that indicate the performance that can be obtained. In the first part of this paper, we review the theoretical background involving modelling of silicon cells to elucidate the concepts that our experimental work is addressing. This review is followed by a discussion of both low recombination n+ and p÷ regions, and of the importance of surface recombination. Cell optics are next presented, and a discussion of lighttrapping is followed by cell thickness considerations. We conclude with a discussion of the present technology, cell performance achievements and a description of research needed to attain efficiency that more closely approaches the theoretical limit.

2. Theoretical background In 1980, Spitzer et al. [1] calculated the upper limit to the efficiency of silicon cells operating in low injection (one-sun) conditions. It was shown that if the surface recombination velocity is low (about 100 cm s-1 ), then by also employing optical confinement (so-called light trapping), the cell could be made thin, with a consequent increase in Voc and no decrease in shortcircuit current Jsc- The improvement of Vo¢ obtains because by making the cell thin, bulk recombination is minimized; however, such an approach requires control of both surface recombination and optical losses. In practice, the substructures that control these losses will not be perfect and the optimum design will probably reflect a trade-off between bulk and surface loss mechanisms. Figure 1 serves both to clarify our terminology and to illustrate the cell configuration upon which this work is based. The base width W and junction depth xj are the distances between the back and front surfaces (characterized by surface recombination velocities SB and SF) and the space-charge-region edges. The reflection of light incident upon the front surface as a function of wavelength k, is given by R(k). Measured reflectance data are used in all of our modelling. The back surface reflector (BSR) is characterized by a reflectance RB, and we will describe the manner in which we measure RB in a later section. We assume t h a t some fraction of the photons that are reflected from the BSR will be trapped by total internal reflection when they

137

~-~S B

SF ----~

~-II~RF R (x) BACK SURFACE REFLECTOR : --•111 Xj

W

~1

Fig. 1. Solar cell design employed in theoretical calculations.

reach the front surface. The fraction that is trapped by total internal reflection is approximated by a constant RF, independent of wavelength or other factors. A number of authors have addressed the manner in which this lighttrapping m a y be obtained [8-10] and we shall return to an examination of some of these considerations in Section 5. Control of the surface recombination velocity may be obtained by using minority carrier mirrors, so-termed because they reflect minority carriers before the carriers reach the actual surface of the solar cell. Such reflection is sometimes achieved by forming p-p++ or n - n ++ built-in fields. The low-high emitter and back surface field are examples of such structures. In our opinion, the n - n ++ and p-p++ structures are of limited value in highly efficient cells because the apparent surface recombination velocity is low only when the p and n layers are doped at concentrations of less than 10 16 cm-3. Moreover, the p++ and n ++ regions are heavily doped and consequently introduce deleterious recombination. These factors limit the applicability of this approach. In principle, a better approach would comprise the use of an isotype heterojunction formed between Si and a wide-bandgap material, as illustrated in Fig. 2. In the particular case shown, a barrier to minority carriers (electrons) is formed at the back of a cell between the silicon and the metal contact by interposing a material chosen to provide a discontinuity in the minority carrier band edge (thus forming a quantum mechanical barrier) while imposing no discontinuity in the majority carrier band edge. The barrier therefore reflects minority carriers but passes majority carriers. Such a barrier m a y be provided at the front of the cell, in which case the large bandgap precludes absorption of incoming light within the minority carrier mirror. In our modelling, we view the minority carrier mirror as a surface characterized by a recombination velocity s. The value of s is one of the boundary conditions placed upon the diffusion equation which governs minority carrier transport in the quasi-neutral regions of the solar cell. A computer code has been developed which evaluates the solutions of the

138

1

EVAC

[

WBS

XSi

~m

Ec I

WIDE METAL .-=.I.,~BANDGAP SEMI CONDUCTOR

~.Ef Ev

: !=

SILICON

=I

Fig. 2. Conceptual band diagram of the ideal minority carrier mirror (MCM). The case shown is the p-type MCM, in which a barrier to minority carrier (electron) transport is introduced in the conduction band. Xsi is the electron affinity of the silicon and Cm is the work function of metal.

diffusion equation in the emitter and base regions of the device, and integrates these solutions over the solar spectrum. This code is described in detail in ref. 11 and will not be discussed further here. The principal finding of the calculations is that for the case of perfect optical and minority carrier mirrors, and in the absence of shadow and reflection loss, the limit to the efficiency is about 27% [1]. If we assume that actual cells will be characterized by 4% metal coverage and 2% reflection loss, the upper limit is reduced to a practical limit of 25%. We view the practical limit as a target that might be achieved if the surface recombination could be completely controlled and the light-trapping made highly efficient. We will present further modelling results for non-ideal cases and discuss experiments aimed at attaining high performance in the sections to follow.

3. Transparent n + and p+ regions As we have noted in the previous section, recombination must be controlled, not only at the surfaces, but also in the thin highly doped n + and p+ regions that are used to form p/n junctions. In such regions, undesirable heavy-doping effects may obtain if the n ÷ and p+ regions are doped to solid solubility limits [12]. We have overcome this problem to a large extent by limiting the doping concentrations to values that achieve a good compromise between majority carrier conductivity and minority carrier recombination. Our approach is entirely based upon using the ion implantation process [13] to carefully limit the n + and p+ doping. Unlike diffusion, it is possible

139

with ion implantation to obtain surface dopant concentrations ranging from a b o u t 1017 cm -a to over 1020 cm -a, simply by changing the dose. In several recent papers [2, 1 4 - 1 6 ] , we showed that the peak dopant concentration exerts a strong influence on solar cell performance. Our results can be summarized by the following general findings: for the case of n + emitters, the optimal peak donor concentration is in the range 1 X 1019 a m -3 . 3 X 1019 c m -3. For p+ emitters, the optimal peak acceptor concentration is in the range of 4 X 1018 c m -3 to 6 X 10 Is c m -a. Transparent regions require low surface recombination velocity [7], otherwise the benefits of transparency are lost. Indeed, since transparent regions can supply carriersto the surface faster than non-transparent regions, the presence of surface recombination can be m o r e deleterious than the bulk recombination that obtains in the non-transparent heavily-doped conventional approach. In the next section, we will discuss successful results obtained by using SiO2 to limit surface recombination.

4. Minority carrier mirrors Figure 2 illustrated the ideal minority carrier mirror formed by an isot y p e heterojunction. In practice, we have achieved low surface recombination velocity by growing a thin (100 A) layer of SiO2 on the surface. To quantify h o w transparent n + and p+ regions perform when provided with a low recombination surface, we have measured solar cell quantum efficiency with an without the SiO2 passivation. Figure 3 illustrates the result of

1.0 0.9 f~ 0.8 Z W LL LU

0.7

WITH

0.6 Z) I,,--

z

0.5

0

0.4

.-I

(IC W

0.3

I- 0.2 X ILl

0.1 I

400

I

I

!

I

600 800 WAVELENGTH (nm)

I

/

1000

Fig. 3. External quantum efficiency of an n+-p-p + cellwith and without SiO2 passivation. (Cell 46033-15D; no AR-coating; AJsc, 1.1 m A c m -2 ( A M 1).)

140

removing the front surface passivation from the n + - p - p + cell, and Fig. 4 shows the same effect in a p + - n - n + cell. It can be seen that the blue response is sensitive to the front surface recombination velocity, indicating that in both cases the emitters are transparent and that the SiO2 reduces the surface recombination velocity. This effect is completely corroborated by dark I-V, Voc and Jse measurements. Details of the processing and characterization are reported fully in ref. 15. Unfortunately, SiO2 is not conductive and does not transmit majority carriers without impediment. Therefore, contact must be made by removing the oxide beneath the metal grid; thus, since part (4%) of the surface is in contact with metal, part is characterized by a high value of s. This areal inhomogeneity [17, 18] introduces a serious loss mechanism. Figure 5 replicates Voc data from an experiment to quantify the areal inhomogeneity effect as a function of ion implantation dose (see refs. 15 and 16). It can be seen that as the dose is reduced, the Voc increases, corresponding to a decrease in heavy-doping effects. The Voc, however, appears to be limited to 635 mV, owing to recombination at the metal/Si interface. However, if the metal interaction area is reduced to 0.1% by placing most of the contact grid on top of the oxide, the Voc is increased to over 650 mV, and in subsequent experiments, we have achieved Voc values of up to 660 mV. We emphasize that attainment of these Voc values requires either reduction of the recombination velocity at the metal/Si interface, or reduction of the area of the metal/Si interface. Unfortunately, reduction of the area of the contact can introduce deleterious series resistance. Passivation has been combined with both n + and p+ transparent emitters in the fabrication of highly efficient solar cells. We replicate in Table 1

1.0 >- 0.9 O Z - 0.8

_o

uu. 0.7 uJ

WITH SiO 2

0.6 I-

~ 0.5 0 0.4 ,<

~ 0.3 t.u I--

J

REMOVED

x 0.2 LU 0.1 I 400

I

I 600

I

Wavelength

I 800 (nm)

J

I 1000

Fig. 4. External q u a n t u m efficiency of a p+-n-n + cell with and w i t h o u t SiO2 passivation. (Cell 4 6 2 5 - 1 A ; no A R - c o a t i n g ; AJsc , 1.9 m A cm -2 (AM 1).)

141

> 660 E

\!

OHMIC CONTACT A.EA=O.1,

640

o

-[

0 62G

~

-°

IH

5

re

t

si02 OPENING

H

IN SiO2 ~ [~GRID ~

°°'--

GRIDLINE I 10TM

OHMIC CONTACT

/REA-4%

\

I 1019

1 1020

PEAK BORON CONCENTRATION (cm -3)

Fig. 5. Voc as a f u n c t i o n o f ion i m p l a n t a t i o n dose for s t a n d a r d c o n t a c t s a n d for c o n t a c t s w i t h r e d u c e d o h m i c c o n t a c t area. T h e m e t h o d for r e d u c t i o n o f o h m i c c o n t a c t area is s h o w n s c h e m a t i c a l l y in t h e inset. (No A R - c o a t i n g s ; 1 0 0 m W e m -2 ( A M 1); t e m p e r a t u r e , 28 °C.)

TABLE 1 Cell p e r f o r m a n c e o f c o m p a r a b l e n ÷ - p - p ÷ a n d p + - n - n ÷ solar cells a

Cell 4625-5f 4625-7a 4532-7a 4532-8b

Type p+-n-n + p*-n-n + n+-p-p + n*-p-p +

Voc

J~

FF

EFF.

(mY)

( m A c m -2)

(%)

(%)

657 639 637 634

34.0 34.2 33.7 33.7

75.2 79.6 81.0 79.1

16.8 17.4 17.4 16.9

a I n s o l a t i o n , s i m u l a t e d A M 1, 1 0 0 m W c m -2 ; cell area, 4 c m 2 ; cell t e m p e r a t u r e , 28 °C. Measurements courtesy of Sandia National Laboratories.

data from ref. 16 obtained with both n+-p-p + and p÷-n-n + structures. It can be seen that high efficiency is obtained with either structure. Thus it is demonstrated that p+ and n + regions may be combined with SiO2 passivation to produce low-recombination d o p e d regions and surfaces. The problem of recombination at the metal/Si interface remains unsolved, but other promising approaches to minority carrier mirror formation include use of GaP/Si or ZnS0.9Se0.1/Si isotype heterojunctions. Both GaP and ZnS0. 9 Se0.1 are nearly lattice matched to Si and have the requisite electron affinity for minority carrier reflection [19], and so may form minority carrier mirrors of the t y p e described by Fig. 2. Work has also been reported on SIPOS [20] and poly-Si [21] technology with promising results. Further work in these areas is required.

142

5. Optical considerations The attainment of the limit efficiency requires that front surface reflectance be minimized. Surface texture, combined with an anti-reflection coating, can be used to attain extremely low values of reflectance over the entire spectral range of interest [22]. Figure 6 replicates reflectance data obtained at Sandia on a Spire texture-etched cell. It can be seen that reflection is reduced to very low levels. Similar results have been reported with multi-layer anti-reflection coatings applied to polished cells [3, 4]. Texture also causes oblique propagation of the photons within the crystal, owing to refraction that occurs upon incidence [22]. This refraction enhances the absorption and improves J~c- In thin Si, the refraction may be particularly beneficial, since the p h o t o n pathlength may be made longer than the cell thickness. The optical thickness can be further increased by utilizing BSRs. Figure 7 replicates BSR reflectance data obtained by illuminating the front of the cell and by collecting reflected beam with an integrating sphere. Reflectance of the sub-band-gap photons indicates that a polished (specular) back yields a back reflection coefficiency RB of 0.9. (No correction has been made to separate front surface reflection.) Note that the etched backs yield lower values of reflectance, probably owing to light-trapping. Indeed, in Fig. 8 we show data for samples having textured fronts with various back configurations. By comparison to Fig. 7, we infer that the oblique propagation of the light increases the light trapping significantly. The light-trapping optics can be modelled by considering the infinite sum of reflected rays from the front and back. It can be shown [11] that for a cell having a width W and a front internal reflection coefficient R F , the number of photons as a function of position y in the cell is given by e -~y + R B e -~(2W-y) N ( y ) = No

1--RFRs

(1)

e -2aW

I00

z

0

0

t

400

=

i

i i i

l l =

i

.

.

600

.

.

.

.

.

.

.

.

!

.

.

.

.

800

i

900

.

.

.

.

i

I000

.

.

.

.

I I00

WAVELENGTH (nm)

Fig. 6. Reflectance of a textured and A R - c o a t e d silicon solar cell (courtesy of Sandia National Laboratories).

143 I00

POLISHEDBACKWITHS i O 2 ~ .

I - -- - ~ -

90 80

70 60

l

/f

ETCHEDBACK

50

,ill

40 3C

i/ i/

20 I0

0900 ,~oo l,~o ;2~,o ,3'00,4'oo ,~o ,,;oo ,-~oo ,800 WAVELENGTH (nanometers)

Fig. 7. Front surface reflectance of a polished, AR-coated sample with various Al back surface preparations. The sub-band-gap reflectivity provides an indication of the performance of the BSR.

I00

90 80

70 60 50 40

30

POLISHED BACK

f---/

ETCHED BACK

20 I0

0

/

i

~

- __TEX__TLI_RE_D_BACK_. . . . .

i

i

9o0 ,ooo ,,oo ,2'00,300 ,,~o ,~oo ,ioo ,~

,800

WAVELENGTH (nanometers)

Fig. 8. Front surface reflectance of a textured AR-coated sample with various AI back surface preparations.

144 Here N o is the incident flux, ~ is the absorption coefficient and y is the distance f r o m the f r o n t surface. This expression can be d i r e c t i o n a l l y differentiated to determine the generation rate. It can also be shown that the n u m b e r o f p h o t o n s NF reflected by the BSR but n o t trapped is given by No(1-- RF)RB

NF =

e -2~W

1 - - R B R F e -2~W

(2)

The n u m b e r of p h o t o n s NB absorbed at the non-ideal BSR is given by N0(1 - - R B ) e -~w g B --

1 - - R B R F e -2aw

(3)

These equations can be used to interpret the data shown in Fig. 8. For sub-bandgap p h o tons , aW ~ 0. Thus, if the reflection from the f r o n t surface is low, the measured total reflectance R will be given by R -

NF

RB(1 -- RF)

No

1 - RB RF

(4)

and this expression m a y be used to find RF. F o r example, Fig. 7 indicates that R s = 0.9; th e r ef or e evaluation of eqn. (4) for the t e x t u r e d cells described by Fig. 8 yields: R F (polished back), ~ 0 . 9 5 ; R F (etched back), ~ 0 . 9 6 ; R F (textured back), ~ 0 . 9 7 . Note that we have not measured the reflectance of a t e x t u r e d AR-coated semi-infinite sample to correct for f r o n t surface reflection, and t hat we are obtaining only an approxi m at i on here. Similar analysis applied to a polished cell with an etched back (shown in Fig. 7) reveals that the diffuse scattering f r om the back implies R F -- 0.79. These calculations are based in all cases on a value of RB of 0.9 (except for the textured back, in which case RB ->RB2); we must note that this value o f RB has only been established by measurement for the specular BSR case, and we are assuming that R B is the same for polished and etched backs. With this caveat, we conclude that for a solar cell that is t e x t u r e d on the fr o n t and etched on the back, the values R F -~ 0.96 and RB = 0.9 are achievable with present technology.

6. Cell thickness The foregoing discussion indicates t hat practical cells may be made thin w i t h ou t serious optical losses if light-trapping is employed. In cells in which W ~ L, calculations show t hat surface r e c o m b i n a t i o n emerges as the most i m p o r tan t loss mechanism. Figure 9 presents the results of a calculation of efficiency as a function of thickness for various levels of back surface recombination, assuming a realistic bulk diffusion length of 150 pm, and reflectance values from the previous section. This modelling shows that the light-trapping is indeed effective, but high efficiency is obtained only when

145

22

s = 100 cm/s

>(~ 2 0

s = l o 3 cm/s

w. w 18

16 S = 106 cm/s

i

i

i

i

i

50

100

150

200

250

i 300

BASE THICKNESS (pm)

Fig. 9. T h e o r e t i c a l e f f i c i e n c y for a t e x t u r e d light-trapping cell as a f u n c t i o n o f t h i c k n e s s for various values of S B. (p, 0.3 ~ cm; Le, 150 pm; RB, 0.90 ; RF, 0.96.)

TABLE 2 Average p e r f o r m a n c e of BSF test solar cells a

Back surface

Cell thickness (tzm)

No BSF

Voc

Jsc

FF

EFF.

(mY)

( m A c m -2)

(%)

(%)

380

619 (3)

22.9 (0.3)

80.3 (0.6)

11.4 (0.2)

BSF

380

628 (4)

24.4 (0.1)

80.3 (0.6)

12.3 (0.2)

No BSF

250

615 (3)

23.2 (0.2)

80.6 (0.8)

11.5 (0.1)

BSF

250

621 (4)

24.3 (0.2)

80.7 (0.3)

12.2 (0.2)

a N o A R - c o a t i n g s or t e x t u r e e m p l o y e d : insolation, simulated AM 1, 100 mW cm -2 ; area, 4 cm 2 ; T, 28 °C; standard deviations are s h o w n in parentheses.

the back surface recombination velocity is reduced to levels that are less than bulk transport velocity [23], which is in this case about 103 cm s -1. The importance of cell thickness has been examined experimentally. Table 2 lists data obtained with ordinary BSF test solar cells having thicknesses of 250 ttm and 380 tzm, formed from p-type silicon having a resistivity of 0.3 ~t cm. The fact that Voc decreases as thickness is decreased is a strong indication that the BSF is not providing low surface recombination velocity. A better approach may be to reduce the BSF doping and add

146

SiO2 passivation to the back. Experiments on cells of this type are presently in progress. Experiments have also been carried out with textured BSF-BSR cells. Data taken prior to application of AR-coatings are shown in Table 3. It can be seen that making the cells thinner has a small detrimental effect on performance. These cells also have low F F owing to anomalous leakage, but this is not regarded as fundamental. AR-coatings were applied to selected cells and data are provided in the next section.

7. Solar cell performance measurements In the course of the research described above, we have fabricated highly efficient cells. These cells all incorporate oxide passivation and transparent doped layers. Typical efficiency is approximately 18%. Independent measurements have been obtained from a number of sources, but owing to recent changes in calibration, general agreement is at present lacking. For this reason, we will cite our measurements as well as measurements made in other laboratories. Table 4 lists cell performance for a variety of cells. The first two entries (4579-7E, 7F) are for AR-coated textured BSF-BSR cells (see Table 3). Better performance can be achieved by reducing leakage current to improve F F , and by improving the BSF to raise Js¢ and Vo¢. We believe that with this cell design a reasonable target performance is Vo¢ = 645 mV, Js¢ = 37 mA cm -2 and F F = 0.81, and we are presently developing such cells. Cells 4593-7C, 4606-7C and 4737-18D were fabricated in the course of our ongoing research to improve our understanding of the devices. We point out that these cells all have remarkably high q u a n t u m efficiency. Figure 10 replicates internal q u a n t u m efficiency measurements made at Sandia National Laboratories on cell 4579-7F; note that the q u a n t u m

TABLE 3 Average performance of textured BSF-BSR cells a T h i c k hess

(pro)

380 250

Voc (mV)

Jsc ( m A c m -2)

FF

EFF.

(%)

(%)

629 (2) 623 (1)

34.8 (0.4) 34.1 (0.1)

76.7 (0.6) 76.9 (0.8)

16.8 (0.4) 16.3 (0.2)

a No AR-coatings employed; insolation, simulated A M 1 , 100 m W c m - 2 ; temperature, 28 °C; standard deviations are shown in parentheses.

area, 4 cm2;

147 TABLE 4 Cell performance measurements Cell

Measurement source

Area

(cm 2)

Voc (mV)

Jsc (mA cm -2)

FF

EFF.

(%)

(%)

4579-7E

Spire Sandia

4.00 4.00

627 628

37.6 36.6

78.2 78.3

18.5 18.0

4579-7F

Spire SERI Sandia

4.00 4.00 4.00

628 626 625

37.8 35.8 36.5

78.6 78.4 78.2

18.7 17.6 17.8

4593-7C

Spire JPL

4.00 4.00

641 643

36.2 35.6

79.0 78.8

18.3 18.0

4606-7C

Spire JPL JPL b

4.00 4.00 4.00

627 625 626

37.7 35.4 36.5

79.7 80.1 79.8

18.8 17.7 18.2

4737-18D

Spire SERI

4.00 4.02

631 635

37.8 36.3

81.5 81.6

19.4 18.8

4626-11

Spire JPL

53.04 53.04

609 612

38.0 37.1

77.9 78.8

18.0 17.9

a Cell temperature, 28 °C, except for JPL measurements, which were at 25 °C. b Indicates AM 1.5 global spectrum, all others were direct, 100 mW em -2 .

~ z

E w

100

8O

60

~ 0

4O

z

20

~

0

,~o's~o'sGo'700'

. 8".oo . . 9oo' looo'

WAVELENGTH (nm)

Fig. 10. Internal quantum efficiency of cell 4579-7F. The data at 600 nm are attributed to instrumental errors at the point where the diffraction grating is changed.

e f f i c i e n c y is 1 0 0 % b e t w e e n 4 0 0 n m a n d 7 0 0 n m a n d o v e r 9 0 % b e t w e e n 350nm and 900nm. The short-wavelength response indicates that the t r a n s p a r e n t e m i t t e r is p r o v i d i n g c o l l e c t i o n e f f i c i e n c y o f n e a r l y 1 0 0 % . C e l l 4 6 2 6 - 1 1 is a l a r g e - a r e a t e x t u r e d c e l l t h a t h a s b e e n f a b r i c a t e d f r o m 1 . 5 ~ c m f l o a t z o n e s i l i c o n . T h i s c e l l h a s a B S R as w e l l as o x i d e p a s s i v a t i o n a n d a t r a n s p a r e n t e m i t t e r . T h e Vo~ is c o n t r o l l e d m a i n l y b y t h e b a s e r e s i s t -

148 ivity, but the greater diffusion length for this resistivity provides high Js¢. This cell demonstrates that high efficiency can be obtained not only from laboratory-scale cells but also from full-size module cells. We expect that under global irradiance the efficiency will be even greater. The cell design principles discussed in this paper have also been applied to point-focus silicon concentrator cells [24]. Textured AR-coated BSFBSR cells having transparent n ÷ emitters and oxide passivation were fabricated. These cells were tested at Sandia National Laboratories at an insolation level of 3 W c m - : (30 suns); temperature was maintained at 28 °C. The best cell yielded an efficiency of 20.7%, indicating that the high-efficiency techniques described in this paper are applicable to concentrator cells as well. 8. Conclusions This paper has reviewed performance calculations and results of experimental research on highly efficient cells, with a view to estimating the best performance that can be achieved with present-day technology. We have shown experimentally that although transparent emitters and SiO2 passivation introduce only very small amounts of recombination, metal/Si interface recombination may limit Voc in the best cells. An important area for future research therefore comprises the development of front minority carrier mirrors that can be applied at this interface. We have indicated that thin cells can be made efficient if light-trapping is used to confine the generation to regions between the minority carrier mirrors. Front surface texture combined with a BSR appears to be one good way of achieving light-trapping. We find, however, that the back surface recombination velocity must be reduced to less than 10 a cm s-1, in order to make thinning of the base useful. Since the BSF does not appear to be useful for this purpose, we require the development of a minority carrier mirror that may be applied between the silicon and the BSR. Calculations indicate that if such minority carrier mirrors are developed, an efficiency of over 20% may be obtained, even though the minority carrier diffusion length is only 150 pm. Finally, we have shown experimentally that this work is applicable to both large-area flat-plate and point-focus concentrator cells.

Acknowledgments The authors would like to thank Dr. Ajeet Rohatgi {Westinghouse Electric Corporation) and Dr. Ben Rose (Sandia National Laboratories) for many helpful discussions and measurements. One of us (M.B.S.) is indebted to Professor J . J . Loferski {Brown University) for motivating and encouraging this work in its early stages at Brown. This research was supported by the United States Department of Energy under contracts with SERI (ZB-3-02090), Sandia National Laboratories {58-1430) and the Jet Propulsion Laboratory (956641).

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