An investigation of the properties of interdigatated back contact solar cells

Sola~ Cells, 25 (1988) 53 - 60 53

AN INVESTIGATION OF THE PROPERTIES OF INTERDIGITATED BACK CONTACT SOLAR CELLS

MUNAWAR AHMAD

National Insti tute o f Silicon Technology, Islarnabad (Pakistan)

(Received September 21, 1987; accepted May 4, 1988)

Summary

The interdigitated back contact (IBC) solar cell has shown itself to be promising for concentrator applications. In this study examples of this type of cell are tested against varying finger width ratios of p+ and n ÷ back con- tacts. The current-voltage characteristics of the cells under a concentrated light of 7 suns are measured and solar cell equation parameters such as Rs, Rsh, Is, Iph and A are evaluated for cells having Lp÷ to Ln÷ finger width ratios of 1:1 to 5:1. The cell parameters are computed from experimental data on fourth~cluadrant characteristics on the basis of a single-exponential solar cell model. Opt imum values of these parameters are found for an IBC cell with a finger width ratio of 5:1.

1. Introduct ion

Among the various solar cells designed for operation under concen- trated light, the interdigitated back contact (IBC) solar cell possesses several interesting features [1 - 3]. Its potential advantages, for example no shadow- ing loss due to the collecting grid, lower electrode resistance and the absence of sheet resistance effects arising from lateral current flow in diffused regions [4], are the main reasons for the intensive research into this type of cell. However, to date it has not been tested against varying Lp÷ to Ln* finger width ratio under concentrated light to evaluate solar cell equation param- eters. These parameters, such as series resistance Rs, shunt resistance Rsh , photon~enera ted current Iph, reverse saturation current Is and ideality factor A, are calculated on the basis of a computat ional analysis of a single-expo- nential solar cell model. The parameters are significant for evaluating the per- formance of a solar cell and optimizing IBC cell design.

The cells under test were fabricated on 3-in n-type silicon (100) slices of thickness 380/~m and polished on both sides. The cells are square and a little over 3 mm in dimension, with an interdigitation repeat distance of 200 pro. They have a range of finger width ratios Lp÷:Ln÷ varying from 1:1 to 5:1 and are designated IBC1 - IBCs respectively.

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54

The areas follows IBC1 : p+, 2.41 IBC2: p+, 3.07 IBC3 : p+, 3.39 IBC4: p+, 3.59 IBCs: p+, 3.71

(cm 2) of the p+ and n + active regions in these cells are as

X 10 -2 ; n +, 2.41 X 10 -2 ; X 10 -2 ; n +, 1.74 X 10 -2 ; X 10-2; n +, 1.42 X 10-2; X 10-2; n +, 1.22 X 10-2; X 10 -2 ; n +, I.i0 X 10 -2.

On every slice there are 22 complete groups of structures, each group consisting of five solar cells and test-structures [5]. The slice layout and active regions of one of the test cells is shown in Fig. 1; the test structures shown in Fig. 2 were fabricated at the Southampton University SERC Micro- electonics Centre.

2. Initial considerations A number of factors, both positive and negative, apply specifically to

the IBC cell. The general layout is shown in a simplified way in Fig. 3 to- gether with a cross-section in Fig. 4. The cell's principal advantages are as follows.

(a) There is no contact grid shadowing. (b) Because of the large area available, series resistance in the metallic

contacts to the emitters can be much reduced. (c) Internal series resistance is reduced due to the closeness of the p+

and n + regions. (d) These lower series resistances mean that there is less degradation of

performance in the event of nonuniform illumination.

PRIMARY FLAT (EDGE A)

Gh . . . . /

y y 1 " ~ Gg \

1 2 3 G~

1 2 3 4 5 Ge

1 2 3 4 5 Gd

1 2 3 Z, 5 Gc

o h _ - - \ ' 2 /

Go . . . . .

220 UM

200 U M - -

50 U M - -

SECONDARY FLAT (a) (FDGE B) (b)

" II I I =

II i , ,

,tl

,, I IJ ' 1 t j Jl

2730 UM

Fig. 1. (a) Ar rangement of groups in a single slice. (b) n +, p+ active regions in the IBC 3 device: areas are p+, 3.39 x 10 -2 cm and n +, 1.42 x 10 -2 cm.

II] l]U l]lJ I]U rig UI d_J LS!_515LSIh

10-TERM I NAL RESISTOR N*

3- TERMINAL RESISTOR N*

SHEET RESISTANCE N ÷

IN LINE FOUR PROBE GATE CONTROLLED DIODE

Fig. 2. Test structures used.

qggg p IDDUOUBUDDnl

IO-TER M INAL RESISTOR p÷

'3- TERMINAL RESISTOR p,

SHEET RESISTA N CE p*

HAYNES --SHOCK LEY

55

SlO 2

Fig. 3. A p ÷ - n ÷ solar cell.

~ S I G 2

[ I [ ,R . I I .__l lp . l i p IL.__IIN p I =--s~oz

H I G H . L I F E T I M E BULK REGION

Fig. 4 Cross sect ion o f tile c e l l

(e) There is no need for very shallow emitter diffusion to attain high collection efficiency in the blue part of spectrum.

The series resistance is more important at higher concentration. The disadvantages of the cell are as follows.

(a) A low surface recombination velocity is needed over all surfaces; in particular, this is necessary at the edges and in the interdigital spaces.

(b) Relatively high bulk lifetimes are required if the cells are to be of manageable thickness.

(c) Cell mounting has to combine good electrical isolation with excel- lent thermal contact to a heat sink.

(d) The cell geometry makes it difficult to obtain adequate gettering during cell processing.

In fact, all of these problems are process related.

56

3. Preliminary proglmmme

The main questions that emerged from the initial analysis concerned firstly the finger-width ratio, secondly the interface between emitter and metallization and thirdly the maintenance of high bulk lifetime by gettering. It was also important to obtain direct experience of silicon solar cell pro- cessing. For this reason it was decided that the first experiments should be directed to the finger-width ratio and validation of the test structures; this would automatically also provide information on the correctness of the process chosen.

The test structures were designed to provide separate data on the most important post-processing properties o f the semiconductor. These may be listed as substrate resistivity, p+ and n ÷ sheet resistance, carrier mobility, lifetime, surface recombination velocity and contact resistances. To obtain these data, nine test structures were incorporated in the chip layout: (i) a symmetrical gate-controlled diode for the determination of lifetime and SRV; (ii) an in-line four probe using n ÷ dots to measure bulk resistivity; (iii) a Haynes-Shockley structure with p+ emitter and collector to determine hole mobility; (iv) and (v) van der Pauw configurations for sheet resistance measurements on the p÷ and n ÷ regions; (vi) and (vii) three-terminal resistors; {viii) and (ix) ten-terminal " ladder" resistors for contact resistance measure- ment.

4. Experimental technique

The slice was mounted on a specially designed probe table and one particular cell in a group was illuminated carefully at the front polished surface (without antireflection coating) by means of point-spot concentrated light reflected from a mirror mounted at an adjustable angle underneath the slice. The light spot dimensions were slightly bigger than the cell area and the spot was homogeneous with respect to light intensity. The light source was a 250 W, 24 V tungsten-halogen lamp with ellipsoidal dichroic reflector of diameter 50 mm and focal distance 32 ram. To avoid damage to the lamp reflector and diffuser, a cooling fan was fixed behind the lamp housing for air circulation. The principle of the light concentration is shown in Fig. 5; in practice this was accomplished by using a combination of piano-convex lenses. The incident power at the plane of measurement was determined with reference to a very sensitive Centronic 1 mm 2 photodiode (OSD5-1) and found to be 7 suns.

5. Solar cell equation parameters of the test cells

The illuminated characteristics of some groups of IBC cells over the slice were measured. To determine the solar cell equation parameters from

57

AIR EXHAUST ~ S O

CO OL)NG FAN DIFFUSER / / J ~ _ _ - - r" ~ IU~N COLUMATOR ~ / - - I [ [ .

r - - - - - ~ - ~ ~ - _ - - -

l ~ ' d . ~ ANO CO MIRROR L-J I J LAMP HOUSING

POWER SUPPLY (H}GH AMP)

Fig. 5. Experimental set-up for illuminating IBC solar cells.

these characteristics a single-exponential solar cell model [6] is used. The implicit I - V relation is given by

V + R s I I = Iph Is [exp{B(Y + Rs/)} -- 1] (1)

Rsh

where B = q / A K T . From the open-circuit voltage Voc and short-circuit cur- rent I~, it may be deduced from eqn. (1) that

Isc(1 + R s / R ~ h ) - - Voc /Rsh Is = ( 2 )

A1 --A2

and

Yoo Iph = Is (A1 - - 1) + (3)

Rsh

where A1 = e x p ( B V o c ) a n d A 2 = e x p ( B R s I s ¢ ) ; through differentation of eqn. (1) and consideration of the values

and

expressions for Rs and R ~ can be obtained

1 Rs = Rso -- (4)

1/Rsh + B I s A 1

1

and

R s h = 1/(Rsho - - R s ) - - B I s A 2

58

TABLE 1

Parameters calculated for IBC cells using the single-exponential model

Group Lp*:Ln÷ Rs Rsh Iph Is A V m a x Imax (~-~) (X10 6 ~-~) (X10 -4 A) (A) (V) (XI0 -4 A)

Ge-2 5:1 4.199 3.409 46.8 2.559 X 10 - i i 1.34 0.400 43.2 4:1 4.366 3.056 46.2 2.286 X 10 -11 1.33 0.400 42.8 3:1 6.301 3.101 44.5 2.75 X10 12 1.16 0.394 42.6 2:1 3.108 3.102 42.4 8.11 X10 -12 1.25 0.400 40.1 1:1 5.504 3.104 32.8 1.315 X10 - l i 1.31 0.394 30.9

5:1 3.306 3.221 67.1 6.70 X10 ii 1.36 0.408 56.1 4:1 3.361 3.107 61.1 2.52 X10 - l l 1.32 0.405 55.6 3:1 3.741 3.101 56.4 2.78 X10 -12 1.17 0.410 52.0 2:1 4.958 3.05 52.7 1.33 X10 - i l 1.29 0.405 48.8 1:1 5.115 3.101 39.9 3.166 ×10 -12 1.19 0.410 36.4

G f-3

TABLE 2

Parameters of IBC test cells measured from dark characteristics

Group Lp÷:Ln÷ Rs A Is X 10 -10 KT/q (~ ) (A) (mV)

Gd-3 1:1 3.11 1.32 9.14 25.0 2:1 4.37 1.26 9.23 24.5 3:1 3.47 1.15 9.48 24.6 4:1 4.50 1.24 9.72 24.1 5:1 3.75 1.13 9.9 24.9

Gf-3 1:1 4.54 1.23 8.03 24.5 2:1 3.87 1.21 8.14 24.6 3:1 3.56 1.25 8.32 25.2 4:1 3.71 1.34 8.41 25.0 5:1 4.02 1.28 8.63 25.2

Gc-4 1:1 4.03 1.18 8.38 24.9 2:1 4.32 1.22 8.41 25.0 3:1 3.24 1.23 8.65 25.0 4:1 3.37 1.23 8.9 25.0 5:1 5.40 1.16 9.05 25.2

Ge-2 1:1 3.85 1.30 8.25 24.6 2:1 3.15 1.63 9.59 24.7 3:1 5.76 1.23 9.05 24.7 4:1 4.45 1.26 9.21 24.7 5:1 5.63 1.64 9.62 24.8

Us ing t h e s e e q u a t i o n s , t h e p a r a m e t e r s o f t e s t cel ls w i t h v a r i o u s f i n g e r w i d t h

r a t i o s are c a l c u l a t e d , as l i s t ed in T a b l e 1. S o m e p a r a m e t e r s d e r i v e d f r o m t h e

d a r k c h a r a c t e r i s t i c s o f t h e t e s t cel ls are p r e s e n t e d in T a b l e 2. T h e s e t a b l e s

i n d i c a t e t h a t t h e i d e a l i t y f a c t o r o f t h e t e s t cel ls l ies b e t w e e n 1 a n d 2 a n d

t h a t t h e ser ies r e s i s t a n c e r anges f r o m 2 t o 6 ~ . T h e f i l l f a c t o r o f t h e s e cel ls

59

is found to lie in the range 0.71 - 0.78, while the reverse saturation current Is is of the order of 3.1 × 10 -12 A - 6.70 × 10 -11 A. The shunt resistance is approximately (3.1 - 3 .5)× 106 ~2 and the open-circuit voltage ranges from 0.52 to 0.58 V.

6. Results and discussion

The experimental results show that the test design for the IBC cells exhibits very high values of series resistance, which strongly affects device performance. The measured series resistances of the test design might have been comparatively lower than the values reported here if the two illumina- tion level method had been chosen to evaluate this parameter. In analyses based on the single-exponential model, errors can often occur in measuring exactly and precisely the dynamic resistances Rso and Rsho. The values of shunt resistance, ideality and fill factors for the test IBC cells are quite good.

The dark characteristics of these cells were measured by manually varying the bias voltage and then recording forward and reverse currents. The diode reverse saturation current Is found from the dark characteristics is higher in magnitude than that computed using the single-exponential model. The R s values derived using these two approaches also differ slightly. In fact, manual measurement of the dark characteristics of the test IBC cells does not provide accurate and consistent results. Moreover, the solar cell junct ion temperature also increases upon reverse biasing, which eventually causes Is to increase.

The test design for the IBC cell (a device of very small area) showed energy conversion efficiencies of 4%. The maximum efficiency so far re- ported for the IBC cell (area 1.3 cm 2) is 18% at 30 suns [7].

The nine test structures designed and fabricated under the same phys- ical conditions as those used for the IBC test cells provided very important results. The gate-controlled diode showed very low lifetimes of up to 90 ns, with surface recombination velocity down to 1000 cm s -1 . These results and the high series resistances for the test design of the IBC cell provide suffi- cient explanation for the poor efficiencies of these cells. The bulk resistivity was found to be 0.6 ~ cm and the contact resistances at the Al-n ÷ interfaces were about ten times higher than those at the Al-p ÷ contacts.

7. Conclusions

Of the IBC cells with varying finger width ratio studied, cell IBCs ha~ shown the highest energy-conversion efficiency of 4.37%. It also has the highest values of Isc,/max, Vma~ and fill factor of all the test designs exam- ined. In some groups over the slice, cell IBC4 showed performance compar- able with that of IBCs, but most of the results pointed in favour of IBCs.

The parameter values deduced from computations using the single- exponential model are acceptably good except in the case of the series

60

resistance. Rs for the test device can be controlled by optimization in cell design and processing. In conclusion, cell IBCs has shown the best perfor- mance and efficiency of the various finger width ratio cells and hence it may be recommended for further research and concentrator applications.

Acknowledgments

The author would like to thank Prof. J. E. Parrot, UWIST (U.K.) for helpful suggestions; thanks also go to Southampton University SERC Micro- electronics Centre for collaboration in the fabrication of test devices.

References

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21 - 26, 1985, IEEE, New York, 1985 , p. 441. 6 J. P. Charles, M. Abde lk r im, Y. H~ Muoy and P. Mialhe, Sol. Cells, 4 ( 1 9 8 1 ) 169 - 173. 7 C. M. Garner , R. D. Nasby, F. W. Sex ton , J. L. Rodr iguez and D. P. Norwood , Proc.

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