High efficiency, large-area p+-n-n+ silicon solar cells

Solid-State Electronics Vol. 30, No. 4, pp. 397401, 1987

0038-I 101/87 $3.00 + 0.00

Printed in Great Britain. All rights reserved

HIGH

Copyright 0

EFFICIENCY, SILICON ANDREI SILARD,’

‘Department ‘IPRS-BFmeasa,

LARGE-AREA SOLAR CELLS

FLORIN PERA’

and

GABRIEL

p +-n-n

1987 Pergamon JournalsLtd

+

NANI~

of Electronics, Polytechnic Institute, Bucharest, 0.P.16, 76206 Romania and (Enterprise for Radio Components & Semiconductors), Bucharest, 72996 Romania (Received

19 February

1986; in revised form

I June 1986)

Abstract-The work reports the development of high-efficiency, large-area, preferentially currentgenerating p +-n-n + silicon solar cells. The distinctive trait of the developed cells is their extremely high value of the short-circuit current density J,,- under AMI conditions (100 mW/cm2 insolation). Two main factors had shaped the unique features of these large-area (17.2 cm3 cells. One of the factors is the peculiar impurity profile of the p +-, junction with a depth of X, = 0.15-0.25 pm, obtained through adequate technological means. The second factor is a novel, optimized front grid metallization pattern, which reduces the area1 inhomogeneities typical of large-area devices. The combined action of these factors has reduced the coverage ratio to about 5.6% and confined the series resistance R, to 60-100 mR range. The conversion efficiency of cells exceeds 17%. The results of this work were obtained in industrial environment, not laboratory conditions. It is noteworthy that none of the usual refinements (double AR layer, textured surface, etc.) were used in the fabrication process. Thus, this work shows clearly that a simple, but adequately designed/processed large-area p +-n-n + cell could possess parameters similar or even superior to those of sophisticated laboratory samples elaborated on small (2 x 2 cm) silicon chips.

1. INTRODUCTION During the past two years, remarkable progress has been made in the area of “back-surface-field” (BSF) single crystalline silicon solar cells [l-7]. Both diffused [l] and ion-implanted [2] n+-p-p+ silicon photovoltaic cells achieved conversion efficiencies r] of 17-18% under AM1 conditions (100 mW/cm’ insolation). Elaborated laboratory samples reached conversion efficiencies of 18-19% [3]. It was also shown [4-71 that by masked ion-implantation one could more easily control the main electro-optical characteristics of BSF-type silicon solar cells/optical sensors. All reported laboratory results [l-3] were obtained either by diffusion or ion implantation of phosphorus (P) into small area p-wafers (usually 1 cm2 or 2 x 2 cm chips). To increase the conversion efficiency q, relatively sophisticated and costly refinements were used (double-antireflection layer, textured surface, back-surface passivation, etc.) [l-3]. It is noteworthy that since 1980 [8] no notable attempts to achieve high-efficiency p +-, or p +-n-n + silicon solar cells were reported in the literature. To the best of the authors’ knowledge, the excellent laboratory results [l-3] on n+-p-p+ cells were not reproduced in industrial environment on large-area wafers, wherein the area1 inhomogeneities [9] play an important role. The present work overcomes the reported gap of the past years by disclosing the development of high-efficiency (17-18%) silicon solar cells fabricated on large-area (50 mm) n-wafers. The distinctive trait of developed cells is their extremely large short-circuit

397

current density under AM 1 conditions (100 mW/cm2 insolation). The devices were fabricated by controlled-boron diffusion into large-area (50mm) n-wafers with a starting resistivity of l-2 Rem. A novel front grid metallization pattern, optimized as a function of p+-layer sheet resistance, was employed for the developed devices. The combined action of technological/design factors had reduced the converage ratio to about 5.6% and confined the series resistance R, to 60-100mR range.

2. CELL DESIGN

The developed cells structure was p +-n-n + backsurface-field (BSF)-type. The devices were fabricated on 50 mm, (11 l), Czochralski n-silicon wafers with a starting resistivity in the l-2 Rem range. The thickness of the wafer was 220pm. Two main design/technological factors had shaped the unique features of these cells. One of the factors is the peculiar impurity profile of the p +-n junction with a depth of x, = 0.15-0.25 pm, obtained through adequate technological means. The projected boron impurity profile had a high surface concentration (= 102’ cm-‘). The obtained impurity profile, with a shape close to that of phosphorus (P)-diffusion detailed in Ref. [3], had the following features: 1. high concentration of boron (B) in the proximity of the light-incident surface; 2. rapidly falling B-concentration in the proximity of the surface. The high surface concen-

ANDREI SILAKD

398

tration of boron decreases the contribution of the front contact region to the resistance R, of the cell. It is also considered [3] that the high dopant concentration at the surface reduces the minority-carriers concentration therein and, hence, decreases the surface recombination rates. The rapidly falling boron diffusion profile in the proximity of the surface undoubtedly reduces the recombination (especially the Auger processes) in that region, a fact usually yielding a good blue response of the cells. These features of diffusion profile in the surface region combined with a junction depth x, = 0.15-0.25 nm were expected to produce a sheet resistance of the p ’ -layer in the IO0 + 200 R/square range. To obtain such a profile a boron diffusion from a boron nitride source with hydrogen injection similar to that described in Ref. [IO] was used. To enhance the collection efficiency of the front grid metallization and to reduce the impact of areal inhomogeneities, typical of large area device [9] on the collection efficiency, a novel “spider web”-type (Fig. I) front grid metallization pattern was developed. The pattern was optimized for one-sun insolation and for an p+-emitter sheet resistance of 200 R/square. The computer-aided design of the pattern (Fig. I) was based on a method correlating the collection efficiency with the sheet resistance of p +-layer according to methodologies described in the literature [ll-151. The contact geometry (Fig. 1) consisted of eight concentric rows of collecting pads disposed on the 17.2 cm’-area of finished devices. Two mutually perpendicular busbars (Fig. I) each approximately 350pm wide with a 2.5 x 2.5 mm bounding pad were used. The shading loss is approximately 5.6%. The metallization resistance and the specific contact resistance loss amounted-according to computations to 2.26%. The total contact loss is therefore about 7.86%. The actual sheet resistance of p+-layer was in the 80-120 Q/square range (see Section 3). Consequently, there is a considerable

et ul

scope for improvement of the front grid metallization pattern (Fig. I). The front surfaces of the cells were passivatcd with a thermally-grown, thin SiOz layer. No passivation was provided for back surface of cells.

3. FABRICATION

An initial oxidation in wet oxygen (O?) at II80 C was performed to obtain a silicon dioxide (SiO,) layer I /irn thick. Following a deoxidation of the wafers rear, the backside n-n + contact was formed by means of phosphorus diffusion at I 100 C from POCI, source. The depth of the n-n+ junction was 0.60.8~m. A photolithographic process was used to open the windows for the front junction formation. To this end, the 1 pm thick oxide on the front surfaces was selectively etched down. except for the 1.6 mm wide circular periphery of the wafers. As a result, of 19.63 cm2 total area of 50 mm wafers, the useful light-incident surface including the front grid metallization amounted to 17.2 cm’. The front p ++I junction was formed by boron (B) diffusion from a boron nitride source with hydrogen injection [IO]. To this end, the wafers were introduced into a nitrogen-oxygen ambient at 820 C. The temperature was then raised to 920 C in approximately IO min. A I min injection of hydrogcn/nitrogcn at constant temperature (920’ C) followed. Subsequent drive-in diffusion was carried out in a nitrogen ambient without the boron nitride wafer for approximately 25 min. The temperature was subsequently lowered to 820’ C. A 15 min oxidation at 750 C in wet oxygen/nitrogen atmosphere followed. The last step consisted of a chemical etch of the surface in a buffer solution. The measured sheet resistance of the formed p +-layer was in the 80-120 R/square range. To passivate the front surface of the wafers. a I5 min oxidation at 750 C in wet oxygen/nitrogen ambient was carried out. The thickness of the formed SiO, layer was about 100 A. Standard Ti -Ag metallizations were employed for the front and backs of the wafers. An electrochemical deposition of nickel followed. A 900 8, thick SiO antireflection (AR) layer was deposited on the front surface of the finished devices. Contact adherence was insured by sintcring the cells for IO min at 450 C in nitrogen ambient.

4.

Fig. I. Photograph of the developed front grid metallization pattern for large-area, high-efficiency silicon p +%I tz + solar cells.

CHARACTERIZATION

A Karl Zeiss Jena quartz prism monochromator with a Xenon arc lamp as source (simulator of the AM I spectrum with 100 mW/cm’ insolation) coupled to a Tektronix 3114661 data acquisition system was used to measure the relative spectral response (RSR) of the test devices (Fig. 2.). The main optical parameters: bandwidth B, measured at 0.5 S ,,,,,,, peak responsivity wavelength >.,. quantum efficiency at

High-efficiency, large-area p+-n-n + silicon solar cells 10

Wavelength

Fig. 2. Typical relative sentative test

spectral

399

r

O

Lnm)

response

p +-n-n + silicon

(RSR) of a represolar

cell.

= 13.4

x 10-gA/cm2

= 2.96

x 10~gA/cm’

Table 1. Optical bandwidth i7, peak responsivity wavelength ,I, and the quantum efficiency at relevant wavelengths of the incident light for relevant test p+--n-n + silicon solar cells

Quantum efficiency (%) at

1,

B

Cell 420 535 490 560 595 495 415 470 465 490

109 110 112 113 116 120 121 130 133 134

0.7 pm

0.5pm

(nm)

56 58 59.5 59.6 64.5 56 54 56.2 57.5 57

800 800 860 800 800 860 860 860 860 860

0.9 JLm

78 79 80 81 82 82 78 79 78 II

96 95 94.6 95 97 95 95 94.7 95 94.7

different wavelengths of the incident light, are synoptically shown in Table 1. The AMI, 25”C, illuminated and dark current (Itvoltage (V) characteristics are shown in Figs 3 and 4, respectively. The I-V plots were measured

I--

= JSc =39.56 Voc = 593

,-

FF

= 77.32

‘)

=16.15%

RS

Forward

Fig. 4. Plots of p+-n-n+ silicon age of the tangent magnitude of the

I 400

I 500

voltage

I 600

I 700

(mV)

the dark J-V curve for a relevant test solar cell. The extrapolation to zero voltto the n = I portion of the curve gives the diffusion component J,,d of the saturation

current density [8].

using a Tektronix DM 501-3 l/4661 system. The main parameters of the cells are shown in Table 2 (shortcircuit current density J,,, open-circuit voltage VO,, fill-factor FF, conversion efficiency 9). The diode ideality factor n, the diffusion Jo,, and recombination Jo, components of the reverse saturation current density determined according to the method outlined in Ref. [8] are shown in Table 3 together with the series resistance R,. The values of the latter were determined from the difference between the dark and AM1 illuminated I-Vcurves [16]. The n-base lifetime measured according to method outlined in Ref. [17] is also shown in Table 3. The subsequent discussion is restricted to ten representative cells. The rest of test devices have failed to

Table

P,

=312mW = 17.2

I 200

I 300

ma

=4o/Ls

25’C,AMl,

I 100

Fig. 3. AMI,

=72

I 200

I 100

%

TB

Area I-

mA/cm’ mV

Ki”1

2. Main

AMI

electro-optical

p+-Ml+

silicon

parameters

JU

cm2 100mW/cm2

I 300

I 400

Voltage

(mV)

I 500

L 600

illuminated J-V characteristic of a relevant test p +-n-n + silicon solar cell.

Cell

(mAjcm2)

109 110 II2 113 116 120 I21 130 133 134

38.54 39.68 39.36 39.58 39.46 39.56 39.60 38.84 38.95 38.37

of

relevant

solar cells

(2) 590 590 585 593 592 588 589 584 583 581

77.70 74.75 74.48 77.32 73.40 77.81 75.60 76.14 77.38 76.70

17.67 17.50 17.15 18.15 17.14 18.10 17.64 17.50 17.57 17.10

304 301 295 312 295 311 303 301 302 294

test

ANDREI &LARD et al.

400

Table 3. The man parameters of the dark J-V characteristics, the series resistance R, and the n-base lifetime 7B for relevant test cells of Table 2

109 110 II2 113 II6 120 I21 130 133 134

57 85 88 72 99 68 76 76 60 75

59 66 36 40 66 80 40 85 32 50

I.41 I .58 1.51 I .39 I .44 I.58 I.41 1.34 1.36 I .56

4.93 5.07 6.10 4.50 4.67 5.46 5.26 6.26 6.50 7.50

4.05 24.44 13.40 2.96 5.37 25.57 4.10 2.00 2.70 23.00

have both values of J,, and 9 above 38 mA/cm* and 17%, respectively. Cell parameters measurements, performed at I.P.R.S.-Baneasa (Enterprise for Radio Components and Semiconductors) were subsequently confirmed at the 1.C.E.F.I.Z (Central Institute of Physics, Bucharest). At both locations, a cross-calibration with control cells was performed for the test units. 5.

COMMENTS

ON

CELLS

PERFORMANCE

The following aspects concerning cells performance are noteworthy: (a) The developed devices are “preferentially current-generating” [S] i.e. their relevant output signal is the current: the amount of short-circuit current delivered by the 17.2 cm2 area cells was usually in the 660-683 mA range (Table 2). The following factors endowed the devices with the highest value of shortcircuit current density J,, (Fig. 3 and Table 2) ever reported in the literature for any type of BSF-type cells: (i) the relatively large optical bandwidth B (Fig. 2 and Table 1); (ii) the high quantum efficiency, especially at longer wavelengths (0.7-l. 1 pm) of the incident light (Fig. 2 and Table 1); (iii) the relatively high n-base lifetime 7B = 3&80 ps (Table 3) which has as a result that a large amount of carriers, optically generated in the bulk of the samples with high quantum efficiency (See ii), do actually “survive” and contribute to the photocurrent generation; (iv) the implementation of the novel front grid metallization pattern (Fig. I), a step which diminished the shading losses and reduced the impact of area1 inhomogeneities on the cell performance; (v) the front surface passivation and the peculiar impurity profile of the p +-n junction, which both increased the contribution of the p+-emitter to the generated photocurrent. It should be noted that the test cells exhibited higher than usual [ 1,2,4-71 quantum efficiency for photons with energies E < 1.2 eV (Fig. 2 and Table 1) and that any eventual losses in the blue response were more than compensated by an increase in the infrared response, especially at wavelengths J 2 1.0 pm. (b) Although the series resistance R, was reduced drastically in comparison with similar BSF-type cells,

being confined to 60-100R range (Table 3) the fill-factor FF did not exceed 78% (Table 2). Even in these circumstances, with an open-circuit voltage of V,, = 581-593 mV (Table 2), the values of the conversion efficiency q were in the 17-18% range, with two cells (113 and 120) exceeding the superior limit (Table 2). The peak power P, delivered into a properly-matched load impedance was for all relevant cells in the P,, = (300 k 2%) mW range (Table 2). One of the main factors which affected the value of FF is precisely the “preferentially currentgenerating” nature of the developed cells. Thus, at higher output currents, there is an additional voltage drop on the series resistance R, induced by the “excess” current which, in its turn, increases the distance between V,,, and the voltage coordinate V,,, corresponding to the maximum power P, rectangle. The latter phenomenon leads to an 1 + 2% fall in the value of FF. (c) The diode ideality factor was n = 1.5 ) IO%, while the diffusion component J,,,, of the doubleexponential dark I-V characteristics was located in the 467.5 x 10-‘2A/cm2 range (Table 3). This indicates that there is still scope for further reduction of Jm,, which might eventually lead to an increase of v,,. 6. CONCLUDING

REMARKS

This work has demonstrated for the first time that the p ‘G-n + single-crystal silicon solar cells could possess AM 1 conversion efficiencies q similar to those of the best n +-P-P + photovoltaic devices reported to date [l-3]. The test cells possessed the highest value of the short-circuit current density J,, ever reported in the literature for BSF-type photovoltaic devices, It is worth noting that even if the whole area of the wafers (19.63 cm2 instead of 17.2 cm2) is taken into the calculations, the values of J,, and n for the test cells (Table 2) are located in the 33.6-34.7 mA/cm2 and 15-16% ranges, respectively. Even these values of J,, and p are superior to those reported for small-area p +-n-n + cells [8, 161. The main factors shaping the unique features of the developed cells have been presented in detail. It was also noted that the optimization of the front grid metallization pattern (Fig. 1) could further improve the performance of these cells. Additional work is now being performed to increase V,,, and FF in these cells and to produce high-efficiency photovoltaic devices on 3-inch wafers. All reported results were obtained in an industrial environment, not in laboratory conditions. It is noteworthy that none of the usual refinements (textured surface, double AR layer, backside passivation, etc.) were used in the fabrication process. Thus, this work shows clearly that an adequately designed/processed large-area p +-n-n + cells fabricated on relatively thin wafers could possess parameters similar or superior to those of sophisticated laboratory samples elabo-

High-efficiency, large-area p+-n n + silicon solar cells rated on small-area silicon chips. The implementation of the aforementioned refinements could further im-

prove the electro-optical oped silicon solar cells.

performance

of the devel-

401

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