20 000 PERL silicon cells for the ‘1996 World Solar Challenge’ solar car race

20 000 PERL Silicon Cells forthe `1996 World SolarChallenge' Solar Car RaceJ. Zhao, A. Wang, F. Yun, G. Zhang, D. M. Roche, S. R. Wenham and M. A. Green�

Photovoltaics Special Research Centre, University of New South Wales, Sydney 2052, Australia

High-e�ciency large-area PERL (passivated emitter, rear locally di�used) cells

have been produced at the University of New South Wales in a su�cient quantity to

supply three cars for the 1996 World Solar Challenge (WSC) solar car race. Almost

20 000 cells were fabricated, with designated illumination area e�ciency ranging up

to 24%. A ¯at-plate module made from 50 such cells fabricated early in the produc-

tion process and having an average e�ciency of just over 23% has demonstrated a

record e�ciency of 22.7%. The energy conversion e�ciency of a typical cell fabri-

cated late in the production process was subsequently measured as 23.7% at Sandia

National Laboratories under standard test conditions �1 kW mÿ2; global AM1.5

spectrum at 258C) based on the designated illumination area of 21.6 cm2. Honda's

Dream and Aisin Seiki's Aisol III were two vehicles using these PERL cells, and were

placed ®rst and third, respectively, in the race. Honda also set a new record by

reaching Adelaide in 4 days with an impressive average speed of 90 km hÿ1 over the3010-km course. # 1997 John Wiley & Sons, Ltd.

Prog. Photovolt. Res. Appl., 5, 269±276 (1997)

No. of Figures: 7. No. of Tables: 1. No. of References: 7.

INTRODUCTION

The World Solar Challenge (WSC) solar car race has attracted worldwide attention to solar-relatedtechnologies,1 considerably improving public perception of their potential. The ®rst WSC in 1987 waswon by General Motors' Sunraycer. The highly favoured Honda team entered the second WSC in 1990,but ®nished second behind the Spirit of Biel/Bienne II from Switzerland. By the time of the third WSC in1993, silicon cell e�ciencies had improved considerably. Both the Honda and Biel teams entered superblyengineered vehicles, with the Honda Dream using backside contact (BSC) solar cells made by SunPowerCorporation and having an average e�ciency of 21.2%.2 Dream went on to win the race with a recordaverage speed of 84:96 km hÿ1:

On 27 October 1996, the fourth WSC commenced. Forty-®ve solar cars left Darwin in a bid to reachAdelaide, 3010 km away, powered only by sunshine. Four of these vehicles were considered as potentialwinners: the Honda Dream, Aisin Seiki's Aisol III, Aurora Vehicle Association's Aurora 101 and theUnited High Schools of Biel's sCHooler. With the exception of sCHooler, these vehicles were all usingPERL (passivated emitter, rear locally di�used) cells manufactured at the University of New South Wales(UNSW). The Honda Dream (Figure 1) was again considered the race favourite, primarily because of the

CCC 1062±7995/97/040269±08$17.50 Received 18 February 1997# 1997 John Wiley & Sons, Ltd. Revised 9 April 1997

� Correspondence to: M. A. Green, Photovoltaics Special Research Centre, University of New South Wales, Sydney 2052, Australia

Contract grant sponsor: Australian Research Council

PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS, VOL. 5, 269±276 (1997)

Applications

team's meticulous preparation and the vehicle's extremely high array power, made possible through theuse of the highest e�ciency PERL cells and a two-seat layout to allow a larger array area. Dream went onto win the event, breaking the race record and ®nishing in 4 days with an average speed of 89:76 km hÿ1:sCHooler ®nished second and Aisol III third.Aurora 101 was forced to withdraw from the race following amechanical failure: an extremely unfortunate result given the very high speed potential of this vehicle. Thesecond-placed sCHooler team also believed that they could have beaten Honda had they been using PERLcells. In this paper, the fabrication of these high-e�ciency PERL cells for 1996 WSC teams will bediscussed.

CELL DESIGN

Small-area silicon PERL solar cells, as shown in Figure 2, demonstrated a record e�ciency of 24.0% in1994.3 However, there have been doubts about the feasibility of manufacturing large quantities of suchcells, owing to the complexity of the cell process. The high cost of manufacturing PERL cells has also beenseen to limit the ability of these cells to compete with other lower cost cell technologies. The PERL cellswere ®rst produced in a reasonably large quantity of 41000 cells for the 1993 WSC (cell area 45 cm2).It was not possible at the time to produce su�cient cells for one single solar car because of the lowthroughput of the research equipment set-up.

Each PERL cell made for the earlier event had an SiO2 single-layer antire¯ection coating. The highsurface re¯ection due to this coating was one of the major e�ciency-limiting factors at the time. The cellsalso used a non-optimal top metallization scheme to make them interchangeable with the buried contactcells also supplied for the 1993 event. Despite these limitations, PERL cells made in 1993 demonstratedrecord module and large-area cell e�ciencies of 20.6% and 21.6%, respectively.4 The success of the 1993project also demonstrated our capability of greatly increasing the production quantity of high-e�ciencyPERL cells for the 1996 WSC.

Figure 1. Honda Dream in the 1996 World Solar Challenge solar car race

Prog. Photovolt. Res. Appl., 5, 269±276 (1997) # 1997 John Wiley & Sons, Ltd.

270 J. ZHAO ET AL.

After the 1993 race, an e�ort was made to improve the PERL cell performance by introducing, forexample, double-layer antire¯ection coatings. This quickly resulted in 24.0% e�cient, double-layer-coated, small-area (4 cm2) PERL cells.3

These small-area research PERL cells were redesigned into large-area (21.6 cm2 designatedillumination area) cells for the 1996 WSC. A long and narrow cell layout of 6:2� 3:8 cm2 was chosento allow shorter and narrower grid lines. This design allows two cells to be produced from one 100-mmdiameter wafer. Figure 3 shows the front pattern of such a cell. This new design has the followingmodi®cations and improvements:

(i) The cells were designed to be shingled to form an array as shown in Figure 4. This allows thecell busbar area to be placed under an adjacent cell when interconnecting cells prior to encap-sulation. Lateral current ¯ow along the busbar is also eliminated because current ¯ows directly

Figure 2. Passivated emitter, rear locally di�used (PERL) cell with a double-layer antire¯ection coating

Figure 3. Front metal layout of 21.6 cm2 (designated illumination area) PERL cell

Figure 4. `Brickwork' shingling arrangement

# 1997 John Wiley & Sons, Ltd. Prog. Photovolt. Res. Appl., 5, 269±276 (1997)

THE 1996 WORLD SOLAR CHALLENGE SOLAR CAR RACE 271

into the back of the adjacent cell. Hence, the cell busbar does not cause any shading or resistanceloss.

(ii) The busbar is designed to be on top of a thick inactive ®eld-oxide, isolated from the substrate asshown in Figure 5. Hence, the busbar area does not contribute to an increased dark saturationcurrent of the cell.

(iii) E�orts were made to keep di�used junctions well away from the scribed cell edges where highrecombination losses occur. The narrow long shape also helps to reduce the recombination loss atthe busbar end. The high recombination at the scribed edges has been a problem for all high-e�ciency cells with very high minority carrier di�usion lengths. Sinton et al.5 have independentlysuggested overlapping the cell edges and using lower-resistance substrates to reduce this recombina-tion for SunPower's BSC cells. The PERL cells were already being fabricated on low-resistivitysubstrates of 1 O � cm; however, the edge recombination loss was still estimated to be quite high.6

Hence, this shingling method is important for maintaining high e�ciencies with these large-areascribed PERL cells.

(iv) The rectangular cell design also allows short metal ®ngers. This considerably reduces the ®ngerresistance loss, which is proportional to the square of the metal length, hence increasing the cell ®llfactors.

The rear metal used for the small-area research cells has been changed to Ag for solderability. A bu�ermetal of Ti±Pd is used on top of the rear Al, which considerably increases the processing workload for thevacuum coating equipment. A total of six masking steps were required for cell production. Owing to thecomplexity of cell processing, the skills of the processing personnel were critical.

The other improved features of double-layer-coated PERL cells3 have been incorporated into the newdesign. Hence, the newly designed large-area cells have demonstrated performance levels that match thosepreviously achieved only for small-area research cells.

CELL PRODUCTION

Production set-up

Almost 20 000 cells were fabricated at the UNSW during this production period. Possible bottleneckswere analysed in advance. Some old equipment was modi®ed, some special jigs were made and some newequipment was purchased to avoid these bottlenecks. This equipment included an SVG-8126 PC-RDautomatic track coater (for coating photoresist on silicon wafers), two high-throughput Varian Delco3120 vacuum coaters and an in-house-designed high-throughput Ag plating bench. This equipmentproved vital for alleviating the forecast bottlenecks.The other usual processing facilities, such as for wafer cleaning, developing and resist removal, were

also modi®ed to accommodate high-throughput production. Two large nitrogen boxes were purchased to

Figure 5. The busbar of a shingled cell is isolated by a thick layer of SiO2


272 J. ZHAO ET AL.

store the half-processed wafers. Many other modi®cations were made throughout the processing areas tofurther improve throughput, and by the end of production it was found that cells ran through the linesmoothly with no obvious bottlenecks.

Two high-temperature processing areas, operating in parallel, were established for this project. One ofthese areas (area W), previously used for research, was devoted to producing the highest performancecells. Another area (area P), previously used for technology transfer of the Centre's buried contact celltechnology, was set up to have a much higher throughput than area W, but was expected to produce cellsof slightly lower performance than area W because of the more basic equipment and less experiencedprocessing personnel. However, area P performed much better than expected, producing cells of similare�ciencies to those produced in area W.

Personnel training

Personnel training has been vital for this production. In contrast to other countries such as the USA, inAustralia it is di�cult to ®nd highly skilled workers with experience in semiconductor processing. Hence,many new processors were freshly trained for this project.

Owing to the complexity of the processing sequences, any mistake or poor-quality processing mayreduce the cell performance. Hence, particularly during the early stages of production, which was also thetraining period for some of the new processors, area W usually produced cells of much better performancethan area P. However, by the end of production, the performance and yields of area P had greatlyimproved, with most cells from both areas having designated area e�ciencies of 423%: Earlier availa-bility of trained processors would have assisted the early production yields.

Production throughput

Two work shifts were used throughout production. A production throughput of 1000 cells per week(500 wafers per week) was maintained for most of production, which is equivalent to about one car's arrayof cells per month. The total throughput time during the middle stages of production was about 10 weeks.E�orts were made to improve the speed of production as the project progressed, resulting in an overallthroughput of 41500 cells per week and a total throughput time as low as 4 weeks towards the end ofproduction. These improvements indicate the capacity to make further increases in the volume ofproduction for future projects.

CELL AND MODULE PERFORMANCE

Throughout production, PERL cells were sent to Sandia National Laboratories for measurement. Thesecells were sent back to the UNSW to be used as reference cells. Table I lists the electrical parameters of onesuch cell tested at Sandia National Laboratories under the standard 1 kW mÿ2 global AM1.5 spectrum at258C. The designated area e�ciency of 23.7% is the highest con®rmed e�ciency reported to date for anysilicon cell of a similar size.

These large-area PERL cells showed open-circuit voltages up to 706 mV, which were very close to those ofthe unscribed small-area research PERL cells of 709 mV.3 Owing to the relatively small edge to area ratio,

Table I. Sandia measurements of 21.6 cm2 (designated illumination area) PERL cell

under the standard 1 kW mÿ2 global AM1.5 spectrum at 258C

VOC JSC Fill factor E�ciency

Cell (mV) �mA cmÿ2� (%) (%)

W139-07a 704 41.5 81.0 23.7



edge recombination does not greatly reduce the cell voltage. Furthermore, because of this reduced edgerecombination loss, these cells demonstrated very high short-circuit current densities of �41:5 mA cmÿ2:During the early stages of production, 32 full-size (21.6 cm2) and eight half-size (10.8 cm2) cells were

interconnected and encapsulated into a ¯at-plate photovoltaic module with the `brickwork' shinglingmethod shown in Figure 4. This module was measured outdoors at Sandia National Laboratories anddemonstrated an e�ciency of 22.7%,7 which is the highest e�ciency ever achieved for a ¯at-plate photo-voltaic module made from any material. Further increases in module e�ciency are expected to be made byusing better encapsulation techniques and higher e�ciency cells from the later production stages.

Owing to the di�culty of forming a brickwork-shingled array over a curved surface, the requirementsfor high voltage output and the need for small subarray areas to minimise mismatch, all three cars usingPERL cells had their cells shingled in single rows rather than in a brickwork arrangement. Although thishad little e�ect on the packing density, it increased the risk of mismatch from a single poorly performingcell.

PRODUCTION RESULTS

Figure 6 shows the cell e�ciency distributions after each month of production. The earlier months show areasonably wide distribution, owing to the factors previously mentioned. Nevertheless, most cells madeduring these months have e�ciencies of 422%; which is higher than for any cell previously available incommercial quantities.2

An interesting result is noted for May, where a bimodal distribution of cell e�ciencies is apparent. Thisresult allowed us to identify the previously unsuspected importance of a particular metallization step oncell performance. Not only did this improve the average performance of subsequently processed cells, butit is also expected to result in the e�ciency record for small-area PERL cells being increased well beyondthe present mark of 24.0%.

The distributions for July and August show that, after accumulating su�cient production experience,the overall cell performance improved to give 460% of all cells fabricated with designated area e�ci-encies of 423%:

The performance of a batch of cells, typical of those made towards the end of the project, is shown inthe e�ciency distribution of Figure 7. Most cells from this batch have designated area e�ciencies close to

Figure 6. Distribution of designated area e�ciencies of PERL cells after each month of production


274 J. ZHAO ET AL.

24%. The uniform cell performance for this batch shows the capability of making further improvementsin PERL technology. Any future projects for high-volume manufacture of PERL cells are expected toproduce large numbers of batches of similar or better performance to this batch, which, with otherimprovements to be implemented, is likely to result in signi®cant quantities of cells being produced withe�ciencies in excess of 24%.

CONCLUSIONS

High-volume and high-yield production of large-area PERL cells has been successfully demonstrated atthe UNSW in the process of supplying cells for the 1996 World Solar Challenge. A modi®ed cell design,use of a double-layer antire¯ection coating and adoption of a shingling technique have been majorcontributions to improved cell and module e�ciencies. A very high yield of cells with designated areae�ciencies of 423% has been demonstrated. This e�ort is expected to lead to even higher performanceresearch cells in the future, demonstrating the e�ectiveness of the World Solar Challenge in acceleratingthe development of photovoltaic technology.

Acknowledgements

The authors would like to thank the other members of the Photovoltaics Special Research Centre, inparticular David Jordan, Ebrahim Abbaspour-Sani, the processing sta�, Mark Silver and the othermembers of the laboratory development team, and Keith McIntosh and the other testing sta�. We wouldalso like to thank the sta� of Sandia National Laboratories for their contributions to this work. Thesupport and management provided by Unisearch Ltd and the Faculty of Engineering is gratefullyacknowledged. The Photovoltaics Special Research Centre was established and is supported under theAustralian Research Council's Research Centres Program.

REFERENCES

1. M. A. Green, `World Solar Challenge 1993: the trans-Australian solar car race', Prog. Photovolt. Res. Appl., 2(1),73±79 (1994).

2. P. J. Verlinden, R. M. Swanson and R. A. Crane, `7000 High-e�ciency cells for a Dream', Prog. Photovolt. Res.Appl., 2(2), 143±152, (1994).

3. J. Zhao, A. Wang, P. Altermatt and M. A. Green, `Twenty-four percent e�cient silicon solar cells with doublelayer antire¯ection coatings and reduced resistance loss', Appl. Phys. Lett., 66(26), 3636±3638, (1995).

Figure 7. Distribution of designated area e�ciencies of typical batch of cells at end of production



4. J. Zhao, A. Wang, M. Taouk, S. R. Wenham, M. A. Green and D. L. King, `20% E�cient photovoltaic module',IEEE Electron. Dev. Lett., 14, 539±541, (1993).

5. R. A. Sinton, P. J. Verlinden, R. M. Swanson, R. A. Crane, K. Wickham and J. Perkins, `Improvements to siliconbackside-contact solar cells for high-value one-sun application', 13th European Photovoltaic Solar EnergyConference, 1995, pp. 1586±1589.

6. A. G. Aberle, P. Altermatt, G. Heiser, S. Robinson, A. Wang, J. Zhao, U. Krumbein and M. A. Green,`Loss mechanisms in high e�ciency PERL solar cells', J. Appl. Phys., 77(7), 3491±3504, (1995).

7. J. Zhao, A. Wang, D. M. Roche, S. R. Wenham and M. A. Green, `Pilot production of high e�ciency PERLsilicon solar cells for the World Solar Challenge solar car race', Technical Digest of 9th International PhotovoltaicScience and Engineering Conference, Miyazaki, Japan, 11±15 November 1996, pp. 65±66.


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20 000 PERL silicon cells for the ‘1996 World Solar Challenge’ solar car race

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