High-Volume Manufacturing of Flexible and Lightweight CIGS Solar Cells

J.S. Britt, S. Wiedeman, U. Schoop, D. Verebelyi Global Solar Energy, Inc., Tucson, AZ 85747

ABSTRACT

Progress in scaling up CIGS thin film solar cell pro-

duction at a new Global Solar manufacturing line is re-ported. A new plant with high throughput tools will in-crease capacity from 4MWp/yr to 40MWp/yr in 2008. The processes applied to make the product and the status of the manufacturing processes during the start-up stage will be reported.

Investigations of tool capability will be presented.

The tool cycle times and coating uniformity have been investigated to ensure production readiness. Satisfactory capability has been demonstrated for all coating proc-esses and large area conversion efficiencies greater than 10% have been independently demonstrated through substitution in the first-generation production line.

INTRODUCTION

In 1996 Global Solar began developing the technolo-

gies for roll-to-roll manufacture of products based on CIGS deposited on a flexible substrate. The technologies required include thin film coatings, roll-to-roll processing equipment, thin film solar cells, cell electrical interconnec-tion, packaging, and product reliability testing. In practice, it is difficult to successfully address the required technolo-gies separately due to the cross-dependencies and occa-sionally conflicting goals (e.g. cost vs. performance). Without an existing model to follow, much of the develop-ment had to be conducted iteratively as advancements within each technology were achieved [1,2].

Figure 1. Global Solar 10,220m

2 Tucson manufacturing

facility. After successfully operating a 4.2MWp/yr CIGS pro-

duction line for several years, Global Solar initiated an ambitious scale-up plan in 2006. Now in the final stages of

the first phase of the plan, 75MWp/yr of combined manu-facturing capacity is being installed at new production facilities in Tucson, AZ and Berlin, Germany. A 40MWp/yr production line is being installed in a 10,220m

2 production

facility in Tucson, Arizona (Fig. 1). Equipment installation is planned to be completed in this factory in August 2008. Tool installation in the 35MWp/yr Berlin production line will begin in May 2008 and should be completed in Q3 2009.

The new production tools represent an evolutionary

progression of the Global Solar CIGS technology. The primary design goal for the new tools was decreased manufacturing cost. Manufacturing cost will be reduced by increased automation, higher material utilization, and greater tool capacity. Greater tool capacity leads to re-duced capital expenses applied to unit production and a smaller factory floor area requirement. Although the Tuc-son factory tool installation is not yet complete, a com-plete line has been installed and process engineering for each tool set has begun. Preliminary results of individual process evaluations have been generated.

PRODUCTION PROCESSES

Global Solar employs a batch manufacturing process

based on webs 670m to 1000m in length and approxi-mately 32cm in width. Substrate webs are stainless steel approximately 25µm thick. Each web is processed inde-pendently through the coating steps. Each web and man-drel combination weighs between 40kg and 60kg. Be-cause of the sizable weights, webs are loaded and unloaded into coaters by crane and conveyed between processes on carts.

The batch-style production facilitates independent process optimization, and in particular allows each proc-ess to be developed for optimal speed. Production bottle-necks can be easily addressed by placing additional tools where the bottlenecks arise. Batch production also miti-gates the effect of unscheduled downtime if multiple tool sets for each production step are available. Finally, batch production permits off-line characterization between steps for improved quality control.

Back Electrode

The back electrode structure is Chromium and Mo-lybdenum. A thin Chromium coating is applied to enhance adhesion of the Molybdenum coating to the stainless steel web. Both metals are deposited by pulsed-DC sputtering.

Absorber

CIGS is deposited by multi-source co-evaporation of the elements (Fig. 2). The effusion sources are loaded with the least expensive forms of the metals (shot, wire,

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etc.). In practice, the effusion source control reactivity is small due to the large source thermal masses of the sources. The total deposition time for the CIGS coating (thickness: 1.7µm) is 2.6 minutes.

Figure 2. CIGS deposition by co-evaporation

Buffer

The buffer layer is CdS and is deposited from solution by a proprietary technique. The targeted CdS thickness is approximately 80nm. The process effluent is treated by a purification system to reduce metals and other contami-nants below levels considered hazardous. Only solid waste is generated.

Front Electrode

The front electrode is a transparent conducting oxide. The oxide is deposited by pulsed-DC sputtering (Fig. 3). The total TCO coating thickness is approximately 100nm.

Figure 3. Front electrode deposition by sputtering (a simi-lar tool is applied for the back electrode)

Collection Grid and Slitting

The collection grid is formed by roll-to-roll screen-printing of a silver ink (Fig. 4). The ink is thermally cured in the same step, prior to re-wrapping the web. The nomi-nal cell dimensions are 210mm x 100mm. Three cells are

printed across the width of the web. After printing, the web is slit into three separate reels, so that each reel is the width of a single solar cell, and contains between 3200 and 4800 solar cells.

Figure 4. Collection grid printing

Stringing

Single reels of printed cells are input to the stringer. In the stringer, the cells are separated and then serially attached to one another by bonding conductive ribbons between the collection grid of one cell and the backside of an adjacent cell. There are three ribbons per cell (Fig. 5). Strings are comprised of up to 18 cells and have a maxi-mum length of 2m and width 210mm. The strings are electrically characterized and binned according to their output characteristics. Finally, strings are packaged and shipped to module manufacturers for integration into their products. The majority of production in the new GSE fac-tories will be CIGS strings. The strings are designed to look like traditional Si solar cell strings to increase their appeal to module manufacturers with products designed for those strings.

Pmp (W): 39.5

Vmp (A): 7.3

Imp (A): 5.4

Voc (V): 10.3

Isc (A): 6.7

Figure 5. CIGS String and nominal string electrical char-acteristics

PROCESS QUALIFICATION

The first tools for all process steps have been in-

stalled into the Tucson production line. For several proc-ess steps, multiple tools have been installed. In the first stage of process development, the capability of coating uniformity has been demonstrated for each process. In the following stage, efforts shift to process integration to

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optimize cell performance and meeting manufacturing goals on the new production line.

Back Electrode

The thickness of the Mo back electrode should be op-timized for maximum performance and minimal cost [1]. At Global Solar, the Mo coating thickness is characterized by XRF (Fig. 6). Along the majority of the web length, the Mo typical thickness uniformity is +/- 3% and the thinnest coating occurs in the web center. Thinner Mo coating is frequently observed in the first 100m of deposition. The source of this effect is under investigation.

Figure 6. Contour plot of Mo thickness (a.u.) down and across the web as measured by XRF (distance weighted least squares fit)

Absorber

Coating uniformity of copper, gallium, and indium is critical to achieving optimal CIGS string performance and high yields. In practice, thermal evaporation cross-web uniformity is more difficult to achieve than uniformity down the web length. The cross-web CIGS uniformity is chiefly determined by the design of the effusion sources, deposi-tion zone geometry (location of sources and shielding), and zone pressure. However, the new CIGS coaters and effusion sources have been designed with increased de-grees of freedom to permit better control of coating thick-ness across the web than was allowed by the previous generation of ClGS coaters.

Figure 7. Equivalent thicknesses of copper, indium, and gallium in a CIGS film (across the web width) as meas-ured by ex-situ XRF

The cross-web profiles of the elements are similar for a typical CIGS film deposited in the new coaters (Fig. 7). All elements are deposited from identical effusion sources in nearly the same environment, so it not surprising that the profiles are similar. When combined to make CIGS, the thickness varies across the web width, but the com-posite ratios Cu/(Ga+In) and Ga/(Ga+In) are relatively uniform (Fig. 8).

Figure 8. Atomic ratios and thickness of a CIGS film (across the web width) as measured by ex-situ XRF

The coating uniformity of Cu, Ga, and In along the web length has been evaluated by XRF for CIGS-coated webs up to 670m in length (Fig. 9). In this instance, the web was sampled at identical cross-web locations down the length of the web. The effusion sources contain rela-tively large charges of the elements. The large thermal masses provide stable evaporation rates within the re-sponse time of the control loop, and uniformity along the length of the web is generally excellent.

Figure 9. Equivalent thicknesses of copper, indium, and gallium in a CIGS film (down the web length) as measured by ex-situ XRF at the web center

Buffer

If the CdS coating thickness is less than optimal, Voc and fill factor are reduced. If the coating thickness ex-ceeds the optimal value, Jsc is reduced due to increased absorption of light within the CdS coating. Global Solar has developed a non-destructive optical technique for qualification of the CdS coating thickness on production webs. The characterization is performed after the CdS has been applied on the CIGS coating, and before the TCO coating has been applied. The CdS coating thick-

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ness is typically within specification in the utilized portion of the web (web edges are not utilized) (Fig. 10).

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Figure 10. Optical confirmation of CdS coating thickness within specification down and across a web

Collection Grid

The collection grid has been designed to minimize re-sistive losses, cell shading, and required Ag ink volume. To first order, the design targets can be achieved through proper screen design and tool setup, but other variables such as ink pot life, screen wear, and environmental con-ditions can lead the process out of control. Resistive losses can be severe if the grid finger print geometry de-viates substantially below the design goals for height and width. On the other hand, excessive ink application adds unnecessary product cost.

The ink print process has been characterized by opti-cal profilometry. Cells were extracted at intervals from a single printed reel 350m in length. The grid finger print height and width were characterized at two locations on each cell. The mean ink height and width down the web was determined to be within acceptable limits (Fig. 11).

Figure 11. Collection grid finger average print characteris-tics along the web length

Process Development

The first-generation production tools have been ap-plied for process development utilizing a “hook and loop” test methodology. In the initial “hook and loop” tests, indi-

vidual processes in the new tools have been evaluated against first-generation tool processes. All thin film proc-esses in the new tools have been demonstrated capable of producing large-area solar cells (68cm

2) with conver-

sion efficiency greater than 9% (Table 1). In the final stage of the “hook and loop” tests, near completion now, multiple new tool process steps are being evaluated on single lots. The tests will conclude with the demonstration of all new processes together in the final string form.

Table 1. Best cell electrical characteristics generated in “hook and loop” tests

Process Voc (V) Isc (mA) FF (%) η (%)

Back electrode 0.570 2209 61.3 11.2

CIGS 0.504 2657 50.9 9.9

Buffer 0.592 2284 61.6 12.1

Front electrode 0.535 2379 55.4 10.3

SUMMARY

Global Solar is in the process of starting up new fac-tories in Tucson, AZ and Berlin, Germany. In the first phase, the annual capacity of the plants will be 40MWp/yr and 35MWp/yr. A complete line of new tools has been installed in Tucson and additional lines will be brought up in Tucson and Berlin during the coming months. The new tools are based on the prior generation of production tools. The earlier tools have demonstrated average con-version efficiency 10% over many production runs. The improved control and capability of the new tools should ultimately lead to even higher performance.

Tool capability has been assessed and found ade-

quate for process development to begin. “Hook and loop” style tests have been conducted, and the results indicate that conversion efficiencies >10% are achievable by all coating processes in the new tools. Integrated process development is now underway. String production will commence after manufacturing robustness and repeat-ability have been demonstrated.

ACKNOWLEDGEMENTS We gratefully acknowledge support from NREL in this

effort under the Thin Film Partnership Subcontract ZXL-6-44205-13 (TFPPP).

REFERENCES

[1] J.S. Britt, R. Huntington, J. VanAlsburg, S. Wiede-man, M. E. Beck, “Cost Improvement for Flexible CIGS-Based Product”, 4

th World Conference on Photovoltaic

Energy Conversion (IEEE), 2006, pp. 388-391. [2] J.S. Britt, E. Kanto, S. Lundberg, M. E. Beck, “CIGS Device Stability on Flexible Substrates”, 4

th World Confer-

ence on Photovoltaic Energy Conversion (IEEE), 2006, pp. 352-355.

978-1-4244-1641-7/08/$25.00 ©2008 IEEE