Solar

Cell Assembly: Low-cost, Continuous and Ecological Foil Cleaning for Photovoltaics

Issue 4 and Volume 2.

Atmospheric plasma cleaning protocols are gaining early adoption within thin film PV foil cleaning processes. APT prevents chemical waste effluents. Etch, cleaning and bonding trial data confirm system efficacies.

Atmospheric plasma cleaning protocols are gaining early adoption within thin film PV foil cleaning processes, with some of the key drivers being its use of a continuous, roll-to-roll (R2R) process, significantly lower production floor footprint, and lower capital cost. The highly efficient method for removing organic surface contaminations from PV foils prevents generation of chemical waste effluents found with wet-cleaning processes. Rory A. Wolf, Enercon Industries Corporation, examines plasma cleaning systems and details etching, cleaning and bonding trial data confirming system efficacies.

Metal foils are in widespread use in photovoltaic (PV) applications, and particularly with copper indium gallium selenide (CIGS) cells in the form of polycrystalline thin films. CIGS PV manufacturers require specific metal foil alloy formulations and dimensions, which are not uncommon formulations for metal foil providers. With the use of foil-based cells, copper and other materials replace silicon as the semiconductors. Key advantages of solar cells constructed with flexible foils include their ability to withstand high temperatures during further processing; they experience low impact from evaporation; they are highly etchable; and they can contain side electrodes that act as contacts for powering auxiliary units. However, moisture transport, adhesion, and corrosion protection of PV module packaging materials rely in part on clean foil surfaces for improving adhesion to glass and polymer (encapsulant, backsheet) component surfaces to prevent ingress and maximize efficiency. Atmospheric plasma pre-cleaning of foils in continuous roll-to-roll processes has been found to be a low cost, dry, and highly efficient method for removing organic surface contaminations from PV foils without the generation of chemical waste effluents compared to wet-cleaning processes. This paper examines these systems and details etching, cleaning and bonding trial data confirming system efficacies.

Atmospheric plasma systems

The use of plasma surface modification technology in PV cell manufacturing has heretofore been used primarily in applications such as the deposition of amorphous hydrogenated silicon nitride (SiN) layers in a vacuum plasma-enhanced chemical vapor deposition (PECVD) process to create anti-reflection and surface (and bulk) passivation on thin-film solar cells, or the use of vacuum plasma etching to perform edge isolation in some remaining fabrication processes. As PV cell manufacturing processes evolve, and with the added pressures of increasing hazardous chemical waste disposal costs, there has been interest in atmospheric plasma systems as efficient dry etching, surface cleaning and adhesion promotion process tools.

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CIGS evolution

Thin film PV cells fabricated from CIGS technology, for example, have the potential to produce energy at a higher efficiency than crystalline Si (c-Si) and GaAs solar cell technologies. CIGS solar cells also have excellent chemical stability and a tolerance against high radiation. Because CIGS solar cell panels are manufactured from smaller cells, which are connected by labor-intensive welding processes, cost advantages materialize when CIGS thin films are deposited on metal foils by using continuous roll-to-roll manufacturing processes. This method of fabricating these lightweight PV cells enables flexible application of these cells to a much broader range of supported surfaces. To further monolithic CIGS-based platforms with foils, new developments involving the use of glass as an insulating layer on foils offer the opportunity to support high processing temperature resistance (up to 550°C) and high dielectrics. Insufficient cleaning of these foils can sub-optimize the glass-foil bond and create delamination during in-process thermal expansion, as well as pin-holing effects.

Metals being integrated into thin film PV systems include stainless steel, aluminum, copper, iron, nickel, silver, zinc, molybdenum, stainless/copper alloy, copper/nickel alloy, and other alloys or multilayers. Economic preferences have elected aluminum, iron, copper, and alloys of these materials. Considering both performance and cost, aluminum, electroplated iron, and electroplated copper rank high with respect to economic preferences. Surface etchings of these metal foils typically employ Lewis acids and Bronsted acids. Specifically, copper is typically etched by ferric chloride, nitric acid, or sulfuric acid. Aluminum can be etched by caustic soda. The preferred foil thickness range for etching is roughly about 5 to 50µm, with most PV foils found between 1-500µm.

Figure 1. Wet chemical cleaning system.

Wet chemical cleaning processes

 Surface cleaning of these foils by wet cleaning processes for thin film solar cells will employ de-ionized (DI) water and tenside surfactants in a sequence similar to the process shown in Fig. 1.
The total system cycle time can be 10-15min, and is dependent on the processes employed. The typical wet cleaning system footprint is 8m long, 3-4m wide (depending upon the foil width) and 3m high.

Figure 2. Homogeneous atmospheric plasma discharge between planar and roll electrodes.

Dry atmospheric plasma cleaning process

 Atmospheric plasma treatment (APT) devices allow for completely homogenous surface modification without filamentary discharges (known as streamers), because a uniform and homogenous high-density plasma at atmospheric pressure and low temperature is produced (Fig. 2).

The APT process modifies material surfaces similarly to vacuum plasma treatment processes; the surface energy of treated materials increases substantially, corresponding to enhancements in surface cleanliness, wettability, printability, and adhesion properties. The APT process consists of exposing a polymer to a low-temperature, high-density glow discharge (i.e., plasma). The resulting plasma is a partially ionized gas consisting of a mixture of neutral molecules, electrons, ions, excited atomic and free radical species. Excitation of the gas molecules is accomplished by subjecting the gas to an electric field, typically at high frequency. Free electrons gain energy from the imposed high-frequency electric field, colliding with neutral gas molecules and transferring energy, dissociating the molecules to form numerous reactive species. Interaction of electrons, UV radiation and excited species with solid surfaces placed in opposition to the plasma results in the chemical and physical modification of the material surface (Fig. 3).

Figure 3. Species of an atmospheric plasma.

Plasma’s effect on a given material is determined by the chemistry of the reactions between the surface and the reactive species present in the plasma. At the low exposure energies typically used for surface treatment, the plasma surface interactions only change the atomic surface of the material; the effects are confined to a region only several molecular layers deep and do not change the bulk properties of the substrate. The surface is subjected to ablation and activation processes (Fig.  4). Activation is a process where surface functional groups are replaced with different atoms or chemical groups chosen to react within the plasma.

Figure 4. Plasma activation of polymer surface by creation of free radicals through substitution.

Bombardment of the polymer surface with energetic particles and radiation of plasma produces the ablation effect. Where bond strength is required, atmospheric plasma’s highly reactive species significantly increase the creation of polar groups on the surface of materials so that strong covalent bonding between the substrate and its immediate interface (i.e., adhesives, coatings) takes place.

Figure 5. Micrograph of PET film (L) untreated with low molecular weight organic contamination, (C) after corona discharge cleaning, and (R) after oxygen-based atmospheric plasma cleaning.

Surface cleaning via atmospheric plasma techniques reduces organic contamination on the surface in the form of residues, anti-oxidants, carbon residues and other organic compounds. Oxygen-based atmospheric plasmas in particular are effective in removing organics whereby mono-atomic oxygen (O+, O-) reacts with organic species resulting in plasma volatilization and removal (Fig. 5).
 
Solar cell processes transferrable to atmospheric pressure plasma processes are therefore dry etching, surface cleaning, etching, and activation. Layer reductions of organics using hydrogen-based atmospheric glow discharge plasmas is also therefore an employable aspect of the technology, and therefore suitable for cleaning of metal foils.

Experimental details

Cleaning and functionalizing the surface of flexible foils and polymer films in a continuous process prior to thin film PV cell fabrication can be critical in achieving new levels of output efficiencies. Moreover, as Table 1 outlines, avoiding the use of wet chemical cleaning solutions in favor of “green” process techniques which do not generate VOCs or waste effluents can also significantly improve commercial returns. 

Table 1. Wet vs. dry solar cell base material surface cleaning.
Metal foil cleaning process Caustic chemical cleaning (wet) APT cleaning (dry)
Active cleaning agent(s) Water-based sulfuric acid Ion/electron/photon bombardment
  Alkaline solutions Inert and non-hazardous reactive gas
  Alcohol solutions  
 
Dissolved within caustic chemical Rolling oils n/a
  Oxides  
  Water-soluble derivatives  
 
Entrained in process exhaust n/a Volatilized hydrocarbon particles
    Aluminum oxide particles
    Water-soluble aluminum derivatives
 
Emissions Water laden with chemical waste 15 ppm ozone
    Inert gas (98%)
    reactive gas (90%)
    <10 ppm CO2
    <10 ppm water vapor
    Volatilized surface particulates
 
Recurring process costs Fresh water, additional chemicals Process gases
  Handling and disposal costs  


 
Given the process benefits of APT above, an experimental was performed employing this continuous process. Referring to Table 2, a microcosm of PV cell foils were exposed to an APT process for the specified treatment purposes for cleaning surface contamination, thereby optimizing interfacial adhesion and improving PV cell output efficiency.

Table 2. Metal foil pretreatment protocols.
PV foil material APT surface treatment purpose Treatment protocol
Stainless steel/copper alloy Clean foil of organic contamination Helium/O2 plasma
Copper/nickel alloy Clean foil of organic contamination Helium/O2 plasma
Stainless steel Clean foil of organic contamination Argon/O2 plasma

Relative to cleanliness benchmarks, pre-specified low level organic particle contamination concentrations were established to optimize lamination adhesions. The specified plasma gas mixtures applied to each protocol were predetermined relative to the required surface effect by laboratory trials on commercial continuous roll-to-roll and tangential atmospheric plasma surface treatment systems at the Enercon Industries pilot facility.

Figure 6. Minimum plasma power density requirement for specific metal foil alloys.

As can be seen in Fig. 6, the minimum power densities required to achieve a goniometer surface contact angle of <5° averaged between 76W/m2/min. and 120W/m2/min. The contact angle benchmark, although non-quantitative, is an effective indicator of relative surface cleanliness. An idealized, perfectly clean metal PV foil surface would have a contact angle of 0°, which is impossible to obtain in laboratory air. A contaminated foil would have a high contact angle, such as 90° or more. The effective plasma discharge area was 38mm in width and with a length corresponding to the foil web width. The atmospheric plasma treatment was conducted at a foil conveyance speed of 10mpm.

Conclusion

Atmospheric plasma cleaning protocols are gaining early adoption within thin film PV foil cleaning processes. The key drivers for this trend of conversion from chemical wet cleaning processes include continuous, roll-to-roll process; significantly lower production floor footprint and lower capital cost; no VOCs or chemical effluent disposal costs; and no water supply costs.
 
Rory A. Wolf received his MBA from Marquette U. and is the VP of Business Development at Enercon Industries Corp., W140 N9572 Fountain Blvd., Menomonee Falls, WI, 53051, USA; 262-255-6070; [email protected].