Plasma Surface Modification of Polymer Backsheets: Origins of Future Interfacial Barrier/Backsheet Failure 

Joel W. Pankow, Stephen H. Glick National Renewable Energy Laboratory • Golden, CO  80401 

Abstract Objectives X-ray Photoelectron Spectroscopy 

XPS of Polyethylene Terephthalate 

Plasma Treatment of PET Angle Resolved XPS of Plasma-Treated PET 

Thermal Desorption XPS of Plasma-Treated PET 

Interfacial Failure Analysis of PET/SiOxNy/EVA/TPE 

Depth Profile of SiOxNyBarrier Film 

Conclusions 

Future Directions 

Acknowledgements 

The information contained in this poster is subject to a government license. 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion • Waikoloa, Hawaii • May 7-12, 2006  • NREL/PO-520-39954 

Flexible polymer substrates coated with inorganic oxide moisture barriers are a potential replacement for glass backsheets in thin-film PV (photovoltaic) modules.  Silicon oxynitride (SiOxNy) deposited by plasma enhanced chemical vapor deposition (PECVD) on polyethylene terephthalate (PET) represents one potential new backsheet candidate. Barrier deposition runs at NREL have included a nitrogen-rich plasma pretreatment prior to barrier deposition with the intention of cleaning the PET surface and enhancing adhesion of the SiOxNy barrier film to PET; however, test coupons of PET/barrier/EVA/TPE failed after damp-heat exposure. (EVA is ethylene vinyl acetate and TPE is Tedlar®-PET-EVA).  PET substrates exposed to plasma conditions similar to those used in pretreatment were examined by X-ray photoelectron spectroscopy (XPS) to reveal that new low molecular weight PET fragments were created at the PET surface. These fragments are responsible for barrier/PET interfacial failure and barrier transfer to the EVA encapsulant side following damp heat exposure. 

• Glass is typically used in thin film PV modules as both the frontsheet and backsheet material.1 

• Polymer films are an alternative backsheet:  advantages are decreased weight, cost and better handleability. 

• Unlike glass, polymer films are not inherently impervious to moisture and need a moisture barrier. 

• Silicon oxynitride (SiOxNy) coatings can provide moisture intrusion protection. 

• PET was coated with SiOxNy moisture barrier films using a Pernicka PECVD deposition system at NREL.2 

GLASS 

PV ABSORBER 

ENCAPSULANT 

BACKSHEET MOISTURE BARRIER 

• Surface sensitive – information depth is <100 Å. 

• Qualitative and quantitative information – elements identified by the photoelectron binding energies and signal is proportional to concentration. 

• Chemical state information – binding energy sensitive to valence state and chemical environment. 

• Depth profiling – sequential sputtering and analysis. 

• Angle resolved XPS (ARXPS) – grazing take-off angle (TOA) enhances information from near-surface species. 

photoelectrons to analyzerX-rays (E=hν) 

Ar+ θ (TOA) 

INFORMATION DEPTH < 100 Å 

• High resolution XPS scans show various carbon and oxygen functional groups resolved in expected 3:1:1 and 1:1 ratios. 

O2 O1 C1 C2 

C3 

• PET was exposed in the Pernicka PECVD deposition chamber to 60W plasma power, 500 sccm N2 and 100 sccm N2O (14% N2O in N2) for 10 min @ 40oC. 

• Samples were sealed in vials in a glove bag attached to the Pernicka load-lock and then mounted and transported to the XPS instrument under inert/vacuum conditions. 

• Terminal carbonyl, carboxylic acid and phenolic –OH groups appeared in plasma treated PET consistent with a chain scission mechanism resulting in low molecular weight PET fragments. 

• Approximately 5 atomic percent nitrogen was also detected. 

• Variation of photoelectron take off angle (TOA) alters XPS sampling depth and is non-destructive. 

• Sampling depth varied from approximately 36 Å to 80 Å. • Grazing TOA spectra contain significant contribution to 

oxygen lineshapes from the PET fragments.  High TOA shows predominantly bulk, native PET. 

• Plasma-modified layer layer is less than ~ 55 Å. 

• Plasma-treated PET sample was heated in the XPS chamber with periodic scanning of carbon, oxygen and nitrogen. 

• Nitrogen concentration decreased beginning at 70oC and C1s and O1s lineshapes gradually resembled those for pure PET. 

• Nitrogen was non-detectable by 148oC and C1s and O1s lineshapes were identical to those for pure PET. 

• PET fragments are volatile or diffuse into bulk indicating low molecular weight; soaking sample in water achieves same results indicating fragments have a high polarity. 

•SiOxNy barrier coated PET sample as used for laminate coupon was analyzed by XPS depth profiling. •A N2O/N2 plasma pre-treatment was used. Nitrogen incorporation at the PET/barrier interface is evident. •Only trace levels of nitrogen incorporation during phases 1 and 2 of the deposition despite N2O and N2 presence during these phases. 

• Plasma treatment of PET induces chain scission to occur resulting in formation of nitrogen-containing, low molecular weight, polar PET fragments (thickness < 55 Å). 

• Fragments are either thermally desorbed or diffused into bulk PET at elevated temperature. 

• PET fragments are water soluble. • Depth profiles show retention of PET fragments at the 

barrier/PET interface due to low substrate temperature conditions. 

• Depth profiles show a 2:1 oxygen to silicon ratio at the PET/phase 1 interface similar to that found for the EVA side of delaminated coupons indicating failure near the PET/phase 1 interface. 

• Damp heat exposure of coupons causes failure at the PET/barrier interface with barrier transfer to the EVA side.  This is consistent with the water soluble nature of polar PET fragments at this interface. 

• Utilize higher substrate temperatures to thermally desorb PET fragments produced via intentional or incidental plasma exposure. 

• Conduct mapping experiments to determine compositional and thickness uniformity for current Pernicka conditions. 

• Examine relationship between film composition and barrier efficacy and adhesion. 

• Evaluate impact of film thickness on barrier efficacy. • Evaluate thermal properties of polymer substrates and impact 

on adhesion. 

The authors appreciate the contributions and support from NREL NCPV staff and management on this joint Materials and Characterization/Module Reliability project particularly Gary Jorgensen, Kent Terwilliger, Joseph DelCueto, Tom McMahon, Sally Asher, Pete Sheldon and Carol Riordan.  

This NREL work was performed under DOE contract DE-AC36-99-GO10337. 

References 1G.J. Jorgensen, et al., “Moisture transport, adhesion, and corrosion protection of PV module packaging materials”; paper to be submitted to Progress in Photovoltaics. 

2S.H. Glick, J.A. del Cueto, K.M. Terwilliger, G.J. Jorgensen, J.W. Pankow, B.M. Keyes, L.M. Gedvilas, F.J. Pern, “Silicon oxynitride thin film barriers for PV packaging”, DOE 2005 Program Review Proceedings. 

POSSIBLE CHAIN SCISSION SITE 

107400321503 

67404121502 

6740881501 

10 (60 Watts) 50001000Plasma pre-treat 

Time (min) N2O2N2OSiH4Phase 

• Note: Moisture barriers to date have yielded reasonable water vapor transmission values; it is adhesion at the PET/barrier interface that fails. Depth profiles of barriers produced without plasma pretreatment do not show enhancement of nitrogen at the barrier/PET interface by nitrogen-containing PET fragments and adhesion of coupons after damp heat exposure is improved. 

Conclusions (cont.) 

• Barriers were grown using a nitrogen-rich plasma pretreatment. • Test coupons demonstrated reasonable initial adhesion but 

delaminated after 100 hours of damp heat exposure (85oC/85%RH). 

• XPS analysis suggests barrier transfer to EVA encapsulant side.