1. Novel Depth Profiling for Monitoring Degradation of Organic Photovoltaic Materials James McGettrick, Emily Speller, Zhe Li, Wing Chung Tsoi, James Durrant, Trystan Watson 8 th April 2016 PVSAT-12 Liverpool

2. Agenda 1)XPS & depth profiling: a very brief introduction. If it’s so good, why don’t OPV researchers (widely) use it already? 2)Photo-oxidation of PC 60 BM

3. 1)XPS & depth profiling: a very brief introduction. If it’s so good, why don’t OPV researchers (widely) use it already?

4. A tiny bit of science background KE=hv-BE-Ø If you’re not familiar with X-Ray Photoelectron Spectroscopy (XPS)… think inverse EDX/EDS. X-Rays in. Electrons out. e - travel <10 nm in solid. Peak position = electron binding energy. ( Sensitive to oxidation state) Peak intensity α no. of atoms. (Hence quantitative.. At% = Stoichiometry) UHV & can’t see hydrogen.

5. Depth profiles

6. Depth profiles: why don’t more people do it? “You can’t etch organics with argon – it destroys the chemistry.” -A non-UK instrument manufacturer at Tuesday’s EPSRC Photoelectron Spectroscopy Roadmap meeting.

7. Simple answer: artefacts Etching a PCBM layer with Ar + : an argon peak develops over time… As most researchers don’t use argon in their cells, this is a problem…

8. Simple answer: artefacts Even worse: destroys chemical information – e.g. loss of oxygen… Ultimately becomes graphite like… not helpful for studying oxidation… C-O, C=O, π→π* visible initially On etching, these features are obliterated

9. Gas cluster ion sources (GCIS) eliminate these Traditional Ar + Single atom, accelerated at 5 kV Very aggressive, penetrates into the bulk Fast & widely available GCIS Ar 500 + 500 atoms, accelerated at 2.5 kV ‘splash’ on surface Slower & not widely available.

10. Right tool for right job Inorganics Metals CIGS/CdTe etc. Organics PCDTBT PEDOT:PSS Fragile inorganics Traditional Ar + GCIS Ar 500 +

11. 2) Photo-oxidation of PC 60 BM

12. Materials studied Materials examined (thanks to Emily Speller, Zhe Li, Wing Chung Tsoi): Fresh PS(Polystyrene)-PCBM (40 wt%). Aged PS-PCBM after 15h 20m under AM1.5 1sun illumination in air. Can depth profiles give chemical information that will help to elucidate failure mechanisms? Can be more broadly applied to other material sets, e.g. P3HT, PEDOT:PSS, inorganic systems (of course!) etc. Note – all etch depths confirmed by profilometer after XPS analysis.

13. Tricky combination PS is very stable under these conditions compared to other options: 0.9 At% oxygen on aging. No oxygen in the bulk. C-O ether-like carbon gives one distinct peak Very similar peaks, heavily overlapping O-C=O ester carbon gives another distinct peak

14. New gas cluster ion sources eliminate artefacts e.g. typical spectra after 270s etch compare well to synchotron data see: Richter et al, BioNanoSci 2012, 2, 59-65 π→π* O-C-O C-O C-CC-C Benzene, C 60 C=O C-O-C

15. Allowing us to study chemistry changes… π→π* O-C-O C-O C-CC-C Benzene, C 60 e.g. typical spectra after 270s etch compare well to synchotron data see: Richter et al, BioNanoSci 2012, 2, 59-65 Benzene, C 60 C-CC-C O-C-O π→π* Fresh Aged C-O

16. A series of etches gives depth information… Fresh Aged As produced material is refreshingly boring… But on aging, the oxidation penetrates deep into the bulk.

17. And hence stoichiometric information… 70 nm = 4.5 nm/h penetration

18. Conclusions XPS is a widely used surface tool sampling <10 nm. Combined with argon etching, allows chemistry through a whole film to be analysed. New GCIS sources allow fragile materials to be examined. PS-PCBM shows oxidation through bulk. Generally applicable to all layered solar systems

19. Thanks to… Emily Speller (supply of materials and initial inspiration) Zhe Li & Wing Chung Tsoi (insight into OPV construction, physics and failure mechanisms) The wider PV team at SPECIFIC for tolerating my XPS enthusiasm…