“Improving the sustainability of photovoltaic materials” – Dr Patrick Isherwood, Loughborough University, presenting at the Net Zero Conference 2022, ‘Research Journeys in/to Net Zero: Current and Future Research Leaders in the Midlands, UK’ (on Friday 24th June 2022 at De Montfort University)

1. Improving the Sustainability of
Photovoltaic Materials
Dr. Patrick Isherwood
CREST, Wolfson school, Loughborough University

2. Photovoltaics crash course
• What are they?
– Silicon
– CdTe
– CIGS
– Organometal halide perovskites
– …Others?
• Semiconductor-based devices for direct conversion of light into
electrical energy
• Most common form is silicon p-n junction

3. Photovoltaics crash course
• Why do we care?
– Incoming solar resource is about 174 PW
– More than 10,000 times humanity’s annual energy usage
– Plentiful and effectively infinite clean energy source
• But…
– Energy is not always available when needed
– Dispersed nature makes it more challenging to capture
– Although increasingly cost-effective, solar modules are still relatively
expensive

4. Geologist to solar energy researcher
• 2004-8: MSci in Geoscience from Durham
• 2009-10: MSc in Engineering Geology from Newcastle
• 2011-15: PhD in Electrical Engineering from Loughborough
So why the change?
• Unemployment leaves plenty of time to contemplate your
surroundings!
• Winter 2010 was particularly cold, and the spring and summer were
very warm and green in comparison…

5. Doctoral research: TCOs and
negative results

6. Transparent conducting oxides
• One of those things everyone uses, but no-one has ever heard of!
• Used in anything which needs a visually transparent electrical
contact
• Computer monitors, smartphones, flat-screen TVs, smart windows,
double glazing, touchscreens…
• Consist of wide bandgap metal oxide semiconductors which are
degenerately doped to make them conductive
• Materials which combine the highest transparency with the best
electrical conductivity are mostly based on indium oxide

7. TCOs: extracting current
• Widely used as front transparent contacts in thin film
cells​
• Many experimental devices use fluorine-doped tin oxide
coated glass as substrates​
• Vital to the development of next-generation concepts
such as tandem and multijunction devices
• But… All commercially available examples are n-type

8. Why p-type transparent conductors?
General uses:
• High work-function transparent contacts
• Transparent electronics
Photovoltaic applications:
• Tandem and multijunction cell interconnects
• Bifacial cells
• Back contacts for OPV, CdTe etc.
• Hole transport materials for perovskites
However, it turns out that there is a problem…
THEY DON’T WORK!
At least, not very well.

9. Research problems, new
materials and concepts

10. Photovoltaics materials problems
• Silicon is the second most abundant material in the Earth’s crust
• Refining process involves reducing SiO2 using carbon
• Production of solar grade (very high purity) silicon is very energy-
intensive
• Alternatives include CdTe, CIGS and organometal halide
perovskites
• Of these, only CdTe remains commercially viable

11. Photovoltaics materials problems
• CdTe and CIGS both contain rare and toxic elements
• High-efficiency devices are usually deposited using high vacuum
methods, which are expensive
• Room-pressure techniques have been developed, particularly for
CIGS, but…
https://en.wikipedia.org/wiki/Hydrazine

12. Photovoltaics materials problems
• Alternative solution approach developed at CREST using a
combination of a dithiol and a diamine as the solvents
• Enables direct dissolution of Cu2S, In2Se3 and Ga. Excess Se and S
are also added
• Current lab record of 12%
• Still involves rare materials, but the process also works for more
Earth-abundant technologies

13. Possible research directions
• Development of cheap, Earth-abundant, atmospherically
processable and long-term stable absorber materials
– Metal-organic complexes (tannates, curcumin complexes, dyes…)
– Alternative chalcogen materials (Cu2S, FeS2…)
– N-type metal oxide absorbers (Copper tungstate)
• Alternative low-energy, low cost and environmentally friendly means
for refining of silicon

14. Photovoltaic modules problems
• Installed capacity has been increasing near-exponentially for over a
decade
• Modules typically have a stated lifetime of 25 years
• The number of modules needing to be recycled will increase nearly
exponentially within the coming decade!

15. Photovoltaic modules problems
• The recycling problem is mostly economic not technological
• All glass, metal and silicon components of a silicon module can
theoretically be fully recycled and reused effectively infinitely
• All glass, metal and semiconductor components in both CIGS and
CdTe can theoretically be recovered, fully recycled and reused
• CdTe modules are already almost fully recycled
• All modules contain polymeric materials which are not currently
recyclable
https://www.flickr.com/photos/dullhunk/28251201308
https://doi.org/10.3390/su14031676
https://doi.org/10.3390/su14031676

16. Possible research directions
• Silicon is likely to remain the primary photovoltaic technology for the
foreseeable future
– Development of low-cost methods for refining high-grade silicon from
defunct cells
• Technology-agnostic research possibilities:
– Glass-glass modules which contain no polymeric materials
– Design modules to make them more readily recyclable

17. Conclusions
• Photovoltaic technology, and particularly silicon PV, is an increasingly cheap means
for generating electricity
• Silicon is abundant, but is energy-intensive to refine
• The current alternatives all have problems – rare and/or toxic materials, energy-
intensive processing or significant atmospheric instability
• Modules are designed to last 25 years or more, making them difficult to dismantle.
This is made worse by the widespread use of non-recyclable polymeric materials
• Plenty of new materials to research!
• May be ways to refine silicon without using carbon
• It is theoretically possible to make a glass-backed module containing no polymers
• New modules should be designed to be both durable and simple to recycle

18. Any questions?