“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)
“Improving the sustainability of photovoltaic materials” – Dr Patrick Isherwood, Loughborough 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…
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.
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