1. Recycling Photons to Break the Efficiency
Limit of Solar Cells
Cambridge Society for Applied Research
Churchill College, Cambridge
16th of May, 2016
Luis M. Pazos Outon

2. Photovoltaic efficiency – What is that?
Efficiency = Electrical Power Output Light Power Input
Theoretical Limit ≈ 33%

3. Photovoltaic efficiency – Why is it important?
If we replace a 20% efficient silicon module with a 25% efficient one:
- Fabrication costs become 20% cheaper.
- …

4. Photovoltaic efficiency – Why is it important?
If we replace a 20% efficient silicon module with a 25% efficient one: - Fabrication costs become 20% cheaper. - Transportation costs become 20% cheaper. - Storage costs become 20% cheaper. - Land acquisition becomes 20% cheaper. - Cleaning costs become 20% cheaper… To get cheap photovoltaics, we must invest in the most efficient technologies!

5. Photovoltaic efficiency – Perovskite Solar Cells
Earth abundant materials.
Cheap roll-to-roll scalable production.
Efficiencies comparable to crystalline Silicon.

6. Photovoltaic efficiency – How can we improve it?

7. Photovoltaic efficiency – How can we improve it?
Eli Yablonovitch. “Lead Halides join the top optoelectronics league“, , pp 1401, Science (2016).

8. Photovoltaic efficiency – How can we improve it?
Perovskites show photon recycling.
Its photovoltaic efficiency could
surpass crystalline silicon.
Even smarter:
Combining them with silicon could
lead to efficiencies exceeding 30%.
Pazos-Outon et al. “Photon recycling in lead-iodide perovskite solar cells“, , pp 1430, Science (2016).

9. Conclusions
Improving the efficiency of solar cells is key to reduce their cost per watt.
Our observation of photon recycling proofs that their optoelectronic properties are comparable to the best semiconductors known to man.
This shows their potential for several applications: PV, LEDs, lasers, thermophotonics, laser refrigeration, …

10. Acknowledgments University of Cambridge: Collaborators:
Richard Friend
Felix Deschler
Henning Sirringhaus
Neil Greenham
Erwin Reissner
Monika Szumilo
Robin Lamboll
Johannes Richter
Micaela Crespo
Mojtaba Abdi
Harry Beeson
Milan Vrucinic
Mejd Alsari
Collaborators:
Henry Snaith – Oxford University
Bruno Ehrler – AMOLF Institute, Amsterdam.
Ullrich Steiner – Adolphe Merkle Institute, Switzerland.
Funding Agencies:
Cambridge Home European Scholarship Scheme .
Nano Doctoral Training Center.
King Abdulaziz City for Science and Technology.
The worshipful company of the armourers and brasiers.

11. Additional Slides

12. Conditions for (relevant) photon recycling
1.- High internal PLQEs:
Direct bandgap
Low disorder -> Sharp Urbach Tail.
Fast bi-molecular recombination and slow monomolecular
2.- High refractive index (to achieve confinement)
3.- Small or non-existing Stokes-shift.
normalized
Figure from: Stefaan de Wolf et al. “Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance”, Journal of Physical Chemistry Letters (2014).

13. Spatial decay of long-range emission beyond Beer-Lambert absorption

14. Electrodeposit PEDOT/TiO2
Effect of photon recycling on long-range excitation transport and photocurrent
Pattern ITO
Electrodeposit PEDOT/TiO2
IBC Solar Cell

15. Experimental decay of photocurrent modelled with photon recycling
1015
1014
1013
𝑑𝑛 𝑑𝑡 =𝐷 𝛻 2 𝑛+𝐺+ 𝑐 𝑛 𝑠 𝜆 𝛼 𝜆 𝛾 𝜆 − 𝑘 1 𝑛− 𝑘 2 𝑛 2
Charge transport:
𝑑 𝛾 𝜆 𝑑𝑡 = 𝐷 𝜆 𝛻 2 𝛾 𝜆 − 𝑐 𝑛 𝑠 𝛼 𝜆 𝛾 𝜆 + (𝑘 2 𝑛 2 𝑃 𝑠𝑡𝑎𝑦 )𝑃 𝜆
Photon transport:

16. Conclusions
High Internal PLQEs + Low trap densities + No Stokes shift = Photon Recycling
Consequences:
1.- Energy (electron + hole) transport in perovskites is not limited by diffusive charge transport. It can occur over long distances through multiple absorption-diffusion-emission events.
2.- This process creates high excitation densities within the active perovskite layer, and, as with GaAs solar cells, allows high open circuit voltages.
L.M. Pazos-Outon, M. Szumilo, R. Lamboll, J. Richter, M. Crespo-Quesada, M. Abdi-Jalebi, H.J. Beeson, M. Vrucinic, M. Alsari, H. J. Snaith, B. Ehrler, R. H. Friend, F. Deschler. “Photon Recycling in Lead Iodide Perovskite Solar Cells“, , pp 1430, Science (2016).