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

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

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

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!

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

Photovoltaic efficiency – How can we improve it?

Photovoltaic efficiency – How can we improve it? Eli Yablonovitch. “Lead Halides join the top optoelectronics league“, 351-6280, pp 1401, Science (2016).

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“, 351-6280, pp 1430, Science (2016).

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, …

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.

Additional Slides

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).

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

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

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:

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“, 351-6280, pp 1430, Science (2016).