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SUMMARY:Excitonic Light Management for Solar Cells beyond Shockley-Queisse
 r - Timothy W. Schmidt  ARC Center of Excellence in Exciton Science\, Scho
 ol of Chemistry\, UNSW Sydney\, NSW\, Australia
DTSTART:20190416T150000Z
DTEND:20190416T160000Z
UID:TALK122896@talks.cam.ac.uk
CONTACT:Alexandra Clark
DESCRIPTION:Single-junction solar cells are limited to an energy conversio
 n efficiency of 33.7% under unconcentrated sunlight. Crystalline silicon\,
  with a band gap of 1.12 eV\, is limited to 29%\, and UNSW researchers and
  others have pushed this beyond 25%. To go beyond 30% requires careful man
 agement of photon energies.\nThe principal inefficiencies of a solar cell 
 can be divided into two phenomena: The inability to absorb photons below t
 he band gap\, and the thermalisation of carriers generated with photon ene
 rgies exceeding the bandgap. Both of these shortcomings can be addressed u
 sing excitonic phenomena in organic materials.\n\nUpconversion: Sub-bandga
 p losses can be remedied by the application of photochemical upconversion\
 , whereby transmitted light is converted to light of higher energy\, which
  can then be harvested by the cell and contribute to current generation. W
 e achieved unprecedented upconversion efficiencies and incorporated upconv
 erters into amorphous silicon and organic solar cells. These proof-of-prin
 ciple experiments motivated the development of more efficient upconverters
  which harvest light deep in the infrared – beyond silicon. In our most 
 recent work using quantum dot sensitizers we achieved upconversion from be
 low the silicon bandgap for the first time\, therefore establishing a low 
 energy record for photochemical upconversion. This was achieved in the pre
 sence of oxygen\, usually the nemesis of molecular triplet states.\n\nSing
 let fission is a process whereby two triplet excitons can be produced from
  one photon\, potentially increasing the efficiency of photovoltaic device
 s by mitigating thermalisation losses. Endothermic singlet fission is desi
 red for a maximum energy-conversion efficiency\, and such systems have bee
 n considered to form an excimer-like state with multiexcitonic character p
 rior to the appearance of triplets. However\, the role of the excimer as a
 n intermediate has\, until now\, been unclear. Here we show that\, rather 
 than acting as an intermediate\, the excimer serves to trap excited states
  to the detriment of singlet-fission yield. We show that singlet fission a
 nd its conjugate process\, triplet–triplet annihilation\, occur at a lon
 ger intermolecular distance than an excimer intermediate would impute. The
 se results establish that an endothermic singlet-fission material must be 
 designed to avoid excimer formation\, thus allowing singlet fission to rea
 ch its full potential in enhancing photovoltaic energy conversion\n\nRefer
 ences\n[1]  Elham M. Gholizadeh\, Shyamal K. K. Prasad\, Zhi-Li Teh\, Thil
 ini Ishwara\, Sarah Norman\, Anthony J. Petty II\, John E. Anthony\, Shuju
 an Huang and T. W. Schmidt\, ChemRxiv\, DOI: 10.26434/chemrxiv.7834838\n[2
 ]  Cameron B. Dover\, Joseph K. Gallaher\, Laszlo Frazer\, Patrick C. Tapp
 ing\, Anthony J. Petty II\, Maxwell J. Crossley\, John E. Anthony\, Tak W.
  Kee and Timothy W. Schmidt\, Nature Chemistry 2018\, 10\, 305-310.\n
LOCATION:Wolfson Lecture Theatre\,  Department of Chemistry\, Lensfield Ro
 ad
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