Limits for Recombination in a Low Energy Loss Organic Heterojunction.
Donor-acceptor organic solar cells often show high quantum yields for charge collection, but relatively low open-circuit voltages (VOC) limit power conversion efficiencies to around 12%. We report here the behavior of a system, PIPCP:PC61BM, that exhibits very low electronic disorder (Urbach energy less than 27 meV), very high carrier mobilities in the blend (field-effect mobility for holes >10-2cm2V-1s-1), and a very low driving energy for initial charge separation (50 meV). These characteristics should give excellent performance, and indeed, the VOCis high relative to the donor energy gap. However, we find the overall performance is limited by recombination, with formation of lower-lying triplet excitons on the donor accounting for 90% of the recombination. We find this is a bimolecular process that happens on time scales as short as 100 ps. Thus, although the absence of disorder and the associated high carrier mobility speeds up charge diffusion and extraction at the electrodes, which we measure as early as 1 ns, this also speeds up the recombination channel, giving overall a modest quantum yield of around 60%. We discuss strategies to remove the triplet exciton recombination channel.
Department of Physics, Cavendish Laboratory, University of Cambridge , J.J. Thompson Avenue, Cambridge CB3 0HE, United Kingdom.
S Matthew
SM
Menke
Department of Physics, Cavendish Laboratory, University of Cambridge , J.J. Thompson Avenue, Cambridge CB3 0HE, United Kingdom.
Aditya
A
Sadhanala
Department of Physics, Cavendish Laboratory, University of Cambridge , J.J. Thompson Avenue, Cambridge CB3 0HE, United Kingdom.
Mark
M
Nikolka
Center for Polymers and Organic Solids, University of California , Santa Barbara, California 93106, United States.
Niva A
NA
Ran
Solar & Photovoltaics Engineering Research Center, Division of Physical Science and Engineering, King Abdullah University of Science and Technology , Thuwal 23955-6900, Saudi Arabia.
Mahesh Kumar
MK
Ravva
Solar & Photovoltaics Engineering Research Center, Division of Physical Science and Engineering, King Abdullah University of Science and Technology , Thuwal 23955-6900, Saudi Arabia.
Safwat
S
Abdel-Azeim
Department of Physics, Cavendish Laboratory, University of Cambridge , J.J. Thompson Avenue, Cambridge CB3 0HE, United Kingdom.
Hannah L
HL
Stern
Center for Polymers and Organic Solids, University of California , Santa Barbara, California 93106, United States.
Ming
M
Wang
Department of Physics, Cavendish Laboratory, University of Cambridge , J.J. Thompson Avenue, Cambridge CB3 0HE, United Kingdom.
Henning
H
Sirringhaus
Center for Polymers and Organic Solids, University of California , Santa Barbara, California 93106, United States.
Thuc-Quyen
TQ
Nguyen
Solar & Photovoltaics Engineering Research Center, Division of Physical Science and Engineering, King Abdullah University of Science and Technology , Thuwal 23955-6900, Saudi Arabia.
Jean-Luc
JL
Brédas
Center for Polymers and Organic Solids, University of California , Santa Barbara, California 93106, United States.
Guillermo C
GC
Bazan
Department of Physics, Cavendish Laboratory, University of Cambridge , J.J. Thompson Avenue, Cambridge CB3 0HE, United Kingdom.
Richard H
RH
Friend
charge recombination; charge transfer states; energy loss; high mobility; organic solar cell