Speaker
Mark W.B. Wilson
(Massachusetts Institute of Technology)
Description
Singlet exciton fission is the process in organic semiconductors by which one Coulombically-bound electron-hole pair (exciton) with net singlet spin evolves into a pair of spin-triplet excitons. Although the phenomenon has long been a curiosity in organic crystals[1], we recently demonstrated that singlet fission proceeds rapidly (80fs-400ps)[2-4] and efficiently (approaching quantitative (`200%') yield)[4-5] in thin films made from a range of organic molecules that satisfy the energetic criteria: $E_{sing} \geq 2\cdot E_{trip}$[4].
Critically, singlet fission is now attracting wider interest[6] because it could boost the power conversion efficiency of photovoltaic devices. Specifically, as first envisioned by Dexter[7], a blue-absorbing singlet fission material could sensitize a red-absorbing solar cell, enabling the generation of additional photocurrent from high-energy photons. However, though we have recently reported fission-based photovoltaic devices where the peak photon-to-electron conversion efficiency is greater than unity [8-9], their overall power conversion performance remains unremarkable, as they rely on immature solar technologies (e.g. organic semiconductors) for the red-absorbing cell.
Here, by detecting a characteristic magnetic field dependence using steady-state and transient spectroscopies, we demonstrate that fission-generated triplet excitons readily undergo exchange-mediated (Dexter) transfer to PbS colloidal quantum dots. Because the spin-states of the quantum dots are mixed at room temperature, this renders the fission-generated excitons emissive. Thus, we consider that longer-range Förster energy transfer may permit the direct sensitization of a conventional back-contacted silicon solar cell with an organic singlet fission material.
[1] Swenberg, C.E., & Geacintov, N.E. **Org. Mol. Photophys.** (1973)
[2] Wilson, M.W.B., *et al.* **J. Am. Chem. Soc.** (2011)
[3] Wilson, M.W.B., *et al.* **J. Am. Chem. Soc.** (2013)
[4] Yost, S.R., Lee, J., Wilson, M.W.B., *et al.*, **Nat. Chem.** (In Press)
[5] Thompson, N.J., *et al.* **Adv. Mater.** (2013)
[6] Smith, M.B., & Michl, J. **Annu. Rev. Phys. Chem.** (2013)
[7] Dexter, D.L. **J. Lumin.** (1979)
[8] Congreve, D.N., Lee, J., Thompson, N.J., *et al.* **Science** (2013)
[9] Thompson, N.J. *et al.* **Appl. Phys. Lett.** (2013)
Authors
Mark W.B. Wilson
(Massachusetts Institute of Technology)
Nicholas J. Thompson
(Massachusetts Institute of Technology)
Co-authors
Jennifer M. Scherer
(Massachusetts Institute of Technology)
Prof.
Marc A. Baldo
(Massachusetts Institute of Technology)
Prof.
Moungi G. Bawendi
(Massachusetts Institute of Technology)
Thomas S. Bischof
(Massachusetts Institute of Technology)
