Harvesting Singlet Fission for Solar Energy Conversion: One- versus Two-Electron Transfer from the Quantum Mechanical Superposition

Abstract

Singlet fission, the creation of two triplet excitons from one singlet exciton, is being explored to increase the efficiency of solar cells and photo detectors based on organic semiconductors, such as pentacene and tetracene. A key question is how to extract multiple electron-hole pairs from multiple excitons. Recent experiments in our laboratory on the pentacene/C(60) system (Chan, W.-L.; et al. Science 2011, 334, 1543-1547) provided preliminary evidence for the extraction of two electrons from the multiexciton (ME) state resulting from singlet fission. The efficiency of multielectron transfer is expected to depend critically on other dynamic processes available to the singlet (S(1)) and the ME, but little is known about these competing channels. Here we apply time-resolved photoemission spectroscopy to the tetracene/C(60) interface to probe one- and two-electron transfer from S(1) and ME states, respectively. Unlike ultrafast (~100 fs) singlet fission in pentacene where two-electron transfer from the multiexciton state resulting from singlet fission dominates, the relatively slow (~7 ps) singlet fission in tetracene allows both one- and two-electron transfer from the S(1) and the ME states that are in a quantum mechanical superposition. We show evidence for the formation of two distinct charge transfer states due to electron transfer from photoexcited tetracene to the lowest unoccupied molecular orbital (LUMO) and the LUMO+1 levels in C(60), respectively. Kinetic analysis shows that ~60% of the S(1) ⇔ ME quantum superposition transfers one electron through the S(1) state to C(60) while ~40% undergoes two-electron transfer through the ME state. We discuss design principles at donor/acceptor interfaces for optimal multiple carrier extraction from singlet fission for solar energy conversion.

Harvesting singlet fission for solar energy conversion via triplet energy transfer

“…The time constant of singlet fission is fixed at t SF ¼ 7ps, as determined in our TR-2PPE experiments 11,17 . We fit the parameters t CT and t RC to the rise and decay of the CT state in both bilayer samples, and t TET to the slow rise in CuPc T 0 1 population from the 2-nm CuPc/20-nm Tc sample, as shown in Fig.…”

“…A limitation of this approach is that CT processes from both the singlet and the triplet pair are possible 11 ; the former does not contribute to any increase in power conversion efficiency beyond the ShockleyQueisser limit and is difficult to avoid for most electron acceptors. Another difficulty is that the electron (or hole) acceptor material may not necessarily serve as an efficient light absorber in the long wavelength region, as required for a singlet fission solar cell.…”

“…It avoids the direct interface between the singlet fission material and the electron acceptor, thus, in principle, eliminating the less efficient channel of electron transfer from singlet excitons in A to C (ref. 11). This scheme is enabled by the relatively long-lived triplet excitons 12 that can be transferred over long distances 13 from A through B before eventually reaching the charge-separating interface with C. Recently, Baldo and co-workers used the Tc/CuPc/C 60 trilayer structure and suggested the transport of triplets from singlet fission in Tc through the CuPc layer to reach C 60 for enhanced photocurrent, but the overall power conversion efficiency of the device remained low 7 .…”

Localization length scales of triplet excitons in singlet fission materials

“…[3][4][5][6][7][8][9] Particularly important for solar cell applications has been the process of singlet fission which enables rapid conversion of a photogenerated singlet exciton into two triplet excitons. 5,6,[10][11][12][13] This provides a route to circumvent the traditional thermalization loss limit which hinders energy harvesting, 14 as well new opportunities to explore and utilize optically generated triplet excitons.…”

Singlet fission in linear chains of molecules

“…44 This agrees with previous studies, motivating the competition between SF and exciton dissociation at the interface, for C60-pentacene and C60-tetracene systems, and showing that SF is favored for pentacene but not for tetracene. 45 However, a critical point of non-adiabatic dynamics is given by the fact that it forces the system into one of the adiabatic potential energy surfaces via a sequence of fast hops, while, for very fast processes it is more likely for the system to be in a linear combination of electronic states.…”