Molecular Design Strategies for Efficient Intramolecular Singlet Exciton

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Molecular Design Strategies for Efficient Intramolecular Singlet Exciton Fission K C Krishnapriya, Andrew J. Musser, and Satish Patil ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01833 • Publication Date (Web): 26 Nov 2018 Downloaded from http://pubs.acs.org on December 2, 2018

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ACS Energy Letters

Molecular Design Strategies for Efficient Intramolecular Singlet Exciton Fission K C Krishnapriya,1 Andrew J Musser*2 and Satish Patil*1 1Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012

2Department of Physics and Astronomy, University of Sheffield, Sheffield, United Kingdom

Communicating authors: [email protected] and [email protected]

Abstract The process of carrier multiplication via singlet fission can potentially exceed Shockley–Queisser limit on the efficiency of single-junction photovoltaics. In the recent past, theoretical analysis provided the principal guidelines on molecular design strategies for singlet fission. In this perspective, we focus instead on correlating experimental results for different classes of reported singlet fission materials to identify principles to aid in the design of new molecules for efficient intramolecular singlet fission. Building on an evaluation of several series of multichromophoric and polymeric singlet fission materials, we extract new suggested strategies for molecular design.

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Singlet exciton fission (SEF) is the collapse of a photoexcited singlet exciton into two triplets localized on different chromophores. This process therefore produces two triplet excitons at the expense of one photon.1 This conversion is mediated by a ‘triplet-pair state’ (1TT) in which the two triplets are coupled into an overall spin-zero configuration. The process is thus spin-conserving and can be ultrafast in nature, given suitable energetics. Though SEF was first observed in anthracene crystals in 19652 and studied in tetracene through the 1970s3,4, further progress in the field was marginal for the next ~40 years. However, Dexter’s novel proposal5 to harness SEF to improve silicon solar cells was revisited in 2006 (Figure 1a)6. This study generalized the concept of exciton multiplication solar cells and demonstrated that SEF could be used to improve single-junction solar cell efficiencies beyond the Shockley-Queisser limit of ~33% to 41.9%6 (as high as 45.9% when factoring in the possibility of endothermic SEF, Figure 1b7). In the same period, important photophysical studies revived the exploration of SEF in acene materials8 and demonstrated unexpectedly fast fission in pentacene9,10.


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ACS Energy Letters

Pentacene-based solar cells were observed to have surprisingly high efficiency11; this result was later attributed to SEF, and this exciton multiplication effect was harnessed to create photodetectors with quantum efficiency >100%12. Together, these theoretical proposals for enhanced efficiency, the observation of SEF in new materials and its implementation in preliminary devices inspired a wave of interest in the field. Since these pioneering works SEF research has advanced dramatically.

Figure 1. (a) Solar spectrum with representative absorptions of low bandgap conventional semiconductor and high bandgap SEF material (b) Theoretical power-conversion efficiency for a solar cell incorporating endothermic SEF for exciton multiplication, as a function of the bandgaps of the ‘red’ solar cell Er and of the ‘blue’ SEF



component Eb. The white line corresponds to calculations performed by Hanna and Nozik6. Below that line, SEF is endothermic. Adapted from Tayebjee et al.7 (c) A schematic of SEF-enhanced inorganic photovoltaic device. In such a device, the inorganic photovoltaic serves as the low bandgap semiconductor. The triplet excitons generated by SF are either dissociated via charge transfer or transferred directly into the inorganic material via energy transfer. Adapted from Rao and Friend13, used with permission.

There have been promising results on the incorporation of SEF into working solar cells, demonstrating that the concept has real potential to boost power conversion efficiencies14–16. The focus of the field to date has been on understanding the detailed mechanism of triplet formation, discussion of which is beyond the scope of this perspective. In this perspective, we briefly summarize key findings that inform the concepts of molecular design. Ultrafast spectroscopic measurements have established a growing consensus that SEF is a multi-step process, where initial conversion of S1 into a distinct intermediate 1TT

is followed by triplet-pair separation into T1+T1 (spin coherence between these may proceed

through longer timescales)17–19. This model in principle allows the two steps to be treated separately and potentially independently optimized through chemical design. Most debates have centered on the initial S1à1TT conversion and the nature of the interactions that govern this process. It may be accomplished through the ‘direct’ two-electron coupling between S1 and 1TT, but this is typically very weak1,20. The current consensus is that coupling to charge-transfer states (i.e. S1àCT and CTà1TT) is much stronger and likely to drive SEF1,20, even when CT is energetically inaccessible and can only serve as a ‘virtual’ intermediate21. It has not been possible to directly verify this model in thin films, though recent results on covalent dimers in solution reveal that both ‘direct’ and CT-mediated channels can yield efficient SEF22–24. Rather than couplings, experiments typically probe conversion rates and in particular how these are affected by interchromophore interactions (inferred from absorption spectra). As the coupling increases, 1TT formation is generally accelerated24–26. However, it has been shown that when the coupling is too strong the resulting 1TT pair remains bound and rapidly annihilates26,27. Unfortunately, the primary mechanism of 1TT separation – triplet hopping17,18 – is governed by similar


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ACS Energy Letters

orbital overlap elements as the interchromophore coupling. Separate optimization of these SEF stages is thus difficult, and the essential tension between efficient 1TT formation and efficient separation into T1+T1 remains a foremost challenge in the field. In the context of eventual applications, an even greater challenge is to broaden the pool of eligible chromophores. Nearly all SEF mechanistic and device studies are based on tetracene, pentacene and their derivatives (2,3). While these are convenient systems for fundamental study they are not among the most desirable materials for solar cell applications, exhibiting poor photochemical stability and low extinction coefficients19. SEF-sensitized solar cell optimization would also greatly benefit from the ability to tune other properties such as singlet and triplet energies, carrier and energy transport, morphological stability, processability, frontier orbital energy levels, etc. SEF solar cells are themselves very complex and there are several distinct device concepts being explored, each with its own materials requirements (see discussion below and a recent detailed review13). At this time, we believe that the problems of developing efficient SEF materials and incorporating them into devices are best tackled separately. In practice this will require building a library of SEF systems, ideally beyond the acenes, to exploit in solar cells. We outline below some of the key design concepts and outstanding problems in the field. We note that in doing so we assume that the principles and mechanisms extracted from acene systems are not unique to those materials; that question also merits further study as new materials are developed.

Monomer energetics The most straightforward approach to SEF materials design is to break the problem down into monomer energetics and electronic coupling. The essential energetic requirements, as laid out in previous reviews1,20, are (1) E(S1)≥2E(T1) and (2) E(T2)≥2E(T1). Condition (1) ensures that SEF is energetically accessible and prevents radiative triplet-triplet annihilation T1+T1àS1+S0, which in the



context of SEF solar cells would constitute a loss channel. However, if the ‘re-fission’ rate effectively outcompetes S1 emission the loss may be minimal and the intervening singlet may aid triplet transport.28 Moreover, current understanding in light of endothermic SEF systems where E(S1)

Molecular Design Strategies for Efficient Intramolecular Singlet Exciton

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