Engineering Molecular Ligand Shells on Quantum Dots for Quantitative Harvesting of Triplet Excitons Generated by Singlet Fission

Abstract

Singlet fission is an exciton multiplication process in organic molecules in which a photogenerated spin-singlet exciton is rapidly and efficiently converted to two spin-triplet excitons. This process offers a mechanism to break the Shockley−Queisser limit by overcoming the thermalization losses inherent to all single-junction photovoltaics. One of the most promising methods to harness the singlet fission process is via the efficient extraction of the dark triplet excitons into quantum dots (QDs) where they can recombine radiatively, thereby converting high-energy photons to pairs of low-energy photons, which can then be captured in traditional inorganic PVs such as Si. Such a singlet fission photon multiplication (SF-PM) process could increase the efficiency of the best Si cells from 26.7% to 32.5%, breaking the Shockley−Queisser limit. However, there has been no demonstration of such a singlet fission photon multiplication (SF-PM) process in a bulk system to date. Here, we demonstrate a solution-based bulk SF-PM system based on the singlet fission material TIPS-Tc combined with PbS QDs. Using a range of steady-state and timeresolved measurements combined with analytical modeling we study the dynamics and mechanism of the triplet harvesting process. We show that the system absorbs >95% of incident photons within the singlet fission material to form singlet excitons, which then undergo efficient singlet fission in the solution phase (135 ± 5%) before quantitative harvesting of the triplet excitons (95 ± 5%) via a low concentration of QD acceptors, followed by the emission of IR photons. We find that in order to achieve efficient triplet harvesting it is critical to engineer the surface of the QD with a triplet transfer ligand and that bimolecular decay of triplets is potentially a major loss pathway which can be controlled via tuning the concentration of QD acceptors. We demonstrate that the photon multiplication efficiency is maintained up to solar fluence. Our results establish the solution-based SF-PM system as a simple and highly tunable platform to understand the dynamics of a triplet energy transfer process between organic semiconductors and QDs, one that can provide clear design rules for new materials.

Mechanisms of triplet energy transfer across the inorganic nanocrystal/organic molecule interface

“…Interestingly, the spectral features of TCA on NC surfaces are narrower than those of TCA in toluene solution. This suggests TCA molecules likely aggregate in solution, which is expected for extended π-systems and is consistent with previous reports on singlet fission observed for tetracene derivatives in solution 14,38 . The aggregation is also implied by the lack of clear vibronic structures on the PL spectrum of TCA in toluene ( Supplementary Fig.…”

“…Recently, there has been growing interest in combining the light harvesting and emitting capability of inorganic semiconductors with the SF and TTA-UC processes in organic molecules via interfacial excitation energy transfer to explore new functionalities [7][8][9][10][11] . For example, researchers have pursued the ideas of harvesting the SF-induced triplets of polycyclic aromatic hydrocarbons (PAHs) such as tetracene and pentacene using either silicon solar cells 12,13 or lead chalcogenide nanocrystals (NCs) 7,8,14 for efficiency-doubled light conversion or emission. Alternatively, inorganic semiconductors, and in particular their NCs, can be used to sensitize the triplets of PAHs for TTA-UC 9,[15][16][17][18][19] .…”

Prospects of Coupled Organic–Inorganic Nanostructures for Charge and Energy Transfer Applications

“…[127] Rao et al used TIPS-tetracene as a solution-based SF active chromophore: After an efficient SF process in solution, TET to PbSe NCs takes places, with surface-associated tetracene ligands key to mediate an efficient transfer. [128] Similarly, SF and TET take place when the chromophore is directly bound to the PbSe NC. [129] From a fundamental point of view, the NC to organic transfer direction is especially interesting: The NC band gap can be tuned, and even energetically high-lying naphthalene triplets are accessible via visible-light-driven TET from CsPbBr 3 NCs.…”

“…[54] It has thus been argued that the main challenge in optimizing triplet exciton harvesting is increasing the efficiency of TET, and designing strategies for tailored ligand molecules for QDs. [128] In contrast, shallow surface states of QDs can even be beneficial during TET by acting as a temporary storage place for triplets before they are transferred to the acceptor. [33] Due to their low transition dipole moment, TET between the sensitizer and the acceptor is dominated by the Dexter mechanism, where two individual charges are consecutively transferred and spin is exchanged.…”

“…[12,196] Sufficient light absorption and a high enough concentration of SF chromophores to harvest most photons is difficult to realize in bilayer structures. [128] COIN structures might allow a high enough chromophore and sufficiently low NC concentration to make practical devices possible. For an efficient SF process, a spatially (and electronically) intimate arrangement is needed to support the bimolecular excimer mechanism.…”