Triplet Energy Transfer from PbS(Se) Nanocrystals to Rubrene: the Relationship between the Upconversion Quantum Yield and Size

Abstract

Photon upconversion has attracted enormous attention due to its wide range of applications in biological imaging, photocatalysis, and especially photovoltaics. Here, the effect of quantum confinement on the efficiency of Dexter energy transfer from PbS and PbSe nanocrystals (NCs) to a rubrene acceptor is studied. A series of experiments exploring the relationship between NC size and the upconversion quantum yield (QY) in this hybrid platform show that energy transfer occurs in the Marcus normal regime. By decreasing the NC diameter from 3.5 to 2.9 nm for PbS and from 3.2 to 2.5 nm for PbSe, the relative upconversion QY is enhanced about 700 and 250‐fold respectively. In addition, the dynamic Stern–Volmer constant (KSV) for the quenching of PbSe NCs by rubrene increases approximately fivefold with a decrease in NC diameter from 3.2 to 2.5 nm to a value of 200 m−1. This work shows that high quality, well‐passivated, small NCs are critical for efficient triplet energy transfer to molecular acceptors.

Designing Transmitter Ligands That Mediate Energy Transfer between Semiconductor Nanocrystals and Molecules

“…For TET from an NC to a bound transmitter, ΔG 0 can be tuned by varying the size of the NC, and experimentally, higher upconversion QYs are obtained from smaller NCs. 34,35 This suggests that TET is in the Marcus normal region. Along these lines, within the Marcus normal region, the triplet energy levels of the transmitter should be lower than the dark excitonic state of the NC, thus providing a driving force for higher Φ TET .…”

Morphology of Passivating Organic Ligands around a Nanocrystal

“…Excitonic energy transfer, relevant in solar cells and light emitters, usually occurs through Forster resonant energy transfer (FRET) [1,21,[25][26][27][28][29][30] or Dexter processes [31][32][33]; these are also dependent on distance. Since the organic ligand shell is usually composed of insulating alkane chains, they behave as a spacer layer that can determine that closest approach distance [13,[19][20][21]34].Ligand exchange reactions [35][36][37][38] give us in situ synthetic access to the ligand shell, and using this design space it is possible to achieve fine control of the aforementioned electronic processes.Recently, the ability to control the energy gap and energy transfer has been exploited for novel optoelectronic devices [8][9][10], allowing for down-conversion of a high energy UV photon into two lower energy photons [26,32,33], and up-conversion of two low energy IR photons into one higher energy photon [39][40][41]. The ability to up-, and down-convert photon energy can allow solar cells to capture more of the solar spectrum, thereby circumventing the Shockley-Queisser limit [42][43][44].…”

“…Recently, the ability to control the energy gap and energy transfer has been exploited for novel optoelectronic devices [8][9][10], allowing for down-conversion of a high energy UV photon into two lower energy photons [26,32,33], and up-conversion of two low energy IR photons into one higher energy photon [39][40][41]. The ability to up-, and down-convert photon energy can allow solar cells to capture more of the solar spectrum, thereby circumventing the Shockley-Queisser limit [42][43][44].…”

Triplet transport in thin films: fundamentals and applications

“…However, an ongoing challenge in this field is to translate these encouraging results from solution into commercially relevant thin films, ideally made by low-cost printing methods. Therefore, it would be interesting to fabricate thin films that can harness triplet excitons created by singlet fission, chargetransfer (CT) states at the polymer:fullerene interface, [23][24][25][26][27][28][29] or nanocrystal photosensitization, [30][31][32][33][34][35][36] and extract the energy contained in these excitons either through TTA via photon upconversion or improved PCEs in solar cells. While singlet fission enhanced organic solar cells have been demonstrated, any improvement in the PCE of the solar cell by the enhanced external quantum efficiency (EQE) from the down-converted photons was offset by the poor exciton or charge transport in existing thin films.…”