High Photon Upconversion Efficiency with Hybrid Triplet Sensitizers by Ultrafast Hole‐Routing in Electronic‐Doped Nanocrystals

Abstract

Low‐power photon upconversion (UC) based on sensitized triplet–triplet annihilation (sTTA) is considered as the most promising upward wavelength‐shifting technique to enhance the light‐harvesting capability of solar devices. Colloidal nanocrystals (NCs) with conjugated organic ligands have been recently proposed to extend the limited light‐harvesting capability of molecular absorbers. Key to their functioning is efficient energy transfer (ET) from the NC to the triplet state of the ligands that sensitize free annihilator moieties responsible for the upconverted luminescence. The ET efficiency is typically limited by parasitic processes, above all nonradiative hole‐transfer to the ligand highest occupied molecular orbital (HOMO). Here, a new exciton‐manipulation approach is demonstrated that enables loss‐free ET by electronically doping CdSe NCs with gold impurities that introduce a hole‐accepting intragap state above the HOMO energy of 9‐anthracene acid ligands. Upon photoexcitation, the NC photoholes are rapidly routed to the Au‐level, producing a long‐lived bound exciton in perfect resonance with the ligand triplet. This hinders hole‐transfer leading to ≈100% efficient ET that translates into an upconversion quantum yield as high as ≈12% (≈24% in the normalized definition), which is the highest performance for NC‐based upconverters based on sTTA to date and approaches the record efficiency of optimized organic systems.

Ultra-small PbS nanocrystals as sensitizers for red-to-blue triplet-fusion upconversion

“…42,78 However such trapping is not unique to PbS, 76,79 nor it is exclusively observed in ultra-small particles, 64 so we do not consider that its presence is the primary origin of the slow ($5.4 ms) TET dynamics that we observe. Still, ongoing 77,80 and future work to reveal the chemical nature of electronic trap states in these materials may establish whether a sequential mechanism would have stronger coupling or an overall efficiency benet.…”

Ultra-small PbS nanocrystals as sensitizers for red-to-blue triplet-fusion upconversion

“…41,76 However such trapping is not unique to PbS, 74,77 nor it is exclusively observed in ultra-small particles, 62 so we do not consider that its presence is the primary origin of the slow (∼5.4 μs) TET dynamics that we observe. Still, ongoing 75,78 and future work to reveal the chemical nature of electronic trap states in these materials may establish whether a sequential mechanism would have stronger coupling or an overall efficiency benefit. Closer inspection of the TRPL trace with our model provides additional support that TET from PbS to CPA is endothermic in this system.…”

Application of Triplet–Triplet Annihilation Upconversion in Organic Optoelectronic Devices: Advances and Perspectives

“…According to these above requirements, many materials including heavy-metal complexes, organic dyes (including TADF materials), quantum dots (QDs), and perovskite have been successfully developed as triplet sensitizers for efficient TTA upconversion. [94,[119][120][121][122][123][124][125][126][127][128] Charge transfer states between electron donor and acceptor moieties can also act as sensitizers. [129,130] For an efficient TTA upconversion system, the annihilators should meet below requirements: 1) a singlet energy level at just below two times of the first excited triplet state energy level; 2) a long triplet lifetime; 3) the second triplet excited-state should not be energetically accessible from the TTA event to ensure high TTA yields.…”