Efficient NIR-to-Visible Upconversion of Surface-Modified PbS Quantum Dots for Photovoltaic Devices

Abstract

Conversion of low-energy (infrared) photons into high-energy (visible) photons has potential application in the field of solar photovoltaics to boost up the power conversion efficiency beyond the Shockley−Queisser limit. In this paper, we investigated the near-infrared photon upconversion (UC) based on triplet−triplet annihilation (TTA) of direct-attaching emitter [5,11bis(triethylsilylethynyl)anthradithiophene, TES-ADT] to PbS quantum dots (QDs) with three different lengths of ligand passivating the PbS QD. Upconversion quantum yield (UC-QY) was measured for the mixture solution of QDs and emitter, which was found to be maximized at the middle-length ligand with the values of 0.29% at 785 nm excitation and 0.06% at 975 nm excitation. Measurements of QD emission decay and UC emission rise indicated that triplet energy transfer (TET) from QDs to emitter accelerates with the shorter ligand. Meanwhile, it was found that the ligand-exchange process creates a trap state that is inert for TET in PbS QDs and only a fraction of PbS QDs could contribute to TET and then upconversion. Competition between the acceleration of TET and quenching of the excited state causes the optimization of UC-QY at the middle-length ligand. Furthermore, a model is proposed to illustrate the complex behavior with the ligand-exchanged PbS QDs in context to the photon upconversion.

Near-infrared-to-visible upconversion from 980 nm excitation band by binary solid of PbS quantum dot with directly attached emitter

“…The result shows that efficient TET is possible by the transmitter mechanism, where the TES-ADT is attached to the QD surface and acts as transmitter of triplet energy. Moreover, it is also noted here that the triplet decay time constant (t D ) of TES-ADT in the cast solid is in the range 2-5 ms, which is also much faster than the reported values in solution This journal is © The Royal Society of Chemistry 2022 system (10-20 ms), 42 probably due to the stronger intermolecular interaction in the solid.…”

“…Previously, TES-ADT was demonstrated as a directly attached emitter in solution system. 40,42 Two mechanisms are proposed to explain how energy is transferred from the bound TES-ADT on the QD surface to another TES-ADT diffusing in solution (free TES-ADT) after the bound one receives the energy from the QD. One is the transmitter mechanism, where transfer occurs when a free TES-ADT collides with the bound TES-ADT on the QD surface.…”

“…The I th values found here were 2-6 W cm À2 and one order of magnitude lower than the solution system of PbS QD/TES-ADT. 42 The low I th value in solid compared to the solution shows the solid has potential for operation under weaker light and is preferable for application. I th is formulated as 11,12,43…”

“…The TET time constant (t) is faster than 200 ns for all PbS QD:TES-ADT samples, which is one order of magnitude faster than the natural (i.e., not quenched) decay time constant of PbS QD (t 0 B3 ms). 42 The TET quantum yield is estimated from the convention F TET = 1 À t/t 0 B93%. The result shows that efficient TET is possible by the transmitter mechanism, where the TES-ADT is attached to the QD surface and acts as transmitter of triplet energy.…”

“…The I th values found here were 2–6 W cm −2 and one order of magnitude lower than the solution system of PbS QD/TES-ADT. 42 The low I th value in solid compared to the solution shows the solid has potential for operation under weaker light and is preferable for application. I th is formulated as 11,12,43 I th ∝ ( α Φ ISC Φ TET ) −1 ( k 2 3 A / k 2 ),where α is the absorption coefficient of the sensitizer and Φ ISC and Φ TET are quantum yield of the intersystem crossing (ISC) of the sensitizer (can be taken as unity for PbS QD) and TET, respectively.…”