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

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 [5,11-bis(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.