Electronic State Engineering in Perovskite‐Cerium‐Composite Nanocrystals toward Enhanced Triplet Annihilation Upconversion

Abstract Wavelength conversion based on hybrid inorganic–organic sensitized triplet–triplet annihilation upconversion (TTA‐UC) is promising for applications such as photovoltaics, light‐emitting‐diodes, photocatalysis, additive manufacturing, and bioimaging. The efficiency of TTA‐UC depends on the population of triplet excitons involved in triplet energy transfer (TET), the driving force in TET, and the coupling strength between the donor and acceptor. Consequently, achieving highly efficient TTA‐UC necessitates the precise control of the electronic states of inorganic donors. However, conventional covalently bonded nanocrystals (NCs) face significant challenges in this regard. Herein, a novel strategy to exert control over electronic states is proposed, thereby enhancing TET and TTA‐UC by incorporating ionic‐bonded CsPbBr3 and lanthanide Ce3+ ions into composite NCs. These composite‐NCs exhibit high photoluminescence quantum yield, extended single‐exciton lifetime, quantum confinement, and uplifted energy levels. This engineering strategy of electronic states engendered a comprehensive impact, augmenting the population of triplet excitons participating in the TET process, enhancing coupling strength and the driving force, ultimately leading to an unconventional, dopant concentration‐dependent nonlinear enhancement of UC efficiency. This work not only advances fundamental understanding of hybrid TTA‐UC but also opens a door for the creation of other ionic‐bonded composite NCs with tunable functionalities, promising innovations for next‐generation optoelectronic applications.


Synthesis of NCs-cerium composite with different sizes
Preparation of cesium oleate solution: Cs2CO3 (0.814 g) was loaded into 250 mL 3neck flask along with ODE (30 mL) and oleic acid (2.5 mL), dried for 1 h at 120 °C, and then heated under nitrogen atmosphere to 150 °C until all Cs2CO3 reacted.The solution was kept at 100 °C before injection.

Preparation of precursor solution:
First put the mixture of PbBr2 (0.188mmol) and ODE (5ml) into a 25mL three-necked flask and degas for 60 minutes.At the same time, the OAm needed for the experiment was also degassed until it was added to the precursor solution.Different molar ratios of CeBr3 (0-400% to that of PbBr2) were dissolved in the OA solution in advance and stirred at 70°C until completely dissolved.
The precursor solution was prepared by injecting pre-dissolved CeBr3: OA solution (1 ml) and degassed OAm (1 ml) into a 25 ml three-neck flask containing PbBr2 mixed with ODE and reacted at 120°C for 10 minutes under nitrogen atmosphere.It should be noted that due to the solubility of CeBr3, precipitation will occur with the increase of the molar ratio, which will make it difficult to control the doping concentration of the final synthesized NCs.To ensure sufficient dissolution, OA and OAm were increased to 2ml when feeding CeBr3 higher than 150% molar ratios.

Hot injection reaction:
The reaction was finally carried out by hot injection.After all the precursors were completely dissolved, the precursor solution was rapidly heated to 190°C under nitrogen atmosphere with vigorous stirring.Take 0.4 ml of Cs-oleate solution and quickly inject it into the vigorously stirred precursor solution, and it can be observed that green luminescent perovskite NCs are rapidly generated.After maintaining the temperature for 1 min, put the three-necked flask into a cold-water bath.
The reaction was quenched, and the reaction was completed when the temperature dropped below 30°C.
Purification: the crude solution was centrifuged at 7000 rpm for 5 min to remove the unreacted salts.Both precipitate and supernatant were retained and were subjected to subsequent purification respectively.This is because as the molar ratio of doping increases, the size distribution of nanocrystals will shift to small sizes, resulting in a decrease in the concentration of large-sized nanocrystals that can be generated, and the extraction of smaller-sized NCs need additional steps.The collected precipitate was dispersed with 5 ml of toluene, and then centrifuged at 12,000 rpm for 5 minutes, and the obtained supernatant was the larger size NCs synthesized at this temperature.It should be noted that when feeding at 0-100% molar ratio, the content of localized quantum dot is negligible.Then, the supernatant of the crude solution was added to ~8ml of methyl acetate until the solution was slightly crude, then centrifuged at 10,000 rpm for 5 minutes to get the precipitate and redispersed into ~5ml of toluene solution.
The resulting solution was perovskite NCs with stronger quantum confinement.
Ligand exchange on NCs: The process of replacing the native OA ligand with PCA transmitters was carried out in a glovebox.For samples with varying dopant concentrations, 100 μL of the original solution was diluted with dry toluene to achieve an absorbance of approximately 0.2 in the cuvette with 10 mm optical path.Subsequently, 2 mL of this solution was mixed with 2 mg of PCA and stirred at room temperature overnight.Insoluble ligands were removed using a 0.22-micron syringe filter.Then, 80 μL of DPA toluene solution (10 mg/mL) was added to obtain the final upconversion samples, which were transferred to hermetically sealed cuvettes for analysis.

Characterizations
TEM images were acquired on a Hitachi HT-7700 transmission electron microscope with an accelerating voltage of 120 kV.The X-ray diffraction results were recorded using Rigaku Smartlab X-ray diffractometer equipped with 9kV Cu Kα source.The ultraviolet−visible (UV−vis) absorption spectra were collected by a UV-Vis-NIR spectrometer (UH-5700, Hitachi, Japan).The absolute PLQY of NCs-cerium composite toluene solutions was obtained by a UV-NIR absolute photoluminescence quantum yield spectrometer (Quantaurus-QY Plus C13534-12, Hamamatsu Photonics, Japan), where the error is within 1%, all solutions were properly diluted to ensure little reabsorption effect.The ultraviolet photoelectron spectra (UPS) were carried out with a 21.2 eV He-I source and a −9.8 eV bias.Samples for UPS were prepared by drop casting doped and undoped NCs on a 10 mm × 10 mm ITO substrate.The ultrafast transient absorption (TA) uses a femtosecond amplified Ti:sapphire laser source with pulse duration of ∼100 fs and repetition rate of 1 kHz.The laser output is divided into two parts, and the major part is used to generate pump pulses at 450 nm (8μJ/cm 2 ) in an optical parametric amplifier.The pump beam is modulated at 500 Hz by a mechanical chopper.Another part of the laser is used to generate white light through a CaF2 crystal plate.The white light can be focused onto the sample by an off-axis parabolic mirror and detected by a silicon detector.The time delay between pump and probe beams is controlled by a motorized stage.The PL decay curves were monitored at the PL center wavelength of each sample using FL920 fluorescence spectrometer (Edinburgh Instruments, UK) under ambient conditions with 470 nm picosecond laser excitation.The data collected was based on time-correlated single photon counting (TCSPC).The excitonic absorptions of all the TTA-UC solution samples were controlled around ~0.2 (10 mm optical path).PL lifetime is obtained by double exponential fitting.All the element ratios were determined using an Agilent 720ES inductively coupled plasma−optical emission spectrometer (ICP−OES).The TTA-UC upconversion measurement was carried out with a home-made upconversion measurement system with a power-tunable cw laser of 450 nm.The absolute and filtered fluorescence (450 short-pass filter) were collected by an Ocean-Optics USB2000+ spectrometer.All the excitation thresholds were extracted through powerdependent upconversion fluorescence measurements.The photo of upconversion fluorescence was taken by mobile phone through a 425nm short-pass filter.
The efficiency and excitation threshold are analyzed by integrating the DPA fluorescence spectra.Since the upconverted emission of DPA is a linear optical process, the effective range over which the upconverted emission can be integrated is from 400 nm to ~440 nm.By comparing the DPA spectrum excited by down-conversion, the spectral integral intensity in this interval plus the loss of filter transmittance accounts for 52.5% of the overall spectral intensity.As a result, the recovered integrated upconversion intensity was used to calculate the TTA-UC efficiency and excitation threshold.For the estimation of the TTA-UC efficiency, the following equation was used: where F, A, I, and η represent the quantum yield, absorbance at the excitation wavelength, integrated photoluminescence spectral profile, excitation intensity, and the refractive index of the solvent, respectively.Subscripts std and UC are Coumarin 307 and sample, respectively.Coumarin 307 was used as the reference whose PL quantum yield is 75% in ethanol.To better compare with the upconversion efficiency reported in recent studies, a pre-factor 2 is not introduced, so that the mentioned TTA-UC efficiency in the main text should refer to the unnormalized upconversion efficiency.As discussed above and in the main text, for the CeBr3 feeding ratio lower than 100%, there is no quantum confinement and the size change of NCs is not significant.ICP-MS results showed that the Ce/(Pb+Ce) molar ratio is much lower than that using more than 150% CeBr3 feeding.Thus, the decrease of TTA-UC should be attributed to the effects of passivation caused by excess Br -, indicating that the low concentration of the doped Ce 3+ ions and high concentration of Br -ions may be one of the reasons that lead to the low efficiency of TTA-UC at the doping concentration of 50%.Although surface passivation brought by Br -and Ce 3+ ions are beneficial for radiative recombination, we clarify that the Br -passivation may negatively influence on TTA-UC.In contrast, Ce 3+ is much more difficult to incorporate into the CsPbBr3 lattice than Br -, which is why Ce concentration rises significantly and positively influences the TTA-UC after doping with 150% or more CeBr3.Therefore, we believe that further doping NCs with over 400% CeBr3-PbBr2 should not be a wise choice for enhancing TTA-UC.(c) XRD pattern of 400Ce-NC, where an obvious peak at 11.3° and 13.5° can be detected.This may relate to Cs3CeBr6 reported previously [3] .

Figure S4 .
Figure S4.Illustration of band structure determined by UPS.HOMO level of AMP was also shown because AMP was selected as hole-transfer acceptor.

Figure
Figure S6.(a) sharp absorption peak can be observed in 400Ce-NC below 400 nm.We have experimentally checked this absorption peak and it originated from the excess CeBr3.After washing, the optical density of this peak can be reduced.(b) This electronic transition is harmful to both PLQY and TTA-UC due to the strong re-absorption.

Figure S7 .Figure
Figure S7.Auger effect in the early time in TA, and the relative transient kinetics fitting for (a) 150Ce-NC, and (b) 300Ce-NC.

Table S1 .
Fitting parameters of time-resolved PL lifetime from CsPbBr3 NCs with different CeBr3 doping mole ratios.

Table S2 .
Single-exciton emission lifetime fitting.The fitting lifetime increases with higher doping ratio.These fittings were conducted by removing the early 100ps kinetics in transient absorption, which was believed that the multiexciton Auger process had completely decayed.The resulting kinetics can be fit by single exponential, representing the single-exciton lifetime.