Quasi-thresholdless Photon Upconversion in Metal–Organic Framework Nanocrystals

Abstract

Photon upconversion based on sensitized triplet−triplet annihilation (sTTA) is considered as a promising strategy for the development of light-managing materials aimed to enhance the performance of solar devices by recovering unused low-energy photons. Here, we demonstrate that, thanks to the fast diffusion of excitons, the creation of triplet pairs in metal−organic framework nanocrystals (nMOFs) with size smaller than the exciton diffusion length implies a 100% TTA yield regardless of the illumination condition. This makes each nMOF a thresholdless, single-unit annihilator. We develop a kinetic model for describing the upconversion dynamics in a nanocrystals ensemble, which allows us to define the threshold excitation intensity I th box required to reach the maximum conversion yield. For materials based on thresholdless annihilators, I th box is determined by the statistical distribution of the excitation energy among nanocrystals. The model is validated by fabricating a nanocomposite material based on nMOFs, which shows efficient upconversion under a few percent of solar irradiance, matching the requirements of real life solar technologies. The statistical analysis reproduces the experimental findings, and represents a general tool for predicting the optimal compromise between dimensions and concentration of nMOFs with a given crystalline structure that minimizes the irradiance at which the system starts to fully operate.

Nanostructured Polymers Enable Stable and Efficient Low‐Power Photon Upconversion

“…When two emitter triplets are simultaneously created in such a confined space, they can decay only by annihilation with a resulting TTA efficiency of 100% [ 25 ] and potentially reach the maximum conversion efficiency at excitation intensities lower than in the classical bulk counterpart. [ 6 ] The occurrence of confined‐TTA in the nanostructured host is demonstrated by the time‐resolved PL data shown in the inset of Figure 3b. In contrast to the bulk‐TTA regime, where the excitation intensity determines k TTA and the UC‐PL signal decay dynamics, [ 14 ] no changes in the UC‐PL time‐resolved spectrum are observed when I exc is varied by two orders of magnitude around I th .…”

“…Specifically, it is worth noting that the QY uc versus I exc behavior in our new nanophase‐separated polymers differs from the one of previously reported nano‐upconverters, which strictly follows a binomial distribution of the excited states (Figure 3b, dashed line). [ 6 ] This discrepancy can be ascribed to the fact that the polymer nanostructuring is a homogenous process, which results in a log‐normal distribution of the liquid domain size. [ 26 ] In order to analyze the QY uc dependence on I exc , and to extrapolate the size distribution of the liquid domains in the polymer matrix, we analyzed the power‐dependent efficiency data with a statistical approach.…”

“…This enables both hopping‐assisted ET and TTA and, importantly, an effective localization and confinement of interacting excitons, which has a direct impact on the s TTA‐UC kinetics enhancing the upconversion performance at low power densities of the nanostructured polymer with respect to its homogeneous counterpart. [ 6 ] The new upconverting nanomaterials show a remarkably high optical quality, an outstanding upconversion quantum yield QY uc of ≈23%, and excellent stability in air, which is a prerequisite for the development of upconversion‐based technologies.…”

Photon upconverting bioplastics with high efficiency and in-air durability

“…However, fabricating efficient solidstate TTA-UC materials faces several challenges, such as aggregation-induced quenching of emitter fluorescence and triplet quenching by molecular oxygen in air. 45 Various solidstate TTA-UC materials developed to date include polymers doped with dye molecules, [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] dye nano/microparticles, [46][47][48] dye-bridged metal-organic frameworks (MOFs), [49][50][51][52][53][54][55][56] dye-based glasses, 57,58 and crystals. [59][60][61][62] Among these, the molecular diffusion (MD) based TTA-UC in soft synthetic polymers has been a simple and effective approach to achieve high TTA-UC efficiency even in air.…”

Triplet–Triplet Annihilation Upconversion in a MOF with Acceptor‐Filled Channels

“…Other examples combineM OF annihilators with external mobile sensitizers. [27] In this study,w ec ombine the sensitizer andt he DPAa nnihilator within ac rystalline mesoporous MOF using an inverted design, namely immobilizing palladium porphyrin linkers Pd(TCPP) as sensitizers in the channel walls and placing the mobile DPAi nt he channels of the MOF (Figure 1b). To provide an optimum microenvironment for DPAe nabling efficient TTET and TTAp rocesses, the channels of the chosen MOF with Zr 6 (m 3 -OH) 8 (OH) 8 SBUs and PCN-222 structure [28] were modified with caprylic acid (CA;o ctanoic acid) acting as as olventf or DPA( Figure 1b (7) ; c = 17.143(2) [28b] ).…”