Jacopo Pedrini

Reversed oxygen sensing using colloidal quantum wells towards highly emissive photoresponsive varnishes

Colloidal quantum wells combine the advantages of size-tunable electronic properties with vast reactive surfaces that could allow one to realize highly emissive luminescent-sensing varnishes capable of detecting chemical agents through their reversible emission response, with great potential impact on life sciences, environmental monitoring, defence and aerospace engineering. Here we combine spectroelectrochemical measurements and spectroscopic studies in a controlled atmosphere to demonstrate the ‘reversed oxygen-sensing’ capability of CdSe colloidal quantum wells, that is, the exposure to oxygen reversibly increases their luminescence efficiency. Spectroelectrochemical experiments allow us to directly relate the sensing response to the occupancy of surface states. Magneto-optical measurements demonstrate that, under vacuum, heterostructured CdSe/CdS colloidal quantum wells stabilize in their negative trion state. The high starting emission efficiency provides a possible means to enhance the oxygen sensitivity by partially de-passivating the particle surfaces, thereby enhancing the density of unsaturated sites with a minimal cost in term of luminescence losses.

Unraveling Triplet Excitons Photophysics in Hyper-Cross-Linked Polymeric Nanoparticles: Toward the Next Generation of Solid-State Upconverting Materials

The technological application of sensitized upconversion based on triplet-triplet annihilation (TTA) requires the transition from systems operating in liquid solutions to solid-state materials. Here, we demonstrate that the high upconversion efficiency reported in hyper-cross-linked nanoparticles does not originate from residual mobility of the embedded dyes as it happens in soft hosts. The hyper-reticulation from one side blocks the dyes in fixed positions, but on the other one, it suppresses the nonradiative spontaneous decay of the triplet excitons, reducing intramolecular relaxations. TTA is thus enabled by an unprecedented extension of the triplet lifetime, which grants long excitons diffusion lengths by hopping among the dye framework and gives rise to high upconversion yield without any molecular displacement. This finding paves the way for the design of a new class of upconverting materials, which in principle can operate at excitation intensities even lower than those requested in liquid or in rubber hosts.

Permanent excimer superstructures by supramolecular networking of metal quantum clusters

Long-life excimer-like structures Metal quantum clusters have ideal properties for medical applications such as imaging. The challenge is to prolong their transient properties for the fabrication of useful devices. Santiago-Gonzalez et al. arranged gold clusters in a supramolecular lattice held together by hydrogen bonding and showed that this material can be used for imaging of fibroblast cells. In the superstructure, the gold molecules can come together in the excited state as excimers and then dissociate to emit radiation. Because they are within a lattice, this behavior shows long-term stability. Furthermore, the lattice superstructure scavenges reactive oxygen species and reduces cell damage. Science, this issue p. 571 Excimer-like superstructures that emerge from hydrogen bond networking of gold clusters can scavenge reactive oxygen species. Excimers are evanescent quasi-particles that typically form during collisional intermolecular interactions and exist exclusively for their excited-state lifetime. We exploited the distinctive structure of metal quantum clusters to fabricate permanent excimer-like colloidal superstructures made of ground-state noninteracting gold cores, held together by a network of hydrogen bonds between their capping ligands. This previously unknown aggregation state of matter, studied through spectroscopic experiments and ab initio calculations, conveys the photophysics of excimers into stable nanoparticles, which overcome the intrinsic limitation of excimers in single-particle applications—that is, their nearly zero formation probability in ultra-diluted solutions. In vitro experiments demonstrate the suitability of the superstructures as nonresonant intracellular probes and further reveal their ability to scavenge reactive oxygen species, which enhances their potential as anticytotoxic agents for biomedical applications.

Role of Nonradiative Defects and Environmental Oxygen on Exciton Recombination Processes in CsPbBr3 Perovskite Nanocrystals

Lead halide perovskite nanocrystals (NCs) are emerging as optically active materials for solution-processed optoelectronic devices. Despite the technological relevance of tracing rational guidelines for optimizing their performances and stability beyond their intrinsic resilience to structural imperfections, no in-depth study of the role of selective carrier trapping and environmental conditions on their exciton dynamics has been reported to date. Here we conduct spectro-electrochemical (SEC) experiments, side-by-side to oxygen sensing measurements on CsPbBr3 NCs for the first time. We show that the application of EC potentials controls the emission intensity by altering the occupancy of defect states without degrading the NCs. Reductive potentials lead to strong (60%) emission quenching by trapping of photogenerated holes, whereas the concomitant suppression of electron trapping is nearly inconsequential to the emission efficiency. Consistently, oxidizing conditions result in minor (5%) brightening due to suppressed hole trapping, confirming that electron traps play a minor role in nonradiative decay. This behavior is rationalized through a model that links the occupancy of trap sites with the position of the NC Fermi level controlled by the EC potential. Photoluminescence measurements in controlled atmosphere reveal strong quenching by collisional interactions with O2, which is in contrast to the photobrightening effect observed in films and single crystals. This indicates that O2 acts as a scavenger of photoexcited electrons without mediation by structural defects and, together with the asymmetrical SEC response, suggests that electron-rich defects are likely less abundant in nanostructured perovskites than in the bulk, leading to an emission response dominated by direct interaction with the environment.

Recent advances in the application triplet–triplet annihilation-based photon upconversion systems to solar technologies

Abstract. Upconversion (UC) of low-energy photons into high-energy light can in principle increase the efficiency of solar devices by converting photons with energies below the energy absorption threshold into radiation that can be utilized. Among UC mechanisms, the sensitized triplet–triplet annihilation-based upconversion (sTTA-UC) is the most recent that would be applied in solar technologies. sTTA-UC was demonstrated using sunlight in 2006, and due to its high efficiency at low excitation intensities with noncoherent light, it is considered a promising strategy for the recovering of subbandgap photons. Here, we describe briefly the working principle of the sTTA-UC, and we review the most recent advances of its use in solar applications.

Cascade sensitization of triplet–triplet annihilation based photon upconversion at sub-solar irradiance

In triplet-triplet annihilation based upconversion, high-energy photons are generated through the annihilation of fluorophore triplets, populated via energy transfer from a light-harvesting sensitizer. However, the absorption band of common sensitizers is narrow, limiting the fraction of recoverable photons. We overcome this issue using a third chromophore as an additional light-harvester in the transparency window between the upconverted luminescence and the sensitizer absorption. The third component transfers the extra-collected energy to sensitizers, realizing a cascade-sensitized upconversion that shows a 20% increment of the high-energy photon output and a conversion yield of 10% at solar irradiance.

Broadband sensitized photon up-conversion at subsolar irradiance(Conference Presentation)

A crucial limit of solar devices is their inability to harvest the full solar spectrum. Currently, sensitized up-conversion based on triplet-tripled annihilation (STTA-UC) in bi-component organic systems is the most promising technique to recover sub-bandgap photons, showing good efficiencies also at excitation intensities comparable to the solar irradiance. In STTA-UC, high-energy light is generated through annihilation of metastable triplet states of molecules acting as emitters, which are populated via resonant energy transfer from a light-harvesting sensitizer. However, suitable sensitizers show narrow absorption bands, limiting the fraction of recoverable photons, therefore preventing the application of STTA-UC to real-world devices. Here we demonstrate how to overcome the described limit by using multiple sensitizers that work cooperatively to broaden the overall system absorption band. This is obtained using an additional sensitizer that transfers the extra harvested energy to the main one (sensitization of the sensitizer), or a set of properly designed complementary absorbing sensitizers all able to excite simultaneously the same emitter (multi-sensitizers). In both cases STTA-UC performances result strongly enhanced compared to the corresponding mono-sensitizer system, increasing the up-converted light intensity generated at AM 1.5 up to two times. Remarkably, by coupling our light converters to a DSSC we prove its operation by exploiting exclusively sub-bandgap photons. A detailed modeling of the photophysical processes involved in these complex systems allows us to draw the guidelines for the design of the next generation STTA-UC materials, encouraging their application to photovoltaic technologies.