Nature Photonics: Research on near-infrared photon up-conversion and solar light synthesis

Wu Kaifeng research team of Dalian Institute of Chemical Physics, Chinese Academy of Sciences has made important progress in the photochemical research of quantum dots, taking the lead in realizing the efficient upconversion of near-infrared to visible light sensitized by low-toxicity quantum dots, and integrating this system with organic photocatalysis to achieve efficient and rapid solar synthesis, Relevant research was published in Nature Photonics with the title of "Near-infrared photo upconversion and solar synthesis using lead-free nanocrystals".

Image source: Visual China

Photon conversion from near infrared (NIR) to visible light has broad application prospects. In this paper, researchers reported that Zn-doped CuInSe2 nanocrystals, as a low-cost and lead-free substitute, realized the upconversion from near-infrared to visible light, and the external quantum efficiency reached 16.7%. When directly combined with photoreduction catalysis, the system can realize efficient near-infrared driven organic synthesis and polymerization, thus solving the problem of reabsorption loss of sensitized upconversion of nanocrystals. In addition, the broadband light capture of these nanocrystals can react very quickly in the indoor sunlight. Expanding the scope of "solar synthesis" to near infrared may realize the dream of using sunlight for high-value-added chemical transformation for a century.

Sensitized triplet fusion, or triplet-triplet annihilation photon upconversion (TTA-UC), is a practical way to upconvert incoherent continuous wave photons, such as solar photons. In such a system, the photosensitizer captures the incident low-energy photons, and then transfers their excited state energy to the molecular triplet. Through the fusion of two triplets, a high-energy photon is emitted. Over the years, photochemical scientists have developed various photosensitizers, including but not limited to organic molecules, organometallic complexes, colloidal nanocrystals (NCs) and lead halide perovskite films or organic heterojunction films. However, materials that can sensitize near-infrared to visible upconversion are not only scarce, but also have their own defects. Organic metal complexes with this ability usually contain noble metals, such as platinum (Pt), palladium (Pd) and osmium (Os). Pure organic near-infrared sensitizers usually lead to very low upconversion efficiency. The lead halide perovskite film is highly toxic and has limited spectral response in the near infrared region (starting at~750 nm). Colloidal NCs are more diversified in principle in terms of material composition and spectral adjustability; However, up to now, the only NCs family that can sensitize near-infrared to visible TTA-UC is lead sulfide compounds.

Copper indium selenide (CuInSe2) NCs and their derivatives have been widely studied as substitutes for lead sulfide compounds. Previously, these NCs have been successfully applied to solar cells and light-emitting solar concentrators. In this paper, researchers used Zn-doped and ZnS coated CuInSe2 NCs (ZCISe NCs) as three-state photosensitizers to sensitize TTA-UC from near-infrared to visible light.

Characterization of ZCISe NCs

These NCs adopt chalcopyrite phase and are coated with 0.6 nm thick ZnS to form ZCISe/ZnS NCs with an average diameter of 4.0 nm (Fig. 1a). It improves the stability of nc and enables them to TET. In addition, ZnS can be used as an encapsulation layer to prevent elements from leaking from the core, thus making NCs more environmentally friendly.

Figure 1: Excited state dynamics and energy transfer of ZCISe NCs.

Upconversion from near infrared to visible light
On the basis of ZCISe NCs' highly effective three-state sensitization of TCA, the researchers further added rubrene molecules to extract the three-state energy from TCA ligands for near-infrared to visible TTA-UC. As shown in Figure 2. As mentioned at the beginning, the 16.7% efficiency here is an "external" efficiency, which is attenuated by the reabsorption of TTA-UC system itself, especially by ZCISe NCs. The upconversion of near-infrared photoreduction catalyst is shown in Figure 3.

Figure 2: Near infrared to visible TTA-UC.

Figure 3: Photooxidation and reduction driven by near-infrared and solar energy using TTA-UC.

As shown in Figure 4a, in the presence of TMPTA and additives, the polymer gel can be completely formed in 12 minutes when the up conversion system of researchers is irradiated with 808 nm laser. The control experiment conducted without TCA showed that ZCISe NCs did not form polymer gel.

Figure 4: Near infrared and solar-driven photopolymerization using TTA-UC.

The excellent performance of the up-conversion photoreduction system studied by the researchers in the sunlight can be attributed to the panchromatic absorption of ZCISe NCs, which starts at near infrared. This is fundamentally different from the previous up-conversion-photoredox systems that use Pd or Pt as sensitizers. These systems have only narrow absorption bands in the near-infrared region and very weak absorption in the visible region. The single state produced by the three-state annihilation is in the form of organic synthesis rather than in the form of photon emission, which can effectively alleviate the photon reabsorption loss especially unfavorable to NCs-sensitized TTA-UC. Based on these considerations, the combination of lead-free NCs sensitized upconversion and photoredox catalysis provides a new research direction for solar synthesis.

Article source: https://www.nature.com/articles/s41566-023-01156-6