New antenna for a better photon harvest

Scientists from Italy’s University of Milano-Bicocca claim to have developed an organic nanomaterial which offers more effective photon-sensitized upconversion – a process by which low-energy photons can be converted into high-energy ones to offer better light harvesting in solar cells.

The paper, Engineering Porous Emitting Framework Nanoparticles with Integrated Sensitizers for Low‐Power Photon Upconversion by Triplet Fusion study, published in Advanced Materials, states the nanomaterials used – emitting porous aromatic frameworks (ePAF) – are easy to embed in PV devices due to their architecture featuring a triplet sensitizer and an annihilator/emitter. The former acts as an antenna for the absorption of low energy photons in the porous nanoparticle and the annihilator/emitter converts the photons to a high energy state.

“To date, the challenge to achieve solid-state upconverters suitable to fabricate solar devices for industrial processing, is still open since there are several difficulties, especially with regard to the organization and accessibility of the two complementary components,” the report noted.

Pioneering engineering

The combination of nanoparticles of robust ePAFs with the metallo-porphyrin sensitizers used for triplet-triplet annihilation (sTTA) enabled green-to-blue photon upconversion with a record yield of 15% in condensed-phase annihilators, the researchers claimed.

The team added that its proposed system is the first example of engineered fully organic autonomous nanoparticles for sTTA-based upconversion. “The synthetic route developed exploits optically active moieties as building blocks for fabricating a homogeneous covalent organic network with a highly controlled density of chromophores without affecting the electronic properties of the individual emitters,” the researchers wrote.

That particular configuration is said to allow better diffusion and annihilation of the triplet excitons crucial for the triplet-triplet annihilation process responsible for generating high energy photons.

“The great advantage of this system,” said research co-author Angiolina Comotti, “is the possibility of exploiting the high degree of porosity of the lattice to incorporate the desired amount of antenna molecule by controlling the composition of the final material that regulates the efficiency of the conversion process.”