Low-power photon upconversion through triplet–triplet annihilation in polymers

Abstract

Low-power upconversion via sensitized triplet-triplet annihilation (TTA-UC) is a useful and versatile process that allows for the conversion of optical radiation into photons of higher energy. While this effect had been known to occur in solution for some 50 years, it was only very recently realized for the first time in polymers. The possibility to realize the TTA-UC process in solid materials is important for a variety of applications that range from the optimization of photovoltaic devices to bioimaging. This article provides a brief introduction to the field, summarizes the underlying photophysical aspects, reviews the design concepts for TTA-UC in polymers, and summarizes the recent advances in the development of such materials.

Photochemical upconversion: present status and prospects for its application to solar energy conversion

“…140 Recent studies imply that this indeed increases the UC yield in liquid solutions, 141 while earlier studies have found an increase of the TTA photon yield of a porphyrin sensitizer end-capped poly-pentaphenylene solid TTA emitter 142 as compared to a porphyrin-doped host. Although others have argued that the gain in UC efficiency in this system was minute and relate this finding to a possible exciton back-diffusion, 58 these strategies point in the right direction but require much more effort in order to realize efficient and robust TTA systems tailored for solar energy applications.…”

“…The reader is referred to the review article of Simon and Weder, who discuss the pending issues in great detail. 58 Strategies that might allow such control will be discussed below in Section 4.2.3.…”

“…57 Concerning the attainable efficiency of such blended solid-state systems, aggregation or even phase-segregation of the dye species, at concentrations above a few percent, has been found to be a serious issue which has so-far prevented the realization of higher UC quantum yields as compared to the liquid systems. 58 A strategy that might help to overcome part of this problem is the use of a polymer host which, at the same time, acts as the TTA emitter species, and is doped with only one triplet sensitizing species. Several groups have investigated these materials, including porphyrin-doped polyfluorene 59 , porphyrindoped poly-pentaphenylene 60 and porphyrin-doped superyellow.…”

Kinetically controlled crystal growth approach to enhance triplet energy migration-based photon upconversion

“…[1][2][3][4][5][6][7][8][9][10] In the typical TTA-UC process, (1) a triplet excited state of the donor is formed by intersystem crossing (ISC) from a photoexcited singlet state, (2) acceptor triplet excited states are populated by triplet energy transfer (TET) from the donor triplets, and (3) annihilation between two acceptor triplets (TTA) generates an acceptor singlet excited state, from which upconverted delayed fluorescence is emitted [ Fig. 1(a)].…”

Photochemical upconversion of light for renewable energy and more

“…[1][2][3][4][5] In PUC, the material or chemical composition is irradiated at a longer wavelength than the ensuing (delayed) fluorescence. As such, it has potential applications in areas including: Photovoltaics, in recovering below-bandgap photon losses; [6][7][8][9][10] Photoelectrochemistry, similarly generating more usable photons; [11][12][13] Biological Imaging, in circumventing autofluorescence; [14][15][16][17] Drug Activation, stimulating drug release inside the body through the tissue window; 18,19 and Water Purification, generating short wavelengths to degrade water impurites.…”