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Enhanced Triplet–Triplet Annihilation Upconversion in Dual-Sensitizer Systems: Translating Broadband Light Absorption to Practical Solid-State Materials

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journal contribution
posted on 19.12.2016, 21:30 by Anna L. Hagstrom, Fan Deng, Jae-Hong Kim
Photochemical upconversion (UC) of low-energy photons that would otherwise be wasted could drastically improve the efficiency of solar technologies by allowing them to harness a greater fraction of the solar spectrum. Although UC through the triplet–triplet annihilation (TTA) mechanism operates efficiently under low-power irradiation such as sunlight, its ability to improve solar device efficiencies is limited by the narrow light absorption bands of its sensitizer chromophores. This bottleneck on UC performance can be overcome by employing multiple sensitizers in tandem, but such an approach has thus far been studied exclusively in solution-based TTA-UC systems requiring intensive deoxygenation and sealing procedures. This study presents the first dual-sensitizer TTA-UC system in a solid-state host suitable for practical applications. We fabricate thin polyurethane films containing two benchmark TTA-UC sensitizers in a range of different concentrations and characterize their red-to-blue and green-to-blue UC performance as a function of excitation intensity. The broadband absorption of the dual-sensitizer films significantly enhances their performance under simultaneous low-intensity excitation of the two sensitizers, giving rise to anti-Stokes fluorescence surpassing the combined anti-Stokes fluorescence of the films’ single-sensitizer analogues. We circumvent trade-offs between light absorption and TTA-UC performance at high sensitizer concentrations by harnessing the films’ unique versatility to produce an alternative “multijunction” TTA-UC system comprising overlaid single-sensitizer films, thereby achieving strong broadband light absorption and superior TTA-UC performance.