No abstract

Photochemical upconversion is performed, whereby emitter triplet states are produced through triplet energy transfer from sensitizer molecules excited with low energy photons. The triplet emitter molecules undergo triplet-triplet annihilation to yield excited singlet states which emit upconverted fluorescence. Experiments comparing the 560 nm prompt fluorescence when rubrene emitter molecules are excited directly, using 525 nm laser pulses, to the delayed, upconverted fluorescence when the porphyrin sensitizer molecules are excited with 670 nm laser pulses reveal annihilation efficiencies to produce excited singlet emitters in excess of 20%. Conservative measurements reveal a 25% annihilation efficiency, while a direct comparison between the prompt and delayed fluorescence yield suggests a value as high as 33%. Due to fluorescence quenching, the photon upconversion efficiencies are lower, at 16%.

Upconversion (UC) via triplet-triplet annihilation (TTA) is a promising concept to improve the energy conversion efficiency of solar cells by harvesting photons below the energy threshold. Here, we present a kinetic study of the delayed fluorescence induced by TTA to explore the maximum efficiency of this process. In our model system we find that more than 60% of the triplet molecules that decay by TTA produce emitters in their first excited singlet state, so that the observed TTA effiency exceeds 40% at the point of the highest triplet emitter concentration. This result thoroughly disproves any spin-statistical limitation for the annihilation efficiency and thus has crucial consequences for the applicability of an upconvertor based on TTA, which are discussed.SECTION Kinetics, Spectroscopy S ingle threshold photovoltaic devices suffer from their inability to harvest photons below an energy threshold. Upconversion (UC), the combination of two low energy photons into a higher energy photon, can be utilized to address this shortfall. 1 A promising concept is the usage of long-lived triplet states to store low energy quanta. 2 Lowenergy photons are absorbed by sensitizer molecules, which undergo very efficient intersystem crossing (ISC) to their triplet state T 1 upon S 1 rS 0 excitation. In the next step, the sensitizer molecules transfer their triplet energy to emitter molecules with long triplet lifetimes and large energy gaps between the first triplet state (T 1 ) and the first excited singlet state (S 1 ). Upon the encounter of two triplet emitter molecules, triplet-triplet annihilation (TTA) can result in one emitter in its ground state (S 0 ) and one in its S 1 state (Figure 1). Consequently, delayed fluorescence from the emitter S 1 state is observed. 3 When this is at shorter wavelengths than the originally absorbed light, upconversion is manifested. Since this process does not rely on the coherence of the exciting radiation, 4 it is of interest for improving the energy conversion efficiency of single threshold solar cells. Recently, several new molecular systems have been reported to undergo TTA-UC. [5][6][7][8][9][10][11][12][13] However, to be cost-effective, the upconvertor has to attain a certain efficiency. In the case of TTA as the UC process, the underlying mechanism is usually understood in terms of the complex formation between two triplet emitter molecules 3 M * . [14][15][16][17] As a consequence of the tensor product of the initial spin states of the molecules, the encounter complex can be of singlet, triplet, or quintet multiplicity: [14][15][16][17] 3 M Ã þ 3 M Ã

Single-threshold solar cells are fundamentally limited by their ability to harvest only those photons above a certain energy. Harvesting below-threshold photons and re-radiating this energy at a shorter wavelength would thus boost the efficiency of such devices. We report an increase in light harvesting efficiency of a hydrogenated amorphous silicon (a-Si:H) thin-film solar cell due to a rear upconvertor based on sensitized triplet-triplet-annihilation in organic molecules. Low energy light in the range 600 − 750 nm is converted to 550 − 600 nm light due to the incoherent photochemical process. A peak efficiency enhancement of (1.0 ± 0.2)% at 720 nm is measured under irradiation equivalent to (48 ± 3) suns (AM1.5). We discuss the pathways to be explored in adapting photochemical UC for application in various single threshold devices.