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.

The efficiency of thin-film solar cells with large optical band gaps, such as organic bulk heterojunction or amorphous silicon solar cells, is limited by their inability to harvest the (infra)red part of the solar spectrum. Photochemical upconversion based on triplet–triplet annihilation (TTA-UC) can potentially boost those solar cells by absorbing sub-bandgap photons and coupling the upconverted light back into the solar cell in a spectral region that the cell can efficiently convert into electrical current. In the present study we augment two types of organic solar cells and one amorphous silicon (a-Si:H) solar cell with a TTA-upconverter, demonstrating a solar cell photocurrent increase of up to 0.2% under a moderate concentration (19 suns). The behavior of the organic solar cells, whose augmentation with an upconverting device is so-far unreported, is discussed in comparison to a-Si:H solar cells. Furthermore, on the basis of the TTA rate equations and optical simulations, we assess the potential of TTA-UC augmented solar cells and highlight a strategy for the realization of a device-relevant current increase by TTA-upconversion.

Photon upconversion (UC) by triplet-triplet annihilation (TTA-UC) is employed in order to enhance the response of solar cells to sub-bandgap light. Here, we present the first report of an integrated photovoltaic device, combining a dye-sensitized solar cell (DSC) and TTA-UC system. The integrated device displays enhanced current under sub-bandgap illumination, resulting in a figure of merit (FoM) under low concentration (3 suns), which is competitive with the best values recorded to date for nonintegrated systems. Thus, we demonstrate both the compatibility of DSC and TTA-UC and a viable method for device integration.

Electronic excitation energy transfer (EET) rates in rylene diimide dyads are calculated using second-order approximate coupled-cluster theory and time-dependent density functional theory. We investigate the dependence of the EET rates on the interchromophoric distance and the relative orientation and show that Forster theory works quantitatively only for donor-acceptor separations larger than roughly 5 nm. For smaller distances the EET rates are over- or underestimated by Forster theory depending on the respective orientation of the transition dipole moments of the chromophores. In addition to the direct transfer rates we consider bridge-mediated transfer originating from oligophenylene units placed between the chromophores. We find that the polarizability of the bridge significantly enhances the effective interaction. We compare our calculations to single molecule experiments on two types of dyads and find reasonable agreement between theory and experiment.

Photochemical upconversion is applied to a hydrogenated amorphous silicon solar cell in the presence of a back-scattering layer. A custom-synthesized porphyrin was utilized as the sensitizer species, with rubrene as the emitter. Under a bias of 24 suns, a peak external quantum efficiency (EQE) enhancement of ∼ 2% was observed at a wavelength of 720 nm. Without the scattering layer, the EQE enhancement was half this value, indicating that the effect of the back-scatterer is to double the efficacy of the upconverting device. The results represent a figure of merit of 3.5 × 10 −4 mA cm −2 sun −2 , which is the highest reported to date.

Conventional photochemical upconversion (UC) through homo-geneous triplet-triplet annihilation (TTA) is subject to several enthalpic losses that limit the UC margin. Here, we address one of these losses: the triplet energy transfer (TET) from the sensitizer to the emitter molecules. Usually, the triplet energy level of the emitter is set below that of the sensitizer. In our system, the triplet energy level of the emitter exceeds that of the sensitizer by ∼600 cm(-1). Choosing suitable concentrations for the sensitizer and emitter molecules, we can exploit entropy as a driving force for the migration of triplet excitation from the sensitizer to the emitter manifolds. Thereby we obtain a new record for the peak-to-peak TTA-UC energy margin of 0.94 eV. A modified Stern-Volmer analysis yields a TET rate constant of 2.0 × 10(7) M(-1) s(-1). Despite being relatively inefficient, the upconverted fluorescence is easily visible to the naked eye with irradiation intensities as low as 2 W cm(-2).

A dual-emitter upconvertor is applied to thin-film solar cells for the first time, generating record figures of merit.