Harnessing Sunlight via Molecular Photon Upconversion

Abstract

Molecular photon upconversion via triplet−triplet annihilation (TTA-UC) is an intriguing strategy to harness sub-bandgap photons and surpass the Shockley−Queisser (SQ) limit for solar energy conversion. In this perspective, we briefly summarize the progress to date harnessing TTA-UC in solar cells using both optically and electrically coupled schemes. We then highlight the efficiency limiting processes for these schemes and outline possible paths toward upconverted photocurrent contributions of >1 mA/cm 2 . Further progress in red-shifting absorption, coupling to high-energy light harvesting motifs, photon management, sensitizer/ annihilator design, and more are necessary for the realization of a viable TTA-UC solar cell that can pass the SQ limit.

Recent advances in materials for and applications of triplet–triplet annihilation-based upconversion

“…[108][109][110] According to recent research, harnessing TTA-UC has made a lot of progress in applications to solar cells by optical or electrical coupling. 111 Both approaches have the advantages of long triplet lifetimes and appropriate energy matching between new sensitizers with long triplet lifetimes and a new annihilator with high TTA yields but it needs further improvements due to device-relevance thresholds and commercial applications. 112 To optimize hybrid research of TTA-UC and solar cells, it should focus on minimizing the reverse energy transfer from the annihilator to the sensitizer and maximizing the photon output coupling yield from the UC film to the solar cell beyond the high-efficiency UC quantum efficiency.…”

Harnessing near-infrared lightviaS0to T1sensitizer excitation in a molecular photon upconversion solar cell

“…Nonetheless, this work demonstrates that NIR, S 0 to T 1 * excitation can be harnessed in an integrated TTA-UC solar cell ( J sc ≈ 3.5 μA cm −2 ) with a five-fold increase in photocurrent compared to the sum of its parts, but this is still well below device relevant photocurrent contributions (>100 μA cm −2 ). 2 Further improvements can be readily achieved using (1) sensitizers with increased triplet excited state lifetimes, (2) an annihilator with increased driving force and decreased spatial separation between the chromophoric unit and the TiO 2 surface for increased electron injection yields, and (3) a balance of driving force and distance between the sensitizer and annihilator molecule for maximized triplet energy transfer and minimized back, singlet energy transfer.…”

“…1 A promising strategy to surpass this limit is to harness the transmitted, low energy photons using photon upconversion via triplet–triplet annihilation (TTA-UC). 2 TTA-UC combines two, low energy photons to generate one higher energy excited state and can be achieved even under low intensity, non-coherent irradiation. 3 Theoretical analyses indicate that if harnessed in a solar cell TTA-UC could increase the maximum efficiency limit to >43%.…”

Near-Infrared to Visible Photon Upconversion by Palladium(II) Octabutoxyphthalocyanine and Rubrene in the Solid State

“…A popular strategy to overcome this loss mechanism is the upconversion of low-energy photons to match the absorption profile of the solar cell. The first attempts in this direction employed rare earth metal ions as the upconverting material, but in recent years, photochemical upconversion, using upconverters based on molecules, has increased in popularity due to their comparatively high upconversion efficiencies under sunlight and tunable photophysical properties. − …”

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In this study, we observe near-infrared-to-visible photon upconversion via sensitized triplet−triplet annihilation in a solid-state film based on neat palladium(II) 1,4,8,11,15,18,22, 8 ] and 5 ,6,11,12-tetraphenylnapthacene (rubrene). By continuous excitation at 730 nm, the system reaches a maximum upconversion quantum yield of 3.6 × 10 −2 % at power densities below 0.2 W cm −2 .

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