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Integrating molecular photon upconversion via triplet–triplet annihilation (TTA-UC) directly into a solar cell offers a means of harnessing sub-bandgap, near infrared (NIR) photons and surpassing the Shockley–Queisser limit. However, all integrated TTA-UC solar cells to date only harness visible light. Here, we incorporate an osmium polypyridal complex (Os) as the triplet sensitizer in a metal ion linked multilayer photoanode that is capable of harnessing NIR light via S 0 to T 1 * excitation, triple energy transfer to a phosphonated bis(9,10-diphenylethynyl)anthracene annihilator (A), TTA-UC, and electron injection into TiO 2 from the upcoverted state. The TiO 2 -A-Zn-Os devices have five-fold higher photocurrent (∼3.5 μA cm −2 ) than the sum of their parts. IPCE data and excitation intensity dependent measurements indicate that the NIR photons are harvested through a TTA-UC mechanism. Transient absorption spectroscopy is used to show that the low photocurrent, as compared to visible light harnessing TTA-UC solar cells, can be atributed to: (1) slow sensitizer to annihilator triplet energy transfer, (2) a low injection yield for the annihilator, and (3) fast back energy transfer from the upconverted state to the sensitizer. Regardless, these results serve as a proof-of-concept that NIR photons can be harnessed via an S 0 to T 1 * sensitizer excited, integrated TTA-UC solar cell and that further improvements can readily be made by remedying the performance limiting processes noted above.
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Exploring Sensitized Photon Upconversion – From Past to Present
The conversion of low energy photons into high energy photons via triplet-triplet annihilation (TTA) photon upconversion (UC) has become a promising avenue for furthering a wide range of optoelectronic applications. Through the decades of research, many combinations of triplet sensitizer species and annihilator molecules have been investigated unlocking the entire visible spectrum upon proper pairings of sensitizer and annihilator identities. Here, we reflect upon the seminal works which lay the foundation for TTA-UC originating from solution-based methods and highlight the recent advances made within the solid state primarily focusing on perovskite-based triplet generation.
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- PAR ID:
- 10536305
- Publisher / Repository:
- CHIMIA
- Date Published:
- Journal Name:
- CHIMIA
- Volume:
- 78
- Issue:
- 7-8
- ISSN:
- 0009-4293
- Page Range / eLocation ID:
- 518 to 524
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Triplet‐triplet annihilation upconversion (TTA‐UC) is a photophysical process in which two low‐energy photons are converted into one higher‐energy photon. This type of upconversion requires two species: a sensitizer that absorbs low‐energy light and transfers its energy to an annihilator, which emits higher‐energy light after TTA. In spite of the multitude of applications of TTA‐UC, few families of annihilators have been explored. In this work, we show dipyrrolonaphthyridinediones (DPNDs) can act as annihilators in TTA‐UC. We found that structural changes to DPND dramatically increase its upconversion quantum yield (UCQY). Our optimized DPND annihilator demonstrates a high maximum internal UCQY of 9.4 %, outperforming the UCQY of commonly used near‐infrared‐to‐visible annihilator rubrene by almost double.more » « less
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Abstract Triplet–triplet annihilation‐based photon upconversion (TTA‐UC) can efficiently generate higher energy photons at low relative fluences. Bulk metal halide perovskites have offered promise in efficiently sensitizing molecular triplet states in the solid state, necessary for the integration of TTA‐UC into device‐based applications. Recent work focused on TTA‐UC from a rubrene triplet annihilator sensitized by perovskite thin films has established relatively efficient charge extraction from the perovskite, forming the triplet exciton in rubrene. Yet, the specifics underpinning charge transfer at the perovskite/rubrene interface are not fully elucidated. To improve device performance and study the properties governing charge transfer at the interface, various organic solvents are explored to treat the perovskite surface. Scanning tunneling microscopy and spectroscopy show a difference in the electronic band structure, where both n‐ and p‐type terminated perovskite surfaces are observed depending on the solvent used. Supported by optical spectroscopy, the impact of the perovskite electronic structure is monitored, indicating that n‐type perovskite sensitizers feature higher TTA‐UC efficiencies due to favorable band bending resulting in efficient hole‐mediated triplet formation. Overall, the tuning of the electronic structure of the perovskite sensitizer through solvent treatment is shown to be a key force in tuning the mechanism of efficient triplet generation.more » « less
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Abstract Triplet‐triplet annihilation photon upconversion (TTA‐UC) converts low‐energy photons to higher‐energy ones under low‐intensity incoherent excitation, thus enabling applications in fields ranging from medicine to solar energy conversion. Silylethynyl mono‐ and di‐substitution of acenes offers an attractive route to creating new annihilators that operate with minimal energy loss. Here, it is demonstrated that this approach can be extended to pyrene, yielding annihilators that display efficient red‐to‐blue upconversion. While pyrene is the namesake of P‐type delayed fluorescence, the original name for triplet‐triplet annihilation, it is known to be a poor annihilator due to its propensity for forming excimers. By tetra‐substituting pyrene with silylethynyl groups, excimer formation is substantially hindered while simultaneously minimizing the energy gap between the singlet and triplet pair states that participate in TTA‐UC, yielding outstanding annihilators for red‐to‐blue upconversion that operate with quantum yields of upward of 19% (29% when corrected for inner filter effects). Further, it is found that reducing the bulkiness of the silyl substituents is key to achieving high TTA‐UC quantum yields, which highlights the importance of annihilator side group selection when optimizing photon upconversion.more » « less
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The development of efficient solid-state photon upconversion (UC) devices remains paramount for practical applications of the technology. In recent years, the incorporation of perovskite thin films as triplet sensitizers for triplet–triplet annihilation (TTA)-based UC has provided a promising solution. In the pursuit of finding an “ideal annihilator” to maximize the apparent anti-Stokes shift, we investigate naphtho[2,3-a]pyrene (NaPy) as an annihilator in both solution-based and perovskite-sensitized TTA-UC systems. Surprisingly, we observe different emission behaviors of NaPy in the solid state based on the excitation wavelength. Under direct excitation, a high-energy transition S1' dominates the emission spectrum, while UC results in increased emission from a lower lying state S1''. We propose that this is the result of aggregation-related lowering of the singlet excited state thus changing the fundamental energetic landscape underlying TTA. Aggregation decreases the singlet energy below the energy level of the triplet pair state 1(TT), yielding energetically favorable emission from the aggregated singlet state S1'' and weak emission from the higher lying singlet state S1' through thermally or entropically driven TTA-UC.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.2533/chimia.2024.518
