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Reflections on hosting summer undergraduate researchers in the midst of a pandemic
A. O. Gallegos, G. H. Ahmed, T. H. Schloemer, D. N. Congreve*. 

Abstract: The COVID-19 pandemic continues to impact nearly every aspect of our lives, including academic research. In this Matter of Opinion, we reflect on hosting both in-person and virtual undergraduate students during these challenging times.
Photon Upconversion in Aqueous Nanodroplets
Samuel N. Sanders, Mahesh K. Gangishetty, Matthew Y. Sfeir, and Daniel N. Congreve*

Abstract: Triplet fusion upconversion, the conversion of two low-energy photons into one higher-energy photon via excitonic intermediates, has the potential to revolutionize fields as diverse as biological imaging, photovoltaics, and optogenetics. However, important hurdles to widespread application still exist; for example, the vast majority of demonstrations are in nonpolar solvents, limiting applications. Furthermore, the necessary high concentrations of dyes limit optical penetration depth. Efforts toward aqueous solutions utilizing micelles and other nanoencapsulants have been limited by poor efficiencies or scatter from the nanoparticles. Here, we demonstrate a facile micellular fabrication method that drives a high boiling point solvent into the core of a block copolymer micelle, greatly reducing molecular aggregation. We show that this simple preparation is scalable and provides benefits across five different colors of photon upconversion. We expect this simple, user-friendly, and high-performance system to aid a multitude of photon upconversion applications, in particular, for optogenetics, photodynamic therapy, and photochemistry.
Mn2+ Doping Enhances the Brightness, Efficiency, and Stability of Bulk Perovskite Light-Emitting Diodes
Mahesh K. Gangishetty, Samuel N. Sanders, and Daniel N. Congreve*

Abstract: Interest in organic–inorganic hybrid perovskite (ABX3) LEDs has exploded over the past several years, yet significant gains in stability, efficiency, and brightness are required before commercialization is possible, particularly for blue devices. The perovskite composition has been shown to play a crucial role in its performance, yet to date nearly all existing reports focus on tuning the A-site composition. Here, we find that doping the B-site with manganese allows us to achieve bright, efficient, and stable LEDs regardless of A or X composition. By doping with Mn, we demonstrate ultrabright sky-blue, green, and red perovskite LEDs with a maximum brightness of 11800, 97000, and 1470 cd/m2 and quantum efficiencies of 0.58%, 3.2%, and 5.1%, respectively. Crucially, these devices show excellent operational stability, with the sky-blue devices lasting for 20 min and red devices over 5 h with strong spectral stability. Moreover, the green devices showed over 1% efficiency even at higher current densities, ∼2000 mA/cm2. Mn doping allows for universal improvement in perovskite performance and stability, opening the door to a huge number of applications.
Annihilator Dimers Enhance Triplet Fusion Upconversion
Andrew B. Pun, Samuel N. Sanders, Matthew Y. Sfeir, Luis M. Campos*, and Daniel N. Congreve*

Abstract: Optical upconversion is a net process by which two low energy photons are converted into one higher energy photon. There is vast potential to exploit upconversion in applications ranging from solar energy and biological imaging to data storage and photocatalysis. Here, we link two upconverting chromophores together to synthesize a series of novel tetracene dimers for use as annihilators. When compared with the monomer annihilator, TIPS–tetracene, the dimers yield a strong enhancement in the triplet fusion process, also known as triplet–triplet annihilation, as demonstrated via a large increase in upconversion efficiency and an order of magnitude reduction of the threshold power for maximum yield. Along with the ongoing rapid improvements to sensitizer materials, the dimerization improvements demonstrated here open the way to a wide variety of emerging upconversion applications.
Tunable Emission from Triplet Fusion Upconversion in Diketopyrrolopyrroles
​Andrew B. Pun, Luis M. Campos*, Daniel N. Congreve*

Abstract: Optical upconversion based on triplet fusion (TF), also known as triplet–triplet annihilation, is a process by which two or more low-energy photons are converted to one higher energy photon. This process requires two components, a sensitizer which absorbs the incident low-energy photons and an annihilator which emits the higher energy photons. While much attention has been given to the investigation of new types of sensitizers, very little work has been done on the exploration of new annihilators. In this work, we show that the singlet energy of diketopyrrolopyrroles (DPPs) can be altered by modifying the pendant aryl substituents to the core. This allows us to meet the energetic requirements necessary for TF upconversion and demonstrates DPPs as a new class of annihilator molecules. Using this new DPP platform, the output wavelength from upconversion can easily be tuned, which will greatly diversify the number of applications of DPPs in upconversion technologies.
Photoredox Catalysis using Infrared Light via Triplet Fusion Upconversion
​Benjamin D. Ravetz, Andrew B. Pun, Emily M. Churchill, Daniel N. Congreve*, Tomislav Rovis*, Luis M. Campos*

Abstract: Recent advances in photoredox catalysis have made it possible to execute a multitude of challenging synthetic transformations, polymerizations, and surface modifications. In all cases, these transformations require ultraviolet or visible light stimuli. The use of visible light irradiation has intrinsic challenges. For example, the penetration of visible light is very low through most reaction media, leading to problems in large scale reactions. Moreover, reactants can compete with the photocatalysts for incident light absorption, limiting scope. Near infrared (NIR) radiation can overcome many of these fundamental problems. NIR light is known to have a much higher penetration depth through a variety of media, notably biological tissue. Here, we demonstrate a variety of photoredox transformations under infrared radiation by utilizing the photophysical process of triplet fusion upconversion. This generates molecules in their singlet excited state that can perform single electron transfer (SET), serving as photocatalysts or photoinitiators.  We illustrate that this is a general strategy applicable to a wide range of photoredox reactions. We tune the upconversion components to adjust the output light, accessing both orange light and blue light from low-energy infrared (IR) light, by pair-wise manipulation of the sensitizer and annihilator. We further demonstrate that the annihilator itself can be used as a photocatalyst, simplifying the reaction. This approach allows us to execute catalysis through several barriers that are impenetrable by visible light, expanding the landscape of photocatalysis to a variety of new materials and environments. The results demonstrated here allow high-energy transformations from low-energy IR light with all the benefits that the latter affords.
Efficient Blue and White Perovskite Light Emitting Diodes via Manga​nese D​oping
Hou, S., Gangishetty, M.K., Quan, Q.*, Congreve, D.N.*, (2018) Joule 2, 1–13

Abstract: Despite heavy research, blue perovskite nanocrystal LEDs have struggled to match the high efficiencies of their red and green cousins, particularly at wavelengths blue enough to meet the NTSC blue standard. One of the most critical problems is the low photoluminescence yield of the nanocrystals in thin films. Recently, manganese doping has been shown to increase the photoluminescence yield of the perovskite even as a decay pathway to a long-lived emissive state on the manganese ion is introduced. Here, we employ a two-step synthetic approach to carefully tune the manganese doping to increase the blue photoluminescence while preventing significant manganese emission, allowing for blue perovskite LEDs with quantum efficiencies over 2% that meet the NTSC standard. Manganese doping increases the photoluminescence yield and lifetime, reduces trap states, and makes the dots more monodisperse, reducing the emission bandwidth. Finally, we utilize perovskite nanocrystal downconverters to build an all-perovskite white LED.
Reducing Architecture Limitations for Efficient Blue Perovskite Light Emitting Diodes
Gangishetty, M.K., Hou, S., Quan, Q., Congreve, D.N.*, (2018) Advanced Materials, 1706226. 

​Abstract: Light-emitting diodes utilizing perovskite nanocrystals have generated strong interest in the past several years, with green and red devices showing high efficiencies. Blue devices, however, have lagged significantly behind. Here, it is shown that the device architecture plays a key role in this lag and that NiOx, a transport layer in one of the highest efficiency devices to date, causes a significant reduction in perovskite luminescence lifetime. An alternate transport layer structure which maintains robust nanocrystal emission is proposed. Devices with this architecture show external quantum efficiencies of 0.50% at 469 nm, seven times higher than state-of-the-art devices at that wavelength. Finally, it is demonstrated that this architecture enables efficient devices across the entire blue-green portion of the spectrum. The improvements demonstrated here open the door to efficient blue perovskite light emitting diodes.
Triplet Harvesting from Intramolecular Singlet Fission in Polytetracene
A. B. Pun, S. N. Sanders, E. Kumarasamy, M. Y. Sfeir*, D. N. Congreve*, L. M. Campos*. (2017) Advanced Materials, 1701416.

Abstract: Singlet fission (SF), a promising mechanism of multiple exciton generation, has only recently been engineered as a fast, efficient, intramolecular process (iSF). The challenge now lies in designing and optimizing iSF materials that can be practically applied in high-performance optoelectronic devices. However, most of the reported iSF systems, such as those based on donor–acceptor polymers or pentacene, have low triplet energies, which limits their applications. Tetracene-based materials can overcome significant challenges, as the tetracene triplet state is practically useful, ≈1.2 eV. Here, the synthesis and excited state dynamics of a conjugated tetracene homopolymer are studied. This polymer undergoes ultrafast iSF in solution, generating high-energy triplets on a sub-picosecond time scale. Magnetic field-dependent photocurrent measurements of polytetracene-based devices demonstrate the first example of iSF-generated triplet extraction in devices, exhibiting the potential of iSF materials for use in next-generation devices.

Other Publications

*contributed equally
Sanders, Samuel N.*, Tracy H. Schloemer*, Mahesh K. Gangishetty, Daniel Anderson, Michael Seitz, Arynn O. Gallegos, R. Christopher Stokes, and Daniel N. Congreve. "Triplet Fusion Upconversion Nanocapsules for Volumetric 3D Printing." arXiv preprint arXiv:2109.02780 (2021). 
Seitz, Michael, Marc Meléndez, Peyton York, Daniel A. Kurtz, Alvaro J. Magdaleno, Nerea Alcázar, Mahesh K. Gangishetty, Rafael Delgado-Buscalioni, Daniel N. Congreve, and Ferry Prins. "Halide mixing inhibits exciton transport in two-dimensional perovskites despite phase purity." arXiv preprint arXiv:2107.06560 (2021).
Feldmann, Sascha, Mahesh K. Gangishetty, Ivona Bravić, Timo Neumann, Bo Peng, Thomas Winkler, Richard H. Friend, Bartomeu Monserrat, Daniel N. Congreve, and Felix Deschler. "Charge Carrier Localization in Doped Perovskite Nanocrystals Enhances Radiative Recombination." Journal of the American Chemical Society (2021).
Seitz, Michael, Marc Meléndez, Nerea Alcázar‐Cano, Daniel N. Congreve, Rafael Delgado‐Buscalioni, and Ferry Prins. "Mapping the Trap‐State Landscape in 2D Metal‐Halide Perovskites Using Transient Photoluminescence Microscopy." Advanced Optical Materials (2021): 2001875.
Before Stanford: 
Fallon, Kealan J., Emily M. Churchill, Samuel N. Sanders, James Shee, John L. Weber, Rinat Meir, Steffen Jockusch et al. "Molecular Engineering of Chromophores to Enable Triplet–Triplet Annihilation Upconversion." Journal of the American Chemical Society (2020).
Hoye, Robert LZ, Azhar Fakharuddin, Daniel N. Congreve, Jianpu Wang, and Lukas Schmidt-Mende. "Light emission from perovskite materials." APL Materials (2020)

M.H. Futscher, M.K. Gangishetty, D.N. Congreve, B. Ehrler, “Quantifying mobile ions in perovskite-based devices with temperature-dependent capacitance measurements: frequency versus time domain”. (2019)
C.F. Perkinson, D.P. Tabor, M. Einzinger, D. Sheberla , H. Utzat , T. Lin, D.N. Congreve, M.G. Bawendi,  A. Aspuru-Guzik , M.A. Baldo, “Discovery of blue singlet exciton fission molecules via a high-throughput virtual screening and experimental approach”, (2019) Journal of Chemical Physics 151, 121102.
M. Einzinger, T. Wu, H.L. Smith, C.F. Perkinson, L. Nienhaus, S. Wieghold, D.N. Congreve, A. Kahn, M.G. Bawendi, M.A. Baldo. “Sensitization of silicon by singlet exciton fission in tetracene”, (2019) Nature 571, 90–94.

Congreve, D.N. et al. “Tunable Light-Emitting Diodes Utilizing Quantum-Confined Layered Perovskite Emitters”, (2017) ACS Photonics, 4 (3), 476-481.

Wu, M.*, Congreve, D.N.*, Wilson, M.W.B.*, et al. “Solid-state infrared-to-visible upconversion sensitized by colloidal nanocrystals”, (2016) Nature Photonics, 10(1), 31-34.
Deotare, P.B.*, Chang, W.*, Hontz, E.*, Congreve, D.N.*, Shi, L.*, et al. “Nanoscale transport of charge transfer states in organic donor-acceptor blends”, (2015) Nature Materials, 14 (11), 1130-1134.

Hontz, E., Chang, W., Congreve, D.N., Baldo, M.A., Van Voorhis, T. “The role of electron-hole separation in thermally activated delayed fluorescence in donor-acceptor blends”, (2016) Journal of Physical Chemistry C, 119 (45), 25591-25597.
Wu, T.C, Congreve, D.N., Baldo, M.A. “Solid state photon upconversion utilizing thermally-activated-delayed-fluorescence molecules as triplet sensitizer”, (2015) Applied Physics Letters, 107 (3), 031103.
Chang, W.*, Congreve, D.N.*, et al. “Spin dependent charge transfer state design rules in organic photovoltaics”, (2015) Nature Communications, 6 (6415).
Thompson, N.J.*, Wilson, M.W.B.*, Congreve, D.N.*, et al., “Energy harvesting of non-emissive triplet excitons in tetracene by emissive PbS nanocrystals”, (2014) Nature Materials 13, pp. 1039-1043.
Wu, T.C., Thompson, N.J., Congreve, D.N., et al., “Singlet fission efficiency in tetracene-based organic solar cells”, (2014) Applied Physics Letters, 104 (19).
Thompson, N.J.*, Hontz, E.*, Congreve, D.N., et al., “Nanostructured singlet fission photovoltaics subject to triplet-charge annihilation”, (2014) Advanced Materials, 26 (9), pp. 1366-1371.
Yost, S.R., Lee, J., Wilson, M.W.B., Wu, T., McMahon, D.P., Parkhurst, R.R., Thompson, N.J., Congreve, D.N., et al., “A transferable model for singlet-fission kinetics”, (2014) Nature Chemistry, 6 (6), pp. 492-497.
Congreve, D.N.*, Lee, J.*, Thompson, N.J.*, et al., “External quantum efficiency above 100% in a singlet-exciton-fission-based organic photovoltaic cell”, (2013) Science, 340 (6130), pp. 334-337.
Lee, J., Jadhav, P., Reusswig, P.D., Yost, S.R., Thompson, N.J., Congreve, D.N., et al., “Singlet exciton fission photovoltaics”, (2013) Accounts of Chemical Research, 46 (6), pp. 1300-1311.
Thompson, N.J.*, Congreve, D.N.*, Goldberg, D., et al., “Slow light enhanced singlet exciton fission solar cells with a 126% yield of electrons per photon”, (2013) Applied Physics Letters, 103 (26).
Reusswig, P.D., Congreve, D.N., Thompson, N.J., Baldo, M.A. “Enhanced external quantum efficiency in an organic photovoltaic cell via singlet fission exciton sensitizer” (2012) Applied Physics Letters, 101 (11).