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Trade-off between efficiency and stability in Mn2+-doped perovskite light-emitting diodes
Fernández, Sebastian, William Michaels, Manchen Hu, Pournima Narayanan, Natalia Murrietta, Arynn Gallegos, Ghada Ahmed, Junrui Lyu, Mahesh Gangishetty, and Daniel Congreve*

Abstract:  Although perovskite light-emitting diodes (PeLEDs) have demonstrated external quantum efficiencies (EQEs) well over 20%, their instability limits their commercial viability. Incorporating transition-metal dopants has previously improved the brightness, stability, and efficiency of PeLEDs. Here, we dope Mn2+ ions into a quasi-bulk 3D perovskite and introduce tris(4-fluorophenyl)phosphine oxide (TFPPO) to achieve a 14.0% peak EQE and 128,000 cd/m2 peak luminance. Whereas incorporating TFPPO into PeLEDs dramatically increases their EQE, it also severely compromises their stability. At a 5 mA/cm2 electrical current bias, PeLEDs fabricated without TFPPO (2.97% EQE) and with TFPPO (14.0% EQE) decay to half their maximum luminance in 37.0 and 2.54 min, respectively. In order to investigate this trade-off in EQE and stability, we study both photophysical and optoelectronic characteristics before and after PeLED electrical operation. Although Mn2+-doped PeLEDs hold the potential to enable bright and efficient lighting, device stability degradation mechanisms require further investigation
Water additives improve the efficiency of violet perovskite light-emitting diodes
Hu, Manchen, Sebastian Fernandez, Qi Zhou, Pournima Narayanan, Balreen Saini, Tracy H. Schloemer, Junrui Lyu, Arynn O. Gallegos, Ghada H. Ahmed, and Daniel N. Congreve*

Abstract: High external quantum efficiencies (EQEs) have been achieved for blue, green, red, and near-infrared perovskite light-emitting diodes (PeLEDs), and their energy efficiencies are approaching the efficiencies of III-V-based LEDs. Beyond the visible regime, ultraviolet light offers great promise for many applications such as disinfection. However, PeLEDs demonstrate poor performance in the violet/ultraviolet region, with reports of violet PeLED performance hindered by poor thin-film quality. In this work, we improve the uniformity of perovskite films by adding water into the precursor solution to engineer the crystallization process of spin-coated 2D perovskites. The resulting improved film uniformity, coupled with the reduction in nanoplate size, reduces leakage current and promotes faster recombination rates. The fabricated PeLEDs deliver bright violet emission at 408 nm with a maximum external quantum efficiency of 0.41%, a 5-fold increase over control devices. This work demonstrates viable steps toward cost-effective, efficient ultraviolet PeLEDs.
Controlling the Durability and Optical Properties of Triplet-Triplet Annihilation Upconversion Nanocapsules
Schloemer, Tracy H., Samuel Sanders, Pournima Narayanan, Qi Zhou, Manchen Hu, and Daniel Congreve. "Controlling the Durability and Optical Properties of Triplet-Triplet Annihilation Upconversion Nanocapsules." Nanoscale (2023).

Abstract: Deep penetration of high energy photons by direct irradiation is often not feasible due to absorption and scattering losses, which are generally exacerbated as photon energy increases. Precise generation of high energy photons beneath a surface can circumvent these losses and significantly transform optically controlled processes like photocatalysis or 3D printing. Using triplet–triplet annihilation upconversion (TTA-UC), a nonlinear process, we can locally convert two transmissive low energy photons into one high energy photon. We recently demonstrated the use of nanocapsules for high energy photon generation at depth, with durability within a variety of chemical environments due to the formation of a dense, protective silica shell that prevents content leakage and nanocapsule aggregation. Here, we show the importance of the feed concentrations of the tetraethylorthosilicate (TEOS) monomer and the methoxy poly(ethyleneglycol) silane (PEG-silane) ligand used to synthesize these nanocapsules using spectroscopic and microscopy characterizations. At optimal TEOS and PEG-silane concentrations, minimal nanocapsule leakage can be obtained which maximizes UC photoluminescence. We also spectroscopically study the origin of inefficient upconversion from UCNCs made using sub-optimal conditions to probe how TEOS and PEG-silane concentrations impact the equilibrium between productive shell growth and side product formation, like amorphous silica. Furthermore, this optimized fabrication protocol can be applied to encapsulate multiple TTA-UC systems and other emissive dyes to generate anti-Stokes or Stokes shifted emission, respectively. These results show that simple synthetic controls can be tuned to obtain robust, well-dispersed, bright upconverting nanoparticles for subsequent integration in optically controlled technologies.
Triplet Fusion Upconversion Nanocapsule Synthesis
Schloemer, T.H., Sanders, S.N., Zhou, Q., Narayanan, P., Hu, M., Gangishetty, M.K., Anderson, D., Seitz, M., Gallegos, A.O., Stokes, R.C. and Congreve, D.N., 2022. Triplet Fusion Upconversion Nanocapsule Synthesis. JoVE (Journal of Visualized Experiments), (187), p.e64374.

Abstract: Triplet fusion upconversion (UC) allows for the generation of one high energy photon from two low energy input photons. This well-studied process has significant implications for producing high energy light beyond a material's surface. However, the deployment of UC materials has been stymied due to poor material solubility, high concentration requirements, and oxygen sensitivity, ultimately resulting in reduced light output. Toward this end, nanoencapsulation has been a popular motif to circumvent these challenges, but durability has remained elusive in organic solvents. Recently, a nanoencapsulation technique was engineered to tackle each of these challenges, whereupon an oleic acid nanodroplet containing upconversion materials was encapsulated with a silica shell. Ultimately, these nanocapsules (NCs) were durable enough to enable triplet fusion upconversion-facilitated volumetric three-dimensional (3D) printing. By encapsulating upconversion materials with silica and dispersing them in a 3D printing resin, photopatterning beyond the surface of the printing vat was made possible. Here, video protocols for the synthesis of upconversion NCs are presented for both small-scale and large-scale batches. The outlined protocols serve as a starting point for adapting this encapsulation scheme to multiple upconversion schemes for use in volumetric 3D printing applications.
Luminescence Enhancement Due to Symmetry Breaking in Doped Halide Perovskite Nanocrystals
Ahmed, Ghada H., Yun Liu, Ivona Bravić, Xejay Ng, Ina Heckelmann, Pournima Narayanan, Martin S. Fernández, Bartomeu Monserrat, Daniel N. Congreve, and Sascha Feldmann. "Luminescence Enhancement Due to Symmetry Breaking in Doped Halide Perovskite Nanocrystals." Journal of the American Chemical Society 144, no. 34 (2022): 15862-15870.

Abstract: Metal-halide perovskite nanocrystals have demonstrated excellent optoelectronic properties for light-emitting applications. Isovalent doping with various metals (M2+) can be used to tailor and enhance their light emission. Although crucial to maximize performance, an understanding of the universal working mechanism for such doping is still missing. Here, we directly compare the optical properties of nanocrystals containing the most commonly employed dopants, fabricated under identical synthesis conditions. We show for the first time unambiguously, and supported by first-principles calculations and molecular orbital theory, that element-unspecific symmetry-breaking rather than element-specific electronic effects dominate these properties under device-relevant conditions. The impact of most dopants on the perovskite electronic structure is predominantly based on local lattice periodicity breaking and resulting charge carrier localization, leading to enhanced radiative recombination, while dopant-specific hybridization effects play a secondary role. Our results suggest specific guidelines for selecting a dopant to maximize the performance of perovskite emitters in the desired optoelectronic devices.
Triplet Fusion Upconversion Nanocapsiles for Volumetric 3D Printing
Sanders, Samuel N., Tracy H. Schloemer, Mahesh K. Gangishetty, Daniel Anderson, Michael Seitz, Arynn O. Gallegos, R. Christopher Stokes, and Daniel N. Congreve. Nature 604, no. 7906 (2022): 474-478.

Abstract: Three-dimensional (3D) printing has exploded in interest as new technologies have opened up a multitude of applications, with stereolithography a particularly successful approach. However, owing to the linear absorption of light, this technique requires photopolymerization to occur at the surface of the printing volume, imparting fundamental limitations on resin choice and shape gamut. One promising way to circumvent this interfacial paradigm is to move beyond linear processes, with many groups using two-photon absorption to print in a truly volumetric fashion. Using two-photon absorption, many groups and companies have been able to create remarkable nanoscale structures, but the laser power required to drive this process has limited print size and speed, preventing widespread application beyond the nanoscale. Here we use triplet fusion upconversion to print volumetrically with less than 4 milliwatt continuous-wave excitation. Upconversion is introduced to the resin by means of encapsulation with a silica shell and solubilizing ligands. We further introduce an excitonic strategy to systematically control the upconversion threshold to support either monovoxel or parallelized printing schemes, printing at power densities several orders of magnitude lower than the power densities required for two-photon-based 3D printing.
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
Schloemer, Tracy, Pournima Narayanan, Qi Zhou, Emma Belliveau, Michael Seitz, and Daniel N. Congreve. "Nanoengineering Triplet–Triplet Annihilation Upconversion: From Materials to Real-World Applications." ACS nano (2023).
Burroughs, Michael C., Tracy H. Schloemer, Daniel N. Congreve, and Danielle J. Mai. "Gelation Dynamics during Photo-Cross-Linking of Polymer Nanocomposite Hydrogels." ACS Polymers Au (2023).
Hu, Manchen, Emma Belliveau, and Daniel N. Congreve. "Interfacial charge transfer states enable efficient solid-state upconversion." Matter 5, no. 8 (2022): 2542-2545.
Fakharuddin, Azhar, Mahesh K. Gangishetty, Mojtaba Abdi-Jalebi, Sang-Hyun Chin, Abd Rashid bin Mohd Yusoff, Daniel N. Congreve, Wolfgang Tress, Felix Deschler, Maria Vasilopoulou, and Henk J. Bolink. "Perovskite light-emitting diodes." Nature Electronics 5, no. 4 (2022): 203-216. 
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).