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  • Academic
    frohnak@stanford.edu
    University - Scholar Department: Materials Science and Engineering Position: Postdoctoral Scholar

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All Publications

  • Multifunctional sulfonium-based treatment for perovskite solar cells with less than 1% efficiency loss over 4,500-h operational stability tests. Nature energy Suo, J., Yang, B., Mosconi, E., Bogachuk, D., Doherty, T. A., Frohna, K., Kubicki, D. J., Fu, F., Kim, Y., Er-Raji, O., Zhang, T., Baldinelli, L., Wagner, L., Tiwari, A. N., Gao, F., Hinsch, A., Stranks, S. D., De Angelis, F., Hagfeldt, A. 2024; 9 (2): 172-183

    Abstract

    The stabilization of grain boundaries and surfaces of the perovskite layer is critical to extend the durability of perovskite solar cells. Here we introduced a sulfonium-based molecule, dimethylphenethylsulfonium iodide (DMPESI), for the post-deposition treatment of formamidinium lead iodide perovskite films. The treated films show improved stability upon light soaking and remains in the black α phase after two years ageing under ambient condition without encapsulation. The DMPESI-treated perovskite solar cells show less than 1% performance loss after more than 4,500 h at maximum power point tracking, yielding a theoretical T80 of over nine years under continuous 1-sun illumination. The solar cells also display less than 5% power conversion efficiency drops under various ageing conditions, including 100 thermal cycles between 25 °C and 85 °C and an 1,050-h damp heat test.

    View details for DOI 10.1038/s41560-023-01421-6

    View details for PubMedID 38419691

    View details for PubMedCentralID PMC10896729

  • Nanoscale chemical heterogeneity dominates the optoelectronic response of alloyed perovskite solar cells NATURE NANOTECHNOLOGY Frohna, K., Anaya, M., Macpherson, S., Sung, J., Doherty, T. S., Chiang, Y., Winchester, A. J., Orr, K. P., Parker, J. E., Quinn, P. D., Dani, K. M., Rao, A., Stranks, S. D. 2022; 17 (2): 190-196

    Abstract

    Halide perovskites perform remarkably in optoelectronic devices. However, this exceptional performance is striking given that perovskites exhibit deep charge-carrier traps and spatial compositional and structural heterogeneity, all of which should be detrimental to performance. Here, we resolve this long-standing paradox by providing a global visualization of the nanoscale chemical, structural and optoelectronic landscape in halide perovskite devices, made possible through the development of a new suite of correlative, multimodal microscopy measurements combining quantitative optical spectroscopic techniques and synchrotron nanoprobe measurements. We show that compositional disorder dominates the optoelectronic response over a weaker influence of nanoscale strain variations even of large magnitude. Nanoscale compositional gradients drive carrier funnelling onto local regions associated with low electronic disorder, drawing carrier recombination away from trap clusters associated with electronic disorder and leading to high local photoluminescence quantum efficiency. These measurements reveal a global picture of the competitive nanoscale landscape, which endows enhanced defect tolerance in devices through spatial chemical disorder that outcompetes both electronic and structural disorder.

    View details for DOI 10.1038/s41565-021-01019-7

    View details for Web of Science ID 000721470000001

    View details for PubMedID 34811554

  • Hybrid perovskites for device applications HANDBOOK OF ORGANIC MATERIALS FOR ELECTRONIC AND PHOTONIC DEVICES, 2ND EDITION Frohna, K., Stranks, S. D., Ostroverkhova, O. 2019: 211-256

    View details for DOI 10.1016/B978-0-08-102284-9.00007-3

    View details for Web of Science ID 000488247800008

  • Inversion symmetry and bulk Rashba effect in methylammonium lead iodide perovskite single crystals NATURE COMMUNICATIONS Frohna, K., Deshpande, T., Harter, J., Peng, W., Barker, B. A., Neaton, J. B., Louie, S. G., Bakr, O. M., Hsieh, D., Bernardi, M. 2018; 9: 1829

    Abstract

    Methylammonium lead iodide perovskite (MAPbI3) exhibits long charge carrier lifetimes that are linked to its high efficiency in solar cells. Yet, the mechanisms governing these unusual carrier dynamics are not completely understood. A leading hypothesis-disproved in this work-is that a large, static bulk Rashba effect slows down carrier recombination. Here, using second harmonic generation rotational anisotropy measurements on MAPbI3 crystals, we demonstrate that the bulk structure of tetragonal MAPbI3 is centrosymmetric with I4/mcm space group. Our calculations show that a significant Rashba splitting in the bandstructure requires a non-centrosymmetric lead iodide framework, and that incorrect structural relaxations are responsible for the previously predicted large Rashba effect. The small Rashba splitting allows us to compute effective masses in excellent agreement with experiment. Our findings rule out the presence of a large static Rashba effect in bulk MAPbI3, and our measurements find no evidence of dynamic Rashba effects.

    View details for DOI 10.1038/s41467-018-04212-w

    View details for Web of Science ID 000431630400011

    View details for PubMedID 29739939

    View details for PubMedCentralID PMC5940805

  • High carrier mobility along the [111] orientation in Cu2O photoelectrodes. Nature Pan, L., Dai, L., Burton, O. J., Chen, L., Andrei, V., Zhang, Y., Ren, D., Cheng, J., Wu, L., Frohna, K., Abfalterer, A., Yang, T. C., Niu, W., Xia, M., Hofmann, S., Dyson, P. J., Reisner, E., Sirringhaus, H., Luo, J., Hagfeldt, A., Grätzel, M., Stranks, S. D. 2024; 628 (8009): 765-770

    Abstract

    Solar fuels offer a promising approach to provide sustainable fuels by harnessing sunlight1,2. Following a decade of advancement, Cu2O photocathodes are capable of delivering a performance comparable to that of photoelectrodes with established photovoltaic materials3-5. However, considerable bulk charge carrier recombination that is poorly understood still limits further advances in performance6. Here we demonstrate performance of Cu2O photocathodes beyond the state-of-the-art by exploiting a new conceptual understanding of carrier recombination and transport in single-crystal Cu2O thin films. Using ambient liquid-phase epitaxy, we present a new method to grow single-crystal Cu2O samples with three crystal orientations. Broadband femtosecond transient reflection spectroscopy measurements were used to quantify anisotropic optoelectronic properties, through which the carrier mobility along the [111] direction was found to be an order of magnitude higher than those along other orientations. Driven by these findings, we developed a polycrystalline Cu2O photocathode with an extraordinarily pure (111) orientation and (111) terminating facets using a simple and low-cost method, which delivers 7 mA cm-2 current density (more than 70% improvement compared to that of state-of-the-art electrodeposited devices) at 0.5 V versus a reversible hydrogen electrode under air mass 1.5 G illumination, and stable operation over at least 120 h.

    View details for DOI 10.1038/s41586-024-07273-8

    View details for PubMedID 38658685

    View details for PubMedCentralID 6965123

  • Multifunctional sulfonium-based treatment for perovskite solar cells with less than 1% efficiency loss over 4,500-h operational stability tests NATURE ENERGY Suo, J., Yang, B., Mosconi, E., Bogachuk, D., Doherty, T. S., Frohna, K., Kubicki, D. J., Fu, F., Kim, Y., Er-Raji, O., Zhang, T., Baldinelli, L., Wagner, L., Tiwari, A. N., Gao, F., Hinsch, A., Stranks, S. D., De Angelis, F., Hagfeldt, A. 2024

    View details for DOI 10.1038/s41560-023-01421-6

    View details for Web of Science ID 001136714500001

  • Integration of Metal Meshes as Transparent Conducting Electrodes into Perovskite Solar Cells ADVANCED MATERIALS INTERFACES Ongaro, C., Roose, B., Fleury, J., Schneider, R., Frohna, K., Ooi, Z., Heier, J., Stranks, S. D., Schuler, A. 2023

    View details for DOI 10.1002/admi.202300923

    View details for Web of Science ID 001124249000001

  • Self-supervised deep learning for tracking degradation of perovskite light-emitting diodes with multispectral imaging NATURE MACHINE INTELLIGENCE Ji, K., Lin, W., Sun, Y., Cui, L., Shamsi, J., Chiang, Y., Chen, J., Tennyson, E. M., Dai, L., Li, Q., Frohna, K., Anaya, M., Greenham, N. C., Stranks, S. D. 2023

    View details for DOI 10.1038/s42256-023-00736-z

    View details for Web of Science ID 001099196000001

  • Imaging Light-Induced Migration of Dislocations in Halide Perovskites with 3d Nanoscale Strain Mapping ADVANCED MATERIALS Orr, K. P., Diao, J., Lintangpradipto, M., Batey, D. J., Iqbal, A. N., Kahmann, S., Frohna, K., Dubajic, M., Zelewski, S. J., Dearle, A. E., Selby, T. A., Li, P., Doherty, T. S., Hofmann, S., Bakr, O. M., Robinson, I. K., Stranks, S. D. 2023: e2305549

    Abstract

    In recent years, halide perovskite materials have been used to make high-performance solar cells and light-emitting devices. However, material defects still limit device performance and stability. Here, synchrotron-based Bragg coherent diffraction imaging is used to visualize nanoscale strain fields, such as those local to defects, in halide perovskite microcrystals. Significant strain heterogeneity within MAPbBr3 (MA = CH3 NH3 + ) crystals is found in spite of their high optoelectronic quality, and both 〈100〉 and 〈110〉 edge dislocations are identified through analysis of their local strain fields. By imaging these defects and strain fields in situ under continuous illumination, dramatic light-induced dislocation migration across hundreds of nanometers is uncovered. Further, by selectively studying crystals that are damaged by the X-ray beam, large dislocation densities and increased nanoscale strains are correlated with material degradation and substantially altered optoelectronic properties assessed using photoluminescence microscopy measurements. These results demonstrate the dynamic nature of extended defects and strain in halide perovskites, which will have important consequences for device performance and operational stability.

    View details for DOI 10.1002/adma.202305549

    View details for Web of Science ID 001119392100001

    View details for PubMedID 37735999

  • Methylammonium-free co-evaporated perovskite absorbers with high radiation and UV tolerance: an option for in-space manufacturing of space-PV? RSC ADVANCES Lang, F., Chiang, Y., Frohna, K., Ozen, S., Neitzert, H. C., Denker, A., Stolterfoht, M., Stranks, S. D. 2023; 13 (31): 21138-21145

    Abstract

    With a remarkable tolerance to high-energetic radiation and potential high power-to-weight ratios, halide perovskite-based solar cells are interesting for future space PV applications. In this work, we fabricate and test methylammonium-free, co-evaporated FA0.7Cs0.3Pb(I0.9Br0.1)3 perovskite solar cells that could potentially be fabricated in space or on the Moon by physical vapor deposition, making use of the available vacuum present. The absence of methylammonium hereby increased the UV-light stability significantly, an important factor considering the increased UV proportion in the extra-terrestrial solar spectrum. We then tested their radiation tolerance under high energetic proton irradiation and found that the PCE degraded to 0.79 of its initial value due to coloring of the glass substrate, a typical problem that often complicates analysis. To disentangle damage mechanisms and to assess whether the perovskite degraded, we employ injection-current-dependent electroluminescence (EL) and intensity-dependent VOC measurements to derive pseudo-JV curves that are independent of parasitic effects. This way we identify a high radiation tolerance with 0.96 of the initial PCE remaining after 1 × 1013 p+ cm-2 which is beyond today's space material systems (<0.8) and on par with those of previously tested solution-processed perovskite solar cells. Together our results render co-evaporated perovskites as highly interesting candidates for future space manufacturing, while the pseudo-JV methodology presents an important tool to disentangle parasitic effects.

    View details for DOI 10.1039/d3ra03846g

    View details for Web of Science ID 001026436800001

    View details for PubMedID 37449029

    View details for PubMedCentralID PMC10337721

  • Template Pore Size and A-Site Cation Management Dictate Luminescence Efficiency, Stability, and Wavelength in Confined Perovskite Nanostructures ADVANCED OPTICAL MATERIALS da Costa, V. P., Frohna, K., Stranks, S. D., Coffer, J. L. 2023; 11 (15)

    View details for DOI 10.1002/adom.202202755

    View details for Web of Science ID 001000027600001

  • Vacuum-Deposited Wide-Bandgap Perovskite for All-Perovskite Tandem Solar Cells ACS ENERGY LETTERS Chiang, Y., Frohna, K., Salway, H., Abfalterer, A., Pan, L., Roose, B., Anaya, M., Stranks, S. D. 2023; 8 (6): 2728-2737

    Abstract

    All-perovskite tandem solar cells beckon as lower cost alternatives to conventional single-junction cells. Solution processing has enabled rapid optimization of perovskite solar technologies, but new deposition routes will enable modularity and scalability, facilitating technology adoption. Here, we utilize 4-source vacuum deposition to deposit FA0.7Cs0.3Pb(IxBr1-x)3 perovskite, where the bandgap is changed through fine control over the halide content. We show how using MeO-2PACz as a hole-transporting material and passivating the perovskite with ethylenediammonium diiodide reduces nonradiative losses, resulting in efficiencies of 17.8% in solar cells based on vacuum-deposited perovskites with a bandgap of 1.76 eV. By similarly passivating a narrow-bandgap FA0.75Cs0.25Pb0.5Sn0.5I3 perovskite and combining it with a subcell of evaporated FA0.7Cs0.3Pb(I0.64Br0.36)3, we report a 2-terminal all-perovskite tandem solar cell with champion open circuit voltage and efficiency of 2.06 V and 24.1%, respectively. This dry deposition method enables high reproducibility, opening avenues for modular, scalable multijunction devices even in complex architectures.

    View details for DOI 10.1021/acsenergylett.3c00564

    View details for Web of Science ID 001012180100001

    View details for PubMedID 37324541

    View details for PubMedCentralID PMC10262197

  • The Electronic Disorde Landscape of Mixed Halide Perovskites ACS ENERGY LETTERS Liu, Y., Banon, J., Frohna, K., Chiang, Y., Tumen-Ulzii, G., Stranks, S. D., Filoche, M., Friend, R. H. 2023; 8 (1): 250-258

    Abstract

    Band gap tunability of lead mixed halide perovskites makes them promising candidates for various applications in optoelectronics. Here we use the localization landscape theory to reveal that the static disorder due to iodide:bromide compositional alloying contributes at most 3 meV to the Urbach energy. Our modeling reveals that the reason for this small contribution is due to the small effective masses in perovskites, resulting in a natural length scale of around 20 nm for the "effective confining potential" for electrons and holes, with short-range potential fluctuations smoothed out. The increase in Urbach energy across the compositional range agrees well with our optical absorption measurements. We model systems of sizes up to 80 nm in three dimensions, allowing us to accurately reproduce the experimentally observed absorption spectra of perovskites with halide segregation. Our results suggest that we should look beyond static contribution and focus on the dynamic temperature dependent contribution to the Urbach energy.

    View details for DOI 10.1021/acsenergylett.2c02352

    View details for Web of Science ID 000898678000001

    View details for PubMedID 36660372

    View details for PubMedCentralID PMC9841609

  • Taking a closer look - how the microstructure of Dion-Jacobson perovskites governs their photophysics JOURNAL OF MATERIALS CHEMISTRY C Kahmann, S., Duim, H., Rommens, A. J., Frohna, K., ten Brink, G. H., Portale, G., Stranks, S. D., Loi, M. A. 2022; 10 (46): 17539-17549

    Abstract

    Scarce information is available on the thin film morphology of Dion-Jacobson halide perovskites. However, the microstructure can have a profound impact on a material's photophysics and its potential for optoelectronic applications. The microscopic mechanisms at play in the prototypical 1,4-phenylenedimethanammonium lead iodide (PDMAPbI4) Dion-Jacobson compound are here elucidated through a combination of hyperspectral photoluminescence and Raman spectro-microscopy supported by x-ray diffraction. In concert, these techniques allow for a detailed analysis of local composition and microstructure. PDMAPbI4 thin films are shown to be phase-pure and to form micron-sized crystallites with a dominant out-of-plane stacking and strong in-plane rotational disorder. Sample topography, localised defects, and a strong impact of temperature-variation create a complex and heterogeneous picture of the luminescence that cannot be captured by a simplified bulk-semiconductor picture. Our study highlights the power of optical microscopy techniques used in combination, and underlines the danger of conceptual oversimplification when analysing the photophysics of perovskite thin films.

    View details for DOI 10.1039/d2tc04406d

    View details for Web of Science ID 000885653400001

    View details for PubMedID 36561307

    View details for PubMedCentralID PMC9714182

  • Local nanoscale phase impurities are degradation sites in halide perovskites NATURE Macpherson, S., Doherty, T. S., Winchester, A. J., Kosar, S., Johnstone, D. N., Chiang, Y., Galkowski, K., Anaya, M., Frohna, K., Iqbal, A. N., Nagane, S., Roose, B., Andaji-Garmaroudi, Z., Orr, K. P., Parker, J. E., Midgley, P. A., Dani, K. M., Stranks, S. D. 2022; 607 (7918): 294-+

    Abstract

    Understanding the nanoscopic chemical and structural changes that drive instabilities in emerging energy materials is essential for mitigating device degradation. The power conversion efficiency of halide perovskite photovoltaic devices has reached 25.7 per cent in single-junction and 29.8 per cent in tandem perovskite/silicon cells1,2, yet retaining such performance under continuous operation has remained elusive3. Here we develop a multimodal microscopy toolkit to reveal that in leading formamidinium-rich perovskite absorbers, nanoscale phase impurities, including hexagonal polytype and lead iodide inclusions, are not only traps for photoexcited carriers, which themselves reduce performance4,5, but also, through the same trapping process, are sites at which photochemical degradation of the absorber layer is seeded. We visualize illumination-induced structural changes at phase impurities associated with trap clusters, revealing that even trace amounts of these phases, otherwise undetected with bulk measurements, compromise device longevity. The type and distribution of these unwanted phase inclusions depends on the film composition and processing, with the presence of polytypes being most detrimental for film photo-stability. Importantly, we reveal that both performance losses and intrinsic degradation processes can be mitigated by modulating these defective phase impurities, and demonstrate that this requires careful tuning of local structural and chemical properties. This multimodal workflow to correlate the nanoscopic landscape of beam-sensitive energy materials will be applicable to a wide range of semiconductors for which a local picture of performance and operational stability has yet to be established.

    View details for DOI 10.1038/s41586-022-04872-1

    View details for Web of Science ID 000823196000001

    View details for PubMedID 35609624

    View details for PubMedCentralID 4936376

  • Extracting Decay-Rate Ratios From Photoluminescence Quantum Efficiency Measurements in Optoelectronic Semiconductors PHYSICAL REVIEW APPLIED Bowman, A. R., Macpherson, S., Abfalterer, A., Frohna, K., Nagane, S., Stranks, S. D. 2022; 17 (4)

    View details for DOI 10.1103/PhysRevApplied.17.044026

    View details for Web of Science ID 000789248100003

  • Understanding Performance Limiting Interfacial Recombination in <i>pin</i> Perovskite Solar Cells ADVANCED ENERGY MATERIALS Warby, J., Zu, F., Zeiske, S., Gutierrez-Partida, E., Frohloff, L., Kahmann, S., Frohna, K., Mosconi, E., Radicchi, E., Lang, F., Shah, S., Pena-Camargo, F., Hempel, H., Unold, T., Koch, N., Armin, A., De Angelis, F., Stranks, S. D., Neher, D., Stolterfoht, M. 2022; 12 (12)

    View details for DOI 10.1002/aenm.202103567

    View details for Web of Science ID 000753274300001

  • Perovskite Solar Cells with Carbon-Based Electrodes - Quantification of Losses and Strategies to Overcome Them ADVANCED ENERGY MATERIALS Bogachuk, D., Yang, B., Suo, J., Martineau, D., Verma, A., Narbey, S., Anaya, M., Frohna, K., Doherty, T., Mueller, D., Herterich, J. P., Zouhair, S., Hagfeldt, A., Stranks, S. D., Wuerfel, U., Hinsch, A., Wagner, L. 2022; 12 (10)

    View details for DOI 10.1002/aenm.202103128

    View details for Web of Science ID 000749637800001

  • Overcoming Nanoscale Inhomogeneities in Thin-Film Perovskites via Exceptional Post-annealing Grain Growth for Enhanced Photodetection NANO LETTERS Du, T., Richheimer, F., Frohna, K., Gasparini, N., Mohan, L., Min, G., Xu, W., Macdonald, T. J., Yuan, H., Ratnasingham, S. R., Haque, S., Castro, F. A., Durrant, J. R., Stranks, S. D., Wood, S., McLachlan, M. A., Briscoe, J. 2022; 22 (3): 979-988

    Abstract

    Antisolvent-assisted spin coating has been widely used for fabricating metal halide perovskite films with smooth and compact morphology. However, localized nanoscale inhomogeneities exist in these films owing to rapid crystallization, undermining their overall optoelectronic performance. Here, we show that by relaxing the requirement for film smoothness, outstanding film quality can be obtained simply through a post-annealing grain growth process without passivation agents. The morphological changes, driven by a vaporized methylammonium chloride (MACl)-dimethylformamide (DMF) solution, lead to comprehensive defect elimination. Our nanoscale characterization visualizes the local defective clusters in the as-deposited film and their elimination following treatment, which couples with the observation of emissive grain boundaries and excellent inter- and intragrain optoelectronic uniformity in the polycrystalline film. Overcoming these performance-limiting inhomogeneities results in the enhancement of the photoresponse to low-light (<0.1 mW cm-2) illumination by up to 40-fold, yielding high-performance photodiodes with superior low-light detection.

    View details for DOI 10.1021/acs.nanolett.1c03839

    View details for Web of Science ID 000748018400001

    View details for PubMedID 35061402

    View details for PubMedCentralID PMC9007526

  • Stabilized tilted-octahedra halide perovskites inhibit local formation of performance-limiting phases SCIENCE Doherty, T. S., Nagane, S., Kubicki, D. J., Jung, Y., Johnstone, D. N., Iqbal, A. N., Guo, D., Frohna, K., Danaie, M., Tennyson, E. M., Macpherson, S., Abfalterer, A., Anaya, M., Chiang, Y., Crout, P., Ruggeri, F., Collins, S., Grey, C. P., Walsh, A., Midgley, P. A., Stranks, S. D. 2021; 374 (6575): 1598-+

    Abstract

    Efforts to stabilize photoactive formamidinium (FA)–based halide perovskites for perovskite photovoltaics have focused on the growth of cubic formamidinium lead iodide (α-FAPbI3) phases by empirically alloying with cesium, methylammonium (MA) cations, or both. We show that such stabilized FA-rich perovskites are noncubic and exhibit ~2° octahedral tilting at room temperature. This tilting, resolvable only with the use of local nanostructure characterization techniques, imparts phase stability by frustrating transitions from photoactive to hexagonal phases. Although the bulk phase appears stable when examined macroscopically, heterogeneous cation distributions allow microscopically unstable regions to form; we found that these transitioned to hexagonal polytypes, leading to local trap-assisted performance losses and photoinstabilities. Using surface-bound ethylenediaminetetraacetic acid, we engineered an octahedral tilt into pure α-FAPbI3 thin films without any cation alloying. The templated photoactive FAPbI3 film was extremely stable against thermal, environmental, and light stressors.

    View details for DOI 10.1126/science.abl4890

    View details for Web of Science ID 000736589300030

    View details for PubMedID 34941391

  • An open-access database and analysis tool for perovskite solar cells based on the FAIR data principles NATURE ENERGY Jacobsson, T., Hultqvist, A., Garcia-Fernandez, A., Anand, A., Al-Ashouri, A., Hagfeldt, A., Crovetto, A., Abate, A., Ricciardulli, A., Vijayan, A., Kulkarni, A., Anderson, A. Y., Darwich, B., Yang, B., Coles, B. L., Perini, C. R., Rehermann, C., Ramirez, D., Fairen-Jimenez, D., Di Girolamo, D., Jia, D., Avila, E., Juarez-Perez, E. J., Baumann, F., Mathies, F., Gonzalez, G., Boschloo, G., Nasti, G., Paramasivam, G., Martinez-Denegri, G., Nasstrom, H., Michaels, H., Kobler, H., Wu, H., Benesperi, I., Dar, M., Pehlivan, I., Gould, I. E., Vagott, J. N., Dagar, J., Kettle, J., Yang, J., Li, J., Smith, J. A., Pascual, J., Jeronimo-Rendon, J. J., Montoya, J., Correa-Baena, J., Qiu, J., Wang, J., Sveinbjornsson, K., Hirselandt, K., Dey, K., Frohna, K., Mathies, L., Castriotta, L. A., Aldamasy, M. H., Vasquez-Montoya, M., Ruiz-Preciado, M. A., Flatken, M. A., Khenkin, M., Grischek, M., Kedia, M., Saliba, M., Anaya, M., Veldhoen, M., Arora, N., Shargaieva, O., Maus, O., Game, O. S., Yudilevich, O., Fassl, P., Zhou, Q., Betancur, R., Munir, R., Patidar, R., Stranks, S. D., Alam, S., Kar, S., Unold, T., Abzieher, T., Edvinsson, T., David, T., Paetzold, U. W., Zia, W., Fu, W., Zuo, W., Schroeder, V. F., Tress, W., Zhang, X., Chiang, Y., Iqbal, Z., Xie, Z., Unger, E. 2022; 7 (1): 107-115

    View details for DOI 10.1038/s41560-021-00941-3

    View details for Web of Science ID 000729687900004

  • Manipulating Color Emission in 2D Hybrid Perovskites by Fine Tuning Halide Segregation: A Transparent Green Emitter ADVANCED MATERIALS Zanetta, A., Andaji-Garmaroudi, Z., Pirota, V., Pica, G., Kosasih, F., Gouda, L., Frohna, K., Ducati, C., Doria, F., Stranks, S. D., Grancini, G. 2022; 34 (1): e2105942

    Abstract

    Halide perovskite materials offer an ideal playground for easily tuning their color and, accordingly, the spectral range of their emitted light. In contrast to common procedures, this work demonstrates that halide substitution in Ruddlesden-Popper perovskites not only progressively modulates the bandgap, but it can also be a powerful tool to control the nanoscale phase segregation-by adjusting the halide ratio and therefore the spatial distribution of recombination centers. As a result, thin films of chloride-rich perovskite are engineered-which appear transparent to the human eye-with controlled tunable emission in the green. This is due to a rational halide substitution with iodide or bromide leading to a spatial distribution of phases where the minor component is responsible for the tunable emission, as identified by combined hyperspectral photoluminescence imaging and elemental mapping. This work paves the way for the next generation of highly tunable transparent emissive materials, which can be used as light-emitting pixels in advanced and low-cost optoelectronics.

    View details for DOI 10.1002/adma.202105942

    View details for Web of Science ID 000707863100001

    View details for PubMedID 34658076

  • Proton-Radiation Tolerant All-Perovskite Multijunction Solar Cells ADVANCED ENERGY MATERIALS Lang, F., Eperon, G. E., Frohna, K., Tennyson, E. M., Al-Ashouri, A., Kourkafas, G., Bundesmann, J., Denker, A., West, K. G., Hirst, L. C., Neitzert, H., Stranks, S. D. 2021; 11 (41)

    View details for DOI 10.1002/aenm.202102246

    View details for Web of Science ID 000697613600001

  • Unraveling the varied nature and roles of defects in hybrid halide perovskites with time-resolved photoemission electron microscopy ENERGY & ENVIRONMENTAL SCIENCE Kosar, S., Winchester, A. J., Doherty, T. S., Macpherson, S., Petoukhoff, C. E., Frohna, K., Anaya, M., Chan, N. S., Madeo, J., Man, M. L., Stranks, S. D., Dani, K. M. 2021; 14 (12): 6320-6328

    Abstract

    With rapidly growing photoconversion efficiencies, hybrid perovskite solar cells have emerged as promising contenders for next generation, low-cost photovoltaic technologies. Yet, the presence of nanoscale defect clusters, that form during the fabrication process, remains critical to overall device operation, including efficiency and long-term stability. To successfully deploy hybrid perovskites, we must understand the nature of the different types of defects, assess their potentially varied roles in device performance, and understand how they respond to passivation strategies. Here, by correlating photoemission and synchrotron-based scanning probe X-ray microscopies, we unveil three different types of defect clusters in state-of-the-art triple cation mixed halide perovskite thin films. Incorporating ultrafast time-resolution into our photoemission measurements, we show that defect clusters originating at grain boundaries are the most detrimental for photocarrier trapping, while lead iodide defect clusters are relatively benign. Hexagonal polytype defect clusters are only mildly detrimental individually, but can have a significant impact overall if abundant in occurrence. We also show that passivating defects with oxygen in the presence of light, a previously used approach to improve efficiency, has a varied impact on the different types of defects. Even with just mild oxygen treatment, the grain boundary defects are completely healed, while the lead iodide defects begin to show signs of chemical alteration. Our findings highlight the need for multi-pronged strategies tailored to selectively address the detrimental impact of the different defect types in hybrid perovskite solar cells.

    View details for DOI 10.1039/d1ee02055b

    View details for Web of Science ID 000695967600001

    View details for PubMedID 35003331

    View details for PubMedCentralID PMC8658252

  • Multimodal Microscale Imaging of Textured Perovskite-Silicon Tandem Solar Cells. ACS energy letters Tennyson, E. M., Frohna, K., Drake, W. K., Sahli, F., Chien-Jen Yang, T., Fu, F., Werner, J., Chosy, C., Bowman, A. R., Doherty, T. A., Jeangros, Q., Ballif, C., Stranks, S. D. 2021; 6 (6): 2293-2304

    Abstract

    Halide perovskite/crystalline silicon (c-Si) tandem solar cells promise power conversion efficiencies beyond the limits of single-junction cells. However, the local light-matter interactions of the perovskite material embedded in this pyramidal multijunction configuration, and the effect on device performance, are not well understood. Here, we characterize the microscale optoelectronic properties of the perovskite semiconductor deposited on different c-Si texturing schemes. We find a strong spatial and spectral dependence of the photoluminescence (PL) on the geometrical surface constructs, which dominates the underlying grain-to-grain PL variation found in halide perovskite films. The PL response is dependent upon the texturing design, with larger pyramids inducing distinct PL spectra for valleys and pyramids, an effect which is mitigated with small pyramids. Further, optimized quasi-Fermi level splittings and PL quantum efficiencies occur when the c-Si large pyramids have had a secondary smoothing etch. Our results suggest that a holistic optimization of the texturing is required to maximize light in- and out-coupling of both absorber layers and there is a fine balance between the optimal geometrical configuration and optoelectronic performance that will guide future device designs.

    View details for DOI 10.1021/acsenergylett.1c00568

    View details for PubMedID 34307879

    View details for PubMedCentralID PMC8291767

  • Revealing Nanomechanical Domains and Their Transient Behavior in Mixed-Halide Perovskite Films ADVANCED FUNCTIONAL MATERIALS Mela, I., Poudel, C., Anaya, M., Delport, G., Frohna, K., Macpherson, S., Doherty, T. S., Scheeder, A., Stranks, S. D., Kaminski, C. F. 2021; 31 (23)

    View details for DOI 10.1002/adfm.202100293

    View details for Web of Science ID 000635442200001

  • Relaxed Current Matching Requirements in Highly Luminescent Perovskite Tandem Solar Cells and Their Fundamental Efficiency Limits ACS ENERGY LETTERS Bowman, A. R., Lang, F., Chiang, Y., Jimenez-Solano, A., Frohna, K., Eperon, G. E., Ruggeri, E., Abdi-Jalebi, M., Anaya, M., Lotsch, B., Stranks, S. D. 2021; 6 (2): 612-620

    Abstract

    Perovskite-based tandem solar cells are of increasing interest as they approach commercialization. Here we use experimental parameters from optical spectroscopy measurements to calculate the limiting efficiency of perovskite-silicon and all-perovskite two-terminal tandems, employing currently available bandgap materials, as 42.0% and 40.8%, respectively. We show luminescence coupling between subcells (the optical transfer of photons from the high-bandgap to low-bandgap subcell) relaxes current matching when the high-bandgap subcell is a luminescent perovskite. We calculate that luminescence coupling becomes important at charge trapping rates (≤106 s-1) already being achieved in relevant halide perovskites. Luminescence coupling increases flexibility in subcell thicknesses and tolerance to different spectral conditions. For maximal benefit, the high-bandgap subcell should have the higher short-circuit current under average spectral conditions. This can be achieved by reducing the bandgap of the high-bandgap subcell, allowing wider, unstable bandgap compositions to be avoided. Lastly, we visualize luminescence coupling in an all-perovskite tandem through cross-section luminescence imaging.

    View details for DOI 10.1021/acsenergylett.0c02481

    View details for Web of Science ID 000619803400040

    View details for PubMedID 33614966

    View details for PubMedCentralID PMC7887871

  • Unraveling the antisolvent dripping delay effect on the Stranski-Krastanov growth of CH<sub>3</sub>NH<sub>3</sub>PbBr<sub>3</sub> thin films: a facile route for preparing a textured morphology with improved optoelectronic properties PHYSICAL CHEMISTRY CHEMICAL PHYSICS Kumar, J., Kumar, R., Frohna, K., Moghe, D., Stranks, S. D., Bag, M. 2020; 22 (45): 26592-26604

    Abstract

    Inorganic-organic hybrid perovskite materials have been a topic of interest for the last few years due to their superior optoelectronic properties. However, the optical properties of perovskite materials are strongly dependent on the film morphology. A textured film morphology is expected to have higher light absorption as well as light out-coupling efficiency compared to a smooth film. There have been numerous methods for controlling and optimizing the film morphology to achieve high efficiency in solar cells and light emitting diodes. Here we have demonstrated that controlled anti-solvent treatment at low temperature can lead to Stranski-Krastanov growth in CH3NH3PbBr3 thin films with superior optical and electronic properties for light emitting diode applications. We have studied their photoluminescence properties at the micro- to nano-scale via fluorescence microscopy, hyper-spectral imaging and scanning near-field optical microscopy. We have demonstrated that the nanostructured micro-islands are highly emissive because of large quasi-Fermi level splitting (QFLS) due to the localization of free charges in the smaller crystals. We have shown that the photoluminescence as well as electroluminescence can be improved by at least seven-fold due to the presence of micro-islands on a smooth background film enhancing light out-coupling. Photo-induced photoluminescence enhancement is also observed in smooth films while micro-islands show photo-degradation.

    View details for DOI 10.1039/d0cp05467d

    View details for Web of Science ID 000592983900049

    View details for PubMedID 33201960

  • Impact of Mesoporous Silicon Template Pore Dimension and Surface Chemistry on Methylammonium Lead Trihalide Perovskite Photophysics ADVANCED MATERIALS INTERFACES da Costa, V. P., Gonzalez-Rodriguez, R., Frohna, K., Delport, G., Stranks, S. D., Canham, L. T., Coffer, J. L. 2020; 7 (21)

    View details for DOI 10.1002/admi.202001138

    View details for Web of Science ID 000568699000001

  • Stable Hexylphosphonate-Capped Blue-Emitting Quantum-Confined CsPbBr<sub>3</sub> Nanoplatelets ACS ENERGY LETTERS Shamsi, J., Kubicki, D., Anaya, M., Liu, Y., Ji, K., Frohna, K., Grey, C. P., Friend, R. H., Stranks, S. D. 2020; 5 (6): 1900-1907

    Abstract

    Quantum-confined CsPbBr3 nanoplatelets (NPLs) are extremely promising for use in low-cost blue light-emitting diodes, but their tendency to coalesce in both solution and film form, particularly under operating device conditions with injected charge-carriers, is hindering their adoption. We show that employing a short hexyl-phosphonate ligand (C6H15O3P) in a heat-up colloidal approach for pure, blue-emitting quantum-confined CsPbBr3 NPLs significantly suppresses these coalescence phenomena compared to particles capped with the typical oleyammonium ligands. The phosphonate-passivated NPL thin films exhibit photoluminescence quantum yields of ∼40% at 450 nm with exceptional ambient and thermal stability. The color purity is preserved even under continuous photoexcitation of carriers equivalent to LED current densities of ∼3.5 A/cm2. 13C, 133Cs, and 31P solid-state MAS NMR reveal the presence of phosphonate on the surface. Density functional theory calculations suggest that the enhanced stability is due to the stronger binding affinity of the phosphonate ligand compared to the ammonium ligand.

    View details for DOI 10.1021/acsenergylett.0c00935

    View details for Web of Science ID 000541766000024

    View details for PubMedID 32566752

    View details for PubMedCentralID PMC7296617

  • Proton Radiation Hardness of Perovskite Tandem Photovoltaics JOULE Lang, F., Jost, M., Frohna, K., Kohnen, E., Al-Ashouri, A., Bowman, A. R., Bertram, T., Morales-Vilches, A., Koushik, D., Tennyson, E. M., Galkowski, K., Landi, G., Creatore, M., Stannowski, B., Kaufmann, C. A., Bundesmann, J., Rappich, J., Rech, B., Denker, A., Albrecht, S., Neitzert, H., Nickel, N. H., Stranks, S. D. 2020; 4 (5): 1054-1069

    Abstract

    Monolithic [Cs0.05(MA0. 17FA0. 83)0.95]Pb(I0.83Br0.17)3/Cu(In,Ga)Se2 (perovskite/CIGS) tandem solar cells promise high performance and can be processed on flexible substrates, enabling cost-efficient and ultra-lightweight space photovoltaics with power-to-weight and power-to-cost ratios surpassing those of state-of-the-art III-V semiconductor-based multijunctions. However, to become a viable space technology, the full tandem stack must withstand the harsh radiation environments in space. Here, we design tailored operando and ex situ measurements to show that perovskite/CIGS cells retain over 85% of their initial efficiency even after 68 MeV proton irradiation at a dose of 2 × 1012 p+/cm2. We use photoluminescence microscopy to show that the local quasi-Fermi-level splitting of the perovskite top cell is unaffected. We identify that the efficiency losses arise primarily from increased recombination in the CIGS bottom cell and the nickel-oxide-based recombination contact. These results are corroborated by measurements of monolithic perovskite/silicon-heterojunction cells, which severely degrade to 1% of their initial efficiency due to radiation-induced recombination centers in silicon.

    View details for DOI 10.1016/j.joule.2020.03.006

    View details for Web of Science ID 000535806100011

    View details for PubMedID 32467877

    View details for PubMedCentralID PMC7238692

  • Structural and spectroscopic studies of a nanostructured silicon-perovskite interface NANOSCALE Gonzalez-Rodriguez, R., Costa, V. P., Delport, G., Frohna, K., Hoye, R. Z., Stranks, S. D., Coffer, J. L. 2020; 12 (7): 4498-4505

    Abstract

    While extensively investigated in thin film form for energy materials applications, this work investigates the formation of APbBr3 structures (A = CH3NH3+ (MA), Cs+) in silicon and oxidized silicon nanotubes (SiNTs) with varying inner diameter. We carefully control the extent of oxidation of the nanotube host and correlate the relative Si/Si oxide content in a given nanotube host with the photoluminescence quantum efficiency (PLQE) of the perovskite. Complementing these measurements is an evaluation of average PL lifetimes of a given APbBr3 nanostructure, as evaluated by time-resolved confocal photoluminescence measurements. Increasing Si (decreasing oxide) content in the nanotube host results in a sensitive reduction of MAPbBr3 PLQE, with a concomitant decrease in average lifetime (τave). We interpret these observations in terms of decreased defect passivation by a lower concentration of oxide species surrounding the perovskite. In addition, we show that the use of selected nanotube templates leads to more stable perovskite PL in air over time (weeks). Taken in concert, such fundamental observations have implications for interfacial carrier interactions in tandem Si/perovskite photovoltaics.

    View details for DOI 10.1039/c9nr09622a

    View details for Web of Science ID 000531408100026

    View details for PubMedID 32031192

  • Photodoping through local charge carrier accumulation in alloyed hybrid perovskites for highly efficient luminescence NATURE PHOTONICS Feldmann, S., Macpherson, S., Senanayak, S. P., Abdi-Jalebi, M., Rivett, J. H., Nan, G., Tainter, G. D., Doherty, T. S., Frohna, K., Ringe, E., Friend, R. H., Sirringhaus, H., Saliba, M., Beljonne, D., Stranks, S. D., Deschler, F. 2020; 14 (2): 123-+

    View details for DOI 10.1038/s41566-019-0546-8

    View details for Web of Science ID 000511124400013

  • Charge-Carrier Recombination in Halide Perovskites CHEMICAL REVIEWS deQuilettes, D. W., Frohna, K., Emin, D., Kirchartz, T., Bulovic, V., Ginger, D. S., Stranks, S. D. 2019; 119 (20): 11007-11019

    Abstract

    The success of halide perovskites in a host of optoelectronic applications is often attributed to their long photoexcited carrier lifetimes, which has led to charge-carrier recombination processes being described as unique compared to other semiconductors. Here, we integrate recent literature findings to provide a critical assessment of the factors we believe are most likely controlling recombination in the most widely studied halide perovskite systems. We focus on four mechanisms that have been proposed to affect measured charge carrier recombination lifetimes, namely: (1) recombination via trap states, (2) polaron formation, (3) the indirect nature of the bandgap (e.g., Rashba effect), and (4) photon recycling. We scrutinize the evidence for each case and the implications of each process on carrier recombination dynamics. Although they have attracted considerable speculation, we conclude that multiple trapping or hopping in shallow trap states, and the possible indirect nature of the bandgap (e.g., Rashba effect), seem to be less likely given the combined evidence, at least in high-quality samples most relevant to solar cells and light-emitting diodes. On the other hand, photon recycling appears to play a clear role in increasing apparent lifetime for samples with high photoluminescence quantum yields. We conclude that polaron dynamics are intriguing and deserving of further study. We highlight potential interdependencies of these processes and suggest future experiments to better decouple their relative contributions. A more complete understanding of the recombination processes could allow us to rationally tailor the properties of these fascinating semiconductors and will aid the discovery of other materials exhibiting similarly exceptional optoelectronic properties.

    View details for DOI 10.1021/acs.chemrev.9b00169

    View details for Web of Science ID 000492802500001

    View details for PubMedID 31496228

  • A Highly Emissive Surface Layer in Mixed-Halide Multication Perovskites ADVANCED MATERIALS Andaji-Garmaroudi, Z., Abdi-Jalebi, M., Guo, D., Macpherson, S., Sadhanala, A., Tennyson, E. M., Ruggeri, E., Anaya, M., Galkowski, K., Shivanna, R., Lohmann, K., Frohna, K., Mackowski, S., Savenije, T. J., Friend, R. H., Stranks, S. D. 2019; 31 (42): e1902374

    Abstract

    Mixed-halide lead perovskites have attracted significant attention in the field of photovoltaics and other optoelectronic applications due to their promising bandgap tunability and device performance. Here, the changes in photoluminescence and photoconductance of solution-processed triple-cation mixed-halide (Cs0.06 MA0.15 FA0.79 )Pb(Br0.4 I0.6 )3 perovskite films (MA: methylammonium, FA: formamidinium) are studied under solar-equivalent illumination. It is found that the illumination leads to localized surface sites of iodide-rich perovskite intermixed with passivating PbI2 material. Time- and spectrally resolved photoluminescence measurements reveal that photoexcited charges efficiently transfer to the passivated iodide-rich perovskite surface layer, leading to high local carrier densities on these sites. The carriers on this surface layer therefore recombine with a high radiative efficiency, with the photoluminescence quantum efficiency of the film under solar excitation densities increasing from 3% to over 45%. At higher excitation densities, nonradiative Auger recombination starts to dominate due to the extremely high concentration of charges on the surface layer. This work reveals new insight into phase segregation of mixed-halide mixed-cation perovskites, as well as routes to highly luminescent films by controlling charge density and transfer in novel device structures.

    View details for DOI 10.1002/adma.201902374

    View details for Web of Science ID 000485748200001

    View details for PubMedID 31489713

  • Influence of Grain Size on Phase Transitions in Halide Perovskite Films ADVANCED ENERGY MATERIALS Stavrakas, C., Zelewski, S. J., Frohna, K., Booker, E. P., Galkowski, K., Ji, K., Ruggeri, E., Mackowski, S., Kudrawiec, R., Plochocka, P., Stranks, S. D. 2019; 9 (35)

    View details for DOI 10.1002/aenm.201901883

    View details for Web of Science ID 000489733800011

  • Minimizing Current and Voltage Losses to Reach 25% Efficient Monolithic Two-Termina Perovskite-Silicon Tandem Solar Cells ACS ENERGY LETTERS Bush, K. A., Manzoor, S., Frohna, K., Yu, Z. J., Raiford, J. A., Palmstrom, A. F., Wang, H., Prasanna, R., Bent, S. F., Holman, Z. C., McGehee, M. D. 2018; 3 (9): 2173–80

    View details for DOI 10.1021/acsenergylett.8b01201

    View details for Web of Science ID 000445052900023

  • Compositional Engineering for Efficient Wide Band Gap Perovskites with Improved Stability to Photoinduced Phase Segregation ACS ENERGY LETTERS Bush, K. A., Frohna, K., Prasanna, R., Beal, R. E., Leijtens, T., Swifter, S. A., McGehee, M. D. 2018; 3 (2): 428–35

    View details for DOI 10.1021/acs.energy.lett.7b01255

    View details for Web of Science ID 000425560900026

  • Optical and Compositional Engineering of Wide Band Gap Perovskites with Improved Stability to Photoinduced Phase Segregation for Efficient Monolithic Perovskite/Silicon Tandem Solar Cells Bush, K. A., Palmstrom, A. F., Yu, Z. J., Frohna, K., Manzoor, S., Ali, A., Ali, W., Prasanna, R., Beal, R. E., Leijtens, T., Bent, S. F., Holman, Z., McGehee, M. D., IEEE IEEE. 2018: 0189–91

    View details for Web of Science ID 000469200400043

https://orcid.org/0000-0002-2259-6154