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Kilian Lohmann
D.Phil. Candidate
Clarendon Laboratory Room 245
Phone (office): +44 (0) 1865 272339
Phone (lab): +44 (0) 1865 282649
Email:
kilian.lohmann@physics.ox.ac.uk
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Research interests
Vapour depostion of solar cells
Publications
- In situ nanoscopy of single-grain nanomorphology and ultrafast carrier dynamics in metal halide perovskites,
M Zizlsperger, S Nerreter, Q Yuan, KB Lohmann, F Sandner, F Schiegl, C Meineke, YA Gerasimenko, LM Herz, T Siday, MA Huber, MB Johnston, R Huber
Nat. Photon., 18:21 (2024)
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pdf | doi:10.1038/s41566-024-01476-1 ]
Designing next-generation light-harvesting devices requires a detailed understanding of the transport of photoexcited charge carriers. The record-breaking efficiencies of metal halide perovskite solar cells have been linked to effective charge-carrier diffusion, yet the exact nature of charge-carrier out-of-plane transport remains notoriously difficult to explain. The characteristic spatial inhomogeneity of perovskite films with nanograins and crystallographic disorder calls for the simultaneous and hitherto elusive in situ resolution of the chemical composition, the structural phase and the ultrafast dynamics of the local out-of-plane transport. Here we simultaneously probe the intrinsic out-of-plane charge-carrier diffusion and the nanoscale morphology by pushing depth-sensitive terahertz near-field nanospectroscopy to extreme subcycle timescales. In films of the organic-inorganic metal halide perovskite FA0.83Cs0.17Pb(I1-xClx)3 (where FA is formamidinium), domains of the cubic alpha-phase are clearly distinguished from the trigonal delta-phase and PbI2 nano-islands. By analysing deep-subcycle time shifts of the scattered terahertz waveform after photoexcitation, we access the vertical charge-carrier dynamics within single grains. At all of the measured locations, despite topographic irregularities, diffusion is surprisingly homogeneous on the 100 nm scale, although it varies between mesoscopic regions. Linking in situ carrier transport with nanoscale morphology and chemical composition could introduce a paradigm shift for the analysis and optimization of next-generation optoelectronics that are based on nanocrystalline materials. Transient visible-pump terahertz-probe near-field microscopy enables the simultaneous retrieval of the local chemical composition, crystallographic structure, topography and out-of-plane charge-carrier diffusion in perovskite films. - Thermally stable perovskite solar cells by all-vacuum deposition,
QM Yuan, KB Lohmann, RDJ Oliver, AJ Ramadan, SY Yan, JM Ball, MG Christoforo, NK Noel, HJ Snaith, LM Herz, MB Johnston
ACS Appl. Mater. Interfaces, 15:772-781 (2023)
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pdf | doi:10.1021/acsami.2c14658 ]
Vacuum deposition is a solvent-free method suitable for growing thin films of metal halide perovskite (MHP) semiconductors. However, most reports of high-efficiency solar cells based on such vacuum-deposited MHP films incorporate solution-processed hole transport layers (HTLs), thereby complicating prospects of industrial upscaling and potentially affecting the overall device stability. In this work, we investigate organometallic copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) as alternative, low-cost, and durable HTLs in all-vacuum-deposited solvent-free formamidinium-cesium lead triodide [CH(NH2)2]0.83Cs0.17PbI3 (FACsPbI3) perovskite solar cells. We elucidate that the CuPc HTL, when employed in an "inverted" p-i-n solar cell configuration, attains a solar-to-electrical power conversion efficiency of up to 13.9%. Importantly, unencapsulated devices as large as 1 cm2 exhibited excellent longterm stability, demonstrating no observable degradation in efficiency after more than 5000 h in storage and 3700 h under 85 degrees C thermal stressing in N2 atmosphere. - Controlling intrinsic quantum confinement in formamidinium lead triiodide perovskite through cs substitution,
KA Elmestekawy, AD Wright, KB Lohmann, J Borchert, MB Johnston, LM Herz
ACS Nano, 16:9640-9650 (2022)
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pdf | doi:10.1021/acsnano.2c02970 ]
Lead halide perovskites are leading candidates for photovoltaic and light-emitting devices, owing to their excellent and widely tunable optoelectronic properties. Nanostructure control has been central to their development, allowing for improvements in efficiency and stability, and changes in electronic dimensionality. Recently, formamidinium lead triiodide (FAPbI3) has been shown to exhibit intrinsic quantum confinement effects in nominally bulk thin films, apparent through above-bandgap absorption peaks. Here, we show that such nanoscale electronic effects can be controlled through partial replacement of the FA cation with Cs. We find that Cs-cation exchange causes a weakening of quantum confinement in the perovskite, arising from changes in the bandstructure, the length scale of confinement, or the presence of delta H-phase electronic barriers. We further observe photon emission from quantum-confined regions, highlighting their potential usefulness to light-emitting devices and single-photon sources. Overall, controlling this intriguing quantum phenomenon will allow for its suppression or enhancement according to need. - Solvent-Free Method for Defect Reduction and Improved Performance of {p-i-n} Vapor-Deposited Perovskite Solar Cells,
KB Lohmann, SG Motti, RDJ Oliver, AJ Ramadan, HC Sansom, QM Yuan, KA Elmestekawy, JB Patel, JM Ball, LM Herz, HJ Snaith, MB Johnston
ACS Energy Lett., 7:1903-1911 (2022)
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pdf | doi:10.1021/acsenergylett.2c00865 ]
As perovskite-based photovoltaics near commercialization, it is imperative to develop industrial-scale defect-passivation techniques. Vapor deposition is a solvent-free fabrication technique that is widely implemented in industry and can be used to fabricate metal-halide perovskite thin films. We demonstrate markably improved growth and optoelectronic properties for vapor-deposited [CH(NH2)2]0.83Cs0.17PbI3 perovskite solar cells by partially substituting PbI2 for PbCl2 as the inorganic precursor. We find the partial substitution of PbI2 for PbCl2 enhances photoluminescence lifetimes from 5.6 ns to over 100 ns, photoluminescence quantum yields by more than an order of magnitude, and charge-carrier mobility from 46 cm2/(V s) to 56 cm2/(V s). This results in improved solar-cell power conversion efficiency, from 16.4% to 19.3% for the devices employing perovskite films deposited with 20% substitution of PbI2 for PbCl2. Our method presents a scalable, dry, and solvent-free route to reducing nonradiative recombination centers and hence improving the performance of vapor-deposited metal-halide perovskite solar cells. - Atomic-scale microstructure of metal halide perovskite,
MU Rothmann, JS Kim, J Borchert, KB Lohmann, CM O'Leary, AA Sheader, L Clark, HJ Snaith, MB Johnston, PD Nellist, LM Herz
Science, 370:eabb5940 (2020)
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pdf | doi:10.1126/science.abb5940 ]
Hybrid organic-inorganic perovskites have high potential as materials for solar energy applications, but their microscopic properties are still not well understood. Atomic-resolution scanning transmission electron microscopy has provided invaluable insights for many crystalline solar cell materials, and we used this method to successfully image formamidinium lead triiodide [CH(NH2)(2)Pbl(3)] thin films with a low dose of electron irradiation. Such images reveal a highly ordered atomic arrangement of sharp grain boundaries and coherent perovskite/Pbl(2) interfaces, with a striking absence of long-range disorder in the crystal. We found that beam-induced degradation of the perovskite leads to an initial loss of formamidinium [CH(NH2)(2)] ions, leaving behind a partially unoccupied perovskite lattice, which explains the unusual regenerative properties of these materials. We further observed aligned point defects and climb-dissociated dislocations. Our findings thus provide an atomic-level understanding of technologically important lead halide perovskites. - Light absorption and recycling in hybrid metal halide perovskite photovoltaic devices,
JB Patel, AD Wright, KB Lohmann, K Peng, CQ Xia, JM Ball, NK Noel, TW Crothers, J Wong-leung, HJ Snaith, LM Herz, MB Johnston
Adv. Energy Mater., 10:1903653 (2020)
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pdf | doi:10.1002/aenm.201903653 ]
The production of highly efficient single- and multijunction metal halide perovskite (MHP) solar cells requires careful optimization of the optical and electrical properties of these devices. Here, precise control of CH3NH3PbI3 perovskite layers is demonstrated in solar cell devices through the use of dual source coevaporation. Light absorption and device performance are tracked for incorporated MHP films ranging from approximate to 67 nm to approximate to 1.4 mu m thickness and transfer-matrix optical modeling is utilized to quantify optical losses that arise from interference effects. Based on these results, a device with 19.2% steady-state power conversion efficiency is achieved through incorporation of a perovskite film with near-optimum predicted thickness (approximate to 709 nm). Significantly, a clear signature of photon reabsorption is observed in perovskite films that have the same thickness (approximate to 709 nm) as in the optimized device. Despite the positive effect of photon recycling associated with photon reabsorption, devices with thicker (>750 nm) MHP layers exhibit poor performance owing to competing nonradiative charge recombination in a "dead-volume" of MHP. Overall, these findings demonstrate the need for fine control over MHP thickness to achieve the highest efficiency cells, and accurate consideration of photon reabsorption, optical interference, and charge transport properties. - Control over Crystal Size in Vapor Deposited Metal-Halide Perovskite Films,
KB Lohmann, JB Patel, MU Rothmann, CQ Xia, RDJ Oliver, LM Herz, HJ Snaith, MB Johnston
ACS Energy Lett., 5:710-717 (2020)
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pdf | doi:10.1021/acsenergylett.0c00183 ]
Understanding and controlling grain growth in metal halide perovskite polycrystalline thin films is an important step in improving the performance of perovskite solar cells. We demonstrate accurate control of crystallite size in CH3NH3PbI3 thin films by regulating substrate temperature during vacuum co-deposition of inorganic (PbI2) and organic (CH3NH3I) precursors. Films co-deposited onto a cold (−2 °C) substrate exhibited large, micrometer-sized crystal grains, while films that formed at room temperature (23 °C) only produced grains of 100 nm extent. We isolated the effects of substrate temperature on crystal growth by developing a new method to control sublimation of the organic precursor, and CH3NH3PbI3 solar cells deposited in this way yielded a power conversion efficiency of up to 18.2%. Furthermore, we found substrate temperature directly affects the adsorption rate of CH3NH3I, thus impacting crystal formation and hence solar cell device performance via changes to the conversion rate of PbI2 to CH3NH3PbI3 and stoichiometry. These findings offer new routes to developing efficient solar cells through reproducible control of crystal morphology and composition.
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