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Dr James Ball
Postdoctoral Research Fellow
Robert Hooke Room G26
Phone (office): +44 (0) 1865 282327
Phone (lab): +44 (0) 1865 282649
Email:
james.ball@physics.ox.ac.uk
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Research interests
Metal halide perovskite semiconductors.
Publications
- 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. - Intermediate-phase engineering via dimethylammonium cation additive for stable perovskite solar cells,
DP McMeekin, P Holzhey, SO Funrer, SP Harvey, LT Schelhas, JM Ball, S Mahesh, S Seo, N Hawkins, JF Lu, MB Johnston, JJ Berry, U Bach, HJ Snaith
Nat. Mater., 22:73--83 (2023)
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pdf | doi:10.1038/s41563-022-01399-8 ]
Achieving the long-term stability of perovskite solar cells is arguably the most important challenge required to enable widespread commercialization. Understanding the perovskite crystallization process and its direct impact on device stability is critical to achieving this goal. The commonly employed dimethyl-formamide/dimethyl-sulfoxide solvent preparation method results in a poor crystal quality and microstructure of the polycrystalline perovskite films. In this work, we introduce a high-temperature dimethyl-sulfoxide-free processing method that utilizes dimethylammonium chloride as an additive to control the perovskite intermediate precursor phases. By controlling the crystallization sequence, we tune the grain size, texturing, orientation (corner-up versus face-up) and crystallinity of the formamidinium (FA)/caesium (FA)(y)Cs1-yPb(IxBr1-x)(3) perovskite system. A population of encapsulated devices showed improved operational stability, with a median T80 lifetime (the time over which the device power conversion efficiency decreases to 80% of its initial value) for the steady-state power conversion efficiency of 1,190 hours, and a champion device showed a T80 of 1,410 hours, under simulated sunlight at 65 & DEG;C in air, under open-circuit conditions. This work highlights the importance of material quality in achieving the long-term operational stability of perovskite optoelectronic devices. - 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, Q 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. - Metal composition influences optoelectronic quality in mixed-metal lead-tin triiodide perovskite solar absorbers,
MT Klug, RL Milot, JB Patel, T Green, HC Sansom, MD Farrar, AJ Ramadan, S Martani, ZP Wang, B Wenger, JM Ball, L Langshaw, A Petrozza, MB Johnston, LM Herz, HJ Snaith
Energy Environ. Sci., 13:1776-1787 (2020)
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pdf | doi:10.1039/d0ee00132e ]
Current designs for all-perovskite multi-junction solar cells require mixed-metal Pb-Sn compositions to achieve narrower band gaps than are possible with their neat Pb counterparts. The lower band gap range achievable with mixed-metal Pb-Sn perovskites also encompasses the 1.3 to 1.4 eV range that is theoretically ideal for maximising the efficiency of single-junction devices. Here we examine the optoelectronic quality and photovoltaic performance of the ((HC(NH2)(2))(0.83)Cs-0.17)(Pb1-ySny)I(3)family of perovskite materials across the full range of achievable band gaps by substituting between 0.001% and 70% of the Pb content with Sn. We reveal that a compositional range of "defectiveness" exists when Sn comprises between 0.5% and 20% of the metal content, but that the optoelectronic quality is restored for Sn content between 30-50%. When only 1% of Pb content is replaced by Sn, we find that photoconductivity, photoluminescence lifetime, and photoluminescence quantum efficiency are reduced by at least an order of magnitude, which reveals that a small concentration of Sn incorporation produces trap sites that promote non-radiative recombination in the material and limit photovoltaic performance. While these observations suggest that band gaps between 1.35 and 1.5 eV are unlikely to be useful for optoelectronic applications without countermeasures to improve material quality, highly efficient narrower band gap absorber materials are possible at or below 1.33 eV. Through optimising single-junction photovoltaic devices with Sn compositions of 30% and 50%, we respectively demonstrate a 17.6% efficient solar cell with an ideal single-junction band gap of 1.33 eV and an 18.1% efficient low band gap device suitable for the bottom absorber in all-perovskite multi-junction cells. - 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. - Dual-source coevaporation of low-bandgap $FA_{1-x}Cs_xSn_{1-y}Pb_yI_3$ perovskites for photovoltaics,
JM Ball, L Buizza, HC Sansom, MD Farrar, MT Klug, J Borchert, J Patel, LM Herz, MB Johnston, HJ Snaith
ACS Energy Lett., 4:2748-2756 (2019)
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pdf | doi:10.1021/acsenergylett.9b01855 ]
Perovskite halides are well-suited to monolithic multijunction photovoltaics, promising low-cost solar-to-electrical power conversion. Critical to all-perovskite multijunction fabrication is the deposition of a low-bandgap absorber without damaging other device layers. Vapor deposition is thus an attractive method, obviating the need for optically lossy protective interlayers, but is challenging for multicomponent perovskites. Here, we demonstrate a method to dual-source coevaporate low-bandgap perovskite films and devices. We used mixtures formed by melting of metal halides as a single-crucible source of Cs, Pb, and Sn cations. Surprisingly, when this melt was coevaporated with formamidinium iodide (FM), uniform and dense perovskite films in the family( )FA(1-x)Cs(x)Sn(1-y)Pb(y)I(3 )were formed. Inclusion of SnF2 in the melt helped to regulate the perovskite's optoelectronic quality, leading to a steady-state power conversion efficiency of similar to 10% in a solar cell. This represents a new processing paradigm for evaporated perovskite alloys, which is an important step toward all-perovskite multijunction photovoltaics. - Solution-processed all-perovskite multi-junction solar cells,
DP McMeekin, S Mahesh, NK Noel, MT Klug, J Lim, JH Warby, JM Ball, LM Herz, MB Johnston, HJ Snaith
Joule, 3:387-401 (2019)
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pdf | doi:10.1016/j.joule.2019.01.007 ]
Multi-junction device architectures can increase the power conversion efficiency (PCE) of photovoltaic (PV) cells beyond the single-junction thermodynamic limit. However, these devices are challenging to produce by solution-based methods, where dissolution of underlying layers is problematic. By employing a highly volatile acetonitrile(CH3CN)/methylamine(CH3NH2) (ACN/MA) solvent-based perovskite solution, we demonstrate fully solution-processed absorber, transport, and recombination layers for monolithic all-perovskite tandem and triple-junction solar cells. By combining FA(0.83)Cs(0.17)Pb(Br0.7I0.3)(3) (1.94 eV) and MAPbI(3) (1.57 eV) junctions, we reach two-terminal tandem PCEs of more than 15% (steady state). We show that a MAPb(0.75)Sn(0.25)I(3) (1.34 eV) narrow band-gap perovskite can be processed via the ACN/MA solvent-based system, demonstrating the first proof-of-concept, monolithic all-perovskite triple-junction solar cell with an open-circuit voltage reaching 2.83 V. Through optical and electronic modeling, we estimate the achievable PCE of a state-of-the-art triple-junction device architecture to be 26.7%. Our work opens new possibilities for large-scale, low-cost, printable perovskite multi-junction solar cells. - Elucidating the long-range charge carrier mobility in metal halide perovskite thin films,
J Lim, MT Horantner, N Sakai, JM Ball, S Mahesh, NK Noel, YH Lin, JB Patel, DP McMeekin, MB Johnston, B Wenger, HJ Snaith
Energy Environ. Sci., 12:169-176 (2019)
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pdf | doi:10.1039/c8ee03395a ]
Many optoelectronic properties have been reported for lead halide perovskite polycrystalline films. However, ambiguities in the evaluation of these properties remain, especially for long-range lateral charge transport, where ionic conduction can complicate interpretation of data. Here we demonstrate a new technique to measure the long-range charge carrier mobility in such materials. We combine quasi-steady-state photo-conductivity measurements (electrical probe) with photo-induced transmission and reflection measurements (optical probe) to simultaneously evaluate the conductivity and charge carrier density. With this knowledge we determine the lateral mobility to be approximate to 2 cm(2) V-1 s(-1) for CH3NH3PbI3 (MAPbI(3)) polycrystalline perovskite films prepared from the acetonitrile/methylamine solvent system. Furthermore, we present significant differences in long-range charge carrier mobilities, from 2.2 to 0.2 cm(2) V-1 s(-1), between films of contemporary perovskite compositions prepared via different fabrication processes, including solution and vapour phase deposition techniques. Arguably, our work provides the first accurate evaluation of the long-range lateral charge carrier mobility in lead halide perovskite films, with charge carrier density in the range typically achieved under photovoltaic operation. - Plasmonic-induced photon recycling in metal halide perovskite solar cells,
M Saliba, W Zhang, VM Burlakov, SD Stranks, Y Sun, JM Ball, MB Johnston, A Goriely, U Wiesner, HJ Snaith
Adv. Funct. Mater., 25:5038-5046 (2015)
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pdf | doi:10.1002/adfm.201500669 ]
Organic-inorganic metal halide perovskite solar cells have emerged in the past few years to promise highly efficient photovoltaic devices at low costs. Here, temperature-sensitive core-shell Ag@TiO2 nanoparticles are successfully incorporated into perovskite solar cells through a low-temperature processing route, boosting the measured device efficiencies up to 16.3%. Experimental evidence is shown and a theoretical model is developed which predicts that the presence of highly polarizable nanoparticles enhances the radiative decay of excitons and increases the reabsorption of emitted radiation, representing a novel photon recycling scheme. The work elucidates the complicated subtle interactions between light and matter in plasmonic photovoltaic composites. Photonic and plasmonic schemes such as this may help to move highly efficient perovskite solar cells closer to the theoretical limiting efficiencies. - Optical properties and limiting photocurrent of thin-film perovskite solar cells,
JM Ball, SD Stranks, MT Horantner, S Huttner, W Zhang, EJW Crossland, I Ramirez, M Riede, MB Johnston, RH Friend, HJ Snaith
Energy Environ. Sci., 8:602-609 (2015)
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pdf | doi:10.1039/C4EE03224A ]
Metal-halide perovskite light-absorbers have risen to the forefront of photovoltaics research offering the potential to combine low-cost fabrication with high power-conversion efficiency. Much of the development has been driven by empirical optimisation strategies to fully exploit the favourable electronic properties of the absorber layer. To build on this progress a full understanding of the device operation requires a thorough optical analysis of the device stack providing a platform for maximising the power conversion efficiency through a precise determination of parasitic losses caused by coherence and absorption in the non-photoactive layers. Here we use an optical model based on the transfer-matrix formalism for analysis of perovskite-based planar heterojunction solar cells using experimentally determined complex refractive index data. We compare the modelled properties to experimentally determined data and obtain good agreement revealing that the internal quantum efficiency in the solar cells approaches 100percent. The modelled and experimental dependence of the photocurrent on incidence angle exhibits only a weak variation{,} with very low reflectivity losses at all angles highlighting the potential for useful power generation over a full daylight cycle.
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