|
Juliane Borchert
D.Phil. Candidate
Clarendon Laboratory Room 245
Phone (office): +44 (0) 1865 272339
Phone (lab): +44 (0) 1865 282649
Email:
juliane.borchert@physics.ox.ac.uk
|
Research interests
Vapour deposition of Perovskites
Publications
- 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)
[
|
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. - Limits to electrical mobility in lead-halide perovskite semiconductors,
CQ Xia, JL Peng, S Ponce, JB Patel, AD Wright, TW Crothers, MU Rothmann, J Borchert, RL Milot, H Kraus, QQ Lin, F Giustino, LM Herz, MB Johnston
J. Phys. Chem. Lett., 12:3607-3617 (2021)
[
|
pdf | doi:10.1021/acs.jpclett.1c00619 ]
Semiconducting polycrystalline thin films are cheap to produce and can be deposited on flexible substrates, yet high-performance electronic devices usually utilize single-crystal semiconductors, owing to their superior charge-carrier mobilities and longer diffusion lengths. Here we show that the electrical performance of polycrystalline films of metal-halide perovskites (MHPs) approaches that of single crystals at room temperature. Combining temperature-dependent terahertz conductivity measurements and ab initio calculations we uncover a complete picture of the origins of charge-carrier scattering in single crystals and polycrystalline films of CH3NH3PbI3. We show that Frohlich scattering of charge carriers with multiple phonon modes is the dominant mechanism limiting mobility, with grain-boundary scattering further reducing mobility in polycrystalline films. We reconcile the large discrepancy in charge-carrier diffusion lengths between single crystals and films by considering photon reabsorption. Thus, polycrystalline films of MHPs offer great promise for devices beyond solar cells, including light-emitting diodes and modulators. - Halide segregation in mixed-halide perovskites: influence of a-site cations,
AJ Knight, J Borchert, RDJ Oliver, JB Patel, PG Radaelli, HJ Snaith, MB Johnston, LM Herz
ACS Energy Lett., 6:799-808 (2021)
[
|
pdf | doi:10.1021/acsenergylett.0c02475 ]
Mixed-halide perovskites offer bandgap tunability essential for multijunction solar cells; however, a detrimental halide segregation under light is often observed. Here we combine simultaneous in situ photoluminescence and X-ray diffraction measurements to demonstrate clear differences in compositional and optoelectronic changes associated with halide segregation in MAPb(Br0.5I0.5)(3) and FA(0.83)Cs(0.17)Pb(Br0.4I0.6)(3) films. We report evidence for low-barrier ionic pathways in MAPb(Br0.5I0.5)(3), which allow for the rearrangement of halide ions in localized volumes of perovskite without significant compositional changes to the bulk material. In contrast, FA(0.83)Cs(0.17)Pb(Br0.4I0.6)(3) lacks such low-barrier ionic pathways and is, consequently, more stable against halide segregation. However, under prolonged illumination, it exhibits a considerable ionic rearrangement throughout the bulk material, which may be triggered by an initial demixing of A-site cations, altering the composition of the bulk perovskite and reducing its stability against halide segregation. Our work elucidates links between composition, ionic pathways, and halide segregation, and it facilitates the future engineering of phase-stable mixed-halide perovskites. - Intrinsic quantum confinement in formamidinium lead triiodide perovskite,
AD Wright, G Volonakis, J Borchert, CL Davies, F Giustino, MB Johnston, LM Herz
Nat. Mater., 19:1201 (2020)
[
|
pdf | doi:10.1038/s41563-020-0774-9 ]
Understanding the electronic energy landscape in metal halide perovskites is essential for further improvements in their promising performance in thin-film photovoltaics. Here, we uncover the presence of above-bandgap oscillatory features in the absorption spectra of formamidinium lead triiodide thin films. We attribute these discrete features to intrinsically occurring quantum confinement effects, for which the related energies change with temperature according to the inverse square of the intrinsic lattice parameter, and with peak index in a quadratic manner. By determining the threshold film thickness at which the amplitude of the peaks is appreciably decreased, and through ab initio simulations of the absorption features, we estimate the length scale of confinement to be 10-20 nm. Such absorption peaks present a new and intriguing quantum electronic phenomenon in a nominally bulk semiconductor, offering intrinsic nanoscale optoelectronic properties without necessitating cumbersome additional processing steps. Oscillatory features in the absorption spectra of formamidinium lead triiodide perovskite thin films reveal the occurrence of intrinsic quantum confinement effects with confinement on the scale of tens of nanometres. - 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)
[
|
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. - Charge-carrier trapping dynamics in bismuth-doped thin films of {MAPbBr}$_3$ perovskite,
AM Ulatowski, AD Wright, B Wenger, LRV Buizza, SG Motti, HJ Eggimann, KJ Savill, J Borchert, HJ Snaith, MB Johnston, LM Herz
J. Phys. Chem. Lett., 11:3681-3688 (2020)
[
|
pdf | doi:10.1021/acs.jpclett.0c01048 ]
Successful chemical doping of metal halide perovskites with small amounts of heterovalent metals has attracted recent research attention because of its potential to improve long-term material stability and tune absorption spectra. However, some additives have been observed to impact negatively on optoelectronic properties, highlighting the importance of understanding charge-carrier behavior in doped metal halide perovskites. Here, we present an investigation of charge-carrier trapping and conduction in films of MAPbBr(3) perovskite chemically doped with bismuth. We find that the addition of bismuth has no effect on either the band gap or exciton binding energy of the MAPbBr(3) host. However, we observe a substantial enhancement of electron-trapping defects upon bismuth doping, which results in an ultrafast charge-carrier decay component, enhanced infrared emission, and a notable decrease of charge-carrier mobility. We propose that such defects arise from the current approach to Bi-doping through addition of BiBr3, which may enhance the presence of bromide interstitials. - 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)
[
|
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. - Impurity Tracking Enables Enhanced Control and Reproducibility of Hybrid Perovskite Vapor Deposition,
J Borchert, I Levchuk, LC Snoek, MU Rothmann, R Haver, HJ Snaith, CJ Brabec, LM Herz, MB Johnston
ACS Appl. Mater. Interfaces, 11:28851-28857 (2019)
[
|
pdf | doi:10.1021/acsami.9b07619 ]
Metal halide perovskite semiconductors have the potential to enable low-cost, flexible, and efficient solar cells for a wide range of applications. Physical vapor deposition by co-evaporation of precursors is a method that results in very smooth and pinhole-free perovskite thin films and allows excellent control over film thickness and composition. However, for a deposition method to become industrially scalable, reproducible process control and high device yields are essential. Unfortunately, to date, the control and reproducibility of evaporating organic precursors such as methylammonium iodide (MAI) have proved extremely challenging. We show that the established method of controlling the evaporation rate of MAI with quartz microbalances (QMBs) is critically sensitive to the concentration of the impurities MAH2PO3 and MAH2PO2 that are usually present in MAI after synthesis. Therefore, controlling the deposition rate of MAI with QMBs is unreliable since the concentration of such impurities typically varies from one batch of MAI to another and even during the course of a deposition. However once reliable control of MAI deposition is achieved, we find that the presence of precursor impurities during perovskite deposition does not degrade the solar cell performance. Our results indicate that as long as precursor deposition rates are well controlled, physical vapor deposition will allow high solar cell device yields even if the purity of precursors changes from one run to another. - Impact of the organic cation on the optoelectronic properties of formamidinium lead triiodide,
CL Davies, J Borchert, CQ Xia, RL Milot, H Kraus, MB Johnston, LM Herz
J. Phys. Chem. Lett., 9:4502-4511 (2018)
[
|
pdf | doi:10.1021/acs.jpclett.8b01628 ]
Metal halide perovskites have proven to be excellent light-harvesting materials in photovoltaic devices whose efficiencies are rapidly improving. Here, we examine the temperature-dependent photon absorption, exciton binding energy, and band gap of FAPbI(3) (thin film) and find remarkably different behavior across the beta-gamma phase transition compared with MAPbI(3). While MAPbI(3) has shown abrupt changes in the band gap and exciton binding energy, values for FAPbI(3) vary smoothly over a range of 100-160 K in accordance with a more gradual transition. In addition, we find that the charge-carrier mobility in FAPbI(3) exhibits a clear T-0.5 trend with temperature, in excellent agreement with theoretical predictions that assume electron-phonon interactions to be governed by the Frohlich mechanism but in contrast to the T-0.5 dependence previously observed for MAPbI(3). Finally, we directly observe intraexcitonic transitions in FAPbI(3) at low temperature, from which we determine a low exciton binding energy of only 5.3 meV at 10 K. - Large-Area, Highly Uniform Evaporated Formamidinium Lead Triiodide Thin Films for Solar Cells,
J Borchert, RL Milot, JB Patel, CL Davies, AD Wright, L Martinez Maestro, HJ Snaith, LM Herz, MB Johnston
ACS Energy Lett., 2:2799-2804 (2017)
[
|
pdf | doi:10.1021/acsenergylett.7b00967 ]
Perovskite thin-film solar cells are one of the most promising emerging renewable energy technologies because of their potential for low-cost, large-area fabrication combined with high energy conversion efficiencies. Recently, formamidinium lead triiodide ($rm {FAPbI_3}$) and other formamidinium (CH(NH$_2$)$_2$) based perovskites have been explored as interesting alternatives to methylammonium lead triiodide ($rm {MAPbI_3}$), because they exhibit better thermal stability. However at present a major challenge is up-scaling of perovskite solar cells from small test-cells to full solar modules. We show that co-evaporation is a scalable method for the deposition of homogeneous $rm FAPbI_3$ thin-films over large areas. The method allows precise control over film thickness and results in highly uniform, pin-hole free layers. Our films exhibited a high charge-carrier mobility of 26,$rm cm^2 V^{-1}s^{-1}$, excellent optical properties and a bimolecular recombination constant of $7times10^{-11}$,cm$^3$s$^{-1}$. Solar cells fabricated using these vapor-deposited layers within a regular device architecture produced stabilized power conversion efficiencies of up to 14.2,$rm%$. Thus we demonstrate that efficient $rm FAPbI_3$ solar cells can be vapor-deposited, which opens up a pathway towards large-area stable perovskite photovoltaics. - Structural investigation of co-evaporated methyl ammonium lead halide perovskite films during growth and thermal decomposition using different PbX2 (X = I, Cl) precursors,
J Borchert, H Boht, W Fraenzel, R Csuk, R Scheer, P Pistor
J. Mater. Chem. A, 3:19842--19849 (2015)
[
|
pdf | doi:10.1039/C5TA04944J ]
While the progress in device development of perovskite solar cells is rapidly evolving, details of the film formation and the interplay of processing parameters, structural and compositional properties of deposited phases and their stability are still under dispute. Here we present a detailed structural analysis of methylammonium lead halide (I, Cl) films by in situ X-ray diffraction during their growth and thermal recrystallization up to their decomposition. MAPbI3 films grown by co-evaporating MAI and PbI2 are compared to MAPbI3(Cl) films derived from an evaporation route using MAI and PbCl2 precursors. The main differences observed between the two routes are varying crystal structures at room temperature and differently limited process windows, but similar overall growth, recrystallization and decomposition features. The preferential orientation of the pure MAPbI3 is shown to depend on the applied molar precursor flux ratio and can additionally be modified by thermal annealing.
|
|
