Quick links:

Group

• Homepage
• People
• Vacancies

Research

• Publications
• Introduction to THz Spectroscopy
• Virtual THz Spectrometer
• Collaborations
• Support

Teaching

• Condensed Matter Physics Major Option
• CCC Tutorials [ox only]

Contact details

• Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, UK
• How to find us including a map
• Interactive map
• University of Oxford contact search

Resources

• Intranet
• Useful Links

Research interests

Terahertz spectroscopy. Perovksite solar cells.

Publications

1. Ultrafast photo-induced phonon hardening due to pauli blocking in {MAPbI}$_3$ single-crystal and polycrystalline perovskites,
   CQ Xia, S Ponce, JL Peng, AM Ulatowski, JB Patel, AD Wright, RL Milot, H Kraus, QQ Lin, LM Herz, F Giustino, MB Johnston
   J. Phys-Mater., 4:044017 (2021) [ abstract | pdf | doi:10.1088/2515-7639/ac22b9 ]
   Metal-halide perovskite semiconductors have attracted intense interest over the past decade, particularly for applications in photovoltaics. Low-energy optical phonons combined with significant crystal anharmonicity play an important role in charge-carrier cooling and scattering in these materials, strongly affecting their optoelectronic properties. We have observed optical phonons associated with Pb-I stretching in both MAPbI(3) single crystals and polycrystalline thin films as a function of temperature by measuring their terahertz conductivity spectra with and without photoexcitation. An anomalous bond hardening was observed under above-bandgap illumination for both single-crystal and polycrystalline MAPbI(3). First-principles calculations reproduced this photo-induced bond hardening and identified a related lattice contraction (photostriction), with the mechanism revealed as Pauli blocking. For single-crystal MAPbI(3), phonon lifetimes were significantly longer and phonon frequencies shifted less with temperature, compared with polycrystalline MAPbI(3). We attribute these differences to increased crystalline disorder, associated with grain boundaries and strain in the polycrystalline MAPbI(3). Thus we provide fundamental insight into the photoexcitation and electron-phonon coupling in MAPbI(3).
2. 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) [ abstract | 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.
3. Nanotechnology for catalysis and solar energy conversion,
   U Banin, N Waiskopf, L Hammarstrom, G Boschloo, M Freitag, EMJ Johansson, J Sa, H Tian, MB Johnston, LM Herz, RL Milot, MG Kanatzidis, W Ke, I Spanopoulos, KL Kohlstedt, GC Schatz, N Lewis, T Meyer, AJ Nozik, MC Beard
   Nanotechnology, 32:042003 (2021) [ abstract | pdf | doi:10.1088/1361-6528/abbce8 ]
   This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the application of nanotechnology in addressing the current challenges of energy conversion: 'high efficiency, stability, safety, and the potential for low-cost/scalable manufacturing' to quote from the contributed article by Nathan Lewis. This roadmap focuses on solar-to-fuel conversion, solar water splitting, solar photovoltaics and bio-catalysis. It includes dye-sensitized solar cells (DSSCs), perovskite solar cells, and organic photovoltaics. Smart engineering of colloidal quantum materials and nanostructured electrodes will improve solar-to-fuel conversion efficiency, as described in the articles by Waiskopf and Banin and Meyer. Semiconductor nanoparticles will also improve solar energy conversion efficiency, as discussed by Boschloo et al in their article on DSSCs. Perovskite solar cells have advanced rapidly in recent years, including new ideas on 2D and 3D hybrid halide perovskites, as described by Spanopoulos et al 'Next generation' solar cells using multiple exciton generation (MEG) from hot carriers, described in the article by Nozik and Beard, could lead to remarkable improvement in photovoltaic efficiency by using quantization effects in semiconductor nanostructures (quantum dots, wires or wells). These challenges will not be met without simultaneous improvement in nanoscale characterization methods. Terahertz spectroscopy, discussed in the article by Milot et al is one example of a method that is overcoming the difficulties associated with nanoscale materials characterization by avoiding electrical contacts to nanoparticles, allowing characterization during device operation, and enabling characterization of a single nanoparticle. Besides experimental advances, computational science is also meeting the challenges of nanomaterials synthesis. The article by Kohlstedt and Schatz discusses the computational frameworks being used to predict structure-property relationships in materials and devices, including machine learning methods, with an emphasis on organic photovoltaics. The contribution by Megarity and Armstrong presents the 'electrochemical leaf' for improvements in electrochemistry and beyond. In addition, biohybrid approaches can take advantage of efficient and specific enzyme catalysts. These articles present the nanoscience and technology at the forefront of renewable energy development that will have significant benefits to society.
4. 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) [ abstract | 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.
5. Charge-carrier dynamics, mobilities, and diffusion lengths of 2d-3d hybrid butylammonium-cesium-formamidinium lead halide perovskites,
   LRV Buizza, TW Crothers, ZP Wang, JB Patel, RL Milot, HJ Snaith, MB Johnston, LM Herz
   Adv. Funct. Mater., 29:1902656 (2019) [ abstract | pdf | doi:10.1002/adfm.201902656 ]
   Perovskite solar cells (PSCs) have improved dramatically over the past decade, increasing in efficiency and gradually overcoming hurdles of temperature- and humidity-induced instability. Materials that combine high charge-carrier lifetimes and mobilities, strong absorption, and good crystallinity of 3D perovskites with the hydrophobic properties of 2D perovskites have become particularly promising candidates for use in solar cells. In order to fully understand the optoelectronic properties of these 2D-3D hybrid systems, the hybrid perovskite BA(x)(FA(0.83)Cs(0.17))(1-x)Pb(I0.6Br0.4)(3) is investigated across the composition range 0 <= x <= 0.8. Small amounts of butylammonium (BA) are found that help to improve crystallinity and appear to passivate grain boundaries, thus reducing trap-mediated charge-carrier recombination and enhancing charge-carrier mobilities. Excessive amounts of BA lead to poor crystallinity and inhomogeneous film formation, greatly reducing effective charge-carrier mobility. For low amounts of BA, the benevolent effects of reduced recombination and enhanced mobilities lead to charge-carrier diffusion lengths up to 7.7 mu m for x = 0.167. These measurements pave the way for highly efficient, highly stable PSCs and other optoelectronic devices based on 2D-3D hybrid materials.
6. The effects of doping density and temperature on the optoelectronic properties of formamidinium tin triiodide thin films,
   RL Milot, MT Klug, CL Davies, Z Wang, H Kraus, HJ Snaith, MB Johnston, LM Herz
   Adv. Mater., 30:1804506 (2018) [ abstract | pdf | doi:10.1002/adma.201804506 ]
   Optoelectronic properties are unraveled for formamidinium tin triiodide (FASnI(3)) thin films, whose background hole doping density is varied through SnF2 addition during film fabrication. Monomolecular charge-carrier recombination exhibits both a dopant-mediated part that grows linearly with hole doping density and remnant contributions that remain under tin-enriched processing conditions. At hole densities near 10(20) cm(-3), a strong Burstein-Moss effect increases absorption onset energies by approximate to 300 meV beyond the bandgap energy of undoped FASnI(3) (shown to be 1.2 eV at 5 K and 1.35 eV at room temperature). At very high doping densities (10(20) cm(-3)), temperature-dependent measurements indicate that the effective charge-carrier mobility is suppressed through scattering with ionized dopants. Once the background hole concentration is nearer 10(19) cm(-3) and below, the charge-carrier mobility increases with decreasing temperature according to approximate to T-1.2, suggesting that it is limited mostly by intrinsic interactions with lattice vibrations. For the lowest doping concentration of 7.2 x 10(18) cm(-3), charge-carrier mobilities reach a value of 67 cm(2) V-1 s(-1) at room temperature and 470 cm(2) V-1 s(-1) at 50 K. Intraexcitonic transitions observed in the THz-frequency photoconductivity spectra at 5 K reveal an exciton binding energy of only 3.1 meV for FASnI(3), in agreement with the low bandgap energy exhibited by this perovskite.
7. Raman spectrum of the organic-inorganic halide perovskite ch3nh3pbi3 from first principles and high-resolution low-temperature raman measurements,
   MA Perez-Osorio, QQ Lin, RT Phillips, RL Milot, LM Herz, MB Johnston, F Giustino
   J. Phys. Chem. C, 122:21703-21717 (2018) [ abstract | pdf | doi:10.1021/acs.jpcc.8b04669 ]
   We investigate the Raman spectrum of the low temperature orthorhombic phase of the organic-inorganic halide perovskite CH3NH3PbI3, by combining first-principles calculations with high-resolution low-temperature Raman measurements. We find good agreement between theory and experiment and successfully assign each of the Raman peaks to the underlying vibrational modes. In the low frequency spectral range (below 60 cm(-1)), we assign the prominent Raman signals at 26, 32, 42, and 49 cm(-1) to the Pb-I-Pb bending modes with either A(g) or B-2g symmetry and the signal at 58 cm(-1) to the librational mode of the organic cation. Owing to their significant intensity, we propose that these peaks can serve as clear markers of the vibrations of the [PbI3](-) network and of the CH3NH3+ cations in this perovskite, respectively. In particular, the ratios of the intensities of these peaks might be used to monitor possible deviations from the ideal stoichiometry of CH3NH3PbI3.
8. Interplay of structural and optoelectronic properties in formamidinium mixed tin-lead triiodide perovskites,
   ES Parrott, T Green, RL Milot, MB Johnston, HJ Snaith, LM Herz
   Adv. Funct. Mater., 28:1802803 (2018) [ abstract | pdf | doi:10.1002/adfm.201802803 ]
   Mixed lead-tin triiodide perovskites are promising absorber materials for low bandgap bottom cells in all-perovskite tandem photovoltaic devices. Key structural and electronic properties of the FAPb(1-x)Sn(x)I(3) perovskite are presented here as a function of lead: tin content across the alloy series. Temperature-dependent photoluminescence and optical absorption measurements are used to identify changes in the bandgap and phase transition temperature. The large bandgap bowing parameter, a crucial element for the attainment of low bandgaps in this system, is shown to depend on the structural phase, reaching a value of 0.84 eV in the low-temperature phase and 0.73 eV at room temperature. The parabolic nature of the bowing at all temperatures is compatible with a mechanism arising from bond bending to accommodate the random placement of unevenly sized lead and tin ions. Charge-carrier recombination dynamics are shown to fall into two regimes. Tin-rich compositions exhibit fast, monoexponential recombination that is almost temperature-independent, in accordance with high levels of electrical doping. Lead-rich compositions show slower, stretched-exponential charge-carrier recombination that is strongly temperature-dependent, in accordance with a multiphonon assisted process. These results highlight the importance of structure and composition for control of bandgap bowing and charge-carrier recombination mechanisms in low bandgap absorbers for all-perovskite tandem solar cells.
9. 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) [ abstract | 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.
10. Bimolecular recombination in methylammonium lead triiodide perovskite is an inverse absorption process,
    CL Davies, MR Filip, JB Patel, TW Crothers, C Verdi, AD Wright, RL Milot, F Giustino, MB Johnston, LM Herz
    Nat. Commun., 9:293 (2018) [ abstract | pdf | doi:10.1038/s41467-017-02670-2 ]
    Photovoltaic devices based on metal halide perovskites are rapidly improving in efficiency. Once the Shockley-Queisser limit is reached, charge-carrier extraction will be limited only by radiative bimolecular recombination of electrons with holes. Yet, this fundamental process, and its link with material stoichiometry, is still poorly understood. Here we show that bimolecular charge-carrier recombination in methylammonium lead triiodide perovskite can be fully explained as the inverse process of absorption. By correctly accounting for contributions to the absorption from excitons and electron-hole continuum states, we are able to utilise the van Roosbroeck-Shockley relation to determine bimolecular recombination rate constants from absorption spectra. We show that the sharpening of photon, electron and hole distribution functions significantly enhances bimolecular charge recombination as the temperature is lowered, mirroring trends in transient spectroscopy. Our findings provide vital understanding of band-to-band recombination processes in this hybrid perovskite, which comprise direct, fully radiative transitions between thermalized electrons and holes.
11. 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) [ abstract | 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.
12. Near-infrared and short-wavelength infrared photodiodes based on dye-perovskite composites,
    QQ Lin, ZP Wang, M Young, JB Patel, RL Milot, LM Maestro, RR Lunt, HJ Snaith, MB Johnston, LM Herz
    Adv. Funct. Mater., 27:1702485 (2017) [ abstract | pdf | doi:10.1002/adfm.201702485 ]
    Organohalide perovskites have emerged as promising light-sensing materials because of their superior optoelectronic properties and low-cost processing methods. Recently, perovskite-based photodetectors have successfully been demonstrated as both broadband and narrowband varieties. However, the photodetection bandwidth in perovskite-based photodetectors has so far been limited to the near-infrared regime owing to the relatively wide band gap of hybrid organohalide perovskites. In particular, short-wavelength infrared photodiodes operating beyond 1 mu m have not yet been realized with organohalide perovskites. In this study, narrow band gap organic dyes are combined with hybrid perovskites to form composite films as active photoresponsive layers. Tuning the dye loading allows for optimization of the spectral response characteristics and excellent charge-carrier mobilities near 11 cm(2) V-1 s(-1), suggesting that these composites combine the light-absorbing properties or IR dyes with the outstanding charge-extraction characteristics of the perovskite. This study demonstrates the first perovskite photodiodes with deep near-infrared and short-wavelength infrared response that extends as far as 1.6 mu m. All devices are solution-processed and exhibit relatively high responsivity, low dark current, and fast response at room temperature, making this approach highly attractive for next-generation light-detection techniques.
13. Photon reabsorption masks intrinsic bimolecular charge-carrier recombination in {CH$_3$NH$_3$PbI$_3$} perovskite,
    TW Crothers, RL Milot, JB Patel, ES Parrott, J Schlipf, P Muller-buschbaum, MB Johnston, LM Herz
    Nano Lett., 17:5782-5789 (2017) [ abstract | pdf | doi:10.1021/acs.nanolett.7b02834 ]
    An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CH3NH3PbI3 are strongly affected by photon reabsorption that masks a much larger intrinsic bimolecular recombination rate constant. By investigating a set of films whose thickness varies between 50 and 533 nm, we find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon reabsorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient of value 6.8 X 10(-10) cm(3)s(-1) is common to all samples irrespective of film thickness. Hence, we postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, whereas thinner films or index-matched surrounding layers can reduce the possibility for photon reabsorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement.
14. Band-tail recombination in hybrid lead iodide perovskite,
    AD Wright, RL Milot, GE Eperon, HJ Snaith, MB Johnston, LM Herz
    Adv. Funct. Mater., 27:1700860 (2017) [ abstract | pdf | doi:10.1002/adfm.201700860 ]
    Traps limit the photovoltaic efficiency and affect the charge transport of optoelectronic devices based on hybrid lead halide perovskites. Understanding the nature and energy scale of these trap states is therefore crucial for the development and optimization of solar cell and laser technology based on these materials. Here, the low-temperature photoluminescence of formamidinium lead triiodide (HC(NH2)(2)PbI3) is investigated. A power-law time dependence in the emission intensity and an additional low-energy emission peak that exhibits an anomalous relative Stokes shift are observed. Using a rate-equation model and a Monte Carlo simulation, it is revealed that both phenomena arise from an exponential trap-density tail with characteristic energy scale of approximate to 3 meV. Charge-carrier recombination from sites deep within the tail is found to cause emission with energy downshifted by up to several tens of meV. Hence, such phenomena may in part be responsible for open-circuit voltage losses commonly observed in these materials. In this high-quality hybrid perovskite, trap states thus predominantly comprise a continuum of energetic levels (associated with disorder) rather than discrete trap energy levels (associated, e.g., with elemental vacancies). Hybrid perovskites may therefore be viewed as classic semiconductors whose band-structure picture is moderated by a modest degree of energetic disorder.
15. {Cs}$_2${InAgCl}$_6$: a new lead-free halide double perovskite with direct band gap,
    G Volonakis, AA Haghighirad, RL Milot, WH Sio, MR Filip, B Wenger, MB Johnston, LM Herz, HJ Snaith, F Giustino
    J. Phys. Chem. Lett., 8:772- (2017) [ abstract | pdf | doi:10.1021/acs.jpclett.6b02682 ]
    A(2)BB'X-6 halide double perovskites based on bismuth and silver have recently been proposed as potential environmentally friendly alternatives to lead-based hybrid halide perovskites. In particular, Cs2BiAgX6 (X = Cl, Br) have been synthesized and found to exhibit band gaps in the visible range. However, the band gaps of these compounds are indirect, which is not ideal for applications in thin film photovoltaics. Here, we propose a new class of halide double perovskites, where the B3+ and B+ cations are In3+ and Ag+, respectively. Our first-principles calculations indicate that the hypothetical compounds Cs2InAgX6 (X = Cl, Br, I) should exhibit direct band gaps between the visible (I) and the ultraviolet (Cl). Based on these predictions, we attempt to synthesize Cs2InAgCl6 and Cs2InAgBr6, and we succeed to form the hitherto unknown double perovskite Cs2InAgCl6. X-ray diffraction yields a double perovskite structure with space group Fm3m. The measured band gap is 3.3 eV, and the compound is found to be photosensitive and turns reversibly from white to orange under ultraviolet illumination. We also perform an empirical analysis of the stability of Cs2InAgX6 and their mixed halides based on Goldschmidts rules, and we find that it should also be possible to form Cs2InAg(Cl1xBrx)(6) for x < 1. The synthesis of mixed halides will open the way to the development of lead-free double perovskites with direct and tunable band gaps.
16. Photovoltaic mixed-cation lead mixed-halide perovskites: links between crystallinity, photo-stability and electronic properties,
    W Rehman, DP McMeekin, JB Patel, RL Milot, MB Johnston, HJ Snaith, LM Herz
    Energy Environ. Sci., 10:361-369 (2017) [ abstract | pdf | doi:10.1039/c6ee03014a ]
    Lead mixed halide perovskites are highly promising semiconductors for both multi-junction photovoltaic and light emitting applications due to their tunable band gaps, with emission and absorption energies spanning the UV-visible to near IR regions. However, many such perovskites exhibit unwanted halide segregation under photo-illumination, the cause of which is still unclear. In our study, we establish crucial links between crystal phase stability, photostability and optoelectronic properties of the mixed-cation lead mixed-halide perovskite Cs(y)FA((1-y)) Pb(BrxI(1-x))(3). We demonstrate a region for caesium content between 0.10 < y < 0.30 which features high crystalline quality, long charge-carrier lifetimes and high charge-carrier mobilities. Importantly, we show that for such high-quality perovskites, photo-induced halide segregation is strongly suppressed, suggesting that high crystalline quality is a prerequisite for good optoelectronic quality and band gap stability. We propose that regions of short-range crystalline order aid halide segregation, possibly by releasing lattice strain between iodide rich and bromide rich domains. For an optimized caesium content, we explore the orthogonal halide-variation parameter space for Cs(0.17)FA(0.83)Pb(BrxI(1-x))(3) perovskites. We demonstrate excellent charge-carrier mobilities (11-40 cm(2) V-1 s(-1)) and diffusion lengths (0.8-4.4 mm) under solar conditions across the full iodide-bromide tuning range. Therefore, the addition of caesium yields a more photo-stable perovskite system whose absorption onsets can be tuned for bandgap-optimized tandem solar cells.
17. Perovskite-perovskite tandem photovoltaics with optimized band gaps,
    GE Eperon, T Leijtens, KA Bush, R Prasanna, T Green, JTW Wang, DP McMeekin, G Volonakis, RL Milot, R May, A Palmstrom, DJ Slotcavage, RA Belisle, JB Patel, ES Parrott, RJ Sutton, W Ma, F Moghadam, B Conings, A Babayigit
    Science, 354:861-865 (2016) [ abstract | pdf | doi:10.1126/science.aaf9717 ]
    We demonstrate four-and two-terminal perovskite-perovskite tandem solar cells with ideally matched band gaps. We develop an infrared-absorbing 1.2-electron volt band-gap perovskite, FA(0.75)Cs(0.25)Sn(0.5)Pb(0.5)I(3), that can deliver 14.8% efficiency. By combining this material with a wider-band gap FA(0.83)Cs(0.17)Pb(I0.5Br0.5)(3) material, we achieve monolithic two-terminal tandem efficiencies of 17.0% with > 1.65-volt open-circuit voltage. We also make mechanically stacked four-terminal tandem cells and obtain 20.3% efficiency. Notably, we find that our infrared-absorbing perovskite cells exhibit excellent thermal and atmospheric stability, not previously achieved for Sn-based perovskites. This device architecture and materials set will enable "all-perovskite" thin-film solar cells to reach the highest efficiencies in the long term at the lowest costs.
18. Charge-carrier dynamics in {2D} hybrid metal-halide perovskites,
    RL Milot, RJ Sutton, GE Eperon, AA Haghighirad, JM Hardigree, L Miranda, HJ Snaith, MB Johnston, LM Herz
    Nano Lett., 16:7001 (2016) [ abstract | pdf | doi:10.1021/acs.nanolett.6b03114 ]
    Hybrid metal-halide perovskites are promising new materials for use in solar cells; however, their chemical stability in the presence of moisture remains a significant drawback. Quasi two-dimensional (2D) perovskites that incorporate hydrophobic organic interlayers offer improved resistance to degradation by moisture, currently still at the cost of overall cell efficiency. To elucidate the factors affecting the optoelectronic properties of these materials, we have investigated the charge transport properties and crystallographic orientation of mixed methylammonium (MA)-phenylethylammonium (PEA) lead iodide thin films as a function of the MA-to-PEA ratio and, thus, the thickness of the "encapsulated" MA lead halide layers. We find that monomolecular charge-carrier recombination rates first decrease with increasing PEA fraction, most likely as a result of trap passivation, but then increase significantly as excitonic effects begin to dominate for thin confined layers. Bimolecular and Auger recombination rate constants are found to be sensitive to changes in electronic confinement, which alters the density of states for electronic transitions. We demonstrate that effective charge-carrier mobilities remain remarkably high (near 10 cm(2)V-(1)s(-1)) for intermediate PEA content and are enhanced for preferential orientation of the conducting lead iodide layers along the probing electric field. The trade-off between trap reduction, electronic confinement, and layer orientation leads to calculated charge-carrier diffusion lengths reaching a maximum of 2.5 mu m for intermediate PEA content (50%).
19. Radiative monomolecular recombination boosts amplified spontaneous emission in $HC(NH_2)_2SnI_3$ perovskite films,
    RL Milot, GE Eperon, T Green, HJ Snaith, MB Johnston, LM Herz
    J. Phys. Chem. Lett., 7:4178-4184 (2016) [ abstract | pdf | doi:10.1021/acs.jpclett.6b02030 ]
    Hybrid metal-halide perovskites have potential as cost-effective gain media for laser technology because of their superior optoelectronic properties. Although lead-halide perovskites have been most widely studied to date, tin-based perovskites have been proposed as a less toxic alternative. In this Letter, we show that amplified spontaneous emission (ASE) in formamidinium tin triiodide (FASnI(3)) thin films is supported by an observed radiative monomolecular charge recombination pathway deriving from its unintentional doping. Such a radiative component will be active even at the lowest charge-carrier densities, opening a pathway for ultralow light-emission thresholds. Using time-resolved THz photoconductivity analysis, we further show that the material has an unprecedentedly high charge-carrier mobility of 22 cm(2) V-1 s(-1) favoring efficient transport. In addition, FASnI(3) exhibits strong radiative bimolecular recombination and Auger rates that are over an order of magnitude lower than for lead halide perovskites. In combination, these properties reveal that tin-halide perovskites are highly suited to light-emitting devices.
20. Electron-phonon coupling in hybrid lead halide perovskites,
    AD Wright, C Verdi, RL Milot, GE Eperon, MA Pérez-Osorio, HJ Snaith, F Giustino, MB Johnston, LM Herz
    Nat. Commun., 7:11755 (2016) [ abstract | pdf | doi:10.1038/ncomms11755 ]
    Phonon scattering limits charge-carrier mobilities and governs emission line broadening in hybrid metal halide perovskites. Establishing how charge carriers interact with phonons in these materials is therefore essential for the development of high-efficiency perovskite photovoltaics and low-cost lasers. Here we investigate the temperature dependence of emission line broadening in the four commonly studied formamidinium and methylammonium perovskites, HC(NH2)(2)PbI3, HC(NH2)(2)PbBr3, CH3NH3PbI3 and CH3NH3PbBr3, and discover that scattering from longitudinal optical phonons via the Frohlich interaction is the dominant source of electron-phonon coupling near room temperature, with scattering off acoustic phonons negligible. We determine energies for the interacting longitudinal optical phonon modes to be 11.5 and 15.3 meV, and Frohlich coupling constants of similar to 40 and 60 meV for the lead iodide and bromide perovskites, respectively. Our findings correlate well with first-principles calculations based on many-body perturbation theory, which underlines the suitability of an electronic band-structure picture for describing charge carriers in hybrid perovskites.
21. Effect of structural phase transition on charge-carrier lifetimes and defects in ch3nh3sni3 perovskite,
    ES Parrott, RL Milot, T Stergiopoulos, HJ Snaith, MB Johnston, LM Herz
    J. Phys. Chem. Lett., 7:1321-1326 (2016) [ abstract | pdf | doi:10.1021/acs.jpclett.6b00322 ]
    Methylammonium tin triiodide (MASnI(3)) has been successfully employed in lead-free perovskite solar cells, but overall power-conversion efficiencies are still significantly lower than for lead-based perovskites. Here we present photoluminescence (PL) spectra and time resolved PL from 8 to 295 K and find a marked improvement in carrier lifetime and a substantial reduction in PL line width below similar to 110 K, indicating that the cause of the hindered performance is activated at the orthorhombic to tetragonal phase transition. Our measurements therefore suggest that targeted structural change may be capable of tailoring the relative energy level alignment of defects (e.g., tin vacancies) to reduce the background dopant density and improve charge extraction. In addition, we observe for the first time an above-gap emission feature that may arise from higher-lying interband transitions, raising the prospect of excess energy harvesting.
22. Formation Dynamics of CH$_3$NH$_3$PbI$_3$ Perovskite Following Two-Step Layer Deposition,
    JB Patel, RL Milot, AD Wright, LM Herz, MB Johnston
    J. Phys. Chem. Lett., 7:96-102 (2016) [ abstract | pdf | doi:10.1021/acs.jpclett.5b02495 ]
    Hybrid metal-halide perovskites have emerged as a leading class of semiconductors for optoelectronic devices because of their desirable material properties and versatile fabrication methods. However, little is known about the chemical transformations that occur in the initial stages of perovskite crystal formation. Here we follow the real-time formation dynamics of MAPbI3 from a bilayer of lead iodide (PbI2) and methylammonium iodide (MAI) deposited through a two-step thermal evaporation process. By lowering the substrate temperature during deposition, we are able to initially inhibit intermixing of the two layers. We subsequently use infrared and visible light transmission, X-ray diffraction, and photoluminescence lifetime measurements to reveal the room-temperature transformations that occur in vacuum and ambient air, as MAI diffuses into the PbI2 lattice to form MAPbI3. In vacuum, the transformation to MAPbI3 is incomplete as unreacted MAI is retained in the film. However, exposure to moist air allows for conversion of the unreacted MAI to MAPbI3, demonstrating that moisture is essential in making MAI more mobile and thus aiding perovskite crystallization. These dynamic processes are reflected in the observed charge-carrier lifetimes, which strongly fluctuate during periods of large ion migration but steadily increase with improving crystallinity.
23. Charge-Carrier Dynamics and Mobilities in Formamidinium Lead Mixed-Halide Perovskites,
    W Rehman, RL Milot, GE Eperon, C Wehrenfennig, JL Boland, HJ Snaith, MB Johnston, LM Herz
    Adv. Mater., 27:7938--7944 (2015) [ abstract | pdf | doi:10.1002/adma.201502969 ]
    The mixed-halide perovskite FAPb(BryI1–y)3 is attractive for color-tunable and tandem solar cells. Bimolecular and Auger charge-carrier recombination rate constants strongly correlate with the Br content, y, suggesting a link with electronic structure. FAPbBr3 and FAPbI3 exhibit charge-carrier mobilities of 14 and 27 cm2 V&#8722;1 s&#8722;1 and diffusion lengths exceeding 1 &#956;m, while mobilities across the mixed Br/I system depend on crystalline phase disorder.
24. Vibrational Properties of the Organic−Inorganic Halide Perovskite $CH_3NH_3PbI_3$ from Theory and Experiment: Factor Group Analysis, First-Principles Calculations, and Low-Temperature Infrared Spectra,
    MA Perez-Osorio, RL Milot, MR Filip, JB Patel, LM Herz, MB Johnston, F Giustino
    J. Phys. Chem. C, 119:25703--25718 (2015) [ abstract | pdf | doi:10.1021/acs.jpcc.5b07432 ]
    In this work, we investigate the vibrational properties of the hybrid organic/inorganic halide perovskite MAPbI3 (MA = CH3NH3) in the range 6−3500 cm−1 by combining first-principles density-functional perturbation theory calculations and low-temperature infrared (IR) absorption measurements on evaporated perovskite films. By using a group factor analysis, we establish the symmetry of the normal modes of vibration and predict their IR and Raman activity. We validate our analysis via explicit calculation of the IR intensities. Our calculated spectrum is in good agreement with our measurements. By comparing theory and experiment, we are able to assign most of the features in the IR spectrum. Our analysis shows that the IR spectrum of MAPbI3 can be partitioned into three distinct regions: the internal vibrations of the MA cations (800−3100 cm 180 cm−1), and the internal vibrations of the PbI3 network (<100 cm−1). The low-frequency region of the IR spectrum is dominated by Pb−I stretching modes of the PbI3 network with Bu symmetry and librational modes of the MA cations. In addition, we find that the largest contributions to the static dielectric constant arise from Pb−I stretching and Pb−I−Pb rocking modes, and that one low-frequency B2u Pb−I stretching mode exhibits a large LO−TO splitting of 50 cm−1.
25. Temperature-dependent charge-carrier dynamics in {CH$_3$NH$_3$PbI$_3$} perovskite thin films,
    RL Milot, GE Eperon, HJ Snaith, MB Johnston, LM Herz
    Adv. Funct. Mater., 25:6218-6227 (2015) [ abstract | pdf | doi:10.1002/adfm.201502340 ]
    The photoluminescence, transmittance, charge-carrier recombination dynamics, mobility, and diffusion length of CH3NH3PbI3 are investigated in the temperature range from 8 to 370 K. Profound changes in the optoelectronic properties of this prototypical photovoltaic material are observed across the two structural phase transitions occurring at 160 and 310 K. Drude-like terahertz photoconductivity spectra at all temperatures above 80 K suggest that charge localization effects are absent in this range. The monomolecular charge-carrier recombination rate generally increases with rising temperature, indicating a mechanism dominated by ionized impurity mediated recombination. Deduced activation energies E-a associated with ionization are found to increase markedly from the room-temperature tetragonal (E-a approximate to 20 meV) to the higher-temperature cubic (E-a approximate to 200 meV) phase adopted above 310 K. Conversely, the bimolecular rate constant decreases with rising temperature as charge-carrier mobility declines, while the Auger rate constant is highly phase specific, suggesting a strong dependence on electronic band structure. The charge-carrier diffusion length gradually decreases with rising temperature from about 3 m at -93 degrees C to 1.2 m at 67 degrees C but remains well above the optical absorption depth in the visible spectrum. These results demonstrate that there are no fundamental obstacles to the operation of cells based on CH3NH3PbI3 under typical field conditions.
26. Influence of free-carrier absorption on terahertz generation from {ZnTe}(110),
    SM Harrel, RL Milot, JM Schleicher, CA Schmuttenmaer
    J. Appl. Phys., 107:033526 (2010) [ abstract | pdf | doi:10.1063/1.3296064 ]
    ZnTe(110) is widely used as a source of terahertz radiation generated by optical rectification. However, when ZnTe(110) is excited with 800 nm light, optical rectification is not the only process which can occur. Specifically, second harmonic generation and two-photon absorption are also possibilities. In addition, free carriers generated by two-photon absorption can absorb terahertz radiation, further reducing the efficiency of optical rectification. We have used terahertz emission spectroscopy to study these effects by analyzing the dependence of the terahertz waveform on excitation fluence. At high excitation fluences, the overall efficiency is reduced and the trailing edge of the waveform is attenuated. A simple model reproduces the measured behavior.
27. Synergistic effect between anatase and rutile TiO2 nanoparticles in dye-sensitized solar cells,
    GH Li, CP Richter, RL Milot, L Cai, CA Schmuttenmaer, RH Crabtree, GW Brudvig, VS Batista
    Dalton Trans., 45:10078-10085 (2009) [ abstract | pdf | doi:10.1039/b908686b ]
    A synergistic effect between anatase and rutile TiO2 is known, in which the addition of rutile can remarkably enhance the photocatalytic activity of anatase in the degradation of organic contaminants. In this study, mixed-phase TiO2 nanocomposites consisting of anatase and rutile nanoparticles (NPs) were prepared for use as photoanodes in dye-sensitized solar cells (DSSCs) and were characterized by using UV-vis spectroscopy, powder X-ray diffraction and scanning electron microscopy. The addition of 10-15% rutile significantly improved light harvesting and the overall solar conversion efficiency of anatase NPs in DSSCs. The underlying mechanism for the synergistic effect in DSSCs is now explored by using time-resolved terahertz spectroscopy. It is clearly demonstrated that photo-excited electrons injected into the rutile NPs can migrate to the conduction band of anatase NPs, enhancing the photocurrent and efficiency. Interfacial electron transfer from rutile to anatase, similar to that in heterogeneous photocatalysis, is proposed to account for the synergistic effect in DSSCs. Our results further suggest that the synergistic effect can be used to explain the beneficial effect of TiCl4 treatment on DSSC efficiency.