Publications by Jay Patel


Temperature-Dependent Refractive Index of Quartz at Terahertz Frequencies

JOURNAL OF INFRARED MILLIMETER AND TERAHERTZ WAVES 39 (2018) 1236-1248

CL Davies, JB Patel, CQ Xia, LM Herz, MB Johnston


Modification of the fluorinated tin oxide/electron-transporting material interface by a strong reductant and its effect on perovskite solar cell efficiency

MOLECULAR SYSTEMS DESIGN & ENGINEERING 3 (2018) 741-747

F Pulvirenti, B Wegner, NK Noel, G Mazzotta, R Hill, JB Patel, LM Herz, MB Johnston, MK Riede, HJ Snaith, N Koch, S Barlow, SR Marder


Bimolecular recombination in methylammonium lead triiodide perovskite is an inverse absorption process.

Nature communications 9 (2018) 293-

CL Davies, MR Filip, JB Patel, TW Crothers, C Verdi, AD Wright, RL Milot, F Giustino, MB Johnston, LM Herz

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.


Modelling and simulation of photovoltaic module for micro inverter application

Proceedings - International Conference on Trends in Electronics and Informatics, ICEI 2017 2018-January (2018) 82-85

M Patel, H Surati, J Patel

© 2017 IEEE. This paper presents on a program developed in MATLAB/Simulink of photovoltaic module for micro inverter application. This program is based on mathematical equations and is defined through an equivalent circuit including a photocurrent source, and a diode. The developed program allows the prediction of PV module behaviour under different temperature and radiation. Effect of two environmental parameters of temperature and irradiance variations could be observed from simulated characteristics. The boost converter is to be used along with a Maximum Power Point Tracking topology. The MPPT is responsible for extracting the maximum possible power from the photovoltaic and feed it to the load via the boost converter which is used to steps up the voltage to required magnitude.


Photocurrent Spectroscopy of Perovskite Solar Cells Over a Wide Temperature Range from 15 to 350 K.

The journal of physical chemistry letters 9 (2018) 263-268

JB Patel, Q Lin, O Zadvorna, CL Davies, LM Herz, MB Johnston

Solar cells based on metal halide perovskite thin films show great promise for energy generation in a range of environments from terrestrial installations to space applications. Here we assess the device characteristics of the prototypical perovskite solar cells based on methylammonium lead triiodide (CH3NH3PbI3) over a broad temperature range from 15 to 350 K (-258 to 77 °C). For these devices, we observe a peak in the short-circuit current density and open-circuit voltage at 200 K (-73 °C) with decent operation maintained up to 350 K. We identify the clear signature of crystalline PbI2 contributing directly to the low-temperature photocurrent spectra, showing that PbI2 plays an active role (beyond passivation) in CH3NH3PbI3 solar cells. Finally we observe a blue-shift in the photocurrent spectrum with respect to the absorption spectrum at low temperature (15 K), allowing us to extract a lower limit on the exciton binding energy of 9.1 meV for CH3NH3PbI3.


Highly Crystalline Methylammonium Lead Tribromide Perovskite Films for Efficient Photovoltaic Devices

ACS ENERGY LETTERS 3 (2018) 1233-1240

NK Noel, B Wenger, SN Habisreutinger, JB Patel, T Crothers, Z Wang, RJ Nicholas, MB Johnston, LM Herz, HJ Snaith


Influence of Interface Morphology on Hysteresis in Vapor-Deposited Perovskite Solar Cells

ADVANCED ELECTRONIC MATERIALS 3 (2017) ARTN 1600470

JB Patel, J Wong-Leung, S Van Reenen, N Sakai, JTW Wang, ES Parrott, M Liu, HJ Snaith, LM Herz, MB Johnston


Large-Area, Highly Uniform Evaporated Formamidinium Lead Triiodide Thin Films for Solar Cells

ACS ENERGY LETTERS 2 (2017) 2799-2804

J Borchert, RL Milot, JB Patel, CL Davies, AD Wright, LM Maestro, HJ Snaith, LM Herz, MB Johnston


Photon Reabsorption Masks Intrinsic Bimolecular Charge-Carrier Recombination in CH3NH3PbI3 Perovskite.

Nano letters 17 (2017) 5782-5789

TW Crothers, RL Milot, JB Patel, ES Parrott, J Schlipf, P Müller-Buschbaum, MB Johnston, LM Herz

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 × 10-10 cm3s-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.


Crystallization Kinetics and Morphology Control of Formamidinium-Cesium Mixed-Cation Lead Mixed-Halide Perovskite via Tunability of the Colloidal Precursor Solution.

Advanced materials (Deerfield Beach, Fla.) 29 (2017)

DP McMeekin, Z Wang, W Rehman, F Pulvirenti, JB Patel, NK Noel, MB Johnston, SR Marder, LM Herz, HJ Snaith

The meteoric rise of the field of perovskite solar cells has been fueled by the ease with which a wide range of high-quality materials can be fabricated via simple solution processing methods. However, to date, little effort has been devoted to understanding the precursor solutions, and the role of additives such as hydrohalic acids upon film crystallization and final optoelectronic quality. Here, a direct link between the colloids concentration present in the [HC(NH2 )2 ]0.83 Cs0.17 Pb(Br0.2 I0.8 )3 precursor solution and the nucleation and growth stages of the thin film formation is established. Using dynamic light scattering analysis, the dissolution of colloids over a time span triggered by the addition of hydrohalic acids is monitored. These colloids appear to provide nucleation sites for the perovskite crystallization, which critically impacts morphology, crystal quality, and optoelectronic properties. Via 2D X-ray diffraction, highly ordered and textured crystals for films prepared from solutions with lower colloidal concentrations are observed. This increase in material quality allows for a reduction in microstrain along with a twofold increase in charge-carrier mobilities leading to values exceeding 20 cm2 V-1 s-1 . Using a solution with an optimized colloidal concentration, devices that reach current-voltage measured power conversion efficiency of 18.8% and stabilized efficiency of 17.9% are fabricated.


Unveiling the Influence of pH on the Crystallization of Hybrid Perovskites, Delivering Low Voltage Loss Photovoltaics

Joule 1 (2017) 328-343

NK Noel, M Congiu, AJ Ramadan, S Fearn, DP McMeekin, JB Patel, MB Johnston, B Wenger, HJ Snaith

© 2017 Elsevier Inc. Impressive power conversion efficiencies coupled with the relative ease of fabrication have made perovskite solar cells a front runner for next-generation photovoltaics. Although perovskite films and optoelectronic devices have been widely studied, relatively little is known about the chemistry of the precursor solutions. Here, we present a study on the hydrolysis of N,N-dimethylformamide, correlating how pH changes related to its degradation affect the crystallization of MAPbI3−xClx perovskite films. By careful manipulation of the pH, and the resulting colloid distribution in precursor solutions, we fabricate perovskite films with greatly improved crystallinity, which when incorporated into photovoltaic devices reproducibly yield efficiencies of over 18%. Extending this method to the mixed cation, mixed halide perovskite FA0.83MA0.17Pb(I0.83Br0.17)3, we obtain power conversion efficiencies of up to 19.9% and open-circuit voltages of 1.21 V for a material with a bandgap of 1.57 eV, achieving the lowest yet reported loss in potential from bandgap to a VOC of only 360 mV. Metal halide perovskites have shown tremendous promise in optoelectronic devices and are of particular interest as absorber materials in solar cells, having achieved remarkable power conversion efficiencies in a staggeringly short period of time. Although improvements in deposition techniques have greatly increased the quality of perovskite films and have allowed perovskite solar cells to dominate the class of emerging photovoltaic technologies, relatively little focus has been placed on understanding the chemistry of the precursor solutions. Here, we elucidate how the hydrolysis and thermal decomposition of N,N-dimethylformamide, the most commonly used solvent for perovskites, has far-reaching effects on the crystallization and optoelectronic properties of perovskite films and show how controlling the degradation of this solvent allows us to achieve record low voltage losses in highly efficient perovskite solar cells. The degradation of N,N-dimethylformamide results in the formation of formic acid and dimethylamine. The changes in pH that occur as a result of this solvent degradation can be correlated to changes in the colloid concentration in perovskite precursor solutions. By tuning the pH and hence colloid concentration of these solutions, we improve the crystallization and optoelectronic quality of the perovskite films, resulting in solar cells with a record low loss in potential from bandgap to VOC of 360 mV.


Efficient and Air-Stable Mixed-Cation Lead Mixed-Halide Perovskite Solar Cells with n-Doped Organic Electron Extraction Layers.

Advanced materials (Deerfield Beach, Fla.) 29 (2017)

Z Wang, DP McMeekin, N Sakai, S van Reenen, K Wojciechowski, JB Patel, MB Johnston, HJ Snaith

Air-stable doping of the n-type fullerene layer in an n-i-p planar heterojunction perovskite device is capable of enhancing device efficiency and improving device stability. Employing a (HC(NH2 )2 )0.83 Cs0.17 Pb(I0.6 Br0.4 )3 perovskite as the photoactive layer, glass-glass laminated devices are reported, which sustain 80% of their "post burn-in" efficiency over 3400 h under full sun illumination in ambient conditions.


Near-Infrared and Short-Wavelength Infrared Photodiodes Based on Dye-Perovskite Composites

ADVANCED FUNCTIONAL MATERIALS 27 (2017) ARTN 1702485

Q Lin, Z Wang, M Young, JB Patel, RL Milot, LM Maestro, RR Lunt, HJ Snaith, MB Johnston, LM Herz


Photovoltaic mixed-cation lead mixed-halide perovskites: links between crystallinity, photo-stability and electronic properties

ENERGY & ENVIRONMENTAL SCIENCE 10 (2017) 361-369

W Rehman, DP McMeekin, JB Patel, RL Milot, MB Johnston, HJ Snaith, LM Herz


Perovskite-perovskite tandem photovoltaics with optimized band gaps.

Science (New York, N.Y.) 354 (2016) 861-865

GE Eperon, T Leijtens, KA Bush, R Prasanna, T Green, JT-W 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, H-G Boyen, S Bent, F Giustino, LM Herz, MB Johnston, MD McGehee, HJ Snaith

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, FA0.75Cs0.25Sn0.5Pb0.5I3, that can deliver 14.8% efficiency. By combining this material with a wider-band gap FA0.83Cs0.17Pb(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.


Efficient perovskite solar cells by metal ion doping

ENERGY & ENVIRONMENTAL SCIENCE 9 (2016) 2892-2901

JT-W Wang, Z Wang, S Pathak, W Zhang, DW deQuilettes, F Wisnivesky-Rocca-Rivarola, J Huang, PK Nayak, JB Patel, HAM Yusof, Y Vaynzof, R Zhu, I Ramirez, J Zhang, C Ducati, C Grovenor, MB Johnston, DS Ginger, RJ Nicholas, HJ Snaith


Structured Organic-Inorganic Perovskite toward a Distributed Feedback Laser.

Advanced materials (Deerfield Beach, Fla.) 28 (2016) 923-929

M Saliba, SM Wood, JB Patel, PK Nayak, J Huang, JA Alexander-Webber, B Wenger, SD Stranks, MT Hörantner, JT-W Wang, RJ Nicholas, LM Herz, MB Johnston, SM Morris, HJ Snaith, MK Riede

A general strategy for the in-plane structuring of organic-inorganic perovskite films is presented. The method is used to fabricate an industrially relevant distributed feedback (DFB) cavity, which is a critical step toward all-electrially pumped injection laser diodes. This approach opens the prospects of perovskite materials for much improved optical control in LEDs, solar cells, and also toward applications as optical devices.


Bandgap-Tunable Cesium Lead Halide Perovskites with High Thermal Stability for Efficient Solar Cells

ADVANCED ENERGY MATERIALS 6 (2016) ARTN 1502458

RJ Sutton, GE Eperon, L Miranda, ES Parrott, BA Kamino, JB Patel, MT Horantner, MB Johnston, AA Haghighirad, DT Moore, HJ Snaith


Formation Dynamics of CH3NH3PbI3 Perovskite Following Two-Step Layer Deposition.

The journal of physical chemistry letters 7 (2016) 96-102

JB Patel, RL Milot, AD Wright, LM Herz, MB Johnston

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.


Enhanced Amplified Spontaneous Emission in Perovskites Using a Flexible Cholesteric Liquid Crystal Reflector.

Nano letters 15 (2015) 4935-4941

SD Stranks, SM Wood, K Wojciechowski, F Deschler, M Saliba, H Khandelwal, JB Patel, SJ Elston, LM Herz, MB Johnston, APHJ Schenning, MG Debije, MK Riede, SM Morris, HJ Snaith

Organic-inorganic perovskites are highly promising solar cell materials with laboratory-based power conversion efficiencies already matching those of established thin film technologies. Their exceptional photovoltaic performance is in part attributed to the presence of efficient radiative recombination pathways, thereby opening up the possibility of efficient light-emitting devices. Here, we demonstrate optically pumped amplified spontaneous emission (ASE) at 780 nm from a 50 nm-thick film of CH3NH3PbI3 perovskite that is sandwiched within a cavity composed of a thin-film (∼7 μm) cholesteric liquid crystal (CLC) reflector and a metal back-reflector. The threshold fluence for ASE in the perovskite film is reduced by at least two orders of magnitude in the presence of the CLC reflector, which results in a factor of two reduction in threshold fluence compared to previous reports. We consider this to be due to improved coupling of the oblique and out-of-plane modes that are reflected into the bulk in addition to any contributions from cavity modes. Furthermore, we also demonstrate enhanced ASE on flexible reflectors and discuss how improvements in the quality factor and reflectivity of the CLC layers could lead to single-mode lasing using CLC reflectors. Our work opens up the possibility of fabricating widely wavelength-tunable "mirror-less" single-mode lasers on flexible substrates, which could find use in applications such as flexible displays and friend or foe identification.

Pages