Publications


Efficient Deep Red Light-Sensing All-Polymer Phototransistors with p-type/n-type Conjugated Polymer Bulk Heterojunction Layers.

ACS Appl Mater Interfaces 9 (2017) 14983-14989

S Nam, J Seo, H Han, H Kim, DDC Bradley, Y Kim

Here we demonstrate deep red light-sensing all-polymer phototransistors with bulk heterojunction layers of poly[4,8-bis[(2-ethylhexyl)-oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7) and poly[[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)] (P(NDI2OD-T2)). The device performances were investigated by varying the incident light intensity of the deep red light (675 nm), while the signal amplification capability was examined by changing the gate and drain voltages. The result showed that the present all-polymer phototransistors exhibited higher photoresponsivity (∼14 A/W) and better on/off photoswitching characteristics than the devices with the pristine polymers under illumination with the deep red light. The enhanced phototransistor performances were attributed to the well-aligned nanofiber-like morphology and nanocrystalline P(NDI2OD-T2) domains in the blend films, which are beneficial for charge separation and charge transport in the in-plane direction.


Carbon Nanotubes in Perovskite Solar Cells

ADVANCED ENERGY MATERIALS 7 (2017) ARTN 1601839

SN Habisreutinger, RJ Nicholas, HJ Snaith


Controlling Nucleation and Growth of Metal Halide Perovskite Thin Films for High-Efficiency Perovskite Solar Cells.

Small (Weinheim an der Bergstrasse, Germany) 13 (2017)

N Sakai, Z Wang, VM Burlakov, J Lim, D McMeekin, S Pathak, HJ Snaith

Metal halide perovskite thin films can be crystallized via a broad range of solution-based routes. However, the quality of the final films is strongly dependent upon small changes in solution composition and processing parameters. Here, this study demonstrates that a fractional substitution of PbCl2 with PbI2 in the 3CH3 NH3 I:PbCl2 mixed-halide starting solution has a profound influence upon the ensuing thin-film crystallization. The presence of PbI2 in the precursor induces a uniform distribution of regular quadrilateral-shaped CH3 NH3 PbI3 perovskite crystals in as-cast films, which subsequently grow to form pinhole-free perovskite films with highly crystalline domains. With this new formulation of 3CH3 NH3 I:0.98PbCl2 :0.02PbI2 , this study achieves a 19.1% current-voltage measured power conversion efficiency and a 17.2% stabilized power output in regular planar heterojunction solar cells.


Large-area, Highly Uniform Evaporated Formamidinium Lead Triiodide Thin-films for Solar Cells

ACS Energy Letters American Chemical Society (2017)

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.


Charge-Carrier Mobilities in Metal Halide Perovskites: Fundamental Mechanisms and Limits

ACS ENERGY LETTERS 2 (2017) 1539-1548

LM Herz


Polarisation-controlled single photon emission at high temperatures from InGaN quantum dots.

Nanoscale 9 (2017) 9421-9427

T Wang, TJ Puchtler, T Zhu, JC Jarman, LP Nuttall, RA Oliver, RA Taylor

Solid-state single photon sources with polarisation control operating beyond the Peltier cooling barrier of 200 K are desirable for a variety of applications in quantum technology. Using a non-polar InGaN system, we report the successful realisation of single photon emission with a g(2)(0) of 0.21, a high polarisation degree of 0.80, a fixed polarisation axis determined by the underlying crystallography, and a GHz repetition rate with a radiative lifetime of 357 ps at 220 K in semiconductor quantum dots. The temperature insensitivity of these properties, together with the simple planar epitaxial growth method and absence of complex device geometries, demonstrates that fast single photon emission with polarisation control can be achieved in solid-state quantum dots above the Peltier temperature threshold, making this system a potential candidate for future on-chip applications in integrated systems.


High-temperature performance of non-polar (11-20) InGaN quantum dots grown by a quasi-two-temperature method

PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS 254 (2017) ARTN 1600724

T Wang, TJ Puchtler, T Zhu, JC Jarman, RA Oliver, RA Taylor


Monolithic Wide Band Gap Perovskite/ Perovskite Tandem Solar Cells with Organic Recombination Layers

The Journal of Physical Chemistry C (2017)

R Sheng, MT Hoerantner, Z Wang, Y Jiang, W Zhang, A Agosti, S Huang, X Hao, AWY Ho-Baillie, MA Green, HJ Snaith


Efficient and Stable Perovskite Solar Cells Using Molybdenum Tris(dithiolene)s as p-Dopants for Spiro-OMeTAD

ACS ENERGY LETTERS 2 (2017) 2044-2050

A Pellaroque, NK Noel, SN Habisreutinger, Y Zhang, S Barlow, SR Marder, HJ Snaith


V-Shaped Hole-Transporting TPD Dimers Containing Troger's Base Core

JOURNAL OF PHYSICAL CHEMISTRY C 121 (2017) 10267-10274

T Braukyla, N Sakai, M Daskeviciene, V Jankauskas, E Kamarauskas, R Komskis, T Malinauskas, S Jursenas, HJ Snaith, V Getautis


Two-Dimensional Excitonic Photoluminescence in Graphene on a Cu Surface.

ACS nano 11 (2017) 3207-3212

Y Park, Y Kim, CW Myung, RA Taylor, CCS Chan, BPL Reid, TJ Puchtler, RJ Nicholas, LT Singh, G Lee, C-C Hwang, C-Y Park, KS Kim

Despite having outstanding electrical properties, graphene is unsuitable for optical devices because of its zero band gap. Here, we report two-dimensional excitonic photoluminescence (PL) from graphene grown on a Cu(111) surface, which shows an unexpected and remarkably sharp strong emission near 3.16 eV (full width at half-maximum ≤3 meV) and multiple emissions around 3.18 eV. As temperature increases, these emissions blue shift, displaying the characteristic negative thermal coefficient of graphene. The observed PL originates from the significantly suppressed dispersion of excited electrons in graphene caused by hybridization of graphene π and Cu d orbitals of the first and second Cu layers at a shifted saddle point 0.525(M+K) of the Brillouin zone. This finding provides a pathway to engineering optoelectronic graphene devices, while maintaining the outstanding electrical properties of graphene.


Efficient ambient-air-stable solar cells with 2D-3D heterostructured butylammonium-caesium-formamidinium lead halide perovskites

Nature Energy 2 (2017)

Z Wang, Q Lin, FP Chmiel, N Sakai, LM Herz, HJ Snaith

© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. Perovskite solar cells are remarkably efficient; however, they are prone to degradation in water, oxygen and ultraviolet light. Cation engineering in 3D perovskite absorbers has led to reduced degradation. Alternatively, 2D Ruddlesden-Popper layered perovskites exhibit improved stability, but have not delivered efficient solar cells so far. Here, we introduce n-butylammonium cations into a mixed-cation lead mixed-halide FA 0.83 Cs 0.17 Pb(I y Br 1-y ) 3 3D perovskite. We observe the formation of 2D perovskite platelets, interspersed between highly orientated 3D perovskite grains, which suppress non-radiative charge recombination. We investigate the relationship between thin-film composition, crystal alignment and device performance. Solar cells with an optimal butylammonium content exhibit average stabilized power conversion efficiency of 17.5 ± 1.3% with a 1.61-eV-bandgap perovskite and 15.8 ± 0.8% with a 1.72-eV-bandgap perovskite. The stability under simulated sunlight is also enhanced. Cells sustain 80% of their post burn-in' efficiency after 1,000 h in air, and close to 4,000 h when encapsulated.


Polyacetylene-based polyelectrolyte as a universal interfacial layer for efficient inverted polymer solar cells

ORGANIC ELECTRONICS 48 (2017) 61-67

S Nam, J Seo, M Song, H Kim, M Ree, Y-S Gal, DDC Bradley, Y Kim


Terahertz Spectroscopy Study of Weak Base-Treated Conducting Polymer Films and Applications for Polymer Solar Cells

IEEE Journal of Selected Topics in Quantum Electronics 23 (2017)

J Seo, S Nam, S Park, C Lee, J Park, H Kim, Y Kim

© 2017 IEEE. Terahertz spectroscopy was employed to investigate the charge carrier dynamics of aniline-treated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films. The measured charge carrier lifetime was correlated with the performances of organic solar cells with the aniline-treated PEDOT:PSS hole-collecting layers. The result showed that the charge carrier lifetime was sensitive to the aniline content. The longest charge carrier lifetime was achieved for 1.2 mol.% aniline, which led to the highest power conversion efficiency of 8.83%. In particular, the trend of the charge carrier lifetime was found to be strongly affected by the surface morphology of the aniline-treated PEDOT:PSS layers.


Direct generation of linearly polarized single photons with a deterministic axis in quantum dots

NANOPHOTONICS 6 (2017) 1175-1183

T Wang, TJ Puchtler, SK Patra, T Zhu, M Ali, TJ Badcock, T Ding, RA Oliver, S Schulz, RA Taylor


Carbazole-based enamine: Low-cost and efficient hole transporting material for perovskite solar cells

NANO ENERGY 32 (2017) 551-557

M Daskeviciene, S Paek, Z Wang, T Malinauskas, G Jokubauskaite, K Rakstys, KT Cho, A Magomedov, V Jankauskas, S Ahmad, HJ Snaith, V Getautis, MK Nazeeruddin


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


Processing Solvent-Dependent Electronic and Structural Properties of Cesium Lead Triiodide Thin Films.

The journal of Physical Chemistry Letters (2017) 4172-4176

AJ Ramadan, LA Rochford, S Fearn, HJ Snaith

Cesium lead triiodide (CsPbI3) is an attractive material for photovoltaic applications due to its appropriate band gap, strong optical absorption, and high thermal stability. However, the perovskite phase suffers from moisture induced structural instability. Previous studies have utilized a range of solvent systems to establish the role of solvent choice in structural instabilities. Despite this, effects of different solvents on the electronic structure of this material have not been compared. We report substantial chemical and compositional differences in thin films of CsPbI3 prepared from a range of solvent systems. We confirm via X-ray diffraction thin films formed from DMF, DMSO, and a mixture of these solvent systems share the same crystal structure. However, secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and low energy ion scattering measurements reveal significant differences between films processed via different solvent systems. Our findings reveal the critical impact solvents have upon compositional stoichiometry and thin-film morphology.


Interplay between many body effects and Coulomb screening in the optical bandgap of atomically thin MoS2.

Nanoscale 9 (2017) 10647-10652

Y Park, SW Han, CCS Chan, BPL Reid, RA Taylor, N Kim, Y Jo, H Im, KS Kim

Due to its unique layer-number dependent electronic band structure and strong excitonic features, atomically thin MoS2 is an ideal 2D system where intriguing photoexcited-carrier-induced phenomena can be detected in excitonic luminescence. We perform micro-photoluminescence (PL) measurements and observe that the PL peak redshifts nonlinearly in mono- and bi-layer MoS2 as the excitation power is increased. The excited carrier-induced optical bandgap shrinkage is found to be proportional to n4/3, where n is the optically-induced free carrier density. The large exponent value of 4/3 is explicitly distinguished from a typical value of 1/3 in various semiconductor quantum well systems. The peculiar n4/3 dependent optical bandgap redshift may be due to the interplay between bandgap renormalization and reduced exciton binding energy.

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