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


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.


The entangled triplet pair state in acene and heteroacene materials.

Nature communications 8 (2017) 15953-

CK Yong, AJ Musser, SL Bayliss, S Lukman, H Tamura, O Bubnova, RK Hallani, A Meneau, R Resel, M Maruyama, S Hotta, LM Herz, D Beljonne, JE Anthony, J Clark, H Sirringhaus

Entanglement of states is one of the most surprising and counter-intuitive consequences of quantum mechanics, with potent applications in cryptography and computing. In organic materials, one particularly significant manifestation is the spin-entangled triplet-pair state, which mediates the spin-conserving fission of one spin-0 singlet exciton into two spin-1 triplet excitons. Despite long theoretical and experimental exploration, the nature of the triplet-pair state and inter-triplet interactions have proved elusive. Here we use a range of organic semiconductors that undergo singlet exciton fission to reveal the photophysical properties of entangled triplet-pair states. We find that the triplet pair is bound with respect to free triplets with an energy that is largely material independent (∼30 meV). During its lifetime, the component triplets behave cooperatively as a singlet and emit light through a Herzberg-Teller-type mechanism, resulting in vibronically structured photoluminescence. In photovoltaic blends, charge transfer can occur from the bound triplet pairs with >100% photon-to-charge conversion efficiency.


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

ACS ENERGY LETTERS 2 (2017) 1539-1548

LM Herz


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


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.


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


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.


Strong Composition Effects in All-Polymer Phototransistors with Bulk Heterojunction Layers of p-type and n-type Conjugated Polymers.

ACS applied materials & interfaces 9 (2017) 628-635

H Han, C Lee, H Kim, J Seo, M Song, S Nam, Y Kim

We report the composition effect of polymeric sensing channel layers on the performance of all-polymer phototransistors featuring bulk heterojunction (BHJ) structure of electron-donating (p-type) and electron-accepting (n-type) polymers. As an n-type component, poly(3-hexylthiopehe-co-benzothiadiazole) end-capped with 4-hexylthiophene (THBT-4ht) was synthesized via two-step reactions. A well-studied conjugated polymer, poly(3-hexylthiophene) (P3HT), was employed as a p-type polymer. The composition of BHJ (P3HT:THBT-4ht) films was studied in detail by varying the THBT-4ht contents (0, 1, 3, 5, 10, 20, 30, 40, and 100 wt %). The best charge separation in the P3HT:THBT-4ht films was measured at 30 wt % by the photoluminescence (PL) study, while the charge transport characteristics of devices were improved at the low THBT-4ht contents (<10 wt %). The photosensing experiments revealed that the photosensivity of all-polymer phototransistors was higher than that of the phototransistors with the pristine P3HT layers and strongly dependent on the BHJ composition. The highest (corrected) responsivity (RC) was achieved at 20 wt %, which can be attributable to the balance between the best charge separation and transport states, as investigated for crystal nanostructures and surface morphology by employing synchrotron-radiation grazing-incidence wide-angle X-ray scattering, high-resolution/scanning transmission electron microscopy, and atomic force microscopy.


Band-Tail Recombination in Hybrid Lead Iodide Perovskite

ADVANCED FUNCTIONAL MATERIALS 27 (2017) ARTN 1700860

AD Wright, RL Milot, GE Eperon, HJ Snaith, MB Johnston, LM Herz


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 MAPbI 3−x Cl x 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 FA 0.83 MA 0.17 Pb(I 0.83 Br 0.17 ) 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 V OC 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 V OC of 360 mV.


Modulation of terahertz polarization on picosecond timescales using polymer-encapsulated semiconductor nanowires

Optics InfoBase Conference Papers Part F41-CLEO_SI 2017 (2017)

SA Baig, JL Boland, DA Damry, H Hoe Tan, C Jagadish, HJ Joyce, MB Johnston

© 2017 OSA. We exploit the photoconductivity of semiconductor nanowires to achieve ultrafast broad-bandwidth modulation of THz pulses. A modulation depth of -8 dB was exhibited by a polarizer consisting of 14 layers of nanowires encapsulated in polymer.


Solution-Processed Cesium Hexabromopalladate(IV), Cs2PdBr6, for Optoelectronic Applications.

Journal of the American Chemical Society 139 (2017) 6030-6033

N Sakai, AA Haghighirad, MR Filip, PK Nayak, S Nayak, A Ramadan, Z Wang, F Giustino, HJ Snaith

Lead halide perovskites are materials with excellent optoelectronic and photovoltaic properties. However, some hurdles remain prior to commercialization of these materials, such as chemical stability, phase stability, sensitivity to moisture, and potential issues due to the toxicity of lead. Here, we report a new type of lead-free perovskite related compound, Cs2PdBr6. This compound is solution processable, exhibits long-lived photoluminescence, and an optical band gap of 1.6 eV. Density functional theory calculations indicate that this compound has dispersive electronic bands, with electron and hole effective masses of 0.53 and 0.85 me, respectively. In addition, Cs2PdBr6 is resistant to water, in contrast to lead-halide perovskites, indicating excellent prospects for long-term stability. These combined properties demonstrate that Cs2PdBr6 is a promising novel compound for optoelectronic applications.


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


How to Avoid Artifacts in Surface Photovoltage Measurements: A Case Study with Halide Perovskites.

The journal of physical chemistry letters 8 (2017) 2941-2943

I Levine, G Hodes, HJ Snaith, PK Nayak


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


Broadband single-nanowire photoconductive terahertz detectors

Optics InfoBase Conference Papers Part F41-CLEO_SI 2017 (2017)

K Peng, P Parkinson, Q Gao, JL Boland, Z Li, F Wang, YC Wenas, CL Davies, L Fu, MB Johnston, HH Tan, C Jagadish

© 2017 OSA. Broadband photoconductive terahertz detectors based on undoped InP single nanowires were demonstrated. By further design and growth of an axial n + -i-n + structure to reduce the contact resistance, highly-sensitive n + -i-n + InP single-nanowire terahertz detectors were achieved.

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