Publications


High electron mobility and insights into temperature-dependent scattering mechanisms in InAsSb nanowires

Nano Letters American Chemical Society 18 (2018) 3703–3710-

JL Boland, F Amaduzzi, S Sterzl, H Potts, LM Herz, AFI Fontcuberta i Morral, M Johnston

InAsSb nanowires are promising elements for thermoelectric devices, infrared photodetectors, high-speed transistors, as well as thermophotovoltaic cells. By changing the Sb alloy fraction the mid-infrared bandgap energy and thermal conductivity may be tuned for specific device applications. Using both terahertz and Raman noncontact probes, we show that Sb alloying increases the electron mobility in the nanowires by over a factor of 3 from InAs to InAs0.65Sb0.35. We also extract the temperature-dependent electron mobility via both terahertz and Raman spectroscopy, and we report the highest electron mobilities for InAs0.65Sb0.35 nanowires to date, exceeding 16,000 cm2 V–1 s–1 at 10 K.


Engineering interactions in QDs–PCBM blends: a surface chemistry approach

Nanoscale Royal Society of Chemistry 10 (2018) 11913-11922

A Privitera, M Righetto, F Carraro, L Bolzonello, C Ferrante, L Franco, R Bozio

Here we present a comprehensive study on the photophysics of QDs–fullerene blends, aiming to elucidate the impact of ligands on the extraction of carriers from QDs. We investigated how three different ligands (oleylamine, octadecanethiol and propanethiol) influence the dynamics of charge generation, separation, and recombination in blends of CdSe/CdS core/shell QDs and PCBM. We accessed each relevant process directly by combining the results from both optical and magnetic spectroscopies. Transient absorption measurements revealed a faster interaction dynamics in thiol-capped ligands. Through phenomenological modeling of the interaction processes, i.e., energy transfer and electron transfer, we estimated the suppression of exciton migration and the enhancement of electron transfer processes when alkyl–thiols are employed as ligands. Contextually, we report the profound impact of the ligands’ alkyl chain length, leading to strengthened interactions with PCBM acceptors. Quantitatively, we measured a 10-fold increase in the electron transfer rate when oleylamine ligands were exchanged with propanethiol ligands. EPR spectroscopy gave access to subtle details regarding both the enhanced charge generation and lower binding energy of charge-transfer states in blends compared to PCBM alone. Moreover, through pulsed EPR techniques, we inferred the localization of deep electron traps in localized sites close to QDs in the blends. Therefore, our thorough characterization evidenced the essential role of ligands in determining QD interactions. We believe that these discoveries will contribute to the efficient incorporation of QDs in existing organic PV technologies.


Present status and future prospects of perovskite photovoltaics.

Nature materials 17 (2018) 372-376

HJ Snaith


Balancing charge carrier transport in a quantum dot P–N Junction toward hysteresis-free high-performance solar cells

ACS Energy Letters American Chemical Society 3 (2018) 1036-1043

Y Cho, B Hou, J Lim, S Lee, S Pak, J Hong, P Giraud, A-R Jang, Y-W Lee, J Lee, JE Jang, HJ Snaith, S Morris, J Sohn, S Cha, JM Kim

In a quantum dot solar cell (QDSC) that has an inverted structure, the QD layers form two different junctions between the electron transport layer (ETL) and the other semiconducting QD layer. Recent work on an inverted-structure QDSC has revealed that the junction between the QD layers is the dominant junction, rather than the junction between the ETL and the QD layers, which is in contrast to the conventional wisdom. However, to date, there have been a lack of systematic studies on the role and importance of the QD heterojunction structure on the behavior of the solar cell and the resulting device performance. In this study, we have systematically controlled the structure of the QD junction to balance charge transport, which demonstrates that the position of the junction has a significant effect on the hysteresis effect, fill factor, and solar cell performance and is attributed to balanced charge transport.


Hybrid Perovskites: Prospects for Concentrator Solar Cells.

Advanced science (Weinheim, Baden-Wurttemberg, Germany) 5 (2018) 1700792-1700792

Q Lin, Z Wang, HJ Snaith, MB Johnston, LM Herz

Perovskite solar cells have shown a meteoric rise of power conversion efficiency and a steady pace of improvements in their stability of operation. Such rapid progress has triggered research into approaches that can boost efficiencies beyond the Shockley-Queisser limit stipulated for a single-junction cell under normal solar illumination conditions. The tandem solar cell architecture is one concept here that has recently been successfully implemented. However, the approach of solar concentration has not been sufficiently explored so far for perovskite photovoltaics, despite its frequent use in the area of inorganic semiconductor solar cells. Here, the prospects of hybrid perovskites are assessed for use in concentrator solar cells. Solar cell performance parameters are theoretically predicted as a function of solar concentration levels, based on representative assumptions of charge-carrier recombination and extraction rates in the device. It is demonstrated that perovskite solar cells can fundamentally exhibit appreciably higher energy-conversion efficiencies under solar concentration, where they are able to exceed the Shockley-Queisser limit and exhibit strongly elevated open-circuit voltages. It is therefore concluded that sufficient material and device stability under increased illumination levels will be the only significant challenge to perovskite concentrator solar cell applications.


Cubic or orthorhombic? Revealing the crystal structure of metastable black-phase CsPbI3 by theory and experiment

ACS Energy Letters American Chemical Society 3 (2018) 787–1794-

R Sutton, M Filip, A-A Haghighirad, N Sakai, B Wenger, F Giustino, H Snaith

Room-temperature films of black-phase cesium lead iodide (CsPbI3) are widely thought to be trapped in a cubic perovskite polymorph. Here, we challenge this assumption. We present structural refinement of room-temperature black-phase CsPbI3 in an orthorhombic polymorph. We demonstrate that this polymorph is adopted by both powders and thin films of black-phase CsPbI3, fabricated either by high- or low-temperature processes. We perform electronic band structure calculations for the orthorhombic polymorph and find agreement with experimental data and close similarities with orthorhombic methylammonium lead iodide. We investigate the structural transitions and thermodynamic stability of the various polymorphs of CsPbI3 and show that the orthorhombic polymorph is the most stable among its other perovskite polymorphs, but it remains less stable than the yellow nonperovskite polymorph.


Understanding Thermal Admittance Spectroscopy in Low-Mobility Semiconductors

JOURNAL OF PHYSICAL CHEMISTRY C 122 (2018) 9795-9803

S Wang, P Kaienburg, B Klingebiel, D Schillings, T Kirchartz


Preface to special topic: frontiers on THz photonic devices

APL Photonics AIP Publishing 3 (2018) 051501-1

S Atakaramians, M Johnston, W Padilla, R Mendis

Terahertz (THz) photonic devices are now exploiting emerging materials systems, while novel device designs utilise plasmonic effects, nanophotinics, and metamaterials. The scope of this special topic highlights and reviews the recent cutting-edge THz photonic devices which have been revolutionised from the advances in the above research areas.


Perovskite/Colloidal Quantum Dot Tandem Solar Cells: Theoretical Modeling and Monolithic Structure

ACS ENERGY LETTERS 3 (2018) 869-874

A Karani, L Yang, S Bai, MH Futscher, HJ Snaith, B Ehrler, NC Greenham, D Di


Nonspiro, Fluorene-Based, Amorphous Hole Transporting Materials for Efficient and Stable Perovskite Solar Cells.

Advanced science (Weinheim, Baden-Wurttemberg, Germany) 5 (2018) 1700811-

Š Daškevičiū Tė, N Sakai, M Franckevičius, M Daškevičienė, A Magomedov, V Jankauskas, HJ Snaith, V Getautis

Novel nonspiro, fluorene-based, small-molecule hole transporting materials (HTMs) V1050 and V1061 are designed and synthesized using a facile three-step synthetic route. The synthesized compounds exhibit amorphous nature with a high glass transition temperature, a good solubility, and decent thermal stability. The planar perovskite solar cells (PSCs) employing V1050 generated an excellent power conversion efficiency of 18.3%, which is comparable to 18.9% obtained with the state-of-the-art Spiro-OMeTAD. Importantly, the devices based on V1050 and V1061 show better stability compared to devices based on Spiro-OMeTAD when aged without any encapsulation under uncontrolled humidity conditions (relative humidity around 60%) in the dark and under continuous full sun illumination.


Key Tradeoffs Limiting the Performance of Organic Photovoltaics

Advanced Energy Materials (2018)

I Ramirez, M Causa', Y Zhong, N Banerji, M Riede

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. 2017 saw the publication of several new material systems that challenge the long-held notion that a driving force is necessary for efficient exciton dissociation in organic photovoltaics (OPVs) and that a loss of ≈0.6 eV between the energy of the charge transfer state E ct and the energy corresponding to open circuit is general. In light of these developments, the authors combine insights from device physics and spectroscopy to review the two key tradeoffs limiting OPV performances. These are the tradeoff between the charge carrier generation efficiency and the achievable open circuit voltage (V oc ) and the tradeoff between device thickness (light absorption) and fill factor. The emergence of several competitive nonfullerene acceptors (NFAs) is exciting for both of these. The authors analyze what makes these materials compare favorably to fullerenes, including the potential role of molecular vibrations, and discuss both design criteria for new molecules and the achievable power conversion efficiencies.


Interplay of structural and optoelectronic properties in formamidinium mixed tin-lead triiodide perovskites

Advanced Functional Materials Wiley 28 (2018)

ES Parrott, T Green, RL Milot, MB Johnston, HJ Snaith, LM Herz

Mixed lead-tin triiodide perovskites are promising absorber materials for low band-gap bottom cells in all-perovskite tandem photovoltaic devices. Key structural and electronic properties of the FAPb1-xSnxI3 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 band-gap and phase transition temperature. The large band-gap bowing parameter, a crucial element for the attainment of low band-gaps 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, mono-exponential 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 multi-phonon assisted process. These results highlight the importance of structure and composition for control of band-gap bowing and charge-carrier recombination mechanisms in low band-gap absorbers for all-perovskite tandem solar cells.


Enabling reliability assessments of pre-commercial perovskite photovoltaics with lessons learned from industrial standards

NATURE ENERGY 3 (2018) 459-465

HJ Snaith, P Hacke


Correction to "Exciton-Dominated Core-Level Absorption Spectra of Hybrid Organic-Inorganic Lead Halide Perovskites".

The journal of physical chemistry letters 9 (2018) 3193-

C Vorwerk, C Hartmann, C Cocchi, G Sadoughi, SN Habisreutinger, R Félix, RG Wilks, HJ Snaith, M Bär, C Draxl


Enhanced photovoltage for inverted planar heterojunction perovskite solar cells

Science American Association for the Advancement of Science 360 (2018) 1442-1446

D Luo, W Yang, Z Wang, A Sadhanala, Q Hu, R Su, R Shivanna, GF Trindade, JF Watts, Z Xu, T Liu, K Chen, F Ye, P Wu, L Zhao, J Wu, Y Tu, Y Zhang, X Yang, W Zhang, RH Friend, Q Gong, HJ Snaith, R Zhu

The highest power conversion efficiencies (PCEs) reported for perovskite solar cells (PSCs) with inverted planar structures are still inferior to those of PSCs with regular structures, mainly because of lower open-circuit voltages (Voc). Here we report a strategy to reduce nonradiative recombination for the inverted devices, based on a simple solution-processed secondary growth technique. This approach produces a wider bandgap top layer and a more n-type perovskite film, which mitigates nonradiative recombination, leading to an increase in Voc by up to 100 millivolts. We achieved a high Voc of 1.21 volts without sacrificing photocurrent, corresponding to a voltage deficit of 0.41 volts at a bandgap of 1.62 electron volts. This improvement led to a stabilized power output approaching 21% at the maximum power point.


Linearly polarized photoluminescence of InGaN quantum disks embedded in GaN nanorods.

Scientific reports 8 (2018) 8124-8124

Y Park, CCS Chan, L Nuttall, TJ Puchtler, RA Taylor, N Kim, Y Jo, H Im

We have investigated the emission from InGaN/GaN quantum disks grown on the tip of GaN nanorods. The emission at 3.21 eV from the InGaN quantum disk doesn't show a Stark shift, and it is linearly polarized when excited perpendicular to the growth direction. The degree of linear polarization is about 39.3% due to the anisotropy of the nanostructures. In order to characterize a single nanostructure, the quantum disks were dispersed on a SiO2 substrate patterned with a metal reference grid. By rotating the excitation polarization angle from parallel to perpendicular relative to the nanorods, the variation of overall PL for the 3.21 eV peak was recorded and it clearly showed the degree of linear polarization (DLP) of 51.5%.


Exciton-Dominated Core-Level Absorption Spectra of Hybrid Organic-Inorganic Lead Halide Perovskites.

The journal of physical chemistry letters 9 (2018) 1852-1858

C Vorwerk, C Hartmann, C Cocchi, G Sadoughi, SN Habisreutinger, R Félix, RG Wilks, HJ Snaith, M Bär, C Draxl

In a combined theoretical and experimental work, we investigate X-ray absorption near-edge structure spectroscopy of the I L3 and the Pb M5 edges of the methylammonium lead iodide (MAPbI3) hybrid inorganic-organic perovskite and its binary phase PbI2. The absorption onsets are dominated by bound excitons with sizable binding energies of a few hundred millielectronvolts and pronounced anisotropy. The spectra of both materials exhibit remarkable similarities, suggesting that the fingerprints of core excitations in MAPbI3 are essentially given by its inorganic component, with negligible influence from the organic groups. The theoretical analysis complementing experimental observations provides the conceptual insights required for a full characterization of this complex material.


Template-directed synthesis of a conjugated zinc porphyrin nanoball

Journal of the American Chemical Society American Chemical Society 140 (2018) 5352-5355

J Cremers, R Haver, M Rickhaus, J Gong, L Favereau, M Peeks, T Claridge, L Herz, H Anderson

We report the template-directed synthesis of a π-conjugated 14-porphyrin nanoball. This structure consists of two intersecting nanorings containing six and 10 porphyrin units. Fluorescence upconversion spectroscopy experiments demonstrate that electronic excitation delocalizes over the whole three-dimensional π system in less than 0.3 ps if the nanoball is bound to its templates or over 2 ps if the nanoball is empty.


Carrier confinement effects of InxGa1-xN/GaN multi quantum disks with GaN surface barriers grown in GaN nanorods

OPTICAL MATERIALS 78 (2018) 365-369

Y Park, CCS Chan, RA Taylor, N Kim, Y Jo, SW Lee, W Yang, H Im


Highly crystalline methylammonium lead tribromide perovskite films for efficient photovoltaic devices

ACS Energy Letters American Chemical Society 3 (2018) 1233−1240-

N Noel, B Wenger, S Habisreutinger, J Patel, T Crothers, Z Wang, R Nicholas, M Johnston, L Herz, H Snaith

The rise of metal-halide perovskite solar cells has captivated the research community, promising to disrupt the current energy landscape. While a sizable percentage of the research done on this class of materials has been focused on the neat and iodide-rich perovskites, bromide-based perovskites can deliver substantially higher voltages because of their relatively wide band gaps of over 2 eV. The potential for efficient, high-voltage devices makes materials such as these incredibly attractive for multijunction photovoltaic applications. Here, we use the acetonitrile/methylamine solvent system to deposit smooth, highly crystalline films of CH3NH3PbBr3. By using choline chloride as a passivating agent for these films, we achieve photoluminescence quantum efficiencies of up to 5.5% and demonstrate charge-carrier mobilities of 17.8 cm2/(V s). Incorporating these films into photovoltaic devices, we achieve scanned power conversion efficiencies of up to 8.9%, with stabilized efficiencies of 7.6%, providing a simple route to realizing efficient, high-voltage CH3NH3PbBr3 planar-heterojunction devices.

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