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CMP
Credit: Jack Hobhouse

Robin Nicholas

Emeriti

Sub department

  • Condensed Matter Physics
Robin.Nicholas@physics.ox.ac.uk
Telephone: 01865 (2)72250
Clarendon Laboratory, room 148
  • About
  • Publications

Chemical Interaction at the MoO3/CH3NH3PbI3-xClx Interface.

ACS applied materials & interfaces 13:14 (2021) 17085-17092

Authors:

Xiaxia Liao, Severin N Habisreutinger, Sven Wiesner, Golnaz Sadoughi, Daniel Abou-Ras, Marc A Gluba, Regan G Wilks, Roberto Félix, Marin Rusu, Robin J Nicholas, Henry J Snaith, Marcus Bär

Abstract:

The limited long-term stability of metal halide perovskite-based solar cells is a bottleneck in their drive toward widespread commercial adaptation. The organic hole-transport materials (HTMs) have been implicated in the degradation, and metal oxide layers are proposed as alternatives. One of the most prominent metal oxide HTM in organic photovoltaics is MoO3. However, the use of MoO3 as HTM in metal halide perovskite-based devices causes a severe solar cell deterioration. Thus, the formation of the MoO3/CH3NH3PbI3-xClx (MAPbI3-xClx) heterojunction is systematically studied by synchrotron-based hard X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. Upon MoO3 deposition, significant chemical interaction is induced at the MoO3/MAPbI3-xClx interface: substoichiometric molybdenum oxide is present, and the perovskite decomposes in the proximity of the interface, leading to accumulation of PbI2 on the MoO3 cover layer. Furthermore, we find evidence for the formation of new compounds such as PbMoO4, PbN2O2, and PbO as a result of the MAPbI3-xClx decomposition and suggest chemical reaction pathways to describe the underlying mechanism. These findings suggest that the (direct) MoO3/MAPbI3-xClx interface may be inherently unstable. It provides an explanation for the low power conversion efficiencies of metal halide perovskite solar cells that use MoO3 as a hole-transport material and in which there is a direct contact between MoO3 and perovskite.
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Giant fine structure splitting of the bright exciton in a bulk MAPbBr3 single crystal

Nano Letters American Chemical Society 19:10 (2019) 7054-7061

Authors:

M Baranowski, K Galkowski, A Surrente, JM Urban, Ł Klopotowski, S Mackowski, DK Maude, R Ben Ben Aich, K Boujdaria, M Chamarro, C Testelin, Pabitra Nayak, M Dollmann, HJ Snaith, Robin Nicholas, P Plochocka

Abstract:

Exciton fine structure splitting in semiconductors reflects the underlying symmetry of the crystal and quantum confinement. Since the latter factor strongly enhances the exchange interaction, most work has focused on nanostructures. Here, we report on the first observation of the bright exciton fine structure splitting in a bulk semiconductor crystal, where the impact of quantum confinement can be specifically excluded, giving access to the intrinsic properties of the material. Detailed investigation of the exciton photoluminescence and reflection spectra of a bulk methylammonium lead tribromide single crystal reveals a zero magnetic field splitting as large as ~200μeV. This result provides an important starting point for the discussion of the origin of the large bright exciton fine structure observed in perovskite nanocrystals.
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Revealing the nature of photoluminescence emission in the metal-halide double perovskite Cs2AgBiBr6

Journal of Materials Chemistry C Royal Society of Chemistry 7:27 (2019) 8350-8356

Authors:

SJ Zelewski, JM Urban, A Surrente, DK Maude, A Kuc, Laura Schade, Roger Johnson, Markus Dollmann, Pabitra Nayak, Henry Snaith, Paolo Radaelli, R Kudrawiec, Robin Nicholas, P Plochocka, M Baranowski

Abstract:

Double perovskite crystals such as Cs2AgBiBr6 are expected to overcome the limitation of classic hybrid organic–inorganic perovskite crystals related to the presence of lead and the lack of structural stability. Perovskites are ionic crystals in which the carriers are expected to strongly couple to lattice vibrations. In this work we demonstrate that the photoluminescence (PL) emission in Cs2AgBiBr6 is strongly influenced by the strong electron–phonon coupling. Combining photoluminescence excitation (PLE) and Raman spectroscopy we show that the PL emission is related to a color center rather than a band-to-band transition. The broadening and the Stokes shift of the PL emission from Cs2AgBiBr6 is well explained using a Franck–Condon model with a Huang–Rhys factor of S = 11.7 indicating a strong electron–phonon interaction in this material.

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Solubilization of carbon nanotubes with ethylene-vinyl acetate for solution-processed conductive films and charge extraction layers in perovskite solar cells

ACS Applied Materials and Interfaces American Chemical Society 11:1 (2018) 1185-1191

Authors:

Giulio Mazzotta, Markus Dollmann, Habisreutinger, Greyson Christoforo, Zhiping Wang, Henry Snaith, Moritz Riede, Robin Nicholas

Abstract:

Carbon nanotube (CNT) solubilization via non-covalent wrapping of conjugated semiconducting polymers is a common technique used to produce stable dispersions for depositing CNTs from solution. Here, we report the use of a non-conjugated insulating polymer, ethylene vinyl acetate (EVA), to disperse multi- and single-walled CNTs (MWCNT and SWCNT) in organic solvents. We demonstrate that despite the insulating nature of the EVA, we can produce semitransparent films with conductivities of up to 34 S/cm. We show, using photoluminescence spectroscopy, that the EVA strongly binds to individual CNTs, thus making them soluble, preventing aggregation, and facilitating the deposition of high-quality films. To prove the good electronic properties of this composite, we have fabricated perovskite solar cells using EVA/SWCNTs and EVA/MWCNTs as selective hole contact, obtaining power conversion efficiencies of up to 17.1%, demonstrating that the insulating polymer does not prevent the charge transfer from the active material to the CNTs.
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Carbon nanotubes for quantum dot photovoltaics with enhanced light management and charge transport

ACS Photonics American Chemical Society 5:12 (2018) 4854-4863

Authors:

Yujiro Tazawa, S Habisreutinger, Nanlin Zhang, Daniel AF Gregory, G Nagamine, Sameer Kesava, Giulio Mazzotta, Hazel Assender, Moritz K Riede, L Padilha, Robin J Nicholas, Andrew AR Watt

Abstract:

Colloidal quantum dot (CQD)-based photovoltaics are an emerging low-cost solar cell technology with power conversion efficiencies exceeding 10%, i.e., high enough to be interesting for commercialization. Well-controlled and understood charge carrier transport through the device stack is required to make the next step in efficiency improvements. In this paper, polymer-wrapped single-walled carbon nanotube (SWNT) films embedded in an insulating poly(methyl methacrylate) (PMMA) matrix and capped by a thermally evaporated Au electrode are investigated as a composite hole transport layer and optical spacer. Employing transient absorption spectroscopy we show that the SWNTs enhance the charge transfer rate from CQD to CQD, ZnO, or SWNT. In order to pinpoint the underlying mechanism for the improvement, we investigate the energetics of the junction by measuring the relative alignment of the band edges, using Kelvin probe and cyclic voltammetry. Measuring the external quantum efficiency and absorption we find that the improvement is not mainly from electronic improvements but from enhanced absorption of the CQD absorber. We demonstrate experimentally and theoretically, by employing a transfer-matrix model, that the transparent PMMA matrix acts as an optical spacer, which leads to an enhanced absorption in the absorber layer. With these electronic and optical enhancements, the efficiency of the PbS CQD solar cells improved from 4.0% to 6.0%.
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