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CMP
Credit: Jack Hobhouse

David McMeekin

PDRA

Sub department

  • Condensed Matter Physics

Research groups

  • Snaith group
david.mcmeekin@physics.ox.ac.uk
Telephone: 01865 (2)82327
Robert Hooke Building, room G24
  • About
  • Publications

Electronic traps and phase segregation in lead mixed-halide Perovskite

ACS Energy Letters American Chemical Society 4:1 (2018) 75-84

Authors:

Alexander J Knight, Adam D Wright, Jay B Patel, David P McMeekin, Henry J Snaith, Michael B Johnston, Laura M Herz

Abstract:

An understanding of the factors driving halide segregation in lead mixed-halide perovskites is required for their implementation in tandem solar cells with existing silicon technology. Here we report that the halide segregation dynamics observed in the photoluminescence from CH3NH3Pb(Br0.5I0.5)3 is strongly influenced by the atmospheric environment, and that encapsulation of films with a layer of poly(methyl methacrylate) allows for halide segregation dynamics to be fully reversible and repeatable. We further establish an empirical model directly linking the amount of halide segregation observed in the photoluminescence to the fraction of charge carriers recombining through trap-mediated channels, and the photon flux absorbed. From such quantitative analysis we show that under pulsed illumination, the frequency of the modulation alone has no influence on the segregation dynamics. Additionally, we extrapolate that working CH3NH3Pb(Br0.5I0.5)3 perovskite cells would require a reduction of the trap-related charge carrier recombination rate to ≲105s–1 in order for halide segregation to be sufficiently suppressed.
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A generic interface to reduce the efficiency-stability-cost gap of perovskite solar cells.

Science (New York, N.Y.) 358:6367 (2017) 1192-1197

Authors:

Yi Hou, Xiaoyan Du, Simon Scheiner, David P McMeekin, Zhiping Wang, Ning Li, Manuela S Killian, Haiwei Chen, Moses Richter, Ievgen Levchuk, Nadine Schrenker, Erdmann Spiecker, Tobias Stubhan, Norman A Luechinger, Andreas Hirsch, Patrik Schmuki, Hans-Peter Steinrück, Rainer H Fink, Marcus Halik, Henry J Snaith, Christoph J Brabec

Abstract:

A major bottleneck delaying the further commercialization of thin-film solar cells based on hybrid organohalide lead perovskites is interface loss in state-of-the-art devices. We present a generic interface architecture that combines solution-processed, reliable, and cost-efficient hole-transporting materials without compromising efficiency, stability, or scalability of perovskite solar cells. Tantalum-doped tungsten oxide (Ta-WO x )/conjugated polymer multilayers offer a surprisingly small interface barrier and form quasi-ohmic contacts universally with various scalable conjugated polymers. In a simple device with regular planar architecture and a self-assembled monolayer, Ta-WO x -doped interface-based perovskite solar cells achieve maximum efficiencies of 21.2% and offer more than 1000 hours of light stability. By eliminating additional ionic dopants, these findings open up the entire class of organics as scalable hole-transporting materials for perovskite solar cells.
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Unveiling the influence of pH on the crystallization of hybrid perovskites, felivering low voltage loss photovoltaics

Joule Cell Press 1:2 (2017) 328-343

Authors:

Nakita Noel, M Congiu, Alexandra J Ramadan, S Fearn, David P McMeekin, Jay B Patel, Michael B Johnston, Bernard Wenger, Henry J Snaith

Abstract:

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 MAPbI3xClx 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.
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Crystallization kinetics and morphology control of formamidinium-cesium mixed-cation lead mixed-halide perovskite via tunability of the colloidal precursor solution

Advanced Materials Wiley 29:29 (2017) 1-8

Authors:

David McMeekin, Zhiping Wang, Waqaas Rehman, F Pulvirenti, Jay B Patel, Nakita K Noel, Michael B Johnston, Marder, Laura Herz, Henry J Snaith

Abstract:

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 cm(2) 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.
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23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability

Nature Energy Springer Nature 2:4 (2017) 17009

Authors:

Kevin A Bush, Axel F Palmstrom, Zhengshan J Yu, Mathieu Boccard, Rongrong Cheacharoen, Jonathan P Mailoa, David P McMeekin, Robert LZ Hoye, Colin D Bailie, Tomas Leijtens, Ian Marius Peters, Maxmillian C Minichetti, Nicholas Rolston, Rohit Prasanna, Sarah Sofia, Duncan Harwood, Wen Ma, Farhad Moghadam, Henry J Snaith, Tonio Buonassisi, Zachary C Holman, Stacey F Bent, Michael D McGehee
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