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CMP
Credit: Jack Hobhouse

Prof Henry Snaith FRS

Professor of Physics

Sub department

  • Condensed Matter Physics

Research groups

  • Photovoltaic and optoelectronic device group
Henry.Snaith@physics.ox.ac.uk
Robert Hooke Building, room G21
  • About
  • Publications

Thermal management enables stable perovskite nanocrystal light-emitting diodes with novel hole transport material

Small Wiley (2023) e2303472

Authors:

Xinyu Shen, Seon Lee Kwak, Woo Hyeon Jeong, Ji Won Jang, Zhongkai Yu, Hyungju Ahn, Hea Jung Park, Hyosung Choi, Sung Heum Park, Henry J Snaith, Do-Hoon Hwang, Bo Ram Lee

Abstract:

The severely insufficient operational lifetime of perovskite light-emitting diodes (LEDs) is incompatible with the rapidly increasing external quantum efficiency, even as it approaches the theoretical limit, thereby significantly impeding the commercialization of perovskite LEDs. In addition, Joule heating induces ion migration and surface defects, degrades the photoluminescence quantum yield and other optoelectronic properties of perovskite films, and induces the crystallization of charge transport layers with low glass transition temperatures, resulting in LED degradation under continuous operation. Here, a novel thermally crosslinked hole transport material, poly(FCA<sub>60</sub> -co-BFCA<sub>20</sub> -co-VFCA<sub>20</sub> ) (poly-FBV), with temperature-dependent hole mobility is designed, which is advantageous for balancing the charge injection of the LEDs and limiting the generation of Joule heating. The optimised CsPbI<sub>3</sub> perovskite nanocrystal LEDs with poly-FBV realise approximately a 2-fold external quantum efficiency increase over the LED with commercial hole transport layer poly(4-butyl-phenyl-diphenyl-amine) (poly-TPD), owing to the balanced carrier injection and suppressed exciton quenching. Moreover, because of the Joule heating control provided by the novel crosslinked hole transport material, the LED utilising crosslinked poly-FBV has a 150-fold longer operating lifetime (490 min) than that utilizing poly-TPD (3.3 min). The study opens a new avenue for the use of PNC LEDs in commercial semiconductor optoelectronic devices.
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Chloride-Based Additive Engineering for Efficient and Stable Wide-Bandgap Perovskite Solar Cells.

Advanced materials (Deerfield Beach, Fla.) 35:30 (2023) e2211742

Authors:

Xinyi Shen, Benjamin M Gallant, Philippe Holzhey, Joel A Smith, Karim A Elmestekawy, Zhongcheng Yuan, PVGM Rathnayake, Stefano Bernardi, Akash Dasgupta, Ernestas Kasparavicius, Tadas Malinauskas, Pietro Caprioglio, Oleksandra Shargaieva, Yen-Hung Lin, Melissa M McCarthy, Eva Unger, Vytautas Getautis, Asaph Widmer-Cooper, Laura M Herz, Henry J Snaith

Abstract:

Metal halide perovskite based tandem solar cells are promising to achieve power conversion efficiency beyond the theoretical limit of their single-junction counterparts. However, overcoming the significant open-circuit voltage deficit present in wide-bandgap perovskite solar cells remains a major hurdle for realizing efficient and stable perovskite tandem cells. Here, a holistic approach to overcoming challenges in 1.8 eV perovskite solar cells is reported by engineering the perovskite crystallization pathway by means of chloride additives. In conjunction with employing a self-assembled monolayer as the hole-transport layer, an open-circuit voltage of 1.25 V and a power conversion efficiency of 17.0% are achieved. The key role of methylammonium chloride addition is elucidated in facilitating the growth of a chloride-rich intermediate phase that directs crystallization of the desired cubic perovskite phase and induces more effective halide homogenization. The as-formed 1.8 eV perovskite demonstrates suppressed halide segregation and improved optoelectronic properties.
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Photovoltaic performance of FAPbI3 perovskite is hampered by intrinsic quantum confinement

ACS Energy Letters American Chemical Society 8:6 (2023) 2543-2551

Authors:

Karim A Elmestekawy, Benjamin M Gallant, Adam D Wright, Philippe Holzhey, Nakita K Noel, Michael B Johnston, Henry J Snaith, Laura M Herz

Abstract:

Formamidinium lead trioiodide (FAPbI3) is a promising perovskite for single-junction solar cells. However, FAPbI3 is metastable at room temperature and can cause intrinsic quantum confinement effects apparent through a series of above-bandgap absorption peaks. Here, we explore three common solution-based film-fabrication methods, neat N,N-dimethylformamide (DMF)–dimethyl sulfoxide (DMSO) solvent, DMF-DMSO with methylammonium chloride, and a sequential deposition approach. The latter two offer enhanced nucleation and crystallization control and suppress such quantum confinement effects. We show that elimination of these absorption features yields increased power conversion efficiencies (PCEs) and short-circuit currents, suggesting that quantum confinement hinders charge extraction. A meta-analysis of literature reports, covering 244 articles and 825 photovoltaic devices incorporating FAPbI3 films corroborates our findings, indicating that PCEs rarely exceed a 20% threshold when such absorption features are present. Accordingly, ensuring the absence of these absorption features should be the first assessment when designing fabrication approaches for high-efficiency FAPbI3 solar cells.

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Exciton Formation Dynamics and Band-Like Free Charge-Carrier Transport in 2D Metal Halide Perovskite Semiconductors

ADVANCED FUNCTIONAL MATERIALS Wiley (2023)

Authors:

Silvia G Motti, Manuel Kober-Czerny, Marcello Righetto, Philippe Holzhey, Joel Smith, Hans Kraus, Henry J Snaith, Michael B Johnston, Laura M Herz

Abstract:

Metal halide perovskite (MHP) semiconductors have driven a revolution in optoelectronic technologies over the last decade, in particular for high-efficiency photovoltaic applications. Low-dimensional MHPs presenting electronic confinement have promising additional prospects in light emission and quantum technologies. However, the optimisation of such applications requires a comprehensive understanding of the nature of charge carriers and their transport mechanisms. This study employs a combination of ultrafast optical and terahertz spectroscopy to investigate phonon energies, charge-carrier mobilities, and exciton formation in 2D (PEA)2PbI4 and (BA)2PbI4 (where PEA is phenylethylammonium and BA is butylammonium). Temperature-dependent measurements of free charge-carrier mobilities reveal band transport in these strongly confined semiconductors, with surprisingly high in-plane mobilities. Enhanced charge-phonon coupling is shown to reduce charge-carrier mobilities in (BA)2PbI4 with respect to (PEA)2PbI4. Exciton and free charge-carrier dynamics are disentangled by simultaneous monitoring of transient absorption and THz photoconductivity. A sustained free charge-carrier population is observed, surpassing the Saha equation predictions even at low temperature. These findings provide new insights into the temperature-dependent interplay of exciton and free-carrier populations in 2D MHPs. Furthermore, such sustained free charge-carrier population and high mobilities demonstrate the potential of these semiconductors for applications such as solar cells, transistors, and electrically driven light sources.
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Understanding the Degradation of Methylenediammonium and Its Role in Phase-Stabilizing Formamidinium Lead Triiodide.

Journal of the American Chemical Society 145:18 (2023) 10275-10284

Authors:

Elisabeth A Duijnstee, Benjamin M Gallant, Philippe Holzhey, Dominik J Kubicki, Silvia Collavini, Bernd K Sturdza, Harry C Sansom, Joel Smith, Matthias J Gutmann, Santanu Saha, Murali Gedda, Mohamad I Nugraha, Manuel Kober-Czerny, Chelsea Xia, Adam D Wright, Yen-Hung Lin, Alexandra J Ramadan, Andrew Matzen, Esther Y-H Hung, Seongrok Seo, Suer Zhou, Jongchul Lim, Thomas D Anthopoulos, Marina R Filip, Michael B Johnston, Robin J Nicholas, Juan Luis Delgado, Henry J Snaith

Abstract:

Formamidinium lead triiodide (FAPbI3) is the leading candidate for single-junction metal-halide perovskite photovoltaics, despite the metastability of this phase. To enhance its ambient-phase stability and produce world-record photovoltaic efficiencies, methylenediammonium dichloride (MDACl2) has been used as an additive in FAPbI3. MDA2+ has been reported as incorporated into the perovskite lattice alongside Cl-. However, the precise function and role of MDA2+ remain uncertain. Here, we grow FAPbI3 single crystals from a solution containing MDACl2 (FAPbI3-M). We demonstrate that FAPbI3-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA2+ is not the direct cause of the enhanced material stability. Instead, MDA2+ degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI3 crystals grown from a solution containing HMTA (FAPbI3-H) replicate the enhanced α-phase stability of FAPbI3-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA+ is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H+). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H+ is selectively incorporated into the bulk of both FAPbI3-M and FAPbI3-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability.
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