Publications by Pabitra Nayak

Revealing the nature of photoluminescence emission in the metal-halide double perovskite Cs2AgBiBr6


SJ Zelewski, JM Urban, A Surrente, DK Maude, A Kuc, L Schade, RD Johnson, M Dollmann, PK Nayak, HJ Snaith, P Radaelli, R Kudrawiec, RJ Nicholas, P Plochocka, M Baranowski

Photovoltaic solar cell technologies: analysing the state of the art

Nature Reviews Materials Nature Research 4 (2019) 269-285

P Nayak, S Mahesh, HJ Snaith, D Cahen

Giant Fine Structure Splitting of the Bright Exciton in a Bulk MAPbBr3 Single Crystal.

Nano letters (2019)

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

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.

Direct Observation of Ultrafast Exciton Dissociation in Lead Iodide Perovskite by 2D Electronic Spectroscopy

ACS PHOTONICS 5 (2018) 852-860

A Jha, H-G Duan, V Tiwari, PK Nayak, HJ Snaith, M Thorwart, RJD Miller

Impact of Bi3+ heterovalent doping in organic-inorganic metal halide perovskite crystals

Journal of the American Chemical Society American Chemical Society 140 (2018) 574-577

P Nayak, M Sendner, B Wenger, Z Wang, K Sharma, A Ramadan, R Lovrinčić, A Pucci, PK Madhu, H Snaith

Intrinsic organic-inorganic metal halide perovskites (OIHP) based semiconductors have shown wide applications in optoelectronic devices. There have been several attempts to incorporate heterovalent metal (e.g., Bi3+) ions in the perovskites in an attempt to induce electronic doping and increase the charge carrier density in the semiconductor. It has been reported that inclusion of Bi3+ decreases the band gap of the material considerably. However, contrary to the earlier conclusions, despite a clear change in the appearance of the crystal as observed by eye, here we show that the band gap of MAPbBr3 crystals does not change due the presence of Bi3+ in the growth solution. An increased density of states in the band gap and use of very thick samples for transmission measurements, erroneously give the impression of a band gap shift. These sub band gap states also act as nonradiative recombination centers in the crystals.

The effect of ionic composition on acoustic phonon speeds in hybrid perovskites from Brillouin spectroscopy and density functional theory


IV Kabakova, I Azuri, Z Chen, PK Nayak, HJ Snaith, L Kronik, C Paterson, AA Bakulin, DA Egger

Consolidation of the optoelectronic properties of CH3NH3PbBr3 perovskite single crystals.

Nature Communications Springer Nature 8 (2017) 590-

B Wenger, P Nayak, X Wen, SV Kesava, NK Noel, HJ Snaith

Ultralow trap densities, exceptional optical and electronic properties have been reported for lead halide perovskites single crystals; however, ambiguities in basic properties, such as the band gap, and the electronic defect densities in the bulk and at the surface prevail. Here, we synthesize single crystals of methylammonium lead bromide (CH3NH3PbBr3), characterise the optical absorption and photoluminescence and show that the optical properties of single crystals are almost identical to those of polycrystalline thin films. We observe significantly longer lifetimes and show that carrier diffusion plays a substantial role in the photoluminescence decay. Contrary to many reports, we determine that the trap density in CH3NH3PbBr3 perovskite single crystals is 1015 cm-3, only one order of magnitude lower than in the thin films. Our enhanced understanding of optical properties and recombination processes elucidates ambiguities in earlier reports, and highlights the discrepancies in the estimation of trap densities from electronic and optical methods.Metal halide perovskites for optoelectronic devices have been extensively studied in two forms: single-crystals or polycrystalline thin films. Using spectroscopic approaches, Wenger et al. show that polycrystalline thin films possess similar optoelectronic properties to single crystals.

Solution-processed cesium hexabromopalladate(IV), Cs2PdBr6, for optoelectronic applications

Journal of the American Chemical Society 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.

Synthesis, photophysical, electrochemical and electroluminescence studies of red emitting phosphorescent Ir(III) heteroleptic complexes

Journal of Chemical Sciences 129 (2017) 1391-1398

F Ali, PK Nayak, N Periasamy, N Agarwal

© 2017, Indian Academy of Sciences. Abstract: Five heteroleptic, cyclometalated (C∧N) Iridium(III) complexes of acetylacetone (acac) and 1-phenyl-isoquinoline (piq) derivatives, Ir(acac)(piq)2, Ir(acac)(2,4-difluoro-piq)2, Ir(acac)(4-trifluoromethyl-piq)2, Ir(acac)(4-N,N-dimethyl-piq)2, Ir(acac)(4-acetyl-piq)2, were synthesized and characterized. The (C ∧N ) 2Ir(acac) complexes in toluene showed phosphorescence (λ max= 598 nm to 658 nm) with quantum yields (0.1 to 0.32) and microsecond lifetimes (0.43 to 1.9 μ s). The complexes were non-luminescent in thin films due to self-quenching but luminescent when lightly doped (5%) in a host organic material, 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP). The HOMO levels determined using cyclic voltammetric oxidation potentials were in the range −5.48 to −5.80 eV. Electroluminescence properties and performance of the Ir complexes doped in CBP (active layer) were studied in a multilayer (ITO/F4TCNQ/TPD/doped CBP/BCP/LiF/Al) organic light emitting device (OLED). The electroluminescense (EL) spectra of the device matched with the phosphorescent spectra of the Ir complexes. The turn-on voltage at ∼ 4.5 V, maximum brightness of 7600 cd/m 2 and current efficiency of ∼ 7.0 cd/A at a brightness of ∼ 100 cd/m 2 indicate that these are promising OLED materials. GRAPHICAL ABSTRACT: Synopsis. Heteroleptic, cyclometalated (C∧N) Iridium(III) complexes of acetylacetone (acac) and 1-phenyl-isoquinoline were synthesized and their photophysical, electrochemical and electroluminescence properties were studied. The OLED of Ir complex as emitting material showed turn-on voltage at ∼ 4.5 V, maximum brightness of 7600 cd/m 2 and current efficiency of ∼ 7.0 cd/A at a brightness of ∼ 100 cd/m 2.[Figure not available: see fulltext.].

How to avoid artifacts in surface photovoltage measurements: a case study with halide perovskites

Journal of Physical Chemistry Letters American Chemical Society 8 (2017) 2941–2943-

G Hodes, I Levine, HJ Snaith, P Nayak

Interface-dependent ion migration/accumulation controls hysteresis in MAPbI3 solar cells

Journal of Physical Chemistry C American Chemical Society 120 (2016) 16399-16411

I Levine, P Nayak, JT-W Wang, N Sakai, S Van Reenen, TM Brenner, S Mukhopadhyay, H Snaith, G Hodes, D Cahen

Hysteresis in the current-voltage characteristics of hybrid organic-inorganic perovskite-based solar cells is one of the fundamental aspects of these cells that we do not understand well. One possible cause, suggested for the hysteresis, is polarization of the perovskite layer under applied voltage and illumination bias, due to ion migration within the perovskite. To study this problem systemically, current-voltage characteristics of both regular (light incident through the electron conducting contact) and so-called inverted (light incident through the hole conducting contact) perovskite cells were studied at different temperatures and scan rates. We explain our results by assuming that the effects of scan rate and temperature on hysteresis are strongly correlated to ion migration within the device, with the rate-determining step being ion migration at/across the interfaces of the perovskite layer with the contact materials. By correlating between the scan rate with the measurement temperature, we show that the inverted and regular cells operate in different hysteresis regimes, with different activation energies of 0.28 ± 0.04 eV and 0.59 ± 0.09 eV, respectively. We suggest that the differences observed between the two architectures are due to different rates of ion migration close to the interfaces, and conclude that the diffusion coefficient of migrating ions in the inverted cells is 3 orders of magnitude higher than in the regular cells, leading to different accumulation rates of ions near the interfaces. Analysis of VOC as a function of temperature shows that the main recombination mechanism is trap-assisted (Shockley-Read Hall, SRH) in the space charge region, similar to what is the case for other thin film inorganic solar cells.

Optical phonons in methylammonium lead halide perovskites and implications for charge transport

MATERIALS HORIZONS 3 (2016) 613-620

M Sendner, PK Nayak, DA Egger, S Beck, C Mueller, B Epding, W Kowalsky, L Kronik, HJ Snaith, A Pucci, R Lovrincic

Efficient perovskite solar cells by metal ion doping


JT-W Wang, Z Wang, S Pathak, W Zhang, DW deQuilettes, F Wisnivesky-Rocca-Rivarola, J Huang, PK Nayak, JB Patel, HAM Yusof, Y Vaynzof, R Zhu, I Ramirez, J Zhang, C Ducati, C Grovenor, MB Johnston, DS Ginger, RJ Nicholas, HJ Snaith

Structured organic–inorganic perovskite toward a distributed feedback laser

Advanced Materials Wiley 28 (2015) 923-929

M Saliba, S Wood, J Patel, P Nayak, J Huang, J Alexander-Webber, B Wenger, S Stranks, M Hörantner, J Wang, R Nicholas, L Herz, M Johnston, S Morris, H Snaith, M Riede

A general strategy for the in-plane structuring of organic-inorganic perovskite films is presented. The method is used to fabricate an industrially relevant distributed feedback (DFB) cavity, which is a critical step towards all-electrially pumped injection laser diodes. This approach opens the prospects of perovskite materials for much improved optical control in LEDs, solar cells and also toward applications as optical devices.

Mechanism for rapid growth of organic-inorganic halide perovskite crystals

Nature Communications Nature Publishing Group 7 (2016) 13303-

PK Nayak, DT Moore, B Wenger, S Nayak, AA Haghighirad, A Fineberg, NK Noel, OG Reid, G Rumbles, P Kukura, K Vincent, HJ Snaith

Optoelectronic devices based on hybrid halide perovskites have shown remarkable progress to high performance. However, despite their apparent success, there remain many open questions about their intrinsic properties. Single crystals are often seen as the ideal platform for understanding the limits of crystalline materials, and recent reports of rapid, high-temperature crystallization of single crystals should enable a variety of studies. Here we explore the mechanism of this crystallization and find that it is due to reversible changes in the solution where breaking up of colloids, and a change in the solvent strength, leads to supersaturation and subsequent crystallization. We use this knowledge to demonstrate a broader range of processing parameters and show that these can lead to improved crystal quality. Our findings are therefore of central importance to enable the continued advancement of perovskite optoelectronics and to the improved reproducibility through a better understanding of factors influencing and controlling crystallization.

Shunt‐blocking layers for semitransparent perovskite solar cells

Advanced Materials Interfaces Wiley 3 (2016) 1500837-

MT Hörantner, P Nayak, S Mukhopadhyay, K Wojciechowski, C Beck, D McMeekin, B Kamino, GE Eperon, H Snaith

<p style="text-align:justify;"> Perovskite solar cells have shown phenomenal progress and have great potential to be manufactured as low‐cost large area modules. However, perovskite films often suffer from pinholes and the resulting contact between hole‐ and electron transporting layers provides lower resistance (shunt) pathways, leading to decreased open‐circuit voltage and fill factor. This problem is even more severe in large area cells and especially in the case of neutral color semitransparent cells, where a large absorber‐free area is required to provide the desired transparency. Herein, a simple, inexpensive, and scalable wet chemical method is presented to block these “shunting paths” via deposition of transparent, insulating molecular layers, which preferentially bind to the uncovered surface of the electron collecting oxide, without hindering charge extraction from the perovskite to the charge collection layers. It is shown that this method improves the performance in semitransparent cells, where the enhancement in open‐circuit voltage is up to 30% without negatively impacting the photocurrent. Using this method, we achieved an efficiency of 6.1% for a neutral color semitransparent perovskite cell with 38% average visible transmittance. This simple shunt blocking technique has applications in improving the yield as well as efficiency of large area perovskite solar cells and light emitting devices. </p>

Formation of thin films of organic-inorganic perovskites for high-efficiency solar cells.

Angewandte Chemie (International ed. in English) 54 (2015) 3240-3248

SD Stranks, PK Nayak, W Zhang, T Stergiopoulos, HJ Snaith

Organic-inorganic perovskites are currently one of the hottest topics in photovoltaic (PV) research, with power conversion efficiencies (PCEs) of cells on a laboratory scale already competing with those of established thin-film PV technologies. Most enhancements have been achieved by improving the quality of the perovskite films, suggesting that the optimization of film formation and crystallization is of paramount importance for further advances. Here, we review the various techniques for film formation and the role of the solvents and precursors in the processes. We address the role chloride ions play in film formation of mixed-halide perovskites, which is an outstanding question in the field. We highlight the material properties that are essential for high-efficiency operation of solar cells, and identify how further improved morphologies might be achieved.

Enhanced optoelectronic quality of perovskite thin films with hypophosphorous acid for planar heterojunction solar cells


W Zhang, S Pathak, N Sakai, T Stergiopoulos, PK Nayak, NK Noel, AA Haghighirad, VM Burlakov, DW deQuilettes, A Sadhanala, W Li, L Wang, DS Ginger, RH Friend, H Snaith

Solution-processed metal halide perovskite semiconductors, such as CH3NH3PbI3, have exhibited remarkable performance in solar cells, despite having non-negligible density of defect states. A likely candidate is halide vacancies within the perovskite crystals, or the presence of metallic lead, both generated due to the imbalanced I/Pb stoichiometry which could evolve during crystallization. Herein, we show that the addition of hypophosphorous acid (HPA) in the precursor solution can significantly improve the film quality, both electronically and topologically, and enhance the photoluminescence intensity, which leads to more efficient and reproducible photovoltaic devices. We demonstrate that the HPA can reduce the oxidized I2 back into I(-), and our results indicate that this facilitates an improved stoichiometry in the perovskite crystal and a reduced density of metallic lead.

Generic synthesis of a variety of nanocrystalline metal oxides at room temperature

Journal of Materials Chemistry 18 (2008) 3636-3639

YS Chaudhary, D Chinthalapelly, UM Bhat, PK Nayak, D Khushalani

Generic synthesis of nanocrystals of TiO2, ZrO2 and V2O5 under ambient conditions at room temperature is presented. For the first time, it employs a water-octyl-β-d- glucopyranoside-ethylene glycol diethyl ether based microemulsion and a unique set of glycolate precursors. © The Royal Society of Chemistry.