HyPe-2017: A Discussion Meeting on Hybrid Perovskites he first meeting in India on hybrid perovskites (HyPe2017) took place at S. N. Bose National Centre for Basic Sciences, Kolkata on Dec 14−15, 2017, and was organized by Manoranjan Kumar and Priya Mahadevan. The meeting brought together leading scientists (Figure 1) working on various aspects of the physics and chemistry of these materials that combine organic and inorganic building blocks, an area that was almost dormant after the initial explorations by David Mitzi for potential optoelectronic applications. Tsutomu Miyasaka’s group first explored the feasibility of using hybrid perovskites as solar cell materials, followed by Nam-Gyu Park’s group. However, it was not until the publication of more than 10% efficiency achieved in a solar cell reported by the group of Henry Snaith did the community take any great notice of these materials, and since then, it has never looked back, leading to an unprecedented hype across the world. There were two distinct areas of topical importance covered in various talks at the conferenceone of course related to photovoltaic applications and the other related to primarily optical properties, without any reference to the photovoltaic aspects. Prashant Kamat (University of Notre Dame) spoke on mixed halide perovskites, which allow bandgap tuning through halide composition with a variation in the ratio of Cl:Br and Br:I, resulting in controlled absorption and emission in the visible region. However, phase segregation takes place, creating iodiderich and bromide-rich regions in MAPbBrxI3−x mixed halides under light exposure, limiting the use of mixed halides in solar cells and display devices. Hence, he emphasized that efforts are needed to identify the origin of such halide ion mobility under photoirradiation and ways to suppress the migration of halide ions. Another important issue in these materials is to find alternate lead-free hybrid perovskites with the same performance as with their Pb counterparts. David Mitzi (Duke University) in his talk focused on the structural and electronic flexibility of the double perovskites as a route to Pb-free halide semiconductors. Complex organic cations were introduced to synthesize 2D perovskites with increased functionality. An aspect that becomes important in the context of highperformance perovskite films uses the self-assembling nature of the hybrid perovskites. This allows an array of processing approaches that include additive engineering, melt processing, as well as matrix-assisted pulsed laser evaporation. On the topic of structure, an important aspect was addressed in the presentation by D. D. Sarma (IISc), who examined the role of the dipoles associated with the organic moieties in the hybrid perovskites in the phase relevant for photovoltaic applications. While all experimental results pointed to a structure in which the dipoles were disordered, they also explored and eliminated the possibility of a locally polar structure in the photoexcited state using pump−probe experiments to explore a time interval of ∼100 fs to 2 ns from the photoexcitation. New results were also presented to reliably estimate the excitonic binding energy
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in these systems using two-photon absorption-induced photoluminescence studies. Pankaj Mandal (IISER Pune) examined the charge dynamics of CsPbBr3 nanocrystals in colloidal dispersions as well as in thin films using time-resolved THz spectroscopy. The time scale associated with the carrier transfer from the nanocrystals to a carrier acceptor was found to be sub −300 fs. On the theoretical front, one needs methods to be able capture of the structural as well as electronic properties of these materials. Volker Blum (Duke University) covered in his talk electronic structure formalisms that are scalable to large systems and are sufficiently reliable to capture the structural and carrier properties of layered hybrid perovskites with complex, active organic functionalities. The role of the atom at the A site in hybrid perovskites of the form ABX3 in determining the structure and consequently modifying the band gap was discussed by Priya Mahadevan (S. N. Bose Center), revisiting some solid-state chemistry concepts that have emerged in the context of inorganic perovskites and how they get modified when one has an organic molecule at the A site. Sayan Battacharyya (IISER Kolkata) demonstrated strategies of improving power conversion efficiency and ambient stability of photovoltaic devices for inorganic halide perovskite quantum dots as a function of the halide composition, metal ion doping, nanodimensionality of the perovskites, and filling of pinholes. He also showed that the charge collection in polymer solar cells can be boosted by immobilizing the quantum dots on polymer nanostructures. K. Narayan (JNCASR) showed that the degradation and aging processes in hybrid perovskites appear in the form of noise fluctuations in the photocurrent. Spatial mapping of the photocurrent revealed different levels of photocurrent and noise spectra from degraded regions with characteristic 1/f type behavior. The first step before any study of the physical properties involves the synthesis and growth of these materials. In contrast to conventional routes employed in the synthesis of these materials, several speakers showcased novel approaches used for the synthesis and growth. Narayan Pradhan (IACS) discussed how one could use classical ideas of crystal growth by controlling the kinetics as well as thermodynamic considerations to not only restrict the size of the lead halide nanocrystals but also expand their stability range. A theoretical approach to stabilizing the (100) surface in Cs−Pb-based halide perovskites was proposed by Yong Hyun Kim (KAIST) using organic ligands. Sameer Sapra (IITD) showed mechanochemistry at work and demonstrated that just by grinding with a mortar and pestle they could optimize APbX3 (A = Cs, MA, FA, Received: February 11, 2018 Accepted: February 13, 2018
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DOI: 10.1021/acsenergylett.8b00247 ACS Energy Lett. 2018, 3, 733−734
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Cite This: ACS Energy Lett. 2018, 3, 733−734
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Figure 1. Group photo taken at HyPe-2017.
dynamics are of relevance in the performance in solar cells. Vijayakumar C. Nair (CSIR-NIIST) was able to obtain an extended fluorescence lifetime (∼4.2 μs) for self-assembled close-packed perovskite nanocrystals in a thin film compared to isolated ones in solution. On the applications perspective, Dinesh Kabra (IITB) spoke on the recent advances made in his group on light-emitting diodes (LEDs), discussing the challenges in red and blue perovskite LEDs as well as a picture to understand the long-lived photoluminescence time observed. This was explained by them using a model of shallow defect states.
EA and X = Cl, Br, I) as well as dope Mn. Continuing on the same approach, Kanishka Biswas (JNCASR) demonstrated scalable synthesis of various inorganic perovskite halides with varying connectivity as well as transformations between them. Anion exchange has been widely used in the synthesis of nanoparticles by colloidal chemistry routes. Somobrata Acharya (IACS) extended those ideas to this class of materials and provided a route to transform PbS nanocrystals into PbI2 by anion exchange and then into hybrid perovskite nanocrystals by controlled exposure to CH3NH3Br. Characterization of the anion exchanged nanocrystals was explored by Pralay Santra (CeNS) who used variable-energy hard X-ray photoelectron spectroscopy on pure and anion exchanged CsPbX3 (X = Br, I) to quantify the distribution of anion at various stages of the exchange process. The toxicity of Pb requires its replacement with alternate elements that are less toxic and whose compounds have the same functionality as Pb. Angshuman Nag (IISER Pune) discussed various Pb-free hybrid perovskites/double perovskites synthesized by colloidal methods and their optoelectronic properties. The optoelectronic properties of the hybrid perovskites were the focus of several other talks. Satish Ogale (IISER Pune) explored small molecular synthetic manipulation of organic−inorganic hybrid perovskites for control/enhancement of the quantum yield, giving several examples, one of which was an improvement from 1 to 30% achieved by quartenary alkyl ammonium salt treatment. Janardan Kundu (NCL) demonstrated a 40% quantum yield associated with Mn emission when doped into (butyl ammonium)2PbBr4, which could be explored for applications as a phosphor. The carrier
Priya Mahadevan
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S. N. Bose National Centre for Basic Sciences, JD Block, Sector 3, Salt Lake, Kolkata 700106, India
AUTHOR INFORMATION
ORCID
Priya Mahadevan: 0000-0003-0240-4490 Notes
Views expressed in this Energy Focus are those of the author and not necessarily the views of the ACS. The author declares no competing financial interest.
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DOI: 10.1021/acsenergylett.8b00247 ACS Energy Lett. 2018, 3, 733−734
