Hot carrier extraction in CH3NH3PbI3 unveiled by pump-push-probe spectroscopy

Halide perovskites are promising materials for development in hot carrier (HC) solar cells, where the excess energy of above-bandgap photons is harvested before being wasted as heat to enhance device efficiency. Presently, HC separation and transfer processes at higher-energy states remain poorly understood. Here, we investigate the excited state dynamics in CH3NH3PbI3 using pump-push-probe spectroscopy. It has its intrinsic advantages for studying these dynamics over conventional transient spectroscopy, albeit complementary to one another. By exploiting the broad excited-state absorption characteristics, our findings reveal the transfer of HCs from these higher-energy states into bathophenanthroline (bphen), an energy selective organic acceptor far above perovskite's band edges. Complete HC extraction is realized only after overcoming the interfacial barrier formed at the heterojunction, estimated to be between 1.01 and 1.08 eV above bphen's lowest unoccupied molecular orbital level. The insights gained here are essential for the development of a new class of optoelectronics

Highly Efficient Thermally Co-evaporated Perovskite Solar Cells and Mini-modules

The rapid improvement in the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has prompted interest in bringing the technology toward commercialization. Capitalizing on existing industrial processes facilitates the transition from laboratory to production lines. In this work, we prove the scalability of thermally co-evaporated MAPbI3 layers in PSCs and mini-modules. With a combined strategy of active layer engineering, interfacial optimization, surface treatments, and light management, we demonstrate PSCs (0.16 cm2 active area) and mini-modules (21 cm2 active area) achieving record PCEs of 20.28% and 18.13%, respectively. Un-encapsulated PSCs retained ∼90% of their initial PCE under continuous illumination at 1 sun, without sample cooling, for more than 100 h. Looking toward tandem and building integrated photovoltaic applications, we have demonstrated semi-transparent mini-modules and colored PSCs with consistent PCEs of ∼16% for a set of visible colors. Our work demonstrates the compatibility of perovskite technology with industrial processes and its potential for next-generation photovoltaics

PaLM 2 Technical Report

We introduce PaLM 2, a new state-of-the-art language model that has better multilingual and reasoning capabilities and is more compute-efficient than its predecessor PaLM. PaLM 2 is a Transformer-based model trained using a mixture of objectives. Through extensive evaluations on English and multilingual language, and reasoning tasks, we demonstrate that PaLM 2 has significantly improved quality on downstream tasks across different model sizes, while simultaneously exhibiting faster and more efficient inference compared to PaLM. This improved efficiency enables broader deployment while also allowing the model to respond faster, for a more natural pace of interaction. PaLM 2 demonstrates robust reasoning capabilities exemplified by large improvements over PaLM on BIG-Bench and other reasoning tasks. PaLM 2 exhibits stable performance on a suite of responsible AI evaluations, and enables inference-time control over toxicity without additional overhead or impact on other capabilities. Overall, PaLM 2 achieves state-of-the-art performance across a diverse set of tasks and capabilities. When discussing the PaLM 2 family, it is important to distinguish between pre-trained models (of various sizes), fine-tuned variants of these models, and the user-facing products that use these models. In particular, user-facing products typically include additional pre- and post-processing steps. Additionally, the underlying models may evolve over time. Therefore, one should not expect the performance of user-facing products to exactly match the results reported in this report

Spectroscopy study of hot carrier cooling dynamics in halide perovskites

Hot carrier solar cells (HCSCs) hold one of the greatest promises to break the fundamental limit behind the power conversion efficiencies (PCE) of single-junction solar cells. A theoretical limit known as the Shockley-Queisser limit places a barrier of about 33 % PCE for a single-junction solar cell. A class of materials known as halide perovskites (HPs) have recently emerged as frontrunner materials for several optoelectronic applications such as solar cells and light-emitting diodes. PCEs of perovskite solar cells (PSCs) have seen unprecedented growth in the past decade but recent increments have decelerated considerably as the PCEs creep towards the fundamental limit. However, the slow hot carrier cooling (HCC) properties reported in HPs sparked renewed interest towards the possibility in developing a perovskite HCSC, as such slow HCC properties are one of the prerequisites for potential HC absorber materials. This thesis aims to advance the understanding of the HCC dynamics in HPs using various optical spectroscopy methods. One other aim of this thesis is proposing methods for systematic determination of HC metrics that will aid the development of HCSCs. The first part of this thesis re-establishes the reported HCC dynamics from ultrafast transient absorption (TA) spectroscopy. We show that the well-established method of extracting the carrier temperature, which is one of the most important HC metrics, has fundamental flaws and compromises comparability of values between studies. We evaluate the underlying assumptions behind this method and found them to be principally inappropriate. We propose an alternative full-spectrum fit approach to reproduce the TA spectrum of HPs and show that the extracted HC metrics reproduce the expected physics behind HCC of HPs. Our full-spectrum fit method that is systematic and free from ambiguities can describe compositions of HPs well. In the next part, we study the HCC dynamics from the photoluminescence (PL) spectra of HPs. PL-based techniques that are less commonly employed, can also be used to monitor the HCC dynamics of HPs. The procedure for extracting relevant HC metrics from the PL spectra of HPs is largely like that adopted for the TA spectra. Motivated by the ambiguities of the standard fitting method, we sought to develop a full-spectrum fitting method for the complementary PL-based techniques. A full-spectrum fitting procedure based on the generalized Planck's law for radiation from semiconductors is proposed for extracting the HC metrics. By adopting our approach, the PL spectra of several compositions of HPs were studies and we show that the slow HCC effects also manifest under steady-state conditions in the form of low thermalization coefficients. The results provide the connection between the widely reported ultrafast slow HCC phenomena and the steady state. An evaluation of the viability of halide perovskites for HCSCs operating under continuous illumination is also presented. One other approach to achieve slower HCC rates is through quantum confinement. Halide perovskite nanocrystals (HPNCs) possess a unique property in that the quantum confinement induced cooling bottleneck is retained in these materials compared to conventional group II-VI semiconductors like CdSe and PbSe. However there remains an unresolved controversy regarding the existence of such a bottleneck in size-dependent studies of HPNCs. The final part of the thesis examines the influence of quantum confinement effects on the HCC rates of HPNCs in a bid to resolve the existing controversy. Through a series of independent ultrafast spectroscopy techniques, we study the hot carrier cooling properties of weakly and strongly confined HPNCs to identify any quantum confinement induced changes. Our findings provide an overall picture on the evolution of quantum confinement effects on HPNCs. The findings in this thesis advance the understanding behind the HCC behaviour of HPs on the ultrafast and the steady state. The proposed procedures for determining relevant HC metrics in this thesis pushes towards their systematic and unambiguous determination, facilitating their comparability between studies. These findings are valuable in the direction towards the development of next-generation perovskite photovoltaic technologies.Doctor of Philosoph

Spin dynamics in organic-inorganic lead halide perovskites

Organic-inorganic hybrid lead halide perovskites are a new class of revolutionary materials that have great potential for several applications in optoelectronics due to their properties including facile processing, tunable bandgap and long charge carrier diffusion lengths. Their excellent optoelectronic properties have led them to be incorporated into light-harvesting (photovoltaics) and light-emitting (light emitting diodes, lasers, etc.) applications with great efficiencies, amongst several others. Recently, various novel spin phenomena have been demonstrated in this material including tunable spin-selective optical Stark effects, large Rashba splitting and the ability to optically generate spin polarised carriers easily. These novel spin phenomena have been thought to result from the strong SOC present in the system. However, such strong SOC also limits the spin lifetime of organic-inorganic hybrid lead halide perovskites to a few picoseconds, which is detrimental for spintronics devices. One of the main aims of this work is hence to investigate whether we can overcome this limitation of short spin lifetimes by for example, transmitting the spin information at even shorter timescales before the spin dephases. Herein, we employed various ultrafast and steady state spectroscopy techniques to study the general spectral features as well as the spin related dynamics of the various sub-categories of lead halide perovskites. In the 3D perovskite CH3NH3PbI3, we deduced that the primary photogenerated species were free carriers due to its weak exciton binding energy. For the 2D and Ruddlesden-Popper perovskites with dimensionality n = 2 & n = 4, we found that their primary photogenerated species were excitons due to large exciton binding energy. In addition, we verified the existence of a funnelling mechanism in the Ruddlesden-Popper perovskites. Following, we studied the spin dynamics in these perovskites using spin-selective transient absorption spectroscopy. Using a simple model, we were able to determine the electron and hole spin lifetimes in the 3D perovskite and verified that they were only a few picoseconds, in a far shorter timescale compared to carrier recombination which is in the nanosecond timescale. For the Ruddlesden-Popper system, we constructed a similar model for excitons and successfully modelled the spin dynamics. We found that in the n = 2 & n = 4 systems, exciton relaxation occurs primarily in the direct mechanism, where both the spins of the hole and electron flip simultaneously. Of particular interest is that the spin polarisation of the Ruddlesden-Popper perovskites are preserved much better than in the 3D perovskite. We attributed this to the presence of an ultrafast spin funnelling mechanism in which excitons can funnel down multiple bands, bypassing momentum scattering processes greatly and preserving their spin information. Our findings demonstrated the possibility of transmitting spin information before the spin relaxes in the system, hence overcoming the short spin lifetimes in organic-inorganic hybrid lead halide perovskites and validating the viability of using this material in spintronics applications.Bachelor of Science in Physic

Spotlight on hot carriers in halide perovskite luminescence

Harnessing hot carriers’ (HC) excess energy is an attractive approach to surpass the Shockley-Queisser limit. Halide perovskites possess desirable slow HC cooling properties for developing next-generation solar cells. Their HC cooling properties on the ultrafast timescale are well-reported using a myriad of techniques. However, there remains a significant gap between the manifestations of such ultrafast phenomena into the steady state, which is crucial towards translation into real-world efficiency enhancements. Here, we illuminate the connection between these two realms in their steady-state photoluminescence spectra with a unified model that retrieves essential HC metrics like carrier temperature and thermalization coefficient under non-equilibrium conditions. Our findings reveal that perovskites’ thermalization coefficients are an order of magnitude lower than incumbent absorbers. Importantly, our direct approach deepens our understanding of HC contributions to efficiency enhancements and enables wider accessibility to the HC research community, which will help accelerate the development of perovskite HC solar cells.Ministry of Education (MOE)National Research Foundation (NRF)Accepted versionThis research project was supported by the Ministry of Education under its AcRF Tier 2 grants (MOE2019-T2-1-006, MOE2019-T2-1-097, and MOE-T2EP50120-0004) for the material synthesis and characterization and the National Research Foundation (NRF) Singapore under its NRF Investigatorship (NRF-NRFI-2018-04) for the photophysics studies

Origins of the long-range exciton diffusion in perovskite nanocrystal films : photon recycling vs exciton hopping

The outstanding optoelectronic performance of lead halide perovskites lies in their exceptional carrier diffusion properties. As the perovskite material dimensionality is reduced to exploit the quantum confinement effects, the disruption to the perovskite lattice, often with insulating organic ligands, raises new questions on the charge diffusion properties. Herein, we report direct imaging of >1 μm exciton diffusion lengths in CH3NH3PbBr3 perovskite nanocrystal (PNC) films. Surprisingly, the resulting exciton mobilities in these PNC films can reach 10 ± 2 cm2 V-1 s-1, which is counterintuitively several times higher than the carrier mobility in 3D perovskite films. We show that this ultralong exciton diffusion originates from both efficient inter-NC exciton hopping (via Förster energy transfer) and the photon recycling process with a smaller yet significant contribution. Importantly, our study not only sheds new light on the highly debated origins of the excellent exciton diffusion in PNC films but also highlights the potential of PNCs for optoelectronic applications.Ministry of Education (MOE)National Research Foundation (NRF)Published versionWe acknowledged Dr. Pio John S. Buenconsejo from the Facility for Analysis Characterization Testing and Simulation (FACTS), Nanyang Technological University, Singapore, for help with GISAXS measurements. This research/project was supported by Nanyang Technological University under its start-up grants (M4080514, M4081630); the Ministry of Education under its AcRF Tier 1 grant (RG91/19) and Tier 2 grants (MOE2016-T2-1-034, MOE2017-T2-1-001, and MOE2017-T2-2-002); and the National Research Foundation (NRF) Singapore under its NRF Investigatorship (NRF-NRFI-2018-04) and Competitive Research Programme (NRF-CRP14-2014-03). Author information Author notes These authors contributed equally: David Giovanni, Marcello Righetto Affiliations Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University (NTU), 21 Nanyang Link, Singapore, 637371, Singapore David Giovanni, Marcello Righetto, Qiannan Zhang, Jia Wei Melvin Lim, Sankaran Ramesh & Tze Chien Sum Energy Research Institute @NTU (ERI@N), Interdisciplinary Graduate School, Nanyang Technological University, 50 Nanyang Avenue, S2-B3a-01, Singapore, 639798, Singapore Jia Wei Melvin Lim & Sankaran Ramesh Contributions D.G. and M.R. conceived the idea. M.R. synthesized the samples. D.G. and M.R. performed the optical spectroscopy measurements and analysis of perovskite samples. Q.N. performed the AFM and GISAXS measurements and analysis. M.R. and J.W.M.L. performed the TEM measurement and analysis. D.G. and S.R. performed CdSe sample fabrication and measurement. T.C.S. led the project. All authors were involved in writing the manuscript. Corresponding author Correspondence to Tze Chien Sum. Ethics declarations Conflict of interest The authors declare that they have no conflict of interest. Supplementary information Supplementary Information for Origins of the Long-Range Exciton Diffusion in Perovskite Nanocrystal Films: Photon Recycling vs Exciton Hopping Rights and permissions Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Reprints and Permissions About this article Verify currency and authenticity via CrossMark Cite this article Giovanni, D., Righetto, M., Zhang, Q. et al. Origins of the long-range exciton diffusion in perovskite nanocrystal films: photon recycling vs exciton hopping. Light Sci Appl 10, 2 (2021). https://doi.org/10.1038/s41377-020-00443-z Download citation Received19 July 2020 Revised13 November 2020 Accepted23 November 2020 Published01 January 2021 DOIhttps://doi.org/10.1038/s41377-020-00443-z Share this article Anyone you share the following link with will be able to read this content

Hot carriers perspective on the nature of traps in perovskites

Amongst the many spectacular properties of hybrid lead halide perovskites, their defect tolerance is regarded as the key enabler for a spectrum of high-performance optoelectronic devices that propel perovskites to prominence. However, the plateauing efficiency enhancement of perovskite devices calls into question the extent of this defect tolerance in perovskite systems; an opportunity for perovskite nanocrystals to fill. Through optical spectroscopy and phenomenological modeling based on the Marcus theory of charge transfer, we uncover the detrimental effect of hot carriers trapping in methylammonium lead iodide and bromide nanocrystals. Higher excess energies induce faster carrier trapping rates, ascribed to interactions with shallow traps and ligands, turning these into potent defects. Passivating these traps with the introduction of phosphine oxide ligands can help mitigate hot carrier trapping. Importantly, our findings extend beyond photovoltaics and are relevant for low threshold lasers, light-emitting devices and multi-exciton generation devices.Ministry of Education (MOE)Nanyang Technological UniversityNational Research Foundation (NRF)Published versionThis research was supported by Nanyang Technological University under its start-up grant (M4080514) and its JSPS-NTU Joint Research Project (M4082176); by the Ministry of Education under its AcRF Tier 2 grants (MOE2016-T2-1-034 and MOE2017-T2-2- 002); and by the National Research Foundation (NRF) Singapore under its NRF Investigatorship (NRF-NRFI-2018-04)

Insights to carrier-phonon interactions in lead halide perovskites via multi-pulse manipulation

A fundamental understanding of the hot-carrier dynamics in halide perovskites is crucial for unlocking their prospects for next generation photovoltaics. Presently, a coherent picture of the hot carrier cooling process remains patchy due to temporally overlapping contributions from many-body interactions, multi-bands, band gap renormalization, Burstein-Moss shift etc. Pump-push-probe (PPP) spectroscopy recently emerges as a powerful tool complementing the ubiquitous pump-probe (PP) spectroscopy in the study of hot-carrier dynamics. However, limited information from PPP on the initial excitation density and carrier temperature curtails its full potential. Herein, this work bridges this gap in PPP with a unified model that retrieves these essential hot carrier metrics like initial carrier density and carrier temperature under the push conditions, thus permitting direct comparison with traditional PP spectroscopy. These results are well-fitted by the phonon bottleneck model, from which the longitudinal optical phonon scattering time τLO , for MAPbBr3 and MAPbI3 halide perovskite thin film samples are determined to be 240 ± 10 and 370 ± 10 fs, respectively.Ministry of Education (MOE)National Research Foundation (NRF)Submitted/Accepted versionThis research/project is supported by the Ministry of Education under its AcRF Tier 2 grants (MOE2019-T2-1-006, MOE-T2EP20120-0013 and MOE-T2EP50120-0004); and the National Research Foundation (NRF) Singapore under its NRF Investigatorship (NRF-NRFI2018-04)

Zero-field quantum beats and spin decoherence mechanisms in CsPbBr₃ perovskite nanocrystals

Coherent optical manipulation of exciton states provides a fascinating approach for quantum gating and ultrafast switching. However, their coherence time for incumbent semiconductors is highly susceptible to thermal decoherence and inhomogeneous broadening effects. Here, we uncover zero-field exciton quantum beating and anomalous temperature dependence of the exciton spin lifetimes in CsPbBr3 perovskite nanocrystals (NCs) ensembles. The quantum beating between two exciton fine-structure splitting (FSS) levels enables coherent ultrafast optical control of the excitonic degree of freedom. From the anomalous temperature dependence, we identify and fully parametrize all the regimes of exciton spin depolarization, finding that approaching room temperature, it is dominated by a motional narrowing process governed by the exciton multilevel coherence. Importantly, our results present an unambiguous full physical picture of the complex interplay of the underlying spin decoherence mechanisms. These intrinsic exciton FSS states in perovskite NCs present fresh opportunities for spin-based photonic quantum technologies.Ministry of Education (MOE)National Research Foundation (NRF)Published versionNRF-CRP25-2020-000