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

Engineering carrier dynamics in lead halide perovskites

Organic-inorganic hybrid perovskites have attracted immense attention primarily due to its outstanding photovoltaic and light emission properties. Specifically, certified power conversion efficiencies exceeding 20 % have been demonstrated in perovskite solar cells – attributed to their large absorption coefficients and long, balanced, ambipolar diffusion lengths, large grains and its unique defect tolerance. Careful morphological control is needed to form dense, uniform films essential for high performance devices. However, the resultant fundamental optoelectronic properties of such process controls are not well understood. In addition, the higher excited states that can aid in breaking the detailed balance limit is also unexplored. This thesis reports on a series of studies using ultrafast optical spectroscopy on the archetypal bulk 3D perovskite, MAPI, to provide insights on the photophysics and recombination dynamics of this class of materials. Here, process controls in fabricating perovskite thin films refers to treatments to the film, in the form of solvent engineering and additives, and to the substrate for hydrophilicity. Our findings reveal that solvent engineering of MAPI, i.e., dripping of toluene during spincoating which is key to improving film morphologies and subsequent solar cell efficiencies, had resulted in increased trap densities. We attribute this anomalous behaviour to an interplay of factors where the improved film morphology had also resulted in the lengthening of the carrier recombination lifetimes. Interestingly, depending on the combination of treatments, the charge extraction interface can go from injecting to non-injecting. Exceptionally low carrier recombination rates in lead halide perovskites is crucial for its high performance. By careful optimisation of the additive concentration, improvements to device performance were observed when H2O was added to MAPI as an additive. Trace amounts of H2O passivates the trap states leading to reduced recombination rates, markedly improved carrier lifetimes and Jsc. At the optimal additive concentration of 1 vol% H2O, higher order carrier recombination is suppressed, and the greatly reduced monomolecular and bimolecular recombination rates was correlated with an increase in power conversion efficiencies. Process controls have all but ensured the unprecedented growth in record efficiencies of perovskite solar cells. However, it slows as it approaches the detailed balance limit of solar cells. One way to surpass this limit is to exploit the concept of hot carriers -- photoexcited carriers in higher excited states. A technique to directly probe these states is presented, and through this, the broad photoinduced absorption band was revealed to be attributed to the promotion of photoexcited carriers to higher energy states. However, the observed sub-picosecond thermalisation times of these higher excited states may prove difficult for hot carrier extraction. Importantly, our results underscores the importance of judiciously choosing process controls to optimise optoelectronic properties of perovskite devices.Doctor of Philosoph

Quo vadis, perovskite emitters?

Halide perovskites hold great promise for next generation printable optoelectronic devices. Within a decade of their debut in photovoltaics, these amazing materials proliferate beyond solar cells to applications such as light-emitting devices, lasers, radiation detectors, and memristors. Such versatility stems from perovskites’ favorable optoelectronic properties that are highly exceptional for a facile solution-processed system. Halide perovskite emitters have made significant inroads, in particular, perovskite light emitting device (PeLED) efficiencies have risen from 20% within 5 years, and perovskite continuous wave amplified spontaneous emission has also been demonstrated recently. This perspective distills the photophysical mechanisms underpinning the various approaches in enhancing their radiative efficiencies. Selected works are highlighted to detail the milestones and to chart the direction the field is heading. Challenges and opportunities for solid-state PeLEDs are discussed. A clear understanding of their basic photophysics and structure-function relations holds the key to rationalizing strategies and streamlining efforts to realize high efficiency PeLEDs and perovskite lasers.NRF (Natl Research Foundation, S’pore)ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore)Accepted versio

The photophysics of Ruddlesden-Popper perovskites : a tale of energy, charges, and spins

Quasi two-dimensional halide perovskites (also known as Ruddlesden-Popper or RPs) are the most recent and exciting evolution in the perovskite field. Possessing a unique combination of enhanced moisture and material stability, whilst retaining the excellent optoelectronic properties, RPs are poised to be a game changer in the perovskite field. Spurred by their recent achievements in solar cells, light-emitting diodes and spintronic devices, these materials have garnered a mounting interest. Herein, we critically review the photophysics of RPs and distil the science behind their structure-property relations. We first focus on their structure and morphology by highlighting the crucial role of large cations: dictating the RPs’ layered structure and the statistical distribution of thicknesses (i.e., n-phases). Next, we discuss how optoelectronic properties of RPs differ from conventional halide perovskites. Structural disorder, stronger excitonic and polaronic interaction shape the nature of photo-excitations and their fate. For example, faster recombinations and hindered transport are expected for charge carriers in thinner n-phases. However, the complex energetic landscape of RPs, which originates from the coexistence of different n-phases, allows for funnelling of energy and charges. Presently, the photophysics of RPs is still nascent, with many recent exciting discoveries from coherence effects in the above-mentioned funnelling cascade to spin effects. Giant Rashba spin-orbit coupling, also observed in RPs, dictates their spin dynamics and provides exciting spintronics opportunities. To leverage these propitious RPs, future research must entail a cross-disciplinary approach. While materials engineering will unlock new chiral RPs and Dion-Jacobson variants, novel characterization techniques such as in situ synchrotron-based X-ray diffraction, ultrafast electron microscopy, and multi-dimensional electronic spectroscopy etc. are essential in unravelling their secrets and unleashing their full potential.Ministry of Education (MOE)Nanyang Technological UniversityNational Research Foundation (NRF)Accepted versionThis research was supported by Nanyang Technological University under its start-up grant (M4080514); the Ministry of Education under its AcRF Tier 1 grant (RG91/19) and Tier 2 grants (MOE2017-T2-1-001, MOE2017-T2-2-002 and MOE2019-T2-1-006); and the National Research Foundation (NRF) Singapore under its Competitive Research Program (NRFCRP14- 2014-03) and NRF Investigatorship (NRF-NRFI-2018-04)

Role of water in suppressing pecombination pathways in CH3NH3PbI3 perovskite solar cells

Moisture degradation of halide perovskites is the Achilles heel of perovskite solar cells. A surprising revelation in 2014 about the beneficial effects of controlled humidity in enhancing device efficiencies overthrew established paradigms on perovskite solar cell fabrication. Despite the extensive studies on water additives in perovskite solar cell processing that followed, detailed understanding of the role of water from the photophysical perspective remains lacking; specifically, the interplay between the induced morphological effects and the intrinsic recombination pathways. Through ultrafast optical spectroscopy, we show that both the monomolecular and bimolecular recombination rate constants decrease by approximately 1 order with the addition of an optimal 1% H2O by volume in CH3NH3PbI3 as compared to the reference (without the H2O additive). Correspondingly, the trap density reduces from 4.8 × 1017 cm-3 (reference) to 3.2 × 1017 cm-3 with 1% H2O. We obtained an efficiency of 12.3% for the champion inverted CH3NH3PbI3 perovskite solar cell (1% H2O additive) as compared to the 10% efficiency for the reference cell. Increasing the H2O content further is deleterious for the device. Trace amounts of H2O afford the benefits of surface trap passivation and suppression of trap-mediated recombination, whereas higher concentrations result in a preferential dissolution of methylammonium iodide during fabrication that increases the trap density (MA vacancies). Importantly, our study reveals the effects of trace H2O additives on the photophysical properties of CH3NH3PbI3 films.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Contour Based Path Planning with B-Spline Trajectory Generation for Unmanned Aerial Vehicles (UAVs) over Hostile Terrain

This research focuses on trajectory generation algorithms that take into account the stealthiness of autonomous UAVs; generating stealthy paths through a region laden with enemy radars. The algorithm is employed to estimate the risk cost of the navigational space and generate an optimized path based on the user-specified threshold altitude value. Thus the generated path is represented with a set of low-radar risk waypoints being the coordinates of its control points. The radar-aware path planner is then approximated using cubic B-splines by considering the least radar risk to the destination. Simulated results are presented, illustrating the potential benefits of such algorithms

Hot dipping post treatment for improved efficiency in micro patterned semi-transparent perovskite solar cells

Perovskite solar cells have emerged as a new semi-transparent PV technology for urban infrastructures that demands an explicit trade-off between power conversion efficiency (PCE) and average visible transparency (AVT) which can be adjusted by various modifications in the absorber layer. Here, we introduce a scalable and facile “one and a half” step deposition route for mixed cation perovskites patterned in a sub-micron sized grid structure for semi-transparent solar cells. The initial perovskite phase is formed in one step using a grid pattern, while the additional step involves dipping of the pre-deposited perovskite grid in a hot solution of formamidinium iodide (FAI) in isopropanol (IPA). Detailed analysis suggests that the additional step increases pore filling, crystal quality, and grain size and lowers the content of residual PbI2 as well as reveals improved photo physical properties. An average PCE ∼10% with an AVT of 28% is attained with a gold contact for the champion semi-transparent solar cell. The proposed deposition route can be generalized for all other types of perovskite based devices to yield better efficiency.NRF (Natl Research Foundation, S’pore

Cation influence on carrier dynamics in perovskite solar cells

Rubidium and Cesium cations (Rb + and Cs + ) incorporation recently emerged as a viable strategy to enhance perovskite solar cells (PSCs) efficiency. However, a clear understanding of the impact of these cations on the structure-function relationship in relation to the device performance is severely lacking. Here, we systematically investigate the influence of Rb + and Cs + on the carrier dynamics using transient optical spectroscopy and correlate with solar cell performance. Unlike Rb + , Cs + integrates well with methylammonium (MA + ) and formamidinium (FA + ) yielding increased perovskite grain size, longer charge carrier lifetimes and improved power conversion efficiency (PCE). Concomitant incorporation of Cs + /Rb + cooperatively retards radiative recombination by ~60% in the quaternary-cation based perovskite system (RbCsMAFA) compared to the dual-cation MAFA samples. By suppressing the defect density, PCEs around 20% are obtained along with more balanced charge carrier diffusion length and comparable photoluminescence quantum yield in quaternary-cation perovskites. While the synergistic addition of Rb + and Cs + is attractive for controlling defects and recombination losses in efficient solar cells development, sole incorporation of Rb + is still an engineering challenge. Importantly, our study explicates the underlying mechanisms behind the synergistic combination of cations to minimize the charge carrier losses and achieve high efficiency perovskite solar cells.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

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)

Modulating Carrier Dynamics through Perovskite Film Engineering

Precise morphological control in perovskite films is key to high performance photovoltaic and light emitting devices. However, a clear understanding of the interplay of morphological effects from substrate/perovskite antisolvent treatments on the charge dynamics is still severely lacking. Through detailed ultrafast optical spectroscopy, we correlate the morphology-kinetics relationship in a combination of substrate/film treated samples (i.e., plasma-cleaned vs piranha-etched substrates and solvent (toluene)-engineered (or toluene anti-solvent treated) perovskite films). Our findings reveal that toluene-dripped treatment has a more pronounced influence on the morphology of perovskite films prepared on plasma-cleaned substrates over those on piranha-etched substrates. Surprisingly, the highly effective toluene-dripping/washing approach reported in the literature increases the surface trap densities of perovskite films. Despite the marked improvements in the surface morphology of the toluene-dripped films, there is only a slight improvement in the carrier relaxation lifetimes – likely due to the competition between the morphology improvements and the increased surface traps densities. In addition, the injection of photoexcited holes to spiro-OMeTAD from toluene-dripped films on piranha-etched substrates is inhibited, possibly due to a realignment of the energy bands. Nonetheless, piranha-etching of the substrates could possibly offer an approach to improve the balance between the electron and hole diffusion lengths in the perovskite film. Importantly, our findings would help unravel the complex relationship of substrate/film treatments on the morphology and charge kinetics in perovskite thin films.NRF (Natl Research Foundation, S’pore)ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore)Accepted versio