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Ruddlesden-Popper (RP) perovskites have been gaining traction in the development of high-efficiency or blue-emitting perovskite light emitting diodes (PeLEDs) due to the unique energy funneling mechanism, which enhances photoluminescence intensity, and dimensional control, which enables spectral tuning. In a conventional p-i-n device structure, the quality of RP perovskite films, including grain morphology and defects, as well as device performance can be significantly influenced by the underlying hole-transport layer (HTL). Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is commonly used in several PeLEDs as an HTL because of its high electrical conductivity and optical transparency. Nonetheless, the energy level mismatch and exciton quenching caused by PEDOT:PSS often compromises PeLED performance. Herein, we investigate the mitigation of these effects through addition of work-function-tunable PSS Na to the PEDOT:PSS HTL and assess the impact on blue PeLED performance. Surface analysis of the modified PEDOT:PSS HTLs reveals a PSS-rich layer that alleviates exciton quenching at the HTL/perovskite interface. At an optimal concentration of 6% PSS Na addition, an improvement in the external quantum efficiency is observed, with champion blue and sky-blue PeLEDs achieving 4% (480 nm) and 6.36% (496 nm), respectively, while operation stability is prolonged by fourfold. Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version This research is supported by the National Research Foundation (NRF), Singapore, under its Competitive Research Program (CRP) (NRF-CRP25-2020-0002) and the Ministry of Education (MOE2019-T2-2-097).

Defect management strategies are vital for enhancing the performance of perovskite-based optoelectronic devices, such as perovskite-based light-emitting diodes (PeLEDs). As additives can fucntion both as acrystallization modifier and/or defect passivator, a thorough study on the roles of additives is essential, especially for blue emissive Pe-LEDs, where the emission is strictly controlled by the n-domain distribution of the Ruddlesden–Popper (RP, L2An–1PbnX3n+1, where L refers to a bulky cation, while A and X are monovalent cation, and halide anion, respectively) perovskite films. Of the various additives that are available, octyl phosphonic acid (OPA) is of immense interest because of its ability to bind with uncoordinated Pb2+ ( notorious for nonradiative recombination) and therefore passivates them. Here, with the help of various spectroscopic techniques, such as X-ray photon-spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and photoluminescence quantum yield (PLQY) measurements, we demonstrate the capability of OPA to bind and passivate unpaired Pb2+ defect sites. Modification to crystallization promoting higher n-domain formation is also observed from steady-state and transient absorption (TA) measurements. With OPA treatment, both the PLQY and EQE of the corresponding PeLED showed improvements up to 53% and 3.7% at peak emission wavelength of 485 nm, respectively. Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version This research was funded by National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Competitive Research Programme (CRP Award NRFCRP14-2014-03) and Ministry of Education, Singapore (MOE2019-T2-2-097). The photophysics studies were supported by the grants funded by the Singapore Ministry of Education under its AcRF Tier 2 grant (MOE-T2EP50120- 0004) and the NRF under NRF Investigatorship (NRF-NRFI2018-04).

The rapid emergence of organic-inorganic lead halide perovskites for low-cost and high-efficiency photovoltaics promises to impact new photovoltaic concepts. Their high power conversion efficiencies, ability to coat perovskite layers on glass via various scalable deposition techniques, excellent optoelectronic properties and synthetic versatility for modulating transparency and colour, allows perovskite solar cells (PSCs) to be an ideal solution for building-integrated photovoltaics (BIPV) which transforms windows or facades into electric power generators. In this review, the unique features and properties of PSCs for BIPV application are accessed. Device engineering and optical management strategies of active layers, interlayers and electrodes for semi-transparent, bifacial and colourful PSCs are also discussed. The performance of PSCs under conditions that are relevant for BIPV such as different operational temperature, light intensity and light incident angle are also reviewed. Recent outdoor stability testing of PSCs in different countries and other demonstration of scalability and deployment of PSCs are also spotlighted. Finally, the current challenges and future opportunities for realising perovskite based BIPV are discussed. This article is protected by copyright. All rights reserved.

Surface imperfections created during fabrication of halide perovskite (HP) films could induce formation of various defect sites that affect device performance and stability. In this work, all-organic surface modifiers consisting of alkylammonium cations and alkanoate anions are introduced on top of the HP layer to passivate interfacial vacancies and improve moisture tolerance. Passivation using alkylammonium alkanoate does not induce formation of low-dimensional perovskites species. Instead, the organic species only passivate the perovskite's surface and grain boundaries, which results in enhanced hydrophobic character of the HP films. In terms of photovoltaic application, passivation with alkylammonium alkanoate allows significant reduction in recombination losses and enhancement of open-circuit voltage. Alongside unchanged short-circuit current density, power conversion efficiencies of more than 18.5 % can be obtained from the treated sample. Furthermore, the unencapsulated device retains 85 % of its initial PCE upon treatment, whereas the standard 3D perovskite device loses 50 % of its original PCE when both are subjected to ambient environment over 1500 h. National Research Foundation (NRF) The authors would like to acknowledge funding from the Singapore National Research Foundation through the Intra-CREATE Collaborative Grant (NRF2018-ITC001-001), Energy Innovation Research Program (NRF2015EWT-EIRP003-004, Solar CRP: S18-1176-SCRP) and the Competitive Research Program (NRF-CRP14-2014-03).

In the past few years, a large variety of perovskite solar cells (PSCs) with vivid and well-distinguished color hues have been demonstrated. In this Perspective, we compare different strategies employed to realize colorful PSCs both in opaque and semitransparent designs. The approaches used to modulate the PSCs' colorful appearance can be divided into two main categories: the first one based on the modifications of their internal layers (i.e., absorber, electron- and/or hole-transporting layers, and electrodes), while the second is based on the addition of external colored or nanostructured films to the standard PSCs. The advantages and bottlenecks of each strategy are discussed in terms of PSCs' color tunability, transparency, photovoltaic performances, fabrication processes feasibility, and scalability, in view of suitable applications in an urban context for building-integrated photovoltaics. National Research Foundation (NRF) Accepted version This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under Energy Innovation Research Program (Grant numbers: NRF2015EWT-EIRP003-004, NRF-CRP14-2014-03, Solar CRP: S18-1176-SCRP, NRF2018- ITC001-001).

Metal lead halide perovskite nanocrystals have emerged as promising candidates for optoelectronic applications. However, the inclusion of toxic lead is a major concern for the commercial viability of these materials. Herein, we introduce a new family of non-toxic reduced dimension Rb2CuX3 (X= Br, Cl) colloidal nanocrystals with one-dimensional crystal structure consisting [CuX4]3- ribbons isolated by Rb+ cations. These nanocrystals were synthesised using a room-temperature method under ambient conditions, which makes them cost effective and scalable. Phase purity quantification was confirmed by Rietveld refinement of powder x-ray diffraction and corroborated by 87Rb MAS NMR technique. Both samples also exhibited high thermal stability up to 500°C, which is essential for optoelectronic applications. Rb2CuBr3 and Rb2CuCl3 displays PL emission peaks at 387 nm and 400 nm with high PLQYs of ~100% and ~49%, respectively. Lastly, the first colloidal synthesis of quantum-confined rubidium copper halide-based nanocrystals opens up a new avenue to exploit their optical properties in lighting technology as well as water sterilisation and air purification. National Research Foundation (NRF) P.V. acknowledges a Presidential Postdoctoral Fellowship from Nanyang Technological University (NTU), Singapore via grant 04INS000581C150OOE01. M.K., N.M., S.G.M., and T.W. acknowledge financial support from the Singapore National Research Foundation, Prime Minister's Office, through the Competitive Research Program (CRP Award No. NRF-CRP14-2014-03). T.C.S and D.G. acknowledge the financial support from the NRF Investigatorship (NRF-NRFI-2018-04) and the Ministry of Education under its AcRF Tier 1 grant (RG91/19) and Tier 2 grant MOE2019-T2-1-006. The authors would like to acknowledge the Facility for Analysis, Characterization, Testing and Simulation (FACTS) at NTU, Singapore, for use of their electron microscopy and X-ray diffraction facilities. We would also like to acknowledge the NTU Centre of High Field NMR Spectroscopy and Imaging for the use of their NMR facilities. We thank Mr. Sai S. H. Dintakurti and Dr. Ankit, NTU, Singapore.

Inorganic cesium lead halide perovskite nanocrystals are promising materials for optoelectronic applications as they exhibit high thermal stability alongside precise color tunability and high color purity; however, their optical properties are degraded by surface defects. This work demonstrates a room temperature synthesis of CsPbBr3 nanocrystals facilitating in situ surface passivation via the incorporation of Zn2+ cations. The facile incorporation ZnBr2 into the precursor solution facilitates Zn2+ and Br− substitution into the nanocrystal surface/subsurface layers to induce passivation of existing Pb2+ and Br– vacancies and increase the photoluminescence quantum yield from ∼48 to 86%. The XPS and solid-state 1H MAS NMR techniques show that the key modification is a reduction of the octylamine:oleic acid ratio leading to a near-neutral surface charge; this is accompanied by the appearance of larger nanosheets and nanowires observed by quantitative powder XRD and HR-TEM. The suitability of these perovskite nanocrystals for electrically driven applications was confirmed by the fabrication of light-emitting diodes, which demonstrate that the in situ Zn2+ passivation strategy enhanced the external quantum efficiency by ∼60%. National Research Foundation (NRF) PV acknowledges a Presidential Postdoctoral Fellowship from Nanyang Technological University (NTU), Singapore via grant 04INS000581C150OOE01. We acknowledge financial support from the Singapore National Research Foundation, Prime Minister’s Office, through the Competitive Research Program (CRP Award No. NRF-CRP14-2014-03). The authors would like to acknowledge the Facility for Analysis, Characterization, Testing and Simulation (FACTS) at NTU, Singapore, for use of their electron microscopy and X-ray diffraction facilities. We would also like to acknowledge the NTU Center of High Field NMR Spectroscopy and Imaging for the use of their NMR facilities. The authors would also like to thank Prof. Nripan Mathews and Dr. Nur Fadilah Jamaludin for the valuable discussion, Mr. Sai S. H. Dintakurti for discussions with aspects of the crystallographic analysis and Mr. Gautam V. Nutan for assistance with the XPS measurements. JVH acknowledges financial support for the solid state NMR instrumentation at Warwick used in this research which was funded by EPSRC (grants EP/M028186/1 and EP/K024418/1), the University of Warwick, and the Birmingham Science City AM1 and AM2 projects which were supported by Advantage West Midlands (AWM) and the European Regional Development Fund (ERDF). AAMB acknowledges the Tizard studentship from the Faculty of Engineering and Physical Sciences at University of Southampton.

Recent advances in Ruddlesden–Popper (RP) perovskites have shown highly efficient light-emitting diodes (LEDs) due to the energy confinement by an energy cascade from the quasi-2D (donors) to 3D (acceptors) states. This is achieved by forming mixed-dimensional phases using 2D templating (spacer) molecules. The effect of the spacer molecular design on the efficacy of light emission is hitherto unknown, thus motivating this study into eight different spacer molecules ranging from the naphthyl to phenyl alkyl families. The phase distribution, which influences the resultant energy landscape, can simply be modulated by the choice of this spacer molecule. It was found that a high acceptor-to-donor ratio is required to achieve the best LED efficiencies. A simple absorption spectroscopy measurement can also be employed to screen potential RP systems. The findings are of significance to other halide perovskites and lay down a useful guideline to assess new spacer molecules for more efficient RP perovskite LEDs. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version This research was supported by the National Research Foundation, Prime Minister’s Office, Singapore, under its Competitive Research Programme (CRP Award No. NRFCRP14- 2014-03) and Intra-CREATE Collaborative Grant (NRF2018-ITC001-001). T.C.S. and D.G. acknowledge the financial support from the Tier 2 grant MOE2017-T2-2-002; and from the Singapore National Research Foundation Investigatorship NRF-NRFI-2018-04.

Lead-free halide perovskite semiconductors are necessary due to the atmospheric instability and lead toxicity associated with the 3D lead halide perovskites. However, a stable lead-free perovskite with an ideal band gap (1.2-1.4 eV) for photovoltaics is still missing. In this work, we synthesized organic-inorganic gold halide double perovskites ((CH3NH3)2Au2X6, X = Br, I) through a solution-processed route that offers an ideal direct band gap for photovoltaic applications. Density functional theory calculations confirm the direct nature of the band gap with reasonable absorption coefficients in the visible range and excellent charge transport properties. In addition, the Au-halide perovskites show high chemical stability and photoresponse. These combined properties demonstrate that Au-based halide perovskites can be a promising group of compounds for optoelectronic applications. National Research Foundation (NRF) Accepted version This research was supported by the National Research Foundation, Prime Minister’s Office, Singapore, under its Competitive Research Programme (CRP Award No. NRF- 482 CRP14-2014-03) and Intra-CREATE Collaborative Grant (NRF2018-ITC001-001). We would also like to thank Dr. Li Yongxin for helping us solve the crystal structures of the materials presented herein.

High performance organic nano-floating gate transistor memory (NFGTM) has important prerequisites of low processing temperature, solution–processable layers and charge trapping medium with high storage capacity. We demonstrate organic NFGTM using black phosphorus quantum dots (BPQDs) as a charge trapping medium by simple spin-coating and low processing temperature (< 120 °C). The BPQDs with diameter of 12.6 ± 1.5 nm and large quantum confined bandgap of ~2.9 eV possess good charge trapping ability. The organic memory device exhibits excellent memory performance with a large memory window of 61.3 V, write-read-erase-read cycling endurance of 10 3 for more than 180 cycles and reliable retention over 10,000 sec. In addition, we successfully improved the memory retention to ON/OFF current ratio > 10 4 over 10,000 sec by introducing PMMA as the tunneling layer. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

Perovskite semiconductors have experienced meteoric rise in a variety of optoelectronic applications. With a strong foothold on photovoltaics, much focus now lies on their light emission applications. Rapid progress in materials engineering have led to the demonstration of external quantum efficiencies that surpass the previously established theoretical limits. However, there remains much scope to further optimize the light propagation inside the device stack through careful tailoring of the optical processes that take place at the bulk and interface levels. Photon recycling in the emitter material followed by efficient outcoupling can result in boosting external efficiencies up to 100%. In addition, the poor ambient and operational stability of these materials and devices restrict further commercialization efforts. With best operational lifetimes of only a few hours reported, there is a long way to go before perovskite LEDs can be perceived as reliable alternatives to more established technologies like organic or quantum dot-based LED devices. This review article starts with the discussions of the mechanism of luminescence in these perovskite materials and factors impacting it. It then looks at the possible routes to achieve efficient outcoupling through nanostructuring of the emitter and the substrate. Next, we analyze the instability issues of perovskite-based LEDs from a photophysical standpoint, taking into consideration the underlying phenomena pertaining to defects, and summarize recent advances in mitigating the same. Finally, we provide an outlook on the possible routes forward for the field and propose new avenues to maximally exploit the excellent light-emitting capabilities of this family of semiconductors. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) National Research Foundation (NRF) Published version This research was supported by National Research Foundation, Prime Minister’s Office, Singapore, under its Competitive Research Programme (CRP Award No. NRFCRP14-2014-03), and Intra-CREATE Collaborative Grant (NRF2018-ITC001-001), as well as Ministry of Education (MOE) under AcRF Tier 2 grants (2018-T2-1-075 and 2019-T2-2-106), and A*STAR National Robotics Programme (W1925d0106).

Despite the rapid development of perovskite light-emitting diodes (PeLEDs) in recent years, blue PeLEDs’ efficiencies are still inferior to those of their red and green counterparts. The poor performance is associated with, among other factors, halide segregation in bromide-chloride materials and energy funneling to lowest bandgaps in multilayered Ruddlesden–Popper (RP) systems. This study reports that compositional engineering through prudent selection of the A-site cation in a pure bromide RP system can result in a narrow distribution of layered domains. With a narrow distribution centered around the desired RP domain, efficient energy cascade to yield blue emission is ensured. Coupled with rapid nucleation induced by an antisolvent deposition technique, record efficiencies of 2.34 and 5.08%, corresponding to color-stable deep blue (∼465 nm) and cyan (∼493 nm), respectively, were attained. This composition and process engineering to design favorable structural landscape is transferrable to other material systems, which paves the way for high-performance PeLEDs. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

Slow hot carrier cooling in halide perovskites holds the key to the development of hot carrier (HC) perovskite solar cells. For accurate modeling and pragmatic design of HC materials and devices, it is essential that HC temperatures are reliably determined. A common approach involves fitting the high-energy tail of the main photobleaching peak in a transient absorption spectrum with a Maxwell-Boltzmann distribution. However, this approach is problematic because of complications from the overlap of several photophysical phenomena and a lack of consensus in the community on the fitting procedures. Herein, we propose a simple approach that circumvents these challenges. Through tracking the broadband spectral evolution and accounting for bandgap renormalization and spectral line width broadening effects, our method extracts not only accurate and consistent carrier temperatures but also other important parameters such as the quasi-Fermi levels, bandgap renormalization constant, etc. Establishing a reliable method for the carrier temperature determination is a step forward in the study of HCs for next-generation perovskite optoelectronics. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

Metal halide perovskites have affirmed their pedigree as extraordinary semiconducting materials, exhibiting properties rivalling those observed in single crystal compound semiconductors. Perovskites show tremendous versatilities in both structure and composition tuning, and therefore applications ranging from optoelectronics to X-ray imaging and spintronics, neuromorphic electronics are emerging. Moreover, when their dimensions become comparable to the exciton Bohr radius, perovskite nanostructures and layered systems display remarkable properties because of quantum confinement. Nanostructured and lower dimensional layered perovskites exhibit properties that are yet to be fully exploited such as extraordinarily high luminescence, narrow emissions, high exciton binding energies, strong non-linear phenomena, and carrier cascade characteristics. This review, while highlighting the frontier phenomena that continue to be unravelled, outlines how confined structures of these materials have demonstrated properties that promise to unlock exceptional quantum phenomena to challenge the optoelectronic limits. Accepted version

Successful adoption of defect management and carrier confinement strategies in Ruddlesden-Popper (RP) perovskites has driven the impressive improvements to performance of perovskite-based light-emitting diodes (PeLEDs) seen to date. Although functional additives have been advantageous in mitigating defects, their influence over crystallization behavior of RP (L2Am-1PbmX3m+1) perovskites has yet to be fully studied. This is especially important for blue-emitting mono-halide RP perovskites, where stringent control over m domain distribution is needed for efficient PeLEDs. Herein, we investigate the effect of tri-phenyl-phosphine-oxide (TPPO) on crystallization behaviour of blue RP (PBA2Csm-1PbmBr3m+1) perovskites. Despite TPPO addition, its absence in the resulting film eliminates its role as a passivating agent. Instead, TPPO acts as crystallization and phase distribution modulator – promoting the formation of a narrow distribution of higher m domains with higher Br content. In doing so, an enhancement of ~35% was noted with champion device yielding efficiency of 3.8% at λ of 483 nm. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version This research was funded by Ministry of Education, Singapore (MOE2018-T2-2-083) and (MOE2019-T2-2-097). The photophysical measurements are made possible through the support from the Ministry of Education, Singapore under its AcRF Tier 2 grant MOE-T2EP50120-0004, and the National Research Foundation, Singapore under its NRF Investigatorship (NRF-NRFI2018-04). We acknowledge the Facility for Analysis, Characterization, Testing and Simulation, Nanyang Technological University, Singapore, for use of their electron microscopy/X-ray facilities.

Co-evaporated perovskite solar cells have shown outstanding scalability, intrinsic stability, high efficiency and adaptability on rough surfaces. Here we present an optimal hybrid method to wide the co-evaporated MAPbI3's bandgap by a MABr treatment.

Hybrid metal halide perovskite solar cells (PSCs) have consistently demonstrated high power conversion efficiency (PCE), although the best performing PSCs mostly employ high-temperature (500 oC) processed compact and mesoporous TiO2. Instead, low-temperature processed PSCs are desirable for implementation on flexible substrates and tandem solar cells. Here, we present a new method to achieve high efficiency flexible planar PSCs based on a low-temperature processed nonaqueous sol-gel route synthesized TiO2 and a guanidinium iodide (GuaI) salt passivation treatment of the perovskite film. We fabricate both rigid and flexible triple-cation perovskite (Cs0.05 (MA0.17FA0.83)0.95Pb(I0.85Br0.15)3, Eg ~1.58 eV) PSCs, achieving PCEs of 19.8% and 17.0% on glass and polyethylene naphtholate, (PEN) substrates respectively. At the same time, rigid and flexible high-bandgap double cation (FA0.85Cs0.15Pb(I0.7Br0.3)3, Eg ~1.72 eV) PSCs reached a PCE of 18.0 % and of 15.8%. Moreover, large area (1cm2) ~1.58 eV and ~1.72 eV-PSCs achieved PCEs of 18.2% and 16.7% PCE on glass substrates and of 16.2% and 13.9% on PEN substrates demonstrating the high uniformity of all the solar cell layers. NRF (Natl Research Foundation, S’pore) Accepted version

Recent progress of vapor-deposited perovskite solar cells (PSCs) has proved the feasibility of this deposition method in achieving promising photovoltaic devices. For the first time, it is probed the versatility of the co-evaporation process in creating perovskite layers customizable for different device architectures. A gradient of composition is created within the perovskite films by tuning the background chamber pressure during the growth process. This method leads to co-evaporated MAPbI3 film with graded Fermi levels across the thickness. Here it is proved that this growth process is beneficial for p-i-n PSCs as it can guarantee a favorable energy alignment at the charge selective interfaces. Co-evaporated p-i-n PSCs, with different hole transporting layers, consistently achieve power conversion efficiency (PCE) over 20% with a champion value of 20.6%, one of the highest reported to date. The scaled-up p-i-n PSCs, with active areas of 1 and 1.96 cm2, achieved the record PCEs of 19.1% and 17.2%, respectively, while the flexible PSCs reached a PCE of 19.3%. Unencapsulated PSCs demonstrate remarkable long-term stability, retaining ≈90% of their initial PCE when stored in ambient for 1000 h. These PSCs also preserve over 80% of their initial PCE after 500 h of thermal aging at 85 °C. © 2021 Wiley-VCH GmbH

The absence of adequate modern sources of energy impedes the presence of decent living conditions. This very lack of modern sources of energy is Energy Poverty. Lack of availability, accessibility, and affordability are the main reasons behind this problem. In the literature, all three aspects were not covered together. In this paper, we have developed a composite index of energy poverty to measure the magnitude of energy poverty in the ASEAN region and India, where we capture all three elements. The measure revealed the degree of problem in each country, with Cambodia being the most energy poor amongst the countries in the study and Thailand being the least energy poor. To dig deeper into the problem at the micro level and to deliberate the policy implications, we developed detailed village-level case studies and analysed region-specific energy poverty. We thus identify the key root causes and propose effective solutions to eradicate the prevailing problem in these regions.

Here we report an efficient and reproducible multifunctional additive engineering strategy via methoxysilane cross-linking agents functionalized by the different terminal group, moderate electron-d...

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 ef...

Next-generation displays and lighting technologies require efficient optical sources that combine brightness, color purity, stability, substrate flexibility. Metal halide perovskites have potential use in a wide range of applications, for they possess excellent charge transport, bandgap tunability and, in the most promising recent optical source materials, intense and efficient luminescence. This review links metal halide perovskites' performance as efficient light emitters with their underlying materials electronic and photophysical attributes. Accepted version

The power conversion efficiency (PCE) of hole conductor free carbon-based perovskite solar cells (PSCs) is restricted by the poor charge extraction and recombination losses at the carbon-perovskite interface. For the first time we successfully demonstrated incorporation of thin layer of copper doped nickel oxide (Cu:NiOx) nanoparticles in carbon-based PSCs, which helps in improving the performance of these solar devices. Cu:NiOx nanoparticles have been synthesized by a facile chemical method, and processed into a paste for screen printing. Extensive X-ray Absorption Spectroscopy (XAS) analysis elucidates the co-ordination of Cu in a NiOx matrix and indicates the presence of around 5.4% Cu in the sample. We fabricated a monolithic perovskite module on a 100 cm2 glass substrate (active area of 70 cm2) with a thin Cu:NiOx layer (80 nm), where the champion device shows an appreciated power conversion efficiency of 12.1% under an AM 1.5G illumination. To the best of our knowledge, this is the highest reported efficiency for such a large area perovskite solar device. I-V scans show that the introduction of Cu:NiOx mesoporous scaffold increases the photocurrent, and yields fill factor (FF) values exceeding 57% due to the better interface and increased hole extraction efficiency. Electrochemical Impedance Spectroscopy (EIS) results reinforce the above results by showing the reduction in recombination resistance (Rrec) of the PSCs that incorporates Cu:NiOx interlayer. The perovskite solar modules with a Cu:NiOx layer are stable for more than 4500 h in an ambient environment (25 °C and 65% RH), with PCE degradation of less than 5% of the initial value. NRF (Natl Research Foundation, S’pore) Accepted version

We utilized two organic dications containing, respectively, a pyridinium and an imidazolium core to construct new n = 1 (where n refers to the number of contiguous two-dimensional (2D) inorganic layers, i.e., not separated by organic cations) 2D lead-iodide perovskites 1 and 2. The former material exhibits a (100)- and the latter a very rare 3 � 3 (110)-structural type. Compared with primary ammonium functionality, their constituent ring-centered positive charges have lower charge density. As a result, PbI 6 4- interoctahedral distortions of the inorganic lattice in 1 and 2 are reduced (Pb-I-Pb bond angles are as high as 166° and 174°, respectively). This results in bathochromically shifted optical features. In addition, the compact nature of the dications produces extremely short lead-iodide sheet separations, with respective iodide-iodide (I···I) distances as small as 4.149 and 4.278 à . These are among the shortest separations of adjacent lead-iodide layers ever reported for such materials. When crystallized as thin films on top of substrates, the resulting 2D perovskite layers do not adopt a regular growth direction parallel to the surface. Instead, the crystallites grow with no fixed orientation. As a consequence of their proximate inorganic distances and unusual crystallization tendencies, the resulting 2D perovskites exhibit low excitonic activation energies (93.59 and 96.53 meV, respectively), enhanced photoconductivity in solar cells, and unprecedented incident photon-to-current conversion rates of up to 60%. More importantly, mesoporous 2D layered perovskite solar cells with power conversion efficiencies of 1.43 and 1.83% were achieved for 1 and 2, respectively. These are the highest values obtained thus far for pure n = 1 lead-iodide perovskites and more than 20 times higher than those obtained for materials templated by more conventional cations such as phenylethylammonium (0.08%). Copyright © 2019 American Chemical Society.

Herein, we report Sn2+ doping in FAPbI3 NCs to stabilize the α-phase, while using propionic acid as a co-ligand. The Sn2+ doping enhances the emission quantum yield from 35% to 63% and dramatically improves the colloidal and phase stability. Also, we demonstrated the use of Sn doped FAPbI3 NCs in near-infrared (NIR) LEDs. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

Halide perovskites possess enormous potential for various optoelectronic applications. Presently, a clear understanding of the interplay between the lattice and electronic effects is still elusive. Specifically, the weakly absorbing tail states and dual emission from perovskites are not satisfactorily described by existing theories based on the Urbach tail and reabsorption effect. Herein, through temperature-dependent and time-resolved spectroscopy on metal halide perovskite single crystals with organic or inorganic A-site cations, we confirm the existence of indirect tail states below the direct transition edge to arise from a dynamical Rashba splitting effect, caused by the PbBr6 octahedral thermal polar distortions at elevated temperatures. This dynamic effect is distinct from the static Rashba splitting effect, caused by non-spherical A-site cations or surface induced lattice distortions. Our findings shed fresh perspectives on the electronic-lattice relations paramount for the design and optimization of emergent perovskites, revealing broad implications for light harvesting/photo-detection and light emission/lasing applications., The weak effects induced by lattice disorder on the optoelectronic properties of halide perovskites still remain elusive. Here Wu et al. confirm the indirect transition tail states in perovskite crystals which explain their low photoluminescence quantum yield, dual emission peaks and difficulties in realizing lasing.

In less than a decade, organic-inorganic metal halide perovskites (MHPs) have shown tremendous progress in the field of light-emitting applications. Perovskite light-emitting diodes (PeLEDs) have reached external quantum efficiencies (EQE) exceeding 20 % and they have been recognized as a potential contender of the commercial display technologies. However, perovskite thin films in PeLEDs are generally deposited via a spin-coating process, which is not favourable for large area device fabrication. Despite the great success of solution-processed PeLEDs, very few articles have been reported on vacuum processed PeLEDs and the improvements in their optoelctronic performances are also progressing slowly. On the other hand, vacuum processing techniques are mostly used in organic LED technology as they can guarantee (i) the absence of solvent during thin-film growth, (ii) process scalability over large area substrates, and (iii) precise thin-film thickness control. This thin-film growth process is suitable for application in the advancement of a large variety of display technologies. In this Review, we present an overview of current research advances in the field of perovskite thin films grown via vacuum techniques, a study of their photophysical properties, and integration in PeLEDs for the generation of different colors. We also highlight the current challenges and future prospects for the further development of vacuum processed PeLEDs.

Surface passivation has been proven to be an effective strategy to improve power conversion efficiency and stability of perovskite solar cells. However, the rationale for choosing an appropriate passivator, in terms of the type of interaction with the underlying perovskite layer, is still not clear yet. Here, we purposively choose two molecules as passivators for perovskites, fluorinated phosphonic acid and its corresponding phosphonate salt, to monitor the extent of interaction between these passivators and the perovskite surface. The effect of passivation on film stability and device performance is also determined. Higher photoluminescence intensity and longer carrier lifetime are observed in perovskite films that treated with phosphonium salt passivation because of the stronger interaction with perovskites. The corresponding device shows enhancement in power conversion efficiency from 18.27 to 19.44%. Furthermore, the water contact angle of passivated perovskite films exceeds 110.9° as compared to the pristine, untreated perovskite (74.5°). This superhydrophobic nature imparted by fluorinated phosphonium salt passivation enables excellent long-term stability of devices, allowing over 90% of their initial efficiency to be retained even after 90 days' storage under ambient conditions with 30% relative humidity. Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) The authors acknowledge funding from the National Research Foundation, Prime Minister's Office, Singapore, under its Intra-CREATE Collaborative Grant (NRF2018-ITC001-001), Competitive Research Program (CRP Award No. NRF-CRP14-2014-03), and Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE) CREATE Program, Office of Naval Research Global (ONRG-NICOP-N62909-17-1-2155), Nanyang Technological University start-up grants (M4080514 and M4081293); the Ministry of Education Academic Research Fund Tier 1 grants (RG184/14, RG166/16 and RG101/15), and Tier 2 grants (MOE2016-T2-1-100, MOE2014-T2-1-044, and MOE2015-T2-2-015). G. Han would like to acknowledge the support of the Fundamental Research Funds of Shandong University numbered 2019GN002.

Room-temperature perovskite nanocrystal syntheses have previously lacked the size tunability attainable through high-temperature methods. Herein, we outline a scalable approach whereby the nucleation and growth of CsPbBr3 nanocrystals (NCs) can be decoupled and controlled at room temperature by utilizing different ligands. We employed octylphosphonic acid (OPA) ligands to regulate the critical radius and the NC growth rate. The subsequent addition of a bulkier didodecyldimethylammonium bromide ligand quenches the NC growth, defining the reaction duration. Management of these three variables enables precise tuning of the NC diameter between 6.8 and 13.6 nm. The photoluminescence quantum yield of the NCs remains above 80% for all sizes even after thorough antisolvent purification. The use of hydrogen-bonding OPA ligands enhances quantum confinement effects, characterized by strong, well-resolved absorption peaks. Solution and solid-state nuclear magnetic resonance spectra confirmed the effective removal of unbound ligands during purification and the presence of a hydrogen-bonded network of OPA ligands on the surface of the purified NCs. Overall, this approach has the potential to facilitate a broad range of future endeavors from studies of hot carrier dynamics to both optically and electrically driven device applications. Accepted version

Carbon-based perovskite solar cells (C–PSCs) have attracted significant attention from the scientific community owing to their improved stability, low-cost fabrication, and potential scalability with screen printing technology. In this study, we examine the effect of three most commonly used carbon blacks, i.e. vulcan carbon (VC), meso carbon (MC) and super P (SP), on the properties of the resultant carbon electrode and the effect on photovoltaic performance as well as device stability. It shows that VC based carbon electrode promotes decent wettability to perovskite precursor solution, excellent infiltration and strong adhesion within the device stacks due to its high porosity and high pore volume. Furthermore, counter electrodes consisting of highly conductive VC exhibit sheet resistance of as low as 11.0 Ohm/□ as compared to those of MC (15.3 Ohm/□) and SP (22.0 Ohm/□). C–PSCs fabricated using VC based carbon electrode display champion power conversion efficiency of 12.55% with almost no decrease in efficiency under ambient conditions (~75% relative humidity) for 30 days without encapsulation. This work highlights the importance of carbon paste formulation for developing highly efficient and stable perovskite solar cells. National Research Foundation (NRF) N.M., S.G.M., would like to acknowledge funding from the Singapore National Research Foundation through the Intra-CREATE Collaborative Grant (NRF2018-ITC001-001).

Despite a meteoric rise in the efficiency and promising scalability aspects, the operational stability of halide perovskites poses a serious concern for the commercialization of this technology. A paradigm shift from thermally unstable MA+ (methylammonium)-based perovskites to stable FA+ (formamidinium) and Cs+ (cesium)-based mixed halide perovskite variants is a step in this direction. However, phase stabilization of mixed-cation halide perovskites within a triple-layer scaffold remains a major challenge. In this work, we demonstrate two-step sequential fabrication of FA+- and Cs+-based halide perovskites with formulation Cs0.05FA0.95Pb(IBr)3 in a triple-mesoscopic scaffold with a carbon layer as the back electrode. A strong but reversible performance degradation is observed under light illumination. Addition of hypophosphorous acid (HPA) into the perovskite precursor solution improves the operational stability of the cells. A striking correlation between phase- and operational stability was observed. From structural analysis, it was found that HPA tends to suppress the formation of a hexagonal yellow phase and promotes trigonal black phase formation. Further optical analysis of the cells showed the improvement in the optoelectronic properties in terms of defects and carrier recombination in the perovskite formed by HPA addition supported by external quantum efficiency and photoluminescence measurements. A stable 12% power conversion efficiency was achieved by tuning the composition and optimizing the process conditions for Cs0.05FA0.95Pb(IBr)3-based triple-mesoscopic perovskite solar cells. National Research Foundation (NRF) N.M. and S.G.M. would like to acknowledge funding from the Singapore National Research Foundation through the IntraCREATE Collaborative Grant (NRF2018-ITC001-001) and the Competitive Research Program: NRF-CRP14-2014-03.

Heterostructures, combining perovskite nanocrystals (PNC) and chalcogenide quantum dots, could pave a path to optoelectronic device applications by enabling absorption in the near-infrared region, tailorable electronic properties, and stable crystal structures. Ideally, the heterostructure host material requires a similar lattice constant as the guest which is also constrained by the synthesis protocol and materials selectivity. Herein, we present an efficient one-pot hot-injection method to synthesize colloidal all-inorganic cesium lead halide-lead sulfide (CsPbX3 (X = Cl, Br, I)-PbS) heterostructure nanocrystals (HNCs) via the epitaxial growth of the perovskite onto the presynthesized PbS nanocrystals (NCs). Optical and structural characterization evidenced the formation of heterostructures. The embedding of PbS NCs into CsPbX3 perovskite allows the tuning of the absorption and emission from 400 to 1100 nm by tuning the size and composition of perovskite HNCs. The CsPbI3-PbS HNCs show enhanced stability in ambient conditions. The stability, tunable optical properties, and variable band alignments accessible in this system would have implications in the design of novel optoelectronic applications such as light-emitting diodes, photodetectors, photocatalysis, and photovoltaics. Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) Accepted version M.J. acknowledges the National Research Foundation, Prime Minister’s Office, Singapore, under its Competitive Research Programme (CRP Award NRF-CRP14-2014-03). We acknowledge the Facility for Analysis, Characterization, Testing and Simulation, Nanyang Technological University, Singapore, for use of their electron microscopy/X-ray facilities. M.J. acknowledge National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2020R1A4A1019455). P.V. acknowledges a Presidential Postdoctoral Fellowship from Nanyang Technological University (NTU), Singapore, via Grant 04INS000581C150. We also acknowledge the NTU Centre of High Field NMR Spectroscopy and Imaging for the use of their NMR facilities. T.C.S. and J.W.M.L. acknowledge the financial support from the Singapore National Research Foundation through the NRF Investigatorship (NRF-NRFI-2018-04) and the Ministry of Education under its AcRF Tier 1 Grant RG91/19 and Tier 2 Grant MOE2019-T2-1-006.

One of the major bottlenecks of perovskite photovoltaic modules fabrication is the homogeneous deposition of perovskite material on large-area substrates. Here, we show that slot-die coating technique combined with synergistic gas quenching and substrate heating can produce compact, homogenous and reproducible Cs0.16FA0.84Pb(I0·88Br0.12)3 perovskite films. We demonstrate the fabrication of perovskite solar cells (PSCs) in a planar (n-i-p) device configuration and attain power conversion efficiency (PCE) of 18% over 0.09 cm2 device active area. The versatility of this crystallization strategy, which eliminates the need for complex solvents or additive engineering, was studied using planar SnO2- and TiO2-coated FTO substrates. Our study provides greater insights into achieving controlled coating and homogeneous crystallization of perovskite films over large-area substrates (~10 × 10 cm2) necessary for the commercialization of this technology. Ministry of Education (MOE) Nanyang Technological University National Research Foundation (NRF) Submitted/Accepted version This research was supported by NTU start-up grant (M4081866), Ministry of Education (MOE) under AcRF Tier 2 grants (2018-T2-1-075 and 2019-T2-2-106) and National Research Foundation, Prime Minister’s Office, Singapore, under Energy Innovation Research Program (grant numbers: NRF2015EWT-EIRP003-004 and Solar CRP: S18-1176- SCRP). This research was supported by a CEA CTBU funding, under PTC PrintRose project, and by the IDEX Mobility grant from Grenoble Alpes University (Com-UGA).

Despite extensive reports on highly efficient perovskite light-emitting diodes, rules governing the design of suitable two-dimensional (2D) perovskite templating cation to facilitate formation of optimal emitter landscape for energy cascade remain largely elusive. With factors such as structure, size, functionalization, and charge capable of influencing the distribution of multidimensional perovskite phases, the importance of 2D templating cation design in determining film optoelectronic properties is indisputable. However, typical mono-functionalized 2D templating cations often result in larger lead halide octahedral spacing, which impedes effective charge transport. This has fueled investigation into the use of multiple cations for optimal domain distribution and improved charge transfer kinetics to the emitting species. In this study, we attempt to impart enhanced charge transfer characteristics to the resultant multidimensional perovskite by employing two bi-functionalized aromatic cations, namely, pyridinium ethyl ammonium and imidazolium ethyl ammonium, reminiscent of mono-functionalized phenyl ethyl ammonium, a widely used 2D perovskite templating cation. Although it is proposed that greater intermolecular bonding would enhance charge transfer rates, the simultaneous increase in lead halide octahedral distortion results in quenching of their corresponding 2D and multidimensional perovskite luminescence properties, correlated with increased defect density within the material. This manifests in the form of shorter PL decay lifetimes, lower PLQY, and device performance arising from inferior energy funneling. This study highlights the importance of designing 2D perovskite templating cations offering better transport and reduced octahedral distortion for the development of energy cascade-efficient, multidimensional perovskites. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version This research was primarily supported by the National Research Foundation under its Competitive Research Programme (CRP Award No. NRF-CRP14-2014-03) and the Ministry of Education under MOE2018-T2-2-083. The photophysical measurements are supported by National Research Foundation Investigatorship (NRF-NRFI-2018-04) and by the Ministry of Education under MOE Tier 2 grant MOET2EP50120- 0004.

Halide perovskite nanocrystals are a promising candidate for lighting applications. However, the production of white light emitting diodes (LEDs) is still a major challenge due to halide ion segregation. In this work, it is demonstrated that reducing the thickness of the perovskite layer in an LED stack can modulate the recombination zone, such that a tunable emission can be obtained. This comprises of an orange electromer emission from a hole-transport layer (HTL), green electroluminescence from the perovskite active layer, and a blue monomer emission from the same HTL. Overall, a pure white emission can be achieved after successful device optimization, which is particularly challenging for LEDs in which the emission originates solely from perovskite layer. It is anticipated that this methodology could be employed on any type of green-emitting nanocrystals to fabricate white LEDs. Nanyang Technological University National Research Foundation (NRF) P.V. acknowledges a NTU, Singapore Presidential Postdoctoral Fellowship via grant 04INS00581C150. The authors acknowledge financial support from the Singapore National Research Foundation, Prime Minister’s Office, through the Competitive Research Program (CRP Award No. NRF-CRP14-2014-03).

Thermal evaporation is a very successful and widely adopted coating technique for the deposition of organic and inorganic materials on rough and textured surfaces and over large areas. Indeed, this technique is extensively used in the semiconductor industry for the fabrication of organic light emitting diodes (OLEDs) and is commonly used in displays. In the last few years, thermal evaporated perovskite solar cells (PSCs) have also shown the potential to reach high power conversion efficiency (PCE) both on small and over large area devices. In this work, we present a detailed optimization of the potassium-based surface treatment used to improve the performances of our MAPbI PSCs fabricated using the thermal co-evaporation technique. Small area planar n-i-p PSCs with an active area of 0.16 cm achieved PCEs above 19% and the large area PSCs with an active area of 1 cm reached 18.1%. These un-encapsulated PSCs also proved an excellent long-term shelf stability maintaining 90% of their initial PCEs for over six months when stored at ambient temperature. National Research Foundation (NRF) Published version This research is supported by the National Research Foundation, Prime Minister’s Office, Singaporeunder Energy Innovation Research Program (Grant numbers: NRF2015EWT-EIRP003-004, NRF-CRP14-2014-03, Solar CRP: S18-1176-SCRP, NRF2018-ITC001-001).

In this progress report, recent improvements to the room temperaturesyntheses of lead halide perovskite nanocrystals (APbX3, X = Cl, Br, I) are assessed, focusing on various aspects which influence the commercial viability of the technology. Perovskite nanocrystals can be prepared easily from low-cost precursors under ambient conditions, yet they have displayed near-unity photoluminescence quantum yield with narrow, highly tunable emission peaks. In addition to their impressive ambipolar charge carrier mobilities, these properties make lead halide perovskite nanocrystals very attractive for light-emitting diode (LED) applications. However, there are still many practical hurdles preventing commercialization. Recent developments in room temperature synthesis and purification protocols are reviewed, closely evaluating the suitability of particular techniques for industry. This is followed by an assessment of the wide range of ligands deployed on perovskite nanocrystal surfaces, analyzing their impact on colloidal stability, as well as LED efficiency. Based on these observations, a perspective on important future research directions that can expedite the industrial adoption of perovskite nanocrystals is provided. National Research Foundation (NRF) Accepted version A.A.M.B. gratefully acknowledges the Tizard studentship from the Faculty of Engineering and Physical Sciences at University of Southampton. This research was also supported by the National Research Foundation, Prime Minister’s Office, Singapore, under its Competitive Research Program (CRP Award No. NRF-CRP14-2014-03).

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Realization of reduced ionic (cationic and anionic) defects at the surface and grain boundaries (GBs) of perovskite films is vital to boost the power conversion efficiency of organic-inorganic halide perovskite (OIHP) solar cells. Although numerous strategies have been developed, effective passivation still remains a great challenge due to the complexity and diversity of these defects. Herein, a solid-state interdiffusion process using multi-cation hybrid halide perovskite quantum dots (QDs) is introduced as a strategy to heal the ionic defects at the surface and GBs. It is found that the solid-state interdiffusion process leads to a reduction in OIHP shallow defects. In addition, Cs+ distribution in QDs greatly influences the effectiveness of ionic defect passivation with significant enhancement to all photovoltaic performance characteristics observed on treating the solar cells with Cs0.05 (MA0.17 FA0.83 )0.95 PbBr3 (abbreviated as QDs-Cs5). This enables power conversion efficiency (PCE) exceeding 21% to be achieved with more than 90% of its initial PCE retained on exposure to continuous illumination of more than 550 h. Nanyang Technological University National Research Foundation (NRF) N.M., S.G.M. would like to acknowledge funding from the Singapore National Research Foundation through the Intra-CREATE Collaborative Grant (NRF2018-ITC001-001). P.V. acknowledges Presidential Postdoctoral Fellowship visa grant M408070000.