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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.

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.

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.

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

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.

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.

Halide perovskites are of great interest for light-emitting diodes (PeLEDs) in recent years due to their excellent photo- and electroluminescence properties. However, trap/defects and ion migration of devices under high external driving voltage/current are yet overcome. In this work, it is found that upon potassium (K) addition to a CsPbBr3/Cs4PbBr6 (3D:0D = 0.85:0.15) perovskite, a locally-disordered 0D Cs4-xKxPbBr6 phase is formed with nearly 0.35:0.65 admixture of 0D:3D, along with an unreacted KBr phase potentially passivating the surface and grain boundaries. The formation of CsPbBr3 nanocrystals (~10nm) confined within the Cs4-xKxPbBr6 matrix accompanied by larger CsPbBr3 grains (~50nm) is further confirmed by high-resolution transmission electron microscopy. In addition, the kinetics of ion migration were characterized with Auger electron spectroscopy and double-layer polarization using capacitive-frequency measurements, revealing significantly lower hysteresis, halide ion migration and accumulation for the K-incorporated samples during device operation, resulting in substantial improvements in LED performances and stability. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

The synthesis of all-inorganic cesium lead halide perovskite quantum dots (QDs) typically requires high temperatures, stringent conditions, large quantities of surface ligands, and judicious purification steps to overcome ligand-induced charge injection barriers in optoelectronic devices. Low-temperature syntheses generally require lower ligand concentrations, but are severely limited by the low solubility of the Cs precursor. We describe an innovative and general approach under ambient conditions to overcome these solubility limitations, by employing crown ethers. The crown ethers facilitate complete dissolution of the CsBr precursor, rendering CsPbBr3 QD inks practical for device fabrication. The resultant LEDs displayed bright green emission, with a current efficiency, and external quantum efficiency of 9.22 cd A-1 and 2.64%, respectively. This represents the first LED based on CsPbBr3 QDs prepared at room temperature. Lastly, the crown ethers form core-shell structures, opening new avenues to exploit their strong coordination strength. NRF (Natl Research Foundation, S’pore) Accepted version

Research into perovskite-based light-emitting diodes (PeLEDs) has been rapidly gaining momentum since the initial reports of green-emitting methylammonium lead bromide (CH3NH3PbBr3)-based devices were published. However, issues pertaining to its stability and morphological control still hamper progress toward high performing devices. Solvent engineering, a technique typically employed to modulate film crystallization, offers little opportunity for scale-up due to the tendency for inhomogeneous film growth and low degree of reproducibility. Here, we propose and show a simple gas-facilitated process to deposit a stable, all-inorganic perovskite CsPbBr3 film. The formation of smaller and less percolated grains, which gives rise to enhanced optical properties, highlights the importance of spatial charge confinement in the film. Consequently, the performance of our PeLEDs shows great improvement, with luminance as high as 8218 cd m–2 and turn-on voltage as low as 2.4 V. Concomitantly, the current efficiency and EQE of our device were increased to 0.72 cd A–1 and 0.088%, respectively. High reproducibility in the performance of PeLEDs fabricated using this process opens the path for large-area devices. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Published version

The original version of this article incorrectly listed the present address of Bo Wu as ‘Present address: Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, Guangdong Province 510006, China’. This is the author’s primary affiliation. This has been corrected in both the PDF and HTML versions of the article.

Identified as emerging light absorbers due to their plethora of unique optoelectronic properties, perovskites have also been touted as a promising candidate for light emission. However, despite the effortless transition of perovskites into the current organic light-emitting diodes (OLEDs), misalignment of energy levels at the hole transporting material (HTM) and perovskite interface limits the efficacy of interfacial charge transport. Herein, it is shown that by incorporating a small organic molecule, bathophenanthroline (BPhen), into the CH3NH3PbBr3 emitter via a solvent engineering technique, the energy band levels of the perovskite can be tailored and the energy mismatch at the HTM/perovskite interface can be ameliorated through the formation of a graded emitter layer and accompanying morphological improvements. With a BPhen concentration of 0.500 mg mL−1, more than ten-fold enhancement of device luminance and efficiency was achieved, thus demonstrating a facile and viable approach for fabricating high-performance perovskite light-emitting diodes (PeLEDs). Accepted version

Emulation of brain‐like signal processing is the foundation for development of efficient learning circuitry, but few devices offer the tunable conductance range necessary for mimicking spatiotemporal plasticity in biological synapses. An ionic semiconductor which couples electronic transitions with drift‐diffusive ionic kinetics would enable energy‐efficient analog‐like switching of metastable conductance states. Here, ionic–electronic coupling in halide perovskite semiconductors is utilized to create memristive synapses with a dynamic continuous transition of conductance states. Coexistence of carrier injection barriers and ion migration in the perovskite films defines the degree of synaptic plasticity, more notable for the larger organic ammonium and formamidinium cations than the inorganic cesium counterpart. Optimized pulsing schemes facilitates a balanced interplay of short‐ and long‐term plasticity rules like paired‐pulse facilitation and spike‐time‐dependent plasticity, cardinal for learning and computing. Trained as a memory array, halide perovskite synapses demonstrate reconfigurability, learning, forgetting, and fault tolerance analogous to the human brain. Network‐level simulations of unsupervised learning of handwritten digit images utilizing experimentally derived device parameters, validates the utility of these memristors for energy‐efficient neuromorphic computation, paving way for novel ionotronic neuromorphic architectures with halide perovskites as the active material. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

Metal halide perovskites have demonstrated breakthrough performances as absorber and emitter materials for photovoltaic and display applications respectively. However, despite the low manufacturing cost associated with solution-based processing, the propensity for defect formation with this technique has led to an increasing need for defect passivation. Here, we present an inexpensive and facile method to remedy surface defects through a postdeposition treatment process using branched alkylammonium cation species. The simultaneous realignment of interfacial energy levels upon incorporation of tetraethylammonium bromide onto the surface of CH

Film morphology has a major influence on the performance of halide perovskite semiconductors, where poor coverage and pinholes are generally detrimental and result in undesirable current leakage. All‐inorganic cesium lead bromide (CsPbBr3) has an advantage of improved stability especially under device operation, but the low solubility of the ionic precursor CsBr limits the coverage of the solution‐processed film. To tackle this, a sequential deposition technique is proposed whereby PbBr2 is first deposited on a planar substrate prior to exposure to a CsBr solution to initiate conversion to the desired CsPbBr3 phase. This approach essentially nullifies the solubility problem of CsBr and the final perovskite film coverage now chiefly depends on the initial PbBr2 concentration and the duration of exposure to the CsBr solution. With over 90 % film coverage achieved, a proof of concept perovskite light‐emitting diode (PeLED) with green emission at 527 nm has also been achieved, thereby demonstrating the feasibility of this methodology for future device fabrication. NRF (Natl Research Foundation, S’pore) Accepted version

The incorporation of phenylethylammonium bromide (PEABr) into a fully inorganic CsPbBr3 perovskite framework led to the formation of mixed-dimensional perovskites, which enhanced the photoluminescence due to efficient energy funnelling and morphological improvements. With a PEABr : CsPbBr3 ratio of 0.8 : 1, PeLEDs with a current efficiency of 6.16 cd A−1 and an EQE value of 1.97% have been achieved. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

Traps exert an omnipotent influence over the performance of halide perovskite optoelectronic devices. A clear understanding of the origin and nature of the traps in halide perovskites is the key to controlling them and realizing optimal devices. Herein, the role of localized traps on the optical properties of lead bromide perovskite films is investigated. In the low-temperature orthorhombic phase of CH3NH3PbBr3 perovskite, band-edge carrier dynamics exhibit a power-law decay due to the presence of structural-disorder-induced localized traps, which has a depth of ≈40 meV. The continuous distribution of these localized traps gives rise to a broad sub-band-gap emission that becomes more prominent in thicker films with a larger trap density. The presence of this emission only from the hybrid organic–inorganic perovskites points to the vital role of organic dipoles in localized trap states formation. This study explicates the nature of these localized traps as well as their nontrivial role in carrier recombination kinetics, which is of fundamental importance in perovskites optoelectronics. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

A new protocol for the synthesis of a highly stable (over 2 months under ambient conditions) solution-processed core–shell type structure of mixed methylammonium–octylammonium lead bromide perovskite nanoparticles (5–12 nm), having spherical shape, color tunability in the blue to green spectral region (438–521 nm) and a high photoluminescence quantum yield (PLQY) of up to 92% is described. The color tunability, high PLQY and stability are due to the quantum confinement imparted by the crystal engineering associated with core–shell nanoparticle formation during growth. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version