In Situ Fabricated Perovskite Nanocrystals: A

PROGRESS REPORT Perovskite Nanocrystals

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In Situ Fabricated Perovskite Nanocrystals: A Revolution in Optical Materials Shuai Chang, Zelong Bai, and Haizheng Zhong* obvious excitonic features with enhanced photoluminescence (PL) properties due to the well-known size-dependent quantum confinement effects.[14–17] The combination of color saturated emissions, super high quantum yield (QY), as well as easy processing inspired the intensive exploration of PNCs as light emitters, which is now under spotlights in the field of optical materials. The first perspective aim of PNCs is to alternate the state-of-the-art CdSe or InP quantum dots (QDs) as new generation luminescence materials.[18,19] As shown in Figure 1, these QD materials have been well demonstrated to be potential functional components for photonic and optoelectronic applications, and are currently on the way to industrialization.[20–23] Specifically, QDs can be employed as fluorescence labels,[24] laser gain media,[25] LED phosphors,[26] and down-shifting films for LCD backlights.[27,28] They can also be processed into thin film for optoelectronic devices including solar cells,[29] photodetectors,[30,31] transistors[32] and LEDs.[33] The PNCs outcompete these conventional QDs in terms of narrower full width at half maximum (FWHM) and low fabrication cost, but most importantly, the ease of in situ preparation.[34,35] Herein, this progress report highlight the developments of in situ fabricated PNCs. Colloidal semiconductor QDs are typically synthesized ex situ in flasks by hot injection method, stabilized by surface ligands.[36–39] To incorporate such solution based materials into applications usually requires purification, redispersion and surface engineering. For instance, ligand exchange is often employed to disperse QDs into aimed solvent, inducing defects that inevitably derogating the PLQYs.[40] Moreover, the organic ligands themselves also suffer from the risk of disabsorbing, reducing the stability of the QDs and thus the final devices.[41] Because halide perovskite are ionic compounds that can be well dissolved into polar solvents, PNCs can be fabricated through either ex situ routes in flask or in situ strategies on substrates. The in situ fabricated PNCs are adaptable to devices integration due to their scalable and low-cost manufacturing. In this regard, more and more efforts have been made in terms of the controlling synthesis, physical properties and device applications of PNC materials. In this progress report, we first start with a brief description of the features of PNCs and their development history, and then summarize the recent developments of in situ fabricated PNCs with a focus on the strategies and principles. Moreover,

After over 30 years of development, colloidal quantum dots have now become mature luminescent materials in photonic and optoelectronic operations and start to hit the market. The emerging perovskite nanocrystals are expected to promote large-scale commercialization due to their superior optical properties as well as low cost and easy fabrication. This progress report is focused on the developments of in situ fabricated perovskite nanocrystals with attractive features of excellent superior photoluminescence and electroluminescence performance, as well as the solution processability, easy integration, and scale-up fabrication. These advantages make them suitable candidates as down-shifting materials for photonics or emissive component for optoelectronics. Recently, the first television demons using composite films based on in situ fabricated perovskite nanocrystals were demonstrated at CES 2018 exhibition, and the performance of electroluminescence devices based on in situ fabricated perovskite nanocrystals is leading the study. It is believed that the in situ fabricated perovskite nanocrystrals will bring a revolution in the field of optical materials after further developments.

1. Introduction Since 2009, the investigations of organic–inorganic hybrid and all-inorganic halide perovskites as low-cost solution-processed semiconductors are invoked[1] and impressive progresses have been made in high-performance photovoltaic as well as other optoelectronics including light-emitting didoes (LED), lasers, photodetectors, and transistors.[2–9] The in-depth understanding of the fundamental properties of such star material further promoted the advanced designs to meet the requirements toward practical conditions.[10–13] It has been demonstrated that the small exciton binding energy and hence efficient dissociation in bulk halide perovskites, long carrier diffusion length, and an absorption range covering the visible solar spectrum, collectively enable the high performance of photoelectricity conversion. Compared to the bulk materials, perovskite nanocrystals or quantum dots (usually denoted as PNCs or PQDs) show Prof. S. Chang, Dr. Z. L. Bai, Prof. H. Z. Zhong Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems School of Materials Science & Engineering Beijing Institute of Technology 5 Zhongguancun South Street, Haidian District, Beijing 100081, China E-mail: [email protected] The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adom.201800380.

DOI: 10.1002/adom.201800380

Adv. Optical Mater. 2018, 1800380

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© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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the up-to-date applications in display technology evolving in situ fabricated PNCs are exhaustively surveyed and the challenges of potential use in photonic and optoelectronics are also discussed.

2. Functionalized Metal Halide Perovskites: From Single Crystal to Nanocrystals 2.1. Categories and Properties of Functionalized Metal Halide Perovskites Metal halide perovskite are compounds built up of all corner sharing octahedra [BX6]4−, and an organic or inorganic cation A.[42] When the size of A is small and can be filled into the voids created by four neighboring octahedra, such as Cs, CH3NH3 (MA), NH2CHNH2 (FA), the resulted compound forms a 3D network with cubic structure, adopting the formula ABX3 (ACs, MA, FA, etc.; BPb, Sn, Ge, etc.; XCl, Br, I). For certain A, B, and X combinations, A2BX5 and A4BX6 structures could also be obtained by adjusting the stoichiometric ratio. The structure of the perovskites would evolve from cubic structure to less symmetric tetragonal or orthorhombic structure by introducing large organic ammonium (OA) cation into the 3D structure that beyond the tolerance range. 2D or quasi 2D quantum well like perovskites are formed, adopting a formula of L2An−1BnX3n+1, where L is the large organic cation and n is the value of BX64− octahedral layers in the nanocrystallite (n = 1 for pure 2D structure; n = ∞ for 3D structure). Generally speaking, functionalized metal halide perovskite can be categorized by their spatial scales: single crystals with size ranging from hundreds of micrometers to a few centimeters, polycrystals with densely packed grains in the size from hundreds of nanometers to several micrometers, and nanocrystals below the size of 100 nm, where quantum confinement effect should be considered. Large single crystals in centimeter size and polycrystalline films have been intensely investigated due to the remarkable photovoltaic performance.[43–46] It has been discovered that even using low-cost solution-based fabrication methods, polycrystalline films show long carrier diffusion lengths that exceed many state-of-the-art inorganic and organic semiconductors.[12] Furthermore, single crystal metal halide perovskites with much lower interface defects reveal extremely long charge carrier diffusion lengths of tens of microns.[47] For bulk metal halide perovskite single crystals at macro scale, the influence of the surface becomes negligible. On the other hand, the absence of deep levels inside the sample ensures a high carrier lifetime thus long carrier diffusion length. Moreover, the long PL lifetimes[12] obtained from time-resolved photoluminescence measurements confirm that the PL emissions mainly originate from the radiative recombination of free electrons and holes. In the case of perovskite polycrystalline thin films, the influence of defects at the grain boundaries need to be taken into consideration. Surface/interface induced trap states diminish the PLQY at low excitation powers and when such traps are predominantly filled at high excitation density, a jump of the PL efficiency can be observed.[13,48] This phenomena is also observed in PNCs.[49]


Shuai Chang obtained his B.Sc. degree in applied physics in 2011 from the University of Science and Technology of China, and received his Ph.D. degree in materials science and engineering in 2015 from the Chinese University of Hong Kong. He currently works as an assistant professor in the School of Materials Science & Engineering at Beijing Institute of Technology (BIT). His research interests include quantum dots and metal halide perovskites for photo-/electroluminescent devices. Zelong Bai received his B.E. and Ph.D. degrees respectively in 2011 and 2018 from the Beijing Institute of Technology (BIT), Beijing, China. Currently, he is working as a postdoc with Prof. Haizheng Zhong and Prof. Liangyu Zhao in BIT. His project is focused on the research of quantum dot based composites for photonics and optoelectronics applications. Haizheng Zhong obtained his B.E. degree in 2003 from Jilin University, and then undertook his Ph.D. studies at the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) with Prof. Yongfang Li from 2003 to 2008. After that, he worked as a postdoc with Prof. Greg Scholes at the University of Toronto during 2008–2010. He joined School of Materials Science & Engineering at Beijing Institute of Technology (BIT) as an associate professor in 2010 and was promoted to full professor in 2013. His current research interests are in the area of colloidal quantum dots for photonics and optoelectronics. On the other hand, the recombination kinetics in PNCs are quite the opposite to the bulk counterparts due to remarkably enhanced exciton binding energy.[50] As a result, reported PNCs present PL emissions with high quantum yield, especially in the form of colloidal QDs with diameter less than 10 nm.[17,51] The strong excitonic feature of perovskite NCs and their tunable emission wavelengths make them promising emitters in LED and laser operations. Early demonstrations of perovskite LED (PeLED) using polycrystalline films suffer from low external quantum efficiency (EQE) due to nonideal PLQY of the

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Figure 1. Potential photonic and optoelectronic applications based on traditional quantum dots and perovskite nanocrystals. The inset table compares their typical properties.

bulk halide perovskites, while PNCs present potential alternatives toward efficient and bright light-emitting devices.[52–57]

2.2. The Roadmap of Perovskite Nanocrystals As illustrated in Figure 2, since the demonstration of PL emission from in situ fabricated PNCs in mesoporous templates in 2012,[58] the luminescence properties of PNCs have attracted more and more research efforts. The first breakthrough is the earlier attempts of fabrication PNCs via hot injection method borrowed from the preparation of conventional QDs.[51,59] Precursors were injected into the hot/warm solution for the growth of nanosized PNCs, stabilized by surface ligands with long alkyl chains. In 2015, the colloidal synthesis was greatly promoted by the ligand assisted precipitation (LARP) method,[17] which are commonly adopted for the fast and efficient synthesis of perovskite quantum dots.[60–62] To facilitate the purification of resulting QDs from colloidal solution for optoelectronic devices, an alternative emulsion synthesis for perovskite QDs is demonstrated later.[53] By employing ligands engineering, the in situ fabrication of PNCs also emerged to meet the requirement for high efficient electroluminescent devices. Practically, a mixture of small and large organic cations is applied to the perovskite structure. In most cases, quasi 2D “Ruddlesden-Popper (RP)” phase PNCs were reported,[56,63–66] which can be treated as a mixture of multilayered PNCs while nanosized 3D PNCs were reported elsewhere.[55] Here, the selection and incorporation concentration of bulky organic cations, the controlling of spin-coating process as well as the antisolvent play an important role in the formation mechanism. Besides molecular ligands, polymers can serve as natural nanosized scaffold for the growth of PNCs. In situ fabricated


PNCs can be obtained by simply spin-coating a mixture of perovskite and polymer precursors[67–70] or through a controlled drying process[71] or swelling technique.[72,73] Such PNCs in polymer matrix (also denoted as PQD embedded composite films, PQDCF) take advantages in terms of simple processing and good film formability. The good water and oxygen resisting property of polymers make them promising for improving the stability of the yielded PNCs. The PQDCF can be easily adapted for large-scale roll-to-roll manufacturing, which fits well with the requirements for LCD industry. Recently, there have been a number of studies on such phase perovskite due to the observation of abnormal strong and visible PL phenomena in cesium based Cs4PbBr6 perovskite macro- or microcrystals.[74,75] Contradictory and controversial explanations were proposed, a clarified mechanism behind the PL performance of Cs4PbBr6 is highly required.[76] We will clarify in the following sections that the fabrication procedure can be described as in situ growth of strong emissive CsPbBr3 PNCs in the matrix of single Cs4PbBr6 crystal due to the nonstoichiometry induced partial phase transformation process. From the developing roadmap of PNCs, it is clearly manifested that in situ fabricated PNCs gradually becomes predominant because of the increasingly popular tendencies of application-oriented research in this field.

3. In situ Fabrication Strategies From the viewpoint of material science, the essence of in situ fabrication of PNCs is to confine the size and dimensions within the growth process. Under such guiding principles, we illustrate in Figure 3 the main strategies and corresponding methodologies.

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Figure 2. The developing roadmap of perovskite nanocrystals. Ex-situ and in-situ fabrication approaches are labeled with red and blue font color, respectively. Reproduced with permission.[17,51,53,59,67,78,88] Copyright 2014–2017, American Chemical Society;[57] Copyright 2015, American Association for the Advancement of Science;[58] Copyright 2012, The Chemical Society of Japan;[63,65] Copyright 2016, Springer Nature;[62,70–72,117,121] Copyright 2015–2018, Wiley-VCH.

3.1. Hard Template Assisted In situ Fabrication Owing to its solution-processing feature, halide perovskites can be readily incorporated into mesoporous inorganic templates. These templates physically confined the growth of perovskites, producing PNCs inside the mesoporous structures. In 2012, Kojima et al. observed strong luminescence when spin-coating diluted MAPbBr3 on mesoporous Al2O3 or ZrO2 films.[58] By diluting the perovskite precursor solution to 1 wt%, MAPbBr3 NCs as large

Figure 3. Summative chart of the main strategies and specific methodologies for in situ fabrication of the PNCs.


as 5 nm was generated simultaneously on the surface of the mesoporous particles upon spin-coating. This can be denoted as the first report of in situ fabricated PNCs. The observed PL enhancement from these PNCs in mesoporous templates opens up the exploration of metal halide perovskites based composites. It is also possible to confine the size of metal halide perov skites using mesoporous silica templates.[77,78] The fabrication process can be carried out by simply mixing the mesoporous silica and perovskite precursor solution under annealing. The resultant PNCs were well embedded into the pores of the mesoporous silica with their size matches perfectly with the pore size from 7.1 nm down to 3.3 nm. Apparent quantum confinement effects depending on the size of the in situ fabricated PNCs were observed such as the increasing of optical bandgaps and blue shifting of PL peaks along with the shrinking of crystal size. Therefore the size distribution as well as optical properties of PNCs can be regulated simply by manipulating the template.[77] For traditional QDs, the template fabrication inevitably induced large amount of surface defects, heavily deteriorating the emission performance of the materials. Fortunately, it is elucidated that intrinsic defects have minimal effects on the recombination kinetics of the PNCs due to their ability of forming only shallow levels near the band edge.[79,80] As a result, the solution-processed PNCs with silica template show impressively high quantum efficiency beyond 50%.[78] In another approach, Anaya et al. designed periodically

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Figure 4. a) Schematics of NCP process during spin-coating to fabricate MAPbBr3 nanograin films. Reproduced with permission.[57] Copyright 2015, American Association for the Advancement of Science. b) Schematic illustrations of electron injection efficiency differences in CF-treated and TPBitreated MAPbBr3. Reproduced with permission.[83] Copyright 2017, Elsevier Ltd.

mesostructured films through sol–gel treatment of TiO2 and SiO2 precursors within organic templating agents. The resulted films with 3D pore network can be employed as nanosized templates for the synthesis of PNCs.[81] Although in situ template synthesis provided a facile approach for in situ growth of PNCs, the preparation of aimed templates complicates the fabrication process and limits their applications in many aspects. Nontemplate synthesis of PNCs is hence of great importance for the final implementation in practical uses.

The size of the resulting PNCs was further reduced to about 100 nm with a modified additive-based NCP (A-NCP), where an organic molecule as well as typical electron transport material, 2,2′,2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBI) was added into the antisolvent. This additive can infiltrate into the grain boundaries at the top side of the resulted film, which passivates the interface defects and facilitates the electron injection thus meliorating the charge injection balance (Figure 4b).[83] The NCP technique was adopted solely or in combination with other techniques to ensure the controllability of the in situ fabrication.

3.2. In situ Fabrication through Molecular Ligand Engineering 3.2.2. Organic Ammonium Assisted In situ Fabrication 3.2.1. Nanocrystal Pinning Metal halide perovskite tends to form cuboid polycrystals through either solution processing or vapor deposition method. Intriguingly, PNCs can be obtained by applying an antisolvent treatment within the process of spin-coating. Antisolvent engineering has been proved an effective approach for the fabrication of dense and uniform perovskite films for photovoltaic purposes.[82] In the case of in situ fabricated PNCs, Lee and co-workers developed a nanocrystal pinning (NCP) method by flushing out the original solvent of the perovskite precursors with an antisolvent (chloroform) during the spin-coating process, which initiates fast crystallization due to pinning of the nucleation on the substrate,[57] as depicted in Figure 4a. With such NCP technique, densely packed perovskite nanograins can be achieved with size ranging from 100 to 250 nm, which can be considered as in situ formed PNC thin films.


The A-site cation plays an important role in the in situ fabricated PNCs. It is found that by increasing the stoichiometric ratio of cation A can confine the size of the perovskite grains in situ. The role of excess AX in the process of in situ fabrication of PNCs is under scrutiny by Tan and co-workers. As depicted in Figure 5a, by tuning the molar ratio of perovskite precursor MAX and PbX2 from 1:1 to 3:1, the average grain size of resulted perovskite film was reduced from micrometer scale to nanometer scale, such film morphology can be treated as in situ fabricated PNCs.[84] Hardly no fluorescent effect was detected from the stoichiometric perovskite films while the film with excess MAX showed strong PL emission range from 453 to 642 nm through the substitution of halide ions. The distinctive optical property was attributed to the heavily confined excitons within the PNCs and to the passivation of the excess MAX surrounding the PNCs. Park and co-workers also

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Figure 5. a) Schematic representation showing crystallization and carrier recombination mechanisms for x = 1 and 3. NP represents nanoparticle. For the excess MABr case, a type I structure was formed with larger bandgap MABr (4.0 eV) and a smaller one for MAPbBr3 (≈2.2 eV). Reproduced with permission.[85] Copyright 2017, American Chemical Society. b) Cross-sectional TEM image of MAPbI3 films without (left) and with (right) FPMAI additives. Reproduced with permission.[89] Copyright 2017, American Chemical Society. c) Unit cell structure of quasi-2D (PEA)2(MA)n−1PbnI3n+1 perovskites with different 〈n〉 values, showing the evolution of dimensionality from 2D (n = 1) to 3D (n = ∞). Reproduced with permission.[63] Copyright 2016, Springer Nature. d) Schematic of cascade energy transfer in quasi 2D PNC films. Reproduced with permission.[65] Copyright 2016, Springer Nature.

demonstrated that by adding excess MABr to the 3D perovskite structure together with an antisolvent facilitated crystallization technique, MAPbBr3 NC films instead of polycrystalline films were achieved. The redundant MABr and the antisolvent washing during the spin-coating process lead to confined crystal size and enhanced radiative recombination rates.[85] As stated above, by further increasing the size of the cation A, quantum well like 2D layered perovskite structures are formed. The inorganic divalent cation B and halogen anion X form a monolayer of corner-sharing [BX6]4− octahedra layer which is in-plane covalently bonded, while the bulky organic cations can only link to the outer edge of the octahedra set. The inorganic and organic layers are stacked in an alternate manner and bonded by van der Waals force. Distinct excitonic behavior can be observed in 2D metal halide perovskites, such as excitonic absorption features and blueshifted PL emission.[86] Giving the insulating nature of the long organic chains, the charge transport is heavily inhibited in pure 2D structures, which limits their implementation in optoelectronic devices. Actually, as early as 1994, the first 2D structured perovskite based LED was


reported using phenylethylammonium (PEA) as the organic cation,[87] however, due to the severe exciton quenching at room temperature and poor charge injection efficiency, the device can only be operated at liquid-nitrogen temperature with a operation voltage as high as 24 V. An eclectic strategy is to synthesize PNC structures by using large and small cations jointly. The resulted PNCs can be categorized as nanometer-sized 3D crystallites or quasi 2D RP phases according to the incorporation proportion of the large OA cations. Rand and co-workers found that, with the percentage of loadings of butylammonium (BA) no more than 20 mol% as compared to the molar amount of the 3D perovskites, the bulky BA cations only play the role of surface ligands that confines the growth rather than participate in the crystallization of the PNC grains.[55,88] Similarly, 3D PNCs of MAPbI3 and MAPbBr3 with 5.4 and 6.4 nm in size were also realized at an incorporation molar ratio of 20% with fluorophenylmethylammonium (FPMA) and PEA, respectively (Figure 5b).[89] At higher incorporation amount, however, multilayered RP phases begin to appear with the large OA cations intercalate into the crystal

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structure, which is reflected by multiple absorption peaks that can be assigned to quasi 2D structures with different layers (Figure 5c). As mentioned above, the quasi 2D PNC structure can be denoted as L2An−1BnX3n+1. Plenty of investigations have been conducted on the choices of bulky cation L, including PEA,[56,63,64,89–92] BA,[55,88,93–95] ethylammonium (EA),[96] pheny lmethylamine (PMA),[97] phenylbutylammonium (PBA),[98,99] naphthylmethylamine (NMA)[65,100] and 2-Phenoxyethylamine (POEA).[101] Similar to the pure 2D structure, with the space dimension reduced to nanosized 3D or quasi 2D, the PNCs exhibit strong excitonic behavior. As calculated, the exciton binding energy of these PNCs can increase from a few tens of meV to more than 300 meV[17,102,103] while their excitonic monomolecular recombination rates is almost two orders of magnitude higher than that of the bulk 3D counterparts.[104] Furthermore, an unusual excitation behavior for the RP structure containing PNCs with different layer numbers is discovered, instead of displaying multiple light emission peaks associated with the 2D structures upon excitation, only the emission of the PNCs with the largest n value, i.e., the smallest bandgap is observed, which is in close proximity to that of the 3D perovskite materials. An energy transfer mechanism is therefore proposed, that is, rather than self-recombination or quenching, the excitons in the PNCs with lower n values undergo a funneling effect through an energy cascade to the nanocrystal grains with the highest n, as shown in Figure 5d. Such energy funneling process can be ascribed to the multiple quantum wells energy landscape of quasi 2D PNCs with spatial adjacencies to each other. According to the transient absorption and time-resolved PL analysis, the energy transfer process takes place on a timescale of about 0.5 to 10 ps. The extremely fast and efficient energy transfer is preferable for both the PL and EL performance of the quasi 2D PNCs since only the trap states of thickest PNCs need to be filled as the excitons are mostly concentrated in these parts. It is also found using density functional theory (DFT) that the incorporation of large OA cations brought non-negligible van der Waals interactions and raised the formation energy of the quasi 2D structures.[91] In other words, the stability of the quasi 2D PNCs is improved in comparison to the 3D counterparts, which is verified by the device demonstration in many works.[55,89] NCP process was applied in the large OA assisted in situ PNC fabrication in most cases. The boiling point of the selected antisolvent has distinct influences on the final distribution of the multiple quantum wells. High boiling point antisolvent such as toluene may induce a flat energy landscape with most PNC domains approaching highest n value while low boiling point antisolvent such as chloroform help to shape an engineered energy landscape, enhancing the effect of energy funneling.[64] Moreover, graded energy alignment can be achieved by tuning the proportions of large and small cations to achieve an optimized distribution of quasi 2D domains, which ultimately leads to remarkably promoted PL and EL performances. Nevertheless, it seems in some works that large OA cations can solely assist the in situ fabrication of PNCs without the participation of antisolvent.[65,98] Therefore, in contrast to the great success of film optimization, the understanding into the role of OA cations as well as antisolvents in the crystalline kinetics falls behind and needs to be clarified.


3.3. In Situ Fabrication in Polymer Matrix Adding polymer as a growth matrix provides one possible way of nontemplate size confinement of halide perovskite for fabricating composite films. The choices for polymers should fulfill the following requirements: same solvent solubility with perovskite precursors to ensure effective phase mixing; strong interaction between polymer and perovskites; selected dielectric property that compatible for EL device operation.

3.3.1. Blended Spin-Coating The simplest way of employing polymer for PNC fabrication is by spin-coating of blended polymer and perovskite precursors, followed by moderate annealing to initiate the crystallization of the perovskite within the polymer matrix, as depicted in Figure 6a. The pioneer work was done by Friend and co-workers.[105] 4,4-bis(N-carbazolyl)-1,1-biphenyl (CBP) and aromatic polyimide precursor (PIP) were selected as organic matrix and blended with perovskite precursors.[67] Upon spin coating, the viscous polymer confines the growth space of the perovskite precursor, leading to a more uniform and smooth morphology. The benefits of incorporating CBP and PIP include pinhole-free film morphology and improved PL or EL properties. The size of in situ fabricated PNCs can also be controlled under a selfassembling effect within such matrix. As shown in Figure 6b, by controlling the blending ratio of PIP, the size of resulting PNCs can be tuned from 200 to 60 nm. However, the relatively high imidization temperature of PIP and insulating nature of the above polymers hinder further optimization of such composite system. Regarding the requirements for EL applications, Yu and co-workers introduced poly(ethylene oxide) (PEO) as polymer matrix for the in situ fabrication of PNCs based composite films.[70] Upon PEO incorporation, the resulted composite evolves from discrete microcrystal islands to continuous and smooth films containing PNCs, as shown in Figure 6c. As a ionic conductive polymer, PEO facilitates the migration of the ionic species in the nanocomposite under the applied bias electric field, spontaneously forming a p–i–n homojunction structure within the PEO/perovskite nanocomposite layer regardless of the layer thicknesses.[106] Such built-in p–i–n homojunction helps the hole/electron injection from the anode/cathode side and finally gives rise to a single layer structured LED without the use of charge injection layers. In an opposite approach, nonionic dielectric polymers such as poly(2-ethyl-2-oxazoline) (PEtOz)[68] or poly ethylene glycol (PEG)[107] were employed for the fabrication of PNC-polymer composite films. Here, the nonionic polymers mainly play the role of confining the grain sizes of the PNCs as well as passivating the defects at the grain boundaries, however, the composite films have to be thin enough to ensure efficient charge transport in the operation of EL devices.

3.3.2. Controllable Drying Spin coating makes a facile way of thin film demonstration at lab scale, however, it is not compatible with large area manufacturing. In industrial field, tape casting is commonly

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Figure 6. a) Schematic of solution-based synthesis of PNC-polymer composite films using spin coating followed by thermal annealing. Reproduced with permission.[130] Copyright 2016, Wiley-VCH. b) Top view SEM image of perovskite only, 1/10 PIP/perovskite, and 1/2 PIP/perovskite on PEDOT:PSS coated silicon. Inset in the rightmost Figure shows an enlarged image of 1/2 PIP/perovskite film. Scale bars represent 1 µm.Reproduced with permission.[67] Copyright 2015, American Chemical Society. c) SEM cross-sectional image of perovskite only (top) and 1:1 ratio perovskite-PEO composite thin film (bottom) on a silicon/SiO2 (50 nm) substrate. Reproduced with permission.[70] Copyright 2015, Wiley-VCH.

adopted for large-scale film fabrication. A tape casting approach for polymer-perovskite composite film is thereby developed. In a tape casting approach for polymer based films, typically three steps are included. The first step is to prepare the mixed solution with uniform dispersion, the second step is to cast wet film on the substrate, and the final step is a curing process to obtain the resultant solid films. In order to achieve the uniform dispersion of perovskite nanocrystals within the polymer matrix, Zhong et al. demonstrated the in situ fabrication of PQDs (≈5 nm) based composite films by separating of the crystallization processes of PVDF matrix and MAPbX3 QDs through controllable evaporation of dimethylformamide (DMF). It should be also noted that the strong interactions between PVDF and perovskites plays an important role in determining the size and size distributions of PQDs. More specifically, a typical fabrication of MAPbBr3 QDs embedded PVDF composite film involved three stages shown in Figure 7a. Stage I: a mixture of MABr, PbBr2, and PVDF was first dissolved in DMF under vigorous stirring to form a transparent precursor solution (see inset in the left of Figure 7a). After that, a fixed amount of precursor solution was poured onto a smooth glass substrate to obtain a uniform precursor coating layer. Stage II: the glass substrate with precursor coating was placed into a vacuum oven to accelerate the evaporation of DMF under vacuum pumping. Finally, the coated precursor changed into colorless precursor films (see inset in the middle of Figure 7a). Stage III: the colorless precursor films were moved out from the oven to remove the residual DMF and gradually changed into green emissive composite films (see inset in the right of Figure 7a). As for the controllable drying process, in stage II, DMF was quickly removed from the casted precursor solution under


vacuum drying, resulting in the crystallization of PVDF into colorless film as an intermediate state. In the end of stage II, a typical colorless composite film contain 75.5 wt% PVDF, 8.5 wt% MAPbBr3, and 16 wt% residual DMF. The calculated concentration of MAPbBr3 in the composite films is less than the reported critical concentration of MAPbBr3 in DMF solution. With the slow removing of residual DMF in stage III, the concentration of MAPbBr3 approached to the critical concentration and subsequently induced the crystallization process. The resultant composite films show brilliant emissions even under ambient light. The good transparency of these composite films implies the formation of nanosized PQDs with uniform distributions in the polymeric matrix (Figure 7b). The transmittance of PQDCF is about 90% for the free-standing film. Moreover, PQDs exhibit the same or narrower emission spectra with an FWHM of 25 nm and high PLQYs up to 95%. In addition, this in situ fabrication method can be extended for red and blue fluorescent composite films by simply modifying the halogen constitutions of the perovskite precursors. Color-tunable PQDCFs were achieved with emissive wavelengths ranging from 440 to 730 nm (Figure 7c–e). In the following part, we also present the scale up fabrication of PQDCF through roll-to-roll process.

3.3.3. Swelling–Deswelling Microencapsulation It is known that polymers can swell in good solvent, bringing in the solvents as well as the solutes. Based on this phenomenon, Dong and co-workers invented an alternative method for the scalable fabrication of polymer-perovskite composite films (Figure 7f).[72] Perovskite precursor solutions were prepared in DMF solvent. The swelling process is conducted by spin-coating

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Figure 7. a) Schematic illustration of the in situ fabrication of MAPbBr3 NCs embedded PVDF composite films. b) A typical TEM image of MAPbBr3 NCs in PVDF matrix. The inset is the HRTEM image of a typical MAPbBr3 NC. c) Optical images under a UV lamp (365 nm) of color-tunable MAPbX3 /PVDF composite films with different halogen constitutions on glass substrates (S1: MAPbClBr2; S2: MAPbCl0.5Br2.5; S3: MAPbBr3; S4: MAPbBr2.7I0.3; S5: MAPbBr2.5I0.5; S6: MAPbBr2I; S7: MAPbBr1.5I1.5; S8: MAPbI3). d) Transmittance spectra of the correlated composite films on glass substrate. e) PL spectra of the composite films. Reproduced with permission.[71] Copyright 2016, Wiley-VCH. f) Scheme of MAPbBr3-polymer composite film formation process through swelling–deswelling. Reproduced with permission.[72] Copyright 2016, Wiley-VCH.

or brushing the precursor solution onto the aimed polymer substrates. Typical commercialized polymers with certain swelling ability in DMF were selected and tested accordingly, including polystyrene (PS), polycarbonate (PC), acrylonitrile butadiene styrene, cellulose acetate (CA), polyvinyl chloride, and poly(methyl methacrylate) (PMMA). Perovskite precursor solutes were brought into the polymeric matrix during the swelling process. After the complete swelling in the DMF based precursor solution, the resulted substrates were annealed for the activation of deswelling. PNCs were thereby generated in situ upon the evaporation of solvent. Cross-section TEM showed that the size and density of the as-grown PNCs are strictly dependent on the embedding-depth in the polymer substrate. Thanks to the weak size dependent optical properties of low-dimensional perovskites, the PNCs exhibited quite narrow FWHM below 24 nm, which is comparable to the ex situ fabricated colloidal PNCs. The water


resistant ability is remarkably enhanced for these polymer-PNC composite films, except for CA and PMMA, mainly due to the higher water permeability or lower DMF swelling ability, respectively. The highest water and thermal stability is observed in PS and PC based composite films, as they maintained more than 80% of their original PLQY after immersing into boiling water.

3.3.4. Microfluidic Spinning Electrospinning and microfluidic spinning are conventional approaches for the fabrication of microfibers.[108] Ma et al. demon strated in situ fabrication of CsPbBr3 NCs combined with a microfluidic spinning method.[109] PMMA was chosen as carrier material as its high transmission in the visible range. Cation precursors such as Cs-oleate and Pb-oleate and anion

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precursor Br-oleate was mixed with PMMA, separately. The anion precursor/PMMA mixture was spun onto the cation precursor/PMMA films through a syringe with controlled flow rate, forming uniform microfibers. PNCs were generated spontaneously at the contact area of fiber and film. The resulted nanocomposite microfibers provided excellent optical properties and improved stability. After grinding into powders, the nanocomposites can be integrated with blue emissive chip for LCD backlights.

3.3.5. In Situ Polymerization from Monomers and Perovskite Precursors Instead of using polymerized molecule as raw material for PNC synthesis, Xin et al. developed an in situ fabrication strategy for PNCs on the basis of bulk monomers.[110] Perovskite precursors dissolved in good solvent DMF were dropped into the bulk monomer solvents such as styrene methyl methacrylate (MMA) and butyl methacrylate (BMA). Emissive PNCs were generated immediately upon dropping, which is quite similar to the solution based LARP process. After which, polymerization of the monomers can be initiated via either thermal annealing or UV radiation in the presence of a thermal initiator (2,2’-azobis(2-methylpropionitrile), AIBN) or a UV-light initiator (diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, TPO). Finally, PNCs embedded in the PS, PMMA, or poly(butyl methacrylate) (PBMA) polymer matrix can be produced. It is found that the PNC-polymer composite materials obtained through thermal annealing suffered from very low PLQYs mainly due to aggregation and decomposition of the PNCs upon heating, while UV polymerization addresses such issue and gives rise to a high PLQY over 50% for PNC-PMMA and PNC-PBMA composite materials.

3.4. In Situ Fabricated PNCs Embedded in Crystalline Matrix Single crystals of halide perovskite are also attractive candidates for solar cells and photodetectors because of their low defect density and long carrier diffusion length. Even that per ovskite single crystals show better stability and easy processability for application exploration, they are not suitable for light emission uses due to their weak PL response at room temperature. Recently, Cs4PbBr6 single crystallites with micro or macro sizes gained increasing attention due to abnormal strong visible light emission properties at room temperature,[75,111,112] however, the nanosized NCs of this kind of material are nonluminescent.[113–115] The emissive Cs4PbBr6 can be achieved through antisolvent vapor-assisted crystallization,[74] precursor injection at room[116] or elevated temperature,[115] spin-coating with nonstoichiometric precursors[116,117] or inhomogeneous interface reaction (IIR).[118] In the A4BX6 structured perovskite, the [BX6]4− octahedra are completely separated by A cations without any sharing corners. The decoupling of octahedra in A4BX6 leads to remarkably increasing of its bandgap. Owing to the isolated [BX6]4− octahedra, the A4BX6 phase is also described as 0D perovskite. Therefore the rhombic prism Cs4PbBr6 was usually considered as a wide-gap semiconductor Adv. Optical Mater. 2018, 1800380

with a bandgap of about 3.9 eV,[113,119] the green emission from pristine Cs4PbBr6 single crystallites was suspected as an effect of defects or impurities, due to lack of evidence for the existence of CsPbBr3 phases.[74,112] On the other hand, it was confirmed that endotaxial synthesis of CsPbBr3 NCs within the Cs4PbBr6 compound was feasible since the lattice structure of CsPbBr3 matches well with the lattice of Cs4PbBr6 when rotates at a certain angle.[116] By elaborate experimental design, the in situ growth of CsPbBr3 NCs embedded in Cs4PbBr6 microcrystals can be observed. Therefore, it is more reasonable to deduce that nanosized CsPbBr3 NCs were formed regardless of the fabrication techniques of Cs4PbBr6 micro or macro crystals.[120] Very recently, we developed a HBr-assisted slow cooling method to fabricate Cs4PbBr6 single crystallites with embedded CsPbBr3 NCs.[121] as depicted in Figure 8a,b. PbBr2 and CsBr with molar ratio of 1:4 was dissolved in a fixed mixture of DMF and HBr (Volume ratio 5:4). The solution was first maintained at 1 °C above saturation point and the seed crystal was placed into the solution, then the temperature of the solution was decreased below the saturation point to launch the growth of Cs4PbBr6 single crystals. The highest PLQY of the as-prepared Cs4PbBr6 single crystal reached up to 97%. The large-sized green emissive crystals were grinded into powders and dispersed in toluene for TEM characterization. Small nanosized NCs with a lattice spacing of 2.7 Å corresponding to (200) phase for CsPbBr3 was observed within the grinded particles of Cs4PbBr6 (Figure 8c,d). This provides direct evidence for the formation of CsPbBr3 NCs embedded Cs4PbBr6 crystals in the green emissive crystals. The existence of CsPbBr3 NCs was also supported by the XPS analysis. Two Br 3d binding energy peaks were obtained, the binding energy peak at 67.7 eV can be ascribed to the Br atoms in isolated [PbBr6]4− of Cs4PbBr6[122] while the peak at 68.7 eV may be correlated with the Br atoms of CsBr impurity or corner-sharing [PbBr6]4− of CsPbBr3. The molar ratio of Cs:Pb:Br determined using inductively coupled plasma mass spectrometry gives a value of 4.5:1:6.4. The stoichiometric deviation from Cs4PbBr6 can be explained by the existence of Pb defects. Based on the above characterizations, it is clear that the nonstoichiometry with Pb vacancies induced partial in situ transformation from initial Cs4PbBr6 crystals and the process controlled by thermodynamic, as illustrated in Figure 8e. Especially, the resulting CsPbBr3 NCs embedded Cs4PbBr6 crystals have particularly good thermal stability that quite stable under 220 °C.

4. Applications The integration of in situ fabricated PNCs into photonic or optoelectronic devices is the ultimate goal of material research. In fact, at most circumstances it is the requirement for application pushed the investigation of in situ synthesis, which saves the incorporation step from as-prepared samples to targeting device structures. With the narrow and clean light emission at high efficiency, the most intuitive application of PNCs is for improve the performance quality of display fixtures. Similar to QDs, the in situ fabricated PNCs can be incorporated into the display devices either as down shifting materials or as emissive

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Figure 8. a) Schematic illustration of the setup for crystal growth. b) Solubility curves of Cs4PbBr6 in mixed HBr-DMF solution, inset: the as-fabricated green emissive crystal. The grid in the inset is 1 mm. c) Typical TEM images of the particles processed from green emissive CsPbBr3 nanocrystals embedded Cs4PbBr6 crystals. d) HRTEM image of a typical particle. The inset in the bottom right corner is the corresponding FFT image of CsPbBr3 and the top right corner is the corresponding FFT image of CsPbBr3 nanocrystals embedded Cs4PbBr6 crystals. e) Schematic illustration of crystal growth mechanism. Reproduced with permission.[121] Copyright 2018, Wiley-VCH.

components in EL devices. Besides display, other potential applications of PNCs such for UV enhanced Si photodetection was also explored.

4.1. Backlighting Units for Liquid Crystal Display 4.1.1. On-Chip Incorporation Wide color gamut (WCG) is a crucial indicator to evaluate highend display panels. Colloidal QDs exhibit narrow FWHM and high PLQY, which can significantly enhance the color gamut of LCD monitors from 70% to 110% NTSC by replacing or partially replacing the traditional rare earth phosphors in the


LED based backlighting units (BLUs).[123] Generally, there are mainly three structures to incorporate QDs into BLUs: on-chip structure where QDs are encapsulated on the blue LED chip to replace the rare earth phosphors for white LED, on-edge structure where QDs are loaded in an vacuum glass tube and placed upon the blue LED light strips to achieve white light source and on-surface structure by adding a quantum dots enhancement film (QDEF) which is placed between the light guide plate and brightness enhancement film and converts blue light into WCG white light. Among them, on-chip structure provides a direct means to examine the feasibility of QDs for display application, although the materials always suffer from thermal degradation. By combining the green emissive PQDCF and red emissive K2SiF6:Mn4, our group demonstrated the prototype white

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Figure 9. a) Large scale PQDCF fabrication through roll-to-roll technique. b) The structure diagram of PQDCF-LCD TV prototype. c) Comparison of BLU structures and film structures of QDEF and PQDCF. d) Image of the 55″ PQDCF-LCD TV panel. e) The color gamut and white color coordinates of the PQDCF-LCD TV prototype in CIE 1931 diagram.

LED devices with high luminous efficiency up to 109 lm W−1 at 20 mA and color gamut of 121% of NTSC 1931 standard. Considering the color quality and luminous efficiency, the performance is much better than the previous reported ex situ fabricated PNCs. More inspiring, the color quality and luminous efficiency was further enhanced by the strong emissive Cs4PbBr6 crystals embedded CsPbBr3 QDs. By combining the highly luminescent Cs4PbBr6 crystals as green emitters and commercial K2SiF6:Mn4+ phosphor as red emitters with blue emitting GaN chips, high quality white lights with luminous efficiency of ≈151 lm W−1 and color gamut of 90.6% Rec. 2020 at 20 mA was achieved. To our knowledge, this is the best reported values in lab for LCD backlights.

4.1.2. On-Surface Incorporation Currently, on-surface structure has become main approach to incorporate QDs BLU.[28] So far, CdSe and InP QDs are commonly used in most of commercialized QDEF products (e.g., QDEF by Nanosys and 3M) to achieve WCG white light. Unfortunately, the complicated manufacturing process of CdSe and InP QDs and QDEF lead to an extremely high cost and inevitable brightness loss. Meanwhile, the high content of heavy metal cadmium does not meet European Restriction of Hazardous Substances (EU RoHS) requirement. Recently, Zhijing Nanotech and Lucky cooperation have developed the roll-to-roll process for scale-up fabrication of PQDCF (Figure 9a) and they worked with TCL to demo nstrate the first PQDCF integrated for LCD backlights.[71] Figure 9b illustrates the structure diagram of PQDCF-LCD


TV prototype, the blue light emitted from LED chip is partly converted into red and green light by red K2SiF6:Mn4+ phosphor and green PQDCF, respectively. Compared with CdSe QDs based QDEF, PQDCF has significant advantages in terms of low costs, high brightness, and low heavy metal content (Figure 9c). In CES 2018, TCL published the first PQDCF based 55” TV panel with a color gamut of 101% and a highest brightness of 500 nits, as shown in Figure 9d,e. In another approach, based on the polymer-PNC composite films through swelling–deswelling microencapsulation, Dong and co-workers also demonstrated backlight for LCD with a color gamut of 89–91% Rec. 2020 standard.[73] It is known that under adequate tensile force, the PVDF molecules may be stretched and aligned along the force direction. The alignment of PVDF molecules consequently leads to the spatial alignment of the inclusions within the composite film. Taking advantage of this unique mechanical property and combined it with the in situ fabrication, PQDCF with strong polarized PL emission was realized.[124] Through controllable stretching at typical time nodes before and after the crystallization of the perovskite precursors during the in situ fabrication process, the precursors or PQDs are migrated and rearranged along the extending direction. The PQD-aligned wires caused strong Coulomb interaction of the inclusive PQDs and the dielectric confinement of optical electric field, resulting in redistribution of the transition dipole moment of the exciton to orient along the stretching direction. PL emission showed no polarization for the unstretched composite films while the polarization ratio of the stretched films reaches up to 0.33 with a high PLQY of 80%. Simulation showed that with a PL polarization ratio of 0.3, the light transmittance of polarizers in LCD panel could be potentially improved from 50% to 65%.

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Although the polarization ratio still needs to be improved, the stretching strategy of the PQDCFs provides a facile approach for achieving large-area polarized light source.

4.2. Electroluminescence Since the commercialization of organic light emitting diodes (OLEDs) in display panels, handheld devices and cell phones, the electroluminescence oriented new generation display technologies have been widely studied. Electroluminescent devices based on QDs (QLEDs) and metal halide perovskites have emerged with sky-rocketing device parameters that outperform OLED in many aspects.[125–127] Compared to OLED and QLED, PeLEDs possess advantages in terms of narrower FWHM thus potentially higher color gamut in display application, high external quantum efficiency especially for the green emission devices, and the possibility of in situ fabrication that provides an efficient and convenient way for scalable production. The investigation of PeLEDs based on in situ fabricated PNC materials is advancing quickly. The NCP process and OA manipulation, polymeric encapsulation and imbedded CsPbBr3 NCs in Cs4PbBr6 are all available for electroluminescent investigations. Table 1 summarizes the up-to-date device parameters of in situ fabricated PNC based PeLEDs. For the molecular ligand assisted in situ fabricated PNC films, Cho et al reported the fabrication MAPbBr3 PNCs with average grain size lower than 100 nm through the A-NCP process and achieved recorded device with a current efficient (CE) of 42.9 cd A−1.[57] With the assistance of excess MAX in the in situ spin-coating process, the size of the PNC grains can be shrined to merely 21.7 nm, the device incorporating MAPbBr3 NCs revealed a maximum luminance over 8000 cd m−2 and a CE of 5.1 cd A−1.[84] By employing a solvent-vacuum drying (SVD) technique instead of conventional thermal evaporation in the in situ fabrication process, the CE of the final device was achieved beyond 34 cd A−1.[85] Upon the incorporation of large OA molecules, Band’s group further reduce the size of MAPbX3 NCs down to ≈10 nm and obtained ameliorative device performance.[55] Furthermore, by inserting the large OA molecules into the 3D perovskite structure, the resulted RP phase PNCs demonstrated PeLEDs with dramatically improved CE and EQE due to the energy funneling effect.[63–65,98,100] Very recently, a high efficient green emission PeLED employing quasi 2D PNCs was reported with record CE of 62.4 cd A−1 and EQE of 14.36%.[92] Through lowmagnification high-angle annular dark-field, an ordered spatial distribution of the RP phases where the majority of large-n phases are located close to the hole transporting layer is discovered by Wang and co-workers.[65] which is favorable for the charge injection process in the electroluminescent devices. For the polymer-PNC nanocomposite films, the formation of p–i–n homojunction under applied bias as mentioned above using ionic polymer was explained by time-dependent discharging current evolution,[70] transient light emission response,[70] alternating-current impedance measurements,[106] distribution of the built-in electric fields[128] and junction propagation along with layer thickness.[106] Such interior p–i–n band structure is beneficial for charge injection through a


sharp self-formed tunneling barrier. A single layer device with the simplified structure and a striking maximum brightness beyond 590000 cd m−2.[129] Efforts have also been made to explain the effectiveness of PEO incorporation in terms of improving the film morphology,[129–131] incorporation ratio,[132] and flexible/stretchable devices.[69,133] It is discovered that by introducing a common surfactant, poly(vinylpyrrolidone) (PVP), the high polarity of the pyrrolidone group on the PVP molecules facilitates the dispersion of PNCs, leading to an improved film quality toward a dense and pinhole free state. Thanks to the simplified device structure, fully printed flexible PeLEDs composed of the polymer-PNC nanocomposite sandwiched between single-walled carbon nanotube anode and silver nanowire cathode was realized and could survive after a number of bending cycles. Utilizing the stretchability of polymers, stretchable PeLEDs were also demonstrated, where the device can be stretched to 40% larger than its original length and can still work after 100th stretching cycles. As for the nonionic dielectric polymers, a maximum luminance of 36600 cd m−2 and CE of 19 cd A−1 is achieved for the ultrathin PEG-CsPbBr3 nanocomposite film with a thickness of 30 nm[107] and near-infrared emissive PeLED was demonstrated using PEtOz-MAPbI3 nanocomposite thin films.[68] It is also possible to blend the polymeric charge transporting materials such as polyvinyl carbazole (PVK)[134,135] into the perovskite films to control the film morphology and facilitate the charge transport. The electroluminescent uses of in situ fabricated CsPbBr3 NCs in Cs4PbBr6 was also explored.[117] Compared to the polycrystalline CsPbBr3 film based PeLED, the highest luminance and CE of the device employing Cs4PbBr6 with CsPbBr3 nanoinclusions were improved from 20 to 850 cd m−2 and 3.33 × 10−3 to 0.30 cd A−1, respectively. However, these device parameters are still relatively lower than the PeLEDs based on other approaches of in situ fabricated PNCs. The achieved record parameters of EL performance verifies the superiority of in situ fabricated PNCs. Compared to the EL applications adopting polycrystalline perovskites films in the earlier study,[2] the in situ fabricated PNC films with much enhanced excitonic features prevails in both EL intensity and efficiency. For the devices with emitting layer composed of colloidal perovskite QDs,[53] the in situ fabrication technique not only simplify the device preparation procedure but also eliminate the possible material defects or film pin-holes induced by the reassembling of QDs. The introduced organic molecules or polymeric matrix further strengthens the stability of the final devices. Therefore, in situ fabricated PNCs play an increasingly irreplaceable role in the optoelectronic device applications.

4.3. Enhanced UV Response of Silicon Photodetectors The strong UV absorption and visible PL emission with tunable wavelengths and high efficiency of PNCs also inspired their application for enhancing the UV response of Silicon (Si) based photodetectors. Before the invention of PNCs, organic dyes,[136,137] rare earth doped compounds[138] and colloidal QDs[139,140] have been explored as down-shifting materials for enhancing the UV response of Si based charge coupled device (CCD), CMOS devices, and solar cells. However, the severe UV

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Table 1. The reported device parameters of in situ PNC based PeLEDs. Publication year/ month

Peak EQE [%] EL peak [nm] Maximum luminance/ Peak current radiance efficiency [cd A−1]

Ref.

Methodology

Emissive material

2015/02

Polymeric matrix

PIP:MAPbBr3 (1:1 w/w)

534

≈600 cd m−2

N/A

1.2

[67]

2015/08

Polymeric matrix

PEO:MAPbBr3 (0.75:1 w/w)

532

4064 cd m−2

0.38

0.083

[70]

2015/10

Additive-based nanocrystal pinning (A-NCP)

MAPbBr3 NCs

541

≈15000 cd m−2

42.9

8.53

[57]

2015/12

Polymeric matrix


545

21014 cd m−2

4.91

1.1

[69]

0.6

0.14

PEO:MAPbBr3 (0.75:1 w/w) in a flexible device structure

545

360 cd

m−2

2016/06

OA incorporation (2D)

(PEA)2PbBr4 NP

410

N/A

N/A

0.04

[90]

2016/06

OA incorporation (Quasi 2D)

MAPbBr3(40 wt%):(PEA)2PbBr4(10 wt%) = 1:6 (MPEA16)

≈520

2935 cd m−2

4.90

N/A

[56]

2016/06


(PEA)2(MA)n−1PbnI3n+1 (avg. n = 5)

≈750

80 W m−2 sr−1

N/A

8.8

[63]

2016/07

Excess OA (3D NC)

MAPbClxBr3−x NCs (Cl:Br = 1:1)

490

154 cd m−2

0.08

N/A

[84]

MAPbBr3 NCs

528

8794 cd m−2

5.1

N/A

MAPbBrxI3−x NCs (Br:I = 1:1)

645

346 cd m−2

0.23

N/A


683

384 cd

m−2

0.08

N/A


707

222 cd m−2

0.08

N/A


(4-PBA)2Csn−1PbnBr3n+1 (PCPbB)

491

186 cd m−2

N/A

0.015

[99]

2016/08

Polymeric matrix

PEO:CsPbBr3 (0.086:1 w/w)

521

53525 cd m−2

15.67

4.26

[130]

2016/09

OA incorporation

(BA)2(MA)2Pb3I10

700

0.1

2.29

[95]

2016/07

(Quasi 2D)

2016/09

2016/09


Polymeric matrix

214 cd

m−2 m−2

(BA)2(MA)4Pb5Br16

523

3.48

1.01

(BA)2(MA)2Pb3Br7Cl3

468

21 cd m−2

0.006

0.01

(NMA)2FAPb2I7 (NFPI7)

786

55 W m−2 sr−1

N/A

9.6

(NMA)2FAPb2I6Br (NFPI6B)

763

82 W m−2 sr−1

N/A

11.7 7.1

2246 cd

[65]

(NMA)2FAPb2I5Br2 (NFPI5B2)

736

N/A

N/A


685

N/A

N/A

1.7


664

N/A

N/A

0.05


611

N/A

N/A

0.01

(NMA)2FAPb2IBr7 (NFPIB7)

518

N/A

N/A

0.1

CsPbBr3:PEO:PVP (100:50:5 w/w)

522

591197 cd m−2

21.5

5.7

[129]

N/A

0.76

[68] [101]

m−2 sr−1

2016/11

Polymeric matrix

PEtOz: MAPbI3 (5% PEtOz (weight ratio))

760

2016/12


(PEOA)2(MA)n−1PbnBr3n+1 (10%-POEA (molar ratio))

524

2146 cd m−2

1.21

0.31

(PEOA)2(MA)n−1PbnBr3n+1 (40%-POEA (molar ratio))

480, 494, 508

19 cd m−2

2.1

1.1

12.31 W

2017/01


(NMA)2CsPb2I7 (NCPI7)

688

440 cd m−2

N/A

3.7

[100]

2017/01

OA incorporation (3D NC/ Quasi 2D)a)

3D MAPbI3 NCs with BAI additives (BA:MA (20:100 molar ratio))

748

≈50 cd m−2

0.09

10.4

[55]

3D MAPbBr3 NCs with BABr additives (BA:MA (20:100 molar ratio))

513

≈8000 cd m−2

17.1

9.3


520

30100 cd m−2

2017/01 2017/02

Polymeric matrixb) OA incorporation (Quasi 2D)


(BA)2(MAPbBr3)n−1PbBr4 (BA:MA (2:1 molar ratio))

≈443, 460, 476

(BA)2(MAPbBr3)n−1PbBr4 (BA:MA (1:1 molar ratio))

≈502, 508

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15.1

4.0

[132]

m−2

≈0.007

≈0.005

[94]

≈0.8 cd m−2

≈0.015

≈0.006

≈1.5 cd




Table 1. Continued. Publication year/ month

2017/03

2017/03

Methodology

OA incorporation (3D NC/ Quasi 2D)c)

Excess OA (3D NC)d)

Emissive material

Peak EQE [%] EL peak [nm] Maximum luminance/ Peak current radiance efficiency [cd A−1]

Ref.

(BA)2(MAPbI3)n−1PbI4 (BA:MA (1.8:1 molar ratio))

≈586, 607

≈3 cd m−2

≈0.02

≈0.009

(BA)2(MAPbI3)n−1PbI4 (BA:MA (1.4:1 molar ratio))

≈646

≈0.05 cd m−2

≈0.0015

≈0.004

3D MAPbI3 NCs with FPMAI additives (FPMA:MA (20:100 molar ratio))

749

72 W m−2 sr−1

N/A

7.9

3D MAPbBr3 NCs with PEABr additives (PEA:MA (20:100 molar ratio))

≈520

11400 cd m−2

N/A

7.0

MAPbBr3 NCs

530

6950 cd m−2

34.46

8.21

[85]

m−2

9.45

2.28

[135]

2017/04

Polymeric matrix

[PVK:TPBi (1:2 w/w) in toluene]: MAPbBr3 in DMF (1:5 volume ratio)

534

7263 cd

2017/04

Polymeric matrixe)

PEO:MAPbBr3 (0.5:1 w/w) before stretch

547

15960 cd m−2

2.7

0.62

PEO:MAPbBr3 (0.5:1 w/w) with 20% stretch

10148 cd m−2

2.9

N/A

PEO:MAPbBr3 (0.5:1 w/w) with 40% stretch

7340 cd m−2

3.2

N/A

[89]

2017/05


(PEA)2(MA)4Pb5Br16 (Graded energy landscape)

526

8400 cd m−2

N/A

7.4

[64]

2017/05

Polymeric matrix

PVK:MAPbBr3 (2:20 w/w)

524

1427 cd m−2

6.50

1.88

[134]

m−2

2017/05

Polymeric matrix

PEO-CsPbBr3

525

51890 cd

21.38

4.76

[131]

2017/05

Polymeric matrix

MAPbBr3:PEO:PVP (100:50:0.25 w/w) with film thickness of 450 nm

538

104954 cd m−2

3.8

0.9

[106]

2017/07


(BA)2(MA)n−1PbnBr3n+1 (BA:MA (3:3 molar ratio))

515

> 1000 cd m−2

3.3

N/A

[93]

2017/08


(EA)2(MA)n−1PbnBr3n+1

473, 485

200 cd m−2

N/A

2.6

[96]

2017/08

Polymeric matrix

PEG:CsPbBr3 (0.034:1 w/w)

522

36600 cd m−2

19.0

5.34

[107]

m−2

N/A

8.79

[83] [98]

2017/10

Additive-based nanocrystal pinning (A-NCP)

MAPbBr3 NC film (0.1 wt% TPBI)

541

> 10000 cd

2017/10


(PBA)2(CsPbBr3)n−1PbBr4

514

≈14000 cd m−2

N/A

9.7

(PBA)2(CsPbI3)n−1PbI4

683

N/A

N/A

7.3

0.3

2.4 × 10−5

[117]

m−2

2017/10

Embedded NCs

CsPbBr3 NCs in Cs4PbBr6 host

514

≈850 cd

2018/02

OA incorporation (Quasi 2D)f)

PEA2(FAPbBr3)n−1PbBr4 (n = 3 composition)

532

9120 cd m−2

62.43

14.36

[92]

2018/03

OA incorporation (3D NC)g)

MAPbBr3 NCs (drop PMA 0.50 vol% in chlorobenzene)

≈535

55400 cd m−2

55.2

12.1

[97]

a)It is confirmed that that at lower OA incorporation ratio, the structure of the resulted materials is still nanosized 3D crystallites; b)This result was achieved along with a 4,4′,4″-Tri(9-carbazoyl) triphenylamine (TCTA) layer treatment; c)Same as footnote a); d)This result was achieved along with a solvent-vacuum drying (SVD) process; e)A stretchable PeLED is demonstrated in this work, the device parameters at three stretching stages are presented; f)This result was achieved along with a trioctylphosphine oxide (TOPO) treatment; g)Same as footnote a).

degradation issue[141,142] of organic dyes and rare earth ions doped compounds, the aggregation induced scattering loss and PL quenching[143] problem of colloidal QD based enhancing films hinders further optimization of these techniques. The capacity of in situ fabrication of PNCs within transparent and protective polymer wrappage provides a unique pathway that may bypass the above issues. Similar to the application in the LCD backlights, PQDCF was integrated on the surface of a Si photodiode by spin coating and vacuum drying. Owing to the high PLQY over 94% and high transparency in the visible


range, apparent UV responding enhancement was observed with neglectable influences on the original photodetecting region. By coating the PQDCF on a Si photodiodes, response spectra was extended to UV region of 200–400 nm and the EQE was improved from near zero up to 50.6% at 290 nm.[144] Furthermore, the PQDCF can be well adapted for front-illuminated electron multiplying CCD (EMCCD) image sensors without significant resolution and response loss. By mounting solar-blind UV filter in front of the lens, we further demonstrated prototype solar-blind UV image sensor. In all, in situ fabricated PQDCF

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provide low-cost method to achieve UV enhanced Si photodetectors toward broadband and solar-blind light detection.

5. Conclusion and Outlook The unique in situ processing properties make PNCs a game changer in the optical materials. Based on the above description, the in situ fabricated PNCs not only facilitates the high-performance devices for photonic and optoelectronic applications, but also reduces the fabrication cost. Due to the diverse routes of preparation methods and achievable high performance in photonic and optoelectronic applications, in situ fabricated PNCs has aroused intense interests for researchers with various backgrounds ranging from materials chemistry, spectroscopy, semiconductor devices as well as optical engineering. By far fruitful research outcomes have been achieved, however, there are not only critical issues need to be solved but also important directions need to be explored. In the following, we tried to present a few of important tasks for future study from our perspective.

5.1. The Formation Mechanism Underline In Situ Fabrication There is no doubt that the in situ fabrication provides great benefits for simplifying the integration process. To clarify the structural-property correlations, it is much desired to understand the formation mechanism underline the in situ fabrication process. Unlike the reaction in flask, the in situ fabricated PNCs experienced different nucleation and growth process. First, the supersaturation that drives the formation of in situ fabricated PNCs was usually determined by the solvent evaporation and/or solvent mixing on substrates, which is more complicated to draw the nucleation and growth mechanism. In addition, the in situ fabricated PNCs are thin films or composite films, their structural characterizations are of great challenge. One of the main tasks will be the development of suitable techniques to monitor the evolution process.

means to characterize exciton diffusions as well as their recombination kinetics. Moreover, the OA molecules and polymeric matrix in the PNC based films are roughly considered as encapsulating media in most cases, therefore their potential interactions with the PNC structures and the exciton behavior in the presence of various OA ligands or polymers remain unclear.

5.3. Device Integration For LCD backlights, there are already very successful prototype demons. Although the stability issues discussed in the following need to be solved, there is great motivation to explore the use of these PQDCF in other photonics. As described in the last part, the integration into Si detectors has been successfully demonstrated, showing great potential as down-shifting materials. It is also expected that these PQDCF can be suitable candidates as down-shifting materials for PV enhancement, laser gain media and intelligent optical sensors for other technology. For the EL applications, despite the successful demonstration of green emission PeLEDs with comparable performances to the OLED and QLED devices, the investigation and optimization of red and blue PeLEDs still lag behind. It is found that the device optimization strategies for the bromide based green emission PNCs are not applicable for the iodide or chloride based red and blue emission PNCs, probably due to the changed electronic properties of materials after halide substitution. Specific device structural designs are required as well as the material optimization towards enhanced emission efficiency. The potential of in situ fabricated PNCs for other optoelectronic devices such as photovoltaic devices, photodetectors and lasers also need to be addressed. A few works were carried out on the use of perovskite-polymer nanocomposite for solar cell applications to control the growth and morphology of the films.[145,146] Nevertheless, more and more efforts should be made for the function extension of in situ fabricated PNCs.

5.4. Long-Term Stability 5.2. Fundamental Optical Properties Because the incorporation of QDs into composites films usually bring PL quenching or aggregated induced scattering effects, previous spectroscopic studies mainly focus on the colloidal QDs solution. Considering this, the high transparency of in situ fabricated PQDCF provide advantages for spectroscopic study. It has been reported that colloidal PNCs show very interesting nonlinear optical properties. The available in situ fabricated PQDCF can be more suitable candies for nonlinear optics. It is known that free charge carriers hold for the majority population in the 3D bulk metal halide perovskites, and the carrier diffusion length can be extremely long that exceed many state-of-the-art inorganic and organic semiconductors.[12,47] While in the case of PNCs, especially when the size of the materials goes down to the quantum confinement regime, the exciton binding energy is tremendously increased, leading to an exciton dominated environment. There is still a lack of direct


Intrinsic instability of metal halide perovskites has become the greatest obstacle that hinders the practical implementation. The in situ fabrication of PNCs partially addressed this issue. The incorporation with polymers and the encapsulation of a wide bandgap A4PbX6 phase help to raise the water and oxygen resistance of the PNCs, however the thermal stability remains to be a significant problem. The increased formation energy of large OA induced RP phase PNCs show better material stability, while that the ion migration effect should be taken into consideration under the operation of EL devices, which is detrimental to the long-term stability of the devices. In all, stability will be always one of predominant topics for functional materials and the solution should be resulted from a combined engineering design of molecular, composition, structure, surface, and systems. We believed that the in situ fabricated PNCs hold the potential to give the first perovskite based commercial product.

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Acknowledgements This work was supported by the National Key R&D Program (No. 2017YFB0404600) and the National Natural Science Foundation of China (Nos. 21603012 and 61722502). This work is dedicated for the 70th birthday of Prof. Yongfang Li at Institute of Chemistry, Chinese Academy of Sciences.

Conflict of Interest The authors declare no conflict of interest.

Keywords displays, electroluminescence, nanocrystals, photoluminescence

in

situ

fabrication,

perovskite

Received: March 22, 2018 Revised: May 15, 2018 Published online:

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