Chen Weijie

Triarylamine/bithiophene copolymers (TABT), synthesized via iron-catalyzed C−H/C−H coupling, have been applied as hole-transporting materials for perovskite solar cells (PVSCs). Owing to the film uniformity, enhanced quinoidal conjugation in radical cation state, and excellent ionization potential matching, a PVSC using a mesityl substituted TABT polymer outperformed devices using reference standards such as PTAA and Spiro-OMeTAD.

Abstract

Polytriarylamine is a popular hole-transporting materials (HTMs) despite its suboptimal conductivity and significant recombination at the interface in a solar cell setup. Having noted insufficient conjugation among the triarylamine units along the polymer backbone, we inserted a bithiophene unit between two triarylamine units through iron-catalyzed C−H/C−H coupling of a triarylamine/thiophene monomer so that two units conjugate effectively via four quinoidal rings when the molecule functions as HTM. The obtained triarylamine/bithiophene copolymer (TABT) used as HTM showed a high-performance in methylammonium lead iodide perovskite (MAPbI₃) solar cells. Mesityl substituted TABT forms a uniform film, shows high hole-carrier mobility, and has an ionization potential (IP=5.40 eV) matching that of MAPbI₃. We fabricated a solar cell device with a power conversion efficiency of 21.3 % and an open-circuit voltage of 1.15 V, which exceeds the performance of devices using reference standard such as poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) and Spiro-OMeTAD.

The impact of the commonly used tin fluoride (SnF₂) additive in Sn-based perovskite solar cells on the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/perovskite interface are analyzed. SnF₂ is found to preferably precipitate at this interface where it forms a SnS interlayer of approximately 1.2 nm thickness induced by a chemical reaction with sulfur-containing groups at the PEDOT:PSS surface.

Abstract

Tin-based perovskites are promising alternative absorber materials for lead-free perovskite solar cells but need strategies to avoid fast tin (Sn) oxidation. Generally, this reaction can be slowed down by the addition of tin fluoride (SnF₂) to the perovskite precursor solution, which also improves the perovskite layer morphology. Here, this work analyzes the spatial distribution of the additive within formamidinium tin triiodide (FASnI₃) films deposited on top of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transporting layers. Employing time-of-flight secondary ion mass spectrometry and a combination of hard and soft X-ray photoelectron spectroscopy, it is found that SnF₂ preferably accumulates at the PEDOT:PSS/perovskite interface, accompanied by the formation of an ultrathin SnS interlayer with an effective thickness of ≈1.2 nm.

A small molecule [2-(9-H-Carbazol-9-yl) ethyl] phosphonic acid (2PACz) is introduced into low donor and acceptor ratio (1:3) semitransparent organic solar cells. Due to the mitigated charge recombination and strengthened charge extraction, a remarkable efficiency of 15.2% is achieved with an average visible transmittance (AVT) of 19.2%, which represents the highest efficiency in semitransparent devices with AVT around 20%.

Abstract

Semitransparent organic solar cells (ST-OSCs) have promising prospects for building or vehicle integrated solar energy harvesting with energy generation and see-through function. How to achieve both an adequate average visible transmittance (AVT) and high-power conversion efficiency (PCE) is always the key issue. Herein, a simple but effective strategy for constructing high performance ST-OSCs by introducing a small molecule [2-(9-H-Carbazol-9-yl) ethyl] phosphonic acid (2PACz) into a low-donor content active layer is reported. The fill factor is improved from 70.5% to 75.5% and correlated to the mitigated charge recombination and strengthened charge extraction, further ascribed to the enhanced build-in potential, reduced charge transport resistance, and favorable film morphology. By combining the unique nature of 2PACz, that can spontaneously form in situ self-organized hole transport interlayers under bulk-heterojunction films, PEDOT-free ST-OSCs with a PCE of 15.2%, amongst the highest values in this field, is achieved with an AVT of 19.2%. Moreover, an outstanding light utilization efficiency of 3.39% is also obtained with an AVT of 30.0% and a PCE of 11.3% in a translucent device by tuning the electrode. The work demonstrates a new and simple strategy for achieving excellent AVT and PCE in ST-OSCs with simplified device structure.

High-performance ternary organic solar cells with a decreased bandgap and increased open-circuit voltage are fabricated with the small-molecule donor BTID-2F featuring a higher highest occupied molecular orbital energy level than PM6 as the third component in the PM6:Y6 system. Significant improvements in ternary device performance are realized, and an impressive power conversion efficiency of 18.52% is obtained.

Abstract

Ternary architecture is a promising strategy to further boost the performance of organic solar cells (OSCs). Reducing the bandgap of the active layer materials not only widens the absorption wavelength range and enhances the short-circuit current (J_sc ) of the OSC, but also decreases the open-circuit voltage (V_oc ) of the device, leading to a trade-off situation for the optimization of the material system. Herein, a small-molecule donor BTID-2F, featuring a narrower bandgap than that of PM6, is introduced into a PM6:Y6 based system. The redshift in external quantum efficiency indicates the narrower bandgap and better aggregation in the ternary blends than those of binary ones. Interestingly, lower energy disorder and energy loss are also attained for the ternary devices, leading to higher V_oc . Furthermore, owing to the suppressed recombination and morphological optimization, a simultaneous enhancement in the J_sc and fill factor boosts the power conversion efficiency (PCE) of ternary OSC to 17.9% compared to 16.62% for the binary device. Likewise, replacing the acceptor with the L8-BO molecule further improves the ternary PCE to 18.52%. This work indicates an emerging approach for fabricating high-performance ternary OSCs with a decreased bandgap and increased V_oc .

The gallium and titanium doped indium oxide (IO:GT) translucent electrode has a shallow work function (−4.23 eV) with a well energy matching that of inverted structure semitransparent perovskite solar cells (ST-PSCs). This suggested IO:GT contributes to attaining a certified conversion efficiency of 17.53% with a competitive averaged visible transmission of 21.9%, which provides the best efficient light utilization among ST-PSCs reported to date.

Abstract

Transparent electrodes are essential to allow optical transparency for realizing semitransparent perovskite solar cells (ST-PSCs). This study addresses gallium- and titanium-doped indium oxide (IO:GT) between the electron transport layer (ETL) and top electrode to potentially replace conventional indium tin oxide (ITO) used in inverted ST-PSCs. The shallower work function (−4.23 eV) of IO:GT than that (−4.69 eV) of conventional ITO contributes to suppressing the formation of the Schottky barrier and enhancing the charge transport at the ETL/cathode interface. By adopting IO:GT, the ST-PSC exhibits an enhancement in power conversion efficiency (PCE) from 8.59% to 17.90% (certified 17.53%) with an average visible transmittance (AVT) of 21.9%, which is the record PCE at similar AVT among all ST-PSCs reported to date. Moreover, combining these ST-PSCs as the top cell, a four-terminal perovskite–perovskite tandem solar cell is realized, showing a high PCE of 23.35%. Furthermore, the stability of the ST-PSCs is confirmed excellent, maintaining over 96% of the initial PCE after 1864 h (≈77 days) in air ambient without encapsulation, which is better than the device employing a metal cathode. Therefore, these results demonstrate that the adoption of IO:GT can be a promising route for efficient and stable inverted ST-PSCs with preferred transparency.

In this manuscript, by exploiting dual-modal ferroelectric photovoltaic effect in a Dion–Jacobson hybrid perovskite (BDA)(EA)₂Pb₃Br₁₀ (1, BDA is 1,4-butadiammonium, EA is ethylammonium), high-performance self-powered polarization photodetection is realized in the visible and infrared region with a polarization ratio up to 4.2 (visible region) and 4.8 near infrared region. This work prophesies the tremendous potential of hybrid ferroelectrics in various optoelectronic applications.

Abstract

Ferroelectric materials, particularly the emerging layered hybrid ferroelectrics, have shown great potential for high-sensitive polarization photodetection owing to their striking bulk photovoltaic effect (BPVE). Despite recent great achievements, the linear photoresponse range based on single-mode BPVE is still limited in the shortwave region due to the large intrinsic bandgaps. Herein, first, the realization of self-powered visible–infrared polarization photodetection by exploiting dual-modal BPVE in a newly developed layered Dion–Jacobson (D-J) hybrid ferroelectric (BDA)(EA)₂Pb₃Br₁₀ (1, BDA is 1,4-butadiammonium, EA is ethylammonium) is reported. Crystallographic investigations indicate that 1 adopts a typical trilayered D-J perovskite structure with a fascinating ferroelectric feature and a giant two-photon absorption coefficient as giant as 4.73 cm MW^–1. Meanwhile, the bulk single crystal device of 1 exhibits excellent self-powered direct detection performance under both visible light (405 nm) and near-infrared light (800 nm), with a current on/off ratio as high as 10³. More intriguingly, the device displays high sensitivity to the polarization of illuminated light, showing a considerable anisotropy up to 4.2 (405 nm) and 4.8 (800 nm), which are much larger than the detectors achieved by geometry anisotropy. The realization of self-powered visible-infrared dual-modal polarization photodetection in 1 indicates the tremendous potential of hybrid ferroelectrics in various optoelectronic applications.

A highly-oriented Pb-Sn alloyed perovskite film is achieved via a surface ligand anchoring strategy. The (100)-orientation-dominated perovskite film demonstrates lower trap density, higher carrier mobilities, longer carrier lifetimes, as well as much-suppressed Sn²⁺ oxidation, favoring an excellent power conversion efficiency exceeding 20%.

Abstract

The narrow bandgap (≈1.2 eV) Pb-Sn alloyed perovskite solar cell is a promising bottom component cell for all-perovskite tandem devices that are expected to offer higher efficiency than the theoretical Shockley–Queisser limit of the single-junction solar cells. The density functional theory (DFT) study reveals that the Pb-Sn perovskite film with the (100) orientation would render significantly reduced trap density, which is a critical figure-of-merit for perovskite device performance. Alkyl diamine is therefore designed to first anchor onto the surface as a nucleation agent to modulate the Pb-Sn perovskite growth to proceed preferentially along with the (100) orientation. It is observed that the diamine cations not only effectively induced the crystal growth at the nucleation stage, but also remained on the crystal surface to eventually passivate the resultant perovskite film. As a result, the diamine-based films show (100) preferred orientation with superior optoelectronic properties, as predicted by the DFT investigation. Consequently, the champion power conversion efficiency of 20.03% is achieved, one of the highest for this type of device. These findings provide a practicable strategy to theoretically design surface nucleation to induce preferential growth of perovskite material for better optoelectronic performance.

A photovoltaic layer consisting of bicontinuous phase-separated networks of polymer donor and Y6 acceptor along with highly ordered nano-sized Y6 aggregates can effectively enhance charge generation via hole transfer by increasing the intra-moiety excited states from acceptor domains, which enables a high efficiency of 17.98% in D18:Y6-based organic solar cells.

Abstract

The single bulk-heterojunction active layer based on non-fullerene acceptors (NFAs) has dominated the power conversional efficiencies above 18% in state-of-the-art organic solar cells (OSCs). However, a deep understanding of the relationship between charge carrier process and film microstructure remains unclear for emerging NFA OSCs. Herein, with the superstar PM6:Y6 blend as a model, the charge generation process in active layers is successfully manipulated by designing three different film microstructures, and they are correlated with the final photovoltaic performance in OSC devices. The amount of intermediate intra-moiety excited states from the nanoscale Y6 aggregates can be effectively enhanced by controlling the phase separation domains and film crystallinity in the bicontinuous PM6:Y6 networks. This robustly improves the hole transfer, and thus promotes charge generation. As a result, the optimal films show superior device performance, that is, the high efficiencies of 16.53% and 17.98% for PM6:Y6- and D18:Y6-based single junction OSCs, respectively. The results presented here give a rational guide for optimizing the charge carrier process through controlling morphological microstructures toward high-performance NFA OSCs.

SiO₂-coated gold nanorods (GNR@SiO₂) are introduced into perovskite film and take advantage of the plasmonic local heating effect for in situ strain relaxation. The GNR@SiO₂-incorporated perovskite solar cells (PVSCs) achieve a highest power conversion efficiency over 23% in plasmonic-incorporated PVSCs. Moreover, the intrinsic stability of the resulting PVSCs is greatly improved especially in the day-night working mode in a real-life situation.

Abstract

The residual strain induced during the growth of perovskite film is an intrinsic issue that significantly affects the efficiency and stability of perovskite solar cells (PVSCs). Inspired by the flipped annealing method to release strain in perovskite thin films, SiO₂-coated gold nanorods (GNR@SiO₂) are introduced into perovskite film and advantage of the plasmonic local heating effect is taken for in situ strain relaxation. The GNR@SiO₂-incorporated inverted PVSCs exhibit a champion power conversion efficiency (PCE) over 23%, which is the highest PCE in plasmonic-incorporated PVSCs. Moreover, the intrinsic stability of the resulting PVSCs is greatly improved for the nonencapsulated device and retains 84% of its initial PCE after 2800 h aging under continuous illumination at 65 ± 5 °C in an N₂ glove box and nearly 90% after 1000 h repetitive 12 h light on–off cycles. This work provides an efficient yet easy-to-implement plasmonic heating strategy for simultaneously enhancing the efficiency and stability of PVSCs.

A multifunctional sulfonate surfactant anchoring strategy is introduced for regulating perovskite precursor solution surface tension, retarding the perovskite crystallization and hindering the tilting of the grains, leading to full-coverage uniform large-area perovskite layers with high crystallinity as well as low trap densities. The target modules achieve power conversion efficiencies of 21.05% (stabilized power output around 20.4%) on active areas of 25.98 cm².

Abstract

Formamidinium lead triiodide-based perovskite solar cells have emerged as one of the most promising candidates that can be potentially used to develop photovoltaic technologies in the future. The commercial use of perovskite solar cell modules (PSCMs) is limited as it is challenging to fabricate high-quality, efficient, and stable large-area perovskite light-absorbing films. Heptadecafluorooctanesulfonic acid tetraethylammonium salt (HFSTT), containing fluorinated long alkyl chains as hydrophobic tails and sulfonic acid groups (SO3⁻) as hydrophilic heads, which exhibit a great synergistic potential in large-area film uniform fabrication, crystallization orientation modulation, defect passivation, and device operation stability enhancement, are introduced. The HFSTT-modified films exhibit a prominent (100) orientation and lower trap-state density as well as enhanced carrier mobilities and diffusion lengths, facilitating a champion unit device with an impressive power conversion efficiency (PCE) of 23.88% (0.14 cm²) and 22.52% (1 cm²) with a low voltage deficit around 0.341 V. The unencapsulated device retains ≈70% of its initial efficiency after 1000 h under heat damping test (60 °C and ≈60% RH). Moreover, the PSCMs exhibiting PCEs of 21.05% (with notable fill factor 0.79) and 18.27% are characterized by the active areas of 25.98 and 60.68 cm², respectively.

The piezoelectric properties of two types of metal halide perovskite solar cells (PSCs), the 3D and the 3D/2D structure are investigated. It is found that the 3D/2D structure has a considerably more intense piezoelectric features than the 3D-only one. The deep level transient spectroscopy measurement unveils that the Pb_Br defects play a crucial role in enhancing piezoelectric properties for the PSCs of the 3D/2D heterostructure, which unravels the correlation between the deep level defects and piezoelectric properties in PSCs.

Abstract

Metal halide perovskite solar cells (PSCs) have been considered to be one of the most promising next-generation energy harvesters over the past decades due to remarkably rapid improvement of power conversion efficiency in photovoltaics. However, energy harvesters based on the solar energy source have an intrinsic environment limitation for indoor applications. A feasible solution to the limitation is to add non-solar energy harvesting functions to the solar energy harvesters. Here, the piezoelectric properties of two types of metal halide PSCs are investigated, the 3D only and the 3D/2D structure, showing PCEs of 21.3% and 23.2%, respectively. Piezo-response force microscopy and synchrotron-based X-ray diffraction demonstrate that both types of PSC sample have piezoelectricity. Remarkably, the 3D/2D structure has considerably higher piezoelectric amplitude than the 3D-only. The deep level transient spectroscopy results reveal that the enhancement in the piezoelectricity of the 3D/2D structure originates from Pb_Br defects. This study unravels the role of defects in the piezoelectricity of metal halide PSCs and provides a direction to develop the multi-function energy harvesters based on the PSCs.

A versatile manufacturing technology, solvent-annealing assisted thermal evaporation (SATE), is introduced to effectively modulate organic film morphology as well as optoelectronic properties. The SATE method produces undoped spiro-OMeTAD layers with high density, good film homogeneity, enhanced conductivity, and remarkable film stability, which leads to a 36% enhancement of power conversion efficiency to 20.02% and remarkable stability. This work demonstrates that SATE can be generally applicable to controllable fabrication of organic thin film and reliable devices.

Abstract

Thermal evaporation (TE) as a scalable and low-cost technique for fabrication of organic hole transport materials (HTMs) typically produces low photovoltaic performance and poor device reproducibility in the application of perovskite solar cells (PSCs), and there is a clear need to understand the weaknesses of TE. Here, a versatile manufacturing technology, solvent-annealing assisted thermal evaporation (SATE), enabling effective modulation of organic film morphology as well as optoelectronic properties, is introduced. The SATE method produces undoped spiro-OMeTAD layers with high density, good film homogeneity, enhanced conductivity, and remarkable film stability, all of which are superior to that made by conventional TE. In addition, SATE films eliminate the dopant induced degradation mechanism and simultaneously improve the electrical conductivity of undoped HTMs. Significantly, the resulting devices yield a 36% enhancement of power conversion efficiency (PCE) from 14.68% (TE) to 20.02% (SATE), which is the highest reported PCE for evaporated HTMs in n–i–p PSCs. Moreover, unencapsulated PSC devices with SATE demonstrate an impressive environmental and thermal stability by maintaining 85% of initial performance after 2500 h in air with 30% humidity. The high efficiency with simultaneously improved stability demonstrates SATE can be generally applicable to controllable fabrication of organic thin film and reliable devices.

Phase-transfer-catalyzed LiTFSI doping in Spiro-OMeTAD is developed to address the negative impacts of doping-induced hygroscopicity and ion diffusion. Crowning Li⁺ ions in the hole-transporting layer enables perovskite solar cells with enhanced power conversion efficiency and significantly improves stability under humid and thermal conditions.

Abstract

State-of-the-art perovskite solar cells (PSCs) exhibit comparable power conversion efficiency (PCE) to that of silicon photovoltaic devices. However, the device stability remains a major obstacle that restricts widespread application. Doping-induced hygroscopicity, ion diffusion, and use of polar solvents in the hole-transport layer are detrimental factors for performance degradation of PSCs. Here, phase-transfer-catalyzed LiTFSI doping in Spiro-OMeTAD is developed to address these negative impacts. 12-Crown-4 as an efficient phase-transfer catalyst promotes the dissolution of LiTFSI without requiring acetonitrile. A combined experimental and theoretical study demonstrates the host–guest interaction between Li⁺ ions and 12-crown-4. Crowning Li⁺ ions by forming more stable and less diffusive crown-ether–Li⁺ complexes retards the generation of hygroscopic lithium oxides and mitigates Li⁺-ion migration. Optimized PSCs deliver enhanced PCE and significantly improved stability under humid and thermal conditions compared with a control device. This method can also be applied to dope π-conjugated polymer. The findings provide a facile avenue to improve the long-term stability of PSCs.

Oxygen vacancies (OVs) were managed by the lattice Mg and the interstitial Mg which modulated the formation and ionization of OVs for self-doping in SnO₂. OV management optimized the capability of high-temperature mesoporous SnO₂ to serve as well-performed ETLs for printable PSCs with efficiency enhanced from 6.62 % to 17.25 %.

Abstract

The planar SnO₂ electron transport layer (ETL) has contributed to the reported power conversion efficiency (PCE) record of perovskite solar cells (PSCs), while the high-temperature mesoporous SnO₂ ETL (mp-SnO₂) brings poor device performance. Herein, we report the application of mp-SnO₂ for efficient printable PSCs via oxygen vacancy (OV) management by introducing magnesium (Mg) into the paste. We find that high-temperature annealing suppresses self-doping of SnO₂ by reducing OVs. The introduced Mg occupies both the Sn site and interstitial site of SnO₂ and promotes the formation of OVs. Lattice Mg tends to induce neutral OVs and interstitial Mg could promote the ionization of neutral OVs for self-doping. The synergy effect on OVs increases the carrier density and upshifts the Fermi level energy of mp-SnO₂, ensuring its capability as the well-performed ETL with trap-less charge transport and suppressed surface recombination for dramatic improved device PCE from 6.62 % to 17.25 %.

Featuring two hydroxyls and one carbonyl, 1,3-dihydroxypropan-2-one is introduced to passivate the uncoordinated lead cations and organic components on the surface of perovskite films via coordination and hydrogen bonding interactions. The device efficiency is improved from 20.21% to 23.26%, as well as better storage and thermal stability.

To reduce the surface defects of perovskite film and hole extraction loss, an organic small molecule, 1,3-dihydroxypropan-2-one (DHA), is introduced at the perovskite/hole transport layer interface. Fourier transform infrared spectroscopy is conducted to reveal the interactions (coordination and hydrogen bonding) between DHA and perovskite. A variety of characterizations (including photoluminescence (PL), time-resolved photoluminescence (TRPL), electrochemical impedance spectroscopy (EIS), etc.) are performed to disclose the effect of DHA on device performance. Consequently, a high efficiency of 23.26% is obtained for the DHA-treated device, while the control device shows only a companion efficiency of 20.21%. Moreover, the DHA-treated devices exhibit improved storage and thermal stability.

Herein, a poly(3-hexylthiophene) (P3HT)-based all-polymer solar cell (all-PSC) with a record efficiency of 7.35% is achieved by blending P3HT with a thermodynamically miscible polymer acceptor DCNBT-IDT. Morphology characterization and the Flory–Huggins model unravel that the formation of well-mixed morphology with fibrillary structures due to the appropriate miscibillity between P3HT and DCNBT-IDT is the key to the unprecedented efficiency.

Poly(3-hexylthiophene) (P3HT) is the most classical conjugated polymer for organic photovoltaics due to its low-cost and synthetic scalability. However, P3HT-based organic photovoltaics suffer from inferior device performance with respect to donor–acceptor copolymers. Particularly, the device performance of P3HT-based all-polymer solar cells (all-PSCs) is rather poor due to the challenges in reaching ideal bulk-heterojunction morphology. Herein, highly efficient P3HT-based all-PSCs by blending P3HT with a thermodynamic miscible polymer acceptor are reported. Among the three state-of-the-art polymer acceptors (N2200, PYT, and DCNBT-IDT), N2200 and PYT are thermodynamically immiscible with P3HT and thus led to excessive phase separation when blended with P3HT, whereas DCNBT-IDT displayed proper thermodynamic miscibility with P3HT and generated the formation of well-mixed fibrillary active layer morphology. As a result, a power conversion efficiency of 7.35% has been achieved by P3HT:DCNBT-IDT blend, which is a new record for P3HT-based all-PSCs and largely higher than any previous results. Broad implication for further efficiency enhancement of P3HT-based all-PSCs is provided in the results and a promising pathway to realize highly efficient yet cost-effective solar energy production is suggested.

Nature Photonics, Published online: 07 April 2022; doi:10.1038/s41566-022-00985-1

The nucleation process is more thermodynamically difficult by involving a higher surface molecular unit ratio than the growth process of solution-processed halide perovskite (HP) crystals. Nucleation occurs under a higher supersaturation (C > C _min) while growth can occur when C is over C _s. Typical preparation methods for nucleation-dominated HP nanocrystals, growth-dominated HP single crystals, and nucleation-and-growth-balanced HP thin films are reviewed.

Abstract

Halide perovskite semiconductors with extraordinary optoelectronic properties have been fascinatedly studied. Halide perovskite nanocrystals, single crystals, and thin films have been prepared for various fields, such as light emission, light detection, and light harvesting. High-performance devices rely on high crystal quality determined by the nucleation and crystal growth process. Here, the fundamental understanding of the crystallization process driven by supersaturation of the solution is discussed and the methods for halide perovskite crystals are summarized. Supersaturation determines the proportion and the average Gibbs free energy changes for surface and volume molecular units involved in the spontaneous aggregation, which could be stable in the solution and induce homogeneous nucleation only when the solution exceeds a required minimum critical concentration (C _min). Crystal growth and heterogeneous nucleation are thermodynamically easier than homogeneous nucleation due to the existent surfaces. Nanocrystals are mainly prepared via the nucleation-dominated process by rapidly increasing the concentration over C _min, single crystals are mainly prepared via the growth-dominated process by keeping the concentration between solubility and C _min, while thin films are mainly prepared by compromising the nucleation and growth processes to ensure compactness and grain sizes. Typical strategies for preparing these three forms of halide perovskites are also reviewed.