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The nature of precursor solutions not only impacts the nucleation rate and crystallization kinetics of perovskite crystals, but also influences the physical properties of perovskite thin films. This Review presents the comprehensive understanding on the nature of perovskite precursor solutions and the formation mechanism of perovskite thin films from these precursor solutions.

Abstract

The composition, crystallinity, morphology, and trap‐state density of halide perovskite thin films critically depend on the nature of the precursor solution. A fundamental understanding of the liquid‐to‐solid transformation mechanism is thus essential to the fabrication of high‐quality thin films of halide perovskite crystals for applications such as high‐performance photovoltaics and is the topic of this Review. The roles of additives on the evolution of coordination complex species in the precursor solutions and the resulting effect on perovskite crystallization are presented. The influence of colloid characteristics, DMF/DMSO‐free solutions and the degradation of precursor solutions on the formation of perovskite crystals are also discussed. Finally, the general formation mechanism of perovskite thin films from precursor solutions is summarized and some questions for further research are provided.

Stability and toxicity are bottlenecks for halide perovskite solar cells despite their remarkable efficiency. Double halide perovskites with heterovalent metal cations pave a way for lead‐free‐based devices for enhanced stability. This Review summarizes the theoretical and experimental progress of lead‐free double perovskite. The issues, challenges, applications, and future prospects are integrated to provide a full picture.

Perovskite solar cells (PSCs) have achieved a high power conversion efficiency (PCE) with a credible certified value over 25%. More efforts have been devoted to the development of stable and ecofriendly perovskite materials. Lead‐free double perovskites (LFDPs) are a noteworthy choice as a photoactive layer because of their favorable photovoltaic (PV) properties, intrinsic chemical stability, and environmental friendliness. This Review presents various LFDP materials whose structural stability and optoelectronic properties are predicted by theoretical calculations. The synthesis and experimental properties of LFDPs and their applications in PSCs and optoelectronics in pursuing high performance, low toxicity, and functional stability are also reviewed. Perovskites active layers are critical for PSCs, and their appropriate properties are responsible for achieving a high PCE. On the other side, the stability of PSCs under working conditions is a critical requirement for their practical applications. Defect‐ordered perovskites are also presented to provide another outlook on lead‐free perovskite‐based PVs. The introduction and interest toward LFDP in PSCs can represent a viable solution to the toxicity issue, stimulate further research, and bring a real impact to future PV technologies.

A method of combined electron transporting bilayer is reported to reduce energy loss and inhibit defects in the perovskite solar cells (PSCs) by combining the commercially accessible SnO₂ and home‐made TiO₂ nanoparticles. Consequently, the PSCs devices acquire a high efficiency of 20.50%, which is superior to that based on SnO₂ layers with a efficiency of 18.09%.

Herein, commercially accessible SnO₂ and home‐made TiO₂ nanoparticles as a combined electron transporting bilayer (ETBL) are applied to achieve highly efficient planar perovskite solar cells (PSCs). With the formed cascade‐aligned energy levels from the proper stacking of SnO₂ and TiO₂ layers and the excellent defect‐passivation ability of TiO₂, SnO₂/TiO₂ ETBLs effectively reduce energy loss and inhibit defects formation both at the electron transporting layers (ETL)/perovskite interfaces and within the bulk of perovskite layer as revealed by a comprehensive analysis of photoelectric characteristic analysis, including ultraviolet photoelectron spectroscopy, photoluminescence, and electrochemical impedance spectroscopy. Consequently, the PSC devices acquired a power conversion efficiency (PCE) of 20.50% with a V _oc of 1.10 V, a J _sc of 24.2 mA cm⁻² and an fill factor of 77%, which are superior to the values of the control device based on single SnO₂ layer with a PCE of 18.09% (a 13.3% boosting on PCE). Moreover, there was no degradation after 49 days, indicating the great stability after adding TiO₂ layer. Herein, it is demonstrated that the cascaded alignment of energy levels between the electrode and perovskite layer by ETBLs could be an effective approach to improve the photovoltaic performance of the PSCs with excellent long‐term stability.

A two‐step annealing method is developed for studying the water effect on different kinds of perovskites. It is demonstrated that 60 °C is favorable to the formation of hydrate phase which leads to a reconstruction process in the second annealing stage. The corresponding water effects highly depend on the cations of the perovskite itself.

Water effect on perovskite solar cells has received growing interest in recent years. A widely accepted view is that moderate water content induces the formation of hydrate phase which enhances the recrystallization of the perovskite. However, the underlying factors which influence the formation of hydrate phase are yet to be investigated. Herein, by controlling the annealing temperature, it is demonstrated that 60 °C is the most suitable temperature for the formation of hydrated perovskite. After further annealing at 120 °C, the resulting perovskite film reveals enhanced crystallinity with a more uniform morphology, contributing to device efficiency above 20%. In addition, the water effect on different types of perovskites is studied and it is concluded that the formation of hydrated perovskite is mainly determined by the cations of the perovskite itself. The findings in this work elucidate the conditions for the formation of hydrated perovskite, contributing to the fabrication of highly efficient perovskite solar cells.

By combining the nonvolatile complexant solvent and a spin‐assisted solvent extraction crystallization method, high‐quality, large‐area perovskite films are prepared in air reproducibly to achieve high efficiency.

Perovskite solar cells (PSCs) are one of the highly promising new‐generation photovoltaic technologies. One of the remaining challenges for commercializing PSCs is to prepare high‐quality, large‐area perovskite films in an ambient atmosphere reproducibly using less toxic materials. Herein, a nonvolatile, less toxic substance, 2‐pyrrolidinone (2P), is used as a complexant solvent for the perovskite precursor solution. Combining this with a newly developed spin‐assisted solvent extraction (SASE) crystallization method, a high‐quality perovskite film is prepared in air reproducibly. The nonvolatile nature of 2P and the formation of MAI–PbI₂–2P adduct widen the time window for antisolvent engineering, and SASE enhances the solvent exchange rate of the precursor film to form a high‐quality perovskite film in air. An inverted cell (based on the postsolvent‐treated perovskite absorber) exhibits a power conversion efficiency of more than 18%. This film preparation method is feasible for depositing a high‐quality mixed‐cation or mixed‐cation, mixed‐halide perovskite film on both PEDOT:PSS and TiO₂ surfaces to fabricate inverted and regular PSCs, respectively, to achieve efficiencies of more than 16%. More importantly, high‐quality, large‐area MAPbI₃ film is prepared reproducibly in air for fabricating perovskite submodules to achieve an efficiency of 14% (verified at 13.04% after the cell is packed for efficiency evaluation).

Two novel nonfullerene acceptors (NFAs) 4TFIC‐4F and 4TCIC‐4F are designed based on fluorene and carbazole. Compared with 4TFIC‐4F, 4TCIC‐4F exhibits higher lowest unoccupied molecular orbital (LUMO) level and narrower optical bandgap. Therefore, polymer solar cells based on PBDB‐T‐2Cl:4TCIC‐4F achieve a high‐power conversion efficiency of 13.02%, which is the highest value for the carbazole‐containing NFAs‐based devices.

It is important to tune the energy levels of nonfullerene acceptors (NFAs) to achieve more balanced open‐circuit voltage (V _oc) and short‐circuit current density (J _sc) to improve the device performance. Herein, two novel NFAs are designed via fusing fluorene or carbazole with two thieno[3,2‐b]thiophene and end capped with INIC‐2F, namely, 4TFIC‐4F and 4TCIC‐4F, respectively. The impact of the fluorene and carbazole unit on the PSC performance is systematically studied. Compared with 4TFIC‐4F, 4TCIC‐4F exhibits a higher lowest unoccupied molecular orbital (LUMO) energy level of −3.95 eV and a narrower optical bandgap of 1.51 eV owing to the stronger electron‐donating capacity of fused‐carbazole ring core. Consequently, the 4TCIC‐4F device achieves a high power conversion efficiencies (PCE) of 13.02% with a higher V _oc of 0.94 V and a larger J _sc of 18.98 mA cm⁻², whereas the 4TFIC‐4F device shows a PCE of 11.24%. The PCE of 13.02% is the highest value so far reported with the carbazole‐containing NFAs‐based PSCs. More importantly, the 4TCIC‐4F device shows good film thickness insensitive and long‐term thermal stability. The investigation demonstrates that the fused‐carbazole ring is a superior option to fused‐fluorene ring as electron‐donating core for designing high‐performance NFAs by improving V _oc and J _sc simultaneously.

Using fluorine‐substituted benzotriazole (BTA) as the core building block, two novel donor–accepter–donor (D–A–D) structured hole‐transporting materials, 2FBTA‐1 and 2FBTA‐2, are synthesized and applied into perovskite solar cells, achieving a high power conversion efficiency of 17.94%.

Two novel donor–accepter–donor structured hole‐transporting materials based on fluorine‐substituted benzotriazole (BTA) core building blocks (2FBTA‐1 and 2FBTA‐2) are designed and synthesized through molecular regulation. Applying these materials into perovskite solar cells, power conversion efficiencies of 7.55% and 17.94% are obtained for 2FBTA‐1 and 2FBTA‐2, respectively. The better photovoltaic performance of 2FBTA‐2 is attributed to its more suitable energy level, more planar molecular configurations, and higher hole mobility. Moreover, the devices with 2FBTA‐2 as hole transport material (HTM) show good stability in air. The results indicate that BTA is a promising building block for future HTM design.

Hole‐transport material based on dibenzo[b,d]thiophene (DBTMT) is synthesized with low costs. A champion power conversion efficiency of the optimized p–i–n planar perovskite solar cells based on dopant‐free DBTMT reaches 21.12% with a high fill factor of 83.25%, due to good hole‐transport properties and the passivation effect of DBTMT.

N ²,N ²,N ⁸,N ⁸‐tetrakis(4‐(methylthio)phenyl)dibenzo[b,d]thiophene‐2,8‐diamine (DBTMT) is synthesized from three commercial monomers for application as a promising dopant‐free hole‐transport material (HTM) in perovskite solar cells (pero‐SCs). The intrinsic properties (optical properties and electronic energy levels) of DBTMT are investigated, proving that DBTMT is a suitable HTM for the planar p–i–n pero‐SCs. The champion power conversion efficiency (PCE) of the optimized pero‐SCs (with structure as ITO/pristine DBTMT/MAPbI₃/C₆₀/BCP/Ag) reaches 21.12% with a fill factor (FF) of 83.25%, which is among the highest PCEs and FFs reported for planar p–i–n pero‐SCs based on dopant‐free HTMs. The Fourier‐transform infrared spectroscopy, X‐ray diffraction, and X‐ray photoelectron spectroscopy spectra of MAPbI₃ and DBTMT–MAPbI₃ films demonstrate that there is an interaction between DBTMT and MAPbI₃ at the interface through the sulfur atoms in DBTMT to passivate the defects, which is corresponding to the higher FF and PCE of the corresponding device.

Herein, three polyfluorene copolymers (TFB, PFB, and PFO) are investigated as hole‐transport materials (HTMs) for the construction of inverted perovskite solar cells. The photovoltaic performance of the device is found to be closely correlated with the electronic properties of HTMs. The TFB‐based device exhibits the highest efficiency of 18.48% due to its high mobility and favored energy‐level alignment.

Inverted perovskite solar cells (PSCs) that can be entirely processed at low temperatures have attracted growing attention due to their cost‐effective production. Hole‐transport materials (HTMs) play an essential role in achieving efficient inverted PSCs, as they determine the effectiveness of charge extraction and recombination at interfaces. Herein, three polyfluorene copolymers (TFB, PFB, and PFO) are investigated as HTMs for construction of inverted PSCs. It is found that the photovoltaic performance of the solar cells is closely correlated with the electronic properties of the HTMs. Due to its high mobility along with the favored energy‐level alignment with perovskite, TFB shows superior charge extraction and suppressed interfacial recombination than PFB‐ and PFO‐based devices, which delivers a high efficiency of 18.48% with an open‐circuit voltage (V _OC) of up to 1.1 V. In contrast, the presence of a large energy barrier in the PFO‐based devices results in substantial losses in both V _OC and photocurrent. These results demonstrate that TFB can serve as a superior HTM for inverted PSCs. Moreover, it is anticipated that the performance of the three HTMs identified here might guide the molecular design of novel HTMs for the manufacture of highly efficient inverted PSCs.

In article number https://doi.org/10.1002/aenm.2019012681901268, Ning Cai, Sai‐Wing Tsang and co‐workers present a dopant‐free coplanar D‐π‐D hole‐transporting material which is deployed in inverted planar perovskite solar cells, achieving a high fill factor of 81.7% and an excellent power conversion efficiency of 19.16%. By comparison with a counterpart, a correlation between the molecular packing, hole mobility and device performance is revealed, providing insights for the rational design of hole‐transporting materials.

Spiro‐OMeTAD [2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine) 9,9′‐spirobifluorene], the most used solid‐state hole conductor in perovskite solar cells (PSCs), is usually processed in solution, but requires dopants for efficient charge transport. Here, dopant‐free Spiro‐OMeTAD layers prepared by vacuum sublimation are reported. Temperature control of the samples during evaporation induces crystalline and microstructural changes. The implementation in PSCs demonstrates unprecedented superior stability with respect to solution‐processed devices.

Abstract

The main handicap still hindering the eventual exploitation of organometal halide perovskite‐based solar cells is their poor stability under prolonged illumination, ambient conditions, and increased temperatures. This article shows for the first time the vacuum processing of the most widely used solid‐state hole conductor (SSHC), i.e., the Spiro‐OMeTAD [2,2′,7,7′‐tetrakis (N,N‐di‐p‐methoxyphenyl‐amine) 9,9′‐spirobifluorene], and how its dopant‐free crystalline formation unprecedently improves perovskite solar cell (PSC) stability under continuous illumination by about two orders of magnitude with respect to the solution‐processed reference and after annealing in air up to 200 °C. It is demonstrated that the control over the temperature of the samples during the vacuum deposition enhances the crystallinity of the SSHC, obtaining a preferential orientation along the π–π stacking direction. These results may represent a milestone toward the full vacuum processing of hybrid organic halide PSCs as well as light‐emitting diodes, with promising impacts on the development of durable devices. The microstructure, purity, and crystallinity of the vacuum sublimated Spiro‐OMeTAD layers are fully elucidated by applying an unparalleled set of complementary characterization techniques, including scanning electron microscopy, X‐ray diffraction, grazing‐incidence small‐angle X‐ray scattering and grazing‐incidence wide‐angle X‐ray scattering, X‐ray photoelectron spectroscopy, and Rutherford backscattering spectroscopy.

The efficiencies of semitransparent perovskite device and four‐terminal perovskite/silicon multijunction/tandem solar cells rise to 18.3% and 27.0%, respectively. This is the highest recorded efficiency for semitransparent perovskite solar cells thus far. The high efficiencies originate from good transparency and high conductivity of the nanomesh‐structured gold top electrode.

Abstract

Multijunction/tandem solar cells have naturally attracted great attention because they are not subject to the Shockley–Queisser limit. Perovskite solar cells are ideal candidates for the top cell in multijunction/tandem devices due to the high power conversion efficiency (PCE) and relatively low voltage loss. Herein, sandwiched gold nanomesh between MoO₃ layers is designed as a transparent electrode. The large surface tension of MoO₃ effectively improves wettability for gold, resulting in Frank–van der Merwe growth to produce an ultrathin gold nanomesh layer, which guarantees not only excellent conductivity but also great optical transparency, which is particularly important for a multijunction/tandem solar cell. The top MoO₃ layer reduces the reflection at the gold layer to further increase light transmission. As a result, the semitransparent perovskite cell shows an 18.3% efficiency, the highest reported for this type of device. When the semitransparent perovskite device is mechanically stacked with a heterojunction silicon solar cell of 23.3% PCE, it yields a combined efficiency of 27.0%, higher than those of both the sub‐cells. This breakthrough in elevating the efficiency of semitransparent and multijunction/tandem devices can help to break the Shockley–Queisser limit.

Gap states present a new approach to develop multi‐photon upconversion light emission in quasi‐2D perovskite films under continuous‐wave infrared excitation. Magneto‐photoluminescence (PL) and polarization‐dependent PL reveal that the gap states are essentially spatially extended states involved in orbit–orbit interaction toward generating multi‐photon excitation in quasi‐2D perovskite films.

Abstract

A new approach to generate a two‐photon up‐conversion photoluminescence (PL) by directly exciting the gap states with continuous‐wave (CW) infrared photoexcitation in solution‐processing quasi‐2D perovskite films [(PEA)₂(MA)₄Pb₅Br₁₆ with n = 5] is reported. Specifically, a visible PL peaked at 520 nm is observed with the quadratic power dependence by exciting the gap states with CW 980 nm laser excitation, indicating a two‐photon up‐conversion PL occurring in quasi‐2D perovskite films. Decreasing the gap states by reducing the n value leads to a dramatic decrease in the two‐photon up‐conversion PL signal. This confirms that the gap states are indeed responsible for generating the two‐photon up‐conversion PL in quasi‐2D perovskites. Furthermore, mechanical scratching indicates that the different‐n‐value nanoplates are essentially uniformly formed in the quasi‐2D perovskite films toward generating multi‐photon up‐conversion light emission. More importantly, the two‐photon up‐conversion PL is found to be sensitive to an external magnetic field, indicating that the gap states are essentially formed as spatially extended states ready for multi‐photon excitation. Polarization‐dependent up‐conversion PL studies reveal that the gap states experience the orbit–orbit interaction through Coulomb polarization to form spatially extended states toward developing multi‐photon up‐conversion light emission in quasi‐2D perovskites.

The elastic modulus of 3D perovskite is very close to that of human bones and the elastic modulus of 2D perovskite with long chains is close to that of cartilage. By reconstructing a crystal lattice with different A cations at the surface of perovskite films, a nature “bone‐joint” configuration is built in perovskite, which provides a cushioning effect to external stresses.

Abstract

To improve the photovoltaic performance (both efficiency and stability) in hybrid organic–inorganic halide perovskite solar cells, perovskite lattice distortion is investigated with regards to residual stress (and strain) in the polycrystalline thin films. It is revealed that residual stress is concentrated at the surface of the as‐prepared film, and an efficient method is further developed to release this interfacial stress by A site cation alloying. This results in lattice reconstruction at the surface of polycrystalline thin films, which in turn results in low elastic modulus. Thus, a “bone‐joint” configuration is constructed within the interface between the absorber and the carrier transport layer, which improves device performance substantially. The resultant photovoltaic devices exhibit an efficiency of 21.48% with good humidity stability and improved resistance against thermal cycling.