Chen Weijie

The role of cation and halide mixing is revealed using in situ X‐ray scattering measurements during spin‐coating. Modulating the cation/halide composition directly impacts the lifetime of the sol–gel precursor film and its easy and reproducible conversion to the perovskite phase to yield solar cells with 20% power conversion efficiency.

Abstract

Perovskite solar cells increasingly feature mixed‐halide mixed‐cation compounds (FA_{1− x − y} MA _x Cs _y PbI_{3− z} Br_z) as photovoltaic absorbers, as they enable easier processing and improved stability. Here, the underlying reasons for ease of processing are revealed. It is found that halide and cation engineering leads to a systematic widening of the anti‐solvent processing window for the fabrication of high‐quality films and efficient solar cells. This window widens from seconds, in the case of single cation/halide systems (e.g., MAPbI₃, FAPbI₃, and FAPbBr₃), to several minutes for mixed systems. In situ X‐ray diffraction studies reveal that the processing window is closely related to the crystallization of the disordered sol–gel and to the number of crystalline byproducts; the processing window therefore depends directly on the precise cation/halide composition. Moreover, anti‐solvent dripping is shown to promote the desired perovskite phase with careful formulation. The processing window of perovskite solar cells, as defined by the latest time the anti‐solvent drip yields efficient solar cells, broadened with the increasing complexity of cation/halide content. This behavior is ascribed to kinetic stabilization of sol–gel state through cation/halide engineering. This provides guidelines for designing new formulations, aimed at formation of the perovskite phase, ultimately resulting in high‐efficiency perovskite solar cells produced with ease and with high reproducibility.

Chlorine‐substituted graphdiyne (ClGD) is employed into electron transport layers of MAPbI₃‐based perovskite solar cells. It is experimentally and theoretically demonstrated that the interactions of derivated graphdiyne and PCBM stem from four types of noncovalent bonds, which contribute to the improved device performance. Perovskite solar cells based on the ClGD‐PCBM obtain an enhanced power conversion efficiency (PCE) of 20.34%.

Chlorine‐substituted graphdiyne (ClGD) is employed into electron transport layers (ETLs) of MAPbI₃‐based perovskite solar cells for the first time, forming a high‐quality film with superior film morphology and electrical conductivity as compared with pristine [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) film. Strikingly, a champion power conversion efficiency of 20.34% is achieved, showing a 19% enhancement compared with the counterparts (17.08%). Simultaneously, ClGD‐PCBM‐based devices show suppressed J–V hysteresis. It is experimentally and theoretically demonstrated that the interactions of derivated graphdiyne and PCBM stem from four types of noncovalent bonds, which contribute to the improved device performance. The results suggest that derivated graphdiyne‐based interfacial material is promising for the applications in solar cells and other photoelectric devices.

Nature Energy, Published online: 17 June 2019; doi:10.1038/s41560-019-0400-8

Publication date: August 2019

Source: Nano Energy, Volume 62

Author(s): Sanping Wu, Jingjing Jiang, Shaotang Yu, Yuancai Gong, Weibo Yan, Hao Xin, Wei Huang

Graphical abstract

The different aggregation forms of hole‐transporting materials (HTMs) affect intermolecular charge transfer and hole transporting in achieving highly efficient dopant‐free perovskite solar cells. The combination of twisted periphery groups with planar core units shows an efficient approach to regulate the state of molecular aggregation after a systematical investigation of 6,12‐dihydroindeno[1,2‐b]fluorine (IDF)‐HTMs with the same IDF core and the different periphery groups.

Abstract

Although several hole‐transporting materials (HTMs) have been designed to obtain perovskite solar cells (PSCs) devices with high performance, the dopant‐free HTMs for efficient and stable PSCs remain rare. Herein, a rigid planar 6,12‐dihydroindeno[1,2‐b]fluorine (IDF) core with different numbers of bulky periphery groups to construct dopant‐free HTMs of IDF‐SFXPh, IDF‐DiDPA, and IDF‐TeDPA is modified. Thanks to the contributions of the planar IDF core and the twisted SFX periphery groups, the dopant‐free IDF‐SFXPh‐based PSCs device achieves a device performance of 17.6%, comparable to the doped 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD)‐based device (17.6%), with much enhanced device stability under glovebox and ambient conditions.

Sequentially processed organic photovoltaics (OPVs) using temperature‐dependent aggregation polymers where the acceptor materials have been processed using various nonorthogonal solvents provide almost similar performance in every single case. The superior performance when compared to their blend‐casting counterparts can be attributed to better control in morphology, which is critical for the large‐area scale‐up of OPVs.

Abstract

The conventional method to prepare bulk‐heterojunction organic photovoltaics (OPVs) is a one‐step method from the blend solution of donor and acceptor materials, known as blend‐casting (BC). Recently, an alternative method was demonstrated to achieve high efficiencies (13%) comparable to state‐of‐the‐art BC devices. This two‐step‐coating method, known as “sequential processing,” (SqP) involves sequential deposition of the donor and then the acceptor from two orthogonal solvents. However, the requirement of orthogonal solvents to process the donor and acceptor constrains the choice of materials and processing solvents. In this paper, an improved version of SqP method without the need for using orthogonal solvents is reported. The success is based on donor polymers with strong temperature‐dependent aggregation properties whose solution can be processed at a high temperature, but the resulting film becomes completely insoluble at room temperature, which allows for the processing of overlying acceptors from a wide range of nonorthogonal solvents. With this approach, efficient SqP OPVs is demonstrated based on a range of donor/acceptor materials and processing solvents, and, in every single case, SqP OPVs can outperform their BC counterparts. The results broaden the solvent choices and open a much larger window to optimize the processing parameters of SqP method.

O‐IDTBCN is a new nonfullerene acceptor that uses dicyanovinyl end groups to improve the electron mobility in blends with PTB7‐Th, relative to its predecessor, O‐IDTBR. Blends with O‐IDTBCN possess more balanced charge carrier mobilities, resulting in longer charge carrier lifetimes, which ultimately manifests in the attainment of fill factors of over 70% in devices.

Abstract

High fill factors have only recently become commonplace in nonfullerene‐based organic solar cells, with the balance of charge carrier mobilities often cited as the contributing factor. Here an end‐group modification to a commonly used nonfullerene acceptor (O‐IDTBR) is reported, in which the rhodanine end groups are replaced with dicyano moieties, resulting in the acceptor O‐IDTBCN. This new acceptor affords significant improvement in the fill factor (73%) and photocurrent (19.8 mA cm⁻²) in organic solar cells with the low bandgap polymer PTB7‐Th. A narrowing of the bandgap, as a result of greater push–pull hybridization, allows complementary absorption to the donor and thus improved photon harvesting. Additionally, the measurement of charge carrier mobilities and lifetimes in both systems reveal that the PTB7‐Th:O‐IDTBCN blend possesses more balanced charge carrier mobilities, and longer lifetimes. Morphology studies reveal a slightly greater degree of molecular mixing of the O‐IDTBCN when blended with PTB7‐Th, despite the greater and more balanced charge carrier mobilities in this blend.

In this contribution, the detailed pathways for the sequential deposition of FAPbI₃ are investigated using in situ X‐ray techniques and the influence of additive ions on the crystallization and grain orientation of the resultant perovskite films is revealed; the optimal film preparation conditions are obtained through in situ experimental results.

Abstract

Metal halide perovskites have revolutionized the development of highly efficient, solution‐processable solar cells. Further advancements rely on improving perovskite film qualities through a better understanding of the underlying growth mechanism. Here, a systematic in situ grazing‐incidence X‐ray diffraction investigation is performed, facilitated by other techniques, on the sequential deposition of formamidinium lead iodide (FAPbI₃)‐based perovskite films. The active chemical reaction, composition distribution, phase transition, and crystal grain orientation are all visualized following the entire perovskite formation process. Furthermore, the influences of additive ions on the crystallization speed, grain orientation, and morphology of FAPbI₃‐based films, along with their photovoltaic performances, are fully evaluated and optimized, which leads to highly reproducible and efficient perovskite solar cells. The findings provide key insights into the perovskite growth mechanism and suggest the fabrication of high‐quality perovskite films for widespread optoelectronic applications.

Metal oxides are used as charge transporting layers to effectively separate the photogenerated electrons and holes in perovskite solar cells (PSCs). The metal oxide layers require a wide bandgap, a good charge mobility, and a compatible band alignment with the perovskite layers. This review summarizes and correlates the preparation and performance of the various metal oxides used in PSCs.

Abstract

Currently, the efficiency of perovskite solar cells (PSCs) is ≈24%. For the fabrication of such high efficiency PSCs, it is necessary to use both electron and hole transport layers to effectively separate the charges generated by light absorption of the perovskite layer and selectively transfer the separated electrons and holes. In addition to the efficiency, the materials used for transporting charges must be resilient to light, heat, and moisture to ensure long‐term stability of PSCs; furthermore, low‐cost fabrication is required to form a charge transport layer at low temperatures by a solution process. For this purpose, metal oxides are best suited as charge transport materials for PSCs because of their advantages such as low cost, long‐term stability, and high efficiency. In this Review, the metal oxide electron and hole transport materials used in PSCs are reviewed and preparation of these materials is summarized. Finally, the challenges and future research direction for metal oxide‐based charge transport materials are described.

An energetic cascade between mixed and pure regions assists in suppressing recombination losses in nonfullerene acceptor (NFA)‐based organic solar cells. The impact of polymer–NFA blend composition upon film morphology, energetics, charge carrier recombination kinetics, and photocurrent properties is studied.

Abstract

Here, it is investigated whether an energetic cascade between mixed and pure regions assists in suppressing recombination losses in non‐fullerene acceptor (NFA)‐based organic solar cells. The impact of polymer‐NFA blend composition upon morphology, energetics, charge carrier recombination kinetics, and photocurrent properties are studied. By changing film composition, morphological structures are varied from consisting of highly intermixed polymer‐NFA phases to consisting of both intermixed and pure phase. Cyclic voltammetry is employed to investigate the impact of blend morphology upon NFA lowest unoccupied molecular orbital (LUMO) level energetics. Transient absorption spectroscopy reveals the importance of an energetic cascade between mixed and pure phases in the electron–hole dynamics in order to well separate spatially localized electron–hole pairs. Raman spectroscopy is used to investigate the origin of energetic shift of NFA LUMO levels. It appears that the increase in NFA electron affinity in pure phases relative to mixed phases is correlated with a transition from a relatively planar backbone structure of NFA in pure, aggregated phases, to a more twisted structure in molecularly mixed phases. The studies focus on addressing whether aggregation‐dependent acceptor LUMO level energetics are a general design requirement for both fullerene and NFAs, and quantifying the magnitude, origin, and impact of such energetic shifts upon device performance.

Publication date: August 2019

Source: Nano Energy, Volume 62

Author(s): Daniele Benetti, Efat Jokar, Che-Hsun Yu, Amir Fathi, Haiguang Zhao, Alberto Vomiero, Eric Wei-Guang Diau, Federico Rosei

Graphical abstract

Two novel hole‐transporting materials (HTMs) based on 9‐(4‐methoxyphenyl) carbazole and benzodithiophene cores are synthesized. The impact of these cores on the physicochemical properties and performance of perovskite solar cells (PSCs) based on these HTMs are investigated. The newly developed N1,N1′‐(9‐(4‐methoxyphenyl)‐9H‐carbazole‐3,6‐diyl)bis(N1‐(4‐(bis(4‐methoxyphenyl)amino)phenyl)‐N4,N4‐bis(4‐methoxyphenyl)benzene‐1,4‐diamine) (PhCz‐4MeOTPA)‐based PSC exhibits a power conversion efficiency of 16.04% along with enhanced stability under heat and illumination.

Perovskite solar cells (PSCs) possess both high‐power conversion efficiency (PCE) and good operation stability for future application. Although many different types of hole‐transporting materials (HTMs) are assessed, few dopant‐free small organic molecule HTMs‐based PSC cells exist, which exhibit excellent stability under both heat and illumination. Herein, two novel HTMs that are based on 9‐(4‐methoxyphenyl) carbazole and benzodithiophene cores are synthesized and named N1,N1′‐(9‐(4‐methoxyphenyl)‐9H‐carbazole‐3,6‐diyl)bis(N1‐(4‐(bis(4‐methoxyphenyl)amino)phenyl)‐N4,N4‐bis(4‐methoxyphenyl)benzene‐1,4‐diamine) (PhCz‐4MeOTPA) and N1,N1′‐(benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl)bis(N1‐(4‐(bis(4‐methoxyphenyl)amino)phenyl)‐N4,N4‐bis(4‐methixyphenyl)benzene‐1,4‐diamine) (BDT‐4MeOTPA). Of the two HTMs, PhCz‐4MeOTPA possesses a lower level of planarity than that of BDT‐4MeOTPA, which inhibits molecular stacking to improve film quality and increases hole‐transport mobility and charge transport. A PCE of 16.04% is achieved with the application of dopant‐free PhCz‐4MeOTPA in PSCs, which is higher than that of dopant‐free BDT‐4MeOTPA. The unencapsulated PSC devices based on PhCz‐4MeOTPA maintain 82% of their initial values under continuous sun illumination in an ambient environment at 40–45 °C after 672 h and 92% of their initial values at 80 °C in an ambient environment after 1200 h in the dark.

Perovskite solar cells are found to be exceptionally susceptible to potential‐induced degradation (PID) with performance losses up to 95% after 18 h of high‐voltage stress. Still, most of the lost performance can be regained by reversing the polarity of the applied high voltage.

In recent years, metal halide perovskite solar cells have become a major competitor in the run to lower the levelized cost of electricity (LCOE) of photovoltaic (PV) systems. Commercialization of this new technology mainly depends on the long‐term stability of such devices, for which potential‐induced degradation (PID) may represent a factor of detrimental impact. As PID can trigger rapid and significant losses in PV systems, it is generally considered among the most critical failure modes with a high financial repercussion. Herein, the results of PID tests on perovskite solar cells are reported for the very first time. The solar cells are found to be extremely susceptible to PID: 18 h of high‐voltage stress, according to the PID test standard IEC 62804‐1 TS (foil method at 60 °C), shows a performance degradation of up to 95%, which mainly results from a decrease in the short‐circuit current. These results also uncover near full PID recoverability and pave the way toward further research into its mechanisms, kinetics, and mitigation.

A kinetically enduring Lewis acid–base reaction between the additives from the solution‐processed hole‐transport layer occurs, and perovskite manipulates both the energy‐band alignment and the charge transfer at the interface, accounting for the short‐term evolution of figures of merit in perovskite solar cells.

Device stability is the most important issue that hinders perovskite solar cell (PSC) commercialization, given the achieved efficiency of PSC that exceeds 23%. The perovskite materials and contact layers throughout the device stack are scrutinized with regard to stability, but research has mainly focused on the examination of long‐term behavior. Herein, the impacts on the short‐term stability of PSCs, which are always overlooked, are investigated. The short‐term stability of the PSCs is correlated to the additives of the solution‐processed hole‐transport layer. These additives exert a critical impact underneath the perovskite layer. A kinetically enduring Lewis acid–base reaction between the additives and the perovskite manipulates both the energy‐band alignment and the charge transfer at the interface, accounting for short‐term evolution of figures of merit in PSCs. Our revelation of the impacts on the short‐term stability of PSCs calls for a re‐evaluation of interface modification induced by solution‐process engineering, thereby compensating for the overall device stability and allowing for progress toward commercialization.

Sulfur‐incorporated bismuth‐based perovskite films are obtained by a low‐pressure vapor‐assisted solution process (LP‐VASP) method. A homogeneous and highly compact MBI film with a narrower bandgap of 1.67 eV is successfully achieved. In addition, the obtained film has a low trap‐state density of 1.9 × 10¹⁶ cm⁻³ and the optimal PCE of MA₃Bi₂I_9‐2x S _x PSCs reached 0.152%.

Methylammonium (MA) bismuth iodide ((CH₃NH₃)₃Bi₂I₉) is a promising perovskite material for solar cell application considering the air stability and the nontoxic lead‐free molecular constitution. However, the further improvement of the device performances is prohibited by the wide bandgap (≈2.1 eV) and unsatisfied crystallinity of the (CH₃NH₃)₃Bi₂I₉ films. Herein, a developed low‐pressure vapor‐assisted solution process (LP‐VASP) method is applied to obtain the sulfur‐incorporated bismuth‐based perovskites films. Due to the presence of sulfur, both the crystal quality and the energy band property are improved effectively in the as‐fabricated lead‐free perovskite films. After a systematic study of the influence of the reaction time on the device performances, the optimized reaction time is found to be 30 min, under which, the sulfur‐incorporated MA₃Bi₂I_9‐2x S _x perovskite films exhibit a reduced bandgap of 1.67 eV and a compact morphology. The corresponding optimal PCE reaches 0.152%. This study provides a new way for the incorporation of sulfur in the lead‐free bismuth‐based perovskite solar cells.

Photostability is one of the most vital challenges for perovskite solar cells (PSCs). With the embedding of black phosphorus (BP), well known for its self‐healing and superior property to regulate charge recombination, into MAPbI₃ perovskites, the associated devices exhibit significant enhancement in photostability. The incorporation of BP effectively inhibits Pb⁰ defect formation and retards hot carrier recombination.

Photostability is one of the most vital challenges for organic–inorganic hybrid perovskite solar cells (PSCs). With the incorporation of black phosphorus (BP), well known for self‐healing and its superior property to regulate charge recombination, into CH₃NH₃PbI₃ perovskites (MAPbI₃/BP), the associated PSCs exhibit significant enhancement in photostability in addition to the photovoltaic (PV) performance. The MAPbI₃/BP‐based PSCs retain ≈94% of initial efficiency after 1000 h continuous white light LED illumination in a dry N₂ glovebox whereas their counterparts without the incorporation of BP decrease to ≈30%. Although BP has very small influence on the morphology and structure of the perovskite crystals, Pb⁰ defects are effectively inhibited and hot carrier recombination is found to be retarded as confirmed by femtosecond optical spectroscopy. The utilization of the material to simultaneously inhibit Pb⁰ defect formation and retard charge recombination, such as BP, is a promising strategy to enhance the photostability of organic–inorganic hybrid perovskite‐based PSCs and their siblings.

Eliminating the use of toxic, halogenated antisolvents in perovskite film preparation is long desired. This work demonstrates the use of a halogen‐free, mixed antisolvent composed of ethyl acetate and hexane to boost the efficiency of tin oxide (SnO₂)–triple cation system beyond 20% without the use of additional passivation layer.

Abstract

Solution‐processed triple‐cation perovskite solar cells (PSCs) rely on complex compositional engineering or delicate interfacial passivation to balance the trade‐off between cell efficiency and long‐term stability. Herein, the facile fabrication of highly efficient, stable, and hysteresis‐free tin oxide (SnO₂)‐based PSCs is demonstrated with a champion cell efficiency of 20.06% using a green, halogen‐free antisolvent. The antisolvent, composed of ethyl acetate (EA) solvent and hexane (Hex) in different proportions, works exquisitely in regulating perovskite crystal growth and passivating grain boundaries, leading to the formation of a crack‐free perovskite film with enlarged grain size. The high quality perovskite film inhibits carrier recombination and substantially improves the cell efficiency, without requiring an additional enhancer/passivation layer. Furthermore, these PSCs also demonstrate remarkable long‐term stability, whereby unencapsulated cells exhibit a power conversion efficiency (PCE) retention of ≈71% after >1500 hours of storage under ambient condition. For encapsulated cells, an astounding PCE retention of >98% is recorded after >3000 hours of storage in air. Overall, this work realizes the fabrication of SnO₂‐based PSCs with a performance greater or comparable to the state‐of‐the‐art PSCs produced with halogenated antisolvents. Evidently, EA–Hex antisolvent can be an extraordinary halogen‐free alternative in maximizing the performance of PSCs.

Germanium bromide hybrid perovskites are demonstrated as a promising, nontoxic alternative to lead‐based ones. The Ruddledsen–Popper series (CH₃(CH₂)₃NH₃)₂(CH₃NH₃) _{n −1}Ge _n Br_{3 n +1} is studied with a combination of experiments and first principle calculations, highlighting the peculiarity of this materials class, where the optical bandgap is weakly affected by 2D confinement and the highly stereochemically active 4s² lone pair preludes to ferroelectricity.

Abstract

Metal halide perovskites are maturing as materials for efficient, yet low cost solar cells and light‐emitting diodes, with improving operational stability and reliability. To date however, most perovskite‐based devices contain Pb, which poses environmental concerns due to its toxicity; lead‐free alternatives are of importance to facilitate the development of perovskite‐based devices. Here, the germanium‐based Ruddledsen–Popper series (CH₃(CH₂)₃NH₃)₂(CH₃NH₃) _{n −1}Ge _n Br_{3 n +1} is investigated, derived from the parent 3D (n = ∞) CH₃NH₃GeBr₃ perovskite. Divalent germanium is a promising, nontoxic alternative to Pb²⁺ and the layered, 2D structure appears promising to bolster light emission, long‐term durability, and moisture tolerance. The work, which combines experiments and first principle calculations, highlights that in germanium bromide perovskites the optical bandgap is weakly affected by 2D confinement and the highly stereochemically active 4s² lone pair preludes to possible ferroelectricity, a topic still debated in Pb‐containing compounds.

In this contribution, the detailed pathways for the sequential deposition of FAPbI₃ are investigated using in situ X‐ray techniques and the influence of additive ions on the crystallization and grain orientation of the resultant perovskite films is revealed; the optimal film preparation conditions are obtained through in situ experimental results.

Abstract

Metal halide perovskites have revolutionized the development of highly efficient, solution‐processable solar cells. Further advancements rely on improving perovskite film qualities through a better understanding of the underlying growth mechanism. Here, a systematic in situ grazing‐incidence X‐ray diffraction investigation is performed, facilitated by other techniques, on the sequential deposition of formamidinium lead iodide (FAPbI₃)‐based perovskite films. The active chemical reaction, composition distribution, phase transition, and crystal grain orientation are all visualized following the entire perovskite formation process. Furthermore, the influences of additive ions on the crystallization speed, grain orientation, and morphology of FAPbI₃‐based films, along with their photovoltaic performances, are fully evaluated and optimized, which leads to highly reproducible and efficient perovskite solar cells. The findings provide key insights into the perovskite growth mechanism and suggest the fabrication of high‐quality perovskite films for widespread optoelectronic applications.

Metal oxides are used as charge transporting layers to effectively separate the photogenerated electrons and holes in perovskite solar cells (PSCs). The metal oxide layers require a wide bandgap, a good charge mobility, and a compatible band alignment with the perovskite layers. This review summarizes and correlates the preparation and performance of the various metal oxides used in PSCs.

Abstract

Currently, the efficiency of perovskite solar cells (PSCs) is ≈24%. For the fabrication of such high efficiency PSCs, it is necessary to use both electron and hole transport layers to effectively separate the charges generated by light absorption of the perovskite layer and selectively transfer the separated electrons and holes. In addition to the efficiency, the materials used for transporting charges must be resilient to light, heat, and moisture to ensure long‐term stability of PSCs; furthermore, low‐cost fabrication is required to form a charge transport layer at low temperatures by a solution process. For this purpose, metal oxides are best suited as charge transport materials for PSCs because of their advantages such as low cost, long‐term stability, and high efficiency. In this Review, the metal oxide electron and hole transport materials used in PSCs are reviewed and preparation of these materials is summarized. Finally, the challenges and future research direction for metal oxide‐based charge transport materials are described.

2‐Thiophenemethylammonium spacer cations are successfully embedded into formamidinium iodide (FAI)‐ and methylammonium iodide (MAI)‐based 3D perovskites, and these cations can induce the crystalline growth and orientation of the obtained 2D/3D hybrid perovskite. A champion efficiency of 21.49% is demonstrated for a 2D/3D perovskite device, which is combined with a dramatically improved stability in comparison with that of the control device.

Abstract

Highly efficient and stable 2D/3D hybrid perovskite solar cells using 2‐thiophenemethylammonium (ThMA) as the spacer cation are successfully demonstrated. It is found that the incorporation of ThMA spacer cation into 3D perovskite, which forms a 2D/3D hybrid structure, can effectively induce the crystalline growth and orientation, passivate the trap states, and hinder the ion motion, resulting in improved carrier lifetime and reduced recombination losses. The optimized device exhibits a power conversion efficiency (PCE) of 21.49%, combined with a high V _OC of 1.16 V and a notable fill factor (FF) of 81%. More importantly, an encapsulated 2D/3D hybrid perovskite device sustains ≈99% of its initial PCE after 1680 h in the ambient atmosphere, whereas the control 3D perovskite device drops to ≈80% of the original performance. Importantly, the device stability under continuous light soaking (100 mW cm⁻²) is enhanced significantly for 2D/3D perovskite device in comparison with that of the control device. These results reveal excellent photovoltaic properties and intrinsic stabilities of the 2D/3D hybrid perovskites using ThMA as the spacer cation.

In this contribution, the detailed pathways for the sequential deposition of FAPbI₃ are investigated using in situ X‐ray techniques and the influence of additive ions on the crystallization and grain orientation of the resultant perovskite films is revealed; the optimal film preparation conditions are obtained through in situ experimental results.

Abstract

Metal halide perovskites have revolutionized the development of highly efficient, solution‐processable solar cells. Further advancements rely on improving perovskite film qualities through a better understanding of the underlying growth mechanism. Here, a systematic in situ grazing‐incidence X‐ray diffraction investigation is performed, facilitated by other techniques, on the sequential deposition of formamidinium lead iodide (FAPbI₃)‐based perovskite films. The active chemical reaction, composition distribution, phase transition, and crystal grain orientation are all visualized following the entire perovskite formation process. Furthermore, the influences of additive ions on the crystallization speed, grain orientation, and morphology of FAPbI₃‐based films, along with their photovoltaic performances, are fully evaluated and optimized, which leads to highly reproducible and efficient perovskite solar cells. The findings provide key insights into the perovskite growth mechanism and suggest the fabrication of high‐quality perovskite films for widespread optoelectronic applications.

Metal oxides are used as charge transporting layers to effectively separate the photogenerated electrons and holes in perovskite solar cells (PSCs). The metal oxide layers require a wide bandgap, a good charge mobility, and a compatible band alignment with the perovskite layers. This review summarizes and correlates the preparation and performance of the various metal oxides used in PSCs.

Abstract

Currently, the efficiency of perovskite solar cells (PSCs) is ≈24%. For the fabrication of such high efficiency PSCs, it is necessary to use both electron and hole transport layers to effectively separate the charges generated by light absorption of the perovskite layer and selectively transfer the separated electrons and holes. In addition to the efficiency, the materials used for transporting charges must be resilient to light, heat, and moisture to ensure long‐term stability of PSCs; furthermore, low‐cost fabrication is required to form a charge transport layer at low temperatures by a solution process. For this purpose, metal oxides are best suited as charge transport materials for PSCs because of their advantages such as low cost, long‐term stability, and high efficiency. In this Review, the metal oxide electron and hole transport materials used in PSCs are reviewed and preparation of these materials is summarized. Finally, the challenges and future research direction for metal oxide‐based charge transport materials are described.