Chen Weijie

Au@CdS fills in the perovskite grain boundaries to form an Au@CdS–PbI₂ intermediate, which increases the valence band maximum of Spiro‐OMeTAD for a more favorable energy alignment with perovskite. With the help of the localized surface plasmon resonance of Au@CdS, the holes easily overcome the interface barrier through the bridge of the intermediate Au@CdS–PbI₂. The corresponding device shows high performance.

Abstract

The plasmonic characteristic of core–shell nanomaterials can effectively improve exciton‐generation/dissociation and carrier‐transfer/collection. In this work, a new strategy based on core–shell Au@CdS nanospheres is introduced to passivate perovskite grain boundaries (GBs) and the perovskite/hole transport layer interface via an antisolvent process. These core–shell Au@CdS nanoparticles can trigger heterogeneous nucleation of the perovskite precursor for high‐quality perovskite films through the formation of the intermediate Au@CdS–PbI₂ adduct, which can lower the valence band maximum of the 2,2,7,7‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine)9,9‐spirobifluorene (Spiro‐OMeTAD) for a more favorable energy alignment with the perovskite material. With the help of the localized surface plasmon resonance effect of Au@CdS, holes can easily overcome the barrier at the perovskite/Spiro‐OMeTAD interface (or GBs) through the bridge of the intermediate Au@CdS–PbI₂, avoiding the carrier accumulation, and suppress the carrier trap recombination at the Spiro‐OMeTAD/perovskite interface. Consequently, the Au@CdS‐based perovskite solar cell device achieves a high efficiency of over 21%, with excellent stability of ≈90% retention of initial power conversion efficiencies after 45 days storage in dry air.

This study pictures the complex morphological evolution of perovskite thin films when organic cations of different size and concentration are blended together and proposes an effective solution, enables stable performance for tens of hours at the maximum power point, without encapsulation, at 50% relative humidity.

Abstract

The intrinsic instability of lead halide perovskite semiconductors in an ambient atmosphere is one of the most critical issues that impedes perovskite solar cell commercialization. To overcome it, the use of bulky organic spacers has emerged as a promising solution. The resulting perovskite thin films present complex morphologies, difficult to predict, which can directly affect the device efficiency. Here, by combining in‐depth morphological and spectroscopic characterization, it is shown that both the ionic size and the relative concentration of the organic cation, drive the integration of bulky organic cations into the crystal unit cell and the thin film, inducing different perovskite phases and different vertical distribution, then causing a significant change in the final thin film morphology. Based on these studies, a fine‐engineered perovskite is constructed by employing two different large cations, namely, ethyl ammonium and butyl ammonium. The first one takes part in the 3D perovskite phase formation, the second one works as a surface modifier by forming a passivating layer on top of the thin film. Together they lead to improved photovoltaic performance and device stability when tested in air under continuous illumination. These findings propose a general approach to achieve reliability in perovskite‐based optoelectronic devices.

By introduction of a multiple ligand (pentaerythritol tetrakis(3‐mercaptopropionate)), uncoordinated Pb²⁺ and Pb⁰ defects are simultaneously passivated. Meanwhile, better energy level matching between the valence band of perovskite and the highest occupied molecular orbital of the HTM is achieved. As a result, perovskite solar cell efficiency increases from 19.0% to 21.4% after surface passivation by multiple ligands.

Abstract

In the past decade, the efficiency of perovskite solar cells quickly increased from 3.8% to 25.2%. The quality of perovskite films plays vital role in device performance. The films fabricated by solution‐process are usually polycrystalline, with significantly higher defect density than that of single crystal. One kind of defect in the films is uncoordinated Pb²⁺, which is usually generated during thermal annealing process due to the volatile organic component. Another detrimental kind of defect is Pb⁰, which is often observed during the film fabrication process or solar cell operation. Because the open circuit voltage has a close relation with the defect density, it is thus desirable to passivate these two kinds of defects. Here, a molecule with multiple ligands is introduced, which not only passivates the uncoordinated Pb²⁺ defects, but also suppresses the formation of Pb⁰ defects. Meanwhile, such a treatment improves the energy level alignment between the valence band of perovskite and the highest occupied molecular orbital of spiro‐OMeTAD. As a result, the performance of perovskite solar cells significantly increases from 19.0% to 21.4%.

Impermeable electron transport layers (ETLs) are shown to enable the deposition of semitransparent AgNW electrodes from green aqueous dispersions on top of perovskite solar cells (PSCs) without damage. Semitransparent PSCs with an efficiency of 17.4% are shown, which represents the highest efficiency of semitransparent p‐i‐n PSCs with an AgNW top electrode.

Abstract

Several applications of perovskite solar cells (PSCs) demand a semitransparent top electrode to afford top‐illumination or see‐through devices. Transparent conductive oxides, such as indium tin oxide (ITO), typically require postdeposition annealing at elevated temperatures, which would thermally decompose the perovskite. In contrast, silver nanowires (AgNWs) in dispersions of water would be a very attractive alternative that can be deposited at ambient conditions. Water is environmentally friendly without safety concerns associated with alcohols, such as flammability. Due to the notorious moisture sensitivity of lead‐halide perovskites, aqueous processing of functional layers, such as electrodes, on top of a perovskite device stack is elusive. Here, impermeable electron transport layers (ETLs) are shown to enable the deposition of semitransparent AgNW electrodes from green aqueous dispersions on top of the perovskite cell without damage. The polyvinylpyrrolidone (PVP) capping agent of the AgNWs is found to cause a work–function shift and an energy barrier between the AgNWs and the adjacent ETL. Thus, a high carrier density (≈10¹⁸ cm⁻³) in the ETL is required to achieve well‐behaved J/V characteristics free of s‐shapes. Ultimately, semitransparent PSCs are demonstrated that provide an efficiency of 17.4%, which is the highest efficiency of semitransparent p‐i‐n perovskite solar cells with an AgNW top electrode.

Conductive atomic force microscopy can realize a real‐space visualization of topography coupled with electronic properties on the microscopic scale and thereby demonstrates a unique ability to probe local effects of perovskite materials and devices. This manuscript comprehensively reviews the applications in perovskite solar cells for electronic transport behavior, ion migration and hysteresis, ferroelectric polarization, and facet orientation investigation.

Abstract

Metal halide perovskite materials, benefiting from a combination of outstanding optoelectronic properties and low‐cost solution‐preparation processes, show tremendous potential for optoelectronics and photovoltaics. However, the nanoscale inhomogeneities of the electronic properties of perovskite materials cause a number of difficulties, such as recombination, stability, and hysteresis, all of which seriously restrict device performance. Scanning probe microscopy, as a high‐resolution imaging technique, has been widely used to connect local properties and micro‐area morphologies to overall device performance. Conductive atomic force microscopy (C‐AFM) can realize a real‐space visualization of topography coupled with optoelectronic properties on a microscopic scale and thereby is uniquely suited to probe the local effects of perovskite materials and devices. The fundamental principles, alternative operation modes, and development of C‐AFM are comprehensively reviewed, and applications in perovskite solar cells (PSCs) for electronic transport behavior, ion migration and hysteresis, ferroelectric polarization, and facet orientation investigation are discussed. A comprehensive understanding and summary of up‐to‐date applications in PSCs is beneficial to further fully exploit the potential of such an emerging technique, so as to provide a novel and effective approach for perovskite materials analysis.

Publication date: May 2020

Source: Nano Energy, Volume 71

Author(s): Chong Wang, Shuaicheng Lu, Sen Li, Siyu Wang, Xuetian Lin, Jun Zhang, Rokas Kondrotas, Kanghua Li, Chao Chen, Jiang Tang

Publication date: May 2020

Source: Nano Energy, Volume 71

Author(s): Yuhao Liu, Bo Yang, Muyi Zhang, Bing Xia, Chao Chen, Xueping Liu, Jie Zhong, Zewen Xiao, Jiang Tang

Publication date: May 2020

Source: Nano Energy, Volume 71

Author(s): Xixia Liu, Yuanhang Cheng, Baoshan Tang, Zhi Gen Yu, Mengsha Li, Fen Lin, Siwen Zhang, Yong-Wei Zhang, Jianyong Ouyang, Hao Gong

Light-matter interactions in semiconductors are uniformly treated within the electric dipole approximation; multipolar interactions are considered "forbidden." We experimentally demonstrate that this approximation inadequately describes light emission in two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs), solution processable semiconductors with promising optoelectronic properties. By exploiting the highly oriented crystal structure, we use energy-momentum spectroscopies to demonstrate that an exciton-like sideband in 2D HOIPs exhibits a multipolar radiation pattern with highly directed emission. Electromagnetic and quantum-mechanical analyses indicate that this emission originates from an out-of-plane magnetic dipole transition arising from the 2D character of electronic states. Symmetry arguments and temperature-dependent measurements suggest a dynamic symmetry-breaking mechanism that is active over a broad temperature range. These results challenge the paradigm of electric dipole–dominated light-matter interactions in optoelectronic materials, provide new perspectives on the origins of unexpected sideband emission in HOIPs, and tease the possibility of metamaterial-like scattering phenomena at the quantum-mechanical level.

A polar nonconjugated small molecule ultrathin layer with an intrinsic dipole moment is introduced to modify the work function of indium tin oxide and to optimize the front interface energy level alignment, which contributes to suppressed energy loss and results in a 20.55% efficient electron transport layer–free perovskite solar cell with enhanced open‐circuit voltage short circuit current density and fill factor, simultaneously.

Abstract

Efficient electron transport layer–free perovskite solar cells (ETL‐free PSCs) with cost‐effective and simplified design can greatly promote the large area flexible application of PSCs. However, the absence of ETL usually leads to the mismatched indium tin oxide (ITO)/perovskite interface energy levels, which limits charge transfer and collection, and results in severe energy loss and poor device performance. To address this, a polar nonconjugated small‐molecule modifier is introduced to lower the work function of ITO and optimize interface energy level alignment by virtue of an inherent dipole, as verified by photoemission spectroscopy and Kelvin probe force microscopy measurements. The resultant barrier‐free ITO/perovskite contact favors efficient charge transfer and suppresses nonradiative recombination, endowing the device with enhanced open circuit voltage, short circuit current density, and fill factor, simultaneously. Accordingly, power conversion efficiency increases greatly from 12.81% to a record breaking 20.55%, comparable to state‐of‐the‐art PSCs with a sophisticated ETL. Also, the stability is enhanced with decreased hysteresis effect due to interface defect passivation and inhibited interface charge accumulation. This work facilitates the further development of highly efficient, flexible, and recyclable ETL‐free PSCs with simplified design and low cost by interface electronic structure engineering through facile electrode modification.

Nanoporous Au film is successfully introduced into perovskite solar cells to replace the typical thermal deposition of metal electrode with a high efficiency of 19.0% on rigid substrate and sustains an excellent bending durability of 98.5% even after 1000 cycles testing on a flexible device, while its facile and recycled utilization significantly reduces the device fabrication cost, noble metal consuming, and environmental pollution.

Abstract

Perovskite solar cells (PSCs) using metal electrodes have been regarded as promising candidates for next‐generation photovoltaic devices because of their high efficiency, low fabrication temperature, and low cost potential. However, the complicated and rigorous thermal deposition process of metal contact electrodes remains a challenging issue for reducing the energy pay‐back period in commercial PSCs, as the ubiquitous one‐time use of a contact electrode wastes limited resources and pollutes the environment. Here, a nanoporous Au film electrode fabricated by a simple dry transfer process is introduced to replace the thermally evaporated Au electrode in PSCs. A high power conversion efficiency (PCE) of 19.0% is demonstrated in PSCs with the nanoporous Au film electrode. Moreover, the electrode is recycled more than 12 times to realize a further reduced fabrication cost of PSCs and noble metal materials consumption and to prevent environmental pollution. When the nanoporous Au electrode is applied to flexible PSCs, a PCE of 17.3% and superior bending durability of ≈98.5% after 1000 cycles of harsh bending tests are achieved. The nanoscale pores and the capability of the porous structure to impede crack generation and propagation enable the nanoporous Au electrode to be recycled and result in excellent bending durability.

Publication date: May 2020

Source: Nano Energy, Volume 71

Author(s): Yuxi Dou, Ziwen Liu, Zhengli Wu, Yifan Liu, Jing Li, Chongqian Leng, De Fang, Guijie Liang, Junyan Xiao, Wei Li, Xingzhan Wei, Fuzhi Huang, Yi-Bing Cheng, Jie Zhong

Solvation of PbI₂ promotes the intercalation of solvent molecules with formamidinium iodide to form the perovskite phase of FAPbI₃ directly at room temperature. Subsequent annealing completes the conversion and yields high‐quality perovskite films with reduced trap state density and a high power conversion efficiency.

Abstract

The two‐step conversion process consisting of metal halide deposition followed by conversion to hybrid perovskite has been successfully applied toward producing high‐quality solar cells of the archetypal MAPbI₃ hybrid perovskite, but the conversion of other halide perovskites, such as the lower bandgap FAPbI₃, is more challenging and tends to be hampered by the formation of hexagonal nonperovskite polymorph of FAPbI₃, requiring Cs addition and/or extensive thermal annealing. Here, an efficient room‐temperature conversion route of PbI₂ into the α‐FAPbI₃ perovskite phase without the use of cesium is demonstrated. Using in situ grazing incidence wide‐angle X‐ray scattering (GIWAXS) and quartz crystal microbalance with dissipation (QCM‐D), the conversion behaviors of the PbI₂ precursor from its different states are compared. α‐FAPbI₃ forms spontaneously and efficiently at room temperature from P₂ (ordered solvated polymorphs with DMF) without hexagonal phase formation and leads to complete conversion after thermal annealing. The average power conversion efficiency (PCE) of the fabricated solar cells is greatly improved from 16.0(±0.32)% (conversion from annealed PbI₂) to 17.23(±0.28)% (from solvated PbI₂) with a champion device PCE > 18% due to reduction of carrier recombination rate. This work provides new design rules toward the room‐temperature phase transformation and processing of hybrid perovskite films based on FA⁺ cation without the need for Cs⁺ or mixed halide formulation.

Impermeable electron transport layers (ETLs) are shown to enable the deposition of semitransparent AgNW electrodes from green aqueous dispersions on top of perovskite solar cells (PSCs) without damage. Semitransparent PSCs with an efficiency of 17.4% are shown, which represents the highest efficiency of semitransparent p‐i‐n PSCs with an AgNW top electrode.

Abstract

Several applications of perovskite solar cells (PSCs) demand a semitransparent top electrode to afford top‐illumination or see‐through devices. Transparent conductive oxides, such as indium tin oxide (ITO), typically require postdeposition annealing at elevated temperatures, which would thermally decompose the perovskite. In contrast, silver nanowires (AgNWs) in dispersions of water would be a very attractive alternative that can be deposited at ambient conditions. Water is environmentally friendly without safety concerns associated with alcohols, such as flammability. Due to the notorious moisture sensitivity of lead‐halide perovskites, aqueous processing of functional layers, such as electrodes, on top of a perovskite device stack is elusive. Here, impermeable electron transport layers (ETLs) are shown to enable the deposition of semitransparent AgNW electrodes from green aqueous dispersions on top of the perovskite cell without damage. The polyvinylpyrrolidone (PVP) capping agent of the AgNWs is found to cause a work–function shift and an energy barrier between the AgNWs and the adjacent ETL. Thus, a high carrier density (≈10¹⁸ cm⁻³) in the ETL is required to achieve well‐behaved J/V characteristics free of s‐shapes. Ultimately, semitransparent PSCs are demonstrated that provide an efficiency of 17.4%, which is the highest efficiency of semitransparent p‐i‐n perovskite solar cells with an AgNW top electrode.

A steric engineering strategy to impede ion migration in perovskite thin films is demonstrated where ion migration is effectively hindered by localized lattice distortions induced by incorporation of oversized A site cations. The steric engineering approach improves the operational lifetime of perovskite solar cells by more than nine‐fold from 222 h to 2011 h.

Abstract

The operational instability of perovskite solar cells (PSCs) is known to mainly originate from the migration of ionic species (or charged defects) under a potential gradient. Compositional engineering of the “A” site cation of the ABX₃ perovskite structure has been shown to be an effective route to improve the stability of PSCs. Here, the effect of size‐mismatch‐induced lattice distortions on the ion migration energetics and operational stability of PSCs is investigated. It is observed that the size mismatch of the mixed “A” site composition films and devices leads to a steric effect to impede the migration pathways of ions to increase the activation energy of ion migration, which is demonstrated through multiple theoretical and experimental evidence. Consequently, the mixed composition devices exhibit significantly improved thermal stability under continuous heating at 85 °C and operational stability under continuous 1 sun illumination, with an extrapolated lifetime of 2011 h, compared to the 222 h of the reference device.

Hole transport material (HTM) plays important roles in n–i–p type perovskite solar cells. It affects both efficiency and the stability. After the recognition of its importance, a number of HTMs have been developed. This review summarizes various types of HTMs and discusses their development.

Abstract

With the application of organic–inorganic hybrid perovskites to liquid‐type solar cells, the unprecedented development of perovskite solar cells (Per‐SCs) has been boosted by the introduction of solid‐state hole transport materials (HTMs). The removal of liquid electrolyte has lead to improved efficiency and stability. Supported by high‐quality perovskite films, the certified efficiency of Per‐SCs has reached 25.2%. For Per‐SCs assembled in a conventional structure (n–i–p), the hole transport layer (HTL) plays an extra role in preventing the perovskite layer from external stimuli. In summary, the successful design and fabrication of the HTL must meet various requirements in terms of solubility, hole transport, recombination prevention, stability, and reproducibility, to name but a few. Many research strategies are focused on the development of high‐performance HTMs to meet such requirements. Such strategies for the development of HTMs employed in conventional n–i–p solar cells are reviewed herein. A vision of the future HTMs is proposed in this review based on the already proposed solutions and current trends.

This study pictures the complex morphological evolution of perovskite thin films when organic cations of different size and concentration are blended together and proposes an effective solution, enables stable performance for tens of hours at the maximum power point, without encapsulation, at 50% relative humidity.

Abstract

The intrinsic instability of lead halide perovskite semiconductors in an ambient atmosphere is one of the most critical issues that impedes perovskite solar cell commercialization. To overcome it, the use of bulky organic spacers has emerged as a promising solution. The resulting perovskite thin films present complex morphologies, difficult to predict, which can directly affect the device efficiency. Here, by combining in‐depth morphological and spectroscopic characterization, it is shown that both the ionic size and the relative concentration of the organic cation, drive the integration of bulky organic cations into the crystal unit cell and the thin film, inducing different perovskite phases and different vertical distribution, then causing a significant change in the final thin film morphology. Based on these studies, a fine‐engineered perovskite is constructed by employing two different large cations, namely, ethyl ammonium and butyl ammonium. The first one takes part in the 3D perovskite phase formation, the second one works as a surface modifier by forming a passivating layer on top of the thin film. Together they lead to improved photovoltaic performance and device stability when tested in air under continuous illumination. These findings propose a general approach to achieve reliability in perovskite‐based optoelectronic devices.

By introduction of a multiple ligand (pentaerythritol tetrakis(3‐mercaptopropionate)), uncoordinated Pb²⁺ and Pb⁰ defects are simultaneously passivated. Meanwhile, better energy level matching between the valence band of perovskite and the highest occupied molecular orbital of the HTM is achieved. As a result, perovskite solar cell efficiency increases from 19.0% to 21.4% after surface passivation by multiple ligands.

Abstract

In the past decade, the efficiency of perovskite solar cells quickly increased from 3.8% to 25.2%. The quality of perovskite films plays vital role in device performance. The films fabricated by solution‐process are usually polycrystalline, with significantly higher defect density than that of single crystal. One kind of defect in the films is uncoordinated Pb²⁺, which is usually generated during thermal annealing process due to the volatile organic component. Another detrimental kind of defect is Pb⁰, which is often observed during the film fabrication process or solar cell operation. Because the open circuit voltage has a close relation with the defect density, it is thus desirable to passivate these two kinds of defects. Here, a molecule with multiple ligands is introduced, which not only passivates the uncoordinated Pb²⁺ defects, but also suppresses the formation of Pb⁰ defects. Meanwhile, such a treatment improves the energy level alignment between the valence band of perovskite and the highest occupied molecular orbital of spiro‐OMeTAD. As a result, the performance of perovskite solar cells significantly increases from 19.0% to 21.4%.

Impermeable electron transport layers (ETLs) are shown to enable the deposition of semitransparent AgNW electrodes from green aqueous dispersions on top of perovskite solar cells (PSCs) without damage. Semitransparent PSCs with an efficiency of 17.4% are shown, which represents the highest efficiency of semitransparent p‐i‐n PSCs with an AgNW top electrode.

Abstract

Several applications of perovskite solar cells (PSCs) demand a semitransparent top electrode to afford top‐illumination or see‐through devices. Transparent conductive oxides, such as indium tin oxide (ITO), typically require postdeposition annealing at elevated temperatures, which would thermally decompose the perovskite. In contrast, silver nanowires (AgNWs) in dispersions of water would be a very attractive alternative that can be deposited at ambient conditions. Water is environmentally friendly without safety concerns associated with alcohols, such as flammability. Due to the notorious moisture sensitivity of lead‐halide perovskites, aqueous processing of functional layers, such as electrodes, on top of a perovskite device stack is elusive. Here, impermeable electron transport layers (ETLs) are shown to enable the deposition of semitransparent AgNW electrodes from green aqueous dispersions on top of the perovskite cell without damage. The polyvinylpyrrolidone (PVP) capping agent of the AgNWs is found to cause a work–function shift and an energy barrier between the AgNWs and the adjacent ETL. Thus, a high carrier density (≈10¹⁸ cm⁻³) in the ETL is required to achieve well‐behaved J/V characteristics free of s‐shapes. Ultimately, semitransparent PSCs are demonstrated that provide an efficiency of 17.4%, which is the highest efficiency of semitransparent p‐i‐n perovskite solar cells with an AgNW top electrode.

Mixed‐halide perovskites are essential for use in all‐perovskite or perovskite–silicon tandem solar cells. Through photoluminescence measurements and electric field application, three distinct defect species are found responsible for charge‐carrier trapping, halide segregation, and electric field screening, respectively, within MAPb(Br_0.5I_0.5)₃ materials. External quantum efficiency measurements highlight that charge‐carriers can be extracted from the low‐bandgap regions of the phase‐segregated perovskite formed under illumination.

Abstract

Mixed‐halide perovskites are essential for use in all‐perovskite or perovskite–silicon tandem solar cells due to their tunable bandgap. However, trap states and halide segregation currently present the two main challenges for efficient mixed‐halide perovskite technologies. Here photoluminescence techniques are used to study trap states and halide segregation in full mixed‐halide perovskite photovoltaic devices. This work identifies three distinct defect species in the perovskite material: a charged, mobile defect that traps charge‐carriers in the perovskite, a charge‐neutral defect that induces halide segregation, and a charged, mobile defect that screens the perovskite from external electric fields. These three defects are proposed to be MA⁺ interstitials, crystal distortions, and halide vacancies and/or interstitials, respectively. Finally, external quantum efficiency measurements show that photoexcited charge‐carriers can be extracted from the iodide‐rich low‐bandgap regions of the phase‐segregated perovskite formed under illumination, suggesting the existence of charge‐carrier percolation pathways through grain boundaries where phase‐segregation may occur.

A novel solvent‐free and low‐temperature melting encapsulation strategy enables the full encapsulating operations under an ambient environment. It is found that the strategy not only removes residual oxygen and moisture to prevent the perovskite from phase segregation, but also suppresses the species volatilization to impede absorber decomposition, enabling a perovskite solar cell device with good thermal, moisture, and maximum power point stability.

Abstract

Improving device lifetime is one of the critical challenges for the practical use of metal halide perovskite solar cells (PSCs), wherein a reliable encapsulation is indispensable. Herein, based on an in‐depth understanding of the degradation mechanism for the PSCs, a solvent‐free and low‐temperature melting encapsulation technique, by employing low‐cost paraffin as the encapsulant that is compatible with perovskite absorbers, is demonstrated. The encapsulation strategy enables the full encapsulating operations to be undertaken under an ambient environment. It is found that the strategy not only removes residual oxygen and moisture to prevent the perovskite from phase segregation, but also suppresses the species volatilization to impede absorber decomposition, enabling a PSC devices with good thermal and moisture stability. As a result, the as‐encapsulated PSCs achieve a 1000 h operational lifetime for the encapsulated device at continuous maximum power point output under an ambient environment. This work paves the way for scalable and robust encapsulation strategy feasible to hybrid perovskite optoelectronics in an economic manner.

It is found that unique adducts form between CsI and dimethyl sulfoxide (DMSO) and certain antisolvents, such as methyl acetate, during film formation of the all‐inorganic perovskite CsPbI₃. These adducts significantly influence crystallization and the power conversion efficiency of the resulting solar cells.

Abstract

The excellent optoelectronic properties demonstrated by hybrid organic/inorganic metal halide perovskites are all predicated on precisely controlling the exact nucleation and crystallization dynamics that occur during film formation. In general, high‐performance thin films are obtained by a method commonly called solvent engineering (or antisolvent quench) processing. The solvent engineering method removes excess solvent, but importantly leaves behind solvent that forms chemical adducts with the lead‐halide precursor salts. These adduct‐based precursor phases control nucleation and the growth of the polycrystalline domains. There has not yet been a comprehensive study comparing the various antisolvents used in different perovskite compositions containing cesium. In addition, there have been no reports of solvent engineering for high efficiency in all‐inorganic perovskites such as CsPbI₃. In this work, inorganic perovskite composition CsPbI₃ is specifically targeted and unique adducts formed between CsI and precursor solvents and antisolvents are found that have not been observed for other A‐site cation salts. These CsI adducts control nucleation more so than the PbI₂–dimethyl sulfoxide (DMSO) adduct and demonstrate how the A‐site plays a significant role in crystallization. The use of methyl acetate (MeOAc) in this solvent engineering approach dictates crystallization through the formation of a CsI–MeOAc adduct and results in solar cells with a power conversion efficiency of 14.4%.

Surface coating of 3D perovskite with alkylammonium bulky cations passivates the surface defects and improves the perovskite solar cell performance, while incorporating those cations into the bulk negatively affects the crystallinity and reduces the device performance. Using the surface‐coating strategy, four‐terminal perovskite‐silicon tandem reaches an efficiency of 27.7% with interdigitated back contact silicon bottom cells.

Abstract

Mixed‐dimensional perovskite solar cells combining 3D and 2D perovskites have recently attracted wide interest owing to improved device efficiency and stability. Yet, it remains unclear which method of combining 3D and 2D perovskites works best to obtain a mixed‐dimensional system with the advantages of both types. To address this, different strategies of combining 2D perovskites with a 3D perovskite are investigated, namely surface coating and bulk incorporation. It is found that through surface coating with different aliphatic alkylammonium bulky cations, a Ruddlesden–Popper “quasi‐2D” perovskite phase is formed on the surface of the 3D perovskite that passivates the surface defects and significantly improves the device performance. In contrast, incorporating those bulky cations into the bulk induces the formation of the pure 2D perovskite phase throughout the bulk of the 3D perovskite, which negatively affects the crystallinity and electronic structure of the 3D perovskite framework and reduces the device performance. Using the surface‐coating strategy with n‐butylammonium bromide to fabricate semitransparent perovskite cells and combining with silicon cells in four‐terminal tandem configuration, 27.7% tandem efficiency with interdigitated back contact silicon bottom cells (size‐unmatched) and 26.2% with passivated emitter with rear locally diffused silicon bottom cells is achieved in a 1 cm² size‐matched tandem.

A soft template‐controlled growth (STCG) method is proposed for the fabrication of a pinhole‐free CsPbI₃ film and the device exhibits an efficiency of 16.04%. By suppressing the inductive effect of defects on the phase transition and utilizing the unique reversibility of the phase transition, the STCG‐based all‐inorganic solar cell retains 90% of its initial efficiency after 3000 h of light soaking and heating.

Abstract

The unfavorable morphology and inefficient utilization of phase transition reversibility have limited the high‐temperature‐processed inorganic perovskite films in both efficiency and stability. Here, a simple soft template‐controlled growth (STCG) method is reported by introducing (adamantan‐1‐yl)methanammonium to control the nucleation and growth rate of CsPbI₃ crystals, which gives rise to pinhole‐free CsPbI₃ film with a grain size on a micrometer scale. The STCG‐based CsPbI₃ perovskite solar cell exhibits a power conversion efficiency of 16.04% with significantly reduced defect densities and charge recombination. More importantly, an all‐inorganic solar cell with the architecture fluorine‐doped tin oxide (FTO)/NiO _x /STCG‐CsPbI₃/ZnO/indium‐doped tin oxide (ITO) is successfully fabricated to demonstrate its real advantage in thermal stability. By suppressing the inductive effect of defects during the phase transition and utilizing the unique reversibility of the phase transition for the high‐temperature‐processed CsPbI₃ film, the all‐inorganic solar cell retains 90% of its initial efficiency after 3000 h of continuous light soaking and heating.