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The issue of poor long-term stability against moisture is still a key challenge hindering perovskite solar cells for practical applications. Here, we report an approach to sequentially apply a typical one-step solution formulation—self-seeding growth (SSG)—to realize high-quality perovskite thin films with reduced defect density, fewer apparent grain boundaries, improved charge-carrier transport and lifetime, and enhanced hydrophobicity for enhanced stability. Using FA/MA/Cs-based perovskite, SSG devices showed improved efficiency from 17.76% (control) to 20.30% (SSG), with an unencapsulated device retaining >80% of its initial power conversion efficiency over 4,680-h storage in an ambient environment with high relative humidity. In addition, SSG can be applied to different substrates (e.g., SnO₂ versus TiO₂; planar versus mesoporous) and perovskite compositions, making it a viable method for preparing high-quality perovskite thin films for device applications.

Hybrid organic-inorganic lead halide perovskite solar cells have shown a remarkable rise in power conversion efficiency over a short period of time; however, long-term stability remains a key challenge hindering the practical application of these cells. Here, we report an approach to sequentially apply a typical one-step solution formulation—self-seeding growth (SSG)—to realize high-quality perovskite thin films with reduced defect density, fewer apparent grain boundaries, improved charge-carrier transport and lifetime, and enhanced hydrophobicity for enhanced stability. Using FA-MA-Cs-based perovskite, SSG devices showed improved efficiency from 17.76% (control) to 20.30% (SSG), with an unencapsulated device retaining >80% of its initial efficiency over 4,680-h storage in an ambient environment with high relative humidities. The SSG devices also exhibited much improved thermal and operational stabilities. In addition, SSG can be applied to different substrates and perovskite compositions, which makes it a viable method for preparing high-quality perovskite thin films for device applications.

N-type silicon wafers exhibit superior electrical properties compared to their p-type counterparts, such as higher minority carrier lifetime and absence of light-induced degradation, resulting in a higher efficiency potential and increased reliability of photovoltaic devices. However, most of the commonly used metals (e.g., Al and Ag) cannot form an ohmic contact on the lightly doped n-type silicon wafers, retarding the development of an n-type analog to the Al-back-surface-field p-type solar cell. Herein, we present a dual-function, electron-conductive contact based on titanium nitride (TiN) for n-type silicon solar cells. By implementing the SiO₂/TiN contact, which acts simultaneously as a surface passivating layer and metal electrode, an efficiency of 20% was achieved by an n-type silicon solar cell with a simplified fabrication flow. This work demonstrates the path forward to develop efficient n-type silicon solar cells with dual-function metal nitride contacts at a low cost.

High-performance passivating contact is a prerequisite for high-efficiency crystalline silicon (c-Si) solar cells. In this work, an electron-conductive, hole-blocking contact based on titanium nitride (TiN) deposited by reactive magnetron sputtering is presented. Quasi-metallic TiN combined with an ultrathin SiO₂ passivation layer (SiO₂/TiN) is demonstrated to be an effective electron-selective contact on c-Si, featuring a low-contact resistivity of 16.4 mΩ.cm² and a tolerable recombination current parameter of ∼500 fA/cm². By implementing the dual-function SiO₂/TiN contact, which acts simultaneously as a surface passivating layer and metal electrode, an efficiency of 20% is achieved by an n-type c-Si solar cell with a simple structure. This work not only demonstrates a way to develop efficient n-type c-Si solar cells with dual-function metal nitride contacts at a low cost but also expands the pool of available carrier transport materials, from metal oxides to metal nitrides, for photovoltaic devices.

Research into organic solar cells has gone from pure scientific curiosity to a topic of commercial relevance in the past few years, as a result of rapid development of non-fullerene acceptors. This transition is mainly driven by the development of new materials. Recently in Joule, Zou and co-workers developed a new acceptor material and reached a record efficiency for single-junction organic solar cells.

Perovskite solar cells have been the fastest advancing photovoltaic technology in the last decade. The versatility of the material in terms of ease of fabrication and band-gap tunability opens up various types of applications such as single-junction, flexible, semi-transparent, and multi-junction tandem solar cells. While the rate of improvement for organic lead halide perovskite solar cells is slowing, there is a dramatic increase in cell efficiencies and in the number of cell demonstrations for inorganic cesium lead halide perovskite (e.g., CsPbIXBr3-X) solar cells in the last 2 years. The higher band gap and thermal stability of CsPbIXBr3-X are attributes desirable for tandem solar cell applications and other optoelectronic devices.

This paper provides a comprehensive review of the development of CsPbIXBr3-X solar cells, including their challenges such as meta-stable phases and halide segregation. This is followed by an analysis of demonstrated devices in terms of their performance relative to their theoretical limits. While the cells perform well optically, with some reaching 90% of their theoretical current output limits, the low voltage outputs and fill factors of these cells limit their power conversion efficiencies to only 60% of their theoretical limits.

To further improve cell performance, the appropriate choice and engineering of electron and hole transport layers with the aim of producing desirable valance and conduction band offsets, increasing carrier lifetimes from nanosecond to microsecond range, and reducing surface recombination velocity from 105 cm/s to 103 cm/s are paramount, allowing thicker absorber devices to be fabricated. This will bring the outputs of inorganic perovskite cells to match those of organic metal halides—reaching 75% of the SQ efficiency limits. Therefore, it is not entirely impossible for efficiencies of CsPbI3, CsPbI2Br, and CsPbIBr2 cells to reach 21.7%, 19.0%, and 16.6%, respectively, in the near term.

While the rate of improvement for organic lead halide perovskite solar cells is slowing, there has been a dramatic increase in cell efficiencies and in the number of cell demonstrations for inorganic cesium lead halide perovskite (e.g., CsPbIXBr3-X) solar cells in the last 2 years. The higher band gap and thermal stability of CsPbIXBr3-X are desirable for tandem solar cell applications and other optoelectronic devices. It is apparent that these cells are performing well optically, with some reaching 90% of their theoretical current output limits. However, low carrier lifetime and high surface recombination limit the voltages and fill factors of these cells, limiting their performance to only 60% of their theoretical efficiency limits. Appropriate transport layer designs (producing positive band offsets), reducing surface recombination velocities (to 103 cm/s), and improving lifetimes (10 μs) are effective strategies for improving efficiencies, allowing cells with thick absorbers to be fabricated, and achieving efficiencies above 80% of their theoretical limits.

Silicon-based solar cells are dominating today’s solar energy market. However, their efficiencies will soon reach their maximum practical limit. Without any gains in efficiency, price reductions will become increasingly difficult to achieve. Tandem and multi-junction architectures can overcome this single-junction efficiency limit. Perovskite materials offer both band-gap tunability and solution processability. This unique combination of properties allows for fabrication of multi-junction solar cells using high-throughput deposition techniques such as blade coating, roll-to-roll, gravure coating or inkjet printing. However, these solar cells have yet to be fabricated using these deposition techniques due to difficulties in sequentially depositing these semiconductors. By utilizing an acetonitrile/methylamine-based solvent, we demonstrate the first monolithic all-perovskite multi-junction solar cells fabricated via solution processing of all active layers, apart from the electrodes.

Multi-junction device architectures can increase the power conversion efficiency (PCE) of photovoltaic (PV) cells beyond the single-junction thermodynamic limit. However, these devices are challenging to produce by solution-based methods, where dissolution of underlying layers is problematic. By employing a highly volatile acetonitrile(CH₃CN)/methylamine(CH₃NH₂) (ACN/MA) solvent-based perovskite solution, we demonstrate fully solution-processed absorber, transport, and recombination layers for monolithic all-perovskite tandem and triple-junction solar cells. By combining FA_0.83Cs_0.17Pb(Br_0.7I_0.3)₃ (1.94 eV) and MAPbI₃ (1.57 eV) junctions, we reach two-terminal tandem PCEs of more than 15% (steady state). We show that a MAPb_0.75Sn_0.25I₃ (1.34 eV) narrow band-gap perovskite can be processed via the ACN/MA solvent-based system, demonstrating the first proof-of-concept, monolithic all-perovskite triple-junction solar cell with an open-circuit voltage reaching 2.83 V. Through optical and electronic modeling, we estimate the achievable PCE of a state-of-the-art triple-junction device architecture to be 26.7%. Our work opens new possibilities for large-scale, low-cost, printable perovskite multi-junction solar cells.

The recent rise of halide perovskites (HPs) has revolutionized solar cells and (opto)electronic device research. Numerous studies have implied that structural and compositional characteristics of HPs at various length scales govern the device performance, but much remains poorly understood. In this context, transmission electron microscopy (TEM) has emerged as a powerful characterization tool to accelerate our fundamental understanding of HP materials and devices, with the potential of having a significant impact on the development of HP-based solar cells and (opto)electronic devices. This perspective discusses the important unsolved materials-science problems surrounding HPs and the few available examples of TEM studies of HP materials and devices in the literature. It also highlights the urgency of application of TEM-based techniques for tackling these unsolved issues. Further research in this direction is crucial for advancing the science in the increasingly important HP materials and devices field.

Transmission electron microscopy (TEM)-based techniques are uniquely suited for site-specific structural and analytical characterization of halide perovskites (HPs) at atomic, nanometer, and micrometer length scales. TEM-based studies hold the key to understanding the nature and functionality of these fascinating materials that are at the heart of emerging solar cells and (opto)electronic devices. While TEM-based techniques have made several groundbreaking discoveries that have resulted in astonishing advancements in the field of materials science in general over the past decades, their application to HPs has been relatively sparse. Here, we provide a perspective on TEM-based studies of HPs that have been conducted so far and project a vision for how these powerful characterization techniques can be brought to bear on research problems in the field of HPs. An outlook discussing important challenges and opportunities that lay ahead is also presented.

Perovskite solar cells (PSCs) have attracted tremendous academic and industrial interests because of their rapidly increased power conversion efficiency (PCE) in the past few years, but the intrinsic instability of commonly used 3D perovskites induces the issue of low device stability. Ruddlesden-Popper (RP) phase 2D layered perovskites have recently been reported to show enhanced stability. However, weak van der Waals interactions between interlayers cannot sufficiently stabilize their 2D layered structure. By removing the van der Waals gaps in the RP case, we herein develop a series of Dion-Jacobson phase (DJ) 2D layered perovskites with higher structural stability for PSCs. A maximum PCE of 13.3% is achieved from the DJ phase 2D PSCs, and unencapsulated devices are extremely stable, retaining more than 95% of initial PCE upon exposure to ambient air (4,000 hr), damp heat (85°C and 85% RH, 168 hr), and continuous light illumination (3,000 hr).

2D layered perovskites have emerged as potential alternates to traditional 3D analogs to solve the stability issue of perovskite solar cells (PSCs). However, van der Waals gaps in reported Ruddlesden-Popper (RP) phase 2D perovskites with monoammonium cations provide weak interactions between layers, potentially destabilizing the layered perovskite structure and thus the device. Here we eradicate such gaps by incorporating diammonium cations into MAPbI₃, developing a series of Dion-Jacobson phase 2D perovskites that afford a cell efficiency of 13.3% with ultrahigh device stability. Unencapsulated devices retain over 95% efficiency upon exposure to various harsh stresses including ambient air (40%–70% relative humidity [RH]) for 4,000 hr, damp heat (85°C and 85% RH) for 168 hr, and continuous light illumination for 3,000 hr. The improved device stability over the RP counterpart is attributed to alternating hydrogen bonding interactions between diammonium cations and inorganic slabs, strengthening the 2D layered perovskite structure.

Monolithic perovskite/Si tandem cells have attracted huge interest because of their potential as a solution to overcome the theoretical efficiency limit of single-junction silicon solar cells. However, there are some critical issues, which are caused by a spectral mismatch or an intrinsic structural problem, to be resolved for accurate characterization of tandem solar cells. Therefore, establishing a protocol for comprehensive measurement is crucial to achieving highly efficient monolithic tandem cells. We demonstrate that the photovoltaic properties of each subcell in monolithic perovskite/Si tandem cells, including electron dynamics, can be correctly measured using three-terminal architecture. In particular, the external quantum efficiency of each subcell can be accurately collected without any complicating bias. We then demonstrate the optimization process, including the optical and band-gap engineering, for highly stable and efficient monolithic tandem cells.

Measuring perovskite/Si tandem cells’ photovoltaic properties is challenging due to intrinsic and extrinsic issues such as the monolithic series connection feature and the spectral mismatch of most commercial solar simulators. Here, we report a simple but effective strategy that involves the application of three-terminal (3-T) architecture to monolithic perovskite/Si tandem cells for comprehensive characterization while circumventing the spectral mismatch issue. We demonstrate that the current density-voltage characteristics and the external quantum efficiency (EQE) of each subcell can be measured independently using 3-T architecture without any light/potential bias. In addition, a comprehensive study of electron dynamics, such as charge recombination kinetics of subcells, has been performed. As a result of reducing optical losses and precise current matching, the monolithic perovskite/Si tandem cell exhibits a power-conversion efficiency of 23.5% (23.1% corrected by EQE) and remarkable stability by maintaining 97% of its initial value after 100 days.

Dr. Kai Wang joined CEHMS, Virginia Tech as a Postdoctoral Associate in 2017 after his graduation from The University of Akron. In the fall of 2018, Kai joined Pennsylvania State University as a Research Assistant Professor in the College of Earth and Mineral Sciences, Department of Materials Science and Engineering. His research interests include halide perovskite photovoltaics, two-dimensional multiple quantum well physics, and bioelectronics.

Dr. Shashank Priya currently serves as the Associate Vice President for Research and Director, Strategic Initiatives at Pennsylvania State University. He is a professor in the Department of Materials Science and Engineering at Pennsylvania State University and Adjunct Professor in the Department of Mechanical Engineering at Virginia Tech. Priya's research focuses on the intersection of multifunctional materials, bio-inspired systems and technologies, and energy harvesting and storage. As the principal investigator, he leads multiple programs targeting the development of thermoelectrics, photovoltaics, piezoelectrics, and other emerging energy-conversion and storage devices.

Dr. Dong Yang worked with Professor Shengzhong (Frank) Liu in Shaanxi Normal University, China since 2014 and became a full professor in 2017. Dong joined Virginia Tech in 2017 and moved to Pennsylvania State University in the fall of 2018 as Research Assistant Professor. His research interests include solar cells, semiconductor materials, materials science, and engineering of graphene carbon materials.

Dr. Congcong Wu has led the solar cell team in CEHMS, Virginia Tech since 2014. In the fall of 2018, Congcong joined Pennsylvania State University as Research Associate Professor. His research mainly focuses on developing next-generation photovoltaic systems for clean and efficient energy conversion.

Dr. Joe Shapter received his PhD in Reaction Dynamics from the University of Toronto in 1990. He subsequently held an NSERC Fellowship at The University of Western Ontario before moving to Australia in 1996 to take up a position at Flinders University. Joe served as Dean of the School of Chemical and Physical Sciences for 6.5 years and headed the Flinders involvement in both the Australian Microscopy and Microanalysis Research Facility (AMMRF) and the Australian National Fabrication Facility (ANFF), and was SA Director for AMMRF. His major interests are in the area of novel nanomaterial production, nanometer-scale characterization of these materials, and their applications in, for example, sensors or solar cells.