Tin and Mixed Lead–Tin Halide Perovskite Solar Cells: Progress and their Application in Tandem Solar Cells

Metal halide perovskites have recently attracted enormous attention for photovoltaic applications due to their superior optical and electrical properties. Lead (Pb) halide perovskites stand out among this material series, with a power conversion efficiency (PCE) over 25%. According to the Shockley–Queisser (SQ) limit, lead halide perovskites typically exhibit bandgaps that are not within the optimal range for single-junction solar cells. Partial or complete replacement of lead with tin (Sn) is gaining increasing research interest, due to the promise of further narrowing the bandgaps. This enables ideal solar utilization for single-junction solar cells as well as the construction of all-perovskite tandem solar cells. In addition, the usage of Sn provides a path to the fabrication of lead-free or Pb-reduced perovskite solar cells (PSCs). Recent progress in addressing the challenges of fabricating efficient Sn halide and mixed lead–tin (Pb–Sn) halide PSCs is summarized herein. Mixed Pb–Sn halide perovskites hold promise not only for higher efficiency and more stable single-junction solar cells but also for efficient all-perovskite monolithic tandem solar cells.     

From Lead Halide Perovskites to Lead‐Free Metal Halide Perovskites and Perovskite Derivatives

Despite the exciting progress on power conversion efficiencies, the commercialization of the emerging lead (Pb) halide perovskite solar cell technology still faces significant challenges, one of which is the inclusion of toxic Pb. Searching for Pb‐free perovskite solar cell absorbers is currently an attractive research direction. The approaches used for and the consequences of Pb replacement are reviewed herein. Reviews on the theoretical understanding of the electronic, optical, and defect properties of Pb and Pb‐free halide perovskites and perovskite derivatives are provided, as well as the experimental results available in the literature. The theoretical understanding explains well why Pb halide perovskites exhibit superior photovoltaic properties, but Pb‐free perovskites and perovskite derivatives do not.     

Halide Perovskites: Poor Man's High‐Performance Semiconductors

Halide perovskites are a rapidly developing class of medium‐bandgap semiconductors which, to date, have been popularized on account of their remarkable success in solid‐state heterojunction solar cells raising the photovoltaic efficiency to 20% within the last 5 years. As the physical properties of the materials are being explored, it is becoming apparent that the photovoltaic performance of the halide perovskites is just but one aspect of the wealth of opportunities that these compounds offer as high‐performance semiconductors. From unique optical and electrical properties stemming from their characteristic electronic structure to highly efficient real‐life technological applications, halide perovskites constitute a brand new class of materials with exotic properties awaiting discovery. The nature of halide perovskites from the materials' viewpoint is discussed here, enlisting the most important classes of the compounds and describing their most exciting properties. The topics covered focus on the optical and electrical properties highlighting some of the milestone achievements reported to date but also addressing controversies in the vastly expanding halide perovskite literature.     

Structural and Functional Diversity in Lead‐Free Halide Perovskite Materials

Lead halide perovskites have emerged as promising semiconducting materials for different applications owing to their superior optoelectronic properties. Although the community holds different views toward the toxic lead in these high‐performance perovskites, it is certainly preferred to replace lead with nontoxic, or at least less‐toxic, elements while maintaining the superior properties. Here, the design rules for lead‐free perovskite materials with structural dimensions from 3D to 0D are presented. Recent progress in lead‐free halide perovskites is reviewed, and the relationships between the structures and fundamental properties are summarized, including optical, electric, and magnetic‐related properties. 3D perovskites, especially A₂B⁺B³⁺X₆‐type double perovskites, demonstrate very promising optoelectronic prospects, while low‐dimensional perovskites show rich structural diversity, resulting in abundant properties for optical, electric, magnetic, and multifunctional applications. Furthermore, based on these structure–property relationships, strategies for multifunctional perovskite design are proposed. The challenges and future directions of lead‐free perovskite applications are also highlighted, with emphasis on materials development and device fabrication. The research on lead‐free halide perovskites at Linköping University has benefited from inspirational discussions with Prof. Olle Inganäs.     

Mixed Lead–Tin Halide Perovskites for Efficient and Wavelength‐Tunable Near‐Infrared Light‐Emitting Diodes

Near‐infrared (NIR) light‐emitting diodes (LEDs), with emission wavelengths between 800 and 950 nm, are useful for various applications, e.g., night‐vision devices, optical communication, and medical treatments. Yet, devices using thin film materials like organic semiconductors and lead based colloidal quantum dots face certain fundamental challenges that limit the improvement of external quantum efficiency (EQE), making the search of alternative NIR emitters important for the community. In this work, efficient NIR LEDs with tunable emission from 850 to 950 nm, using lead–tin (Pb‐Sn) halide perovskite as emitters are demonstrated. The best performing device exhibits an EQE of 5.0% with a peak emission wavelength of 917 nm, a turn‐on voltage of 1.65 V, and a radiance of 2.7 W Sr^?1 m^?2 when driven at 4.5 V. The emission spectra of mixed Pb‐Sn perovskites are tuned either by changing the Pb:Sn ratio or by incorporating bromide, and notably exhibit no phase separation during device operation. The work demonstrates that mixed Pb‐Sn perovskites are promising next generation NIR emitters.     

Interface and Defect Engineering for Metal Halide Perovskite Optoelectronic Devices

Metal halide perovskites have been in the limelight in recent years due to their enormous potential for use in optoelectronic devices, owing to their unique combination of properties, such as high absorption coefficient, long charge‐carrier diffusion lengths, and high defect tolerance. Perovskite‐based solar cells and light‐emitting diodes (LEDs) have achieved remarkable breakthroughs in a comparatively short amount of time. As of writing, a certified power conversion efficiency of 22.7% and an external quantum efficiency of over 10% have been achieved for perovskite solar cells and LEDs, respectively. Interfaces and defects have a critical influence on the properties and operational stability of metal halide perovskite optoelectronic devices. Therefore, interface and defect engineering are crucial to control the behavior of the charge carriers and to grow high quality, defect‐free perovskite crystals. Herein, a comprehensive review of various strategies that attempt to modify the interfacial characteristics, control the crystal growth, and understand the defect physics in metal halide perovskites, for both solar cell and LED applications, is presented. Lastly, based on the latest advances and breakthroughs, perspectives and possible directions forward in a bid to transcend what has already been achieved in this vast field of metal halide perovskite optoelectronic devices are discussed.     

Combining Efficiency and Stability in Mixed Tin–Lead Perovskite Solar Cells by Capping Grains with an Ultrathin 2D Layer

The development of narrow-bandgap (E_g ≈ 1.2 eV) mixed tin–lead (Sn–Pb) halide perovskites enables all-perovskite tandem solar cells. Whereas pure-lead halide perovskite solar cells (PSCs) have advanced simultaneously in efficiency and stability, achieving this crucial combination remains a challenge in Sn–Pb PSCs. Here, Sn–Pb perovskite grains are anchored with ultrathin layered perovskites to overcome the efficiency-stability tradeoff. Defect passivation is achieved both on the perovskite film surface and at grain boundaries, an approach implemented by directly introducing phenethylammonium ligands in the antisolvent. This improves device operational stability and also avoids the excess formation of layered perovskites that would otherwise hinder charge transport. Sn–Pb PSCs with fill factors of 79% and a certified power conversion efficiency (PCE) of 18.95% are reported—among the highest for Sn–Pb PSCs. Using this approach, a 200-fold enhancement in device operating lifetime is achieved relative to the nonpassivated Sn–Pb PSCs under full AM1.5G illumination, and a 200 h diurnal operating time without efficiency drop is achieved under filtered AM1.5G illumination.     

Mutual Insight on Ferroelectrics and Hybrid Halide Perovskites: A Platform for Future Multifunctional Energy Conversion

An insight into the analogies, state‐of‐the‐art technologies, concepts, and prospects under the umbrella of perovskite materials (both inorganic–organic hybrid halide perovskites and ferroelectric perovskites) for future multifunctional energy conversion and storage devices is provided. Often, these are considered entirely different branches of research; however, considering them simultaneously and holistically can provide several new opportunities. Recent advancements have highlighted the potential of hybrid perovskites for high‐efficiency solar cells. The intrinsic polar properties of these materials, including the potential for ferroelectricity, provide additional possibilities for simultaneously exploiting several energy conversion mechanisms such as the piezoelectric, pyroelectric, and thermoelectric effect and electrical energy storage. The presence of these phenomena can support the performance of perovskite solar cells. The energy conversion using these effects (piezo‐, pyro‐, and thermoelectric effect) can also be enhanced by a change in the light intensity. Thus, there lies a range of possibilities for tuning the structural, electronic, optical, and magnetic properties of perovskites to simultaneously harvest energy using more than one mechanism to realize an improved efficiency. This requires a basic understanding of concepts, mechanisms, corresponding material properties, and the underlying physics involved with these effects.     

300% Enhancement of Carrier Mobility in Uniaxial‐Oriented Perovskite Films Formed by Topotactic‐Oriented Attachment

Organic–inorganic perovskites with intriguing optical and electrical properties have attracted significant research interests due to their excellent performance in optoelectronic devices. Recent efforts on preparing uniform and large‐grain polycrystalline perovskite films have led to enhanced carrier lifetime up to several microseconds. However, the mobility and trap densities of polycrystalline perovskite films are still significantly behind their single‐crystal counterparts. Here, a facile topotactic‐oriented attachment (TOA) process to grow highly oriented perovskite films, featuring strong uniaxial‐crystallographic texture, micrometer‐grain morphology, high crystallinity, low trap density (≈4 × 10¹⁴ cm^?3), and unprecedented 9 GHz charge‐carrier mobility (71 cm² V^?1 s^?1), is demonstrated. TOA‐perovskite‐based n‐i‐p planar solar cells show minimal discrepancies between stabilized efficiency (19.0%) and reverse‐scan efficiency (19.7%). The TOA process is also applicable for growing other state‐of‐the‐art perovskite alloys, including triple‐cation and mixed‐halide perovskites.     

Two‐Dimensional Materials for Halide Perovskite‐Based Optoelectronic Devices

Halide perovskites have high light absorption coefficients, long charge carrier diffusion lengths, intense photoluminescence, and slow rates of non‐radiative charge recombination. Thus, they are attractive photoactive materials for developing high‐performance optoelectronic devices. These devices are also cheap and easy to be fabricated. To realize the optimal performances of halide perovskite‐based optoelectronic devices (HPODs), perovskite photoactive layers should work effectively with other functional materials such as electrodes, interfacial layers and encapsulating films. Conventional two‐dimensional (2D) materials are promising candidates for this purpose because of their unique structures and/or interesting optoelectronic properties. Here, we comprehensively summarize the recent advancements in the applications of conventional 2D materials for halide perovskite‐based photodetectors, solar cells and light‐emitting diodes. The examples of these 2D materials are graphene and its derivatives, mono‐ and few‐layer transition metal dichalcogenides (TMDs), graphdiyne and metal nanosheets, etc. The research related to 2D nanostructured perovskites and 2D Ruddlesden–Popper perovskites as efficient and stable photoactive layers is also outlined. The syntheses, functions and working mechanisms of relevant 2D materials are introduced, and the challenges to achieving practical applications of HPODs using 2D materials are also discussed.     

Tailoring the Performances of Lead Halide Perovskite Devices with Electron‐Beam Irradiation

Lead halide perovskites are intensively studied in past few years due to their potential applications in optoelectronic devices such as solar cells, photodetectors, light‐emitting diodes (LED), and lasers. In addition to the rapid developments in material synthesis and device fabrication, it is also very interesting to postsynthetically control the optical properties with external irradiations. Here, the influences of very low energy (10–20 keV) electron beam of standard electron beam lithography are experimentally explored on the properties of lead halide perovskites. It is confirmed that the radiolysis process also happens and it can selectively change the photoluminescence, enabling the direct formation of nanolaser array, microsized light emitter array, and micropictures with an electron beam writer. Interestingly, it is found that discontinuous metallic lead layers are formed on the top and bottom surfaces of perovskite microplate during the radiolysis process, which can act as carrier conducting layers and significantly increase the photocurrent of perovskite photodetector by a factor of 217%. By using the electron beam with low energy to modify the perovskite, this method promises to shape the emission patterns for micro‐LED with well‐preserved optical properties and improves the photocurrent of photodetector.     

Large‐Grain Tin‐Rich Perovskite Films for Efficient Solar Cells via Metal Alloying Technique

Fast research progress on lead halide perovskite solar cells has been achieved in the past a few years. However, the presence of lead (Pb) in perovskite composition as a toxic element still remains a major issue for large‐scale deployment. In this work, a novel and facile technique is presented to fabricate tin (Sn)‐rich perovskite film using metal precursors and an alloying technique. Herein, the perovskite films are formed as a result of the reaction between Sn/Pb binary alloy metal precursors and methylammonium iodide (MAI) vapor in a chemical vapor deposition process carried out at 185 °C. It is found that in this approach the Pb/Sn precursors are first converted to (Pb/Sn)I₂ and further reaction with MAI vapor leads to the formation of perovskite films. By using Pb–Sn eutectic alloy, perovskite films with large grain sizes up to 5 µm can be grown directly from liquid phase metal. Consequently, using an alloying technique and this unique growth mechanism, a less‐toxic and efficient perovskite solar cell with a power conversion efficiency (PCE) of 14.04% is demonstrated, while pure Sn and Pb perovskite solar cells prepared in this manner yield PCEs of 4.62% and 14.21%, respectively. It is found that this alloying technique can open up a new direction to further explore different alloy systems (binary or ternary alloys) with even lower melting point.     

Lead‐Free Organic–Inorganic Hybrid Perovskites for Photovoltaic Applications: Recent Advances and Perspectives

Organic–inorganic hybrid halide perovskites (e.g., MAPbI₃) have recently emerged as novel active materials for photovoltaic applications with power conversion efficiency over 22%. Conventional perovskite solar cells (PSCs); however, suffer the issue that lead is toxic to the environment and organisms for a long time and is hard to excrete from the body. Therefore, it is imperative to find environmentally‐friendly metal ions to replace lead for the further development of PSCs. Previous work has demonstrated that Sn, Ge, Cu, Bi, and Sb ions could be used as alternative ions in perovskite configurations to form a new environmentally‐friendly lead‐free perovskite structure. Here, we review recent progress on lead‐free PSCs in terms of the theoretical insight and experimental explorations of the crystal structure of lead‐free perovskite, thin film deposition, and device performance. We also discuss the importance of obtaining further understanding of the fundamental properties of lead‐free hybrid perovskites, especially those related to photophysics.     

Perovskite solar cells: must lead be replaced – and can it be done?

Perovskite solar cells have recently drawn significant attention for photovoltaic applications with a certified power conversion efficiency of more than 22%. Unfortunately, the toxicity of the dissolvable lead content in these materials presents a critical concern for future commercial development. This review outlines some criteria for the possible replacement of lead by less toxic elements, and highlights current research progress in the application of low-lead halide perovskites as optically active materials in solar cells. These criteria are discussed with the aim of developing a better understanding of the physio-chemical properties of perovskites and of realizing similar photovoltaic performance in perovskite materials either with or without lead. Some open questions and future development prospects are outlined for further advancing perovskite solar cells toward both low toxicity and high efficiency.     

Bandgap Engineering of Stable Lead‐Free Oxide Double Perovskites for Photovoltaics

Despite the rapid progress in solar power conversion efficiency of archetype organic–inorganic hybrid perovskite CH₃NH₃PbI₃‐based solar cells, the long‐term stability and toxicity of Pb remain the main challenges for the industrial deployment, leading to more uncertainties for global commercialization. The poor stabilities of CH₃NH₃PbI₃‐based solar cells may not only be attributed to the organic molecules but also the halides themself, most of which exhibit intrinsic instability under moisture and light. As an alternative, the possibility of oxide perovskites for photovoltaic applications is explored here. The class of lead‐free stable oxide double perovskites A₂M(III)M(V)O₆ (A = Ca, Sr, Ba; M(III) = Sb³⁺ or Bi³⁺; M(V) = V⁵⁺, Nb⁵⁺, or Ta⁵⁺) is comprehensively explored with regard to their stability and their electronic and optical properties. Apart from the strong stability, this class of double perovskites exhibits direct bandgaps ranging from 0.3 to 3.8 eV. With proper B site alloying, the bandgap can be tuned within the range of 1.0–1.6 eV with optical absorptions as strong as CH₃NH₃PbI₃, making them suitable for efficient single‐junction thin‐film solar cell application.     

Hybrid Organic–Inorganic Perovskite Photodetectors

Hybrid organic–inorganic perovskite materials garner enormous attention for a wide range of optoelectronic devices. Due to their attractive optical and electrical properties including high optical absorption coefficient, high carrier mobility, and long carrier diffusion length, perovskites have opened up a great opportunity for high performance photodetectors. This review aims to give a comprehensive summary of the significant results on perovskite‐based photodetectors, focusing on the relationship among the perovskite structures, device configurations, and photodetecting performances. An introduction of recent progress in various perovskite structure‐based photodetectors is provided. The emphasis is placed on the correlation between the perovskite structure and the device performance. Next, recent developments of bandgap‐tunable perovskite and hybrid photodetectors built from perovskite heterostructures are highlighted. Then, effective approaches to enhance the stability of perovskite photodetector are presented, followed by the introduction of flexible and self‐powered perovskite photodetectors. Finally, a summary of the previous results is given, and the major challenges that need to be addressed in the future are outlined. A comprehensive summary of the research status on perovskite photodetectors is hoped to push forward the development of this field.     

Slow Hot‐Carrier Cooling in Halide Perovskites: Prospects for Hot‐Carrier Solar Cells

Rapid hot‐carrier cooling is a major loss channel in solar cells. Thermodynamic calculations reveal a 66% solar conversion efficiency for single junction cells (under 1 sun illumination) if these hot carriers are harvested before cooling to the lattice temperature. A reduced hot‐carrier cooling rate for efficient extraction is a key enabler to this disruptive technology. Recently, halide perovskites emerge as promising candidates with favorable hot‐carrier properties: slow hot‐carrier cooling lifetimes several orders of magnitude longer than conventional solar cell absorbers, long‐range hot‐carrier transport (up to ≈600 nm), and highly efficient hot‐carrier extraction (up to ≈83%). This review presents the developmental milestones, distills the complex photophysical findings, and highlights the challenges and opportunities in this emerging field. A developmental toolbox for engineering the slow hot‐carrier cooling properties in halide perovskites and prospects for perovskite hot‐carrier solar cells are also discussed.     

Perovskite Photovoltaics: The Significant Role of Ligands in Film Formation,Passivation, and Stability

Due to their outstanding optoelectronic properties, metal halide perovskites have been intensively studied in recent years. The latest certificated efficiency of 23.3% recently achieved in perovskite solar cells (PVSCs) enables them to be used as a very promising candidate for next‐generation photovoltaics. The morphology, defect density, and water resistance of perovskite films have an enormous impact on the performance and stability of PVSCs. Ligands, with coordinating capability, have been widely developed to improve the quality and stability of perovskite materials significantly. In the first section of this review, the role of ligands in fabricating perovskite films by different methods (one‐step, two‐step, and postdeposition treatment) is discussed. In the second section, the progress on ligand‐passivated perovskites via post‐treatment, in situ passivation during perovskite formation, and modifying the substrates before perovskite formation is reviewed. In the third section, a discussion of ligand‐stabilized perovskite films from the perspectives of crystal crosslinking, dimensionality engineering, and interfacial modification is presented. Finally, a summary and an outlook are given.     

Fluorinated 2D Lead Iodide Perovskite Ferroelectrics

Hybrid perovskite materials are famous for their great application potential in photovoltaics and optoelectronics. Among them, lead‐iodide‐based perovskites receive great attention because of their good optical absorption ability and excellent electrical transport properties. Although many believe the ferroelectric photovoltaic effect (FEPV) plays a crucial role for the high conversion efficiency, the ferroelectricity in CH₃NH₃PbI₃ is still under debate, and obtaining ferroelectric lead iodide perovskites is still challenging. In order to avoid the randomness and blindness in the conventional method of searching for perovskite ferroelectrics, a design strategy of fluorine modification is developed. As a demonstration, a nonpolar lead iodide perovskite is modified and a new 2D fluorinated layered hybrid perovskite material of (4,4‐difluorocyclohexylammonium)₂PbI₄, 1 , is obtained, which possesses clear ferroelectricity with controllable spontaneous polarization. The direct bandgap of 2.38 eV with strong photoluminescence also guarantees the direct observation of polarization‐induced FEPV. More importantly, the 2D structure and fluorination are also expected to achieve both good stability and charge transport properties. 1 is not only a 2D fluorinated lead iodide perovskite with confirmed ferroelectricity, but also a great platform for studying the effect of ferroelectricity and FEPV in the context of lead halide perovskite solar cells and other optoelectronic applications.     

Metal‐Halide Perovskite Transistors for Printed Electronics: Challenges and Opportunities

Following the unprecedented rise in photovoltaic power conversion efficiencies during the past five years, metal‐halide perovskites (MHPs) have emerged as a new and highly promising class of solar‐energy materials. Their extraordinary electrical and optical properties combined with the abundance of the raw materials, the simplicity of synthetic routes, and processing versatility make MHPs ideal for cost‐efficient, large‐volume manufacturing of a plethora of optoelectronic devices that span far beyond photovoltaics. Herein looks beyond current applications in the field of energy, to the area of large‐area electronics using MHPs as the semiconductor material. A comprehensive overview of the relevant fundamental material properties of MHPs, including crystal structure, electronic states, and charge transport, is provided first. Thereafter, recent demonstrations of MHP‐based thin‐film transistors and their application in logic circuits, as well as bi‐functional devices such as light‐sensing and light‐emitting transistors, are discussed. Finally, the challenges and opportunities in the area of MHPs‐based electronics, with particular emphasis on manufacturing, stability, and health and environmental concerns, are highlighted.