Synthetic Approaches for Halide Perovskite Thin Films

Composite Films of CsPbBr3 Perovskite Nanocrystals in a Hydrophobic Fluoropolymer for Temperature Imaging in Digital Microfluidics

A composite film material that combines CsPbBr₃ perovskite nanocrystals with a Hyflon AD 60 fluoropolymer was developed and utilized for high-resolution optical temperature imaging. It exhibited bright luminescence and, most importantly, long-term stability in an aqueous medium. CsPbBr₃ nanocrystal-Hyflon films immersed in aqueous solutions showed stable luminescence over at least 4 months and exhibited a fully reversible pronounced temperature sensitivity of 1.2% K^-1 between 20 and 80 °C. They were incorporated into a digital microfluidic (electrowetting on dielectric) platform and were used for spatially resolved temperature measurements during droplet movements. Thermal mapping with a CsPbBr₃ nanocrystal-Hyflon sensing layer in a room temperature environment (22.0 °C) revealed an increase in local temperatures of up to 40.2 °C upon voltage-driven droplet manipulations in a digital microfluidic system, corresponding to a local temperature change of up to 18.2 °C.

Boosting Multiple Interfaces by Co-Doped Graphene Quantum Dots for High Efficiency and Durability Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs), as the most rapidly developing next-generation thin-film photovoltaic technology, have attracted extensive research interest, yet their efficiency, scalability, and durability remain challenging. α-Fe₂O₃ could be used as an electron transporting layer (ETL) of planar PSCs, which exhibits a much higher humidity and UV light-stability compared to TiO₂-based planar PSCs. However, the photovoltaic conversion efficiency (PCE) of the Fe₂O₃-based device was still below 15% because of poor interface contact between α-Fe₂O₃ and perovskite and poor crystal quality of perovskites. In this work, we have engineered the interfaces throughout the entire solar cell via incorporating N, S co-doped graphene quantum dots (NSGQDs). The NSGQDs played remarkable multifunctional roles: (i) facilitated the perovskite crystal growth; (ii) eased charge extraction at both anode and cathode interfaces; and (iii) induced the defect passivation and suppressed the charge recombination. When assembled with a α-Fe₂O₃ ETL, the planar PSCs exhibited a significantly increased efficiency from 14 to 19.2%, with concomitant reductions in hysteresis, which created a new record of the PCE for Fe₂O₃-based PSCs to date. In addition, PSCs with the entire device interfacial engineering showed an obviously improved durability, including prominent humidity, UV light, and thermal stabilities. Our interfacial engineering methodology via graphene quantum dots represents a versatile and effective way for building high efficiency as well as durable PSCs.

Enhanced optical path and electron diffusion length enable high-efficiency perovskite tandems

Tandem solar cells involving metal-halide perovskite subcells offer routes to power conversion efficiencies (PCEs) that exceed the single-junction limit; however, reported PCE values for tandems have so far lain below their potential due to inefficient photon harvesting. Here we increase the optical path length in perovskite films by preserving smooth morphology while increasing thickness using a method we term boosted solvent extraction. Carrier collection in these films - as made - is limited by an insufficient electron diffusion length; however, we further find that adding a Lewis base reduces the trap density and enhances the electron-diffusion length to 2.3 µm, enabling a 19% PCE for 1.63 eV semi-transparent perovskite cells having an average near-infrared transmittance of 85%. The perovskite top cell combined with solution-processed colloidal quantum dot:organic hybrid bottom cell leads to a PCE of 24%; while coupling the perovskite cell with a silicon bottom cell yields a PCE of 28.2%.

In Situ Light-Initiated Ligands Cross-Linking Enables Efficient All-Solution-Processed Perovskite Light-Emitting Diodes

Solution process has been considered to be an effective method to fabricate an emitting layer (EML) in light-emitting diodes (LEDs). However, the fabrication of the charge transport layer (CTL) above the perovskite EML by solution processing is challenging. Herein, we incorporated polymerizable molecules, conjugated linoleic acid (CLA), as surface ligands to passivate perovskite QDs. The polymerized CLA can create a cross-linked QD film, which allows the solution deposition of subsequent CTLs. The theoretical calculations reveal that the binding energy of polymerized CLA with QDs increased, and the strong ligands' binding state can better passivate the surface and improve the stability of QDs. As a result, all-solution-processed multilayer perovskite LEDs were fabricated with performance of a max luminance of 2470 cd/m² for CsPbBr₃-based devices and a peak EQE of 2.67% for CsPbI₃-based devices. These results demonstrate that the in situ light-initiated ligands cross-linking can be an effective strategy in all-solution-processed optoelectronic devices.

Sub-1.4eV bandgap inorganic perovskite solar cells with long-term stability

State-of-the-art halide perovskite solar cells have bandgaps larger than 1.45 eV, which restricts their potential for realizing the Shockley-Queisser limit. Previous search for low-bandgap (1.2 to 1.4 eV) halide perovskites has resulted in several candidates, but all are hybrid organic-inorganic compositions, raising potential concern regarding device stability. Here we show the promise of an inorganic low-bandgap (1.38 eV) CsPb_0.6Sn_0.4I₃ perovskite stabilized via interface functionalization. Device efficiency up to 13.37% is demonstrated. The device shows high operational stability under one-sun-intensity illumination, with T₈₀ and T₇₀ lifetimes of 653 h and 1045 h, respectively (T₈₀ and T₇₀ represent efficiency decays to 80% and 70% of the initial value, respectively), and long-term shelf stability under nitrogen atmosphere. Controlled exposure of the device to ambient atmosphere during a long-term (1000 h) test does not degrade the efficiency. These findings point to a promising direction for achieving low-bandgap perovskite solar cells with high stability.

Current research focus on the perovskites solar cells (PSCs) is mainly limited to the lead-based ones with bandgaps above 1.5 eV. Here Hu et al. report efficient and stable inorganic tin-containing PSCs, opening doors to exploring abundant perovskite materials with bandgaps lower than 1.4 eV.

Liquid-phase growth and optoelectronic properties of two-dimensional hybrid perovskites CH3NH3PbX3 (X = Cl, Br, I)

The hybrid perovskite CH3NH3PbX3 (X = Cl, Br, I) is a promising material for developing novel optoelectronic devices. Due to its intrinsic non-layered crystal structure, it remains challenging to synthesize two-dimensional (2D) single-crystalline CH3NH3PbX3 with nanoscale thickness. Here, we report a bottom-up approach to fabricate large CH3NH3PbX3 2D crystals via liquid-phase growth on a mica substrate. The strong potassium-halogen interactions at the perovskite/mica interface decrease the interface energy, driving the striking in-plane growth of the perovskite. The grown 2D CH3NH3PbBr3 crystal was characterized as 8 nm in thickness and hundreds of micrometers in lateral size. Weak exciton binding energy was crucial for improving the photoelectric performance of 2D CH3NH3PbBr3. A visible-light photodetector with a metal/insulator/perovskite configuration was finally achieved with a photoresponsivity of 126 A W-1 and a bandwidth exceeding 80 kHz. Our work proves that the liquid-phase growth on mica is a controllable method to grow 2D hybrid CH3NH3PbX3 perovskites, which can facilitate both device applications and fundamental investigations.

Materials chemistry and engineering in metal halide perovskite lasers

The invention and development of the laser have revolutionized science, technology, and industry. Metal halide perovskites are an emerging class of semiconductors holding promising potential in further advancing the laser technology. In this Review, we provide a comprehensive overview of metal halide perovskite lasers from the viewpoint of materials chemistry and engineering. After an introduction to the materials chemistry and physics of metal halide perovskites, we present diverse optical cavities for perovskite lasers. We then comprehensively discuss various perovskite lasers with particular functionalities, including tunable lasers, multicolor lasers, continuous-wave lasers, single-mode lasers, subwavelength lasers, random lasers, polariton lasers, and laser arrays. Following this a description of the strategies for improving the stability and reducing the toxicity of metal halide perovskite lasers is provided. Finally, future research directions and challenges toward practical technology applications of perovskite lasers are provided to give an outlook on this emerging field.

High voltage vacuum-processed perovskite solar cells with organic semiconducting interlayers

In perovskite solar cells, the choice of appropriate transport layers and electrodes is of great importance to guarantee efficient charge transport and collection, minimizing recombination losses. The possibility to sequentially process multiple layers by vacuum methods offers a tool to explore the effects of different materials and their combinations on the performance of optoelectronic devices. In this work, the effect of introducing interlayers and altering the electrode work function has been evaluated in fully vacuum-deposited perovskite solar cells. We compared the performance of solar cells employing common electron buffer layers such as bathocuproine (BCP), with other injection materials used in organic light-emitting diodes, such as lithium quinolate (Liq), as well as their combination. Additionally, high voltage solar cells were obtained using low work function metal electrodes, although with compromised stability. Solar cells with enhanced photovoltage and stability under continuous operation were obtained using BCP and BCP/Liq interlayers, resulting in an efficiency of approximately 19%, which is remarkable for simple methylammonium lead iodide absorbers.

The effect of n-type interlayers and electrodes on the voltage and stability of fully vacuum-deposited perovskite solar cells is evaluated.

Modeling Thin Film Solar Cells: From Organic to Perovskite

Device model simulation is one of the primary tools for modeling thin film solar cells from organic materials to organic-inorganic perovskite materials. By directly connecting the current density-voltage (J-V) curves to the underlying device physics, it is helpful in revealing the working mechanism of the heatedly discussed organic-inorganic hybrid perovskite solar cells. Some distinctive optoelectronic features need more phenomenological models and accurate simulations. Herein, the application of the device model method in the simulation of organic and organic-inorganic perovskite solar cells is reviewed. To this end, the ways of the device model are elucidated by discussing the metal-insulator-metal picture and the equations describing the physics. Next, the simulations on J-V curves of organic solar cells are given in the presence of the space charge, interface, charge injection, traps, or exciton. In the perovskite section, the effects of trap states, direct band recombination, surface recombination, and ion migration on the device performance are systematically discussed from the perspective of the device model simulation. Suggestions for designing perovskite devices with better performance are also given.

Vacuum-Deposited Blue Inorganic Perovskite Light-Emitting Diodes

Perovskite light-emitting diodes (PeLEDs) have drawn great research attention because of their outstanding electroluminescence performance by solution processing. PeLEDs made by thermal evaporation are relatively rarely explored but are compatible to existing organic light-emitting diode industrial lines. Blue-emitting PeLEDs are all based on organic-containing perovskites, rather than more stable all-inorganic perovskites because of their poor solubility, too fast crystallization, uneven discrete films, and unattainable pure blue emission. Here, we report all-inorganic, vacuum-processed blue PeLEDs. High-throughput combinatorial approaches are employed to optimize Cs-Pb-Br-Cl composition in our dual-source co-evaporation system to achieve the balance between film photoluminescence and injection efficiency. The as-deposited perovskite films demonstrated excellent intrinsic stability against heat, UV-light, and humidity attack. A series of PeLEDs were obtained covering the standard blue spectral region with a best luminance of 121 cd/m² and an external quantum efficiency of 0.38%. We believe that the vacuum processing strategy demonstrated here provides a very promising alternative way to produce efficient and stable all-inorganic blue-emitting PeLEDs.

Mechanoperovskites for Photovoltaic Applications: Preparation, Characterization, and Device Fabrication

Hybrid organic-inorganic metal halide perovskites (MHPs) have emerged as excellent absorber materials for next generation solar cells owing to their simple solution-processed synthesis and high efficiency. This breakthrough in photovoltaics along with an accompanying impact in light-emitting applications prompted a renaissance of interest in the broad family of MHPs. Notably, the optoelectronic properties and the photovoltaic parameters of MHPs are highly sensitive to the adopted synthetic strategy. The preparation of MHPs has commonly relied on solution-based methods requiring elevated temperatures for homogeneity of reaction mixtures. While the solution-based approach is relatively versatile, it faces challenges such as limitations in compositional engineering of MHPs or their long-term storage among others. Therefore, there is a continuous great challenge to develop efficient synthetic strategies affording various high-quality MHP materials for numerous technological optoelectronic applications. In the past decade, mechanochemistry has appeared as a green alternative to traditional synthesis. This solid-state, re-emerging efficient synthetic methodology mediated by direct absorption of mechanical energy is growing explosively across organic and inorganic chemistry and materials science. In this Account, we describe our shared interest in the productive use of mechanical force in chemistry of MHPs, as well as assembly of the respective solar cell devices. We highlight the milestones achieved by our groups along with the seminal contributions by other groups. In particular, we demonstrate that mechanochemistry efficiently allows the formation of various phase pure hybrid lead and lead-free halide perovskite compositions (called hereafter "mechanoperovskites"). The progress in solvent-free solid-state synthesis is greatly enhanced by the integration of advanced methods of solid-state analysis like powder X-ray diffraction (pXRD), solid-state nuclear magnetic resonance (ss-NMR) and UV-vis spectroscopies, and we aim to illustrate this ongoing integration through appropriate examples. Furthermore, we show that thin films based on mechanoperovskites have the advantage of providing a higher degree of control of the stoichiometry and higher reproducibility, stability, and material phase purity. The impact of using powdered mechanoperovskite as a precursor for thin film formation on the electrochemical and photovoltaic properties of the solar cells is also discussed. Finally, our view of current challenges and future directions in this emerging interdisciplinary area of research is provided.

Engineering Halide Perovskite Crystals through Precursor Chemistry

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

Contrasting Effects of Organic Chloride Additives on Performance of Direct and Inverted Perovskite Solar Cells

Perovskite solar cells (PSCs) have demonstrated encouraging progress in recent years. Additive engineering, where diverse additives are incorporated into the perovskite layer, has been widely adopted to tune the perovskite grains, reduce defect density and charge recombination. Here, we observe a universal phenomenon that organic chloride additives enhance the open circuit voltage (V_OC) and power conversion efficiencies (PCEs) of direct PSCs but decrease the V_OC, short-circuit current (J_SC), and PCE of inverted PSCs, regardless of the choice of charge transport materials. The polyTPD-based direct device incorporating trimethylammonium chloride (TACl) additive delivery improved PCE from 17.8 to 20.0%, arising from the enhanced V_OC from 1.03 to 1.12 V. With the same content of TACl, the best PCE of the polyTPD-based inverted device decreased from 20.2 to 18.5% because of the reduced V_OC (1.05-1.01 V) and J_SC (23.2-22.5 mA/cm²). Our investigation confirms that organic chloride will p-dope perovskites and elevate the work functions, which lead to favorable/unfavorable charge transfer between perovskite films and its upper transport layers in direct and inverted devices. This work provides an insight into the rational design of the device structure when applying additives which can dope the perovskite to affect charge transfer at the perovskite/charge transport layer interface.

Multiple Roles of Cobalt Pyrazol-Pyridine Complexes in High-Performing Perovskite Solar Cells

Chemical doping is a ubiquitously applied strategy to improve the charge-transfer and conductivity characteristics of spiro-OMeTAD, a hole-transporting material (HTM) used widely in solution-processed perovskite solar cells (PSCs). Cobalt(III) complexes are commonly employed HTM dopants, whose major role is to oxidize spiro-OMeTAD to provide p-doping for improved conductivity. The present work discloses additional, previously unknown important functions of cobalt complexes in the HTM films that influence the photovoltaic performance. Specifically, it is demonstrated that commercial p-dopant FK269 (bis(2,6-di(1H-pyrazol-1-yl)pyridine) cobalt(III) tris(bis(trifluoromethylsulfonyl)imide)) reduces the interfacial recombination and alleviates the decomposition of the perovskite layer under the action of tert-butylpyridine and lithium bis(trifluoromethanesulfonyl)imide. These effects are demonstrated for 1 cm² perovskite solar cells that achieve a stabilized power conversion efficiency of 19% under 1 sun irradiation.

Long-Term Chemical Aging of Hybrid Halide Perovskites

Because the power conversion efficiency (PCE) of hybrid halide perovskite solar cells (PSCs) could exceed 24%, extensive research has been focused on improving their long-term stability for commercialization in the near future. In a previous study, we reported that the addition of a number of ionized iodide (triiodide: I₃^-) ions during perovskite film formation significantly improved the efficiency of PSCs by reducing deep-level defects in the perovskite layer. Understanding the relationship between the concentration of these defects and the long-term chemical aging of PSCs is important not only for obtaining fundamental insight into the perovskite materials but also for studying the long-term chemical stability of PSCs. Herein we aim to identify the origin of the natural decay in PCE during long-term chemical aging of PSCs in the dark based on formamidinium lead triiodide by comparing the performance of control and low-defect (LD) devices. After aging for 200 days, the change in the PCE of the LD devices (1.3%) was found to be half that of the control devices (2.6%). We investigated this difference using grazing incidence wide-angle X-ray scattering, deep-level transient spectroscopy, scanning photoelectron microscopy, and high-resolution photoemission spectroscopy. The addition of I₃^- was found to reduce the amounts of hydroxide and O_x in the halide perovskites (HPs), affecting the migration of defects and the structural transformation of the HPs.

Material and Interface Engineering for High-Performance Perovskite Solar Cells: A Personal Journey and Perspective

Hybrid organic-inorganic perovskite solar cells (PSCs) have become a shining star in the photovoltaic field due to their spectacular increase in power conversion efficiency (PCE) from 3.8 % to over 23 % in just few years, opening up the potential in addressing the important future energy and environment issues. The excellent photovoltaic performance can be attributed to the unique properties of the organometal halide perovskite materials, including high absorption coefficient, tunable bandgap, high defect tolerance, and excellent charge transport characteristics. The authors entered this field when pursuing research on dye-sensitized solar cells (DSCs) by leveraging nanorods arrays for vectorial transport of the extracted electrons. Soon after, we and others realized that while the organometal halide perovskite materials have excellent intrinsic properties for solar cells, interface engineering is at least equally important in the development of high-performance PSCs, which includes surface defect passivation, band alignment, and heterojunction formation. Herein, we will address this topic by presenting the historical development and recent progress on the interface engineering of PSCs primarily of our own group. This review is mainly focused on the material and interface design of the conventional n-i-p, inverted p-i-n and carbon electrode-based structure devices from our own experience and perspective. Finally, the challenges and prospects of this area for future development will also be discussed.

Thermodynamic Stability and Structural Insights for CH3NH3Pb1-xSixI3, CH3NH3Pb1-xGexI3, and CH3NH3Pb1-xSnxI3 Hybrid Perovskite Alloys: A Statistical Approach from First Principles Calculations

The recent reaching of 20% of conversion efficiency by solar cells based on metal hybrid perovskites (MHP), e.g., the methylammonium (MA) lead iodide, CH3NH3PbI3 (MAPbI3), has excited the scientific community devoted to the photovoltaic materials. However, the toxicity of Pb is a hindrance for large scale commercial of MHP and motivates the search of another congener eco-friendly metal. Here, we employed first-principles calculations via density functional theory combined with the generalized quasichemical approximation to investigate the structural, thermodynamic, and ordering properties of MAPb1-xSixI3, MAPb1-xGexI3, and MAPb1-xSnxI3 alloys as pseudo-cubic structures. The inclusion of a smaller second metal, as Si and Ge, strongly affects the structural properties, reducing the cavity volume occupied by the organic cation and limitating the free orientation under high temperature effects. Unstable and metaestable phases are observed at room temperature for MAPb1-xSixI3, whereas MAPb1-xGexI3 is energetically favored for Pb-rich in ordered phases even at very low temperatures. Conversely, the high miscibility of Pb and Sn into MAPb1-xSnxI3 yields an alloy energetically favored as a pseudo-cubic random alloy with tunable properties at room temperature.

Two-Dimensional Halide Perovskites in Solar Cells: 2D or not 2D?

Two-dimensional (2D) halide perovskites have recently emerged as a more stable and more versatile family of materials than three-dimensional (3D) perovskite solar cell absorbers. Although solar cells made with 2D perovskites have yet to improve their power conversion efficiencies to compete with 3D perovskite solar cells, their immense diversity offers great opportunities and avenues for research that will likely close the gap between these two. Further, 2D perovskites can have various roles within a solar cell, either as the primary light absorber, as a capping layer, passivating layer, or within a mixed 2D/3D perovskite solar cell absorber. In this Minireview, we will review the history of 2D perovskites in solar cells, the relevant properties of such materials, the different roles that they can play in a solar cell, as well as current trends and challenges.

Surprising discoveries on the way to an old compound: four transient iodido antimonates

During the synthesis of the literature-known iodido antimonate [Cu(MeCN)4]4[Sb3I11]2 (MeCN = acetonitrile), four transient compounds, [Cu(MeCN)4]4[Sb6I22]·2MeCN (1), [Cu(MeCN)4]4[Sb7I25]·MeCN (2), [Cu(MeCN)4]4[Sb10I34] (3) and [Cu(MeCN)4]4[Sb8I28] (4), were identified. The compounds appeared within hours or days and subsequently re-dissolved in the mother liquor, leading to [Cu(MeCN)4]4[Sb3I11]2 as the final product. Single crystal X-ray analysis showed that all four compounds feature unprecedented anion motifs.

Performance enhancement of perovskite solar cells via material quality improvement assisted by MAI/IPA solution post-treatment

Improved perovskite film quality by MAI/IPA solution post-treatment.

Mechanochemical synthesis of 0D and 3D cesium lead mixed halide perovskites

A simplified mechanochemical synthesis approach for Cs-containing mixed halide perovskite materials of lower and higher dimensionality (0D and 3D, respectively) is presented with stoichiometric control from their halide salts, CsX and PbX2 (X = Cl, Br, I). Excellent optical bandgap tunability through halide substitution is supported by property measurements and changes to the materials' structure. Complementary NMR and XRD methods, along with support from DFT calculations, reveal highly crystalline 0D and 3D solid solutions with a complex arrangement of [PbX6-xXx']4- pseudooctahedra caused by halide distribution about the Pb centre.

A polar-hydrophobic ionic liquid induces grain growth and stabilization in halide perovskites

A polar-hydrophobic ionic liquid additive enlarges and functionalizes halide perovskite grains.