Narrow Bandgap Metal Halide Perovskites for All-Perovskite Tandem Photovoltaics.

All-perovskite tandem solar cells are attracting considerable interest in photovoltaics research, owing to their potential to surpass the theoretical efficiency limit of single-junction cells, in a cost-effective sustainable manner. Thanks to the bandgap-bowing effect, mixed tin-lead (Sn-Pb) perovskites possess a close to ideal narrow bandgap for constructing tandem cells, matched with wide-bandgap neat lead-based counterparts. The performance of all-perovskite tandems, however, has yet to reach its efficiency potential. One of the main obstacles that need to be overcome is the─oftentimes─low quality of the mixed Sn-Pb perovskite films, largely caused by the facile oxidation of Sn(II) to Sn(IV), as well as the difficult-to-control film crystallization dynamics. Additional detrimental imperfections are introduced in the perovskite thin film, particularly at its vulnerable surfaces, including the top and bottom interfaces as well as the grain boundaries. Due to these issues, the resultant device performance is distinctly far lower than their theoretically achievable maximum efficiency. Robust modifications and improvements to the surfaces of mixed Sn-Pb perovskite films are therefore critical for the advancement of the field. This Review describes the origins of imperfections in thin films and covers efforts made so far toward reaching a better understanding of mixed Sn-Pb perovskites, in particular with respect to surface modifications that improved the efficiency and stability of the narrow bandgap solar cells. In addition, we also outline the important issues of integrating the narrow bandgap subcells for achieving reliable and efficient all-perovskite double- and multi-junction tandems. Future work should focus on the characterization and visualization of the specific surface defects, as well as tracking their evolution under different external stimuli, guiding in turn the processing for efficient and stable single-junction and tandem solar cell devices.

An open-cage bis[60]fulleroid as an electron transport material for tin halide perovskite solar cells.

An open-cage bis[60]fulleroid (OC) was applied as an electron transport material (ETM) in tin (Sn) halide perovskite solar cells (PSCs). Due to the reduced offset between the energy levels of Sn-based perovskites and ETMs, the power conversion efficiency (PCE) of Sn-based PSCs with OC reached 9.6% with an open-circuit voltage (VOC) of 0.72 V. Additionally, OC exhibited superior thermal stability and provided 75% of the material without decomposition after vacuum deposition. The PSC using vacuum-deposited OC as the ETM could afford a PCE of 7.6%, which is a big leap forward compared with previous results using vacuum-deposited fullerene derivatives as ETMs.

Anchoring Charge Selective Self-Assembled Monolayers for Tin-Lead Perovskite Solar Cells.

Self-assembled monolayers (SAMs) have displayed great potential for improving efficiency and stability in p-i-n perovskite solar cells (PSCs). The anchoring of SAMs at the conductiv metal oxide substrates and their interaction with perovskite materials must be rationally tailored to ensure efficient charge carrier extraction and improved quality of the perovskite films. Herein, SAMs molecules with different anchoring groups and spacers to control the interaction with perovskite in the p-i-n mixed Sn-Pb PSCs are selected. It is found that the monolayer with the carboxylate group exhibits appropriate interaction and has a more favorable orientation and arrangement than that of the phosphate group. This results in reduced nonradiative recombination and enhanced crystallinity. In addition, the short chain length leads to an improved energy level alignment of SAMs with perovskite, improving hole extraction. As a result, the narrow bandgap (≈1.25 eV) Sn-Pb PSCs show efficiencies of up to 23.1% with an open-circuit voltage of up to 0.89 V. Unencapsulated devices retain 93% of their initial efficiency after storage in N2 atmosphere for over 2500 h. Overall, this work highlights the underexplored potential of SAMs for perovskite photovoltaics and provides essential findings on the influence of their structural modification.

Stabilizing a non-IPR C2(13333)-C74 cage with Lu2C2/Lu2O: the importance of encaged non-metallic elements.

A difference in encaged non-metallic element (i.e., C2versus O) leads to a clear change of intramolecular interactions and shifts in redox potentials of Lu2C2@C2(13333)-C74 and Lu2O@C2(13333)-C74, as a result of their distinct molecular orbital energy levels. Different from these two endoherals whose HOMOs are located on the cage, experimentally absent Lu2@C2(13333)-C74 possesses a HOMO predominantly delocalized on the internal Lu-Lu bond, accompanied by a much smaller HOMO-LUMO gap, suggesting that the presence of a non-metallic unit broadens the electrochemical gaps and consequently improves the kinetic stability. These findings shed light on the role of non-metallic moieties in clusterfullerenes, providing valuable insights into the stability and properties of metallofullerenes.

Tin Halide Perovskite Solar Cells with Open-Circuit Voltages Approaching the Shockley-Queisser Limit.

The power conversion efficiency of tin-based halide perovskite solar cells is limited by large photovoltage losses arising from the significant energy-level offset between the perovskite and the conventional electron transport material, fullerene C60. The fullerene derivative indene-C60 bisadduct (ICBA) is a promising alternative to mitigate this drawback, owing to its superior energy level matching with most tin-based perovskites. However, the less finely controlled energy disorder of the ICBA films leads to the extension of its band tails that limits the photovoltage of the resultant devices and reduces the power conversion efficiency. Herein, we fabricate ICBA films with improved morphology and electrical properties by optimizing the choice of solvent and the annealing temperature. Energy disorder in the ICBA films is substantially reduced, as evidenced by the 22 meV smaller width of the electronic density of states. The resulting solar cells show open-circuit voltages of up to 1.01 V, one of the highest values reported so far for tin-based devices. Combined with surface passivation, this strategy enabled solar cells with efficiencies of up to 11.57%. Our work highlights the importance of controlling the properties of the electron transport material toward the development of efficient lead-free perovskite solar cells and demonstrates the potential of solvent engineering for efficient device processing.

Prospects for Tin-Containing Halide Perovskite Photovoltaics.

Tin-containing metal halide perovskites have enormous potential as photovoltaics, both in narrow band gap mixed tin-lead materials for all-perovskite tandems and for lead-free perovskites. The introduction of Sn(II), however, has significant effects on the solution chemistry, crystallization, defect states, and other material properties in halide perovskites. In this perspective, we summarize the main hurdles for tin-containing perovskites and highlight successful attempts made by the community to overcome them. We discuss important research directions for the development of these materials and propose some approaches to achieve a unified understanding of Sn incorporation. We particularly focus on the discussion of charge carrier dynamics and nonradiative losses at the interfaces between perovskite and charge extraction layers in p-i-n cells. We hope these insights will aid the community to accelerate the development of high-performance, stable single-junction tin-containing perovskite solar cells and all-perovskite tandems.

BAr2 -Bridged Azafulvene Dimers with Tunable Energy Levels for Photostable Near-Infrared Dyes.

Organic dyes with strong absorption in the near-infrared (NIR) region are potentially useful in medical applications, such as tumor imaging and photothermal therapy. In this work, new NIR dyes combining BAr2 -bridged azafulvene dimer acceptors with diarylaminothienyl donors in a donor-acceptor-donor configuration were synthesized. Surprisingly, it was found that in these molecules the BAr2 -bridged azafulvene acceptor adopts a 5-membered, rather than 6-membered ring structure. The influence of the aryl substituents on the HOMO and LUMO energy levels of the dye compounds was assessed from electrochemical and optical measurements. Strong electron-withdrawing fluorinated substituents (Ar=C6 F5 , 3,5-(CF3 )2 C6 H3 ) lowered the HOMO energy while preserving the small HOMO-LUMO energy gap, resulting in promising NIR dye molecules that combine strong absorption bands centered around 900 nm with good photostability.

Tripodal Triazatruxene Derivative as a Face-On Oriented Hole-Collecting Monolayer for Efficient and Stable Inverted Perovskite Solar Cells.

Hole-collecting monolayers have drawn attention in perovskite solar cell research due to their ease of processing, high performance, and good durability. Since molecules in the hole-collecting monolayer are typically composed of functionalized π-conjugated structures, hole extraction is expected to be more efficient when the π-cores are oriented face-on with respect to the adjacent surfaces. However, strategies for reliably controlling the molecular orientation in monolayers remain elusive. In this work, multiple phosphonic acid anchoring groups were used to control the molecular orientation of a series of triazatruxene derivatives chemisorbed on a transparent conducting oxide electrode surface. Using infrared reflection absorption spectroscopy and metastable atom electron spectroscopy, we found that multipodal derivatives align face-on to the electrode surface, while the monopodal counterpart adopts a more tilted configuration. The face-on orientation was found to facilitate hole extraction, leading to inverted perovskite solar cells with enhanced stability and high-power conversion efficiencies up to 23.0%.

Synergistic Surface Modification of Tin-Lead Perovskite Solar Cells.

Interfaces in thin-film photovoltaics play a pivotal role in determining device efficiency and longevity. In this work, the top surface treatment of mixed tin-lead (≈1.26 eV) halide perovskite films for p-i-n solar cells is studied. Charge extraction is promoted by treating the perovskite surface with piperazine. This compound reacts with the organic cations at the perovskite surface, modifying the surface structure and tuning the interfacial energy level alignment. In addition, the combined treatment with C60 pyrrolidine tris-acid (CPTA) reduces hysteresis and leads to efficiencies up to 22.7%, with open-circuit voltage values reaching 0.90 V, ≈92% of the radiative limit for the bandgap of this material. The modified cells also show superior stability, with unencapsulated cells retaining 96% of their initial efficiency after >2000 h of storage in N2 and encapsulated cells retaining 90% efficiency after >450 h of storage in air. Intriguingly, CPTA preferentially binds to Sn2+ sites at film surface over Pb2+ due to the energetically favored exposure of the former, according to first-principles calculations. This work provides new insights into the surface chemistry of perovskite films in terms of their structural, electronic, and defect characteristics and this knowledge is used to fabricate state-of-the-art solar cells.

A Universal Surface Treatment for p-i-n Perovskite Solar Cells.

Perovskite interfaces critically influence the final performance of the photovoltaic devices. Optimizing them by reducing the defect densities or improving the contact with the charge transporting material is key to further enhance the efficiency and stability of perovskite solar cells. Inverted (p-i-n) devices can particularly benefit here, as evident from various successful attempts. However, every reported strategy is adapted to specific cell structures and compositions, affecting their robustness and applicability by other researchers. In this work, we present the universality of perovskite top surface post-treatment with ethylenediammonium diiodide (EDAI2) for p-i-n devices. To prove it, we compare devices bearing perovskite films of different composition, i.e., Sn-, Pb-, and mixed Sn-Pb-based devices, achieving efficiencies of up to 11.4, 22.0, and 22.9%, respectively. A careful optimization of the EDAI2 thickness indicates a different tolerance for Pb- and Sn-based devices. The main benefit of this treatment is evident in the open-circuit voltage, with enhancements of up to 200 mV for some compositions. In addition, we prove that this treatment can be successfully applied by both wet (spin-coating) and dry (thermal evaporation) methods, regardless of the composition. The versatility of this treatment makes it highly appealing for industrial application, as it can be easily adapted to specific processing requirements. We present a detailed experimental protocol, aiming to provide the community with an easy, universal perovskite post-treatment method for reliably improving the device efficiency, highlighting the potential of interfaces for the field.

Perovskite/Perovskite Tandem Solar Cells in the Substrate Configuration with Potential for Bifacial Operation.

Perovskite/perovskite tandem solar cells have recently exceeded the record power conversion efficiency (PCE) of single-junction perovskite solar cells. They are typically built in the superstrate configuration, in which the device is illuminated from the substrate side. This limits the fabrication of the solar cell to transparent substrates, typically glass coated with a transparent conductive oxide (TCO), and adds constraints because the first subcell that is deposited on the substrate must contain the wide-bandgap perovskite. However, devices in the substrate configuration could potentially be fabricated on a large variety of opaque and inexpensive substrates, such as plastic and metal foils. Importantly, in the substrate configuration the narrow-bandgap subcell is deposited first, which allows for more freedom in the device design. In this work, we report perovskite/perovskite tandem solar cells fabricated in the substrate configuration. As the substrate we use TCO-coated glass on which a solution-processed narrow-bandgap perovskite solar cell is deposited. All of the other layers are then processed using vacuum sublimation, starting with the charge recombination layers, then the wide-bandgap perovskite subcell, and finishing with the transparent top TCO electrode. Proof-of-concept tandem solar cells show a maximum PCE of 20%, which is still moderate compared to those of best-in-class devices realized in the superstrate configuration yet higher than those of the corresponding single-junction devices in the substrate configuration. As both the top and bottom electrodes are semitransparent, these devices also have the potential to be used as bifacial tandem solar cells.

Steering Lu3N clusters in C76-78 cages: cluster configuration dominated by cage transformation.

While the strong interaction between the internal unit and the fullerene cage inside metallofullerenes is widely acknowledged, how the cage transformation interacts with the cluster configuration remains elusive. For this purpose, we herein synthesized three metallofullerene molecules with an easy-to-compare cluster configuration and cage arrangement, namely Lu3N@Cs(17 490)-C76, Lu3N@C2(22 010)-C78, and Lu3N@D3h(5)-C78. The three lutetium-based nitride clusterfullerenes (NCFs) with small C76-78 carbon cages were synthesized by a modified arc-discharge method and their structures were unambiguously confirmed by X-ray crystallography. Notably, the cage transformation from Cs(17 490)-C76 to C2(22 010)-C78via a simple C2-unit insertion leads to a remarkable configuration change of the encapsulated Lu3N cluster from an unusual asymmetric plane to a common symmetric one. This close correlation between the cluster configuration and cage transformation is further confirmed by the pyramidal Lu3N cluster in Lu3N@D3h(5)-C78 other than the symmetric planar Lu3N unit in Lu3N@C2(22 010)-C78, as a result of an even larger difference in the cage arrangement. Astonishingly, such a cluster shrinkage, accompanied by an increase in the cage size from Cs(17 490)-C76 to D3h(5)-C78, is dramatically opposite to the cluster expansion with cage elongation found in La2C2- or Y2C2-based metallofullerenes.

Cluster-Geometry-Associated Metal-Metal Bonding in Trimetallic Carbide Clusterfullerenes.

Geometry configurations of the metallic clusters play a significant role in the involved bonding nature. Herein, we report the crystallographic characterization of unprecedented erbium-based trimetallic clusterfullerenes, namely, Er3C2@Ih(7)-C80, in which the inner Er3C2 cluster presents a lifted bat ray configuration with the C2 unit elevated by ∼1.62 Å above the Er3 plane. Within the plane, the Er···Er distances for Er1···Er2, Er1···Er2A, and Er2···Er2A are 3.4051(15), 3.4051(15), and 3.3178(15) Å, respectively, falling into the range of the metal-metal bonding. Density functional theory calculations unveil the three-center-one-electron Er-Er-Er bond in Er3C2@Ih(7)-C80 with one electron shared by three metals, and thus, its exceptional electronic structure can be expressed as (Er3)8+(C2)2-@C806-. Interestingly, with the further observation on the geometry configurations of the encapsulated clusters in M3C2@C2n (M = Sc, Y, and Lu) series, we find that the lifted bat ray configuration of the inner cluster is explicitly associated with the formation of the bonding interactions between the inner metals. This finding provides insights into the nature of metal-metal bonding and gives guidelines for the design of the single-molecule magnet.

Mixed lead-tin perovskite films with >7 µs charge carrier lifetimes realized by maltol post-treatment.

Mixed lead-tin (Pb-Sn) halide perovskites with optimum band gaps near 1.3 eV are promising candidates for next-generation solar cells. However, the performance of solar cells fabricated with Pb-Sn perovskites is restricted by the facile oxidation of Sn(ii) to Sn(iv), which induces self-doping. Maltol, a naturally occurring flavor enhancer and strong metal binding agent, was found to effectively suppress Sn(iv) formation and passivate defects in mixed Pb-Sn perovskite films. When used in combination with Sn(iv) scavenging, the maltol surface treatment led to high-quality perovskite films which showed enhanced photoluminescence intensities and charge carrier lifetimes in excess of 7 µs. The scavenging and surface treatments resulted in highly reproducible solar cell devices, with photoconversion efficiencies of up to 21.4% under AM1.5G illumination.

Sn(IV)-free tin perovskite films realized by in situ Sn(0) nanoparticle treatment of the precursor solution.

The toxicity of lead perovskite hampers the commercialization of perovskite-based photovoltaics. While tin perovskite is a promising alternative, the facile oxidation of tin(II) to tin(IV) causes a high density of defects, resulting in lower solar cell efficiencies. Here, we show that tin(0) nanoparticles in the precursor solution can scavenge tin(IV) impurities, and demonstrate that this treatment leads to effectively tin(IV)-free perovskite films with strong photoluminescence and prolonged decay lifetimes. These nanoparticles are generated by the selective reaction of a dihydropyrazine derivative with the tin(II) fluoride additive already present in the precursor solution. Using this nanoparticle treatment, the power conversion efficiency of tin-based solar cells reaches 11.5%, with an open-circuit voltage of 0.76 V. Our nanoparticle treatment is a simple and broadly effective method that improves the purity and electrical performance of tin perovskite films.

Crystallographic Characterization of Er2C2@C80-88: Cluster Stretching with Cage Elongation.

Six dierbium carbide endohedral metallofullerenes have been synthesized and chromatographically isolated. Single-crystal X-ray diffractometry unambiguously ascertains their structures as Er2C2@C2v(5)-C80, Er2C2@Cs(6)-C82, Er2C2@Cs(15)-C84, Er2C2@C2v(9)-C86, Er2C2@Cs(15)-C86, and Er2C2@Cs(32)-C88, respectively. The Er···Er distances of the major erbium sites inside the Cs(6)-C82, C2v(5)-C80, Cs(15)-C84, Cs(15)-C86, C2v(9)-C86, and Cs(32)-C88 cages are 3.801, 3.860, 4.062, 4.066, 4.307, and 4.372 Å, respectively, which show a linear tendency with an increase in the major axis of the fullerene cages (8.064, 8.238, 8.508, 8.582, 8.815, and 8.953 Å, respectively). Furthermore, the electrochemical and molecular orbital analyses reveal that the redox chemistry of the Er2C2@C80-88 isomers is associated with the carbon cage, which is different from the situations found for typical dimetallofullerenes, such as Y2@C82, Er2@C82-84, and Lu2@C82,86 isomers, which show metal-dependent oxidation processes, indicating the importance of C2 insertion in carbide cluster metallofullerenes.

Endohedral Metallofullerenes: New Structures and Unseen Phenomena.

Endohedral metallofullerenes (EMFs), namely fullerenes with metallic species encapsulated inside, represent an ideal platform to investigate metal-metal or metal-carbon interactions at the sub-nanometer scale by means of single-crystal X-ray diffraction (XRD) crystallography. Herein, recent progress in the identification of new structures and unprecedented properties are discussed according to the categories of monometallofullerenes, dimetallofullerenes, carbide clusterfullerenes, and nitride clusterfullerenes. In particular, the dimerization and the cage-isomer dependent oxidation state of the inner metal atom are summarized in terms of pristine monometallofullerenes. Metal-metal bonds involving lanthanide-lanthanides or actinide-actinides are discussed based on both experimental and theoretical studies. The cluster-cage matching and/or mutual selections, as well as the rarely seen M=C double bonds, are discovered in M2 C2 @C2n , U2 C@C80 , M2 TiC@C80 , and Ti3 C3 @C80 . Subsequently, the geometries of different M3 N clusters in various cages are discussed, revealing size-matching between the internal M3 N cluster and the outer cage induced by the planarity of the cluster. Finally, an outlook regarding the future developments of the molecular structures and applications of EMFs is presented.

Diels-Alder Cycloaddition on Nonisolated-Pentagon-Rule C2v(19 138)-C76 and YNC@C2v(19 138)-C76: The Difference in Regioselectivity Caused by the Inner Metallic Cluster.

Diels-Alder reactions of cyclopentadiene to C2v(19 138)-C76 and YNC@C2v(19 138)-C76 violating the isolated pentagon rule have been systematically studied by means of density functional theory calculations. As for the free fullerene, the pentalene-type [5,5]-bond in the adjacent pentagon pair is the most favorable from thermodynamic and kinetic viewpoints, which is attributed to the highly strained carbon atoms accompanied by the suitable lowest unoccupied molecular orbital shape with a large distribution to interact with cyclopentadiene. Upon encapsulating the YNC cluster, a corannulene-type [5,6]-bond and a pyracylene-type [6,6]-bond become the two most reactive addition sites under thermodynamic and kinetic conditions, which possess similar reaction energies and energy barriers. Especially, the [5,6]-bond exhibits a larger reaction energy and a lower energy barrier than that on the free fullerene, which should be ascribed to its shorter bond length and larger π-orbital axis vector value after trapping the metallic cluster. The suitable unoccupied molecular orbital lobes with large distributions on the [5,6]- and [6,6]-bonds are also an advantage of cycloadditions. This work presents the first example that the most favorable addition site is remote from the adjacent pentagon pair in the fullerene cage after encapsulating a metallic cluster.

Crystallographic characterization of Er2C2@C2(43)-C90, Er2C2@C2(40)-C90, Er2C2@C2(44)-C90, and Er2C2@C1(21)-C90: the role of cage-shape on cluster configuration.

For endohedral metallofullerenes (EMFs), that is, fullerenes encapsulating metallic species, cage size is known to be an important factor for cluster configuration adoption; however, the impact of the cage shape on the cluster geometry fitting remains poorly understood. Herein, for the first time, four dierbium-carbide EMFs with C90 cages, namely, Er2C2@C2(43)-C90, Er2C2@C2(40)-C90, Er2C2@C2(44)-C90, and Er2C2@C1(21)-C90, were successfully synthesized and fully characterized using a combination of mass spectrometry, single-crystal X-ray diffractometry, vis-NIR, Raman and photoluminescence spectroscopies, and cyclic voltammetry. In particular, the fullerene cages of C2(43)-C90 and C2(44)-C90 are crystallographically identified for the first time. Interestingly, the ErEr distance of the major sites in Er2C2@C2(43)-C90, Er2C2@C2(40)-C90, Er2C2@C2(44)-C90, and Er2C2@C1(21)-C90 is 3.927, 4.058, 4.172, and 4.651 Å, respectively, which increases gradually with an increase in the major axis of the cage. Moreover, the bond length of the inner C2-unit decreases progressively with an increase in the ErEr distance, indicating that the inserted C2-unit can serve as a molecular spring to support the strong metal-cage interactions within cages with the same size but different shapes. Hence, the role of cage shape on the cluster configuration is unveiled safely for the as-obtained Er2C2@C90 isomers.

Crystallographic characterization of Er3N@C2n (2n = 80, 82, 84, 88): the importance of a planar Er3N cluster.

A series of Er-based nitride clusterfullerenes (NCFs), Er3N@C80-88, have been successfully synthesized and isolated. In particular, Er3N@Ih(7)-C80, Er3N@D5h(6)-C80, Er3N@C2v(9)-C82, Er3N@Cs(51365)-C84, and Er3N@D2(35)-C88 have been characterized by single-crystal X-ray diffraction (XRD) for the first time. The planar configuration of the inserted Er3N cluster is identified unambiguously and the Er-N distances increase in accordance with cage expansion to maintain strong metal-cage interactions. Additionally, the electrochemical properties of the Er3N@C80-88 series are studied by means of cyclic voltammetry. It is found that the first reduction potentials are roughly similar for all compounds under study, while the first oxidation potentials are cathodically shifted along with the increase of the cage size in the Er3N@C2n (2n = 80, 84, 86, 88) series, leading to a decrease in the corresponding electrochemical band gaps. Nevertheless, for Er3N@C2v(9)-C82, a good electron donating ability is manifested by its relatively small first oxidation potential, which results from the relatively higher energy level of the highest occupied molecular orbital. The redox behaviors observed in such Er3N-based NCFs may promise their great potential applications in donor-acceptor systems.