Tetrapodal Hole-Collecting Monolayer Materials Based on Saddle-Like Cyclooctatetraene Core for Inverted Perovskite Solar Cells.

Hole-collecting monolayers have greatly advanced the development of positive-intrinsic-negative perovskite solar cells (p-i-n PSCs). To date, however, most of the anchoring groups in the reported monolayer materials are designed to bind to the transparent conductive oxide (TCO) surface, resulting in less availability for other functions such as tuning the wettability of the monolayer surface. In this work, we developed two anchorable molecules, 4PATTI-C3 and 4PATTI-C4, by employing a saddle-like indole-fused cyclooctatetraene as a p-core with four phosphonic acid anchoring groups linked through propyl or butyl chains. Both molecules form monolayers on TCO substrates. Thanks to the saddle shape of a cyclooctatetraene skeleton, two of the four phosphonic acid anchoring groups were found to point upward, resulting in hydrophilic surfaces. Compared to the devices using 4PATTI-C4 as the hole-collecting monolayer, 4PATTI-C3-based devices exhibit a faster hole-collection process, leading to higher power conversion efficiencies of up to 21.7% and 21.4% for a mini-cell (0.1 cm2) and a mini-module (1.62 cm2), respectively, together with good operational stability. This work represents how structural modification of multipodal molecules could substantially modulate the functions of the hole-collecting monolayers after being adsorbed onto TCO substrates.

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.

In Situ Thermal Cross-Linking of 9,9'-Spirobifluorene-Based Hole-Transporting Layer for Perovskite Solar Cells.

A novel 9,9'-spirobifluorene derivative bearing thermally cross-linkable vinyl groups (V1382) was developed as a hole-transporting material for perovskite solar cells (PSCs). After thermal cross-linking, a smooth and solvent-resistant three-dimensional (3D) polymeric network is formed such that orthogonal solvents are no longer needed to process subsequent layers. Copolymerizing V1382 with 4,4'-thiobisbenzenethiol (dithiol) lowers the cross-linking temperature to 103 °C via the facile thiol-ene "click" reaction. The effectiveness of the cross-linked V1382/dithiol was demonstrated both as a hole-transporting material in p-i-n and as an interlayer between the perovskite and the hole-transporting layer in n-i-p PSC devices. Both devices exhibit better power conversion efficiencies and operational stability than devices using conventional PTAA or Spiro-OMeTAD hole-transporting materials.

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.

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.

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.

Additive-free, Cost-Effective Hole-Transporting Materials for Perovskite Solar Cells Based on Vinyl Triarylamines.

A series of cost-effective hole-transporting materials (TOP-HTMs) for perovskite solar cells (PSCs) was designed and synthesized. The molecules, composed of multiple 4,4'-dimethoxytriphenylamines linked to a benzene core via trans-vinylene units, can be manufactured from inexpensive materials through a simple synthetic route. The photophysical, electrochemical, and thermal properties, as well as hole mobilities, were strongly influenced by the position and number of vinyl triarylamine substituents on the core benzene ring. CH3NH3PbI3-based solar cells using the X-shaped TOP-HTM 3 with additives gave a high power conversion efficiency of 17.5% (forward scan)/18.6% (reverse scan). Crucially, TOP-HTMs gave high working device efficiency without the need for conduction-enhancing additives. The power conversion efficiency for the device with additive-free TOP-HTM 3 was 16.0% (forward scan)/16.6% (reverse scan). Device stability is also enhanced and is superior to the reference HTM, 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD).

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.

A Purified, Solvent-Intercalated Precursor Complex for Wide-Process-Window Fabrication of Efficient Perovskite Solar Cells and Modules.

A high-purity methylammonium lead iodide complex with intercalated dimethylformamide (DMF) molecules, CH3 NH3 PbI3 ⋅DMF, is introduced as an effective precursor material for fabricating high-quality solution-processed perovskite layers. Spin-coated films of the solvent-intercalated complex dissolved in pure dimethyl sulfoxide (DMSO) yielded thick, dense perovskite layers after thermal annealing. The low volatility of the pure DMSO solvent extended the allowable time for low-speed spin programs and considerably relaxed the precision needed for the antisolvent addition step. An optimized, reliable fabrication method was devised to take advantage of this extended process window and resulted in highly consistent performance of perovskite solar cell devices, with up to 19.8 % power-conversion efficiency (PCE). The optimized method was also used to fabricate a 22.0 cm2 , eight-cell module with 14.2 % PCE (active area) and 8.64 V output (1.08 V/cell).

Influence of Alkoxy Chain Length on the Properties of Two-Dimensionally Expanded Azulene-Core-Based Hole-Transporting Materials for Efficient Perovskite Solar Cells.

A series of two-dimensionally expanded azulene-core-based π systems have been synthesized with different alkyl chain lengths in the alkoxy moieties connected to the partially oxygen-bridged triarylamine skeletons. The thermal, photophysical, and electronic properties of each compound were evaluated to determine the influence of the alkyl chain length on their effectiveness as hole-transporting materials (HTMs) in perovskite solar cells (PSCs). All the synthesized molecules showed promising material properties, including high solubility, the formation of flat and amorphous films, and optimal alignment of energy levels with perovskites. In particular, the derivatives with methyl and n-butyl in the side chains retained amorphous stability up to 233 and 159 °C, respectively. Such short alkoxy chains also resulted in improved electrical device properties. The PSC device fabricated with the HTM with n-butyl side chains showed the best performance with a power conversion efficiency of 18.9 %, which compares favorably with that of spiro-OMeTAD-based PSCs (spiro-OMeTAD=2,2',7,7'-tetrakis[N,N-bis(p-methoxyphenyl)amino]-9,9'-spirobifluorene).

Alternative Face-on Thin Film Structure of Pentacene.

Pentacene attracts a great deal of attention as a basic material used in organic thin-film transistors for many years. Pentacene is known to form a highly ordered structure in a thin film, in which the molecular long axis aligns perpendicularly to the substrate surface, i.e., end-on orientation. On the other hand, the face-on oriented thin film, where the molecular plane is parallel to the substrate, has never been found on an inert substrate represented by SiO2. As a result, the face-on orientation has long been believed to be generated only on specific substrates such as a metal single crystal. In the present study, the face-on orientation grown on a SiO2 surface has first been identified by means of visible and infrared p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) together with two-dimensional grazing incidence X-ray diffraction (2D-GIXD). The combination of the multiple techniques readily reveals that the face-on phase is definitely realized as the dominant component. The face-on film is obtained when the film growth is kinetically restricted to be prevented from transforming into the thermodynamically stable structure, i.e., the end-on orientation. This concept is useful for controlling the molecular orientation in general organic semiconductor thin films.

Phthalimide-Based Transparent Electron-Transport Materials with Oriented-Amorphous Structures: Preparation from Solution-Processed Precursor Films.

A star-shaped molecule featuring three phthalimide units attached to a triazine core was designed as an electron-transport material. Solution processing was achieved by preparing a precursor molecule bearing tert-butoxycarbonyl (Boc) substituents, which are afterwards removed by annealing. The annealed film is transparent to visible light, with an absorption edge of 339 nm. Two-dimensional grazing incidence X-ray diffraction (2D-GIXD) and p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) measurements showed that the molecular orientation changed from random to face-on during the cleaving of Boc groups while keeping the overall crystallinity low. This "oriented-amorphous" structure is desirable for transport layers in devices such as organic light-emitting diodes (OLEDs) and solar cells.

Donor-acceptor polymers containing thiazole-fused benzothiadiazole acceptor units for organic solar cells.

Two p-type semiconducting donor-acceptor polymers were designed and synthesized for use in organic solar cells. The polymers combine a benzodithiophene (BDT) donor and a thiazole-fused benzothiadiazole (TzBT) acceptor. Two TzBT acceptor units are compared, one with an alkylthio group (P1) and the other with a more strongly electron-withdrawing alkylsulfonyl group (P2) at the fused thiazole ring. The strongly electron-accepting nature of the TzBT unit lowers the lowest unoccupied molecular orbital (LUMO) energy of P1 and P2 relative to that of the BT analog (PBDT-BT), without altering the energy of the highest occupied molecular orbital (HOMO). Despite the smaller optical band gaps, bulk heterojunction organic solar cells fabricated using these polymers in a PC71BM blend showed high open-circuit voltages. The power conversion efficiency (PCE) of the P1-based device reached 6.13%. Though efficiency of the P2-based device was lower, photoelectric conversion extended into the near-IR region up to 950 nm.

Lead-Free Solar Cells based on Tin Halide Perovskite Films with High Coverage and Improved Aggregation.

Two simple methods to improve tin halide perovskite film structure are introduced, aimed at increasing the power conversion efficiency of lead free perovskite solar cells. First, a hot antisolvent treatment (HAT) was found to increase the film coverage and prevent electrical shunting in the photovoltaic device. Second, it was discovered that annealing under a low partial pressure of dimethyl sulfoxide vapor increased the average crystallite size. The topographical and electrical qualities of the perovskite films are substantively improved as a result of the combined treatments, facilitating the fabrication of tin-based perovskite solar cell devices with power conversion efficiencies of over 7 %.

Accurate Molecular Orientation Analysis Using Infrared p-Polarized Multiple-Angle Incidence Resolution Spectrometry (pMAIRS) Considering the Refractive Index of the Thin Film Sample.

Infrared (IR) p-polarized multiple-angle incidence resolution spectrometry (pMAIRS) is a powerful tool for analyzing the molecular orientation in an organic thin film. In particular, pMAIRS works powerfully for a thin film with a highly rough surface irrespective of degree of the crystallinity. Recently, the optimal experimental condition has comprehensively been revealed, with which the accuracy of the analytical results has largely been improved. Regardless, some unresolved matters still remain. A structurally isotropic sample, for example, yields different peak intensities in the in-plane and out-of-plane spectra. In the present study, this effect is shown to be due to the refractive index of the sample film and a correction factor has been developed using rigorous theoretical methods. As a result, with the use of the correction factor, organic materials having atypical refractive indices such as perfluoroalkyl compounds ( n = 1.35) and fullerene ( n = 1.83) can be analyzed with high accuracy comparable to a compound having a normal refractive index of approximately 1.55. With this improved technique, we are also ready for discriminating an isotropic structure from an oriented sample having the magic angle of 54.7°.

Voltage stress induced reversible diode behavior in pentacene thin films.

The current-voltage characteristics of a vacuum-deposited 100 nm pentacene thin film have been measured in situ under ultrahigh vacuum. Despite using bottom contact geometry with titanium for both electrodes, the I-V curves are asymmetric and the direction and degree of the diode-like behavior vary with sample and measurement history. After careful examination we have found that applying a high positive or negative bias voltage for about 24 h at elevated temperatures was sufficient to completely switch the diode forward direction. The switching action is fully reversible and the diode behavior, once switched, remains stable to repeated measurements at least over a period of several weeks.

In situ conductance measurements of copper phthalocyanine thin film growth on sapphire [0001].

The current flowing through a thin film of copper phthalocyanine vacuum deposited on a single crystal sapphire [0001] surface was measured during film growth from 0 to 93 nm. The results, expressed as conductance vs. nominal film thickness, indicate three distinct film growth regions. Conductive material forms below about 5 nm and again above 35 nm, but in the intermediate thicknesses the film conductance was observed to decrease with increasing film thickness. With the aid of ac-AFM topology images taken ex situ, the conductance results are explained based on the Stranski-Krastanov (2D + 3D) film growth mechanism, in which the formation of a thin wetting layer is followed by the growth of discrete islands that eventually coalesce into an interpenetrating, conductive network.