Semi‐Locked Tetrathienylethene as a Building Block for Hole‐Transporting Materials: Toward Efficient and Stable Perovskite Solar Cells

The construction of state‐of‐the‐art hole‐transporting materials (HTMs) is challenging regarding the appropriate molecular configuration for simultaneously achieving high morphology uniformity and charge mobility, especially because of the lack of appropriate building blocks. Herein a semi‐locked tetrathienylethene (TTE) serves as a promising building block for HTMs by fine‐tuning molecular planarity. Upon incorporation of four triphenylamine groups, the resulting TTE represents the first hybrid orthogonal and planar conformation, thus leading to the desirable electronic and morphological properties in perovskite solar cells (PSCs). Owing to its high hole mobility, deep lying HOMO level, and excellent thin film quality, the dopant‐free TTE‐based PSCs exhibit a very promising efficiency of over 20 % with long‐term stability, achieving to date the best performances among dopant‐free HTM‐based planar n‐i‐p structured PSCs.     

Cove‐Edge Nanoribbon Materials for Efficient Inverted Halide Perovskite Solar Cells

Two cove‐edge graphene nanoribbons hPDI2‐Pyr‐hPDI2 ( 1 ) and hPDI3‐Pyr‐hPDI3 ( 2 ) are used as efficient electron‐transporting materials (ETMs) in inverted planar perovskite solar cells (PSCs). Devices based on the new graphene nanoribbons exhibit maximum power‐conversion efficiencies (PCEs) of 15.6 % and 16.5 % for 1 and 2 , respectively, while a maximum PCE of 14.9 % is achieved with devices based on [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM). The interfacial effects induced by these new materials are studied using photoluminescence (PL), and we find that 1 and 2 act as efficient electron‐extraction materials. Additionally, compared with PC₆₁BM, these new materials are more hydrophobic and have slightly higher LUMO energy levels, thus providing better device performance and higher device stability.     

Highly Conductive Anion‐Exchange Membranes from Microporous Tröger's Base Polymers

The development of polymeric anion‐exchange membranes (AEMs) combining high ion conductivity and long‐term stability is a major challenge for materials chemistry. AEMs with regularly distributed fixed cationic groups, based on the formation of microporous polymers containing the V‐shape rigid Tröger's base units, are reported for the first time. Despite their simple preparation, which involves only two synthetic steps using commercially available precursors, the polymers provide AEMs with exceptional hydroxide conductivity at relatively low ion‐exchange capacity, as well as a high swelling resistance and chemical stability. An unprecedented hydroxide conductivity of 164.4 mS cm^?1 is obtained at a relatively a low ion‐exchange capacity of 0.82 mmol g^?1 under optimal operating conditions. The exceptional anion conductivity appears related to the intrinsic microporosity of the charged polymer matrix, which facilitates rapid anion transport.     

Indeno[1,2‐b]carbazole as Methoxy‐Free Donor Group: Constructing Efficient and Stable Hole‐Transporting Materials for Perovskite Solar Cells

With perovskite‐based solar cells (PSCs) now reaching efficiencies of greater than 20 %, the stability of PSC devices has become a critical challenge for commercialization. However, most efficient hole‐transporting materials (HTMs) thus far still rely on the state‐of‐the‐art methoxy triphenylamine (MOTPA) donor unit in which methoxy groups usually reduce the device stability. Herein, a carbazole‐fluorene hybrid has been employed as a methoxy‐free donor to construct organic HTMs. The indeno[1,2‐b]carbazole group not only inherits the characteristics of carbazole and fluorene, but also exhibits additional advantages arising from the bulky planar structure. Consequently, M129, endowed with indeno[1,2‐b]carbazole simultaneously exhibits a promising efficiency of over 20 % and superior long‐term stability. The hybrid strategy toward the methoxy‐free donor opens a new avenue for developing efficient and stable HTMs.     

An Easy and Efficient Synthesis of Bisindolylmethanes and Tetraindolylmethane Tröger's Base Catatlyzed by AgBF4

This work presents a highly efficient and simple method for the synthesis of bisindolylmethanes, catalyzed by AgBF₄, with excellent yields. Various substituted aldehydes and ketones with indole under this reaction condition is elucidated. This reaction occurs efficiently under mild reaction conditions, such as are applied with methoxy or furfural, which commonly undergoes cleavage under strongly acidic reaction conditions. The presented approach was suitable for the synthesis of complex systems, such as tetraindolylmethane Tröger's Base.     

Efficient Hole‐Transporting Materials with Triazole Core for High‐Efficiency Perovskite Solar Cells

Efficient hole‐transporting materials (HTMs), TAZ‐[MeOTPA]₂ and TAZ‐[MeOTPATh]₂ incorporating two electron‐rich diphenylamino side arms, through direct linkage or thiophen bridges, respectively, on the C3‐ and C5‐positions of a 4‐phenyl‐1,2,4‐triazole core were synthesized. These synthetic HTMs with donor–acceptor type molecular structures exhibited effective intramolecular charge transfer for improving the hole‐transporting properties. The structural modification of HTMs by thiophene bridging might increase intermolecular π–π stacking in the solid state and afford a better spectral response because of their increased π‐conjugation length. Perovskite‐based cells using TAZ‐[MeOTPA]₂ and TAZ‐[MeOTPATh]₂ as HTMs afforded high power conversion efficiencies of 10.9 % and 14.4 %, respectively, showing a photovoltaic performance comparable to that obtained using spiro‐OMeTAD. These synthetically simple and inexpensive HTMs hold promise for replacing the more expensive spiro‐OMeTAD in high‐efficiency perovskite solar cells.     

Formation of Stable Tin Perovskites Co‐crystallized with Three Halides for Carbon‐Based Mesoscopic Lead‐Free Perovskite Solar Cells

We synthesized and characterized methylammonium (MA) mixed tri‐halide tin perovskites (MASnIBr_2−x Cl_x) for carbon‐based mesoscopic solar cells free of lead and hole‐transporting layers. Varied SnCl₂/SnBr₂ ratios yielded tin perovskites with three halides (I, Br, and Cl) co‐crystallized inside the tin‐perovskite. When the SnCl₂ proportion was ≥50 % (x ≥1), phase separation occurred to give MASnI_3−y Br_y and MASnCl_3−z Br_z in the stoichiometric proportions of their precursors, confirmed by XRD. A device with MASnIBr_1.8Cl_0.2 (SnCl₂=10 %) showed the best photovoltaic performance: J _SC=14.0 mA cm⁻², V _OC=380 mV, FF=0.573, and PCE=3.1 %, and long‐term stability. Electrochemical impedance spectra (EIS) show superior charge recombination and dielectric relaxation properties for the MASnIBr_1.8Cl_0.2 cell. Transient PL decays showed the intrinsic problem of tin‐based perovskites with average lifetimes less than 100 ps.     

Observing Defect Passivation of the Grain Boundary with 2-Aminoterephthalic Acid for Efficient and Stable Perovskite Solar Cells

Metal halide perovskite solar cells (PSCs), with their exceptional properties, show promise as photoelectric converters. However, defects in the perovskite layer, particularly at the grain boundaries (GBs), seriously restrict the performance and stability of PSCs. Now, a simple post-treatment procedure involves applying 2-aminoterephthalic acid to the perovskite to produce efficient and stable PSCs. By optimizing the post-treatment conditions, we created a device that achieved a remarkable power conversion efficiency (PCE) of 21.09 % and demonstrated improved stability. This improvement was attributed to the fact that the 2-aminoterephthalic acid acted as a cross-linking agent that inhibited the migration of ions and passivated the trap states at GBs. These findings provide a potential strategy for designing efficient and stable PSCs regarding the aspects of defect passivation and crystal growth.     

Thermally Rearranged Polymer Membranes Containing Tröger's Base Units Have Exceptional Performance for Air Separations

The influence of segmental chain motion on the gas separation performance of thermally rearranged (TR) polymer membranes is established for TR polybenzoxazoles featuring Tröger's base (TB) monomer subunits as exceptionally rigid sites of contortion along the polymer backbone. These polymers are accessed from solution‐processable ortho‐acetate functionalized polyimides, which are readily synthesized as high‐molecular‐weight polymers for membrane casting. We find that thermal rearrangement leads to a small increase in d‐spacing between polymer chains and a dramatic pore‐network reconfiguration that increases both membrane permeability and O₂/N₂ selectivity, putting its performance above the 2015 upper bound.     

A Cation‐Exchange Approach for the Fabrication of Efficient Methylammonium Tin Iodide Perovskite Solar Cells

Tin‐based halide perovskite materials have been successfully employed in lead‐free perovskite solar cells, but the overall power conversion efficiencies (PCEs) have been limited by the high carrier concentration from the facile oxidation of Sn²⁺ to Sn⁴⁺. Now a chemical route is developed for fabrication of high‐quality methylammonium tin iodide perovskite (MASnI₃) films: hydrazinium tin iodide (HASnI₃) perovskite film is first solution‐deposited using presursors hydrazinium iodide (HAI) and tin iodide (SnI₂), and then transformed into MASnI₃ via a cation displacement approach. With the two‐step process, a dense and uniform MASnI₃ film is obtained with large grain sizes and high crystallization. Detrimental oxidation is suppressed by the hydrazine released from the film during the transformation. With the MASnI₃ as light harvester, mesoporous perovskite solar cells were prepared, and a maximum power conversion efficiency (PCE) of 7.13 % is delivered with good reproducibility.     

Hole‐Transport Materials for Perovskite Solar Cells

The pressure to move towards renewable energy has inspired researchers to look for ideas in photovoltaics that may lead to a major breakthrough. Recently the use of perovskites as a light harvester has lead to stunning progress. The power conversion efficiency of perovskite solar cells is now approaching parity (>22 %) with that of the established technology which took decades to reach this level of performance. The use of a hole transport material (HTM) remains indispensable in perovskite solar cells. Perovskites can conduct holes, but they are present at low levels, and for efficient charge extraction a HTM layer is a prerequisite. Herein we provide an overview of the diverse types of HTM available, from organic to inorganic, in the hope of encouraging further research and the optimization of these materials.     

Benzotrithiophene‐Based Hole‐Transporting Materials for 18.2 % Perovskite Solar Cells

New star‐shaped benzotrithiophene (BTT)‐based hole‐transporting materials (HTM) BTT‐1, BTT‐2 and BTT‐3 have been obtained through a facile synthetic route by crosslinking triarylamine‐based donor groups with a benzotrithiophene (BTT) core. The BTT HTMs were tested on solution‐processed lead trihalide perovskite‐based solar cells. Power conversion efficiencies in the range of 16 % to 18.2 % were achieved under AM 1.5 sun with the three derivatives. These values are comparable to those obtained with today's most commonly used HTM spiro‐OMeTAD, which point them out as promising candidates to be used as readily available and cost‐effective alternatives in perovskite solar cells (PSCs).     

Highly Efficient Perovskite Solar Cells Employing an Easily Attainable Bifluorenylidene‐Based Hole‐Transporting Material

The 4,4′‐dimethoxydiphenylamine‐substituted 9,9′‐bifluorenylidene ( KR216 ) hole transporting material has been synthesized using a straightforward two‐step procedure from commercially available and inexpensive starting reagents, mimicking the synthetically challenging 9,9′‐spirobifluorene moiety of the well‐studied spiro‐OMeTAD. A power conversion efficiency of 17.8 % has been reached employing a novel HTM in a perovskite solar cells.     

Antioxidant Grain Passivation for Air‐Stable Tin‐Based Perovskite Solar Cells

Tin‐based perovskites with excellent optoelectronic properties and suitable band gaps are promising candidates for the preparation of efficient lead‐free perovskite solar cells (PSCs). However, it is challenging to prepare highly stable and efficient tin‐based PSCs because Sn²⁺ in perovskites can be easily oxidized to Sn⁴⁺ upon air exposure. Here we report the fabrication of air‐stable FASnI₃ solar cells by introducing hydroxybenzene sulfonic acid or its salt as an antioxidant additive into the perovskite precursor solution along with excess SnCl₂. The interaction between the sulfonate group and the Sn²⁺ ion enables the in situ encapsulation of the perovskite grains with a SnCl₂–additive complex layer, which results in greatly enhanced oxidation stability of the perovskite film. The corresponding PSCs are able to maintain 80 % of the efficiency over 500 h upon air exposure without encapsulation, which is over ten times longer than the best result reported previously. Our results suggest a possible strategy for the future design of efficient and stable tin‐based PSCs.     

3,4‐Phenylenedioxythiophene (PheDOT) Based Hole‐Transporting Materials for Perovskite Solar Cells

Two new electron‐rich molecules based on 3,4‐phenylenedioxythiophene (PheDOT) were synthesized and successfully adopted as hole‐transporting materials (HTMs) in perovskite solar cells (PSCs). X‐ray diffraction, absorption spectra, photoluminescence spectra, electrochemical properties, thermal stabilities, hole mobilities, conductivities, and photovoltaic parameters of PSCs based on these two HTMs were compared with each other. By introducing methoxy substituents into the main skeleton, the energy levels of PheDOT‐core HTM were tuned to match with the perovskite, and its hole mobility was also improved (1.33×10^?4 cm² V^?1 s^?1, being higher than that of spiro‐OMeTAD, 2.34×10^?5 cm² V^?1 s^?1). The PSC based on MeO‐PheDOT as HTM exhibits a short‐circuit current density (J_sc) of 18.31 mA cm^?2, an open‐circuit potential (V_oc) of 0.914 V, and a fill factor (FF) of 0.636, yielding an encouraging power conversion efficiency (PCE) of 10.64 % under AM 1.5G illumination. These results give some insight into how the molecular structures of HTMs affect their performances and pave the way for developing high‐efficiency and low‐cost HTMs for PSCs.