Non‐Planar and Flexible Hole‐Transporting Materials from Bis‐Xanthene and Bis‐Thioxanthene Units for Perovskite Solar Cells

Two new hole‐transporting materials (HTMs), BX‐OMeTAD and BTX‐OMeTAD , based on xanthene and thioxanthene units, respectively, and bearing p‐methoxydiphenylamine peripheral groups, are presented for their use in perovskite solar cells (PSCs). The novelty of the newly designed molecules relies on the use of a single carbon‐carbon bond ‘C?C’ as a linker between the two functionalized heterocycles, which increases the flexibility of the molecule compared with the more rigid structure of the widely used HTM spiro‐OMeTAD. The new HTMs display a limited absorbance in the visible region, due to the lack of conjugation between the two molecular halves, and the chemical design used has a remarkably impact on the thermal properties when compared to spiro‐OMeTAD. BX‐OMeTAD and BTX‐OMeTAD have been tested in ([(FAPbI₃)_0.87(MAPbBr₃)_0.13]_0.92[CsPbI₃]_0.08)‐based PSC devices exhibiting power conversion efficiencies of 14.19 and 16.55 %, respectively. The efficiencies reached, although lower than those measured for spiro‐OMeTAD (19.63 %), are good enough to consider the chemical strategy used as an interesting via to design HTMs for PSCs.     

Direct C−H Arylation Meets Perovskite Solar Cells: Tin‐Free Synthesis Shortcut to High‐Performance Hole‐Transporting Materials

In contrast to the traditional multistep synthesis, we demonstrate herein a two‐step synthesis shortcut to triphenylamine‐based hole‐transporting materials (HTMs) through sequential direct C?H arylations. These hole‐transporting molecules are fabricated in perovskite‐based solar cells (PSCs) that exhibit promising efficiencies up to 17.69 %, which is comparable to PSCs utilizing commercially available 2,2′,7,7′‐tetrakis[N,N‐di(4‐methoxyphenyl)amino]‐9,9′‐spirobifluorene (spiro‐OMeTAD) as the HTM. This is the first report describing the use of step‐saving C?H activations/arylations in the facile synthesis of small‐molecule HTMs for perovskite solar cells.     

Inexpensive Hole‐Transporting Materials Derived from Tröger's Base Afford Efficient and Stable Perovskite Solar Cells

The synthesis of three enamine hole‐transporting materials (HTMs) based on Tröger's base scaffold are reported. These compounds are obtained in a three‐step facile synthesis from commercially available materials without the need of expensive catalysts, inert conditions or time‐consuming purification steps. The best performing material, HTM3, demonstrated 18.62 % PCE in PSCs, rivaling spiro‐OMeTAD in efficiency, and showing markedly superior long‐term stability in non‐encapsulated devices. In dopant‐free PSCs, HTM3 outperformed spiro‐OMeTAD by a factror of 1.6. The high glass‐transition temperature (T_g=176 °C) of HTM3 also suggests promising perspectives in device applications.     

Self‐Assembly of Hybrid Oxidant POM@Cu‐BTC for Enhanced Efficiency and Long‐Term Stability of Perovskite Solar Cells

The controllable oxidation of spiro‐OMeTAD and improving the stability of hole‐transport materials (HTMs) layer are crucial for good performance and stability of perovskite solar cells (PSCs). Herein, we report an efficient hybrid polyoxometalate@metal–organic framework (POM@MOF) material, [Cu₂(BTC)_4/3(H₂O)₂]₆[H₃PMo₁₂O₄₀]₂ or POM@Cu‐BTC, for the oxidation of spiro‐OMeTAD with Li‐TFSI and TBP. When POM@Cu‐BTC is introduced to the HTM layer as a dopant, the PSCs achieve a superior fill factor of 0.80 and enhanced power conversion efficiency 21.44 %, as well as improved long‐term stability in an ambient atmosphere without encapsulation. The enhanced performance is attributed to the oxidation activity of POM anions and solid‐state nanoparticles. Therefore, this research presents a facile way by using hybrid porous materials to accelerate oxidation of spiro‐OMeTAD, further improving the efficiency and stability of PSCs.     

Dopant‐Free Squaraine‐Based Polymeric Hole‐Transporting Materials with Comprehensive Passivation Effects for Efficient All‐Inorganic Perovskite Solar Cells

Development of high‐performance dopant‐free hole‐transporting materials (HTMs) with comprehensive passivation effects is highly desirable for all‐inorganic perovskite solar cells (PVSCs). Squaraines (SQs) could be a candidate for dopant‐free HTMs as they are natural passivators for perovskites. One major limitation of SQs is their relatively low hole mobility. Herein we demonstrate that polymerizing SQs into pseudo two dimensional (2D) p–π conjugated polymers could overcome this problem. By rationally using N,N‐diarylanilinosquaraines as the comonomers, the resulting polysquaraine HTMs not only exhibit suitable energy levels and efficient passivation effects, but also achieve very high hole mobility close to 0.01 cm^?2 V^?1 s^?1. Thus as dopant‐free HTMs for α‐CsPbI₂Br‐based all‐inorganic PVSCs, the best PCE reached is 15.5 %, outperforming those of the doped‐Spiro‐OMeTAD (14.4 %) based control devices and among the best for all‐inorganic PVSCs.     

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.     

Efficient Hole Transporting Materials with Two or Four N,N‐Di(4‐methoxyphenyl)aminophenyl Arms on an Ethene Unit for Perovskite Solar Cells

Novel steric bulky hole transporting materials (HTMs) with two or four N,N‐di(4‐methoxyphenyl)aminophenyl units have been synthesized. When the EtheneTTPA was used as a hole transporting material in perovskite solar cell, the power conversion efficiency afforded 12.77 % under AM 1.5 G illumination, which is comparable to the widely used spiro‐OMeTAD based solar cell (13.28 %).     

Facile Synthesis of a Furan–Arylamine Hole‐Transporting Material for High‐Efficiency,Mesoscopic Perovskite Solar Cells

A novel hole‐transporting molecule (F101) based on a furan core has been synthesized by means of a short, high‐yielding route. When used as the hole‐transporting material (HTM) in mesoporous methylammonium lead halide perovskite solar cells (PSCs) it produced better device performance than the current state‐of‐the‐art HTM 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD). The F101‐HTM‐based device exhibited both slightly higher J_sc (19.63 vs. 18.41 mA cm^?2) and V_oc (1.1 vs. 1.05 V) resulting in a marginally higher power conversion efficiency (PCE) (13.1 vs. 13 %). The steady‐state and time‐resolved photoluminescence show that F101 has significant charge extraction ability. The simple molecular structure, short synthesis route with high yield and better performance in devices makes F101 an excellent candidate for replacing the expensive spiro‐OMeTAD as HTM in PSCs.     

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).     

A Simple 3,4‐Ethylenedioxythiophene Based Hole‐Transporting Material for Perovskite Solar Cells

We report a novel electron‐rich molecule based on 3,4‐ethylenedioxythiophene (H101). When used as the hole‐transporting layer in a perovskite‐based solar cell, the power‐conversion efficiency reached 13.8 % under AM 1.5G solar simulation. This result is comparable with that obtained using the well‐known hole transporting material 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9′‐spirobifluorene (spiro‐OMeTAD). This is the first heterocycle‐containing material achieving >10 % efficiency in such devices, and has great potential to replace the expensive spiro‐OMeTAD given its much simpler and cheaper synthesis.     

Efficient Blue‐Colored Solid‐State Dye‐Sensitized Solar Cells: Enhanced Charge Collection by Using an in Situ Photoelectrochemically Generated Conducting Polymer Hole Conductor

A high power conversion efficiency (PCE) of 5.5 % was achieved by efficiently incorporating a diketopyrrolopyrrole‐based dye with a conducting polymer poly(3,4‐ethylenediothiophene) (PEDOT) hole‐transporting material (HTM) that was formed in situ, compared with a PCE of 2.9 % for small molecular spiro‐OMeTAD‐based solid‐state dye solar cells (sDSCs). The high PCE for PEDOT‐based sDSCs is mainly attributed to the significantly enhanced charge‐collection efficiency, as a result of the three‐order‐of‐magnitude higher hole conductivity (0.53 S cm^?1) compared with that of the widely used low molecular weight HTM spiro‐OMeTAD (3.5×10^?4 S cm^?1).     

Helicenes as All‐in‐One Organic Materials for Application in OLEDs: Synthesis and Diverse Applications of Carbo‐ and Aza[5]helical Diamines

A set of eight helical diamines were designed and synthesized to demonstrate their relevance as all‐in‐one materials for multifarious applications in organic light‐emitting diodes (OLEDs), that is, as hole‐transporting materials (HTMs), EMs, bifunctional hole transporting + emissive materials, and host materials. Azahelical diamines function very well as HTMs. Indeed, with high T_g values (127–214 °C), they are superior alternatives to popular N,N′‐di(1‐naphthyl)‐N,N′‐diphenyl‐(1,1′‐biphenyl)‐4,4′‐diamine (NPB). All the helical diamines exhibit emissive properties when employed in nondoped as well as doped devices, the performance characteristics being superior in the latter. One of the carbohelical diamines (CHTPA) serves the dual function of hole transport as well as emission in simple double‐layer devices; the efficiencies observed were better by quite some margin than those of other emissive helicenes reported. The twisting endows helical diamines with significantly high triplet energies such that they also function as host materials for red and green phosphors, that is, [Ir(btp)₂acac] (btp=2‐(2′‐benzothienyl)pyridine; acac=acetylacetonate) and [Ir(ppy)₃] (ppy=2‐phenylpyridine), respectively. The results of device fabrications demonstrate how helicity/ helical scaffold may be diligently exploited to create molecular systems for maneuvering diverse applications in OLEDs.     

Lowering Molecular Symmetry To Improve the Morphological Properties of the Hole‐Transport Layer for Stable Perovskite Solar Cells

Inspired by the structural feature of the classical hole‐transport material (HTM), Spiro‐OMeTAD, many analogues based on a highly symmetrical spiro‐core were reported for perovskite solar cells (PSCs). However, these HTMs were prone to crystallize because of the high molecular symmetry, forming non‐uniform films, unfavorable for the device stability and large‐area processing. By lowering the symmetry of spiro‐core, we report herein a novel spirobisindane‐based HTM, Spiro‐I, which could form amorphous films with high uniformity and morphological stability. Compared to the Spiro‐OMeTAD‐based PSCs, those containing Spiro‐I exhibit similar efficiencies for small area but higher ones for large area (1 cm²), and especially much higher air stability (retaining 80 % of initial PCE after 2400 h storage without encapsulation). Moreover, the Spiro‐I can be synthesized from a cheap starting material bisphenol A and used with a small amount for the device fabrication.     

Emerging of Inorganic Hole Transporting Materials For Perovskite Solar Cells

Hole transporting material (HTM) is a significant component to achieve the high performance perovskite solar cells (PSCs). Over the years, inorganic, organic and hybrid (organic‐inorganic) material based HTMs have been developed and investigated successfully. Today, perovskite solar cells achieved the efficiency of 22.1 % with with 2,2’,7,7’‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine) 9,9‐spirobifluorene (spiro‐OMeTAD) as HTM. Nevertheless, synthesis and cost of organic HTMs is a major challenging issue and therefore alternative materials are required. From the past few years, inorganic HTMs showed large improvement in power conversion efficiency (PCE) and stability. Recently CuO_x reached the PCE of 19.0% with better stability. These developments affirms that inorganic HTMs are better alternativesto the organic HTMs for next generation PSCs. In this report, we mainly focussed on the recent advances of inorganic and hybrid HTMs for PSCs and highlighted the efficiency and stability of PSCs improved by changing metal oxides as HTMs. Consequently, we expect that energy levels of these inorganic HTMs matches very well with the valence band of perovskites and improved efficiency helps in future practical deployment of low cost PSCs.     

Dithieno[3,2‐b:2′,3′‐d]pyrrole Cored p‐Type Semiconductors Enabling 20 % Efficiency Dopant‐Free Perovskite Solar Cells

Organic p‐type semiconductors with tunable structures offer great opportunities for hybrid perovskite solar cells (PVSCs). We report herein two dithieno[3,2‐b:2′,3′‐d]pyrrole (DTP) cored molecular semiconductors prepared through π‐conjugation extension and an N‐alkylation strategy. The as‐prepared conjugated molecules exhibit a highest occupied molecular orbital (HOMO) level of ?4.82 eV and a hole mobility up to 2.16×10^?4 cm² V^?1 s^?1. Together with excellent film‐forming and over 99 % photoluminescence quenching efficiency on perovskite, the DTP based semiconductors work efficiently as hole‐transporting materials (HTMs) for n‐i‐p structured PVSCs. Their dopant‐free MA_0.7FA_0.3PbI_2.85Br_0.15 devices exhibit a power conversion efficiency over 20 %, representing one of the highest values for un‐doped molecular HTMs based PVSCs. This work demonstrates the great potential of using a DTP core in designing efficient semiconductors for dopant‐free PVSCs.     

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.     

Metallated Macrocyclic Derivatives as a Hole – Transporting Materials for Perovskite Solar Cells

Spiro‐OMeTAD is widely used as thehole‐transporting material (HTM) in perovskite solar cells (PSC), which extracts positive charges and protects the perovskite materials from metal electrode, setting a new world‐record efficiency of more than 20 %. Spiro‐OMeTAD layer engross moisture leading to the degradation of perovskite, and therefore, has poor air stability. It is also expensive therefore limiting scale‐up, so macrocyclic metal complex derivatives (MMDs) could be a suitable replacement. Our review covers low‐cost, high yield hydrophobic materials with minimal steps required for synthesis of efficient HTMs for planar/mesostructured PSCs. The MMDs based devices demonstrated PCEs around 19 % and showed stability for a longer duration, indicating that MMDs are a promising alternative to spiro‐OMeTAD and also easy to scale‐up via solution approach. Additionally, this review describes how optical and electrical properties of MMDs change with chemical structure, allowing for the design of novel hole‐mobility materials to achieve negligible hysteresis and act as effective functional barriers against moisture which results in a significant increase in the stability of the device. We provide an overview of the apt green‐synthesis, characterization, stability and implementation of the various classes of macrocyclic metal complex derivatives as HTM for photovoltaic applications.     

Amorphous Hole‐Transporting Material based on 2,2′‐Bis‐substituted 1,1′‐Biphenyl Scaffold for Application in Perovskite Solar Cells

Perovskite solar cells are considered a promising technology for solar‐energy conversion, with power conversion efficiencies currently exceeding 20 %. In most of the reported devices, Spiro‐OMeTAD is used for positive‐charge extraction and transport layer. Although a number of alternative hole‐transporting materials with different aromatic or heteroaromatic fragments have already been synthesized, a cheap and well‐performing hole‐transporting material is still in high demand. In this work, a two‐step synthesis of a carbazole‐based hole‐transporting material is presented. Synthesized compounds exhibited amorphous nature, good solubility and thermal stability. The perovskite solar cells employing the newly synthesized material generated a power conversion efficiency of 16.5 % which is slightly lower than that obtained with Spiro‐OMeTAD (17.5 %). The low‐cost synthesis and high performance makes our hole‐transport material promising for applications in perovskite‐based optoelectronic devices.     

A Novel Spiro[acridine‐9,9′‐fluorene] Derivatives Containing Phenanthroimidazole Moiety for Deep‐Blue OLED Application

Typical π–π stacking and aggregation‐caused quenching could be suppressed in the film‐state by the spiro conformation molecular design in the field of organic light‐emitting diodes (OLEDs). Herein, a novel deep‐blue fluorescent material with spiro conformation, 1‐(4‐(tert ‐butyl)phenyl)‐2‐(4‐(10‐phenyl‐10H ‐spiro[acridine‐9,9′‐fluoren]‐2‐yl)phenyl)‐1H ‐phenanthro[9,10‐d ]imidazole ( SAF‐BPI ), was designed and synthesized. The compound consists of spiro‐acridine‐fluorene (SAF) as donor part and phenanthroimidazole (BPI) as acceptor part. Owing to the rigid SAF skeleton, this compound exhibits a high thermal stability with a glass transition temperature (T _g) of 198 °C. The compound exhibits bipolar transporting characteristics demonstrated by the single‐carrier devices. The non‐doped OLEDs based on the SAF‐BPI as the emitting layer shows maximum emission at 448 nm, maximum luminance of 2122 cd m⁻², maximum current efficiency (CE) of 3.97 cd A⁻¹, and a maximum power efficiency of 2.08 lm W⁻¹. The chromaticity coordinate is stable at (0.15, 0.10) at the voltage of 7–11 V. The device shows a slow efficiency roll‐off with CE of 3.35 and 2.85 cd A⁻¹ at 100 and 1000 cd m⁻², respectively.     

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.