A Two-Dimensional Hole-Transporting Material for High-Performance Perovskite Solar Cells with 20 % Average Efficiency

In Situ Photogenerated Radicals of Hydroxyl Substituted Pyrene-Based Triphenylamines with Enhanced Transport and Free Doping/Post-Oxidation for Efficient Perovskite Solar Cells

The high-performance hole transporting material (HTM) is one of the most important components for the perovskite solar cells (PSCs) in promoting power conversion efficiency (PCE). However, the low conductivity of HTMs and their additional requirements for doping and post-oxidation greatly limits the device performance. In this work, three novel pyrene-based derivatives containing methoxy-substituted triphenylamines units (PyTPA, PyTPA-OH and PyTPA-2OH) are designed and synthesized, where different numbers of hydroxyl groups are connected at the 2- or 2,7-positions of the pyrene core. These hydroxyl groups at the 2- or 2,7-positions of pyrene play a significantly role to enhance the intermolecular interactions that are able to generate in situ radicals with the assistance of visible light irradiation, resulting in enhanced hole transferring ability, as well as an enhanced conductivity and suppressed recombination. These pyrene-core based HTMs exhibit excellent performance in PSCs, which possess a higher PCE than those control devices using the traditional spiro-OMeTAD as the HTM. The best performance can be found in the devices with PyTPA-2OH. It has an average PCE of 23.44% (PCEmax = 23.50%), which is the highest PCE among the reported PSCs with the pyrene-core based HTMs up to date. This research offers a novel avenue to design a dopant-free HTM by the combination of the pyrene core, methoxy triphenylamines, and hydroxy groups.

3D Conjugated Hole Transporting Materials for Efficient and Stable Perovskite Solar Cells and Modules

The orthogonal structure of the widely used hole transporting material (HTM) 2,2',7,7'-tetrakis(N, N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD) imparts isotropic conductivity and excellent film-forming capability. However, inherently weak intra- and inter-molecular π-π interactions result in low intrinsic hole mobility. Herein, a novel HTM, termed FTPE-ST, with a twist conjugated dibenzo(g,p)chrysene core and coplanar 3,4-ethylenedioxythiophene (EDOT) as extended donor units, is designed to enhance π-π interactions, without compromising on solubility. The three-dimensional (3D) configuration provides the material multi-direction charge transport as well as excellent solubility even in 2-methylanisole, and its large conjugated backbone endows the HTM with a high hole mobility. Moreover, the sulfur donors in EDOT units coordinate with lead ions on the perovskite surface, leading to stronger interfacial interactions and the suppression of defects at the perovskite/HTM interface. As a result, perovskite solar cells (PSCs) employing FTPE-ST achieve a champion power conversion efficiency (PCE) of 25.21% with excellent long-time stability, one of the highest PCEs for non-spiro HTMs in n-i-p PSCs. In addition, the excellent film-forming capacity of the HTM enables the fabrication of FTPE-ST-based large-scale PSCs (1.0 cm2) and modules (29.0 cm2), which achieve PCEs of 24.21% (certificated 24.17%) and 21.27%, respectively.

Dopant-Free Pyrene-Based Hole Transporting Material Enables Efficient and Stable Perovskite Solar Cells

Dopant-free hole transporting materials (HTMs) is significant to the stability of perovskite solar cells (PSCs). Here, we developed a novel star-shape arylamine HTM, termed Py-DB, with a pyrene core and carbon-carbon double bonds as the bridge units. Compared to the reference HTM (termed Py-C), the extension of the planar conjugation backbone endows Py-DB with typical intermolecular π-π stacking interactions and excellent solubility, resulting in improved hole mobility and film morphology. In addition, the lower HOMO energy level of the Py-DB HTM provides efficient hole extraction with reduced energy loss at the perovskite/HTM interface. Consequently, an impressive power conversion efficiency (PCE) of 24.33 % was achieved for dopant-free Py-DB-based PSCs, which is the highest PCE for dopant-free small molecular HTMs in n-i-p configured PSCs. The dopant-free Py-DB-based device also exhibits improved long-term stability, retaining over 90 % of its initial efficiency after 1000 h exposure to 25 % humidity at 60 °C. These findings provide valuable insights and approaches for the further development of dopant-free HTMs for efficient and reliable PSCs.

Benzothieno[3,2-b]thiophene-Based Noncovalent Conformational Lock Achieves Perovskite Solar Cells with Efficiency over 24

Organic semiconductors with noncovalently conformational locks (OSNCs) are promising building blocks for hole-transporting materials (HTMs). However, lack of satisfied neighboring building blocks negatively impacts the optoelectronic properties of OSNCs-based HTMs and imperils the stability of perovskite solar cells (PSCs). To address this limitation, we introduce the benzothieno[3,2-b]thiophene (BTT) to construct a new OSNC, and the resulting HTM ZS13 shows improved intermolecular charge extraction/transport properties, proper energy level, efficient surface passivation effect. Consequently, the champion devices based on doped ZS13 yield an efficiency of 24.39 % and 20.95 % for aperture areas of 0.1 and 1.01 cm2 , respectively. Furthermore, ZS13 shows good thermal stability and the capability of inhibiting I- ion migration, thus, leading to enhanced device stability. The success in neighboring-group engineering can triggered a strong interest in developing thienoacene-based OSNCs toward efficient and stable PSCs.

Two-dimensional (n = 1) ferroelectric film solar cells

Molecular ferroelectrics that have excellent ferroelectric properties, a low processing temperature, narrow bandgap, and which are lightweight, have shown great potential in the photovoltaic field. However, two-dimensional (2D) perovskite solar cells with high tunability, excellent photo-physical properties and superior long-term stability are limited by poor out-of-plane conductivity from intrinsic multi-quantum-well electronic structures. This work uses 2D molecular ferroelectric film as the absorbing layer to break the limit of multiple quantum wells. Our 2D ferroelectric solar cells achieve the highest open-circuit voltage (1.29 V) and the best efficiency (3.71%) among the 2D (n = 1) Ruddlesden-Popper perovskite solar cells due to the enhanced out-of-plane charge transport induced by molecular ferroelectrics with a strong saturation polarization, high Curie temperature and multiaxial characteristics. This work aims to break the inefficient out-of-plane charge transport caused by the limit of the multi-quantum-well electronic structure and improve the efficiency of 2D ferroelectric solar cells.

Organic Hole-Transport Layers for Efficient, Stable, and Scalable Inverted Perovskite Solar Cells

Hole-transporting layers (HTLs) are an essential component in inverted, p-i-n perovskite solar cells (PSCs) where they play a decisive role in extraction and transport of holes, surface passivation, perovskite crystallization, device stability, and cost. Currently, the exploration of efficient, stable, highly transparent and low-cost HTLs is of vital importance for propelling p-i-n PSCs toward commercialization. Compared to their inorganic counterparts, organic HTLs offer multiple advantages such as a tunable bandgap and energy level, easy synthesis and purification, solution processability, and overall low cost. Here, recent progress of organic HTLs, including conductive polymers, small molecules, and self-assembled monolayers, as utilized in inverted PSCs is systematically reviewed and summarized. Their molecular structure, hole-transport properties, energy levels, and relevant device properties and resulting performances are presented and analyzed. A summary of design principles and a future outlook toward highly efficient organic HTLs in inverted PSCs is proposed. This review aims to inspire further innovative development of novel organic HTLs for more efficient, stable, and scalable inverted PSCs.

Interfacial Passivation Engineering for Highly Efficient Perovskite Solar Cells with a Fill Factor over 83

Charge carrier nonradiative recombination (NRR) caused by interface defects and nonoptimal energy level alignment is the primary factor restricting the performance improvement of perovskite solar cells (PSCs). Interfacial modification is a vital strategy to restrain NRR and enable high-performance PSCs. We report here two interfacial materials, PhI-TPA and BTZI-TPA, consisting of phthalimide and a 2,1,3-benzothiadiazole-5,6-dicarboxylicimide core, respectively. The difunctionalized BTZI-TPA with imide and thiadiazole shows higher hole mobility, better aligned energy levels, and stronger interaction with uncoordinated Pb²⁺ on the perovskite surface, suppressing NRR and carrier accumulation at the interface of perovskite/spiro-OMeTAD and yielding enhanced open-circuit voltage and fill factor. Consequently, the PSC based on BTZI-TPA delivers a high efficiency of 24.06% with an excellent fill factor of 83.10%, superior to that (21.47%) of the reference cell without an interfacial layer, and 21.45% efficiency for the device with a scaled-up area (1.00 cm²). These results underscore the potential of imide and thiadiazole groups in developing interfacial layers with strong passivation capability, effective charge transport property, and fine-tuned energetics for stable and efficient PSCs.

Oxidation of Spiro-OMeTAD in High-Efficiency Perovskite Solar Cells

2,2',7,7'-Tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD), as an organic small molecule material, is the most commonly employed hole transport material (HTM) in perovskite solar cells (PSCs) because of its excellent properties that result in high photovoltaic performances. However, the material still suffers from low conductivity, leading to the necessary use of dopants and oxidative processes to overcome this issue. The spiro-OMeTAD oxidation process is highlighted in this review, and the main parameters involved in the process have been studied. Furthermore, the best alternatives aiming to improve the spiro-OMeTAD electrical properties have been discussed. Lastly, this review concludes with suggestions and outlooks for further research directions.

The evolution of triphenylamine hole transport materials for efficient perovskite solar cells

In recent years, the dramatic increase in power conversion efficiency (PCE) coupled with a decrease in the total cost of third-generation solar cells has led to a significant increase in the collaborative research efforts of academic and industrial researchers. Such interdisciplinary studies have afforded novel materials, which in many cases are now ready to be brought to the marketplace. Within this framework, the field of perovskite solar cells (PSCs) is currently an important area of research due to their extraordinary light-harvesting properties. In particular, PSCs prepared via facile synthetic procedures, containing hole transport materials (HTMs) with versatile triphenylamine (TPA) structural cores, amenable to functionalization, have become a focus of intense global research activity. To optimize the efficiency of the solar cells to achieve efficiencies closer to rival silicon-based technology, TPA building blocks must exhibit favourable electrochemical, photophysical, and photochemical properties that can be chemically tuned in a rational manner. Although PSCs based on TPA building blocks exhibit attractive properties such as high-power efficiencies, a reduction in their synthetic costs coupled with higher stabilities and environmental considerations still need to be addressed. Considering the above, a detailed summary of the most promising compounds and current methodologies employed to overcome the remaining challenges in this field is provided. The objective of this review is to provide guidance to readers on exploring new avenues for the discovery of efficient TPA derivatives, to aid in the future development and advancement of TPA-based PSCs for commercial applications.

Triarylamine/Bithiophene Copolymer with Enhanced Quinoidal Character as Hole-Transporting Material for Perovskite Solar Cells

Polytriarylamine is a popular hole-transporting materials (HTMs) despite its suboptimal conductivity and significant recombination at the interface in a solar cell setup. Having noted insufficient conjugation among the triarylamine units along the polymer backbone, we inserted a bithiophene unit between two triarylamine units through iron-catalyzed C-H/C-H coupling of a triarylamine/thiophene monomer so that two units conjugate effectively via four quinoidal rings when the molecule functions as HTM. The obtained triarylamine/bithiophene copolymer (TABT) used as HTM showed a high-performance in methylammonium lead iodide perovskite (MAPbI₃ ) solar cells. Mesityl substituted TABT forms a uniform film, shows high hole-carrier mobility, and has an ionization potential (IP=5.40 eV) matching that of MAPbI₃ . We fabricated a solar cell device with a power conversion efficiency of 21.3 % and an open-circuit voltage of 1.15 V, which exceeds the performance of devices using reference standard such as poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) and Spiro-OMeTAD.

The Evolution of Classical Spiro-OMeTAD: Synthesis of Arylamine Endcapped Indenone Spirofluorene

Spiro-OMeTAD is the well-known hole transporting material (HTM) in perovskite solar cells. In this work, its derivatives, namely four D-A shaped triphenylamine or biphenylamine endcapped indenone spirofluorene (SFD-TPA, SFD-OMeTPA, SFD-TAD, and SFD-OMeTAD), were designed and synthesized. With the introduction of electron-donating moieties and the extension of conjugation length, a series of changes in photophysical and electrochemical properties could be detected. Notably, in comparison with the optical gap (2.96 eV) of the reported spiro-OMeTAD, SFD-OMeTAD presents an optical gap as low as 1.87 eV. Moreover, density functional theory simulations were employed to further investigate their geometric and electronic structures. Finally, steady-state photoluminescence measurements proved the efficient charge separation and collection processes at the perovskite/HTM interface. It can be predicted that all four compounds with enhanced sunlight absorption capability and suitable frontier energy levels can be used as hole-transporting materials for perovskite solar cells.

Effect of Thiophene Insertion on X-Shaped Anthracene-Based Hole-Transporting Materials in Perovskite Solar Cells

In this work, two novel tetra-substituted X-shaped molecules X1 and X2 that were constructed with anthracene as the central core and arylamine as the donor groups have been synthesized. The HTMs X1 and X2 were synthesized in two steps from industrially accessible and moderately reasonable beginning reagents. These new HTMs are described in terms of utilization of light absorption, energy level, thermal properties, hole mobility (µ_h), and film-forming property. The photovoltaic performances of these HTMs were effectively assessed in perovskite solar cells (PSCs). The devices based on these HTMs accomplished an overall efficiency of 16.10% for X1 and 10.25% for X2 under standard conditions (AM 1.5 G and 100 mW cm⁻²). This precise investigation provides another perspective on the use of HTMs in PSCs with various device configurations.

Designs from single junctions, heterojunctions to multijunctions for high-performance perovskite solar cells

Hybrid metal-halide perovskite solar cells (PVSCs) have drawn unprecedented attention during the last decade due to their superior photovoltaic performance, facile and low-cost fabrication, and potential for roll-to-roll mass production and application for portable devices. Through collective composition, interface, and process engineering, a comprehensive understanding of the structure-property relationship and carrier dynamics of perovskites has been established to help achieve a very high certified power conversion efficiency (PCE) of 25.5%. Apart from material properties, the modified heterojunction design and device configuration evolution also play crucial roles in enhancing the efficiency. The adoption and/or modification of heterojunction structures have been demonstrated to effectively suppress the carrier recombination and potential losses in PVSCs. Moreover, the employment of multijunction structures has been shown to reduce thermalization losses, achieving a high PCE of 29.52% in perovskite/silicon tandem solar cells. Therefore, understanding the evolution of the device configuration of PVSCs from single junction, heterojunction to multijunction designs is helpful for the researchers in this field to further boost the PCE beyond 30%. Herein, we summarize the evolution and progress of the single junction, heterojunction and multijunction designs for high-performance PVSCs. A comprehensive review of the fundamentals and working principles of these designs is presented. We first introduce the basic working principles of single junction PVSCs and the intrinsic properties (such as crystallinity and defects) in perovskite films. Afterwards, the progress of diverse heterojunction designs and perovskite-based multijunction solar cells is synopsized and reviewed. Meanwhile, the challenges and strategies to further enhance the performance are also summarized. At the end, the perspectives on the future development of perovskite-based solar cells are provided. We hope this review can provide the readers with a quick catchup on this emerging solution-processable photovoltaic technology, which is currently at the transition stage towards commercialization.

A Phenanthrocarbazole-Based Dopant-Free Hole-Transport Polymer with Noncovalent Conformational Locking for Efficient Perovskite Solar Cells

Adequate hole mobility is the prerequisite for dopant-free polymeric hole-transport materials (HTMs). Constraining the configurational variation of polymer chains to afford a rigid and planar backbone can reduce unfavorable reorganization energy and improve hole mobility. Herein, a noncovalent conformational locking via S-O secondary interaction is exploited in a phenanthrocarbazole (PC) based polymeric HTM, PC6, to fix the molecular geometry and significantly reduce reorganization energy. Systematic studies on structurally explicit repeats to targeted polymers reveals that the broad and planar backbone of PC remarkably enhances π-π stacking of adjacent polymers, facilitating intermolecular charge transfer greatly. The inserted "Lewis soft" oxygen atoms passivate the trap sites efficiently at the perovskite/HTM interface and further suppress interfacial recombination. Consequently, a PSC employing PC6 as a dopant-free HTM offers an excellent power conversion efficiency of 22.2 % and significantly improved longevity, rendering it as one of the best PSCs based on dopant-free HTMs.

Low-Cost Hole-Transporting Materials Based on Carbohelicene for High-Performance Perovskite Solar Cells

Two hole-transporting materials (HTMs) based on carbohelicene cores, CH1 and CH2, are developed and used in fabricating efficient and stable perovskite solar cells (PSCs). Owing to the rigid conformation of the helicene core, both compounds possess unique CH-π interactions in the crystalline packing pattern and good phase stability, which are distinct from the π-π intermolecular interactions of conventional planar and spiro-type molecules. PSCs based on CH1 and CH2 as HTMs deliver excellent device efficiencies of 19.36 and 18.71%, respectively, outperforming the control device fabricated with spiro-OMeTAD (18.45%). Furthermore, both PSCs exhibit better ambient stability, with 90% of initial performance retained after aging with a 50-60% relative humidity at 25 °C for 500 h. Due to the low production cost of both compounds, these newly designed carbohelicene-type HTMs have the potential for the future commercialization of PSCs.

Dopant-Free π-Conjugated Hole Transport Materials for Highly Stable and Efficient Perovskite Solar Cells

Current high-efficiency hybrid perovskite solar cells (PSCs) have been fabricated with doped hole transfer material (HTM), which has shown short-term stability. Doping applied in HTMs for PSCs can enhance the hole mobility and PSCs' power conversion efficiency, while the stability of PSCs will be significantly decreased due to inherent hygroscopic properties and chemical incompatibility. Development of dopant-free HTM with high hole mobility is a challenge and of utmost importance. In this review, a series of selected and typical π-conjugated dopant-free hole transport materials, mainly regarding small molecules, are reviewed, which could consequently help to further design high-performance dopant-free HTMs. In addition, an outline of the molecular design concept and also the perspective of ideal dopant-free HTMs were explored.

Fluorinating Dopant-Free Small-Molecule Hole-Transport Material to Enhance the Photovoltaic Property

For the stability and commercial development of the perovskite solar cells (PVK-SCs), synthesizing high-efficiency dopant-free hole-transport materials (DF-HTMs) and exploring how the DF-HTM structure affects the photovoltaic performance is inevitable. Two small-molecule DF-HTMs based on 2,2'-bithiophene as a central part (denoted by BT-MTP and DFBT-MTP) were designed and synthesized. DFBT-MTP, with two more fluorine atoms substituted on the 2,2'-bithiophene group, exhibited enhanced photovoltaic property as DF-HTMs, including larger backbone planarity, declining highest occupied molecular orbit (HOMO) energy level, increasing hole transportation, more effective passivation, and efficient charge extraction. With fluorinated DFBT-MTP being applied as DF-HTMs in p-i-n PVK-SCs, an efficiency of 20.2% was achieved, showing ∼35% efficiency increase compared with the nonfluorinated BT-MTP-based devices. The leading power conversion efficiency (PCE) indicates that the fluorinated compounds should be a promising direction for exploring high-performance DF-HTMs in the p-i-n PVK-SCs.

Side-Chain Engineering of Diketopyrrolopyrrole-Based Hole-Transport Materials to Realize High-Efficiency Perovskite Solar Cells

The design and synthesis of a stable and efficient hole-transport material (HTM) for perovskite solar cells (PSCs) are one of the most demanding research areas. At present, 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-MeOTAD) is a commonly used HTM in the fabrication of high-efficiency PSCs; however, its complicated synthesis, addition of a dopant in order to realize the best efficiency, and high cost are major challenges for the further development of PSCs. Herein, various diketopyrrolopyrrole-based small molecules were synthesized with the same backbone but distinct alkyl side-chain substituents (i.e., 2-ethylhexyl-, n-hexyl-, ((methoxyethoxy)ethoxy)ethyl-, and (2-((2-methoxyethoxy)ethoxy)ethyl)acetamide, designated as D-1, D-2, D-3, and D-4, respectively) as HTMs. The variation in the alkyl chain has shown obvious effects on the optical and electrochemical properties as well as on the molecular packing and film-forming ability. Consequently, the power conversion efficiency (PCE) of the PSC under one sun illumination (100 mW cm^-2) is shown to increase in the order of D-1 (8.32%) < D-2 (11.12%) < D-3 (12.05%) < D-4 (17.64%). Various characterization techniques reveal that the superior performance of D-4 can be ascribed to the well-aligned highest occupied molecular orbital energy level with the counter electrode, the more compact π-π stacking with a higher coherence length, and the excellent hole mobility of 1.09 × 10^-3 cm² V^-1 s^-1, thus providing excellent energetics for effective charge transport with minimal charge-carrier recombination. Furthermore, the addition of the dopant Li-TFSI in D-4 is shown to deliver a remarkable PCE of 20.19%, along with a short-circuit current density (J_SC), open-circuit voltage (V_OC), and fill factor (FF) of 22.94 mA cm^-2, 1.14 V, and 73.87%, respectively, and superior stability compared to that of other HTMs. These results demonstrate the effectiveness of side-chain engineering for tailoring the properties of HTMs, thus offering new design tactics to fabricate for the synthesis of highly efficient and stable HTMs for PSCs.

A star-shaped cyclopentadithiophene-based dopant-free hole-transport material for high-performance perovskite solar cells

A new cyclopentadithiophene (CPDT)-based organic small molecule serves as an efficient dopant-free hole transport material (HTM) for perovskite solar cells (PSCs). Upon incorporation of two carbazole groups, the resulting CPDT-based HTM (C-CPDT) shows an impressive power conversion efficiency (PCE) of 19.68% with better stability compared with those of spiro-OMeTAD.

A Coplanar π-Extended Quinoxaline Based Hole-Transporting Material Enabling over 21 % Efficiency for Dopant-Free Perovskite Solar Cells

Developing dopant-free hole transporting materials (HTMs) is of vital importance for addressing the notorious stability issue of perovskite solar cells (PSCs). However, efficient dopant-free HTMs are scarce. Herein, we improve the performance of dopant-free HTMs featuring with a quinoxaline core via rational π-extension. Upon incorporating rotatable or chemically fixed thienyl substitutes on the pyrazine ring, the resulting molecular HTMs TQ3 and TQ4 show completely different molecular arrangement as well as charge transporting capabilities. Comparing with TQ3, the coplanar π-extended quinoxaline based TQ4 endows enriched intermolecular interactions and stronger π-π stacking, thus achieving a higher hole mobility of 2.08×10^-4  cm²  V^-1  s^-1 . It also shows matched energy levels and high thermal stability for application in PSCs. Planar n-i-p structured PSCs employing dopant-free TQ4 as HTM exhibits power conversion efficiency (PCE) over 21 % with excellent long-term stability.

Introducing ammonium salt into hole transporting materials for perovskite solar cells

The developed ammonium salt-containing hole transporting material could passivate perovskite defects and transport holes, and exhibits better performance compared with the non-ammonium salt counterpart.

Conformational and Compositional Tuning of Phenanthrocarbazole-Based Dopant-Free Hole-Transport Polymers Boosting the Performance of Perovskite Solar Cells

Conjugated polymers are regarded as promising candidates for dopant-free hole-transport materials (HTMs) in efficient and stable perovskite solar cells (PSCs). Thus far, the vast majority of polymeric HTMs feature structurally complicated benzo[1,2-b:4,5-b’]dithiophene (BDT) analogs and electron-withdrawing heterocycles, forming a strong donor–acceptor (D–A) structure. Herein, a new class of phenanthrocarbazole (PC)-based polymeric HTMs (PC1, PC2, and PC3) has been synthesized by inserting a PC unit into a polymeric thiophene or selenophene chain with the aim of enhancing the π–π stacking of adjacent polymer chains and also to efficiently interact with the perovskite surface through the broad and planar conjugated backbone of the PC. Suitable energy levels, excellent thermostability, and humidity resistivity together with remarkable photoelectric properties are obtained via meticulously tuning the conformation and elemental composition of the polymers. As a result, PSCs containing PC3 as dopant-free HTM show a stabilized power conversion efficiency (PCE) of 20.8% and significantly enhanced longevity, rendering one of the best types of PSCs based on dopant-free HTMs. Subsequent experimental and theoretical studies reveal that the planar conformation of the polymers contributes to an ordered and face-on stacking of the polymer chains. Furthermore, introduction of the “Lewis soft” selenium atom can passivate surface trap sites of perovskite films by Pb–Se interaction and facilitate the interfacial charge separation significantly. This work reveals the guiding principles for rational design of dopant-free polymeric HTMs and also inspires rational exploration of small molecular HTMs.

A Ladder-like Dopant-free Hole-Transporting Polymer for Hysteresis-less High-Efficiency Perovskite Solar Cells with High Ambient Stability

Perovskite solar cells (PSCs) have received high attention in the past few years due to their terrific photovoltaic performance and potentially low production cost. However, the use of hole transport materials (HTMs) with hygroscopic dopants, which cause the inevitable instability of device performance, has hampered commercialization. Herein, a dopant-free polymeric HTM with functional aromatic rings was used to optimize the HTM/perovskite interface and employed in a planar n-i-p configuration. Poly(1,4-(2,5-bis((2-butyloctyloxy)phenylene)-2,7-(5,5,10,10-tetrakis(4-hexylphenyl)-5,10-dihydro-s-indaceno[2,1-b:6,5-b']dithiophene)) (IDTB) co-polymer constructed with indaceno[1,2-b:5,6-b']dithiophene and bis(alkyloxy)benzene units adopts an S⋅⋅⋅O intramolecular bond linked ladder-like planar conjugated polymer backbone. Without any dopant, the hole mobility of IDTB is in the same order of magnitude as a doped 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD). Also, the hydrophobic nature of IDTB facilitated the long-term stability of the perovskite underneath. The unencapsulated PSC devices made of IDTB-based HTM achieved a power conversion efficiency of 19.38 % with a high moisture stability, retaining above 80 % of initial power conversion efficiency at 65 % relative humidity for more than 10 days. The superior passivation effect to perovskite surface made a hysteresis of 0.44 % was almost the least reported for regular planar undoped polymer HTM PSCs.

N-Substituted Phenothiazines as Environmentally Friendly Hole-Transporting Materials for Low-Cost and Highly Stable Halide Perovskite Solar Cells

Most of the high-performing halide perovskite solar cells (PSCs) leverage toxic chlorinated solvents (e.g., o-dichlorobenzene or chlorobenzene) for the hole-transporting material (HTM) processing and/or antisolvents in the perovskite film fabrication. To minimize the environmental and health-related hazards, it is highly desirable, yet at the same time demanding, to develop HTMs and perovskite deposition processes relying on nonhalogenated solvents. In this work, we designed two small molecules, AZO-III and AZO-IV, and synthesized them via simple and environmentally friendly Schiff base chemistry, by condensation of electron-donating triarylamine and phenothiazine moieties connected through an azomethine bridge. The molecules are implemented as HTMs in PSCs upon processing in a nonchlorinated (toluene) solvent, rendering their synthesis and film preparation eco-friendly. The enhancement in the power conversion efficiency (PCE) was achieved when switching from AZO-III (9.77%) to AZO-IV (11.62%), in which the thioethyl group is introduced in the 2-position of the phenothiazine ring. Additionally, unencapsulated PSCs based on AZO-III displayed excellent stabilities (75% of the initial PCEs is retained after 6 months of air exposure for AZO-III to be compared with a 48% decrease of the initial PCE for Spiro-OMeTAD-based devices). The outstanding stability and the extremely low production cost (AZO-III = 9.23 $/g and AZO-IV = 9.03 $/g), together with the environmentally friendly synthesis, purification, and processing, make these materials attractive candidates as HTMs for cost-effective, stable, and eco-friendly PSCs.

Lewis-base containing spiro type hole transporting materials for high-performance perovskite solar cells with efficiency approaching 20

Owing to excellent performance and dopability, spiro-OMeTAD remains an irreplaceable hole transporting material (HTM) in perovskite solar cells (PSCs). In order to further improve the performance of spiro-OMeTAD based PSCs, a Lewis base can be introduced into the structure of spiro-OMeTAD wisely, which can keep the advantages of spiro-OMeTAD while incorporating the functionality of a Lewis base in passivating the surface of the perovskite. Therefore, spiro-type HTMs (spiro-CN-OMeTAD with a dicyano group and spiro-PS-OMeTAD with a thiocarbonyl group) were synthesized and confirmed by density functional theory (DFT) calculations and X-ray single-crystallographic diffraction. Spiro-CN-OMeTAD as an HTM is certified to have a suitable interfacial band alignment with the perovskite, good film quality and effective defect passivation, which facilitate the resulting device to achieve an efficiency of 19.90% with a high open-circuit voltage, low hysteresis, and improved stability. This study provides an alternative strategy for the molecular design of better HTMs in high-performance PSCs.

In Situ Observation of Vapor-Assisted 2D-3D Heterostructure Formation for Stable and Efficient Perovskite Solar Cells

The heterogeneous stacking of a thin two-dimensional (2D) perovskite layer over the three-dimensional (3D) perovskite film creates a sophisticated architecture for perovskite solar cells (PSCs). It combines the remarkable thermal and environmental stabilities of 2D perovskites with the superior optoelectronic properties of 3D materials which resolves the chronic stability issue with no compromise on efficiency. Herein, we propose the vapor-assisted growth strategy to fabricate high-quality 2D/3D heterostructured perovskite films by introducing long-chain organoamine gases in which the 2D layers have a uniform and tunable thickness. The 3D to 2D transformation of the widely adopted MAPbI₃ (MA = methylammonium) film is initiated by the butylamine vapor and monitored through the in situ grazing-incidence X-ray diffraction technique. A variety of 2D species are observed and rationalized by the different collapsing and reconstruction models of the Pb-I octahedra. The PSC devices based on the optimized 2D/3D heterostructures show significant improvements in photovoltaic performances, owing to better energy level alignments, longer carrier lifetimes, and less defects as compared to their 3D analogues. In addition, both the butylamine vapor-treated perovskite films and the derived PSC devices demonstrate exceptional long-term stabilities.

Tailoring of the core structure towards promising small molecule hole-transporting materials for perovskite solar cells: a theoretical study

The influence of the core on the performance of small molecule HTMs is investigated by using quantum chemistry methods, and potential HTMs are proposed.

Introducing fluorene into organic hole transport materials to improve mobility and photovoltage for perovskite solar cells

A new small molecular hole-transporting material, 1,3,6,8-tetrakis[N-(p-methoxyphenyl)-N'-(9,9'-dimethyl-9H-fluoren-2-yl)-amino]pyrene (TFAP) was synthesized and applied in CH3NH3PbI3-perovskite solar cells. A best power conversion efficiency of 19.7% with a photovoltage of 1.11 V has been achieved.

Europium and Acetate Co-doping Strategy for Developing Stable and Efficient CsPbI2 Br Perovskite Solar Cells

All-inorganic perovskite solar cells have developed rapidly in the last two years due to their excellent thermal and light stability. However, low efficiency and moisture instability limit their future commercial application. The mixed-halide inorganic CsPbI₂ Br perovskite with a suitable bandgap offers a good balance between phase stability and light harvesting. However, high defect density and low carrier lifetime in CsPbI₂ Br perovskites limit the open-circuit voltage (V_oc < 1.2 V), short-circuit current density (J_sc < 15 mA cm^-2 ), and fill factor (FF < 75%) of CsPbI₂ Br perovskite solar cells, resulting in an efficiency below 14%. For the first time, a CsPbI₂ Br perovskite is doped by Eu(Ac)₃ to obtain a high-quality inorganic perovskite film with a low defect density and long carrier lifetime. A high efficiency of 15.25% (average efficiency of 14.88%), a respectable V_oc of 1.25 V, a reasonable J_sc of 15.44 mA cm^-2 , and a high FF of 79.00% are realized for CsPbI₂ Br solar cells. Moreover, the CsPbI₂ Br solar cells with Eu(Ac)₃ doping demonstrate excellent air stability and maintain more than 80% of their initial power conversion efficiency (PCE) values after aging in air (relative humidity: 35-40%) for 30 days.

Unraveling the Structure-Property Relationship of Molecular Hole-Transporting Materials for Perovskite Solar Cells

Clarifying the structural basis and microscopic mechanism lying behind electronic properties of molecular semiconductors is of paramount importance in further material design to enhance the performance of perovskite solar cells. In this paper, three conjugated quasilinear segments of 9,9-dimethyl-9H-fluorene, 9,9-dimethyl-2,7-diphenyl-9H-fluorene, and 2,6-diphenyldithieno[3,2-b:2',3'-d]thiophene are end-capped with two bis(4-methoxyphenyl)amino groups for structurally simple molecular semiconductors Z1, Z2, and Z3, which crystallize in the monoclinic P2₁/n, triclinic P1̅, and monoclinic C2/c space groups, respectively. The modes and energies of intermolecular noncovalent interactions in various closely packed dimers extracted from single crystals are computed based on the quantum theory of atoms in molecules and energy decomposition analysis. Transfer integrals, reorganization energies, and center-of-mass distances in these dimers as well as band structures of single crystals are also calculated to define the theoretical limit of hole transport and microscopic transport pictures. Joint X-ray diffraction and space-charge-limiting current measurements on solution-deposited films suggest the dominant role of crystallinity in thin-film hole mobility. Photoelectron spectroscopy and photoluminescence measurements show that an enhanced interfacial interaction between the perovskite and Z3 could attenuate the adverse impact of reducing the energetic driving force of hole extraction. Our comparative studies show that the molecular semiconductor Z3 with a properly aligned highest occupied molecular orbital energy level and a high thin-film mobility can be employed for efficient perovskite solar cells, achieving a good power conversion efficiency of 20.84%, which is even higher than that of 20.42% for the spiro-OMeTAD control.

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.

Dopant-Free Small-Molecule Hole-Transporting Material for Inverted Perovskite Solar Cells with Efficiency Exceeding 21

Hole-transporting materials (HTMs) play a critical role in realizing efficient and stable perovskite solar cells (PVSCs). Considering their capability of enabling PVSCs with good device reproducibility and long-term stability, high-performance dopant-free small-molecule HTMs (SM-HTMs) are greatly desired. However, such dopant-free SM-HTMs are highly elusive, limiting the current record efficiencies of inverted PVSCs to around 19%. Here, two novel donor-acceptor-type SM-HTMs (MPA-BTI and MPA-BTTI) are devised, which synergistically integrate several design principles for high-performance HTMs, and exhibit comparable optoelectronic properties but distinct molecular configuration and film properties. Consequently, the dopant-free MPA-BTTI-based inverted PVSCs achieve a remarkable efficiency of 21.17% with negligible hysteresis and superior thermal stability and long-term stability under illumination, which breaks the long-time standing bottleneck in the development of dopant-free SM-HTMs for highly efficient inverted PVSCs. Such a breakthrough is attributed to the well-aligned energy levels, appropriate hole mobility, and most importantly, the excellent film morphology of the MPA-BTTI. The results underscore the effectiveness of the design tactics, providing a new avenue for developing high-performance dopant-free SM-HTMs in PVSCs.

Side-Chain Polymers as Dopant-Free Hole-Transporting Materials for Perovskite Solar Cells-The Impact of Substituents' Positions in Carbazole on Device Performance

Side-chain polymers have the potential to be excellent dopant-free hole-transporting materials (HTMs) for perovskite solar cells (PSCs) because of their unique characteristics, such as tunable energy levels, high charge mobility, good solubility, and excellent film-forming ability. However, there has been less research focusing on side-chain polymers for PSCs. Here, two side-chain polystyrenes with triphenylamine substituents on carbazole moieties were designed and characterized. The properties of the side-chain polymers were tuned finely, including the photophysical and electrochemical properties and charge mobilities, by changing the positions of triphenylamine substituents on carbazole. Owing to the higher mobility and charge extraction ability, the polymer P2 with the triphenylamine substituent on the 3,6-positions of the carbazole unit showed higher performance with power conversion efficiency (PCE) of 18.45%, which was much higher than the PCE (16.78%) of P1 with 2,7-positions substituted. These results clearly demonstrated that side-chain polymers can act as promising HTMs for PSC applications and the performance of side-chain polymers could be optimized by carefully tuning the structure of the monomer, which provides a new strategy to design new kinds of side-chain polymers and obtain high-performance dopant-free HTMs.

Efficiency vs. stability: dopant-free hole transporting materials towards stabilized perovskite solar cells

In the last decade, perovskite solar cells have been considered a promising and burgeoning technology for solar energy conversion with a power conversion efficiency currently exceeding 24%. However, although perovskite solar cells have achieved high power conversion efficiency, there are still several challenges limiting their industrial realization. The actual bottleneck for real uptake in the market still remains the cost-ineffective components and instability, to which doping-induced degradation of charge selective layers may contribute significantly. This article overviews the highest performance molecular and polymeric doped and dopant-free HTMs, showing how small changes in the molecular structure such as different atoms and different functional groups and changes in substitution positions or the length of the π-conjugated systems can affect photovoltaic performance and long-term stability of perovskite solar cells.

Probing effects of molecular conformation on the electronic and charge transport properties in two- and three-dimensional small molecule hole-transporting materials: a theoretical investigation

Thiophene/benzene-fused π-conjugated systems are normally employed as the core units of two- and three-dimensionally expanded small molecule hole-transporting materials (HTMs) to improve their electronic and charge transport properties, whereas comparison studies between two-dimensional and three-dimensional core conformations are less reported. To further find useful clues for the design of highly-efficient small molecule HTMs and to find new core units, in this work, four HTM molecules are designed by employing triphenylene, benzotrithiophene, triptycene, and thiophenetriptycene as the core units, and simulated with density functional theory combined with the Marcus hopping model. Our results show that all the considered HTMs display appropriate molecular energy levels, less optical absorption in the visible light region and large Stokes shifts, and high hole mobilities (9.80 × 10-2 cm2 V-1 s-1). Compared with the two-dimensional core structures, the three-dimensional cores exhibit evident superiorities with the same chemical components. Meanwhile, we also find that the quasi-degenerate HOMO energy levels will be helpful to enlarge the transfer integrals between adjacent molecules, and further to promote the hole transport in HTMs. By considering the various elements simultaneously, these investigated HTMs (S-1-S-4) with thiophene- and benzene-fused cores can be expected as potential promising candidates to help create more efficient solar cells.

A Rutile TiO2 Electron Transport Layer for the Enhancement of Charge Collection for Efficient Perovskite Solar Cells

Interfacial charge collection efficiency has demonstrated significant effects on the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Herein, crystalline phase-dependent charge collection is investigated by using rutile and anatase TiO₂ electron transport layer (ETL) to fabricate PSCs. The results show that rutile TiO₂ ETL enhances the extraction and transportation of electrons to FTO and reduces the recombination, thanks to its better conductivity and improved interface with the CH₃ NH₃ PbI₃ (MAPbI₃ ) layer. Moreover, this may be also attributed to the fact that rutile TiO₂ has better match with perovskite grains, and less trap density. As a result, comparing with anatase TiO₂ ETL, MAPbI₃ PSCs with rutile TiO₂ ETL delivers significantly enhanced performance with a champion PCE of 20.9 % and a large open circuit voltage (V_OC ) of 1.17 V.

An Oxa[5]helicene-Based Racemic Semiconducting Glassy Film for Photothermally Stable Perovskite Solar Cells

Attaining the durability of high-efficiency perovskite solar cells (PSCs) operated under concomitant light and thermal stresses is still a serious concern before large-scale application. It is crucial to maintain the phase stability of the organic hole-transporting layer for thermostable PSCs across a range of temperatures sampled during device operation. To address this issue, we propose a racemic semiconducting glassy film with remarkable morphological stability, exemplified here by a low-molecular symmetry oxa[5]helicene-centered organic semiconductor (O5H-OMeDPA). The helical configuration of O5H-OMeDPA confers the trait of multiple-dimension charge transfer to the solid, resulting in high hole mobility of 6.7 × 10^-4 cm² V^-1 s^-1 of a solution-processed glassy film. O5H-OMeDPA is combined with a triple-cation dual-halide lead perovskite to fabricate PSCs with power conversion efficiencies of 21.03%, outperforming the control cells with spiro-OMeTAD (20.44%). Moreover, the cells using O5H-OMeDPA exhibit good long-term stability during full-sunlight soaking at 60°C.