Stable Hole-transporting Materials Research Articles

Investigation of Structural, Morphological, and Optical Properties of Novel Electrospun Mg-Doped TiO2 Nanofibers as an Electron Transport Material for Perovskite Solar Cells.

Perovskite solar cells (PSCs) are quickly becoming efficient solar cells due to the effective physicochemical properties of the absorber layer. This layer should ideally be placed between a stable hole transport material (HTM) layer and a conductive electron transport material (ETM) layer. These outer layers play a critical role in the current densities and cell voltages of solar cells. In this work, we successfully fabricated Mg-doped TiO2 nanofibers as ETM layers via electrospinning. This study systematically investigates the morphological and optical features of Mg-doped nanofibers as mesoporous ETM layers. The existence of the Mg element in the lattice was confirmed by XRD and XPS. These optical characterizations indicated that Mg doping widened the energy band gap and shifted the edge of the conduction band minimum upward, which enhanced the open circuit voltage (Voc) and short current density (Jsc). The electron-hole recombination rate was lowered, and separation efficiency increased with Mg doping. The results have demonstrated the possibility of improving the efficiency of PSCs with the use of Mg-doped nanofibers as an ETM layer.

Exploring the performance of perovskite solar cells with dual hole transport layers via SCAPS-1D simulation

The potential of perovskite solar cells (PSCs) to produce high efficiency (over 25 %) is well known, but the poor stability of these solar cells is still a concern for practical applications. Thus, improving the stability is essential for furthering the development of PSCs. To address this issue, researchers found that incorporating inorganic materials into the hole transport layers (HTLs) can improve the stability of PSCs, yet charge recombination at the interface between the perovskite and inorganic HTLs can still reduce the efficiency of the PSCs. This study simulated the performance and mechanism of methyl ammonium lead iodide (MAPbI3) PSCs with a dual HTL using Solar Cell Capacitance Simulator-one Dimension (SCAPS-1D). A dual HTL formed by combining Spiro-OMeTAD (spiro), a highly efficient hole transport material, and copper zinc tin sulfide (CZTS), a highly stable hole transport material, leads to a PSC with a power conversion efficiency (PCE) of 23.47 % and significantly improved stability. Simulation results of the electric field at the interfaces of PSCs indicated that the PSC with spiro/CZTS as dual HTL have enhanced charge separation and reduced recombination. Furthermore, the simulation results have also revealed the influence of different parameters, such as absorber thickness, absorber defect density, HTL thickness, HTL/absorber interface defects, temperature, series resistance, and shunt resistance, on solar cell’s characteristics.

Dopant-free NiOx Nanocrystals: A Low-cost and Stable Hole Transport Material for Commercializing Perovskite Optoelectronics.

Advancing inverted (p-i-n) perovskite solar cells (PSCs) is critical for commercial applications given their compatibility with different bottom cells for tandem photovoltaics, low-temperature processability (≤100 °C), and promising operational stability. Although inverted PSCs have achieved an efficiency of over 25 % using doped or expensive organic hole transport materials (HTMs), their synthesis cost and stability still cannot meet the requirements for their commercialization. Recently, dopant-free and low-cost non-stoichiometric nickel oxide nanocrystals (NiOx NCs) have been extensively studied as a low-cost and effective HTM in perovskite optoelectronics. In this minireview, we summarize the synthesis and surface-functionalization methods of NiOx NCs. Then, the applications of NiOx NCs in other perovskite optoelectronics beyond photovoltaics are discussed. Finally, we provide a perspective for the future development of NiOx NCs for the commercialization of perovskite optoelectronics.

Rationally heteroarylated pyridines as hole transport materials for OLEDs.

The advancement in developing highly efficient hole transport materials for OLED devices has been a challenge over the past several years. For an efficient OLED device, there should be an efficient promotion of charge carriers from each electrode and effective confinement of triplet excitons in the emissive layer of the phosphorescent OLED (PhOLED). Thus, the development of stable and high triplet energy hole transport materials is in urgent demand for high-performing PhOLED devices. The present work demonstrates the development of two hetero-arylated pyridines as high triplet energy (2.74-2.92 eV) multifunctional hole transport materials to reduce the exciton quenching and to enhance the extent of charge carrier recombination in the emissive layer. In this regard, we report the design, synthesis, and theoretical modeling with electro-optical properties of two molecules, namely PrPzPy and MePzCzPy, with suitable HOMO/LUMO energy levels and high triplet energy, by incorporating phenothiazine as well as other donating units into a pyridine scaffold, and finally developing a hybrid phenothiazine-carbazole-pyridine based molecular architecture. The natural transition orbital (NTO) calculations were done to analyze the excited state sensation in these molecules. The long-range charge transfer characteristics between the higher singlet and triplet states were also analyzed. The reorganization energy of each molecule was calculated to examine their hole transportability. The theoretical calculations for PrPzPy and MePzCzPy revealed that these two molecular systems could be promising materials for the hole transport layer of OLED devices. As a proof of concept, a solution-processed hole-only device (HOD) of PrPzPy was fabricated. The increase in current density with an increase in operating voltage in the range of ∼3-10 V supported that the suitable HOMO energy of PrPzPy can facilitate the hole transportation from the hole injection layer (HIL) to the emissive layer (EML). These results indicated the promising hole transportability of the present molecular materials.

Unveiling and overcoming the interfacial degradation between CuSCN and metal electrodes in perovskite solar cells

The interfacial degradation at the CuSCN/Au interface due to Au–Cu alloying can be improved by replacing Au with carbon electrodes, delivering high device stability under light illumination.

Modulating donor assemblies of D-π-D type hole transport materials for perovskite solar cells

Donor-π-donor (D-π-D) type organic semiconductors have been demonstrated to be excellent hole transporting materials (HTMs) in perovskite solar cells (PSCs). However, a systematic understanding of the impact from peripheral donors on the physicochemical properties and photovoltaic performance of HTMs is still scarce. Herein, through a synergistic modulation of the donor inner/outer arms, a series of novel D-π-D type HTMs are designed, synthesized and successfully applied in PSCs. It is found that substituting phenothiazine with carbazole as the inner arm could well improve the structural planarity and charge delocalization of HTMs. Moreover, via introducing fluorene units into the outer arm of the donor structure, HTM YT-KMPF exhibits excellent film morphology and hydrophobicity, making PSCs achieve a power conversion efficiency of 21.40% as well as excellent long-term stability. This work reveals the significance of donor modulation for designing efficient and stable HTMs.

Janus dione derivatives: Novel high-mobility hole transport materials for perovskite solar cells

Developing efficient and stable hole-transporting materials (HTMs) is very important for improving the stability and increasing the power conversion efficiency of perovskite solar cells (PSCs). Herein, novel Janus dione based HTMs with X-shaped structure were investigated using quantum chemical calculations. The results showed that introducing appropriate groups on both sides of the core led to a suitable HOMO level, good optical properties, and satisfactory stability. Importantly, due to strong inter-molecular electronic coupling, the calculated hole mobility of JT6, is as high as 7.92 cm2 V−1 s−1, which is 50 times higher than that of Spiro-OMeTAD. Simulating the adsorption properties on perovskite material CH3NH3PbI3 has also shown that these high mobility HTMs are easy to adsorb on the surface of perovskite material and can achieve good charge transfer between surface layers. The present findings highlight the potential of Janus dione core based HTMs for efficient PSCs and provide a meaningful practice for the design of HTMs with controllable HOMO levels.

Inhibited Aggregation of Lithium Salt in Spiro-OMeTAD for Perovskite Solar Cells

High-efficiency and stable hole transport materials (HTMs) play an essential role in high-performance planar perovskite solar cells (PSCs). 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobi-fluorene (Spiro-OMeTAD) is often used as HTMs in perovskite solar cells because of its excellent characteristics, such as energy level matching with perovskite, good film-forming ability, and high solubility. However, the accumulation and hydrolysis of the common additive Li-TFSI in Spiro-OMeTAD can cause voids/pinholes in the hole transport layer (HTL), which reduces the efficiency of the PSCs. In order to improve the functional characteristics of HTMs, in this work, we first used CsI as a dopant to modify the HTL and reduce the voids in the HTL. A small amount of CsI is introduced into Spiro-OMeTAD together with Li-TFSI and 4-tert-butylpyridine (TBP). It is found that CsI and TBP formed a complex, which prevented the rapid evaporation of TBP and eliminated some cracks in Spiro-OMeTAD. Moreover, the uniformly dispersed TBP inhibits the agglomeration of Li-TFSI in Spiro-OMeTAD, so that the effective oxidation reaction between Spiro-OMeTAD and air produces Spiro-OMeTAD+ in the oxidation state, thereby increasing the conductivity and adjusting the HTL energy. Correspondingly, the PCE of the planar PSC of the CsI-modified Spiro-OMeTAD is up to 13.31%. In contrast, the PSC without CsI modification showed a poor PCE of 10.01%. More importantly, the PSC of Spiro-OMeTAD treated with CsI has negligible hysteresis and excellent long-term stability. Our work provides a low-cost, simple, and effective method for improving the performance of hole transport materials and perovskite solar cells.

Performance of trans perovskite solar cells improved by finely adjusting the particle size of nickel oxide nanoparticles through pH value

As a low-cost, high stable hole transport material, nickel oxide has been widely used in inverted structure perovskite solar cells in recent years. By far, the most common method of preparing nickel oxide hole transport layers is spin-coating pre-prepared nickel oxide nanoparticles (NiO<sub><i>x</i></sub> NPs), which puts forward high requirement for the particle sizes and solution processing capabilities of NiO<sub><i>x</i></sub> NPs. In this work, the sizes of NiO<sub><i>x</i></sub> NPs are precisely controlled by adjusting the pH value of the system in the synthesis process, and high-quality nickel oxide hole transport layers are then prepared. The experimental results exhibit that the NiO<i>x</i> NPs with sizes of 5–10 nm are obtained at a pH value in a range of 9.5–9.8. More interestingly, the obtained NiO<sub><i>x</i></sub> NPs have good dispersion stability and can achieve long-term dispersion in aqueous solution. Furthermore, the structural composition analysis of NiO<sub><i>x</i></sub> NPs shows that the pH value of the synthesis system does not have a significant effect on the material structure nor composition of the NiO<sub><i>x</i></sub> NP. Surface morphological analysis shows that the NiO<sub><i>x</i></sub> film prepared by the pH-controlled NiO<sub><i>x</i></sub> NPs is rather dense and particularly flat with small surface roughness. It is also found that the film exhibits good hole extraction capability. We also fabricate an inverted perovskite solar cell based on the NiO<sub><i>x</i></sub> film. The device structure is ITO/NiO<sub><i>x</i></sub>/CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>/PC61BM/Bphen/Ag. It yields a good photovoltaic conversion efficiency (17.39%). In addition, the device is almost hysteresis-free. Our experimental results exhibit that the performance of perovskite solar cells can be effectively improved by precisely controlling the sizes of NiO<sub><i>x</i></sub> NPs through pH values. Our work is expected to facilitate the development of NiO<sub><i>x</i></sub>-based perovskite solar cells.

Performance of trans perovskite solar cells improved by finely adjusting the particle size of nickel oxide nanoparticles through pH value

As a low-cost, high stable hole transport material, nickel oxide has been widely used in inverted structure perovskite solar cells in recent years. By far, the most common method of preparing nickel oxide hole transport layers is spin-coating pre-prepared nickel oxide nanoparticles (NiO<sub><i>x</i></sub> NPs), which puts forward high requirement for the particle sizes and solution processing capabilities of NiO<sub><i>x</i></sub> NPs. In this work, the sizes of NiO<sub><i>x</i></sub> NPs are precisely controlled by adjusting the pH value of the system in the synthesis process, and high-quality nickel oxide hole transport layers are then prepared. The experimental results exhibit that the NiO<i><sub>x</sub></i> NPs with sizes of 5–10 nm are obtained at a pH value in a range of 9.5–9.8. More interestingly, the obtained NiO<sub><i>x</i></sub> NPs have good dispersion stability and can achieve long-term dispersion in aqueous solution. Furthermore, the structural composition analysis of NiO<sub><i>x</i></sub> NPs shows that the pH value of the synthesis system does not have a significant effect on the material structure nor composition of the NiO<sub><i>x</i></sub> NP. Surface morphological analysis shows that the NiO<sub><i>x</i></sub> film prepared by the pH-controlled NiO<sub><i>x</i></sub> NPs is rather dense and particularly flat with small surface roughness. It is also found that the film exhibits good hole extraction capability. We also fabricate an inverted perovskite solar cell based on the NiO<sub><i>x</i></sub> film. The device structure is ITO/NiO<sub><i>x</i></sub>/CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>/PC<sub>61</sub>BM/Bphen/Ag. It yields a good photovoltaic conversion efficiency (17.39%). In addition, the device is almost hysteresis-free. Our experimental results exhibit that the performance of perovskite solar cells can be effectively improved by precisely controlling the sizes of NiO<sub><i>x</i></sub> NPs through pH values. Our work is expected to facilitate the development of NiO<sub><i>x</i></sub>-based perovskite solar cells.

Pyridine functionalized phenothiazine derivatives as low-cost and stable hole-transporting material for perovskite solar cells

Heterocyclic phenothiazine (PTZ) has been proven effective in designing organic hole-transporting materials for perovskite solar cells (PSCs) as it has excellent electron-donating properties, straightforward synthesis, and feasible structure modifications. However, the resulting photovoltaic performance is still unsatisfactory, and the long-term stability has not been studied. Herein, we report a couple of PTZ-based hole transporting materials (HTMs) (PT-H and PH-HP) to investigate the efficacy of a pyridine functionalization on improving film morphology, hole mobility, and device performance, as well as long-term stability. The covalent incorporation of a pyridine unit in PT-HP does not alters the absorption, emission, and energy levels of PTZ-based HTMs. However, it can effectively facilitate the dispersion of lithium salt additive to form a uniform thin film, which is generally achieved by an additional additive of 4-tert-butylpyridine. This strategy remarkably improves the film quality and stability of spin-coated HTM for application in PSCs. As a result, PSCs based on 4-tert-butylpyridine-free PT-HP achieve a power conversion efficiency of 19.09%, significantly outperforming the reference HTM of PT-H (15.20%). More importantly, the stability of PT-HP based PSCs is successfully enhanced. The work paves the way to develop low-cost, efficient, and stable PTZ-based HTMs for PSCs.

Recent progress of doping strategies for organic hole transport materials in perovskite solar cells

<p indent=0mm>Organic-inorganic hybrid<bold> </bold>perovskite materials demonstrate outstanding optoelectronic properties, including high light absorption capacity, ambipolar charge transport, high defects tolerance. The application of perovskite materials in solar cells shows impressive progress by increasing the power conversion efficiencies (PCEs) from 3.8% to over 25%. Moreover, perovskite solar cells (PSCs) have the advantages of low materials cost and simple fabrication, giving rise to high potentials for future applications. The fast improvement in the photovoltaic performance of PSCs is mainly attributed to the intensive optimization in perovskite composition, device architecture, defects passivation, contact materials and perovskite crystallization kinetics. Contact materials, especially hole transport materials (HTMs), are key components in PSCs, which can not only affect the PCEs of PSCs, but also greatly influence the stability of devices. 2,2′,7,7′-Tetrakis[<italic>N</italic>,<italic>N</italic>-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD) is one of the most popularly used HTMs in PSCs and widely seen as a standard HTM for comparison. However, the synthetic process of spiro-OMeTAD is tedious and complex, leading to a high material cost. Moreover, spiro-OMeTAD has relatively low mobility, and chemical doping is one of the most efficient ways to improve the hole conductivity. So far different types of inorganic and organic dopants have been reported and their effects on photovoltaic performance are investigated. On the one hand, the introduction of chemical dopants can induce extra positive charges in HTMs and therefore enhance the conductivity. In this respect, doping efficiency is one critical factor for developing new dopants. On the other hand, research shows that chemical dopants can induce side effects in stability of HTMs and interfaces, limiting the lifetime of the PSCs devices. In this review, we first discuss the categories of HTMs and their specific advantages. Compared with inorganic HTMs, organic HTMs attract great attention because of high structural tunability, low cost and solution processability. However, organic molecules generally have low mobility, limiting their charge transport capacity. Therefore, doping by inducing redox reaction is the most effective way to enhance the conductivity of organic materials. Also, the doping mechanism slightly varies with different dopants and doping procedures are different. We focus on the progress of different chemical dopants in PSCs, and their roles and doping efficiencies in HTMs are briefly compared. We use spiro-OMeTAD as a reference HTM and conclude the influences of dopants on the conductivity and photovoltaic parameters in PSCs. The dopants include metal-based salts, ionic liquid and other molecules. Therein, dopants based on lithium bis-trifluoromethanesulfonimide (LiTFSI) and 4-tert-butylpyridine (TBP) are widely used in organic HTMs, but recent work shows that these additives have deleterious effects on device stability. Specifically, it is found LiTFSI migrates and accumulates at interfaces under the conditions of bias potentials. TBP has a relatively low boiling point and can easily evaporate at high temperatures, and also causes corrosion at perovskite interfaces. Therefore, developing alternative dopants and investigating their stability are crucial for future scalable applications in PSCs. Finally, we discuss the potentials of dopants in terms of future applications, and believe that developing new dopants with high stability is highly desired. Alternatively, designing efficient dopant-free HTMs is another strategy for obtaining stable HTMs for PSCs.

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 (JSC), open-circuit voltage (VOC), 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.

Stannite Quaternary Cu2M(M = Ni, Co)SnS4 as Low Cost Inorganic Hole Transport Materials in Perovskite Solar Cells

In this study, inorganic stannite quaternary Cu2M(M = Ni, Co)SnS4 (CMTS) is explored as a low-cost, earth abundant, environmentally friendly and chemically stable hole transport material (HTM). CMTS nanoparticles were synthesized via a facile and mild solvothermal method and processed into aggregated nanoparticle inks, which were applied in n-i-p perovskite solar cells (PSCs). The results show that Cu2NiSnS4 (CNiTS) is more promising as an HTM than Cu2CoSnS4 (CCoTS), showing efficient charge injection as evidenced by considerable photoluminescence quenching and lower series resistance from Nyquist plots, as well as higher power conversion efficiency (PCE). Moreover, the perovskite layer coated by the CMTS HTM showed superior environmental stability after 200 h light soaking in 50% relative humidity, while organic HTMs suffer from a severe drop in perovskite absorption. Although the obtained PCEs are modest, this study shows that the cost effective and stable inorganic CMTSs are promising HTMs, which can contribute towards PSC commercialization, if the field can further optimize CMTS energy levels through compositional engineering.

Advancement in Inorganic Hole Transport Materials for Inverted Perovskite Solar Cells

Organometallic halide-perovskite solar cells have undergone massive improvements in power conversion efficiency in the past decade, from around 4% in 2009 to 24% in 2019. A hotly debated issue in this field involves the investigation of economical, stable and power-efficient hole transport materials (HTMs) and electron transport materials in order to improve overall device performance and feasibility of mass production in the coming years. Even though the conventional (n-i-p) structure continues to be the most commonly used in perovskite solar cells, research in the field has shown that its potential for further commercial application is limited due to the higher J–V hysteresis and need for high temperature during fabrication. To address this issue, inverted (p-i-n) perovskite structures have been seriously examined because of their straightforward processability at low and moderate temperatures. These investigations have established that the HTMs are a significant part of the inverted (p-i-n) perovskite structure, which can render shape to a specific contact. They are perfect for reducing charge recombination and effective hole collection to enhance the overall performance of the device. This article examines in minute detail the different characteristics of inorganic hole transport materials used in inverted perovskite structures over the past decade, including power conversion efficiency, device configuration, energy band position and synthesis methods. It goes on to briefly discuss the stability analysis conducted to identify the factors which make perovskite unstable, so that possible ways to further optimize the performance parameters may be derived from the observations.

A theoretical study for high-performance inverted p-i-n architecture perovskite solar cells with cuprous iodide as hole transport material

Inverted perovskite solar cells (p-i-n PSCs) have been fascinated due to rapid progress of performance in recent years. PEDOT:PSS is commonly used hole transport material (HTM) in p-i-n PSCs having hygroscopic and acidic nature that leads towards poor performance of device. Therefore, it is necessary to replace PEDOT:PSS with stable HTM. In this paper, theoretical study is carried out to investigate physical parameters that can affect the performance of p-i-n PSCs with copper iodide (CuI) as HTM. These parameters include the effect of doping density of electron transport material (ETM), absorber and HTM as well as defect density and thickness of absorber on the performance of p-i-n PSCs. Hole mobility and thickness of HTM are also investigated. It is found that performance of p-i-n PSCs is strongly dependent on defect density and thickness of absorber. Upon final optimization, device attains PCE of more than 21% which is encouraging.

Tetra and octa substituted Zn(II) and Cu(II) phthalocyanines: Synthesis, characterization and investigation as hole-transporting materials for inverted type-perovskite solar cells

Metallophthalocyanines (MPcs) are used as cost-effective and thermally stable hole-transporting materials (HTMs) in the production of perovskite solar cells (PSCs). Unlike the mostly used n-i-p type PSCs, there are a few studies based on p-i-n type PSC with pthalocyanine HTMs. Here, for the first time, both tetra and octa substituted zinc and copper phthalocyanines have been studied as hole transporting materials in the inverted-type (p-i-n) PSCs without using any dopant material. The developed tetra substituted ZnPc and CuPc possess four dichlorophenylthio groups, and the octa-substituted Pcs have eight dichlorophenylthio groups in their peripheral position which enhance their solubility in perovskite-friendly solvent and improved thin film quality. In the inverted solar cell (ITO/HTM/CH3NH3Pbl3/PCBM/Al), methylammonium lead iodide (MAPI) was used as the active layer and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the electron transport material. Devices based on tetra substituted zinc and copper Pcs, as dopant-free HTMs, indicated significant efficiency enhancement and cell stability compared to similar octa substituted Pcs. While the open-circuit voltage (Voc) for the reference PEDOT:PSS was measured 0.77 V, for tetra substituted CuPc was found to be 0.83 V. Due to higher Voc value, tetra substituted CuPc is an appropriate candidate that can replace expensive and unstable HTMs that are currently used in PSCs.

Fluorinated triphenylamine-based dopant-free hole-transporting material for high-performance inverted perovskite solar cells

A fluorinated methoxytriphenylamine-based small molecule H2 is demonstrated as an effective hole-transporting material (HTM) to simultaneously enhance the efficiency and stability of the dopant-free inverted perovskite solar cells (PSCs). The fluorine-methoxytriphenylamine hybrid strategy in H2 is found to improve energy level alignment, hole extraction, and most importantly, the film morphology, which enables the growth of high quality perovskite active layer and superior interfacial contact. Consequently, the dopant-free H2-based inverted PSCs achieve an excellent efficiency of 18.69% with negligible hysteresis and good ambient stability, representing one of the best dopant-free organic-small-molecule HTMs for conventional perovskite MAPbI3 PSCs reported thus far. These results demonstrate the promising applications of fluorine-methoxytriphenylamine hybrid strategy in designing efficient and stable HTMs for dopant-free PSCs.

Phenothiazine-Based Hole Transport Materials for Perovskite Solar Cells.

The promising photovoltaic solar cells based on the perovskite light-harvesting materials have attracted researchers with their outstanding power conversion efficiencies (over 23% certified). The perovskite work has geared up in just under a decade and is competing with well-established semiconductor technologies such as silicon (Si), copper indium gallium selenide (CIGS), and cadmium telluride (CdTe). To commercialize the perovskite solar cells, their stability is the major concern. To address the stability issue, several factors need to be taken into account, and one of them is developing stable hole transport materials (HTMs), which are the essential building blocks. In this mini-review, we will discuss the important features of the HTMs, such as design and development of phenothiazine-based HTMs. Since phenothiazine is a low cost and stable molecule compared to the spiro-OMeTAD, it can be modified further via molecular engineering.

Development of Dopant‐Free Organic Hole Transporting Materials for Perovskite Solar Cells

AbstractThere has been considerable progress over the last decade in development of the perovskite solar cells (PSCs), with reported performances now surpassing 25.2% power conversion efficiency. Both long‐term stability and component costs of PSCs remain to be addressed by the research community, using hole transporting materials (HTMs) such as 2,2′,7,7′‐tetrakis(N,N′‐di‐pmethoxyphenylamino)‐9,9′‐spirbiuorene(Spiro‐OMeTAD) and poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine] (PTAA). HTMs are essential for high‐performance PSC devices. Although effective, these materials require a relatively high degree of doping with additives to improve charge mobility and interlayer/substrate compatibility, introducing doping‐induced stability issues with these HTMs, and further, additional costs and experimental complexity associated with using these doped materials. This article reviews dopant‐free organic HTMs for PSCs, outlining reports of structures with promising properties toward achieving low‐cost, effective, and scalable materials for devices with long‐term stability. It summarizes recent literature reports on non‐doped, alternative, and more stable HTMs used in PSCs as essential components for high‐efficiency cells, categorizing HTMs as reported for different PSC architectures in addition to use of dopant‐free small molecular and polymeric HTMs. Finally, an outlook and critical assessment of dopant‐free organic HTMs toward commercial application and insight into the development of stable PSC devices is provided.