Hole-transporting Materials For Perovskite Solar Cells Research Articles

Dopant‐Free Phthalocyanine Hole Conductor with Thermal‐Induced Holistic Passivation for Stable Perovskite Solar Cells with 23% Efficiency

AbstractSimultaneous passivation of the defects at the surface and grain boundaries of perovskite films is crucial to achieve efficient and stable perovskite solar cells (PSCs). It is highly desirable to accomplish the above passivation through rational engineering of hole transport materials (HTMs) in combination with appropriate procedure optimization. Here, methylthiotriphenylamine‐substituted copper phthalocyanine (SMe‐TPA‐CuPc) is reported as a dopant‐free HTM for PSCs, exhibiting excellent efficiency, and stability. After thermal annealing, SMe‐TPA‐CuPc molecules diffused into the bulk of the perovskite film and effectively passivated the defects in the bulk and at the interface of the perovskite, owing to the strong interaction between the methylthio moiety and undercoordinated lead. The best‐performing annealed SMe‐TPA‐CuPc‐based device shows efficiency of 21.51%, which is higher than the unannealed SMe‐TPA‐CuPc‐based device (power‐conversion efficiency (PCE) of 20.75%) and reference doped spiro‐OMeTAD‐based device (PCE of 20.61%). Further modification of the perovskite of the annealed SMe‐TPA‐CuPc‐based device by the QAPyBF4 additive result in even higher efficiency of 23.0%. It also shows excellent stability, maintaining 96% of its initial efficiency after 3624 h aging at 85 °C. This work highlights the great potential of phthalocyanine‐based dopant‐free HTMs and the defect passivation by thermal‐induced molecular diffusion strategy for developing highly efficient and stable PSCs.

Analytical Review of Spiro‐OMeTAD Hole Transport Materials: Paths Toward Stable and Efficient Perovskite Solar Cells

The hole transport material (HTM) of organic–inorganic perovskite solar cells (PVSCs) plays a very important role for achieving high power conversion efficiency and long‐term stability. 2,2’,7,7’‐tetrakis[N,N‐di(4‐methoxyphenyl)amino]‐9‐9’‐spirobifluorene (spiro‐OMeTAD) is the first solid‐state HTM used in PVSCs and has gained tremendous attention during the last decade. Herein, the concept of spirolinkage for synthesis of spiro‐based HTMs is discussed, followed by an overview of the desirable optical and electrical properties of spiro‐OMeTAD. Recent progress in efficiency improvements of spiro‐based PVSCs is analyzed systematically, and the impacts of interface engineering, dopant‐free spiro‐OMeTAD, and novel spiro‐based HTMs are reviewed in detail. The hole mobility of spiro‐OMeTAD depends on the types of dopants and doping concentration. Commonly used lithium bis(trifluoromethylsulfonyl)imide and 4‐tert‐butylpyridine additives reduce the PVSC stability due to hygroscopicity and corrosiveness, respectively. The effects of additives on device stability and the techniques to improve the long‐term stability of spiro‐based PVSCs are also discussed. The review and analysis of various methods and strategies presented is useful for the perovskite research community, providing guidance and directions toward the further development of spiro‐based HTMs for PVSCs with improved efficiency and stability.

An asymmetrically substituted dithieno[3,2-b:2′,3′-d]pyrrole organic small-molecule hole-transporting material for high-performance perovskite solar cells

Hole-transporting materials play a vital role in terms of the performance of perovskite solar cells (PSCs). The dithieno[3,2- b :2′,3′- d ]pyrrole (DTP), an S,N-heterocyclic building block, has been proved to be desirable for molecular design of hole-transporting materials in PSCs. We developed an asymmetrically substituted DTP small-molecule (JW12) and a reference compound (JW11). The asymmetrical structure of JW12 leads to different absorption properties and electron distribution. The device in a planar n-i-p architecture using JW12 shows a much higher PCE (18.07%) than that based on JW11 (15.46%), which is also better than the device based on spiro-OMeTAD (17.47%). We hope our research can provide a new perspective in molecular design of organic HTMs for perovskite solar cells.

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.

Dopant Free Triphenylamine‐Based Hole Transport Materials with Excellent Photovoltaic Properties for High‐Performance Perovskite Solar Cells

Recently, scientists are more devoted to designing and synthesizing organic perovskite solar cells for attaining high power conversion efficiency (PCE). Herein, a series of small molecules as hole transport materials with A–π–D–π–A framework, namely (DFA1, DFA2, DFA3, and DFA4) having triphenylamine (TPA)‐based central donor core unit (A1–A4) end‐capped acceptor moieties linked via thiophene spacer is designed theoretically to study optoelectronic and photovoltaic properties. MPW1PW91 hybrid functional in conjunction with 6‐31G** basis set is found the best method to investigate the optoelectronic and photovoltaic properties. All hole transport materials (HTMs) possess a high open‐circuit voltage (0.98–1.41 V) with downshifted highest occupied molecular orbital energy values (−4.98 to −5.41 eV) by tailoring of electron‐withdrawing acceptor moieties. Meanwhile, appropriate elongation of π‐conjugated acceptor units is advantageous for enhancing the molar absorption coefficient and intermolecular electronic coupling. The highest hole mobility and charge transfer integral owing to lower hole reorganization energies indicate these molecules are best for HTMs. DFA1‐DFA4 has a power conversion efficiency of up to 24% which authenticates that these HTMs have excellent photovoltaic attributes compared to the reported reference DFH. The present computational investigation validates the efficacy of the designed techniques and opens a new route for designing high‐performance dopant‐free HTMs in perovskite solar cells.

Corannulene-based hole-transporting material for efficient and stable perovskite solar cells

Core structures with linear, planar, and spiral conformations have been designed as triphenylamine (TPA)-based hole-transporting materials (HTMs), which are the most prevalent small molecular HTMs in perovskite solar cells (PSCs). However, most of the reported TPA-based HTMs cannot achieve sufficient balance between efficiency and stability, which is governed by core structures. Herein, a sym -penta( N,N -bis(4-methoxyphenyl)aniline)corannulene (cor-OMePTPA) featuring a corannulene core and five TPA peripheral arms is designed as alternative HTM. Planar negative-intrinsic-positive (n-i-p) PSCs with cor-OMePTPA exhibit champion efficiencies of 20% and maintain 86% of their initial performances for more than 1,000 h after thermal annealing at 60°C. Compared with spiro-OMeTAD, cor-OMePTPA-based PSCs show slightly lower efficiencies but much better thermal stabilities. The main merit of cor-OMePTPA lies in its bimolecular interpenetration capability with noncovalent interactions to modulate the HTM configurations from single-molecular curvature to bimolecular planarity, thereby providing promising opportunities to achieve excellent balance between efficiency and stability. • Corannulene-based hole-transporting material for perovskite solar cells • Face-to-face interpenetrating dimer structures with high glass-transition temperature • Devices based on cor-OMePTPA exhibit high efficiency and excellent thermal stability An et al. report a corannulene-based hole-transporting material (cor-OMePTPA) for perovskite solar cells. Cor-OMePTPA forms face-to-face interpenetrating dimer structure with high glass transition temperature. Devices based on cor-OMePTPA exhibit efficiency of 20% and maintain 86% of their initial performance for more than 1,000 h after thermal annealing at 60°C.

Tuning Alkyl Chain Lengths of Oxasmaragdyrins-B(OR)2 for Optimizing Hole-Transport and Efficiency in Perovskite Solar Cells

A major challenge toward commercialization of perovskite solar cells (PSCs) is the development of cost-effective hole-transport materials (HTMs) with good hole mobility and long-term stability. Porphyrinoids such as metal-free oxasmaragdyrins as alternative HTMs in PSCs show promising power conversion at lower costs. In this study, a difluoroboryl oxasmaragdyrin, SM09, has been modified by introducing alkoxy chains with different chain lengths (methoxy (OMe), ethoxy (OEt), butoxy (OBu), and octyloxy (OOct)) at the central core position, affecting molecular packing, varying intermolecular distances, and consequently altering the hole mobility. These modified oxasmaragdyrins were used as HTMs for the planar PSCs. The device performance was evaluated and correlated with the alkyl chain length and was rationalized by photophysical characterizations. The device efficiencies decrease with an increase in the alkyl chain length from the highest PCE of 15.47% for SM-OMe to 13.35% for SM-OOct. The best performance was obtained in SM-OMe, due to its higher hole mobility (3.75 × 10–4 cm2 V–1 s–1) and stronger p-type character. In the future, a simple tuning of alkyl chain length at central core positions of oxasmaragdyrin HTMs can be an effective strategy to enhance the power conversion efficiency (PCE) of PSCs.

Structurally‐tuned benzo[1,2‐b:4,5:b']dithiophene‐based polymer as a dopant‐free hole transport material for perovskite solar cells

AbstractFor highly efficient and stable perovskite solar cells (PSCs), hole transport material (HTM) should be designed and synthesized to afford suitable energy levels, high charge transport, efficient passivation ability, and high device stability. Here, we systematically modulated benzo[1,2‐b:4,5:b']dithiophene‐based polymer by finely controlling the thienyl and pyridyl contents within the conjugated backbone in order to develop a high performance dopant‐free HTM for PSCs. We found that the optimized copolymer with 25% of pyridine content exhibits improved energy level, charge transport, and morphology compared with control homopolymers. As a result, remarkably high power conversion efficiencies up to 21.1% were achieved by employing the optimized polymer as a dopant‐free HTM in PSCs.

Synthesis and properties of 3,4‐dioxythiophene and 1,4‐dialkoxybenzene based copolymers via direct CH arylation: Dopant‐free hole transport material for perovskite solar cells

AbstractDirect CH arylation coupling reaction has gained significant importance in synthesis of conjugated polymers for organic electronic applications. We report here a facile and straightforward method called “direct CH arylation” reaction to synthesize conjugated 3,4‐dioxythiophene and 1,4‐dialkoxybenzene based copolymers as hole transport material (HTM) for perovskite solar cells. Two electron‐rich conjugated polymers P1‐2 were synthesized, in which 1,4‐dibromo‐2,5‐bis(dodecyloxy)benzene and 3,4‐dialkoxy‐thiophene units were used for polymerization. The resulting polymers were characterized and exhibited high solubility in organic solvents. Electrochemical and optical characterizations were carried out by cyclic voltammetry and UV–Vis–NIR absorption spectroscopy and found that these polymers show higher‐lying HOMO energy levels with wide band gap. Density functional theory calculation was performed on these polymers (P1‐2) and correlated with our experimental results. Finally, perovskite solar cells were fabricated by solution‐processable deposition of P1‐2 as dopant‐free HTM with device geometry ITO/SnO2/Perovskite/HTM(P1/P2)/Ag and achieved a maximum power conversion efficiency of 5.28%. This study provides information on designing and simple preparation by direct CH arylation reaction of higher‐lying HOMO energy level polymer as HTM for perovskite solar cells.

Azatriphenylene-based D-A-D-typed hole-transporting materials for perovskite solar cells with tunable energy levels and high mobility

Computational study on relationship between molecular structure and property of small molecule hole-transporting materials (HTMs) is an efficient pathway to acquire potential HTMs for perovskite solar cells (PSCs). Herein, by conjugating the acceptor groups of electron-deficient diazatriphenylene (DAT), tetraazatriphenylene (TAT) and hexaazatriphenylene (HAT), and p-methoxy-substituted triphenylamine (MeOTPA) electron-donors, six novel donor–acceptor-donor (D-A-D) typed HTMs (SM11 ~ SM16) are designed. Furthermore, three thiophene-substituted acceptors (SM17 ~ SM19) are also investigated. The influences of acceptor moiety on the performance of HTMs are studied by carrying out theoretical chemical calculations. Compared with the reported HTM with triphenylene unit (TPH-T), the new tailored HTMs (SM11 ~ SM19) exhibit more negative highest occupied molecular orbital (HOMO) energy levels and slightly red-shifted light absorption due to the increased electron-deficient property of acceptor group and linked positions with the MeOTPA-donor. Meanwhile, our results show that the small torsion angle between the acceptor and the donor is beneficial to promote the hole transport in HTMs, and the thiophene units on acceptor may also be helpful to enhance the charge mobility of HTMs. In addition, the large charge transfer amounts and small exciton binding energies are found for designed HTMs, which will be favorable to facilitate the electron-hole separation in HTMs. The stability of HTMs may be decreased based on the calculated results of electrostatic surface potential and absolute hardness. We hope this work could highlight the potential of azatriphenylene acceptor-based D-A-D typed HTMs for efficient PSCs.

Dopant‐Free Hole‐Transporting Material with Enhanced Intermolecular Interaction for Efficient and Stable n‐i‐p Perovskite Solar Cells

AbstractDeveloping low‐cost, efficient, and stable dopant‐free hole‐transporting materials (HTMs) in perovskite solar cells (PVSCs) is essential to their commercial deployment. Herein, the synthesis of a novel spirofluorene‐dithiolane based small molecular HTM, SFDT‐TDM, through facile and low‐cost synthetic routes is reported. The CH…π interactions in adjacent SFDT‐TDM are beneficial for high hole mobility and the methylthio groups in SFDT‐TDM can serve as Lewis bases to passivate the defects on the surface of perovskite films, leading to suppressed non‐radiative recombination and enhanced charge extraction at the perovskite/HTM interface. As a result, CsxFA1−xPbI3 based PVSCs with SFDT‐TDM as the HTM realize champion power conversion efficiencies (PCEs) of 21.7% and 20.3% for small‐area (0.04 cm2) and large‐area (1.0 cm2) devices with negligible photocurrent hysteresis, respectively. Additionally, all‐inorganic CsPbI3−xBrx based PVSCs with SFDT‐TDM demonstrate an impressive PCE of 17.1% along with excellent stability. This work highlights the great potential of the spirofluorene core for exploring low‐cost and dopant‐free HTMs for PVSCs with high efficiency and stability.

Novel Triarylamine-Based Hole Transport Materials: Synthesis, Characterization and Computational Investigation.

Three novel triarylamine-based electron-rich chromophores were synthesized and fully characterized. Compounds 1 and 2 were designed with electron-rich triphenylamine skeleton bearing two and four decyloxy groups namely, 3,4-bis(decyloxy)-N,N-diphenylaniline and N-(3,4-bis(decyloxy)phenyl)-3,4-bis(decyloxy)-N-phenylaniline, respectively. The well-known electron-rich phenothiazine was introduced to diphenylamine moiety through a thiazole ring to form N,N-bis(3,4-bis(decyloxy)phenyl)-5-(10H-phenothiazin-2-yl)thiazol-2-amine (Compound 3). These three novel compounds were fully characterized and their UV–vis absorption indicated their transparency as a favorable property for hole transport materials (HTMs) suitable for perovskite solar cells. Cyclic voltammetry measurements revealed that the HOMO energy levels were in the range 5.00–5.16 eV for all compounds, indicating their suitability with the HOMO energy level of the perovskite photosensitizer. Density functional theory (DFT) and time-dependent DFT (TD-DFT) have been used to investigate the possibility of the synthesized compounds to be utilized as HTMs for perovskite solar cells (PSCs). The computational investigation revealed that the hole mobility of Compound 1 was 1.08 × 10−2 cm2 V−1 s−1, and the substitution with two additional dialkoxy groups on the second phenyl ring as represented by Compound 2 significantly boosted the hole mobility to reach the value 4.21 × 10−2 cm2 V−1 s−1. On the other hand, Compound 3, in which the third phenyl group was replaced by a thiazole-based phenothiazine, the value of hole mobility decreased to reach 5.93 × 10−5 cm2 V−1 s−1. The overall results indicate that these three novel compounds could be promising HTMs for perovskite solar cells.

D-π-D type fluorescent compounds based on dehydroabietic acid triarylamine: Synthesis, photophysical properties and application as hole-transporting materials in perovskite solar cells

D-π-D hole-transporting materials have the potential to be excellent dopant-free hole-transporting materials (HTMs) for perovskite solar cells (PSCs) because of their tunable energy levels, high charge mobility, good solubility, and excellent film-forming properties. In this paper, two series of D-π-D type compounds, Series 1 (2TPA-CZ1~2TPA-CZ3) and Series 2 (2DTPA-CZ1~2DTPA-CZ3), were synthesized by linking a carbazole derivative (as a π-bridge) to triaryamine or dehydroabietic acid triarylamine groups (the donor groups) via C–C coupling reactions. The optical, electrochemical, thermal, and photovoltaic properties in PSCs of these compounds were fully investigated. The two series of compounds exhibit good ultraviolet absorption, fluorescence emission, and electrochemical and thermal properties. In addition, their highest occupied molecular orbital levels and energy gaps make them suitable for use as hole-transporting materials in PSCs. Among the PSCs developed, Device 1, based on 2TPA-CZ1, showed the best performance, with a power conversion efficiency (PCE) of 12.27%, a open circuit voltage (VOC) of 1.13 V, a short-circuit photocurrent density (JSC) of 15.10 mA cm−2, and a fill factor (FF) of 71.61%, which is comparable to that of a device based on the conventional hole-transporting material Spiro-OMeTAD. While the PCE of Devices 4–6 was lower than the PCE of Devices 1–3, Devices 4–6 exhibit better stability. This may be due to the higher Tg and better film-forming properties of compounds formed using the dehydroabietic acid skeleton. In additional, the introduction of alkyl groups on the carbazole moiety decreased the efficiency of the PSC devices. These results demonstrate that the 2TPA-CZ1 compounds are promising HTMs for PSC applications.

C[tbnd]N-based carbazole-arylamine hole transporting materials for perovskite solar cells: Substitution position matters

Hole transporting materials (HTMs) containing passivating groups for perovskite materials have attracted much attention for efficient and stable perovskite solar cells (PSCs). Among them, CN-based molecules have been proved as efficient HTMs. Herein, a series of novel CN functionalized carbazole-arylamine derivatives with variable CN substitution positions (para, meta, and ortho) on benzene-carbazole skeleton (on the adjacent benzene of carbazole) were synthesized (p-HTM, m-HTM and o-HTM). The experimental results exhibit that the substitution positions of the CN unit on HTMs have minor difference on the HOMO energy level and hydrophobicity. m-HTM has a relatively lower glass transition temperature compared with that of p-HTM and o-HTM. The functional theory calculations show that the CN located on meta position exposed very well, and the exposure direction is also the same with the methoxy. Upon applying these molecules as HTMs in PSCs, their device performance is found to sensitively depend on the substitution position of the CN unit on the molecule skeleton. The devices using m-HTM and o-HTM exhibit better performance than that of p-HTM. Moreover, m-HTM-based devices exhibit better light-soaking performance and long-term stability, which could be resulted from better interaction with the perovskite according to DFT results. Moreover, we further prepared a HTM with two CN units on the symmetrical meta position of molecular skeleton (2m-HTM). Interestingly, 2m-HTM-based devices exhibit relatively inferior performance compared with that of the m-HTM, which could be resulted from weak negative electrical character of CN unit on 2m-HTM. The results give some new insights for designing ideal HTM for efficient and stable PSCs.

Device Modeling and Design of Inverted Solar Cell Based on Comparative Experimental Analysis between Effect of Organic and Inorganic Hole Transport Layer on Morphology and Photo-Physical Property of Perovskite Thin Film.

It is crucial to find a good material as a hole transport layer (HTL) to improve the performance of perovskite solar cells (PSCs), devices with an inverted structure. Polyethylene dioxythiophene-poly (styrene sulfonate) (PEDOT:PSS) and inorganic nickel oxide (NiOx) have become hotspots in the study of hole transport materials in PSCs on account of their excellent properties. In our research, NiOx and PEDOT: PSS, two kinds of hole transport materials, were prepared and compared to study the impact of the bottom layer on the light absorption and morphology of perovskite layer. By the way, some experimental parameters are simulated by wx Analysis of Microelectronic and Photonic Structures (wxAMPS). In addition, thin interfacial layers with deep capture levels and high capture cross sections were inserted to simulate the degradation of the interface between light absorption layer and PEDOT:PSS. This work realizes the combination of experiment and simulation. Exploring the mechanism of the influence of functional layer parameters plays a vital part in the performance of devices by establishing the system design. It can be found that the perovskite film growing on NiOx has a stronger light absorption capacity, which makes the best open-circuit voltage of 0.98 V, short-circuit current density of 24.55 mA/cm2, and power conversion efficiency of 20.01%.

Chromium trioxide modified spiro-OMeTAD for highly efficient and stable planar perovskite solar cells

Hole transporting materials (HTMs) play an unparalleled role in heightening the stability and photovoltaic performance of perovskite solar cells (PSCs). The organic small molecule spiro-OMeTAD is frequently utilized for HTM in PSCs. However, the raw spiro-OMeTAD without dopant would be harmful to the development of highly efficient PSCs, due to its unsatisfied hole mobility and conductivity. Therefore, we introduce an inorganic dopant (chromium trioxide, CrO3) into the lithium-salt doped spiro-OMeTAD. Because of the exclamatory oxidizability of CrO3, it can accelerate the oxidation of spiro-OMeTAD and thereby enhancing the hole mobility of HTM. The introduction of CrO3 not only substantially decreases the density of defects, but also adjusts spiro-OMeTAD energy band, and thus effectively suppresses the hysteresis and improving stability of PSCs. In the end, we obtained a power conversion efficiency (PCE) as high as 22.6% after doping CrO3 in spiro-OMeTAD. The facile, low cost and outstanding photovoltaic performance render CrO3 an excellent dopant for HTMs in PSCs.

Tetraphenylethylene-Arylamine Derivatives as Hole Transporting Materials for Perovskite Solar Cells.

A series of hole transporting materials (HTMs) with fused tetraphenylethylene cores (9,9'-bifluorenylidene and dibenzo[g,p]chrysene) as well as different substitution positions of arylamine side arms has been designed and synthesized. A reference HTM with a non-fused tetraphenylethylene core is also prepared for a comparative study. It is noted that fused tetraphenylethylene molecules show a bathochromic spectral shift, electronegative character, and lower reorganization energies than the non-fused ones. Furthermore, the molecules with side arms located on the meta-position on the tetraphenylethylene core in terms of a double bond exhibit a deeper highest occupied molecular orbital level than those of the para-position-based ones whether tetraphenylethylene is fused or not. Moreover, the reorganization energies of fused meta-position-based HTMs are lower than those of para-position-based HTMs. Fused tetraphenylethylene HTMs own a better hole-extraction capability than the non-fused ones. When used in perovskite solar cells, all devices with fused tetraphenylethylene HTMs display better performance than those of the non-fused ones. The HTMs based on dibenzo[g,p]chrysene exhibit better performance than those of bifluorenylidene. Moreover, the devices with HTMs with side arms located on the meta-position on the tetraphenylethylene core display higher power conversion efficiency than those of the para-position-based ones. The results give some new insight and reference to develop ideal HTMs for perovskite solar cells.

Low-cost molecular glass hole transport material for perovskite solar cells

The availability of low-cost hole transport materials (HTMs) that are easy to process is crucial for the eventual commercialization of perovskite solar cells (PSCs), as the commonly used HTM (Spiro-OmeTAD) is expensive, and its processing is complex. In this study, we synthesized an amorphous molecular material (termed as TPA-glass) from the condensation of 2-mexylamino-4-methylamino-6-(4-aminophenylamino)-1,3,5-triazine and N-(4-formylphenyl)diphenylamine with a low-cost and easy process, and applied as an HTM in PSCs. We investigated the effect of TPA-glass thin-films with varying thickness, as well as their corresponding solar cell’s properties. The preliminary performance data indicate that TPA-glass thin-film can be a potential HTM candidate for planar 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.

Introducing an Organic Hole Transporting Material as a Bilayer to Improve the Efficiency and Stability of Perovskite Solar Cells

Although perovskite solar cells (PSCs) have achieved high power conversion efficiency (PCE) by utilizing 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-Spirobifluorene (Spiro-OMeTAD) as hole transporting material (HTM), the reproducibility and stability of PSCs are still a pressing concern. Herein, we introduced a solvent processed organic-organic bilayer based on 2-(4-(7-(9,9-dimethylacridin-10(9H)-yl)-9,9-diethyl-9H-fluoren-2-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (AFpPPI) and Spiro-OMeTAD in layer to layer as HTM in PSCs. The devices configured with AFpPPI/Spiro-OMeTAD bilayer achieved a maximum PCE of 19.9% in mesoporous-TiO2 (mp-TiO2) structure with perovskite absorber of Cs0.05Rb0.05(FAPbI3)0.76 (MAPbBr3)0.14. The properties of the bilayer structure were analyzed with steady-state photoluminescence, ultra-violet photoelectron and impedance spectroscopy. The AFpPPI/Spiro-OMeTAD improved open-circuit voltage (Voc) by lowering the quasi-Fermi energy level for holes and reducing the charge recombination, resulting in high Voc (1.14 V in the champion cell) and high fill factor (FF) that lead to high PCE. The addition of AFpPPI layer improves the quality of Spiro-OMeTAD and provides pinhole-free film. Moreover, the stability is improved in controlled temperature and humid conditions. This work affords a new approach for commercial applications of PSCs with better stability.