Hole-transporting Materials For Perovskite Solar Cells Research Articles

Evidences of De-Doped Spiro-OMeTAD Employing Tert-Butyl Pyridine As Additive in Hole-Transporting Layers for n-i-p Perovskite Photovoltaics

Spiro-OMeTAD is the mostly employed molecular hole transporting material (HTM) in n-i-p perovskite solar cells (PSCs). Ease of processing from solution and good filmability on top of the perovskite photo-active layer are characteristics that make this HTM outstanding and incomparable for the role. However, both chemical doping with tert-butylpyridine (tBP) and lithium bis(trifluoromethylsulfonyl)-imide (LiTFSI), coupled to further oxidation steps, are required in order to achieve high hole mobility and conductivity. Previous investigations revealed that tBP is fundamental for addressing the best morphology in the hole-transporting layer during processing. We provide here for the first time a spectroscopic evidence of the detrimental impact on long-term conservation of Spiro-OMeTAD structural and electrical properties when tBP is used as additive. These aspects are crucial for the future design and understanding of new molecular HTMs for PSCs.

Optimizing electron-rich arylamine derivatives in thiophene-fused derivatives as π bridge-based hole transporting materials for perovskite solar cells.

Based on the observations of thienothiophene derivatives as π-bridged small molecule hole transporting materials (HTMs), adjusting their electron-rich arylamine derivatives is an effective approach to obtain the alternative HTMs for perovskite solar cells (PSCs). In this work, starting from a new electron-rich arylamine derivative and different π-bridged units of thienothiophene derivatives, a series of arylamine derivative-based HTMs were designed, and their properties were investigated using density functional theory combined with the Marcus charge transfer theory. Compared with the parental Z26 material, the designed H01–H04 exhibit appropriate frontier molecular orbitals, good optical properties, better solubility, good stability and higher hole mobilities. H01–H04 materials with high hole mobility (∼× 10−2) can serve as promising HTMs for improving the efficiency of PSCs. The results confirm that the design strategy of adjusting the electron-rich arylamine derivatives in thienothiophene derivatives as π-bridged HTMs is a reliable approach to obtain the promising HTMs for PSC applications.

Novel π-extended porphyrin-based hole-transporting materials with triarylamine donor units for high performance perovskite solar cells

Efficient hole transporting materials (HTMs) for perovskite solar cells (PrSCs) is highly desired in emerging solar cell technologies. Three HTMs comprising a coplanar porphyrin core with different substituted electron donating triarylamine (TAA) groups (coded as SGT-061, SGT-062, and SGT-063) were successfully developed and used as HTMs for PrSCs. The porphyrin-based HTMs with more bulky donor groups (SGT-062 and SGT-063) possess the larger dihedral angle between alkoxy-substituted phenyl ring and the donor TAA core, leading to the greater interference for the dense molecular arrangement and reducing the π-π stacking of HTM molecules. But SGT-061 with a less bulky donor enables a tight molecular arrangement to increase the π-π stacking between molecules. This renders SGT-061 to have a higher hole mobility than that of SGT-062 and SGT-063 HTMs with bigger donor groups. Moreover, the dynamic charge transfer process of perovskite/various HTM films was studied by time-resolved photoluminescence decay. And SGT-061 exhibited more efficient hole extraction ability than that of SGT-062 and SGT-063. This could be ascribed to better hole mobility and better co-facial π-π stacking between molecules. These results indicate that a less bulky electron donor unit is a preferred peripheral group for developing porphyrin based HTMs for efficient perovskite solar cells.

Molecular Engineering of Copper Phthalocyanines: A Strategy in Developing Dopant‐Free Hole‐Transporting Materials for Efficient and Ambient‐Stable Perovskite Solar Cells

AbstractCopper (II) phthalocyanines (CuPcs) have attracted growing interest as promising hole‐transporting materials (HTMs) in perovskite solar cells (PSCs) due to their low‐cost and excellent stability. However, the most efficient PSCs using CuPc‐based HTMs reported thus far still rely on hygroscopic p‐type dopants, which notoriously deteriorate device stability. Herein, two new CuPc derivatives are designed, namely CuPc‐Bu and CuPc‐OBu, by molecular engineering of the non‐peripheral substituents of the Pc rings, and applied as dopant‐free HTMs in PSCs. Remarkably, a small structural change from butyl groups to butoxy groups in the substituents of the Pc rings significantly influences the molecular ordering and effectively improves the hole mobility and solar cell performance. As a consequence, PSCs based on dopant‐free CuPc‐OBu as HTMs deliver an impressive power conversion efficiency (PCE) of up to 17.6% under one sun illumination, which is considerably higher than that of devices with CuPc‐Bu (14.3%). Moreover, PSCs containing dopant‐free CuPc‐OBu HTMs show a markedly improved ambient stability when stored without encapsulation under ambient conditions with a relative humidity of 85% compared to devices containing doped Spiro‐OMeTAD. This work thus provides a fundamental strategy for the future design of cost‐effective and stable HTMs for PSCs and other optoelectronic devices.

High isotropic dispiro structure hole transporting materials for planar perovskite solar cells

Abstract Two novel fluorene-based hole transporting materials (HTMs) were synthesized to be used in perovskite solar cells (PSCs). C102 was designed based on C101 by simply linking the two carbon–carbon single bonds to compose a “dispiro” structure. Their typically similar structures cause them sharing almost the same energy levels. However, their photovoltaic performances are quite different due to the small variations. The PSC that contained the “dispiro” structure, C102, reached a power conversion efficiency (PCE) of 17.4%, while the device contained C101, obtained a lower PCE of 15.5%. Electrochemical properties and Photovoltaic characterization of the two materials have been investigated to explain the result. It is shown that C102 has a stronger ability to transport holes and resist the charge recombination. Thus, the dispiro structure should be more appropriate being used as HTM in PSCs.

Thermo-evaporated pentacene and perylene as hole transport materials for perovskite solar cells

Thermo-evaporated organic semiconductors with suitable frontier molecular orbitals are potential hole transport materials (HTMs) for perovskite solar cells (PSCs), owing to their high crystallinity associated with high carrier mobility. In this study, we employ typical organic semiconductors, namely pentacene and perylene, as the HTMs in PSCs with an inverted structure. We gradually regulate the crystallographic morphologies of pentacene and perylene thin films by elevating the target substrate temperature from room temperature to 120 °C and 80 °C, respectively. We observe a clear dependence of the photovoltaic performance of inverted PSCs on the substrate temperature. We obtain the optimal power conversion efficiency (PCE) for pentacene and perylene at the substrate temperatures of 40 °C and 60 °C, respectively. The PSCs based on perylene show the best PCE of 13.5%, which is much higher that based on pentacene (5.7%). Perylene-based device shows better stability than PEDOT:PSS-based one. This study substantially deepens our knowledge regarding PSCs based on thermo-evaporated HTMs.

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

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

An Efficient Amphiphilic-Type Triphenylamine-Based Organic Hole Transport Material for High-Performance and Ambient-Stable Dopant-Free Perovskite and Organic Solar Cells.

A new set of simply structured triphenylamine-based small molecules are synthesized and evaluated as dopant-free hole transporting materials (HTMs) for high-performance perovskite solar cells (PSCs) and bulk heterojunction inverted organic solar cells (BHJ IOSCs). Surprisingly, the new amphiphilic-type HTM-1 (with internal hydrophilic groups and peripheral hydrophobic alkyl tails) showed better compatibility and performance than the actual target molecule, that is, HTM-2 in PSCs and BHJ IOSCs. Importantly, the HTM-1-based dopant-free PSCs and BHJ IOSCs exhibited high power conversion efficiencies (PCEs) of 11.45 % and 8.34 %, respectively. These performances are superior and comparable to those of standard HTMs Spiro-OMeTAD (2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene) and PEDOT:PSS (poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate) in PSCs and BHJ IOSCs, respectively. The enhanced device performance of the HTM-1-based PSCs is ascribed to its strong affinity towards the perovskite, properly aligned energy levels with respect to the perovskite valence band, and excellent hole transporting behavior. In addition, the well-organized energy levels of the HTMs showed excellent compatibility in BHJ IOSCs. The new amphiphilic-type HTM-based photovoltaic devices also showed long-term air stability over 700 h. These promising results offer new and unexpected prospects for engineering the interface between the photoactive material and HTMs in PSCs and BHJ IOSCs.

Advances in the Synthesis of Small Molecules as Hole Transport Materials for Lead Halide Perovskite Solar Cells.

Over hundreds of new organic semiconductor molecules have been synthesized as hole transport materials (HTMs) for perovskite solar cells. However, to date, the well-known N2, N2, N2', N2', N7, N7, N7', octakis-(4-methoxyphenyl)-9,9-spirobi-[9,9'-spirobi[9 H-fluorene]-2,2',7,7'-tetramine (spiro-OMeTAD) is still the best choice for the best perovskite device performance. Nevertheless, there is a consensus that spiro-OMeTAD by itself is not stable enough for long-term stable devices, and its market price makes its use in large-scale production costly. Novel synthetic routes for new HTMs have to be sought that can be carried out in fewer synthetic steps and can be easily scaled up for commercial purposes. On the one hand, synthetic chemists have taken, as a first approach, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the spiro-OMeTAD molecule as a reference to synthesize molecules with similar energy levels, although these HOMO and LUMO energy levels often have been measured indirectly in solution using cyclic voltammetry. On the other hand, the "spiro" chemical core has also been studied as a structural motif for novel HTMs. However, only a few molecules incorporated as HTMs in complete functional perovskite solar cells have been capable of matching the performance of the best-performing perovskite solar cells made using spiro-OMeTAD. In this Account, we describe the advances in the synthesis of HTMs that have been tested in perovskite solar cells. The comparison of solar cell efficiencies is of course very challenging because the solar cell preparation conditions may differ from laboratory to laboratory. To extract valuable information about the HTM molecular structure-device function relationship, we describe those examples that always have used spiro-OMeTAD as a control device and have always used identical experimental conditions (e.g., the use of the same chemical dopant for the HTM or the lack of it). The pioneering work was focused on well-understood organic semiconductor moieties such as arylamine, carbazole, and thiophene. Those chemical structures have been largely employed and studied as HTMs, for instance, in organic light-emitting devices. Interestingly, most research groups have reported the hole mobility values for their novel HTMs. However, only a few examples have been found that have measured the HOMO and LUMO energy levels using advanced spectroscopic techniques to determine these reference energy values directly. Moreover, it has been shown that those molecules, upon interacting with the perovskite layer, often have different HOMO and LUMO energies than the values estimated indirectly using solution-based electrochemical methods. Last but not least, porphyrins and phthalocyanines have also been synthesized as potential HTMs for perovskite solar cells. Their optical and physical properties, such as high absorption and good energy transfer capabilities, open new possibilities for HTMs in perovskite solar cells.

Interfacial Morphology Addresses Performance of Perovskite Solar Cells Based on Composite Hole Transporting Materials of Functionalized Reduced Graphene Oxide and P3HT

The development of novel hole transporting materials (HTMs) for perovskite solar cells (PSCs) that can enhance device's reproducibility is a largely pursued goal, even to the detriment of a very high efficiency, since it paves the way to an effective industrialization of this technology. In this work, we study the covalent functionalization of reduced graphene oxide (RGO) flakes with different organic functional groups with the aim of increasing the stability and homogeneity of their dispersion within a poly(3‐hexylthiophene) (P3HT) HTM. The selected functional groups are indeed those recalling the two characteristic moieties present in P3HT, i.e., the thienyl and alkyl residues. After preparation and characterization of a number of functionalized RGO@P3HT blends, we test the two containing the highest percentage of dispersed RGO as HTMs in PSCs and compare their performance with that of pristine P3HT and of the standard Spiro‐OMeTAD HTM. Results reveal the big influence of the morphology adopted by the single RGO flakes contained in the composite HTM in driving the final device performance and allow to distinguish one of these blends as a promising material for the fabrication of highly reproducible PSCs.

New Metal−Free Porphyrins as Hole−Transporting Materials in Mesoporous Perovskite Solar Cells.

AbstractTwo A2B2 type free base porphyrins have been synthesized and examined as hole−transporting materials (HTMs) in perovskite solar cells (PSCs). These molecules contain a porphyrin ring as a π−spacer, two N,N’‐dimethylphenyl groups and either two phenylcarboxylic acid or two phenyl‐methyl‐ester units respectively. These molecules have been chosen as alternative HTMs to the well−known Spiro‐MeOTAD (N2,N2,N2′,N2′,N7,N7,N7′,N7′‐octakis(4‐methoxyphenyl)‐9,9′‐spirobi[9H‐fluorene]‐2,2′,7,7′‐tetramine) since porphyrins exhibit high versatility in molecular engineering towards the best performance in solar cells. Thus, they were characterized in terms of their electrochemical properties as the HOMO and LUMO levels are very important regarding their behaviour as HTMs. Mesoporous PSCs were constructed using both molecules and compared with standard ones based on Spiro‐MeOTAD. To the best of our knowledge this is the first study in which metal−free porphyrins are employed as HTMs in PSCs. The cells that incorporated the phenylcarboxylic acid based porphyrin showed similar current density (23 mA cm−2) with those based on Spiro‐MeOTAD but lower open circuit voltage which overall constitutes a promising result consolidating metal−free porphyrins as a new class of HTMs.

Effects of thiophene substituents on hole-transporting properties of dipolar chromophores for perovskite solar cells

We present a theoretical investigation of thiophene substituent effects on the electrochemical properties of dipolar chromophores (TCNE, TCNE22 and TCNE24) as hole-transporting materials (HTMs) in perovskite solar cells (PSCs). Herein, the material properties in crystalline phases are explored by using the first-principle calculations combined with Marcus theory. The results show that the increased number of thiophene substituents for TCNE, TCNE22 and TCNE24 results in a redshift of the absorption spectrum (27–46 nm). Furthermore, both TCNE22 and TCNE24 have maximum absorption peaks at a wavelength of 400 nm. Most importantly, the molecular planarity is improved effectively, which generates strong intermolecular face-to-face π–π packing interaction. The higher hole mobility of TCNE24 (2.069 × 10−1 cm2 V−1 s−1) with four thiophene substituents is obtained due to the face-to-face π–π packing. The new designed TCNE24 not only has excellent spectral property, but also has strong hole mobility. Therefore, TCNE24 is a promising organic small-molecule HTMs. Our work provides theoretical guidance for designing higher-performance HTMs in PSCs.

Polymeric hole-transport materials with side-chain redox-active groups for perovskite solar cells with good reproducibility.

Two monomers, M:OO and M:ON, and their corresponding polymers, P:OO and P:ON, were prepared from styrene derivatives N,N-diphenyl-4-vinyl-aniline with different substituents (-OCH3 and -N(CH3)2) in the N-phenyl para positions. The polymers were synthesised and fully characterised to study their function as hole transport materials (HTMs) in perovskite solar cells (PSCs). The thermal, optical and electrochemical properties and performance of these monomers and polymers as HTMs in PSCs were compared in terms of their structure. The polymers form more stable amorphous glassy states and showed higher thermal stability than the monomers. The different substituent in the para position influenced the highest occupied molecular orbital (HOMO) level, altering the oxidation potential. Both monomers and polymers were employed as HTMs in perovskite solar cells with a device configuration FTO/bl-TiO2/mp-TiO2/CH3NH3PbI3/HTM/Au resulting in power conversion efficiencies of 7.48% for M:OO, 5.14% for P:OO, 5.28% for P:ON and 3.52% for M:ON. Although showing comparatively low efficiencies, the polymers showed much superior reproducibility in comparison with Spiro-OMeTAD or the monomers, suggesting further optimisation of polymeric HTMs with redox side groups is warranted.

Effects of Cyclic Tetrapyrrole Rings of Aggregate-Forming Chlorophyll Derivatives as Hole-Transporting Materials on Performance of Perovskite Solar Cells

Organic hole-transporting materials (HTMs) are essential components of high-performance perovskite solar cells (PSCs). Three zinc-coordinated chlorophyll derivatives with bacteriochlorin, chlorin, and porphyrin macrocycles, namely, ZnBChl, ZnChl, and ZnPor, respectively, were newly synthesized and employed as HTMs in PSCs. The difference in the π backbones of these HTMs causes differences in their photophysical properties, and thus different hole-extraction abilities, as revealed by steady-state photoluminescence spectra. The power conversion efficiencies (PCEs) of PSCs with a typical mesoporous structure, fluorine-doped tin oxide/compact TiO2/mesoporous TiO2/CH3NH3PbI3/HTM/Ag, are 8.26%, 11.88%, and 0.68% for ZnBChl, ZnChl, and ZnPor, respectively. The small PCE of the ZnPor-based PSC is partially attributed to the small energy gap of the highest occupied molecular orbital (HOMO) levels between ZnPor and CH3NH3PbI3 perovskite. Therefore, we increased this energy gap slightly by shifting the HOMO level of...

Inorganic Hole‐Transporting Materials for Perovskite Solar Cells

AbstractIn the last few years, inorganic–organic metal halide perovskite solar cells (PSCs) have attracted a great deal of attention as a promising next‐generation solar‐cell technology because of their high efficiencies and low production cost. Hole‐transporting materials (HTMs) play an essential role in effective charge extraction and thus in achieving high overall efficiency. Therefore, searching for an efficient, stable, and low‐cost HTM in PSCs has been one of the hottest research topics in this field. Inorganic p‐type semiconductors that possess several appealing characteristics, such as suitable energy levels, high hole mobility, and high chemical stability, as well as low production cost, etc., are promising HTM candidate materials in PSCs. Here, specific attention is paid to the recent progress in inorganic HTMs being explored for PSCs. A variety of methods developed for the fabrication of these inorganic HTMs are summarized in detail, together with their corresponding performance in PSCs. Finally, an outlook on further enhancements of highly efficient PSCs based on inorganic HTMs is presented.

The Importance of Pendant Groups on Triphenylamine‐Based Hole Transport Materials for Obtaining Perovskite Solar Cells with over 20% Efficiency

AbstractTremendous progress has recently been achieved in the field of perovskite solar cells (PSCs) as evidenced by impressive power conversion efficiencies (PCEs); but the high PCEs of >20% in PSCs has so far been mostly achieved by using the hole transport material (HTM) spiro‐OMeTAD; however, the relatively low conductivity and high cost of spiro‐OMeTAD significantly limit its potential use in large‐scale applications. In this work, two new organic molecules with spiro[fluorene‐9,9′‐xanthene] (SFX)‐based pendant groups, X26 and X36, have been developed as HTMs. Both X26 and X36 present facile syntheses with high yields. It is found that the introduced SFX pendant groups in triphenylamine‐based molecules show significant influence on the conductivity, energy levels, and thin‐film surface morphology. The use of X26 as HTM in PSCs yields a remarkable PCE of 20.2%. In addition, the X26‐based devices show impressive stability maintaining a high PCE of 18.8% after 5 months of aging in controlled (20%) humidity in the dark. We believe that X26 with high device PCEs of >20% and simple synthesis show a great promise for future application in PSCs, and that it represents a useful design platform for designing new charge transport materials for optoelectronic applications.

Polypyridyl Iron Complex as a Hole-Transporting Material for Formamidinium Lead Bromide Perovskite Solar Cells

An efficient hole-transporting material (HTM) is indispensable for high-performing perovskite solar cells (PSCs), which have recently emerged as a breakthrough photovoltaic technology. Here, we demonstrate the capacity of the transition metal complex (6,6′-bis(1,1-di(pyridin-2-yl)ethyl)-2,2′-bipyridine)-iron(II/III) trifluoromethanesulfonate ([Fe(bpyPY4)](OTf)2+x) to act as an additive-free, solution-processable HTM in PSCs based on the formamidinium lead bromide absorber. State-of-the-art physical methods have been employed to characterize [Fe(bpyPY4)](OTf)2+x and, in particular, to demonstrate its significantly higher conductivity compared to that of the conventional HTM spiro-OMeTAD. A maximum power conversion efficiency of 2.2% was obtained for a device employing [Fe(bpyPY4)](OTf)2+x, which is the first evidence of the applicability as a HTM in a PSC of a solid material in which conductivity is provided by a redox transformation of a transition metal.

Two dimethoxyphenylamine-substituted carbazole derivatives as hole-transporting materials for efficient inorganic-organic hybrid perovskite solar cells

Perovskite solar cells (PSCs) are currently under both of intense academic and industrial investigations. Here, two dimethoxyphenylamine-substituted carbazole derivatives (SYN1 and PARA1), as hole-transporting materials (HTMs) for PSCs, have been synthesized via two-step synthesis with low cost and simple operation. PSCs devices based on SYN1 and PARA1 showed power conversion efficiencies (PCE) of 11.4% and 13.1%, respectively, outperforming that of Spiro-OMeTAD (12.0%), demonstrating the dimethoxyphenylamine-substituted carbazole derivatives are good candidates as HTMs for perovskite solar cells. Time-resolved photoluminescence (TRPL) and J-V test under dark indicate that the hole injecting efficiency and dark current affect the cell performance seriously.

4‐Tert‐butylpyridine Free Organic Hole Transporting Materials for Stable and Efficient Planar Perovskite Solar Cells

Abstract4‐Tert‐butylpyridine (tBP) is an important additive in triarylamine‐based organic hole‐transporting materials (HTMs) for improving the efficiency and steady‐state performance of perovskite solar cells (PVSCs). However, the low boiling point of tBP (196 °C) significantly affects the long‐term stability and device performance of PVSCs. Herein, the design and synthesis of a series of covalently linked Spiro[fluorene‐9,9′‐xanthene] (SFX)‐based organic HTMs and pyridine derivatives to realize efficient and stable planar PVSCs are reported. One of the tailored HTMs, N2,N2,N7,N7‐tetrakis(4‐methoxyphenyl)‐3′,6′‐bis(pyridin‐4‐ylmethoxy) spiro[fluorene‐9,9′‐xanthene]‐2,7‐diamine (XPP) with two para‐position substituted pyridines that immobilized on the SFX core unit shows a high power conversion efficiency (PCE) of 17.2% in planar CH3NH3PbI3‐based PVSCs under 100 mW cm−2 AM 1.5G solar illumination, which is much higher than the efficiency of 5.5% that using the well‐known 2,2′,7,7′‐tetrakis‐(N,N‐di‐p‐methoxy‐phenyl‐amine)9,9′‐spirobifluorene (Spiro‐OMeTAD) as HTM (without tBP) under the same condition. Most importantly, the pyridine‐functionalized HTM‐based PVSCs without tBP as additive show much better long‐term stability than that of the state‐of‐the‐art HTM Spiro‐OMeTAD‐based solar cells that containing tBP as additive. This is the first case that the tBP‐free HTMs are demonstrated in PVSCs with high PCEs and good stability. It paves the way to develop highly efficient and stable tBP‐free HTMs for PVSCs toward commercial applications.

Highly π-extended copolymer as additive-free hole-transport material for perovskite solar cells

Organolead halide perovskite solar cells have achieved a certified power-conversion efficiency (PCE) of 22.1% and are thus among the most promising candidates for next-generation photovoltaic devices. To date, most high-efficiency perovskite solar cells have employed arylamine-based hole-transport materials (HTMs), which are expensive and have a low mobility. The complicated doping procedures and the potentially stability-adverse dopants used in these HTMs are among the major bottlenecks for the commercialization of perovskite solar cells (PSCs). Herein, we present a polythiophene-based copolymer (PDVT-10) with a hole mobility up to 8.2 cm2·V−1·s−1 and a highest occupied molecular orbital level of −5.28 eV as a hole-transport layer (HTL) for a PSC. A device based on this new HTM exhibited a high PCE of 13.4% under 100 mW·cm−2 illumination, which is one of the highest PCEs reported for the dopant-free polymer-based HTLs. Moreover, PDVT-10 exhibited good solution processability, decent air stability, and thermal stability, making it a promising candidate as an HTM for PSCs.