Effects of heteroatom substitution in spiro-bifluorene hole transport materials

Three new spirofluorene-based hole transport materials, Spiro-S, Spiro-N, and Spiro-E, are synthesized by replacing the para-methoxy substituent in 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-MeOTAD) with methylsulfanyl, N,N-dimethylamino and ethyl groups. Their properties as hole transport materials in perovskite solar cells are investigated. The impact of replacing the para-methoxy substituent on bulk properties, such as the photophysical properties, HOMO/LUMO energy level, hole extraction properties and morphologies of perovskite thin films are investigated. Their optoelectronic and charge-transport properties and performance in perovskite solar cells are compared with the current benchmarked and structurally-related hole transport material (HTM) Spiro-MeOTAD. Surprisingly, the methylsulfanyl substituted spirofluorene shows the highest power conversion efficiency of 15.92% among the investigated spirofluorenes, which is an over 38% increase in PCE compared with that of Spiro-MeOTAD under similar device fabrication conditions.

Donor-π-donor type hole transporting materials: marked π-bridge effects on optoelectronic properties, solid-state structure, and perovskite solar cell efficiency

Donor–π-bridge–donor type oligomers (D–π–D) have been studied intensively as active materials for organic optoelectronic devices.

Donor–π-bridge–donor type oligomers (D–π–D) have been studied intensively as active materials for organic optoelectronic devices. In this study, we introduce three new D–π–D type organic semiconductors incorporating thiophene or thienothiophene with two electron-rich TPA units, which can be easily synthesized from commercially available materials. A thorough comparison of their optoelectronic and structural properties was conducted, revealing the strong influence of the extent of longitudinal π-bridge conjugation on both the solid structure of the organic semiconductive materials and their photovoltaic performance when applied as hole transporting materials (HTM) in perovskite solar cells. Single-crystal measurements and time-resolved photoluminescence (TRPL) studies indicate that these coplanar donor–π–donor type HTMs could be promising alternatives to state-of-the-art spiro-OMeTAD, due to the multiple intermolecular short contacts as charge transporting channels and efficient charge extraction properties from the perovskite layer. The optimized devices with PEH-9 exhibited an impressive PCE of 16.9% under standard global AM 1.5 illumination with minimized hysteretic behaviour, which is comparable to that of devices using spiro-OMeTAD under similar conditions. Ambient stability after 400 h revealed that 93% of the energy conversion efficiency was retained for PEH-9, indicating that the devices had good long-term stability.

Efficiency enhancement of perovskite solar cells via incorporation of phenylethenyl side arms into indolocarbazole-based hole transporting materials

Small-molecule hole transporting materials based on an indolocarbazole core were synthesized and incorporated into perovskite solar cells, which displayed a power conversion efficiency up to 15.24%. The investigated hole transporting materials were synthesized in three steps from commercially available and relatively inexpensive starting materials without using expensive catalysts. Various electro-optical measurements (UV-vis, CV, hole mobility, DSC, TGA, ionization potential) have been carried out to characterize the new hole transporting materials.

Benzotrithiophene-Based Hole-Transporting Materials for 18.2 % Perovskite Solar Cells

New star-shaped benzotrithiophene (BTT)-based hole-transporting materials (HTM) BTT-1, BTT-2 and BTT-3 have been obtained through a facile synthetic route by crosslinking triarylamine-based donor groups with a benzotrithiophene (BTT) core. The BTT HTMs were tested on solution-processed lead trihalide perovskite-based solar cells. Power conversion efficiencies in the range of 16 % to 18.2 % were achieved under AM 1.5 sun with the three derivatives. These values are comparable to those obtained with today's most commonly used HTM spiro-OMeTAD, which point them out as promising candidates to be used as readily available and cost-effective alternatives in perovskite solar cells (PSCs).

A New 1,3,4-Oxadiazole-Based Hole-Transport Material for Efficient CH3 NH3 PbBr3 Perovskite Solar Cells

A new hole-transport material (HTM) based on the 1,3,4-oxadiazole moiety (H1) was prepared through a single-step synthetic pathway starting from commercially available products. Thanks to a deep HOMO level, H1 was used as HTM in CH3 NH3 PbBr3 perovskite solar cells yielding an efficiency of 5.8%. The reference HTM (Spiro-OMeTAD), under the same testing conditions, furnished a lower efficiency of 5.1%. Steady-state and time-resolved photoluminescence of the thin films showed good charge-extraction dynamics for H1 devices. In addition, H1 shows a large thermal stability and completely amorphous behavior (as evaluated by thermal gravimetric analysis and differential scanning calorimetry).

Development of simple hole-transporting materials for perovskite solar cells

Three low-cost propeller-shaped small molecules based on a triphenylamine core and the high-performance donor molecule 7,7′-[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl]bis[6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole] (DTS(FBTTh2)2) were investigated as hole-transporting materials in perovskite solar cells. Each hole-transporting material was designed with highly modular side arms, allowing for different bandgaps and thin-film properties while maintaining a consistent binding energy of the highest occupied molecular orbitals to facilitate hole extraction from the perovskite active layer. Perovskite solar cell devices were fabricated with each of the three triphenylamine-based hole-transporting materials and DTS(FBTTh2)2 and were compared to devices with 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) hole-transporting layers. Each of our triphenylamine hole-transporting materials and DTS(FBTTh2)2 displayed surface morphologies that were considerably rougher than that of spiro-OMeTAD; a factor that may contribute to lower device performance. It was found that using inert, insulating polymers as additives with DTS(FBTTh2)2 reduced the surface roughness, resulting in devices with higher photocurrents.

Hydrophobic Organic Hole Transporters for Improved Moisture Resistance in Metal Halide Perovskite Solar Cells

Solar cells based on organic-inorganic perovskite semiconductor materials have recently made rapid improvements in performance, with the best cells performing at over 20% efficiency. With such rapid progress, questions such as cost and solar cell stability are becoming increasingly important to address if this new technology is to reach commercial deployment. The moisture sensitivity of commonly used organic-inorganic metal halide perovskites has especially raised concerns. Here, we demonstrate that the hygroscopic lithium salt commonly used as a dopant for the hole transport material in perovskite solar cells makes the top layer of the devices hydrophilic and causes the solar cells to rapidly degrade in the presence of moisture. By using novel, low cost, and hydrophobic hole transporters in conjunction with a doping method incorporating a preoxidized salt of the respective hole transporters, we are able to prepare efficient perovskite solar cells with greatly enhanced water resistance.

Decoupling Charge Transfer and Transport at Polymeric Hole Transport Layer in Perovskite Solar Cells

Tailoring charge extraction interfaces in perovskite solar cells (PeSCs) critically determines the photovoltaic performance of PeSCs. Here, we investigated the decoupling of two major determinants of the efficient charge extraction, the charge transport and interfacial charge transfer properties at hole transport layers (HTLs). A simple physical tuning of a representative polymeric HTL, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), provided a wide range of charge conductivities from 10(-4) to 10(3) S cm(-1) without significant modulations in their energy levels, thereby enabling the decoupling of charge transport and transfer properties at HTLs. The transient photovoltaic response measurement revealed that the facilitation of hole transport through the highly conductive HTL promoted the elongation of charge carrier lifetimes within the PeSCs up to 3 times, leading to enhanced photocurrent extraction and finally 25% higher power conversion efficiency.

Hole-Transporting Materials Based on Twisted Bimesitylenes for Stable Perovskite Solar Cells with High Efficiency

A new class of hole-transport materials (HTMs) based on the bimesitylene core designed for mesoporous perovskite solar cells is introduced. Devices fabricated using two of these derivatives yield higher open-circuit voltage values than the commonly used spiro-OMeTAD. Power conversion efficiency (PCE) values of up to 12.11% are obtained in perovskite-based devices using these new HTMs. The stability of the device made using the highest performing HTM (P1) is improved compared with spiro-OMeTAD as evidenced through long-term stability tests over 1000 h.

Perovskite Solar Cells: Influence of Hole Transporting Materials on Power Conversion Efficiency

The recent advances in perovskite solar cells (PSCs) created a tsunami effect in the photovoltaic community. PSCs are newfangled high-performance photovoltaic devices with low cost that are solution processable for large-scale energy production. The power conversion efficiency (PCE) of such devices experienced an unprecedented increase from 3.8 % to a certified value exceeding 20 %, demonstrating exceptional properties of perovskites as solar cell materials. A key advancement in perovskite solar cells, compared with dye-sensitized solar cells, occurred with the replacement of liquid electrolytes with solid-state hole-transporting materials (HTMs) such as 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD), which contributed to enhanced PCE values and improved the cell stability. Following improvements in the perovskite crystallinity to produce a smooth, uniform morphology, the selective and efficient extraction of positive and negative charges in the device dictated the PCE of PSCs. In this Review, we focus mainly on the HTMs responsible for hole transport and extraction in PSCs, which is one of the essential components for efficient devices. Here, we describe the current state-of-the-art in molecular engineering of hole-transporting materials that are used in PSCs and highlight the requisites for market-viability of this technology. Finally, we include an outlook on molecular engineering of new functional HTMs for high efficiency PSCs.

Simple biphenyl or carbazole derivatives with four di(anisyl)amino substituents as efficient hole-transporting materials for perovskite solar cells

Power conversion efficiencies of 13.6% and 11.5% were achieved in perovskite solar cells with two simple and readily accessible biphenyl or carbazole derivatives as hole-transporting materials.

A perylene diimide-based non-fullerene acceptor as an electron transporting material for inverted perovskite solar cells

Herein we introduce a new perylene diimide dimer (diPDI) as a non-fullerene electron transporting layer (ETL) material for inverted perovskite solar cells.

Diketopyrrolopyrrole or benzodithiophene-arylamine small-molecule hole transporting materials for stable perovskite solar cells

Two new hole transporting materials with diketopyrrolopyrrole or benzodithiophene moieties were developed for stable perovskite solar cells.

Molecular engineering of the hole-transporting material spiro-OMeTAD via manipulation of alkyl groups

Aliphatic substituent effects on the HOMO energy levels and the ability to transport charge and form stable molecular glasses of systematically modified spiro-OMeTAD analogues were investigated.

Modeling the degradation and recovery of perovskite solar cells

Degradation and recovery can be modelled differently for every bias range.

Investigation on a dopant-free hole transport material for perovskite solar cells

In this work, we demonstrate a dopant free hole transport material for planar perovskite solar cells using a tetraphenylethene derivative, delivering an overall power conversion efficiency of 9.12% in the absence of additives.

Probing the energy levels of perovskite solar cells via Kelvin probe and UV ambient pressure photoemission spectroscopy

We present a study of the energy levels present in a perovskite solar cell using Kelvin probe and UV air photoemission measurements. By constructing a detailed map of the energy levels in the system we are able to predict the maximum open circuit voltage of the solar cell.

Recent progress on stability issues of organic–inorganic hybrid lead perovskite-based solar cells

Over the past few years, substantial progress has been made in research on organic–inorganic halide perovskite solar cells.

High conductivity Ag-based metal organic complexes as dopant-free hole-transport materials for perovskite solar cells with high fill factors

Two Ag-based metal organic complexes (HA1 and HA2) are employed as a new class of dopant-free HTMs for the application in PSCs. The cell based on HA1 achieved high PCE of 11.98% under air conditions, which is comparable to the PCE of the cell employing the doped spiro-MeOTAD (12.27%) under the same conditions.

Hole-transport materials (HTMs) play an important role as hole scavenger materials in the most efficient perovskite solar cells (PSCs). Here, for the first time, two Ag-based metal organic complexes (HA1 and HA2) are employed as a new class of dopant-free hole-transport material for application in PSCs. These HTMs show excellent conductivity and hole-transport mobility. Consequently, the devices based on these two HTMs exhibit unusually high fill factors of 0.76 for HA1 and 0.78 for HA2, which are significantly higher than that obtained using spiro-OMeTAD (0.69). The cell based on HA1-HTM in its pristine form achieved a high power conversion efficiency of 11.98% under air conditions, which is comparable to the PCE of the cell employing the well-known doped spiro-MeOTAD (12.27%) under the same conditions. More importantly, their facile synthesis and purification without using column chromatography makes these new silver-based HTMs highly promising for future commercial applications of PSCs. These results provide a new way to develop more low-cost and high conductivity metal-complex based HTMs for efficient PSCs.

Novel Carbazole-Based Hole-Transporting Materials with Star-Shaped Chemical Structures for Perovskite-Sensitized Solar Cells

Novel carbazole-based hole-transporting materials (HTMs), including extended π-conjugated central core units such as 1,4-phenyl, 4,4'-biphenyl, or 1,3,5-trisphenylbenzene for promoting effective π-π stacking as well as the hexyloxy flexible group for enhancing solubility in organic solvent, have been synthesized as HTM of perovskite-sensitized solar cells. A HTM with 1,3,5-trisphenylbenzene core, coded as SGT-411, exhibited the highest charge conductivity caused by its intrinsic property to form crystallized structure. The perovskite-sensitized solar cells with SGT-411 exhibited the highest PCE of 13.00%, which is 94% of that of the device derived from spiro-OMeTAD (13.76%). Time-resolved photoluminescence spectra indicate that SGT-411 shows the shortest decay time constant, which is in agreement with the trends of conductivity data, indicating it having fastest charge regeneration. In this regard, a carbazole-based HTM with star-shaped chemical structure is considered to be a promising candidate HTM.

Organic Charge Carriers for Perovskite Solar Cells

The photovoltaic field is currently experiencing the "perovskite revolution". These materials have been known for decades, but only recently have they been applied in solid-state solar cells to obtain outstanding power conversion efficiencies. Given that the variety of perovskites used so far is limited, a lot of attention has been devoted to the development of suitable organic charge-transport materials to improve device performance. In this article, we will focus on the most promising materials able to transport electrons or holes from a structural point of view. Thereby, we focus on organic materials owing to their ease of preparation and manipulation, and this is nicely combined with the potential tuning of their properties through chemical synthesis.

A Methoxydiphenylamine-Substituted Carbazole Twin Derivative: An Efficient Hole-Transporting Material for Perovskite Solar Cells

The small-molecule-based hole-transporting material methoxydiphenylamine-substituted carbazole was synthesized and incorporated into a CH3NH3PbI3 perovskite solar cell, which displayed a power conversion efficiency of 16.91%, the second highest conversion efficiency after that of Spiro-OMeTAD. The investigated hole-transporting material was synthesized in two steps from commercially available and relatively inexpensive starting reagents. Various electro-optical measurements (UV/Vis, IV, thin-film conductivity, hole mobility, DSC, TGA, ionization potential) have been carried out to characterize the new hole-transporting material.

Phosphonium Halides as Both Processing Additives and Interfacial Modifiers for High Performance Planar-Heterojunction Perovskite Solar Cells

Organic halide salts are successfully incorporated in perovskite-based planar-heterojunction solar cells as both the processing additive and interfacial modifier to improve the morphology of the perovskite light-absorbing layer and the charge collecting property of the cathode. As a result, perovskite solar cells exhibit a significant improvement in power conversion efficiency (PCE) from 10% of the reference device to 13% of the modified devices.

Enhancing Thermal Stability and Lifetime of Solid-State Dye-Sensitized Solar Cells via Molecular Engineering of the Hole-Transporting Material Spiro-OMeTAD

Thermal stability of hybrid solar cells containing spiro-OMeTAD as hole-transporting layer is investigated. It is demonstrated that fully symmetrical spiro-OMeTAD is prone to crystallization, and growth of large crystalline domains in the hole-transporting layer is one of the causes of solar cell degradation at elevated temperatures, as crystallization of the material inside the pores or on the interface affects the contact between the absorber and the hole transport. Suppression of the crystal growth in the hole-transporting layer is demonstrated to be a viable tactic to achieve a significant increase in the solar cell resistance to thermal stress and improve the overall lifetime of the device. Findings described in this publication could be applicable to hybrid solar cell research as a number of well-performing architectures rely heavily upon doped spiro-OMeTAD as hole-transporting material.

Subphthalocyanine as hole transporting material for perovskite solar cells

A boron subphthalocyanine has been studied as hole transporting material in perovskite solar cells.

Rational design of triazatruxene-based hole conductors for perovskite solar cells

Solution processable, triazatruxene based hole-transporting materials were synthesized using inexpensive precursors with state forward synthetic steps and integrated in perovskite based solar cells.

Novel spiro-based hole transporting materials for efficient perovskite solar cells

Three spiro-acridine-fluorene based hole transporting materials (HTMs), namely CW3, CW4 and CW5, are employed in the fabrication of organic–inorganic hybrid perovskite solar cells.

AgAl alloy electrode for efficient perovskite solar cells

We demonstrate an efficient mixed halide perovskite solar cell employing a thermally evaporated AgAl alloy as a back electrode.

Efficient, symmetric oligomer hole transporting materials with different cores for high performance perovskite solar cells

This communication provides information about the study and development of the properties of novel hole transporting materials (HTMs) for perovskite solar cells. This communication describes the synthesis of three HTMs incorporating 3,4-ethylenedioxythiophene (EDOT) and 2,1,3-benzothiadiazole (BTD) cores.

Improvement of dye-sensitized solar cells performance through introducing different heterocyclic groups to triarylamine dyes

The overall power conversion efficiency (PCE) of DSSCs based on TTR1–3 with chenodeoxycholic acid (CDCA) coadsorbant are 5.20%, 5.71% and 6.30%, respectively, and the value of TTR3 is close to that of N719 (6.62%).

Research progress of perovskite materials in photocatalysis- and photovoltaics-related energy conversion and environmental treatment

Perovskite materials are shown to be active in the applications of photocatalysis- and photovoltaics-related energy conversion and environmental treatment.

The theoretical investigation on the 4-(4-phenyl-4-α-naphthylbutadieny)-triphenylamine derivatives as hole transporting materials for perovskite-type solar cells

The hole mobility of hole transport materials is improved by the face-to-face packing mode, and phenyl is an outstanding substituent group for improving hole mobility.

Organic–inorganic halide perovskite based solar cells – revolutionary progress in photovoltaics

This review outlines the latest progress in perovskite-based solar cells, including device achievements and underlying insights and mechanisms of the perovskite materials.

Hole-transport materials with greatly-differing redox potentials give efficient TiO2–[CH3NH3][PbX3] perovskite solar cells

Hole-transport materials 0.44 V different in redox potential give perovskite solar cells with only 0.12 V difference in V_OC and similar PCEs.

Indolocarbazole based small molecules: an efficient hole transporting material for perovskite solar cells

Compared to Spiro-OMeTAD, an improved photovoltaic performance of the C12-carbazole based device is obtained due to the better hole extraction ability of the C12-carbazole.

Diarylamino-substituted tetraarylethene (TAE) as an efficient and robust hole transport material for 11% methyl ammonium lead iodide perovskite solar cells

We report the synthesis and characterisation of tetra{4-[N,N-(4,4′-dimethoxydiphenylamino)]phenyl}ethene (TAE-1) as an efficient and robust hole transport material for its application in methyl ammonium lead iodide (MAPI) perovskite solar cells.