Dopant-free Hole-transporting Materials Research Articles

A chlorinated polythiophene-based polymer as a dopant-free hole transport material in perovskite solar cells

We report a chlorine-substituted polythiophene-based, dioxobenzodithiophene-containing conjugated polymer (P2T-Cl) as a promising dopant-free hole transport material in lead halide perovskite solar cells.

Dopant-free small-molecule hole-transport material for low-cost and stable perovskite solar cells

Dopant-free hole-transporting materials (HTMs) aim to improve efficiency and stability simultaneously, and are a promising direction for efficient perovskite solar cells (PSCs).

Efficiency enhancement of dopant-free perovskite solar cells by employing fluoro-substituted electron donor–electron acceptor type polymeric hole-transporting materials

PBCz-FBTz with fluoro-substituents exhibits superior hole transport properties and passivation effects to PBCz-BTz. The devices employing PBCz-FBTz as dopant-free hole-transporting material show a champion PCE of 20.71% with excellent stability.

Green-solvent-soluble, highly efficient dopant-free hole-transporting material for perovskite solar cells

Halogenated solvents (e.g. chlorobenzene and chloroform) are typically used for hole-transporting materials (HTMs) in perovskite solar cells (PSCs); however, their use should be avoided as they are known to be hazardous to the environment. Herein, we synthesized a nonhalogenated-solvent-soluble, dopant-free HTM, SF62. When depositing HTMs for PSCs, SF62 could be dissolved with a nonhalogenated and green solvent, ethyl acetate. It is one of the most common organic solvents and is known to have a low environmental impact. Non-doped-SF62-based PSCs exhibited higher power-conversion efficiency (18.6%) than doped 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene (Spiro-OMeTAD)-based ones (18.3%), with enhanced stability.

Novel Push-Pull Benzodithiophene-Containing Polymers as Hole-Transport Materials for Efficient Perovskite Solar Cells

Donor-acceptor conjugated polymers are considered advanced semiconductor materials for the development of thin-film electronics. One of the most attractive families of polymeric semiconductors in terms of photovoltaic applications are benzodithiophene-based polymers owing to their highly tunable electronic and physicochemical properties, and readily scalable production. In this work, we report the synthesis of three novel push-pull benzodithiophene-based polymers with different side chains and their investigation as hole transport materials (HTM) in perovskite solar cells (PSCs). It is shown that polymer P3 that contains triisopropylsilyl side groups exhibits better film-forming ability that, along with high hole mobilities, results in increased characteristics of PSCs. Encouraging a power conversion efficiency (PCE) of 17.4% was achieved for P3-based PSCs that outperformed the efficiency of devices based on P1, P2, and benchmark PTAA polymer. These findings feature the great potential of benzodithiophene-based conjugated polymers as dopant-free HTMs for the fabrication of efficient perovskite solar cells.

Underlying Interface Defect Passivation and Charge Transfer Enhancement via Sulfonated Hole-Transporting Materials for Efficient Inverted Perovskite Solar Cells.

To date, numbers of polymeric hole-transporting materials (HTMs) have been developed to improve interfacial charge transport to achieve high-performance inverted perovskite solar cells (PSCs). However, molecular design for passivating the underlying surface defects between perovskite and HTMs is a neglected issue, which is a major bottleneck to further enhance the performance of the inverted devices. Herein, we design and synthesize a new polymeric HTM PsTA-mPV with the methylthiol group, in which a lone pair of electrons of sulfur atoms can passivate the underlying interface defects of the perovskite more efficiently by coordinating Pb2+ vacancies. Furthermore, PsTA-mPV exhibits a deeper highest occupied molecular orbital (HOMO) level aligned with perovskite due to the π-acceptor capability of sulfur, which improves interfacial charge transfer between perovskite and the HTM layer. Using PsTA-mPV as a dopant-free HTM, the inverted PSCs show 20.2% efficiency and long-term stability, which is ascribed to surface defect passivation, well energy-level matching with perovskite, and efficient charge extraction.

Dopant-Free Bithiophene-Imide-Based Polymeric Hole-Transporting Materials for Efficient and Stable Perovskite Solar Cells.

The development of hole-transport materials (HTMs) with high mobility, long-term stability, and comprehensive passivation is significant for simultaneously improving the efficiency and stability of perovskite solar cells (PVSCs). Herein, two donor-acceptor (D-A) conjugated polymers PBTI and PFBTI with alternating benzodithiophene (BDT) and bithiophene imide (BTI) units are successfully developed with desirable hole mobilities due to the good planarity and extended conjugation of molecular backbone. Both copolymers can be employed as HTMs with suitable energy levels and efficient defect passivation. Shortening the alkyl chain of the BTI unit and introducing fluorine atoms on the BDT moiety effectively enhances hole mobility and hydrophobicity of the HTMs, leading to improved efficiency and stability of PVSCs. As a result, the organic-inorganic hybrid PVSCs with PFBTI as the HTM deliver a power conversion efficiency (PCE) of 23.1% with enhanced long-term operational and ambient stability, which is one of the best efficiencies reported for PVSCs with dopant-free polymeric HTMs to date. Moreover, PFBTI can be applied in inorganic PVSCs and perovskite/organic tandem solar cells, achieving a high PCE of 17.4% and 22.2%, respectively. These results illustrate the great potential of PFBTI as an efficient and widely applicable HTM for cost-effective and stable PVSCs.

Self-Powered and Low-Noise Perovskite Photodetector Enabled by a Novel Dopant-Free Hole-Transport Material with Bottom Passivation for Underwater Blue Light Communications.

Designing dopant-free hole-transport materials (HTMs) is a facile and effective strategy to realize high-performance organic-inorganic hybrid perovskite (OIHP) photodetectors. Herein, a novel phenothiazine polymer, poly[4-(10H-phenothiazin-10-yl)-N,N-bis(4-methoxyphenyl)aniline] (PPZ-TPA), was synthesized and employed as a promising HTM in OIHP photodetectors. The triphenylamine donor unit was combined with a phenothiazine core, furnishing the polymer with a suitable highest occupied molecular orbital level, favorable thermal stability, and appropriate film morphology. The sulfur atom in the phenothiazine functional group can intentionally passivate the undercoordinated Pb2+ of OIHP films, suppressing nonradiative recombination and yielding an ultralow dark current density of 1.26 × 10-7 A cm-2 under -0.1 V, as well as a low-noise current of 3.75 × 10-13 A Hz-1/2 at 70 Hz. Encouragingly, the self-powered PPZ-TPA-based OIHP photodetectors were successfully integrated into a blue light communication system for the first time, demonstrating their application for receiving and transmitting light signals with a transmission rate of 300 bps. In addition, the PPZ-TPA-based devices exhibit nearly 1 year shelf stability without obvious degradation. We believe that PPZ-TPA demonstrates great potential to achieve high-performance perovskite photodetectors, also providing a strategy for the design of novel HTMs.

Dopant-free small molecule hole transport materials based on triphenylamine derivatives for perovskite solar cells

In the past decade, perovskite solar cells have become a promising candidate in the photovoltaic industry owing to their high power conversion efficiency that surpasses 25%. However, there are certain limitations that have hindered the development and full-scale practical application of these cells, including the high cost and degradation of perovskite caused by the dopants. Hence, there is an urgent need to develop dopant-free hole transport materials (HTMs). In recent years, HTMs based on triphenylamine (TPA-HTMs) are receiving growing interest owing to their high hole mobility, excellent film formation, and suitable energy levels. The literature here covers work relevant to TPA-HTMs in the last five years. They have been classified according to different core types. The correlations between performance and structure are summarized, and the future development trend of TPA-HTMs is highlighted.

Molecular Self-Assembly Regulated Dopant-Free Hole Transport Materials for Efficient and Stable n-i-p Perovskite Solar Cells and Scalable Modules.

Dopant-free organic hole transport materials (HTMs) remain highly desirable for stable and efficient n-i-p perovskite solar cells (pero-SCs) but rarely succeed. Here, we propose a molecular assembly strategy to overcome the limited optoelectronic properties of organic HTMs by precisely designing a linear organic small molecule BDT-DPA-F from the atomic to the molecular levels. BDT-DPA-F can assemble into a fibril network, showing an obviously improved hole mobility and decreased energy disorder. The resultant pero-SCs showed a promising efficiency of 23.12 % (certified 22.48 %), which is the highest certified value of pero-SCs with dopant-free HTMs, to date. These devices also showed a weak-dependence of efficiency on size, enabling a state-of-the-art efficiency of 22.50 % for 1-cm2 device and 20.17 % for 15.64-cm2 module. For the first time, the pero-SCs based on dopant-free HTMs realized ultralong stabilities with T80 lifetimes over 1200 h under operation or thermal aging at 85 °C.

Bacteriochlorin aggregates as dopant-free hole-transporting materials for perovskite solar cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted great interests not only of academic field but also of industrial world toward practical applications in the last decade due to their extremely low production cost and excellent photo-physical properties. Hole-transporting layer (HTL) as an integral component of PSC plays an important role in elevating its performance. In this work, five bacteriochlorophyll- a derivatives characterized by different C17-propionate residue were prepared and their J- type aggregates were investigated as HTLs of PSCs. It was confirmed that the nature of the introduced C17-propionate side chains have negligible effects on the electrochemical properties of main dye unit, such as the electronic absorption spectra and energy levels. However, the different lipophilicity, hydrophilicity, or hydrophobicity of chain characters affect the solubility of each dye component, resulting in diverse surface morphologies of their aggregate films as revealed by AFM observations. Furthermore, the five solid films prepared by spin-coating method showed discrepant charge extraction and transport abilities as HTLs, which were supported by both photo luminescence spectra and electrochemical impedance spectroscopy measurement results and were concluded to be the fundamental reason of the different PSC performance. • Five bacteriochlorophyll- a derivatives were investigated as HTLs of PSCs. • PSC employing BChl-2 bearing a dodecyl ester as an HTL gives the highest PCE. • Charge transfer process and interfacial contact are the essential factors for HTLs.

Management of Donor and Acceptor Building Blocks in Dopant-Free Polymer Hole Transport Materials for High-Performance Perovskite Solar Cells.

The dominant hole transport material (HTM) in state-of-the-art perovskite solar cells (PSCs) is Spiro-OMeTAD, which needs to be doped using hydrophilic dopants to improve its hole mobility and conductivity, resulting in inferior device stability. Here, we propose an effective molecular design strategy to construct dopant-free polymer HTMs by selecting four structurally related polymers and investigating their structure-property relationship. It is found that the donor and acceptor units with longitudinal conjugate extension, such as BDT-T and BDD, could not only enhance the planarity of the conjugated polymer backbone and tune the energy levels but also promote the face-on orientation, resulting in superior charge extraction and transport. The optimized device utilizing dopant-free polymer HTM shows a high open-circuit voltage of 1.19 V and a champion efficiency of 24.04 % with greatly improved operational stability, making it among the best performance PSCs based on dopant-free HTMs.

Tetrathiafulvalene Derivatives as Hole-Transporting Materials in Perovskite Solar Cell.

Over a couple of decades perovskite solar cells have become a highly promising photovoltaic technology. Choosing a dopant-free Hole-Transporting Material (HTM) that offers protection to a perovskite layer from oxidation is one of the viable strategies while addressing the stability of perovskite solar cell. In this line of interest, tetrathiafulvale (TTF) derivatives have shown promise in the past. However, studies that focus on small-molecule TTF derivatives as potential HTM options are scarce. The present study is an attempt to explore the applicability of a few TTF derivatives as HTM in a perovskite solar cell. Here four TTF derivatives, namely, TTF-1 (experimentally reported in a previous study), TTF-2, DBTTF1, and TMTSF1, were studied through electronic structure calculations. The properties concerning HTM, such as impact of adsorption on molecular structure, absorption spectra, distribution of frontier molecular orbitals, interaction energy between TTF derivative and MAPbI3 surface, and charge transfer at an interface, were analyzed. Results show that TTF-2 has the expected energy-level alignment, transparency in the visible range of solar spectrum, and good charge-injection ability at the interface with a perovskite layer. Hence, TTF-2 could be a potential hole-transporting material for a perovskite solar cell, and it can perform better than TTF-1.

Dopant-free hole-transporting materials based on a simple nonfused core with noncovalent conformational locking for efficient perovskite solar cells

Developing dopant-free hole-transporting materials (HTMs) toward perovskite solar cells (PVSCs) is essential to their commercial deployment. In this work, two dopant-free organic HTMs (Q20 and Q21) based on a simple nonfused core with noncovalent conformational locking have been design and synthesized. It is found that, upon incorporation of the thiophene−3,4-ethylenedioxythiophene (EDOT)−thiophene, both Q20 and Q21 exhibit a high intrinsic hole mobility over 10 −4 cm 2 V −1 s −1 , thus leading to the desirable electronic properties in dopant-free PSCs. Owing to its high hole mobility, deep lying HOMO level, and excellent thin film quality, the dopant-free Q21-based PSCs exhibit an efficiency of 18.05% with negligible hysteresis. Notably, the devices based on dopant-free Q21 can retain more than 90% of their initial PCEs after being stored at ambient conditions for 1000 h. These results demonstrate that the good potential of the nonfused core with S–O conformational locking as building block for developing dopant-free small organic HTMs toward high-performance PSCs. • Two new dopant-free small organic HTMs have been designed and synthesized. • The thiophene−3,4-ethylenedioxythiophene−thiophene group was used as the π-bridge. • S–O conformational locking endows the HTM with high mobility over 10 −4 cm 2 V −1 s −1 . • An efficiency of 18.05% was achieved by dopant-free PSC based on Q21.

Probing the effect of acceptor engineering in benzothiadiazole-based D-A-D-typed hole-transporting materials for perovskite solar cells

Hole-transporting materials (HTMs) is very important for improving the stability and efficiency of perovskite solar cells (PSCs) because it plays a crucial role for the exciton dissociation at the interface and the following hole transport. To improve the performance of HTMs, the strategy of extension and rigidification of the π-conjugated acceptor in donor-acceptor-donor-typed HTMs is evaluated in this work. Theoretical calculations indicate that all the predicted HTMs display the suitable energy levels to ensure the effortless hole transfer at the perovskite/HTM interface. More importantly, our results reveal that extending the acceptor group is a good strategy to promote the hole transport in HTMs. Meanwhile, the bent molecular conformation by rigidifying the conjugated acceptor can also improve the hole mobility of HTMs due to the better molecular planarity and enhanced intermolecular stacking and orbital overlapping. All the predicted HTMs display the higher hole mobility than that of the YN1. The higher mobility and matched energy levels of predicted HTMs are favorable for improving the performance of PSCs. In addition, the better optical property and solution property can also be anticipated for the new tailored HTMs. In summary, this work provides a useful strategy for the design of high-efficient HTMs, and three new designed HTMs are proposed as the potential candidates toward more efficient dopant-free HTMs.

Ionic Dopant-Free Polymer Alloy Hole Transport Materials for High-Performance Perovskite Solar Cells.

The dominated hole transport material (HTM) used in state-of-the-art perovskite solar cells (PSCs) is Spiro-OMeTAD, which needs to be doped to improve its conductivity and mobility. The inevitable instability induced by deliquescent dopants and the necessary oxidation process in air hinders the commercialization of this technology. Here, an alloy strategy using two conjugated polymers with highly similar structures but different crystallinities for dopant-free HTM and high-performance PSCs has been demonstrated. We found that the polymeric packing and crystallinity of a polymer alloy could be regulated finely by blending the polymer PM6 and our developed polymer PMSe, which exhibits a shorter π-π stacking distance due to the improved planarity of the polymer backbone with strong C═O···Se noncovalent interactions. The structure-property relationship of the polymer alloy is investigated by theoretical and experimental analyses. The optimized PSCs using the polymer alloy HTM without any ionic dopants feature an excellent power conversion efficiency of 24.53% and a high open circuit voltage (VOC) of 1.19 V with much improved stability. This efficiency is much higher than that of the control device using doped Spiro-OMeTAD HTM (PCE = 22.54%). Our work provides a very effective strategy to design and construct dopant-free hole transport materials for highly efficient perovskite solar cells and other applications.

Highly Efficient Dopant-Free Cyano-Substituted Spiro-Type Hole-Transporting Materials for Perovskite Solar Cells

The hole-transporting material (HTM) is an important component of perovskite solar cells (PSCs) because it plays a crucial role in achieving high performance. The most frequently used HTM is 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene (Spiro-OMeTAD). Spiro-OMeTAD requires dopants such as Li(TFSI) to realize high power conversion efficiencies (PCEs). However, these dopants cause severe instability issues in PSCs. The Spiro-OMeTAD layer also tends to undergo severe morphological deformation at high temperature, which produces large voids and further reduces the cell performance. These drawbacks must be overcome for the commercialization of PSCs. Therefore, it is strongly desirable to develop high-performance dopant-free HTMs to replace Spiro-OMeTAD. Here, we synthesize two cyano-substituted spirobifluorene-based HTMs, SF27 and SF48, for this purpose. These compounds are derived from Spiro-N, which is a Spiro-OMeTAD analogue obtained via the replacement of the p-methoxy substituents with strong electron-donating p-N,N-dimethylamino groups. The influence of cyano substituents on the optoelectronic properties, PCEs, and charge transport behavior in PSCs are investigated. These compounds have a highest occupied molecular orbital energy level that is well-matched to the valence band of perovskite layers. The hole mobility of SF48 (o-cyano-substituted) was determined to be higher than that of SF27 (m-cyano-substituted), although it is lower than those of Spiro-OMeTAD and the reference compound Spiro-N. Mesoporous PSCs were fabricated using these new HTMs without dopants under relatively low HTM concentration conditions, and the cell performance was compared to that of devices with Spiro-OMeTAD and Spiro-N. The PSC with non-doped SF48 exhibited a high PCE of 18.7%, which is comparable to that of the reference PSCs with doped Spiro-OMeTAD (18.6%). In addition, the thermal stability of SF48 at 85 °C in air was superior to that of Spiro-OMeTAD, both with and without dopants. Therefore, the SF48 spirobifluorene-based compound is determined to be quite effective as a high-performance dopant-free HTM for PSCs.

A multifunctional interlayer for highly stable and efficient perovskite solar cells based on pristine poly(3- hexylthiophene)

Poly (3-hexylthiophene) (P3HT) has been recognized to be a potential hole-transport material (HTM) for stable and efficient perovskite solar cells (PSCs) due to its low cost, high hole mobility, and solution processability. However, the uncontrolled configuration of the polymer chains and the poor contact on the perovskite film restrain the charge extraction and transport in the devices. Here, we demonstrate a simple and versatile strategy for the fabrication of stable and efficient P3HT-based PSCs, using 1-hexyl-2,5-dimethyl-1H-pyrrole-3-carboxylic acid (HPCA) as interlayer between perovskite and P3HT layers. The carboxyl group in HPCA can effectively suppress anion-vacancy defects at the grain boundaries and surface of the perovskite film, reducing the trap state density and improving the carrier lifetimes. Moreover, the peripheral hexyl-pyrrole in HPCA can improve the interface contact between the perovskite and P3HT, and regulate P3HT orientation and improve charge extraction and transport in the device. As a result, the PSC using the dopant-free P3HT HTM yields a power conversion efficiency (PCE) of 20.8%, along with superior stability at a relative humidity of 60%, heat at 80 °C, or under continuous light illumination.

Transfer-Printed Cuprous Iodide (CuI) Hole Transporting Layer for Low Temperature Processed Perovskite Solar Cells.

Perovskite solar cells (PSCs) have achieved significantly high power-conversion efficiency within a short time. Most of the devices, including those with the highest efficiency, are based on a n–i–p structure utilizing a (doped) spiro-OMeTAD hole transport layer (HTL), which is an expensive material. Furthermore, doping has its own challenges affecting the processing and performance of the devices. Therefore, the need for low-cost, dopant-free hole transport materials is an urgent and critical issue for the commercialization of PSCs. In this study, n–i–p structure PSCs were fabricated in an ambient environment with cuprous iodide (CuI) HTL, employing a novel transfer-printing technique, in order to avoid the harmful interaction between the perovskite surface and the solvents of CuI. Moreover, in fabricated PSCs, the SnO2 electron transport layer (ETL) has been incorporated to reduce the processing temperature, as previously reported (n–i–p) devices with CuI HTL are based on TiO2, which is a high-temperature processed ETL. PSCs fabricated at 80 °C transfer-printing temperature with 20 nm iodized copper, under 1 sun illumination showed a promising efficiency of 8.3%, (JSC and FF; 19.3 A/cm2 and 53.8%), which is comparable with undoped spiro-OMeTAD PSCs and is the highest among the ambient-environment-fabricated PSCs utilizing CuI HTL.

Tailored Polymeric Hole-Transporting Materials Inducing High-Quality Crystallization of Perovskite for Efficient Inverted Photovoltaic Devices.

For achieving high-performance p-i-n perovskite solar cells (PSCs), hole transporting materials (HTMs) are critical to device functionality and represent a major bottleneck to further enhancing device stability and efficiency in the inverted devices. Three dopant-free polymeric HTMs are developed based on different linkage sites of triphenylamine and phenylenevinylene repeating units in their main backbone structures. The backbone curvatures of the polymeric HTMs affect the morphology and hole mobility of the polymers and further change the crystallinity of perovskite films. By using PTA-mPV with moderate molecular curvature, p-i-n PSCs with high efficiency of 19.5% and long-term stability can be achieved. The better performance is attributed to the more effective hole extraction ability, higher charge-carrier mobility, and lower interfacial charge recombination. Furthermore, these three polymeric HTMs are synthesized without any noble metal catalyst, and show great advantages in future application owing to the low-cost.