Development Of Hole-transporting Materials Research Articles

Dopant-free hole-transporting materials featuring intramolecular π-π interactions for efficient and stable perovskite solar cells

The development of hole-transporting materials (HTMs), especially dopant-free ones, is of great significance for simultaneously improving the efficiency and stability of perovskite solar cells (PSCs). Herein, two molecularly engineered HTMs (BFM-C1 and BFM-C6) featuring intramolecular π-π interactions are designed based on the well-known spiro-OMeTAD and synthesized via simple synthetic routes. Unlike the spiro-OMeTAD with orthogonal conformation, both BFM-C1 and BFM-C6 are revealed to adopt a “face-to-face” stacking orientation with evident intramolecular π-π interactions, and display about five times higher hole mobilities (≈ 10-4 cm2V-1s−1) than pristine spiro-OMeTAD. Moreover, both the HTMs exhibit high hole extraction capacity likely owing to good electronic contact with the perovskite. Notably, the incorporated long alkyl chains endow BFM-C6 with better film morphology and superior hydrophobicity, as compared with BFM-C1, thereby resulting in enhanced charge extraction at the perovskite/HTM interface as well as the moisture stability of the perovskite underneath. Consequently, the PSCs employing BFM-C6 as HTM without dopants achieve a power conversion efficiency of 19.06%, along with preferable durability. Furthermore, BFM-C6-based PSCs also exhibit improved thermal stability as compared to the devices based on doped spiro-OMeTAD. This work not only enriches the variety of dopant-free HTMs for PSCs, but also provide a new strategy for developing high-performance dopant-free HTMs.

Side-chain engineering on triphenylamine derivative-based hole-transport materials for perovskite solar cells: Theoretical simulation and experimental exploration

The development of hole-transport materials (HTMs) is a significant approach to promote the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Here, based on the triphenylamine (TPA) derivatives and patulous TPA derivatives as side-chains, WR1 and WR2 are designed and explored by density functional theory (DFT), time-dependent DFT (TD-DFT) in combination with Marcus electron transfer theory. The calculated results show that the WR1 exhibits matching energy levels with perovskite and better hole transporting ability in comparison with these of WR2 can save as a potential HTM for PSCs applications. In order to confirm screening results of molecular design, the WR1 as HTM in PSC device reveals that the WR1-based PSC device obtained the PCE of 20.04% higher than that of the typical Spiro-OMeTAD-based device (18.84%). Moreover, the experimental results can well verify the data of theoretical simulations. The strategy of side-chain modification on TPA derivatives-based materials is a viable method to exploit new HTMs.

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.

Photophysical and DFT investigation of imidazole-based hole transporting materials for phosphorescent OLEDs with high current efficiency

Organic small molecules have played a significant role in the field of organic light-emitting diodes (OLEDs) technology. Improving the stability of OLEDs continues to be a major task in organic electronics. To overcome this issue, it is essential to enhance the carrier injection into the emissive layer for better charge balance. Though OLEDs are highly desirable for the solution process, enabling simple and cost-effectively manufacturing to fabricate large-area devices, the development of hole-transporting materials (HTMs) still a considerable challenge. Herein the IMzN (1-(4-methoxyphenyl)-2-(4-nitrophenyl)-4,5-diphenyl-1H-imidazole) and IMzB (4-(1-(4-methoxyphenyl)-4,5-diphenyl-1H-imidazol-2-yl)benzoic acid) with excellent solution processability and stability suitable for efficient OLED devices using a simple and efficient microwave-assisted synthetic procedure. Thermogravimetric analysis of IMzN and IMzB suggested that the material be thermally stable up to 320–345 °C. No glass transition has occurredas confirmed by DSC indicating the amorphous nature of the materials, which is well supported by XRD analysis. Materials acquire the appropriate HOMO levels for favorable hole injection as well as superior hole mobility. As a result, both HTMs show better device performance than the commercially used TAPC. The IMzB as HTL shows remarkable current efficiency of 91.13 cd A-1 and EQE of 21.11%. This is the highest current efficiency reported using solution-processed green phosphorescent OLEDs. Devices with IMzB as HTL also exhibit relatively better stability under continuous stress.

Chalcogen-Containing Hole Transporting Materials

Abstract This review summarizes our recent achievements in the development of new chalcogen-containing materials employed as hole-transporting materials (HTMs) in efficient perovskite solar cells (PSCs). Following a simple and inexpensive synthetic methodology we prepared new heterocycle-based HTMs with comparable photovoltaic (PV) behaviour to the widely used spiro-OMeTAD. In particular, new star-shaped HTMs have been obtained through an easy synthetic route by crosslinking electron-donor groups with a central scaffold. As sulfur-containing cores, benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene (BTT) and the corresponding isomers (bbb-BTT and bbc-BTT), thieno[3,2-b]thiophene (TbT), cyclooctatetrathiophene (CoTh), anthra[1,2-b:4,3-b′:5,6-b′′:8,7-b′′′]tetrathiophene (ATT), dibenzothieno[1,2-b:4,3-b′:6,7-b′′:9,8-b′′′]quinquethiophene (DBQT), dibenzothieno[3,2-b]thiophen[1,2-b:4,3-b′:6,7-b′′:9,8-b′′′]sexithiophene (DBST) and thioxanthone have been employed. To extend the comparison, HTMs with heteroatoms such as oxygen or selenium in the central unit, namely xanthone (BX), benzotrifuran (BTF) or benzotriselenophene (BTSe), were also designed, synthesized and employed in PSCs. Currently, there is no doubt that organic compounds are an important part of the PSCs architecture. Nevertheless, the future commercialization of PSCs is driven by the development of HTMs away from the comprehension of structure-property relationships. Therefore, our main goal is to contribute to a better understanding of the chemistry behind competitive HTMs and provide a clear picture of the effect of chalcogen-containing HTMs in device performances.

Insights into the hole transport properties of LiTFSI-doped spiro-OMeTAD films through impedance spectroscopy

Hole transport materials are crucial for efficient charge extraction in perovskite solar cells to achieve high power conversion efficiency and stability. Herein, the hole transport properties of the 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene (spiro-OMeTAD) thin films with a dopant lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) are investigated through impedance spectroscopy. Upon doping, we observe a dispersive hole transport with nearly a 100-fold increase in the hole mobility compared with the pristine spiro-OMeTAD films. The hole mobilities slightly decrease with increasing electric fields for both films, exhibiting a negative electric field dependence of mobility due to the positional disorder. Subsequently, the charge carrier density of the LiTFSI-doped spiro-OMeTAD film is three orders of magnitude higher than that of the pristine film. The LiTFSI dopant induces two different electrical regions in the doped thin film, which can be reflected through impedance spectroscopy. The presented investigation through impedance spectroscopy is of high practical interest for the development of hole transport materials and the optimization of the transport layer doping in perovskite solar cells.

Phenothiazine Functionalized Multifunctional A-π-D-π-D-π-A-Type Hole-Transporting Materials via Sequential C-H Arylation Approach for Efficient and Stable Perovskite Solar Cells.

Three phenothiazine-based A-π-D-π-D-π-A-type small molecules containing various terminal acceptor units, which act as Lewis base blocks, have been synthesized via an efficient and step-economical, direct C-H arylation strategy in the aim toward the development of hole-transporting materials (HTMs) with multifunctional features (such as efficient hole extraction layer, trap passivation layer, and hydrophobic protective layer) for perovskite solar cells (PrSCs). Optical-electrochemical correlation and density functional theory studies reveal that dicyanovinylene acceptor in SGT-421 downshifted the highest occupied molecular orbital (HOMO) level (-5.41 eV), which is more proximal to the valence band (-5.43 eV) of the perovskite, whereas N-methyl rhodanine in SGT-420 and 1,3-indanedione (IND) in SGT-422 destabilized the HOMO, leading to an increased interfacial energy-level offset. SGT-421 exhibits superior properties in terms of a sufficiently low-lying HOMO level and favorable energy-level alignment, intrinsic hole mobility, interfacial hole transfer, hydrophobicity, and trap passivation ability over spiro-OMeTAD as a benchmark small-molecule HTM. As envisaged in the design concept, SGT-421-based PrSC not only yields a comparable efficiency of 17.3% to the state-of-art of spiro-OMeTAD (18%), but also demonstrates the enhanced long-term stability compared to the spiro-OMeTAD because of its multifunctional features. More importantly, the synthetic cost of SGT-421 is estimated to be 2.15 times lower than that of spiro-OMeTAD. The proposed design strategy and the study of acceptor-property relationship of HTMs would provide valuable insights into and guidelines for the development of new low-cost and efficient multifunctional HTMstowardthe realization of efficient and long-term stable PrSCs.

Hole‐Transporting Materials in Inverted Planar Perovskite Solar Cells

Hybrid organic–inorganic halide‐perovskite‐based solar cells have achieved notable progress. A hot topic in this field is exploring inexpensive, stable and effective hole‐transporting materials (HTMs) in order to improve the device performance and be favorable for large‐scale production in the future. The HTMs have been proven to be an important component of perovskite solar cells, which can form selective contact being favorable for reducing charge recombination and effective hole collection, thus resulting in the enhancement of the open‐circuit voltage and the fill factor. Here, an overview of the design and development of HTMs is given, mainly divided into conductive polymers, inorganic p‐type semiconductors in inverted‐structure‐based planar perovskite solar cells. The influences of their mobility, work function and film property on device performance are discussed.

Metal-Interrupted Perylene Diimide: Toward a New Class of Tunable n-Type Inorganic−Organic Hybrid Semiconductors

In organic thin-film transistors (OTFTs), organic electron-transport materials (n-type semiconductors) are well behind the advances in development of hole-transport materials (p-type semiconductors). Currently, one class of organic n-type semiconductor materials that is widely utilized is N,N'-dialkyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-R) derivatives with high electron affinities (EAs), such as N,N'-dioctyl-3,4,9,10-perylenetetracarboxylic diimide with a reported EA as high as 4.4 eV. The PTCDI-R derivatives have been manipulated by adding substituents on the perylene moiety or at the amine position to afford more stable compounds and higher EAs. On the basis of these materials, we have developed metal-containing perylenediimide analogues, placing a salpen ligand for metal ion chelation between two n-isobutylnaphthalimides. We demonstrate here that the electronic properties of this class of materials can be systematically tuned in a divergent manner by simply changing the metal center. The synthesis, characterization, electrochemistry, and band-gap analysis are discussed herein.