Efficient Hole-transporting Materials Research Articles

Stable and efficient perovskite solar cells based on tert-butyl peroxy-2-ethylhexanoate modified hole transport layer

Nowadays, perovskite solar cells (PSCs) have gained significant attention as one of the most promising new energy technologies. However, the application of spiro-OMeTAD, a highly efficient organic hole transport material (HTM) in PSCs, is limited due to its inherent weaknesses in conductivity and hole mobility. Here, a free radical initiator, tert-butyl peroxy-2-ethylhexanoate (TBPEH) is introduced to induce the rapid and controllable oxidation of Spiro. TBPEH-modified hole transport layer (HTL) produces more free radical cations, increasing carrier mobility and electrical conductivity. A smoother surface of the modified HTL improves the charge transport efficiency. Additionally, trap-state density and defect-induced nonradiative recombination of TBPEH-modified device are decreased. In addition, better energy level matching between the HTL and perovskite layer enables more efficient hole transport. Consequently, the PSC doped with TBPEH achieves a champion power conversion efficiency (PCE) of 24.33%, surpassing the PCE of 21.72% obtained from the pristine device. Furthermore, the TBPEH-modified devices show better stability. Storing in the atmosphere at 30% relative humidity for 42 days, the TBPEH-modified device maintains 90.8% of its highest efficiency, compared to the 73.8% of the pristine one.

Rational designing of phenothiazine dioxide based hole transporting materials for efficient perovskite solar cells

In this study, we design five novel non-fullerene end capped acceptor based efficient hole transporting materials (HTMs) from phenothiazine dioxide-based core. MPW1PW91 functional with a 6-31G basis set was employed for all DFT calculations and simulation analysis. By substituting acceptor groups in PDO2, numbers of parameters like band gaps, the frontier molecular orbitals (FMOs), higher absorption parameters, and electronic properties like ionization potential, electron affinity, binding energy, and light harvesting efficiency (LHE) were well tuned. In addition, significant variations in other geometrical parameters such as dipole moment, and reorganizational energy parameters were also noticed for the derived molecules compared to the reference molecule (R). Device performance was analyzed by performing calculations of open circuit voltage (VOC) and fill factor (FF). Consequently, all the designed derivatives demonstrated an improved VOC of 0.84 V for an optimized designed structure. In comparison, it was limited to a VOC of 0.57 V for the reference (R) PDO2, displaying an increment in efficiency which is essential to use these molecules as highly efficient HTMs for perovskite solar cell devices in the future.

Improvement in Dibenzofuran-Based Hole Transport Materials for Flexible Perovskite Solar Cells.

The π-conjugated system and the steric configuration of hole transport materials (HTMs) could greatly affect their various properties and the corresponding perovskite solar cells' efficiencies. Here, a molecular engineering strategy of incorporating different amounts of p-methoxyaniline-substituted dibenzofurans as π bridge into HTMs was proposed to develop oligomer HTMs, named mDBF, bDBF, and tDBF. Upon extending the π-conjugation of HTMs, their HOMO energy levels were slightly deepened, significantly increasing the thermal stability and hole mobility. The incorporation of p-methoxyaniline bridges built one or two additional triphenylamine propeller structures, resulting in a denser film. Here, the tDBF-based n-i-p flexible perovskite solar cells createdchampion efficiency, giving a power conversion efficiency of 19.46%. And the simple synthesis and purification process of tDBF contributed to its low manufacturing cost in the laboratory. This work provided a reference for the development of low-cost and efficient HTMs.

Theoretical study of optoelectronic performance of hole-transporting material quinoxaline-based with architecture (D-A-D) in perovskite solar cells: A DFT method

Developing ideal small-molecule hole-transporting materials (HTMs) is one of the most effective methods to improve the performance of perovskite solar cells (PSCs). Meanwhile, designing new molecules with theoretical chemistry methods and gaining a more fundamental understanding of the structure-property relationship is significant for developing highly efficient HTMs. In this study, new HTMs were designed based on quinoxaline core. Based on time-dependent density functional theory (TD-DFT) and density functional theory (DFT), the electron transfer mechanism was investigated by hole and electron analysis and inter-fragment charge transfer (IFCT). The results show that HTMs (TQ2, TQ3, TQ5, TQ7, and TQ8) exhibit suitable energy levels. The matched energy levels with MAPbI3 are helpful for the interfacial charge transfer. IFCT and hole-electron distribution indices show that TQ5 has a higher centroid distance (D = 1.74 Å), a greater degree of spatial expansion (H = 4.58 Å), the smaller overlap area (Sr = 0.62), the highest degree separation (t = −0.11 Å), more transmission of net charge (Q = 0.60 e−), and the highest hole contribution (71.38 %). Due to matched energy levels with MAPbI3, superior hole-electron distribution indices, good solubility, and better hole mobility, it is suggested that TQ5 has desirable properties as HTM in perovskite solar cells with (n-i-p) architecture.

Cu2(Thiourea)Br2 Complex as a Multifunctional Interfacial Layer for Reproducible PTAA‐Based p–i–n Perovskite Solar Cells

Poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine (PTAA) is one of the most efficient hole transport materials for p–i–n structured perovskite solar cells (PSCs). However, the hydrophobicity of PTAA causes wettability problems and is thereby associated with low yield and poor reproducibility. Herein, a new strategy for improving the perovskite precursor wettability on PTAA is developed. A Cu2(Thiourea)Br2 complex interfacial layer is introduced on the PTAA, improving the coverage of perovskite precursors and thereby the perovskite film. Partially dissolved Cu2(Thiourea)Br2 in the perovskite absorber further enhances band alignment and carrier extraction between layers. With the addition of Cu2(Thiourea)Br2 layer on nondoped PTAA, PSCs in p–i–n structure with a bandgap of 1.63 eV have achieved over 21% efficiency both on 0.1 and 1.0 cm2 sized devices with a significantly improved yield and reproducibility. Unencapsulated devices retain 100% of their initial efficiency after 2000 h at 65 °C.

Conformation Tailoring of Diphenylfluorene‐Cored Isomers as Hole‐Transport Materials for Perovskite Solar Cells

The fine structural–functional construction of organic semiconductor materials provides an abundant candidate store for the hole‐transport materials (HTMs) in perovskite solar cells (PSCs). Whereas, challenges remain in unveiling the clews behind the molecular conformation engineering and desired properties realizing for efficient HTMs. In this work, a couple of simple organic diphenylfluorene‐cored isomers (FH27 and FH99) with different spatial integration strategies of molecular conformation are judiciously devised as HTMs in the PSCs. The theoretical simulation and experimental analyses decipher that the excellent electronic/energetic distribution, intermolecular stacking geometry, intrinsic solid‐crystal film morphology, and charge‐extraction‐transport ability are demonstrated for FH27, ascribing to its harmonious intersectional couplings for the planar/orthogonal molecular conformation. The FH27‐based device achieves a high efficiency of 23.61% with benign life span, both superior to the FH99‐based device.

Efficient dopant-free small-molecule hole transport material with D-A-D structure for inverted perovskite solar cells

Efficient dopant-free small-molecule hole transport material with D-A-D structure for inverted perovskite solar cells

Benzodithiophene‐Thienothiadiazole Copolymers as Dopant‐Free Hole‐Transporting Layers Enabling Efficient and Stable Perovskite Solar Cells

The exploration of dopant‐free hole‐transporting materials (HTMs) with excellent optoelectronic properties and defect passivation ability is of great significance for simultaneously improving the efficiency and stability of perovskite solar cells (PSCs). Herein, two donor–acceptor type conjugated polymers PTTDZ‐F and PTTDZ‐Cl with alternating benzodithiophene (BDT) and thienothiadiazole (TTD) units are successfully developed with desirable hole mobilities and conductivities. The formation of non‐covalent interaction (S···N) between thiophene bridge and TTD units significantly improves the co‐planarity of the molecular backbone and promotes the intramolecular charge transfer from BDT to TTD. The hydrophobicity and energy levels of the polymers are finely regulated via introducing different halogen atoms on the BDT moiety. Moreover, the functional groups in polymers efficiently passivate the surface charged traps in perovskite. As a result, champion power conversion efficiencies of 21.50% and 22.20% along with negligible hysteresis have been achieved for devices based on dopant‐free PTTDZ‐F and PTTDZ‐Cl, respectively. The unencapsulated devices also demonstrate excellent long‐term operational stability. Over 93% of their initial efficiencies can be retained after 320 h of maximum output power point tracking under 1 sun illumination. Herein, it paves a new avenue for developing highly stable and efficient HTMs for PSCs.

Constructing High‐Performance Solar Cells and Photodetectors with a Doping‐Free Polythiophene Hole Transport Material

AbstractThe emerging solution‐based solar cells and photodetectors have gained worldwide research interest over the past decades. Hole transport materials (HTMs) have greatly advanced the progress of these solution‐based electronics. Nevertheless, developing low‐cost and efficient HTMs is far from satisfactory. In this contribution, poly(3‐pentylthiophene) (P3PT) is introduced as a facile, low‐cost, and versatile dopant‐free polymer HTM for both quantum dot (QD) and perovskite electronic devices. Compared to the broadly used poly(3‐hexylthiophene), P3PT presents the reduced molecular aggregation and preferential face‐on orientation, which can markedly enhance the hole‐carrier transport in optoelectronic devices. Accordingly, P3PT can deliver the substantial improvement of photovoltaic performance from ∼8.6% to ∼9.5% for QD/polythiophene solar cells and from ∼16% to ∼18.8% for perovskite/polythiophene solar cells, which are both among the topmost values in the corresponding fields. Furthermore, P3PT HTMs can also significantly enhance the photodetection performance of QD and perovskite photodetectors by a factor of ∼3, indicating its great application potential in a variety of emerging optoelectronics.

Molecular engineering of novel dithiophene-fused hole-transporting materials for highly efficient perovskite solar cells

Understanding the relationship between structure and property of hole-transporting materials (HTMs) plays an important role in developing highly efficient perovskite solar cells (PSCs). Herein, we report a density functional theory study (DFT) combined with Marcus theory on the structural, electronic, optical, electrochemical and hole-transporting properties of designed molecules. The calculated results demonstrate that extending π-conjugation of the center by replacing CDT with IDT can lower the HOMO level, increase Stokes shift, enhance the solubility, strengthen the defect passivation ability and improve the hole mobility of molecules. The HOMO and LUMO energy alignments of the designed molecules properly match with the VBM and CBM of conventional perovskite CH3NH3PbI3 or mixed perovskite (FAPbI3)0.85(CH3NH3PbBr3)0.15, respectively. The introduction of electron-accepting groups in the π-linker results in the blue-shift of absorption edge into the UV region, which has no competition with the absorption of perovskites. Besides possessing lower HOMO levels, better solubilities and spectral properties, three designed molecules para-IDT-CN (1.76 × 10−1 cm2 V−1 s−1), meta-IDT-O (2.09 × 10−2 cm2 V−1 s−1) and meta-IDT-CN (7.53 × 10−3 cm2 V−1 s−1) exhibit higher hole mobility than the reported HTMs CDT (4.15 × 10−3 cm2 V−1 s−1) and CDT-CN (1.15 × 10−3 cm2 V−1 s−1), which ensure that these novel molecules have potentials to be efficient HTMs in the application of PSCs.

Methoxy management of side chains on the carbazole-diphenylamine derivatives based hole transport materials for perovskite solar cells: A theoretical design and experimental research

Molecular design is the important approach for realizing the improvement of performance for hole transport materials (HTMs) in perovskite solar cells (PSCs). In this work, starting from the carbazole-diphenylamine derivatives based HTMs, three molecules (CX1-CX3) are designed through methoxy management on the side chains. The calculated results indicated that CX1-CX3 yield different molecular arrangements through methoxy management on the side chains, resulting in adjusting the intermolecular coupling strength and hole transport capabilities. In comparison with the designed CX2 and CX3, CX1 exhibits a higher hole mobility, stronger interfacial interactions and more efficient charge transfer at perovskite/HTMs interface. Hence, under the same conditions, PSC devices based on CX1 yields a PCE of 22.13% higher than those of other molecules CX2 (20.96%) and CX3 (19.72%). These findings highlight the significance of methoxy management on the side chains in HTMs for the photovoltaic performance of PSCs, and reveal the potential of molecular engineering for the design of efficient HTMs.

Elucidating modelling of C≡N-based carbazole-arylamine hole transporting materials for efficient organic and perovskite solar cells

Recently, the scientific community has shown great interest in hole transport materials (HTMs) to develop efficient and stable organic and perovskite solar cells. To boost the photovoltaic performance of these solar cell devices, scientists have been investigating the use of modified HTMs. In this study, we have developed a distinctive and proficient approach to building a better photovoltaic material by modifying the endcaps of the reference (MA). Specifically, we designed five new efficient HTMs (MA1-MA5) through molecular engineering on a terminal of reference molecule MA. Using density functional theory (DFT) and time-dependent-DFT methods, we theoretically characterized the key characteristics of these newly designed materials, including the orientation of frontier molecular orbitals (FMO), absorption maxima, the partial density of states (PDOS), binding energies, reorganization energy, transition density matrix, fill factor (FF), open-circuit voltages (Voc) and power conversion efficiencies (PCEs) for these materials. Our theoretical representations revealed that newly constructed materials (MA1-MA5) have a highly red-shifted absorption spectrum with lower binding energies and narrow energy gaps, making them ideal for charge shifting. Among these designed MA1-MA5 materials, the MA1 has the potential to produce as much as higher 23.24% PCE when it will be used in fabricating solar cell devices, whereas, the other designed materials MA2-MA4 has also very comparable and much higher PCE values than the refence molecule MA. These findings demonstrate our efficient designing approach to tune the photovoltaic and optoelectronic properties of the HTMs, and we recommend these engineered materials for the future developments of an efficient solar cell devices.

Exploring the performance of perovskite solar cells with dual hole transport layers via SCAPS-1D simulation

The potential of perovskite solar cells (PSCs) to produce high efficiency (over 25 %) is well known, but the poor stability of these solar cells is still a concern for practical applications. Thus, improving the stability is essential for furthering the development of PSCs. To address this issue, researchers found that incorporating inorganic materials into the hole transport layers (HTLs) can improve the stability of PSCs, yet charge recombination at the interface between the perovskite and inorganic HTLs can still reduce the efficiency of the PSCs. This study simulated the performance and mechanism of methyl ammonium lead iodide (MAPbI3) PSCs with a dual HTL using Solar Cell Capacitance Simulator-one Dimension (SCAPS-1D). A dual HTL formed by combining Spiro-OMeTAD (spiro), a highly efficient hole transport material, and copper zinc tin sulfide (CZTS), a highly stable hole transport material, leads to a PSC with a power conversion efficiency (PCE) of 23.47 % and significantly improved stability. Simulation results of the electric field at the interfaces of PSCs indicated that the PSC with spiro/CZTS as dual HTL have enhanced charge separation and reduced recombination. Furthermore, the simulation results have also revealed the influence of different parameters, such as absorber thickness, absorber defect density, HTL thickness, HTL/absorber interface defects, temperature, series resistance, and shunt resistance, on solar cell’s characteristics.

Novel Spiro-Core Dopant-Free Hole Transporting Material for Planar Inverted Perovskite Solar Cells.

Hole-transporting materials (HTMs) have demonstrated their crucial role in promoting charge extraction, interface recombination, and device stability in perovskite solar cells (PSCs). Herein, we present the synthesis of a novel dopant-free spiro-type fluorine core-based HTM with four ethoxytriisopropylsilane groups (Syl-SC) for inverted planar perovskite solar cells (iPSCs). The thickness of the Syl-SC influences the performance of iPSCs. The best-performing iPSC is achieved with a 0.8 mg/mL Syl-SC solution (ca. 15 nm thick) and exhibits a power conversion efficiency (PCE) of 15.77%, with Jsc = 20.00 mA/cm2, Voc = 1.006 V, and FF = 80.10%. As compared to devices based on PEDOT:PSS, the iPSCs based on Syl-SC exhibit a higher Voc, leading to a higher PCE. Additionally, it has been found that Syl-SC can more effectively suppress charge interfacial recombination in comparison to PEDOT:PSS, which results in an improvement in fill factor. Therefore, Syl-SC, a facilely processed and efficient hole-transporting material, presents a promising cost-effective alternative for inverted perovskite solar cells.

Star-like, dopant-free, corannulene-cored hole transporting materials for efficient inverted perovskite solar cells

Hole transporting materials (HTMs), owing to their unique bottom position in inverted perovskite solar cells (PSCs), exert a considerable influence on the hole-transporting process and the morphology/performance of the perovskite layers. Considering the well-known interfacial problems associated with commonly-used NiOx and poly(triarylamine) HTMs in traditional inverted PSCs, there is an urgent need for exploring efficient HTMs exhibiting good film formability/processability and interfacial contact to meet the specific requirements of high-performance and stable inverted PSCs. Herein, we fabricated inverted PSCs involving two star-like, dopant-free, corannulene-cored HTMs with O-terminals (sym-penta(N, N-bis(4-methoxyphenyl)aniline)corannulene, namely Cor-OMePTPA) and S-terminals (sym-penta(N, N-bis(4-(methylthio)phenyl)aniline)corannulene, namely Cor-SMePTPA) for the first time. The Cor-SMePTPA HTM exhibited a stronger hole-transporting ability and a better interfacial chemical linkage when compared to the Cor-OMePTPA HTM. The device containing Cor-SMePTPA HTM delivered a power conversion efficiency of 21.70%, which was the highest among inverted PSCs based on methylthio (SMe)-terminated HTMs. Furthermore, comprehensive experimental and theoretical characterizations clearly showed that the core and outer terminals (O or S) affected the optoelectric and chemical properties of the HTMs as well as the photovoltaic performance and stability of the corresponding PSCs, highlighting the superiority of the design involving a corannulene core and SMe groups in HTMs for inverted PSCs. We believe that such dopant-free corannulene-cored HTMs can have considerable potential for realizing highly efficient and stable PSCs in the future.

Copper‐Doped InxGa2−xO3 Nanocrystals as Efficient Hole Transport Materials of Perovskite Solar Cells by Regulating Energy Levels

The hole transport layers are critical for collecting holes and suppressing recombination in perovskite solar cells, and metal oxides are superior due to their green synthesis and good ambient stability. Although gallium oxide is rarely applied as the hole transport layers of devices for the mismatch of valence bands with perovskite materials, copper dopants can provide an additional transport channel via their impurity levels. Herein, InxGa2−xO3 nanocrystals with copper dopants are synthesized as the hole transport material of perovskite solar cells by the hydrothermal method, and uniform hole transport layers can be deposited by spin‐coating at a low temperature of 100 °C. The energy position of impurity levels and the hole transport ability of hole transport layers can be regulated by incorporating indium ions. In the optimized condition, the efficiencies of devices increase from 19.6% to 21.4%, which is related to the improvement of the charge extraction process and carrier recombination process. And the photostability of devices can be improved simultaneously. This work provides an efficient hole transport material for inverted perovskite solar cells and demonstrates the regulation of transport channels via impurity levels.

Designing Donor‐Acceptor‐Donor (D‐A‐D) Type Molecules for Efficient Hole‐Transporting in Perovskite Solar Cells – A DFT Study

AbstractWe designed eight hole‐transporting materials (HTMs) of D‐A‐D type comprising an acceptor (A) unit flanked between two triphenylamine (TPA) donor (D) units. The structural, electronic, and absorption properties of designed molecules are studied using density functional theory (DFT) and time‐dependent DFT (TD‐DFT) methods. The MPW1PW91 functional with a 6‐31g(d,p) basis set was chosen, followed by a careful benchmark of DFT functionals. We identified dibenzothiophene‐triphenylamine (DBT‐TPA) and dibenzo‐1,4‐dioxaspiro[4,4]nonane‐triphenylamine (DBDN‐TPA) as the most promising molecules due to their unique properties, such as finite π‐conjugation, suitable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), deep blue absorption, and lower reorganization energy than the recently reported anthradithiophene–triphenylamine (ADT‐TPA) based HTMs. Furthermore, the newly proposed D‐(L‐A‐L)‐D type molecules with a linker (L) display the enhanced features required for improved hole mobility. Our work demonstrates the fine‐tuning of electronic interactions between the donor and acceptor units, with a linker (L), in achieving the planarity, absorption, and hole mobility essential for efficient HTMs in perovskite solar cells.

Design of new bithieno thiophene (BTTI) central core-based small molecules as efficient hole transport materials for perovskite solar cells and donor materials for organic solar cells

This present study focuses on designing five bithienothiophene (BTTI) central core-based small molecules (MPA-BTTI-A1-MPA-BTTI-A5) in exploring the optoelectronic properties to ensure their potentiality as donor candidates for organic solar cells (OSCs) and HTMs. MPA-BTTI-A1 and MPA-BTTI-A3 are tailored by the replacing thiophene-bridge end-capped acceptors on one side terminal of reference (MPA-BTTI-R). Similarly, MPA-BTTI-A2, MPA-BTTI-A4 and MPA-BTTI-A5 have been designed by employing the same strategy on both side terminals. To ascertain stable geometric structures, CAM-B3LYP/6-311G (d,p) is selected and used in all computational simulations. The MPA-BTTI-A1 exhibits the least energy gap of 1.69 eV with greatest λmax at 722 nm in dichloromethane (DCM) and highest dipole moment of 19.59 D in DCM solvent, indicating excellent miscibility as compared to the model/reference. All newly fabricated molecules (MPA-BTTI-A1- MPA-BTTI-A5) suggest excellent operational efficiency due to their higher Voc also relatively high-power conversion efficiency (PCE) in comparison to reference. Also, all fabricated molecules are attributed with a greater estimated PCE of range (20.74–40.3%), than reference (PCE = 16%). The highest electron mobility of 0.0871 eV is observed in MPA-BTTI-A3 andMPA-BTTI-A2 exhibits the highest hole mobility of 0.1334 eV. Hence, the modification scheme is a promising way in designing proficient photovoltaic materials, paving ways for new design and advancement of a small-molecule HTMs for donor contributors for organic solar cells (OSCs) and in PSCs.

Modulating the strength of acceptor in D-A-D type hole transport materials for efficient inverted perovskite solar cells

We report herein a design strategy to improve the hole mobility of D-A-D-based hole transport material (HTM) by modifying the accepting core of the reference HTMs based on triphenylamine (TPA) and bithieno thiophene (BTTI) units. Eight HTMs are designed by replacing the BTTI unit in BTTI-TPA HTM with several commonly available diones and imides as acceptors. The structural, electronic, optical, and charge transport properties are studied using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The designed molecules are extensively examined for their structural, electronic, and absorption properties to extract the features that improve their hole mobility using the MPW1PW91/6–31 g(d,p) method. We identified four carefully designed BTTD-TPA, DBTT-TPA, NDTTI-TPA, and NDTI-TPA molecules as the most promising HTMs. This work exemplifies the role of the methyl group on electron acceptors on hole reorganization energies. Results obtained from this study provide valuable guidelines for designing efficient HTMs.

Cation exchange strategy to construct nanopatterned Zn:NiOx electrode with highly conductive interface for efficient inverted perovskite solar cells

Inorganic hole-transporting materials (HTMs) with excellent hole extraction and transfer properties are highly desirable for inverted perovskite solar cells (PSCs). However, the widely employed oxide HTMs (e.g., NiOx nanocrystal) often suffer from the low p-type conductivity and severe trap-assisted recombination at HTMs/perovskite interface, which significantly limits the final performance of PSCs. Herein, the facile template-assisted cation exchange strategy is developed to fabricate nanopatterned Zn:NiOx as efficient HTMs for PSCs, for the first time. Promisingly, by virtue of the advantageous 1D nanoscale architecture and synergistic substitutional Zn doping, high conductivity, high hole mobility and low interfacial charge trap states density have been achieved by such nanopatterned Zn:NiOx HTMs. These features endow the highly conductive interface for PSCs to accelerate hole extraction and minimize recombination loss. As a consequence, PSCs with nanopatterned Zn:NiOx deliver a champion efficiency of ≈20 % with open-circuit voltage of as high as 1.14 V. This work demonstrates and paves a new insight and avenue for the rational design of efficient oxide HTMs.