Air-Stable and Eco-Friendly Symmetrical Imine with Thiadiazole Moieties in Neutral and Protonated form for Perovskite Photovoltaics

This paper proposes molecular and supramolecular concepts for potential application in perovskite solar cells. New air-stable symmetrical imine, with thiadiazole moieties PPL2: (5E,6E)-N2,N5-bis(4-(diphenylamino)benzylidene)-1,3,4-thiadiazole-2,5-diamine), as a hole-transporting material was synthesised in a single-step reaction, starting with commercially available and relatively inexpensive reagents, resulting in a reduction in the cost of the final product compared to Spiro-OMeTAD. Moreover, camphorsulfonic acid (CSA) in both enantiomeric forms was used to change the HOMO-LUMO levels and electric properties of the investigated imine-forming complexes. Electric, optical, thermal, and structural studies of the imine and its complexes with CSA were carried out to characterise the new material. Imine and imine/CSA complexes were also characterised in depth by the proton Nuclear Magnetic Resonance 1H NMR method. The position of nitrogen in the thidiazole ring influences the basicity of donor centres, which results in protonation in the imine bond. Simple devices of ITO/imine (with or without CSA(-) or CSA(+))/Ag/ITO architecture were constructed, and a thermographic camera was used to find the defects in the created devices. Electric behaviour was also studied to demonstrate conductivity properties under the forward current. Finally, the electrical properties of imine and its protonated form with CSA were compared with Spiro-OMeTAD. In general, the analysis of thermal images showed a very similar response of the samples to the applied potential in terms of the homogeneity of the formed organic layer. The TGA analysis showed that the investigated imine exhibits good thermal stability in air and argon atmospheres.

Asymmetric Triphenylethylene-Based Hole Transporting Materials for Highly Efficient Perovskite Solar Cells

Molecular hole-transporting materials (HTMs) having triphenylethylene central core were designed, synthesized, and employed in perovskite solar cell (PSC) devices. The synthesized HTM derivatives were obtained in a two- or three-step synthetic procedure, and their characteristics were analyzed by various thermoanalytical, optical, photophysical, and photovoltaic techniques. The most efficient PSC device recorded a 23.43% power conversion efficiency. Furthermore, the longevity of the device employing V1509 HTM surpassed that of PSC with state-of-art spiro-OMeTAD as the reference HTM.

Counterion Engineering toward High-Performance and pH-Neutral Polyoxometalates-Based Hole-Transporting Materials for Efficient Organic Optoelectronic Devices

Although protonated polyoxometalates (POMs) are promising hole-transporting layer (HTL) materials for optoelectronic devices owing to their excellent hole collection/injection property, pH neutrality, and noncorrosiveness, POMs are seldom used as high-performance HTL materials. Herein, we designed and synthesized a series of mixed-additive POMs with pH-neutral counterions (NH4+, K+, and Na+) as HTL materials. X-ray photoelectron spectroscopy and single-crystal X-ray analyses indicated that the use of the lacunary heteropolyanion [P2W15O56]12- as an intermediate ensured successful incorporation of the counterions into the mixed-addenda POMs without causing deterioration of the POM frameworks. The hole-transporting layer performance of POM-NH4, which was characterized by a high work function and good conductivity and could be prepared using a low-cost method surpassed those of its protonated counterpart POM-4 and many classic HTL materials. An organic solar cell (OSC) modified with POM-NH4 delivered a power conversion efficiency of 18.0%, which was the highest photovoltaic efficiency achieved by POM-based OSCs to date. Moreover, an HTL material based on POM-NH4 reduced the turn-on voltage of an organic light-emitting diode from 4.2 to 3.2 V. The results of this study suggest that POMs are promising alternatives to the classic HTL materials owing to their excellent hole-collection ability, low costs, neutral nature, and high-chemical stability.

Highly Solvent Resistant Small-Molecule Hole-Transporting Materials for Efficient Perovskite Quantum Dot Light-Emitting Diodes

Perovskite quantum dot light-emitting diodes (Pe-QLEDs) have been shown as promising candidates for next-generation displays and lightings due to their unique feature of wide color gamut and high color saturation. Hole-transporting materials (HTMs) play crucial roles in the device performance and stability of Pe-QLEDs. However, small-molecule HTMs have been less studied in Pe-QLEDs due to their poor solvent resistance and low hole mobility. In this work, three novel small-molecule HTMs employing benzimidazole as the center core, named X4, X5, and X6, were designed and synthesized for application in Pe-QLEDs. One of the tailored HTM-X6 exhibits excellent solvent resistant ability to the perovskite quantum dot (QD) inks due to its proper solubility and low surface energy. Our result clearly demonstrated that the synergistic effect of poor solubility and low surface energy facilitates the achievement of good solvent resistance to perovskite QD inks. As a result, a promising maximal external quantum efficiency (EQE) of 14.1% is achieved in X6-based CsPbBr3 Pe-QLEDs, which is much higher than that of X4 (9.16%) and X5 (6.60%)-based devices, which is comparable to the PTAA reference (EQE ∼ 15.8%) under the same conditions. To the best of our knowledge, this is the first example that a benzimidazole-based small-molecule HTM demonstrated a good application in Pe-QLEDs. Our work provides new guidance for the rational design of small-molecule HTMs with high solvent resistance for efficient Pe-QLEDs and other photoelectronic devices.

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 J_sc = 20.00 mA/cm², V_oc = 1.006 V, and FF = 80.10%. As compared to devices based on PEDOT:PSS, the iPSCs based on Syl-SC exhibit a higher V_oc, 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.

Indolocarbazole-Based Photo-Crosslinkable Hole-Transporting Layer for Efficient Solution-Processed Organic Light-Emitting Diodes

We designed and synthesized a new indolocarbazole-based polymer, poly(N,N-diphenyl(5,11-dihexylindolo[3,2,1-jk]carbazol-2-yl)amine) (PICA), for solution-processed organic light-emitting diodes (OLEDs). The highest occupied and lowest unoccupied molecular orbital energy levels of this polymer, -5.25 and -2.46 eV, respectively, are suitable for hole transport from the anode to the emissive layer. PICA was photo-crosslinked by UV irradiation with ethane-1,2-diyl bis(4-azido-2,3,5,6-tetrafluorobenzoate) (FPA) as the photoinitiator. Successful crosslinking was confirmed by a decreased intensity in the azide-stretching FT-IR peak and solvent test with toluene (a suitable solvent for PICA). The PICA film photo-crosslinked with 3 wt% FPA showed enhanced solvent resistance (90%) compared to the non-crosslinked neat PICA film (<20%). Moreover, OLED devices using PICA-based hole-transporting layers exhibited better device performance (EQE/LE/PE: 8.88%/12.97/8.12 in red devices, 10.84%/38.47 cd/A/25.06 lm/W in green devices) than those using poly-TPD:FPA. We demonstrated that the photo-crosslinked PICA can be applied as a hole-transporting layer in solution-processed OLEDs.

Multifunctional Narrow Band Gap Terpolymer-Enabled High-Performance Dopant-Free Perovskite and Additive-Free Organic Solar Cells with Long-Term Stability

The optoelectronic devices endowing multifunctionality while utilizing a single low-cost material have always been challenging. For this purpose, we adopted a random ternary copolymerization strategy for designing two terpolymers, namely TP-0.8-EG and TP-0.8-TEG comprising a benzothiadiazole (BT)-benzo[1,2-b:4,5-b']dithiophene-diketopyrrolo[3,4-c]pyrrole (A₁-π-D-π-A₂) backbone. The figure of merits of the narrow band gap TP-0.8-EG terpolymer include deepened frontier energy levels, high hole mobility, better film formability, enriched multifunctionality, and passivation capability. Accordingly, the suitable electronic properties of TP-0.8-EG revealed that it can function as a dopant-free hole-transporting material in perovskite solar cells (PSCs) as well as the third component in organic solar cells (OSCs). Remarkably, TP-0.8-EG outperforms by exhibiting a higher power conversion efficiency (PCE) of 20.9% over TP-0.8-TEG (PCE of 18.3%) and BT-UF (PCE of 14.6%) in dopant-free PSCs. Interestingly, TP-0.8-EG fabricated along with PM6:Y7 displayed a high PCE of 16.52% in ternary OSCs. Also, TP-0.8-EG established good device storage stabilities (85 and 83% of their initial PCEs for 1200 and 500 h) in dopant-free PSC as well as OSC devices. Notably, the devices with TP-0.8-EG showed excellent thermal and moisture stabilities. To the best of our knowledge, A₁-π-D-π-A₂ terpolymer performing both in PSCs and OSCs with decent efficiencies and good device stabilities is a rare scenario.

Nickel(II) Nitrate Hole-Transporting Layers for Single-Junction Bulk Heterojunction Organic Solar Cells with a Record 19.02% Efficiency

A facile strategy was developed here to improve the film quality of nickel-based hole transporting layer (HTL) for efficient organic solar cell (OSC) applications. To prevent the agglomeration of Ni(NO3)2 during film deposition, acetylacetonate was added into the precursor solution, which led to the formation of an amorphous and glass-like state. After thermal annealing (TA) treatment, the film-forming ability could be further improved. The additional UV-ozone (UVO) treatment continuously improved the film quality and increased the work function and conductivity of such HTL. The resulting TA & UVO modified Ni(NO3)2 & Hacac HTL produced highly efficient organic solar cells with exciting power conversion efficiencies of 18.42% and 19.02% for PM6:BTP-eC9 and D18:BTP-Th devices, respectively, much higher than the control PEDOT:PSS devices.

De Novo Synthesis of α-Oligo(arylfuran)s and Its Application in OLED as Hole-Transporting Material

Tuning the photophysical properties of π-conjugated oligomers by functionalization of skeleton, to achieve an optically and electronically advantageous building block for organic semiconductor materials is a vital yet challenging task. In this work, a series of structurally well-defined polyaryl-functionalized α-oligofurans, in which aryl groups are introduced precisely into each of the furan units, are rapidly and efficiently synthesized by de novo metal-free synthesis of α-bi(arylfuran) monomers for the first time. This new synthetic strategy nicely circumvents the cumbersome substituent introduction process in the later stage by the preinstallation of the desired aryl groups in the starting material. The characterization of α-oligo(arylfuran)s demonstrates that photoelectric properties of coplanar α-oligo(arylfuran)s can be tuned through varying aryl groups with different electrical properties. These novel α-oligo(arylfuran)s have good hole transport capacity and can function as hole-transporting layers in organic light-emitting diodes, which is indicative of significant breakthrough in the application of α-oligofurans materials in OLEDs. And our findings offer an avenue for the ingenious use of α-oligo(arylfuran)s as p-type organic semiconductors for OLEDs.

Effect of Zn/Sn Ratio on Perovskite Solar Cell Performance Applying Off-Stoichiometric Cu2ZnSnS4/Carbon Hole-Collecting Electrodes

Low-cost inorganic hole-transporting materials (HTMs) accompanied by a printable carbon electrode is an efficient approach to address the limitation of material cost of perovskite solar cells (PSCs) and get this technology closer to commercialization. The present work is focused on optimizing the Zn/Sn ratio of Cu₂ZnSnS₄/carbon hole collectors in n-i-p structured PSCs, where CuInS₂/carbon is applied as the reference hole collector. This composition regulation is a solution to address the challenge of composition-related defects of the Cu₂ZnSnS₄ (CZTS) material. The Zn/Sn ratio was tuned by the initial proportion of the zinc precursor during the nanoparticle (NP) synthesis using a heating-up procedure. It was found that the enhancement of the Zn/Sn ratio leads to a gradual increase of the optical band gap. More importantly, an increased density of B-type defect clusters [2Zn_Cu + Zn_Sn] is confirmed using Raman results. Additionally, results from the cyclic voltammetry measurement show that by increasing the Zn/Sn value, the highest occupied molecular orbital (HOMO) of HTM is pulled down. These data match the upward trend of photovoltage. CZTS HTM with an optimal Zn/Sn ratio of 1.5 has a compatible energy level, along with the features of uniform and smooth coverage. The best efficiency of about 14.86% was obtained for optimal CZTS/carbon-based PSCs, which reaches from 14.86 to 15.49% after 25 days of aging.

Efficient and Stable Methylammonium-Free Tin-Lead Perovskite Solar Cells with Hexaazatrinaphthylene-Based Hole-Transporting Materials

Incorporating non-aqueous hole-transporting materials (HTMs) to replace the widely used PEDOT:PSS is favorable for improving the stability of tin-lead perovskite solar cells (Sn-Pb PSCs). Herein, hexaazatrinaphthylene (HATNA) is found to be a promising HTM building block for Sn-Pb PSCs. By introducing triphenylamine (TPA) and methoxy-triphenylamine into the HATNA core, molecular energy levels and surface wettability can be well regulated, and a high hole mobility and thermal stability can be maintained. Moreover, a homogeneous Sn-Pb perovskite film with low Sn⁴⁺ contents and vertically orientated grains can be prepared on the substrate TPA-HATNA. Compared with PEDOT:PSS, the optimal TPA-HATNA-based methylammonium-free device enables a 70 mV increase in V_OC, delivering a remarkable PCE exceeding 18% (certified 16.4%). Impressively, the TPA-HATNA-based devices without encapsulation retain 90% efficiency after aging for 600 min under maximum-power-point tracking. Our work provides alternative HTMs for boosting the performance of Sn-Pb PSCs.

Hole-Transporting Materials for Perovskite Solar Cells Employing an Anthradithiophene Core

A decade after the report of the first efficient perovskite-based solar cell, development of novel hole-transporting materials (HTMs) is still one of the main topics in this research field. Two of the main advance vectors of this topic lie in obtaining materials with enhanced hole-extracting capability and in easing their synthetic cost. The use of anthra[1,9-bc:5,10-b'c']dithiophene (ADT) as a flat π-conjugated frame for bearing arylamine electroactive moieties allows obtaining two novel highly efficient HTMs from very cheap precursors. The solar cells fabricated making use of the mixed composition (FAPbI₃)_0.85(MAPbBr₃)_0.15 perovskite and the novel ADT-based HTMs show power conversion efficiencies up to 17.6% under 1 sun illumination compared to the 18.1% observed when using the benchmark compound 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD). Detailed density functional theory calculations allow rationalization of the observed opto-electrochemical properties and predict a flat molecular structure with a low reorganization energy that supports the high conductivity measured for the best-performing HTM.

Molecular Weight Effects of Biscarbazole-Based Hole Transport Polymers on the Performance of Solid-State Dye-Sensitized Solar Cells

The leakage and volatilization of liquid electrolytes limit the commercialization of dye-sensitized solar cells (DSCs). As solid-state (ss) hole-transporting materials, free from leakage and volatilization, biscarbazole-based polymers with different molecular weights (PBCzA-H (21,200 g/mol) and PBCzA-L (2450 g/mol)) were applied in combination with additives to produce ssDSCs. An ssDSC with PBCzA-H showed a better short-circuit current (J_sc), open-circuit voltage (V_oc), and fill factor (FF) than a device with PBCzA-L, resulting in 38% higher conversion efficiency. Compared to the PBCzA-L, the PBCzA-H with a higher molecular weight showed faster hole mobility and larger conductivity, leading to elevations in J_sc via rapid hole transport, V_oc via rapid hole extraction, and FF via lowered series and elevated shunt resistances. Thus, it is believed that PBCzA-H is a useful candidate for replacing liquid electrolytes.

Carbazole-Based Spiro[fluorene-9,9'-xanthene] as an Efficient Hole-Transporting Material for Perovskite Solar Cells

For the practical application of perovskite solar cells (PSC), it is desirable to have high efficiency, long-term stability, and low manufacturing cost. Therefore, it is required to develop inexpensive and well-performing hole-transporting materials (HTMs). In this study, we synthesized SFXDAnCBZ, which is a new carbazole-based spiro[fluorene-9,9'-xanthene] (SFX) derivative, where the central core and end-cap units consist of SFX and N3,N6-bis(di-4-anisylamino)-9H-carbazole (DAnCBZ), respectively, as an efficient and low-cost HTM for PSCs. Photoluminescence quenching at the SFXDAnCBZ/perovskite interface was more effective than at the perovskite/Spiro-OMeTAD (2,2',7,7'-tetrakis-(N,N-di-p-methoxy-phenyl-amine) 9,9'spiro-bifluorene) interface. We fabricated a PSC with a power conversion efficiency (PCE) of 20.87% under 1 sun illumination (100 mW cm^-2) using SFXDAnCBZ as an HTM. This value is comparable to that measured for the benchmark Spiro-OMeTAD. Thus, this result confirms that SFX core-based materials can be a new kind of HTMs for high-efficiency and low-cost PSCs.

Carbazole-Terminated Isomeric Hole-Transporting Materials for Perovskite Solar Cells

A set of novel hole-transporting materials (HTMs) based on π-extension through carbazole units was designed and synthesized via a facile synthetic procedure. The impact of isomeric structural linking on their optical, thermal, electrophysical, and photovoltaic properties was thoroughly investigated by combining the experimental and simulation methods. Ionization energies of HTMs were measured and found to be suitable for a triple-cation perovskite active layer ensuring efficient hole injection. New materials were successfully applied in perovskite solar cells, which yielded a promising efficiency of up to almost 18% under standard 100 mW cm^-2 global AM1.5G illumination and showed a better stability tendency outperforming that of 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene. This work provides guidance for the molecular design strategy of effective hole-conducting materials for perovskite photovoltaics and similar electronic devices.

Effects of Annealing on Characteristics of Cu2ZnSnSe4/CH3NH3PbI3/ZnS/IZO Nanostructures for Enhanced Photovoltaic Solar Cells

This paper presents new photovoltaic solar cells with Cu₂ZnSnSe₄/CH₃NH₃PbI₃(MAPbI₃)/ZnS/IZO/Ag nanostructures on bi-layer Mo/FTO (fluorine-doped tin oxide) glasssubstrates. The hole-transporting layer, active absorber layer, electron-transporting layer, transparent-conductive oxide layer, and top electrode-metal contact layer, were made of Cu₂ZnSnSe₄, MAPbI₃ perovskite, zincsulfide, indium-doped zinc oxide, and silver, respectively. The active absorber MAPbI₃ perovskite film was deposited on Cu₂ZnSnSe₄ hole-transporting layer that has been annealed at different temperatures. TheseCu₂ZnSnSe₄ filmsexhibitedthe morphology with increased crystal grain sizesand reduced pinholes, following the increased annealing temperature. When the perovskitefilm thickness was designed at 700 nm, the Cu₂ZnSnSe₄ hole-transporting layer was 160 nm, and the IZO (indium-zinc oxide) at 100 nm, and annealed at 650 °C, the experimental results showed significant improvements in the solar cell characteristics. The open-circuit voltage was increased to 1.1 V, the short-circuit current was improved to 20.8 mA/cm², and the device fill factor was elevated to 76.3%. In addition, the device power-conversion efficiency has been improved to 17.4%. The output power P_max was as good as 1.74 mW and the device series-resistance was 17.1 Ω.

Side-Chain Polymers as Dopant-Free Hole-Transporting Materials for Perovskite Solar Cells-The Impact of Substituents' Positions in Carbazole on Device Performance

Side-chain polymers have the potential to be excellent dopant-free hole-transporting materials (HTMs) for perovskite solar cells (PSCs) because of their unique characteristics, such as tunable energy levels, high charge mobility, good solubility, and excellent film-forming ability. However, there has been less research focusing on side-chain polymers for PSCs. Here, two side-chain polystyrenes with triphenylamine substituents on carbazole moieties were designed and characterized. The properties of the side-chain polymers were tuned finely, including the photophysical and electrochemical properties and charge mobilities, by changing the positions of triphenylamine substituents on carbazole. Owing to the higher mobility and charge extraction ability, the polymer P2 with the triphenylamine substituent on the 3,6-positions of the carbazole unit showed higher performance with power conversion efficiency (PCE) of 18.45%, which was much higher than the PCE (16.78%) of P1 with 2,7-positions substituted. These results clearly demonstrated that side-chain polymers can act as promising HTMs for PSC applications and the performance of side-chain polymers could be optimized by carefully tuning the structure of the monomer, which provides a new strategy to design new kinds of side-chain polymers and obtain high-performance dopant-free HTMs.

Tuning the energy level of TAPC: crystal structure and photophysical and electrochemical properties of 4,4'-(cyclohexane-1,1-diyl)bis[N,N-bis(4-methoxyphenyl)aniline]

The energy level of a hole-transporting material (HTM) in organic electronics, such as organic light-emitting diodes (OLEDs) and perovskite solar cells (PSCs), is important for device efficiency. In this regard, we prepared 4,4'-(cyclohexane-1,1-diyl)bis[N,N-bis(4-methoxyphenyl)aniline] (TAPC-OMe), C₄₆H₄₆N₂O₄, to tune the energy level of 4,4'-(cyclohexane-1,1-diyl)bis[N,N-bis(4-methylphenyl)aniline] (TAPC), which is a well-known HTM commonly used in OLED applications. A systematic characterization of TAPC-OMe, including ¹H and ¹³C NMR, elemental analysis, UV-Vis absorption, fluorescence emission, density functional theory (DFT) calculations and single-crystal X-ray diffraction, was performed. TAPC-OMe crystallized in the triclinic space group P-1, with two molecules in the asymmetric unit. The dihedral angles between the central amine triangular planes and those of the phenyl groups varied from 26.56 (9) to 60.34 (8)° due to the steric hindrance of the central cyclohexyl ring. This arrangement might be induced by weak hydrogen bonds and C-H...π(Ph) interactions in the extended structure. The emission maxima of TAPC-OMe showed a significant bathochomic shift compared to that of TAPC. A strong dependency of the oxidation potentials on the nature of the electron-donating ability of substituents was confirmed by comparing oxidation potentials with known Hammett parameters (σ).

Powder Pressed Cuprous Iodide (CuI) as A Hole Transporting Material for Perovskite Solar Cells

This study focuses on employing cuprous iodide (CuI) as a hole-transporting material (HTM) in fabricating highly efficient perovskite solar cells (PSCs). The PSCs were made in air with either CuI or 2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD) as HTMs. A simple and novel pressing method was employed for incorporating CuI powder layer between perovskite layer and Pt top-contact to fabricate devices with CuI, while spiro-OMeTAD was spin-coated between perovskite layer and thermally evaporated Au top-contact to fabricate devices with spiro-OMeTAD. Under illuminations of 100 mW/cm² with an air mass (AM) 1.5 filter in air, the average short-circuit current density (J_SC) of the CuI devices was over 24 mA/cm², which is marginally higher than that of spiro-OMeTAD devices. Higher J_SC of the CuI devices can be attributed to high hole-mobility of CuI that minimizes the electron-hole recombination. However, the average power conversion efficiency (PCE) of the CuI devices were lower than that of spiro-OMeTAD devices due to slightly lower open-circuit voltage (V_OC) and fill factor (FF). This is probably due to surface roughness of CuI powder. However, optimized devices with solvent-free powder pressed CuI as HTM show a promising efficiency of over 8.0 % under illuminations of 1 sun (100 mW/cm²) with an air mass 1.5 filter in air, which is the highest among the reported efficiency values for PSCs fabricated in an open environment with CuI as HTM.

Electronic Structure Characterization of Cross-Linked Sulfur Polymers

Cross-linked polymers of elemental sulfur are of potential interest for electronic applications as they enable facile thin-film processing of an abundant and inexpensive starting material. Here, we characterize the electronic structure of a cross-linked sulfur/diisopropenyl benzene (DIB) polymer by a combination of soft and hard X-ray photoelectron spectroscopy (SOXPES and HAXPES). Two different approaches for enhancing the conductivity of the polymer are compared: the addition of selenium in the polymer synthesis and the addition of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) during film preparation. For the former, we observe the incorporation of Se into the polymer structure resulting in a changed valence-band structure. For the latter, a Fermi level shift in agreement with p-type doping of the polymer is observed and also the formation of a surface layer consisting mostly of TFSI anions.

Donor-Acceptor-Type S,N-Heteroacene-Based Hole-Transporting Materials for Efficient Perovskite Solar Cells

Two new donor-acceptor (D-A)-substituted S,N-heteroacene-based molecules were developed and investigated as hole-transporting material (HTM) for perovskite solar cells (PSCs). Optical and electrochemical characterization brought out that the energy levels of both HTMs are suitable for their use in PSCs. Consequently, a power-conversion efficiency of 17.7% and 16.1% was achieved from PSCs involving the HTM-1 and HTM-2, respectively. The optoelectronic properties in terms of series resistance, conductivity, and charge carrier recombination were further examined to unfold the potential of these new HTMs. Time-resolved photoluminescence spectroscopy brought out that the hole injection from the valence band of perovskite into HTMs follows the trend, which is in accordance with the position of the highest occupied molecular orbital. Overall, our findings underline the potential of S,N-heteroacene co-oligomers as promising HTM candidates for PSCs.

Oxasmaragdyrins as New and Efficient Hole-Transporting Materials for High-Performance Perovskite Solar Cells

The high performance of the perovskite solar cells (PSCs) cannot be achieved without a layer of efficient hole-transporting materials (HTMs) to retard the charge recombination and transport the photogenerated hole to the counterelectrode. Herein, we report the use of boryl oxasmaragdyrins (SM01, SM09, and SM13), a family of aromatic core-modified expanded porphyrins, as efficient hole-transporting materials (HTMs) for perovskite solar cells (PSCs). These oxasmaragdyrins demonstrated complementary absorption spectra in the low-energy region, good redox reversibility, good thermal stability, suitable energy levels with CH₃NH₃PbI₃ perovskite, and high hole mobility. A remarkable power conversion efficiency of 16.5% (V_oc = 1.09 V, J_sc = 20.9 mA cm^-2, fill factor (FF) = 72%) is achieved using SM09 on the optimized PSCs device employing a planar structure, which is close to that of the state-of-the-art hole-transporting materials (HTMs), spiro-OMeTAD of 18.2% (V_oc = 1.07 V, J_sc = 22.9 mA cm^-2, FF = 74%). In contrast, a poor photovoltaic performance of PSCs using SM01 is observed due to the interactions of terminal carboxylic acid functional group with CH₃NH₃PbI₃.

Hole-Transporting Materials for Printable Perovskite Solar Cells

Perovskite solar cells (PSCs) represent undoubtedly the most significant breakthrough in photovoltaic technology since the 1970s, with an increase in their power conversion efficiency from less than 5% to over 22% in just a few years. Hole-transporting materials (HTMs) are an essential building block of PSC architectures. Currently, 2,2',7,7'-tetrakis-(N,N'-di-p-methoxyphenylamine)-9,9'-spirobifluorene), better known as spiro-OMeTAD, is the most widely-used HTM to obtain high-efficiency devices. However, it is a tremendously expensive material with mediocre hole carrier mobility. To ensure wide-scale application of PSC-based technologies, alternative HTMs are being proposed. Solution-processable HTMs are crucial to develop inexpensive, high-throughput and printable large-area PSCs. In this review, we present the most recent advances in the design and development of different types of HTMs, with a particular focus on mesoscopic PSCs. Finally, we outline possible future research directions for further optimization of the HTMs to achieve low-cost, stable and large-area PSCs.

Non-Conjugated Polymer as an Efficient Dopant-Free Hole-Transporting Material for Perovskite Solar Cells

A new non-conjugated polymer (PVCz-OMeDAD) with good solution processability was developed to serve as an efficient dopant-free hole-transporting material (HTM) for perovskite solar cells (PSCs). PVCz-OMeDAD was simply prepared by the free-radical polymerization of vinyl monomers, which were synthesized from low-cost raw materials through three high-yield synthesis steps. The combination of the flexible non-conjugated polyvinyl main chain and hole-transporting methoxydiphenylamine-substituted carbazole side chains endowed PVCz-OMeDAD with excellent film-forming ability, a suitable energy level, and high hole mobility. As a result, by using an ultra-thin (≈30 nm) PVCz-OMeDAD film as cost-effective dopant-free polymer HTM, the conventional n-i-p-type PSCs demonstrated a power conversion efficiency (PCE) up to 16.09 %, suggesting the great potential of the polymer film for future low-cost, large-scale, flexible PSCs applications.

Study of Arylamine-Substituted Porphyrins as Hole-Transporting Materials in High-Performance Perovskite Solar Cells

To develop new hole-transporting materials (HTMs) for efficient and stable perovskite solar cells (PSCs), 5,10,15,20-tetrakis{4-[N,N-di(4-methoxylphenyl)amino-phenyl]}-porphyrin was prepared in gram scale through the direct condensation of pyrrole and 4-[bis(4-methoxyphenyl)amino]benzaldehyde. Its Zn(II) and Cu(II) complexes exhibit excellent thermal and electrochemical stability, specifically a high hole mobility and very favorable energetics for hole extraction that render them a new class of HTMs in organometallic halide PSCs. As expected, ZnP as HTM in PSCs affords a competitive power conversion efficiency (PCE) of 17.78%, which is comparable to that of the most powerful HTM of Spiro-MeOTAD (18.59%) under the same working conditions. Meanwhile, the metal centers affect somewhat the photovoltaic performances that CuP as HTM produces a lower PCE of 15.36%. Notably, the PSCs employing ZnP show a much better stability than Spiro-OMeTAD. Moreover, the two porphyrin-based HTMs can be prepared from relatively cheap raw materials with a facile synthetic route. The results demonstrate that ZnP and CuP can be a new class of HTMs for efficient and stable PSCs. To the best of our knowledge, this is the best performance that porphyrin-based solar cells could show with PCE > 17%.

High-performance and environmentally stable planar heterojunction perovskite solar cells based on a solution-processed copper-doped nickel oxide hole-transporting layer

An effective approach to significantly increase the electrical conductivity of a NiOx hole-transporting layer (HTL) to achieve high-efficiency planar heterojunction perovskite solar cells is demonstrated. Perovskite solar cells based on using Cu-doped NiOx HTL show a remarkably improved power conversion efficiency up to 15.40% due to the improved electrical conductivity and enhanced perovskite film quality. General applicability of Cu-doped NiOx to larger bandgap perovskites is also demonstrated in this study.

Enhanced Hole Extraction in Perovskite Solar Cells Through Carbon Nanotubes

Here, we report the use of polymer-wrapped carbon nanotubes as a means to enhance charge extraction through undoped spiro-OMeTAD. With this approach a good solar cell performance is achieved without the implementation of conventional doping methods. We demonstrate that a stratified two-layer architecture of sequentially deposited layers of carbon nanotubes and spiro-OMeTAD, outperforms a conventional blend of the hole-conductor and the carbon nanotubes. We also provide insights into the mechanism of the rapid hole extraction observed in the two-layer approach.