Dopant-Free Pyrene-Based Hole Transporting Material Enables Efficient and Stable Perovskite Solar Cells

Dopant-free hole transporting materials (HTMs) is significant to the stability of perovskite solar cells (PSCs). Here, we developed a novel star-shape arylamine HTM, termed Py-DB, with a pyrene core and carbon-carbon double bonds as the bridge units. Compared to the reference HTM (termed Py-C), the extension of the planar conjugation backbone endows Py-DB with typical intermolecular π-π stacking interactions and excellent solubility, resulting in improved hole mobility and film morphology. In addition, the lower HOMO energy level of the Py-DB HTM provides efficient hole extraction with reduced energy loss at the perovskite/HTM interface. Consequently, an impressive power conversion efficiency (PCE) of 24.33 % was achieved for dopant-free Py-DB-based PSCs, which is the highest PCE for dopant-free small molecular HTMs in n-i-p configured PSCs. The dopant-free Py-DB-based device also exhibits improved long-term stability, retaining over 90 % of its initial efficiency after 1000 h exposure to 25 % humidity at 60 °C. These findings provide valuable insights and approaches for the further development of dopant-free HTMs for efficient and reliable PSCs.

In Situ Photogenerated Radicals of Hydroxyl Substituted Pyrene-Based Triphenylamines with Enhanced Transport and Free Doping/Post-Oxidation for Efficient Perovskite Solar Cells

The high-performance hole transporting material (HTM) is one of the most important components for the perovskite solar cells (PSCs) in promoting power conversion efficiency (PCE). However, the low conductivity of HTMs and their additional requirements for doping and post-oxidation greatly limits the device performance. In this work, three novel pyrene-based derivatives containing methoxy-substituted triphenylamines units (PyTPA, PyTPA-OH and PyTPA-2OH) are designed and synthesized, where different numbers of hydroxyl groups are connected at the 2- or 2,7-positions of the pyrene core. These hydroxyl groups at the 2- or 2,7-positions of pyrene play a significantly role to enhance the intermolecular interactions that are able to generate in situ radicals with the assistance of visible light irradiation, resulting in enhanced hole transferring ability, as well as an enhanced conductivity and suppressed recombination. These pyrene-core based HTMs exhibit excellent performance in PSCs, which possess a higher PCE than those control devices using the traditional spiro-OMeTAD as the HTM. The best performance can be found in the devices with PyTPA-2OH. It has an average PCE of 23.44% (PCEmax = 23.50%), which is the highest PCE among the reported PSCs with the pyrene-core based HTMs up to date. This research offers a novel avenue to design a dopant-free HTM by the combination of the pyrene core, methoxy triphenylamines, and hydroxy groups.

Reducing the Depletion Region Width at the Anode Interface via a Highly Doped Conjugated Polyelectrolyte Composite for Efficient Organic Solar Cells

In the realm of organic solar cells (OSCs), the width of the depletion region at the anode interface is a critical factor that adversely impacts the open-circuit voltage (Voc) and the power conversion efficiency (PCE). To address this challenge, a novel approach involving a conjugated polyelectrolyte (CPE)-based composite, PCP-2F-Li:POM, has been developed. This composite serves as a solution-processed hole transport layer (HTL), effectively minimizing the depletion region width in high-performance OSCs. The innovative aspect of PCP-2F-Li:POM lies in its "mutual doping" mechanism. Polyoxometalate (POM) is utilized as a dopant, facilitating the formation of p-doped CPE and n-doped POM within the composite. This results in a substantial increase in doping density, nearly 2 orders of magnitude higher than that observed in unmodified CPE. Consequently, the width of depletion region is markedly reduced, shrinking from 76.4 to 6.0 nm. This reduction plays a pivotal role in enhancing hole transport via the tunneling effect. The practical impact of this development is notable. It leads to an increase in Voc from 0.84 to 0.86 V, thereby contributing significantly to an impressive PCE of 18.04% in OSCs. Moreover, the compatibility of PCP-2F-Li:POM with large-area processing techniques underscores its potential as a viable HTL material for future practical applications. Additionally, its contribution to the enhanced long-term stability of OSCs further bolsters its suitability for practical applications.

2D-Self-Assembled Organic Materials in Undoped Hole Transport Bilayers for Efficient Inverted Perovskite Solar Cells

Interfaces between photoactive perovskite layer and selective contacts play a key role in the performance of perovskite solar cells (PSCs). The properties of the interface can be modified by the introduction of molecular interlayers between the halide perovskite and the transporting layers. Herein, two novel structurally related molecules, 1,3,5-tris(α-carbolin-6-yl)benzene (TACB) and the hexamethylated derivative of truxenotris(7-azaindole) (TTAI), are reported. Both molecules have the ability to self-assemble through reciprocal hydrogen bond interactions, but they have different degrees of conformational freedom. The benefits of combining these tripodal 2D-self-assembled small molecular materials with well-known hole transporting layers (HTLs), such as PEDOT:PSS and PTAA, in PSCs with inverted configuration are described. The use of these molecules, particularly the more rigid TTAI, enhanced the charge extraction efficiency and reduced the charge recombination. Consequently, an improved photovoltaic performance was achieved in comparison to the devices fabricated with the standard HTLs.

Di(arylcarbazole) Substituted Oxetanes as Efficient Hole Transporting Materials with High Thermal and Morphological Stability for OLEDs

A group of di(arylcarbazole)-substituted oxetanes has been prepared in Suzuki reactions by using the key starting material 3,3-di[3-iodocarbazol-9-yl]methyloxetane and various boronic acids (fluorophenylboronic acid, phenylboronic acid or naphthalene-1-boronic acid). Full characterization of their structure has been presented. The low molar mass compounds represent materials having high thermal stability with 5% mass loss thermal degradation temperatures in the range of 371-391 °C. Glass transition temperatures of the materials are also very high and range from 107 °C to 142 °C, which is a big advantage for formation of stable amorphous layers for optoelectronic devices, i.e., organic light emitting diodes. Hole transporting properties of the prepared materials were confirmed in formed organic light emitting diodes with tris(quinolin-8-olato)aluminium (Alq3) as a green emitter, which also served as an electron transporting layer. In the device's materials, 3,3-di[3-phenylcarbazol-9-yl]methyloxetane (5) and 3,3-di[3-(1-naphthyl)carbazol-9-yl]methyloxetane (6) demonstrated superior hole transporting properties than that of material 3,3-di[3-(4-flourophenyl)carbazol-9-yl]methyloxetane (4) based device. When material 5 was used in the device structure, the OLED demonstrated rather low turn-on voltage of 3.7 V, luminous efficiency of 4.2 cd/A, power efficiency of 2.6 lm/W and maximal brightness exceeding 11670 cd/m². HTL of 6 based device also showed exclusive OLED characteristics. The device was characterized by turn-on voltage of 3.4 V, maximum brightness of 13193 cd/m², luminous efficiency of 3.8 cd/A and power efficiency of 2.6 lm/W. An additional hole injecting-transporting layer (HI-TL) of PEDOT considerably improved functions of the device with HTL of compound 4. The modified OLED with a layer of the derivative 4 demonstrated exclusive characteristics with turn-on voltage of 3.9 V, high luminous efficiency of 4.7 cd/A, power efficiency of 2.6 lm/W and maximal brightness exceeding 21,000 cd/m². These observations confirmed that the prepared materials have a big potential in the field of optoelectronics.

Dopant-Free Hole Transporting Material Based on Nonconjugated Adamantane for High-Performance Perovskite Solar Cells

A new dopant-free hole transporting material (HTM) 4′,4‴,4‴'',4‴''''-(adamantane-1,3,5,7-tetrayl)tetrakis(N,N-bis(4-methoxyphenyl)-[1,1′-biphenyl]-4-amine) (Ad-Ph-OMeTAD) (named FDY for short), which consists of a nonconjugated 3D bulky caged adamantane (Ad) as the core, triphenyl amines as side arms, and phenyl units as a linking bridge, is synthesized and applied in an inverted planar perovskite solar cell (PSC). As a result, the champion device with FDY as HTM yields an impressive power of conversion efficiency (PCE) of 18.69%, with J_SC = 22.42 mA cm⁻², V_OC = 1.05 V, and FF = 79.31% under standard AM 1.5G illumination, which is ca. 20% higher than that of the device based on PEDOT:PSS (only 15.41%). Notably, the stability of PSC based on FDY is much better than that of devices based on PEDOT:PSS, and the corresponding devices retain over 90% of their initial PCEs after storing for 60 days in a nitrogen glove box without any encapsulation. Even when stored in an open air condition with 50–60% relative humidity for 188 h, the retained PCE is still over 81% of its initial one. All these results demonstrate that the new design strategy by combing the bulky and nonconjugated (aliphatic) adamantane unit as the core and triphenyl amines as side arms can efficiently develop highly efficient HTMs for PSCs, which is different from the traditional way based on conjugated backbones, and it may open a new way for scientists to design small-molecule HTMs for PSCs.

Dendritic-Like Molecules Built on a Pillar[5]arene Core as Hole Transporting Materials for Perovskite Solar Cells

Multi-branched molecules have recently demonstrated interesting behaviour as charge-transporting materials within the fields of perovskite solar cells (PSCs). For this reason, extended triarylamine dendrons have been grafted onto a pillar[5]arene core to generate dendrimer-like compounds, which have been used as hole-transporting materials (HTMs) for PSCs. The performances of the solar cells containing these novel compounds have been extensively investigated. Interestingly, a positive dendritic effect has been evidenced as the hole transporting properties are improved when going from the first to the second-generation compound. The stability of the devices based on the best performing pillar[5]arene material has been also evaluated in a high-throughput ageing setup for 500 h at high temperature. When compared to reference devices prepared from spiro-OMeTAD, the behaviour is similar. An analysis of the economic advantages arising from the use of the pillar[5]arene-based material revealed however that our pillar[5]arene-based material is cheaper than the reference.

New Electroactive Polymers with Electronically Isolated 4,7-Diarylfluorene Chromophores as Positive Charge Transporting Layer Materials for OLEDs

A group of polyethers containing electroactive pendent 4,7-diarylfluorene chromophores have been prepared by the multi-step synthetic route. Full characterization of their structures has been presented. The polymeric materials represent derivatives of high thermal stability with initial thermal degradation temperatures in a range of 392-397 °C. Glass transition temperatures of the amorphous polymers range from 28 °C to 63 °C and depend on structures of the 4,7-diarylfluorene chromophores. Electron photoemission spectra of thin layers of the electroactive derivatives showed ionization potentials in the range of 5.8-6.0 eV. Hole injecting/transporting properties of the prepared polymeric materials were confirmed during formation of organic light-emitting diodes with tris(quinolin-8-olato)aluminium (Alq₃) as a green emitter, which also serves as an electron transporting layer. The device using hole-transporting polymer with electronically isolated 2,7-di(4-biphenyl)fluorene chromophores demonstrated the best overall performance with low turn on voltage of 3 V, high current efficiency exceeding 1.7 cd/A, and with maximum brightness over 200 cd/m². The organic light-emitting diode (OLED) characteristics were measured in non-optimized test devices. The efficiencies could be further improved by an optimization of device structure, formation conditions, and encapsulation of the devices.

Asymmetric Benzotrithiophene-Based Hole Transporting Materials Provide High-Efficiency Perovskite Solar Cells

In this study, we synthesized a series of small-molecule benzotrithiophenes (BTTs) and used them as hole transporting materials (HTMs) in perovskite solar cells (PSCs). The asymmetric benzo[2,1-b:-3,4-b':5,6-b″]trithiophene unit was used as the central core to which were appended various donor groups, namely, carbazole (BTT-CB), thieno thiophene (BTT-FT), triphenylamine (BTT-TPA), and bithiophene (BTT-TT). The extended aromatic core in the asymmetric BTT provided full planarity, thereby favoring intermolecular π-stacking and charge transport. The physical, optical, and electrical properties of these small-molecule HTMs are reported herein. BTT-TT displayed good crystallinity and superior hole mobility, when compared with those of the other three HTMs, and formed smooth and uniform surfaces when covering the perovskite active layer. Accordingly, among the devices prepared in this study, a PSC incorporating BTT-TT as the HTM achieved the highest power conversion efficiency (18.58%). Moreover, this BTT-TT-containing device exhibited good stability after storage for more than 700 h. Thus, asymmetric BTTs are promising candidate materials for use as small-molecule HTMs in PSCs.

V2O5 as Hole Transporting Material for Efficient All Inorganic Sb2S3 Solar Cells

This research demonstrates that V₂O₅ is able to serve as hole transporting material to substitute organic transporting materials for Sb₂S₃ solar cells, offering all inorganic solar cells. The V₂O₅ thin film is prepared by thermal decomposition of spin-coated vanadium(V) triisopropoxide oxide solution. Mechanistic investigation shows that heat treatment of V₂O₅ layer has crucial influence on the power conversion efficiency of device. Low temperature annealing is unable to remove the organic molecules that increases the charge transfer resistance, while high temperature treatment leads to the increase of work function of V₂O₅ that blocks hole transporting from Sb₂S₃ to V₂O₅. Electrochemical and compositional characterizations show that the interfacial contact of V₂O₅/Sb₂S₃ can be essentially improved with appropriate annealing. The optimized power conversion efficiency of device based on Sb₂S₃/V₂O₅ heterojunction reaches 4.8%, which is the highest power conversion efficiency in full inorganic Sb₂S₃-based solar cells with planar heterojunction solar cells. Furthermore, the employment of V₂O₅ as hole transporting material leads to significant improvement in moisture stability compared with the device based organic hole transporting material. Our research provides a material choice for the development of full inorganic solar cells based on Sb₂S₃, Sb₂(S,Se)₃, and Sb₂Se₃.

BODIPY-Based Conjugated Polymers for Use as Dopant-Free Hole Transporting Materials for Durable Perovskite Solar Cells: Selective Tuning of HOMO/LUMO Levels

Recently, perovskite solar cells (PSCs) have emerged as an excellent photovoltaic device owing to the outstanding power conversion efficiency (PCE). Nevertheless, device instability remains a critical issue in this field. To overcome device instability without deteriorating PCE, dopant-free hole transporting materials (HTMs) are needed to separate the air-sensitive perovskite layer from extrinsic factors, which induce its degradation. Herein, we developed novel conjugate polymers of benzo[1,2- b:4,5- b']dithiophene (BDT) and 4,4-difluoro-4-bora-3 a,4 a-diaza- s-indacene (BODIPY) for use as HTMs without dopants. The pBDT-BODIPY polymer allows individual "dialing" of the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) levels with small modifications to the molecular structure, enabling study of the impact of the frontier molecular orbital on PSC performance. Different alkyl chains on BDT can minutely adjust the HOMO level, and meso-substituents on BODIPYs can selectively set the LUMO level of the resulting polymers. Application of BODIPY-containing polymer into the perovskite solar cell as an HTM leads to a high PCE value (16.02%) and exceptional solar cell stability shown by the fact that over 80% of its original PCE value was maintained after 10 days under ambient air conditions.

Development of Dopant-Free Donor-Acceptor-type Hole Transporting Material for Highly Efficient and Stable Perovskite Solar Cells

In perovskite solar cells (PSCs), overlying hole transporting materials (HTMs) are important for achieving high efficiencies as well as protecting perovskite active layers from degradation factors. This study reports the synthesis of a dopant-free HTM based on a D'-A-D-A-D-A-D' (D, D': electron donor, A: electron acceptor) conjugated structure and incorporation of the HTM into a PSC. The resulting PSC exhibits a high efficiency of 17.3%, which is comparable to that of the device based on doped spiro-OMeTAD HTM, and exhibits much improved stability: without encapsulation, the PSC based on the new HTM was found to retain 80% of its initial performance over 500 h under the conditions of 60% relative humidity/1 sun light-soaking without encapsulation. The high performance is attributed to efficient hole-extraction/collection and hole transport. We demonstrate that the extended π-structure of the D'-A-D-A-D-A-D'-type HTM slows moisture intrusion and protects the perovskite layer better than smaller D-A-type molecules. The improved stability is primarily due to the hydrophobic nature of the HTM; the relatively large π-conjugated molecule forms denser films, which effectively decrease the spaces between the molecules and retard water intrusion. The dopant-free D-A-type HTM with an extended π-structure is effective not only in improving device efficiency, but also device stability.

Employing PEDOT as the p-Type Charge Collection Layer in Regular Organic-Inorganic Perovskite Solar Cells

Organic-inorganic halide perovskite solar cells have recently emerged as high-performance photovoltaic devices with low cost, promising for affordable large-scale energy production, with laboratory cells already exceeding 20% power conversion efficiency (PCE). To date, a relatively expensive organic hole-conducting molecule with low conductivity, namely spiro-OMeTAD (2,2',7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine) 9,9'- spirobifluorene), is employed widely to achieve highly efficient perovskite solar cells. Here, we report that by replacing spiro-OMeTAD with much cheaper and highly conductive poly(3,4-ethylenedioxythiophene) (PEDOT) we can achieve PCE of up to 14.5%, with PEDOT cast from a toluene based ink. However, the stabilized power output of the PEDOT-based devices is only 6.6%, in comparison to 9.4% for the spiro-OMeTAD-based cells. We deduce that accelerated recombination is the cause for this lower stabilized power output and postulate that reduced levels of p-doping are required to match the stabilized performance of Spiro-OMeTAD. The entirely of the materials employed in the perovskite solar cell are now available at commodity scale and extremely inexpensive.