Organic Electron Transport Materials Research Articles

Non-Fullerene Organic Electron Transport Materials toward Stable and Efficient Inverted Perovskite Photovoltaics.

Inverted perovskite solar cells (PSCs) attract continuing interest due to their low processing temperature, suppressed hysteresis, and compatibility with tandem cells. Considerable progress has been made with reported power conversion efficiency (PCE) surpassing 26%. Electron transport Materials (ETMs) play a critical role in achieving high-performance PSCs because they not only govern electron extraction and transport from the perovskite layer to the cathode, but also protect the perovskite from contact with ambient environment. On the other hand, the non-radiative recombination losses at the perovskite/ETM interface also limits the future development of PSCs. Compared with fullerene derivatives, non-fullerene n-type organic semiconductors feature advantages like molecular structure diversity, adjustable energy level, and easy modification. Herein, the non-fullerene ETM is systematically summarized based on the molecular functionalization strategy. Various types of molecular design approaches for producing non-fullerene ETM are presented, and the insight on relationship of chemical structure and device performance is discussed. Meantime, the future trend of non-fullerene ETM is analyzed. It is hoped that this review provides insightful perspective for the innovation of new non-fullerene ETMs toward more efficient and stable PSCs.

Interface defect formation for atomic layer deposition of SnO2 on metal halide perovskites

With the rapidly advancing perovskite solar cell (PSC) technology, dedicated interface engineering is critical for improving device stability. Atomic layer deposition (ALD) grown metal oxide films have drawn immense attention for the fabrication of stable PSC. Despite the advantages of ALD, the deposition of metal oxides directly on bare perovskite has so far not been achieved without damaging the perovskite layer underneath. In addition, the changes to the physicochemical and electronic properties at the perovskite interface upon exposure to the ALD precursors can alter the material and hence device functionality. Herein, we report on a synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES) investigation of the interface between metal halide perovskite (MHP) absorber and ALD-SnO2 electron transport layer. We find clear evidence for the formation of new chemical species (nitrogen compound, lead dihalides) and an upward band bending in the MHP and downward band bending in the SnO2 towards the MHP/ALD-SnO2 interface. The upward bending at the interface forms an electron barrier layer of ∼400 meV, which is detrimental to the PSC performance. In addition, we assess the effectiveness of introducing a thin interlayer of the organic electron transport material Phenyl-C61-butyric acid methyl ester (PCBM) between MHP and ALD-SnO2 to mitigate the effects of ALD deposition.

Single‐Layer Carbon Nitride as an Efficient Metal‐Free Organic Electron‐Transport Material with a Tunable Work Function

AbstractSingle‐layer carbon nitride (SLCN) is introduced as a metal‐free organic electron transport material (ETL) for organic solar cells, delivering a performance comparable to that of the benchmark nanocrystalline ZnO. SLCN is produced in the form of stable aqueous inks and can serve as an efficient electron transport layer for three different types of active layers, showing high stability to photodegradation. A lower conductivity of SLCN films as compared to ZnO is counter‐balanced by their higher transparency and lower light scattering losses. The SLCN ETL provides a unique opportunity to tune the work function from ca. −4.1 to −4.6 eV by varying the annealing temperature due to partial thermal oxidation of the SLCN film. The PL band position follows the oxidation‐induced changes of the work function allowing PL properties of SLCN to be used to predict the PV efficiency. The 2D structure of SLCN coupled with unique structural and compositional variability, and tunability of the work function show a high promise for emerging organic PV devices.

The impact of ester groups on 1,8-naphthalimide electron transport material in organic solar cells

Recently, 1,8-naphthalimide (NI) molecules have emerged as a new category of organic electron transport materials (ETMs) employed in organic solar cells (OSCs). Among them, NI molecules with a "NI-π-NI" molecular structure commonly exhibit over aggregation in films owing to their planar molecular structures, which is unfavorable for solution-processed film fabrication in OSC devices. In this work, we successfully synthesized a new NI molecule named NDT-2, employing a strategy that incorporates a solubilizing ester group modification to induce twisted molecular planarity, thereby reducing intermolecular over-aggregation and optimizing quality of the film. Interestingly, the NDT-2 modified by ester groups as a π bridge exhibits an enlarged molecular dihedral angle, enhanced alcohol solubility, a smoother film surface morphology, and improved film forming quality. Ultimately, OSCs incorporating NDT-2 as the ETM achieved a notable power conversion efficiency (PCE) of 17.1%. This study introduces a design strategy that utilizes ester functional groups to improve film forming quality of ETM, offering valuable insights for the development of advanced, alcohol-soluble NI molecules as efficient ETMs in OSCs.

Benzoimidazolium-derived dimeric and hydride n-dopants for organic electron-transport materials: impact of substitution on structures, electrochemistry, and reactivity.

1,3-Dimethyl-2,3-dihydrobenzo[d]imidazoles, 1H, and 1,1',3,3'-tetramethyl-2,2',3,3'-tetrahydro-2,2'-bibenzo[d]imidazoles, 12, are of interest as n-dopants for organic electron-transport materials. Salts of 2-(4-(dimethylamino)phenyl)-4,7-dimethoxy-, 2-cyclohexyl-4,7-dimethoxy-, and 2-(5-(dimethylamino)thiophen-2-yl)benzo[d]imidazolium (1g-i+, respectively) have been synthesized and reduced with NaBH4 to 1gH, 1hH, and 1iH, and with Na:Hg to 1g2 and 1h2. Their electrochemistry and reactivity were compared to those derived from 2-(4-(dimethylamino)phenyl)- (1b+) and 2-cyclohexylbenzo[d]imidazolium (1e+) salts. E(1+/1•) values for 2-aryl species are less reducing than for 2-alkyl analogues, i.e., the radicals are stabilized more by aryl groups than the cations, while 4,7-dimethoxy substitution leads to more reducing E(1+/1•) values, as well as cathodic shifts in E(12•+/12) and E(1H•+/1H) values. Both the use of 3,4-dimethoxy and 2-aryl substituents accelerates the reaction of the 1H species with PC61BM. Because 2-aryl groups stabilize radicals, 1b2 and 1g2 exhibit weaker bonds than 1e2 and 1h2 and thus react with 6,13-bis(triisopropylsilylethynyl)pentacene (VII) via a "cleavage-first" pathway, while 1e2 and 1h2 react only via "electron-transfer-first". 1h2 exhibits the most cathodic E(12•+/12) value of the dimers considered here and, therefore, reacts more rapidly than any of the other dimers with VII via "electron-transfer-first". Crystal structures show rather long central C-C bonds for 1b2 (1.5899(11) and 1.6194(8) Å) and 1h2 (1.6299(13) Å).

Recent progress in perovskite solar cells: material science

Perovskite solar cells represent a promising third-generation photovoltaic technology with low fabrication cost and high power conversion efficiency. In light of the rapid development of perovskite materials and devices, a systematic survey on the latest advancements covering a broad range of related work is urgently needed. This review summarizes the recent major advances in the research of perovskite solar cells from a material science perspective. The discussed topics include the devices based on different type of perovskites (organic-inorganic hybrid, all-inorganic, and lead-free perovskite and perovskite quantum dots), the properties of perovskite defects, different type of charge transport materials (organic, polymeric, and inorganic hole transport materials and inorganic and organic electron transport materials), counter electrodes, and interfacial materials used to improve the efficiency and stability of devices. Most discussions focus on the key progresses reported within the recent five years. Meanwhile, the major issues limiting the production of perovskite solar cells and the prospects for the future development of related materials are discussed.

Evaluating the performance of Cs2PtI6−xBrx for photovoltaic and photocatalytic applications using first-principles study and SCAPS-1D simulation

All inorganic free-lead halide double perovskites are attractive materials in solar energy harvesting applications. In this study, density functional theory calculations have been used to predict the structures, band structures, and density of states of Cs2PtI6−xBrx with (x = 0, 2, 4, and 6). The optical properties (reflectivity, refractive index, absorption, dielectric function, conductivity, and loss function) of these materials have been predicted and discussed. The band edges calculations showed that the Cs2PtI6−xBrx may be an efficient visible-light photocatalyst for water splitting and CO2 reduction. The calculated bandgap value of Cs2PtI6 exhibited a great match with the reported experimental values. It has been seen that increasing the doping content of Br− in Cs2PtI6−xBrx (x = 0, 2, 4, and 6) increases the bandgaps from 1.4 eV to 2.6 eV and can be applied in single junction and tandem solar cells. Using Solar Cell Capacitance Simulator (SCAPS), a 1D device modelling has been performed on Cs2PtI6 inorganic lead-free solar cells. For the fully inorganic device, the effect of replacing organic hole transport materials (HTL) and electron transport materials (ETL) with inorganic ones is investigated while keeping high efficiencies and stabilities of solar cell devices. From the obtained results, it was found that WS2 as ETL and Cu2O as HTL were the most suitable materials compared to the others. Further investigation studies are performed on the effect of changing metal back contact work function, absorber layer thickness, doping density, and defect density on the power conversion efficiency (PCE) of the solar cell. The optimized suggested structure (FTO/WS2/Cs2PtI6/Cu2O/Carbon) obtained a PCE of 17.2% under AM1.5 solar illumination.

Designing Organic Electron Transport Materials for Stable and Efficient Performance of Perovskite Solar Cells: A Theoretical Study.

In this article, electron transporting layer (ETL) materials are designed to enhance the performance and stability of methyl ammonium lead iodide (MAPbI3) perovskite solar cells (PSCs). The optical and electronic properties of the designed ETLs are investigated using density functional theory. The designed ETLs show better charge mobility compared to nickel phthalocyanines (NiPcs). The NiPc, a hole transporting layer material, shows ETL-like behavior for PSCs with the substitution of different electron withdrawing groups (X = F, Cl, Br, and I). The stability and electron injection behavior of the designed ETLs are improved. The Br16NiPc shows the highest charge mobility. Further, the stability of the designed ETLs is relatively better compared to NiPc. Due to the hydrophobic nature, the designed ETLs act as a passivation layer for perovskites and prevent the absorber materials from degradation in the presence of moisture and provide extra stability to the PSCs. The effect of designed ETLs on the performance of MAPbI3 solar cells is also investigated. The PSCs designed with Br16NiPc as an ETL shows a relatively better (23.23%) power conversion efficiency (PCE) compared to a TiO2-based device (21.55%).

Doping effects and stability of calcium in organic electron-transport materials

Abstract Ca was co-evaporated and incorporated into different organic electron-transport materials as a dopant, but effective n-type doping was only achieved in Bathophenanthroline (BPhen) which has an electron affinity greater than the work function of Ca. Both electron transport and injection in BPhen were markedly improved as the Ca doping concentration was raised above 4.1 wt%. The doping effects were attributed to facile electron transfer and limited gap state formation. Blue fluorescent organic light-emitting diodes (OLEDs) with a Ca-doped BPhen electron-transport layer exhibited a lower voltage, higher luminance and a lifetime increase by up to 22 times. Contrary to the general belief, stressing tests showed that the Ca dopant is very stable and a slow diffuser in the BPhen matrix under current injection.

Molecular engineering of an electron-transport triarylphosphine oxide-triazine conjugate toward high-performance phosphorescent organic light-emitting diodes with remarkable stability

Organic electron-transport materials are an essential component to boost performances and stability of organic light-emitting diodes. We present a robust organic electron-transport compound 3-(6-(3-(4,6-bis(4-biphenylyl)-l,3,5-triazin-2-yl)phenyl)pyridin-2-yl)phenyldiphenylphosphine oxide by facilely coupling the triphenylphosphine oxide moiety to the 2-phenyl-4,6-bis(4-biphenylyl)-1,3,5-triazine unit via a 2,6-pyridinylene linker. It is well soluble in weakly polar solvents and possesses a high Tg of 123 °C with an exceptional Td≈470 °C at 1% weight loss and deep HOMO/LUMO levels of ca. −6.45/-3.06 eV. The phos-phorescent spectrum measured in solid state at 77 K reveals a notable triplet energy of 2.88 eV. n-Doping with 8-hydroxyquinolatolithium (Liq) produces the electron mobility value of 4.66×10−5-3.21×10−4 cm V−1s−1 @(2–5)×10−5 V cm−1. Moreover, the contrasting solubility of the bromo reaction intermediate and the new compound in alcoholic solvents facilitates separation. The characterizations of bottom- and top-emission green phosphorescent OLEDs involving this single Liq-doped electron-transport layer reveal long stability. In particular, the latter provides outstanding performances with 77.4 cd A−1 (corresponding to an EQE of 18.7%) and 86.8 lm W−1@ca. 1000 cd m−2, based on the green emitter bis(2-phenylpyridine)(2-(4-methyl-3-phenylphenyl)pyridine)iridium(III). Moreover, driven by a constant current for ca. 640 h, the initial luminance of 1000 cd m−2 appears almost no decay.

Recent advances of non-fullerene organic electron transport materials in perovskite solar cells

The advances of non-fullerene ETMs in PerSCs and their design strategies were systematically summarized.

Improved performance of quantum dot light-emitting diodes by hybrid electron transport layer comprised of ZnO nanoparticles doped organic small molecule

Abstract ZnO nanoparticles (NPs) are widely employed as the electron transport layer (ETL) material in quantum dot light-emitting diodes (QLEDs) due to their excellent electron transport properties and inherent stability. However, the unbalanced carrier injection caused by the redundant electron injection is a common phenomenon existing in QLEDs with ZnO ETL. Here, based on the lower electron mobility and shallower lowest unoccupied molecular orbit (LUMO) of most organic electron transport ·materials than ZnO NPs, we report a strategy to modulate the electron transporting properties of ZnO NPs by doping organic small molecular electron transport materials. Several kinds of small molecular dopants including LiQ (lithium 8-quinolate), TPBi (1,3,5-tris(N-phenylbenzimidazol-2,yl) benzene), and BPhen (4,7-diphenyl-1,10-phenanthroline) are separately introduced to modify ZnO. The resulting device with LiQ-doped ZnO ETL at a doping ratio of 7 wt% exhibits the optimal performance at peak current efficiency (CE) and external quantum efficiency (EQE) up to 8.07 cd/A and 7.74%, which are about 1.93- and 1.94-fold higher than 4.19 cd/A and 3.98% of the device with pristine ZnO ETL, respectively. The improvement of performances can be mainly attributed to the inhibition of electron injection and interfacial exciton quenching by the use of LiQ-doped ZnO ETLs. This work may offer a promising method for fabricating high performance QLEDs and promote their application development in the display fields.

Band Alignment Boosts Charge-Carrier Collection in Sn-based Perovskite over Pb Counterparts.

The diverse elemental compositions endow metal halide perovskites with tailorable electronic structures and broad optoelectronic applications. For Sn-based perovskites, their bandedge positions, which govern interfacial charge-carrier transport, are less well studied than their Pb counterparts. In this work, the valence band maximum (VBM) of CsSnBr3 was experimentally and theoretically determined to be -5.2 eV, to which Au forms a good contact. The conduction band minimum (CBM) of CsSnBr3 at -3.4 eV is matched by 1,3,5-tris(4-phenylquinolin-2-yl)benzene (TQB), an organic electron transport material and a ligand to Sn(II). Thanks to proper band alignment, the device structure Al/TQB/CsSnBr3/Au constitutes a photodetector responsive to the entire visible spectrum without a bias voltage and outperforms Pb-based devices under similar conditions. Our results highlight the advantage of combined experimental and theoretical tools in understanding intrinsic material properties and guiding device fabrication.

Materials and structures for the electron transport layer of efficient and stable perovskite solar cells

The electron transport layer plays a vital function in extracting and transporting photogenerated electrons, modifying the interface, aligning the interfacial energy level and minimizing the charge recombination in perovskite solar cells. This review summarizes the recent research progress on electron transport materials of metal oxides, organic molecules and multilayers. The doped metal oxides as electron transport materials in regular perovskite solar cells show improved device performance relative to their non-doped counterpart due to enhanced electron mobility and energy level alignment. The non-fullerene organic electron transport materials with better electron mobility and tunable energy level alignment need to be further designed and developed despite their advantages of mechanical flexibility and wide range tunability. The multilayer electron transport materials are suggested to be an important direction of research for efficient and stable perovskite solar cells because of their favorable synergistic interaction.

Theoretical studies on 4H-cyclopenta[2,1-b:3,4-b′]dithiophene-based Windmill-shaped nanogrids with low reorganization energies

A new kind of organic charge transport nanomolecule, 4H-cyclopenta[2,1-b:3,4-b′]dithiophene-based windmill-shaped nanogrid called WG-CPDT, and its corresponding derivatives substituted by ethynyl, cyano, fluorine and methyl groups at 1-position of CPDT moiety, was designed and studied theoretically. The density functional theory (DFT) was then employed to investigate the molecular structures and electronic properties such as molecular orbitals, ionization potentials (IPs), electron affinities (EAs), reorganization energies (λ). Furthermore, the noncovalent interaction analysis and the normal mode analysis were utilized to estimate the intramolecular weak interactions and the contributions of each vibrational modes to reorganization energies, respectively. The results indicate that WG-CPDT possesses a small reorganization energy in the electron transfer process, and that WG-CPDT is a kind of potential organic electron transport material. This is especially notable when WG-CPDT is modified with cyano on the 1-position of the CPDT moiety and the electron reorganization energies decrease to 0.099 eV. In addition, the delocalization of LUMO near C(sp3) position is consistent with the S⋯π and CH⋯π weak interactions, which favors good electron transport properties. This work provides new insights into a type of bulky macrocycle in charge transport materials.

Electron‐Transport Materials in Perovskite Solar Cells

AbstractPerovskite solar cells (PSCs) based on organic–inorganic hybrid materials are a rising technology offering an alternative to silicon‐based solar cells. Electron‐transport materials (ETMs) are important for PSCs and have received much attention. Here, first, the development of the structure of PSCs, from which the ETM requirements would be derived, is briefly discussed. Second, the progress of ETMs in mesoscopic PSCs, as well as regular (n–i–p) and inverted (p–i–n) planar PSCs is surveyed and analyzed, in terms of the material requirements, inorganic ETMs, organic ETMs, and interfacial materials. Third, the advancement of PSCs without ETMs is discussed. Finally, a summary and outlook on the current challenges and future development of ETMs in PSCs is given.

Lead-free (CH3NH3)3Bi2I9 perovskite solar cells with fluorinated PDI films as organic electron transport layer

Fluorinated perylene diimide (FPDI) was used as organic electron transport material in lead-free bismuth based perovskite solar cells fabricated by sequential vacuum vapor deposition. The influences of convenient solvent treatments and substrate temperature on the morphology and structure of methyl-ammonium bismuth (III) iodide (MA3Bi2I9) perovskite films were studied and the properties of the according solar cells were investigated. It was found that the power conversion efficiency of the device was 0.06% when the electron transport layer (ETL) of FPDI treated by solvent vapor annealing with chloroform and the substrate temperature at 75 °C. The device exhibits limited performance degradation after 17 days' storage in ambient atmosphere. The results provide the probability of utilizing FPDI as organic electron transport material in bismuth based perovskite solar cells with low toxicity and high stability.

Appending triphenyltriazine to 1,10-phenanthroline: a robust electron-transport material for stable organic light-emitting diodes

There has been an increasing demand for high-performance and cost-effective organic electron-transport materials for organic light-emitting diodes (OLEDs). In this contribution, we present a simple compound 3-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-1,10-phenanthroline through the facile Pd-catalyzed coupling of a triphenyltriazine boronic ester with 3-bromo-1,10-phenanthroline. It shows a high Tg of 112 °C. The ultraviolet photoelectron spectroscopy measurements reveal a deep HOMO level of −6.5 eV. The LUMO level is derived as −3.0 eV, based on the optical bandgap. The low-temperature solid-state phosphorescent spectrum gives a triplet energy of ∼2.36 eV. n-Doping with 8-hydroxyquinolatolithium (Liq, 1:1) leads to considerably improved electron mobility of 5.2 × 10−6–5.8 × 10−5 cm2 V−1 s−1 at E = (2–5) × 105 V cm−1, in contrast with the triarylphosphine oxide-phenantroline molecular conjugate we reported previously. It has been shown that through optimizing the device structure and hence suppressing polaron-exciton annihilation, introducing this single Liq-doped electron-transport layer could offer high-efficiency and stable phosphorescent OLEDs.

UV- and NIR-Protective Semitransparent Smart Windows Based on Metal Halide Solar Cells

In this study, a solution-processable lead iodide semiconductor having a wide band gap was investigated as a light absorbing material for various organic electron transport materials, in a search for low-cost semiconductor materials allowing the facile fabrication of efficient photovoltaic devices. A Tauc plot suggested a wide intrinsic optical band gap of 2.4 eV for a thin film of PbI2, while X-ray diffraction revealed that the spin-coated PbI2 thin film had a hexagonal crystalline structure with preferable orientation along the (001) plane. The effect of the light intensity on the values of Voc and Jsc was studied to investigate the charge recombination mechanism of fabricated devices. An efficient bifacial solar cell was prepared featuring a thin Ag film sandwiched between BCP and MoO3 layers as a transparent rear electrode. The whole device featuring the BCP/Ag/MoO3 electrode exhibited a maximum transmittance of approximately 60% in the visible region, less than 15% in the UV region, and less than 25% in the NIR region. A power conversion efficiency of 2.19% was achieved for a device featuring an opaque electrode (Ca/Al), while the corresponding device featuring the transparent electrode (BCP/Ag/MoO3) provided values of 0.75% and 0.67% when illuminated from the front and rear, respectively. Thus, wide band gap metal halide materials potentially open up a new path for fabricating efficient and transparent photovoltaic devices having applications as building-integrated smart windows. It also effectively prevents the penetration of UV and NIR light, which is harmful for human health, into the building.

Research progress in electron transport layer in perovskite solar cells

Since perovskite solar cells appeared in 2009, its simple preparation process, high photoelectric conversion efficiency and the characteristic of low cost in preparation process let it become the hot spot of both at-home and abroad. Owing to the constant efforts of scientists, the conversion efficiency of perovskite solar cells is more than 20% now. Perovskite solar cells are mainly composed of conductive glass, electron transport layer and hole transport layer, perovskite layer and electrode parts. This paper will briefly introduce the working principle and working process about the electron transport layer of perovskite solar cells. The paper focuses on aspects such as material types (e.g., inorganic electron transport materials, organic small molecule electron transport materials, surface modified electron transport materials and doped electron transport materials), preparation technology of electron transport layer, the effects of electron transport layer on the photovoltaic performance of the devices, and the electron transport layer in the future research.