Tunable Energy Levels Research Articles

Perylenediimide derivatives with branched imide substituents: Aggregation behaviour and impact on photovoltaic properties

Although organic photovoltaics has witnessed significant growth in their power conversion efficiency still there is need to develop novel electron transport materials that can improve the electron extraction through energy level tuning. Perylenediimide (PDI) is one of the known n-type semiconductor which forms highly ordered self-assembled aggregates by intermolecular interactions. Considering the facile processability of these, herein we report perylenediimide (PDI) derivatives with branched end groups via Michael addition reaction. This work describes the synthesis of two PDI derivatives, i.e., MA-PDI-EDA and MA-PDI-PPD via two-step process and these were further studied for the effect of branched end groups on aggregation behaviour as well as its correlation with the opto-electronic properties. The self-assembling and charge transport properties of the synthesized molecules are fine tuned by branched imide substituent which allows improved performance of these materials as electron transport layer in organic solar cell devices in comparison to their PDI precursors. MA-PDI-EDA forming J-type aggregate showed improved power conversion efficiency in comparison to MA-PDI-PPD undergoing H-type aggregation. Ultrafast transient absorption spectroscopy was also performed for understanding the charge carrier dynamics in the two MA products. A complete rational for the observation is discussed using the theoretical fitting of device I-V characteristics.

Annealing Controls Ultrafast Dynamics of Carrier Production in Organic Photovoltaics Incorporating a Nonfullerene Acceptor

Nonfullerene acceptors have emerged as leading candidates in organic photovoltaics pushing past some of the limitations of their fullerene counterparts with absorption extending in the visible and NIR, decreased air-sensitivity, and tunable energy levels. Significant progress has been made in demonstrating the potential of nonfullerene acceptor-based devices to reach high efficiencies; however, a photophysical and mechanistic understanding of charge generation in these nonfullerene acceptors lags behind. In particular, the effects of annealing on carrier production have not been previously examined. Here, we use transient absorption spectroscopy and atomic force microscopy to examine the effect of solvent vapor annealing on a perylene diimide-based nonfullerene acceptor, PDI-DPP-PDI. We find that when mixed with the donor PTB7-Th, the effect of solvent vapor annealing on PDI-DPP-PDI is to reduce geminate recombination, leading to balanced exciton generation and charge separation.

Organic Hole-Transport Layers for Efficient, Stable, and Scalable Inverted Perovskite Solar Cells.

Hole-transporting layers (HTLs) are an essential component in inverted, p-i-n perovskite solar cells (PSCs) where they play a decisive role in extraction and transport of holes, surface passivation, perovskite crystallization, device stability, and cost. Currently, the exploration of efficient, stable, highly transparent and low-cost HTLs is of vital importance for propelling p-i-n PSCs toward commercialization. Compared to their inorganic counterparts, organic HTLs offer multiple advantages such as a tunable bandgap and energy level, easy synthesis and purification, solution processability, and overall low cost. Here, recent progress of organic HTLs, including conductive polymers, small molecules, and self-assembled monolayers, as utilized in inverted PSCs is systematically reviewed and summarized. Their molecular structure, hole-transport properties, energy levels, and relevant device properties and resulting performances are presented and analyzed. A summary of design principles and a future outlook toward highly efficient organic HTLs in inverted PSCs is proposed. This review aims to inspire further innovative development of novel organic HTLs for more efficient, stable, and scalable inverted PSCs.

Designing easily synthesizable non-fused small acceptors for organic solar cells

A new strategy of implementing intramolecular noncovalent interactions (INCIs) has been adopted to design readily synthesizable acceptors for enhancing power conversion efficiency (PCE) and interfacial modifications of organic solar cells (OSCs). Computational approaches like density functional theory (DFT) have contributed to enterprise and analyze in what manner distinct chromophore components interact to produce optical transitions as well as orbital energy levels. This research focuses on a thorough and general overview of five designed conjugated molecular systems (BTZM1-BTZM5) possessing non-covalent conformational locking as well as evaluations using DFT. After incorporating INCIs (SO, SN, and S-F) into structured backbones, the resultant acceptor materials (BTZM1-BTZM5) showed tunable energy levels, reduced band gaps, enhanced absorption maximum, minimum binding energy, improved dipole moments and charge mobilities. Upon coupling BTZM5 acceptor with P3HT donor the blended film created a uniform surface and tighter π-π stacking ensuing VOC of 2.20 eV. Various computational analysis of molecules has been conducted to further confirm the nature of interactions such as TDM, DOS, FMOs, and RDG. Thus, the occurrence of many INCIs to closely lock the conformation of organic π-conjugated molecules is essential to improve the effectiveness of OSCs.

Design of Pyrene-Based Small Molecules Constructed Organic Photocatalyst with Tunable Energy Level and Adjustable Electron Transfer for Efficient Photocatalytic CO2 Reduction

Organic semiconductor materials recently have been widely used in photocatalytic CO2 reduction due to their diverse synthesis methods, good light absorption, and tunable band structure, while their uncontrollable synthesis processes and exact structures have hindered the study for the relationships of structure–property over these organic photocatalysts. Herein, a series of pyrene-based conjugated small molecules were synthesized via the strategy of precise design. The conjugation of these pyrene-based molecules was modified by the introduction of unsaturated acetylene bonds and the benzene rings through classic cross-coupling reactions, which induces the different localization of photogenerated carriers on themselves and achieves an enhanced intermolecular electron transfer to the catalytic center of metal complexes. Based on this strategy, 1,3,6,8-tetraphenylpyrene (TPPy) exhibits a remarkable performance of 0.96 mmol g–1 h–1 with a selectivity of 79% in photocatalytic CO2 reduction, while 1,6-diphenylpyrene (BPPy) possesses an enhanced selectivity of 94% but a halved performance in photocatalytic CO2 reduction (0.44 mmol g–1 h–1) compared to the TPPy. The further characterizations and in situ studies reveal that the performance and selectivity of photocatalytic CO2 reduction are highly dependent on the optimized electron transfer and the modified energy levels, respectively. The purpose of this work is to not only promote the development of organic photocatalysts to reduce CO2 emissions but also provide new vision for the design of pyrene-based photocatalysts with tunable energy band levels and tunable electron transfer.

Grain boundaries passivation of high efficiency and stable perovskite photodetector by polyoxometalate-based composite SiW 11@ZIF-8

Polyoxometalates (POMs) can enable energy level tuning to match perovskite layers. They are considered electronic bridges to modify perovskite and improve the performance of photovoltaic devices. Therefore, in the present work, we dispersed the vacancy POMs K₈[α-SiW₁₁O₃₉]·H₂O ({SiW₁₁}) in the metal-organic frameworks (MOFs) to modify the perovskite layer. {SiW₁₁} could adjust the energy level between the layers of the perovskite photodetector. Moreover, the hydrogen bonds formed between SiW₁₁@ZIF-8 and perovskite effectively passivated the grain boundaries (GBs) of the perovskite layer. X-ray diffraction spectroscopy (XRD) showed that the crystallinity of perovskite was significantly improved. In addition, scanning electron microscopy (SEM) images demonstrated that the average size of perovskite grains increased from 254.50 to 719.27 nm, proving the effective passivation of the GBs. Furthermore, a series of tests such as infrared spectroscopy (IR), N₂ adsorption/desorption isotherms, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) also proved that {SiW₁₁} could be successfully loaded into the pores of ZIF-8 through electrostatic interactions. The photocurrent of the SiW₁₁@ZIF-8 doped device reached 41.95 μA, about three times as high as that of the blank device (14.41 μA). Also, under unencapsulated conditions, it could still maintain more than 90% stability for nearly 700 h. This work demonstrates the potential application of POM@MOF-type composites in the field of perovskite photodetectors.

Environmentally compatible 3‐dimensional star‐shaped donor materials for efficient organic solar cells

Small molecule (SM)-based star-shaped materials are getting tremendous attention to be employed as hole transport materials for organic/perovskite solar cells (OSCs) due to their tunable energy levels, intense absorption ability, and their three-dimensional (3D) charge transport properties. Herein, four-armed SM donors (BD1-BD10) with benzodithiophene (BDT) as the central-core unit have been efficiently designed and then characterized theoretically to investigate their optical and optoelectronic behavior. These four-armed designed star-shaped (BD1-BD10) materials exhibited deeper HOMO levels and higher extinction coefficients, which tend to offer better phase separation morphology during blend formation. These molecules (BD1-BD10) and reference (R) have been fully characterized theoretically with advanced quantum chemical approaches. The photophysical and optoelectronic characteristics have been investigated with density functional theory (DFT) and time-dependent (TD-DFT) calculations. The alignment of frontier molecular orbitals (FMOs), optical characteristics, open-circuit voltages, the density of states (DOS), transition density matrix (TDM), and reorganization energies of holes and electrons in these materials have been investigated. BD3 shows the highest absorption (λmax) of 571.33 nm, with an optical bandgap at 3.30 eV, respectively. Moreover, a complex study of BD3/PC61BM unveiled the process of remarkable charge shifting at the donor-acceptor interface. Therefore, our proposed strategy is a prerequisite for designing desirable photovoltaic molecules for efficient organic/perovskite SCs and light-emitting diodes (LEDs).

Impact of charge character on anionic cyanine-based organic salt photovoltaics

Small bandgap organic compounds with absorption in the near-infrared are exciting materials for a variety of applications ranging from light harvesters in photovoltaics to active agents in photodynamic therapy. Organic salts, a class of small molecule organic compounds comprised of an ionic chromophore and a counterion, have been used in opaque and transparent photovoltaics, primarily as donor materials in bilayer architectures. They possess excellent molecular extinction coefficients with near-infrared selective absorption, adjustable bandgaps, and tunable energy levels. To approach organic salt photovoltaics from a new perspective, we fabricated devices with an unexplored group of anionic salts comprised of a near-infrared absorbing chromophore paired with a varying number of cationic counterions. We observed different donor and acceptor decay trends in external quantum efficiencies that allowed us to separate and independently quantify exciton diffusion and charge transfer for each salt. Increased charge character on the chromophore greatly improves hole transport, as anions with a net −3 charge have charge collection lengths greater than four times those of corresponding singly charged chromophores. This presents an interesting platform for independent quantification of exciton diffusion and charge transport of an active material in a single photovoltaic device and demonstration of the important role of charge on the chromophore. The dependence of charge transport capabilities on charge character of the chromophore will be a useful tool in the design of future organic salts to engineer materials for higher efficiency transparent photovoltaics.

A series of selenium-containing non-fullerene acceptors with side chain engineering for organic solar cells

Non-fullerene acceptors (NFAs) have been crucial in improving the efficiency of organic solar cells (OSCs) due to their attractive properties, such as strong light-harvesting ability, easily tunable energy levels, and high crystallinity. Nevertheless, general NFAs have limited light absorption, and studies on the design of NFAs that can absorb a broad range of the solar spectrum and that offer complementary absorption to that of donors should be conducted. Because selenium, a chalcogen element, can induce strong intermolecular Se⋯Se interactions, planarity, and extended effective conjugated length in OSC materials, NFAs with selenium usually exhibit redshifted absorption. Here, we synthesized a series of selenium-containing non-fullerene acceptors with different inner alkyl chain lengths, named Se-Y6-BO (2,2'-((2Z, 2′Z)-((12,13-bis(2-butyloctyl)-3,9-dinonyl-12,13-dihydroselenopheno[2″,3'':4′,5']thieno[2′,3':4,5]pyrrolo[3,2-g]selenopheno[2′,3':4,5]thieno [3,23,2-b] [1,2,5]thiadiazolo [3,43,4-e]indole-2,10-diyl)bis(methanylylidene))bis(5,6-dichloro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) and Se–Y6-HD (2,2'-((2Z, 2′Z)-((12,13-bis(2-hexyldecyl)-3,9-dinonyl-12,13-dihydroselenopheno[2″,3'':4′,5']thieno[2′,3':4,5]pyrrolo[3,2-g]selenopheno[2′,3':4,5]thieno [3,23,2-b] [1,2,5]thiadiazolo [3,43,4-e]indole-2,10-diyl)bis(methanylylidene))bis(5,6-dichloro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile). The optical absorption of two selenium-containing NFAs was redshifted compared with that of Y6 due to the effect of selenium atoms in the backbone. The PM6:Se-Y6-BO-based devices achieved a power conversion efficiency (PCE) of 13.8% with an open-circuit voltage (VOC) of 0.81 V, short-circuit current density (JSC) of 24.4 mA cm−2, and a fill factor of 0.70. Similarly, the OSCs based on PM6:Se–Y6-HD achieved a PCE of 13.6% and a JSC of 23.7 mA cm−2.

Mixed valence Ce-doped TiO2 with multiple energy levels and efficient charge transfer for boosted SERS performance

Considering the variable valence characteristics of rare earth elements, they can be in a variety of valence forms coexistence. Doping of rare earth element with different valence states may produce different energy levels to tune the semiconductor energy band structure. We utilize rare earth element Ce doping TiO2 for the development of high-performance semiconductor surface-enhanced Raman scattering (SERS) substrates based on an energy-level tuning strategy. Ce doping not only forms multiple energy levels including Ce3+ and Ce4+ metal doping energy levels in the bandgap of TiO2, but also enriches the surface state level of TiO2 itself, which together promote the separation of photogenerated carriers and improve charge transfer efficiency between substrates and absorbed molecules. This endows TiO2 semiconductor substrate with a higher SERS enhancement factor, which can reach 2.2 × 106. The detectable concentration of methylene blue can be as low as 10−10 mol/L. Moreover, the semiconductor substrate exhibits excellent uniformity and stability. This study not only provides a new strategy to develop excellent semiconductor SERS substrate with multiple energy levels, but also lays the foundation for promising practical application of semiconductor substrate.

Engineering Periodic Dinuclear Lanthanide‐Directed Networks Featuring Tunable Energy Level Alignment and Magnetic Anisotropy by Metal Exchange (Small 22/2022)

Lanthanide Multinuclear Networks In article number 2107073, Sofia O. Parreiras, David Écija, and co-workers demonstrate the tuning of the electronic and magnetic properties of dinuclear lanthanide metal–organic networks by metal exchange while preserving the same structural architecture. The exchange between Er and Dy metallic centers leads to a shift in the energy level alignment and allows the tailoring of both the intensity and orientation of magnetic anisotropy.

NiOx Nanocrystals with Tunable Size and Energy Levels for Efficient and UV Stable Perovskite Solar Cells

AbstractSolution‐processed nickel oxide nanocrystals (NiOx NCs) ink can be facilely applied to deposit NiOx thin films as the hole transport layer (HTL) for perovskite solar cells (PSCs). Both the efficiency and stability of the corresponding PSCs depend significantly on the size and the energy levels of the as‐synthesized NiOx NCs; however, previous studies have shown that these two aspects can be hardly controlled synchronously to maximize the device performance. Herein, a novel synthesis of highly dispersed NiOx NCs is demonstrated by employing tetraalkylammonium hydroxides (TAAOHs, alkyl = methyl, ethyl, propyl, butyl) as precipitating bases, where the varied alkyl chain lengths of TAAOHs enable the size control of the NiOx NCs and the subsequent altering of their Ni3+ contents, leading to tunable energy levels of the NiOx thin films. With the longest butyl chain, the smallest crystal size and the optimal energy level alignment at the NiOx/perovskite interface are achieved. After further passivating the detrimental Ni3+ species on the surface of NiOx HTL, a remarkable power conversion efficiency (PCE) approaching 23% is obtained, which is one of the highest PCEs reported for NiOx‐based inverted PSCs. Furthermore, the unencapsulated device exhibits excellent ultraviolet stability, which maintains ≈87% of its PCE after 200 h exposure.

Engineering Periodic Dinuclear Lanthanide-Directed Networks Featuring Tunable Energy Level Alignment and Magnetic Anisotropy by Metal Exchange.

The design of lanthanide multinuclear networks is an emerging field of research due to the potential of such materials for nanomagnetism, spintronics, and quantum information. Therefore, controlling their electronic and magnetic properties is of paramount importance to tailor the envisioned functionalities. In this work, a multidisciplinary study is presented combining scanning tunneling microscopy, scanning tunneling spectroscopy, X-ray absorption spectroscopy, X-ray linear dichroism, X-ray magnetic circular dichroism, density functional theory, and multiplet calculations, about the supramolecular assembly, electronic and magnetic properties of periodic dinuclear 2D networks based on lanthanide-pyridyl interactions on Au(111). Er- and Dy-directed assemblies feature identical structural architectures stabilized by metal-organic coordination. Notably, despite exhibiting the same +3 oxidation state, there is a shift of the energy level alignment of the unoccupied molecular orbitals between Er- and Dy-directed networks. In addition, there is a reorientation of the easy axis of magnetization and an increment of the magnetic anisotropy when the metallic center is changed from Er to Dy. Thus, the results show that it is feasible to tune the energy level alignment and magnetic anisotropy of a lanthanide-based metal-organic architecture by metal exchange, while preserving the network design.

Toward high-performance organic photovoltaics: the new cooperation of sequential solution-processing and promising non-fullerene acceptors.

Organic photovoltaics (OPVs) have long been a hot topic due to their light weight, low cost, and flexibility. Simple blend-based OPVs have sufficient donor/acceptor (D/A) interfaces and high exciton dissociation efficiency, which result in certified high power conversion efficiency (PCE) exceeding 18%. However, the difficult morphology control and poor device stability limit further progress toward higher PCE and future application. Sequential solution-processing with tunable vertical phase distribution, D/A interfaces, and charge transportation pathways not only benefit device stability but can also overcome the up-scaling challenge. In recent years, the development of non-fullerene acceptors (NFAs) has been very rapid, which is attributed to their tunable energy levels, bandgaps, planarity, and crystallinity. In this minireview, the opportunities for the cooperation of sequential solution-processing and NFAs are revealed based on their characteristics, such as diverse molecular shapes, abundant functional groups and heteroatoms, and various aggregation states for NFAs; independent active layer processing, controllable D/A interfaces, and excellent device stability for sequential solution-processing. Few but important existing examples are discussed to display the prospects of sequential solution-processed fullerene-free OPVs toward high PCE, good device stability, high semitransparency, and large-area industrial manufacture. Finally, some possible research directions are predicted and the main issues that need to be overcome are proposed for sequential solution-processed fullerene-free OPVs toward higher performance.

Energy level tuning at the interface of inorganic and organic semiconductors

Optimizing the functionality of semiconductor heterojunctions

Opportunities for energy level tuning at inorganic/organic semiconductor interfaces

The aim of this Perspective is to provide an overview of approaches that can be employed to tune the energy level alignment at interfaces between inorganic and organic semiconductors for use in electronic and optoelectronic devices. The approaches include tailoring intramolecular dipolar bond distribution, controlling molecular orientation at interfaces, and the insertion of a molecularly thin interlayer that abruptly shifts the electrostatic potential between the two semiconductors and, thus, affords level tuning. With these state of the art methods, the frontier energy levels at an inorganic/organic heterojunction can be varied up to ca. 3 eV, i.e., covering the energy gap of most semiconductors. By combining two or more of these approaches or by employing interfacial molecular switches, it is envisioned that unconventional and dynamically switchable interfacial energy level scenarios can be created, enabling expanded or superior device functionality.

Polyarylether-Based 2D Covalent-Organic Frameworks with In-Plane D-A Structures and Tunable Energy Levels for Energy Storage.

The robust fully conjugated covalent organic frameworks (COFs) are emerging as a novel type of semi‐conductive COFs for optoelectronic and energy devices due to their controllable architectures and easily tunable the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO) levels. However, the carrier mobility of such materials is still beyond requirements due to limited π‐conjugation. In this study, a series of new polyarylether‐based COFs are rationally synthesized via a direct reaction between hexadecafluorophthalocyanine (electron acceptor) and octahydroxyphthalocyanine (electron donor). These COFs have typical crystalline layered structures, narrow band gaps as low as ≈0.65 eV and ultra‐low resistance (1.31 × 10−6 S cm−1). Such COFs can be composed of two different metal‐sites and contribute improved carrier mobility via layer‐altered staking mode according to density functional theory calculation. Due to the narrow pore size of 1.4 nm and promising conductivity, such COFs and electrochemically exfoliated graphene based free‐standing films are fabricated for in‐plane micro‐supercapacitors, which demonstrate excellent volumetric capacitances (28.1 F cm−3) and excellent stability of 10 000 charge–discharge cycling in acidic electrolyte. This study provides a new approach toward dioxin‐linked COFs with donor‐acceptor structure and easily tunable energy levels for versatile energy storage and optoelectronic devices.

Clusterization-Triggered Color-Tunable Room-Temperature Phosphorescence from 1,4-Dihydropyridine-Based Polymers.

A series of poly(1,4-dihydropyridine)s (PDHPs) were successfully synthesized via one-pot metal-free multicomponent polymerization of diacetylenic esters, benzaldehyde, and aniline derivatives. These PDHPs without traditional luminescent units were endowed with tunable triplet energy levels by through-space conjugation from the formation of different cluster sizes. The large and compact clusters can effectively extend the phosphorescence wavelength. The triplet excitons can be stabilized by using benzophenone as a rigid matrix to achieve room-temperature phosphorescence. The nonconjugated polymeric clusters can show a phosphorescence emission up to 645 nm. A combination of static and dynamic laser light scattering was conducted for insight into the structural information on formed clusters in the host matrix melt. Moreover, both the fluorescence and phosphorescence emission can be easily tuned by the variation of the excitation wavelength, the concentration, and the molecular weight of the guest polymers. This work provides a unique insight for designing polymeric host-guest systems and a new strategy for the development of long wavelength phosphorescence materials.

Toward High‐Performance Semitransparent Organic Photovoltaics with Narrow‐Bandgap Donors and Non‐Fullerene Acceptors

AbstractSolar energy offers an alternative solution to the global community's growing energy demands. Semitransparent organic photovoltaics (ST‐OPVs) have received tremendous attention due to their tunable energy levels and rising power conversion efficiency (PCE). Because of its transparency, ST‐OPVs are able to serve as the power‐generating roof of the greenhouse, and color‐tunable walls/windows for modern buildings or façades. With the rapid development of narrow‐bandgap semiconductors to absorb near‐infrared photons, the performances of ST‐OPVs has progressed with PCEs over 12% with average visible transmittances over 20%. Here, recent developments in ST‐OPVs based on narrow‐bandgap donors and non‐fullerene acceptors are reviewed. Several strategies for chemical structures design have been reported to lower bandgaps semiconductor materials. The recent developments of non‐fullerene acceptor structures for ST‐OPVs are categorized into A‐D‐A, A‐π‐D‐π‐A, and A‐DAD‐A by the structure alignment. From device perspectives, the strategies such as ternary blend, distributions of donors and acceptors in active layers, tandem and transparent conductive electrodes for high‐performance ST‐OPVs are summarized. To conclude, some insightful guidelines for future developments in ST‐OPVs from both materials and device points of views are provided.

Solution-processed flexible nonvolatile organic field-effect transistor memory using polymer electret

Flexible organic field-effect-transistor (OFET) memory is one of the promising candidates for next-generation wearable nonvolatile data storage due to its low price, solution-processability, light-weight, mechanically flexibility, and tunable energy level via molecular tailoring. In this paper, we report flexible nonvolatile OFET memory devices fabricated with solution-processed polystyrene-brush electret and organic semiconductor blends of p-channel 6, 13-bis-(triisopropylsilylethynyl)pentacene (TIPS-PEN) and n-channel poly-{[N,N′-bis(2- octyldodecyl)-naphthalene-1,4,5,8-bis-(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P-(NDI2OD-2T); N2200). Fabricated flexible OFET memory devices exhibited high memory window (30 V) and ON/OFF current ratio (memory ratio) over 103. Furthermore, we obtained reliable memory ratio (~103) over retention time of 108 s, 100 times of repeated programming/erasing cycles, and 1000 times of bending tests at a radius of 3 mm.