Highest Occupied Molecular Orbital Level Research Articles

Asymmetric Wide-Bandgap Polymers Simultaneously Improve the Open-Circuit Voltage and Short-Circuit Current for Organic Photovoltaics.

A trade-off between open-circuit voltage (V OC ) and high short-circuit (J SC ) becomes one of the most vital problems limiting further improvement in polymer solar cells' (PSCs) efficiency. In this work, two asymmetric polymer donors PBDT-F-2TC and PBDT-SF-2TC are designed and synthesized. When blended with a state-of-the-art acceptor IT-4F with low lowest-unoccupied molecular orbital level, simultaneously high V OC (up to 0.94V) and J SC (up to 20.73mA cm-2 ) are obtained for both copolymers. Note that the V OC value of 0.94V is the highest value of PSCs based on IT-4F reported so far. The simultaneously improved V OC and J SC in resulting devices are discovered from the deep highest-occupied molecular orbital levels (-5.5 to -5.7eV) and the hyperchromic effect of the polymers, the small driving force, and the small energy loss during the charge transfer, due to the synergistic effect of asymmetric carboxylate unit and fluorine/sulfur atoms. More importantly, thanks to the asymmetric 2TC, both PBDT-F-2TC- and PBDT-SF-2TC-based PSCs can be successfully processed by non-halogenated solvent 1,2,4-trimethylbenzene (TMB) to yield device efficiencies of 10.29% and 10.39%, respectively, which are the maximum values for non-fullerene PSCs fabricated using the eco-friendly solvent TMB.

NIR Emission and Acid-Induced Intramolecular Electron Transfer Derived from a SOMO–HOMO Converted Non-Aufbau Electronic Structure

Some organic radicals violate the Aufbau principle and have unique electronic structures in which the energy level of the singly occupied molecular orbital (SOMO) is formally lower than that of the highest occupied molecular orbital (HOMO). We synthesized a novel organic radical with a SOMO–HOMO converted electronic structure, TPA-R•, a novel donor–acceptor hybrid of triphenylamine (an electron donor), and a stable polychlorinated diphenyl(4-pyridyl)methyl radical (an electron acceptor). TPA-R• exhibited fluorescence in the near-infrared region (λmax = 910 nm) in cyclohexane from a polar intramolecular charge-transfer excited state. Cyclic voltammetry, absorption spectroscopy, and density functional theory calculation revealed the inversion of the SOMO and HOMO levels in the electronic structure of TPA-R•. Addition of trifluoromethanesulfonic acid to TPA-R• caused a two-step change. Protonation initially occurred on the diphenylpyridylmethyl radical moiety to form TPA-[RH]•+. This protonation induced larg...

Introduction of Fluorine Into spiro[fluorene‐9,9′‐xanthene]‐Based Hole Transport Material to Obtain Sensitive‐Dopant‐Free, High Efficient and Stable Perovskite Solar Cells

Despite the substantial development of efficient hole transporting materials (HTMs) for high‐performance perovskite solar cells (PSCs), optimization of the HTMs to sensitive‐dopant‐free HTMs for high efficient PSCs with prominent stability have rarely been reported. Herein, a low‐cost fluorinated spiro[fluorene‐9,9′‐xanthene] based HTM termed 2mF‐X59 is designed and synthesized. In comparison with its reported nonfluorinated counterpart X59, 2mF‐X59 shows lowered highest occupied molecular orbital (HOMO) level, improved hole mobility, and hydrophobicity. Aided by 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) to further lower the HOMO level of 2mF‐X59 and improve its hole transfer, the optimized 2mF‐X59 based PSCs show a maximum power conversion efficiency (PCE) of 18.13% without the use of any sensitive‐dopants (e.g., lithium salt/4‐tert‐butylpyridine), which is comparable to the Spiro‐OMeTAD based PSCs (18.22%) with sensitive dopants. More importantly, the sensitive‐dopant‐free 2mF‐X59 based PSCs maintain 95% of their initial performance for more than 500 h under air exposure, showing much better long‐term stability than control PSCs based on Spiro‐OMeTAD with sensitive dopanst. This is the first case that a sensitive‐dopant‐free HTM is demonstrated in PSCs with a high PCE (>18%) and good stability by optimizing the literature HTM. This work could pave a new way to develop low‐cost sensitive‐dopant‐free HTMs for highly efficient and stable PSCs for practical applications.

Dual hole transport layers for blue-light-emitting PLED: Suppress the formation of exciplex towards high device performance and color purity

The realization of high-performance blue polymer light-emitting diodes typically requires a hole transport layer to facilitate hole injection, primarily due to the deep highest occupied molecular orbital (HOMO) energy levels of the blue light-emitting polymers. However, the widely-used hole transport layers (HTLs) are prone to induce exciplex with longer wavelength emission when cooperating with those n-type blue-light-emitting layers, due to the HOMO and lowest unoccupied molecular orbital (LUMO) energy levels offsets. To avoid formation of exciplex and improve device performance of blue light-emitting, we employed two hole-transport layers consisting of a cross-linkable poly(indenofluorene-co-triphenylamine) copolymer and an additional poly(9-vinylcarbazole) (PVK) layer. The devices with dual HTLs have an enhanced luminous efficiency of 7.67 cd A−1 while keep the electroluminescent spectrum identical to the device based on PVK devices. The resulting blue light-emitting device based on such two-layer HTL exhibited remarkable stability, with luminous efficiency remained about 6.6 cd A−1 at a luminance of 10000 cd m−2. Our results shown that the dual HTLs device structure is a promising strategy for constructing high performance solution-processed blue polymer light-emitting diodes.

Elucidating the impact of N-arylanilino substituents of squaraines on their photovoltaic performances

Abstract Squaraines have attracted increasing research attention for organic photovoltaic applications because of their facile and low-cost synthesis, and intense and broad absorption in the visible and near-infrared (Vis-NIR) regions. Herein, three new squaraines, N-phenylanilino substituted SQ-EP, N-2-naphthylanilino substituted SQ-EN, and N-fluorenylanilino substituted SQ-EF, were developed for improving the hole mobility of aniline-based squaraines. The substitution effect of N-arylanilino groups on the optoelectronic properties of squaraines was investigated. These squaraines exhibited similar absorbance spectra, HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels, and hole mobilities in their neat films. However, compared to their neat films, the hole mobility of the SQ-EP-based BHJ blend film showed the smallest decrease when blending with PC71BM ([6,6]-phenyl C71 butyric acid methyl ester). Besides, morphological studies have shown that a phase separation of 20–30 nm can only be observed in the SQ-EP/PC71BM blend film. Consequently, a solution-processed bulk-heterojunction (BHJ) organic photovoltaic (OPV) device fabricated with the as-cast SQ-EP/PC71BM blend film exhibited obviously higher power conversion efficiency (PCE: 5.4%) than those of the SQ-EN/PC71BM (PCE: 4.3%) or SQ-EF/PC71BM systems (PCE: 2.8%). This is one of the highest recorded PCE values in aniline-based squaraines single-junction BHJ-OPV devices. This much-enhanced PCE can be attributed to the significant increases in short-circuit current density (Jsc) and fill factor (FF) of the SQ-EP/PC71BM-based OPV device.

How to stabilize the HOMO levels and to improve the charge transport properties of hole-transporting materials? Introducing a symmetrical core unit

Designed with a symmetrical core naphthatetrathiophene (NTT) and diphenylamine (DPA)-based side arms, a series of novel organic small molecule hole-transporting materials are simulated for perovskite solar cells (PSCs) by using density functional theory (DFT) and Marcus theory of electron transfer. As a fundamental understanding, the energy level alignments and the charge transport properties are explored for their potential applications. Our results show that, by introducing a symmetrical NTT core and regulating the terminal groups, the highest occupied molecular orbital (HOMO) levels of new predicted molecules are obviously down-shifted from -4.96 eV to -5.25 eV, which is beneficial for enlarging the open circuit voltage of PSCs. Moreover, we also find that rigidifying the core structure can make the HOMO down-shift and the lowest unoccupied molecular orbital (LUMO) up-shift simultaneously. On the other hand, the quasi-planar molecular architecture and the delocalized frontier molecular orbitals can effectively enhance the electronic coupling between adjacent molecules, and coupled with the lower reorganization energies, the hole mobilities of molecules 3–5 are clearly increased with the extension of the π-conjugated core. In summary, through systemic investigations, several potential candidates are proposed toward more efficient PSCs, and we hope that our work could provide some new clues for the rational design of organic small molecule materials.

Blue Phosphorescent Platinum Complexes Based on Tetradentate Bipyridine Ligands and Their Application to Organic Light-Emitting Diodes (OLEDs)

Blue phosphorescent Pt(II) complexes with 2,3′-bipyridine-based tetradentate C^N chelating ligands have been designed and synthesized. To investigate the effect of the substitution of an electron-donating/withdrawing unit on the solid-state structure and photo- and electroluminescence, either fluorine or methoxy substituents were incorporated into the C-coordinating pyridine ring. The molecular structures of all the complexes were fully characterized by different spectroscopic methods, including X-ray diffraction analysis. There are no Pt(II)···Pt(II) intermolecular interactions in crystal packing. Pt(II) complexes bearing fluorine substituents emit bright blue phosphorescence with λmax 460 nm, while methoxy-functionalized Pt(II) complexes display red-shifted and bluish green emissions with λmax 490 nm. The electron-withdrawing group at the C-coordinating pyridine ring produced a deeper HOMO (highest occupied molecular orbital) level in comparison to its electron-donating counterpart. However, the substit...

A1‐A2 Type Wide Bandgap Polymers for High‐Performance Polymer Solar Cells: Energy Loss and Morphology

Introducing electron‐withdrawing groups onto donor‐acceptor (D‐A) type conjugated materials is a commonly used method for lowering their highest occupied molecular orbital (HOMO) energy level to achieve higher open circuit voltage (Voc) in polymer solar cells (PSCs). However, this method is rather costly due to the tedious synthesis and low yield involved in preparing the target monomers. Here, a novel design concept of using two different acceptor units to construct acceptor1‐acceptor2 (A1‐A2) type polymers with a deep HOMO level is proposed. Two A1‐A2 type wide bandgap (WBG) polymers, PB24‐3TDC and PB68‐3TDC, were designed for PSCs. The developed polymers possess proper energy levels and complementary absorption with an efficient electron acceptor ITIC‐Th. More importantly, by slightly regulating the alkyl side‐chains, molecular stacking and photoluminescence (PL) emission energy loss of polymers can be alternated significantly. As a result, tuned Voc from 0.9 to 1.0 V and short‐circuit current (Jsc) from 9.4 to 17.0 mA cm−2 can be achieved. The device based on PB24‐3TDC:ITIC‐Th exhibits a higher power conversion efficiency (PCE) of 10.3% compared to PB68‐3TDC:ITIC‐Th based device with a PCE of 7.88%. These results show that the design concept of A1‐A2 type polymer donors have great potential for blending with non‐fullerene acceptors for achieving high performance PSCs.

The effects of electronic structures of four benzodithiophene-based copolymers on their photovoltaic performances

The electronic and structural properties of the four copolymers, PBDPQU, PBDPBT, PBDPDPP, and PBDPTP, are investigated by density functional theory and time-dependent density functional theory. The research purpose is to rationalize the relationship between observed experimental performances and structural properties. The properties that can affect open-circuit voltage (VOC) and short-circuit current (JSC) are comprehensively investigated, which refer to absorption spectra, frontier molecular orbital, charge transport properties, and exciton dissociation abilities at the copolymer/PCBM interface. The results indicate that the favourable photovoltaic performances of PBDPBT are due to large VOC, lower the highest occupied molecular orbital (HOMO), and small effective mass (m*). Furthermore, the unfavourable performance of PBDPTP mainly originates from the higher HOMO level and weak ability of exciton dissociation at interface, while the intermediate properties aforementioned for PBDPQU and PBDPDPP determine their intermediate photovoltaic performances. In conclusion, the theoretical results agree well with the experimental ones.

Donor–Acceptor Conjugated Polymers Based on Bisisoindigo: Energy Level Modulation toward Unipolar n-Type Semiconductors

Four bisisoindigo (bis-IID) derivatives containing different numbers of electron withdrawing fluorine (F) or nitrogen (N) atoms and eight alternating copolymers of them with 2,2′-bithophene (BT) or 3,3′,4,4′-tetrafluoro-2,2′-bithiophene (4FBT) were synthesized. Depending on the numbers of F and N atoms incorporated, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the polymers were feasibly tuned in the ranges of −5.51 to −6.06 eV and −3.64 to −4.01 eV, respectively. Top gate and bottom contact (TGBC) organic thin-film transistors (OTFTs) were fabricated to study the semiconducting properties of the polymers. Five polymers, i.e., P0F-BT, P2F-BT, P4F-BT, P2N2F-BT, and P0F-4FBT that contain 0–4 F and N atoms in the repeating unit, with HOMO and LUMO energy levels above −5.90 and −3.80 eV, respectively, all showed ambipolar OTFT characteristics. Three other polymers P2F-4FBT, P4F-4FBT, and P2N2F-4FBT that contain 6 or 8 F and N atoms with HOMO ...

Bithiazole-based copolymer with deep HOMO level and noncovalent conformational lock for organic photovoltaics

To match with the state-the-art-of non-fullerene acceptor for polymer solar cells (PSCs), it is urgent to develop high performance wide-bandgap (WBG) copolymer donors with deep highest occupied molecular orbital (HOMO) level and highly planar backbone to further promote the device efficiency. A new WBG copolymer PBDTBTz, poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithi ophene))-alt-(5,5’-(-4,4’-dinonyl-2,2’-bithiazole))], which is based on benzodithiophene (BDT) as donor unit and 4,4’-dinonyl-2,2’-bithiazole (BTz) as acceptor unit is prepared for non-fullerene PSCs. Due to the strong electron-withdrawing effect, BTz unit can dramatically lower the HOMO level of PBDTBTz to −5.60 eV. Notably, double noncovalent conformational locks (N⋯S) of BTz are formed in the backbone to reduce the steric hindrance and favor a highly planar geometry for efficient charge transport and molecular packing. As a result, the device based on PBDTBTz as donor and 3,9-bis(2-methylene-((3-(1,1-dic-yanomethylene)-6,7-difluoro)-indanone))-5,5,11, 11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d']-s-indaceno[1,2-b:5,6-b']dithiophen e(IT-4F) as acceptor afforded a high open circuit voltage (Voc) of 0.92 V, which is the highest value for IT-4F-based PSCs reported so far. Furthermore, the device operated well with a negligible driving force (ΔEHOMO) of as small as 0.06 eV. These results revealed that combination of electron-withdrawing bithiazole and double noncovalent conformational lock of N⋯S is a promising molecular design concept of polymer donor with deep and planar structure for high performance PSCs.

Bis(carboxylate) substituted benzodithiophene based wide-bandgap polymers for high performance nonfullerene polymer solar cells

To develop wide bandgap donor materials for high performance nonfullerene polymer solar cells (NF PSCs), bis(carboxylate) substituted benzo[1,2-b:4,5-b]dithiophene (BDT) weak electron acceptor (A) unit was used to polymerize with 5-alkylthiophene-2-yl-substituted BDT or 5-alkylthiothiophene-2-yl-substituted BDT donor unit to construct donor-acceptor (D-A) alternative polymers PBDT-BDTC and PBDTS-BDTC, respectively. PBDT-BDTC and PBDTS-BDTC exhibit wide bandgaps over 1.90 eV and low lying highest occupied molecular orbital (HOMO) levels below −5.50 eV. The absorption and energy levels of these two polymers match well with those of the typical low bandgap acceptor ITIC-Th, resulting in a good complementary absorption from 300 to 900 nm and a low HOMO level offset of about 0.15 eV. NF PSCs with an inverted device structure were fabricated and power conversion efficiencies (PCEs) higher than 7% were achieve. Furthermore, PBDT-BDTC based PSCs exhibit much better photovoltaic performance than PBDTS-BDTC based ones after the treatment with 0.5% 1,8-diiodooctane (DIO). The optimized devices gave a PCE of 8.32% with an open-circuit voltage (Voc) of 0.92 V, a short-circuit current density (Jsc) of 13.91 mA/cm2, and a fill factor (FF) of 65.21%, indicating that PBDT-BDTC is a promising polymer donor for NF PSCs. To the best of our knowledge, a PCE of 8.32% is the highest record for bis(carboxylate) substituted BDT based polymers.

Recent Progress in Fused-Ring Based Nonfullerene Acceptors for Polymer Solar Cells.

The progress of bulk-heterojunction (BHJ) polymer solar cells (PSCs) is closely related to the innovation of photoactive materials (donor and acceptor materials), interface engineering, and device optimization. Especially, the development of the photoactive materials dominates the research filed in the past decades. Photoactive materials are basically classified as p-type organic semiconductor donor (D) and an n-type organic semiconductor acceptor (A). In the past two decades, fullerene derivatives are the dominant acceptors for high efficiency PSCs. Nevertheless, the limited absorption and challenging structural tunability of fullerenes hinder further improve the efficiency of PSCs. Encouragingly, the recent progresses of fused-ring based A-D-A type nonfullerene acceptors exhibit great potential in enhancing the photovoltaic performance of devices, driving the power conversion efficiency to over 13%. Such kind of nonfullerene acceptors is usually based on indacenodithiophene (IDT) or its extending backbone core and end-caped with strong electron-withdrawing group. Owing to the strong push-pulling effects, the acceptors possess strong absorption in the visible-NIR region and low-lying HOMO (highest occupied molecular orbital) level, which can realize both high open-circuit voltage and short-circuit current density of the devices. Moreover, the photo-electronic and aggregative properties of the acceptors can be flexibly manipulated via structural design. Many strategies have been successfully employed to tune the energy levels, absorption features, and aggregation properties of the fused-ring based acceptors. In this review, we will summarize the recent progress in developing highly efficient fused-ring based nonfullerene acceptors. We will mainly focus our discussion on the correlating factors of molecular structures to their absorption, molecular energy levels, and photovoltaic performance. It is envisioned that an analysis of the relationship between molecular structures and photovoltaic properties would contribute to a better understanding of this kind of acceptors for high-efficiency PSCs.

How to stabilize the HOMO levels and to improve the charge transport properties of hole-transporting materials? Probing the effects of molecular symmetry

Abstract Considering the effects of molecular symmetries, a series of novel organic small molecule hole-transporting materials are simulated by using density functional theory (DFT) and Marcus theory of electron transfer. As a fundamental understanding, the energy level alignments and the charge transport properties are explored for their potential applications. Our results show that, compared with the dissymmetric cases, the symmetric molecules exhibit obvious advantages with more deep energy levels, more delocalized frontier molecular orbitals, and more blue-shifted absorption spectra. By adding oxygen-bridge and sulfur-bridge in the core unit of spiro-OMeTAD, the highest occupied molecular orbital (HOMO) levels of new designed molecules are obviously down-shifted from −5.08 eV to −5.20 eV, whereas the linked nitrogen-bridge makes the HOMOs up-shifted due to its strong electron-donating capacity. Meanwhile, our results also indicate that the delocalized frontier molecular orbitals in symmetric cases can effectively enhance the electronic coupling between adjacent molecules, and coupled with the lower reorganization energies, the high hole mobilities are obtained. In addition, inserting linked atoms in core unit or changing molecular symmetry only have slightly influences on the aspects of electron-hole dissociation, solubility and stability. Through systemic investigations, several promising candidates are proposed toward more efficient PSCs, and we hope that our work could provide some clues for the experimentalists to design and synthesize new small molecule materials.

SYNTHESIS OF NEW NITROBENZYLIDENE DERIVATIVES AND PYRROLE-BASED COPOLYMERS FOR DYE-SENSITIZED SOLAR CELLS: EFFECT OF SUBSTITUENT ON OPTO-ELECTRICAL PROPERTIES OF DIP-COATED THIN FILMS

The mastery of the optoelectronic properties of conjugated copolymers by substituting their radicals is a promising way for increasing the light absorption and charge career transport in the organic devices active layer. In this paper, we present the chemical synthesis of four different conjugated benzaldehyde derivatives and pyrrole-based copolymers (P–P:B) followed by their conception in thin films on glass substrates by dip coating root from a solution in dichloromethane. UV–Vis measurements showed absorption in good part of the visible region, with an optical gap around 2 eV. Morphological properties observed by scanning electron microscope of the four P–P:B based thin films illustrated homogenous and continuous surfaces with roughness and surface shape that can be modulated according to Nitrobenzylidene derivative that contains the copolymer. First oxidation ([Formula: see text]) and reduction ([Formula: see text]) potentials of synthetized copolymers have been estimated by cyclic voltammetry which led us to estimate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively. The HOMO and LUMO energy level diagram of P–P:B compared with the most commonly used organic materials as donor/acceptor couples showed a cascade shape, which allowed us to opt for organic solar cells based on multiple active layers in our aim to improve its performance.

An Expansive Exploration for Novel Liquid Electrolytes through Data-Driven Physics-Based Modeling

Electrolyte development plays a key role in the design of current and future battery chemistries. Almost all of the advanced battery technologies such as lithium metal, high voltage cathodes, dual ion batteries, metal air batteries and other beyond lithium ion battery technologies are bottlenecked by the performance of the electrolytes.[i] ,[ii],[iii] There is an increasing need for electrolytes with large electrochemical windows, wide temperature operating windows, very good ionic conductivity for operation at high rates of charge and discharge, low viscosity, good solubility and low flammability and toxicity from a safety standpoint. Thus, there are a large number of constraints on the electrolytes and they are becoming increasingly stringent. To add to the complexity of electrolyte design, there exist multiple trade-offs between all these properties as shown in previous literature.[iv] Although various solid electrolytes, ionic liquids and polymer electrolytes are being researched, organic liquid electrolytes still outperform the other electrolytes when we consider all the properties required. The current approach of new electrolyte design is largely intuition based using certain structural features in the organic molecules. Currently only a few hundred solvents and salts have been demonstrated in all the battery technologies while there are over 20 million organic molecules discovered. The richness of liquid organic compounds is multiplied even further if we start considering mixtures as most of the properties vary non-linearly as you mix two organic compounds. In this work, we suggest a much better data-driven systematic exploration of new electrolytes. The methodology employs determination of various electronic structure features calculated from simple and scalable density functional theory (DFT) calculations and topological features of the molecules. We will identify unique parameters which are non-linear functions of the above features for each of the properties above along with features that govern mixtures. We currently have structures for over 1 million molecules with over 100,000 data points for each property. This gives a unique capability to identify parameters accurately, explore multiple trade-offs and different methods of overcoming these trade-offs. The cathodic stability of the electrolyte can be described in terms of the highest occupied molecular orbital (HOMO) energy level of each of the components. The HOMO level will be estimated using GGA level DFT, quantitative structure-property relationships (QSPR) and higher order theories and comparisons will be shown. The solubility of a salt is governed by the solvation of the individual ions and the dielectric constant of the solvent as shown by the extended Debye- Hückel and other models. We have developed a modified Ising model to describe solvation shell around the Li+ ions, and the model has been solved under mean field approximations to derive the fraction of all species in the Li+ solvation shell.[v] The model can also be extended to describe other cations and anions and is applicable to mixtures. The Ising model coefficients used for the model are the ion-solvent interactions, ion-ion interactions and solvent-solvent interactions can be calculated from DFT. The dielectric constant, important for solubility is correlated to the dipole moment of molecule. This relationship is quite qualitative as in a liquid, the molecules exist as multiple conformers. To provide more quantitative data, we have identified the volume of molecule calculated using DFT along with number of rotatable bonds as additional geometric features to determine the dielectric constant. The dielectric constant along with the solvation energies will be used to give a feature for solubility. The viscosity of an electrolyte is dependent on the solvation shell structure of ions and the viscosity of the solvent which is related to intermolecular interactions and will be explored. We will also show the relationship between the Li+ solvation energy and charge transfer resistance. We believe this electrolyte exploration method is much more systematic, quantitative, scalable and also able to identify outliers to certain trends. This method and database will create a new frontier for electrolyte discovery for most electrochemical systems and also other areas of science. In the future, we will add features for other physical and chemical properties and also extend these to inorganic compounds.

Binary hole transport materials blending to linearly tune HOMO level for high efficiency and stable perovskite solar cells

To maximize the photovoltaic performance of perovskite solar cells (PVSCs）by developing new hole-transport layer (HTL) materials, the precise tuning of their energy levels especially the highest occupied molecular orbital (HOMO) is highly desirable. Here, a simple binary strategy for the first time is proposed to acquire ideal HOMO level by optimizing the composition of binary blend HTLs including CZ-TA (HOMO = −5.170 eV) and CZ-STA (HOMO = −5.333 eV). By adding 10 wt% CZ-STA, the binary HTM (HOMO = −5.199 eV) based perovskite solar cells achieve a maximum power conversion efficiency of 19.85% (18.32% for CZ-TA). The introducing of S atom in CZ-STA not only downshifts HOMO level but also forms stronger Pb-S interaction with perovskites than Pb-O in CZ-TA, leading to better device performance and reduced hysteresis. Importantly, the un-encapsulated PVSCs using CZ-TA:CZ-STA (90:10, w/w) binary HTL exhibit good environment stability in ambient air, maintaining over 82% of their initial efficiency after 60 days’ storage with a relative humidity around 50%. Therefore, this strategy provides new insights on HTL development to push forward the progress of the emerging PVSCs

1,4-Di(3-alkoxy-2-thienyl)-2,5-difluorophenylene: A Building Block Enabling High-Performance Polymer Semiconductors with Increased Open-Circuit Voltages

A new building block, 1,4-di(3-alkoxy-2-thienyl)-2,5-difluorophenylene (DOTFP) with several desirable features such as high backbone planarity, suitably lying highest occupied molecular orbital (HOMO), and good solubility, was developed by inserting an electron-deficient difluorophenylene into the 3,3′-dialkoxy-2,2′-bithiophene (BTOR) unit. Three regioregular D-A1-D-A2 type polymers based on DOTFP and benzothiadiazole (BT) derivatives were synthesized and characterized by comparing with a D–A type BTOR-based polymer. The content of highly electron-rich alkoxythiophene is reduced by half in the DOTFP-based polymers versus that of the BTOR-based polymer analogue, which results in a deeper HOMO level and benefits high open-circuit voltage (Voc) in polymer solar cells (PSCs). Consequently, the DOTFP-ffBT-based solar cells exhibited a significantly improved power conversion efficiency (PCE) of 8.7% and an increased Voc of 0.84 V compared to the BTOR-ffBT-based solar cells with a PCE of 2.6% and a Voc of 0.49 V...

Improved Efficiency of Polymer Solar Cells by Modifying the Side Chain of Wide-Band Gap Conjugated Polymers Containing Pyrrolo[3,4- f]benzotriazole-5,7(6 H)-dione Moiety.

Two novel wide-band gap donor-acceptor-type conjugated copolymers, PTzBI-S and PTzBI-Ph, are designed and synthesized, based on alkylthio-thienyl- or alkylphenyl-substituted benzodithiophene (BDT) derivatives as the electron-donating unit and pyrrolo[3,4- f]benzotriazole-5,7(6 H)-dione as the electron-withdrawing unit. The as-generated copolymers show the comparable optical and electrochemical properties. The alkylthio-thienyl-substituted BDT unit facilities a benign decrease of the highest occupied molecular orbital (HOMO) levels. This consequently enhances open-circuit voltages ( VOC) over 0.9 V in relevant solar cells with the fullerene acceptor ([6, 6]-phenyl-C71-butyric acid methyl ester, PC71BM) or the nonfullerene acceptor (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3- d:2',3'- d']- s-indaceno[1,2- b:5,6- b']dithiophene, ITIC). The combination studies of Fourier transform photocurrent spectroscopy and electroluminescence further rationalize the VOC difference between solar cells with fullerene and nonfullerene acceptors. An impressively high power conversion efficiency of 10.19% is obtained for the device based on PTzBI-Ph:ITIC, outperforming the 8.84% achieved by the PC71BM-based device. Our results demonstrate that the modification of substituents of BDT units can effectively decrease the HOMO level and consequently improve VOC, ultimately allowing the attainment of high-efficiency polymer solar cells.

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.