Low-lying HOMO Energy Levels Research Articles

Computational study of new small molecules D-A based on triphenylamines for bulk heterojunction solar cells (BHJ)

In this computational study, four donor–acceptor (D–A) small molecules were constructed using indacenodithiophene triphenylamine (IDTTPA) as the donor, with modifications made to the acceptor units intended for use in organic bulk heterojunction (BHJ) solar cells. Their electrical and optical properties were determined through density functional theory (DFT) and time-dependent DFT (TD-DFT) techniques. To ascertain the impact of acceptor group modifications on intramolecular electron delocalization and light absorption capabilities, several key variables were examined. Based on the results, it was determined that molecule SM4 demonstrated the best performance among the designed compounds. It exhibited a maximum wavelength of 594 nm, a narrow energy gap of 1.91 eV, a low-lying HOMO energy level of −4.512 eV in its absorption spectra, and a theoretical power conversion efficiency (PCE) of 8 %. This research provides valuable insights for the development of efficient D-A compounds for use in organic solar cells.

Efficient Ternary Organic Solar Cells with Suppressed Nonradiative Recombination and Fine-Tuned Morphology via IT-4F as Guest Acceptor.

The large open circuit voltage (VOC) loss is currently one of the main obstacles to achieving efficient organic solar cells (OSCs). In this study, the ternary OSCs comprising PM6:BTP-eC9:IT-4F demonstrate a superior efficiency of 18.2 %. Notably, the utilization of the medium bandgap acceptor IT-4F as the third component results in an exceptionally low nonradiative recombination energy loss of 0.28 V. The desirable energy level cascade is formed among PM6, BTP-eC9, and IT-4F due to the low-lying HOMO and LUMO energy levels of IT-4F. More importantly, the VOC of PM6:BTP-eC9:IT-4F OSCs can reach as high as 0.86 V, which is higher than both binary OSCs without sacrificing JSC and FF. Besides, this strategy proved that IT-4F can not only broaden the absorption range but also work as a morphology modifier in PM6:BTP-eC9:IT-4F OSCs, and there also exists efficient energy transfer between BTP-eC9 and IT-4F. This result provides a promising way to suppress the nonradiative recombination energy loss and realize higher VOC than the two binary OSCs in ternary OSCs to obtain high power conversion efficiencies.

A novel [1,2,4]triazolo[1,5-a]pteridine framework: Synthesis, photophysical properties and material studies

The synthesis of novel [1,2,4]triazolo[1,5-a]pteridine (TPt) derivatives was carried out starting from [1,2,4]triazolo[1,5-a]pyrimidine-6,7-diamines and 1,2-dicarbonyl compounds. For obtained TPt DFT/TD-DFT calculations were performed with subsequent complex photophysical and electrochemical studies. The substituent in the pyrazine ring of TPt framework proved to have a higher effect on photophysical properties than that in the triazole ring. The fluorescence quantum yields of studied compounds were generally low (3–23%). However, it was shown that some of TPt was sensitive to trace amounts of organic peroxides (about 100 ppm), which will make it possible to create test systems based on them. On the other hand, low-lying HOMO energy levels and the energy gap values of TPt (2.63 eV < E0-0 < 2.97 eV) as well as the electron mobility in solid-state films measured by the conventional CELIV technique (approximately 10−4 cm2 V−1 s−1) make these materials promising for the further use in organic electronic devices.

Fluorination strategy on π-bridge of polymer donor for efficient photovoltaic performance

Although fluorine-contained polymers have been adopted in efficient polymer solar cells, complicated reaction methods and costly fluorination reagents have impeded large-scale application. Here, a novel fluorinated polymer donor, PM6-F, was designed and synthesized via a newly developed 3-fluoro-2-iodithiophene to simplify synthesis procedures and minimise reaction costs from modest raw materials. Compared with the polymer analogue PM6 without any fluorine substituent on the main chain, it was found that PM6-F with a better configuration exhibited more orderly molecular packing, stronger and broader absorption, a narrower optical band gap, and a lower-lying HOMO energy level. The PM6-F:IT-4F-based PSCs obtained an excellent PCE of 14.90% with a VOC of 0.91 V, a JSC of 23.25 mA cm−2, and an FF of 70.42%. Notably, the value of 14.90% was among the highest recorded in the literature for IT-4F-based PSCs. This research demonstrated that modifying π-bridges with fluorine is a promising strategy for developing high-efficiency polymer donors.

Functionalized Benzothiadiazole Non-Fused A-D-A'-D-A Small Molecules for Effective Electron Mobilities and Metal-free Photocatalysis.

Three non-fused A-D-A'-D-A π-conjugated metal-free small molecules BT-IT1, BT-IBT2 and BT2F-IBT3 have been synthesized and their absorption characteristics and redox properties, as well as charge carrier mobilities have been investigated. The resultant molecules exhibited broad absorption in the range of 325-600 nm in solutions and in thin films the absorption range is 350-700 nm. These new conjugated small molecules showed low-lying HOMO energy levels (-5.49 to -5.50 eV), deep LUMO energy levels (-3.67 to -3.69 eV) and narrow bandgaps of (Eg CV ) ∼0.79 eV. Appreciable electron mobilities for compounds BT-IBT2 and BT2F-IBT3 were 4.56×10-3 and 1.12×10-3 cm2 V-1 s-1 , respectively, obtained by space-charge limited current (SCLC) measurements. Additionally, excellent photocatalytic performances of BT-2T (6), BT2F-2T (10), BT-IT1, BT-IBT2 and BT2F-IBT3 for highly selective oxidation of thioanisole to desired sulfoxides under visible light irradiation were observed.

Single junction binary and ternary polymer solar cells-based D–A structured copolymer with low lying HOMO energy level and two nonfullerene acceptors

The power conversion efficiency of the ternary PSCs (16.32%) is higher than that for binary counterparts, i.e., 13.16% and 12.62% for P(DTB-BDD):DBTBT-IC and P(DTB-BDD):Y6, respectively.

Tetrathienopyrrole-based hole-transporting materials for highly efficient and robust perovskite solar cells

Developing coplanar π-extended backbone is essential for simultaneously achieving high hole transport capability and thermal stability for hole-transporting material (HTMs). In this work, the large coplanar tetrathienopyrrole (TTTP) building block is employed to construct two novel linear organic HTMs (WH01 and WH02). With respect to the dithieno[3,2-b:2′,3′-d]pyrrole core, the extended π-conjugation of the TTTP endows low-lying HOMO energy levels, improved interfacial contact and defect passivation, as well as enhanced hole extraction/transport capacity for TTTP-based HTMs. As a result, the TTTP core shows good compatibility to either planar or twisted donor. Perovskite solar cells (PSCs) based on both WH01 and WH02 realize power conversion efficiencies (PCEs) of around 21%. The optimized PSCs adopting WH01 exhibit a champion PCE of 21.54%, which significantly outperforms the reference M130 (18.85%). Furthermore, the un-encapsulated devices with TTTP-series HTMs demonstrate excellent operational stability under ambient air. This systematic study highlights that TTTP-based molecules are promising potential candidates as HTMs for achieving highly efficient and stable PSCs.

Ester-functionalized quinoxaline-based polymers for application in organic photovoltaics

Quinoxaline (Qx) is an interesting building unit for π-conjugated polymers as it allows us to easily control the electronic property by selecting the substituents. In this study, we synthesized ester-functionalized Qx (QE) and its polymers with octyl (PTQE-C8) and 2-ethylhexyl groups (PTQE-EH). Due to the electron-withdrawing property of the ester group, QE had strong electron-deficient nature and thereby the polymers had relatively low-lying HOMO energy levels of around −5.5 eV and relatively small optical bandgaps of narrower than 1.6 eV. It was found that PTQE-C8 consistently exhibited higher photovoltaic performance than PTQE-EH in both cells based on fullerene (PC61BM) and non-fullerene (Y6), in which the efficiencies were 5.8% and 9.2%, respectively. We ascribed the higher performance of PTQE-C8 compared to PTQE-EH to the higher crystallinity of the materials in the blend films. Interestingly, we also found that the polymers functioned as n-type materials. This work demonstrates that QE can be an interesting building unit for both p-type and n-type polymers.

Novel wide bandgap benzodithiophene-based polymer donors with electron-withdrawing indolin-2-one side chains for efficient organic solar cells with high open circuit voltage

Two new π-conjugated polymers with donor backbone and π-conjugated indolin-2-one side chains, PBDTTI and PBDTTIF, are designed and synthesized as wide bandgap donors for non-fullerene acceptor-based organic solar cells (OSCs). The monomers containing electron accepting indolin-2-one side chains, (Z)-3-((2,5-dibromothiophen-3-yl) methylene)-1-methylindolin-2-one (M1) and (Z)-3-((2,5-dibromothiophen-3-yl) methylene)-5-fluoro-1-methylindolin-2-one (M2), can be easily synthesized via Knoevenagel condensation between 2,5-dibromothiophene-3-carbaldehyde and 2-oxindole or 5-fluoro-2-oxindole, respectively. Stille coupling polymerization of the electron donating benzodithiophene (BDT)-containing monomer 1,1′-[4,8-bis [5-(2-ethylhexyl)-2-thienyl]benzo [1,2-b:4,5-b′]dithiophene-2,6-diyl]bis [1,1,1-trimethylstannane] and M1 or M2 produced PBDTTI or PBDTTIF, respectively. The strong electron accepting π-conjugated 1-methylindolin-2-one and 5-fluoro-1-methylindolin-2-one side chains can achieve low-lying HOMO energy levels of −5.59 eV for PBDTTI and −5.60 eV for PBDTTIF, which is beneficial for realizing high open circuit voltage (VOC) of the resulting OSCs. On the other hand, since the electron acceptor units are on the side chains, the polymer backbone containing only electron donor units could maintain wide bandgaps of 1.91 eV and 1.89 eV for PBDTTI and PBDTTIF, respectively. When PBDTTI and PBDTTIF were used as donors and a small bandgap non-full acceptor ITIC as an acceptor, the resulting OSCs devices achieved VOC of 0.97 and 1.00 V, short circuit current densities (JSC) of 15.60 and 13.70 mAcm-2, and fill factors (FF) of 0.60 and 0.59, resulting in power conversion efficiencies of 8.00 and 7.70%, respectively.

Fluorination of the π-bridge in a polymer skeleton enables a significant improvement in photovoltaic performance

An easy and efficient approach for the synthesis of an important photoelectric intermediate 3-fluoro-2-iodithiophene is developed. Therefrom, a polymer J52FF, featuring fluorination of the π-bridges on the backbone and side chains of the classical polymer, J52, is synthesized. J52FF displays a coplanar molecular configuration with the dihedral angles smaller than 2° in skeleton, a low-lying HOMO energy level of −5.62 eV, which is nearly aligned with the acceptor IT-4F, and a broad absorption range from 300 nm to 650 nm. The J52FF:IT-4F (1:0.8) blend shows a strong and complementary spectral absorption, a uniform fibrillar textured morphology and high and balanced charge mobilities. The inverted device based on J52FF:IT-4F blend obtained an encouraging power conversion efficiency (PCE) of 14.83% with the open-circuit voltage (Voc) of 0.90 V, the short circuit current (Jsc) of 23.21 mA cm−2, and the fill factor (FF) of 0.71 under the optimal conditions, which is one of the best PCE value among the ever reported BDT-alt-BTz derivatives or IT-4F based non-fullerene organic solar cells (NF–OSCs). It demonstrates that fluorination of polymer in skeleton is an efficient strategy for obtaining high performance.

Insight into the impact of the substituent modification on the photovoltaic performance of ferrocenyl chalcones based DSSCs

The design, synthesis, spectroscopic, computational and solar cell performance studies of two new ferrocenyl chalcones (Fc-O1 and Fc-S2) were synthesized by the Claisen-Schmidt condensation reaction. In the synthesized molecules, the enone moiety acts as an efficient π-bridge that links the donor ferrocene derivative with methyl-furan (Fc-O1) and bithiophene (Fc-S2) substituents which permits good viability in fine-tuning the intramolecular charge transfer (ICT) and optical absorption spectrum. The chemical structure of the compounds have been elucidated by proton and carbon NMR as well as FTIR spectroscopies. The comparison study between the structures of these ferrocenyl chalcones were done by XRD and computational investigations. The substitution of bithiophene derivative on the Fc-S2 revealed the improvement of the structure planarity as compared to slightly twisted structure of Fc-O1. Fc-O1 and Fc-S2 exhibited large maximum absorption wavelength within the UV–Vis region (~485 ​nm), suggesting their potential as dye sensitizer in DSSC application. The compounds also possessed suitable energy levels of HOMO and LUMO for utilization as sensitizer in DSSC to ensure the process of electron injection and dye regeneration. A good ICT due to its planarity, large absorption maximum in visible range and low-lying HOMO energy level of Fc-S2 give rise to the higher conversion efficiency in DSSC study as in comparison to Fc-O1.

Naphthalenothiophene imide-based polymer exhibiting over 17% efficiency

Summary Developing novel electron-deficient monomers is crucially important for the future development of non-fullerene organic solar cells. Here, we report a new electron-deficient monomer naphthalenothiophene imide (NTI) and NTI-based copolymers PNTB and PNTB-2T. The strong electron-withdrawing ability of NTI could effectively lower the HOMO level of the polymer that facilitates the tuning of the polymer’s physical properties while simultaneously maintaining the low-lying HOMO value. Polymer chains modification by introducing extra thiophenes results in the closed π−π stacking and ordered polymer chain packing of PNTB-2T and is proven to be an effective strategy to increase the device performance. Thus, solar cell devices for PNTB-2T:Y6 exhibit an efficiency of 16.72%, which is further enhanced to 17.35% by the third component PC71BM, whereas that for PNTB:Y6 is only 3.81%. Importantly, PNTB-2T-based devices exhibit excellent batch-to-batch reproducibility. These features make this family of polymer donors very promising and the design strategies will enlighten future development of high-performance polymer donors for organic solar cells (OSC).

Impact of Chalcogenophenes on Donor-Acceptor Copolymers for Bulk Heterojunction Solar Cells

Three new selenophene-based conjugated copolymers having different ratios of the monomeric units were designed, synthesized and thoroughly characterized. The introduction of an electron-poor and surfaced building moiety like selenathiazole was highly efficient in tuning the bandgap and polymer properties. The chalcogenophene-based medium-bandgap polymers demonstrated low-lying HOMO energy levels (∼5.87 eV), which is benign for use in multi-junction polymer solar cell applications. The representative polymers with heavy atoms revealed the change in electronegativity and atomic size that highly affected the molecular property, its topological features, and photovoltaic properties in polymer solar cells. The selenium-substituted (0.5:0.5) polymer donors showed power conversion efficiencies above 3% when combined with [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) acceptors in a quintessential bulk-heterojunction solar cell.

N-Type Thin-Film Transistors Based on Diketopyrrolopyrrole Derivatives: Role of Siloxane Side Chains and Electron-Withdrawing Substituents.

The physical properties, packing, morphology, and semiconducting performance of a planar π-conjugated system can be effectively modified by introducing side chains and substituent groups, both of which can be complementary to the π framework in changing the intermolecular association, frontier molecular orbital energy, optical band gap, and others. We herein show that installation of end-capped electron-withdrawing groups (EWGs), such as dicyanovinyl (-DCV), 3-ethylrhodanine (-RD), and 2-(3-oxo-indan-1-ylidene)-malononitrile (-INCN), together with siloxane side chains to the backbones of dithienyldiketopyrrolopyrrole (DPPT), such as DPPT-Si-DCV, DPPT-Si-RD, and DPPT-Si-INCN, can greatly improve its solubility, air stability, and film morphology to serve as an n-channel in thin-film transistor fabrication. The EWGs attached to the DPPT core narrowed the optical band gap (Egopt) and changed the highest occupied molecular orbital and the lowest unoccupied molecular orbital energies (EHOMO and ELUMO), making them suitable for n-channel field-effect transistor (FET) applications. The benefits of introducing siloxane side chains to the DPPT core include enhanced solubility, low crystallization barrier, enantiotropic phase behavior, and much improved FET performance. The DPPT-Si-INCN film displayed low-lying HOMO (-5.82 eV) and LUMO (-4.60 eV) energy levels and an optical band gap as low as 1.22 eV, all of which suggest that this derivative can be quite resistant toward aerial oxidation. Thin films of these derivatives were prepared by the solution-shear method. A comparison of the solution-sheared films indicated that the molecular packing motif of DPPT-Si-INCN film was somehow different from that of DPPT-Si-DCV and DPPT-Si-RD, in which the π-π stacking tended to align orthogonally to the shearing direction. This specific π-π stacking alignment could have an impact on the electron mobility (μe) values in transistors based on the solution-sheared films.

Indacenodithieno[3,2-b]thiophene-Based Wide Bandgap D-π-A Copolymer for Nonfullerene Organic Solar Cells.

Herein, a wide-bandgap (2.02 eV) donor-π-acceptor (D-π-A) polymer PIDTT-DTffBTA, composed of a rigid indacenodithieno[3,2-b]thiophene (IDTT) and fluorinated benzo[d][1,2,3]triazole (ffBTA) units as D and A units, respectively, is synthesized. In comparison with its analogue benzodithiophene-alt-benzotriazole copolymer J52 with classic benzodithiophene (BDT) as the D unit, PIDTT-DTffBTA demonstrates a lower-lying HOMO energy level and higher carrier mobilities when paired with a nonfullerene acceptor (NFA) Y6 based on a ladder-type dithienothiophen[3.2-b]-pyrrolobenzothiadiazole central unit. Thus, PIDTT-DTffBTA:Y6 based organic solar cells (OSCs) exhibit an improved power conversion efficiency (PCE) of 11.05% than that of J52:Y6 (7.15%), which is also the highest value for IDTT-based photovoltaic polymers. This result proves that the IDTT unit is also a promising building block to construct not only NFAs but also p-type photovoltaic polymers.

Dopant-free benzothiadiazole bridged hole transport materials for highly stable and efficient perovskite solar cells

D-A-D typed hole transport materials have been designed and synthesized with benzothiadiazole acceptor unit by introducing electron-withdrawing group fluorine atoms and electron-donating group alkoxy as side group. It is found that the three hole transport materials have a low-lying HOMO energy level and higher hole mobility. Furthermore, when these materials are applied in perovskite solar cell, the higher hole mobility (1.02 × 10−3 cm2 V−1 s−1) make the B3, which the fluorine atoms on core acceptor without dopants, get a power conversion efficiency of 12.1%, while spiro-OMeTAD based dopant-free perovskite solar cell only shows a 7.96% efficiency. In addition, the long-term stability of the perovskite solar cells based all the three hole transport materials improved greatly, especially B1-based cells show a favorable long-term stability, which keeps a 90% initial efficiency after 10 days at a relative humidity of 30%. Therefore, the D-A-D typed hole transport materials have a good application prospects in the field of perovskite solar cells with the incorporation of appropriate side group.

Syntheses, structures and properties of asymmetric thiophene-containing pentacene-like heteroacenes organic semiconductors

Two thiophene-containing pentacene-like semiconductors, benzo[d]naphtho[2,1-b]thiophene (BNT) and benzo[d]phenanthro[9,10-b]thiophene (BPT), were synthesized in a new route, and they were characterized by means of ultraviolet–visible absorption spectrum, thermo- and electro-chemistry analysis. Single crystals were obtained by vapor diffusion of solvent to analyze the correlation between charge transport and structure properties. The large band gaps and low-lying HOMO energy levels could result in much better stability. It was observed that the molecular packing in the herringbone motif is similar to that in pentacene, which is the benchmark of organic semiconductors. The BNT and BPT molecules form columnar π-stacking along the crystallographic a-axis direction with an interplanar separation of 3.535 and 3.442 Å, respectively, which suggests that the molecules adopt compact packing in solid state. The distances indicate the presence of π−π interactions along the intermolecular stacking axis in the two materials. Face-to-face columnar π-stacking increases the effective dimensionality of the electronic structure and benefits charge transport. Multicolor emission property was observed for both BNT and BPT thin films.

Multi-Donor Random Terpolymers Based on Benzodithiophene and Dithienosilole Segments with Different Monomer Compositions for High-Performance Polymer Solar Cells

We synthesized and characterized a series of new random terpolymers which are composed of two electron-rich segments, thienyl-substituted benzo[1,2-b:4,5-b’]dithiophene (BDT) and dithieno[3,2-b:2’,3’-d]silole (DTS), and one electron- deficient segment, thieno[3,4-c]pyrrole-4,6-dione (TPD), with different ratio of DTS segment to BDT segment (25%, 50%, and 75% DTS). The compositional effect of the random terpolymers on physicochemical properties and photovoltaic performances were studied. The different compositions of BDT and DTS segments in the conjugated backbone of the terpolymers had a crucial effect on the electrochemical properties of the random terpolymers. PTPD-BDT75-DTS25 (P1) with monomeric composition of BDT and DTS (75:25) segments showed excellent light harvesting ability, high charge carrier mobility, and low-lying HOMO energy level. Moreover, the photovoltaic performance of the random terpolymer based BHJ PSCs was strongly influenced by the composition of BDT and DTS segments in the conjugated backbones of the terpolymers. The inverted BHJ PSCs based on the PTPD-BDT75- DTS25 exhibited a V oc of 0.94 V, a J sc of 10.83 mA/cm2, and a FF of 56.52%, leading to a high PCE of 5.78%.

STFTYT: A simple and broadly absorbing small molecule for efficient organic solar cells with a very low energy loss

Abstract A simple and new A-π-D-π-A small molecule denoted as STFTYT was designed and synthesized employing spiro[cyclopenta[1,2-b:5,4-b′]dithiophene-4,9′-fluorene] (STF) as the central donor (D) unit, ethylhexylthiophene as the π-conjugated bridge, indenedione as the acceptor (A) unit, for application as an electron donor material in solution-processable organic solar cells (OSCs). Results demonstrate that the target material exhibits an incredibly broad absorption extended to 813 nm, a small bandgap of 1.53 eV, and a low-lying HOMO energy level of −5.26 eV. Theory calculations reveal that the shortened π-bridge is beneficial to improving molecular planarity and enhancing the intramolecular charge transfer (ICT) effect between the donor and acceptor groups, and ultimately contributing to a broad absorption and small bandgap. Bulk-heterojunction OSCs based on STFTYT as donor and PC71BM as acceptor were prepared and the photovoltaic properties were optimized. A considerable power conversion efficiency (PCE) of 6.17% was achieved along with an extremely high open-circuit voltage (Voc) of 0.97 V and a very low energy loss (Eloss) of 0.56 eV.

Tuning the central donor core via intramolecular noncovalent interactions based on D(A-Ar)2 type small molecules for high performance organic solar cells

Two narrow band-gap small molecules with D(A-Ar)2 framework, namely DMPh(DPP-Py)2 and DFPh(DPP-Py)2, were designed and synthesized for high-performance small molecule organic solar cells (SMOSC), in which the 1,4-dimethoxybenzene (DMPh) and 1,4-difluorobenzene (DFPh) were employed as rigid donor cores, respectively, and the pyrenere (Py) unit is selected as terminal-capping groups on an electron-deficient diketopyrrolopyrrole (DPP)-based linear backbone. The impacts of the fluorine-sulfur (F⋯S) atoms and oxygen-sulfur (O⋯S) atoms noncovalent interaction on their absorption spectra, molecular energy levels, morphological properties, hole mobilities and photovoltaic properties were investigated thoroughly. The fluorinated DFPh(DPP-Py)2 possess a relatively lower-lying HOMO energy level, better miscibility of the blend with PC71BM, as well as higher mobility in comparison with those of the methoxyled DMPh(DPP-Py)2. As a consequence, the OSCs devices based on DMPh(DPP-Py)2 and DFPh(DPP-Py)2 exhibited PCEs of 5.47% and 7.54%, respectively. Obviously, the device based on DFPh(DPP-Py)2 presented a better performance, which should be ascribed to the improved simultaneously Voc of 0.77 V, Jsc of 15.3 mA cm−2, and FF of 64%. The results indicated that the choice of the fluorination designation on the molecular backbone is an effective approach to develop D(A-Ar)2 type small molecule donors for highly efficient solar cell applications.