Low Band Gap Small Molecule Research Articles

Synergistic strategies to unveil the dual influence of additive and annealing treatment on the morphology of isatin-derived low bandgap small molecule donor in solution-processed organic solar cells

A novel organic small molecule, 5-(4-(diphenylamino)phenyl)-1-octylindoline-2,3-dione (TPI) containing an alkyl chain substituted electron accepting isatin (indoline-2,3-dione) and strong electron withdrawing N, N-diphenylaniline units have been synthesized through Suzuki coupling reaction. The molecular structure of TPI is confirmed by NMR, FT-IR, and Mass spectroscopy techniques. The TPI molecule exhibits a low band gap of approximately ∼ 2.03 eV which is also supported by the DFT results. The influence of additive and heat treatment on the electronic, morphological, and photovoltaic performance of organic solar cells was investigated meticulously. The TPI donor exhibits complementary light absorption with fullerene acceptor in combination with DIO and TA which efficiently absorb the photon in the 300–730 nm range. The TPI was later used as an electron donor material for the fabrication of organic solar cells. Remarkably, the morphology-dependent optimized TPI:PC71BM (1:3.0) exposed excellent power conversion efficiency (PCE) of about ∼ 5.56 % with enhanced VOC of ∼ 0.811 V, JSC of 11.81 mA cm−2 and FF of 55.3 % fabricated in addition with DIO and thermal annealing treatment. Most importantly, the unencapsulated devices maintained > 96 % of the initial PCE value after 240 h under ambient temperature. Our findings indicate that isatin-derived small molecules with simple architecture propose great potential for promising BHJ-OSCs material.

Precise Side Chain Engineering Enabling High‐Performance Wide‐Bandgap Polymer Donors

AbstractA series of wide bandgap (WBG) conjugated polymers PTFBDT‐C, PTFBDT‐O and PTFBDT‐S are designed and synthesized with alkyl, alkoxy and thioalkyl side chains. By selecting benzodithiophene derivatives as the donor units and benzothiadiazole derivatives as the acceptor units, PTFBDT‐C, PTFBDT‐O and PTFBDT‐S are all of wide optical bandgaps. Furthermore, low bandgap small molecule L8‐BO‐4F is used as the acceptor to fabricate organic solar cells (OSCs). According to the results, PTFBDT‐C:L8‐BO‐4F blend film displays appropriate phase separation, high hole and electron mobilities, etc. Thus, the power conversion efficiency (PCE) of PTFBDT‐C:L8‐BO‐4F based OSCs is 14.02%, which is much higher than those of PTFBDT‐O:L8‐BO‐4F based (9.06%) and PTFBDT‐S:L8‐BO‐4F based ones (10.45%). These results show that the side chain engineering is an effective strategy to improve photovoltaic performance of wide bandgap polymers.

Small D-π-A-π-D organic dyes for near-infrared emitting OLEDs with excellent external quantum efficiency

Organic-light emitting diodes (OLEDs) with near-infrared (NIR) emission have an extremely broad range of attractive applications. Compounds containing donor (D) and acceptor (A) fragments are promising materials for use as active emitting layer in OLEDs due to their fast luminescence and because their luminescence properties can be fine-tuned at the synthesis stage. In this work, we synthesized and investigated two D-π-A-π-D NIR-emitting dyes based on a new pyridazine-thiadiazole electron acceptor with two different π linkers. The introduction of π linkers, together with the geometry modification, provides an additional redshift of the emission maximum from 700 to 850 nm. These dyes demonstrate moderate quantum yields up to 15% in chloroform solutions and to 4% in the solid state. Spectroscopic studies were supported by Time Dependent Density Functional Theory (TD-DFT) and multireference (XMCQDPT/CASSCF) calculations. A series of OLEDs with electroluminescence at 700 and 850 nm was fabricated. One of the devices exhibited maximum optical power 76 μW/cm2 and maximum external quantum efficiency (EQE) 1.2%, which is close to the theoretical limit calculated for fluorescent NIR OLEDs based on low band gap small organic molecules.

Low-bandgap small molecule acceptors with asymmetric side chains

Low-bandgap ADA small molecule acceptors A1 and A2 with asymmetric side chains show good performance in organic solar cells.

Synthesis of low-bandgap small molecules by extending the π-conjugation of the termini in quinoidal compounds

A feasible strategy to construct low-bandgap π-conjugated small molecules was proposed.

Synthesis and photovoltaic properties of organic molecules based on difluoroquinoxaline derivatives for OPVs

We designed and synthesized two novel low band gap small molecule donors (M1 and M2) that consist of benzo[1,2-b:4,5-b′]dithiophene as the electron-rich core and 6,7-difluoroquinoxaline as the electron-deficient end-group. M1 and M2 with A-D-A type structure were synthesized via Stille coupling reaction for use in OSCs. The absorption maximum of the M1 and M2 in solid thin films exhibited at 536 and 540 nm and absorption onset at 582 and 592 nm, corresponding to band gaps of 2.31 and 2.10 eV, respectively. Although the absorption of the M1 with branched alkyl chain was blue shifted as compared to M2, the branched alkyl chain in benzodithiophene core provide appropriate solubility to provide good OSC performance.

Organic Solar Cells Based on Non-fullerene Small-Molecule Acceptors: Impact of Substituent Position

Summary Molecular engineering of non-fullerene small-molecule acceptors (SMAs) plays a key role in enhancing the performance of organic solar cells. An effective strategy is to introduce functional groups into SMA end groups to tune the electronic and morphological properties of polymer/SMA blends. Here, molecular dynamics simulations and long-range corrected density functional theory calculations are combined to examine the impact of the position of methoxy substitution in the SMA end groups. As representative systems, blends of the IT-OM small-molecule acceptor with the PBDB-T polymer donor are explored; three different positions of the methoxy substitution of the IT-OM end groups are examined. By considering intermolecular mixing and packing, the energetic distribution of the charge-transfer electronic states, the exciton-dissociation and non-radiative recombination processes, and the electron-transfer rates among adjacent acceptors, we provide a comprehensive molecular-scale rationalization of the significant experimental variations in device performance for PBDB-T/IT-OM-based solar cells as a function of methoxy position.

Effects of optical interference and optimized crystallinity in organic photovoltaic cells with a low-bandgap small molecule fabricated by dry process

Low-bandgap molecules have been used in the photoactive layer of organic photovoltaic (OPV) cells to achieve higher open-circuit voltage and absorb light with longer wavelength for improving power conversion efficiency (PCE). Herein, we report on the effect of the film characteristics of N-(n-hexyl)-tetracene carboxylic acid 5,6-imide-11,12-disulfide (Hexyl-TIDS) as a p-type organic semiconductor by introducing an n-type organic semiconductor C60 as an underlayer. Also, we investigate the effects on the performance of OPV devices by optimizing the device structure. Regarding the inverted structure, the crystallinity of Hexyl-TIDS is decreased by adding C60. The incident photon-to-current conversion efficiency is increased owing to the optical interference effect over the absorption range of C60. Sixfold enhancement of the PCE is achieved due to the optical interference effects and low-crystallinity of Hexyl-TIDS film compared to the conventional structure.

Facile Synthetic Route of a Solution-Processable, Thieno[3,4-c]pyrrolo-4,6-dione-Based Conjugated Small Molecule and Control of the Optoelectronic Properties via Processing Additives

In this study, a new type of low-bandgap small molecule has been synthesized with a thieno[3,4-c]pyrrole-4,6-dione (TPD) derivative for application in bulk heterojunction (BJH) solar cells. The series of solar cells were fabricated by blending the TPD-based small molecule (M1) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). In order to optimize the performance of solar cells, the nanoscale morphologies of the BHJ layers were controlled via processing additives with 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN). Therefore, we demonstrated that the use of CN successively suppressed molecular aggregation and demonstrated suitable phase separation, in addition to increasing the power conversion efficiency from 0.36% to 1.86%.

Indigo-Based Acceptor Type Small Molecules: Synthesis, Electrochemical and Optoelectronic Characterizations.

In this paper, we report design and synthesis of novel low bandgap small molecules, indigo-benzimidazole (Tyr-3) and indigo-schiff base (Tyr-4) type acceptors. In these structures, tert-butoxycarbonyl (t-BOC) group has been attached to indigo nitrogen atom in order to increase the solubility. UV-vis absorption spectra of Tyr-3 and Tyr-4 dyes exhibit wide absorption bands ranging from 350 to 600nm, indicating the relatively low bandgap giving around 2.07eV for each. Excitation of both Tyr-3 and Tyr-4 dyes at 485nm displays characteristic emission features of indigo moiety and also intramolecular charge transfer complex (ICT) related with their subunits. Besides increasing fluorescence quantum yields as compared to model compound, biexponential decay times for fluorescence life times were also obtained for Tyr-3 and Tyr-4 dyes. Their appropriate energy levels along with low HOMO levels are desired for light harvesting acceptors for organic solar cells when blended with poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2',1',3'-benzothiadiazole] (PCDTBT) as donor polymer. Photovoltaic behavior of the synthesized dyes were examined in bulk heterojunction concept and achieved photovoltaic conversion efficiencies were discussed.

Achieving high short-circuit current and fill-factor via increasing quinoidal character on nonfullerene small molecule acceptor

Recently, the fused-ring based low band gap (LBG) small molecule acceptors (SMAs) have emerged as efficient nonfullerene acceptors. So far, these LBG SMAs are mainly designed with IC (2-methylene-(3-(1,1-dicyanomethylene)indanone)) or its analogs, the benzo-type electron-accepting (A) units. Compared to benzene, thiophene is less aromatic and thus the thiophene-involving semiconducting molecule has more quinoidal character, which effectively reduces the energy gap between the highest occupied molecular orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO). Herein, we show that replacing the IC units in ITIC with the CT (cyclopenta[c]thiophen-4-one-5-methylene-6-(1,1-dicyanomethylene)), a thiophene-fused A unit, the quinoidal character is enhanced from 0.0353 on ITIC to 0.0349 on ITCT, the CT-ended SMA. The increase in the quinoidal character reduces the optical band gap and enhances the near IR absorptivity. When blended with the wide band gap (WBG) polymer donor, PBDB-T, an average power conversion efficiency of 10.99% is obtained with a short-circuit current-density (Jsc) of 17.88mA/cm2 and a fill-factor (FF) of 0.723. For comparisons, the Jsc is of 16.92mA/cm2, FF is of 0.655 and PCE is of 9.94% obtained from the ITIC:PBDB-T device. This case indicates that the replacement of the benzene ring on the IC unit with a more polarizable five-member ring such as thiophene is an effective way to enhance the absorption of the near IR solar photons towards designing high-performance nonfullerene polymer solar cells.

Synthesis of low bandgap small molecules containing fluorinated benzothiadiazole and phenothiazine for photovoltaic applications

ABSTRACTA series of small molecules (SMs) containing fluorinated benzothiadiazole (BT) and phenothiazine (PT) were synthesized via the Suzuki coupling reaction. For exploring the effect of auxiliary electron-accepting moieties on the chemical structures, one or two fluorine atoms were selectively incorporated into the central electron-withdrawing BT unit of the non-fluorinated standard small molecule (BDP) to afford BDP-F and BDP-FF, respectively. Because of the favorable contributions of the fluorine atoms, the optimized inverted-type photovoltaic devices containing the SMs as active components exhibited improvement in PCE with increasing fluorine substitution, i.e., 0.49%, 0.54%, and 0.78% for BDP, BDP-F, and BDP-FF, respectively.

High-performance nonfullerene polymer solar cells with open-circuit voltage over 1 V and energy loss as low as 0.54 eV

In this work, we developed high efficiency nonfullerene PSCs based on a wide bandgap polymer donor PBPD-Th with a structure of meta-alkoxyphenyl benzodithiophene-alt-thienyl benzodithiophene-4,8-dione and a low bandgap small molecule acceptor ITIC. The PBPD-Th shows a strong absorption in the short wavelength of 300–650nm with a bandgap of 1.91eV, which is complementary with that of ITIC (1.55eV). Moreover, PBPD-Th possesses a deep HOMO level of −5.42eV, which is beneficial to obtain high open-circuit voltage (Voc) in PSCs. As a result, the optimal PSCs based on PBPD-Th: ITIC showed a power conversion efficiency (PCE) of 10.8% with a Voc of up to 1.01V, a high short-circuit current density (Jsc) of 18.3mAcm−2 and a fill factor (FF) of 59%, under the illumination of AM 1.5G, 100mWcm−2. Notably, the energy loss (Eloss) of the optimal PSCs is 0.54eV, which is smaller than the empirically low threshold of 0.6eV. The PCE of 10.8% is one of the highest values reported in the literatures for the PSCs with Voc over 1.0V.

Design of diblock co-oligomers as low bandgap small molecules for organic solar cells

The properties of a particular kind of small molecule that is built from two oligomers of different monomers, i.e. a diblock co-oligomer, as the electron donor in the active layer of organic solar cells are investigated theoretically. For these molecules, this work shows that it is possible to predict the energies of the frontier molecular orbitals by knowing the same energies for the oligomers that constitute the diblock, opening the possibility of designing new materials with optimal energy levels and optical properties. Furthermore, it was observed that the optical absorption bands of these diblock co-oligomers were broader than that of the constituent oligomers and also of the homopolymers, allowing greater absorption of photons and possibly an improved electric current in the device. It was also shown that these phenomena are size-dependent.

An Azulene-Containing Low Bandgap Small Molecule for Organic Photovoltaics with High Open-Circuit Voltage.

A simple azulene-containing squaraine dye (AzUSQ) showing bandgap of 1.38 eV and hole mobility up to 1.25×10(-4) cm(2) V(-1) s(-1) was synthesized. With its low bandgap, an organic photovoltaic (OPV) device based on it has been made that exhibits an impressive open-circuit voltages (Voc ) of 0.80 V. Hence, azulene might be a promising structural unit to construct OPV materials with simultaneous low bandgap, high hole mobility and high Voc .

Solvent Annealing Effects in Dithieno[3,2-b:2′,3′-d]pyrrole–5,6-Difluorobenzo[c][1,2,5]thiadiazole Small Molecule Donors for Bulk-Heterojunction Solar Cells

Low-bandgap small molecule (SM) donors that can be solution-processed with fullerene acceptors (e.g., PC61/71BM) are proving to be particularly promising in bulk-heterojunction (BHJ) solar cells. Compared to their π-conjugated polymer counterparts, SM donors are well-defined (monodisperse) and more synthetically modular, with relatively wide ranges of bandgaps that can be achieved in stepwise couplings of various donor and acceptor motifs. However, the optimization of SM–fullerene morphologies and BHJ device efficiencies relies more specifically on the use of processing additives, postprocessing thermal, or solvent vapor annealing (SVA) approaches, and achieving adequate interpenetrating networks and structural order in BHJ thin films can be challenging. In this report, we examine the correlated effects of molecular structure and postprocessing SVA on the BHJ solar cell performance of a set of π-extended SM donors composed of dithieno[3,2-b:2′,3′-d]pyrrole (DTP) and 5,6-difluorobenzo[c][1,2,5]thiadiazole ...

Design and Synthesis of a Low Bandgap Small Molecule Acceptor for Efficient Polymer Solar Cells.

A novel non-fullerene acceptor, possessing a very low bandgap of 1.34 eV and a high-lying lowest unoccupied molecular orbital level of -3.95 eV, is designed and synthesized by introducing electron-donating alkoxy groups to the backbone of a conjugated small molecule. Impressive power conversion efficiencies of 8.4% and 10.7% are obtained for fabricated single and tandem polymer solar cells.

Synthesis and characterization of acceptor–donor–acceptor-based low band gap small molecules containing benzoselenadiazole

Acceptor–donor–acceptor-type compounds 5′,5″-(benzo[c][1,2,5]selenadiazole-4,7-diyl)bis(3′-dodecyl-2,2′-bithiophene-5-carbonitrile) (9) and 4,4′-(5,5′-(benzo[c][1,2,5]selenadiazole-4,7-diyl)bis(3-dodecylthiophene-5,2-diyl))dibenzonitrile (10) were designed and synthesized. These compounds differ in terminal positions, compound 9 with a thiophene-containing nitrile group and compound 10 with a phenyl-containing nitrile group. Both compounds have shown good thermal stability and low band gap. The band gaps of compounds 9 and 10 were 1.74 and 1.83 eV, respectively. These results indicate that they are promising materials for use in optoelectronics.

Influence of the positions of thiophenes and side chains on diketopyrrolopyrrole based narrow band-gap small molecules for organic solar cells

A series of low band-gap small molecules with multiple donor-acceptor type backbones, which contained diketopyrrolopyrrole as a bulk core, thiophene/benzene as donor moiety and isoindigo/thienoisoindigo as end acceptor moiety, were designed and synthesized. All the four compounds show broad absorption from 300 nm to 900 nm and the optical band gaps are all lower than 1.5 eV. The absorption spectra and the energy levels of the four compounds are finely tuned by changing the donor unit of benzene with thiophene. X-ray diffraction measurement indicated that there are differences between the crystallinity of the four materials. The effects of the positions of thiophenes and side chains on morphology, charge transport and photovoltaic properties of these narrow band-gap small molecules were also studied. The highest power conversion efficiency of these compounds is 2.01% after thermal annealing. Ternary solar cells were fabricated to investigate the effect of the second donor on the power conversion efficiency.

NIR absorbing D-π-A-π-D structured diketopyrrolopyrrole-dithiafulvalene based small molecule for solution processed organic solar cells.

A new low band gap small molecule with a D-π-A-π-D molecular structure composed of a dithiafulvalene (DTF) donor and a diketopyrrolopyrrole (DPP) acceptor was synthesized and tested for organic solar cells. Using the small molecule as an electron donor, and PC71BM as an acceptor a high power conversion efficiency (PCE) of 4.3% was achieved.