Narrow Band-gap Small Molecules Research Articles

Quinoxaline-Based D-A Copolymers for the Applications as Polymer Donor and Hole Transport Material in Polymer/Perovskite Solar Cells.

Polymer solar cells (PSCs) have achieved great progress recently, benefiting from the rapid development of narrow bandgap small molecule acceptors and wide bandgap conjugated polymer donors. Among the polymer donors, the D-A copolymers with quinoxaline (Qx) as A-unit have received increasing attention since the report of the low-cost and high-performance D-A copolymer donor based on thiophene D-unit and difluoro-quinoxalline A-unit in 2018. In addition, the weak electron-deficient characteristic and the multiple substitution positions of the Qx unit make it an ideal A-unit in constructing the wide bandgap polymer donors with different functional substitutions. In this review article, recent developments of the Qx-based D-A copolymer donors, including synthetic method of the Qx unit, backbone modulation, side chain optimization, and functional substitution of the Qx-based D-A copolymers, are summarized and discussed. Furthermore, the application of the Qx-based D-A copolymers as hole transport material in perovskite solar cells (pero-SCs) is also introduced. The focus mainly on the molecular design strategies and structure-properties relationship of the Qx-based D-A copolymers, aiming to provide a guideline for developing high-performance Qx-based D-A copolymers for the applications as donor in PSCs and as hole transport material in pero-SCs.

Design and structural modification of narrow-bandgap small molecules based on asymmetric porphyrin-diketopyrrolopyrrole backbone for solution-processed organic solar cells

Three novel organic small molecules based on asymmetric porphyrin-diketopyrrolopyrrole (DPP) backbone, namely ZnPTDPP, ZnPTDPPCN and ZnPTDPPO, have been designed and synthesized for solution-processed organic solar cells (OSCs). The terminal units of 4-cyanophenyl and 4-methoxyphenyl were introduced into ZnPTDPPCN and ZnPTDPPO respectively, and the structural modification effects on photoelectric and photovoltaic performance were fully investigated. Depending on the characteristics of porphyrin and DPP, all three molecules possess narrow bandgaps of lower than 1.50 eV, showing excellent absorption properties. Structural modification of terminal groups can effectively regulate energy levels of ZnPTDPPCN and ZnPTDPPO. As expected, extended molecular structure from D-A type to D-A-A1 or D-A-D1 type, was conducive to enlarging the π-conjugation and affecting the push-pull effect of intramolecular electrons, thus affecting the photoelectric character of materials. Solution-processed OSCs employing these molecules as the donor materials and PC61BM as the acceptor material were fabricated. It was worth mentioning that devices based on materials ZnPTDPPCN and ZnPTDPPO achieved moderate power conversion efficiencies (PCEs) of 4.26% and 4.24%, respectively, while ZnPTDPP showed an inferior PCE of 1.46%. To our knowledge, the work based on asymmetric porphyrin-DPP backbone for solution-processed OSCs, is first reported. The work will provide valuable hints for rational design of asymmetric porphyrin-based donor materials.

All-small-molecule organic solar cells with over 14% efficiency by optimizing hierarchical morphologies

The high efficiency all-small-molecule organic solar cells (OSCs) normally require optimized morphology in their bulk heterojunction active layers. Herein, a small-molecule donor is designed and synthesized, and single-crystal structural analyses reveal its explicit molecular planarity and compact intermolecular packing. A promising narrow bandgap small-molecule with absorption edge of more than 930 nm along with our home-designed small molecule is selected as electron acceptors. To the best of our knowledge, the binary all-small-molecule OSCs achieve the highest efficiency of 14.34% by optimizing their hierarchical morphologies, in which the donor or acceptor rich domains with size up to ca. 70 nm, and the donor crystals of tens of nanometers, together with the donor-acceptor blending, are proved coexisting in the hierarchical large domain. All-small-molecule photovoltaic system shows its promising for high performance OSCs, and our study is likely to lead to insights in relations between bulk heterojunction structure and photovoltaic performance.

Nonfullerene Small-Molecule Acceptors with Extended Optical Absorption Based on the “Spliced” Strategy for Organic Solar Cells

How to broaden the optical absorption of photovoltaic materials is one of the key issues in the design of high-performance organic solar cells. Nowadays, the sunlight of 400–550 nm wavelength range is not effectively utilized for most small-molecule nonfullerene acceptors. In this work, we proposed the “spliced” strategy of combining the acceptor–donor–acceptor type narrow band-gap small molecules and wide-band-gap perylene diimide (PDI) moieties via a flexible alkyl chain linkage, which could give the superposition effect of the absorption spectra, and three small-molecule acceptors (S1, S2, and S3) were designed based on various end-capping groups with different electron withdrawing abilities. Encouragingly, the as-constructed molecules can well make use of 400–550 nm sunlight with two independent absorption regions. Meanwhile, the aggregation of S1 with a highly planar end-capping group was dominated by both the PDI unit and main skeleton, while S2 and S3 exhibited PDI-controlled aggregation. When fabricated into organic solar cells, S1-based devices achieved a superior efficiency of 3.41% in comparison with those of the other two. The poor photovoltaic performance could be attributed to severe PDI aggregation, which can hinder the charge transfer through the main skeletons. This work could provide a new perspective to modulate optical absorption through the spliced strategy.

Effects of Short‐Axis Alkoxy Substituents on Molecular Self‐Assembly and Photovoltaic Performance of Indacenodithiophene‐Based Acceptors

AbstractThe effects of central alkoxy side chain length of a series of narrow bandgap small molecule acceptors (SMAs) on their physicochemical properties and on the photovoltaic performance of the SMA‐based polymer solar cells (PSCs) are systematically investigated. It is found that the ordered aggregation of these SMAs in films is enhanced gradually with the increase of alkoxy chain length. The single‐crystal structures of these SMAs further reveal that small changes in the side chain length can have a dramatic impact on molecular self‐assembly. The short‐circuit current density and power conversion efficiency values of the corresponding PSCs increase with the increase of the side chain length of the SMAs. The π–π coherence length of the SMAs in the active layers is increased with the increase of the side chain length, which could be the reason for the increase of the Jsc in the PSCs. The results indicate that small changes in side chain length can have a dramatic impact on the molecular self‐assembly, morphology, and photovoltaic performance of the PSCs. The structure–performance relationship established in this study can provide important instructions for the side chain engineering and for the design of efficient SMAs materials.

Small molecule acceptors with indacenodithiophene–benzodithiophene–indacenodithiophene as donating cores for solution-processed non-fullerene solar cells

Abstract Two acceptor-donor1-donor2-donor1-acceptor (A-D1-D2-D1-A)-type narrow bandgap small molecules BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 with indacenodithiophene–benzodithiophene-indacenodithiophene (BDT-(IDT)2) as donating cores and 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (IC) or 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (2FIC) as end groups, have been synthesized and investigated as non-fullerene acceptors in solution-processed polymer solar cells (PSCs). The optical bandgaps of BDT(IDT-IC)2 and BDT(IDT-IC-2F)2 are 1.59 eV and 1.54 eV, respectively. Using PBDB-T as donor, the power conversion efficiencies (PCEs) of 4.98% and 6.21% are achieved by PSCs based on BDT(IDT-IC)2 and BDT(IDT-IC-2F)2, respectively.

Oligothiophene-Indandione-Linked Narrow-Band Gap Molecules: Impact of π-Conjugated Chain Length on Photovoltaic Performance.

Solution-processed organic solar cells (OSCs) based on narrow-band gap small molecules hold great promise as next-generation energy-converting devices. In this paper, we focus on a family of A-π-D-π-A-type small molecules, namely, BDT- nT-ID ( n = 1-4) oligomers, consisting of benzo[1,2- b:4,5- b']dithiophene (BDT) as the central electron-donating (D) core, 1,3-indandione (ID) as the terminal electron-accepting (A) units, and two regioregular oligo(3-hexylthiophene)s ( nT) with different numbers of thiophene rings as the π-bridging units, and elucidate their structure-property-function relationships. The effects of the length of the π-bridging nT units on the optical absorption, thermal behavior, morphology, hole mobility, and OSC performance were systematically investigated. All oligomers exhibited broad and intense visible photoabsorption in the 400-700 nm range. The photovoltaic performances of bulk heterojunction OSCs based on BDT- nT-IDs as donors and a fullerene derivative as an acceptor were studied. Among these oligomers, BDT-2T-ID, incorporating bithiophene as the π-bridging units, showed better photovoltaic performance with a maximum power conversion efficiency as high as 6.9% under AM 1.5G illumination without using solvent additives or postdeposition treatments. These favorable properties originated from the well-developed interpenetrating network morphology of BDT-2T-ID, with larger domain sizes in the photoactive layer. Even though all oligomers have the same A-D-A main backbone, structural modulation of the π-bridging nT length was found to impact their self-organization and nanostructure formation in the solid state, as well as the corresponding OSC device performance.

Computational Designing of Low Energy Gap Small Molecule Acceptors for Organic Solar Cells

In this study, effort is done to design a series of narrowband-gap small molecule acceptors for organic solar cells. We have predicated the electronic and optical properties using theoretical methods. Results show that the orbital spatial distribution, HOMO/LUMO energy levels, band gap and optical properties can be systematically changedby modification of terminal acceptor units and conjugated system. Most of the acceptors show low energy gaps reveal thermodynamical more stability. Conjugated system help to tune the electronic properties and decrease the band gap of small molecules. Finally, we have identified potential terminal acceptor groups for proficient organic solar cell materials.

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.

Efficient chemical structure and device engineering for achieving difluorinated 2,2′-bithiophene-based small molecular organic solar cells with 9.0% efficiency

Three novel narrow band gap small molecules (SMs) with D(A–Ar)2 molecular frameworks, based on fluorinated 2,2′-bithiophene (FBT) as an electron-donating (D) central core coupled with different electron-acceptor (A) bridge units, and 2-octylthiophene (Ar) units as end-capping units, were designed and synthesized for application as donor materials in solution-processed SM-OSCs. Under optimized conditions, a high PCE of 9.00% with a very high Jsc of 16.14 mA cm−2 and a FF of 73.52% for FBT(TDPP-T)2/PC71BM is obtained in single layer OSCs, which is among the highest reported for D(A–Ar)2-type SM based solar cells with a PCE up to 9% so far.

High-Performance As-Cast Nonfullerene Polymer Solar Cells with Thicker Active Layer and Large Area Exceeding 11% Power Conversion Efficiency.

In this work, a nonfullerene polymer solar cell (PSC) based on a wide bandgap polymer donor PM6 containing fluorinated thienyl benzodithiophene (BDT-2F) unit and a narrow bandgap small molecule acceptor 2,2'-((2Z,2'Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (IDIC) is developed. In addition to matched energy levels and complementary absorption spectrum with IDIC, PM6 possesses high crystallinity and strong π-π stacking alignment, which are favorable to charge carrier transport and hence suppress recombination in devices. As a result, the PM6:IDIC-based PSCs without extra treatments show an outstanding power conversion efficiency (PCE) of 11.9%, which is the record value for the as-cast PSC devices reported in the literature to date. Moreover, the device performances are insensitive to the active layer thickness (≈95-255 nm) and device area (0.20-0.81 cm2 ) with PCEs of over 11%. Besides, the PM6:IDIC-based flexible PSCs with a large device area of 1.25 cm2 exhibit a high PCE of 6.54%. These results indicate that the PM6:IDIC blend is a promising candidate for future roll-to-roll mass manufacturing and practical application of highly efficient PSCs.

High Efficiency Nonfullerene Polymer Solar Cells with Thick Active Layer and Large Area

In this work, high-efficiency nonfullerene polymer solar cells (PSCs) are developed based on a thiazolothiazole-containing wide bandgap polymer PTZ1 as donor and a planar IDT-based narrow bandgap small molecule with four side chains (IDIC) as acceptor. Through thermal annealing treatment, a power conversion efficiency (PCE) of up to 11.5% with an open circuit voltage (Voc ) of 0.92 V, a short-circuit current density (Jsc ) of 16.4 mA cm-2 , and a fill factor of 76.2% is achieved. Furthermore, the PSCs based on PTZ1:IDIC still exhibit a relatively high PCE of 9.6% with the active layer thickness of 210 nm and a superior PCE of 10.5% with the device area of up to 0.81 cm2 . These results indicate that PTZ1 is a promising polymer donor material for highly efficient fullerene-free PSCs and large-scale devices fabrication.

Energy level alignment at C60/DTDCTB/PEDOT:PSS interfaces in organic photovoltaics

The electronic structure of a narrow band gap small molecule ditolylaminothienyl–benzothiadiazole–dicyanovinylene (DTDCTB), possessing a donor-acceptor-acceptor configuration, was investigated with regard to its application as an efficient donor material in organic photovoltaics (OPVs). The interfacial orbital alignment of C60/DTDCTB/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was determined using in situ ultraviolet photoelectron and inverse photoelectron spectroscopic methods. The ionization energy and electron affinity values of DTDCTB were measured to be 5.27eV and 3.65eV, respectively, and thus a very small transport gap of 1.62eV was evaluated. Large band bending of DTDCTB on PEDOT:PSS was observed, resulting in a low hole extraction barrier. Additionally, the photovoltaic gap between the highest occupied molecular orbital level of the DTDCTB donor and the lowest unoccupied molecular orbital level of the C60 acceptor was estimated to be 1.30eV, which is known to be the theoretical maximum open-circuit voltage in OPVs employing the C60/DTDCTB active layer. The unique electronic structures of DTDCTB contributed toward the recently reported excellent power conversion efficiencies of OPVs containing a DTDCTB donor material.

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.

Facile synthesis of naphthodithiophenediimide based small molecules and polymers via direct arylation coupling

Naphthodithiophenediimide (NDTI) as a new electron-accepting building block has been used to synthesize narrow bandgap small molecules and polymers for ambi-polar organic field-effect transistors and organic solar cells. Herein, we report the first application of direct arylation coupling to synthesize a series of NDTI-based small molecules and polymers. The regioselectivity was examined in the direct arylation coupling of NDTI with 2-bromo-9,9-dihexylfluorene. Two NDTI-based narrow bandgap polymers, one with benzothiadiazole and the other with 9,9-dioctylfluorene, were synthesized via direct arylation polymerization. The chemical structures and optoelectronic properties of these molecules and polymers were characterized.

A dual-functional benzobisthiadiazole derivative as an effective theranostic agent for near-infrared photoacoustic imaging and photothermal therapy.

Nanoparticles (NPs) with integrated functionalities of targeting, therapy, imaging contrast and biocompatibility have shown promise for application in improved disease diagnosis and therapy. Herein, we report a theranostic agent based on a narrow-bandgap small molecule, benzo[1,2-c;4,5-c']bis[1,2,5]thiadiazole-4,7-bis(9,9-dioctyl-9H-fluoren-2-yl)thiophene (denoted as BBT-2FT), with the strong absorption of near-infrared (NIR) light. Colloidal nanoparticles composed of BBT-2FT showed a photoacoustic signal intensity ten times higher than that of blood, and high photothermal conversion efficiency (η = 40%) under the irradiation of 808 nm laser light, which killed over 90% of HeLa cells in 10 min.

Series of Quinoidal Methyl-Dioxocyano-Pyridine Based π-Extended Narrow-Bandgap Oligomers for Solution-Processed Small-Molecule Organic Solar Cells

Small molecules with narrow bandgap of <1.6 eV can harvest the visible and near-infrared solar photons. In this Article, we report a new method to achieve narrow bandgap small molecule donors by using electron-deficient quinoidal methyl-dioxocyano-pyridine (MDP) to induce possible quinoidal resonance structure along the conjugated A-pi-D-pi-A backbone. Practically, two MDP moieties are covalently linked onto an electron-rich benzodithiophene (BDT) through the oligothiophene (OT-5T) pi-bridge. The affording small molecules, namely, nTBM, exhibit broad and strong absorption bands covering the visible and near-infrared region from 400 to 870 nm. The estimated optical bandgap is down to 1.4 eV. The narrow bandgap is associated with the low-lying lowest unoccupied molecular orbital (LUMO) energy level (about -3.7 eV) and the high-lying highest occupied molecular orbital (HOMO) energy level (around -5.1 eV). Density-functional theory calculations reveal that the HOMO and LUMO energy levels, with the increase of the size of the oligothiophene bridge, become localizations in different moieties, i.e., the central electron-donating and the terminal electron-withdrawing units, respectively, which provides necessary driving force for the delocalization of the excited electrons and formation of the quinoidal resonance structure. The quinoidal structure enhances the photoinduced intramolecular charge-transfer, leading to the absorbance enhancement of the low-energy absorption band. With the increase of the size of the oligothiophene from 0 to 5 thienyl units and the change of the direction of the alkyl chains on the bridged thiophene from outward to inward, the crystalline nature, fibril length, and phase size of the blend films as well as the cell performance are all fine-tuned, also. With the inward alkyl chains, the terthiophene bridged molecule is amorphous, while the pentathiophene bridged one is relatively crystalline. Both molecules form nanoscale interpenetrating networks with a phase size of 15-20 nm when blended with PC71BM, showing the higher hole mobility and promising electric performance.

All-organic and fully-printed semitransparent photodetectors based on narrow bandgap conjugated molecules.

All-organic, fully-printed and semitransparent photodetectors with a broad wavelength band response, based on a ternary blend comprising narrow band-gap small molecules, are demonstrated. The ternary blend with a semiconducting polymer allows for the optimal printing of small molecules, suppressing strong phase segregation, and uncontrolled crystallization. The insertion of a suitable interlayer enables the adoption of polymer, transparent, top and bottom printed electrodes, thus making light detection possible from both device sides.

Facile synthesis of 1-(2,6-diisopropylphenyl)-2,5-di(2-thienyl)pyrrole-based narrow band gap small molecules for solar cell applications

In this study, 1-(2,6-diisopropylphenyl)-2,5-di(2-thienyl)pyrrole-based two new low band gap organic small molecules, TPT-SM1 and TPT-SM2, were prepared and their optical, electrical and photovoltaic properties were investigated. TPT-SM1 was synthesized by simply linking the electron rich 1-(2,6-diisopropylphenyl)-2,5-di(2-thienyl)pyrrole and electron deficient tricyanovinyl (−C(CN)C(CN)2) units. On the other hand, TPT-SM2 was prepared by attaching the electron donating non planar triphenyl amine and electron accepting tricyanovinyl groups respectively on both side of 1-(2,6-diisopropylphenyl)-2,5-di(2-thienyl)pyrrole unit. Interestingly, the absorption band of TPT-SM1 and TPT-SM2 was found to be located exactly at the maximum solar flux region of the solar spectrum and their strong absorption band appears at the region of 470–800nm and 490–900nm, respectively, in film state. The optical band gap of TPT-SM1 and TPT-SM2 was calculated to be 1.58eV and 1.42eV, respectively, and the electrochemical studies revealed that the HOMO energy level was located at around −5.15eV. The solution processed small molecule organic solar cells (SMOSCs) prepared by using TPT-SM1 and TPT-SM2 as an electron donor and PC71BM as an electron acceptor at 1:2wt% donor:acceptor blend ratio showed maximum power conversion efficiency (PCE) of 1.19% and 1.68%, respectively.

Near-infrared response photovoltaic device based on novel narrow band gap small molecule and PCBM fabricated by solution processing

In this paper, we report on synthesis of a novel diphenylaminofluorenyl-capped thiadiazoloquinoxaline (6,7-dimethyl-4,9-di{5-[9,9-bis(octyl)-7-diphenylaminofluorene-2-yl]-4-hexylthien-2-yl}[1,2,5]-thiadiazolo[3,4- g]quinoxaline (DPAFL-TDOX-DPAFL which is further abbreviated to TDOX), Scheme 1) and its use as donor material for the realization of efficient bulk-heterojunction organic solar cell by spin coating from blend with PCBM as acceptor. TDOX showed very good solution processibility and film uniformity up to weight ratio TDOX:PCBM=1:4. Spectral response was extended to near-infrared region (at around 1000 nm) with energy conversion efficiency (ECE) 0.75% and short-curcuit current density 1.95 mA/cm 2.