Dithienogermole Research Articles

Dithienogermole-Based Nonfullerene Acceptors: Roles of the Side-Chains’ Direction and Development of Green-Tinted Efficient Semitransparent Organic Solar Cells

Inspired by the encouraging properties of Ge-fused heterocyclic dithienogermole (DTG) in optoelectronic applications, we here report two narrow-bandgap acceptor–donor′–donor–donor′–acceptor (A–D′–D...

Application of small molecules based on a dithienogermole core in bulk heterojunction organic solar cells and perovskite solar cells

We report the synthesis and characterization of a dithienogermole (DTGe) based small molecule, named Ge-PO-2CN, as a donor material for bulk heterojunction (BHJ) organic solar cells (OSCs) and as a hole transport material (HTM) for dopant-free perovskite solar cells (PSCs).

Intramolecular Energy Transfer in Dithienogermole Derivatives.

Dithienogermole (DTG) has been applied as a useful building unit of optical/semiconducting materials for organic optoelectronic devices because of its extended conjugation, high chemical stability, and good emissive properties. Although DTG has two substituents on the Ge atom, the substituents have been limited to simple alkyl and aryl groups in previous work. In this work, to further uncover the new functionalities of this useful building unit, various π-conjugated groups were introduced on Ge of DTG. It was expected that the introduction of π-conjugated groups would give rise to efficient energy transfer between the substituents and the DTG core, which are in proximity and linked by a Ge atom. The thus-prepared DTG compounds with fluorene, terthiophene, and pyrene units on Ge possessed well-separated frontier orbitals on the substituents and the DTG core, as proved by the absorption spectra and DFT calculations. The substituted DTG derivatives showed clear emission only from the energy acceptor even though the energy donor was photoexcited. This indicated the highly efficient energy transfer in these compounds. We also prepared more π-extended compound DTGFl2-Ph with phenyl groups on the DTG thiophene rings. DTGFl2-Ph showed strong emission in the visible region with efficient energy transfer properties. These results clearly indicate the potential application of the present DTG system as optical functional materials.

Direct comparison of dithienosilole and dithienogermole as π-conjugated linkers in photosensitizers for dye-sensitized solar cells.

Dithienosilole (DTS) and dithienogermole (DTG) are useful building units of π-conjugated organic materials. In the present work, donor-π-acceptor (D-π-A) dyes with bis(dihexyloxyphenyl)aminophenyl, DTS or DTG, and pyridine or cyanoacrylic acid as the donor (D), the π-conjugated linker (π), and the acceptor (A) units, respectively, were prepared and their optical properties were investigated. The D-π-A dyes exhibited strong absorption in the visible region, indicating efficient intramolecular donor-acceptor interaction. The addition of trifluoroacetic acid to solutions of pyridine-containing dyes led to red-shifts of the absorption bands as a result of pyridinium salt formation. Similar red-shifts were observed for cyanoacrylic acid dyes, which were due to the enhanced formation of neutral dyes relative to the separated ion pairs. The D-π-A dyes, however, showed similar absorption spectra when attached to the TiO2 surface, indicating that the dye-TiO2 electronic interaction was rather weak. In contrast to the finding that these dyes exhibited similar optical properties regardless of the π-linker (i.e., DTS or DTG), dye-sensitized solar cells (DSSCs) based on DTG-containing dyes exhibited superior performance compared to those based on DTS-containing dyes. Electrochemical impedance spectroscopy measurements supported the higher performance of the DSSCs with DTG-containing dyes.

Dithienogermole-containing D–π–A–π–A Photosensitizers for Dye-sensitized Solar Cells

Recently, we prepared D–π1–A1–π2–A2-type dyes, containing dithienogermole (DTG) as π1-spacer, and demonstrated their use as DSSC sensitizers. In this study, alkoxy units were introduced on the triphenylamine D fragment, which would enhance the hydrophobicity on the TiO2 surface in DSSC. We also introduced a benzene ring as the π2-spacer, in place of thiophene to control the energy levels.

Effect of Heterocyclic Anchoring Sequence on the Properties of Dithienogermole-Based Solar Cells.

The synthesis and characterization of two new small molecular donor materials, DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2, are presented for application in organic solar cells. These two materials represent structural evolutions of the high-efficiency, dithienogermole (DTGe)-cored small molecule DTGe(FBTTh2)2, in which the conjugation length in the backbone was extended by incorporating additional thiophene units. Using the same molecular framework, we have evaluated how the anchoring sequence of heterocyclic units influences material properties and function in solar cell devices. It was found that incorporating additional thiophene units into the backbone, regardless of the position in the molecular platform, caused a small reduction in band gaps; however, both highest occupied molecular orbitals and lowest unoccupied molecular orbital energy bands were at lower energies when the thiophenes were incorporated near the terminus of the molecule. The film morphologies of both materials could be controlled by either thermal or solvent vapor annealing to yield phase separation on the order of tens of nanometers and improved crystallinity. Peak power-conversion efficiencies of 3.6% and 3.1% were obtained using DTGe(ThFBTTh2)2 and DTGe(FBTTh3)2, after solvent vapor treatment and thermal annealing, respectively. Our study provides a detailed analysis of how the ordering sequence of heterocyclic building blocks influences the properties and function of organic solar cells.

Dithienogermole-based solution-processed molecular solar cells with efficiency over 9.

A molecular donor of intermediate dimensions based on dithienogermole (DTG) as the central electron rich unit, coded as DTG(FBT2Th2)2, was designed and synthesized for use in bulk heterojunction, solution-processed organic solar cells. Under optimized conditions, a maximum power conversion efficiency (PCE) of 9.1% can be achieved with [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the acceptor semiconductor component.

Synthesis, Properties, and Polymerization of Spiro[(dipyridinogermole)(dithienogermole)

Compounds with spiro-condensed dithienogermole (DTG) and dipyridinogermole (DPyG) units were synthesized, and their optical and electrochemical properties were investigated. The reaction of tetrachlorogermane and 3,3′-dilithio-5,5′-bis(trimethylsilyl)bithiophene, followed by treatment of the resulting mixture containing 1,1-dichlorodithienogermole with 3,3′-dilithio-4,4′-bipyridyl, gave spiro{(dipyridinogermole) [bis(trimethylsilyl)dithienogermole]} (sDPyDTG1). The UV–vis spectrum of sDPyDTG1 showed absorption bands that were due to both electronically isolated DTG and DPyG units. In contrast, the photoluminescence (PL) spectrum showed only the band ascribed to the DTG unit even when the DPyG unit was irradiated, indicating intramolecular photoenergy transfer. It was also suggested that photoinduced electron transfer from DTG to DPyG occurred to suppress the PL quantum yield. Bromination of sDPyDTG1 with NBS provided the dibromide, and the subsequent Stille coupling of the resulting dibromide with distann...

End Capping Does Matter: Enhanced Order and Charge Transport in Conjugated Donor–Acceptor Polymers

Optimized microstructure through control of both intra- and intermolecular interactions in organic semiconductors is critical for enhancing and optimizing charge transport for the realization of next-generation low-cost, mechanically flexible, and easy to process high performance, organic field effect transistors (OFETs). Herein, we report donor–acceptor alternating copolymers of dithienogermole (DTG) with 2,1,3-benzothiadiazole (BTD) and probe the importance of end groups on the control of molecular order and microstructure as it relates to the enhancement of charge carrier transport. Partial end-capping reactions, confirmed by 1H NMR and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) analyses, on the DTG–BTD copolymer provided significant improvement in grazing incidence wide angle X-Ray scattering (GIWAXS) determined polymer ordering in thin films. Consequently, OFETs exhibited charge-carrier mobilities up to 0.60 cm2/(Vs) for the end-capped copolymer, which are an order of ma...

Dithienogermole‐Containing Small‐Molecule Solar Cells with 7.3% Efficiency: In‐Depth Study on the Effects of Heteroatom Substitution of Si with Ge

Two small molecule donor materials (DTGe(FBTTh2)2 and DTGe(FBTBFu)2) incorporating the dithienogermole (DTGe) moiety with fluorobenzothiadiazole (FBT) and bithiophene (Th2) or benzofuran (BFu) end‐capping groups are synthesized and their properties as donor materials in small molecule bulk heterojunction type (BHJ) solar cells are investigated. The DTGe(FBTTh2)2 with Th2 end groups shows outstanding solar cell characteristics with efficiencies up to 6.4% using a standard BHJ architecture and 7.3% using a ZnO optical spacer, while the BFu end‐capped DTGe(FBTBFu)2 has slightly wider band gaps and yields slightly higher open circuit voltage (VOC) at the expense of short circuit current (JSC) and fill factor (FF). In this study, the DTGe‐based molecules are systematically compared to the dithienosilole (DTSi)‐based analogues, which are currently among the highest power conversion efficiency (PCE) small molecule solar cell donor materials known. The JSC produced by the DTGe molecule is found to be similar to, or slightly higher than the Si analogue, despite similar absorption characteristics, however, the PCE is similar to the Si analogues due to small decreases in VOC and FF. This report marks the first small molecule BHJ based on a Ge‐containing heterocycle with PCE over 7%.

Synthesis of dithienogermole-containing oligo- and polysilsesquioxanes as luminescent materials.

Dithienogermole (DTG)-containing oligo- and polysilsesquioxanes were prepared by hydrolysis/condensation of DTGs bearing one () or two trialkoxysilyl group(s) (). The reaction of gave a cage-type octasilsesquioxane with eight DTG groups at the edges () as a viscous oil, whereas the reaction of yielded a network polymer () as a self-standing film. showed a photoluminescence (PL) quantum yield (Φ) of 56% in THF. This value was as high as that of (Φ = 58%), in spite of the accumulation of DTG units in the molecule, as characteristics of the POSS structure. The PL of in THF was suppressed by contact with nitrobenzene, showing the potential of for sensing nitroaromatic explosives. Polymer exhibited a relatively low Φ of 2% as a film, but Φ was improved to 38% by copolymerization with trimethoxymethylsilane. was also copolymerized with a trimethoxysilyl-substituted carbazole derivative () to provide polysilsesquioxanes with DTG and carbazole units, which showed efficient photo-energy transfer from carbazole to DTG in the films. Similar copolymerization of with in the presence of poly(9-vinylcarbazole) provided a composite material with hole-transporting electroluminescence properties, applicable in multi-layered organic light emitting diodes.

Investigations of the Conjugated Polymers Based on Dithienogermole (DTG) Units for Photovoltaic Applications

Conjugated polymers based on the dithienogermole (DTG) units showed promising properties for the applications in polymer solar cells (PSCs), so that the investigations of the natures and photovoltaic properties of the DTG-based polymers with varied backbone structures would be of great interest. In this work, four DTG-based polymers named as PDTG-BDD, PDTG-IID, PDTG-BT, and PDTG-DPP were synthesized and characterized. The results indicate that the DTG-based polymers show varied absorption bands and molecular energy levels. In X-ray diffraction measurements, these polymers show different laminar packing and π–π stacking distances in solid films. The PSC devices based on the four DTG-based polymers were fabricated, and their photovoltaic properties were characterized. The results show that different device fabrication conditions are needed to get optimal photovoltaic performance of these four polymers. The device of PDTG-BDD shows a PCE of 6.3% with a high Voc of 0.935 V, a FF of 65.0%, and a Jsc of 10.3 mA/cm2, which is the highest one in these four polymers; the devices of PDTG-DPP showed a miserably low Jsc of 3.19 mA/cm2 due to the unfavorable morphologies of the polymer:PC71BM blend. Overall, the comparisons among these four polymers provide fundamental information for understanding the correlations among molecular structures and photovoltaic properties of the DTG-based polymers, and how to control or modulate the bandgaps, molecular energy levels, and morphologies of the DTG-polymers will be the key to fully explore their potentials as photovoltaic materials.

Efficient Synthesis of Dithienogermole (DTG) Derivatives via Olefin Cross-Metathesis

4,4'-Bis-(4-pentenyl)-dithieno[3,2-b:2',3'-d]germole was synthesized as a functional building block for the efficient preparation of dithienogermole (DTG) derivatives with varying alkyl chain lengths and pendant functionalities in excellent yields. These derivatives were efficiently isolated via olefin cross-metathesis followed by hydrogenation.

A comparison between dithienosilole and dithienogermole donor–acceptor type co-polymers for organic bulk heterojunction photovoltaic devices

We report the synthesis of peripherally alkylated dithienosilole (DTS) and dithienogermole (DTG) monomers that possess side chains of different lengths (methyl vs. butyl) attached to a bridging heavy atom (Si or Ge) and the resulting copolymers with benzothiadiazole (BT) units. We study the optoelectronic and charge transport properties of these copolymers, with a particular focus on their use for bulk heterojunction photovoltaic devices in blends with phenyl-C70-butyric acid methyl ester (PC70BM). Enhanced charge carrier mobility is observed by substituting Si atom with Ge atom and better mixing of copolymers with PC70BM desirable for better charge generation is obtained by shortening the side chain length of copolymers from butyl to methyl groups. We observe a very high open circuit voltage (Voc = ∼0.94 V) in single layer blend devices (polymer:PC70BM = 1 : 4 by weight in DCB) with good device performance (PCE = 2.66%, Jsc = 6.11 mA cm−2, FF = 0.46) using Ge substituted copolymers with methyl side groups. Neither additional pre- or post-thermal annealing steps nor additives to the photoactive blend layer are needed to achieve such device performance. Based on detailed investigation including absorption, emission, charge carrier mobility and quantum chemical calculations for molecular geometry and electronic energy levels, we clarify the role of the heavy atom substitution in the donor–acceptor bridged copolymers and discuss their effects on device performance in bulk heterojunction photovoltaic cells.

Dithienogermole As a Fused Electron Donor in Bulk Heterojunction Solar Cells

We report the synthesis and bulk heterojunction photovoltaic performance of the first dithienogermole (DTG)-containing conjugated polymer. Stille polycondensation of a distannyl-DTG derivative with 1,3-dibromo-N-octyl-thienopyrrolodione (TPD) results in an alternating copolymer which displays light absorption extending to 735 nm, and a higher HOMO level than the analogous copolymer containing the commonly utilized dithienosilole (DTS) heterocycle. When polyDTG-TPD:PC(70)BM blends are utilized in inverted bulk heterojunction solar cells, the cells display average power conversion efficiencies of 7.3%, compared to 6.6% for the DTS-containing cells prepared in parallel under identical conditions. The performance enhancement is a result of a higher short-circuit current and fill factor in the DTG-containing cells, which comes at the cost of a slightly lower open circuit voltage than for the DTS-based cells.