Impact of Thermal Annealing on Organic Photovoltaic Cells Using Regioisomeric Donor-Acceptor-Acceptor Molecules

Unprecedented short-circuit current density and efficiency of vacuum-deposited organic solar cells based on 8H-thieno[2',3':4,5]thieno[3,2-b] thieno[2,3-d]pyrrole

Despite the many advantages for industrial mass production, vacuum-deposited organic solar cells (OSCs) suffer from low efficiency, primarily due to the limited molecular library of small-molecule donor and acceptor materials, which remains a significant challenge. Herein, two donor-acceptor-acceptor (D-A-A)-configured small-molecule donors, named TTBTDC and TTBTDC-F were synthesized, using 8H-thieno[2',3':4,5]thieno[3,2-b]thieno[2,3-d]pyrrole (TTP) as a new fused-ring donor unit. Benefiting from the strong electron-donating ability of the TTP moiety and the adoption of the D-A-A molecular configuration, these molecules exhibited strong visible and near-infrared absorption as well as deep-lying highest occupied molecular orbital (HOMO) energy levels. Consequently, OSCs based on TTBTDC achieved an unprecedented power conversion efficiency (PCE) of 10.28 % (certified value of 10.05 %) with a short-circuit current density (Jsc) up to 17.78 mA cm-2, representing the highest PCE and Jsc values reported to date for vacuum-deposited OSCs. In contrast, OSCs based on TTBTDC-F exhibited an inferior PCE of 8 % with slightly higher open-circuit voltage (Voc) but lower Jsc. By systematically investigating the relationships between molecular structure and properties, we found that the high performance of TTBTDC devices results from extended absorption up to 900 nm, stronger cofacial antiparallel π-π interactions, and superior charge transport with suppressed recombination. Besides, both TTBTDC and TTBTDC-F devices exhibited excellent device stabilities, including storage stability, thermal stability, and photo-stability. This work demonstrates the great potential of TTP as a building block for constructing small-molecule donors and sheds light on the molecular design of small-molecule donors for high-efficiency and stable vacuum-deposited OSCs.

Remarkable Enhancement of Hole Mobility of Novel DA-D'-AD Small Molecules by Thermal Annealing: Effect of the D'-Bridge Block

Conjugated small molecules are advanced semiconductor materials with attractive physicochemical and optoelectronic properties enabling the development of next-generation electronic devices. The charge carrier mobility of small molecules strongly influences the efficiency of organic and hybrid electronics based on them. Herein, we report the synthesis of four novel small molecules and their investigation with regard to the impact of molecular structure and thermal treatment of films on charge carriers' mobility. The benzodithiophene-containing compounds (BDT) were shown to be more promising in terms of tuning the morphology upon thermal treatment. Impressive enhancement of hole mobilities by more than 50 times was found for annealed films based on a compound M4 comprising triisopropylsilyl-functionalized BDT core. The results provide a favorable experience and strategy for the rational design of state-of-the-art organic semiconductor materials (OSMs) and for improving their charge-transport characteristics.

Thieno[3,2-b]indole–benzo[b]thieno[2,3-d]thiophen-3(2H)-one-based D–π–A molecules as electron transport materials for perovskite solar cells

New small molecule D–π–A compounds, bearing thieno[3,2-b]indole and benzo[b]thieno[2,3-d]thiophen-3(2H)-one scaffolds, were prepared, characterized and utilized as electron transport materials in perovskite solar cells.

Ultrasmooth Organic Films Via Efficient Aggregation Suppression by a Low-Vacuum Physical Vapor Deposition

Organic thin films with smooth surfaces are mandated for high-performance organic electronic devices. Abrupt nucleation and aggregation during film formation are two main factors that forbid smooth surfaces. Here, we report a simple fast cooling (FC) adapted physical vapor deposition (FCPVD) method to produce ultrasmooth organic thin films through effectively suppressing the aggregation of adsorbed molecules. We have found that thermal energy control is essential for the spread of molecules on a substrate by diffusion and it prohibits the unwanted nucleation of adsorbed molecules. FCPVD is employed for cooling the horizontal tube-type organic vapor deposition setup to effectively remove thermal energy applied to adsorbed molecules on a substrate. The organic thin films prepared using the FCPVD method have remarkably ultrasmooth surfaces with less than 0.4 nm root mean square (RMS) roughness on various substrates, even in a low vacuum, which is highly comparable to the ones prepared using conventional high-vacuum deposition methods. Our results provide a deeper understanding of the role of thermal energy employed to substrates during organic film growth using the PVD process and pave the way for cost-effective and high-performance organic devices.

Mobility of Small Molecules in Solid Polymer Film for π-Stacked Crystallization

Crystallization or π-stacked aggregation of small molecules is an extensively observed phenomenon which favors charge transport along the crystal axis and is important for the design of organic optoelectronic devices. Such a process has been reported for N,N′-Bis(1-ethylpropyl)-3,4,9,10-perylenebis(dicarboximide) (EPPTC). However, the π-stacking mechanism requires solution–air or solution–solid interfaces. The crystallization or aggregation of molecules doped in solid films is generally thought to be impossible, since the solid environment surrounding the small molecules does not allow them to aggregate together into π-stacked crystals. In this work, we demonstrate that the movement of the EPPTC molecules becomes possible in a solid polymer film when it is heated to above the glass transition temperature of the polymer. Thus, crystal particles can be produced as a doped matrix in a thin solid film. The crystallization process is found to be strongly dependent on the annealing temperature and the annealing time. Both the microscopic and spectroscopic evaluations verify such discoveries and characterize the related properties of these crystals.

A cascade deprotonation/intramolecular aldol reaction of α-carbonyl sulfonium ylides with 2-mercaptoindole-3-carbaldehydes and 2-mercaptobenzaldehydes to access thieno[2,3-b]indoles and benzothiophenes

The first catalyst-free cascade deprotonation/intramolecular aldol reaction of α-carbonyl sulfonium ylides with 2-mercaptoindole-3-carbaldehydes and 2-mercaptobenzaldehydes was developed. A series of thieno[2,3-b]indoles and benzothiophenes were smoothly obtained in high to excellent yields. The salient features of the protocol include catalyst-free conditions, an environment-friendly solvent, broad substrate scope, and large-scale synthesis.

A 9.16% Power Conversion Efficiency Organic Solar Cell with a Porphyrin Conjugated Polymer Using a Nonfullerene Acceptor

A new low-molecular-weight porphyrin-based polymer, PPPyDPP, with pyridine-capped diketopyrrolopyrrole (DPP) has been synthesized, and its optical and electrochemical properties were investigated. The polymer is prepared with a low content of homocoupling units and gives a widely spread absorption from 400 to 900 nm with a narrow optical band gap of 1.46 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are respectively located at -5.27 and -3.78 eV, respectively. PPPyDPP was used as the electron donor, whereas [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) and bis(rhodanine)indolo-[3,2-b]-carbazole (ICzRd₂), a nonfullerene small molecule, were used as acceptors for the fabrication of solution-processed bulk heterojunction polymer solar cells. Overall power conversion efficiencies (PCEs) of 7.31 and 9.16% (record high for porphyrin-containing polymers) were obtained for PC₇₁BM and ICzRd₂, respectively. A high V_oc of 1.01 V and a low E_loss of 0.45 eV may explain this new record.

A new convenient synthetic route towards 2-(hetero)aryl-substituted thieno[3,2-b]indoles using Fischer indolization

A number of 2-(hetero)aryl-substituted thieno[3,2-b]indoles have been successfully prepared using an efficient transition-metal-free strategy, involving the Fiesselmann synthesis of methyl 5-(hetero)aryl-3-hydroxythiophene-2-carboxylates from 2-bromo-3-(hetero)arylacrylates and methyl thioglycolate, and the transformation of the synthesized 3-hydroxyesters into the corresponding thiophen-3(2H)-ones, followed by their treatment with arylhydrazines to directly form the targeted structures via Fischer indolization. At the same time, structural variety of the obtained thieno[3,2-b]indoles has been achieved due to a wide range of available starting materials, including both 2-bromo-3-(hetero)arylacrylates and arylhydrazines. In addition, two π-extended molecules, namely 1,4-bis(4H-thieno[3,2-b]indol-2-yl)benzene and 2,5-bis(4H-thieno[3,2-b]indol-2-yl)thiophene, have been synthesized in line with the current approach towards 2-(hetero)arylated thieno[3,2-b]indoles.

Synthesis of Thieno[3,2-b]indoles via Halogen Dance and Ligand-Controlled One-Pot Sequential Coupling Reaction

A two-pot synthesis of thieno[3,2-b]indole from 2,5-dibromothiophene is described. A halogen dance of 2,5-dibromothiophene was performed with LDA, and subsequent Negishi coupling was performed with 2-iodoaniline derivatives to provide the corresponding coupling products. The resulting two bromo groups have different reactivities, which were utilized for the one-pot Suzuki-Miyaura coupling/intramolecular Buchwald-Hartwig amination to produce thieno[3,2-b]indole via an assisted tandem catalysis that involved in situ ligand exchange.

Mesogenic D–A fluorophores based on cyanovinyl and benzothiadiazole

Synthesized cyanovinyl and BTD based fluorophores displayed LC, Gel as well as optical waveguide and chemosensor properties.