Low-Bandgap Small-Molecule Donor Material Containing Thieno[3,4-b]thiophene Moiety for High-Performance Solar Cells

Tuning the Properties of Donor-Acceptor and Acceptor-Donor-Acceptor Boron Difluoride Hydrazones via Extended π-Conjugation

Molecular materials with π-conjugated donor-acceptor (D-A) and acceptor-donor-acceptor (A-D-A) electronic structures have received significant attention due to their usage in organic photovoltaic materials, in organic light-emitting diodes, and as biological imaging agents. Boron-containing molecular materials have been explored as electron-accepting units in compounds with D-A and A-D-A properties as they often exhibit unique and tunable optoelectronic and redox properties. Here, we utilize Stille cross-coupling chemistry to prepare a series of compounds with boron difluoride hydrazones (BODIHYs) as acceptors and benzene, thiophene, or 9,9-dihexylfluorene as donors. BODIHYs with D-A and A-D-A properties exhibited multiple reversible redox waves, solid-state emission with photoluminescence quantum yields up to 10%, and aggregation-induced emission (AIE). Optical band gaps (or highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps) determined for these compounds (2.02-2.25 eV) agree well with those determined from cyclic voltammetry experiments (2.05-2.42 eV). The optoelectronic properties described herein are rationalized with density functional theory calculations that support the interpretation of the experimental findings. This work provides a foundation of understanding that will allow for the consideration of D-A and A-D-A BODIHYs to be incorporated into applications (e.g., organic electronics) where fine-tuning of band gaps is required.

Size Effect of Two-Dimensional Conjugated Space in Photovoltaic Polymers' Side Chain: Balancing Phase Separation and Charge Transport

Various two-dimensional (2D) side-chain-substituted benzo(1,2-b:4,5-b')dithiophene (BDT) blocks have been used to construct donor polymers, whereas the size effect of the side chains on the photovoltaic performance was overlooked in the past few years. In this work, three size-varied conjugated spaces (benzene, naphthalene, and biphenyl) were introduced into the corresponding polymers PBDB-Ph, PBDB-Na, and PBDB-BPh. This space engineering has a significant impact on the extent of phase separation in the active layer which blended with the polymer and the acceptor ITCPTC and preserved the desired morphology. The varied space size in the side chains lead to distinct balance mobility ratios of holes to electrons (benzene, 0.21; naphthalene, 0.75; and biphenyl, 0.57). Finally, PBDB-Na-based polymer solar cells (PSCs) delivered the highest power conversion efficiency of 12.52% when compared to the PSC performances of PBDB-Ph (8.48%) and PBDB-BPh (11.35%). The method in tailoring the side chain structures could fabricate a balance between phase separation and charge transport, providing an enlightenment for the development of photovoltaic devices.

Thieno[3,4-b]thiophene-Based Novel Small-Molecule Optoelectronic Materials

Because of the tailorable photoelectric properties derived from judicious molecular design and large-area and low-temperature processability especially on flexible substrates, design and synthesis of new organic π-functional materials is always a central topic in the field of organic optoelectronics, which siginificantly contributed to the development of high-performance optoelectronic devices such as organic photovoltaics (OPVs), organic field-effect transistors (OFETs), and organic light-emitting diodes (OLEDs). Compared with polymers, small molecules with well-defined molecular structures benefit the establishment of structure-property relationships, which may provide valuable guidelines for the design of new optoelectronic materials to further promote the device performance. New building blocks are essential for the construction of optoelectronic materials. As is well recognized, thiophene-based functional materials have played an indispensable role in the development of organic optoelectronics. Compared with six-membered benzene, five-membered thiophene shows weaker aromaticity and lower steric hindrance and may provide extra sulfur-sulfur interactions in solid state. Among various thiophene building blocks, thieno[3,4-b]thiophene (TbT) is an asymmetric fused bithiophene containing four functionalization positions, in which the proaromatic thiophene can effectively stabilize the quinoidal resonance of the aromatic thiophene. Thus, TbT exhibits a unique characteristic of quinoid-resonance effect that is powerful to modulate electronic structures. Although the application of TbT in polymer donor materials represented by PTB-7 has achieved a great success, its application in small-molecule optoelectronic materials is almost an untouched field. In this Account, we summerize the rational design of a series of TbT-based small-molecule optoelectronic materials designed and optimized by quinoid-resonance effect, regiochemistry, and side-chain engineering and demonstrate the crucial effect of TbT building blocks on the electronic structures, photophysical and charge transport properties, and photovoltaic performance. With well-defined regioregular oligothieno[3,4-b]thiopenes, we revealed the quinoid-resonance effect of the TbT moiety and its geometric origin. TbT-based small molecules exhibit full-color tunable emissions in the visible to near-infrared regions and excellent performance in OFETs and OPVs. For instance, TbT-based quinoidal molecules with near-infrared fluorescence quantum yields up to 53.1% and TbT-based aromatic molecules with full-color-tunable emissions and high fluorescence quantum yields approaching 100% in polar solvent were designed and synthesized. Solution-processable ambient-stable n-channel organic thin-film transistors based on two-dimensional π-expanded quinoidal terthiophenes with distal or proximal sulfur orientations (2DQTTs) realized a record electron mobility of 5.2 cm² V^-1 s^-1. Furthermore, TbT-based electron donor and electron acceptor materials were successfully designed for OPV applications delivering high power conversion efficiencies up to 9.26% and 10.07%, respectively. We believe that new TbT-based small-molecule materials designed by a synergy of molecular engineering strategy may not only further promote OFET and OPV performance but also realize more unique applications.

Nonfullerene-Acceptor All-Small-Molecule Organic Solar Cells Based on Highly Twisted Perylene Bisimide with an Efficiency of over 6

Two twisted singly linked perylene bisimide (PBI) dimers with chalcogen bridges in the PBI cores, named C4,4-SdiPBI-S and C4,4-SdiPBI-Se, were synthesized as acceptors for nonfullerene all-small-molecule organic solar cells (NF all-SMSCs). A moderate-band-gap small-molecule DR3TBDTT used as the electron donor displayed complementary absorption with C4,4-SdiPBI-S and C4,4-SdiPBI-Se. It was found that solvent-vapor annealing (SVA) played a critical role in the photovoltaic performance in NF all-SMSCs, which improves the crystallinity of the donor and acceptors, promotes the proper phase segregation domain size, and therefore enhances charge transport. The power conversion efficiencies (PCEs) of NF all-SMSC devices based on DR3TBDTT/C4,4-SdiPBI-S and DR3TBDTT/C4,4-SdiPBI-Se increased from 2.52% to 5.81% (J_SC = 11.12 mA cm^-2, V_OC = 0.91 V, and FF = 57.32%) and from 2.65% to 6.22% (J_SC = 11.55 mA cm^-2, V_OC = 0.92 V, and FF = 58.72%), respectively, after exposure to chloroform vapor. The best efficiency of 6.22% is one of the highest PCEs for NF all-SMSC-based PBI acceptors so far. The studies illustrate that highly efficient NF all-SMSCs can be achieved by using a PBI acceptor with a suitable SVA process.

Investigation of Fluorination on Donor Moiety of Donor-Acceptor 4,7-Dithienylbenzothiadiazole-Based Conjugated Polymers toward Enhanced Photovoltaic Efficiency

It is known that fluorination on π-conjugated donor-acceptor (D-A) polymers can significantly affect the optoelectronic properties and fluorination on A moiety has been well established for design of efficient photovoltaic materials. For example, polymers based on 4,7-dithienyl-5,6-difluorobenzothiadiazole (DTffBT) have been intensively investigated and exhibited excellent performance, but the corresponding DTBT-based polymers without fluorine often display an unfavorable efficiency. With the purpose of improving photovoltaic efficiency of DTBT-based D-A polymers, we design three polymers PDTBT-TxfBT (x = 0, 1, 2) with fluorination on D moiety (TxfBT) and systematically investigate fluorination on the photophysical/electrochemical and photovoltaic properties. The results show that polymer solar cells (PSCs) based on PDTBT-TBT exhibit moderate power conversion efficiency (PCE) of 5.84%. However, the bis-fluorination on TffBT moiety (PDTBT-TffBT) can greatly enhance the molecular planarity and intermolecular interaction, improve the charge transport and heterojunction morphology, and further suppress the charge recombination losses. PSCs based on PDTBT-TffBT demonstrate obviously improved photovoltaic efficiency with the best PCE up to 7.53% without any processing additives, which ranks among the top DTBT-based PSCs. However, it should be noted that unsymmetrical fluorination on TfBT moiety (PDTBT-TfBT) impairs the regularity of polymer backbone and intermolecular interaction, increases the recombination losses, and seriously reduces the short-circuit current density and efficiency (5.44%). The results exhibit that fluorination on D moiety is a helpful strategy for design high-performance photovoltaic materials and the regularity of fluorination is crucial to improving efficiencies.

Following the TRMC Trail: Optimization of Photovoltaic Efficiency and Structure-Property Correlation of Thiophene Oligomers

Semiconducting conjugated oligomers having same end group (N-ethylrhodanine) but different central core (thiophene: OT-T, bithiophene: OT-BT, thienothiophene: OT-TT) connected through thiophene pi-linker (alkylated terthiophene) were synthesized for solution processable bulk-heterojunction solar cells. The effect of the incorporation of an extra thiophene to the central thiophene unit either through C-C bond linkage to form bithiophene or by fusing two thiophenes together to form thienothiophene on the optoelectronic properties and photovoltaic performances of the oligomers were studied in detail. Flash photolysis time-resolved microwave conductivity (FP-TRMC) technique shows OT-TT has significantly higher photoconductivity than OT-T and OT-BT implying that the former can outperform the latter two derivatives by a wide margin under identical conditions in a bulk-heterojunction solar cell device. However, the initial photovoltaic devices fabricated from all three oligomers (with PC71BM as the acceptor) gave power conversion efficiencies (PCEs) of about 0.7%, which was counterintuitive to the TRMC observation. By using TRMC results as a guiding tool, solution engineering was carried out; no remarkable changes were seen in the PCE of OT-T and OT-BT. On the other hand, 5-fold enhancement in the device efficiency was achieved in OT-TT (PCE: 3.52%, VOC: 0.80 V, JSC: 8.74 mA cm(-2), FF: 0.50), which was in correlation with the TRMC results. The structure-property correlation and the fundamental reasons for the improvement in device performance upon solvent engineering were deduced through UV-vis absorption, atomic force microscopy, bright-field transmission electron microscopy, photoluminescence quenching analysis and two-dimensional grazing incidence X-ray diffraction studies.

A D–π–A1–π–A2 push–pull small molecule donor for solution processed bulk heterojunction organic solar cells

Herein, benzothiadiazole (BTD), as an acceptor A1, has been used as a backbone to link triphenylamine (TPA) as donor and naphthalimide (NPI) as acceptor (A2) moieties through ethylene linkers to design a small molecule.