Structure-property relationship study of substitution effects on isoindigo-based model compounds as electron donors in organic solar cells

Analysis of the Performance of Narrow-Bandgap Organic Solar Cells Based on a Diketopyrrolopyrrole Polymer and a Nonfullerene Acceptor

The combination of narrow-bandgap diketopyrrolopyrrole (DPP) polymers and nonfullerene acceptors (NFAs) seems well-matched for solar cells that exclusively absorb in the near infrared but they rarely provide high efficiency. One reason is that processing of the active layer is complicated by the fact that DPP-based polymers are generally only sufficiently soluble in chloroform (CF), while NFAs are preferably processed from halogenated aromatic solvents. By using a ternary solvent system consisting of CF, 1,8-diiodooctane (DIO), and chlorobenzene (CB), the short-circuit current density is increased by 50% in solar cells based on a DPP polymer (PDPP5T) and a NFA (IEICO-4F) compared to the use of CF with DIO only. However, the open-circuit voltage and fill factor are reduced. As a result, the efficiency improves from 3.4 to 4.8% only. The use of CB results in stronger aggregation of IEICO-4F as inferred from two-dimensional grazing-incidence wide-angle X-ray diffraction. Photo- and electroluminescence and mobility measurements indicate that the changes in performance can be ascribed to a more aggregated blend film in which charge generation is increased but nonradiative recombination is enhanced because of reduced hole mobility. Hence, while CB is essential to obtain well-ordered domains of IEICO-4F in blends with PDPP5T, the morphology and resulting hole mobility of PDPP5T domains remain suboptimal. The results identify the challenges in processing organic solar cells based on DPP polymers and NFAs as near-infrared absorbing photoactive layers.

Synthesis and Characterization of Isoindigo-Based Push-Pull Chromophores

Symmetrical and unsymmetrical chromophores of isoindigo 3-7 were designed and synthesized, in which isoindigo was used as the central unit (electron acceptor unit A), triphenylamine as the end capping unit (electron donor group D), 1,1,4,4-tetracyanobutadiene (TCBD, A') and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (A″) as the acceptor unit. The effects of multiacceptor units on photophysical, electrochemical, and computational studies were investigated. The photophysical properties of isoindigo 6 and 7 exhibit a strong intramolecular charge transfer (ICT) absorption band in the near IR region. The isoindigo 4-7 shows multi-redox waves with a low electrochemical band gap, which signifies the tuning of highest occupied molecular orbital-lowest unoccupied molecular orbital energy levels and enhance the π-conjugation. The computational studies demonstrate that there is a good agreement with experimental data. The molecular design and synthesis of isoindigo 4-7 gives a new avenue for the development of building blocks in organic electronics.

Oligo(ethylene glycol) side chain effect on the physical properties and molecular arrangement of oligothiophene-isoindigo based conjugated polymers

Oligo(ethylene glycol) (OEG) side chains are widely used in donor-acceptor conjugated polymers (D-A CPs) and enable the polymers to dissolve and be processed in environmentally friendly and cost-effective nonchlorinated solvents, such as water. However, the OEG effect on the physical properties of D-A CPs has not been thoroughly studied and sometimes the results are controversial. In this study, two oligothiophene-isoindigo based conjugated polymers, P3TI and P4TI, are selected as model polymers to investigate the OEG effect. PnTI has octyl side chains on the oligothiophene unit and 2-hexyldecyl side chains on the isoindigo unit. The replacement of an alkyl side chain with OEG not only changes the optical and thermal properties but also the molecular arrangements of the polymers such as π-π d-spacing, crystallinity, and packing orientation. The domination of the crystallization behavior changes from the oligothiophene unit to the isoindigo unit when the bulky alkyl group is replaced by the flexible and linear OEG. The packing changes from edge-on to face-on orientation. The results are intriguing and provide new insights into this class of polymers.

Synthesis and Density Functional Theory Studies of Azirinyl and Oxiranyl Functionalized Isoindigo and (3Z,3'Z)-3,3'-(ethane-1,2-diylidene)bis(indolin-2-one) Derivatives

The design and synthesis of functionalized isoindigo compounds by reaction of isoindigo with (S)-glycidyl tosylate, epibromohydrin, 2-(bromomethyl)-1-(arylsulfonyl)aziridine, and 2-(bromomethyl)-1-(alkylsulfonyl)aziridine in the presence of MeONa proceed under mild conditions in moderate yields. (3Z,3’Z)-3,3’-(Ethane-1,2-diylidene)bis(1-(oxiran-2-ylmethyl)indolin-2-one), with an extended central olefin π-conjugated moiety was also reacted with methyl-oxiranes to give the corresponding N,N’-disubstituted derivative. Calculations with DFT and TD-DFT of hypothetical isoindigo-thiophene DA molecules with various electron withdrawing substituents, including aziridine, oxirane, nitrile, carbonyl, and sulfonate, indicated that the proximity and strength of the functional group have a significant effect on the HOMO, LUMO, vertical excitation energy, and oscillator strength of the π–π* transitions.

Molecular Engineering of a Donor-Acceptor Polymer To Realize Single Band Absorption toward a Red-Selective Thin-Film Organic Photodiode

Herein, we explore the strategy of realizing a red-selective thin-film organic photodiode (OPD) by synthesizing a new copolymer with a highly selective red-absorption feature. PCZ-Th-DPP, with phenanthrocarbazole (PCZ) and diketopyrrolopyrrole (DPP) as donor and acceptor units, respectively, was strategically designed/synthesized based on a time-dependent density functional theory calculation, which predicted the significant suppression of the band II absorption of PCZ-Th-DPP due to the extremely efficient intramolecular charge transfer. We demonstrate that the synthesized PCZ-Th-DPP exhibits not only a high absorption coefficient within the red-selective band I region, as theoretically predicted, but also a preferential face-on intermolecular structure in the thin-film state, which is beneficial for vertical charge extraction as an outcome of a glancing incidence X-ray diffraction study. By employing PCZ-Th-DPP as a photoactive layer of Schottky OPD, to fully match its absorption characteristic to the spectral response of the red-selective OPD, we demonstrate a genuine red-selective specific detectivity in the order of 10¹² Jones while maintaining a thin active layer thickness of ∼300 nm. This work demonstrates the possibility of realizing a full color image sensor with a synthetic approach to the constituting active layers without optical manipulation.

Adjusting Aggregation Modes and Photophysical and Photovoltaic Properties of Diketopyrrolopyrrole-Based Small Molecules by Introducing B←N Bonds

The packing mode of small-molecular semiconductors in thin films is an important factor that controls the performance of their optoelectronic devices. Designing and changing the packing mode by molecular engineering is challenging. Three structurally related diketopyrrolopyrrole (DPP)-based compounds were synthesized to study the effect of replacing C-C bonds by isoelectronic dipolar B←N bonds. By replacing one of the bridging C-C bonds on the peripheral fluorene units of the DPP molecules by a coordinative B←N bond and changing the B←N bond orientation, the optical absorption, fluorescence, and excited-state lifetime of the compounds can be tuned. The substitution alters the preferential aggregation of the molecules in the solid state from H-type (for C-C) to J-type (for B←N). Introducing B←N bonds thus provides a subtle way of controlling the packing mode. The photovoltaic properties of the compounds were evaluated in bulk heterojunctions with a fullerene acceptor and showed moderate performance as a consequence of suboptimal morphologies, bimolecular recombination, and triplet-state formation.

Efficient and Stable Perovskite Solar Cells Using Low-Cost Aniline-Based Enamine Hole-Transporting Materials

Metal-halide perovskites offer great potential to realize low-cost and flexible next-generation solar cells. Low-temperature-processed organic hole-transporting layers play an important role in advancing device efficiencies and stabilities. Inexpensive and stable hole-transporting materials (HTMs) are highly desirable toward the scaling up of perovskite solar cells (PSCs). Here, a new group of aniline-based enamine HTMs obtained via a one-step synthesis procedure is reported, without using a transition metal catalyst, from very common and inexpensive aniline precursors. This results in a material cost reduction to less than 1/5 of that for the archetypal spiro-OMeTAD. PSCs using an enamine V1091 HTM exhibit a champion power conversion efficiency of over 20%. Importantly, the unsealed devices with V1091 retain 96% of their original efficiency after storage in ambient air, with a relative humidity of 45% for over 800 h, while the devices fabricated using spiro-OMeTAD dropped down to 42% of their original efficiency after aging. Additionally, these materials can be processed via both solution and vacuum processes, which is believed to open up new possibilities for interlayers used in large-area all perovskite tandem cells, as well as many other optoelectronic device applications.

Synthesis and photophysical characterization of isoindigo building blocks as molecular acceptors for organic photovoltaics

Five isoindigo-based donor-acceptor-donor (D-A-D) type small molecules have been synthesized in order to investigate their intramolecular charge transfer characteristics. UV-vis absorption of these dyes exhibits a wide absorption band ranging from 300 to 650 nm with two distinct bands, giving the narrow bandgaps between 1.72 and 1.85 eV. Taking into account their HOMO-LUMO energy levels and bandgaps, isoindigo dyes have been used in the active layer of organic solar cell (OSC) devices. When these small molecule semiconductors were used as acceptors with the donor poly(3-hexylthiophene-2,5-diyl (P3HT) polymer in the inverted OSC devices, the highest power conversion efficiency (PCE) was obtained as 0.10% for pyrene-substituted isoindigo derivative.

Nanoscale isoindigo-carriers: self-assembly and tunable properties

Over the last decade isoindigo derivatives have attracted much attention due to their high potential in pharmacy and in the chemistry of materials. In addition, isoindigo derivatives can be modified to form supramolecular structures with tunable morphologies for the use in drug delivery. Amphiphilic long-chain dialkylated isoindigos have the ability to form stable solid nanoparticles via a simple nanoprecipitation technique. Their self-assembly was investigated using tensiometry, dynamic light scattering, spectrophotometry, and fluorometry. The critical association concentrations and aggregate sizes were measured. The hydrophilic-lipophilic balance of alkylated isoindigo derivatives strongly influences aggregate morphology. In the case of short-chain dialkylated isoindigo derivatives, supramolecular polymers of 200 to 700 nm were formed. For long-chain dialkylated isoindigo derivatives, micellar aggregates of 100 to 200 nm were observed. Using micellar surfactant water-soluble forms of monosubstituted 1-hexadecylisoindigo as well as 1,1'-dimethylisoindigo were prepared for the first time. The formation of mixed micellar structures of different types in micellar anionic surfactant solutions (sodium dodecyl sulfate) was determined. These findings are of practical importance and are of potential interest for the design of drug delivery systems and new nanomaterials.

2,6-Di(thiophenyl)-1,5-dihydrodipyrrolopyrazine (DT-DPP) structural isomers as donor–acceptor–donor molecules and their optoelectronic investigation

We report the synthesis and characterization of two new D–A–D molecules via Pd catalyzed C–C and C–N coupling reactions. This illustrates the potential of DT-DPP for promising optoelectronic applications.

Structural engineering of porphyrin-based small molecules as donors for efficient organic solar cells

Porphyrin-based small molecules as donors have long been ignored in bulky heterojunction organic solar cells due to their unfavorable aggregation and the low charge mobility. With the aim of striking a delicate balance between molecular design, morphology, interfacial layer and device fabrication to maximize the power conversion efficiency (PCE) of organic solar cells, three comparable porphyrin-based small molecules with an acceptor-donor-acceptor configuration have been developed for use as donor materials in solution processed small molecule bulk heterojunction organic solar cells. In these molecules, electron-deficient 3-ethylrhodanine is introduced into the electron-rich porphyrin core through 5,15-bis(phenylethynyl) linkers. Structural engineering with 10,20-bis(2-hexylnonyl) aliphatic peripheral substituent on the porphyrin core, instead of the aromatic substituents such as 10,20-bis[3,5-di(dodecyloxyl)phenyl], and 10,20-bis(4-dodecyloxylphenyl), can simultaneously facilitate stronger intermolecular π-π stacking and higher charge transfer mobility in the film, leading to a maximum PCE of 7.70% in a conventional device. The inverted devices have also been demonstrated to have long-term ambient stability and a comparable PCE of 7.55%.

Influence of structure–property relationships of two structural isomers of thiophene-flanked diazaisoindigo on carrier-transport properties

Two structural isomers of thiophene-flanked diazaisoindigo, 6,6′-substituted 6,6′-T-DAII and 5,5′-substituted 5,5′-T-DAII, have been synthesized to study the influence of the connecting modes on the carrier-transport properties.

A detailed study on the thermal, photo-physical and electrochemical properties and OFET applications of D–π–A–π–D structured unsymmetrical diketopyrrolopyrrole materials

Seven unsymmetrical DPP-derivatives with a D–π–A–π–D architecture have been synthesized and the effect of the electron donating ability and extent of electronic conjugation of the end capping units on their properties are explored.

Photocurrent generation of nanofibers constructed using a complex of a gelator and a fullerene derivative

Two-component xerogel film constructed by a carboxylic acid derivative and fullerene derivative could generate large photocurrent under light irradiation.

Isoindigo-Containing Molecular Semiconductors: Effect of Backbone Extension on Molecular Organization and Organic Solar Cell Performance

We have synthesized three new isoindigo-based small molecules by extending the conjugated length through the incorporation of octyl-thiophene units between the isoindigo core and benzothiophene terminal units. Both UV-vis and Grazing incidence X-ray diffraction experiments show that such extension of the π-conjugated backbone can induce H-aggregation, and enhance crystallinity and molecular ordering of these isoindigo-based small molecules in the solid state. Compared to two other isoindigo-based derivatives in the series, the derivative with two octyl-thiophene units, BT-T2-ID, is the most crystalline and ordered, and its molecular packing motif appears to be substantially different. Devices utilizing these new extended isoindigo-based small molecules as the electron donor exhibit higher performance than those utilizing nonextended BT-ID as the electron donor. Particularly, devices containing BT-T2-ID in an as-cast blend with PC₆₁BM show power conversion efficiencies up to 3.4%, which is comparable to the best devices containing isoindigo-based molecular semiconductors and is a record among devices containing isoindigo-based small molecules that were processed in the absence of any additives.