Solar cell efficiency, self-assembly, and dipole-dipole interactions of isomorphic narrow-band-gap molecules

Study of Vertical and Lateral Charge Transport Properties of DPP-Based Polymer/PC61BM Films Using Space Charge Limited Current (SCLC) and Field Effect Transistor Methods and their Effects on Photovoltaic Characteristics

We studied the vertical and lateral charge transport characteristics of a diketopyrrolopyrrole polymer donor (D)–PC61BM acceptor (A) system by measuring the space charge limited current (SCLC) mobility and field-effect mobility respectively. It was found that with an increase in annealing temperature, the SCLC hole mobility decreased for the pure polymer (PDBFBT) but increased for the PDBFBT:PC61BM blends, which could be explained by changes in the crystallinity and crystal orientation (edge-on versus face-on). The pure PDBFBT and most blend films showed the maximum field-effect hole mobility (µh) when annealed at 100°C, which then declined as the annealing temperature was further increased. Surprisingly, the D/A = 1/1 blend films annealed at high temperatures exhibited an abrupt increase in the field-effect µh. This unusual phenomenon was interpreted by the antiplasticization effect of PC61BM, which promoted the molecular organization of the polymer. The effect of annealing on the carrier mobility was further correlated with the performance of inverted organic solar cell devices with the PDBFBT:PC61BM blend (D/A = 1/3). Thermal annealing at high temperatures (>100°C) was found to obstruct electron transport and cause the device performance to significantly deteriorate.

Thiophene-based push–pull chromophores for small molecule organic solar cells (SMOSCs)

A concise review on small molecules organic solar cells based on π-conjugated thiophene scaffolds.

Maximizing the dielectric response of molecular thin films via quantum chemical design

Developing high-capacitance organic gate dielectrics is critical for advances in electronic circuitry based on unconventional semiconductors. While high-dielectric constant molecular substances are known, the mechanism of dielectric response and the fundamental chemical design principles are not well understood. Using a plane-wave density functional theory formalism, we show that it is possible to map the atomic-scale dielectric profiles of molecule-based materials while capturing important bulk characteristics. For molecular films, this approach reveals how basic materials properties such as surface coverage density, molecular tilt angle, and π-system planarity can dramatically influence dielectric response. Additionally, relatively modest molecular backbone and substituent variations can be employed to substantially enhance film dielectric response. For dense surface coverages and proper molecular alignment, conjugated hydrocarbon chains can achieve dielectric constants of >8.0, more than 3 times that of analogous saturated chains, ∼2.5. However, this conjugation-related dielectric enhancement depends on proper molecular orientation and planarization, with enhancements up to 60% for proper molecular alignment with the applied field and an additional 30% for conformations such as coplanarity in extended π-systems. Conjugation length is not the only determinant of dielectric response, and appended polarizable high-Z substituents can increase molecular film response more than 2-fold, affording estimated capacitances of >9.0 μF/cm2. However, in large π-systems, polar substituent effects are substantially attenuated.

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.

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

We designed and synthesized a series of isoindigo-based derivatives to investigate how chemical structure modification at both the 6,6'- and 5,5'-positions of the core with electron-rich and electron-poor moieties affect photophysical and redox properties as well as their solid-state organization. Our studies reveal that 6,6'-substitution on the isoindigo core results in a stronger intramolecular charge transfer band due to strong electronic coupling between the 6,6'-substituent and the core, whereas 5,5'-substitution induces a weaker CT band that is more sensitive to the electronic nature of the substituents. In the solid state, 6,6'-derivatives generally form J-aggregates, whereas 5,5'-derivatives form H-aggregates. With only two branched ethylhexyl side chains, the 6,6'-derivatives form organized lamellar structures in the solid state. The incorporation of electron-rich benzothiophene, BT, substituents further enhances ordering, likely because of strong intermolecular donor-acceptor interactions between the BT substituent and the electron-poor isoindigo core on neighboring compounds. Collectively, the enhanced photophysical properties and solid-state organization of the 6,6'-benzothiophene substituted isoindigo derivative compared to the other isoindigo derivatives examined in this study resulted in solar cells with higher power conversion efficiencies when blended with a fullerene derivative.

Mapping orientational order in a bulk heterojunction solar cell with polarization-dependent photoconductive atomic force microscopy

New methods connecting molecular structure, self-organization, and optoelectronic performance are important for understanding the current generation of organic photovoltaic (OPV) materials. In high power conversion efficiency (PCE) OPVs, light-harvesting small-molecules or polymers are typically blended with fullerene derivatives and deposited in thin films, forming a bulk heterojunction (BHJ), a self-assembled three-dimensional nanostructure of electron donors and acceptors that separates and transports charges. Recent data suggest micrometer-scale orientational order of donor domains exists within this complex nanomorphology, but the link to the optoelectronic properties is yet unexplored. Here we introduce polarization-dependent, photoconductive atomic force microscopy (pd-pcAFM) as a combined probe of orientational order and nanoscale optoelectronic properties (∼20 nm resolution). Using the donor 7,7'-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(6-fluoro-4-(5'-hexyl[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole), p-DTS(FBTTh2)2, we show significant spatial dependence of the nanoscale photocurrent with polarized light in both pristine and BHJ blends (up to 7.0% PCE) due to the local alignment of the molecular transition dipoles. By mapping the polarization dependence of the nanoscale photocurrent, we estimate the molecular orientation and orientational order parameter. Liquid crystalline disclinations are observed in all films, in agreement with complementary electron microscopy experiments, and the order parameter exceeds 0.3. The results demonstrate the utility of pd-pcAFM to investigate the optical/structural anisotropy that exists within a well-performing BHJ system and its relationship to optoelectronic properties on both the nanometer and micrometer length scales.

Hydrogen bonding in bulk heterojunction solar cells: a case study

Small molecules with dithieno[3,2-b;2',3'-d]thiophene as central building block and octyl cyanoacetate and octyl cyanoacetamide as different terminal building blocks have been designed and synthesized. The amide containing small molecule can form intermolecular hydrogen bonding between N-H...O = C of the amide group. The photovoltaic properties and active layer morphologies of the two molecules in bulk heterojunction solar cells are compared to study the influence of hydrogen bonding on the active layer morphology. New methanofullerene compound containing amide group has also been synthesized and compared with conventional fullerene electron acceptors.

Nanoscopic management of molecular packing and orientation of small molecules by a combination of linear and branched alkyl side chains

We synthesized a series of acceptor-donor-acceptor-type small molecules (SIDPP-EE, SIDPP-EO, SIDPP-OE, and SIDPP-OO) consisting of a dithienosilole (SI) electron-donating moiety and two diketopyrrolopyrrole (DPP) electron-withdrawing moieties each bearing linear n-octyl (O) and/or branched 2-ethylhexyl (E) alkyl side chains. X-ray diffraction patterns revealed that SIDPP-EE and SIDPP-EO films were highly crystalline with pronounced edge-on orientation, whereas SIDPP-OE and SIDPP-OO films were less crystalline with a radial distribution of molecular orientations. Near-edge X-ray absorption fine structure spectroscopy disclosed an edge-on orientation with a molecular backbone tilt angle of ∼22° for both SIDPP-EE and SIDPP-EO. Our analysis of the molecular packing and orientation indicated that the shorter 2-ethylhexyl groups on the SI core promote tight π-π stacking of the molecular backbone, whereas n-octyl groups on the SI core hinder close π-π stacking to some degree. Conversely, the longer linear n-octyl groups on the DPP arms facilitate close intermolecular packing via octyl-octyl interdigitation. Quantum mechanics/molecular mechanics molecular dynamics simulations determined the optimal three-dimensional positions of the flexible alkyl side chains of the SI and DPP units, which elucidates the structural cause of the molecular packing and orientation explicitly. The alkyl-chain-dependent molecular stacking significantly affected the electrical properties of the molecular films. The edge-on oriented molecules showed high hole mobilities in organic field-effect transistors, while the radially oriented molecules exhibited high photovoltaic properties in organic photovoltaic cells. These results demonstrate that appropriate positioning of alkyl side chains can modulate crystallinity and molecular orientation in SIDPP films, which ultimately have a profound impact on carrier transport and photovoltaic performance.

Unexpected but convenient synthesis of soluble meso-tetrakis(3,4-benzoquinone)-substituted porphyrins

A new route to 3,4-benzoquinone-substituted porphyrins is reported. In attempted nitration reactions on the copper(II) or nickel(II) complexes of 5,10,15,20-tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)porphyrin using lithium nitrate in acetic anhydride-acetic acid/chloroform no nitration products could be detected with the main products being the corresponding complexes of 5,10,15,20-tetrakis(3,4-dioxo-5-t-butylcyclohexa-1,5-dienyl)porphyrin. These o-quinone-substituted porphyrins are available in reasonable yield (> 50%), their synthesis is simple and they are of good solubility. The electrochemical and spectroelectrochemical properties of representative o-quinone-substituted Cu ( II ) and Ni ( II ) porphyrin derivatives are also reported.

Dipole-dipole interactions in solution mixtures probed by two-dimensional synchronous spectroscopy based on orthogonal sample design scheme

Two-dimensional (2D) synchronous spectroscopy together with a new approach called "Orthogonal Sample Design Scheme" was used to study the dipole-dipole interactions in two representative ternary chemical systems (N,N-dimethyllformamide (DMF)/CH3COOC2H5/CCl4 and C60/CH3COOC2H5/CCl4). For the first system, dipole-dipole interactions among carbonyl groups from DMF and CH3COOC2H5 are characterized by using the cross peak in 2D Fourier Transform Infrared Radiation (FT-IR) spectroscopy. For the second system, intermolecular interaction among π-π transition from C60 and vibration transition from the carbonyl band of ethyl acetate is probed by using 2D spectra. The experimental results demonstrate that "Orthogonal Sample Design Scheme" can effectively remove interfering part that is not relevant to intermolecular interaction. Additional procedures are carried out to preclude the possibilities of producing interfering cross peaks by other reasons, such as experimental errors. Dipole-dipole interactions that manifest in the form of deviation from the Beer-Lambert law generate distinct cross peaks visualized in the resultant 2D synchronous spectra of the two chemical systems. This work demonstrates that 2D synchronous spectra coupled with orthogonal sample design scheme provide us an applicable experimental approach to probing and characterizing dipole-dipole interactions in complex molecular systems.

Evaluation of heterocycle-modified pentathiophene-based molecular donor materials for solar cells

Two novel solution-processable acceptor-donor-acceptor (A-D-A)-structured organic small molecules with diketopyrrolopyrrole (DPP) as terminal acceptor units and pentathiophene (PTA) or pyrrole-modified pentathiophene (NPTA) as the central donor unit, namely, DPP2(PTA) and DPP2(NPTA), were designed and synthesized. We examined the effects of changing the central bridging heteroatoms of the five-ring-fused thienoacene core identity from sulfur [DPP2(PTA)] to nitrogen [DPP2(NPTA)] in the small-molecule donor material. Replacement of the bridging atom with a different electronic structure has a visible effect on both the optical and electrical properties: DPP2(NPTA), which contains much more electron-rich pyrrole in the central thienoacene unit, possesses red-shifted absorption and a higher HOMO level relative to DPP2(PTA) with the less electron-rich thiophene in the same position. More importantly, substitution of the bridging atoms results in a change of the substituting alkyl chains due to the nature of the heteroatoms, which significantly tailored the crystallization behavior and the ability to form an interpenetrating network in thin-film blends with an electron acceptor. Compared to DPP2(PTA) with no alkyl chain substituting on the central sulfur atom of the PTA unit, DPP2(NPTA) exhibits improved crystallinity and better miscibility with PC71BM probably because of a dodecyl chain on the central nitrogen atom of the NPTA unit. These features endow the DPP2(NPTA)/PC71BM blend film higher hole mobility and better donor/acceptor interpenetrating network morphology. Optimized photovoltaic device fabrication based on DPP2(NPTA)/PC71BM (1.5:1, w/w) has resulted in an average power conversion efficiency (PCE) as high as 3.69% (the maximum PCE was 3.83%). This study demonstrates that subtle changes and tailoring of the molecular structure, such as simply changing the bridging heteroatom in the thienoacene unit in D/A-type small molecules, can strongly affect the physical properties that govern their photovoltaic performances.

Transient photocurrent response of small-molecule bulk heterojunction solar cells

Transient photocurrent measurements are used to investigate the effects of processing additives on charge transport in small molecule bulk heterojunction solar cells. The additive decreased carrier recombination rates and improved carrier mobility, both of which are beneficial to carrier extraction. Geminate recombination of charge transfer excitons is ruled out by the data.

Additive-assisted control over phase-separated nanostructures by manipulating alkylthienyl position at donor backbone for solution-processed, non-fullerene, all-small-molecule solar cells

A non-fullerene, all-small-molecule solar cell (NF-SMSC) device uses the blend of a small molecule donor and a small molecule acceptor as the active layer. Aggregation ability is a key factor for this type of solar cell. Herein, we used the alkylthienyl unit to tune the aggregation ability of the diketopyrrolopyrrole (DPP)-based small molecule donors. Replacing two alkoxyl units in BDT-O-DPP with two alkylthienyl units yields BDT-T-DPP, and further introducing another two alkylthienyl units into the backbone produces BDT-T-2T-DPP. With the introduction of alkylthienyl, the backbone becomes twisted. As a result, the ππ-stacking strength, aggregation ability, and crystallite size all obey the sequence of BDT-O-DPP > BDT-T-DPP > BDT-T-2T-DPP. When selected a reported perylene diimide dimer of bis-PDI-T-EG as acceptor, the best NF-SMSC device exhibits a power conversion efficiency of 1.34, 2.01, and 1.62%, respectively, for the BDT-O-DPP, BDT-T-DPP, and BDT-T-2T-DPP based system. The BDT-T-DPP/bis-PDI-T-EG system yields the best efficiency of 2.01% among the three combinations. This is due to the moderate aggregation ability of BDT-T-DPP yields moderate phase size of 30-50 nm, whereas the strong aggregation ability of BDT-O-DPP gives a bigger size of 50-80 nm, and the weak aggregation ability of BDT-T-2T-DPP produces a smaller size of 10-30 nm. The BDT-T-DPP/bis-PDI-T-EG combination exhibits balanced hole/electron mobility of 0.022/0.016 cm(2)/(V s), whereas the BDT-O-DPP/bis-PDI-T-EG and the BDT-T-2T-DPP/bis-PDI-T-EG blend show a hole/electron mobility of 0.0011/0.0057 cm(2)/(V s) and 0.0016/0.11 cm(2)/(V s), respectively.

Design and synthesis of molecular donors for solution-processed high-efficiency organic solar cells

Organic semiconductors incorporated into solar cells using a bulk heterojunction (BHJ) construction show promise as a cleaner answer to increasing energy needs throughout the world. Organic solar cells based on the BHJ architecture have steadily increased in their device performance over the past two decades, with power conversion efficiencies reaching 10%. Much of this success has come with conjugated polymer/fullerene combinations, where optimized polymer design strategies, synthetic protocols, device fabrication procedures, and characterization methods have provided significant advancements in the technology. More recently, chemists have been paying particular attention to well-defined molecular donor systems due to their ease of functionalization, amenability to standard organic purification and characterization methods, and reduced batch-to-batch variability compared to polymer counterparts. There are several critical properties for efficient small molecule donors. First, broad optical absorption needs to extend towards the near-IR region to achieve spectral overlap with the solar spectrum. Second, the low lying highest occupied molecular orbital (HOMO) energy levels need to be between -5.2 and -5.5 eV to ensure acceptable device open circuit voltages. Third, the structures need to be relatively planar to ensure close intermolecular contacts and high charge carrier mobilities. And last, the small molecule donors need to be sufficiently soluble in organic solvents (≥10 mg/mL) to facilitate solution deposition of thin films of appropriate uniformity and thickness. Ideally, these molecules should be constructed from cost-effective, sustainable building blocks using established, high yielding reactions in as few steps as possible. The structures should also be easy to functionalize to maximize tunability for desired properties. In this Account, we present a chronological description of our thought process and design strategies used in the development of highly efficient molecular donors that achieve power conversion efficiencies greater than 7%. The molecules are based on a modular D(1)-A-D(2)-A-D(1) architecture, where A is an asymmetric electron deficient heterocycle, which allowed us to quickly access a library of compounds and develop structure-property-performance relationships. Modifications to the D1 and D2 units enable spectral coverage throughout the entire visible region and control of HOMO energy levels, while adjustments to the pendant alkyl substituents dictate molecular solubility, thermal transition temperatures, and solid-state organizational tendencies. Additionally, we discuss regiochemical considerations that highlight how individual atom placements can significantly influence molecular and subsequently device characteristics. Our results demonstrate the utility of this architecture for generating promising materials to be integrated into organic photovoltaic devices, call attention to areas for improvement, and provide guiding principles to sustain the steady increases necessary to move this technology forward.

Synthesis and photovoltaic performances in solution-processed BHJs of oligothiophene-substituted organocobalt complexes [(η4-C4(nT)4)Co(η5-C5H5)]

New cobalt complexes substituted by four oligothiophene arms have been synthesized. Solution processed solar cells based on CpCoCb(3T)₄ exhibit conversion efficiencies of up to 2.1%.

Dithieno[3,2-b:2′,3′-d]pyran-containing organic D–π–A sensitizers for dye-sensitized solar cells

Organic regioisomeric D–π–A sensitizers incorporating dithieno[3,2-b:2′,3′-d]pyran (DTP) unit have been designed and evaluated for DSSCs.

25th anniversary article: a decade of organic/polymeric photovoltaic research

Organic photovoltaic (OPV) technology has been developed and improved from a fancy concept with less than 1% power conversion efficiency (PCE) to over 10% PCE, particularly through the efforts in the last decade. The significant progress is the result of multidisciplinary research ranging from chemistry, material science, physics, and engineering. These efforts include the design and synthesis of novel compounds, understanding and controlling the film morphology, elucidating the device mechanisms, developing new device architectures, and improving large-scale manufacture. All of these achievements catalyzed the rapid growth of the OPV technology. This review article takes a retrospective look at the research and development of OPV, and focuses on recent advances of solution-processed materials and devices during the last decade, particular the polymer version of the materials and devices. The work in this field is exciting and OPV technology is a promising candidate for future thin film solar cells.

The role of additive in diketopyrrolopyrrole-based small molecular bulk heterojunction solar cells

A new diketopyrrolopyrrole-based small molecule for solution-processed OPVs is synthesized. The chemical additive has a profound effect in refining the morphology and, thus, significantly increases the efficiencies of the devices. The role of the additive is characterized by various techniques and the additive-driven structure-property relationship is established, which reveals that the additive affects the crystallization, PCBM aggregation, and phase separation.

Tri-diketopyrrolopyrrole molecular donor materials for high-performance solution-processed bulk heterojunction solar cells

Two new high-performance DPP-containing donor molecules employing a molecular architecture with three DPP chromorphores (tri-DPP) in conjugated backbones are synthesized and characterized. The two tri-DPP molecules with only a structural difference on alkyl substitutions, when blended with PC71 BM, lead to power conversion efficiencies up to 4.8 and 5.5%, respectively.

Effects of stereoisomerism on the crystallization behavior and optoelectrical properties of conjugated molecules

Three stereoisomers of DPP(TBFu)2 are separated and identified to investigate the effects of stereoisomerism on crystal structures and the optoelectrical properties. The crystal structures and FET mobility are sensitive to stereoisomers, in which the mesomer possesses the highest carrier mobility and the greatest crystallization tendency to dominate the crystallization in spin-cast films of the as-synthesized stereoisomeric mixture.

Constructing high-efficiency D-A-π-A-featured solar cell sensitizers: a promising building block of 2,3-diphenylquinoxaline for antiaggregation and photostability

Controlling the sensitizer morphology on a nanocrystalline TiO2 surface is beneficial to facilitating electron injection and suppressing charge recombination. Given that the grafted alkyl chain on a π-bridge thiophene segment for preventing π aggregation can deteriorate its intrinsic photostability, we incorporate a promising building block of 2,3-diphenylquinoxaline as the additional acceptor to construct a novel D-A-π-A-featured dye IQ4, which exhibits several characteristics: (i) efficiently decreasing the molecular HOMO-LUMO energy gap by extending its absorption bands; (ii) showing a moderate electron-withdrawing capability for an ideal balance in both promising photocurrent and photovoltage; (iii) endowing an ideal morphology control with strong capability of restraining the intermolecular aggregation and facilitating the formation of a compact sensitizer layer via two twisted phenyl groups grafted onto the quinoxaline unit. The coadsorbent-free dye-sensitized solar cell (DSSC) based on dye IQ4 exhibits very promising conversion efficiency as high as 9.24 ± 0.05%, with a short-circuit current density (Jsc) of 17.55 mA cm(-2), an open-circuit voltage (Voc) of 0.74 V, and a fill factor (FF) of 0.71 under AM 1.5 illumination (100 mW cm(-2)). IQ4-based DSSC devices with an ionic liquid electrolyte can keep constant performance during a 1000 h aging test under 1 sun at 60 °C. Because of spatial restriction, the two phenyl groups grafted onto the additional electron-withdrawing quinoxaline are demonstrated as efficient building blocks, not only improving its photostability and thermal stability but also allowing it to be a successful antiaggregation functional unit. As a consequence, the incorporated 2,3-diphenylquinoxaline unit can realize a facile structural modification for constructing organic coadsorbent-free D-A-π-A-featured sensitizers, thus paving a way to replace the common, stability-deleterious grafted alkyl chain on the thienyl bridge.

Remarkable order of a high-performance polymer

We directly image the rich nanoscale organization of the high performance, n-type polymer poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2)) using a combination of high-resolution transmission electron microscopy and scanning transmission electron microscopy. We demonstrate that it is possible to spatially resolve "face-on" lamella through the 2.4 nm alkyl stacking distance corresponding to the (100) reflection. The lamella locally transition between ordered and disordered states over a length scale on the order of 10 nm; however, the polymer backbones retain long-range correlations over length-scales approaching a micrometer. Moreover, we frequently observe overlapping structure implying a number of layers may exist throughout the thickness of the film (~20 nm). The results provide a simple picture, a highly ordered lamella nanostructure over nearly the entire film and ordered domains with overlapping layers providing additional interconnectivity, which unifies prior seemingly contradictory conclusions surrounding this remarkable, high-mobility material.

Efficient solution-processed small-molecule solar cells with inverted structure

We successfully demonstrate inverted structure small-molecule (SM) solar cells with an efficiency of 7.88% using ZnO and PEIE as an interfacial layer. Modification of ZnO with a cost-effective PEIE thin layer increases the efficiency of the inverted cell as a result of reducing the work function of the cathode and suppressing the trap-assisted recombination. In addition to the high efficiency, the inverted SM solar cells are relatively stable in air compared to conventional cells.

Influences of the non-covalent interaction strength on reaching high solid-state order and device performance of a low bandgap polymer with axisymmetrical structural units

A high organic field-effect transistor mobility (0.29 cm(2) V(-1) s(-1) ) and bulk-heterojunction polymer solar cell performance (PCE of 6.82%) have been achieved in a low bandgap alternating copolymer consisting of axisymmetrical structural units, 5,6-difluorobenzo-2,1,3-thiadiazole. Introducing the fluorine substituents enhanced intermolecular interaction and improved the solid-state order, which consequently resulted in the highest device performances among the 2,1,3-thiadiazole-quarterthiophene based alternating copolymers.