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Morphological control over the bulk heterojunction (BHJ) microstructure of a high-efficiency small molecule photovoltaic system composed of a quinquethiophene based molecule (DRCN5T) as electron donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC 70 BM) as electron acceptor is demonstrated using three different post-processing strategies, including thermal annealing (TA), solvent vapor annealing (SVA), and two-step annealing (TA-SVA) treatments. We systematically analyze the processing condition-microstructure-device property relationships, explore the corresponding morphology evolution and their effects on carrier transport and recombination dynamics in BHJs as well as understand the nature of phase-separation process resulting in light-induced degradation mechanisms. Within the investigated results, the causative relations between annealing sequence, photovoltaic parameters, morphology evolution and charge carrier dynamics are for the first time delineated. In addition, the observed trade-offs in device efficiency and stability with respect to the well-defined morphologies are highlighted. The in-depth picture of the bulk microstructure formation and its kinetic evolution as a function of the specific post-processing approaches is a valuable asset for the design of new photovoltaic materials and thin film nanoscale architectures that are more efficient and better aid future commercialization efforts. [ABSTRACT FROM AUTHOR]

We report a large bandgap (1.9 eV) donor–acceptor copolymer (named PffT2-FTAZ) that enables polymer solar cells with a high power conversion efficiency of 7.8%. An important structural feature of the PffT2-FTAZ polymer is a difluorinated donor unit (3,3′-difluoro-2,2′-bithiophene, or, ffT2) that introduces several surprising and/or beneficial effects. By comparing PffT2-FTAZ with the analog polymer (PT2-FTAZ) without fluorination on the bithiophene donor unit, it is found that the ffT2 unit effectively lowers the HOMO and LUMO energy levels of the polymer and slightly reduces optical bandgap. It also introduces strong interchain aggregation for the polymer in solution, which leads to a highly crystalline polymer film and reasonably high hole transport mobility. On the other hand, the PffT2-FTAZ: fullerene blend still exhibits a reasonably small polymer domain size suitable for polymer solar cell operation. All these positive factors combined leads to dramatically enhanced performance for the polymer solar cells with the power conversion efficiency increasing from 2.8% for PT2-FTAZ to 7.8% for f PffT2-FTAZ. The high PSC performance of PffT2-FTAZ makes it a promising candidate for high efficiency tandem PSCs. [ABSTRACT FROM AUTHOR]

High-performance indoor organic photovoltaics (IOPV) require large-bandgap material systems to absorb visible light efficiently and reduce energy loss. However, state-of-the-art non-fullerene acceptors (NFAs) have absorptions in the near-infrared region and are thus not suitable for IOPV applications. Herein, we report a series of large-bandgap (>1.70 eV) NFAs named FCC-Cl-C8, FCC-Cl-4Ph and FCC-Cl-6Ph by modifying the alkyl side chains with alkylphenyl chains partially or completely. Results show that the bulky alkylphenyl side chains can finely tune the absorption properties of the NFAs and also affect their morphological properties. Interestingly, the best-performing NFA is the one (named FCC-Cl-4Ph) with partial alkyl and alkylphenyl substitutions, which blue-shift the absorption of the NFAs while minimizing the negative morphological effect of the bulky alkylphenyl chains. As a result, FCC-Cl-4Ph can achieve excellent indoor efficiencies over 29% under a 3000 K LED lamp at 1000 lux and show better solution processability over FCC-Cl-C8. More importantly, FCC-Cl-4Ph can maintain high indoor performance (29.7–26.8% at 1000 lux) under a wide range of indoor lighting spectra (2600, 3000, 4000, and 6500 K LED lamps), which should be due to the blue-shifted spectra of FCC-Cl-4Ph and better matching with various indoor conditions. This work reveals an interesting structure-property relationship and offers useful strategies for the further design of NFAs toward efficient IOPV devices. [Display omitted] • Side chain modifications of non-fullerene acceptors yield indoor efficiency approaching 30%. • Reduced spectral mismatch enables indoor devices with decent performance from 2600 K to 6000 K. • Stable FCC-Cl-4Ph-based cells are suitable for large-area devices driving low-power sensors. [ABSTRACT FROM AUTHOR]

The utilization of near edge x-ray absorption fine structure spectroscopy (NEXAFS) in achieving strong, novel contrast for soft x-ray microscopy and scattering methods has been afforded significant success in elucidating outstanding issues in organic materials systems due to the unique combination of high sensitivity to chemical functionality and thus composition, moderately high spatial resolution and moderate radiation damage. We illustrate the basic operating principles of NEXAFS spectroscopy, scanning transmission x-ray microscopy, and resonant soft x-ray scattering, and exemplify the impact by discussing a few recent applications. The focus of this perspective will be the characterization of synthetic organic materials, with a further emphasis on applications involving semi-conducting polymers. We also provide a brief perspective of future instrument and method developments. [Copyright &y& Elsevier]

Abstract: Near Edge X-ray Absorption Fine Structure (NEXAFS) spectromicroscopy, resonant scattering and resonant reflectivity are specialized, synchrotron radiation based, soft X-ray characterization tools that provide moiety-specific contrast and either real-space imaging at ∼30nm spatial resolution, or scattering signals which can be inverted to provide chemically sensitive information at an even higher spatial resolution (<5nm). These X-ray techniques complement other real and reciprocal space characterization tools such as various microscopies and conventional electron, X-ray and neutron scattering. We provide an overview of these synchrotron based tools, describe their present state-of-the-art and discuss a number of applications to exemplify their unique aspects. [Copyright &y& Elsevier]

In addition to high power conversion efficiency (PCE) and good stability, the low-cost of photovoltaic materials is also very important for the practical application of organic solar cells (OSCs). Herein, we synthesized a carboxylate substituted pyrazine-based electron-deficient building block (DTCPz) with a simple structure and low synthetic cost, and then developed a novel wide bandgap polymer donor PFBCPZ. Due to the synergistic electron-withdrawing effects of the fluorination in donor unit (BDT-TF) and esterification and C=N double-bond in DTCPz unit, PFBCPZ shows a deeper HOMO level of −5.60 eV, a strong intermolecular π-π interaction, good crystallinity and stacking, and high hole-mobility of 2.11 × 10−3 cm2 V−1 s−1. Matched with a low bandgap acceptor IT-4F, excellent charge transfer, weak recombination, and small non-radiative energy loss in OSCs was achieved, resulting in an impressive fill factor of 0.785 and a high open-circuit voltage of 0.92 V. As a result, a PCE of up to 15.3% is obtained in OSCs, which is the highest value in the IT-4F-based binary OSCs so far and indicates that low-cost DTCPz with a simple structure is a promising building block to construct high-performance polymer donors for application in efficient OSCs. [Display omitted] • A novel wide bandgap polymer donor PFBCPZ was synthesized. • The novel building block (DTCPz) showed easily synthesized routs with cheap raw material. • The OSC based on PFBCPZ:IT-4F exhibited excellent performance with 15.3% efficiency. • The efficiency is the highest value in the IT-4F-based binary OSCs so far. [ABSTRACT FROM AUTHOR]

Despite significant advances in the design and synthesis of photovoltaic materials, halogenated solvent additives have been predominately used in the device fabrication of state-of-the-art materials. Also, little attention has been paid to replace these additives with halogen-free ones. Aimed at addressing this problem, this study shows the effectiveness of a hydrocarbon solvent additive, namely 1-phenylnaphthalene (PN) in two high-efficiency polymer:nonfullerene blends (PTB7-Th:IEICS-4F and PM6:Y6) and importantly presents a detailed understanding of the role of PN by combined X-ray scattering and calorimetric analysis. Tuning the concentration of PN allows us to control the film crystallinity and domain correlation characteristics of polymer:nonfullerene blend films, thereby resulting in distinct device performances. Inclusion of 0.5% volume amount of PN dramatically enhances the nonfullerene ordering in the blend films, as evidenced by sharp reflection peaks and high melting enthalpies. The promoted paracrystalline order together with the highest composition variation related to domain purity result in ~35% improvement in the device efficiency of PTB7-Th:IEICS-4F over reference devices. Applying the same treatment results in enhanced efficiency in the PM6:Y6 device, which outperforms its additive-free control device. This mechanistic understanding will be of great significance to the future development of OPVs towards eco-friendly fabrication. Image 1 ● Profound effect of solvent additive in efficient polymer:nonfullerene solar cells ● Revealing how sensitive the additive concentration alters film morphology ● 1-phenylnaphthalene is capable of tuning morphology and performance [ABSTRACT FROM AUTHOR]

Inner side-chain engineering on Y6 has been proven successful in improving short-circuit current density (J SC) through fine-tuning aggregated structures of acceptors. However, it fails in tuning the lowest unoccupied molecular orbital level (LUMO) and open-circuit voltage (V OC). In this paper, we turn to focus on engineering the outer side chains on the flanking thienothiophene units with 4-hexylphenyl (PhC6) and 6-phenylhexyl (C6Ph) chains. Use of PhC6 enhances the steric effect between the attached phenyl and the ending group, which in combination with the additional conjugation effect provided by the linking phenyl leads to upshifted energy levels and increased V OC as a result. Again, substitution with the bulkier PhC6 unprecedentedly improves film-morphology with reduced paracrystalline disorder and long period and increased root-mean-square composition variations as well, leading to increased electron and hole mobilities and suppressed monomolecular recombination with J SC and fill-factor (FF) simultaneously enhanced. The PM6:BTP-PhC6-based devices yield a higher efficiency value of 16.7% than the PM6:BTP-C6Ph-based one (15.5%). Therefore, this study shows a conceptual advance in materials design towards reducing the conflict between V OC and J SC in binary blended organic solar cells, which can be achieved by introducing bulkier chains to twist the backbone and simultaneously enhance the packing order. Image 1 • Two isomeric nonfullerene acceptors, BTP-C6Ph and BTP-PhC6 are reported. • This work shows a conceptual advance in materials design towards reducing the conflict between V OC and J SC. • 4-hexylphenyl (PhC6) as the outer side chains leads to upshifted energy levels and increased V OC. • The bulkier PhC6 chains provided enhanced hindrance acts as conformation-locking to reduce the number of stereoisomers. • Higher PCE of 16.7% was obtained on the PM6:BTP-PhC6-based devices. [ABSTRACT FROM AUTHOR]

Simultaneously broadening the spectral response and reducing the energy loss are challenging tasks in the material design of organic solar cells (OSCs). Herein, a novel asymmetrically noncovalently fused-ring electron acceptor (NFEA) with unilateral alkylthio-substituted thiophene π-bridge, namely IDST-4F, is synthesized. IDST-4F exhibits a broader absorption, higher-lying energy levels, larger dipole moments and suppressed crystallinity than its symmetric counterpart (ID-4F) without the π-bridge. Compared to the devices of PM6:ID-4F, the optimized PM6:IDST-4F-based devices display simultaneously enhanced current density and photovoltage, resulting in an excellent power conversion efficiency (PCE) of 14.3%, which is the highest value among the OSCs based on NFEAs reported in the literature to date. More importantly, the PM6:IDST-4F-based OSCs possess excellent thermal stability with 82% of the initial PCE after thermal treatment at 150 °C for 1200 min. In summary, this study indicates that asymmetrically NFEAs are promising to achieve high efficiency with excellent thermal stability. Image 1 • A novel acceptor IDST-4F, consisting of an asymmetric fused-ring core locked by a noncovalent interaction is developed. • The PM6:IDST-4F-based OSCs display a significantly improved efficiency of 14.3%. • The PM6:IDST-4F-based PSCs exhibit lower energy loss of 0.59 eV and excellent thermal stability. [ABSTRACT FROM AUTHOR]

Abstract Poly(3-hexylthiophene) (P3HT) is a broadly used donor material for polymer solar cells (PSCs) due to its simple molecular structure and low production cost. In recent years, the P3HT-based non-fullerene PSCs have been extensively studied as the rapid development of the non-fullerene small molecular acceptors (NFSMAs). However, the active layers based on the P3HT and NFSMAs blend usually exhibit unfavorable phase separation morphologies, resulting in relatively low short-circuit currents (J SC s) and power conversion efficiencies (PCEs) of the corresponding PSC devices. Herein, we utilized the chloroform (CF)/anisole solvent mixture to adjust the aggregation effect of P3HT in solution state to optimize the phase separation morphology of the active layers. As a result, the combination of CF, anisole, and 1,8-diiodooctane (DIO) affords an enhanced J SC of 14.23 mA cm−2 for a P3HT:IEICO-based PSC. Graphical abstract Image 1 Highlights • The aggregation effect of P3HT in solution state was modulated by mixed solvent. • The phase separation morphology of the P3HT:IEICO film was optimized. • Device processed by CF/anisole/DIO showed higher J SC than the one processed by CF. [ABSTRACT FROM AUTHOR]

Polymer acceptors based on perylene diimide (PDI) with three symmetrical S-heteroacene backbone units of different sizes were synthesized for use in all-polymer solar cells (all-PSCs). The effects of varying the size of the heteroacene unit on the backbone of the PDI polymer are evident in the absorption spectra and their energy level offsets, which are correlated with their morphological and photovoltaic properties. These newly synthesized polymers were employed as acceptors with the polymer poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)] (PPDT2FBT) as the donor in all-PSCs that were found to exhibit modest power conversion efficiencies. The variations in photovoltaic properties of all-PSCs are investigated by characterizing the charge generation, carrier mobilities and recombination. The morphological disorder at the polymer/PPDT2FBT interface and average composition variations are revealed by using grazing incidence wide angle X-ray scattering (GIWAXS) and resonance soft X-ray scattering (R-SoXS) characterizations. [ABSTRACT FROM AUTHOR]

Molecular miscibility is a defining property of mixtures of electron-donating polymers and electron-accepting fullerenes used in the photoactive layer of bulk heterojunction (BHJ) organic solar cells. Even though miscibility of fullerene in the polymer is a commonly observed property, quantitative measurements have unknown connections with other morphological properties such as domain size and domain purity. By varying the amount of fullerene loading in BHJ thin films, we show that morphological properties are related to miscibility via straightforward relationships. In particular, miscibility influences the sensitivity of domain spacing to fullerene loading and determines the domain volume fractions and the residual fullerene content in the mixed polymer-rich phase in devices. Not only does this work show the relationships between miscibility and morphology, but it also highlights the general importance in determining miscibility levels in BHJ systems in order to target optimum morphologies and domain purities. [ABSTRACT FROM AUTHOR]

Abstract: We have shown that the addition of resorcinol di(phenyl phosphate) (RDP)-coated starch can improve the compatibility to either Ecoflex or poly(lactic acid) (PLA). The increased compatibilization enhanced the tensile properties such as yield strength and impact toughness. In particular, we examined the effect of addition of RDP-coated starch on thermal responses of a blend of Ecoflex/PLA. We found that the combination of RDP-coated starches with nanoclays could render the blends self-extinguishing since they are formed as a shell-like chars on the exposure surface against heat, which can prevent the melt polymers against dripping. With an examination on the scanning transmission X-ray microscopy (STXM) images of the blends, the Ecoflex domains were well dispersed in the PLA matrix, while the domains became smaller when the RDP-coated starch was added. Moreover, we demonstrated that the introduction of either flat-like or tube-like clays could provide an increase of interfacial area on the RDP-coated starch surfaces, where each polymer chain preferentially segregates to either the starch or the clay surface. Thus, large complex in-situ grafts with polymers can be formed at the interfaces. Additionally, the complex in-situ grafts could influence flammability of the blends. We have shown that the addition of RDP-coated clays can decrease the mass loss rate of Ecoflex/Starch blends, while a lot of nanofiber are formed on the chars surface, which are entangled each other with the clay platelets. The mechanical properties of the chars structures were examined by nano-indentation, where a good elastic chars formation could keep the internal pressures built up with decomposed gases from melt polymers as well as ductility of the chars could play an important role on releasing the internal gases through small vents on its surface, steadily where a good elastic and ductile chars formation could require keeping the internal pressures built up with decomposed gases from melt polymers. [Copyright &y& Elsevier]