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For the first time, formamidinium–caesium perovskite thin films were successfully synthesised from a lead acetate-based precursor. Efficient perovskite solar cells (21.0%) and modules (18.8%) have been produced using blade coating techniques.

A systematic spectroelectrochemical approach is presented to precisely determine frontier orbital energies of PM6:Y6 blends in device-relevant films and results are discussed regarding their impact on solar cell performance.

A range of advanced imaging techniques are employed to study the micron- and nano-scale morphology of a polymer/non-fullerene acceptor blend. Cryo-electron microscopy in particular reveals nanoscale variations in molecular orientation and order.

The way in which conjugated polymers pack in the solid state strongly affects the performance of polymer-based optoelectronic devices. However, even for the most crystalline conjugated polymers the precise packing of chains within the unit cell is not well established. Here we show that by performing resonant X-ray diffraction experiments at the sulfur K-edge we are able to resolve the tilting of the planar backbones of crystalline poly(3-hexylthiophene) (P3HT) within the unit cell. This approach exploits the anisotropic nature of the X-ray optical properties of conjugated polymers, enabling us to discern between different proposed crystal structures. By comparing our data with simulations based on different orientations, a tilting of the planar conjugated backbone with respect to the side chain stacking direction of 30 ± 5° is determined.

The random double-cable conjugated polymers pendent with near-infrared acceptors with tunable contents have been synthesized, exhibiting a high efficiency of 9.4% in single-component organic solar cells.

While the strategy of blending commodity insulating polymers with conjugated polymers has been previously applied to modify the properties of organic field-effect transistors (OFETs), comprehensive...

This study is jointly supported by MOST (2017YFA0204702, 2018YFA0208504,) and NSFC (52073016, 51773207, 51973031, 21905018, 21905158) of China. This work was further supported by the Fundamental Research Funds for the Central Universities (buctrc201828, XK1802-2), Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm202043), Jiangxi Provincial Department of Science and Technology (No. 20192ACB20009) and Hong Kong Scholars Program (Grant No. XJ2020051). The research reported in this publication was supported by funding from King Abdullah University of Science and Technology (KAUST). A.C.Y.L. acknowledges support from the Australian Research Council (FT180100594). The authors acknowledge the use of instruments and assistance at the Monash Ramaciotti Centre for Cryo-Electron Microscopy, a Node of Microscopy Australia, including the Titan Krios (Australian Research Council LE120100090). This work was performed in part at the SAXS/WAXS beamtline at the Australian Synchrotron, part of ANSTO. Z. T. acknowledges the Shanghai Pujiang Program (Grant No. 19PJ1400500).

Conjugated polymer field-effect transistors are emerging as an enabling technology for flexible electronics due to their excellent mechanical properties combined with sufficiently high charge carrier mobilities and compatibility with large-area, low-temperature processing. However, their electrical stability remains a concern. ON-state (accumulation mode) bias-stress instabilities in organic semiconductors have been widely studied, and multiple mitigation strategies have been suggested. In contrast, OFF-state (depletion mode) bias-stress instabilities remain poorly understood despite being crucial for many applications in which the transistors are held in their OFF-state for most of the time. Here, we present a simple method of using an anti-solvent treatment to achieve significant improvements in OFF-state bias-stress and environmental stability as well as general device performance for one of the best performing polymers, solution-processable indacenodithiophene-co-benzothiadiazole (IDT-BT). IDT-BT is weakly crystalline, and we attribute the notable improvements to an anti-solvent-induced, increased degree of crystallinity, resulting in a lower probability of electron trapping and the removal of charge traps. Our work highlights the importance of the microstructure in weakly crystalline polymer films and offers a simple processing strategy for achieving the reliability required for applications in flexible electronics. Validerad;2023;Nivå 2;2023-07-05 (hanlid);Funder: Engineering and Physical Sciences Research Council (EP/R031894/1); EPSRC Centre for Doctoral Training (EP/L015889/1); EPSRC (EP/S030662/1)

Funder: China Scholarship Council; Id: http://dx.doi.org/10.13039/501100004543, Funder: Royal Society Newton International Fellowship, Heavy heteroatom substitution of the backbone is an effective strategy to improve molecular packing and charge delocalization in polymer semiconductors. Such a backbone modification also facilitates oxidative doping as a result of reduced ionization potential (IP). Here, the effect of single‐atom selenium substitution on doping and charge transport properties of a class of polythiophene copolymers is explored. The room temperature (RT) conductivities of the doped polymers are significantly enhanced by the selenium substitution for both molecular doping and ion exchange doping. The enhanced conduction is rationalized by the better crystallinity of the selenium‐containing system, which can be reinforced by a chain‐extended ribbon‐phase morphology induced by thermal annealing, which is robust toward doping. The resulting increase in the charge delocalization of the doped selenium‐containing system is evidenced by temperature‐dependent conductivities. In ion exchange doped films the maximum conductivity of ≈700 S cm−1 and a high thermoelectric (TE) power factor (PF) of 46.5 μW m−1 K−2 is achieved for the doped selenophene polymer and signatures of a metal‐insulator (M–I) transition are observed that are characteristics for heterogeneous conduction systems. The results show that single‐atom selenium substitution is an effective molecular design approach for improving the charge transport and TE properties of conjugated polymers.

The combination of computational methods and advanced characterization techniques is used to highlight the role of the intramolecular hydrogen bond in thienyldiketopyrrolopyrrole (ThDPPTh) copolyme...

The performance of optoelectronic devices based on conjugated polymers is critically dependent upon molecular packing; however, the paracrystalline nature of these materials limits the amount of information that can be extracted from conventional X-ray diffraction. Resonant diffraction (also known as anomalous diffraction) occurs when the X-ray energy used coincides with an X-ray absorption edge in one of the constituent elements in the sample. The rapid changes in diffraction intensity that occur as the X-ray energy is varied across an absorption edge provide additional information that is lost in a conventional nonresonant experiment. Taking advantage of the fact that many conjugated polymers contain sulfur as heteroatoms, this work reveals pronounced resonant diffraction effects at the sulfur K-edge with a particular focus on the well-studied electron transporting 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). The observed behavior is found to be consistent with the theory of resonant diffraction, and by simulating the energy-dependent peak intensity based on proposed crystal structures for P(NDI2OD-T2), we find that resonant diffraction can discriminate between different crystalline packing structures. The utilization of resonant diffraction opens up a new way to unlock important microstructural information about conjugated polymers for which only a handful of diffraction peaks are typically available.

The performance of bulk heterojunction (BHJ) organic solar cells is well-known to be influenced by the properties of the donor polymer employed such as its molecular weight (MW) and regioregularity. In this study, four different molecular weight batches of high regioregularity (100%) poly(3-hexylthiophene) (DF-P3HT) are investigated. Unlike other studies, here the RR of the P3HT is fixed (to 100%) and the MW of the polymer is varied to understand the influence of MW on P3HT physical properties such as its electrical, optical and thermal properties and microstructure using X-ray synchrotron techniques. Significantly, it is found that annealing has less of an influence the properties of pristine films of P3HT when the RR of the P3HT is very high. A similar approach is used to examine the physical properties and microstructure of P3HT:PCBM blend films. The properties of the blend films for different MW are correlated with the performance of the BHJ solar cells fabricated using P3HT:PCBM blends. A record high average efficiency of 3.8% for as-cast devices (no annealing or solvent additive) with best devices reaching performance over 4% is obtained for DF-P3HT:PCBM with MW of 44 kDa. Though there is a marginal variation in the performance of the devices with change in MW, no systematic variation in device performance as a function of MW is observed in contrast to other MW studies employed P3HT of lower RR. These findings are attractive in the light of the mass production of polymer solar cells such as via roll-to-roll printing where thermal annealing is not possible or desired.

Atom-economic protocols for the synthesis of poly(indacenodithiophene-alt-benzothiadiazole) (PIDTBT) are presented in which all C–C coupling steps are achieved by direct arylation. Using two different synthetic pathways, PIDTBT copolymers with different side chains (hexylphenyl, octylphenyl, dodecyl, methyl/2-octyldodecylphenyl, 2-octyldodecylphenyl/2-octyldodecylphenyl) and molecular weight (MW) are prepared. Route A makes use of direct arylation polycondensation (DAP) of indacenodithiophene (IDT) and 4,7-dibromo-2,1,3-benzothiadiazole (BTBr2) leading to PIDTBT in high yields, with adjustable MW and without indications for structural defects. Route B starts from a polyketone precursor also prepared by DAP following cyclization. While route B allows introduction of asymmetric side chains at the IDT unit, polymer analogous cyclization gives rise to defect formation. The absorption coefficient of PIDTBT with alkylphenyl side chains made by route A increases with MW. Field-effect hole mobilities around ∼10−2 cm2 V−1 s−1 are molecular weight-independent, which is ascribed to a largely amorphous thin film morphology. PIDTBT with linear dodecyl side (C12) chains exhibits a bathochromic shift (20 nm), in agreement with theory, and more pronounced vibronic contributions to absorption spectra. In comparison to alkylphenyl side chains, C12 side chains allow for increased order in thin films, a weak melting endotherm and lower energetic disorder, which altogether explain substantially higher field-effect hole mobilities of ∼ 10−1 cm2 V−1 s−1.

Three n-type fused lactam semiconducting polymers were synthesized for thermoelectric and transistor applications via a cheap, highly atom-efficient, and nontoxic transition-metal free aldol polycondensation. Energy level analysis of the three polymers demonstrated that reducing the central acene core size from two anthracenes (A-A), to mixed naphthalene-anthracene (A-N), and two naphthalene cores (N-N) resulted in progressively larger electron affinities, thereby suggesting an increasingly more favorable and efficient solution doping process when employing 4-(2,3-dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI) as the dopant. Meanwhile, organic field effect transistor (OFET) mobility data showed the N-N and A-N polymers to feature the highest charge carrier mobilities, further highlighting the benefits of aryl core contraction to the electronic performance of the materials. Ultimately, the combination of these two factors resulted in N-N, A-N, and A-A to display power factors (PFs) of 3.2 μW m-1 K-2, 1.6 μW m-1 K-2, and 0.3 μW m-1 K-2, respectively, when doped with N-DMBI, whereby the PFs recorded for N-N and A-N are among the highest reported in the literature for n-type polymers. Importantly, the results reported in this study highlight that modulating the size of the central acene ring is a highly effective molecular design strategy to optimize the thermoelectric performance of conjugated polymers, thus also providing new insights into the molecular design guidelines for the next generation of high-performance n-type materials for thermoelectric applications.

The crystallization and aggregation behaviors of semiconducting polymers play a critical role in determining the ultimate performance of optoelectronic devices based on these materials. Due to the soft nature of polymers, crystallite imperfection exists ubiquitously. To this aspect, crystallinity is often used to represent the degree of crystallite imperfection in a reciprocal relation. Despite of the importance, the discussion on crystallinity is still on the phenomenological level and ambiguous in many cases. As two major contributors to crystallite imperfection, crystallite size and paracrystallinity are highly intertwined and hardly separated, hindering more accurate and trustworthy structural analysis. Herein, with the aid of synchrotron-based X-ray diffraction, combined with environmentally controlled heating capability, the evolution of crystallite size and paracrystallinity of two prototypical polythiophene-based thin films have been successfully measured. Strikingly, the paracrystallinity of poly(3-hexylthiophene-2,5-diyl) (P3HT) crystallites remains unchanged with annealing, while the paracrystallinity of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) becomes diminished with crystallite growth. This work delivers a promising gesture to semiconducting polymers community, confirming that it is possible to experimentally separate crystallite size and paracrystallinity, both of which are highly intertwined. With this progress, investigation on the correlation between further detailed microstructural parameters and device performance can be achieved.

The nanomorphology of bulk heterojunction (BHJ) blends based on poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediy...

Interfacial out-of-plane molecular orientation critically influences the electronic performance of organic semiconductor thin films. The appearance of a lyotropic liquid crystalline (LC) mesophase ...

Introducing layered quasi-2D perovskite phases into a conventional 3D perovskite light-absorbing matrix is a promising strategy for overcoming the limited environmental stability of 3D perovskite solar cells. Here, we present a simple drop-casting method for preparing hybrid perovskite films comprising both quasi-2D and quasi-3D phases, formed using phenylethylammonium or iso-butylammonium as spacer cations. The film morphology, phase purity, and crystal orientation of the hybrid quasi-2D/3D perovskite films are improved significantly by applying a simple N2 blow-drying step, together with inclusion of methylammonium chloride as an additive. An enhanced power conversion efficiency of 16.0% is achieved using an iso-butylammonium-based quasi-2D/3D perovskite layer which, to our knowledge, is the highest recorded to date for a quasi-2D/3D perovskite solar cells containing a non-spin-cast perovskite layer prepared under ambient laboratory conditions. Perovskite solar cells have substantial potential for solar conversion, but developing simple and scalable fabrication processes is challenging. Here, a drop-casting process compatible with roll-to-roll production of quasi-2D/3D perovskite layers is developed, with a conversion efficiency of up to 16%.

Sequential deposition is one of the most promising approaches toward achieving scalable fabrication of metal halide perovskite thin films. However, this fabrication approach conventionally lends itself to the incomplete conversion of the lead halide, which impacts the stability, performance, and reproducibility of functional devices featuring such thin films. In this work, we have overcome this limitation by using a simple solvent and process engineering approach. We show that through the use of an optimized dimethylformamide and dimethyl sulfoxide solvent mixture in the precursor solution, and through the judicious control of this solution, the substrate, and final annealing temperatures, highly uniform and mesoporous PbI2 thin films can be deposited. The porous structure of these films is found to accelerate the interdiffusion of CH3NH3I (MAI) during the second-step process when carried out at room temperature, enabling their complete conversion into CH3NH3PbI3 perovskite. Detailed investigations using scanning electron microscopy, X-ray diffraction, grazing-incidence wide-angle X-ray scattering, thermogravimetric analysis, UV–vis absorption, photoluminescence, and time-of-flight secondary ion mass spectrometry have been used to provide mechanistic insights into the porous PbI2 film formation and the interdiffusion process. Solar cells based on planar fluorine-doped tin oxide (FTO)/TiO2/CH3NH3PbI3/spiro-OMeTAD/Au device architectures yield optimized device efficiencies of 19%, which is among the highest for this device structure and perovskite absorber material. The applicability of this enhanced sequential deposition method to other perovskite systems has been further validated through the fabrication of efficient FAxMA1–xPbIxBr3–x and CsPbIxBr3–x solar cells.

Raman spectroscopy is a powerful technique for the study of materials chemistry and nanostructure. This nondestructive technique is highly sensitive to molecular and crystal lattice vibrations, whi...

Two indenothiophene-based non-fullerene small molecule acceptors (NFSMAs), FTBT and FTTBT, were designed and synthesized to investigate the photovoltaic effect of fusing a thiophene into the core of NFSMAs. Compared with the none-fused FTBT, the thiophene-fused FTTBT achieves a much higher power conversion efficiency (PCE) of 9.79% with a Voc of 0.934 V, a Jsc of 16.01 mA cm−2 and an FF of 65.49%, when it was blended with PM6 polymer donor to fabricate bulk-heterojunction solar cells. Combined photophysical, electrochemical, photovoltaic property and morphology analysis indicates that the boosted device performance mainly lies in two reasons: (i) the incorporation of an electron-donating thiophene ring narrows the optical bandgap by extending π-conjugation, which contributes to a large short-circuit current; (ii) the incorporation of a single thiophene ring transforms the axisymmetrical molecular configuration into a centrosymmetrical one, which decreases the crystallinity and optimizes the packing feature in the blend. It results in a transport-favorable blending morphology and contributes to a high fill factor. The work clarifies an effective molecular design strategy for performance enhancement of organic solar cells.

In this study we report on a series of 12 small molecular p-type penta- and hexathiophene semiconductors end-capped with either malononitrile (MN), hexyl rhodanine (HR) or dicyano hexyl rhodanine (CHR) acceptors and functionalized with alkyl sidechains of two different lengths, orientated away from the central core in a regioregular manner. Organic photovoltaic devices were fabricated, and annealed using conditions optimized for each material to produce a wide range of power-conversion efficiencies (PCEs) from 1–8%. We find that achieving high PCEs is critically dependent on matching the conjugation length to specific end-group acceptors, and also find a slight correlation between improved performances and longer sidechains. To understand this behavior we performed an in-depth analysis of the structure–property relationships of these materials on intermolecular packing through grazing-incidence wide-angle X-ray scattering (GIWAXS) studies, which yielded several key relationships between molecular structure, bulk crystallinity and optoelectronic properties.

Summary Flexible organic solar cells (OSCs) with high power conversion efficiency (PCE) and excellent mechanical properties are considered a promising power source for wearable electronic devices. However, simultaneously achieving high efficiency and robust mechanical stability is still challenging because highly crystalline or aggregated microstructures that are thought to be critical for enabling efficient device operation render the active layer brittle. In this study, we demonstrate 3-μm-thick ultraflexible OSCs by utilizing a mixed fullerene/non-fullerene acceptor that can achieve an efficiency of 13% (certified value of 12.3%) with 97% retention in the PCE after 1,000 bending cycles (bending radius of 0.5 mm). In addition, although ultraflexible OSCs cannot survive under the intrinsic tensile test with a large strain, they exhibit excellent mechanical behavior under the cyclic compression-stretching test via the formation of a buckling device structure, yielding an 89% retention in the PCE after 1,000 cycles (45% compression and bending radius of 10 μm). A facile approach introducing a small amount of high-electron-mobility fullerene acceptor into a non-fullerene binary blend enhances charge transport, improves exciton separation, and optimizes the blend morphology with more amorphous regions, thus producing a more efficient and mechanically robust device.