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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 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.

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...

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.

Ruddlesden–Popper layered perovskites have emerged as a promising solution for overcoming the moisture instability of three-dimensional hybrid perovskite materials. Given that the optoelectronic properties of these layered perovskites strongly depend on the dimensionality (n) of the phases present, understanding of the microstructure and order in such materials is important. Typically, the dimensionality of phases present is inferred from optical measurements rather than diffraction measurements which are a more direct probe of structural order. Here we use a combination of grazing-incidence transmission wide-angle X-ray scattering and transmission wide-angle X-ray scattering techniques to probe the in-plane microstructure of highly textured Ruddlesden–Popper hybrid perovskite films with a target dimensionality of n = 4. By being able to resolve the in-plane diffraction peaks corresponding to the periodic repeating of layered phases that are often obscured by the substrate horizon in grazing incidence measurements, we are able to directly characterise structural order in such films. Despite having a target dimensionality of n = 4, only diffraction peaks corresponding to n = 2, 3 and ∞ phases are observed. Observations from X-ray measurements are combined with optical absorption measurements to show that there is increasing structural disorder in low-n phases with increasing dimensionality. Further analysis of peak positions and peak widths in the X-ray scattering patterns indicate that stacking fault defects are the origin of structural disorder in layered perovskite films, with increasing disorder with increasing n. We also present a complete indexing of grazing-incidence wide-angle X-ray scattering patterns of highly textured layered perovskite films and confirm the structurally ordered n = 3 and ∞ phases as the dominant crystalline phases in such films. This work provides guidance for the reliable characterisation of structural order and orientation in layered perovskite thin films from X-ray scattering measurements which will assist with assessing n-phase purity and photovoltaic device optimisation.

Hybrid organic-inorganic perovskites with appealing optoelectronic properties have attracted significant interest for photovoltaic application. The use of chloride (Cl-)-containing species to induce improved perovskite thin-film microstructures and improved optoelectronic properties is well-established. However, the mechanism for the formation of perovskite films with highly textured, micron-sized grains in the presence of Cl- is not well established. Using synchrotron-based in situ two-dimensional grazing incidence wide-angle X-ray scattering complemented by scanning electron microscopy imaging, we present an oriented attachment mechanism via mineral bridge formation for the microstructural evolution of perovskite films post-treated with methylammonium chloride. We have identified the crucial role of the chlorine-containing intermediate phase as the mineral bridge, which enables the reorientation of primary, nanoscale perovskite grains followed by fusion into uniaxial oriented quasi-single crystal grains. The resulting perovskite films exhibit micron-sized grains with preferential orientation of the tetragonal (110) direction perpendicular to the substrate, resulting in improved solar cell efficiency attributed to improved charge collection. Our findings help to understand the fundamental mechanisms of microstructure evolution via soft processing in hybrid perovskite films.

We report two newly synthesized naphthalene diimide (NDI)-based conjugated polymers, poly[(E)-2,7-bis(2-decyltetradecyl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone-vinylene-thiophene-viny...

The determination of the onset temperature for molecular reorganization in conjugated polymer thin films is highly relevant for understanding microstructural evolution and device optimization. In s...

All-polymer solar cells have gained attention in recent years with a solar cell performance of over 11% power conversion efficiency (PCE) recently demonstrated. The n-type polymer PNDIT2, also known as N2200 or P(NDI2OD-T2), has been extensively used for both photovoltaic and field-effect transistor applications. When paired with donor materials that have appropriately aligned energy levels, PNDIT2 exhibited device efficiencies over 10% PCE, and organic field-effect transistors fabricated with PNDIT2 exhibited mobilities over 1 cm2/V s. Thionation of the naphthalene diimide (NDI) moiety, which is the substitution of imide oxygen with sulfur atoms, has been shown to improve the field-effect transistor performance of NDI-based small molecules. By applying this strategy to PNDIT2, we explored the effect that thionation, in a 2S-trans configuration, has on the performance of all-polymer solar cells fabricated with the donor polymer PTB7-Th. Solar cells were fabricated with the original polymer, PNDIT2, as a r...

We have synthesised new materials comprised of 9,9′-bifluorenylidene and dithienodiketopyrrolopyrrole units. While 9,9′-bifluorenylidene has been primarily used in non-fullerene acceptors, when used in combination with the diketopyrrolopyrrole moiety it can form donor materials that can be used in conjunction with fullerene acceptors. The compounds differ in the substituents on the 9,9′-bifluorenylidene (protonated = 1A and dimethoxy = 1B) moiety. The structure of both the neat and blend films with PC70BM were found to be strongly dependent on the processing solvent used. In particular, Grazing Incidence Wide Angle X-ray Scattering measurements of films of 1A or 1B blended with PC70BM prepared from chloroform or chloroform with o-dichlorobenzene as an additive showed that the donor material had no particular ordering. However, when 1,8-diiodooctane was added to the processing solvent the 1A blends showed liquid crystalline ordering while 1B formed well-defined crystallites with three-dimensional ordering. The difference in film structure had a profound effect on the device properties. For 1A the optimised blend ratio with PC70BM was 1:4, but when 1,8-diiodooctane was used as the additive the best ratio of 1A to fullerene was 1:1. The films containing the well-defined crystallites of 1B all performed poorly, which was ascribed to a lack of a percolation pathway for hole extraction. The best performing device was comprised of a 1:1 blend of 1A and PC70BM, which had a power conversion efficiency of 2.6%.

Summary Molecular Kekule diradicals have been demonstrated to show unique optoelectronic properties as a function of their diradical character. A series of thienoquinoidal oligothiophenes from dimer to pentamer and substituted with an odd and even number of pyrrolo-dione groups have been prepared and proven to be n-dopable materials showing outstanding ambient stability and excellent electrical and thermoelectric behavior. Going from dimer to pentamer, a progressive change in the diradical character and aggregation mode is observed, with the tetramer showing an optimal diradical character that allows favorable intermolecular contacts with π−π multi-bonding features, while the presence of the two dione groups promotes a cholesteric-like π−π stacking. Both features synergistically contribute to form a material with exceptional ambient stability for an n-doped system exhibiting high electrical conductivities and thermoelectric performance.

Naphthalene diimide (NDI) with tertiary amine side chains is used to n-dope a series of NDI derivatives of varying energy levels. We demonstrate a photoinduced, intermolecular redox-doping process in which a dimethylpropyl amine side chain attached to one NDI reduces another NDI derivative to form radical anions. The influence of the aromatic core substituents on energy levels, doping efficacy and radical anion stability is studied by cyclic voltammetry, UV-Vis and electron paramagnetic resonance (EPR) spectroscopy. In general, the HOMO energy level of the NDI is responsible for the doping process and the LUMO for air stability of the resulting radical anion. The most electron deficient NDI derivative having two cyano substituents displays the highest doping yield and yields air stable radical anions for both light- and thermally-induced doping. Thermal doping is further accompanied by morphologic changes that stabilize radical anions in air.

Chiral alkyl chains are widely utilized to ensure the solubility of solution-processed organic semiconductors (OSCs), while intrinsic defects due to the mixture of several stereoisomers in one material are frequently neglected. Herein, through introducing an optically pure pendant into the molecular backbone of a core-expanded naphthalene diimide (NDI-DTYM2), enantiopure semiconductor materials (1-R/1-S) and the corresponding racemate (1-rac) were designed and synthesized to investigate the impact of enantiopure and racemic OSCs on charge transport in organic field-effect transistors (OFETs). Surprisingly, a 2–4 times increase in electron mobility (from 0.15 cm2 V−1 s−1 to 0.6 cm2 V−1 s−1 for as-cast devices and from 0.42 cm2 V−1 s−1 to 0.98 cm2 V−1 s−1 for thermally-annealed devices) was discovered in solution-processed OFETs simply by switching the active layer material from racemic to the enantiopure form. Grazing-incidence wide-angle X-ray scattering (GIWAXS), atomic force microscopy (AFM) and were utilized to investigate the crystallization, molecular micro-organization and film morphology of 1-R, 1-S and 1-rac, demonstrating that the lower mobility of racemic material (1-rac) might be attributed to interfacial defects among different crystalline domains as well as subtle changes in molecular packing.

Sidechain engineering of the benzodithiophene-based high-performance organic solar cell (OSC) material BTR generated four novel p-type small-molecule semiconducting materials to investigate structure–property relationships in p-type organic semiconductors. Through depth-dependent grazing incidence wide-angle X-ray scattering (GIWAXS) studies, it is found that our best performing materials possess an active-layer morphology comprised of a vertically graded structure with different crystallite orientations. We further establish a correlation between a high solubility in the spin-coating deposition solution and the occurrence of this vertical distribution, whereas materials with lower solubility are found to possess a predominantly edge-on molecular orientation. These results further the understanding of this high-performing class of materials and suggest the importance of solubility considerations to OSC morphology.

Metal-halide perovskites have emerged as one of the most promising semiconductor materials for photovoltaic and other optoelectronic applications. Recently, mixed halide hybrid perovskites where I− is partly substituted by Br− have driven a paradigm shift in perovskite solar cells, combining tunability of absorption onset with high performance. However, poor long-term device stability has been limiting their practical application and the mechanisms of degradation have not been fully understood. In this work, we study light induced degradation in mixed halide perovskite CH3NH3PbI3−xBrx. Although it is well-known that such perovskites undergo a reversible light-induced phase segregation, the subsequent degradation process under continued, and in particular high intensity laser irradiation has not been systematically studied. By utilizing in situ laser excited photoluminescence spectroscopy it is shown that such mixed halide perovskites degrade via light-induced degradation of the I-rich domain after this initial phase segregation. Significantly, the oxygen sensitivity of the I-rich domain is identified as a key factor to inducing this irreversible decomposition as supported by in situ Raman spectroscopy. Our work provides insight into the degradation of mixed-halide perovskites under high light intensities and direction for the development and processing of compositionally and environmentally stable perovskite materials.

Its inherent strong tendency to aggregate in solution is used in the following study to prepare highly anisotropic films of the n-type copolymer 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)). Solvent vapor annealing (SVA) allows to tune the size of oriented domains in spherulite-like superstructures with alignment up to several hundreds of micrometers. Blade coating (BC), on the other hand, yields square centimeter large perfectly oriented films with dichroic ratios of 18 and charge transport anisotropies up to 14. On the nanometer scale highly oriented fibers of form I are visible in the oriented areas with the fiber long axis parallel to the chain direction. We give experimental evidence that structure formation does involve liquid crystal (LC) mesophases at high solution concentrations which are frozen upon solvent removal. Temperature post-treatment of the oriented films gives, on the other hand, evidence for a classical semi...

Bias stress degradation in conjugated polymer field-effect transistors is a fundamental problem in these disordered materials and can be traced back to interactions of the material with environmental species,1,2,3 as well as fabrication-induced defects.4,5 However, the effect of the end groups of the polymer gate dielectric and the associated dipole-induced disorder on bias stress stability has not been studied so far in high-performing n-type materials, such as N2200.6,7 In this work, the performance metrics of N2200 transistors are examined with respect to dielectrics with different end groups (Cytop-M and Cytop-S8). We hypothesize that the polar end groups would lead to increased dipole-induced disorder, and worse performance.1,9,10 The long-time annealing scheme at lower temperatures used in the paper is assumed to lead to better crystallization by allowing the crystalline domains to reorganize in the presence of the solvent.11 It is hypothesized that the higher crystallinity could narrow down the range at which energy carriers are induced and thus decrease the gate dependence of the mobility. The results show that the dielectric end groups do not influence the bias stress stability of N2200 transistors. However, long annealing times result in a dramatic improvement in bias stress stability, with the most stable devices having a mobility that is only weakly dependent on or independent of gate voltage., Comment: The following article has been accepted by APL Materials. After it is published, it will be found at https://doi.org/10.1063/5.0044785

The extensive use of halomethanes (CH3X, X = F, Cl, Br, I) as refrigerants, propellants, and pesticides has drawn serious concern due to their adverse biological and atmospheric impact. However, there are currently no portable rapid and accurate monitoring systems for their detection. This work introduces an approach for the selective and sensitive detection of halomethanes using photoluminescence spectral shifts in cesium lead halide perovskite nanocrystals. Focusing on iodomethane (CH3I) as a model system, it is shown that cesium lead bromide (CsPbBr3) nanocrystals can undergo rapid (

The low dielectric constant of organic semiconductors limits the efficiency of organic solar cells (OSCs). In an attempt to improve the dielectric constant of conjugated polymers, we report the synthesis of three semiconducting copolymers by combining the thiophene-substituted diketopyrrolopyrrole (TDPP) monomer with three different monomeric units with varying electron donating/accepting strengths: benzodithiophene (BBT-3TEG-TDPP), TDPP (TDPP-3TEG-TDPP), and naphthalene diimide (P(gNDI-TDPP)). Among the series, BBT-3TEG-TDPP and P(gNDI-TDPP) exhibited the highest dielectric constants (∼5) at 1 MHz frequency, signifying the contribution of dipolar polarization from TEG side-chains. Furthermore, transient absorption spectroscopic studies performed on these polymers indicated low exciton diffusion length as observed in common organic semiconducting polymers. Our findings suggest that utilizing the polar side-chains enhances the dielectric constant in a frequency regime of megahertz. However, it is not sufficient to reduce the Coulombic interaction between hole and electron in excitonic solar cells.

The charge transport–limiting factors in conjugated polymers without single-bond linkages in the backbone have been identified., We investigate the charge transport physics of a previously unidentified class of electron-deficient conjugated polymers that do not contain any single bonds linking monomer units along the backbone but only double-bond linkages. Such polymers would be expected to behave as rigid rods, but little is known about their actual chain conformations and electronic structure. Here, we present a detailed study of the structural and charge transport properties of a family of four such polymers. By adopting a copolymer design, we achieve high electron mobilities up to 0.5 cm2 V−1 s−1. Field-induced electron spin resonance measurements of charge dynamics provide evidence for relatively slow hopping over, however, long distances. Our work provides important insights into the factors that limit charge transport in this unique class of polymers and allows us to identify molecular design strategies for achieving even higher levels of performance.

Two fused ladder type non-fullerene acceptors, DTCCIC and DTCCIC-4F, based on an electron-donating alkylated dithienocyclopentacarbazole core flanked by electron-withdrawing non-fluorinated or fluorinated 1,1-dicyanomethylene-3-indanone (IC or IC-4F), are prepared and utilized in organic solar cells (OSCs). The two new molecules reveal planar structures and strong aggregation behavior, and fluorination is shown to red shift the optical band gap and down shift energy levels. OSCs based on DTCCIC-4F exhibit a power conversion efficiency of 12.6 %, much higher than that of DTCCIC based devices (6.2 %). Microstructural studies reveal that while both acceptors are highly crystalline, bulk heterojunction blends based on the non-fluorinated DTCCIC result in overly coarse domains, while blends based on the fluorinated DTCCIC-4F exhibit a more optimal nanoscale morphology. These results highlight the importance of end group fluorination in controlling molecular aggregation and miscibility.

3D/2D hybrid perovskite systems have been intensively investigated to improve the stability of perovskite solar cells (PSCs), whereas undesired crystallization of 2D perovskite during the film formation process could undermine the structural stability of 2D perovskite materials, which causes serious hysteresis of PSCs after aging. This issue is, however, rarely studied. The stability study for 3D/2D hybrid systems to date is all under the one-direction scan, and the lack of detailed information on the hysteresis after aging compromises the credibility of the stability results. In this work, by correlating the hysteresis of the hybrid PSCs with the 2D crystal structure, we find that the prompt 2D perovskite formation process easily induces numerous crystal imperfections and structural defects. These defects are susceptible to humidity attack and decompose the 2D perovskite to insulating long-chain cations and 3D perovskite, which hinder charge transfer or generate charge accumulation. Therefore, a large hysteresis is exhibited after aging the 3D/2D hybrid PSCs in an ambient environment, even though the reverse-scan power conversion efficiency (PCE) is found to be well-preserved. To address this issue, alkali cations, K

Doped organic semiconductors are required for applications such as organic solar cells, organic light-emitting diodes, and thermoelectric generators. To further establish structure–property relatio...

The synthesis of a naphthalene diimide bithiophene copolymer P(EO-NDIT2) with branched, base-stable, and purely ether-based side chains is presented. Stille polycondensation leads to high molecular ...

Interface engineering, especially the realization of Ohmic contacts at the interface between organic semiconductors and metal contacts, is one of the essential preconditions to achieve high-efficiency organic electronic devices. Here, the interface structures of polymer/fullerene blends are correlated with the charge extraction/injection properties of working organic solar cells. The model system-poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM)-is fabricated using two different degrees of P3HT regioregularity to alter the blend interchain order and molecular packing, resulting in different device performances. Investigations by electroabsorption spectroscopy on these devices indicate a significant reduction (≈1 V) in the built-in potential with an increase in the P3HT regioregularity. This observation is also supported by a change in the work function (WF) of high regioregular polymer blends from photoelectron spectroscopy measurements. These results confirm the presence of a strong dipole layer acting as a δ-hole-transporting layer at the polymer/MoO3/Ag electrode interface. Unipolar hole-only devices show an increase in the magnitude of the hole current in high regioregular P3HT devices, suggesting an increase in the hole injection/extraction efficiency inside the device with a δ-hole-transporting layer. Microscopically, near-edge X-ray absorption fine structure spectroscopy was conducted to probe the surface microstructure in these blends, finding a highly edge-on orientation of P3HT chains in blends made with high regioregular P3HT. This edge-on orientation of P3HT chains at the interface results in a layer of oriented alkyl side chains capping the surface, which favors the formation of a dipole layer at the polymer/MoO3 interface. The increase in the charge extraction efficiency due to the formation of a δ-hole-transporting layer thus results in higher short circuit currents and fill factor values, eventually increasing the device efficiency in high regioregular P3HT devices despite a slight decrease in cell open circuit voltage. These findings emphasize the significance of WF control as a tool for improved device performance and pave the way toward interfacial optimization based on the modulation of fundamental polymer properties, such as polymer regioregularity.

The use of a ternary blend is a promising strategy to enhance the power conversion efficiency of organic solar cells. However, an active layer thickness of ∼100 nm is typically required to achieve optimized performance in ternary blend organic solar cells. The efficiency of a thicker ternary blend film is limited by the low exciton diffusion length and charge carrier mobility of organic semiconductors, which leads to significant energy loss. In this work, we have employed a thick layer (∼300 nm) of ternary blend, featuring a donor–acceptor type diketopyrrolopyrrole (2DPP-BDT) based small molecule along with P3HT and PC71BM and established the role of Forster resonance energy transfer (FRET) to improve the power conversion efficiency (PCE). A dramatic enhancement (27%) in PCE was observed for the ternary blend organic solar cell compared to the binary blend solar cell containing P3HT:PC71BM as active layer. The performance enhancement is attributed to extended light absorption by the ternary blend photoact...

Organic thermoelectrics are attractive for the fabrication of flexible and cost-effective thermoelectric generators (TEGs) for waste heat recovery, in particular by exploiting large-area printing of polymer conductors. Efficient TEGs require both p- and n-type conductors: so far, the air instability of polymer n-type conductors, which typically loose orders of magnitude in electrical conductivity ({\sigma}) even for short exposure time to air, has impeded processing under ambient conditions. Here we tackle this problem in a relevant class of electron transporting, naphthalene-diimide co-polymers, by substituting the imide oxygen with sulphur. n-type doping of the thionated co-polymer gives rise to a higher {\sigma} with respect to the non-thionated one, and most importantly, owing to a reduced energy level of the lowest-unoccupied molecular orbital, {\sigma} is substantially stable over 16 h of air exposure. This result highlights the effectiveness of chemical tuning to improve air-stability of n-type solution-processable polymer conductors and shows a path towards ambient large-area manufacturing of efficient polymer TEGs.

PNNT has been prepared as a polymeric electron acceptor for organic solar cells. The polymer has an A–A′–A acceptor motif linked alternatively with thiophene and vinyl moieties. The A′-unit is a naphthalene diimide, while the A groups are thiazoles. PNNT films were found to have an estimated electron affinity of ≈4.3 eV and an electron mobility of the order of 10–4 cm2 V–1 s–1. Its relatively low solubility in common chlorinated solvents at ambient temperature allowed the manufacture of sequentially deposited (SD) devices, which were found to have significantly higher efficiency than that of bulk heterojunction (BHJ) solar cells containing the same materials. Grazing-incidence wide-angle X-ray scattering measurements indicated that the SD films retained the ordering of the individual polymers to a greater extent compared to the BHJ films. The best SD devices were found to have a power conversion efficiency of up to 4.5%, with stable performance under thermal stress.

Electron injection layers (EILs) based on a simple polymer blend of polyethyleneimine ethoxylated (PEIE) and poly[(9,9-bis(3'-(( N, N-dimethyl)- N-ethylammonium)-propyl)-2,7-fluorene)- alt-2,7-(9,9-dioctylfluorene)] (PFN-Br) can suppress the dependence of organic light-emitting device (OLED) performance on thickness variation compared with single PEIE or PFN-Br EILs. PEIE and PFN-Br were compatible with each other and PFN-Br uniformly mixed in the PEIE matrix. PFN-Br in PEIE formed more fluorene-fluorene pairs than PFN-Br alone. In addition, PEIE:PFN-Br blends reduced the work function (WF) substantially compared with single PEIE or PFN-Br polymer. PEIE:PFN-Br blends were applied to EILs in fluorescent polymer-based OLEDs. Optimized PEIE:PFN-Br blend EIL-based devices presented lower driving voltages and smaller dependences of device performance on EIL thickness than single PEIE or PFN-Br-based devices. These improvements were attributed to electron-transporting fluorene moieties, increased fluorene-fluorene pairs working as channels of electron transport, and the large WF reduction effect of PEIE:PFN-Br blends.

The depth dependence of crystalline structure within thin films is critical for many technological applications but has been impossible to measure directly using common techniques. In this work, by monitoring diffraction peak intensity and location and utilizing the highly angle-dependent waveguiding effects of X-rays near grazing incidence, we quantitatively measure the thickness, roughness, and orientation of stratified crystalline layers within thin films of a high-performance semiconducting polymer. In particular, this diffractive X-ray waveguiding reveals a self-organized 5 nm thick crystalline surface layer with crystalline orientation orthogonal to the underlying 65 nm thick layer. While demonstrated for an organic semiconductor film, this approach is applicable to any thin film material system with stratified crystalline structure where orientation can influence important interfacial processes such as charge injection and field-effect transport.

The design, synthesis, characterization, and application of a novel series of copolymers based on the electron deficient thieno[3,4-c]pyrrole-4,6-dione building block, copolymerized with either thieno[3,2-b]thiophene (PTPDTT) or thiophene (PTPDT), are reported. High molecular weights were obtained for PTPDTT via Stille polycondensation. For the PTPDTs, different molecular weights were achieved by varying the polymerization conditions. The increase in molecular weight (PTPDT-2) favors face-on alignment and increases the charge carrier mobility. Grazing-incidence wide-angle X-ray scattering measurements reveal higher crystallinity for PTPDTT with up to 5 orders of lamellar stacking compared to PTPDTs. All polymers show ambipolar charge transport with highly balanced hole and electron mobilities in organic field effect transistors (OFETs), which improve considerably upon thermal annealing. A shift of comonomer from simple thiophene in PTPDT-2 to planar and electron-dense thienothiophene in PTPDTT drastically...

Recent demonstration of mobilities in excess of 10 cm2 V–1 s–1 have energized research in solution deposition of polymers for thin film transistor applications. Due to the lamella motif of most soluble, semiconducting polymers, the local mobility is intrinsically anisotropic. Therefore, fabrication of aligned films is of interest for optimization of device performance. Many techniques have been developed to control film alignment, including solution deposition via directed flows and deposition on topologically structured substrates. We report device and detailed structural analysis (ultraviolet–visible absorption, IR absorption, near-edge X-ray absorption (NEXAFS), grazing incidence X-ray diffraction, and atomic force microscopy) results from blade coating two high performing semiconducting polymers on unpatterned and nanostructured substrates. Blade coating exhibits two distinct operational regimes: the Landau–Levich or horizontal dip coating regime and the evaporative regime. We find that in the evapora...

The replacement of the imide oxygen atoms with sulfur atoms in the naphthalene diimide (NDI) framework is an attractive strategy for tuning of the electronic properties of such molecules. Here we report the synthesis and characterization of the full series of six thionated NDI molecules with branched side chains. In order to provide a direct comparison with the unthionated parent molecule, a top-gate, bottom contact geometry is adopted, with work function-modified gold electrodes facilitating electron injection. The highest electron mobility was observed for the S2-cis molecule, with a mobility of 0.20 cm2/Vs compared to 0.068 cm2/Vs for the parent molecule. Mobilities slightly lower than that achieved for the S2-cis molecule were also achieved for the S2-trans and S3 derivatives, with the S1, S4 and S2-cis imide derivatives showing mobilities lower than the parent molecule. The thin film microstructure of these molecules has also been explored using a combination of grazing-incidence wide-angle X-ray scattering and near-edge X-ray absorption fine-structure spectroscopy. The performance and microstructure of thin films deposited either by spin-coating or blade coating are also compared.

Polymer analogous thionation of the n-type conjugated polymer PNDIT2 is investigated using Lawesson’s reagent (LR). Detailed high-temperature NMR spectroscopic investigations show that due to the copolymer structure, two out of the four available carbonyl groups present in the naphthalene diimide (NDI) comonomer are sterically less hindered and react preferentially. This leads to regioselective thionation in the trans-configuration even for a large excess of LR. For high degrees of O/S conversion, signals of minor intensity show up in addition pointing to undesired side reactions. These signals could not be eliminated despite further optimized reaction conditions including different aromatic solvents and reaction temperatures. Compared to PNDIT2, the resulting 2S-trans-PNDIT2 features strong aggregation, lower solubility, an 80 nm bathochromic shift of the charge-transfer band, a by 0.22 eV lower LUMO energy level, a lower thermal stability, and higher melting temperatures (Tm). As the combination of the ...

Highly fluorescent perovskite materials have attracted considerable interest for fundamental research and potential applications. In this work, we demonstrate the recoverable PL quenching of methylammonium lead halides (MAPbX3, where X is Cl, Br or I) upon exposure to gaseous ammonia that enables the use of hybrid perovskites in gas-sensing applications. XRD analysis confirmed that the MA cations in the perovskite material were replaced by NH3 to form NH4PbX3·MA, thereby resulting in distinct changes in the crystalline structure and, consequently, PL quenching. However, as a weak coordination complex, NH4PbX3·MA can be easily thermally decomposed to recover the starting product MAPbX3 with the release of ammonia. An in-depth understanding of the reversible chemical and structural changes of the perovskites by exposing them to polar molecules such as ammonia can advance the development of hybrid perovskite sensors and provides insights into the mechanisms of how perovskites coordinate with polar molecules.

Here, we systematically study the effect of fluorination on the performance of all-polymer solar cells by employing a naphthalene diimide (NDI)-based polymer acceptor with thiophene-flanked phenyl co-monomer. Fluorination of the phenyl co-monomer with either two or four fluorine units is used to create a series of acceptor polymers with either no fluorination (PNDITPhT), bifluorination (PNDITF2T), or tetrafluorination (PNDITF4T). In blends with the donor polymer PTB7-Th, fluorination results in an increase in power conversion efficiency from 3.1 to 4.6% despite a decrease in open-circuit voltage from 0.86 V (unfluorinated) to 0.78 V (tetrafluorinated). Countering this decrease in open-circuit voltage is an increase in short-circuit current from 7.7 to 11.7 mA/cm2 as well as an increase in fill factor from 0.45 to 0.53. The origin of the improvement in performance with fluorination is explored using a combination of morphological, photophysical, and charge-transport studies. Interestingly, fluorination is ...

Organic thermoelectrics offer the potential to deliver flexible, low-cost devices that can directly convert heat to electricity. Previous studies have reported high conductivity and thermoelectric power factor in the conjugated polymer poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT). Here, we investigate the thermoelectric properties of PBTTT films in which the polymer chains were aligned uniaxially by mechanical rubbing, and the films were doped by a recently developed ion exchange technique that provides a choice over the counterions incorporated into the film, allowing for more optimized morphology and better stability than conventional charge transfer doping. To optimize the polymer alignment process, we took advantage of two Design of Experiment (DOE) techniques: regular two-level factorial design and central composite design. Rubbing temperature Trub and post-alignment annealing temperature Tanneal were the two factors that were most strongly correlated with conductivity. We were able to achieve high polymer alignment with a dichroic ratio >15 and high electrical conductivities of up to 4345 S/cm for transport parallel to the polymer chains, demonstrating that the ion exchange method can achieve conductivities comparable/higher than conventional charge transfer doping. While the conductivity of aligned films increased by a factor of 4 compared to unaligned films, the Seebeck coefficient (S) remained nearly unchanged. The combination of DOE methodology, high-temperature rubbing, and ion exchange doping provides a systematic, controllable strategy to tune structure–thermoelectric property relationships in semiconducting polymers.

We report the morphological and photovoltaic evaluation of a novel fully conjugated donor/acceptor block copolymer system based on the P3HT-b-PFTBT scaffold. The incorporation of hydrophilic tetraethylene glycol side-chains into the PFTBT acceptor block generates an amphiphilic species whose properties provide demonstrable benefits over traditional systems. This design strategy facilitates isolation of the block copolymer from homopolymer impurities present in the reaction mixture, and we show that this purification leads to better-defined morphologies. The chemical disparity introduced between donor and acceptor blocks causes spontaneous microphase separation into well-defined domains, which we demonstrate with a combination of spectroscopy, microscopy, and X-ray scattering. The morphological advantages of this system are significant; however, preliminary device characterization indicates a loss of electron mobility in the hydrophilic acceptor block.

High molecular symmetry is always observed in high-performance organic semiconductors. However, whether it is an essential factor for molecular design is unclear. In this work, we designed and synthesized three quinoidal isomers, QBTT-o, QBTT-i, and QBTT-s, with different sulfur orientations and a stable E configuration to investigate the relationship between the structure symmetry and organic thin-film transistor performance. We found that QBTT-o and QBTT-i with high C2h symmetry exhibit electron mobilities of 0.02 and 0.15 cm2 V–1 s–1, respectively, while QBTT-s exhibits an unexpectedly high electron mobility of 0.32 cm2 V–1 s–1 with Ion/Ioff ratios of ≤106. The enhanced electron mobilities from QBTT-o and QBTT-i to QBTT-s can be attributed to the different sulfur orientations, especially, molecular symmetry. The thin-film microstructures of three QBTTs were systematically investigated by grazing incidence wide-angle X-ray scattering, near-edge X-ray absorption fine structure spectroscopy, atomic force ...

In solution-processed organic bulk heterojunction (BHJ) solar cells, the purity of the phase-separated domains is known to play an important role in determining device function. While the effects of domain purity have been investigated by tuning of the BHJ morphology, such tuning typically results in several parameters (for example domain size and crystallinity) being varied at once. Here we show that by varying the time between spin-coating and the application of an anti-solvent treatment, the domain purity of the polymer-rich phase in PBDTTT-EFT: PC71BM blends can be tuned while keeping other morphological parameters constant. This unique approach enables the effect of domain purity on device function to be isolated and quantified. Over the purity range explored, solar cell power conversion efficiency is observed to monotonically increase from 7.2% to 9.6% with increasing domain purity, with the cell fill factor most affected by changes in domain purity. Employing transient photovoltage measurements we find that purer phases result in a reduction in the rate constant of bimolecular recombination. A more thorough treatment is also presented on the relationship between the total scattering intensity (derived from resonant soft X-ray scattering measurements) and domain purity. In particular it is shown that domain purity does not scale linearly with total scattering intensity requiring an initial estimate of absolute domain composition.

A series of copolymers based on thieno[3,4-c]pyrrole-4,6-dione and thiophene-phenyl-thiophene with varying degrees of fluorination on the phenyl unit was synthesized by Stille polycondensation. The influence of the degree of fluorination on the optical, thermal and electrochemical properties of these polymers is systematically studied. Additionally, the charge transport in organic field effect transistors (OFETs) as well as the thin film alignment are investigated. After thermal annealing the non-fluorinated as well as difluorinated polymers show ambipolar charge transport in OFETs. In contrast, tetrafluorination results in exclusively n-type behaviour with an electron mobility of 3.7 × 10−4 cm2 V−1 s−1. GIWAXS measurements of as-cast and annealed films reveal a very interesting structural alignment in thin films. All polymers show the rarely observed “end-on” orientation, where the polymer chains stand on the substrate. This may be explained by the low molecular weights of these polymers in relation to the film thickness. Fluorination does not influence the microstructural properties, only the propensity of crystallization is increased.

Control of molecular ordering and packing of π-conjugated molecules in the solid state is crucial for enhancing the charge transport properties in organic electronics. A series of quinoidal materials based on different alkyl-chain branching positions on the thieno[3,4-c]pyrrole-4,6-dione moiety flanked with unsymmetric thieno[3,4-b]thiophenes (2DQTT-n) are synthesized. By the combination of organic thin-film transistor performances and thin-film characterization, we clarified the influence of the branching position on the film microstructure/molecular packing and charge transport properties. Air-stable solution-processable n-channel 2DQTT-n derivatives show dramatic changes in film morphology and molecular packing, which leads to disparate electron mobilities ranging from ∼0.34 to 4.5 cm2 V−1 s−1. 2DQTT-1 with a branching point at the two-position in the alkyl side chain results in a 3D molecular packing with a lamellate morphology, and an electron mobility of up to 4.5 cm2 V−1 s−1 using an annealing temperature of just 80 °C. In contrast, the other three materials exhibit polymorphs and 2DQTT-3 and 2DQTT-4 even show mix-oriented crystallites which are highly disadvantageous to charge transport. These results demonstrate that variation of the alkyl-chain branching point is a powerful strategy to tune the stacking modes in the thin-film state, which enables high charge transport properties.

The performance of bulk heterojunction (BHJ) organic solar cells (OSCs) strongly depends on the intermolecular packing of donor:acceptor (D:A) molecules, as it directly influences the photo-physics and charge transfer properties of the blend. Intermolecular packing can be tuned by solvent additives and/or the chemical structure of molecules to optimize the performance of OSCs. Three high efficiency polymer:fullerene systems are investigated based on the donor polymers PCPDTBT, PTB7 and PTB7-Th (also known as PBDTTT-EFT). For the PCDPTBT:PC71BM system, we demonstrate that the use of the solvent additive diiodooctane (DIO) which is employed to optimize the solar cell efficiency actually increases interfacial disorder by 13 meV despite lowering bulk disorder by 35 meV. In contrast, the Urbach energy (bulk and interfacial charge transfer (CT) states) and reorganization energy (λ) are minimally influenced by the addition of DIO into PTB7:PC71BM & PTB7-Th:PC71BM devices, where PTB7-Th is a chemical analogue of PTB7. However, PTB7-Th devices show remarkably low voltage losses (∼100 meV) with respect to other systems studied here, suggesting a favorable energy landscape at the D:A interface for charge separation. Hence, optimizing the molecular structure of the donor polymer can help to overcome the voltage losses even when solvent additive treatment is applied.

In this work, for the first time we used two novel fused aromatic conjugated electron withdrawing moieties 9-fluorenone and 9,10-anthraquinone, respectively, to design two non-fullerene acceptors and evaluated their viability in solution-processable organic solar cells (OSCs). 9-Fluorenone and 9,10-anthraquinone were used as core electron withdrawing blocks in combination with another common strong electron accepting diketopyrrolopyrrole (DPP) end-capping group. The compounds 6,6′-(5,5′-(9-oxo-9H-fluorene-2,7-diyl)bis(thiophene-5,2-diyl))bis(2,5-bis(2-butyloctyl)-3-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (DPP-FN-DPP) and 6,6′-(5,5′-(9,10-dioxo-9,10-dihydroanthracene-2,6-diyl)bis(thiophene-5,2-diyl))bis(2,5-bis(2-butyloctyl)-3-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (DPP-ANQ-DPP) were synthesized via a Suzuki coupling reaction and characterized completely. The new acceptors exhibit good solubility in common organic solvents and good thermal stability with 5% weight loss above 360 °C. DPP-FN-DPP and DPP-ANQ-DPP possess a broad absorption band at 300–700 nm with optical band-gaps of 1.75 and 1.71 eV, respectively. The use of different core acceptor building blocks resulted in a difference in LUMO and HOMO energy levels. Inverted OSC devices employing P3HT as the donor polymer and DPP-FN-DPP and DPP-ANQ-DPP as acceptors yielded quite high open-circuit voltages (VOC) of 0.85–0.98 V, benefiting from the relatively low-lying LUMO energy levels of the two acceptors. Among both, OSC devices based on DPP-FN-DPP as acceptor exhibits the highest performance with a VOC of 0.97 V, a short-circuit current density (JSC) of 3.2 mA cm−2, a fill factor (FF) of 37%, and an overall power conversion efficiency of 1.2%.

Perovskite solar cells can be produced by a solution process and have achieved power conversion efficiency over 20% as well as improving long-term stability, offering great potential for a low cost, high efficiency photovoltaic technology. An increasing effort has been shifted to Lab-to-Fab translation, where device manufacture is accomplished by using a fully scalable printing process. One remarkable bottleneck for upscaling the device is, however, the lack of scalable hole-transport materials (HTMs) that can form the desired morphology during the printing fabrication. In this manuscript, we apply a twisted but fully π-conjugated 2,2′,7,7′-tetrakis(N,N-di- p -methoxyphenyl)amine-9,9′-bifluorenylidene (Bifluo-OMeTAD) into slot-die coated devices, which exhibits excellent film forming properties and outperforms the well-known Spiro-OMeTAD HTM. The improved film forming properties of Bifluo-OMeTAD are achieved via molecular design, with the chemical structure of Bifluo-OMeTAD effectively suppressing crystallization during printing. A power conversion efficiency of 14.7% is achieved in the fully slot-die coated devices based on Bifluo-OMeTAD, outperforming previous reported values for all-printed perovskite solar cells. Therefore, Bifluo-OMeTAD has attractive potential to replace Spiro-OMeTAD for the large scale roll-to-roll production of fully slot-die coated perovskite solar cells.