Morphology Of Blend Films Research Articles

Effect of guest concentration on carrier transportation in blends of conjugated polymers

In this paper, the charge transport in pure poly (9,9-dioctylfluorene-2, 7-diyl) (PFO) and its blend with poly (5-methoxy-2-2-ethyl-hexylthio)-p-phenylenevinylene (MEH-PPV) is studied. The mobility (μeff) and diffusivity (D) of the pure and blend thin films have been calculated using electroluminescence transient (ELT) technique. MEH-PPV concentration was varied from 0.8 to 15 wt%. It is found that at relatively low concentration of MEH-PPV, less than 1.2 wt%, the mobility of PFO:MEH-PPV blend increases with the concentration of MEH-PPV, and after that, it starts decreasing. The field dependence of effective hole mobility follows the Poole–Frenkel (P–F) plot of mobility (μ) as a function applied electric field with positive slope (βPF > 0) up to 5.0 wt% concentration of MEH-PPV. A negative P-F type dependence is then observed for 8.0–15 wt% MEH-PPV concentration. Diffusivity (D) of blends is following the same trends as mobility i.e. blend with 1.2 wt% MEH-PPV shows the highest diffusivity. These results have been correlated with the morphology of pure and blend thin films. Gaussian Disorder Model (GDM) alone is not able to explain the change in βPF with the MEH-PPV concentration, guest induced crystallization of PFO plays an important role at low concentrations of MEH-PPV. At higher concentrations of MEH-PPV (≥8 wt%), crystallization is suppressed, and position disorder induced behavior of polymer determines the charge transport in the blends.

Enhanced thermal stability of organic photovoltaics via incorporating triphenylamine derivatives as additives

In this work, we prepared four star-shaped conjugated small molecules, the triphenylamine dithiophene (TBT) derivatives, namely TBT-H, TBT-Br, TBT-OH, and TBT-N3 presenting hydride, bromide, hydroxyl, and azide terminal functional groups, respectively. These TBT derivatives were used as additives in the active layers of organic photovoltaics to investigate the effect of intermolecular interactions (TBT-H, TBT-OH) or crosslinking (TBT-N3, TBT-Br) on the long-term thermal stability of the devices. From analyses of blend film morphologies, and optoelectronic and device performance, we observed significant enhancements in thermal stability during accelerated heating tests at 150°C for the devices incorporated with the additives TBT-N3 and TBT-Br. These two additives functioned as crosslinkers, and constructed local borders that effectively impeded heat-promoted fullerene aggregation, thereby leading to highly stable morphologies. When compared with corresponding normal devices, the TBT-N3–derived devices based on poly(3-hexylthiophene) exhibited greater stability, with the power conversion efficiency (PCE) remaining as high as 2.5% after 144h at 150°C. Because of this enhancement, a device based on an amorphous low-bandgap polymer, namely poly(thieno[3,4-b]thiophene-alt-benzodithiophene), with the addition of TBT-N3 was fabricated. We observed a significant improvement in device stability, retaining approximately 60% (from 5.0 to 3.3%) of its initial PCE under accelerated heating (150°C). In contrast, the PCE of the corresponding normal device decayed to 0.01% of its initial value.

Ternary polymer solar cell based on two donors and one acceptor for improving morphology and power conversion efficiency

We synthesized isoindigo-based two donor-acceptor (D-A) copolymers to optimize the morphology in ternary blend films with two p-type polymers and n-type phenyl-C61-butyric acid methyl ester (PC61BM). Compared to binary systems with a single copolymer and PC61BM, the ternary polymer solar cell with two copolymers and PC61BM showed highly enhanced power conversion efficiency of 5.13% with a high short circuit current of 13.23mAcm−2. These results were supported by atomic force microscopy (AFM) and transmission electron microscopy (TEM) observations of the surface and internal morphology of the thin film, respectively.

High‐Performance All‐Polymer Photoresponse Devices Based on Acceptor–Acceptor Conjugated Polymers

Three acceptor–acceptor (A–A) type conjugated polymers based on isoindigo and naphthalene diimide/perylene diimide are designed and synthesized to study the effects of building blocks and alkyl chains on the polymer properties and performance of all‐polymer photoresponse devices. Variation of the building blocks and alkyl chains can influence the thermal, optical, and electrochemical properties of the polymers, as indicated by thermogravimetric analysis, differential scanning calorimetry, UV–vis, cyclic voltammetry, and density functional theory calculations. Based on the A–A type conjugated polymers, the most efficient all‐polymer photovoltaic cells are achieved with an efficiency of 2.68%, and the first all‐polymer photodetectors are constructed with high responsivity (0.12 A W−1) and detectivity (1.2 × 1012 Jones), comparable to those of the best fullerene based organic photodetectors and inorganic photodetectors. Photoluminescence spectra, charge transport properties, and morphology of blend films are investigated to elucidate the influence of polymeric structures on device performances. This contribution demonstrates a strategy of systematically tuning the polymeric structures to achieve high performance all‐polymer photoresponse devices.

Impact of solvent additive on exciton dissociation in P3HT : EP-PDI blend film via controlling morphology

Suitable solvent additives provide an effective means to control the morphology of polymer blend films. In this work, we systematically investigate the impact of the solvent additive chloronaphthalene (CN) on the morphology of P3HT : EP-PDI blends. The optimum volume fraction of solvent additive CN was found to be 0.5 vol% by atom force microscopy and transmission electron microscopy. UV–visible absorption spectroscopy and grazing incidence x-ray diffraction indicate that the crystallinity of both the P3HT and EP-PDI domains significantly decreased in the blends with 0.5 vol% CN. Grazing incidence small-angle x-ray scattering results show that the size of the small EP-PDI aggregation decreases from 44–22 nm with the addition of CN. Time-resolved photoluminescence measurement reveals that the decreased EP-PDI domains give rise to increased donor–acceptor interfacial areas, which not only facilitate the exciton dissociation, but suppresses the formation of the EP-PDI intermolecular state.

Diketopyrrolopyrrole based A2-D-A1-D-A2 type small molecules for organic solar cells: Effects of substitution of benzene with thiophene

A series of small molecules based on acceptor-donor-acceptor-donor-acceptor molecular skeletons, which contained a diketopyrrolopyrrole core as the central acceptor unit, two oligomeric benzene or thiophene as the donor, and two isoindigo or benzothienoisoindigo as the weaker acceptor, were synthesized. The influence of the number of thiophene on optical physical, crystallinity, charge carrier mobility, the morphologies of blend films, electrochemical and photovoltaic properties was investigated. The absorption of the diketopyrrolopyrrole based compounds showed a general trend of red-shift with increased number of thiophene. The highest occupied molecular orbital energy levels of the materials increased as a result of substitution benzene with thiophene as donor. X-ray diffraction measurement indicated that all these materials have strong crystallinity. Characterizations of morphology by atomic force microscopy and carrier mobilities revealed that the number of thiophene units influenced the microstructure of the blend films and the performance of organic solar cells.

Optimization and Analysis of Conjugated Polymer Side Chains for High‐Performance Organic Photovoltaic Cells

Optimization and analysis of conjugated polymer side chains for high‐performance organic photovoltaic cells (OPVs) reveal a critical relationship between the chemical structure of the side chains and photovoltaic properties of polymer‐based bulk heterojunction OPVs. In particular, the impact of the alkyl side chain length on the π‐bridging (thienothiophene, TT) unit is considered by designing and synthesizing a series of benzodithiophene derivatives (BDT(T)) and thieno[3,2‐b]thiophene‐π‐bridged thieno[3,4‐c]pyrrole‐4,6(5H)‐dione (ttTPD) alternating copolymers, PBDT(T)‐(R2)ttTPD, with alkyl chains of varying length on the TT unit. Using a combination of 2D X‐ray diffraction, Raman spectroscopy, and electrical device characterization, it is elucidated in detail how these subtle changes to the chemical structure affect the molecular conformation, thin film molecular packing, blend film morphology, optoelectronic properties, and hence overall photovoltaic performance. For copolymers employing both the alkoxy or alkylthienyl‐substituted BDT motifs, it is found that octyl side chains on TT unit yield the maximum degree of molecular backbone coplanarity and result in the highest quality of molecular packing and optimized hole mobility. Inverted devices fabricated using this PBDTT‐8ttTPD: polymer/[6,6]‐phenyl‐C71‐butylic acid methyl ester active layer show a maximum power conversion efficiency (PCE) of 8.7% with large area cells (0.64 cm2) maintaining a PCE of 7.5%.

The molecular regioregularity induced morphological evolution of polymer blend thin films

The polymer regioregularity has a dramatic effect on device performance of organic photovoltaics in terms of material crystallinity and phase separated morphology. In this study, we investigated the molecular regioregularity induced morphological evolution of P3HT and PF12TBT blend thin films processed by solvents with different solubility and boiling point. After cast by good solvents chloroform (CF) and o-dichlorobenzene (oDCB), the regioregular (RR) and regiorandom (RA)-P3HT/PF12TBT blend films showed the similar phase separated morphology, namely fast solidification induced unsharp morphology for CF blend films and long-range demixing via spinodal decomposition within extending drying process for oDCB blend films, while the domain size for RA blend films were larger than those of RR blend films which could be attributed to easily disentanglement and diffusion of RA-P3HT molecules facilitated by smaller aggregation size in RA blend solutions. In addition, the morphology of RR blend films exhibited micro change while the RA blend films developed into long-range phase separation after thermal annealing for CF blend films which also confirmed the molecular diffusion induced morphological evolution. For marginal solvents p-xylene (XY), the RR P3HT showed obvious molecular aggregation in XY solution due to the intensive solvent–polymer interaction (Δδ) and high regioregularity and therefore the value of aggregation size (Ra) for RR-P3HT is as three times as that of RA-P3HT. After cast from XY solution, the RA blend films displayed droplet and bicontinuous morphology based on various blend ratios via spinodal decomposition. Meanwhile, the nanofiber morphology could be observed when the weight fraction of RR-P3HT beyond 10% in RR blend films, which resulting from the continuous growth of P3HT aggregations formed anterior of the phase separation in the film solidification process.

All-Polymer Solar Cell Performance Optimized via Systematic Molecular Weight Tuning of Both Donor and Acceptor Polymers

The influence of the number-average molecular weight (Mn) on the blend film morphology and photovoltaic performance of all-polymer solar cells (APSCs) fabricated with the donor polymer poly[5-(2-hexyldodecyl)-1,3-thieno[3,4-c]pyrrole-4,6-dione-alt-5,5-(2,5-bis(3-dodecylthiophen-2-yl)thiophene)] (PTPD3T) and acceptor 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); N2200) is systematically investigated. The Mn effect analysis of both PTPD3T and N2200 is enabled by implementing a polymerization strategy which produces conjugated polymers with tunable Mns. Experimental and coarse-grain modeling results reveal that systematic Mn variation greatly influences both intrachain and interchain interactions and ultimately the degree of phase separation and morphology evolution. Specifically, increasing Mn for both polymers shrinks blend film domain sizes and enhances donor-acceptor polymer-polymer interfacial areas, affording increased short-circuit current densities (Jsc). However, the greater disorder and intermixed feature proliferation accompanying increasing Mn promotes charge carrier recombination, reducing cell fill factors (FF). The optimized photoactive layers exhibit well-balanced exciton dissociation and charge transport characteristics, ultimately providing solar cells with a 2-fold PCE enhancement versus devices with nonoptimal Mns. Overall, it is shown that proper and precise tuning of both donor and acceptor polymer Mns is critical for optimizing APSC performance. In contrast to reports where maximum power conversion efficiencies (PCEs) are achieved for the highest Mns, the present two-dimensional Mn optimization matrix strategy locates a PCE "sweet spot" at intermediate Mns of both donor and acceptor polymers. This study provides synthetic methodologies to predictably access conjugated polymers with desired Mn and highlights the importance of optimizing Mn for both polymer components to realize the full potential of APSC performance.

Understanding the morphology of solution processed fullerene-free small molecule bulk heterojunction blends.

Bulk-heterojunction (BHJ) molecular blends prepared from small molecules based on diketopyrrolopyrrole (DPP) and perylene-diimide (PDI) chromophores have been studied using optical absorption, cyclic voltammetry, photoluminescence quenching, X-ray diffraction, atomic force microscopy, and current-voltage measurements. The results provided useful insights into the use of DPP and PDI based molecules as donor-acceptor composites for organic photovoltaic (OPV) applications. Beside optoelectronic compatibility, the choice of active layer processing conditions is of key importance to improve the performance of BHJ solar cells. In this context, post-production treatments, viz. thermal and solvent vapour annealing, and the use of 1,8-diiodooctane as a solvent additive were employed to optimize the morphology of blend films. X-ray diffraction and atomic force microscopy indicated that the aforementioned processing strategies led to non-optimal composite morphologies with significantly large crystallites in comparison to exciton diffusion lengths. Although the open circuit voltage of the OPV devices was satisfactory (0.78 V), it was anticipated that the bulky domains hamper charge dissociation and transport, which resulted in low photovoltaic performance.

Poly(3-hexylthiophene)/polystyrene (P3HT/PS) blends based organic field-effect transistor ammonia gas sensor

Ammonia (NH3) gas sensors based on organic field-effect transistor (OFET) using poly(3-hexylthiophene) (P3HT) blended with polystyrene (PS) as a semiconductor layer were fabricated. An optimized composition of 1.6wt% P3HT in PS matrix exhibited the best performance to various concentrations of NH3, which is comparable to that of pure P3HT (8wt%). The results showed the percentage responses of saturation current were 52% and 16% under 50ppm and 5ppm NH3, respectively. Also, it showed that there was a remarkable shift in the field-effect mobility after exposed to NH3 gas. By analyzing the morphologies of blend films and the electrical characteristics of OFET sensors, it was found that the film of P3HT blended with PS has more interface to interact with NH3, resulting in more efficient detection to NH3 even in the range of low concentration. Besides, the PS matrix would prevent the gas from diffusing into the semiconductor/dielectric interface directly, which was beneficial to the selectivity of P3HT/PS blend OFET sensor. Moreover, the sensing property was related to the solvents and molecular weight of PS. In addition, the environmental stability of OFET sensors was measured after storing the sensors under ambient atmosphere for 40 days, and the device with the blend semiconducting layer exhibited the superior stability.

Solution-Processed Diketopyrrolopyrrole-Containing Small-Molecule Organic Solar Cells with 7.0% Efficiency: In-Depth Investigation on the Effects of Structure Modification and Solvent Vapor Annealing

A family of narrow band gap extended π-conjugated D2-A2-D1-A1-D1-A2-D2 type small molecules based on diketopyrrolopyrrole derivatives as the stronger acceptor core (A1) coupled with indacenodithiophene (IDT; D1) and difluorobenzothiadiazole (A2) are synthesized, and their properties as donor materials in solution-processed small-molecule organic solar cells are investigated. The impacts of replacing the thiophene ring by a more electron-deficient thiazole ring and inserting thiophene spacer between electron-donating (D1) and electron-accepting (A1 and A2) aromatic moieties on bulk properties, such as the photophysical properties, the HOMO/LUMO energy level, charge carrier mobilities, and the morphologies of blend films, as well as optimization on device performance via solvent vapor annealing are investigated. NDPPFBT shows outstanding efficiencies up to 7.00% after THF vapor annealing for 60 s because of a very high fill factor (FF) of 0.73 and high Voc of 0.89 V. The reported efficiency is among one of ...

Deep absorbing porphyrin small molecule for high-performance organic solar cells with very low energy losses.

We designed and synthesized the DPPEZnP-TEH molecule, with a porphyrin ring linked to two diketopyrrolopyrrole units by ethynylene bridges. The resulting material exhibits a very low energy band gap of 1.37 eV and a broad light absorption to 907 nm. An open-circuit voltage of 0.78 V was obtained in bulk heterojunction (BHJ) organic solar cells, showing a low energy loss of only 0.59 eV, which is the first report that small molecule solar cells show energy losses <0.6 eV. The optimized solar cells show remarkable external quantum efficiency, short circuit current, and power conversion efficiency up to 65%, 16.76 mA/cm(2), and 8.08%, respectively, which are the best values for BHJ solar cells with very low energy losses. Additionally, the morphology of DPPEZnP-TEH neat and blend films with PC61BM was studied thoroughly by grazing incidence X-ray diffraction, resonant soft X-ray scattering, and transmission electron microscopy under different fabrication conditions.

Side Chain Influence on the Morphology and Photovoltaic Performance of 5-Fluoro-6-alkyloxybenzothiadiazole and Benzodithiophene Based Conjugated Polymers.

Three conjugated polymers (P1-P3) with benzodithiophene derivatives as the donor unit, 5-fluoro-6-(2-hexyldecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5] thiadiazole as the acceptor unit and thiophene as the spacer were designed, synthesized, and used as donor materials for polymer solar cells (PSCs). The influence of side chains at the benzodithiophene unit on the performance of PSCs was investigated. PSCs with the blend of P2:PC71BM (1:2, by weight) as the active layer show the highest power conversion efficiency (PCE) of 6.88%, with an open circuit voltage (Voc) of 0.76 V, a short circuit current (Jsc) of 14.67 mA/cm(2), and a fill factor (FF) of 0.62. Our research revealed that the variation of side chains had a great influence on the morphology of blend films, which is crucial to the performance of PSCs. As indicated by transmission electron microscopy, the blends of P1:PC71BM (1:2) and P2:PC71BM (1:2) formed nanofibers, whereas the blends of P3:PC71BM (1:2) formed spherical domains. Therefore, we concluded that formation of a more interpenetrating phase-separated donor-acceptor network with a larger interfacial area and proper percolation in the blends from P1 to P2 is mainly responsible for better performance in the corresponding devices.

Rational Design of Small Molecular Donor for Solution‐Processed Organic Photovoltaics with 8.1% Efficiency and High Fill Factor via Multiple Fluorine Substituents and Thiophene Bridge

A series of tetrafluorine‐substituted small molecules with a D1‐A‐D2‐A‐D1 linear framework based on indacenodithiophene and difluorobenzothiadiazole is designed and synthesized for application as donor materials in solution‐processed small‐molecule organic solar cells. The impacts of thiophene π‐bridge and multiple fluorinated modules on the photophysical properties, the energy levels of the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), charge carrier mobility, the morphologies of blend films, and their photovoltaic properties as electron donor material in the photoactive layer are investigated. By incorporating multiple fluorine substituents of benzothiadiazole and inserting two thiophene spacers, the fill factor (FF), open‐circuit voltage, and short‐circuit current density are dramatically improved in comparison with fluorinated‐free materials. With the solvent vapor annealing treatment, further enhancement in charge carrier mobility and power conversion efficiency (PCE) are achieved. Finally, a high PCE of 8.1% with very‐high FF of 0.76 for BIT‐4F‐T/PC71BM is achieved without additional additive, which is among one of the highest reported for small‐molecules‐based solar cells with PCE over 8%. The results reported here clearly indicate that high PCE in solar cells based small molecules can be significantly increased through careful engineering of the molecular structure and optimization on the morphology of blend films by solvent vapor annealing.

Effect of thermal annealing on the performance of ternary organic photovoltaics based on PTB7:PC71BM:F8BT

The effect of thermal annealing on the inverted PTB7:PC71BM:F8BT ternary organic photovoltaics (OPVs) was systematically investigated, which was the first study about the effect of thermal annealing on the performance of PTB7-based OPVs. The results showed that the devices thermal annealed at 120 °C leading to 60 % increase of power conversion efficiency (PCE) compared with the as-spun ternary OPVs and 3 % enhancement compared with the reference PTB7:PC71BM binary OPVs coated from chlorobenzene. The performance improvement was mainly attributed to formation of fine nanomorphology of the ternary blend film which could secure both charge separation and charge percolation path. However, the PCE decreased to 1.90 % with temperature higher than 140 °C, which was due to the disruptive morphology change and low quantum yield. The results also showed that the thermal annealing process could not only maintain optimized self-assembled morphology of the ternary blend film, but also influence the quantum yield and improve charge transfer efficiency further.

Morphological Evolution and Its Impacts on Performance of Polymer Solar Cells

In this paper, the role of fullerene loading on the nanomorphology and photovoltaic performance of alternating copolymer poly{2-octyldodecyloxy-benzo[1,2-b;3,4-b] dithiophene-alt-5,6-bis(dodecyloxy)-4,7bis(thiophen-2-yl)-benzo[c] [1,2,5]-thiadiazole} (PBDT-ABT-1) blend films was investigated. The morphology of blend films with different Phenyl C-60-butyric acid methyl ester (PCBM) mixing ratios and solvent additives was studied using atomic force microscopy (AFM) and energy-filtered transmission electron microscopy (EFTEM). AFM and EFTEM images showed difference in the intermixing of polymer with fullerene between 1:1, 1:2, and 1:3 weight ratios. Polymer/PCBM intermixed domain size increases with higher PCBM weight ratios. X-ray diffraction measurements on the pristine polymer and blend films cast without additives did not show any peaks, suggesting an amorphous nature of PBDT-ABT-1. EFTEM images from the donor/acceptor composite showed intermixed polymer-PCBM domains separated by the polymer boundary. Furthermore, EFTEM images for di-iodooctane (DIO) additive cast film revealed purer polymer domain. Photo-charge extraction by linearly increasing voltage measurement exhibited that charge extraction is highest in the nanomorphology sample with a weight ratio of 1:2, corresponding to the lowest bimolecular recombination and the highest charge carrier mobility.

Improved open-circuit voltage of benzodithiophene based polymer solar cells using bulky terthiophene side group

Terthiophene, including one α–α and one branching α–β connection of the thiophene units, is introduced as benzodithiophene (BDT) side chain to build a novel two-dimensional (2D) conjugated BDT block. By copolymerizing this BDT block with three electron acceptors (DTTz (bis(thiophene-2-yl)-tetrazine), DPP (diketopyrrolopyrrole), DTffBT (4,7-bis(4-hexylthienyl)-5,6-difluoro-2,1,3-benzothiadiazole)) and one electron donor (TTT (2,5-Di(2-thienyl)thiophene)), four terthiophene side-chained benzodithiophene based copolymers were synthesized. Due to the difference in electron affinity among DTTz, DPP, DTffBT and TTT, these four polymers show different UV–vis absorption spectra and optical band gaps (1.3–2.0eV), while fortunately they all remain deep highest occupied molecular orbital (HOMO) energy levels (−5.3 to 5.6eV) which is very favorable to high open-circuit voltage (Voc) polymer solar cells (PSCs). By comparing the photovoltaic properties with polymers which have same backbone but do not have the bulky 2D side group in the literatures, our polymer solar cells devices show higher Voc. Especially for PQ3 (a copolymer of benzodithiophene and diketopyrrolopyrrole), the donor photon energy loss (Eg–eVoc) is 0.51eV which is almost the lowest value achieved by the researchers. It can be concluded that: the bulky terthiophene side group helps to improve Voc of the PSCs devices. The overall performance of solar cells devices is correlated with the molecule conformation, polymer hole mobility and polymer/PCBM blend film morphology.

Phase Diagram of Conjugated Polymer Blend P3HT/PF12TBT and the Morphology-Dependent Photovoltaic Performance

In this study, we systematically investigated the morphological evolution of poly(3-hexylthiophene) (P3HT) and poly{2,7-(9,9-didodecyl-fluorene)-alt-5,5-[4′,7′-bis(2-thienyl)-2′,1′,3′-benzothiadiazole]} (PF12TBT) blend thin films cast from o-dichlorobenzene (oDCB) solution and its effect on photovoltaic performance. Blend ratio and film drying time both played a crucial role in determining polymer blend film morphologies, including phase-separated structure and domain size. An apparent morphological transformation from uniform morphology to droplet and bicontinuous structure could be observed with the P3HT/PF12TBT blend ratio ranging from 10/90 to 90/10. Uniform morphology resulted from one component becoming dominated, and thus no obvious phase separation could be observed. The formation of bicontinuous morphology via a spinodal decomposition mechanism which emerged at nearly equal blend ratio could be attributed to the similar and favorable solubility for both polymers in processing solvent, while the asymmetric composition led to the formation of droplet morphology via a spinodal decomposition mechanism. In addition, the increased domain size which resulted from coarsening of adjacent domains could be observed with the extending film drying time. Further expanding drying time, the thin films exhibited unsharp morphology with large domains which could be attributed to the mixing of polymer blend with the formation of vast P3HT crystallites and the aggregation of both donor and acceptor via molecular diffusion. On the basis of the varying morphologies, an approximate phase diagram of P3HT/PF12TBT blend was depicted. In order to demonstrate the effect of various morphologies on photovoltaic performance, devices based on films with three kinds of blend ratios (40/60, 50/50, and 60/40) while undergoing different drying time (20, 30, and 45 s) were prepared. As the drying time extended, a gradual decreased power conversion efficiency (PCE) could be observed for all blend ratio devices, which resulted from the dramatic morphological transformation from small domains to intermediate domains and then upsharp morphology with large domains ultimately.

The structure-property relationships of LLDPE–EVOH blend films fabricated by multiplication extrusion

The blend of linear low density polyethylene (LLDPE) and ethylene vinyl alcohol (EVOH) with weight fraction of 50–50 is extruded through a series of multipliers to fabricate thin films. Different numbers of multipliers are utilized to tailor the morphology of the extruded blend films. We discover that as the number of multipliers increases, the blend film morphology evolves dramatically. The elongated and layer-like structure is gradually replaced by well-dispersed nano-blend morphology when the number of multipliers reaches eight. The domain size of each component decreases due to the effect of multiplication process. This is because during the multiplication process, multipliers behave similar to static mixers that physically break the elongated and layer-like structure of LLDPE and EVOH into tiny domains. Along with the morphological evolution, the physical properties of the extruded blend films change greatly. Atomic force microscopy (AFM) and wide angle X-ray scattering (WAXS) are utilized to investigate the morphology and crystalline structure of the blend films. Oxygen gas permeability and water vapor transport rate (WVTR) are measured by MOCON units. The transmission rate and mechanical properties are studied by the UV–vis and a mechanical tensile stretcher (MTS) respectively. We report that both oxygen permeability and WVTR increase as the number of multipliers increases. The transparency is improved as more multipliers are used. The last but not the least, tensile mechanical behaviors gradually become isotropic at different deformation directions as the morphology changes.