Blend Thin Films Research Articles

New conjugated molecules with two and three dithienyldiketopyrrolopyrrole (DPP) moieties substituted at meta positions of benzene toward p- and n-type organic photovoltaic materials.

Two conjugated molecules, TADPP3 and TADPP2-TT, are reported, in which three and two dithienyldiketopyrrolopyrrole (DPP) moieties, respectively, are substituted at the meta positions of benzene. Based on cyclic voltammetry and absorption data, TADPP3 and TADPP2-TT possess similar HOMO and LUMO energies of about -5.2 and -3.4 eV, respectively. Thin films of TADPP3 and TADPP2-TT exhibit p-type semiconducting behavior with hole mobilities of 2.36×10(-3) and 3.76×10(-4) cm(2) V(-1) s(-1) after thermal annealing. Molecules TADPP3 and TADPP2-TT were utilized as p-type photovoltaic materials to fabricate organic solar cells after blending with phenyl C71 butyric acid methyl ester (PC71BM) and phenyl C61 butyric acid methyl ester (PC61BM). The relatively low JSC and fill factor values can be attributed to poor film morphologies based on AFM and XRD studies. A solar cell with a thin film of TADPP3 with PC71BM in a weight ratio of 1:2 exhibits a high open-circuit voltage (VOC) of 0.99 V and a power conversion efficiency (PCE) of 2.47 %. Interestingly, TADPP3 can also be employed as an n-type photovoltaic material. The blended thin film of TADPP3 with P3HT in a weight ratio of 1:2 gave a high VOC of 1.11 V and a PCE of 1.08 % after thermal annealing.

Evaluation of Heterocycle-Modified Pentathiophene-Based Molecular Donor Materials for Solar Cells

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

Role of Organoclay in Controlling the Morphology and Crystal‐Growth Behavior of Biodegradable Polymer‐Blend Thin Films Studied Using Atomic Force Microscopy

This study reports the effect of organically modified nanoclay on the morphology and crystal‐growth behavior of biodegradable polylactide/poly[(butylene succinate)‐co‐adipate] (PLA/PBSA) blend thin films with the average thickness of 280 nm. The neat and organically modified blended thin films were cast on silicon (001) substrate using a spin coater. Blended composite thin films with three different types of organically modified clays were prepared, and the impact of the nature of the pristine clays, their organic modifications and their initial d001‐spacing on the morphology and crystal‐growth behavior of the PLA/PBSA blend were studied. An atomic force microscopy equipped with a hot‐stage scanner was used to examine the crystalline morphology of the thin films. The morphology and crystal‐growth behavior of the films were investigated during in situ annealing and cooling from melt. The results indicated that the initial d001‐spacing of the organoclay plays a pivotal role in controlling the morphology of the dispersed phase and hence the crystal‐growth behavior.

Study of electrical properties of polyvinylpyrrolidone/polyacrylamide blend thin films

Electrical properties of polyvinylpyrrolidone, polyacrylamide and their blend thin films have been investigated as a function of temperature and frequency. The films were prepared using solution casting method and the measurements on films were carried out at different temperatures ranging from 305 to 345 K covering a frequency range from 102 to 105 Hz. The conductivity of film samples was found to increase upon increasing the temperature. Lowering of activation energy by increasing the polyvinylpyrrolidone percentage may be due to the predominance of ion conduction mechanism caused by polyvinylpyrrolidone in the blend. The permittivity (𝜖 r) and dielectric loss (𝜖 i) were found to decrease upon increasing frequency. Temperature and frequency dependence of impedance, relaxation time and electric modulus of thin film samples have also been studied. From electric modulus formalism, polarization and conduction relaxation behaviour in the film samples have been discussed.

Characterization of spray-coating methods for conjugated polymer blend thin films

We examine the characteristics and functionality of conjugated polymer thin films, based on blends of poly(9,9-dioctylfluorene-2,7-diyl-co-bis-N,NN′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine) (PFB) and poly(9,9-dioctylfluorene-2,7-diyl-co-benzothiadiazole) (F8BT), using a spray-coating deposition technique suitable for large areas. The morphological properties of these blend films are studied in detail by atomic force microscopy (AFM) methods, showing that favourable results, in terms of layer deposition rate and uniformity, can be achieved using a 5:1 blend of o-dichlorobenzene and chlorobenzene as the solvent medium. A photoluminescence quenching efficiency of above 80 % is also observed in such blend films. As a feasibility study, prototypical photovoltaic devices exhibit open circuit voltages of up to 1.0 V under testing, and solar power conversion efficiencies in the 0.1–1 % order of magnitude; metrics which are comparable with those reported for spin-coated cells of the same active blend and device architecture.

Self-organization and nanostructural control in thin film heterojunctions

In spite of more than two-decades of studies of molecular self-assembly, the achievement of low cost, easy-to-implement and multi-parameter bottom-up approaches to address the supramolecular morphology in three-dimensional (3D) systems is still missing. In the particular case of molecular thin films, the 3D nanoscale morphology and function are crucial for both fundamental and applied research. Here we show how it is possible to tune the 3D film structure (domain size, branching, etc.) of thin film heterojunctions with nanoscale accuracy together with the modulation of their optoelectronic properties by employing an easy two-step approach. At first we prepared multi-planar heterojunctions with a programmed sequence of nanoscopic layers. In a second step, thermal stimuli have been employed to induce the formation of bulk heterojunctions with bicontinuous and interdigitated phases having a size below the exciton diffusion length. Importantly, the study of luminescence quenching of these systems can be considered as a useful means for the accurate estimation of the exciton diffusion length of semiconductors in nanoscale blends. Finally, nearly a thousand times lower material consumption than spin coating allows a drastic reduction of material wasting and a low-cost implementation, besides the considerable possibility of preparing thin film blends also by employing materials soluble in different solvents.

Substantial photovoltaic response and morphology tuning in benzo[1,2-b:6,5-b′]dithiophene (bBDT) molecular donors

The influence of solubilizing substituents on the photovoltaic performance and thin-film blend morphology of new benzo[1,2-b:6,5-b']dithiophene (bBDT) based small molecule donor semiconductors is investigated. Solar cells based on bBDT(TDPP)2-PC71BM with two different types of side chains exhibit high power conversion efficiencies, up to 5.53%.

Enhancing the Performance of Solution‐Processed n‐Type Organic Field‐Effect Transistors by Blending with Molecular “Aligners”

A novel approach to enhancing the performance of solution-processed n-type organic field-effect transistors by using trace amounts of molecular "aligners" to manipulate the assembly of "matrix" molecules in thin films is demonstrated. The device performance is one order of magnitude higher in 1wt% blended thin films than that in neat films, which correlates to an induced change of preferred orientation of the in-plane π-stacking molecules upon blending.

RETRACTED: Effect of thermal annealing on P3HT:PCBM bulk-heterojunction organic solar cells: A critical review

The main focus of this review article is to introduce the reader to the advantage and limitation of organic solar cells and to understand how the thermal annealing process could improve performance of organic solar cells. The article explores the operation principles of organic photovoltaic (OPV) device and reviews the effects of thermal annealing on bulk-heterojunction organic solar cells based on a thin film blend of poly (3-hexylthiophene-2,5 diyl) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). It focuses on changes in the physical film properties, the electronic performance of devices, as well as the link between film structure and electronic performance during a thermal annealing process. Analysis of film and electrical characteristics, as a function of thermal annealing parameters, indicates that a balancing is established. The balancing between charge conduction and excitonic dissociation leads to the establishment of optimal annealing conditions for maximized electronic performance. Additionally, the report highlights research areas that require attention for future development of this technology.

New Insights into the Mechanisms of Photodegradation/Stabilization of P3HT:PCBM Active Layers Using Poly(3-hexyl-d13-Thiophene)

The photo-oxidation mechanism of thin-film blends based on poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) upon irradiation with ultraviolet–visible light (UV-Vis) was studied. The use of deuterated P3HT, i.e., poly(3-hexyl-d13-thiophene) (P3HdT), permitted discrimination of carbon originating from the hexyl-d13 chain and carbon originating from PCBM and the nondeuterated thiophene unit. The photo-oxidation of both components of the blend was monitored using the combination of various analytical techniques to probe the bulk and the surface of the deposits. The results show that the stabilization of P3HT by PCBM is due to a morphological reorganization between P3HT and PCBM. This change occurs at a low temperature (ca. 42 °C) and increases the lifetime of the primary property, i.e., the ability of the active layer to absorb light. However, this is counterbalanced by the enhanced formation of oxidized PCBM molecules, which may act as electrons traps. It is shown that UV light is harmful for P3HT, PCBM, and P3HT:PCBM blend stabilities, even if PCBM provides a filter effect that is strongest at short wavelengths. It is proposed that the photochemical behavior of the chromophoric species involved in the chain radical oxidation of P3HT is a key characteristic in the underlying mechanism. The results obtained in this work advance the understanding of active layer stability and will help improve the design of long lifetime organic solar cells thanks to the use of cutoff filter in the substrate or encapsulation of the devices.

Spherulitic crystallization in binary thin films under solvent-vapor annealing. I. A sharp-interface theory

We present a thermodynamically consistent theory for solvent-vapor induced spherulitic crystallization in binary thin-film blends, including those which consist of polymeric or organic small-molecule semiconductors. Under the proposed theory, spherulitic growth is interface driven, with no diffusion of any species. The thermodynamic driving force at the interface between the spherulite and amorphous phase is identified, and a kinetic relation that delivers a constant growth rate is proposed.

Breakup of a Transient Wetting Layer in Polymer Blend Thin Films: Unification with 1D Phase Equilibria

We show that lateral phase separation in polymer blend thin films can proceed via the formation of a transient wetting layer which breaks up to give a laterally segregated film. We show that the growth of lateral inhomogeneities at the walls in turn causes the distortion of the interface in the transient wetting layer. By addressing the 1D phase equilibria of a polymer blend thin film confined between selectively attracting walls, we show that the breakup of a transient wetting layer is due to wall-blend interactions; there are multiple values of the volume fraction at the walls which solve equilibrium boundary conditions. This mechanism of lateral phase separation should be general.

Combination of guided mode and photometric optical metrology methods for precise determination of refractive index dispersion: application to polymer blend and ceramic thin films for gas sensors

Two optical techniques, m-lines and spectroscopic ellipsometry, are compared for their suitability for obtaining the wavelength and the temperature dispersion of the refractive index of thin-film layers used in gas detector devices. Two types of materials that are often integrated into gas sensors are studied: a polymer organic–inorganic blend deposited by spin coating typically used in near-infrared waveguides and the ceramic semiconductor SrTi 1−x Fe x O 3 (strontium titanate) doped with iron at concentrations x=0.075 and 0.1 deposited by electron beam deposition. The refractive index dispersion obtained by m-lines and ellipsometry is compared, and the differences between the measured parameters for the two materials are discussed. The chromatic dispersion will be represented by a three-term Cauchy law. An intuitive method for verifying the measured indices using an integrating sphere and reflexion coefficient modeling techniques are also demonstrated. Thermo-optic coefficients in the order of −1×10 −4 /K for both materials are reported, and very low chromatic dispersions are also measured, thanks to the high sensitivity of the m-lines technique. The uniaxial anisotropic properties of the polymer-blend films are measured and discussed in the case of the semiconductor films.

Triplet Formation by Charge Recombination in Thin Film Blends of Perylene Red and Pyrene: Developing a Target Model for the Photophysics of Organic Photovoltaic Materials

Photoinduced charge separation in a mixture of Perylene Red (N,N'-bis(2,6-di-isopropylphenyl)-1,6,7,12-tetraphenoxyperylene-3,4:9,10-tetracarboxylic acid bis-imide) and pyrene, organized in thin solid film on quartz, was studied by means of steady-state absorption and emission spectroscopy and by femtosecond transient absorption spectroscopy. Steady state spectroscopy shows substantial interaction between the two chromophores in the ground and excited states. Luminescence quenching indicates charge transfer interaction. Global and target analysis of the transient absorption data indicates a complex photophysical behavior with the formation of long-lived charges (free charge carriers) and of a triplet excited state species (with rates of ∼10.4 × 10(9) and 72.1 × 10(6) s(-1)) via charge recombination pathways from charged states. A generally applicable target model for the analysis of photophysical data of photovoltaic blends is developed.

Deviations from bulk morphologies in thin films of block copolymer/additive binary blends

Deviations from bulk morphologies in thin films of binary blends of alkyne-functionalized diblock copolymer poly(ethylene oxide)-block-poly(n-butyl methacrylate-random-propargyl methacrylate) (PEO-b-P(nBMA-r-PgMA)) and Rhodamine B azide are reported, where thermal click reaction between the two components leads to microphase separated morphologies. Both in the bulk and in thin films, increasing the azide loading ratio resulted in the transition from a lamellar microdomain morphology to a hexagonally packed cylindrical mircodomain morphology. However, in thin films the lamellae-cylinder transition was observed at a different azide loading ratio, which was determined by film thickness. As a result, significant deviations from the bulk morphology were observed. These results indicate that surface interactions and confined geometry can play an important role in dictating the morphology in thin films of BCP/additive binary blends.

Charge Carrier Dynamics of Vapor-Deposited Small-Molecule/Fullerene Organic Solar Cells

Although small-molecule organic solar cells (SMOSCs) have shown increasingly promising prospects as a source of solar power, there have been few studies concerning the photophysics of these systems. Here, we report the time scale and efficiency of charge separation and recombination in a vapor-deposited SMOSC material that produces 5.81% power conversion efficiency. Transient absorption and time-resolved photoluminescence (trPL) studies of thin film blends comprising DTDCTB, a narrow-band gap electron donor, and either C60 or C70 as an electron acceptor show that charge separation occurs in ~100 fs, while charge recombination takes place over sub-ns and ns time scales. trPL indicates a donor electron-hole pair lifetime of ~33 ps in the neat film and reveals that ~20% of donors fail to charge separate in donor-acceptor mixed films, likely owing to some spatially extended donor-rich regions that interact poorly with acceptors. Our results suggest that morphological manipulations of this material could further improve device efficiency.

Effects of solvent additive on inverted structure PCPDTBT:PC71 BM bulk heterojunction organic solar cells

This work investigates the effects of the solvent additive, 1,8-octanedithiol (1,8-ODT), on solution-processed inverted bulk heterojunction (BHJ) organic solar cells based on a thin film blend of poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]] (PCPDTBT) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). In this work, varying concentrations of 1,8-ODT were added into the polymer solution in order to optimise the device performance. The UV–Vis spectrometer measurements showed that adding 1,8-ODT can improve the magnitude of light absorption in the blend film, as well as inducing an ∼40 nm red shift for long wavelengths. External quantum efficiency (EQE) measurements displayed improvements at all wavelengths for 1–4 vol% 1,8-ODT concentration, when compared to the reference device. Current density–voltage (J–V) measurements confirmed this addition of photocurrent, as the short circuit current density (Jsc) increases by 23%, from 9.89 mA cm−2 for the reference to 12.14 mA cm−2 for the 1 vol% additive cell. The highest average device efficiency of 2.59% was obtained using a 1,8-ODT concentration of 3 vol%. Further increasing additive concentration caused a reduction in efficiency. Additionally, the effect of drying the additive processed films in a vacuum was investigated. Drying in a vacuum had little effect on films processed with low additive concentration. However, the vacuum drying process led to a significant efficiency enhancement for high 1,8-ODT concentrations. The efficiency of the 4 vol% 1,8-ODT devices increased by 25% as a result of drying in a vacuum. This was due to the removal of excess residual additive from the active layer.

Dewetting Process of Deuterated Polystyrene and Poly(vinyl methyl ether) Blend Thin Films via Phase Separation

We studied the structure development in the dewetting process of deuterated polystyrene (dPS) and poly(vinyl methyl ether) (PVME) thin films, which were prepared by a spin-coating method on a quartz substrate, after the temperature jump into the two-phase region, using time-resolved specular and off-specular neutron reflectivity (NR), specular X-ray reflectivity (XR), light scattering (LS), and optical microscope (OM) measurements. The OM and specular XR measurements showed that there clearly existed an incubation period before dewetting. In the incubation period, composition fluctuations between dPS and PVME were developed due to phase separation. The specular NR results revealed that composition fluctuations began to occur just after the temperature jump along the depth direction over the film, giving rise to the composition gradient heterogeneously over the film. Composition fluctuations were also observed in the in-plane direction by the off-specular NR and LS measurements: the off-specular NR measure...

Evidence for Kinetically Limited Thickness Dependent Phase Separation in Organic Thin Film Blends

We present depth-resolved grazing incidence x-ray diffraction, grazing incidence small angle scattering and x-ray reflectivity studies on the structure of mixed C(60) and diindinoperylene (DIP) films as a function of the mixing ratio. We observe enhanced out-of-plane order and smoothing of the mixed films compared to pure films upon coevaporation of DIP:C(60) thin films (in different mixing ratio) which otherwise phase separate. The mixing ratio of molecules can be tuned to alter the in-plane crystallite size as well as the interisland distances of the mixing molecules. Real-time in situ grazing incidence x-ray diffraction measurements show the kinetics and thickness dependence of phase separation, which appears to proceed only after a certain thickness. The crystallite grain size of the individual phase separated components is significantly larger at the top of the film than at the bottom with implications for the understanding of devices.

Graded Absorption Layers in Bulk Heterojunction Organic Solar Cells

Bulk heterojunction organic solar cells with a vertical variation of the C60:ZnPc composition within the absorption layer have been fabricated. The resulting gradient layer has been characterized by UV/vis transmission and reflection spectroscopy. Depending on the mixing strategy, the formation of higher aggregated ZnPc species can be initialized earlier in a blended thin film. The gradient strength has been varied and its influence on the solar cell performance has been determined. A variation of the absorption layer thickness has been carried out to investigate a possible charge carrier transportation improvement. In order to explain the positive effect of a graded structure within the absorption layer, detailed voltage-dependent spectral response measurements have been performed. It is shown that not only the absorption behavior of the cell is improved, but also the charge carrier transportation properties.