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Investigation of the effects of substrate annealing on the properties of polymer blends
Materials Chemistry and Physics, 2010
In this manuscript, we investigated the effect of substrate annealing prior to the deposition of the active layer on the morphological, structural and optoelectrical properties of two variations of organic polymer blends of poly (3hexylthiophene):C 60 fullerene and poly (3-hexylthiophene):[6,6]-phenyl C 61 butyric acid methyl ester films using atomic force microscopy, field emission scanning microscopy, x-ray diffraction and Hall effect measurements. P3HT films deposited on pre-annealed substrates exhibit an enhancement in the crystallization, and an increase in the electrical conductivity and Hall mobility of p-type P3HT. The microscopic morphology reveals self-assembled fibrillar structures due to the suppressed growth of fullerenes clusters induced by the controlled evaporation rate of the solvent. It is proposed that pre-substrate annealing controls the crystallization of P3HT, the phase separation and diffusion of the acceptor material (C 60 or PCBM).
Journal of Materials Chemistry C, 2013
We report the preparation of films of poly(3-hexylthiophene) nanofibers suitable for fabrication of efficient multilayer solar cells by successive deposition of donor and acceptor layers from the same solvent. The nanofibers are obtained by addition of di-tert-butyl peroxide (DTBP) to a solution of P3HT in chlorobenzene. Interestingly, by varying the concentration of DTBP we are able to control both crystallinity and film retention of the spin-cast films. We also investigate the influence of the DTBPinduced crystallization on charge transport by thin-film transistor measurements, and find a more than five-fold increase in the hole mobility of nanofiber films compared to pure P3HT. We attribute this effect to the synergistic effects of increased crystallinity of the fibers and the formation of micrometersized fiber networks. We further demonstrate how it is possible to make use of the high film retention to fabricate photovoltaic devices by subsequent deposition of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) from a chlorobenzene solution on top of the nanofiber film. The presence of a relatively large crystalline phase strongly affects the diffusion behavior of PCBM into the P3HT film, resulting in a morphology which is different from that of common bulk heterojunction solar cells and resembles a bilayer structure, as can be inferred from comparison of the external quantum efficiency spectra.
Soft Matter, 2010
The power conversion efficiency in a conjugated polymer-functionalized fullerene bulk heterojunction organic photovoltaic (OPV) device is dependent both on the electronic properties of the constituent materials and on the nanoscale morphology of the active semiconductor layer thin-film. Here we use in situ ellipsometry and grazing incidence X-ray scattering (GI-XS) to study molecular self-organization in poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend films in real time, during the drying process as they are cast from solution. We illustrate the evolution of the extinction coefficient from a solution to a solid, semi-crystalline state. We show that once the solvent fraction in the film falls below 50%, the P3HT undergoes rapid crystallization via heterogeneous nucleation; a process that is complete in seconds. We also evidence a rapid, dynamic self-annealing process that reduces the characteristic lamella spacing in the P3HT crystallites. The mechanistic understanding of film-formation demonstrated here is an important component in optimizing deposition processes suitable for large-area OPV manufacture. †Electronic supplementary information (ESI) available: 3D cartoon scheme of the in situ system set-up, methods of extracting film thickness and optical constants during the P3HT:PCBM film drying process via ellipsometry, GI-WAXS of P3HT:PCBM blend, and other additional results supporting the discussion. See
Solar Eng Mater Solar Cells 2013 Nanoparticles in P3HT-PCBM
a b s t r a c t Nanoparticles (NPs) having different surface capping agent, variant electrical conductivity and sunlight absorption have been studied for the ternary hybrid containing poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) in bulk heterojunction (BHJ) organic photovoltaics (OPVs). These NPs are composed of conducting gold, semiconducting CdS or PbS, or insulating cage-like molecular silica (POSS). We use a series of microscopic methods including TEM, AFM, SEM, optical, and fluorescence microscopy to estimate NP size and to probe the agglomeration and/or aggregation of NPs in P3HT/PCBM blends. Surface capping agent aromatic thiophenol (SPh) was found to be poor in the dispersion of NPs in P3HT/PCBM blends. The light harvesting of these NPs ranges from transparent (POSS NP), near transparent (CdS NP), visible light absorbing (Au NP), to near-infrared absorbing (PbS NP). Nevertheless, the absorbance of these NPs is all too small relative to that of P3HT polymer. Concerning the charge separation of P3HT exciton, the LUMO energy levels of these NPs have been determined by the combination of optical band-gap energy and HOMO energy levels. By the transient photocurrent time-offlight method, charge carrier mobility of P3HT/PCBM/NP(CdS-SPh) ternary hybrid was found to be improved, although fluorescence quenching studies imply insufficient or ineffective contact between P3HT and all NPs in the present study. NPs hybrid P3HT/PCBM BHJ OPVs were fabricated by solution process. Regardless of conductivity or sunlight absorbance, all NPs show no improvement on power conversion efficiency of ternary hybrid OPVs, which is 3.03-3.91% less than 4.0-4.1% of P3HT/PCBM OPVs without NPs. Based on the present study, a few problems that associate with the inferior performance of NPs hybrid P3HT/PCBM BHJ OPVs are delineated.
Solar Energy Materials and Solar Cells, 2012
The rapidly increasing power conversion efficiencies of organic solar cells are an important prerequisite towards low cost photovoltaic fabricated in high throughput. In this work we suggest indane as a nonhalogenated replacement for the commonly used halogenated solvent o-dichlorobenzene. Indane was blended with the higher volatile solvents chloroform or toluene or o-xylene in order to improve wettability and to reduce drying time. The combination of high volatile solvents with the less volatile host solvent indane allows for an increased fabrication speed due to a reduction of the overall drying time and provides films with good light absorption behavior and high polymer crystallinity. For the solvent mixture toluene-indane, solar cell performance is comparable to the o-dichlorobenzene reference device indicating this mixture as a suitable replacement for increased productivity without drawbacks in nanomorphology as investigated by atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXD). This study provides a fundamental understanding on solvent mixture drying kinetics and can aid the ink formulation.
A stable morphology in the photoactive layer is a prerequisite for increasing the lifetime of organic solar cells. Intense research efforts focusing on this research topic have typically resorted to complicated synthetic methods to reach this goal. Herein, the authors present a facile approach to directly achieve efficient polymer solar cells with a remarkably enhanced thermally stable morphology by constructing densely distributed poly(3-hexylthiophene) (P3HT) nanofibers in the pristine composite films with PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) from solution without any post treatments. Controlled experiments reveal that the presence of numerous preformed P3HT nanofibers in the pristine films, with much larger size than P3HT and PCBM molecules, provides a fixed and rigid network to spatially confine the diffusion of PCBM molecules during thermal annealing, thus preventing the formation of large-scale PCBM crystals. This simple method represents a ''one-step'' way to prepare high performance photovoltaic devices with thermally stable morphologies and no necessary post treatments.
Solar Energy Materials and Solar Cells, 2012
Films of poly(3-hexyl thiophene) (P3HT):[6,6]-phenyl C 61 -butyric acid methyl ester (PCBM) were controllably exposed to CS 2 vapor in a column with a linear solvent vapor pressure gradient. Changes in the morphology of the P3HT:PCBM thin film were monitored and correlated to the ability of this thin film to act as the active layer in an organic solar cell. The results show that the crystallinity and crystal size of the P3HT increase initially with solvent vapor pressure and annealing time, but longer exposure to solvent decreases P3HT crystallinity and photovoltaic efficiency. Neutron reflectivity indicates that the PCBM segregates to the Si substrate in the as-cast thin film, but distributes throughout the film with solvent annealing. The changes in crystallinity and the depth profile of the P3HT:PCBM mixture differ from those induced by thermal annealing. The structural variation with solvent exposure is correlated to photovoltaic function, demonstrating that the solvent annealing provides a window of optimum efficiency, which depends on solvent exposure. Moreover, the control of depth profile and structure should be generally applicable to a broad range of polymer-nanoparticle mixtures and thus these results provide fundamental information that can be used to control the depth profile, morphology and function of thin film nanocomposites.