PCBM Thin Films Research Articles

Effect of solvent annealing on the crystallinity of spray coated ternary blend films prepared using low boiling point solvents

The effects of different post treatments on the spray coated P3HT:PPV:PCBM for an active layer in polymer solar cell were investigated using atomic force microscopy (AFM), UV–vis spectroscopy, photoluminescence (PL) spectroscopy and X-ray diffractometer (XRD). The annealing temperatures were varied from 130°C to 150°C and the annealing time was kept constant for 10min. The RMS roughness of the sample that dried in the ambient condition was lower compared to other samples. This explained that the samples that undergo solvent and thermal annealing treatment had higher crystallinity of P3HT or phase separation in these samples is mostly favored. The peak absorption for P3HT:PPV:PCBM thin film is higher compared to P3HT:PCBM thin film. The better defined shoulders can be observed in the samples with solvent annealing treatment. The XRD spectra showed that the structure of the samples will evolve with high thermal annealing temperature (150°C) due to a weak intermolecular force in P3HT molecules. This will contribute to the low crystallinity of the films which consequently affects the absorption properties of the films.

Synthesis and characterization of novel fulleropyrrolidine in P3HT blended bulk heterojunction solar cells

Synthesis of novel fullerene derived electron acceptors and characterization of their organic photovoltaic (OPV) properties is important for advancing fundamental knowledge towards developing next generation organic solar cells. We report the synthesis of a novel fulleropyrrolidine derivative C60-fused N-(3-methoxypropyl)-2-(carboxyethyl)-5-(4-cyanophenyl)fulleropyrrolidine (NCPF) by 1,3-dipolar cycloaddition reaction and characterization of NCPF by 1H NMR, 13C NMR, MALDI-TOFMS, FT-IR, UV–Vis and CV. The synthesized NCPF fullerene derivative showed good solubility in common organic solvents such as chlorobenzene and 1,2 dichlorobenzene important for film formation, with optical absorbance and electronic properties comparable to PCBM. Optical micrographs of P3HT:PCBM thin films reveal formation of sparse, phase segregated needle shape PCBM micro-crystalline aggregates after 1 h of annealing at 150 °C whose length follows nucleation and growth kinetics over 24 h. In contrast, the P3HT:NCPF thin films exhibit homogeneity over 24 h, possibly due to weaker interparticle vanderWaals forces and/or stronger interactions with P3HT. This long term morphological stability of P3HT:NCPF is important for extended use in OPV applications. At an order of magnitude smaller scale, AFM of as cast and 10 min annealed at 150 °C P3HT:PCBM and P3HT:NCPF films reveal mostly smooth surfaces, with some NCPF cluster formation. Grazing incidence wide angle X-ray scattering (GIWAXS) measurements of P3HT:NCPF films indicate an increase of P3HT crystallinity with thermal annealing, leading to improvement in device performance. Photovoltaic devices fabricated with the active layer of P3HT:NCPF and P3HT:PCBM sandwiched between ITO/PEDOT:PSS and Al layer showed comparable performance upon short term annealing.

Absorption and scattering effects by silver nanoparticles near the interface of organic/inorganic semiconductor tandem films

We experimentally and theoretically characterize back-scattering and extinction of Ag nanoparticle (AgNP) arrays on both Si wafer substrates and optically-thick Ag substrates with and without organic poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) bulk-heterojunction thin film coatings. A strong red-shift in back-scattered light wavelength occurs from AgNP arrays on Si as a function of increasing mean nanoparticle diameter (ranging from 30 to 90 nm). Back-scattering from the AgNP array is notably quenched in the wavelength range of strong P3HT absorption when the organic layer is applied. However, back-scattering is enhanced to a degree relative to the uncoated AgNP array on Si at wavelengths greater than the absorption band edge of P3HT. For comparison, the optical properties of AgNPs on an optically-thick Ag substrate are reported with and without P3HT:PCBM thin film coatings. On the reflective Ag substrates, a significant enhancement (by a factor of 7.5) and red-shift of back-scattered light occurred upon coating of the AgNPs with the P3HT:PCBM layer. Additionally, red-edge extinction was enhanced in the P3HT:PCBM layer with the presence of the AgNPs compared to the planar case. Theoretical electromagnetic simulations were carried out to help validate and explain the scattering and extinction changes observed in experiment. Both increasing nanoparticle size and an increasing degree of contact with the Si substrate (i.e., effective index of the nanoparticle environment) are shown to play a role in increasing back- and forward-scattering intensity and wavelength, and in increasing absorption enhancements in both the organic and Si layers. AgNPs placed at the P3HT:PCBM/Si interface give rise to absorption increases in P3HT of up to 18 %, and only enhance Si absorption at wavelengths longer than the absorption band edge of P3HT (by almost 90 % in the 660–1,200 nm wavelength range). These results provide insight into how metal nanoparticles placed near an organic/inorganic interface can be employed for light management in tandem or hybrid organic/inorganic thin-film semiconductor configurations for solar energy harvesting applications or light detection applications.

Effects of processing additives on nanoscale phase separation, crystallization and photovoltaic performance of solar cells based on mesogenic phthalocyanine

A study on the effects of processing additives on the nanoscale phase separation, crystallization, and photovoltaic performance of bulk heterojunction (BHJ) thin films made of 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH2) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) via spin-casting for photovoltaic applications is reported. By incorporating various solvents as processing additives to a volume of a few percent, the separation of donor and acceptor phases in C6PcH2:PCBM thin films, which discussed by taking the photoluminescence quenching, Davydov splitting at the Q-band of the absorbance spectra and the surface nanomorphology into consideration, is improved, and the crystallinity of the discotic C6PcH2 molecules with hexagonal structures is reinforced. Photovoltaic cells with the optimum phase-separated BHJ materials and high crystallinity of the discotic C6PcH2 molecules are demonstrated to have a power conversion efficiency of 4.2%.

A study of donor/acceptor interfaces in a blend of P3HT/PCBM solar cell: Effects of annealing and PCBM loading on optical and electrical properties

Optical, structural and electrical properties based on blend of P3HT: PCBM thin films have been investigated for several PCBM loading whilst exposed to air. Experimental evidence indicates that the peak absorption for PCBM with highest loading concentration show greater reduction in intensity upon annealing temperature of 175°C. This provides useful information regarding PCBM segregation associated with annealing process. Comparison of GIXRD patterns indicates an increase in intraplanar d spacing and polymer grain size upon thermal annealing. Variation in open circuit voltage and short circuit current with annealing temperatures, suggests that the latter remains almost constant up to around 100°C and improves gradually with temperature thereafter, whilst the former improves with temperature and the onset of deterioration in current density is linked to percentage PCBM loading. These results are further explained or supported by optical absorption spectroscopy to account for PCBM clustering. A power conversion efficiency of 3.38% was achieved at 175°C for devices fabricated and tested in air.

Effect of process parameters on the morphology and nanostructure of roll-to-roll printed P3HT:PCBM thin films for organic photovoltaics

Roll-to-roll (R2R) gravure exhibits significant advantages such as high precision and throughput for the printing of photoactive and conductive materials and the fabrication of flexible organic electronics such as organic photovoltaics (OPVs). Since the photoactive layer is the core of the OPV, it is important to investigate and finally control the process parameters and mechanisms that define the film morphology in a R2R process. The scope of this work is to study the effect of the R2R gravure printing and drying process on the nanomorphology and nanostructure of the photoactive P3HT:PCBM thin films printed on PEDOT:PSS electrodes towards the fabrication of indium tin oxide (ITO)-free flexible OPVs. In order to achieve this, P3HT:PCBM blends of different concentration were R2R printed under various speeds on the PEDOT:PSS layers. Due to the limited drying time during the rolling, an amount of solvent remains in the P3HT:PCBM films and the slow-drying process takes place which leads to the vertical and lateral phase separation, according to the Spectroscopic Ellipsometry and Atomic Force Microscopy analysis. The enhanced slow-drying leads to stronger phase separation, larger P3HT crystallites according to the Grazing Incidence X-Ray Diffraction data and to weaker mechanical response as it was shown by the nanoindentation creep. However, in the surface of the films the P3HT crystallization is controlled by the impinged hot air during the drying, where the more the drying time the larger the surface P3HT crystallites. The integration of the printed P3HT:PCBM and PEDOT:PSS layers in an OPV device underlined the feasibility of fabricating ITO-free flexible OPVs by R2R gravure processes.

Polymer-regulated epitaxial crystallization of methanofullerene on mica

This work focuses on both the structure manipulation and the crystallizing mechanism investigation of the well-known methanofullerene, [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM). PCBM crystals with two novel structures, i.e., five-fold twinned and cubic crystals are obtained by the introduction of the mica substrate and the polymer blenders (P3HT and PS) into PCBM thin films under thermal annealing. The morphology and nanostructure of these crystals have been well investigated with AFM, TEM and XRD techniques. The roles of the mica substrate and the polymer blenders have been studied by varying the annealing temperature, the substrate, the polymer benders and the blending ratio. It has been proved that both the PCBM intermolecular and PCBM-mica interactions influence the PCBM crystallization process. The mica substrate has been proved to have the epitaxial effect on PCBM crystallization. The polymer blenders have been suggested to weaken the PCBM intermolecular interaction and limit PCBM molecular diffusion.

Reduced Surface Roughness of P3HT: PCBM Thin Films with Different Ratios by Electrospray Deposition Methods

Reduced Surface Roughness of P3HT: PCBM Thin Films with Different Ratios by Electrospray Deposition Methods

Structural and electrical properties of P3HT:PCBM/PEDOT:PSS thin films deposited through electrohydrodynamic atomization technique

This paper presents the deposition of solution based composite P3HT:PCBM thin film on PEDOT:PSS coated ITO substrate via electrohydrodynamic atomization (EHDA) technique. Deposition conditions such as stable cone jet formation, flow rate and applied potential have been investigated. X-ray diffraction anlaysis revealed the presence of P3HT polymer in the as deposited film. Surface morphology investigated using a field emission-scanning electron microscope confirms that a uniform deposition of thin films was achieved. The current–voltage characteristics of the as deposited films show a rectifying behavior. The capacitance–voltage study indicated capacitance of the as deposited thin films depended on the frequency .

Improvement of the Solar Efficiency of Polymer Solar Cells by using 1, 3, 5-Trichlorobenzene as Co-solvent

Bulk-heterojunction (BHJ) solar cells utilizing P3HT:PCBM composite using 2.5, 3.5, 5.5 and 7.5 mg of 1, 3, 5-trichlorobenzene (TCB) as co-solvent were fabricated. The co-solvent method can induce a good quality of P3HT:PCBM thin film nanostructure, leading to enhanced absorption. With the optimized TCB quantity, the power conversion efficiency (PCE) achieved with the solar cell was up to 15% better than that attained by a similar device without the presence of TCB. The devices load with and without Niobium doped ZnO Nanoparticles (Nb/ZnO NPs) produced by flame spray pyrolysis (FSP) were also compared.

The impact of controlled solvent exposure on the morphology, structure and function of bulk heterojunction solar cells

Films of poly(3-hexyl thiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) were controllably exposed to CS2 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.

Effect of Electric Field Treatment on the Performance of Organic Salt Doped Solar Cell

High efficiency is one of the characteristics of a good solar cell. The objective of the experiment is to investigate the effect of electric field treatment on the performance of organic salt tetrabutylammonium hexafluorophosphate (TBAPF6) doped solar cell with ITO/MEHPPV:PCBM/Al structure where indium tin oxide (ITO) was used as anode, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) as donor, (6,6)-phenyl-C61 butyric acid methyl ester (PCBM) as acceptor and aluminium (Al) as cathode. The devices were build by depositing 20 % of TBAPF6 (weight percentage with respect to MEHPPV) doped MEHPPV:PCBM thin film onto the ITO by spin coating technique, and followed by deposition of Al by using electron gun evaporation technique. The performance of the devices was analyzed through the current-voltage (I-V) curve under illumination with a solar simulator at 100 mW/cm2. Electric field treatment was given by applying different constant voltages of 6 V, 8 V and 10 V for 60 s to the devices before the I-V characterization. A device without electric field treatment was also investigated for comparison. The efficiency of the devices increased with constant voltage given and reached an optimum value at treated voltage of 8 V. The optimum device exhibited the highest efficiency of 0.16% with short circuit current density of 0.88 mA/cm2, open circuit voltage of 0.60 V and fill factor of 30%.

The miscibility and depth profile of PCBM in P3HT: thermodynamic information to improve organic photovoltaics

Recent work has shown that poly(3-hexylthiophene) (P3HT) and the surface-functionalized fullerene 1-(3-methyloxycarbonyl)propyl(1-phenyl[6,6])C(61) (PCBM) are much more miscible than originally thought, and the evidence of this miscibility requires a return to understanding the optimal morphology and structure of organic photovoltaic active layers. This manuscript describes the results of experiments that were designed to provide quantitative thermodynamic information on the miscibility, interdiffusion, and depth profile of P3HT : PCBM thin films that are formed by thermally annealing initial bilayers. It is found that the resultant thin films consist of a 'bulk' layer that is not influenced by the air or substrate surface. The composition of PCBM in this 'bulk' layer increases with increased PCBM loading in the original bilayer until the 'bulk' layer contains 22 vol% PCBM. The introduction of additional PCBM into the sample does not increase the amount of PCBM dispersed in this 'bulk' layer. This observation is interpreted to indicate that the miscibility limit of PCBM in P3HT is 22 vol%, while the precise characterization of the depth profiles in these films shows that the PCBM selectively segregates to the silicon and near air surface. The selective segregation of the PCBM near the air surface is ascribed to an entropic driving force.

Effects of direct solvent exposure on the nanoscale morphologies and electrical characteristics of PCBM-based transistors and photovoltaics

We investigated the effects of direct solvent exposure on the properties of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films and poly(3-hexylthiophene) (P3HT)/PCBM blend films employed as active layers in, respectively, organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). The crystallinity, morphology, and OFET characteristics of the PCBM thin films were significantly influenced by direct exposure to solvent, especially to select alcohols. Control over the nanoscale morphology of the PCBM film, achieved via direct solvent exposure, yielded highly efficient poly(3-hexylthiophene) (P3HT)/PCBM OPVs with a short-circuit current density of 10.2 mA cm−2, an open-circuit voltage of 0.64 V, and a power conversion efficiency of 3.25% under AM 1.5 illumination with a light intensity of 100 mW cm−2. These results indicated that optimal phase separation in the P3HT/PCBM films could be obtained simply by exposing the active layer films for a few seconds to solvent.

The Effects of Substrate Temperature on the Morphology and Photovoltaic Cell Performance of P3HT:PCBM Thin Film Fabricated by Doctor-Blading Technology

The Effects of Substrate Temperature on the Morphology and Photovoltaic Cell Performance of P3HT:PCBM Thin Film Fabricated by Doctor-Blading Technology

Morphological Characterization of Spray-Coated PCBM Thin Films

Morphological Characterization of Spray-Coated PCBM Thin Films

Co-solvent effects on the morphology of P3HT:PCBM thin films

Morphology control of the active layer in bulk hetero junction devices is essential for obtaining high-energy conversion efficiency. For poly(3-hexylthiophene) (P3HT)–[6-6]-phenyl-C61-butyric acid methyl ester (PCBM) based organic photovoltaic cells, the low performance is assigned to the formation of PCBM aggregates, whose process has not been clearly established. In this work, we have investigated P3HT:PCBM composite films, which were prepared by dissolving the mixture in co-solvents of chlorobenzene and toluene of different proportions. The solvents have been chosen for their close boiling points and for their different polarities, which impact on the solubility of organic components. Optical (absorption, photoluminescence), surface (Atomic Force Microscopy, X-ray photoelectron spectroscopy) and structural (X-ray diffraction) characterization techniques have been performed on blend films prepared by co-solvents. The result analysis has clearly pointed out the key role of the PCBM solubility on the film organization. Conversely, the regioregularity of P3HT would not influence the film morphology when using dominant chlorobenzene solvents but strongly affected it when using dominant toluene solvents.

Evolution of vertical phase separation in P3HT:PCBM thin films induced by thermal annealing

The achievement of the desirable morphology at the nanometer scale of bulk heterojunctions consisting of a conjugated polymer with fullerene derivatives is a prerequisite in order to optimize the power conversion efficiency of organic solar cells. The various experimental conditions such as the choice of solvent, drying rates and annealing have been found to significantly affect the blend morphology and the final performance of the photovoltaic device. In this work, we focus on the effects of post deposition thermal annealing at 140 °C on the blend morphology, the optical and structural properties of bulk heterojunctions that consist of poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM). The post thermal annealing modifies the distribution of the P3HT and the PCBM inside the blend films, as it has been found by Spectroscopic Ellipsometry studies in the visible to far-ultraviolet spectral range. Phase separation was identified by AFM and GIXRD as a result of a slow drying process which took place after the spin coating process. The increase of the annealing time resulted to a significant increase of the P3HT crystallinity at the top regions of the blend films.

Bulk heterojunction organic photovoltaic cell fabricated by the electrospray deposition method using mixed organic solvent

AbstractA high‐efficiency bulk heterojunction organic photovoltaic cell (OPV) was achieved by the electrospray deposition method. The surface roughness of the P3HT:PCBM thin film can be reduced using the mixed solvent consisting of o‐dichlorobenzene (o‐DCB) and acetone. The effect of acetone concentration is related to its dielectric constant. Under an optimized concentration of acetone in o‐DCB (20 vol%), the P3HT/PCBM active layer with a smooth surface can be formed, and the power conversion efficiency of the OPV was 1.9%.magnified image(© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Raman spectroscopic identification of fullerene inclusions in polymer/fullerene blends

AbstractThin films of the conjugated polymer poly(3‐hexylthiophene) (P3HT) and blends of the soluble fullerene derivative[6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) with P3HT—a well studied but not completely understood donor–acceptor system for organic solar cells—have been studied by means of UV–visible absorption and resonant Raman spectroscopy. Additionally, we have employed atomic force microscopy phase imaging to characterize the nanomorphology of the P3HT : PCBM thin film, revealing a close intermixing of two phases with domain sizes ranging from a few to several tens of nanometers. A systematic analysis of pristine polymer and blend Raman spectra provides evidence that features attributable to PCBM, possibly even depending on the charge state of the fullerene molecule, can be observed. Hence our results suggest that fullerene inclusions in polymer/fullerene blends can be identified via Raman spectroscopy. Copyright © 2011 John Wiley & Sons, Ltd.