PCBM Thin Films Research Articles

Polymer:Fullerene Bimolecular Crystals for Near-Infrared Spectroscopic Photodetectors.

Spectroscopic photodetection is a powerful tool in disciplines such as medical diagnosis, industrial process monitoring, or agriculture. However, its application in novel fields, including wearable and biointegrated electronics, is hampered by the use of bulky dispersive optics. Here, solution-processed organic donor-acceptor blends are employed in a resonant optical cavity device architecture for wavelength-tunable photodetection. While conventional photodetectors respond to above-gap excitation, the cavity device exploits weak subgap absorption of intermolecular charge-transfer states of the intercalating poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bimolecular crystal. This enables a highly wavelength selective, near-infrared photoresponse with a spectral resolution down to 14 nm, as well as dark currents and detectivities comparable with commercial inorganic photodetectors. Based on this concept, a miniaturized spectrophotometer, comprising an array of narrowband cavity photodetectors, is fabricated by using a blade-coated PBTTT:PCBM thin film with a thickness gradient. As an application example, a measurement of the transmittance spectrum of water by this device is demonstrated.

Unraveling the multifunctional capabilities of PCBM thin films in inverted-type CH3NH3PbI3 based photovoltaics

Comprehensive studies were carried out to explore the roles of phenyl-C61-butyric acid methyl ester (PCBM) thin films in inverted-type CH3NH3PbI3 (MAPbI3) based photovoltaics, including the surface morphologies, transmittance spectra, photoluminescence spectra, X-ray diffraction (XRD) patterns, Raman scattering spectra and thin-film residual stress analysis. The reduction in the photoluminescence line width indicates that the crystallinity of the MAPbI3 thin film can be increased by covering it with the PCBM thin film. The XRD patterns and Raman scattering spectra show a reduction in the compressive stress of MAPbI3 thin film when covered by the PCBM thin film. In addition, it is noted that the residual stress at the contact interface between the hydrophilic MAPbI3 and the hydrophobic PCBM can be ignored as confirmed by the results obtained with a home-made Twyman-Green interferometer. Consequently, the superior optoelectronic properties of the perovskite materials, as well as the use of a multifunctional fullerene-based thin film as the capping layer allow for the high-efficiency perovskite photovoltaics.

Influence of high energy ion irradiation on fullerene derivative (PCBM) thin films

The modifications produced by 55MeV Si4+ swift heavy ion irradiation on the phenyl C61 butyric acid methyl ester (PCBM) thin films (thickness∼100nm) has been enlightened. The PCBM thin films were irradiated at 1×1010, 1×1011 and 1×1012ions/cm2 fluences. After ion irradiation, the decreased optical band gap and FTIR band intensities were observed. The Raman spectroscopy reveals the damage produced by energetic ions. The morphological variation were investigated by atomic force microscopy and contact angle measurements and observed to be influenced by incident ion fluences. After 1011ions/cm2 fluence, the overlapping of ion tracks starts and produced overlapping effects.

Impact of solution phase behaviour and external fields on thin film morphology: PCBM and RRa-P3HT model system.

We report the impact of the ternary solution phase behaviour on the film morphology and crystallization of a model polymer:fullerene system. We employ UV-Vis absorption spectroscopy, combined with sequential filtration and dilution, to establish the phase diagram for regio-random poly(3-hexylthiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester (PCBM) in chlorobenzene. Films are systematically cast from one- and two-phase regions decoupling homogeneous and heterogenous nucleation, and the role of pre-formed aggregates from solutions. Increasing annealing temperature from 120 to 200 °C reveals a highly non-monotonic nucleation profile with a maximum at 170 °C, while the crystal growth rate increases monotonically. UV ozonolysis is employed to vary substrate energy, and found to increase nucleation rate and to promote a binary crystallization process. As previously found, exposure to light, under an inert atmosphere, effectively suppresses homogeneous nucleation; however, it has a considerably smaller effect on heterogeneous nucleation, either from solution aggregates or substrate-driven. Our results establish a quantitative link between solution thermodynamics, crystallization and provide insight into morphological design based on processing parameters in a proxy organic photovoltaic system.

Poly[(9,9-dioctylfluorene)-2,7-diyl-alt-(4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole)-2′,2″-diyl]:PCBM thin films: Molecular self-assembly, hexyl substituent groups and sheet resistance relationships

Poly[(9,9-dioctylfluorene)-2,7-diyl-alt-(4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole)-2′,2′′-diyl] (F8TBT) is a conjugated polymer with peculiar characteristics due to its D-A type architecture. F8TBT ambipolar feature has allowed the confection of F8TBT/PCBM ([6,6]−Phenyl-C61-butyric acid methyl ester) and polymer/F8TBT heterojunctions for solar cells. However, the use of PCBM as electron acceptor in photovoltaic devices is still sovereign because of its great electron affinity and compatibility with a large amount of conjugated polymers.The effects of PCBM content (0, 33, 50, 67, 75 and 80wt%) and amount of hexyl substituent groups in F8TBT polymer backbone on morphology and Hall sheet resistance (Rs) of F8TBT:PCBM thin films are evaluated quantitatively by optical microscopy and image processing. Circular-like cavities are formed on F8TBT films by molecular self-assembly, such structures on F8TBT film surface were identified by AFM. The statistical distributions of the self-assembled cavity diameters can be fitted by Lorentz, Boltzmann sigmoidal and Gauss statistics functions, depending on the amount of hexyl substituent groups and PCBM. The absence of these alkyl groups on the polymer chains causes the following effects: increasing of the Rs value on 69% and modification of the cavity size distribution. Moreover, there is significant elevation of Rs due to the cavity density on polymer and nanocomposite thin films.

Molecular ordering of spin-coated and electrosprayed P3HT:PCBM thin films and their applications to photovoltaic cell

Poly(3-hexylthiophene-2,5-diyl) (P3HT) is one of the most popular donor polymers for organic photovoltaic cells; however, annealing-induced molecular ordering is necessary for realizing high photoconversion efficiency. In this study, the ratio of ordered-disordered P3HT Raman signals was calculated as the crystallinity parameter, which can be evaluated as the molecular ordering of P3HT. In the case of spin-coated devices, the crystallinity parameter increases with increasing annealing temperature, and this agrees with the photovoltaic performance. Furthermore, the direct molecular ordering of P3HT during electrospray deposition is reported, detailing the relation between the solvent evaporation time and the crystallinity of P3HT, and is evaluated by Raman spectroscopy and grazing incidence X-ray diffraction. To observe the solvent evaporation phenomena of the electropsray process, in situ measurement of solvent evaporation time was also successfully realized for the first time by placing a CCD camera below the substrate in electrospray deposition. The solvent evaporation time was controlled from 0.036 to 2.8s by changing the applied voltage and solvent. By investigating the relationship between the solvent evaporation time and the molecular ordering of P3HT, the long solvent evaporation time caused the high crystallinity of P3HT. In addition, the population of P3HT crystallinity with an edge-on orientation also increased with increasing solvent evaporation time by evaluating the grazing incidence X-ray diffraction pattern, in good agreement with the estimated crystallinity from Raman spectroscopy. Finally, a photoconversion efficiency of 2.0% was achieved by electrospray deposition without post thermal annealing.

Electronic excitation induced modifications of optical and morphological properties of PCBM thin films

Phenyl C61 butyric acid methyl ester (PCBM) is a fullerene derivative and most commonly used in organic photovoltaic devices both as electron acceptor and transporting material due to high electron mobility. PCBM is easy to spin caste on some substrate as it is soluble in chlorobenzene. In this study, the spin coated thin films of PCBM (on two different substrate, glass and double sided silicon) were irradiated using 90MeV Ni7+ swift heavy ion beam at low fluences ranging from 1×109 to 1×1011ions/cm2 to study the effect of ion beam irradiation. The pristine and irradiated PCBM thin films were characterized by UV–visible absorption spectroscopy and fourier transform infrared spectroscopy (FTIR) to investigate the optical properties before and after irradiation. These thin films were further analyzed using atomic force microscopy (AFM) to investigate the morphological modifications which are induced by energetic ions. The variation in optical band gap after irradiation was measured using Tauc’s relation from UV–visible absorption spectra. A considerable change was observed with increasing fluence in optical band gap of irradiated thin films of PCBM with respect to the pristine film. The decrease in FTIR band intensity of C60 cage reveals the polymerization reaction due to high energy ion impact. The roughness is also found to be dependent on incident fluences. This study throws light for the application of PCBM in organic solar cells in form of ion irradiation induced nanowires of PCBM for efficient charge carrier transportation in active layer.

Determination of diffusion lengths in organic semiconductors: Correlation with solar cell performances

Organic semiconductors are promising candidates for future applications in solar energy conversion. Recent investigations of bulk heterojunction (BHJ) semiconductors have suggested a density of states and transport mechanisms by multiple trapping close to those observed in disordered inorganic thin films. That is why we have applied to BHJ thin films experiments that are currently used for disordered semiconductors. In addition to the steady state photoconductivity we have tested the ability of the steady state photocarrier grating (SSPG) technique to provide information on the minority carrier diffusion length. We found that SSPG can be applied to P3HT:PCBM thin films leading, for the best sample, to a diffusion length of the order of 125 nm. From the comparison of the transport parameters obtained on thin films with the performances of the devices integrating the latter, we conclude that SSPG is a very powerful tool for optimizing the BHJ thin film properties before their incorporation in solar devices.

Effect of nano particle sizes on the third-order optical non-linearities and nanostructure of copolymer P3HT:PCBM thin film for organic photovoltaics

Organic solar cells are based on (3-hexylthiophene):[6,6]-phenyl C61-butyric acid with methyl ester Bulk Heterojunction. An inverted structure has been fabricated using nano-anatase crystalline titanium dioxide, as the electron transport layer, which was prepared on either the Indium Tin Oxide coated glass (ITO—glass), or Silicon wafer, as well as on glass substrates by the sol–gel method, at different spin speed, using the spin-coating system. The effect of thickness on the surface morphology and on the optical properties of TiO2 layer, was investigated using the Atomic Force Microscopy (AFM), X-ray diffraction, and UV–visible spectrophotometer. The samples were examined to feature currents and voltages, in the darkness and light extraction efficiency of the solar cell. The highest open-circuit voltage, Voc, and power conversion efficiency were 0.66% and 0.39%, fabricated with 90nm, respectively. The non-linear optical properties of nano-anatase TiO2 sol–gel, were investigated at different particle sizes, using the z-scan technique.

Study of Electrical Transport Properties of Thin Films Used as HTL and as Active Layer in Organic Solar Cells, through Impedance Spectroscopy Measurements

Impedance spectroscopy (IS) is used for studying the electrical transport properties of thin films used in organic solar cells with structure ITO/HTL/active layer/cathode, where PEDOT:PSS (poly(3,4-ethylenedioxythiophene):polystyrene sulfonic acid) and CuPC (tetrasulfonated copper-phthalocyanine) were investigated as HTL (hole transport layer) and P3HT:PCBM (poly-3-hexylthiophene:phenyl-C61-butyric acid methyl ester) blends prepared from mesitylene and chlorobenzene based solutions were studied as active layer and Ag and Al were used as cathode. The study allowed determining the influence of the type of solvent used for the preparation of the active layer as well as the speed at which the solvents are removed on the carriers mobility. The effect of exposing the layer of P3HT to the air on its mobility was also studied. It was established that samples of P3HT and P3HT:PCBM prepared using mesitylene as a solvent have mobility values significantly higher than those prepared from chlorobenzene which is the solvent most frequently used. It was also determined that the mobility of carriers in P3HT films strongly decreases when this sample is exposed to air. In addition, it was found that the electrical properties of P3HT:PCBM thin films can be improved by removing the solvent slowly which is achieved by increasing the pressure inside the system of spin-coating during the film growth.

Solution Aging Effects on the Performance and Morphology of P3HT: PCBM Bulk Heterojunction Solar Cells

In a bulk heterojunction photovoltaic device, the morphology of the active layer strongly affects the device performance. By comparing the device performance from P3HT:PCBM solutions that were aged for different times, we notice that aging time has an influence on device power conversion efficiency. We prove the morphological difference in P3HT:PCBM thin films is caused by different aging time, and demonstrate a relationship between the P3HT:PCBM phase behavior and BHJ device performance through UV-Vis absorption spectrum and atomic force microscope.

Capillary force induced phase separation in P3HT: PCBM thin films during selective solvent vapor annealing

In this paper, we report an efficient method for the phase separation of thin films composed of organic semiconductors. The phase behaviors of P3HT and PCBM blended films in confined geometries have been investigated under solvent selective vapor annealing. Anisole was used to generate the solvent vapor due to its differential activation of P3HT and PCBM. After the anisole vapor interacted with the mixed film, the molecular chains of the PCBM preferentially diffused and aggregated as a result of the capillary forces to form an ordered arrangement on the films surface. Ordered micron- and nano-scale phase separation structures were obtained using this methodology.

EFFECT OF SPRAY DEPOSITION TIME ON OPTICAL AND MORPHOLOGICAL PROPERTIES OF P3HT: PCBM THIN FILMS

This paper investigated the effect of spray deposition time on optical and morphological properties of P3HT: PCBM thin films. The effects of spray deposition time on the optical and morphological properties of thin films were investigated using optical microscopy, UV-Vis spectrophotometry and atomic force microscopy (AFM). The AFM spectra show that the thin films prepared at 10s spray deposition time are more uniform while the 15s and 20s samples presented coffee ring shapes with inhomogeneous surface formation. The ridge-like features can be observed in the surface for all samples and become more pronounced with increasing spray deposition time. The root mean square (RMS) roughness of the samples increased with increasing spray deposition time. Based on the absorption results, it is concluded that higher spray coating times result in lower crystallinity of the thin film. The 10 s spray deposition time is the most suitable deposition time for producing thin films with good morphology and crystallinity for polymer solar cells (PSCs) with improved power conversion efficiency (PCE).

X-ray reflectivity and topography of the solvent-treated P3HT:PCBM thin films

P3HT:PCBM (poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester) thin films are widely used as an active layer in organic solar cells. The influence of solvent (1,8-diiodooctane) was studied on electron density and topography of thin spin-coated P3HT:PCBM films. The structural properties were studied by grazing incidence X-ray diffraction technique. It was found that the crystallinity of the investigated films was significantly increased after solvent treatment. Using X-ray reflectivity (XRR) analysis, data from P3HT:PCBM/air and P3HT:PCBM/glass interfaces were collected and compared with a theoretical model. The electron density, film thickness, and interface roughness were determined with a few angstroms accuracy by simulating the XRR curves. The XRR data were then analyzed using a scheme based on the distorted-wave Born approximation. It was found that the addition of diiodooctane did not change the absorption coefficient but changed electron density and roughness. The XRR simulation results were in good agreement with the experimental results obtained from AFM measurements.

Fundamental Study on the Effect of Spray Parameters on Characteristics of P3HT:PCBM Active Layers Made by Spray Coating

This paper is an attempt to elucidate the effects of the important spray characteristics on the surface morphology and light absorbance of spray-on P3HT:PCBM thin-films, used as an active layer in polymer solar cells (PSCs). Spray coating or deposition is a viable scalable technique for the large-scale, fast, and low-cost fabrication of solution-processed solar cells, and has been widely used for device fabrication, although the fundamental understanding of the underlying and controlling parameters, such as spray characteristics, droplet dynamics, and surface wettability, is still limited, making the results on device fabrication not reproducible and unreliable. In this paper, following the conventional PSC architecture, a PEDOT:PSS layer is first spin-coated on glass substrates, followed by the deposition of P3HT:PCBM using an automatic ultrasonic spray coating system, with a movable nozzle tip, to mimic an industrial manufacturing process. To gain insight, the effects of the spray carrier air pressure, the number of spray passes, the precursor flow rate, and precursor concentration are studied on the surface topography and light absorbance spectra of the spray-on films. Among the results, it is found that despite the high roughness of spray-on films, the light absorbance of the film is satisfactory. It is also found that the absorbance of spray-on films is a linear function of the number of spray passes or deposition layers, based on which an effective film thickness is defined for rough spray-on films. The effective thickness of a rough spray-on P3HT:PCBM film was found to be one-quarter of that of a flat film predicted by a simple mass balance.

Absorption properties enhancement using Montmorillonite (MMT) as filler in spray-coated P3HT:PCBM thin films

This paper has investigated the effects of montmorillonite (MMT) as filler and post treatment in optical properties of bulk heterojunction P3HT:PCBM thin films. Annealing temperatures varied from 130 to 150 °C while annealing time was kept constant for 10 min. The absorption data has shown an increment in peak absorption at around 602 nm, suggesting that the addition of MMT improved the degree of crystallinity of P3HT. The addition of MMT exhibited absorption in the infrared region, suggesting that MMT improved the optical properties of the thin films. The post treatment effects have shown interesting results, with 130 °C demonstrating the highest degree of PL quenching with acceptable light absorption.

The effect of oxygen induced degradation on charge carrier dynamics in P3HT:PCBM and Si-PCPDTBT:PCBM thin films and solar cells

Photooxidation of organic solar cells reduces charge extraction rates more strongly than recombination rates, thus causing electrical performance loss.

Preheated solvent exposure on P3HT:PCBM thin film: A facile strategy to enhance performance in bulk heterojunction photovoltaic cells

In this study, we explored the ability of a preheated solvent (methanol) to induce characteristic changes at the organic active layer/metal interface, thereby improving the performance of fabricated organic photovoltaic (OPV) cells composed of poly(3-hexylthiopene) (P3HT) and a [6,6]-phenyl-C71-butyric acid methyl ester (PCBM) photoactive blend. Our results demonstrate that exposure to methanol (at room temperature, or preheated at 45 °C or 65 °C) improves the performance of the fabricated OPV cells. After preheated methanol exposure, the P3HT:PCBM thin films were tested for crystallinity, morphology, mobility, and photovoltaic characteristics. Our results revealed that use of the preheated solvent on the organic active layer significantly influences the micro/nano scale morphology and phase segregation of the P3HT:PCBM thin films, as well as the charge carrier mobility. It is hypothesized that the side chain ordering of P3HT and redistribution of PCBM could be results of the modified active layer. Consequently, OPV cells modified with the methanol preheated at 65 °C exhibited a power conversion efficiency (PCE) of 3.36%, with open-circuit voltage of 0.59 V, short-circuit current density of 13.83 mA/cm2, and fill-factor of 0.41. In contrast, the unmodified P3HT:PCBM thin film (without methanol exposure) showed a PCE of only 2.13%.

Characterization of P3HT:PCBM Thin Film Interfaces by Doubly Resonant Sum-Frequency Generation Spectroscopy

The surface structures and the electronic states of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blend films are studied by using doubly resonant infrared-visible sum frequency generation (DR-SFG) spectroscopy. The SFG spectra of P3HT indicate the co-existence of the ordered and disordered P3HT at the interface. In the P3HT:PCBM blend film, the peak located at 1460 cm−1, which is originated from the charge transfer state of PCBM, is observed.

Effective absorption enhancement in dielectric thin-films with embedded paired-strips gold nanoantennas.

This study focuses on determining the optimized thickness of an absorbing thin-film with embedded gold nanoantennas, for absorption enhancement. Gold paired-strips nanoantennas with small gaps have been proposed for light trapping because of the high localized electric field in the gap due to resonance. Paired-strips nanoantennas with small gaps produce higher effective absorption compared to single-strip gratings. From the average absorption two-dimensional map, the absorption enhancement may increase by a factor of up to 20 for gold paired-strips nanoantennas embedded in a 100 nm thick P3HT:PCBM thin-film.