Device Physics of Polymer:Fullerene Bulk Heterojunction Solar Cells

Plastic solar cells bear the potential for large‐scale power generation based on materials that provide the possibility of flexible, lightweight, inexpensive, efficient solar cells. Since the discovery of the photoinduced electron transfer from a conjugated polymer to fullerene molecules, followed by the introduction of the bulk heterojunction (BHJ) concept, this material combination has been extensively studied in organic solar cells, leading to several breakthroughs in efficiency, with a power conversion efficiency approaching 5 %. This article reviews the processes and limitations that govern device operation of polymer:fullerene BHJ solar cells, with respect to the charge‐carrier transport and photogeneration mechanism. The transport of electrons/holes in the blend is a crucial parameter and must be controlled (e.g., by controlling the nanoscale morphology) and enhanced in order to allow fabrication of thicker films to maximize the absorption, without significant recombination losses. Concomitantly, a balanced transport of electrons and holes in the blend is needed to suppress the build‐up of the space–charge that will significantly reduce the power conversion efficiency. Dissociation of electron–hole pairs at the donor/acceptor interface is an important process that limits the charge generation efficiency under normal operation condition. Based on these findings, there is a compromise between charge generation (light absorption) and open‐circuit voltage (V_OC) when attempting to reduce the bandgap of the polymer (or fullerene). Therefore, an increase in V_OC of polymer:fullerene cells, for example by raising the lowest unoccupied molecular orbital level of the fullerene, will benefit cell performance as both fill factor and short‐circuit current increase simultaneously.     

Grafting of buckminsterfullerene onto polythiophene: novel intramolecular donor-acceptor polymers

Conjugated polymer-fullerene composites have shown efficient photoinduced charge transfer. The attachment of a fullerene moiety to the conducting polymer backbone is expected to lead to materials with more intimate association of the donor/acceptor sites. Two approaches to the attachment of fullerenes onto polythiophene derivatives have been explored. In the first case, fullerene has been bonded to a bithiophene derivative which can be electropolymerized. In the second, solvent processable polythiophene copolymers were prepared and functionalized with fullerene. Both these systems exhibit electrochemical and optoelectrochemical properties for fullerene and the conducting polymer.     

Radiation-Induced Synthesis of Fullerene-Silica Hybrid Nanomaterials

C₆₀ fullerene has been radiation grafted on bare and functionalized silica surface with mercaptopropyl moieties in toluene solution. The resulting fullerene-silica hybrid nanomaterials have been characterized by FT-IR and solid state ¹³C CP-MAS NMR spectroscopy and compared to the radiolysis products of C₆₀ in toluene. It has been found that fullerene is grafted on silica surface but, because the radiation grafting has been conducted in toluene, fullerene results arylated from radicals derived from toluene radiolysis. The fullerene-silica hybrid nanomaterials cannot be extracted by warm decalin, demonstrating that the fullerene cage is chemically bonded on the silica surface. The thermal behavior of fullerene-silica hybrid nanomaterials has been studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) in air flow.     

Dispersed state of C60 fullerene in some polymers

The character of C₆₀ fullerene dispersion and its temperature dependence in various polymer matrices has been studied by UV-Vis spectroscopy and thermodesorption mass spectrometry. It is established that the degree of fullerene aggregation in these composites varies from molecular-dispersed state to largesize clusters. For the polymers studied, the initial dispersed state is most stable with respect to temperature in the case of poly-α-methylstyrene.

     

Remarkable Improvement in Efficiency of Filtration Method for Fullerene Purification

Filtration of fullerene extract through thin layer of powdered activated carbon fiber was reported as a very practical process for fullerene purification. In order to make it more efficient, the conditions of the filtration were further investigated. As a result, 1,2,4-trimethylbenzene with high fullerene solubility was found to be employed as solvent to give pure C₆₀ and C₇₀ in moderarte to excellent yields, reducing a volume of solvent, an amount of activated carbon and a time for purification. The efficiency of the improved filtration method presented here is considered to be comparable to or even better than that of the previous methods.     

SYNTHESES AND PHOTOPHYSICAL PROPERTIES OF FULLEROPYRROLIDINES CONTAINING PHOTOACTIVE UNITS

The syntheses and a systematic spectroscopic study of a series of fulleropyrrolidines containing polycyclic aromatic hydrocarbon are reported. The fulleropyrrolidines were characterized by UV, IR, ¹H-NMR, ¹³C-NMR and ESI-MS. According to the absorption spectra and the electrochemical data, no significant interaction was observed between the polycyclic aromatic rings and fullerene moiety in the ground state of fulleropyrrolidines. However, the fluorescence and nanosecond transient absorption spectra indicate that these fulleropyrrolidines have sensitized fluorescence of fullerene due to the intramolecular energy transfer, and that their excited triplet state production are considerably reduced, which in turn result in a lower optical limiting performance compared with that of C₆₀.     

Synthesis, Photophysical and Photovoltaic Properties of New [60]Fullerene Pyrrolidine Derivatives

Two new [60]fullerene pyrrolidine derivatives 2 and 3 were synthesized and characterized by ¹H NMR, ¹³C NMR and MS. Their photophysical processes have been investigated by using laser flash photolysis. The experiments show quenching of oligomer singlet excited state and the evidence of the fullerene singlet excited emission. And the intramolecular of electron transfer and also energy transfer of 2 and 3 occurred and the triplet excited state of fullerene moiety in 2 and 3 have different lifetime due to their different structures. A photovoltaic device using compound 2 as the only photoactive material has also been investigated which showed the energy conversion efficiency is 0.011%.     

Increased Exciton Delocalization of Polymer upon Blending with Fullerene

Interfaces between donor and acceptor in a polymer solar cell play a crucial role in exciton dissociation and charge photogeneration. While the importance of charge transfer (CT) excitons for free carrier generation is intensively studied, the effect of blending on the nature of the polymer excitons in relation to the blend nanomorphology remains largely unexplored. In this work, electroabsorption (EA) spectroscopy is used to study the excited‐state polarizability of polymer excitons in several polymer:fullerene blend systems, and it is found that excited‐state polarizability of polymer excitons in the blends is a strong function of blend nanomorphology. The increase in excited‐state polarizability with decreased domain size indicates that intermixing of states at the interface between the donor polymers and fullerene increases the exciton delocalization, resulting in an increase in exciton dissociation efficiency. This conclusion is further supported by transient absorption spectroscopy and time‐resolved photoluminescence measurements, along with the results from time‐dependent density functional theory calculations. These findings indicate that polymer excited‐state polarizability is a key parameter for efficient free carrier generation and should be considered in the design and development of high‐performance polymer solar cells.     

Synthesis of mono-dispersed nano-scale fullerene (C60) crystals

Fullerene (C60), which has a unique molecular structure, was used in the preparation of crystalline organic nano-crystals. Fullerene was dissolved in toluene and this fullerene solution was mixed with water drastically. During this process, fullerene transferred from toluene to water phase. The significantly different solubility of fullerene in a toluene/water solvent system played an important role in the self-assembly of single fullerene nano-crystals, as it is called drowning-out crystallization. In addition, pH of water was controlled to carry out the interfacial transference of fullerene. An optical spectrum analysis showed that the fullerene was transferred by a hydrolysis reaction from toluene to water, depending on the pH and toluene involved in the crystal structure. During the interfacial transference, the growth of nano-scale fullerene occurred at pH > 7. Importantly, fullerene nanocrystals were formed with a mono-dispersed square structure on a nano-scale (104 nm average size and 1.03 +/- 0.24 aspect ratio) at pH 10.     

MALDI matrices for biomolecular analysis based on functionalized carbon nanomaterials

When used in small molar ratios of matrix to analyte, derivatized fullerenes and single wall nanotubes are shown to be efficient matrices for matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. The mixing of an acidic functionalized fullerene with a solution of bioanalyte, depositing a dried droplet, and irradiating with a pulsed nitrogen laser yields protonated or cationized molecular ions. Derivatized fullerenes could offer several advantages over conventional MALDI matrices: a high analyte ionization efficiency, a small molar ratios (less than 1) of matrix/analyte, and a broader optical absorption spectrum, which should obviate specific wavelength lasers for MALDI acquisitions. The major disadvantage to the use of fullerenes is the isobaric interference between matrix and analyte ions; however, it is overcome by using MALDI-ion mobility time-of-flight (IM-oTOF) mass spectrometry to preseparate carbon cluster ions from bioanalyte ions prior to TOF mass analysis. However, an alternative to the dried droplet preparation of fullerene MALDI samples is the aerosolization of matrix-analyte solutions (or slurries) followed by impacting the aerosol onto a stainless surface. We also demonstrate that the fullerene matrices can be used to acquire spectra from rat brain tissue.     

Glucose/Fe-doped C70 fullerene hybrid biosensor: theoretical study

Abstract

The interaction between glucose molecule and C70 fullerene (pristine and Fe-doped) was investigated with the aid of density functional theory. Our results provide useful information that uncover the chemical and biological activities of C70 when interact with glucose molecule. To mimic real situations, all possible configurations of glucose with respect to fullerene were investigated under both vacuum and distilled water mediums. As expected, no significant change in the calculated energy gaps for all pristine configurations regardless the simulated medium, which indicates that the interaction between glucose molecule and pristine Fullerene is weak physisorption. However, doping fullerene with Fe atom significantly reduced the energy gap for all configurations of the complex which in turn offers an advantage over pristine fullerene for measuring glucose in real situations such as blood.     

Evidences of Rubber Grafting on Activated Carbon Surfaces Containing Fullerene‐like Structures

Abstract

It is shown that graphite is converted into an high disordered carbon black by prolonged ball milling. The kinetics of this transformation has been followed by powder x‐ray diffraction, measurements of the crystallinity and of the surface area. Ball milling is able to introduce an high concentration of defective sites in the pristine graphite including the fullerene‐like structures. By mixing with natural rubber both the pristine and the ball‐milled graphite, it is shown by bound rubber measurements that the amount of rubber grafted (chemically linked) on the pristine graphite surface is negligible but reaches a very high level in the ball‐milled graphite. Similarly, ball‐milling of N660 carbon black causes a deep activation of its surface activity which can be measured by a significant increase in the bound rubber level and in the amount of grafted rubber in comparison to the pristine N660 sample. The bound rubber measurement has been performed also on a natural rubber masterbatch with extracted fullerene carbon black (EFCB). Also in this case extremely high levels of rubber grafting have been achieved in comparison to graphite. It is discussed and demonstrated that the fullerene‐like nanostructures in carbon blacks play a key role in the formation of bound rubber phenomenon and in grafting natural rubber on carbon black surface.     

Photoluminenscence Blinking Dynamics of Colloidal Quantum Dots in the Presence of Controlled External Electron Traps

The effect of the external charge trap on the photoluminescence blinking dynamics of individual colloidal quantum dots is investigated with a series of colloidal quantum dot–bridge–fullerene dimers with varying bridge lengths, where the fullerene moiety acts as a well‐defined, well‐positioned external charge trap. It is found that charge transfer followed by charge recombination is an important mechanism in determining the blinking behavior of quantum dots when the external trap is properly coupled with the excited state of the quantum dot, leading to a quasi‐continuous distribution of ‘on' states and an early fall‐off from a power‐law distribution for both ‘on' and ‘off' times associated with quantum dot photoluminescence blinking.     

Ground and Excited State Electronic Interactions in Push–Pull-Chromophore–[60]Fullerene Conjugates

A variety of push–pull-chromophore–[60]fullerene conjugates connected with different spacers was successfully synthesized by applying “click chemistry” to the corresponding acetylene-appended fullerene precursors. Direct connection of the fullerene cages to push–pull motifs gives rise to ground state electronic interactions, which were characterized by electrochemical studies. On the other hand, when the two moieties were linked through pyrrolidine rings, no interactions occurred between the C₆₀ units and the push–pull motifs in the ground states. Instead, an electron transfer proceeded upon light exitation, giving the charge-separated states, which was corroborated by time-resolved transient absorption measurements.     

INFLUENCE OF DESIGN AND OPERATIONAL PARAMETERS ON YIELD OF FULLERENES

Fullerene production by arc-discharge method using graphite electrodes has been studied with respect to influence of different design and operational parameters on fullerene yield in a constant arc fullerene reactor. The design parameters like reactor length, diameter, heat transfer area and operational parameters like voltage, current, pressure, coolant flow rate, graphite evaporation rate and electrode diameter etc. have been experimentally studied in detail to establish a relationship between these parameters and fullerene yield. All the parameters affecting the yield have been correlated by dimensional analysis and an equation to calculate the fullerene yield is derived. It was observed experimentally as well as by dimensional analysis that many favorable parameters for getting good yield are linked with other parameters which also get changed if the favorable parameters are changed and thus it is difficult to make a substantial change in the yield of fullerenes.  The relationship established between the yield and parameters is however useful in optimising fullerene yield in a reactor and also helpful in designing a futuristic fullerene reactor of improved yield and productivity. The fullerene yield from different designs of reactors is obtained in the range of 4% to 20%.     

Effects of plasma parameters on increasing the purity of nitrogen atom encapsulated fullerene in an RF plasma

Relatively high purity of nitrogen atom encapsulated fullerene (N@C₆₀) has been synthesized by an electron beam superimposed radio frequency (RF) discharge plasma method. Nitrogen species are characterized by an optical emission spectroscopy (OES); and a relationship between optical emission spectra and the purity of N@C₆₀ has been examined. It is observed that the increased amount of nitrogen molecule ions impinging on the sublimated fullerenes enhance the synthesis of N@C₆₀. Here, it is cleared that the efficient synthesis of N@C₆₀ is possible by controlling the parameters of electron beam superimposed RF plasma. As a consequence, comparatively high purity of about 0.08% of N@C₆₀ has been obtained.     

The significance of fullerene electron acceptors in organic solar cell photo-oxidation

Stability of organic solar cells requires development before their commercialisation is possible. This review will give a brief overview of organic solar cells and their stability, before focussing on the photochemical stability of the active layer. The photo-oxidation of the donor polymers will be looked at first which has been studied quite extensively and then fullerene electron acceptors, such as widely used phenyl-C61-butyric acid methyl ester, which has been considerably less studied. It has been shown that oxidation of the fullerene cage on phenyl-C61-butyric acid methyl ester results in oxides with a deeper lowest unoccupied molecular orbital (LUMO) level than the fresh electron acceptor. These oxides act as electron traps, leading to deterioration of the blend photoconductivity. The significance of fullerene photo-oxidation on device stability has been indirectly shown via research on: photoconductivity; organic solar cells made with an oxidised fullerene derivative and organic field effect transistors. Techniques that could be developed to increase photochemical stability of fullerene electron acceptor resistance to photo-oxidation include: reducing its LUMO level; increasing its crystallinity or aggregation and changing its chemical structure. Improving the photochemical stability of organic solar cells would move us one step closer to a more accessible solar power.     

INFLUENCE OF DESIGN AND OPERATIONAL PARAMETERS ON YIELD OF FULLERENES

ABSTRACT

Fullerene production by arc-discharge method using graphite electrodes has been studied with respect to influence of different design and operational parameters on fullerene yield in a constant arc fullerene reactor. The design parameters like reactor length, diameter, heat transfer area and operational parameters like voltage, current, pressure, coolant flow rate, graphite evaporation rate and electrode diameter etc. have been experimentally studied in detail to establish a relationship between these parameters and fullerene yield. All the parameters affecting the yield have been correlated by dimensional analysis and an equation to calculate the fullerene yield is derived. It was observed experimentally as well as by dimensional analysis that many favorable parameters for getting good yield are linked with other parameters which also get changed if the favorable parameters are changed and thus it is difficult to make a substantial change in the yield of fullerenes. The relationship established between the yield and parameters is however useful in optimising fullerene yield in a reactor and also helpful in designing a futuristic fullerene reactor of improved yield and productivity. The fullerene yield from different designs of reactors is obtained in the range of 4% to 20%.     

Mechanosynthesis of nanostructured composites copper-fullerite,copper-graphite

Comparative studies of the influence of carbon allotropic form on the formation of the structural-phase composition of copper-based composites have been performed by means of Scanning Electron Microscopy, X-ray and Thermal analysis and Raman spectroscopy. It has been stated that the kinetics of solid state reactions in the systems Cu-C_60/70 and Cu-C_g obtained in process of mechanosynthesis depend on deformational stability and oxidation-reduction properties of fullerite and graphite. It has been shown that partial destruction of fullerene molecules in the Cu-C_60/70 sample results in the formation of an amorphous fullerite-like phase, copper oxide Cu₂O, and supersaturated solid solution Cu(C, O). Total destruction of fullerene molecules in the system Cu-C_60/70 results in the formation of supersaturated solid solution Cu(C), just like in the case of the composite Cu-C_g.     

Efficient Organic Solar Cells with Non‐Fullerene Acceptors

Fullerene‐free OSCs employing n‐type small molecules or polymers as the acceptors have recently experienced a rapid rise with efficiencies exceeding 12%. Owing to the good optoelectronic and morphological tunabilities, non‐fullerene acceptors exhibit great potential for realizing high‐performance and practical OSCs. In this Review, recent exciting progress made in developing highly efficient non‐fullerene acceptors is summarized, mainly correlating factors like absorption, energy loss and morphology of new materials to their correspondent photovoltaic performance.