T. John S. Dennis

Isomer-Pure Bis-PCBM-Assisted Crystal Engineering of Perovskite Solar Cells Showing Excellent Efficiency and Stability

A fullerene derivative (α-bis-PCBM) is purified from an as-produced bis-phenyl-C61 -butyric acid methyl ester (bis-[60]PCBM) isomer mixture by preparative peak-recycling, high-performance liquid chromatography, and is employed as a templating agent for solution processing of metal halide perovskite films via an antisolvent method. The resulting α-bis-PCBM-containing perovskite solar cells achieve better stability, efficiency, and reproducibility when compared with analogous cells containing PCBM. α-bis-PCBM fills the vacancies and grain boundaries of the perovskite film, enhancing the crystallization of perovskites and addressing the issue of slow electron extraction. In addition, α-bis-PCBM resists the ingression of moisture and passivates voids or pinholes generated in the hole-transporting layer. As a result, a power conversion efficiency (PCE) of 20.8% is obtained, compared with 19.9% by PCBM, and is accompanied by excellent stability under heat and simulated sunlight. The PCE of unsealed devices dropped by less than 10% in ambient air (40% RH) after 44 d at 65 °C, and by 4% after 600 h under continuous full-sun illumination and maximum power point tracking, respectively.

13 C NMR spectroscopy of C76, C78, C84 and mixtures of C86–C102; anomalous chromatographic behaviour of C82, and evidence for C70H12

Using high pressure liquid chromatography, the fullerenes C76, C78. C84, and C86–C102 have been separated (in some cases partially) from soot produced by the arc-discharge procedure. The 13C NMR spectrum for C76 confirms that reported previously, whilst the spectrum for C78 indicates the presence of C2v, D3, C2v, isomers in the approximate relative yields (there is a small batch dependence) of 52 : 30 : 18%. The spectrum for the D3 isomer (confirmed from two separate batches) differs from that given in the literature with respect to the location of one peak. The relative peak heights in the C8413C NMR spectrum are similar to those reported for samples produced under different conditions. The stabilities of the component isomers must therefore be similar and so produced in a similar ratio irrespective of conditions; this indicates them to be the D2and a O2d isomers. The spectrum also contains over 70 minor peaks due to other isomers of C84. Eleven of the peaks are more intense than the others, consistent with the presence of the D3d and D6h isomers, predicted to be of relatively high stability. Two fractions (giving yellow solutions in hexane) eluted after C84: the first was separated into two components, shown by mass spectrometry to consist of minor amounts of C86–C92, and mainly C90 and C92 with lesser amounts of C94–C,102, respectively. The 13C NMR spectrum of the C94–C102 component indicated the presence of a fullerene isomer of high symmetry. The second yellow fraction consisted largely of C82 together with traces of fullerenes up to C108, but the quantity was insufficient for the 13C NMR spectrum to be obtained. The retention time of C82 is inconsistent with values for the other fullerenes indicating that it consists of the C3v isomer, predicted to be polar. C90 eluted with two different retention times suggesting that the second fraction is also a polar isomer. One sample of C84 contained a coeluent, identified by mass spectrometry as C70H12; proposals are made concerning its possible origin, structure, and stability relative to other hydrogenated derivatives.

Enthalpies of formation of buckminsterfullerene (C60) and of the parent ions C60+, C602+, C603+ and C60–

The standard enthalpy of formation of buckminsterfullerene (C60) in the crystalline state has been determined as 2278.1 ± 14.4 kJ mol–1 using a microcombustion calorimeter. Statistical mechanical calculations of the heat capacity of gaseous C60 as a function of the temperature and a critical survey of the literature data for the enthalpy of sublimation, the heat capacity in the crystalline state, the first, second and third ionization energies and the electron affinity of C60, enabled the enthalpy of formation of gaseous C60 and the enthalpies of formation of the ions C60+, C602+, C603+ and C60– to be derived.

Isolation and characterisation of the two major isomers of [84]fullerene (C84)

We report the first successful separation of the two major isomers of C84, i.e. [84-D2]fullerene and [84-D2d(II)]- fullerene, and 13C NMR and UV–VIS–near IR absorption spectra of the purified materials.

Fluorination of buckminsterfullerene

Fluorination of solid C60 with fluorine gas proceeds in a stepwise manner to give, after twelve days (10 mg scale), a colourless derivative indicated by a single line 19F NMR spectrum to be C60F60; reaction time is quantity dependent.

Addition of Carbene to the Equator of C70 To Produce the Most Stable C71H2 Isomer: 2 aH‐2(12)a‐Homo(C70‐D5h(6))[5,6]fullerene

China by the 973 Program [2006CB300402]; NSFC [20821003]; NSF [CHE-0716718, 0701525]; US Environmental Protection Agency (EPA) [RD-83385601]; Royal Society

A mass spectrometric–NMR study of fullerene-78 isomers

Three isomers of C78 have been detected by 13C NMR spectroscopy; one of the signals ascribed in the literature to the D3 isomer is absent from our spectrum.

Modification of the conductance of single fullerene molecules by endohedral doping

We use scanning tunneling microscopy to establish controlled contacts to single molecules of endohedrally doped Ce2@C80 fullerenes with C60 as a reference. The stability of the experimental setup allows for the determination of the conductance of Ce2@C80 relative to the conductance of C60. The endohedral doping reduces the conductance of Ce2@C80 by a factor of about five with respect to C60. Ab initio calculations show that the reason for this reduced conductance is the absence of electron orbitals delocalized over the cage of Ce2@C80 in the energy window of the conductance measurement.

Ultraviolet photoelectron spectra of metallofullerenes, two Ca@C82 isomers

Abstract Ultraviolet photoelectron spectra (UPS) of two Ca@C 82 isomers (III and IV) were measured with a synchrotron radiation light source. The photoelectron onset energies of isomers III and IV were 0.7 and 0.8 eV below the Fermi level, respectively, which indicates their semiconductive nature. When the excitation energy is tuned, spectral intensity changes as other fullerenes have shown. Their upper valence band (0–5 eV) spectra are different from those of other metallofullerenes such as La@C 82 ,Sc@C 82 and Gd@C 82 as well as those of empty C 82 but their lower valence band spectra (below 5 eV) are almost identical. Comparison between the UPS and ab initio calculation assuming transfer of two electrons from encapsulated calcium atom to the cage (i.e. Ca +2 @C 82 2− ) suggests C 2 (c) geometry for isomer III and C s for isomer IV.

Hot electron production and diffuse excited states in C70, C82, and Sc3N@C80 characterized by angular-resolved photoelectron spectroscopy

Angular-resolved photoelectron spectroscopy using wavelength-tuneable femtosecond laser pulses is presented for a series of fullerenes, namely, C70, C82, and Sc3N@C80. The photoelectron kinetic energy distributions for the three molecules show typical thermal electron spectra with a superimposed peak structure that is the result of one-photon ionization of diffuse low-angular momenta states with electron density close to the carbon cage and that are related to so-called super atom molecular orbitals. Photoelectron angular distributions confirm this assignment. The observed structure is less prominent compared to the thermal electron background than what was observed in C60. It can be concluded that hot electron emission is the main ionization channel for the larger and more complex molecules for these excitation conditions.