C60 Layer Research Articles

Efficient planar CH3NH3PbBr3 perovskite solar cells prepared at room temperature with ionic-liquids/fullerene as an electron transport bilayer

A room temperature solution processed ionic liquids (1-benzyl-3-methylimidazolium chloride)/C60 bilayer were used as the electron transport layer in planar n-i-p structure CH3NH3PbBr3 perovskite solar cells. The ionic liquids could effectively decrease the work function and improve the wettability of the perovskite precursor solution on the C60 layer. Moreover, C60 guarantees the excellent electron mobility. In comparison with devices with different ETLs, the IL/C60-based solar cells show greatly improved performance, achieving the PCE of 5.88% with a Jsc of 6.14 mA/cm2, a Voc of 1.39 V, and a FF of 69.94%.

Peculiar covalent bonding of C60/6H-SiC(0 0 0 1)-(3 × 3) probed by photoelectron spectroscopy

High resolution photoemission with synchrotron radiation was used to study the interface formation of a thin layer of C60 on 6H-SiC(0 0 0 1)-, characterized by protruding Si-tetramers. The results show that C60 is chemisorbed by orbital hybridization between the highest-occupied molecular orbital (HOMO) and the pz orbital of Si adatom at the apex of the tetramers. The covalent nature of the bonding was inferred from core level as well as valence band spectra. The Si 2p spectra reveal that a large fraction (at least ) of the Si adatoms remain unbound despite the reactive character of the associated dangling bonds. This is consistent with a model in which each C60 is attached to the substrate through a single covalent C60–Si bond. A binding energy shift of the core levels associated with sub-surface Si or C atoms indicates a decrease of the SiC band bending caused by a charge transfer from the C60 molecules to the substrate via the formation of donor-like interface states.

Perovskite solar cells with versatile electropolymerized fullerene as electron extraction layer

An electropolymerized layer of a C60 fullerene derivative was successfully used as selective contact and electron transport layer in perovskite solar cells. The electropolymeric film was formed over FTO electrodes by an electrochemical methodology in just one step. The light emission of perovskite films formed over the electropolymer was quenched, confirming an efficient electron transfer from the perovskite to the electropolymerized C60 layer. Solar cells constructed with C60 polymer layer showed a much better performance compared with the same cell without the fullerene containing electropolymer layer. The best non-optimized device presented an efficiency of 11.0%, with an open circuit voltage of 969 mV, a short circuit current of 17 mA/cm2, and a fill factor of 65%. These results demonstrated that the use of an electrochemical methodology in the formation of an organic electron transport layers as replacement of metal oxides in perovskite solar cells opens a new approach in the fabrication of efficient energy conversion systems.

Two-Component Fullerene Molecular Layers Assembled on the Au(111) Substrate

Self-assembly of binary and multicomponent colloidal crystals is an effective approach for the fabrication of structured new materials. The formation of colloidal crystals in most cases is controlled by optimized space occupation by the particles. C60 and C70 molecules, when treated as the smallest “colloidal particles”, can be used as model systems to study the formation of multicomponent molecular crystals in which the intermolecular forces are mainly of the van der Waals type. By deposition of C60 and C70 molecules onto Au(111), the mixing of the two molecules has been investigated. Both C60 and C70 form close-packed layers on Au(111), but with different lattice constants. At room temperature, the molecules within the molecular layer have complete freedom to rotate. The boundary between the C60 and the C70 domains consists of row dislocations. Adding C60 onto a C70 layer has an interesting effect in that C60 molecules can push their way into the C70 layer and force the C70 molecules to take a fixed upright configuration where the long axis of the molecule is perpendicular to the interface. Significant interlayer diffusion takes place at elevated temperatures. When C70 is added to a C60 layer, there is little evidence of C70 getting into the C60 layer. The second layer C70 forms a lattice-matched structure by taking the upright orientation.

A C60 Modification Layer Using a Scalable Deposition Technology for Efficient Printable Mesoscopic Perovskite Solar Cells

Carbon‐based, hole‐conductor‐free, full‐printable, mesoscopic perovskite solar cells (PMPSCs) have attracted much attention due to their superior stability, low cost, and potential scalability. However, TiO2 generally shows a relatively low electron mobility, which may result in charge accumulation and recombination at the TiO2/perovskite interface, thus hindering the improvements from the PMPSCs performance. In this study, a simple strategy to deposit a pristine C60 layer using a spray method is reported and the low electrical conductivity of TiO2 in PMPSCs is modified. The preferable electron extraction and transportation properties of pristine C60 and the up‐shifted lowest unoccupied molecular orbital (LUMO) of the C60 modified TiO2, that is, TiO2(C60) layer, efficiently promotes the charge transfer at the interface between the electron transport layer (ETL) and the perovskite. As a result, a significantly improved power conversion efficiency (PCE) of 15.4% is yielded, which is much higher than that of the control device (13.7%). In addition, the TiO2(C60)‐based device presents a lower hysteresis effect than that of the TiO2‐based device, which can likely be attributed to the reduced space charge accumulation at the perovskite/ETL interface. The present work provides a simple approach to boost the efficiency of PMPSCs and paves the way for the industrialization of PMPSCs.

Orientational Epitaxy of van der Waals Molecular Heterostructures.

The shape of individual building blocks is an important parameter in bottom-up self-assembly of nanostructured materials. A simple shape change from sphere to spheroid can significantly affect the assembly process due to the modification to the orientational degrees of freedom. When a layer of spheres is placed upon a layer of spheroids, the strain at the interface can be minimized by the spheroid taking a special orientation. C70 fullerenes represent the smallest spheroids, and their interaction with a sphere-like C60 is investigated. We find that the orientation of the C70 within a close-packed C70 layer can be steered by contacting a layer of C60. This orientational steering phenomenon is potentially useful for epitaxial growth of multilayer van der Waals molecular heterostructures.

Halide Perovskites for Selective Ultraviolet-Harvesting Transparent Photovoltaics

Summary Halide perovskite materials have emerged as a potential Si replacement with excellent photovoltaic properties. There has been growing interest in applying halide perovskites to semitransparent and spatially segmented transparent photovoltaics (TPVs) to enable a greater range of deployment routes. However, the continuous-band absorption of these semiconductors prevents near-infrared selective harvesting typically targeted for TPVs with the highest efficiency and transparency needed to meet the aesthetic demands of many potential applications. In this work, we demonstrate TPVs based on perovskite semiconductors where the band gap is sensitively tuned with a range of compositions to selectively harvest only ultraviolet photons with band gaps between 410 and 440 nm. This approach offers theoretical efficiencies up to 7% with >99% visible transparency when precisely targeting band gaps around 435 nm. Practical optimization of these perovskite cells could quickly yield TPVs with power conversion efficiencies rivaling state-of-the-art near-infrared harvesting TPVs today while also providing a route to higher-efficiency multi-junction TPVs.

C60 layer growth on intact and Tl-modified Si(1 1 1)5 [formula omitted] 2-Au surfaces

The Si(1 1 1)5 × 2-Au surface comprises a quasi-one-dimensional surface reconstruction consisting of Au stripes mediated by honeycomb Si chains. In the present study, using STM observations we explored adsorption of thallium (Tl) onto Si(1 1 1)5 × 2-Au surface as well as growth of C60 layers on Tl-modified and intact Si(1 1 1)5 × 2-Au surfaces. Analysis of the results was based on the recently proposed KK model of Si(1 1 1)5 × 2-Au surface [Kwon and Kang, PRL 113 (2014) 086101]. It was found that intact Si(1 1 1)5 × 2-Au surface produces a poor template effect on the overgrown C60 layers which have a close-packed-like structure without distinct ordering. In contrast, the C60 layers grown on the Tl-adsorbed Si(1 1 1)5 × 2-Au surface where Tl atoms occupy various adsorption sites on the honeycomb Si chains have a well-ordered quasi-one-dimensional structure consisting of straight C60 chains. Ordering of the C60 molecules is a result of an intricate interplay between adsorbed C60 molecules with each other and with a substrate surface. Separation between neighboring C60 chains is dictated by the periodicity of the underlying Si(1 1 1)5 × 2-Au template, while the C60-C60 spacing within the chains of ∼12 Å that is ∼3.2a0 reflects the peculiarity of the C60-C60 intermolecular interaction on the Tl-adsorbed Si(1 1 1)5 × 2-Au surface. Structural information on (Au, Tl)/Si(1 1 1) and C60/(Au, Tl)/Si(1 1 1) systems gained in the present study can be useful for the prospective works on elucidating the ways to control electronic properties of the Si(1 1 1)5 × 2-Au surface using atomic and molecular adsorbates.

Inverted near-infrared organic photodetector with oriented lead (II) phthalocyanine molecules via substrate heating

We developed a lead (II) phthalocyanine (PbPc)-based inverted organic photodetector (OPD) detecting near-infrared (NIR) wavelength near 940 nm. Such inverted OPDs can be combined with n-channel silicon-based integrated circuits. Operation near 940 nm is crucial in terms of daylight applications because the intensity of sunlight is low near that wavelength. A spectral response to 1000 nm was achieved by depositing PbPc on a heated C60 layer to form a triclinic rather than a monoclinic phase. This exhibited a much higher responsivity (35.8 mA/W at 960 nm) compared to a device fabricated without substrate heating (8.4 mA/W). The external quantum efficiency also increased from 0.9 to 4.4%.

Facile Method for the Investigation of Temperature-Dependent C60 Diffusion in Conjugated Polymers.

We developed a novel all-optical method for monitoring the diffusion of a small quencher molecule through a polymer layer in a bilayer architecture. Experimentally, we injected C60 molecules from a C60 layer into the adjacent donor layer by stepwise heating, and we measured how the photoluminescence (PL) of the donor layer becomes gradually quenched by the incoming C60 molecules. By analyzing the temporal evolution of the PL, the diffusion coefficient of C60 can be extracted, as well as its activation energy and an approximate concentration profile in the film. We applied this technique to three carbazole-based low-bandgap polymers with different glass temperatures with a view to study the impact of structural changes of the polymer matrix on the diffusion process. We find that C60 diffusion is thermally activated and not driven by WFL-type collective motion above Tg but rather by local motions mediated by the sidechains. The results are useful as guidance for material design and device engineering, and the approach can be adapted to a wide range of donor and acceptor materials.

ZnO-Assisted Growth of CH3NH3PbI3- xCl x Film and Efficient Planar Perovskite Solar Cells with a TiO2/ZnO/C60 Electron Transport Trilayer.

Appropriate electron transport layers (ETL) are essential in perovskite solar cells (PSCs) with high power conversion efficiency (PCE). Herein, a TiO2/ZnO/C60 trilayer fabricated on a transparent fluorine-doped tin oxide (FTO) glass substrate is used as a compound ETL in planar PSCs. The trilayer shows positive effects on both perovskite synthesis and device performance. The ZnO layer assists growth of CH3NH3PbI3- xCl x ( x ≈ 0) annealed at a lower temperature and with a shorter time, which is due to a more rapid and easier decomposition of the intermediate CH3NH3PbCl3 phase in the growth of CH3NH3PbI3- xCl x. All three materials in the trilayer are important for obtaining PSCs with a high PCE. ZnO is critical for enhancing the open circuit voltage by ensuring proper energy alignment with the TiO2 and C60 layers. C60 enhances carrier extraction from the CH3NH3PbI3- xCl x layer. TiO2 eliminates charge recombination at the FTO surface and ensures efficient electron collection. The best-performing PSC based on the TiO2/ZnO/C60 electron transport trilayer features a PCE of 18.63% with a fill factor of 79.12%. These findings help develop an understanding of the effects of ZnO-containing ETLs on perovskite film synthesis and show promise for the future development of high-performance PSCs with compound ETLs.

Enhancement of Photoswitchable Dielectric Property by Conducting Electron Donors on Plasmonic Core–Shell Gold-Fluorenyl C60 Nanoparticles

A new nanomaterial design was made by the construction of a tetralayered core–shell configuration, containing a magnetic core of nanospherical γ-FeOx particle (NP) encapsulated by one plasmonic gold subshell, one middle layer of electron-polarizable C60(>DPAF-Cn)x-derived fullerosome membrane, and an outer shell of organic electron donors. This class of multilayered core–shell nanospherical materials was recently reported to be excellent microwave absorbers. We found that this analogous of multilayered NPs was capable of inducing photoswitchable dielectric property (permittivity) amplification at the microwave frequency range of 1.0–4.0 GHz. The enhanced phenomena were further investigated by the use of six variable organic molecular electron donors and two conducting conjugated polymers, serving as the supply of photoinduced transferrable electrons at the outer shell layer, for comparison. The design largely increased the number of polarizable charges, leading to a maximum of 387% amplification of the re...

Effect of Band Bending and Energy Level Alignment at the Donor/Acceptor Interface on Open-Circuit Voltage in Organic Solar Cells.

Open-circuit voltage ( VOC) is a key parameter governing the power conversion efficiency of organic solar cells. We clarified the effect of band bending in organic semiconductor films on the VOC of phthalocyanine (H2Pc)/fullerene (C60) planar heterojunction (PHJ) OSCs. The VOC was significantly affected by the H2Pc layer thickness, varying from 0.20 V at 2 nm to 0.49 V at 100 nm thickness. The large VOC change is due to the band bending in the H2Pc layer, where the difference in H2Pc layer thickness led to significant changes in the degree of the vacuum-level shift in the C60 layer because of the Fermi level ( EF) difference between the H2Pc and C60 layers. This result demonstrated that a new influential factor affecting the VOC is the position of the EF that is dependent on band bending in organic semiconductor films.

Evaluation of ambipolar carrier mobility in alkyl-substituted phthalocyanine thin film

Ambipolar carrier mobility in a thin film of hexyl-substituted phthalocyanine (C6PcH2), which is a promising donor material for solution-processed organic thin-film solar cells, has been studied by measuring photogenerated charge carriers extracted under a linearly increasing voltage. As the extraction transient current of the C6PcH2 thin film could not be observed in a conventional device structure composed of a single organic layer, an evaporated layer of C60 or pentacene was introduced as a charge-separation layer to determine the hole or electron mobility, respectively. An extraction transient current clearly appeared owing to the efficient photo-induced charge generation at the interface between the C6PcH2 and charge-separation layers. The hole and electron mobilities were evaluated by a proposed analytical model, which took into account the carrier generation at the interface and the carrier diffusion during the delay time.

The effects of mixture ratios of CuPc:C60 in nanostructured organic hybrid planar–mixed heterojunction photovoltaic cells

Efficient organic hybrid planar–mixed heterojunction photovoltaic cells based on copper phthalocyanine (CuPc) and fullerene (C60) were fabricated. This structure, in addition to the high efficiency in exciton diffusion of a mixed heterojunction, has low resistance in charge transporting of a planar heterojunction. The effects of different CuPc:C60 mixing ratios in two stages and by varying the CuPc and C60 thicknesses, respectively, were investigated. Also, we illustrate the charge transport mechanisms and specify the charge injection models for the cells. For the optimized cell, trap-charge-limited current (TCLC) model was dominant. MoO3 is one of the common materials as anodic buffer layer which is used for modifying the ITO electrode and improving the charge extraction. According to the AFM results, after depositing MoO3 layer on ITO, the surface root mean square (RMS) roughness decreases from 8.31 to 4.14 nm. Optimal thickness for both CuPc and C60 layers was obtained 20 nm. Finally, the greatest value in power conversion efficiency was achieved 3.81% using the 1:1 CuPc:C60 mixing ratio. Moreover, short-circuit current density (Jsc), fill factor (FF), and open-circuit voltage (Voc) for this optimized cell were found 10.21 (mA/cm2), 0.64 (V), and 0.58, respectively.

Surface modified fullerene electron transport layers for stable and reproducible flexible perovskite solar cells

We report flexible planar perovskite solar cells with robust electron-transport layers (ETLs) processed at low temperature. A poly(allylamine) (PAA; 0.08 wt%) solution was deposited on a C60 layer and heated at 150 °C for 60 s, resulting in the formation of an insoluble robust C60–PAA electron-transport layer (ETL) on the flexible substrate. The flexible planar perovskite solar cell with the C60–PAA ETL exhibited excellent properties with 83% efficiency retention (η = 15.2% without hysteresis) after 600 cycles of bending. This performance is superior to that of the flexible device with a C60 ETL fabricated without the use of PAA (65% efficiency retention; η = 9.8% with some hysteresis).

Effect of magnetic fullerene on magnetization reversal created at the Fe/C60 interface

Probing the hybridized magnetic interface between organic semiconductor (OSC) and ferromagnetic (FM) layers has drawn significant attention in recent years because of their potential in spintronic applications. Recent studies demonstrate various aspects of organic spintronics such as magnetoresistance, induced interface moment etc. However, not much work has been performed to investigate the implications of such OSC/FM interfaces on the magnetization reversal and domain structure which are the utmost requirements for any applications. Here, we show that non-magnetic Fullerene can obtain non-negligible magnetic moment at the interface of Fe(15 nm)/C60(40 nm) bilayer. This leads to substantial effect on both the magnetic domain structure as well as the magnetization reversal when compared to a single layer of Fe(15 nm). This is corroborated by the polarized neutron reflectivity (PNR) data which indicates presence of hybridization at the interface by the reduction of magnetic moment in Fe. Afterwards, upto 1.9 nm of C60 near the interface exhibits magnetic moment. From the PNR measurements it was found that the magnetic C60 layer prefers to be aligned anti-parallel with the Fe layer at the remanant state. The later observation has been confirmed by domain imaging via magneto-optic Kerr microscopy.

Electronic Structure of C60/Zinc Phthalocyanine/V₂O₅ Interfaces Studied Using Photoemission Spectroscopy for Organic Photovoltaic Applications.

The interfacial electronic structures of a bilayer of fullerene (C60) and zinc phthalocyanine (ZnPc) grown on vanadium pentoxide (V2O5) thin films deposited using radio frequency sputtering under various conditions were studied using X-ray and ultraviolet photoelectron spectroscopy. The energy difference between the highest occupied molecular orbital (HOMO) level of the ZnPc layer and the lowest unoccupied molecular orbital (LUMO) level of the C60 layer was determined and compared with that grown on an indium tin oxide (ITO) substrate. The energy difference of a heterojunction on all V2O5 was found to be 1.3~1.4 eV, while that on ITO was 1.1 eV. This difference could be due to the higher binding energy of the HOMO of ZnPc on V2O5 than that on ITO regardless of work functions of the substrates. We also determined the complete energy level diagrams of C60/ZnPc on V2O5 and ITO.

Contrasting behavior of covalent and molecular carbon allotropes exposed to extreme ultraviolet and soft x-ray free-electron laser radiation

All carbon materials, e.g., amorphous carbon (a-C) coatings and C60 fullerene thin films, play an important role in short-wavelength free-electron laser (FEL) research motivated by FEL optics development and prospective nanotechnology applications. Responses of a-C and C60 layers to the extreme ultraviolet (SPring-8 Compact SASE Source in Japan) and soft x-ray (free-electron laser in Hamburg) free-electron laser radiation are investigated by Raman spectroscopy, differential interference contrast, and atomic force microscopy. A remarkable difference in the behavior of covalent (a-C) and molecular (C60) carbonaceous solids is demonstrated under these irradiation conditions. Low thresholds for ablation of a fullerene crystal (estimated to be around 0.15 eV/atom for C60 vs 0.9 eV/atom for a-C in terms of the absorbed dose) are caused by a low cohesive energy of fullerene crystals. An efficient mechanism of the removal of intact C60 molecules from the irradiated crystal due to Coulomb repulsion of fullerene-cage cation radicals formed by the ionizing radiation is revealed by a detailed modeling.

High-Stability, Self-Powered Perovskite Photodetector Based on a CH3NH3PbI3/GaN Heterojunction with C60 as an Electron Transport Layer

We reported first photodetectors based on a CH3NH3PbI3/GaN heterojunction with or without an electron transport layer (ETL). Through the investigation, the device without an ETL showed good sensitivity, high stability, and repeatability. Particularly, when C60 was applied as the ETL, the device showed a self-powered characteristic with high stability. The best self-powered device with 6 nm C60 displayed an on/off ratio of greater than 5000, peak responsivity of 0.198 A/W, and detectivity of 7.96 × 1012 cm Hz1/2/W, which is comparable with the other reports. The high performance mentioned above was attributed mainly to the C60 layer, which can reduce the density of trap states and passivate the grain boundaries of the CH3NH3PbI3 absorbing layer to facilitate intermolecular charge transport.