Photoelectric investigations of charge-transferring metal-doped [60] fullerenes

Physical Jet Deposition method was used to form the C₆₀Ni films, one of the metal-doped [60] fullerenes films, which were made into the Al/C₆₀Ni/ ITO layered structure photovoltaic cells. The photovoltaic effect and the current–voltage characteristic in dark with the devices were taken. Compared with the Al/C₆₀/ITO layer structure devices, the enhancement of photovoltage and rectification rate of C₆₀Ni devices was found. Possible reasons were discussed.     

Photoelectron spectroscopy and atomic force microscopy study of 1,2-dicyano-methanofullerene C60(CN)2 thin film for photovoltaic applications

1, 2-dicyano-methanofullerene (C₆₀(CN)₂) is a soluble fullerene derivative that has been reported to have stronger electron affinity than parent C₆₀. Ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) experiments were carried out on C₆₀(CN)₂ thin films spin coated on heavily doped n-type Si substrate. UPS spectra enabled the determination of the vacuum shift at the fullerene derivative/Si interface and the onset of the highest occupied molecular orbital (HOMO). From the UV-vis absorption spectra of C₆₀(CN)₂ thin films spin coated on quartz substrates, the optical band gap (E_g) and the onset of absorption were determined. These measurements allowed the determination of the lowest occupied molecular orbital (LUMO) position. The morphology of the deposited film was probed by AFM and reveals non-uniformity of the thin film. Open circuit voltage (V_oc) measurements on P3HT/C₆₀(CN)₂ based organic solar cell device are compared to the commonly used P3HT/PCBM device.     

Stability and photodegradation mechanisms of conjugated polymer/fullerene plastic solar cells

Degradation studies of poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene-vinylene) (MDMO-PPV), fullerenes ((6,6)-phenyl C₆₁- butyric acid methyl ester (PCBM) and C₆₀), and mixtures, which are the photoactive components in plastic solar cells, are shown. The degradation processes of the individual components and of a 1 : 3 mixture are characterized by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and by current/voltage (I–V) measurements of devices under the influence of light and oxygen. A faster degradation rate was found for the polymer compared with C₆₀. In composites with fullerenes, the stability of MDMO-PPV is enhanced due to the fast electron transfer to C₆₀.     

Microstructures and photovoltaic properties of C60 based solar cells with copper oxides,CuInS2, phthalocyanines,porphyrin, PVK,nanodiamond, germanium and exciton diffusion blocking layers

Abstract

Organic solar cells have a potential for use in lightweight, flexible, inexpensive and large scale solar cells. However, significant improvements of photovoltaic efficiencies are mandatory for use in future solar power plants. One of the improvements is donor–acceptor proximity in the devices, which are called bulk heterojunction solar cells. Bulk heterojunction is an efficient method to generate free charge carriers, and the charge transfer is possible at the semiconductor interface. The purpose of the present work is to fabricate and characterise C₆₀ based solar cells with copper oxides, CuInS₂, phthalocyanines, porphyrin, poly-vinylcarbazole, nanodiamond, germanium and exciton diffusion blocking layers. In the present work, C₆₀ and fullerenol [C₆₀(OH)_10–12] were used for n-type semiconductors, and metal copper oxides, metal phthalocyanine derivative, porphyrin and poly-vinylcarbazole were used for p-type semiconductors. In addition, nanodiamond and germanium based molecules were added into the active layers of the solar cells. The novel aspect of the research is to investigate the relation between properties and microstructures of the solar cells using transmission electron microscopy, X-ray diffraction and electronic structure calculation. The impact of the research concerns the study of organic solar cells by means of microstructural analysis, property measurements and theoretical calculations.     

Hybrid inverted organic photovoltaic cells based on nanoporous TiO2 films and organic small molecules

Solar cells based on nanoporous TiO₂ films with an inverted structure of indium tin oxide (ITO)/TiO₂/copper phthalocyanine (CuPc):fullerene (C₆₀)/CuPc/poly(3,4-oxyethyleneoxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/Au were fabricated. The best overall photovoltaic performance undergoing a series of device optimization was achieved with the device of ITO/dense TiO₂ (30 nm)/nanoporous TiO₂ (130 nm)/C₆₀:CuPc (1:6 weight) (20 nm)/CuPc (20 nm)/PEDOT:PSS (50 nm)/Au (30 nm). The device using the nanoporous TiO₂ films has better photovoltaic properties compared to those using dense TiO₂ films. Higher photovoltaic performances were obtained by introducing a coevaporated layer of C₆₀:CuPc between TiO₂ and CuPc. The stability of inverted structure was better than that of the normal device, which gives a promising way for fabrication of solar cells with improved stability.     

Electronic structure and optical properties of CuGaS2 and CuInS2 solar cell materials

We report energy bands, density of states and optical properties of CuGaS₂ and CuInS₂ chalcopyrites. The electronic structure has been computed using linear combination of atomic orbitals (LCAO) scheme within density functional theory (DFT) and full-potential linearised augmented plane wave method. The energy bands, density of states, components of dielectric tensors and absorption coefficients are compared with the available data. It is seen that the present LCAO-DFT calculations reproduce the electronic properties of both the chalcopyrites in a reasonable way. The optical properties show more absorption of solar radiations for CuGaS₂ chalcopyrite, depicting its more usefulness in the solar cells.     

Zirconium nitride based transparent heat mirror coatings —preparation and characterisation

Transparent heat mirror coatings based on thin zirconium nitride films have been prepared using reactive magnetron sputtering. The zirconium nitride films have been sandwiched between layers of zirconium oxide. It is shown that the multilayer configuration ZrO₂/ZrN/ZrO₂ can be used as solar control coatings on window glazings. A visible transmittance of around 60% and a thermal emittance lower than 0.2 can be obtained, and the ratio between visible transmittance and total solar transmittance can be as high as 1.7. The influence of substrate temperature on the optical quality of the films is evaluated and it is shown that the crystal structure of the first oxide layer is of importance for the optical quality of the nitride. The influence of preparation conditions and accelerated ageing has been modelled using the optical constants of thin films prepared under identical conditions as the films in the multilayer coatings.     

Titania–germanium nanocomposite as a photovoltaic material

Titania–germanium (TiO₂–Ge) nanocomposite, which comprises Ge nanodots in the TiO₂ matrix, is an interesting optoelectronic material. We can easily tailor the structural, optical and electronic properties of the nanocomposite thin films in a wide range by customizing the density and size of Ge dots. The promising properties encouraged us to fabricate solar cells with TiO₂–Ge nanocomposites. We fabricated p-Si/TiO₂–Ge heterojunction solar cells. The preliminary characterizations of these devices show us the promise of TiO₂–Ge nanocomposites for the photovoltaic application.     

Fabrication of three-dimensional nanocomposite with open cage fullerene polymer and MoS2 for enhanced electrochemical hydrogen evolution activity

Development of advanced materials for electrocatalytic water splitting to generate hydrogen is the key in utilization of renewable energy. In the present work, we constructed three-dimensional (3D) nanoparticles (P-PC₆₀@MoS₂) based on an open-cage fullerene polymer (P-PC₆₀), on the surface of which two-dimensional (2D) molybdenum disulfide (MoS₂) are grafted vertically. The rational design endows the nanocomposite materials with combined advantages of open-cage fullerenes and MoS₂. Especially, compared with traditional fullerenes, polymeric fullerene derivatives are expected to have remarkable electronic, optical, or catalytic properties due to the formation of an interconnected continuous network. The results indicate that the few-layer MoS₂ slices arranged vertically on the surface of the fullerene, showing an expanded layer spacing of 6.4 Å, which is beneficial for maximally exposing active edge sites. The hybrid P-PC₆₀@MoS₂ with proper synthetic engineering possesses superior catalytic kinetics with surpassing overpotential of 59 mV, Tafel slope of 37 mV dec^?1, which are much lower than those of pure MoS₂ and P-PC₆₀. The small values indicate fast electrochemical hydrogen evolution reaction (HER) kinetics.     

Thermal-induced changes on the properties of spin-coated P3HT:C60 thin films for solar cell applications

The thermal transition behaviour, optical and structural properties of spin-coated P3HT:C₆₀ blended films with different C₆₀ ratios were investigated using differential scanning calorimetry (DSC), thermo-gravimetric analysis (TGA), ultraviolet-visible (UV-vis) spectroscopy, photoluminescence (PL), Fourier transform infrared absorption (FT-IR) spectroscopy and Raman spectroscopy. DSC analysis showed that the P3HT:C₆₀ blends have quite different thermal characteristics. The absorption spectra of the annealed P3HT:C₆₀ (1:1 wt%) films becomes enhanced and red shifted. This feature is evident in the photoluminescence measurements where the formation of polymer crystallites upon annealing is observed. Raman spectroscopy showed a substantial ordering in the polymer film during annealing. It was found that the performance of a P3HT:C₆₀ (1:1 wt%) device was dramatically improved by annealing.     

Engineering of hybrid interfaces in organic photovoltaic devices

In this study, we engineer and investigate the interface structure and chemistry at the indium tin oxide (ITO) anode (front-side electrode) as well as at the Mg−Ag cathode (back-side electrode) in metal phthalocyanine (MePc)/C₆₀ organic solar cells (OSCs).For the front-side electrode, Zn-phthalocyaninetetraphosphonic acid (Zn-PTPA) and Sn-phthalocyanine axially substituted with tartaric acid (Sn-PTA) have been used for the surface termination of ITO coated glass substrates. Both terminations yielded OSCs with higher fill factors and open circuit voltages, thus increasing the power conversion efficiency by 33% and 67%, respectively. A possible influence of a chemisorbed Zn-PTPA on the film growth of the adjacent ZnPc absorber in the vicinity of the hybrid interface is discussed using X-ray reflectivity and near edge X-ray absorption fine structure data. Distinct effects of the Zn-PTPA and Sn-PTA terminations on the electronic properties of the ITO surface were found by X-ray photoelectron spectroscopy (XPS) measurements at the valence band edge. We demonstrate the possibility to engineer the hybrid interface without additional buffer.For the back-side electrode we report the formation of buffer-free charge carrier selective Mg−Ag cathodes, which are applied for bulk heterojunction organic absorbers consisting of copper phthalocyanine (CuPc) donor and fullerene C₆₀ acceptor materials. The chemical and structural properties of the CuPc:C₆₀/Mg−Ag interface are investigated by element depth profiling using secondary ion mass spectrometry (SIMS), grazing incidence X-ray diffraction analysis (GI-XRD) and XPS.We demonstrate that an optimum charge carrier selectivity is achieved with Mg:Ag/Ag cathode structures, where the Mg:Ag alloy layer has a composition close to that of Ag₃Mg. In addition, Mg diffusion into CuPc:C₆₀ layer is observed. As a result, an interaction between Mg and Cu²⁺ with a concurrent change in oxidation state of both metals takes place. However, no formation of MgPc is observed.The findings of this work are discussed against the background of the performance and electrical properties of the corresponding MePc/C₆₀-based organic solar cells.     

Some physical properties of Cd1−xSnxS films used as window layer in heterojunction solar cells

CdS has been proved to be an ideal material for use as the window layer for heterojunction solar cells especially with n-CdS/p-CdTe. CdS, Cd_0.9Sn_0.1S and Cd_0.8Sn_0.2S films were deposited onto glass substrates at 300 °C substrate temperature by using ultrasonic spray pyrolysis technique (USP). The effect of Sn concentration on some structural, optical and electrical properties of the films was presented. The crystal structure and orientation of the films were investigated by X-ray diffraction (XRD) patterns. XRD patterns showed that films have polycrystalline nature with a hexagonal structure. The grain size of the films decreased with increasing x values. The optical band gap values were obtained from optical absorption spectra of the films. The optical band gap values of the films were found to be between 2.44 and 2.45 eV. The variations of conductivity of Cd_1−xSn_xS (0 ≤ x ≤ 0.2) films have been investigated depending on applied voltage in dark and under illumination. The resistivity significantly decreased with increasing tin concentration and under illumination.     

Higher fullerenes as electron acceptors for polymer solar cells: A quantum chemical study

In the present study, we have used quantum chemical methods to study the energy levels of the frontier orbitals of higher fullerene derivatives (from C₇₀ to C₈₄ and having the same addend as in [6,6]-phenyl C₆₁-butyric acid methyl ester) with the aim to understand if they can be used as electron acceptors in bulk heterojunction polymer–fullerene solar cells. Higher fullerenes have a stronger and broader absorption compared to C₆₀ and they can improve the current output of the corresponding devices. The geometries of all the compounds were optimized with the density functional theory at the B3LYP/3-21G* level of calculation. The lowest unoccupied molecular orbital (LUMO) levels of the investigated compounds correlate well with the reduction potentials (obtained by cyclic voltammetry) of the already prepared species. We found that the LUMO level depends not only on the fullerene size (number of carbons of the cage) and constitutional isomer, but also on the position and, in some cases, the addend orientation. This issue should be considered because for a proper device operation, a well-defined LUMO is required. The position of the LUMO level of some higher fullerene derivatives can be suitable for low-bandgap polymers.     

Modeling the optical absorption within conjugated polymer/fullerene-based bulk-heterojunction organic solar cells

In this paper, we report our results on the modeling of the optical properties of the bulk-heterojunction “plastic solar cells”, consisting of a solid-state blend of the conjugated polymer poly-[2-(3,7-dimethyloctyloxy)-5-methyloxy]-para-phenylene-vinylene and the fullerene C₆₀ derivative 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C₆₁. Upon illuminating these cells with the standard AM 1.5 solar spectrum, the short circuit current can be determined for any given internal quantum efficiency as a function of the active layer thickness. In addition, the depth profiles of photoinduced charge generation rates are calculated. Based on the agreement of this modeling with experimentally determined efficiencies of these solar cells, an internal quantum efficiency of about 80% has been estimated.     

Improvement of a-Si solar cell properties by using SnO2:F TCO films coated with an ultra-thin TiO2 layer prepared by APCVD

TiO₂-overcoated SnO₂:F transparent conductive oxide films were prepared by atmospheric pressure chemical vapor deposition (APCVD) and an effect of TiO₂ layer thickness on a-Si solar cell properties was investigated. The optical properties and the structure of the TiO₂ films were evaluated by spectroscopic ellipsometry and X-ray difractometry. a-Si thin film solar cells were fabricated on the SnO₂:F films over-coated with TiO₂ films of various thicknesses (1.0, 1.5 and 2.0 nm) and I–V characteristics of these cells were measured under 1 sun (100 mW/cm² AM-1.5) illumination. It was found that the TiO₂ film deposited by APCVD has a refractive index of 2.4 at 550 nm and anatase crystal structure. The conversion efficiency of the a-Si solar cell fabricated on the 2.0 nm TiO₂-overcoated SnO₂:F film increased by 3%, which is mainly attributed to an increase in open circuit voltage (V_oc) of 30 mV.     

Sc/Ti-decorated and B-substituted defective C60 as efficient materials for hydrogen storage

Doping heteroatoms and producing defects are perfect methods to improve the hydrogen storage property of TM-decorated carbon materials. In this view, four novel Sc/Ti-decorated and B- substituted defective C₆₀ fullerenes (B₂₄C₂₄) are explored. The special stability, large specific surface, uniform distribution of the metal and positively charged states make these four fullerenes have high hydrogen storage capacities. Especially, each Sc atom in Sc₆B₂₄C₂₄(B₄) can adsorb up to five H₂ molecules with a storage capacity of 6.80 wt %. The adsorbed H₂ molecules in Sc₆B₂₄C₂₄(B₄)–30H₂ begin to relax at 190 K and are 100% released at 290 K. Moreover, a comparative study is carried out for hydrogen storage properties of Sc-decorated B₄, C₄, or N₄ coordination environments. These results provide a new focus on the nature of B-, and N-substituted defective carbon nanomaterials.     

Organic donor, acceptor and buffer layers of small molecules prepared by OVPD technique for photovoltaics

Organic vapour-phase deposition (OVPD^®) is used for the growth of the organic solar cell component materials such as the donor copper phthalocyanine (CuPc), the acceptor fullerene C₆₀, and electron-conducting buffer layers of bathocuproine (BCP) on Si1 0 0 wafers and indium tin oxide (ITO) substrates on areas as large as 15×15 cm². By means of X-ray diffraction (XRD) analysis we show that under continuous operating conditions the source materials possess long-term stability. The CuPc, C₆₀ and BCP thin film morphology and structure are characterised using scanning electron microscopy and XRD analysis. We demonstrate CuPc thin films with a highly folded surface morphology suitable for the preparation of solar cells with an interpenetrating donor–acceptor interface. The XRD diffraction patterns of the CuPc and C₆₀ layers deposited under conditions appropriate for the preparation of organic solar cells show spectra typical for these materials. Mixed CuPc:C₆₀ layers with controlled constituent ratios and layer thickness are deposited for the preparation of organic solar cells. First ITO/CuPc:C₆₀/Al organic photovoltaic devices are prepared with an efficiency of 1% (conditions AM1.5).     

High performance sputtered Ni : SiO2 composite solar absorber surfaces

A solar selective absorber can be prepared by dispersing suitably sized metallic particles in an insulating host matrix. Absorption is assisted by controlling optical interference within the composite film. Graded index thin films of metallic nickel in quartz (Ni : SiO₂) were made by co-sputtering with metal volume fractions ranging from 10–90% from top (anti reflecting coating) to bottom (base layer) of the structure, to minimise optical interference peaks. The films are 100–170 nm thick with an additional 70 nm anti-reflection (AR) coating. Coatings of different thickness, metal volume fraction and compositional gradient were investigated. Substrates were Al and Cu and films were deposited on either the bare substrate or substrates coated with evaporated nickel. The influence of substrate choice on the optical properties was studied. Films with solar absorptance, α, in the range 0.90–0.96 and thermal emittance, ,=0.03–0.14 were achieved. The dependence of these properties on thickness, film composition and gradient and substrate were determined. A computer programme which calculates the solar absorptance and thermal emittance based on the assumptions of both Maxwell Garnett and Bruggeman theories for metal fill factors below and above 0.3, respectively, was used to design the structure of the composite films. The theoretical results are not presented here.     

Ellipsometric spectroscopy study of cobalt oxide thin films deposited by sol–gel

Due to their unique optical properties, solar selective coatings enhance the thermal efficiency of solar photothermal converters. Hence it seems to be interesting to study the optical properties of promising materials as solar selective coatings. In an earlier work, it was demonstrated that sol–gel deposited cobalt oxide thin films possess suitable optical properties as selective coatings. In this work, cobalt oxide thin films were prepared by same technique and their optical properties were analyzed as a function of the dipping time of the substrate in the sol, using the spectroscopy ellipsometry, atomic force microscopy and X-ray photoelectron spectroscopy techniques. The optical constants (n and k) for these films, in the 200–800 nm range, are reported as a function of the dipping time. The fitting of ellipsometric data, I_s and I_c, for the glass substrate and the cobalt oxide thin film, as modeled with the Lorentz and Tauc–Lorentz dispersion relations, indicated that the film microstructure resembles a multilayer stack with voids. From these results, the Co₃O₄ and void percentages in the film were estimated. Both, thin film thickness and void/Co₃O₄ percentage ratio, were determined to be strongly dependent on the immersion time. Furthermore, the total thickness of a multilayered film was found to be the sum of thickness of each individual layer.