Water and oxygen induced degradation of small molecule organic solar cells

Small molecule organic solar cells were studied with respect to water and oxygen induced degradation by mapping the spatial distribution of reaction products in order to elucidate the degradation patterns and failure mechanisms. The active layers consist of a 30 nm bulk heterojunction formed by the donor material zinc-phthalocyanine (ZnPc) and the acceptor material Buckminsterfullerene (C₆₀) followed by 30 nm C₆₀ for additional absorption. The active layers are sandwiched between 6 nm 4,7-diphenyl-1,10-phenanthroline (Bphen) and 30 nm N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine p-doped with C₆₀F₃₆ (MeO-TPD:C₆₀F₃₆), which acted as hole transporting layer. Indium-tin-oxide (ITO) and aluminum served as hole and electron collecting electrode, respectively. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) in conjunction with isotopic labeling using H₂¹⁸O and ¹⁸O₂ provided information on where and to what extent the atmosphere had reacted with the device. A comparison was made between the use of a humid (oxygen free) atmosphere, a dry oxygen atmosphere, and a dry (oxygen free) nitrogen atmosphere during testing of devices that were kept in the dark and devices that were subjected to illumination under simulated sunlight. It was found that water significantly causes the device to degrade. The two most significant degradation mechanisms are diffusion of water through the aluminum electrode resulting in massive formation of aluminum oxide at the BPhen/Al interface, and diffusion of water into the ZnPc:C₆₀ layer where ZnPc becomes oxidized. Finally, diffusion from the electrodes was found to have no or a negligible effect on the device lifetime.     

Kinetic improvement of H2 absorption and desorption properties in Mg/MgH2 by using niobium ethoxide as additive

The hydrogen absorption and desorption properties of a MgH₂ – 1 mol.% Nb(V) ethoxide mixture are reported. The material was prepared by hand mixing the additive with previously ball-milled MgH₂. Nb ethoxide reacts with MgH₂ during heating, releasing C₂H₆ and H₂, and producing MgO and Nb or Nb hydride. Hydriding and dehydriding are greatly enhanced by the use of the alkoxide. At 250 °C the material with Nb takes up 1.8 wt% in 30 s compared with 0.1 wt% of pure Mg, and releases 4.2 wt% in 30 min, whereas MgH₂ without Nb does not appreciably desorb hydrogen. The absorption and desorption activation energies are reduced from 153 kJ/mol H₂ to 94 kJ/mol H₂, and from 176 kJ/mol H₂ to 75 kJ/mol H₂, respectively. The hydrogen sorption properties remain stable after 10 cycles at 300 °C. The kinetic improvement is attributed to the fine distribution of amorphous/nanometric NbH_x achieved by the dispersion of the liquid additive.     

Analytical study of PPV-oligomer- and C60-based devices for optimising organic solar cells

A blend of a 5-ring n-octyloxy-substituted oligo(p-phenylene vinylene) and C₆₀, sandwiched between two electrodes, has been used as the active layer for an organic solar cell. It delivered external quantum efficiencies up to 60% in the visible and 70% in the UV part of the spectrum. To unambiguously determine which parts of the bulk heterojunction structure are responsible for the rectifying behaviour and which can be considered as ohmic, the I–V characteristics of several other devices were investigated. It is found that the presence of C₆₀ in a bulk heterojunction solar cell introduces fill factor reducing shunting paths.     

Reversible hydrogen absorption in sodium intercalated fullerenes

The hydrogen absorption of sodium intercalated fullerenes (Na_xC₆₀) was determined and compared to pure fullerenes (C₆₀). Up to 3.5 mass% hydrogen can reversibly be absorbed in Na_xC₆₀ at 200 °C and a hydrogen pressure of 200 bar. The absorbed amount of hydrogen is significantly higher than for the case when only the sodium would be hydrogenated (∼1 mass% for x = 10). At 200 bar the onset of hydrogen absorption is observed at 150 °C. At a pressure of 1 bar hydrogen the major desorption starts at 250 °C and is completed at 300 °C (heating rate 1 °C min⁻¹). This absorption and desorption temperatures are significantly reduced compared to pure C₆₀, either due to a catalytic reaction of hydrogen on sodium or due to the negatively charged C₆₀. The hydrogen ab/desorption is accompanied by a partial de/reintercalation of sodium. A minor part of the hydrogen is ionically bonded in NaH and the major part is covalently bonded in C₆₀H_x. The sample can be fully dehydrogenated and no NaH is left after desorption. In contrast to C₆₀, where the fullerene cages for high hydrogen loadings are destroyed during the sorption process, the Na_xC₆₀ sample stays intact. The samples were investigated by X-ray, in-situ neutron powder diffraction and infrared spectroscopy. Na_xC₆₀ was synthesized by reacting sodium azide (NaN₃) with C₆₀ (molar ratio of Na:C₆₀ is 10:1).     

Precursor route poly(thienylene vinylene) for organic solar cells: Photophysics and photovoltaic performance

Photophysical studies and photovoltaic devices on a low bandgap, high-charge carrier mobility poly(thienylene vinylene) (PTV), prepared from a soluble precursor polymer synthesised via the “dithiocarbamate route”, are reported. In composites with an electron acceptor ([6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM), a soluble fullerene derivative), photoinduced absorption characteristic for charged excitations together with photoluminescence quenching are observed indicating photoinduced electron transfer. The “bulk heterojunction” photovoltaic devices using PTV and PCBM composites show short-circuit currents up to 4 mA/cm² under AM 1.5 white-light illumination. The photocurrent spectrum of the photovoltaic device shows an onset about 1.65 eV (750 nm), which corresponds to the absorption spectrum of the polymer.     

Dicyanovinyl substituted oligothiophenes: Thermal stability, mobility measurements, and performance in photovoltaic devices

A series of dicyanovinyl-oligothiophenes are investigated concerning their thermal stability, absorption in thin film, and hole mobility. Due to very high extinction coefficients, these materials are interesting for application as donor in solar cells. The quinquethiophene DCV₂-5T, which shows a hole mobility of 2.2×10^-5 cm²/Vs, is used as donor material in a flat heterojunction organic small molecule solar cells. Despite a very thin donor layer of only 6 nm, these devices exhibit in a planar heterojunction with 15 nm C₆₀ an efficiency of up to 2.8% with a fill factor of up to 58%, a short circuit current density of 5.2 mA/cm², an open circuit voltage of 1.03 V, and an external quantum efficiency of 30% in the green spectral range.     

A phenylcarbazole functionalized ruthenium dye for efficient dye-sensitized solar cells

A new heteroleptic Ru(II) complex of [Ru(Hcpip)(Hdcbpy)(NCS)₂]⁻·[N(C₄H₉)₄]⁺·H₂O {where Hcpip = 2-(4-(9H-carbazol-9-yl)phenyl)-1H-imidazo[4,5-f] [1,10]phenanthroline, Hdcbpy = 4-carboxylic acid-4′-carboxylate-2,2′-bipyridine} has been synthesized and demonstrated to function as an efficient sensitizer for nanocrystalline TiO₂-based dye-sensitized solar cell (DSSC). The DSSC based on this Ru(II) complex showed a short-circuit photocurrent density of 19.2 mA cm⁻², an open-circuit photovoltage of 630 mV, a fill factor of 57.7%, corresponding to an overall light to electricity conversion efficiency of 6.98% under simulated solar light irradiation at 100 mW cm⁻². This efficiency value is 2.81- and 1.08-fold efficiency values of 2.48% and 6.47% observed for carbazole-free parent complex [Ru(Hpip)(Hdcbpy)(NCS)₂]⁻·[N(C₄H₉)₄]⁺·H₂O {where Hpip = 2-phenyl-1H-imidazo[4,5-f][1,10]phenanthroline}- and cis-bis(isothiocyanato)bis(4,4′-dicarboxylic acid-2,2′-bipyridine)ruthenium(II) N3-based solar cells respectively, under identical experimental conditions. The molecular structures and electronic properties of the Ru(II) complexes were also investigated by means of density functional theory calculations in an effort to understand the device performance observed.     

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.     

Biindene-C60 adducts for the application as acceptor in polymer solar cells with higher open-circuit-voltage

Two C₆₀ derivatives, biindene-C₆₀ monoadduct (BC₆₀MA) and biindene-C₆₀ bisadduct (BC₆₀BA), were synthesized by an one-pot reaction of 1, 1′-biindene and C₆₀, for the application as acceptor materials in Polymer Solar Cells (PSCs). The two C₆₀ derivatives possess good solubility in toluene and o-dichlorobenzene, and the solubility of BC₆₀BA is even better than that of BC₆₀MA. The electrochemical properties and the LUMO energy levels of the two fullerene derivatives were investigated by cyclic voltammetry. The LUMO energy levels of BC₆₀MA and BC₆₀BA were 0.06 and 0.17 eV higher (up-shifted) than that of PCBM, respectively. The PSCs based on P3HT as donor and BC₆₀MA or BC₆₀BA as acceptor exhibited higher V_oc of 0.68 and 0.82 V, respectively, which is benefited from the higher LUMO energy levels of the C₆₀ derivatives. The power conversion efficiency of the PSC based on P3HT/BC₆₀MA was 2.21% after annealing at 140 °C for 5 min.     

The hydrogen evolution reaction on single crystal gold electrode

As one of the important candidate of power sources for the future, the research and production of hydrogen gas has a significant importance. In this article, the emphasis is on the influence of impurities on hydrogen evolution reaction, i.e., the influence of an addition of decacyclene, C₁₂H₃₅C₆H₄SO₄Na, CH₃CH₂OH, chromanone, H₂SO₄, HNO₃, 4,4′-biphenediol and 1,2,3,4-tetraphenyl-1,3-cyclopentadiene was studied by electrochemical impedance technique. The adsorption structure for some organics was measured by scanning tunneling spectroscopy techniques. Superstructure of adsorbed decacyclene on Au(111) surface was captured. The ordered adsorption structure of 4,4′-biphenyldiol on Au(111) and (100) was also observed. The addition of decacyclene has shown an opposite effects on hydrogen evolution for Au(111) and (100) surface, i.e., it inhibits the reaction at Au(100) but enhances the one at Au(111). The results show that the addition of C₁₂H₃₅C₆H₄SO₄Na and HNO₃, especially the latter, can improve the hydrogen evolution. In the article the adsorption structure and hydrogen evolution reaction have been studied in order to give some useful information about the relation between the adsorption structure and the properties. The purpose of this article is to attempt to find the relation between electrochemical performance and the adsorption structure, and to explore the effect of some additives.     

Nanostructured polymer blends (P3HT/PMMA): Inorganic titania hybrid photovoltaic devices

We have fabricated a photovoltaic (PV) device based on the polymer blends of (poly(3-hexylthiophene) (P3HT)/polymethylmethacrylate (PMMA)) and inorganic TiO₂ nanorod bulk heterojunction. The optimized photovoltaic device with 1.6 wt% PMMA concentration has a power conversion efficiency of 0.65% under simulated AM 1.5 illumination (100 mW/cm²), which is 38% more efficient than the device without the incorporation of PMMA. Furthermore, the PMMA-included device gives a short-circuit current density of 2.57 mA/cm², an open-circuit voltage of 0.53 V, and a fill factor of 0.48. Our studies have shown that having optimal PMMA concentration in the photovoltaic devices helps to smoothen the surface of the hybrid thin film, broaden the absorption spectrum, and improve the electrical conductivity. The results implying improvement in cell performance can be illustrated using atomic force microscopy (AFM), a UV/vis spectrophotometer and electrical measurements.     

Lifetimes of organic photovoltaics: Combining chemical and physical characterisation techniques to study degradation mechanisms

Degradation mechanisms of a photovoltaic device with an Al/C₆₀/C₁₂-PSV/PEDOT:PSS/ITO/glass geometry was studied using a combination of in-plane physical and chemical analysis techniques: TOF-SIMS, AFM, SEM, interference microscopy and fluorescence microscopy. A comparison was made between a device being stored in darkness in air and a device that had been subjected to illumination under simulated sunlight (1000  W m^–2, AM1.5) in air. It was found that oxygen diffuses through pinholes in the aluminium electrode. If stored in air in the dark the oxidation is limited to the C₆₀ layer. Illumination accelerates the oxidation/degradation and thus expands the process to involve at least the underlying layer of C₁₂-PSV. Furthermore, it was found that particles are formed in the device during storage.     

Optimization of inverted tandem organic solar cells

Inverted tandem organic solar cells, consisting of two bulk heterojunction sub-cells with identical poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C₆₁ (PCBM) active layer and a MoO₃/Ag/Al/Ca intermediate layer, have been presented and optimized. Indium tin oxide (ITO) modified by Ca acts as a cathode for electron collection and Ag is used as the anode for hole collection for the tandem device. A proper thickness of Ca (3 nm) forms a continuous layer, working as a cathode for the top sub-cell. MoO₃ as the anode buffer layer prevents exciton quenching and charge loss at the anode side, which could result in increase in interfacial resistance. The variance of sub-cell thickness adjusts the optical field distribution in the entire device, facilitating light absorption and good current matching in both sub-cells. The optimal inverted tandem device achieves a maximum power conversion efficiency of 2.89% with a short-circuit current density of 4.19 mA/cm², an open-circuit voltage of 1.17 V, and a fill factor of 59.0% under simulated 100 mW/cm² (AM 1.5G) solar irradiation.     

Comparison of solar cell performance of conducting polymer dyes with different functional groups

Conductive polymer precursors, including carboxylic acid, cyano groups, amino groups, 5,2′:5′,2″-terthiophene-3′-carboxylic acid (TTCA), 3′-cyano-5,2′:5′,2″-terthiophene (CTT), and 3′,4′-diamino-2,2′:5′,2″-terthiophene (DATT) are synthesized. Electrochemically polymerized films of the precursors on a nanocrystalline TiO₂ layer are examined as photo sensitizers, and the cell performance is compared. The photovoltaic cells are assembled with a polymer-coated TiO₂ layer treated with TiCl₄ as an anode and a Pt layer as a cathode in a propionitrile solution containing an iodide ion-based redox electrolyte. The charge-transfer processes of polymer-dyed cells are studied using impedance spectroscopy. The polymer dyes on the TiO₂ surfaces are characterized by scanning electron microscope (SEM), atomic force microscope (AFM), transmission electron microscope (TEM), and X-ray photoelectron spectroscopy (XPS). XPS results show that the conducting polymer dye, bearing a carboxylic acid group, is more strongly bound to the TiO₂ layer in comparison with other groups. Various experimental parameters affecting the cell efficiency are optimized, including the scan rate, number of potential cycles, and terthiophene monomer concentration. Of these polymers, the best cell efficiency is attained for poly-TTCA containing a carboxylic acid group. The optimized cell with the poly-TTCA dye shows a short-circuit current of 6.78 mA cm⁻², an open-circuit voltage of 0.54 V, and a fill factor of 63.6. An energy conversion efficiency of 2.32% is obtained with a cell area of 0.24 cm² under an air mass 1.5 solar simulated light irradiation of 100 mW cm⁻².     

Organic solar cells based on the spin-coated blend films of TPA-th-TPA and PCBM

In this paper, the optical and photo-induced electron transfer properties of a new conjugated molecule, 4,7-bis{5′-[4″,4″-N,N-diphenylamino-styryl]thiphen-2′-yl} -benzo[1,2,5-thiadiazole] (simplified as TPA-th-TPA), were investigated. Using TPA-th-TPA as a photoactive layer, organic solar cells with three different architectures were fabricated by spin-coating method. The photosensitive layers of these architectures comprise pure TPA-th-TPA layer, heterojunction of bi-layered TPA-th-TPA and C₆₀, and bulk-heterojunction of TPA-th-TPA and [6, 6]-phenyl C₆₁-butyric acid methyl ester (PCBM) blend. Furthermore, towards the bulk-heterojunction devices, the effect of the cathode materials (Mg, Ca, LiF/Al, Ba) on the performance of the devices was studied. The power conversion efficiency reached 0.26% for the device based on the blend of TPA-th-TPA and PCBM with Ba/Al as the cathode.     

Hydrogen transport properties of several vanadium-based binary alloys

Vanadium-based alloys are an emerging alternative to palladium alloys for use in hydrogen-selective alloy membranes. The tendency of vanadium to embrittle, due to its high hydrogen absorption, means it lacks the robustness required for industrial hydrogen separation applications. Alloying vanadium with certain elements reduces hydrogen absorption, but also influences the diffusivity of hydrogen through the bulk material. Consequently, diffusivity and absorption data must be decoupled in order to fully evaluate the influence of various alloying additions on the hydrogen transport properties of vanadium alloys. To address this need, the hydrogen transport properties of V–Al (V₉₅Al₅, V₉₀Al₁₀, V₈₅Al₁₅, V₈₀Al₂₀, V₇₅Al₂₅, expressed as atom%) and V–Cr (V₉₅Cr₅, V₉₀Cr₁₀, V₈₅Cr₁₅) alloys have been compared through a series of absorption and flux measurements. Pd-coated alloy disks were formed from arc melted and sectioned ingots, and each alloy was subjected to a microstructural analyses and a detailed examination of hydrogen absorption and permeation properties. Additions of Al and Cr reduce the hydrogen absorption and diffusivity of vanadium, with V–Cr alloys exhibiting the greatest hydrogen diffusivity for a given hydrogen feed pressure. The diffusivity of each alloy showed strong concentration dependence. Diffusivity-concentration results have been overlayed with an isoflux curve corresponding to a target flux of 1.0 mol m⁻² s⁻¹, enabling prediction of the thickness and pressure required to achieve this target flux target for a given alloy.     

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.     

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.     

Environment-friendly energy from all-carbon solar cells based on fullerene-C60

An efficient single layer organic solar cell based on plain buckminsterfulerence (C₆₀) has been fabricated. By inserting a very thin N,N′-bis(naphthalene-1-yl)-N,N′-bis(phenyl)benzidine layer between the indium tin oxide and single C₆₀ active layer, a short-circuit current of 1.98 mA/cm² and an open-circuit voltage of 0.52 V are obtained under 100 mW/cm² AM1.5G simulated illumination. The highest power conversion efficiency of 0.414% based on plain C₆₀ is thus demonstrated, which is the first step to realize an environment-friendly energy source.     

An algorithm for sensor-less fuel control of direct methanol fuel cells

We report an algorithm for real-time control of the fuel of a DMFC. The MEA voltage decay coefficients [e₁, e₂], and I-V-T, M′-I-T, and W′-I-T curves (where I is the current, V the voltage, T the temperature, and M′ and W′ the methanol and water consumption rates, respectively) of n fuels with specified methanol concentrations C_M,k (k = 1, 2,…, n) are pre-established and form (I,V,T), (M′,I,T), and (W′,I,T) surfaces for each C_M,k. The in situ measured (I,V,T)_u after voltage decay correction is applied to the n preset (I,V,T) surfaces to estimate C_M,u (the C_M corresponding to (I,V,T)_u) using an interpolation procedure. The C_M,u is then applied to the n preset (M′,I,T) and (W′,I,T) surfaces to estimate cumulated “methanol” and “water” consumed quantities . Thus in a real-time system, the C_M and total quantity of fuel can be controlled using the estimated C_M,u and cumulated “methanol” and “water” consumed quantities.