• Energy Research

Lanthanum strontium manganate (LSM) micro-electrodes with the nominal composition of (La0.75Sr0.25)0.95MnO3 were deposited on yttria stabilized zirconia (YSZ). The diameter varied from 20 – 100 μm and the thickness was kept constant at ca. 0.5 μm. Electrochemical characterization was carried out in situ at temperatures from 660 to 860 °C using controlled atmosphere high temperature scanning probe microscope (CAHT-SPM) set-up for measurements of impedance spectroscopy and potential sweep. The oxygen partial pressure, pO2, was varied. Further, ex situ surface analysis by time of flight secondary mass spectrometry (TOF-SIMS) and structure examination by scanning electron microscopy (SEM) were performed. Segregation of Sr and La oxides to LSM surfaces and Mn rich oxide to the three phase boundary (TPB) was observed. YSZ and LSM attract different oxides/impurities. The oxygen electrode kinetics are discussed in light of the surface compositions and compared with the literature on microelectrode and composite LSM-YSZ electrode kinetics.

Abstract 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 (C 60 ) followed by 30 nm C 60 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 C 60 F 36 (MeO-TPD:C 60 F 36 ), 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 H 2 18 O and 18 O 2 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:C 60 layer where ZnPc becomes oxidized. Finally, diffusion from the electrodes was found to have no or a negligible effect on the device lifetime.

Abstract Degradation mechanisms of a photovoltaic device with an Al/C 60 /C 12 -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 C 60 layer. Illumination accelerates the oxidation/degradation and thus expands the process to involve at least the underlying layer of C 12 -PSV. Furthermore, it was found that particles are formed in the device during storage.

Inverted all polymer solar cells based on a blend of a perylene diimide based polymer acceptor and a dithienosilole based polymer donor were fabricated from small area devices to roll-to-roll (R2R) coated and printed large area modules. The device performance was successfully optimized by using solvent additive to tune the phase separation. By adding 2% chloronaphthalene as solvent additive for small area (0.25 cm2) devices, a power conversion efficiency (PCE) up to 0.63% was achieved for inverted geometry, higher than that (0.39%) of conventional geometry. This polymer blend showed excellent solution processibility and R2R coated and printed large area (4.2 cm2) solar cells exhibited a PCE of 0.20%.

This work is a study of the impact of short-term strong cathodic polarization in a Ni|YSZ model system using Ni probes as working microelectrodes in a high temperature scanning probe microscope at 650°C in humidified 9% H2 in N2. Impedance spectroscopy revealed one to three orders of magnitude decrease in the high frequency resistance and four to five orders of magnitude decrease in the low frequency impedance with polarization from −1.06 V to −3.06 V vs E°(O2), indicating introduction of electronic conductivity and expansion of the reaction zone around the Ni microelectrode. The effect on the Ni|YSZ interface included formation of electronic conductance, reaction between Ni and YSZ and accumulation of impurities around the Ni|YSZ contact as verified by conductance scans of the polarized area. Cyclic voltammetry was used to compare three systems with different impurity levels and showed that the presence of silicates reduces the current, i.e. lowers the performance of the electrode reaction.

AbstractOrganic photovoltaics (OPVs) evolve in an exponential manner in the two key areas of efficiency and stability. The power conversion efficiency (PCE) has in the last decade been increased by almost a factor of ten approaching 10%. A main concern has been the stability that was previously measured in minutes, but can now, in favorable circumstances, exceed many thousands of hours. This astonishing achievement is the subject of this article, which reviews the developments in stability/degradation of OPVs in the last five years. This progress has been gained by several developments, such as inverted device structures of the bulk heterojunction geometry device, which allows for more stable metal electrodes, the choice of more photostable active materials, the introduction of interfacial layers, and roll‐to‐roll fabrication, which promises fast and cheap production methods while creating its own challenges in terms of stability.

Abstract Degradation mechanisms in organic and polymer photovoltaics are addressed through the study of an organic photovoltaic molecule based on a single phenylene–vinylene-type oligomer molecule. The synthesis of such a model compound with different end-groups is presented that allows for assignment of degradation products from different parts of the molecule. Photovoltaic devices with and without C 60 have been prepared and their characteristics under AM1.5 conditions are reported. The degradation of the active phenylene–vinylene compound in darkness and after 20 h of illumination were investigated using a mass spectrometric technique (time-of-flight secondary ion mass spectrometry) allowing elucidation of the oxidative degradation pathways.