A short lifetime is the main factor hindering the wider implementation of low-cost organic photovoltaics in large-area and outdoor applications. Ingress of oxygen and water vapour through non-ideal encapsulation layers is a known cause of degradation for polymer/fullerene based solar cells. To better understand the origin of this performance degradation, we study the effect of intentional exposure of the photo-active layer to simulated sunlight (AM1.5) in air both on the solar cell performance and on the molecular semiconductor materials. Cathode-free thin films of a blend of the electron donor polymer poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1) and the electron acceptor fullerene derivative [6,6]-phenyl-C₇₀-butyric acid methyl ester (PC₇₀BM) were exposed to simulated sunlight in air. Fourier-transform infrared spectra demonstrate the formation of carbonyl photo-oxidation products in the blend films, as well as in the pristine polymer and fullerene films. Solar cells prepared with photo-oxidized active layers show increasingly degraded electrical performance (lower short circuit current, open circuit voltage and fill factor) with increasing exposure time. The increased diode ideality factor indicates that trap-assisted recombination hinders device operation after exposure. The external quantum efficiency decreases drastically with increasing exposure time over the whole photon energy range, while the UV-vis absorption spectra of the blend films only show a mild photo-induced bleaching. This demonstrates that not only the photo-induced degradation of the solar cell performance is not predominantly caused by the loss in light absorption, but charge transport and collection are also hampered. This is explained by the fact that photo-oxidation of PC₇₀BM causes bonds in its conjugated cage to break, as evidenced by the decreased ∏* intensity in C1s-NEXAFS spectra of PC₇₀BM films. This degradation of unoccupied states of PC₇₀BM will hinder the transport of photo-generated electrons to the electrode. Surface photovoltage spectroscopy gives direct evidence for gap states at the surface of a PC₇₀BM film, formed after 2 hours of exposure and resulting in upward band bending at the PC₇₀BM/air surface. These observations indicate that the photo-oxidation of PC₇₀BM is likely to be the main cause of the performance degradation observed when the photoactive layer of a TQ1:PC₇₀BM solar cell is intentionally exposed to light in air.

The fullerene derivatives PC₆₀BM and PC₇₀BM are widely used as electron accepting components in the active layer of polymer solar cells. Here we compare their photochemical stability by exposing thin films of PC₆₀BM and PC₇₀BM to simulated sunlight in ambient air for up to 47 h, and study changes in their UV–vis and FT-IR spectra. We quantify the photo-degradation by tracking the development of oxidation products in the transmission FT-IR spectra. Results indicate that PC₆₀BM photodegrades faster than PC₇₀BM. The rate of photo-oxidation of the thin films is dependent on the rate of oxygen diffusion in to the film and on the photo-oxidation rate of a single molecule. Both factors are dependent on the nature of the fullerene cage. The faster photo-oxidation of PC₆₀BM than of PC₇₀BM is in agreement with its slightly lower density and its higher reactivity. The use of PC₇₀BM in solar cells is advantageous not only because of its absorption spectrum, but also because of its higher stability.

In the quest towards more durable solution-processed solar cells, the stability of the active layer materials under operation conditions is important. While lifetimes of several years have been demonstrated for encapsulated organic solar cells, it is generally known that degradation events can be accounted for by air components (O₂ and/or water vapour) leaking into the cell through a non-ideal sealing. Here we present a fundamental study of intentional photo-degradation of the electron-acceptor PC₆₀BM ([6,6]-phenyl-C₆₁-butyric acid methyl ester) in air, with the purpose of improving the understanding of the electronic effects of fullerene photo-oxidation. We have studied spincoated thin films of PC₆₀BM by X-ray Photoelectron Spectroscopy, Near-edge X-ray Absorption Fine Structure spectroscopy, and Fourier Transform Infrared Spectroscopy, before and after exposing them to simulated sunlight in air. The changes observed in the spectra obtained by these complementary methods were compared with calculated spectra of a large set of possible oxidation products of PC₆₀BM where oxygen atoms have been attached to the C₆₀ cage. The best fit with experimental IR spectra of photodegraded PC₆₀BM films was obtained for a linear combination of calculated spectra for two degradation products, a dicarbonyl and an anhydride, both with open cages with 58 carbon atoms, and the pristine PC₆₀BM molecule. From this comparison, we conclude that the conjugation of the fullerene cage is disturbed by the formation of several carbonyl-based derivatives on the C₆₀ cage, accompanied by a transition from sp² to sp³-hybridized carbon. The π* resonance in the C1s NEXAFS spectrum was found to be a very sensitive probe for small changes to the fullerene cage, and FT-IR was needed in combination with O1s NEXAFS, to identify the oxidation products.

Controlled distribution of ZnO nanoparticles on Si/SiO2 surfacesIn recent years the properties and applications of ZnO nanomaterials has been extensively examined, partly due to the potential of ZnO as UV and visible light emitter and detector. Much of the previous work concern synthesis and growth of ZnO in different forms. In this survey, we have primarily studied commercially available nanoparticles and how they can be distributed on surfaces, aiming for future applications in e.g. photovoltaic devices. Later custom synthesised ZnO particles will be used in collaboration with Prof. Gunnar Westin at Uppsala University.ZnO nanoparticles, non-coated and organo-silane coated, with an average size of 70 nm from Alfa Aesar GmbH & Co KG, Germany, have been used in different dispersions for application on surfaces and the aim has been to achieve a controllable distribution of particles. In order to achieve this, different substrate preparations, solvents for dispersion and application methods have been used. Characterisation of the particle distribution has been done with Optical Microscopy (OM), Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The chemical and the crystallographic properties of the particles have also been investigated using Auger Electron Spectroscopy (AES) and X-ray Diffraction (XRD).The substrates used are Si(001) with different preparations to create hydrophobic or hydrophilic surfaces. Nanoparticles were applied to the substrates using drop-coating and spin-coating. Several different solvents, including water, 1,2-PMA and chloroform, have been used for dispersions, yielding the possibility to use vapour pressure and solubility parameters to control the distribution. Separated particles are observed in all of the examined samples below a certain ZnO concentration. The amount of separated particles is dependent on the surface used, the solvent and the preparation procedure. A trend that can be followed from micro-scale to nano-scale is that smaller agglomerates correspond to more separated particles. Particles of different forms have been identified for later characterization.

The surface morphology and electronic structure of hexagonal ZnO nanoplates have been studied by Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM). It was found that these nanoplates are terminated by their polar (0001) surfaces. The AFM investigation was performed in the ambient conditions with the nanocrystals “as grown”. Surprisingly, the AFM images of the top surfaces revealed an interesting triangular reconstruction, which was earlier observed only after cycles of sputtering and annealing of the ZnO(0001) surface in Ultra High Vacuum (UHV) systems. The surface atomic and electronic structures of these nanoplates have been further studied by STM and Scanning Tunneling Spectroscopy (STS) in UHV. The STM images also showed a triangular structure with single atomic steps. In addition, a 2x2 surface reconstruction has been observed with high resolution STM. This reconstruction agrees well with the recently proposed model that involves the removal of 1/4 of the topmost Zn atoms on the ZnO(0001) surface.

The understanding of surfaces of materials is of crucial importance to all of us. Considering nanocrystals (NCs), that have a large surface to bulk ratio, the surfaces become even more important. Therefore, it is important to understand the fundamental surface properties in order to use NCs efficiently in applications. In the work reported in this thesis ZnO NCs were studied.

At MAX-lab in Lund, synchrotron radiation based Spectroscopic Photoemission and Low Energy Electron Microscopy (SPELEEM) and X-ray Photoelectron Spectroscopy (XPS) were used. At Karlstad University characterization was done using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), Scanning Tunnelling Microscopy (STM), Auger Electron Spectroscopy (AES), and XPS.

The fundamental properties of ZnO surfaces were studied using distributions of ZnO NCs on SiO₂/Si surfaces. The conditions for distribution of ZnO NCs were determined to be beneficial when using ethanol as the solvent for ultrasonically treated dispersions. Annealing at 650 °C in UHV cleaned the surfaces of the ZnO NCs enough for sharp LEEM imaging and chemical characterization while no sign of de-composition was found. A flat energy band structure for the ZnO/SiO₂/Si system was proposed after 650 °C. Increasing the annealing temperature to 700 °C causes a de-composition of the ZnO that induce a downward band bending on the surfaces of ZnO NCs.

Flat ZnO NCs with predominantly polar surfaces were grown using a rapid microwave assisted process. Tuning the chemistry in the growth solution the growth was restricted to only plate-shaped crystals, i.e. a very uniform growth. The surfaces of the NCs were characterized using AFM, revealing a triangular reconstruction of the ZnO(0001) surface not seen without surface treatment at ambient conditions before. Following cycles of sputtering and annealing in UHV, we observe by STM a surface reconstruction interpreted as 2x2 with 1/4 missing Zn atoms.

Thermal cleaning in ultrahigh vacuum of ZnO nanocrystals distributed on SiO₂/Si surfaces has been studied using spectroscopic photoemission and low energy electron microscopy (SPELEEM). This study thus concern weakly bound ZnO nanocrystals covering only 5%–10% of the substrate. Chemical properties, crystallinity, and distribution of nanocrystals are used to correlate images acquired with the different techniques showing excellent correspondence. The nanocrystals are shown to be clean enough after thermal cleaning at 650 °C to be imaged by LEEM and x-ray PEEM as well as chemically analyzed by site selective x-ray photoelectron spectroscopy (μ-XPS). μ-XPS shows a sharp Zn 3d peak and resolve differences in O 1s states in oxides. The strong LEEM reflections together with the obtained chemical information indicates that the ZnO nanocrystals were thermally cleaned, but do not indicate any decomposition of the nanocrystals. μ-XPS was also used to determine the thickness of SiO₂ on Si. This article is the first to our knowledge where the versatile technique SPELEEM has been used to characterize ZnO nanocrystals.

Characterization of nano-ZnO using SPELEEM and STMLeif Ericsson1, Kjell Magnusson1 and Alexei Zakharov21) Karlstad University, Sweden, 2) MAX-lab, Lund University, SwedenFuture functional applications of nanostructured ZnO, a wide bandgap semiconductor, include optical, electronic, optoelectronic, photovoltaic and piezoelectric devices. Much of the previous research has mainly concerned synthesis and growth of ZnO in different forms. In a previous survey, we have studied commercially available nanoparticles (Alfa Aesar GmbH, Germany) with an average size of 70 nm, in dispersions spin coated on SiO2/Si(001) surfaces. Using 2-10 mg/ml in ethanol and using ultrasound, we have prepared surfaces with a varying number of separated individual nanoparticles. These particles have been characterized with SEM and AFM, to reveal variations in morphology and size, and degree of agglomeration.In the present study, we have used Low-Energy Electron Microscopy (LEEM) and Photoemission Electron Microscopy (PEEM), the latter using synchrotron radiation, to further characterize the ZnO nanoparticles in terms of crystallinity, chemical state and electronic properties. The experiments have been done at MAX-lab, Lund, Sweden, at Beamline I311, using a system supplied by ELMITEC GmbH, Germany. In addition to microscopy, X-ray Photoelectron Spectroscopy from selected areas down to 1,3 micrometer, micro-XPS, has been used, yielding detailed spectroscopic information with this degree of lateral resolution.After a low-temperature anneal at 500 °C, LEEM images from 25 micrometer show strong (0,0)-reflections from separated areas of sizes down to less than 100 nm, in accordance with expectations for the size of the ZnO particles. Different electron energies show reflections from different grains due to orientation of the nanoparticles. Further annealing at 650 °C improves the LEEM image, yielding evidence for a fair degree of crystallinity and surface cleanliness in accordance with peak intensities from micro-XPS.The chemical state and electronic properties of both ZnO particles and SiO2/Si(001) substrate were studied with micro-XPS and PEEM, yielding well-resolved peaks from Zn 3d and Si 2p, which were used to create PEEM images with a perfect complementarity between Zn 3d and Si 2p based images. micro-XPS from ZnO nanoparticle agglomerates, ZnO free SiO2/Si(001) surface as well as mixed areas has been collected, showing differences in binding energies.For characterization of atomic structure and electronic structure with yet higher lateral resolution, Scanning Tunneling Microscopy and Spectroscopy, STM-STS, has been applied and preliminary result will be presented.

ZnO nanocrystals distributed by spin-coating on SiO2/Si surfaces were annealed in UHV and studied in situ by synchrotron radiation based X-ray Photoelectron Spectroscopy. Changes in chemical composition and electronic structure of ZnO nanocrystal surfaces were found with increasing annealing temperatures. Annealing at 650 °C reduces the surface contaminant levels without any observed de-composition of ZnO. After annealing at 700 °C an initial de-composition of ZnO together with further reduction of contaminants was observed. As a result, 650 °C is found to be the optimal annealing temperature for thermal cleaning of ZnO nanocrystals. Chemical changes and induced point defect formation cause changes in the band structure of the ZnO/SiO2/Si system. An upward band bending of 0.7 eV on the surfaces of the ZnO nanocrystals was found after annealing at 300 °C. The bands on the surfaces of ZnO nanocrystals gradually bend downwards with increasing annealing temperatures. A downward band bending of 1.4 eV is the result after annealing at 750 °C for 1 h. This large downward band bending is explained as due to the change in balance of oxygen vacancies and zinc vacancies on the surfaces of ZnO nanocrystals.

Organic photovoltaic cells based on ternary blends of materials with complementary properties represent an approach to improve the photon-absorption and/or charge transport within the devices. However, the more complex nature of the ternary system, i.e. in diversity of materials' properties and morphological features, complicates the understanding of the processes behind such optimizations. Here, organic photovoltaic cells with wider absorption spectrum composed of two electron-donor polymers, F8T2, poly(9,9-dioctylfluorene-alt-bithiophene), and PTB7, poly([4,8-bis[(2'-ethylhexyl) oxy] benzo[1,2-b: 4,5-b'] dithiophene-2,6-diyl][3-fluoro-2-[(2'-ethylhexyl) carbonyl] thieno[3,4-b] thiophenediyl]), mixed with [6,6]-phenyl-C-61-butyric acid methyl ester (PC61BM) are investigated. We demonstrate an improvement of 25% in power conversion efficiency in comparison with the most efficient binary blend control devices. The active layers of these ternary cells exhibit gross phase separation, as determined by Atomic Force Microscopy (AFM) and Synchrotron-based Scanning Transmission X-ray Microscopy (STXM).

We have studied the photo-degradation in air of a blend of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (TQ1), and how the photo-degradation affects the solar cell performance. Using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, changes to the electronic structure of TQ1 and PCBM caused by illumination in ambient air are investigated and compared between the pristine materials and the blend. The NEXAFS spectra show that the unoccupied molecular orbitals of TQ1 are not significantly changed by the exposure of pristine TQ1 to light in air, whereas those of PCBM are severely affected as a result of photo-induced degradation of PCBM. Furthermore, the photo-degradation of PCBM is accelerated by blending it with TQ1. While the NEXAFS spectrum of TQ1 remains unchanged upon illumination in air, its valence band spectrum shows that the occupied molecular orbitals are weakly affected. Yet, UV-Vis absorption spectra demonstrate photo-bleaching of TQ1, which is attenuated in the presence of PCBM in blend films. Illumination of the active layer of TQ1: PCBM solar cells prior to cathode deposition causes severe losses in electrical performance.

Blend thin films, prepared by dip-coating, of polyfluorene (F8 or PFO), acting as an electron donor, and [6,6]-phenyl-C-61-butyric acid methyl ester (PC60BM), acting as the electron acceptor, have been characterized by UV/VIS absorption spectroscopy, static and dynamic fluorescence, and atomic force microscopy. Four different solvents were used for the film preparation; the monohalogenated fluorobenzene and chlorobenzene and their dihalogenated counterparts o-difluorobenzene and o-dichlorobenzene, respectively. Fluid mechanics calculations were used to determine the withdrawal speed for each solvent, in order to prepare wet films of comparable thicknesses. The resulting dry films were also of similar thicknesses. It was found that the choice of solvent influences the ability for F8 to form its beta-phase.

The electronic structures of the Sn/Si(111)-2√3×2√3 surface and deposited 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) have been studied by use of high-resolution photoelectron spectroscopy and scanning tunneling microscopy. On deposition, PTCDA molecules form a 4√3×2√3 periodicity superposed on the substrate. The new reconstruction is caused by a charge transfer between the Sn/Si(111)-2√3×2√3 surface and the molecules, as indicated by a new component of the Sn 4d core level that is shifted toward higher binding energy. In contrast to earlier reports, the charge provided by Sn is given to carbonyl C atoms instead of O atoms. This is evidenced by a new component in the C 1s core-level spectra, which is shifted toward lower binding energy. The charge transfer also induces a splitting in the highest occupied molecular orbital level of PTCDA seen in the valence band structure.