Highly Oriented Atomically Thin Ambipolar MoSe2

Transition metal dichalcogenides (TMDCs), together with other two-dimensional (2D) materials, have attracted great interest due to the unique optical and electrical properties of atomically thin layers. In order to fulfill their potential, developing large-area growth and understanding the properties of TMDCs have become crucial. Here, we have used molecular beam epitaxy (MBE) to grow atomically thin MoSe2 on GaAs(111)B. No intermediate compounds were detected at the interface of as-grown films. Careful optimization of the growth temperature can result in the growth of highly aligned films with only two possible crystalline orientations due to broken inversion symmetry. As-grown films can be transferred onto insulating substrates, allowing their optical and electrical properties to be probed. By using polymer electrolyte gating, we have achieved ambipolar transport in MBE-grown MoSe2. The temperature-dependent transport characteristics can be explained by the 2D variable-range hopping (2D-VRH) model, indicating that the transport is strongly limited by the disorder in the film.

Electrical and optical properties of Ta-Si-N thin films deposited by reactive magnetron sputtering

David Oezer, Rosendo Sanjines

The electrical and optical properties of TaxSiyNz thin films deposited by reactive magnetron sputtering from individual Ta and Si targets were studied in order to investigate the effects of nitrogen and silicon contents on both properties and their correlation to the film microstructure. Three sets of fcc-TaxSiyNz thin films were prepared: sub-stoichiometric TaxSiyN0.44, nearly stoichiometric TaxSiyN0.5, and over-stoichiometric TaxSiyN0.56. The optical properties were investigated by near-normal-incidence reflectivity and ellipsometric measurements in the optical energy range from 0.375 eV to 6.8 eV, while the d.c. electrical resistivity was measured in the van der Pauw configuration from 20 K to 300 K. The optical and electrical measurements were interpreted using the standard Drude-Lorentz model and the so-called grain boundary scattering model, respectively. The electronic properties were closely correlated with the compositional and structural modifications of the TaxSiyNz films due to variations in the stoichiometry of the fcc-TaNz system and the addition of Si atoms. According to the nitrogen and silicon contents, fcc-TaxSiyNz films can exhibit room temperature resistivity values ranging from 10(2) mu Omega cm to about 6 x 10(4) mu Omega cm. The interpretation of the experimental temperature-dependent resistivity data within the Grain Boundary Scattering model, combined with the results from optical investigations, showed that the mean electron transmission probability G and the free carriers concentration, N, are the main parameters that control the transport properties of these films. The results indicated that the correlation between electrical and optical measurements with the chemical composition and the nanostructure of the TaxSiyNz thin films provides a pertinent and consistent description of the evolution of the Ta-Si-N system from a solid solution to a nanocomposite material due to the addition of Si atoms. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4766904]

Amer Inst Physics2012

Window Layers for Silicon Heterojunction Solar Cells

Johannes Peter Seif

Currently, crystalline silicon (c-Si) wafer-based solar cells dominate the photovoltaic market (80-90%). In this thesis we concentrate on silicon heterojunction (SHJ) solar cells that--in contrast to diffused homojunction cells--rely on the application of amorphous silicon (a-Si:H) thin films. Unlike standard homojunction devices, which are typically limited by their highly recombination-active semiconductor-to-metal contacts, SHJ devices exhibit excellent surface passivation enabled by intrinsic and doped a-Si:H films. These a-Si:H layers, however, entail drawbacks for optical performance and carrier transport, two topics that will be addressed in this work. To this end we investigate non-traditional materials for SHJ devices, with the goal of replacing the a-Si:H or the transparent electrodes. These materials include microcrystalline silicon (uc-Si:H) and organic semiconductors for contact formation; amorphous silicon suboxides (a-SiOx:H) for surface passivation; and transparent electrodes applied by atomic layer deposition (ALD) as protective layers against subsequent processing steps. Along with the optical and electrical properties of these materials, we study the impact on device performance associated with their deposition. For this we test the devices under standard testing conditions (25 °C) and at elevated temperatures closer to those encountered in the field. For the investigations on uc-Si:H layers, we vary process parameters (including temperature, pressure, power, excitation frequency and hydrogen dilution) as well as pre-treatments, gas variations and nucleation layers. We assess the suitability of these approaches for SHJ solar cells and apply selected measures in devices. Thereby we demonstrate a gain in short-circuit current density in the range of 0.5-1 mA.cm-2 and good fill factor values of up to 79.2% using either n- and p-type uc-Si:H layers. Furthermore, with the goal of reducing optical losses, we test wide-bandgap a-SiOx:H layers for passivation. In terms of current gain without negative side effects on transport, we argue that these layers are best applied to the electron-collecting contact--put at the front of the device--as their application to the hole-collecting contact introduces a transport barrier for holes. This barrier deteriorates the device performance at 25 °C, but shows a beneficial effect on the temperature coefficient of the device, yielding coefficients as low as -0.1%/°C. In some cases--compared to standard devices--devices with a-SiOx:H layers exhibit superior performance at elevated temperatures, which can be of interest in warmer climates. In parallel to these main topic we also study aluminium-doped zinc oxide (ZnO:Al) layers deposited by ALD as a protective layer against sputter-induced damage and organic semiconductors as transparent electrodes for the hole-collecting contact. In both cases we observe a gain in terms of surface passivation, which indicates that these materials may be beneficial for the contact formation in future device structures. In addition to these material-related investigations, we unravel the temperature-dependence of each individual cell parameter and present a brief comparison of state-of-the-art technologies and their respective temperature dependence.

EPFL2015

High performance solid-state mesoscopic solar cells

Julian Burschka

Mesoscopic sensitized solar cells are one of the most promising third-generation photovoltaic technologies. Dye-sensitized solar cells (DSCs), imitating the photosynthesis of green plants, were the first photovoltaic devices to utilize a mesoscopic heterojunction for the conversion of solar irradiation into electrical power. Solid-state dye-sensitized solar cells (ssDSCs), that employ an organic hole-transporting material in place of a liquid redox electrolyte, have evolved as viable contenders to conventional liquid DSCs. A typical ssDSC is composed of a mesostructured wide-bandgap metal oxide semiconductor that is sensitized with a light-absorbing chromophore and infiltrated with a molecular organic hole-transporter, usually by solution-processing. In this device, photoexcitation of the sensitizer and subsequent ultrafast electron injection into the conduction band of the metal oxide semiconductor is followed by hole transfer from the oxidized sensitizer to the organic hole-transporter. Charge transport of both the electron and the hole through the two bicontinuous phases, followed by charge migration through the external circuit, completes the photovoltaic operation of the cell. Despite more than 15 years of development, ssDSCs have always been lagging behind their liquid counterparts in terms of both power conversion efficiency and long-term stability. My thesis presents three different approaches that are aimed at contributing to the development of high-performance solid-state mesoscopic solar cells. Firstly, I present a new class of Co(III) complexes as solution-processable p-type dopants for triarylamine-based hole-conductors such as the commonly employed 2,2’,7,7’-tetrakis-N,N-di-para-methoxyphenylamine-9,9’-spirobifluorene (spiro-MeOTAD). The proposed Co(III) complexes were characterized in detail using optical and electrochemical techniques. The application of the new p-dopants in ssDSC rendered it possible to directly relate the conductivity of the doped hole-transporter to the photovoltaic performance of the devices. The work shows that chemical p-doping is a powerful tool to control the charge transport properties of spiro-MeOTAD in ssDSCs, capable of replacing the commonly employed photo-doping, i.e. the light-assisted one-electron oxidation of spiro-MeOTAD by ambient oxygen. Combining this strategy with a state–of–the–art organic D–π–A sensitizer allowed us to achieve power conversion efficiencies of up to 7.2%—a new record for this kind of device architecture. Secondly, I investigated a series of new molecular hole-transporters based on the triarylamine-substituted 9,9’-spirobifluorene core. In total, we synthesized seven new materials and characterized them in detail using electrical and electrochemical techniques. Organic-field effect transistors were fabricated employing these materials in order to evaluate their hole mobilities. From the comparison of different substituents on the hole-conducting triarylamine moieties, a structure–property relationship was derived, highlighting the importance of processability and hole mobility of the hole-transporting material for an efficient application in a ssDSC configuration. We encountered the purity of the new materials as a key parameter for a proper functioning of the device and identified the use of functionalized metal-scavenging polystyrene beads as a valuable purification step. Thirdly, a new technique is introduced that allows to realize functional composites of a mesoporous metal oxide and the organic–inorganic hybrid perovskite CH3NH3PbI3. CH3NH3PbI3 has recently attracted great attention as a light-harvesting pigment in mesoscopic solar cells. We propose a two-step technique to deposit the perovskite absorber: PbI2 is first applied on a substrate by solution-processing and subsequently exposed to a solution of CH3NH3I. It was evidenced that the desired hybrid perovskite forms within seconds after contacting the two precursors. Scanning electron microscopy was employed to elucidate the changes in crystal morphology during the transformation reaction. The kinetics of the reaction was investigated in detail using X-ray diffraction and optical spectroscopy. Employing the two-step technique for the fabrication of mesoscopic perovskite-based solar cells enabled new record power conversion efficiencies of up to 15%. By following three different strategies, this thesis could contribute significantly to the development of high-performance solid-state mesoscopic solar cells. The power conversion efficiencies of 7.2% and 15%, achieved with a molecular dye and an organic–inorganic perovskite pigment, respectively, represent important milestones for the design of third-generation photovoltaic cells that hopefully one day can compete with the conventional silicon-based solar cell technology.

EPFL2013