R. Könenkamp

Vertical nanowire light-emitting diode

We report room-temperature, white-color electroluminescence in vertically oriented ZnO nanowires. Excitonic luminescence around 380 nm is observed as a shoulder on a broader defect-related band covering all of the visible range and centered at 620 nm. The ZnO nanowires are grown in a low-temperature process on SnO2-coated glass substrates, employing a technique that is suitable for large-area applications. The nanowires are robustly encapsulated in a thin polystyrene film deposited from high-molecular-weight solutions. Electron injection occurs through the transparent SnO2 layer, while hole injection is mediated by a p-doped polymer and an evaporated Au contact. Stable device operation is observed at ambient conditions on the time scale of 1 h.

Flexible inorganic nanowire light-emitting diode.

We report a highly flexible light-emitting device in which inorganic nanowires are the optically active components. The single-crystalline ZnO nanowires are grown at 80 degrees C on flexible polymer-based indium-tin-oxide-coated substrates and subsequently encapsulated in a minimal-thickness, void-filling polystyrene film. A reflective top contact serving as the anode in the diode structure is provided by a strongly doped p-type polymer and an evaporated Au film. The emission through the polymer side of this arrangement covers most of the visual region. Electrical and optical properties as well as performance limitations of the device structure are discussed.

Thin film semiconductor deposition on free-standing ZnO columns

We report the deposition of a-Si:H on thin films of free-standing single crystalline ZnO columns. The ZnO columns have a height of several μm and a diameter between 100 and 200 nm. The ZnO films are prepared in electrodeposition and have considerable potential for use in photoelectric thin film devices. Morphology, electronic parameters, and basic optical behavior, such as reflectance and light trapping efficiency, are reported. Amorphous silicon is deposited on the columns as a continuous smooth film with conformal coverage. Some possibilities of using these films in devices are discussed.

Solar cell with extremely thin absorber on highly structured substrate

We report the optical, structural and photovoltaic properties of an extremely-thin-absorber (eta) solar cell on highly porous TiO2 substrates. Nano-crystalline absorber layers with a local thickness of only 150 nm have been prepared. This small absorber thickness allows good carrier collection even for absorber material with poor transport properties. The morphology of the highly structured TiO2 substrate produces strong internal light scattering, resulting in an enhancement of the optical path length by a factor 5. With 150 nm thick CdTe absorber layers, the eta solar cells produce an open-circuit voltage of 0.67 V and a short-circuit current of 8.9 mA cm−2 for 100 mW cm−2 illumination. Alloying the CdTe with Hg improves the short-circuit current to 15 mA cm−2.

The eta-solar cell with CuInS2: A photovoltaic cell concept using an extremely thin absorber (eta)

Diffusion length of charge carriers within the absorbing material is one of the important restricting properties for the efficiency of solar cell devices. A new cell design using an extremely thin absorber (eta-solar cell) is prepared to obtain an effective separation of charge carriers within the depletion layer. It could be figured out that the properties of CuInS2 (CIS) strongly depend on the porosity of the base layer. Multiple scattering within the porous structure is evident. Moreover, it can be demonstrated that there is a maximum in short-circuit current density for a medium thickness of the absorbing layer.

Hexagonal nanotubes of ZnS by chemical conversion of monocrystalline ZnO columns

Monocrystalline ZnO columns grown in electrodeposition were converted to ZnS using ion exchange reactions in H2S or S vapor. At ∼400 °C the reaction with H2S only affects a thin layer of 10–30 nm thickness at the surface of the ZnO crystallites, and ZnS-coated ZnO columns are produced. Exploiting the large difference in etch resistance between ZnS and ZnO, the ZnO core of the columns can be removed, and a tubular structure of ZnS can be prepared. Typical dimensions of the ZnS tubes are a length of 1–3 μm, a diameter of 100–300 nm, and a wall thickness of 10–30 nm. The ZnS tubes have the same distribution, alignment, and surface morphology as the original ZnO columns. The reaction in S vapor is suitable to produce solid ZnS columns.

Nano-structures for solar cells with extremely thin absorbers

Abstract We have developed several methods to prepare nano-structured semiconductor and conducting oxide layers for large area applications. Due to their unique optical and electrical properties these layers have considerable potential for the fabrication of a novel solar cell type with an extremely thin absorber (eta-cell). Preparation methods, structural and optical properties are reported.

Contacts to a solar cell with extremely thin CdTe absorber

Abstract The concept for a solar cell with extremely thin absorber (eta-cell) comprises a porous TiO2 substrate covered by a CdTe film with a local thickness of 150–250 nm. At the present stage of development this type of cell exhibits a photovoltage between 600 and 700 mV and a photocurrent of 5–9 mA/cm2 under AM1.5 illumination, indicating the feasibility of the concept. The fill factor is, however, only in the range of 20%, due to a strong voltage-dependence of the photocurrent. This paper discusses the influence of the front and back contacts on the limited cell performance and shows that the insertion of a buffer layer at the front contact gives an improved fill-factor.

A novel deposition technique for compound semiconductors on highly porous substrates: ILGAR

Abstract ILGAR (ion layer gas reaction), a novel low-cost technology for the preparation of sulfidic thin layers is described, which can be analogously applied for other chalcogenides. The process consists of three steps: (1) application of a precursor solution on a substrate by dipping or spraying, (2) drying in an inert gas stream, (3) sulfurization of the solid precursor (e.g. a metal halide) by hydrogen sulfide gas. This cycle is repeated until the desired layer thickness is obtained. Not only on smooth, but also on structured and porous substrates the method allows the deposition of homogenous thin films following the microscopic structure, where other methods often have problems with shading. Once the film is closed, the growth per dip cycle is constant and reproducible during the process. The binary compounds CdS, Cu 2 S, In 2 S 3 and also the ternary CuInS 2 have been prepared by ILGAR on glass and on porous TiO 2 or SiO 2 . The layers were characterised by XRD, SEM and EDX.

Vertical nanowire transistor in flexible polymer foil

Fabrication and operation of a vertical nanowire field-effect transistor is reported. The device is prepared by growing vertical wires in the cylindrical pores of a polymer foil stack. The nanowire diameter is approximately 100 nm, the packing density up to 108 cm−2. The polymer foil stack consists of two polymer layers and an intermediate metal layer. Cylindrical holes are prepared in this stack by using fast ion irradiation and subsequent etching. Well-defined cylindrical openings with diameters between 50 and 150 nm are obtained. The semiconductor growth involves electrodeposition of the p-type quaternary compound CuSCN. Electrical measurements on first devices show transistor action with some gate leakage, which may be improved in future designs. The arrangement of inorganic device material embedded in soft polymer matrix is structurally robust, and the devices show low sensitivity to mechanical strain of the foil. Single electron effects may be expected in these devices, when the dimensions are furth...