Lukas Schmidt-Mende

ZnO - nanostructures, defects, and devices

ZnO has received much attention over the past few years because it has a wide range of properties that depend on doping, including a range of conductivity from metallic to insulating (including n-type and p-type conductivity), high transparency, piezoelectricity, wide-bandgap semiconductivity, room-temperature ferromagnetism, and huge magneto-optic and chemical-sensing effects. Without much effort, it can be grown in many different nanoscale forms, thus allowing various novel devices to be achieved. We review recent studies of ZnO nanostructures, fabrication, novel device applications, and its potential as an electron-acceptor material in hybrid solar cells. Control of its rich defect chemistry, which is critical for controlling properties but has not been widely addressed in the context of novel applications, is also discussed.

Photocatalytic Reduction of CO2 on TiO2 and Other Semiconductors

Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the ensuing effects on the global climate and future energy supply. Utilizing the abundant solar energy to convert CO2 into fuels such as methane or methanol could address both problems simultaneously as well as provide a convenient means of energy storage. In this Review, current approaches for the heterogeneous photocatalytic reduction of CO2 on TiO2 and other metal oxide, oxynitride, sulfide, and phosphide semiconductors are presented. Research in this field is focused primarily on the development of novel nanostructured photocatalytic materials and on the investigation of the mechanism of the process, from light absorption through charge separation and transport to CO2 reduction pathways. The measures used to quantify the efficiency of the process are also discussed in detail.

Nanostructured Organic and Hybrid Solar Cells

This Progress Report highlights recent developments in nanostructured organic and hybrid solar cells. The authors discuss novel approaches to control the film morphology in fully organic solar cells and the design of nanostructured hybrid solar cells. The motivation and recent results concerning fabrication and effects on device physics are emphasized. The aim of this review is not to give a summary of all recent results in organic and hybrid solar cells, but rather to focus on the fabrication, device physics, and light trapping properties of nanostructured organic and hybrid devices.

Control of dark current in photoelectrochemical (TiO2/I−–I3−) and dye-sensitized solar cells

The ruthenium complex bis-tetrabutylammonium cis-dithiocyanato-N,N-bis-2,2-bipyridine-4-carboxylic acid, 4-carboxylate ruthenium(II), N-719, was found to block the dark current of dye sensitized solar cells (DSC), based on mesoporous TiO2 films deposited on a F-doped tin oxide electrode and the effect was compared to surface treatment by TiCl4 and the introduction of a compact TiO2 blocking layer.

Strong Efficiency Improvements in Ultra-low-Cost Inorganic Nanowire Solar Cells

The need for sustainable power generation has encouraged research into a variety of photovoltaic materials and structures, with a greater emphasis being placed on a balance between performance and cost. The stability of many semiconducting oxides relative to other inexpensive solar cell technologies, such as organic [ 1 ] and dye-sensitized [ 2 ] cells, makes them an attractive alternative. Yet low-cost, non-toxic, inorganic solar cell technologies have received comparatively little attention. In a recent report, nine inorganic semiconductors were identifi ed as having both the potential for annual electricity production in excess of worldwide demand and material extraction costs less than that of crystalline silicon. [ 3 ] Further to materials costs, a recent study examined the high cost of modern vacuum deposition methods and highlighted the need for low-temperature, atmospheric, solution-based synthesis. [ 4 ] Solution-based synthesis of several of the nine, promising inorganic materials has been demonstrated previously. [ 5–7 ] Copper (I) oxide (Cu 2 O), in particular, has been synthesized extensively in polycrystalline form by electrodeposition from solutions near room temperature. [ 5 , 8 , 9 ]

A simple low temperature synthesis route for ZnO-MgO core-shell nanowires

We report a hydrothermal synthesis method for MgO shell coatings directly onto the surface of ZnO nanowire arrays. The entire process can be carried out below 100u2009°C. The MgO shells are produced by the addition of 10xa0mM magnesium nitrate with 0.2xa0M sodium hydroxide in water, resulting in a shell thickness of up to 8xa0nm, verified by high resolution transmission electron microscopy. The viability of the MgO layer as a functional element of optoelectronic devices was tested on solid-state organic hole-transporter based dye-sensitized solar cells. Incorporation of the MgO shell into the solar cell resulted in substantive efficiency improvements of over 400% in comparison to the pristine ZnO nanowire based photovoltaics, indicating that electrons can efficiently tunnel through the insulating MgO shell.

Nanostructured interfaces in polymer solar cells

The morphology in organic photovoltaics plays a key role in determining the device efficiency. We propose a method to fabricate bilayer devices with controlled nanostructured interfaces by combining nanoimprinting and lamination techniques. This technique allows us to achieve a network structure of donor-acceptor material with a ∼80u2002nm periodicity and ∼40u2002nm width. These structures have an abrupt interface between the donor and acceptor materials and show an increased effective interfacial area and photovoltaic performance compared to bilayer solar cells. In contrast to blend films, they will allow an in depth analysis of the influence of morphology on interfacial physical processes.

Highly absorbing solar cells - a survey of plasmonic nanostructures

Plasmonic light trapping in thin film solar cells is investigated using full-wave electromagnetic simulations. Light absorption in the semiconductor layer with three standard plasmonic solar cell geometries is compared to absorption in a flat layer. We identify near-field absorption enhancement due to the excitation of localized surface plasmons but find that it is not necessary for strong light trapping in these configurations: significant enhancements are also found if the real metal is replaced by a perfect conductor, where scattering is the only available enhancement mechanism. The absorption in a 60 nm thick organic semiconductor film is found to be enhanced by up to 19% using dispersed silver nanoparticles, and up to 13% using a nanostructured electrode. External in-scattering nanoparticles strongly limit semiconductor absorption via back-reflection.

Efficient organic photovoltaics from soluble discotic liquid crystalline materials

Two different types of soluble discotic liquid crystalline materials and a crystalline perylene dye have been used to create, directly from solution, photovoltaic devices which are compared in this work. Self-organisation of the soluble electron-accepting perylene derivative and the soluble liquid crystalline (LC) discotic material which is stable in a LC phase at room temperature (HBC-PhC12) leads to segregated structures optimised for charge separation and transport in photovoltaic device structures. High external quantum efficiencies up to 34% near 490nm have been reached. The high efficiencies result from efficient photo-induced charge transfer between the materials as well as effective transport of electrons and holes to the cathode and anode through segregated perylene and the discotic peri-hexabenzocoronene p-system. Atomic force microscopy and device characteristics suggest that the driving force for phase separation and surface energy effects during spin coating of the HBC-PhC12:perylene blend result in a spontaneous vertical segregation of the HBC and the perylene normal to the plane of the spun film. This represents a nearly ideal, self-organised structure in which vertical segregation of charge transport layers coexist with a high interfacial area between the two charge transfer components. This vertical segregation has not been observed in the spin-coated blends where the HBC-PhC12 is replaced by HBC-C8∗. One probable reason for this may be the different phase stability of the LC phase in the HBCs, which leads to different film-forming properties and film morphologies.

Organic thin film photovoltaic devices from discotic materials

Since the first demonstration of organic photovoltaic devices much progress has been made. Organic solar cells reach now power conversion efficiencies of up to 3% over the solar spectrum. The morphology of the active film is very important for efficient devices. Films spin-coated from blend solutions phase separate. The scale of the phase separation depends on the solvent, solubility of the materials and parameters of the spin-coating process such as speed, temperature, etc. If the morphology could be controlled on a molecular scale the efficiency of charge separation and transport could be expected to be substantially higher. The use of discotic liquid crystalline materials might help to reach this goal, because of their capacity to self-organise into columnar stacks. In this work we describe photovoltaic devices made with discotic liquid crystalline hexabenzocoronene and perylene dye molecules. Thin films have been produced by spin coating blends directly from solution. Devices with an external quantum efficiency (incident photon to current efficiency) of up to 34% at monochromatic illumination at 490 nm have been achieved with a blend of hexaphenyl-substituted hexabenzocoronene (HBC-PhC 12 ) and a perylene diimide. Photovoltaic devices with other hexabenzocoronene derivatives as hole conductor show lower efficiencies. We attribute the lower device performance of the latter to the different film morphology occurring from spin coating of these materials. The aim of this work is to exploit the advantageous self-organising properties of HBC-perylene blends for solar cells.