Many different photovoltaic technologies are being developed for large-scale solar energy conversion. The wafer-based first-generation photovoltaic devices have been followed by thin-film solid semiconductor absorber layers sandwiched between two charge-selective contacts and nanostructured (or mesostructured) solar cells that rely on a distributed heterojunction to generate charge and to transport positive and negative charges in spatially separated phases. Although many materials have been used in nanostructured devices, the goal of attaining high-efficiency thin-film solar cells in such a way has yet to be achieved. Organometal halide perovskites have recently emerged as a promising material for high-efficiency nanostructured devices. Here we show that nanostructuring is not necessary to achieve high efficiencies with this material: a simple planar heterojunction solar cell incorporating vapour-deposited perovskite as the absorbing layer can have solar-to-electrical power conversion efficiencies of over 15 per cent (as measured under simulated full sunlight). This demonstrates that perovskite absorbers can function at the highest efficiencies in simplified device architectures, without the need for complex nanostructures.

Efficient planar heterojunction perovskite solar cells by vapour deposition

Physical properties of ZnO:F obtained from a fresh and aged solution of zinc acetate and zinc acetylacetonate

Nanoscale metal/oxide/metal switches have the potential to transform the market for nonvolatile memory and could lead to novel forms of computing. However, progress has been delayed by difficulties in understanding and controlling the coupled electronic and ionic phenomena that dominate the behaviour of nanoscale oxide devices. An analytic theory of the ‘memristor’ (memory-resistor) was first developed from fundamental symmetry arguments in 1971, and we recently showed that memristor behaviour can naturally explain such coupled electron–ion dynamics. Here we provide experimental evidence to support this general model of memristive electrical switching in oxide systems. We have built micro- and nanoscale TiO2 junction devices with platinum electrodes that exhibit fast bipolar nonvolatile switching. We demonstrate that switching involves changes to the electronic barrier at the Pt/TiO2 interface due to the drift of positively charged oxygen vacancies under an applied electric field. Vacancy drift towards the interface creates conducting channels that shunt, or short-circuit, the electronic barrier to switch ON. The drift of vacancies away from the interface annilihilates such channels, recovering the electronic barrier to switch OFF. Using this model we have built TiO2 crosspoints with engineered oxygen vacancy profiles that predictively control the switching polarity and conductance. Nanoscale metal/oxide/metal devices that are capable of fast non-volatile switching have been built from platinum and titanium dioxide. The devices could have applications in ultrahigh density memory cells and novel forms of computing.

Memristive switching mechanism for metal/oxide/metal nanodevices.

Transparent conductors—A status review

Stability/degradation of polymer solar cells

Thin film phenomena

Thin film solar cells (TFSC) are a promising approach for terrestrial and space photovoltaics and offer a wide variety of choices in terms of the device design and fabrication. A variety of substrates (flexible or rigid, metal or insulator) can be used for deposition of different layers (contact, buffer, absorber, reflector, etc.) using different techniques (PVD, CVD, ECD, plasma-based, hybrid, etc.). Such versatility allows tailoring and engineering of the layers in order to improve device performance. For large-area devices required for realistic applications, thin-film device fabrication becomes complex and requires proper control over the entire process sequence. Proper understanding of thin-film deposition processes can help in achieving high-efficiency devices over large areas, as has been demonstrated commercially for different cells. Research and development in new, exotic and simple materials and devices, and innovative, but simple manufacturing processes need to be pursued in a focussed manner. Which cell(s) and which technologies will ultimately succeed commercially continue to be anybody's guess, but it would surely be determined by the simplicity of manufacturability and the cost per reliable watt. Cheap and moderately efficient TFSC are expected to receive a due commercial place under the sun.

Thin-film solar cells: an overview

Why Thin Film Solar Cells?- Basic Physical Processes in Solar Cell Materials- Photovoltaic Behavior of Junctions- Photovoltaic Measurements, Junction Analysis, and Material Characterization- Thin Film Deposition Techniques- Properties of Thin Films for Solar Cells- Cu2S Based Solar Cells- Polycrystalline Thin Film Silicon Solar Cells- Emerging Solar Cells- Amorphous Silicon Solar Cells- Photoelectrochemical Cells- Novel Concepts in Design of High-Efficiency Solar Cells

Thin Film Solar Cells

The effect of hydrogen plasma treatment on indium tin oxide (ITO), fluorine‐doped tin oxide (FTO), and indium‐doped zinc oxide (IZO) films has been studied. X‐ray photoelectron spectroscopy analysis shows that ITO and FTO surfaces get reduced to yield elemental indium and tin, respectively. Annealing of the plasma treated films in air leads to re‐oxidation of the reduced surface and the electro‐optical properties are recovered. In contrast, IZO films are not reduced by plasma treatment and show no changes in the electrical and optical properties. The surface of plasma treated IZO films shows a higher binding energy O(1s) peak probably due to OH or OH...O species which appear to form a protective layer against plasma degradation.

K. L. Chopra

Papers

Transparent conductors—A status review

Thin film phenomena

Thin-film solar cells: an overview

Thin Film Solar Cells

Effect of hydrogen plasma treatment on transparent conducting oxides