The first observations of the photoelectric effect date back to the early 19th century from work by Alexandre Edmond Becquerel, Heinrich Hertz, Wilhelm Hallwachs and J J Thomson. The theory behind the phenomena was clarified in a seminal paper by Einstein in 1905 and became an archetypical feature of the wave–particle description of light. A different manifestation of quantised electron excitation, whereby electrons are not emitted but excited into the valence band of the material, is what we call the photoconductive effect. As well as providing an extension to theories in fundamental physics, the phenomenon has spawned a field with enormous ramifications in the energy industry through the development of solar cells.Among advances in photovoltaic technology has been the development of organic photovoltaic technology. These devices have many benefits over their inorganic counterparts, such as light-weight, flexible material properties, as well as versatile materials' synthesis and low-cost large-scale production—all highly advantageous for manufacturing. The first organic photovoltaic systems were reported over 50 years ago [1], but the potential of the field has escalated in recent years in terms of efficiency, largely through band offsetting. Since then, great progress has been made in studies for optimising the efficiency of organic solar cells, such as the work by researchers in Germany and the Netherlands, where investigations were made into the percentage composition and annealing effects on composites of poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) [2].Hybrid devices that aim to exploit the advantages of both inorganic and organic constituents have also proven promising. One example of this is the work reported by researchers in Tunisia and France on a systematic study for optimising the composition morphology of TiO₂ nanoparticles in poly(N-vinylcarbazole) (PVK), which also led to insights into the charge transport mechanism and trap distribution in these composites [3]. An advantage of investigating solar cell technology based on organic materials rather than silicon is that silicon photovoltaics requires high-purity silicon, whereas the material demands of organic technology are not nearly so strict. Work by researchers in Denmark and Germany highlights the simplicity and tolerance to ambient conditions of organic photovoltaic fabrication in the demonstration of a nanostructured polymer solar cell made from a thermocleavable polymer material and zinc oxide nanoparticles. All the manipulations during device preparation could be carried out in air at around 20 °C and 35% humidity [4].A possible route to enhancing cell performance is through the improvment of the transport efficiency. Researchers in Taiwan demonstrate how effectively this can be implemented in a hybrid device comprising TiO₂ nanorods and poly[2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) [5]. In addition, inorganic semiconductor nanocrystals that have tunable optical bandgaps can be combined with organic semiconductors for the fabrication of hybrid photovoltaic devices with broad spectral sensitivity. A collaboration of researchers in the UK and the US has now developed a near-infrared sensitive hybrid photovoltaic system with PbS nanocrystals and C60. The reported improvement in device performance is attributed to increased carrier mobility of the PbS nanocrystal film [6]. In this issue, Patrick G Nicholson and Fernando A Castro from the National Physical Laboratory in the UK present a topical review on the principles and techniques for the characterization of organic photovoltaics [7]. The review presents a comprehensive picture of the current state-of-the-art understanding of the working mechanisms behind organic solar cells, and also describes electronic morphological considerations relevant to optimizing the devices, as well as different nanoscale techniques for investigating organic solar cell technology. In spring 2011, Nanotechnology launches a new section wholly dedicated to the coverage of new and stimulating research into energy sources based on nanoscale science and technology. There is at present considerable concern over how to fuel the planet in a sustainable manner with the increasingly energy-thirsty human population. Yet the Earth receives more solar energy in one hour than the world population consumes in one year [8]. No wonder research into photovoltaics and ways of increasing the efficiency with which this energy can be harnessed continues to hold so much fascination.References[1] Levin I and White C E 1949 J. Chem. Phys. 18 417[2] Chirvase D, Parisi J, Hummelen J C and Dyakonov V 2004 Nanotechnology 15 1317[3] Kwong C Y, Choy W C H, Djurišić A B, Chui P C, Cheng K W and Chan W K 2004 Nanotechnology 15 1156[4] Krebs F C, Thomann Y, Thomann R and Andreasen J W 2008 Nanotechnology 19 424013[5] Zeng T-W, Lin Y-Y, Lo H-H, Chen C-W, Chen C-H, Liou S-C, Huang H-Y and Su W-F 2006 Nanotechnology 17 5387[6] Dissanayake D M N M, Hatton R A, Lutz T, Curry R J and Silva S R P 2009 Nanotechnology 20 245202[7] Nicholson P G and Castro F A 2010 Nanotechnology 21 492001[8] http://www.solarenergyworld.org/solar-energy-facts/