Towards new efficient dye-sensitised solar cells Julien Preat, * Denis Jacquemin and Eric A. Perp ete Received 8th January 2010, Accepted 9th March 2010 First published as an Advance Article on the web 14th May 2010 DOI: 10.1039/c000474j The technology to convert sunlight into electric current by employing organic photovoltaic systems has been an issue of an intensive research over the last two decades and nowadays receives an increasing industrial interest. Dye-sensitised solar cells (DSSCs) provide a technically and economically credible alternative to the present well-known p–n junction or thin film photovoltaic devices. In this Perspective, we review the basic concepts of DSSCs and we specifically discuss the recent theoretical research aimed at optimising DSSC’s properties, as well as several of the controversial and emerging issues in this field. 1. Introduction The present energetic and environmental crisis has stimulated interest in the design of renewable energy sources. Indeed, with the foreseen 10 13 watts of new power needed for the coming 50 years, the carbon dioxide concentration resulting from the fossil fuel-based energy generation will exceed the current level (400 ppm) that should be maintained if not decreased. The development of carbon-free sources of energy that are also scalable in order to meet increasing societal demands is probably the most important scientific challenge of this century. In this framework, solar photovoltaic devices are likely to be leading technologies in a promising ‘‘low-carbon level’’ future. 1–8 The photovoltaic effect, i.e. the conversion of light into elec- tricity, was first reported in the work of Chaplin and coworkers 9 in which a silicon-based single p–n junction device showed a Solar Power Conversion Efficiency (SPCE) 10,11 of about 6%. This innovative study led to the first-generation photovoltaics that currently dominate the solar technologies market as wafer-size single-junction cells based on crystalline silicon assemblies. This generation is expensive but is very efficient in terms of conversion (with a maximal SPCE of 33%). Though in 2007, first-generation solar cells accounted for 89.6% of the commercial production, the is market share is now declining. The manufacturing processes that are used to produce these cells are inherently expensive, meaning that they may take years to pay for their purchasing and energetic costs. Nevertheless, major efforts have been performed in order to significantly reduce the cost of power production by : (i) increasing the SPCE; (ii) modifying the semiconductor (SC) nature in order to reduce the amount of light-absorbing compounds and; (iii) lowering the assembly cost of the solar cells modules. 4,12–15 Second-generation solar cells (also referred to as thin film tech- nology) which were under intense development in the 1990s and early 2000s, are significantly cheaper to produce than first-gener- ation cells, but present lower efficiencies. These are most frequently associated with thin film solar cells, that use minimal materials and cheap manufacturing processes. The most popular materials used for this second-generation designs are copper, indium, gallium, selenide and cadmium telluride (e.g. II–VI semiconductors), combined with amorphous silicon and micromorphous silicon. Whilst the controlled production of second-generation cells remains a challenge, their commercial use is still growing, due to one of the great advantages of thin films: their flexibility. Also, this thin film technology has spurred lightweight, aesthetically pleasing solar innovations such as solar shingles or solar panels that can be rolled out onto a roof or on other surfaces. 16,17 Concurrently, over the past 20 years, the science and design of organic semiconducting materials has progressed very rapidly, leading to the production of a large panel of organic-based solid- state devices with multiple applications like light-emitting diodes, 18 photodiodes and transistors, 19,20 as well as photovoltaic cells. 21 The ability to significantly modify the chemical and physical properties of organic compounds by a fine tuning of their structures constitutes the principal reason for expanding research and increased industrial interest in organic photo- voltaics. The organic-based approach, and all those which are not directly related to a single p–n junction are referred to as third-generation devices and they include: (i) multijunction cells derived from group IV and III–V semiconductors; 22 (ii) hybrid approaches that are related to the combination of National Fund for Scientific Research, Unit e de Chimie Physique Th eorique et Structurale (UCPTS), Facult es Universitaires Notre-Dame de la Paix (FUNDP), rue de Bruxelles, 61, 5000 Namur, Belgium. E-mail: julien.preat@fundp.ac.be Julien Preat Julien Preat is currently a post- doctoral researcher of the Belgian National Fund for Scientific Research (FNRS) at the University of Namur. From 2008 to 2009, he was a post- doctoral fellow in the Depart- ment of Innovation in Materials and Molecular Engineering at the Universitat Polit ecnica de Catalunya in Barcelona. He obtained his BA, MS and PhD in chemistry from the University of Namur. His research interests and expertise currently center on the theoretical investigation of the third-generation DSSCs at the QM, MM and MD level. This journal is ª The Royal Society of Chemistry 2010 Energy Environ. Sci., 2010, 3, 891–904 | 891 PERSPECTIVE www.rsc.org/ees | Energy & Environmental Science Published on 14 May 2010. Downloaded by University of Saskatchewan on 31/07/2013 17:06:04. View Article Online / Journal Homepage / Table of Contents for this issue