ISOPHOS 2008

European Photovoltaic Industry Association (EPIA) identifies the development of new materials and concepts as a crucial step for achieving the goals of 2020, besides increasing the efficiency of silicon modules. Among these new concepts, particular emphasis has been given to organic cells and in particular to Dye Sensitized Solar Cells (DSC). EPIA considers the latter as the most mature amongst the organic technologies and thus pushes for a decisive plan for its research and development that can deliver the first production lines in the next future.Compared to traditional photovoltaics, DSC has several differentiation advantages, such as low dependence on angle of light, stable operating voltage in all light conditions, optional transparency.

Moving from classical semiconductors, in the last years the search for new materials to maximize photovoltaic output and reduce manufacturing costs has been more and more extended into the field of organic molecules and polymers, which offer several technological advantages compared to classical semiconductors.

To better understand the specific characteristics of DSC technology and potentialities with respect to silicon, a synthetic overview of the characteristics of solar radiation and cell performances measurements is given.

Part of the industry sector is indeed involved in moving market understanding away from the ‘Watt Peak’ promotion used by the traditional silicon industry to real time performance rating of actual photovoltaic products. Manufacturers of standard silicon solar panels have educated the market to understand performance rated under laboratory conditions at controlled temperature and artificial illumination. However, under normal operating conditions, true performance of silicon solar cells is considerably lower than “Wp” and is characterized by the number of kWh produced per annum (kWh/SqM/annum).

After a general presentation of the current trends in organic photovoltaics, the main relevant aspect of DSC technology will be discussed, with a special attention to the specific characterization issues of DSC recently underlined in the literature.

A comparison between the morphologies of fully organic and
dye-sensitized solar cells shows the advantages and disadvantages of
these two different type of solar cell structures. The aim will be to
overcome the disadvantages, which directed our research towards
nanostructured solar cells. The control of the morphology will be a key
issue for further improvements. Approaches which might allow to achieve
this control will be discussed. Especially we will focus on different
metal oxide nanostructures, their synthesis and possible application in
hybrid solar cells.

The advancement of low cost photovoltaics is a key step towards a clean future energy technology. Solar energy systems harness the inexhaustible resource of sunlight and can reduce the impact of global warming by reducing fossil-fuel consumptions and the related pollution. In order to meet the worldwide energy demands with photovoltaic energy, substantial improvements must be achieved in areas of cost processability of the devices and power-conversion efficiency and here Nanosciences and Nanotechnologies will play a crucial role. The ability to manipulate the structure and composition at the nano-scale opens new horizons and opportunities to create materials with superior performance. The novel chemical, physical, optical and electronic properties of nanocomposite materials will enable photovoltaic technologies based on them to reach in the long term new levels of performance and cost effectiveness that could never be paralleled by the existing traditional technologies. This can succeed if we will able to make novel devices based on purposely nanostructured materials with controlled architectures in a cost effective manner. In this lecturer novel hybrid organic-inorganic solar cells (HSC) based on spherical, branched and iperbranched Colloidal Nanocrystals blended with polymeric materials will be presented. The fundamental working mechanisms of a HSC cell, the state of the art and the activities carried out at the National Nanotechnology Laboratory of CNR-INFM will be reported.

We discuss some physical and photoelectrochemical techniques that are useful for understanding and improving the electronic processes in nanostructured semiconductors, dye-sensitized solar cells (DSC), and all organic devices (plastic solar cells, OLEDs). Carrier transport in networks of semiconductor particles shielded with an ionic or hole conductor is discussed, in relation with DSC. We consider mainly impedance spectroscopy and related techniques (IMPS, time transients). In these methods the nanostructured electrode is accessed from the contacts, manipulating Fermi level, or electronic current. The result of the measurement by a certain peturbation, is a global property of the nanostructured assembly. We first provide a tutorial presentation of the meaning of the central time constants that determine the kinetic processess in the solar cell: the transit time (related to the diffusion coefficient), and the lifetime. We show how to access fundamental electronic-kinetic parameters from the measurement, as well as device characterisctics, using appropriate models. We revise the diffusion model, the modification due to presence of traps, the chemical capacitance, and the electron diffusion length. We comment the application of these methods to interpret experimental results on dye-sensitized solar cells, and how the methods are used for fundamental research and also for technological application.

Science and technology ever seek, on the one hand, to see and understand matter at progressively smaller scale and, on the other hand, to build structures of progressively smaller size. Nanotechnology indicates the ability to individually address, control, and modify structures, materials and devices with nanometre precision, and the synthesis of such structures into systems of micro- and macroscopic dimensions such as MEMS based devices. By manipulating the arrangements and bonding of atoms, materials can be today designed and realized with a far wider range of physical, chemical and biological properties than has been previously conceived. In such a context, it is fundamental to characterize the relationship between starting composition, which can be controlled, with the resulting structure and properties of a nanoscale-designed material that has superior and unique performance. Microscopy is then essential to the development of nanotechnology, serving as its eyes and hands.
The growth of nanotechnology is really impressive around the world and the importance of microscopy will continue to increase, being undoubtedly the most powerful and intriguing engine for discovery and fundamental understanding of nanoscale phenomena and structures. The lecture will give a general overview concerning the different microscopy techniques and some examples regarding the state-of-the-art in some electron and scanning probe microscopy techniques to observe, characterize, measure and manipulate materials on the nanometer scale.

Dye sensitized solar cell technology is an interesting, inexpensive, and promising alternative to traditional photovoltaics. The process of extending the sensitivity of transparent materials such as titanium dioxide to the visible spectra is called spectral sensitization. Therefore the sensitizing dye is one of the key components in DSC and its performance is generally recognised as a fundamental parameter. In order to ensure high efficiency and long term stability to cells, a large number of dyes have been proposed and studied in deep in the last twenty years. The structural optimisation of photoactive component is a delicate settlement among: (i) spectral properties, in order to ensure maximal visible light absorption, (ii) redox properties in the ground and excited state, (iii) photophysical properties-types and number of excited states, their lifetimes and quantum yields, (iv) effectiveness of anchoring groups, (v) choice of counterions and degree of protonation (overall charge), (vi) choice of peripheral groups (i.e.: axial ligands, substituents,…) and so on. In this talk, without going in deep in the history of sensitization, the progress made on dyes for DSC will be reviewed. The intention is to make a literature survey on both organometallic and organic dyes pointing out on features and drawbacks of the two different categories. Structural characteristics that enables to reach particular goal (i.e: pancromatism, IR absorption, long term stability, high molar extinction coefficient, etc.) will be discussed also from the synthetic point of view. Finally some information on purification methods and costs will be given.

I will present the potential of computer simulation by ab initio methods applied to the investigation of the structural, electronic and spectroscopic properties of Ru(II)-dyes and nanoscale systems of interest in the field of Dye-Sensitized Solar Cells. We devised over time a computational strategy based on DFT and its Time Dependent extension (TDDFT) which allows the accurate calculation of excited state properties of large systems in solution. The main ingredients of our computational strategy are discussed. In particular, our efficient implementation of the Car-Parrinello method, allows us to calculate the structures and dynamics of realistic systems; and the inclusion of solvation effects provides a direct connection of the calculated properties to the experimental quantities. Due to these methodological advances, theory is today a predictive tool, allowing us to scrutinize the properties of several organic and inorganic dyes before their synthesis. A computational tool for the in silico screening of dye-sensitizers is therefore presented. Finally, I will present realistic models of DSSC devices, obtained by adsorbing dyes on TiO₂ nanoparticles, simulating the fundamental processes of light-harvesting and charge injection in DSSCs. Modelling of the optical properties of ZnO nanostructures in relation to DSSCs applications will also be presented.

Photovoltaics (solar electricity) is the renewable energy source with the highest potential. In an ambitious scenario PV covers 20% of global electricity consumption by 2040. The global PV sector has grown by an average 50% p.a. over the past five years. This has become possible by successful market development in an increasing number of countries (e.g. Germany, Spain, France, etc.). An investment report published in 2004 by Credit Lyonnais Security Asia forecasts that the photovoltaics sector has a realistic potential to expand from 5.8 billion EUR in 2004 to 25 billion EUR in 2010 corresponding to 5.3 GW in annual sales. The world-wide photovoltaic industry has the potential to create more than 2 million jobs by 2020. The total number of jobs in the PV sector in Europe was estimated to be 55,000 in 2007, of which 40,000 in Germany.
PV stands a chance of becoming an alternative to fossil fuels. PV will have considerable impacts on bettering the environment, providing energy security, and readying critical disaster and crisis insurance. Now, they are also critical times because decisions made now will enable the potentially enormous photovoltaic contributions that should be realised in this first part of our XXI century. Failure to aggressively seize the opportunities and resolutely address the challenges now that face PV - as well as other clean energy sources - would damage the ability of both current and future generations to impact positively the environment, establish energy security, and future.
The paper review current status and perspectives of the developing PV market.