Dr. Idriss Bedja

Welcome to My Research Website. I hope we can share interesting exchanges on new PV generation likely Dye Sensitized Solar Cells, Quantum-Dots Solar Cells and recently Perovskite Solar Cells.

My Research Focus:
- To study Photophysics and Photoelectrochemistry of  Nanocrystalline  Semiconductor Systems including Dye Sentized Solar cells, Quamtum Dots solar cells and recently Perovskite Solar Cells.  Mechanistic studies of Photosensitization and Modulation of Electron Transfer Kinetics  
-To elucidate the Elctrochromic and Photoelectrochromic effects in nanocrysttaline metal oxide Semiconductors.
 
Nano-Solar Cells:
For the conception of the new generation of Nano-Organic solar Cells Or Dye-sensitized nanocrystalline semiconductor cells, these are some activity areas that are need for the full study.
1. Preparation of nanostructured semiconductor film substrates;
2. Surface analysis of nano-semiconductor films;
3.  Surface modification with dyes and Q-sized chalcogenides-clusters;
4.  Spectroelectrochemical and PEC measuremenst of Si-DSSC;
5.  Investigation of kinetics of electron-transfer and electron-trapping by means of transient
    spectroscopies;
 
 Why Nanoparticles in Solar Energy Conversion?
Today's solar panels are made with silicon. The silicon usually has impurities, which limits its efficiency. Purifying a chemical is too expensive. For that reason, smaller is better. One can fit as many nanoparticles into a golf ball as one can fit beach balls into the earth. Only a tiny percentage of a piece of material has impurities. If the entire chunk of material makes one crystal in a solar panel, the crystal will not work. But if that chunk is broken up into 100 tiny nanoparticles, then only the few unlucky nanoparticles with the impurities  will not function. All the other nanoparticles will be pure and therefore will work. It comes thus the idea to make large, high-output solar voltaic panels that are dirt cheap to produce. It's only then that the price starts to become competitive with burning fossil fuels.
 
How does DSSC Work?
            DSSC, Dye Sensitized Semiconductor Cell is simply composed of
            1. a sensitized dye,
            2. a dye absorber: a substrat of nanostructured film actsas electron transporter,
            3. an electrolyte (filling the pores between nanocrystallites)
            4. front and back electrodes.

The basic scheme of a DSSC:  A layer of light-sensitive dye is attached to the surfaces of the 15-20 nm
nanoparticles of TiO2 in a 5-20 microns thick. As the dye molecules are hit by light, The excited electrons in the dye are transmitted to TiO2 and then migrate to the front electrode (a transparent ITO glass) which can be  
extracted as an external current. The dye molecules are then electrically reduced to their initial states by electrons transferred from redox couple in the electrolyte. The oxidized ions in the electrolyte, diffuse to the back electrode to receive electrons.
 
  What is the Main Functional Difference of DSSC Compared To Conventional Cells? 

The main difference of DSSC cells compared to conventional cells is that the functional element, which is responsible for light absorption (the dye), is separated from the charge carrier transport itself (here TiO2). In the case of the n-type semiconductor TiO2 (band gap 3.2 eV), this results in a working cycle starting with the dye excitation by an absorbed photon at the TiO2/ electrolyte interface and an electron injection into the TiO2. The major advantage of the concept of dye sensitization is the fact that the conduction mechanism is based on a majority carrier transport as opposed to the minority carrier transport of conventional inorganic cells. This means that bulk or surface recombination of the charge carriers in the TiO2 semiconductor cannot happen. Thus, impure starting  materials and a simple cell processing without any clean room steps are permitted, yet resulting in promising conversion efficiencies of 7–11% and the hope of a low-cost device for photoelectrochemical solar energy conversion. The most important issue of the dye-sensitized cells is the stability over the time and the temperature range which occurs under outdoor conditions.
 
One main effect of the nanostructured film is to greatly amplify the light-sensitive surface. The actual surface area in a 10 micron thick film is 1 000 times greater than that projected; because of the small size of the particles, these films are highly transparent. Such nanostructured films form an important new class of electronic materials and are called mesoporous films of wide-band gap semiconductor oxides. They are constituted by a network of nanocrystalline particles of oxides, such as TiO2, SnO2 or ZnO, sintered together to allow for charge carrier transport to take place. The pores between the nanoparticles are filled with an electrolyte or a solid state organic hole conductor forming an interpenetrating heterojunction of very large contact area. These junctions exhibit extraordinary opto-electronic properties due to their large surface area to volume ratio leading to applications in different domains, such as photovoltaics, intercalation batteries, electrochromic and electroluminescent displays, photocatalysis and chemical sensors.
 
What are the Benefits of DSSC over Conventional Silicon-based Cells?
 
 

 
What are the Major Issues Confronting Practical Applications of Dye -Sensitization Up To Date?

 An increase of the open-circuit voltage by suppression of recombination could increase the efficiency by only an additional few percent.  However, in order to increase the efficiency of DSSC to a level comparable to that of the silicon-based solar cell,broadening of the spectral response becomes an essential requirement. DSSC's have narrower spectral response corresponding to the absorption spectrum of the dye. Synthesis of dyes with broader spectral response has been attempted as a possible strategy. The other method is to use more than one pigment. Unfortunately, the straightforward way of doing this by using mixtures of dyes is unsuccessful owing to the quenching and insulating effect of thick dye layers composed of many components. Direct application of dye mixtures, in almost all cases result lowering of both energy and quantum conversion efficiencies compared to at least that of the cell based on the best single dye.  Duleepa et al. have reported an enhanced efficiency by coupling of two dye molecules to each other and to n- and p-type semiconductors.  But still methods have to be found to avoid extraneous interactions between coupled molecules.
 
Mechanism of Charge Separation in Thin Semiconductor Nanoparticulate Films

For bulk semiconductors in contact with electrolyte, the charge transfer occurs only in presence of electron acceptor or donor which may create a zone of depletion in the semiconductor.  For the ultra-small (nano-) particles, the curved band is almost flat, and the charge separation occurs in this case only by diffusion.  Under bandgap illumination, electon-hole pairs are generated and oriented randomly in the optical space.  These charges recombine and diffuse towards the surface where redox reactions take place.  The time of transit of the diffusion given by  tD = r02/p2D is about 3 ps for a TiO2 nanoparticle (6 nm of size). Compared to the charge recombination process (about 100 ps), the diffusion (3ps) is very fast which suggests an optical quantum yield of a redox reaction approaching unity. 
As indicated in earlier studies, semiconductor nanocrystallites in a thin film prepared from nanoparticles are in close contact in each other and capable of exhibiting phetoelectrochemical (PEC) properties similar to a polycrystalline semiconductor film.  The PEC behavior is similar to te Schottky Barrier type photovoltaic cells.  Although an ideal Schottky barrier is absent in nanocrystalline semiconductor films, a potential gradient arising from varying degree of electron accumulation within the semiconductor particles acts as a driving force for the transport of injected electrons across the films.