Grätzel and o’Regan first demonstrated a photoelectrochemical solar cell in 1991. Since then, development based upon these initial results has been taking place around the globe. Material scientists in China, Europe, Japan and the US are focused on how best to mass produce DSC (Dye-sensitized Solar Cells). The lower costs of manufacturing are expected to give dye-sensitized solar cells an important niche in a rapidly growing market.
In shopping for a PV (Photo Voltaic) system, one weighs the amount of electricity generated against the cost. Thus, developers are striving not only to lower the cost of fabrication, but also increase the efficiency of TFPV (Thin Film Photo Voltaic) cells. They want to increase their competitiveness with more established, more efficient PV from silicon crystals.
For a significant portion of the PV market, the only thin film photovoltaic technology, in Jim Fraser’s opinion, to surpass crystalline silicon PV, in terms of lower system price, is Cadmium telluride. Still, there is considerable research with III-V compounds (molecules made from elements in the III and V columns of the Periodic Table of Elements), plus crossover development from the fields of semiconductors and energy storage devices.
For instance, this blog previously reported upon breakthroughs in quantum dot photo voltaic cells by 1) Rice University researchers, who used CdSe (cadmium selenide) and 2) Dutch researchers, who used PbSe (lead selenide). Such semiconducting nanocrystals can sustain a cascade effect, whereby one photon triggers the release two or three electrons instead of just one.
Compared with organic dyes, quantum dots exhibit outstanding photoelectric properties. “Due to the quantum confinement their emission color can be continuously tuned from the ultraviolet to the near infrared range by changing the size and chemical composition. They exhibit a broad absorption spectrum, a narrow emission band and large absorption cross sections.”
Unfortunately, the fabrication of such semiconductor nanocrystals are more complex and expensive compared to organic dyes. Another approach is with hybrid materials, which could be used in the future for low cost fabrication of better performing, TVPV (Thin Film Photo Voltaic) cells.
Image of a nanoparticle surrounded by organic ligands
Science Daily1 now reports that IMDEA Nanoscience Instituto Madrileño de Estudios Avanzados en Nanociencia) in Madrid is collaborating with the University of Hamburg in development of composite materials.
Thanks to a remarkable effort in the synthetic activities in the last 20 years, scientists can now produce nanoparticles of different materials controlling their size, shape, and surface properties. Examples of nanoparticles produced by non hydrolytic colloidal synthetic methods are CdS, CdTe, InP, GaAs, PbS, or PbSe. However, the most studied system is CdSe, with tunable emission from blue to red. Due to the synthetic approach (hot injection method), the surface of these nanoparticles is capped with an organic shell that protects them and makes them stable in non-polar organic solvents.
It is also possible to control replacement of the initial organic shell for water compatible ones. The organic shell plays a relevant role in the quantum efficiency of the nanoparticles and their stability in different media. However, this shell prevents high electrical conduction.
Image of a carbon nanotubes
Carbon Nanotubes
Carbon nanotubes are another example of nanomaterials with extraordinary electrical properties. They consist of one or several rolled up graphene layers. In the case of a single layer they are called single-wall and multi-wall when several layers are rolled-up. Hybrid materials composed of semiconductor nanoparticles and carbon nanotubes combine the high absorption properties of the former and the high electrical conductivity of the latter.
One of the main drawbacks in the formation of such hybrid structures focuses on the type of interaction between them. Most of the existing procedures involve the growth of nanoparticles on previous defect sites provoked on the surface or edges of carbon nanotubes by aggressive chemical means. These aggressive treatments render an oxidized nanotube surface or even structural damage that deteriorates their outstanding electrical, mechanical, and optical properties significantly. Thus, supramolecular or electrostatic functionalisations are better approaches for photovoltaic applications.
Dr Beatriz H. Juárez, from IMDEA Nanoscience, is working on the preparation of hybrid materials that exhibit high coverage without modifying the electrical properties of the tubes. Furthermore, the monodispersity of the nanoparticles with high crystallographic quality and a close contact between nanoparticles and nanotubes are also under investigation. The composites show photoelectrical response, injecting charge carriers in the nanotubes upon nanoparticle excitation.
