Review of Organic Solar Cell Technology

Organic solar cells: An overview
Harald Hoppea and Niyazi Serdar Sariciftci, J. Mater. Res., Vol. 19, No. 7, Jul 2004
(Excellent Review with an extensive reference list)

• Organic polymers are currently made by wet solution processing or dry thermal evaporation of the organic constituents.
• The potential of semiconducting organic materials to transport electric current and to absorb light in the ultraviolet (UV)-visible part of the solar spectrum is due to the sp2-hybridization of carbon atoms.
• Most of the organic semiconductors are hole conductors and have an optical band gap around 2 eV.
• 2 main challanges for organic solar cell technology: conversion efficiency and lifetime & stability
• Conversion of light into electric current in 4 Steps: (i) Absorption of a photon leading to the formation of an excited state, the electron-hole pair (exciton). (ii) Exciton diffusion to a region, where (iii) the charge separation occurs. (iv) Finally the charge transport to the anode (holes) and cathode (electrons), to supply a direct current for the consumer load.
• Mechanism of charge transport: To reach the electrodes, the charge carriers need a net driving force, which generally results from a gradient in the electrochemical potentials of electrons and holes. Two “forces” contribute to this: internal electric fields and concentration gradients of the respective charge carrier species. The first leads to a field induced drift and the other to a diffusion current. Thin film devices (<100>

Gratzel Cells.. the future?

Dye-sensitized cell - "GREEN Solar Cell":

• Made of low-cost materials and easy to manufacture
• Similar to photosynthesis (dye molecule has porphyrin ring as in Chlorophyll or Heamoglobin, hence non-toxic)
  
• Can be incorporated into tinted windows that trap light to generate electricity.
• Environmentally friendly because made from titanium dioxide – a plentiful, renewable and non-toxic white mineral, already used in toothpaste, white paints and cosmetics.
• In limited production by Konarka (Lowell, MA)

http://en.wikipedia.org/wiki/Dye-sensitized_solar_cells
http://www.publicaddress.net/default,4117.sm#post4117
http://www.technologyreview.com/read_article.aspx?id=17490&ch=biztech&sc=&pg=1
B. O’Regan and M. Gratzel: A low cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 353, 737 (1991).
M. Gratzel: Photoelectrochemical cells. Nature 414, 338 (2001).
M. Gratzel: Dye-sensitized solar cells. J. Photochem. Photobiol. C4, 145 (2003).
U. Bach, D. Lupo, P. Comte, J.E. Moser, F. Weissortel, J. Salbeck, H. Spreitzer, and M. Gratzel: Solid-state dyesensitized mesoporous TiO2 solar cells with high photon-toelectron conversion efficiencies. Nature 395, 583 (1998).

Quantum Dots - hopeful but how??

News Article: June 2006
Depending on the quantum dot's composition and size, more than 1 electron-hole pair (exciton) can be created by an incident photon. The practical potential is huge, but the fundamental mechanism is not clear.. hmm!!

Points to ponder about:

• High energy photons cause impact ionization..
• QDs have higher bandgaps than bulk semiconductors, hence absorb high freq light
• Since the QD is much smaller than the wavelength of an electron inside it, the dot immobilizes the electron.
• Nozik figured that a dot's grip on an electron would nullify the motion-related subtleties that squelched the impact ionization process at larger scales.
• multiple excitons appear so quickly—within less than 50 femtoseconds (fs).. however, impact ionization proceeds sequentially.. so is it impact ionization?
  
• Alexander L. Efros has instead invoked quantum theory to propose that a photon hitting a quantum dot instantaneously creates a novel quantum object that's simultaneously both one and many excitons.
• Vladimir M. Agranovich suggests that a so-called virtual exciton springs into existence for a moment after the photon hits. Armed briefly with more energy than physics ordinarily permits, it spawns the multiple excitons simultaneously.
• Alex Zunger's team's calculations indicate that impact ionization can account for the experimental findings.. debatable!!
  

Experiments on 8 nm dia lead selenide quantum dots have given the best results so far: Ultraviolet-light photons—albeit at a wavelength found sparingly in sunlight—released seven electrons apiece!!
Relevant details/references in: http://www.sciencenews.org/articles/20060603/bob8.asp

Transporting Solar Energy..

Photogeneration Cell: Electrolysis of water to hydrogen and oxygen gas occurs when the anode is irradiated with EM radiation. This has been suggested as a way of converting solar energy into a transportable form, namely hydrogen. The photogeneration cells passed the 10 percent economic efficiency barrier. Lab tests confirmed the efficiency of the process. The main problem is the corrosion of the semiconductors which are in direct contact with water. Research is going on to meet the DOE requirement, a service life of 10000 hours.
http://en.wikipedia.org/wiki/Photoelectrochemical_cell

Why polymer solar cells??

Plastic solar cells can be successfuly realized as "bulk heterojunctions" between an organic polymer and organic molecule as electron acceptor, eg fullerene embedded into a conjugated polymer.

Advantages:

• lightweight (which is important for small autonomous sensors)
• disposable
• inexpensive to fabricate
• flexible
• designable on the molecular level
• have little potential for environmental impact.

Challanges:

• low efficiencies (~ 5%) so far
• degradation effects: efficiency decreases due to environment effects: UV rays, moisture etc

An important difference to inorganic solid-state semiconductors lies in the generally poor (orders of magnitudes lower) charge-carrier mobility in these materials, which has a large effect on the design and efficiency of organic semiconductor devices.

However, organic semiconductors have relatively strong absorption coefficients (usually >= 10E5 cm^-1), which partly balances low mobilities, giving high absorption in even <100 nm thin devices.

Another important difference to crystalline, inorganic semiconductors is the relatively small diffusion length of primary photoexcitations (called excitons) in these rather amorphous and disordered organic materials. Exciton binding energies usually exceeding those of inorganic semiconductors. These features of organic semiconducting materials lead generally to devices with very small layer thicknesses of the order <=100 nm.

http://en.wikipedia.org/wiki/Polymer_solar_cell
http://en.wikipedia.org/wiki/Conductive_polymers
Harald Hoppea and Niyazi Serdar Sariciftci: Organic solar cells: An overview, J. Mater. Res., Vol. 19, No. 7, Jul 2004, p1924

Flexible Solar Cells w/o organic polymers!

News Article: Feb 2003
http://www.newscientist.com/article.ns?id=dn3380
Unlike conventional solar cells, the new, cheap material has no rigid silicon base. Instead, it is made of thousands of inexpensive silicon beads sandwiched between two thin layers of aluminium foil and sealed on both sides with plastic. Each bead functions as a tiny solar cell, absorbing sunlight and converting it into electricity. The aluminium sheets give the material physical strength and act as electrical contacts.

The manufacturing process uses waste silicon from the chip-making industry, which is melted down and shaped into spheres about one millimetre across. Next, the cores of the silicon spheres are doped with boron atoms, which turn it into a "p-type" (positive) semiconductor. Then phosphorus atoms are diffused into the outer layer of the beads, converting it into a negative "n-type" material. The bumpy surface presented by the spheres offers a large area for absorbing light, giving the material an overall efficiency of 11 per cent.

Hello!

This blog is intended to help me organize my thoughts as I embark on my PhD project related to understanding fundamental processes (eg. exciton and charge-carrier generation & transport) in organic semiconductors. Femtosecond laser spectroscopy will be used to follow the dynamics of photoexcitations. A possible application is in the area of photovoltaics (solar cells). Working in this area might give me the ultimate opportunity to do something useful for the planet.. hooray!!