About polymer photovoltaics

Polymer photovoltaics are a potentially cheaper alternative to solar energy than traditional inorganic photovoltaics made from such materials as silicon and gallium arsenide. Inorganics require high temperature, high vacuum processing conditions, such as molecular beam epitaxy. Up to 40% of the cost of a silicon photovoltaic lies within the material processing. Polymers, however, are liquid processable at room temperature and may be deposited on large sheets using ink-jet printing, screen-printing, or spin-casting. They are also mechanically flexible, able to withstand bending that would crack a silicon panel. They are also color tuneable, to a certain extent, so that they can be tweaked to emit or absorb in a variety of colors.

The challenges to polymer photovoltaics are low efficiencies and short lifetimes. The current record power efficiency for a polymer photovoltaic is 3.5% -- a full order of magnitude lower than the record power efficiency for silicon photovoltaics. However, silicon photovoltaics have enjoyed over 20 years of dedicated R&D, whereas polymer photovoltaics are a relatively new application.

How do polymers conduct? The answer lies in their conjugated structure. Conjugated polymers have alternating single- and double- carbon-carbon bonds along the polymer backbones. The carbons along the backbone are sp-2 hybridized, which leaves one unhybridized p-z orbital sticking up out of the plane of the polymer. The electrons in these pi-orbitals form a delocalized electron cloud, which is free to conduct.

The alternating bond length of the single- and double- carbon-carbon bonds creates an energy gap at the fermi level, giving rise to a conduction band and a valence band, and thus semiconducting behavior.

A polymer photovoltaic takes advantage of this fact by promoting an electron from the valence to conduction band, which then diffuses as a neutral exciton to an interface where it may be split into an electron and a hole. The charge carriers are then swept out by an internal field created by the workfunction difference of the electrodes.

Conjugated polymers exhibit nonlinear optical properties. Nonlinear optics concerns matter's reaction to light. In linear systems, when light hits some material, the medium's polarization is linearly related to the electric field by the susceptibility chi. In all systems, one can expand the polarization to look like: P - xE + x(2)E^2 + x(3)E^3...etc. where x is the susceptibility. In non-linear systems, other terms besides the first one in that series dominate. For example, if the E^2 term dominates, those non-linear materials are called chi 2 materials. So, basically anything that can polarize can exhibit non-linearities. In conjugated polymers, it is the delocalization of the pi electrons down the backbone that give rise to non linear optics. That allows them to polarize when hit by an incoming field. There are lots of characteristics
of the polymers that increase or decrease their non-linear terms (those things are all pretty complicated) such as chain length and end substituents.

Useful references:

Fairley, Peter. Solar on the cheap. Technology Review, January/February 2001 (pdf) - Popular press article on polymer photovoltaics

Halls, J.J.M. and R.H. Friend, Organic photovoltaic devices, in Clean electricity from photovoltaics, M.D. Aracher and R.D. Hill, Editors. 2001, Imperial College Press: London. p. 377-445. Excellent review of the field, from an academic standpoint.

Heeger, A.J., Semiconducting and metallic polymers: the fourth generation of polymeric materials. Synth. Met., 2002. 125: p. 23-42. Heeger's Nobel-prize acceptance paper serves as a great review of conducting polymer theory and experiment.

Polymer LEDs work on the same principles as polymer photovoltaics (above) but with an opposite goal. Instead of trying to separate the neutral exciton at an interface, polymer LEDs take advantage of the exciton's short lifetime. When the exciton radiatively recombines it emits light, with a color specific to the bandgap of that polymer. The ability to color-tune the emission of polymer LEDs is a huge advantage over traditional inorganics such as GaAs.

Useful references:

Bergh, A., et al., The promise and challenge of solid-state lighting. Physics Today, 2001. Popular press article outlining the field.

Heeger, A.J., Semiconducting and metallic polymers: the fourth generation of polymeric materials. Synth. Met., 2002. 125: p. 23-42. Heeger's Nobel-prize acceptance paper serves as a great review of conducting polymer theory and experiment.

About Polymer Electrochromic Devices

Electrochromics (ECDs) are materials whose color changes upon applying a potential difference across the material. Because these color changes can be reversible, these materials have been studied and utilized for devices such as “smart windows” or windows that can change variably from a completely transmissive state to a completely non-transmissive state. Currently, commercialized ECDs are made of inorganic materials such as transition metal oxides and prussian blue. However, in order to reduce production costs and increase color tunability there is a push to understand the electrochromic properties of some organic materials such as conducting polymers for such devices.

We study electrochromic devices that utilize the conductive properties of these conjugated polymers. The structure of the device in our lab is that of a ‘solid-state’ electrochemical cell. The device consists of an anode and a cathode (both ITO on glass substrates) that sandwich a conducting polymer layer and a polymer gel electrolyte layer. Upon applying a voltage, the polymer is electrochemically doped that is, the anode (cathode) oxidizes (reduces) the polymer layer. This changes the structure of the polymer and thus its color.