Scientists are researching organic photovoltaic cells as a cheaper alternative energy source to traditional silicon solar cells. Organic cells use carbon-based conjugated polymers and molecules that can be easily processed and are flexible. Various architectures for organic solar cells have been investigated, including single layer cells, bilayer cells with donor and acceptor materials, and bulk heterojunction cells with the donor and acceptor mixed together. Researchers aim to increase the power conversion efficiency of polymer-fullerene bulk heterojunction solar cells to 8-10% through improved materials and device design.

1. PHOTOVOLTAIC CELLS
BASED ON ORGANIC MATERIALS
The increasing global energy demand has determined scientists to look for new
methods to provide cheap energy. Thereby, in recent decades, the research on
alternative energy sources has experienced a rapid development. I have chosen for
further study the solar energy branch, namely the one based on organic cells, because I
believe that it will shortly become the most widely used source of energy, given the low
production costs and their increasing effectiveness.
A photovoltaic cell (also called a solar cell) is an electrical device that converts
the energy of light directly into electricity by the photovoltaic effect. It is a form
of photoelectric cell, which, when exposed to light, can generate and support an electric
current without being attached to any external voltage source. Photovoltaics is one of
the fastest growing of all the renewable energy technologies.
At present, the active materials used for the fabrication of solar cells are mainly
inorganic materials, such as silicon (Si), gallium-arsenide (GaAs), cadmium-telluride
(CdTe), and cadmium-indium-selenide (CIS). The power conversion efficiency for these
solar cells varies from 8 to 29%.The current status of photovoltaics is that it hardly
contributes to the energy market, because it is far too expensive. The large production
costs for the silicon solar cells are one of the major obstacles.
Hence, another approach is based on solar cells made of entirely new materials,
conjugated polymers and molecules. They have the immense advantage of facile,
chemical tailoring to alter their properties, such as the band gap. Conjugated polymers
combine the electronic properties known from the traditional semiconductors and
conductors with the ease of processing and mechanical flexibility of plastics. Therefore,
this new class of materials has attracted considerable attention owing to its potential of
providing environmentally safe, flexible, lightweight, inexpensive electronics.
Photovoltaic cell configurations based on organic materials differ from those
based on inorganic semiconductors, because the physical properties of inorganic and
organic semiconductors are significantly different. Organic materials have a low
dielectric constant and the exciton binding energy is generally large, while inorganic
semiconductors have a higher dielectric constant and a lower exciton binding energy.
Various architectures for organic solar cells have been investigated in recent
years. Generally, for a successful organic photovoltaic cell, four important processes
have to be improved to obtain a high conversion efficiency of solar energy into electrical
energy:
 Absorption of light
 Charge transfer and separation of the opposite charges
 Charge transport
 Charge collection
For an efficient collection of photons, the absorption spectrum of the photoactive
organic layer should match the solar emission spectrum and the layer should be
sufficiently thick to absorb the whole incident light. A better overlap with the solar
emission spectrum is obtained by lowering the band gap of the organic material, but this
will ultimately have some bearing on the open-circuit voltage. Increasing the layer
thickness is convenient for light absorption, but burdens the charge transport.

2. Creation of charges is one of the key steps in conversion of solar light into
electrical energy. In most organic solar cells, charges are created by photoinduced
electron transfer. In this reaction an electron is transferred from an electron donor, a p-
type semiconductor, to an electron acceptor, an n-type semiconductor, with the aid of
the additional input energy of an absorbed photon.
There are several types of organic photovoltaic cells:
 Single layer organic photovoltaic cells are the simplest of the various forms of
organic photovoltaic cells. These cells are made by sandwiching a layer of organic
electronic materials between two metallic conductors. In practice, this kind of cells
doesn’t work well. They have low quantum efficiencies (<1%) and low power
conversion efficiencies (<0.1%).
 Bilayer organic photovoltaic cells contain two different layers in between the
conductive electrodes. These two layers have different electron
affinity and ionization energy, therefore electrostatic forces are generated at the
interface between the two layers. These local electric fields are stronger and they
break up the excitons much more efficiently than the single layer photovoltaic. The
layer with higher electron affinity and ionization potential is the electron acceptor,
and the other layer is the electron donor. This structure is also called a planar
donor-acceptor heterojunction. The problem is that the diffusion length of excitons in
organic electronic materials is typically on the order of 10 nm. In order for most
excitons to diffuse to the interface of layers and break up into carriers, the layer
thickness should also be in the same range as the diffusion length. However,
typically a polymer layer needs a thickness of at least 100 nm to absorb enough
light. At such a large thickness, only a small fraction of the excitons can reach the
heterojunction interface.
 Bulk heterojunction photovoltaic cell - the electron donor and acceptor are mixed
together, forming a polymer blend. If the length scale of the blend is similar to the
exciton diffusion length, most of the excitons generated in either material may reach
the interface, where excitons break efficiently. Electrons move to the acceptor
domains then were carried through the device and collected by one electrode, and
holes were pulled in the opposite direction and collected at the other side.
 Graded Heterojunction photovoltaic cells - in this type of photovoltaic cell, the
electron donor and acceptor are mixed together, like in the bulk heterojunction, but
in such a way that the gradient is gradual. This architecture combines the short
electron travel distance in the dispersed heterojunction with the advantage of the
charge gradient of the bilayer technology
The prospect that lightweight and flexible polymer solar cells can be produced by
roll-toroll production, in combination with high energy-conversion efficiency, has spurred
interests from research institutes and companies. In the last five years there has been
an enormous increase in the understanding and performance of polymer-fullerene
bulkheterojunction solar cells. Comprehensive insights have been obtained in crucial
materials parameters in terms of morphology, energy levels, charge transport, and
electrode materials. To date, power conversion efficiencies close to 3% are routinely
obtained and now it is aimed the increase of the efficiency to 8–10%. By combining

3. synthesis, processing, and materials science with device physics and fabrication there
is hope that these appealing levels of performance will be achieved in the near future.
References
René Janssen (Eindhoven University of Technology) -Introduction to organic solar cells
Dieter Wohrle and Dieter Meissner - Organic Solar Cells **
Kaltenbrunner, M. - Ultrathin and lightweight organic solar cells with high flexibility
http://en.wikipedia.org/wiki/Organic_solar_cell
http://en.wikipedia.org/wiki/Polymer_solar_cell
http://www2.warwick.ac.uk /pressreleases/organic_solar_cells/