Editorial

Polymer solar cells..!!!

      New photovoltaic (PV) energy technologies can contribute to environmentally friendly,

renewable energy production, and the reduction of the carbon dioxide emission associated

with fossil fuels and biomass. One new PV technology, plastic solar cell technology, is based

on conjugated polymers and molecules. Polymer solar cells have attracted considerable

attention in the past few years owing to their potential of providing environmentally safe,

flexible, lightweight, inexpensive, efficient solar cells. Especially, bulk-heterojunction solar

cells consisting of a mixture of a conjugated donor polymer with a methanofullerene

acceptor are considered as a promising approach. Here a brief introduction and overview is

given of the field of polymer solar cells.

        It is expected that the global energy demand will double within the next 50 years. Fossil

fuels, however, are running out and are held responsible for the increased concentration of

carbon dioxide in the earth’s atmosphere. Hence, developing environmentally friendly,

renewable energy is one of the challenges to society in the 21st century. One of the

renewable energy technologies is photovoltaics (PV), the technology that directly converts

daylight into electricity. PV is one of the fastest growing of all the renewable energy

technologies, in fact, it is one of the fastest growing industries at present.1 Solar cell

manufacturing based on the technology of crystalline, silicon devices is growing by

approximately 40% per year and this growth rate is increasing.1 This has been realized

mainly by special market implementation programs and other government grants to

encourage a substantial use of the current PV technologies based on silicon. Unfortunately,

financial support by governments is under constant pressure.

      

       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% (Table 1). With regard to the technology used, these solar cells can be divided

into two classes. The crystalline solar cells or silicon solar cells are made of either (mono- or

poly-) crystalline silicon or GaAs. About 85% of the PV market is shared by these crystalline

solar cells.1 Amorphous silicon, CdTe, and CI(G)S are more recent thin-film technologies.

        The current status of PV is that it hardly contributes to the energy market, because it is far

too expensive. The large production costs for the silicon solar cells is one of the major

obstacles. Even when the production costs could be reduced, large-scale production of the

current silicon solar cells would be limited by the scarcity of some elements required, e.g.

solar-grade silicon. To ensure a sustainable technology path for PV, efforts to reduce the

costs of the current silicon technology need to be balanced with measures to create and

sustain variety in PV technology. It is, therefore, clear that ‘technodiversity’, implying new

solar cell technologies, is necessary.2 In the field of inorganic thin-films, technologies based

on cheaper production processes are currently under investigation.

       Another approach is based on solar cells made of entirely new materials, conjugated

polymers and molecules. Conjugated materials are organics consisting of alternating single

and double bonds. The field of electronics based on conjugated materials started in 1977

when Heeger, MacDiarmid, and Shirakawa discovered that the conductivity of the

conjugated polymer polyacetylene (PA, Figure 1) can be increased by seven orders of

magnitude upon oxidation with iodine,3 for which they were awarded the Nobel Prize in

Chemistry in 2000.4,5,6,7 This discovery led, subsequently, to the discovery of

electroluminescence in a poly(p-phenylene vinylene) (PPV, Figure 1) by Burroughes et al. in

1990.8,9 The first light-emitting products based on electroluminescence in conjugated

polymers have already been launched at the consumer market by Philips (The Netherlands)

in 2002, whereas light-emitting products based on conjugated molecules have been

introduced by the joint venture of Kodak and Sanyo (Japan). Going from discovery to

product within a little bit more than one decade truly holds a huge promise for the future of

plastic electronics. Other emerging applications are coatings for electrostatic dissipation and

electromagnetic-interference shielding.10

         Conjugated polymers and molecules have the immense advantage of facile, chemical

tailoring to alter their properties, such as the band gap. Conjugated polymers (Figure 1)

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.

           The cost reduction mainly results from the ease of processing from solution. Solution

processing requires soluble polymers. Poly[p-phenylene vinylene] (PPV, Figure 1) is hardly

soluble. Attachment of side-groups to the conjugated backbone, as in poly[2-methoxy-5-

(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV, Figure 1), enhances the

solubility of the polymer enormously. Furthermore, the nanoscale morphology, affecting the

opto-electronic properties of these polymer films, can be controlled by proper choice of the

position and nature of these side-groups.

           Recent developments in ink-jet printing, micro-contact printing, and other soft lithography

techniques have further improved the potential of conjugated polymers for low-cost

fabrication of large-area integrated devices on both rigid and flexible substrates.

Architectures to overcome possible electronic scale-up problems related to thin film organics

are being developed.11 In contrast to conjugated polymers, conjugated molecules are mainly

thermally evaporated under high vacuum. This deposition technique is much more

expensive than solution processing and, therefore, less attractive.

          The necessity for new PV technologies together with the opportunities in the field of

plastic electronics, such as roll-to-roll production, have drawn considerable attention to

plastic solar cells in the past few years. Apart from application in PV, it is expected that

plastic solar cells will create a completely new market in the field of cheap electronics.

Basic processes in an organic solar cell :

       Various architectures for organic solar cells have been investigated in recent years. In

general, for a successful organic photovoltaic cell four important processes have to be

optimized 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 all 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 advantageous for light absorption,

but burdens the charge transport.

      Creation of charges is one of the key steps in photovoltaic devices in the 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 (D), a p-type semiconductor, to an electron acceptor (A), an n-type semiconductor,

with the aid of the additional input energy of an absorbed photon (hν). In the photoinduced

electron transfer reaction the first step is excition of the donmor (D*) or the acceptor (A*),

followed by creation of the charge-separated state consisting of the radical cation of the

donor (D•+) and the radical anion of the acceptor 

(A•-).D + A + hν → D* + A (or D + A*) → D•+ + A•-

For an efficient charge generation, it is important that the charge-separated state is the

thermodynamically and kinetically most favorite pathway after photoexcitation. Therefore,

it is important that the energy of the absorbed photon is used for generation of the chargeseparated state and is not lost via competitive processes like fluorescence or non-radiative decay. In addition, it is of importance that the charge-separated state is stabilized, so that the photogenerated charges can migrate to one of the electrodes. Therefore, the back electron transfer should be slowed down as much as possible.

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 some

laboratories have reported power conversion efficiencies of ~4–544 and now aim at

increasing the efficiency to 8–10%. By combining synthesis, processing, and materials science

with device physics and fabrication there is little doubt that these appealing levels of

performance will be achieved in the near future.