2-4-1 Homojunctions
The simplest device structure is a layer of organic materials sandwiched between two different conducting contacts, typically indium tin oxide (ITO) and a low work function metal such as Ag, Al, Ca, or Mg (Figure 2.6). The difference in work function provides an
electric field which drives separated charge carriers towards the respective contacts. This electric field is seldom sufficient to break up the photo generated exciton. Instead the exciton diffuses within the organic layer until it reaches a contact, where it may be broken up supply separate charges, or recombine. Since exciton diffusion lengths are short, typically 1-10 nm, exciton diffusion limits charge carrier generation in such a device.
Photo carrier generation is therefore a function not only of bulk optical absorption, but also of available mechanisms for exciton dissociation. Other loss factors are non-radiative recombination at the interfaces and non-geminate recombination at impurities or trapped charges.
Single layer solar cells of this type typically deliver quantum efficiencies (QE) of less than 1 % and power conversion efficiencies of less than 0.1 %. QE is the ratio of electrons delivered to the external circuit per incident photon of a given wavelength, and is the figure of merit in organic photo voltaics. High QE is a necessary, through not sufficient, condition for high photovoltaic efficiency. In organic devices the value is still far from the values of 80-90 % typically in inorganic solar cell.
2-4-2 Heterojunction
Most of the developments that have improved performance of organic photovoltaic devices are based on donor-acceptor heterojunctions. At the interface between two different materials, electrostatic forces result from the differences in electron affinity and isolation potential. If both electron affinity and ionization potential are greater in one materials than the other then the interfacial electric field drives charge separation (Figure 2.7). These local electric fields are strong and may break up photo generated excitions provided that the differences in potential energy are larger than the exciton binding energy.
In a planar heterojunction, or “bi-layer” device, the organic donor-acceptor interface
separates excitons much more efficiently than the organic-metal interfaces in a single layer device and with very high purity materials, photovoltaic devices with high QE may be made.
2-4-3 Dispersed Heterojunctions
A revolutionary development in organic photovoltaic came in the mid 1990s with the introduction of a dispersed heterojunction, where an electron accepting and an electron donating materials are blended together. If the domain size in either material is similar to the exciton diffusion length, then wherever an exciton is photogenerated in that material, it is likely to diffuse to an interface and break up. If continuous paths exist in each material from the interface to the respective electrodes, then the separated charge carriers may travel to the contacts and deliver current to the external circuit (Figure 2.8). This effect was reported independently by several groups [33-35] for a blend of two conjugated polymer.
The blend improved QE t around 6-8 % from less than 1 % for either polymer alone.
Around the same time, Yu and co-workers reported a QE of 29 % for a blend of the hole transporter, poly-phenylene vinylene (PPV) with a derivative of C60 [36], where the C60
acts as the electron transporting component (Figure 3(b)).
This was followed by observation of enhanced QE in heterojunctions made from conjugated polymers with inorganic nanocrystals [37-38] and organic dye crystals [39].
The demonstration of improved QE with dispersed heterojunction represents a departure from the device physics of conventional solar cells and has led to new device and materials designs. The principles of operation are shared by dye sensitized solar cells which are discussed in reference [40].
2-5 Organic solar cells materials
Organic electronic materials are conjugated solids where both optical absorption and charge transport are dominated by partly delocalized π and π* orbits. Candidates for photovoltaic applications include crystalline or polycrystalline films of small molecules, amorphous films of small molecules prepared by vacuum deposition or solution, and combinations of any of these either with order organic solids or with inorganic materials. A comprehensive discussion of the development of organic solids for photovoltaic applications is given by Hall [41].
Organic photovoltaic materials differ from inorganic semiconductors in the following important respects [42].
1. Photo generated excitons are strongly bound and do not spontaneously dissociate pairs of energy of ~100 meV compared to a few meV for a crystalline semiconductor. This means that charge carrier separation generation dose not necessarily result from the absorption of light.
2. Charge transport proceeds by hopping between localized states, rather than transport within a band, which results in low mobilities.
3. The spectral range of optical absorption is relatively narrow compared to the solar spectrum.
4. Absorption coefficients are high (~105 cm-1) so that high optical densities can be achieved, at peak wavelength, with films less than 100 nm thick.
5. Many organic materials are susceptible to degradation in the presence of oxygen or water.
6. As one-dimensional semiconductors, their electronic and optical properties can be highly anisotropic. This is potentially useful for devices design.
The first two features are due to the fact that the intermolecular van der Waals forces
in organic solids are weak compared to bonds in inorganic crystals and much weaker than the intramolecular bonds. As a consequence all electronic states are localized on single molecules and do not from bonds. Low mobility is aggravated by the high degree of disorder present in many organic solids. The optical excitations accessible to visible photons are usually π to π* transitions. Most conjugated solids absorb in the blue or green, absorption in the red or infrared is harder to achieve. However, the absorption bandwidth depends on the degree of conjugation and wider spectral sensitivity can be achieved in highly conjugated dye molecules.
These properties impose some constraints on organic photovoltaic devices:
1. A strong driving force should be present to break up the photo generated excitons.
2. Low charge carrier mobilities limit the useful thickness of devices.
3. Limited light absorption across the solar spectrum limits the photocurrent.
4. Very thin devices mean interference effects can be important.
5. Photocurrent may be sensitive to temperature through hopping transport and thermal dissociation of excitons.