A common thiophene-based donor material is poly(3-hexylthiophene), or P3HT. It is soluble in several organic solvents, which makes it compatible with low-cost solution processing. Another donor material is poly(3,3///-didodecyl quaterthiophene), or PQT-12. Their molecular structures are shown in Figure 6.34. The optical absorption spectra of these compounds (Figure 6.35) show that their absorption bands are limited to the green and red parts of the solar spectrum.
Acceptor materials must provide good electron conductivity but optical absorption is not desired. A popular material is a C60 derivative. C60 is a fullerene, or a molecule composed entirely of carbon. The graphite-like surface of C60 allows effective electron transport through the delocalized electrons in the molecular orbitals, which result in the LUMO level. C60 as well as its derivative [6,6]-phenyl-C6i-butyric acid methyl ester (PCBM) are shown in Figure 6.36. The modification allows the fullerene derivative PCBM to be solution-processed, which reduces manufacturing costs, whereas C60 must be vacuum deposited.
Fullerenes are excellent acceptors since they have a LUMO level with electron energy well below the vacuum level and more specifically somewhat below the LUMO levels of a variety of donor molecules as required for solar cells. This is a requirement of an acceptor molecule, as illustrated in Figure 6.31. Each C60 actually has the ability to accept multiple electrons due to the number of available vacancies in the LUMO level. Electron transport within a single C60 molecule is very fast and efficient. The electrons are metastable within
the molecules and are therefore readily transferred to the solar cell electrode. Finally these fullerenes have an absorption spectrum that peaks in the ultraviolet part of the spectrum, which means that they exhibit only minimal absorption in the important parts of the solar spectrum, leaving the donor layer free to absorb the visible or infrared solar radiation.
The carbon nanotube is another acceptor material of considerable interest. A promising development is the use of carbon nanotubes as rod-like acceptors to form structures similar to that shown in Figure 6.33b. Nanotubes, being composed of rolled-up graphite sheets, have electronic properties that are similar to the fullerenes, allowing them to function
effectively as acceptors. The achievement of bulk heterojunctions with oriented nanotubes is required. A carbon nanotube is shown in Figure 6.37.
There are numerous new materials being studied and the list of these is growing steadily. The field of organic solar cells is in its infancy and research is underway in materials and nanostructures to optimize optical absorption, carrier collection efficiency, operational stability and large-scale manufacturability. The measured efficiency of organic solar cells is rising rapidly (see Figure 4.6). Very recent results not shown in Figure4.6 have demonstrated over 8% efficiency for organic solar cells using newly developed materials.
292 Principles of Solar Cells, LEDs and Diodes
3 cm 3 + 2.50 x 103 cm s 1 x 2.25 x 103 cm 3) 1 + 1
The attentive reader will notice that based on Figure 1.11 p = . The factor of 2 is omitted in the denominator because the
boundary condition applied to Equation 1.27 is altered for travelling electron waves rather than standing waves. The relevant boundary condition applied to Equation 1.27 becomes kx = X for travelling electron waves and hence p = h k = h x ^ = Ь. See Problem 2.12.
where IL, the current optically generated by sunlight, has three components, from the n-
side, from the depletion region, and from the p-side respectively. Using Equation 4.5 as well as the second terms from Equations 4.4a and 4.4b at xn — xp — 0, we obtain
which confirms that Figure 4.2 is valid and the I – V characteristic is shifted vertically (by amount IL) upon illumination. Of the three terms in Equation 4.7, the second term is generally smallest due to small values of W compared to carrier diffusion lengths, and since electron mobility and diffusivity values are higher than for holes the first term will be larger than the third term. It is reasonable that diffusion lengths Ln and Lp appear in Equation 4.7: carriers must cross over the depletion region to contribute to solar cell output current. They have an opportunity to diffuse to the depletion region before they drift across it, and the diffusion lengths are the appropriate length scales over which this is likely to occur.
Full molecular names such as Poly(2,5-dialkoxy) paraphenylene vinylene will not generally be listed in this chapter but may be found in Suggestions For Further Reading, Z. Li et al.