One sophisticated approach to produce photovoltaic cells (PV) involves the sole use of organic based materials — molecules built of carbon, with small amounts of oxygen, nitrogen, or phosphorus. Here the p-, electron-acceptor element is a chemically modified fullerene and the n-, electron-donor element is a semiconductive polymer. In one technology, fullerene molecules are modified with organic arms [2] to make them more soluble in the polymer matrix, increasing PV output. Because the two materials can be dissolved the same organic solvent and spin-coated, large photoactive polymeric layers can be easily obtained, enabling the inexpensive production of large-area photocells.
These organic (plastic) solar cells are widely studied due to their light weight, flexibility, and low-cost fabrication of large areas, as compared to conventional silicon photovoltaics. However, the maximum reported efficiency of fullerene/polymer PVs is about 5-6 %, less than half that obtained by silicon cells. [3] [J.Y. Kim, K. Lee, N.E. Coates, D. Moses, T.-Q. Nguyen, M. Dante, A. J. Heeger, “Efficient tandem polymer solar cells fabricated by all-solution processing”, Science, 317, 222-5 (2007)] The main factor limiting this efficiency is poor electronic hopping or transport between the fullerene molecules of the semiconductor. Recently Prof. Rachel Yerushalmi-Rozen of the Ben-Gurion University Dept. of Chemical Engineering and Eugene Katz initiated a multi-disciplinary research program to investigate the use of carbon nanotubes (CNTs) to replace modified fullerenes as the electron acceptor in plastic PVs. There are several advantages of CNTs over fullerenes, including the ability to modify tube diameters, providing a wide range of electron absorption band gaps (from 0 to 1.2 eV) that can accurately match the solar energy spectrum and improve energy conversion efficiencies. Moreover, the ability of electrons (or holes) to rapidly move up and down the molecularly long tubes (whose length is over 1000 times their diameter) reduces the electron scattering experienced with fullerenes and further raises efficiencies. An additional major advantage is the high surface area (~1600m2/g) of CNTs, providing a high CNT/polymer interfacial area for charge separation. Therefore, both critical processes governing the operation of conjugated polymer solar cells, i.e. charge separation and charge transport, are expected to be enhanced in CNT/conjugated polymer PVs.
Another bottleneck in the potential use of organic PVs is the cell’s lower stability under operational conditions. There are several well known mechanisms by which the conjugated semiconductive polymers used in plastic cells break down and lose their photovoltaic activity. While this phenomenon have been studied in the laboratory by accelerated indoor testing, the first long-term study of plastic cell voltaic stability under natural sunlight was carried out by Eugene Katz and co-workers at the BIDR Department of Solar Energy and Environmental Physics.
Because of the ideal climatic and unsullied atmospheric conditions, the Sede Boqer campus of the BIDR is ideal for optimal PV field testing. For this reason, large area encapsulated plastic cells designed by Dr. Frederik C. Krebs of the Danish Polymer Center at the RIS״ National Laboratory in Denmark were sent to BIDR for evaluation in natural sunlight for a 32 day period. Two of these cells types involved a polyphenylenevinylene [4] or poly-3-alkylthiophene [5] plastic containing a dispersed C61 fullerene [6] derivative. The third type was formed from alternative layers of poly(3-carboxythiophene-co-thiophene) [7] plastic and sublimed C60.
The researchers found that the normalized efficiency of all cells decreased during daylong exposure to sunlight. However, this decrease partially reversed itself following overnight resting, and morning power output efficiency was higher than that measured in the late afternoon. Nevertheless, there were major differences between the three types of cells. The polyphenylenevinylene bulk cells had such poor overnight recovery that it stopped functioning entirely over the 32 days of monitoring. The 3-aklylthiophene cell, on the other hand — while still operating — decreased in efficiency by over 90%. The most stable cell was the layered variety, which still lost some 2/3 of its original efficiency during the study. This field study demonstrates that there is still much work before completely organic cells will be able to compete with alternative approaches.
In Conclusion
From work carried out at the BIDR, it appears that the most likely photovoltaic approach to reach wide commercial energy application in the nearest future will be the use of concentrator PV with multi-junction ultra-efficient solar cells. However, some nontraditional PV materials and devices are also on the way.
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