Sito originale :Barry Rand
http://www.princeton.edu/~ocmweb/projects/Organic/Org%20PV/Stacked%20PV/clusters.htm
The structure of an organic tandem photovoltaic (PV) cell is illustrated below. Indium tin oxide (ITO) coated glass is used as the anode. The donor (D) copper phthalocyanine (CuPc), acceptor (A) 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI), and a Ag cathode are deposited by thermal evaporation. The tandem cell is a series connection of two organic PV cells which allows for an increase in open circuit voltage to twice that of a single device. Upon light absorption, excitons are formed in both photovoltaic subcells. After dissociation at a DA interface, the hole in the front PV cell and electron in the back PV cell are collected at the adjacent electrodes. To prevent build-up of charge within the cells, the electron in the front PV cell and hole in the back PV cell diffuse to the metal nanocluster layer where they recombine. The advantage of this structure is that individual devices can be made thin to allow for a large percentage of excitons to contribute to photocurrent, while the device itself is thick to allow for a large absorption efficiency.
This tandem device structure, however, led to an increase in power efficiency which was more than twice that of a single device, from 1.1% for a single DA heterojunction device to 2.5% for the tandem device. Here we show that the optical field enhancement due to plasmon generation on the cluster surfaces is responsible for the high efficiencies observed in tandem organic PV cells whose individual subcells are separated by ultrathin metal cluster layers.
The figure above shows that the 10 Å thick Ag island film (dotted curve) has a 100 nm peak (full width at half maximum) centered at a wavelength of λp = 440 nm due to surface plasmon excitation of the nanoclusters. Absorption of a 70 Å thick film of CuPc (dashed curve) is also shown. The solid curve corresponds to a 70 Å CuPc film deposited on top of the 10 Å thick Ag island film. The most noticeable feature is the increase of CuPc absorption at wavelengths λ > 470 nm. This broadband, nonresonant enhancement leads to an approximately 15% increase in the efficiency of tandem PV cells over that expected simply by combining the efficiencies of several stacked CuPc/PTCBI bilayers.
References:
"Long-range absorption enhancement in organic thin film solar cells containing silver nanoclusters," B. P. Rand, P. Peumans and S. R. Forrest, J. Appl. Phys., 96, 7519 (2004).
"High photovoltage multiple-heterojunction organic solar cells incorporating interfacial metallic nanoclusters," A. Yakimov and S. R. Forrest, Appl. Phys. Lett. 80, 1667 (2002).
"Small molecular weight organic thin-film photodetectors and solar cells," P. Peumans, A. Yakimov, and S. R. Forrest, J. Appl. Phys. 93, 3693 (2003).