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Dye-sensitized Solar Cells (DSSCs), introduced by B. O’Regan and M. Grätzel in 1991 (Fig.1), have reached high solar to electric power conversion efficiency currently exceeding 11%. The DSC low cost, due the use of inexpensive materials and processing, coupled to their high performance have raised substantial impulse to industrialization of this technology. A key advantage of DSSCs is that, unlike conventional Si solar cells, their conversion efficiency does not decrease under non-ideal illumination conditions, such as angled or diffuse light. Therefore, they are particularly suited for Europe climate and integration into building façades. Moreover, DSSCs can be realized in different colours by varying the nature of the dye making them particularly appealing for aesthetic and architectural applications. Assuming a 6% efficiency on an 80% active area, a 1 m2 panel would produce ca. 40 watt, leading to a cost of the order of 2 euro per watt peak, which is already competitive with the Si-based technology. This cost can be further reduced by increasing the efficiency of the single cell and of the modules and by considering an economy of scale.
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In addition to matching the absorber bandgap to the solar spectrum and selecting syntheses with high quantum yield, the material and synthesis cost, the global material availability and the potential environmental and health impacts are also important factors to consider along the road to developing nanocrystal photovoltaics that have the potential to impact the global energy economy. In particular regarding material availability and cost, a landmark study was recently performed in which 9 semiconductor material systems were identified that could in principle supply world-wide electricity demand via photovoltaic devices at material costs substantially lower than silicon. In particular FeS2 was the highest ranked in this regard, followed by Cu2S, Cu2ZnSnS4, and PbS. Toxicity has also found to be a significant hindrance for commercialization of Cd or Pb based technology. |
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Most of the organic semiconductors and nanocrystals (such as CdSe) absorb light in the visible range, whereas the sun emits radiation from 300 to 2100nm approximately, and the desirable nanocrystal should be the one which can absorb as much as possible in the red region of the electromagnetic radiation. Colloidal PbSe nanocrystals can absorb also the IR radiation of sunlight, moreover it has been recently reported that colloidal PbSe quantum dot can produce multiple excitons when irradiated at photon energies 4 to 7.8 times the NC´s band gap. Although the use of low bang gap nanoparticles such as PbSe helps in improving the photocurrent generation up to near infrared region, it tends to decrease the open circuit voltage. The tandem cells architecture, a multilayer that is equivalent to two photovoltaic cells in series, offers a number of advantages. In particular because the two cells are in series, the open circuit voltage (Voc) is increased to the sum of the Voc´s of the individual cells. Moreover the use of two semiconductors with different band gaps enables absorption over a broad range of photon energies within the solar emission spectrum. |
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Realization of organic-inorganic blends in which the inorganic component consists of a 3D network of cross-linked nanorods/tetrapods has been recently reported. Efficient blending with polymers and/or creation of such networks in-situ could greatly improve the performance of state-of-art solar cells based on spin-coated polymer-nanocrystal blends. The device architecture consists of a polymer-nanocrystal blend where the nanocrystals (either nanorods or branched nanocrystals) form a cross-linked “inorganic” polymer. In this blend, the organic polymer acts as the hole-transporting medium, while the “inorganic” polymer is the electron transporting medium. Charge collection and separation can be optimized in the inorganic polymer phase, as this can be made of individual units of different nanocrystals, which will be therefore assembled together to form an “inorganic block-copolymer”. This establishes direct electrical contact of a very large number of nanocrystals and creates percolating networks for electrons throughout the whole layer. It also avoids dead-ends in electron transport, which is a typical drawback of traditional hybrid plastic-additive cells. |
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Energy