Category Third Generation Photovoltaics
Lanthanides have also been employed in upconverters attached to the back of bifacial silicon solar cells. Trivalent erbium is ideally suited for up conversion of NIR light due to its ladder of nearly equally spaced energy levels that are multiples of the 4I15/2 to 4I13/2 transition (1540 nm) (see also Fig. 3). Shalav et al. (2005) have demonstrated a 2.5% increase of external quantum efficiency due to up conversion using NaYF4:20% Er3+...Read More
Upconversion was, like DC also suggested in the 1950s, by Bloembergen (1959), and was related to the development of IR detectors: IR photons would be detected through sequential absorption, as would be possible by the arrangement of energy levels of a solid. However, as Auzel (2004) pointed out the essential role of energy transfer was only recognized nearly 20 years later. Several types of upconversion mechanisms exist (Auzel 2004), of which the APTE (addition de photon par transferts d’energie) or, in English, ETU (energy transfer upconversion) mechanism is the most efficient; it involves energy transfer from an excited ion, named sensitizer, to a neighbouring ion, named activator (see Fig. 5).
Chung et al. (Chung et al. 2007) reported downshifting phosphor coatings consisting of Y2O3:Eu3+ or Y2O2S:Eu3+ dispersed in either polyvinyl alcohol or polymethylmethacrylate on top of mc_Si solar cells: an increase in conversion efficiency was found of a factor of 14 under UV illumination by converting the UV radiation (for which the response of c_Si is low) to 600 nm emission from the Eu3+ ion. The used solar cells used were encapsulated in an epoxy that absorbs photons with energy higher that ~3 eV. This protective coating can remain in place and down converters or shifters can easily be added. A 2...Read More
Downshifting (DS) or photoluminescence is a property of many materials, and is similar to downconversion, however, only one photon is emitted and energy is lost due to nonradiative relaxation, see Fig. 2. Therefore the quantum efficiency is lower than unity. DS can be employed to overcome poor blue response of solar cells (Hovel et al. 1979), due to, e. g., non-effective front surface passivation for silicon solar cells. Shifting the incident spectrum to wavelengths where the internal quantum efficiency of the solar cell is higher than in the blue can effectively enhance the overall conversion efficiency by ~10% (Van Sark et al. 2005). Improvement of front passivation may make down shifters obsolete, or at least less beneficial...Read More
The most promising systems for downconversion rely on lanthanide ions. The unique and rich energy level structures of these ions allow for efficient spectral conversion, including up – and downconversion processes mediated by resonant energy transfer between neighboring lanthanide ions (Auzel 2004, Wegh et al. 1999). Considering the energy levels of all lanthanides, as shown in the Dieke energy level diagram (Dieke 1968, Peijzel et al. 2005, Wegh et al. 2000) it is immediately evident that the energy level structure of Yb3+ is ideally suited to be used in down conversion for use in c-Si solar cells. The Yb3+ ion has a single excited state (denoted by the term symbol 2F5/2) some 10,000 cm-1 above the 2F7/2 ground state, corresponding to emission around 1000 nm...Read More
Downconversion was theoretically suggested first by Dexter in the 1950s (Dexter 1953, 1957), and shown experimentally 20 years later using the lanthanide ion praseodymium Pr3+ in an yttrium fluoride YF3 host (Piper et al. 1974, Sommerdijk et al. 1974). A VUV photon (185 nm) is absorbed in the host, and its energy is transferred into the 1S0 state of the Pr3+ ion, from where two photons (408 and 620 nm) are emitted in a two-step process (1S0^3Pj at 408 nm followed by 3Pj^3F2 at 620 nm). In this way a single absorbed high energy photon results in the emission of two visible photons and a higher-than-unity quantum efficiency is realized. Another frequently used ion is gadolinium Gd3+ (Wegh et al. 1997), either single or co-doped (Wegh et al. 1999)...Read More
W. G.J. H.M. van Sark1, A. Meijerink2 and R. E.I. Schropp3
1Utrecht University, Copernicus Institute, Science, Technology and Society, Utrecht Utrecht University, Debye Institute for NanoMaterials Science, Condensed Matter and Interfaces, Utrecht Utrecht University, Debye Institute for NanoMaterials Science, Nanophotonics – Physics of Devices, Utrecht
The possibility to tune chemical and physical properties in nanosized materials has a strong impact on a variety of technologies, including photovoltaics. One of the prominent research areas of nanomaterials for photovoltaics involves spectral conversion...Read More
The quest for alternative sustainable energy sources has led us to the third generation of photovoltaics. The feasibility for PV to become a major contributor in the electricity mixture of the U. S. and other major energy consuming countries has been proven. The production and sales of photovoltaics have increased by an average of 45% per year over the last fifteen years. As with all energy-generation technologies, PV deployment is subsidized but the expectation is that, within a decade, the cost of PV electricity generation will be in parity with electricity from the grid, making additional subsidies unnecessary.
For PV grid cost parity we need high efficiencies and low production costs...Read More