Photovoltaics



PHOTOVOLTAICS AND ENERGETICS

Senior investigator: Dr. Isabella Concina

This activity mainly deals with two lines, one on nanowire based third-generation solar cells and one on solar concentration and spectral splitting (carried out at Ferrara). An additional seed activity is starting on energy harvesting through piezoelectric materials.

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Nanowire based third-generation solar cells

This research activity is addressed to the investigation of the physico-chemical features affecting the efficiency and long term stability of third generation solar cells.
Such innovative devices could allow significant improvement on the overall quality of solar energy conversion, with respect to traditional photovoltaic devices.
In particular, the following improvement can be envisaged [1]:

  • Enhanced photovoltaic conversion efficiency
  • “Green energy” and low impact at environmental level, thanks to employment of not toxic materials
  • Low cost for fabrication of devices thanks to simple and cheap technologies
  • One of the most promising ways to be successful in the development of third generation solar cells is application of quasi one-dimensional (1D) nanostructures, as explored in dye sensitized solar cells (DSC) [2,3].

Various types of photoelectrochemical systems are under investigation at SENSOR Lab:

  • DSCs based on traditional polycrystalline TiO2 photoanode
  • Nanowire-integrated photoanode
  • Quantum dot solar cells, QDSC (in collaboration with Universitat Jaume I, Castelló de la Plana)
  • New dyes (in collaboration with CNR-ISMAC, Milano)
  • Nanowire-based all inorganic solar cells
  • Innovative transparent conducting oxides (Cd2SnO4) at the photoelectrodes

A fabrication & test facility for third generation solar cells has been set-up and is now operating at SENSOR Lab.

Nanowire-integrated photoanodes

Single crystalline nanowires exhibit electron mobility up to 2 orders of magnitude higher than the polycrystalline counterpart employed at present in commercial DSC. This characteristic provides direct path for charge collection, thus reducing the recombination processes that are the main responsible for reduction of the overall cell efficiency.
Innovative photoanodes are developed at SENSOR Lab using ZnO and SnO2 single crystalline nanowires and TiO2 nanotubes.


Figure 1. (left, center) Scheme of two different photoanodes based on traditional polycrystalline network, and innovative network composed on nanoparticles embedded in nanowire array. (right) Impedance spectroscopy of the two systems integrated in a DSC.

Figure 2. Left to right: ZnO nanowire array; ZnO nanoparticles; ZnO nanowire bundle filled by ZnO nanoparticles; high magnification image of a single nanowire with a conformal coverage of nanoparticles.

Quantum Dot Solar Cells

(in collaboration with Universitat Jaume I, Castelló de la Plana)

QDSCs are based on QD excitons for light trapping and charge transfer to the photoanode [4].   They have the potential to induce a revolution in the field of photoconversion and electric current generation [5].  Recent theoretical investigations indicate that it should be possible to obtain a photoconversion efficiency up to 45% [6],  thanks to two main principal photogeneration processes: (a) multiple exciton generation (MEG) thanks to the absorption of a photon with sufficient energy; (b) presence of intra-gap energy bands, which allow the absorption of sub-bandgap photons and creation of electron-hole pairs. QDs are produced on a regular basis at SENSOR using the Successive Ionic Layer Absorption and Reaction (SILAR) technique and integrated in QDSCs.

Figure 3. CdSe QDs under Vis irradiation (top) and UV irradiation (bottom). The emission wavelength is depending on the QD size.

One strategy to enhance the photoconversion efficiency is application of QDs that can absorb in the NIR region. Composite PbS-CdS system has been successfully investigated for the purpose.

Figure 4. UV-Vis absorption of CdS (left) and CdS-PbS (right) QDs. Widening of the absorption spectrum has been promoted by PbS QDs.

Figure 5. JV curves of composite CdS-PbS QDSCs under 1 sun irradiation (1.5 AM G). Maximum efficiency of 2.21% has been achieved.

Cd2SnO4 TCO

The modified flatband potential of Cd2SnO4 with respect to the traditional FTO (SnO2:F) can allow enhanced VOC, resulting in improved functional properties of the SC, as demonstrated in prototype systems at SENSOR Lab.


Figure 6. Enhanced VOC in DSC based on Cd2SnO4 TCO, with respect to the traditional SnO2:F.

References

  1. For a perspective view of the third-generation solar cells see the books of abstracts (& proceedings) of the 2 world conferences:
    33 rd IEEE Photovoltaic Specialists Conference, San Diego , CA , May 11-16, 2008 .
    23 rd European Photovoltaic Solar Energy Conference and Exhibition, Valencia , Spain , September 1-5, 200
  2. J.B. Baxter and E.S. Aydil Appl. Phys. Lett. 2005 , 86 , 053114.
  3. M. Law et al. Nature Materials 2005 , 4 , 455.
  4. P.V. Kamat J. Phys. Chem. C 111, 2834 (2007); P.V. Kamat J. Phys. Chem. C 112, 18737 (2008).
  5. A. Luque et al. MRS Bull. 32, 236 (2007).
  6. A.J. Nozik, Physica E 14, 115 (2002).

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Solar concentration and spectral splitting

Regarding the photovoltaic activity, maintaining solar concentration and spectral splitting as the leit-motive of the research affords to evaluate high technology approaches to high efficiency concentrated radiation converters. While pursuing the development of Virtual Substrates and of the InGaP cells on the top of it, it is important to explore different approaches to obtain spectrally specialized photovoltaic converters capable of operating under concentration. Approaches based on Chalcogenides or on structures characterized by quantum confinement will be considered.
At the same time approaches based on the spectral transformation, by the use of spectral down converters, of the short wavelength spectrum part to be, then, feed to usual silicon converters are being evaluated. A promising approach based on Quantum Confinement (e.g. quantum dots) as high efficiency spectral down converter with Multiple Excitons Generation will be evaluated.
As a counterpart of down-converters, up converters, capable of producing 1 high energy photon from two low energy ones, will be explored considering that these process may benefit from the high radiation fluxes attainable under solar concentration.
Engineering of the Primary concentrator and of the receiver will continue to be developed to maximize the panel thermal performances and the energy transfer and beam characteristics of the concentrator.

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Energy harvesting transducers

An additional field of current investigation for possible applications of micro-electromechanical devices, especially based on piezoelectric materials, is the conversion of waste mechanical energy from vibrations into electrical energy (energy harvesting). Energy harvesting is considered a key step toward the development of innovative energy-autonomous sensing nodes, and advances in micro-electromechanical energy converters are crucially demanded to enable the growth of next-generation microsensors.