Experiments

Advanced functional semiconductors for photovoltaics and energetics

This activity mainly deals with two lines, one on engineered metal oxide nanostructures to be applied as efficient anodes in third-generation solar cells and one on solar concentration and spectral splitting (carried out at Ferrara).

Engineered metal oxide nanostructures
This research activity is focused on the design and fabrication of engineered metal oxide structures to be applied as high efficiency photoanodes in third generation photovoltaics (PV), namely in dye- and quantum dot- sensitized solar cells (DSSCs and QDSSCs, respectively). By a proper modulation of the electron transport layer, the capability of solar energy conversion of these devices may be significantly improved, by improving electron lifetime and reducing exciton recombination within the working electrode.
In particular, the following goals are pursued [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
▪ 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
▪ Integration of 1D and 2D carbon materials (carbon nanotubes and graphene) in DSSCs (in collaboration with INRS-EMT, Canada and CNR-IMM, Bologna)
▪ Investigation of metal oxides alternative to TiO2 as electrode scaffold (ZnO and SnO2)
▪ Quantum dot solar cells
▪ New metal free dyes (in collaboration with CNR-ISMAC, Milano and CNR-ISTM, Milano)
A fabrication & test facility for third generation solar cells is operating at SENSOR Lab, including solar simulator, incident photon to current conversion spectroscopy, electrochemical impedance spectroscopy.

Integration of 1D and 2D carbon materials (carbon nanotubes and graphene) in DSSCs
A potentially appealing alternative to one-dimensional oxide nanostructures is represented by the incorporation of carbon nanotubes (CNTs) and graphene sheets in TiO2 traditional photoanodes. [4,5] CNTs, as well as other carbonaceous materials (e.g., graphene, graphene oxide and fullerenes) have been proposed as possible key elements in directing the flow of photogenerated electrons as well as favoring charge injection/ extraction in solar cell-based technologies.
Innovative photoanodes constituted by hybrid CNT (graphene)/TiO2 nanoparticles are developed and tested at SENSOR.

Quantum Dot Solar Cells
QDSCs are based on QD excitons for light trapping and charge transfer to the photoanode [7]. They have the potential to induce a revolution in the field of photoconversion and electric current generation [8]. Recent theoretical investigations indicate that it should be possible to obtain a photoconversion efficiency up to 45% [9], 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.
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.

References
1 For a perspective view of the third-generation solar cells see the books of abstracts (& proceedings) of the 2 world conferences:
2 33 rd IEEE Photovoltaic Specialists Conference, San Diego , CA , May 11-16, 2008 .
3 23 rd European Photovoltaic Solar Energy Conference and Exhibition, Valencia , Spain , September 1-5, 200
4 Kongkanand, A.; Martínez Domínguez, R.; Kamat, P. V. Nano Lett. 2007, 7, 676−680.
5 Chan, Y.-F.; Wang, C.-C.; Chen, B.-H.; Chen, C.-Y. Prog. Photovoltaics 2013, 21, 47−57.
6 (14) Sawatsuk, T.; Chindaduang, A.; Sae-Kung, C.; Pratontep, S.; Tumcharern, G. Diamond Relat. Mater. 2009, 18, 524−527.
7 J.B. Baxter and E.S. Aydil Appl. Phys. Lett. 2005 , 86 , 053114.
8 M. Law et al. Nature Materials 2005 , 4 , 455.
9 A.J. Nozik, Physica E 14, 115 (2002).

Solar concentration and spectral splitting (Ferrara)
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.

(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.