Sviluppo di fotoanodi a base di ossido di Zinco e di nuove molecole organiche per celle solari a colorante ad alta efficienza

Responsabili di progetto

Alberto Vomiero, Eric Diau

Accordo

TAIWAN - NSTC - National Science and Technology Council

Bando

CNR/NSC 2014-2015

Dipartimento

Scienze fisiche e tecnologie della materia

Area tematica

Scienze fisiche e tecnologie della materia

Stato del progetto

Nuovo

Proposta di ricerca

Photoelectrochemical systems[1] are promising devices for environmentally compatible, cheap, and large scale solar energy conversion, hence representing a concrete alternative to commercial solid-state semiconductor solar cells. Dye-sensitized solar cells (DSSCs)[2] exhibit the best performance in terms of long-term stability and energy conversion efficiency, despite the latter remains below 13 % due to the intrinsic limitation in charge transport. The photoanode consists of a mesoporous wide-band-gap oxide semiconductor film with high specific surface deposited on a transparent conducting oxide (TCO). The highest photoconversion efficiency (PCE) has been achieved with film consisting of 20 nm TiO2 nanocrystallites sensitized by different dye molecules.[3,4] Other n-type metal oxide semiconductors can be used to replace of TiO2; ZnO is the most promising one[5] due to its electronic band structure similar to TiO2 and higher electron mobility. The preparation of the photoelectrode is crucial to enhance PCE and various geometries and network structures have been applied, like one-dimensional nanostructures,[6] hybrid structures[7] or combination of nanoparticles and nanowires.[8] Furthermore, synthetic routes for the fabrication of photoanodes play a dominant role with respect to other possible approaches based on light confinement or electron transport in specific geometry configuration: one of the highest PCE values reported for a ZnO-based DSSC (6.58%)[9] has indeed been obtained by using ZnO nanoparticles. Larger values (7.5%) have been very recently obtained by introducing a buffer layer with the overall result of inhibiting electron recombination during the transport and collection process.[10]

Due to the poor chemical stability of ZnO in acidic dye solution and the formation of Zn2+/dye complexes that could decrease the electron injection rate from the dye molecules to the semiconducting electrodes, we propose a series of novel metal-free organic dyes as potential sensitizers for ZnO-based DSSC. Differing from the ruthenium complexes widely applied for DSSC, metal-free organic dyes only have one carboxylic acid anchoring group so that the chemical stability of the ZnO photoanodes can be improved in the acidic dye solutions.[11] In this project we propose the development of highly efficiency DSSC based on ZnO photoanodes and suitably prepared dye molecules optimized for ZnO. CNR of Italy will be in charge for the development of ZnO photoanodes, while NCTU of Taiwan will develop new dye molecules.

The wide possibilities in terms of obtainable shapes and easiness of synthesis offered by the wet-chemistry approach make the choice of liquid phase highly desirable and effective.[12] Exploitation of well established techniques (spray deposition and template-sustained aqueous growth) will allow obtaining a wide range of ZnO structures simply tuning either the working parameters or the amine exploited as template (in case of aqueous growth). Both synthetic approaches exploit low cost solvents with no (or very reduced) toxicity, and mild reaction conditions (preparation temperatures varying from 95 to 270 °C), together with a negligible waste production. One additional advantage of this strategy relies on the reduced environmental impact resulting from the material preparation. The design of the cell includes optimization of optical and electrical behavior of the oxide network.



The general idea to design a metal-free organic dye is to have the dye with a D-p-A configuration, i.e., a donor group (D) for pushing the electrons, an acceptor group (A) for pulling the electrons, the p-conjugated bridge to connect between D and A species, and an anchoring group linked to A to attach to the surface of ZnO. In this project we will use the triphenylamine derivatives for donor substitutes and cyanoacrylic acid or rhodanine-3-acetic acid groups for acceptor substitutes.

The new photoanodes will be sensitized with the new dye molecules and the corresponding devices will be assembled and tested. Benchmarking commercial dyes on ZnO photoanodes and the new dyes on commercial TiO2 photoanodes will be also applied to investigate the improved functional properties of the new ZnO-based DSSCs with respect to consolidated systems.

To optimize the cell performance and understand their electron-transport mechanism, photovoltaic and kinetics characterizations will be performed with frequency-domain (EIS/IMPS/IMVS) and time-domain (time resolved fluorescence, transient absorption and photocurrent/photovoltage decays) measurements.

Aside to the complementary activities on fabrication of ZnO photoanodes (CNR) and synthesis of new dyes (NCTU), round-robin benchmarking tests will be carried out in the two labs for cross check of the results obtained on the characterization of the devices. This aspect is indeed of fundamental relevance to assess the device reproducibility, which is a mandatory feature for possible fabrication scale-up.

The expected results, in terms of application of new dyes and new ZnO photoanodes can give concrete contribution to overcome the intrinsic limits of the state of the art excitonic cells (with overall efficiency around 13%), allowing the exploitation of all the potential of the excitonic solar cells, whose efficiency is not limited by the Queisser limit.

References
1 M. Graetzel, Nature, 2001, 414, 338.
2 B. O'Regan, M. Graetzel, Nature, 1991, 353, 737.
3 A. Yella et al. Science, 2011 334, 629.
4 F. Sauvage et al. J. Am. Chem. Soc., 2011, 133, 9304
5 A R. Rao, V Dutta, Nanotechnol., 2008, 19, 445712
6 M. Law et al. Nat. Mater. 2005, 4, 455
7 S. H. Ko et al. Nano Lett. 2011, 11, 666
8 S. Yodyingyong et al. Appl. Phys. Lett. 2010, 96, 073115
9 M. Saito, S. Fujihara, Energy Environ. Sci. 2008, 1, 280
10 N. Memarian, et al. Angew. Chem. Int. Ed. 2011, 50, 12321
11 W. Zhang, et al. Appl. Phys. Lett. 2009, 95, 043304
12 C. Wöll, Progr. Surf. Sci., 2007, 82, 55; L. Vayssieres, Adv. Mat. 2003, 15, 464; C. Pacholski et al. Angew. Chem. 2002, 41, 1188
13 D. S. Boyle et al. Chem. Comm. 2002, 80

Obiettivi della ricerca

Key objective is the improvement of PCE in DSSCs based on ZnO photoanodes and accordingly designed dye molecules. The goals are organized at three levels:
1) Development of new ZnO photoanodes;
2) Development of new metal-free organic dyes for sensitizer.
3) Fabrication and testing of the devices and benchmark with traditional systems.
The new photoanodes will seek for improved light scattering, high specific surface and electron blocking layer for inhibition of back electron recombination.
For the sensitizers, we will design and synthesize new series of metal-free organic dyes with broad absorption spectra and high absorption coefficients. In order to retard the charge recombination and to reduce the degree of dye aggregation, we will introduce long alkoxyl chains in the donor substitutes to form a blocking layer for the purpose of getting a better charge collection yield and a higher electron injection yield. The concept to design metal-free organic dyes and their possible components are summarized in the following scheme.

Charge transport kinetics will be investigated through transient photocurrent/photovoltage decay and EIS measurements. Charge injection properties from the new dyes to the ZnO photoanode will be investigated using ultrafast transient optical spectroscopies. Direct correlation between the optical properties of dyes and ZnO photoanodes, charge transport properties and external quantum efficiency of the device will allow optimization of photoanode and dye.

Ultimo aggiornamento: 23/05/2025