The morphology of the layers showed, both in dense and in nanoporous layers the nanosize of the grains, with a different compaction degree, Fig. 1a and b.

Fig. 1b Nanoporouse anatase matrix

The TiO2 anatase structure was identified in the XRD spectra, Fig. 2. The thin dense layer (100 nm), developed at a lower temperature, shows a peak corresponding to Ti8Oi5, a compound involving also Ti3+. The nanoporous thicker layers (1000 nm) consist only of anatase.

The efficiency of the final solar cell depends on the good conduction along each layer but limitative is the back contact therefore the structure of the dense layer must be free of pinholes for avoiding shunts. The I-V curves, Fig. 3, confirmed that the layers are electron conductive and that thin dense films without pinholes can be obtained.

Experiments proved that layers thinner than 100 nm, deposited on TEC8, exhibit discontinuities and have a poor conduction.

The defect concentration was calculated in the dense films using the Mott Schotky, Fig. 4. Using the simplified formula (1) the donor density in the space charge region is calculated from the slope of the curve Csc"2 as a function of electrode potential (V), representing the Mott Schottky plot:

1 _ 2 j_

Nd A2eeQ6r H

where H is now the slope of the Mott-Schottky plot and the values used for the constants in eq. (1) are : e=1,610-19C; eo=8.8542i0-12 Fm-1; er=55 and A=3.1410-4 m2.

Fig. 5 The impedance spectrum of the dense TiO2 Fig. 6 The equivalent circuit of the

dense TiO2

The impedance spectrum of the dense anatase film, annealed in oxygen (air), Fig. 5, revealed the existence of the shallow defects (high frequency measurements 1 MHz) while the low frequency signals (10 kHz) are the consequence of the deep defects. The RC equivalent circuit will be modified accordingly, Fig. 6.

The subsequent layers in the 3D cell are deposited at high temperature in an oxygen-free atmosphere. In order to test the behaviour of the anatase layers in the reductive environment, annealing in H2 was investigated.

Annealing in hydrogen increases the oxygen vacancies concentration. Annealing in oxygen exhibits the lowest oxygen vacancies concentration and according to the calculations based on the Mott Schottky plot, the value of the donor density is of 1016/cm3. The Raman spectra of both types of layers did not exhibit any change after hydrogen treatments confirming that the n-type anatse structure is preserved, Fig. 7.

The defect concentration is, however, sensitive to hydrogen annealing, as the photoluminescence measurements show, Fig. 8. The broad peak corresponding to the dense or nanoporous layers may be correlated with the defects existent in the anatase structure as grown via SPD: oxygen vacancies and interstitial titanium ions (+3 and +4). The sharper peaks and the shift of the maximum of the PL spectra to lower values after annealing will than be correlated with the modification of the defects ratio mainly due to the modification in the oxygen vacancies concentration. The XRD measurements showed no increase of the Ti8O15 peak after annealing confirming that interstitial titanium is practically not reduced in the hydrogen atmosphere, in the working conditions.

The photovoltaic response of a 3D cell with the structure: TEC8/dense n-type anatase/ nanoporous n-type anatase/ Al2O3 + In2S3 buffer / p — type nanoporous CuInS2/Au was recently reported, Fig. 9, [1]. The energy conversion efficiency of the cell with a geometrical area of 3.14 x 10-2 cm2 is about 4%. Under AM 1.5 irradiation from a calibrated source the cell has the open circuit voltage VOC of 0.49V, a short-circuit current, ISC, of 18mA/cm2 and a fill factor of 0.44.

Fig. 8

Photoluminescence
of TiO2 layers,

T= 10K

Conclusions

Using Spray Pyrolisys Deposition the porosity of nanostructurated n-type anatase thin layers can be controlled. By modifying the temperature and precursors composition dense and nanoporous layers can be obtained; the layers are not chemically reactive at high temperature, in hydrogen atmosphere. The layers are electron conductive and can be consequently used for developing 3D solid state solar cells using a complete aerosol technique.

References

5. M. Nanu, J. Schoonman, A. Gossens, Adv. Mat., 2004, 16, 5, 453

2. K. Tennakone, G. R.R. A. Kumara, I. R.M. Kottegoda, V. P.S. Perera, G. M.L. Aponsu, J. Phys. D, 1998, 31, 2326

3. B. O’Reagan, M. Gratzel, Nature, 1991, 353, 737

4. K. Ernst, A. Belaidi, R. Koenenkampf, Semicond. Sci. Technol. 2003, 18, 475

6. M. Nanu, L. Reijnen, B. Meester, J. Schoonman, A. Goossens, Chem. Vap. Deposition, in press.

7. A. Duta, M. Nanu, I. Visa, Sol. Energ. Mat. Sol. Cells, submitted for publication