CHARACTERISATION OF TIO2 LAYERS. FOR THREE DIMENSIONAL SOLAR CELLS

Anca DUTA, Transilvania University of Brasov, Center for Sustainable Development, Romania

Marian NANU, Delft University of Technology, the Netherlands

Introduction: The concept of three dimensional(3D) solar cells lately developed, […], represents a promising alternative to silicon solar cell. This type of solar cell is based on a composite p-n heterojunction mixed on a nanometric scale.

Dense and nanoporouse n-type anatase layers are obtained by Spray Pyrolisys. These films solve the electron conduction in a 3D solar cell with the structure TEC8/dense n-TiO2/nanoporous n-TiO2/ p-CuInS2/Au.

Thin layers of TiO2 were obtained using titanium tetra-isoprpoxyde (TTIP) as precursor. The morphology, crystallinity and conductive properties are discussed in conjunction with the deposition parameters.

The photovoltaic response of the 3D solar cell is presented.

The Three — Dimensional Solar Cell

The development of energy-friendly alternatives to the silicon solar cells, efficient and with short payback time, represents a research topic formulated in the past 15 years. The solar cells involving only solid state compounds, not silicon based, are now-a-days developed: the ETA cells, [2] (where a thin nanoporous n-type and p-type semiconductors) represents an advent of the dye-sensitized (Gratzel type), [3] solar cell.

With the advent of polymer bulk heterojunction and ETA solar cells, a three dimensional (3D) solar cell represents a further development where the n-type nanoporous wide band gap semiconductor thin layer is infiltrated with a p-type, light absorbing nanoporous semiconductor. The large interpenetration nanoporous heterojunction overcomes the drawback of the low conduction in solid state.

In a 3D solar cell the n-type semiconductor is usually anatase TiO2, chosen because its inert chemical behavior in various environment. This condition is necessary to be fulfilled since the p-type semiconductor, CuInS2 (with the band gap of 1.55 eV) is obtained in deposition and reductive annealing conditions that can affect a more reactive oxide (e. g. ZnO considered state-of-art in the ETA cell). For avoiding shunts at the back contact interface, the anatase consists of two layers: a dense thin layer, with low flexibility (100 nm) and the nanoporous matrix (1000 nm) able to be infiltrated with CuInS2.

Literature reports different techniques for obtaining solid state solar cells: The ETA cells with the highest efficiency (2%) are obtained by dipping of microporous anatase TiO2 in liquid precursors of Cd and Te, [4]; CuInS2 for solar application was obtained by vaporization of the metal precursors followed by annealing. Recently, CVD and AlD deposition was reported for CuInS2 in a 3D solar cell, [1].

Spray Pyrolisys Deposition (SPD) is a simple and attractive technique and the aim of developing a 3d solar cell using only this procedure is already formulated.

This paper investigates the possibility of obtaining dense and nanoporous anatase TiO2 layers using SPD. The deposition parameters are discussed in conjunction with the morphology, structure and conduction properties of the obtained layers.

Experimental

Absolute ethanol (EtOH, 99.99% Aldrich) solutions of TTIP (97% Aldrich) were prepared and acetylacetonate (AcAc 99+%, Aldrich) was added in order to obtain regular morphologies.

Dense thin TiO2 films were prepared by spraying, at 350oC, a mixture with a volume ratio TTIP : AcAc : EtOH = 1 : 1.5 : 22.5.

The nanoporous films were prepared by SPD at 450oC, using a mixture with the volume ratio: TTIP : AcAc : EtOH = 1.5 : 1 : 18.33. The substrate was conductive TCO/TEC8 glass (SnO2:F) with a low internal resistance but a rough surface morphology.

A complete ETA cell was developed having a dense n-type anatase layer deposited by SPD, a buffer layer of In2O3 and the absorber — p type CuInS2 both layers being deposited using Atomic Layer Deposition (ALD) as described elsewhere, [5].

The films were characterised as such and after annealing in reductive atmosphere for six hours (hydrogen gas, at 450oC and 1 mbar).

The crystalline structure was investigated by XRD (Bruker D8, CuKal ).

The morphology of the platinum sputtered layers was investigated by Scanning Electron Microscopy (Joel JM 5800 LV). Luminescence spectra were recorded with a home-built set-up (TU Delft) using a Spectra Physics Millennia Nd:YVO4 laser with a wavelength of 532 nm. The recordings were done in a backscattering mode, using a set of notch filters to remove the Rayleigh scattering and a liquid nitrogen flow cooled the CCD camera (Princeton Instruments LN/CCD-1100PB). A Spex 340E monochromator equipped with a 100 grooves/mm grating was used. Corrections for the filters, the sensitivity of the CDD camera and the monochromator are applied. The same set-up, used in a backscattering mode using a set of notch filters and a Spex 340 E monochromator equipped with a 1800 grooves/mm grating, was used for the Raman spectra.

A Solartron 1286 Electrochemical Interface was used for potentiostatic control and to conduct the Mott Schottky and flat band measurements.

For the ETA cell, the current-voltage (I-V) curves are recorded with a DC source Meter (Keithley, Model 2400) in tha dark and under illumination. A calibrated solar simulator, SolarConstant 1200 (K. H. Steuernagel Lichtechnik GmbH) is used as artificial light source.

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