The front contact

In order to produce a suitable n-type front contact for solar cells, it is necessary to have materials characterized by a high optical transparency, at least 90% in the visible region of the solar spectrum, together with a good electronic conductivity. In fact, these materials must be transparent, for letting the incident light pass through them and reach the beneath p-n junction and, at the same time, they are required to have a low electrical resistivity, for allowing a good extraction of photo-generated carriers.

Such materials can be obtained by producing an electron degeneracy in wide-gap oxides, with energy gap higher than 3 eV , by introducing in their lattice suitable non-stoichiometric defects and/or proper dopants.

In general, the electrical properties of oxides critically depend upon the oxidation state of their metal components (oxide stoichiometry) and on the nature and quantity of impurities

incorporated into the films.

Perfectly stoichiometric oxides are either insulators or ionic conductors. These latest ones are of no interest as transparent conductors due to the high activation energy required for ionic

conductivity.

The way to obtain good conducting oxides is doping them with dopants, which must have the same size as, or be smaller than the host ions replaced and don’t have to form compounds with the host oxide.

In particular, Indium oxide In2O3 is a semiconductor oxide with an energy gap of 3.5 eV.

As grown, it generally lacks stoichiometry because of Oxygen vacancies VO into the lattice, which act as double donors and each one provide two electrons to the material electrical conductivity. This material can be represented like In2O3-x where x is the VO concentration and should be a mixed conductor, having both electronic and O^2- ion conduction.

The donor carriers can be generated in it by doping this material with elements of the group IV, like Tin. In fact, Sn dopes n-type the In2O3 by substituting In, since In has three valence

electrons while Sn has four, providing so an “almost free” electron into the material lattice per each substitution.

Increasing the amount of Sn concentration into the material, it is possible to increase the n-doping up to very high values of carrier concentration, on the order of 5-10∗10²⁰ cm^-3 . At these high doping levels, the material has so many free carriers that a donor level, full of electrons, starts forming near the bottom of the conduction band, or even into the conduction band itself.

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Chapter 4Aaa Fabrication of a Cu(In,Ga)Se2/CdS thin film solar cella aa aaa aaaaaa

In this condition the material is called “degenerated” and since its donor energy levels are already into the conduction band, they need a very small energy to contribute to its electrical conduction. This is the reason of the metal-like high electrical conductivity of this material.

The In2O3:Sn, Indium Tin Oxide (ITO) has also a very high transparency, more than 90%, in the range of light which can be normally converted by photovoltaic devices (visible and near infra-red) and so, because of its very good optical and electrical properties, it is one of the most interesting material among the Transparent Conducting Oxides (TCOs) suitable to be used as front contact in solar cells.

Nevertheless, for a too high doping level the carrier concentration of ITO films decreases; this implies that a part of Tin remains electrically inactive, because, for an excessive Sn

concentration, there is a compensation of such donors and also a strong scattering of free charge-carriers. The high impurity amount introduced into the material results also in a distorted crystal lattice, which contribute to reduce either the concentration or the mobility of carriers.

This problem can be solved increasing the material deposition temperature.

It has been seen that, as the substrate temperature increases, the material carrier concentration increases too and this phenomenon may be due to an enhancement in the Sn atom diffusion and their better distribution into the In2O3 lattice. Giving them more energy, they are allowed to thermalize reaching the best energetic conditions, like donors from interstitial locations and grain boundaries into the proper In location sites.

At the same time, an increase in carrier mobility normally happens and it is regarded as the result of a better crystalline quality of the film deposited at higher temperatures, of about 400-450°C, which promote the enlargement in its grains size and reduce the grain-boundary scattering mechanisms. In this way the electrical-resistivity of the film decreases (Figure 33).

Quite similar effects have been observed also performing an annealing of ITO films at high temperature, even if they had been deposited at lower temperatures.

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64

Chapter 4Aaa Fabrication of a Cu(In,Ga)Se2/CdS thin film solar cella aa aaa aaaaaa

Even if ITO is considered an excellent transparent electrical contact, the high doping level makes this semiconductor degenerate, implying a near-infrared light absorption by its free carriers.

In fact, the material normally shows a band edge, in the UV spectral range, which is determined by its energy gap electronic transitions; instead, within the visible region its transmittance is very high and exhibits such extremes of minimum and maximum which are due to interference

effects.

More characteristic is the infrared region, in which the film material enters into a reflecting regime with metallic-like properties. The strong enlargement of this absorption and reflection region, called plasma edge, is associated to the excitations of the many free electrons present into the conduction band (Figure 34).

Figure 34. T) transmittance and R) reflectance of the ITO film with thickness of 1.656 μm [60].

To overcome this drawback, Zirconium doped ITO could be used, since it was discovered that for dielectric oxides, their permittivity can be increased by the addition of higher-permittivity oxides such as ZrO2 or HfO2 . It is also known that the permittivity of dielectric oxides increases rapidly with even small additions of a higher-permittivity constituent and this is especially true for high-frequency permittivity ε∞ . The increase in ε∞ due to Zr addition, can shift the plasma resonance wavelength λp , to a longer wavelength and this makes possible to improve the Near Infra Red (NIR) transmission significantly (Figure 35), without altering the material properties like its carrier concentration or its mobility.

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65

Chapter 4Aaa Fabrication of a Cu(In,Ga)Se2/CdS thin film solar cella aa aaa aaaaaa

Figure 35. T) transmittance, A) absorption and R) reflectance of ITO and ITO:Zr (ITZO) thin films [61].