Study of structure and electronic properties of high performance CdTe solar cells by electrical investigation.

Electrical characterization is a powerful investigation method for semiconductor devices. Compared to other types of characterization, its main advantage consists in the possibility to analyze the finished devices. For many kinds of technologies this issue is mandatory to understand deeply the device structure, its operation mechanism and how the materials can change during the fabrication process. Therefore, electrical characterization techniques represent probably the most important feedback in the device development. In this thesis a possible methodology to investigate thin films solar cells by means of some electrical characterization techniques will be described. Moreover I will show how these methodologies have been used to extract important information for CdS/CdTe solar cells fabricated in our laboratories. This information has been very useful to develop and optimize a low temperature (< 450 °C) production process so to be able to achieve CdTe solar cells with efficiency exceeding 14 % (best cases over 15 %). In general, the investigations which constitute the chapters of this thesis, have been approached changing in reasonable manner important process parameters and, then, analyzing the resulting effects on the electronic and structural properties of the materials and consequently on the devices. It is known that CdTe needs a special “activation treatment” to perform high efficiency devices. This treatment has been studied to further assess the “magic” benefits on the CdTe semiconductor properties. Two different activation treatments have been optimized in our labs. The first is based on a mixture of gases Ar and difluorochlorometane (Freon®), already used by other researchers, who demonstrated its effectiveness on CdTe cells fabricated at high temperature. The second is based on the deposition of a liquid solution of CdCl2 in methanol and a subsequent annealing in air. Solar cells with CdTe treated in these two different ways were fabricated and analyzed also by means of electrical characterization. Results were compared also with cells fabricated at high temperature kindly provided by Parma University. CdCl2 treatment was able to recrystallize the low temperature deposited CdTe also by improving the electrical properties, while the gaseous treatment was demonstrated to be weak in increasing the carriers concentration but, at the same time, too invasive in affecting the intermixed layer at the CdS/CdTe interface, with an excess of sulfur diffusion. The treatment based on liquid CdCl2 has been further investigated modulating its effectiveness. Under-treated, sub-optimum, optimum and over-treated samples were prepared and analyzed, in order to address the changes involved by the activation treatment on the films composing the devices. It has been demonstrated that a strong connection between the treatment effectiveness and the defect concentration in CdTe polycrystals is present. The CdTe carrier concentration increases as treatment increases but at the same time recombination is also enhanced by the deep defects close to the junction, the optimum treatment represent the best tradeoff between this two phenomena. Another important issue in thin film devices is the absorber thickness, which is desired to be as small as possible. Unfortunately the scaling is usually challenging. By preparing several numbers of cells with different CdTe thickness, it has been demonstrated that the problems connected with thickness are not only light absorption and films homogeneity, but most important they are mainly generated by different materials composition and different transport mechanism. Within this study solar cells with 1.5 µm of CdTe and efficiency exceeding 10 % have been fabricated. Finally, the performance degradation of CdTe solar cells with Cu/Au back contact has also been investigated by electrical characterization. Identical samples were stressed for long time in different condition of light, temperature and electrical bias. Different Cu migration has been observed for the different kinds of stresses, excluding the hypothesis that at different stresses it corresponds just only a different diffusion speed. Moreover we conclude that probably Cu in our samples does not migrate as positively ionized like it has been proposed by other authors, but in negative or neutral configuration generating middle band defects, which enhance recombination.