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Study of structure and electronic properties of high performance CdTe solar cells by electrical investigation.
No full textVedasi abstract in lingua ingleseElectrical 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
Effects of activation treatment on the electrical properties of low temperature grown CdTe devices
No full textCdTe has shown a significant potential for high mass production, resulting to be one of the cheapest photovoltaic technology available. Efficiencies exceeding 17% have been obtained by the application of high temperature CdTe deposition.
However a very different role of the CdCl2 activation treatment for the low and high temperature deposited CdTe is known: requiring a strong grain size enhancement only in the first case.
In our labs, vacuum evaporated CdTe is typically treated by CdCl2-methanol saturated solution. Different amounts of this solution have been applied on CdTe layers. Final conversion efficiencies are ranging, depending on the different treatments, between 6 and 13%. The electrical properties of these finished devices have been studied by means of current-voltage, capacitance-voltage, drive level capacitance profiling and admittance spectroscopy in order to connect the electrical properties of the device with the different activation treatments.
Admittance spectroscopy shows a different defect energy distribution depending on the applied activation treatment, drive level capacitance profiling the different doping profile and the different transport mechanisms are addressed by studying the behavior of dependence of I-V characteristics with temperature
Ageing of CdTe Devices by Copper Diffusion
No full textCdTe solar cells are widely used in industrial production and they currently have the lowest cost per Watt available in the market thanks to its simple and scalable technology. One of the main engineering challenges for these devices is to provide a suitable back contact for CdTe due to its high electron affinity which requires a material with a very high work function. Solar cells with copper based back contacts have shown the highest efficiencies, however it is well known that their performance reduces with time,
mainly connected with diffusion of Cu through the absorber. Working conditions (i.e. light intensity, temperature and applied bias) can dramatically affect the degradation speed. In order to study the impact of the bias on the Cu diffusion, then on performance degradation, three different kinds of stress were applied on identical CdTe solar cells with Cu/Au back contact (Dark without bias, light-temperature without bias, light temperature with bias). The degraded devices have been periodically analyzed by currentvoltage, capacitance-voltage, drive level capacitance profiling and admittance spectroscopy. A detailed analysis of defect characterization and distribution has
been addressed
Influence of CdTe thickness on structural and electrical properties of CdTe/CdS solar cells
No full textDue to its high scalability and low production cost, CdTe solar cells have shown a very strong potential for large scale energy production. Although the number of modules produced could be limited by tellurium scarcity, it has been reported that
reducing CdTe thickness down to 1.5 microns would solve this issue. There are, however, issues to be considered when reducing thickness, such as formation of pinholes, lower crystallization, and different possible effects on material diffusion within the interfaces. In this work, we present the study of CdTe solar cells
fabricated by vacuum evaporation with different CdTe thicknesses. Several cells with CdTe thickness from 0.7 to 6 microns have been fabricated. The deposition process has been optimized accordingly and their physical and electrical properties have been studied. Thin cells show a different electrical behavior in terms of open circuit voltage and fill factor. Efficiencies range from 7 % for thin
CdTe cells up to 13.5 % for standard thickness
Study of difluorochloromethane activation treatment on low substrate temperature deposited CdTe solar cells
No full textn this paper we describe a low substrate temperature CdTe fabrication process by vacuum evaporation (VE) where the recrystallization step avoids the use of CdCl2, which is a carcinogenic material. In our process CdS and CdTe are deposited at pressures of 10−6 mbar in the same chamber by thermal evaporation and with substrate temperature of 150 °C and 340 °C, respectively. The CdTe/CdS/TCO stacks are then put in a quartz chamber and, after having previously pumped the chamber down to 10−5 mbar, a controlled mixture of argon and chlorine containing gas is fluxed with a substrate temperature in a range between 400 °C and 450 °C. The morphological properties of the single layers are studied by X-ray diffraction (XRD), atomic force microscopy (AFM), cathodo-luminescence (CL) and the electrical properties of the finished devices are presented by means of current–voltage (I–V) and capacitance–voltage (C–V) measurements
Etching effect of CdTe absorber on the stability of thin film solar cell devices
No full textCu seems to be necessary to achieve best
efficiencies, however it is strongly connected with
performance degradation due to its tendency to diffuse.
The Cu benefits and drawbacks are still not completely
clear. Other studies have shown a direct connection
between Cu and defects concentration, however it is still
not clear if the Cu induced degradation is due to a
compensation or enhanced recombination due to the
formation of new defects in bulk CdTe. Within this study
many samples with Cu/Au back-contact have been
prepared with different etching processes applying
thermal, luminous and electrical stresses. We have
analyzed the aging effects of those stresses by means of
current-voltage, capacitance-voltage, drive level
capacitance profiling characterization techniques
Improved stability of CdTe solar cells by absorber surface etching
No full textFor CdTe solar cells copper seems to be necessary to achieve best energy conversion efficiencies, whilst it is known to be the main reason of cell performance degradation due to its tendency to diffuse through the bulk. Some studies have shown a direct connection between defect concentration and copper, but little has been discussed about its relation to the CdTe etching. Within this study many samples with Cu/Au back-contact have been prepared with different etching times and tested applying thermal, luminous and electrical stresses. We have analyzed the aging effects on the cell performance and on the nature and concentration of the defects by means of a variety of characterization techniques, like atomic force microscopy, Raman, current-voltage, capacitance-voltage, drive level capacitance profiling and admittance spectroscopies. Results of the accelerated lifetime tests show that different performance degradation is observed for cells with differently etched absorber. Solar cells made with optimized etching have very low degradation while the strongest performance reduction is detected for unetched cells despite their initial efficiency is as high as for the case of etched absorbers
Device and Physical Properties of Solar Cells with Thin CdTe Absorbers
No full textTellurium is considered as a rare element and there are controversial reports on its real amount available on the earth. It has been reported that with the current CdTe technology in the market it would be difficult to produce more than about 30 GW per year. On the other hand reducing dramatically CdTe thickness, the
production could increase substantially up to 150/250 GW per year (depending on the thickness reduction of the absorber). In our laboratory we have prepared solar cells by depositing CdTe with different thicknesses: from 0.7 to 6 m. Ultra-thin absorber
(between one and two microns) devices have demonstrated efficiencies around 10%, higher efficiencies are reported for thicker CdTe absorbers.
The structural properties of the absorbers have been studied by X-ray diffraction spectroscopy and by atomic force microscopy. Finished solar cells were analyzed by electrical characterization (currentvoltage,voltage, drive level capacitance profiling, capacitancevoltage and admittance spectroscopy)
Superior stability of ultra thin CdTe solar cells with simple Cu/Au back contact
No full textDue to its high scalability and low production cost, CdTe has shown a significant potential for high mass production,
resulting to be one of the cheapest photovoltaic technologies available. Efficiencies exceeding 20% have been
obtained by the application of high temperature CdTe deposition.
However tellurium scarcity is a limitation for mass production and one of the possibilities to overcome this is the
reduction of absorber thickness.
We have already demonstrated efficiencies above 11% for devices with 1.5 μm thick CdTe. Nowadays we have
fabricated ultra-thin absorber devices performing more than 13% efficiencies. But what is most interesting is
that we have observed a different electrical operation and stability, connected to the fact that the depletion region
takes a very large part of the device.
In this work many CdTe solar cells with a standard Cu/Au back contact, made with different absorber thicknesses,
were prepared, stored in dark and tested at different aging times, showing different reactions to the aging and in
particular a remarkable stability as CdTe thickness reduces
STUDY OF ELECTRO-OPTICAL PROPERTIES OF CdTe SOLAR CELLS PREPARED BY A LOW TEMPERATURE FABRICATION PROCESS
No full textCdTe solar cells are prepared by a low temperature vacuum evaporation, a process suitable for
growing flexible cells on polymer substrates, together with an alternative activation post-deposition thermal treatment
that avoids the use of dangerous CdCl2. Usual CdCl2 treatment is also employed in order to compare the effect of
Freon and CdCl2 annealing procedure on electro-optical properties of the solar cells. The obtained CdTe/CdS/ITO
structures are studied as function of Freon annealing temperatures and time. For both treatment procedures the CdTe
grain size increase due to the annealing is assessed by SEM analyses. SEM-cathodoluminescence studies show a
large increase in the radiative recombination efficiency of the treated CdTe. The luminescence intensities of CL
bands, of treated samples, increase of about one order of magnitude, respect to the untreated ones, demonstrating an
improvement of the crystalline quality and a decrease of the non-radiative recombination centres in the annealed
materials. EBIC analyses showed that no correlation between grain size and signal exists, in addition the comparison
with cathodoluminescence mapping evidences that these two contributions are coming from different regions of the
sample
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