3,148 research outputs found
The Importance of Exploring and Developing a Wide Variety of Photovoltaic Technologies
No full textAccording to the latest Copernicus report [1], 2024 is expected to be the warmest year on record and the first year with temperatures more than 1.5 °C above pre-industrial levels; however, this growth is concerningly in line with the temperatures recorded last year and in previous years [2]. Furthermore, in recent decades, the effects of climate change have become increasingly evident. The impact of global warming on ecosystems also affects the lives and livelihoods of millions of people worldwide, making natural disasters more frequent and powerful while also exacerbating existing social and economic inequalities. It is well known that the natural climate has oscillated between warm periods and ice ages for the past million years. This fluctuation is closely related to Milankovitch cycles. However, the scientific community agrees, based on numerous studies, data, and simulations, that the changes in global temperature during the twentieth century can only be explained if both natural and anthropogenic processes are considered [3]. In particular, global warming is associated with the increasing concentration of greenhouse gases in the atmosphere. The Earth’s temperature is determined by the energy balance between absorbed solar radiation and emitted infrared radiation, and greenhouse gases absorb infrared radiation from the Earth’s surface. Without them, the Earth’s temperature would be close to minus 20 °C, making life as we know it impossible. On the other hand, an increase in greenhouse gases in the atmosphere reduces the emission of infrared radiation, creating an imbalance that results in an increase in temperature [4]. Approximately three-quarters of emissions of these gases come from energy use and consumption [5]; this is why replacing conventional energy sources with clean, renewable energy technologies is paramount. Furthermore, energy demand is expected to rise exponentially in the coming years, making it increasingly necessary to resort to energy sources that, unlike fossil fuels, are not at risk of depletion [6].
Renewable energy sources, such as solar, wind, hydropower, ocean and geothermal energy, biomass, and biofuels, are inexhaustible and cleaner alternatives to fossil fuels. They can lower greenhouse gas emissions, diversify our energy options, and reduce our dependence on volatile fossil fuel prices. Exploiting all these sources and utilizing every possible technology to diversify energy production is essential. Many of these sources are not present everywhere, nor are they constant over time. On the other hand, each source can be efficient for energy production if utilized in the right location and for the most suitable application. Therefore, it is essential not to focus on just one alternative energy source; the combined development and use of multiple sources can solve this pressing issue.
One of the technologies that harness renewable energy sources, specifically solar energy, is photovoltaic (PV) PV technologies have been classified, depending on their development over time, into first-, second-, third-, and fourth-generation devices:
The first generation is based on monocrystalline and polycrystalline silicon and gallium arsenide.
The second generation focuses on developing thin-film PV technologies, such as CdTe and CIGS.
The third generation includes “emerging technologies”, characterized by low manufacturing costs, non-toxicity, and elemental abundance of their constituent, such as perovskites and organic cells, as well as multi-junction devices.
The fourth generation refers to a new hybrid technology under development that uses nanoparticles or organic nanomaterials such as graphene, carbon nanotubes, and graphene derivatives.
Subsequent generations have been developed, in some cases, to reduce production costs and, in others, to address new technological needs or applications, without replacing or interrupting research on earlier technologies. To date, the market is still dominated by panels from the first two generations: approximately 97% consist of monocrystalline silicon, with the remainder consisting of CdTe thin-film devices [7]. This does not mean that research on new materials is useless; rather, it is essential to differentiate technologies and develop them all to ensure the proper technology is available for each purpose. Research is now focused not only on the development of traditional photovoltaic panels but also on, for example, devices for powering the Internet of Things (IoT) [8], and cells integrated into construction elements, building materials (building-integrated photovoltaics, BIPV), or space applications. Therefore, it is essential to develop materials that enable the development of devices with diverse characteristics, such as semi-transparent or colored cells, materials that can be deposited into light and flexible substrates, devices in superstrate or substrate configurations, etc
New paradigms for advances in efficiency and stability of CdTe solar cells
No full textPolycrystalline thin film CdTe continues to be a leading material for the development of cost effective and reliable photovoltaics. Although research in CdTe dates back to the 1950s, it is a very innovative and promising technology. Thanks to the continuous development, in 2016 an efficiency of 22.1 % has been achieved, a gain of about 5 percentage point compared to the efficiency of 2012. In these last years many innovations have been made, but yet the theoretical efficiency has not been obtained and there is still a large room for improvement. Moreover many physical and electronic mechanisms in the devices are still not well understood, the understanding of which would lead to further improvement. Thus the research work on CdTe is far from being finished. In our laboratory, superstrate configuration CdTe solar cells, fabricated by low temperature process, have achieved efficiencies exceeding 16 %. However, the continuation of our studies and the optimization of our processes will lead to even better efficiencies. In this thesis I have presented the latest innovations we have introduced in the standard CdTe device: in particular our work and our studies concerning the window layer, the activation treatment, and the back contact. We did not just reproduce the changes introduced in the cells by the world leader First Solar; we looked for and followed different directions. Since it is very difficult to predict what direction will lead to the best results (as achieving the maximum theoretical efficiency and the improvement of the cells stability) it is very important to follow also alternative ways. Study of alternative window/buffer layer The treatment in difluorochloromethane gas of a thin CdS layer, in the order of 30-80 nm, allows to improve the short circuit current of the devices, reducing the open circuit voltage loss. While, generally, cells with thin CdS layers are affected by micro-pinholes resulting in lower open circuit voltage. The insertion of a high resistance transparent HRT layer of Mg-doped ZnO between ZnO or ITO and CdS leads to an improved band alignment with the CdS buffer layer, compared to ITO or ITO/ZnO stacks. In fact, as the band gap of MZO is tunable by changing the amount of Mg in the compound, it is possible to optimize the band alignment. This is probably the cause of the FF improvement up to 74 %, which allows to reach efficiencies up to 16.2%. Because of the tunable band gap of MZO, it is even theoretically possible to tune the MZO band gap and electron affinity in a way to have an optimum match with CdTe, forming MZO/CdTe heterojunction. Thus CdS layer has been totally removed, dramatically increasing the short circuit current. This has been confirmed by the EQE response, with a significant absorption improvement in short-wavelength region. Good efficiencies close to 13 % have been obtained, with Voc above 885 mV and short-circuit current of 27.0 mA/cm2, but low fill factor. This is probably the symptom of the low quality of the MZO/CdTe junction, which could be caused by the presence of a large amount of defects due to the reticular mismatch and possibly by an insufficient doping of MZO. Despite the results obtained so far, regarding the window/buffer layer innovations, a further optimization of the MZO layer and engineering of the MZO/CdTe heterojunction is the simplest way to increase the samples’ efficiency. Study of the alternative MgCl2 activation treatment Solar cells with efficiencies exceeding 14 % have been obtained by replacing the traditional treatment agent CdCl2, with MgCl2. MgCl2 has demonstrated to be a good alternative to the conventional CdCl2, with the advantage to be non-toxic and less expensive. However devices produced with CdCl2 still exhibit a higher efficiency and this could be related to the lower charge density and to a different structure of defects for the MgCl2 treated samples. For this reason, currently CdCl2 activation treatment remains the most effective. Study of alternative back contact The insertion of MoOx at the back contact has led to the fabrication of samples with efficiencies exceeding 13%. However, in order to achieve these efficiencies, the presence of copper at the back contact is still necessary. MoOx deposited by reactive sputtering does not make a good ohmic contact with low substrate temperature CdTe. The insertion of MoOx improves neither the efficiency of our superstrate CdTe solar cells nor their stability. Reducing the thickness of the copper layer in the cells from 2.0 nm (our standard) to 0.1 nm, deposited by vacuum evaporation, the samples average efficiency decreases from 15.6 % to 13.3 %. However the analysis suggest that a 0.1 nm thick Cu layer is enough to dope a 7 μm thick CdTe layer, while the additional copper amount leads to the formation of compensating defects. The fill factor is the main parameter which negatively affects the J-V characteristics of the low Cu samples. It is clearly caused by a marked roll over, symptom of a high back contact barrier. Thus, despite a very thin copper layer succeeds in CdTe doping, this thickness is insufficient to favor the perfect ohmicity of the contact between CdTe and gold. This study leads to interesting conclusions about cells degradation related to copper content. In devices with a large amount of copper (a 1-2 nm thick layer), a higher amount of Cu diffusion from the back contact leads to the formation of a larger amount of compensating defects, as an excessive Cu diffusion can compensate the CuCd- acceptors with Cui donors. While in cells with a 0.1-0.5 nm thick Cu layer, the degradation is mainly due to the decay of the CuCd acceptor into the donor acceptor pair Cui VCd. A CuCl2 wet deposition method for inserting in the CdTe structure a quantity of copper equivalent to a 0.1 nm thick Cu layer has been developed. This process allows reduction of the incorporated copper quantity without any loss in performance compared to a standard Cu contacting route and simple and rapid depositing evenly over the entire area of the sample. It allows the formation of Cu-Cd acceptor defects, reducing the formation of the compensating donors Cui. Moreover, a dramatic improvement in performance stability has been achieved. The introduction of this method in the industrial production could really improve the lifetime of CdTe panels, without having too much efficiency losses
CdTe-Based Thin Film Solar Cells: Past, Present and Future
No full textCdTe is a very robust and chemically stable material and for this reason its related solar cell thin film photovoltaic technology is now the only thin film technology in the first 10 top producers in the world. CdTe has an optimum band gap for the Schockley-Queisser limit and could deliver very high efficiencies as single junction device of more than 32%, with an open circuit voltage of 1 V and a short circuit current density exceeding 30 mA/cm2. CdTe solar cells were introduced at the beginning of the 70s and they have been studied and implemented particularly in the last 30 years. The strong improvement in efficiency in the last 5 years was obtained by a new redesign of the CdTe solar cell device reaching a single solar cell efficiency of 22.1% and a module efficiency of 19%. In this paper we describe the fabrication process following the history of the solar cell as it was developed in the early years up to the latest development and changes. Moreover the paper also presents future possible alternative absorbers and discusses the only apparently controversial environmental impacts of this fantastic technolog
CdTe-Based Thin Film Solar Cells: Present Status and Future Developments
No full textCdTe solar cells are the most successful thin film photovoltaic technology of the last ten years. It was one of the first being brought into production together with amorphous silicon (already in the mid 90 s Solar Cells Inc. in USA, Antec Solar and BP Solar in Europe were producing 60 × 120 cm modules), and it is now the largest in production among thin film solar cells. CdTe solar cells stand out for the robustness of the absorber material: its high chemical stability and the large variety of successful preparation methods available make them suitable for large area module production. Compared to other thin film absorber materials, CdTe has an optimum band gap of 1.5 eV so that it could deliver efficiencies above 27%, with an open- circuit voltage of 1 V and a short-circuit current density of 30.5% mA/cm2. In this chapter, we will follow the history of the fabrication process together with each and every improvement explaining the discoveries and achievements that have brought to the record efficiency of 22.1%. Moreover, the environmental impact, the future applications and the possible evolutions of this technology will be also described
Low substrate temperature CdTe solar cells: A review
No full textCdTe photovoltaic technology is one of the first being brought into production together with amorphous silicon (already in the mid 90s Solar Cells Inc. in USA, Antec Solar and BP Solar in Europe were producing 60 × 120cm modules) and it is now the largest in production among thin film solar cells (Photovoltaics Report, 2014).CdTe has high chemical stability and a large variety of successful preparation methods available, which makes this technology one of the most suitable for large area module production.Historically there are two large categories of CdTe photovoltaic devices depending on the substrate deposition temperature, typically low temperature processes are considered when substrate temperature is below 450 °C.In this paper we will describe the last progress of CdTe based thin film solar cells, fabricated with low substrate temperature process, and their pros and cons
Cadmium telluride as a potential conversion surface
No full textIn instruments for low energetic neutral atom imaging of space plasmas, a charge state conversion surface (CS) is used to convert neutral atoms into ions for detection. We investigated a cadmium telluride (CdTe) coated sample as a novel material candidate regarding its suit- ability to be used as a CS. We measured the efficiency of converting H and O atoms into negative ions by surface scattering, as well as their angular scattering distribution, for energies from 195 eV to 1 keV at 8 incidence angle. Also, the energy distribution of scattered particles was recorded for incident Oþ2 ions, which confirms that molecules are mainly scattered as single atoms. The mean energy loss per atom was about 45%. The negative ion yield from scattering off CdTe was up to 13% for O and about 2% for H, which is comparable to other CS coat- ings in use. CdTe shows a nearly circular angular scattering cone of width comparable to established CS materials. We conclude that CdTe is a viable CS coating material for ENA instruments in space applications
Analysis of the drying process for precursors of Cu2ZnSn(S,Se)4 layers by low cost non vacuum fabrication technique
No full textCu2ZnSn(S,Se)4 solar cells were fabricated by a non-vacuum solution-based spin coating technique followed by annealing in selenium atmosphere without H2S treatment. The effects of the drying time after spin coating on physical and electrical properties of the cells were investigated. Structural and morphological properties were investigated by X-ray diffraction, Raman spectroscopy, energy dispersive X-ray analysis, atomic force microscopy and scanning electron microscopy. The physical properties, in terms of stoichiometry, crystal structure, morphology and orientation of the grains were shown to be influenced by the different drying times. Consequently, current-voltage characteristics show that the drying time of the precursors has a significant involvement in the device performance. The highest efficiency has been measured for 5 min drying time (with a 4.73% efficiency is obtained with Voc = 320 mV, Jsc = 28.6 mA/cm2 and FF = 52%), while samples with longer or shorter drying time have lower efficiencies. A further increase in efficiency, exceeding 5%, is obtained by inserting a contact grid on the front contact. Also accelerated stability tests (performed in a black box with one sun illumination at 80 degrees C) show different effects in light soaking, almost no degradation is observed for all the three cases. In this paper we highlight the connection of the different physical to the electrical properties generated just by the different drying times of the precursor for the CZTS absorbers
STUDY OF CDSETE/CDTE DEVICES FABRICATED BY THERMAL EVAPORATION
No full textOur group fabricates and studies CdTe based solar cells deposited by thermal evaporation method at low substrate temperature (below 450 °C). In this paper we present and study CdSeTe/CdTe cells, where the CdSeTe compound is formed by evaporating in sequence a CdSe and a CdTe layer, subsequently mixed by a CdCl2 activation treatment. These devices, with an absorber thickness of only 2 μm, show current densities above 27 mA/cm2. Earlier we also introduced an innovative method to develop CdSeTe by treating a thin CdTe layer at high temperature in selenium atmosphere (selenization), which gets subsequently mixed with a thicker CdTe layer. We study the structure of the new devices completely manufactured by evaporation and we compare them with the samples fabricated by selenization. This reveals how the manufacturing method of the CdSeTe layer affects the device properties
Superior stability of Sb-doped CdSeTe/CdTe devices
No full textOne of the primary activities in CdTe solar cell research is identifying an alternative to copper doping. Copper has limited solubility in the CdTe matrix, which restricts the achievement of higher open-circuit voltages (Voc). It is also a fast diffuser, making it a major contributor to device degradation. Finding a suitable alternative doping element could enhance Voc, improve cell efficiency, and increase stability. This study introduces a new approach for inserting Sb in CdSeTe/CdTe as a dopant: a thin Sb2Se3 layer is deposited on CdTe, and a CdCl2 treatment is used as a conveyor for Sb in the CdTe matrix. This simple and easy method proves to be effective, as Sb-doped cells achieve efficiencies comparable to those of Cu-doped cells. Furthermore, in accelerated stress tests without encapsulation, Sb-doped samples demonstrate much greater stability than Cu-doped cells, equivalent to undoped devices
ANALYSIS OF SELENIZATION TEMPERATURE FOR THE PERFORMANCE IMPROVEMENT OF SPIN COATED CZTSSe SOLAR CELLS
No full textIn our laboratory, Cu2ZnSn(S,Se)4 (CZTSSe) films are prepared by selenization of spin coated CZTS precursors. In this study, different temperatures are tested for the selenization process and their effects on the device are investigated. The influence of the selenization temperature on the Se incorporation and structural properties of the absorber have been analyzed by X-ray diffraction, Raman spectroscopy and scanning electron microscopy methods. Current-voltage analysis reveals a fill factor reduction with increasing temperature, concurrently with an improvement in short circuit current and efficiency. SLG/Mo/CZTSSe/CdS/iZnO/ITO/Au cells fabricated with selenization process at 550 °C reach an efficiency of 6.3 %
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