rus
In the area of flexible electronics and photonics such areas were developed as the organic light sources, the films with controlled optical properties as well as flexible solar power panels.
In addition to commonness of the targeted functions, they are similar in some cases from the points of view of the material science and producing technologies. Considered
solutions and advanced developments harmonize with
the new industrial human environment.
In addition to commonness of the targeted functions, they are similar in some cases from the points of view of the material science and producing technologies. Considered
solutions and advanced developments harmonize with
the new industrial human environment.
Теги: electrochromic structures flexible solar power panels organic light-emitting diode photochromic structure гибкие солнечные источники энергии органический светодиод фотохромные структуры электрохромные структуры
This publication aims to systematise the information on the modern science, technology and industry solutions in the field of flexible photonics in the context of the present problems of artificial and natural lighting, dimming, and conversion of solar energy in the traditional human environment (private housing, public and industrial buildings, vehicles). Considered engineering solutions and advanced developments harmonize with the new industrial environment.
Organic Light-Emitting Diodes (OLED)
Organic Light-Emitting Diode is one of the key solutions for lighting in the future. The shape, size, design, colour and light output, all of these properties of OLEDs can be varied and picked up in an extremely large number of combinations.
OLED is a solid-state semiconductor light source based on the functional layer of organic material sandwiched between the anode and cathode layers on the substrate (fig.1). The functional layer has by a rather complex thin-film structure which is schematically shown in fig.2.
The anode and cathode are the sources of charge carriers, holes and electrons (fig.2, a). Under the influence of an applied electric potential they are moved respectively in the injection layers of holes 1 and electrons 7 whose properties allow the acceleration of the charge carriers. Through the hole conduction layers 2 and electronic conductivity layers 6 with a minimal of recombination, charge carriers fall into the electron blocking layers 3 and hole blocking layers 5 (fig.2, b). This ensures control of their passage as well as the spatial restriction of the recombination area to reduce non-emissive losses. Recombination of electron-hole pairs occurs in the emission layer 4 (fig.2, c) with the emission of photons (fig.2, d).
Such thin film structure needs a reliable sealing for protection against the influence of oxygen and water vapour with a glass lid or multilayer film coating for structures on a flexible substrate.
Thus the OLED is an ultra-planar emitting light source, the most important difference of which is the diffusion (Lambertian), spatially distributed nature of the emission not requiring the using of any additional optical elements.
The luminescence spectrum of the OLED is rather narrow (50-100 nm). The position of spectral peaks is determined by the characteristics of the materials used. The spectrum covers almost the entire range of the visible area without any pronounced local extrema that provides the comfortable psychophysical perception of the light emitted.
To control the spectral and light characteristics, three main design types of OLEDs are used, i.e. multilayer, which is the easiest to manufacture; dual, which provides an increased efficiency; banded, which allows you to adjust the colour including changing the shade of white. Adjustable OLED are in fact the only developed devices that can implement a wide range of colour temperatures from 2300K to 8200K, thus potentially overlap almost the entire spectrum of solar radiation.
In producing of OLEDs two areas were developed: deposition from the gas phase and a solution coating.
The first one is developed for the SM-LEDs based on small molecules, which evaporate well and are slightly soluble. In practice both thermal spraying in vacuum and vapour transfer in the highly purified carrier gas are used. Both methods are technologically well developed for a wide range of different semiconductor materials that let you create separate layers and topological structure of a given purity and uniformity to ensure the required efficiency. The most significant disadvantages of the gas-phase deposition are the high cost due to the complexity of the equipment as well as a relatively small area of the derived OLEDs.
Solution coating is used for the P-LEDs based on conjugated polymers which are soluble and non-volatile. In this case, along with the inkjet printing methods, to create OLED lighting systems of a large area and especially flexible ones, roll-to-roll technologies are of the greatest interest. In fact, that is an analogue of offset printing which is based on the formation of layers by transferring the organic material solution onto a substrate using a printing cylinder. Such organisations as Fraunhofer COMEDD, General Electric Global Research and Novaled actively advance this technological trend. The key advantages of the method include a high rate of application of organic materials at a relatively low cost.
The formulas of most materials used in modern designs are a trade secret and therefore not disclosed. OLED-structures are usually produced using the group-arrangement processes, a sheet type for the deposition methods and a roll type for the roll-to-roll method.
In assessing the prospects for the development and production of OLED lamps one should take into account the optical performance standards of the light sources (table 1). The main advantages of OLED lighting are shown in table 2.
Development of solid-state LED lighting products goes in parallel with OLED, and a technological gap is at least 5-7 years. This gap as well as the presence of a number of technological problems limit the industrial-scale production of inexpensive OLED.
Current market prices are $0.002 – $0.005/lm for a packaged LED and $0.3 – $0.5/lm for OLED. A substantial proportion of the cost accounts for the glass substrate, thus giving priority to the development of flexible polymer-based structures.
Basic requirements for the optical characteristics of commercial OLED lighting systems are presented in table 3.
The OLED lighting products are represented by a number of well-known companys (table 4). The comparison of the actual parameters of OLED-structures from table 4 with the desired values for a wide commercial use (table 3) shows that, given the high cost, the practical use of OLED can be of only the limited nature.
However manufacturers in collaboration with leading designers in the field of lighting equipment offer a number of solutions for general lighting, installations and luminaires based on OLEDs. Typically they are characterised by the relative minimalism of style, emphasising the unique properties of light itself.
Fig.3 shows one of the developments of the Verbatim company, the dynamic wall lighting with the VELVE OLED modules commercially available. The characteristic features of the installation are the soft light output, colour management and integrated calibration.
The developments of Fraunhofer COMEDD, along with TABOLA modules, also include the colour adjustable OLED which can simulate natural lighting depending on the time of day, blue light in the morning, white colour at around noon, warm reddish or yellowish colours in the evening.
The Japanese Research Institute for Organic Electronics (RIOE) demonstrated OLED-panels (fig.4) which are composed of almost transparent elements. Excluding wiring conductors the declared transparency reaches 70-75%.
The Lumiotec company produces serial OLED-modules in the radiation of which there is absolutely no infrared or ultraviolet components, thus making them particularly attractive for use in museums.
It can be stated that OLED lighting are presented in the commercial market by a whole range of finished products and solutions. Local markets have become more stable, that means that the phase of initial commercialisation is reached. Investments in R&D are currently becoming more sustainable, and that suggests that the OLED lighting market will continue to evolve in the next few years.
Films with controlled optical properties
A significant role in creating a comfortable environment can be attributed to factors such as natural light and heat balance of the premises. The large areas of glazing typical of modern architecture require effective solutions for managing light transmission and heat transfer processes. Control over the intensity of solar radiation transmittance can be both active and passive.
The active management of transparency is characteristic of the electrochromic structures. There are two main physical and technological trends in the development thereof. The first trend is based on the use of the multilayer structure of metal oxide films where a change in transparency occurs due to reversible chemical reactions. The second trend is focused on creating flexible structures and uses the so-called smart film as active elements, LC (liquid crystals), PDLS (dispersed polymer liquid crystals) and SPD (suspended particle devices).
The operating principle of the LC and PDLC structures is illustrated in fig.5. With no applied voltage, the liquid-crystal film is opaque since liquid crystals are in a disordered state. AC arrange crystals in rows , which makes the structure transparent. The SPD system operates in a similar manner with the only difference that rod-like particles of the laminated material are placed in the polymer matrix.
To create flexible electrochromic structures three types of films are used:
EVA is an ethylene-vinyl-acetate film. The advantages are good adhesion to glass and plastic and the low cost of production, and the disadvantages are a relatively low transparency, the tendency to delamination and sensitivity to humidity.
PVB is a polyvinyl butyral film characterised by a high adhesion to glass, and a low adhesion to plastics. It occupies a middle place as far as the quality-price ratio is concerned.
TPU is a thermoplastic polyurethane film which is characterised by the best adhesion to glass and plastics. This is the most high-quality film impervious to moisture, mechanical loads and hostile environments, but also the most expensive in the production.
The systems on smart films, e.g. LC and PDLC are characterised by an increase or decrease in transparency without significant changes in light transmission; that means that glass becoming opaque does not darken the room. Modern liquid-crystal films of the third generation (3G) in an opaque state have a high degree of light scattering, which allows, in addition to the basic functions, using them as a screen to obtain high-quality images with virtually no loss of colour.
The systems on the smart films like SPD are characterised by the simultaneous changes in transparency and light transmittance under the influence of the control voltage.
Currently the industrial production of electrochromic structures is established. As seen from table 5, the main disadvantage of the commercially produced products has a very short service life especially of flexible structures due to aging.
In practice electrochromic structures can be used both outside and inside buildings (fig.6). Hybrid and virtually energy-independent solutions, which combine, for example, a flexible solar energy panel and an electrochromic film in a single glass design, have a good potential.
The passive management of light transmission is characteristic of the photochromic structures. The light transmittance of the photochromic glass (fig.7) changes due to reversible chemical reactions depending on the intensity of solar radiation exposure.
The light transmittance value is inversely proportional to the magnitude of the intensity of solar radiation. Furthermore, for photochromic structures are typical the UV-reflection and IR radiation delay, that allows using them to regulate the thermal parameters too.
Photochromic films usually are not included in the composition of glass but are glued onto it. The photochromic films offered in the market are characterised by the following parameters:
reflection of solar radiation – up to 58%;
infrared radiation delay – up to 99%;
reflection of ultraviolet radiation – up to 99%;
visible light transmission – up to 78%.
If electrochromic structure due to the managed properties and flexibility of the structure can be effectively used in intelligent comfort control systems, the photochromic ones may only be used in stand-alone solutions.
The key obstacle preventing a wide spread of films with controlled optical properties is a very short lifetime due to aging of the active substance, and the high cost.
Flexible solar power panels
Flexible solar power panels are also potentially interesting from the point of view of comfortable environment. Environmental friendliness and power generation safety, lightness of the structure, the possibility of placing on any exterior surface as well as the prospects for integration with other flexible photonic devices are the core advantages designed to promote the flexible solar energy. The most significant problem remains a significantly lower (more than doubled compared with the single-crystal silicon panels) efficiency concerning the conversion of solar energy into electrical energy.
The working principle of flexible solar power devices is based on the photoelectric effect, the electric current generation under the influence of radiation in solar cells based on inhomogeneous semiconductor structures such as p-n junctions, hetero-junctions and variband systems.
To forming a solar cell structure (fig.8) on the polymer substrate 1 the contact layer 2, the layers of p-type 3, buffer layer 4, the layers of n-type 5 and the transparent contact layer 6 are successively applied.
The efficiency of conversion is determined primarily by the electrophysical and optical properties of the materials used. A decrease in the efficiency of solar cells is mainly due to the reflection of solar radiation, its selective absorption, heat dissipation and recombination of electron-hole pairs in the structure. The efficiency is increased by selecting the materials with optimal properties, creating the multilayer structures made of materials with different forbidden-band widths, applying multifunctional optical coatings and applying nanotechnology and nanostructures to increase the internal quantum efficiency and the use of infrared radiation in the process of energy conversion.
Solar energy sources are classified according to the peak power. One peak watt is the value of the generated power under the condition that the solar radiation of 1 kW/m2 falls on an element at 25ºC. However sunlight rarely reaches this value. Moreover, during the operation an element is heated significantly above its rated temperature thereby reducing its capacity. Brownout, in turn, causes a drop in the output voltage due to losses in the unlit side.
Flexible solar power panels has gone through several stages of the technological development. To create the first single-crystal battery, monocrystal technologies based on the ultra-pure silicon were used. At the highest efficiency their main disadvantages the high cost of raw materials and equipment for production, sophisticated technology and criticality to the light conditions and temperature requirements.
In the second generation of solar cells, cost reduction and the ability to create flexible structures were determined using materials such as polycrystalline silicon, cadmium selenide, selenide, copper-indium-gallium (CIGS). But in this case the applied processes require the use of toxic materials, and the finished products contain harmful substances which subsequently require expensive disposal procedures. In addition, the basic materials are relatively rare and rather expensive thus also leading to relatively high production costs.
However, according to the available data, up to 90% of solar energy facilities are currently equipped with devices of the first and second generation.
The third generation is represented by the thin-film solar batteries. They are making progress but their share in the solar power panels market until now is relatively small. One of the actively developed priority areas is the use of organic technologies. According to experts, they have a maximum commercial potential and will promote the development of solar energy in the coming years. These technologies are most environmentally friendly, use relatively inexpensive materials, and they initially focused on the creation of flexible devices.
Some commercially available and developed flexible solar power panels are presented in table 6.
Like in the case of OLED-lighting devices, the thermal deposition technologies and high-efficiency roll-to-roll processes are increasingly used as the methods for producing flexible solar power panels of the second and third generation.
The fourth generation, which still represents an upcoming trend, is the hybrid structures that combine technological materials of previous generations, innovative nanotechnologies and nanostructures. Some developing flexible solar energy sources of the fourth generation are presented in table 7. Such solutions are designed to primarily increase the efficiency of absorption and conversion of the solar energy and use cheaper raw materials.
For example, the flexible solar power panels of the Solo Power and EMPA companies are shown in fig.9.
Thus, we can say that both the research and manufacturing of flexible solar energy sources are in progress. For more practical use including in integrated solutions with other flexible photonic devices, an increase in service life and efficiency, as well as cheapening and debugging of mass production will be required.
Perspectives
All the presented areas of flexible photonics, e.g. OLED lighting, the films with controlled optical properties and solar power panels have common typical features. Primarily this involves a widespread use of nanotechnologies providing the required product specifications. The advanced level of such technologies is characterised by an increased use of environmentally friendly materials and technical processes, which do not constitute any hazardous waste in the recycling process after the end of a product lifecycle.
The industrial production of OLED lighting, the films with controlled optical properties and flexible solar energy has already reached the stage of creation of the dedicated full-cycle enterprises. Joint projects and investments are implemented within the policy agreed between individual businesses, i.e. developers and manufacturers, and the governmental programmes for the long-term technological and economic development.
Despite the limited practical application of the considered systems, primarily because of their still very high cost, there is a demand in the international market, which significantly improves the prospects for successful commercialization of the emerging developments. A high level of investment and a good pace of scientific and technological development in these areas suggest that in the near future the flexible photonic devices will become ordinary elements of life. A set of the achieved and planned parameters of specific devices allow integrating them into intelligent interfaces designed to ensure a comfortable living environment. ■
Organic Light-Emitting Diodes (OLED)
Organic Light-Emitting Diode is one of the key solutions for lighting in the future. The shape, size, design, colour and light output, all of these properties of OLEDs can be varied and picked up in an extremely large number of combinations.
OLED is a solid-state semiconductor light source based on the functional layer of organic material sandwiched between the anode and cathode layers on the substrate (fig.1). The functional layer has by a rather complex thin-film structure which is schematically shown in fig.2.
The anode and cathode are the sources of charge carriers, holes and electrons (fig.2, a). Under the influence of an applied electric potential they are moved respectively in the injection layers of holes 1 and electrons 7 whose properties allow the acceleration of the charge carriers. Through the hole conduction layers 2 and electronic conductivity layers 6 with a minimal of recombination, charge carriers fall into the electron blocking layers 3 and hole blocking layers 5 (fig.2, b). This ensures control of their passage as well as the spatial restriction of the recombination area to reduce non-emissive losses. Recombination of electron-hole pairs occurs in the emission layer 4 (fig.2, c) with the emission of photons (fig.2, d).
Such thin film structure needs a reliable sealing for protection against the influence of oxygen and water vapour with a glass lid or multilayer film coating for structures on a flexible substrate.
Thus the OLED is an ultra-planar emitting light source, the most important difference of which is the diffusion (Lambertian), spatially distributed nature of the emission not requiring the using of any additional optical elements.
The luminescence spectrum of the OLED is rather narrow (50-100 nm). The position of spectral peaks is determined by the characteristics of the materials used. The spectrum covers almost the entire range of the visible area without any pronounced local extrema that provides the comfortable psychophysical perception of the light emitted.
To control the spectral and light characteristics, three main design types of OLEDs are used, i.e. multilayer, which is the easiest to manufacture; dual, which provides an increased efficiency; banded, which allows you to adjust the colour including changing the shade of white. Adjustable OLED are in fact the only developed devices that can implement a wide range of colour temperatures from 2300K to 8200K, thus potentially overlap almost the entire spectrum of solar radiation.
In producing of OLEDs two areas were developed: deposition from the gas phase and a solution coating.
The first one is developed for the SM-LEDs based on small molecules, which evaporate well and are slightly soluble. In practice both thermal spraying in vacuum and vapour transfer in the highly purified carrier gas are used. Both methods are technologically well developed for a wide range of different semiconductor materials that let you create separate layers and topological structure of a given purity and uniformity to ensure the required efficiency. The most significant disadvantages of the gas-phase deposition are the high cost due to the complexity of the equipment as well as a relatively small area of the derived OLEDs.
Solution coating is used for the P-LEDs based on conjugated polymers which are soluble and non-volatile. In this case, along with the inkjet printing methods, to create OLED lighting systems of a large area and especially flexible ones, roll-to-roll technologies are of the greatest interest. In fact, that is an analogue of offset printing which is based on the formation of layers by transferring the organic material solution onto a substrate using a printing cylinder. Such organisations as Fraunhofer COMEDD, General Electric Global Research and Novaled actively advance this technological trend. The key advantages of the method include a high rate of application of organic materials at a relatively low cost.
The formulas of most materials used in modern designs are a trade secret and therefore not disclosed. OLED-structures are usually produced using the group-arrangement processes, a sheet type for the deposition methods and a roll type for the roll-to-roll method.
In assessing the prospects for the development and production of OLED lamps one should take into account the optical performance standards of the light sources (table 1). The main advantages of OLED lighting are shown in table 2.
Development of solid-state LED lighting products goes in parallel with OLED, and a technological gap is at least 5-7 years. This gap as well as the presence of a number of technological problems limit the industrial-scale production of inexpensive OLED.
Current market prices are $0.002 – $0.005/lm for a packaged LED and $0.3 – $0.5/lm for OLED. A substantial proportion of the cost accounts for the glass substrate, thus giving priority to the development of flexible polymer-based structures.
Basic requirements for the optical characteristics of commercial OLED lighting systems are presented in table 3.
The OLED lighting products are represented by a number of well-known companys (table 4). The comparison of the actual parameters of OLED-structures from table 4 with the desired values for a wide commercial use (table 3) shows that, given the high cost, the practical use of OLED can be of only the limited nature.
However manufacturers in collaboration with leading designers in the field of lighting equipment offer a number of solutions for general lighting, installations and luminaires based on OLEDs. Typically they are characterised by the relative minimalism of style, emphasising the unique properties of light itself.
Fig.3 shows one of the developments of the Verbatim company, the dynamic wall lighting with the VELVE OLED modules commercially available. The characteristic features of the installation are the soft light output, colour management and integrated calibration.
The developments of Fraunhofer COMEDD, along with TABOLA modules, also include the colour adjustable OLED which can simulate natural lighting depending on the time of day, blue light in the morning, white colour at around noon, warm reddish or yellowish colours in the evening.
The Japanese Research Institute for Organic Electronics (RIOE) demonstrated OLED-panels (fig.4) which are composed of almost transparent elements. Excluding wiring conductors the declared transparency reaches 70-75%.
The Lumiotec company produces serial OLED-modules in the radiation of which there is absolutely no infrared or ultraviolet components, thus making them particularly attractive for use in museums.
It can be stated that OLED lighting are presented in the commercial market by a whole range of finished products and solutions. Local markets have become more stable, that means that the phase of initial commercialisation is reached. Investments in R&D are currently becoming more sustainable, and that suggests that the OLED lighting market will continue to evolve in the next few years.
Films with controlled optical properties
A significant role in creating a comfortable environment can be attributed to factors such as natural light and heat balance of the premises. The large areas of glazing typical of modern architecture require effective solutions for managing light transmission and heat transfer processes. Control over the intensity of solar radiation transmittance can be both active and passive.
The active management of transparency is characteristic of the electrochromic structures. There are two main physical and technological trends in the development thereof. The first trend is based on the use of the multilayer structure of metal oxide films where a change in transparency occurs due to reversible chemical reactions. The second trend is focused on creating flexible structures and uses the so-called smart film as active elements, LC (liquid crystals), PDLS (dispersed polymer liquid crystals) and SPD (suspended particle devices).
The operating principle of the LC and PDLC structures is illustrated in fig.5. With no applied voltage, the liquid-crystal film is opaque since liquid crystals are in a disordered state. AC arrange crystals in rows , which makes the structure transparent. The SPD system operates in a similar manner with the only difference that rod-like particles of the laminated material are placed in the polymer matrix.
To create flexible electrochromic structures three types of films are used:
EVA is an ethylene-vinyl-acetate film. The advantages are good adhesion to glass and plastic and the low cost of production, and the disadvantages are a relatively low transparency, the tendency to delamination and sensitivity to humidity.
PVB is a polyvinyl butyral film characterised by a high adhesion to glass, and a low adhesion to plastics. It occupies a middle place as far as the quality-price ratio is concerned.
TPU is a thermoplastic polyurethane film which is characterised by the best adhesion to glass and plastics. This is the most high-quality film impervious to moisture, mechanical loads and hostile environments, but also the most expensive in the production.
The systems on smart films, e.g. LC and PDLC are characterised by an increase or decrease in transparency without significant changes in light transmission; that means that glass becoming opaque does not darken the room. Modern liquid-crystal films of the third generation (3G) in an opaque state have a high degree of light scattering, which allows, in addition to the basic functions, using them as a screen to obtain high-quality images with virtually no loss of colour.
The systems on the smart films like SPD are characterised by the simultaneous changes in transparency and light transmittance under the influence of the control voltage.
Currently the industrial production of electrochromic structures is established. As seen from table 5, the main disadvantage of the commercially produced products has a very short service life especially of flexible structures due to aging.
In practice electrochromic structures can be used both outside and inside buildings (fig.6). Hybrid and virtually energy-independent solutions, which combine, for example, a flexible solar energy panel and an electrochromic film in a single glass design, have a good potential.
The passive management of light transmission is characteristic of the photochromic structures. The light transmittance of the photochromic glass (fig.7) changes due to reversible chemical reactions depending on the intensity of solar radiation exposure.
The light transmittance value is inversely proportional to the magnitude of the intensity of solar radiation. Furthermore, for photochromic structures are typical the UV-reflection and IR radiation delay, that allows using them to regulate the thermal parameters too.
Photochromic films usually are not included in the composition of glass but are glued onto it. The photochromic films offered in the market are characterised by the following parameters:
reflection of solar radiation – up to 58%;
infrared radiation delay – up to 99%;
reflection of ultraviolet radiation – up to 99%;
visible light transmission – up to 78%.
If electrochromic structure due to the managed properties and flexibility of the structure can be effectively used in intelligent comfort control systems, the photochromic ones may only be used in stand-alone solutions.
The key obstacle preventing a wide spread of films with controlled optical properties is a very short lifetime due to aging of the active substance, and the high cost.
Flexible solar power panels
Flexible solar power panels are also potentially interesting from the point of view of comfortable environment. Environmental friendliness and power generation safety, lightness of the structure, the possibility of placing on any exterior surface as well as the prospects for integration with other flexible photonic devices are the core advantages designed to promote the flexible solar energy. The most significant problem remains a significantly lower (more than doubled compared with the single-crystal silicon panels) efficiency concerning the conversion of solar energy into electrical energy.
The working principle of flexible solar power devices is based on the photoelectric effect, the electric current generation under the influence of radiation in solar cells based on inhomogeneous semiconductor structures such as p-n junctions, hetero-junctions and variband systems.
To forming a solar cell structure (fig.8) on the polymer substrate 1 the contact layer 2, the layers of p-type 3, buffer layer 4, the layers of n-type 5 and the transparent contact layer 6 are successively applied.
The efficiency of conversion is determined primarily by the electrophysical and optical properties of the materials used. A decrease in the efficiency of solar cells is mainly due to the reflection of solar radiation, its selective absorption, heat dissipation and recombination of electron-hole pairs in the structure. The efficiency is increased by selecting the materials with optimal properties, creating the multilayer structures made of materials with different forbidden-band widths, applying multifunctional optical coatings and applying nanotechnology and nanostructures to increase the internal quantum efficiency and the use of infrared radiation in the process of energy conversion.
Solar energy sources are classified according to the peak power. One peak watt is the value of the generated power under the condition that the solar radiation of 1 kW/m2 falls on an element at 25ºC. However sunlight rarely reaches this value. Moreover, during the operation an element is heated significantly above its rated temperature thereby reducing its capacity. Brownout, in turn, causes a drop in the output voltage due to losses in the unlit side.
Flexible solar power panels has gone through several stages of the technological development. To create the first single-crystal battery, monocrystal technologies based on the ultra-pure silicon were used. At the highest efficiency their main disadvantages the high cost of raw materials and equipment for production, sophisticated technology and criticality to the light conditions and temperature requirements.
In the second generation of solar cells, cost reduction and the ability to create flexible structures were determined using materials such as polycrystalline silicon, cadmium selenide, selenide, copper-indium-gallium (CIGS). But in this case the applied processes require the use of toxic materials, and the finished products contain harmful substances which subsequently require expensive disposal procedures. In addition, the basic materials are relatively rare and rather expensive thus also leading to relatively high production costs.
However, according to the available data, up to 90% of solar energy facilities are currently equipped with devices of the first and second generation.
The third generation is represented by the thin-film solar batteries. They are making progress but their share in the solar power panels market until now is relatively small. One of the actively developed priority areas is the use of organic technologies. According to experts, they have a maximum commercial potential and will promote the development of solar energy in the coming years. These technologies are most environmentally friendly, use relatively inexpensive materials, and they initially focused on the creation of flexible devices.
Some commercially available and developed flexible solar power panels are presented in table 6.
Like in the case of OLED-lighting devices, the thermal deposition technologies and high-efficiency roll-to-roll processes are increasingly used as the methods for producing flexible solar power panels of the second and third generation.
The fourth generation, which still represents an upcoming trend, is the hybrid structures that combine technological materials of previous generations, innovative nanotechnologies and nanostructures. Some developing flexible solar energy sources of the fourth generation are presented in table 7. Such solutions are designed to primarily increase the efficiency of absorption and conversion of the solar energy and use cheaper raw materials.
For example, the flexible solar power panels of the Solo Power and EMPA companies are shown in fig.9.
Thus, we can say that both the research and manufacturing of flexible solar energy sources are in progress. For more practical use including in integrated solutions with other flexible photonic devices, an increase in service life and efficiency, as well as cheapening and debugging of mass production will be required.
Perspectives
All the presented areas of flexible photonics, e.g. OLED lighting, the films with controlled optical properties and solar power panels have common typical features. Primarily this involves a widespread use of nanotechnologies providing the required product specifications. The advanced level of such technologies is characterised by an increased use of environmentally friendly materials and technical processes, which do not constitute any hazardous waste in the recycling process after the end of a product lifecycle.
The industrial production of OLED lighting, the films with controlled optical properties and flexible solar energy has already reached the stage of creation of the dedicated full-cycle enterprises. Joint projects and investments are implemented within the policy agreed between individual businesses, i.e. developers and manufacturers, and the governmental programmes for the long-term technological and economic development.
Despite the limited practical application of the considered systems, primarily because of their still very high cost, there is a demand in the international market, which significantly improves the prospects for successful commercialization of the emerging developments. A high level of investment and a good pace of scientific and technological development in these areas suggest that in the near future the flexible photonic devices will become ordinary elements of life. A set of the achieved and planned parameters of specific devices allow integrating them into intelligent interfaces designed to ensure a comfortable living environment. ■
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