Periodic Reporting for period 2 - PISCO (Photochromic Solar Cells: Towards Photovoltaic Devices with Variable and Self-Adaptable Optical Transmission)
Periodo di rendicontazione: 2021-03-01 al 2022-08-31
Among emerging photovoltaic technologies, Dye-Sensitized Solar Cells (DSSCs) represent one of the most promising approaches. Indeed, this generation of photovoltaic devices can show efficiencies higher than 14% in laboratory or 10% in small modules, they exhibit remarkable performance in low light conditions and their stability can exceed the 10 years of outdoor operation. Their manufacturing is simple, environmentally acceptable, compatible with industrial requirements and large-scale production, and the raw materials are relatively inexpensive. They are also fully customizable and they can be designed semi-transparent and colourful making them appealing elements for applications in buildings. However, in order to convert efficiently light into electricity, solar cells have to harvest maximum of light emitted by the sun therefore a trade-off has to be found between transparency and efficiency, especially when applications in façades of buildings are targeted. Besides, the actual state-of-the-art only allows for the fabrication of semi-transparent solar cells showing a given optical transmission determined and fixed during the fabrication process. For the development of photovoltaic windows and the massive integration of this technology in buildings, solar cells with variable and self-adaptable optical properties are now necessary.
The main objective of PISCO is to develop a new class of solar cells, transparent under low light conditions that would tune their absorption under higher incident light intensity to produce energy. Such solar cells would be no longer considered as devices that simply convert light in electricity but they would be regarded as versatile active architectural elements, capable to manage their light transmission, change their colours, and even tune heat transfers, depending on external conditions.
The amount of energy that could be generated in our society would be huge if all passive elements such as windows in buildings, cars, or public transports, could generate electricity, while offering the comfort for the users to self-adjust their light transmission with the intensity of the daylight.
To develop such new class of solar cells, the strategy is to replace classical dyes used in dye-sensitized solar cells by photochromic dyes, which can reversibly change colour upon illumination. In PISCO we develop new photochromic dyes and we implement them for the fabrication of semi-transparent dye sensitized solar cells and modules.
The main objective of PISCO is to develop a new class of solar cells, transparent under low light conditions that would tune their absorption under higher incident light intensity to produce energy. Such solar cells would be no longer considered as devices that simply convert light in electricity but they would be regarded as versatile active architectural elements, capable to manage their light transmission, change their colours, and even tune heat transfers, depending on external conditions.
The amount of energy that could be generated in our society would be huge if all passive elements such as windows in buildings, cars, or public transports, could generate electricity, while offering the comfort for the users to self-adjust their light transmission with the intensity of the daylight.
To develop such new class of solar cells, the strategy is to replace classical dyes used in dye-sensitized solar cells by photochromic dyes, which can reversibly change colour upon illumination. In PISCO we develop new photochromic dyes and we implement them for the fabrication of semi-transparent dye sensitized solar cells and modules.
To achieve the ambitious goal of PISCO we have designed, synthesized and characterized novel push-pull dyes embedding various photochromic units. First, we carried out theoretical studies to assess that the new dyes envisioned could fulfil the optoelectronic criteria required for being used as sensitizers in DSSCs.
Calculations and modelling helped us to identify the most favourable dye’s chemical structures and to predict their absorption properties. Then, we developed innovative synthetic routes to access to the photochromic dyes. The optoelectronic and photochromic properties of the compounds were fully investigated by spectroscopic and electrochemical techniques allowing us to establish novel structure-properties relationships for these novel class of photosensitizers. We obtained photochromic dyes exhibiting a good reversibility of the photochromic process and showing a high fatigue resistance and a good photo-colourability.
Then, the new dyes were implemented in DSSCs to evaluate their photochromic behaviour and their photovoltaic properties in device configuration. Several fabrication parameters were investigated with the goal to optimize the performances of the solar cells. We developed specific characterization techniques such as impedance spectroscopy techniques and electrical characterizations under various light sources to fully unravel and understand the interplay between photochromic and photovoltaic properties of these dyes when incorporated in solar cells.
Finally, we demonstrated that under solar irradiation, DSSCs containing our photochromic dyes can vary their colour (turning from light yellow to deep green), self-adapt their visible light transmission to the daylight. Simultaneously they deliver a photocurrent that increases upon irradiation when the solar cells are fully coloured, thus yielding a power conversion efficiency of up to 4.2%. We also fabricated photochromic semi-transparent mini-modules with an active surface of 14 cm2 showing a 32.5 mW power output.
Calculations and modelling helped us to identify the most favourable dye’s chemical structures and to predict their absorption properties. Then, we developed innovative synthetic routes to access to the photochromic dyes. The optoelectronic and photochromic properties of the compounds were fully investigated by spectroscopic and electrochemical techniques allowing us to establish novel structure-properties relationships for these novel class of photosensitizers. We obtained photochromic dyes exhibiting a good reversibility of the photochromic process and showing a high fatigue resistance and a good photo-colourability.
Then, the new dyes were implemented in DSSCs to evaluate their photochromic behaviour and their photovoltaic properties in device configuration. Several fabrication parameters were investigated with the goal to optimize the performances of the solar cells. We developed specific characterization techniques such as impedance spectroscopy techniques and electrical characterizations under various light sources to fully unravel and understand the interplay between photochromic and photovoltaic properties of these dyes when incorporated in solar cells.
Finally, we demonstrated that under solar irradiation, DSSCs containing our photochromic dyes can vary their colour (turning from light yellow to deep green), self-adapt their visible light transmission to the daylight. Simultaneously they deliver a photocurrent that increases upon irradiation when the solar cells are fully coloured, thus yielding a power conversion efficiency of up to 4.2%. We also fabricated photochromic semi-transparent mini-modules with an active surface of 14 cm2 showing a 32.5 mW power output.
Before PISCO, mainly two concepts were proposed to tackle the challenge of semi-transparent solar cells with dynamic optical properties. The first one relies on the combination of a photovoltaic cell with an electrochromic film. In such system, the light-absorption process in the photovoltaic cell is separate from the coloration process in the electrochromic cell. Therefore, the manufacturing can be difficult since both processes need to be optimized individually. The coloration process of the electrochromic film requires electrical current, thus consuming a part of the electricity generated by the solar cell; finally, the system cannot be fully transparent.
The second concept used photochromic materials to tune the optical transmission. However, the photochromic molecules that were developed and used in solar cells before the PISCO project did not show a reversible photochromic behaviour, or they led to very poor performances. None of these strategies had succeeded in providing a simple and efficient way to access solar cells with self-adjustable optical properties.
Within PISCO, we have demonstrated that photochromic and photovoltaic properties can be easily reconciled in the same device using photochromic dyes with a proper chemical design. We have demonstrated this disruptive concept by fabricating solar cells that exhibit self-adjusting colour change and light transmission depending on sunlight conditions. Our results set a state of the art: we obtained semi-transparent cells with transparency in the visible range varying from 60% to 30% and conversion efficiencies around 4%. The process of coloration-decolouration is fully reversible and can be observed over several months for this first generation of devices. We demonstrated the industrial potential by manufacturing semi-transparent photo-chromo-voltaic mini-modules with an active surface of 14 cm². Our work brings a significant progress in the synthesis of photochromic dyes and the understanding of their optoelectronic properties. In the field of solar cells, our devices are the first ones ever reported capable of autonomously self-adjusting their transparency and energy production with the intensity of the daylight.
We target now to develop new photochromic dyes allowing fabricating semi-transparent photochromic devices with faster coloration and decolouration rate, showing a neutral hue in their coloured state and demonstrating performances comparable to classical organic dyes in DSSCs.
The second concept used photochromic materials to tune the optical transmission. However, the photochromic molecules that were developed and used in solar cells before the PISCO project did not show a reversible photochromic behaviour, or they led to very poor performances. None of these strategies had succeeded in providing a simple and efficient way to access solar cells with self-adjustable optical properties.
Within PISCO, we have demonstrated that photochromic and photovoltaic properties can be easily reconciled in the same device using photochromic dyes with a proper chemical design. We have demonstrated this disruptive concept by fabricating solar cells that exhibit self-adjusting colour change and light transmission depending on sunlight conditions. Our results set a state of the art: we obtained semi-transparent cells with transparency in the visible range varying from 60% to 30% and conversion efficiencies around 4%. The process of coloration-decolouration is fully reversible and can be observed over several months for this first generation of devices. We demonstrated the industrial potential by manufacturing semi-transparent photo-chromo-voltaic mini-modules with an active surface of 14 cm². Our work brings a significant progress in the synthesis of photochromic dyes and the understanding of their optoelectronic properties. In the field of solar cells, our devices are the first ones ever reported capable of autonomously self-adjusting their transparency and energy production with the intensity of the daylight.
We target now to develop new photochromic dyes allowing fabricating semi-transparent photochromic devices with faster coloration and decolouration rate, showing a neutral hue in their coloured state and demonstrating performances comparable to classical organic dyes in DSSCs.