Periodic Reporting for period 4 - PISCO (Photochromic Solar Cells: Towards Photovoltaic Devices with Variable and Self-Adaptable Optical Transmission)
Periodo di rendicontazione: 2024-03-01 al 2025-08-31
Dye-sensitized solar cells (DSSCs) are among the most promising emerging photovoltaic technologies. They perform exceptionally well in low light, maintain stability for over a decade outdoors, and are simple to manufacture. The raw materials are inexpensive, and the cells can be customised, for example, to be semi-transparent and available in various colours, making them particularly suitable for building-integrated photovoltaic (BIPV) applications.
However, maximising light absorption for efficient electricity generation creates a trade-off between transparency and efficiency, particularly for façades. Before PISCO, semi-transparent solar cells could only be produced with fixed optical transmission determined during manufacturing. To enable large-scale integration of photovoltaic windows, it was therefore desirable to develop solar cells with variable, self-adaptable optical properties. Such environmentally friendly technologies could accelerate decarbonisation and reduce the residential sector’s energy demand.
The main objective of PISCO was to develop solar cells that remain transparent in low light but dynamically adjust absorption under brighter conditions. Rather than serving merely as electricity-generating devices, these cells function as versatile architectural elements capable of managing light transmission, changing colour, and regulating heat transfer according to external conditions.
If passive elements such as building windows or vehicles could generate electricity while allowing users to adjust light transmission, the potential energy yield for society would be enormous. To realise this vision, PISCO replaced conventional DSSC dyes with photochromic dyes that change colour reversibly upon illumination. New families of these dyes were developed and successfully implemented in semi-transparent solar cells and modules.
Through PISCO, we expanded knowledge on photochromic dyes and developed new materials, demonstrating their potential for solar cells and modules with sunlight-adaptive optical properties. The work also advanced DSSC technology, including the development of new electrode materials and electrolytes, and innovative recycling methods.
However, maximising light absorption for efficient electricity generation creates a trade-off between transparency and efficiency, particularly for façades. Before PISCO, semi-transparent solar cells could only be produced with fixed optical transmission determined during manufacturing. To enable large-scale integration of photovoltaic windows, it was therefore desirable to develop solar cells with variable, self-adaptable optical properties. Such environmentally friendly technologies could accelerate decarbonisation and reduce the residential sector’s energy demand.
The main objective of PISCO was to develop solar cells that remain transparent in low light but dynamically adjust absorption under brighter conditions. Rather than serving merely as electricity-generating devices, these cells function as versatile architectural elements capable of managing light transmission, changing colour, and regulating heat transfer according to external conditions.
If passive elements such as building windows or vehicles could generate electricity while allowing users to adjust light transmission, the potential energy yield for society would be enormous. To realise this vision, PISCO replaced conventional DSSC dyes with photochromic dyes that change colour reversibly upon illumination. New families of these dyes were developed and successfully implemented in semi-transparent solar cells and modules.
Through PISCO, we expanded knowledge on photochromic dyes and developed new materials, demonstrating their potential for solar cells and modules with sunlight-adaptive optical properties. The work also advanced DSSC technology, including the development of new electrode materials and electrolytes, and innovative recycling methods.
To achieve PISCO’s objectives, we designed, synthesised, and characterised a new generation of dyes incorporating photochromic units. The project began with theoretical studies to verify that the proposed dyes met the optoelectronic criteria required for use as sensitizers in dye-sensitized solar cells (DSSCs). Modelling and simulations identified the most promising molecular structures and predicted their absorption properties. Based on these insights, we developed innovative synthetic routes, successfully obtaining several families of dyes adapted for solar cells, two of which were patented. These dyes exhibited excellent reversibility, high fatigue resistance, and strong photo-colourability, and their optical and electronic properties were studied using advanced characterisation techniques developed for the project.
The dyes were incorporated into DSSCs to evaluate photochromic behaviour and photovoltaic performance. Fabrication parameters were optimised, and electrical characterisation under different lighting conditions provided new insights into the interaction between photochromic and photovoltaic properties. This work significantly advanced our understanding of photochromic materials, revealing new relationships between molecular structure and device performance.
For the first time, we demonstrated fully reversible photochromic solar cells that change colour in response to light intensity. DSSCs with our dyes adapted their visible light transmission to daylight while simultaneously generating electricity, with fully coloured cells producing higher photocurrents and power conversion efficiencies (Nature Energy 2020, DOI: 10.1038/s41560-020-0624-7).
Building on this new knowledge, molecular engineering produced dyes with faster colouration and decolouration kinetics, lower recombination losses, improved efficiency, higher stability, and in some cases excellent colour rendering in semi-transparent devices. Data-driven and machine-learning approaches were also developed to accelerate the design of new electrolytes tailored to this class of dyes.
To demonstrate technological potential, minimodules up to 25 cm² were fabricated with Solaronix (Switzerland). Large-area photochromic modules of 600 cm² were produced with the University of Tor Vergata (Italy), confirming scalability and robustness. Manufacturing and recycling methods were also developed to reduce carbon footprint and costs, opening new industrial collaboration opportunities.
PISCO’s work has resulted in over 20 publications in high-impact journals, including Nature Energy, Advanced Energy Materials, Advanced Materials, Chemical Science, and Materials Horizons. The project also led to a patent, more than 30 invited presentations, and several awards, notably the Materials Horizons Prize (RSC, 2025). It has received extensive media coverage on specialised scientific platforms and in national newspapers and magazines, highlighting its technological and societal relevance.
The dyes were incorporated into DSSCs to evaluate photochromic behaviour and photovoltaic performance. Fabrication parameters were optimised, and electrical characterisation under different lighting conditions provided new insights into the interaction between photochromic and photovoltaic properties. This work significantly advanced our understanding of photochromic materials, revealing new relationships between molecular structure and device performance.
For the first time, we demonstrated fully reversible photochromic solar cells that change colour in response to light intensity. DSSCs with our dyes adapted their visible light transmission to daylight while simultaneously generating electricity, with fully coloured cells producing higher photocurrents and power conversion efficiencies (Nature Energy 2020, DOI: 10.1038/s41560-020-0624-7).
Building on this new knowledge, molecular engineering produced dyes with faster colouration and decolouration kinetics, lower recombination losses, improved efficiency, higher stability, and in some cases excellent colour rendering in semi-transparent devices. Data-driven and machine-learning approaches were also developed to accelerate the design of new electrolytes tailored to this class of dyes.
To demonstrate technological potential, minimodules up to 25 cm² were fabricated with Solaronix (Switzerland). Large-area photochromic modules of 600 cm² were produced with the University of Tor Vergata (Italy), confirming scalability and robustness. Manufacturing and recycling methods were also developed to reduce carbon footprint and costs, opening new industrial collaboration opportunities.
PISCO’s work has resulted in over 20 publications in high-impact journals, including Nature Energy, Advanced Energy Materials, Advanced Materials, Chemical Science, and Materials Horizons. The project also led to a patent, more than 30 invited presentations, and several awards, notably the Materials Horizons Prize (RSC, 2025). It has received extensive media coverage on specialised scientific platforms and in national newspapers and magazines, highlighting its technological and societal relevance.
Prior to PISCO, two main approaches were proposed to create semi-transparent solar cells with dynamic optical properties. The first combined a photovoltaic cell with an electrochromic film. In this system, light absorption in the photovoltaic cell is separate from the colouring process in the electrochromic film, making manufacturing complex since both processes must be optimised individually. Moreover, the colouration process requires electrical current, consuming part of the electricity generated and preventing full transparency. The second approach used photochromic materials to tune optical transmission, but molecules developed prior to PISCO either showed poor reversibility or low performance. Neither strategy provided a simple, efficient solution for solar cells with self-adjustable optical properties.
Within PISCO, we demonstrated that photochromic and photovoltaic functions can be effectively combined in a single device by carefully designing the chemical composition of photochromic dyes. Several families of dyes were developed, revealing new correlations between molecular structure and optoelectronic and photovoltaic properties. Molecular engineering produced dyes with unprecedented characteristics, including a high colour rendering index and panchromatic absorption.
We validated this disruptive concept by fabricating solar cells capable of self-adjusting their colour and light transmission in response to sunlight. Semi-transparent cells with visible-range transparency varying from 60% to 20% achieved maximum power conversion efficiencies of around 5% in the best devices. For the first time, fully reversible photochromic behaviour was demonstrated in solar cells, observable over several years in this first generation.
The industrial potential was confirmed through the production of semi-transparent photo-chromo-voltaic mini-modules and larger modules ranging from 25 cm² to 600 cm².
Finally, our work on DSSCs led to manufacturing innovations, including machine-learning approaches to accelerate the design of efficient, highly transparent electrolytes, and recycling methodologies potentially adaptable to other solar cell technologies.
Within PISCO, we demonstrated that photochromic and photovoltaic functions can be effectively combined in a single device by carefully designing the chemical composition of photochromic dyes. Several families of dyes were developed, revealing new correlations between molecular structure and optoelectronic and photovoltaic properties. Molecular engineering produced dyes with unprecedented characteristics, including a high colour rendering index and panchromatic absorption.
We validated this disruptive concept by fabricating solar cells capable of self-adjusting their colour and light transmission in response to sunlight. Semi-transparent cells with visible-range transparency varying from 60% to 20% achieved maximum power conversion efficiencies of around 5% in the best devices. For the first time, fully reversible photochromic behaviour was demonstrated in solar cells, observable over several years in this first generation.
The industrial potential was confirmed through the production of semi-transparent photo-chromo-voltaic mini-modules and larger modules ranging from 25 cm² to 600 cm².
Finally, our work on DSSCs led to manufacturing innovations, including machine-learning approaches to accelerate the design of efficient, highly transparent electrolytes, and recycling methodologies potentially adaptable to other solar cell technologies.