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Reporting period: 2020-12-01 to 2022-05-31

Transparent photovoltaics (TPV) possesses a huge untapped potential in the harvesting of solar energy where it readily can be embedded in buildings applications worldwide to significant reduce CO2 emissions, and support the needed development of nearly zero-energy buildings. TPV will increase the utilization of renewable energy directly where it is needed, and play a crucial role for the sustainable transformation of the energy sector in large cities. In the CITYSOLAR project, a new breakthrough concept for TPV will be developed by exploiting the combined use of emerging technologies, namely multi-junction solar modules developed from near-ultraviolet perovskite solar cells (NUV-PSC) and near-infrared organic solar cells (NIR-OPV). Using advanced concepts within light management such as photonic crystals, nanophotonics and photon recycling and advanced module integration schemes, CITYSOLAR will radically change performance limits for TPV by significantly reducing losses related to light absorption and scale-up from individual solar cells to multi-junction modules.
In the first period (M1-M18) we performed an assessment and synthesis of complementary pairs of electron donor and acceptor materials for use as the photoactive blend components in visibly transparent devices. Several photoactive polymers and small molecules have been synthesised and purified at KAUST, and BM. Eni made available its patented background regarding photoactive polymeric materials for OPV and the most promising materials have been selected and tested in near-infrared organic solar cells. Concerning the transporting layer for near-ultraviolet perovskite solar cell, several molecular families have been screened by means of experimental and theoretical methods in order to identify the most promising one in terms of optoelectronic properties and synthetic versatility by the CNR. A promising family has been identified and test are performing in perovskite cell.

Several partners of the CITYSOLAR consortium have set up a complete and comprehensive characterisation platform, aimed at investigating fundamental properties and device operating principle of photovoltaic materials and cells. Experimental findings were also supported by optical simulations performed by FAU in order to find design guidelines to achieve high efficiency, AVT and CRI. Specifically, CNRS and EDF performed multidimensional photoluminescence imaging while CNR performed ultrafast dynamics of the photoexcited charges, XRD, AFM, EDXR, high-resolution photoemission spectroscopy using synchrotron radiation.

UTV and CNR worked on the development and optimization of fabrication processes involving solution processing and physical deposition. We found that state-of-art PCE and AVT values can be achieved using planar NIP architecture. UNITOV demonstrates the huge potential of FAPbBr3 perovskite from solution processing technique like spin-coating and blade coating. CNR activity was also focused on the development of perovskite films by physical depositions such as Pulsed Laser Ablation and Molecular Beam Epitaxy.

Bulk heterojunction solar cells with high transparency, color neutrality, and high photoelectrical conversion efficiency have been developed by FAU. For the best commercially available materials, simulations predict PCEs of up to 9 % PCE and AVT of 58 % when the top electrode is not included in the calculation, and of up to 6 % PCE and AVT of 38 % for the complete device stack. The fabrication of such NIR-OPV showed PCE and AVT very similar to the one predicted by the simulations. In order to further improve these values, light management systems were included into the simulations.

To this end, Distributed Bragg Reflectors (DBR) made from alternating low and high refractive index oxide materials have been developed. The oxide materials are done from a reactive sputtering process that can be scaled through Roll-to-Roll (R2R) vacuum sputtering. The materials chosen are abundant, non-toxic and show low-environmental impact. DBRs that are specifically addressing the NIR-OPV and NUV-PSC have been developed, showing in both optimized cases reflectance values above 90% in the targeted NIR and NUV ranges, and AVT values close to 100%. Furthermore, an integration route with both the NIR organic solar cells and NUV perovskite solar cells have been demonstrating.

A collection of potential lamination materials was identified, and their suitability as laminating materials for integrating the CITYSOLAR NUV perovskite and NIR organic solar cells was studied. The materials were short-listed based on their processability conditions, such as temperature, pressure, and UV dosage for curing, to match with the semi-transparent PV processing conditions. Emphasis was given to epoxy (UV curable) based and pressure sensitive adhesive (PSA) materials.
The work carried out in project allowed to identify some different classes of promising photo-active polymeric materials which have different absorption characteristics. New donor and acceptor materials for NIR-OPV as well new hole transporting layers for NUV-PSC have been synthetized for the specific goal of the project. In particular, the materials proposed by Eni are also characterized by a synthetic simplicity in order to obtain an easier scale up.

NUV-PSC with state-of-art performances in terms of PCE and AVT has been achieved by UNITOV and supported by the characterization platform (CNRS, CNR) and electro-optical simulations (FAU, UNITOV). Scaling up is also feasible and has been demonstrated by using out of glove-box blade coating. CNR developed new physical deposition methods for the production of Bromide perovskite by using PLD and MBE, not yet demonstrated in literature.

The development of NIR-OPV permitted to achieve cells with PCE and AVT that are promising for the project and to identify the important aspect that need to be optimized to achieve the goal of the project. In particular, the light management strategy planned in the project and the development of specific absorbers will contribute to this objective. Optimization of the scaling up was also performed permitting to produce large area modules with efficiencies similar to those obtained with small area lab cells.

Distributed Bragg Reflectors (DBR) showing above 90% reflectance in targeted NIR and NUV wavelength ranges while demonstrating AVT close to 100% have been developed, using solely low-environmental impact and non-toxic materials systems. Furthermore, the DBRs are fully scalable using low-cost Roll-to-Roll (R2R) fabrication techniques. The enhancement in short-circuit current of 19% for semi-transparent NUV perovskite solar cells using these DBRs represent to the best of our knowledge progress beyond state-of-the-art for such devices.

The lamination process of tandem cells will be carried out in a cost-effective manner using roll lamination. We expect to develop the lamination with minimal optical loss so that it will not have a negative effect on achieving 15% power conversion efficiency with 50% average visible transmittance for the CITYSOLAR tandem solar modules. The lamination is expected to ease the process of achieving two terminals (2T) for the targeted CITYSOLAR multijunction module. The needed structural integrity for the targeted CITYSOLAR module is expected from the lamination.

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