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New technological advances for the third generation of Solar cells

Periodic Reporting for period 2 - GOTSolar (New technological advances for the third generation of Solar cells)

Reporting period: 2017-01-01 to 2018-12-31

In the last years, new energy policies are being developed and some were enforced aiming at more energy-sustainable urban environments. Under these guidelines a new concept of buildings arises - the Nearly Zero Energy Building (NZEB), Directive 2010/31/EU - in which the building should have nearly zero net energy consumption and nearly zero carbon emissions over the course of one year. Efficient photovoltaic (PV) panels have a great potential for architectural integration and so they are an obvious choice for renewable electricity generation. The emerging perovskite sensitized solar cells (PSCs) are very attractive candidates to fulfil all these requirements and they are marching ahead in the emerging photovoltaic efficiency race. In only three years, organic/inorganic lead halide PSCs have leapt from around 10 % in 2013 to a certified value of 22.1 % of power conversion efficiency (PCE) in 2016. This meteoric rise in performance attracted intense attention from the scientific community, and in particular from GOTSolar consortium. So, this project proposes disruptive approaches for the development of highly efficient PSCs aiming to face the challenge of bringing this new and promising technology to the commercial level for the first time ever. The foreseen innovations that go substantially beyond the state-of-the-art are: 1) development of new advanced materials targeting 24 % of efficiency for lab-size (ca. 25 mm2) PSCs; 2) assess and solve PSC durability issues, namely, developing a new hermetically laser assisted glass encapsulation process to enable high-durability and tested under accelerated aging conditions (stable for 500 h under 80 °C); and 3) framing the early stage scale-up processes by assembling a device of 10 × 10 cm2 used for demonstrating the scalability of the developments for producing the first perovskite solar module with potential for 20 years of lifetime.
Within the first year of project, the work packages responsible for materials development (WP2 to WP4) initiated the preparation of the selected family of materials with enhanced properties. EPFL synthesized a triple Cs/MA(methylammonium)/ FA(formamidinium) cation perovskite achieving high efficient PSCs with a stabilized PCE of 21.1 %. These triple cation perovskites showed to be more robust to subtle variations during the fabrication process enabling a breakthrough in terms of reproducibility. Concerning lead-free absorber materials, two research lines were followed in parallel. IChF PAN synthesized by mechanochemistry bismuth based materials to replace lead in perovskites due to the same electron configuration of Pb2+ and Bi3+ ions. Present materials with composition of A3Bi2I9 like Cs3Bi2I9 or MA3Bi2I9 exhibit bandgaps in the range of 1.9 eV – 2.1 eV. A theoretical research line, based on photophysics properties of lead-free perovskites, have been extensively studied by CNRS partner, who identified tin-based divalent compounds likely to be the most relevant for lead free perovskite solar cells. Moreover, it was concluded that Cs–based compounds (namely CsSnI3) present two additional advantages: they have a slightly reduced band gap and are free from two scattering mechanisms present in hybrid halide perovskite compounds, namely molecular quasielastic relaxations and rotation-translation couplings. These two research lines will converge for assessing the lead-free absorber that really allows reaching the forecasted 16 % efficiency. Finally, mechanochemistry developed by partner IChF PAN was also applied to prepare high purity MAxFA1-xPbI3 hybrid perovskite absorbers and to assess the contour proportions of reactants for obtaining single-phase perovskites. Within WP3, it has been successfully synthesized and cha¬racterized the first series of low band gap HTMs: the S,N-heteroacene-based D-π-A HTMs, which showed excellent power conversion efficiencies close to 18 %. The second series was designed to lead to large band gap materials, where a synthetic route for the preparation of spiro-cyclopentadithiophene-based HTMs has been successfully established. The acridine-based HTM will be finished soon and the synthesis of the thioxanthene-based HTM has already started. Novel alternative to TiO2 scaffold materials that can act as electron contacts were developed within WP4. IChF PAN partner prepared novel zinc oxide (ZnO) nanostructures including: (i) carboxylate oligoethylene glycol (OEG-carboxylate) coated ZnO nanocrystals, as well as (ii) mesoporous structures composed of ZnO nanoparticles. Towards increasing the PSC device performance, polymer-templated nucleation and crystal growth of perovskite films were applied by EPFL partner. The incorporation of rubidium cations into perovskite solar cells allowed to reach a stabilized power conversion efficiency of 21.6 % with an open circuit voltage of 1.24 V by applying lithium doped TiO2 as scaffold material. Finally, UPORTO developed within WP5 a laser-assisted glass sealing process for bonding TCO-TCO glass substrates, soda-lime glass to TCO glass substrates and to bond glass to titanium coated glass substrates. The process temperature for sealing the above mentioned substrates was 120 °C and the substrates dimension was gradually increased from 5 × 5 cm2 up to 9 × 9 cm2 aiming the scale up of the laser-sealing process. The hermeticity and long-term stability laser-sealed empty cells was assessed, showing leak rates complying with the pertinent standards.
Perovskite PV technology is at the early stage of development for commercial use. Assessing the environmental impact profile of the technology is one of the activities defined in the project. The fabrication techniques and materials contain the lowest embedded energy of any other solar cell platform and materials with low environmental impact – special interest will be given to further develop lead-free devices. This will minimize the lifecycle and environmental impact of products arising from commercial-scale manufacture of perovskite solar cell materials. In addition, perovskite PV modules serve different markets when compared to c-Si, and would be advantageous for development of BIPV applications. Accelerated ageing tests and field trials will ensure the reliability of the proposed technology for real-world applications. GOTSolar targets a set of breakthrough developments critical for making this technology commercial. These developments are: a) laser-assisted glass sealing of the substrates, which allow dramatically more stable devices even using oxygen/humidity sensitive components such as lead-free perovskite absorbers; b) new components for more efficient devices such as new scaffolds with better energy level alignments and high electrical conductivity and more efficient absorbers; c) new hole transport materials (HTM) more thermally stable than spiro-MeOTAD and though with good energy level alignments and high electrical conductivity; d) new preparation methods more suitable with the industrial production, namely R2R processing of large area flexible metal substrate devices is addressed and; e) fundamental research essential for future developments. Significant work was developed in the first year of project in this different topics for fulfilling the forecasted objectives. The consortium is confident that the output results of the project will be of critical relevance for the European energy development and the implementation of the NZEB directive.
Sample photograph by ICHF PAN: synthetic laboratory