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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - MOLEMAT (Molecularly Engineered Materials and process for Perovskite solar cell technology)

Teaser

Summary

The societal need to fulfil the energy demand of our planet is a pressing issue and considerable efforts are being made to find decarbonized process for energy conversion. The overarching aim of the project is the development of high performance materials, process identification to promote photovoltaic (PV) properties. Currently Si and CdTe based PV can deliver such requirements but they suffer from grid parity issue and their unavailability for widespread usage. Silicon based solar cells utilizes around 200 microns thick layer in order to effectively capture light, while perovskites are exceptionally strong light absorbers and can absorb the same amount of light with a thickness of only 0.5 microns. Thus the cost of active materials is just a couple of euros per square meter, and the PV panel will costs half or less compared to the current technology, which also does not required high capital cost due to solution processing.
Hybrid perovskite based solar cell, as emerged as strong contender in thin film PV technology as it offers to harvest light at grid parity, currently >24% light to electricity, power conversion efficiencies (PCEs), are being measured, which has well positioned it at par with mature thin film PV technologies.[i] Further push in PCE and stability requires new approaches, to ultimately enable this technology ready for manufacturing. The main objectives of this project is a) to design materials by engineering at a molecular level for p-type and n-type charge collection, adding functionality to the perovskites and b) to find a process for large area deposition to promote photovoltaic properties. For new perovskite formation, direction in research is being laid for new stoichiometry compositional engineering to enhance the stability and absorption onset, crystal size control (anti-solvent approach), optimized thickness, doping or/and crosslinking of perovskite. However synergistic interactions between molecules and its interface need also to be identified to control electrical properties. We are also designing new charge transport materials and fine tune its ability to deliver optimal PV results.

The societal appetite for green and clean technology, which should be innovative and cost-effective are increasing. This has allowed to investigate innovative materials, which can convert solar rays into electricity at cut rate price. MOLEMAT is a multidisciplinary project and addresses different area of expertise such as synthetic chemistry, materials science, electro-optical characterization and device fabrication.
MOLEMAT is a multidisciplinary project and has the potential to intensely contribute for the competitiveness of the European photovoltaic based industry. The project will contribute to the tree of knowledge, giving in-depth analysis, designing efficient materials and creates know how. The project will pave way to compete European industry successfully on the global stage, by training highly qualified multidisciplinary research staff in the field of materials chemistry in an Academia-Industria environment. Creating new products, processes and technology is one of the principal driving forces of economic growth, competitiveness and employment. The transformation to a greener economy can also generate 15-20 million additional jobs globally over the next decade, and directly or indirectly it will benefit half of the world workforce. The on-going commitment to renewable energies and measures for improved energy efficiency has already started creating plenty of jobs in Europe.

OBJECTIVES:
1- Rational design of selective contacts
2- Tuning of perovskite for stability
3- Interface optimization
4- Process for layer deposition
5- Encapsulation and outdoor testing

Work performed

Solution processed perovskite solar cells have made stunning progress in a short time frame, though they also suffer from intrinsic issues. Device long-term instability, triggered by thermal and moisture-induced degradation, remains a challenging task. We have discovered the use of a hydrophobic ionic liquid which can act as a ubiquitous dopant for hole transport materials (HTM) such as Spiro-OMeTAD and can rival the use of state-of-the-art dopant. This generic route can be applied to other organic semiconductors to increase the charge carrierâ€™s density and control the degradation of the underlying water-sensitive perovskite layer. The use of hygroscopic lithium salts as a dopant in charge-selective layer compromises device stability.
Three-dimensional hybrid perovskite materials (CH3NH3PbI3) suffer from intrinsic instability owing to organic cation evaporation and ion migration. The doping of imidazolium iodide as an organic cation in CH3NH3PbI3 matrix, acts as a reservoir to control the spontaneous loss of iodide. The introduction of imidazolium iodide in amounts <20% has an impact on the crystallization process but do not alter the optical properties while reducing non-radiative recombination and improves the open-circuit voltage of the solar cells.
Passivating strategy was employed by placing a thin layer (20-30nm) of imidazolium iodide, which will effectively passivates the surface at the perovskite/hole transporting layer interface. By tuning the passivation layer thickness, the photovoltaic parameters, open circuit voltage and fill-factor were improved. The placement of imidazolium iodide layer on top of perovskite retards the growth of superoxide formation. Photoluminescence (PL) emission exhibits significant blue shift due to passivation layers confirming reduced number of surface traps, while the PL decay shows a similar trend as of MAPbI3.
Thiophene-based p-type molecules are being investigated in opto-electrical devices due to their intriguing semiconducting properties. We have synthesised 4H-cyclopenta[1,2-b:5,4-b0]dithiophene-based core, having methoxy-substituted triphenylamine side arms as donor groups. These rationally designed molecules were obtained through easy cross-coupling reactions, in few synthetic and purification steps. The synthesized molecules showed excellent thermal stability, and the fabricated perovskite solar with these HTMs gave on par performance as of state-of-the-art Spiro-OMeTAD. PL of perovskite layers coated with these HTMs show relatively high quenching suggesting the injection of holes from the valence band of the perovskite into the HOMO of the HTM. The estimated production cost was calculated and found to be a fraction of that of commercially available state-of-the-art Spiro-OMeTAD.

Final results

New materials synthesis and fundamental studies
Despite the tremendous growth and evolution of the research line in these direction, there is pressing requirement of using cost effective materials and eliminate the use exotic materials. Molecular engineering or functionalization is the key to new materials architecture for our societal needs, as they can be employed as novel electron and hole transporting materials in a variety of devices. Understanding the way how the assembling of these molecules on electrode is of deep curiosity for physical and materials scientist. The know- how created, by elucidating the role of anchoring group on device electrical properties can also be translated in other application, apart from its holistic integration into our PV devices. The PV market today is rapidly increasing and it is envisioned to reach $ 350 billion by 2020. Cutting down the product costs can be a major attributes to bring it to the masses. MOLEMAT provides innovative materials which are cost effective as strategy involve the use of any earth abundant elements.
Result 2: Identification of process for large area fabrication of PV module
The identification and validation of process will benefit advance materials based companies to improve their competitiveness and manufacturing companies to adapt to the market needs for new markets. The market for thin film PV products has been estimated to cross 80 billion $ in 2020. It is decisive to develop innovative process which allows Europe to be major player in PV industry for high potential market. It is evident that perovskites can deliver required power conversion efficiency, but cannot full fill the stringent technological demand due to their limited stability and process know how. MOLEMAT will impact PV and semiconductor industry, as this technology is based on abundant materials and utilizes minimum quantity of materials. One of the drawbacks, Europe is facing today is, translating its knowledge base in key enabling technology into marketable goods and service and t related manufacturing is decreasing in EU. Thin-film based third generation PV is on the beginning to make an industrial impact. The successful commercialization of second generation cost effective PV with CdTe, has paved way for this new technology. The project is timely proposed and capitalizes the opportunities arising from the government commitment to depend more on renewable energy sources and recent breakthrough. The project aims towards the practical realization of thin film solar cells, beyond Si and Cadmium technology.