PEROVSKITE SOLAR PANEL
The groups’ focus is Molecular Engineering of Functional Materials for Photovolatic and Light emitting applications. In the field of molecular-based photovoltaic devices, dye-sensitized solar cells (DSCs) have reached an efficiency of over 13%. This efficiency level, coupled with the use of inexpensive materials and processing has stimulated momentum to industrialize this technology. In these cells, the sensitizer, located at the junction between electron and hole transporting phases, absorbs sunlight and injects an electron and a hole into the n- and p-type materials, respectively. The former is an inorganic n-type wide bandgap oxide semiconductor (typically TiO2 anatase) and the latter is a liquid electrolyte or p-type hole transporter. The generated free charge carriers, travel through the nanostructured oxide to be collected as current at the external contacts. The significant advantage of DSCs is that they achieve the separation of light harvesting, and charge carrier transport, thus the maximum power point is virtually independent of light level therefore useful in all climate conditions. The general losses in dye-sensitized solar cells are due to the lack of sensitizer absorption in the near IR region, and the loss-in-potential from the optical band gap to the open-circuit voltage. The goal of the group is to engineer at molecular level novel panchromatic sensitizers and functionalized hole-transporting materials to achieve power conversion efficiency over 18%.
Recently organohalide lead perovskites have revolutionised the scenario of emerging photovoltaic (PV) technologies, with recently certified efficiency of 20.1% based on a perovskite solar cell. Such high efficiency is largely due to panchromatic absorption (down to ca. 800 nm), high absorption coefficient (1.4*104 cm-1 at 550 nm), low exciton binding energy (25 meV in MAPbI3), large mobility (66 cm2/Vs for MAPbI3), ambipolar charge transport, and large electron and hole diffusion lengths (175 um). The group is aiming to enhance power conversion efficiency of perovskite solar cells beyond 20%, and stability using functionalized electron and hole transporting materials. Our approach unfolds in three strategies: i) Interface engineering of new electron transporting materials; ii) Design and develop molecularly engineered novel family of large band gap hole transporting materials; and iii) Optimize the perovskite deposition techniques. The perovskite solar cell technology has been already proven to be remarkably efficient and has scope to compete with the very best crystalline semiconductor and thin film photovoltaic systems while offering the very lowest potential cost for materials and solution processed manufacturing. The groups’ ambitious goal of reaching power conversion efficiency 23% under 100 mW cm−2 will be realized by developing novel materials.
Using solar cells to generate hydrogen, and reduction of CO2 to feuls is ultimately an environmental friendly process that reduces greenhouse gases. Therefore the group will explore H2 generation via water splitting using sunlight, and CO2 reduction to liquid fuels using molecularly engineered transition metal complexes. These tailored metal complexes can act as inner sphere electron transfer agent to activate CO2.
The inverse process of dye-sensitized solar cell is producing light from electricity in organic light emitting diodes (OLEDS), which is also one of the main focuses of the group. Here the goal is to engineer highly phosphorescent emitters with blue, green and red colors for display and lighting applications.
Perovskite Solar Cell Technology
Photovoltaics (PVs), where electricity is generated directly from sunlight, represents an ideal solution to supply sustainable, environmentally-friendly and grid-free electricity. Silicon solar cells have dominated the research and the PV market since the ’70s, leading to power conversion efficiency (PCE) of 26%. However, presently several new PV technologies are the focus of intense research, aiming for alternatives with a lower cost and higher accessibility to the general population. In 2009 a new technology, Perovskite Solar Cells (PSCs), revolutionized the PV research showing an impressive improvement on PCE from 3.8% to 23% in only 7 years, a record for a recently-developed technology, while the dominant silicon solar technology efficiency reaching 26%.2 Perovskite Solar Cells (PSC) are based on hybrid crystalline material which possess a three-dimensional (3D) crystal structure ABX3 where A is an organic cation (typically R-NH3+), B is a metal cation (typically Pb+2 or Sn+2), and X is a halide anion, forming an octahedral in the unit cell (see Fig. 2a). The hybrid perovskite materials have significant advantages for optoelectronic applications due to their combined electrical and optical properties in terms of high absorption coefficient, ambipolar charge transport properties, long carrier diffusion lengths and extremely low exciton binding energy. Additionally, the ability to tune the perovskite band gap by simple chemical substitution (i.e. by manipulation of the “A” cation and “X” halide substitution) together with their facility to be solution-processable from inexpensive precursors and simple fabrication methods, make them competitive with the existing PV materials such as silicon or Cu(InGa)Se2 (CIGS) in terms of efficiency and, more importantly, lower cost. Typical PSC exploits a sandwich n-i-p configuration where the perovskite is deposited on top of a mesoporous titanium dioxide (TiO2) or tin dioxide (SnO2) layer, which behaves as electron transport material (ETM), and covered by a hole transporting material (HTM), crucial to facilitate the extraction of holes from perovskite to the back contact. Other sandwiched structure has also been developed such as n-i-p planar or inverted (p-i-n) architecture. Both ETM and HTM and their interfaces with the perovskite are of paramount importance in governing charge collection and extraction to the contacts, and thus the overall device performances. The highest PSC efficiency in Nazeeruddin’s laboratory is at present 21.3% (see Fig. 2b) comparable to what is published so far in literature (PCE=22.1%) based on n-i-p mesoscopic architecture. These results emanate from the compositional engineering of the cations (A) and anions (X), using a nonstoichiometric lead iodide precursor and a solvent-engineering method to grow the over layer of perovskite on the mesoporous layer.
Figure 2a. Perovskite crystal structure of ABX3, where A is the organic cation (green), B is the metal Pb2+ cation (grey) and X is the halide anion (purple); b. J-V data of (FAPbI3)0.85(MAPbBr3)0.15 perovskite with PTAA as HTL.
Mesoscopic PSC employs typically solution processing, in particular, spin coating or screen-printing. A schematic drawing of the cross-section is shown in Figure 3. Perovskite technology is uniquely leveraged by the numerous architectures, and with rigid and flexible substrates to generate electricity.
Figure 3a. A nano-composite embodiment of a PSC where the perovskite is infiltrated in the mesoscopic TiO2 scaffold.
Figure 3b. Flexible PSC based on n-i-p configuration.
Perovskite solar cells function well, and so are well-positioned for a variety of applications, including exterior surfaces of all kinds. Rigid and flexible (plastic) substrates, and building integrated PV are part of the business model. The fabricated modules on a flexible substrate will be lightweight (~10g/m2) and expected to produce power >100 W/m2. Similarly, the colour of the modules can be modified at the cost of efficiency (limited light absorption translates to limited power output), but a range of colours and transparencies could allow for more widespread usage. The various steps of the manufacturing processes are shown in Figure 4. By utilizing low-temperature curing techniques, it will be possible to fabricate flexible Perovskite Solar Cells and Modules for a wide variety of applications.
Figure 4. The manufacturing process we will develop can be represented schematically:
Validation of Technology:
Perovskite solar cells are produced at low temperature using simple solution based processes. Hence, the energy payback time will be less than 1 month (for Si solar cells this is 2 years). Even though the perovskite layers contain Pb, the amounts are well below the EU directives (RoHS) for PV and even for electrical and electronic equipment when integrated into a complete module. In favour of the highly efficient perovskite cells, is the offset in pollution by the adoption of such a renewable technology. To enable the uptake of new technology, we study long-term operational performance following the international standard for certifying photovoltaic operational stability IEC61215. We will ensure investors’ confidence by quickly developing scale-up and pilot production. VPS advance the perovskite thin-film photovoltaic technology towards readiness for manufacture as a cost-effective and resource efficient high-performance technology. VPS targets cell efficiency reaching 20% and stability; IEC61215 protocol, explain less than 10 % efficiency decrease after 1000 hours at 80 C.3 In addition, we will assess both solutions processed large area and inkjet (100 cm2) manufacturing methodologies (capable of producing 1000 m2 per year already in 2021), and make a key decision upon with deposition methodologies will deliver the most price competitive (< 0.05 Euro/ kWh) fabrication process, which include considerations of performance, yield and manufacturing costs. A key aspect of the work-programme is to deliver industrially acceptable module stability to enable the pilot-scale manufacturing.
Risk management and evaluation protocol for materials and processes will be set-up at the start of the project. This will ensure that only scalable and recyclable materials will be used with up-scalable deposition processes. Critical issues for upscaling towards industrially suitable environment will be assessed, such as the environmental impact of the solvents. The processing steps employed will exclusively be performed using processes that are proven in large volume manufacturing and using materials and chemicals which are industrially acceptable.
VPS - VALAIS PEROVSKITE SOLAR
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1951 Sion, Valais