Research at the Laboratory of Photonics and Interfaces EPFL Graetzel group (LPI)
The main focus of research at LPI is on photo-systems that generated electric power or fuels from sunlight. The inverse process of producing light from electricity in organic light emitting diodes (OLEDS) is also being investigated. The great majority of devices examined in our laboratories employs mesoscopic structures composed of nano sized particles as a key substrate element. In fact, it was the Graetzel group at LPI that pioneered the use of such mesoscopic architectures for the solar production of electricity and fuels. The choice of mesoscopic oxides is supported by our extensive studies of photo induced electron- and energy-transfer processes of nanocrystalline systems of various kinds. The advantages of mesoscopic oxide substrates are manifold. Advances in our understanding on how to control/manipulate interfacial charge transfer enables to design efficient photo chemical systems that effect overall conversion of solar energy as electric power as in solar cells or as solar fuels such as H2 from photoelectrochemical water splitting and reduction of CO2 to methanol and other C1 products.
A new and revolutionary thin film solar cell invented by our laboratory is the Dye Sensitized Solar Cells (DSC). The use of surface-bound dyes on mesoscopic oxides as a key electron capturing substrate permits design of very high efficiency photovoltaic thin layer solar cells in two complementary designs. Widely studied and relative mature embodiment involves a “liquid form” of redox electrolyte using iodide-triiodide or cobalt complexes as redox couple. The efficiency of these dye solar cells has reached 13% and commercial production has started. The other version involves organic and inorganic solid-state hole transport materials replacing the electrolyte. The efficiency of this solid-state version of the DSC has seen very dramatic improvements in the last few years to over 17% due to the advent of metal halide perovskites such as CH3NH3PbI3 and CsSnI3 as light harvester. Methylammonium and formamidinium lead iodide perovskites have emerged as outstanding candidates in reducing the active layer thickness of the photovoltaic to a few hundred nanometers while achieving solar conversion efficiency of over 17%. Current emphasis hence is to exploit this new and very promising class of materials in solar cells to increase further their performance and ascertaining log term stability.
A second major theme of research at LPI is the generation of solar fuels with photo-electrochemical systems that employ mesoscopic forms of semiconducting oxides as light absorbers. Total and cyclic decomposition of water to H2 and O2 can be achieved with remarkably high efficiency in electrochemical solar cells using p- and n-type oxides such as p-Cu2O and Fe2O3. Tandem design solar cells are also being examined. Recently it has been found that 2 perovskites cells connected in series can afford sustained photochemical water splitting reaching a solar to hydrogen (chemical) conversion efficiency of 15 %.
Some Specific research projects in the dye sensitized (DSC) and perovskite (PSC) solar cell field
The goal of the below projects is to enhance the industrial development of sensitized solar cells on a large scale. There are at least two applications where DSCs offers unique selling propositions: i) flexible light weight embodiments to supply electric power in particular for portable electronic devices and ii) Look through electricity glass panels for BIPV. Commercial sales of dye-sensitized solar cells have started in these two markets and some examples are shown in the figures below.
i) The conception and synthesis of a new class of sensitizers that achieve high device performance and durability while responding at the same time to the strong market demand for DSSC glass panels for specific colors in particular green and blue. These sensitizers will be used in BIPV end products, which will be marketed by g2e for glazing and building façade applications. In addition to ruthenium complexes, organic donor acceptor dyes will be explored in this direction. We shall develop new routes for preparing an intensely blue organic dye whose synthesis can be upscaled to the 100 kg quantities at a cost of < 100 CHF/g. We shall also endow the currently used green porphyrin dye with stronger anchoring groups and simplify their synthesis. Further more we shall design and synthesize highly stable novel organic sensitizers, which response in the visible and near IR regions having absorption coefficients of over 100,000 M-1 cm-1. The target sensitizers are phthalocyanines and novel donor- chromophore-acceptor organic dyes
ii) The development of novel ionic liquid electrolytes (ILE) that achieve high solar to electric power conversion efficiencies (PCEs) for currently used formulations while offering the high stability and transparency required for applications of DSSC glass panels in the building industry. Alternate new ionic liquids are selected from the collaborative work with Lonza and Merck companies and the electrolyte compositions will be formulated to achieve targeted high device performances. The synthesis of these new ionic liquids is cheaper than the state of the art ionic liquid. New solid state formulations employing gel forming agents and inorganic nano-composites will be realized that ascertain excellent prospects for large scale applications and will accelerate market penetration of the electric power generating glass panels produced by industrial partners.
iii) The development of new nanostructured titania films, whose pore size, shape and porosity will be engineered to sustain photocurrent densities of up to 20 mA/cm2 without suffering mass transport limitations when used in conjunction with the new ionic liquid electrolytes. This is a necessary condition to meet the ambitious efficiency goals set for ionic liquids.
iv) Development of perovskite solar cells (PVCs). LPI maintains a leading position in the field metal halide perovskite solar cells whose meteoric rise in efficiency to a current certified level of 17.9% has stunned the PV community. A major thrust of our current research is dedicated to exploit the full strength of these low cost and solution processed photovoltaics which have the potential to reach or even surpass power conversion efficiencies of 20%. Nevertheless, this technology is still in its infancy and there remain major challenges such as ascertaining long time stability under outdoor conditions, which we shall address. (M.Grätzel Nature Materials 2014, 13, 838).
These goals will be reached through introducing revolutionary new concepts in the choice of the three materials that play a key role for their performance and durability of the DSSC, i.e., the light haresting pigment, the electrolyte or hole conductor and the mesoporous TiO2 scaffold. Our research output will pave the way to enhance the production and sales of a new generation of durable and highly efficient glass panels that meet the esthetic and high quality demand of the market.
The new Swiss-Tech Convention Center in Lausanne Switzerland features an electricity producing glass façade made of dye sensitized solar cells.
Electric power generating colored look-through glass panels produced by g2e (www.g2e.ch) shown at the HOPV14 congress exhibition.
Back pack equipped with G24Power flexible DSCs to produce continuously electricity form ambientt light stored in a battery pack for powering electric equipments such as portable telephones.