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Förster resonant energy transfer for high efficiency quantum dot solar cells

Final Report Summary - F-LIGHT (Förster resonant energy transfer for high efficiency quantum dot solar cells)

Current research directives in solar energy conversion are investigating new methods, and, in particular, new materials and processes that will render integrated and more efficient power harvesters from solar energy. Moreover, from a technological point of view, low-cost fabrication processes using available, inexpensive and health and environmental friendly materials are required. Excitonic solar cells (XSCs), which belong to the so-called third-generation solar cells, thanks to their low cost, easy fabrication methodologies, low toxicity of the applied materials, are at present one of the most promising technologies to gain an efficient and cost effective solar energy conversion and can have the potential to make affordable solar power a reality. In fact, the thermodynamic limit of the photoconversion efficiency (the so-called Shockley-Queisser limit) of a single-junction first- or second-generation solar cell under one sun irradiation (AM 1.5G) is 32.9%. Excitonic solar cells aim at enhancing the photoconversion efficiency beyond the Shockley-Queisser limit through photoconversion mechanisms, which are not present in first- and second-generation, like multiple exciton generation by impact ionization, intra-band transitions, optical up and down-converters, multi-junction cells. However, the large scale production and commercialization of the excitonic solar cells is still limited, mainly due to the low photoconversion efficiency gained up to now (not higher than 13%), which is far below the theoretical predictions (up to 45%).

Exciton harvesting is of fundamental importance for the efficient operation of third generation photovoltaic devices. The quantum efficiencies of many organic and hybrid organic-inorganic devices are still limited by low excitation harvesting efficiencies. Therefore, to achieve new record efficiencies, light absorption must be somehow extended, possibly in the 350-1000 nm range. Various strategies have been explored to extend the spectral response of a cell, like tandem structure or co-sensitization with complementary dyes. However, despite the promising results, no significant improvement in photoconversion efficiency has been obtained with respect to a single dye cell. A significant enhancement in the light harvesting and photocurrent generation in XSCs can be obtained harnessing the Förster resonance energy transfer (FRET). The introduction of a third component (donor) able to absorb high energy photons and efficiently transfer the energy to the anchored primary absorber (acceptor) can increase the absorption bandwidth of the solar cell and, hopefully, the photoconversion efficiency.
The project F-LIGHT aims at exploiting in innovative way the Förster resonant energy transfer (FRET) process in excitonic solar cells, by adding proper donor/acceptor couples, which lead to broadening of the absorption spectral range and improve the photoconversion efficiency. The D/A couples will be composed of commercially available dye molecules and colloidal and non-colloidal quantum dots (QDs).
The main project objective is to demonstrate that suitably tailored light harvesters can significantly boost photoconversion efficiency in excitonic solar cells by properly regulating exciton dynamics, which lead to current photogeneration and cell operation.
In the framework of the project F-LIGHT new composite nanomaterials were synthesized and studied. We both focused on the oxide photoanode itself (by proposing new nanomaterials which increase charge collection and inhibit charge recombination), and on light harvesters (mainly composite QDs).
We thoroughly investigated the static and dynamic optical properties of the QDs before and after grafting with the oxide, and characterized the final solar cells, clearly correlating the photoconversion efficiency of the final devices to the optical properties of the nanomaterials.
We demonstrated that composite harvesters can offer improved optical properties, compared to traditional QDs or commercial dyes, highlighting the importance of tuning the electronic band-structure of donor-acceptor couple to maximize the efficiency of the final device.

The deployment of nanotechnology is a major driver for the trend to improve existing products by creating smaller components and better (functional and environmental) performance materials.
Energy (including energy conversion, efficiency, storage and transportation) is a field in which nanotechnology will lead to real breakthroughs. Third generation photovoltaic QDSSCs could make a profound impact to meet major societal challenges, such as sustainable energy supply and consumption.
To realize this scenario the design and exploitation of easily scalable processing techniques for mass materials production is of paramount importance. This project is addressing such issues by developing viable and cost effective synthetic routes for producing the next generation QDSSCs. The expected results will advance the knowledge and processing of nanostructured oxides and QDs, with special emphasis on the full control of their electrical/optical properties, thus leading to a better understanding of scaling problems and phenomena occurring in QDSSCs. This project is timely and our research aims towards the practical realization of cheap (in energy and money) thin film PV, beyond Si technology. Our technical objectives have been chosen for impact in the future thin-film PV industry, that is manufacturing, supply of manufacturing tools, supply of raw materials, cost competitiveness of PV, sustainability of the PV market, stability of the solar cells. Downstream impacts are expected in growth of the PV market, PV systems, de-carbonization and climate stabilization and the critical need for sustainable energy in emerging economies.