Joint research project

Development of a Modular Integrated Device for Solar Energy Conversion

Project leaders
Andrea Barbieri1, Tarek Ghaddar
Agreement
LIBANO - CNRS-L - National Council for Scientific Research of Lebanon
Call
CNR-CNRS-L 2016-2017
Department
Chemical sciences and materials technology
Thematic area
Chemical sciences and materials technology
Status of the project
New

Research proposal

The basic requirement for any solar conversion device is harvesting as many photons as possible from sunlight. However, photovoltaic (PV) devices lose a substantial portion of the solar spectrum with photon energies below the semiconductor band-gap, which ultimately limits the overall photon-to-current efficiency (PCE). The recovery of low energy photons in the near-infrared (NIR) region of the spectrum can be addressed by using an up-conversion (UC) device that can generate one blue/green photon from two red/NIR photons. Additionally, it is possible to obtain substantial enhancement of the device performance by using solar concentrators, even without major changes in solar cell design. In particular, Luminescent Solar Concentrators (LSCs) can achieve high optical concentrations without solar tracking. This proposal targets a modular solar light-to-electricity device consisting of three key components that, in concert, optimize sunlight harvesting. They are as follows.
A dye sensitized solar cell (DSSC): it represents the core of our modular solar energy conversion system. The use of dye-sensitized semiconductors as light harvesters in mesoscopic solar cells has represented a major breakthrough in the development of cost-effective and efficient solar energy conversion devices. Liquid and solid (e.g. perovskite-based) DSSCs are of great relevance in PV devices due to their panchromatic light absorption and ambipolar behavior. Record power conversion efficiencies of 13-14% have been reported for liquid-based DSSCs with Co(II/III) electrolytes. On the other hand, the efficiency of perovskite-based solar cells has doubled in less than two years and the present record certified PCE is 15%. The key component in a liquid-based DSSC is either an organic or inorganic based dye, whose structure has a strong impact on the light harvesting, electron injection, collection and regeneration efficiencies and thus on the total DSSC efficiency. Similarly, perovskites utilized in PV devices are synthetic hybrid organic-inorganic methylammonium lead halide materials of general formula CH3NH3MX3 (where M = Pb, Sn and X = I, Br, Cl). Perovskites have wide direct band gaps which can be tuned either by changing the alkyl group, the metal atom or the halide. Thus, size, structure, conformation, and charge of the organic cations dictate the final structure of the material and its properties. Perovskites display large absorption coefficients, high charge carrier mobilities, solution processability, and tunable optical and electronic properties. This project will exploit the easy tunability of both dye and perovskite material properties to match the characteristics of the LSC and TTA-UC modules. The influence of the nature of the starting materials and of the processing conditions will be assessed and correlated with the performances of the PV cell.
The Luminescent Solar Concentrator: LSCs are cost-effective complements to semiconductor PVs that enable the integration of PV-active architectural elements into buildings (e.g. PV windows). The basic LSC design allows sunlight to penetrate the top surface of an inexpensive and transparent thin plastic plate, in which luminescent molecules (e.g. organic dyes, inorganic phosphors, quantum dots) are dispersed. The incident solar photons absorbed by the dyes are then isotropically re-emitted at longer wavelengths. Part of the light emitted by the luminescent species is guided by total internal reflection towards the solar cells positioned at the edge of the LSC. Since the edge area is smaller than the receiving one, this allows for concentration of sunlight without the need for solar tracking. LSC photoactive materials must exhibit a combination of properties: broad and intense absorption across the visible range, large Stokes' shift to limit self-absorption, high photoluminescence quantum yield, matching of the re-emitted photons with the bandgap of the PV cell, good solubility in matrix materials, long thermal and photochemical stability. This project targets multicomponent systems in which the final emitter is a commercially available organic fluorophore with unit quantum yield or an organometallic complex with large Stokes' shift and high emission efficiency. In both systems, the absorption of solar light and the energy transfer cascade are provided by a multichromophoric array of properly matched organic dyes.
The Up-Conversion system: UC devices can harvest the unused sub-threshold light behind the solar cell and generate one higher energy photon out of two transmitted low-energy photons. The former is radiated back towards the solar cell, widening the exploitation of the solar spectrum. UC can be accomplished at low irradiation rate by triplet-triplet-annihilation (TTA) with organic chromophores, taking advantage of the longevity of molecular triplet states. TTA-UC requires the combination of two molecules: (i) a sensitizer that absorbs the low energy photons, undergoes intersystem crossing to its lowest triplet state and then transfers its energy into the lowest triplet state of an emitter by fast triplet-triplet energy transfer; (ii) two emitter molecule interacting via TTA, one of which is promoted to its higher lying excited singlet state and the other one is quenched to the ground state. The TTA-UC system will be based on a red-NIR absorbing organometallic complex with long triplet lifetime as sensitizer and a fluorescent emitter with properly matched energy levels and high fluorescence quantum yield in the visible range.
At last, the individual modules will be assembled into the final device architecture and tested for their overall performance. This original modular approach and system implementation represent a breakthrough in solar energy conversion systems. The generation of novel concepts and fundamental knowledge will enable a significant improvement in the realization of a new generation of solar cells capable of rivalling the Si-based technology.

Research goals

The primary objective of this collaborative project is the development of a new generation of photovoltaic devices with enhanced photon-to-current conversion efficiency, based on a modular integrated architecture, comprising a luminescent solar concentrator and an up-converting modules coupled to an optimized dye sensitized solar cell. Secondary objectives are multifold: (i) Environmentally-friendly preparation of simple light-harvesting materials based on earth abundant elements; (ii) Identification of a library of suitable dyes to be used as active materials in luminescent solar concentrators and development of simple procedures for the preparation of the concentrator module; (iii) Identification and matching of pairs of dyes targeted to solar up-converting devices with high efficiency; (iv) Assembly and optimization of the constituent modules in an integrated device with improved solar energy conversion efficiency. Particular attention will be devoted to the use of easily available and cheap materials, simple and low energy demanding manufacturing methods, and long-term stability of the device. The programmed training and exchange of early stage and experienced researchers during the project, through secondments in the laboratories of the network, will promote the establishment of a new collaboration link between CNR and CNRS-L on solar energy conversion, a topic of key strategic importance for the participants.

Last update: 27/11/2021