Bio-inspired dipolar dyes for solution processing of organic solar cells – Chalcones

Scientifically and technologically, but also economically, dye chemistry has represented a major field of activity in chemical industry and academic research since the 19th century. Nowadays, dye sciences are facing new exciting challenges. Given the announced shortage in fossil fuels, alternative energy supplies such as photovoltaics represent realistic solutions that require efficient materials for sun photons capture and conversion into electricity. Among the vast majority of dye families, those combining electron donor (D) and acceptor (A) groups, such as D-A dipolar compounds, occupy a central place because presence of alternating electron-rich/electron-deficient units reduces the bandgap and facilitates intrachain charge transfer. First incorporated into polymeric backbones, the D-A concept has now gained much interest with the recent discovery that low molecular weight dye structures with a net dipole moment can lead to the rational design of new active materials for organic solar cells. Therefore, the in-depth investigation of both optical and electronic properties of low molecular weight D-A molecules in the solid-state represents a fundamental challenge of tremendous importance in view of assessing the performance of new generations of low band gap materials. Moreover there is still a critical need for the design of small molecular semiconductors in order to further explore structure-property correlations and ultimately overcome the technological barriers that concern cost-effective materials, easy processability, large-area device efficiency, and end-products mass production. This project proposes a rational study based on the implementation of chalcone and curcuminoid dyes in which borondifluoride complexation is used as a simple way to impart the molecular structure with extremely strong ground- and excited-state dipole moments. Such systems have the D-A and D-A-D structures, respectively. A preliminary study showed chalcones compounds display promising photovoltaic properties when used as donor materials in the presence of [6,6]phenyl-C61-butyric acid methyl ester (PC61BM) in bulk heterojunction solar cells. The project aims at unravelling the factors that control solar cell efficiency combining chemical, spectroscopic, theoretical, and technological approaches. It includes the synthesis of new dyes with optimized electronic energy levels, improved charge transport properties, and allowing the control of thin film morphology at the nanoscale. Quantum chemical calculations will be used to unravel structure-properties, and next, to predict the spectroscopic features of new molecular structures. Device fabrication will allow identifying the best candidates and will focus on the optimization of efficient solution-processed single junction. In a final stage, materials with the best photovoltaic performance will be tested in the fabrication of large area solar cells using an industrial prototype for solution printing, which will need the development of ink formulation. The project gathers three academic teams, two of chemists – experimental and theoretician – and one of physicists, and an industrial partner. This project is mainly devoted to chalcones. Indeed these compounds can be obtained via simple, efficient preparation and easy purification procedures. They represent archetypal examples of organic compounds that can be obtained according to green chemistry, such as in solvent-free reactions. Ultimately, a long-term goal would be to identify the potential of the novel molecular structures in advanced applications with the idea of fostering green chemical approaches.