Strong light-matter coupling (SC) is increasingly proposed as a powerful tool for post-synthetic control of the optoelectronic properties of organic materials. This technology aims to exploit the easily tuneable polariton states arising from the SC between confined light fields and excitons in organic materials to rewrite molecular energy landscapes and redirect physical pathways. Singlet fission (SF) is a promising technology for improving the efficiency of photovoltaic solar cells beyond their theoretical limit. The SF process consists of the splitting of a singlet excited state into two entangled triplet-triplet states that later become two independent triplets, yielding up to two excited states per absorbed photon –hence, more efficient solar cells. Despite its great potential, SF has been observed only in a limited number of organic compounds and in many cases with a low efficiency, being the synthesis of new derivatives a huge challenge. Recently, some theoretical studies proposed SC as a post-synthesis solution to enhance the SF performance of inefficient materials, by controlling their energy landscape. However, the growing difficulty in reproducing key results in the field of Organic Polaritonics (OP) suggests a poor understanding of the involved phenomena. The major research ambition of this MSCA proposal is to understand the working principles in the OP field and demonstrate that SC can be exploited to enhance the SF efficiency. The implementation of this MSCA proposal will provide a deep knowledge of SC at the molecular scale and how to control it at the macroscale within polaritonic devices, realizing the post-synthetic control of the molecular properties. This achievement will lead to important breakthroughs in Materials Science and Photonics, setting the basis for the OP field. Besides, the proposed research and training activities will expand my experience, research expertise and networks, providing a boost to my career as an independent researcher.