In the last years, the massive evolution of modern technologies has gradually created an alarming gap between the production and the consumption of energy. Traditional energy resources are no longer sufficient to satisfy the demand of energy without spoiling earth environment. Solar photovoltaics represents a highly promising technology to tackle this global energy issue. The thorough scientific discussion on this fundamental topic gave rise to interesting results and the organic solar cells (OSCs) are one of these achievements.
One major reason of the development and the increasing interest in this new technology is its eco-friendliness and the potentially low-cost production of solar modules on flexible (plastic) substrates. Furthermore, new applications are expected by flexible or semitransparent organic solar cells. Nevertheless, two main problems must be overcome before this promising technology replaces the long-established silicon solar cells: the low power conversion efficiency and the scarce stability. In order to tackle these fundamental issues the research efforts must be focused towards both the development of new materials and their detailed photophysical and morphological characterization.
Recently the application of nanostructured architectures within the active layers of OSCs has demonstrated to be an efficient alternative to boost solar cell efficiency. Indeed, the nanometric miniaturization of materials opened a huge amount of possibilities to tune and bolster their optical and electrical properties.
In this thesis work, the potentialities of the nanostructured architectures are explored. In particular, the attention of this work is addressed towards the development and the photophysical characterization of new hybrid nanostructured photoactive materials. Three different families of nanostructures, colloidal Quantum Dots, Carbon Dots, and hybrid organic/inorganic perovskite nanoparticles, are blended with organic photovoltaic materials. The thorough investigation of the photo-physical and morphological interactions between the nanostructures and the organic materials aims to investigate these nanocomposite as new photoactive materials for next-generation solar cells.
The first step of the work focuses on the investigation of a prototypical active layer consisting in binary blends of the fullerene derivative PCBM and CdSe/CdS core-shell Quantum Dots (QDs) capped with different ligands (namely, oleylamine, octadecanethiol, and propanethiol). The double purpose is both to demonstrate that QDs do not influence only the morphology of the active layers, as it is often reported in literature, but also its photophysics and to unravel the pivotal role of QDs ligands on the electron transfer process, which is fundamental for organic solar cells. Through the combined use of steady-state, time resolved and pulsed electron paramagnetic resonance (EPR) techniques the photophysical role of QDs in OSCs is clarified and the possibility to tailor the electron transfer process through the proper choice of QDs ligands is demonstrated.
The second part of the work aims at promoting the application of carbon dots (CDs) as electron donor materials for OSCs. CDs seem to be a good alternative to colloidal QDs, thanks to their low toxicity, good biocompatibility and peculiar photo-physical properties, however their poor solubility in organic solvents and mediocre electron-donor properties hampered their photovoltaic application. To tackle these critical issues, the synthesis and photo-physical characterization of N-doped CDs functionalized with two different thiophene-containing groups is carried out in this work. The functionalization intends to enhance the electron donating properties of the CDs and improve their solubility in organic solvents. The increased solubility allows to investigate the photoinduced interactions of functionalized CDs with the PCBM in solution and in solid blends. Through the combined cyclic voltammetry, optical and EPR analysis the enhanced electron donor capabilities of the functionalized CDs are demonstrated and the electron transfer process is characterized in detail.
Finally, the last part of the work concentrates on the hybrid organic inorganic perovskite nanostructures. These recent nanostructures are definitely the best candidate to compete with silicon solar cells since their bulk counterpart has already provided record photovoltaic efficiencies in less than five years. However, the application of perovskite nanoparticles (PNPs) in organic solar cells has been scarcely investigated so far. Therefore, in this thesis work the synthesis of PNPs and the investigation of their interaction with both the PCBM and the semiconducting polymer P3HT is carried out. After the confirmation of the obtained synthesis through optical spectroscopy, X-ray diffraction and XPS analysis, the electron transfer from PNPs to PCBM is investigated. In particular, the effect of the ligand length on the electron transfer is examined, probing the process with two different PNPs ligands: octylamine and oleylamine. Successively, the role of the PNPs in blend with P3HT is studied. A triple effect of PNPs on the polymer properties is observed: (1) an increment of the dimension of P3HT crystalline domains, (2) a p-doping of the P3HT, and (3) an enhanced interchain order.
The results of this work underpin the relevance of applying nanostructured architectures in organic photovoltaic materials, highlighting their beneficial role not only in morphology, but also in the main photo-physical processes that take place in solar cells. Additionally, the relevant role of the tailored surface engineering of nanostructures in the process of solar energy conversion is evidenced. All these observations aim at providing guidelines for the design and the fabrication of highly efficient solar cells
