Solution-processable perylene-based hybrid electron acceptors for perovskite solar cells

Abstract

Perovskite solar cells (PSC) are devices based on organic-inorganic perovskite-structured semiconductors that convert sunlight directly into electricity. Such solar devices have become a significant competitor in the photovoltaic field and overall in the renewable-energy race due to rapidly reaching a conversion efficiency of 25%, already comparable to the commonly used Silicon technology. Besides the unique and favourable optoelectronic properties of perovskite absorbers, their ease of synthesis and versatility has opened the possibility to produce lightweight, portable and cheap photovoltaic devices. However, most of the research still relies on inherited materials from preceding technologies, such as TiO₂ or PCBM electron transport materials (ETM), which hinders the use of a wide range of substrates and increases the manufacturing costs. Nowadays, much effort is being made by the research community to enhance further the conversion efficiency, stability and manufacturing scalability of PSCs. This work aims to investigate alternative organic and hybrid electron acceptors as ETMs in PSCs. To this purpose, six different Perylene-based organic semiconductors were designed, synthesised, characterised and assessed as ETMs and interlayers in PSCs.

In Chapter 3, a series of three perylene derivatives (PDI-1 to -3) with different functional groups attached to the bay-position of the molecule’s core was successfully synthesised through a five-step synthesis procedure. The optical, electrochemical and computational characterisation was carried out through different characterisation techniques. The effect that the electronic nature of the substituents has on the optoelectronic properties of the molecules was studied. The results showed that all the PDI derivatives have suitable optical and electrochemical properties, with reversible cathodic redox potential and suitable lowest unoccupied molecular orbital (LUMO) for favourable charge transfer, comparable to the values of PCBM used in an inverted PSC architecture. The PV characterisation showed comparable performance between a PDI-3 device and a PCBM device in a triple cation mixed halide perovskite. Additional photoluminescence quenching measurements showed favourable energy alignment between MAPI and PDI-1 to -3 films.

In Chapter 4, the commonly used TiO₂ as ETM in conventional PSCs architecture was substituted for a low-temperature ZnO ETM prepared through a sol-gel route. Due to the chemical instability of the perovskite/ZnO interface, derivatives PDI-4 to -6 were designed as an n-type interlayer to protect the perovskite from degradation by direct contact with ZnO while avoiding detrimental effects in charge extraction due to their semiconductor nature. All molecules were fully characterised through optical, electrochemical, and computational methods. Perovskite films showed excellent stability when using the PDI interlayer deposited on top of the ZnO film. Photoluminescence quenching confirmed the charge extraction and the suitable match of LUMO levels of PDI derivatives. Photovoltaic characteristics of final devices showed that PDI-5 and -6 had comparable performance to those using the wellestablished TiO₂ ETM.

Finally, based on the encouraging results obtained from the previous experience with interfacial materials. In Chapter 5, hybrid ZnO: PDI materials were synthesised to explore a “one-step interlayer” approach by incorporating PDI derivatives in the synthetic route of ZnO. The hybrid materials were studied through two different procedures: One, by mixing a PDI solution in different concentrations with a ZnO sol-gel precursor solution. The other by the hydrolysis of PDI derivatives and functionalisation of ZnO nanoparticles. Although stability results in the sol-gel procedure were not encouraging, it opens the possibility of the tailored design of PDI derivatives to dissolve and readily incorporate in the solvent system. On the other hand, the functionalised ZnO nanoparticles showed better stability; nevertheless, optimisation in film deposition and particle size is required for full coverage and proper charge extraction.

In total, this project produced six different perylene-based organic semiconductors to be assessed as ETMs and interlayer materials, and also two different approaches to synthesise hybrid ZnO: PDI materials.