Experimental characterization and numerical modeling of ionic and electronic dynamics in nanostructured hybrid materials for photoconversion

Abstract

The development of alternative, low emissions energy sources has gained importance for society as the worldwide energy demand increases while the environmental impact of the accumulated use of fossil fuels becomes more evident, reflected in climate change. As a response to this problem, the scientific community has focused on researching new energy sources. One of the technologies that has driven more attention is photovoltaic technology to directly exploit the vast amount of energy reaching the Earth¿s surface as sunlight. While silicon-based solar cells have dominated the photovoltaic landscape for many years, the search for alternatives decreasing the need for scarce materials or high energy cost manufacturing processes has led to the development of the third-generation photovoltaics. In this context, dye-sensitized solar cells were a breakthrough in the field as they are made of abundant and cheap materials and comprise relatively simple manufacturing processes.

Furthermore, their adaptability and variety of elements make them very appealing for emerging markets and new applications such as building integrated photovoltaics or indoor applications. Perovskite solar cells rapidly emerged from a particular application of dye-sensitized solar cells to a brand new photovoltaic technology in their own right, reaching outstanding efficiencies thanks to their excellent optoelectronic properties. In addition, the natural abundance of the precursors involved in the synthesis of the material also makes them an up-and-coming technology. However, the lack of stability under environmental conditions, the use of expensive materials, along with other technical limitations such as the need for inert environments in the manufacturing process have kept this technology from deep market penetration and widespread implementation. For this reason, fundamental knowledge of the electronic and ionic properties behind the positive and negative aspects of these materials is highly needed to help optimize them.

The main aim of this thesis is to understand the ionic and electronic dynamics and the physicochemical processes that determine the photovoltaic performance under operating conditions and the long-term stability of these hybrid nanostructured materials. To accomplish this objective, small-signal

perturbation optoelectronic techniques have been used, together with numer- ical drift-diffusion simulations. Considered together, it helps to cast light

on the electronic and ionic phenomena that determine the functioning of the device as well as the key interplay between the two: electronics and ionics. In this context, the similarities between already understood systems such as normal dye-sensitized solar cells with the materials studied in this thesis are used to identify and interpret the different signals obtained from small signal perturbation optoelectronic techniques. In addition, combining these experimental techniques with numerical simulations has proven to be, in this thesis, an instrumental approach to understanding the physical meaning of the elements identified in experimental spectra allowing for their interpretation and understanding their role in determining the photovoltaic properties of the device under operation conditions.