Moises Espindola-Rodriguez

Optical techniques provide a fast and flexible path to material inspection. In particular, the investigation of key metrics during material production can help obtain the optimal parameters to achieve desired material functionality as well as to ensure product quality. Here we present an optical imaging setup designed for fast non-contact material inspection in a roll-to-roll deposition environment. The method allows for robust simultaneous spatial mapping of thickness in the range from 0.3 µm to 110 µm of multiple layers of transparent or semi-transparent materials in the form of a free film or a film deposited on a thicker transparent substrate. The PEDOT:PSS and ZnO materials chosen for this proof of concept are relevant for the fabrication of organic solar cells. Additionally, the method may be used to characterize birefringence.

A method is presented to manipulate the final morphology of roll‐to‐roll slot‐die coated poly(3‐hexylthiophene) (P3HT) by optically exciting the p‐type polymer in solution while coating. These results provide a comprehensive picture of the entire knowledge chain, from demonstrating how to apply the authors’ method to a fundamental understanding of the changes in morphology and physical properties induced by exciting P3HT while coating. By combining results from density functional theory and molecular dynamics simulations with a variety of X‐ray experiments, absorption spectroscopy, and THz spectroscopy, the relationship between morphology and physical properties of the thin film is demonstrated. Specifically, in P3HT films excited with light during deposition, changes in crystallinity and texture with more face‐on orientation and increased out‐of‐plane charge mobility are observed.

Fabrication on transparent soda-lime glass/FTO substrates opens the way to advanced applications for kesterite solar cells such as semitransparent, bifacial and tandem devices which are key to the future of the PV market. However, the complex behavior of the p-kesterite/n-FTO back interface potentially limits the power conversion efficiency of such devices. Overcoming this issue requires careful interface engineering. This work empirically explores the use of transition metal oxides (TMOs) and Mo-based nanolayers to improve the back interface of Cu₂ZnSnSe₄, Cu₂ZnSnS₄ and Cu₂ZnSn(S,Se)₄ solar cells fabricated on transparent glass/FTO substrates. Although the use of TMOs alone is found to be highly detrimental for the devices inducing complex current blocking behaviors, the use of Mo:Na nanolayers and their combination with the n-type TMOs TiO₂ and V₂O₅ are shown to be a very promising strategy to improve the limited performance of kesterite devices fabricated on transparent substrates. The optoelectronic, morphological, structural and in-depth compositional characterization performed on the devices suggests that the improvements observed are related to a combination of shunt insulation and recombination reduction. This way, record efficiencies of 6.1%, 6.2% and 7.9% are obtained for Cu₂ZnSnSe₄, Cu₂ZnSnS₄ and Cu₂ZnSn(S,Se)₄ devices, respectively, giving proof of the potential of TMOs for the development of kesterite solar cells on transparent substrates.

In order to deepen the knowledge of the vibrational properties of two-dimensional (2D) MoS ₂ atomic layers, a complete and systematic Raman scattering analysis has been performed using both bulk single-crystal MoS ₂ samples and atomically thin MoS ₂ layers. Raman spectra have been measured under non-resonant and resonant conditions using seven different excitation wavelengths from near-infrared (NIR) to ultraviolet (UV). These measurements have allowed us to observe and identify 41 peaks, among which 22 have not been previously experimentally observed for this compound, and characterize the existence of different resonant excitation conditions for the different excitation wavelengths. This has also included the first analysis of resonant Raman spectra that are achieved using UV excitation conditions. In addition, the analysis of atomically thin MoS ₂ layers has corroborated the higher potential of UV resonant Raman scattering measurements for the non-destructive assessment of 2D MoS ₂ samples. Analysis of the relative integral intensity of the additional first- and second-order peaks measured under UV resonant excitation conditions is proposed for the non-destructive characterization of the thickness of the layers, complementing previous studies based on the changes of the peak frequencies.