Solar photovoltaics (PV) offer viable and sustainable solutions to satisfy the growing energy demand and to meet the pressing climate targets. The deployment of conventional PV technologies is one of the major contributors of the ongoing energy transition in electricity power sector. However, the diversity of PV paradigms can open different opportunities for supplying modern systems in a wide range of terrestrial, marine, and aerospace applications. Such ubiquitous and versatile applications necessitate the development of PV technologies with customized design capabilities. This involves multifunctional characteristics such as aesthetic appearance, visual comfort, and heat insulation. To enable on‐demand adaptation to the requirements of distributed applications, tunable solar cells (SC) feature exceptional degrees of freedom in the manipulation of their intrinsic properties via adjusted materials engineering. The pertinent tuning abilities include but are not limited to bandgap energy, transparency, color, and thermal management. In this review, the main principles of different tuning approaches are specified and an overview of relevant concepts of tunable SC technologies is presented. Then, the recent integrations of cutting‐edge tunable PV adapted to versatile applications are systematically summarized. In addition, current challenges and insightful perspectives into potential future opportunities for omnipresent tunable PV are discussed.

An optical study based on spectroscopic ellipsometry, performed on ultrathin hydrogenated amorphous silicon (a-Si:H) layers, is presented in this work. Ultrathin layers of intrinsic amorphous silicon have been deposited on n-type mono-crystalline silicon (c-Si) wafers by plasma enhanced chemical vapor deposition (PECVD). The layer thicknesses along with their optical properties –including their refractive index and optical loss- were characterized by spectroscopic ellipsometry (SE) in a wavelength range from 250 nm to 850 nm. The data was fitted to a Tauc-Lorentz optical model and the fitting parameters were extracted and used to compute the refractive index, extinction coefficient and optical bandgap. Furthermore, the a-Si:H film grown on silicon was etched at a controlled rate using a TMAH solution prepared at room temperature. The optical properties along with the Tauc-Lorentz fitting parameters were extracted from the model as the film thickness was reduced. The etch rate for ultrathin a-Si:H layers in TMAH at room temperature was found to slow down drastically as the c-Si interface is approached. From the Tauc-Lorentz parameters obtained from SE, it was found that the a-Si film exhibited properties that evolved with thickness suggesting that the deposited film is non-homogeneous across its depth. It was also found that the degree of crystallinity and optical (Tauc) bandgap increased as the layers were reduced in thickness and coming closer to the c-Si substrate interface, suggesting the presence of nano-structured clusters mixed into the amorphous phase for the region close to the crystalline silicon substrate. Further results from Atomic Force Microscopy and Transmission Electron Microscopy confirmed the presence of an interfacial transitional layer between the amorphous film and the underlying substrate showing silicon nano-crystalline enclosures that can lead to quantum confinement effects. Quantum confinement is suggested to be the cause of the observed increase in the optical bandgap of a-Si:H films close to the a-Si:H/cSi interface.

Hydrogenated amorphous silicon (a-Si : H) layers deposited by chemical vapour deposition provide an attractive route to achieve high-performance crystalline silicon (c-Si) solar cells due to their deposition at low temperatures and their superior passivation quality. Hydrogen certainly plays an additional crucial role by passivating the dangling bonds, and thus improving the electrical and optical properties. In this work, we present the variation of the effective lifetime with the hydrogen dilution ratio R = (H₂/SiH₄). We find that at lower hydrogen dilution rates (R < 2), the polymerization reaction of silane molecules in the plasma bulk as well as the excess of dihydride (Si–H₂) incorporation in as-deposited layers result in higher microvoid density and worse passivation quality. In contrast, the deposition at higher hydrogen dilution rates (R > 5) leads to a degradation in the film quality with very low hydrogen content and crystalline epitaxial growth at the a-Si/c-Si interface. Thus, the best material quality is obtained just below the onset of amorphous-to crystalline transition (R = 3), which favours further improvement of the passivation during a post-deposition annealing.