Main DSSC components, along with their most critical materials and related issues are reviewed and feasible alternatives are envisaged.

Almost ten years after their first use in the photovoltaic (PV) field, perovskite solar cells (PSCs) are now hybrid devices that, in addition to having reached silicon performance, can accelerate the energy transition and boost the use of abundant elements for their manufacturing process. However, noble metals (in particular gold) represent the most typically used sources for back electrode fabrication, and this issue has been intensively considered by the research community in the last five years. This review shows how the most promising solution, considering also the need to develop a large-scale production process, is based on the use of carbon-based materials for the preparation of back electrodes. Graphite, carbon black, graphene and carbon nanotubes (CNTs) have been proposed, functionalized and characterized, leading to laboratory-scale solar cells and modules capable of providing excellent efficiencies and ensuring stability greater than those of gold-based devices. Strengthened by these results and its hydrophobizing properties, carbon has also started to be used as an electron transporting material (ETM), with excellent results on both rigid and flexible substrates. This review discusses the major advances and the updated state-of-the-art in the carbon-based PSC scenario, keeping a solid trajectory where the accessibility, low cost, high electrical conductivity, chemical stability and controllable porosity of carbon are highlighted and exploited in the design of upscalable hybrid solar cells. Broader contextToday, more than 75% of the world energy demand is met by traditional, non-sustainable energy resources, mainly based on coal, natural gas and oil. Photovoltaics represents a concrete action to mitigate fossil fuel consumption, and among all the developed solar cells, perovskite-based ones have shown the highest increase in terms of efficiency (currently above 25%). Halide perovskites, as a family of new-generation semiconductor materials, are also exploitable for light-emitting devices, photodetectors and memristors, but their use in photovoltaics gives rise to outstanding performances. Here, photogenerated charges pass through electron/hole transporting materials and are transferred to the external circuit by front and back electrodes. While the front electrode is a common conductive glass, many issues are now under study concerning the back electrode, traditionally made of gold. Indeed, replacing this expensive metal with a much cheaper alternative is vital for realizing affordable solar technologies. Carbon, in its multiple forms, represents the winning solution and the scientific community has recently shown its large scale processability, its power as a stability booster and some interesting effects at the perovskite/electrode interface. This review shows how carbon is becoming the winning ingredient for the scaling up and worldwide diffusion of perovskite solar cells.

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In the quest for sustainable materials for quasi-solid-state (QS) electrolytes in aqueous dye-sensitized solar cells (DSSCs), novel bioderived polymeric membranes were prepared in this work by reaction of preoxidized kraft lignin with poly(ethylene glycol)diglycidylether (PEGDGE). The effect of the PEGDGE/lignin relative proportions on the characteristics of the obtained membranes was thoroughly investigated, and clear structure–property correlations were highlighted. In particular, the glass transition temperature of the materials was found to decrease by increasing the amount of PEGDGE in the formulation, indicating that polyethylene glycol chains act as flexible segments that increase the molecular mobility of the three-dimensional polymeric network. Concurrently, their swelling ability in liquid electrolyte was found to increase with the concentration of PEGDGE, which was also shown to influence the ionic transport efficiency within the membrane. The incorporation of these lignin-based cross-linked systems as QS electrolyte frameworks in aqueous DSSCs allowed the preparation of devices with excellent long-term stability under UV–vis light, which were found to be superior to benchmark QS-DSSCs incorporating state-of-the-art carboxymethylcellulose membranes. This study provides the first demonstration of lignin-based QS electrolytes for stable aqueous DSSCs, establishing a straightforward strategy to exploit the potential of lignin as a functional polymer precursor for the field of sustainable photovoltaic devices.