Among the emerging photovoltaic (PV) technologies, Dye-Sensitized Solar Cells (DSSCs) appear especially interesting in view of their potential for unconventional PV applications. In particular, DSSCs have been proven to provide excellent performances under indoor illumination, opening the way to their use in the field of low-power devices, such as wearable electronics and wireless sensor networks, including those relevant for application to the rapidly growing Internet of Things technology. Considering the low intensity of indoor light sources, efficient light capture constitutes a pivotal factor in optimizing cell efficiency. Consequently, the development of novel dyes exhibiting intense absorption within the visible range and light-harvesting properties well-matched with the emission spectra of the various light sources becomes indispensable. In this review, we will discuss the current state-of-the-art in the design, synthesis, and application of organic dyes as sensitizers for indoor DSSCs, focusing on the most recent results. We will start by examining the various classes of individual dyes reported to date for this application, organized by their structural features, highlighting their strengths and weaknesses. On the basis of this discussion, we will then draft some potential guidelines in an effort to help the design of this kind of sensitizer. Subsequently, we will describe some alternative approaches investigated to improve the light-harvesting properties of the cells, such as the co-sensitization strategy and the use of concerted companion dyes. Finally, the issue of measurement standardization will be introduced, and some considerations regarding the proper characterization methods of indoor PV systems and their differences compared to (simulated) outdoor conditions will be provided.

Two novel organic sensitizers for Dye-Sensitized Solar Cells (DSSC), called TP1 and TP2, based on the electron-poor thieno[3,4-b]pyrazine (TPz) π-bridge and the electron-rich N,N-bis(4-(hexylthio)phenyl)aniline (TPA) were designed following two different approaches: the classical D-A-π-A design and a symmetric structure with double anchoring functions. Both compounds were prepared exploiting short synthetic pathways based on direct arylation strategies and possibly one-pot desymmetrization. The two novel dyes displayed opposite optical properties: a broad and intense light absorption over the entire visible spectrum for TP1, and a localized absorption that peaked in the center of the visible region for TP2, resulting in a pitch-dark coloration and a green tone, respectively. When assembling the photovoltaic devices, different electrolyte compositions were explored to enhance the optical properties of the dyes. Power conversion efficiencies as high as 5.2% under full sun intensity were recorded for small test devices. The composition of the light transmitted through the TP2-containing transparent DSSC fits well with the human eye sensitivity spectrum, thus fulfilling the transparency requirements for building-integrated photovoltaics (BIPV).

The preparation and spectroscopic characterization of some symmetrical small molecules containing the disubstituted (E)-3,3′-bifuranylidene-2,2′-dione chromophore (the so-called Pechmann lactone) and featuring an extended conjugation are reported. The synthetic approach of such compounds is based on the Stille–Migita coupling reaction, which is carried out in very mild conditions, suitable to be applied with the very sensitive Pechmann core. The absorption and emission spectra of the new molecules obtained have been recorded in solution and clearly show how their photophysical properties can be modulated by a proper choice of the (hetero)aromatic terminal groups. The compounds prepared in this study (or close analogues thereof) have interesting optical properties and could find application as semiconductors in organic electronics or as near-IR fluorescent dyes.

Three organic conjugated donor–acceptor dyes with hydrophobic side chains were studied as potential photosensitizers for dye-sensitized photoelectrochemical cells (DS-PECs). The dyes were anchored on TiO₂ electrodes and investigated using steady-state and femtosecond transient absorption spectroscopies. Optical studies revealed intramolecular charge transfer (ICT) as the dominant excitation pathway, with AD418 showing single-state emission and BTD-DTP1/TTZ5 displaying dual ICT emission. On TiO₂, ultrafast electron injection (1–2 ps) leads to long-lived mono-cationic species, indicating effective charge separation.