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).

In this work, the highly conjugated pentacyclic [1]benzothieno[3,2‐ b ]thieno[2,3‐ f ][1]benzothiophene ( TBTBT ) moiety has been introduced, for the first time, as a mild donor group in the structure of three donor‐acceptor organic dyes, named PCD1‐3 . After dye design assisted by DFT and TD‐DFT computational studies, the fused TBTBT system with the appropriate substitution pattern was synthesized by means of a three‐step sequence, featuring a Pd‐catalyzed direct arylation of 1‐bromo‐2‐(methylsulfinyl)benzene and a subsequent interrupted Pummerer reaction as the key steps. Further elaboration of the dyes’ structures was then achieved by application of additional C(sp ² )‐C(sp ² ) direct arylation protocols, thereby making the overall pathway particularly concise. Spectroscopic and electrochemical characterization revealed that dyes PCD1‐3 presented the appropriate properties for employment as photosensitizers in dye‐sensitized photovoltaic cells (DSSCs) for indoor applications. Preliminary tests conducted on DSSCs fabricated with the new dyes highlighted differences in their compatibility with electrolyte solutions of different compositions, as well as in their response to different indoor and outdoor (simulated) light sources, with the best results obtained under illumination with a cold white LED (900 lux).