Phenothiazine dyes have attracted significant attention as metal-free sensitizers for dye-sensitized solar cells since their introduction in 2007. Since then, over 430 phenothiazine dyes have been prepared and investigated. Despite the ever-increasing molecular variation, the overall improvement in the power conversion efficiency has been slow. In this review, we identify the traits of successful phenothiazine dyes and shed light on inherent molecular limitations. We include all phenothiazine dyes reported to date, and the big dataset is used to look for trends in photophysical, electrochemical and photovoltaic performance. The dataset used in this study is curated to contain only photovoltaic performance data that is validated using a novel credibility assessment. We compare the distinct sub-geometries of phenothiazine dyes, and investigate the effects of auxiliary donors, the use of π-spacers, DSSC electrolyte compatibility, N-alkyl vs. N-aryl substituents and mono- vs. multianchored vs. tethered dyes. Lastly, the phenothiazine dyes are compared to their closest dye analogue, the phenoxazines. Based on the findings in this review, we believe phenothiazine dyes are unlikely to be the main sensitizers of high-efficiency DSSCs in the future, due to low oxidation potentials, low extinction coefficients and inherent limitations restricting the further optimization of their photophysical properties.

Auxiliary donors are common design motifs for phenothiazine sensitizers for dye-sensitized solar cells. Despite this, there are only a few reports on their overall contribution to the photon-to-electron conversion efficiency. Twelve sensitizers have been prepared and investigated, including ten different auxiliary donors in addition to a control with no auxiliary donor. The various auxiliary donors improved the PCE by a modest 4–11%, and pyrene (AFB-19) was determined to be the most efficient auxiliary donor, with the best cell delivering a PCE of 5.36%. Included in the dye series was also the champion dye within the phenothiazine class. With a reported PCE of 12.1%, it would be an excellent phenothiazine reference dye. The high V_OC of 0.83 V of this dye is worth further investigation, but the absorption and photovoltaic performance in this work does not correlate with the previously reported PCE of 12.1%.

Chenodeoxycholic acid (CDCA) is the most used antiaggregation additive in dye‐sensitized solar cells since its introduction to the field in 1993. However, effective suppression of dye aggregation comes at the cost of reduced dye loading, a lower open‐circuit voltage, and limited control of dye/additive distribution when cosensitizing with free CDCA. To combat this, herein, a novel dye design concept that uses the covalent attachment of a CDCA moiety to triarylamine sensitizers is reported. The CDCA substituents do not affect the photophysical or electrochemical properties of the sensitizers but have a positive effect on the photovoltaic performance with [Cu^+/2+(tmby)₂](TFSI)_1/2 electrolyte (tmby = 4,4′,6,6′‐tetramethyl‐2,2′‐bipyridine, TFSI = bis(trifluoromethanesulfonyl)imide). By ensuring a one‐to‐one ratio of dye and CDCA, paired with isotropic distributions of each component, this approach results in a higher‐quality dye monolayer. Compared with the reference system, the novel approach reported herein gives a higher open‐circuit voltage and power conversion efficiency (PCE). The best device is fabricated with the dye C₆–CDCA, delivering a PCE of 6.84% (8 μm TiO₂, 1 mm CDCA, J_SC = 8.64 mA cm⁻², V_OC = 1007 mV, and FF = 0.77).

Conjugated π-spacers are used to improve the light harvesting properties of sensitizers for dye-sensitized solar cells. In several recent works, furan has outperformed other popular π-spacers such as thiophene and phenyl. One of the best performing phenothiazine dyes in the literature has no π-spacer, and two polar triethylene oxide methyl ether (TEOME) chains. Herein we report the synthesis and evaluation of three novel sensitizers based on EO3, investigating the effect of introducing a furan linker and stepwise removal of the TEOME chains. The furan linker redshifted the onsets in the IPCE spectra by 50 nm and improved the PCE by up to 29% compared to EO3, despite lowering the V_OC by more than 60 mV. The best sensitizer of the study was AFB-30, with an average PCE of 5.86% (J_SC = 10.41 mA cm⁻², V_OC = 783 mV, FF = 0.73) under 1 sun AM 1.5G illumination.