Dye‐sensitized solar cells (DSCs) are a promising source of renewable energy. However, the power conversion efficiency (PCE) of devices has been limited largely by the difficulty of producing electricity using photons from the near‐infrared (NIR) spectral region. Metal‐free organic sensitizers frequently employ strong electron‐donating or ‐withdrawing moieties to tune the optical band gap to allow the absorption of lower energy wavelengths in charge‐transfer systems, whereas porphyrins and phthalocyanines use substituents to shift the Soret and Q bands toward lower energy absorption. Very few devices employing precious metal‐free dyes have achieved panchromatic and NIR photon conversion for electricity generation at wavelengths >750 nm despite a tremendous number of sensitizers published over the last 25 years. This Minireview seeks to compile a summary of these sensitizers to encourage assimilation, analysis, and development of efficient future sensitizers with absorption extending into the NIR. Herein, we discuss common synthetic strategies, optical properties, and electronic properties of the most successful panchromatic organic sensitizers.

A series of near-infrared (NIR) organic emissive materials were synthesized and the photophysical properties analyzed. The donor-acceptor-donor materials were designed with thienopyrazine and thienothiadiazole acceptor groups with thiophene-, furan-, and triphenylamine-based donor groups. The absorption and emission spectra were found to be widely tunable on the basis of the donor and acceptor groups selected. Computational analysis confirms these materials undergo an intramolecular charge-transfer event upon photoexcitation. Large Stokes shifts of ∼150 nm were observed and rationalized by computational analysis of geometry changes in the excited state. Fluorescence studies on the dye series reveal maximum peak emission wavelengths near 900 nm and a quantum yield exceeding 16% for 4,6-bis(2-thienyl)thieno[3,4-c][1,2,5]thiadiazole. Additionally, several dyes were found to have reasonable quantum yields within this NIR region (>1%), with emission wavelengths reaching 1000 nm at the emission curve onset. Photostability studies were conducted on these materials in an ambient oxygen environment, revealing excellent stability in the presence of oxygen from all the dyes studied relative to a benchmark cyanine dye (ICG) during photoexcitation with exceptional photostability from the 4,6-bis(5'-dodecyl-[2,2'-bithiophene]-5-yl)thieno[3,4-c][1,2,5]thiadiazole derivative.

Four D-π-A dyes (D=donor, A=accpetor) based on a 3,4-thienothiophene π-bridge were synthesized for use in dye-sensitized solar cells (DSCs). The proaromatic building block 3,4-thienothiophene is incorporated to stabilize dye excited-state oxidation potentials. This lowering of the excited-state energy levels allows for deeper absorption into the NIR region with relatively low molecular weight dyes. The influence of proaromatic functionality is probed through a computational analysis of optimized bond lengths and nucleus independent chemical shifts (NICS) for both the ground- and excited- states. To avoid a necessary lowering of the TiO2 semiconductor conduction band (CB) to promote efficient dye-TiO2 electron injection, strong donor functionalities based on triaryl- and diarylamines are employed in the dye designs to raise both the ground- and excited-state oxidation potentials of the dyes. Solubility, aggregation, and TiO2 surface protection are addressed by examining an ethylhexyl alkyl chain in comparison to a simple ethyl chain on the 3,4-thienothiophene bridge. Power conversion efficiencies of up to 7.8 % are observed.

Porphyrins are attractive chromophores for application in dye-sensitized solar cells (DSCs), as judicious tuning of donor–acceptor properties can enable excellent near-infrared (NIR) absorption and exceptional device performance. Here, we report a porphyrin-based dye (SM85) conjugated to the planar strong electron donor, indolizine, designed to extend absorption further into the NIR region by inducing π–π interactions such as head-to-tail dye aggregation. The optoelectronic consequences of indolizine incorporation in SM85 include raising the ground-state oxidation potential and broadening and red-shifting ultraviolet–visible–NIR absorptions, along with increased molar absorptivity when compared to the dye SM315. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations confirm the push–pull character of SM85, which features an overlap of frontier occupied and unoccupied orbitals. Steady-state spectrophotometric analyses reveal the presence of solution aggregates via absorption and emission spectroscopies. Aggregate modes were probed by DFT and TD-DFT analyses, and plausible models are presented. SM85-based DSC devices demonstrate a 5.7% power conversion efficiency (PCE) at full sun (7.4% PCE at 10% sun) with an exceptional improvement to the incident photon-to-current conversion onset at ∼850 nm. Current dynamics measurements, time-correlated single photon counting, and computational analyses are used to better understand device performances. This study puts forward a novel intramolecular charge-transfer porphyrin system with a dramatic shift into the NIR region, as is needed for nonprecious metal-based sensitizers, and provides an example of controlled, donor–acceptor-mediated aggregation as a complementary strategy to traditional donor–acceptor modifications to single-molecule π-systems in accessing enhancements in long wavelength light harvesting in molecular-based optoelectronic devices.

A sensitizer incorporating a heavily alkylated surface blocking indolizine donor exhibits excellent light absorption and diminished recombination rates in dye-sensitized solar cells (DSCs). DSC device efficiencies (up to 8%) using either I(-)/I3(-) or Co(bpy)3(2+/3+) redox shuttles were obtained, which compare favourably to the known excellent surface coverage co-sensitization dye, .