The relatively large voltage loss (V loss ) in excitonic type solar cells severely limits their power conversion efficiencies (PCEs). Here, we report a comprehensive control of V loss through efficacious engineering of the sensitizer and redox mediator, making a breakthrough in the PCE of dye-sensitized solar cells (DSSCs).The targeted down-regulation of V loss is successfully realized by three valid channels: (i) reducing the driving force of electron injection through dye molecular engineering, (ii) decreasing the dye regeneration overpotential through redox mediator engineering, and (iii) suppressing interfacial electron recombination.Significantly, the ''trade-off'' effect between the dye optical band gap and the open-circuit voltage (V OC ) is minimized to a great extent, achieving a distinct enhancement in photovoltaic performance (PCE 4 11.5%with V OC up to 1.1 V) for liquid junction cells. The solidification of the best-performing device leads to a PCE of 11.7%, which is so far the highest efficiency obtained for solid-state DSSCs. Our work inspires further development in highly efficient excitonic solar cells by comprehensive control of V loss . Broader contextExcitonic type solar cells, including dye-sensitized solar cells (DSSCs), perovskite solar cells (PSCs) and organic solar cells (OSCs), are potential alternatives for photovoltaic applications. To further advance their power conversion efficiencies (PCEs), it is crucial to reduce the so called voltage loss (V loss ) as much as possible. In DSSCs, the V loss mainly takes place in the electron injection, dye regeneration and interfacial recombination processes, whereas the targeted decreasing of V loss in all these aspects has been rarely reported. To address this issue, we herein demonstrate a comprehensive control of V loss , achieving 11.7% efficiency for solid-state DSSCs. This work is a good example of radically improving the PCE through rationally reducing V loss .

1,3-Phenylene-bridged perylenebisimide dimer (PBI2) and trimer (PBI3) were prepared along with monomer reference (PBI1) using perylene imide-anhydride 5 as a key precursor. 3,3-Dimethylbut-1-yl substituents were introduced at the 2,5-positions of perylenebisimide (PBI) to improve the solubilities of PBI oligomers. Actually, no serious aggregation of PBI2 and PBI3 was detected in their dilute CH(2)Cl(2) solutions. Under these conditions, intramolecular electronic interactions among PBI chromophores have been revealed by measuring the photophysical properties at their ensemble and single-molecule levels. The excitation energy transfer times of PBI2 (0.16 ps) and PBI3 (0.60 ps) were determined from the two different observables, anisotropy depolarization, and singlet-singlet annihilation, respectively, which are considered as the incoherent Förster-type energy hopping (EEH) times as compared with the EEH time constant (1.97 ps) calculated on the basis of the Förster mechanism. The relatively short EEH times compared to similar PBI oligomers can be attributed to 1,3-phenylene linker, which assures a short distance between the chromophores and, as a consequence, makes it hard to treat the PBI unit as a point dipole. The limitation of point-dipole approximation to describe the PBI oligomers and additional through-bond type interactions can be attributed as the causes of the discrepancies in excitation energy transfer times. Considering these photophysical properties, we can suggest that 1,3-phenylene-linked PBI oligomers have potentials as molecular photonic devices including the artificial light-harvesting system.

The conformation of embedded molecule in a polymer matrix is sensitive to the local nano-environment that the molecule experiences. Particularly, single molecule spectroscopic methods have been utilized to visualize each molecular conformation in local sites of the polymer matrix by monitoring rotational diffusion and fluctuating fluorescence of the molecule. Here, we have performed single molecule spectroscopic experiments on a π-stacked perylenediimide (PDI) dimer and trimer, in which enhanced π-π interaction in π-stacked PDIs makes the fluorescence lifetime longer, embedded in two different polymers, namely poly(methyl methacrylate) (PMMA) and poly(butyl methacrylate) (PBMA), to reveal the conformational change depending on the polymer matrix. The fluorescence lifetimes of π-stacked PDIs are influenced by polymer surroundings because their molecular conformations are dependent on their interactions with the local environment in the polymer matrix. Furthermore, from an in-depth analysis of autocorrelation functions of fluorescence intensity trajectories, we could assign that the first autocorrelation value (lag 1) is larger as the intensity trace becomes more fluctuating. Thus, we expect that π-stacked PDIs, a model system for the formation of PDI excimer-like states, can be utilized to probe the surrounding nano-environment by monitoring the conformational change in real time.

The development of push−pull organic dyes utilizing the donor−(πconjugated bridge)−acceptor (D−π−A) motif has induced a paradigm shift in the design of efficient nanocrystalline dye-sensitized solar cells (DSSCs), offering control over the aesthetic properties and performance. Because of the large transient dipole moment characterizing this type of dye upon photoexcitation, these molecules are intrinsically subjected to significant lateral electronic interactions once adsorbed on a solid surface, which can greatly affect the efficiency of the electron injection in DSSCs. Here, we investigated the detailed intermolecular interactions upon photoexcitation of D−π−A diketopyrrolopyrrole (DPP)-based dye molecules, denoted (E)-3-(5-(4-(4-(5-(4-(bis(2′,4′-dibutoxy-[1,1′-biphenyl]-4-yl)amino)phenyl)thiophen-2-yl)-2,5-bis-(2-ethylhexyl)-3,6-dioxo-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)phenyl)furan-2yl)-2-cyanoacrylic acid (DPP_A) and commercially available under the name "Dyenamo Blue", using ultrafast transient absorption spectroscopy. A comparison of the excited-state properties of the molecule with those of a similar dye lacking the triarylamine donor moiety (DPP_R) revealed the critical role played by the donor in determining the mechanisms of intermolecular interactions. The results showed that the intermolecular interactions between DPP_A molecules in solution caused partial delocalization of excitons in a process involving the participation of the triarylamine moiety. Lateral charge transfer (CT) between DPP_A molecules adsorbed on the redox-inactive surface of Al 2 O 3 was observed upon photoexcitation and yielded CT excitons between neighboring dyes. Furthermore, on the basis of the experimental evidence obtained from the pump fluence-dependent transient absorption and excitation spectra, the DPP_R molecules adsorbed on the Al 2 O 3 film exhibited intermolecular π−π interactions, inducing the formation of excimer-like excited states. These results suggest that lateral intermolecular electronic interactions between dye sensitizer molecules adsorbed on the surface of a semiconductor can have a strong influence on the dynamics of electron injection in DSSCs and should thus be considered in the molecular design of new efficient dyes.

Doubly and triply N-confused hexaphyrins revealed quite unique photophysical properties arising from confusion of pyrrole rings in the macrocycle, and the molecular shape when compared with their parent regular hexaphyrin molecules.