We report two new molecularly engineered push-pull dyes, i.e., YA421 and YA422, based on substituted quinoxaline as a π-conjugating linker and bulky-indoline moiety as donor and compared with reported IQ4 dye. Benefitting from increased steric hindrance with the introduction of bis(2,4-dihexyloxy)benzene substitution on the quinoxaline, the electron recombination between redox electrolyte and the TiO2 surface is reduced, especially in redox electrolyte employing Co(II/III) complexes as redox shuttles. It was found that the open circuit photovoltages of IQ4, YA421, and YA422 devices with cobalt-based electrolyte are higher than those with iodide/triiodide electrolyte by 34, 62, and 135 mV, respectively. Moreover, the cells employing graphene nanoplatelets on top of gold spattered film as a counter electrode (CE) show lower charge-transfer resistance compared to platinum as a CE. Consequently, YA422 devices deliver the best power conversion efficiency due to higher fill factor, reaching 10.65% at AM 1.5 simulated sunlight. Electrochemical impedance spectroscopy and transient absorption spectroscopy analysis were performed to understand the electrolyte influence on the device performances with different counter electrode materials and donor structures of donor-π-acceptor dyes. Laser flash photolysis experiments indicate that even though the dye regeneration of YA422 is slower than that of the other two dyes, the slower back electron transfer of YA422 contributes to the higher device performance.

Spiro is beautiful. Two spiro-type molecules are compared at molecular levelviasingle-crystallography. Through molecular engineering, we synthesized new hole transporting material PST1 which works efficiently in perovskite solar cells without cobalt dopant.

The photophysical properties of a tetrahedral molecule with naphthalene diimide (NDI) moieties and of two model compounds were investigated. The absorption and fluorescence spectra of dialkyl-substituted NDI are in agreement with literature. While the absorption spectra of phenyl-substituted molecules are similar to all other NDIs, their fluorescence showed a broad band between 500 and 650 nm. This band is sensitive to the polarity of the solvent and is attributed to a CT state. The absorption spectra and lifetime (10+/-2 ps) of the electronically excited singlet state of a dialkyl-substituted NDI was determined by femtosecond transient absorption spectroscopy, and the latter was confirmed by picosecond fluorescence spectroscopy. Nanosecond flash photolysis showed the subsequent formation of the triplet state. The presence of a phenyl substituent on the imide nitrogen of NDI resulted in faster deactivation of the singlet state (lifetime 0.5-1 ps). This is attributed to the formation of a short-lived CT state, which decays to the local triplet state. The faster deactivation was confirmed by fluorescence lifetime measurements in solution and in a low-temperature methyl-tetrahydrofuran glass.