Effect of Alkyl Chain Length on the Sensitizing Action of Substituted Non-Symmetric Squaraines for p-Type Dye-Sensitized Solar Cells

KuQuinones: a ten years tale of the new pentacyclic quinoid compound

Quinones are widespread in nature, as they participate, mainly as redox mediators, in several biochemical processes. Up to now, various synthetic quinones have been recommended in the literature as leading molecules in energy, biomedical and catalytic fields. In this brief review, we retraced our research activity in the last ten years, mainly dedicated to the study of a new class of peculiar pentacyclic conjugated quinoid compounds, synthesized in our group. In particular, their application as sensitive materials in photoelectrochemical devices and in biosensors, as photocatalysts in selective oxidation reactions, and their anticancer activity is here reviewed.

Experimental and theoretical studies of the influence of alkyl groups on the photovoltaic properties of (E)-6-((2, 3-dihydroxylnaphthalene)diazenyl)-1H-benzoisoquinoline-1,3-dione-based organic solar cell

The manipulation of the active dye material for application in dye-sensitized solar cell (DSSC) using simple or bulky group substituents is necessary for improved dye performance. Herein, we carried out a combined experimental and theoretical studies of different alkylated novel reactive (E)-6-(2,3-dihydroxyl naphthalene diazinyl)-1H-benzoisoquinoline-1,3-dione azo-based dyes using spectral (FTIR, UV-visible, and NMR) analysis and electronic structure theory method based first principle density functional theory (DFT) calculations to investigate the molecular electronic properties, structural analysis, excitation behavior, and the theoretical potential application in photovoltaic cell. The synthesized azo dye (azoD) was theoretically modeled by varying the number of alkyl chains denoted as AzoD₁, AzoD₂, AzoD₃, and AzoD₄ to represent azo dyes having ten (10), twelve (12), fourteen (14), and sixteen (16) alkyl chain length respectively. From the natural bond orbital (NBO) analysis, the higher stabilization energies, 227.80 and 227.77 kcal/mol respectively, recorded for AzoD₁ and AzoD₄ may be due to extra orbital contribution by π*(N₂₁-N₂₂) to π*C₅₄-C₅₆ 31.19 eV for AzoD₁ and π*(N₂₁-N₂₂) → π*(C₅₃-C₅₅) 31.43 eV AzoD₄ confirming that chain length affected the orbital interaction of the molecules. The driving force (ΔG_inject) of electron injection into the TiO₂ surface (- 1.92 to - 1.93) shown in this study is indicative that alkylated azo dyes are good for improved DSSCs performance. Again, the open circuit voltage (V_oc) of 1.090 (AzoD₁), 1.092 (AzoD₂), 1.093 (AzoD₃), and 1.095 (AzoD₄) are also evidence of the suitability of azo dyes as photosensitizers. All the spectroscopic analysis, FTIR, UV-visible, and NMR combined with theoretical calculations, provided accurate data for characterizing the titled azo dye compound and showed that it has good photophysical properties. The presence of alkyl groups and chain length promoted the stability of the dyes thereby making them suitable for application in DSSCs. Increase in chain length as well enhanced the electron injection into the conduction band of the semiconductor.

Toward Current Matching in Tandem Dye-Sensitized Solar Cells

The tandem pn-type dye-sensitized solar cells (pn-DSCs) have received much attention in the field of photovoltaic technologies because of their great potential to overcome the Shockley-Queisser efficiency limitation that applies to single junction photovoltaic devices. However, factors governing the short-circuit current densities (J_sc) of pn-DSC remain unclear. It is typically believed that J_sc of the pn-DSC is limited to the highest one that the two independent photoelectrodes can achieve. In this paper, however, we found that the available J_sc of pn-DSC is always determined by the larger J_sc that the photoanode can achieve but not by the smaller one in the photocathode. Such experimental findings were verified by a simplified series circuit model, which shows that a breakdown will occur on the photocathode when the photocurrent goes considerably beyond its threshold voltage, thus leading to an abrupt increase in J_sc of the circuit. The simulation results also suggest that a higher photoconversion efficiency of the pn-DSCs can be only achieved when an almost equivalent photocurrent is achieved for the two photoelectrodes.

Electrochemically Deposited NiO Films as a Blocking Layer in p-Type Dye-Sensitized Solar Cells with an Impressive 45% Fill Factor

The enhancement of photoelectrochemical conversion efficiency of p-type dye-sensitized solar cells (p-DSSCs) is necessary to build up effective tandem devices in which both anode and cathode are photoactive. The efficiency of a p-type device (2.5%) is roughly one order of magnitude lower than the n-type counterparts (13.1%), thus limiting the overall efficiency of the tandem cell, especially in terms of powered current density. This is mainly due to the recombination reaction that occurs especially at the photocathode (or Indium-doped Tin Oxide (ITO))/electrolyte interface. To minimize this phenomenon, a widely employed strategy is to deposit a compact film of NiO (acting as a blocking electrode) beneath the porous electrode. Here, we propose electrodeposition as a cheap, easy scalable and environmental-friendly approach to deposit nanometric films directly on ITO glass. The results are compared to a blocking layer made by means of sol-gel technique. Cells embodying a blocking layer substantially outperformed the reference device. Among them, BL_1.10V shows the best photoconversion efficiency (0.166%) and one of the highest values of fill factor (approaching 46%) ever reported. This is mainly due to an optimized surface roughness of the blocking layer assuring a good deposition of the porous layer. The effectiveness of the implementation of the blocking layer is further proved by means of Electrochemical Impedance Spectroscopy.

Variation in hydrophobic chain length of co-adsorbents to improve dye-sensitized solar cell performance

Three compounds based on the phenyltetrazole system, 5-(4-hydroxyphenyl)tetrazole (LTz-1), 5-(4-methoxyphenyl)tetrazole (LTz-2) and 5-(4-hexyloxyphenyl)tetrazole (LTz-3), were synthesized and characterized as co-adsorbents in dye-sensitized solar cells (DSSCs). The effects of hydrophobic chain length of the co-adsorbent and the effects of tetrazole anchoring group on the properties of DSSCs containing the previously reported dye HD-2 were compared with the benchmark deoxycholic acid (DCA). The charge-transfer resistance of the dye/TiO₂ interface followed the order HD-2-DCA > HD-2-LTz-2 > HD-2-LTz-3 > HD-2-LTz-1. However, the V_oc of the dye HD-2 with co-adsorbent DCA was 0.66 V, for the dye HD-2 with co-adsorbent LTz-1, it was 0.70 V, for the dye HD-2 with co-adsorbent LTz-2, it was 0.68 V and for the dye HD-2 with co-adsorbent LTz-3, it was 0.67 V. Co-adsorbents LTz-1, LTz-2 and LTz-3 achieved a mean solar-to-power conversion efficiency (%η), for the three devices, of 8.29, 7.63 and 8.49, respectively, compared to 7.76 for DCA under the same experimental device conditions. For the LTz-3 co-adsorbent, the results can be attributed to the repellent effect of the long alkyl chain. For LTz-1 and LTz-2 co-adsorbents, it is possible that the more compact layer formed improves electron-injection efficiency into TiO₂.

Sodium Hydroxide Pretreatment as an Effective Approach to Reduce the Dye/Holes Recombination Reaction in P-Type DSCs

We report the synthesis of a novel squaraine dye (VG21-C12) and investigate its behavior as p-type sensitizer for p-type Dye-Sensitized Solar Cells. The results are compared with O4-C12, a well-known sensitizer for p-DSC, and sodium hydroxide pretreatment is described as an effective approach to reduce the dye/holes recombination. Various variable investigation such as dipping time, dye loading, photocurrent, and resulting cell efficiency are also reported. Electrochemical impedance spectroscopy (EIS) was utilized for investigating charge transport properties of the different photoelectrodes and the recombination phenomena that occur at the (un)modified electrode/electrolyte interface.