Saddle-shaped porphyrins for dye-sensitized solar cells: new insight into the relationship between nonplanarity and photovoltaic properties.

We report on the theoretical and experimental studies of the new dye-sensitized solar cells functionalized with 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin zinc(II) complexes bearing 2- and 8-bromo substituents at the ß positions. In agreement with the results of TD-DFT calculations, the absorption maxima of di- and octa-brominated Zn(II) complexes, ZnTCPPBr2 and ZnTCPPBr8, exhibited large red-shift compared to that of the non-brominated free base porphyrin (H2TCPP). Furthermore, DFT calculations showed that the higher stabilization of the LUMO levels relative to the HOMO ones makes the HOMO-LUMO gap of the brominated Zn-porphyrins models smaller compared to that of the nonbrominated counterparts, which explains the red shifts of the Soret and Q bands of the brominated compounds. Solar cells containing the new saddle-shaped Zn(II) porphyrins were subjected to analysis in a photovoltaic calibration laboratory to determine their solar to electric energy conversion. In this regard, we found that the overall conversion efficiency of ZnTCPPBr8 adsorbed on TiO2 nanocrystalline films was 5 times as large as that of ZnTCPPBr2 adsorbed on the same films. The effect of the increasing number of Br groups on the photovoltaic performance of the complexes was compared to the results of computational methods using ab initio DFT molecular dynamics simulations and quantum dynamics calculations of electronic relaxation to investigate the interfacial electron transfer (IET) in TCPPBrx/TiO2-anatase nanostructures. Better IET in ZnTCPPBr8 compared to ZnTCPPBr2, and in H2TCPP was evaluated from interfacial electron transfer (IET) simulations. The IET results indicate that electron injection in ZnTCPPBr8-TiO2 (τ = 25 fs) can be up to 5 orders of magnitude faster than ZnTCPPBr2-TiO2 (τ = 125 fs). Both experimental and theoretical results demonstrate that the increase of the number of bromo-substituents at the ß-pyrrole positions of the porphyrin macrocycle created a new class of porphyrin-based DSSC, which exhibits a remarkable increase in the photovoltaic performance compared to non-brominated porphyrins.

Ultrafast interfacial charge transfer from the LUMO+1 in ruthenium(ii) polypyridyl quinoxaline-sensitized solar cells.

This paper describes the implementation of robust and modular sensitizers containing aromatic-amphiphilic ligands to provide new insights into the relationship between the molecular structure and electron injection process governing the efficiency of dye-sensitized solar cells (DSSCs). The significance of this work lies in the combination of favorable experimental and theoretical results in a new class of Ru(ii) polypyridyl complexes with the molecular formula of [Ru(E101)(Dicnq)x(Y)] which is named M101-M104 when X = 1 and Y = bpy, X = 1 and Y = phen, X = 2, and X = 1 and Y = 2 NCS, respectively. E101 and Dicnq ligands are 1,10-phenanthroline-5,6 heptan ammin and 6,7-dicyanodipyrido[2,2-d:2',3'-f]quinoxaline, respectively. The good agreement between the experimental and the time-dependent density functional theory (TDDFT)-calculated absorption spectra of the M101-104 sensitizers allowed us to provide a detailed assignment of the main spectral features of the investigated dyes. M102 which contained phen as an ancillary ligand had the best photovoltaic performance which can be attributed to the higher light harvesting of M102 in the visible light region. A DSSC based on complex M102 without the E101 ligand did not show any observable power conversion efficiency (PCE), indicating the importance of the amphiphilic ligand, E101. Transient absorption studies indicated that the ratio of kreg/krec (krec = the rate constant of the recombination of the dye and kreg = the rate constant of regeneration in the presence of the electrolyte) for M101-104 is 1.1, 2.9, 1.3, and 1.2, clearly confirming a weak competition between dye regeneration and recombination. Therefore, because this ratio for M101, 103, and 104 is small, kreg ≈ krec, the operation of the device has been limited by back electron transports, subsequently enhancing the recombination process. However, the rate of recombination is relatively normal for an efficient DSSC, while the rate of regeneration is very low. Subsequently, the PCE will be poor, confirming the role of aliphatic chains in reducing the recombination process. To obtain a deeper insight into the charge transfer process in the investigated devices, ab initio DFT molecular dynamics simulations and quantum dynamics of electronic relaxation were carried out, clearly showing that the interfacial electron transfer (IET) time scale particularly depends on the type of ancillary ligand. The IET results substantially proved that M102 has the fast lifetime of 2.3 ps and 90 fs for the LUMO and LUMO+1, respectively, indicating the experimentally higher PCE of M102 compared to the other three investigated sensitizers.