Efficient Light Harvesting by Using Green Zn-Porphyrin-Sensitized Nanocrystalline TiO₂ Films

Abstract

A series of novel zinc metalloporphyrins, cyano-3-(2'-(5',10',15',20'-tetraphenylporphyrinato zinc(II))yl)-acrylic acid (Zn-3), 3-(trans-2'-(5',10',15',20'-tetraphenylporphyrinato zinc(II))yl)-acrylic acid (Zn-5), 2-cyano-5-(2'-(5',10',15',20'-tetraphenylporphyrinato zinc(II))yl)-penta-2,4-dienoic acid (Zn-8), 4-(trans-2'-(2' '-(5' ',10' ',15' ',20' '-tetraphenylporphyrinato zinc(II))yl)ethen-1'-yl))-1,2-benzenedicarboxylic acid (Zn-11), and 2-cyano-3-[4'-(trans-2' '-(2' ''-(5' '',10' '',15' '',20' ''-tetraphenylporphyrinato zinc(II))yl) ethen-1' '-yl)-phenyl]-acrylic acid (Zn-13) were synthesized and characterized by using various spectroscopic techniques. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations show that key molecular orbitals (MOs) of porphyrins Zn-5 and Zn-3 are stabilized and extended out onto the substituent by pi-conjugation, causing enhancement and red shifts of visible transitions and increasing the possibility of electron transfer from the substituent. The porphyrins were investigated for conversion of sunlight into electricity by constructing dye-sensitized TiO(2) solar cells using an I(-)/I(3)(-) electrolyte. The cells yield close to 85% incident photon-to-current efficiencies (IPCEs), and under standard AM 1.5 sunlight, the Zn-3-sensitized solar cell demonstrates a short circuit photocurrent density of 13.0 +/- 0.5 mA/cm(2), an open-circuit voltage of 610 +/- 50 mV, and a fill factor of 0.70 +/- 0.03. This corresponds to an overall conversion efficiency of 5.6%, making it the most efficient porphyrin-sensitized solar cell reported to date.

DFT/TD-DFT molecular design of porphyrin analogues for use in dye-sensitized solar cells

“…A weighted average method was used to determine the absorption peak of the theoretical model in order to compare it with the experimental 20,22 results. The excitation energies were grouped based on the simulated absorption spectra.…”

“…The LUMO of ZnTPP-A is closer to the conduction band (CB) of titanium oxide than that of ZnTPP-B by 0.34 eV, which could increase the charge transfer behavior of this analogue. This is one of the reasons why ZnTPP-A has a higher cell efficiency (Z = 5.6%) 20 compared with ZnTPP-B (4.1%). 22 The molecular orbital (MO) contribution is very important in determining the charge-separated states of porphyrin analogues.…”

“…Consequently, research is now directed toward finding cheaper and safer alternatives, such as organic-based dyes that do not involve Ru or any noble metal. [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Among organic-based dyes, porphyrins have attracted a great deal of attention. They have a natural role in light harvesting, notably in the range of 400-700 nm.…”

“…However, the added substituent or molecule causes only minor changes in its band positions and molar absorption coefficients, which have little effect on its overall efficiency, suggesting that aggregation in porphyrin analogues is not a significant factor influencing the efficiency of the cell. 17,20 Before attempting any syntheses, computations using semiempirical, density functional theory (DFT) and time-dependent DFT (TD-DFT) provide a useful theoretical basis for designing and screening candidate analogues, based on molecular energy levels, absorption spectra, and spatial orientation. 28,[34][35][36][37] Most DFT calculations use Becke's three-parameter hybrid functional 38 with the nonlocal Lee-Yang-Parr correlation functional 39 (B3LYP).…”

Studying the dynamic behaviour of porphyrins as prototype functional molecules by scanning tunnelling microscopy close to room temperature

“…4 The importance of porphyrins is highlighted by their omnipresence as main functional building blocks in nature 5 examples are iron porphyrin in heme 6 or magnesium porphyrin in chlorophyll 7 -but also due to their application in sensor 8,9 and solar cell technology. [10][11][12][13][14][15] The enormous potential of porphyrins for the fabrication of tailor-made functional molecular architectures on well-defined substrates has stimulated significant activities in fundamental research. 4,[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] In this contribution, we review recent advances in the understanding of the dynamic behaviour of porphyrins, as representatives of organic molecules, adsorbed on a Cu(111) single crystal surface in ultra-high vacuum (UHV).…”