Optimizing porphyrins for dye sensitized solar cells using large-scale ab initio calculations

Abstract

In the search for sustainable energy sources, dye sensitized solar cells (DSSCs) represent an attractive solution due to their low cost, relatively high efficiencies, and flexible design. Porphyrin-based dyes are characterized by strong absorption in the visible part of the spectrum and easy customization allowing their electronic properties to be controlled by structural variations. Here we present a computational screening study of more than 5000 porphyrin-based dyes obtained by modifying the porphyrin backbone (metal center and axial ligands), substituting hydrogen by fluorine, and adding different side and anchoring groups. Based on the calculated frontier orbital energies and optical gaps we quantify the energy level alignment with the TiO2 conduction band and different redox mediators. An analysis of the energy level-structure relationship reveals a significant structural diversity among the dyes with the highest level alignment quality, demonstrating the large degree of flexibility in porphyrin dye design. As a specific example of dye optimization, we show that the level alignment of the high efficiency record dye YD2-o-C8 [Yella et al., Science, 2011, 334, 629-634] can be significantly improved by modest structural variations. All the presented data have been stored in a publicly available database.

Electronic state dependence of heterogeneous electron transfer: injection from the S1 and S2 state of phlorin into TiO2

“…43 It should be noted that the level alignment for porphyrins does not change significantly upon substitution of side groups, 43 or removal of the central metal atom. 44 Therefore, we assume a HOMO offset of 1.65 eV for our system and deduce the position of the S 1 and S 2 state from the respective optical gaps. This results in the level alignment shown in Fig.…”

Design of two-photon molecular tandem architectures for solar cells by ab initio theory

“…The atomic and electronic structures of 5000+ porphyrins are taken from our public database 22 , 23 (; http://cmr.fysik.dtu.dk) containing quantum mechanical calculations based on the use of density functional theory (DFT) 30 with the PBE 31 exchange–correlation functional as implemented in the GPAW code. 32 The calculations use consistently a basis set of numerical atomic orbitals 33 with double- ζ and polarization, a grid-spacing of 0.18 Å and a unit cell with a 5.0 Å vacuum added on all sides of the molecules.…”

“…This mechanism puts some constraints on the two dyes to be used, but using our database of functionalized porphyrins, 22 , 23 we have identified around 800 suitable dye pairs (see Fig. 2 for an illustration of this process) made from 9 unique dyes for use as dye 2, all functionalized with highly donating side groups.…”

Calculations of the light absorption spectra of porphyrinoid chromophores for dye-sensitized solar cells

“…The carboxylate group of porphyrinoids is deprotonated and coordinated to one or two Ti atoms. 22,[71][72][73][74] The optimized geometries of the ZnPBAT molecule, and that of the ZnPBA, also studied in this section, are shown in Fig. 3.…”