Direct vs. indirect injection mechanisms in perylene dye-sensitized solar cells: A DFT/TDDFT investigation
Graphical abstract
Different electron injection mechanisms are predicted for two analogous perylene dye-sensitized TiO2, as a function of the different dye anchoring group.
Introduction
Within today’s global challenge to capture and utilize solar energy for a sustainable development on a grand scale, dye-sensitized solar cells (DSSCs) represent a particularly promising approach to the direct conversion of light into electrical energy at low cost and with high efficiency [1], [2], [3], [4], [5], [6]. In these devices, a dye sensitizer absorbs the solar radiation and transfers the photoexcited electron to a wide band-gap semiconductor electrode consisting of a mesoporous oxide layer composed of nanometer-sized particles, while the concomitant hole is transferred to the redox electrolyte or to a hole conductor. Ruthenium(II) complexes are widely employed as dye sensitizers, delivering record efficiencies exceeding 11% [7]. The remarkable performance of the [cis-(dithiocyanato)-Ru-bis(2,2′-bipyridine-4,4′-dicarboxylate)] complex (N3) and its doubly protonated analogue (N719) had a central role in significantly advancing the DSSC technology [8]. In these complexes, the thiocyanate ligands ensure fast regeneration of the photo-oxidized dye by the redox mediator, while the bipyridine ligands functionalized in their 4–4′ positions by carboxylic groups ensure stable anchoring to the TiO2 surface, allowing at the same time for the strong electronic coupling required for efficient excited state charge injection. For further progress, however, higher DSSC conversion efficiencies need to be achieved. To this end, new sensitizers and a deeper understanding of the interaction between the dye and the TiO2 nanoparticle are essential.
Due to their large structural variety, organic dyes posses a number of assets which make them valuable alternatives compared to their transition metal analogues, and efficiencies exceeding 9% have already been achieved [9], [10], [11]. These systems can be designed to show adjustable absorption spectral response, high molar extinction coefficient, and environmental compatibility. In addition they are readily available and are endowed with low-cost manufacturing and usually offer an efficient way to optimize the dye ground and excited state energies in relation to the redox mediator and to the TiO2 energy levels, thus maximizing dye regeneration and electron injection [9]. A key feature of organic dyes is the bridging and anchoring group, which electronically connects the dye excited state to the TiO2 unoccupied states. Dyes bearing conjugated acrylic and unconjugated rhodanine bridging groups in conjunction to carboxylic anchoring groups have been extensively investigated, suggesting a different behaviour of the two types of dyes when employed in DSSC devices [12], [13].
The generally proposed DSSC mechanism involves photoexcitation to a dye excited state, from which an electron is transferred to the TiO2 conduction band (c.b.) [1]. This indirect injection mechanism is inferred from the similarity of the free dye absorption spectrum and that of the DSSC device [1], and requires a large density of unoccupied semiconductor states. In this case, the electron transfer usually happens in the non-adiabatic regime, i.e. electron tunneling through the potential barrier at the heterointerface between the dye and the semiconductor [14]. For selected inorganic and organic dyes on TiO2, on the other hand, direct photoexcitation to TiO2c.b. was suggested by appearance of new low-energy absorption bands [15]. We also recently showed that intermediate regimes between purely indirect and direct injection mechanisms can also coexist in the same dye, by just varying the number of sensitizer protons [16]. This was the case of the N719 dye, in which strong coupling was observed upon protonation of the TiO2 surface, involving at least a partial contribution from a direct injection mechanism from the singlet excited state.
The class of perylene dyes has been extensively investigated in this respect as case studies for fundamental research concerning the injection mechanism in DSSCs [17], [18], [19], [20], [21], [22] and have recently shown very promising efficiency when employed in solid state devices [23] and in liquid electrolyte DSSCs [24].
A recent comprehensive ultra-fast spectroscopic investigation of two perylene dyes differing for their anchoring group, 1 and 2 in Fig. 1, has shown a distinctively different excited state behaviour of the two systems with respect to the excited state electron injection into TiO2 nanoparticles [20]. Dye 1, bearing an unconjugated propionic acid anchoring group, showed almost unmodified absorption spectra in solution and upon dye adsorption onto both ZnO and TiO2 surfaces. Dye 2, bearing a conjugated acrylic acid anchoring group, showed on the other hand a red-shift and broadening of the absorption spectra upon adsorption onto TiO2, while relatively smaller changes compared to the solution were observed on ZnO. Shorter injection times (ca. a factor 6) were also observed for dye 2 on TiO2 compared to dye 1. Furthermore, for dye 2, the excited state decay occurred with the same time constant as the rise of the dye cationic species. These results, along with a detailed analysis of time-resolved experiments, were interpreted as being originated from a different coupling of the two dyes with the TiO2 unoccupied electronic states, with dye 2 possibly showing at least a partial contribution from direct electron excitation from the dye ground state to TiO2 unoccupied states [20].
Various computational investigations have been performed on perylene dyes adsorbed on TiO2 surfaces by the Persson group [25], [26], [27]. Periodic boundary conditions and cluster calculations have been performed on sensitization of both rutile and anatase TiO2 surfaces by perylene dyes 1 and 2[25], [26]. Even though no excited states were computed for the sensitized semiconductor systems, inspection of the lowest unoccupied band states or molecular orbitals of those systems effectively revealed an increased electronic coupling for 2 compared to 1, which translated also in a reduced estimate of the electron injection time for the former. For an analogous perylene dye with the carboxylic group directly linked to the perylene aromatic moiety, the same group also simulated the electron transfer dynamics occurring at the heterointerface of a dye-sensitized semiconductor nanocluster [27].
Here we investigate in detail the excited state changes occurring upon adsorption of the perylene dyes 1 and 2 on a model TiO2 nanoparticle, by performing large-scale excited state Time-Dependent DFT (TDDFT) calculations on the two perylene dyes in solution and adsorbed onto the TiO2 surface. We show that by varying the anchoring group from 1 to 2, i.e. increasing the dye conjugation in the anchoring group, the excited state of the combined dye/semiconductor systems acquires a substantial direct charge-transfer character, suggesting a shift from a purely indirect in 1 to a partially direct injection regime in 2. We also highlight, on the technical side, the difficulties associated to the calculation of the dye excited states when these are immersed in the quasi-continuum of the semiconductor unoccupied states.
Section snippets
Computational details
The experimental perylene dyes are modeled by omitting the tert-butyl substituents, for computational convenience. This approximation was verified to lead to exactly the same transition energies for the two dyes. To model the TiO2 nanoparticle, we used a (TiO2)38 cluster [15], [28], obtained by appropriately ‘cutting’ an anatase slab exposing the majority (1 0 1) surface [29]. All atomic structures were optimized by means of Car–Parrinello (CP) molecular dynamics [30], using the PBE
Results and discussion
We start our analysis by briefly describing the electronic structure and absorption spectra of dyes 1 and 2 in solution. The experimental absorption spectra of 1 and 2 in methanol solution display absorption bands at 2.81 and 2.69 eV, respectively [20]. Since organic dyes bearing acidic carboxylic anchoring groups are often deprotonated in solution [13], [38], we investigated here both the protonated and deprotonated forms of 1 and 2. For the protonated (deprotonated) perylene dyes 1 and 2 in
Conclusions
We have investigated in detail the excited state changes occurring upon adsorption of two prototype perylene dyes on a model TiO2 nanoparticle, by performing large-scale excited state Time-Dependent DFT calculations on the two perylene dyes in solution and adsorbed onto the TiO2 surface. We have shown that by varying the anchoring group from a non-conjugated –(CH2–CH2)–COOH in dye 1 to a conjugated –(CH–CH)–COOH in dye 2, the excited states of the combined dye/semiconductor systems acquire a
Acknowledgments
The author thank Fondazione Istituto Italiano di Tecnologia, Project SEED 2009-HELYOS, for financial support. Dr. Andreas Bartelt is gratefully acknowledged for helpful discussions.
References (40)
- et al.
Chem. Phys. Lett.
(2009) - et al.
Nature
(1991) - et al.
J. Phys. Chem. B
(1996) - et al.
Chem. Rev.
(1995) Nature
(2001)Inorg. Chem.
(2006)J. Phys. Chem. C
(2007)J. Am. Chem. Soc.
(2005)J. Am. Chem. Soc.
(1993)Adv. Mater.
(2006)
Chem. Commun.
Chem. Commun.
J. Phys. Chem. C
Energy Environ. Sci.
J. Am. Chem. Soc.
J. Am. Chem. Soc.
J. Am. Chem. Soc.
J. Phys. Chem.
J. Phys. Chem. B
J. Phys. Chem. B
Cited by (124)
First principles studies of some polymer–PCBM complexes for PV cells
2024, Journal of Physics and Chemistry of SolidsAccelerating the discovery of N-annulated perylene organic sensitizers via an interpretable machine learning model
2024, Journal of Molecular StructureIn silico investigation of catechol-based sensitizers for type II dye sensitized solar cells (DSSCs)
2021, Inorganica Chimica ActaCitation Excerpt :Thus, in this work, a series of Cat-sensitizers (Cat-I to Cat-XV) with electron-donating and electron-withdrawing moieties is computationally investigated. We explored the effect of substituents in different positions, as well as the effect of enhancing the conjugation by introducing an ethylene spacer as π bridge between the substituents and the catechol unit (Fig. 1(c)), knowing that type II character can also increase by expanding the dye conjugation in the anchoring group [27]. In particular, Cat-I is the 5,6-dihydroxyindole, generally known as DHI, where the pyrrole is considered as the electron-donating moiety; Cat-II and Cat-III contain the 3,4-(2,2-dimethylpropylenedioxy)thiophene and the 2,4-dimethoxy-1,3,5-triazine as electron-donating substituents, respectively, both linked to the Cat unit through the π spacer.
Absorption mechanism of dopamine/DOPAC-modified TiO<inf>2</inf> nanoparticles by time-dependent density functional theory calculations
2021, Materials Today EnergyCitation Excerpt :Moreover, the chelated adsorption mode was found to present bands at lower energies than the other modes [39]. Moving to bigger TiO2 clusters, a (TiO2)38 exposing the majority (101) surface was extensively used by De Angelis et al. with Ru-based dyes [33,40], perylene [31], [Fe(CN)6] [41], and phenothiazine [42]; whereas, Zhao et al. used the same cluster to study a series of novel metal-free dyes based on the C217 [43]. Sanchez's group has used a cluster of 90 units of TiO2 to study the charge injection mechanism of different dyes in a coupled dye−TiO2 NP using time-dependent self-consistent density functional tight-binding (TD-DFTB) [34,35].