Cosensitized Porphyrin System for High-Performance Solar Cells with

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Cosensitized Porphyrin System for High Performance Solar Cells with TOF-SIMS Analysis Wenjun Wu, Huaide Xiang, Wei Fan, Jinglin Wang, Haifeng Wang, Xin Hua, Zhaohui Wang, Yi-Tao Long, He Tian, and Wei-Hong Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 20 Apr 2017 Downloaded from http://pubs.acs.org on April 21, 2017

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ACS Applied Materials & Interfaces

Cosensitized Porphyrin System for High Performance Solar Cells with TOF-SIMS Analysis Wenjun Wu,† Huaide Xiang,† Wei Fan,‡ Jinglin Wang,† Haifeng Wang,† Xin Hua,† Zhaohui Wang,‡ Yitao Long,† He Tian,† and Wei-Hong Zhu*,† †

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, Centre for Computational Chemistry, Shanghai Key Laboratory of Functional Materials Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237. E-mail: [email protected].

‡

Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.

ABSTRACT: Up to date, development of organic sensitizers has been predominately focused on light harvesting, HOMO and LUMO energy levels, and electron transferring process. In contrast, their adsorption mode as well as dynamic loading behaviour onto nanoporous TiO2 is rarely considered. Herein we have employed the time of flight secondary ion mass spectrometry (TOF-SIMS) to take insight into the competitive dye adsorption mode and kinetics in cosensitized porphyrin system. Using the novel porphyrin dye FW-1 and D–A–π–A featured dye WS-5, the different bond-breaking mode in TOF-SIMS and dynamic dye-loading amount during the coadsorption process are well compared with two different anchoring groups such as benzoic acid and cyanoacrylic acid. With the bombardment mode in TOF-SIMS spectra, we have speculated that the cyano group grafts onto nanoporous TiO2 as tridentate binding for the common anchoring unit of cyanoacrylic acid, and confirmed it through extensive first-principles density functional theory (DFT) calculation by anchoring either carboxyl or cyano group, which shows that cyano group can efficiently participate the adsorption of WS-5 molecule onto TiO2 nanocrystal. The grafting reinforcement interaction between cyano group and TiO2 in WS-5 can well explain the rapid adsorption characteristics. A strong coordinate bond between the lone pair of electrons on nitrogen or oxygen atom and the Lewis acid sites of TiO2 can increase electron injection efficiencies with respect to that between benzoic acid group and the Brønsted acid sites of TiO2 surface. Upon optimization of coadsorption process with dye WS-5, the photoelectric conversion efficiency (PCE) based on porphyrin dye FW-1 is increased from 6.14 to 9.72%. The study on adsorption dynamic of organic sensitizers with TOF-SIMS analysis might provide a new venue for improvement of cosensitized solar cells. Keywords: Solar cells, organic sensitizes, cosensitization, TOF-SIMS, photovoltaics

INTRODUCTION The development of organic chromophores for dye-sensitized solar cells (DSSCs) is mainly concentrated on absorption spectra, matchable energy levels, as well as electron injection or transferring process.1-10 Since Grätzel et al. brought up the possible dye adsorption mode onto nanoporous TiO2 in DSSCs,11 most organic sensitizers have been developed from the essential concept of stable bidentate or less stable monodentate binding, especially with anchoring units such as carboxylic and cyanoacrylic acid.12-19 The cosensitization based on two or multiple dyes as “dye cocktails” is considered as a convenient shortcut to build panchromatic dye sensitized solar cells (DSSCs).20-28 However, in the cosensitization system, the competitive dye adsorption mode as well as dynamic loading behaviour onto nanoporous TiO2 is rarely considered because of the limitation to availably efficient in situ monitoring methods. Time of flight secondary ion mass spectrometry (TOF-SIMS)29-30 is an efficient surface analysis technique which can impressively provide the existing element, chemical state and molecular information from the surface of solid materials, serving as a powerful tool for adsorption characterization. For deep insight into competitive grafting behaviours of organic sensitizers with various anchoring groups in cosensitized system, here we

explored TOF-SIMS for unravelling adsorption dynamics in panchromatic light-harvesting cosensitization based on a novel D-π-A porhyrin dye FW-1 and D–A–π–A featured dye WS-5 (Figure 1). With TOF-SIMS, the different ionic peaks and their distributions for FW-1 and WS-5 onto TiO2 surface were well compared which clearly revealed the relative loading amount in the competitive adsorption process following co-sensitization time. In contrast with FW-1 containing both molecular ion peak [M] + at 1756 m/z and fragment peak at 1712 m/z (without carboxyl, [M - CO2] +), WS-5 only exhibits the latter carboxyl-losing fragment peak at 610 m/z. Based on the observed different peak intensity, bond-breaking mode in TOF-SIMS and the first-principles density functional theory (DFT) calculations, we attempted to make clear whether the cyano group participates in adsorption grafting or not for the most common anchoring unit of cyanoacrylic acid in WS-5. With TOF-SIMS, we have quantified the competitive adsorption in cosensitization system and speculate the tridentate binding for cyanoacrylic acid upon additional anchoring from cyano group. The grafting reinforcement interaction between cyano group and TiO2 in WS-5 can induce the rapid and strong adsorption characteristics. As an efficient clue to optimization of the coadsorption process for dye

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WS-5, the photoelectric conversion efficiency (PCE) based on porphyrin dye FW-1 is increased from 6.14 to 9.72%.

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choice of cosensitized dyes, we unravelled a D-π-A dye (7H-dibenzo[c,g]carbazole fused Zn-porphyrin (FW-1, Figure 1). In the molecular configuration of FW-1, 7H-dibenzo[c,g]carbazole serves as electron donor,31 Zn-porphyrin as π-spacer, and benzoic acid as acceptor and anchoring unit, thus bestowing a “push-pull” feature. Furthermore, the long alkyl chains as well as alkoxy-substituted phenyl groups are chosen to surmount the solubility problem and reduce dye aggregation, which is very sensitive to extraordinary photovoltaic performances. Moreover, the generation and transferring of photo-excited electrons as well as dye regeneration processes are thermodynamically favourable for FW-1 and WS-5 (Table S1 in Supporting Information, SI). As expected, FW-1 exhibits typical porphyrin absorption characteristics with a strong Soret band in the ranges of 400–500 nm and mild Q-bands in the range of 600–700 nm, but very weak absorption between 350–400 nm and 500–600 nm (Figure 2a),32-34 which is one of the major stumbling blocks to restrict the enhancement of photovoltaic performances. To address the challenge, the co-sensitization method, utilizing cosensitizers to simultaneously fill up the porphyrin absorption valleys, becomes an inevitable choice.35 Recently, fascinating investigations have converged on a series of D–A–π–A indoline dyes (WS series) with auxiliary acceptors as high performance sensitizers in DSSCs.36-47 Among these dyes, WS-545 with high light-harvesting capability in the regions of 350–420 nm and 450–650 nm has obtained promising PCEs in application of cosensitization systems.35,49 In this cosensitized system, WS-5 was chosen as FW-1 alternatives with complementary spectra (Figure 2a). Upon cosensitization with WS-5 on TiO2 film, the absorption spectrum of FW-1 is efficiently complemented (Figure 2b), showing a strong absorption or light harvesting from 350 nm to 600 nm. In traditional non-porphyrin organic dyes, cyanoacrylic acid is widely used as efficient anchoring unit. Due to the large planar porphyrin chromophore, benzoic acid in dye FW-1 is preferably chosen as the anchoring units for the sake of linear molecular conformation to avoid the charge recombination.32-35 In contrast with the focus on light harvesting, HOMO and LUMO energy levels, and electron transferring process, here we shed light on their adsorption grafting mode as well as dynamic loading behaviour50-52 onto nanoporous TiO2 from viewpoints of different anchoring units such as benzoic acid and cyanoacrylic acid in dyes FW-1 and WS-5. We checked the absorbance as a function of dipping time with FW-1 and WS-5 to optimize the cosensitization procedure. As depicted in Figures 2c and 2d, FW-1 needs 12 h to reach its saturated state, while for WS-5, the saturated adsorption duration is only around 6 h, showing a much faster adsorption rate or more readily to adsorb onto TiO2 surface with respect to FW-1.

Figure 1. a) Molecular structures of dyes FW-1 and WS-5, and b) schematic adsorption grafting modes for different anchoring units onto nanoporous TiO2: bidentate binding for benzoic acid, and tridentate binding for cyanoacrylic acid upon additional anchoring from cyano group. The carboxylic acid (–COOH) prefers to adsorb at two adjacent titanium sites with a bidentate configuration which is traditionally attributed as the main adsorption mode, and the less stable monodentate adsorption mode is not shown for clarity. For WS-5, the strong electron withdrawing capability of cyano group weakens the σ-bond between the carboxyl and molecular body, facilitating the carboxyl lost under ion bombardment. With this bombardment mode, the relevant molecular fragments coincide with the test results of TOF-SIMS.

Next, we sought to probe the process of competitive adsorption of FW-1 and WS-5 onto TiO2 photoanode with TOF-SIMS analysis, which can take insight into molecular information from surfaces of solid materials. As shown in Figure 3, TOF-SIMS analysis was conducted for FW-1-sensitized TiO2 dipped in WS-5 solution for different time. After 12 h of adsorption in FW-1 (Figure 3a), an ionic peak appeared at 1756 m/z corresponding to its molecular ion peak [M]+ in the negative ion TOF-SIMS mode. After cosensitizing with WS-5 for only 0.5 h, the molecular ion peak of FW-1 decreased drastically, which was in correlation with a sharp decrease in adsorbance of FW-1 due to the replacement of WS-5. It should be noted that the peak intensity

RESULTS AND DISCUSSION The adsorption dynamic of anchoring group in organic sensitizers can efficiently guide the optimization in cosensitization system, which has direct effect upon the photovoltaic behaviours. For the

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Figure 2. (a) Absorption spectra of FW-1 and WS-5 in THF solution, multiplying absorbance of WS-5 five times for clarity, (b) FW-1, WS-5 and cosensitizers (FW-1+WS-5) adsorbed onto 3 µm transparent TiO2 films, and absorbance as a function of dipping time for (c) FW-1 and (d) WS-5 on a 12 µm transparent TiO2 film. During the dynamic process of competitive adsorption, FW-1 can be easily replaced with WS-5, and the adsorption amount of WS-5 increases with the cosensitizing time. with TOF-SIMS can be utilized to reflect the change of relative adsorbance or loading amount for two competitive dyes. Upon cosensitization for 0, 0.5, 1 and 2 h, the fragment peak intensity of WS-5 (Figure 3) is 0, 4932, 6585 and 8509 m/z, and that of FW-1 is 1590, 104, 87 and 60 m/z, respectively. Evidently, the adsorption amount of FW-1 decreases, while that of WS-5 increases with the cosensitization time. Obviously, FW-1 can be easily replaced with WS-5 during the process of competitive adsorption. And then the amount of adsorbed dye on TiO2 (Table S2) was determined by a spectroscopic method through measuring the concentration of FW-1, WS-5 and FW-1 & WS-5 with different dipping time desorbed on the titania surface into a solution of 0.1 M NaOH in THF/H2O (1/1). Following the dipping time, the adsorption amount of FW-1 decreases from 5.24 × 10-7 mol cm-2 to 1.4 × 10-8 mol cm-2 and that of WS-5 increases from zero to 1.11 × 10-7 mol cm-2 which is consistent with the forementioned conclusions.

anchoring unit of benzoic acid with cyanoacrylic acid, the predominant difference is the cyano group in the latter. Thus the issue whether the cyano group participates in the grafting adsorption or not is need to be answered. In this regard with WS-5, the ion beam from TOF-SIMS can only cut carboxyl group instead of bombarding the whole molecule, which is distinctive from FW-1 having both ionic peaks with and without carboxyl group. This can be attributed that the strong electron-withdrawing capability of CN weakens the σ-bond between the carboxyl and molecular body, thus facilitating the carboxyl lost under ion bombardment (Figure 1b). Comparing with the relatively strong intensity ratio from the fragment to molecular ionic peak for FW-1, the single carboxyl-losing fragment peak for WS-5 is a good indicator index to the cyano group involved in the molecular grafting onto nanoporous TiO2 surface. It should be noted that the energy of the source ions in TOF-SIMS measurement was quit high. The prior breaking of the σ-bond suggests a strong anchoring-strength of dye WS-5 relative to FW-1, which could be the origin of the different dye-loading capability as observed in Figure 2.

Intriguingly in TOF-SIMS analysis, there existed a new peak at 610 m/z instead of 654 m/z (the molecular weight of WS-5, with a shift of 44 m/z). Given that FW-1 loses a unit peak of 44 m/z at 1712 m/z (Figure 3b), it can be corroborated that this is the fragment peak resulted from losing carboxyl group. Compared with FW-1 containing both molecular ion peak at 1756 m/z and fragment peak at 1712 m/z (without carboxyl), WS-5 only exhibits the latter carboxyl-losing fragment peak. We expected that the observed different bond-breaking mode in TOF-SIMS might be related with the different adsorption grafting modes onto nanoporous TiO2. Generally, the carboxyl group is always believed to the bidentate-grafting mode.12-14 When compared the

The grafting reinforcement interaction between cyano group and TiO2 in WS-5 can also induce the above-mentioned rapid adsorption characteristics. Here the oxygen (O) and nitrogen (N) atoms can chelate to titanium ions onto the TiO2 surface, thus achieving strong electron coupling between the excited-state energy level of the dyes and the conduction band of TiO2. That is, a strong coordinate bond between the lone pair of electrons on the nitrogen or oxygen atom and the Lewis acid sites of TiO2 can give

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Figure 3．．TOF-SIMS profiles of FW-1 + WS-5 co-sensitized DSSCs (Bi3++ species) with negative polarity: co-sensitized with WS-5 for 0 h (a), 0.5 h (b), 1.0 h (c) and 2.0 h (d). Note: Comparing with the relatively strong intensity ratio from the fragment to molecular ionic peak for FW-1, the single carboxyl-losing fragment peak for WS-5 is a good indicator index to the cyano group involved in the molecular grafting onto nanoporous TiO2 surface. electron injection efficiencies comparable to that between benzoic acid group and the Brønsted acid sites of TiO2 surface.50-52 We speculate that, the grafting reinforcement interaction between cyano group and TiO2 in WS-5 can also induce the above-mentioned rapid adsorption characteristics. Accordingly, in case of dye WS-5, apart from the auxiliary electron-withdrawing effect for the intermolecular charge transfer (ICT) process, the cyano group in the anchoring unit of cyanoacrylic acid can realize rapid adsorption characteristics through participating the tridentate-grafting procedure (Figure 1).

coordinatively unsaturated 5(2)-fold Ti(O) atoms, Ti6c(O3c) and Ti5c(O2c), respectively. Among them, Ti5c and the oxygen vacancy at O2c site in the case of reduced TiO2 surface could usually serve as the preliminary adsorption sites. Possible adsorption modes of WS-5 are illustrated in Figure 4b, which presents the carboxyl group sitting at Ti5c sites or O2c vacancies in a mono- or bi-dentate configuration with and without participation of cyano group. All the DFT calculations were Perdew-Burke-Ernzerhof (PBE) function within the generalized gradient approximation using the VASP code (see details in SI).

To further study on the adsorption mode speculated in cosensitization process for WS-5, we carried out the extensive first-principles density functional theory (DFT) calculations on examining its possible adsorption configurations by anchoring either carboxyl or cyano group. Herein, a simplified molecular structure, which retains the crucial fragment of WS-5 (Figure S1 in SI), was taken to mimic the complicated WS-5 dye for the calculation accuracy-cost balance, and the most exposed (101) surface was used to describe the anatase-typed TiO2 nanoparticle. TiO2(101) surface exhibits a sawtooth-like surface corrugation (Figure 4a) containing both fully saturated 6(3)-fold and

On the stoichiometric TiO2(101) surface, we firstly calculated the traditionally conceived adsorption structure with the carboxyl group (-COOH) as the anchoring site. It was found that the – COOH preferred to adsorb at two adjacent Ti5c sites along the [010] direction with a bidentate configuration (Figure 5a), and its H readily released to the neighboring O2c site, which corresponds to an dissociative adsorption energy of -1.75 eV, being 0.63 eV more stable than the monodentate adsorption configuration (Figure 5b and Table S3 in SI). Secondly, we examined the possibility of cyano group (-CN) participating adsorption, and the obtained stable structures upon optimization were shown in

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Furthermore, we considered the adsorption of WS-5 molecule on the defected TiO2(101) surface containing oxygen vacancy. On the partially reduced TiO2(101) surface, which was simulated with introducing an O2c vacancy on the surface, it was found that the O2c vacancy can accommodate the carboxyl or cyano group more readily compared to the stoichiometric TiO2(101) surface. As shown in Figure 5e and Table S3 in SI, carboxyl group can efficiently adsorb on reduced TiO2(101) surface with one O inserting in the O2c vacancy and the other O sitting at the nearest neighboring Ti5c site, corresponding to an adsorption energy as large as -3.59 eV. By comparison, the adsorption configuration that –CN binds in the O2c vacancy while –COO¯ adopts a monodentate configuration at the nearest Ti5c (Figure 5f) was less stable with an adsorption energy of -3.06 eV, indicating that the O2c vacancy can preferentially bind carboxyl group over the –CN group. Nevertheless, starting with the configuration of bidentate – COO¯ adsorption (Figure 5e), we tested a series of possible adsorption configurations involving –CN at the adjacent Ti5c site. Figure 5g shows the most stable one, which gives a weakened adsorption (-3.28 eV) relative to the original bidentate –COO¯ adsorption (Figure 5e), owing to molecular structural distortion of WS-5 in this case which destroys the coplanar configuration of cyano and carboxyl group. Interestingly, when the –COO¯ group adsorbs at the O2c vacancy with a mono- or bi-dentate configuration (corresponding to Figure 5g and 5i, respectively), the adsorption of –CN at Ti5c site can greatly result in a strong adsorption of the whole WS-5 adsorption, which give adsorption energies of -3.53 and -3.61 eV, respectively. Especially, the coadsorption configuration of –CN and –COO¯ group shown in Figure 5i presents the most stable adsorption mode.

Figure 5c and 5d. Figure 5c describes WS-5 adsorption on TiO2(101) surface through the monodentate –COO¯ adsorption together with –CN binding at Ti5c, which gives an adsorption energy of -1.71 eV, indicating that –CN can facilitate the adsorption compared with the monodentate –COO¯ adsorption only (-1.12 eV, Figure 4b). Notably, this configuration shows similar adsorption energy with the bidentate –COO¯ adsorption, which is traditionally attributed as the main adsorption mode. Moreover, it is worth discussing the above mentioned coadsorption of –CN and the bidentate –COO- configuration, that is, the tridentate binding for cyanoacrylic acid upon additional anchoring from cyano group. As shown in Figure 5d and Table S3 in SI, it shows also a relatively strong adsorption (-1.66 eV), despite being a little weak in comparison with the most stable bidentate carboxyl group adsorption (Figure 5a) as a result of structural distortion of WS-5 in the coadsorption process. In other words, the cyano group can probably participate the adsorption of WS-5 molecule in reality.

Figure 4. (a) Calculated structures (side view) of anatase TiO2 (101) surface, (b) the simple diagram of the conceived adsorption modes of carboxyl group in dye WS-5 with bidentate or monodentate modes, as well as whether the cyano group participates in adsorption or not, which was indicated by the red dashed line.

Figure 5. Optimized adsorption configurations (side view) of dye WS-5 on (a-d) stoichiometric anatase TiO2 (101) surface, (e-i) partially reduced surface and (j-l) highly reduced surface. The red and gray represent O and Ti of TiO2, and the pink, blue, black, white and yellow represent the oxygen of carboxyl group, nitrogen of cyano group, carbon, hydrogen and sulfur of dye WS-5, respectively.

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Figure 6. (a) J–V characteristics of FW-1+WS-5 cosensitized DSSCs with different dipping time of WS-5 based on iodine electrolyte, and (b) optimized IPCE spectra of DSSCs sensitized by FW-1, WS-5 and FW-1+WS-5 with dipping time of 12 h, 6 h and 12 + 1.5 h, respectively. In addition, the highly reduced TiO2(101) surface with two adjacent oxygen vacancies introduced simultaneously was examined. Compared with the bidentate –COO¯ adsorption occupying two O2c vacancies (Figure 5j), or with one O sitting at the O2c vacancy and the other one sitting at the nearest Ti5c, the adsorption structure with –CN sitting at an O2c vacancy and the – COO¯ adopting the monodentate configuration at the O2c vacancy (Figure 5l) is evidently more stable (-3.18 eV), further indicating that –CN group can efficiently participate the adsorption of WS-5 molecule on TiO2 nanocrystal. As consequence, the different bond-breaking mode in TOF-SIMS and dynamic change of loading amount in the coadsorption process lays a solid foundation for further deriving the binding mechanism and adsorption dynamic for photoanodes in cosensitized system.

Figure 7. Curves under a series potential bias of DSSCs based on dye FW-1 sensitization and FW-1+WS-5 cosensitization with iodine redox electrolyte: (a) cell capacitance (Cµ), (b) calculated electron lifetime (τ) and (c) VOC decay curves of FW-1 and FW-1+WS-5 with dipping time of 12 h and 12 + 1.5 h, respectively. For cosensitization in DSSCs, the PCE improvement is our main target. As presented in Table 1 and Figure 6, the photovoltaic performances of dye FW-1 sensitized and FW-1+WS-5 cosensitized solar cells were tested under AM 1.5 G irradiation (100 mW cm-2), with more details listed in Table S2 in SI. Figure 6a illustrates J–V characteristics of FW-1+WS-5 cosensitized DSSCs with different dipping time of WS-5 based on I-/I3− electrolytes. As a result, after cosensitized with WS-5 for only 0.5 h, the short circuit current density (JSC) increased significantly from 12.38 to 18.12 mA cm-2. Along with the

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recombination rate. 12 To gain insight into the effect of cosensitization on VOC, the single and cosensitization DSSCs based on FW-1 and FW-1 + WS-5 were characterized by electrochemical impedance spectroscopy (EIS). As derived from Figure 7a, the logarithm of Cµ increases linearly with increasing bias potential with the same slope and values, indicating that ECB of both devices are at the same level. However, compared to FW-1, the cosensitization with FW-1 + WS-5 presents a longer electron lifetime (Figure 7b, enhanced about 1 time around the open-circuit voltage), indicating that the introduction of cosensitizer WS-5 can suppress the charge recombination on the surface of TiO2 and thus increase injected electrons. Figure 7c shows the representative VOC decay curves of FW-1 and FW-1 + WS-5 devices, which can remarkably reflect the charge recombination rate in cell devices. Apparently, the VOC decay rate of FW-1 single-sensitized cell is much higher than that of FW-1+WS-5 cosensitized cell, showing that the lower recombination rate for cosensitized DSSCs, which is in good agreement with EIS result. It is expected that upon choosing WS-5 as a cosensitizer, the adsorption mode in the dual grafting action of cyano and carboxyl groups can guarantee the strong electron coupling between the excited-state energy level of the dyes and the conduction band of TiO2 as the results from T O F S I M S .

increase of dipping time from 0.5 to 1.5 h, JSC slightly reached 18.41 mA cm-2. The improvement in JSC is mainly attributed to the complemented light response area of FW-1 with WS-5. However, JSC decreased when dipping time extended for 1.5 h. After cosensitized for 2.5 h, JSC decreased to 16.68 mA cm-2, which might be attributed that a great loading amount of FW-1 is replaced by WS-5 (Figure 3), finally resulting in a huge decrease of IPCE in the long wavelength region (Figure 6b). Meanwhile, it is noteworthy that open circuit voltage (VOC) plays a pivotal role in the development of DSSCs. To be noticed, along with the increase of dipping time from 0 to 2.5 h, VOC increased from 689 to 762 mV. Consequently, a promising PCE of 9.72% (VOC of 754 mV, JSC of 18.15 mA cm-2 and FF of 0.71) is achieved under irradiation of AM 1.5 simulated solar light (100 mW cm-2). Here the combination of two dyes FW-1 and WS-5 with significant difference in the adsorption rate shows promising cosensitization characteristics. Table 1. Photovoltaic performances of FW-1 sensitized and FW-1 + WS-5 cosensitized DSSCs under irradiation of AM 1.5 simulated solar light (100 mW cm-2) Dyea

JSC (mA cm-2)

VOC (mV)

FF

N719

17.36±0.08

732±5

0.70±0.02

8.90±0.1

FW-1

12.38±0.20

689±3

0.72±0.02

6.14±0.2

PCE (%)

WS-5

15.12±0.14

749±4

0.72±0.01

8.15±0.1

FW-1+WS-5

18.15±0.21

754±4

0.71±0.01

9.72±0.2

CONCLUSIONS

a

Cyanoacrylic acid is the most widely used as efficient anchoring unit in building organic sensitizers, in which the carboxylic acid (–COOH) is always considered as a bidentate configuration adsorbed at two adjacent titanium sites. In this work, we focused on the adsorption dynamic of two different anchoring groups such as benzoic acid (porphyrin dye FW-1) and cyanoacrylic acid (dye WS-5) with TOF-SIMS analysis. Interestingly, they showed different bond-breaking mode in TOF-SIMS and dynamic change of loading amount in the coadsorption process. In contrast with FW-1 containing both molecular ion peak at 1756 m/z and fragment peak at 1712 m/z (without carboxyl), WS-5 only exhibits the latter carboxyl-losing fragment peak at 610 m/z. The DFT simulation shows that cyano group can efficiently participate the adsorption of WS-5 molecule on TiO2 nanocrystal in view of the traditionally conceived adsorption structure, especially for the defected TiO2(101) surface containing oxygen vacancy and highly reduced TiO2(101) surface. We have figured out that the cyano group in cyanoacetic anchor participates in the graft process like a tridentate ligand with TOF-SIMS. The grafting reinforcement interaction between cyano group and TiO2 in WS-5 can induce the rapid and strong adsorption characteristics. As a whole, TOF-SIMS analysis can make possible in the insight into the process of competitive adsorption onto TiO2 photoanode, and achieve molecular information from surfaces of solid materials, thus providing a solid foundation for further deriving the binding mechanism and dynamic for photoanodes in cosensitized system, especially for the step-by-step cosensitization.

Electrolyte: 0.1 M LiI, 0.05 M I2, 0.6 M 1-methyl-3-propyl-imidazolium iodide (PMII), and 0.5 M 4-tert-butylpyridine (TBP) in a mixture of acetonitrile and valeronitrile (volume ratio of 85: 15) and the dipping time of FW-1, WS-5 and FW-1 + WS-5 is 12 h, 6 h and 12+1.5 h, respectively. Given that the fill factor FF has not changed significantly (Table 1), the PCE of photovoltaic devices mainly depends on JSC and VOC. Generally, JSC is always dependent upon its IPCE spectrum. In an effort to provide a further insight into cosensitization system, we explored the contribution of co-sensitization on JSC with IPCE spectra. As shown in Figure 5b, two IPCE valleys of FW-1 are located in the ranges of 350–410 and 500–600 nm, respectively. In addition, the IPCE values of WS-5 is above 70% within a wide range of wavelength, proving evidently that WS-5 serves as a perfect cosensitizer for FW-1 by filling up both of the responding valleys. As a result, the IPCE valleys of FW-1 around 350 nm and 550 nm are indeed filled up, with IPCE values lying above 80% roughly within a wide wavelength range of 380–580 nm. Notably, JSC estimated from IPCE spectra are smaller than those obtained from the J–V curves, for instance, integration of IPCE curve for FW-1 + WS-5 affords a calculated JSC of 17.76 mA cm-2, which is slightly lower than the experimental value of 18.15 mA cm-2. Similar observations have also been reported for a number of porphyrin dyes.53-54 These observations may be interpreted in terms of more efficient charge transport and collection55,56 and stronger thermal effect associated with the full sunlight irradiation.57

ASSOCIATED CONTENT Supporting Information

As another considerable parameter, VOC obviously increases along with the passage of dipping time of WS-5 in the cosensitized system (Figure 7 and Table 1). With fixed redox species, VOC is only determined by Fermi level (EFn) of TiO2, which is affected by conduction band (E CB ) and charge

The Supporting Information is available free of charge on the ACS Publications website at DOI:

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Experimental section, fabrication of dye-sensitized solar cells, physical test and photovoltaic performance, computational details about adsorption model of WS-5.

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AUTHOR INFORMATION Corresponding Author *[email protected]

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Notes The author declare no competing financial interest.

ACKNOWLEDGMENTs

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This work was supported by National Key Research and Development Program (2016YFA0200300), NSFC for Creative Research Groups (21421004), Distinguished Young Scholars (21325625), Key Project (21636002), NSFC/China, Oriental Scholarship, Fundamental Research Funds for the Central Universities (WJ1416005 and WJ1315025), the Scientific Committee of Shanghai (14ZR1409700 and 15XD1501400), and Programme of Introducing Talents of Discipline to Universities (B16017).

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Table of contents (TOC)


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Cosensitized Porphyrin System for High-Performance Solar Cells with

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