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Conjugated or Broken: the Introduction of Isolation Spacer ahead of the Anchoring Moiety, and the Improved Device Performance Zhaofei Chai, Sushu Wan, Cheng Zhong, Ting Xu, Manman Fang, Jinfeng Wang, Yujun Xie, Yu Zhang, Anyi Mei, Hongwei Han, Qian Peng, Qianqian Li, and Zhen Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10030 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 6, 2016

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ACS Applied Materials & Interfaces

Conjugated or Broken: the Introduction of Isolation Spacer ahead of the Anchoring Moiety, and the Improved Device Performance Zhaofei Chai†, Sushu Wan†, Cheng Zhong†, Ting Xu†, Manman Fang†, Jinfeng Wang†, Yujun Xie†, Yu Zhang†, Anyi Mei‡, Hongwei Han‡, Qian Peng§, Qianqian Li*†, and Zhen Li*† †

Department of Chemistry, Hubei Key Lab on Organic and Polymeric Opto-Electronic Materials,

Wuhan University, Wuhan 430072, China. ‡

Michael Grätzel Center for Mesoscopic Solar Cells, Wuhan National Laboratory for

Optoelectronics, Huazhong University of Science and Technology, Wuhan,430072, China. §

Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Science, Institute

of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China KEYWORDS: dye-sensitized solar cells, isolation spacers, electronic coupling, electron injection and recombination, the structure-property relationship ABSTRACT: Acceptors in traditional dyes are generally designed closed to TiO2 substrate to form a strong electronic coupling with each other (e.g. cyanoacrylic acid), in order to enhance the electron injection for the high performance of the corresponding solar cells. But, some newly developed dyes with chromophores or main acceptors isolated from anchoring groups also

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exhibit comparable or even higher performances. To investigate the relatively untouched electronic coupling effect in dye-sensitized solar cells, a relatively precise method is proposed, in which the strength is adjusted gradually by changing isolation spacers between main acceptors and anchoring groups to partially control the electronic interaction. After an analysis of three different groups of sensitizers including eleven ones, it is inferred that the electronic coupling should keep in a suitable level to balance the electron injection and recombination. Based on a reference dye LI-81 possessing a cyanoacrylic acid as acceptor and anchoring group, both photocurrent and photovoltage are synergistically improved after changing the properties of isolation spacers through the adjustment of the length, steric hindrance and push-pull electronic characteristic. Accordingly, the rationally designed dye LI-87 with an isolation spacer of thiophene ethylene gives an efficiency of 8.54 % and further improved to 9.07 % in the presence of CDCA, showing a new way to develop efficient sensitizers. 1. INTRODUCTION Dye-sensitized solar cells (DSCs) have been extensively investigated for their potential of becoming a clean and affordable source of renewable energy.1-2 One of the most critical elements within a DSC is the sensitizer, because this constituent not only determines the light-harvesting properties, but also mainly controls key electron transfer processes at the TiO2/dye/electrolyte interfaces as confirmed by many cases reported in literatures.3-7 Based on initial forms of this scientific exploration by using ruthenium(II)-based dyes, noble metal-free dyes have gained considerable attention over the last two decades, because of their potentially lower production costs, easier and more versatile synthesis, and much larger molar absorption coefficients.8-10 These dyes commonly feature an electron-donating moiety connected via a π-conjugated bridge unit to an electron acceptor for light harvesting and an anchoring group for the linkage onto TiO2.


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Usually, acceptors and anchoring groups are quite close to each other in traditional dyes (such as cyanoacrylic acid ), which make the intramolecular charge transfer (ICT) of sensitizers increase from the donor region to that in the vicinity of the TiO2 substrate, to enhance the electronic communication between the excited dyes and the semiconductor, resulting in the promoted electron-injection process.11-12 However, remarkable achievements have also been attained for sensitizers with main acceptors isolated from anchoring groups by isolation spacers (IS), which was a particularly effective approach to develop outstanding dyes containing porphyrin rings13-16, additional electron-withdrawing units17-21 and even the traditional ones with cyanoacrylic acid22 (Some selected examples of the two systems were shown in Chart S1). From the perspective of modulation for bandgaps and energy levels alone, it is difficult to clearly explain the lower efficiencies of some low bandgap dyes with sound energy levels, which could not be employed to make DSCs with high external quantum efficiencies. Considering the above typical examples carefully, to our opinions, perhaps, another important factor concerning the working principle between dyes and TiO2, is the spatial distributions of molecular orbitals (particularly for the terminal of sensitizer), which undoubtedly influence the electronic coupling with TiO2 and then the corresponding photovoltaic performances (Figure 1A). Nevertheless, in comparison with much interest in the development of donors and π-spacers to extend the absorption range, match the redox shuttles and improve the arrangement of self-assembled dye layers, the research on the electronic coupling was limited, and few efforts have been devoted to the investigation of the relationship between acceptor and anchoring groups to control its strength, which indeed influenced the electron injection and recombination.4, 23-26 Fortunately, an efficient dye LI-86 with a twisted structure, which was designed to partially control the electron injection and recombination processes, inspired us a new way to analyse the


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electronic coupling process.27 Thus, ideally, by changing the isolation spacers (ISs) including conjugation units, steric hindrance and cyano moiety, the front π system should be decoupled from the anchoring group (Ac) of carboxyl unit gradually, accompanying with the adjustment of the electronic interaction between acceptor and TiO2. Here, for convenient comparison, eleven sensitizers were divided into three groups by different aromatic-ring isolation spacers (rISs) (Figure 1B). The framework for this series of dyes consisted of a main light harvesting part of triarylamine-thiophene-BTD (green part) connected via different isolation spacers to carboxyl unit. From Group 1 to Group 3, the conjugation degree between the BTD part and -COOH weakened subsequently by changing rISs from thienyl, phenyl to 3, 5-dimethylphenyl one. For each group, the cyano group was firstly removed, and then the distance between the BTD part and anchoring group (L1) was shortened, in which the tendency of the interaction strength is as follows: first decreasing, then increasing (and finally decreasing in Group 2). Thus, we could fully investigate how the variation of the coupling strength influences the photovoltaic performances, which would help us to deeply understand the structure-property relationships, and then simplify molecular structures and improve device performances. Really, the experimental results confirmed our idea, and it is true that dyes needed an appropriate electronic communication to balance the relationship between electron injection and recombination. Accordingly, with the appropriate adjustment of the electronic coupling, LI-87 gave the highest solar-to-electric power conversion efficiency of 8.54% in its corresponding DSC, which was 1.7 times than that of LI-81 (5.06%) and could further increase to 9.07% with the aid of trace CDCA.


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Figure 1． (A) Schematic representation of sensitizer engineering; (B) Structures, grouping situations and their corresponding efficiencies 2. RESULTS AND DISCUSSION 2.1 Design and Synthesis of the Sensitizers In order to minimize spectral differences caused by distinct molecular structures, the basic π system of BTD was preserved similarities. Cyanoacrylic acid was used as the anchoring group for the first sensitizers (LI-81, LI-85, and LI-86) in each group. As reported, both -COOH and CN could adsorb onto TiO2, which will pull LUMO orbitals of the dyes towards TiO2 to form a strong electronic coupling interaction (the extended π system could overlap with Ti 3d orbitals).28-30 And then the cyano group was removed while keeping the same L1, definitely accompanied with the decrease of electronic communication (in LI-87, LI-89, and LI-93). Following this, L1 was shortened by eliminating -CH=CH- spacer, resulting in the enhancement of electronic coupling again (in LI-88, LI-90, and LI-94). In our initial design, a panoramic understanding of different linkage modes were expected to be built, thus including saturated


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carbon ones for LI-91 and LI-92, which have been successfully used in some efficient dyes, such as rhodanine31-34, pyridinium and their derivatives35-38. For their similar conjugation skeletons, these two sensitizers were assigned to Group 2. Scheme 1. Synthetic routes of sensitizers.

Reagents and conditions: a: i) n-BuLi, -78 oC, 0.5 h; ii) CO2, -56 oC, 2 h. b: H2SO4, EtOH, 12 h. c: PhP3=CHCOOEt, THF, 48 h. d: (Bpin)2, AcOK, Pd(dppf)Cl2, DMSO, 80 oC, 18 h. e: Pd(PPh3)4, K2CO3, THF/H2O, reflux, overnight. f: KOH, THF/H2O, reflux, overnight. The synthetic routes of all the dyes were depicted in Scheme 1. Most of aromatic halides were first transformed to their corresponding boronates by Miyaura borylation reaction, and then reacted with Ar-Br through the Suzuki coupling reaction. Besides, compounds 5 and 17 were converted to acrylic ester by Wittig reaction in high yields. In the end, all the precursor carboxylic esters (compound 18-24) were hydrolyzed to yield final dyes in the presence of KOH.


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2.2 Optical Properties

3 2 1 0 300 1.2

400

500

600

700

Wavelength (nm) LI-81 LI-87 LI-88 LI-85 LI-89 LI-90 LI-91 LI-92 LI-86 LI-93 LI-94

(B)

1.0 0.8 0.6 0.4 0.2 0.0

(C) LI-94

Group 3

4 -1 -1 10 mol cm )

4

LI-93 LI-86

LI-92 LI-91

Group 2

LI-81 LI-87 LI-88 LI-85 LI-89 LI-90 LI-91 LI-92 LI-86 LI-93 LI-94

(A)

LI-90 LI-89 LI-85 LI-88

Group 1

5

Normalized absorbption

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


LI-87 LI-81

400

500

600

Wavelength (nm)

700

-2.0 -1.5 -1.0 -0.5

0.0

0.5

1.0

1.5

Potential (V) vs Ag/AgCl

Figure 2. (A) Absorption spectra of sensitizers in CH2Cl2 solution (concentrations: 3×10-5 mol L1

); (B) Absorption spectra of dyes coated on TiO2 films; (C) Cyclic voltammograms of

sensitizers in CH2Cl2. The absorption spectra of the sensitizers were shown in Figure 2A, with the corresponding photophysical data summarized in Table 1. In the visible region, all the sensitizers exhibited two distinct absorption bands. The absorption bands shorter than 400 nm were assigned to a π-π* transition, while those at longer wavelength were originated from the intramolecular charge transfer (ICT) process. Among the three groups, the minimum isolation of thienyl unit with the lower resonance energy showed the best conjugation between BTD and anchoring groups, indicating that the cyano group and L1 would have the greatest impact on their optical properties. Compared to the maximum absorption wavelength (λmax) of LI-81 in Group 1, LI-87 and LI-88


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showed blue-shifts of 30 nm and 42 nm, respectively. When the thienyl spacer was replaced by a phenyl one with higher resonance energy and a little larger size, the influence of IS (mainly the electronic effect) would decrease the interaction between BDT and anchoring groups, as reflected in their corresponding absorption behaviors. For Group 2, the blue-shifts of λmax in LI89 and LI-90 were 15 and 6 nm, respectively, with reference to that of LI-85. The tiny redshifted λmax of LI-90 than that of LI-89 suggested that the shorter distance would be favor to the electronic interaction between the BTD and -COOH unit in this group. As expected, when rIS was further changed to 3,5-dimethylphenyl unit to form a twisted configuration, the electronic effect of IS nearly had no influence on the absorption spectra, that was, the λmaxs of the three dyes (LI-86, LI-93 and LI-94) in Group 3 were almost the same to ArBr (Figure S2). Interestingly, the λmaxs of LI-91 and LI-92 in Group 2 with the conjugation interrupted by the saturated carbon atom(s) were almost the same with those for dyes in Group 3, indicating that the phenyl unit contributed a little to the overall π system from the spectrum standpoint only. Upon adsorpting onto TiO2 films (Figure 2B), all the sensitizers exhibited the hypsochromic shift in different degrees. Among them, LI-81 showed a large hypsochromic shift by 37 nm from 569 to 532 nm, probably due to the deprotonation of cyanoacrylic acid group. Although LI-87 and LI88 showed good conjugation with thienyl unit, they exhibited little hypsochromic shifts (∆λ0.3 eV).49-52 However, it could not be concluded that LI-91 and LI-92, showing a lower IPCE in comparison


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with dyes in Group 3, could inject electrons into TiO2 films efficiently, possibly due to a worse electronic communication between them, despite thermodynamically possible. Thus, it was deduced that the remaining effect of variation in charge collection efficiencies may contribute to their IPCE height difference (except for LI-91 and LI-92). The charge collection could be calculated through the following equation17: 𝜂𝑐𝑜𝑙𝑙 =

𝑅𝑐𝑡

(2)

𝑅𝑐𝑡 +𝑅𝑡

Where Rct stands for a charge transfer resistance and Rt is a transport resistance. As a matter of fact, the charge collection efficiency is the result of competition between the charge recombination and collection. For the devices fabricated under same conditions, the charge recombination may play a key role in the efficiency of the charge collection. Through the optimized structures of sensitizers (Figure S3), it could be found that all the similar-looking dyes have the –COOH anchoring group rather horizontal to the body of the dye, after being adsorbed, an almost perpendicular position of the dye was aligned with respect to the TiO2. Thus, the orientation of all dyes makes donor away from the surface of the TiO2, and then reduces the possible electron recombination with dye cations. The organization of sensitizers on the TiO2 surface is also very important.53 Generally, the larger dye loading amount would form more densely packed dye layers to block electrolyte close to TiO2, leading to the improved device performance. In Group 1, owing to the similar dye loading amount, the smaller charge transfer resistances for LI-81 and LI-88 (Figure S4) were probably arisen from the faster charge recombination with D+ cations, because of their strong electronic coupling compared to LI-87, which resulted in the much lower ηcoll (Figure 8B). In Group 2, apart from the smaller dye loading amount, the lower ηcoll of LI-85 may also be caused by the same reasons for the cases of


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LI-81 and LI-88. Meanwhile, LI-91 and LI-92-based cells showed almost the same ηcoll with LI-89 and LI-90 based cells. So, the lower IPCEs could easily be explained by the insufficient electron injection with the combination of above analysis. In Group 3, all the dyes showed almost identical ηcolls, suggesting that the higher IPCE and corresponding JSC of LI-86-based dye were attributed to its higher electron injection due to the presence of cyano group. Combined with the barycenter positions of LI-91-LI-94, it could be assumed that the conjugated isolations were more favorable to transfer electrons than that of saturated carbon ones. All the results corroborated well with the outcome and conjecture in J-V characterization. (B) (A)

Group 1

1.0 LI-81 LI-88

LI-87 LI-85 LI-89 LI-90 LI-91 LI-92 LI-86 LI-93 LI-94

0.8 0.6 0.4

0.8 LI-81

LI-87

LI-88

0.6 0.40 coll (%)

1.0

LHE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.45

0.50

0.55

0.60

0.8

LI-85

0.70

LI-89

LI-90

0.6 0.45

LI-91

0.50

LI-92

0.55

0.60

0.65

1.0

0.2

0.65 Group 2

1.0

0.70

0.75

Group 3

0.8 0.0 400

500

600

700

LI-86

0.6 0.50

LI-93

0.55

Wavelength (nm)

LI-94

0.60

0.65

0.70

0.75

0.80

Potential (V)

Figure 8. (A) LHE spectra calculated from the absorption spectra of dye-loaded TiO2 films. (B) Charge collection efficiencies as a function of the potential. Besides JSC, VOC was another very important parameter to evaluate the photovoltaic performance. It is widely recognized for a DSC with the same redox electrolyte, the voltage generated under illumination is relevant to the electron quasi-Fermi-level (EF,n) in titania, which was controlled by the titania conduction band edge (ECB) and electron density. At a fixed carrier photogeneration flux, the electron density depended on the interfacial recombination rate of titania electrons with electron accepting species in the electrolyte and/or D+ cations. Thereby we further performed the charge extraction (CE) method and intensity-modulated photovoltage


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spectroscopy (IMVS) measurement, to gain further insight into electronic origins of the aforesaid VOC fluctuation.54,55 In Group 1, LI-88 featured a lower extracted charge (Q) at the same VOC (Figure 9A), compared to LI-87 and LI-88-based cells, suggesting that ECB of the former cell was up-shifted. Nonetheless, the relatively higher conduction-band edge of the LI-88-based cell did not lead to an improved VOC, owing to the reduced τ at a given VOC as illustrated in Figure 9B. LI-87-based cell showed an about two magnitudes of τ at the same VOC relatively to that of LI88, giving an about a 90 mV enhancement of VOC. In Group 2, ECB of the LI-90-based cell had a small downward move, but showed comparable electron lifetime compared to LI-85 and LI-89based cells, probably due to its higher dye loading amount. However, the five dyes did not exhibit regular changes, especially for LI-91 and LI-92, which were expected to show a longer electron lifetime as previously reported with the increase of L1 37, 56, possibly stemmed from their alignment difference in TiO2 films caused by the flexible anchoring groups. Somehow, the broken-linking type is still a promising way to improve VOC, but need to pay attention to the length of -(CH2)n- and the electron-withdrawing ability of acceptors. Very surprisingly, LI-86based cell displayed a very long electron lifetime not just within Group 3 and more superior to the aforementioned linking type of -(CH)n-, highlighting the important properties of a cyano group in this series of dyes. (A)

(B)

1

Group 1

LI-87

1E17 0.65

1E18

0.40

0.45

0.50

LI-90

0.50

LI-85 LI-91

1E18

LI-89 LI-92

2E18

0.1

0.55 LI-85

1

0.55

0.60

0.65

LI-89 LI-91

LI-90 LI-92

Group 2

0.01 0.45

0.50

0.55

0.60

0.65

0.70

0.75

LI-86

1

Group 3

LI-87 LI-88

0.01

Group 2

0.60

0.65

LI-81

Group 1

0.1

LI-81 LI-88

s

0.60 0.55 0.50 0.45 0.40

VOC (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


LI-93 LI-94

0.60 0.55

LI-86

8E17

-3 dn (cm )

LI-93

1.6E18

LI-94

0.1

Group 3

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

VOC (V)


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Figure 9. (A) Comparison of the charges extracted from the dyes grafted titania films at a certain open-circuit photovoltage and (B) the electron lifetimes at a certain charge extracted from dyes grafted titania films. In the above presentations, LI-87 and LI-90-based cells demonstrated very good photovoltaic performances, which attracted our interest to further optimize their performances by the coadsorption strategy. Accordingly, chenodeoxycholic acid (CDCA) was incorporated to hinder the formation of sensitizer aggregates and insulate TiO2 films from electrolyte oxidant.57 After the preliminary attempt, LI-87 with 5 mM CDCA and LI-90 with 10 mM CDCA showed best performances, giving efficiencies of 9.07% and 8.60% under standard conditions, respectively, which possibly stemmed from the inhibition of the unfavorable π-π stacking interaction and facilitation of the electron injection. However, the efficiencies decreased upon further increase of the CDCA concentration (Table S1 and Figure S6). The reduced JSC was attributed to the occupation of CDCA in TiO2 films, and thus the coverage of dyes would decrease, as reported in literatures. But the zero or slow growths of VOC should be ascribed to the H+ left by absorption of CDCA, resulting in the negative shift of the conduction band edge, as confirmed by the CE method. As shown in Figure S7, both LI-87 and LI-90 showed upward shifts upon the addition of CDCA, but did not further improve, indicating that protons in the presence of large amounts of CDCA would prevent the further enhancement of VOCs.58,59 The J-V curves of the best devices for each dye under lower light intensity were also shown in Figure S6, and the data was summarized in Table S1. LI-87 showed a higher η of 9.36% under 0.496 sun and LI-90 with a η of 8.85% under 0.701 sun, mainly owing to the excellent fill factors. Compared to LI-86, these two new sensitizers were more easily synthesized with relatively simple synthetic routes and higher overall yields, which were preferred in large-scale applications.


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3. CONCLUSIONS In summary, we succeeded in the design of eleven dyes with subtly modified structures, providing a comparatively accurate model to analyze the electronic coupling between the sensitizers and TiO2 films. Combined with the fundamental tests and DFT calculations, it was found that the electronic coupling strength could be adjusted by changing the properties of isolation groups including lengths, steric hindrance and electronic characteristics. Unlike traditional view, it was recommended to maintain a moderate level to balance the two processes of electron injection and charge recombination in order to achieve the maximum conversion efficiency. The cyano unit, as one of the electronic coupling regulators, should be used sparingly, which was thought to be of critical importance in traditional systems. So, additional attention for distributions of molecular orbitals should be paid to the future active explorations on efficient dyes. This perspective would greatly help us to interpret the high efficiencies of some efficient dyes with relatively bad conjugation and deepen the understanding of electron transfer between dyes and semiconductor. Moreover, LI-87 and LI-90 based cells exhibited high efficiencies of 9.07 and 8.60%, respectively, indicating a successful methodology to design new dyes. ASSOCIATED CONTENT Supporting Information. Selected examples from the literatures, the photophysical properties of ArBr, charge transfer resistances, the characterization of LI-87/CDCA and LI-90/CDCA sensitized solar cells, detailed synthetic procedures, the fabrication and measurement of DSCs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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Corresponding author. Phone: 86-27-62254108; Fax: 86-27-68756757; E-mail: [email protected] (Q. Li); [email protected] or [email protected] (Z. Li). Funding Sources We are grateful to the National Science Foundation of China (21372003 and 21325416), the Natural Science Foundation of Hubei Province (2014CFB698) for ﬁnancial support. Notes The authors declare no competing financial interest.

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