Mechanistic and Time Resolved Single-Photon Counting Analysis for

Subscriber access provided by READING UNIV

Article

Mechanistic and Time Resolved Single-Photon Counting Analysis for Light Harvesting Characteristics Depending on the Adsorption Mode of Organic Sensitizers in DSSCs Hyo Jeong Jo, Jung Eun Nam, Hyo Jung Heo, Dae-Hwan Kim, Jae Hong Kim, and Jin-Kyu Kang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05376 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

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

The Journal of Physical Chemistry

Mechanistic and Time Resolved Single-Photon Counting Analysis for Light Harvesting Characteristics Depending on the Adsorption Mode of Organic Sensitizers in DSSCs H. J. Joa, J. E. Nama, H. Heob, D-H. Kima, J. H. Kimb*, and J-K Kanga* a

DGIST, 333, Techno Jungang Daero, Hyeonpung-Myeon, Dalseong-Gun, Daegu, 711-873,

Republic of Korea b

Department of Display and Chemical Engineering, Yeungnam University, 214-1, Dae-dong,

Gyeongsan, Gyeongbuk 712-749, Republic of Korea

■ AUTHOR INFORMATION Corresponding Authors a

*E-mail: [email protected]

b


1

ACS Paragon Plus Environment


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

Page 2 of 28

ABSTRACT: This study investigated the origin of improved photovoltaic performance of bibranched phenothiazine derivatives in DSSCs, in terms of the adsorption modes of the π-bridge dyes as sensitizers. Two novel twin acceptor type organic dyes with various π-bridge systems were designed and synthesized. We mainly focused on binding mode of dye on the TiO2 surface and investigated the effect of adsorption mode on the characteristic of solar cells. A stronger chemical bonding and a fast electron injection were found to originated from the bridge bidentate mode. This adsorption mode was also found to have the fastest electron transfer bonding of a πconjugation anchoring group for organic dyes with 3,4-ethylenedioxythiophene (EDOT). Meanwhile, using thiophene as π-conjugated anchoring groups with monodentate adsorption mode was found out to produce slower electron transfer. The structural differences due to the πconjugated anchoring groups affect their electron injection/recombination properties. Further, we found that the DSSCs based on organic sensitizers containing EDOT exhibited significantly enhanced long-term stability.

2


Page 3 of 28

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


1. Introduction Dye-sensitized solar cells (DSSCs) are recognized as a potential alternative to the high-cost solar cells using inorganic solar cell (ex. Silicon) due to low production costs and limitless variety of dyes available.1-5 Several recent main developments in the structure of dyes and electrolytes for DSSCs have improved power conversion efficiency.2 However, the rapid development of perovskite solar cells has raised some questions as to whether the efficiency of DSSCs can be sufficiently improved through current strategies. Nevertheless, there is still many room for improvement in DSSC properties in term of the theoretical reported solar cell performance. The main strategy for increasing Jsc is to develop organic dyes to improve the light harvesting efficiency.7-13 For high light harvesting efficiency, most reports used cocktail-formed DSSCs (Co-sensitization) that mixed a ruthenium metal dye or a porphyrin metal dye with organic dyes.6,14-15 Co-sensitization of the TiO2 electrodes by two or more dyes with supplemently spectral profiles have been demonstrated in an effective way to achieve mimic solar spectrum response and increase current density in organic sensitizers-based DSSCs. Furthermore, the dye in a co-sensitized system can play the role of the co-adsorbent to achieve synergetic effects, such as panchromatic sensitization and preventing dye aggregation.16 However, most reports on cosensitized DSSCs show that the relatively poor performance was due to the aggregation by π- π stacking on the TiO2 surface, which make to put out of photo-excited states, fast electron recombination, and low stability.17-19 This indicates that the device performances can be further enhanced by structural engineering of the organic dyes for enhanced light harvesting efficiency. In order to achieve high photon-to-current efficiency in DSSCs, the dyes should possess a wide 3



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

Page 4 of 28

absorption range, high molar extinction coefficients, passivation to the TiO2 and strong binding mode. In particular, the binding mode of the dye and the passivation of the alkyl length of the hydrocarbon silane on the TiO2 surface can prevent charge recombination, which affects the electron transfer and long term stability.45-46 Recently, the importance of electronic binding mode between carboxylic acid groups and the TiO2 surface was studied by Kim et al., suggesting the influence on electron injection.20 In this study, we designed two phenothiazine derivatives with different π-conjugation anchoring groups, in order to control their adsorption modes on TiO2 surface (Fig. 1). Many organic dyes, such as triphenylamine, tetrahydroquinoline, carbazole, iminodibenzyl, and phenothiazine21 have been researched, and their molecular structures tuned to increase the performance of DSSCs. However, the effects of the dye adsorption mode controlled by the πconjugation anchoring group have not been reported for such organic dyes. Phenothiazine dyes with electron-rich nitrogen and sulfur atoms have an advantage in DSSCs, because of their stronger electron-donating properties and less planar structures compared to other dyes. Specifically, here we synthesized two phenothiazine dyes, PRSCN2 and PREDCN2, with different π-conjugated anchoring groups (two thiophene units and 3,4-ethylenedioxythiophene (EDOT) units, respectively). The influence of the π-conjugated anchoring groups on the electron injection efficiency, charge recombination, defect density, and the photovoltaic performance was investigated for the first time. In addition, both synthesized dyes have N-phenyl alkyl group on the phenothiazine moiety to control their bandgap and prevent their aggregation. PREDCN2 dye with EDOT units as π-conjugated anchoring groups exhibited a higher photon-to-current efficiency of 8.32% and showed significantly enhanced Jsc compared to PRSCN2 dye with 4


Page 5 of 28

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


thiophene units under the same conditions. Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) spectroscopy, Time-Correlated Single Photon Counting (TCSPC), and Density Of State (DOS) studies were performed to understand why the DSSC based on PREDCN2 showed higher performance compared to that based on PRSCN2.

Figure 1 Molecular structures of the two organic dyes. 2. Results and discussion 2.1 Synthesis The PRSCN2 and PREDCN2 organic dyes were prepared in moderate yields according to the literature,22 using the five steps illustrated in Scheme S1 (Supporting Information). The donor moiety was synthesized from 10-(4-hexyl-phenyl)-10H-phenothiazine, which was added using Palladium(II) acetate via the Buchwald-Hartwig coupling reaction. The resulting donor group was then attached onto thiophene or EDOT units by Stille coupling. Phenothiazine aldehyde compounds were produced by the Vilsmeier-Haack reaction. Upon reaction with cyanoacetic acid in the face of a piperidine catalyst, these aldehydes synthesized the phenothiazine dyes as discussed in Supporting Information (S1 and S2). 2.2 Optical and electrochemical properties

5



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

Page 6 of 28

Fig. 2 shows the absorption and fluorescence spectrums of the organic dyes in the solution state and on the TiO2 thin film. The optical and electrochemical properties of the organic sensitizers are shown in Table 1.

Figure 2 Absorption spectra (a) and fluorescence spectra (b) of the different organic sensitizers in dimethylformamide (DMF) solution and (b) on TiO2 film. From PRSCN2 to PREDCN2, the maximum absorption wavelength (λmax) due to the π-π* transitions in the conjugated molecules was red-shifted from 445 to 459 nm, and the molar extinction coefficient (εmax) increased from 2.6 × 104 to 3.4 × 104 M-1·cm-1, as listed in Table 1. The PREDCN2 dye containing EDOT in the chromophore exhibited a slightly increased λmax compared to PRSCN2. This was attributed to the increased electron donating ability and planarity of the EDOT framework.23-24 Also, the N-phenyl alkyl group of 10H-phenothiazine increased the electron donating ability, resulting in an elevated Highest Occupied Molecular Orbital (HOMO) level. These organic sensitizers calculation results are given in Figs. S1. The stronger electron donor-acceptor conjugated π-chromophore and enhanced molar extinction coefficient of PREDCN2 can increase the light-harvesting efficiency (or photo-generated electron-hole pairs) of the organic dye. Fig. 2(b) shows that the absorption peak of the organic 6


Page 7 of 28

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


sensitizers adsorbed on TiO2 films are slightly widen and red-shifted compared to those in solution. This is due to dye aggregation through molecular π-π stacking and dye-TiO2 interactions, which can lead to widening and red-shifting of the absorption.25-29 To examine the capability of electron injection from the LUMO level of the organic sensitizers into the TiO2 conduction band) and the dye regeneration by electrolytes, the electrochemical motions of organic dyes were examined by cyclic voltammetry in DMF solution. Figs. S2 and table 1 show the results of cyclic voltammograms (CVs) were investigated with respect to an internal reference such as ferrocene (Fc/Fc+). Optical band gaps (Egopt) were used to determine the Lowest Unoccupied Molecular Orbital (LUMO) energy levels. The results indicated that both organic sensitizers had more positive HOMO levels than the redox potential level of I-/I3- in the electrolyte: 0.687 and 0.625 V for PRSCN2 and PREDCN2, respectively. This results that the oxidized dyes can be easily play back by the I3-/I- redox couple. Table 1 Absorption and electrochemical parameters for organic dyes (NHE: normal hydrogen electrode).

dye PRSCN2

λ maxa (nm) εmaxa -1 -1 (M cm ) Soln. TiO2 26204 445 450

PREDCN2 34265 a

459

471

E0–0 (eV)b (abs)

Eoxc (V vs. NHE)

Eox-E0–0d (V vs. NHE)

HOMO (eV)

LUMO (eV)

2.38

0.687

-1.693

-4.997

-2.617

2.33

0.625

-1.705

-4.935

-2.605

b

Lambda max absorption and extinction coefficients in DMF. E0–0 (band gap) was determined from the tangent

of the absorption spectra in DMF. c Oxidation potential (EHOMO) of the dye was measured using cyclic voltammetry in DMF. d Eox - E0–0 = ELUMO.

2.3 Effect of dye adsorption mode on electron injection The adsorption mode on the TiO2 surface of the dye is reported to be as follows.47 Figure 4(a), (b) show the planarity difference due to the dihedral angle of EDOT and thiophene in the 7



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

Page 8 of 28

molecules48,49. The planarity of dye can determine the adsorption form on the TiO2 surfaces. In addition, the adsorption mode of dye anchoring groups to the TiO2 surface, which is related to interfacial electron transfer, was examined by ATR-FTIR spectroscopy. In particular, the stronger chemical bonding and faster electron transfer could originate from the bridged bidentate mode. Fig. 3 shows the ATR-FTIR spectra for neat dyes (in KBr) and dye-sensitized TiO2 electrodes. The major IR bands of neat dyes are identified according to refs.30-32

Figure 3 ATR-FTIR spectra for (a) PRSCN2 (b) PREDCN2 obtained in photoacoustic mode using solid samples. Neat dyes were in the protonated form, since a carbonyl stretching band in −COOH was observed at 1722 and 1730 cm-1 for PRSCN2 and PREDCN2, respectively. The wavenumber assigned to −C≡N stretching in organic dyes (~2250 cm-1) did not change in the spectra before or after dye adsorption on the TiO2 surface, suggesting that there is no interaction between the cyano part of the dye and the TiO2 surface during the adsorption process. The residue carbonyl 8


Page 9 of 28

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


peak was observed in the spectra of PREDCN2 adsorbed on the TiO2 surface, suggesting a bidentate binding mode. On the other hand, a carbonyl peak was observed for adsorbed PRSCN2 but it was shifted to higher energies (1743 cm-1), suggesting a mono-dentate binding mode or

non-bonded dye molecules.33-34 Therefore, we assumed that the bi-dentate binding mode of PREDCN2 dye is related to the stronger adsorption properties.

Figure 4 Forms for possible surface chemical bonding of organic dyes anchored on TiO2: (a) monodentate, (b) bidentate, (c) PRSCN2, and (d) PREDCN2. To confirm this hypothesis, we conducted TCSPC (time correlated single photon counting) analysis. This description was used to determine the electron transfer efficiency, through the excited state lifetimes of the dissolved state of the solvent and the state of the organic dyes adsorbed on the TiO2 surface,35 as shown in Fig. 5.

9



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

Figure 5 The fitted TCSPC fluorescence spectras of organic dyes dissolved in EtOH (a,b) and adsorbed on the TiO2 film (c,d). The rapid exponential decay for dyes adsorbed on TiO2 films provides for faster electron transfer into the semiconductor. The electron lifetime of organic dyes in solution state was confirmed to be τ_solution is 2.18 ns for PRSCN and 2.45 ns for PREDCN2, respectively. For organic dye adsorbed on the TiO2 surface, the electron lifetime is rapidly reduced due to electron transfer from the conduction band to the semiconductor. In the adsorbed dyes on the TiO2 films, the two dyes confirmed that the slope of -2.63 for PRSCN2 had a lifetime (τ_TiO2 should be 1/2.63 (ns) = 380 ps), while slope of 10


Page 10 of 28

Comment [u1]: “Author reply” I deleted the table on the graph according to the comment of the reviewer.

Page 11 of 28

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


-2.88 for PREDCN2 had a shorter lifetime (τ_TiO2 = 1/2.88 (ns) = 347 ps). From τ_solution and τ_TiO2, time constants of electron injection, τ_inj can be estimated as 1/τ_inj =1/τ_TiO2 1/τ_solution.54 Based on this relation, τ_inj is estimated to be 461 ps for PRSCN2 and 405 ps for PREDCN2. The electron injection efficiency calculated using the following equation was slightly higher for PREDCN2 (85.8%) than for PRSCN2 (82.5%).

ηinj = 1-

ૌ_‫۽ܑ܂‬૛ ૌ_‫ܖܗܑܜܝܔܗܛ‬

(1)

2.4 Effect of the dye adsorption mode on increasing Voc The conduction band edge potential (quasi Fermi level, EF,n) of TiO2 in corresponding solar cells is affected by the efficiency of electron injection. The EF,n level of TiO2 will control the Voc value, and it can be described by formula (2) below:36 EF,n = ECB + KBT Ln(n/NCB)

(2)

Here, ECB is the conduction band edge of TiO2, T is the absolute temperature, KB is the Boltzmann constant, NCB is the density of states, and n is the number of electrons in TiO2 associated with the carrier transport processes (containing the electron injection and recombination). Based on eqn (2), EF,n can be raised by a positive shift of the vacuum level of the ECB or by an increased electron population of the electron injection of TiO2, thereby improving

the Voc. In order to figure out the mechanisms of Voc increase, cyclic voltammetry was performed to change the ECB and distribution of electrons in the film. In order to investigate the change of the trap state due to the adsorption mode of the organic dye and to show the change of the Voc, the organic dyes were adsorbed on the TiO2 films and cyclic voltammetry was performed, as 11



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

Page 12 of 28

shown in Fig. 6. For experimental on surface trap sites, as shown in supporting information S2.3740, 50-53

Figure 6 Surface electron distributions and energy levels for organic dyes at the TiO2/LiClO4 interface, measured by cyclic voltammetry using a 0.05 V s-1 scan rate. Fig. 6 shows the current capacity of the electrode with organic dyes at the TiO2/LiClO4 interface, which represented progressive onsets under a forward potential. The electrode containing PRSCN2 exhibited a slightly decreased onset compared to PREDCN2. The downshift of onset means the conduction band edge of TiO2 was consistent with the trend of the negative shift and the variation of Voc. Moreover, the slope dQ/dV (where Q is the total number of surface 12


Page 13 of 28

trapping sites and V is the potential applied to the electrode) of the electrode with PRSCN2 was larger than that with PREDCN2. A decrease in the number of trapping sites indicates that the charge recombination between the electrode and the electrolyte is reduced. Thus, this high injection efficiency and positive shift in trap state can increase Voc and Jsc in devices containing PREDCN2, which is consistent with the solar cell performance characteristics in Section 2.5. To further confirm these observations, we conducted X-ray photoelectron spectroscopy (XPS) analysis. The XPS experiment can relatively confirm the electron recombination rate between the electrons on the TiO2 surface and the electrolyte.41

TiO2 PRSCN2 PREDCN2 I/a.u.

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


457

458

459 460 Binding energy/eV

461

Figure 7 Ti2p3/2 optoelectronic signal obtaining from the titania substrate. The decreased Ti2p signal indicates an increased average thickness of the adsorbed organic dyes layer on titania surface.

Fig. 7 compares the thickness of the coated dye through the amount of photoelectron emission from titania to vacuum. In comparison with an the substrate without dye coating, The intensity of the Ti2p3/2 signal from the dye-coated substrate is lowered because the adsorbed dye layer can 13



prevent electrons from escaping from the titania to the vacuum. A comparison of the intensities allows a relative comparison of the adsorption of the dye on the TiO2 surface. The organic PREDCN2 coating is thicker than that of PRSCN2; thus indicating a slower charge recombination rate and the resulting higher Voc in the former. 2.5 Photovoltaic performance The photovoltaic properties of the organic dyes in the DSSCs were investigated, as shown in Fig. 8. In addition, the device parameters are listed in Table 2. Details of the cell fabrication methods and instumentation are described in Supporting Information S2. (a)

15 10 5

0.8 Quantum efficiency

20 Jsc(mA/cm2)

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

Page 14 of 28

N719_dark PRSCN2_dark PREDCN2_dark N719 PRSCN2 PREDCN2

0.2

0.4 Voc(V)

0.6

0.4

0.2

0 -5 0.0

N719 PRSCN2 PREDCN2

(b)

0.6

0.8

0.0 300

400

500 600 Wavelength(nm)

700

800

Figure 8 Current density-voltage properties for DSSCs with organic dyes under 1 Sun (AM1.5, 100 mW/cm2). (b) IPCE data for DSSCs using organic dyes. Under the standard global AM1.5 solar irradiation, the PREDCN2-sensitized cell showed Jsc = 19.75 mA·cm-2, Voc = 0.676 V, and FF = 62.33%; corresponding to an overall conversion efficiency (η) of 8.32%. Under the same experiment conditions, DSSCs based on PRSCN2 exhibited Jsc = 17.23 mA·cm-2, Voc = 0.645 V, and FF = 64.47%; corresponding to η = 7.16%. Both dyes were compared with a reference dye, N719, which yielded η = 9.10%. The higher efficiency of PREDCN2 than PRSCN2 can be explained by the increased HOMO energy level 14


Page 15 of 28

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


(thus the decreased band gap) and stronger adsorption of PREDCN2 due to the introduction of EDOT. The EDOT device of PREDCN2 can improve electron injection efficiency to the TiO2 electrode in the excited state due to the planarity of the π-conjugated structure and the adsorption structure of the bidentate type.42 To verify the effect of the dye-TiO2 interaction, the amount of organic dye adsorbed on the TiO2 films was measured by the absorbance of the desorbed dye solution, and the concentration of PRSCN2 (1.29 × 10-5 mmol·cm-2) was lower than that of PREDCN2 (1.85 × 10-5 mmol·cm-2). This result indicates that a stronger chemical bonding of the dye can increase the amount of dye adsorbed on TiO2, which in gradually can effect the photocurrent (Fig. 8(a)). The PREDCN2 increases the light harvesting efficiency by the increased photocurrent, which can be confirmed by the photocurrent-current conversion efficiency (IPCE) spectrum(Fig. 8(b)). The onset wavelength of the IPCE spectrum for the PREDCN2-based DSSCs was 800 nm, and the maximum IPCE (higher than 70%) was observed in the wavelength range of 350–650 nm. In contrast, the IPCE performance of the DSSCs containing PREDCN2 was considerably improved at longer wavelengths, which is corresponding with the increased current density shown in Fig. 8(a). The enhanced cell efficiency with PREDCN2 containing the EDOT unit may be owing to the high molar extinction coefficients, efficient electron extraction paths, and electron injection from the dye to the TiO2 conduction band in the long-wavelength region, which increased the Jsc values compared to the other dye under the same conditions, as shown in Table 2. Table 2 Photovoltaic properties of the DSSCs.a dyeb PRSCN2

Jsc (mA cm- Voc/V 2 ) 17.23 0.645

FF(%)

η /%

dye loading capacity (mmol·cm-2)

64.47

7.16

1.29 x10-5 15



PREDCN2 N719 a

19.75 17.72

0.676 0.739

62.33 69.47

1.85 x10-5 -

8.32 9.10

Photovoltaic performance under AM1.5 irradiation of the DSSCs containing PRSCN2 and PREDCN2 dyes, based

on the electrolyte containing 3-propyl-1-methyl-imidazolium iodide (PMII, 1 M), lithium iodide (LiI, 0.2 M), iodide (I2, 0.05 M), and tert-butylpyridine (TBP, 0.5 M) in acetonitrile/valeronitrile (85:15, v/v).

b

Dye bath: ethanol

-4

solution (3 × 10 M).

2.6 Electron transport, diffusion length, and recombination To further understand the variation in electron injection efficiency by dye adsorption mode, we employed optical impedance experiment, such as Intensity Modulated Photocurrent Spectroscopy (IMPS)/ Intensity Modulated photoVoltage Spectroscopy (IMVS).43 The IMPS provides information

on the electron transport time (τd) through the TiO2 electrode under short-circuit current conditions. The IMVS also measures the recombination lifetime (τe) of electrons recombining with electrolytes or oxidation dyes under open-voltage conditions. Fig. 9 shows that the electron transport time is very distinction in DSSCs with the two dyes at the same short-circuit current

Charge transport time (ms)

density. The detailed parameters are listed in Table 3.

Lifetime (ms)

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

Page 16 of 28

0.1

0.01 0.01

PRSCN2 PREDCN2 0.1 Jsc (mA/cm2)

1

16


Page 17 of 28

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


Figure 9 Electron transport time (τd, solid line) and electron lifetime (τe, dash line) for DSSCs of different organic dyes as a values for photocurrent density (Jsc).

The device with PREDCN2 has the highest electron diffusion coefficient (8.54×10-3). Therefore, it can be confirmed that the electron transfer is promoted by the efficient electron extraction path due to the adsorption characteristics of PREDCN2, which increases the charge injection and collection efficiency. Furthermore, the τe values of PREDCN2 were enhanced compared with those of PRSCN2. This is due to the retarded electron recombination from the faster electron transfer, which prevents combination between the electrons on the TiO2 surface and I3- ions in the electrolyte. To identify the influence of electron lifetimes, we conducted electrochemical impedance spectroscopy (EIS) and open circuits voltage decay (OCVD)analysis, and the results are shown in supporting information S5 and S6. From these results, the values of τe and De may be used to calculate the electron diffusion length (Ln) in TiO2 films, defined as Ln = (Deτe)0.5. The Ln values of PREDCN2 are higher and closer to the film thickness, compared to the case of PRSCN2 in which Ln is smaller than the film thickness (11 µm). This implies that the PRSCN2-based device suffers from poor collection efficiency of injected electrons, which decreases the Jsc. As a result, the charge-collection efficiency (ηcc), described by the relationship ηcc =1-(τd/τe), is much greater for PREDCN2 devices than PRSCN2 devices (about 12% higher according to Table 3). Therefore, PREDCN2 not only enhances the photocurrent density (Jsc) but also increases the charge-collection efficiency and the light-harvesting efficiency. Table 3 The IMPS/IMVS properties of DSSCs with organic dyes. 17



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

dye

Dea (cm2/s)

Lnb (µm)

ηccc (%)

PRSCN2

6.78×10-3

7.4

71.8

PREDCN2

8.54×10

9.67

83.7

a

-3

Page 18 of 28

De, diffusion coefficient. b Ln, diffusion length. c ηcc, charge-collection efficiency. d ηcc, charge-collection efficiency.

2.7 Stability We conducted photo- and thermal-aging experiments to investigate the stability of the dyes with different adsorption modes. Fig. 10 compares the time evolution of performance parameters under 1 sun light while soaking at 30 °C for 2000 h. The DSSC containing PREDCN2 exhibited much higher long-term stability compared with that containing PRSCN2. The long-term stability of the DSSCs was effected significantly more by Jsc than by other factors such as Voc and FF. This indicated that the bidentate binding mode (PREDCN2) is superior to the monodentate binding mode (PRSCN2) in terms of the stability and interfacial quantum yields of electron injection, owing to the direct contact with the semiconductor surface.44 The significantly improved stability of the DSSCs containing PREDCN2 can be aspected to the stronger chemical bonding afforded by the binding mode of the EDOT units within the dye.

18


Page 19 of 28

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


Figure 10 Stability test results of DSSCs with organic dyes under 1 sun light while soaking at 30 °C for 2000 h. 3. Conclusions The origin of simultaneous improvement in photocurrent and photo-voltage in DSSCs between one phenothiazine-based dye with novel conjugated groups to another was investigated. UV-visible absorption and cyclic voltammetry results indicate that the increased λmax and positive HOMO level of the dye PREDCN2 contribute to its high light harvesting efficiency. Photo-optical measurements (ATR-FTIR, TCSPC, DOS, and XPS) suggest this dye has increased photocurrent due to fast electron transfer, which is attributed to the bi-dentate binding mode, while the enhanced light harvesting and electron injection efficiencies increased the Voc simultaneously. Finally, we demonstrated that the DSSCs devices containing PREDCN2 dye with bi-dentate adsorption mode display simultaneously improvements in efficiency factors and stability. Although most of the factors are made with positive contributions to efficiency, which 19



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

Page 20 of 28

are also complementary and careful consideration is needed to determine the contributions to each factors.

■ ASSOCIATED CONTENT Supporting Information. Materials synthesis; theoretical calculation; detailed solar cell fabrication, characteristics, and electrochemical properties. This information is available free of charge from the Journal of Physical Chemistry C.

■ AUTHOR INFORMATION Corresponding Authors a


b


■ NOTES The authors declare no competing financial interests.

■ ACKNOWLEDGMENT This study was supported by a grant from the Fundamental R&D program for Core Technology of Materials (10050966) funded by the Ministry of Knowledge Economy, Republic of Korea. This work was also supported by the DGIST R&D Programs of the Ministry of Science, ICT & Future Planning of Korea (17-ET-01).

20


Page 21 of 28

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


■ REFERENCES (1)

Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338–344.

(2)

O’Regan, B.; Grätzel, M. A Low-cost, High-efficiency Solar Cell Based on Dye-

sensitized Colloidal TiO2 Films. Nature 1991, 353, 737–7403. (3)

Hagfeldt, A.; Grätzel, M. Molecular Photovoltaics. Acc. Chem. Res. 2000, 33, 269–277.

(4)

Grätzel, M. Recent Advances in Sensitized Mesoscopic Solar Cells. Acc. Chem. Res.

2009, 42, 1788–1798. (5)

Hagfeldt, A.; Boschloo, G.; Sun, L. C.; Kloo L.; Pettersson, H. Dye-sensitized Solar

Cells. Chem. Rev. 2010, 110, 6595–6663. (6)

Yella, A.; Lee, H. W.; Tsao, H. N.; Yi, C. Y.; Chandiran, A. K.; Nazeeruddin, M. K.;

Diau, E. W. G.; Yeh, C. Y.; Zakeeruddin, S. M.; Grätzel, M. Porphyrin-sensitized Solar Cells With Cobalt (II/III)–based Redox Electrolyte Exceed 12 Percent Efficiency. Science 2011, 334, 629–634. (7)

Tian, H.; Yang, X.; Chen, R.; Hagfeldt, A.; Sun, L. A Metal-free “Black Dye” for

Panchromatic Dye-sensitized Solar Cells. Energy Environ. Sci. 2009, 2, 674–677. (8)

Kim, S.; Mor, G. K.; Paulose, M.; Varghese, O. K.; Baik, C.; Grimes, C. A. Molecular

Design of Near-IR Harvesting Unsymmetrical Squaraine Dyes. Langmuir 2010, 26, 13486– 13492. (9)

Yum, J. H.; Walter, P.; Huber, S.; Rentsch, D.; Geiger, T.; Nuesch, F.; Angelis, F. D.;

Grätzel, M.; Nazeeruddin, M. K. Efficient Far Red Sensitization of Nanocrystalline TiO2 Films by an Unsymmetrical Squaraine Dye. J. Am. Chem. Soc. 2007, 129, 10320 –10321. 21



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

(10)

Page 22 of 28

Sudeep, P. K.; Takechi, K.; Kamat, P. V. Harvesting Photons in the Infrared. Electron

Injection from Excited Tricarbocyanine Dye (IR-125) into TiO2 and Ag@ TiO2 Core−Shell Nanoparticles. J. Phys. Chem. C 2007, 111, 488–494. (11)

Kolemen, S.; Bozdemir, O. A.; Cakmak, Y.; Barin, G.; Erten-Ela, S.; Marszalek, M.;

Yum, J. H.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. et al. Optimization of DistyrylBodipy Chromophores for Efficient Panchromatic Sensitization in Dye Sensitized Solar Cells. Chem. Sci. 2011, 2, 949–954. (12)

Hua, Y.; Wang, H. D.; Zhu, X. J.; Islam, A.; Han, L. Y.; Qin, C. J.; Wong, W. Y. New

Simple Panchromatic Dyes Based on Thiadiazolo[3,4-c]pyridine Unit for Dye-sensitized Solar Cells. Dyes Pigm. 2014, 102, 196–203. (13)

Qin, C. J.; Mirloup, A.; Leclerc, N.; Islam, A.; El-Shafei, A.; Han, L. Y.; Ziessel, R.

Molecular Engineering of New Thienyl-bodipy Dyes for Highly Efficient Panchromatic Sensitized Solar Cells. Adv. Energy Mater. 2014, 4, 1400085. (14)

Han, L. Y.; Islam, A.; Chen, H.; Malapaka, C.; Chiranjeevi, B.; Zhang, S. F.; Yang, X.

D.; Yanagida, M. High-efficiency Dye-sensitized Solar Cell with a Novel Co-adsorbent. Energy Environ. Sci. 2012, 5, 6057–6060. (15)

Lan, C. M.; Wu, H. P.; Pan, T. Y.; Chang, C. W.; Chao, W. S.; Chen, C. T.; Wang, C. L.;

Lin, C. Y.; Diau, E. W. G. Enhanced Photovoltaic Performance with Co-sensitization of Porphyrin and an Organic Dye in Dye-sensitized Solar Cells. Energy Environ. Sci. 2012, 5, 6460–6464. (16)

Kim, Y. R.; Yang, H. S.; Ahn, K.-S.; Kim, J. H. Enhanced Performance of Dye Co-

sensitized Solar Cells by Panchromatic Light Harvesting. J. Kor. Phys. Soc. 2014, 6, 904–909. 22


Page 23 of 28

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

(17)


Funabiki, K.; Mase, H.; Saito, Y.; Otsuka, A.; Hibino, A.; Tanaka, N.; Miura, H.;

Himori, Y.; Yoshida, T.; Kubota, Y. et al. Design of NIR-absorbing Simple Asymmetric Squaraine Dyes Carrying Indoline Moieties for Use in Dye-Sensitized Solar Cells with Pt-Free Electrodes. Org. Lett. 2012, 14, 1246–1249. (18)

Paek, S.; Choi, H.; Kim, C.; Cho, N.; So, S.; Song, K.; Nazeeruddine, M. K.; Ko J.

Efficient and Stable Panchromatic Squaraine Dyes for Dye-sensitized Solar Cells. Chem. Commun. 2011, 47, 2874–2876. (19)

Qin, C. J.; Wong, W. Y.; Han, L. Y. Squaraine Dyes for Dye-Sensitized Solar Cells:

Recent Advances and Future Challenges. Chem. Asian J. 2013, 8, 1706–1719. (20)

Park, J. K.; Lee, H. R.; Chen, J.; Shinokubo, H.; Osuka, A.; Kim, D.

Photoelectrochemical Properties of Doubly β-Functionalized Porphyrin Sensitizers for DyeSensitized Nanocrystalline-TiO2 Solar Cells. J. Phys. Chem. C 2008, 112, 16691–16699. (21)

Huang, Z.–S.; Meier, H.; Cao, D. Phenothiazine-based Dyes for Efficient Dye-sensitized

Solar Cells. J. Mater. Chem. C 2016, 4, 2404–2426. (22)

Jo, H. J.; Nam, J. E.; Kim, D.-H..; Kim, H. J.; Kang, J.-K. A Comparison of the

Electronic and Photovoltaic Properties of Novel Twin-anchoring Organic Dyes Containing Varying Lengths of π-bridges in Dye-sensitized Solar Cells. Dyes Pigm. 2014, 102, 285–292. (23)

Li, W.; Liu, B.; Wu, Y.; Zhu, S.; Zhang, Q.; Zhu, W. Organic Sensitizers Incorporating

3,4-Ethylenedioxythiophene as the Conjugated Bridge: Joint Photophysical and Electrochemical Analysis of Photovoltaic Performance. Dyes Pigm. 2013, 99, 176–184. (24)

Yang, Z.; Liu, C.; Shao, C.; Lin, C.; Liu, Y. First-Principles Screening and Design of

Novel Triphenylamine-Based D−π–A Organic Dyes for Highly Efficient Dye-Sensitized Solar Cells. J. Phys. Chem. C 2015, 119, 21852–21859. 23



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

(25)

Page 24 of 28

Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J.-H.; Fantacci, S.; De Angelis, F.; Di

Censo, D.; Nazeeruddin, K. M.; Grätzel, M. Molecular Engineering of Organic Sensitizers for Solar Cell Applications. J. Amer. Chem. Soc. 2006, 128, 16701–16707. (26)

Hagberg, D.; Yum, J. H.; Lee, H.; Angelis, F. D.; Marinado, T.; Karlsson, K. M.;

Humphry-Baker, R.; Sun, L.; Hagfedt, A.; Grätzel, M. et al. Molecular Engineering of Organic Sensitizers for Dye-sensitized Solar Cell Applications. J. Am. Chem. Soc. 2008, 130, 6259–6266. (27)

Hara, K.; Wang, Z.-S.; Sato, T.; Furube, A.; Katoh, R.; Sugihara, H.; Dan-oh, Y.;

Kasada, C.; Shinpo, A.; Suga, A. Oligothiophene-Containing Coumarin Dyes for Efficient DyeSensitized Solar Cells. J. Phys. Chem. B 2005, 109, 15476–15482. (28)

Kim, D. H.; Lee, J. K.; Kang, S. O.; Ko, J. J. Molecular Engineering of Organic Dyes

Containing N-Aryl Carbazole Moiety for Solar Cell. Tetrahedron 2007, 63, 1913–1922. (29)

Cho, M. J.; Park, S. S.; Yang, Y. S.; Kim, J. H.; Choi, D. H. Molecular Design of

Donor–acceptor-type Cruciform Dyes for Efficient Dyes-sensitized Solar Cells. Synth. Met. 2010, 160, 1754–1760. (30)

Bae, E.; Choi, W.; Park, J. W.; Shin, H. S.; Kim, S. B.; Lee, J. S. Effects of Surface

Anchoring Groups (Carboxylate vs Phosphonate) in Ruthenium-Complex-Sensitized TiO2 on Visible Light Reactivity in Aqueous Suspensions. J. Phys. Chem. B 2004, 108, 14093–14101. (31)

Nazeeruddin, M. K.; Amirnasr, M.; Comte, P.; Mackay, J. R.; McQuillan, A. J.; Houriet,

R.; Grätzel, M. Adsorption Studies of Counterions Carried by the Sensitizer cisDithiocyanato(2,2‘-bipyridyl-4,4‘-dicarboxylate) Ruthenium(II) on Nanocrystalline TiO2 Films. Langmuir 2000, 16, 8525–8528.

24


Page 25 of 28

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

(32)


Qu, P.; Thompson, D. W.; Meyer, G. J. Temperature-Dependent Electron Injection from

Ru(II) Polypyridyl Compounds with Low Lying Ligand Field States to Titanium Dioxide. Langmuir 2000, 16, 4662–4671. (33)

Qin, P.; Yang, X.; Chen, R.; Sun, L. Influence of π-Conjugation Units in Organic Dyes

for Dye-Sensitized Solar Cells. J. Phys. Chem. C 2007, 111, 1853–1860. (34)

Leon, C. P.; Kador, L.; Peng, B.; Thelakkat, M. Characterization of the Adsorption of

Ru-bpy Dyes on Mesoporous TiO2 Films with UV−Vis, Raman, and FTIR Spectroscopies. J. Phys. Chem. B 2006, 110, 8723–8730. (35)

Liu, J.; Li, R. Oligothiophene Dye-sensitized Solar Cells. Energy Environ. Sci. 2010, 3,

1924–1928. (36)

Huang, S. Y.; Schlichthörl, G.; Nozik, A. J.; Grätzel, M.; Frank, A. J. Charge

Recombination in Dye-Sensitized Nanocrystalline TiO2 Solar Cells. J. Phys. Chem. B 1997, 101, 2576–2582. (37)

Zhang. A.; Zakeeruddin. S. M.; O’Ragan. B.C.; Humphry-Baker. R.; Grätzel, M.

Influence of 4-Guanidinobytyric Acid as Coadsorbent in Reducing Recombination in DyeSensitized Solar Cells. J. Phys. Chem. B 2005, 109, 21818-21824. (38)

J. Morris. A.; J. Meyer. G. TiO2 Surface Functionalization to control the Density of

States. J. Phys. Chem. C 2008, 112, 18224-18231. (39)

Lyon, L. A.; Hupp, J. T. Energetics of the Nanocrystalline Titanium Dioxide/Aqueous

Solution Interface: Approximate Conduction Band Edge Variations between H0 = −10 and H- = +26. J. Phys. Chem. B 1999, 103, 4623-4628. (40)

van de Krol, R.; Goossens, A.; Schoonman, J. Spatial extent of Lithium Intercalation in

Anatase TiO2. J. Phys. Chem. B 1999, 103, 7151-7159. 25



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

(41)

Page 26 of 28

Hofmann, S. Auger- and X-Ray Photoelectron Spectroscopy in Materials Science,

Springer, 2012. (42)

Hagberg, D. P.; Marinado, T.; Karlsson, K. M.; Nonomura, K.; Hagfeldt, A.; Sun, L.

Tuning the HOMO and LUMO Energy Levels of Organic Chromophores for Dye Sensitized Solar Cells. J Org. Chem. 2007, 72, 9550–9556. (43)

Jennings, J. R.; Peter, L. M. A Reappraisal of the Electron Diffusion Length in Solid-

State Dye-Sensitized Solar Cells. J. Phys. Chem. C 2007, 111, 16100–16104. (44)

Kohjiro. H.; Koji. M.; Yoshimoto. A.; Masatoshi. Y. Electron Transport in Coumarin-

Dye-Sensitized Nanocrystalline TiO2 Electrodes. J. Phys. Chem. B 2005, 109, 23776–23778. (45)

Sewvandi. G. A.; Tao. Z.; Kusunose. T.; Tanaka. Y.; Nakanishi, S.; Feng. Q.

Modification of TiO2 Electrode with Organic Silane Interposed Layer for High-Performance of Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 5818-5826. (46)

Sewvandi. G. A.; Chen. C.;Ishii. T.; Kusunose. T.; Tanaka. Y.; Nakanishi, S.; Feng. Q.

Interplay between Dye Coverage and Photovoltaic Performances of Dye-Sensitized Solar Cells Based on Organic Dyes. J. Phys. Chem. B 2014, 118, 20184-20192. (47)

K. E. Lee,; M.A. Gomez, S. Elouatik,; G. Demopoulos. Further Understanding of the

Adsorption Mechanism of N719 Sensitizer on Anatase TiO2 Films for DSSC Applications Using Vibrational Spectroscopy and Confocal Raman Imaging Langmuir 2010, 26, 9575-9583. (48)

K. D. Seo,; I. T. Choi,; Y. G. Park.; S.W. Kang.; J. Y. Lee.; H. K. Kim.; Novel D-A-π-A

coumarin dyes containing low band-gap chromophores for dye-sensitized solar cells J. dyepig. 2012, 94, 469-474.

26


Page 27 of 28

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

(49)


W. Li,; B. Liu,; Y. Wu,; S. Zhu,; Q. Zhang,; W. Zhu.; Organic sensitizers incorporating

3,4-ethylenedioxythiophene as the conjugated bridge: Joint photophysical and electrochemical analysis of photovoltaic performance J. dyepig. 2013, 99, 176-184. (50)

Q. Wang,; S.M. Zakeerudding,; J. Cremer,; P. Bauerle,; T. Humphry-Baker,; M.

Grätzel.; Cross surface Ambipolar charge percolation in Molecular triads on mesoscopic oxide films J. AM. CHEM. SOC. 2005, 127, 5706-5713. (51)

J. Bisquert,; F. FAbregat-Santiago,; I. Mora-Sero,; G. Garcia-Belmonte,; E. M. Barea,;

E. Palomares.; A review of recent results on electrochemical determination of the density of electronic states of nanostructured metal-oxide semiconductors and organic hole conductors Inorganica Chimica Acta. 2008, 361, 684-698. (52)

Md. Rahman,; NarChandra Deb Nath,; K.-Mo. Noh,; J. Kim,; J.-J. Lee.; A Facile

Synthesis of Granular ZnO Nanostructures for Dye-Sensitized Solar Cells International Journal of photoenergy. 2013, 2013. (53)

M.Molayem,; M.Springborg,; B.Kirtman.; Enhanced performance of dye-sensitized

solar cells with dual-function coadsorbent: reducing the surface concentration of dye–iodine complexes concomitant with attenuated charge recombination phys. Chem. Chem. Phys. 2015, 17, 22985-22990. (54)

A.Kathiravan,; V.Raghavendra,; R. Ashok Kumar,; P. Ramamurthy.; Protoporphyrin

IX on TiO2 electrode: A spectroscopic and photovoltaic investigation J. dyepig. 2013, 96, 196203.

27



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

Page 28 of 28

TOC GRAPHICS

28


Mechanistic and Time Resolved Single-Photon Counting Analysis for

Recommend Documents