Enhanced Photocurrent Density by Spin-Coated NiO Photocathodes

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Enhanced Photocurrent Density by Spin-Coated NiO Photocathodes for N-annulated Perylene Based p-Type Dye-Sensitized Solar Cells Xing Li, Fengtao Yu, Sebastian Stappert, Chen Li, Ying Zhou, Ying Yu, Xin Li, Hans Ågren, Jianli Hua, and He Tian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04007 • Publication Date (Web): 15 Jul 2016 Downloaded from http://pubs.acs.org on July 20, 2016

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ACS Applied Materials & Interfaces

Enhanced Photocurrent Density by Spin-Coated NiO Photocathodes for N-annulated Perylene Based pType Dye-Sensitized Solar Cells Xing Li, a Fengtao Yu, a Sebastian Stappert, b Chen Li, b Ying Zhou, a Ying Yu, a Xin Li, c* Hans Ågren, c Jianli Hua a* and He Tian a

a

Key Laboratory for Advanced Materials, Institute of Fine Chemicals, School of Chemistry and

Molecular Engineering, East China University of Science & Technology, 130 Meilong Road, Shanghai, 200237, PR China. Fax: 8621-64250940. b

c

Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany Division of Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal Institute

of Technology, SE-10691 Stockholm, Sweden

KEYWORDS: p-type dye-sensitized solar cells, spin-coated NiO photocathode, N-annulated perylene, quinoxaline, organic sensitizer

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ABSTRACT: The low photocurrent density of p-type dye-sensitized solar cells (p-DSSCs) has limited the development of high-efficiency tandem cells due to the inadequate light-harvesting ability of sensitizers and the low hole mobility of semiconductors. Hereby, two new “push-pull” type organic dyes (PQ-1 and PQ-2) containing N-annulated perylene as electron donor have been synthesized, where the PQ-2-based p-DSSCs show higher photoelectric conversion efficiency (PCE) of 0.316 % owing to the higher molar extinction compared to of that PQ-1. Additionally, the photocurrent densities were remarkably increased from 2.20 to 5.85 mA cm-2 for PQ-1and 2.45 to 6.69 mA cm-2 for PQ-2 by spin-coated NiO photocathode based on PQ-1 / PQ-2, respectively. This results are ascribed to the enhancement of hole transport rate, dye-loading amounts and transparency of NiO films in comparison to that prepared by screen-printing method. Electrochemical impedance spectroscopy and theoretical calculations studies indicate that the molecular dipole moment approaching closer to the NiO surface shifts the quasi-Fermi level to more positive levels, improving open-circuit voltage (Voc). Intensity-modulated photocurrent spectroscopy illustrates that the hole transit time in NiO films prepared in spincoating is shorter than that prepared by screen-printing method.

INTRODUCTION The p-type dye-sensitized solar cells (p-DSSCs) have recently attracted extensive attention due to considerable scientific interest not only in their latent capacity to develop efficiency tandem cells with n-DSSCs1-3 and hydrogen generation devices,4-6 but also for the development of new photovoltaic materials, such as organic sensitizers,7-10 semiconductor materials,11-13 etc. In general, the open-circuit voltage (Voc) could be easily and effectively improved by tandem cells. However, the low photocurrent density of p-DSSCs has become a key constrain on the development of high-efficiency tandem cells with n-DSSCs. So far, the highest photocurrent


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density (Jsc) for p-DSSCs remains around 8.0 mA cm-2,

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which is far less than the Jsc of n-

DSSCs that has reached 15-20 mA cm-2. 14-16 Since the light-harvesting material is one of the main points in getting an effective photocurrent density, the development of rationally designed sensitizers and semiconductors holds great promise to overcome this bottleneck. The general design principle of an organic sensitizer for p-DSSCs consists of a “push-pull” configuration, increasing intramolecular charge transfer (ICT) for better light absorption and reducing holes recombination between the semiconductor and the electrolytes or reduced dyes, and then increasing the photocurrent density.8,17,18 In 2009, an oligothiophene-functionalized push-pull dye was designed and named as PMI-6T-TPA.19 It contains an oligothiophene connected to the electron donor of triphenylamine and a perylenemonoimide (PMI) unit as the electron acceptor, and remains one of the best sensitizers reported for p-DSSCs until now.20-22 Recently, a series of high power conversion efficiencies (PCEs) of over 12 % have been reported for n-DSSCs based on the dyes of N-annulated perylene unit as electron donor by Wang and coworkers,23-25 since the perylene moiety with coplanar molecular structure is ordinarily given a high molar absorption coefficient, large luminescence yield, and attractive photostability.26, 27 In addition, quinoxaline, as an auxiliary acceptor with strong electron-withdrawing ability, not only is beneficial to broaden absorption band of sensitizers, but also enhances the power conversion efficiencies and stability.28 In our previous studies, quinoxaline with D-π-A organic dyes showed promising results for n-DSSCs.29-32 Those advantages of N-annulated perylene and quinoxaline inspired us to explore further into their application in p-DSSCs. The most studied semiconductor in p-DSSCs is nickel oxide (NiO), which shows a wide band gap of 3.5 eV with excellent chemical and thermal stability and transparency. However, the preparation procedure for NiO has significant effects on light-harvesting ability, mainly owing


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to the difference in dye-loading amounts, transparency and stability.17 For instance, the mechanical stability of NiO films prepared in screen-printing method often is very poor.17 Although some attempts (altering treatment temperature 22,33 or preparing in doctor-blading 34,35, etc.) have been made to enhance the quality of NiO films, the Jsc has still not been improved dramatically for p-DSSCs due to its low hole mobility. For the sake of studying the effect of dye molecular structure on absorption, energy levels, and photovoltaic performances, we have synthesized two “push-pull” type organic sensitizers, PQ-1 and PQ-2, employing N-annulated perylene as the donor, quinoxaline (QA) as the auxiliary acceptor, cyclopentadithiophene (CPDT) as the π-bridge, PMI as the electron acceptor, and – COOH as anchoring group to achieve good coupling between dye sensitizer and the NiO electrode. PQ-1 and PQ-2 are isomers with the same molecular formula but the positions of the CPDT and QA units between the donor and acceptor groups are interchanged (see Scheme 1). These two dyes were subsequently employed in p-DSSCs based on NiO films prepared by screen-printing method, and the PQ-2-based p-DSSCs exhibited a better PCE of 0.122% using the iodide/triiodide redox electrolyte. To further improve the photovoltaic performance of the PQ-1 / PQ-2 based p-DSSCs, we introduced a significantly increase in photocurrent by using spin-coating a precursor solution of NiCl2 with F68 (triblock co-polymer) as template onto FTO glass with scotch tape on each side. As a result, the PQ-1 / PQ-2 based p-DSSCs made with 4 layers of NiO films (~ 2 µm) prepared in spin-coating exhibits the best PCEs of 0.244 % / 0.316 % resulting from the short current (Jsc) with a dramatic increase to 5.85 and 6.69 mA cm-2 for PQ-1 and PQ-2, respectively. Although spin-coating method to make NiO films have been reported, 36, 37 the preparation process of the NiO thin films are tedious for coating source by solgel method. In addition, electrochemical impedance spectroscopy (EIS) was used to investigate


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the effect of altering the position of the CPDT-QA core on the holes recombination and interfacial charge transfer processes in the p-DSSCs. Intensity-modulated photocurrent spectroscopy (IMPS) was also been employed to estimate hole transport dynamics in NiO films prepared by both spin-coating and screen-printing method.

Scheme 1. The structures of PQ-1 and PQ-2 RESULTS AND DISCUSSION Sensitizers Synthesis. In this article, molecular structure research was conducted by altering the sequence of synthetic procedures based on the core of CPDT-QA to obtain two isomeric structure organic dyes (PQ-1 and PQ-2). The electron donor part of the N-annulated perylene was synthesized according to literature

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and 2-ethyl hexyl was comprised to improve its

solubility in organic solvents and block molecular aggregation on NiO surface, the same roles played in the CPDT unit. The synthetic detailed procedures of organic dyes PQ-1 and PQ-2 are described in Scheme S1, followed by a series chemical reactions of Suzuki coupling, Stille coupling, bromination and carboxyester hydrolysis. It is important to highlight the sequence of Suzuki coupling reaction for the core (CPDT-QA). When the core reacts with the donor part (N-


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annulated perylene) firstly, then the final product is PQ-1. On the contrary, it is the prerequisite for giving PQ-2 that the CPDT-QA unit reacts first with acceptor group (PMI). All intermediates and two target sensitizers were confirmed with 1H NMR, 13C NMR, HRMS and MALDI-TOFMS. Optical and Electrochemical Properties. The optical absorption spectra of PQ-1 and PQ-2 in tetrahydrofuran (THF) and on 2 µm NiO films are shown in Figure 1, and their maximum absorptions, molar extinction coefficients are listed in Table 1. Obviously, the position alteration of the CPDT-QA moiety leads to significant difference in the wavelength range and the intensity of the absorption band. As a result, the absorption peaks resulting from the intramolecular charge transfer (ICT) process in the visible region are located at λmax = 564 and 528 nm for PQ-1 and PQ-2 respectively, demonstrating that D-A-π-A configuration facilitates to broaden the absorption band than D-π-A-A configuration through altering the position of the CPDT-QA unit. However, PQ-1 shows one other major absorption peak at around 450 nm, which can be attributed to the charge transition from the donor moiety to the quinoxaline unit, and suggests that reversing the position of the CPDT-QA unit also has great influence on the shape of the absorption spectra. It could be noted that the charge transfer process of PQ-2 from the donor moiety to the quinoxaline unit may be contained in the ICT process, since quinoxaline unit and PMI moiety can be integrated as one wholes acceptor group in PQ-2. Compared with the spectra in solution, both absorption spectra recorded for PQ-1 and PQ-2 adsorbed on NiO films in Figure 1(b) show a weak absorption band between 650-750 nm, which might be attributed to the formation of J-aggregation. To investigate the feasibility of holes injection from the oxidized dyes into the valence band (VB) of NiO, and electron transfer from the excited dyes to the redox couple in the electrolyte,


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cyclic voltammograms of PQ-1 and PQ-2 were recorded in THF solution (Figure 2a) to measure the

Table 1. Photophysical and electrochemical parameters of dyes PQ-1 and PQ-2. λmaxa/nm (ε, M cm )

λmaxb/nm (on NiO)

E0oxc/V (vs. NHE)

E0redc/V (vs. NHE)

E0–0d/eV

E*red f/V

PQ-1

564 (39078)

525

1.06

-0.90

1.85

0.95

PQ-2

528 (46782)

Dye

a

-1

-1

490 0.95 -1.07 2.19 1.12 b Absorption maximum in THF at room temperature. Absorption maximum on 2 µm NiO films. c

All potentials were measured in THF with 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte, calibrated with E0Fc+/Fc = + 0.63 V vs. NHE. dThe zero-zero transition energy (E0-0) was calculated from the intersection wavelength (λ) of normalized emission and absorption curves by 1240/λ. fThe excited state reduction potential (E*red) was calculated by E0red plus E0-0. redox potentials and the corresponding data are summarized in Table 1. The positions of ground state oxidation potentials (E0ox) for PQ-1 and PQ-2 are located at 1.06 and 0.95 V vs. NHE, respectively. Interestingly, the E0ox of PQ-2 is less positive than PQ-1, which is attributed to CPDT unit and N-annulated perylene moiety could be integrated as one wholes donor group in PQ-2, leading to an increase in ground state electron density (see Table 2). On the other hand, the ground state reduction potentials (E0red) of PQ-1 and PQ-2 are -0.90 and -1.07 V vs. NHE, respectively. The E0red of the two dyes are placed sufficiently higher than the redox potential of iodide/triiodide (~0.40 V vs. NHE), indicating sufficient thermodynamic force for electron transfer from the excited dyes to the redox couple. The zero-zero transition energy (E0-0), determined from the intersection of normalized emission and absorption curves, were 1.85 eV for PQ-1 and 2.19 eV for PQ-2 (Figure S1). This indicated that the position exchange of CPDTQA moiety between donor and acceptor unit can be significantly changed the energy gap of the


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dyes. Moreover, the excited state reduction potential (E*red) was calculated by E0red plus E0-0. And the values for PQ-1 and PQ-2 are 0.95 and 1.12 V, respectively, which are more positive than the valence band (VB) of NiO (~0.54 V vs. NHE), guaranteeing ample driving force for dye regeneration.

Figure 1. (a) Absorption spectra of PQ-1 and PQ-2 in THF. (b) Normalized absorption spectra of dyes PQ-1 and PQ-2 on 2 µm NiO films.


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Figure 2. (a) Cyclic volatmmograms of PQ-1 and PQ-2; (b) Schematic diagram of energy levels and charge transfer processes in p-DSSCs (solid lines: desired charge transfer processes; dashed lines: undesired charge transfer processes). Theoretical Approach. We employed density functional theory (DFT) calculations to optimize the ground state geometries of the dyes, using the hybrid B3LYP functional and the 6-31G(d,p) basis set.39-41 Time-dependent (TD) DFT calculations were carried out to study the low-lying excited states at the optimized geometries, using the range-separate CAM-B3LYP density functional and the 6-311+G(d, p) basis set.42,43 The polarizable continuum model was used to study the solvent effects of THF throughout the calculations.44 All calculations were accomplished using the Gaussian 09 program package.45 Table 2. Frontier molecular orbitals and energy levels of the PQ-1 and PQ-2. Dye

HOMO-2

HOMO-1

HOMO

LUMO

LUMO+1

LUMO+2

-5.57 eV

-5.09 eV

-4.83 eV

-2.81 eV

-2.32 eV

-1.85 eV

-5.44 eV

-5.21 eV

-4.77 eV

-2.73 eV

-2.35 eV

-1.97 eV

PQ-1

PQ-2

Theoretical calculations suggest that the dihedral angles between the QA unit and adjacent polycyclic aromatic ring are 55.5° and 57.9° for PQ-1 and PQ-2, respectively, while those for the corresponding CPDT unit are relatively smaller (41.6° and 44.9°, see Table S1). The overall planarity of compound PQ-1 is slightly better than that of PQ-2, demonstrating that PQ-1 possesses wider absorption range compared to the PQ-2. The frontier molecular orbitals suggest a comparable HOMO-LUMO gap for both compounds (~2.0 eV), while the energy level of the


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HOMO-1 shows a notable difference (Table 2), owing to opposite positions of the CPDT-QA unit. PQ-1 and PQ-2 molecules exhibit push-pull electron effects, where the HOMO and LUMO are largely contributed by the donor and acceptor moieties, respectively. In both compounds, the HOMO-1 orbital receives contributions from the electron-rich donor and the electron-deficient acceptor. In compound PQ-1, however, due to conjugation with two adjacent electron-rich moieties (i.e. the donor and the CPDT unit), the electron-deficient QA unit contributes more to the HOMO than the HOMO-1 (Table 2). On the contrary, the QA unit in PQ-2 contributes more to HOMO-1. The consequence is that the energy levels of PQ-1’s HOMO and PQ-2’s HOMO-1 become lowered, leading to a notably larger gap between HOMO and HOMO-1 in PQ-2 (see Figure S3). Table 3. Computed excitation energy, oscillator strength and molecular orbital compositions for the lowest excited states of PQ-1 and PQ-2. Excited Dyes Compositiona Excitation energy Oscillator strength state

PQ-1

PQ-2

a

S1

42% H-1L 27% HL 19% H-2L

2.28 eV, 544 nm

2.270

S2

62% HL+1 14% H-2L

2.65 eV, 467 nm

0.725

S3

45% HL+2 20% HL+1 11% H-1L+2

2.93 eV, 423 nm

0.509

S1

36% H-1L 29% H-2L 13% HL

2.39 eV, 518 nm

2.576

S2

43% HL+1 19% H-2L

2.61 eV, 475 nm

0.548

S3

61% HL+2 17% H-1L+1

2.87 eV,431 nm

0.501

H = HOMO, L = LUMO


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TD-DFT calculations show that the HOMO-1 → LUMO transition, instead of the HOMO → LUMO transition, contributes dominantly to the maximum absorption band (Table 3). This is reasonable because there is very little spatial overlap between the HOMO and LUMO in both PQ-1 and PQ-2 molecules, such that they contribute very little to the overall oscillator strength. The HOMO-1 → LUMO transition, however, show a mixing between long-range charge transfer and local excitation, and the resultant oscillator strength for the lowest excited state is significant. The calculated excitation energy is also lower for PQ-1, owing to the smaller HOMO-1– LUMO gap and the greater portion of HOMO-1 → LUMO transition in the lowest excited state. This explains experimentally observed broadening of absorption spectrum of PQ-1 in comparison with PQ-2. Photovoltaic Properties. The p-DSSCs performances based on the screen-printed NiO films of the two dyes were measured under AM 1.5G irradiation (100 mWcm-2). Photocurrent density-voltage curves (J-V) of the p-DSSCs based on the two dyes are given in Figure 3a and the corresponding data are summarized in Table 4. Herein, the influence of CPDT-QA core position variation on the p-DSSCs characteristics is put into focus. It is obvious that the device made with PQ-2 shows both higher Jsc and Voc values, in turn, to higher efficiency. In general, the Jsc values are positively correlated to the light-harvesting ability of the dyes. Although the PQ-2 exhibits the narrower absorption spectrum, Jsc of the p-DSSCs based on PQ-2 is higher than that of the PQ-1. This may be attributed to the higher dye-loading amounts and molar extinction coefficient of PQ-2, which exhibits stronger light-harvesting ability ranging from around 450 to 550 nm. This result can also be illustrated by the fact that the PQ-2 based device shows the higher IPCE values than the PQ-1 based device in the range of 350~600 nm (see Figure 3b). In addition, the position exchange of CPDT-QA moiety between donor and


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acceptor unit is also highlighted in terms of the changes in Voc. This may be caused by the CPDT unit approaches to the electron donor group of PQ-2 and then two 2-ethyl hexyl chains in CPDT unit could be a better use of inhibiting molecular aggregation on NiO surface, which decreases the possibility of hole recombination processes at NiO/dye/electrolyte interface and improves the Voc.

Figure 3. (a) J-V curves and (b) IPCE spectra for p-DSSCs prepared in screen-printing method based on PQ-1 and PQ-2 with iodide/triiodide redox electrolyte.

Table 4. Photovoltaic performance of PQ-1 and PQ-2 with iodide/triiodide redox electrolytea. Jsc

Voc

FF

PCE

PQ-1

Dye-loading density [mol cm-2] [mol cm-2-7] 2.94×10

[mA cm-2] 2.20 ± 0.05

[mV] 135 ± 2

[%] 29.59 ± 1

[%] 0.088 ± 0.005

PQ-2

3.15×10-7

2.45 ± 0.05

145 ± 1

34.25 ± 1

0.122 ± 0.006

Dye

a

The electrolyte contained 0.03 M I2, 0.5 M tBP, 0.6 M BMII and 0.1 M GuSCN in acetonitrile.

(tBP = tert-butylpyridine, BMII = 1-butyl-3-methylimidazolium iodide. Average values are based on four replicate measurements).


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Electrochemical Impedance Analysis. In general, Voc is influenced by the valence band (Evb) position and the holes recombination processes at the NiO/dye/electrolyte interface. To better understand the difference in Voc among the two dyes, EIS was used to explore the holes recombination and charge transfer processes at NiO/dye/electrolyte interface. A series of bias potentials ranging from 20 to 160 mV were applied to p-DSSCs with the two dyes respectively under dark conditions and the transmission line model was used for equivalent circuits of data fitting.

Figure 4. For the p-DSSCs prepared in screen-printing method based on PQ-1 and PQ-2: (a) charge transfer resistance (Rrec) and chemical capacitance (Cchem); (b) holes lifetime (τh) plotted against the bias potential. Figure 4a depicts the charge transfer resistance (Rrec) (between the NiO films and the electrolyte) and chemical capacitance (Cchem) of the devices based on the two dyes as a function of the bias potential. It is should be noted that the minor difference in the Rrec values of the devices based on the two dyes, which may be attributed to the similar molecular structures (isomeric structure) of PQ-1 and PQ-2 resulting in slightly different effectiveness as a


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compacting layer between the NiO films and the electrolyte. In addition, Chemical capacitance that positively correlated with the quasi-Fermi level (EFn), lies on the hole state density distribution above the Evb of the NiO photocathode electrode. Strikingly, larger Cchem values is obtained for the PQ-2 based cell, which represents higher capacitive response of the NiO photocathode than that of PQ-1 and leads to higher hole density in NiO valence band. This phenomenon can be confirmed by the corresponding molecular dipole moment of PQ-1 and PQ2 from theoretical calculations (see Table 5). A dipole moment approaching the NiO surface will lead to an increase in the energy gap between the quasi-Fermi level for holes in NiO and that for electrons in the redox couple, resulting in a larger Voc. Accordingly, the longer hole lifetime (τh = Rct × Cchem) in NiO films, the higher the hole density state will be ensured, which might result in higher Voc (Figure 4b). Clearly, PQ-2 based cell exhibits longer τh values than that of PQ-1 under the same bias potentials, which agrees well with the measured Voc trend of the devices. Table 5. Ground state and excited state dipole moment of PQ-1 and PQ-2. Dye

S0

S1

5.31 Debye

16.91 Debye

7.24 Debye

17.17 Debye

PQ-1

PQ-2


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Optimization of NiO Films. Nonetheless, the IPCE values of PQ-1 and PQ-2 were limited to 15 % and 20 %, respectively, as a result of the relatively low charge collection efficiency due to the lower hole transport rate of the screen-printed NiO films. So more well-distributed and compacter mesoporous NiO films should be prepared to obtain higher currents. In this case, the new NiO films were made by spin-coating a precursor solution of NiCl2 based on the triblock copolymer F68 as template onto FTO glass with scotch tape on each side. As shown in the Table 6, the Jsc values of p-DSSCs made with spin-coating method increase with the thickness of NiO films from 1 to 4 layers (~ 2 µm), and then decrease with further increasing NiO films thickness to 5 layers, whereas minor changes are observed in corresponding Voc. The PQ-2 based p-DSSCs made with 4 layers of NiO films prepared in spin-coating exhibits the best PCE of 0.316 % with the Voc of 135 mV, Jsc of 6.69 mA cm-2 and fill factor (FF) of 35.03 %. The p-DSSCs made with PQ-1 also achieved a significant improvement in photovoltaic performance of 0.244 % with the Voc of 126 mV, Jsc of 5.85 mA cm-2 and fill factor (FF) of 33.16 % under the optimal operating conditions. In addition, we have measured the amount of dye adsorption by the desorption method (Tables 4 and 6). It can be seen from the results that PQ-1 loading amounts with screenprinted and spin-coated NiO films (both of the thickness are around 2 µm) are 2.94×10-7 and 4.92×10-7

mol cm-2, respectively. A remarkable enhancement has also been observed in

corresponding loading amounts for PQ-2 from 3.15×10-7 to 4.83×10-7 mol cm-2. Clearly, spincoated NiO films is beneficial for access to greater dye-loading capacity, which contributed in part to the increased Jsc.

Table 6. Photovoltaic parameters of p-DSSCs employing PQ-1 and PQ-2 with various layers of NiO films by spin-coating method. Dyes

NiO films a

Dye-loading density [mol cm-2]

Jsc [mA cm-2]

Voc [mV]

FF [%]

PCE [%]

PQ-1

1 layer

1.55×10-7

2.43 ± 0.03

128 ± 1

32.90 ± 1

0.102 ± 0.005


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2 layers 2.80×10-7 3.97 ± 0.03 3 layers 3.78×10-7 5.03 ± 0.04 4 layers 4.92×10-7 5.85 ± 0.02 5 layers 5.36×10-7 5.10 ± 0.02 1 layer 1.52×10-7 3.41 ± 0.03 2 layers 2.71×10-7 4.93 ± 0.02 PQ-2 3 layers 3.96×10-7 6.42 ± 0.05 4 layers 4.83×10-7 6.69 ± 0.02 5 layers 5.61×10-7 6.64 ± 0.02 a The film thickness of one layer is ca. 500 nm.

123 ± 1 124 ± 2 126 ± 2 130 ± 1 135 ± 1 134 ± 1 137 ± 2 135 ± 1 131 ± 1

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32.46 ± 1 31.93 ± 1 33.16 ± 1 32.15 ± 1 30.04 ± 1 32.35 ± 1 33.35 ± 1 35.03 ± 1 34.30 ± 1

0.159 ± 0.007 0.200 ± 0.009 0.244 ± 0.009 0.213 ± 0.009 0.138 ± 0.009 0.214 ± 0.006 0.293 ± 0.007 0.316 ± 0.009 0.298 ± 0.009

To better investigate the natures that contribute to the improvement of photocurrent, the NiO films prepared by screen-printing and spin-coating were characterized with a field-emission scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis. As can be seen in Figure 5, the particles were more compact and smaller at the surfaces and uniformly distributed throughout the film made with spin-coating, while the film made with screen-printing appears to be more porous of surface morphology. The cross-sections of both films revealed mesoporous layers of particles; boundaries could be observed where the film prepared by spin-coating with layer-by-layer appears to be more denser than that prepared by screen-printing method, leading to a better mechanical stability of the former. In Figure 6, XRD figure reveals high-purity of the overall phase and fantastic crystallinity of the NiO films with different preparation processes. XRD data suggest that this spin-coating processing causes the major peaks to broaden because of the reduced average grain size. It is notable that the largely enhancement of photocurrent for spin-coated NiO films, implying that the increased surface area of the NiO films induced a distinct improvement of charge collection process. Besides, the Brunauer-Emmett-Teller (BET) and t-plot model were used to determine the specific surface area and t-plot micropore area of the samples, respectively (Table S2). Obviously, surface area of NiO prepared by spin-coating is larger than that of screen-printing method, which is consistent with the dye-loading capacity. It cannot be ignored that the amorphous scattering present in the spin-coated films arising from the


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calcination under high temperature was not thorough or perfect, since the larger amount of NiCl2 precursor solution was coated onto the FTO glass for meeting the XRD measurement.

Figure 5. SEM images of surface and cross-section of NiO films prepared in (a)/(c) screenprinting and (b)/(d) spin-coating method.

Figure 6. XRD images of NiO films prepared in (a) screen-printing and (b) spin-coating method. The IPCE is a function of three components which are light-harvesting efficiency LHE(λ), charge collection efficiency (ηcol) and the charge injection efficiency (Φinj) from the oxidized dye


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to the NiO photocathode substrate: IPCE = LHE(λ) × ηcol × Φinj. Compared with the screenprinting method, the NiO films prepared by spin-coating is more dye-loading capacity and transparent (Figure 8 and Figure S2), indicating that the LHE of photocathode electrode made by spin-coating is greater than that made by screen-printing. Intensity-modulated photocurrent spectroscopy (IMPS) has also been used to evaluate electron transport dynamics in our cells, which shows hole transit times (τtr) in the NiO as function of the light density under short-circuit conditions. It should be noted that the τtr in NiO films prepared by spin-coating are shorter than that prepared by screen-printing for PQ-1 / PQ-2, suggesting higher ηcol values for spin-coating method (Figure 9). In addition, almost the same energy gaps between NiO valence band and HOMO levels of PQ-1 / PQ-2 for two types of film preparation, the Φinj values from the oxidized dyes to the valence band of NiO films prepared by spin-coating and screen-printing are nearly identical through thermodynamics. As a result, the IPCE values of PQ-1 / PQ-2 based p-DSSCs prepared by spin-coating are higher than that prepared by screen-printing, leading to higher Jsc (Figure 7).

Figure 7. IPCE spectra for p-DSSCs prepared in screen-printing and spin-coating method based on PQ-1 (a) and PQ-2 (b) with iodide/triiodide redox electrolyte.


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Figure 8. Absorption of NiO films prepared in screen-printing and spin-coating method.

Figure 9. The hole transit time as a function of light intensity employing the PQ-1 (a) and PQ-2 (b) based p-DSSCs prepared in screen-printing and spin-coating method.

CONCLUSIONS In this work, two new N-annulated perylene-based organic dyes (PQ-1 and PQ-2) have been designed and synthesized. We have studied the effects of the opposite positions of the CPDT-QA unit between the donor and acceptor parts on UV absorption, electrochemical properties and the


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performance of p-DSSCs with iodide/triiodide redox electrolyte. Also, TD-DFT calculations have been used to study the absorption and energy level characteristics of the dyes. And the theoretical calculations studies show good agreement with experimental observations, where the absorption bands are found to be dominated mainly by charge transfer excitations from the HOMO, HOMO-1 and HOMO-2 orbitals to the LUMO. It is worth nothing that PQ-2 based device shows both higher Jsc and Voc values despite that the PQ-1 exhibits a wider absorption band. Moreover, new NiO films were made by spin-coating a precursor solution of NiCl2 based on the triblock co-polymer F68 as template onto FTO glass with scotch tape on each side. Consequently, the IPCE values of PQ-1 / PQ-2 based p-DSSCs prepared by spin-coating method increased dramatically in comparison to that prepared by screen-printing method, owing to the improvement of hole transport rate, dye-loading amounts and transparency of the NiO films according to the IMPS and films absorption results, leading to the higher Jsc. EIS and theory calculation results indicate that molecular dipole moment approaching closer to the NiO surface will increase the energy gap between the quasi-Fermi level for holes in the NiO and that for electrons in the redox couple, which leads to a larger Voc. This work emphasizes the molecular engineering of the N-annulated perylene containing “push-pull” dyes in striking the balance between the spectral absorption range and the device performance. What’s more, our results demonstrate that the NiO films prepared in spin-coating method would represent efficient semiconductor for application in p-DSSCs and other organic optoelectronic devices (such as perovskite solar cells, hydrogen generation devices, etc.).


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ASSOCIATED CONTENT Supporting Information. Synthetic procedures and characterization data of all key intermediates and two target dyes. Preparation of NiO Films, fabrication and characterization of the devices. AUTHOR INFORMATION Corresponding Author [email protected] (J.H.); [email protected] (X.L.) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS For financial support of this research, we thank the Science Fund for Creative Research Groups (21421004) and NSFC/China (21172073, 21372082, 2116110444 and 91233207). X.L. and H.Å. acknowledge computational resources provided by the Swedish National Infrastructure for Computing (SNIC 2014/11-31).

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Enhanced Photocurrent Density by Spin-Coated NiO Photocathodes

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