Comparison of UV Irradiation and Sintering on Mesoporous

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Comparison of UV-Irradiation and Sintering on Mesoporous Spongelike ZnO Films Prepared from PS-b-P4VP Templated Sol-Gel Synthesis Kun Wang, Senlin Xia, Wei Cao, Nuri Hohn, Sebastian Grott, Lucas Kreuzer, Matthias Schwartzkopf, Stephan V. Roth, and Peter Muller-Buschbaum ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02039 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 18, 2018

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ACS Applied Nano Materials

Comparison of UV-Irradiation and Sintering on Mesoporous Sponge-like ZnO Films Prepared from PS-b-P4VP Templated Sol-Gel Synthesis Kun Wanga, Senlin Xiaa, Wei Cao a, Nuri Hohn a, Sebastian Grott a, Lucas P. Kreuzer a, Matthias Schwartzkopf b, Stephan V. Roth b,c, Peter Müller-Buschbaum a,d*

aLehrstuhl

für Funktionelle Materialien, Physik-Department, Technische Universität München,

James-Franck-Strasse 1, 85748 Garching, Germany bDeutsches cKTH

Elektronen-Synchrontron (DESY), Notkestrasse 85, 22603 Hamburg, Germany

Royal Institute of Technology, Department of Fibre and Polymer Technology,

Teknikringen 56-58, SE-100 44 Stockholm, Sweden dHeinz

Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Lichtenbergstr. 1,

85748 Garching, Germany

ABSTRACT Mesoporous ZnO films with large surface-area-to-volume-ratio show great promise in multiple applications, among which solid-state dye-sensitized solar cells (ssDSSCs) have attracted great attention in the field of photovoltaics. An appropriate mesopore size in the nanostructured ZnO films significantly plays an indispensable role in improving the device efficiency resulted from an efficient penetration of dye molecules and solid hole transport material. In the present work, mesoporous sponge-like ZnO films are prepared using sol-gel synthesis templated by a diblock

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copolymer polystyrene-block-poly(4-vinylpyridine). Two different template removal techniques, UV-irradiation and high temperature sintering, are used to compare their respective impact on the pore sizes of the final ZnO thin films. Both, the surface morphology and the inner morphology show that mesopores obtained via UV-irradiation are smaller as compared to their sintered counterparts. Moreover, increasing the template-to-ZnO precursor ratio is found to further enlarge present mesopores. Accordingly, a strong correlation between the pore sizes of sol-gel synthesized ZnO films and photovoltaic performance of fabricated ssDSSCs is demonstrated. In contrast with the devices fabricated from the UV-irradiated ZnO films, those obtained from sintered samples show more than two times higher efficiency.

KEYWORDS: UV-irradiation, sintering, mesopores, solid-state DSSCs, grazing-incidence small-angle X-ray scattering

1. INTRODUCTION Zinc oxide (ZnO) shows great promise for application in catalysis,1 sensors,2 drug delivery3 and optoelectronics4-5 due to a broad diversity of accessible morphologies and multiple functionalities. Nanostructured ZnO including nanorods,6 nanowires,7 nanospheres8 and nanoflowers9 are well fabricated. These structures in nanometer length scale provide a tremendous increase in surface-area-to-volume-ratio, especially for the nanostructures with mesoporous morphology, which is indispensable for application in a variety of fields. One of the most important applications for the nanostructured ZnO films with mesoporous morphology is dye-sensitized solar cells (DSSCs). One essential part of DSSCs in general is a


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mesoporous, inorganic semiconductor with a wide-bandgap, on which a monolayer of dye molecules is dispersed and brought into contact with a liquid electrolyte. As an n-type semiconductor with a wide bandgap, ZnO is generally used as an electron transport material (ETM), which is a promising alternative to titanium dioxide (TiO2), due to a higher carrier mobility and a lower crystallization temperature.5,

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Nevertheless, the majority of mesoporous

thin films used in DSSCs are TiO2 films and the efficiencies based on ZnO films are still far below those on TiO2 in literature due to the severe charge recombination at the interface of ZnO/dye/electrolyte and the corrosion of ZnO resulted from the reaction with the electrolyte solutions.10-12 For improving the photovoltaic performance of DSSCs based on ZnO films, ZnO with a mesoporous interconnected network structure is favorable to increase the interface area between ZnO and dye molecules and also to provide a good pathway for electron transport to the corresponding electrode, thus promoting the exciton separation and hindering the charge recombination. Moreover, to solve the corrosion problems, solid-state DSSCs (ssDSSCs) emerge as an attempt to replace conventional DSSCs. Instead of liquid electrolytes, ssDSSCs work on the basis of solid-state p-type conducting polymers acting as hole transport material (HTM). Thus, every layer of ssDSSC devices is in a solid state, which results in an improved stability and fabrication of these devices. Furthermore, ssDSSCs profit from a reduced weight due to less massive encapsulation, which diversifies potential applications.13 However, despite all efforts dedicated to ssDSSCs, their photovoltaic performance still remains modest and cannot reach that of conventional DSSCs. One major issue, which especially limits the device performance, originates from a poor backfilling of dye molecules and the HTM into the mesoscopic nanopores of the inorganic semiconductor. Consequently, only a poor electronic contact between dye molecules and charge carrier transport materials can be obtained. One way


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to overcome these challenges is to replace the conjugated polymers with conducting small molecules, which have fewer difficulties in penetrating pores.14-17 However, the availability of highly suitable small organic conductors appears to be limited as also within such approach the conventional DSSCs cannot be outperformed. Therefore, a precise control over the pore size of the mesoporous inorganic scaffolds tends to be a promising route towards obtaining highly efficient ssDSSCs. Establishing an optimum pore size is significant, since increased pore sizes have the tendency to enhance the backfilling capability, but reversely reduce the surface-area-tovolume-ratio, which is unfavorable for exciton separation at the nanostructured inorganic semiconductor and dye molecule interface. In the literature many different routes for preparing mesoporous inorganic semiconductor ZnO films were reported. Among these routes the sol-gel synthesis using small precursor molecules is very attractive since it is a wet chemical process, which could be compatible with many large scale deposition methods. In particular, the block copolymer assisted sol-gel synthesis of inorganic mesoporous films is very promising.18-21 Templating inorganic semiconductors by diblock copolymers turns out to be an efficient way to tune the pore size of the ETM for DSSCs.22-25 Via a diblock copolymer templating, various morphologies can be obtained, such as foam-like, worm-like and sphere-like structure.26 Many parameters can have an influence on the pore size, such as the weight ratio of inorganic semiconductors to the block copolymer used for templating,22 different weight fractions of the two blocks in the block copolymer,23-24 and solvent or thermal annealing processes applied to the films.25 However, as far as we know, no effort has been made to study the influence of different post-treatment techniques on the pore size of the final polymer-free inorganic films. In the literature, the strain changes of mesoporous ZnO structures after organic surfactant decomposition via UV-irradiation and thermal annealing were


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compared to accurately calculate the strain change in the ZnO crystallization process.27 In general, UV-irradiation and high temperature sintering are two commonly used methods for the diblock copolymer removal from polymer-metal oxide composite films prepared via sol-gel synthesis.24-25,

28-31

During UV-irradiation or sintering, the polymer template is removed. The

initial hybrid films turn into an inorganic one which has a mesoporous thin film morphology. By variation of the template removal method, different collapsed states of the inorganic thin films can be obtained, which can significantly influence the final morphology of the mesoporous thin film.31-35 However, so far the influence of the applied post-treatment method on the final morphology of the synthesized mesoporous ZnO films is only rarely reported. In the present work, mesoporous sponge-like ZnO films are prepared by sol-gel synthesis in combination

with

templating

with

the

diblock

copolymer

polystyrene-block-poly(4-

vinylpyridine) (PS-b-P4VP). In the obtained sponge-like structure, the interconnected network provides a good pathway for electron transport to the corresponding electrode. The partially ordered mesopores perpendicular to the substrate, lead to a good infiltration of the HTM into the ZnO scaffold, which is expected to result in more ordered polymer structures and consequently should give rise to a higher conductivity.36-37, Moreover, the mesoporous films are favorable to increase the interface area between p- and n-type materials after backfilling with the dye molecules which contributes to a better exciton separation. To remove the polymer template, two post-treatment methods, namely UV-irradiation and high temperature sintering are used to compare both the surface and inner morphology of the resulted samples systematically. These different post-treatments result in different pore sizes of the ZnO films. Moreover, upon variation of the weight ratio of the polymer template to the precursor of ZnO in the sol-gel synthesis, the pore size of the final ZnO films can be tuned. Scanning electron microscopy (SEM) and atomic


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force microscopy (AFM) measurements provide an evidence for increasing surface pore sizes with increasing template-to-precursor ratio. To investigate the pores buried in the films, grazingincidence small-angle X-ray scattering (GISAXS) is used. GISAXS reveals that the two posttreatment techniques have a similar effect on the inner pore sizes as found at the surface of the thin films. Furthermore, the influence of ZnO films with different pore sizes on the photovoltaic performance of ssDSSCs is investigated.

2. EXPERIMENTAL SECTION 2.1 Materials. Zinc acetate dihydrate (ZAD, 99.999% trace metals basis, density: 1.84 g cm-3), 5-[[4-[4-(2,2-diphenylethenyl)phenyl]-1,2,3,3a,4,8b-hexahydrocyclopent[b]indol-7yl]methylene]-2-(3-octyl-4-oxo-2-thioxo-5-hiazolidinylidene)-4-oxo-3-thiazolidineacetic

acid

(D205), N, N-dimethylformamide (DMF, 99.8%), ethanolamine (MEA, 98%), 2-methoxyethnal (99.8%) and chlorobenzene (99.8%) were purchased from Sigma-Aldrich. The diblock copolymer, polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP with Mn: 11.8-b-10.8 kg mol1),

was purchased from Polymer Source Inc., Canada, with a polydispersity of 1.12.

N2,N2,N2’,N2’,N7,N7,N7’,N7’-octakis(4-methoxyphenyl)-9,9’-spirobi[9H-fluorene]-2,2’,7,7’tetramine (spiro-OMeTAD) was purchased from Merck KGaA. All the materials were used as received. Glass or silicon (100) (with a thin layer of silicon dioxide layer on top) was pre-cleaned in an acid bath as described elsewhere.38 2.2 Preparation of Polymer/ZnO Composite Films. ZAD (120 mg) was dissolved in DMF (0.5 mL), stirred for 30 min and then filtered using 0.45 µm Teflon filters. Separately, different amounts of PS-b-P4VP (10 mg, 30 mg, 50 mg, and 70 mg) was dissolved in DMF (0.5 mL) and stirred for 30 min and then filtered using 0.45 µm Teflon filters, followed by adding MEA (32


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µL) to the PS-b-P4VP solution and stirring for additional 30 min, which leads to the formation of PS-b-P4VP micelles in the good-poor solvent pair in the mixture. Afterwards, the ZAD solution and the polymer solution were mixed together and then stirred for 1 h. During this period, ZnO sol-gel preferentially incorporated in the P4VP block. All the solutions were prepared and processed at ambient conditions. For film deposition, spin-coating was used on pre-cleaned silicon (100) or glass substrates using spin-coating parameters: 1000 rpm for 60 s. The obtained thin films were thermally annealed at 160 °C or 240 °C for 1h (160 °C for all the following samples except for those specially labeled). 2.3 Polymer Removal. To remove the template of PS-b-P4VP, two methods were applied. One was UV-irradiation for 24 h, which was proven to be an efficient way to remove the polymer template.31, 39 The other one was sintering, which was carried out at 400 °C for 30 min with a heating rate of 375 °C h-1. Afterwards, the mesoporous ZnO films were obtained. 2.4 Solar Cell Fabrication and Characterization. For fabricating ssDSSCs, a fluorine-doped tin oxide (FTO)-coated glass was used as a cathode, with part of the FTO etched with zinc powder and hydrochloric acid (HCl, 12 M) to form a desirable electrode. First, the etched FTO was cleaned with Alconox solution, ethanol, acetone and 2-propanol via ultrasonication and then followed by an oxygen plasma treatment for 10 min. A compact ZnO layer was deposited as a hole blocking layer in a way as stated in our previous work.26 For the nanoporous ZnO layer, the prepared films annealed at 160 °C with either UV-irradiation or sintering were performed as mentioned above. After being treated with oxygen plasma again, the samples were immediately immersed in D205 solution (0.3 mM D205 and 0.6 mM chenodeoxycholic acid in a mixture of acetonitrile and tert-butanol at a volume ratio of 1:1) for 20 h. Afterwards, the spiro-OMeTAD solution (320 mg of spiro-OMeTAD, 4 mL chlorobenzene, 114 μL of 4-tert-butylpyridine, and 70 μL of bis(trifluoromethane)sulfonamide lithium salt (Li-TFSI) stock solution (520 mg/mL in


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acetonitrile)) was spin-coated at 3000 rpm for 60 s. For the metal electrode, a thin layer of Au was thermally evaporated on top of the films.

2.5 Characterization and Measurements. X-ray diffraction (XRD) measurements were performed with a Bruker D8 ADVANCE X-ray diffractometer. A CuKα radiation was used to generate an X-ray with the wavelength of λ = 1.54 Å. Fourier-transform infrared (FT-IR) spectra were carried out with a Bruker Equinox FTIR spectrometer equipped with a DTGS detector. For SEM measurements, a Zeiss Gemini Ultra Plus field emission was used at an electron accelerating voltage of 3 kV. For AFM investigations, a MFD-3D AFM (Asylum Research) was used in a tapping mode. The used tip, with a curvature radius of 7 nm, was mounted onto a cantilever (OMCL-AC240TS-R3, Asylum Research). GISAXS measurements were carried out at the P03 beamline40 of the PETRA III storage ring at DESY (Hamburg, Germany) with the wavelength of 0.954 Å. A Pilatus 1M (Dectris Ltd.; pixel size of 172 µm × 172 µm, 981 by 1043 array) was used to detect the scattered signal. To investigate the desirable mesopore sizes, a sample-detector distance of 4.3 m was used. An incident angle of 0.35° was chosen, which is above the critical angle of ZnO and thereby enables the probing of the inner structure. Transmittance was measured using a Lambda 35 UV/Vis spectrometer from PerkinElmer. Film thicknesses were investigated by Bruker DektakXT stylus profiler. To characterize the photovoltaic performance, a simulated AM1.5 solar illumination (100 mW cm−2, Solar Constant by K. H. Steuernagel Lichttechnik GmbH) was used for illumination and a Keithley 2400 sourcemeter was used to record the J-V curves. Electrochemical impedance spectra (EIS) measurements were performed with an electrochemical workstation under the same illumination as above at open-circuit potential.


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3. RESULTS AND DISCUSSION To prepare mesoporous ZnO films, a sol-gel synthesis is carried out using PS-b-P4VP as a template (Figure 1). In the sol-gel synthesis, N, N-dimethylformamide (DMF) is used as a good solvent, which efficiently dissolves the zinc precursor zinc acetate dihydrate (ZAD) (Figure 1a) as well as both blocks of the polymer template (PS and P4VP blocks, Figure 1b). In contrast, ethanolamine (MEA) acts as a poor solvent for the PS block and is a selective solvent for the P4VP block. Micelles in solution with hydrophobic PS cores and hydrophilic P4VP coronas are formed with ZAD preferentially incorporating in the P4VP block (Figure 1c). After spin-coating the solution onto a cleaned silicon (100) or glass substrate (Figure 1d), annealing is performed to improve the self-assembly of the nanostructure (Figure 1e). High temperature sintering (Figure 1f) and UV-irradiation (Figure 1g) are used to efficiently remove the polymer template and to obtain mesoporous ZnO films (Figure 1h).24-25, 28-31, 41 Fourier-transform infrared spectroscopy (FT-IR) is used to determine the residual PS-b-P4VP after template removal. It is shown in Figure S1 that the polymer template is removed completely for ZnO films treated with either sintering or UV-irradiation as compared with the ZnO/PS-b-P4VP composite films before templet removal. A more detailed description about the features in the FT-IR spectra is given in the Supporting Information. The respectively obtained morphologies of the polymer-free mesoporous ZnO films are discussed with focus on surface and inner structures. The influence of the post-treatment method and the polymer template-to-ZAD ratio on the structure of the mesoporous ZnO films as well as on the photovoltaic performance of the corresponding ssDSSCs are shown below.


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Figure 1. Schematic representation of the sol-gel synthesis in combination with templating with PS-b-P4VP to prepare ZnO/PS-b-P4VP composite thin films, followed by post-treatment (high temperature sintering or UV-irradiation) to remove the polymer template for gaining mesoporous ZnO films.

3.1 Influence on ZnO film structure. To investigate the crystal phase of the ZnO films, XRD data of ZnO/PS-b-P4VP and mesoporous ZnO films post-treated with sintering and UV-irradiation are shown in Figure S2. The peaks (labeled by diamonds in the range of 30 deg. to 37 deg.) indicate the crystal planes of the wurtzite phase. The presence of the broad XRD peaks for the ZnO/PS-b-P4VP samples indicates the formation of wurtzite ZnO for films annealed at 160 °C.42-43 After UV-irradiation treatment, no difference is observed in the XRD data, suggesting that the crystallization is not improved by UV-irradiation treatment. The peaks for the sintered ZnO films show a narrower full width at


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half maximum than those observed for the UV-irradiated samples, indicating a better crystallization for the sintered films. The surface morphologies of ZnO films are detected with SEM measurement (Figure 2). All ZnO films show a sponge-like structure, independently of the applied template-removal method. This structure provides a large surface-area-to-volume-ratio, which favors the adsorption of the dye molecules onto the surface of the ZnO domains. Moreover, the interconnected network structure promotes the charge carrier transport along the pathway of the mesoporous ZnO layer. For the sintered ZnO films, the pore sizes increase when increasing the ratio of the polymer template to ZAD (ωPS-b-P4VP:ωZAD) from 1:12 to 7:12. With the ratio increasing, small pores grow to larger ones and more mesopores tend to interconnect, preferentially forming larger pores (about 140 nm for the sample of 5:12 and 170 nm for 7:12, marked by red boxes in Figure 2c and 2d). The same tendency is observed for the films treated with UV-irradiation. Moreover, the pore sizes are found to be smaller for the UV-irradiated films as compared to the sintered counterparts. Despite that the small pores of UV-irradiated films become larger at high ratios (some pore sizes reach about 50 nm for sample 5:12 and 120 nm for 7:12, respectively, as marked by red boxes in Figure 2g and 2h), they are still smaller than the sintered counterparts. When the template is removed from the films, the initial ZnO scaffold will inevitably collapse which leads to decreased pore sizes.24-25, 31 Although the ZnO films treated by these two methods show similar sponge-like structure, the pores of UV-irradiated films are smaller than the sintered counterparts, which indicates the more serious collapse of UV-irradiated samples when the template is extracted from the composite films. An incomplete template removal via UVillumination can be ruled out according to the reported work.26, 31


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Figure 2. SEM images of mesoporous ZnO films with polymer removal via (a-d) sintering and (e-h) UV-irradiation with different weight ratios of polymer template to ZAD (ωPS-b-P4VP:ωZAD): (a, e) 1:12, (b, f) 3:12, (c, g) 5:12, (d, h) 7:12. The red rectangles in (c), (d), (g), (h) indicate the presence of connected pores.


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AFM measurements are used to further investigate the surface morphology of the ZnO films after block copolymer template removal. Figure 3 shows the topography of mesoporous ZnO films post-treated by sintering and UV-irradiation. The mesoporous structure is clearly observed in all the AFM images. With increasing the template-to-ZAD ratio, the pore sizes of ZnO films become larger no matter which post-treatment is used. Compared with the sintered films, the UV-irradiated samples appear to be more compact with an appearance of smaller pore sizes. These observations are consistent with the SEM results. As discussed above, the smaller pore size of the UV-irradiated samples may be resulted from the more serious collapse of the ZnO scaffold. To further illustrate changes of the morphology when increasing the template-to-ZAD ratio, the surface morphology of ZnO films annealed at 240 °C is shown in Figures S3 and S4 and a detailed description is given in the Supporting Information.


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Figure 3. AFM images of mesoporous ZnO films with polymer removal via (a-d) sintering and (e-h) UV-irradiation with different weight ratios of polymer template to ZAD (ωPS-b-P4VP:ωZAD): (a, e) 1:12, (b, f) 3:12, (c, g) 5:12, (d, h) 7:12.

Investigating the nanostructure buried inside the ZnO films is also of critical importance, as the inner morphology can be different from that at the surface.44-45 Specifically, for ssDSSCs, an inner porous structure is absolutely necessary to allow the dye molecules and HTM to be backfilled into the pores of ZnO films. Accordingly, GISAXS measurements are performed at an incident angle of 0.35°, which is larger than the critical angle of the ZnO. As a result, the X-rays can penetrate the whole ZnO thin film and detect potential buried nanostructures with statistical significance.46 The 2D GISAXS data of the ZnO films prepared from four different ratios via both, high temperature sintering and UV-irradiation, are shown in Figure 4. All recorded 2D GISAXS data show two prominent Bragg scattering rods along the qz direction, which is a strong indication for the presence of an ordered structure perpendicular to the substrate. The partially vertically oriented ZnO structure is favorable for electron transport to the transparent electrode.47 Moreover, the partially vertically oriented mesopores also help to form ordered structure for the HTMs after backfilling, which favors to improve the conductivity.36-37 This is expected to favor the charge transport to the corresponding electrode when using it in ssDSSCs. Independent of the method used for polymer template removal, the position of the Bragg scattering rods tends to shift to a lower qy value when increasing the template-to-ZAD ratio. This observation proves, that both, surface and inner morphology, show the similar tendency to form larger structures with increasing the template-to-ZAD ratio.


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Figure 4. 2D GISAXS data of mesoporous ZnO films with polymer removal via (a-d) sintering and (e-h) UV-irradiation with different weight ratios of polymer template to ZAD (ωPS-bP4VP:ωZAD):

(a, e) 1:12, (b, f) 3:12, (c, g) 5:12, (d, h) 7:12. The specular peak is shielded by a

beamstop. The black dashed arrow in (a) indicates the Yoneda peak position where horizontal line cuts for all the scattering patterns are made.

For a quantitative analysis, horizontal line cuts of the 2D GISAXS data are performed along the Yoneda peak which is a high intensity peak at the critical angle of the respective material (marked by the black dashed line in Figure 4a).48 Information of characteristic length scales parallel to the substrate can be gained through fitting these horizontal line cuts (Figure 5a and


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5c). Figure 5a shows the horizontal line cuts of ZnO films post-treated by sintering. A prominent peak at around 0.2 nm-1 is observed (marked by a blue box in Figure 5a) and it moves to lower qy values, which indicates the formation of larger center-to-center distances. For the ZnO films treated by UV-irradiation, a similar trend for the length scales is observed. The horizontal line cuts are fitted within the effective interface approximation of the Distorted-Wave Born Approximation (DWBA).49-52 A cylindrical geometry is assumed to represent the form factor of the ZnO domains with a Gaussian distribution of the cylinder radii. The structure factors describes the center-to-center distances of two neighboring cylinders based on a 1D paracrystalline model. Figure S5 sketches this used model. Thus, the average pore sizes in the ZnO films can be calculated by the difference of center-to-center distance with the diameter of the domains via modelling the cuts as explained in the Supporting Information.25 Three sizes of cylinders are needed to fit the horizontal line cuts within the local monodisperse approximation (LMA). More details on the GISAXS data modeling are given in the Supporting Information. The fitted curves are shown with red lines in Figure 5a and 5c. The average sizes of small and middle pores are extracted and plotted in Figure 5b (sintering) and Figure 5d (UV-irradiation) due to their more important role in ssDSSCs as compared with the large pores. As described above, when increasing the template-to-ZAD ratio, more mesopores tend to interconnect to form larger pores (labeled by the red rectangles in Figure 2). Therefore, the middle pores obtained from the GISAXS fitting correspond to the interconnected pores. It is shown in Figures 5b and 5d that the sizes of the small and middle pores from the GISAXS fitting are consistent with those of the small and interconnected pores observed in the SEM images, respectively. As a general trend, when increasing the ratio of the template, the small- and middle-sized pores become larger independently of the post-treatment method. Moreover, the small-sized pores in the sintered


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films are larger than the corresponding ones with UV-treatment and the same trend is observed for the middle-sized pores. This trend in the pore sizes coincides with that observed via SEM and AFM, which means that the inner and surface structures follow the same trends.

Figure 5. Horizontal line cuts obtained from the 2D GISAXS measurements for ZnO films with different ratios of polymer template to ZAD with ωPS-b-P4VP:ωZAD=1:12, 3:12, 5:12, and 7:12 from bottom to top. The polymer is removed by (a) sintering, and (c) UV-irradiation. The solid red lines indicate the fits to the data. Curves are shifted along the intensity axis for clarity of the presentation. Average pore size of mesoporous ZnO films with polymer removal via (b) sintering


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and (d) UV-irradiation extracted from the fits are plotted as a function of the ratio of template to ZAD. Triangles indicate small-sized pores (blue) and circles middle-sized pores (red). The dashed lines in (b) and (d) are guides to the eye.

The film collapse upon polymer template removal can also be investigated by the change of film thickness (Figure 6). With increasing the ratio of the template, the thickness of ZnO films increases obviously. This indicates that the amount of diblock copolymer has a great influence on the film thickness, despite the total amount of used ZAD is constant. Besides, the thickness of the ZnO films prepared with sintering and UV-irradiation is different. In brief, the sintered films are thicker than the corresponding UV-irradiated ones. This indicates that the more severe collapse via UV-irradiation leads to thinner films compared with sintering treatment.

Figure 6. Film thickness of nanoporous ZnO films plotted as a function of the ratio of polymer template to ZAD. The dashed lines are guides to the eye.


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To illustrate the pore size tuning of the ZnO films via the applied post-treatment method, a schematic representation of the mesoporous ZnO films is shown in Figure 7. Upon introducing the good- poor solvent pair to the diblock copolymer the micro-phase separation process happens, which leads to the formation of the polymer micelles with precursor molecules enriched in one block of the polymer template.20, 53-54 With increasing the template-to-ZAD ratio, more polymer chains aggregate to form larger micelles, resulting in larger core sizes of the micelles. As the ZAD molecules prefer being incorporated in the P4VP block, the mobility of the deposited films is hindered to a large extent during self-assembly in the annealing process.55 After polymer removal, the pores show similar changing tendency when increasing the ratio of the diblock copolymer. Increased pore sizes are observed when the ratio increases from ωPS-b-P4VP:ωZAD=1:12 to 7:12. For the two different polymer removal methods, UV-irradiation and sintering, the process of polymer degradation is different and complicated. For thermal degradation, two main processes, namely material stabilization at low temperatures and actual degradation at high temperatures, take place.56 In the pre-sintering process, the low temperature increase rate helps to stabilize the films, thereby decreasing the collapse of the ZnO films.27 However, under UVirradiation, photooxidative degradation directly occurs, which results in breaking of the polymer chains into free radicals and small molecules.57 Therefore, a more serious collapse and smaller pore sizes are observed in the final UV-irradiated ZnO films. UV-Vis measurement is a useful tool to detect the transparency and the band gap of the ZnO films. Figure S6 shows the transmittance of the ZnO films with different template-to-ZAD ratios post-treated by high temperature sintering (Figure S6a) or UV-irradiation (Figure S6b). All the ZnO films show high transmittance in the visible region, which makes it possible for the


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adsorbed dye molecules to absorb more photons. For the direct band gap of the wurtzite phase, which is confirmed in our earlier work,26, 58 the Tauc’s equation is used αhʋ = A(hʋ-Eg)n

(1)

where A is a constant, hʋ is the photon energy, Eg is the allowed band gap and n = 1/2 is for direct transition.59 Figure S6c and 6d show the curves by plotting (αhʋ)2 as a function of photon energy and the corresponding linear fits (dashed lines) at the absorption edge. All the ZnO films show a similar band gap of around 3.3 eV, which is similar to the reported value of ZnO.60-62

Figure 7. Schematic illustration of the nanostructures formed by ZnO/PS-b-P4VP micelles in solution (top row) and in the mesoporous ZnO films treated via sintering (middle row) and UVirradiation (bottom row) for different polymer ratios.

3.2 Influence on photovoltaic performance. In order to investigate the influence of the mesopore size of the sponge-like structure on the photovoltaic performance of the complete devices, ssDSSCs based on ZnO films with different


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pore sizes are fabricated. A device layout is shown in Figure 8a. It is: FTO/compact ZnO/mesoporous

ZnO/indoline

dye

D205/N2,N2,N2’,N2’,N7,N7,N7’,N7’-octakis(4-

methoxyphenyl)-9,9’-spirobi[9H-fluorene]-2,2’,7,7’-tetramine (spiro-OMeTAD)/Au. A SEM cross-section image of the corresponding ssDSSC is shown in Figure S7. Figure 8b depicts the current density–voltage characteristics (J-V curves) of ssDSSCs based on sintered ZnO films prepared from different ratios of the template and ZAD molecules. The obtained photovoltaic parameters are shown in Figure S8. Although the obtained overall efficiency is not as high as those reported, which is mainly due to the much thinner ZnO films used,63-66 the trend in the photovoltaic performance can be well correlated with the ZnO films discussed above. In Figure S8, the open-circuit voltage (Voc) raises from 0.44 ± 0.01 eV to 0.66 ± 0.01 eV when increasing the ratio (ωPS-b-P4VP:ωZAD) from 1:12 to 7:12, which indicates that larger pore sizes and thicker films may promote more effective adsorption of dye molecules in the scaffold, and thus contributes to the improvement of Voc to some extent.67-68 The power conversion efficiency (PCE) first increases and then decreases, reaching a highest PCE around 0.47 % for the samples of ωPS-b-P4VP:ωZAD=5:12. The short current density (Jsc) changes with the same trend. With increasing the ratio from 1:12 to 5:12, the pore sizes increase, which enhances the possibility for dye molecules and HTM to be backfilled into the ZnO films and thereby improves the connection at the interfaces of ZnO/dye/HTM. Moreover, the increased ZnO film thickness also promotes to improve the amount of dye molecules adsorbed at the interface of mesoporous ZnO films, thereby increasing the generated free charge carrier density.67 However, ZnO films (with the ratio of 7:12) with too large pores show a negative effect on the photovoltaic performance parameters (Jsc, FF and PCE) in spite of the thicker ZnO film, which might result from the decreased interface areas between dye molecules with ZnO and HTM. Besides, it also indicates


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that the influence of the ZnO pore sizes on the photovoltaic performance is more noticeable than the film thickness. Figure 8c shows the J-V curves of photovoltaic devices utilizing two different post-treated ZnO films with the ratio of ωPS-b-P4VP:ωZAD=5:12. The PCE values of the devices from sintered ZnO films are much higher (0.47 ± 0.01 %) than those from UV-irradiated ZnO films (0.2 ± 0.04 %), which mainly originates from the improvement of Jsc. In this case the devices from sintered ZnO films exhibit a Jsc, which is more than twice of those from UVirradiated ZnO films. The detailed photovoltaic parameters are displayed in Table S1. The improved photovoltaic performance may be explained by a better penetration of dye molecules and HTM. Therefore, a modified contact of ZnO/dye/spiro-OMeTAD interfaces can be obtained which is induced from the larger pore sizes of the sintered ZnO films compared with those of the UV-irradiated samples as indicated by both surface and inner morphology. Apart from this, the better crystallization of the sintered ZnO films, indicated from the sharper XRD peaks (Figure S2), is expected to decrease the charge traps in the semiconductor oxide. This favors the speed up of the charge transport process, thereby decreasing the possibility of exciton recombination at the ZnO/dye interface and leading to a better photovoltaic performance of the ssDSSCs.


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Figure 8. (a) Schematic representation of the used ssDSSC device geometry; (b) Current density– voltage characteristics of ssDSSCs based on sintered mesoporous ZnO films prepared from different template-to-ZAD ratios; (c) Current density–voltage characteristics of ssDSSCs based on different post-treated nanoporous ZnO films prepared from ωPS-b-P4VP:ωZAD=5:12; (d) Stability of ssDSSCs based on sintered mesoporous ZnO films prepared from ωPS-b-P4VP:ωZAD=5:12. The dashed lines are guides to the eye.

To compare the kinetics of the interfacial charge carrier transfer process, electrochemical impedance spectra (EIS) measurements are performed (Figure S9). It is observed that the ssDSSCs based on sintered ZnO films present a smaller charge transfer resistance (Rct) which is attributed from the charge transfer process at the interface of ZnO/dye/HTM when compared with that of the UV-irradiated ZnO based ssDSSCs. The smaller charge carrier resistance demonstrates a lower possibility of the hole-electron recombination action in the ssDSSCs


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fabricated from the sintered ZnO films. Figure S9b shows the Bode phase plot of the EIS spectra for the ssDSSCs based on sintered and UV-irradiated ZnO films. The characteristic frequency peak of the sintered samples tends to shift to lower frequency in contrast with that for the UVirradiated samples. Therefore, the electron life time for the recombination (τ) of ssDSSCs fabricated from sintered ZnO films is larger than that from UV-irradiated films due to the following equation 1

1

(2)

τ = 𝜔𝑚𝑎𝑥 = 2𝜋𝑓𝑚𝑎𝑥

where 𝑓𝑚𝑎𝑥 is the corresponding characteristic low-frequency peaks.69 It implies that the electron recombination can be reduced for the ssDSSCs based on the sintered samples, therefore contributing to the improvement of the photovoltaic performance. For photovoltaic devices, the long-term stability is of great significance as it is required for solar cells to generate electricity over a long period. Figure S10 and Figure 8d show the J-V curves measured after several tens of days and the stability of the photovoltaic parameters obtained from the J-V curves of the ssDSSC based on sintered ZnO films prepared from the ratio of ωPS-bP4VP:ωZAD=5:12,

respectively. Even though all steps (including fabrication, storage and

characterization) of the ssDSSCs are conducted under ambient air conditions, it is observed that the Voc is stable over time. Moreover, despite the fluctuation of PCE, still 92.6 % of the initial efficiency remains after 80 days. The fluctuation in the PCE coincides with those of Jsc and FF. The initial increased PCE might originate from the improved penetration of the HTM into the complete ZnO films and the activation of the entire electrode.70

4. CONCLUSION


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In this work, mesoporous sponge-like ZnO films templated with the diblock copolymer PS-bP4VP are prepared via sol-gel synthesis. We demonstrate that the mesopore sizes of the ZnO films are influenced by the applied post-treatment methods, namely UV-irradiation and high temperature sintering. SEM and AFM reveal that ZnO films with UV-irradiation show smaller pore sizes at the surface as compared with those treated with sintering. Furthermore, GISAXS investigation on the inner film morphology also shows smaller pore sizes for the UV-irradiated ZnO films. The more serious collapse of UV-irradiated samples might be the main reason for this as seen in the change in film thickness. Moreover, the surface and inner morphology show that the pore sizes of ZnO films increase with increasing the ratio of the template to the ZnO precursor. Apart from the pore size of the ZnO films, XRD data reveal a better crystallization of the wurtzite ZnO phase for the sintered films than the UV-irradiated samples. UV-Vis measurements indicate a band gap of 3.3 eV for the prepared ZnO films, demonstrating the successful preparation of semiconductive ZnO with a suitable band gap for an electron transport layer. Accordingly, the influence of the mesopore size of the ZnO films on the photovoltaic performance of fabricated ssDSSCs illustrates a great correlation of the structure and the photovoltaic performance. With increasing pore sizes of the sintered ZnO films, the efficiency of the corresponding devices first increases and then decreases, showing an optimal efficiency at the ratio of ωPS-b-P4VP:ωZAD=5:12. The devices based on the sintered films show a better photovoltaic performance, which might be due to the larger mesopore size and a better crystallization as compared with those based on UV-irradiated samples. Besides, ssDSSCs based on the optimal pore sizes show a good long-term stability, with only slight loss even after 80 days at ambient condition.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +49 (0)89 289 12473. Tel: +49 (0)89 289 12451. Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by funding from TUM.solar in the context of the Bavarian Collaborative Research Project “Solar Technologies Go Hybrid” (SolTech), International Research Training Groups 2022 Alberta/Technical University of Munich International Graduate School for Environmentally Responsible Functional Hybrid Materials (ATUMS) and the Nanosystems Initiative Munich (NIM). K.W., S.X. and W.C acknowledge the China Scholarship Council (CSC). The authors thank Yu Tong from Ludwig-Maximilians-Universität München for helping with the SEM measurements as well as Professor Alexander Holleitner and Peter Weiser for the access to AFM measurements. Portions of this research were carried out at the synchrotron light source PETRA III at DESY. DESY is a member of the Helmholtz Association (HGF).

REFERENCES 1. Evgenidou, E.; Fytianos, K.; Poulios, I. Semiconductor-sensitized photodegradation of dichlorvos in water using TiO2 and ZnO as catalysts. Appl. Catal. B 2005, 59, 81-89. 2. Fan, Z.; Lu, J. G. Gate-refreshable nanowire chemical sensors. Appl. Phy. Lett. 2005, 86, 123510.


26

Page 27 of 31 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


3. Martínez-Carmona, M.; Gun’ko, Y.; Vallet-Regí, M. ZnO Nanostructures for drug delivery and theranostic applications. Nanomaterials 2018, 8, 268. 4. Liang, F.-X.; Gao, Y.; Xie, C.; Tong, X.-W.; Li, Z.-J.; Luo, L.-B. Recent advances in the fabrication of graphene–ZnO heterojunctions for optoelectronic device applications. J. Mater. Chem. C 2018, 6, 3815-3833. 5. Lin, C.-Y.; Lai, Y.-H.; Chen, H.-W.; Chen, J.-G.; Kung, C.-W.; Vittal, R.; Ho, K.-C. Highly efficient dye-sensitized solar cell with a ZnO nanosheet-based photoanode. Energy Environ. Sci. 2011, 4, 3448-3455. 6. Yao, Q.; Wang, C.; Fan, B.; Wang, H.; Sun, Q.; Jin, C.; Zhang, H. One-step solvothermal deposition of ZnO nanorod arrays on a wood surface for robust superamphiphobic performance and superior ultraviolet resistance. Sci. Rep. 2016, 6, 35505. 7. Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nanowire dye-sensitized solar cells. Nat. Mater. 2005, 4, 455. 8. Zhang, Y.; Chung, J.; Lee, J.; Myoung, J.; Lim, S. Synthesis of ZnO nanospheres with uniform nanopores by a hydrothermal process. J. Phys. Chem. Solids 2011, 72, 1548-1553. 9. Fang, B.; Zhang, C.; Zhang, W.; Wang, G. A novel hydrazine electrochemical sensor based on a carbon nanotube-wired ZnO nanoflower-modified electrode. Electrochim. Acta 2009, 55, 178-182. 10. Tian, J.; Zhang, Q.; Zhang, L.; Gao, R.; Shen, L.; Zhang, S.; Qu, X.; Cao, G. ZnO/TiO2 nanocable structured photoelectrodes for CdS/CdSe quantum dot co-sensitized solar cells. Nanoscale 2013, 5, 936-943. 11. Wu, D.; Gao, Z.; Xu, F.; Chang, J.; Tao, W.; He, J.; Gao, S.; Jiang, K. Hierarchical ZnO aggregates assembled by orderly aligned nanorods for dye-sensitized solar cells. CrystEngComm 2013, 15, 1210-1217. 12. Ai, X.; Anderson, N. A.; Guo, J.; Lian, T. Electron injection dynamics of Ru polypyridyl complexes on SnO2 nanocrystalline thin films. J. Phys. Chem. B 2005, 109, 7088-7094. 13. Chung, I.; Lee, B.; He, J.; Chang, R. P. H.; Kanatzidis, M. G. All-solid-state dyesensitized solar cells with high efficiency. Nature 2012, 485, 486-489. 14. Hagen, J.; Schaffrath, W.; Otschik, P.; Fink, R.; Bacher, A.; Schmidt, H.-W.; Haarer, D. Novel hybrid solar cells consisting of inorganic nanoparticles and an organic hole transport material. Synth. Met. 1997, 89, 215-220. 15. Senadeera, G. K. R.; Jayaweera, P. V. V.; Perera, V. P. S.; Tennakone, K. Solid-state dye-sensitized photocell based on pentacene as a hole collector. Sol. Energy Mater. Sol. Cells 2002, 73, 103-108. 16. Salbeck, J.; Yu, N.; Bauer, J.; Weissörtel, F.; Bestgen, H. Low molecular organic glasses for blue electroluminescence. Synth. Met. 1997, 91, 209-215. 17. Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weissörtel, F.; Salbeck, J.; Spreitzer, H.; Grätzel, M. Solid-state dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies. Nature 1998, 395, 583-585. 18. Orilall, M. C.; Wiesner, U. Block copolymer based composition and morphology control in nanostructured hybrid materials for energy conversion and storage: solar cells, batteries, and fuel cells. Chem. Soc. Rev. 2011, 40, 520-535. 19. Guldin, S.; Huttner, S.; Tiwana, P.; Orilall, M. C.; Ulgut, B.; Stefik, M.; Docampo, P.; Kolle, M.; Divitini, G.; Ducati, C.; Redfern, S. A. T.; Snaith, H. J.; Wiesner, U.; Eder, D.; Steiner, U. Improved conductivity in dye-sensitised solar cells through block-copolymer confined TiO2 crystallisation. Energy Environ. Sci. 2011, 4, 225-233.


27


Page 28 of 31

20. Cheng, Y.-J.; Gutmann, J. S. Morphology phase diagram of ultrathin anatase TiO2 films templated by a single PS-b-PEO block copolymer. J. Am. Chem. Soc. 2006, 128, 4658-4674. 21. Rawolle, M.; Niedermeier, M. A.; Kaune, G.; Perlich, J.; Lellig, P.; Memesa, M.; Cheng, Y.-J.; Gutmann, J. S.; Müller-Buschbaum, P. Fabrication and characterization of nanostructured titania films with integrated function from inorganic-organic hybrid materials. Chem. Soc. Rev. 2012, 41, 5131-5142. 22. Liu, Z.; Li, Y.; Zhao, Z.; Cui, Y.; Hara, K.; Miyauchi, M. Block copolymer templated nanoporous TiO2 for quantum-dot-sensitized solar cells. J.Mater. Chem. 2010, 20, 492-497. 23. Lim, J. Y.; Lee, C. S.; Lee, J. M.; Ahn, J.; Cho, H. H.; Kim, J. H. Amphiphilic blockgraft copolymer templates for organized mesoporous TiO2 films in dye-sensitized solar cells. J. Power Sources 2016, 301, 18-28. 24. Sarkar, K.; Rawolle, M.; Niedermeier, M. A.; Wang, W.; Herzig, E. M.; Korstgens, V.; Buffet, A.; Roth, S. V.; Müller-Buschbaum, P. A quantitative approach to tune metal oxide network morphology based on grazing-incidence small-angle X-ray scattering investigations. J. Appl. Crystallogr. 2014, 47, 76-83. 25. Sarkar, K.; Schaffer, C. J.; Gonzalez, D. M.; Naumann, A.; Perlich, J.; MüllerBuschbaum, P. Tuning the pore size of ZnO nano-grids via time-dependent solvent annealing. J. Mater. Chem. A 2014, 2, 6945-6951. 26. Wang, K.; Körstgens, V.; Yang, D.; Hohn, N.; Roth, S. V.; Müller-Buschbaum, P. Morphology control of low temperature fabricated ZnO nanostructures for transparent active layers in all solid-state dye-sensitized solar cells. J. Mater. Chem. A 2018, 6, 4405-4415. 27. Hong, M.-H.; Shim, D. I.; Cho, H. H.; Park, H.-H. Effect of mesopore-induced strain/stress on the thermoelectric properties of mesoporous ZnO thin films. Appl. Surf. Sci. 2018, 446, 160-167. 28. li, X.; Fu, J.; Steinhart, M.; Ha Kim, D.; Knoll, W. Au/titania composite nanoparticle arrays with controlled size and spacing by organic-inorganic nanohybridization in thin film block copolymer templates. Bull. Korean Chem. Soc. 2007, 28, 1015-1020. 29. Haseloh, S.; Choi, S. Y.; Mamak, M.; Coombs, N.; Petrov, S.; Chopra, N.; Ozin, G. A. Towards flexible inorganic "mesomaterials": one-pot low temperature synthesis of mesostructured nanocrystalline titania. Chem. Commun. 2004, 1460-1461. 30. Sun, Z.; Kim, D. H.; Wolkenhauer, M.; Bumbu, G. G.; Knoll, W.; Gutmann, J. S. Synthesis and photoluminescence of titania nanoparticle arrays templated by block-copolymer thin films. ChemPhysChem 2006, 7, 370-378. 31. Song, L.; Abdelsamie, A.; Schaffer, C. J.; Körstgens, V.; Wang, W.; Wang, T.; Indari, E. D.; Fröschl, T.; Hüsing, N.; Haeberle, T.; Lugli, P.; Bernstorff, S.; Müller-Buschbaum, P. A low temperature route toward hierarchically structured titania films for thin hybrid solar cells. Adv. Funct. Mater. 2016, 26, 7084-7093. 32. Choi, S. Y.; Mamak, M.; Speakman, S.; Chopra, N.; Ozin, G. A. Evolution of nanocrystallinity in periodic mesoporous anatase thin films. Small 2005, 1, 226-232. 33. Stefik, M.; Song, J.; Sai, H.; Guldin, S.; Boldrighini, P.; Orilall, M. C.; Steiner, U.; Gruner, S. M.; Wiesner, U. Ordered mesoporous titania from highly amphiphilic block copolymers: tuned solution conditions enable highly ordered morphologies and ultra-large mesopores. J. Mater. Chem. A 2015, 3, 11478-11492. 34. Fattakhova-Rohlfing, D.; Zaleska, A.; Bein, T. Three-dimensional titanium dioxide nanomaterials. Chem. Rev. 2014, 114, 9487-9558.


28

Page 29 of 31 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


35. Song, L.; Wang, W.; Körstgens, V.; González, D. M.; Yao, Y.; Minar, N. K.; Feckl, J. M.; Peters, K.; Bein, T.; Fattakhova-Rohlfing, D.; Santoro, G.; Roth, S. V.; Müller-Buschbaum, P. Spray deposition of titania films with incorporated crystalline nanoparticles for all-solid-state dye-sensitized solar cells using P3HT. Adv. Funct. Mater. 2016, 26, 1498-1506. 36. Coakley, K. M.; Srinivasan, B. S.; Ziebarth, J. M.; Goh, C.; Liu, Y.; McGehee, M. D. Enhanced hole mobility in regioregular polythiophene infiltrated in straight nanopores. Adv. Funct. Mater. 2005, 15, 1927-1932. 37. Nilsson, E.; Furusho, H.; Terasaki, O.; Palmqvist, A. E. C. Synthesis of nanoparticulate anatase and rutile crystallites at low temperatures in the Pluronic F127 microemulsion system. J.Mater. Res. 2011, 26, 288-295. 38. Müller-Buschbaum, P. Influence of surface cleaning on dewetting of thin polystyrene films. Eur. Phys. J. E 2003, 12, 443-448. 39. Cummins, C.; Bell, A. P.; Morris, M. A. Creating active device materials for nanoelectronics using block copolymer lithography. Nanomaterials 2017, 7, 304:1-12. 40. Buffet, A.; Rothkirch, A.; Dohrmann, R.; Korstgens, V.; Abul Kashem, M. M.; Perlich, J.; Herzog, G.; Schwartzkopf, M.; Gehrke, R.; Müller-Buschbaum, P.; Roth, S. V. P03, the microfocus and nanofocus X-ray scattering (MiNaXS) beamline of the PETRA III storage ring: the microfocus endstation. J. Synchrotron Radiat. 2012, 19, 647-653. 41. Cummins, C.; Gangnaik, A.; Kelly, R. A.; Borah, D.; O'Connell, J.; Petkov, N.; Georgiev, Y. M.; Holmes, J. D.; Morris, M. A. Aligned silicon nanofins via the directed selfassembly of PS-b-P4VP block copolymer and metal oxide enhanced pattern transfer. Nanoscale 2015, 7, 6712-6721. 42. Khan, M. F.; Ansari, A. H.; Hameedullah, M.; Ahmad, E.; Husain, F. M.; Zia, Q.; Baig, U.; Zaheer, M. R.; Alam, M. M.; Khan, A. M.; AlOthman, Z. A.; Ahmad, I.; Ashraf, G. M.; Aliev, G. Sol-gel synthesis of thorn-like ZnO nanoparticles endorsing mechanical stirring effect and their antimicrobial activities: Potential role as nano-antibiotics. Sci. Rep. 2016, 6, 27689. 43. M. El-Agez, T.; El Tayyan, A.; Al-Kahlout, A.; Taya, S.; abdel-latif, M., Dye-sensitized solar cells based on ZnO films and natural dyes. Int. J. Mater. Chem. 2012, 2, 105-110. 44. Einollahzadeh-Samadi, M.; Dariani, R. S.; Paul, A. Tailoring morphology, structure and photoluminescence properties of anodic TiO2 nanotubes. J. Appl. Crystallogr. 2017, 50, 11331143. 45. Hoppe, H.; Sariciftci, N. S. Morphology of polymer/fullerene bulk heterojunction solar cells. J. Mater. Chem. 2006, 16, 45-61. 46. Müller-Buschbaum, P. The active layer morphology of organic solar cells probed with grazing incidence scattering techniques. Adv. Mater. 2014, 26, 7692-7709. 47. Umang, V. D.; Chengkun, X.; Jiamin, W.; Di, G. Solid-state dye-sensitized solar cells based on ordered ZnO nanowire arrays. Nanotechnology 2012, 23, 205401. 48. Yoneda, Y. Anomalous surface reflection of x-rays. Phys. Rev. 1963, 131, 2010-2013. 49. Sinha, S. K.; Sirota, E. B.; Garoff, S.; Stanley, H. B. X-ray and neutron scattering from rough surfaces. Phys. Rev. B 1988, 38, 2297-2311. 50. Holy´, V.; Kuběna, J.; Ohli´dal, I.; Lischka, K.; Plotz, W. X-ray reflection from rough layered systems. Phys. Rev. B 1993, 47, 15896-15903. 51. Holý, V.; Baumbach, T. Nonspecular x-ray reflection from rough multilayers. Phys. Rev. B 1994, 49, 10668-10676.


29


Page 30 of 31

52. Baumbach, G. T.; Holy, V.; Pietsch, U.; Gailhanou, M. The influence of specular interface reflection on grazing incidence X-ray diffraction and diffuse scattering from superlattices. Physica B: Condens. Matter 1994, 198, 249-252. 53. Swann, J. M. G.; Topham, P. D. Design and application of nanoscale actuators using block-copolymers. Polymers 2010, 2, 454-469. 54. Yoo, S. I.; Sohn, B.-H.; Zin, W.-C.; An, S.-J.; Yi, G.-C. Self-assembled arrays of zinc oxide nanoparticles from monolayer films of diblock copolymer micelles. Chem. Commun. 2004, 2850-2851. 55. Lee, J. Y.; Thompson, R. B.; Jasnow, D.; Balazs, A. C. Effect of nanoscopic particles on the mesophase structure of diblock copolymers. Macromolecules 2002, 35, 4855-4858. 56. Rasmussen, K.; Grampp, G.; Eesbeek, M. V.; Rohr, T. Thermal and UV degradation of polymer films studied in situ with ESR spectroscopy. ACS Appl. Mater. Interfaces 2010, 2, 1879-1883. 57. Yousif, E.; Haddad, R. Photodegradation and photostabilization of polymers, especially polystyrene: review. SpringerPlus 2013, 2, 398. 58. Wang, K.; Bießmann, L.; Schwartzkopf, M.; Roth, S. V.; Müller-Buschbaum, P. Tuning of the morphology and optoelectronic properties of ZnO/P3HT/P3HT-b-PEO hybrid films via spray deposition method. ACS Appl. Mater. Interfaces 2018, 10, 20569-20577. 59. Yu, C.-F.; Sung, C.-W.; Chen, S.-H.; Sun, S.-J. Relationship between the photoluminescence and conductivity of undoped ZnO thin films grown with various oxygen pressures. Appl. Surf. Sci. 2009, 256, 792-796. 60. Anderson, J.; Chris, G. V. d. W. Fundamentals of zinc oxide as a semiconductor. Rep. Prog. Phys. 2009, 72, 126501. 61. Zak, A. K.; Razali, R.; Majid, W. H. A.; Darroudi, M. Synthesis and characterization of a narrow size distribution of zinc oxide nanoparticles. Int. J. Nanomed. 2011, 6, 1399-1403. 62. Abou Chaaya, A.; Viter, R.; Bechelany, M.; Alute, Z.; Erts, D.; Zalesskaya, A.; Kovalevskis, K.; Rouessac, V.; Smyntyna, V.; Miele, P. Evolution of microstructure and related optical properties of ZnO grown by atomic layer deposition. Beilstein J. Nanotechnol. 2013, 4, 690-698. 63. Xu, C.; Wu, J.; Desai, U. V.; Gao, D. High-efficiency solid-state dye-sensitized solar cells based on TiO2-coated ZnO nanowire arrays. Nano Lett. 2012, 12, 2420-2424. 64. Lancelle-Beltran, E.; Prené, P.; Boscher, C.; Belleville, P.; Buvat, P.; Sanchez, C. Allsolid-state dye-sensitized nanoporous TiO2 hybrid solar cells with high energy-conversion efficiency. Adv. Mater. 2006, 18, 2579-2582. 65. Cao, Y.; Saygili, Y.; Ummadisingu, A.; Teuscher, J.; Luo, J.; Pellet, N.; Giordano, F.; Zakeeruddin, S. M.; Moser, J. E.; Freitag, M.; Hagfeldt, A.; Grätzel, M. 11% efficiency solidstate dye-sensitized solar cells with copper(II/I) hole transport materials. Nat. Commun. 2017, 8, 15390:1-8. 66. Chergui, Y.; Nehaoua, N.; Mekki, D. E. Photovoltaic characteristics of ZnO nanotube dye-sensitized solar cells and TiO2 nanostructure. Res. Rev.: J. Mater. Sci. 2016, 01, 18-24. 67. Chang, W.-C.; Lee, C.-H.; Yu, W.-C.; Lin, C.-M. Optimization of dye adsorption time and film thickness for efficient ZnO dye-sensitized solar cells with high at-rest stability. Nanoscale Res. Lett. 2012, 7, 688-688. 68. Li, B.; Wang, L.; Kang, B.; Wang, P.; Qiu, Y. Review of recent progress in solid-state dye-sensitized solar cells. Sol. Energy Mater. Sol. Cells 2006, 90, 549-573.


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69. Yang, W.-G.; Wan, F.-R.; Chen, Q.-W.; Li, J.-J.; Xu, D.-S. Controlling synthesis of wellcrystallized mesoporous TiO2 microspheres with ultrahigh surface area for high-performance dye-sensitized solar cells. J. Mater. Chem. 2010, 20, 2870-2876. 70. Venkata-Haritha, M.; Gopi, C. V. V. M.; Kim, S.-K.; Lee, J.-c.; Kim, H.-J. Solutionprocessed morphology-controllable nanosphere structured highly efficient and stable nickel sulfide counter electrodes for dye- and quantum dot-sensitized solar cells. New J. Chem. 2015, 39, 9575-9585. TOC Figure


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Comparison of UV Irradiation and Sintering on Mesoporous

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