Enhanced Photoelectric Properties in Dye-Sensitized Solar Cells

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Enhanced Photoelectric Properties in Dye Sensitized Solar Cells Using TiO Pyramid Arrays 2

Wei Jiang, Hongzhong Liu, Lei Yin, Yongsheng Shi, and Bangdao Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02687 • Publication Date (Web): 21 Apr 2016 Downloaded from http://pubs.acs.org on April 26, 2016

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The Journal of Physical Chemistry

Enhanced Photoelectric Properties in Dye Sensitized Solar Cells Using TiO2 Pyramid Arrays Wei Jiang, Hongzhong Liu*, Lei Yin, Yongsheng Shi, Bangdao Chen

State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, China, Fax: 86-29-83399508; Tel: 86-29-83399508. *To whom correspondences should be addressed. E-mail: [email protected]

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Abstract In this study, the improved device performance by introducing pyramid structured photoanodes has been investigated. The periodical pyramid arrays in the TiO2 active layer are fabricated by soft imprint lithography. And the pyramid structured photo-anodes are characterized by scanning electron microscope, UV-Vis spectroscopy and electrochemical impedance spectroscopy. The experimental results demonstrate that pyramid structured photoanodes can reduce the transmission light loss over a broad wavelength region and result in increased light reflection and scattering effect. Hence, dye sensitized solar cells with periodical pyramid arrays can increase the light absorption in the active layer and enhance the photovoltaic performance. The overall efficiency of such structured devices is increased by 20% in comparison with the device without periodical pyramid arrays and scattering layers. Fabrication of periodical pyramid arrays of mesoporous inorganic oxide films also can be applied for other various devices with micro/nano-structures at low cost using readily available pastes and soft imprint lithography.


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1. Introduction Nowadays the growing consumption of fossil fuel and increasing environmental awareness are driving people to look for new alternative energy resource. The solar energy is clean and non-pollution, which can be used without limit. Photovoltaic technology is regarded as one of the most efficient light utilization technologies.1-2 And the new photovoltaic production technologies with low cost and simple processes are of great interest to academia and industry.3 For the past few decades, as one of next generation solar cells, dye sensitized solar cells (DSSCs) have attracted more and more researchers’ attention owing to its inexpensive and environmental friendly processes.4-6 However, the photovoltaic efficiency of current DSSCs has not reached the theoretical level.7-8 The device performance strongly depends on the joint effect of different combination of photo-anode morphologies, sensitizers, electrolytes and counter electrodes.9-10 With regard to the photo-anode, it is mainly charged with the dye molecules uptake, incident light capture, and providing electrons and electrolyte ions pathway. Therefore, the photo-anode plays an important and decisive role in DSSCs operation principle.11 Generally, the photo-anode is composed by mesoporous TiO2 nanoparticle networks, which have a high surface area and an average particle size less than 30 nm. But the thin planar nanoparticles film has high light transparency leading to negligible light scattering effect, and the thick planar nanoparticles film has more complicated nanoparticle grain boundaries resisting the mobility of photo-induced electrons.12 Hence, the above mentioned drawbacks significantly limit the further enhancement of the device efficiency. In order to solve above issues, some kind of bilayer photo-anodes13, in which small size TiO2 nanoparticles as under-layer and large size TiO2 nanoparticles14 or other nanostructures aggregates15-16, 17 as over-layer, have been demonstrated to be efficient methods to improve the performance of DSSCs. And also some other hierarchical structured photo-anodes including spherical/core–shell aggregates have been introduced to enhance the photoelectric conversion efficiency of DSSCs.18-19 For instance, the 3D macroporous TiO2 sphere with 3 ACS Paragon Plus Environment


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controlled macropore sizes are synthesized by using well-arrayed polymethyl methacrylate with different diameters.20-21 However, in order to further expedite the electrons transport and reduce the costs, the next generation DSSCs will require a much thinner active layer, resulting in most of the above mentioned methods ineffective. Aiming at these problems, the light trapping strategies have been developed to enhance the light absorption of the limited film thickness of the active layer. In conventional silicon based solar cells, light trapping technologies have been generally used to enhance the photoelectrical properties by pattering silicon substrate with low reflection and reduced resistivity. These light trapping structures can substantially reduce reflection loss as well as significantly increase the effective optical pathlength by total internal reflection within the cell. A large augmentation achieving to 24.4% in silicon cell efficiency by integrated light trapping structures has been reported.22 Accordingly, some light-trapping strategies have also been developed in DSSCs to enhance the light absorption for the limited film thickness of the active layer. For example, Ki Seok Kim23 et al. reported a functionalized photo-anode with periodically aligned ZnO hemisphere crystals. And nano-patterned photo-anodes obtained from etched transparent conducting glasses24 and nanoimprinting TiO2 nanoparticles25-26 film by the patterned master also have been introduced to enhance the light absorption of the photo-anodes in some public reports. Therefore, the light trapping structures can both result in cheaper devices and better performance by strengthening light absorption ability and producing larger current. Similarly, in order to further increase the competitive advantages of DSSCs, efforts on maximizing the light absorption ability of the photo-anodes are still strongly expected. Pyramid structures have been extensively used to fabricate optoelectronic devices, particularly in photovoltaic fields over the decades owing to the multiple internal light reflection and absorption on the angular surface.27-31 The pyramid structures can increase the optical pathlength by factors as high as 40 and have been experimentally demonstrated to be one of the most efficient light 4 ACS Paragon Plus Environment

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trapping structures.31-32 Teck Kong Chong et al.33 confirmed that 91% of the ideal light trapping limit (the Lambertian limit) could be achieved by a skewed silicon pyramid grating placed on the front of a thin silicon solar cell. Accordingly, some efforts should also been tried on integrating the pyramid structures into the DSSCs photo-anodes. And the pyramid structures are expected to increase the light harvesting capacity of the active layer for the optimum performance. In this paper, we report on the fabrication of pyramid structured photo-anodes prepared by soft imprint lithographic technique for DSSCs application. The pyramid structured TiO2 nanoparticle film not only reduces incident light transmission loss but also increases the cell’s effective optical thickness. Meanwhile, the enhanced interface area between the scattering layer and active layer, can lead to more incident light be scattered and absorbed by enlarged interface area. The schematic cell based on periodical pyramid arrays is shown in Figure 1. The photovoltaic performance was investigated by optoelectronic and electrochemical measurements in detail. The characteristics of the pyramid structured DSSCs were compared with those of flat reference DSSCs in the motivation to explore the effectiveness of pyramid structured photo-anodes and soft imprint lithographic technologies. In the present scenario of photoelectric research, this report may be useful for the researchers to fabricate some efficient patterned structures on electrodes via cost effective methods and to achieve the improved efficiency. 2. Experimental 2.1. Fabrication of the inverted pyramid polydimethylsiloxane(PDMS) Stamp The masterplate with periodically inverted pyramid arrays were fabricated on silicon substrate by using conventional micro-fabrication techniques based on traditional photolithography and wet etching process. The wet etching process was proceeded in a solution containing 30 g NaOH and 1000 ml water at 80℃ ℃. The complete fabrication process flow is described as follow steps, which is shown in Figure 2. As the first step of 5 ACS Paragon Plus Environment


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this process, a 200 nm thick Cr layer was deposited on a silicon wafer by magnetic sputtering. After depositing Cr layer, the periodical holes were patterned in photoresist on the surface of Cr layer by photolithography. The following pattern transferring onto the Cr layer was performed by a Ce(NH4)2(NO3)6 (ammonium ceric nitrate) solution etching process. Subsequently, the wafer was etched in NaOH solution to fabricate the pyramid structures. The reason for applying NaOH solution was that it can etch silicon anisotropically yielding a self-aligned pyramid structure. And the morphologies of the generated pyramid structure can be tuned by varying the period, exposure and etching conditions. Then, the Cr mask was removed by Ce(NH4)2(NO3)6 solution after the etching process. And the resultant pyramid structures were cleaned with acetone, isopropyl alcohol, and ethanol in sequence. In order to reduce the damage to the pyramid mold during replication process, the obtained pyramid mold was further treated with trimethylchlorosilane to lower the surface energy and realize the easy PDMS peel-off in the following steps. During PDMS replication process, the precursor and cross-linker mixture (a 10:1 ratio, Sylgard 184, Dow Corning) was stirred together and deposited on the pyramid master. After degassing under vacuum for 10 min and curing at 100℃ ℃ for 1h, the solidified PDMS pyramid stamp was detached from the pyramid silicon mold. 2.2. Assembly of DSSCs The pyramid structured TiO2 nanoparticle (P25 Degussa) film acting as active layer was prepared by soft imprint lithographic technique, and the inverted pyramid structured large particle (200nm TiO2 nanoparticle) film acting as scattering layer was prepared by conformal contact deposition. The detailed fabrication process is described as follows and shown in Figure 3. Firstly, the TiO2 paste was coated on the transparent conductive substrate, and all the samples with or without patterns have the same initial content of TiO2 paste. Then, the thin PDMS stamp was manually placed on the top of the wetted TiO2 paste with proper pressure 6 ACS Paragon Plus Environment

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and maintained at room temperature for 12 h, which can allow the solvent to be evaporated fully and drive the TiO2 paste to penetrate into the pyramid space in the PDMS stamp. After the stamp was peeled off, the obtained pyramid structured TiO2 film was sintered at 450 ℃ for half an hour. The scattering layer was prepared by coating the large TiO2 particles onto the surface of periodical TiO2 pyramid arrays. Then, the solidified TiO2 films were heated at 500 °C for 30 min to remove the organics completely. As for the photo-anode sensitization, the cooled electrode was impregnated with the N719 dye solution (0.3 mM) for 24 hours. In terms of the electrolyte, it comprised 0.6 M 1,2-dimethylimidazolium iodide(DMPII), 0.1 M LiI, 0.05 M I2, 0.5 M tert-butylpyridine. On the part of the counter electrode, Pt nanoparticles were deposited on the surface of the transparent conductive substrate by the rf-magnetron sputtering technique. The photo-anode and counter electrode were adhered together by a 50 µm thick urlyn polymer. And the electrolyte solution was drawn into the cell under a vacuum through the drilled hole on the counter electrode. Finally, the punched hole was sealed with a 25 µm thick urlyn polymer film and a glass cover by hot pressing. The detailed fabrication process follow our prievous report.34 2.3. Characterizations and measurements of DSSCs The surface morphologies of the pyramid structured photo-anodes were observed by using a SU8000 field emission SEM. The optical properties of the prepared samples were characterized by a UV-Vis-NIR spectrophotometer (Shimadzu UV3600, Shimadzu, Japan). The current density–voltage (J-V) curves were characterized by using a digital source meter (Keithley 2612) and a 300 W Xenon light source (AM 1.5, 100 mW cm−2). The electrochemical impedance spectra (EIS) of the cells were measured by employing the CHI-660D in the frequency range from 0.05 Hz to105 Hz. The applied bias voltages were set at the open-circuit voltage of the cells with an AC amplitude of 10 mV. 3. Results and Discussion 3.1. Morphologies of pyramid patterned mold and TiO2 photo-anodes 7 ACS Paragon Plus Environment


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The morphologies of the pyramid mastertemplate were investigated by SEM as shown in Figure 4. It can be seen from the figure that the arrayed structures with inverted pyramidshaped profiles were successfully obtained with a period of 4 µm and a depth of 1.4 µm approximately in the resultant template. The top view SEM images of the open aperture arrays in Cr layer and the resulting pyramid arrays in silicon substrate are shown in Figure 4(a) and Figure 4(c), which is related to the inverted pyramids etched with lithographic Cr mask layer and the following Cr layer removed in the experiment respectively. The Cr layer mask exhibits translucent, intact and perforated morphologies in NaOH wet etching process (shown in Figure 4(b)), which effectively protected the underlying silicon from NaOH solution corrosion. The unprotected silicon surface was etched resulting in a typical inverted-pyramid structure after NaOH treatment. Figure 5 shows the SEM images of the patterned TiO2 photo-anodes with periodical pyramid arrays (4 µm period) with and without light scattering layer. Figure 5a-c shows the SEM graphs of TiO2 pyramid patterns in active layer after annealing, which was carried out by a pyramid PDMS stamp with a period of about 4 µm and depth of about 1.4 µm. As we can see from the figures that the pyramid structures were successfully transferred from PDMS stamp to the TiO2 active layer, which verifies that the flexibility and permeability of the PDMS stamp can effectively make the conformal contact intact and compact between the stamp and the TiO2 film, thus to obtain a perfect TiO2 pyramid replication and reduce the damage to the TiO2 film. As shown in Figure 5a, 5b and 5c, the acquired patterned pyramid arrays are well uniform and evenly distributed with mesoporous TiO2 nanoparticles network. To characterize the light-scattering and charge-generating properties with comparative studies, we also prepared a double layer photo-anode using large sized particles as scattering layer. Figure 5d-f shows the cross-sectional SEM images of the bilayer photo-anodes based on pyramid structured TiO2 active layer/light scattering layer. As can be seen from the Figure 5d and 5e, the double layer structured photo-anode, under-layer composed by a 4 µm periodical 8 ACS Paragon Plus Environment

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TiO2 pyramid arrays, and top-layer consisting of large sized TiO2 particles with a 5 µm thickness, can be successfully fabricated. The overlayer exhibits good adhesion to the nanocrystalline underlayer and no peeling is observed after annealing. In addition, Figure 5f shows that the TiO2 nanoparticles can fully fill into the spacing between the adjacent pyramid grating structures. The morphologies of the patterned TiO2 pyramid arrays are unaffected by the coating large sized TiO2 particles, and the covered large sized TiO2 particles network are tight and crackless. 3.2. UV-Vis optical absorption The main motivation for making patterned pyramid structures is to increase the optical pathlength within the sensitized TiO2 electrodes by optical diffraction and light scattering effects. The incident light that reaches the surface of periodical pyramid arrays is diffracted and scattered in deflected beams travelling in different directions. Thus, the residence time of incident photons travelling in the active layer is prolonged, leading to the enhanced light absorption. These different optical behaviors based on patterned pyramid structures could be further explained by the schematic diagram in Figure 6. As can be seen from the Figure 6a, the large amount of incident light travelling in the photo-anode would pass through the planar TiO2 film due to the small particle size. This drawback results in a great loss of incident photons. In Figure 6b, periodical pyramid arrays are distributed on the surface of the planar TiO2 film, but still some incident light is missed after passing through the patterned structures due to the incomplete light diffraction and scattering effects. However, as large TiO2 particles deposited on the surface of the arrayed pyramid structures (as shown in Figure 6c), the formed inverted pyramid scattering layer can effectively scatter a large proportion of the incident light returning into the nanocrystalline active layer and strongly trap the incident photons within the photo-anode, which could largely enhance the light harvesting efficiency and compensate for the loss of incident photons. These features were further experimentally confirmed by dye



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sensitized films with various structures for larger area (1 cm × 1 cm) and the corresponding UV–Vis absorption spectra is shown in Figure 7. As we can see from the Figure 7, the planar and pyramid patterned photo-anodes with and without scattering layer are presented. The pure planar TiO2 nanopartcle photo-anode is denoted as TP, the pyramid patterned TiO2 nanoparticle film is denoted as TP-PY, and the TP film and TP-PY film attached with scattering layer are denoted as TP-SC and TP-PY-SC, respectively. The absorption peaks spanning from 300 nm to 400 nm can be owing to the absorption of the TiO2 nanoparticles, and the absorption band during the wavelength range of 450 nm to 650 nm can be mainly attributed to the additional dye absorption. Interestingly, among the various photo-anodes, the PYTP-PYSC photo-anode demonstrates the best optical absorption with an intense and wide absorption band ranging from 400nm to 800nm. The TiO2 photo-anodes without scattering layer exhibit lower optical absorption than the TiO2 photo-anodes with scattering layer for visible wavelength, which indicates that the scattering layer can reduce the light transmission loss and reflect the incident light inner the film due to their relatively larger particles. The result also shows that the optical absorption of the pyramid patterned TiO2 photo-anode is greater than that of a planar photo-anode in the range of 400–800 nm. The enhanced light absorption of the samples with periodical pyramid structures is owing to the joint effects of the antireflection and scattering functions acquired by combination of the larger dimensions and grating diffraction properties of the periodical pyramid arrays. Incident light can be reflected inner the pyramid active layer before reaching the scattering layer, thus, leading to much longer optical path lengths. Furthermore, for the photo-anodes with scattering layer group and without scattering layer group, the various photo-anodes in the same group have the same amount of TiO2 nanoparticles and dye loading due to the same initial TiO2 film deposited by the doctor blading method. Although the planar photo-anodes adsorbed the same amount of dyes, the pyramid structured photo-anodes were found to absorb more incident photons than the planar photo-anodes. It indicates that the 10 ACS Paragon Plus Environment

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periodical pyramid arrays play a critical role in the light absorption of the DSSCs. And in order to further investigate the optical properties of periodical pyramid arrays, we also show the other two kinds of periodical pyramid photo-anodes in the supplementary materials. 3.3. Effects of periodical TiO2 pyramid arrays on the performance of DSSCs To investigate the influences of the pyramid patterned structures on the device performance, the photovoltaic characteristics of prepared samples (three cells for each kind) were measured under illumination of AM 1.5 100 mW cm2. The corresponding J–V curves for different kinds of photo-anodes based cells are presented in Figure 8. The short-circuit current density of the cell based on 4µm periodical pyramid structured TiO2 film (PYTP) and single planar TiO2 film (TP) are 9.68 mA/cm2 and 8.31 mA/cm2, and the corresponding efficiencies are 4.81% and 3.96%. Especially, we noted that the Jsc of pyramid structured cell is increased over 16% in comparison with the conventional planar TiO2 cell. The improved current density indicates that the periodical pyramid arrays in the TiO2 active layer can increase the optical pathlength within the sensitized photo-anode by diffraction effects. In addition, the short-circuit current density of the cell based on pyramid structured bilayer structure (PYTP-PYSC) and planar bilayer structure (TP-SC) are 12.65 mA/cm2 and 11.12 mA/cm2, and the corresponding efficiencies are 6.39% and 5.64%. The further improved light trapping design exhibits over 13% enhancement in comparison with the TP-SC cell, which indicates that the periodical pyramid arrays attached with conformal contacted scattering layer can further increase the optical pathlengths and prolong the residence time of photons in the active layer leading to the improved device performance. 3.4. Incident photon to current conversion efficiency (IPCE) The corresponding IPCE curves of the planar and pyramid structured photo-anodes with and without scattering layer are shown in Figure 9. Generally, the profile of IPCE curve is similar as that of the absorption spectrum, because the IPCE curves reflect the electron generation rate in external circuit at a given incident light wavelength. The variation in Jsc can be tested 11 ACS Paragon Plus Environment


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and verified by IPCE measurements. As can be seen from the figure, the pyramid structured photo-anode with scattering layer (PYTP-PYSC) has the maximum IPCE values, following the trend showed in the J–V curves. In comparison with the single-layer photo-anodes (TP, PYTP), the double-layer photo-anodes (TP-SC, PYTP-PYSC) present substantially enhanced IPCE values in the whole wavelength range owing to the addition of the scattering layers. Furthermore, it also can be seen from the figure that pyramid patterned devices presented enhanced IPCE values than those of flat device (PYTP versus TP, PYTP-PYSC versus TPSC). The improvement of the pyramid structured cells primarily arise from the marginally superior optical performance of the periodical pyramid arrays augmented by navigating more number of incident photons into the active layer and yielding more photo-electrons. Thus, the IPCE spectra demonstrate a direct evidence for the enhanced effective light absorption of the periodical TiO2 pyramid arrays. 3.5. EIS In order to analyze the interfacial carrier transfer behaviors, which are significantly related to the device performance, EIS measurements were implemented at dark with 0.8 V bias by testing the cells based on planar and pyramid structured photo-anodes with and without scattering layer, as shown in Figure 10. All the EIS spectra are made up of two semicircles distributed in high and medium frequency regimes. The semicircle in the medium frequency region represents the carrier transfer processes at the TiO2/dye/electrolyte interface according to the previous report, and the size of the semicircle at dark condition mainly reflects the carrier recombination resistance in the photo-anode.35-36 As shown in Figure 10 (b), the second semicircle of the cell with the pyramid structured electrode (PYTP) is larger than that of the cell with flat electrodes (TP), reflecting the suppressing carrier recombination in the pyramid electrode. The suppressing recombination rate can be ascribed to the increased contact area at the interface of pyramid structured TiO2 film/electrolyte as well as effective electrolyte penetration. 12 ACS Paragon Plus Environment

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The Bode phase graph exhibited in Figure 10 (c), presents the characteristic frequency values during the charge transfer processes. The lifetime of the electrons travelling in the device can be estimated from the characteristic frequencies of the impedance semicircle at medium frequencies, according to the equation τe=1/(2πfmax), where fmax is the maximum frequency of the medium frequency peaks. The fmax of the cell with the pyramid structured electrode (PYTP) slightly shifts to a much lower value in comparison with the flat TiO2 electrode (TP). The characteristic frequency shifting from a high value to a low value reveals a much faster carrier transport process. All above analyses suggest that the photo-anodes with integrated pyramid patterned TiO2 nanoparticles film can effectively enhance the carrier transfer at the interface of the photo-anode/electrolyte and resist the corresponding interfacial electron recombination. 4. Conclusion In summary, we have successfully fabricated the periodical pyramid arrays directly in the mesoporous TiO2 nanoparticle film. The periodical TiO2 pyramid arrays were prepared by the soft imprint lithography technique. Among the different types of photo-anodes employed, the pyramid-shaped TiO2 photo-anode exhibits the improved light absorption ability and hence the enhanced photovoltaic performance. There is a strong evidence that the periodical pyramid arrays can confine the incident light within the active layer more effective without causing any destructive side effects such as aggravating carrier recombination. The enhanced light absorption properties of periodical pyramid arrays make pyramid structured photoanodes promising candidates as effective photo-anodes for photovoltaic applications. This work also further provides us with some new chances for fabricating high-performance photovoltaic devices based on micro/nano-structures design and operation of the lighttrapping structures. Supporting Information Available



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The supporting information includes the SEM images of the pyramid molds, the pyramid patterned photo-anodes, and the corresponding UV–Vis absorption spectra in 20 µm and 40 µm periods.

Acknowledgements This work is supported by the Major Research Plan of National Natural Science Foundation on Nanomanufacturing (No. 91323303), National Natural Science Foundation of China (No. 51275400),

National

Science

and

Technology

Project

(Nos.

2011ZX04014-071,

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Figure Captions Figure 1. A schematic diagram of DSSCs for periodical pyramid structured photo-anodes.

Figure 2. A schematic representation of the preparation of inverted pyramid PDMS stamp.

Figure 3. The schematic illustration for the fabrication of pyramid structured photo-anode with conformal contacted scattering layer.

Figure 4. (a) SEM images of micro-hole arrays with 4um period on Cr layer; (b) NaOH etched inverted pyramid arrays with Cr mask; (c) Obtained inverted pyramid arrays mold with removed Cr layer.

Figure 5. (a) The overview SEM image of pyramid patterned TiO2 nanoparticles film; b) the top view image of the pyramid arrays with a 4um period; c) the enlarged image of the single pyramid composed by TiO2 nanoparticles. d) The cross sectional overview image of pyramid structured photo-anode with scattering layer; e) the enlarged cross sectional image; f) the interface between the active layer and scattering layer.

Figure 6. The optical schematic diagrams for (a) planar TiO2 photo-anode, (b) pyramid structured photo-anode without scattering layer, (c) pyramid structured photoanode with conformal contacted scattering layer.

Figure 7. UV-Vis spectra of absorption from different photo-anodes.

Figure 8. J–V curves of the DSSCs based on planar and pyramid structured DSSCs. 19 ACS Paragon Plus Environment


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Figure 9. IPCE spectra of the DSSCs based on the TP, PYTP, TP-SC and PYTP-PYSC photo-anodes.

Figure 10. (a) Equivalent circuit used to fit the EIS spectra for DSSCs: Rs is the series resistance; R1 and CPE1 represent the charge transfer resistance and corresponding constant phase element at the TiO2/dye/electrolyte interface; Zw is the electrolyte diffusion resistance; RPt and CPEPt are the charge-transfer resistance and corresponding constant phase element at the counter electrode; (b) Nyquist plots (c) and Bode phase plots for solar cells based on TP and PYTP photo-anodes.


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Table of Contents



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Figure 1. A schematic diagram of DSSCs for periodical pyramid structured photo-anodes. 79x55mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Figure 2. A schematic representation of the preparation of inverted pyramid PDMS stamp. 150x135mm (300 x 300 DPI)



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Figure 3. The schematic illustration for the fabrication of pyramid structured photo-anode with conformal contacted scattering layer. 111x81mm (300 x 300 DPI)


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Figure 4. (a) SEM images of micro-hole arrays with 4um period on Cr layer; (b) NaOH etched inverted pyramid arrays with Cr mask; (c) Obtained inverted pyramid arrays mold with removed Cr layer. 145x36mm (300 x 300 DPI)



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Figure 5. (a) The overview SEM image of pyramid patterned TiO2 nanoparticles film; b) the top view image of the pyramid arrays with a 4um period; c) the enlarged image of the single pyramid composed by TiO2 nanoparticles. d) The cross sectional overview image of pyramid structured photo-anode with scattering layer; e) the enlarged cross sectional image; f) the interface between the active layer and scattering layer. 130x65mm (300 x 300 DPI)


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Figure 6. The optical schematic diagrams for (a) planar TiO2 photo-anode, (b) pyramid structured photoanode without scattering layer, (c) pyramid structured photo-anode with conformal contacted scattering layer. 170x52mm (300 x 300 DPI)



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Figure 7. UV-Vis spectra of absorption from different photo-anodes. 144x101mm (300 x 300 DPI)


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Figure 8. J–V curves of the DSSCs based on planar and pyramid structured DSSCs. 144x101mm (300 x 300 DPI)



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Figure 9. IPCE spectra of the DSSCs based on the TP, PYTP, TP-SC and PYTP-PYSC photo-anodes. 144x101mm (300 x 300 DPI)


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Figure 10. (a) Equivalent circuit used to fit the EIS spectra for DSSCs: Rs is the series resistance; R1 and CPE1 represent the charge transfer resistance and corresponding constant phase element at the TiO2/dye/electrolyte interface; Zw is the electrolyte diffusion resistance; RPt and CPEPt are the chargetransfer resistance and corresponding constant phase element at the counter electrode; (b) Nyquist plots (c) and Bode phase plots for solar cells based on TP and PYTP photo-anodes. 82x146mm (300 x 300 DPI)


Enhanced Photoelectric Properties in Dye-Sensitized Solar Cells

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