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Mesoporous titania–vertical nanorod films with interfacial engineering for high performance dye-sensitized solar cells

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Nanotechnology Nanotechnology 26 (2015) 105401 (8pp)

doi:10.1088/0957-4484/26/10/105401

Mesoporous titania–vertical nanorod films with interfacial engineering for high performance dye-sensitized solar cells Irfan Ahmed1, Azhar Fakharuddin1, Qamar Wali1, Ayib Rosdi Bin Zainun2, Jamil Ismail1 and Rajan Jose1 1

Nanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, 26300, Malaysia 2 Faculty of Electrical and Electronic Engineering, Universiti Malaysia Pahang, 26600, Malaysia E-mail: [email protected] Received 9 November 2014, revised 14 January 2015 Accepted for publication 14 January 2015 Published 17 February 2015 Abstract

Working electrode (WE) fabrication offers significant challenges in terms of achieving highefficiency dye-sensitized solar cells (DSCs). We have combined the beneficial effects of vertical nanorods grown on conducting glass substrate for charge transport and mesoporous particles for dye loading and have achieved a high photoconversion efficiency of (η) > 11% with an internal quantum efficiency of ∼93% in electrode films of thickness ∼7 ± 0.5 μm. Controlling the interface between the vertical nanorods and the mesoporous film is a crucial step in attaining high η. We identify three parameters, viz., large surface area of nanoparticles, increased light scattering of the nanorod–nanoparticle layer, and superior charge transport of nanorods, that simultaneously contribute to the improved photovoltaic performance of the WE developed. S Online supplementary data available from stacks.iop.org/NANO/26/105401/mmedia Keywords: nanoenergy, nanowire supported films, interfacial engineering, light scattering, photovoltaics (Some figures may appear in colour only in the online journal) However, excessive electron diffusion paths in the film and the short electron diffusion length (LD ∼ 10–50 μm) as restricted by defects in the mesoporous particles [9, 10] offer significant barriers to further improvement of the PV performance of these devices. Many approaches have been undertaken to improve LD: (i) one-dimensional (1D) arrays, ordered [11–14] or random [15–17], on transparent conducting glass substrates (TCO); (ii) core–shell structures to suppress recombination with the electrolyte [18, 19]; (iii) doping [20–22]; (iv) channeling the transport by controlling the diffusion volume [23, 24]; (v) novel morphologies such as flowers [25, 26] and hierarchical structures [27, 28]; (vi) nanowire/nanoparticle composites [29]; and so on. Among them ordered 1D arrays on TCO have the highest LD due to directed charge channels, increased light scattering due to their micrometer length scale, and the possibility of a space charge to increase charge mobility [30].

1. Introduction Dye-sensitized solar cells (DSCs) have emerged as low-cost photovoltaic (PV) devices due to their workability under lowlight conditions, easy integration into buildings as solar windows, low cost, and high PV conversion efficiency (η) [1– 5]. A mesoporous TiO2 film is at the heart of the devices with respect to providing a large surface for hosting the dyes and hence determining the amount of light absorbance [6, 7]. In addition, the photoexcited electrons in the dye molecules are separated into free carriers at the TiO2–dye interface and are then transported through the mesoporous film for collection. Photoconversion efficiency as high as ∼13% has been achieved in DSCs to date by employing TiO2 nanoparticles, owing to their high surface area (150 m2 g−1), but using panchromatic dyes with a higher absorption cross-section and electrolytes with lower electrochemical potential [8]. 0957-4484/15/105401+08$33.00

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© 2015 IOP Publishing Ltd Printed in the UK

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Scheme 1. (a) Structure of the NR–NP electrode; an isolated NR–NP configuration is in (b). The NRs facilitate the channeling of electron

transport and improve light scattering, whereas the NPs enable the anchoring of a large amount of dyes. The figure is drawn free of scale.

These salient features have given rise to such 1D nanostructures as photoanodes in DSCs. However, their lengths are limited to a few micrometers, typically an order of magnitude lower than their LDs [11, 14], thereby limiting dye loading and overall light absorption. Considerable success has been achieved in growing ordered TiO2 nanowire and TiO2 nanotube composite structures for DSCs with appreciable lengths (∼30 to 33 μm) and η ∼ 10% [31–33]; however, all such methods suffer from long processing time, tedious multiple processing, loss of transparency at the preferred side of illumination, and scalability. There are reports of TiO2 nanorod/nanoparticle composite structures, but the efficiency is η ∼ 3.25%, with a reduced fill factor [34, 35]. On the other hand, short nanorods (NRs) of a few microns in length (∼2–4 μm) are routinely produced on TCO, although with low η (2–3%) [36, 37]. Herein, we report a simple electrode preparation strategy for high-efficiency DSCs (η > 11%) using short nanorods as in Scheme 1. By developing a mesoporous TiO2 film (NP) on vertical rutile TiO2 nanorods grown on TCO (NR–NP layer) (figure 1), we have achieved internal quantum efficiencies (QE) > 90% and η > 11% in electrodes of thickness ∼7 μm. The DSCs based on NR–NP layers give more current and voltage than the total currents and voltages of the individual NR- and NP-based devices. Three effects, viz., large surface area of nanoparticles, increased light scattering, and LD of NRs, simultaneously contributed to the observed increase in the PV parameters. At the same time, electrode preparation for high-η DSCs involves sophisticated deposition of different functional TiO2 layers for hole-blocking, absorbing, and scattering layers [38]. The new strategy considerably simplifies the electrode preparation procedures for achieving high η in DSCs.

(a)

(110)

(200)

TiO2 NR FTO

Intensity

(101)

(301) (002) (211) (101)

(110)

(301)

(211) (310)

20

30

40

50

60

70

2θ Figure 1. The XRD pattern of synthesized TNRs.

In a typical synthesis, FTO was selectively etched to get an area of 0.14 cm2 (see supporting information, S13) and sequentially cleaned ultrasonically by deionized (DI) water, acetone, and ethanol for 15 min each and then dried in air. A 30 ml DI water and 30 ml HCl (37%) solution was mixed and magnetically stirred for 10 min, followed by the addition of 1.2 ml of titanium isopropoxide (TTiP), and again stirred for 10 min to get a transparent solution. FTOs (1.5 cm × 2 cm) were placed in the solution at an angle of ∼45° in a Teflon socket with the conducting side facing down. An autoclave containing the Teflon socket was kept at 150 °C for 4 h in an electric oven to achieve TiO2 NRs of the desired thickness. The autoclave was cooled to room temperature, and the samples were rinsed three times with DI water and ethanol and dried in air. A film of mesoporous TiO2 paste was coated 3

Electronic supplementary information (ESI) available: SEM images showing the surface morphology and packing density of the TiO2 nanorods; TEM and SAED images of the single-crystalline TiO2 nanorods; absorption spectra of the dye, dissolved in NaOH, of the devices, indicating the dye loading on the electrodes; details of calculation of current density from the IPCE spectra; the method of spin-coating binder-free TiO2 paste onto the nanorods; the electrical-equivalent circuit of dye-sensitized solar cells according to the diffusion–recombination model; and a sample impedance spectrum showing the transport parameters. This document is available freely through the Internet via stacks.iop.org/NANO/26/105401/mmedia.

2. Experiment 2.1. Synthesis of rutile TiO2 nanorods

Rutile TiO2 nanorods (NRs) were directly grown on fluorinedoped tin oxide substrate (FTO) by a hydrothermal process. 2

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Figure 2. Cross-sectional view of (a) NRDSC, (b) NRDSC (magnified view), (c) NRNPDSC and (d) NRNPDSC (magnified view).

onto the as-synthesized TiO2 nanorods by the doctor-blade technique.

2.2. Dye-sensitized solar cell fabrication and testing

The DSCs were fabricated on pre-cleaned fluorine-doped tin oxide–coated glass substrates (FTOs). The DSCs were fabricated using the following photoanodes: (i) hydrothermally grown TiO2 nanorod arrays on FTO (NRDSCs); (ii) commercial TiO2 nanoparticle paste (purchased from Solaronix) coated onto the FTO by the doctor-blade technique (NPDSCs); (iii) commercial TiO2 paste coated onto TiO2 nanorods (NRNPDSCs); and (iv) commercial TiO2 paste coated onto TiO2 nanorods (NRNPDSCs) with an intermediate thin TiO2 layer (NRTLNPDSCs). The coated substrates were annealed at 450 °C for 30 min at a heating rate of 2 °C min−1. The samples were removed from the furnace at 80 °C and immediately immersed in 0.5 m of Mcis-bis(isothiocyanato)bis(2,20-bipyrridyl-4-40-dicarboxylato) ruthenium(II)bis-tetrabutylammo-nium dye (N719, Solaronix) for 24 h. The DSCs were fabricated by using a pre-drilled platinum electrode as a counter-electrode. A Surylin spacer of ∼25 μm (meltonix 1170—25PF) was used to avoid shortcircuiting between the working electrodes and counter-electrodes. Electrolyte (iodide/tri-iodide) was injected through the

Figure 3. Absorption spectra of dye-anchored photoanodes of the

three devices.

pre-drilled hole via vacuum back filling into the gap between the two electrodes. The devices were analyzed using field emission scanning electron microscopy (FESEM, JEOL, USA) to investigate the morphology and thickness of all photoanodes. Absorption spectra of the fabricated devices were measured by a UV–vis NIR spectrophotometer (UV– 2600 Shimadzu). The incident photon-to-current conversion efficiency of the devices was recorded via CEP–2000, Bukoh Keiki, Japan. Photocurrent measurements of the assembled 3

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Figure 4. (a) Photovoltaic performance and (b) IPCE spectra of the

three devices.

Figure 5. (a) Cross-sectional view of NRNP with spin-coated

intermediate layer; (b) magnified view showing adhesion and infiltration of NPs into NR. Table 1. Statistics for photovoltaic properties of the DSCs.

Electrode NRDSCs NPDSCs NRNPDSCs

JSC (mAcm−2)

VOC (V)

η (%)

FF (%)

IPCE (%)

Mean

Best

Mean

Best

Mean

Best

Mean

Best

2.59 ± 0.23 10.96 ± 0.57 16.56 ± 0.56

2.86 11.42 17.1

0.73 ± 0.01 0.71 ± 0.01 0.75 ± 0.01

0.74 0.72 0.76

0.66 ± 0.02 0.62 ± 0.01 0.55 ± 0.02

0.7 0.64 0.58

1.27 ± 0.08 4.85 ± 0.17 6.88 ± 0.34

1.37 5.01 7.37

DSCs were taken using a solar simulator (SOLAR LIGHT, model 16-S 150) employing a single port simulator with power supply (XPS 400) under AM1.5G conditions. I–V curves were obtained using a potentiostat (Autolab PGSTAT30, Eco Chemie B.V., the Netherlands) employing NOVA® software.

40 63 86

deposited on FTO substrate is a rutile TiO2 crystal structure with lattice parameters a = b = 4.59 Å and c = 2.95 Å; the three peaks (101), (211), and (002), corresponding to rutile tetragonal TiO2 (ICDD 00-001-1292), have been identified at 36.3°, 54.68°, and 64.72°, respectively. The (002) peak intensity is significantly enhanced, indicating that the TiO2 NRs are growing with the (101) plane parallel to the FTO substrate and are oriented in the (001) direction, with the grown axis perpendicular to the surface of the FTO. The NRs have diameters of ∼100–150 nm, with a packing density of ∼4.19 × 1010 cm−2 (see supporting information, figure S23). The rutile is a phase more easily grown than the most preferred anatase for DSC application [36, 37]. Furthermore, the TEM studies show that the NRs are single

3. Results and discussion To investigate the structure of the TiO2 NRs, we carried out x-ray diffraction (XRD) of grown NRs on the substrate. Figure 1 shows the XRD pattern of (i) FTO and (ii) as-synthesized TiO2 NRs. The XRD pattern reveals that the film 4

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(>650 nm) side of the NR electrodes shows complete transparency. On the other hand, the NP and NRNP electrodes show increased absorbance and extended absorption onsets (700 and 800 nm, respectively) compared with the other electrodes. Large absorbance of the NP electrodes may be attributable to increased dye loading (106 nmolcm−2). The NRNP electrodes display larger absorbance and more-extended absorption onset than the NP electrodes despite their lower dye loading (∼88 nmolcm−2). This increased absorption cross-section (i.e., the product of peak absorbance and absorbing wavelength range) of the NRNP electrodes may be attributable to (i) significantly higher dye loading from the NP electrodes than the NR electrodes and (ii) significantly enhanced light scattering from the NR electrodes relative to the NP electrodes. Thus, the absorption spectra of the electrodes clearly demonstrate the enhanced light-harvesting properties of the NRNP electrodes. Larger particles (>100 nm) and nanowires are routinely used to increase light scattering in DSCs [38, 39]; however, we achieved it by growing vertical nanowires on FTO, which should contribute to improving charge transport also. Figure 4 shows the photocurrent density (J)–photovoltage (V) characteristics and incident photon-to-current conversion efficiency (IPCE) spectra of the best performing devices; statistics regarding their PV parameters are given in table 1. The η of the best-performing DSCs fabricated using NRs, NP electrodes, and NRNP electrodes is 1.37, 4.76, and 7.37%, respectively. The variation in the η of the devices arises mainly from their significantly different short-circuit current density (JSC). The JSC and η of the NRDSCs are the lowest, owing to their manyfold lower dye loading (4.2 nmolcm−2) compared with the other two devices. Among all the devices, NPDSCs show the highest dye loading (106 nmolcm−2), but their JSC and η are significantly lower than those of the NRNPDSCs. We further note that the JSC of the NRNPDSCs (17.1 mAcm−2) is 30% higher than the sum (13.17 mAcm−2) of the JSCs of the NRDSCs (JSC ∼ 2.86 mAcm−2) and NPDSCs (JSC ∼ 10.31 mAcm−2). One of the evident reasons for this increment in the NRNPDSCs is their increased scattering cross-section, as observed in figure 3. The open-circuit voltage (VOC) shows only a marginal variation among all devices, whereas the fill factor (FF) is practically constant for the NPDSCs and NRDSCs but significantly lowered in the NRNPDSCs. The lower FF is obviously from the charge recombination with the iodide electrolyte, the source of which is identified as the poor bonding at the NR–NP interface. We have solved this issue, which we will discuss after discussing the IPCE spectra of the aforementioned three devices. Variations similar to those observed in the J–V characteristics are also displayed in the IPCE spectra of the three devices (table 1). The lower surface area of the NRs has resulted in lower IPCE (∼40%). A bimodal IPCE is observed in the NRDSCs similar to the absorption spectra of the corresponding dye-anchored electrode; the first maximum is at 380 nm (IPCE ∼40%), and the next is at 540 nm (IPCE ∼22%), corresponding to the band edge absorbance of the NRs and N719 dye, respectively. The IPCE of the

Figure 6. (a) Photovoltaic characteristics and (b) IPCE spectra of

NRTLNPDSC.

crystalline with extended crystallinity, as revealed by the high-resolution lattice images and the electron diffraction pattern (see supporting information, figure S33). Figures 2(c) and (d) are the SEM images of the cross-sections of the sintered NRNP layers. The NP layers have formed uniformly on the NRs, and the total thickness of the electrode is ∼7 ± 0.5 μm. Four devices were developed in each category. Prior to discussing their photovoltaic properties, we discuss the lightharvesting properties of the dye-anchored electrodes. The dye loading was measured using the desorption test (see supporting information, figure S43), and the light scattering property of the electrode was studied by UV–vis absorption spectroscopy (figure 3). The dye loading measured for the NR, NRNP, and NP electrodes was 4.2, 88, and 106 nmolcm−2, respectively. The higher dye loading of the NP electrodes is due to their larger specific surface area. The extremely low dye loading of the NR electrodes is due to their larger diameter (100–150 nm) and shorter length (2 μm). Figure 3 shows the absorption spectra of the dyeanchored electrodes. The NR electrodes show bimodal absorption behavior with higher absorbance at wavelengths 10% has been achieved in electrodes of thickness 1 kHz); (ii) resistance to charge recombination (RCT), which occurs at intermediate frequencies (1 kHz < f < 1 Hz); and resistance to ion diffusion, which occurs at lower frequencies (