Photo-electrodes with tailored three-dimensional nanostructures offer a large enhancement in light harvesting capability for various optoelectronic devices enabled by strong light scattering in the nanostructures as well as improved charge transport. Here we present an array of three-dimensional titanium dioxide (TiO2) nanohelixes fabricated by the oblique angle deposition method as a multifunctional photoanode for CdSe quantum dot sensitized solar cells (QDSSCs). The CdSe QDSSC with a TiO2 nanohelix photoanode shows a 100% higher power conversion efficiency despite less light being absorbed in CdSe QDs when compared with a conventional TiO2 nanoparticle photoanode. We attribute the higher power conversion efficiency to strong light scattering by the TiO2 nanohelixes and much enhanced transport and collection of photo-generated carriers enabled by the unique geometry and near-single crystallinity of the TiO2 nanohelix structure.
© 2014 Optical Society of America
Dye sensitized solar cells (DSSCs) have received tremendous attention due to their much reduced manufacturing costs, capability for achieving a high power conversion efficiency (PCE), and relative ease of fabrication [1–5]. One of the factors limiting the performance of DSSCs is the lack of available dyes with an extended long-wavelength absorption edge, which motivates the use of quantum dots (QDs); such QDs have a high extinction coefficient, a tunable peak absorption energy that can be adjusted by controlling the QD size, and the capability for multiple exciton generation [6–11]. As a consequence, QD-sensitized solar cells (QDSSCs) are attracting great interest for highly promising next-generation photovoltaic devices [12–15].
Solar cells generate electricity through two sequential steps; first, the generation of charge carrier pairs via light absorption and, second, the transport and collection of the carriers. Especially in (Q)DSSCs, the photoanode plays a crucial role in both sequential steps, thereby strongly affecting the PCE. The geometry of a photoanode determines the degree of light scattering and absorption, and thus, the light harvesting efficiency, while the crystallinity of a photoanode affects the electrical conductivity, and thus, the transport of photo-generated charge carriers. Thin films made of sintered TiO2 nanoparticles (~20nm diameter) are widely used as photoanodes for (Q)DSSCs due to their large surface area that is amenable for the adsorption of dye molecules and QDs . However, there are two significant drawbacks of these conventional sintered TiO2 photoanodes. First, the scattering of incident light which generally increases the probability of photon-QDs interaction, is negligible because the size of the TiO2 nanoparticles is much smaller than the wavelength range of the absorbed solar irradiation . Thus, additional scattering layers consisting of big TiO2 nanoparticles whose diameter is comparable to the wavelength of sun light, are incorporated on top of the film consisting of sintered small nanoparticles. Second, the transport of carriers through entangled complex networks of sintered nanoparticles is poor due to long percolated carrier travel lengths for charge collection [18,19], and numerous grain boundaries between sintered nanoparticles that promote recombination and very slow trap-limited diffusion of carriers [19–22].
Various one-dimensional nanostructures such as nanowire [23–26], nanorod [20,27–30] and nanotube [31–34] have been demonstrated as photoanodes for improving the transport of electrons by providing direct conduction pathways with reduced trapping and recombination at the grain boundaries . Although these one-dimensional nanostructures showed improved carrier transport properties, the smaller surface area of nanoparticle electrodes caused less absorption of QDs/dye molecules, i.e. less light harvesting . This drawback has been partly overcome by the hybridization of nanoparticles and one-dimensional nanostructures [37–41], however, such hybrid photoanodes still suffer from very limited light scattering and little enhancement in transport properties; thus a lower-than-expected PCE has been attained.
In this study, we present a new multifunctional photoanode based on an array of three-dimensional TiO2 nanohelixes for strongly enhanced electron transport properties as well as stronger light scattering, and consequently for a remarkably increased PCE of Cadmium Selenide (CdSe) QDSSCs. Structural properties of three-dimensional TiO2 nanohelixes fabricated by oblique angle deposition are investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Optical and electrical properties of the photoanodes are systematically investigated by simulations and experiments. CdSe QDSSCs based on the TiO2 nanohelix array showed 100% higher PCE over those with the conventional nanoparticle photoanode despite lower amount of absorbed CdSe QDs.
2. Results and discussion
2.1 Fabrication of QDSSCs with TiO2 nanohelix array
Figure 1(a) shows a schematic description of the QDSSC investigated here. It consists of a photoanode based on an array of TiO2 nanohelixes sensitized with QDs on a FTO coated glass substrate, a CuS-coated FTO counter electrode, and infiltrated electrolyte between the electrodes. Figure 1(b) shows a cross-sectional SEM image of an array of ~5μm-thick TiO2 nanohelixes grown vertically on the substrate. As shown in the SEM top view image in the inset, the TiO2 nanohelix array provides space sufficiently open for the penetration of QDs and the redox electrolyte.
2.2 Structural properties of TiO2 nanohelix array
The micro-structure of the TiO2 nanohelixes strongly affects the transport and collection of photo-generated charge carriers. The micro-structural evolution of the TiO2 nanohelixes upon thermal annealing was investigated by XRD, as shown in Fig. 2. The XRD peaks from the as-deposited sample are all identified as those of the FTO substrate; there was no single peak assignable to TiO2-related phases. New diffraction peaks appeared after thermal annealing at 500°C for 30minutes and they can be indexed as the anatase phase of TiO2. The XRD results indicate that the as-deposited amorphous phase TiO2 nanohelixes were transformed to the anatase phase during thermal annealing.
The microstructure and crystallinity of the annealed TiO2 nanohelixes were characterized by using TEM. Figures 3(a) and 3(b) show a bright-field TEM image of a bundle of TiO2 nanohelixes and corresponding diffraction pattern, respectively. The spot diffraction pattern as well as pronounced diffraction contrast of the TEM image indicates the high crystallinity of anatase TiO2 nanohelixes. Dark-field TEM images formed with separate diffraction spots [c, d, and e indicated by yellow circles in Fig. 3(b)] allows us to estimate the size of grains constituting the nanohelixes [Figs. 3(c) and 3(e)] since a similar intensity contrast region in a dark field image indicates that the orientations of the lattice planes are similar to the selected diffraction spot. Note that each nanohelix is composed of large TiO2 grains with the average size of 413 ± 25nm along the growth direction, which corresponds to ~1.5 times the pitch of a helix [the pitch of nanohelix is marked in Fig. 3(a)]. The high-resolution TEM (HRTEM) image taken from one of the grains shows near-single crystalline lattice fringes which are indexed as the lattice planes of the anatase phase of TiO2 [Fig. 3(f)]. TiO2 nanohelixes consisting of such large grains with high crystallinity can act as “express way” for the transport of photo-generated electrons; in other words, the photo-generated electrons can be efficiently transported through the nanohelixes with a much lower probability of recombination at grain boundaries when compared with a sintered TiO2 nanoparticle network that typically have a much smaller grain size.
The CdSe QDs loaded TiO2 nanohelixes were indeed confirmed in the high-angle annular dark-field (HAADF) mode of scanning TEM [Fig. 4(a)]. The HAADF imaging mode is useful to distinguish CdSe from TiO2 since the difference in average atomic numbers of CdSe and TiO2 causes a difference in HAADF intensities. In the HAADF shown in Fig. 4a, CdSe QDs are clearly distinguishable from the TiO2 nanohelix, showing that the QDs are densely packed and dispersed rather uniformly over the helix surface. The energy dispersive X-ray spectrum (EDS) obtained from one of the bright spots [see inset in Fig. 4(a)] further confirmed the presence of the elements Cd and Se. Note that Ti and O are detected as well since the electron probe transmits electrons also into the TiO2, through the CdSe QDs, thereby generating X-rays from both crystals. Figure 4b shows a HRTEM image of a QD-loaded TiO2 nanohelix. The lattice fringes of both CdSe and TiO2 are clearly resolved. In addition, Moiré fringes are frequently observed due to the overlap of the two lattices (CdSe and TiO2), indicating that both crystals have the excellent crystallinity. Based on the microstructural analyses by XRD and TEM, the TiO2 nanohelixes consisting of large grains with uniformly covered by densely packed CdSe QDs are expected to enhance the generation and transport of photo-generated carriers.
2.3 Optical properties of TiO2 nanohelix array
Absorbance values of a CdSe QDs-loaded TiO2 nanohelix array and a TiO2 nanoparticle reference layer with same thickness on FTO/glass substrates were measured as a function of wavelength, as shown in Fig. 5. The absorbance of the nanohelix array is much higher than that of the nanoparticle over the entire visible wavelength range. There are two representative factors determining the absorbance of photoanodes – the amount of loaded QDs and the light scattering effect. In order to figure out which factor makes major contribution to the higher absorbance in the nanohelix array, we estimated the surface area of both photoanodes considering the geometrical features observed by SEM and the porosities estimated by ellipsometry. The estimated surface area in 3 × 3 × 3μm3 volume of the nanoparticle layer is 6.0 × 10−9m2, ~22 times larger than that of the nanohelixes layer (2.7 × 10−10m2) Since, the amount of the loaded QDs is generally proportional to the surface area of the structures , thus, it can be regarded that the nanoparticle layer contains much larger amount of loaded QDs than the nanohelix array. Despite less QDs loading, the nanohelix array shows much higher absorbance even outside of the QDs absorption band, which is attributed to strong light scattering effect caused by the three-dimensional TiO2 nanohelix array.
The effect of light scattering in various photoanode structures was simulated using the commercial finite element method software COMSOL Multiphysics. Figures 6(a)–6(d) show different geometries of the modelled structures (such as a stack of TiO2 nanoparticles, a TiO2 nanorod, a TiO2 nanotube, and a TiO2 nanohelix) and the corresponding electric field distribution at the surfaces and surroundings of each structure when an x-polarized electromagnetic wave is incident in normal direction from the FTO substrate side. Perfect electric conductors (PEC) condition was imposed on two boundary surfaces perpendicular to x-axis, and perfect magnetic conductors (PMC) condition was applied to two boundary surfaces perpendicular to y-axis for considering the periodicity of the structures. [Note that the scale for the stacked nanoparticles in Fig. 6(a) is smaller than for the other structures due to much smaller size of the nanoparticles (~20nm).] The incident electromagnetic wave (with wavelength of 550nm) passing through the stack of nanoparticles, the nanorod, and the nanotube seems to be guided along the structures maintaining its periodicity. However, much more complex distribution of electric field with ~3 times higher amplitude than other structures is observed in both the surrounding region and the surface area of the nanohelix, which is attributed to the strong light scattering effect of the three-dimensional nanohelix structure. On the other hand, the electromagnetic wave propagates through the stack of nanoparticles with negligible light scattering because the size of nanoparticles is much smaller than the wavelength of incident light . Meanwhile, one-dimensional nanorods and nanotubes having comparable sizes (diameters) to the incident wavelength have a lower ability to scatter the incident light than the three-dimensional nanohelix. The probability for propagating photons to encounter an interface between the TiO2 nanostructures and air, in other words, the light scattering cross-section, is much greater for the three-dimensional nanostructure array than for the one-dimensional nanostructure arrays.
In order to experimentally verify the enhanced light scattering by the three-dimensional TiO2 nanohelix array, the diffuse transmittance of the 5μm-thick TiO2 nanohelix array layer and the 5μm-thick TiO2 nanoparticle layer on FTO glass and bare FTO glass were measured. Inset of Fig. 7(a) shows a schematic diagram of specular and diffused optical transmission. The total transmittance through the samples was measured by using an integrated sphere, so that both diffusely and specularly transmitted light were collected, whereas the specular transmittance was measured without an integrated sphere.
In results, the bare FTO substrate sample shows the highest total transmittance and specular transmittance among the three samples as shown in Fig. 7(a). With a 5 μm-thick TiO2 nanoparticle layer on top of the FTO substrate sample, both the total transmittance and the specular transmittance decrease by a similar ratio and tendency with respect to the wavelength of incident light. In case of the TiO2 nanohelixes array/FTO sample, both transmittances are much lower than for the other two samples. Then, the diffuse transmittance was estimated by subtracting the specular transmittance from the total transmittances of each sample. Figure 7(b) shows the diffuse transmittance curves together with the absorption curve of the CdSe QD sensitizer (shown by the yellow shaded region) as a function of wavelength. Diffuse transmittance through the TiO2 nanohelix array layer is higher than the TiO2 nanoparticle layer and bare FTO glass over the entire visible wavelength region, indicating a much stronger light scattering effect – i.e. much more spreading of incident light – by the TiO2 nanohelixes. Note that the diffuse transmittance through the nanohelix array layer is 4.3 times higher than through the TiO2 nanoparticle layer near the band edge of the CdSe QD sensitizer. The diffuse transmittance of the nanoparticle layer is slightly higher than that of the FTO glass, indicating that light scattering by the nanoparticles exists, but the amount is negligible. A photograph made of the three samples located on a string of red characters (“POSTECH”) is shown in the inset of Fig. 7(b). Enhanced light scattering by the nanohelix array obstructs the visibility of characters while the characters are clearly visible through the nanoparticle sample and the bare FTO glass. Interestingly, the diffuse transmittance of the nanoparticle layer and bare FTO glass sample decreases exponentially with the wavelength of the incident light, whereas that of the nanohelix array shows a quite different trend, which, we believe, is attributed to different light scattering mechanisms involved, i.e. Rayleigh scattering for the nanoparticle sample and Mie scattering for the nanohelix array sample. The nanostructure size enabling Mie scattering is approximately half of the incident light’s wavelength while Rayleigh scattering occurs for nanostructures even if their feature size is much smaller size than the wavelength . Therefore, the origin of the wavelength-dependent diffuse transmittance through the nanoparticle layer and bare substrate is presumed to be Rayleigh scattering in which scattering cross section is inversely proportional to the 4th power of the wavelength. On the other hand, in case of nanohelix array, Mie scattering presumably is predominant. Such enhanced light scattering by the three-dimensional photoanode nanostructures increases the travelling length of light in the active layer which increases the probability of photons to interact with the sensitizers; this desirable effect can compensate for the lower light absorption ability caused by relatively smaller surface area of the nanohelix photoanode when compared to conventional nanoparticle-based photoanode.
2.4 Photovoltaic performance
The photocurrent-density-versus-voltage (J-V) characteristics of CdSe QDSSCs based on a 5μm-thick TiO2 nanohelix array and 5μm-thick TiO2 nanoparticles reference were measured under simulated AM1.5 (100mW/cm2) solar illumination, as shown in Fig. 8(a). The photovoltaic parameters calculated from the J-V characteristics are summarized in Table 1.The 5μm nanohelix array based QDSSC shows a 1.6 times higher short-circuit current density (Jsc) (5.74mA/cm2) than nanoparticle layer based cell (3.60mA/cm2) with same thickness. Although the nanohelixes have much less surface area for the adsorption of QDs than the nanoparticles, the former induce a strong light scattering effect by their unique three-dimensional geometry enabling much higher light harvesting and short-circuit photocurrent density. The open circuit voltage (Voc) of the QDSSC with nanohelix array (0.49V) compares favourably to the Voc of the QDSSC with nanoparticle layer (0.40V) because the former has a high diffusion coefficient and thus low charge-carrier recombination.
Electron transport properties of each photoanode were evaluated by measuring the electron transport time by means of intensity modulated photocurrent spectroscopy with a 530nm illuminating light source using a wide range of light intensities, ranging from 5 to 100W/m2. The electron transport time in the 3.4μm nanohelix layer based photoanode is one order of magnitude faster than in the 10.8μm nanoparticle layer based photoanode. In order to consider the different thicknesses of the photoanodes for a fair evaluation of charge transport characteristics, the electron diffusion coefficient (Dn) was calculated using the equation: Dn = d2/4τc where d is the thickness of the film in which electrons travel and τc is electron transport time [43,44]. Fig. 8(b) shows the electron transport time and diffusion coefficient of the QDSSCs as function of incident 530nm laser light intensity. The results demonstrate that the promotion of electron transport in the near-single crystalline TiO2 nanohelix makes electron transport time shorter due to a four times higher diffusion coefficient when compared with a conventional nanoparticle layer. In addition, the openly structured film composed of nanohelixes makes mass transport resistance small , in other words, a better penetration of the electrolyte into the porous nanohelix array favourably affects the Voc to yield a high Voc . The low surface area of nanohelixes makes the probability of unwanted re-combination between photo-generated electron charges and electrolyte or QDs low and also results in a strongly enhanced Voc. Consequently, in spite of the smaller amount of adsorbed QDs (due to the smaller specific surface area for the nanohelix photoanode), the CdSe QDSSC based on the TiO2 nanohelix array shows a two-fold improvement in overall solar energy conversion efficiency (1.34%) over the QDSSC based on the nanoparticle layer (0.66%). Although the PCE values of our QDSSC are somewhat lower than that of recent state-of-the-art QDSSCs with newly developed QDs for wide absorption range, electrolytes, and appropriate post-treatments, we believe that our concept can make further improvement in the PCE of such high-performance QDSSCs.
3. Experimental section
3.1 Preparation of TiO2 photoanodes
In order to fabricate the TiO2 nanohelix array, a cleaned fluorine-doped SnO2 (FTO) glass substrate was masked to define the active region (2 × 5mm2); then the TiO2 nanohelix array was formed by oblique angle deposition [47, 48] using an electron beam evaporator. Oblique angle deposition is a very attractive method that is able to fabricate a variety of nanostructures due to its great versatility, position/shape-controllability, reproducibility, and compatibility with conventional micro-electronics processes. In oblique angle deposition, the substrate is tilted with respect to the incident vapor flux unlike typical physical vapor deposition methods. At the initial stage of the deposition, nuclei form on the substrate, producing self-shadowed regions with respect to the obliquely incident vapor flux. Subsequent vapor flux cannot deposit in the self-shadowed regions, but preferentially deposits on the high point (tip) of the nuclei, thereby creating an array of slanged nanorods. In addition, by rotating the substrate during the oblique deposition, various three-dimensional nanostructures (such as nanohelixes and nano zig-zags), can be fabricated. Although oblique angle deposition by electron-beam evaporation for fabricating three-dimensional nanohelixes array may be more costly than typical doctor-blade method used for nanoparticles coating, its advantages and performance enhancements enabled by various three-dimensional nanostructures would eliminate the potential cost trade-off. In addition, e-beam evaporation is a well-matured technique compatible with current microelectronics technology, which enables uniform, high-quality deposition of any evaporable materials by a batch process, thus, the capability of large area deposition on multiple wafer (or cell) scale can be achieved by appropriate chamber size and design. Various applications utilizing the advantages of the oblique angle deposition have been demonstrated [49–53]. For fabricating a TiO2 nanohelix array, the substrate was tilted against vapor flux at 80°, and repeated rotation with a speed of 1rpm for 2 seconds followed by a pause for 12 seconds; the film deposition rate was 5Å/s. Then, the as-deposited TiO2 nanohelix array on the FTO glass substrate was annealed at 500°C for 30 minutes under air ambient. The reference sample has a 5µm-thick TiO2 nanoparticle layer that was formed by doctor blade-printing with a commercial TiO2 paste (Ti-Nanoxide, T20/SP: average nano-particle size of about 20nm).
3.2 Fabrication and measurements of QDSSCs
For fabricating QDSSCs based on TiO2 nanohelix arrays and nanoparticle layer photoanodes, TiO2 nanohelix arrays and doctor-bladed nanoparticles on FTO glass substrate were dipped into a prepared CdSe QD solution for 12 hours, so that the QDs adsorbed on the TiO2 helixes by the electrostatic attractive force between negatively charged CdSe QDs and positively charged TiO2 . CdSe QDs were used as sensitizer due to its matching conduction band edge alignment with TiO2, and excellent photo- and chemical stability in polymide electrolytes. The loading of the CdSe QDs on the TiO2 nanohelix surface was investigated by a scanning TEM equipped with an energy dispersive spectrometer. After QDs loading, the working electrode was assembled into a sandwich type cell using a CuS on a FTO glass counter electrode . The cell was sealed by a hot-pressing process using a 50µm thick polymer sheet (Himilan of Mitsui-DuPont poly chemical). Finally, polysulfide electrolyte solution with 1.0M Na2S and 1.0M S in water was injected into the solar cell’s gap between two electrodes through a pre-drilled hole, and the hole was sealed by hot pressing. For solar cell performance characterization, the cell was irradiated by 100mW/cm2 of light from a 300W xenon lamp of a solar simulator (model 66902, Newport). The electron transport properties were measured by intensity modulated photocurrent spectroscopy (IMPS).
In summary, we have demonstrated a three-dimensional TiO2 nanohelix array, fabricated by the oblique angle deposition method, as a multifunctional photoanode for high performance quantum dot (CdSe) sensitized solar cells (QDSSCs). The TiO2 nanohelix array based QDSSCs showed much improved photovoltaic characteristics with a Voc of 0.49V, a Jsc of 5.74mA/cm2, and consequently two times higher PCE than conventional nanoparticle based QDSSCs having same thickness. The enhanced performance of nanohelix array based QDSSC is attributed to two important sequential improvements; first, a boost in the number of photo-generated carriers due to strong light scattering and, second, a very efficient collection of photo-generated carriers through near-single crystalline nanohelix structures. The array of aligned nanohelixes consisting of reduced number of inter-crystallite grain boundaries allows for injected photo-generated carriers to be rapidly transported due to a high diffusion coefficient in the photoanode (four times higher than conventional nanoparticle layer based photoanode). In addition, the three-dimensional nanohelix structure effectively scatters incident light, so that the interaction of light with QD sensitizers increases, resulting in strongly enhanced light harvesting which in turn results in a high Jsc. Therefore, more photons are absorbed by the QDs and more carriers survive the unwanted charge recombination, which leads to the improved PCE of the QDSSC. These favorable results inspire the use of three-dimensional metal oxide nanostructures to extend their application in optoelectronic devices particularly photoelectrochemical cells and organic photovoltaics.
The authors gratefully acknowledge supports by International Collaborative R&D Program funded by Korean Ministry of Knowledge Economy and Korea Institute for Advancement of Technology (KIAT), Golbal Research Network program (2011-220-D00064) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0094036).
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