1.

Introduction

The development of hybrid (inorganic:organic) photovoltaic devices (h-PV) is an attractive prospect because of the suitability of such devices for low-cost solution processing. However, wide-spread use is still limited by two significant challenges; conversion efficiency and cost.¹ In this paper a simple method is outlined for the preparation of well-defined nanostructured ZnO scaffolds that can be used to improve device efficiency. In principle, an organic component, e.g., a conjugated polymer with controlled morphology, electrical and absorption properties, combined with an appropriate inorganic semiconductor, e.g., ZnO, in which structure, transport, and surface properties can be readily tuned, should provide dramatic improvements over organic photovoltaics (OPV) devices. However, to date h-PV devices have demonstrated low conversion efficiencies,^{2, 3} normally attributed to the limited exciton diffusion length in conjugated polymers (5 to 10 nm).⁴ Charge separation occurs at the heterojunction between the ZnO (acceptor) and the polymer (donor), therefore the ideal structure can be envisaged as an interconnected-array in which exciton generation occurs in the vicinity of an interface.⁵ The intimate intermixing of donor-acceptor species can be achieved in a number of ways but an engineered heterojunction ensures direct pathways for the free charge carriers to the electrodes, ensuring that efficient transport and collection occurs. In h-PV devices the morphology control in the donor-acceptor composite may be achieved by first constructing the inorganic scaffold with the appropriate layout and dimensions.⁶ This combination of materials presents an exciting opportunity for the development of h-PV devices that can overcome the current challenges and realize performance improvement by controlling the morphology of the active layer, which is considered to be a crucial step necessary for the evolution of efficient h-PV devices.²

ZnO is attractive for use in photovoltaic devices as it offers a direct electron pathway, effective light-scattering centers,¹ is low cost, and can be formed in a wide variety of nanostructures from solution. In addition to PV applications^{7, 8, 9, 10, 11} nanostructured ZnO has been widely used in a range of applications including light emitting diodes,^{12, 13, 14} thin film transistors,¹⁵ and as chemical sensors.^{16, 17} Nanostructured materials offer significantly larger interfacial areas than bulk films, however the challenge of preparing nanostructures with tailored and well-defined structures with high surface-area-to-volume ratios that maximize light harvesting while being compatible with the short polymer exciton diffusion length has continued to be problematic. Although several publications detail the formation of h-PV devices based on ZnO nanorods, reported efficiencies remain modest compared with OPV devices.^{3, 18, 19} The formation of metal oxide nanorod scaffolds has led to performance improvements compared with planar devices.^{7, 8, 18} More recently it has been shown that blended ZnO:P3HT h-PV devices with efficiencies of 2% can be prepared,⁶ stimulating further h-PV research.

Clearly a dispersed nanorod array fulfills many of the requirements necessary to improve device performance. Indeed there have been numerous techniques investigated in an effort to achieve this goal, including; electrochemical deposition,^{20, 21} template directed growth,²² pulsed laser deposition,²³ and hydrothermal methods.^{24, 25, 26, 27} While solution-based methods tend to favor the formation of a large number of rods, control of their aspect ratio, spacing, and orientation has yet to be demonstrated. Vacuum-based methods show potential for morphological control but are inherently unsuitable for scale-up.

There is a requirement for the preparation of tailored ZnO structures, in which the rod dimensions, spacing and orientation can be controlled. Such structures would open up a pathway for further improving h-PV device performance through compatibility with exciton diffusion length, in addition to providing direct pathways to the electrodes to minimize carrier transport time and the probability of back electron transfer.¹⁹ In this paper a method for the reproducible preparation of ZnO nanorod arrays using entirely solution-based methods is discussed. In such arrays the rod aspect ratio can be tuned by varying the growth conditions resulting in uniform length and vertically aligned structures. More importantly, the rod spacing can also be controlled which allows structures to be prepared that are ideal for preparing tailored h-PV devices with controlled separation between donor-acceptor centers.

2.

Experimental

ZnO nanorods were created via a two-step solution-based process. First, a seed layer of ZnO was deposited by spin-coating a 0.75 M solution of zinc acetate dihydrate [Zn(O₂CCH₃). 2(H₂O), Sigma-Aldrich] and 2-Aminoethanol [H₂N(CH₂)₂OH, Sigma-Aldrich] in 2-methoxyethanol [HO(CH₂)₂OCH₃), Sigma-Aldrich]²⁸ onto indium tin oxide (ITO)-coated glass substrates (PsiOteC UK Ltd; 12 to 16 Ω/sq). A dense uniform layer was prepared by loading at 500 rpm and increasing the spin speed to 2000 rpm for 30 s. The process was repeated three times with the substrates briefly heated to 300°C between coats. A final 60 min anneal at 450°C was carried out after the layers had been deposited.

In the second step, the as-prepared substrates with ZnO seed layers were suspended upside down in an equimolar (25 mM) solution of HMT [NH₂(CH₂)₆NH(CH₂)₆NH₂, Sigma-Aldrich] and zinc nitrate [(Zn(NO₃)₂, Sigma-Aldrich] at 95°C in a similar method described by Vayssieres.³⁰ The additives 10 mM polyethylenimine [H(NHCH₂CH₂)_nNH_2, Sigma-Aldrich] and/or 0.1 M potassium chloride (KCl, Sigma-Aldrich) were added directly to the solution. Following rod deposition the films were rinsed thoroughly with deionized water and allowed to dry at 95°C.

Scanning electron microscope (SEM) images were obtained using a Leo 1525 field emission scanning electron microscope. Cross-sectional images of the rod assemblies were obtained to determine rod length, by scratching or cleaving the substrate. Surface images were used to measure rod diameter. Data analysis was carried out using ImageJ software.

X-ray diffraction (XRD) patterns were obtained using a Panalytical X’Pert Pro diffractometer equipped with a monochromated Cu Kα source and an accelerator detector. The voltage and current were set at 40 kV and 40 mA, respectively.

3.

Results and Discussion

3.1.

Seed Layer Growth

Thin ZnO seed layers have been deposited onto ITO substrates by spin-coating a mixed precursor solution. The method has been adapted to lower the annealing temperature (450°C versus 600°C). Differential scanning calorimetry measurements carried out on the zinc acetate precursor show two distinct regions of mass loss on heating and a sharp crystallization peak at ∼250°C on cooling. The technique results in the formation of polycrystalline thin films, confirmed by SEM (Fig. 1) and XRD (Fig. 2). Variations in chemical bath conditions and seed layer morphology have been shown to control rod density, diameter, and morphology.^{25, 27, 29}

Fig. 1

SEM micrographs showing top-surface rod morphology and cleaved cross sections (insets) for ZnO rods grown by hydrothermal method under the conditions, (a) additive free growth, (b) addition of PEI (10 mM), (c) addition of KCl (100 mM), and (d) combined addition of PEI (10 mM) and KCl (100 mM). Main scale markers 200 nm, inset 500 nm.

Fig. 2

X-ray diffraction patterns obtained from ZnO seed layers and nanorod arrays, curves labelled from bottom to top; seed layer on ITO (* indicates peaks from ITO substrate), additive-free nanorod growth, addition of PEI (10 mM), addition of KCl (100 mM), and combined addition of PEI (10 mM) and KCl (100 mM).

3.5.

UV-Visible Spectroscopy

The optical transparency of the films was measured from 300 to 900 nm against an air background; the results are shown in Fig. 3. The ITO/glass substrate shows a strong absorption at <320 nm that rises sharply to give the substrate a transparency of >90% over the 400 to 900 nm range. The ZnO absorption band (360 nm) is apparent when the seed layer is added to the ITO and the transparency remains comparable to the bare substrate. The additive-free growth and the growth with PEI produce films that have significant scattering losses, which we attribute to the misalignment of the rods. The addition of KCl reduces the scattering and the KCl/PEI mixture produces a film with similar optical properties to only the seed layer. These films have fewer scattering losses owing to the morphology of the rods. The aligned structures are also beneficial for h-PV applications as they have more rods per unit area (increased interfacial area), and with less overlap between rods there are fewer obstacles to complete polymer infiltration.

Fig. 3

UV-visible data for ITO substrate, seed layer, and nanorod arrays.

4.

Conclusions

The development of a nanostructured ZnO template for h-PV applications is described. Hydrothermal growth has been identified as an appropriate method for the formation of nanorod arrays. Unmodified growth results in the formation of ZnO rods with a broad size distribution and a low density. Furthermore the rods are poorly aligned and overlapping. Orientated rods are necessary for h-PV applications; more rods per unit area increase interfacial area and matches closely with the exciton diffusion length in typical organic components. In addition, with less overlap between rods there are fewer obstacles to complete polymer infiltration, and voids are seen to be present when filling disordered ZnO structures.^{11, 18}

To improve the quality of the rod arrays the influence of additives, PEI and KCl, to the hydrothermal growth bath was investigated. PEI is reported as an absorbent on the nonpolar faces of the growing ZnO nanorod while the role of KCl has until now been unreported. The addition of the surfactant results in the formation of short, tapered, and misaligned rods. The wetting behavior of the rods is nonideal and the UV-visible spectra show increased scattering. The addition of KCl significantly improves the uniformity of rod length and width and results in arrays composed of rods with improved alignment, and well-defined faceted rods are formed. The improvement in the wetting behavior of these arrays is attributed to the stabilization of the polar (002) surface at the tip of the rods. The combination of PEI and KCl addition results in ZnO arrays with the most desirable properties. Rods with a narrow size distribution and a high number density are prepared; additionally the rods are highly aligned and are terminated by (002) surfaces that gives rise to the improved wetting behavior. The highly aligned rods reduce scattering losses, which will be beneficial for h-PV devices, as it will ensure maximum photon density in the conjugated polymer.