Dye-sensitized solar cells based on semiconductor morphologies with ZnO nanowires
Introduction
Widespread use of conventional inorganic semiconductor solar cells based on the p–n junction remains limited by high production costs mainly due to the high temperatures and high vacuum required to reduce defect levels in the materials. Organic and hybrid organic–inorganic solar cells provide an inexpensive means of achieving photovoltaic energy conversion, but efficiencies of these devices are currently limited to about 3% [1], [2], [3]. Titanium dioxide nanoparticle-based dye-sensitized solar cells (DSSCs) provide higher efficiencies, above 10%, with low demand for energy-intensive processing steps and could provide a cost effective alternative to inorganic solar cells based on the p–n junction [4], [5], [6], [7]. After an initial breakthrough of 7% efficiency [8], continual improvements in DSSCs have been made with much of the current research focusing on extending the range of spectral absorbance by modifying the dye [9], on improving hole transport and stability by replacing the liquid electrolyte with ionic solids or conducting polymers [10], [11], [12], [13], and on improving electron transport and reducing recombination by the use of alternative semiconductor materials or core–shell structures [14], [15].
In DSSCs, the choice of semiconductor is constrained by several factors. First, the semiconductor must have conduction band energy and density of states that allow efficient electronic coupling with the dye energy levels to facilitate charge separation and minimize recombination. Second, the semiconductor morphology must present high surface area to maximize light absorption by the dye monolayer while maintaining good electrical connectivity to the substrate. Currently these constraints are most successfully satisfied by a 10 μm thick porous (∼50%) film made up of 15 nm diameter TiO2 nanoparticles abutted against each other.
Electron transport in nanoparticle-based DSSCs has been proposed to occur either by a series of hopping events between trap states on neighboring particles [16] or by diffusive transport within extended states slowed down by trapping/detrapping events [7]. During its traversal of the photoelectrode an electron is estimated to cross 103 to 106 particles [16]. Time-resolved terahertz spectroscopy measurements have shown that electrons within a nanoparticle are highly mobile, but transport through the bulk film is slowed by the disorder of the nanoparticle network [17]. Electron transport in the nanoparticle network is surprisingly efficient, possibly due to low probability of encounter between a trapped electron and the I3− that exists in low concentration in the electrolyte surrounding the nanoparticles. However, a causal link between electron transport and recombination has been suggested with transport limiting the recombination rate; as a consequence, any improvement in the transport rate also increases the recombination rate and solar cell performance remains unaffected [18]. This link may be due to the nanoparticle morphology and it may be beneficial to explore significantly different semiconductor morphologies for DSSC photoelectrodes. For example, the electron transport mechanism may be altered and improved by changing the semiconductor morphology from a nanoparticle film to a nanowire array. A dense array of single crystal nanowires would provide a direct path from the point of electron injection to the substrate via the semiconductor conduction band while maintaining the high surface area necessary for dye adsorption [19]. A schematic of a nanowire-based DSSC is shown in Ref. [19]. The nanowires need not be ordered and perfectly perpendicular to the substrate. Randomly nucleated and coiled wires on a transparent conducting electrode would still carry the injected electron to the anode.
High aspect ratio (length-to-diameter) nanostructures have been used in nanorod–polymer blend solar cells where the external quantum efficiency was improved by a factor of three by increasing the length of 7 nm diameter nanorods from 7 to 60 nm [3]. The authors suggested that further enhancement in performance can be achieved by aligning the nanorods perpendicular to the substrate and further increasing their length. Furthermore, it has been shown through both simulation and experimental tuning of the porosity that the network connectivity in nanoparticle films greatly affects the transport time for an electron to reach the electrode [16]. Replacing a nanoparticle network with an array of nanowires, where every possible point of electron injection is directly connected to the substrate with a minimal number of interfaces and grain boundaries in between, could further improve charge transport in these devices.
ZnO has nearly the same band gap and electron affinity as TiO2, making it a possible candidate as an effective DSSC semiconductor. ZnO nanoparticle DSSCs have shown the second highest efficiencies after TiO2 [20] and ZnO is often used as the shell in core-shell nanoparticle structures [21]. Additionally, there is a wealth of information regarding the processing and properties of ZnO nanowires and a large variety of morphologies that are accessible by either vapor deposition [22], [23] or solution growth methods [24], [25], [26].
In this article, we report the growth of ZnO nanowires with dendritic structure and the use of these nanowires as the wide band gap semiconductor in DSSCs. Our objective is to introduce and demonstrate new possibilities in designing the semiconductor morphology in DSSCs. While we focus on ZnO, similar strategies can be used with TiO2 and the possibilities are only limited by availability of synthesis methods to produce the appropriate nanostructures.
Section snippets
Nanowire growth by MOCVD
ZnO nanowires were grown by metalorganic chemical vapor deposition (MOCVD) from zinc acetylacetonate hydrate [Zn(C5H7O2)2·xH2O, or Zn(AcAc)2] and oxygen gas on conducting glass substrates (F:SnO2 with sheet resistance 15 Ω/□) heated to 550 °C in vacuum. Zn(AcAc)2 powder was heated to 75 °C and transported into the MOCVD chamber with 20 sccm of Ar carrier gas. The gas inlet tube (4 mm inner diameter) is 20 mm above the substrate and produces a stagnation point flow across the substrate. O2 gas was
Nanowire growth
In a narrow temperature range (500–600 °C), nanowires grow spontaneously on F:SnO2 substrates. The morphology and the density of nanowires depend on the growth conditions and duration. The nanowires nucleate and grow from a thin polycrystalline film (∼10–20 nm) that grows on the substrate during the initial stages of the deposition. The water in the hydrated precursor desorbs during the initial stages of deposition, leading to a rise followed by a drop in the water partial pressure in the reactor
Summary and conclusions
ZnO nanowire-based DSSCs were constructed using dendritic nanowires grown by MOCVD. The nanowires provide a direct pathway from the point of electron injection to the collecting electrode and may offer improved transport compared to sintered nanoparticle films in which electrons must cross many interfaces to reach the substrate. The current densities and efficiencies of the nanowire solar cells increase by over two orders of magnitude upon growth of dense, branched nanowires as compared to
Acknowledgements
This work was supported by the University of California Energy Institute's Energy Science and Technology Program and made use of MRL Central Facilities supported by the MRSEC Program of the National Science Foundation under Award no. DMR00-80034. J.B.B. was supported by a National Science Foundation Graduate Fellowship. The authors would also like to thank Tom Jaramillo for assistance in absorbance and IPCE measurements.
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