SOLAR ENERGY
The increasing energy demand in the near future has forced us to seek environmentally clean and economically viable alternative energy resources. Among various candidates, solar energy has been shown as the viable choice to meet our energy demands. But despite being clean and inexhaustible, the energy produced by solar has been less than 0.01% of the total energy demand and a change will requires new initiatives to harvest solar radiation with improved efficiency. During the past decade, nanomaterials have emerged as a building block for constructing next generation of photovoltaic devices. In our lab, we have been working on several critical aspects of photovoltaic technology.
Nanowire based Solar cells
Nanowire based semiconductor solar cells, among other candidates, have shown potential to produce high efficiency solar cells. Its advantages includes natural strain relief to eliminate lattice-match constraints resulting in more choice of the materials, multiple bounces for light trapping due to dense nanowire array, efficient charge collector and ability to tune the absorption threshold of individual structures.
Towards this initiative, we are currently fabricating various nanowire based solar cell to achieve improvement in performance. Figure 1 (a) shows the solar radiation spectrum with absorption threshold marked for various semiconductors. Figure 1 (b) shows the schematic of the nanowire material structure we are currently fabricating. Figure 1 (c)-(f) shows the various semiconducting nanowires grown on different substrates using SA-MOCVD as the preliminary step for making heterojunctions and tandem solar cell.
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Figure 1 (a) Solar Radiation Spectrum (b) Schematic illustration of nanowire structure (c) InP on InP (111) A, (d) GaAs on GaAs (111) B, (e) InAs on GaAs (111) B and (f) Si on GaAs (111) B.
In addition to achieving high efficiency, minimizing the cost of these solar cells will be required to make it economically viable. We are currently employing several high throughput nano-patterning techniques to fabricate these nanowires on a large scale. We are currently using techniques such as Diblock copolymer patterning, nanosphere lithography and traditional E-beam lithography to make a template for growing nanowire using MOCVD.
Another area of research is to develop synthesis protocols to grow aligned TiO2 nanowires array directly on FTO substrates. The motivation behind this work comes from the fact that an oriented nanowire array would provide vectorial transport pathways to the separated charge carriers, in a dye- or quantum dot (QD)-sensitized solar cells. Highly crystalline TiO2 nanowires array comprises of the most optimal anode architecture that not only provides high surface area to enhance the dye loading, it also improves the charge collection efficiency of the cell significantly. Towards this objective, we have synthesized arrays of rutile-phase TiO2 nanowires on arbitrary substrates and have demonstrated their application in traditional dye-sensitized solar cell. A 3um TiO2 nanowire array of DSSC gives us a performance of 3%.
We are currently working on replacing dye molecules with different nanocrystal quantum dots to harvest different regions of sun’s spectrum. The major difficulty of DSSC is the volatile liquid electrolyte. The I-/I3- is corrosive, and the stability and packaging would be the issue for commercialize the cell. Effort has been made to use solid state Hole Transport Layers (HTM), such as P3HT and Spiro-OMeTAD. However, the deposition utilize drop cast or spin cast which rely on capillary action for uptake HTM molecules and require subsequent solvent removal from deep within the nanostructure, as a result, the transverse motion affect the interface between dye and HTM layers. Here we investigated novel organic molecules as HTMs and developed a physical vapor deposition technique to create a solid state Dye-Sensitized Solar cell. The deposition is scalable and relatively inexpensive. The deposition is completely solvent free so we can generate intimate contact between HTM and dyes which would be more favorable for HTM deposition and industry use. Initial measurements on our solid state dye solar cells are very impressive and encouraging. With a layer of doped NNP layer as the hole transporting media, we achieve a fill factor of ~ 60% and an efficiency of ~ 0.5% using 200 nm long TiO2 nanowire arrays. Presently, we are working on utilizing longer nanowire array to enhance the photocurrent.
Develop cheap and high efficiency solar cell is the ultimate goal for photovoltaic research. Two routes should be considered to achieve the goal: in materials aspect, eliminating the usage of rare earth material such as In, Te, which would likely have shortage of supply and therefore hinder the development of such solar cell market. In processing aspect, avoiding high vacuum deposition technique and utilized more salable solution deposition which are adaptable to large scale manufacturing such as roll to roll printing is essential for photovoltaic to be cheap enough to compete with other renewable energies. Copper Zinc Tin Sulfide (CZTS) has been one of the promising materials that meets the standard. It is non-toxic and earth abundant, and CZT(S,Se) has reached the conversion efficiency of 9.6%. In our group we are investigating a cheap, printable, and scalable solution process to fabricate CZTS solar cell, and we look forward to its bright future for cheap solar energy harvesting.
Another area of focus in our lab is to look for materials that can replace expensive and brittle ITO films which are used as transparent conductive electrodes in different photovoltaic devices. We have recently used a random network of silver nanowires as transparent and conductive electrodes. Silver nanowire films exhibit high performance that rival ITO films suggesting that these metallic nanostructures might hold a key to the ITO problems. We have developed a facile transfer method to print nanowire networks on both rigid and flexible substrates and have obtained films with a sheet resistance of 10 Ohms/sq at 85% transparency.
HYBRID NANOSTRUCTURED ENERGY STORAGE AND CONVERSION DEVICES
In recent years, due to the depletion of fossil fuels and the increasing concerns of environmental issues, the interests of developing alternative energy sources and devices, including solar cells, fuel cells, supercapacitors, and lithium ion batteries, have been increased to greater extent.
With the advantage of short diffusion ion length, nanostructures can be one of the most promising materials to overcome the kinetic problems (i.e. slow cation and electron diffusion) existing in conventional electrode materials for the applications in electrochemical energy conversion and storage cells. In considerations of the solid-state ionic diffusion of in electrode materials, the mean diffusion time, τeq, can be determined by the diffusion coefficient (D), and the diffusion length (L), according the following equation [1],
And, the diffusion coefficient can be expressed by using the Stokes-Einstein relation of D= (kB.T.μ)/(Z.e) , which is proportional to the material mobility (μ). According to the above equations, there are two approaches to overcome the kinetic problems of electrode materials. One is to enhance the diffusion coefficient by enhancing the mobility of electrode materials (e.g., via a doping process), which also leads to improve material conductivity. The other method is to further decrease the cation and electron diffusion length by using nanostructured materials. For instance, a reduction of diffusion length from 10 μm (the typical value of conventional electrode materials) to 50 nm (average diameter of 1-D nanostructures), for a LiCoO2 electrode with diffusion coefficient of 10-11.6 cm2/s from electrochemical impedance spectroscopy [2], the τeq can dramatically reduce from ~ 50,000 seconds to ~1 second, which leads to fast charging/discharging behavior in the electrochemical cells. Thus, the “going nano” effect is so significant that most of research efforts have been devoted in developing new nanostructures.
In addition to short ion diffusion length, large surface area (A) can be another advantage from nanostructures. According to the Cottrell equation,
where n is the number of electron exchange between oxidation and reduction, F is Faraday constant (9.65 ×104 C/mol), C is the solution concentration, and t is diffusion time. The charging/discharging current is proportional to the surface area of electrode materials; therefore results in a fast charging/discharging rate and high power density in nanostructure based electrochemical cells
Last but not least, nanostructures offer the other advantage of providing better accommodation of strain from cation insertion/removal process, which significantly improve the cycle life of electrochemical batteries. For example, in lithium ion batteries, Si thin film electrodes usually exhibit a 4-fold of volume expansion after lithium ion inserting into the crystal structures, which results in a permanent damage of electrodes and a loss of active materials; therefore decrease the specific capacity within short cycling numbers. The problem cannot be solved until recently Chan et al. adapted Si nanowires as the anode material and successfully showed good capacity retention even after 50 cycles of charging/discharging measurements [3].
However, nanostructures also can lead to some disadvantages, including an increase of undesirable electrode/electrolyte reactions due to large surface area, lower volumetric energy density because of inferior packing of materials, and potentially more complex synthesis. Among them, undesirable electrode/electrolyte reactions can cause self-discharging, poor cycling and calendar life, which are still one of unsolved issues to nanostructures applied in electrochemical conversion and storage devices.
Comparing all of these advantages and disadvantages, our work focuses on synthesize, characterize and explore novel hybrid nanostructured energy storage and conversion device, generally including the following projects:
i. Preparation and Characterization of Flexible Asymmetric Supercapacitors Based on Transition-Metal-Oxide Nanowire/Single-Walled Carbon Nanotube Hybrid Thin-Film Electrodes. [4]
In the work described in this paper, we have successfully fabricated flexible asymmetric supercapacitors (ASCs) based on transition-metal-oxide nanowire/single-walled carbon nanotube (SWNT) hybrid thin-film electrodes. These hybrid nanostructured films, with advantages of mechanical flexibility, uniform layered structures, and mesoporous surface morphology, were produced by using a filtration method. Here, manganese dioxide nanowire/SWNT hybrid films worked as the positive electrode, and indium oxide nanowire/SWNT hybrid films served as the negative electrode in a designed ASC. In our design, charges can be stored not only via electrochemical double-layer capacitance from SWNT films but also through a reversible faradic process from transition-metal-oxide nanowires. In addition, to obtain stable electrochemical behavior during charging/discharging cycles in a 2 V potential window, the mass balance between two electrodes has been optimized. Our optimized hybrid nanostructured ASCs exhibited a superior device performance with specific capacitance of 184 F/g, energy density of 25.5 Wh/kg, and columbic efficiency of 90%. In addition, our ASCs exhibited a power density of 50.3 kW/kg, which is 10-fold higher than obtained in early reported ASC work. The high-performance hybrid nanostructured ASCs can find applications in conformal electrics, portable electronics, and electrical vehicles.
ii. Inkjet Printing of Single-Walled Carbon Nanotube/RuO2 Nanowire Supercapacitors on Cloth Fabrics and Flexible Substrates. [5]
Single-walled carbon nanotube (SWNT) thin film electrodes have been printed on flexible substrates and cloth fabrics by using SWNT inks and an off-the-shelf inkjet printer, with features of controlled pattern geometry (0.4-6 cm2), location, controllable thickness (20-200 nm), and tunable electrical conductivity. The as-printed SWNT films were then sandwiched together with a piece of printable polymer electrolyte to form flexible and wearable supercapacitors, which displayed good capacitive behavior even after 1,000 charge/discharge cycles. Furthermore, a simple and efficient route to produce ruthenium oxide (RuO2) nanowire/SWNT hybrid films has been developed, and it was found that the knee frequency of the hybrid thin film electrodes can reach 1,500 Hz, which is much higher than the knee frequency of the bare SWNT electrodes (similar to 158 Hz). In addition, with the integration of RuO2 nanowires, the performance of the printed SWNT supercapacitor was significantly improved in terms of its specific capacitance of 138 F/g, power density of 96 kW/kg, and energy density of 18.8 Wh/kg. The results indicate the potential of printable energy storage devices and their significant promise for application in wearable energy storage devices.
iii. Hybrid Si-Carbon nanostructure as anode of Lithium-ion battery
We are interested in a broad range of hybrid structures of Si-Carbon working as anode in Lithium-ion batteries, like core-shell and thin film. With our own glove-box, potential station, and multi-channel battery test system, we have developed a set of synthetic and fabrication techniques to obtain designed nanostructure materials with composition, size and shape control.
Here are our facilities on lithium-ion batteries.
REFERENCES
[1] Guo Y, Hu J, Wan L. Nanostructured materials for electrochemical energy conversion and storage devices. Advanced Materials 2008; 20 (23): 2878-2887.
[2] Okubo M, Hosono E, Kim J, Enomoto M, Kojima N, Kudo T, Zhou H, Honma I. Nanosize effect on high-rate Li-ion intercalation in LiCoO2 electrode. Journal of the American Chemical Society 2007; 129 (23): 7444-7452.
[3] Chan C K, Peng H, Liu G, McIlwrath K, Zhang X F, Huggins R A, Cui Y. High-performance lithium battery anodes using silicon nanowires. Nature Nanotechnology 2008; 3(1): 31-35.
[4] Chen P C, Shen G Z, Shi Y, Chen H T, Zhou C W. Preparation and Characterization of Flexible Asymmetric Supercapacitors Based on Transition-Metal-Oxide Nanowire/Single-Walled Carbon Nanotube Hybrid Thin-Film Electrodes. ACS NANO 2010; 4(8): 4403-4411
[5] Chen P C, Chen H T, Qiu J, Zhou C W. Inkjet Printing of Single-Walled Carbon Nanotube/RuO2 Nanowire Supercapacitors on Cloth Fabrics and Flexible Substrates. Nano Research 2010; 3(8): 594-603