The present invention relates generally to solar energy generation. More particularly, the present invention relates to systems and methods for improving the efficiency of dye-sensitized solar cells and the use of dye-sensitized solar cells for generating fuel.
This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Dye-sensitized solar cells (DSSCs) differ from conventional solar cells in that they rely on a large area nanoparticle network to achieve sufficient absorption of sunlight. This approach limits the opportunities to further increase DSSC power efficiency because it necessarily restricts the choice of redox shuttles to those compatible with the long electron transit times and ample recombination opportunities inherent to the nanoparticle-based architecture. In one embodiment, the present invention is directed to a unity roughness photoelectrode design that affords adequate light absorption under resonance conditions while dramatically reducing the parasitic dark current density. As will be further described below, the following demonstrates a resonantly coupled cavity scheme on a planar, thin-film DSSC with a polarized, monochromatic incident photon-to-current efficiency (IPCE) of 17% from a single monolayer of a conventional Ru-dye. Upon illumination on resonance, open-circuit voltages reach 1 V, thereby approaching the theoretical limit for open-circuit voltage set by the dye and redox shuttle energy levels.
DSSCs have been optimized through nearly two decades of worldwide research to reach a local optimum that enables photoconversion of nearly all incident light above the dye optical energy gap. Further increasing the power conversion efficiency of these cells requires either extending the dye absorption toward longer wavelengths to absorb more of the solar spectrum or reducing the loss in potential of photoexcited carriers—that is, increasing the open circuit voltage (Voc). This loss in potential is especially large in DSSCs as compared to crystalline inorganic cells: the most efficient DSSCs 10 have Voc=0.75 to 0.85 V, representing approximately a 50% reduction from their optical bandgap of approximately 1.6 eV, whereas for a GaAs cell with Voc=1.11 V and a bandgap of 1.4 eV, the loss is only approximately 25%.
Nearly all of the excess voltage loss in DSSCs can be traced back to the large roughness of the photoelectrode, which usually consists of an approximately 10 μm thick film of sintered, metal oxide wide-bandgap semiconducting nanoparticles. This porous framework is necessary to obtain the approximate 1000-fold surface area enhancement required to chemically bind enough dye to absorb greater than 90% of the incident above-bandgap sunlight. Although this architecture has led to DSSC power conversion efficiencies (ηj) reaching 11%, it is also the chief obstacle to improving upon the current local optimization in DSSC performance.
In recent analyses of the loss in potential of DSSCs, two factors are found to account for the majority of losses. First, a significant loss can be avoided if the heterogeneity of electron injection could be reduced such that all electrons are injected at the potential at which the average electron is currently injected. This heterogeneity is due in part to energetic disorder among the different dye binding sites that stems from the nanoparticle nature of the semiconductor photoelectrode. The greatest voltage loss, however, is due to the large overpotential for dye regeneration by the I−/I3− redox couple, which amounts to approximately 0.5-0.6 eV. Although not optimal from an energy loss standpoint, this multi-electron, multi-step redox couple is uniquely suited to inhibiting the dark reaction across the very large liquid/solid interface. Several promising alternative redox shuttles capable of regenerating dyes with smaller overpotential have been reported. Unfortunately, to date, a significant increase in the dark current densities result in a voltage loss, as determined by the diode equation, largely offsetting the original reduction in dye regeneration overpotential.
A planar, thin-film photoelectrode provides a simple path to improving performance in both of these respects by potentially narrowing the energetic distribution of dye binding sites and reducing the specific solid/liquid heterojunction area (in which recombination is roughly first order) by several orders of magnitude. The challenge with this strategy is to maintain sufficient optical absorption with the correspondingly lower volume of surface bound dye. To this end, there has been significant effort to design a “super-chromophore” that exhibits the broadband absorption of traditional Ru-based dyes but with greater extinction. Research in this area has resulted in effective dyes with peak extinction coefficients (ε) up to 300,000 M−1 cm−1 but, unfortunately, the increased ε is largely offset by reduced packing density and lower ε in other regions of the visible spectrum such that the required roughness of the photoelectrode is of the same order (1000x) as that required of the Ru-based dye (N719) (ε˜14,000 M−1 cm−1) used in the most efficient DSSC.
An alternative approach involves optical concentration on the nanoscale. This has previously been explored using localized surface plasmon resonances (LSPR) supported by metal nanoparticles, which enhance the electromagnetic field in the vicinity of traditional DSSC chromophores and lead to a 7-fold improvement in the IPCE conversion efficiency relative to that of a comparable photoelectrode. While this approach is promising, a route to planar DSSCs with peak IPCE>1% has remained elusive.
DSSCs are one of the least expensive solar cell technologies, but they are relatively inefficient at converting solar energy into electric energy. While research in recent years has focused on improving efficiency, it has remained at approximately 11%. Improving the efficiency of DSSCs would provide a scalable cost-effective solar energy generation technology.
Various embodiments of the present invention relate to systems and methods for improving the efficiency of dye-sensitized solar cells. In general, embodiments of the device include a transparent substrate, a photoelectrode, a dye layer, an electrolyte, and a cathode. In one embodiment, a DSSC is provided. The DSSC generally comprises a transparent substrate, an anode layer, an oxide layer, a dye layer, a cathode, and an electrolyte. The anode layer is deposited on a surface of the transparent substrate. The oxide layer is deposited on the anode layer and the dye is deposited on the oxide layer. A cathode is disposed adjacent to the dye layer and an electrolyte is disposed between the anode layer and the cathode.
These and other features of the invention, together with the organization and manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the several drawings described below.
a)-(b) illustrate current density versus applied potential (J-V) in the dark (solid black line) and under illumination on resonance (solid gray line) for a ZnO-based device. The J-V is characteristic of a flat, ZnO control device built on a TCO under broadband illumination at normal incidence (dashed gray).
a)-(b) illustrate current density versus applied potential (J-V) in the dark (solid black line) and under illumination on resonance (solid gray line) for a bilayer-based (ZnO/TiO2) planar device. The J-V of a nanoparticle TiO2 DSSC under broadband normally (e.g. perpendicular) incident light (dashed gray line, scaled by 1/10 for clarity in
In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and made part of this disclosure.
An embodiment relates to a new route to improve the LHE of planar DSSCs 10 by coupling monochromatic light to guided modes supported by the semiconductor thin-film photoelectrode 12. In coupling to guided modes, dye 22 absorption is maximized through a resonant increase of the electromagnetic field intensity in the plane of the surface-bound dye 22. This approach achieves an IPCE of 17% from a single dye 22 monolayer for ZnO-based cells and Voc=1.0 V for TiO2/ZnO bilayer cells. The latter result approaches the theoretical limit for Voc set by the energetics of the N719 dye 22 and I−/I3− redox shuttle. By dramatically reducing the interfacial area over which the dark current flows, planar electrodes provide an exciting direction for device design and new opportunities for understanding losses in DSSCs 10.
The oxide layer 20 is deposited on a surface of the anode layer 18. The oxide layer 20 comprises a metal oxide wide-bandgap semiconducting thin film of sub-micron thickness. While the oxide layer 20 illustrated in
With the photoelectrode 12 fabricated, the at least one spacer 26 is disposed between the photoelectrode 12 and the cathode 28, creating a gap, which is filled with an electrolyte 24 as illustrated in
At normal (e.g. perpendicular) incidence, the photoelectrode 12 exhibits more than 90% reflectivity, however, upon prism coupling in the Kretschmann configuration (see
The surface plasmon polariton and higher order transverse magnetic (TM) and transverse electric (TE) guided modes are accessible by tuning the thickness of each layer in the stack. The complex refractive indices of each layer were experimentally determined for glass, Ag, ZnO, TiO2, N719, and the conventional I−/I3− acetonitrile-based electrolyte 24. A standard transfer matrix approach modeled the optical fields in the multilayer structure and the adjacent electrolyte 24. Initially, a genetic algorithm was employed to identify optimal layer thickness combinations that maximize the energy absorbed by the dye 22 monolayer. The largest couplings occur for TE modes in photoelectrodes 12 consisting of Ag and ZnO. This is because the TE boundary conditions and the smaller real component of the ZnO refractive index (n=2.0), as compared to that of TiO2 (n =2.4), lead to smaller modal overlap with the Ag anode 18 and hence less parasitic absorption. Little difference in dye 22 absorption is predicted among the various TE modes. Therefore, experiments focused on the lowest order (TE0) mode since it minimizes both the layer resistance and any potential loss due to residual color in the ZnO.
The oxide layer 20 used to protect the underlying Ag film were grown by atomic layer deposition (ALD) and characterized using variable-angle spectroscopic ellipsometry (VASE). As a “soft” and layer-by-layer deposition technique, ALD is used to deposit high quality, pinhole free materials with sub-nanometer thickness control without damaging the underlying Ag film. The large number of metal oxides accessible by ALD (including, but not limited to, TiO2, ZnO, SnO2, ZrO2, and NiO) makes the technique further applicable for the development of future photoelectrochemical systems. Detailed properties of the resulting polycrystalline films, including complex indices of refraction and film thickness are determined through VASE and transmission measurements, and serve as inputs for the transfer matrix modeling.
As shown in
According to the model, the balance of non-reflected power (79%) that is not absorbed by the dye 22 (55%) is lost to parasitic absorption by the adjacent Ag anode 18, which competes effectively as a power dissipation channel due to the low optical density of the dye 22. Nevertheless, the advantage of resonant coupling is significant, representing a 35-fold enhancement over the IPCE=0.5% measured (0.6% predicted,
As illustrated in
The dark parameters were held constant when fitting the photocurrent, only the photogenerated current, Jphoto was fit. Table 1 summarizes the fit parameters used in the fits plotted in
With reference to
Photocurrent densities at short circuit (Jsc) of 1.1 mA cm−2 were measured on resonance and would extrapolate to 3.6 mA/cm2 under an AM1.5-equivalent photon flux (again, integrated out to the optical bandgap of the dye 22). A fill factor (FF) of 0.42 results in a monochromatic power efficiency of 2.3% under TE-polarized illumination at the peak resonant condition.
Although the model suggests that the lower index of ZnO is suited to maximize optical coupling to the dye 22, TiO2 is the preferred material for efficient DSSCs 10. Nanoparticle ZnO and TiO2 devices both show JSC as approximately 18 mA cm−2 but the FF and Voc are consistently higher in TiO2 heterojunctions relative to their ZnO equivalent. Both pure TiO2 and ZnO/TiO2 bilayers were considered for use in cavity-mode-enhanced DSSCs 10. ZnO/TiO2 bilayer films were pursued, however, because (1) TiO2 exhibits a propensity for pinholes, and (2) the dye 22 optical coupling predicted for pure TiO2 devices is less than 15% due to strong modal confinement and field penetration of the Ag anode 18 that result from the high TiO2 refractive index. Ideally, a TiO2 shell would allow for increased FF and Voc while only marginally reducing the predicted maximum dye 22 coupling to 18%.
The J-V curve under resonant illumination in
Although the strategy described herein exhibits several practical shortcomings, most prominently that the IPCE enhancement occurs over a small angle of incidence for each monochromatic illumination wavelength and applies to only half of an unpolarized light source, the benefits of a flat dye-sensitized photoelectrode 12 are revealing. For example, there is an important kinetic competition between the electron survival time and the relatively long electron transport time through the nanoporous framework that determines charge collection efficiency. In the limit of these thin film photoelectrodes 12, the orders of magnitude shorter transport time renders this competition void. Similarly, the choice of redox shuttle is typically limited to highly soluble species whose high concentration sufficiently lowers the mass transport resistance to “hole” transport across the 25 micron electrode separation. By using a flat photoelectrode 12 this span may be decreased to approximately 500 nm, at which point the guided mode fields would begin to overlap with and leak into the platinized cathode. In the case of such a small electrode gap, the need for preformed oxidized species in solution is minimized. As the dark current roughly scales with the oxidized species concentration, this will pay dividends on well-known redox solutions and creates the opportunity to employ new couples. Therefore, these results are expected to dramatically expand the multivariable space over which DSSCs 10 can be explored.
While conventional DSSC 10 components (dyes 22, redox shuttles, spacers 26) have been used for the purpose of comparison, the design may benefit significantly from improved chemistries already demonstrated for use in DSSCs 10. For example, using high extinction dyes 22 such as, e.g. squaraines at their absorbance peak, may achieve an on-resonance LHE>62% due to more favorable competition with parasitic power dissipation in the Ag anode 18. Any practical application of this approach will, of course, require a route to broadband, polarization-independent enhancement at normal incidence, similar to solar radiation. Antennae-based approaches provide one potential solution, in which light is absorbed by an adjacent, optically thick absorber layer and then predominantly re-emitted into the waveguide modes of the planar DSSC 10 analogous to that demonstrated here. Alternatively, in-plane structural periodicity or the use of metal nanoparticles could be used to efficiently scatter incident light into waveguide modes. More generally, the wealth of knowledge developed in the context of traditional thin-film solar cell light management is applicable to the architecture studied here, raising the prospect for efficient DSSCs 10 with modest roughness and the benefits revealed herein. These results supply new insight into processes presently limiting DSSCs 10 and point to novel strategies to overcome these losses.
While the forgoing description has described improving the efficiency of converting solar energy into electric energy, another alternate embodiment is directed to converting solar energy into a fuel. In this embodiment, by exposing the photoelectrode 12 (with or without a dye 22) to catalysts, solar energy is directly converted into a fuel. This process is described in detail in Michael G. Walter et al., Solar Water Splitting Cells, Chem. Rev., 2010, 110, 6446-6473, and Roel van de Krol et al., Solar Hydrogen Production with Nanostructured Metal Oxides, J. Mater. Chem., 2008, 18, 2311-2320, which are hereby fully incorporated by reference.
A DSSC employing this improved light harvesting approach was made by first solvent-cleaning corning 1737 display grade glass and blow-drying the glass with a particle-filtered N2 stream. A thin (2.0 nm) Ge film was evaporated from a W boat under 5×10−7 Torr vacuum at a rate of 0.01 nm/s. The Ge film serves as an adhesion layer for and reduces the roughness of the subsequent Ag film (38 nm, 0.05 nm/s) deposited from a Mo boat without breaking vacuum as more fully described in Vj, L.; Kobayashi, N. P.; Islam, M. S.; Wu, W.; Chaturvedi, P.; Fang, N. X.; Wang, S. Y.; Williams, R. S. Nano Lett. 2009, 9, 178, incorporated herein by reference. The films are annealed overnight under flowing Ar at 450° C. (2° C. deg/min ramp rates, 30 min soak).
After fully cooling, the samples are promptly transferred to the 60° C. sample stage of an ALD tool (Savannah 200, Cambridge Nanotech). Metal oxides (Al2O3, ZnO or TiO2) were deposited by dosing trimethyl aluminum (TMA), diethyl zinc (DEZ), or titanium isopropoxide (TTIP), respectively, alternately with water. The (metal precursor)-(N2 purge)-(H2O)-(N2 purge) timings were x-60-0.015-60 where x=0.015 s for DEZ and TMA or 0.15 s for TTIP. Two cycles of Al2O3 were first deposited to improve adhesion and promote oxide nucleation.
Subsequently, 4 nm of ZnO was deposited to further protect the Ag film before ramping the temperature up to 140° C. for ZnO and finally 225° C. for TiO2. The remainder of each film thickness was deposited at these higher temperatures with purge times reduced to 20 s for ZnO or 7 s for TiO2. After cooling back to 80° C. the samples were removed, transferred to a quartz tube furnace and annealed overnight under flowing O2 at 400° C. (2° C. deg/min ramp rates, 10 min soak). Both the Ag and metal oxide film quality and thickness were monitored by ex-situ variable angle spectroscopic ellipsometry (VASE) throughout the fabrication procedure. It should be noted that the entire photoelectrode fabrication is a careful balance between preserving the ultra-smooth Ag film and sufficiently oxidizing the overlying metal oxide for efficient DSSC operation. Once optimized, batch yields greater than 80% were found to be of device quality pinhole free.
Finally, each sample was soaked in an ethanoic solution of 0.5 mM (Bu4N)2[Ru(4-(COOH),4′-(COO)-2,2′-bipyridine)2(NCS)2] (“N719”, Dyesol, B2 dye) for 30 min (ZnO) or 2.5 hours (bilayer ZnO/TiO2) and rinsed with dry acetonitrile before the devices were assembled according to established methods as more fully set forth in Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Bessho, T.; Gratzel, M. J. Am. Chem. Soc. 2005, 127, 16835, incorporated herein by reference. Briefly, a 25 μm Surlyn spacer (Solaronix SX1170-25) was sandwiched between the photoelectrode and a platinized fluorine doped tin oxide (FTO) dark electrode. A 0.07 cm2 active area was defined by a spacer, which softens at 80° C. to seal the device. Planar control devices without cavity mode enhancement were constructed on FTO without Ge or Ag layers. Nanoparticle control devices were constructed on FTO by doctor-blading a transparent and opaque nanoparticle paste and firing to 500° C. under flowing O2. The total nanoparticle film thickness was ˜8 um.
A solution of 0.6 M butylmethylimidizolium iodide (TCI America), 0.1 M lithium iodide, 0.03 M I2, and 0.25 M tert-butylpyridine in acetonitrile was introduced into the cell via vacuum backfilling through a hole in the platinized FTO electrode. A second Surlyn spacer and microscope coverslip were sealed over the hole with a soldering iron. All chemicals were used as received from Sigma-Aldrich unless otherwise specified.
Normal incidence illumination for spectral response experiments was achieved with an IPCE measurement kit (Newport) that consists of an excitation monochromator coupled to a 300W Xe lamp with AM1.5 filter and calibrated with a Si photodiode. IPCE and broadband (Iint=83 mW/cm2) normal incidence J-V data were measured with a potentiostat (Metrohm, Autolab III) in a two-electrode configuration. Reflectivity measurements were conducted by prism-coupling the 543 nm line of a HeNe laser into the device, which was mounted on a computer controlled rotation stage, with a Si photodiode and lock-in amplifier used for detection.
The LHE of dyed photoelectrodes was derived from the subtraction of absorption spectra acquired before and after dye removal (with 0.1 M KOH in H2O) on a UV-Vis-NIR spectrophotometer (Varian Cary 5000). As illustrated in
The foregoing description of embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the present invention. The embodiments were chosen and described in order to explain the principles of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments, and with various modifications, as are suited to the particular use contemplated.
The United States Government claims certain rights in this invention pursuant to Contract No. DE-ACO2-06CH11357 between the United States Department of Energy and UChicago Argonne, LLC, representing Argonne National Laboratory.