Bilayer dye sensitized solar cell and fabrication method thereof

Abstract

A photovoltaic cell comprises a first electrode that includes a first transparent conductive substrate, a first layer having a plurality of first semiconductor nanofibers, and a second layer having a plurality of second semiconductor super-fine fibers, the first semiconductor nanofibers having an average diameter smaller than an average diameter of the second semiconductor super-fine fibers, a light absorbing material adsorbed to at least some of the first semiconductor nanofibers and second semiconductor super-fine fibers, a second electrode includes a second transparent conductive substrate, and electrolytes dispersed in the first and second layers.

Claims

1. A photovoltaic cell comprising: a first electrode includes: a first transparent conductive substrate, and a bilayer, nanofiber mat structure comprising a first light harvesting layer overlying said first transparent conductive substrate, said first light harvesting layer consisting of a plurality of randomly oriented first semiconductor nanofibers, and a second light scattering and light harvesting layer formed directly on said first light harvesting layer, said second light scattering and light harvesting layer consisting of a plurality of randomly oriented second semiconductor nanofibers said first and second layers of said bilayer mat structure being electrospun from the same material to form a unitary, interconnected mat structure having a total thickness (H) of 12 microns or less and wherein the ratio (r.sub.h) of the thickness of the first layer to the second layer is no less than 1 and no more than 6, said plurality of nanofibers in said first layer having an average diameter of between 50 nm and 80 nm and said plurality of nanofibers in said second layer having an average diameter of between 80 nm and 120 nm; a light absorbing material adsorbed to at least some of the first semiconductor nanofibers and second semiconductor nanofibers; a second electrode includes a second transparent conductive substrate; and electrolytes dispersed in the first and second layers, wherein said bilayer, nanofiber mat structure forms a permeable, porous, and interconnected structure for electron transport within said cell.

2. The photovoltaic cell according to claim 1, wherein the first semiconductor nanofibers and the second semiconductor nanofibers are titanium dioxide fibers.

3. The photovoltaic cell according to claim 1, further includes a nanoparticle layer between the first transparent conductive substrate and the first layer, the nanoparticle layer includes a matrix of semiconductor nanoparticles.

4. The photovoltaic cell according to claim 3, wherein the semiconductor nanoparticles are titanium dioxide nanoparticles.

5. The photovoltaic cell according to claim 1, wherein the first transparent conductive substrate and the second transparent conductive substrate are indium tin oxide glass or fluorine-doped tin oxide glass.

6. The photovoltaic cell according to claim 1, wherein the first layer and the second layer have a thickness ratio (thickness of the first layer/thickness of second layer) of 4 to 5.

7. The photovoltaic cell according to claim 1, wherein the light absorbing material is ruthenium based dyes.

8. The photovoltaic cell according to claim 1, wherein the second electrode is a counter electrode.

9. The photovoltaic cell according to claim 3, wherein the first layer and the second layer have a thickness ratio (thickness of the first layer/thickness of second layer) of 4 to 5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

(2) FIG. 1 illustrates the principle of operation of nanocrystalline TiO.sub.2 dye-sensitized solar cells.

(3) FIG. 2 illustrates a schematic of a bilayer semiconductor nanofiber DSSC in accordance with one embodiment of the present invention.

(4) FIG. 3 illustrates an exemplary process for fabricating a bilayer semiconductor nanofiber photoanode.

(5) FIGS. 4A and 4B illustrate Scanning Electron Microscopy (SEM) images of the SNF and BNF layer.

(6) FIG. 5 illustrates a graph of UV-Visible transmission spectra of SNF and BNF layer having the same thickness.

(7) FIG. 6 illustrates a graph of power conversion efficiency versus combined thickness of photoanode for a constant r.sub.t or constant r.sub.h.

DESCRIPTION OF THE EMBODIMENTS

(8) The present invention provides a high efficiency DSSC using two layers of semiconductor super-fine fibers as photoanode. The fabrication of the DSSC is simple, fast, and cost effective.

(9) FIG. 2 illustrates a schematic of a bilayer semiconductor dye-sensitized solar cell (DSSC) in accordance with one embodiment of the present invention. Referring to FIG. 2, the DSSC 200 includes a semiconductor electrode 270 and a counter electrode 280. The semiconductor electrode 270 includes a transparent conductive substrate 210, a nanoparticle layer 215, a first nanofiber layer 220, and a second super-fine fiber layer 230. The counter electrode 280 includes a platinum layer 240, and a transparent conductive layer 250. In one embodiment, the first nanofiber layer 220 and second super-fine fiber layer 230 are made with titanium dioxide fibers, which are adsorbed with ruthenium based dye molecules. The void space of the first and second layers is filled with a mediator such as electrolyte (not shown). The electrolytes are adapted to transport electrons from the counter electrodes to the first and second layer to replenish the sensitized dyes. In one embodiment, the transparent conductive substrate 210 and 250 are fluorine-doped tin oxide (FTO) or indium tin oxide (ITO) glasses.

(10) First, fabrication of the bilayer fibrous electrode can be performed by controlled processes (e.g. electrospinning, hydro-thermal processing, etc.) in one step. Thus, the production method is fast, simple and cost-effective. The fabrication process will be discussed in more details in connection with FIG. 3. Second, there is a specific relationship between the fiber diameter of the bilayer for, respectively, the energy harvesting layer and the reflector layer. The first nanofiber layer 220 with the smaller diameter nanofibers (SNF) in the photoanode has an average diameter nominally of 10-300 nm with a preferred average diameter of around 50-80 nm, while the second super-fine fiber layer 230 (reflector layer) with a bigger diameter nanofibers (BNF) or super-fine fibers that has an average diameter of 50 nm-2 microns with a preferred diameter around 80-120 nm. The SNF layer with high surface area is adapted to adsorb sufficient dye molecules and directly transport electrons reducing loss due to recombination.

(11) The BNF layer primarily functions as a light scattering (light reflector) layer, ensuring adequate light is trapped in the device. Also, the BNF layer is also adapted to (a) harvest light as it also have dyes adsorbed onto the BNF, and (b) provide a permeable, porous, and well-connected structure for electrolytes (e.g. iodide ions I.sup. and I.sup..sub.3) transport, carrying electrons from the counter electrode to regenerate the sensitized dyes in the SNF.

(12) There is also a thin layer (e.g., a monolayer) of semiconductor nanoparticles 215 (e.g., TiO.sub.2 nanoparticles), typically 5-20 nm, which is coated on the conductive glass 210 (i.e., ITO or FTO glass) for (i) providing good attachment (avoiding any cracks/gaps) of the SNF-BNF layers onto the conductive glass, and (ii) blocking any electrons in escaping through gaps/cracks which results in efficiency loss. In another embodiment, an adhesive may be used in lieu of the semiconductor nanoparticles.

(13) FIG. 3 illustrates an exemplary process for fabricating a bilayer semiconductor nanofiber photoanode. To begin the fabrication process, a piece of FTO glass is prepared in S302. Next, in S304, a first layer of TiO.sub.2/PVP composite nanofibers are electrospun on the FTO glass from a precursor solution which contains titanium isopropoxide (TIP, 1.2 g), polyvinyipyrrolidone (PVP, 1 g), acetic acid (1 g) and ethanol (30 mL). Other materials with good affinity to titanium dioxide such as polyvinylacetate, polyvinylalcohol, polyethyleneoxide and the like may also be used to prepare the precursor solution. After a first layer of nanofibers is dispersed on the FTO glass, a second layer of super-fine fibers with larger diameter is electrospun on the first layer of nanofibers to create a bilayer structure.

(14) The diameter of the electrospun fibers can be influenced by processing parameters, and the diameter of the semiconductor (TiO.sub.2) fibers is controlled by adjusting the composition of electrospinning solution, collector distance, and voltage. The electrospinning process is continued until the fibrous layer of a desired thickness is reached. For instance, a voltage of 70 Kv is first applied on an electrospinning apparatus over a collecting distance of approximately 19 cm, and after a predetermined amount of time, the voltage is changed to 55 Kv. By changing the voltage during the electrospinning process, two fibrous layers with different diameters can be fabricated. The thicknesses of the two layers are controlled by their respective electrospinning time.

(15) Next, a calcination step is performed on the bilayer fiber in 450 C. for 2 h (S306). After calcination, the bilayer fiber is peeled off from the FTO glass due to shrinking effect and poor adhesion in step S308. Subsequently, another piece of FTO glass is prepared (S310) and a thin layer of TiO.sub.2 nanoparticles is formed on the FTO glass by doctor blading in step (S312). Thereafter, the pilled bilayer semiconductor fiber is placed on top of the nanoparticles layer in which the nanoparticles layer serves as a hole-blocking layer as well as a bonding layer. Subsequently this new photoanode is calcinated at 450 C. for 2 h (S314). The calcinated photoanode is further treated with an aqueous solution of TiCl.sub.4 (40 mM) at 60 C. for 15 min. Next, in step S316, the obtained bilayer TiO.sub.2 photoanode is sensitized in a solution of 0.03 mM Ru dye (N719) solution in absolute ethanol at 50 C. for 24 h. The soaked photoanode is then washed with ethanol to remove unanchored dye molecules and then the photoanode is left dried (S318).

(16) Platinum-sputtered FTO or conductive glass (not shown) is used as a counter electrode of the photoanode created in step. The counter electrode and dye anchored TiO.sub.2 photoanode are assembled into a sandwich structure with Surlyn (DuPont, 25 m). An electrolyte is filled in the photoanode, which is composed of 0.6M 1-methyl-3-propylimidazolium iodide (PMII), 0.05 M LiI, 0.05 M I.sub.2, and 0.5M 4-tert-butyl pyridine (TBP) in acetonitrile.

(17) FIGS. 4A and 4B are the Scanning Electron Microscopy (SEM) images of SNF and BNF layer, respectively. From these SEM images, it can be seen that electrospun nanofibers are randomly distributed in the layer with an average diameter of about 60 nm and 100 nm respectively.

(18) In a DSSC according to the present invention, the BNF layer acts as a light scattering layer which causes incident light on the light harvesting layer (i.e., SNF layer) to be sufficiently scattered, thus increase the optical path length in the DSSC device, and enhancing the light absorption in the device. As a result, by installing the light scattering layer (i.e., BNF layer) in the photoanode, the power conversion efficiency (PCE) of DSSC can be significantly improved to various levels above-and-beyond that of a single layer.

(19) FIG. 5 illustrates the UV-Visible transmission spectra of SNF and BNF layer with the same thickness. From UV-Visible transmission spectra, it can be seen that both of these layers show relatively high transmission (higher than 30%), and at the long wavelength range, the transmission higher than 50%. This transmission spectrum also shows that BNF scattering layer exhibiting lower transmission than that of the SNF layer at the wavelength range from 400 to 800 nm. This indicates more light is reflected in the BNF layer, and hence, there is less optical loss in the BNF layer which benefits from light harvesting.

(20) Because the individual thickness of the respective SNF and BNF layers can be controlled simply by their electrospinning time, an object of the present invention is to monitor the ratio r.sub.t of the electrospinning time for the SNF to that of the BNF to obtain the optimal performance. This ratio of the electrospinning time r.sub.t can be held in constant while the combined thickness H is also monitored and can be changed accordingly. H is the combined thickness of the SNF layer h.sub.SNF and BNF layer h.sub.BNF, i.e. H=h.sub.SNF+h.sub.BNF, where h.sub.SNF denotes the thickness of the SNF layer and h.sub.BNF denotes the thickness of the BNF layer.

(21) In another embodiment, one can monitor the thickness ratio r.sub.h, (i.e., the ratio of the thickness of BNF to that of the SNF) as used for non-electrospinning production of nanofibers for which formation time may not be conveniently measured. In conjunction with the above, the combined thickness H can also be monitored.

(22) FIG. 6 shows a graph of PCE as a function of H for a given ratio r.sub.t based on the electrospinning time. Similar curve can be drawn for constant ratio r.sub.h based on thickness. The behavior is typically an inverted concave behavior. At small thickness, PCE increases with thickness as there is increasing surfaces for dye to harvest solar energy, while at large thickness recombination rate becomes important and PCE reduces with further increase in thickness. There is a maximum efficiency for each ratio r.sub.t referred as PCE_max.

Example

(23) In this example, a photoanode having a bilayer structure, with an average SNF diameter of 60 nm and an average BNF diameter of 100 nm, is produced using the method described above in connection with FIG. 3. The ratio of electrospinning time of SNF (t.sub.SNF) to BNF (t.sub.BNF) can be in the range of 1 to 20. Table 1 shows the maximum efficiency obtained for different r.sub.t. As can be seen when r.sub.t varies between 2 and 5, the maximum efficiency rises above 8.4% with a preferred condition that at r.sub.t=5, the PCE_max jumps to 9.5%. For reference, this should be compared to the performance of a single-layer of nanofiber photoanode with fiber diameter of 60 nm and with a total thickness between 8-12 microns, which is 7.14% obtained also from our study. When r.sub.t increases above 5, the reflector nanofiber layer becomes small and ineffective, and the PCE_max falls back to 6.8%. This is almost equivalent that of a single nanofiber layer at 7.14%. The ratio of the thickness between the small nanofibers to that of the bigger nanofibers can be between 1 and 20, and an ideal range between 4 and 5.

(24) TABLE-US-00001 TABLE 1 Bilayer Nanofiber Photoanode (H = 8-12 microns) r.sub.t = t.sub.SNF/t.sub.BNF 2 3 4 5 6 r.sub.h = h.sub.SNF/h.sub.BNF 1-2 2-3 3-4 4-5 5-6 PCE_max 8.5% 8.4% 8.5% 9.5% 6.8%

(25) Table 2 compares the present invention with existing technologies that also utilize a reflector layer in the photoanode. Table 2 shows that our bilayer is much better than the one using nanoparticle-nanofiber (NP-NF) arrangement that was reported in Efficient dye-sensitized solar cells using electrospun TiO.sub.2 nanofibers as a light harvesting layer by Yoshikawa et al. (2008) at 7.1% (34% improvement), as well as earlier result disclosed in Influence of scattering layers on efficiency of dye-sensitized solar cells by Hore et al. (2006) on nanoparticle-nanoparticle, which is 6.8%.

(26) TABLE-US-00002 TABLE 2 Comparing present invention with prior arts, H = 6-12 microns Photoanode NF-NF with Bilayer NP-NF NP-NP reflectance (present (Yoshikawa, (Hore, layer invention) 2008) 2006) PCE 9.5% 7.1% 6.8% % Improvement 34% increase 100% / % Improvement 40% increase / 100%

(27) Also, another advantage is that the total layer thickness H of the present invention is usually less than 12 microns, predominantly 8-10 microns. This is much below that when nanoparticles are being used which increases up to 15-20 microns. In other words, a thinner layer (almost half as thick) means even lower cost in fabrication due to less materials being used.

(28) While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications and equivalent structures and functions.