Quantum dot-sensitized solar cell and method of making the same

The quantum dot-sensitized solar cell (QDSSC) includes a photoelectrode, a counter electrode, and an electrolyte sandwiched between the photoelectrode and the counter electrode. The photoelectrode is formed from a titanium dioxide (TiO2) layer, a cadmium sulfide (CdS) quantum dot sensitizer layer, and a tin dioxide (SnO2) nanograss layer sandwiched between the titanium dioxide (TiO2) layer and the cadmium sulfide (CdS) quantum dot sensitizer layer.

BACKGROUND

The disclosure of the present patent application relates to quantum dot-sensitized solar cells (QDSSCs), and particularly to a quantum dot-sensitized solar cell having a photoelectrode which includes a tin dioxide (SnO2) nanograss buffer layer.

2. Description of the Related Art

There is presently great interest in third-generation solar cells, such as quantum dot sensitized solar cells (QDSSCs), due to their inorganic structures, high theoretical efficiencies, ease of fabrication and relative low cost. Various metal chalcogenide-semiconductors, such as CdS, CdSe, PbS, CdSexS1-x, CuInS2, CuInZnS, etc. have been investigated as promising sensitizers for the QDSSCs due to their tunable band gap, high absorption coefficient and multiple exciton generation (MEG). A typical structure of the QDSSC is composed of a QD-sensitized TiO2photoelectrode, a polysulfide electrolyte (S2−/Sn2−), and a counter electrode, which leads to various hetero-interfaces. Although quantum dots (QDs) have unique characteristics, thus far, QDSSCs have achieved a low power conversion efficiency (PCE) of 12%, due to the recombination of charges that occur at the photoanode/QDs/electrolyte interfaces. Due to the potential that QDSSCs show, it would be desirable to be able to overcome this limitation. Thus, a quantum dot-sensitized solar cell and a method of making the same solving the aforementioned problems are desired.

SUMMARY

The quantum dot-sensitized solar cell (QDSSC) includes a photoelectrode, a counter electrode, and an electrolyte sandwiched between the photoelectrode and the counter electrode. The photoelectrode is formed from a titanium dioxide (TiO2) layer, a cadmium sulfide (CdS) quantum dot sensitizer layer, and a tin dioxide (SnO2) nanograss layer sandwiched between the titanium dioxide (TiO2) layer and the cadmium sulfide (CdS) quantum dot sensitizer layer. As non-limiting examples, the counter electrode may be made of copper sulfide (CuS), and the electrolyte may be a polysulfide electrolyte.

The quantum dot-sensitized solar cell is made by applying a titanium dioxide paste on a fluorine-doped tin oxide substrate. The titanium dioxide paste and the fluorine-doped tin oxide substrate are heated to form a mesoporous titanium dioxide electrode. Tin dioxide nanograss is then grown on the mesoporous titanium dioxide electrode using chemical bath deposition to form a pre-loaded electrode. The cadmium sulfide quantum dots are loaded on the pre-loaded electrode using successive ionic layer adsorption and reaction cycles to form the photoelectrode. The electrolyte is then sandwiched between the photoelectrode and the counter electrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As illustrated inFIG. 1, the quantum dot-sensitized solar cell (QDSSC)10includes a photoelectrode18, a counter electrode22, and an electrolyte20sandwiched between the photoelectrode18and the counter electrode22. The photoelectrode18is formed from a titanium dioxide (TiO2) layer12, a cadmium sulfide (CdS) quantum dot sensitizer layer16, and a tin dioxide (SnO2) nanograss layer14sandwiched between the titanium dioxide (TiO2) layer12and the cadmium sulfide (CdS) quantum dot sensitizer layer16. As non-limiting examples, the counter electrode22may be made of copper sulfide (CuS), and the electrolyte20may be a polysulfide electrolyte. As a further non-limiting example, the polysulfide electrolyte may be formed from 1 M Na2S, 2 M S and 0.1 M KCl in methanol and water solution (7:3). The electrolyte20may be sealed with the photoelectrode18and the counter electrode22using any suitable type of sealant, such as Meltonix 1170-60, manufactured by Solaronix SA of Switzerland.

In order to prepare photoelectrode18, prior to the deposition, fluorine-doped tin oxide (FTO, 13Ω sq−2) glasses (1.3×1.5 cm2) were washed with acetone, ethanol and deionized (DI) water for 10 minutes each. Mesoporous TiO2electrodes were prepared by doctor-blading a TiO2paste (20 nm diameter, manufactured by Solaronix SA of Switzerland) onto the FTO substrates. The samples were maintained at 450° C. for 30 minutes. Then, tin dioxide nanograss (SnO2NGs) layer14was grown on the TiO2surface via a facile chemical bath deposition (CBD) reaction. Particularly, 0.05 M SnCl2.2H2O and 0.15 M hexamethylenetetramine (HMTA) were mixed in 70 mL of DI water under magnetic stirring for 30 minutes. The as-prepared TiO2samples were dipped horizontally into the SnO2growth solution and heated at 95° C. for 10 h. The HMTA in the precursor solution not only serves as a surfactant by reducing the surface tension between the growth solution and the substrate, but it also supports the growth of the SnO2layer. After deposition, the SnO2NGs loaded FTO/TiO2films were rinsed with DI water and ethanol, and then heated at 450° C. for 30 mins, producing TiO2/SnO2NGs electrodes.

Next, the cadmium sulfide quantum dots (CdS QDs) were loaded on the TiO2/SnO2NGs by a facile successive ionic layer adsorption and reaction (SILAR) method. Particularly, the TiO2/SnO2NGs films were dipped in aqueous 0.1 M Cd(CH3COO)2.2H2O solution for 5 mins, rinsed with ethanol, then dipped in aqueous 0.1 M Na2S solution for 5 mins, and then again rinsed with ethanol. This process is a single SILAR cycle, and a total of eight SILAR cycles were repeated to produce TiO2/SnO2NGs/CdS films. Photoelectrode18can be formed from the TiO2/SnO2NGs/CdS film on the FTO substrate.

The CuS counter electrodes22were prepared on FTO substrates using a CBD method. The QDSSCs10were formed by sandwiching the TiO2/SnO2NGs/CdS photoelectrodes18and the CuS counter electrodes22using a sealant (Meltonix 1170-60, manufactured by Solaronix SA of Switzerland) with the polysulfide electrolyte20. In experiments, polysulfide electrolyte20was formed from 1 M Na2S, 2 M S and 0.1 M KCl in a methanol and water solution (7:3).

As will be discussed in detail below, the crystalline phase, elemental analysis and morphology of the electrodes were investigated using X-ray diffraction (XRD) with a D/max 2400 X-ray diffractometer manufactured by Rigaku® of Japan, X-ray photon spectroscopy (XPS) using an ESCALAB 250 analyzer manufactured by Thermo Scientific® of Massachusetts, and a scanning electron microscope (SEM) using an S-2400 model SEM, manufactured by Hitachi® of Japan, with energy-dispersive X-ray spectroscopy (EDX, 15 kV) mapping characterizations. The optical absorption properties of the as-prepared electrodes were analyzed by UV-visible absorption spectra using a 3220UV UV-vis spectrophotometer manufactured by Optizen. The photocurrent-voltage (J-V) measurements were conducted using a solar simulator manufactured by Abet Technologies, which exhibits an intensity of 100 mW cm−2. Electrochemical impedance spectroscopy (EIS) was carried out using an SP-150 electrochemical workstation manufactured by BioLogic, with a frequency range of 500 kHz to 0.1 Hz and an AC amplitude of 10 mV.

The XRD patterns of the as-prepared TiO2and TiO2/SnO2NGs are shown inFIG. 2. As seen inFIG. 2, both the TiO2and TiO2/SnO2NGs samples exhibit peaks at 20=25.1°, 37.5°, 47.8°, 53.9° and 62.3, which are well-matched with the TiO2tetragonal anatase structure (JCPDS:21-1272). After the deposition of SnO2over the TiO2surface, new diffraction peaks were observed at 2θ=26.7°, 33.6°, 51.9°, 54.9°, 61.5° and 65.8°, which are consistent with the SnO2tetragonal structure (JCPDS:41-1445).

XPS characterization was performed to investigate the valence states of elements in the TiO2/SnO2NGs electrode. The XPS survey spectra of the TiO2/SnO2NGs sample is shown inFIG. 3A, which clearly shows the presence of only Ti, Sn, O and C elements. The presence of the carbon element in the total survey scan spectrum is ascribed to the exposure of the sample to air. As shown inFIG. 3B, the XPS spectrum of Ti 2p exhibits the Ti 2p3/2and Ti 2p1/2peaks that are centered at 458.7 eV and 464.5 eV, respectively. These spectra are consistent with the presence of a Ti4+valence state. Due to the spin-orbit coupling of Ti 2p, the obtained peaks are separated by 5.8 eV. The Sn 3d spectrum shown inFIG. 3Cexhibits two peaks at 486.8 eV and 495.3 eV, which are indexed to Sn 3d5/2and Sn 3d3/2, respectively, and agree with the Sn4+state in SnO2. As shown inFIG. 3D, the spectrum of O 1s displays a peak that is centered at 530.9 eV, corresponding to O2in the TiO2and SnO2crystal lattices. The XRD and XPS results confirmed the formation of SnO2NGs on the TiO2surface.

The surface morphologies of the as-prepared TiO2and TiO2/SnO2NGs electrodes were studied using SEM analysis, and the corresponding images are shown inFIGS. 4A and 4Bfor TiO2on a FTO substrate, and inFIGS. 4C and 4Dfor SnO2on the TiO2surface (TiO2/SnO2) NGs. As shown inFIG. 4A, the TiO2nanoparticles are uniformly distributed throughout the substrate. It can be clearly seen fromFIG. 4B, that the TiO2exhibits spherical nanoparticle morphology with diameters in the range of ˜16 to ˜20 nm. As discussed above, the CBD method was used to grow the SnO2nanostructures on the surface of the TiO2. It can be clearly observed inFIG. 4Cthat the SnO2exhibits the nanograss (NGs) morphology. The diameter of the SnO2NGs was estimated to be in the range of ˜21 to ˜46 nm (FIG. 4D). The SEM images clearly reveal that the SnO2NGs nanostructures were successfully grown on the TiO2surface. In addition, SEM-EDX color mapping was carried out to examine the distribution of elements in the TiO2/SnO2NGs, revealing the homogeneous distribution of Ti, Sn and O elements in the as-prepared TiO2/SnO2NGs electrode.

UV-vis absorption measurements were carried out to demonstrate the optical properties of the TiO2, TiO2/SnO2NGs, TiO2/CdS and TiO2/SnO2NGs/CdS samples, and the results are plotted inFIG. 5in the range of 300 to 800 nm. The TiO2sample exhibits an absorption edge at around 378 nm. The growth of SnO2NGs on the TiO2surface increased the absorption intensity. In addition, the absorption onset of TiO2/SnO2NGs increased to 388 nm. The absorption onsets of the TiO2/CdS and Ti2/SnO2NGs/CdS samples were measured to be 510 nm and 525 nm, respectively. The absorption onset of the Ti2/SnO2NGs/CdS electrode was increased and had a higher absorbance when compared to Ti2/CdS. These observations can be attributed to the growth of SnO2NGs passivation/barrier layer over the TiO2surface. Further, the SnO2NGs interlayer supports the nucleation and growth of CdS QDs, which results in the high rate of absorption of the TiO2/SnO2NGs/CdS electrode.

To evaluate the impact of the SnO2NGs interlayer on the photoanodes, the photovoltaic behaviors of TiO2/CdS and TiO2/SnO2NGs/CdS-based QDSSCs were investigated. The J-V profiles of the as-fabricated QDSSCs were measured under one sun illumination, and the related photovoltaic considerations are shown inFIG. 6Aand Table 1 below. The TiO2/CdS QDSSC exhibits a short-circuit current density (JSC) of 6.64 mA cm−2, an open circuit voltage (VOC) of 0.614 V, and a fill factor (FF) of 0.530, resulting in a power conversion efficiency (PCE) of 2.16%. When a SnO2NGs layer was grown on a TiO2surface (TiO2/SnO2NGs/CdS), the JSC, VOCand FF were considerably improved to 8.92 mA cm2, 0.619 V, and 0.570, respectively, resulting in an elevated PCE of 3.15%, which is much higher than the PCE of a TiO2/CdS (2.16%) photoelectrode. The enhanced VOCand JSCof the TiO2/SnO2NGs/CdS-based QDSSCs are attributed to the reduced charge recombination and the fast charge transfer at the TiO2/QDs/electrolyte interfaces. Thus, the PCE of the QDSSC has improved from 2.16% to 3.15% by the growth of the SnO2NGs interlayer over the TiO2surface. Furthermore, an incident photon to current conversion efficiency (IPCE) study was carried out to illustrate the light absorption and electron generation behaviors in the QDSSCs.FIG. 6Bshows the IPCE plots of the as-prepared QDSSCs. It can be seen from the IPCE spectra that the introduction of the SnO2NGs interlayer enhanced the IPCE response from 66% to 75% and also enlarged the IPCE response edge from 548 nm to 568 nm. The TiO2/SnO2NGs/CdS-based QDSSCs exhibit a higher IPCE response than the TiO2/CdS, which is in good agreement with the JSCof the J-V plots.

TABLE 1Photovoltaic parameters of QDSSCs based on TiO2/CdS andTiO2/SnO2 NGs/CdS photoelectrodes under one sun illuminationVOCJSC(mAPCEτe(V)cm−2)FF%Rct(Ω)Cμ(μm)(ms)TiO2/CdS0.6146.640.5302.168.78135811.92TiO2/SnO20.6198.920.5703.1527.94218360.99NGs/CdS

In order to demonstrate the impact of the SnO2-NGs interlayer on the photoelectrode films, electrical impedance spectroscopy (EIS) measurements were conducted under forward bias (VOC) and illumination. The Nyquist plots of the TiO2/CdS and TiO2/SnO2NGs/CdS-based QDSSCs are shown inFIG. 6C. The Nyquist plots of both devices exhibit the two semicircle-type shapes, in which the first (small) semicircle is obtained in the high-frequency region and represents the charge transfer resistance (RCE), the chemical capacitance (CCE) at the interface of the counter electrode/electrolyte, and the series resistance (RS) which is obtained in the high-frequency region, where the phase is zero. The recombination resistance (Rct) at the TiO2/QD/electrolyte interface and the chemical capacitance (Cμ) indexed to the bigger semicircle, which is obtained at mid/low frequency. Based on the equivalent circuit model (inset ofFIG. 6C) and using Z-view software, the Nyquist plots were fitted to obtain the Rctvalues, and the fitting results are summarized in Table 1 above. There is no considerable change in the RCE, which is due to the usage of similar electrolytes and counter electrodes in the fabrication of the QDSSCs. However, the Rctof the TiO2/SnO2NGs/CdS-based QDSSC (27.94 Ωcm2) is much higher than that of the TiO2/CdS (8.78 Ωcm2). The higher Ra of the TiO2/SnO2NGs/CdS-based QDSSC demonstrates that the growth of SnO2NGs over the TiO2surface successfully suppresses the electron recombination at the TiO2/QDs/electrolyte interfaces. Thus, the growth of the SnO2NGs interlayer hinders the interfacial charge recombination and enhances the photovoltaic performance (JSCand FF). Moreover, the QDSSC based on the TiO2/SnO2NGs/CdS photoelectrode achieves higher Cμ(2183 μF) than that of the TiO2/CdS (1358 μF) system. The higher Cμof the Ti2/SnO2NGs/CdS system demonstrates the improved collection of photo-excited electrons into the conduction band of the photoanode, which is mainly due to the hindered recombination at the TiO2/QDs/electrolyte interfaces. Further, the electron life time (τc) of the QDSSCs can be obtained as τc=Rct×Cμ. It should be noted that the T of the TiO2/CdS with SnO2NGs interlayer (60.99 ms) is much higher than that of the SnO2NGs-free device (11.92 ms), which reveals the higher charge collection efficiency of the TiO2/SnO2NGs/CdS system.

Under dark conditions, the J-V plots of the TiO2/CdS and TiO2/SnO2NGs/CdS-based QDSSCs were obtained and are shown inFIG. 6D. The electron recombination occurs at the interfaces of TiO2/QDs and TiO2/electrolyte, and this is the source of the dark current. Thus, a dark current study is a useful indicator of charge recombination. With the introduction of the SnO2NGs interlayer, the TiO2/SnO2NGs/CdS device delivers the reduced dark current, which is lower compared with that of the TiO2/CdS device. The reduction of the dark current arose from an enhancement in electron transport with a decrease in the internal resistance. As a result, the electron recombination was effectively reduced by the introduction of the SnO2NGs interlayer, and this contributed to the enhanced current and decreased electron loss.

Furthermore, to investigate the impact of the SnO2NGs interlayer on the performance of the QDSSCs, EIS tests were conducted at various bias applied voltages under dark conditions in the 500 kHz to 100 mHz frequency range. The obtained recombination resistance (Rrec) and chemical capacitance (Cμ) from the corresponding EIS tests are shown inFIGS. 7A and 7B, respectively. The Rrecand Cμreveal that the charge recombination process occurs at the TiO2/QDs/electrolyte interfaces. A higher Rrecrepresents the low recombination rate and greater Cμvalues denote the Fermi level upward shift, yielding the enhanced VOC. It can be seen fromFIG. 7Athat the Rrecvalues of both devices decrease with the increment of the forward bias voltage due to the increased Fermi level of TiO2at the forward bias. Also, TiO2/SnO2NGs/CdS QDSSCs exhibit higher Rrecvalues than the Ti2/CdS device under identical bias voltages. The recombination rate is inversely proportional Rrec. Moreover, the higher Cμvalues of the TiO2/SnO2NGs/CdS device denote the upward shift of the Fermi level of TiO2, resulting in a large VOC. These EIS studies reveal the reduced recombination rate for the TiO2/SnO2NGs/CdS device, which is favorable for the improved JSCand FF.

The excited electrons life time was examined using the open-circuit voltage (VOC) decay studies with time. Initially, the QDSSCs were irradiated with one sun illumination to a steady voltage, then the illumination was turned off and the VOCdecay data was obtained.FIG. 8Ashows the VOCdecay profiles of the TiO2/CdS and TiO2/SnO2NGs/CdS-based QDSSCs. The VOCdecay plots clearly exhibit the continued monitoring of VOCvalues under illumination and approach to decay after switching off the illumination. It is evident from the VOCdecay plots that the TiO2/SnO2NGs/CdS-based QDSSCs exhibit a slower voltage decay rate than the TiO2/CdS photoanode. This behavior is mainly attributed to the SnO2NGs interlayer, which efficiently suppresses the charge recombination at the Ti2/QDs/electrolyte interfaces and also promotes efficient electron transfer. Further, the electron life time (τe) can be estimated as

τe=-(kB⁢Te)⁢(dVOCdt)-1,
where kB, T and e are the Boltzmann constant, temperature and electron charge, respectively.FIG. 8Bshows the plot of electron life time (τe, in log sign) as a function of VOC. The TiO2/SnO2NGs/CdS-based QDSSCs deliver longer τevalues than that of TiO2/CdS, implying a suppressed recombination of the photo-generated electrons, leading to the efficient charge transfer, which agrees well with the EIS analysis.

FIGS. 9A and 9Bshow the impact of the SnO2NGs interlayer on the charge transfer mechanism in QDSSCs. As shown inFIG. 9A, upon illumination, CdS captures the photons and produces the electron-hole pairs (i.e., the excitons). Then, the electrons transfer into the TiO2conduction band, while the holes are reduced by the polysulfide electrolyte. Simultaneously, the possibility of charge recombination also takes place at the interfaces of TiO2/CdS QDs/electrolyte, which results in poor photovoltaic performance. Thus, the SnO2NGs interlayer is introduced between the TiO2and CdS QDs (TiO2/SnO2NGs/CdS) to prevent the charge recombination (FIG. 9B). The introduction of the SnO2NGs interlayer on the TiO2surface retards the back injection of the electron from the TiO2to the QDs, and also prevents the injection of the excited electron from the TiO2to redox couple, resulting in the high photovoltaic performance. Therefore, introducing the SnO2NGs interlayer over the TiO2surface achieves desirable properties, such as a wide solar light-harvesting ability, and an enhanced charge transfer and suppressed charge recombination at the TiO2/QDs/electrolyte interfaces.

It is to be understood that the quantum dot-sensitized solar cell and the method of making the same are not limited to the specific embodiments described above, but encompasses any and all embodiments within the scope of the generic language of the following claims enabled by the embodiments described herein, or otherwise shown in the drawings or described above in terms sufficient to enable one of ordinary skill in the art to make and use the claimed subject matter.