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

Abstract

A quantum dot sensitized solar cell (QDSSC) includes a highly catalytic Ni-doped CuS thin film as a counter electrode (CE). The Ni-doped CuS CE can deliver outstanding electrocatalytic activity, conductivity, and low-charge transfer resistance at the CE/electrolyte interface. As a result, the QDSSC can achieve higher efficiency (=4.36%) than a QDSSC with a bare CuS CE (3.24%).

Claims

1. A quantum dot-sensitized solar cell, consisting of: a quantum dot-sensitized photoanode, the photoanode consisting of TiO.sub.2, CdS, and CdSe; a redox couple including a liquid polysulfide electrolyte; a photo cathode including a Ni-doped CuS counter electrode, wherein the Ni-doped CuS is in the form of a plurality of nanoflake structures, each nanoflake structure comprising a nanoflake having an array of nanoparticles grown on a surface thereof, and a sealant.

2. The quantum dot-sensitized solar cell as recited in claim 1, wherein the counter electrode delivers a charge transfer resistance of 7.82.

3. A method of preparing a quantum dot-sensitized solar cell, comprising: preparing a Ni-doped CuS solution; immersing a plurality of fluorine-doped tin oxide substrates vertically in the Ni-doped CuS solution, wherein the substrates immersed in the Ni-doped CuS solution are heated in a hot air oven at about 60 C. for about 90 minutes; heating the substrates immersed in the Ni-doped CuS solution to provide a Ni-doped CuS counter electrode, wherein the Ni-doped CuS is in the form of a plurality of nanoflake structures, each nanoflake structure comprising a nanoflake having an array of nanoparticles grown on a surface thereof; providing a photoelectrode consisting of TiO.sub.2, CdS, and CdSe; assembling the counter electrode and the photoelectrode together using a sealant; and filling a space between the photoelectrode and the counter electrode with a polysulfide electrolyte.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph showing the XRD pattern of CuS and NiCuS films on the FTO substrate.

(2) FIGS. 2A-2D show XPS patterns of 2(A) survey spectrum and high-resolution spectra; (2B) Cu 2p; (2C) Ni 2p; and (2D) S 2p signals.

(3) FIG. 3 is a schematic showing the QDSSCs based on CuS and NiCuS CEs and the electron charge transfer mechanisms at the interface of the CE/electrolyte.

(4) FIGS. 4A-4B show FIG. 4(A) EIS and 4(B) Tafel polarization plots of symmetrical cells based on CuS and NiCuS electrodes.

(5) FIGS. 5A-5B show 5(A) J-V and 5(B) IPCE measurements of QDSSCs based on CuS and NiCuS counter electrodes.

(6) Similar reference characters denote corresponding features consistently throughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(7) A quantum dot-sensitized solar cell (QDSSC) includes a highly catalytic Ni-doped CuS thin film as a counter electrode (CE). The CE has high electrocatalytic activity and low charge transfer resistance. The Ni-doped CuS CE thin film can be fabricated on a fluorine-doped tin oxide (FTO) substrate via a facile chemical bath deposition method.

(8) In an embodiment, the QDSSC, according to the present teachings includes a quantum dot-sensitized photoanode, a redox couple including a liquid electrolyte and a photocathode including the Ni-doped CuS counter electrode (NiCuS CE) thin film. In an embodiment, the photoanode includes TiO.sub.2, CdS, and CdSe. In an embodiment, the redox couple includes a polysulfide electrolyte.

(9) As described herein, the NiCuS counter electrode exhibits a higher electrocatalytic activity than bare CuS, i.e., without Ni doping. It was also found that doping in the CE material offers abundant active sites for the catalytic reactions and an enhanced pathway for fast electron transport, which results in a lower charge transfer resistance at the interface of CF/electrolyte. The NiCuS delivers a higher electrocatalytic activity than CuS for S'S redox couple. Further, the QDSSC with the Ni-doped CuS counter electrode provides a higher efficiency (4.36%) than the bare CuS (3.24%).

(10) In an embodiment, a method of preparing a quantum dot-sensitized solar cell can include preparing a Ni-doped CuS solution; immersing a plurality of fluorine-doped tin oxide substrates vertically in the Ni-doped CuS solution; heating the substrates immersed in the Ni-doped CuS solution to provide a Ni-doped CuS thin film counterelectrode; providing a photoelectrode including TiO.sub.2, CdS, and CdSe; assembling the counterelectrode and the photoelectrode together using a sealant; and filling a space between the photoelectrode and the counterelectrode with a polysulfide electrolyte. In an embodiment, substrates immersed in the Ni-doped CuS solution are heated in a hot air oven at about 60 C. for about 90 minutes.

(11) The present teachings are illustrated by the following examples.

Example 1

Preparation of CuS and Ni-Doped CuS CEs on FTO Substrate

(12) Prior to deposition, FTO substrates (Hartford Glass, 1.61.3 cm.sup.2) were thoroughly ultrasonically cleaned with acetone, ethanol and distilled (DI) water for 10 min each. The Ni-doped CuS and bare CuS counter electrodes were prepared on the FTO substrate through a facile chemical bath deposition (CBD) route. Briefly, a Ni-doped CuS solution was prepared by mixing 0.1 M of CuSO.sub.4.5H.sub.2O, 10 mM NiSO.sub.4.6H.sub.2O, 0.4 M of C.sub.2H.sub.5NS and 0.4 M of CH.sub.4N.sub.2O in 50 mL DI water to form a mixture. The mixture was stirred for 15 min to form a clear Ni-doped CuS solution. The cleaned FTO substrates were placed vertically in the Ni-doped CuS growth solution and kept in a hot air oven at 60 C. for 90 min. After deposition, Ni-doped CuS films (active area of 0.7 cm.sup.2) were taken from the solution and rinsed several times with DI water and ethanol. The as-prepared thin film was labelled NiCuS. The bare CuS CE was also prepared without the addition of NiSO.sub.4.6H.sub.2O using a similar approach and labelled CuS.

Example 2

Fabrication of QDSSCs and Symmetric Cells

(13) TiO.sub.2/CdS/CdSe photoelectrodes were fabricated using the procedure described in C. V. V. M. Gopi, S. S. Rao, S. K. Kim, D. Punnoose, H. J. Kim, Highly effective nickel sulfide counter electrode catalyst prepared by optimal hydrothermal treatment for quantum dot-sensitized solar cells. J. Power Sources 275, 547-556 (2015). Based on this process, TiO.sub.2 nanoparticles were first deposited on the FTO substrate using the doctor blade method (0.27 cm.sup.2 active area). Then, TiO.sub.2 films were in situ sensitized with CdS and CdSe using a facile SILAR method. Finally, the NiCuS CEs and TiO.sub.2/CdS/CdSe photoelectrodes were assembled using a sealant (SX 1170-60, Solaronix) at 100 C. and the space between the electrodes was filled with a polysulfide electrolyte including 1 M Na.sub.2S, 2 M S and 0.2 M KCl in methanol and water at a ratio of 7:3.

(14) Two identical NiCuS and CuS electrodes were assembled using a sealant (SX 1170-60, Solaronix) at 100 C. to fabricate symmetrical cells and these cells were filled with the polysulfide redox couple.

Example 3

Characterization

(15) The surface morphology, structure, crystal nature and chemical composition of NiCuS and CuS thin films were examined using a scanning electron microscope (SEM, S-2400, Hitachi), X-ray diffraction (XRD, D/Max-2400, Rigaku) and X-ray photon spectroscopy (XPS, VG Scientific ESCALAB 250). The photovoltaic current-voltage (J-V) characteristics were measured using an ABET Technologies (USA) solar simulator under one sun illumination (AM 1.5G, 100 mW cm.sup.2). The EIS (in the frequency range of 0.01 mHz to 500 kHz) and Tafel polarization (at a scan rate of 10 mV s.sup.1) measurement were conducted using a SP-150 BioLogic instrument for the symmetrical cells (NiCuS//NiCuS and CuS//CuS) in dark conditions.

(16) The CuS and NiCuS CEs on FTO substrate were prepared using the facile CBD route described previously. Initially, the phase structure and composition of the CuS and NiCuS electrodes were examined by XRD characterization. FIG. 1 depicts the XRD pattern of the CuS and NiCuS thin films on the FTO substrate. Due to the background of the FTO substrate, the FTO peaks are obtained in the XRD pattern and are denoted with the diamond symbol. The diffraction peaks of CuS and NiCuS films at 20 values of 27.7, 29.4, 32.0 and 47.9 are indexed to the (101), (102), (103) and (110) crystal planes of the hexagonal CuS phase (JCPDS no. 06-0464). The peak intensity changes observed in the XRD spectra suggest that the Ni dopant can introduce increased disorder into the CuS material structure and thereby improve the electrocatalytic reduction of polysulfide electrolytes in the QDSSC. These results confirm the successful deposition of CuS and NiCuS compounds on the FTO substrates.

(17) Furthermore, the valence state of the elements in the NiCuS thin film was investigated using X-ray photoelectron spectroscopy (XPS). The survey spectra of NiCuS film and high-resolution spectrum of Cu 2p, Ni 2p and S 2p elements are shown in FIGS. 2A-2D. The survey spectra of NiCuS shows the existence of Cu, Ni, S, O, and C signals, where the O Is signal is due to oxygen on the sample surface in air (FIG. 2A). As shown in FIG. 2B, the high-resolution Cu 2p spectrum shows the two peaks located at binding energy of 932.8 eV for Cu 2p.sub.3/2 and 952.4 eV for Cu 2p.sub.1/2, and there is no satellite peak in the Cu 2p spectrum, which reveals CuI+ state in the NiCuS film. As depicted in FIG. 2C, the Ni 2p spectra exhibits the peaks including Ni 2p.sub.3/2 at 853.4 eV, Ni 2p.sub.1/2 at 870.8 eV, and a satellite peak at 858.1 and 877.2 eV, which is consistent with the presence of Ni.sup.2+ state in the NiCuS film. With respect to the S 2p spectrum in FIG. 2D, the two strong peaks observed at 161.6 and 162.9 eV were attributed to the binding energies of S 2p.sub.3/2 and S 2p.sub.1/2, which can be ascribed to the S2 signal. A weak satellite peak located at 168.0 eV can be attributed to the SO.sub.4.sup.2 signal due to the sulfur oxidation, confirming the presence of surface oxidation. The XPS and XRD studies indicate that the NiCuS is successfully deposited on the surface of the FTO substrate.

(18) The surface morphology of the CuS and NiCuS CEs were examined using SEM and the corresponding low and high-magnification SEM images. The CuS thin film on FTO substrate and the nanoflake structures were grown on the surface of the FTO substrate. When the Ni content was doped to the CuS, the surface became denser and the voids in between the nanoflakes were reduced with uniform deposition. An array of nanoparticles were grown on the nanoflake surface with the Ni doping of CuS. The nanoparticles of the NiCuS thin film offers abundant electroactive sites for the reaction of the polysulfide redox couple, supports the facial electron transfer and provides the enhanced electrocatalytic activity, which results in low charge transfer resistance at the CE/electrolyte interface.

(19) FIG. 3 depicts the structure of the QDSSCs and the charge transfer process that occurred in the solar cells based on CuS and NiCuS CEs. Upon 1 sun illumination (AM 1.5G, 100 mW cm.sup.2), excitons were generated by the excited state of QDs and the electrons were driven toward the TiO.sub.2 conduction band and finally transferred to the FTO substrate, while the hole transferred into the electrolyte and oxidized the polysulfide redox couple. The injected electrons flowed through the semiconductor network to the back contact, transferred to the nanoflake CuS CE, and then participated in the reduction reaction of Sx.sup.2 at the CuS CE/electrolyte interface. However, after doping with Ni in CuS CE (NiCuS), the favorable surface morphology of nanoparticles over nanoflake structures served as an excellent electrical tunnel for rapid electron transport from the external circuit, greatly enhanced the contact between NiCuS film and FTO, and also provided rich redox reactions, which resulted in high current density.

(20) EIS and Tafel polarization measurements were conducted on symmetrical cells in dark conditions to investigate the charge transfer mechanism and electrocatalytic property of the CEs. EIS Nyquist plots of the CuS and NiCuS symmetrical cells are shown in FIG. 4A. The Nyquist plot shows the series resistance (RS) at the intercept of the X-axis in the high-frequency region. The semicircle in the middle frequency region corresponds to the charge transfer resistance (Rct) and chemical capacitance (C) at the CE/electrolyte interface, while the low-frequency region represents the Warburg diffusion impedance (ZW) of the polysulfide electrolyte. The obtained Nyquist plots were analyzed using Z-software with the equivalent circuit model (inset of FIG. 4A). The corresponding Nyquist plots data are shown in Table 1. The NiCuS CE exhibits a lower RS (5.11) than that of the bare CuS CE (6.43). The higher series resistance is due to the network shape of NiCuS, resulting in a longer transport pathway for the electrons. More importantly, the NiCuS CE delivers the lowest R.sub.ct (7.82) compared to bare CuS CE (41.07), denoting that the NiCuS renders outstanding electrocatalytic activity. Further, NiCuS CE archives a higher C value (681.27 F) than the bare CuS CE (129.48 F), representing that the NiCuS CE has larger active areas for the reduction of Sx.sup.2 to S.sup.2, consistent with the R.sub.ct trends of CuS<Ni-CuS. In addition, the NiCuS CE exhibited a smaller ZW of 2.99 than CuS CE (9.83), revealing much more efficient diffusion of the polysulfide electrolyte at the CE/electrolyte interface.

(21) Furthermore, a Tafel polarization study is a useful measurement to examine the electrocatalytic activity of CEs. The extrapolated intercept of the anodic and cathodic branches of the Tafel plot reveals an exchange current density (J.sub.O) (FIG. 4B), which is directly related to the electrocatalytic activity of CE. NiCuS CE delivered a higher exchange current density (J.sub.O) than that of bare CuS CE, revealing that a lower activation energy is needed by the reduction process of S.sup.2/S.sub.n.sup.2 for NiCuS CE, which demonstrates the higher electrocatalytic activity of the NiCuS CE. In addition, the J.sub.O is directly related to the R.sub.ct using the following equation (1)]:
J_.sub.O=RT/(nFR_.sub.ct)(1),

(22) where R, n, T, and F have their usual meanings. R.sub.ct is the charge-transfer resistance obtained from the EIS study at the interface of CE/electrolyte. A higher J.sub.O for NiCuS CE contributes to lower R.sub.ct values in the EIS study.

(23) In addition, NiCuS CE delivers a higher limiting current density (J.sub.lim) than the bare CuS CE, indicating a higher diffusion velocity for the NiCuS CE in polysulfide redox couple. Based on equation (2), the J.sub.lim is directly related to the diffusion coefficient (D), which represents the diffusion behavior of the S.sup.2/S.sub.n.sup.2 redox couple in the electrolyte.
D=J_.sub.lim/2nFC(2),

(24) where D, l, n, F and C have their usual meanings. Thus, the charge transfer property and electrocatalytic activity of NiCuS CE is much better than that of CuS CE. The Tafel polarization study is consistent with EIS results.

(25) To investigate the performance of NiCuS as the CE of QDSSCs, TiO.sub.2/CdS/CdSe was employed as the photoelectrode and S.sup.2/S.sub.x.sup.2 as the electrolyte. FIGS. 5A-5B depict the J-V profiles of TiO.sub.2/CdS/CdSe QDSSC based on NiCuS and bare CuS CEs, observed under the One Sun illumination (AM 1.5, 100 mW cm.sup.2). The corresponding photovoltaic parameters, such as photocurrent density (J.sub.SC, mA cm.sup.2), open-circuit voltage (V.sub.OC, V), fill factor (FF), and power conversion efficiency () are summarized in Table 1.

(26) TABLE-US-00001 TABLE 1 Photovoltaic parameters of QDSSCs assembled with the TiO.sub.2/CdS/CdSe/ZnS photoanode and the CuS and NiCuS CEs in the presence of the polysulfide electrolyte. J.sub.SC V.sub.OC (mA R.sub.s R.sub.ct C.sub. Z.sub.w Cell (V) cm.sup.2) FF % () () (F) () CuS 0.559 10.63 0.546 3.24 6.43 41.07 129.48 9.83 NiCuS 0.567 13.78 0.558 4.36 5.11 7.82 681.27 2.99

(27) EIS and Tafel polarization measurements were conducted on symmetrical cells in dark conditions to investigate the charge transfer mechanism and electrocatalytic property of the CEs. Table 1 shows the EIS results of symmetrical cells fabricated with CuS and NiCuS CEs, and the cells filled with a polysulfide electrolyte. As depicted in Table 1, the QDSSC with NiCuS CE achieved excellent photovoltaic performance with an i of 4.36%, a J.sub.SC of 13.78 mA cm.sup.2, a V.sub.OC of 0.559 V, and a FF of 0.546, while the QDSSC with bare CuS CE had of 3.24%. From the J-V results, the enhanced J.sub.SC and FF of the QDSSC with NiCuS can be attributed to the lower R.sub.ct as aforementioned in the EIS results and higher electrocatalytic activity of NiCuS. Hence, the J-V results demonstrate that Ni doping of CuS CE achieved good electrocatalytic activity for polysulfide redox couple and is very suitable to use as an effective CE for QDSSC. The IPCE measurement was also conducted to examine the outstanding performance of NiCuS as a CE. FIG. 5B depicts the IPCE spectra of the QDSSCs with CuS and NiCuS CEs as a function of wavelength. As shown in the figure, the QDSSC based on NiCuS CE exhibits a maximum IPCE value (74%) in the range of 350-700 nm compared with that of bare CuS CE (64%), which is consistent with the abovementioned J-V measurement, illustrating the outstanding performance of the NiCuS CE for the polysulfide electrolyte.

(28) It is to be understood that the quantum dot-sensitized solar cell is 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.