We report a quantum dot sensitized solar cell (QDSSC) with a thioglycolic acid (TGA) capped CdSe quantum dot (QD) sensitized ZnO nanorod photoanode. As revealed by UV-Vis absorption spectrum and transmission electron microscopy, the quantum dots can be effectively adsorbed onto ZnO nanorods. By studying the emission decay, the quenching of the CdSe QDs by ZnO nanorod was verified, and an electron transfer (from QD to ZnO) rate constant of 1 x 108 s−1 was obtained. The efficiency of the as-prepared QDSSC was 0.66% and an incident power conversion efficiency of 22% at 400 nm was achieved.
© 2010 OSA
Zinc oxide (ZnO) is a multifunctional material with a wide direct band gap (3.37 eV) and large exciton binding energy (60 meV) , which has attracted tremendous attention as a potential candidate for applications in filed emissions , light-emitting diode , biosensors , electrochromic display , dye-sensitized solar cells (DSSCs) , and quantum dot sensitized solar cells (QDSSCs) . Besides a proper energy level and high electronic mobility, ZnO has other unique advantages suitable for QDSSC photoanode. For example, ZnO can be easily fabricated in nanowire form; a morphology that helps to improve the electron transport by avoiding particle-to-particle hopping that occurs in the TiO2 network (used in conventional QDSSC) . ZnO nanorod array also acts as an efficient antireflection coating layer to increase light coupling in solar cells . To aid the readers, the basic working mechanism of QDSSC with ZnO photoanode is briefed as follow. Under sunlight irradiation, electron-hole pairs are firstly generated in QDs. With proper band alignment, some electrons can be injected from the QD excited state into the ZnO conduction band (before thermal de-excitation or any other quenching process occurs). Then some injected electrons are transported through ZnO and collected by the external circuit. The holes are scavenged by the electrolyte, which is connected by a counter electrode and linked to the external circuit.
However, few ZnO nanostructure based QDSSCs have been reported and the efficiency is rather low. Zhang et al. applied a chemical deposition method to load QDs onto ZnO nanowires and obtained an efficiency of 0.34% . Leschkies et al. performed plasma treatment on ZnO nanowire to effectively adsorb mercaptopropionic acid (MPA) capped CdSe and the power conversion efficiency obtained was 0.4% .
In this letter, we shall report a QDSSC with ZnO-nanorod photoanode fabricated by low temperature hydrothermal decomposition. CdSe QDs were loaded onto ZnO nanorods through two capping ligands, thioglycolic acid (TGA) and MPA, which can form chemical bonds with ZnO leading to a large loading of QDs. We show that the electron transfer is faster in ZnO-TGA-CdSe system due to a shorter chain length of TGA (HS-CH2-COOH) compared to MPA (HS-(CH2)2-COOH) molecule.
To fabricate TGA- and MPA-capped CdSe QDs, a 1 mL purified OA-capped CdSe QD toluene solution containing approximately 50 mg of CdSe QDs was firstly mixed with 1mL TGA (MPA) and 1 mL acetone in a vial. The OA-capped QDs synthesis can be found elsewhere . The mixture solution was continuously stirred for 30 - 40 min (some ethanol was added). The solution was then centrifuged at 3500 rpm for 5 min with the supernatant discarded. The resulting precipitates containing TGA-capped CdSe QDs were dissolved in ethanol solution and some tetramethylammonium hydroxide (TMAOH) in methanol was added to make it clear. ZnO nanorod was grown by a hydrothermal method at 95°C . In brief, a 10 nm ZnO seed layer was firstly deposited on a fluorine-doped SnO2 (FTO) (15Ω/square, Solaronix) glass substrate by ultrasonic spray pyrolysis. Then the FTO substrate was immersed in a solution containing 0.01 M zinc nitrate and 0.01 M hexamethylenetetramine (HMT) in a bottle with autoclavable screw cap. The solution was kept at 95°C for 12 hours to grow ZnO nanorods. Then the as-grown nanorods were treated by oxygen plasma for 2 min. After plasma treatment, the ZnO nanorod was kept in TGA- (or MPA) capped CdSe ethanol solution for one day to load QDs. QDSSCs were assembled with the QD loaded ZnO nanorod substrate and a 200Å platinum-coated indium tin oxide (ITO) glass counter electrode. A 60 µm thermal-plastic spacer was used to separate the two electrodes. Iodide/triiodide (I- / I3 -) electrolyte composed of 0.1 M I2, 0.1 M LiI, 0.5 M terbutylpyridine, and 0.6 M 1-hexyl-3-methylimidazolium iodide in methoxy-acetnnitrile was injected into the cell by capillary effect. The solar cells with TGA- and MPA-capped CdSe sensitizers were named as sample A, and B respectively. The OA-capped QDs were not used to construct solar cells, as their adsorption to ZnO is physical only.
3. Results and discussion
Figure 1(a) shows the absorption spectra of OA-, TGA- and MPA-capped CdSe solutions which exhibit a characteristic sharp peak at their band edges. Since the CdSe QDs can be well dissolved in nonpolar (chloroform for OA-capped) and polar (ethanol for both TGA- and MPA-capped) solvent, the ligand capped CdSe should be hydrophobic with OA-capping and hydrophilic with TGA- and MPA-capping. From the excitonic transition wavelength of 570 nm, the sizes of these QDs are estimated as 3.2 nm . Figure 1(b) shows the absorption spectra of ZnO nanorods before and after sensitization with 3.2 nm TGA- and MPA-capped CdSe QDs. After loading QDs onto ZnO nanorods, the featured peaks of QDs remain unchanged for both TGA- and MPA-capped CdSe QDs [Fig. 1(a)].
Figure 2(a) and (b) show the scanning electron microscopy (SEM) images of the ZnO nanorods before and after adsorbing TGA-capped CdSe QDs. The QDs adsorbed onto the surface of ZnO nanorod via the carboxylate group in TGA. After sensitization by QDs, the surface of ZnO nanorod becomes rougher [Fig. 2(b)]. Figure 2(c) shows a transmission electron microscopy (TEM) image of the TGA-capped CdSe QDs. It can be seen that the diameter of the QDs is about 3 to 4 nm, consistent with the absorption spectra [Fig. 1(a)]. Figure 2(d) shows a TEM image of TGA-capped CdSe QDs adsorbed on ZnO surface. It has been reported that QDs selectively adsorb onto ZnO surface via the linker, and there is an abrupt transition between the lattice planes of ZnO and the lattice planes of CdSe QD .
Figure 3 (a) and (b) show the current – voltage (I-V) characteristics and incident photon to current conversion efficiency (IPCE) of samples A and B at simulated one Sun (AM1.5G, 100 mW / cm2). It can be seen that the current density (Jsc) and open circuit voltages (Voc) of the two samples are 2.8 and 2.2 mA/cm2, and 0.58 and 0.558V, for sample A and B respectively. Both the fill factor (FF) and power conversion efficiency (η) of sample A (40.4% and 0.66% respectively) are higher than those of sample B (33.2% and 0.41% respectively). The improvement in η is 38%. The better performance of sample A is possibly due to the shorter chain length of TGA and faster electron transfer rate in ZnO film with TGA-capped CdSe QDs, which helps to reduce the opportunity of electrons and holes recombination.
The IPCE of unsensitized ZnO nanorod solar cell was also shown in Fig. 3(b). From Fig. 3(b), the sensitization effect of QD is clearly seen. The IPCE peak at 570 nm of samples A and B matches exactly with the first excitonic absorption of CdSe QDs [Fig. 1(a)]. The IPCE at 400 nm and 570 nm are 22% and 16.4%, and 13.6% and 10%, for sample A and B respectively. The higher IPCE of sample A leads to a higher photogenerated current density of sample A, which is consistent with Fig. 3(a).
Lastly, to understand the electron transfer dynamics, we performed time-resolved photoluminescence measurement for the TGA- and MPA-capped CdSe QDs deposited on glass and adsorbed on ZnO nanorods. Figure 4 (a) and (b) show the emission decays recorded for TGA- and MPA-capped CdSe QDs respectively. The triexponential decay kinetics was found suitable in determining the emission lifetimes . The triexponential fitting parameters of the TGA- and MPA-capped CdSe QDs were tabulated in Table 1 . The average lifetime of CdSe emission was estimated using13].It can be seen from Table 1 that the average lifetime of TGA- and MPA-capped CdSe QDs on glass without ZnO is 8.7 ns and 9.6 ns respectively, and it decreases to 4.8 ns and 6.6 ns for TGA- and MPA-capped CdSe QDs on ZnO, respectively. The decrease in lifetime can be attributed to the charge transfer from QD to ZnO, and the charge-transfer rate constant can be estimated by , where and are the average emission lifetimes of CdSe-ZnO and CdSe samples, respectively. Using the data from Table 1, the electron-transfer rate constant can be calculated as 1.0 × 108 and 0.48 × 108 s−1 for TGA- and MPA-capped CdSe QDs respectively. As a result, the electrons transfer is faster in ZnO-TGA-CdSe system compared to ZnO-MPA-CdSe system. A faster electron transfer reduces electron and hole recombination. Kamat and associates have reported that the rate constant of the electron transfer depended on the size of CdSe QDs and the energy offset between the conduction bands of CdSe and TiO2 . For the same size of QDs, the electron-transfer rate constant is higher in TiO2-CdSe system than that of ZnO-CdSe system (the rate for TiO2-CdSe system is about 108 ~109 for 3.2 nm CdSe QD) . Rachel et al. have studied the relationship between the linker chain length and the electron injection efficiency in CdS-TiO2 system . They observed that, as the linker chain length was increased, the electron injection efficiency was reduced, which is consistent with our result. In ZnO-CdSe system here, TGA has a shorter chain length compared to MPA, leading to a faster electron transfer rate, and a better solar cell performance [Fig. 3(a)].
It is worth mentioning that the PCE obtained here is much lower compared to the start-of-the-art inorganic solar cells. It is clear that electron transfer rate is one of the causes. By engineering the molecular binding, the rate can be improved (as shown here). Moreover, a complete coverage of the ZnO electrode by a monolayer of QDs is important. However, we were not able to achieve complete coverage as shown in Fig. 2(d).
In conclusion, ZnO based TGA- and MPA-capped CdSe QDSSCs were studied comparatively. Due to a shorter chain length of TGA, the faster electron transfer rate was faster for CdSe-TGA-ZnO system compared to CdSe-MPA-ZnO. As a result, a PCE of 0.66% was achieved for CdSe-TGA-ZnO cell, which accounts for 38% improvement compared to the CdSe-MPA-ZnO one.
We thank E. K. L. Yeow and X. Y. Wu from the Division of Chemistry & Biological Chemistry, School of Physical and Mathematics, Nanyang Technological University for useful discussions and help in fluorescence decay measurement. The sponsorship from Nanyang Technological University (RGM 44/06) is gratefully acknowledged. The project is also supported by National Science Foundation of China (No: 50872022), Foundation of Doctoral Program of Ministry of Education (20030286003), Program for New Century Excellent Talents in University, Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No.2009102TSJ0122), and Scientific Research Foundation of Graduate School of Southeast UniversityYBJJ0927.
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