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Improved conversion efficiency of Ag2S quantum dot-sensitized solar cells based on TiO2 nanotubes with a ZnO recombination barrier layer.

Chen C, Xie Y, Ali G, Yoo SH, Cho SO - Nanoscale Res Lett (2011)

Bottom Line: We improve the conversion efficiency of Ag2S quantum dot (QD)-sensitized TiO2 nanotube-array electrodes by chemically depositing ZnO recombination barrier layer on plain TiO2 nanotube-array electrodes.It is found that for the prepared electrodes, with increasing the cycles of Ag2S deposition, the photocurrent density and the conversion efficiency increase.In addition, as compared to the Ag2S QD-sensitized TiO2 nanotube-array electrode without the ZnO layers, the conversion efficiency of the electrode with the ZnO layers increases significantly due to the formation of efficient recombination layer between the TiO2 nanotube array and electrolyte.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong, Yuseong, Daejeon 305-701, Republic of Korea. socho@kaist.ac.kr.

ABSTRACT
We improve the conversion efficiency of Ag2S quantum dot (QD)-sensitized TiO2 nanotube-array electrodes by chemically depositing ZnO recombination barrier layer on plain TiO2 nanotube-array electrodes. The optical properties, structural properties, compositional analysis, and photoelectrochemistry properties of prepared electrodes have been investigated. It is found that for the prepared electrodes, with increasing the cycles of Ag2S deposition, the photocurrent density and the conversion efficiency increase. In addition, as compared to the Ag2S QD-sensitized TiO2 nanotube-array electrode without the ZnO layers, the conversion efficiency of the electrode with the ZnO layers increases significantly due to the formation of efficient recombination layer between the TiO2 nanotube array and electrolyte.

No MeSH data available.


Energy diagram and dark current. (a) Energy diagram of Ag2S-sensitized ZnO/TNT solar cells and (b) the dark current of the Ag2S(4)/ZnO/TNT and Ag2S(4)/TNT electrodes.
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Figure 6: Energy diagram and dark current. (a) Energy diagram of Ag2S-sensitized ZnO/TNT solar cells and (b) the dark current of the Ag2S(4)/ZnO/TNT and Ag2S(4)/TNT electrodes.

Mentions: It is clearly seen from Figure 5 that for a chemical bath deposition (CBD) cycle n and an applied potential, the photocurrent density of the Ag2S(n)/ZnO/TNT electrode is higher than that of the Ag2S(n)/TNTs without ZnO layer. This can be explained from the increased absorbance of the Ag2S(n)/ZnO/TNT electrode shown in Figure 4 and the energy diagram of Ag2S-sensitized ZnO/TNT solar cells presented in Figure 6a. Due to the formation of ZnO energy barrier layer over TNTs, the charge recombination with either oxidized Ag2S quantum dots or the electrolyte in the Ag2S-sensitized ZnO/TNT electrode is suppressed compared to the Ag2S-sensitized TNTs. This explanation can be supported by the dark current density-applied potential characteristics of the Ag2S(n)/ZnO/TNTs and Ag2S(n)/TNTs because the dark current represented the recombination between the electrons in the conduction band and the redox ions of the electrolyte. As an example, Figure 6b shows the curves of dark density vs. the applied potential for the Ag2S(4)/ZnO/TNTs and Ag2S(4)/TNTs. Apparently, for the Ag2S-sensitized TNTs with ZnO-coated layers, the dark current density decreases significantly. In addition, it is found that for both Ag2S-sensitized ZnO/TNT and TNT electrodes, the photocurrent density at an applied potential increases with increasing CBD cycles, which can be attributed to a higher incorporated amount of Ag2S that can induce a higher photocurrent density. This result is consistent with the observed UV-vis absorption spectra shown in Figure 4. Similar results have been obtained in CdS-sensitized QDSSCs [31]. Moreover, it should be noted that although the conduction band (CB) level of ZnO is slightly higher than that of TiO2 (Figure 6a), it seems that the electron transfer efficiency from Ag2S to ZnO is not much lower than that from Ag2S to ZnO because the photocurrent density of the Ag2S/ZnO/TNTs is more higher than that of the Ag2S/TNTs. This phenomenon can be explained as follows. According to Marcus and Gerischer's theory [32-34], the rate of electron transfer from electron donor to electron acceptor depends on the energetic overlap of electron donor and acceptor which are related to the density of states (DOS) at energy E relative to the conductor band edge, reorganization energy, and temperature. Therefore, in our case, even though The CB level of electron donor (Ag2S) is lower than that of electron acceptor (TiO2 or ZnO), the electron transfer may also happen if there is an overlap of the DOS of Ag2S and TiO2 (or ZnO), which may be the reason for the photocurrent generation in Ag2S-sensitized TNT electrodes. The more important thing is that for semiconductor nanoparticles, the DOS may be strongly influenced by the doped impurity [35], the size of the nanoparticles [36], and the presence of surrounding media such as liquid electrolyte (i.e., Na2S electrolyte in our case) [37]. This means that the DOS of semiconductor nanoparticles may distribute in a wide energy range. Recently, the calculation results [38] showed that the DOS of Ag2S can distribute in a wide energy range from about -14 to 5 eV, indicating that the electron can probably transfer from Ag2S to TiO2 or ZnO due to the overlap of the electric states of Ag2S and TiO2 or ZnO. Besides, considering that the difference between the CB level of TiO2 and that of ZnO is not so large, it may be possible that the electron transfer rate from Ag2S to ZnO is not much lower than that from Ag2S to TiO2. The photocurrent and photovoltage of Ag2S QD-sensitized TiO2 electrode have been experimentally found not only by us but also by others [10,25].


Improved conversion efficiency of Ag2S quantum dot-sensitized solar cells based on TiO2 nanotubes with a ZnO recombination barrier layer.

Chen C, Xie Y, Ali G, Yoo SH, Cho SO - Nanoscale Res Lett (2011)

Energy diagram and dark current. (a) Energy diagram of Ag2S-sensitized ZnO/TNT solar cells and (b) the dark current of the Ag2S(4)/ZnO/TNT and Ag2S(4)/TNT electrodes.
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Figure 6: Energy diagram and dark current. (a) Energy diagram of Ag2S-sensitized ZnO/TNT solar cells and (b) the dark current of the Ag2S(4)/ZnO/TNT and Ag2S(4)/TNT electrodes.
Mentions: It is clearly seen from Figure 5 that for a chemical bath deposition (CBD) cycle n and an applied potential, the photocurrent density of the Ag2S(n)/ZnO/TNT electrode is higher than that of the Ag2S(n)/TNTs without ZnO layer. This can be explained from the increased absorbance of the Ag2S(n)/ZnO/TNT electrode shown in Figure 4 and the energy diagram of Ag2S-sensitized ZnO/TNT solar cells presented in Figure 6a. Due to the formation of ZnO energy barrier layer over TNTs, the charge recombination with either oxidized Ag2S quantum dots or the electrolyte in the Ag2S-sensitized ZnO/TNT electrode is suppressed compared to the Ag2S-sensitized TNTs. This explanation can be supported by the dark current density-applied potential characteristics of the Ag2S(n)/ZnO/TNTs and Ag2S(n)/TNTs because the dark current represented the recombination between the electrons in the conduction band and the redox ions of the electrolyte. As an example, Figure 6b shows the curves of dark density vs. the applied potential for the Ag2S(4)/ZnO/TNTs and Ag2S(4)/TNTs. Apparently, for the Ag2S-sensitized TNTs with ZnO-coated layers, the dark current density decreases significantly. In addition, it is found that for both Ag2S-sensitized ZnO/TNT and TNT electrodes, the photocurrent density at an applied potential increases with increasing CBD cycles, which can be attributed to a higher incorporated amount of Ag2S that can induce a higher photocurrent density. This result is consistent with the observed UV-vis absorption spectra shown in Figure 4. Similar results have been obtained in CdS-sensitized QDSSCs [31]. Moreover, it should be noted that although the conduction band (CB) level of ZnO is slightly higher than that of TiO2 (Figure 6a), it seems that the electron transfer efficiency from Ag2S to ZnO is not much lower than that from Ag2S to ZnO because the photocurrent density of the Ag2S/ZnO/TNTs is more higher than that of the Ag2S/TNTs. This phenomenon can be explained as follows. According to Marcus and Gerischer's theory [32-34], the rate of electron transfer from electron donor to electron acceptor depends on the energetic overlap of electron donor and acceptor which are related to the density of states (DOS) at energy E relative to the conductor band edge, reorganization energy, and temperature. Therefore, in our case, even though The CB level of electron donor (Ag2S) is lower than that of electron acceptor (TiO2 or ZnO), the electron transfer may also happen if there is an overlap of the DOS of Ag2S and TiO2 (or ZnO), which may be the reason for the photocurrent generation in Ag2S-sensitized TNT electrodes. The more important thing is that for semiconductor nanoparticles, the DOS may be strongly influenced by the doped impurity [35], the size of the nanoparticles [36], and the presence of surrounding media such as liquid electrolyte (i.e., Na2S electrolyte in our case) [37]. This means that the DOS of semiconductor nanoparticles may distribute in a wide energy range. Recently, the calculation results [38] showed that the DOS of Ag2S can distribute in a wide energy range from about -14 to 5 eV, indicating that the electron can probably transfer from Ag2S to TiO2 or ZnO due to the overlap of the electric states of Ag2S and TiO2 or ZnO. Besides, considering that the difference between the CB level of TiO2 and that of ZnO is not so large, it may be possible that the electron transfer rate from Ag2S to ZnO is not much lower than that from Ag2S to TiO2. The photocurrent and photovoltage of Ag2S QD-sensitized TiO2 electrode have been experimentally found not only by us but also by others [10,25].

Bottom Line: We improve the conversion efficiency of Ag2S quantum dot (QD)-sensitized TiO2 nanotube-array electrodes by chemically depositing ZnO recombination barrier layer on plain TiO2 nanotube-array electrodes.It is found that for the prepared electrodes, with increasing the cycles of Ag2S deposition, the photocurrent density and the conversion efficiency increase.In addition, as compared to the Ag2S QD-sensitized TiO2 nanotube-array electrode without the ZnO layers, the conversion efficiency of the electrode with the ZnO layers increases significantly due to the formation of efficient recombination layer between the TiO2 nanotube array and electrolyte.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong, Yuseong, Daejeon 305-701, Republic of Korea. socho@kaist.ac.kr.

ABSTRACT
We improve the conversion efficiency of Ag2S quantum dot (QD)-sensitized TiO2 nanotube-array electrodes by chemically depositing ZnO recombination barrier layer on plain TiO2 nanotube-array electrodes. The optical properties, structural properties, compositional analysis, and photoelectrochemistry properties of prepared electrodes have been investigated. It is found that for the prepared electrodes, with increasing the cycles of Ag2S deposition, the photocurrent density and the conversion efficiency increase. In addition, as compared to the Ag2S QD-sensitized TiO2 nanotube-array electrode without the ZnO layers, the conversion efficiency of the electrode with the ZnO layers increases significantly due to the formation of efficient recombination layer between the TiO2 nanotube array and electrolyte.

No MeSH data available.