Abstract
Metal oxide semiconductor/chalcogenide quantum dot (QD) heterostructured photoanodes show photocurrent densities >30 mA/cm2 with ZnO, approaching the theoretical limits in photovoltaic (PV) cells. However, comparative performance has not been achieved with TiO2. Here, we applied a TiO2(B) surface passivation layer (SPL) on TiO2/QD (PbS and CdS) and achieved a photocurrent density of 34.59 mA/cm2 under AM 1.5G illumination for PV cells, the highest recorded to date. The SPL improves electron conductivity by increasing the density of surface states, facilitating multiple trapping/detrapping transport, and increasing the coordination number of TiO2 nanoparticles. This, along with impeded electron recombination, led to enhanced collection efficiency, which is a major factor for performance. Furthermore, SPL-treated TiO2/QD photoanodes were successfully exploited in photoelectrochemical water splitting cells, showing an excellent photocurrent density of 14.43 mA/cm2 at 0.82 V versus the Reversible Hydrogen Electrode (RHE). These results suggest a new promising strategy for the development of high-performance photoelectrochemical devices.
Highlights
Chalcogenide quantum dots (QDs) have attracted much attention as building blocks for next-generation light-harvesting devices due to their outstanding optical characteristics such as a wide light absorption range over the near-IR regions and high extinction coefficient.[1−5] Typical examples of such lightharvesting devices include photovoltaic (PV) cells and photoelectrochemical (PEC) water splitting cells, consisting of light-harvesting materials deposited on a mesoporous n-type semiconductor and a counter ellaeycetrroodfe.6T−i8O2, ZnO, orSnO2; an electrolyte; Over the last 5 years, high photocurrent densities greater than 30 mA/cm[2] have been reported for PV cells using metal oxide/chalcogenide QD heterostructured photoanodes having high light-harvesting ability (Figure 1)
To estimate the density of surface states in the TiO2 films with and without surface passivation layer (SPL), electrochemical methods of cyclic voltammetry (CV) and impedance spectroscopy (IS) were used with a three-electrode system and an aqueous electrolyte consisting of 0.25 M Na2S and 0.35 M Na2SO3, which was the same composition used for PEC water splitting cells under dark conditions
An SPL whose thickness ranged from 2 to 5 nm was identified at the surface of the mesoporous TiO2 films by transmission electron microscope (TEM), high-resolution TEM (HR-TEM), and X-ray photoelectron spectroscopy (XPS) (Figures 2a and S1)
Summary
Chalcogenide quantum dots (QDs) have attracted much attention as building blocks for next-generation light-harvesting devices due to their outstanding optical characteristics such as a wide light absorption range over the near-IR regions and high extinction coefficient.[1−5] Typical examples of such lightharvesting devices include photovoltaic (PV) cells and photoelectrochemical (PEC) water splitting cells, consisting of light-harvesting materials deposited on a mesoporous n-type semiconductor and a counter ellaeycetrroodfe.6T−i8O2, ZnO, or. To design the optimal architecture of photoanodes with TiO2/chalcogenide QDs for high photocurrents, enhancement of both charge transfer kinetics at the TiO2/chalcogenide QD interface and charge transport via the TiO2 films should be simultaneously considered.[21] As a general strategy, the introduction of a surface passivation layer (SPL) on TiO2 film has led to significant improvements in sensitized PV cells.[22,23] TiCl4 treatment to form a TiO2 SPL at the interface of TiO2/light-harvesting materials has led to an about 10−30% photocurrent density increase.[24] In dyesensitized solar cells (DSSCs) with a SPL, performance. We achieved a photocurrent density of 34.59 mA/ cm[2] in PV cells with a 0.18 cm[2] active area and 14.43 mA/cm[2] in PEC water splitting cells with an active area of 1.33 cm[2] at 1 sun condition with about 20% enhancement (compared to the reference samples) by hydrothermal treatment of TiCl4 to form an SPL on the TiO2/PbS-CdS QD photoanodes. We conclude that the main effects of the SPL on the photocurrent density of TiO2/PbS-CdS QD photoanodes are related to improved charge collection efficiency (ηcc), driven by enhancement of electron transport and suppression of electron recombination
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