The effect of CdS on the charge separation and recombination dynamics in PbS/CdS double-layered quantum dot sensitized solar cells

doi:10.1016/j.chemphys.2016.03.014

Chemical Physics

Volume 478, 20 October 2016, Pages 159-163

https://doi.org/10.1016/j.chemphys.2016.03.014 Get rights and content

Abstract

Quantum dot sensitized solar cells (QDSSCs) have attracted much interest due to their theoretical efficiency, predicted to be as high as 44%. However, the energy conversion efficiency of QDSSCs is still a lot lower than the theoretical value, one reason for which is the number of surface defects on the QDs. In order to improve the conversion efficiency, surface passivation of the QDs has been applied to QDSSCs. Studying the mechanism of how the surface passivation influences the photoexcited carrier dynamics is very important. In this paper, we clarify the effects of CdS passivation on electron injection, trapping and recombination in CdS passivated PbS QDSSCs (called PbS/CdS double-layered QDSSCs). We found that electron trapping and recombination can be suppressed effectively, and that the electron injection efficiency can be increased significantly by surface passivation with CdS on PbS QDSSCs. Our findings provide a better understanding of the effects of surface passivation on QDSSCs, which will prove beneficial for making further improvements in the photovoltaic properties of QDSSCs.

Introduction

Quantum dots (QDs) can be used as excellent light harvesting materials, because of their unique properties, such as the capability of tuning the optical absorption spectrum by controlling their size, the large optical absorption coefficient, and the possibility of multiple exciton generation (MEG) [1], [2], [3]. The maximum theoretical efficiency of QD based solar cells is expected to be as high as 44% [4], which is about 15% higher than the theoretical efficiencies of dye sensitized solar cells (DSSCs) [5] and traditional silicon solar cells [6]. QD sensitized solar cells (QDSSCs) have attracted much attention because they are easy to produce. However, the energy conversion efficiency of QDSSCs is still less than 12% [7], [8]. One of the reasons for the low energy conversion efficiency is the defect levels on the QD surfaces. Therefore, QD surface passivation is crucial for decreasing the surface defect levels and improving the energy conversion efficiency of QDSSCs [9]. In our earlier studies, our group and several others have found that CdS passivation on PbS QDSCCs can greatly improve the photocurrent and energy conversion efficiency [10], [11], [12], [13]. Nevertheless, the mechanisms for the effects of CdS, especially how CdS passivation affects the photoexcited carrier dynamics, such as charge separation and recombination in the PbS QDSSCs, are still not very clear.

Generally, the photovoltaic properties of solar cells, such as the short circuit current and the open circuit voltage, are very dependent on the photoexcited carrier dynamics. To improve the photovoltaic properties of QDSSCs, it is vital to understand the mechanisms of the charge transfer dynamics at the TiO₂/QDs, TiO₂/redox, and QDs/QDs interfaces on different timescales. In the timescales from femtoseconds to nanoseconds, there are several relaxation processes for photoexcited electrons, such as electron injection from the QDs to the TiO₂ electrode [14], [15], electron trapping at the defect levels [16], [17], and recombination of the electrons with holes in the QDs [18]. On the other hand, in the timescales from microseconds to milliseconds, there are some charge recombination processes, such as recombination of electrons injected into the TiO₂ with holes remaining in the QDs and/or in the electrolyte. Up till now, we have succeeded in measuring the photoexcited carrier dynamics in various kinds of solar cells such as QDSSCs, DSSCs, inorganic–organic hybrid solar cells and perovskite solar cells [19], [20], [21], [22], [23], [24], [25] using the transient grating (TG) and transient absorption (TA) methods.

In this study, we studied the photoexcited carrier dynamics in both PbS and PbS/CdS double-layered QDSSCs, including electron injection and charge recombination, using TA measurements. We clarified how the CdS outer layer affects the charge separation and recombination processes as well as the photovoltaic properties of PbS QDSSCs.

Section snippets

Experimental

The method used to prepare the TiO₂ electrodes was reported in a previous paper [26]. An anatase TiO₂ paste (PST-18NR, diameter 20 nm, JGC Catalysts and Chemicals Ltd.) was cast onto a glass substrate coated with fluorine doped tin oxide (FTO, 10 Ohm, Asahi Glass) using scotch tape as a frame and spacer, and raking off the excess solution with a glass rod (Squeegee technique). The TiO₂ electrodes were dried in air at room temperature for 10 min, and annealed at 450 °C for 30 min in a furnace, before

Results and discussion

We prepared three different types of sample, CdS/TiO₂, PbS/TiO₂, and CdS/PbS/TiO₂ samples. The optical absorption spectra of these were measured using a PA technique and the PA spectra are shown in Fig. 1. The PA signal intensities for PbS/TiO₂ and CdS/PbS/TiO₂ increase from about 1.0 eV. Since the band gap energies of TiO₂ (3.0 eV) and CdS (2.42 eV) are higher than 1.0 eV, the PA signals for PbS/TiO₂ and CdS/PbS/TiO₂ result from the PbS QDs. As shown in Fig. 1, PbS/TiO₂ and CdS/PbS/TiO₂ have

Conclusion

We studied the photoexcited carrier dynamics of PbS and PbS/CdS double-layered QDSSCs over timescales from ps to ms. We found that CdS passivation reduces the photoexcited electron trapping and increases the efficiency of the electron injection from the PbS QDs to the TiO₂ electrode. In addition, charge recombination at the interfaces between the QDs and the TiO₂ electrodes was suppressed greatly by the CdS passivation. As a result, the charge separation efficiency and the charge collection

Conflict of interest

There is no conflict of interest in this paper.

Acknowledgement

This research was supported by the Japan Science and Technology Agency (JST) CREST program and MEXT KAKENHI Grant Number 26286013.

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¹: Co-corresponding author.

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The effect of CdS on the charge separation and recombination dynamics in PbS/CdS double-layered quantum dot sensitized solar cells

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Conflict of interest

Acknowledgement

Chem. Phys. Lett.

Thin Solid Films

Sol. Energy

Thin Solid Films

J. Phys. Chem. C

ChemPhysChem

J. Appl. Phys.

J. Phys. Chem.

J. Appl. Phys.

J. Phys. Chem. C

J. Am. Chem. Soc.

J. Appl. Phys.

J. Mater. Chem.

J. Phys. Chem. Lett.

J. Phys. Chem. C