Publication date: 1st April 2013
Quantum dots (QDs) have attracted attention in the solar cell community due to a number of unique properties, such as size-tunable absorption spectrum, and high absorption cross-section. However, the most distinct feature of QDs is the presence of multiple exciton generation (MEG). Excitation of several electron–hole pairs through absorption of a single photon enables efficient harvesting of a broad range of the light spectrum – thus it can boost the efficiency of solar cells.
The most commonly used solar cell architecture employing QDs is the concept of QD-sensitized solar cells. The QDs act as an absorber sensitizing a metal oxide surface. During the last years the related research has mainly been focused on the study of electron transfer from QDs to metal oxide – the principal process responsible for charge separation. However, very little attention has been paid so far to the role of electron injection on MEG. Although it has been proven that the MEG effect can enhance solar cell efficiency, electron injection rates needed for this remained unclear.
The rapid electron injection is in fact a crucial factor, as the MEG benefits can be lost due to Auger recombination – the inverse process to MEG. The Auger recombination in QDs takes place already on timescales of tens of picoseconds, or even shorter. Therefore it poses a real threat to the MEG efficiency.
We will present a study of competition between Auger recombination and electron injection in the system of CdSe QDs attached to ZnO. By comparing the kinetics of the processes we prove that the rapid electron injection is able to inhibit the unwanted Auger recombination and enable multiple exciton harvesting. In particular we will focus on the role of the linker molecule attaching QDs to the ZnO surface.
Our results confirm that the linker molecule affects greatly electron injection – and hence also MEG exploitation. Previously the linker layer has been mostly described as a tunneling barrier for electrons being injected into metal oxide. However, our results show that the structure of the linker has a big impact on the injection process as well. Therefore the tunneling picture is an oversimplification, which might be misleading for some molecules. Our findings open a possibility to cleverly choose a linker molecule for the particular case.
(A) Scheme of the studied system. (B) Transient absorption kinetics of electron injection – comparison of quantum dots attached and not attached to ZnO. (C) Auger recombination kinetics studied by transient absorption for various excitation intensities normalized on the long-lived single exciton kinetics. (D) Scheme of competition between electron injection and Auger recombination in multiply-excited quantum dots. Kinetics were measured for exc. 470 nm, probe 520–540 nm.
(1) Žídek, K.; Zheng, K.; Ponseca, C. S.; Messing, M. E.; Wallenberg, L. R.; Chábera, P.; Abdellah, M.; Sundström, V.; Pullerits, T. Electron Transfer in Quantum-Dot-Sensitized ZnO Nanowires: Ultrafast Time-Resolved Absorption and Terahertz Study. J. Am. Chem. Soc. 2012, 134, 12110–7. (2) Žídek, K.; Zheng, K.; Abdellah, M.; Lenngren, N.; Chábera, P.; Pullerits, T. Ultrafast Dynamics of Multiple Exciton Harvesting in the CdSe-ZnO System: Electron Injection versus Auger Recombination. Nano Lett. 2012, 12, 6393.
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