Study On The Photo-induced Charge Transfer Dynmaics At Interfaces In Coupled Quantum Dot Films

With the development of multiple exciton theory and the support of experiments in quantum dots(QDs), the study of QD solar cells with high quantum efficiency become an international hot spot gradually. The process about the dissociation of the exciton by charge transfer at interfaces and the charges transport to the electrodes is a key physical issue to understand the high quantum efficiency in QD solar cells. Recently, the reported experimental results by applying photoelectric and spectroscopic methods simultaneously to study the charge transfer dynamics at interfaces in QD films on different time scales according to the actual working conditions of the photovoltaic devices are still lacking, which have a great scientific significance for the design of high efficiency photovoltaic devices.This paper shows the charge transfer dynamics at interfaces between QDs and the acceptors and that between two layers of QDs with different sizes in single and double Cd Se coupled QD thin film devices, mainly using transient photocurrent(TOF) and time-resolved fluorescence spectroscopy. The corresponding contents are as follows:First, we design and prepare two types of Cd Se coupled QD thin film devices according to energy level matching of different materials. For the photovoltaic device containing electrodes, the transient photocurrent characteristics on long time scale are studied, while for the other one only with QD films and acceptors, the time-resolved fluorescence spectral characteristics on short time scale are studied. Two methods are used to study the charge transfer dynamics at interfaces on ms and ns time scale respectively. And the results are in agreement with each other.Secondly, we study the charge transfer dynamics at interfaces between QDs and the acceptors. By varying the thickness of QD thin film in prototype photovoltaic device, we find that the TOF signal mainly comes from carriers transport in acceptors after charge transfer across the interfaces between QDs and the acceptors. While the TOF signal in the device without exciton blocking layer(acceptor) indicates the charge transfer dynamics and combination at interfaces between QDs and the electrodes. For further experimental support, we collect and do global-fit on the time-resolved fluorescence spectroscopy. As a result, for QD thin film device, three fluorescent components are confirmed, however, for the QD/acceptor thin film device, the fourth fluorescence component is found, which is considered as the charge transfer dynamics at QD/acceptor interfaces. The corresponding kinetic parameters are obtained.Finally, we study the charge transfer dynamics at interfaces between QDs of different sizes. After analyzing the TOF results of the double QD thin film photovoltaic devices, we identified the contribution of charge transfer at interfaces between different QD layers, and characterize its kinetic parameters. Meanwhile, TOF results under different bias conditions also support the related conclusions.