In situ interface engineering for probing the limit of quantum dot photovoltaic devices

Here, we report an in-situ fabrication and investigation of single nanowire/QD heterojunction solar cell using a custom-designed photoelectric transmission electron microscope holder, which is good for in situ photoelectrical studies and provides valuable insight in nanoscale devices.

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Quantum dot heterojunction solar cells (QDHSCs) have attracted considerable attention for several significant merits, such as tunable band gap, high extinction coefficient, multiple exciton generation effect, and high stability against oxidative deterioration, which represents an important new generation solar cell towards the commercial application.

However, the experimentally achieved efficiency to date remains far from ideal, largely due to non-ideal interfaces that induces undesired carrier recombination. To push the limit of PCE up to the theoretical value, a fundamental understanding of the physics of nanoscale heterojunctions is necessary. In-situ TEM studies of these energy devices can offer valuable insight for fundamental understanding to further enhance the device performance. Yet, to the best of our knowledge, no such study has been carried out on the in-situ assembly and observation of photoelectric devices, mainly due to the complexity in designing a suitable in-situ photoelectric TEM stage.

   Here we report the design of a state-of-the-art TEM holder by incorporating a light source within the holder without affecting the quality of high-resolution TEM (HRTEM) imaging. Using this TEM sample holder, we built a single nanowire QDHSC, with the simplest cell structure and a minimum number of interfaces. The two electrodes have different work functions, which enable the directional movement of charge carriers in the solar cell (Fig. 1a). When the LED is switched on, as illustrated in Fig. 1b, the QDHSC was exposed to white light from the LED.

Fig. 1| Configuration and performance of single nanowire quantum dot heterojunction solar cell| a, Schematic diagram of the in situ fabrication of an individual TiO2 /CdSe nanowire quantum dot solar cell based on a heterojunction. b ,The shematic diagram of the QDHSC exposed to white light from the LED.

To elucidate the effect of interfaces and defects on the photovoltaic performance of the QDHSC, a direct in situ control over the interface area can be precisely controlled by the movement of the Pt tip, as shown in Fig.2 a and b. This QDHSC design offers a valuable model system to tailor and probe the impacts of interfacial electron loss on the power conversional efficiency (PCE) to approach the theoretical limit. The results led to a better understanding of the intrinsic mechanisms associated with the high PCE of the single TiO2 nanowire QDHSC.

Fig. 2| The dependence of the photocurrent on the interface area. a–b, Typical TEM images of QDHSC devices with the different interface areas (the interface is highlighted using false colour).

Our study establishes a robust “nanolab” platform in a TEM for in situ photoelectrical studies and provides valuable insight into the interfacial effects in nanoscale solar cells. The methodologies developed here can also be extended to the design and investigation of other types of solar cells and optoelectronic devices.

More details can be found in our paper "In situ interface engineering for probing the limit of quantum dot photovoltaic devices" published in Nature Nanotechnology: DOI : 10.1038/s41565-019-0526-7

Litao Sun

Professor, Dean , School of Electronic Science and Engineering, Southeast University

Litao Sun is Changjiang Distinguished Professor and the head of School of Electronic Science and Engineering, Southeast University (SEU), the director of SEU-FEI Nano-Pico Center, and the director of Center for Advanced Materials and Manufacture, Joint Research Institute of Southeast University and Monash University. He is the chairman of In-situ Microscopy Chapter, Electron Microscopy Society of China. Currently, his research interests focus on: (1) Dynamic in-situ experimentation in the electron microscope (Setting up a Nanolab inside a TEM for nanomaterials); (2) Novel behaviors/properties from sub-10nm materials; (3) Applications of nanomaterials in environment, renewable energy and nanoelectromechanical systems. He is the author and co-author of around 200 papers on international journals including 2 in Science, 13 in Nature and Nature series journals, etc. He holds around 80 patents and has given more than 160 invited presentations. He is the Review Panel member of Graphene Flagship, European Union and Member of European Science Foundation College of Expert Reviewers. He has obtained National Science Fund for Distinguished Young Scholars of China, New Century Excellent Talents in University, Young Leading Talent in Science and Technology Innovation etc.