Photoinduced Dipoles in Core@Shell Quantum Dot Sensitized Solar Cells: Insights from First Principles

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Poster, Jon M. Azpiroz, 124
Publication date: 5th February 2015

Due to their outstanding optical properties, semiconductor quantum dots (QDs) have earned lots of attention as sensitizers in the solar cell community. However, they have worked quite disappointingly so far, with record efficiencies of 7 % [1]. Part of the reason for such a poor performance is the highly negative redox potential of the commonly used polysulfide electrolyte. A route for optimizing the functioning of QD Sensitized Solar Cells (QDSSCs) implies the addition of molecular dipoles, which negatively shift the Conduction Band (CB) edge of TiO₂ when adsorbed on its surface [2].Such an electrostatic effect is however small, because dipoles are based only on partial charges. A possible solution to this shortcoming lies in the use of type-II core@shell QDs, composed of a shell localizing the photoexcited electron and a core localizing the hole. When irradiated, a dipole is created between the newly injected electrons and the holes trapped in the QD core (photoinduced dipole or PID), which gives rise to a strong electrostatic effect [3]. To shed light on this process, two kind of devices have been simulated by first-principles: the standard cell (C1 in Figure), in which TiO₂ is sensitized with a CdS layer; and the modified cell (C2 in Figure), in which the ZnSe@CdS QD is docked on top of the CdS slab. In their ground state, C1 and C2 share an almost identical TiO₂ CB edge. In the excited state, instead, C2 displays a more pronounced upward shift of the CB, resulting in a higher open circuit voltage, as experimentally found. From our calculations, the electrostatic effect produced by the hole confined in the QD is responsible for this particular behavior. Our studies provide insight on the physical processes occurring at the QD/TiO₂ interface, paving the way for future works in the field.
Figure. CdS@TiO2 (C1, blue) and QD@CdS@TiO2 (C2, red) systems, representing the standard and the modified solar cells, along with their excited state dipoles and conduction band edges.
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