Update: 2014-12-30 18:16:31      Author: yangjuan@csrc.ac.cn

The increasing demand of renewable energy supply has motivated the search for high efficiency photovoltaic devices. Semiconductor nanostructured photovoltaic devices have emerged as a new generation of devices due to their potential applications in low cost devices with high power conversion efficiencies (PCEs). Recent studies found that the nanowire-based solar cell is able to achieve high PCE, which breaks the traditional limit. Due to the different temporal and spatial scales involved, it remains a great challenge to model the processes in photovoltaic energy conversion. Conventionally, two major approaches for optoelectronic device simulations are the continuum equation model and the discrete dynamic Monte Carlo (DMC) method, which are both based on classical models relying on empirical parameters.

Recently we developed a quantum mechanical simulation method to model directly the photo-induced current of photovoltaic structure. The method adopts a DFTB description of the electronic structure, non-equilibrium Green’s function (NEGF) formalism to model the quantum transport and many-body theory for electron-photon interaction. Our work presents a full quantum mechanical simulation of photovoltaic devices, with all processes included simultaneously. Our method captures quantum mechanically all the electronic and optical properties of a photovoltaic device under the radiation of an external electromagnetic source. All the photovoltaic process, such as photon absorption, electron-hole generation and recombination, and subsequent electron-hole separation and charge transport, are treated on the equal footing. It presents an important step towards the prediction of the properties of photovoltaic devices and the ultimate design of these devices from first-principles.

A full quantum mechanical study of a silicon nanowire-based photovoltaic device of the order of ten thousand atoms was carried out. In Fig. 1 the local density of states (LDOS) is plotted in the two-dimensional space of energy and coordinate. The LDOS forms the band-like structures in real space. The contribution from individual dopants can be clearly seen. In Fig. 1a, the bending of the band structure across the PN junction is clearly observed, from which a built-in potential is formed. Upon the photo-excitation, an electron is excited to the conduction band and a hole is created simultaneously in the valence band. The electron moves to the right and the hole moves to the left, producing the photo-induced current. As the forward bias voltage is increased to the open-circuit voltage, the directions of the electron and hole movements are reversed (Fig. 1b). Based on the analysis of the LDOS and the underlying dynamic processes, our method provides detailed insights into the fundamental processes such as photo-absorption, electron-hole separation/recombination and charge transport in photovoltaic devices. Fig. 2 plots the current-voltage characteristics of a silicon nanowire. The black and red lines are dark currents and illuminated currents, respectively. A monochromatic light illumination with photon energy 2.5 eV was applied to the system. The calculated short-circuit currents is 2.3 3mA/cm2 and the open circuit voltages is 0.62 V.

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