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Multiscale Simulation of Solar Cells with Nanostructure Components

A wide range of novel solar cell devices contains nanostructures or inhomogeneities on the nanometer scale, which are introduced in order to adjust the optoelectronic properties to the specific requirements of the device concepts. Representatives of nanostructure components that are under active experimental development at IEK-5 are the passivation and contact layers of the silicon heterojunction solar cell (AG Ding) and the nanoparticle absorbers from earth-abundant and non-toxic materials (AG Carius).

The presence of nanostructure components exhibiting the desired deviation of the physical behaviour from that of the bulk material introduces new challenges for the modelling of the resulting photovoltaic devices: while the conventional macroscopic continuum models for the bulk materials do not provide a valid description of the physics of the nanostructures, a complete microscopic picture with atomic resolution is neither feasible - due to the huge computational costs of such a description – nor appropriate, since only the nanostructure related parts deviate from the bulk picture. In this situation, new multiscale simulation approaches are required, which combine the macroscopic description of the bulk material with the consideration of nanostructure properties on a microscopic level, thereby providing a physically valid picture of the impact of nanostructure components on the photovoltaic device characteristics.

On the level of the absorber and conductor materials, microscopic approaches for the description of the local microstructure, e.g., ab-initio methods such as DFT or empirical formalisms like tight-binding can be used in combination with molecular dynamics to obtain the electronic and vibrational states of the nanostructures. These states can then be used to parametrize mesoscopic Hamiltonians for quantum-kinetic models of the charge carrier dynamics, such as the Non-equilibrium Green's function formalism (NEGF), which can already provide local photovoltaic device characteristics. For the simulation of complete solar cells with realistic dimensions, either the microscopically determined material parameters of the nanostructures are used in a 3D multi-physics (optical-electronic-thermal) device simulation environment, or the mesoscopically described regions are integrated in such a macroscopic simulation by the means of suitable boundary conditions.

One of the focus activities of the multiscale simulation group lies on the developments of simulation methods at the mesoscopic scale, i.e., in the extension of the NEGF formalism to new processes and nanostructure components relevant for the operation of novel solar cell devices, such as, e.g., non-radiative recombination via defects and the Auger effect as well as nanocrystal quantum dots or ultra-thin absorber layers. These developments are directly implemented in simulation approaches for various third generation high-efficiency solar cell concepts, such as quantum well solar cells, quantum dot intermediate band solar cells, tunnel junctions for multijunction solar cells, etc.

Nanoparticle absorber solar cellMultiscale simulation of a nanoparticle absorber solar cell