NEGF theory of quantum photovoltaic devices
Many high-efficiency photovoltaics concepts require an advanced control and manipulation of the optoelectronic properties of the active device structure, leading to a prominent role of low dimensional absorbers such as quantum wells, wires and dots in the implementation of these concepts. However, the quantum effects governing the optoelectronic characteristics of the nanostructures not only provide the desired design degrees of freedom, but also request new models for the description of the photovoltaic operation, since conventional macroscopic theories of generation, transport and recombination do not allow for a consistent consideration of such effects and the related device behaviour.
One of the common features of quantum photovoltaic devices is the dominant contribution from the strongly localized states of the low dimensional absorbers to generation and recombination, while transport is mediated mainly via extended states. This means that one either has to find the ideal degree of localization which provides the best compromise between efficient transport and strong absorption, as indicated e.g. in the case of quantum well or
quantum dot superlattices used in multi-junction solar cells, where miniband transport is required, or to optimize the processes that couple maximally localized absorbing states with maximally extended current carrying states, which corresponds to the situation encountered in quantum well solar cells.
The conventional approach to the problem described above is to combine a microscopic model for the physical processes involving confined states with a macroscopic, semiclassical theory for charge transport via the use of detailed balance rates determined within the microscopic theory. The resulting hybrid approach often provides effective fitting models able to quantitatively reproduce experimental device characteristics. However, at the same time, the large number of required ad-hoc assumptions tends to obscure the underlying mechanisms of quantum photovoltaic device operation, especially concerning the crucial processes of carrier escape and capture which couple localized and extended states.
To go beyond the existing approaches in capturing the essential physics of quantum photovoltaic devices, a microscopic theory based on the non-equilibrium Green's function formalism (NEGF) for the electronic, optical and vibrational degrees of freedom was developed and applied to the simulation of quantum well solar cells. The theory is capable of describing both optical and transport properties including quantum effects such as confinement and
tunneling in an open non-equilibrium system under consideration of elastic and inelastic scattering effects leading to incoherence and relaxation.
In this presentation, originally give at the 2010 Manchester CECAM workshop, after an introduction to the theoretical framework of the NEGF for quantum photovoltaics, the approach is illustrated on the example of generation, escape and capture of charge carriers in quantum well solar cells.
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