Nonequilibrium Green's function modeling of superlattice solar cells

As quoted from a recent speech, ``climate change is real, it is happening right now, it is the most urgent threat facing our entire species, and we need to work collectively together and stop procrastinating.'' Addressing climate change requires renewing several processes in diverse contexts, from food to waste disposal, from transport to heating/cooling. Many of these actions are related to the ultimate challenge for humankind, i.e., the need of clean, sustainable energy sources. It is clear that the cleanest energy source available on Earth is the Sun. Despite the huge efforts invested in this direction, solar energy production is still limited to the 8% of the global energy production. Among the causes behind this issue, one of the toughest issues concerns the low efficiency of photovoltaics. In particular, a significant problem is the non-optimal absorption of the solar spectrum by the available single band gap materials. A possible solution proposed to increase the efficiency is to use multi quantum well solar cell, where the presence of the lower band gap material can increase the absorption of the cell in the infra-red spectrum. However, the presence of the wells can have a detrimental effect on the transport of carriers limiting the amount collected, lowering the efficiency reachable. Therefore, a careful design must be implemented when characterizing these devices to ensure that there is a gain in efficiency with respect to the bulk case. The semiconductor simulation tools available in the market are mostly based on the Poisson-drift-diffusion (PDD) model, which is not able to consider quantum effects natively. In this type of structures, the carriers are generated inside the wells’ confined states, which then can leave either by thermoionic emission or by resonant tunnelling through the barriers separating the wells. Therefore, quantum effects are significant in the definition of the behaviour of these devices. On the other hand, genuine quantum-kinetic approaches such as nonequilibrium Green's Function (NEGF) methods allow a much more rigorous description of carrier transport. Yet, NEGF implementations lead to extreme computational costs. In this thesis, this problem is addressed with two different lines of attack. The first, is to simplify the most critical point NEGF issue, which is the formulation of self-energies evaluating energy relaxation phenomena realistically, but in a more efficient way. This is carried out by employing self-energies inspired by the concept of Büttiker probes and describing interband generation/recombination phenomena in a semiclassical way. The second approach is to ennoble Poisson-drift-diffusion by introducing quantum corrections inspired by the idea of the localization landscape method. It is presented how combining these two approaches promises to be the ultimate simulation strategy for III-N MQW solar cells: relatively short devices (about 30 QWs) can be studied with NEGF. This provides useful data to fit the PDD electron and hole mobilities, enabling the study of much larger structures. The thesis is concluded presenting the results of several parametric simulation campaigns on the various geometrical and material parameters. The focus of this thesis will be a multi-quantum well solar cell in GaAs/InGaAs, which was one of the first devices proposed, to validate the working principle of the model. The thesis is concluded presenting the results of several parametric simulation campaigns on the various geometrical and material parameters.