The need for more efficient, cheaper, easily producible solar cells is growing as this combination of attributes can decrease the levelized cost of energy (LCOE). A two­terminal (2T) Perovskite and galliuminidium­gallium­selenide (Pvk/CIGS) tandem cell is an excellent candidate, as it can have a power conversion efficiency (PCE) of +30%, can be flexible (making it suitable for roll­-to­-roll production), and
the deposition of a Pvk solar cell (PSC) on top of established techniques like CIGS will only add one extra step to the production process, thereby improving the PCE significantly

This thesis is part of LAFLEX­-2T project which aims to design and engineer a highly efficient flexible 2T thin­-film solar device based on the Pvk/CIGS tandem configuration. The aim of this thesis is to develop an opto­-electrical model of the tandem configuration which replicates the inner physics of a reference solar cell. This can be used to give insights on the current losses occurring in the cell and on the limitations in the charge carrier transport towards the electrodes. Based on this analysis, a route of improvements is proposed with could result in more efficient solar cells.

The simulation template uses optical and electrical simulations based on GENPRO4 and TCAD Sentaurus, respectively. An extensive model for Pvk/CIGS tandem cells is presented and validated using experimentally obtained J­-V curve measurements. It was found that charge transport in the tunnel recombination junction (TRJ) depends on direct energy and in­direct energy transfer, in terms of two tunneling mechanisms: band to band tunneling (B2BT) and trap assisted tunneling (TAT). For TAT, a non­-local model facilitated by trap states is successfully implemented.
Simulation results reveal that the transport in the TRJ of the reference solar cell is based on TAT. The loss analysis points out that reflectance losses are responsible for a loss of 10.92 mA/cm². Similarly, losses due to parasitic absorption are equal to 5.32 mA/cm². The CIGS bottom was identified as the current limiting layer. The dominant recombination mechanisms in the Pvk and CIGS absorber layers are Shockley-­Read­-Hall (SRH) and surface recombination calculated as 4.71 mA/cm² and 1.59 mA/cm² for top Pvk and bottom CIGS, respectively. An increase in losses in the absorber layers in maximum power point (MPP) compared to short circuit (SC) conditions exposed charge transport issues in the collecting path of charge carriers.
After assessing the loss mechanisms, a road­map for efficiency improvements is proposed. After implementation, an increase in the PCE of the reference tandem cell was observed from 10.63% to 26.69%.

High-performance perovskite solar cells (PSCs) have attracted great attention from researchers around the world. Rapid researches and developments of PSCs are shown by the increase of its efficiency from 3.8% in 2009 to a new world record of 22.4% in 2017. Hysteresis phenomenon in current density voltage (J-V) is one of the challenges occurring in some specific PSCs that should be overcome to continue the improvement of PSCs. This phenomenon occurs when a voltage is swept with different scan directions from negative to positive value (forward) and positive to negative value (backward). Hysteresis effect results in a different maximum power point on the J-V characteristic leading to under- or overestimation of PSCs efficiency.

Furthermore, when the applied voltage is abruptly changed, a transient current density response is introduced implying a capacitive behaviour. Due to this behaviour, there is an indication that PSCs cannot be represented by the conventional equivalent circuit. Thus, the purpose of this project is to investigate hysteresis phenomenon in PSC by electrical modelling. In this project, a PSC sample was fabricated by Solliance Solar Research. Time-resolved J-V measurement was done to obtain more insight of J(t) as a function of applied voltage. The hysteresis phenomenon was analyzed in different voltage scan direction and various scan rates. Simulation of band diagram in dark condition was done to understand working principle of PSC device. Two predicted equivalent circuit of the cell were derived from the simulated band diagram. These equivalent circuit models considered the charge accumulation at the bulk of perovskite and at the interface between charge transport layers and perovskite. Furthermore, two additional equivalent circuit models were proposed to represent the hysteresis effect. J(t) curve fitting of measurement results and simulation was employed to verify the equivalent circuit. This insight might help to get a better understanding of hysteresis effect in PSCs.

Two dimensional plasmonic back reflectors have shown the ability to match the
scattering performance of standard back reflectors. However, three dimensional
plasmonic structures have not yet been thoroughly investigated. Here, the scat-
tering properties of a 3D plasmonic structure are approached at three levels of
system. First, the scattering properties of different types of particles are inves-
tigated. An optimum particle size is found for both spherical and cylindrical
silver particles. The interaction of two particles is then studied. By comparing
numerical and analytic results, the strength of different effects is explored. It is
found that dipole-dipole interactions can not be ignored within the structure.
Finally, the effect of the lattice structure on the scattering properties is probed.
A new lattice summation method is developed, revealing that only particles near
the point of interest must be considered. Several 3D devices were fabricated to
investigate the properties of self forming metal island films. It was found that
the surface morphology of layers deposited on top of these films, do not depend
on the properties of the film.