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%.