Abstract-Optoelectronic two dimensional (2D) technology computer aided design (TCAD) simulation of unconstrained four terminal hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) tandem solar cells have been carried out. Tandem solar cell approach is one of the promising approaches to achieve high efficiency solar cell by reducing lattice thermalization loss, but needs extensive optimization as the efficiency depends on a number of parameters. The optoelectronic properties of various materials involved have been taken into consideration and a range of parameters such as top ARC (anti reflection coating) thickness, bottom ARC thickness, a-Si:H solar cell thickness, optical connecting layer thickness, a-Si:H solar cell doping, crystalline silicon solar cells doping etc. have been optimized. The optimization resulted in a simulated efficiency of 19.29% for an untextured planar solar cell.