Silicon heterojunction rear emitter solar cells: Less restrictions on the optoelectrical properties of front side TCOs

In this paper we address the fundamental challenge of balancing the trade-off between the optical and electrical properties of the front side transparent conductive oxide (TCO) in silicon heterojunction solar cells. We present two-dimensional numerical device simulations as well as experimental investigations concerning n-type silicon heterojunction (SHJ) solar cells featuring either a front emitter or rear emitter. By varying both, the pitch of the front side metal grid electrode and the sheet resistance of the front side TCO, the fill factor losses associated with the two-dimensional carrier transport are investigated. We prove experimentally that in the case of the rear emitter design a substantial part of the lateral carrier transport is shifted from the front side TCO into the absorber, thus, lowering the effective sheet resistance of the front side. Owing to this additional lateral current path the trade-off between the optical and electrical properties of the front side TCO are less pronounced for a rear emitter cell design compared to the traditional front emitter cell design. This leads to fewer restrictions for the choice of front side TCOs and the design of the front side metal grid electrode. Depending on the optimization route, this should allow for higher solar cell efficiencies or the application of more economic TCOs.