Modeling and Implementation of a Graded Emitter With a Full 2-D Phosphorus Dopant Optimization

Abstract

A selective emitter attempts to optimize emitter losses by using two doping levels: high doping under the metal finger and low doping between the fingers. While a selective emitter is an improvement over a homogeneous doped emitter, it is not a full optimization. A graded emitter is introduced here, in which the doping is continually varied between the finger and midpoint between the fingers. The two main emitter power loss mechanisms, recombination and lateral resistance ( I 2 R ), vary in severity from the metal finger to the midpoint between two fingers. Thus, in a graded emitter, the doping level is also varied to minimize the total power loss in a continuous manner. The benefits of the graded emitter include a higher efficiency of up to 0.5% abs compared with a homogeneous emitter. In comparison with a typical POCl3 diffused selective emitter, the ion-implanted graded emitter can achieve a higher efficiency by 0.2%abs at a wider finger pitch, representing a reduction of the front silver paste consumption by 8%. Finally, an industrially feasible method of manufacturing graded emitters using a commercially available ion implantation tool for solar cells is described.