Improving the a-Si:H(p) rear emitter contact of n-type silicon solar cells
In this paper we address the fundamental challenge of forming an efficient contact between the p-type a-Si:H layer and n-type TCO's as contact layers in amorphous/crystalline silicon heterojunction (SHJ) solar cells. We point out the capability of Suns-V-oc measurements to give valuable insights into the formation of Schottky barriers and its influence on the solar cell fill factor FF. The influence of the a-Si:H(p) doping on the Schottky characteristic is shown for test structures and on device level. Test structures are used to probe the influence of various contact layers on the effective work function at the a-Si:H/contact layer interface. A very good correlation between the vacuum work function of different contact layers and the open-circuit voltage is observed for test structures. Therefore, we could demonstrate the work function mismatch between a-Si:H and ITO and a-Si:H and various metals as contact layers. For small area n-type silicon solar cells featuring an a-Si:H(p) rear emitter and a diffused front surface field (FSF), it is shown that by improving the carrier transport between the a-Si:H(p) and the contacting layer, ITO(n) or metal, FF above 80% can be obtained. Furthermore, we demonstrate that a TCO is not mandatory for the rear SHJ emitter, which simplifies the cell structure and allows for proper junction engineering. We obtained high internal rear side reflection with a single metal layer and an efficiency of 22.8% for these TCO-less SHJ emitter solar cells. As these solar cells feature FF of up to 81.5%, they clearly demonstrate the high FF potential of the silicon heterojunction which can be achieved by proper junction engineering.