Contact Stack Evaluation for SHJ Solar Cells and Process Development of IBC-SHJ Solar Cells

Abstract

Nowadays, silicon heterojunction (SHJ) solar cell is one of the most promising photovoltaic technologies thanks to the outstanding passivation quality from the a-Si:H layers. Together with the interdigitated-back-contacted (IBC) architecture, it enables the highest efficient, 26.7%, single junction c-Si solar cell. However, the mass production of such high efficient solar cells is limited, due to the complexity of the solar cell processes and the involved expensive TCO layer(s).
The objective of this thesis is to develop high efficiency, simple processed IBC-SHJ solar cells. To accomplish this goal, a comprehensive study ‘from layer to device’ is conducted: firstly, the focus is on the contact stacks deposited via PECVD, which includes intrinsic and doped hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon oxide (nc-SiOx:H) thin-film layers; Then the optimized passivation contact stacks are used in the front back contacted (FBC) solar cells, with which the factors that limit the fill factor (FF) and open-circuit voltage (VOC) are identified; Lastly, a simplified process is developed to fabricate tunneling IBC-SHJ solar cells.
The influences of PECVD deposition parameters on passivation quality and carrier selectivity of the passivation and contact layer stacks were intensively studied. With the optimized 6 nm thick intrinsic a-Si:H layer an effective lifetime over 3 ms and implied-VOC (iVOC) beyond 720 mV are achieved on double side textured c-Si. Enhanced passivation qualities with iVOC of 729 mV is obtained by adding the field effect passivation from the optimized n-type a-Si:H and nc-SiOx:H layers on top of the excellent chemical passivation induced by the optimized 10 nm thick intrinsic a-Si:H layer. On the other hand, with the optimized intrinsic a-Si:H passivation layer, the deposition of p-type a-Si:H or nc-SiOx:H does not deteriorate the overall passivation quality. Besides that the optimized p-type nc-SiOx:H layer exhibits excellent activation energy of 51.4 meV, which closes to the optimal value for a high efficient hole selectivity, and a dark conductivity of 0.174 S/cm, which is high enough to facilitate the hole transport.
Research on FBC-SHJ solar cells reveals that thicker p-type nc-SiOx:H layer is essential to ensure a smaller/no drop from SunsVoc to VOC, which is related to minority carrier collection. Besides, such a thick doped nc-SiOx:H layers can effectively shield the device precursor from the influence of ITO’s field effect and keep the overall passivation quality after ITO sputtering. Accordingly, the best FBC device shows promising results in terms of SunsVoc with 727 mV and 734 mV, pFF of 0.862 and 0.841, measured before and after metallization, respectively. By implementing 3 nm n-type nc-Si:H instead of directly n-type nc-SiOx:H in contact with ITO, FF improves from 0.56 to 0.73. The best manufactured FBC-SHJ solar cell (7.84 cm2) exhibits VOC of 710 mV, EQE short-circuit current density (JSC,EQE) of 39.4 mA/cm2, FF of 0.73 and efficiency of 20.4%.
For IBC-SHJ solar cells manufacture, the lift-off patterning approach is proved to be not suitable for processing double side textured cells, mainly due to the fact that the doped nc-SiOx thin film alloys is not HF resistant. However, by applying this non-HF resistant property of the doped nc-SiOx alloys, a novel wet-chemical approach for processing tunneling IBC-SHJ solar cell is developed. This patterning approach allows to simplify the process. And the tunneling structure avoids the patterning step of the p-type nc-SiOx:H layer. The first preliminary IBC device demonstrated with this approach exhibits VOC of 659 mV, JSC of 41.30 mA/cm2, FF of 0.67, and efficiency of 18.2%. Further optimization on the thickness of the intrinsic a-Si:H layers induces an excellent VOC of 719 mV with an average VOC of 715 mV over 7 cells, JSC over 41 mA/cm2. However, the low FF (<0.60) is limiting the cells performance, which is mainly attribute to the low conductivity of the n-type nc-SiOx:H layers.