Advanced Bifacial Solar Cell with Poly-Si Passivating Contacts

Abstract

As a new face of PV industry, bifacial technology offers the utmost utilization of reflected light while efforts are still required to further improve its cell efficiency. The objective of this thesis project is to fabricate bifacial solar cell with poly-Si passivating contacts only underneath metal grids. An advanced bifacial architecture is presented combining carrier-selective n/p+ doped poly-Si passivating contacts to quench recombination at c-Si/metal interface, and lightly doped n/p type c-Si surface to ensure high optical transparency on both sides.
LPCVD based doped poly-Si works together with wet-chemically grown ultrathin oxide, providing both field-effect and chemical passivation for metal contacts. By investigation into poly-Si thickness and thermal budget, symmetric test samples show a good passivation of 5.4fA/cm2 J0 for n doped poly-Si and 10.9fA/cm2 J0 for p doped poly-Si.
An optimal n+ c-Si surface passivation of 14.5 fA/cm2 J0 is achieved with PECVD deposited a-Si:H/SiNX stack on textured wafers. For p+ c-Si surface passivation, the influence of thermal ALD Al2O3 film thickness and Forming Gas Annealing on Al2O3/SiNX stack is studied. Also an optimal p+ surface with 123 Ω/sq sheet resistance is formed by boron ion implantation approach, which provides space to play with the trade-off between surface passivation and lateral carrier transport for emitter and front/back surface field.
Applying optimized results, bifacial solar cell fabrication enables only one-time high temperature annealing for both highly doped poly-Si and lightly doped c-Si activation. Following such flowchart, n/p bulk rear/front junction test PeRFeCT cells were fabricated, stressing the importance of FSF passivation on solar cell VOC performance.
A good passivated bifacial cell precursor is also prepared with iVOC of 714mV while BHF, poly-etch and TMAH developer in bifacial cell fabrication is proved to over-etch poly-Si passivaitng material, resulting in a poor performance. For further improvement with smooth processing and delicate control of etching steps, a good performed bifacial solar cell with poly-Si passivaing contacts is expected to be fabricated.