High efficiency IBC solar cells with poly-Si implanted passivating contacts

Abstract

Crystalline silicon (c-Si) interdigitated back contacted (IBC) solar cell with poly-Si passivating contacts is one of the most promising approaches to achieve high conversion efficiency solar cells. The fabrication of IBC silicon-based solar cells provided by poly-Si passivating contacts is investigated in this thesis. The passivating poly-Si contacts structure used in this work is based on an ultra-thin layer of t-SiOX with optimized poly-Si thicknesses of 250 nm and the implantation parameters of Phosphorous and Boron doping with 6e15 and 5e15 ions/cm2 respectively, at a fixed implantation energy of 20 keV. The influence of the post-implantation annealing conditions is discussed with experimental results, obtained on symmetrical test structures with thermal SiOx/doped poly-Si on each side. The bestobtained passivation result for n+ -poly-Si contact was 728 mV at annealing conditions of 1050°C for 1 minute, while for the p+ poly-Si contact at the same annealing conditions 699 mV is obtained. To enhance the passivation, PECVD deposited SiNX capping layer and annealing in forming gas as hydrogenation processes are carried out. Then, FBC solar cells are fabricated to evaluate the electrical performances, in terms of passivation and carrier transport, of the poly-Si contacts structures, which are prepared with different annealing conditions, in terms of temperature and time. The results obtained from the FBC cells show that increasing the annealing temperature leads to an increase in the passivation thus the VOC. The best-obtained VOC was 706mV for the wafer annealed at 1050°C for 1 minute. To enhance the optical properties of the IBC cells, the surface is textured and the influence of doping dose on passivation of the front surface field (FSF) is evaluated. Four different passivation stack layers SiNX, AlOX/SiNX, a-Si/SiNX, and double SiNy /SiNX are evaluated for FSF implanted with dose of 1e14 ions/cm2. The best-obtained passivation result is from FSF sample passivated with a-Si/SiNX. It shows excellent passivation property with iVOC of 714 mV and a J0 of 7.5 fA/cm2. while the other stacks show lower passivation of iVOC 708 mV for the AlOX/SiNX sample and iVoc=686 mV for the SiNy /SiNX sample. The influence of FGA on the FSF passivation quality is also evaluated. The results show that the sample passivated with a-Si/SiNX experienced a sharp decrease of 35 mV in the iVOC after FGA due to the low thermal stability of the a-Si:H. On the other hand, other FSF samples with passivation stacks of SiNX, AlOX/SiNX, and SiNX /SiNX show a positive influence upon the FGA on their passivation quality as a result of the extra hydrogen diffusion during the FGA which saturates more defects on the c-Si surface. The implantation of the optimized thermal SiOx/ doped poly-Si structure and FSF with a-Si/SiNX passivating layer into the IBC solar cells leads to high-efficiency IBC solar cells. The record cell has a conversion efficiency of 21.04% and 22.15% after the post metallization annealing, VOC of 681 mV, and FF of 78,9 with Jsc of 37.5 mA/cm2