Ultra-thin LPCVD SiNx/n+poly-Si passivated contacts: A possibility?
Gurleen Kaur1,2, Tanmay Dutta1,3, Zheng Xin2, Zhi Peng Ling2, Marvic John Santos Naval2, MSM Saifullah3, Rolf Stangl2, Aaron Danner1,2
1Spin and Energy Lab, Department of Electrical and Computer Engineering, National University of Singapore, Singapore, Singapore
/2Solar Energy Research Institute of Singapore, National Unversity of Singapore, Singapore, Singapore
/3Institute of Materials Research and Engineering, A*STAR (Agency for Science and Technology Research), Singapore, Singapore

Metal induced recombination losses at the contact are a major contributor to the losses in a solar cell. The use of passivated contacts i.e. deploying passivating tunnel layers with highly doped capping layers between Si and metal have resulted in improved efficiencies. They successfully passivate the defects at the Si surface and enhance the selectivity of the contact. The SiOx/poly-Si passivated contacts with moderate fixed charge has led to cell efficiencies > 25%. This creates a case for exploring the possibility of using ultrathin silicon nitride (SiNx) films exhibiting a higher positive fixed charge in a SiNx/n+poly-Si passivating contact.
 
In this work, we optimized the factors strongly influencing the passivation performance of ultrathin SiNx layer, deposited by industrial, LPCVD, including (i) film thickness, (ii) annealing conditions (temperature, time and ambient), and (iii) surface pre-treatment, for application in a passivating contact. Using methodology developed by Brendel et al., we quantify the efficiency potential of these contacts by measuring the recombination current density, Jo,contact and the tunnelling resistance, ρcontact.
 
Our preliminary experiments reveal excellent surface passivation and low recombination current density, Jo (45 fA.cm-2) by ~1.5 nm thick LPCVD SiNx films when subjected to an air ambient anneal at 800°C for 30 mins. This is due to the formation of high positive fixed charge density (1.5 ´10 12 cm-2). Air ambient annealed (465 μs) samples also have a higher lifetime when compared to the forming gas annealed (208 μs) samples. These passivating SiNx films were further integrated into SiNx/n+poly-Si contacts and characterized for Jo,contact and tunneling resistance, rcontact­. The best SiNx/n+poly-Si passivated contact in this study has Jo, contact = 5.9 fA.cm-2, rcontact­ = 0.525 Ω.cm2 and an efficiency potential > 22.75%. According to our knowledge, it is the first report confirming the formation of passivated contacts with SiNx as the dielectric tunnel layer.