Chapters authored
Double-Sided Passivated Contacts for Solar Cell Applications: An Industrially Viable Approach Toward 24% Efficient Large Area Silicon Solar Cells
Tunnel layer passivated contacts have been successfully demonstrated for next-generation silicon solar cell concepts, achieving improved device performance stemming from the significantly reduced contact recombination of the solar cell contacts. However, these carrier-selective passivated contacts are currently deployed only at the rear side of the silicon solar cell, while the front side adopts a conventional diffused junction and contacting scheme. In this work, we report on the novelty and feasibility of deploying tunnel layer passivated contacts on both sides of a silicon wafer-based solar cell, featuring a textured front surface and a planar rear surface. In particular, we demonstrate that silicon solar cells incorporating our in-house developed electron-selective thermal-SiOx/poly-Si(n+) and hole-selective thermal-SiOx/poly-Si(p+) passivated contacts have a solar cell efficiency potential approaching 24%. Deploying contact passivation only at the rear side of the solar cell, we have reached a solar cell efficiency of 21.7%, using conventional screen printing for metallization. We present a systematic approach of evaluating our in-house developed electron-selective and hole-selective passivated contacts on both textured and planar lifetime test structures, as well as dark I–V test structures, to extract the recombination current density j0 and the contact resistance Rc of the passivated contact, which is used for process optimization as well as for subsequent efficiency potential prediction. The two key challenges aiming at a double-sided integration of passivated contacts are (1) parasitic absorption within the front-side highly doped poly-Si capping layer, requiring the processing of ultrathin (≤10-nm) contact passivation layers. This has been quantified by numerical simulation (using SunSolve™) and also solved experimentally, i.e., processing ultrathin 3-/10-nm hole/electron extracting SiOx/poly-Si(p+/n+) passivated contact layers, reaching an implied open-circuit voltage of 690/703 mV on a planar/textured silicon surface, which will even further enhance after SiNx capping. (2) Ensuring process compatibility with conventional screen printing: Screen printing on electron extracting poly-Si(n+) seems feasible; however, screen printing on hole-extracting poly-Si(p+) is still a challenge. Solar cell precursors have been processed, showing excellent pre-metallization results (implied-VOC ∼ 713 mV). According to our efficiency potential prediction (using the measured j0 and Rc values of our developed contact passivation layers), bifacial double-sided passivated contact solar cells (efficiency potential of ∼23.2%, using our layers) can clearly outperform rear-side-only passivated contact solar cells (efficiency potential of ∼22.5%).
Part of the book: Silicon Materials