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This paper reports on in-depth understanding, modeling, and fabrication of 23.8% efficient 4 cm2 n-type Float Zone (FZ) silicon cells with a selective boron emitter and photolithography contact on front and tunnel oxide passivating contact on the back. Tunnel oxide passivating contact composed of a very thin chemically grown silicon oxide (∼15 A) capped with plasma-enhanced chemical vapor deposition (PECVD) grown 20 nm n+ poly Si gave excellent surface passivation and carrier selectivity with very low saturation current density (∼5 fA/cm2). A high-quality boron selective emitter was formed using ion implantation and solid source diffusion to minimize metal recombination and emitter saturation current density. Process optimization resulted in a cell $V_{{\rm{oc}}}$ of 712 mV, $J_{{\rm{sc}}}$ of 41.2 mA/cm2, and FF of 0.811. A simple methodology is used to model these cells which replaces tunnel oxide passivating contact region by electron and hole recombination velocities extracted from measured saturation current density of tunnel oxide passivating contact region and analysis. Using this approach and two-dimensional device modeling gave an excellent match between the measured and simulated cell parameters and efficiency, supporting excellent passivation and carrier selectivity of these contacts. Extended simulations showed that 26% cell efficiency can be achieved with this cell structure by further optimization of wafer quality, emitter profile, and contact design.

Carrier selective passivated contacts composed of thin oxide, n + polycrystalline Si and metal on top of a n-Si absorber can significantly lower the recombination current density (Jorear ≤8 fA/cm2) under the contact while providing excellent specific contact resistance (5–10 mΩ-cm2); 25.1% efficient small area cells with photolithography front contacts on boron doped selective emitter and Fz wafers have been achieved by Fraunhofer ISE using their tunnel oxide passivated contact (TOPCon) approach. This paper shows a methodology to model such passivated contact cells using Sentaurus device model, which involves replacing the TOPCon region by carrier selective electron and hole recombination velocities to match the measured Jorear of the TOPCon region as well as all the light IV values of the cell. We first validated the methodology by modeling a 24.9% reference cell. The model was then extended to assess the efficiency potential of large area TOPCon cells on commercial grade n-type Cz material with screen-printed front contacts. To use realistic input parameters, a 21% n-type PERT cell was fabricated on Cz wafer (5 Ω-cm, 1.5-ms lifetime). Modeling showed that the cell efficiency will improve to only 21.6% if the back of this cell is replaced by the above TOPCon, and the performance is limited by the homogenous emitter. Efficiency was then modeled to improve to 22.6% with the implementation of selective emitter (150/20 Ω/sq). Finally, it is shown that screen printing of 40-µm-wide lines and improved bulk material (10 Ω-cm, 3-ms lifetime) can raise the single side TOPCon Cz cell efficiency to 23.2%. Copyright © 2016 John Wiley & Sons, Ltd.

In this paper, we demonstrate the formation of a boron selective emitter using single-step B implantation through laser patterned dielectric layer. Optimized dielectric thickness, implantation dose and annealing results in a high sheet resistance of ∼170 ohm/D was formed in the oxide mask area and low sheet resistance of∼60 ohm/D in the laser ablated regions. Screen printed metal grid lines were aligned inside the laser exposed p++ highly doped region to complete the fabrication of large area, screen printed n-type bifacial selective emitter cells. The samples with laser patterned grid lines gave cell efficiency of 20.75% compared to 20.5% cells without laser pattering. Detailed cell analysis showed that the performance of 20.75% efficiency cell is limited by the recombination in metallized homogeneous phosphorus BSF. Application of this technology for the formation of phosphorus selective BSF, optimization of the laser ablation conditions to reduce laser damage, and reducing in the width of selective region can give much higher cell efficiency.

Fabrication and analysis of a 21% efficient baseline n-PERT cell in this paper revealed that its performance was limited by its large J 0 values on the front and back side, highlighting the need for a bifacial structure with selective emitters to decrease J 0 . To determine the practical efficiency limit of such a cell, Sentaurus 2-D modeling was conducted using doping profiles previously reported for high efficiency cells. The maximum efficiency was calculated to be 22.7% with self-alignment of the contacts to the heavily doped regions. Modeling was then conducted to investigate the losses incurred from the required alignment tolerances associated with the heavily doped region and metal contacts made with today's large-scale manufacturing equipment. Through careful design of the selective emitter and back surface field doping profiles, we show that for 50 μm wide metal contacts screen printed on a 300 μm wide heavily doped region on a 10 Ω-cm base wafer with bulk lifetime of 3 ms, the loss can be decreased by 0.65%, with practical screen-printed cell efficiencies approaching 22.5%. Projected future technology improvements show efficiencies up to 23% are possible with this structure.

This paper reports on the effect of screen printed metallization on the passivation quality of a boron doped poly-Si/SiOx passivated contact (PC) structure composed of a very thin Si oxide (∼15 A) capped with boron doped poly-Si. Our boron doped poly-Si/SiOx passivated contact (p-Poly Si/SiOx PC) with a SiNx capping layer gave excellent surface passivation with a very low saturation current density of ∼5 fA/cm2. After screen printed metallization on poly-Si with a metal coverage of ∼10%, this value increased to ∼17 fA/cm2. This paper also demonstrates the fabrication of screen printed, large area (239 cm2), high efficiency (∼21%) n-base bifacial back junction Si solar cells with p-Poly-Si/SiOx PC on the rear and a phosphorus implanted n++-n+ selective front surface field. Detailed analysis is performed to quantify recombination and extract the saturation current density contributions (J0) from each layer of the cell including the metallized front surface field and the tunnel oxide passivated contact. Finally, 2D device modeling of this back junction cell is performed by implementing a simple approach which replaces the p-Poly-Si/SiOx PC by an equivalent p-n junction with the same J0 and gives a good match between the measured and simulated cell parameters using the extracted J0 and recombination velocity values.