Black silicon (b-Si) surfaces typically have a high density of extreme nanofeatures and a significantly large surface area. This makes high-quality surface passivation even more critical for devices such as solar cells with b-Si surfaces. It has been hypothesized that conformal dielectrics with a high fixed charge density (Q f ) are preferred as the nanoscale features of b-Si result in a significant enhancement of field-effect passivation. This article uses 1-D, 2-D, and 3-D numerical simulations to study surface passivation of b-Si, where we particularly focus on the charge carrier control by |Q f | up to 1 × 10 13 cm −2 under accumulation conditions. We will show that there is a significant space charge region compression in b-Si nanofeatures, which affects the charge carrier population control for moderate |Q f | up to ≈1 × 10 12 cm −2 . The average surface minority charge carrier density can be reduced by 70% in some cases, resulting in an equivalent reduction in area-normalized surface recombination losses if the effective surface recombination velocity (S eff ) is limited by minority carriers. This provides a possible solution for the empirical S eff ∝ 1/Q 4 f reported previously. We will also show that the situation is more complicated for surface passivation films where the ratio between the electron and hole capture cross section (σ n / σ p ) is higher than 10 for p-type surfaces. For commonly used surface passivation films with a |Q f | larger than ≈1 × 10 12 cm −2 , there is little space charge compression for b-Si. Consequently, S eff simply scales with the surface area, i.e., there is no enhanced reduction of surface recombination by field-effect passivation on b-Si.
In this work, we report on a significant breakthrough in fabricating the critical tunnel oxide layer of tunnel oxide passivated contacts (TOPCon) high-efficiency solar cells compatible with high-volume manufacturing. We show that the tunnel oxide can be controlled at the atomic scale, enabled by an innovative tube-type industrial plasmaassisted atomic layer deposition (PEALD) method. In combination with an in situ doped poly-Si (n + ) layer grown by plasma-enhanced chemical vapor deposition, a uniform, ultrathin $1.3 nm SiO x layer is obtained at the c-Si/SiO x /poly-Si (n + ) interface.Extremely low recombination current densities down to 2.8 fA/cm 2 and an implied open-circuit voltage (iV oc ) as high as 759 mV are achieved, comparable to state-ofthe-art laboratory results. The developed tube-type PEALD SiO x is applied to industrial TOPCon solar cells resulting in a solar cell efficiency and open-circuit voltage of up to 24.2% and 710 mV, respectively. The tunnel oxide process window is about 2.4 Å, highlighting the importance of precisely controlling the tunnel oxide thickness at the atomic scale for TOPCon solar cells. The newly developed tube-type industrial PEALD SiO x method opens up a promising new route toward mass production of high-efficiency industrial TOPCon solar cells. Furthermore, the developed tube-type PEALD method can easily be integrated with the industrial tube-type plasmaenhanced chemical vapor deposition (PECVD) method, thus enabling the deposition of all thin film layers in TOPCon solar cells in one integrated PEALD/PECVD system. This significantly simplifies manufacturing complexity and fosters the commercialization of next-generation high-efficiency industrial TOPCon solar cells.
In this work, we present a breakthrough in boronsilicate glass (BSG) passivated industrial tunnel oxide passivated contact (i‐TOPCon) solar cells. We find that a high‐temperature firing process significantly improves the front side BSG passivation quality; however, the use of such high‐temperatures is undesirable for metallization as it could lead to more junction damage by the metal paste spikes. In this study, we present a simple and industrially viable method to resolve this dilemma. With a high‐temperature industrial firing activation step to maximize the potential of BSG passivation, a low emitter saturation current (J0e) of 34 fA/cm2 has been achieved, demonstrating excellent boron emitter passivation that is comparable to state‐of‐the‐art SiO2 and Al2O3‐based passivation methods on similar structures and boron emitters. Applying this solution to cell device, the open‐circuit voltage (Voc) is improved by about 6 mV, corresponding to an absolute cell efficiency improvement of about 0.2%. Furthermore, after activating the BSG passivation, a lower temperature paste could be used at the rear side which further improves the Voc by around 3 mV. Combined together, an overall improvement of Voc close to 10 mV is achieved, propelling the cell Voc into the 690‐mV era. The effectiveness of this solution was also verified in a mass production line, with average cell efficiencies of around 23.2% (0.5% more than the baseline) and a maximum cell efficiency and Voc of 23.4% and 693 mV, respectively. This work opens new routes for further improving conventional solar cell efficiencies, in particular for BSG‐passivated TOPCon solar cells.
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