Outstanding Surface Passivation for Highly Efficient Silicon Solar Cells Enabled by Innovative Al_yTiO_x/TiO_x Electron‐Selective Contact Stack

Abstract

Passivating contacts based on transition metal oxides (TMOs) have the potential to overcome existing performance limitations in high‐efficiency crystalline silicon (c‐Si) solar cells, which is a significant driver for continuing cost/Watt reductions of photovoltaic electricity. Herein, innovative stacks of Al‐alloyed TiO x (Al y TiO x ) and pure TiO x as transparent electron‐selective passivating contacts for n‐type c‐Si surfaces are explored. An optimized stack of 2 nm Al y TiO x and 2 nm TiO x is shown to provide both record‐quality surface passivation and excellent electrical contact, with a surface recombination current density prefactor J 0 of 2.4 fA cm−2 and a specific contact resistivity ρ c of 15.2 mΩ cm2. The performance of this innovative stack significantly exceeds previously reported values for pure or doped TiO x single layers, SiO x /TiO x stacks, a‐Si:H/TiO x stacks, and other transparent contact technologies. Furthermore, an excellent efficiency of 21.9% is attained by incorporating the optimized stack as a full‐area rear contact in an n‐type c‐Si solar cell. The findings set a new benchmark for the passivation performance of metal oxide‐based passivating contacts, bringing it to a level on par with state‐of‐the‐art SiO x /poly‐Si contacts while greatly improving optical transparency.

Dramatic Reduction of Silicon Surface Recombination by ALD TiOx Capping Layer from TiCl4 and H2O: The Role of Chlorine

“…Therefore, the integration of such films into c-Si solar cells has been intensively investigated, as summarized in Figure , in order to demonstrate their performance at the device level and to benefit from their high transparency compared to Si-based contacts (a-Si:H and poly-Si) that cause parasitic absorption losses. Indeed, TiO x -based contacts have proven their ability to be integrated into a wide range of high-efficiency c-Si solar cell architectures, including either as full-area or partial-area rear contacts for front homojunction cells, ,– electron- or hole-selective passivating contacts in silicon heterojunction (SHJ) cells, , electron-selective passivating contacts in interdigitated back contact (IBC) cells, as well as antireflection coatings (ARC) for front homojunction cells . Unfortunately, the efficiency of such cells remains significantly below that of c-Si solar cells using Si-based contacts (a-Si:H and poly-Si), which have enabled recent record efficiencies in architectures including front-junction (FJ) and back-junction (BJ) tunnel oxide passivated contact (TOPCon) cells, SHJ, SHJ IBC, , and IBC , that are close to the theoretical limit for single-junction c-Si solar cells, as shown in Figure …”

“…29 Furthermore, we have recently observed that exceptional surface passivation can be accomplished by capping 2 nm of Al y TiO x with 2 nm of TiO x (using TiCl 4 and H 2 O) at 75 °C to form an Al y TiO x /TiO x stack, in contrast to a reduced passivation quality obtained for single layers of TiO x or Al y TiO x , or the mirrored stack (TiO x / Al y TiO x ) on planar Si surfaces. 8 We have recently further improved upon this stack by incorporating ZnO to form an Al y TiO x /ZnO/TiO x structure, with similarly exceptional surface passivation but improved contact characteristics. 20 These enhancements in surface passivation performance after capping Al 2 O 3, Al y TiO x , or Al y TiO x /ZnO with a TiO x layer (using TiCl 4 and H 2 O) were argued to be due to atomic Cl being liberated during the deposition process and diffusing across the stack, accumulating at the Si surface and passivating Si dangling bonds.…”

Effect of Al Electrodes on Surface Passivation of TiOx Selective Heterocontacts for Si Solar Cells

“…As an evolutionary innovation from TiO x , the addition of Al doping (TiO x :Al) has demonstrated enhanced surface passivation and thermal stability. [ 26,27 ]…”

“…Such a level of passivation is in line with our as‐deposited TiO x :Al layer ( J 0 = 26.5 fA cm −2 shown in Figure S8, Supporting Information) and also reported in the literature for samples passivated by a single layer of TiO x (without metallization). [ 42 ] It is noted that the reported V oc of cells based on a rear full‐area TiO x contact is in the range of 630–665 mV [ 13,27,42 ] corresponding to a J 0 value >300 fA cm −2 in Figure 8b, which implies that the good surface passivation of as‐deposited TiO x deteriorates in the device after metallization. The simulation highlights that strategies of maintaining good surface passivation of passivating contact after metallization are critical to achieving high V oc and high efficiency.…”

“…As an evolutionary innovation from TiO x , the addition of Al doping (TiO x :Al) has demonstrated enhanced surface passivation and thermal stability. [26,27] To investigate the impact of Al deposition on the sample lifetime, TiO x :Al passivated Si samples with different combinations of LiF x and Al were fabricated. Seven small samples were cut from the same wafer which had been symmetrically passivated with %4 nm TiO x :Al as displayed by the wafer sketch in Figure 1.…”