REC Solar Holdings As v. Maxeon Solar Pte Ltd

1. Case Identification

• Case #: IPR2025-00595
• Patent #: 11,251,315
• Filed: February 14, 2025
• Petitioner(s): REC Solar Holdings AS
• Patent Owner(s): Maxeon Solar Pte. Ltd.
• Challenged Claims: 16 and 19

2. Patent Overview

• Title: Solar Cells with Improved Lifetime, Passivation and/or Efficiency
• Brief Description: The ’315 patent describes back-contact solar cells featuring a silicon substrate with a lightly doped front portion (concentration ≤ 2x10¹⁸ cm⁻³). The backside includes a dielectric layer, over which first and second doped polysilicon emitter regions are formed, which are claimed to contain gettered metal impurities to improve cell performance.

3. Grounds for Unpatentability

Ground 1: Obviousness over Li in view of King, Swanson-352, or Marvin - Claims 16 and 19

• Prior Art Relied Upon: Li (Application # 2011/0214719), King (a 1990 journal article), Swanson-352 (Patent 4,234,352), and Marvin (a 1989 paper).
• Core Argument for this Ground:
  • Prior Art Mapping: Petitioner argued that Li disclosed a back-contact solar cell with a structure substantially identical to that claimed, including a silicon substrate, a backside dielectric layer (tunnel oxide), and overlying P-type and N-type doped polysilicon layers corresponding to the first and second emitter regions. Li, however, did not specify the front-side dopant concentration. Petitioner asserted that King, Swanson-352, and Marvin all explicitly teach that the optimal front-side surface dopant concentration for a high-efficiency back-contact solar cell is approximately 1x10¹⁸ cm⁻³, which satisfies the claimed limitation of "less than or equal to 2x10¹⁸ cm⁻³." Furthermore, Petitioner contended that the limitation requiring "metal impurities" in the emitter regions was inherently disclosed by Li. Li's process involves heating at 950°C, a temperature sufficient to cause metal impurities known to be present in silicon substrates to getter through the thin dielectric layer and become trapped in the backside polysilicon emitter regions. Finally, Petitioner argued that the metal contacts required by the claims, while not illustrated in Li, were an obvious and necessary component of any functional back-contact solar cell as taught by the general knowledge in the art and specifically by Swanson-352 and Marvin.
  • Motivation to Combine: A Person of Ordinary Skill in the Art (POSITA) reviewing Li's cell, which aimed to improve efficiency, would have been motivated to implement the optimal front-side dopant concentration taught by King, Swanson-352, or Marvin. These references directly addressed the optimization of this specific parameter in analogous back-contact solar cells to achieve the lowest surface recombination and thus the highest efficiency.
  • Expectation of Success: A POSITA would have had a reasonable expectation of success in achieving the claimed concentration using the known gaseous diffusion methods described in Li. The combination merely involved applying a well-known, optimal process parameter to a known solar cell structure to achieve a predictable improvement in performance.

Ground 2: Obviousness over Li in view of Muller - Claims 16 and 19

• Prior Art Relied Upon: Li (Application # 2011/0214719) and Muller (a 2014 journal article).

• Core Argument for this Ground:

  • Prior Art Mapping: This ground presented Muller as an alternative to King, Swanson-352, or Marvin for teaching the claimed front-side dopant concentration. Muller described a high-efficiency N-type back-contact solar cell with a front portion doped via ion implantation to a peak concentration of approximately 2x10¹⁸ cm⁻³. Petitioner argued that combining Muller’s teachings with Li’s foundational structure would render the claims obvious. Muller also provided explicit disclosure of first and second metal contacts formed on the backside doped regions, reinforcing the argument that adding such contacts to Li's structure would have been obvious. As in Ground 1, Petitioner maintained that the "metal impurities" limitation was inherently met by Li’s high-temperature fabrication process.
  • Motivation to Combine: A POSITA would combine Li with Muller to gain the demonstrated benefits of higher conversion efficiency and superior stability against UV degradation, which Muller reported for its ion-implanted front floating emitter compared to other designs. Muller’s use of ion implantation was also known to simplify fabrication, providing a further incentive for its adoption.
  • Expectation of Success: Success was expected because fabricating the front and back sides of a solar cell were known to be largely independent processes. A POSITA could readily integrate Muller's front-side ion implantation process with Li's backside structure without substantial modification or undue experimentation.

• Additional Grounds: Petitioner asserted further obviousness challenges (Grounds 3 and 4) based on the primary combinations of Li with King/Swanson-352/Marvin or Li with Muller, additionally citing Hu (Patent 4,053,335) and Ramappa (a 1999 journal article). These grounds relied on Hu and Ramappa to explicitly support the argument that gettering metal impurities into a backside polysilicon layer through a thin silicon dioxide layer at high temperatures was a well-known and predictable phenomenon, thereby reinforcing the inherency argument for the "metal impurities" limitation.

4. Arguments Regarding Discretionary Denial

• Petitioner argued that discretionary denial would be inappropriate. The petition was filed concurrently with a motion for joinder to an existing IPR (IPR2024-01194), which conserves Board resources and avoids inconsistent results.
• To address Fintiv factors, Petitioner stipulated that if the IPR is instituted, it will not pursue in the parallel district court litigation any invalidity grounds that were raised or reasonably could have been raised in the petition.
• Petitioner also argued that denial under §325(d) is unwarranted because the prior art references and combinations asserted in the petition were not considered by the Examiner during the prosecution of the ’315 patent.

5. Relief Requested

• Petitioner requests institution of inter partes review and cancellation of claims 16 and 19 of Patent 11,251,315 as unpatentable.