REC Solar Holdings As v. Maxeon Solar Pte Ltd

1. Case Identification

• Case #: IPR2025-00593
• Patent #: 8,878,053
• Filed: February 14, 2025
• Petitioner(s): REC Solar Holdings As
• Patent Owner(s): Maxeon Solar Pte. Ltd.
• Challenged Claims: 9-20

2. Patent Overview

• Title: Front Contact Solar Cell with Formed Emitter
• Brief Description: The ’053 patent discloses methods for fabricating a front-contact solar cell featuring a backside junction. The method involves forming a polysilicon layer over an oxide layer on the back surface of a silicon substrate, diffusing dopants into the polysilicon to create a backside emitter junction, diffusing dopants into the front surface, and forming metal contacts on the front surface.

3. Grounds for Unpatentability

Ground 1: Claims 9-11, 14, 16-17, and 19-20 are obvious over Froitzheim and Gan

• Prior Art Relied Upon: Froitzheim (European Patent Publication No. EP1732142A1) and Gan (a 1990 IEEE conference paper).
• Core Argument for this Ground:
  • Prior Art Mapping: Petitioner argued that Froitzheim taught a backside junction solar cell with all the claimed process steps except for the use of a deposited polysilicon emitter; Froitzheim’s emitter was formed by diffusing dopants directly into the substrate. Gan, which analyzed polysilicon emitters, expressly taught that solar cell performance is improved by replacing such a diffused-in-substrate emitter with a deposited polysilicon emitter having an interfacial oxide layer. The combination of Froitzheim’s base process with Gan’s improved emitter structure rendered independent claims 9 and 16 obvious. For claim 16, Gan explicitly taught the claimed sequence of forming an oxide layer before depositing the polysilicon layer.
  • Motivation to Combine: A POSITA would combine Froitzheim and Gan because Gan explicitly suggested the modification to improve the performance of solar cells, including backside contact cells like Froitzheim's. Gan provided data showing the benefits of using a polysilicon emitter with an interfacial oxide layer, predicting more than a one percent absolute increase in cell efficiency.
  • Expectation of Success: A POSITA would have had a high expectation of success, as the combination involved substituting one known type of emitter for another improved, known type of emitter to achieve the predictable result of enhanced cell performance.

Ground 2: Claims 9-11, 14, 16-17, and 19-20 are obvious over Froitzheim and Kwark

• Prior Art Relied Upon: Froitzheim (EP1732142A1) and Kwark (a 1984 International Electron Devices Meeting paper).
• Core Argument for this Ground:
  • Prior Art Mapping: This ground presented an alternative to Gan. Petitioner asserted that Kwark, like Gan, taught the benefits of using a polysilicon emitter with an intervening oxide layer. Kwark demonstrated that adding a chemically grown oxide layer (“chemox”) between the polysilicon and the substrate “greatly decreases” emitter saturation current, a key factor for improving solar cell efficiency. A POSITA would have incorporated Kwark’s high-efficiency emitter structure into Froitzheim’s backside junction solar cell design.
  • Motivation to Combine: Kwark’s disclosure that its polysilicon-oxide structure leads to "potential improvements of energy conversion efficiency for solar cells" would have motivated a POSITA to apply this teaching to Froitzheim’s solar cell. The desire for higher efficiency is a primary driver in solar cell design.
  • Expectation of Success: The combination was presented as a predictable application of a known technique (Kwark's emitter) to improve a similar device (Froitzheim's cell), with a clear expectation of success in achieving higher efficiency.

Ground 3: Claims 11-13, 15, and 18 are obvious over Froitzheim and Gan (or Kwark) in further view of Hartiti or Sivoththaman

• Prior Art Relied Upon: Froitzheim (EP1732142A1), Gan (1990 paper), Hartiti (a 1994 IEEE conference paper), and Sivoththaman (a 1996 IEEE conference paper).
• Core Argument for this Ground:
  • Prior Art Mapping: Petitioner argued that while Froitzheim taught front-surface doping from a gas phase, dependent claims 12, 13, and 18 require forming a solid "second dopant source layer" (e.g., phosphorus-doped silicon dioxide) on the front surface. Hartiti and Sivoththaman taught this exact step, disclosing the use of phosphosilicate glass (PSG) films as a diffusion source for front-surface doping. Furthermore, these references taught a single-step, simultaneous "co-diffusion" of front (phosphorus) and back (boron) dopants, which meets the "in situ" limitation of claim 15.
  • Motivation to Combine: A POSITA would combine these references to simplify the manufacturing process and reduce costs. Hartiti and Sivoththaman explained that co-diffusion in a single thermal step reduces the number of high-temperature processes, saving energy and cost, which is a significant motivation in solar cell manufacturing.
  • Expectation of Success: The techniques were compatible. Froitzheim disclosed that various known diffusion methods could be used, including from solid phases as taught by Hartiti and Sivoththaman, creating a strong expectation of success.

4. Key Claim Construction Positions

• "in situ" (Claim 15): Petitioner argued this term, as defined in the ’053 patent, means performing the front and back diffusion steps after a "single manual loading" of the substrate into a processing tool. Petitioner contended that the single-step, simultaneous co-diffusion processes disclosed in Hartiti and Sivoththaman met this construction, as they occurred within a single rapid thermal processing unit after a single loading.

5. Arguments Regarding Discretionary Denial

• Petitioner argued that discretionary denial would be inappropriate.
  • §325(d): Denial was argued to be unwarranted because the core prior art (Froitzheim, Gan, Kwark, Hartiti, Sivoththaman) was never considered during prosecution and addressed the specific reasons for allowance.
  • Fintiv Factors: Petitioner argued against denial based on parallel district court litigation by stipulating that, if the inter partes review (IPR) is instituted, it will not pursue in court any invalidity ground that was raised or reasonably could have been raised in the IPR.
  • General Plastic Factors: Petitioner contended this was not an improper follow-on petition, as it was filed to seek joinder with a pending IPR and was Petitioner’s first challenge to the ’053 patent.

6. Relief Requested

• Petitioner requested institution of an IPR and cancellation of claims 9-20 of Patent 8,878,053 as unpatentable.