PTAB

IPR2025-01411

Mundra Solar PV Ltd v. First Solar Inc

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Petition
petition

1. Case Identification

2. Patent Overview

  • Title: High-efficiency Solar Cell Structures and Methods of Manufacture
  • Brief Description: The ’732 patent describes a method for fabricating a solar cell. The method involves depositing a single amorphous silicon-containing compound, comprising both an oxygen dopant and a conductive dopant, and then using a thermal treatment to diffuse the oxygen to form a distinct interface passivation layer while the remaining compound forms a passivating film.

3. Grounds for Unpatentability

Ground 1: Obviousness over Yablonovitch and Kwark - Claims 1-4 and 6-9 are obvious over Yablonovitch in view of Kwark.

  • Prior Art Relied Upon: Yablonovitch (a 1985 scientific article titled "A 720 mV open circuit voltage SiOx:c-Si:SiOx double heterostructure solar cell") and Kwark (a 1984 doctoral dissertation titled "SIPOS heterojunction contacts to silicon").
  • Core Argument for this Ground:
    • Prior Art Mapping: Petitioner argued that Yablonovitch disclosed all steps of independent claim 1. Yablonovitch taught a method for fabricating a solar cell by providing a silicon substrate, thermally growing a thin oxide layer (the interface passivation layer), and depositing a phosphorus-doped semi-insulating polycrystalline silicon (SIPOS) layer over it (the passivating film with a passivating material and conductive dopant). The SIPOS layer, as deposited, was an amorphous silicon-containing compound with both oxygen and phosphorus (conductive) dopants. Yablonovitch then performed a high-temperature anneal. Petitioner asserted that Kwark explained the physical phenomena occurring during Yablonovitch's anneal step: the thermal treatment caused oxygen from the SIPOS layer to diffuse toward the substrate, forming an additional oxide layer at the interface. This combined oxide layer defined the interface passivation layer, while the remaining crystallized SIPOS defined the passivating film. Furthermore, the anneal caused the conductive phosphorus dopant to diffuse throughout the interface passivation layer, creating pinholes and conductive pathways that facilitated electrical connection to the substrate.
    • Motivation to Combine: A Person of Ordinary Skill in the Art (POSITA) would combine the references because Yablonovitch described a high-performance solar cell but acknowledged that the functioning of the interfacial oxide structures was "complex" and formed "inadvertently," omitting a detailed analysis. Kwark, which addressed the same field of SIPOS heterojunctions, provided the exact physical explanations and experimental data needed to understand and optimize the process Yablonovitch disclosed. Petitioner argued a POSITA would have looked to Kwark to understand the critical trade-offs between oxide thickness, passivation quality, and contact resistance to predictably implement and improve upon Yablonovitch's design.
    • Expectation of Success: Petitioner contended that the art was mature and predictable by 2009. The fabrication of polysilicon emitters, thermal annealing, dopant diffusion, and interface engineering were well-understood disciplines. The behavior of thin oxide layers during high-temperature processing was an area of extensive research. Therefore, a POSITA would have had a high expectation of success in applying Kwark’s detailed teachings to optimize Yablonovitch’s method, viewing it as routine engineering rather than inventive discovery.

Ground 2: Obviousness over Yablonovitch, Kwark, and Batra - Claim 5 is obvious over Yablonovitch in view of Kwark and Batra.

  • Prior Art Relied Upon: Yablonovitch, Kwark, and Batra (a 1993 scientific article titled "Study of Lateral Non-Uniformity as a Function of Junction Depth...").
  • Core Argument for this Ground:
    • Prior Art Mapping: This ground built upon the combination of Yablonovitch and Kwark, which Petitioner argued rendered underlying claim 2 obvious. Claim 5 added the limitation that the interface passivation layer "protects the substrate from crystallization or re-crystallization" during the thermal treatment. Petitioner asserted that while Yablonovitch and Kwark taught the formation of the layers and the annealing process, they did not explicitly detail the effect of the crystallization of the passivating film on the underlying single-crystal substrate. Batra was introduced to supply this teaching. Batra studied the effects of annealing on such structures and provided transmission electron microscopy (XTEM) micrographs showing that the crystallization of a polysilicon layer did not create damage in the underlying substrate. Petitioner argued this lack of damage was due to the protective interfacial oxide, thus directly teaching the limitation of claim 5.
    • Motivation to Combine: A POSITA implementing the high-temperature (900°C) annealing step from Yablonovitch would have been concerned about potentially damaging the crystalline structure of the silicon wafer substrate. To resolve this concern and ensure the process's viability, the POSITA would have been motivated to consult a reference like Batra, which specifically investigated the substrate's integrity during annealing of overlying polycrystalline silicon films. Batra provided the assurance that the interfacial layer protects the substrate, making its teachings directly relevant and beneficial to the Yablonovitch/Kwark process.
    • Expectation of Success: The expectation of success was high because Batra was in the same field of semiconductor structures and directly addressed the technical question of substrate damage during annealing. A POSITA would have reasonably expected that the protective effects shown by Batra would apply to the analogous structure in Yablonovitch, as both involved annealing a silicon-based film on a silicon substrate with an interfacial oxide.

4. Relief Requested

  • Petitioner requests institution of an inter partes review and cancellation of claims 1-9 of the ’732 patent as unpatentable.