Rubidium Chloride (RbCl) Charge-Enhanced Induced-Junction (MISIL) Solar Cells for Low-Temperature Silicon Photovoltaics
Rubidium Chloride (RbCl) Charge-Enhanced Induced-Junction (MIS/IL) Solar Cells for Low-Temperature Silicon Photovoltaics

Rubidium Chloride (RbCl) Charge-Enhanced Induced-Junction (MIS/IL) Solar Cells for Low-Temperature Silicon Photovoltaics

A practical process workflow for R&D and engineering teams to build metal–insulator–semiconductor / inversion-layer (MIS/IL) solar cells using a Rubidium Chloride (RbCl) charge-enhancement layer to raise fixed charge density and improve passivation.

Application: Silicon PV Architecture: MIS/IL (Induced Junction) Key Material: Rubidium Chloride (RbCl) Goal: High fixed charge, low interface states

1) Overview

Induced-junction (MIS/IL) solar cells form the functional equivalent of a p–n junction by placing a thin dielectric stack with high fixed charge on a semiconductor surface. The fixed charge drives band bending and induces an inversion layer near the surface, enabling carrier separation without relying on high-temperature dopant diffusion.

This cell structure uses a p-type semiconductor substrate with a backside metal electrode and a front-side “induction layer” stack: transition layer (SiOx or SiOxNy), a charge-enhancement layer (e.g., RbCl), and an anti-reflection layer (SiNx). With an optimized charge-enhancement layer, the fixed positive charge density in the induction layer can reach the 1012–1013 cm-2 range while maintaining low interface-state density, improving surface passivation and device performance.

What RbCl does in this stack RbCl acts as a thin, inorganic charge-enhancement source that boosts fixed charge in the overlying dielectric (e.g., SiNx), strengthening inversion formation and reducing recombination when interface states are controlled.
Why engineers like this approach Lower thermal budget, simpler layer stack tuning, and process compatibility with common silicon PV tools (PECVD, evaporation, sputtering, anneal).

2) Detailed Experimental Procedure

Target device stack (front to back):

  • Top electrode: Al (or Ni/Cr/Au/Ti/Pd/Ag), patterned via mask or lithography
  • Anti-reflection layer (AR): SiNx (typical thickness ~70 nm via PECVD)
  • Charge-enhancement layer: RbCl (single or combined with CaCl2/KCl/MgCl2)
  • Transition layer: SiOx or SiOxNy
  • p-type substrate: preferred p-type monocrystalline Si (0.1–10 Ω·cm, often 0.3–1 Ω·cm)
  • Bottom electrode: Al (or Ti/Pd/Ag, Ti/Ag depending on deposition flow)

Recommended RbCl thickness for the charge-enhancement layer: 0.5–10 nm (commonly 1–3 nm for tight electrical/optical balance).

Process Route A: Back Electrode First + Anneal-Grown SiOx Transition Layer (Example-Oriented)

  1. Substrate preparation (front side and back side):
    • Start with a cleaned p-type silicon wafer (e.g., ~0.3 Ω·cm).
    • Use an industry-standard silicon PV cleaning sequence to remove organics/particles/metal ions.
  2. Deposit bottom electrode (back side):
    • Deposit metal on the backside via thermal evaporation, sputtering, or e-beam evaporation.
    • Example: thermal evaporation of Al ~5 μm on the backside.
  3. First anneal (forms high-quality SiOx on the front side while protecting back metal):
    • Anneal at 400–700°C for 10–120 min in H2, NH3, inert gas, or mixtures (often NH3 favored).
    • Example: 550°C in NH3 for 40 min.

    This anneal route is designed to avoid oxidizing the backside Al while still forming a useful front transition oxide.

  4. Apply RbCl charge-enhancement layer on the front side (spin-coating option):
    • Prepare a saturated RbCl solution (control insolubles; filter if needed for uniform films).
    • Dispense at the front-side center and spin-coat:
      • Low-speed spread: 800 rpm, 6 s
      • High-speed thin-down: 2000–3000 rpm, 20–40 s (typical: 2500 rpm, 30 s)
    • Soft bake: 75°C for 20 min.
  5. Deposit top electrode (front side):
    • Deposit metal (often Al) by sputtering/thermal evaporation/e-beam evaporation (e-beam common).
    • Pattern via shadow mask or photolithography (dense grid is commonly used for lower series resistance).
  6. Deposit SiNx anti-reflection layer (uncovered regions):
    • Grow SiNx by PECVD (or other PVD/CVD variants) on areas not covered by the top electrode.
    • Example PECVD gas set: silane + ammonia + argon; one typical flow ratio: 20:40:5
    • Example process: 300°C, 20 Pa, thickness ~70 nm
  7. Second anneal (final passivation enhancement):
    • Anneal at 150–500°C for 5–60 min in H2, NH3, inert gas, or mixtures.
    • Example: 200°C in H2 for 25 min.

Process Route B: Front Transition Layer by PECVD SiOxNy + Ultra-Thin RbCl by Evaporation

  1. Substrate preparation:
    • Clean p-type silicon wafer (e.g., ~0.5 Ω·cm) using standard PV-grade cleaning.
  2. Deposit transition layer (front side SiOxNy):
    • Use PECVD (preferred for tool compatibility), sputtering, direct oxidation/nitridation, or ion implantation approaches.
    • PECVD example: SiH4 + N2O with flow ratio in 0.1–1 range (often 0.2–0.5), temperature 200–400°C (often 300–350°C), pressure 10–50 Pa (often 15–40 Pa).
    • Example: SiH4:N2O = 2.5:10, 320°C, 20 Pa.
  3. Deposit bottom electrode (back side):
    • Form backside metal stack by e-beam evaporation, sputtering, or thermal evaporation.
    • Example stack: p-Si/Ti/Pd/Ag.
  4. Deposit RbCl charge-enhancement layer (front side, PVD option):
    • Deposit an ultra-thin RbCl film by thermal evaporation (commonly preferred among PVD options), sputtering, or e-beam evaporation.
    • Example: ~1 nm RbCl on the transition layer.

    For RbCl evaporation, raw material purity and moisture control matter: low Na/K contamination and low water uptake help stabilize electrical behavior and repeatability.

  5. Deposit and pattern top electrode:
    • Deposit Al (or alternative metals) via e-beam evaporation and pattern via mask/lithography.
  6. Deposit SiNx anti-reflection layer:
    • PECVD example: silane + ammonia + argon; one typical flow ratio: 20:40:10, 320°C, 15 Pa, thickness ~70 nm.
  7. Second anneal:
    • Example: 220°C in H2 for 20 min.

Process control checklist (engineer-focused):

  • RbCl layer thickness: keep within 0.5–10 nm (often 1–3 nm) to balance fixed charge and optical absorption/scattering.
  • Moisture management: RbCl is hygroscopic; control ambient humidity during solution handling and prior to vacuum steps.
  • Contamination: limit Na/K and particulates (filter solutions; verify residue/insolubles) to reduce variability in interface states.
  • Anneal atmospheres: use NH3/H2/inert as specified to tune interface chemistry while protecting metal layers.
  • Tool compatibility: PECVD for transition/AR layers and evaporation for electrodes are compatible with common silicon PV lines.

3) Comparison: This Production Method vs Traditional Approaches

Traditional mass-market silicon solar cells typically create the built-in field using a high-temperature dopant diffusion process (e.g., phosphorus diffusion on p-type Si). While mature, that route can introduce issues such as heavy emitter doping side effects, diffusion-induced surface defects, “dead layer” behavior that reduces spectral response, and thermal degradation of minority-carrier lifetime.

Item Traditional Diffused p–n Junction RbCl-Enhanced MIS/IL Induced Junction
Built-in field formation Formed by dopant diffusion to create a p–n junction Formed by fixed-charge dielectric stack that induces inversion (junction-like behavior)
Thermal budget High-temperature diffusion and related thermal cycles Lower-temperature stack engineering + two anneals (first: 400–700°C, second: 150–500°C), avoiding diffusion-heavy steps
Surface damage / recombination Diffusion can introduce defects and recombination centers near surface Designed to reduce interface states and enhance passivation via high fixed charge + controlled anneals
Passivation strategy Often relies on emitter optimization + dielectric passivation Relies on dielectric fixed charge density (1012–1013 cm-2) and low interface-state density for strong field-effect passivation
Manufacturing complexity Mature but diffusion adds high-temp equipment and process constraints Thin-layer deposition (spin/PVD/PECVD) + anneals; compatible with PECVD and evaporation tools
Cost lever Higher thermal energy and diffusion consumables Lower-cost thin inorganic chlorides as charge enhancers + simplified junction formation pathway

A key practical distinction is the first anneal: conventional MIS/IL routes may oxidize backside Al in dry oxygen (creating insulating Al2O3 that later requires removal), whereas the described workflow uses protective atmospheres to avoid metal oxidation while still forming a functional front transition oxide. The added second anneal is used to further reduce interface-state density and strengthen passivation.

4) Why Use Rubidium Chloride (RbCl) in This Application

In induced-junction solar cells, the induction layer’s electrical quality is the performance engine. The charge-enhancement layer is where Rubidium Chloride (RbCl) becomes strategically valuable: it is a thin, inorganic chloride that can be deposited by spin coating (from saturated solution) or physical vapor deposition (thermal evaporation/e-beam/sputter) to help the dielectric stack reach high fixed charge density while supporting low interface-state density after appropriate annealing.

Engineering advantages of RbCl as a charge-enhancement raw material:

  • High fixed-charge enabling behavior: supports induction-layer fixed charge in the 1012–1013 cm-2 regime, strengthening inversion formation and carrier separation.
  • Lower interface-state density pathway: with the two-anneal strategy, interface-state density can be driven down further, improving field-effect and chemical passivation synergy.
  • Non-toxic alternative to cesium-based enhancers: avoids toxic cesium-compound handling while still achieving strong charge enhancement.
  • Process flexibility: compatible with both solution (spin) and vacuum (PVD) methods, letting teams choose based on line capability and uniformity targets.
  • Cost-down potential: inexpensive inorganic chlorides used at nanometer-scale thickness can reduce material cost contribution while enabling simpler junction formation.
  • PV toolchain compatibility: integrates with standard PV equipment (PECVD, evaporation/sputter, anneal furnaces) without requiring dopant diffusion infrastructure for junction formation.

Performance indicators you can monitor to validate RbCl effectiveness:

  • Fixed charge density: track C–V or related characterization; target 1012–1013 cm-2 order.
  • Interface-state density (Dit): ensure charge enhancement does not come at the cost of higher Dit; the two-anneal sequence is intended to push Dit lower.
  • Minority-carrier lifetime: use lifetime testing (e.g., WT-1000 class tools) as a direct proxy for passivation quality improvements.
  • Optical response: verify that the AR layer and electrode layout preserve short-wavelength response benefits typical of MIS/IL designs.

For reproducibility, prioritize RbCl raw material specifications that matter in thin-film PV: low moisture, low Na/K, and low insolubles to stabilize interface chemistry and electrical dispersion. The synthesis method mentioned in this article references patent document number CN202010849725.3