Near 13% efficient semitransparent Cu(In,Ga)S2 solar cells with band gap of 1.6 eV on transparent back contact

This study demonstrates that wide-gap (1.6 eV) semitransparent Cu(In,Ga)S₂ solar cells with In₂O₃:Sn transparent back contacts can achieve efficiencies near 13% by optimizing sodium supply and high-temperature growth, while balancing GaOₓ interfacial layer thickness to prevent current blocking.

The Gist

Imagine you are trying to build a two-story solar power plant.

In a traditional solar panel, the roof is made of a material that catches all the sunlight, but it wastes a lot of energy because it tries to catch every color of light, even the ones it can't use efficiently. To fix this, scientists are building tandem solar cells. Think of this as a two-story house:

• The Top Floor (Top Cell): Needs to catch the high-energy, blue light and let the lower-energy, red light pass through to the floor below.
• The Bottom Floor (Bottom Cell): Catches the red light that the top floor let through.

The problem is that the "Top Floor" material (called Cu(In,Ga)S₂ or CIGS) is tricky to build. It needs to be transparent at the bottom so light can pass through, but it also needs to be a perfect electrical conductor to collect the energy. Usually, the back of a solar cell is made of a shiny, opaque metal (like a mirror), which blocks light. For a tandem cell, we need a Transparent Back Contact (TBC)—like a clear glass floor that still conducts electricity.

The researchers in this paper tried to build this "Top Floor" using a clear material called ITO (Indium Tin Oxide) as the back contact. They hit a few snags, but they figured out how to fix them to get a very efficient result (nearly 13% efficiency, which is a big deal for this specific type of cell).

Here is the story of how they did it, using some simple analogies:

1. The Heat Problem: Cooking the Perfect Cake

To make the solar cell material (the "cake") high quality, you need to bake it at a very high temperature (630°C).

• The Issue: When you bake at this high heat, the clear glass floor (ITO) starts reacting with the cake ingredients. Specifically, a layer of "rust" (Gallium Oxide, or GaOx) forms between the cake and the floor.
• The Analogy: Imagine baking a cake on a glass plate. If the plate is too thick and the oven is too hot, a thick layer of burnt sugar forms between the cake and the plate. This burnt layer acts like a traffic jam. The electricity (the "cars") gets stuck trying to leave the cake and cross the floor, causing the solar cell to perform poorly.

2. The Secret Ingredient: Sodium (Na)

The researchers discovered that adding Sodium (Na) is like adding a special spice that fixes the texture of the cake.

• Where it comes from: Usually, the glass substrate (the baking tray) naturally leaks a little bit of sodium into the cake.
• The Experiment: They tried adding extra sodium (by sprinkling NaF powder) and also tried baking on thinner glass plates (thinner ITO layers).
• The Result:
  • Thick ITO + High Heat: Created a thick "rust" layer (GaOx). Even with extra sodium, the traffic jam remained. The electricity got stuck, and the efficiency dropped.
  • Thin ITO + High Heat: The "rust" layer was very thin. The electricity could flow through easily.
  • The Magic: Even without adding extra sodium, the thin ITO allowed enough natural sodium from the glass to seep in. This sodium acted like a traffic police officer, clearing the road and fixing the "traffic jam" caused by the rust.

3. Fixing the Cracks (Defects)

Solar cells have tiny cracks and imperfections (defects) where energy gets lost.

• Low Heat (575°C): The cake was undercooked. It had small grains and many cracks. The energy leaked out.
• High Heat (630°C): The cake was perfectly baked. The grains grew large and merged together, sealing the cracks.
• The Sodium Effect: Adding sodium helped "heal" the remaining cracks. The researchers measured how much light the material glowed with (Photoluminescence). The high-heat, sodium-treated samples glowed 100 times brighter than the low-heat ones, meaning they were losing almost no energy to defects.

The Big Win

By combining high heat (to make a strong, defect-free cake) with a thin transparent back contact (to keep the "rust" layer thin) and just the right amount of sodium (to clear the traffic), they achieved a breakthrough:

• They built a solar cell with a bandgap of 1.6 eV (perfect for the top floor of a tandem cell).
• It reached 12.7% efficiency.
• Most importantly, it worked well even with the transparent back contact, proving that this material is ready to be the "Top Floor" in the next generation of super-efficient solar panels.

In summary: They figured out how to bake a high-performance solar cell on a clear glass floor without letting the heat create a traffic-jamming layer of rust. They did this by using a thinner glass floor and letting natural sodium do the heavy lifting of cleaning up the interface. This paves the way for solar panels that can catch more sunlight than ever before.

Technical Summary

1. Problem Statement

Wide-bandgap chalcopyrite solar cells, specifically Cu(In,Ga)S₂ (CIGS), are critical candidates for the top cell in tandem photovoltaic devices (e.g., CIGS/Si tandems). To function effectively in a tandem configuration, the top cell requires a Transparent Back Contact (TBC) to allow unabsorbed near-infrared light to reach the bottom cell. Conventional opaque Molybdenum (Mo) back contacts are unsuitable for this application.

However, replacing Mo with Transparent Conductive Oxides (TCOs) like Indium Tin Oxide (ITO) introduces significant challenges:

• Interfacial Barriers: High-temperature processing often leads to the formation of a Gallium Oxide (GaOx) interlayer at the CIGS/ITO interface, which can act as a carrier extraction barrier, causing S-shaped current-voltage (J-V) curves and reduced Fill Factor (FF).
• Defect Density: Wide-bandgap absorbers (Eg > 1.5 eV) often suffer from high non-radiative recombination due to deep defects, limiting the Quasi-Fermi Level Splitting (QFLS) and open-circuit voltage (VOC).
• Sodium (Na) Management: Na is essential for high efficiency in CIGS, but its diffusion through TCOs is complex. Excessive Na can alter interface chemistry, while insufficient Na leads to poor grain growth and high defect density.
• Scarcity of Data: While TBCs are well-studied for Se-based CIGSe, reports on Sulfide-based CIGS with TBCs are scarce, particularly regarding high-efficiency devices with bandgaps suitable for tandems (~1.6 eV).

2. Methodology

The researchers fabricated semitransparent CIGS solar cells using a three-stage co-evaporation process on soda-lime glass (SLG) substrates coated with ITO.

• Substrates: Two ITO thicknesses were compared: 250 nm (high sheet resistance ~14 Ω/sq) and 100 nm (lower sheet resistance ~30 Ω/sq).
• Growth Conditions: Absorbers were grown at two substrate temperatures: 575°C (low) and 630°C (high).
• Sodium Supply Strategies:
  1. Diffusion only: Na diffusing naturally from the SLG substrate through the ITO.
  2. Co-evaporation: Additional Na supplied via NaF co-evaporation during the second stage of growth (durations of 7 and 18 minutes).
• Characterization:
  • Structural/Compositional: SEM (morphology), XRD (crystallinity), GDOES (depth profiling of Ga/In/Na), and ToF-SIMS (interface analysis of GaOx and Na distribution).
  • Optoelectronic: Absolute Photoluminescence (PL) to determine Photoluminescence Quantum Yield (YPL) and QFLS.
  • Device Performance: J-V characteristics under AM1.5G, External Quantum Efficiency (EQE), and rear-illumination testing.

3. Key Contributions

• Record Efficiency for Sulfide TBCs: The study reports a 12.7% efficiency for a wide-bandgap (1.6 eV) CIGS solar cell on a transparent back contact, representing the highest reported efficiency for chalcopyrite materials with Eg > 1.5 eV on a TBC.
• Decoupling Na and GaOx Effects: The work elucidates the complex interplay between Na supply, ITO thickness, and the formation of the GaOx interlayer. It demonstrates that while GaOx is inevitable at high temperatures, its thickness and electronic properties can be tuned.
• High-Temperature Optimization: It establishes that high-temperature growth (630°C) significantly improves absorber quality (grain size, QFLS) even on TBCs, provided the interface is managed correctly.
• Thin ITO Advantage: The study highlights that thinner ITO layers (100 nm) facilitate sufficient Na diffusion from the glass while minimizing GaOx thickness, thereby avoiding carrier extraction barriers.

4. Key Results

Structural and Compositional Analysis

• Grain Growth: High-temperature growth (630°C) resulted in significantly larger, columnar grains compared to 575°C. Moderate Na supply (7 min NaF) promoted grain growth, while excessive Na (18 min) reduced grain size and increased grain boundary density.
• Ga Grading: High-temperature samples exhibited flatter Ga gradients and narrower "notches" due to enhanced In/Ga interdiffusion.
• GaOx Formation: A GaOx interlayer formed at the CIGS/ITO interface in all high-temperature samples.
  • Thickness Correlation: GaOx thickness was non-monotonic with Na supply.
    • Thick ITO (250 nm) + No Na: ~53 nm GaOx.
    • Thick ITO + Moderate Na (7 min): ~33 nm GaOx (thinner).
    • Thick ITO + High Na (18 min): ~65 nm GaOx (thickest).
    • Thin ITO (100 nm) + No Na: ~14 nm GaOx (thinnest).
• Na Distribution: Na accumulated at the CIGS/ITO interface. In thin ITO samples, Na diffused sufficiently from the glass to passivate bulk defects without needing co-evaporation.

Optoelectronic Properties

• QFLS Improvement: High-temperature growth increased the QFLS from 880 meV (575°C) to 1060 meV (630°C), a gain of 180 meV.
• Defect Passivation: Na co-evaporation effectively suppressed deep defect emissions (D1 and D2) in PL spectra.
• Quantum Yield: The Photoluminescence Quantum Yield (YPL) improved by two orders of magnitude for high-temperature samples, reaching 1.5 × 10⁻⁵.

Device Performance

• J-V Characteristics:
  • S-Shaped Curves: Samples with thick ITO (250 nm) and high Na (18 min) or no Na showed S-shaped J-V curves, indicating a carrier extraction barrier caused by a thick GaOx layer.
  • Optimal Performance: The sample with Thin ITO (100 nm) and no additional Na achieved the best performance (12.7% efficiency, FF 71.8%). The thin ITO resulted in a thin GaOx layer that did not block carrier transport, while sufficient Na diffused from the glass to ensure bulk quality.
  • Moderate Na (Thick ITO): The T630/(7Na)/ITO250 sample achieved 12.0% efficiency with a high FF (68.5%), showing that moderate Na can mitigate the negative effects of a thicker GaOx layer.
• Bandgap: The champion devices had a bandgap of 1.61 eV, ideal for tandem applications with Si bottom cells.
• Rear Illumination: The device showed 3.2% efficiency under rear illumination, limited by low carrier diffusion lengths and reflection losses, confirming the need for further optimization of carrier transport in tandem stacks.

5. Significance

This work provides a critical roadmap for developing high-efficiency top cells for next-generation tandem solar cells.

• Feasibility of Sulfide Tandems: It proves that sulfide-based CIGS can achieve efficiencies comparable to selenide-based devices (which typically have lower bandgaps) while offering the tunable wide bandgap required for tandems.
• Interface Engineering: The study resolves the trade-off between Na supply and GaOx formation. It suggests that thin ITO layers are a superior strategy for TBCs, as they allow necessary Na diffusion from the substrate while minimizing the formation of blocking oxide layers.
• Scalability: Achieving >12% efficiency on a transparent back contact with a 1.6 eV bandgap demonstrates the viability of these materials for commercial tandem modules, potentially pushing overall tandem efficiencies beyond theoretical limits of single-junction cells.

In conclusion, the authors successfully engineered a wide-bandgap CIGS absorber on a transparent back contact by optimizing growth temperature, ITO thickness, and Na supply, overcoming interfacial barriers to achieve a record-breaking 12.7% efficiency.