Monolithic Selenium/Silicon Tandem Solar Cells: First Demonstration and Analysis

1. Introduction & Overview

Silicon photovoltaics dominate the market but are approaching their single-junction efficiency limit (~26.8%). Tandem solar cells, stacking a wide-bandgap top cell on a silicon bottom cell, offer a clear path to efficiencies >30%. This work presents the first monolithic integration of a Selenium (Se) top cell with a Silicon (Si) bottom cell. Selenium, with a direct bandgap of ~1.8-2.0 eV, high absorption coefficient, and elemental simplicity, is a promising but historically stagnant candidate revived for tandem applications.

2. Device Architecture & Fabrication

2.1 Monolithic Stack Structure

The device is fabricated monolithically, meaning the top and bottom cells are electrically connected in series through a tunnel junction or recombination layer. The general layer stack from bottom to top is:

• Bottom Cell: n-type c-Si substrate with doped poly-Si (n+ and p+) carrier-selective contacts, capped with ITO.
• Interconnect/Tunnel Junction: Critical for low-resistance, optically transparent carrier recombination.
• Top Cell: p-type polycrystalline Selenium (poly-Se) absorber.
• Carrier-Selective Contacts: Electron-selective layer (ZnMgO or TiO2) and hole-selective layer (MoO_x).
• Front Electrode: ITO with a Au grid for current collection.

2.2 Material Selection & Processing

Selenium's low melting point (220°C) enables low-temperature processing compatible with the underlying silicon cell. The choice of carrier-selective contacts is pivotal. Initial devices used ZnMgO, but simulations later identified TiO2 as superior for reducing electron transport barriers.

3. Performance Analysis & Results

3.1 Initial Device Performance

The first monolithic Se/Si tandem demonstrated a promising open-circuit voltage (V_oc) of 1.68 V from suns-V_oc measurements. This high V_oc is a strong indicator of good material quality and effective bandgap pairing, as it approaches the sum of the individual cell voltages.

3.2 Carrier-Selective Contact Optimization

Replacing the initial ZnMgO electron contact with TiO2 led to a 10-fold increase in power output. This dramatic improvement underscores the critical role of interface engineering in tandem cells, where small energy barriers can cause severe current bottlenecks.

3.3 Key Performance Metrics

• Open-Circuit Voltage (V_oc): 1.68 V (suns-V_oc).
• Pseudo Fill Factor (pFF): >80%. This high value, derived from injection-level-dependent V_oc measurements, indicates that the primary losses are parasitic series resistance, not fundamental recombination losses within the absorber.
• Efficiency Limiter: Low fill factor (FF) and current density (J_sc) due to the identified transport barriers.

4. Technical Insights & Challenges

4.1 Transport Barriers & Loss Mechanisms

The core challenge is non-ideal carrier transport across heterointerfaces. SCAPS-1D simulations revealed a significant energy barrier at the electron-selective contact (ZnMgO/Se interface), blocking electron extraction. This manifests as high series resistance, limiting FF and J_sc.

4.2 Simulation-Guided Design (SCAPS-1D)

The use of SCAPS-1D, a standard solar cell capacitance simulator, was instrumental in diagnosing the problem. By modeling the energy band diagram, researchers could pinpoint the exact location and height of the transport barrier, leading to the targeted replacement of ZnMgO with TiO2, which has a more favorable conduction band alignment with Se.

5. Core Analyst Insight: Four-Step Deconstruction

Core Insight: This paper isn't about a high-efficiency champion device; it's a masterclass in diagnostic engineering. The authors have taken a nascent, high-potential material system (Se/Si) and surgically identified its Achilles' heel—interface transport—using a combination of clever metrology and simulation. The real story is the methodology, not the headline efficiency number.

Logical Flow: The logic is impeccable: 1) Build the first-ever monolithic device (a feat in itself). 2) Observe promising V_oc but poor FF. 3) Use suns-V_oc to isolate series resistance as the culprit (pFF >80% is a killer data point). 4) Deploy SCAPS-1D to visualize the offending energy barrier. 5) Swap materials (ZnMgO→TiO2) and achieve a 10x gain. This is textbook problem-solving.

Strengths & Flaws: The strength is the crystal-clear, physics-first approach to device optimization. The flaw, which the authors openly admit, is that this remains a low-current device. The high V_oc is seductive, but without solving optical losses (likely significant in the poly-Se and ITO layers) and further contact engineering, the efficiency ceiling is low. Compared to the rapid, empirical optimization seen in perovskite/Si tandems, this approach is slower but potentially more foundational.

Actionable Insights: For the industry, the message is twofold. First, Se/Si is a viable research path with unique simplicity advantages. Second, the toolkit demonstrated here—suns-V_oc, pFF analysis, SCAPS modeling—should be standard issue for any team developing novel tandem architectures. Investors should watch for follow-up work that addresses optical design and demonstrates a current density >15 mA/cm². Until then, this is a promising but early-stage platform.

6. Original Analysis: Selenium's Renaissance in PV

The resurgence of selenium in photovoltaics, as demonstrated in this work, is a fascinating case of "old materials, new tricks." For decades, selenium was relegated to the history books as the material of the first solid-state solar cells, overshadowed by silicon's industrial dominance. Its recent revival is driven by the specific demands of the silicon tandem paradigm, where a stable, wide-bandgap, and process-simple partner is the holy grail. While perovskite/silicon tandems have stolen the spotlight with their meteoric efficiency rises, they grapple with stability and lead-content issues. As noted in the 2023 NREL Best Research-Cell Efficiency Chart, perovskite/Si tandems lead in efficiency but have a separate column for "emerging PV," highlighting lingering reliability questions.

This work positions selenium as a compelling, if underdog, alternative. Its single-element composition is a fundamental advantage, eliminating the stoichiometric and phase-separation headaches of compound semiconductors like CIGS or perovskites. The reported air stability of selenium films is another critical differentiator, potentially reducing encapsulation costs. The authors' achievement of a 1.68 V V_oc is non-trivial; it indicates that the selenium top cell is not a weak link voltage-wise. This aligns with the Shockley-Queisser detailed balance limit, which shows the optimal top-cell bandgap for a Si bottom cell is around 1.7-1.9 eV—right in selenium's wheelhouse.

However, the path forward is steep. The efficiency gap with perovskite-based tandems is vast. The National Renewable Energy Laboratory (NREL) tracks a perovskite/Si tandem efficiency record over 33%, while this Se/Si device is in its first demonstration phase. The primary challenge, as the authors expertly diagnose, is transport physics at heterointerfaces. This is a common theme in novel PV materials, reminiscent of early organic solar cell research where contact engineering was paramount. The future of Se/Si tandems hinges on developing a library of defect-passivating, band-aligned contact materials—a materials science challenge similar to that faced and partially solved by the perovskite community with compounds like Spiro-OMeTAD and SnO2. If selenium can leverage the interface engineering lessons learned from other emerging PV fields, its inherent stability and simplicity could make it a dark horse contender in the tandem race.

7. Technical Details & Mathematical Formalism

The analysis relies on key photovoltaic equations and simulation parameters:

1. Suns-V_oc Method: This technique measures V_oc as a function of light intensity, decoupling series resistance effects from diode characteristics. The relationship is:
$V_{oc}(S) = \frac{n k T}{q} \ln(S) + V_{oc}(1)$
where $S$ is the suns intensity, $n$ is the ideality factor, $k$ is Boltzmann's constant, $T$ is temperature, and $q$ is the elementary charge. A linear fit reveals the ideality factor.

2. Pseudo Fill Factor (pFF): Derived from the suns-V_oc data, it represents the maximum possible FF in the absence of series resistance ($R_s$) and shunt losses ($R_{sh}$). It is calculated by integrating the extracted diode current-voltage ($J_d-V$) characteristic:
$pFF = \frac{P_{max, ideal}}{J_{sc} \cdot V_{oc}}$
A pFF > 80% indicates the bulk junction quality is high, and losses are primarily resistive.

3. SCAPS-1D Simulation Parameters: Key inputs for modeling the Se/Si tandem include:
- Selenium: Bandgap $E_g = 1.9$ eV, electron affinity $χ = 4.0$ eV, dielectric constant $ε_r ≈ 6$.
- Interfaces: Defect density ($N_t$), capture cross-sections ($σ_n, σ_p$) at heterojunctions.
- Contacts: Work function of ZnMgO (~4.0 eV) vs. TiO2 (~4.2 eV) critically affects the conduction band offset ($ΔE_c$) with Se.

8. Experimental Results & Chart Description

Figure Description (Based on Text): The paper likely contains two key conceptual figures.

Figure 1: Device Architecture Schematic. A cross-sectional diagram showing the monolithic stack: "Ag / poly-Si:H (n+) / c-Si (n) / poly-Si:H (p+) / ITO / [Tunnel Junction] / ZnMgO or TiO2 (n+) / poly-Se (p) / MoO_x / ITO / Au-grid." This illustrates the series connection and the complex material stack required for monolithic integration.

Figure 2: Energy Band Diagrams from SCAPS-1D. This is the critical diagnostic figure. It would show two diagrams side-by-side:
a) With ZnMgO: A pronounced "spike" or barrier in the conduction band at the ZnMgO/Se interface, blocking electron flow from the Se absorber to the contact.
b) With TiO2: A more favorable "cliff" or small spike alignment, facilitating thermionic emission and reducing the electron transport barrier. The lowering of this barrier directly explains the 10x performance improvement.

Implied Current-Voltage (J-V) Curves: The text suggests the initial device would show a characteristic "s-shaped" or severely bent J-V curve due to high series resistance. After replacing ZnMgO with TiO2, the curve would become more square, with improved fill factor and current density, though still limited compared to champion cells.

9. Analysis Framework: A Non-Code Case Study

Case Study: Diagnosing Losses in a Novel Tandem Cell

Scenario: A research group has fabricated a new monolithic tandem cell (Material X on Silicon). It shows a high V_oc but disappointingly low efficiency.

Framework Application (Inspired by this Paper):

1. Step 1 - Isolate Loss Type: Perform suns-V_oc measurement. Result: High pFF (>75%). Conclusion: The photovoltaic junction itself is decent; losses are not primarily from bulk or interface recombination.
2. Step 2 - Quantify Resistive Loss: The difference between the ideal power from pFF and the measured power gives the resistive power loss. A large gap points to high series resistance.
3. Step 3 - Locate the Barrier: Use device simulation software (e.g., SCAPS-1D, SETFOS). Build a model of the stack. Systematically vary the electron affinity/work function of the carrier-selective contact layers. Identify which interface creates a large energy barrier in the band diagram under operating conditions.
4. Step 4 - Hypothesis & Test: Hypothesis: "The electron contact Material Y has a conduction band offset of +0.3 eV with Material X, causing a blocking barrier." Test: Replace Material Y with Material Z, predicted to have a near-zero or negative (cliff) offset.
5. Step 5 - Iterate: Measure the new device. If FF and J_sc improve significantly, the hypothesis was correct. Then, move to the next largest loss (e.g., optical absorption, hole contact).

This structured, physics-based framework moves beyond trial-and-error and is directly applicable to any emerging tandem technology.

10. Future Applications & Development Roadmap

Short-Term (1-3 years):

• Contact Engineering: Discovery and optimization of novel electron/hole transport layers specifically for selenium. Doped metal oxides, organic molecules, and 2D materials should be screened.
• Optical Management: Integrating light-trapping structures (texturing, gratings) and optimizing anti-reflection coatings to boost the current density of the Se top cell, which is likely limited by incomplete absorption or parasitic absorption in contacts.
• Bandgap Tuning: Exploring selenium-tellurium (SeTe) alloys to fine-tune the bandgap closer to the ideal 1.7 eV for Si tandems, potentially improving current matching.

Medium-Term (3-7 years):

• Scalable Deposition: Transitioning from lab-scale thermal evaporation to scalable techniques like vapor transport deposition or sputtering for selenium.
• Tunnel Junction Optimization: Developing a highly transparent, low-resistance, and robust interconnect layer that can withstand processing of the top cell.
• First Efficiency Milestone: Demonstrating a certified Se/Si tandem cell efficiency >15%, proving the concept can move beyond the proof-of-principle stage.

Long-Term & Application Outlook:

• Bifacial & Agri-PV: Leveraging selenium's potential for semi-transparency (by thinning) in bifacial modules or agrivoltaic systems where partial light transmission is desired.
• Space Photovoltaics: Selenium's reported radiation hardness and stability could make Se/Si tandems interesting for space applications, where efficiency and weight are paramount.
• Low-Cost Niche: If manufacturability and efficiency (>20%) can be proven, Se/Si tandems could target market segments where the extreme stability and simple supply chain outweigh the ultimate efficiency crown held by other technologies.

11. References

1. Nielsen, R., Crovetto, A., Assar, A., Hansen, O., Chorkendorff, I., & Vesborg, P. C. K. (2023). Monolithic Selenium/Silicon Tandem Solar Cells. arXiv preprint arXiv:2307.05996.
2. National Renewable Energy Laboratory (NREL). (2023). Best Research-Cell Efficiency Chart. Retrieved from https://www.nrel.gov/pv/cell-efficiency.html
3. Shockley, W., & Queisser, H. J. (1961). Detailed balance limit of efficiency of p-n junction solar cells. Journal of Applied Physics, 32(3), 510-519.
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6. Youngman, T. H., Nielsen, R., Crovetto, A., Hansen, O., & Vesborg, P. C. K. (2021). What is the band gap of selenium? Solar Energy Materials and Solar Cells, 231, 111322.
7. Burgelman, M., Nollet, P., & Degrave, S. (2000). Modelling polycrystalline semiconductor solar cells. Thin Solid Films, 361, 527-532. (SCAPS-1D)