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

1. Introduction and Overview

Silicon-based photovoltaics dominate the market, but the efficiency of their single junctions is approaching the theoretical limit (approximately 26.8%). Tandem solar cells, which stack a wide-bandgap top cell on a silicon bottom cell, provide a clear pathway to achieving efficiencies exceeding 30%. This work demonstrates for the first time theMonolithic IntegrationSelenium, with its direct bandgap of approximately 1.8-2.0 eV, high absorption coefficient, and simple elemental composition, is a promising but historically stagnant candidate material that is now being revived for tandem applications.

2. Device Structure and Fabrication

2.1 Monolithic Stacked Structure

The device is fabricated in a monolithic manner, meaning the top and bottom cells are connected in series via a tunnel junction or recombination layer. The general layer stacking structure from bottom to top is:

• Bottom cell: n-type crystalline silicon (c-Si) substrate with doped polysilicon (n+ and p+) carrier-selective contacts, topped with ITO.
• Interconnect / Tunnel junction: Crucial for achieving low-resistance, optically transparent carrier recombination.
• Top cell: p-type polycrystalline selenium (poly-Se) absorption layer.
• Carrier-selective contact: Electron-selective layer (ZnMgO or TiO₂) and the hole-selective layer (MoO_x).
• Front electrode: ITO and Au grid lines for current collection.

2.2 Material Selection and Process

The low melting point of selenium (220°C) enables the use of low-temperature processes compatible with the underlying silicon cell. The choice of carrier-selective contact is crucial. Initial devices used ZnMgO, but subsequent simulations found TiO₂to be superior in reducing the electron transport barrier.

3. Performance Analysis and Results

3.1 Initial Device Performance

The first monolithic selenium/silicon tandem solar cell demonstrated a high open-circuit voltage of up to 1.68 V through suns-Voc measurement.V_oc) measurement, demonstrating a high value of up to1.68 VOpen-circuit voltage (V_oc). This highV_ocis a strong indicator of good material quality and effective bandgap pairing, as it approaches the sum of the two single-cell voltages.

3.2 Carrier-Selective Contact Optimization

Replacing the initial ZnMgO electron contact with TiO₂After that, the power outputIncreased by 10 times. This significant improvement highlights the critical role of interface engineering in tandem cells, where minor energy barriers can lead to severe current bottlenecks.

3.3 Key Performance Indicators

• Open-circuit voltage (V_oc): 1.68 V (suns-V_ocMeasurement).
• Pseudo fill factor (pFF): >80%。这个高值源自与注入水平相关的V_ocMeasurement indicates that the primary loss isparasitic series resistance, rather than the intrinsic recombination loss within the absorption layer.
• Efficiency limiting factors: Due to the identified transport barriers, resulting in a low fill factor (FF) and current density (J_sc).

4. Technical Insights and Challenges

4.1 Transmission Barriers and Loss Mechanisms

The core challenge lies in the non-ideal carrier transport across heterointerfaces. SCAPS-1D simulations reveal a significant energy barrier at the electron-selective contact (ZnMgO/Se interface), hindering electron extraction. This manifests as high series resistance, limiting FF andJ_sc。

4.2 Simulation-Guided Design (SCAPS-1D)

The standard solar cell capacitance simulator SCAPS-1D played a key role in diagnosing the problem. By simulating the energy band diagram, researchers were able to precisely locate the exact position and height of the transport barrier, thereby enabling the targeted replacement of ZnMgO with TiO₂to replace ZnMgO, because TiO₂has a more favorable conduction band alignment with Se.

5. Core Analytical Insights: Four-Step Deconstruction Method

Core Insights: This article is not about a high-efficiency champion device, but a lesson aboutDiagnostic Engineering. The author adopted an emerging, high-potential material system (Se/Si) and ingeniously combined metrology and simulation to precisely identify its Achilles' heel—interface transport. The real story lies in itsMethodology, rather than the efficiency numbers in the title.

Logical thread: The logic is impeccable: 1) Fabricate the first monolithic device (this in itself is an achievement). 2) Observe promisingV_ocbut the FF is poor. 3) Utilize suns-V_oc将串联电阻分离为罪魁祸首（pFF >80%是关键数据点）。4）部署SCAPS-1D可视化有问题的能量势垒。5）更换材料（ZnMgO→TiO₂) and achieve a 10x gain. This is a textbook problem-solving process.

Strengths and Weaknesses: The strength lies in its clear, physics-based device optimization approach. The weakness, as the authors frankly admit, is that it remains a low-current device. HighV_ocIt is compelling, but its efficiency ceiling is low unless optical losses (likely primarily in the polycrystalline selenium and ITO layers) are addressed and contact engineering is further optimized. Compared to the rapid, empirical optimization in perovskite/silicon tandems, this approach is slower but potentially more fundamental.

Actionable insights: For the industry, the message is twofold. First, selenium/silicon is a viable research path with a unique advantage of simplicity. Second, the toolkit demonstrated in this paper—suns-V_oc、pFF分析、SCAPS建模——应成为任何开发新型叠层架构团队的标准配置。投资者应关注后续解决光学设计问题并展示电流密度>15 mA/cm²的研究工作。在此之前，这是一个有前景但处于早期阶段的平台。

6. Original Analysis: The Revival of Selenium in the Photovoltaic Field

As shown in this work, the revival of selenium in photovoltaics is a fascinating case of "old material, new tricks." For decades, selenium was consigned to history as the material for the first generation of solid-state solar cells, overshadowed by silicon's industrial dominance. Its recent revival is driven by the specific needs of the silicon tandem paradigm, which requires finding astable, wide-bandgap, and simple-to-process partnerAs the holy grail. While perovskite/silicon tandems have attracted significant attention due to their rapid efficiency gains, they face challenges with stability and lead content. As shown in the 2023 NREL Best Research-Cell Efficiency Chart, perovskite/silicon tandems lead in efficiency, but a separate 'Emerging PV' category highlights their persistent reliability issues.

This work positions selenium as a compelling, albeit underdog, alternative. Its single-element composition is a fundamental advantage, eliminating stoichiometry and phase separation challenges common in compound semiconductors like CIGS or perovskites. The reported air stability of selenium thin films is another key differentiator, potentially reducing encapsulation costs. The authors achieved a 1.68 V V_ocis significant; this indicates that the selenium top cell is not a weak link in terms of voltage. This aligns with the Shockley-Queisser detailed balance limit, which shows the optimal top cell bandgap for a silicon bottom cell is approximately 1.7-1.9 eV—precisely within selenium's advantageous range.

However, the road ahead is challenging. The efficiency gap compared to perovskite-based tandems is substantial. The National Renewable Energy Laboratory (NREL) has recorded perovskite/silicon tandem efficiencies exceeding 33%, while this selenium/silicon device is still in its first demonstration phase. As the authors precisely identify, the primary challenge lies inthe transport physics at the heterointerface.This is a common theme in novel photovoltaic materials, reminiscent of early organic solar cell research where contact engineering was crucial. The future of selenium/silicon tandems depends on developing a library of defect-passivating, band-aligned contact materials—a materials science challenge similar to those faced and partially solved in the perovskite field by compounds like Spiro-OMeTAD and SnO₂. If selenium can draw on the interfacial engineering lessons learned from other emerging PV fields, its inherent stability and simplicity could make it a dark horse in the tandem race.

7. Technical Details and Mathematical Formalism

Analysis relies on key photovoltaic equations and simulation parameters:

1. Light intensity-open circuit voltage (suns-V_oc) method: This technique measuresV_ocDecouple the series resistance effect from the diode characteristics with varying light intensity. The relationship is:
$V_{oc}(S) = \frac{n k T}{q} \ln(S) + V_{oc}(1)$
where $S$ is the light intensity (in units of suns), $n$ is the ideality factor, $k$ is Boltzmann's constant, $T$ is the temperature, and $q$ is the elementary charge. Linear fitting can reveal the ideality factor.

2. Pseudo fill factor (pFF): Suns-V_ocData, representing the maximum possible FF in the absence of series resistance ($R_s$) and shunt loss ($R_{sh}$). Calculated from the integrated extracted diode current-voltage ($J_d-V$) characteristics:
$pFF = \frac{P_{max, ideal}}{J_{sc} \cdot V_{oc}}$
pFF > 80% 表明体结质量高，损耗主要是电阻性的。

3. SCAPS-1D simulation parameters: Key inputs for modeling selenium/silicon tandem cells include:
- Selenium: Bandgap $E_g = 1.9$ eV, electron affinity $χ = 4.0$ eV, dielectric constant $ε_r ≈ 6$.
- Interface: Defect density ($N_t$), capture cross-section ($σ_n, σ_p$) at the heterojunction.
- Contact: ZnMgO (approximately 4.0 eV) and TiO₂(approximately 4.2 eV) work function severely affects the conduction band offset ($ΔE_c$) with Se.

8. Experimental Results and Figure Descriptions

Chart Description (Based on Text): The paper may contain two key conceptual diagrams.

Figure 1: Schematic diagram of the device structure. Cross-sectional view showing monolithic stacking: "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 lines." This illustrates the series connection and the complex material stacking required for monolithic integration.

Figure 2: Band diagram from SCAPS-1D. This is a key diagnostic diagram. It will display two graphs side by side:
a) Using ZnMgO: There is a distinct "spike" or barrier at the conduction band of the ZnMgO/Se interface, which hinders electron flow from the selenium absorber layer to the contact layer.
b) Using TiO₂： A more favorable "cliff" or small spike alignment promotes hot electron emission and reduces the electron transport barrier. The reduction of this barrier directly explains the 10-fold performance improvement.

Implied current-voltage (J-V) curve: The text implies that the initial device would show a characteristic "S-shaped" or severely bent J-V curve due to high series resistance. Replacing ZnMgO with TiO₂, the curve becomes more "square", with improved fill factor and current density, although there is still a gap compared to champion cells.

9. Analytical Framework: A Non-Code Case Study

Case Study: Diagnosing Losses in Novel Tandem Solar Cells

Scenario: A research team fabricated a new monolithic tandem cell (Material X on silicon). It exhibits highV_oc, but the efficiency is disappointingly low.

Framework Application (Inspired by this paper):

1. Step 1 - Separate loss types: Execute suns-V_oc测量。结果：高pFF（>75%).Conclusion: The quality of the photovoltaic junction itself is acceptable; the losses are primarily not from bulk or interface recombination.
2. Step 2 - Quantify Resistive Losses: The difference between the ideal power derived from pFF and the measured power is theResistive power loss. A large difference indicates high series resistance.
3. Step 3 - Locate the barrier: Use device simulation software (e.g., SCAPS-1D, SETFOS). Build a stack model. Systematically vary the electron affinity/work function of the carrier-selective contact layer. Identify which interface creates a large energy barrier in the band diagram under operating conditions.
4. Step 4 - Hypothesis and Test: Hypothesis: "Electron contact material Y has a +0.3 eV conduction band offset with material X, creating a blocking barrier." Test: Replace material Y with material Z, which is predicted to have a near-zero or negative (cliff-like) offset.
5. Step 5 - Iteration: Measure the new device. If FF andJ_scsignificant improvement, the hypothesis is 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 can be directly applied to any emerging tandem technology.

10. Future Applications and Development Roadmap

Short-term (1-3 years):

• Contact engineering: Discover and optimize novel electron/hole transport layers specifically for selenium. Doped metal oxides, organic molecules, and two-dimensional materials should be screened.
• Optical management: Integrate light-trapping structures (texturing, gratings) and optimize anti-reflection coatings to enhance the current density of the selenium top cell, which may be limited by incomplete absorption or parasitic absorption in the contact layers.
• Bandgap Tuning: Explore selenium-tellurium (SeTe) alloys to fine-tune the bandgap closer to the ideal value of 1.7 eV for silicon tandem cells, potentially improving current matching.

Medium-term (3-7 years):

• Scalable deposition technology: Transition from lab-scale thermal evaporation to scalable techniques, such as vapor transport deposition or sputtering, for selenium deposition.
• Tunnel junction optimization: Develop highly transparent, low-resistance, and robust interconnect layers capable of withstanding the top cell processing steps.
• First efficiency milestone: 展示认证的硒/硅叠层电池效率>15%，证明该概念可以超越原理验证阶段。

Long-term and Application Outlook:

• Bifacial and Agri-Photovoltaic Complementarity: Utilizing the potential of selenium to achieve semi-transparency through thinning, applied to bifacial modules or agri-photovoltaic systems that require partial light transmission.
• Space Photovoltaics: It is reported that selenium possesses radiation resistance and stability, which could make selenium/silicon tandem cells attractive for space applications, where requirements for efficiency and weight are extremely high.
• Low-cost niche markets: 如果能够证明其可制造性和效率（>20%），硒/硅叠层可以瞄准那些极端稳定性和简单供应链比追求最高效率更重要的细分市场。

11. References

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2. National Renewable Energy Laboratory (NREL). (2023). Best Research-Cell Efficiency Chart. Retrieved from https://www.nrel.gov/pv/cell-efficiency.html
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