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

1. Utangulizi na Muhtasari

Silicon-based photovoltaics dominate the market, but the efficiency of single-junction cells is approaching its theoretical limit (~26.8%). Tandem solar cells, which stack a wide-bandgap top cell on a silicon bottom cell, offer 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. Muundo na Utengenezaji wa Kifaa

2.1 Monolithic Stack Structure

The device is fabricated in a monolithic manner, meaning the top cell and bottom cell 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) na safu ya kuchagua mashimo (MoO_x).
• Elektrodi ya mbele: ITO na mistari ya Au ya gridi inayotumika kukusanya mkondo.

2.2 Material Selection and Process

Kiwango cha chini cha kuyeyuka kwa Selenium (220°C) kinaruhusu matumizi ya utaratibu wa joto la chini unaolingana na seli za chini za silicon. Uchaguzi wa mawasiliano ya kuchagua wabebaji ni muhimu sana. Kifaa cha awali kilitumia ZnMgO, lakini uigizaji uliofuata uligundua kuwa TiO₂ni bora zaidi katika kupunguza kikwazo cha usafirishaji wa elektroni.

3. Uchambuzi wa Utendaji na Matokeo

3.1 Utendaji wa Kifaa cha Awali

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 subcell voltages.

3.2 Uboreshaji wa Mawasiliano ya Uchaguzi wa Wabebaji

Kubadilisha mawasiliano ya elektroni ya awali ya ZnMgO na TiO₂baadaye, pato la nguvuliliongezeka kwa mara 10Uboreshaji huu mkubwa unaonyesha wazi umuhimu wa uhandisi wa kiolesura katika seli za safu nyingi, ambapo vizuizi vidogo vya nishati vinaweza kusababisha vizingiti vikubwa vya mkondo.

3.3 Viashiria Muhimu vya Utendaji

• 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_ocMeasurements indicate that the primary loss isparasitic series resistance, rather than the intrinsic recombination loss within the absorption layer.
• Efficiency limiting factors: Kutokana na vizuizi vya usafirishaji vilivyotambuliwa, kusababisha kipengele cha kujaza (FF) na msongamano wa mkondo (J_sc) kuwa chini.

4. Ufahamu wa Kiufundi na Changamoto

4.1 Kizuizi cha Usafirishaji na Mbinu za Upotezaji

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 the FF andJ_sc。

4.2 Usanifu Unaongozwa na Uigizaji (SCAPS-1D)

The use of 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 aboutUhandisi wa Uchunguziya kozi bora. Mwandishi alitumia mfumo mpya wa nyenzo wenye uwezo mkubwa (Se/Si), na kwa ustadi aliunganisha vipimo na uigizaji ili kutambua hasa kiungo dhaifu chake – usafirishaji wa kiolesura. Hadithi halisi iko katikaMbinurather than the efficiency figures in the title.

Logical thread: The logic is impeccable: 1) Fabricate the first monolithic device (an achievement in itself). 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 candidly admit, is that it remains a low-current device. HighV_ocInamanisho, lakini isipokuwa upotezaji wa mwanga (unaoweza kuwepo hasa katika safu za seleniamu ya poli-kristali na ITO) utatatuliwa na uboreshaji wa mawasiliano uendelezwe zaidi, kiwango cha juu cha ufanisi ni cha chini. Ikilinganishwa na uboreshaji wa haraka na wa kujaribio katika mfumo wa safu-jumuishi ya perovskaiti/silisi, njia hii ni polepole, lakini inaweza kuwa ya msingi zaidi.

Ufahamu unaoweza kutekelezwa: Kwa tasnia, ujumbe ni wa pande mbili. Kwanza, seleniamu/silisi ni njia inayowezekana ya utafiti, yenye faida ya urahisi wa kipekee. Pili, zana zilizowasilishwa katika makala hii — 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 demonstrated in this work, the revival of selenium in the photovoltaic field is a fascinating case of "old material, new applications." For decades, selenium was recorded in history as the material for the first generation of solid-state solar cells, overshadowed by the industrial dominance of silicon. Its recent revival is driven by the specific needs of the silicon tandem paradigm, which requires finding astable, wide-bandgap, and process-simple 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 the selenium thin film is another key differentiator, potentially reducing encapsulation costs. The authors achieved 1.68 V V_ocis significant; this indicates 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 around 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%, whereas 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.. Hili ni mada ya kawaida katika nyenzo mpya za fotovoltiki, inayokumbusha utafiti wa awali wa betri za jua za kikaboni, ambapo uhandisi wa mawasiliano ulikuwa muhimu. Baadaye ya mseto wa Selenium/Silicon inategemea ukuzaji wa mkusanyiko wa nyenzo za mawasiliano zenye kupunguza kasoro na kuweka mpangilio wa bendi ya nishati – hili ni changamoto ya sayansi ya nyenzo, inayofanana na ile iliyokabiliwa na kutatuliwa kwa kiasi na misombo kama vile Spiro-OMeTAD na SnO₂katika nyanja ya perovskite. Ikiwa Selenium inaweza kujifunza kutokana na uzoefu wa uhandisi wa mipaka uliojifunzwa katika nyanja zingine zinazoibuka za fotovoltiki, uthabiti wake wa asili na urahisi wanaweza kuufanya kuwa mpinzani asiyejulikana katika mbio za mseto.

7. Maelezo ya Kiufundi na Umbo la Hisabati

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): Inayotokana na suns-V_ocData, inayowakilisha uwezekano mkubwa zaidi wa FF wakati hakuna upinzani wa mfululizo ($R_s$) na upotezaji wa kugawanya ($R_{sh}$). Imehesabiwa kwa kutumia sifa ya sasa-voltage ya diode ($J_d-V$) iliyotolewa kwa ushirikiano:
$pFF = \frac{P_{max, ideal}}{J_{sc} \cdot V_{oc}}$
pFF > 80% 表明体结质量高，损耗主要是电阻性的。

3. Vigezo vya uigizaji wa SCAPS-1D: Key inputs for modeling the selenium/silicon tandem cell 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 (karibu 4.0 eV) na TiO₂(karibu 4.2 eV) kazi ya kazi inaathiri sana mabadiliko ya bendi ya uendeshaji ($ΔE_c$) na Se.

8. Matokeo ya Uchunguzi na Maelezo ya Michoro

Maelezo ya chati (kulingana na maandishi): Makala yanaweza kujumuisha dhana mbili muhimu za michoro.

Kielelezo 1: Mchoro wa muundo wa kifaa. Onyesha mchoro wa sehemu ya mkusanyiko wa kipande kimoja: "Ag / poly-Si:H (n+) / c-Si (n) / poly-Si:H (p+) / ITO / [tunnel junction] / ZnMgO au 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: Energy band diagram from SCAPS-1D. This is a key diagnostic diagram. It will display two plots side by side:
a) Kutumia ZnMgO: Kuna "kilele" au kizuizi cha uwezo kinachojitokeza wazi kwenye ukanda wa uendeshaji wa kiolesura cha ZnMgO/Se, kinachozuia elektroni kusafiri kutoka kwenye safu ya kunyonya ya seleniamu hadi kwenye safu ya mawasiliano.
b) Kutumia TiO₂： Uunganishaji unaofaa zaidi wa "mwamba" au kilele kidogo, unaoharakisha utoaji wa elektroni zenye joto na kupunguza kizuizi cha usafirishaji wa elektroni. Kupunguzwa huku kwa kizuizi kinachofafanua moja kwa moja ongezeko la utendakazi mara 10.

Implied current-voltage (J-V) curve: The text implies that the initial device would exhibit 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. Mfumo wa Uchambuzi: Uchunguzi wa Kesi Usio na Msimbo

Case Study: Diagnosing Losses in Novel Tandem Solar Cells

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

Framework Application (Inspired by this paper):

1. Hatua ya 1 - Kutenganisha Aina ya Hasara: Tekeleza suns-V_oc测量。结果：高pFF（>75%).Hitimisho: Ubora wa kiungo cha photovoltaic yenyewe unaweza kukubalika; hasara hasi inatokana na mchanganyiko wa mwili au kiolesura.
2. Hatua ya 2 - Kupima hasara za upinzani: Tofauti kati ya nguvu bora inayotokana na pFF na nguvu iliyopimwa niUpotevu wa nguvu kwa sababu ya upinzaniTofauti kubwa inaonyesha upinzani wa mfululizo ulio juu.
3. Hatua ya 3 - Kuanzisha kizuizi: Tumia programu ya kuiga kifaa (kama vile SCAPS-1D, SETFOS). Unda muundo wa safu zilizopangwa. Badilisha kwa utaratibu uwezo wa umeme/kitendaji cha kazi cha safu ya mawasiliano yenye kuchagua wabebaji. Tambua ni kipengele gani cha mawasiliano kinachozalisha kizuizi kikubwa cha nishati katika mchoro wa bendi chini ya hali ya uendeshaji.
4. Hatua ya 4 - Dhana na Uthibitishaji: Dhana: "Nyenzo ya mawasiliano ya elektroni Y ina mabadiliko ya bendi ya uendeshaji ya +0.3 eV ikilinganishwa na nyenzo X, na hii husababisha kizuizi cha kuzuia." Uthibitishaji: Badilisha nyenzo Y kwa nyenzo Z, ikitabiriwa kuwa nyenzo Z ina mabadiliko ya karibu sifuri au hasi (mwinuko).
5. Hatua ya 5 - Urejeshaji: Pima kifaa kipya. Ikiwa FF naJ_scumeboreshwa kwa kiasi kikubwa, basi dhana ni sahihi. Kisha, elekeza kwenye hasara inayofuata kwa ukubwa (mfano, kunyonya kwa mwanga, mawasiliano ya mashimo).

Muundo huu uliowekwa, unaotegemea fizikia, unazidi mbinu ya kujaribu na kukosea, na unaweza kutumika moja kwa moja kwa teknolojia yoyote ya mkusanyiko inayoibuka.

10. Future Applications and Development Roadmap

Muda mfupi (miaka 1-3):

• Uhandisi wa Mawasiliano: Gundua na boresha tabaka za usafirishaji wa elektroni/shimo zinazolenga kwa pekee seleniamu. Inapaswa kuchunguzwa oksidi za metali zilizochanganywa, molekuli za kikaboni na nyenzo za pande mbili.
• Usimamizi wa Mwangaza: Unganisha miundo ya kukamata mwanga (muundo wa nyuzinyuzi, grating) na uboreshwe mipako ya kupinga uakisi, ili kuongeza msongamano wa mkondo wa seli ya juu ya seleniamu, ambao unaweza kuwa mdogo kwa sababu ya unyonyaji usio kamili au unyonyaji wa vimelea katika tabaka ya mawasiliano.
• Udhibiti wa Pengo la Bendi: Chunguza aloi ya seleniamu-teluriamu (SeTe) ili kurekebisha kwa uangalifu pengo la bendi kuwa karibu zaidi na thamani bora ya 1.7 eV ya safu ya silikoni, ambayo inaweza kuboresha mechi ya mkondo.

Mid-term (3-7 years):

• Scalable deposition technology: Kubadilisha uvukizi wa joto wa kiwango cha maabara kwa teknolojia zinazoweza kuongezeka, kama vile uwekaji wa usafirishaji wa gesi au kumwaga, kwa ajili ya uwekaji wa seleniamu.
• Uboreshaji wa makutano ya handaki: Kukuza safu ya viunganishi yenye uwazi mkubwa, upinzani mdogo, na imara inayoweza kustahimili michakato ya usindikaji wa seli ya juu.
• Hatua ya kwanza ya ufanisi: 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。

Mtazamo wa Muda Mrefu na Matumizi:

• Bifacial and Agri-Photovoltaic Complementarity: Utilizing the potential of selenium to achieve semi-transparency through thinning, applicable to bifacial modules or agri-photovoltaic systems requiring partial light transmission.
• Space Photovoltaics: Inaripotiwa seleni ina mali ya kukinga mionzi na uthabiti, ambayo inaweza kufanya safu ya seleni/silika kuwa ya kuvutia katika matumizi ya anga, kwani matumizi ya anga yanahitaji ufanisi na uzito wa hali ya juu.
• Soko dogo la gharama nafuu: 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

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.
4. Green, M. A., Dunlop, E. D., Hohl-Ebinger, J., Yoshita, M., Kopidakis, N., & Hao, X. (2023). Solar cell efficiency tables (Version 61). Progress in Photovoltaics: Research and Applications, 31(1), 3-16.
5. Todorov, T., Singh, S., Bishop, D. M., Gunawan, O., Lee, Y. S., Gershon, T. S., ... & Mitzi, D. B. (2017). Ultrathin high band gap solar cells with improved efficiencies from the world's oldest photovoltaic material. Nature Communications, 8(1), 682.
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)