Figures-of-merit for integrated bypass diode design in passivating-contact back-contact silicon solar cells: A TCAD simulation study

Abstract

The design of an integrated bypass diode employed for hot-spot protection in passivating-contact back-contact (TBC) solar cells involves an inherent tension: the structure must conduct sufficient current under reverse bias to prevent hot-spot formation, yet remain highly resistive under forward bias to avoid degrading power output of cells. However, the complex interplay among band-to-band tunneling (BTBT), trap-assisted tunneling (TAT), interdiffusion, and oxide pinholes that governs reverse conduction and forward leakage remains poorly quantified. A quantitative design framework based on two linked figures of merit (FoM1 and FoM2) is established through comprehensive technology computer-aided design (TCAD) simulations. FoM1 quantifies electrical selectivity as the reverse-to-forward current ratio, while FoM2 translates this selectivity into a geometric design criterion for the overlap region. Systematic parameter exploration reveals phosphorus interdiffusion as the dominant degradation mechanism, suppressing BTBT by over two orders of magnitude even at moderate junction grading. Pinholes exhibit a dual character, enhancing reverse conduction only when interdiffusion is controlled. Design windows ensuring effective hot-spot protection with acceptable forward-bias power loss are established as functions of pinhole density, trap concentration, and interdiffusion depth. A diagnostic methodology distinguishing polysilicon-region degradation from substrate parasitic recombination provides experimentally testable design rules for next-generation TBC cells.

Graphical abstract

Keywords

TBC solar cells / hot-spot prevention / Esaki diode / band-to-band tunneling / trap-assisted tunneling / figures of merit / TCAD simulation

References

[1] Kim K A , Krein P T . Reexamination of photovoltaic hot spotting to show inadequacy of the bypass diode. IEEE Journal of Photovoltaics, 2015, 5(5): 1435–1441

[2] Alonso-García M C , Ruíz J M . Analysis and modelling the reverse characteristic of photovoltaic cells. Solar Energy Materials and Solar Cells, 2006, 90(7–8): 1105–1120

[3] Bishop J W . Microplasma breakdown and hot-spots in silicon solar cells. Solar Cells, 1989, 26(4): 335–349

[4] Dhimish M , Holmes V , Mather P . et al. Novel hot spot mitigation technique to enhance photovoltaic solar panels output power performance. Solar Energy Materials and Solar Cells, 2018, 179: 72–79

[5] Herrmann W, Wiesner W, Vaassen W. Hot spot investigations on PV modules-new concepts for a test standard and consequences for module design with respect to bypass diodes. In: Proceedings of the 26th IEEE Photovoltaic Specialists Conference, Anaheim, USA, 1997, 1129–1132

[6] Meyer E L , van Dyk E E . Assessing the reliability and degradation of photovoltaic module performance parameters. IEEE Transactions on Reliability, 2004, 53(1): 83–92

[7] Silvestre S , Boronat A , Chouder A . Study of bypass diodes configuration on PV modules. Applied Energy, 2009, 86(9): 1632–1640

[8] Shin W G , Ko S W , Song H J . et al. Origin of bypass diode fault in c-Si photovoltaic modules: Leakage current under high surrounding temperature. Energies, 2018, 11(9): 2416

[9] Burgers A R, Eikelboom J A, Schonecker A, et al. Improved treatment of the strongly varying slope in fitting solar cell I–V curves. In: Proceedings of the 25th IEEE Photovoltaic Specialists Conference, Washington, USA, 1996, 569–572

[10] Sinton R A, Verlinden P J, Crane R A, et al. Large-area 21% efficient Si solar cells. In: Proceedings of the 23rd IEEE Photovoltaic Specialists Conference, Louisville, USA, 1993, 157–161

[11] Haase F , Hollemann C , Schäfer S . et al. Laser contact openings for local poly-Si-metal contacts enabling 26.1%-efficient POLO-IBC solar cells. Solar Energy Materials and Solar Cells, 2018, 186: 184–193

[12] Hollemann C , Haase F , Schäfer S . et al. 26.1%-efficient POLO-IBC cells: Quantification of electrical and optical loss mechanisms. Progress in Photovoltaics: Research and Applications, 2019, 27(11): 950–958

[13] Chen R , Tong H B , Zhu H T . et al. 23.83% efficient mono-PERC incorporating advanced hydrogenation. Progress in Photovoltaics: Research and Applications, 2020, 28(12): 1239–1247

[14] Tang H B , Li Y P , Lin H . et al. In-cell bypass diodes for high-efficiency and shading-tolerant back contact silicon photovoltaic modules. Nature Communications, 2026, 17(1): 3360

[15] Morisset A , Cabal R , Grange B . et al. Highly passivating and blister-free hole selective poly-silicon based contact for large area crystalline silicon solar cells. Solar Energy Materials and Solar Cells, 2019, 200: 109912

[16] Padhamnath P , Wong J , Nagarajan B . et al. Metal contact recombination in monoPoly™ solar cells with screen-printed & fire-through contacts. Solar Energy Materials and Solar Cells, 2019, 192: 109–116

[17] Peibst R , Römer U , Larionova Y . et al. Working principle of carrier selective poly-Si/c-Si junctions: Is tunneling the whole story?. Solar Energy Materials and Solar Cells, 2016, 158: 60–67

[18] Seto J Y W . The electrical properties of polycrystalline silicon films. Journal of Applied Physics, 1975, 46(12): 5247–5254

[19] Steinkemper H , Feldmann F , Bivour M . et al. Numerical simulation of carrier-selective electron contacts featuring tunnel oxides. IEEE Journal of Photovoltaics, 2015, 5(5): 1348–1356

[20] Seager C H . Grain boundaries in polycrystalline silicon. Annual Review of Materials Science, 1985, 15: 271–302

[21] van de Loo B W H , Macco B , Melskens J . et al. Silicon surface passivation by transparent conductive zinc oxide. Journal of Applied Physics, 2019, 125(10): 105305

[22] Zhang W Q , Zhang K P , Bai Y H . et al. Passivating pinholes for large-area and high-efficiency silicon solar cells with tunnel oxide passivated contact. Nature Communications, 2026, 17(1): 2490

[23] Tetzlaff D , Krügener J , Larionova Y . et al. A simple method for pinhole detection in carrier selective POLO-junctions for high efficiency silicon solar cells. Solar Energy Materials and Solar Cells, 2017, 173: 106–110

[24] Melskens J , van de Loo B W H , Macco B . et al. Passivating contacts for crystalline silicon solar cells: From concepts and materials to prospects. IEEE Journal of Photovoltaics, 2018, 8(2): 373–388

[25] Glunz S W , Feldmann F . SiO₂ surface passivation layers – A key technology for silicon solar cells. Solar Energy Materials and Solar Cells, 2018, 185: 260–269

[26] Richter A , Benick J , Müller R . et al. Tunnel oxide passivating electron contacts as full-area rear emitter of high-efficiency p-type silicon solar cells. Progress in Photovoltaics: Research and Applications, 2018, 26(8): 579–586

[27] Klaassen D B M . A unified mobility model for device simulation—I. Model equations and concentration dependence. Solid-State Electronics, 1992, 35(7): 953–959

[28] Canali C , Majni G , Minder R . et al. Electron and hole drift velocity measurements in silicon and their empirical relation to electric field and temperature. IEEE Transactions on Electron Devices, 1975, 22(11): 1045–1047

[29] Schenk A . A model for the field and temperature dependence of Shockley-Read-Hall lifetimes in silicon. Solid-State Electronics, 1992, 35(11): 1585–1596

[30] Kane E O . Theory of tunneling. Journal of Applied Physics, 1961, 32(1): 83–91

[31] Hurkx G A M , Klaassen D B M , Knuvers M P G . A new recombination model for device simulation including tunneling. IEEE Transactions on Electron Devices, 1992, 39(2): 331–338

[32] Lahti A , Santonen M , Rad Z J . et al. Polycrystalline silicon, a molecular dynamics study: II. Grains, grain boundaries and their structure. Modelling and Simulation in Materials Science and Engineering, 2024, 32(6): 065026

[33] Fırat M , Wouters L , Lagrain P . et al. Local enhancement of dopant diffusion from polycrystalline silicon passivating contacts. ACS Applied Materials & Interfaces, 2022, 14(15): 17975–17986

[34] Kopecek R , Buchholz F , Mihailetchi V D . et al. Interdigitated back contact technology as final evolution for industrial crystalline single-junction silicon solar cell. Solar, 2023, 3(1): 1–14

[35] Richter A , Glunz S W , Werner F . et al. Improved quantitative description of Auger recombination in crystalline silicon. Physical Review B, 2012, 86(16): 165202

[36] Fell A , Schön J , Muller M . et al. Modeling edge recombination in silicon solar cells. IEEE Journal of Photovoltaics, 2018, 8(2): 428–434

[37] Tang H B , Lin H , Wang G S . et al. Understanding localized current leakage in silicon-based heterojunction solar cells. Progress in Photovoltaics: Research and Applications, 2025, 33(4): 522–530

[38] Hallam B , Chen D , Kim M . et al. The role of hydrogenation and gettering in enhancing the efficiency of next-generation Si solar cells: An industrial perspective. Physica Status Solidi (A), 2017, 214(7): 1700305

[39] Sahakyan A , Harutyunyan V , Grigoryan N . et al. Pico-second pulse electron irradiation damage studies in crystalline silicon solar cells. Solar Energy Materials and Solar Cells, 2026, 295: 113956

[40] Xu M L , Bearda T , Filipič M . et al. Simple emitter patterning of silicon heterojunction interdigitated back-contact solar cells using damage-free laser ablation. Solar Energy Materials and Solar Cells, 2018, 186: 78–83

[41] Hallam B , Sugianto A , Mai L . et al. Hydrogen passivation of laser-induced defects for laser-doped silicon solar cells. IEEE Journal of Photovoltaics, 2014, 4(6): 1413–1420

[42] Lee S H , Bhopal M F , Lee D W . et al. Review of advanced hydrogen passivation for high efficient crystalline silicon solar cells. Materials Science in Semiconductor Processing, 2018, 79: 66–73

[43] Mack S , Schube J , Fellmeth T . et al. Metallisation of boron-doped polysilicon layers by screen printed silver pastes. Physica Status Solidi (RRL) – Rapid Research Letters, 2017, 11(12): 1700334

[44] Hartenstein M B , Stetson C , Nemeth W . et al. Trap-assisted dopant compensation prevents shunting in poly-Si passivating interdigitated back contact silicon solar cells. ACS Applied Energy Materials, 2021, 4(10): 10774–10782

[45] Lima Anderson C , Nemeth W , Guthrey H L . et al. Nanopinhole passivating contact Si solar cells fabricated with metal-assisted chemical etching. Advanced Energy Materials, 2023, 13(11): 2203579

[46] Reichel C , Müller R , Feldmann F . et al. Influence of the transition region between p- and n-type polycrystalline silicon passivating contacts on the performance of interdigitated back contact silicon solar cells. Journal of Applied Physics, 2017, 122(18): 184502

[47] Römer U , Peibst R , Ohrdes T . et al. Ion implantation for poly-Si passivated back-junction back-contacted solar cells. IEEE Journal of Photovoltaics, 2015, 5(2): 507–514

[48] Spinelli P , van de Loo B W H , Vlooswijk A H G . et al. Quantification of pn-junction recombination in interdigitated back-contact crystalline silicon solar cells. IEEE Journal of Photovoltaics, 2017, 7(5): 1176–1183

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