Veröffentlichungen
2021 |
|
S. Schäfer, A. Mercker, A. Köhler, T. Neubert, L. Mettner, B. Wolpensinger, V. Mertens, and R. Peibst Journal of Applied Physics 129 (13), 133103, (2021). Abstract | Links | BibTeX | Schlagwörter: Chemical elements, Chemical processes, electric measurements, Lasers, optical properties, passivation, Semiconductor materials, silicon, Solar Cells, Transmission electron microscopy @article{Schäfer2021,
title = {Role of oxygen in the UV-ps laser triggered amorphization of poly-Si for Si solar cells with local passivated contacts}, author = {S Schäfer and A Mercker and A Köhler and T Neubert and L Mettner and B Wolpensinger and V Mertens and R Peibst}, doi = {10.1063/5.0045829}, year = {2021}, date = {2021-04-07}, journal = {Journal of Applied Physics}, volume = {129}, number = {13}, pages = {133103}, abstract = {In recent years, poly-Si based passivated contacts elevated the conversion efficiencies of crystalline Si solar cells to levels of 26%abs due to their outstanding electrical surface passivation performance and current transport characteristics. A major associated challenge, however, is the large parasitic light absorption within the doped poly-Si, regardless if the contacts are applied on the front and/or on the rear side of the solar cell. It, therefore, might be beneficial to confine the passivated contacts to local regions underneath the metal contacts. We present an effective and flexible laser-based approach to structure the poly-Si layer after its full-area deposition. Laser pulses with a pulse duration of 9 ps and a wavelength of 355 nm trigger an amorphization of the poly-Si surface. The minimum threshold fluence for amorphization is between 89 and 129 mJ/cm2. The a-Si layer, which is laterally homogeneous and up to (33 ± 4) nm in thickness, works as an etch barrier in an alkaline solution. The most robust barrier corresponding to the maximum thickness of the a-Si layer is found for a fluence of (270 ± 30) mJ/cm2. Besides the impact of the laser fluence on the etch resistiveness of the modified poly-Si layer, we study the role of oxygen during the laser process. We find that oxygen becomes incorporated into the material for certain laser fluences, which results in a more robust etch barrier. The amount of oxygen incorporated is below 3 wt. %. Eventually, we present a phenomenological model of our findings.}, keywords = {Chemical elements, Chemical processes, electric measurements, Lasers, optical properties, passivation, Semiconductor materials, silicon, Solar Cells, Transmission electron microscopy}, pubstate = {published}, tppubtype = {article} } In recent years, poly-Si based passivated contacts elevated the conversion efficiencies of crystalline Si solar cells to levels of 26%abs due to their outstanding electrical surface passivation performance and current transport characteristics. A major associated challenge, however, is the large parasitic light absorption within the doped poly-Si, regardless if the contacts are applied on the front and/or on the rear side of the solar cell. It, therefore, might be beneficial to confine the passivated contacts to local regions underneath the metal contacts. We present an effective and flexible laser-based approach to structure the poly-Si layer after its full-area deposition. Laser pulses with a pulse duration of 9 ps and a wavelength of 355 nm trigger an amorphization of the poly-Si surface. The minimum threshold fluence for amorphization is between 89 and 129 mJ/cm2. The a-Si layer, which is laterally homogeneous and up to (33 ± 4) nm in thickness, works as an etch barrier in an alkaline solution. The most robust barrier corresponding to the maximum thickness of the a-Si layer is found for a fluence of (270 ± 30) mJ/cm2. Besides the impact of the laser fluence on the etch resistiveness of the modified poly-Si layer, we study the role of oxygen during the laser process. We find that oxygen becomes incorporated into the material for certain laser fluences, which results in a more robust etch barrier. The amount of oxygen incorporated is below 3 wt. %. Eventually, we present a phenomenological model of our findings.
|
|
M-U. Halbich, and J. Schmidt physica status solidi (RRL) – Rapid Research Letters 15 (4), 2100008, (2021). Abstract | Links | BibTeX | Schlagwörter: carrier lifetimes, depletion region modulation, heterojunctions, PEDOT:PSS, silicon @article{Halbich2021,
title = {Extraction of PEDOT:PSS/c-Si Junction Properties by Modeling of Injection-Dependent Lifetime Measurements Including Depletion Region Modulation}, author = {M-U Halbich and J Schmidt}, doi = {10.1002/pssr.202100008}, year = {2021}, date = {2021-04-01}, journal = {physica status solidi (RRL) – Rapid Research Letters}, volume = {15}, number = {4}, pages = {2100008}, abstract = {The carrier lifetime of n-type silicon wafers coated with the conducting polymer PEDOT:PSS as a function of the excess carrier concentration Δp within the wafer is characterized using the quasi-steady-state photoconductance (QSSPC) method, and a drastic increase in the measured apparent lifetime τapp with decreasing Δp is observed. The observed increase with the depletion region modulation (DRM) effect is explained, as PEDOT:PSS-coated p-type silicon wafers do not show any increase in lifetime toward low injection levels. By modeling the measured τapp(Δp) curves on n-type silicon including interface recombination as well as the DRM effect, the interface recombination velocity as well as the band bending Ψs within the silicon induced by PEDOT:PSS are able to be extracted. The impact of adding sorbitol to the PEDOT:PSS dispersion on the τapp(Δp) curves is examined, and it is demonstrated that the admixture of sorbitol improves chemical interface passivation but leaves the band bending within the silicon bulk toward the PEDOT:PSS/c-Si interface unaffected.}, keywords = {carrier lifetimes, depletion region modulation, heterojunctions, PEDOT:PSS, silicon}, pubstate = {published}, tppubtype = {article} } The carrier lifetime of n-type silicon wafers coated with the conducting polymer PEDOT:PSS as a function of the excess carrier concentration Δp within the wafer is characterized using the quasi-steady-state photoconductance (QSSPC) method, and a drastic increase in the measured apparent lifetime τapp with decreasing Δp is observed. The observed increase with the depletion region modulation (DRM) effect is explained, as PEDOT:PSS-coated p-type silicon wafers do not show any increase in lifetime toward low injection levels. By modeling the measured τapp(Δp) curves on n-type silicon including interface recombination as well as the DRM effect, the interface recombination velocity as well as the band bending Ψs within the silicon induced by PEDOT:PSS are able to be extracted. The impact of adding sorbitol to the PEDOT:PSS dispersion on the τapp(Δp) curves is examined, and it is demonstrated that the admixture of sorbitol improves chemical interface passivation but leaves the band bending within the silicon bulk toward the PEDOT:PSS/c-Si interface unaffected.
|
2020 |
|
L. Helmich, D. C. Walter, D. Bredemeier, and J. Schmidt physica status solidi (RRL) – Rapid Research Letters 14 (12), 2000367, (2020). Abstract | Links | BibTeX | Schlagwörter: Aluminum oxide, defects, diffusion, Hydrogen, silicon @article{Helmich2020,
title = {Atomic-Layer-Deposited Al2O3 as Effective Barrier against the Diffusion of Hydrogen from SiNx:H Layers into Crystalline Silicon during Rapid Thermal Annealing}, author = {L Helmich and D C Walter and D Bredemeier and J Schmidt}, doi = {10.1002/pssr.202000367}, year = {2020}, date = {2020-12-01}, journal = {physica status solidi (RRL) – Rapid Research Letters}, volume = {14}, number = {12}, pages = {2000367}, abstract = {Stacks of hydrogen-lean aluminum oxide, deposited via plasma-assisted atomic-layer-deposition, and hydrogen-rich plasma-enhanced chemical vapor-deposited silicon nitride (SiNx) are applied to boron-doped float-zone silicon wafers. A rapid thermal annealing (RTA) step is performed in an infrared conveyor-belt furnace at different set-peak temperatures. The hydrogen content diffused into the crystalline silicon during the RTA step is quantified by measurements of the silicon resistivity increase due to hydrogen passivation of boron dopant atoms. These experiments indicate that there exists a temperature-dependent maximum in the introduced hydrogen content. The exact position of this maximum depends on the composition of the SiNx layer. The highest total hydrogen content, exceeding 1015 cm−3, is introduced into the silicon bulk from silicon-rich SiNx layers with a refractive index of 2.3 (at λ = 633 nm) at an RTA peak temperature of 800 °C, omitting the Al2O3 interlayer. Adding an Al2O3 interlayer with a thickness of 20 nm reduces the hydrogen content by a factor of four, demonstrating that Al2O3 acts as a highly effective hydrogen diffusion barrier. Measuring the hydrogen content in the silicon bulk as a function of Al2O3 thickness at different RTA peak temperatures provides the hydrogen diffusion length in Al2O3 as a function of measured temperature.}, keywords = {Aluminum oxide, defects, diffusion, Hydrogen, silicon}, pubstate = {published}, tppubtype = {article} } Stacks of hydrogen-lean aluminum oxide, deposited via plasma-assisted atomic-layer-deposition, and hydrogen-rich plasma-enhanced chemical vapor-deposited silicon nitride (SiNx) are applied to boron-doped float-zone silicon wafers. A rapid thermal annealing (RTA) step is performed in an infrared conveyor-belt furnace at different set-peak temperatures. The hydrogen content diffused into the crystalline silicon during the RTA step is quantified by measurements of the silicon resistivity increase due to hydrogen passivation of boron dopant atoms. These experiments indicate that there exists a temperature-dependent maximum in the introduced hydrogen content. The exact position of this maximum depends on the composition of the SiNx layer. The highest total hydrogen content, exceeding 1015 cm−3, is introduced into the silicon bulk from silicon-rich SiNx layers with a refractive index of 2.3 (at λ = 633 nm) at an RTA peak temperature of 800 °C, omitting the Al2O3 interlayer. Adding an Al2O3 interlayer with a thickness of 20 nm reduces the hydrogen content by a factor of four, demonstrating that Al2O3 acts as a highly effective hydrogen diffusion barrier. Measuring the hydrogen content in the silicon bulk as a function of Al2O3 thickness at different RTA peak temperatures provides the hydrogen diffusion length in Al2O3 as a function of measured temperature.
|
|
M. Winter, L. Helmich, D. C. Walter, and J. Schmidt Firing-Triggered LID (FT-LID) of the Carrier Lifetime in Cz-Si Inproceedings WIP (Hrsg.): Proceedings of the 37th European Photovoltaic Solar Energy Conference and Exhibition, 462–467, Online Event, (2020). Abstract | Links | BibTeX | Schlagwörter: defects, Degradation, LID, Lifetime, silicon @inproceedings{Winter2020c,
title = {Firing-Triggered LID (FT-LID) of the Carrier Lifetime in Cz-Si}, author = {M Winter and L Helmich and D C Walter and J Schmidt }, editor = {WIP}, doi = {10.4229/EUPVSEC20202020-2DV.2.15}, year = {2020}, date = {2020-10-28}, booktitle = {Proceedings of the 37th European Photovoltaic Solar Energy Conference and Exhibition}, journal = {37th European Photovoltaic Solar Energy Conference and Exhibition}, pages = {462--467}, address = {Online Event}, abstract = {Light-Induced Degradation’ (LID) effects of the carrier lifetime are present in many different silicon materials used in the production of solar cells. In this contribution, we present an approach to separate the boron-oxygen (BO) LID effect observed in boron-doped Czochralski-grown silicon (Cz-Si) from an additional firing-triggered LID (FT-LID) effect. We perform BO degradation/deactivation cycles by alternating illumination and dark annealing (DA) steps at 200 °C, while recording the changes in the carrier lifetime. Our measurements show that – despite some similarities – the observed FT-LID effect in Cz-Si is different to the ‘Light and elevated Temperature Induced Degradation’ (LeTID) effect reported on multicrystalline silicon (mc-Si) materials. In particular, the time constants of the lifetime degradation differs by at least a factor of 10 under the same experimental conditions. In addition, experiments where multiple alternating light degradation, regeneration and DA steps were performed show a different behavior between FTLID in Cz-Si and LeTID in mc-Si. Although FT-LID as well as LeTID are both triggered by fast-firing, which makes it very likely that hydrogen participates in both defect reactions, the detailed defect physics seems to be different. }, keywords = {defects, Degradation, LID, Lifetime, silicon}, pubstate = {published}, tppubtype = {inproceedings} } Light-Induced Degradation’ (LID) effects of the carrier lifetime are present in many different silicon materials used in the production of solar cells. In this contribution, we present an approach to separate the boron-oxygen (BO) LID effect observed in boron-doped Czochralski-grown silicon (Cz-Si) from an additional firing-triggered LID (FT-LID) effect. We perform BO degradation/deactivation cycles by alternating illumination and dark annealing (DA) steps at 200 °C, while recording the changes in the carrier lifetime. Our measurements show that – despite some similarities – the observed FT-LID effect in Cz-Si is different to the ‘Light and elevated Temperature Induced Degradation’ (LeTID) effect reported on multicrystalline silicon (mc-Si) materials. In particular, the time constants of the lifetime degradation differs by at least a factor of 10 under the same experimental conditions. In addition, experiments where multiple alternating light degradation, regeneration and DA steps were performed show a different behavior between FTLID in Cz-Si and LeTID in mc-Si. Although FT-LID as well as LeTID are both triggered by fast-firing, which makes it very likely that hydrogen participates in both defect reactions, the detailed defect physics seems to be different.
|
|
M. Winter, S. Bordihn, R. Peibst, R. Brendel, and J. Schmidt IEEE Journal of Photovoltaics 10 (2), 423-430, (2020), ISSN: 2156-3403. Abstract | Links | BibTeX | Schlagwörter: Carrier lifetime, Degradation, Poly-Si, Poly-Si on oxide (POLO), silicon, surface passivation @article{Winter2020,
title = {Degradation and Regeneration of n+-Doped Poly-Si Surface Passivation on p-Type and n-Type Cz-Si Under Illumination and Dark Annealing}, author = {M Winter and S Bordihn and R Peibst and R Brendel and J Schmidt}, doi = {10.1109/JPHOTOV.2020.2964987}, issn = {2156-3403}, year = {2020}, date = {2020-03-01}, journal = {IEEE Journal of Photovoltaics}, volume = {10}, number = {2}, pages = {423-430}, abstract = {Degradation and regeneration of recombination parameters can occur in the bulk and at the surfaces of silicon solar cells. This article focuses on the time-resolved analysis of the recombination properties of textured 1.7 Ω cm boron-doped p-type Cz-Si and 5 Ω cm phosphorus-doped n-type Cz-Si wafers, where the surfaces are passivated by n+ poly-Si on interfacial oxide layers exposed to a rapid thermal annealing (RTA) step in a conventional firing furnace. We observe a thermally activated instability in the lifetime over the entire examined injection range. Our experiments show that minority carrier injection (e.g., by illumination) is not required. Degradation in the surface passivation quality of the poly-Si on oxide layer—corresponding to an increase of the saturation current density J0 by up to a factor of five—causes the degradation of the effective lifetime. Interestingly, the surface passivation fully regenerates under prolonged annealing and finally improves even beyond the initial state. Both the extent of the lifetime degradation and the change in J0 depend on the postprocessing treatment temperature which we varied between 80 and 400 °C. Our results indicate that two different processes are responsible for the degradation and the regeneration. Reference samples which did not receive an RTA treatment show no degradation of the surface passivation quality. The RTA treatment applied therefore triggers the degradation effect. A large improvement of the surface passivation quality under prolonged annealing (e.g., at 400 °C) is observed for all samples examined in this study.}, keywords = {Carrier lifetime, Degradation, Poly-Si, Poly-Si on oxide (POLO), silicon, surface passivation}, pubstate = {published}, tppubtype = {article} } Degradation and regeneration of recombination parameters can occur in the bulk and at the surfaces of silicon solar cells. This article focuses on the time-resolved analysis of the recombination properties of textured 1.7 Ω cm boron-doped p-type Cz-Si and 5 Ω cm phosphorus-doped n-type Cz-Si wafers, where the surfaces are passivated by n+ poly-Si on interfacial oxide layers exposed to a rapid thermal annealing (RTA) step in a conventional firing furnace. We observe a thermally activated instability in the lifetime over the entire examined injection range. Our experiments show that minority carrier injection (e.g., by illumination) is not required. Degradation in the surface passivation quality of the poly-Si on oxide layer—corresponding to an increase of the saturation current density J0 by up to a factor of five—causes the degradation of the effective lifetime. Interestingly, the surface passivation fully regenerates under prolonged annealing and finally improves even beyond the initial state. Both the extent of the lifetime degradation and the change in J0 depend on the postprocessing treatment temperature which we varied between 80 and 400 °C. Our results indicate that two different processes are responsible for the degradation and the regeneration. Reference samples which did not receive an RTA treatment show no degradation of the surface passivation quality. The RTA treatment applied therefore triggers the degradation effect. A large improvement of the surface passivation quality under prolonged annealing (e.g., at 400 °C) is observed for all samples examined in this study.
|
|
J. Schmidt, K. Bothe, V. V. Voronkov, and R. Falster Fast and Slow Stages of Lifetime Degradation by Boron–Oxygen Centers in Crystalline Silicon Artikel physica status solidi (b) 257 (1), 1900167, (2020). Abstract | Links | BibTeX | Schlagwörter: defects, Lifetime, light-induced degradation (LID), recombination, silicon @article{Schmidt2020,
title = {Fast and Slow Stages of Lifetime Degradation by Boron–Oxygen Centers in Crystalline Silicon}, author = {J Schmidt and K Bothe and V V Voronkov and R Falster}, doi = {10.1002/pssb.201900167}, year = {2020}, date = {2020-01-01}, journal = {physica status solidi (b)}, volume = {257}, number = {1}, pages = {1900167}, abstract = {A conflict between previous and recently published data on the two-stage light-induced degradation (LID) of carrier lifetime in boron-doped oxygen-containing crystalline silicon is addressed. The previous experiments showed the activation of two boron–oxygen recombination centers with strongly differing recombination properties for the fast and slow stages of LID, whereas more recent studies found only a single center for both stages. To resolve this controversy, the historic silicon samples of these previous examinations are re-examined in this study after more than one decade. It is found that, in the historic samples, the fast stage can be either described by two different centers or a mixture of the two, depending on the duration of previous dark annealing. A possible solution is suggested based on the involvement of different activating impurities in the boron–oxygen defect. In dark-annealed samples, the defect consisting of boron, oxygen, and the activation impurity is present in two latent configurations, which reconfigure during LID at a fast and a slow stage. In the examined historic silicon samples, which did not undergo a gettering pretreatment, a significant concentration of an additional boron–oxygen defect with a different kind of activating impurity attached exists. The historic and modern results are thus reconciled.}, keywords = {defects, Lifetime, light-induced degradation (LID), recombination, silicon}, pubstate = {published}, tppubtype = {article} } A conflict between previous and recently published data on the two-stage light-induced degradation (LID) of carrier lifetime in boron-doped oxygen-containing crystalline silicon is addressed. The previous experiments showed the activation of two boron–oxygen recombination centers with strongly differing recombination properties for the fast and slow stages of LID, whereas more recent studies found only a single center for both stages. To resolve this controversy, the historic silicon samples of these previous examinations are re-examined in this study after more than one decade. It is found that, in the historic samples, the fast stage can be either described by two different centers or a mixture of the two, depending on the duration of previous dark annealing. A possible solution is suggested based on the involvement of different activating impurities in the boron–oxygen defect. In dark-annealed samples, the defect consisting of boron, oxygen, and the activation impurity is present in two latent configurations, which reconfigure during LID at a fast and a slow stage. In the examined historic silicon samples, which did not undergo a gettering pretreatment, a significant concentration of an additional boron–oxygen defect with a different kind of activating impurity attached exists. The historic and modern results are thus reconciled.
|
2019 |
|
J. Schmidt, D. Bredemeier, and D. C. Walter IEEE Journal of Photovoltaics 9 (6), 1497-1503, (2019). Abstract | Links | BibTeX | Schlagwörter: defects, Degradation, Hydrogen, metallic impurity, multicrystalline silicon (mc-Si), silicon, Solar Cells @article{Schmidt2019d,
title = {On the Defect Physics Behind Light and Elevated Temperature-Induced Degradation (LeTID) of Multicrystalline Silicon Solar Cells}, author = {J Schmidt and D Bredemeier and D C Walter}, doi = {10.1109/JPHOTOV.2019.2937223}, year = {2019}, date = {2019-11-01}, journal = {IEEE Journal of Photovoltaics}, volume = {9}, number = {6}, pages = {1497-1503}, abstract = {State-of-the-art solar cells with passivated surfaces fabricated on block-cast multicrystalline silicon (mc-Si) wafers show a pronounced degradation in efficiency under illumination at elevated temperature, as it typically occurs during operation in a solar module. This effect, frequently named `Light and elevated Temperature-Induced Degradation' (LeTID), has been attributed to the activation of a specific, hitherto unrevealed bulk defect in mc-Si. Recent experimental results of several labs have indicated that hydrogen is somehow involved in the responsible defect physics, without however providing any direct evidence so far. In this article, we present experimental data unambiguously showing a direct positive correlation of the extent of LeTID with the hydrogen content introduced into the silicon bulk during firing of the silicon wafers coated with hydrogen-rich silicon nitride (SiN$_rm x$:H) layers. Additional experiments including the pronounced impact of phosphorus gettering on the LeTID extent and the dependence of the degradation and regeneration on the wafer thickness support the involvement of a second species, with most indications pointing towards a metallic impurity. Several approaches of completely avoiding the instability in mc-Si solar cells are derived from the presented defect model, including 1) tuning of the SiN$_rm x$:H layer properties to minimize the in-diffusion of hydrogen into the wafer and 2) the thinning of the mc-Si wafer, improving the getterability of the metal impurity component toward the surfaces.}, keywords = {defects, Degradation, Hydrogen, metallic impurity, multicrystalline silicon (mc-Si), silicon, Solar Cells}, pubstate = {published}, tppubtype = {article} } State-of-the-art solar cells with passivated surfaces fabricated on block-cast multicrystalline silicon (mc-Si) wafers show a pronounced degradation in efficiency under illumination at elevated temperature, as it typically occurs during operation in a solar module. This effect, frequently named `Light and elevated Temperature-Induced Degradation' (LeTID), has been attributed to the activation of a specific, hitherto unrevealed bulk defect in mc-Si. Recent experimental results of several labs have indicated that hydrogen is somehow involved in the responsible defect physics, without however providing any direct evidence so far. In this article, we present experimental data unambiguously showing a direct positive correlation of the extent of LeTID with the hydrogen content introduced into the silicon bulk during firing of the silicon wafers coated with hydrogen-rich silicon nitride (SiN$_rm x$:H) layers. Additional experiments including the pronounced impact of phosphorus gettering on the LeTID extent and the dependence of the degradation and regeneration on the wafer thickness support the involvement of a second species, with most indications pointing towards a metallic impurity. Several approaches of completely avoiding the instability in mc-Si solar cells are derived from the presented defect model, including 1) tuning of the SiN$_rm x$:H layer properties to minimize the in-diffusion of hydrogen into the wafer and 2) the thinning of the mc-Si wafer, improving the getterability of the metal impurity component toward the surfaces.
|
|
D. Walter, D. Bredemeier, R. Falster, V. Voronkov, and J. Schmidt Easy-to-Apply Methodology to Measure the Hydrogen Concentration in Boron-Doped Crystalline Silicon Presentation/Poster Leuven, Belgium, 09.04.2019, (SiliconPV 2019, 9th International Conference on Silicon Photovoltaics). BibTeX | Schlagwörter: Hydrogen, silicon @misc{Walter2019,
title = {Easy-to-Apply Methodology to Measure the Hydrogen Concentration in Boron-Doped Crystalline Silicon}, author = {D Walter and D Bredemeier and R Falster and V Voronkov and J Schmidt}, year = {2019}, date = {2019-04-09}, address = {Leuven, Belgium}, note = {SiliconPV 2019, 9th International Conference on Silicon Photovoltaics}, keywords = {Hydrogen, silicon}, pubstate = {published}, tppubtype = {presentation} } |
|
B. Veith-Wolf, and J. Schmidt Low-Temperature Silicon Surface Passivation for Bulk Lifetime Studies Based on Corona-Charged Al2O3 Presentation/Poster Leuven, Belgium, 08.04.2019, (SiliconPV 2019, 9th International Conference on Silicon Photovoltaics). BibTeX | Schlagwörter: silicon, surface passivation @misc{Veith-Wolf2019,
title = {Low-Temperature Silicon Surface Passivation for Bulk Lifetime Studies Based on Corona-Charged Al2O3}, author = {B Veith-Wolf and J Schmidt}, year = {2019}, date = {2019-04-08}, address = {Leuven, Belgium}, note = {SiliconPV 2019, 9th International Conference on Silicon Photovoltaics}, keywords = {silicon, surface passivation}, pubstate = {published}, tppubtype = {presentation} } |
2018 |
|
B. A. Veith-Wolf, S. Schäfer, R. Brendel, and J. Schmidt Solar Energy Materials and Solar Cells 186 , 194-199, (2018), ISSN: 0927-0248. Abstract | Links | BibTeX | Schlagwörter: Aluminum oxide, Auger recombination, charge carrier lifetime, Intrinsic lifetime, silicon, surface passivation @article{Veith-Wolf2018b,
title = {Reassessment of intrinsic lifetime limit in n-type crystalline silicon and implication on maximum solar cell efficiency}, author = {B A Veith-Wolf and S Schäfer and R Brendel and J Schmidt}, doi = {10.1016/j.solmat.2018.06.029}, issn = {0927-0248}, year = {2018}, date = {2018-11-01}, journal = {Solar Energy Materials and Solar Cells}, volume = {186}, pages = {194-199}, abstract = {Unusually high carrier lifetimes are measured by photoconductance decay on n-type Czochralski-grown silicon wafers of different doping concentrations, passivated using plasma-assisted atomic-layer-deposited aluminum oxide (Al2O3) on both wafer surfaces. The measured effective lifetimes significantly exceed the intrinsic lifetime limit previously reported in the literature. Several prerequisites have to be fulfilled to allow the measurement of such high lifetimes on Al2O3-passivated n-type silicon wafers: (i) large-area wafers are required to minimize the impact of edge recombination via the Al2O3-charge-induced inversion layer, (ii) an exceptionally homogeneous Al2O3 surface passivation is required, and (iii) very thick silicon wafers are needed. Based on our lifetime measurements on n-type silicon wafers of different doping concentrations, we introduce a new parameterization of the intrinsic lifetime for n-type crystalline silicon. This new parameterization has implications concerning the maximum reachable efficiency of n-type silicon solar cells, which is larger than assumed before.}, keywords = {Aluminum oxide, Auger recombination, charge carrier lifetime, Intrinsic lifetime, silicon, surface passivation}, pubstate = {published}, tppubtype = {article} } Unusually high carrier lifetimes are measured by photoconductance decay on n-type Czochralski-grown silicon wafers of different doping concentrations, passivated using plasma-assisted atomic-layer-deposited aluminum oxide (Al2O3) on both wafer surfaces. The measured effective lifetimes significantly exceed the intrinsic lifetime limit previously reported in the literature. Several prerequisites have to be fulfilled to allow the measurement of such high lifetimes on Al2O3-passivated n-type silicon wafers: (i) large-area wafers are required to minimize the impact of edge recombination via the Al2O3-charge-induced inversion layer, (ii) an exceptionally homogeneous Al2O3 surface passivation is required, and (iii) very thick silicon wafers are needed. Based on our lifetime measurements on n-type silicon wafers of different doping concentrations, we introduce a new parameterization of the intrinsic lifetime for n-type crystalline silicon. This new parameterization has implications concerning the maximum reachable efficiency of n-type silicon solar cells, which is larger than assumed before.
|
|
M. Schnabel, M. Rienäcker, E. L. Warren, J. F. Geisz, R. Peibst, P. Stradins, and A. C. Tamboli Equivalent Performance in Three-Terminal and Four-Terminal Tandem Solar Cells Artikel IEEE Journal of Photovoltaics 8 (6), 1584-1589, (2018), ISSN: 2156-3381. Abstract | Links | BibTeX | Schlagwörter: Absorption, Computer architecture, Current measurement, Density measurement, III-V semiconductor materials, Microprocessors, Photovoltaic cells, Power system measurements, silicon @article{Schnabel2018b,
title = {Equivalent Performance in Three-Terminal and Four-Terminal Tandem Solar Cells}, author = {M Schnabel and M Rienäcker and E L Warren and J F Geisz and R Peibst and P Stradins and A C Tamboli}, doi = {10.1109/JPHOTOV.2018.2865175}, issn = {2156-3381}, year = {2018}, date = {2018-11-01}, journal = {IEEE Journal of Photovoltaics}, volume = {8}, number = {6}, pages = {1584-1589}, abstract = {Tandem or multijunction solar cells are a promising method to circumvent the efficiency limit of single-junction solar cells, but there is ongoing debate over how best to interconnect the subcells in a tandem cell. In addition to four-terminal and two-terminal tandem cell architectures, a new three-terminal tandem cell architecture has recently been demonstrated, which features a standard two-terminal (front-back) circuit as well as an interdigitated back contact (IBC) circuit connected to the bottom cell. It has no middle contacts, and thus, maintains some of the simplicity of a two-terminal tandem. In this study, we measure four-terminal GaInP//Si and GaInP/GaAs//Si tandem cells in four-terminal and three-terminal configurations by connecting wires to mimic a three-terminal architecture. We demonstrate that both modes allow the same efficiencies exceeding 30% to be attained. Furthermore, we show that the IBC circuit not only collects excess power from the bottom cell, but that it can inject power into the bottom cell if it is current limiting the front-back circuit, enabling four-terminal performance in monolithic structures, regardless of which cell delivers less current.}, keywords = {Absorption, Computer architecture, Current measurement, Density measurement, III-V semiconductor materials, Microprocessors, Photovoltaic cells, Power system measurements, silicon}, pubstate = {published}, tppubtype = {article} } Tandem or multijunction solar cells are a promising method to circumvent the efficiency limit of single-junction solar cells, but there is ongoing debate over how best to interconnect the subcells in a tandem cell. In addition to four-terminal and two-terminal tandem cell architectures, a new three-terminal tandem cell architecture has recently been demonstrated, which features a standard two-terminal (front-back) circuit as well as an interdigitated back contact (IBC) circuit connected to the bottom cell. It has no middle contacts, and thus, maintains some of the simplicity of a two-terminal tandem. In this study, we measure four-terminal GaInP//Si and GaInP/GaAs//Si tandem cells in four-terminal and three-terminal configurations by connecting wires to mimic a three-terminal architecture. We demonstrate that both modes allow the same efficiencies exceeding 30% to be attained. Furthermore, we show that the IBC circuit not only collects excess power from the bottom cell, but that it can inject power into the bottom cell if it is current limiting the front-back circuit, enabling four-terminal performance in monolithic structures, regardless of which cell delivers less current.
|
|
B. Lim, A. Merkle, R. Peibst, T. Dullweber, Y. Wang, and R. Zhou LID - Free PERC+ Solar Cells with Stable Efficiencies Up to 22.1% Inproceedings WIP (Hrsg.): Proceedings of the 35th European Photovoltaic Solar Energy Conference and Exhibition, 359-365, Brussels, Belgium, (2018). Abstract | Links | BibTeX | Schlagwörter: Czochralski (Cz), LID, PERC, silicon, Simulation @inproceedings{Lim2018,
title = {LID - Free PERC+ Solar Cells with Stable Efficiencies Up to 22.1%}, author = {B Lim and A Merkle and R Peibst and T Dullweber and Y Wang and R Zhou}, editor = {WIP}, doi = {10.4229/35thEUPVSEC20182018-2BO.3.2}, year = {2018}, date = {2018-09-24}, booktitle = {Proceedings of the 35th European Photovoltaic Solar Energy Conference and Exhibition}, pages = {359-365}, publisher = {Brussels, Belgium}, abstract = {e evaluate the potential of advanced B-doped Cz-Si with extremely low oxygen concentration ([Oi]) of 2.6 ppma as well as Ga-doped Cz-Si to avoid light-induced degradation (LID) in state-of-the-art bifacial PERC+ solar cells. We compare these materials to current industry standard B-doped Cz-Si with [Oi] between 12 and 16 ppma.. We measure the solar cell efficiency and the lifetime of samples processed in parallel to the solar cells in three important states: as-processed, after illumination at room temperature (degraded), and after illumination at elevated temperature (regenerated). In the as-processed state, the low [Oi] as well as the Ga-doped solar cells yield 0.2%abs to 0.3%abs higher efficiencies than the industrial B-doped Cz-Si. In addition, their efficiency is stable under illumination at room temperature. Furthermore, the measured bulk lifetimes are used as input parameters in a device simulation. Subsequently, we compare the simulated solar cell efficiencies to the measured efficiencies. For the industry standard B-doped Cz-Si, the simulation predicts 0.7%abs loss due to LID, which fits to the experimental result. After regeneration, the device simulation predicts an increase by 0.4%abs compared to the as-processed state, whereas the measured PERC+ efficiency improves only to the as-processed level. We discuss possible reasons for this discrepancy.}, keywords = {Czochralski (Cz), LID, PERC, silicon, Simulation}, pubstate = {published}, tppubtype = {inproceedings} } e evaluate the potential of advanced B-doped Cz-Si with extremely low oxygen concentration ([Oi]) of 2.6 ppma as well as Ga-doped Cz-Si to avoid light-induced degradation (LID) in state-of-the-art bifacial PERC+ solar cells. We compare these materials to current industry standard B-doped Cz-Si with [Oi] between 12 and 16 ppma.. We measure the solar cell efficiency and the lifetime of samples processed in parallel to the solar cells in three important states: as-processed, after illumination at room temperature (degraded), and after illumination at elevated temperature (regenerated). In the as-processed state, the low [Oi] as well as the Ga-doped solar cells yield 0.2%abs to 0.3%abs higher efficiencies than the industrial B-doped Cz-Si. In addition, their efficiency is stable under illumination at room temperature. Furthermore, the measured bulk lifetimes are used as input parameters in a device simulation. Subsequently, we compare the simulated solar cell efficiencies to the measured efficiencies. For the industry standard B-doped Cz-Si, the simulation predicts 0.7%abs loss due to LID, which fits to the experimental result. After regeneration, the device simulation predicts an increase by 0.4%abs compared to the as-processed state, whereas the measured PERC+ efficiency improves only to the as-processed level. We discuss possible reasons for this discrepancy.
|
|
S. Schäfer, and R. Brendel IEEE Journal of Photovoltaics 8 (4), 1156-1158, (2018), ISSN: 2156-3381. Abstract | Links | BibTeX | Schlagwörter: Efficiency limit, photon recycling, silicon, solar cell @article{Schäfer2018,
title = {Accurate Calculation of the Absorptance Enhances Efficiency Limit of Crystalline Silicon Solar Cells With Lambertian Light Trapping}, author = {S Schäfer and R Brendel}, doi = {10.1109/JPHOTOV.2018.2824024}, issn = {2156-3381}, year = {2018}, date = {2018-07-01}, journal = {IEEE Journal of Photovoltaics}, volume = {8}, number = {4}, pages = {1156-1158}, abstract = {The widely accepted limiting efficiency for crystalline silicon solar cells with Lambertian light trapping under 1 sun was previously calculated to be 29.43% for a 110-μm-thick device by using the commonly applied weak absorption approximation for light trapping. However, the short-circuit current density increases by 0.17 mA/cm2 when modeling the optical absorptance of an ideal Lambertian light trapping scheme exactly. The resulting new 1-sun efficiency limit is 29.56% and holds for a cell that is 98.1 μm in thickness.}, keywords = {Efficiency limit, photon recycling, silicon, solar cell}, pubstate = {published}, tppubtype = {article} } The widely accepted limiting efficiency for crystalline silicon solar cells with Lambertian light trapping under 1 sun was previously calculated to be 29.43% for a 110-μm-thick device by using the commonly applied weak absorption approximation for light trapping. However, the short-circuit current density increases by 0.17 mA/cm2 when modeling the optical absorptance of an ideal Lambertian light trapping scheme exactly. The resulting new 1-sun efficiency limit is 29.56% and holds for a cell that is 98.1 μm in thickness.
|
2017 |
|
B. A. Veith-Wolf, and J. Schmidt physica status solidi (RRL) – Rapid Research Letters 11 (11), 1700235, (2017), ISSN: 1862-6270, (1700235). Abstract | Links | BibTeX | Schlagwörter: Al2O3, Auger recombination, intrinsic recombination, minority carrier lifetime, silicon, surface passivation @article{Veith-Wolf2017,
title = {Unexpectedly High Minority-Carrier Lifetimes Exceeding 20 ms Measured on 1.4-Ω cm n-Type Silicon Wafers}, author = {B A Veith-Wolf and J Schmidt}, doi = {10.1002/pssr.201700235}, issn = {1862-6270}, year = {2017}, date = {2017-11-01}, journal = {physica status solidi (RRL) – Rapid Research Letters}, volume = {11}, number = {11}, pages = {1700235}, publisher = {WILEY?VCH Verlag Berlin GmbH}, abstract = {We measure very high minority‐carrier lifetimes exceeding 20 ms on 1.4‐Ω cm n‐type Czochralski silicon wafers passivated using plasma‐assisted atomic‐layer‐deposited Al2O3 on both wafer surfaces. The measured maximum effective lifetimes are surprisingly high as they significantly exceed the intrinsic lifetime limit previously reported in the literature. We are able to measure such high lifetimes by realizing an exceptionally homogeneous Al2O3 surface passivation on large‐area samples (12.5 × 12.5 cm2). The importance of the homogeneous passivation is demonstrated by comparison with samples of locally reduced passivation quality.}, note = {1700235}, keywords = {Al2O3, Auger recombination, intrinsic recombination, minority carrier lifetime, silicon, surface passivation}, pubstate = {published}, tppubtype = {article} } We measure very high minority‐carrier lifetimes exceeding 20 ms on 1.4‐Ω cm n‐type Czochralski silicon wafers passivated using plasma‐assisted atomic‐layer‐deposited Al2O3 on both wafer surfaces. The measured maximum effective lifetimes are surprisingly high as they significantly exceed the intrinsic lifetime limit previously reported in the literature. We are able to measure such high lifetimes by realizing an exceptionally homogeneous Al2O3 surface passivation on large‐area samples (12.5 × 12.5 cm2). The importance of the homogeneous passivation is demonstrated by comparison with samples of locally reduced passivation quality.
|
|
B. Min, M. Müller, H. Wagner, G. Fischer, R. Brendel, P. P. Altermatt, and H. Neuhaus A Roadmap Toward 24 % Efficient PERC Solar Cells in Industrial Mass Production Artikel IEEE Journal of Photovoltaics 7 (6), 1541-1550, (2017), ISSN: 2156-3381. Abstract | Links | BibTeX | Schlagwörter: Conductivity, Mass production, metallization, Passivated emitter and rear cell (PERC) solar cells, Photovoltaic cells, roadmap, Semiconductor device modeling, silicon, silicon solar cells @article{Min2017c,
title = {A Roadmap Toward 24 % Efficient PERC Solar Cells in Industrial Mass Production}, author = {B Min and M Müller and H Wagner and G Fischer and R Brendel and P P Altermatt and H Neuhaus}, doi = {10.1109/JPHOTOV.2017.2749007}, issn = {2156-3381}, year = {2017}, date = {2017-11-01}, journal = {IEEE Journal of Photovoltaics}, volume = {7}, number = {6}, pages = {1541-1550}, abstract = {Many manufacturers choose the passivated emitter and rear cell (PERC) approach in order to surpass the 20% cell efficiency level in mass production. In this paper, we study the efficiency potential of the PERC approach under realistic assumptions for incremental improvements of existing technologies by device simulations. Based on the most recent published experimental results, we find that the PERC structure is able to reach about 24% cell efficiency in mass production by an ongoing sequence of incremental improvements. As a guideline for future developments, we provide a method to improve cell efficiency most effectively by monitoring the current losses at the maximum power point. By means of numerical device modeling, we identify some key technologies toward 24% efficient PERC cells and provide its technology-related target requirements.}, keywords = {Conductivity, Mass production, metallization, Passivated emitter and rear cell (PERC) solar cells, Photovoltaic cells, roadmap, Semiconductor device modeling, silicon, silicon solar cells}, pubstate = {published}, tppubtype = {article} } Many manufacturers choose the passivated emitter and rear cell (PERC) approach in order to surpass the 20% cell efficiency level in mass production. In this paper, we study the efficiency potential of the PERC approach under realistic assumptions for incremental improvements of existing technologies by device simulations. Based on the most recent published experimental results, we find that the PERC structure is able to reach about 24% cell efficiency in mass production by an ongoing sequence of incremental improvements. As a guideline for future developments, we provide a method to improve cell efficiency most effectively by monitoring the current losses at the maximum power point. By means of numerical device modeling, we identify some key technologies toward 24% efficient PERC cells and provide its technology-related target requirements.
|
|
M. Rienäcker, M. Schnabel, E. Warren, A. Merkle, H. Schulte-Huxel, T. R. Klein, M. F. A. M. van Hest, M. A. Steiner, J. Geisz, S. Kajari-Schröder, R. Niepelt, J. Schmidt, R. Brendel, P. Stradins, A. Tamboli, and R. Peibst Mechanically Stacked Dual-Junction and Triple-Junction III-V/Si-IBC Cells with Efficiencies of 31.5 % and 35.4 % Inproceedings WIP (Hrsg.): Proceedings of the 33rd European Photovoltaic Solar Energy Conference and Exhibition, 1-4, Amsterdam, The Netherlands, (2017), ISBN: 3-936338-47-7. Abstract | Links | BibTeX | Schlagwörter: III-V, Multijunction, silicon @inproceedings{Rienäcker2017,
title = {Mechanically Stacked Dual-Junction and Triple-Junction III-V/Si-IBC Cells with Efficiencies of 31.5 % and 35.4 %}, author = {M Rienäcker and M Schnabel and E Warren and A Merkle and H Schulte-Huxel and T R Klein and M F A M van Hest and M A Steiner and J Geisz and S Kajari-Schröder and R Niepelt and J Schmidt and R Brendel and P Stradins and A Tamboli and R Peibst }, editor = {WIP}, doi = {10.4229/EUPVSEC20172017-1AP.1.2}, isbn = {3-936338-47-7}, year = {2017}, date = {2017-09-25}, booktitle = {Proceedings of the 33rd European Photovoltaic Solar Energy Conference and Exhibition}, pages = {1-4}, address = {Amsterdam, The Netherlands}, abstract = {The theoretical efficiency limit of 29.4 % for single-junction crystalline Silicon (c-Si) solar cells is an insurmountable barrier that is being steadily approached within the last decades. Combining the Si cell with a second absorber material on top in a dual junction tandem or triple junction solar cell is an attractive option to surpass this limit significantly. We demonstrate a mechanically stacked GaInP//Si dual-junction cell with an in-house measured efficiency of 31.5 % and a GaInP/GaAs//Si triple-junction cell with a certified efficiency of 35.4±0.5 % }, keywords = {III-V, Multijunction, silicon}, pubstate = {published}, tppubtype = {inproceedings} } The theoretical efficiency limit of 29.4 % for single-junction crystalline Silicon (c-Si) solar cells is an insurmountable barrier that is being steadily approached within the last decades. Combining the Si cell with a second absorber material on top in a dual junction tandem or triple junction solar cell is an attractive option to surpass this limit significantly. We demonstrate a mechanically stacked GaInP//Si dual-junction cell with an in-house measured efficiency of 31.5 % and a GaInP/GaAs//Si triple-junction cell with a certified efficiency of 35.4±0.5 %
|
|
R. Witteck, B. Min, H. Schulte-Huxel, H. Holst, B. Veith-Wolf, F. Kiefer, M. R. Vogt, M. Köntges, R. Peibst, and R. Brendel physica status solidi (RRL) – Rapid Research Letters 11 (8), 1700178, (2017), ISSN: 1862-6270, (1700178). Abstract | Links | BibTeX | Schlagwörter: ethylene vinyl acetate, PERC solar cells, radiation hardness, silicon, solar modules, UV degradation @article{Witteck2017c,
title = {UV radiation hardness of photovoltaic modules featuring crystalline Si solar cells with AlOx/p+-type Si and SiNy/n+-type Si interfaces}, author = {R Witteck and B Min and H Schulte-Huxel and H Holst and B Veith-Wolf and F Kiefer and M R Vogt and M Köntges and R Peibst and R Brendel}, doi = {10.1002/pssr.201700178}, issn = {1862-6270}, year = {2017}, date = {2017-08-01}, journal = {physica status solidi (RRL) – Rapid Research Letters}, volume = {11}, number = {8}, pages = {1700178}, publisher = {WILEY?VCH Verlag Berlin GmbH}, abstract = {We report on the UV radiation hardness of photovoltaic modules with bifacial n‐type Passivated Emitter and Rear Totally diffused crystalline Si cells that are embedded in an encapsulation polymer with enhanced UV transparency. Modules with front junction cells featuring an AlOx/p+‐type Si passivation interface at the illuminated side are stable for a UV irradiation dose of 598 kWh m−2. In contrast, irradiating modules with back junction cells featuring an a‐SiNy/n+‐type Si passivation interface at the illuminated side reduces the output power by 15%. The quantum efficiency of the a‐SiNy‐passivated module degrades in the spectral range between 300 and 1000 nm, which we ascribe to a degradation of the surface passivation. Modeling the experimental data shows that photons with an energy above 3.4 eV contribute to the degradation effect and enhance the front surface recombination current density by a factor of 15.}, note = {1700178}, keywords = {ethylene vinyl acetate, PERC solar cells, radiation hardness, silicon, solar modules, UV degradation}, pubstate = {published}, tppubtype = {article} } We report on the UV radiation hardness of photovoltaic modules with bifacial n‐type Passivated Emitter and Rear Totally diffused crystalline Si cells that are embedded in an encapsulation polymer with enhanced UV transparency. Modules with front junction cells featuring an AlOx/p+‐type Si passivation interface at the illuminated side are stable for a UV irradiation dose of 598 kWh m−2. In contrast, irradiating modules with back junction cells featuring an a‐SiNy/n+‐type Si passivation interface at the illuminated side reduces the output power by 15%. The quantum efficiency of the a‐SiNy‐passivated module degrades in the spectral range between 300 and 1000 nm, which we ascribe to a degradation of the surface passivation. Modeling the experimental data shows that photons with an energy above 3.4 eV contribute to the degradation effect and enhance the front surface recombination current density by a factor of 15.
|
|
F. Haase, F. Kiefer, S. Schäfer, C. Kruse, J. Krügener, R. Brendel, and R. Peibst Japanese Journal of Applied Physics 56 , 08MB15, (2017). Abstract | Links | BibTeX | Schlagwörter: back-contact, IBC, passivating contact, POLO, silicon, solar cell @article{Haase2017,
title = {Interdigitated back contact solar cells with polycrystalline silicon on oxide passivating contacts for both polarities}, author = {F Haase and F Kiefer and S Schäfer and C Kruse and J Krügener and R Brendel and R Peibst}, doi = {10.7567/JJAP.56.08MB15}, year = {2017}, date = {2017-07-07}, journal = {Japanese Journal of Applied Physics}, volume = {56}, pages = {08MB15}, abstract = {We demonstrate an independently confirmed 25.0%-efficient interdigitated back contact silicon solar cell with passivating polycrystalline silicon (poly-Si) on oxide (POLO) contacts that enable a high open circuit voltage of 723 mV. We use n-type POLO contacts with a measured saturation current density of J_0n = 4 fA cm−2 and p-type POLO contacts with J_0p = 10 fA cm−2. The textured front side and the gaps between the POLO contacts on the rear are passivated by aluminum oxide (AlOx ) with J_0AlOx = 6 fA cm−2 as measured after deposition. We analyze the recombination characteristics of our solar cells at different process steps using spatially resolved injection-dependent carrier lifetimes measured by infrared lifetime mapping. The implied pseudo-efficiency of the unmasked cell, i.e., cell and perimeter region are illuminated during measurement, is 26.2% before contact opening, 26.0% after contact opening and 25.7% for the finished cell. This reduction is due to an increase in the saturation current density of the AlO x passivation during chemical etching of the contact openings and of the rear side metallization. The difference between the implied pseudo-efficiency and the actual efficiency of 25.0% as determined by designated-area light current–voltage (I–V) measurements is due to series resistance and diffusion of excess carriers into the non-illuminated perimeter region.}, keywords = {back-contact, IBC, passivating contact, POLO, silicon, solar cell}, pubstate = {published}, tppubtype = {article} } We demonstrate an independently confirmed 25.0%-efficient interdigitated back contact silicon solar cell with passivating polycrystalline silicon (poly-Si) on oxide (POLO) contacts that enable a high open circuit voltage of 723 mV. We use n-type POLO contacts with a measured saturation current density of J_0n = 4 fA cm−2 and p-type POLO contacts with J_0p = 10 fA cm−2. The textured front side and the gaps between the POLO contacts on the rear are passivated by aluminum oxide (AlOx ) with J_0AlOx = 6 fA cm−2 as measured after deposition. We analyze the recombination characteristics of our solar cells at different process steps using spatially resolved injection-dependent carrier lifetimes measured by infrared lifetime mapping. The implied pseudo-efficiency of the unmasked cell, i.e., cell and perimeter region are illuminated during measurement, is 26.2% before contact opening, 26.0% after contact opening and 25.7% for the finished cell. This reduction is due to an increase in the saturation current density of the AlO x passivation during chemical etching of the contact openings and of the rear side metallization. The difference between the implied pseudo-efficiency and the actual efficiency of 25.0% as determined by designated-area light current–voltage (I–V) measurements is due to series resistance and diffusion of excess carriers into the non-illuminated perimeter region.
|
|
T. F. Wietler, D. Tetzlaff, J. Krügener, M. Rienäcker, F. Haase, Y. Larionova, R. Brendel, and R. Peibst Applied Physics Letters 110 (25), 253902, (2017). Abstract | Links | BibTeX | Schlagwörter: electrical resistivity, Etching, optical multistability, Scanning electron microscopy, silicon @article{Wietler2017,
title = {Pinhole density and contact resistivity of carrier selective junctions with polycrystalline silicon on oxide}, author = {T F Wietler and D Tetzlaff and J Krügener and M Rienäcker and F Haase and Y Larionova and R Brendel and R Peibst}, doi = {10.1063/1.4986924}, year = {2017}, date = {2017-06-01}, journal = {Applied Physics Letters}, volume = {110}, number = {25}, pages = {253902}, abstract = {In the pursuit of ever higher conversion efficiencies for silicon photovoltaic cells, polycrystalline silicon (poly-Si) layers on thin silicon oxide films were shown to form excellent carrier-selective junctions on crystalline silicon substrates. Investigating the pinhole formation that is induced in the thermal processing of the poly-Si on oxide (POLO) junctions is essential for optimizing their electronic performance. We observe the pinholes in the oxide layer by selective etching of the underlying crystalline silicon. The originally nm-sized pinholes are thus readily detected using simple optical and scanning electron microscopy. The resulting pinhole densities are in the range of 6.6 × 10^6 cm−2 to 1.6 × 10^8 cm−2 for POLO junctions with selectivities close to S10 = 16, i.e., saturation current density J0c below 10 fA/cm2 and contact resistivity ρc below 10 mΩcm2. The measured pinhole densities agree with values deduced by a pinhole-mediated current transport model. Thus, we conclude pinhole-mediated current transport to be the dominating transport mechanism in the POLO junctions investigated here.}, keywords = {electrical resistivity, Etching, optical multistability, Scanning electron microscopy, silicon}, pubstate = {published}, tppubtype = {article} } In the pursuit of ever higher conversion efficiencies for silicon photovoltaic cells, polycrystalline silicon (poly-Si) layers on thin silicon oxide films were shown to form excellent carrier-selective junctions on crystalline silicon substrates. Investigating the pinhole formation that is induced in the thermal processing of the poly-Si on oxide (POLO) junctions is essential for optimizing their electronic performance. We observe the pinholes in the oxide layer by selective etching of the underlying crystalline silicon. The originally nm-sized pinholes are thus readily detected using simple optical and scanning electron microscopy. The resulting pinhole densities are in the range of 6.6 × 10^6 cm−2 to 1.6 × 10^8 cm−2 for POLO junctions with selectivities close to S10 = 16, i.e., saturation current density J0c below 10 fA/cm2 and contact resistivity ρc below 10 mΩcm2. The measured pinhole densities agree with values deduced by a pinhole-mediated current transport model. Thus, we conclude pinhole-mediated current transport to be the dominating transport mechanism in the POLO junctions investigated here.
|
|
F. Haase, S. Schäfer, F. Kiefer, J. Krügener, R. Brendel, and R. Peibst Perimeter recombination in 25 %-efficient IBC solar cells with passivating POLO contacts for both polarities Inproceedings IEEE (Hrsg.): Proceedings of the 44th IEEE Photovoltaic Specialists Conference, Washington, DC, USA, (2017). BibTeX | Schlagwörter: silicon @inproceedings{Haase2017c,
title = {Perimeter recombination in 25 %-efficient IBC solar cells with passivating POLO contacts for both polarities}, author = {F Haase and S Schäfer and F Kiefer and J Krügener and R Brendel and R Peibst}, editor = {IEEE}, year = {2017}, date = {2017-06-01}, booktitle = {Proceedings of the 44th IEEE Photovoltaic Specialists Conference}, address = {Washington, DC, USA}, keywords = {silicon}, pubstate = {published}, tppubtype = {inproceedings} } |
|
V. Steckenreiter, D. C. Walter, and J. Schmidt AIP Advances 7 (3), 035305, (2017). Abstract | Links | BibTeX | Schlagwörter: carrier density, illumination, plasma chemical vapor deposition, semiconductor device fabrication, silicon @article{Steckenreiter2017b,
title = {Kinetics of the permanent deactivation of the boron-oxygen complex in crystalline silicon as a function of illumination intensity}, author = {V Steckenreiter and D C Walter and J Schmidt}, doi = {10.1063/1.4978266}, year = {2017}, date = {2017-03-01}, journal = {AIP Advances}, volume = {7}, number = {3}, pages = {035305}, abstract = {Based on contactless carrier lifetime measurements performed on p-type boron-doped Czochralski-grown silicon (Cz-Si) wafers, we examine the rate constant Rde of the permanent deactivation process of the boron-oxygen-related defect center as a function of the illumination intensity I at 170°C. While at low illumination intensities, a linear increase of Rde on I is measured, at high illumination intensities, Rde seems to saturate. We are able to explain the saturation by assuming that Rde increases proportionally with the excess carrier concentration Δn and take the fact into account that at sufficiently high illumination intensities, the carrier lifetime decreases with increasing Δn and hence the slope of Δn(I) decreases, leading to an apparent saturation. Importantly, on low-lifetime Cz-Si samples no saturation of the deactivation rate constant is observed for the same illumination intensities, proving that the deactivation is stimulated by the presence of excess electrons and not directly by the photons.}, keywords = {carrier density, illumination, plasma chemical vapor deposition, semiconductor device fabrication, silicon}, pubstate = {published}, tppubtype = {article} } Based on contactless carrier lifetime measurements performed on p-type boron-doped Czochralski-grown silicon (Cz-Si) wafers, we examine the rate constant Rde of the permanent deactivation process of the boron-oxygen-related defect center as a function of the illumination intensity I at 170°C. While at low illumination intensities, a linear increase of Rde on I is measured, at high illumination intensities, Rde seems to saturate. We are able to explain the saturation by assuming that Rde increases proportionally with the excess carrier concentration Δn and take the fact into account that at sufficiently high illumination intensities, the carrier lifetime decreases with increasing Δn and hence the slope of Δn(I) decreases, leading to an apparent saturation. Importantly, on low-lifetime Cz-Si samples no saturation of the deactivation rate constant is observed for the same illumination intensities, proving that the deactivation is stimulated by the presence of excess electrons and not directly by the photons.
|
|
C. Gemmel, J. Hensen, S. Kajari-Schröder, and R. Brendel IEEE Journal of Photovoltaics 7 (2), 430-436, (2017), ISSN: 2156-3381. Abstract | Links | BibTeX | Schlagwörter: charge carrier lifetime, Epitaxial growth, Epitaxy, Gettering, minority carrier lifetime, porous silicon (PSI), silicon, Substrates, Surface treatment, Temperature measurement @article{Gemmel2017,
title = {4.5 ms Effective Carrier Lifetime in Kerfless Epitaxial Silicon Wafers From the Porous Silicon Process}, author = {C Gemmel and J Hensen and S Kajari-Schröder and R Brendel}, doi = {10.1109/JPHOTOV.2016.2642640}, issn = {2156-3381}, year = {2017}, date = {2017-03-01}, journal = {IEEE Journal of Photovoltaics}, volume = {7}, number = {2}, pages = {430-436}, abstract = {Kerfless silicon wafers epitaxially grown on porous silicon (PSI) and subsequently detached from the growth substrate are a promising candidate for reducing the cost of the silicon wafer, which is particularly important for silicon photovoltaics. However, the carrier lifetime of these epitaxial wafers has to be at least as high as that of today's standard Czochralski (Cz)-grown wafers in order to become competitive. Here, we compare the measured lifetimes of n-type epitaxial silicon wafers that grow on PSI and epitaxial silicon wafers that grow on nonporous surfaces of epi-ready wafers. The latter are subsequently ground to have free-standing epitaxial wafers. Gettering improves the carrier lifetime of the ground wafers up to 4.2 ms. In contrast, PSI wafers show regions with effective lifetimes of 4.5 ms, even without gettering. This lifetime value is a factor of four larger than lifetimes of Cz wafers which are typically employed in today's PERC solar cells. We model the lifetime measurements with three Shockley-Read-Hall (SRH) defects: two defects that exist in the PSI and in the epi-ready wafer and a third defect that is only present in the epi-ready wafer.}, keywords = {charge carrier lifetime, Epitaxial growth, Epitaxy, Gettering, minority carrier lifetime, porous silicon (PSI), silicon, Substrates, Surface treatment, Temperature measurement}, pubstate = {published}, tppubtype = {article} } Kerfless silicon wafers epitaxially grown on porous silicon (PSI) and subsequently detached from the growth substrate are a promising candidate for reducing the cost of the silicon wafer, which is particularly important for silicon photovoltaics. However, the carrier lifetime of these epitaxial wafers has to be at least as high as that of today's standard Czochralski (Cz)-grown wafers in order to become competitive. Here, we compare the measured lifetimes of n-type epitaxial silicon wafers that grow on PSI and epitaxial silicon wafers that grow on nonporous surfaces of epi-ready wafers. The latter are subsequently ground to have free-standing epitaxial wafers. Gettering improves the carrier lifetime of the ground wafers up to 4.2 ms. In contrast, PSI wafers show regions with effective lifetimes of 4.5 ms, even without gettering. This lifetime value is a factor of four larger than lifetimes of Cz wafers which are typically employed in today's PERC solar cells. We model the lifetime measurements with three Shockley-Read-Hall (SRH) defects: two defects that exist in the PSI and in the epi-ready wafer and a third defect that is only present in the epi-ready wafer.
|
|
M. R. Vogt, H. Schulte-Huxel, M. Offer, S. Blankemeyer, R. Witteck, M. Köntges, K. Bothe, and R. Brendel IEEE Journal of Photovoltaics 7 (1), 44-50, (2017), ISSN: 2156-3381. Abstract | Links | BibTeX | Schlagwörter: Glass, Mathematical model, Nominal operating cell temperature (NOCT), operating temperature, passivated emitter rear cell (PERC), photovoltaic (PV) module, Photovoltaic cells, PV module thermal properties, ray tracing, silicon, Temperature measurement, Temperature sensors, Thermal conductivity @article{Vogt2017b,
title = {Reduced module operating temperature and increased yield of modules with PERC instead of Al-BSF solar cells}, author = {M R Vogt and H Schulte-Huxel and M Offer and S Blankemeyer and R Witteck and M Köntges and K Bothe and R Brendel}, doi = {10.1109/JPHOTOV.2016.2616191}, issn = {2156-3381}, year = {2017}, date = {2017-01-01}, journal = {IEEE Journal of Photovoltaics}, volume = {7}, number = {1}, pages = {44-50}, abstract = {We demonstrate a reduced operating temperature of modules made from passivated emitter rear cells (PERCs) compared with modules made from cells featuring an unpassivated fullarea screen-printed aluminum rear side metallization aluminum back surface field (Al-BSF). Measurements on specific test modules fabricated from p-type silicon PERC and Al-BSF solar cells reveal a 4 °C lower operating temperature for the PERC module under 1400 W/m2 halogen illumination, if no temperature control is applied. For detailed analysis of the temperature effect, we perform a 3-D ray tracing analysis in the spectral range from 300 to 2500 nm to determine the radiative heat sources in a photovoltaic (PV) module. We combine these heat sources with a 1-D finite element method model solving the coupled system of semiconductor, thermal conduction, convection, and radiation equations for module temperature and power output. The simulations reveal that the origin of the reduced temperature of the PERC modules is a higher efficiency, as well as a higher reflectivity, of the cells rear side mirror. This reduces the parasitic absorptions in the rear metallization and increases the reflection for wavelengths above 1000 nm. This operating temperature difference is simulated to be linear in intensity. The slope depends on the spectral distribution of the incoming light. Under 1000 W/m2 in AM1.5G, our simulations reveal that the operating temperature difference is about 1.7 °C. The operating temperature can be lowered another 3.2 °C, if all parasitic absorption for wavelengths longer than 1200 nm can be prevented. Standard testing conditions applying a temperature control to the module do not show this effect of enhanced performance of the PERC modules. Yield calculations for systems in the field will thus systematically underestimate their electrical power output unless the inherently lower operating temperature of PERC modules is taken into account.}, keywords = {Glass, Mathematical model, Nominal operating cell temperature (NOCT), operating temperature, passivated emitter rear cell (PERC), photovoltaic (PV) module, Photovoltaic cells, PV module thermal properties, ray tracing, silicon, Temperature measurement, Temperature sensors, Thermal conductivity}, pubstate = {published}, tppubtype = {article} } We demonstrate a reduced operating temperature of modules made from passivated emitter rear cells (PERCs) compared with modules made from cells featuring an unpassivated fullarea screen-printed aluminum rear side metallization aluminum back surface field (Al-BSF). Measurements on specific test modules fabricated from p-type silicon PERC and Al-BSF solar cells reveal a 4 °C lower operating temperature for the PERC module under 1400 W/m2 halogen illumination, if no temperature control is applied. For detailed analysis of the temperature effect, we perform a 3-D ray tracing analysis in the spectral range from 300 to 2500 nm to determine the radiative heat sources in a photovoltaic (PV) module. We combine these heat sources with a 1-D finite element method model solving the coupled system of semiconductor, thermal conduction, convection, and radiation equations for module temperature and power output. The simulations reveal that the origin of the reduced temperature of the PERC modules is a higher efficiency, as well as a higher reflectivity, of the cells rear side mirror. This reduces the parasitic absorptions in the rear metallization and increases the reflection for wavelengths above 1000 nm. This operating temperature difference is simulated to be linear in intensity. The slope depends on the spectral distribution of the incoming light. Under 1000 W/m2 in AM1.5G, our simulations reveal that the operating temperature difference is about 1.7 °C. The operating temperature can be lowered another 3.2 °C, if all parasitic absorption for wavelengths longer than 1200 nm can be prevented. Standard testing conditions applying a temperature control to the module do not show this effect of enhanced performance of the PERC modules. Yield calculations for systems in the field will thus systematically underestimate their electrical power output unless the inherently lower operating temperature of PERC modules is taken into account.
|
2016 |
|
R. Brendel, and R. Peibst Contact selectivity and efficiency in crystalline silicon photovoltaics Artikel IEEE Journal of Photovoltaics 6 (6), 1413-1420, (2016). Abstract | Links | BibTeX | Schlagwörter: Contact resistance, Junctions, passivated emitter and rear cell (PERC), Photovoltaic cells, Photovoltaic systems, principle of solar cells, Resistance, selective contacts, silicon @article{Brendel2016b,
title = {Contact selectivity and efficiency in crystalline silicon photovoltaics}, author = {R Brendel and R Peibst}, doi = {10.1109/JPHOTOV.2016.2598267}, year = {2016}, date = {2016-11-01}, journal = {IEEE Journal of Photovoltaics}, volume = {6}, number = {6}, pages = {1413-1420}, abstract = {Highly doped junctions of a Si solar cell function as membranes that block minority carriers and, at the same time, provide a high conductivity for transporting majority carriers to the contacts. They are thus said to be selective contacts. We propose a quantitative definition for the selectivity of contacts. The selectivity provides a figure of merit for electron and hole contacts that is helpful in designing solar cells and in identifying the efficiency limiting components. Applying the definition of selectivity to poly-Si junctions reveals that the root cause of their high selectivity is the highly asymmetric carrier concentration rather than a specific contact geometry.}, keywords = {Contact resistance, Junctions, passivated emitter and rear cell (PERC), Photovoltaic cells, Photovoltaic systems, principle of solar cells, Resistance, selective contacts, silicon}, pubstate = {published}, tppubtype = {article} } Highly doped junctions of a Si solar cell function as membranes that block minority carriers and, at the same time, provide a high conductivity for transporting majority carriers to the contacts. They are thus said to be selective contacts. We propose a quantitative definition for the selectivity of contacts. The selectivity provides a figure of merit for electron and hole contacts that is helpful in designing solar cells and in identifying the efficiency limiting components. Applying the definition of selectivity to poly-Si junctions reveals that the root cause of their high selectivity is the highly asymmetric carrier concentration rather than a specific contact geometry.
|
|
T. Dullweber, N. Wehmeier, A. Nowack, T. Brendemühl, S. Kajari-Schröder, and R. Brendel physica status solidi (a) 213 (11), 3046-3052, (2016). Abstract | Links | BibTeX | Schlagwörter: Aluminum, boron, boron silicate glass, emitters, silicon, Solar Cells @article{Dullweber2016c,
title = {Industrial bifacial n-type silicon solar cells applying a boron co-diffused rear emitter and an aluminum rear finger grid}, author = {T Dullweber and N Wehmeier and A Nowack and T Brendemühl and S Kajari-Schröder and R Brendel}, doi = {10.1002/pssa.201600346}, year = {2016}, date = {2016-11-01}, journal = {physica status solidi (a)}, volume = {213}, number = {11}, pages = {3046-3052}, abstract = {The solar industry is introducing p‐type monofacial passivated emitter and rear cells (PERC) into mass production. However, the efficiency of p‐type PERC cells is subject to light‐induced degradation (LID). In this paper, we introduce a novel solar cell design which we name BiCoRE as abbreviation of “bifacial co‐diffused rear emitter.” The BiCoRE cell process is very similar to the high volume proven PERC process sequence, but uses LID stable n‐type wafers. A boron silicate glass (BSG) silicon nitride (SiNz) stack at the rear side of the BiCoRE cells acts as protection layer against texturing and POCl3 diffusion, as boron dopant source during the POCl3 co‐diffusion as well as passivation layer. The rear contacts are formed by laser contact opening (LCO) and screen printing of an Al finger grid similar to the recently introduced PERC+ solar cells. The Al finger grid enables bifaciality and results in up to 8.5 μm deep aluminum back surface fields (Al‐BSFs) and up to 21.1% conversion efficiency obtained with n‐type reference solar cells. The multifunctional BSG/SiNz stack demonstrates up to 20.6% conversion efficiency with BiCoRE solar cells. When illuminated from the rear side, the BiCoRE cells exhibit conversion efficiencies up to 16.1% which corresponds to a bifaciality of 78%. }, keywords = {Aluminum, boron, boron silicate glass, emitters, silicon, Solar Cells}, pubstate = {published}, tppubtype = {article} } The solar industry is introducing p‐type monofacial passivated emitter and rear cells (PERC) into mass production. However, the efficiency of p‐type PERC cells is subject to light‐induced degradation (LID). In this paper, we introduce a novel solar cell design which we name BiCoRE as abbreviation of “bifacial co‐diffused rear emitter.” The BiCoRE cell process is very similar to the high volume proven PERC process sequence, but uses LID stable n‐type wafers. A boron silicate glass (BSG) silicon nitride (SiNz) stack at the rear side of the BiCoRE cells acts as protection layer against texturing and POCl3 diffusion, as boron dopant source during the POCl3 co‐diffusion as well as passivation layer. The rear contacts are formed by laser contact opening (LCO) and screen printing of an Al finger grid similar to the recently introduced PERC+ solar cells. The Al finger grid enables bifaciality and results in up to 8.5 μm deep aluminum back surface fields (Al‐BSFs) and up to 21.1% conversion efficiency obtained with n‐type reference solar cells. The multifunctional BSG/SiNz stack demonstrates up to 20.6% conversion efficiency with BiCoRE solar cells. When illuminated from the rear side, the BiCoRE cells exhibit conversion efficiencies up to 16.1% which corresponds to a bifaciality of 78%.
|