Emmerthal/Lausanne: Dr. Felix Haase und Nils Folchert sind für ihre Beiträge bei der internationalen SiliconPV-Konferenz 2018, die vom 19. bis 21. März in Lausanne (Schweiz) stattfand, mit dem SiliconPV-Award geehrt wurden. Diese Auszeichnung wird für die 10 besten Beiträge verliehen. Insgesamt wurden über 200 Beiträge aus über 20 Ländern eingereicht.
Dr. Felix Haase studierte Physik an der Friedrich Alexander Universität Erlangen und kam 2007 als Promotionsstudent ans ISFH. Seit Abschluss seiner Dissertation 2013 zur Entwicklung von hocheffizienten rückkontaktierten Solarzellen auf neuartigen, sägefrei hergestellten Siliziumsubstraten leitete er verschiedene Forschungsprojekte. Im Rahmen des aktuell von ihm geleiteten Verbundprojektes „26+“ konnten das ISFH und das MBE-Institut der Leibniz Universität Hannover mit einer Effizienz von 26,1 % einen neuen Weltrekord für Solarzellen auf Bor-dotiertem Silizium-Material erzielen (siehe Mitteilung vom 6.2.2018). Ein Schlüsselelement für diesen Erfolg war die Verwendung eines industrierelevanten Laserprozesses zur Strukturierung von Schichten auf passivierenden „poly-Si on oxide“ (POLO) Kontakten. Die systematische Entwicklung dieses Laserprozesses stellte den Schwerpunkt des preisgekrönten SiliconPV-Konferenzvortrages von Dr. Felix Haase dar.
Nils Folchert studierte Physik an der Georg August Universität Göttingen und ist seit 2016 Doktorand am ISFH. Im Rahmen seiner Promotion untersucht er die Grenzflächeneigenschaften und die Stromtransportmechanismen in passivierenden POLO-Kontakten. In seinem preisgekrönten Vortrag auf der SiliconPV-Konferenz konnte er zeigen, dass die zwei in Frage kommenden Stromtransportmechanismen in derartigen Kontakten – quantenmechanisches Tunneln und der Stromfluss über Nano-Löcher im Grenzflächenoxid – für den Widerstand des Kontaktes ein entgegengesetztes Temperaturverhalten implizieren. Im Falle von Tunneln wird der Kontaktwiderstand mit zunehmender Temperatur geringer, im Falle von Stromfluss über Nano-Löcher größer. Mit Hilfe dieser Methode kann somit auf den dominierenden Stromtransportmechanismus zurückgeschlossen werden. Das ist für die Praxis deshalb wichtig, weil beide Mechanismen jeweils unterschiedliche Optimierungsanforderungen an die passivierenden Kontakte stellen.
Weitere Details sind in Kürze in der renommierten Fachzeitschrift „Solar Energy Materials and Solar Cells“, in der die mit dem SiliconPV-Award ausgezeichneten Beiträge veröffentlicht werden, nachlesbar.
Die internationale SiliconPV-Konferenz wird von den renommierten europäischen Forschungsinstituten EPFL, Fraunhofer ISE, imec, CEA-INES, Universität Konstanz, ECN und ISFH organisiert und abwechselnd ausgerichtet. 2019 wird sie beim imec in Leuven, Belgien, stattfinden.
Alle Beiträge des ISFH auf der diesjährigen SiliconPV:
Felix Haase : Laser Contact Openings for Local Poly-Si-Metal Contacts
We demonstrate damage free laser contact openings in a silicon oxide layer on polycrystalline silicon on oxide (POLO) passivating contacts. In contrast to firing-through pastes, non-firing-through screen print pastes preserve the passivation quality of the passivating contacts. To increase the rear reflection of the cell, a dielectric layer can be deposited on the polycrystalline silicon layer, which then has to be removed locally for contact formation. The approach of this work is to remove the dielectric layer with a layer selective laser ablation process without compromising the passivation quality of the POLO-junction underneath. Test samples are processed with 160 nm- and 240 nm-thick thermally grown silicon oxide on 150 nm- and 120 nm-thick p-type and n-type POLO-layers respectively. We measure a saturation current density of 6 fA/cm2 and an implied open circuit voltage of 730 mV and 714 mV on n-type and p-type POLO samples respectively. Afterwards, a pulsed laser with a pulse length of 9 ps evaporates the upper part of the polycrystalline silicon layer, lifting the silicon oxide layer on top. Using a wavelength of 355 nm and removing an area fraction of 8.7 % does not change the saturation current density and the implied open circuit voltage in a laser fluence range from 0.08 J/cm2 to 0.10 J/cm2. Using a wavelength of 532 nm either leads to non-reliable contact opening or a degradation in the passivation quality for laser fluences sufficient for ablation This is probably due to the higher absorption depth of the 532 nm laser compared to the 355 nm. The application of this ablation process in an interdigitated solar cells leads to an independently confirmed efficiency of 24.3 %. In a second cell batch currently under work, we measure an implied pseudo-efficiency of 26.1 % after laser ablation / before metallization.
Henning Schulte-Huxel: Yield Analysis and Comparison of GaInP/Si and GaInP/GaAs Tandem Solar Cells
We present a novel model for yield analysis of tandem devices, e.g., Si cells with perovskites, III-V or other top cells designs. Inputs are the IV and QE of the individual subcells and the irradiance-dependent module temperature of the bottom cell. Our model calculates the temperature of the tandem module by taking into account the performance and spectral working range of the different tandem devices, enabling an irradiance- and weather-dependent yield analysis for these modules. We apply the model to compare two types of tandem model, a GaInP top cells on GaAs and Si bottom cells. When the subcells are series connected both technologies perform similarly well within 0.3%. The performance of the GaInP/Si can be significantly improved by 5.6% using 3T devices with a back-contacted bottom cell instead of a 2T configuration. For GaInP/Si, the 3T-device works similarly well as the 4T-device, enabling the integration of monolithic tandem cells into modules at comparable high efficiencies.
Lailah Helmich: In-Situ Characterization of Electron-Assisted Regeneration of Cz-Si Solar Cells
This contribution focuses on improving the fundamental understanding of the carrier lifetime regeneration in boron-doped p-type Czochralski-grown silicon (Cz-Si) due to the deactivation of the boron-oxygen complex. We examine passivated emitter and rear cells (PERCs) fabricated on boron-doped p-type Cz-Si in darkness under constant forward bias voltage (Ubias) in order to isolate the impact of excess electrons on the regeneration kinetics from the possible direct impact of photons. In the solar cell base Ubias defines the electron concentration. Based on these dark regeneration experiments, we address the existing inconsistency regarding the dependence of the regeneration rate constant Rde on the average excess carrier density Δnavg: If the regeneration is performed under illumination, Rde ~ Δnavg has been reported by several research groups [1−2]. A proportional dependence of Rde on Δn seems to be inconsistent with the strictly mono-exponential decay of the effective defect concentration during the regeneration process, as Δn changes strongly during the regeneration. One possible explanation would be that the lifetime is in fact constant at the increased temperature applied during regeneration , which would result in a constant Δn value at regeneration temperature. Using the method of dark regeneration by current injection into solar cells, we are able to measure the total recombination current of the cells at the actual regeneration temperature under Ubias, i.e., at the physically relevant conditions. We observe that the regeneration at practically constant Δn can be described by a double-exponential process, featuring a fast (Rde,fast) and a slow (Rde,slow) component, with only Rde,fast increasing with increasing Ubias at the regeneration temperature. Hence, we identify Rde,fast with the standard regeneration process performed under illumination at elevated temperature. The direct comparison of Rde,fast values measured in the dark under electron injection and under illumination clearly proves that the regeneration is purely electronically stimulated mechanism.
Tobias Wietler (Presented by Dr. Byungsul Min): Formation of a Resistive SiOx Layer at the Interface of Poly-Si to Aluminium-Doped Zinc Oxide
Passivating carrier-selective contacts are key elements for high-efficiency silicon solar cells. Besides a-Si:H heterojunctions, polycrystalline silicon-rich films on a thin interfacial oxide combine excellent surface passivation and low contact resistivity for both electrons and holes. In a simple, non-patterned cell poly-Si would provide the front side emitter and the rear contact. However, absorption losses limit the thickness of poly-Si films used as front contacts to less than 20 nm. In order to achieve sufficient lateral conductivity in the front side emitter contact despite this limitation, transparent conductive films are imperative.
Transparent conductive oxides (TCOs) like indium-doped tin oxide (ITO) or aluminium-doped zinc oxide (AZO) are prominent candidates. The temperature stability of polycrystalline silicon on oxide (POLO) junctions allows high-temperature annealing of the AZO/POLO combination resulting in excellent junction passivation as well as enhanced transparency and conductivity of the TCO. Besides conductivity and transparency, a low contact resistance between the TCO and the adjacent layers is required to translate the low contact resistance of the POLO junctions into high solar cell efficiency. To that end, both the metal/TCO interface as well as the interface between the TCO and the carrier-selective contact material underneath should form stable contacts without any barrier to carrier transport.
We investigate the contact resistance and the structural properties of the interface between sputtered AZO and poly-Si. We identify the formation of an amorphous interfacial layer as a possible cause for high contact resistance. The high spatial resolution of transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS) allows for an analysis of the composition of the interfacial region. It turns out that the interfacial layer is silicon oxide like. Based on the results of this investigation we develop a modified AZO processing sequence to overcome this challenge.
Catherin Gemmel: 8 ms Carrier Lifetime in Kerfless Epitaxial Wafers by n-Type POLO Gettering
After applying phosphorus diffusion gettering, the bulk lifetime of epitaxially grown silicon wafers increases significantly. However, the typically strong diffusion is not ideal for highest solar cell efficiencies. We find that n-type POLO junctions also lead to an increase of the bulk lifetime of epitaxial wafers that is as large as the increase achieved with phosphorus-diffusion gettering. Furthermore, the POLO junctions offer an improved recombination behaviour. This offers an alternative post-wafering treatment to increase the charge carrier lifetime of epitaxial wafers that can be effectively integrated into the processing of solar cells with POLO junctions. We measure an effective lifetime of 8 ms on a 3 Ω cm n-type epitaxial wafer at an injection level of Δp = 1015 cm-3.
Dennis Bredemeier: Lifetime Degradation in Multicrystalline Silicon Under Illumination at Elevated Temperature: The Role of Hydrogen
Within this contribution, we examine the lifetime degradation in multicrystalline silicon (mc-Si) under illumination at elevated temperature, an effect sometimes denoted “LeTID”. Our lifetime analysis of belt-furnace-fired high-performance mc-Si wafers shows that the lightinduced degradation is most pronounced on samples with Al2O3/SiNx-stack passivation. In contrast to that, the degradation on samples coated with Al2O3 single-layers is negligibly small. We identify the presence of SiNx to be a key component to trigger the defect activation process. Our measurements suggest that hydrogen released during the high-temperature firing from the hydrogen-rich PECVD-deposited SiNx into the silicon bulk might play a major role in the defect activation process. Additionally, we find that the magnitude of lifetime degradation increases exponentially with increasing temperature during fast-firing. Comparing this result with recently published results from the literature, we conclude that hydrogen-metal complexes are a likely root cause of LeTID.
Boris Veith-Wolf: Reassessment of Intrinsic Lifetime Limit in n-Type Crystalline Silicon and Implication on Maximum Solar Cell Efficiency
Unusually high carrier lifetimes of 23.7 ms are measured by photoconductance decay on 1.4-Ohm cm n-type Czochralski silicon wafers passivated using plasma-assisted atomic-layer-deposited Al2O3 on both wafer surfaces. The measured effective lifetimes significantly exceed the intrinsic lifetime limit previously reported in the literature (“Richter limit”). Several important 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 detach 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 an adapted parameterization of the intrinsic lifetime for n-type crystalline silicon and discuss the implications concerning the maximum reachable efficiencies in n-type silicon solar cells, which are higher than previously assumed.
Nils Folchert: Temperature-Dependent Contact Resistance Measurements on Carrier-Selective Poly-Si on Oxide Junctions
Carrier selective junctions using a poly-silicon/ silicon oxide stack on crystalline silicon show a remarkable passivation behaviour whilst maintaining low contact resistances 𝜌𝐶. The transport limiting process could be homogeneous tunnelling through the oxide and/or local transport through pinholes with no or negligible oxide thickness. We present an experimental approach to discriminate between these two transport mechanisms by temperature-dependent measurements of contact resistances. Theory predicts opposing temperature dependencies for tunnel and pinhole transport, respectively. This allows identifying the dominant transport process. For the contact resistance of a n+-type poly-Si/ silicon oxide/ c-Si junction with a 2 nm-thick interfacial oxide we find a clear pinhole-type of temperature dependence.
Byungsul Min: Increasing the Photo-Generated Current in Solar Cells with Passivating Contacts by Reducing the Poly-Si Deposition Temperature
The passivating contacts with poly-silicon on oxide (POLO) are a promising technique to suppress the recombination at the interface between metal and silicon. In order to apply the POLO contacts at the cell front side, however, it is important to make the poly-Si transparent enough to avoid significant parasitic absorption losses. We show in this paper that the transparency of poly-Si films can be significantly improved if they are deposited at temperatures below 550 °C. We therefore investigate the dependence of the transparency of undoped poly-Si films on deposition temperature. The optical parameters (n and k) of those films are determined with spectroscopic ellipsometry and applied in ray tracing simulations. Our study shows that the poly-Si deposition at 530 °C leads to an increase of the photo-generated current density up to 0.8 mA/cm² compared to poly-Si films deposited at 610 °C.
Marc-Uwe Halbich: Reduction of Parasitic Absorption in PEDOT:PSS at the Rear of c-Si Solar Cells
The hole-conducting polymer PEDOT:PSS is known to effectively passivate crystalline silicon (c-Si) surfaces and at the same time provide a low contact resistance for holes. PEDOT:PSS can hence be used as a hole-selective contact layer at the rear side of a silicon solar cell. Although cell efficiencies exceeding 20% have already been reported recentently, it was generally observed that the Jsc values of c-Si solar cells with PEDOT:PSS on the rear are fundamentally limited by parasitic absorption of infrared photons in the hole-conducting PEDOT:PSS layer due to free carrier absorption. In this study, we examine two different approaches to reduce the parasitic absorption in the PEDOT:PSS layer at the cell rear: (i) by adjusting the PEDOT:PSS layer thickness, and (ii) by improving the transparency of the PEDOT:PSS layer by adding non-conducting sorbitol to the precursor dispersion. Both approaches are shown to effectively reduce the parasitic absorption losses and thereby increase the Jsc of the solar cells by up to 1.4 mA/cm2, as demonstrated on cell level.
Valeriya Titova: Electron-Selective Atomic-Layer-Deposited TiOx Layers: Impact of Post-Treatments and Implementation Into n-Type Silicon Solar Cells
Atomic-layer-deposited titanium oxide (TiOx) is examined for the application as electron-selective full-area contact for n-type silicon solar cells. Although the surface passivation quality of TiOx-passivated n-type silicon wafers is quite poor directly after deposition of the TiOx, we demonstrate that annealing in ambient environment at only 250°C reduces the surface recombination velocity to values below 10 cm/s over the entire cell-relevant injection range. Application of a hydrogen remote-plasma leads to a further reduction in the surface recombination. By combining lifetime measurements with X-ray diffraction (XRD) characterization we demonstrate that the degradation of the passivation by TiOx during annealing at increased temperature (>350°C) is due to the crystallization of the amorphous TiOx into the anatase phase. Finally, we implement our optimized ALD-TiOx as electron-selective full-area rear contacts into n-type silicon solar cells and reach efficiencies up to 18.4% after low-temperature annealing in our first batch. The extracted Srear from IQE measurements of 110 cm/s is well compatible with efficiencies >20%.