Ultraviolet (UV)-induced degradation is a critical issue for modern photovoltaic (PV) technologies such as passivated emitter and rear cell (PERC), tunnel oxide-passivated contact (TOPCon), and heterojunction (HJT) cell concepts. This study compares the stability against UV radiation of AlOx/SiNy stacks on mini-modules with p-type back junction solar cells. Our cells have a nondiffused textured front surface passivated with an AlOx/SiNy layer stack and feature passivating polysilicon on oxide rear contacts. We compare plasma-enhanced chemical vapor deposition (PECVD) and plasma-enhanced atomic layer deposition (PEALD) processes for the deposition of AlOx layers using the same tube-type deposition system. After a UV dose of 146 kWh/m2 using broadband UV lamps, modules with PECVD-AlOx exhibit an efficiency loss of up to 27% while those with PEALD-AlOx show minimal degradation of 2.5%. This comparison proves that the superior UV stability is achieved with the tube-type PEALD technique. Our findings thus show how UV stability can be improved without extra equipment dedicated solely to depositing ALD-AlOx and without UV absorbing or down converting encapsulants.

Polysilicon on oxide back junction solar cells offer a high efficiency potential with significantly reduced silver consumption by using an aluminum front grid. The cell interconnection typically requires additional silver pads for soldering. Ultrasonic tinning of aluminum generates silver-free tin solder pads which can further decrease the silver consumption. Here, we demonstrate ultrasonic tinning on front side Al pads of polysilicon on oxide back junction solar cells solar cells for a silver-free cell interconnection. We report low damage to the passivation layers with local losses in implied open circuit voltage of 5 mV to 15 mV. On cell level, this results in small open circuit voltage losses of 1 mV to 1.5 mV, or 0.2 %. Single cell mini module fabrication shows moderate cell-to-module losses of 1.3 % to 2 %, reaching up to 20.8 % module efficiency.

ISFH is following a distinct cell development roadmap, which comprises-as a short-term concept-the combination of an n-type doped electron-collecting poly-Si on oxide (POLO) junction with an Al-alloyed p(+) junction for hole collection. This combination can be integrated either in front- and back-contacted back junction cells (POLO-BJ) or in interdigitated back-contacted cells (POLO-IBC). Here, we present recent progress with these two cell concepts. We report on a certified M2-sized 22.9% efficient POLO-BJ cell with a temperature coefficient TC eta of only -(0.3 +/- 0.02) %(rel)/K and a certified 23.7% (4 cm(2) d.a.) efficient POLO-IBC cell. We discuss various specific conceptual aspects of this technology and present a simulation-based sensitivity analysis for quantities related to the quality of the hole-collecting alloyed Al-p(+) junction which are subject to continuous improvement and thus hard to predict exactly. We report that the measured pseudo fill factor values decrease more due to metallization than would be expected from recombination in the metallized regions with an ideality factor of one only. The gap to pseudo fill factor values that are theoretically achievable at the respective open-circuit voltages is 1.1%(abs) (Ga-doped wafer) for POLO-IBC and 1.4%(abs) (B-doped wafer) to 2%(abs) (Ga-doped wafer) for POLO-BJ. With an embedded blocking layer for Ag crystallites in the poly-Si, we present a concept to reduce this gap.

Polysilicon (poly-Si)-on-oxide passivating contact structures (POLO/TOPCon) enable high-efficiency solar cells as they simultaneously provide a very high level of surface passivation and a high conductance for either electrons or holes. The ease of incorporation with existing manufacturing lines and their tolerance for high-temperature processing has increased the wide acceptance of this structure in the PV industry. In this report, we explore the effects of short high-temperature annealing required for effective hydrogenation and formation of ohmic screen-printed contacts across a wide temperature range (636 degrees C-846 degrees C) on the stability of passivating contact structures. We study this on p-type c-Si substrates with phosphorus-doped (n-type) or boron-doped (p-type) polysilicon contacts capped with either an AlOx or SiNx coating. Our experimental results show that irrespective of the poly-Si doping type, AlOx-capped samples suffer a loss in surface passivation across the investigated temperature range, while SiNx-capped samples show an improvement at lower annealing temperatures. Above 744 degrees C, severely ruptured blisters occur for the samples coated with a SiNx layer, leading to lift-off of the poly layer in extreme cases, and in all cases, significant surface passivation losses, up to 99%. A study of the long-term stability of these fired samples under 1-sun illumination @ 140 degrees C shows that they suffer from both bulk and surface-like instabilities. Two degradation cycles were observed: the first, a boron-oxygen light-induced degradation (BO-LID) observed after 5 min, with capture cross-section ratios of 15.8-19.2, and a slower secondary degradation, similar to light and elevated temperature-induced degradation (LeTID), with maximum degradation reached after similar to 14 days. The presence of a silicon nitride layer does not appear to influence the kinetics of post-degradation recovery. Our results suggest that the effect of firing may be influenced by the polarity of the bulk c-Si or perhaps the chemistry of the SiNx film and highlight that passivating contact structures based on p-type c-Si may offer better long-term stability than those based on n-type c-Si.

We conduct numerical device simulations to study to what extend poly-Si on oxide (POLO)(2) IBC solar cells can be optimized. In particular, we evaluate the benefit of the concept of photonic crystals (PCs) for "standard" cell thicknesses compatible with industrial wafer handling. We find that for our current surface passivation quality, implementing PCs and decreasing the wafer thickness down to 15 mu m would increase the efficiency by "only" 1% absolute due to limiting surface recombination losses. We deduce a high c-Si/SiOx interface state density D-it of 2.9 x 10(12) eV(-1)cm(-2) by analyzing special two-terminal IV measurements on small pads that contact the intact interfacial oxide between pinholes with our MarcoPOLO model. Consequently, we improve the hydrogenation process of our POLO junctions by an Al2O3/SiNx/Al2O3 rear-side dielectric layer stack. For n-type POLO (p-type POLO) J(0) is reduced from 4 (10) fA/cm(2) down to 0.5 +/- 0.3 (3.3 +/- 0.7) fA/cm(2). For this improved surface passivation, our numerical device simulations predict an efficiency potential of 29.1% (27.8%) for POLO2 IBC cells with (without) PCs for a standard thickness of 150 mu m. This shows that the "practical limit" for Si solar cells with poly-Si on oxide-based passivating contact schemes is above 27%, and, in general, that the efficiency potential of Si single-junction cells is still far from being exhausted. The first implementation of the improved POLO junctions into cell precursors confirms the predicted improvement on the level of suns -implied open-circuit voltage curves.

The impact of the phosphorus doping density in direct current‐sputtered polysilicon layers on surface passivation and contact resistance by fabricating polysilicon on oxide (POLO) contacts is studied, when applying doping densities ranging from 3 × 10 19 to 4 × 10 20 cm −3 . Hydrogenation is performed either via a hydrogen‐releasing AlO x layer and postdeposition anneals in forming gas using a tube furnace at 400 °C, or by rapid firing of an AlO x /SiN y stack in a conveyor belt furnace at 810 °C. The study shows that the forming gas anneal of the weakly in situ phosphorus‐doped poly‐Si layers with AlO x enables a passivation quality with an implied open‐circuit voltage of up to 734 mV and a recombination current density down to 1.8 fA cm − 2 . For fast firing, a high phosphorus concentration of 4 × 10 20 cm −3 is required for comparably high passivation quality with a recombination current density down to 1.3 fA cm − 2 . A p‐type POLO back‐junction solar cell featuring such ex situ doped sputtered POLO contacts with a cell efficiency of 22.4% and an open‐circuit voltage of 714 mV is fabricated. To our knowledge, this is the highest open‐circuit voltage published so far with sputtered POLO contacts.

Polysilicon based passivating contacts for solar cells are expected to gain significant market share in the future. To ensure successful deployment, understanding the long-term stability of any new product is essential. Degradation and recovery of lifetime can occur in the bulk and surface-related components of a solar cell. In this work, we use planar p-type Czochralski silicon (20 Omega.cm) to fabricate symmetrical lifetime samples featuring p- or n-poly-Si layers capped by AlOx or SiNy to study the impact of the capping layer under different firing conditions, followed by annealing and illumination, on degradation mechanisms. Initially, AlOx samples perform better than SiNy samples. After firing at different temperatures below 792 degrees C, the passivation quality of SiNy samples improves significantly, beyond that of AlOx capped samples. During accelerated degradation (dark annealing and light soaking under high intensity illumination), AlOx samples degrade slightly while SiNy samples demonstrate a significant degradation (directly correlated to the firing temperature), which is followed by recovery. This behaviour is similar to light- and elevated temperature-induced degradation in passivated emitter and rear solar cells, suggesting a possible hydrogen-related mechanism. Lifetime analysis shows that the strongest changes occur in low injection levels, while the dark saturation current remains virtually the same.

Interdigitated back contact (IBC) silicon solar cells with a passivating n‐type poly‐Si on oxide emitter and an aluminum‐doped p + base contact on M2‐sized Ga‐doped p‐type Cz wafers are reported. The Al‐doped base contact forms during the firing of the printed contacts and allows for a lean process flow. The device optimization balances recombination at the base contacts against resistive losses and respects constraints set by the need of interconnecting cells in a module and contacting the cells temporally by a measurement chuck. A special sample holder is designed for measuring the I sc –V oc curve of the IBC cell with a busbar‐less metal grid. The pseudo‐efficiency is 24.7%. All fingers of each polarity are connected with wires and an efficiency of 22.3% is measured. The comparison of simulations and measurements reveals that the cell has 23.4% efficiency without the series resistance losses due to the wires. A huge part of the resistive losses in the cell are the transport losses of the majorities in the base dissipating a power that corresponds to 0.76% abs efficiency and the resistive losses at the Al‐doped base contact (0.29% abs ).

Stability of the passivation quality of poly‐Si on oxide junctions against the conventional mainstream high‐temperature screen‐print firing processes is highly desirable and also expected since the poly‐Si on oxide preparation occurs at higher temperatures and for longer durations than firing. We measure recombination current densities (J0) and interface state densities (Dit) of symmetrical samples with n‐type poly‐Si contacts before and after firing. Samples without a capping dielectric layer show a significant deterioration of the passivation quality during firing. The Dit values are (3 ± 0.2) × 1011 and (8 ± 2) × 1011 eV/cm2 when fired at 620°C and 900°C, respectively. The activation energy in an Arrhenius fit of Dit versus the firing temperature is 0.30 ± 0.03 eV. This indicates that thermally induced desorption of hydrogen from SiH bonds at the poly‐Si/SiOx interface is not the root cause of depassivation. Postfiring annealing at 425°C can improve the passivation again. Samples with SiNx capping layers show an increase in J0 up to about 100 fA/cm2 by firing, which can be attributed to blistering and is not reversed by annealing at 425°C. On the other hand, blistering does not occur in poly‐Si samples capped with AlOx layers or AlOx/SiNy stacks, and J0 values of 2–5 fA/cm2 can be achieved after firing. Those findings suggest that a combination of two effects might be the root cause of the increase in J0 and Dit: thermal stress at the SiOz interface during firing and blistering. Blistering is presumed to occur when the hydrogen concentration in the capping layers exceeds a certain level.

The electrical current through poly-Si on oxide (POLO) solar cells is mediated by tunneling and by nanometer-sized pinholes in the interfacial oxide. To distinguish the two processes, a POLO junction with a measured pinhole density of 1 x 10(7) cm(-2) is contacted by different contact areas ranging from 1 mu m(2) to 2.5 x 10(5) mu m(2), and the temperature-dependent current-voltage curves are measured for the different devices. Model regressions to the measured curves, their temperature dependence, and the quantized value of contact resistances indicate average numbers of pinholes per device corresponding to the expected pinhole density. For the small contacts, the different transport processes can be studied separately, which facilitates further improvements in respect to the present-day POLO junctions. Single-pinhole transport is found for one of the contacts with an area of 1 mu m(2). Random telegraph noise observed for this device in the current-voltage characteristics shows a high sensitivity to single charges.

We demonstrate the fabrication of a fully screen‐printed p‐type silicon solar cell with local hole‐collecting Al‐alloyed (Al‐p + ) contacts with a record open circuit voltage of 716 mV. The solar cell is fabricated by using almost the same process equipment as PERC cells. One of the dominant recombination losses in PERC cells is the recombination in the passivated and in the contacted emitter regions that so far limit the open circuit voltage to values below 700 mV. We eliminate these loss channels by substituting the P‐diffused emitter by a passivating n‐type poly‐Silicon on Oxide (nPOLO) contact. We place this contact on the rear side because of its otherwise strong parasitic absorption. The Al‐p + contacts are also located at the rear side to avoid front‐side shading. This results in a POLO‐IBC cell structure. The efficiency of the best cell so far is 23.0% with a designated area of 4 cm 2 fabricated on a M2‐sized wafer. Scanning electron microscopy reveals an Al‐p + thickness of less than 3.3 μm and only a few 100 nm at the contact ends, which is less than the 5 μm typically for optimized Al‐p + contacts. A comparison of measured and simulated current‐voltage curves over a variation of the contact fraction extracts a high saturation current density of the Al‐p + contact of J 0‐Al ‐p+ = 2,250 fA cm −2 for the current screen‐print conditions and Al‐paste causing an absolute efficiency loss of 0.5% abs . The recombination at the AlO x /SiN y surface and the shunt resistance limits the cell by 0.6% abs each.

We determined the density of defect states of poly-Si/SiOx/c-Si junctions featuring a wet chemical interfacial oxide from lifetime measurements using the MarcoPOLO model to calculate recombination and contact resistance in poly-Si/SiOx/c-Si-junctions. In samples that did not receive any hydrogen treatment, the Di(t),(cSi) is about 2 x 10(12) cm(-2) eV(-1) before firing and rises to 3-7 x 10(12) cm(-2) eV(-1) during firing at measured peak temperatures between 620 degrees C and 863 degrees C. To address the question of why Al0 x /SiN y stacks in contrast to pure SiN y layers for hydrogenation during firing provides better passivation quality, we have measured the hydrogen concentrations at the poly-Si/SiOx/c-Si interface as a function of AlOx layer thickness and compared these to J(0) and calculated si values. We observe an increase of the hydrogen concentration at the SiOx/c-Si interface upon firing as a function of the firing temperature that exceeds the defect concentrations at the interface several times. However, the AlOx layer thickness appears to cause an increase in hydrogen concentration at the SiOx/c-Si interface in these samples rather than exhibiting a hydrogen blocking property.

The setting up of a practical electrically driven light commercial demonstration vehicle with integrated photovoltaics (PV) is reported. The demonstrator vehicle is equipped with 15 modules based on the crystalline Si/amorphous Si heterojunction technology. The nominal total peak power under standard testing conditions is 2180 Wp. Specifically, the PV‐converted energy is fed into the high‐voltage (HV; 400 V) board‐net for a utilization of the large capacity of the HV battery and thus for direct range extension. The demonstrator vehicle is equipped with irradiation, wind, temperature, magnetic, and global positioning system sensors. Irradiation and temperature as well as the energy flows from modules, maximum power point trackers (MPPTs), low‐voltage buffer battery to HV battery via DC/DC, and from the HV battery to the loads during an exemplarily test drive day (May 31, 2021) are monitored. The range extension obtained at this day on our test route (51° 59′ N, 9° 31′ E) was 36 km, the corresponding CO2 savings account for ≈2.3 kg. The chain efficiency of the electronic components from the input side of the MPPTs to the HV output side of the DC/DC was 68.6%, whereas the DC/DC itself has an average efficiency of 90%.

By applying an interdigitated back contacted solar cell concept with poly-Si on oxide passivating contacts an efficiency of 26.1% was achieved recently. In this paper the impact of the implemented initially intrinsic poly-Si region between p-type poly-Si and n-type poly-Si regions is investigated. Two recombination paths are identified: The recombination at the interface between the initially intrinsic poly-Si and the wafer as well as the recombination across the resulting p(i)n diode on the rear side which is aimed to be reduced by introducing an initially intrinsic region. By using test structures, it is demonstrated that the width of the initially intrinsic region ((i) poly-Si region) has a strong influence on the recombination current through the p(i)n diode and that this initially intrinsic region needs to be about 30 μm wide to sufficiently reduce the recombination across the p(i)n diode. Lateral and depth-resolved time of flight secondary ion mass spectrometry analysis shows that the high-temperature annealing step causes a strong lateral inter-diffusion of donor and acceptor atoms into the initially intrinsic region. This diffusion has a positive impact on the passivation quality at the c-Si/SiOx/i poly-Si interface and is thus essential for achieving an independently confirmed efficiency of 26.1% with 30 μm-wide initially intrinsic poly-Si regions.

Passivating contacts formed by poly-Si on oxide (POLO) junctions yield high passivation qualities after an appropriate annealing process at temperatures between 800°C and 1050°C. In today's typical cell process, firing is applied in the end of cell production mainly to form screen printed contacts. Thus, a high stability of the passivation quality against this firing process is required - and also expected since a previous high-temperature process for POLO junction formation implies a much higher thermal budget. However, in this work we found a significant decrease in effective lifetimes of up to 75% for n-type POLO samples with ~ 1.5 nm interfacial oxide at firing temperatures of 620°C to 900°C. This holds without a supply of hydrogen (no capping layers). Experiments with hydrogen-rich dielectric capping layers show, however, that a coating with AlO x :H as opposed to SiN y :H (n = 2.05), can significantly increase the stability of the passivation upon firing. Capacitance-voltage measurements show that the saturation current density correlates to the density of defect states at the SiO x /c-Si interface when varying the firing temperature. Although firing with hydrogen supplying layers such as AlO x :H seems to be viable, our results may indicate that the chemical configuration of the SiO x /Si interface changes from Si-O to Si-H bonds upon firing. If this hypothesis holds true, possible implications on the long-term stability of the passivation quality should be evaluated.

Perimeter recombination is a relevant loss mechanism, in particular for cells with a large perimeter-to-area ratio and with poorly passivated edges, e.g., cut or cleaved solar cells for shingled modules. We experimentally demonstrate that cut edges can be well passivated during front-end processing. The resulting cells have an efficiency of 26%. The designated cell area of our lab-type highly efficient cells is smaller than the total area of the wafer. This causes recombination losses in the masked perimeter region. We separate the active cell area from the wafer on two sides of the cell by slits to reduce the transport of carriers into the perimeter region. We apply a diffusion model to describe impact of the slits on the perimeter recombination. The slits have an effective surface recombination velocity of down to 9 cm/s, depending on the resistivity of the base. For a base resistivity of 80 Qcm, the average cell efficiency increases by 0.7 %(abs) as compared to embedded cells and by 2.3 %(abs) as compared to laser-cut cells due to the passivated slits.

We present experimental results for interdigitated back contacted (IBC) solar cells with passivating POLO contacts for both polarities with a nominal intrinsic poly-Si region between them. We reach efficiencies of 26.1% and 24.9% on a 1.3 omega cm and 80 omega cm p-type FZ wafer and 24.6% on a 2 omega cm n-type Cz wafer, respectively. The initially measured implied efficiency potentials of the cells after passivating the surfaces are very similar, namely, 26.8%, 26.8%, and 26.4%, respectively. We attribute the difference between the efficiency potential and the final current-voltage measurement to degradation, perimeter, and series and shunt resistance losses, which we quantify by lifetime measurements. With these measurements in combination with a finite element simulation, we determine the surface recombination velocity in the nominal intrinsic poly-Si region to be in the range from 13 to 21 cm s(-1). Using the same approach, we analyze the increase of the front surface recombination velocity during cell processing from 2 to 10 cm s(-1) for the 1.3 omega cm and from 0.5 to 2.3 cm s(-1) for the 80 omega cm. This leads to the fact that cells fabricated on lowly doped bulk material are more vulnerable to a process-induced degradation of the surface passivation quality. We further determine the theoretical limits of the cells by firstly idealizing the recombination (28% for 1.3 omega cm and 28.2% for 80 omega cm) and secondly also idealizing the optics of the solar cells (29.4% and 29.5%).

We present a systematic study on the benefit of the implementation of poly‐Si on oxide (POLO) or related junctions into p‐type industrial Si solar cells as compared with the benchmark of Passivated Emitter and Rear Cell (PERC). We assess three aspects: (a) the simulated efficiency potential of representative structures with POLO junctions for none (=PERC+), one, and for two polarities; (b) possible lean process flows for their fabrication; and (c) experimental results on major building blocks. Synergistic efficiency gain analysis reveals that the exclusive suppression of the contact recombination for one polarity by POLO only yields moderate efficiency improvements between 0.23%abs and 0.41%abs as compared with PERC+ because of the remaining recombination paths. This problem is solved in a structure that includes POLO junctions for both polarities (POLO2), for whose realization we propose a lean process flow, and for which we experimentally demonstrate the most important building blocks. However, two experimental challenges—alignment tolerances and screen‐print metallization of p+ poly‐Si—are unsolved so far and reduced the efficiency of the “real” POLO2 cell as compared with an idealized scenario. As an intermediate step, we therefore work on a POLO IBC cell with POLO junctions for one polarity. It avoids the abovementioned challenges of the POLO2 structure, can be realized within a lean process flow, and has an efficiency benefit of 1.59%abs as compared with PERC—because not only contact recombination is suppressed but also the entire phosphorus emitter is replaced by an n+ POLO junction.

We demonstrate damage-free laser contact openings in silicon oxide layers on polycrystalline silicon on oxide (POLO) passivating contacts. A pulsed UV-laser evaporates the upper part of the polycrystalline silicon layer, lifting off the silicon oxide layer on top. On n-type POLO (and p-type POLO, respectively) samples a saturation current density of 2 fA cm(-2) (6 fA cm(-2)) and an implied open-circuit voltage of 733 mV (727 mV) are achieved with a laser contact opening area fraction of 12.3% (8.7%). The application of this ablation process in an interdigitated back contact solar cell leads to an independently confirmed power conversion efficiency of 26.1%. The excellent contact quality of the laser contact openings is proven by the low series resistance of 0.152 Omega cm(2) on the solar cell with a contact area of only 3%.