Search

Sufficiently deep local back-surface-fields (LBSF) are crucial for achieving low contact recombination and thus high solar cell efficiencies in passivated emitter and rear contact (PERC) solar cells. In this article, we investigate spatial variations of LBSF thickness 1) across dashed contacts, 2) between lateral positions within a cell, and 3) from cell to cell. The objective of this experimental study is to proof the impact of LBSF thickness on open-circuit voltage in PERC solar cells. Our measurements and analysis of variance (ANOVA) reveal that variations in LBSF thickness across dashed contacts exceed observed variations in LBSF thickness between lateral positions within a cell as well as cell-to-cell variations. Thus, we statistically treat all LBSF measurements of a single experimental group of PERC solar cells as one population of measurements. The two investigated experimental sets of PERC cells fired using different fast-firing profiles show a difference in LBSF thickness of about 1 μm. The resulting difference in open-circuit voltages arises to 1.3 mV. ANOVA-based analysis shows for our experimental samples that at least 19 random measurements are necessary in order to resolve LBSF thickness difference of 0.5 μm with sufficient statistical significance.

This article investigates the impact of the back-surface-field (BSF) thickness variation within a local aluminum contact on the performance of passivated emitter and rear contact solar cells. A significant difference of BSF thickness between contact endings and the center of dash-shaped contacts is verified experimentally by a comprehensive statistical analysis using scanning electron microscopy. The impact of local BSF thickness differences on the cell performance is studied with 3-D technology computer-aided design (TCAD) device simulations. Several device parameters such as BSF thicknesses, the doping concentration in the BSF profile at rear contacts, or the metallized area fraction at the cell rear side are varied. Our simulation study shows that the open-circuit voltage is mainly affected by locally reduced BSF thicknesses, resulting in an efficiency loss up to 0.14% abs or 0.84% abs , respectively, if an area fraction of 1% or 20% within a local contact has reduced BSF thicknesses. This effect can be minimized either by reducing the metallized area fraction at the cell rear side or by increasing the doping concentration in the BSF profile at aluminum rear contacts. In addition, we demonstrate that the 3-D simulations can be approximated with 2-D simulations by applying a single doping profile with an average BSF thickness, calculated with the harmonic mean.

We present a novel cell concept that combines the tandem cell approach with the passivated emitter and rear cells (PERC) mainstream technology. As an interface between Si bottom and top cell, we utilize passivating n+-type polysilicon on oxide (POLO) contacts and a p+ poly-Si/n+ poly-Si tunneling junction. Our full area PERC+ Si bottom cells are fabricated within a typical industrial process sequence where the POCl3 diffusion and SiNx deposition are replaced by the POLO junction formation processes. The implied open-circuit voltage i V oc that is measured on these devices reaches up to 708 mV (684 mV) under 1 sun (under filtered spectrum to simulated top cell absorption). On sister cells with planar front side, the respective i V oc values are 718 mV (696 mV). In order to understand the device physics of our ultra-abrupt p+ poly-Si/n+ poly-Si tunneling junction, we determined the carrier lifetime in the poly-Si by time-resolved photoluminescence. The extracted lifetimes of 42–54 ps enter as input parameter for numerical Sentaurus Device simulations. These simulations reveal the importance of band-to-band and trap-assisted tunneling for a low tunneling junction resistivity of 2.95 mΩ·cm2. Experimentally, an upper limit for the combined junction resistance of the p+ poly-Si/n+ poly-Si/SiOx stack of 100 mΩ·cm2 is determined.

The determination of the spectral responsivity is an essential part of solar cell characterization. Since solar simulators only approximate the reference spectrum, a spectral mismatch correction has to be performed. This correction procedure requires spectral responsivity data. Apart from the complete differential spectral responsivity procedure, the IEC 60904-8 standard defines four simplifications. In this paper, we provide information on the variations in the spectral responsivity curves for these simplifications. We show that for nonlinear front junction cells, deviations predominantly occur at wavelengths above 700 nm and become largest around 1000 nm. While we found a maximum deviation of 30% for the simplification with lowest requirements in bias irradiance, all other simplifications yield deviations below 10%. For a nonlinear cell measured relative to a world photovoltaic scale reference cell using a class A solar simulator, this transfers to a deviation below 0.01% in the spectral mismatch factor. If one depends on the use of a simplification, we recommend using the multicolor approach. Even though the singlecolor approach might yield lower deviations, this approach requires knowledge about the maximum in the spectral responsivity, which is not generally known in advance of the measurement. Accepting a slightly higher deviation, the white bias approach is a recommendable alternative.

We report on an industrial large area, screen-printed, double-side contacted cell with polysilicon on oxide (POLO) junctions on both sides and an energy conversion efficiency of 22.3% ( A = 244.15 cm 2, V oc = 714 mV, FF = 81.1%, J sc = 38.5 mA/cm2, measured in-house). This cell shows an extraordinarily low series resistance below 0.05 Ω cm2. This confirms the low specific junction resistance observed recently for POLO junctions. The present cell suffers from 1) low short-circuit current due to parasitic absorption in the rather thick poly-Si (30 nm), as well as in the indium tin oxide, 2) deterioration of the recombination behavior upon sputter deposition of a transparent conductive oxide (TCO), and 3) shunts near the edge due to nonadapted TCO edge exclusion. We address all of these limitations experimentally. In particular, we developed a plasma-enhanced chemical vapor deposition process for ZnO:Al, which does not compromise the passivation of the POLO junctions underneath. An estimation of the efficiency potential (based on the two-diode model and the assumption that all these building blocks can be successfully combined on a cell level) shows that 25.3% can be achieved with this cell concept. We also look into potential cost advantages of the POLO junction scheme for this cell structure, such as the usage of p-type Cz-Si material and the omission of Ag fingers.

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.

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 investigate the transport mechanism of poly-Si-based carrier-selective junctions using the two-dimensional numerical semiconductor device simulations. The detailed transport model considers the charge carrier transport through the pinholes as well as tunneling through a very thin silicon oxide simultaneously. For the verification of the simulation model, the complete temperature dependent transfer length method is modeled and its results are verified with measurements of two different samples. By means of rigorous simulations, the influence of different pinhole geometrical and material parameters on junction resistivity are investigated and explained in detail. From the presented results, the fundamental understanding needed for optimizing the poly-Si-based carrier selective junction in respect to the main design parameters such as doping level in poly-Si, annealing time, silicon oxide thickness, and pinhole density is given. The detailed analysis shows the pinhole channel plays the most crucial role in the design of poly-Si-based carrier-selective junctions if the silicon oxide layer thickness is larger than 2 nm.

We measure and discuss the complex refractive index of conventional ethylene vinyl acetate (EVA) and an EVA with enhanced UV-transmission based on spectroscopic ellipsometry, transmission and reflection measurements over the wavelength range from 300-1200 nm. Ray tracing of entire solar cell modules using this optical data predicts a 1.3% increase in short circuit current density (Jsc) at standard test conditions for EVA with enhanced UV transmission. This is in good agreement with laboratory experiments of test modules that result in a 1.4% increase in Jsc by using a UV transparent instead of a conventional EVA. Further, ray tracing simulations with realistic irradiation conditions with respect to angular and spectral distribution reveal an even larger Jsc increase of 1.9% in the yearly average. This increase is largest in the summer months with an increase of up to 2.3%. German Federal Ministry for Economic Affairs and Energy/0325641

The aim of this work is to demonstrate the maturity of the TOPCon technology by conducting a round-robin on symmetrically processed lifetime samples in the leading European PV institutes EPFL, ISC, CEA-INES, ISFH, IMEC and Fraunhofer ISE within the H2020 funded project called HighLite. For all layers, dark saturation current-densities ranging between 2 and 10 fA/cm2 can be reported. Simultaneously, no metal induced recombination for the two lower sintering temperatures have been observed pointing towards a true passivated contact. Furthermore, contact resistivities below 10 mΩcm2 have been achieved. It seems that the industrial passivating contact matured to a fully passivated and conducting contact enabling full efficiency potential. The fact that this can be realized using either PECVD or LPCVD from various manufacturer is expected to drive costs down and contribute to the increased adoption of the TOPCon technology.

This paper investigates the reduction of the parasitic absorption in solar cells with poly-silicon based passivating contacts. We analyze the transparency of poly-Si films fabricated at various deposition temperatures. We observe different surface morphologies by scanning electron microscopy and model their spectroscopic ellipsometry reflection data. Based on the measured optical parameters (n and k) we perform ray tracing simulations for quantifying the expected increase of short-circuit current density of a cell. Our study shows that the poly-Si deposition at 530 °C leads to an increase of the photo-generated current density by up to 0.76 mA/cm² when compared to poly-Si films deposited at 610 °C.This paper investigates the reduction of the parasitic absorption in solar cells with poly-silicon based passivating contacts. We analyze the transparency of poly-Si films fabricated at various deposition temperatures. We observe different surface morphologies by scanning electron microscopy and model their spectroscopic ellipsometry reflection data. Based on the measured optical parameters (n and k) we perform ray tracing simulations for quantifying the expected increase of short-circuit current density of a cell. Our study shows that the poly-Si deposition at 530 °C leads to an increase of the photo-generated current density by up to 0.76 mA/cm² when compared to poly-Si films deposited at 610 °C.

The measured saturation current density J0e of heavily phosphorus-doped emitters of crystalline Si solar cells is analysed by means of sophisticated numerical device modelling. It is concluded that Shockley–Read–Hall (SRH) recombination exceeds Auger recombination significantly; it is caused by inactive phosphorus. This explains the large discrepancies between measured and simulated J0e values, observed persist-ently over the last two decades in industrially fabricated Si solar cells. As a consequence, the heavily phosphorus-diffused emitters still bear a significant potential to contribute to higher Si solar cell efficiency levels, if the amount of inactive phosphorus can be reduced. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

It is commonly assumed in the PV community that Auger recombination is the dominant loss mechanism in heavily phosphorusdoped emitters of industrial Si solar cells. Contrary to this assumption, we show in this work that most of the losses are caused by Shockley-Read-Hall (SRH) recombination via defect states introduced by inactive phosphorus. Using numerical device simulations, we successfully reproduce all the measured J0e values of a series of profiles (with various concentrations of inactive phosphorus) with an effective SRH capture crosses-section for holes, σp = 7.5×10-18 cm2. The composition of inactive phosphorus (e.g. in form of clusters of interstitial phosphorus, fine and rod-shaped precipitates) may vary under different fabrication conditions, hence we do not claim that this σp value is globally valid. However, because the series of profiles is representative for many emitters in mass production, our result implies generally higher SRH than Auger recombination rates and has decisive implications on the improvement of such industrial emitters: many emitters have not yet reached the Auger-limit and they may still be significantly improved by reducing their density of inactive phosphorus. Ministry of Economic Affairs and Energy of the State of North Rhine-Westphalia

This paper investigates process-induced variations of the open-circuit voltage (Voc) using voltage-dependent quantum efficiency measurements. By means of device modelling we show that this method is able to explain the Voc difference of two solar cells, even if they show identical electrical behaviour under short-circuit condition. This paper furthermore explains how the origin of Voc variations can be classified into emitter, base and rear of the solar cell. The simulation results have been experimentally verified with industrial-type passivated emitter and rear cells (PERC) cells made from p-type Czochralski wafers. The proposed analysis method is an attractive way for monitoring Voc variations of solar cells in industrial mass production since there is no need for specially prepared test structures. © 2017 The Authors. Published by Elsevier Ltd.

This work shows the potential of further optimization of phosphorus-doped emitters in p-type silicon solar cells. We investigate the impact of different combinations of phosphorus doping profiles and surface passivation qualities on the saturation current density J 0e by considering boundary conditions based on published experimental data. Our simulation study shows that there are two possible ways to achieve J 0e values below 10 fA/cm 2 . One is the reduction of the electrically active phosphorus concentration n surf at the surface beneath 2×10 19 cm −3 and simultaneously reducing the surface recombination velocity S p to below 10 3 cm/s. The other contrarily increases n surf to values of up to 1×10 21 cm −3 while ensuring full activation of all phosphorus dopants. In the latter case, J 0e values below 10 fA/cm 2 seem possible, even for S p = 10 7 cm/s which is equal to the thermal velocity.

Within the silicon photovoltaics (PV) community, there are many approaches, tools, and input parameters for simulating solar cells, making it difficult for newcomers to establish a complete and representative starting point and imposing high requirements on experts to tediously state all assumptions and inputs for replication. In this review, we address these problems by providing complete and representative input parameter sets to simulate six major types of crystalline silicon solar cells. Where possible, the inputs are justified and up-to-date for the respective cell types, and they produce representative measurable cell characteristics. Details of the modeling approaches that can replicate the simulations are presented as well. The input parameters listed here provide a sensible and consistent reference point for researchers on which to base their refinements and extensions.

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.

Sorting of solar cells is a vital step to achieve the predetermined power out of the photovoltaic module, nevertheless there is a lack of detailed investigations of all relevant parameters defining the global module efficiency. Sorting methods tend to rely on simple electrical parameters such as PMAX , IMPP , and ISC . However, it is crucial to take into consideration that the performance of individual cells will be changed after the encapsulation. Therefore, in this study potential changes in operating parameters such as JSC and Rser are integrated in the sorting step. It is a fact that at low light intensities the impact of Rshunt on module performance is more critical compared to standard test conditions (STC) where the actual measurements are carried out. In addition, the optical parameters of cells are subject to change after the encapsulation. That is why, inclusion of optical parameters such as reflectance into the sorting step is analysed. In this study, innovative sorting methods with the inclusion of Rshunt and reflectance were introduced. The results show that, at low light intensities conventional sorting approach can be extended with a combination of Rshunt and other electrical parameters to achieve higher module efficiencies up to 0.1% absolute. In addition, adding optical reflectance into the sorting parameters set current mismatches in modules can be minimized.

We investigate the enhancement in transparency and conductivity of aluminum doped zinc oxide (ZnO:Al) layers upon high-temperature annealing and its impact on contact resistance as well as on passivation properties of carrier selective junctions based on doped polycrystalline Si on a passivating silicon oxide (POLO). The temperature stability of these junctions allows annealing of the ZnO:Al/POLO combination up to 600°C. We prepare ZnO:Al films by DC magnetron sputtering at room temperature. We determine complex refractive index of ZnO:Al in dependence of post-deposition annealing (PDA) temperature by spectroscopic ellipsometry. High-temperature annealing improves the conductivity and reduces the absorption within ZnO:Al. The optical losses in a ZnO:Al/POLO stack are rather limited by the poly-Si layer than by the ZnO:Al. The sheet resistance improves from roughly 20000W/sq for 80nm thick as-deposited ZnO:Al films to 72W/sq after fast firing at 600°C. At the same time PDA cures the damage induced in the POLO junctions during ZnO:Al deposition. After PDA with AlxOy capping layers, the passivation quality even surpasses the initial level. A transmsission electron microscopy analysis of the interface between the ZnO:Al and the underlying poly-Si reveals the formation of a silicon oxide like interfacial layer after PDA at 400°C. This interfacial layer causes a high contact resistivity of the metal/ZnO:Al/POLO-junction and could limit the thermal budget for cell processing. Our results indicate that after successful process adjustment, ZnO:Al could substitute In-based transparent conductive oxides on POLO cells for cost reasons and also enable a high efficiency potential.

We present a novel cell concept that combines the tandem cell approach with the PERC mainstream technology. As an interface between Si bottom and top cell, we utilize passivating $\text{n}^{\mathbf {+}}$-type polysilicon on oxide (POLO) contacts and a p+ poly-Si / n+ poly-Si tunneling junction. Our full area PERC+ Si bottom cells are fabricated within a typical industrial process $^{\mathbf {sequence\, where\, the\, POCl}}$3$^{\mathbf {-diffusion\, and\, SiN}} \mathbf {x}$deposition are replaced by the POLO junction formation processes. The implied $^{\mathbf {open\, circuit\, voltage\, iV}} \mathbf {oc}$ measured on these devices reaches up to 708 mV (684 mV) under 1 sun (under filtered spectrum), respectively.