Passivated Emitter Rear Contact Research Articles

Thermal Treatment Effects on Flat-band Voltage Shift in Atomic-layer-deposited Alumina or Aluminum Oxide/Silicon Nitride Passivation Stacks

We studied the effects of thermal annealing on the interfacial properties of atomic-layer-deposited alumina (Al2O3) films and aluminum oxide/silicon nitride (AlOx/SiNx) stacks on silicon. Thermal treatment was found to have a significant effect on the lifetime improvement, owing to full activation of field-effect passivation, which can be realized with an increased flat-band voltage (Vfb). The presence of a capping SiNx layer itself and/or during deposition is suspected to cause distribution of unstable negatively charged traps in the AlOx/SiNx stacks, resulting in a slight reduction in lifetime with inversely improved Vfb. The interface trap density, an indicator of chemical passivation, remains at the same order of magnitude after thermal treatment and SiNx deposition, suggesting that atomic-layer deposition induces a high passivation quality. These results suggest that consideration of thermal-annealing effects and SiNx deposition conditions are crucial for improving the quality of passivation stacks at the rear of passivated emitter rear contact (PERC) cells and, consequently, the high performance of PERC cells.

Analysis of Local Aluminum Rear Contacts of Bifacial PERC+ Solar Cells

A recently published industrial passivated emitter rear contact (PERC) solar cell concept called PERC+ enables bifacial applications by printing an aluminum (Al) finger grid instead of the full-area Al layer aligned to the laser contact openings on the rear side. We demonstrate that the rear contacts of these PERC+ solar cells exhibit back-surface field (BSF) depths of around 6 μm over a large range of contact linewidths, whereas PERC cells with full-area Al rear layer show a reduction of the Al-BSF depths for narrower contact lines. Using an existing analytical model for the local contact formation, we show that the measured Al-BSF depths are well described solely by the different volume of Al paste printed on the rear side. Consequently, the open-circuit voltage of PERC+ solar cells improves by up to 5 mV when reducing the contact linewidth only. In contrast, for PERC cells with full-area Al layer, the V oc slightly decreases with narrower contact linewidths due to the thinner Al-BSF depths. We observe a strongly reduced number of voids in the Al–Si eutectic layer for PERC+ cells, compared with PERC. As physical root cause for void formation, we propose the minimization of surface energy of the Al–Si melt.

Impact of the injection‐level‐dependent lifetime on Voc, FF, ideality m, J02, and the dim light response in a commercial PERC cell

AbstractThe injection‐level‐dependent (ILD) lifetime of the silicon wafer impacts many characteristics of the final photovoltaic cell. While efficiency is commonly understood to be impacted by the silicon bulk lifetime (at the maximum power point injection level), this work demonstrates the wide ranging impacts of ILD lifetime on the Voc, the fill factor (FF), the diode ideality factor m, and the dim light response. Instead of a two‐diode model, we utilize a boundary + ILD bulk lifetime model to analyze a commercial passivated emitter rear contact (PERC) cell featuring an AlOx dielectric rear passivation. The ILD lifetime is directly measured and used to calculate the bulk recombination current across injection levels. With this boundary + ILD lifetime model, we demonstrate the role of the ILD lifetime on many cell parameters in this PERC cell. For most high efficiency commercial p‐type monocrystalline solar cells, the typically lower bulk lifetime at the maximum power point versus the lifetime at the open circuit point reduces the measured FF and pseudo‐FF. This work illustrates that for a commercial PERC cell with AlOx rear passivation, the ILD lifetime is the primary mechanism behind reduced FF, ideality factors greater than 1, and the source of the J02 term in the two‐diode model. The crucial implications of this work are not only to better understand commercial PERC cell loss mechanisms but also to encourage a focus on different metrics in cell diagnostics. One such metric is the Voc at 0.1 or 0.05 suns. Copyright © 2016 John Wiley & Sons, Ltd.

Monitoring of Si-solar cell degradation with electroluminescence

Monitoring of the intensity of band-to-band electroluminescence (EL) from B-doped Cz-Si solar cells allowed reliable detection of their degradation under forward bias. Degradation related changes in the open circuit voltage for a cell can be well estimated from the related changes in the EL-intensity value under fixed forward current flow. Kinetics of the degradation could be accessed from the time-dependence of EL-intensity changes. The method of monitoring of the degradation process is fully applicable to the cells with complicated structure, e.g. cells with passivated emitter rear contact (PERC). Comparison of degradation process in standard and PERC solar cells indicated on faster and more severe degradation in PERC cells.

Efficiency Potential of P-Type Al2O3/SiN$_{x}$ Passivated PERC Solar Cells With Locally Laser-Doped Rear Contacts

Technological restrictions on the screen-printed rear-contact feature size on the order of 100 μm are among the limiting factors of the efficiency of p-type passivated emitter rear-contact (PERC) solar cells. Simultaneous contact opening and doping using localized laser processing can overcome these design limitations. We use 3-D numerical device simulations to show that an efficiency gain of 0.3%abs compared with a screen-printed baseline cell, is possible if laser-formed point contacts of 5 μm in size with a contact recombination parameter of 5000 fA·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> and a contact resistance of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> Ω·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> are used. We experimentally demonstrate the implementation of simultaneous rear-surface contact opening and doping on large-area 156 × 156 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> -sized PERC solar cells using ultraviolet (UV) and green laser systems. We achieve efficiencies of up to 19.9% for this process with a 10-nm atomic layer deposited Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /80-nm plasma-enhanced chemical vapor deposited SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> rear-surface dielectric stack.

Sol-gel Synthesized Dielectric Films as a Diffusion Barrier to Rear Side Contact Metal

It is essential to mitigate the costs of cell production in order to achieve an increasing share of the industrial manufacture of advanced and more efficient solar cells. This work aims to contribute to the efficiency enhancement and cost reduction of rear contact cells, such as passivated emitter rear contact (PERC) cells. In order to realize these goals, various dielectric materials are proposed that mainly function as a diffusion barrier to the rear contact metal, i.e. aluminum as well as impurities. Dielectric materials that are formulated as chemical solutions are coated on single-side-polished, monocrystalline silicon. Their barrier function is tested with three different screen-printable aluminum pastes. Among various materials, five inks showed an absolute barrier behavior against different aluminum pastes. Furthermore, the compatibility of these materials with the existing passivation layer, ALD-AlOx, (AlOx film deposited by atomic layer deposition) is imaged by photoluminescence spectroscopy. The minimum film thickness that is necessary for preventing aluminum penetration is determined by scanning electron microscopy (SEM) and ellipsometry. This is found to be 450nm for the cured dielectric material that showed the best overall results.

Series Resistance Analysis of Passivated Emitter Rear Contact Cells Patterned Using Inkjet Printing

For higher-efficiency solar cell structures, such as the Passivated Emitter Rear Contact (PERC) cells, to be fabricated in a manufacturing environment, potentially low-cost techniques such as inkjet printing and metal plating are desirable. A common problem that is experienced when fabricating PERC cells is low fill factors due to high series resistance. This paper identifies and attempts to quantify sources of series resistance in inkjet-patterned PERC cells that employ electroless or light-induced nickel-plating techniques followed by copper light-induced plating. Photoluminescence imaging is used to determine locations of series resistance losses in these inkjet-patterned and plated PERC cells.

Formation of High Efficiency Epitaxial Emitters by APCVD

Improvements in emitter passivation as well as back contact passivation lead to several high efficiency solar cell concepts. PERC (Passivated Emitter Rear Contact) solar cells on monocrystalline wafers above 21% [1] have been achieved, but the diffusion processes with POCL3 and BBr3 have emerged as one of the limiting factors. The formation of high efficiency emitters by APCVD (Atmospheric Pressure Chemical Vapour Deposition) has several advantages compared to the standard diffusion. Epitaxial emitters can be optimised to match the passivation and metallisation of high efficiency concepts as well as industrial approaches like nickel plating. Simulations by PC1D show the potential of epitaxial emitters by featuring low contact resistance in combination with high shunt resistance, a good blue response and low emitter saturation current (J0e) [2,3]. The emitter profiles can be designed in depth and in doping in the range from 1x1017cm-3 up to 1x1020cm-3 for p-type and n-type emitters. Simple reference solar cells with J0e= 46 fA/cm2 and efficiencies of η=17.5% on float zone (FZ) and η=16.1% on multicrystalline (mc) wafers have been processed. Furthermore an epitaxial selective emitter has been developed. The deposition process itself has been enhanced to improve the emitter profiles as well as material quality.

Ni/Cu 전극을 적용한 고효율 실리콘 태양전지의 제작 및 특성 평가

We have applied front contact metallization of plated nickel and copper for high efficiency passivated emitter rear contact(PERC) solar cell. Ni is shown to be a suitable barrier to Cu diffusion as well as desirable contact metal to silicon. The plating technique is a preferred method for commercial solar cell fabrication because it is a room temperature process with high growth rates and good morphology. In this system, the electroless plated Ni is utilized as the contact to silicon and the plated Cu serves as the primary conductor layer instead of traditional solution that are based on Ti/Pd/Ag contact system. Experimental results are shown for over 20 % PERC cells with the Plated Ni/Cu contact system for good performance at low cost.