Ilkay Cesar

Industrially feasible >19% efficiency IBC cells for pilot line processing

Interdigitated Back Contact (IBC) solar cells with >19% efficiencies have been fabricated using n-type silicon wafers and well-demonstrated high-volume solar cell process technologies alone. Excellent current collection is implied by Jsc values as high as 41.6 mA/cm2. High Pseudo Fill Factors (PFF) of above 81% and reduced Fill Factors (FF) of below 72%, suggest that the primary losses are due to series resistance. The process flow described is currently being transferred to a pilot production line for further process development.

Excellent rear side passivation on multi-crystalline silicon solar cells with 20 nm uncapped Al 2 O 3 layer: Industrialization of ALD for solar cell applications

Current bottlenecks for industrialization of Al2O3 deposited by Atomic Layer Deposition (ALD) for crystalline silicon solar cell applications are low growth rate and stability of thin and uncapped layers during co-firing. First results on the performance of a high throughput ALD proto-type, the Levitrack, are presented. Excellent passivation properties have been obtained after firing, for 12 nm thick films deposited on p-Cz (2.3 Ω.cm) with Seff <15cm/s (Δn=3×1015 cm−3). These layers are compatible with solar cells that operate at a maximum open-circuit voltage of 720mV. Furthermore, we report on the passivation of 20nm uncapped aluminum oxide layers on the rear of p-type mc-Si bifacial cells. LBIC measurements unveiled excellent passivation properties on areas covered by 20nm of Al2O3 characterized by an IQE of 91% at 980nm. Remarkably, these lifetime and cell results were obtained without lengthy post-treatments like forming gas anneal.

Industrial application of uncapped Al 2 O 3 and firing-through Al-BSF in open rear passivated solar cells

Current bottlenecks for industrial application of Al<inf>2</inf>O<inf>3</inf> deposited by Atomic Layer Deposition (ALD) on open rear passivated solar cells are low growth rate, firing stability of thin and uncapped layers, and Firing-through BSF formation during co-firing. Long term stability results on the performance of a high throughput ALD prototype, the Levitrack, are presented. Excellent passivation properties have been obtained after firing: for 12 nm thick films deposited on p-Fz (1.8 Ω·cm) a T<inf>eff</inf> of 2.2 ms (Δn=3×10¹⁵ cm⁻³) was obtained that remained constant over a period of 20 weeks. Furthermore, we report on the passivation quality of the firing-through BSF and Al<inf>2</inf>O<inf>3</inf> coating as function of wafer peak temperatures between 790 and 845°C. Bifacial cells covered for 50% Al contacts resulted in local BSF thicknesses of 5 to 7 μm and FF of 78% and 77% for the best mc and mono crystalline cell respectively. For this study 12 nm uncapped aluminum oxide layers are deposited on the rear of p-type mc and mono bifacial cells. LBIC measurements confirm literature reports that uncapped Al<inf>2</inf>O<inf>3</inf> passivation in general is heavily dependent on the wafer peak temperatures during firing in the investigated range.

All-side SiNx passivated mc-Si solar cells evaluated with respect to parasitic shunting

In search of solar cell concepts that allow processing thinner wafers (<150 micron), the conventional full Al rear side is replaced by an open rear metallization combined with a dielectric passivation layer. We show a gain of 2.1% (relative) in the product of Jsc×Voc, when we apply a passivated SiNx dielectric layer and local Al contacts on the rear of p-type mc-Si solar cells instead of a full Al-BSF. To achieve this gain, metallization designs of H-patterns and point-contacts were used with a rear coverage less than 8%. The gain in Jsc×Voc is an improvement over our previously reported results for open rear side cells with rear coverage of 14%. In addition, we propose a new method to quantify parasitic shunting to evaluate the efficiency potential of this cell concept. Experimental evidence shows that the parasitic shunting is one of the main limiting factors and the new method predicts a gain in Jsc and Voc in absence of this phenomenon. Omitting additional resistive losses of the open rear side cell compared to its full-coverage counterpart, we predict a gain in efficiency of about 0.6% absolute if parasitic shunting could be eliminated.

Improved performance of uncapped Al 2 O 3 and local firing-through Al-BSF in Bi-facial solar cells

Silicon solar cells that dominate todays market are H-pattern cells based on p-type silicon wafer material with a full Al Back Surface Field (BSF) as rear contact. ECNs rear passivated bi-facial PASHA (Passivated on all sides H- pattern) and ASPIRe (All Sides Passivated and Interconnected at the Rear, MWT) concepts answer the market pressure to decrease the euro/watt price and increase the efficiency. For optimized cells we estimate 0.5-0.8% absolute higher cell efficiencies compared to the industrial standard due to better rear passivation and reflection, while thinner wafers <;150um) can be processed with limited yield loss. In addition, Al paste consumption can be reduced by 50-70% owing to the open rear metallization. Here we report on the improved performance of PASHA cells passivated by an uncapped Al2O3 layer on the rear, through which Al paste is fired for contact and local aluminum BSF formation. The Al2O3 dielectric layer is deposited in the Levitrack, an industrial-type system for high-throughput Atomic Layer Deposition (ALD) developed by Levitech. On Cz and mc material, a gain in JscxVoc of 1% and 2.5% respectively is obtained compared to the reference, at a rear metal fraction of 30%. Localized IQE mapping shows that the passivation quality of the Al2O3 passivation layer is maintained after firing which is a major improvement as compared to our previous report. Furthermore, reliability tests on single cell laminates (Cz cells) suggest that the passivation layer remains stable during the lifetime of a module.

PASHA: A new industrial process technology enabling high efficiencies on thin and large mc-Si wafers

To maintain high efficiencies for solar cells and reduce the cell bowing, the full Al rear surface of thin conventional solar cells has to be replaced by a more suitable passivating rear surface layer. In our new PASHA-cell (Passivated on All Sides H-patterned cell) we apply a single silicon-nitride (SiNx:H) layer for rear surface passivation in combination with an open, firing through aluminum metallization. This improved processing results in a gain in efficiency of almost 1% absolute compared to full Al BSF, achieving 16.4% on 156 cm2, 200 μm thick mc Si solar cells. Besides this efficiency gain, due to lower consumption of aluminum, there will be a reduction in the costs of cell fabrication. Furthermore, the severe bowing of wafers thinner than 200 μm has been reduced to zero. To test the industrial stability, the processing was applied on a batch of large (243 cm2) and thin (160 μm) wafers which yielded an average cell efficiency of 15.5% with a maximum of 16.1%. This is 0.5% absolute better than the full Al rear reference, even though the rear metallization pattern has not been optimized yet.

Open-rear side H-pattern optimization based on 2D computer simulations

Currently, photovoltaic-cell manufacturers are tending to use thinner wafers in order to reduce material costs. A drawback is that thin wafers with a full aluminum (Al) rear coverage suffer from bowing and therefore have an increased chance of breakage. Another consequence of the thinner wafer is that the surface recombination velocity (SRV) of the rear side is getting more important for the overall cell performance. To overcome these problems an open rear side (i.e. partial metal coverage) can be used and the wafer rear surface can be passivated using an appropriate dielectric layer. In this paper, we will focus on a solar cell with silicon nitride passivation on the rear side and an Al H-patterned rear metallization with two bus bars. The aim is a first validation of the solar cell simulation software package Microtec against experiments and to get a better physics understanding of the open rear side cell. We performed a parameter study, where we calculated the standard output parameters like shortcut current (Isc), open-circuit voltage (Voc) and fill factor (FF) for varying device parameters. The parameter space that has been explored is composed of the finger pitch, the finger width, the back surface field (BSF) depth, the aluminum doping concentration in the BSF and the surface recombination velocity (SRV) of the SiNx in between the contacts.

Designing IBC cells with FFE: Long range effects with circuit simulation

IBC cells with Front Floating Emitter (FFE) pose different design challenges compared to more conventional IBC cells with FSF (Front Surface Field). The FFE enables hole transport over distances that are large compared to the typical BSF or emitter width. The core of the cell design is commonly a device simulation in which, because of the computer resources involved, typically one simulates an as small as possible, but representative part of the solar cell. In an IBC cell this corresponds to 1/2 of the BSF and 1/2 of the emitter. Such a unit cell does not account for important geometric features, such as busbars and pads, edges or interruptions in metallization fingers. We show how to construct an equivalent circuit for our Mercury FFE IBC cells to model features beyond the unit cell efficiently, taking into account the lateral hole transport in the FFE. We compare and calibrate the circuit model against device simulations with quokka.

Efficiency gain for Bi-facial multi-crystalline solar cell With uncapped Al 2 O 3 and local firing-through Al-BSF

The p-type bi-facial cell concept, p-PASHA (Passivated on all sides H-pattern), is developed at ECN and employs an uncapped Al2O3 passivation layer on the rear through which a screen printed H-pattern of aluminium contacts is fired. Here we report a net gain in cell efficiency of 0.2% absolute for the p-PASHA cell vs. industrial reference with the addition of a clean and an ALD step. Even higher gains up to 0.5% abs. are expected after optimization of the cell design and process. Apart from the efficiency gain, the bi-facial cell concept allows for 50-80% reduction in Al paste consumption, the use of thinner wafers, and consists of less processing steps compared to prevalent PERC concepts. The Al2O3 dielectric layer is deposited in the Levitrack, an industrial-type system for high-throughput Atomic Layer Deposition (ALD) developed by Levitech. The efficiency gain is obtained on multi-crystalline wafers, at a rear metal fraction of 40%. Localized IQE mapping, cross-sectional SEM investigation, resistance measurements and 2D simulation relate the efficiency improvement compared to our conventional process to better eutectic and BSF formation at the Al contact edges.