Angel Uruena

On the blistering of atomic layer deposited Al 2 O 3 as Si surface passivation

This work proves that blistering is the partial delamination of a thick enough Al<inf>2</inf>O<inf>3</inf> layer caused by gaseous desorption in the Al<inf>2</inf>O<inf>3</inf> layer upon thermal treatments above a critical temperature: the Al<inf>2</inf>O<inf>3</inf> layer acts as a gas barrier and bubble formation occurs. First, using an atmospheric pressure rapid thermal processor with an atmospheric pressure ionization mass spectrometry, desorbing species upon heating of Si/Al<inf>2</inf>O<inf>3</inf> samples are identified: evident desorption peaks are observed around 400 °C for all spectra. The spectrum for m/e = 18, an indication of H<inf>2</inf>O, illustrates that gaseous desorption from Al<inf>2</inf>O<inf>3</inf> and from the Si substrate itself continues up to 600 °C and 700 °C, respectively. Also, it is shown that in the case of a 30 nm Al<inf>2</inf>O<inf>3</inf> layer, blistering starts at same annealing temperatures as gaseous desorption begins. In the case of a thin enough (< 10 nm) Al<inf>2</inf>O<inf>3</inf> film, blistering does not show. To complete the proof, elastic recoil detection measurements clearly show that after annealing a thick Al<inf>2</inf>O<inf>3</inf> film above 400 °C the H content is higher near the c-Si interface as compared to the near surface. Fortunately, effective lifetime and capacitance voltage measurements show that 5 to 10 nm Al<inf>2</inf>O<inf>3</inf> layers can still be adequate passivation layers after being annealed in N<inf>2</inf> environment at temperatures up to 500–700 °C: (i) interface trap densities (D<inf>it</inf>) can remain below 1×10¹¹ cm⁻² and (ii) fixed charge densities (Q<inf>f</inf>) stay negative and in the order of −3×10¹² cm⁻² Random local Al back surface field (BSF) solar cells, fabricated using a blistered film as rear surface passivation and no additional contact opening step, clearly show that random local BSFs are created upon firing of a blistered rear passivation layer covered by metal. Therefore, it is clear that blistering should be avoided, since it will reduce the overall rear surface passivation. The key to avoid blistering is using 5 to 10 nm Al<inf>2</inf>O<inf>3</inf> passivation layers and performing an annealing step prior to capping and co-firing. Al<inf>2</inf>O<inf>3</inf>/SiN<inf>x</inf> passivated local Al BSF p-type Si solar cells are made using an out-gassing step with temperatures up to 700 °C. For these cells, the reduction in blistering and hence improvement in rear surface passivation is clearly reflected in the gain in average Voc as a function of out-gassing temperature.

The impact of silicon solar cell architecture and cell interconnection on energy yield in hot & sunny climates

Extensive knowledge of the dependence of solar cell and module performance on temperature and irradiance is essential for their optimal application in the field. Here we study such dependencies in the most common high-efficiency silicon solar cell architectures, including so-called Aluminum back-surface-field (BSF), passivated emitter and rear cell (PERC), passivated emitter rear totally diffused (PERT), and silicon heterojunction (SHJ) solar cells. We compare measured temperature coefficients (TC) of the different electrical parameters with values collected from commercial module data sheets. While similar TC values of the open-circuit voltage and the short circuit current density are obtained for cells and modules of a given technology, we systematically find that the TC under maximum power-point (MPP) conditions is lower in the modules. We attribute this discrepancy to additional series resistance in the modules from solar cell interconnections. This detrimental effect can be reduced by using a cell design that exhibits a high characteristic load resistance (defined by its voltage-over-current ratio at MPP), such as the SHJ architecture. We calculate the energy yield for moderate and hot climate conditions for each cell architecture, taking into account ohmic cell-to-module losses caused by cell interconnections. Our calculations allow us to conclude that maximizing energy production in hot and sunny environments requires not only a high open-circuit voltage, but also a minimal series-to-load-resistance ratio.

Large-Area n-Type PERT Solar Cells Featuring Rear p + Emitter Passivated by ALD Al 2 O 3

We present large-area n-type PERT solar cells featuring a rear boron emitter passivated by a stack of ALD Al₂O₃ and PECVD SiO_x. After illustrating the technological and fundamental advantages of such a device architecture, we show that the Al₂O₃/SiO_x stack employed to passivate the boron emitter is unaffected by the rear metallization processes and can suppress the Shockley-Read-Hall surface recombination current to values below 2 fA/cm², provided that the Al₂O₃ thickness is larger than 7 nm. Efficiencies of 21.5% on 156-mm commercial-grade Cz-Si substrates are demonstrated in this study, when the rear Al₂O₃ /SiO_x passivation is applied in combination with a homogeneous front-surface field (FSF). The passivation stack developed herein can sustain cell efficiencies in excess of 22% and V_oc above 685 mV when a selective FSF is implemented, despite the absence of passivated contacts. Finally, we demonstrate that such cells do not suffer from light-induced degradation.

Hydrogen Passivation of Laser-Induced Defects for Laser-Doped Silicon Solar Cells

Hydrogen passivation of laser-induced defects (LasID) is shown to be essential for the fabrication of laser-doped solar cells. On first-generation laser-doped selective emitter solar cells where open-circuit voltages were predominately limited by the full-area back surface field, a 10-mV increase and 0.4% increase in the pseudo-fill factor were observed through hydrogen passivation of defects generated during the laser doping process, resulting in an efficiency gain of 0.35% absolute. The passivation of such defects becomes of increasing importance when developing higher voltage devices and can result in improvements in implied open-circuit voltage on test structures up to 50 mV. On n-type PERT solar cells, an efficiency gain of 0.7% absolute was demonstrated with increases in open-circuit voltage and pseudo-fill factor by applying a short low-temperature hydrogenation process using only hydrogen within the device. This process was also shown to improve the rear surface passivation, increasing the short-circuit current of approximately 0.2 mA/cm2 of wavelengths from 950 to 1200 nm compared with that achieved using an Alneal process. Subsequently, an average efficiency of 20.54% was achieved.

Kerfless Epitaxial Mono Crystalline Si Wafers With Built-In Junction and From Reused Substrates for High-Efficiency PERx Cells

This paper proposes a kerfless wafer structure with built-in p-n junctions in n-type silicon wafers grown using Crystal Solars high throughput epitaxy technology. Compared with a conventional p-type emitter by boron diffusion, ion implantation, or epitaxy, the built-in p-type emitter has a reduced and uniform doping concentration and increased thickness. The epitaxially grown wafers and conventional Czochralski (CZ) n-type wafers were processed into solar cells. A best efficiency of 22.5% with epitaxially grown wafers was achieved, with a 6 mV gain in open-circuit voltage, suggesting a high wafer quality and superiority of the deep epitaxial emitter over a standard boron-diffused emitter. Substrate reuse associated with the kerfless epitaxy technology is studied as well, with respect to its impact on solar cell efficiency. The data suggest no degradation in cell efficiency due to substrate reuse.

Hydrogen passivation of laser-induced defects for silicon solar cells

Hydrogen passivation of laser-induced defects is shown to be essential for the fabrication of laser doped solar cells. On first generation laser doped selective emitter solar cells where open circuit voltages are predominately limited by the full area back surface field, a 10 mV increase and 0.4 % increase in pseudo fill factor is observed through hydrogen passivation of defects generated during the laser doping process resulting in an efficiency gain of 0.35 % absolute. The passivation of such defects becomes of increasing importance when developing higher voltage devices, and can result in improvements on test structures up to 25 mV. On n-type PERT solar cells, an efficiency gain of 0.7 % absolute is demonstrated with increases in open circuit voltage and pseudo fill factor by applying a short low temperature hydrogenation process incorporating minority carrier injection using only hydrogen within the device. This process is also shown to improve the rear surface passivation increasing the short circuit current density from long wavelengths 0.2 mA/cm2 compared to that achieved using an Alneal process. Subsequently an average efficiency of 20.54 % is achieved.

Laser ablation of AlOx and AlOx/SiNx backside passivation layers for advanced cell architectures

In this paper, we investigate laser ablation of aluminum oxide (AlOx) layers and AlOx/SiNx stacks. This laser ablation process is studied in order to be implemented in back side passivation ablation in PERC-type cell process flow. The objective of this work is to define laser conditions for selectively and locally ablating the layers and reducing the laser-induced damages in the Si and in the passivation layer. In addition, different laser ablation patterns have been tested in order to determine the best ablation conditions to reach high efficiency PERC-type cells. Different laser sources and parameters (pulse duration, wavelength, power…) were tested in order to ablate these layers. The quality of the openings was characterized by optical and electronic microscopies and the laser-induced damages were evaluated by QSSPC-calibrated lifetime mapping. Based on this characterization method, we have processed PERC-type cells with AlOx/SiNx back side passivation with different laser ablation patterns. This work allowed us to determine suitable laser conditions and ablation patterns for reaching higher efficiency for AlOx-backside passivated PERC-type cells.